Understanding Liquid Metal Electrode Interfaces for Intermetallic, Metallic, and Semiconductor Growth

Abstract

This thesis presents an atomic-level understanding of liquid metals obtained via in-situ X-ray reflectivity and leverages this insight to develop novel approaches for growing single-crystalline inorganic materials. First, the efficacy of using X-ray reflectivity to probe unary or binary liquid metal/electrolyte interfaces in situ has been described. Second, it introduces the first known method of electrochemical single-crystalline material synthesis at a liquid metal/liquid interface, a process termed ‘quasi’-epitaxial growth. Third, this work outlines how interfacial liquid metal surface ordering directly influences the single-crystalline growth of both known and previously unknown inorganic materials. The first chapter describes the important aspects of liquid metals used in ec-LLS and ec-LPE. First, ec-LLS experiments have similarities in supersaturation behaviors observed with traditional melt growth and vapor-liquid-solid methods but done so electrochemically. Second, the location of nucleation and crystal growth can be controlled. Third, the local atomic structure and liquid metal composition influences the product of ec-LLS. Chapter 2 and Chapter 3 of this thesis details the theory and approach in probing liquid Hg and HgxIn1-x electrode interfaces in-situ using XRR. First, pure Hg in contact with aqueous Na2B4O7 solutions does not exhibit any adsorption at the liquid metal interface. Second, a pure Hg electrode in the presence of dissolved H2GeO3 and/or HGeO3-, at sufficiently positive potentials forms monolayer coverages of polygermanates via a condensation reaction. Additional insights made using HgxIn1-x alloys show that there is Hg surface enrichment beyond what was previously predicted by the Gibbs Adsorption Isotherm for ideal mixtures. Additionally, an intense Yoneda signal was confirmed for HgxIn1-x that shows a noticeable dependence on the radius of curvature and bulk composition. Chapters 4 and 5 introduce a new concept in ec-LLS, referred to as ‘quasi’-epitaxy. Directed crystal growth of metallic material is observed during low flux electrodeposition onto a ‘clean’ liquid metal interface devoid of oxides. Liquid Ga at constant, ambient conditions was used to electrodeposit pure single crystalline forms of Pb[111], Bi[0003], PdGa5[001], and MnGa4[111]. A similar approach was also used for synthesizing previously unreported intermetallic phases, Fe0.14±0.01Ga0.86±0.01 and Fe0.08±0.01Co0.06±0.01Ga0.86±0.01. Chapter 6 describes an alternative approach coined ‘open’ ec-LPE, for observing single-crystal semiconductor growth at room temperature and ambient pressures. Ge(100) epi-films were synthesized electrochemically on a Ge seeding substrate mechanically inserted into a liquid metal electrode. Differences between Hg, Hg35In65, Ga, and e-GaIn are described based on differences in wetting interactions with the seed wafer and solubility toward Ge. The last chapter of this work outlines unresolved work to assist future researchers working with liquid metal electrodes. Attempts at understanding the complex electrochemistry of liquid Ga in alkaline aqueous electrodes are discussed. Additionally, efforts in making a flat liquid Ga electrode by using CuGa2 as a more wettable conductive substrate is presented. Lastly, methods for isolating and electronically characterizing the products of ec-LLS and ec-LPE are discussed. This work paves the way for future studies on interfacial ordering in liquid metal electrodes and its impact on crystal growth while expanding the prospects of liquid metals in synthetic electrochemical applications.