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

Controlling Nucleation and Crystal Growth of Ge in a Liquid Metal Solvent

The electrochemical liquid–liquid–solid (ec-LLS) deposition of crystalline germanium (Ge) in a eutectic mixture of liquid gallium (Ga) and indium (In) was analyzed as a function of liquid metal thickness, process temperature, and flux. Through control of reaction parameters, conditions were identified that allow selective nucleation and growth of crystalline Ge at the interface between e-GaIn and a crystalline Si substrate. The crystal growth rates of Ge by ec-LLS as a function of process temperatures were obtained from time-dependent powder X-ray diffraction measurements of crystalline Ge. The driving force, Δμ, for crystal formation in ec-LLS was estimated through analyses of the experimental data in conjunction with predictions from a finite-difference model. The required Δμ for Ge nucleation was tantamount to a supersaturation approximately 10² larger than the equilibrium concentration of Ge in e-GaIn at the investigated temperatures. These points are discussed both in the context of advancing new, low-temperature synthetic methodologies for crystalline semiconductor films and on understanding semiconductor crystal growth more deeply

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

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

Secondary Functionalization of Allyl-Terminated GaP(111)A Surfaces via Heck Cross-Coupling Metathesis, Hydrosilylation, and Electrophilic Addition of Bromine

The functionalization of single crystalline gallium phosphide (GaP) (111)­A surfaces with allyl groups has been performed using a sequential chlorine-activation/Grignard reaction process. Increased hydrophobicity following reaction of a GaP(111)­A surface with C₃H₅MgCl was observed through water contact angle measurements. Infrared spectra of GaP(111)­A samples after reaction with C₃H₅MgCl showed the asymmetric CC and CC–H modes diagnostic of surface-attached allyl groups. The stability of allyl-terminated GaP(111)­A surfaces under ambient and aqueous conditions was investigated. XP spectra of allyl-terminated GaP(111)­A highlighted a significant resistance against interfacial oxidation both in air and in water relative to the native interface. Electrochemical impedance spectroscopy indicated a change in the flat-band potential of allyl-terminated GaP(111)­A electrodes immersed in water relative to native GaP(111)­A surfaces. Further, the flat-band potentials for allyl-terminated electrodes were insensitive to changes in solution pH. The utility of surface-bound allyl groups for covalent secondary functionalization of GaP(111)­A interfaces was assessed through three separate reactions: Heck cross-coupling metathesis, hydrosilylation, and electrophilic addition of bromine reactions. Addition of aryl groups across the olefins on allyl-terminated GaP(111)­A via Heck cross coupling was performed and confirmed through high-resolution F 1s and C 1s XP spectra and IR spectra. Control experiments with GaP(111)­A surfaces functionalized with short alkanes indicated no evidence for metathesis. Hydrosilylation reactions were separately performed. Si 2s XP spectra, in conjunction with infrared spectra, similarly showed secondary evidence of surface functionalization for allyl-terminated GaP(111)­A but not for CH₃-terminated GaP(111)­A surfaces. Similar analyses showed electrophilic addition of Br₂ across the terminal olefin on an allyl-terminated GaP(111)­A surface after exposure to dilute Br₂ solutions in CH₂Cl₂. The work presented herein establishes a set of secondary reaction strategies utilizing allyl-terminated surfaces to modify chemically protected GaP surfaces

Wet Chemical Functionalization of GaP(111)B through a Williamson Ether-Type Reaction

Functionalization of crystalline gallium phosphide (GaP) (111)­B interfaces has been performed through the formation of P–O–<i>R</i> surface bonds. The approach described herein parallels classical Williamson ether synthesis, where hydroxyl groups on etched GaP(111)B surfaces were reacted with halogenated reactants. Grazing angle total internal reflectance infrared spectra showed increased intensities for −CH₂– and −CH₃ asymmetric and symmetric stretches after reaction with long alkyl halides. Changes in the X-ray photoelectron spectra collected before and after reaction separately corroborated surface attachment to GaP(111)­B. Static sessile drop water contact angle measurements for GaP(111)B separately showed increased hydrophobicity following surface modification with long alkyl chains. The surface functionalization reaction rate was increased by the addition of non-nucleophilic bases, consistent with surface deprotonation as the rate-limiting step. Separately, photoelectrochemical measurements conducted before and after reaction with alkyl halides at long wavelengths (λ > 545 nm) showed surface attachment decreased sub-band-gap photocurrents, implying lowered activity of surface traps. Conversely, photoelectrochemical measurements performed after functionalization of p-GaP(111)B with Coomassie Blue sulfonyl chloride showed evidence of persistent sensitized hole injection from the dye into p-GaP

Concerted Electrodeposition and Alloying of Antimony on Indium Electrodes for Selective Formation of Crystalline Indium Antimonide

The direct preparation of crystalline indium antimonide (InSb) by the electrodeposition of antimony (Sb) onto indium (In) working electrodes has been demonstrated. When Sb is electrodeposited from dilute aqueous electrolytes containing dissolved Sb₂O₃, an alloying reaction is possible between Sb and In if any surface oxide films are first thoroughly removed from the electrode. The presented Raman spectra detail the interplay between the formation of crystalline InSb and the accumulation of Sb as either amorphous or crystalline aggregates on the electrode surface as a function of time, temperature, potential, and electrolyte composition. Electron and optical microscopies confirm that under a range of conditions, the preparation of a uniform and phase-pure InSb film is possible. The cumulative results highlight this methodology as a simple yet potent strategy for the synthesis of intermetallic compounds of interest

Wet Chemical Functionalization of III–V Semiconductor Surfaces: Alkylation of Gallium Arsenide and Gallium Nitride by a Grignard Reaction Sequence

Crystalline gallium arsenide (GaAs) (111)­A and gallium nitride (GaN) (0001) surfaces have been functionalized with alkyl groups via a sequential wet chemical chlorine activation, Grignard reaction process. For GaAs(111)­A, etching in HCl in diethyl ether effected both oxide removal and surface-bound Cl. X-ray photoelectron (XP) spectra demonstrated selective surface chlorination after exposure to 2 M HCl in diethyl ether for freshly etched GaAs(111)­A but not GaAs(111)B surfaces. GaN(0001) surfaces exposed to PCl₅ in chlorobenzene showed reproducible XP spectroscopic evidence for Cl-termination. The Cl-activated GaAs(111)­A and GaN(0001) surfaces were both reactive toward alkyl Grignard reagents, with pronounced decreases in detectable Cl signal as measured by XP spectroscopy. Sessile contact angle measurements between water and GaAs(111)­A interfaces after various levels of treatment showed that GaAs(111)­A surfaces became significantly more hydrophobic following reaction with C_<i>n</i>H_{2<i>n</i>–1}MgCl (<i>n</i> = 1, 2, 4, 8, 14, 18). High-resolution As 3d XP spectra taken at various times during prolonged direct exposure to ambient lab air indicated that the resistance of GaAs(111)­A to surface oxidation was greatly enhanced after reaction with Grignard reagents. GaAs(111)­A surfaces terminated with C₁₈H₃₇ groups were also used in Schottky heterojunctions with Hg. These heterojunctions exhibited better stability over repeated cycling than heterojunctions based on GaAs(111)­A modified with C₁₈H₃₇S groups. Raman spectra were separately collected that suggested electronic passivation by surficial Ga–C bonds at GaAs(111)­A. Specifically, GaAs(111)­A surfaces reacted with alkyl Grignard reagents exhibited Raman signatures comparable to those of samples treated with 10% Na₂S in <i>tert</i>-butanol. For GaN(0001), high-resolution C 1s spectra exhibited the characteristic low binding energy shoulder demonstrative of surface Ga–C bonds following reaction with CH₃MgCl. In addition, 4-fluorophenyl groups were attached and detected after reaction with C₆H₄FMgBr, further confirming the susceptibility of Cl-terminated GaN(0001) to surface alkylation. However, the measured hydrophobicities of alkyl-terminated GaAs(111)­A and GaN(0001) were markedly distinct, indicating differences in the resultant surface layers. The results presented here, in conjunction with previous studies on GaP, show that atop Ga atoms at these crystallographically related surfaces can be deliberately functionalized and protected through Ga–C surface bonds that do not involve thiol/sulfide chemistry or gas-phase pretreatments

Uniform Thin Films of CdSe and CdSe(ZnS) Core(Shell) Quantum Dots by Sol–Gel Assembly: Enabling Photoelectrochemical Characterization and Electronic Applications

Optoelectronic properties of quantum dot (QD) films are limited by (1) poor interfacial chemistry and (2) nonradiative recombination due to surface traps. To address these performance issues, sol–gel methods are applied to fabricate thin films of CdSe and core(shell) CdSe(ZnS) QDs. High-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) imaging with chemical analysis confirms that the surface of the QDs in the sol–gel thin films are chalcogen-rich, consistent with an oxidative-induced gelation mechanism in which connectivity is achieved by formation of dichalcogenide covalent linkages between particles. The ligand removal and assembly process is probed by thermogravimetric, spectroscopic, and microscopic studies. Further enhancement of interparticle coupling <i>via</i> mild thermal annealing, which removes residual ligands and reinforces QD connectivity, results in QD sol–gel thin films with superior charge transport properties, as shown by a dramatic enhancement of electrochemical photocurrent under white light illumination relative to thin films composed of ligand-capped QDs. A more than 2-fold enhancement in photocurrent, and a further increase in photovoltage can be achieved by passivation of surface defects <i>via</i> overcoating with a thin ZnS shell. The ability to tune interfacial and surface characteristics for the optimization of photophysical properties suggests that the sol–gel approach may enable formation of QD thin films suitable for a range of optoelectronic applications