Aqueous Niobium Chemistry at Low Ph: New Precursors for Oxide Thin Films

AN ABSTRACT OF THE DISSERTATION OF

Deok-Hie Park for the degree of Doctor of Philosophy in Chemistry presented on June 14, 2017.

Title: Aqueous Niobium Chemistry at Low pH: New Precursors for Oxide Thin Films

Abstract approved: ______Douglas A. Keszler

Phosphate and peroxide stabilize new oxo-hydroxo niobium clusters in water at low pH. The clusters open a new chapter in aqueous niobium chemistry under acidic conditions. The clusters also produce atomically smooth, amorphous niobium oxide phosphate (NbPOx) thin films. Reaction pathways from cluster solutions to amorphous niobium oxide phosphate solids are elucidated.

Four new peroxoniobium phosphate clusters – H6Nb5P1O13(O2)5 (Nb5P1),

H10Nb7P3O23(O2)7 (Nb7P3), H6Nb4P2O14(O2)4 (Nb4P2), and H10Nb6P4O24(O2)6, (Nb6P4)

– were synthesized at pH < 2. Crystal structures of two clusters as tetramethylammonium (TMA) salts, TMA5[HNb4P2O14(O2)4]·9H2O and

TMA3[H7Nb6P4O24(O2)6]·7H2O, were determined by X-ray crystallography. Solution speciation and cluster stability as a function of pH, phosphate concentration, and counterion (H+ and TMA+) were investigated with electrospray ionization mass spectrometry, dynamic light scattering, and small-angle X-ray scattering. Pair distribution function analysis of niobium oxide phosphate gel powders provides

insights about how cluster condensation leads to amorphous niobium oxide phosphate solids.

Thermochemical and physical properties of NbPOx thin films were explored as a function of Nb:P ratios and anneal temperature via temperature programmed desorption mass spectrometry, X-ray diffraction and reflectivity, spectroscopic ellipsometry, transmission and scanning electron microscopies, optical transmission and reflection spectroscopy, and atomic force microscopy. Wide tunability in refractive index and band gap of the films was demonstrated by controlling the phosphate concentration in precursor solutions. The NbPOx thin films may also be directly patterned via electron- beam exposures, and as an ultra-thin capping layer they enhance dehydration of solution-processed aluminum oxide phosphate (AlPO) thin films.

Aqueous Niobium Chemistry at Low pH: New Precursors for Oxide Thin Films

by Deok-Hie Park

A DISSERTATION

submitted to

Oregon State University

in partial fulfillment of the requirements for the degree of

Doctor of Philosophy

Presented June 14, 2017 Commencement June 2018

Doctor of Philosophy dissertation of Deok-Hie Park presented on June 14, 2017

APPROVED:

Major Professor, representing Chemistry

Chair of the Department of Chemistry

Dean of the Graduate School

I understand that my dissertation will become part of the permanent collection of Oregon State University libraries. My signature below authorizes release of my dissertation to any reader upon request.

Deok-Hie Park, Author

ACKNOWLEDGEMENTS

Looking back over the last five years, I think I enjoyed the challenges in my doctoral program in OSU. Everyday life in foreign country as well as research life have been challenging, but full of fun at the same time. I appreciate that I had this opportunity in my life. It would not have been possible to complete my Ph.D. without the help of many people around.

First and foremost, I would like to thank my academic advisor, Prof. Douglas A.

Keszler. I remember my first meeting with him: A meeting of two people without words.

I was anxious about whether I could communicate well with my advisor at that time.

However, it turned out not a problem. He inspired me with many interesting scientific ideas and critical questions on my research topics. Also, he helped me to stand as an independent scientist by providing me many resources, scientist networks, and freedom.

I also extend much gratitude to my committee members, Profs. May Nyman, Mas

Subramanian, David Cann, and Brady Gibbons. Their lectures during the coursework and insightful questions during my preliminary oral exam helped me to make breakthrough in my research.

I would also like to thank Prof Hie-Joon Kim, my master's advisor in Seoul National

University, for establishing the framework in which I can see the world with a language of chemistry. I am also grateful to Ms. Hye-Sung Cho, my last boss, for challenging me to achieve more in many aspects of my life when I worked in LG Chem. I would like to express my gratitude to Dr. Cheol-Hee Park for introducing me to Prof. Keszler.

I thank the faculty and staff in the department of chemistry at OSU including Rusty

Root, Paula Christie, Sarah Burton, Luanne Johnson, Talley Richardson, Margie Haak,

Michael Burand, and Prof. Rich Carter, who make the department function. I am grateful to professors, especially to Profs. Mike Lerner, Janet Tate, Milo Koretsky, John

Wager, for offering insightful courses, which helped me build up the foundation for interdisciplinary science knowledge.

Over the last five years, current and former Keszler group members made my PhD student life more fertile and productive by supporting each other in many ways. It has been a great pleasure to work with them. When I first joined the group, Shawn Decker introduced all the details I need to know to start my research. Ryan Mansergh and Dr.

Juan Carlos Ramos has been my good friends, brilliant coworkers, and excellent members of team Electrochromics (team EC). Dr. Cory Perkins has been the best post- doc for me to make progress in many projects with his enthusiasm for research. Jenn

Amador and Vasily Gouliouk kindly supported and encouraged me in many ways during rough time. Cheerful Nizan Kenane, who was most recently joined the group, became my best friend in the last year of my PhD. I also would like thank all other members I interacted with, Stephanie Ramos, Dr. Robert Kokenyesi, Dr. Wei Wang,

Dr. Rose Ruther, Dr. Sumit Saha, Yu Huang, Dr. Kris Olsen, and Dr. Jaeseok Heo for their help and support.

I appreciate many CSMC collaborators, especially Dr. Sara Goberna-Ferrón, Dr.

Jung-Ho Son, Nick Landau, and Hans Chiang. My dissertation would not be possible without the help from them.

I spent a lot of time to maintain the TPD-MS and ESI-MS during last few years. I am very thankful for all the help and support from Chris Tasker, Dr. Brendon Flynn, Ryan

Frederick, Nizan Kenane, Trey Diulus, Dr. Pedro Molina Sanchez, Dr. Benjamin (BJ)

Philmus, and Morgan Olsen to keep the mass spectrometers in good working order. I also appreciate Chris Tasker and Rick Presley for all their know-hows on the all instruments in the clean room.

My special thanks go to Dr. Bettye Maddux, Amanda Polley, and Sharon Betterton for all their help which facilitated my PhD student life here in OSU. Another special thanks go to Dr. Judy Giordan for providing me a great experience to complete all three stages of the Lens of the Market program.

My dear friend, Dr. Elena Medina, has been my best and closest friend for last 5 years.

We shared all the joys and sorrows together and won it through. I really appreciate her support for me from the heart. My good old friends, Jungjin Shim and John Kim, I could have not survived in this long journey without them. For all my friends I met here in US and old friends from Korea, although I could not name all of you here, I am very grateful for your support.

Last but not least, I’d like to express my heartfelt thanks to my family for their unconditional support from the other side of the Pacific Ocean during my PhD study.

They used to send me some Korean food and snack occasionally. My cheerful mom,

Myoung-Ok Son, always asked the questions over the phone, ‘did you have a meal today?’, ‘did you sleep well?’, or ‘have you talked to your advisor today?’. These questions reminded me the most important things for me in everyday life as a PhD student. I think my curiosity and interests in science came from my dad, Dong-Hae

Park. He always encouraged me to learn something. My sister, brother, and their families also have been my reliable supporters. 부모님, 감사합니다.

CONTRIBUTION OF AUTHORS

Prof. Douglas A. Keszler has been instrumental to all aspects of the work herein, from scientific inspiration and critical discussions to manuscript writing and editing. Prof.

May Nyman provided invaluable insight into the solution chemistries of polyoxoniobates. Dr. Kai Jiang has laid the foundation for this work by developing a procedure for precursor synthesis detailed in Chapter 2. Ryan H. Mansergh collected all SEM and TEM images and helped editing the manuscript in Chapters 2 and 5. Dr.

Robert S. Kokenyesi helped with collection and interpretation of optical transmission and reflection data described in Chapter 2. Dr. Jung-Ho Son and Prof. Willian H. Casey are major contributors to Chapter 3 not only by isolating crystals of tetramethylammonium salts of peroxoniobophosphate clusters and characterizing the solution speciation but also by writing and editing the manuscript. Shawn R. Decker performed electron-beam patterning work described in Chapter 3. For Chapter 4, Dylan

B. Fast and Prof. Michelle Dolgos collected pair distribution function data and helped with data interpretation. Dr. Sara Goberna-Ferron was responsible for collecting and analyzing small-angle X-ray scattering data. Dr. Milton N. Jackson Jr. and Dolly Zhen collected dynamic light scattering data. Dr. Cory K. Perkins is a major contributor to

Chapter 5 by leading the project with initial idea and critical discussions as well as manuscript writing and editing. Milana C. Thomas, Dr. Charith E. Nanayakkara, and

Prof. Yves J. Chabal were responsible for collection and interpretation of FT-IR data in Chapter 5. Melanie A. Jenkins and Prof. John F. Conley Jr. contributed to the electrical characterization described in Chapter 5.

TABLE OF CONTENTS

Page

Chapter 1: Introduction.……………………………………………………………… 1

References...…………………………………………………………………...9

Chapter 2: New Niobium Oxide Phosphate Amorphous Thin Films for Tuning Refractive Indexes...………………………………………………………………… 16

Abstract.…….…………….…………………………………………………17

Introduction.…….……………………………………………………………18

Experimental...……………………………………………………………….20

Results and Discussion.………………………………………………………25

Conclusion.….……………………………………………………………….52

References...………………………………………………………………….53

Chapter 3: Acid-Stable Peroxoniobophosphate Clusters to Make Patterned Films.…62

Abstract.….…………….……………………………………………………63

Introduction.…….……………………………………………………………64

Results and Discussion.………………………………………………………66

Conclusion...……………………………………………………………….91

Experimental.……………………………………………………………….92

References.…………………………………………………………………...96

Chapter 4: Reaction Pathway: Condensation of peroxoniobium phosphate clusters to amorphous niobium oxide phosphate solids….………………………………………99

Abstract….…………….……………………………………………………100

Introduction...….……………………………………………………………101

Experimental.……………………………………………………………….102

TABLE OF CONTENTS (Continued)

Page

Results and Discussion...……………………………………………………105

Conclusion....……………………………………………………………….122

References.………………………………………………………………….123

Chapter 5: Enhancing Dehydration of Solution-processed Dielectrics at Low- temperatures with Aqueous Deposited Films…….…………………………………125

Abstract….…………….……………………………………………………126

Introduction….……………………………………………………………127

Experimental.……………………………………………………………….128

Results and Discussion……………………………………………………132

Conclusion.……………………………………………………………….145

References.………………………………………………………………….146

Chapter 6: Conclusion………………………………………………………………149

Bibliography.……………………………………………………………………….152

LIST OF FIGURES

Figure Page

1.1 Pourbaix diagram of aqueous niobium (Nb)…………………………………2

8- 1.2 Typical structures of polyoxoniobates. Lindqvist ions (left: [Nb6O19] and 6- 16- middle: [Nb10O28] ) and α-Keggin ion ([TNb12O40] , T = Ge, Si)……………3

2.1 Synthetic procedure of precursor solutions…………………………………26

2.2 ESI-TOF mass spectrum of niobium oxalate stock solution………………….27

2.3 The negative mode ESI-TOF mass spectra for the NbPOx precursor solutions ([Nb] = 3mM) for Nb/P = 0.5, 0.7, 1, 2, and 5………………………………...31

2.4 (a) XRR patterns for NbPOx (Nb/P = 2, 300 °C) thin films deposited from precursors with [Nb] = 0.04 – 0.4 M and (b) XRR-derived film thicknesses and densities as a function of solution concentration of [Nb]….………………….33

2.5 Cross-sectional TEM images of NbPOx (Nb/P = 2) thin films deposited from solution. Films were annealed at (a) 500, (b) 700, (c) 800, and (d) 900 °C. Insets are CEBD image at each temperature except 900 °C..……………………….35

2.6 GIXRD data of NbPOx film for Nb/P = 2 at different anneal temperatures.…36

2.7 Rietveld refinement of NbPOx film for Nb/P = 2 at 800 °C with 81% T-Nb2O5 (Pbam) and 19% H-Nb2O5 (P2/m).….…………………………….………….37

2.8 Top-view SEM image of a NbPOx (Nb/P = 2) thin film annealed at 900 °C….38

2.9 Rietveld refinement of NbPOx film for Nb/P = 2 at 900 °C with 86% H-Nb2O5 (P2/m) and 14% PNb9O25 (I4/m).………………………………….…………39

2.10 EPMA data showing decomposition of phosphate above 800 °C from NbPOx thin film (Nb/P = 2)…………………………………………………………..40

2.11 TPD result showing decomposition of phosphate above 700 °C for the NbPOx (Nb/P = 2, ~ 100 nm) thin film…………….………………………………….41

2.12 (a) XRR patterns for NbPOx (Nb/P = 2, 0.3 M) thin films annealed at 150 – 800 °C for 1h. (b) XRR- and SE-derived film thicknesses and surface roughness from XRR and AFM as a function of anneal temperature. (c) XRR-derived densities and SE-derived refractive indexes (λ = 550 nm) as a function of anneal temperature. Error bars are presented for 300, 500, and 700 °C annealed samples……………………………………………………………….………43

LIST OF FIGURES (Continued)

Figure Page

2.13 TPD results showing (a) thermal decomposition of peroxide and residual ammonia and (b) dehydration of the NbPOx (Nb/P = 2, ~ 100 nm) thin film. The films were annealed at 300, 400, 500, and 600 °C for 1hr in air………….44

2.14 (a) and (b) GIXRD and NbPOx film for Nb/P = 1 and 0.5, respectively, at different annealing temperatures. The peaks marked with asterisks appears from the Si substrate……………………………………………….…………46

2.15 Density of NbPOx films annealed at 300 °C as a function of P/Nb ratio……...48

2.16 (a) Tauc plot and (b) Sellmeier dispersion curve of NbPOx films annealed at 400 °C, and (c) extracted refractive indexes at 550 nm and band baps at 400 °C as a function of Nb/P ratio………………………………………………….51

3.1 Reversible association and dissociation of Nb4P2 and Nb6P4.………………...67

3.2 ESI-MS of Nb4P2 and Nb6P4………...………………………………………69

31 3.3 P MAS NMR spectra of Nb4P2 (amorphous), Nb4P2 (crystalline, middle) and Nb6P4 (bottom)……………………………………….………………………71

31 3.4 P solution NMR of Nb4P2 and Nb6P4….………………………….………...72

1 3.5 H MAS NMR spectra of Nb4P2 and Nb6P4………….…………….………74

3.6 Raman spectra of Nb4P2, Nb6P4, and peroxo-Nb6…...………………………...76

3.7 FT-IR spectra of Nb4P2, Nb6P4, and peroxo-Nb6………………….…………77

3.8 Reaction monitored overtime by mixing peroxo-Nb6 and phosphoric acid (Nb:P=1:0.5)…………………………………………………………………79

3.9 Reaction monitored overtime by mixing peroxo-Nb6 and phosphoric acid (Nb:P=1:1.25)………………………………………………………………..80

3.10 The photograph is of a 15-mM solution of 2 with varying amounts of TMAH added: 1 equiv (far left) to 7 equiv (far right) and UV/Vis spectra of a 0.04 mm solution of Nb6P4 in 0.1m TMACl solution, during titration with TMAH solution.………………………………………………………………………82

3.11 ESI-MS of Nb6P4 solution with stoichiometric amount of TMAH added: 30 min after addition and 3 days after addition………...………………………83

LIST OF FIGURES (Continued)

Figure Page

3.12 ESI-MS of Nb4P2 by varying pH………………………….………………….85

3.13 Conversion of Nb4P2 to Nb6P4 monitored overtime after lowering pH with phosphoric acid……….……………………………….…………………….86

3.14 Cross-sectional SEM image of a spin-coated Nb6P4 solution (10 layers) on Si after annealing at 300 ºC, SEM image of patterned film with increasing dose from 100 μC cm-2 by increment of 40 μC cm-2 and contrast curve following exposure with a 30 kV electron beam and water development..………………89

3.15 Powder XRD of the fabricated film by using Nb6P4 solution after annealing at different temperatures and phase identification.……………………………...90

4.1 Hydrodynamic radius of precursor solutions measured by DLS over time in fridge (top) and at room temperature (bottom)……………………………106

4.2 ESI-TOF mass spectra for freshly prepared NbPOx precursor solutions (Nb:P = 0.5 to 5, top) and % speciation diagram (bottom).…………………………108

4.3 ESI-TOF mass spectra for NbPOx precursor solutions for Nb:P = 2 over time at room temperature……………………………………………………….110

4.4 ESI-TOF mass spectra for NbPOx precursor solutions for Nb:P = 0.5 over time at room temperature.………………………….……………………….…….111

4.5 SAXS scattering curve for TMA-Nb4P2 clusters in water (top) and ESI-MS (bottom)……………………………………………………………………113

4.6 SAXS scattering curve for TMA-Nb4P2 cluster over time………………….114

4.7 ESI-MS spectra showing deprotonation of Nb4P2 cluster in basic solution….115

4.8 Solution PDF for (a) Nb4P2 and (b) Nb6P4 clusters. Black dashed lines represent simulated PDF patterns from crystal structures……………….…………….117

4.9 Powder PDF for various Nb:P ratios at room temperature…………………119

4.10 Powder PDF for Nb:P = 2:1 at various annealing temperatures……………121

LIST OF FIGURES (Continued)

Figure Page

5.1 Cross-sectional TEM micrograph of the NbPOx-capped AlPO annealed at 230 ºC…………………………………………………………….……………133

5.2 FTIR absorption spectra (dashed lines) for 20-nm NbPOx-capped 190-nm AlPO, 20-nm NbPOx, and 190-nm AlPO films after stepwise annealing to 350 °C. The solid lines are difference spectra for samples annealed at 350 ºC referenced to initial spectra of the respective films at 45 ºC.………………...135

5.3 H2O TPD profiles for AlPO only and AlPO capped with 20 nm of NbPOx after soft baking at 230 ºC……………………………………………………….137

5.4 H2O TPD profiles for AlPO only and AlPO capped with 8 nm of NbPOx after soft baking at 230 ºC.……………………………………………………….138

5.5 H2O TPD profiles for 190-nm AlPO only (red), 8-nm (black), 20-nm (blue), 39-nm (purple), and 58-nm (green) NbPOx-capping layers on 190-nm AlPO thin films after soft baking at 230 ºC...………………………………………140

5.6 H2O TPD spectra for AlPO and 8-nm NbPOx-capped AlPO thin films after heating to 550 ºC, then resting in air for 14 days…………….………………141

5.7 Single normalized change in capacitance vs. electric field sweep cycle of MIM capacitors with an AlPO insulator (red) and a NbPOx−AlPO insulator (blue). Both insulating oxide films were annealed at 350 °C for 1 h……...…………144

LIST OF TABLES

Table Page

2.1 List of negative mode ESI-MS peaks of the NbPOx precursor solutions ([Nb] = 3mM) for Nb/P = 0.5, 0.7, 1, 2, and 5.……………………………………. 29

2.2 List of calculated densities of various niobium oxide phosphates from the crystal structures.……………………………………………………………. 49

Aqueous Niobium Chemistry at Low pH: New Precursors for Oxide Thin Films

Chapter 1

Introduction

Niobium clusters in water

Aqueous niobium chemistry mostly has been limited to neutral and basic pH.1 Figure

1.1 shows the Pourbaix diagram of aqueous niobium and the predominance of Nb2O5 at acidic pH.2 When water-soluble niobium complexes are hydrolyzed or when a basic niobate solution is acidified, niobic acid (Nb2O5●xH2O) precipitates with variable water content.3

Figure 1.1 Pourbaix diagram of aqueous niobium (0.001 mol/kg).2

Polyoxoniobates are anionic niobium oxo clusters comprised of distorted NbO6 octahedral building blocks with a terminal Nb=O bond trans to a long Nb–O bond. The polyoxoniobate (PONb) chemistry is characterized by limited redox chemistry and insolubility in acidic pH. Previously reported PONbs include Lindqvist-type ions,

8- 6- hexaniobate, [Nb6O19] , and decaniobate ion, [Nb10O28] , and Keggin-type ions,

16- [TNb12O40] , T = Ge, Si (Figure 1.2). They are only stable in basic conditions (pH =

7–14). Keggin-type dodecaniobates are not very soluble in water due to their high charge, so that they are not effective solution precursors for thin films.1 On the other hand, tetramethylammonium (TMA) salts of hexaniobate and decaniobate produce

4–6 smooth and dense Nb2O5 thin films via solution deposition. Aqueous peroxoniobium

8- 7,8 species, such as peroxohexaniobate, [Nb6O13(O2)6] , and monomeric species,

3- 9 [Nb(O2)4] , are also reported to exist in neutral to basic solutions.

8- Figure 1.2. Typical structures of polyoxoniobates. Lindqvist ions (left: [Nb6O19] and 6- 16- middle: [Nb10O28] ) and α-Keggin ion ([TNb12O40] , T = Ge, Si).

There have been some efforts to extend the pH stability of niobium clusters to acidic conditions. Transition metal (Cr or Mn) substitutions into TMA-decaniobate produce

III 8- 10 III 6- III 8− 11 [Cr 2(OH)4Nb10O30] , [H2Cr Nb9O28] , or [Mn Nb9O28] , which is stable at pH

= 5–11. Vanadium (V) and Phosphorus (P) substitutions in Keggin-type

12 polyoxoniobates produce TMA9 [PV2Nb12O42], which is stable at pH = 4.5–10.

The only inorganic niobium species in acidic solution reported in the literature are

2- 3- peroxo-fluoro-niobates [Nb(O2)F5] and [Nb(O2)2F4] , which are formed by dissolving Nb2O5 in concentrated HF and H2O2 and in the presence of excess counter

+ + + + 9 ion such as Na , K , Rb , or NH4 at pH = 1–3 and ~5, respectively.

Complexes with organic ligands, such as, peroxo-citrato-niobium, (NH4)4

[NbO(O2)(C6H5O7)]2 (formed at pH >7.5) have been reported as aqueous precursors

13 for Nb2O5 nanoparticles.

Why we want to prepare acidic precursor solutions for thin films?

Solution processing is considered to be a next generation thin-film deposition method to replace high-energy vacuum deposition. Solution processing enables large-scale deposition with reduced use of materials and energy. The criteria for ideal solution precursors are summarized by following features: they are highly soluble (1M or higher) in green solvents in order to control film thickness by single coat; they have a minimal number of counterions and no bulky organic ligands that might introduce porosity upon thermal removal; they should not crystallize upon solvent evaporation

(i.e., not simple salts) in order to obtain smooth as-deposited films; and they are stable

5 yet dynamic in solution, ready to crosslink upon mild heating after deposition to yield dense films.14

Subnanometer-size hydroxo/oxo metal clusters are ideal precursors to produce dense and atomically smooth thin films because they are precondensed forms of monomeric species, which are small enough and yet save energy by bypassing a few condensation steps from monomeric species. Undesired counterions of anionic hydroxo/oxo metal clusters can be eliminated or minimized under acidic conditions, and it enables direct cluster-to-solid condensation with minimal volume change. Therefore, dense and atomically smooth thin film oxide films can be prepared by low-temperature solution process.

Motivation for solution-processed niobium oxide (Nb2O5) and niobium oxide phosphate (NbPOx) thin films

Niobium oxide (Nb2O5) thin films have used in a variety of applications including electrochromic devices, optical switches, anti-reflective coatings for solar cells, high permittivity dielectrics, corrosion barrier coatings, and catalysis due to their high refractive index, transparency in the visible range, and chemical stability.15–25 Recently polyoxoniobates or niobium-containing oxides have received additional attention as water-splitting photocatalysts.26–31

Thin-film deposition of Nb2O5 has been done mostly via gas-phase vacuum deposition, such as reactive magnetron sputtering and chemical vapor deposition (CVD).15,20,32,33

Alternative ways to deposit Nb2O5 thin films are solution process, mostly focused on sol-gel process. However, the sol-gel process produces porous and rough thin films,

6 which are not desirable for high-performance applications, due to bulky organic materials in the precursor solutions.34

Niobium oxide phosphate is a well-known catalyst.3,35 Niobium phosphate glasses are used in photonic devices like optical fibers, lenses, and optical switches as well as rare- earth ion hosts for laser materials.36–50 We also find interesting applications of them as intermediate temperature ionic conductors in solid-oxide fuel cells,51–57 radioactive waste immobilization,58–60 and biocompatible glasses such as bone substitute material.61–64

To the best of our knowledge, no NbPOx thin film has prepared by vapor deposition with molecular precursors. The CVD reaction of NbCl5 with cyclohexylphosphine

65 deposits niobium phosphide (NbP) films. CVD utilizing Nb2Cl8(H2PR)4 or a three- precursor system containing niobium pentachloride, phosphorous trichloride, and hydrogen also deposits NbP instead of NbPOx.66 There is a meeting abstract reporting niobium phosphate glass thin films fabricated via sol-gel processing.67

Key questions for a new system.

We have expanded the aqueous chemistry of niobium to acidic pH by dissolving niobic acid in phosphoric acid and hydrogen peroxide. We obtained yellow solutions with different Nb:P ratios at very low pH (<2). For these new aqueous niobium solutions, I ask the following questions.

(1) What is the nature of the solution species under acidic conditions?

(2) Are the species kinetically stable?

(3) How does the speciation change as a function of pH, phosphate concentration, and

counterions (H+ vs. TMA+)?

(4) Can we prepare and realize amorphous niobium oxide phosphate thin films from

these solutions?

(5) What are the limits on the Nb:P ratio?

(6) How does the solution composition affect the optical and electrical properties of

thin films?

(7) How do the cluster structures change and link as the molecular clusters transform

into the amorphous niobium oxide phosphate solid?

In this dissertation, I address these questions. In Chapter 2, I describe the synthesis of the aqueous precursor solutions for niobium oxide phosphate (NbPOx) thin films. From these syntheses, I identified four new clusters – H6Nb5P1O13(O2)5 (Nb5P1),

H10Nb7P3O23(O2)7 (Nb7P3), H6Nb4P2O14(O2)4 (Nb4P2), and H10Nb6P4O24(O2)6,

(Nb6P4). Because these solutions were prepared under acidic condition and had minimal counter ions, I was able to prepare high-density, atomically smooth amorphous

NbPOx thin films via spin coating and annealing. I describe thermochemical and physical properties (highlighting tunability in optical properties) of these NbPOx thin films as functions of Nb:P ratio and annealing temperature.

In Chapter 3, I describe crystal structures of tetramethylammonium (TMA) salts of

Nb4P2 and Nb6P4 clusters. I conducted more detailed solution characterization and determined these two clusters interconvert as a function of pH and phosphoric acid concentration. The NbPOx system is isoelectronic to HafSOx system, which is well

8 studied as inorganic photoresist. Thus, I also demonstrated electron-beam patternability of NbPOx thin films.

In Chapter 4, I focused on studying the equilibrium between the four clusters and the stability of precursor solutions as a function of pH, phosphoric acid concentration, and counterions. I also investigated cluster condensation and formation of amorphous niobium oxide phosphate solids via pair distribution function analysis to understand the reaction pathway from solution to solid.

In Chapter 5, I describe the dehydration character of NbPOx thin films. One of the primary concerns on solution-processed metal oxide thin films for electrical applications is their dehydration. Solution-processed thin films from aqueous precursors intrinsically have residual water after film deposition. To have high- performance metal oxide thin films, it is critical to remove all water and hydroxide in the thin films. I demonstrated that a thin NbPOx capping layer dramatically enhanced dehydration of an underlying aluminum oxide phosphate (AlPO) thin film at low temperature.

References

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(3) Nowak, I.; Ziolek, M. Niobium Compounds: Preparation, Characterization, and Application in Heterogeneous Catalysis. Chem. Rev. 1999, 99, 3603–3624.

(4) Fullmer, L. B.; Mansergh, R. H.; Zakharov, L. N.; Keszler, D. A.; Nyman, M. Nb2O5 and Ta2O5 Thin Films from Polyoxometalate Precursors: A Single Proton Makes a Difference. Cryst. Growth Des. 2015, 15, 3885–3892.

(5) Llordes, A.; Hammack, A. T.; Buonsanti, R.; Tangirala, R.; Aloni, S.; Helms, B. a.; Milliron, D. J. Polyoxometalates and Colloidal Nanocrystals as Building Blocks for Metal Oxide Nanocomposite Films. J. Mater. Chem. 2011, 21, 11631.

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Chapter 2

New Niobium Oxide Phosphate Amorphous Thin Films

for Tuning Refractive Indexes

Deok-Hie Park, Kai Jiang, Ryan H. Mansergha, Robert S. Kokenyesi, Jung-Ho Son,

William H. Casey, and Douglas A. Keszler

In preparation for submission to Journal of Materials Chemistry C

Abstract

We have discovered new peroxo-niobium phosphate clusters under acidic conditions and examined their conversion to thin films. These clusters can be spin coated to produce high-density and atomically smooth thin films of amorphous niobium oxide phosphates (NbPOx) covering the range of Nb/P compositions from 0.5 to 2. Solution speciation was identified by electrospray ionization mass spectrometry. The cluster- to-film transformation was tracked using temperature-programmed desorption mass spectrometry, and the final NbPOx thin-film composition was determined through electron probe microanalysis. Further thin-film characterization was conducted via X- ray diffraction and reflectivity, spectroscopic ellipsometry, transmission and scanning electron microscopies, optical transmission and reflection spectroscopy, and atomic force microscopy to probe the effects of varying annealing temperatures and the Nb/P ratios. The refractive indexes and band gaps of the films can be tuned in the range of

1.7 – 2.2 and 3.4 – 4.0 eV, respectively, by controlling the solution composition. Our findings demonstrate new ways to stabilize peroxo-niobium phosphate clusters at low pH and they can be employed to tune the optical properties of thin films.

Introduction

In this contribution, we report solution-processed amorphous niobium oxide phosphate (NbPOx) thin films from aqueous peroxo-niobium phosphate precursors.

5+ 3- The NbPOx system, which consists of Nb and PO4 , is isoelectronic with peroxo-

1–3 4+ 2- hafnium sulfate (Hf4(OH)6.4(O2)2(SO4)2.8) known as HafSOx (Hf and SO4 ); both possess peroxo ligands on the metal which can enable electron-beam lithography.

HafSOx has been studied as a direct-patternable inorganic hard mask with a single- digit-nm resolution4–7 as well as a dielectric material.1,8 As an isoelectronic material with HafSOx, NbPOx is promising as a candidate for a new patternable material or dielectric. We recently demonstrated its electron-beam patternability, and the NbPOx system shows moderate contrast and a similar sensitivity to HafSOx.9

Niobium phosphate and niobic acid are well-known catalysts,10,11 and recently polyoxoniobates or niobium-containing oxides receive attention as water-splitting photocatalysts.12–18 Niobium phosphate glasses are used in photonic devices like optical fibers, lenses, and optical switches as well as rare-earth ion hosts for laser materials.19–35 We also find interesting applications of them as intermediate temperature ionic conductors in solid-oxide fuel cells,36–42 radioactive waste immobilization,43–46 and biocompatible glasses such as bone substitute material.47–52

There have been few reports of the deposition of NbPOx via conventional vacuum- based techniques. Amorphous niobium phosphate films (NaPO3-Na2B4O7-Nb2O5 and

A2O-Nb2O5-P2O5, A = Li or Na) are prepared by radio frequency or magnetron

53,54 sputtering from a bulk glass as a target. R3PO4 or P(OR)3 have been used as precursors for Ti-P-O, Ca-P-O, Li3PO4, BPO4, and GaPO4 via chemical vapor

55–60 t deposition (CVD) or atomic layer deposition (ALD). [Me2AlO2P(O Bu)2]2 and

i t 61 [Al(O Pr)2O2P(O Bu)2]4 are synthesized as CVD precursors for AlPO4 thin films.

M(CO)5(PH3) (M = Cr, Mo, or W) is also used as precursors for glass-like transition metal phosphate films.62 However, no NbPOx thin film has reported by vapor deposition with molecular precursors. The CVD reaction of NbCl5 with cyclohexylphosphine deposits niobium phosphide (NbP) films.63 CVD utilizing

Nb2Cl8(H2PR)4 or a three-precursor system containing niobium pentachloride, phosphorous trichloride, and hydrogen also deposits NbP instead of NbPOx.64

Solution processing is considered to be a next generation thin-film deposition method to replace conventional vacuum deposition in micro- and macroelectronics industries due to its lower energy consumption, more efficient utilization of materials, and capability of large-area deposition. Polynuclear oxo/hydroxometal clusters in aqueous solutions provide a unique route to the formation of dense and atomically smooth thin oxide films.65–71 One of the primary challenges in fabricating solution-processed metal oxide thin films is to prepare stable aqueous precursors. There has been one meeting abstract reporting niobium phosphate glass thin films fabricated via sol-gel processing.72 A limited number of water-soluble polyoxoniobate clusters have been

5- 6- reported, and the tetramethylammonium (TMA) salt of [H3Nb6O19] and [Nb10O28]

70,73,74 were recently demonstrated as aqueous precursors for Nb2O5 thin films. Niobium clusters are commonly known to be insoluble under acidic conditions. Most of polyoxoniobate chemistry is limited in basic pH,75,76 and lowering pH results in the precipitation of niobic acid.10 However, in combination with hydrogen peroxide and phosphoric acid, niobium is found to be readily soluble at acidic pH, forming unique

20 peroxo-niobium phosphate clusters without extraneous counterions, which enabling direct cluster-to-solid condensation with minimal volume change. Varying the phosphate concentration in solution allows the optical properties of the resultant thin films to be easily tuned.

Experimental

Synthesis of precursor solutions

Figure 2.1 shows for the synthesis of precursor solutions for the deposition of niobium oxide phosphate thin films. A 0.1-M niobium stock solution was prepared by dissolving

0.1 mole of niobium oxalate hydrate, Nb(HC2O4)5•xH2O (Alfa Aesar, 19.9% Nb2O5 min), with 0.5 mole of oxalic acid dihydrate, H2C2O4•2H2O (J.T.Baker, ACS Reagent,

99.5-102.5%), in 1 L of 18.2-MΩ deionized water at 50 °C, then filtering with

Whatman No.1 filter paper. The Nb concentration was determined from the mass of

Nb2O5 produced by drying the solution, then calcining at 800 °C. 120 mL of 5-M

NH3(aq) (Macron Fine Chemicals, ACS Reagent, 28.0 – 30.0% as NH3) was added to

120 mL of the Nb stock solution with vigorous stirring for 30 min at 45 °C to produce a fresh niobic acid suspension. The resulting suspension was equally divided into six

40-mL aliquots in 50-mL centrifuge tubes and centrifuged at 4000 rpm for 3min. The resulting precipitate was washed with 18.2-MΩ deionized water and centrifuged at

4000 rpm for 1h to remove oxalate and ammonia. Rinse and centrifuge steps were repeated six times. A centrifuge tube containing the precipitate was placed in an ice bath for 10 min, and the reaction temperature was kept below 5 °C during the subsequent dissolution step. 4-M phosphoric acid (J.T.Baker, ACS Reagent, 85.0-

87.0% H3PO4) was added to each precipitate to produce precursor mixtures with the molar ratios Nb/P = 0.5, 0.7, 1, 2, and 5. 3 – 4 mL of 30% hydrogen peroxide solution

(Macron Fine Chemicals, ACS Reagent, 29-32% H2O2) was then added to each mixture, which affored complete dissolution of each mixture, except Nb/P = 5. For

Nb/P = 5, the mixture did not fully dissolve, so a 10x excess of hydrogen peroxide was added. It should be noted that adding phosphoric acid prior to hydrogen peroxide keeps the pH of the solution acidic and prevents exothermic decomposition of peroxide, which leads the formation of Nb2O5 nanoparticles. The final Nb concentration for Nb/P

= 5 solution was ~0.03 – 0.04 M Nb, and the other solutions ~0.3 – 0.4 M Nb. Yellow solutions were obtained after a couple of hours. The precursor solutions were stored in the refrigerator before the thin-film deposition.

Characterization of precursor solutions

The CHN elemental analysis for a dried niobic acid precipitate was performed by

Micro Analysis, Inc. (Wilmington, DE). Solution speciation was examined by an electrospray ionization time-of-flight mass spectrometer (ESI-TOF MS) consisting of an Agilent 6230 TOF MS coupled with dual electrospray ionization source and an

Agilent 1260 Infinity HPLC. The mass spectrometer was calibrated in the range of 100

– 3000 m/z (Agilent, G1969-85000, ESI-L Low Concentration Tuning Mix). Solutions were diluted to a Nb concentration of 3 – 4 mM, and 10 μL of each solution were injected at a flow rate of 0.4 mL/min. The mobile phase was 100% water. The drying gas (N2) temperature was set to 325 °C with a flow rate of 5 L/min and a nebulizer pressure of 20 psi. The capillary, fragmentator, skimmer, and octapole 1 RF voltages

22 were set to 3500, 50, 65, and 750 V, respectively. The data were acquired in negative- ion mode.

Thin film preparation

Films were deposited on 2.54 × 2.54 cm2 quartz substrates (GM Associates, Inc.,

Oakland, CA) for band gap measurement, on 100-nm thermally grown SiO2/Si (Silicon

Valley Microelectronics, Inc.) for temperature programmed desorption mass spectrometry (TPD-MS), and on Si substrates (Sumco Oregon Corp) for other measurements. Before film deposition, all substrates were sonicated in acetone and isopropanol, then 18.2-MΩ deionized water for 3min. The substrates were then treated with an oxygen plasma for 3 min at 50 W with an O2 flow rate of 10 sccm in PE-50 plasma cleaning system (Plasma Etch, Inc., Carson City, NV). Films were deposited by using a CEE Model-100 spin coater at 3000 rpm for 30 s after filtering the solutions through a 0.20-μm PTFE filter. The deposited films were baked at 135 °C for 30 s on a hot plate. This procedure was repeated to build the desired thickness. The resulting films were subsequently annealed in air at selected temperatures between 150 and 900

°C in a Neytech Qex furnace for 1 h.

Characterization of thin films

The elemental composition of the thin films was determined by electron probe microanalysis (EPMA) with a Cameca SX100 Electron Microprobe. All samples were coated by 200 nm of carbon. Intensities of Nb, P, O, Si, N, and C were recorded at 3 different accelerating voltages of 5, 10, and 15 kV. Nb, Ca5(PO4)3Cl, MgO, elemental

Si, boron nitride, and graphite served as standards. Film thicknesses (80 – 130 nm), established by ellipsometry, were used for the EPMA analysis. Elemental composition

23 of the thin films was quantified by iterative calculations using StrataGem thin film

77 composition analysis software. A thin film deposited from a TMA3H7Nb6P4O24(O2)6 solution9 served as a standard for determination of Nb/P ratios.

The TPD-MS study was performed on a TPD Workstation (Hiden Analytical) with a quadrupole mass analyzer (3F PIC, Hiden Analytical) under ultrahigh vacuum. The base pressure was < 5 × 10-9 Torr. Thin films on 2.54 × 2.54 cm2 substrates were cleaved into 1 × 1 cm2 and then heated from room temperature to 1000 °C at the heating rate of 30 °C/min. Electron impact (EI) mass spectra were acquired with a 70-eV ionization energy and 20-µA emission current. Selected mass-to-charge (m/z) ratios

(m/z 17, 18, and 32 for NH3, H2O, and O2, respectively) for each sample were monitored in multiple ion detection (MID) mode with a dwell time of 200 ms and a settle time of 50 ms.

Grazing incidence X-ray diffraction (GIXRD) and X-ray reflectivity (XRR) data were collected with a Rigaku Ultima-IV X-ray diffractometer equipped with Cu Kα radiation

(40 kV, 40 mA). The instrument configuration of a 5.0° Soller slit, a 10-mm divergent height limiting (DHL) slit, and a 0.5° parallel-beam slit were used for each measurement. For GIXRD, diffraction patterns were obtained in the range of 10 – 70°

(2θ) with a scan speed of 1°/min and step size of 0.02°. The divergent slit (DS) was set to 0.1 mm, and the scattering slit (SS) and receiving slit (RS) were left open. For XRR, low-angle reflection patterns from 0 – 10° were collected with a scan speed of 0.1°/min and step size of 0.001°. The DS, SS, and RS were set at 0.05, 0.1, and 0.05, respectively, for the lower angles (0 – 1°); 0.2, 0.5, and 0.2 for the middle angles (1 – 4°); and 0.5,

1.0, and 0.5 for the higher angles (4 – 10°). GIXRD and XRR data were analyzed with

PDXL78 and GlobalFit79 software, respectively.

Scanning electron micrographs were obtained on an FEI Nova NanoSEM 230 with a

15-kV electron beam. Transmission electron microscopy (TEM) was performed on an

FEI TITAN G2 80-200 TEM/STEM at 200 kV. For TEM sample preparation, carbon, chromium, and then platinum were deposited on the thin-film samples to provide contrast and to protect the samples during focused-ion-beam (FIB) preparation.

Root-mean-square (RMS) film surface roughness was measured with Veeco Innova atomic force microscope in tapping mode using a silicon probe (Budget Sensors). Scan

2 areas were 10 × 10 μm , and the RMS surface roughness was determined by analyzing nine 1 × 1 μm2 sections within the initial 10 × 10 μm2 scan areas with the Bruker

NanoScope Analysis software package.80

A Cauchy model was used to obtain thickness and refractive index of thin films from data collected by using a J.A. Woollam M-2000 spectroscopic ellipsometer at incident angles of 55, 60, and 65° in the range of 400 – 1000 nm. The ellipsometric data were analyzed by using the CompleteEASE software package.81

Optical transmission and reflection data were collected with an in-house spectrometer equipped with a double grating monochromator, tungsten lamp, and silicon detector.

Absorption coefficients α were calculated from eq. 1, where T is transmission, R is reflection, and d is film thickness:

T/(1-R) = exp(-αd) (1).

The band gaps (Eg) were estimated from Tauc plots, extrapolating the linear regions of

(αhν)1/2 versus hν plots to the abscissa.

Results and discussion

Synthesis of NbPOx Precursor Solutions

We demonstrate a scheme to prepare peroxo niobium phosphate precursor solutions for niobium oxide phosphate (NbPOx) thin films in strongly acidic pH (<2) without dominant counterion. A flow diagram of synthesis of precursor solutions with various

Nb/P ratios, 0.5, 0.7, 1, 2, and 5, is shown in Figure 2.1. In step (1), niobium oxalate hydrate is dissolved in water with excess oxalic acid (0.5 M). The Nb concentration of the resulting solution is ~ 0.1 M, and the pH is < 1.

- The main species in this solution, detected by ESI-TOF MS, are [NbO(C2O4)2] (aq)

- - and excess oxalates, i.e. HC2O4 , and [H(HC2O4)2] (Figure 2.2). Excess oxalate ions in the solution make it difficult to deposit smooth films, as the precursor crystallizes during solvent evaporation. In step (2), the 0.1-M niobium oxalate solution is precipitated with addition of 5-M NH3(aq), and excess oxalate and ammonia are removed by rinses with deionized water. The resulting niobic acid (Nb2O5·zH2O) precipitate was X-ray amorphous. The CHN elemental analysis showed the dried niobic acid precipitate contained 0.19% C, 2.48% H, and 3.63% N by weight, which implies that most of oxalate and ammonia were removed by rinses. The molar ratio of N to Nb in the dried precipitate was 0.4. In step (3), adding phosphoric acid and hydrogen peroxide dissolves the niobic acid solid and produces yellow solutions with pH < 2.

Drying these yellow precursor solutions produces glassy materials.

Figure 2.1. Synthetic procedure of precursor solutions.

Figure 2.2. ESI-TOF mass spectrum of niobium oxalate stock solution.

Solution Speciation

The solution speciation of NbPOx precursor solutions was investigated by ESI-TOF

MS. It should be noted that all the solutions were freshly prepared and stored at low temperature (< 5 °C) before ESI-TOF MS analysis. In the negative mode, four different peroxo-phosphato-niobate clusters were detected. They are singly (1-) to triply charged

n- n- (3-) cluster ions, [H6-nNb5P1O13(O2)5] (Nb5P1), [H10-nNb7P3O23(O2)7] (Nb7P3), [H6-

n- n- nNb4P2O14(O2)4] (Nb4P2), and [H10Nb6P4O24(O2)6] (Nb6P4), n = 1,2, and 3. Table 2.1 summarizes peak assignments. We recently reported crystal structures of

9 tetramethylammonium (TMA) salts of Nb4P2 and Nb6P4. We have yet to successfully crystallize Nb5P1 and Nb7P3, probably due to their low symmetries. Son et al. recently reported Nb5P1 from ESI-MS as one of the intermediates of the interconversion

9- 5- 82 reaction between [H3Nb9P5O41] and [HNb4P2O14(O2)4] . The Nb7P3 is a new peroxoniobium phosphate cluster which has not been reported before. We suggest that

Nb5P1 and Nb7P3 might be structural analogs of Nb4P2 and Nb6P4. The crystal structure

5- 13 of Nb4P2 is similar to the structure of peroxohexaniobate, [H3Nb6O13(O2)6] . Two

Nb(O2) units in the peroxohexaniobate are substituted with two P–O units. We propose that it may produce Nb5P1 when only one Nb(O2) units in the peroxohexaniobate is substituted with a P–O unit.

Table 2.1. List of negative mode ESI-MS peaks of the NbPOx precursor solutions ([Nb] = 3mM) for Nb/P = 0.5, 0.7, 1, 2, and 5.

Species m/z (observed) m/z (calculated) 1- [H3PO4 - H] 96.970 96.970 1- H3PO4 [2H3PO4 - H] 194.947 194.947 1- [3H3PO4 - H] 292.924 292.923 3- [H3Nb5P1O13(O2)5 - H2O] 282.802 282.801 3- [H3Nb5P1O12(O2)6 - H2O] 288.134 288.133 3- [H3Nb5P1O13(O2)5] 288.806 288.805 3- [H3Nb5P1O12(O2)6] 294.138 294.136 Nb5P1 2- [H4Nb5P1O13(O2)5] 433.713 433.711 2- [H4Nb5P1O12(O2)6] 441.710 441.708 1- [H5Nb5P1O13(O2)5] 868.432 868.428 1- [H5Nb5P1O12(O2)6] 884.425 884.423 3- [H3Nb4P2O14(O2)4] 262.830 262.829 2- Nb4P2 [H4Nb4P2O14(O2)4] 394.749 394.747 1- [H5Nb4P2O14(O2)4] 790.504 790.501 3- [H7Nb7P3O23(O2)7] 421.403 447.378 2- Nb7P3 [H8Nb7P3O23(O2)7] 671.572 671.571 1- [H9Nb7P3O23(O2)7] 1344.149 1344.149 3- [H7Nb6P4O24(O2)6] 421.403 421.402 2- Nb6P4 [H8Nb6P4O24(O2)6] 632.607 632.607 1- [H9Nb6P4O24(O2)6] 1266.209 1266.221

In the niobium-rich solution with Nb/P = 5, the main species in the solution is Nb5P1

(Figure 2.3). This cluster is formed during dissolution process of niobic acid by incorporating peroxide and phosphoric acid (eq 2).

2.5Nb2O5 + H3PO4 + 5H2O2  H6Nb5P1O13(O2)5 + 3.5H2O (2)

As phosphate concentrations increase, Nb5P1 concentration decreases while Nb4P2 increases. In the phosphate-rich solutions (Nb/P ≤ 1), Nb5P1 decreases, and the main species are Nb4P2 and excess phosphoric acid (eq 3).

2Nb2O5 + 2H3PO4 + 4H2O2  H6Nb4P2O14(O2)4 + 4H2O (3)

Two minor species, Nb7P3 and Nb6P4, are also detected in the fresh solutions. Further study of speciation for these four peroxoniobium phosphate clusters are beyond the scope of this study. It will be addressed in a forthcoming paper. The NbPOx precursor clusters prepared in this study don’t have bulky organic ligands, they only have protons, small ammonium concentrations, and peroxide. The absence of large-volume ligands enables the preparation of dense and atomically smooth amorphous niobium oxide phosphate thin films at lower annealing temperature (< 300 °C) by solution process.

Figure 2.3. The negative mode ESI-TOF mass spectra for the NbPOx precursor solutions ([Nb] = 3mM) for Nb/P = 0.5, 0.7, 1, 2, and 5. Blue and pink markers n- n- represent [H6-nNb5P1O13(O2)5] (Nb5P1) and [H6-nNb4P2O14(O2)4] (Nb4P2) species, respectively. The numbers near the markers represent the charges (n) of the detected ions. The peaks marked with asterisks correspond to minor species, i.e. n- n- [H10Nb6P4O24(O2)6] (Nb6P4) and [H10-nNb7P3O23(O2)7] (Nb7P3). In case of Nb/P = 5, the solution contains 10x more free peroxide so that additionally peroxidated peaks, n- such as [H6-nNb5P1O12(O2)6] , are found. We believe that this peaks are produced during ionization process due to excess peroxide in the solution. Detailed peak assignment is summarized in Table 2.1.

Thin films

Atomically smooth niobium oxide phosphate thin films were produced by spin coating from freshly prepared precursor solutions with Nb/P = 0.5, 1, and 2. EPMA confirms that molar ratios of Nb/P in these thin films are 0.52, 1.00, and 2.04, respectively, which are comparable to those of the solution. We firstly discuss thin film properties for niobium-rich NbPOx film with Nb/P = 2 and cover phosphate-rich films later. Figure 2.4a shows XRR patterns with strong Kiessig fringes to 8° 2θ, indicating that the resulting films are atomically smooth and uniform. Film thickness increases linearly with metal concentrations of precursor solutions, while the film density changes little (Figure 2.4b). The precursor solutions for [Nb] = 0.04 – 0.4 M produce thin films with the thicknesses of 2.2 – 22 nm per layer when annealed at 300 °C. A linear fit yields equation (4) which describes the relationship between film thickness, t, and niobium concentration, c, of the precursor solution:

t = [57.03c M-1 - 0.56] nm (4).

Figure 2.4. (a) XRR patterns for NbPOx (Nb/P = 2, 300 °C) thin films deposited from precursors with [Nb] = 0.04 – 0.4 M. GlobalFit models are overlaid (black, dashed). (b) XRR-derived film thicknesses (left axis, black, R2 = 0.9914) and densities (right axis, yellow) as a function of solution concentration of [Nb].

Figure 2.5 shows cross-sectional TEM and Convergent Beam Electron Diffraction

(CBED) images of NbPOx thin films (Nb/P = 2). Figure 2.5a and the GIXRD patterns in Figure 2.6 confirm the thin films to be amorphous after annealing at 500 °C. The

TEM image of Figure 2.5b shows phase segregation begins near 700 °C although the sample remains X-ray amorphous (Figure 2.6). The CBED image shows amorphous halo has thickened, and GIXRD shows that broad peak around 20-30° sharpened at 700

°C, which means it starts to crystallize although most of film is still amorphous. At

800 °C, TEM image (Figure 2.5c) shows further phase segregation, and CBED image shows polycrystalline phases, which are identified as crystalline 81% orthorhombic

Nb2O5 phase (T-Nb2O5, Pbam) and 19% monoclinic Nb2O5 (H-Nb2O5, P2/m) by

GIXRD (Figure 2.7). Nb2O5 generally crystallizes at ~ 500 °C into pseudohexagonal

83 (TT-Nb2O5) or orthorhombic (T-Nb2O5) phases. Incorporating phosphate in the film keeps the film amorphous at higher temperature (> 700 °C). At 900 °C, the film further degrades, and it produces voids due to decomposition of phosphate (Figure 2.5d and

2.8). All T-Nb2O5 phase converts to H-Nb2O5, which is the most thermodynamically stable Nb2O5 phase, and rest of the amorphous phase crystallize as tetragonal PNb9O25

(I4/m). Rietveld refinement of a film annealed at 900 °C indicates it is a mixture of

86% H-Nb2O5 and 14% PNb9O25 (Figure 2.9). EPMA shows the relative Nb/P ratio increased from 2 to 20 above 800 °C (Figure 2.10). At elevated temperatures, films can crystallize and lose P4O10. This loss of P4O10 coincides with a decrease in density and formation of porous structures (eq 5). P4O10 (m/z 284) is not detected in the temperature programmed desorption (TPD), but very small TPD signals of PO (m/z 47), P2 (m/z

62), PO2 (m/z 63), P4 (m/z 124), and P4O6 (m/z 220) are detected above 700 °C. (Figure

2.11).

20Nb4P2O15  22Nb2O5 + 4PNb9O25 + 9P4O10↑ (> 700 °C) (5)

Figure 2.5. Cross-sectional TEM images of NbPOx (Nb/P = 2) thin films deposited from solution. Films were annealed at (a) 500, (b) 700, (c) 800, and (d) 900 °C. Insets are CEBD image at each temperature except 900 °C. Italic ‘a’ represents amorphous.

Figure 2.6. GIXRD data of NbPOx film for Nb/P = 2 at different annealing temperatures. The peaks marked with asterisks appears from the Si substrate. (References of T-Nb2O5, H-Nb2O5, and PNb9O25 are from ICSD 1840, ICSD 29, and ICSD 72683, respectively).

Error Residual Meas. data:NbPOX_P05Nb1_4L_800C_X RD_omega029/Data 1 Calc. data:NbPOX_P05Nb1_4L_800C_XR D_omega029/Data 1

2000

[1] [3]

Intensity(cps) 1000

[5]

[8]

[2]

[7]

[4] [6] 0 [9] 20 40 60 80 400

200

-200 Intensity(cps) -400 20 40 60 80

2-theta (deg)

Figure 2.7. Rietveld refinement of NbPOx film for Nb/P = 2 at 800 °C with 81% T- 2 Nb2O5 (Pbam) and 19% H-Nb2O5 (P2/m), Rwp = 5.94, χ = 1.6582.

Figure 2.8. Top-view SEM image of a NbPOx (Nb/P = 2) thin film annealed at 900 °C.

5.0e+003 Error

[3] Residual Meas. data:NbPOX_P05Nb1_4L_900C_X RD_omega024/Data 1 Calc. data:NbPOX_P05Nb1_4L_900C_XR 4.0e+003 D_omega024/Data 1

3.0e+003 [2]

Intensity(cps) 2.0e+003 [10]

1.0e+003 [1]

[7]

[15]

[11]

[13]

[14]

[5]

[16]

[19]

[8]

[6]

[22]

[17]

[12]

[21]

[18] [4] 0.0e+000 [20] 20 40 60 80

500

-500 Intensity(cps)

20 40 60 80

2-theta (deg)

Figure 2.9. Rietveld refinement of NbPOx film for Nb/P = 2 at 900 °C with 86% H- 2 Nb2O5 (P2/m) and 14% PNb9O25 (I4/m), Rwp = 6.39, χ = 2.107.

Figure 2.10. EPMA data showing decomposition of phosphate above 800 °C from NbPOx thin film (Nb/P = 2).

Figure 2.11. TPD result showing decomposition of phosphate above 700 °C for the NbPOx (Nb/P = 2, ~ 100 nm) thin film. The TPD intensity of these signals are 200x smaller than water signal (m/z 18) in Figure 6b. For this results, a higher emission current (200 μA) in EI ionization was used to increase the sensitivity of mass spectrometer.

Figure 2.12 summarizes the film thickness, surface roughness, density, and refractive index of NbPOx (Nb/P = 2) films derived from XRR, SE, and AFM as a function of annealing temperature. The strong Kiessig fringes are preserved from 150 to 700 °C but lost at 800 °C (Figure 2.12a). This result is consistent with the surface roughness data from AFM, which shows the RMS roughness is 0.2 – 0.3 nm below 700 °C but rapidly increases to 1.7 nm at 800 °C (Figure 2.12b). Film thickness decreases with anneal temperature thus the density and refractive index increase (Figure 2.12b-c).

Thicknesses derived from XRR and SE are comparable and show a same trend below

700 °C although it shows ~ 2 nm offset. Refractive index increases with density and is saturated at the value of 2.05 ± 0.01 at 700 °C, then drops to 1.77 at 800 °C due to decomposition of phosphate (Figure 2.12c).

The NbPOx films condense at low temperature by losing peroxo group and residual ammonia as well as constitutional water by 200 °C (Figure 2.13a, eq 6). Dehydration and dehydroxylation continue to 600 °C (eq 7). When the films are annealed in air at

300, 400, 500, and 600 °C for 1 h, 85, 89, 96, and 99% of water and hydroxide are removed from the films, respectively (Figure 2.13b).

H6Nb4P2O14(O2)4 + 1.6NH3 + xH2O  Nb4P2O12(OH)6 + 2O2 + 1.6NH3 + xH2O (6)

Nb4P2O12(OH)6  Nb4P2O15 + 3H2O (7)

At 300 °C, it produces dense NbPOx thin film with density of 3.84 ± 0.03 g/cm3, and film density increases to 3.92 ± 0.07 g/cm3 and 4.20 ± 0.07 g/cm3 at 500 °C and 700

°C, respectively (Figure 2.12c).

Figure 2.12. (a) XRR patterns for NbPOx (Nb/P = 2, 0.3 M) thin films annealed at 150 – 800 °C for 1h. GlobalFit models are overlaid (black, dashed). (b) XRR- and SE- derived film thicknesses (left axis) and surface roughness from XRR and AFM (right axis) as a function of anneal temperature. (c) XRR-derived densities and SE-derived refractive indexes (λ = 550 nm) as a function of anneal temperature. Error bars are presented for 300, 500, and 700 °C annealed samples.

Figure 2.13. TPD results showing (a) thermal decomposition of peroxide and residual ammonia and (b) dehydration of the NbPOx (Nb/P = 2, ~ 100 nm) thin film. The films were annealed at 300, 400, 500, and 600 °C for 1hr in air.

For phosphate-rich NbPOx films (Nb/P ≤ 1), Figure 2.3 shows the precursor solutions contain Nb4P2 clusters and excess phosphoric acid. For Nb/P = 1, excess phosphate didn’t affect the film morphology and crystallization much (Figure 2.14a). This film is also atomically smooth and amorphous to 700 °C, and it crystallizes at 800 °C as T-

Nb2O5 and H-Nb2O5, and then as H-Nb2O5 and PNb9O25 at 900 °C, which is the same as the film with Nb/P = 2. However, with even more phosphate (Nb/P = 0.5) in the film,

Figure 2.14b shows phosphate-rich crystalline phases form at lower temperatures. At

500 °C, small peaks corresponding to NbP1.8O7 (Pa-3) phase, which is ZrP2O7-type

84 pseudo-cubic structure with phosphorus defects , appears, then mainly Nb1.91P2.82O12

(Pbcn) with a small amount of NbP1.8O7 (Pa-3) at 700 °C. The Nb1.91P2.82O12 (Pbcn)

85 can be described as Nb2-xP3-yO12 which is related to Sc2(WO4)3 structure. At 800 and

900 °C, it shows the same phases as niobium-rich samples, which implies decomposition of phosphate.

Figure 2.14. (a) and (b) GIXRD and NbPOx film for Nb/P = 1 and 0.5, respectively, at different annealing temperatures. The peaks marked with asterisks appears from the Si substrate. References of T-Nb2O5, H-Nb2O5, and PNb9O25, NbP1.8O7, and Nb1.91P2.82O12 are from ICSD 1840, ICSD 29, and ICSD 72683, ICSD 80388, and ICSD 79505, respectively. (c) and (d) XRR patterns for NbPOx thin films for Nb/P = 1 and 0.5, respectively, annealed at 150 – 700 °C for 1h. GlobalFit models are overlaid (black, dashed).

Figure 2.15 shows densities of resulting thin films decrease with increasing phosphate content. The green marks in Figure 2.15 shows that H-Nb2O5 and tetragonal PNb9O25 show much higher densities (d ~ 4.5 g/cm3) than the densities of phosphate-rich crystalline niobium oxide phosphates (P/Nb ≥ 1, d < 3.5 g/cm3). These high densities probably come from the edge-shared feature of NbO6 octahedra in H-Nb2O5 and

PNb9O25. H-Nb2O5 and tetragonal PNb9O25 contain ReO3-type blocks containing NbO6 octahedra. Within a block, NbO6 units are connected by corner-sharing, and these blocks share edges with the neighboring blocks and corners with PO4 tetrahedra for

83,86 PNb9O25. On the other hand, for niobium oxide phosphate with 1 ≤ P/Nb ≤ 1.5, such as NbOPO4 and Nb2(PO4)3, NbO6 octahedra and PO4 tetrahedra are connected by

85,87–89 only corner-sharing. For NbP1.8O7 and (NbO)2P4O13, NbO6 octahedra shares

84,90 corners with P2O7 and P4O13, respectively. The small decrease in density for P/Nb

≥ 1 is probably related to small volume changes that occur with an increase in average oxygen coordination number.

Figure 2.15. Density of NbPOx films annealed at 300 °C as a function of P/Nb ratio (black square). Green marks represent calculated densities of various niobium oxide phosphates from reported crystal structures (Table 2.2).

Table 2.2. List of calculated densities of various niobium oxide phosphates from the crystal structures.

Composition Crystal system P/Nb Nb/P Density ICSD

H-Nb2O5 Monoclinic 0 – 4.55 29

PNb9O25 Tetragonal 0.11 9.00 4.49 72683

NbOPO4 Tetragonal 1.00 1.00 4.04 51144

NbOPO4 Monoclinic 1.00 1.00 3.43 93767

NbOPO4 Orthorhombic 1.00 1.00 3.39 71549

Nb3(NbO)2(PO4)7 Monoclinic 1.40 0.71 3.37 67516

Nb2-xP3-xO12 Orthorhombic 1.48 0.68 3.31 79505

Nb2(PO4)3 Trigonal 1.50 0.67 3.24 65658

NbP1.8O7 Cubic 1.80 0.56 3.27 80388

(NbO)2P4O13 Triclinic 2.00 0.50 3.17 56793

Optical properties

Refractive indexes and optical band gaps of the amorphous NbPOx thin films annealed at 400 °C with Nb/P ratios ranging from 0.5 to 2 were measured by SE and optical transmission and reflection measurements, respectively (Figure 2.16). We also prepared niobium oxide thin films without phosphate to compare the optical properties.

We selected 400 °C to compare optical properties of amorphous NbPOx thin films because it starts crystallize at 500 °C for Nb/P = 0.5 and Nb2O5. Figure 2.16b shows the refractive indexes at 550 nm ranges between 1.7 and 2.0 for 0.5 ≤ Nb/P ≤ 2, and amorphous Nb2O5 film shows refractive index of 2.22 at 550 nm when annealed at 400

°C. We found wide-range tunability of refractive indexes of NbPOx thin films from 1.7 to 2.2, although it is somewhat challenging to prepare stable solution precursors with

Nb/P > 2 due to formation of niobium oxide nanoparticles, which affects film roughness. Figure 2.16a shows the optical band gap of the NbPOx thin film is also tunable by controlling phosphate content in the precursor solution. The band bap decreases with increase in niobium content in the films from 4.0 to 3.5 eV for 0.5 ≤

Nb/P ≤ 2, and the band bap of Nb2O5 is 3.4 eV. (Figure 2.16c).

Figure 2.16. (a) Tauc plot and (b) Sellmeier dispersion curve of NbPOx films annealed at 400 °C, and (c) extracted refractive indexes at 550 nm (left axis) and band baps at 400 °C (right axis) as a function of Nb/P ratio.

Conclusion

We have prepared aqueous precursor solutions for deposition of niobium oxide phosphate (NbPOx) thin films. These solutions contain peroxoniobium phosphate clusters and free phosphoric acid. Because these solutions were prepared under acidic condition without bulky counter ions, we were able to prepare high-density, atomically smooth amorphous NbPOx thin films via spin coating. Also by changing solution composition, we were easily able to prepare NbPOx thin films with various Nb/P ratios.

Notably, this work demonstrated the ability to tune the refractive index and band bap of NbPOx thin films by changing the Nb/P ratio in solutions.

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Chapter 3

Acid-Stable Peroxoniobophosphate Clusters to Make Patterned Films

Jung-Ho Son, Deok-Hie Park, Douglas A. Keszler, and William H. Casey

Published in Chemistry A European Journal, 2015, 21, 6727.

Abstract

Two new peroxoniobophosphate clusters were isolated as tetramethylammonium

(TMA) salts having the stoichiometries: TMA5[HNb4P2O14(O2)4]·9H2O and

TMA3[H7Nb6P4O24(O2)6]·7H2O. The former is stable over the pH range: 3

3- solution pH. The [H7Nb6P4O24(O2)6] cluster can be used to fabricate patterned niobium phosphate films by electron-beam lithography after solution deposition.

Introduction

Aqueous metal-oxide clusters are interesting as potential molecular precursors for fabricating large-area films and for making nanopatterned oxide structures with features at the level of single-digit nanometers. Such advances would help enable continued adherence to Moore’s Law in next-generation semiconductor manufacturing.

Some existing examples of solution-processed and patterned films include SiO2 from

1 2 3 4–6 hydrogen silsesquioxane (HSQ), SnO2, Al2O3, TiO2, ZrO2, and HfO2. The solutions in all of these cases contain an oxide cluster that is thought to serve as a precursor to the film. For example, films, formed from peroxo-hafnium-sulfate

(HafSOx) precursor solutions, can be lithographically patterned to produce dense HfO2 nanostructures at resolutions near 10 nm.7 The environmental impact of this method is anticipated to be much less than conventional methods because of the reduction in both resist waste and the number of process steps. The cluster species in the HafSOx solutions are apparently mixed oligomers of uncertain stoichiometry.8

Here we advance the field by showing that peroxoniobophosphate clusters can generate patternable niobium phosphate thin films. Moreover, we have explored the chemistry of peroxoniobate clusters that incorporate phosphate to expand the pH stability range of the niobate clusters to the acidic region. Polyoxoniobate clusters are generally only stable under neutral to basic conditions. Since peroxoniobate clusters have more surface atoms than non-peroxo analogues, and thus lower surface charge density, the former will exhibit lower basicity and thus peroxo substitution might extend the pH range of stability. Phosphate groups can add acidic functionality and solubility. We note that polyoxotungstates substituted with peroxoniobate groups show

65 wide pH stabilities, due to the coexistence of acidic tungstate and basic niobate.9–12

Niobium phosphate glasses also have been of industrial interest for optical applications and as a nuclear waste host.13–16 Niobates and tantalates are known to form various peroxo complexes, and these find use as oxidation catalysts for organic reactions and precursors for metal-oxide films.17 Peroxoniobates have also been studied for their possible anticancer activity.18 Peroxoniobate clusters have been recently synthesized

19,20 and characterized, such as peroxo-Nb6 and peroxo-Ti12Nb6 clusters. More recently, a peroxonioboarsenate Cs2.5H1.5Na2[Nb4As2O14(O2)4] (Nb4As2) cluster has been reported,21 and its structure is similar to one of the peroxoniobophosphate clusters described in this paper.

Here we report the synthesis and characterization of two peroxoniobophosphate clusters as TMA (tetramethylammonium) salts, TMA5[HNb4P2O14(O2)4]·9H2O (1,

Nb4P2) and TMA3[H7Nb6P4O24(O2)6]·7H2O (2, Nb6P4), and demonstrate that atomically smooth, patterned, oxide films can be fabricated by using the Nb6P4 cluster as precursor. These two chemically and structurally related clusters are inter- convertible by suitable choice of the solution conditions.

Results and Discussion

The compounds 1 and 2 can be synthesized at room temperature by simple mixing of a niobate-cluster solution, hydrogen peroxide, and phosphoric acid. First, a TMA salt of decaniobate or hexaniobate is dissolved in water with stirring. Adding excess

19 hydrogen peroxide leads to formation of peroxo-Nb6 ions within a few minutes. After most of the bubbles that arise from excess H2O2 cease evolving, phosphoric acid is added. Both the initial pH of the original niobate solution and the amount of phosphoric acid, thus the final pH, determines whether 1 or 2 form. Generally, 1 forms at solution pH> 3, and 2 forms only at low pH; (pH <3). The solution comprising mostly 1 at high pH is nearly colorless, whereas the solution at pH <3 is intensely yellow. Another viable synthetic route is achieved by adding hydrogen peroxide at room temperature to a hydrothermally reacted solution of hydrous niobium oxide, TMAH, and phosphoric acid.

21 The structure of Nb4P2 is similar to that of Nb4As2, with two phosphate ligands stabilizing the peroxo-Nb4 core (Figure 3.1). Overall, the structures of these clusters

19 are similar to those of the peroxo-Nb6 cluster. Two phosphate groups are substituted

V for two adjacent Nb sites in peroxo-Nb6, thereby reducing the binding of the central oxygen from μ6 to μ4 (Figure 3.1). During the final crystal structure refinement cycles, a proton was found on the surface of the cluster from the electron density map. The proton is located at one of Nb-μ2-O-Nb units, thus the formula of the cluster is

5- represented as [HNb4P2O14(O2)4] . The charge of the Nb4P2 cluster is balanced by five

TMA ions in the crystal structure.

Figure 3.1. Reversible association and dissociation of Nb4P2 and Nb6P4. (Nb: blue; O: red; P: pink; H: gray)

The structure of Nb6P4 is related to that of Nb4P2, in a way that it can form by condensation of two Nb4P2 units, with loss of one corner Nb(O2) site from each (Figure

3.1). The structure of Nb6P4 can be viewed as two thus-formed Nb3P2 units condensed via two Nb-μ2-O-Nb and two P-μ2-O-Nb bridges. The structure of Nb6P4 possesses two

μ3-O units at the center of Nb3P2 subunits. The structure of Nb6P4 exhibits a high degree of protonation, as expected from its low-pH synthesis conditions and relatively high proton affinities. Four Nb-μ2-O-Nb units within the two Nb3P2 subunits are protonated in the Nb6P4 cluster, as found in the electron density map. The linking Nb-μ2-O-Nb and

Nb-μ2-O-P units between the Nb3P2 units are not protonated. The terminal oxygen atoms of two equatorial phosphate groups are protonated; these can be regarded as P-

OH groups. Additionally, one of the axial phosphate groups is protonated and the clusters are linked by this proton via hydrogen bonding to form one-dimensional chains along [010] and [1-10] directions. These protons attached to phosphates were also found in the electron density map. The seven bound protons and three TMA

10- countercations balance the charge of the [Nb6P4O24(O2)6] cluster. The pH of the 15- mM solution of Nb6P4 is approximately 2.0, thus the Nb6P4 cluster behaves more like traditional Mo- or W-based heteropoly acids, but differs distinctly from most of other polyoxoniobate clusters, which are stable at neutral to basic pH conditions. Most notably compound 2 is soluble in distilled water up to 0.02 M, and 1 is very soluble in water, making processing of it environmentally benign. Both 1 and 2 are detected in

ESI-MS (Figure 3.2).

Figure 3.2. ESI-MS of 1 (top) and 2 (bottom)

Selected 31P MAS-NMR spectra of 1 and 2 are shown in Figure 3.3. The 31P NMR spectrum for an amorphous sample of 1 exhibits a single peak at δ = -2.2 ppm that splits into two resolvable peaks at δ = 0.2 and -2.7 ppm when the material is fully crystalline.

Two sites of nearly equal intensity are expected from the two nonequivalent phosphate groups in the structure. Although the two phosphate groups are equivalent within Nb4P2 due to its C2v symmetry, they have different outer environments—one phosphate has two hydrogen-bonded waters, whereas the other phosphate has three hydrogen bonded waters. In the spectra of 2, two different peaks are observed, and we assign the sharper peak at δ = 5.7 ppm to an axial phosphate, and the broader peak at δ = -11.7 ppm to an equatorial phosphate because the axial phosphates have a smaller variation in P–O distances (1.525(3)–1.550(2) Å) compared with those of the bridging equatorial phosphate (1.508(2)–1.561(2) Å). In the solution, 1 exhibits a single peak at δ = 1.7 ppm, and 2 exhibits two peaks at δ = 7.0 and -10.1 ppm (Figure 3.4), which agree with

31P MAS NMR data.

Figure 3.3. 31P MAS NMR spectra of 1 (amorphous, top), 1 (crystalline, middle) and 2 (bottom)

Figure 3.4. 31P solution NMR spectra of 1 (top), and 2 (bottom)

In addition, 1H MAS-NMR spectra were collected to characterize the environments of the protons bound to the clusters (Figure 3.5). For both clusters 1 and 2, a conspicuous large peak arising from the TMA ions is observed near δ = 3.5 ppm; peaks associated with waters of crystallization are observed at δ = 4.6 and 5.6 ppm for 1 and

2, respectively. Both compounds feature additional downfield peaks at δ = 7.5 ppm (1)

1 and d=8.6 ppm (2), which we assign as protons bound to Nb-μ2-O-Nb. H MAS NMR

1 spectra of protonated hexaniobate clusters at the bridging Nb-μ2-O-Nb unit exhibit H peaks either at 1 ppm or between 8 and 10 ppm, depending upon subtle differences in the outer environment and the countercations.22,23 We assume that the inclusion of phosphate groups, and thus the more acidic character of 1 and 2, is responsible for the downfield shifts of the Nb-μ2-O-Nb bound proton peaks. A distinct peak at δ = 14.8 ppm is observed in the 1H MAS-NMR spectra of 2, which is assigned to the acidic protons attached to the phosphate groups. This feature is absent in the spectra of 1, which agrees with the crystal structures.

Figure 3.5. 1H MAS NMR spectra of 1 (top) and 2 (bottom)

Raman spectra of 1, 2, and the unsubstituted peroxo-Nb6 cluster are given in Figure

-1 3.6. Signals from νs(O–O) are clearly seen in the range 800–900 cm with stronger

-1 17 νs(M–O2) peaks located between 500 and 700 cm . A single νs(O–O) peak is observed for peroxo-Nb6, whereas split peaks are observed for Nb4P2 and Nb6P4. From the crystal structure, the peroxo groups in the peroxo-Nb6 cluster are known to be chemically identical. In 1 (Figure 3.1), only two faces of the distorted Nb4 core are capped by phosphate groups. The capping effectively produces chemically distinct peroxo environments. In 2, the peroxo groups are approximately aligned in the planes of the two Nb3 triangles. The asymmetric nature of the phosphate bridging between these triangular moieties (Figure 3.1), however, leads to chemical nonequivalence of the peroxo groups, as evidenced by the split peaks in the Raman spectrum. FT-IR spectra

(Figure 3.7) of 1 and 2 exhibit a number of phosphate bands between 1000 and 1200 cm-1, and 2 exhibits more phosphate bands, consistent with the two chemically and

-1 structurally distinct phosphates in Nb6P4. The νas(O–O) bands near 850 cm in the FT-

IR spectra are split for 1 and 2 similarly to the Raman data, and 2 shows a larger split between the νas(O–O) bands.

Figure 3.6. Raman spectra of 1, 2 and peroxo-Nb6

Figure 3.7. FT-IR spectra of 1, 2 and peroxo-Nb6

To better understand the formation of these new clusters, their interconversion was monitored as a function of time or pH by using ESI-MS. First, the formation of Nb4P2 or Nb6P4 was monitored after adding controlled amounts of phosphoric acid to the solution of the peroxo-Nb6 cluster (Figure 3.8 and 3.9). When the Nb:P ratio was 1:0.5, only Nb4P2 clusters formed (pH 3.8) and the solution remained nearly colorless. When excess phosphoric acid was added to make the Nb:P ratio 1:1.25 (pH 2.2), the Nb6P4 clusters formed and the solution color changed to yellow. When an intermediate amount of phosphoric acid was added, a mixture of Nb4P2 and Nb6P4 formed. The complete formation of Nb4P2 or Nb6P4 took about one day at room temperature.

Figure 3.8. Reaction monitored overtime by mixing peroxo-Nb6 and phosphoric acid (Nb:P=1:0.5)

Figure 3.9. Reaction monitored overtime by mixing peroxo-Nb6 and phosphoric acid (Nb:P=1:1.25)

Conversion of Nb6P4 to Nb4P2 was studied by addition of TMAH to an Nb6P4 solution.

The Nb6P4 cluster has seven protons attached to the surface oxygen atoms, thus we prepared seven different Nb6P4 solutions, comprising one to seven equivalents of

TMAH (Figure 3.10). Each solution was monitored by ESI-MS as a function of time

(Figure 3.11). Although the color change was instant, actual conversion of 2 to 1 was slower. For example, the solution of 2 with seven equivalents of TMAH completely converted to 1 within a day, and the solution of 2 with six equivalents of TMAH nearly converted to 1 only after 3 days. All of the solutions containing one to seven equivalents of TMAH converted to 1 after a few weeks, and all solutions became nearly colorless except the solutions with one or two equivalents of TMAH. This indicates that pH is the determining factor in the hydrolysis of Nb6P4 to Nb4P2, not the amount of base equivalent added. The conversion of 2 to 1 is nearly quantitative, as indicated by peak intensities in the ESI-MS spectra of the solutions.

Figure 3.10. The photograph (top) is of a 15-mM solution of 2 with varying amounts of TMAH added: 1 equiv (far left) to 7 equiv (far right) and UV/Vis spectra (bottom) of a 0.04 mM solution of 2 in 0.1m TMACl solution, during titration with TMAH solution.

Figure 3.11. ESI-MS of Nb6P4 solution with stoichiometric amounts of TMAH added: 30min after addition (left) and 3 days after addition (right)

The Nb4P2 cluster is kinetically stable across a broad range of pH (3–12), as determined by pH-dependent ESI-MS (Figure 3.12), and this point is important for its industrial uses. This wide stability range of Nb4P2 is interesting because most of the known polyoxoniobates form insoluble amorphous niobium-oxide precipitate when the pH is lowered below 4. The wide stability of Nb4P2 is distinct when compared to peroxo-Nb6, which is only stable in the range 6 < pH < 12. When phosphoric acid was added to lower the pH of the Nb4P2 solution below 3, the colorless solution changed to yellow immediately, which is the characteristic color of Nb6P4. From ESI-MS, Nb4P2 completely condenses to Nb6P4 at pH < 2.5 within one hour after addition of phosphoric acid (Figure 3.13). We note that the conversion of Nb4P2 to Nb6P4 is faster (~1 hour) than other inter-cluster conversion processes discussed above (~1 day). When HCl was added to Nb4P2 solution to lower the pH instead of phosphoric acid, ESI-MS peak intensities of newly formed Nb6P4 are lower than those of the solution where phosphoric acid was added. Thus, not only is pH the determining factor for complete conversion of Nb4P2 to Nb6P4, but excess phosphoric acid is required. This excess is

y- partly understandable because each Nb4P2 should lose a monomeric [Nb(O2)Ox] unit after the condensation, which will require excess phosphate to reassemble to form

Nb6P4 clusters.

Figure 3.12. ESI-MS of Nb4P2 by varying pH

Figure 3.13. Conversion of Nb4P2 to Nb6P4 monitored overtime after lowering pH with phosphoric acid

Nb6P4 exhibits a bright yellow color both in solution and in the solid state. The yellow color of Nb6P4 is interesting because both Nb4P2 and peroxo-Nb6 are colorless. The solution of 2 exhibits a peak at λ = 325 nm (ε = 7500) and a stronger ligand-to-metal charge-transfer (LMCT) band below λ = 250 nm (Figure 3.10). The peak at 325 nm tails to the high-energy limits of human vision, producing the yellow color. As the solution was titrated with base, the peak at λ = 325 nm blue shifted (Figure 3.10), suggesting a speciation change. The yellow color of the Nb6P4 solution fades as more

TMAH is added. The color change is immediate, but the speciation change is slower

(hours), as discussed above. This result suggests that the color is more related to protonation, which is fast, rather than changes in speciation, which is much slower.

Even the Nb4P2 solution becomes yellow after addition of phosphoric acid before the conversion to Nb6P4, which completes in hours, indicating that the yellow color is again likely due to protonation. We also note that previously known Keggin- or Dawson-type peroxoniobotungstate clusters exhibit a yellow color as well,9–12 and those clusters are synthesized in acidic conditions and they consist of Nb(O2) groups. Thus, both a Nb(O2) group and acidic conditions (protonation) are required for the yellow color. The loss of yellow color with increasing pH is likely to be due to deprotonation of the peroxoniobophosphate framework.

Using the Nb6P4 cluster as a precursor for the production of niobium phosphate films has been conducted by spin coating its solutions onto silicon substrates. As seen in

Figure 3.14, a uniform amorphous film of Nb6P4O25, exhibiting no pores or cracks, is readily realized. With a root-mean-square roughness near 3 Å, the surface is nearly atomically smooth. The presence of phosphate inhibits crystallization to temperatures

88 above 700 °C, which might be due to the formation of niobium phosphate glass.

Crystalline Nb2O5 is found to separate into phases near 800 °C, followed by crystallization of Nb9PO25 near 900 °C (Figure 3.15). To test patterning, the spin-coated film was exposed with a 30-kV electron beam to produce an array of exposed boxes at increasing dose. After exposure, the patterned structures were developed by dissolving the unexposed areas with water. The use of water instead of widely used TMAH solution in the development step is more environmentally benign. As seen in Figure

-2 3.14, the onset of insolubility begins with an exposure dose of D0 = 120 μC cm with

-2 complete gelation and insolubility occurring at a dose near Df = 320 μC cm . These dosage (sensitivity) values are comparable to those of the HafSOx films.24 The

-1 calculated contrast value (γ = (logDf - logD0) ) from the curve is 2.3, which is in the lower range compared to other solution-processed, inorganic, patterned films.1–7 The pattern formation in this study suggests that inter-cluster polymerization occurs via radiation-induced decomposition of peroxo groups and subsequent condensation like in the HafSOx system. In contrast, the Nb4P2 solution did not produce a patterned film—all the film was washed away during development—indicating that the film was not polymerized by irradiation. These results suggest that the smaller amount of TMA in Nb6P4 (three TMA per cluster) compared to Nb4P2 (five TMA per cluster) is more advantageous for electron-beam induced pattern formation. The TMA can sterically hinder the polymerization. This idea is supported by control experiments where the amount of TMA in the Nb6P4 solutions was augmented separately by adding TMAH.

In these experiments, dose increased and contrast decreased with TMA concentration.

Figure 3.14. Cross-sectional SEM image of a spin-coated Nb6P4 solution (10 layers) on Si after annealing at 300 °C (top), SEM image of patterned film with increasing dose from 100 μC cm-2 by increment of 40 μC cm-2 (bottom left) and contrast curve following exposure with a 30-kV electron beam and water development (bottom right).

900°C

800°C

700°C Intensity Intensity (a.u.) 600°C 500°C 400°C 300°C Soft-baked (230°C)

10 20 30 40 50 60 70 2(°)

900°C

ICSD 72683

PNb9O25 Intensity Intensity (a.u.)

800°C ICSD 1840 Nb2O5

10 20 30 40 50 60 70 2(°)

Figure 3.15. Powder XRD of the fabricated film by using Nb6P4 solution after annealing at different temperatures (top) and phase identification (bottom).

Conclusion

In summary, we find that addition of peroxide and phosphate groups to aqueous NbV chemistry leads to two types of new peroxoniobophosphate clusters, the relative proportion of which depends on pH, including strongly acidic conditions. Switching between Nb4P2 and Nb6P4 clusters can be achieved by simple tuning of the pH. In addition, the Nb6P4 cluster enables the deposition of high-quality patterned thin films from simple aqueous solutions—a first step in evaluating their potential as functional materials for the semiconductor industry. Furthermore, these peroxoniobophosphate clusters have spurred the discovery and development of a rich new family of niobium phosphate glass, a subject of forthcoming contributions.

Experimental

Synthesis of 1

In a PTFE-lined autoclave (23 mL capacity), hydrous niobium oxide (3 g; 80% w/w) was mixed with TMAH solution (8 mL, 25%), and then phosphoric acid (1 g, 85%) was added. The mixture was allowed to hydrothermally react at 130 °C for 16 h. A clear solution formed with pH 10.0. The product solution was diluted with water (ca.

10 mL), and then hydrogen peroxide solution (5 mL, 30%) was added slowly with stirring, and the solution pH was 6.5. The solution was evaporated at room temperature, and the crystalline product which formed after a few weeks was washed with ethanol on a frit. The product was recrystallized in methanol. Yield = 3.04 g (51 %). Elemental analysis calcd (%) for C20H79N5Nb4O31P2: C 18.21, H 6.03, N 5.31, P 4.70, Nb 28.17; found: C 17.51, H 6.04, N 5.12, P 3.86, Nb 26.8.

Synthesis of 2

In a PTFE-lined autoclave (23 mL capacity), hydrous niobium oxide (5 g, 80% w/w) was mixed with TMAH solution (7 mL, 25 %). The mixture was allowed to hydrothermally react at 120 °C for 3 days. The solution contained mostly decaniobate after reaction, as checked with ESI-MS. The solution was washed with isopropanol in a centrifuge tube a few times until a sticky product remained. The product was dissolved in water (ca. 20 mL) and then hydrogen peroxide solution (5 mL, 30%) and phosphoric acid (2.5 mL, 85%) were added with stirring. The solution color became yellow after addition and the solution pH was 2.0. Bright yellow crystals formed during evaporation after half of the solution was evaporated at room temperature. The crystals were filtered on a frit and washed with water. More batches of crystals were harvested

93 repeatedly during the evaporation of washed solution. Combined yield = 4.26 g (53 %).

Elemental analysis calcd (%) for C12H57N3Nb6O43P4: C 8.94, H 3.56, N 2.60, P 7.68,

Nb 34.57; found: C8.68, H 3.65, N 2.54, P 7.72, Nb 35.5.

Crystal data for TMA5[HNb4P2O14(O2)4]·9H2O (1)

C20H79N5Nb4O31P2, M = 1319.46, triclinic, a = 11.358(2), b = 11.596(2), c = 19.303(3)

Å, α = 94.747(3), β = 104.208(3), γ = 99.452(3)°, U = 2410.8(7) Å3, T = 93(2) K, space group P1̅ (no.2), Z = 2, 28091 reflections measured, 10976 unique (Rint = 0.0187) which were used in all calculations. The final wR(F2) was 0.493 (all data).25

Crystal data for TMA3[H7Nb6P4O24(O2)6]·7H2O (2)

C12H55N3Nb6O43P4, M = 1610.93, triclinic, a = 10.202(3), b = 12.883(4), c = 19.392(7)

Å, α = 81.381(5), β = 84.933(5), γ = 68.635(4)°, U = 2345.4(14) Å3, T = 88(2) K, space group P1̅ (no.2), Z = 2, 30333 reflections measured, 10660 unique (Rint = 0.0265) which were used in all calculations. The final wR(F2) was 0.0773 (all data).25

Instrumental Details

Electrospray mass spectrometry (ESI-MS) was performed with Agilent 1100

LC/MSD G1956b model equipped with single quadruple at cone voltage of 20 V. The sample solution was directly injected into the spray chamber with a syringe pump at a speed of 0.1 mL/min. The presented ESI-MS spectra are averaged signals collected for

1 min. For series of ESI-MS dependent on pH, pH of 30 mM solution of 1 or 15 mM solution of 2 were adjusted by using 2.75 M TMAH solution or 6 M HCl solution to minimize volume change. For time-dependent ESI-MS study of reaction of peroxo-Nb6 and phosphoric acid, 15 mM solution of peroxo-Nb6 was prepared and phosphoric acid was added. Portions (5 μL) of solutions at each pH or time lapse were diluted with 1

94 mL of water and directly injected via electrospray nozzle for data acquisition. Solution and solid state NMR spectra were obtained from UC Davis NMR facility with Bruker

NMR spectrometer. UV-Vis spectra were acquired by using Cary UV-Vis spectrometer.

Elemental analyses were done in Galbraith laboratory (Knoxville, TN). Raman spectra were collected by using Renishaw RM1000 research laser Raman microscope equipped with 15 Argon 514-nm laser. FT-IR data were obtained by using Bruker Tensor 27 instrument, with sample dispersed in KBr pellet. Thin-film X-ray diffraction data were collected by using a Rigaku Ultima IV X-ray diffractometer with Cu Kα radiation

(40kV, 40mA) in grazing incidence configuration. The diffraction patterns were obtained in the range from 10 to 70° (2θ) with a scan speed of 1°/min and step size of

0.02°. Scanning electron microscopy (SEM) image was collected on a FEI Nova

NanoSEM 230 with a 15 keV electron beam. Surface roughness was measured by using a Digital Instruments Nanoscope III Multimode atomic force microscope (AFM) in tapping mode 20 using silicon probe (Tap300Al-G, Budget Sensors). A second-order flatten was applied to the AFM images before roughness analysis. Electron beam exposures for pattern formation were conducted with a FEI QUANTA 3D dual beam scanning electron microscope with a Nanopattern Generation System (NPGS). Contrast curve was generated from data collected by using a J.A. Woollam M-2000 spectroscopic ellipsometer at the incident angles of 55, 60, and 65° in the range of 300–

1000 nm. The ellipsometric data were analyzed by using the CompleteEASE® software package. A Cauchy model was used to obtain thickness.

Thin Film Deposition and Patterning

Solutions of 1 and 2 for thin film deposition were prepared by dissolution of powders of 1 and 2, respectively, with deionized water. Niobium concentration of both solutions was 0.1 M. Silicon substrates for spin coating were cleaned by sonication in Decon

Labs Contrad-70 solutions at 45 °C for 45 min and then thoroughly rinsed with deionized water. Solutions were filtered with 0.45 μm filter and 30 spin coated onto silicon substrates at 3000 rpm for 30 s. Films were then baked at 350 °C (for 1) or

230 °C (for 2) for 30 s on a hot plate. This procedure was repeated for multilayer deposition. For patterning, films were baked at 150°C for 10 s on a hot plate and exposed with 30-kV electron-beam to produce a 3x7 array of exposed boxes at increasing dose of 40 μC cm-2 from 0 to 800 μC cm-2 and then developed with deionized water. Finally, the substrates were annealed at 300 °C for 1 hr.

X-ray crystallography

The crystallographic data were collected by using Bruker SMART 1000 or APEX II diffractometer equipped with monochromatic Mo Kα radiation (λ = 0.71073 Å). The diffraction data were reduced by SAINT software26 and absorption correction were applied with SADABS software.27 The crystal structures were solved with direct method by using SHELXTL package28 and refined with SHELXL 2013 program. The methyl H atoms in TMA ions were refined with riding model. Hydrogen atoms attached on the clusters and 40 crystallization water were found on the electron density map during the final refinement stages.

References

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(3) Zanchetta, E.; Della Giustina, G.; Brusatin, G. One-Step Patterning of Double Tone High Contrast and High Refractive Index Inorganic Spin-on Resist. J. Appl. Phys. 2014, 116.

(4) Anderson, J. T.; Munsee, C. L.; Hung, C. M.; Phung, T. M.; Herman, G. S.; Johnson, D. C.; Wager, J. F.; Keszler, D. A. Solution-Processed HafSOx and ZircSOx Inorganic Thin-Film Dielectrics and Nanolaminates. Adv. Funct. Mater. 2007, 17, 2117–2124.

(7) Stowers, J.; Keszler, D. A. High Resolution, High Sensitivity Inorganic Resists. Microelectron. Eng. 2009, 86, 730–733.

(8) Ruther, R. E.; Baker, B. M.; Son, J. H.; Casey, W. H.; Nyman, M. Hafnium Sulfate Prenucleation Clusters and the Hf18 Polyoxometalate Red Herring. Inorg. Chem. 2014, 53, 4234–4242.

(9) Judd, D. A.; Chen, Q.; Campana, C. F.; Hill, C. L. Synthesis, Solution and Solid State Structures, and Aqueous Chemistry of an Unstable Polyperoxo 12- Polyoxometalate: [P2W12(NbO2)6O56] . J. Am. Chem. Soc. 1997, 119, 5461– 5462.

(10) Harrup, M. K.; Kim, G.-S.; Zeng, H.; Johnson, R. P.; VanDerveer, D.; Hill, C. L. Triniobium Polytungstophosphates. Syntheses, Structures, Clarification of Isomerism and Reactivity in the Presence of H2O2. Inorg. Chem. 1998, 37, 5550– 5556.

(11) Kim, G. S.; Zeng, H.; Neiwert, W. A.; Cowan, J. J.; VanDerveer, D.; Hill, C. L.; 7- Weinstock, I. A. Dimerization of A-α-[SiNb3W9O40] by pH-Controlled Formation of Individual Nb-μ-O-Nb Linkages. Inorg. Chem. 2003, 42, 5537– 5544.

(12) Li, S. J.; Liu, S. X.; Li, C. C.; Ma, F. J.; Liang, D. D.; Zhang, W.; Tan, R. K.; Zhang, Y. Y.; Xu, L. Reactivity of Polyoxoniobates in Acidic Solution: Controllable Assembly and Disassembly Based on Niobium-Substituted Germanotungstates. Chem. - A Eur. J. 2010, 16, 13435–13442.

(13) Krishna Mohan, N.; Sahaya Baskaran, G.; Veeraiah, N. Dielectric and Spectroscopic Properties of PbO-Nb2O5-P2O5:V2O5 Glass System. Phys. Status Solidi 2006, 203, 2083–2102.

(14) Teixeira, Z.; Alves, O. L.; Mazali, I. O. Structure, Thermal Behavior, Chemical Durability, and Optical Properties of the Na2O–Al2O3–TiO2–Nb2O5–P2O5 Glass System. J. Am. Ceram. Soc. 2007, 90, 256–263.

(15) Chu, C. M.; Wu, J. J.; Yung, S. W.; Chin, T. S.; Zhang, T.; Wu, F. B. Optical and Structural Properties of Sr-Nb-Phosphate Glasses. J. Non. Cryst. Solids 2011, 357, 939–945.

(16) Yung, S. W.; Huang, Y. S.; Lee, Y.-M.; Lai, Y. S. An NMR and Raman Spectroscopy Study of Li2O–SrO–Nb2O5–P2O5 Glasses. RSC Adv. 2013, 3, 21025.

(17) Bayot, D.; Devillers, M. Peroxo Complexes of niobium(V) and tantalum(V). Coord. Chem. Rev. 2006, 250, 2610–2626.

(18) Thomadaki, H.; Lymberopoulou-Karaliota, A.; Maniatakou, A.; Scorilas, A. Synthesis, Spectroscopic Study and Anticancer Activity of a Water-Soluble Nb(V) Peroxo Complex. J. Inorg. Biochem. 2011, 105, 155–163.

(19) Ohlin, C. A.; Villa, E. M.; Fettinger, J. C.; Casey, W. H. Distinctly Different Reactivities of Two Similar Polyoxoniobates with Hydrogen Peroxide. Angew. Chemie 2008, 120, 8375–8378.

(20) Ohlin, C. A.; Villa, E. M.; Fettinger, J. C.; Casey, W. H. The First I Peroxotitanoniobate Cluster – [N(CH3)4]10[Ti12Nb6O38(O 2)6]. Inorganica Chim. Acta 2010, 363, 4405–4407.

(21) Geng, Q.; Liu, Q.; Ma, P.; Wang, J.; Niu, J. Synthesis, Crystal Structure and Photocatalytic Properties of an Unprecedented Arsenic-Disubstituted Lindqvist- 4.5- Type Peroxopolyoxoniobate Ion: {As2Nb4(O2)4O14H1.5} . Dalt. Trans. 2014, 43, 9843–9846.

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(25) CCDC-1034677 (1) and CCDC-1034678 (2) contain the supplementary crystallographic data for this paper. These data can be obtained free of charge from The Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif.

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(28) Sheldrick, G. M. Acta Cryst. 2008, A64, 112-122.

Chapter 4

Reaction Pathway: Condensation of peroxoniobium phosphate clusters to

amorphous niobium oxide phosphate solids.

Deok-Hie Park, Dylan B. Fast, Sara Goberna-Ferrón, Milton N. Jackson Jr., Dolly

Zhen, Darren W. Johnson, May Nyman, Michelle Dolgos, and Douglas A. Keszler

100

Abstract

Polynuclear oxo/hydroxometal clusters under acidic conditions provide a unique route to deposit dense and atomically smooth thin oxide films. We have prepared aqueous precursor solutions for niobium oxide phosphate thin films, and examined how solution clusters transform to dense thin films to understand the role of phosphate. Detailed solution chemistries of peroxo-niobium phosphate clusters including equilibrium between clusters and stability in solution have been studied by electrospray ionization mass spectrometry, dynamic light scattering, small angle X-ray scattering as a function of pH and phosphate concentration. Condensation of clusters and formation of amorphous solids were studied by pair distribution function analysis. Our findings show new ways to stabilize clusters at low pH and how they convert to high-density thin films.

101

Introduction

Amorphous thin-film oxides are important components in macro and microelectronics applications. Interfaces between the thin films significantly affect their performance.

Crystalline phases of metal oxides have rougher surfaces, and the size of grain boundary of crystalline phase limits transports of electrons, thus device performance.

On the other hand, amorphous metal oxides exhibit atomically smooth interfaces between thin layers, and improve optical and electrical properties. In spite of the importance to understand the relationships between amorphous structures and their properties, most reports focus on structural understanding of amorphous metal oxide thin films.1–3

Niobium oxide (Nb2O5) thin films have been used in a variety of applications including electrochromic devices, optical switches, anti-reflective coatings for solar cells, high permittivity dielectrics, due to their high refractive index, transparency in

4–6 the visible range, and chemical stability. Deposition of Nb2O5 thin films mostly has been done via gas phase vacuum methods, which require potentially more energy than emerging solution processes. Vapor phase deposition also presents difficulties in preparation of thin films with proper stoichiometry.4–7 Alternative ways to deposit

Nb2O5 thin films are various solution processes, which include metal-organic sol-gel methods. But this process commonly produces porous and rough thin films due to bulky organic materials in the precursor solutions.8

We recently reported four novel peroxoniobium phosphate clusters (Nb5P1, Nb4P2,

Nb7P3, and Nb6P4) in strong acidic pH (< 2) in Chapter 2. Also, we isolated crystals of tetramethylammonium (TMA) salts of Nb4P2 and Nb6P4 clusters, and studied

102

9 interconversion between these two species as function of pH. TMA-Nb6P4 is only stable at low pH (pH < 3), while TMA-Nb4P2 is stable over a wide range, 3 < pH <12.

In this study, we conducted more in-depth solution studies of cluster stability as a function of pH, phosphate concentration, and counterions (H+ or TMA+).

Because of the good glass forming characteristics of phosphates, we expected that incorporating phosphate into niobium clusters enables deposition of high-quality amorphous thin films via spin coating, and we demonstrated it (Chapter 2). To understand how the condensation of the peroxoniobium phosphate clusters produces amorphous niobium oxide phosphate (NbPOx) solids, we performed pair distribution function (PDF) analyses of solution precursors and the gel powder from the precursors.

To mimic rapid condensation during spin-coating, we prepared the gel powders by drying precursor solution with a rotary evaporator. Recent reports suggest reaction pathways between bulk and thin films may be different, but these reports have not carefully considered the nature of the initial state.3 In this study, we carefully prepare a gel state that likely represents the initial conditions of a spin-coated film.

Experimental

Synthesis of precursor solutions

The precursor solutions for niobium oxide phosphate (NbPOx) solids (Nb:P = 0.5 to

10) were prepared as described in Chapter 2. To briefly describe, freshly precipitated

Nb2O5•xH2O prepared from hydrolyzing a solution of Nb(HC2O4)5•xH2O dissolved in excess H2C2O4 with 5 M NH3. The precipitate was rinsed with deionized H2O to remove any residual oxalate and ammonium. The precipitate was dissolved in a mixture

103

5+ 3- of H3PO4 and 30% H2O2, to make a solution with Nb :PO4 ratios between 0.5:1 to

10:1. The Nb5+ concentration was 0.3 M for for Nb:P ≤ 2.5 and 0.03 M for Nb:P > 3.3 due to lower solubility.

Characterization of precursor solutions

Hydrodynamic radii of clusters were measured by Wyatt Technology’s Möbiuζ dynamic light scattering (DLS) at room temperature with 532-nm laser light, after passing the solution through a 0.45 μm PTFE filter to remove dust particles.

Solution speciation was examined by an electrospray ionization time-of-flight mass spectrometer (ESI-TOF MS) consisting of an Agilent 6230 TOF MS coupled with dual electrospray ionization source and an Agilent 1260 Infinity HPLC. The mass spectrometer was calibrated in the range of 100 – 3000 m/z (Agilent, G1969-85000,

ESI-L Low Concentration Tuning Mix). Solutions were diluted to a Nb concentration of 3 mM, and 10 μL of each solution were injected at a flow rate of 0.4 mL/min. The mobile phase was 100% water. The drying gas (N2) temperature was set to 325 °C with a flow rate of 5 L/min and a nebulizer pressure of 20 psi. The capillary, fragmentator, skimmer, and octapole 1 RF voltages were set to 3500, 50, 65, and 750 V, respectively.

The data were acquired in negative-ion mode.

Small angle X-ray scattering (SAXS) data were collected on an Anton Paar SAXSess instrument using Cu-Kα radiation (1.54 Å) and line collimation. The instrument was equipped with a 2-D image plate for data collection in the q = 0.018–2.5 Å-1 range.

Solutions were measured in 1.5 mm glass capillaries (Hampton Research). We collected backgrounds for every sample containing water or TMAH 25%. Scattering was measured for 30 min. We used SAXSQUANT software for data collection and

104 treatment (normalization, primary beam removal, background subtraction, desmearing, and smoothing to remove the extra noise created by the desmearing routine). All analysis and fits to determine Rg, size, shape and size distribution were carried out utilizing IRENA10 macros with IgorPro 6.3 (Wavemetrics) software. To simulate scattering data from the crystal structure we used SolX software.11,12

Powder sample preparation

The NbPOx gel powder samples from the precursor solutions (Nb:P = 0.5, 0.7, 1, and

2) were prepared by rapid evaporation and drying of water using a rotary evaporator.

Resulting yellow powders were annealed at various temperature from 100 °C to 900 °C for 1h with ramp rate of 10 °C/min in air in the alumina crucible using a Vulcan® 3-

Series Burnout Furnace.

Pair distribution function (PDF) analysis

TMA5[HNb4P2O14(O2)4]·9H2O and TMA3[H7Nb6P4O24(O2)6]·7H2O crystals were prepared as described in Chapter 3.9 They were dissolved in deionized water to make

[Nb] = 0.1 M and packed into kapton capillaries (1.1 mm inner diameter). Deionized water is used for background correction for solution samples. The gel powder samples were ground and packed into kapton capillaries.

X-ray total scattering experiments were performed at room temperature at beamline

11-ID-B of the Advanced Photon Source, Argonne National Laboratory using a wavelength of 0.2114 Å and the rapid acquisition (RA)-PDF setup. Instrumental parameters were calibrated with a CeO2 standard, and the 2D total scattering data were converted to 1D date using Fit2D.13 PDFs were generated using PDFgetX314 with a

Qmax of 18.4 Å−1. PDF simulation was done in SolX software.11,12

105

Results and Discussion

Stability of precursor solutions

We measured the hydrodynamic radii of the NbPOx precursor solutions with Nb:P ratios of 0.5, 1, and 2 by dynamic light scattering (DLS). The DLS measurement was conducted after decomposing excess peroxide in each solution with Pt foil in fridge overnight. Initial cluster sizes in each solution were 0.745, 0.831, and 0.942 nm for

Nb:P = 0.5, 1, and 2, respectively. Average cluster sizes grow slowly over time. The sizes of clusters after 5 weeks in fridge were 1.281, 1.475, and 8.238 nm for Nb:P =

0.5, 1, and 2, respectively. At room temperature, growth of particle size is accelerated

(Figure 4.1). With high Nb:P, i.e. at lower phosphate concentration, the size of clusters grows faster (Figure 4.1). We attribute this to growth of Nb2O5 nanoparticles. Another possibility would be the condensation of clusters to make dimer and larger polymers.

106

Figure 4.1. Hydrodynamic radius of precursor solutions measured by DLS over time in fridge (top) and at room temperature (bottom)

107

As reported before (Ch 2), when phosphate concentration is low (Nb:P ≥ 5), only

Nb5P1 is observed in ESI-MS, and more phosphate is incorporated into the cluster to produce Nb4P2 as more phosphate is added to the solution (Nb:P < 5). However, adding more phosphate than stoichiometric amount for Nb4P2 does not produce another cluster with more phosphate. Additional phosphate exists in the solution as free phosphate

(Figure 4.2). The rate of cluster formation was much slower when Nb:P > 2.5. I had to use 10 x more hydrogen peroxide to dissolve niobic acid, and the niobium concentration of resulting solution was 10 x lower than higher phosphate concentration.

108

Figure 4.2. ESI-TOF mass spectra for freshly prepared NbPOx precursor solutions (Nb:P = 0.5 to 5, top) and % speciation diagram (bottom).

109

So far, I have discussed speciation in freshly prepared solutions. To understand the condensation chemistry better, we aged each solution at room temperature and monitored changes in solution speciation by ESI-MS. The pH of each fresh solution is

< 2. At this low pH, Nb5P1 and Nb4P2 are not stable and slowly convert to Nb7P3 and

Nb6P4 (Figure 4.3 and 4.4). For Nb5P1 and Nb4P2 to convert to Nb7P3 and Nb6P4, additional phosphoric acid is essential, because it is not a dimerization but a recombination to incorporating more phosphate in the cluster. When there is not additional phosphate source in the solution, Nb2O5 nanoparticles form as a byproduct of this conversion reactions of Nb5P1 and Nb4P2 to Nb7P3 and Nb6P4. (eq 1 and 2.). This formation of Nb2O5 produces the particle-size increase observed in DLS.

3H6Nb5P1O13(O2)5  H10Nb7P3O23(O2)7 + 4Nb2O5 + 4H2O + 4O2 (1)

2H6Nb4P2O14(O2)4  H10Nb6P4O24(O2)6 + Nb2O5(s) + H2O + O2 (2)

When there is excess phosphoric acid in the solution, all species are converted to

Nb6P4 without nanoparticle formation (eq. 3). However, we still see a slow increase of particle size in DLS for Nb:P < 1. This growth probably arises from condensation of

Nb6P4 to dimers or larger polymers (eq. 4).

3H6Nb4P2O14(O2)4 + 2H3PO4  2H10Nb6P4O24(O2)6 + 2H2O (3)

nH10Nb6P4O24(O2)6  H–(H8Nb6P4O23(O2)6)n–OH + (n-1)H2O (4)

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Figure 4.3. ESI-TOF mass spectra for NbPOx precursor solutions for Nb:P = 2 over time at room temperature.

111

Figure 4.4. ESI-TOF mass spectra for NbPOx precursor solutions for Nb:P = 0.5 over time at room temperature.

112

We previously reported the TMA-Nb4P2 cluster to be stable at across a wide pH range

9 3 < pH < 12. Although, it converts to Nb6P4 at low pH (< 2), we found initially Nb4P2 is formed rather than Nb6P2 even at the lower pH. If so, what induces this difference in stability of Nb4P2 between low pH and high pH? We added TMAH to low-pH Nb4P2 solution. For a freshly prepared solution, a simulation of small-angle X-ray scattering

(SAXS) data of the solution closely aligns with the Nb4P2 structure rather than Nb6P4, which is consistent with ESI-MS which shows only Nb4P2 (Figure 4.5).

By aging the solution for two weeks at room temperature, SAXS data do not change

(Figure 4.6). SAXS experiments at lower pH were not successful because of excess peroxide in the solutions, which bursts the sealed capillary during experiment as it converts to H2O and O2(g). We tried to use Pt foil to decompose excess peroxide before loading into the capillary tubes, but we only found cluster was already condensed or aggregated while Pt treatment. Thus, I was unable to collect meaningful SAXS data for an aging study at low pH.

The most significant difference in the mass spectra between low-pH Nb4P2 and high- pH Nb4P2 is deprotonation in high-pH Nb4P2 (Figure 4.7). In high-pH solution, I

2- assigned the peak at m/z 394.24 as H3Nb4P2O14(O2)4 , which has one less proton

2- compared to m/z 394.75 (H4Nb4P2O14(O2)4 ) in the low-pH solution. However, this

2- 5+ ion, H3Nb4P2O14(O2)4 , is not charge-balanced with Nb . One possible explanation would be the reduction of one of four Nb5+ to Nb4+. I suspect this deprotonation may be related to the stability of the cluster. However, we need further study to support this hypothesis. Presence of TMA, which acts as spacer, probably lower the chance of condensation and stabilize the cluster.

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Figure 4.5. SAXS scattering curve for TMA-Nb4P2 clusters in water (top) and ESI-MS (bottom).

114

Figure 4.6. SAXS scattering curve for TMA-Nb4P2 cluster over time.

115

Figure 4.7. ESI-MS spectra showing deprotonation of Nb4P2 cluster in basic solution.

116

To understand how the peroxoniobium phosphate cluster structure transforms to condensed amorphous NbPOx phase, we performed PDF analyses for precursor solutions and the gel powder from the precursor solution. We assume the cluster condensation in the gel powder by rapid evaporation/drying mimics condensation in the thin films. Although, there probably may be differences in reaction pathway between bulk and thin film, it is still useful to understand the reaction pathway in bulk to compare it to information from thin film later. Obtaining and analyzing thin-film

PDF data is much complicated than powder PDF because of the small material volume and interference from the substrate.

Figure 4.8 shows PDF data for Nb4P2 and Nb6P4 clusters in solution. They are well aligned with simulated PDF data based on the crystal structure of the TMA salts. Major peaks are assigned to Nb–O at 2 Å, nearest Nb–Nb and Nb–P at 3.3 Å, next nearest

Nb–Nb at 4 Å. A peak at 3.8 Å, which is labeled as ‘C’ in Figure 4.8 (a), does not appear in the simulated PDF data. I attribute this peak to a corner-shared Nb–O–Nb link resulting from cluster condensation. The solution PDF data for Nb6P4 cluster shows longer Nb–Nb distances which are consistent with the crystal structure.

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Figure 4.8. Solution PDF for (a) Nb4P2 and (b) Nb6P4 clusters. Black dashed lines represent simulated PDF patterns from crystal structures.

118

From the aging study of cluster solution, we found Nb4P2 cluster converts to Nb6P4 cluster over time. Therefore, we expect that the gel powder from cluster solution would have structural information from both Nb4P2 and Nb6P4. In addition to that, we also expect that there should be some amount of cluster condensation during evaporation.

Figure 4.9 shows the powder PDF data from solutions with different Nb:P ratios. We see the increase of peak at 1.5 Å, which is correspond to P–O distance, as Nb:P ratio decreases. Also peak at 6.5 Å increases with decrease in Nb:P, which is consistent with more Nb6P4 cluster content than Nb4P2. The most characteristic difference in the PDF data between four powders is the peak at 3.8 Å. We see bigger peak at 3.8 Å with higher

Nb:P ratio sample, which means more condensation with higher Nb:P ratio. These results are consistent with aging study in solution, which we saw more condensation and more Nb2O5 nanopaticle formation in higher Nb:P solution (Recall the DLS data,

Figure 4.1).

119

Figure 4.9. Powder PDF for various Nb:P ratios at room temperature

120

I investigated how the clusters condense over temperature and disordered amorphous structures evolve to more ordered structures by ex-situ annealed powder PDF studies.

I see the most significant changes in the PDF profile below 200 °C (Figure 4.10). Below

200 °C, I believe the clusters condense to bigger clusters due to dehydration of constitutional water and decomposition of peroxide ligands attached to niobium. I found PDF peaks related to discrete Nb4P2 clusters at 2 (Nb–O), 3.3 (Nb–Nb and Nb–

P), and 4.1 (Nb–Nb) Å decreases, while the corner-shared NbO6 octahedra peak at 3.8

Å increases. Between 200 and 800 °C, changes in PDF profile are insignificant, although increases in peaks around 5–6 Å increase. I attribute it to ordering of the disordered amorphous structure. At 900 °C, we see the longer ranger ordering due to crystallization.

121

Figure 4.10. Powder PDF for Nb:P = 2:1 at various annealing temperatures.

122

Conclusion

The kinetic stability of peroxoniobium phosphate clusters and changes in solution speciation were investigated as a function of pH, phosphate concentration, and

+ + counterions (H and TMA ). During cluster synthesis at low pH, Nb5P1 and Nb4P2 are formed first, then but they convert to Nb6P4 and Nb7P3 over time. With excess phosphoric acid, Nb6P4 is the only stable species at low pH. At low phosphoric acid concentrations (Nb/P ≥ 2) and low pH, solutions are not stable. They produce Nb2O5 nanoparticles. At higher pH, Nb4P2 is the only stable species.

PDF analyses of dried NbPOx gel powders indicate presence of discrete cluster structures as well as partial condensation of the clusters upon drying. Clusters in the gel powder further condense by 200 °C with dehydration of constitutional water and decomposition of peroxide attached to niobium, and it and produces amorphous NbPOx phase. Upon further annealing to 700 °C, amorphous NbPOx slowly orders and finally crystallize above 800 °C.

123

References

(1) Llordés, A.; Wang, Y.; Fernandez-Martinez, A.; Xiao, P.; Lee, T.; Poulain, A.; Zandi, O.; Saez Cabezas, C. A.; Henkelman, G.; Milliron, D. J. Linear Topology in Amorphous Metal Oxide Electrochromic Networks Obtained via Low- Temperature Solution Processing. Nat. Mater. 2016, 15, 1267–1273.

(2) Shyam, B.; Stone, K. H.; Bassiri, R.; Fejer, M. M.; Toney, M. F.; Mehta, A. Measurement and Modeling of Short and Medium Range Order in Amorphous Ta2O5 Thin Films. Sci. Rep. 2016, 6, 32170.

(3) Wood, S. R.; Woods, K. N.; Plassmeyer, P. N.; Marsh, D. A.; Johnson, D. W.; Page, C. J.; Jensen, K. M. Ø.; Johnson, D. C. Same Precursor, Two Different Products: Comparing the Structural Evolution of In-Ga-O “Gel-Derived” Powders and Solution-Cast Films Using Pair Distribution Function Analysis. J. Am. Chem. Soc. 2017, 139, 5607–5613.

(4) Huang, Y.; Zhang, Y.; Hu, X. Structural , Morphological and Electrochromic Properties of Nb2O5 Films Deposited by Reactive Sputtering. 2003, 77, 155–162.

(5) Lai, F.; Li, M.; Wang, H.; Hu, H.; Wang, X.; Hou, J. G.; Song, Y.; Jiang, Y. Optical Scattering Characteristic of Annealed Niobium Oxide Films. Thin Solid Films 2005, 488, 314–320.

(6) Masse, J. P.; Szymanowski, H.; Zabeida, O.; Amassian, A.; Klemberg-Sapieha, J. E.; Martinu, L. Stability and Effect of Annealing on the Optical Properties of Plasma-Deposited Ta2O5 and Nb2O5 Films. Thin Solid Films 2006, 515, 1674– 1682.

(7) Hunsche, B.; Vergöhl, M.; Neuhäuser, H.; Klose, F.; Szyszka, B.; Matthée, T. Effect of Deposition Parameters on Optical and Mechanical Properties of MF- and DC-Sputtered Nb2O5 Films. Thin Solid Films 2001, 392, 184–190.

(8) Aegerter, M. A.; Avellaneda, C. O.; Pawlicka, A.; Atik, M. Electrochromism in Materials Prepared by the Sol-Gel Process. J. Sol-Gel Sci. Technol. 1997, 696, 689–696.

(9) Son, J. H.; Park, D.-H.; Keszler, D. A.; Casey, W. H. Acid-Stable Peroxoniobophosphate Clusters to Make Patterned Films. Chem. - A Eur. J. 2015, 21, 6727–6731.

(10) Ilavsky, J.; Jemian, P. R. Irena: Tool Suite for Modeling and Analysis of Small- Angle Scattering. J. Appl. Crystallogr. 2009, 42, 347–353.

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(11) Zhang, R.; Thiyagarajan, P.; Tiede, D. M. Probing Protein Fine Structures by Wide Angle Solution X-Ray Scattering. J. Appl. Crystallogr. 2000, 33, 565–568.

(12) Tiede, D. M.; Zhang, R.; Chen, L. X.; Yu, L.; Lindsey, J. S. Structural Characterization of Modular Supramolecular Architectures in Solution. J. Am. Chem. Soc. 2004, 126, 14054–14062.

(13) Hammersley, D. A. Fit2D, European Synchrotron Radiation Facility, 12.077 ed. 2005.

(14) Juhás, P.; Davis, T.; Farrow, C. L.; Billinge, S. J. L. PDFgetX3: A Rapid and Highly Automatable Program for Processing Powder Diffraction Data into Total Scattering Pair Distribution Functions. J. Appl. Crystallogr. 2013, 46, 560–566.

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Chapter 5

Enhancing Dehydration of Solution-processed Dielectrics at Low-temperatures

with Aqueous Deposited Films

Cory K. Perkins, Deok-Hie Park, Milana C. Thomas, Charith E. Nanayakkara,

Ryan H. Mansergh, Melanie A. Jenkins, John F. Conley Jr., Yves J. Chabal, and

Douglas A. Keszler

In preparation for submission to Chemistry of Materials

126

Abstract

Aqueous-processed niobium oxide phosphate, Nb4O7(PO4)2 (NbPOx), was studied to determine its effect on the water transport through aluminum oxide phosphate,

Al5O3(PO4)3 (AlPO), dielectric thin films. We studied how AlPO desorbs and absorbs water during heating and cooling, with and without the NbPOx-cap. In-situ Fourier transform infrared spectroscopy showed the NbPOx-capped films contain significantly less water than AlPO-only films after a 1 min anneal at 230 ºC. Temperature programmed desorption measurements confirm the FTIR results, which show capping the AlPO film with NbPOx significantly reduces the amount water desorbed from the films. Further, TPD shows a thickness dependence on the removal of water. The desorbed water signal between 25 and 350 ºC was only 15% of the AlPO-only film when a 39-nm NbPOx-capping layer was used. When an 8-nm thick capping layer was used, the water signal was 91% of the AlPO-only film over the same region. The 8-nm thick capping layer was also able to prevent the hygroscopic AlPO film from resorbing water, functioning an effective moisture barrier.

127

Introduction

Solvent and solute transport through porous and gel media and interfaces is fundamentally important as it affects the functions and processes of many materials.1–

4 For example, it is well established that associated mobile water and hydroxyl protons commonly degrade electronic and device performance.5–12 Thus, as interest continues to grow in aqueous solution processing for electronic materials, the great challenge to remove residual water endures. Consequently, systematic control of water transport through solids is essential to produce functional materials from water based precursors, especially to enable low-temperature processing.

Using only aqueous deposition methods, we recently showed the properties of aluminum oxide phosphate, Al5O3(PO4)3 (AlPO), dielectric thin films could be enhanced by capping them with an ultrathin HfO2 layer at low processing

13 temperatures. With the aid of the ultrathin HfO2 cap, electrical device performance was significantly improved and the relative transient capacitance of a comparable

AlPO-only film was reduced by a factor of 70 in sweeps covering 1.5 MV/cm.

Concurrently, zero-bias hysteresis decreased from 7000 to <100 ppm. These electrical results are consistent with the removal of sluggish mobile protons in the HfO2-capped

AlPO films and an increase in protons in the AlPO-only film, confirmed by chemical analysis via forward hydrogen scattering.13 Using FTIR, we found the AlPO thin films to be highly hydrated even after annealing at high temperatures, due to the hygroscopic

AlPO film reabsorbing water from humid air.14 Rehydration was eliminated by employing an ultrathin HfO2 capping layer. By analysis of the temperature- programmed desorption (TPD) data, we discovered the HfO2 created a diffusion

128 barrier, essentially protecting the AlPO layer from water vapor, rather than inducing dehydration at lower temperatures.14

The HfO2-capped AlPO study motivated us to look at other capping-layer materials as diffusion barrier layers. Here, we examine the effect of a niobium oxide phosphate,

Nb4O7(PO4)2 (NbPOx), capping layer on the H2O transport properties of AlPO films.

Water desorption and resorption are studied dynamically via in-situ Fourier transform infrared (FTIR) spectroscopy and TPD. We find that the addition of the NbPOx capping layer also dramatically reduces the amount of water desorbed in the bilayer films. By

350 ºC, the Nb-based layer converts to a dense, continuous film, which limits water resorption. Hence, solution deposited NbPOx produces an effective ultrathin moisture barrier at modest process temperatures, with a significant reduction in water between

25 and 350 ºC compared to the AlPO-only films. The approach may provide highly effective barrier and encapsulation coatings, where integration supports aqueous processing, along with a decrease in required annealing temperatures for water elimination.

Experimental

AlPO precursor synthesis has been described previously and was prepared with a

3+ 3+ 3- 15 0.95-M Al concentration and an Al :PO4 ratio of 1:0.6. The capping layer,

NbPOx, was prepared using freshly precipitated Nb2O5•xH2O prepared from hydrolyzing a solution of Nb(HC2O4)5•xH2O dissolved in excess H2C2O4 with 5 M

NH3. The precipitate was washed with 500 mL of deionized H2O to remove any residual oxalate and ammonium salts. The precipitate was dissolved in a mixture of

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5+ 3- 5+ H3PO4 and 30% H2O2, to make a solution with an Nb :PO4 ratio of 1:0.5 and a Nb concentration of 0.38 M.

Prior to thin-film deposition, all substrates were cleaned by sonication in a deionized water bath. Following this cleaning, they were treated in a low-energy O2 plasma to create a clean, hydrophilic surface. The films were deposited onto either p-type Si (for the TEM experiment) or 100-nm thermally grown SiO2/Si (for TPD-MS). Films were deposited by spin coating the aqueous precursors at 3000 rpm for 30 s. Each coat was soft baked at 230 °C for 1 min before deposition of additional layers. In each case, two coats of AlPO (0.95 M Al3+) were deposited, resulting in film thicknesses of 180–200 nm. The 0.38 M solution produced films that were 20 nm after softbaking at 230 ºC for

1 min. Subsequent coats were spun to produce thicker films, 2  to make a film 39 nm thick and 3  to prepare a film with a thickness of 58 nm. The solution was diluted to

0.13 M to produce a film with a thickness of 8-nm. Spectroscopic ellipsometry data were collected with a J. A. Woollam M-2000 spectroscopic ellipsometer to determine film thickness. Film thicknesses of the NbPOx and AlPO layers were determined with a Cauchy model via the CompleteEASE software package.16

In-situ FTIR spectroscopy studies were carried out using a nitrogen-purged home- built ALD reactor with a Thermo Nicolet 6700 infrared spectrometer equipped with a liquid nitrogen-cooled broadband mercury cadmium telluride (MCT-B) detector.17 A single-pass transmission at Brewster incidence (~74°) was used to minimize the substrate phonon absorption in the low frequency region (<1000 cm–1) and to increase detection sensitivity of all film chemical components. After heating in flowing N2(g), the films were cooled to 450 °C for data collection. A K-type thermocouple spot-

130 welded onto a tantalum clip was attached at the center of the long edge of the substrate to monitor the sample temperature during analysis. Two gate valves were used to isolate the KBr windows used in the analysis chamber for IR transmission during annealing.

FTIR data analysis involved two parts. To measure water/hydroxide content in the films, the relative absorbance was integrated from the spectra; all films were referenced to a spectrum of a clean SiO2/Si (100) wafer. The amount of water desorbed after annealing was calculated by measuring the integrated area in the range of 3740 – 3000 cm-1 of the absorbance spectra. The baselines of the spectra before and after annealing were not consistent; therefore, absorbance values at 3740 and 3000 cm-1 were assumed as the baseline. Similar to the water content, the initial and final amounts of phosphate binding after the 350 °C anneal were quantified by integrating the area between 1300 and 1250 cm-1. To quantify the phosphate content of the 20-nm NbPOx-capped AlPO film, changes from the NbPOx-cap were subtracted from the spectrum using the band at 1576 cm-1 as the reference.

The TPD study was performed on a Hiden Analytical TPD Workstation with a quadrupole mass analyzer (3F PIC) to monitor gas-phase products released from the thin films upon heating. The measurement was performed under ultra-high vacuum with a base pressure < 5 x 10-9 Torr. Mass spectra were obtained using electron impact ionization with 70 eV ionization potential and 20-µA emission current. The m/z of 18 was selected to monitor of water desorption. Thin films on 2.54 x 2.54 cm2 substrates were cleaved into 1 x 1 cm2 for the TPD analysis. To test the amount of water in each of the samples, the films were heated from room temperature to 550 °C with a ramp

131 rate of 30 °C/min and a dwell time of 5 min at 550 ºC. The samples were then allowed to sit open in air in preparations for tests of water resorption and then ran TPD again heating to 900 ºC. The area under the peaks were integrated to quantify the mass responses for each sample.

Bottom-gate MIMs were fabricated to assess the device bias stability associated with the gate dielectrics. ~50 nm of AlPO thin film or NbPOx-capped-AlPO stack was deposited onto a TaN substrate and thermally evaporated Al contacts were used for the top contacts. All solution deposited layers were annealed at 350 °C for 1 h after the deposition. The transistors were characterized on a heating stage in a dark box with a

Hewlett-Packard 4156C precision semiconductor parameter analyzer.

TEM micrographs were collected using an FEI Titan 80-200 TEM/STEM transmission electron microscope operating at 200 kV. Carbon and chromium coatings were deposited on NbPOx/AlPO bilayers for protection and to enhance sample contrast. After adding a final protective layer of platinum in-situ, a thin cross section of the NbPOx-capped AlPO film was selectively machined using the focused gallium ion beam and an FEI Quanta 3D SEM. The lamella was welded to a copper TEM grid and thinned to approximately 100 nm using the ion beam.

132

Results and Discussion

The NbPOx-AlPO bilayer film features discrete homogeneous layers with a sharp interface between layers (Figure 5.1). The films are deposited from aqueous solutions comprising oxo-hydroxo metal clusters that facilitate partial condensation to produce continuous thin films upon spin coating.15,18,19 The AlPO film must be heated to 230

ºC prior to deposition of additional AlPO layers and the addition of the NbPOx capping layer to yield distinct layers; even after the soft baking step, the AlPO film is still heavily hydroxylated. Herein we focus on the dehydroxylation and hydroxylation and associated desorption and sorption of water, respectively, for both uncapped and capped AlPO to understand the effects of the NbPOx top layer.

133

NbPOx

Cr C AlPO

Figure 5.1. Cross-sectional TEM micrograph of the NbPOx-capped AlPO annealed at 230 ºC.

134

We performed in-situ heated FTIR and temperature programmed desorption (TPD) measurements to examine thermal dehydration of AlPO and NbPOx-capped AlPO films soft baked at 230 ºC for 1 min. For the FTIR studies, films were heated from 45 to 350 ºC in 50 ºC increments, holding each temperature for 5 min. In each case, samples were cooled to 45 ºC prior to data collection. Figure 5.2 shows IR absorption

-1 between 3740 and 3000 cm , corresponding to the OH/H2O vibrational region. The solid lines are difference spectra for samples heated at 80 and 350 ºC. The dips represent dehydration. Percent signal loss is determined from the difference in integrated areas of the dashed-line and solid-line spectra. The single-layer AlPO film loses 68% of its OH signal after annealing to 350 ºC. A significant increase in signal reduction, 98%, occurs for the NbPOx and NbPOx-capped AlPO films. Interestingly, the signal from the soft-baked NbPOx-capped AlPO film is lower than that of the

AlPO-only film, suggesting the 215-nm bilayer film contains much less OH/H2O than a thinner, 190-nm, AlPO film.

135

Figure 5.2. FTIR absorption spectra (dashed lines) for 20-nm NbPOx-capped 190-nm AlPO (blue), 20-nm NbPOx (red), and 190-nm AlPO (black) films after stepwise annealing to 350 °C. The solid lines are difference spectra for samples annealed at 350 ºC referenced to initial spectra of the respective films at 45 ºC.

136

We tracked H2O loss for AlPO and NbPOx-capped AlPO films heated to 550 ºC using

TPD (Figure 5.3). Substantial water evolves below 350 ºC, with the AlPO film losing

83% of its initial H2O content; we note this value is slightly higher than the FTIR experiment (68%), which is possible due to collecting the initial FTIR spectrum at 45

ºC since the TPD curve shows water desorption occurring before 45 ºC (cf Figure 5.3, red line). Conversely, NbPOx-capped AlPO shows much less water desorbed heating to 350 ºC. Over the same region, the NbPOx-capped film loses 58% of the its initial

H2O content; however, the overall water signal corresponding to the 20-nm NbPOx- capped AlPO bilayer is only 42% of the intensity of AlPO-only film up to 350 ºC (cf

Figure 5.3). These results match the FTIR measurements. We note the observed decrease in desorbed water is much less than using a 10-nm thick HfO2-capping layer from our previous report, which desorbed 11% more water from 25 to 350 ºC.14

In our previous study using HfO2, we show thicker capping layers negatively affect the desorption of water from the bilayer film.14 In accordance with that study, we used a thinner NbPOx capping layer here to determine the effect of thickness on water desorption. Figure 5.4 shows water desorption data for a thin, 8-nm NbPOx capping layer. Similar to the thick capping layer, the 8-nm capped bilayer desorbs less water than the AlPO-only film. However, the magnitude of water loss is much greater than when a thicker, 20-nm, cap is used. That is, desorption of water is more than 2  the intensity when the thinner, 8-nm, cap is used, compared to the 20-nm capped film (cf

Figure 5.3). The observed thickness dependence contrasts to our observations in the

HfO2-capped system.

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Figure 5.3. H2O TPD profiles for AlPO only (red) and AlPO capped with 20 nm of NbPOx (blue) after soft baking at 230 ºC.

138

Figure 5.4. H2O TPD profiles for AlPO only (red) and AlPO capped with 8 nm of NbPOx (blue) after soft baking at 230 ºC

139

The results regarding thickness dependence led us to investigate the relationship further. Figure 5.5 shows TPD curves for AlPO-only and bilayer films with varied capping layer thicknesses. There is an apparent trend in the data, as the thickness of the capping layer increases, the amount of water desorbed from the bilayer is decreased.

Capping layers thicker than 39 nm no longer help to dehydrate the AlPO film.

Impressively, the 39-nm NbPOx-capped bilayer desorbs only 15% of water compared with the AlPO-only film from 25 to 350 ºC. We hypothesize the capping layer promotes dehydration via a proton conductive pathway, as niobium phosphates have been shown high temperature proton conductivity.20–22

After annealing, AlPO films exposed to air resorb water. Figure 5.6 shows H2O desorption from a 190-nm AlPO film heated to 550 ºC in the TPD instrument, then rested on the benchtop for 14 days. The film resorbs 100% of the water desorbed in the initial heating to 550 ºC, and the peak desorption signal temperature required to remove the resorbed water increases from 80 to 155 ºC, compare Figures 5.5 and 5.6. Capping the AlPO film with an 8-nm thick NbPOx layer and heating to 550 ºC completely prevents water resorption (cf Figure 5.6, blue line). Similar to the HfO2-capped bilayer system, even a very thin capping layer is able to inhibit the resorption of water.

140

Figure 5.5. H2O TPD profiles for 190-nm AlPO only (red), 8-nm (black), 20-nm (blue), 39-nm (purple), and 58-nm (green) NbPOx-capping layers on 190-nm AlPO thin films after soft baking at 230 ºC.

141

Figure 5.6. H2O TPD spectra for AlPO (red) and 8-nm NbPOx-capped AlPO (blue) thin films after heating to 550 ºC, then resting in air for 14 days.

142

To examine impact of capping on electrical behavior of NbPOx-capped AlPO, we compared capacitance-voltage (C-V) measurements on metal-insulator-metal (MIM) test structures with an Al top contact and TaN bottom electrode for uncapped 50-nm

AlPO films and 50-nm AlPO films capped with a 7-nm thick layer of NbPOx. Prior to

Al top electrode deposition, both films were annealed at 350 ºC in air. CV measurements were first swept to +8MV/cm, then swept back to -8MV/cm, then back to 0V. Using the zero-bias capacitance (C0) of the AlPO only device as reference, we plot the relative capacitance change (C-C0)/C0 as a function of the applied field (Figure

5.7). For an AlPO-only film (red), we observe a 200 ppm increase in C0 following the positive and negative voltage sweeps, demonstrating clear hysteresis and suggesting the presence of sluggish mobile charges in the film.15 When the AlPO layer is capped with NbPOx (blue), we do not observe any zero-bias hysteresis.

Both devices also exhibit a non-linear voltage dependence of capacitance, where capacitance increases with voltage. Capacitance-voltage non-linearity commonly observed in MIM structures is typically characterized by the quadratic voltage coefficient of capacitance, αVCC, which is determined empirically by fitting a quadratic expression to the capacitance vs. voltage measurement:

2 ΔC/C0 = αVCC V + βVCC V, (1)

where C0 is the capacitance density at 0 V, ΔC = C(V) - C0 is the change in capacitance at a given applied field, and βVCC is the linear voltage coefficient of capacitance. A positive αVCC is observed for most dielectrics and it is well established that the "bulk" dielectric material has a dominant effect on the magnitude of αVCC. αVCC increases with increasing dielectric constant and roughly with 1/dox so that thick, low κ insulators will

143

23–25 have a small αVCC. In addition, the αVCC is also influenced by the dielectric interfaces with the electrodes.26,27 For microelectronics applications where high capacitance and ultralow αVCC is required, pairs of positive and negative αVCC insulators

28 may be used for αVCC cancelling. For the AlPO only device the αVCC was found to be

2 approximately 3.0 ppm/V while for the NbPOx capped AlPO film αVCC is decreased

2 to 1.1 ppm/V , a decrease of roughly 63%. The overall low αVCC values in this work are obtained due to a relatively thick AlPO film, which is a low dielectric constant material.

144

Figure 5.7. Single normalized change in capacitance vs. electric field sweep cycle of MIM capacitors with an AlPO insulator (red) and a NbPOx−AlPO insulator (blue). Both insulating oxide films were annealed at 350 °C for 1 h.

145

Conclusion

In-situ FTIR vibrational analysis and TPD measurements reveal the NbPOx capping layer effectively enhances the dehydration of AlPO films, in contrast to HfO2 that essentially only serves as a passivation layer. A thickness dependence of the NbPOx capping layer is noted for dehydrating the AlPO layer, as the an 8-nm layer removes only 58% of the water annealing to 350 ºC, where the 39-nm cap removed 85% over the same temperature range. The thin NbPOx film formed on annealing the bilayer also effectively suppresses water resorption under conditions that uncapped AlPO films sorb copious amounts of water. Solution-processed NbPOx thus enables the low- temperature fabrication of solution-processed dielectrics, while also acting as an effective water-blocking layer. Results suggest other materials, specifically ion- conducting materials, could be used to promote low-temperature dehydration. Overall, we have shown an attractive approach for realizing new high-performance barrier coatings, especially for use where integration is aided by aqueous processing.

146

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Chapter 6

Conclusion

150

This dissertation describes four new peroxoniobium phosphate clusters that form in water. The clusters expand our knowledge on aqueous niobium chemistry at acidic pH.

They function as precursors to deposit new amorphous niobium oxide phosphate thin- film materials with unique optical and chemical properties.

The four novel peroxoniobium phosphate clusters, H6Nb5P1O13(O2)5 (Nb5P1),

H10Nb7P3O23(O2)7 (Nb7P3), H6Nb4P2O14(O2)4 (Nb4P2), and H10Nb6P4O24(O2)6,

(Nb6P4), exist at strong acidic pH (< 2). The crystal structures of the tetramethylammonium (TMA) salts, TMA5[HNb4P2O14(O2)4]·9H2O and

TMA3[H7Nb6P4O24(O2)6]·7H2O, exhibit unique tetrameric and hexameric Nb clusters.

During cluster synthesis at low pH, Nb5P1 and Nb4P2 are formed first, then they convert to Nb6P4 and Nb7P3 over time. With excess phosphoric acid, Nb6P4 is the only stable species at low pH. Without enough phosphoric acid (Nb/P ≥ 2) at low pH, solutions are not stable, and Nb2O5 nanoparticles precipitate as a byproduct of cluster condensation. At higher pH, Nb4P2 is the only stable species. Pair distribution function analyses of dried NbPOx gel powders indicate presence of discrete cluster structures as well as partial condensation of the clusters in the gel powders. Clusters in the gel powder further condense by 200 °C with dehydration of constitutional water and decomposition of peroxide attached to niobium, and it and produces amorphous NbPOx phase. Upon further annealing to 700 °C, amorphous NbPOx slowly orders and finally crystallize above 800 °C.

The peroxoniobium phosphate clusters enable the deposition of high-quality patterned thin films from simple aqueous solutions—a first step in evaluating their potential as

151 functional materials for the semiconductor industry. Furthermore, these clusters have spurred the discovery and development of a rich new family of niobium oxide phosphate (NbPOx) glass. The refractive index and band gap of the amorphous NbPOx thin films vary from 2.2–1.7 and 3.4–4.0, with an increase in the P/Nb ratio from 0 to

2, respectively. Lastly, solution-processed NbPOx enables the low-temperature fabrication of solution-processed dielectrics, while also acting as an effective water- blocking layer. It would provide an attractive approach for realizing new high- performance barrier coatings, especially for use where integration is aided by aqueous processing.

152

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