Electron Transfer Processes At Semiconductor Liquid-Books Pdf

Electron Transfer Processes at Semiconductor Liquid
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Florian Gstrein, All Rights Reserved, This thesis is dedicated to my uncle. Dr H Peter Wagner, Meteorologist, Acknowledgments, A wise and slightly cynical postdoctoral scholar in the Lewis group once told me. as he was pacing up and down the noble hallways of Noyes The only thing people are. really interested in when reading your thesis are the acknowledgements So acknowledge. everyone and everything If this statement is correct I had better do justice to the. expectations of my readership I have done my best in compiling the following list of. people who all have made important contributions to my Ph D thesis and I apologize for. any omissions, It gives me great pleasure to thank my advisor Prof Nathan S Lewis for his. outstanding mentorship and friendship Nate is an exceptional advisor inspiring scientist. and without a shadow of a doubt the smartest man I have ever met The most important. thing I have learned from Nate is that experimental science can only be done by. approaching a scientific question thoroughly and carefully from various angles and by. carrying out the appropriate control experiments, Dr Helfried N fe Max Planck Institute for Metals Research Stuttgart FRG and. Dr Dimitrij I Bronin Institute of High Temperature Electrochemistry Russian Academy. of Sciences Ural Branch Ekaterinburg Russian Federation were the most influential. mentors in my pre Caltech days Their shining example and integrity as researchers left. me with no other choice but to pursue academic research. I would like to thank my co advisors Prof Michael Roukes and Prof James C. Hone who is now at Columbia University Michael not only allowed me to work in his. magnificent laboratory in the Department of Physics but he also fully integrated me into. his research group Not many outsiders belong to the inner circle of the Roukes group and. I am grateful for Michael s genuine and continued interest in my research project Jim the. force extraordinaire behind the NIRT project pursued together with me many projects and. I am looking forward to our collaboration on various levels in the future I also learned. from Jim that you can buy an entire lab on eBay but that you cannot buy a car from him. There are many members of the Lewis and Roukes group who have worked with. me closely on several projects Tom Hamann did an outstanding job in synthesizing many. of the osmium compounds for the ZnO project and in studying their self exchange. properties Dr William Royea and David Michalak worked with me closely on studies of. the surface recombination behavior of silicon In particular Dave carried out most of the. rf conductivity measurements I would like to thank Prof Adrian Lew Stanford. University and Dr Reiner N rnberg Weierstrass Institute Berlin for many useful. discussions regarding the TeSCA simulations The NIRT project was a collaborative effort. of many people Jim s leading role has already been mentioned My friend Dr Srivatsan. Nagarajan came up with many original ideas regarding this project and contributed in. setting up the early instrumentation Working with Leon Bellan who did his senior thesis. work with me was the most pleasant experience I had at Caltech Leon contributed greatly. in setting up the measurement system in its present form I would also like to thank Ali. Hussain Scherer group Carol Garland TEM work and Drs Hongxin Tang Darron. Young Hidehiro Yoshida and Henk Postma all Roukes group for their helping hands. with many NIRT experiments, I am also indebted to many Lewis group members present and past for good.
discussions and their support especially Dr Shawn Sean Briglin Lauren Lorraine. Webb Jordan Katz Prof George Coia the wise old postdoc Prof Sami Anz Dr Will. Royea Dr Rob Rossi Dr Glen Walker Prof Mike Freund Dr Bruce Brunschwig Dr. Jae Joon Lee Dr Arnel Fajardo and two exceptional administrative assistants of Nate s. Nannette Pettis and Sherry Feick Among all Lewis groupies Mr David J Michalak. stands out as an exceptional scientist co worker film maker co actor and great friend. I would like to thank the remaining members of my Ph D committee Prof Rudy. Marcus for all the beautiful theory without which none of the experimental work in this. thesis would have made much sense and Prof Bill Goddard for two exceptionally well. taught classes which convinced me that I better stay far away from computational. chemistry Many of the experiments would have been impossible without Mike Roy and. Guy Duremberg Instrument Shop Tom Dunn Electrical Shop and the great Nils. Apslund Roukes Group, Last but certainly not least I would like to thank the many outstanding people that I. met during graduate school for their friendship love and spiritual support In no particular. order these individuals are Adri Lew and Patri Strass now at Stanford Alex B cker Eva. Peral Veronica Pablito all Pasadena Julian Chaubell Nance Pasadena Mathias. Libedinsky now in Jerusalem Ronen Almog now at the Technion in Haifa Mathias. Zielonka Alejandra Engelberg and Sebastian all Pasadena Eva R ba now in Vienna. Christoph Sonnlechner now in Vienna Ines Schwetz now in Graz and my dear. landlords and friends Robert and Carolyn Volk San Marino. Finally I would like to thank my family back in Austria first and foremost my. parents Hans J rgen Edith Gstrein and my sister Marlis for their relentless moral. support and good humor during all the ups and the rare downs of the past five years. This thesis presents recent progress made in the understanding of charge transfer. kinetics and charge carrier dynamics at semiconductor liquid interfaces and presents a. study of the charge transport characteristics of nanostructured electrodes for the study of. electron tunneling through molecules, It is shown that n ZnO H2O A A junctions where A A corresponds to. Fe CN 6 3 4 Co bpy 3 3 2 or OsL2L 3 2 display energetic and kinetic behavior. of unprecedented ideality for the experimental determination of robust rate constants for. electron transfer from n type semiconductors to electron acceptors in solution The. reorganization energies of these redox couples were for the first time determined. independently through NMR line broadening experiments Semiconductor liquid contacts. with an electrochemical driving force larger than the driving force at the point of optimum. exoergicity showed lower rate constants than contacts with a lower driving force The rate. constant for charge transfer of the junction with the highest driving force increased when. the driving force was lowered This was done by decreasing the pH of the contacting. electrolyte which shifted the conduction band edge of the OH terminated n ZnO to more. positive values This indicates that the contact with the largest driving force operated in. the inverted regime of charge transfer The rate constant of the contact with the lowest. driving force Go 0 7 eV on the other hand decreased when the conduction band. edge was shifted to more positive values which indicates that the low driving force. contact operated in the normal regime of charge transfer These results provide for the first. time a direct and credible experimental indication that semiconductor liquid contacts can. indeed operate in the inverted regime Semiconductor liquid contacts which had a similar. driving force but different reorganization energies showed the expected dependence of the. rate constant on the reorganization energy consistent with Marcus theory. The experimentally observed charge carrier decay dynamics for a variety of. chemically treated Si surfaces can be consistently explained with reference to their. interfacial energetics Low effective surface recombination velocities SRVs were. observed for systems capable of undergoing interfacial charge transfer reactions that. produce an accumulation of holes or an accumulation of electrons at the Si surface In. conjunction with near surface channel conductance measurements it was revealed that the. formation of an accumulation of holes or the formation of an accumulation of electrons. and not a reduced density of electrical trap sites on the surface is primarily responsible for. the long charge carrier lifetimes observed for Si surfaces Digital simulations. incorporating a generalized Shockley Read Hall model for surface recombination. revealed that effective surface recombination velocities 10 cm s 1 can be produced by. surfaces having a density of electrical traps as large as 1012 cm 2 provided that the surface. is in accumulation or inversion due to charge transfer equilibration with the redox active. electrolyte Silicon in contact with aqueous fluoride solutions exhibit low SRVs Some. reports in the past have suggested that the defect density of Si in acidic solutions is. reduced by the protonation of defect sites Alternatively this study shows that an. accumulation of electrons at the surface can lead to low SRVs The degree of band bending. and SRVs of Si 111 in contact with a variety of aqueous fluoride solutions were. determined for the first time at open circuit conditions The accumulation of electrons at. the surface is responsible for the low effective SRVs in NH4F and buffered HF solutions. The reversible protonation of basic defect sites might be important for the low SRV of n. Si 111 H2SO4 aq contacts but plays a minor role in the recombination behavior of Si in. contact with aqueous fluoride solutions, Electromigration induced breaking of metal nanowires is a promising new method. that has recently been employed in the formation of metal molecule metal tunnel. junctions The electrical characteristics of unmodified electrodes have not been thoroughly. studied In this work the current vs voltage J vs E characteristics of electron tunnel. junctions formed by the electromigration of metal nanowires without a molecule bridging. the gap were explored in detail Junctions displayed J vs E characteristics with a variety of. shapes and current magnitudes The low temperature J vs E curves of some junctions. showed regions of zero conductivity near zero bias Such features were absent in the data. collected for other tunnel junctions These differences notwithstanding a common pattern. was discerned in that the low bias resistances of the vast majority of the junctions. decreased by an order of magnitude or more with increasing temperature The features. detected in this study are consistent with the Coulomb blockade effect We attribute the. blockade behavior to metal atom clusters or islands located in the nano sized gap region. This assignment is compatible with the mechanism of electromigration In support of this. interpretation it was found that the low bias resistance vs temperature curves were well. described by Abeles model for electron tunneling in granular metal junctions Additional. support for the assignment was obtained from TEM studies of gold junctions with. chromium adhesion layers where the images showed the presence of a thin granular film. bridging the electrodes, Table of Contents, Acknowledgments iv. Table of Contents xi, List of Figures xiii, List of Tables xv.
Chapter 1 Introduction to Electron Transfer Processes at Semiconductor Liquid. Interfaces and Metal Nanogap Junctions, 1 1 Overview 1 2. 1 2 Energetics of the Semiconductor Liquid Interface 1 3. 1 3 Comparison between Semiconductor and Metal Electrodes 1 12. 1 4 References 1 13, Chapter 2 Energetics of Electron Transfer Reactions at the n type ZnO 0001. Liquid Interface, 2 1 Introduction 2 2, 2 2 Background 2 4. 2 3 Experimental 2 10, 2 3 1 Electrode Preparation 2 10. 2 3 2 Electrochemical Measurements 2 13, 2 3 3 Redox Compounds and Solutions 2 18.
2 4 Results 2 26, 2 4 1 Results of Cdiff E Measurements 2 27. 2 4 2 Results of J E Measurements 2 45, 2 4 3 Determination of Electron Transfer Rate Constants ket 2 54. 2 4 4 Trends in the ket Values for n Zn 0001 Liquid Contacts 2 58. 2 4 5 Rate Constants of n ZnO in Contact with Fe CN 6 3 4 Co bpy 3 3 2 or Ru NH3 5py 3 2. 2 5 Discussion 2 67, 2 5 1 Energetic Behavior of n ZnO 0001 Liquid Contacts 2 67. 2 5 2 Kinetic Behavior of n Zn 0001 Liquid Contacts 2 73. 2 6 Conclusions 2 79, 2 7 References 2 81, Chapter 3 Effects of Interfacial Energetics on the Charge Carrier Dynamics at. Silicon Liquid Contacts, 3 1 Introduction 3 2, 3 2 Background 3 5.
3 3 Experimental 3 10, 3 3 1 Digital Simulations 3 10. 3 3 2 Photoconductivity Decay Measurements 3 13, 3 3 3 Measurement of the Built in Voltage of Si H2O Contacts Using Differential Capacitance Measure. ments 3 15, 3 3 4 Measurement of the Built In Voltage of Si Liquid Contacts Using a Near Surface Channel Conduc. tance Method 3 19, 3 3 5 Materials Chemicals 3 24, 3 4 Results 3 26. 3 4 1 Digital Simulation of Photoconductivity Decay Dynamics for Si Liquid Contacts 3 26. 3 4 2 Photoconductivity Decay Measurements of Hydrogen Terminated Si 111 3 38. 3 4 3 Measurement of the Built In Voltage of n Si 111 H2O Contacts Using Differential Capacitance. Measurements 3 44, 3 4 4 Near Surface Channel Conductance Measurements 3 55.
3 5 Discussion 3 77, 3 5 1 The Influence of Interfacial Energetics on the Surface Recombination Velocity of Inverted n. Si 111 Contacts 3 77, 3 5 2 Comparison between Digital Simulations and Experimental Observations of the Surface Recombi. nation Velocity as a Function of the Redox Potential of the Electrolyte 3 81. Electron Transfer Processes at Semiconductor Liquid Interfaces and Metal Nanogap Junctions Thesis by Florian Gstrein In Partial Fulfillment of the Requirements

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