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Spectroelectrochemistry of Nanomaterials by Anjli Kumar

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Spectroelectrochemistry of Nanomaterials by Anjli Kumar ABSTRACT Spectroelectrochemistry of Nanomaterials by Anjli Kumar As the development and application of novel nanomaterials is continually expanding, so is the need for versatile characterization methods that can probe nanoscale processes as they occur. Such characterization techniques would permit non-destructive and parallel spectroscopy and imaging of nanomaterials. The objective of this work is to address the need for such techniques. First, a new spectroelectrochemical characterization technique, Micro-Extinction Spectroscopy (MExS), is presented, including instrumental design as well as data acquisition and analysis algorithms. Two nanomaterial systems are then studied with MExS: two-dimensional transmission metal dichalcogenides (TMDs) and plasmonic nanoparticles. For TMDs, in situ and hyperspectral reflectance studies coupled with cyclic voltammetry and chronocoulometry enable the time-resolved observation of the process of electroablation, an oxidative technique recently developed for the synthesis of monolayer TMDs. For plasmonic nanoparticles, dark-field microscopy is used in conjunction with chronocoulometry to observe, in a hyperspectral manner, single-particle electrodeposition, a technique developed for nanoparticle surface modification. Together, these two techniques position spectroelectrochemistry as a rich tool for optical nanomaterials, capable of both synthesis and concurrent characterization. 1 2 I would rather have questions that can’t be answered than answers which can’t be questioned. - Richard Feynman Acknowledgments I dedicate this thesis to my parents. Dad introduced me to the world of science and reason at a very young age. Mom always pushed me just one step further. And of course, there’s Khushi, who came into my life at just the right time. She supported me in ways that she may never understand. iii Contents Acknowledgments ............................................................................................................ iii Contents ............................................................................................................................ iv List of Figures .................................................................................................................. vii List of Tables .................................................................................................................. xvi List of Equations ........................................................................................................... xvii Nomenclature .................................................................................................................. 18 Chapter 1 ......................................................................................................................... 19 Introduction ..................................................................................................................... 19 Chapter 2 ......................................................................................................................... 23 Background ..................................................................................................................... 23 2.1. Electrochemistry ..................................................................................................... 23 2.1.1. Fundamentals ................................................................................................... 23 2.1.2. Electroanalytical Techniques ........................................................................... 26 2.2. Transition Metal Dichalcogenides (TMDs) ........................................................... 28 2.2.1. Background of 2D Materials ........................................................................... 28 2.2.2. Chemical properties of TMDs ......................................................................... 28 2.2.3. Opto-Electronic Properties of TMDs ............................................................... 30 2.3. Plasmonic Nanoparticles ........................................................................................ 33 2.3.1. Fundamentals of Plasmonics ........................................................................... 33 2.3.2. Electrochemistry of Plasmonic Systems .......................................................... 36 2.3.3. Plasmonic Spectroelectrochemistry ................................................................. 37 2.3.4. Discrete Dipole Approximation (DDA) .......................................................... 39 Chapter 3 ......................................................................................................................... 41 Micro-Extinction Spectroscopy (MExS): A Versatile Optical Characterization Technique......................................................................................................................... 41 3.1. Abstract .................................................................................................................. 41 3.2. Introduction ............................................................................................................ 42 3.3. Technique Description ........................................................................................... 44 3.3.1. Optical Setup ................................................................................................... 44 iv v 3.3.2. Hyperspectral Data Acquisition ....................................................................... 47 3.3.3. Data Post-Processing ....................................................................................... 50 3.4. Case Studies ........................................................................................................... 52 3.4.1. Case Study: Reflectance Characterization of 2D Materials on Non-Transparent Substrates ................................................................................................................... 52 3.5. Conclusion .............................................................................................................. 54 Chapter 4 ......................................................................................................................... 55 In situ Optical Tracking of Electroablation in Two-Dimensional Transition Metal Dichalcogenides ............................................................................................................... 55 4.1. Abstract .................................................................................................................. 55 4.2. Introduction ............................................................................................................ 56 4.3. Experimental .......................................................................................................... 59 4.3.1. Sample preparation .......................................................................................... 59 4.3.2. In situ spectroelectrochemical cell ................................................................... 59 4.3.3. Reflectance measurements ............................................................................... 60 4.4. Results and Discussion ........................................................................................... 61 4.4.1. Optical detection of the electroablation process .............................................. 61 4.4.2. Determination of the onset of electroablation ................................................. 63 4.4.3. Electroablation of MoS2 .................................................................................. 66 4.4.4. Electroablation of WS2 .................................................................................... 69 4.4.5. Electroablation of MoSe2 ................................................................................. 73 4.4.6. Electroablation of WSe2 .................................................................................. 76 4.5. Conclusion .............................................................................................................. 79 Chapter 5 ......................................................................................................................... 80 Electrodeposition on Plasmonic Nanoparticles Using In Situ Spectroelectrochemistry ................................................................................................. 80 5.1. Introduction ............................................................................................................ 80 5.2. Experimental .......................................................................................................... 82 5.2.1. Au nanoparticle Synthesis ............................................................................... 82 5.2.2. Spectroelectrochemical Cell ............................................................................ 82 5.2.3. Dark-field Optical Measurements .................................................................... 84 5.2.4. Particle Transfer to TEM grid .......................................................................... 84 5.2.5. Statistical Analysis ........................................................................................... 84 vi 5.3. Results and Discussion ........................................................................................... 87 5.3.1. Electrodeposition of Cu ................................................................................... 87 5.3.2. Concurrent Electrodeposition of Cu and Photodeposition
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