Byungdon Min

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.

Al2O3 nanotubes and nanorods fabricated by coating and filling of carbon nanotubes with atomic-layer deposition

Aluminum oxide (Al2O3) nanotubes and nanorods were fabricated by coating and filling of multiwalled carbon nanotubes (MWNTs) with atomic-layer deposition (ALD). Al2O3 material was deposited on the MWNTs at a substrate temperature of 300°C using trimethylaluminum and distilled water. Transmission electron microscopy, high resolution transmission electron microscopy, energy-dispersive X-ray spectroscopy, and selected area electron diffraction of the deposited MWNTs revealed that amorphous Al2O3 material coats the MWNTs conformally and that this material fills the inside of the MWNTs. These illustrate that ALD has an excellent capability to coat and fill any three-dimensional shapes of MWNTs conformally without producing any crystallites.

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.

Photocurrent of undoped, n- and p-type Si nanowires synthesized by thermal chemical vapor deposition

Photocurrent of undoped, n- and p-type Si nanowires synthesized by thermal chemical vapor deposition is investigated in this study. For an undoped Si nanowire biased at 3 V, photocurrent excited by the 633-nm wavelength light is stronger in intensity than that excited by the 325-nm wavelength light, and photoresponses are rapid when the light is switched on and off. In contrast, for the n- and p-type Si nanowires, photocurrent excited by the 633-nm wavelength light is not measurable, although one excited by 325-nm wavelength light is still detectable. And photoresponses obtained for the doped Si nanowires are slower, compared with the undoped Si nanowire. Photocurrent phenomena observed in the undoped, n- and p-type Si nanowires are discussed in this paper.

Aging effect on the optoelectronic properties of a single ZnO nanowire

The effect of aging on the optoelectronic properties of a single ZnO nanowire is investigated in this study. The photoluminescence (PL), photocurrent spectrum, current–voltage characteristics, and photoresponse were measured for the as-grown ZnO nanowire and for the same nanowire exposed to air for three months. For the aged nanowire, the broad PL band is weaker, the intensity of the photocurrent is stronger, and the photoresponse is slower than those of the as-grown nanowire. One of the possible explanations is that the observed aging effect on the PL is due to the reduction in the number of oxygen vacancies within the nanowire and that the aging effect on the photocurrent and photoresponse originates from the formation of oxygen vacancies near the surface.

Synthesis of Single Crystalline In2O3 Nanowires and Their Photoluminescence Characteristics

Single crystalline In2O3 nanowires have been synthesized by thermal evaporation of ball-milled In2O3 powders without any catalysis. The diameter and length range of the synthesized In2O3 nanowires are about 25 nm and 20–30 µm, respectively. Their X-ray diffraction pattern is indexed to bcc structure with a lattice constant of a=1.0126 nm. High-resolution transmission electron microscopy image shows that the inner part of the In2O3 nanowires is free of dislocations, and that any amorphous layers are not formed on the surface of the nanowires. In their PL spectrum, two peaks are observed in the ultraviolet region centered at 380 nm and in the visible region centered at 550 nm.

Sonochemical Synthesis and Photocurrent of HgTe NANOPARTICLES

We report the sonochemical synthesis of mercury telluride (HgTe) nanoparticles and the photocurrents of these nanoparticles and their organic hybrid system. The HgTe nanoparticles were about 5 nm in size and their lattice structure was cubic. Poly(N-vinylcarbazole)(PVK) was added to the HgTe nanoparticles for the formation of an inorganic(HgTe)-organic(PVK) hybrid system. The room-temperature photocurrents of the HgTe nanoparticles and the HgTe-PVK hybrid system were compared in this study. The photocurrent was significantly enhanced for the HgTe-PVK hybrid system.

Structural and Optoelectronic Properties of SnO 2 Nanowires

Structural and optoelectronic properties of as-synthesized SnO nanowires were examined in this study. The SnO nanowires were first synthesized by thermal evaporation of ball-milled SnO powders in argon atmosphere without the presence of any catalysts, arid their structural properties are then investigated by X-ray diffraction, Raman scattering, scanning electron microscopy, and transmission electron microscopy. This investigation revealed that the synthesized SnO nanowires are single-crystalline and that their growth direction is parallel to the [100] direction. In addition, photoresponse of a single SnO nanowire was performed with light with above-gap energy, and different characteristics of photoresponses were obtained for the nanowire at ambient atmosphere and in vacuum. The photoresponse mechanism is briefly discussed in this paper.

Ellipsometric Study of Self-Assembled InAs/GaAs Quantum Dots

We measured the pseudo-dielectric function of self-assembled InAs/GaAs quantum dots at room temperature using spectroscopic ellipsometry. We observed a strong excitonic peak at 0.9 eV, which was attributed to quantum dot transitions. We also observed a plateau from 1.2 eV to 1.4 eV, which arose from steplike joint density of states originating from an InAs wetting layer. Our room temperature data are very similar to the 1.8 K photoluminescence excitation spectra of InAs/GaAs quantum dots reported in the literature. The higher energy dielectric response of the quantum dots enabled us to estimate the morphology of the quantum dots using effective medium analysis. These results were compared to atomic force microscopy measurement results. Effective medium analysis showed that a GaAs cap layer was preferentially grown on the InAs wetting layer rather than on InAs islands.