Solution Grown Se/Te Nanowires: Nucleation, Evolution, and The Role of Triganol Te seeds

We have studied the nucleation and growth of Se–Te nanowires (NWs), with different morphologies, grown by a chemical solution process. Through systematic characterization of the Se–Te NW morphology as a function of the Te nanocrystallines (NCs) precursor, the relative ratio between Se and Te, and the growth time, a number of significant insights into Se–Te NW growth by chemical solution processes have been developed. Specifically, we have found that: (i) the growth of Se–Te NWs can be initiated from either long or short triganol Te nanorods, (ii) the frequency of proximal interactions between nanorod tips and the competition between Se and Te at the end of short Te nanorods results in V-shaped structures of Se–Te NWs, the ratio between Se and Te having great effect on the morphology of Se–Te NWs, (iii) by using long Te nanorods as seeds, Se–Te NWs with straight morphology were obtained. Many of these findings on Se–Te NW growth can be further generalized and provide very useful information for the rational synthesis of group VI based semiconductor NW compounds

Gibbs-Thomson Effect in Planar Nanowires: Orientation and Doping Modulated Growth.

Epitaxy-enabled bottom-up synthesis of self-assembled planar nanowires via the vapor-liquid-solid mechanism is an emerging and promising approach toward large-scale direct integration of nanowire-based devices without postgrowth alignment. Here, by examining large assemblies of indium tin oxide nanowires on yttria-stabilized zirconia substrate, we demonstrate for the first time that the growth dynamics of planar nanowires follows a modified version of the Gibbs-Thomson mechanism, which has been known for the past decades to govern the correlations between thermodynamic supersaturation, growth speed, and nanowire morphology. Furthermore, the substrate orientation strongly influences the growth characteristics of epitaxial planar nanowires as opposed to impact at only the initial nucleation stage in the growth of vertical nanowires. The rich nanowire morphology can be described by a surface-energy-dependent growth model within the Gibbs-Thomson framework, which is further modulated by the tin doping concentration. Our experiments also reveal that the cutoff nanowire diameter depends on the substrate orientation and decreases with increasing tin doping concentration. These results enable a deeper understanding and control over the growth of planar nanowires, and the insights will help advance the fabrication of self-assembled nanowire devices

Nanowire/nanotube array tandem cells for overall solar neutral water splitting

In this study, we report the fabrication and characterization of a novel PEC tandem cell, consisting of p-Si/TiO2/Fe2O3 core/shell/hierarchical nanowire (csh-NW) array photocathode and TiO2/TiO2 core/shell nanotube (cs-NT) array photoanode, for overall solar water splitting in a neutral pH water. The p-Si/n-TiO2/n-Fe2O3 csh-NWs, made mainly by solution-processed methods, offer significantly improved performance in the neutral pH water with a low (positive) onset potential and photoactivity at zero bias, due to the increased reaction surface area, effective energy band alignment among p-Si, n-TiO2 and n-Fe2O3 enhancing the charge separation, improved optical absorption, and enhanced gas evolution. Nitrogen modification (annealing under N2) is used to further enhance the csh-NWs photocathodic performance. The PEC tandem cell is then able to handle overall solar water splitting in the neutral pH water with a solar-to-hydrogen (STH) efficiency of ~0.18%. The achieved results demonstrate initial steps toward the realization of full PEC devices using earth-abundant materials for solar hydrogen generation suggesting competitive performance when solar matched photoanode core material and co-catalysts are used

Engineering Heteromaterials to Control Lithium Ion Transport Pathways

Safe and efficient operation of lithium ion batteries requires precisely directed flow of lithium ions and electrons to control the first directional volume changes in anode and cathode materials. Understanding and controlling the lithium ion transport in battery electrodes becomes crucial to the design of high performance and durable batteries. Recent work revealed that the chemical potential barriers encountered at the surfaces of heteromaterials play an important role in directing lithium ion transport at nanoscale. Here, we utilize in situ transmission electron microscopy to demonstrate that we can switch lithiation pathways from radial to axial to grain-by-grain lithiation through the systematic creation of heteromaterial combinations in the Si-Ge nanowire system. Our systematic studies show that engineered materials at nanoscale can overcome the intrinsic orientation-dependent lithiation, and open new pathways to aid in the development of compact, safe, and efficient batteries