Kyoungah Cho

Photocurrent in ZnO nanowires grown from Au electrodes

ZnO nanowires were grown between two Au electrodes on an Al2O3-deposited Si wafer. Photoresponse, photoresponse spectrum, and current–voltage (I–V) studies were performed for the investigation into photoconduction mechanism in these nanowires. The photoresponse of the nanowires under the continuous illumination of light with above- or below-gap energies was slow, which indicates that photocurrent in the nanowires is surface-related rather than bulk-related. The photoresponse spectrum represents the above- and below-gap absorption bands for the photocurrents. The I–V characteristics under the illumination of the above-gap light are ohmic, but the characteristics under the illumination of the below-gap light are Schottky. This observation indicates that the above-gap light lowers the potential barrier built in the contact between the ZnO nanowires and electrodes, but that the below-gap light does not lower the potential barrier.

Flexible TFTs based on solution-processed ZnO nanoparticles

Flexible electronic devices which are lightweight, thin and bendable have attracted increasing attention in recent years. In particular, solution processes have been spotlighted in the field of flexible electronics, since they provide the opportunity to fabricate flexible electronics using low-temperature processes at low-cost with high throughput. However, there are few reports which describe the characteristics of electronic devices on flexible substrates. In this study, we fabricated flexible thin-film transistors (TFTs) on plastic substrates with channel layers formed by the spin-coating of ZnO nanoparticles and investigated their electrical properties in the flat and bent states. To the best of our knowledge, this study is the first attempt to fabricate fully functional ZnO TFTs on flexible substrates through the solution process. The ZnO TFTs showed n-channel device characteristics and operated in enhancement mode. In the flat state, a representative ZnO TFT presented a very low field-effect mobility of 1.2 x 10(-5) cm(2) V(-1) s(-1), while its on/off ratio was as high as 1.5 x 10(3). When the TFT was in the bent state, some of the device parameters changed. The changes of the device parameters and the possible reasons for these changes will be described. The recovery characteristics of the TFTs after being subjected to cyclic bending will be discussed as well.

Poststroke depression and emotional incontinence Factors related to acute and subacute stages

Objectives: To investigate the characteristics and prevalence of poststroke depression (PSD) and poststroke emotional incontinence (PSEI) and the factors related to these conditions at admission and 3 months after stroke. Methods: We evaluated 508 consecutive patients with acute ischemic stroke for PSD and PSEI at admission and 3 months later. PSD was evaluated using the Beck Depression Inventory, and PSEI was evaluated using Kims criteria. Blood samples were collected and genotyped for the promoter region of the serotonin transporter protein (5-HTTLPR) and the number of tandem repeats within intron 2 (STin2 VNTR). Perceived social support (the ENRICHD Social Support Inventory) was also measured. Results: PSD and PSEI were present in 13.7% and 9.4% of patients, respectively, at admission and in 17.7% and 11.7%, respectively, at 3 months after stroke. Multivariate analyses showed that PSD at admission was associated with the NIH Stroke Scale score at admission (p < 0.001), whereas PSD at 3 months was associated with the presence of microbleeds (p < 0.01) and perceived low social support (p < 0.001). In contrast, only lesion location (p = 0.022) was associated with PSEI at admission, whereas modified Rankin Scale score (p = 0.019), STin2 VNTR (p = 0.040), and low social support (p = 0.042) were related to PSEI 3 months after stroke. Conclusions: Diverse factors such as neurologic dysfunction, lesion location, microbleeds, genetic traits, and social support are differently related to acute and subacute emotional disturbances. Strategies to prevent or manage these problems should consider these differences.

Capacitance characteristics of MOS capacitors embedded with colloidally synthesized gold nanoparticles

In this work, the capacitance characteristics of gold nanoparticle-embedded metal–oxide-semiconductor (MOS) capacitors with Al2O3 control oxide layers are investigated. The capacitance versus voltage (C–V) curves obtained for a representative MOS capacitor embedded with gold nanoparticles synthesized by the colloidal method exhibit large flat-band voltage shifts, which indicate the presence of charge storages in the gold nanoparticles. Their hysteresis characteristics are dependent on the voltage sweep range. The clockwise hysteresis and rightward shift of the flat band voltages observed from the C–V curves imply that electrons are trapped in a floating gate layer consisting of the gold nanoparticles present between SiO2 and Al2O3 layers in the MOS capacitor, and that these trapped electrons originate from the top electrode. In addition, the characteristics of the capacitance versus time curves for the gold nanoparticle-embedded MOS capacitor are discussed in this paper.

Resistance switching memory devices constructed on plastic with solution-processed titanium oxide

Resistance switching memory devices constructed on flexible plastic substrates via the spin-coating of titanium oxide solution were characterized in this study. The resistance switching memory device exhibited a ratio of the high resistance to low resistance states of more than 102, and this large resistance ratio was maintained even after 104 s. These memory characteristics are comparable to those of resistance switching memory devices based on titanium oxide films deposited on Si substrates. Moreover, the endurance of the flexible memory device investigated by means of a continuous substrate bending test revealed that the ratio of the high resistance to low resistance states was negligibly changed up to two hundred cycles. Its resistance switching characteristics were not degraded by the bending of the substrate, due to its short length channel and the high ductility of the electrode.

Bottom- and top-gate field-effect thin-film transistors with p channels of sintered HgTe nanocrystals

Sintered HgTe nanocrystal-based thin-film transistors (TFTs) with bottom- and top-gate geometries were fabricated at a temperature of 150°C by spin coating in this work. The SiO2 bottom- and Al2O3 top-gate field-effect TFTs with p channels composed of sintered HgTe nanocrystals exhibit high carrier mobilities of 0.82 and 2.38cm2∕Vs, respectively. The operating gate voltages for the top-gate transistor with an Al2O3 dielectric layer are actually lower, compared with the SiO2 bottom-gate transistor. The electrical characteristics of these TFTs are discussed in more detail in this letter.

Photocurrent mechanism in a hybrid system of 1-thioglycerol-capped HgTe nanoparticles

Photoluminescence, absorption, and photocurrent measurements were made for a hybrid system of 1-thioglycerol-capped HgTe nanoparticles synthesized by colloidal method to investigate the photocurrent mechanism in this hybrid system. Absorption and photoluminescence spectra taken for the capped HgTe nanoparticles reveal strong exciton peaks in the near-infrared wavelength range. The wavelength dependence of the photocurrent for these capped nanoparticles is very close to that of the absorption spectrum. For the photocurrent mechanism of the hybrid system, on the basis of our experimental results and energy diagram for the 1-thioglycerol-capped HgTe nanoparticles, it is suggested in this letter that holes among electron-hole pairs created by incident photons in the HgTe nanoparticles are transferred to capping 1-thioglycerol while electrons are strongly confined in these nanoparticles and that the holes contribute to the photocurrent flowing in the medium of 1-thioglycerol.

Photocurrent and photoluminescence characteristics of networked GaN nanowires

The dark current, photocurrent and photoluminescence (PL) of networked GaN nanowires were characterized in this study. GaN nanowires were synthesized from Ni particles dispersed on an alumina substrate, and subsequent Ti deposition was performed on the as-synthesized GaN nanowires to form electrodes. For the networked GaN nanowires, a significant dark current was observed and the I–V characteristics of the dark current and photocurrent were independent of temperature in magnitude and shape. These results indicate that the GaN nanowires were networked electrically between the electrodes and that the electronic states of these nanowires were degenerate. In addition, the correlation between the photocurrent and PL of the networked GaN nanowires was examined. The characteristics of the green-band emission were similar to those of the photoresponse, and the wavelength range of the green-band emission exactly overlapped that of the below-gap absorption band in the photocurrent spectra. These results indicate that the green emission and the photoresponse have a common origin.

The transfer of charge carriers photogenerated in ZnO nanoparticles into a single ZnO nanowire.

The transfer of charge carriers, photogenerated in nanoparticles (NPs), into a single nanowire (NW) is demonstrated in this study by conducting a careful comparison of the optoelectronic characteristics of a ZnO NW with ZnO NPs attached to its surface, a bare ZnO NW and a film of close-packed ZnO NPs. Under the illumination of an above-gap light, the photocurrent taken from the NW with the NPs is remarkably higher in magnitude than that obtained from the bare NW, although the photocurrent is substantially lower for the close-packed NPs. The presence of the absorption band of the NPs in the photoresponse spectrum taken from the NW with the NPs reveals that the transfer of the charge carriers photogenerated in the NPs into the NW dramatically enhances the magnitude of the photocurrent flowing in this NW. Nevertheless, during the transfer, the charge carriers experience trapping and detrapping at the interfaces of the NPs and the NW.

Fabrication of single electron transistors with molecular tunnel barriers using ac dielectrophoresis technique

We demonstrate the fabrication and characterization of single electron transistors composed of Au nanoparticles and insulating molecular tunnel barriers. We fabricated these devices by forming a 1,8-octanedithiol self-assembled monolayer on Au nanogap electrode pairs and by bridging them with colloidal Au nanoparticles using ac dielectrophoresis. We observed the typical characteristics of these single electron transistors at the temperature from 4.2 to 300 K. The measured data were consistent with the orthodox theory of Coulomb blockade.