Minhyon Jeon

Graphene counter electrodes for dye-sensitized solar cells prepared by electrophoretic deposition

The graphene counter electrodes (GCEs) for dye-sensitized solar cells (DSSCs) prepared by electrophoretic deposition (EPD) are reported. Thermogravimetric analysis (TGA) was carried out to find the optimum annealing temperatures. It was found that charge transfer resistance (Rct1) related to the interface between electrolyte and counter electrode was significantly reduced by proper annealing. DSSC with GCE annealed at 600 °C demonstrated by the best conversion efficiency of 5.69% under AM 1.5 and 1 sun condition. Once GCE prepared by EPD is fully developed, it could be utilized for a low-cost, high throughput process for DSSCs.

Enhanced field emission from aligned multistage carbon nanotube emitter arrays

In this work we report on the synthesis and field emission properties of carbon nanotube multistage emitter arrays grown on porous silicon by catalytic thermal chemical vapor deposition. The vertically oriented multistage array structures consisted of SWNTs and thin MWNTs grown on MWNTs, confirmed by TEM and Raman analysis. Higher field emission current ∼32 times and low threshold field ∼1.5 times were obtained for these structures in comparison to only MWNT arrays. The enhanced field emission results for these multistage emitters are a consequence of higher field concentration, which is ∼3 times more than MWNTs.

Fabrication and characterization of carbon-based counter electrodes prepared by electrophoretic deposition for dye-sensitized solar cells

Three different carbon-based counter electrodes are investigated in light of catalytic activities such as electrochemical frequencies and interface impedances. We fabricated carbon-based counter electrodes of dye-sensitized solar cells [DSSCs] using graphene, single-walled carbon nanotubes [SWNTs], and graphene-SWNT composites by electrophoretic deposition method. We observed the optical and electrochemical properties of the carbon-based counter electrodes. The DSSC with the graphene-deposited counter electrode demonstrated the best conversion efficiency of 5.87% under AM 1.5 and 1 sun condition. It could be utilized for a low-cost and high-throughput process for DSSCs.

Effects of high potential barrier on InAs quantum dots and wetting layer

Effects of a thin AlAs layer (1 nm) with different position on InAs quantum dots (QDs) and wetting layer have been investigated by transmission electron microscopy (TEM), photoluminescence (PL), and photoreflectance (PR). The PL peak position of InAs QDs directly grown on the thin AlAs is blueshifted from that of InAs QDs grown on the GaAs layer by 171 meV mainly due to the high potential barrier and reduced dot size shown in the TEM image. As the additional GaAs layer (1 and 2 nm) is inserted on top of the AlAs layer, the PL peak position is systematically shifted toward longer wavelength with increase in the thickness. Temperature dependent PL of QD samples shows that a thin AlAs layer significantly influences the thermal activation energy. The wetting layer related peak in PR spectra is changed to lower energy with increase in the thickness of an additional GaAs layer, which is mainly caused by the reduction in the effects of the AlAs layer.

Height-controlled InAs quantum dots by using a thin InGaAs layer

The structural and optical properties of height-controlled InAs quantum dots (QDs) have been investigated by transmission electron microscopy (TEM) and photoluminescence (PL). By depositing 1.4 nm In0.15Ga0.85As and a 1 monolayer (ML) InAs layer with different periods on 3 ML InAs QDs, the height of InAs QDs was systematically controlled with similar lateral size. In TEM images, the indication of dislocations due to the large strain, which can be easily seen in large QDs, is not observed even for the QD sample with the highest aspect ratio (height/width). The PL peak position is shifted toward the longer wavelength with an increase in the aspect ratio of QDs. As the aspect ratio is increased, the full width at half maximum in PL measured at 10 K is decreased from 71 to 34 meV indicating that the inhomogeneous broadening caused by the fluctuation in QD size, especially the height, is significantly reduced.

Carbon nanotube-graphene composite for ionic polymer actuators

In this paper, we develop a new ionic polymer‐metal composite (IPMC) by replacing a typical platinum or gold electrode with a multi-walled carbon nanotube (MWNT)‐graphene based electrode. A solvent of MWNT and graphene is formed on both sides of the ionic polymer membranes as electrodes by means of spray coating and baking. Then, the ionic liquid process is performed for actuating in air. The four kinds of IPMC samples with different MWNT‐graphene ratios are fabricated with the same solid Nafion film. Experimental results show that the IPMC with a pure MWNT based electrode exhibits higher displacement compared to the conventional IPMC with a platinum electrode. Also, the increment of the ratio of graphene to the MWNT‐graphene electrode decreases the resultant displacement but increases the fundamental natural frequency of the polymer actuator. (Some figures may appear in colour only in the online journal)

Enhanced Electrical Conductance of ZnO Nanowire FET by Nondestructive Surface Cleaning

Electrical characteristics of zinc oxide nanowire (ZNW) FETs are investigated by nondestructive surface cleaning, ultraviolet irradiation treatment at high temperature and vacuum. UV-light-stimulated oxygen desorption from the active channel improves the device performance of ZNW-FETs. Results show that charge transport in single ZNW strongly depends on its surface environmental conditions and can be explained by formation of depletion layer at the surface by various surface states present on it. The nondestructive surface cleaning removes these absorbed surface states from the nanowire and the current values increase upto ~ 7 muA from ~ 0.4 muA at a bias voltage of 3 V. ZNW-FETs fabricated in this study exhibit mobility of ~ 28 cm2/Vmiddots and a high I ON ne I OFF ratio of ~ 106.

Effects of the electrical conductivity and orientation of silicon substrate on the synthesis of multi-walled carbon nanotubes by thermal chemical vapor deposition

We studied the effects of the electrical conductivity and orientation of silicon substrate on both catalytic Fe thin film and the structure and morphology of multi-walled carbon nanotube (MWNT) grown by low-pressure chemical vapor deposition. Both p-type Si(100) and Si(111) substrates with three different doping concentrations (high, low, undoped) were used to evaluate the formation of catalytic nanoparticles and the growth of MWNTs. The morphology of catalytic nanoparticles such as size and density was characterized by field-emission scanning electron microscopy, Cs-corrected energy-filtered transmission electron microscopy, and X-ray photoelectron spectroscopy. Structural characteristics of MWNTs grown on different combinations of silicon substrate orientation and electrical conductivities (σ) were also systematically analyzed. Based on the experimental results, growth modes of MWNTs could be controlled by choosing an appropriate combination of σ and orientation of Si substrates.

Growth of heteroepitaxial ZnO thin film and ZnO∕(Mg,Zn)O nanomultilayer by off-axis rf magnetron sputtering

Heteroepitaxial ZnO thin film was deposited on a sapphire (001) substrate using off-axis radio-frequency magnetron sputtering. The crystallinity of the ZnO thin film was affected by deposition pressure, substrate temperature, and postannealing temperature. The low-temperature photoluminescence of heteroepitaxial ZnO thin film grown under optimum conditions of 650°C, 120W, and 10mTorr showed strong UV emission at 3.36eV with a full width at half maximum (FWHM) of 16.0meV. After annealing in an O2 ambient at 950°C, the high-resolution x-ray diffraction rocking curve FWHM of the ZnO thin film markedly decreased from 0.38° to 0.19° although UV emission decreased. These results indicated that a heteroepitaxial ZnO thin film with strong UV emission can be grown by off-axis rf sputtering and that O2 annealing helps enhance the crystallinity of ZnO thin film which can be used as a buffer layer in ZnO∕(Mg,Zn)O multiple quantum well structures. To demonstrate an application of the ZnO thin film, a ZnO∕(Mg,Zn)O nano...

Precise control of chemical vapor deposition graphene layer thickness using NixCu1−x alloys

We investigated a simple but effective method to precisely control the desired number of graphene layers on the NixCu1−x alloy substrates by thermal chemical vapor deposition. Our method could be utilized to precisely control the number of graphene layers without altering growth conditions such as growth temperature and the cooling rate.