INVESTIGATION OF THIN MOLECULAR FILMS BY SURFACE ENHANCED VIBRATIONAL SPECTROSCOPY

DISSERTATION

Presented in Partial Fulfillment of the Requirements for

the Degree Doctor of Philosophy in the Graduate

School of The Ohio State University

By

Hong Tian, M.S.

*****

The Ohio State University 2008

Dissertation Committee: Approved by Professor James V. Coe, Advisor

Professor Richard L. McCreery, Coadviser ______Professor Susan V. Olesik Advisor Graduate Program in Chemistry Professor Heather A. Allen

ABSTRACT

The use of surface enhanced vibrational spectroscopy to characterize structures,

investigate interactions at interface and measure thickness of nanocoatings has been

investigated.

The structural development of pyrolyed photoresist films (PPF) was characterized

by both IR and Raman spectroscopy. The physicochemical properties of these films

changed as a function of the pyrolysis temperature. PPF pyrolyzed at 1000 ℃ formed

highly defective nanocrystalline carbons with a significant contribution from various sp2, sp3 type carbons and aromatic rings of the original polymer.

Dilute solution of photoresist was used as an excellent starting material for the

manufacture of optically transparent pyrolyzed photoresist films (OTPPF). OTPPF offers

the possibility of sensitive, reproducible, and stable measurements in both UV-Vis and

infrared regions of the electromagnetic spectrum. It provides the information of structures

of molecules, interaction of adsorbed molecules and the substrate at air-solid interfaces.

A study of surface enhanced vibrational spectroscopy on 4-nitroazobenzene (NAB)

has been conducted. A new strategy of surface enhanced infrared absorption spectroscopy

(SEIRA) has been developed based on the extraordinary transmission effect of metal microarrays. The intensity and position of transmission resonances of double stacked Ni meshes can be tuned by rotating one mesh relative to the other. When the resonances shift ii to the vibrational frequencies of NAB, the corresponding absorption features are greatly enhanced.

Nanocoatings of TiO2 were applied to one side of free standing Ni meshes. Shifts, attenuation and broadening of the transmission resonances have been observed versus coating thickness. New modes have been developed to correlate the optical response of

Ni mesh with coating thickness. The Ni meshes exhibit potential as sensors for nanoscale coating thickness.

iii

DEDICATION

This work is dedicated to my family and friends. Thank you all for your love, understanding and encouragement.

iv ACKNOWLEDGMENTS

Many people have contributed to this dissertation. I would like to express my gratitude to them for their guidance, support and friendship during my graduate studies at the Ohio State University.

I am very grateful to my advisors, Dr. Richard L. McCreery and Dr. James V. Coe.

I have been fortunate to have two advisors, and both of them offered me guidance and support through these years. I would like to thank Dr. McCreery for his guidance and assistance in getting my graduate career started on the right foot and building my foundation as an analytical chemist. I especially want to thank his cooperation after he moved to University of Alberta. I am particularly grateful for being accepted into Dr.

Coe’s research group. Dr. Coe taught me how to question thoughts and express ideas. He continually stimulated my analytical thinking and interest in research. His patience and support helped me overcome many crisis situations and finish this dissertation.

I would also like to thank all of the members of both Dr. McCreery and Dr. Coe’s research group, especially Belinda Hurley, Haihe Liang, Jing Wu, Jeremy Steach, Aletha

Nowak, Franklin Anariba, William McGovern, Andrew Bonifas, Solomon Ssenyange,

Kenneth Rodriguez, Joseph Heer, Shannon Teeters-Kennedy, Katie Cilwa, and Marin

Malone. They listened and gave me good advices, offered a lot of help in experiments,

v assisted me in adjusting to a foreign country and a new culture, and provided genuine

friendship. I especially want to thank Kenneth Rodriguez and Haihe Liang, for their

cooperation, informative discussion and sharing data with me.

I am also grateful to Dr. Gordon Renkes and Dr. Lisa Hommel for teaching me

how to use their instruments and helping me with experiments. I would like to thank Dr.

Umit Ozkan and her students in the department of chemical engineering who were

willing to help me with Raman experiments even I was a stranger to them.

I extend many thanks to the faculty of this department who have provided good

graduate education to me. I would like to acknowledge graduate studies office, especially

Dr. David Hart, Dr. Robert Coleman, Judith Brown, Jennifer Hambach and Martha

McDowell, for their support and assistance. I am also thankful to all the staff in

instrumentation support group, machine shop, and computer support group for always

providing good services.

Many thanks to my friends, especially Rich Mendola, Hu Lan, Sophia Lee,

Saihong Jiang, Lanhao Yang, Xiaodan Su, Chen Ren, Yali Li, and Ya-Ting Kao. They

have helped me to overcome many difficulties during years of my graduate study at OSU.

I deeply appreciate their support and care. They have been such a blessing to me. There

are many people helped me during the past years not being mentioned in this

acknowledgment, I appreciate your kindness to me, and remember all of you.

Finally, I would like to thank my family. My parents, sister, brother-in-law and nephew have been a constant source of love, concern, joy, strength and support.

vi VITA

May 16, 1978...... Born – Xinxiang, China

1998...... B.S. Zhengzhou University

2001...... M.S. Lanzhou Institute of Chemical Physics, Chinese Academy of Science

2001 – 2002...... Research Associate, Shanghai Institute of Organic Chemistry, Chinese Academy of Science.

2002 – Present...... Graduate Teaching and Reasearch Asstiate, The Ohio State University.

PUBLICATIONS

Research Publications

1. J. V. Coe, J. Heer, S. Teeeters-Kennedy, H. Tian, K. R. Rodriguez. “Extraordinary transmission of metal films with arrays of subwavelength holes.” Annu. Rev. Phys.Chem., 2008, 59, 179-202.

2. H. Tian, A. J. Bergren, R. L. McCreery. “UV-vis Spectroelectrochemistry of Chemisorbed Molecular Layers on Optically Transparent Carbon Electrodes.” Applied Spectroscopy, 2007, 61(11), 1246-1253.

3. H. Liang, H. Tian, R. L. McCreery. “Normal and surface-enhanced raman spectroscopy of nitroazobenzene submonolayers and multilayers on carbon and silver surfaces.” Applied Spectroscopy, 2007, 61(6), 613-620.

4. K. R. Rodriguez, H. Tian, J. M. Heer, J. V. Coe. “Extraordinary Infrared Transmission Resonances of Metal Microarrays for sensing Nanocoating Thickness.” J. Chem.Phys.C, 2007, 111, 12106-12111.

vii 5. K. R. Rodriguez, H. Tian, J. M. Heer, S. Teeters-Kennedy, J. V. Coe. “Interaction of an infrared surface plasmon with an excited molecular vibration.” J. Chem.Phys., 2007, 126(15), 151101/1-151101/5.

6. J. V. Coe, K. R. Rodriguez, S. Teeeters-Kennedy, K. Cilwa, J. Heer, H. Tian, S.M. Williams. “Metal Films with Arrays of Tiny Holes: Spectroscopy with Infrared Plasmonic Scaffolding.” J. Chem.Phys.C, 2007, 111(47), 17459-17472.

FIELDS OF STUDY

Major Field: Chemistry

viii TABLE OF CONTENTS

P a g e

Abstract...... ii

Dedication...... iv

Acknowledgments ...... v

Vita ...... vii

List of Tables...... xii

List of Figures ...... xiii

List of Abbreviations...... xviii

Chapters

1. Introduction ...... 1

1.1 Surface Properties of Pyrolyzed Photoresist Films (PPF) ...... 2 1.2 Optically Transparent Electrodes (OTE) ...... 6 1.3 Extraordinary Transmission of Sub-Wavelength Hole Arrays...... 8 1.3.1 What are surface plasmons ...... 8 1.3.2 Extraordinary transmission through sub-wavelength hole arrays. .. 10 1.3.3 Metal films with periodic sub-wavelength hole arrays...... 12 1.3.4 SP dispersion curves and front-back coupling of metallic arrays of sub-wavelength holes...... 15 1.4 Raman spectroscopy ...... 19 1.4.1 Surface Raman spectroscopy ...... 19 1.4.2 Resonance Raman spectroscopy ...... 21 1.5 Surface enhanced Infrared absorption (SEIRA) ...... 23 1.6 Research objectives ...... 25

ix 2. Characterization of Carbon Film Evolution with Temperature by Infrared and Raman Spectroscopy ...... 38

2.1 Introduction...... 38 2.2 Experimental...... 40 2.3 Raman Spectroscopy of Photoresist Evolution...... 43 2.4 Infrared Spectroscopy of Photoresist Evolution...... 47 2.5 Characterization of Pyrolyzed Photoresist Films Using the Extraordinary Infrared Transmission of Metallic Arrays of Sub-wavelength Aperture . . . .54 2.6 Formation Mechanism of Pyrolyzed Photoresist Films ...... 58 2.7 Conductivity of Pyrolyzed Photoresist Films ...... 59 2.8 Comparison of Pyrolyzed Photoresist Films with Evaporated Carbon Films ...... 59 2.9 Conclusions...... 64

3. Optically Transparent Pyrolyzed Photoresist Film and its Application in Electrochemistry, Uv-Vis and Infrared Spectroscopy...... 73

3.1 Introduction...... 73 3.2 Experimental...... 74 3.3 Optical Properties of Optically Transparent Pyrolyzed Photoresist Films. .. .81 3.4 UV-Vis Spectra of Molecules Chemisorbed onto Optically Transparent Pyrolyzed Photoresist Films ...... 84 3.5 Free and Adsorbed Molecular Spectra ...... 86 3.6 IR Spectroscopy of Chemisorbed Molecules on Optically Transparent Pyrolyzed Photoresist Films...... 90 3.7 Conclusions...... 97

4. Surface Enhanced Vibrational Spectroscopy of 4- Nitroazobenzene ...... 100

4.1 Introduction...... 100 4.2 Experimental...... 101 4.3 UV-Vis Spectra of NAB on Various Substrates ...... 104 4.4 Raman Spectra of NAB on Various Substrates ...... 106 4.5 Infrared Spectra of NAB on Single Mesh...... 109 4.6 Infrared Spectra of NAB on Double Mesh Stacks ...... 111 4.7 Conclusions...... 121

x 5. A New Method of Sensing TiO2 Nanocoatings by Extraordinary Infrared Transmission Resonance of Metal Microarrays...... 126

5.1 Introduction...... 126 5.2 Experimental...... 128 5.3 Characterization of Deposited TiO2 films...... 129 5.4 Resonance Response of Ni mesh Before Coating...... 131 5.5 Resonance Response of Ni mesh toTiO2 Nanocoatings...... 135 5.6 Determination of Shifts in Momentum Space ...... 142 5.7 Comparison to SP-ATR Theory ...... 144 5.8 Calibration Curve Based on Sigmoidal Function ...... 148 5.9 Molecules on TiO2 ...... 154 5.10 Conclusions...... 156

Bibliography...... 160

Appendix

A. Journal Publications ...... 184

xi

LIST OF TABLES

Table Page

2.1 Raman features of photoresist films after heat treatment ...... 46

2.2 Raman features around 1180 cm-1 and possible interpretations of these features . . . 47

2.3 IR absorption features of photoresist (PR) films after heat treatment and their possible assignments ...... 53

2.4 Sheet resistance of photoresist films pyrolyzed at various temperatures ...... 61

3.1 Values for λmax, the transmission (%T) at λmax, and calculated thickness from spectral simulations as a function of photoresist dilution...... 82

3.2 Observed λmax for chemisorbed, solution, and solid molecules...... 90

3.3 Vibrational frequency and assignment for nitroazobenzene ...... 94

4.1 Experimental and calculated frequencies and assignments of 4-nitroazobenzene. . 119

5.1 Transmission resonance peak centers before and after TiO2 coatings ...... 137

5.2 Transmission resonance peak centers, FWHM and heights (% transmission relative to No mesh in the spectrometer) before and after TiO2 coatings ...... 139

5.3 Transmission resonance (1,0)- peak centersbefore and after TiO2 coatings ...... 151

xii LIST OF FIGURES

Figure Page

1.1 Optical constants n (A) and k (B) for polished GC (solid circles), “bulk” (i.e., non- transparent) PPF (open circles), and literature values for GC (open triangle) as a function of wavelength ...... 4

1.2 Schematic diagram of a UV-Vis spectroelectrochemical system, in which WE is working electrode, RE is reference electrode, CE is counter electrode...... 7

1.3 An illustration of surface plasmons at interface between a conductor and an insulator ...... 9

1.4 Dispersion curves for a surface plasmon polariton and a photon in free space ...... 11

1.5 Zero-order transmission spectra of an Ulrich[79] grid with L = 101 μm (green curve), a Coe group grid with L = 12.7 μm (blue curve), and an Ebbesen group[73] grid with L = 0.90 μm (red curve, intensity multiplied by a factor of 8), where L is the lattice spacing ...... 13

1.6 Zero-order FTIR transmission spectrum of a Ni mesh with square holes on a square lattice. Inset is scanning electron microscope (SEM) images ...... 14

1.7 Dispersion diagrams showing the SP dispersion curves (red), the light line (dotted), and region that is accessible by varying the angle of incident light. A. Dispersion diagram for a smooth air/metal interface. B. Using of a prism with a thin metal coating gives rise to additional region (green) accessible by light. C. 1-D array of slits or grating. Periodicity projects the SP dispersion curve in units of 2π/L yielding the orange curves. D. 2D arrays of holes or bi-grating...... 16

1.8 Measured positions of transmission resonances (left) of a single Ni mesh as shown in Figure 1.5 and modeled dispersion curves with the same color coding as in Figure 1.6D...... 18

1.9 Analogy of a stack of two SP-ATR prisms from the work of Welford and Sambles to a single piece of mesh...... 20

xiii 2.1 Components of AZ P4330-RS photoresist ...... 39

2.2 Raman spectra of carbon films after pyrolyzed at (a) 500℃, (b) 700℃, (c) 900℃ and (d) 1000℃...... 44

2.3 IR spectra of photoresist films at (a) room temperature (R.T.) and after pyrolyzed at (b) 100℃ and (c) 300℃...... 48

2.4 IR spectra of photoresist films after pyrolyzed at (a) 500℃, (b) 700℃, (b) 900℃ and (d) 1000℃...... 49

2.5 Calculated thickness of photoresist film (z) as a function of heat-treatment temperature and the corresponding residual weight ({)...... 51

2.6 IR absorption spectra of photoresist films at (a)room temperature (R.T.) and after pyrolyzed at (b) 100℃,(c) 300℃, (d) 500℃, and (e) 700℃...... 52

2.7 IR transmission spectra of photoresist film after pyrolyzed at 1000 ℃...... 55

2.8 IR spectra of zoomed C-H stretch region of photoresist pyrolyzed at 1000 ℃. . . . . 56

2.9 IR absorption spectra of zoomed C-H stretch region of photoresist pyrolyzed at 1000 ℃...... 57

2.10 A possible model for pyrolyzed photoresist films after heat-treatment at 1000 ℃. 60

2.11 Raman spectra of pyrolyzed photoresist films (PPF) after heat-treatment at 1000℃ and evaporated carbon film...... 62

2.12 IR absorption spectra of the C-H stretching region of evaporated carbon films, three different areas of pyrolyzed photoresist films (PPF) after heat treatment at 1000℃, photoresist films after pyrolysis at 300℃, and photoresist films at room temperature...... 63

3.1 Schematics of OTPPF on quartz: Design 1 (top) utilized a gold area adjacent to the OTPPF, while design 2 (bottom) tested a gold border surrounding the OTPPF to reduce ohmic potential errors...... 76

xiv 3.2 Experimental absorbance spectrum of OTPPF (open circles) on quartz prepared using a 5% (v/v) solution of photoresist, with air as reference. The solid line is the calculated spectra using the optical constants for PPF determined with VASE and a 4.8 nm thickness parameter (path length) ...... 83

3.3 (A) UV-Vis absorption spectra of NAB/OTPPF/quartz (dotted curve), with air as a reference. The spectrum for the same OTPPF/quartz sample before bonding of NAB is also shown (solid curve). (B) Absorbance spectra of NAB on OTPPF obtained by subtracting the absorbance of NAB/OTPPF/quartz from that for the same OTPPF/quartz sample prior to modification. The dotted curve (design 1) is for deposition of the Au area adjacent to the OTPPF (see Figure 3.1), while the solid curve (design 2) is for the Au border (see Figure 3.1). Design 2 was used in (A) .. . 85

3.4 UV-Vis spectra of (A) 1 x 10-5 M NAB in cyclohexane, with cyclohexane as a reference, (B) chemisorbed NAB (4 nm thick) on OTPPF, obtained as described for Figure 3.3B (The solid lines are experimental data and the solid circles are peak fitting), and (C) solid NAB film on quartz, prepared by drop casting from ether solution (unknown thickness) ...... 87

3.5 UV-Vis spectra of (A) 1 x 10-4 M AB in cyclohexane, chemisorbed AB (3.2 nm thick) on OTPPF (The solid lines are experimental data and the solid circles are peak fitting), and solid AB on quartz unknown thickness; (B) 1 x 10-4 M NBP in cyclohexane, chemisorbed NBP (1.4 nm thick) on OTPPF, and solid NBP on quartz unknown thickness ...... 89

3.6 IR transmittance spectra of chemisorbed NAB monolayer and multilayer on OTPPF ...... 92

3.7 FT-IR absorption spectra of (a) chemisorbed NAB monolayer on OTPPF, (b) chemisorbed NAB multilayer on OTPPF, and (c) solid NAB in a KBr pellet ...... 93

3.8 FT-IR absorption spectra of (a) chemisorbed NBP on OTPPF, and (b) chemisorbed AB on OTPPF...... 96

4.1 UV-Vis absorption spectra of 1°10-5 M NAB in cyclohexane solution, NAB on PPF, NAB on Ni film, and NAB on Ag film ...... 105

4.2 Raman spectra of NAB in CCl4 solution, NAB on PPF, NAB on Ni film, and NAB on Ag film ...... 107

xv 4.3 IR transmission spectrum of Ni mesh at perpendicular incidence before (dotted trace) and after (solid trace) deposited with NAB multilayer...... 110

4.4 Illustration of Ni mesh converts the photon energy to SPs traveling along (a) the metal surface on single mesh and (b) between the two layers of metal on two mesh stack...... 112

4.5 IR transmission spectra of two pieces of NAB coated Ni mesh stacked at various angles...... 114

4.6 The image of double stacked NAB coated Ni meshes when the absorption features of NAB were observed by FT-IR spectrometry...... 115

4.7 IR absorption spectra of two pieces of NAB coated Ni mesh stacked at various angles...... 116

4.8 IR absorption spectra of double stack NAB coated Ni meshes normalized to the intensity of the 1365 cm-1 band ...... 117

4.9 IR absorption spectra of (a) solid NAB in a KBr pellet, (b) double stack of NAB coated Ni meshes...... 120

5.1 Photograph of TiO2 films with film thicknesses of 20, 40, 60, 80, 100, 120, 140 and 160 nm on Ni mesh ...... 130

5.2 Raman spectra of (a) 160 nm TiO2/Ni, (b) anatase powder and (c) rutile powder. . 132

5.3 SEM images of the dull side, shiny side, flank and a single hole of the mesh...... 133

5.4 Zero-order IR transmission spectra of the Ni mesh at perpendicular incidence before and after coated with TiO2...... 136

5.5 Zero-order IR transmission spectra of the Ni mesh at perpendicular incidence before and after successive coatings of TiO2 ...... 138

5.6 Shifts of transmission resonances vs. thickness of TiO2 coating relative to the uncoated position of the (1,0)+ resonance...... 140

xvi 5.7 Dispersion diagram showing the conversion of shifts in frequency space ( Δν~ ) to

shifts in momentum space ( Δk x ) in order to facilitate the comparison to SP-ATR work...... 143

' 5.8 The neff of Ag film with LiF coatings at different coating thickness versus natural logarithm of coating thickness ...... 150

' 5.9 The neff of Ni mesh with TiO2 coatings at different coating thickness versus natural logarithm of coating thickness...... 153

5.10 Enhanced IR absorption spectra of a variety of molecules on a 105 nm TiO2 coated Ni mesh ...... 155

xvii LIST OF ABBREVIATIONS

AB azobenzene

AFM atomic force microscopy

ATR attenuated total reflection

BAS bioanalytical systems

BWF Breit-Wigner-Fano

CCD charge coupled device

DNQ diazonaphthoquinone

EM electromagnetic

FTIR Fourier transform infrared

FWHM full width at half maximum

GC glassy carbon

HOMO highest occupied molecular orbital

IR infrared

ITO indium-doped tin oxide

LSP localized surface plasmons

LUMO lowest unoccupied molecular orbital

MCT mercury cadmium telluride

MEMS microelectromechanical systems

xviii MSM metal–semiconductor–metal

NAB 4-nitroazobenzene

NBP 4-nitrobiphenyl

NEMS nanoelectromechanical systems

NIR near-infrared

OTE optically transparent electrodes

OTPPF optically transparent pyrolyzed photoresist film

PPF pyrolyzed photoresist films

RAIRS Reflection Absorption Infrared Spectroscopy rms root mean square

SAM self assembled monolayer

SEIRA surface enhanced infrared absorption

SEM scanning electron microscopy

SERS surface enhanced Raman scattering

SEVS surface enhanced vibrational spectroscopy

SP surface plasmon

SPP surface plasmon polaritons

TEM transmission electron microscopy

TGA thermogravimetric analysis

UV ultraviolet

XPS X-ray photoelectron spectroscopy

xix CHAPTER 1

INTRODUCTION

Vibrational spectroscopy is sensitive to the chemical nature and environment of

species adsorbed to surfaces, and can be used as a promising tool for studying surface-

mediated chemical species and reactions. Infrared absorption and Raman scattering are

utilized to detect vibrations in molecules. As optical methods, they are applicable to a

diverse set of conditions such as solids, liquids, vapors, in bulk, as surface layers, as

microscopic particles, in ground or excited states. The techniques have far-ranging

applications and provide solutions to numerous challenging analytical problems.

New methods based on the processes and principles of vibrational spectroscopy

have been developed, and are described in this dissertation. The purpose of this

introductory chapter is to provide sufficient background on subjects of surface vibrational

spectroscopy in the context of the investigation presented in this dissertation. Chapter 1

commences with the background information on pyrolyzed photoresist films (PPF),

which includes its preparation, structure, physical properties and application. After that,

optically transparent electrodes (OTEs) are briefly reviewed. The chapter continues with the introduction of extraordinary transmission of sub-wavelength hole arrays. Then,

1 mechanisms of resonance Raman spectroscopy (RRS) and surface enhanced infrared

absorption (SEIRA) are described. Finally, an outline of the research objectives of this

dissertation is presented.

1.1 Surface Properties of Pyrolyzed Photoresist Films (PPF)

Carbon materials have a wide range of applications in electrochemistry due to

their broad potential window, mechanical stability and low cost[1-5]. They have been

utilized in battery, fuel cells and electrochemical sensors. The precursors of carbon are

very versatile, and include biomass, petroleum, hydrocarbon gases and natural products.

Carbon materials are generally formed from these materials by thermal treatment[6-13].

Over the last two decades, the use of photoresist in the integrated circuit industry for

microfabrication has stimulated the development of carbon materials from

photoresist[14-20]. The advantage of starting with photoresist is the reproducible

behavior and the ability to form microstructures by photolithography techniques. Hence, carbon microstructures produced by pyrolyzing photoresist provide a reliable source of microelectrode fabrication.

Pyrolyzed photoresist films (PPF) are usually prepared by spin-coating

photoresist on a clean and polished substrate, such as quartz, silicon or silicon nitride,

followed by pyrolysis at high temperatures in an oxygen-free environment. By changing the pyrolysis temperature, time and environment, carbon films formed from the pyrolysis of photoresist result in a wide variety of shapes, resistivities and mechanical properties[14, 19-22].

PPF produced by a positive resist AZ4330 has been investigated extensively by

2 the McCreery group[19, 23, 24]. The pyrolysis was carried out in a closed quartz furnace

in a forming gas (95%N2, 5%H2) atmosphere. Several techniques, including

thermogravimetric analysis (TGA), four-point probe measurements, scanning electron

microscopy (SEM), transmission electron microscopy (TEM), atomic force microscopy

(AFM), Raman spectroscopy, X-ray photoelectron spectroscopy (XPS) and ellipsometry

were used to characterize the surface properties of PPF[19, 23].

Pyrolysis of photoresist resulted in a considerable loss of mass as indicted by

TGA measurements[19], with only about 31% of the original mass remaining after pyrolysis to 1200 ℃. After pyrolysis at 1000 ℃ in nitrogen, 20% of the original film

thickness remained. Sheet resistance of carbon films pyrolyzed between 700 and 1100 ℃

were between 845.8 and 51.2 Ω/cm[19]. A pyrolysis temperature of 1000 ℃ was chosen

to obtain highly conductive carbon films.

SEM and TEM images of PPF show a pore-free amorphous film[19]. Tapping

mode AFM images indicate that PPF has a nearly atomically smooth surface with root

mean square (rms) roughness less than 0.5nm[23]. This film is smooth enough to be used

as a substrate for molecular monolayers. The characteristic carbon bands at ~ 1360 cm-1

(disorder band, D) and ~ 1600 cm-1 (graphitic band, G) of Raman spectra confirm the

formation of amorphous graphitic carbon films[19].

The values of refractive index (n) and extinction coefficient (k) of PPF verse λ

were determined by ellipsometry to characterize its optical properties. A plot of n and k

as a function of wavelength for bulk PPF (open circles), GC (filled circles) and literature

values for GC[25] (triangles) is shown in Figure 1.1. The curves of all three cases are

similar in shapes, but the magnitude over most of the spectral range examined is 3

Figure 1.1: Optical constants n (A) and k (B) for polished GC (solid circles), “bulk” (i.e., non-transparent) PPF (open circles), and literature values for GC[25] (open triangle) as a function of wavelength[26].

4 significantly different. Variations in surface roughness produced by hand polishing of the

GC could cause the differences between the literature values[25] and more recent

results[26]. The offsets of the data for PPF relative to GC may be attributable to several factors, including surface roughness, the size and/or distribution of graphitic crystal

grains, and the types and densities of functional groups at the surfaces, etc.

PPF exhibits electrochemical behavior similar to that of glassy carbon, but has a

lower capacitance, background current and oxygen/carbon atomic (O/C) ratio as

compared to glassy carbon[19]. Like glassy carbon and highly ordered pyrolytic graphite,

PPF surfaces can be chemically modified by diazonium salt reduction to yield a

covalently attached monolayer or multilayer[27-29]. Reduction of the diazonium salts

can be carried out by cyclic voltammetry or controlled potential electrolysis in a medium

of acetonitrile or aqueous acid (H2SO4 or HCl, PH < 2)[30, 31]. The mechanism of this

process is as follows:

+ - • R-C H N + e → N + R-C H (1) 6 4 2 2 6 4

• + R-C H → -C H -R (2), 6 4 6 4 carbon surface

where R is a para substituent[32]. The electrochemical reduction of a diazonium ion salt

produces free N2 and a phenyl radical, which irreversibly binds to carbon surfaces via a

carbon-carbon bond. These chemisorbed reactions have been confirmed by cyclic

voltammetry[33, 34], XPS[29, 32] and Raman spectroscopy[24, 29, 32]. Such a modification has been utilized in the preparation of analyte-specific electrodes for various electroanalytical applications[33-37] and the fabrication of molecular electronic

5 junctions[38-45].

1.2 Optically Transparent Electrodes (OTEs)

Optically transparent electrodes (OTEs) have been developed previously for simultaneous spectroscopic and electrochemical studies[46]. The interest in investigating

in situ electrochemically generated species has stimulated the application of OTE. A

variety of OTEs have been developed to meet the requirements for a wide range of

applications. Typical OTEs consist of a transparent substrate (glass or quartz) coated with

indium-doped tin oxide (ITO)[47, 48], or thin metal films of Au[49-52] and Pt[53]. Thin

metal meshes and grids have also been employed[54]. OTEs based on germanium

plates[55, 56], doped diamond[57] or carbon films[58] have been used for IR studies.

By coupling a spectroscopic technique to an electrochemical experiment, spectroelectrochemistry provides information on short-lived unstable intermediates which

are unattainable with normal chemical synthesis. Figure 1.2 shows a schematic diagram

of a computer controlled potentiostat system coupled to an electrochemical cell and a

conventional optical spectrometer. Electrochemically generated intermediates can be

monitored by transmission spectroscopy or internal reflection spectroscopy. As a key

element in spectroelectrochemistry, OTEs have been applied in the studies of reactions

kinetics[59-61], reaction mechanisms[62, 63], identification of reaction intermediates[64-

66], and physiochemical information (such as number of electrons transferred, molar

absorptivities and diffusion coefficients of electrogenerated intermediates or

products)[46].

6

electrolyte Light CE Source Sample Beam WE

RE

Potentiostat

Figure 1.2: Schematic diagram of a UV-Vis spectroelectrochemical system, in which WE is working electrode, RE is reference electrode, CE is counter electrode.

7 1.3 Extraordinary Transmission of Sub-Wavelength Hole Arrays

Surface plasmon polaritons (SPP) and localized surface plasmons (LSP) were

discovered and described in the early 1900s[67, 68]. The observations were connected to

the earlier theoretical studies[69, 70]. After Ritchie’s pioneering work in 1957[71], surface plasmons (SPs) were widely recognized in the field of surface science. Interest in this field has been renewed by recent advances in the optical near field of sub-wavelength dimensions[72, 73]. SPs and their applications will be described in the context of sub-wavelength optics in this section.

1.3.1 What are surface plasmons?

Surface plasmons, also known as surface plasmon polaritons, are collective excitations of conduction electrons which exist at the interface between two media whose

dielectric constants have opposite signs (Figure 1.3)[74]. The frequency-dependent SP

wave vector ksp for a smooth dielectric/metal interface is derived from Maxwell’s

equations under the appropriate boundary conditions, as shown by the following equation:

ω εε dm ksp = (1.1), c + εε dm

where ksp is the wave vector of the SP, ω/c is the wave vector in a vacuum, and εm and εd

are the wavelength dependent complex dielectric constants of metal and dielectric

material, respectively[74]. Surface plasmons (SPs) only exist if εm and εd are of opposite

' signs, and ε m is greater thanε d . This condition is satisfied in the infrared and visible

region of the electromagnetic spectrum by several metals, of which gold and silver are

the most frequently employed[73, 74]. Surface plasmons (SPs) create a field 8

Z

− z zk EZ ~ e

z Dielectric (εd > 0) ● E x + + + ---+ + + ---+ + + Z Hy Metal (εm < 0)

Figure 1.3: An illustration of surface plasmons at interface between a conductor and an insulator.

9 perpendicular to the surface which extends into both dielectric material and metal. The field is evanescent in nature, which decreases exponentially with increasing distance from the interface over a distance on the order of half the wavelength of the light into the dielectric material (Figure 1.3). The decay length into the metal is determined by the skin depth[72, 74].

Surface plasmons (SPs) can be excited by both electrons and light. However, they can not be excited directly by light because energy and momentum conservation cannot be obtained simultaneously. The dispersion curve for the SPs mode lies to the right of the light line (Figure 1.4[72]). The wave vector of surface plasmons (ksp) is greater than that of light wave of the same energy (klight = ω/c), which makes the direct excitation of a surface plasmon mode by light impossible. This can be overcome with the use of prisms, grating couplers or surface roughness [74].

1.3.2 Extraordinary transmission through sub-wavelength hole arrays

The phenomenon of extraordinary transmission through periodic arrays of sub-wavelength holes in metallic films has stimulated new research in optics and photonics since the publication of Ebbesen and co-workers in 1998[75]. In this phenomenon, the transmission of light can be several orders of magnitude greater than that predicted for apertures which are significantly smaller than the optical wavelength[76]. For certain incident wavelengths, the transmitted fraction of the incident light exceeds the open fraction of the array. It is generally accepted that the extraordinary transmission is mainly due to the excitation of SPs at the metal surface[77-79].

Experimental verification of the extraordinary transmission effect has been reported from

10

Figure 1.4: Dispersion curves for a surface plasmon polariton and a photon in free space[72].

11 the far-infrared to the visible regions of the electromagnetic spectrum (Figure 1.5)[75, 80,

81]. Surface plasmon resonance can be tuned by changing the hole-to-hole spacing to

overlap with the electronic transition or molecular vibration at specific frequencies. The

following introduction will focus on surface plasmon enhanced transmission and its

application in the infrared region.

1.3.3 Metal films with periodic sub-wavelength hole arrays

The transmission of light through an aperture of perforated metallic film is extremely small when the diameter of the aperture is much smaller than the wavelength of the incident light. But when the sub-wavelength hole is surrounded by periodic corrugation, the incident radiation can couple to SPs, which leads to an enhanced transmission at wavelengths determined by corrugation period. Figure 1.6 shows a zero order Fourier transform infrared (FTIR) transmission spectrum for a Ni mesh (Precision

Eforming, Coutland, New York) with square holes on a square lattice[80]. The square holes are arranged with a hole-to-hole spacing L of 12.7μm in a 3μm thick Ni film. The

width of the hole is 6.5μm on the smooth side and taper off to 5.2μm on the rough side of

the mesh. The spectrum shows an enhancement of 3.4 (transmittance on the primary

resonance /open fraction of the mesh) on its primary transmission resonance. For

square-symmetry lattice, the approximate transmission resonance at perpendicular

incidence is given by

+ ji 22 ~ （ ） υmax = ' 1.2 , Lneff

' where i and j are steps along the reciprocal lattice and neff is the effective index of 12

L

Figure 1.5: Zero-order transmission spectra of an Ulrich[81] grid with L = 101 μm (green curve), a Coe group grid with L = 12.7 μm (blue curve), and an Ebbesen group[73] grid with L = 0.90 μm (red curve, intensity multiplied by a factor of 8), where L is the lattice spacing. A log scale on the abscissa illustrates the large range of frequencies that can be accessed by tuning L. The percentage of open area is plotted with a gray dotted line for each spectrum.

13

Figure 1.6: Zero-order FTIR transmission spectrum of a Ni mesh with square holes on a square lattice. Inset is scanning electron microscope (SEM) images of the shiny side, dull side, flank of the Ni mesh. The dotted line represents the percentage of the mesh’s open area[80].

14 refraction[79, 82]. The equation indicates that the transmission resonance can be tuned by

adjusting the hole-to-hole spacing L, which has been verified by experiments as shown in

Figure 1.5.

1.3.4 SP dispersion curves and front-back coupling of metallic arrays of sub-wavelength holes

Dispersion curves provide insight into the nature of SP enhanced transmission.

Since our interest is in the infrared spectroscopy, dispersion curves are plotted as wavenumber (υ~ = ω/2π) vs. momentum vector parallel to the surface along the ~ x-direction (kx = 2πυ sin θ, θ is the angle of the incident light relative to the surface

normal) as shown in Figure 1.7[83]. The blue region of the plot is accessible by changing

the angle of the incident light. The light line is represented by the black dotted line, at ~ which incident light is brought parallel to the surface (υ = kx/2π, θ = 90°). The dispersion

curve of a propagating SP is depicted as a solid red line. Figure 1.7A shows the momentum mismatch problem on a smooth air/metal interface, in which the SP dispersion curve lies to the right of the light line. Surface plasmons can not be excited on smooth metal by light at any angle because it has momentum greater than that of a photon on the light line at the same frequency. This problem can be circumvented by using prisms, gratings or surface roughness to couple light to SP. The experimental realization of SP by attenuated total reflection is shown in Figure 1.7B. A prism with a thin metal coating (~ 50 nm thick) has a refractive index larger than 1, which shifts the light line to lower slope. The increased region (green) is accessible by angle tuning of incident light, and overlaps with the SP curve. At fixed wavelengths, incident light can couple with SPs at specific angles. Metal film corrugated with a periodic pattern of slits can excite SPs 15

θ θ n1 = 1 n2 n2 > n1

Figure 1.7: Dispersion diagrams showing the SP dispersion curves (red), the light line (dotted), and region that is accessible by varying the angle of incident light. A. Dispersion diagram for a smooth air/metal interface. B. Using of a prism with a thin metal coating gives rise to additional region (green) accessible by light. C. 1-D array of slits or grating. Periodicity projects the SP dispersion curve in units of 2π/L yielding the orange curves. D. 2D arrays of holes or bi-grating. The y component of surface momentum give rise to the purple curve[83].

16 without a prism. The surface can transfer momentum in units of 2π/L, where L is the

periodicity of the grating. The momentum matching of 1-D grating is given as

2 ' ~ ⎛ i2π ⎞ 2 υπ xieff )( ⎜kkn x += ⎟ (1.3). ⎝ L ⎠

The periodicity of the grating projects the SP dispersion curve within the light line shown as orange curves in Figure 1.7C. The metallic arrays used in this work are periodic in two dimensions. Momentum can be gained in units of 2π/L in either the x or y directions. The

position of the propagating SP resonances for the bi-grating is given as

2 2 ' ~ ⎛ i ⎞ ⎛ j22 ππ ⎞ 2 υπ xieff )( ⎜kkn x += ⎟ + ⎜ ⎟ (1.4), ⎝ L ⎠ ⎝ L ⎠

where i and j are steps along the reciprocal lattice. The y component of momentum

projects into kx giving rise to additional purple curves in Figure 1.7D. The bi-grating

structure has a higher density of resonance than that of 1-D grating.

Because of the coupling of the propagating SPs between the front and back

surfaces of the mesh through the holes, there may be a splitting of each resonance when

the wavelength of the incident light is greater than the thickness of the mesh. Two SP

dispersion curves (Figure 1.8) are obtained when the measured transmission resonance

peak centers are projected by the bi-grating momentum matching equation (1.3). One lies

' on the light line (the “+” curve) with an neff ,+ = 1.000, and the other is displaced to higher

' momentum (the “-” curve) with an neff ,− = 1.061. The “+” and “-” resonances are due to

symmetric and asymmetric front-back coupling states. The intensity of “-” resonances are

much higher than that of “+” resonances (Figure 1.6). The front-back coupling of Ni

mesh is comparable to that of a stack of two SP-ATR prisms with wavelength spacing as 17

Figure 1.8: Measured positions of transmission resonances (left) of a single Ni mesh as shown in Figure 1.5 and modeled dispersion curves with the same color coding as in Figure 1.6D. The peaks were just projected outside the light line with assigned values of i and j. The resulting two dispersion curves illustrate the effect of front-back coupling of SPs through the holes of the mesh[83].

18 shown in Figure 1.9[83, 84]. A single SP-ATR exhibits only one reflectivity resonance.

When a second SP-ATR is placed within the electric field of the first, the two interfaces can couple together. As a result, the splitting of the SP resonance is observed in both reflection and transmission spectra. The similar phenomenon observed on SP-ATR prisms further proves the front-back coupling of the mesh through holes.

1.4 Raman Spectroscopy

Raman spectroscopy is a very good tool for the characterization of molecules on

the surfaces of metals and non-metallic solids. In this thesis, surface Raman spectroscopy

and resonance Raman spectroscopy have been used for analyzing the structures of molecules at surfaces.

1.4.1 Surface Raman spectroscopy

Raman spectroscopy is ideally suited for studying chemical structures at surfaces.

As a vibrational spectroscopy, it is sensitive to the chemical nature and environment of

adsorbed molecules. The spectra observed in most surface Raman experiments are very

similar to those obtained by traditional Raman. However, there are often differences in

the number of modes present and their intensities when covalent bonds are formed

between surface molecules and the immediately neighboring atoms of the substrate. The

modes observed by Raman are dictated by the symmetry of the molecules. Only modes

with nonzero components of the Raman polarizability tensor are Raman active.

Adsorption of molecules onto surfaces breaks the center of symmetry, which modifies the

selection rules. The mutual exclusion rule is no longer applicable upon the loss of the

19

Figure 1.9: Analogy of a stack of two SP-ATR prisms from the work of Welford and Sambles to a single piece of mesh. Two resonance features in both reflectance and transmission are due to coupling of between the two air-metal interfaces[83, 84].

20 center of symmetry, allowing some of the infrared active modes to appear in the surface

Raman spectrum. In principle, the surface selection rules should consider the molecule

and atoms complexed to it as a distinct entity. These simple surface selection rules have

been applied in determination of the orientation of molecules attached to the surface and

explaining the differences between surface Raman spectra and normal Raman spectra.

1.4.2 Resonance Raman spectroscopy

Raman scattering cross sections are rather small. Typically, only one in 106 of the incident photons is scattered inelastically, which limits the application of Raman spectroscopy for surface analysis. This limitation can be overcame by resonance Raman spectroscopy, whose scattering cross sections for molecules with electronic transitions near resonance with the incident laser frequency can be greater than nonresonant cross sections by up to six orders of magnitude.

To obtain resonance Raman scattering, a laser line is chosen which has an excitation frequency close to the electronic transition of a particular chromophoric group in a molecule. Under these conditions, the vibrational modes associated with that particular transition are selectively enhanced. The selectivity can be used for identifying vibrations of this particular chromophore in a complex spectrum or locating its electronic transitions in an absorption spectrum.

Theoretically, the intensity of a Raman band observed at υ0 -υmn is given by

4 2 Imn = constant I 00 −⋅⋅ mn ∑ αυυ ρσ )()( mn (1.5), ρσ

where m and n denote the initial and final states of the electronic ground state,

21 respectively. I0 is the intensity of the incident laser beam of frequencyυ0 . (υ0 −υmn ) is

the frequency of the scattered radiation. α ρσ )( mn represents the change in polarizability

α caused by the m → e → n transition, e represents an electronic transition state involved in Raman scattering, and ρ and σ are x, y and z components of the polarizability tensor.

This term is given by

1 MM enme MM enme α ρσ )( mn = ∑ ( + ) (1.6), h e em 0 iΓ+− e en υυυυ 0 iΓ++ e

where υem and υen are the frequencies corresponding to the energy differences between

the states subscribed and h is Planck’s constant. m is the ground vibronic state, e is a

vibronic state of an excited electronic state, and n is the final vibronic state of the ground

state. m and n are simply the initial and final states of the Raman scattering process. Mme, etc., are the electronic transition moments, such as

M = * dτψμψ (1.7). me ∫ σ em

Here, ψ m and ψ e are total wave functions of the m and e states, respectively, and μσ is

the σ component of the electric dipole moment. Γe is the band width of the eth state.

The iΓe term is called the damping constant, which is inversely proportional to the

lifetime of the state e. In normal Raman scattering, υ0 is selected forυ0 <<υem , so that the

energy of the incident beam is much smaller than that of an electronic transition. The

4 Raman intensity is proportional to 0 −υυ m )( under this condition. The denominator of the

first term in the brackets of equitation (1.6) can become very small as υ0 approachesυem .

This can result in an enormous increase in the intensity of the Raman bandυ0 −υm ,

22 leading to the resonance Raman effect[85].

The competitive process of Raman scattering and fluorescence process limits the

application of resonance Raman spectroscopy to all molecules. This technique has been

used for carotenoids, minerals, pigments, carbon nanotubes and biological systems.

Interested readers can read a recent review by Efremov[86].

1.5 Surface Enhanced Infrared Absorption (SEIRA)

After the discovery of the surface enhanced Raman scattering (SERS) effect, a

new field of surface enhanced spectroscopy was established. As infrared is

complementary to Raman in vibrational spectroscopy, the search of surface enhanced infrared spectroscopy was soon rewarded with success. In 1980, the first SEIRA was observed by Harstein and colleagues, who concluded the IR absorption of molecules chemisorbed on Ag and Au surfaces can be enhanced by a factor of up to 1000[87]. Since its discovery, experimental and theoretical studies have been conducted to understand the

nature and mechanism of SEIRA. As is the case with SERS, at least two different mechanisms, the electromagnetic and chemical mechanisms contribute to the

enhancement. The infrared absorption (A) may be written as

A∝ μ / Q 2 μ / Q Ε∂∂=Ε⋅∂∂ 22 cos2 θ (1.6),

where μ / ∂∂ Q is the derivative of the dipole moment with respect to a normal coordinate

Q, Ε is the electric field that excites the molecule, and θ is the angle between ∂μ / ∂Q and Ε [88]. A vibrational transition can be enhanced by a local field Ε loc which is much

greater than the incident fieldΕ . Experimentally, the electromagnetic field can be

enhanced by using substrates which support SPs. The observations of larger enhancement 23 of chemisorbed molecules than physisorbed molecules suggest chemical contributions to

SEIRA. The molecule-substrate interaction may influence the frequency, intensity and

shape of the observed infrared band which is determined by the partial derivative μ / ∂∂ Q [89]. Based on surface selection rules, chemisorbed molecules also

exhibit orientation effects[90]. The intensities of vibrations that give dipole changes

normal to the surface are enhanced, while those giving dipole changes parallel to the

surface are diminished.

Recent developments in the near field optics provide a new method for

experimental SEIRS. Strong enhancement of optical near-field was obtained by coupling

lattice vibrations (phonons) of polar dielectrics with infrared radiation[91]. The phonon

resonance of 20 nm SiC spherical particles occurs at 920 cm-1. The near-field signal

increases by a factor of 200 within 20 cm-1 of the resonance, and is 20 times greater than that obtained with a gold sample when it is at resonance. The IR absorption of anthracene on polar dielectric nanoparticles of silicon carbide and aluminum oxide was enhanced by about 100-fold when the system was illuminated at the surface resonant frequency[92].

The phonon resonance effect is responsible for the main mechanism of SEIRA in these experiments.

Based on the mechanisms discussed above, metals and semimetals,

semiconductors and dielectric materials may all be used as substrates for SEIRA. The

coinage metals (Ag and Au), which have been widely used in SERS are also employed in

SEIRA. Transition metals, which have been less used in SERS due to their strong

damping of plasmons in the visible and near-infrared regions, are predicted to have an

enhancement as strong as that of coinage metals for SEIRA[90]. Several recent

24 experiments have demonstrated the SEIRA effect on many other metals such as Cu[93],

Ni[94], Pt[95-97], Pd[94], Rh[98], Ir[99], Fe[100], Pt/Ru[101], Sn[102], Pb[103], and

Pt-Fe alloys[104].

1.6 Research Objectives

The overall objective of this research is to develop spectroscopic methods to

characterize structures of molecular thin films, investigate interactions at interface and

measure thickness of nanocoatings.

Chapter 2 presents the structural development of pyrolyed photoresist films (PPF),

which are characterized by both IR and Raman spectroscopy. The physicochemical

properties of these films changed as a function of the pyrolysis temperature are

investigated. The structure of PPF pyrolyzed at 1000 ℃ is compared with that of carbon

films made by evaporation of carbon fiber. These investigations provide important

information about the evolution process of photoresist and surface structures of carbon

films made from pyrolyzed photoresist.

The fabrication of optically transparent pyrolyzed photoresist films (OTPPF) is

described in chapter 3. The OTPPF would be expected to have special optical, physical

and electrical properties. The optical property of OTPPF has been examined by UV-Vis

spectroscopy. The application of OTPPF in characterization of molecular thin films by both UV-Vis and infrared spectroscopy has been conducted.

Surface enhanced vibrational spectroscopy (SEVS) is discussed in chapter 4 using

4-nitroazobenzene (NAB) as a model compound. Both electromagnetic and chemical

effects are investigated by SEVS for NAB. A new strategy of surface enhanced infrared 25 absorption spectroscopy has been developed based on the extraordinary transmission effect of metal microarrays.

Finally, a new method of sensing nanocoating thickness based on extraordinary

infrared transmission resonances of metal microarrays is presented in chapter 5.

Theoretical modes have been developed to correlate the optical response of Ni mesh with

coating thickness.

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37 CHAPTER 2

CHARACTERIZATION OF CARBON FILMS EVOLUTION WITH TEMPERATURE BY INFRARED AND RAMAN SPECTROSCOPY

2.1 Introduction

Carbon materials, ranging from bulk commodity carbons (e.g. coal, graphite,

diamond, etc.) to the specialty carbons (e.g. carbon nanotubes, carbon fibers, fullerenes,

etc.) have been widely used in industry because of their variety of crystalline structures and properties. Carbon is a very versatile engineering material and has more diversity in structures than silicon. Currently, carbon-based microelectromechanical systems (MEMS)

and nanoelectromechanical systems (NEMS), alone or in combination with other

materials, provide solutions for microelectronics and nanoelectronics that have been

mostly based on silicon[1].

Most carbon films used to date in Carbon-MEMS and Carbon-NEMS technology

are fabricated by pyrolysis of patterned photoresist. Common commercial photoresists

contain three basic ingredients: a diazonaphthoquinone (DNQ) type dissolution inhibitor,

a phenolic novolac resin and an organic casting solvent[2]. The components of a

commercial photoresist AZ P4330-RS are listed in Figure 2.1. The DNQ inhibits the

dissolution of the novolac resin, but accelerates dissolution with exposure to light,

38 Cresol novolac resin

Me H+ + H2C O

HO

HO Me

Diazonaphthoquinone sulfonic esters

O N 2

O O

N 2 N O S O 2

O

O S O O S O

O O C Ph O

1-methoxy-2-propanol acetate

OAc

Me CH CH OMe 2

Figure 2.1: Components of AZ P4330-RS photoresist.

39 when the dissolution rate increases even beyond that of pure novolac. The special

property of DNQ provides a photochemical means to image the photoresist by modifying the dissolution rate of the resist in aqueous alkaline developers. The novolac resin is a

film-forming ingredient which provides the physical properties required in the photoresist,

such as pattern stability, etch resistance and thermal properties. The organic solvent is

used to increase the fluidity of the photoresist to form uniform and smooth thin films by

spin coating.

Carbonaceous films obtained by pyrolyzing photoresist at various temperatures

may result in different structures, conductivity, and stability. Therefore, knowing the

crystalline structures and properties of these materials is very important for the

fabrication of carbon based MEMS and NEMS, as well as in applications of pyrolyzed

photoresist in electrochemistry[3-16] and molecular electronics[17-30].

2.2 Experimental

Carbonaceous Film Preparation

Carbonaceous films were prepared on a silicon wafer (thickness ~ 1mm) that has

a thin layer of Si3N4 (thickness ~ 1000Ǻ). The wafer was rinsed rigorously with nanopure water (18 MΩ-cm, Barnsted/Thermolyne, Dubuque, IA), isopropanol (Aldrich Chemical

Company, Inc., Milwaukee, WI), acetone (Aldrich) and acetonitrile (Aldrich), then dried with high purity argon. Positive photoresist AZ P4330-RS (AZ Electronic Materials,

Clariant Corp., Somerville, NJ) was spin-coated onto a piece of clean silicon wafer at

6000 rpm for 30 seconds on a spin coater (WS-400B-6NPP-LITE, Laurell Technologies,

Corp., North Wales, PA). The spin-coated sample was soft-baked at 90 ºC for 20 minutes,

40 then transferred into a tube furnace (Lindberg Blue M) fitted with a 1-inch diameter quartz tube. The tube was flushed with forming gas (95% nitrogen and 5% hydrogen) flowing at 100 ml/min throughout the pyrolysis and cooling processes. For each sample,

the temperature was ramped (10 ºC/min) to the final temperature 100 ºC, 300 ºC, 500 ºC ,

700 ºC, 900 ºC and 1000 ºC respectively, and held at the final temperature for 1 hour,

then cooled to the room temperature.

Carbon film on Ni mesh was prepared by spin coating 5% (v/v) photoresist AZ

P4330-RS solution (diluted by 1-methoxy-2-propanol acetate, Aldrich) on Ni mesh. The photoresist modified Ni mesh was sandwiched between two pieces of quartz plate. The sample was soft-baked at 90 ºC for 20 minutes, and then transferred into a tube furnace

(Lindberg Blue M) fitted with a 1-inch diameter quartz tube. The tube was flushed with forming gas (95% nitrogen and 5% hydrogen) flowing at 100 ml/min throughout the pyrolysis and cooling processes. Samples were heated at 10 ºC/min to 1000ºC, and held at the temperature for 1 hour, then cooled to the room temperature. A thin carbon film was formed on Ni mesh, but the Ni mesh could not be peeled off from the quartz.

Therefore, the Ni mesh and attached two pieces of quartz plate were used as a single sample for IR spectroscopy.

Raman Spectroscopy

Raman spectra were acquired with a custom built line-focused f/2 Raman

spectrometer (Chromex) and a back thinned CCD (Andor) in ambient atmosphere[31].

The focal line was approximately 5 mm × 50 µm. A 514.5 nm argon ion laser (Coherent)

was used as excitation with a power of 30 mW on the sample. The spectra obtained are

41 averages of ten 10-second integrations. The Raman shift axis was calibrated with

benzonitrile. Baseline correction and peak fitting were performed with Mathcad 13

(Mathsoft). All the fit parameters such as peak positions, heights and line widths were

allowed to vary. The combination of Breit-Wigner-Fano (BWF) and Lorentzian functions was used to fit all the peaks. The BWF line has an asymmetric line shape, which should arise from the coupling of a discrete mode to a continuum[32]. The BWF line shape is given by

2 I0 ωω 0 QΓ−+ ]/)(21[ I ω)( = 2 (2.1), ωω 0 Γ−+ ]/)(2[1

where I0 is the peak intensity, ω0 is the peak position, Г is assumed as the full width at

half maximum (FWHM) and Q-1 is the BWF coupling coefficient. The Lorentzian line

shape is recovered in the limit Q-1→0[33].

Infrared Spectroscopy

Infrared spectra of all samples made on silicon wafer were measured by a Bruker

Equinox 55 FT-IR spectrometer with a DTGS detector. The sample chamber was purged

-1 with N2 to remove air. The spectral range covered was 400-5000 cm . For each spectrum,

300 scans were collected at 4 cm-1 resolution in the transmission mode. The Bruker infrared microscope (A 590) with an MCT liquid N2 cooled detector was used in the transmission mode to record the spectra of carbon films on Ni mesh. For each spectrum,

1000 scans were collected at 4 cm-1 resolution.

42 2.3 Raman Spectroscopy of Photoresist Evolution

The great variety of carbon materials arises from the strong dependence of their

physical properties on the ratio of sp2 (graphite-like) to sp3 (diamond-like) sites. In

general, an amorphous carbon can contain any mixture of sp3, sp2 and even sp1

coordinated carbon atoms in a disordered network. A network of sp2-bonded carbons with various degrees of graphitic ordering gives rise to materials as diverse as microcrystalline graphite, glassy carbon and fullerene[34, 35].

Amorphous carbon films have been investigated intensively in the past by Raman

spectroscopy. These films either consist of completely sp2-bonded carbon or both sp2- and sp3-hybridized carbon, and hydrogen to passivate dangling bonds, varying with the

techniques and conditions used for the preparation[33, 36-42]. This thesis deals with the

preparation and characterization of carbon films made from a positive photoresist

material.

In order to analyze the structural development of carbon films, Raman spectra of

the AZ photoresist were acquired after pyrolyzing under various temperatures. The

Raman spectra exhibited multipeak structure, which are shown in Figure 2.2. The “D”

and “G” bands have been studied extensively, which are due to sp2 sites only. The G peak

has been assigned to scattering by optical zone-center phonons (E2g mode) in graphite[41,

43-46], which is due to the bond stretching of all pairs of sp2 atoms in both rings and

chains. The G peak of microcrystalline graphite is expected to occur at 1580-1600

cm-1[47]. The D peak at about 1350 cm-1 has been interpreted to result from scattering of

disorder activated optical zone-edge phonons, which is due to the breakdown of

symmetry occurring at the edges of graphite planes in sp2 carbon materials[48, 49]. This

43

d. 1000 ℃

c. 900 ℃

b. 700 ℃

Intensity (arb.unites)

a. 500 ℃

600 800 1000 1200 1400 1600 1800 2000 Raman Shift (cm-1)

Figure 2.2: Raman spectra of carbon films after pyrolyzed at (a) 500℃, (b) 700℃, (c) 900℃ and (d) 1000℃. The solid circles are the experimental data and the solid lines are peak fitting.

44 mode is forbidden in perfect graphite and only allowed in the presence of disorder[41].

The G peak was fitted by the Breit-Wigner-Fano (BWF) function[50] and the G band

position was taken as the maximum of BWF rather than its center[51]. The BWF line

shape was chosen because it provides a more effective representation of the asymmetric

broadening of the Raman modes in amorphous materials (due to bond-length and bond-

angle fluctuations)[50]. The D and other peaks were fitted using a Lorentzian function.

The combination of BWF and Lorentzian functions provides good fits for all the carbons

at all energies, which gives the positions and intensities of all the peaks (Table 2.1).

Figure 2.2 displays Raman spectra and least-square fitting results for photoresist

films heated at 500℃, 700℃, 900℃ and 1000℃. The unpyrolyzed photoresist and

photoresist pyrolyzed at lower temperature exhibited strong fluorescence, and their

Raman spectra were not obtainable at an excitation wavelength of 514.5 nm. For samples treated at 500℃ , five primary peaks were observed near 1188, 1259, 1365, 1432 and

1606 cm-1. As the heat treatment is increased from 500℃ to 1000℃ , the multiple-peaked

spectrum reduces to a three-peaked spectrum. The peak positions and relative intensities

are summarized in Table 2.1. From 500℃℃ to 1000 , all Raman spectra have a “G” band

at ~ 1600 cm-1 and a “D” band between 1339 and 1365 cm-1. The G band at ~ 1600 cm-1 was assigned to the vibration of all sp2 carbon clusters in both chain of double bonds and

aromatic ring configurations. The existence of a D band indicates the presence of

aromatic rings in amorphous carbon films[36, 40]. Peaks at 1432 cm-1 and 1259 cm-1

were assigned to CH2 deformation mode[52-54] and the stretching mode of Caryl-O[55,

56] respectively. The possible interpretations of peaks between 1150 and 1180 cm-1 are listed in table 2.2.

45 Curing temperature, ℃ Peak center, cm-1 fwhma, cm-1 Intensity

500 1188 32.69 1777

1259 49.95 2395

1365D 64.94 7774

1432 35.30 3219

1606G 34.54 16110

700 1193 26.22 141.6

1339D 91.45 1665

1601G 32.68 2166

900 1164 14.88 22.28

1348D 75.55 657.9

1603G 34.00 659.1

1000 1183 34.83 23.17

1351D 66.41 677.8

1604G 36.06 649.6 afull width at half-maximum; “D” and “G” refer to “D band” and “G band” respectively.

Table 2.1: Raman features of photoresist films after heat treatment.

46 Raman feature, cm-1 Possible interpretation Reference

1180 sp3-rich phase [47, 57, 58]

nanocrystalline diamond [59]

hexagonal diamond [47, 60]

C-H in plane bending [61]

1150 Breathing vib. of the ring [56]

C=C-C=C trans system [38, 62]

990-1290 in plane C-H deformation [63]

Table 2.2: Raman features around 1180 cm-1 and possible interpretations of these features.

The peak position and relative intensity change slightly with pyrolysis temperature. The variations in the width and intensity of the D and G bands imply that they are related to the presence of functional groups of the precursor material, and the growth and size of different carbon phases. Raman results indicate loss of molecular bands related to the precursor material of photoresist at 500 ~ 700℃, then not much change at higher temperature.

2.4 Infrared Spectroscopy of Photoresist Evolution

The IR spectra of photoresist pyrolyzed at different temperatures are shown in 47

1.5

1.2

0.9

Transmittance R.T. 100 ℃ 0.6 300 ℃

0.3 500 1000 1500 2000 2500 3000 3500 4000 4500 5000 Wavenumber (cm-1)

Figure 2.3: IR spectra of photoresist films at (a) room temperature (R.T.) and after pyrolyzed at (b) 100℃ and (c) 300℃.

48 1.4

1.2

1.0

c

ttan e 0.8 500 ℃

smi 700 ℃ 0.6 900 ℃

Tran 1000 ℃

0.4

0.2

0.0 500 1000 1500 2000 2500 3000 3500 4000 4500 5000 Wavenumber (cm-1)

Figure 2.4: IR spectra of photoresist films after pyrolysis at (a) 500℃, (b) 700℃, (b) 900℃ and (d) 1000℃.

49 Figure 2.3 and 2.4. The transmittance of the photoresist films decreases as the pyrolysis

temperature increases, which indicates the photoresist films become denser and less IR

transparent. The film thickness can be estimated from the interference fringe obtained by

IR spectra using the following equation:

N 1 d = (2.2), 2n −υυ 21

where

d = film thickness

n = refractive index of sample

N = number of fringes within a given spectral region

υ1, υ2 = maximum and minimum of values of the wavenumber region

The refractive index of photoresist is approximately 1.5[64]. The calculated film thickness at R.T., 100 ℃ and 300 ℃ are 4.0 μm, 3.5 μm and 2.1 μm respectively. The

decrease in film thickness follows a similar trend of weight loss as temperature increases,

which is shown in Figure 2.5. There is a significant weight loss of 12% and decrease in

film thickness of 48% from room temperature to 300 ℃, indicating the material becomes denser as temperature increases.

IR absorption spectra of photoresist films pyrolyzed at various temperatures are

shown in Figure 2.6. Their absorption peaks and possible assignments are listed in Table

2.3. At 100 ℃, the relative intensity of peaks due to >C=N=N, C=O and C-O-C

stretching and CH3, CH2 deformation decreases, which indicates that the solvent

(1-methoxy-2-propanol acetate) and diazonaphthoquinone inhibitor start to evaporate. At

300 ℃, the absorption peaks of >C=N=N and C-O-C stretching disappeared, implying 50

12.0

4.0 Residual Weight of the Sample (mg) Sample of the Weight Residual

11.5 3.5

3.0 11.0

Thickness (µm) Thickness 2.5

10.5 2.0

0 50 100 150 200 250 300 Temperature (℃)

Figure 2.5: Calculated thickness of photoresist film (z) as a function of heat-treatment temperature and the corresponding residual weight ({)[16].

51

0.004 AU 700 ℃

0.02 AU 500 ℃

0.1 AU 300 ℃

Absorbance 0.1 AU 100 ℃

0.2 AU R.T.

500 1000 1500 2000 2500 3000

Wavenumber (cm-1)

Figure 2.6: IR absorption spectra of photoresist films at (a) room temperature (R.T.) and after pyrolyzed at (b) 100℃, (c) 300℃, (d) 500℃, and (e) 700℃.

52

o.o.p.

(C-H) δ

(C-O-C) (C-O-C) ν

i.p.

(C-H) δ

(C-O) (C-O) ν eir possible assignments.

) 2 ) -1 ,CH 3 (CH

δ

r heat treatment and th Vibration modes (cm (C=C) (C=C)

ν

(C=O) (C=O) ν

, deformation; i.p., in plane; o.o.p., out of plane. δ photoresist (PR) films afte

(>C=N=N) ν ) 2 , stretch; ν ,CH 3

(CH ν

(=C-H) (=C-H)

ν ) ) 3012 2920,2861 2158,2117 1712 1597,1504 1469,1410,1378 1274 1469,1410,1378 2158,2117 1712 2920,2861 1597,1504 ) 3012 1193 1099 813 891 1070 1226 1438 1608 2918,2862 ) 3008 2919 1706 3048 ) 1736 1469,1378 1578,1502 1261 1207,1051 ) 3048 861 ) 1707 882

℃ ℃ ℃ ℃ ℃ ℃ PR PR 3012 1204,1078 2981,2933 1264,1244 1453,1410,1375 1598,1506 2160,2118 1113 1736,1709 815

Vibrational notation used: PR(100 PR(300 PR(500 PR(700 PR(900 Samples PR(1000 Figure 2.3: IR absorption features of 53

the complete evaporation of solvent and inhibitor. At 500 ℃, the relative intensity of

absorption peaks of =C-H and C=C stretching increases, which suggest the formation of

sp2 carbon. At 700 ℃, the strongest peak is due to C-H out of plane deformation, and

weak absorption peaks due to =C-H and C=O stretching were present. Above 700 ℃,

absorption features were not observed. A more sensitive technique is needed to detect the

surface structure of carbon thin films, so extraordinary IR transmission of carbon films on

metal grids was investigated.

2.5 Characterization of Pyrolyzed Photoresist Films Using the Extraordinary Infrared Transmission of Metallic Arrays of Sub-wavelength Aperture

As noted in section 1.3.3, periodic metal microarrays of sub-wavelength aperture

exhibit extraordinary infrared transmission. The phenomenon has been used to obtain

enhanced IR absorption of molecular species at metal surfaces[65-67]. Absorptions due

to the C-H stretching modes of SAMs of alkanethiol are at least 100-fold enhanced over

those reported in RAIRS studies[68-70]. In this study, Ni mesh with periodic arrays of

sub-wavelength aperture was used to investigate the C-H stretching modes of photoresist

films after pyrolysis at 1000 ℃. A piece of photoresist covered Ni mesh was sandwiched

between two pieces of quartz to avoid the deformation of mesh at high temperatures. The

IR transmission spectra of three locations on such a sample after pyrolysis at 1000 ℃ are shown in Figure 2.7. The flat region of the spectra is due to the IR cutoff of quartz below

2000 cm-1. Absorption peaks at ~ 2350 cm-1 and 3756 cm-1 are due to C=O antisymmetric

stretching of carbon dioxide and the antisymmetric stretching of water respectively[71].

The C-H stretch regions of the IR transmission spectra are expanded and shown in Figure 54

area 1

14

12

10 area 2

8

area 3 6

Transmittance (%) 4

2

0 500 1000 1500 2000 2500 3000 3500 4000 4500 5000 Wavenumber (cm-1)

Figure 2.7: IR transmission spectra of photoresist film after pyrolysis at 1000 ℃. Spectra obtained from three different areas of the same sample are displayed.

55 14.75

area 1

14.50

14.25 Transmittance (%)

14.00 2800 2900 3000 3100 3200 Wavenumber (cm-1)

11.25

11.00 area 2

10.75

10.50

(%) Transmittance 10.25

10.00 2800 2900 3000 3100 3200 Wavenumber (cm-1)

8.00

7.75 area 3

7.50

7.25

7.00 Transmittance (%)

6.75

6.50 2800 2900 3000 3100 3200 Wavenumber (cm-1)

Figure 2.8: IR spectra of zoomed C-H stretch region of photoresist pyrolyzed at 1000 ℃. Spectra obtained from three different areas of the same sample are displayed.

56 0.0400

area 3

0.030

0.020

Absorbance

0.010 area 1

area 2

-0.0001 2800 2850 2900 2950 3000 3050 3100 Wavenumber (cm-1)

Figure 2.9: IR absorption spectra of expanded C-H stretch region of photoresist pyrolyzed at 1000 ℃. Spectra obtained from three different areas of the same sample are displayed.

57 2.8. Their corresponding IR absorption spectra are shown in Figure 2.9. The difference in

absorption intensities indicates the nonuniformity of the photoresist film on Ni mesh. The

individual C-H stretch absorption peaks are centered at 2852, 2923, 2955 and 3014 cm-1, which are assigned to CH2 symmetric, CH2 asymmetric, CH3 symmetric and =C-H

stretching modes, respectively. Since the oscillator strength of different C-H stretching

modes is not necessarily equal, it is difficult to quantify the hydrogen content, not even

for estimation of the relative hydrogen content[72].

2.6 Formation Mechanism of Pyrolyzed Photoresist Films

From room temperature to 300 ℃,the decrease of intensities of peaks due to

>C=N=N, C=O, C-O-C stretching and CH3, CH2 deformation indicates the evaporation

of the solvent and diazonaphthoquinone inhibitor. The disappearance of absorption peaks

of >C=N=N and C-O-C stretching indicates the vanishing of solvent and inhibitor. At

500 ℃,domains of both novolac resin and graphite-type structures are present in the

Raman spectra. The IR absorption spectra show that peaks due to =C-H and C=C

stretching shift to higher frequency and their relative intensities increases, also implying

the formation of graphitic carbon rings. At 700 ℃,the Raman features associated with

the novolac resin have disappeared. The IR absorption due to the =C-H stretch and the

C-H out of plane deformation became more pronounced with increasing temperature,

suggesting the graphitic carbon rings are mainly terminated with H atoms. A weak

absorption at 1707 cm-1 indicates the adsorption of carbon-oxygen complexes (C=O) on

carbon surfaces. The IR absorption spectrum of a photoresist film pyrolyzed at 1000 ℃

58 contains CH2 symmetric, CH2 asymmetric, CH3 symmetric and =C-H stretching modes,

which implies the carbon atoms are terminated by methylene (CH2), methyl groups (CH3) or H atoms. A possible model for photoresist films pyrolyzed at 1000 ℃ is shown in

Figure 2.10, based on the spectroscopic observations. The pyrolyzed photoresist films

(PPF) consist of a sp2 carbon network with imperfect sites. The dangling bonds in the

amorphous structure are passivated by methylene, methyl groups and H atoms.

2.7 Conductivity of Pyrolyzed Photoresist Films

From 700 ℃ to 1000 ℃, the Raman spectra indicate that the pyrolyzed

photoresist films are mainly composed of sp2 carbon. The IR transmittance decreases

dramatically for the photoresist film pyrolyzed at 900 ℃ and above, which indicates the

carbon film becomes denser with increasing temperature. The conductivity of the

pyrolyzed photoresist films is expected to increase with heat-treatment temperature due

to the structural changes, which is consistent with the sheet resistance measurements as

shown in Table 2.4[16].

2.8 Comparison of Pyrolyzed Photoresist Films with Evaporated Carbon Films

Raman spectra of PPF and evaporated carbon films are show in Figure 2.11. The

Raman intensity ratio of ID/IG of PPF (1.04) is much greater than that of evaporated

carbon film (0.36), which suggests greater disorder of the sp2 carbons of the PPF. The

Raman spectrum of evaporated carbon films does not exhibit the band at ~1180 cm-1,

which confirms that the band is due to the in-plane C-H bending originated from the

aromatic rings of photoresist component (cresol novolac resin) instead of other forms of 59

… …

… …

… …

Figure 2.10: A possible model for pyrolyzed photoresist films after heat-treatment at 1000 ℃

60 Temperature of pyrolysis (℃) Sheet resistance (Ω/cm)

700 845.8

900 94.3

1000 57.1

Table 2.4: Sheet resistance of photoresist films pyrolyzed at various temperatures[16].

carbon as listed in Table 2.2.

The IR spectra of PPF and evaporated carbon films (Figure 2.12) both display the absorption bands of C-H stretching. The C-H stretching bands of PPF are composed of well resolved CH2 symmetric, CH2 asymmetric, CH3 symmetric and =C-H stretching peaks. Evaporated carbon films exhibit a much broadened band in the C-H stretching region, which results from the superposition of various absorption peaks arising from

m stretching vibrations of different sp CHn configurations, with n, m = 1-3. Due to the lack of correlation between the intensities of the C-H stretching bands and the hydrogen content, the IR absorption peaks cannot be used to determine hydrogen content in carbon films, not even for an estimation of the relative hydrogen content. Based on the CH stretching region in the IR spectrum, it seems that the evaporated carbon film is much more amorphous than PPF.

61

Evaporated carbon film

PPF

unites) (arb. Intensity

1200 1400 1600 Raman Shift (cm-1)

Figure 2.11: Raman spectra of pyrolyzed photoresist films (PPF) after heat-treatment at 1000℃ and evaporated carbon film. The solid circles are the experimental data and the solid lines are peak fitting.

62

Evaporated carbon film

PPF area 1

PPF area 2

PPF area 3

Absorbance Photoresist film after pyrolysis at 300 ℃

Photoresist film at room temperature

2500 2600 2700 2800 2900 3000 3100 3200 3300 3400

Wavenumber (cm-1)

Figure 2.12: IR absorption spectra of the C-H stretching region of evaporated carbon films, three different areas of pyrolyzed photoresist films (PPF) after heat treatment at 1000℃, photoresist films after pyrolysis at 300℃, and photoresist films at room temperature.

63 2.9 Conclusions

Positive photoresists, which are commonly used in the semiconductor industry, were deposited on silicon wafers by spin coating and then pyrolyzed in an inert environment to produce thin carbon films. Raman and IR spectroscopy were used to characterize the structural development of the film. The physicochemical properties of these films changed as a function of pyrolysis temperature. Raman spectra were obtained for samples heated above 500 ℃, which indicate the onset of carbonization. The origin of these Raman bands was discussed. PPF pyrolyzed at 1000 ℃ formed highly defective nanocrystalline carbon with a significant contribution from various sp2 and sp3 type carbons and aromatic rings of the original structure of the polymer. The IR experimental data show that photoresist based carbons undergo a smooth transition from a mostly novolac structure for heat treatment below 500 ℃ to a graphitized structure after pyrolysis at 1000 ℃.The structural information obtained from IR spectroscopy is consistent with Raman results. Compared with vaporized carbon films, PPF exhibited more ordered structures, with better definition of the CH stretch bands for PPF than for vaporized carbon. The aromatic starting materials of PPF provide precursor rings for forming graphite, which could contribute to the more ordered structure of PPF. In contrast, evaporated carbon is mostly atoms and clusters, with no aromatic rings initially.

For both the vaporized carbon film and PPF, the inability to completely graphitize to yield a low D/G band ratio is due to the difficulty of breaking C-C bonds at temperatures in the ~ 1000 ℃ region.

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72 CHAPTER 3

OPTICALLY TRANSPARENT PYROLYZED PHOTORESIST FILM AND ITS APPLICATION IN ELECTROCHEMISTRY, UV-VIS AND INFRARED SPECTROSCOPY

3.1 Introduction

Optically transparent electrodes (OTEs) have been established as a promising material for high technology applications, such as sensors[1-3], metal-semiconductor- metal (MSM) photo-detectors[4], and substrates for in situ UV-Vis and infrared spectroscopy[5]. Traditionally, OTEs have been fabricated by deposition of conductive thin films of indium and tin oxide, noble metals, carbon, and doped diamond on a properly transparent substrate[5-11]. The majority of the work on OTEs has focused on applications in UV-Vis spectroelectrochemistry. Only a few studies have extended applications into the infrared region of the electromagnetic spectrum, which was achieved by doped diamond electrodes[8] or deposition of germanium film on infrared transparent substrates[10].

Optically transparent pyrolyzed photoresist films (OTPPFs) possess attractive qualities as both electrodes and an optically transparent material, making it an obvious choice for application as an OTE in spectroelectrochemical measurements. We

73 previously reported the UV-Vis transmission spectroelectrochemical measurements of

4-nitroazobenzene (NAB), azobenzene (AB) and 4-nitrobiphenyl (NBP) based on

OTPPF[12], which provided information concerning the structural changes taking place in the molecules being studied. OTPPF offered new routes in the characterization of reactions, organic molecules, and even single DNA molecule on carbon surfaces[12, 13].

In this chapter, the fabrication and properties of OTPPF are described. Examples of

UV-Vis and IR transmission spectroscopy measurements made with this new material are demonstrated, which indicate that a better insight into the structure of organic molecules on carbon surfaces can be achieved using OTPPF.

3.2 Experimental

Preparation of Optically Transparent Pyrolyzed Photoresist Films

Optically transparent pyrolyzed photoresist films (OTPPFs) were prepared on a

quartz (1 cm × 5 cm × 1/16 inch) substrate (silicon nitride wafer for IR application),

which was cleaned in a piranha solution [30 % H2O2 (Malinckrodt) : 98 % H2SO4

(Malinckrodt), 1:3 by volume] for thirty minutes, rinsed rigorously with nanopure water

(18 MΩ-cm, Barnsted/Thermolyne, Dubuque, IA), isopropanol (Aldrich Chemical

Company, Inc., Milwaukee, WI), acetone (Aldrich) and acetonitrile (Aldrich), then dried

with high purity argon. Positive photoresist AZ P4330-RS (AZ Electronic Materials,

Clariant Corp., Somerville, NJ) was diluted with 1-methoxy-2-propanol acetate (Aldrich)

to a certain concentration (by volume). The diluted photoresist solution was spin-coated

onto a piece of clean quartz at 6000 rpm for 30 seconds on a spin coater (WS-400B-

6NPP-LITE, Laurell Technologies, Corp., North Wales, PA). The spin-coated sample

74 was soft-baked at 90 ℃ for 10 minutes, and then transferred into a tube furnace

(Lindberg Blue M) fitted with a 1-inch diameter quartz tube. The tube was flushed with

forming gas (95% nitrogen and 5% hydrogen) flowing at 100 ml/min throughout the

pyrolysis and cooling processes. Samples were heated with a ramp of 10 ºC/min to

1000ºC, and held at 1000 ºC for 1 hour, then cooled to the room temperature.

Preparation of Optically Transparent Pyrolyzed Photoresist Film Electrodes

Since the conductivity of transparent PPF is low, it is hard to derivatize a high

density of molecules on the surface due to ohmic potential losses during electrochemical

reduction. These losses can be decreased by depositing gold films on the top of the PPF

surface with a sputter coater (BAL-TEC SCD 050 Sputter Coater). Two designs (Figure

3.1) of gold modification have been attempted. First, on the top of PPF, an area of

1 cm×2 cm was deposited with 10 nm thick gold film (design 1), the gold covered part of this electrode was not immersed in the diazonium salt solution during derivatization.

Second, a 10 nm thick gold film was deposited around a rectangle of 0.4 cm×1.5 cm, the rectangle is 2.5 cm from the top, 1 cm from the bottom, 0.3 cm from both long edges

(design 2). A special electrochemical cell was made for derivatizaion of the central area of the PPF part. The area of PPF exposed to the diazonium solution is 0.3 cm×1.2 cm.

The PPF electrodes of design 2 are expected to have more uniform potential distribution over the exposed PPF. 4-nitroazobenzene (NAB) was derivatized on PPF electrodes of design 1 and design 2 under the same condition (Four voltammetric scans between

+0.4 V and -0.6 V in 1 mM 4-nitroazobenzene diazonium tetrafluoroborate solution and

0.1 M TBABF4 at a scan rate of 0.2 V/s). As will be discussed below, design 2 75

PPF on quartz Design 1

Au Au

molecule PPF on PPF

PPF on quartz Design 2

Au Au

molecule PPF on PPF

Figure 3.1: Schematics of OTPPF on quartz: Design 1 (top) utilized a gold area adjacent to the OTPPF, while design 2 (bottom) tested a gold border surrounding the OTPPF to reduce ohmic potential errors. The hashed regions indicate the area intended for modification. Other materials as indicated.

76 significantly reduced potential error by providing a low resistance current path during electrochemical modification, resulting in uniform, high-coverage films, from which acceptable UV-Vis spectra could be obtained. The gold modified PPF electrodes of design 2 were used for electrochemical and spectroscopic experiments described below.

For IR transparent electrodes, PPF was fabricated on the surface of a silicon nitride wafer that exhibited good conductivity as an electrode. Therefore, gold modification was not needed.

Synthesis of the Diazonium Tetrafluoroborate Derivatives

Approximately 0.01 mol of a particular amino precursor [4-amine-4’-

nitroazobenzene (Disperse Orange 3, Aldrich), 4-phenylazoaniline (Aldrich), or 4-amino-

4’-nitrobiphenyl (Aldrich)] was weighed into a 50 ml three-necked round-bottom flask.

10 mL of 50% fluoroboric acid (HBF4, Fisher Chemicals, Fair Lawn, NJ) was added to dissolve the precursor. The flask was placed into an ice/sodium chloride bath to cool the mixture to below 0°C. A solution of sodium nitrite (NaNO2, Aldrich) was prepared by

dissolving NaNO2 in Nanopure water (18 MΩ-cm, Barnsted/Thermolyne, Dubuque, IA)

at room temperature, in which the NaNO2 was weighed with a mole ratio of 3 NaNO2:

1 amino precursor. The NaNO2 solution was cooled to 0°C. As the mixture was stirred in

the flask, the cold NaNO2 was added drop wise. The temperature of the mixture was kept

below 4˚C during the reaction. After complete addition of NaNO2, the mixture was

stirred continuously for 30 minutes in the ice/sodium chloride bath. The precipitate

formed was filtered in a Buchner funnel with vacuum filtration. The remaining sediments

were removed from the round bottom flask by cold ether (99.9 % assay, Fisher

77 Chemicals). The resulting diazonium salt was recrystallized by dissolving in a minimal amount of cold acetonitrile (Aldrich) and then adding cold anhydrous ether (99.9 % assay,

Fisher Chemicals) drop wise to recover the salt. The purified diazonium salts were stored

in a desiccator held at 4˚C in a freezer. The diazonium tetraflouroborate salts were

characterized by mass spectrometry (ESI-TOF) and nuclear magnetic resonance. The

diazonium salts were used within 1 month of synthesis. Older diazonium salts were

recrystallized before use. Diazonium salt solutions were kept in a freezer and were used

within a few hours of preparation.

Derivatization of Tetrafluoroborate Diazonium Salts on OTPPF Electrodes

Electrochemical experiments were performed on a BAS 100-W potentiostat

(Bioanalytical Systems, West Lafayette, IN), with an Ag+ (0.01 M)/Ag reference

electrode (BAS) and a Pt wire as the auxiliary electrode. Using a 1 mm diameter BAS platinum disc electrode as the working electrode, an Ag+ (0.01 M)/Ag reference electrode

(BAS) was calibrated to the E1/2 of ferrocene. An observed E1/2 for ferrocene of 89 ± 5 mV was accepted. Derivatization of an OTPPF electrode was performed by the electrochemical reduction of the corresponding diazonium salt (1 mM) in acetonitrile

(Aldrich) with n-tetrabutylammonium tetrafluoroborate (0.1 M; Aldrich) as the supporting electrolyte. Tetrafluoroborate diazonium salts of 4-nitroazobenzene (NAB), azobenzene (AB), and 4-nitrobiphenyl (NBP) were used for derivatization. The

diazonium salt solutions were freshly prepared, and had low water content. The

concentration of the diazonium salt solution was 1 mM, and derivatization scans were from +0.4 to -0.6 V versus Ag+/Ag electrode at 0.2 V/s for four cycles. After

78 derivatization, the modified PPF electrodes were immediately rinsed with acetonitrile and then dried with high purity argon.

UV-Vis Spectroscopy

The UV-Vis spectra presented were recorded on a double beam Perkin-Elmer

Lambda 900 UV/VIS/NIR spectrometer (PerkinElmer Inc., Boston, MA). A deuterium

lamp and a tungsten lamp were used as light sources in the ultraviolet and visible regions

respectively. The wavelength accuracy in the UV/VIS region was 0.08 nm. The

wavelength reproducibility was less than 0.02 nm and the noise level in absorbance mode

is less than 0.00007. A photomultiplier was used as the detector in the UV/VIS region.

The spectral slit width of the spectrometer was 2 nm. The scan speed was 57.69 nm/min

with a data interval of 1 nm. Air was used as a reference to obtain absorption spectra.

Spectral subtraction, overlay and processing were carried out by Grams AI, version 6.00

(Galactic Industries, Salem, N.H.). Baseline correction and peak fitting were performed

with Mathcad 13 (Mathsoft). All the fit parameters such as peak positions, heights and

line widths were allowed to vary. The Lorentzian function was used to fit all the peaks.

The optical transmission T of a thin absorbing film on a transparent substrate

(non-absorbing) for normal incidence is given by the following equations[14, 15],

Ax T = +− DxCxB 2 Where

79 22 s += knnA )(16 22 22 s +++++= knnnknB ]))(1][()1[( 2 2222 22 )(1[( s nkknnknC s +−+−+−= cos2)]1(2) ϕ 2222 2 2 (2[ s s +−+++−− knnknnk sin2)]1)(1() ϕ 22 22 s +−−+−= knnnknD ]))(1][()1[( 4πnd ϕ = λ −= αdx )exp( 4πk α = λ

n and k are the real and imaginary parts of the refractive index of the thin film, d is the

film thickness and ns is the real part of the refractive index of the substrate. Multiple reflections inside the substrate are taken into accountant, but these are assumed to be incoherent (i.e. the substrate is sufficiently thick so that it does not give rise to additional

interference effects).

IR Spectroscopy

Infrared spectra were obtained by a Bruker Equinox 55 FT-IR spectrometer with a

DTGS detector. N2 was purged to the sample chamber. The spectral range covered was

400-5000 cm-1. For each spectrum, 3000 scans were collected at 4 cm-1 resolution in the

transmission mode.

80 3.3 Optical Properties of Optically Transparent Pyrolyzed Photoresist Films

Positive photoresist (AZ P4330-RS) solution with various concentrations were

spin coated on quartz to prepare OTPPF samples, resulting in different optical

transmissions as listed in table 3.1. The value of the film transmittance (T %) and the

wavelength of maximum absorbance (λmax) varied monotonically with the concentration

of photoresist, which are in agreement with the results obtained by Donner[13]. The

thicknesses of PPF samples made from various dilutions of photoresist were estimated by

simulating spectra using the optical constants of PPF[12] (measured by J.A. Woollam

VASE). The thickness of the PPF layer was adjusted until the transmittance at λmax (for absorbance) in the calculated spectrum matched the experimental value (results listed in

Table 3.1). As expected, the thickness of the PPF increases with photoresist concentration.

The OTPPF prepared from 5% (v/v) photoresist solution was selected as the carbon substrate for subsequent UV-Vis and IR investigations due to its high transmittance (at least 57% over the range 220 to 800 nm), which maximizes spectroscopic sensitivity. Figure 3.2 displays the average experimental UV-Vis spectrum obtained from ten OTPPF samples. Error bars represent ± one standard deviation, showing that the shape of the absorption curve is reproducible to within 10% over the range from 200 to 800 nm. The wavelength of maximum absorbance (λmax) varies less

than 2% (270 ± 4 nm), while the absorbance value at λmax is 0.23 ± 0.02 (which

corresponds to a minimum %T of 59% ± 3%). The sample-to-sample variation in

absorbance could be caused by small deviations in the thickness of the OTPPF when several samples were prepared under ostensibly identical conditions. The solid curve in

Figure 3.2 is a simulated spectrum using the optical constants of PPF obtained from

81 Photoresist λmax (nm) Thickness (nm)

a concentration (v/v) %T at λmax Experiment Simulation Simulation VASE

50% 0.04% 257 256 100 _

33% 2% 260 260 60 _

25% 26% 265 265 19.5 _

10% 37% 270 268 13 _

5% 57% 272 274 4.8 5.9±0.8

1% 94% 274 274 0.25 _

a Thickness producing the closet match for %T at λmax for spectrum calculated from the optical constants measured by VASE[12].

Table 3.1: Values for λmax, the transmission (%T) at λmax, and calculated thickness from spectral simulations as a function of photoresist dilution.

ellipsometry measurements and a thickness of 4.8 nm, which agrees well with the experimental data. The thickness of the PPF made from 5% (v/v) photoresist solution was also determined by VASE, yielding a thickness of ~ 6 nm with an approximately 10% standard deviation in the ellipsometric data (three samples). This level of variation confirms that the deviation observed in the absorption spectra of OTPPF is caused by thickness variations in the film. This effect is significant when subtracting background spectra of OTPPF from spectra of molecules adsorbed on OTPPF, as discussed below.

82

Absorbance

Wavelength (nm)

Figure 3.2: Experimental absorbance spectrum of OTPPF (open circles) on quartz prepared using a 5% (v/v) solution of photoresist, with air as reference. The error bars represent ± one standard deviation for ten samples. The solid line is the calculated spectra using the optical constants for PPF determined with VASE and a 4.8 nm thickness parameter (path length)[12].

83 3.4 UV-Vis Spectra of Molecules Chemisorbed onto Optically Transparent Pyrolyzed Photoresist Films

OTPPF samples made from a photoresist dilution of 5% (v/v) were chosen for the

following spectroscopic experiments due to their high transparency. Although the

reproducibility of film absorbance at λmax is reasonable (0.24 ± 0.03), it is too large to

permit an accurate subtraction of a standard PPF absorption spectrum from spectra of

molecules chemisorbed onto PPF. Therefore, each OTPPF sample was used as its own

“reference” for optical measurements by recording a spectrum before and after

modification with the molecular layer.

UV-Vis spectra of OTPPF and NAB/OTPPF (with air as a reference) are plotted

in Figure 3.3 A. Although the additional absorbance due to the thin molecular layer is

apparent, it is more obvious after subtraction of the OTPPF spectrum from that of

NAB/OTPPF (for the same OTPPF sample), as shown in Figure 3.3B. This self-

referencing method was applied to more accurately account for the absorbance of the

OTPPF electrode. The absorption band of PPF at ~270 nm does not contribute

significantly to the subtracted spectrum of NAB in Figure 3.3B. The correction also

reduces the overall absorbance to a level that represents the absorbance of the molecular

layer only. Moreover, this correction reduces errors resulting from small differences in the OTPPF thickness, and possible fluctuations in the optical absorbance of each OTPPF substrate caused by other sample-to-sample variations.

Being used as an electrode, OTPPF could exhibit significant ohmic errors due to

its relatively high resistivity. The resistivity of PPF (0.005 Ω-cm) is comparable to that of

glassy carbon, which is much higher than that for most metals. Based on this value, the

84

Absorbance (AU)

Wavelength (nm)

Figure 3.3: (A) UV-Vis absorption spectra of NAB/OTPPF/quartz (dotted curve), with air as a reference. The spectrum for the same OTPPF/quartz sample before bonding of NAB is also shown (solid curve). (B) Absorbance spectra of NAB on OTPPF obtained by subtracting the absorbance of NAB/OTPPF/quartz by that for the same OTPPF/quartz sample prior to modification. The dotted curve (design 1) is for deposition of the Au area adjacent to the OTPPF (see Figure 3.1), while the solid curve (design 2) is for the Au border (see Figure 3.1). Design 2 was used in (A)[12].

85 resistance for an OTPPF electrode with a PPF thickness of 6 nm is estimated to be 8.3 x

103 Ω/m. The thickness and packing density of the NAB films on OTPPF can be affected by the resulting significant potential error during the electrochemical modification process. Figure 3.3B shows UV-Vis spectra of chemisorbed NAB obtained from NAB modified OTPPF electrodes using both of the designs shown in Figure 3.1 (OTPPF was used as its own reference in both cases, as described above). Obviously, a higher density of absorbing species on the surface was obtained from design 2, indicating that without the Au border, Ohmic potential losses lead to lower NAB surface concentrations.

Consequently, design 2 was chosen for all further experiments to ensure a minimal variability in absorbance due to inefficient modification. As noted in the experimental section, molecular layer thicknesses yielded from design 2 (4.0 ±0.7 nm) are comparable to those obtained on bulk PPF (4.5 ±0.7 nm). Figure 3.3 demonstrates that UV-Vis spectra can provide sufficient signal to identify spectral features of molecules chemisorbed on OTPPF.

3.5 Free and Adsorbed Molecular Spectra

In order to investigate the spectral signatures of the molecules and effect of covalent immobilization, spectra of molecules in solution were compared to those of the chemisorbed state. Figure 3.4 displays spectra of NAB in cyclohexane solution (A), chemisorbed onto OTPPF (B), and as a solid film (C). The absorption peaks of chemisorbed NAB were fitted by the Lorentzian function. Several important observations are obtained from the comparison of the spectra in Figure 3.4. First, spectra of chemisorbed NAB shows that absorbance bands due to π-π* and n-π* transitions are both

86

NAB in cyclohexane 0.1 AU

0

chemisorbed NAB 0.01 AU

Absorbance 0

solid NAB 0.1 AU

0 200 300 400 500 600 700 800 Wavelength (nm)

Figure 3.4: UV-Vis spectra of (A) 1 x 10-5 M NAB in cyclohexane, with cyclohexane as a reference, (B) chemisorbed NAB (4 nm thick) on OTPPF, obtained as described for Figure 3.3B (The solid lines are experimental data and the solid circles are peak fitting), and (C) solid NAB film on quartz, prepared by drop casting from ether solution (unknown thickness).

87 broadened and significantly red shifted relative to NAB in cyclohexane. Second, there is

significant absorbance out to at least 800 nm for chemisorbed NAB, while in solution the

absorbance vanishes for wavelengths above ~550 nm. Finally, the spectrum of solid NAB

of unknown thickness (drop-cast from ether) shows considerable absorbance across the

visible range, with a poorly defined peak also broadened and red shifted with respect to

NAB in solution.

Similar results were obtained for AB and NBP to those for NAB. Spectra for AB and NBP in cyclohexane, chemisorbed on OTPPF and solid state are shown in Figure 3.5.

For both AB and NBP, the spectra of chemisorbed molecules show red-shifted and broadened absorbance features, which are similar to the case of NAB. The solid film spectra for these two molecules have much more broadened features than that for NAB.

λmax for all three molecules in the chemisorbed, solution, and solid states are listed in

Table 3.2. Analysis of this table shows a consistent trend: λmax is at the longest

wavelength for chemisorbed films, lowest in the solution, and intermediate (although

closer to the solution value) for the drop-cast film (physisorbed solid).

At least two effects correspond to the observed trends. First, electronic coupling

between the chemisorbed molecules and the graphitic π system may be substantial,

resulting in stronger molecular orbital interactions that lead to decrease in the HOMO-

LUMO gap. Second, intermolecular interactions within the densely packed molecular

film (planarity of molecules in solid state could strengthen the intermolecular interaction) could cause a similar effect, or perhaps reinforce the effect of substrate coupling.

Although the thicknesses of the drop-cast solid films are unknown, these samples were prepared as analogues to the chemisorbed film, but without covalent bonds formed

88

A 0.5 AU AB in cyclohexane

0

0.01 AU chemisorbed AB

Absorbance 0

0.04 AU solid AB

0 200 300 400 500 600 700 800 Wavelength (nm)

B NBP in cyclohexane 1 AU

0

chemisorbed NBP 0.01 AU

Absorbance 0

solid NBP 0.04 AU

0 200 300 400 500 600 700 800 Wavelength (nm)

Figure 3.5: UV-Vis spectra of (A) 1 x 10-4 M AB in cyclohexane, chemisorbed AB (3.2 nm thick) on OTPPF (The solid lines are experimental data and the solid circles are peak fitting), and solid AB on quartz unknown thickness; (B) 1 x 10-4 M NBP in cyclohexane, chemisorbed NBP (1.4 nm thick) on OTPPF, and solid NBP on quartz unknown thickness.

89 Molecule λmax, nm

Chemisorbed Solution (in C6H12) Solid

NAB 356 330 336

AB 340 316 320

NBP 325 296 312

Table 3.2: Observed λmax for chemisorbed, solution, and solid molecules.

between the molecular film and substrate (or between molecules in the layer). The results

in Figure 3.4, Figure 3.5 and Table 3.2 provide important information about the electronic structure of chemisorbed molecules at the OTPPF surface.

3.6 IR Spectroscopy of Chemisorbed Molecules on Optically Transparent Pyrolyzed Photoresist Films

Since thin carbon films are also expected to be transparent in the infrared region,

OTPPFs were fabricated on the surface of Si3N4 wafers (Si wafer with ~ 1000 Å thick

Si3N4) and used as a substrate for IR transmission spectroscopy. NAB molecules were deposited on such a substrate using the same method as for the UV-Vis experiment samples, except the substrate was not modified by Au. The IR spectra were collected in

the 600-5000 cm-1 range but are reported over only a 650-2000 cm-1 spectral range

90 because no absorption bands were observed above 2000 cm-1. Figure 3.6 displays the IR

spectra of NAB monolayer and NAB multilayer obtained in the transmission mode. A

spectrum of an OTPPF on Si3N4 prior to deposition was used as the background

subtracted from the spectra of a monolayer or multilayer of NAB on PPF/Si3N4 (OTPPF

was used as its own reference, as described in the UV-Vis experiment). The IR

-1 absorption band at 1109 cm is due to the Si3N4 wafer.

IR absorption spectra for solid NAB in a KBr pellet, chemisorbed NAB

monolayer and multilayer on OTPPF (converted from the transmittance spectra in Figure

3.6) are shown in Figure 3.7. The absorption band due to Si3N4 was removed by manual

baseline adjustment. Since free NAB has a planar structure, both in-plane and out-of-plane vibrational modes were observed in the IR absorption spectra, as

-1 summarized in Table 3.3. Peaks at 1344 (νs NO2), 1522 (νa NO2), and 1588 (ν C=C) cm correspond to in-plane vibrations. The peak at 860 cm-1 is due to an in-plane ring

breathing vibration coupled with an in-plane phenyl-NO2 bend. The out-of-plane C-H

deformations are responsible for the low frequency bands at 691 and 777 cm-1.

Several important observations result from a comparison of the spectra in Figure

3.7. First, both in-plane and out-of-plane vibrations are observed for solid NAB due to

the random orientation of NAB molecules. Second, the weak bands in the (650-900 cm-1) spectral region indicate that both the chemisorbed NAB monolayer and the NAB multilayer display minor out-of-plane contributions. Third, the C=C stretching mode of solid NAB has two distinct peaks, which are unresolved in the spectra for chemisorbed monolayer and multilayer. Fourth, there is an increase in relative peak intensity of the

C=C stretching band for both the chemisorbed monolayer and multilayer compared to the

91

1.006 NAB multilayer

1.004 NAB monolayer

1.002

Transmittance 1.000

0.998

800 1000 1200 1400 1600 1800 2000 -1 Wavenumber, cm

Figure 3.6: IR transmittance spectra of chemisorbed NAB monolayer and multilayer on OTPPF.

92

1344 1522

691 0.1 860 NAB solid

777

1588 c

0.0004 NAB multilayer

b

Absorbance

0.0002 NAB monolayer a

800 1000 1200 1400 1600 1800 2000

Wavenumber (cm-1)

Figure 3.7: FT-IR absorption spectra of (a) chemisorbed NAB monolayer on OTPPF, (b) chemisorbed NAB multilayer on OTPPF, and (c) solid NAB in a KBr pellet.

93 solid monolayer multilayer assignment

691,m 691,w 691,w γs C-H deformation

777,m 777,w 777,w γa C-H deformation

860,m 860,w 860,w νs (C-NO2) + ring breathing

1344,vs 1344,vs 1344,vs νs (NO2)

1444,w 1456,w 1452,w δa ring deformation + C-N=N

1466,m 1473,w 1466,w N=N stretch + ring deformation

1488,m 1496,m 1496,m N=N stretch + ring deformation

1522,vs 1522,s 1522,s νa (NO2)

1588,m 1593,s 1591,s ν (C=C)

1606,m ν (C=C)

a vs, very strong; s, strong; m, medium; w, weak. b γ, out-of-plane; δ, in-plane; ν, stretch; s, symmetric; a, asymmetric.

Table 3.3: Vibrational frequency and assignment for nitroazobenzene.

94 solid NAB spectrum. Fifth, there is a slight decrease in the relative peak intensity of the

1522 cm-1 in the spectrum of the chemisorbed multilayer verses the chemisorbed

monolayer.

The C=C stretching mode of solid NAB results in two IR absorption peaks at

1588 and 1606 cm-1, which are due to the two phenyl rings in the NAB molecule.

However, the same C=C stretching mode gave only one unresolved band at 1593 and

1591 cm-1 for the NAB monolayer and multilayer respectively. The one band nature

revealed the resonance of the molecule and its strong interaction with the OTPPF

substrate. The IR spectra of the NAB monolayer and the NAB multilayer are very similar

in shape. The absorption intensity of the NAB multilayer spectrum is more intense than

that of the NAB monolayer due to its increased thickness. The small red shift in the C=C stretching mode of the NAB multilayer spectrum relative to the NAB monolayer spectrum implies a more conjugated system in the multilayer. The decrease in the relative peak intensity at 1522 cm-1 of the chemisorbed multilayer could be caused by the

difference in orientation of the additional layers versus the monolayer.

The IR absorption spectra of chemisorbed AB and NBP are also obtainable by the

same method, which are shown in Figure 3.8. For chemisorbed AB, vibrational modes of

(N=N stretch + ring deformation) and C=C stretching are observed. The absorption

spectrum of chemisorbed NBP is very similar to that of NAB, in which in-plane

vibrations of νs NO2, νa NO2, and ν C=C are observed. The quality of the IR absorption

spectra obtained on OTPPF is comparable to that taken by GATR[16], except the

intensity obtained by GATR is a few times stronger, which is presumably due to the advantage of GATR’s optical configuration.

95

0.0001 AB multilayer

b

ce

Absorban 0.0001 NBP multilayer a

600.0 700 800 900 1000 1100 1200 1300 1400 1500 1600 1700 1800 1900 2000.0 -1 Wavenumber (cm )

Figure 3.8: FT-IR absorption spectra of (a) chemisorbed NBP on OTPPF, and (b) chemisorbed AB on OTPPF.

96 3.7 Conclusions

Dilute solutions of photoresist serve as an excellent starting material for the

manufacture of optically transparent pyrolyzed photoresist films (OTPPFs). Optical

transparency is achieved and the conductivity of the film is sufficient for electrochemical deposition. The OTPPF composition, smoothness, and performance are comparable to thick pyrolyzed photoresist films already utilized by the research community. OTPPFs

offer the possibility of sensitive, reproducible, and stable measurements in both UV-Vis

and IR regions of the electromagnetic spectrum. This spectroscopic method is not only

very sensitive but also yields detailed structural information. It allows thin layer

characterization by both UV-Vis and IR spectroscopy. It provides the information of structure and interaction at air-solid interfaces, which is of great interest because of its important role in the understanding of the interactions of adsorbed molecules with the substrate and mechanism of electron transport across molecular junctions. This material has been successfully utilized in observing structural changes of organic molecules chemisorbed on PPF under an applied potential bias by in situ UV-Vis spectroelectrochemistry[12]. It certainly has a promising application in probing the structures of adsorbed molecules or even complete molecular junctions under an applied potential bias using both UV-Vis and IR spectrometry.

97 REFERENCES

1. Choi, C.K., C.H. Margraves, and S.I. Jun, Opto-Electric Cellular Biosensor Using Optically Transparent Indium Tin Oxide (ITO) Electrodes. Sensors, 2008. 8: p. 3257-3270.

2. Shi, Y.N., et al., Spectroelectrochemical sensing based on multimode selectivity simultaneously achievable in a single device.1. Demonstration of concept with ferricyanide. Analytical Chemistry, 1997. 69(18): p. 3679-3686.

3. Mitsubayashi, K., et al., Optical-transparent and flexible glucose sensor with ITO electrode. Biosensors & Bioelectronics, 2003. 19(1): p. 67-71.

4. Chiou, Y.Z., et al., The characteristics of different transparent electrodes on GaN photodetectors. Materials Chemistry and Physics, 2003. 80(1): p. 201-204.

5. Winograd, N. and T. Kuwana, Advances in Electroanalytical Chemistry, ed. D. M. 1974, New York.

6. DeAngelis, T.P., R.W. Hurst, and A.M. Yacynych, Carbon and mercury-carbon optically transparent electrodes. Analytical Chemistry, 1977. 49(9): p. 1395-1398.

7. Haacke, G., Transparent Conducting Coatings. Annual Review of Materials Science, 1977. 7: p. 73-93.

8. Haymond, S., et al., Spectroelectrochemical responsiveness of a freestanding, boron-doped diamond, optically transparent electrode toward ferrocene. Analytica Chimica Acta, 2003. 500(1-2): p. 137-144.

9. Heineman, W.R., F.M. Hawkridge, and H.N. Blount, Spectroelectrochemistry at Optically Transparent Electrodes.2. Electrodes under Thin-Layer and Semi- Infinite Diffusion Conditions and Indirect Coulometric Titrations. Electroanalytical Chemistry, 1984. 13: p. 1-113.

98 10. Mattson, J.S. and C.A. Smith, Optically Transparent Carbon Film Electrodes for Infrared Spectroelectrochemistry. Analytical Chemistry, 1975. 47(7): p. 1122- 1125.

11. Zudans, I., et al., Electrochemical and optical evaluation of noble metal- and carbon-ITO hybrid optically transparent electrodes. Journal of Electroanalytical Chemistry, 2004. 565(2): p. 311-320.

12. Tian, H., A.J. Bergren, and R.L. McCreery, Ultraviolet-visible spectroelectrochemistry of chemisorbed molecular layers on optically transparent carbon electrodes. Applied Spectroscopy, 2007. 61(11): p. 1246-1253.

13. Donner, S., et al., Fabrication of optically transparent carbon electrodes by the pyrolysis of photoresist films: Approach to single-molecule spectroelectrochemistry. Analytical Chemistry, 2006. 78(8): p. 2816-2822.

14. Poelman, D. and P.F. Smet, Methods for the determination of the optical constants of thin films from single transmission measurements: a critical review. Journal of Physics D-Applied Physics, 2003. 36(15): p. 1850-1857.

15. Swanepoel, R., Determination of the Thickness and Optical-Constants of Amorphous-Silicon. Journal of Physics E-Scientific Instruments, 1983. 16(12): p. 1214-1222.

16. Anariba, F., et al., Determination of the structure and orientation of organic molecules tethered to flat graphitic carbon by ATR-FT-IR and Raman spectroscopy. Analytical Chemistry, 2006. 78(9): p. 3104-3112.

99 CHAPTER 4

SURFACE ENHANCED VIBRATIONAL SPECTROSCOPY OF 4-NITROAZOBENZENE

4.1 Introduction

Surface enhanced vibrational spectroscopy deals with the molecular vibrations of

adsorbates on surfaces and interfaces which are capable of enhancing the absorption or

the emission of electromagnetic radiation[1-3]. Surface Raman spectroscopy and surface

enhanced infrared absorption (SEIRA) spectroscopy are the two complementary

components of surface vibrational spectroscopy, which can provide the structural

information of any molecular system, at least in principle.

The surface enhancement effects arise from both electromagnetic and chemical enhancement. Most surface enhanced vibrational spectroscopy happens on noble metal

surfaces consisting of small (much less than the wavelength of light) metal islands or

particles, which greatly enhance the electric field near the metal surface through the

excitation of localized surface plasmons (SPs)[4-6]. Chemical effects (e.g., chemical

attachment of molecules to the metal film) can influence peak positions and relative

intensities of vibrational bands.

Many theories and speculations have been proposed to interpret the phenomena of

100 surface enhanced vibrational spectroscopy [1]. A full understanding of its nature and

mechanism is needed before it can be widely applied. In this chapter, the nature of

surface enhanced vibrational spectroscopy will be investigated by employing a system of

4-nitroazobenzene (NAB) modified Ni microarray. Such studies may provide new insight into the origins of surface enhancement effects, as well as approaches to attaining strong

surface enhancement. The reasons investigating NAB on Ni mesh are as follows: First,

NAB is an active molecule for carbon based molecular junctions. It has been well

characterized on carbon substrates or as a free molecule by Raman, IR and UV-Vis[7-12], which allows comparison of new results with those already published. Second, Ni mesh

is a relatively new material exhibiting SEIRA effects, mainly for self assembled

monolayers (SAMs) thus far[13-17]. The mechanism is under investigation and its

application to a broader field is very promising. Third, spectroscopic investigation of the

metal-phenyl bonds formed by diazonium reduction on metals is rare, NAB-Ag and

NAB-Ni provide examples.

4.2 Experimental

Preparation of Optically Transparent pyrolyzed Photoresist Film (OTPPF), Ni and Ag Film Electrodes

Optically transparent pyrolyzed photoresist film (OTPPF) electrodes were

prepared as described in Chapter 3. To prepare smooth Ag films, 4 nm Cr was deposited

at a rate of 0.1 nm/sec followed by 20 nm of Ag at a rate of 0.1 nm/sec onto Si/SiO2 samples, with a backpressure of ~ 2°10-6 torr. For experiments involving UV-Vis

transmission through the metal/molecule films, quartz was substituted for Si/SiO2[9]. To

prepare Ni films, 10 nm Ni film was deposited at a rate of 0.1 nm/sec onto the quartz 101 samples.

Preparation of the 4-Nitroazobenzene Diazonium Tetrafluoroborate

The 4-nitroazobenzene diazonium reagent was prepared as the tetrafluoroborate

salt as described in Chapter 3, and stored in a freezer. NAB diazonium salt solutions were

prepared fresh daily in acetonitrile.

Derivatization of Electrodes with Tetrafluoroborate Diazonium salts

NAB on OTPPF, normal PPF, Ni film and Ni mesh was deposited by electrochemical reduction. The derivatization scans were from +0.4 to -0.6 V versus

Ag+/Ag electrode at 0.2 V/s for four cycles in 1 mM NAB diazonium salt solution. NAB

was bonded to Ag spontaneously, by exposing the Ag to a 1 mM solution of NAB diazonium tetrafluoroborate in acetonitrile for 2 minutes. The Ag/SiO2 and Ag/quartz

samples were first sonicated for one minute each in acetone, nanopure water, and

isopropanol, then immediately dipped in the NAB diazonium solution[9]. A final rinse

with acetonitrile removed excess diazonium reagent, and high purity argon dried the

samples. The thicknesses of NAB films on smooth Ag, OTPPF and normal PPF were

determined with AFM “scratching”. For two samples of NAB on smooth Ag (2 minute

exposure, 1 mM NAB), the thickness was determined to be 4.6 ±0.2 nm by AFM

scratching, with three 1.5μm x 1.5μm “scratches” analyzed on each sample. The

thicknesses of NAB on OTPPF and normal PPF were determined to be 4.0 ±0.7 nm and

4.5 ±0.7 nm respectively by the same method[9].

102 UV-Vis Spectroscopy

The UV-Vis spectra presented were recorded on a double beam Perkin-Elmer

Lambda 900 UV/VIS/NIR spectrometer (PerkinElmer Inc., Boston, MA). Air was used

as a reference to obtain absorption spectra. The spectra of substrates before modification

are used as the backgrounds subtracted from spectra of NAB modified substrates.

Raman Spectroscopy

Raman spectra were obtained with a 514.5 nm laser and a custom line focused f/2

spectrograph (Chromex) with an Andor back thinned CCD detector cooled to -85 ℃[18].

The line focused configuration significantly reduced the laser power density at the sample,

and reduced radiation damage. Samples observed with a line focus of 50 μm x 5 mm and

2 a power density of 0.12 W/cm exhibited negligible changes in intensity with laser exposure, and completely adequate signal/noise ratio spectra were obtained.

Infrared Spectroscopy

Infrared spectra were measured by an Equinox 55 FTIR spectrometer with a

DTGS detector. N2 was purged in chamber to remove air. The spectral range covered was

400-5000 cm-1. For each spectrum, 1000 scans were collected at 4 cm-1 resolution in the

transmission mode. For the double stack of mesh, a drop of ethanol was added on top of

the sample to assist the angle tuning of the two meshes. The sample was then sandwiched

between shim stocks with an open area of 0.4 cm x 0.4 cm. The spectra were taken five minutes after being put into the chamber to avoid the residual of ethanol.

103 4.3 UV-Vis Spectra of NAB on Various Substrates

In order to study the interaction between adsorbates and substrates, UV-Vis

spectra for molecules in solution are compared to that for chemisorbed states. Figure 4.1

displays spectra of free NAB in solution and chemisorbed NAB on PPF, Ni, and Ag film.

All spectra are background corrected. The chemisorbed spectra were obtained by

subtracting the response for the substrate prior to NAB modification from that for NAB

on the same substrate. The spectrum on Ag has a different shape from that on PPF and Ni

film, and the origin of the double peak is unknown. The lower wavelength absorption

band at 315 nm matches the peak absorption for unmodified Ag/Cr on quartz, which may be a subtraction artifact, and very likely is due to bulk silver plasmon[19]. It is also

observed with biphenyl modified Ag sample, indicating it is not inherent to NAB[9]. The

shapes of the absorption band and peak shifts provide insight into the chemical effect of

the system. Comparison of the spectra reveals that there is a red shift of the UV-Vis

absorbance bands in the chemisorbed NAB layer. The λmax of this band shifts from 330

nm in solution to 356 nm on PPF, 345 nm on Ni, and 356 nm on Ag. NAB-PPF and

NAB-Ag cases exhibit the largest red-shift, implying the greatest electronic interactions

between the NAB and substrate. The n-π* transition band of NAB on PPF is more intense

and shifts to longer wavelength (480 nm) relative to that of free NAB in solution (456

nm). This transition band is not observable for NAB on Ni and Ag substrates. The

UV-Vis absorbances for all chemisorbed NAB are broadened compared to the

corresponding band in solution. The red-shifting and broadening can be caused by at least

two effects. First, orbital energies of molecules could be changed by electronic coupling

between the chemisorbed molecules and the substrate, which leads to decreases in the

104

330 nm

0.1 AU NAB in cyclohexane

456 nm OO N 356 nm 0.02 AU NAB on PPF

e 480 nm

rbanc so

Ab 345 nm N 0.01 AU NAB on Ni N

315 nm 356 nm

0.02 AU NAB on Ag NAB

200 300 400 500 600 700 800

Wavelength (nm)

Figure 4.1: UV-Vis absorption spectra of 1°10-5 M NAB in cyclohexane solution, NAB on PPF, NAB on Ni film, and NAB on Ag film. Pure cyclohexane, unmodified PPF, unmodified Ni film (10 nm thick), and unmodified Ag film (20 nm thick on 4 nm of Cr) were used for background subtraction, respectively. For the UV-Vis spectrum of NAB on PPF, the solid lines are experimental data and the solid circles are peak fitting.

105 HOMO-LUMO gap. Second, intermolecular interactions within the densely packed

molecular film could cause a similar effect (stronger MO interaction than in solution) or perhaps reinforce the effect of substrate electronic coupling. The thickness of NAB on

PPF and Ag measured by AFM are 4.0 ±0.7 nm and 4.6 ±0.2 nm respectively, which are very close to each other. Even though the thickness of NAB on Ni was not measured, the comparable absorbance of NAB on Ni with that of NAB on PPF and Ag indicate they are similar in thickness.

4.4 Raman Spectra of NAB on Various Substrates

Raman spectra of chemisorbed NAB on different substrates and in solution were

obtained in order to investigate possible chemical enhancement. Figure 4.2 shows the

Raman spectra of free NAB in solution and chemisorbed NAB on PPF, and smooth Ag

and Ni films. The spectra of NAB on PPF, Ni mesh and Ag film are significantly

different from that of NAB in solution, implying a strong interaction with NAB and

substrates. This observation agrees with the conclusion derived from the red shift of

absorption peaks of chemisorbed NAB in UV-Vis spectra. The Raman spectra of

chemisorbed NAB are very similar in peak positions, implying that the structure and

bonding of NAB on the substrates are similar. Earlier studies have confirmed the formation of a phenyl-phenyl bond between NAB and PPF[8], presumably phenyl-Ni and phenyl-Ag bonds are formed in the case of Ni and Ag films. The main qualitative differences between solution NAB and chemisorbed NAB are attributable to the additional substitution on the phenyl ring bonded to the surface[8]. It has been reported that diazonium reagents formed either Cu-phenyl or Cu-O-phenyl bonds on copper

106

1470 1141 1450 1492 1183 1341 1591 1108 1401 1002 924 NAB in CCl 4

NAB on PPF 5 e-1 s-1 mW-1

NAB on Ni 1 e-1 s-1 mW-1

1 e-1 s-1 mW-1 NAB on Ag

800 1000 1200 1400 1600 1800 Raman shift, cm-1

Figure 4.2: Raman spectra of NAB in CCl4 solution, NAB on PPF, NAB on Ni film, and NAB on Ag film. Spectra of pure carbon tetrachloride, unmodified PPF, unmodified Ni film, and unmodified Ag film were used for background subtraction.

107 surfaces, depending on the presence of Cu oxide initially[20]. A detailed XPS analysis

was not carried out in the present work, and there is a possibility of forming Ni-C,

Ni-O-C, Ag-C and Ag-O-C bonds at the interface. The NAB spectra and band assignments have been reported previously[8, 9, 11, 12, 21]. Some bands of current

-1 -1 relevance, phenyl-NO2 stretch (1108 cm ), the phenyl-NN stretch (1141 cm ), the NO2

stretch (1341cm-1), the N=N stretches (1401 and 1450 cm-1), and C=C stretch (1591 cm-1)

are investigated here in more detail.

While the spectra of NAB on PPF, Ni and Ag are qualitatively similar, the PPF

spectrum is considerably more intense, by a factor of ~5 for equal laser power, exposure

time, and similar NAB multilayer thickness. PPF is essentially a sp2 hybridized carbon

network, which would not be expected to provide significant EM enhancement. Even

though Ni mesh is able to couple SPs with incident light in the IR region[13-15], it is not

expected to support EM enhancement with UV or visible incidence. If some EM field

enhancement comes from defects on the smooth Ag then truly smooth Ag would be

expected to produce even weaker scattering than what is observed in Figure 4.2. It is

reported that NAB has unusually strong Raman scattering on carbon surfaces, with a

surface cross section about 102 - 103 times greater than the value for free molecules in

solution[21, 22]. A possible reason for this large chemical enhancement is revealed by

the red shift of chemisorbed NAB in Figure 4.1. The shift of NAB absorption band (480

nm) closer to the 514.5 nm laser wavelength could cause the increase in resonance

Raman cross section. Both UV-Vis and Raman spectra imply that the electronic coupling

between NAB and PPF, Ni, or Ag is strong enough to significantly perturb the surface

spectra relative to free NAB in solution. The effect of binding interactions at different

108 interfaces could result in slight differences in the relative intensities of vibration peaks.

Compared with spectra of NAB on PPF, the relative intensities of the NO2 stretch (1341 cm-1) of NAB on Ni and Ag decrease relative to the C=C stretch (1591 cm-1), and the intensity decrease of NAB on Ni is more significant. The decrease in the NO2 stretch implies the partial reduction of NAB[8, 11, 12]. This information is further confirmed by

-1 the decrease of the 1401 cm band (N=N stretch + the ring with NO2 group) of NAB on

Ni. The fact that relative intensities of Raman spectra depend on the binding interaction between the adsorbed molecules and substrate suggests that chemical effects play an important role in the system without EM enhancement.

4.5 Infrared Spectra of NAB on Single Mesh

Metal arrays with periodic sub-wavelength holes are able to couple incident light with SPs, which makes it a promising material for SEIRA. Enhanced IR absorption spectra of alkanethiol SAMs have been obtained on meshes that exhibit extraordinary transmission effect in the IR region[16, 17]. In order to study the mechanism of the enhancement and extend its application, NAB modified Ni mesh is investigated in the following sections. Figure 4.3 displays IR transmission spectra of Ni mesh before (dotted trace) and after (solid trace) modification with NAB. The NAB coating causes a red shift

(~4.6 cm-1) of the (1,0) transmission resonance. However, the molecular absorptions of the NAB coating are not observed. Alkanethiol SAMs are known to be close packed and highly ordered. The alkyl chains form an all-trans extended configuration and are tilted in the range of 0 to 30° from surface normal[23]. The average tilt angle for NAB on PPF surface is 31.0±4.5°[7]. Prepared by the same method, NAB on Ni mesh is assumed to

109

.15

Bare Ni mesh

e c NAB modified Ni mesh

n

a

tt .1

i

m

s

n

a

r

T

.05

1000 1500 2000 2500 3000 3500 4000

Wavenumbers, cm-1

Figure 4.3: IR transmission spectrum of Ni mesh at perpendicular incidence before (dotted trace) and after (solid trace) deposited with NAB multilayer.

110 have similar tilt angle. The absorbance of alkanethiol SAMs obtained by reflection absorption infrared spectroscopy (RAIRS) are ~0.001 in the C-H stretch region[24], which are comparable to the absorbance of NO2 asymmetric stretch of NAB on PPF

(0.0007) measured by IR transmission spectroscopy. An electromagnetic mechanism cannot explain why alkanethiol SAMs are enhanced, while the multilayer of NAB is not.

There could be other contributions to the surface enhancement. The Raman spectra of

NAB on Ni mesh revealed that NAB is partially reduced on Ni, which could result in alteration of charge distribution and dipole moment. It is difficult to give a definite interpretation of this phenomenon at present, but very likely it is a result of chemical changes caused by the formation of the complex between NAB and Ni.

4.6 Infrared Spectra of NAB on Double Mesh Stacks

It has been reported that if two meshes are stacked one upon the other, the stack still exhibits the extraordinary transmission effect[25, 26]. The single Ni mesh shows

77% transmission with only 26% open area, the double stacked mesh shows 60% transmission with only 10% open area, and the quadruple stack mesh shows 22% transmission with an estimated 0.5% open area. In this sequence, the enhancement of transmitted light over the fractional open area increases from 3 to 6 to ~40[26]. These observations suggest that stacking of mesh suppresses the “nonmetallic” diffraction background, and the SP behavior is better isolated. It is likely that Ni mesh converts the photon energy to surface plasmon polaritons traveling along the metal surface and between the two layers of metal (illustrated in Figure 4.4). This technique has the potential to provide electromagnetic enhancement for absorption experiments directed at

111

(a)

(b)

Figure 4.4: Illustration of Ni mesh converts the photon energy to SPs traveling along (a) the metal surface on single mesh and (b) between the two layers of metal on two mesh stack.

112 species on the surface of the Ni mesh.

The IR transmission spectrum for two pieces of stacked NAB coated Ni meshes is

shown in Figure 4.5 (the trace with the highest transmission). Despite stacking, SP

resonances of this system are similar to those of a single piece of mesh, and absorption peaks due to the NAB coatings are still not observable. Since the position and intensity of

SP resonances are determined by the geometry of Ni mesh (hole to hole spacing and hole

widths respectively), it is expected that SP resonances could be tuned by rotation of one

mesh relative to the other. Figure 4.5 displays the IR transmission spectra of double stack

of NAB coated Ni meshes at different rotation angles. The resonance peaks shift and

become narrower as one mesh rotates relative to the other. The absorbance features of

NAB were observed at several rotation angles. An image of the stacked mesh was taken

(Figure 4.6) in order to determine the specific angle which yields maximum NAB

absorption. Analysis of the image reveals that the meshes are rotated one from the other

by ~5°.

Notice that all the absorption features are near the transmission resonances, which

suggests that the near field enhancement of the electric fields increases the infrared

absorption of chemisorbed NAB molecules on Ni mesh. The transmission spectra in

Figure 4.5 were converted into absorption spectra by spline fitting for background

subtraction, and are displayed in Figure 4.7. Note that the strong absorption at 1727 cm-1

due to the carbonyl group is not from the NAB molecule, which implies contaminants in

the studied system. The source and structure of the contaminant are not known, which

makes it difficult to analyze the spectra. Two most intense absorption spectra are

-1 normalized to the intensity of the 1365 cm band (NO2 symmetric stretch), and plotted in

113

2.38

2.2

2.0

1.8 NAB

1.6

) 1.4

1.2 Ni/NiO

1.0 Transmittance (% Transmittance

0.8

0.6

0.4

0.2

0.05 600.0 800 1000 1200 1400 1600 1800 2000.0 Wavenumbers, cm-1

Figure 4.5: IR transmission spectra of two pieces of NAB coated Ni mesh stacked at various angles. Inset is the illustration of the rotation of one mesh relative to the other.

114

Figure 4.6: The image of double stacked NAB coated Ni meshes when the absorption features of NAB were observed by FT-IR spectrometry.

115 -1

cm 1365 0.140 -1

cm 1074 -1 -1 -1

0.120 -1 -1 1727 cm 1727 -1 1042 cm 1042 1445 cm 1445 1175 cm 1175

0.100 cm 1465 -1 1390 cm 1390 -1 -1

0.08 cm 1020 1581 cm 1581 1600 cm 1600

0.06

Absorbance

0.04

0.02

0.00

-0.020 900.0 1000 1100 1200 1300 1400 1500 1600 1700 1800.0

Wavenumbers, cm-1

Figure 4.7: IR absorption spectra of two pieces of NAB coated Ni mesh stacked at various angles.

116 -1

0.1000

1365 cm 0.090 -1

0.080 1074 cm

0.070 -1 -1 -1 -1

0.060 -1

cm 1727 1445 cm 1042 cm 0.050 1175 cm -1 cm 1465 -1

0.040 -1 -1 cm 1390 0.030 1020 cm 1020 1581 cm 1581 cm 1600 Absorbance 0.020

0.010

0.000

-0.010

-0.0200 900.0 950 1000 1050 1100 1150 1200 1250 1300 1350 1400 1450 1500 1550 1600 1650 1700 1750 1800.0 Wavenumbers, cm-1

Figure 4.8: IR absorption spectra of double stack NAB coated Ni meshes normalized to the intensity of the 1365 cm-1 band.

117 Figure 4.8. The absorption band at 1727 cm-1 is ascribed to carbonyl compound, the

contaminant. The differences of absorption bands at 1600, 1581, 1074 cm-1 between each

spectrum are greater than the magnitude of noise, which indicates those bands could be

due to the contaminant only or at least has contribution from the contaminant. The

differences of absorption bands at 1465, 1445, 1390, 1365, 1175, 1042, 1020 cm-1 between each spectrum are smaller than the magnitude of noise, which are ascribed to the absorptions of NAB only.

Table 4.1 summarizes the experimental and calculated vibrational frequencies of

NAB and their assignments. The absorption spectra of solid NAB and chemisorbed NAB

on Ni are plotted in Figure 4.9 for comparison. For chemisorbed NAB on Ni, spectrum

below 900 cm-1 is not plotted since it is noisy and no absorption was observed. The significant shifts of absorption peaks for chemisorbed NAB relative to its solid state suggest that the electronic coupling between NAB and Ni is strong enough to perturb the surface spectra, which are consistent with UV-Vis and Raman observations. The relative

intensities of the enhanced IR absorption peaks depend on the magnitude of electric field

when the vibrational absorption is at resonance. It is difficult to predict the relative

intensity of each vibrational mode since the enhancement factor is different for each

transmission resonance.

In solid NAB, molecules are randomly orientated, and the infrared spectrum

contains both in-plane and out-of-plane vibrations. The orientation of NAB on Ni surface

is not known. As previously mentioned, the average tilt angle for NAB on PPF surface is

known to be 31.0±4.5°[7]. Prepared by the same method, NAB on Ni mesh is assumed to

have similar tilt angle. The SEM image of Ni mesh (Figure 1.5) shows that one side of

118 IR of NABa Solid NAB Raman of DFT calcnb Assignmentc on double stack NAB on Ni Ni mesh (cm-1) (cm-1) mesh (cm-1) (cm-1)

691,m 680 γs C-H deformation

777,m 763 γa C-H deformation

860,m 837 νs (C-NO2) + ring breathing

1020,w δ (C-H) rocking

1042,m δ (C-H) rocking

1103,w 1110 1085 νs (phenyl-NO2) + adjacent

C-H wag

1175,m 1143,w 1140 1126 ν phenyl-NN stretch

1365,s 1345,vs 1340 1337 νs (NO2)

1390,m 1401 1397 N=N + ring deformation

1445,m 1444,w 1448 1459 δa ring deformation + C-

N=N

1465,m 1466,m 1470 N=N stretch + ring

deformation

1488,w 1495 N=N stretch + ring

deformation

1581,w 1521,vs 1554 νa (NO2)

1600,w 1588,m 1592 1589 ν (C=C) a vs, very strong; s, strong; m, medium; w, weak. b calculated with density functional theory B3LYP 6-31 G*, with 0.9613 frequency scaling factor. c γ, out-of-plane; δ, in-plane; ν, stretch; s, symmetric; a, asymmetric.

Table 4.1:Experimental and calculated frequencies and assignments of 4-nitroazobenzene. 119 -1

-1 1344 cm 1344

(a) NAB solid 1522 cm -1 -1 -1

-1 -1 691 cm 691 cm 860 777 cm 777

1606 cm 1588 cm

(b) chemisorbed NAB on double stack Ni mesh -1 -1 -1 -1 -1 -1 -1 -1 -1 1727 cm 1727 -1 1175 cm cm 1042 cm 1365 cm 1445 -1 1465 cm 1465 1074 cm 1074 cm 1390

cm 1020 1581 cm 1600 cm 1600

Figure 4.9: IR absorption spectra of (a) solid NAB in a KBr pellet, (b) double stack of NAB coated Ni meshes.

120 the mesh is very rough and the grain size of Ni particle is much larger than a single molecule. The surface irregularity would allow variation in adsorption geometry. The absence of out-of-plane C-H deformation absorption features for the chemisorbed NAB multilayer on Ni mesh could be because their frequencies are not close to transmission resonances.

The IR absorption of the double stacked NAB coated Ni meshes is about 60 times greater (not fourth as great) than that of NAB on PPF. The electric fields induced by excitation of the surface plasmon resonance of Ni mesh are presumably responsible for the strong enhancement.

4.7 Conclusions

The surface enhanced vibrational spectroscopy of 4-nitroazobenzene has been presented. It is shown that both electromagnetic and chemical effects are important in surface enhancement. In summary, several useful conclusions can be drawn from the current work. First, the chemisorbed NAB molecules exhibit significant red shifts of the

UV-Vis absorption spectra, with the greatest effect observed on a carbon substrate. The shifts imply that significant electronic coupling exists between the chemisorbed NAB and the conducting substrate. In the case of a sp2 hybridized carbon surface, substantial electronic coupling is expected between the phenyl ring of NAB and the phenyl rings in the carbon substrate. For Ag and Ni, this coupling may take place through covalent bonds

(Ag-C or Ni-C) at the interface. Second, Raman scattering from NAB is strongly enhanced on carbon, smooth Ni and Ag, with presumably negligible contribution from electromagnetic field enhancement. The enhancement is considerably greater on the

121 carbon surface, and the larger red shift of the NAB absorption spectrum is consistent with this result. Since the red shift brings the NAB absorption closer to the 514.5 nm laser wavelength of the Raman spectrometer, the unusually strong scattering of surface bound

NAB is likely to be a resonance effect. Third, the transmission resonances of double stacked Ni meshes can be tuned by rotating one mesh relative to the other. When the vibrational modes of NAB are in resonance, the corresponding absorption features are greatly enhanced. Both electromagnetic and chemical mechanisms are operating in the enhanced IR spectra of NAB on Ni mesh, with the EM effect playing a dominant role.

The strategy is versatile and effective for providing EM enhancement in SEIRA. Further work to understand the phenomenon is needed and a general application is expected.

122 REFERENCES

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3. Chang, R.K., Surface Enhanced Raman Scattering, ed. T.E. Furtak. 1982, New York: Plenum.

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5. Hatta, A., Y. Suzuki, and W. Suetaka, Infrared-Absorption Enhancement of Monolayer Species on Thin Evaporated Ag Films by Use of a Kretschmann Configuration - Evidence for 2 Types of Enhanced Surface Electric-Fields. Applied Physics a-Materials Science & Processing, 1984. 35(3): p. 135-140.

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7. Anariba, F., et al., Determination of the structure and orientation of organic molecules tethered to flat graphitic carbon by ATR-FT-IR and Raman spectroscopy. Analytical Chemistry, 2006. 78(9): p. 3104-3112.

8. Itoh, T. and R.L. McCreery, In situ Raman spectroelectrochemistry of electron transfer between glassy carbon and a chemisorbed nitroazobenzene monolayer. Journal of the American Chemical Society, 2002. 124(36): p. 10894-10902.

9. Liang, H.H., H. Tian, and R.L. McCreery, Normal and surface-enhanced Raman spectroscopy of nitroazobenzene submonolayers and multilayers on carbon and silver surfaces. Applied Spectroscopy, 2007. 61(6): p. 613-620.

123 10. Tian, H., A.J. Bergren, and R.L. McCreery, Ultraviolet-visible spectroelectrochemistry of chemisorbed molecular layers on optically transparent carbon electrodes. Applied Spectroscopy, 2007. 61(11): p. 1246-1253.

11. Nowak, A.M. and R.L. McCreery, Characterization of carbon/nitroazobenzene/titanium molecular electronic junctions with photoelectron and Raman spectroscopy. Analytical Chemistry, 2004. 76(4): p. 1089-1097.

12. Nowak, A.M. and R.L. McCreery, In situ Raman spectroscopy of bias-induced structural changes in nitroazobenzene molecular electronic junctions. Journal of the American Chemical Society, 2004. 126(50): p. 16621-16631.

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14. Coe, J.V., et al., Metal Films with Arrays of Tiny Holes: Spectroscopy with Infrared Plasmonic Scaffolding. Journal of Physical Chemistry C, 2007. 111(47): p. 17459-17472.

15. Coe, J.V., et al., Extraordinary IR Transmission with Metallic Arrays of Subwavelength Holes. Analytical Chemistry, 2006. 78(5): p. 1384-1390.

16. Rodriguez, K.R., et al., Enhanced infrared absorption spectra of self-assembled alkanethiol monolayers using the extraordinary infrared transmission of metallic arrays of subwavelength apertures. Journal of Chemical Physics, 2004. 121(18): p. 8671-8675.

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124 20. Hurley, B.L. and R.L. McCreery, Covalent bonding of organic molecules to Cu and Al alloy 2024 T3 surfaces via diazonium ion reduction. Journal of the Electrochemical Society, 2004. 151(5): p. B252-B259.

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125 CHAPTER 5

A NEW METHOD OF SENSING TiO2 NANOCOATINGS BY EXTRAORDINARY INFRARED TRANSMISSION RESONANCE OF METAL MICROARRAYS

5.1 Introduction

Metal films with periodic sub-wavelength holes (grids or meshes) have been shown to exhibit the extraordinary transmission effect in the infrared region[1-9]. The fraction of the incident light transmitted through the mesh is greater than the fraction of the open surface area at one or more resonances. The sub-wavelength features of the mesh are able to couple light with surface plasmons, which propagate along the surface and tunnel through the holes, coming out as photons on the other side of the mesh. The transmission resonances of free standing mesh exhibit similar behaviors to those of metal coated prisms and attenuated total reflection (SP-ATR) devices, which are sensitive to nanoscale changes in coating thickness[10, 11].

Titanium dioxide (TiO2), as a high dielectric constant material, has been used in optical coatings, ultra-thin capacitors, catalyst supports, sensors, photovoltaic cells, and as a basic material for integrated waveguides[12-15]. In this chapter, the effect of the thickness of TiO2 coatings on the zero-order IR transmission resonances of Ni mesh was studied. Unlike the angle-scanned and wavelength-fixed SP-ATR experiments, where SP

126 reflection resonances are obtained by monitoring the fixed-wavelength laser intensity

reflected from a metal coated prism vs. angle, the changes in mesh transmission

resonances with coating thickness are measured by scanning through different

wavelengths at perpendicular incidence. Red shifting, attenuation and broadening of the

predominant resonance curves were observed as a function of coating thickness. Two

models have been developed to describe the relation of mesh resonance position shifts

and coating thickness.

The first model is based on Pockrand’s 2nd order SP-ATR theory for absorbing

coatings[10]. The measurements of Ni mesh resonances at perpendicular incidence can be

converted into shifts at specific wavelengths in momentum space, which makes the

experimental data based on meshes directly comparable to those obtained by SP-ATR

devices. Using SP-ATR theory as a guide, a theoretical model is built to extract explicit thickness and dielectric permittivity information of mesh nanocoatings from the shifts, attenuation, and broadening of mesh resonances in zero-order transmission spectra at perpendicular incidence.

The second model is based on the effective refractive index (neff’) of a coating as a

sigmoidal function of thickness, varying between the uncoated and thickly coated ~ ~ extremes. The value of neff’ at perpendicular incidence is determined by (1/υ L), where υ is the position of (1,0) resonance and the hole-to-hole spacing L is determined independently by the diffraction pattern.

Traditionally, the thickness of nanocoatings is measured by AFM or ellipsometry.

The new method presented in this chapter provides a quick and efficient way to measure

the thickness by infrared spectroscopy.

127 5.2 Experimental

Deposition of TiO2 films on Ni mesh

TiO2 films of 15-105 nm thickness were deposited on the smooth side of a piece

of Ni mesh (Precision Eforming) by a Veeco/Telemark E-Beam Evaporator at 4x10-6

Torr. A 15 nm thick TiO2 film was deposited each time on the same piece of Ni mesh

successively to achieve the desired thickness. A quartz crystal capacitive microbalance

was used to monitor the thicknesses with an uncertainty of ±2 nm.

TiO2 films of 20, 40, 60, 80, 100, 120, 140 and 160 nm thickness were prepared

by the McCreey group at the University of Alberta. A 20 nm thick TiO2 film was

deposited on the smooth side of Ni mesh each time. 20, 40, 60 and 80 nm TiO2 films

were deposited next to each other on a piece of Ni mesh, 100, 120, 140 and 160 nm TiO2 films were prepared by the same method on another piece of Ni mesh.

Measurement of the hole- to-hole spacing L by diffraction pattern

A laser light with a wavelength of 632.8 nm was used as the incident light on a Ni

mesh. A graph paper with 1mm line spacing was put 1.000 m (z) away from the mesh to

measure the distance between two adjacent diffraction spots (x). The hole-to-hole spacing

L can be calculated by the equation

2 ⎛ z ⎞ 2 2λ xzxxL λ222 ≈++−−= ⎜ ⎟ +1λ (5.1). ⎝ x ⎠

The hole-to-hole spacing L of Ni mesh deposited with TiO2 films of 20, 40, 60, 80, 100,

120, 140 and 160 nm thickness is 12.6 μm.

128 Raman Spectroscopy

Raman spectra were recorded using a Renishaw 3000 micro-Raman system with

an Olympus BH-2 microscope. A helium-neon laser operating at 633 nm was used as the

excitation source. The 50 X magnification objective was selected to focus the laser light on the surface of the TiO2 coated Ni mesh. Raman spectra of rutile and anatase powders were taken on a glass slide. The spectral resolution was set at 1.0 cm-1.

Infrared Spectroscopy

Zero-order, FTIR transmission spectra of Ni mesh before and after deposition of

TiO2 films were recorded by a Perkin Elmer Spectrum GX FTIR spectrometer at

perpendicular incidence. N2 purge gas was used to reduce H2O and CO2 in the sample

chamber. For each spectrum, 500 scans were collected at 4 cm-1 resolution with a DTGS

detector. The isolated resonance positions, full-width-at-half-maximum (FWHM), and

intensities of peaks were determined by Perkin Elmer software (Spectrum 5.3.1). Since

both of the “+” resonances are small and the (1,1)+ resonance is a shoulder, the “+”

resonances were isolated from their larger “-” partners by spline modeling of the “-”

resonance wings adjacent to the smaller “+” resonances.

5.3 Characterization of Deposited TiO2 films

White TiO2 beads were used as the source for the deposition of thin TiO2 films.

TiO2 coatings of 20, 40, 60, 80, 100, 120, 140, and 160 nm thicknesses were deposited on

Ni meshes as shown in Figure 5.1. The colors of the TiO2 films on Ni mesh vary with

thickness due to the interference at air/ TiO2 interface and within the TiO2 film.

129

20 nm 40 nm 60 nm 80 nm 100 nm 120 nm 140 nm 160 nm

Figure 5.1: Photograph of TiO2 films with film thicknesses of 20, 40, 60, 80, 100, 120, 140 and 160 nm on Ni mesh.

130 Since Raman spectroscopy is very sensitive to the phase state and composition of

the TiO2 powders, it was used to characterize the TiO2 films deposited by e-beam. Raman

spectra of 160 nm TiO2 film on Ni mesh, anatase and rutile powders are shown in Figure

-1 5.2. Two broad bands near 430 and 620 cm were observed for 160 nm TiO2 film on Ni

mesh. A broad peak around 160 cm-1 was caused by the strong absorption of Rayleigh

scattered light by the notch filter at the excitation wavelength[16]. Compared with the

Raman spectra of anatase and rutile powders, the lack of distinctive peaks and the

broadening of the Raman peaks indicate that the TiO2 films deposited by an e-beam are

amorphous[16].

5.4 Resonance Response of Ni mesh Before Coating

Ni meshes used in this work have biperiodic arrays of uniform square holes with a

hole-to-hole spacing L of 12.7 μm. The holes of ~5 μm width are arranged in a square

pattern on the 2 μm thick Ni film. Scanning electron microscope (SEM) images of the

dull side, shiny side, flank and a single hole of the mesh are shown in Figure 5.3[17].

Two predominant resonances labeled as (1,0) and (1,1) can be observed from the IR transmission spectra of this Ni mesh as shown in Figure 5.4 and 5.5. Each resonance is split into a large and small peak labeled as “-” and “+” respectively. The splitting of each resonance is due to the coupling of the propagating SPs between the front and back surfaces of the mesh through the holes. The splitting occurs more prominently when the thickness of the mesh is less than the wavelength of incident radiation[4, 5]. At perpendicular incidence, resonances of a square lattice occur at

131

c. Rutile powder

b. Anatase powder

a.160nm TiO2/Ni

100 200 300 400 500 600 700 800 -1 Raman shift, cm

Figure 5.2: Raman spectra of (a) 160 nm TiO2 film on Ni mesh, (b) anatase powder and (c) rutile powder.

132

Figure 5.3: SEM images of the dull side, shiny side, flank and a single hole of the mesh. The white bar is 10 μm for the first three images and 1 μm for the last image. The hole-to-hole spacing L is 12.7 μm, the hole width is 5 μm and the thickness of the film is 3 μm[17].

133 22 ~ + ji ν ji ),( ± = ' (5.1), Lneff ,±

where i,j are the integer steps from the origin along the reciprocal lattice, L is the

' hole-to-hole spacing (lattice parameter) of the array, and neff is the real part of the

effective index of refraction of the air/metal perforated interface. The effective index of

' refraction, neff is given by

' neff = { + εεεε smsm )/(Re } (5.2),

in which ε m and ε s are the complex dielectrics of the metal and substrate

' respectively[10, 11]. Based on this equation, neff ,+ is 1.000 for most smooth air/metal

interfaces in IR region. Experimental data of resonance positions can be used to examine

~ -1 the validity of equation (5.1). The position of the (1,0)+ resonance at ν )0,1( + =789.3 cm

' and neff ,+ =1.000 determines a value of L=12.7 μm which is in agreement with SEM

~ ~ measurements. Additionally, the ratio ν + /ν )0,1()1,1( + is 1.416 which is within 0.2% of

the 2 as predicted by equation (5.1). Currently no good theoretical models on mesh can

' be used to predict the quantity neff ,− . The observed position of “-” resonances were used

' to determine values of neff ,− by rearrangement of equation (5.1)

' 22 ~ ' ( eff ,− += /(Ljin ν ji ),( − ) . The observed positions correspond to neff ,− of 1.050 at 752.0

cm-1 and 1.078 at 1035.7 cm-1 for the uncoated mesh.

134 5.5 Resonance Response of Ni mesh to TiO2 Nanocoatings

IR transmission spectra of Ni mesh before and after being coated with 15, 30, 45,

60, 75, 90 and 105 nm TiO2 films are plotted in Figure 5.4. TiO2 coatings were deposited

on three different meshes and the resonance positions of each mesh before and after

coating are listed in Table 5.1. The differences in the flatness of the mesh could cause

systematic errors in IR measurements, which can be minimized by using the same piece

of Ni mesh as substrate. Therefore, all the data analysis are based on measurements on

mesh 3 which has coatings of 60, 75, 90 and 105 nm TiO2 on the same substrate.

Resonances of this Ni mesh responding to TiO2 nanocoatings are shown in Figure 5.5,

which exhibit red shifting, attenuation and broadening. This is similar to Pockrand’s

SP-ATR work on metal films with absorbing coatings[10]. The theoretical model

developed by Pockrand provides important guidance in interpreting mesh results. The

transmission resonance peak centers, full widths at half maximum (FWHM) and heights

of Ni mesh before and after coating are listed in Table 5.2. The shifts in resonance

positions before and after coating relative to the uncoated (1,0)+ position for all of the

resonances as a function of the TiO2 coating thickness are depicted in Figure 5.5.

Quadratic polynomial fits are given in Figure 5.6 to show the trend of the data. All the

resonances respond to TiO2 coatings with red shifting, and the “-” resonances exhibit

more shift than the “+” resonances. Due to the front-back coupling of SPs through the holes, the “-” resonances also have offsets without coatings which are considerably larger than the coating shifts. The (1,1)- resonance responds to TiO2 nanocoatings with the

largest shift, but it is a broad peak (FWHM = ~220 cm-1) that shows more scatter than the

-1 (1,0)- resonance (FWHM = ~50 cm ). Although the “+” resonances display very similar

135 30.00 (1,0) before - 30.00 15 nm (1,0)- 30 nm 45 nm 60 nm 25.0 75 nm 90 nm 25.0 105 nm 20.0

15.0

20.0 10.0 %Transmission (1,0) + 5.0

15.0 0.00 650.0 700 750 800 850.0

%Transmission 10.0 (1,1)-

(1,0)+

5.0 (1,1)+

0.00 600.0 700 800 900 1000 1100 1200 1300 1400 1500 1600 1700 1800 1900 2000.0 ν~(cm-1)

Figure 5.4: Zero-order IR transmission spectra of the Ni mesh at perpendicular incidence before and after coated with TiO2. Measurements were obtained from three different pieces of Ni mesh. The (1,0)± resonance region is expanded in the inset so that the shifts, attenuation, and broadening of the (1,0)- resonance can be better discerned.

136 Samples Resonances (cm-1)

(1,0)- (1,0)+ (1,1)- mesh 1 before coating 754.8 791.3 1038.8

15 nm TiO2/mesh 1 754.1 789.9 1040.1

30 nm TiO2/mesh 1 752.9 788.8 1035.8 mesh 2 before coating 754.5 790.1 1038.9

45 nm TiO2/mesh 2 751.4 787.7 1033.1 mesh 3 before coating 752.0 789.3 1035.7

60 nm TiO2/mesh 3 748.0 786.3 1029.0

75 nm TiO2/mesh 3 746.2 785.2 1025.8

90 nm TiO2/mesh 3 744.2 784.0 1024.4

105 nm TiO2/mesh 3 742.2 783.1 1020.9

Table 5.1: Transmission resonance peak centers before and after TiO2 coatings.

137

Figure 5.5: Zero-order IR transmission spectra of the Ni mesh at perpendicular incidence before and after successive coatings of TiO2. The (1,0)± resonance region is expanded in the inset so that the shifts, attenuation, and broadening of the (1,0)- resonance can be better discerned[17].

138

Coating thickness (nm) Resonances (1,0)- (1,0)+ (1,1)- (1,1)+ center 752.0 cm-1 789.3 cm-1 1035.7 cm-1 1117.8(1120.3) cm-1 0 FWHM 43.4 cm-1 (29.1) cm-1 221 cm-1 (22.1) cm-1 height 35.5% (2.0)% 9.0% (0.31)%

center 748.4 cm-1 786.1 cm-1 1029.0 cm-1 1115.3(1117.1) cm-1 60 FWHM 50.4 cm-1 (22.1) cm-1 214 cm-1 (26.4) cm-1 height 26.1% (0.68)% 8.5% (0.20)%

center 746.2 cm-1 785.2 cm-1 1025.8 cm-1 1114.6(1116.2) cm-1 -1 -1 -1 -1 75 FWHM 53.1 cm (21.2) cm 211 cm (28.2) cm height 24.0% (0.81)% 8.5% (0.23)%

center 744.2 cm-1 784.0 cm-1 1024.4 cm-1 1113.7(1114.1) cm-1 90 FWHM 55.5 cm-1 (20.8) cm-1 212 cm-1 (24.7) cm-1 height 21.6% (0.53)% 8.3% (0.18)%

center 742.2 cm-1 783.1 cm-1 1020.9 cm-1 1113.1(1115.0) cm-1 105 FWHM 58.9 cm-1 (20.6) cm-1 212 cm-1 (28.3) cm-1 height 20.5% (0.43)% 8.4% (0.19)%

*Quantities in parenthesis were determined upon isolating the resonance by subtracting a functional form for the larger nearby resonance.

Table 5.2: Transmission resonance peak centers, FWHM and heights (% transmission relative to No mesh in the spectrometer) before and after TiO2 coatings.

139

Figure 5.6: Shifts of transmission resonances vs. thickness of TiO2 coating relative to the uncoated position of the (1,0)+ resonance. All of the resonances show response to coating thickness, but the “-” resonances show more response than the “+” resonances. The “-” resonances show large shifts without coating that is attributed to radiation damping associated with front-back coupling of SPs through the mesh holes. Note that the radiation damping shift is considerably larger than the shifts due to coating[17].

140 shifts, the (1,1)+ is a small shoulder with uncertainty comparable to its magnitude.

Therefore, the best calibration curve for measuring the thickness of unknown TiO2 nanocoating samples would be based on the response of the most prominent and well defined (1,0)- resonance.

A very thick coating of amorphous TiO2 can shift the (1,0)- resonance from uncoated mesh 752.0 cm-1 to ~313.2 cm-1 (This is estimated based on equation (5.1) and

(5.2), assuming ε m =-1965 andε s =6.3[18]). This large potential frequency shift suggests

that a broad range of experimental conditions exists for analyzing thicker coatings. Since

positions of well defined features can be measured by standard benchtop FTIR to about

0.1 cm-1, there is also potential (with careful setup and signal averaging) to characterize

TiO2 coating thicknesses below 60 nm.

While the widths of the (1,0)+ and (1,1)± resonances change little with coating

-1 thickness, the width of the (1,0)- resonance (FWHM) varies from 43 cm before coating

-1 to 59 cm with the 105 nm thick TiO2 coating. By analogy to SP-ATR results[11], the

attenuation of the (1,0)- resonance and increase of its width are likely due to the

imaginary component of the coating’s dielectric permittivity and the position shifting is

mainly due to the real component of the coating’s dielectric permittivity. Consequently,

these results could potentially be used for measuring the complex dielectric permittivity

of nanoscale coatings on mesh. The infrared SP resonances are sensitive to the thickness

and dielectric properties of nanocoatings of TiO2. However, unlike SP-ATR experiments,

each resonance position of successively thicker coatings corresponds to a different

wavelength. This makes determining thickness and dielectric permittivity from such

results more complicated. 141 5.6 Determination of Shifts in Momentum Space ~ A dispersion diagram plotting ν (wavenumber) vs. kx (surface momentum

wavevector along the holes) of mesh before and after coating is shown in Figure 5.7.

Zero-order IR transmission spectra at perpendicular incidence before (blue) and after

(green) coating are displayed on the left. The momentum wavevector component of light ~ parallel to the surface is k x = 2πν sinθ , where θ is the angle of the incident radiation

from the surface normal along the direction of the holes. At perpendicular incidence, kx=0,

dots are used to indicate measured positions of (1,0)- and (1,0)+ resonances. In order to ~ compare to SP-ATR theory in the next section, ν )0,1( + of the uncoated mesh (very close to

1/L) is used as a reference point for resonance position shifts ( Δν~ ). The shift of the

(1,0)+ resonance upon coating, and the shift of the (1,0)- resonance before and after

coating are depicted. The surface of Ni mesh can transfer momentum in units of 2π/L on

the (1,0) resonance , so the measured resonance positions at kx=0 are matched to points

on SP dispersion curves at kx=2π/L. The SP dispersion curves are split into SP+ and SP- curves due to the coupling of the propagating SPs between the front and back surfaces of the mesh through the holes. The SP+ curve for uncoated mesh has been drawn close to the light line (without any surface oxide it would be right on the light line). Notice that the

SP curves shift to higher momentum wavevector (kx) after coating with TiO2 (green lines). ~ The diagram in Figure 5.6 shows that the frequency shift ( Δν ) and the shift ( Δk x ) in

~ momentum space are related by the equation Δk x = 2πΔν .

142

Figure 5.7: Dispersion diagram showing the conversion of shifts in frequency space ( Δν~ ) to shifts in momentum space ( Δk x ) in order to facilitate the comparison to SP-ATR work. Transmission spectra before (blue) and after coating (green) are shown at left where peak positions correspond to points at kx=0 on the diagram. They are coupled to SP dispersion ~ curves in units of 2π/L and therefore are the same at kx=2π/L. The shifts in Δν are all referenced to the uncoated SP+ dispersion curve which lies very close to the light line. ~ The conversion is simply Δk x = 2πΔν , but the Δk x shifts correspond to different values of ν~ , unlike with SP-ATR work[17].

143 5.7 Comparison to SP-ATR Theory

Pockrand’s SP-ATR experimental evidence on metal films with absorbing and nonabsorbing coatings is very similar to results on mesh with TiO2 coating[10]. His

2nd-order theory correlates the shift of SPR resonances with coating thickness, which

provides important guidance in interpreting mesh results. In terms of shifts from the uncoated SP+ curve, the shifts of the complex SP momentum wavevector with coating is described as

k x =Δ ()(Coating + Radiation damping )+ ( RadandCoatofninteractio .. ) )1( C )2( C )1( R )2( R )2( CR (5.3), ()x x ()x x kkkkk x +++++= ...

where the bottom line uses Pockrand's notation and underlined quantities are complex[10]. Explicit expressions for each of these terms as functions of thickness and dielectric permittivities of the coating, metal, and surrounding dielectrics were derived by

)1( C )2( C st nd Pockrand. The quantity ( x + kk x ) is the 1 and 2 order contribution to the shift

)1( R )2( R st nd due to the coating, the quantity ( x + kk x ) is the 1 and 2 order contribution due to

)2( CR nd radiation damping, and k x is a 2 order term describing the interaction of the

coating and radiation damping. Each of these terms have been indicated for the (1,0)

)1( C )2( C resonances in Figure 5.6. The term ( x + kk x ) is given by 2π times the shifts of the

“+” resonances, shown in the bottom two traces in Figure 5.6. Pockrand’s explicit

expressions may be directly applicable to these shifts (as will be presented shortly). The

)1( R )2( R term ( x + kk x ) is due to radiation damping and is given on the left side of Figure

5.6 at zero thickness by 2π times the shifts of the “-” resonances. In SP-ATR experiments,

144 radiation damping is due to coupling of SPs on the two smooth metal/dielectric interfaces

through the thin metal film. Pockrand’s explicit radiation damping expressions are not

applicable to mesh due to mesh’s specific geometry, but indicate a type of theoretical

modeling that would be very useful on mesh. Notice that the radiation damping shift

-1 -1 before coating of the mesh [2π(36 cm ) for the (1,0)- resonance and 2π(84 cm ) for the

(1,1)- resonance] is much greater than the shifts of any of the resonances due to coating.

)2( CR Finally, the last term of equation (5.3), k x (interaction of radiation and coating

damping), is demonstrated in Figure 5.6 as the increase in response of the “-” resonances

to coating thickness over that of the “+” resonances. The “+” resonances shift 2π(6 cm-1)

-1 -1 -1 cm with a 105 nm coating in momentum space, while the (1,0)- shifts 2π(10 cm ) cm ,

-1 -1 )2( CR and the (1,1)- shifts 2π(15 cm ) cm . Note that the k x term is comparable in

)1( C )2( C importance to the ( x + kk x ) term on mesh, which indicates the greater importance

of radiation damping. Lacking a model for radiation damping on mesh, the shifts of the

“+” resonances are the only recourse for calculating dielectric properties, in spite of the

fact that they are considerably less intense. Using complex dielectrics of the metal,

"' "' coating, and surrounding medium as += iεεε mmm , += iεεε ccc , and = εε dd ,

)1( C )2( C respectively, Pockrand’s explicit expressions for ( x + kk x ) can be rewritten using

his prescription for absorbing coatings and collecting on coating thickness (dc) as

⎛ ε " ⎞ 2 BA )1( C kk )2( C ⎜ 1 −=+ m ⎟ ν~ 2 dAi + ν~ d 23 (5.4), x x ⎜ 2ε ' ⎟ c ' c ⎝ m ⎠ εε dm 4π ' m + εε d

where 145 ' 2 ' ⎛ − εε dc ⎞⎛ εε ⎞ ⎛ − εε mc ⎞ − 2/1 A = π )2( 2 ⎜ ⎟⎜ dm ⎟ ⎜ ⎟()− ' εε (5.5) ⎜ ε ⎟⎜ ' + εε ⎟ ⎜ − εε ' ⎟ dm ⎝ c ⎠⎝ dm ⎠ ⎝ md ⎠

2 2 ' 2 − εε cd + εε B = 2 + dm (5.6). − εεε cdd )( − ε d

The shifts in frequency space at perpendicular incidence are just the real part of equation

(5.4) divided by 2π. A simple expression for the shifts of “+” resonances in frequency space for coating can be written as,

~ ~ 2 ~ 23 =Δ− νν c + ν dDdC c (5.7), where C and D are the real parts of the corresponding terms in equation (5.4) divided by

2π. Accounting for wavelength change is very important since the square and cube of

ν~ are terms of equation (5.7). Equation (5.7) provides a solution for using the shifts of mesh transmission resonances at perpendicular incidence to determine the thickness and dielectric properties of thin coatings. A reasonable fit to equation (5.7) are obtained from the (1,0)+ shifts as,

~ ~ 2 ~ 23 ν )0,1( + ()±=Δ 05.078.0 ν d c ( ±+ 724 )ν d c (5.8),

~ ~ -1 where Δ ν and ν are in cm , dc is the thickness of the TiO2 coating in cm (not nm), the uncertainties are estimated standard deviations (e.s.d.), and the e.s.d of the fit is 0.1 cm-1.

There is a small (but unknown) shift caused by the unavoidable NiO coating of a Ni mesh.

The effect is additive because all of the results reported herein are based on the same mesh with the same NiO thickness. But the total response is intrinsically nonlinear, so experiments with different NiO coatings may result in different C and D fit parameters as defined in equations (5.4-5.7). Notice that an acceptable fit can not be obtained from the

146 (1,1)+ shifts because of its considerably greater uncertainty.

The fit constants, C and D, can be calculated directly given dielectric values of the metal, coating, and surroundings. The unavoidable NiO coating formed on the Ni mesh before TiO2 coating impedes the direct application of the results in the preceding

-1 paragraph. For example, the 6.2 cm shift of the 105 nm TiO2 coating can be simulated

' ' using equations (5.4-5.7) and values of ε m =-58 (for the effective Ni/NiO system), ε s =6.3

(TiO2 films deposited by evaporation using ion-beam source[18]), and ε d =1, producing equation (5.7) constants (C and D) of 0.78 and 22 which are very close to the empirical

' values. The simulated ε m value is much smaller than the pure Ni value[19] of -1965 at

783.1 cm-1. This suggests that the NiO layer significantly reduces of the magnitude of

Ni’s dielectric value for the TiO2 coatings. Since there is no theoretical model for the dielectric constant of microarrays, the validity of the simulated values are still under investigation. Assuming there is no NiO coating and fixing the metal and coating at the

-1 Ni and TiO2 values, these equations predict only 1.5 cm of red shift at thickness of 105 nm, rather than the observed 6.2 cm-1. Therefore, the NiO coating may be enhancing the effect over bare metal by a factor of 4. Similar results are observed for SiO2 coatings with coupled plasmon-waveguide resonators[20]. Finally, the magnitude of second term in equation (5.7) is comparable to the first with 105 nm TiO2 coatings. Films of this thickness on the studied mesh lie just outside the linear region, which indicates 2nd order terms are important.

The radiation damping has great influence on the “-” resonances. Without a TiO2 coating, the (1,0)- and (1,1)- resonances are shifted from the corresponding “+” resonance

147 by 37 and 84 cm-1 in frequency space, respectively. These values are much greater than

-1 the 6, 10, and 15 cm changes observed for the (1,0)+, (1,0)- and (1,1)- resonances, respectively, due to a 105 nm coating. The 2nd term on the r.h.s. of equation (5.3) for

)1( R )2( R st radiation damping, x + kk x , seems to be considerably larger on mesh than the 1

)1( C )2( C term, x + kk x , which is not typically the case with SP-ATR prism systems.

The greater sensitivity to coating thickness of the (1,0)- curve (see Figure 5.6) over the (1,0)+ curve is likely due to the interaction of coating and radiation damping,

rd )2( CR which is represented as the 3 term on the r.h.s. of eq. (2), k x . In order to characterize this type of interaction, shifts of the (1,0)- resonance positions from their uncoated position were also fit to the form of equation (5.7) giving

~ ~ ~ 2 ~ 23 ν − ,)0,1( coated − ν − ,)0,1( uncoated ( ±= 07.074.0 )ν d c ( ±+ 11118 )ν d c (5.9), with a fit e.s.d. of 0.17 cm-1. The difference between equations (5.9) and (5.8),

~ 2 ~ 23 ()()± 09.004.0 ν d c ±+ 1386 ν d c , reveals the enhancement in the response of the (1,0)- resonance due to the interaction of radiation damping and coating (over coating alone). The first term is extremely small suggesting that higher order terms of radiation damping are responsible for the enhancement in sensitivity to coating.

5.8 Calibration Curve Based on Sigmoidal Function

' The neff of Ag film with LiF coatings at different coating thickness are plotted versus the natural logarithm of coating thickness in Figure 5.8 (experimental data were digitized from Pockrand’s paper[10]). Both simple and logistic sigmoidal functions were

148 applied to fit the experimental data. The fitting curves indicate that the logistic sigmoidal function fits better for the experimental data. Pockrand’s SP-ATR experiments

' demonstrate that neff goes as a sigmoidal function of thickness varying between the uncoated and thickly coated extremes[10]. Using it as guidance, a theoretical model based on sigmoidal functions was developed for measuring the thickness of TiO2 on Ni mesh in a broader range.

As discussed in the above sections, resonances of uncoated mesh are defined by

' neff = { εε mm + )1/(Re } + (effect of front-back coupling) (5.10).

A full coating will shift the resonance to

' neff = { + εεεε cmcm )/(Re } + (effect of front-back coupling in medium) (5.11).

' ~ ~ The value of neff can be determined by 1/υ L, where υ is the position of (1,0) resonance at perpendicular incidence and L is determined independently from diffraction pattern

' ~ (12.6 μm). The transmission resonance (1,0)- peak centers and neff (determined by 1/υ L ) of Ni mesh before and after coating are listed in Table 5.3. In a logistic sigmoidal

' function, neff can be represented as

' 0 − nn c neff = P + nc (5.12); ⎛ d ⎞ 1+ ⎜ ⎟ ⎝ B ⎠

' In a simple sigmoidal function, neff can be represented as

− nn n' = c 0 + n (5.13), eff B 0 1+ d

149

1.4

Simple sigmoidal function .1 3564 − .1 0014 n' + .1 0014 1.3 eff = 98.501 1+ d Logistic sigmoidal function .1 0014 − .1 3564 n' + .1 3564 1.2 eff = .1 6038 ⎛ d ⎞ 1+ ⎜ ⎟ ⎝ 98.216 ⎠

' eff n 1.1

1.0

0.9 012345678 ln(d(nm))

' Figure 5.8: The neff of Ag film with LiF coatings at different coating thickness versus natural logarithm of coating thickness. The red dots are experimental data, the black curve is the fitted by logistic sigmoidal function, and the blue curve is the fitted by simple sigmoidal function.

150 Coating thickness (nm) Resonance centers (1,0) (cm-1) ' -1 neff

Sample 1

0 758.9 1.046

20 757 1.048

40 753.4 1.053

60 749.5 1.059

80 745.7 1.064

Sample 2

100 742.9 1.068

120 742 1.070

140 742 1.070

160 740.4 1.072

Sample 3

20 753.8 1.053

40 751.2 1.056

60 747.9 1.061

80 744.5 1.066

Sample 4

100 756.2 1.050

120 752 1.055

140 748.6 1.060

160 746.7 1.063

Sample 5

15 754.1 1.044

30 752.9 1.046

Sample 6

45 751.4 1.048

Sample 6

60 748 1.053

75 746.2 1.055

90 744.2 1.058

105 742.2 1.061

Table 5.3: Transmission resonance (1,0)- peak centers before and after TiO2 coatings.

151 ' ' where nc is neff at thickly coated extremes, n0 is neff before coating, B is a slope parameter, d is the thickness (nm) of the coating, and P is power. n0 is determined by the resonance before coating, which is 1.046 at 758.9 cm-1. Since the contribution of

' front-back coupling in medium to neff is unknown and relatively small compared to the first term in equation (5.11), nc is estimated by the first term of equation (5.11) as 2.658,

' ' assuming ε m =-58 (for the effective Ni/NiO system), ε s =6.3. Based on (1,0)- resonances, a reasonable fit to logistic sigmoidal function equation (5.12) is obtained as

' − 658.2046.1 neff = 9057.0 + 658.2 (5.14). ⎛ d ⎞ 1+ ⎜ ⎟ ⎝16121⎠

The same experimental data were also fitted by a simple sigmoidal function (5.13) as

− 046.1658.2 n' = + 046.1 (5.15). eff 10092 1+ d

' The experimental data and fitted functions are plotted as neff vs. ln(d) in Figure 5.9. Both logistic and simple sigmoid function curves provide a reasonable description of the

' relationship between neff and the thickness of the coating. When the coating thickness d

' is small, the response of neff to ln(d) is near linear. As the coating thickness increases, the response becomes exponential. As saturation begins, the growth slows, and at maturity, growth stops. The method is capable of measuring the thickness of TiO2 in a very broad range. The experimental data only cover a small fraction of the calibration curve. The validation of the method, especially for thick coatings, needs to be investigated further.

There are several possibilities which could cause the deviation of the experimental data

152

3.0

2.5 1.08

1.07

1.06 ' eff 2.0 n

' eff 1.05 n

1.04 1.5 23456 ln(d(nm))

1.0

0246810121416 ln(d(nm))

' Figure 5.9: The neff of Ni mesh with TiO2 coatings at different coating thickness versus natural logarithm of coating thickness. The red dots are experimental data, the black curve is the fitted by logistic sigmoidal function, and the blue curve is the fitted by simple sigmoidal function. The expansion of the plot in the region of the experimental data is expanded in the inset.

153 from the theoretical model. First, resonance response of Ni mesh to materials in holes is more sensitive than to materials outside of holes. TiO2 is deposited mainly on the flat surface of the mesh, and is presumably much thinner inside the holes. The uncertainty of the thickness of TiO2 at edges of holes makes it hard to predict the resonance response of the TiO2 covered Ni mesh. Second, TiO2 coatings with different thickness are deposited on different areas of the mesh. The imperfect stretching of the mesh could cause slight difference in flatness of the mesh, which may influence the resonance positions of IR transmission spectra. The successive deposition on one single mesh, like the sample preparation for our first experiment, could solve the problem by canceling out systematic errors. Third, the coverage of the NiO on mesh surface could vary from sample to sample due to different deposition treatment. Fourth, TiO2 coatings deposited by e-beam have uncertainty of ±2 nm in thickness.

The experimental data are in the linear region of the calibration curve. Adding a thin layer of polymer to the surface of Ni mesh can push the measured signals of TiO2 thin films close to the inflection point, which could be used to improve the sensitivity of this technique.

5.9 Molecules on TiO2

Surface plasmons enhanced infrared spectra of several organic molecules (acetic acid, formic acid, propanoic acid, methanol, and ethanol) on 105 nm TiO2 coated Ni mesh are shown in Figure 5.10. The proton transfer process was observed for ethanol being converted to aldehyde on TiO2 film surface. This technique can be used to explore intermediates and products of reactions on the surface of TiO2 film.

154

Figure 5.10: Enhanced IR absorption spectra of a variety of molecules on a 105 nm TiO2 coated Ni mesh.

155 5.10 Conclusions

Two theoretical models have been developed to investigate the response of IR transmission resonances to nanocoatings. The shifts in the positions of transmission resonances of Ni microarrays (with thin NiO coatings) with TiO2 coatings in the 15-160 nm range of thickness can easily be measured by commercial bench top FTIR spectrometers. The positions of the “+” resonances have been modeled with the same theory used for SP-ATR experiments except for the effects of radiation damping. Ni mesh arrays exhibit considerably stronger radiation damping than their SP-ATR coated prism counterparts, which gives rise to greater sensitivity to TiO2 nanocoatings for the “-” resonances. Theoretical modeling of the relation of radiation damping on mesh to the dielectric properties of coatings is needed for further investigation of Ni mesh. It has been demonstrated that complex dielectric constants of coatings on metal microarrays can potentially be determined using FTIR spectrometer measurements of transmission resonances. Basically, the sigmoidal functions represent a simple and quantitative method, which gives a broader view of the resonance response to coating thickness. The FTIR transmission method has potential to measure thinner and thicker films than studied herein. The two theoretical models proposed in this chapter may be applied to measuring the thickness of both absorbing and nonabsorbing nanocoatings.

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