Chandrashekhar Pendyala

Hybrid Tin Oxide Nanowires as Stable and High Capacity Anodes for Li-Ion Batteries

In this report, we present a simple and generic concept involving metal nanoclusters supported on metal oxide nanowires as stable and high capacity anode materials for Li-ion batteries. Specifically, SnO(2) nanowires covered with Sn nanoclusters exhibited an exceptional capacity of >800 mAhg(-1) over hundred cycles with a low capacity fading of less than 1% per cycle. Post lithiation analyses after 100 cycles show little morphological degradation of the hybrid nanowires. The observed, enhanced stability with high capacity retention is explained with the following: (a) the spacing between Sn nanoclusters on SnO(2) nanowires allowed the volume expansion during Li alloying and dealloying; (b) high available surface area of Sn nanoclusters for Li alloying and dealloying; and (c) the presence of Sn nanoclusters on SnO(2) allowed reversible reaction between Sn and Li(2)O to produce both Sn and SnO phases.

Electrochemical pinning of the Fermi level: mediation of photoluminescence from gallium nitride and zinc oxide.

Charge transfer between diamond and an electrochemical redox couple in an adsorbed water film has recently been shown to pin the Fermi level in hydrogen-terminated diamond. Here we show that this effect is a more general phenomenon and influences the properties of other semiconductors when the band lineup between the ambient and electronic states in the semiconductor is appropriate. We find that the luminescent intensities from GaN and ZnO change in different, but predictable, ways when exposed to HCl and NH3 vapors in humid air. The effect is reversible and has been observed on single crystals, nanowires, flakes, and powders. These observations are explained by electron exchange between the oxygen electrochemical redox couple in an adsorbed water film and electronic states in the semiconductor. This effect can take place in parallel with other processes such as defect formation, chemisorption, and surface reconstruction and may play an important, but previously unrecognized, role when electronic and optical measurements are made in air.

Inorganic nanowires: a perspective about their role in energy conversion and storage applications

There has been tremendous interest and progress with synthesis of inorganic nanowires (NWs). However, much of the progress only resulted in NWs with diameters much greater than their respective quantum confinement scales, i.e. 10?100?nm. Even at this scale, NW-based materials offer enhanced charge transport and smaller diffusion length scales for improved performance with various electrochemical and photoelectrochemical energy conversion and storage applications. In this paper, these improvements are illustrated with specific results on enhanced charge transport with tin oxide NWs in dye sensitized solar cells, higher capacity retention with molybdenum oxide (MoO3) NW arrays and enhanced photoactivity with hematite NW arrays compared with their nanoparticle (NP) or thin film format counterparts. In addition, the NWs or one-dimensional crystalline materials with diameters less than 100?nm provide a useful platform for creating new materials either as substrates for heteroepitaxy or through the phase transformation with reaction. Specific results with single crystal phase transformation of hematite (a-Fe2O3) to pyrite (FeS2) NWs and heteroepitaxy of indium-rich InGaN alloy over GaN NW substrates are presented to illustrate the viability of using NWs for creating new materials. In terms of energy applications, it is essential to have a method for continuous manufacturing of vertical NW arrays over large areas. In this regard, a simple plasma-based technique is discussed that potentially could be scaled up for roll-to-roll processing of NW arrays.

Self-nucleation and growth of group III-antimonide nanowires

In this paper, we show that the growth of III-antimonides can occur via self-catalysis using either group III metal or Sb clusters at their tips. Specific experiments using GaSb and InSb systems show that bulk nucleation and growth of the respective antimonide wires can also occur from mm-sized droplets. The role of equilibrium solubility and the size of the droplet on bulk nucleation versus tip-led growth is discussed.

Nanowires as semi-rigid substrates for growth of thick, InxGa1−xN (x > 0.4) epi-layers without phase segregation for photoelectrochemical water splitting

Here, we show that GaN nanowires (diameter <30 nm) can be used as strain relaxing substrates for the heteroepitaxial growth of stable In(x)Ga(1-x)N alloys of controlled composition and thickness. Thinner nanowires with their smaller interfacial area reduce the heteroepitaxial stress. Also, the limited adatom diffusion length scales on the thinner nanowires aid in reducing the kinetic segregation effects. In addition to being single crystal templates for heteroepitaxial growth, these thick single crystal overlayers on nanowire substrates can provide suitable architectures for photoelectrochemical applications. The stability and crystallinity of the In(x)Ga(1-x)N layers are preserved by the nanowires acting as compliant substrates. Photoelectrochemical water splitting requires In(x)Ga(1-x)N alloys with a 2.2-1.6 eV band gap (i.e. 0.45 < x < 0.65) and 150-200 nm film thickness for efficient light absorption and carrier generation. At such compositions, the In(x)Ga(1-x)N alloys are inherently unstable, the thickness-dependent stress builds up during the commonly employed heteroepitaxial growth methods, and adds to the instability causing phase segregation and property degradation. A dependence of the growth morphology on the GaN nanowire growth orientation was observed and a growth mechanism is presented for the observed orientation dependent growth on a-plane and c-plane GaN nanowires. Photoactivity of GaN and In(x)Ga(1-x)N films on GaN nanowires is also investigated which shows a distinct difference attributable to GaN and In(x)Ga(1-x)N, demonstrating the advantages of using nanowires as strain relaxing substrates.

Searching for new solar-hydrogen materials using nanowires

The global energy and environmental crises have challenged materials scientists, for example to devise methods for solarenergy conversion to fuel via hydrogen production. This challenge cannot be met by current materials. Thus, development of a durable material capable of absorbing light over a large portion of the solar spectrum is urgently needed. Additionally, it must have the correct electronic band structure (band gap) to enable reactions such as water splitting for hydrogen harvesting. Development of materials with band gaps between 1.7 and 2.2eV that exhibit photoresponse (response to light) is currently underway. The first, important step in this search is generation of materials with new compositions and ideal band gaps. Once achieved, the next stage involves determination of their photoresponse. Photoactivity depends on structure, whether monoor polycrystalline or porous. Monocrystalline films are ideal, since they have mobility or optical-absorption-depth scales that are as long as the relevant carrier-diffusion distances (see Figure 1). However, for many metal oxides this is not the case. Growing large, single crystals is difficult. As a result, solarhydrogen materials are predominantly polyor nanocrystalline films. The latter contain many grain boundaries that act as trap sites, where generated carriers are annihilated in a recombination process. Thus, polycrystalline films cannot be reliably used to screen the photoresponses of the materials themselves. They may efficiently generate carriers upon light absorption, but recombination destroys the current, resulting in near-zero measured photoresponse. Thus, other architectures are needed to screen the photoresponse of solar-hydrogen candidates. Nanowires are 1D crystalline materials with micrometer-scale lengths and sub-100nm diameters. In a vertically arranged form (array), the lengths are ideal for light absorption, while the Figure 1. Different length scales involved with different film formats used for photo-electrochemical water splitting. (top) Polycrystalline films with small grain sizes limit carrier-diffusion length and opticalabsorption depth. (middle) Monocrystalline films have the necessary carrier-diffusion length scales, but optical-penetration depth controls film thickness. (bottom) Nanowire arrays, where optical penetration depth and requisite carrier-diffusion length scales can be independently satisfied by length and diameter (dia.) scales, respectively. h : Light energy. H2O, O2: Water, oxygen.