Heon-Jin Choi

Optical and electrical transport properties in silicon carbide nanowires

We report on the optical and electrical transport properties of single-crystalline silicon carbide nanowires (SiC NWs). The NWs were fabricated by a chemical vapor deposition process, and had diameters of <100nm and lengths of several μm. X-ray diffraction and transmission electron microscopy analysis showed the single-crystalline nature of NWs with a growth direction of ⟨111⟩. Photoluminescence characterization showed blue emission at room temperature. The electrical measurements from a field effect transistor structure on individual NWs showed n-type semiconductor characteristics. The resistivity and estimated electron mobility on the NWs are 2.2×10−2Ωcm for 0V of gate voltage and 15cm2∕(Vs), respectively. Our low-resistivity SiC NWs could be applied to a high-temperature operation sensor and actuator due to its own excellent electrical and optical properties.

Large-Scale Assembly of Silicon Nanowire Network-Based Devices Using Conventional Microfabrication Facilities

We present a method for assembling silicon nanowires (Si-NWs) in virtually general shape patterns using only conventional microfabrication facilities. In this method, silicon nanowires were functionalized with amine groups and dispersed in deionized water. The functionalized Si-NWs exhibited positive surface charges in the suspensions, and they were selectively adsorbed and aligned onto negatively charged surface regions on solid substrates. As a proof of concepts, we demonstrated transistors based on individual Si-NWs and long networks of Si-NWs.

Controlled growth of high-quality TiO2 nanowires on sapphire and silica

The vapour?liquid?solid (VLS) growth of TiO2 nanowires (NWs) was performed using a thermally evaporated Ti source and sputter-deposited Au?catalysts under an O2 gas flow. High-density single-crystalline TiO2 NWs having the rutile structure were successfully grown on sapphire (single-crystal ?-Al2O3) and quartz (amorphous SiO2) substrates. Ti buffer layers, deposited on the substrates to prevent undesirable reactions between the Ti vapour and substrates, were identified to promote the TiO2 NW growth by providing supplementary Ti vapour to the Au catalysts. Crystallinity of TiO2 NWs was investigated by x-ray diffraction (XRD) and their morphological features were characterized by field emission scanning electron microscopy (FESEM). High-resolution transmission electron microscopy (HRTEM) and selected area electron diffraction (SAED) analyses reveal growth of the rutile NWs having twofold twin structures along the growth direction on Ti/sapphire and the defect-free single-crystalline rutile NWs on Ti/quartz substrates. TiO2 NWs grown on Ti/quartz showed a short-wavelength (~402?nm) and high-intensity photoluminescence (PL) emission compared to those grown on Ti/sapphire substrates. By introducing a Ti buffer layer and using quartz substrates, the crystallinity and PL properties were successfully improved for VLS-grown TiO2 NWs.

Synthesis of Si Nanosheets by a Chemical Vapor Deposition Process and Their Blue Emissions

We synthesized free-standing Si nanosheets (NSs) with a thickness of about <2 nm using a chemical vapor deposition process and studied their optical properties. The Si NSs were formed by the formation of frameworks first along six different <110> directions normal to [111], its zone axis, and then by filling the spaces between the frameworks along the <112> directions under high flow rate of processing gas. The Si NSs showed blue emission at 435 nm, and absorbance and photoluminescence (PL) excitation measurements indicate that enhanced direct band transition attributes to the emission. Time-resolved PL measurement, which showed PL emission at 435 nm and a radiative lifetime of 1.346 ns, also indicates the enhanced direct band gap transition in these Si NSs. These outcomes indicate that dimensionality of Si nanostructures may affect the band gap transition and, in turn, the optical properties.

Bacterial Recognition of Silicon Nanowire Arrays

Understanding how living cells interact with nanostructures is integral to a better understanding of the fundamental principles of biology and the development of next-generation biomedical/bioenergy devices. Recent studies have demonstrated that mammalian cells can recognize nanoscale topographies and respond to these structures. From this perspective, there is a growing recognition that nanostructures, along with their specific physicochemical properties, can also be used to regulate the responses and motions of bacterial cells. Here, by utilizing a well-defined silicon nanowire array platform and single-cell imaging, we present direct evidence that Shewanella oneidensis MR-1 can recognize nanoscale structures and that their swimming patterns and initial attachment locations are strongly influenced by the presence of nanowires on a surface. Analyses of bacterial trajectories revealed that MR-1 cells exhibited a confined diffusion mode in the presence of nanowires and showed preferential attachment to the nanowires, whereas a superdiffusion mode was observed in the absence of nanowires. These results demonstrate that nanoscale topography can affect bacterial movement and attachment and play an important role during the early stages of biofilm formation.

Thermal conductivities of Si1−xGex nanowires with different germanium concentrations and diameters

The thermal conductivities of Si1−xGex nanowires (NWs) synthesized with Ge concentrations of 0.0%, 0.4%, 4%, and 9% and different diameters were measured from 40 to 420 K. The thermal conductivity of Si1−xGex NWs decreases as the Ge concentration increases due to alloy scattering. As the diameter of the Si1−xGex NWs decreases, the thermal conductivity decreases due to phonon boundary scattering. However, the thermal conductivity dependency on the diameter of the NWs is not significant. This indicates that alloy scattering should be the dominant scattering mechanism in Si1−xGex NWs. This study should provide a basis for designing efficient thermoelectric devices out of Si1−xGex NWs and Si1−xGex nanocomposites.

Ferroelectric Nonvolatile Nanowire Memory Circuit Using a Single ZnO Nanowire and Copolymer Top Layer

IO N Nanowire-based fi eld-effect transistors (FETs) and diodes have been continuously studied along with a great variety of semiconductor nanowires (NWs), including Si, Ge, SiGe, GaN, InP, and ZnO. [ 1–8 ] Among all the NW materials, ZnO appears to have relatively good metal electrode–semiconductor contact, which improves the device fabrication yield. [ 7–17 ] Therefore, using a long ZnO NW it may be possible to realize one-dimensional (1D) NW electronics, which may contain a FET, [ 7 , 10–14 ]

TiO2–CdSe nanowire arrays showing visible-range light absorption

High-density single crystalline TiO2 nanowires (∼50nm diameter) were grown on Ti substrates by chemical vapor deposition, and they were overcoated with the solution containing CdSe nanocrystals (∼5nm diameter) and heat treated at 600°C to form TiO2 hetersotructured nanowire arrays. The TiO2 nanowire arrays showed uniformly distributed CdSe nanocrystals and high crystallinity of rutile and wurtzite from the TiO2 and the CdSe, respectively. Owing to the heterostructure of TiO2, they demonstrate almost full visible-range light absorption, and thus enhanced photocatalytic activity by the charge separation via electron and hole transfers between the CdSe and the TiO2.

Photostable Dynamic Rectification of One-Dimensional Schottky Diode Circuits with a ZnO Nanowire Doped by H during Passivation

For the first time, we demonstrated photostable and dynamic rectification in ZnO nanowire (NW) Schottky diode circuits where two diodes are face-to-face connected in the same ZnO wire. With their properties improved by H-doping from atomic layer deposited Al(2)O(3) passivation, our ZnO NW diode circuits stably operated at a maximum frequency of 100 Hz displaying a good rectification even under the lights. We thus conclude that our results promisingly appoached one-dimensional nanoelectronics.

Low-Temperature Ohmic Contact to Monolayer MoS2 by van der Waals Bonded Co/h-BN Electrodes

Monolayer MoS2, among many other transition metal dichalcogenides, holds great promise for future applications in nanoelectronics and optoelectronics due to its ultrathin nature, flexibility, sizable band gap, and unique spin-valley coupled physics. However, careful study of these properties at low temperature has been hindered by an inability to achieve low-temperature Ohmic contacts to monolayer MoS2, particularly at low carrier densities. In this work, we report a new contact scheme that utilizes cobalt (Co) with a monolayer of hexagonal boron nitride (h-BN) that has the following two functions: modifies the work function of Co and acts as a tunneling barrier. We measure a flat-band Schottky barrier of 16 meV, which makes thin tunnel barriers upon doping the channels, and thus achieve low-T contact resistance of 3 kΩ.μm at a carrier density of 5.3 × 1012/cm2. This further allows us to observe Shubnikov-de Haas oscillations in monolayer MoS2 at much lower carrier densities compared to previous work.