Electrochemical Liquid–Liquid–Solid (ec-LLS) Crystal Growth: A Low-Temperature Strategy for Covalent Semiconductor Crystal Growth

Abstract

This Account describes a new electrochemical synthetic strategy for direct growth of crystalline covalent group IV and III-V semiconductor materials at or near ambient temperature conditions. This strategy, which we call "electrochemical liquid-liquid-solid" (ec-LLS) crystal growth, marries the semiconductor solvation properties of liquid metal melts with the utility and simplicity of conventional electrodeposition. A low-temperature liquid metal (i.e., Hg, Ga, or alloy thereof) acts simultaneously as the source of electrons for the heterogeneous reduction of oxidized semiconductor precursors dissolved in an electrolyte as well as the solvent for dissolution of the zero-valent semiconductor. Supersaturation of the semiconductor in the liquid metal triggers eventual crystal nucleation and growth. In this way, the liquid electrolyte-liquid metal-solid crystal phase boundary strongly influences crystal growth. As a synthetic strategy, ec-LLS has several intrinsic features that are attractive for preparing covalent semiconductor crystals. First, ec-LLS does not require high temperatures, toxic precursors, or high-energy-density semiconductor reagents. This largely simplifies equipment complexity and expense. In practice, ec-LLS can be performed with only a beaker filled with electrolyte and an electrical circuit capable of supplying a defined current (e.g., a battery in series with a resistor). By this same token, ec-LLS is compatible with thermally and chemically sensitive substrates (e.g., plastics) that cannot be used as deposition substrates in conventional syntheses of covalent semiconductors. Second, ec-LLS affords control over a host of crystal shapes and sizes through simple changes in common experimental parameters. As described in detail herein, large and small semiconductor crystals can be grown both homogeneously within a liquid metal electrode and heterogeneously at the interface of a liquid metal electrode and a seed substrate, depending on the particular details chosen for ec-LLS. Third, the rate of introduction of zero-valent materials into the liquid metal is precisely gated with a high degree of resolution by the applied potential/current. The intent of this Account is to summarize the key elements of ec-LLS identified to date, first contextualizing this method with respect to other semiconductor crystal growth methods and then highlighting some unique capabilities of ec-LLS. Specifically, we detail ec-LLS as a platform to prepare Ge and Si crystals from bulk- (∼1 cm(3)), micro- (∼10(-10) cm(3)), and nano-sized (∼10(-16) cm(3)) liquid metal electrodes in common solvents at low temperature. In addition, we describe our successes in the preparation of more compositionally complex binary covalent III-V semiconductors.

Electrochemical Liquid-Liquid-Solid Deposition of Ge at Hg Microdroplet Ultramicroelectrodes

“…10,13,14 In general, the data shown here further emphasize that ec-LLS can be a synthetic materials method at the micro-, meso-, and macro-length scales. However, with liquid Ga or GaIn microdroplets, the growing Ge crystal(s) push the liquid metal nano/microdroplet away from the surface because nucleation and crystal growth occur preferentially at the interface between the liquid metal droplet and the support substrate.…”

“…8,9 A newly identified method is electrochemical liquid-liquid-solid (ec-LLS) crystal growth, where a liquid metal electrode acts both as the source of electrons for heterogeneous reduction reactions and as the solvent for semiconductor crystal formation. 10,11 An advantage of ec-LLS over the aforementioned methods is the capacity to produce crystalline semiconductor micro/nanowires in aqueous solution at ambient temperatures and pressures.…”

Finite‐Sized One‐Dimensional Silica Microstructures (Rods): Synthesis, Assembly, and Applications

“…In vapor–liquid–solid growth, vapors (metal/semiconductor precursors) condense on a liquid metal drop (catalytic center) to make solid nanowires. As in silica rod growth the precursor (TEOS molecules) and catalytic center (emulsion droplet) are both liquid, we can attribute this type of growth as liquid–liquid–solid type growth . PVP makes emulsion droplets by joining with water and citrate.…”

Electroplating for Decorative Applications: Recent Trends in Research and Development

“…Another novel electrodeposition process mimics the vapor-liquid-solid method adopted to grow single crystal semiconductor nanowires. In the electrochemical version, called electrochemical liquid-liquid solid (ELLS) growth [62], the metal ions to be reduced (Si, Ge) are dissolved in a liquid metal and reduced at an electrode. The electrodeposition of Si, Ge in aqueous solutions results in poor crystallinity.…”