Dynamics of hydrogen and low concentration carbon on Au-Ni(111) surface alloys

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Dynamics of Hydrogen and Low Concentration Carbon on Au-Ni(111) Surface Alloys

by Qing Liu

B.S. Chemistry, Tsinghua University (2011) Submitted to the Department of Chemistry

in Partial Fulfillment of the Requirements for the Degree of DOCTOR OF PHILOSOPHY

IN PHYSICAL CHEMISTRY at the

MASSACHUSETTS INSTITUTE OF TECHNOLOGY June 2018

Signature of Author: Certified by: Accepted by: S ETTS INSTITUTE SHNOLOGYl

Signature redacted

Department of Chemistry May 11, 2018

Signature redacted

T Sylfia T. Ceyer

John C. Sheehan Professor of Chemistry Thesis Supervisor

Signature redacted

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This doctoral thesis has been examined by a committee of the Department of Chemistry as follows:

Signature redacted

Professor Keith A. Nelson:

Chairman, Thesis Committee Haslam and Dewey Professor of Chemistry

Professor Sylvia T. Ceyer:

Signature redacted

Thesis jupervisor

John C. Sheehan Professor of Chemistry

Professor Robert G. Griffin:

Signature redacted

Member, Thesis Nm1m-ittee Arthur Amos Noyes Professor of Chemistry

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Dynamics of Hydrogen and Low Concentration Carbon on Au-Ni(111) Surface Alloys

by Qing Liu

Submitted to the Department of Chemistry on May 11, 2018 in partial fulfillment of the requirements for the degree of

Doctor of Philosophy in Physical Chemistry

Abstract

Exposure of Au-Ni( 111) surface alloys to molecular hydrogen results in the dissociative chemisorption of hydrogen to produce H atoms adsorbed on two distinct types of threefold hollow sites, as identified via thermal desorption spectroscopy (TDS), collision induced recombinative desorption (CIRD), and high resolution electron energy loss spectroscopy (HREELS). Hydrogen atoms bind relatively strongly to threefold hollow sites consisting only of Ni atoms, with a binding energy of 63 kcal/mol and characteristic vibrational frequencies of 970 and 1170 cm'. Hydrogen atoms bind relatively weakly to threefold hollow sites consisting of two Ni atoms and one Au atom, with a binding energy of 55 kcal/mol and characteristic vibrational frequencies of 820 and 1050 cm'. Alloying of Au into the Ni(l 11) surface reduces the amount of chemisorbed H upon molecular H2 exposure, but promotes the formation of bulk H upon

atomic H exposure, compared to a Ni(1 11) surface. A robust algorithm for determining the Au coverage of Au-Ni(l 11) surface alloys using Auger electron spectroscopy is also developed.

The observation of three vibrational features at 750, 1470, and 2220 cm' in the HREEL spectra of purportedly clean Au-Ni(1 11) surface alloys is reported. These features are attributed to graphene, formed on the Au-Ni(1 11) surface alloys via segregation of very small amounts of residual carbon in the bulk. The alloyed Au atoms destabilize the carbide phase and promote the nucleation and growth of graphene. The features at 750 and 1470 cm-1 are assigned as the ZO and LO (TO) phonon modes of graphene, respectively. The feature at 2200 cm1 _{is tentatively assigned as a two-phonon}

(ZO + LO/TO) mode. An additional feature at 2990 cm' is identified as the stretching mode of C-H bonds, formed through the interaction between water or hydrogen from the vacuum chamber background pressure and the dangling bonds of carbon atoms along the edges of graphene. The feature at 3670 cm1 is the 0-H stretching mode of isolated water molecules or hydroxyl groups. The addition of Au weakens the interaction between the metal d band and graphene a state, resulting in a stiffening of the ZO mode by 30 ~ 60 cm-', compared to graphene-Ni(1 11). The softening of the LO (TO) mode by 30 cm~1 arises from the slightly larger unit cell of graphene adsorbed on Au-Ni(1 11) as compared to that on Ni(111).

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Acknowledgements

First of all, I would like to thank my research advisor, Professor Sylvia T. Ceyer,

for her mentorship and support over the past six and a half years. Sylvia has great

scientific insight and is extremely knowledgeable. Having discussions with her is always

inspiring and rewarding. She is also kind and full of patience. I've made mistakes such

as accidentally flooding the main chamber with deuterium, and burning through the

molecular beam nozzles. Instead of criticism, I received encouragement and valuable

suggestions from her, and she was truly happy for me when I finally solved these

problems. I was deeply influenced by her persistence for excellence in science. She

would go through my data and writings in great detail, and raise every possible question

and comment, even if it was after midnight. In addition, Sylvia has always been

supportive to my career choice. I had the opportunity to expand my horizon by taking

two internships during my PhD study. None of my accomplishments would have been

possible without Sylvia's support, and I am forever indebted to her.

In addition, I would express my sincere gratitude to other members of my thesis

committee. My thesis committee chair, Professor Keith A. Nelson, has a broad

knowledge across all the disciplines in chemistry. Through our annual meetings, Keith

helped me to look at the problems from a different perspective and provided many useful

suggestions to my work. Professor Robert G. Griffin is thanked for his help with my first

year study of Quantum Mechanics.

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we spent five years working together. I learned a tremendous amount of knowledge of the Little Machine (LM) from him. Beth Hocking joined the Ceyer lab at the same time as I joined and we worked together on a daily basis. Many projects, such as refurbishing the diffusion pumps, would have been much more difficult without her help. Thanasak Sathitwitayakul and Dr. Alexandros Anastasopoulos work on another UHV system called the Big Machine (BM). Thanasak has a strong background in programming and rewrote the data acquisition programs for the LM using LABVIEW and Python, which greatly improved the convenience and reliability for acquiring the LM data. Alex is very knowledgeable about UHV systems and has offered me many valuable suggestions. Previous co-workers on the BM, Daniel Rowlands and Michael Blair, are also thanked. Dr. Yang Dai joined the Ceyer lab in June 2017 as a postdoc and he deserves my special thanks. Although we only spent one year working together, a large portion of the data presented in this thesis were measured with or by Yang. The past year would have been much harder without Yang's help. Yang and his wife Manna are also thanked for inviting me to their home for wonderful dinners.

I must thank the electronic specialist in the Chemistry Department, Dr. Gang Liu. Gang is truly an expert and can fix any electronics, and he always gave our repair requests top priority. Without his help, we would have much longer delay in our experiments. I'd also like to thank Andrew Gallant and Scott Spence from the MIT central machine shop, without whose help the molecular beam nozzles and many parts of the LM would just be paper drawings.

I would like to thank my friends who helped me tremendously over the years. Graduate school was not easy, and I could not have made it through without their support

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and encouragement. Yongbao Sun, Jian Lu, Lei Sun, Jia Liu, Yang Yang, Hang Chen, and Jingjing Ling are my close friends in the Chemistry Department and we spent a great deal of unforgettable time together. Yongbao is specially thanked for his help with my job interview preparations. Guolong Su, Shengxi Huang, Ben Yang, Yumeng Chen, Kai Xiang are my friends from Tsinghua University. I enjoyed playing the game "werewolf' with Qifan Zhang, Jicong Li, Bing Yan, Wei Yu, Jinhu Do, Yicong Ge and many other

friends. The MIT badminton club is a great place for me to stay physically healthy, and I'm grateful for my coach Pashu and friends Yu, Meng, Xiao, Heng, Jeffrey and Alex.

Prof. Steven L. Tait at Indiana University Bloomington, and Prof. Xun Wang, Prof. Yen Wei and Prof. Lin Feng at Tsinghua University are thanked for their guidance during my undergraduate studies.

Finally, I would like to thank my parents for their unconditional support and love throughout my life. I must thank my fiancee, Xi Wang, who has been a wonderful partner and guided me through many important decisions over the past ten years. I love you all.

Qing Liu (

'J

Op) May 20, 2018

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List of Figures

Chapters

Figure 2-1. Mole Fraction of Au vs. Au Coverage ... 32 Figure 2-2. Calculation of bulk contribution factor

P

through layer-by-layer summation. PSD is calculated with the IMFP values given by Seah and Dench, where P, is calculated with IMFP values given by Tanuma, Powell, and Penn. See text for d etail... 3 9 Figure 2-3. Au coverage as a function of Auger intensity ratio, calculated using corrected sensitivity factors, and IMFPs from two sources... 41 Figure 2-4. Examples of Auger spectra acquired in derivative mode. The Au coverage increases from Example 1 to Example 4... 45 Figure 2-5. Examples of Auger curve fitting program output. Curves in various colors

are described in the lower left legend. The upper right yellow boxes contain important results of the curve fitting. See text for detail. ... 48 Figure 3-1. Potential energy diagram of the H-Ni system. The x-axis is the distance from the Ni surface, and the y-axis is potential energy. 9 kcal/mol is the bulk diffusion barrier of H in Ni [8], 4 kcal/mol is the heat of solution of H in Ni [8], 63 kcal/mol is the binding energy of H on Ni [21], and 52 kcal/mol is half of the dissociation energy of H2 [22]. Ea is the activation energy for dissociative chemisorption, and its

value is less than 0.2 kcal/m ol [23, 24]... 58 Figure 3-2. TDS of hydrogen on Ni(1 11). (a) Thermal desorption of 1 ML of surface H. (b) Thermal desorption of 1 ML of surface H and 1.8 ML of bulk H. ... 59 Figure 3-3. HREEL spectra of H on/in the Ni(1 11) surface. (a) Spectrum of surface H formed by exposure of Ni(1 11) to molecular H2. (b) Spectrum of surface H and bulk

H formed by exposure of Ni(1 11) to gas-phase H atoms. Spectra are measured at 77 K, at 12' off specular with 3.5 eV electrons ... 60 Figure 3-4. Potential energy diagram of the H-Au(thin film) system. Values are taken from the work by Stobinski et. al. [39]. 2.2 kcal/mol is the heat of solution of H in the bulk of thin Au film, 2.5 kcal/mol is the heat of adsorption of H on the surface, 6.8 kcal/mol (2.5 kcal/mol + 4.3 kcal/mol) is the activation energy for recombinative desorption, and ~4.3 kcal/mol is the activation energy for dissociative adsorption. All values are expressed with respect to 1/2 H2. . . 62 Figure 3-5. HREEL spectra of a clean Ni(1 11) surface and a clean Au-Ni(1 11) surface. Inset: Auger spectrum of the Au-Ni(1 11) surface, the fitted Au coverage is 0.14 ML. ... 6 6 Figure 3-6. Illustration of background subtraction procedures for TDS. (a) "Chamber background" subtraction to remove background contribution from the chamber. (b) "Rear crystal background" subtraction to remove background contribution from the rear and edge of the crystal. ... 70

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Figure 3-7. Example of a CIRD spectrum. The Xe/He beam enters the main chamber at time = 20 s and lasts for approximately 145 s. ... 72 Figure 3-8. QMS sensitivity measurement and calculation. (a) QMS intensity of mass 2

while leaking in H2 to the main chamber. (b) Linear fit to the average counts and

recorded pressures. The slope and its error are shown. ... 73 Figure 3-9. TDS after molecular hydrogen exposure to Au-Ni(1 11) surface alloy.

Spectra are vertically offset for clarity. (a) TDS with "chamber background" subtraction. (b) TDS with "chamber background" and "rear crystal background" subtraction ... 76 Figure 3-10. TDS measured after H2 exposure to a multi-layer (-2.5 ML) Au covered

surface. Used as the "rear crystal background". See section 3.2.4 for details... 77 Figure 3-11. Hydrogen coverage vs. Au coverage. Au-Ni(1 11) surface alloys were exposed to molecular H2 beam with 80 psig stagnation pressure for 5 minutes... 78

Figure 3-12. Collision induced recombinative desorption of hydrogen from Au-Ni(1 11) surface alloy. Nozzle temperature is 1000 K, and the stagnation pressure is 200 psig of 0.25% X e seeded in H e... 82 Figure 3-13. TDS comparison, dark traces are TDS measured without CIRD, light traces are TDS measured after CIRD. (a) TDS with "chamber background" subtraction. (b) TDS with "chamber background" and "rear crystal background" subtraction. The "rear crystal background" is shown in Figure 3-10. ... 83 Figure 3-14. Comparison of the area under CIRD traces and TD spectra. Blue data points represent the integrated area under CIRD traces, and orange data points represent the difference in the integrated areas of the two thermal desorption spectra show n in Figure 3-13b ... 85 Figure 3-15. Percentage of hydrogen removed by CIRD as a function of Au coverage. Data points in blue are calculated based on the integrated areas of CIRD traces, whereas data points in orange are calculated based on the differences in the integrated areas of TDS before and after CIRD... 86 Figure 3-16. TDS after atomic hydrogen exposure to Au-Ni(111) surface alloy. Molecular H2 is leaked into the chamber to raise the background pressure to 1 x 10-6

Torr. Crystal is placed in front of the H atom filament and held at 130 K. Exposure duration is 300 s. The background from the rear and edge of the crystal is not subtracted . ... 88 Figure 3-17. TDS of hydrogen on a 0.38 ML Au-Ni(11) surface after exposure to atomic H. The red spectrum is taken immediately after the H exposure, whereas the blue spectrum is taken after the H exposure and CIRD. Inset is the associated CIRD trace ... 9 0 Figure 3-18. TDS measured as a function of hydrogen exposure. (a) Exposure of a 0.26 ML Au-Ni( 111) to molecular H2 beam. Hydrogen exposure represented as the

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Figure 3-19. TDS measured as a function of hydrogen exposure. Same spectra as in Figure 3-18 but plot is expanded to better visualize the low-temperature desorption feature. (a) Exposure of a 0.26 ML Au-Ni(1 11) to molecular H2 beam. (b) Exposure

of a 0.52 M L Au-Ni(l 11) surface to atomic H. ... 93 Figure 3-20. Diagram of threefold hollow sites on a Au-Ni(1 11) surface alloy, with different composition of surrounding atoms. The red dot denotes the hollow site.. 95 Figure 3-21. Theoretical percentage of 3-fold hollow sites with differing compositions on a Au-Ni(1 11) surface alloy as a function of Au coverage. ... 96 Figure 3-22. Comparison of experimentally determined H coverage with theoretical percentage of 2 types of 3-fold hollow sites as a function of Au coverage... 97 Figure 3-23. Potential energy diagram of the H-Au-Ni(1 11) surface alloy system, constructed on top of Figure 3-1. 3 kcal/mol is the activation energy of recombinative desorption for H adsorbed on the 3-fold hollow sites surrounded by 2 Ni atoms and 1 Au atom, calculated by this work. See caption of Figure 3-1 for reference to other values... 100 Figure 3-24. HREEL spectra of H on Au-Ni(1 11) surface alloys. The surface alloys were exposed a molecular H2 beam with 80 psig stagnation pressure for 7.5 minutes

at 77 K. All spectra were measured at 20 = 120 off-specular angle with 3 eV electrons. The Au coverages are labeled for each trace... 102 Figure 3-25. HREEL spectra measured on a 0.36 ML Au-Ni(111) surface alloy. (a) After exposure to a molecular H2 beam with 80 psig stagnation pressure for 7.5

minutes at 77 K. (b) After raising the crystal temperature to 190 K. (c) After raising the crystal temperature to 520 K. All spectra were measured at 20 = 120

off-specular angle w ith 3 eV electrons. ... 104 Figure 3-26. Comparison of HREELS of H on a 0.08 ML Au-Ni(111) surface alloy measured with 3.5 eV and 5.5 eV incident electrons. The surface alloy was exposed a molecular H2 beam with 80 psig stagnation pressure for 5 minutes while the

sample was cooling from 773 K to 77 K. All spectra were measured at 26= 120 off-specular angle. The intensity of the red trace (5.5 eV) is multiplied by 0.1... 106 Figure 3-27. HREEL spectra of H on Au-Ni(1 11) surface alloys. The surface alloys were exposed to a molecular H2 beam with 80 psig stagnation pressure for 5 minutes at 77 K. All spectra were measured at 20 = 120 off-specular angle with 5.5 eV electrons. The Au coverages are labeled for each trace. The intensity of trace (a) is m ultiplied by 0.1... 108 Figure 3-28. HREEL spectra measured on a (a) 0.15 ML, and (b) 0.28 ML Au-Ni(1 11) surface alloy. (i) After exposure to a molecular H2 beam with 80 psig stagnation

pressure for 5 minutes at 77 K. (ii) After raising the crystal temperature to 220 K and cooling back to 77 K. All spectra were measured at 20 = 120 off-specular angle w ith 5.5 eV electrons. ... 110 Figure 3-29. HREEL spectra measured on a (a) 0.21 ML and (b) 0.36 ML Au-Ni(1 11) surface alloy. (i) After exposure to a molecular H2 beam with 80 psig stagnation

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and a stagnation pressure of 200 psig Xe/He mixture. All spectra were measured at 20 = 12' off-specular angle with 5.5 eV electrons... 112 Figure 4-1. HREEL spectrum of a Au-Ni( 111) surface with five evenly spaced loss features. The Au coverage is 0.44 ML. Spectrum measured at 20 = 200 off-specular angle with 3 eV electrons while holding the crystal at 77 K. ... 121 Figure 4-2. AES of the same Au-Ni(1 11) surface used for HREELS measurement in

Figure 4-1. Trace amounts of C (272 eV) and 0 (510 eV) are observed... 121 Figure 4-3. The four possible adsorption configurations of graphene on Ni(1 11) surface.

Figure adopted from ref [14]... 126 Figure 4-4. Surface phonon dispersion of graphene on Ni(1 11). Black lines are DFT calculations for isolated graphene from [27]. Red dots are HREELS data measured in the FM direction [25] and 7K direction [26]. Black dots are helium atom scattering (HAS) data from [28]. Dashed blue lines are the longitudinal resonance (LR) and Rayleigh wave (RW) modes of Ni(1 11). Inset is the surface Brillouin zone of graphene. Figure adopted from ref [23]... 128 Figure 4-5. Experimental procedure for studying the effects of temperature and time on

the EELS loss features. Letters from a to j are used to denote each spectrum. ... 131 Figure 4-6. The schematic of HREELS experiments. (a) Specular; (b) Off-specular. The

red arrow (x axis) represents the surface plain of the crystal, and black dot line represents the surface normal direction. See text for other details. ... 133 Figure 4-7. Specular HREEL spectra of a 0.15 ML Au-Ni( 111) surface alloy. Trace labels (a) to () are mapped to experimental procedures described in Figure 4-5. Traces (h) and (j) are measured at 293 K, and all other traces are measured at 77 K. Flash means raising the crystal temperature to 773 K; wait means waiting for -1.5 h o u rs... 13 7 Figure 4-8. Specular HREEL spectra of a 0.27 ML Au-Ni(1 11) surface alloy. Trace labels (a) to (j) are mapped to experimental procedures described in Figure 4-5. Traces (h) and () are measured at 293 K, and all other traces are measured at 77 K. Flash means raising the crystal temperature to 773 K; wait means waiting for -1.5 h o u rs... 13 9 Figure 4-9. Specular HREEL spectra of a 0.38 ML Au-Ni(1 11) surface alloy. Trace labels (a) to () are mapped to experimental procedures described in Figure 4-5. Traces (a), (h) and () are measured at 293 K, and all other traces are measured at 77 K. Flash means raising the crystal temperature to 773 K; wait means waiting for - 1.5 h o u rs... 14 1 Figure 4-10. Specular HREEL spectra of a 0.56 ML Au-Ni(11) surface alloy. Trace labels (a) to () are mapped to experimental procedures described in Figure 4-5. Traces (a), (h) and () are measured at 293 K, and all other traces are measured at 77 K. Flash means raising the crystal temperature to 773 K; wait means waiting for

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Figure 4-11 HREEL spectra of Au-Ni(111) surface alloys measured at different crystal temperatures. The Au coverage and crystal temperature are labeled for each spectrum. For all spectra, Ei = 3 eV, 26 = 200. Trace label (c), (f) and (i) are mapped to experimental procedures described in Figure 4-5. Vertical dot lines are

drawn at 750, 1470, 2200, 2990, and 3670 cm' to guide eyes... 144

Figure 4-12. HREEL spectra of Au-Ni(1 11) surface alloys as a function of Au coverage. Crystal held at 77 K, Ej = 3 eV, 26 = 200. Vertical dotted lines are drawn at 750, 1470, 2200, 2990, and 3670 cm - to guide eyes... 146

Figure 4-13. Simulated Au-Ni( 111) surface alloys at Au coverage from 0.10 ML to 0.70 ML. Grey circles are Ni atoms, yellow circles are Au atoms, black dots are isolated C atoms, and hexagonal structures are graphene "nano-flakes"... 149

Figure 4-14. HREEL spectra of a 0.30 ML Au-Ni(1 11) surface alloy measured with 3 eV electrons at various scattering angles. Crystal held at room temperature... 152

Figure 4-15. HREEL spectra of a 0.31 ML Au-Ni(1 11) surface alloy measured with 6 eV electrons at various scattering angles. Crystal held at room temperature... 153

Figure 4-16. HREEL spectra of a 0.31 ML Au-Ni(1 11) surface alloy measured with 10 eV electrons at various scattering angles. Crystal held at room temperature. ... 154

Figure 4-17 Graphene phonon dispersion on a ~0.30 ML Au-Ni(1 11) surface alloy... 156

Figure 4-18. HREEL spectra of graphene on a 0.35 ML Au-Ni(1 11) surface alloy. (a) After fully growth of graphene, measured at 293 K. (b) After H atom exposure at 77 K, measured at 77 K. (c) After flashing the crystal to 400 K, measured at 293 K. (d) After annealing the crystal at 773 K for 10 min, measured at 293 K. For all spectra, E i = 5.5 eV , 20 = 12 ... 160

Appendices Figure A-1. Main chamber interlock control unit, front and rear view. Green circles represent sensor status LEDs; blue circles represent unit power LEDs, red circles represent error LEDs, red circles (square) on top of grey circles (square) represent reset or bypass buttons. Column name explanations: PRESSURE (foreline pressure), FV (foreline valve), WATER (cooling water to diffusion pump), DP POWER (power/current to diffusion pump), DP (diffusion pump), LN2 (liquid nitrogen), G V (gate valve)... 171

Figure A-2. Circuit Diagram for the Main Chamber Interlock System... 175

Figure A-3. Circuit Diagram for the JKFF Module... 176

Figure B-1. M achine drawing of the cap ... 180

Figure B-2. M achine drawing of the inner tube... 181

Figure B-3. M achine drawing of the outer tube... 182

Figure B-4. Procedure for welding the inner tube and the cap ... 184

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Figure B-6. Diagram of the nozzle with cooling water support. Green arrow shows the movement of the translational stage. Blue arrows indicate directions of water flow. Red arrow represents the gas supply. Drawn courtesy of Yang Dai... 186

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List of Tables

Chapters

Table 3-1. Calculation of activation energy of desorption based on the low-temperature desorption features shown in Figure 3-18a... 99 Table 4-1. Description of phonon modes in graphene... 127 Table 4-2. Comparison of phonon energies in cm' of graphene on different substrates ... 15 7 Appendices

Table A-1. Functionalities of the Interlock System... 169 Table A-2. J- K Flip-Flop Function Table ... 174 Table A-3. Circuit Board Connection Guide for the Main Chamber Interlock System 177 Table B-1. Heating behavior (with cooling water on) of the nozzle ... 187

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Chapter

1 -

Introduction

1.1 Motivation and Background

Surface alloys, produced by alloying a small amount of one element into the surface layer of another base metal, have drawn considerable attention by the surface science community. Variation of the surface composition of surface alloys gives researchers the ability to tune the selectivity and efficiency of the catalysts. Moreover, choosing an inexpensive element as the base metal can lower the cost of surface alloy catalysts. As a result, the studies of surface alloys have greatly accelerated the development of novel and affordable catalytic systems [1].

This thesis explores the unique properties of the Au-Ni(l 11) surface alloy. Our understanding of the structure of the Au-Ni( 111) surface alloy is largely based upon the imaging studies by Nielson [2, 3] using scanning tunneling microscopy. The surface alloy is formed by deposition of Au vapor onto a Ni(1 11) single crystal followed by annealing. This procedure results in displacement of Ni atoms on the top layer by Au atoms, forming a hexagonally closed packed Au-Ni(l 11) surface alloy instead of Au clusters [3]. Note that the replacement of Ni atoms by Au atoms is random. At Au coverages of less than 0.3 monolayers (ML), the two-dimensional structure of the clean Ni surface is preserved. Above 0.3 ML, the top layer lattice constant abruptly increases and a reconstruction of the supporting Ni layer occurs in order to release the surface strain, resulting in periodic triangular misfit dislocations. The details of this surface

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The addition of Au atoms dramatically changes the electronic structure and catalytic properties of Ni(1 11). Nickel by itself is not very useful in catalyzing the CO oxidation reaction [5], because oxygen and CO bind so strongly to the Ni(1 11) surface that no CO2 is produced under UHV conditions [6]. In contrast, a recent study by our

group has shown that CO can be oxidized on the Au-Ni( 11) surface alloy at temperatures as low as 70 K [7]. We demonstrated, via high-resolution electron energy loss spectroscopy (HREELS) and mass spectroscopy, that molecularly adsorbed oxygen (peroxo or superoxo-like species) is stabilized on the Au-Ni( 111) surface alloy and acts as the reactant in the catalytic oxidation of CO. A later study by Knudsen and co-authors also supports this finding [8]. The ability to effectively catalyze CO oxidization at low temperature, along with its low cost, makes the Au-Ni( 111) surface alloy a potential catalyst in catalytic converters for automobiles, and has the advantage that the catalyst is active upon a cold start [9].

1.1.1 Hydrogen on Au-Ni(111) Surface Alloys

The interaction of hydrogen with Ni(1 11) has been studied extensively [10-14]. For example, our group has shown conclusively that bulk H atoms are the reactive species in the hydrogenation of adsorbed methyl radical [12], ethylene [15], and acetylene [16] to gas-phase products on single crystal Ni. However, exploration of the interaction of hydrogen with Au-Ni( 111) surface alloys remains inadequate. Although there have been a few theoretical reports on the H-Au-Ni system [17, 18], current understanding of this system does not lay a solid foundation that allows further study of the effectiveness of Au-Ni( 111) surface alloys for catalyzing hydrogenation reactions. In

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this thesis, the interaction of hydrogen with Au-Ni( 111) surface alloys is investigated spectroscopically, with a focus on identifying the ad/absorption sites of hydrogen and to obtain useful metrics such as vibrational frequencies and binding energies.

One potential application of Au-Ni( 111) surface alloys is to catalyze oxygen reduction reactions, given that the O or 0- species stabilized by Au-Ni( 111) surface alloys are strongly suggested to be key intermediates in the electrochemical reduction of oxygen by hydrogen on Pt electrodes [19]. In addition, the Au-Ni( 111) surface alloy is insensitive to CO poisoning and has a relatively low cost, compared to Pt, a common catalyst used in hydrogen fuel cells. All of the above properties make the Au-Ni(1 11) surface alloy a promising candidate for hydrogen fuel cell applications.

1.1.2 Carbon on Au-Ni(111) Surface Alloys

The interaction of carbon with nickel has been the subject of numerous studies [20, 21]. In the past decade, the potential importance of graphene as a component of solid state devices has led to a boom in the investigations of the graphene-Ni system, since it represents a prototypical example of a strong graphene-metal interface [22]. Graphene can be synthesized through chemical vapor deposition (CVD) of hydrocarbons onto the Ni( 111) surface, but depending on the carbon concentration and sample temperature, the nickel carbide (Ni2C) phase and graphite may also be obtained [22]. In

order to fabricate large-area graphene sheets, the growth mechanisms of graphene need to be thoroughly understood [23, 24].

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exposing the sample to a large amount of hydrocarbons, whether or not graphene can grow in the low concentration carbon regime is not well known. In this thesis, strong evidence for graphene formation at low concentration carbon on the Au-Ni( 111) surface alloy is presented, and the role that Au atoms may play in enhancing the graphene growth process is discussed.

1.2 Outline of Dissertation

In Chapter 2, a method for quantitatively determining the Au coverage using Auger electron spectroscopy (AES), which builds the foundation for studying the Au-Ni(l 11) surface alloys, is discussed. This method consists of two major steps. First, an Auger spectrum is acquired on a Au-Ni(1 11) surface alloy and a curve-fitting algorithm is used to extract the intensity ratio of the Au and Ni transitions from the spectrum. Second, the Au coverage is computed based on the intensity ratio and correction factors, including the Auger sensitivity factors and the bulk contribution factor, which are described in detail in Chapter 2.

Chapter 3 discusses the interaction of hydrogen with Au-Ni(1 11) surface alloys. The hypothesis that exposure of Au-Ni( 111) surface alloys to molecular hydrogen under ultra-high vacuum (UHV) conditions results in the formation of bulk H is tested. The main techniques employed in this investigation include high-resolution electron energy loss spectroscopy (HREELS), thermal desorption spectroscopy (TDS), and collision-induced recombinative desorption (CIRD). Two types of threefold hollow sites for hydrogen atom adsorption are identified. The binding energy and vibrational frequency

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of hydrogen at each site are studied. However, no evidence of bulk H formation was found by exposing Au-Ni( 111) surface alloys to low pressure molecular hydrogen.

In Chapter 4, the observation of five loss features, almost evenly spaced in frequency, in the HREEL spectrum of Au-Ni( 111) surface alloys is reported. These features grow in with time and cannot be easily perturbed. Further study of these features suggests the formation of graphene on the Au-Ni( 111) surface alloys, in the absence of deposition of carbonaceous materials and despite the almost imperceptible concentration of carbon in the Ni crystal. The two most intense features at 750 cm- and 1470 cm-I are attributed to the ZO and LO (TO) phonon modes of graphene, respectively. The origins of other features are also discussed in detail.

Other valuable projects related to this thesis are included in the appendices. Appendix A describes the design and operation of a new interlock control system, which monitors and protects the main chamber. Appendix B illustrates recent modifications to the molecular beam source nozzle. Finally, Appendix C discloses the details of the Python curve-fitting program that is used to determine the Au coverage.

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Ni(111) and on a A u/Ni(111) surface alloy. ACS Nano, 2010. 4(8): p. 43 80-7.

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Hydrogen: Reaction of Hydrogen Embedded in Nickel with Adsorbed CH3.

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Chapter 2 -

Determination

of Au

Coverage

with

Auger

Electron Spectroscopy

2.1 Introduction

The catalytic properties of a solid material are greatly affected by its elemental surface composition. Therefore, determining the surface composition is a critical step in surface studies. This step can be accomplished by a variety of surface sensitive techniques, such as Auger Electron Spectroscopy (AES) and X-ray photoelectron spectroscopy (XPS). In this thesis, AES is used as the analytical tool to determine the Au coverage for Au-Ni(l 11) surface alloys.

The Auger process, first described by physicist Pierre Auger in 1925, is a mechanism for an atom whose core level electron has been ionized to relax to a lower energy state [1]. In this process, an electron from a higher energy shell fills this core vacancy, simultaneously ejecting another electron into the vacuum. This emitted electron is referred to as the Auger electron. Auger electrons have characteristic energies that are distinct for each element. Auger spectra plot the intensity of the detected Auger electrons as a function of electron kinetic energy, and surface compositions can be derived based on the energies and intensities of the Auger electron transitions.

Although qualitative determination of the surface composition is straightforward using Auger electron spectroscopy, quantitative analysis requires care [2]. Part of the challenge is an accurate measurement of the Auger transition intensities from the spectra, because the finite resolution of the spectrometer often leads to transitions that overlap in

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energy. The remaining challenge is construction of a model that converts the measured Auger intensities into a quantitative surface composition. Each of these aspects is discussed in detail below.

The most common measure of Auger transition intensities is the Auger peak-to-peak height (APPH) obtained from derivative spectra [2]. The major advantage of measuring an Auger spectrum in the derivative mode is that the linear component in the background is automatically eliminated, so a background subtraction is no longer necessary. It can also be shown that if the peak shape is independent of the intensity, the APPH is then proportional to the peak area. However, APPH is not straightforward when transitions from two different elements of interest overlap in energy. In such cases, methods like curve fitting or principal component analysis must be used to decompose the overlapping transitions. Usually, AES transitions measured in the normal mode are characterized by a Gaussian or Lorentzian profile [2]. In the following section, a least squares fit to the derivative spectra employing two derivative Gaussians is used to determine the Auger intensities of the Ni transition at 64 eV and Au transition at 74 eV.

Having determined the Auger intensities for each elemental transition, the surface composition can be quantified using elemental relative sensitivity factors that are tabulated in a reference referred to here as the Handbook [1]. These relative sensitivity factors are measured as the ratio of the APPH of a particular transition of element i to that of the Ag (350 eV) transition. However, one major issue with these sensitivity factors is that matrix-dependent factors that affect the Auger intensity, i.e., the "matrix effects", are ignored [3]. Although some matrix effects such as the sputter correction factor, a factor

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neglected in the Au-Ni( 111) surface alloy system, the most important ones need to be corrected. The important matrix effects include the atomic density, which is the total number of atoms per unit volume, and the backscattering factor, which accounts for the increased Auger signal intensity due to additional ionization of core electrons by backscattered secondary electrons [4]. Various models have been published to correct for the matrix effects [5-7].

Another key parameter related to quantitative AES analysis is the inelastic mean free path (IMFP) of electrons, defined as the average distance an electron with a given energy travels between successive inelastic collisions [2]. This parameter is important because it describes how far an electron can travel in a metal before losing its energy, which helps to determine the contribution to the measured Auger intensity from atoms below the surface. In 1979, Seah and Dench proposed a "universal curve" for IMFP as a

function of electron energy, based on an extensive database [8]. Today it is generally accepted that the "universal curve" only provides a rough estimation. More accurate calculation of IMFP can be made through the TPP-2M equation, named after Tanuma, Powell, and Penn [9-11].

Basic formalisms for calculating elemental composition in the homogeneous case and layered structure have been described by Chang [12]. The system of our interest, the Au-Ni(l 11) alloy, is a special case where only the surface layer consists of two metals and the substrate is purely Ni. It is difficult to extend previous formalisms to describe such a dilute alloy system, because additional correction factors need to be considered. In this chapter, a straightforward framework is introduced to calculate the Au coverage for the Au-Ni( 111) alloy, which can also be applied to generic two-component systems.

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The step-wise approach is detailed in the following sections, and the uncertainty of the determined Au coverage is also discussed.

2.2 Formalism

2.2.1 Definition of Au Mole Fraction and Au Coverage

The mole fraction of Au on the surface is defined as p, the number of Au atoms on the surface divided by the total number of atoms on that surface.

N surface P Nsu+ace"Au surface (2.1) NNi +NAu Denote Nsurface rN= Au (2.2) rN Nsurface Ni

The reciprocal of equation (2.1) gives

1 1 _(2.3)

P rN

Gold coverage 0, on the other hand, is the number of Au atoms on the surface divided by the number of Ni atoms if the surface is entirely covered with Ni atoms. For a

surface with both Au atoms and Ni atoms present, the denominator can be expressed as the number of Ni atoms present on the surface, plus the number of Ni atoms that could fit on the surface area covered by Au atoms.

N' surface

0 =Au

Nsurace + Nsurace. Area per Au atom

Ni Au Area per Ni atom

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where dAU = 2.80 A and dNi = 2.49 A are the equilibrium Au-Au and Ni-Ni distances measured previously by scanning tunneling microscopy (STM) of a Au-Ni(1 11) surface alloy [13]. To simplify the notation, define

dAU rd =_dAu-= 1

d dN 0.791

The reciprocal of equation (2.4) in terms of rd is

-= - +

r

0 rN

(2.5)

(2.6)

Combining equations (2.3) and (2.6), the relationship between Au mole fraction and Au coverage is

= +r 2 -1=

+0.264

This relationship is visualized in Figure 2-1 below.

0.8 0.7 0.6 0.5 0.4--0 U 0.3 - 0.2-0.1 nfl 0.0 0.2 0.4 0.6 0.8 Mole Fraction of Au p (2.7) 1.0

Figure 2-1. Mole Fraction of Au vs. Au Coverage

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2.2.2 Derivation of the Expression for Au Coverage

The goal is to determine the Au mole fraction p, or Au coverage 0, based on measurable quantities. The absolute number of Ni atoms and Au atoms on the surface,

Nsuface and Nsurface respectively, are derived from Auger transition intensities. For

example, the measured intensity of the Au transition, measured ,is proportional to Au

and satisfies the expression

m"easured _ jsurface = surfaceS (2.8)

Au Au Au A

The measured Au intensity is equal to the surface Au intensity, Isu"ace because Au atoms are present only on the surface. The factor SAu is a sensitivity factor for the Au Auger

transition that is discussed below. It follows that the intensity of the Ni Auger signal originated from the surface Ni atoms satisfies

Isuface = surface . (2.9)

Ni Ni Ni

where SNi is the Ni sensitivity factor.

It is important to note that unlike the measured Au transition intensity, measured Ni transition intensity Imeasured is not equal to the surface Ni Auger intensityN, Iiace

because of contributions from Ni atoms below the surface layer to the measured intensity. Hence, the measured Ni intensity is a sum of the surface Ni intensity and the bulk Ni intensity, NI"k, as shown in equation (2.10)

'"easured _ jsurface + i "ul (2.10)

Ni Ni Ni

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i = I"'t bulk layer

n=l

= Pue surface layer . exp nd

(2.11)

,

exp_n _{A in} _{out csc}

C[s

Pure surface layer Ni

where ipure surface layer is the intensity from a pure Ni surface layer, d is the Ni(I 11) layer

spacing, n and Aut are the inelastic mean free paths (IMFP) of the incident electron

and emitted Auger electron, respectively, and (p is the acceptance angle of the cylindrical mirror electron energy analyzer. The infinite sum is represented by /, the bulk contribution factor. The value for / is sensitive to the choice of values for the IMFP, and its calculation is presented in section 2.2.4.

For a surface with both Ni and Au atoms, the surface Ni intensity is only a

fraction of INpu surace laYr and is given by

Isurface pure surface layer

Ni Ni

= Jpumr surface layer Ni

= jpure surface layer Ni

Substitution of equations (2.11) and

Pure surface layer and establishment of a

(2.13).

Nsurface_Ni

N surface + Nsurface Area per Au atom

Ni Au Area per Ni atom

1 2 _(2.12)

N surface d

N sufae dl

Ni d

1+ rN

(2.12) into (2.10) results in cancellation of the term relationship between Isurface and Imeasured in equation

Isurface = jmeasured

Ni Ni

1

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Substitution of equation (2.13) into equation (2.9) yields Imeasured

Nsurface = Ni 1 (2.14)

Ni N2 N

Rearrangement of equation (2.8) yields

I measured

Nsurace = Au (2.15)

Au

Division of equation (2.15) by equation (2.14) yields N surface _Au = _r measured N

-Au _Ni _r2 ₊

(216

psurface rN jmeaue

s

(1

' r

Ni Ni Au

Rearrangement of equation (2.16) produces

rN measured2.17)

Ni SAu 2

Imeasured S d

Au Ni

Plugging equation (2.17) into equation (2.2) provides the surface Au mole fraction p, expressed in terms of measurable quantities and constants,

measured

s

Ni Au 2 1measured S d 1 - Au Ni (2.18) p +1

Rearrangement of equation (2.18) yields,

P 1 esue #1+1 -a(#3+1) (.9

measuredmeasured

INi ___ 2+ N1 a[(1r2)+ 1

measured d measured d

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where a = Ni is the ratio of the sensitivity factors. The values of the sensitivity factors

SAu

are given in Reference [1]. The corrections to the tabulated sensitivity factors for the relative density of the material are discussed in the next section.

A similar substitution of equation (2.17) into equation (2.6) yields the Au coverage 6 expressed in terms of measurable quantities and constants,

Sa(#+i) measured Ni +r 2 measured d Au (2.20)

2.2.3 Correction for Relative Sensitivity Factor

Reference [1], called the Handbook for short, provides elemental relative sensitivity factors measured at several different energies of the incident electron beam using the Ag Auger transition at 350 eV as a reference value. According to the description in the Handbook, one of the matrix effects, the backscattering effect, has been taken into account in this compilation of experimentally measured relative sensitivity factors. If no further correction is made, the ratio of the sensitivity factors, a , is

stabulated 1.5287 a =Ni = 1.65528721

tabulated stabulated 2.3433= 0.6524 (2.21) Au

where sNtabulated is the tabulated sensitivity factor for the Ni transition at 64 eV measured

with a 3 keV incident electron beam energy and Stabulated is that for the Au transition at 74 eV. However, the Handbook ignores the difference in atomic densities. That is, it assumes that the same number of atoms are included in the sampling volume of each

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element. To correct for this assumption, equations 12 and 14 from the Handbook are rewritten here as equations (2.22) and (2.23):

S "orrected - Pref x -Sf PX 'ref

I

Stabulated 1i x -S = S x I rf I rf ref ref (2.22) (2.23)

where pref is the atomic density of the reference material, and px is that of the sample. The assumption pref/px =1 is used in the Handbook when calculating the tabulated sensitivity factor, as shown in equation (2.23). Taking the ratio of equations (2.22) and (2.23) yields,

S "rected - Pref S "buIated

Px

The corrected value of a using corrected sensitivity factors becomes

corrected Stabulated

Ni Ni

corrected Scorrected stabulated

Au, Au, 1.5287 2.3433 352.4 pm 407.82 pm stabulated a PAu S Nie Ni stabulated a

PNi SAu (Au

I 3

= 0.4209

where a Au and aNi are lattice constants for Au and Ni, respectively.

In the calculations to follow, acdeed will be used unless otherwise stated. A

comparison of the calculated Au coverage using both tabulated and corrected sensitivity factors is also shown in the discussion section.

(2.24)

)J

3

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2.2.4 Calculation of Bulk Contribution Factor

From equation (2.11), the expression for /, the contribution to the measured Ni Auger intensity from Ni below the surface is

#=Yexp

-nd (2.26)

n=1 n

A k A., Cos p

where d is the Ni(111) layer spacing, Al and )yt are the inelastic mean free paths (IMFP) of the incident electron and emitted Auger electron, respectively, p = 42.30 is the acceptance angle of the CMA. The incoming electron beam is normal to the surface and outgoing beam is approximately 42.3* from the normal due to the CMA geometry, which is why there is a cos(p term in the denominator to account for the longer distance traveled by the outgoing Auger electron with its non-normal geometry. The distance d between the (111) layers of the Ni crystal with it face-centered cubic (FCC) structure is given by

d - 24 = 2._{0 3 5}A (2.27)

where aNi is the Ni lattice constant.

The values for %i and )L, are obtained from two different sources. One source

is the "universal curve" compiled by Seah and Dench largely from many electron scattering experiments [8]. The universal curve assumes that the inelastic mean free path of electrons in all metals is essentially dependent only on the energy of the electron and not the chemical nature of the metal, and can be described as

XSD(nm) 2143 +0.054 - E+ W (2.28)

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where E is the electron kinetic energy in eV, W is the work function of the metal. For Ni(1 11), W = 5.35 eV [15]. Therefore, in our sample, the inelastic mean free path of the

incoming primary electron beam with E = 2.00 x 103 _{eV is A.} ₌_24.2

A,

_{whereas that of}

the outgoing Auger electrons with E =64 eV is AM = 4.79 A. With all the above values,

#

is calculated from equation (2.26) as

#SD

=1.07 (2.29)

Figure 2-2 demonstrates how the value of P progresses by including more and more bulk layers of Ni atoms in the infinite sum. As can be seen, the sum converges at about the 8th bulk layer. In other words, the effective detection depth of the Ni 64 eV Auger electrons excited with a 2 keV incident electron beam is about 1.6 nm. In addition, the value of P is close to 1, which implies that the Ni atoms below the surface layer contribute to the measured Ni intensity equivalently to a pure Ni surface layer.

1.1 0 0.8 -E U 0.6- -SD 1-U1 - pp= 1.03 0.5--1 2 5 4 5 6 7 8 9 10 11 12 13 14

Number of bulk layer

Figure 2-2. Calculation of bulk contribution factor

#

through layer-by-layer summation. is

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Besides the "universal curve", a second source of IMFP values is used to calculate

. Tanuma, Powell, and Penn calculated the IMFPs for elemental solids using experimental optical data and the Penn algorithm [9]. The detail of the TPP-2M equation can be found elsewhere [10, 11, 16]. Here, the tabulated results for Ni in Table 4 of ref [11] are used to yield )Li = 25.3

A

and /lout = 4.59

A.

By plugging in these values in

equation (2.26), we find

#TPP =1.03 (2.30)

A plot of the layer-by-layer summation of 3

TP using the IMFPs calculated by the

TPP-2M method is also included in Figure 2-2. The difference between

#

3

SD and PTP is about

4%.

2.2.5 Quantitative Value of Au Coverage as a Function of Auger Intensities

With values for parameters a and

#

determined, the Au coverage is calculable from equation (2.20), once the ratio of the Auger transition intensities (A. : Ni) is

determined by curve fitting of the experimental intensities. Figure 2-3 plots the Au coverage as a function of IAu 'Ni using acorrected and two different values for P. As can

be seen, when IAu: Ni is less than 0.25, the Au coverage is almost linearly dependent on

IA J :N, with an intercept equal to zero. The relationship becomes more non-linear as

IAU: Ni increases. When IAu NI reaches 1.8 or so, the Au coverage reaches 0.79 ML,

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0.8-- PSD 0.7 - PTPP - 0.6-4)0.5 - 0.4-U ~0.3 0.2- - 0.1-0.0 0.00 0.25 0.50 0.75 1.00 1.25 1.50 1.75 2.00 IAu INi

Figure 2-3. Au coverage as a function of Auger intensity ratio, calculated using corrected sensitivity factors, and IMFPs from two sources.

This model is built on the assumption that Au atoms are present only at the surface. Therefore, it's only suitable for single monolayer or sub-monolayer Au coverage calculation. A calculated Au coverage above 0.79 ML does not accurately reflect the amount of Au on the surface and hence, may only be used for qualitative purposes. We also see that the choice of P, reflecting the method of determining the IMFP values, affects the calculated Au coverage, but the discrepancy is at most 3% at high Au coverages. At Au coverages lower than 0.4 ML, this discrepancy can be neglected. The choice of P introduces a systematic error. A more detailed analysis is found in section 2.4.2.

Dynamics of hydrogen and low concentration carbon on Au-Ni(111) surface alloys

Signature redacted

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Acknowledgements

'J

Table of Contents

List of Figures

P

List of Tables

Chapter

1

-

Introduction

1.2 Outline of Dissertation

Reference

Chapter 2

-

Determination

of Au

Coverage

with

Auger

Electron Spectroscopy

2.1 Introduction

r

+0.264

,

C[s

s

(1

s

I

I

#=Yexp

A,

#

#SD

#

A

A.

#

#