Sriya Banerjee

Frontiers in Applied Atomic Layer Deposition (ALD) Research

Atomic layer deposition (ALD) has been a key player in advancing the science and technology of nanomaterials synthesis and device fabrication. The monolayer (ML) control of growth rate obtained with ALD combined with its ability to self-limit growth reactions at the gas-substrate interface can be exploited in fundamentally new ways to produce novel composite nanomaterials or precisely tailored 3D nanostructures. Fueling the rapid popularity of ALD in nanotechnology research is the relative simplicity of the hardware and exciting new chemistries that allow researchers to deposit a host of new materials including pure metals, metal oxides, sulphides and nitrides and organic thin films with relative ease and superb accuracy. In this review article, we present four impact areas - microelectronics, energy harvesting and energy storage devices and sensors and photonic devices that have benefitted from such an approach. While many excellent review articles are available on the fundamental chemistry of ALD processes, we focus here on the applied science and engineering aspects of cutting edge ALD research

Cationically Substituted Bi0.7Fe0.3OCl Nanosheets as Li Ion Battery Anodes

Cation substitution of Bi3+ with Fe3+ in BiOCl leads to the formation of ionically layered Bi0.7Fe0.3OCl nanosheets. The synthesis follows a hydrolysis route using bismuth(III) nitrate and iron(III) chloride, followed by postannealing at 500 °C. Room temperature electrical conductivity improves from 6.11 × 10-8 S/m for BiOCl to 6.80 × 10-7 S/m for Bi0.7Fe0.3OCl. Correspondingly, the activation energy for electrical conduction reduces from 862 meV for pure BiOCl to 310 meV for Bi0.7Fe0.3OCl. These data suggest improved charge mobility in Bi0.7Fe0.3OCl nanosheets. Density functional theory calculations confirm this behavior by predicting a high density of states near the Fermi level for Bi0.7Fe0.3OCl. The improvement in electrical conductivity is exploited in the electrochemical performance of Bi0.7Fe0.3OCl nanosheets. The insertion capacity of Li+ ions shows an increase of 2.5×, from 215 mAh·.g-1 for undoped BiOCl to 542 mAh·g-1 for Bi0.7Fe0.3OCl after 50 cycles at a current density of 50 mA·g-1. Thus, the direct substitution of Bi3+ sites with Fe3+ in BiOCl results in nanosheets of an ionically layered ternary semiconductor compound which is attractive for Li ion battery anode applications.

Confined anodic aluminum oxide nanopores on aluminum wires

Anodic aluminum oxide (AAO) nanopores were grown on Al wire, 100 μm diameter in oxalic acid at 20, 40 and 70 V. Total charge passed and wire resistance were monitored as a function of anodization time providing precise Al consumption rates. Inductively coupled plasma optical emission spectroscopy was conducted on the used electrolyte to determine ejected Al3+ ion concentration. Reduced growth rates, lower interpore distances, thicker barrier layer Al2O3 and cracks in the AAO > 10 μm thick were observed. Compared to planar AAO at low current densities, at least 22% higher efficiencies and higher volume expansion factors were obtained. These results were explained on the basis of reduced ionic conductivity and increased viscosity of the barrier layer Al2O3 as a result of circumferential compressive stresses generated due to pore growth on convex surfaces.

SnO2 Nanostructured Thin Films for Room-Temperature Gas Sensing of Volatile Organic Compounds

We demonstrated room-temperature gas sensing of volatile organic compounds (VOCs) using SnO2 nanostructured thin films grown via the aerosol chemical vapor deposition process at deposition temperatures ranging from 450 to 600 °C. We investigated the films sensing response to the presence of three classes of VOCs: apolar, monopolar, and biopolar. The synthesis process was optimized, with the most robust response observed for films grown at 550 °C as compared to other temperatures. The role of film morphology, exposed surface planes, and oxygen defects were explored using experimental techniques and theoretical calculations to improve the understanding of the room-temperature gas sensing mechanism, which is proposed to be through the direct adsorption of VOCs on the sensor surface. Overall, the improved understanding of the material characteristics that enable room-temperature sensing gained in this work will be beneficial for the design and application of metal oxide gas sensors at room temperature.