THE SYNTHESIS AND CHARACTERIZATION OF PHOSPHONIC ACIDS FOR THE SURFACE MODIFICATION STUDY ON INDIUM TIN OXIDE A Dissertation Presented to The Academic Faculty by Guanhua Feng In Partial Fulfillment of the Requirements for the Degree Master of Science in the School of Chemistry and Biochemistry Georgia Institute of Technology August 2012 THE SYNTHESIS AND CHARACTERIZATION OF PHOSPHONIC ACIDS FOR THE SURFACE MODIFICATION STUDY ON INDIUM TIN OXIDE Approved by: Dr. Seth Marder, Advisor School of Chemistry and Biochemistry Georgia Institute of Technology Dr. Jean-Luc Brédas School of Chemistry and Biochemistry Georgia Institute of Technology Dr. Joseph Perry School of Chemistry and Biochemistry Georgia Institute of Technology Date Approved: May 04, 2012 ACKNOWLEDGEMENTS I wish to thank my advisor, my committee members, the graduate program coordinator and my lab mates for all the help during the two years. Special thanks for my family and friends for supporting me throughout the years in Georgia Tech. iv TABLE OF CONTENTS Page ACKNOWLEDGEMENTS………………………………………………….................. iv LIST OF TABLES…………………………………………………............................... vii LIST OF FIGURES…………………………………………………............................ viii SUMMARY………………………………………………….......................................... ix CHAPTER 1 INTRODUCTION……………………………………………….……………....1 1.1 Brief Introduction to Organic Electronics………….…………………1 1.2 Processes in Devices and Interface Properties………………………..2 1.2.1 Basic Concepts and Definitions…………………………………….4 1.2.2 Surface Dipole………………………………………………………7 1.2.3 Charge Transfer Dipole……………………………..………………7 1.2.4 Intrinsic Molecular Dipole………………………….………..……..7 1.2.5 Background on Self–Assembled Monolayers (SAMs) …………….8 1.2.6 Work Function Tuning………………………….…………………..9 1.3 Phosphonate and Phosphonic Acid………………………….………10 1.3.1 Why Use Phosphonic Acids Rather Than Other Molecules for Metal Oxide Surface Modification………………………….……………...11 1.3.2 Possible Binding Modes of Phosphonic Acid on Metal Oxide Surface………………………….………………………….………..14 1.3.3 Work function study on phosphonic acids………………………..15 1.4 Organization of the Thesis………………………….………………17 1.5 References………………………….………………………….……17 v 2 SYNTHESIS AND SPECTROSCOPIC CHARACTERIZATION OF PHOSPHONATES AND PHOSPHONIC ACIDS…………………………….. 23 2.1 Synthetic Pathways………………………………………………….. 23 2.1.1 Michaelis – Arbuzov Reaction……………………………………. 23 2.1.2 Organometallic reactions………………………………………….. 24 2.1.3 Hydrolysis of Phosphonates to Form Phosphonic Acids…………..26 2.2 Notes on Synthesis Procedures……………………………………… 27 2.3 Experimental…………………………………………………………29 2.4 References…………………………………………………………....34 3 MODIFICATION OF PLANAR ITO SURFACES AND MEASUREMENTS. …………………………………………………………………………………..37 3.1 Background of Different Planar Substrates that Have Been Modified with Phosphonic Acids……………………………………………… 37 3.2 Methods of Surface Modification…………………………………… 38 3.2.1 Modification of ITO With Phosphonic Acids…………………….. 40 3.3 Photoelectron Spectroscopy………………………………………… 41 3.3.1 X–Ray Photoelectron Spectroscopy………………………………. 41 3.4 Results and Discussion……………………………………………… 42 3.4.1 Work Function Changes of ITO Substrates Using Phosphonic Acids ………………………….………………………….………………42 3.4.2 XPS Characterization………………………………………………45 3.5 Experimental…………………………………………………………48 3.6 References…………………………………………………………... 49 4 THESIS CONCLUSIONS……………………………………………………... 53 4.1 References…………………………………………………………... 60 vi LIST OF TABLES Page Table 3.1: Work function changes (∆Φ) of ITO surfaces modified by different phosphonic acids, obtained form Kelvin Probe measurements. (part a shows data for benzylphosphonic acids and phenylphosphonic acids using DSC modification procedure, part b shows data using DSC/OP procedure)……44 Table 3.2: P/In ratios of ITO surfaces modified by different phosphonic acids. Obtained form XPS. (part a shows data for benzylphosphonic acids and phenylphosphonic acids using DSC modification procedure, part b shows data using DSC/OP procedure)…………………………………………… 46 vii LIST OF FIGURES Page Figure 1.1: Schematic picture of a simplified OLED structure and related energy levels. …………………………………………………………………………...……5 Figure 1.2: Illustration showing the impact of water on the formation of monolayers from silanes. ……………………………………………………………………….12 Figure 1.3: Phosphonic acids used in this thesis. ……………………………………… 14 Figure 1.4: Possible binding scenarios of phosphonic acid on the metal oxide surface...17 Figure 2.1: Mechanism of the Michaelis – Arbuzov reaction. …………………………24 Figure 2.2: Mechanism of the catalytic cycle forming aryl C-P bonds by using palladium salt. ………………………………………………………………………….. 25 Figure 2.3: Optimized reaction condition for palladium catalyzed aryl C–P bond formation. …………………………………………………………………... 26 Figure 2.4: Mechanism of hydrolysis of a dialkylphosphonate with bromotrimethylsilane.……………………………………………………….. 27 Figure 3.1: The T-BAG setup and chemical reactions; colors shown for an example (4TPA).…………………………………… ………………………………... 39 Figure 3.2: Chemical structures of phosphonic acids used in the work function study: 4- nitrobenzylphosphonic acid (pNO2BnPA) (AJG-III-005B), 4- cyanobenzylphosphonic acid (pCNBnPA) (GHF-I-038A), 4- fluorobenzylphosphonic acid (pFBnPA) (AJG-III-11A), benzylphosphonic acid (BnPA) (AJG-III-073D), 4-nitrophenylphosphonic acid (pNO2PPA) (GHF-I-032E), 4-cyanophenylphosphonic acid (pCNPPA) (GHF-I-040A), 4- fluorophenylphosphonic acid (pFPPA) (PJH-III-054C), phenylphosphonic acid (PPA) (Commercially available)……………………………………………. 43 Figure 3.3: The ITO substrate is placed on a vial cap (optional) and then the container filled with the solution of phosphonic acid. Over time the phosphonic acid molecules will be adsorbed onto the ITO surface.………………………….. 49 Figure 4.1: Evolution comparisons……………………………………………………..56 Figure 4.2 Comparison of the work function changes measured by Kelvin Probe as a function of the dipole moment (µ) of the analogous mono substituted benzene of the corresponding phosphonic acids. ……………………………………..58 viii SUMMARY The synthesis and characterization of some phosphonic acids as well as the modification of indium tin oxide (ITO) substrates using these phosphonic acids are presented in this thesis. Phosphonic acids have been known to bind strongly to the surface of a number of metal oxides. ITO substrates were reported to be modified with a variety of surface modifiers. Herein the ITO substrates were modified with the chosen phosphonic acids with different functional groups in order to tune the work function and compare the work function changes with the functional group properties. ix CHAPTER 1 INTRODUCTION The content of this thesis focuses on surface modification of conducting oxides for organic electronics devices, i.e. changing electrode properties by modification of the electrode surfaces with phosphonic acids. Specifically the purpose is to investigate the evolution of the work function changes of indium tin oxide (ITO) upon modification of its surface using phosphonic acids with different head functional groups. This thesis will discuss the design, synthesis and characterization of the surface modifiers – phosphonic acids, characterization of the modified ITO surfaces and the property changes of the surfaces. 1.1 Brief Introduction to Organic Electronics “Organic electronics” represents a branch of study that includes both science and engineering that attempts to exploit the use of organic materials in combination with inorganic materials to be used as one or several active components in electronic and optical devices such as organic light emitting diodes (OLED), organic solar cells (OSC), organic thin film transistors (OFET), organic magnets (OM) for spintronics etc. This field has drawn substantial interests from the science and engineering community as well as considerable research investment. Because of this, technologies based upon organic materials have been advancing rapidly over the last 30 years or so since the discovery of the conductivity of polyacetylene,1,2 before which the possible application of organic molecules in field of molecular electronics3,4 has been realized by researchers in the mid 70’s. 1 Organic electronic devices have several potential advantages with respect to some inorganic counterparts such as the fact that they can be lightweight, flexible, in some cases processed from solution. As a consequence they have promise to be manufactured inexpensively, to achieve excellent device performance by some metrics. However, currently for most applications the conventional inorganic materials still outperform organic materials when judged by parameters such as higher overall efficiency, longer lifetime etc. which are the driving forces for the chemistry and materials science community to continually explore the solution processible, less expensive and stable organic materials that have better device performance. The organic materials used in the organic electronic devices are typically conjugated organic molecules and polymers, which are semiconducting in their undoped state. So the use of organic materials in the devices makes it possible to modify the electronic and optical properties of the device by tailoring the chemical structure of the organic molecule. Some areas of research in this field include exploration of novel semiconducting materials, surface modification (which can be used for matching the charge carrying energy level of the organic material and Fermi level of electrode surface), and optimization of device performance to name a few. The contents