Jongsun Maeng

Biogenic formation of photoactive arsenic-sulfide nanotubes by Shewanella sp. strain HN-41

Microorganisms facilitate the formation of a wide range of minerals that have unique physical and chemical properties as well as morphologies that are not produced by abiotic processes. Here, we report the production of an extensive extracellular network of filamentous, arsenic-sulfide (As-S) nanotubes (20–100 nm in diameter by ≈30 μm in length) by the dissimilatory metal-reducing bacterium Shewanella sp. HN-41. The As-S nanotubes, formed via the reduction of As(V) and S2O32−, were initially amorphous As2S3 but evolved with increasing incubation time toward polycrystalline phases of the chalcogenide minerals realgar (AsS) and duranusite (As4S). Upon maturation, the As-S nanotubes behaved as metals and semiconductors in terms of their electrical and photoconductive properties, respectively. The As-S nanotubes produced by Shewanella may provide useful materials for novel nano- and opto-electronic devices.

Transient reverse current phenomenon in a p-n heterojunction comprised of poly(3,4-ethylene-dioxythiophene):poly(styrene-sulfonate) and ZnO nanowall

We report the characteristics of a p-n heterojunction diode comprised of a poly(3,4-ethylene-dioxythiophene):poly(styrene-sulfonate) (PEDOT:PSS) as the hole-conducting p-type polymer and n-type ZnO nanowall networks. ZnO nanowall networks were synthesized on a GaN/sapphire substrate without metal catalyst using hot-wall type metal organic chemical vapor deposition. The p-n heterojunction diodes of PEDOT:PSS/ZnO nanowall exhibited a space charge limited current phenomena at forward bias and a transient reverse current recovery when a sudden reverse bias was applied from the forward bias condition. The minority carrier lifetime was estimated to be ∼2.5 μs.

The effect of excimer laser annealing on ZnO nanowires and their field effect transistors.

We have investigated the effect of excimer laser annealing on the chemical bonding, electrical, and optical properties of ZnO nanowires. We demonstrate that after laser annealing on the ZnO nanowire field effect transistors, the on-current increases and the threshold voltage shifts in the negative gate bias direction. These electrical results are attributed to the increase of oxygen vacancies as n-type dopants after laser annealing, consistent with the shifts towards higher binding energies of Zn 2p and O 1s in the x-ray photoelectron spectroscopy analysis of as-grown nanowires and laser-annealed ZnO nanowires.

Logic inverters composed of controlled depletion-mode and enhancement-mode ZnO nanowire transistors

We demonstrate ZnO nanowire logic inverters consisting of n-channel depletion-mode (D-mode) transistors and n-channel enhancement-mode (E-mode) transistors that are selectively controlled by smooth- and corrugated-surface ZnO nanowires grown on two different types of substrates via a vapor transport method. Our inverter circuits, by combination of both D-mode and E-mode ZnO nanowire devices, show desired voltage transfer characteristics with a high gain and robust noise margin in a simple circuit design with less power dissipation, which makes them superior to logic inverters based on single-mode nanowire transistors.

Effect of gate bias sweep rate on the electronic properties of ZnO nanowire field-effect transistors under different environments

We report the effects of gate bias sweep rate on the electronic characteristics of ZnO nanowire field-effect transistors (FETs) under different environments. As the device was swept at slower gate bias sweep rates, the current decreased and threshold voltage shifted to a positive gate bias direction. These phenomena are attributed to increased adsorption of oxygen on the nanowire surface by the longer gate biasing time. Adsorbed oxygens capture electrons and cause a surface depletion in the nanowire channel. Different electrical trends were observed for ZnO nanowire FETs under different oxygen environments of ambient air, N2, and passivation.

Electrical properties of ZnO nanowire field effect transistors with varying high-k Al2O3 dielectric thickness

We investigated the electronic properties of ZnO nanowire combined with the scaled high-k Al2O3 dielectrics using metal-oxide-semiconductor and field effect transistor (FET) device structures. We found that Al2O3 dielectric material can significantly reduce leakage currents when the applied voltage was restricted less than the transition voltage of direct tunneling to Fowler–Nordheim tunneling. The ZnO nanowire FETs with Al2O3 dielectrics exhibited the increase in electrical conductance, transconductance, and mobility and the threshold voltage shifted to the negative gate bias direction with decreasing Al2O3 dielectric layer thickness.We investigated the electronic properties of ZnO nanowire combined with the scaled high-k Al2O3 dielectrics using metal-oxide-semiconductor and field effect transistor (FET) device structures. We found that Al2O3 dielectric material can significantly reduce leakage currents when the applied voltage was restricted less than the transition voltage of direct tunneling to Fowler–Nordheim tunneling. The ZnO nanowire FETs with Al2O3 dielectrics exhibited the increase in electrical conductance, transconductance, and mobility and the threshold voltage shifted to the negative gate bias direction with decreasing Al2O3 dielectric layer thickness.

Channel-length and gate-bias dependence of contact resistance and mobility for In2O3 nanowire field effect transistors

We demonstrate the scaling properties of the gate-bias-dependent transfer characteristics of In2O3 nanowire field effect transistors (FETs) studied using a conducting atomic force microscope. The contact resistance was extracted from the scaling of the resistance of an In2O3 nanowire FET with respect to its channel length. This contact resistance was found to be significant for short channel devices and decreased as the gate bias increased. We also investigated the apparent and intrinsic mobilities of the nanowire FET as a function of channel length and gate bias. It was determined that the intrinsic mobility could be corrected by considering the non-negligible contact resistance.

Effects of channel-length scaling on In2O3 nanowire field effect transistors studied by conducting atomic force microscopy

Scaling effects of In2O3 nanowire field effect transistors (FETs) were examined as a function of channel length. The channel length was varied from 1μmto20nm by placing a conducting atomic force microscope tip on the In2O3 nanowire as a movable contact. The In2O3 nanowire FET exhibited a variety of channel-length dependent transfer characteristics in terms of the source-drain current, transconductance, threshold voltage, and mobility. Furthermore, the authors were able to extract the contact resistance and distinguish between apparent mobility and intrinsic mobility. The latter was corrected, taking into account the non-negligible contact resistance for short channel devices.

Transient drain current characteristics of ZnO nanowire field effect transistors

We investigated the characteristics of the time-dependent drain current of ZnO nanowire field effect transistors (FETs). The drain current of ZnO nanowire FETs in ambient air decreases from an initial current level in the microampere range and saturates to the 1–100 nA range in tens of seconds. This transient phenomenon is ascribed to electrically interactive adsorption of oxygen ions to the nanowire surface. Exposure to ambient air during positive gate biasing reduces the conduction channel width by extending the depletion region, resulting in a higher resistivity with conduction only through the narrower nanowire core.

Tuning of operation mode of ZnO nanowire field effect transistors by solvent-driven surface treatment

We report on the adjustment of the operation voltage in ZnO nanowire field effect transistors (FETs) by a simple solvent treatment. We have observed that by submerging ZnO nanowires in isopropyl alcohol (IPA), the surface of the ZnO nanowires is etched, generating surface roughness, and their defect emission peak becomes stronger. In particular, ZnO nanowire FETs before IPA treatment operate in the depletion-mode, but are converted to the enhancement-mode with a positive shift of threshold voltage after submersion in IPA. This solvent treatment can be a useful method for controlling the operation mode of ZnO nanowire FETs for wide applications of nanowire-based electronic devices and circuits.