Low-temperature Electrodeposition of Crystalline Semiconductor Materials.

Crystalline group IV semiconductor materials, silicon (Si) and germanium (Ge) are essential building blocks in energy conversion, storage and optoelectronic devices. The state-of-art material synthetic methods fail to offer a non-energy-intensive solution for producing crystalline group IV semiconductor materials using easy-to-access apparatuses under ambient conditions. The primary aim of this thesis is to develop a cost-effective synthetic method, namely electrochemical liquid-liquid-solid (ec-LLS) growth, for preparing these materials at low temperatures using simple instruments and chemicals. The key innovation of the ec-LLS approach is the utilization of liquid metal electrodes in an electrodeposition process, during which the liquid metal serves simultaneously as a conductive substrate for current flow and as a solvent phase for semiconductor crystallization. The unique combination of electrodeposition and liquid-phase crystallization in this strategy opens new possibility for low temperature preparation of crystalline group IV semiconductors. This thesis will test a few key hypotheses regarding the fundamental and practical aspects of ec-LLS. Chapter 2 focuses on the Ge electrodeposition on liquid pool electrodes with various compositions to demonstrate the versatility of the ec-LLS approach. The significant role of liquid metal electrodes in the crystal formation process will be highlighted by the X-ray diffraction data. Chapter 3 expands the application of ec-LLS strategy to the controlled electrodeposition of Ge nanowires using nano-sized growth catalyst. As-deposited Ge nanowires will also be tested as Li+ battery anodes without further processing. Chapter 4 details the direct epitaxial growth of single-crystalline Ge nanowires at room temperature by the ec-LLS approach. Discrete Ga nanoparticles will be used as the seeding catalyst for the Ge nanowire growth on a single crystal Ge wafer. Electron microscopy evidence supporting the notion of epitaxial growth will be presented. Chapter 5 demonstrates the application of ec-LLS strategy for electrodeposition of crystalline Si at temperature as low as 80 oC from an organic electrolyte. SiCl4 precursor in propylene carbonate will be electrochemically reduced onto liquid Ga pool electrode to form high-coverage elemental Si. In summary, the collected results from this thesis will endorse ec-LLS as a non-energy-intensive synthetic method for producing crystalline group IV semiconductor materials.PHDChemistryUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttp://deepblue.lib.umich.edu/bitstream/2027.42/108916/1/junsigu_1.pd

Imaging 3D Chemistry at 1 nm Resolution with Fused Multi-Modal Electron Tomography

Measuring the three-dimensional (3D) distribution of chemistry in nanoscale matter is a longstanding challenge for metrological science. The inelastic scattering events required for 3D chemical imaging are too rare, requiring high beam exposure that destroys the specimen before an experiment completes. Even larger doses are required to achieve high resolution. Thus, chemical mapping in 3D has been unachievable except at lower resolution with the most radiation-hard materials. Here, high-resolution 3D chemical imaging is achieved near or below one nanometer resolution in a Au-Fe$_3$O$_4$ metamaterial, Co$_3$O$_4$ - Mn$_3$O$_4$ core-shell nanocrystals, and ZnS-Cu$_{0.64}$S$_{0.36}$ nanomaterial using fused multi-modal electron tomography. Multi-modal data fusion enables high-resolution chemical tomography often with 99\% less dose by linking information encoded within both elastic (HAADF) and inelastic (EDX / EELS) signals. Now sub-nanometer 3D resolution of chemistry is measurable for a broad class of geometrically and compositionally complex materials

Meeting of the Society

Direct preparation of crystalline Ge nanowires from dissolved aqueous solutions of GeO 2 via electrodeposition through an electrochemical liquid-liquid-solid (ec-LLS) process using a variety of Ga nano/microdroplets has been demonstrated. Templated silica films featuring regularly spaced openings with defined sizes were prepared on degenerately doped Si substrates by a modified natural lithography patterning process. These platforms were used to electrodeposit Ga droplet arrays with defined sizes and pitch. The Ga nano/microdroplets subsequently facilitated Ge nanowire ec-LLS in water as a function of droplet size. The amperometric responses recorded during ec-LLS depended on the Ga nanodroplet size. However, none of the recorded current-time profiles fit either Cottrell-type or standard nucleation models. Similarly, electron micrographs of the as-produced Ge nanowires showed a strong correlation between the base size of the Ge nanowires and the Ga droplet diameter. All nanowires featured a consistent taper, with a cone angle of approximately 10 • . For the largest Ga droplets, no nanowires were observed under the employed ec-LLS conditions. Instead, a large isotropic Ge deposit covered with small Ge nanowires was observed. These cumulative results provide the first probe of the effects that liquid metal size(volume) have on electrodeposition by ec-LLS. Many of the most promising designs for next generation energy conversion/storage devices like rechargeable batteries and solar cells are based on highly tailored, covalent semiconductor nanowires. 1,2 Accordingly, there is a premium on efficient, scalable, and non-intensive synthetic methods that produce quality semiconductor nanowire materials with precisely defined morphological and electrical properties. For group IV and III-V semiconductor nanowires, a variety of different chemical vapor deposition, 3,4 chemical, • C) in common solvents such as propylene carbonate. 13 In separate reports, we detailed how liquid metal nanodroplets could be used for the ec-LLS process to specifically grow crystalline (poly-and single crystalline) group IV semiconductor nanowires (Scheme 1), 10,12 with the capacity for homo-and heteroepitaxy on certain substrates. ec-LLS as a simple means for preparing a nanostructured Li + battery anode in one step under purely bench-top conditions (i.e. room temperature, in water, in the presence of O 2 ) was also recently detailed. 12 In this paper, we explore directly the hypothesis that the size of the nanodroplet affects the ec-LLS process, the observable electrochemical responses, and the resultant materials' morphological properties. We first show results for an extended natural lithography process with polystyrene spheres and solution cast silica for the preparation of Ga nanodroplet films on a flat conductive support. We present results * Electrochemical Society Active Member. z E-mail: [email protected] from experiments performed with degenerately doped single crystalline Si(100) as the support substrates. In addition to being mostly inert toward H 2 evolution in water, these Si substrates can support crystalline Ge nanowire ec-LLS with heteroepitaxy. 10 Accordingly, results are presented where Ga nandroplet films are used to conduct ec-LLS on n + -Si(100) so as to produce individual crystalline Ge nanowires. The voltammetric responses and current-time transients are reported, as well as evidence indicating factors that impact epitaxy and deposition rates. Further data are presented that show the morphology of electrodeposited Ge follows the size of the Ga nanodroplets and that indicate pertinent aspects of the ec-LLS process. Methods Material

Direct Electrodeposition of Crystalline Silicon at Low Temperatures

An electrochemical liquid–liquid–solid (ec-LLS) process that yields crystalline silicon at low temperature (80 °C) without any physical or chemical templating agent has been demonstrated. Electroreduction of dissolved SiCl₄ in propylene carbonate using a liquid gallium [Ga­(<i>l</i>)] pool as the working electrode consistently yielded crystalline Si. X-ray diffraction and electron diffraction data separately indicated that the as-deposited materials were crystalline with the expected patterns for a diamond cubic crystal structure. Scanning and transmission electron microscopies further revealed the as-deposited materials (i.e., with no annealing) to be faceted nanocrystals with diameters in excess of 500 nm. Energy-dispersive X-ray spectra further showed no evidence of any other species within the electrodeposited crystalline Si. Raman spectra separately showed that the electrodeposited films on the Ga­(<i>l</i>) electrodes were not composed of amorphous carbon from solvent decomposition. The cumulative data support two primary contentions. First, a liquid-metal electrode can serve simultaneously as <i>both</i> a source of electrons for the heterogeneous reduction of dissolved Si precursor in the electrolyte (i.e., a conventional electrode) <i>and</i> a separate phase (i.e., a solvent) that promotes Si crystal growth. Second, ec-LLS is a process that can be exploited for direct production of crystalline Si at much lower temperatures than ever reported previously. The further prospect of ec-LLS as an electrochemical and non-energy-intensive route for preparing crystalline Si is discussed

Electrodeposition of Crystalline GaAs on Liquid Gallium Electrodes in Aqueous Electrolytes

Crystalline GaAs (c-GaAs) has been prepared directly through electroreduction of As₂O₃ dissolved in an alkaline aqueous solution at a liquid gallium (Ga­(l)) electrode at modest temperatures (<i>T</i> ≥ 80 °C). Ga­(l) pool electrodes yielded consistent electrochemical behavior, affording repetitive measurements that illustrated the interdependences of applied potential, concentration of dissolved As₂O₃, and electrodeposition temperature on the quality of the resultant c-GaAs(s). Raman spectra indicated the composition of the resultant film was strongly dependent on both the electrodeposition temperature and dissolved concentration of As₂O₃ but not to the applied bias. For electrodepositions performed either at room temperature or with high (≥0.01 M) concentrations of dissolved As₂O₃, Raman spectra of the electrodeposited films were consistent with amorphous As(s). X-ray diffractograms of As(s) films collected after thermal annealing indicated metallurgical alloying occurred only at temperatures in excess of 200 °C. Optical images and Raman spectra separately showed the composition of the as-electrodeposited film in dilute (≤0.001 M) solutions of dissolved As₂O₃(aq) was pure c-GaAs(s) at much lower temperatures than 200 °C. Diffractograms and transmission electron microscopy performed on as-prepared films confirmed the identity of c-GaAs(s). The collective results thus provide the first clear demonstration of an electrochemical liquid–liquid–solid (ec-LLS) process involving a liquid metal that serves simultaneously as an electrode, a solvent/medium for crystal growth, and a coreactant for the synthesis of a polycrystalline semiconductor. The presented data serve as impetus for the further development of the ec-LLS process as a controllable, simple, and direct route for technologically important optoelectronic materials such as c-GaAs(s)

Electrochemically Gated Alloy Formation of Crystalline InAs Thin Films at Room Temperature in Aqueous Electrolytes

Crystalline InAs films have been prepared directly at room temperature through a new electrochemically induced alloying method by controllably reducing As₂O₃ dissolved in an alkaline aqueous solution at an indium (In) foil electrode. Steady-state Raman spectra, transmission electron microscopy, and selected area electron diffraction indicated that the as-prepared films crystallize in the zincblende phase with no further thermal treatments. Cyclic voltammetry measurements, optical images, and steady-state Raman spectra confirmed that a clean oxide-free interface is critical for the successful formation of the binary InAs phase. The salient feature of this work is the use of simple aqueous electrochemistry to simultaneously remove passive metal oxides from the In(s) metal surface while controllably reducing dissolved arsenic oxide at the interface to drive the In–As alloying reaction. Raman spectral mapping data illustrate that the resulting film coverage and homogeneity are a strong function of the formal As₂O₃ concentration and the duration of the electrodeposition experiment. Potential-dependent in situ Raman spectroscopy was used to implicate the solid-state reaction as the rate-limiting step in InAs film formation over the first 160 min, after which solid-state diffusion dominated the kinetics. The collective results establish a precedent for an alternative synthetic strategy for crystalline InAs thin films that does not require vacuum or sophisticated furnaces, toxic gaseous precursors like arsine, or exotic solvents

Analysis of the Electrodeposition and Surface Chemistry of CdTe, CdSe, and CdS Thin Films through Substrate-Overlayer Surface-Enhanced Raman Spectroscopy

The substrate-overlayer approach has been used to acquire surface enhanced Raman spectra (SERS) during and after electrochemical atomic layer deposition (ECALD) of CdSe, CdTe, and CdS thin films. The collected data suggest that SERS measurements performed with off-resonance (i.e. far from the surface plasmonic wavelength of the underlying SERS substrate) laser excitation do not introduce perturbations to the ECALD processes. Spectra acquired in this way afford rapid insight on the quality of the semiconductor film during the course of an ECALD process. For example, SERS data are used to highlight ECALD conditions that yield crystalline CdSe and CdS films. In contrast, SERS measurements with short wavelength laser excitation show evidence of photoelectrochemical effects that were not germane to the intended ECALD process. Using the semiconductor films prepared by ECALD, the substrate-overlayer SERS approach also affords analysis of semiconductor surface adsorbates. Specifically, Raman spectra of benzenethiol adsorbed onto CdSe, CdTe, and CdS films are detailed. Spectral shifts in the vibronic features of adsorbate bonding suggest subtle differences in substrate-adsorbate interactions, highlighting the sensitivity of this methodology

Sensitization of p‑GaP with CdSe Quantum Dots: Light-Stimulated Hole Injection

The sensitization of p-GaP by adsorbed CdSe quantum dots has been observed. Nondegenerately doped, planar p-GaP(100) photoelectrodes consistently showed sub-band-gap (>550 nm) photoresponsivity in an aqueous electrolyte containing Eu^3+/2+ when CdSe quantum dots (diameters ranging from 3.1 to 4.5 nm) were purposely adsorbed on the surface. Both time-resolved photoluminescence decays and steady-state photoelectrochemical responses supported sensitized hole injection from the CdSe quantum dots into p-GaP. The observation of hole injection in this system stands in contrast to sensitized electron injection seen in other metal oxide/quantum dot material combinations and therefore widens the possible designs for photoelectrochemical energy conversion systems that utilize quantum dots as light-harvesting components

Room-Temperature Epitaxial Electrodeposition of Single-Crystalline Germanium Nanowires at the Wafer Scale from an Aqueous Solution

Direct epitaxial growth of single-crystalline germanium (Ge) nanowires at room temperature has been performed through an electrodeposition process on conductive wafers immersed in an aqueous bath. The crystal growth is based on an electrochemical liquid–liquid–solid (ec-LLS) process involving the electroreduction of dissolved GeO₂(aq) in water at isolated liquid gallium (Ga) nanodroplet electrodes resting on single-crystalline Ge or Si supports. Ge nanowires were electrodeposited on the wafer scale (>10 cm²) using only common glassware and a digital potentiostat. High-resolution electron micrographs and electron diffraction patterns collected from cross sections of individual substrate-nanowire contacts in addition to scanning electron micrographs of the orientation of nanowires across entire films on substrates with different crystalline orientations, supported the notion of epitaxial nanowire growth. Energy dispersive spectroscopic elemental mapping of single nanowires indicated the Ga­(l) nanodroplet remains affixed to the tip of the growing nanowire throughout the nanowire electrodeposition process. Current–voltage responses measured across many individual nanowires yielded reproducible resistance values. The presented data cumulatively show epitaxial growth of covalent group IV nanowires is possible from the reduction of a dissolved oxide under purely benchtop conditions