William T. Nichols

Chemical vapour transport synthesis and optical characterization of MoO3 thin films

MoO3 thin films were successfully prepared through chemical vapour transport (CVT) deposition and post-annealing. These films showed significantly improved optical properties. It was found that the transmittance reaches 80% with low reflectivity due to improved crystallinity and removal of oxygen vacancy states. Optical analysis shows that the index of refraction is around 1.55 with a flat dispersion curve across the visible. Furthermore, the band-gap energy is estimated to be approximately 3.5eV. These properties suggest that CVT may be an effective thin film deposition technique for low-cost, large-area deposition of molybdenum oxides for chromogenic applications. (Some figures in this article are in colour only in the electronic version)

Simple, Large-Scale Patterning of Hydrophobic ZnO Nanorod Arrays

Here we describe a simple, versatile technique to produce large-scale arrays of highly ordered ZnO nanorods. Patterning of three distinct ZnO crystal morphologies is demonstrated through use of different ZnO seed layers. Array formation is accomplished through a simple variation on nanosphere lithography that imprints a thickness variation across a PMMA mask layer. The area of exposed seed layer is controlled through etching time in an oxygen plasma. Subsequent hydrothermal growth from the patterned seed layer produces high-quality ZnO crystals in uniform arrays. The high uniformity of the patterned array is shown to induce a high contact angle hydrophobic state even without the need for chemical modification of the ZnO surface. This technique provides a straightforward way to integrate the optical and electrical properties of high-quality ZnO nanorods with the tunable fluidic properties at the surface of well-ordered arrays.

Improved electrical properties of Pt/HfO2/Ge using in situ water vapor treatment and atomic layer deposition

The effects of water vapor treatment (WVT) on a Ge substrate were investigated in order to understand the improved electrical properties of Pt/HfO2/Ge metal-oxide-semiconductor (MOS) capacitors. The WVT and HfO2 deposition were performed in situ using an atomic layer deposition technique to avoid air exposure. As a result, the WVT on cleaned Ge substrates reduced the native oxide effectively and enhanced the initial growth of the HfO2 film. The improved interface qualities with WVT enhanced Ge-based device performance through a smoother capacitance-voltage curve, less increase in the inversion capacitance, and lower density of interface states.

Growth of single crystal GaAs nanowires by a surface diffusion-mediated solid-liquid-solid process

A facile route to synthesize GaAs nanowires by simply heating Au-coated GaAs substrates to 700 °C under vacuum (∼10−3 Torr) was developed in this study. Detailed structural analyses showed that ultrathin single crystal GaAs nanowires with an average diameter of ∼15 nm were grown outward from the Au metal droplets remaining on the surface of the GaAs substrate. On the other hand, the substrate surface region contacting the metal layer became porous, and the depth of the porous layer increased with increasing processing time. Based on these results, it was concluded that the nanowires were grown by a solid-liquid-solid process involving the surface diffusion of adatoms from the underlying solid substrates to the Au liquid droplets.

Site-specific synthesis of ZnO nanocrystalline networks via a hydrothermal method

ZnO nanocrystalline networks (NCNWs) consisting of percolating nanocrystals with irregular shape and size were synthesized using Al seed layers in a hydrothermal process. Various thicknesses of Al films were used to assess the effects of film thickness on the formation of ZnO NCNWs; the coverage and size of the ZnO nanocrystals increased with an increasing Al film thickness. In addition, by exploiting the seed layer-dependent crystal growth behaviors, two distinctly different ZnO nanostructures, nanorods on ZnO seed and NCNWs on Al seed, could be selectively achieved on the same substrate under the same growth conditions. Spectrally- and spatially-resolved investigations of these two ZnO nanostructures were performed using cathodoluminescence, which provided a significant opportunity to study the effect of the nanostructures on the luminescent characteristics. The ZnO NCNWs have an extremely high surface to volume ratio and sufficient inter-space, which enabled the conversion of the surface property from hydrophilic to superhydrophobic.

Importance of mixing protocol for enhanced performance of composite cathodes in all-solid-state batteries using sulfide solid electrolyte

All-solid-state battery performance is strongly dependent on effective charge transfer at both 1) the interface of the active particles and 2) through the interstitial regions of composite cathode. Design of the composite cathode is further complicated by the necessity to limit the amount of conductor additives in order to attain high energy density. These requirements present a difficult design challenge for the composite cathode. Here we investigate the extent to which the mixing order of the three components in the composite cathode impacts the charge transfer and cell performance. We test a total of 5 mixing protocols and find that the initial discharge capacity and the rate capability varies significantly with mixing order. It is shown that the location of the electron conductive carbon is particularly critical for cell performance due to its limited quantity in the composite cathode. Mixing protocols that concentrate the carbon at the active particle interface lowers the interfacial resistance leading to higher discharge capacity. Mixing protocols that place more carbon in the interstitial regions improves the electron path conductivity and is found to correlate with higher rate capability. Based on these results we demonstrate a mixing protocol that achieves both higher discharge capacity and better rate performance for all-solid-state batteries.

Nanosize Patterning with Nanoimprint Lithography Using Poly(vinyl alcohol) Transfer Layer

Coupling the imprint mold structure having a self-assembled monolayer (SAM) and a buffer oxide layer (BOL) with a poly(vinyl alcohol) (PVA) resin is investigated for thermal nanoimprint lithography on flexible substrates. The mold structure is SAM/BOL/Cr. Among the buffer oxides tested (SiO2, Al2O3, HfO2), SiO2 results in the most hydrophobic character at the SAM surface of the mold. Water-soluble PVA resin is shown to be an excellent pattern transfer layer due to its clean release from the hydrophobic mold and strong barrier to SF6 etching during subsequent substrate patterning. The combination of SAM/SiO2/Cr mold structure with PVA resin is demonstrated to produce high quality, defect-free nanopatterns on both rigid silicon and flexible poly(ethylene terephthalate) and polyimide substrates.