Junghwan Huh

Photoconductance of aligned SnO2 nanowire field effect transistors

We report on the optoelectronic properties of the aligned SnO2 nanowire (NW) field effect transistors (FETs) fabricated via a sliding transfer of NWs grown by chemical vapor deposition. Photocurrent measurements with polarized UV light confirmed a well aligned NWs along the channels. UV photosensitivity of ∼107 at the gate voltage Vg=−40 V was obtained due to a small dark-current at the turn-off state of FET. The dynamic response of the photocurrent became faster for the higher mobility SnO2 NW FETs. We expect our aligned SnO2 NW FETs will be useful as polarized UV detectors with a high sensitivity.

Highly sensitive hydrogen detection of catalyst-free ZnO nanorod networks suspended by lithography-assisted growth

We have successfully demonstrated a ZnO nanorod-based 3D nanostructure to show a high sensitivity and very fast response/recovery to hydrogen gas. ZnO nanorods have been synthesized selectively over the pre-defined area at relatively low temperature using a simple self-catalytic solution process assisted by a lithographic method. The conductance of the ZnO nanorod device varies significantly as the concentration of the hydrogen is changed without any additive metal catalyst, revealing a high sensitivity to hydrogen gas. Its superior performance can be explained by the porous structure of its three-dimensional network and the enhanced surface reaction of the hydrogen molecules with the oxygen defects resulting from a high surface-to-volume ratio. It was found that the change of conductance follows a power law depending on the hydrogen concentration. A Langmuir isotherm following an ideal power law and a cross-over behavior of the activation energy with respect to hydrogen concentration were observed. This is a very novel and intriguing phenomenon on nanostructured materials, which suggests competitive surface reactions in ZnO nanorod gas sensors.

A sensitive diagnostic assay of rheumatoid arthritis using three-dimensional ZnO nanorod structure

We synthesized a three-dimensional nanorod structure of zinc oxide (ZnO) using a simple sol-gel process and systematically investigated properties of the ZnO nanorods regarding protein adsorption and effect on fluorescence emission. As compared to conventional polystyrene plate that has been widely used for strong protein adsorption, the ZnO nanorods had a superior protein adsorption capacity and significantly amplified fluorescence emission, suggesting the ZnO nanorods are attractive for fluorescence-based biomolecular detection assays. When applied to diagnostic assay of rheumatoid arthritis (RA) using cyclic citrullinated peptide (CCP) probe with a RCGRS motif that reportedly has a strong affinity for ZnO, the ZnO nanorods gave apparently high positive signals for all the RA-positive standards and patient sera, whereas upon the detection using conventional polystyrene plate, all the detection signals were relatively negligible. Moreover, the streptavidin-mediated immobilization of well oriented CCP further enhanced sensitivity, even for a 5000-times diluted patient serum. A highly sensitive detection of a very small amount of RA autoantibodies is important because individuals at high risk of developing RA can be identified several years before the clinical onset. Consequently, the fluorescence-based sensitive assay of RA was successfully performed using the three-dimensional ZnO nanorods, owing to the fluorescence amplification and protein/peptide adsorption properties and dimensionality of ZnO nanorods that in turn increases probe accessibility to anti-CCP RA autoantibodies. Although RA was assayed here for proof-of-concept, the ZnO nanorods-based assay can be applied in general to sensitive detection of a wide variety of antibody or protein targets.

A direct measurement of the local resistances in a ZnO tetrapod by means of impedance spectroscopy: The role of the junction in the overall resistance

We have measured the local resistances in the arms and at the junction in a ZnO tetrapod separately by means of ac impedance spectroscopy. The resistance at the junction is found to be even greater than that in the arms although a volume fraction of the junction is negligibly small compared to that of the arms. This result clearly demonstrates that the junction plays a decisive role in the overall electrical property of the ZnO-tetrapod and thus determines the functionality of the ZnO tetrapod electronic devices.

Reduced charge fluctuations in individual SnO2 nanowires by suppressed surface reactions

The interactions between metal oxide nanowires and molecular species can significantly affect the electrical properties of metal oxide nanowires. A passivation process is needed to stabilize the electrical characteristics, regardless of the environmental changes. Herein, we investigated the passivation effects of a polymethyl methacrylate (PMMA) layer on SnO2 nanowire (NW) field-effect transistors (FETs). As a result of the PMMA coating, the electrical properties of the SnO2 NW FETs improved. The electrical noise behavior in both non-passivated and passivated devices can be described with the carrier number fluctuation model associated with the trapping and the release of charge carriers at the surface. The non-passivated devices exhibited higher noise levels than those of the passivated devices. These results demonstrate that surface passivation can lead to the suppression of dynamic responses (electron trapping/release events and scattering fluctuations).

Asymmetric Contacts on a Single SnO2 Nanowire Device: An Investigation Using an Equivalent Circuit Model

Electrical contacts between the nanomaterial and metal electrodes are of crucial importance both from fundamental and practical points of view. We have systematically compared the influence of contact properties by dc and EIS (Electrochemical impedance spectroscopy) techniques at various temperatures and environmental atmospheres (N(2) and 1% O(2)). Electrical behaviors are sensitive to the variation of Schottky barriers, while the activation energy (E(a)) depends on the donor states in the nanowire rather than on the Schottky contact. Equivalent circuits in terms of dc and EIS analyses could be modeled by Schottky diodes connected with a series resistance and parallel RC circuits, respectively. These results can facilitate the electrical analysis for evaluating the nanowire electronic devices with Schottky contacts.

Channel access resistance effects on charge carrier mobility and low-frequency noise in a polymethyl methacrylate passivated SnO2 nanowire field-effect transistors

Channel access resistance (Rsd) effects on the charge carrier mobility (μ) and low-frequency noise (LFN) in a polymethyl-methacrylate (PMMA) passivated tin-oxide nanowire (SnO2-NW) field effect-transistor were investigated. To this end, the Y function method was employed for direct electrical parameters extraction without Rsd influence. Numerical simulation was used to evaluate gate-to-channel capacitance (Cgc) accounting for the electrostatic gate coupling effects through PMMA passivation layer. Furthermore, LFN measurements were carried out to study the SnO2/dielectrics interface. The carrier number fluctuation (CNF) noise model was found appropriate to interpret LFN data provided Rsd influence is included.

Enhanced voltage-current characteristics of GaN nanowires treated by a selective reactive ion etching

In characterizing the electrical properties of individual NWs (nanowires), the amorphous oxide layer on the surface of NWs is known to limit the electrical conductivity owing to the contact barriers between metal electrodes and NWs. To remove the native oxide layer, a systematic reactive ion etching (RIE) was performed, resulting in a gradual decrease of the diameters of NWs. Voltage-current characteristics of the GaN NW devices treated by tuning the RIE process were improved as reflected by a 1000 times increase in conductance, which was in turn attributed to the removal of the thick (d∼3.5nm) contact barrier formed by the native oxide layer.

Degradation pattern of SnO2 nanowire field effect transistors

The degradation pattern of SnO(2) nanowire field effect transistors (FETs) was investigated by using an individual SnO(2) nanowire that was passivated in sections by either a PMMA (polymethylmethacrylate) or an Al(2)O(3) layer. The PMMA passivated section showed the best mobility performance with a significant positive shift in the threshold voltage. The distinctive two-dimensional R(s)-μ diagram based on a serial resistor connected FET model suggested that this would be a useful tool for evaluating the efficiency for post-treatments that would improve the device performance of a single nanowire transistor.

Degradation pattern of SnO 2 nanowire field effect transistors

The degradation pattern of SnO(2) nanowire field effect transistors (FETs) was investigated by using an individual SnO(2) nanowire that was passivated in sections by either a PMMA (polymethylmethacrylate) or an Al(2)O(3) layer. The PMMA passivated section showed the best mobility performance with a significant positive shift in the threshold voltage. The distinctive two-dimensional R(s)-μ diagram based on a serial resistor connected FET model suggested that this would be a useful tool for evaluating the efficiency for post-treatments that would improve the device performance of a single nanowire transistor.