Ken Ha Koh

Low temperature wet chemical synthesis of good optical quality vertically aligned crystalline ZnO nanorods

The growth of ZnO nanorods on Au-coated ITO substrates using a low temperature wet chemical process is presented. Electron microscopy and x-ray diffraction observations reveal that the crystalline ZnO nanorods are preferentially oriented along the c axis. Room temperature photoluminescence (PL) measurements reveal a strong band edge emission at 382 nm, a signature of good crystallinity, with a weak and broad orange-red emission, which is typically attributed to the oxygen interstitials, in the range between 520 and 720 nm. Other than the second order feature of the band edge emission at 760 nm, no red or near-infrared bands are observed. The effect of precursor concentration on the morphological, structural and PL properties are studied, and the results are discussed.

Stable and uniform electron emission from nanostructured carbon films

We have systematically studied the electron-emission characteristics of nanostructured carbon films with various amounts of nanoclusters and degrees of nanotube alignment. According to our observation, the dense packing and/or alignment of nanotubes is detrimental to the low-field emission, which we attributed to the electrostatic screening effect. The best emission was observed from the carbon films dominated by nanoclusters; the turn-on field was 1.6 V/μm and the emission-site density was considerably higher than 5×104 site/cm2. The emission from the nanocluster-dominated film was uniform and stable. Raman spectroscopy identified the nanoclusters as crystalline graphite with some structural defects. It is conceivable that the modification of the boning hybridization at the cluster surface resulted in a diamond-like density of states, and that the corresponding small electron affinity was responsible for the excellent emission the from nanoclusters.

Raman spectra of nano-structured carbon films synthesized using ammonia-containing feed gas

We examined the Raman spectra of nanostructured carbon films grown using the ammonia-containing feed gas at two excitation wavelengths, 514.5 and 1064 nm. The increase of the ammonia concentration in the feed gas resulted in two prominent changes: the complete morphology change (from nanoparticle dominance to nanotube dominance) and the degradation of the graphitic sheet quality. However, the Raman spectra preserved strong resemblance to those of graphite and glassy carbon and showed only systematic variation in the peak position, width, and the intensities. We noted that the ammonia-concentration-dependent broadening of D and G bands, the up-shift of D band, and the increase of the intensity ratio I(D)/I(G) of these films could be interpreted as the signature for the increase of disorder in the graphitic structure and/or the reduction of the graphitic crystallite size in accordance with the similar observation from other carbon-based films. We attributed the observed excitation-energy-dependent shift of ...

Advanced nanosphere lithography for the areal-density variation of periodic arrays of vertically aligned carbon nanofibers

Periodic arrays of vertically aligned isolated carbon nanofibers (CNFs) have been fabricated using self-assembled polystyrene spheres as shadow masks for catalyst-pattern formation. Proper use of monolayer and bilayer masks, and judicial combination of angle-deposition technique with monolayer masks have allowed us to control the dot size and spacing of catalyst patterns. As long as the catalyst-dot size is not too large, isolated single CNF has grown from each catalyst dot. Combining nanosphere lithography with conventional photolithography, we have been able to realize patterned growth of CNF arrays on selected areas.

Triode field emitter with a gated planar carbon-nanoparticle cathode

We fabricated a triode field emitter with a normal gate structure and a planar cathode of carbon nanoparticles (CNPs), which consisted of good quality graphitic sheets encapsulating metal (carbide) cores. For the quantitative analysis of the emission from the CNP triode emitter, we carried out a two-dimensional numerical calculation of electrostatic potential using the finite element method. As it turned out, a radial variation of electric field was very important to account for the emission from a planar emitting layer. By assuming the work function of 5 eV for CNPs, a set of consistent Fowler–Nordheim parameters, together with the radial position of emitting sites, were determined.

Effects of metal buffer layers on the hot filament chemical vapor deposition of nanostructured carbon films

We examined how the addition of different metal buffer layers between the Ni/Fe-alloy-catalyst layer and the silicon substrate affected the growth of nanostructured carbon films; Cr, Ti, Ta, and W were tested as buffer layers. Even when the sputter-deposition of catalytic-metal layers and the hot filament chemical vapor deposition of carbon films were carried out under the identical conditions, different buffer layers resulted in substantially different carbon-film growth. More specifically, carbon-nanoparticle films were produced with the Cr and the W buffer layers, and carbon-nanotube films were produced with the Ti and the Ta buffer layers. X-ray diffraction (XRD) showed a significant and systematic difference between the carbon-nanoparticle and carbon-nanotube films. In the case of the carbon-nanoparticle films deposited with either the Cr or the W buffer layer, the peaks corresponding to the catalytic metal, the carbide phases of the catalytic metal, and the carbide phases of the respective buffer me...

Fabrication of highly efficient and stable doped red organic light-emitting device using 2-methyl-9,10-di(2-napthyl)anthracene and tris(8-hydroxyquinolinato)aluminum as cohost materials

The authors fabricated red organic light-emitting devices using DCJTB as red dopant, and blue-emitting MADN and green-emitting Alq3 as cohost materials for emission layers. The luminance efficiency of 2% DCJTB-doped Alq3(20%)∕MADN(80%) device was 5.42cd∕A at 20mA∕cm2, while that of equivalently doped Alq3 single-host device was 1.79cd∕A, and remained over 5.2cd∕A up to 200mA∕cm2. At the benchmark luminance of 7680cd∕m2, the power efficiency of DCJTB-doped Alq3∕MADN device was 4.1 times better than that of Alq3 single-host device. Moreover, the half-decay lifetime of DCJTB-doped Alq3∕MADN device measured at an initial luminance of 1000cd∕m2 was 14000h.

Bending and bundling of metal-free vertically aligned ZnO nanowires due to electrostatic interaction.

Bending and bundling was observed from vertically aligned arrays of ZnO nanowires with flat (0001) top surfaces, which were synthesized using a vapor-phase method without metal catalysts. Sufficient evidence was found to exclude electron-beam bombardment during scanning electron microscopy as a cause for bending and bundling. We attribute the bending and bundling to electrostatic interactions due to charged (0001) polar surfaces, and also discussed the threshold surface charge densities for the bending and bundling based on a simple cantilever-bending model. Some growth features were indicative of the operation of electrostatic interactions during the growth.

White organic light-emitting devices with mixed interfaces between light emitting layers

White organic light-emitting devices with mixed interfaces between emitting layers (MI-EML WOLEDs) showed luminance and efficiency as large as 26213cd∕m2 and 9.85cd∕A. Efficiencies of MI-EML WOLEDs were about 1.5 times better than those of conventional three-EML WOLEDs for luminance of 1000–5000cd∕m2, and their half-decay lifetime showed 3.1 times improvement. Note that if the authors operate typical active-matrix mobile-phone displays based on combination of WOLED and color filters to produce standard white emission for high definition televisions and illumination sources, MI-EML WOLEDs will have advantages of 25% less power consumption and 2.8 times longer lifetime over conventional three-EML WOLEDs.

Field emission characteristics of defective diamond films

We have studied the field emission characteristics of defective diamond films grown by microwave plasma enhanced chemical vapor deposition. X-ray diffraction, Raman spectroscopy, scanning electron microscopy, atomic force microscopy, and the reflectance measurements have revealed the poor crystal quality and/or small grain sizes of the diamond phase and the inclusion of the non-diamond carbon phases in these films. The degrees of the film defectiveness have been found to depend on the methane concentration. Current-versus-voltage measurements have demonstrated that the defective diamond films have good electron emission characteristics, and, moreover, that the emission characteristics depend on the methane concentration. The observed correlation between the degrees of film defectiveness and the emission characteristics strongly suggests the defect-related electron-emission mechanism. In this study, the lower limits of the field emission current densities, estimated by averaging the total measured emission...