Fu-Ming Pan

Self-organized titanium oxide nanodot arrays by electrochemical anodization

Ordered nanodot arrays of titanium oxides were prepared from TiN/Al films on the silicon substrate by electrochemical anodization of a TiN layer using a nanoporous anodic aluminum oxide film as the template. The microstructure of the nanodot arrays was studied by transmission electron microscopy and scanning electron microscopy, and the chemical composition of nanodots was analyzed by electron energy loss spectroscopy. The as-prepared nanodots are basically composed of amorphous TiOx with a hexagonal arrangement and an average diameter of about 60 nm. Using this approach, it is expected that nanodot arrays of various oxide semiconductors can be achieved.

Plasma treatments of molecularly templated nanoporous silica films

National Nano Device Laboratories, Hsinchu 300, TaiwanOxygen plasma has been successfully utilized to remove the organic template in molecularly templated nanoporous silica films.The resulting nanoporous silica film has a high porosity and a surface roughness of less than 10 A. The hydrophilic film can bemodified to be hydrophobic by reacting with hexamethyldisilazane~HMDS!. Reductive plasmas, such as hydrogen and ammoniaplasmas, have been applied to reduce the leakage current of the nanoporous silica film without deteriorating the dielectricproperties. The dielectric constant of the silica film can be reduced to 1.7 after an HMDS/H

Facile approach to the synthesis of 3D platinum nanoflowers and their electrochemical characteristics

Three-dimensional (3D) platinum nanoflowers have been successfully synthesized by potentiostatic pulse plating method on a silicon substrate, and electrochemical study shows that the nanostructured Pt catalyst has an excellent electrocatalytic activity toward methanol and CO oxidation due to preferential (100) and (110) surface orientations on the Pt nanoflowers.

Interfacial energy levels and related properties of atomic-layer-deposited Al2O3 films on nanoporous TiO2 electrodes of dye-sensitized solar cells

Low-temperature (approximately 150 degrees C), atomic-layer-deposited Al(2)O(3) films on nanoporous TiO2 electrodes of dye-sensitized solar cells (DSSCs) were investigated using electron spectroscopy. The power conversion efficiency (PCE) of the DSSCs was increased from 5.7% to 6.5%, an improvement of 14%, with one monolayer of Al(2)O(3) with a thickness of approximately 0.2 nm. The formation of Ti-O-Al(OH)(2) and interfacial dipole layers exhibited a strong influence on the work function of the Al(2)O(3) over-layers, while the thicker Al(2)O(3) over-layers caused the values of valence band maximum and band gap to approach the values associated with pure Al(2)O(3). A work function difference (Delta Phi(A-T)) of 0.4 eV and a recombination barrier height (epsilon(RB)) of 0.1 eV were associated with the highest PCE achieved by the first monolayer of the Al(2)O(3) layer. Thicker Al(2)O(3) over-layers, however, caused significant reduction of PCE with negative Delta Phi(T-A) and increased interfacial energy barrier height ((*)epsilon(IB)) between the N719 dyes and TiO2 electrodes. It was concluded that the PCE of the DSSCs may correlate with Delta Phi(A-T), epsilon(RB), and (*)epsilon(IB) resulting from various thicknesses of the Al(2)O(3) over-layers and that interfacial reactions, such as the formation of Ti-O-Al(OH)(2) and dipole layers, play an important role in determining the interfacial energy levels required to achieve optimal performance of dye-sensitized TiO2 solar cells.

Preparation and phase transformation of highly ordered TiO2 nanodot arrays on sapphire substrates

Ordered nanodot arrays of titanium oxide (TiO2) were prepared from an epitaxial Al/TiN bilayered film on a sapphire substrate by electrochemical anodization of the TiN layer using a nanoporous anodic aluminum oxide (AAO) film as the template. The nanodots with an average diameter of about 60 nm can faithfully duplicate the size, shape, and hexagonal pore pattern of the AAO nanopores. The phase development of the isolated TiO2 nanodots is very much different from TiO2 thin films and powders. After high temperature annealing, the nanodots are polycrystalline and consist of a mixed phase of anatase and rutile instead of single rutile phase. We expect that TiO2 nanodots with a single phase of anatase can be realized as long as the size of the nanodots is smaller than the critical nuclei size for rutile formation.

Facile synthesis of continuous Pt island networks and their electrochemical properties for methanol electrooxidation.

A two-dimensional continuous Pt island network was successfully synthesized by pulse-potentiostatic electrodeposition on a flat silicon substrate, which showed markedly enhanced catalytic activity toward methanol electrooxidation and high CO tolerance, probably due to the synergistic effect of the Pt island catalyst and surrounding SiO(2) surface layer.

Field emission of carbon nanotubes on anodic aluminum oxide template with controlled tube density

The tube number density of aligned carbon nanotubes (CNTs) grown over the nanoporous anodic aluminum oxide (AAO) template can be directly controlled by adjusting the CH4∕H2 feed ratio during the CNT growth. We ascribe the variation of the tube density as a function of the CH4∕H2 feed ratio to the kinetic competition between outgrowth of cobalt-catalyzed CNTs from the AAO pore bottom and deposition of the amorphous carbon (a-C) overlayer on the AAO template. A pore-filling ratio of 18% to 82% for the nanotubes overgrown out of nanopores on the AAO template can be easily achieved by adjusting the CH4∕H2 feed ratio. Enhanced field emission properties of CNTs were obtained by lowering the tube density on AAO. However, at a high CH4 concentration, a-C by-product deposit on the CNT surface can degrade the field emission property due to a high energy barrier and significant potential drop at the emission site.

Amorphous Carbon Coated Silicon Nanotips Fabricated by MPCVD Using Anodic Aluminum Oxide as the Template

We have used nanoporous anodic aluminum oxide (AAO) as a template to fabricate amorphous carbon (a-C) coated silicon nanotips by microwave plasma chemical vapor deposition (MPCVD). During the preparation of the well-ordered AAO pore channel array, an underlying TiN layer was anodically oxidized as well in the late stage of the AAO anodization, forming titanium oxide nanomasks for Si nanotip fabrication. The titanium oxide nanomasks were then used to transfer the arrangement pattern of the AAO pore channel array to the Si substrate by plasma etch in the MPCVD system and, therefore, a well-ordered Si nanotip array was produced. An α-C layer ∼5 nm thick was in situ deposited on the Si nanotips during the MPCVD process. The α-C layer was rich in nanocrystalline graphitic carbons according to Raman and Auger electron spectroscopies. The nanocrystalline graphitic carbons in the coating and the sharp tip shape made the Si nanotip a good field emitter and a field enhancement factor.

Nanogap formation by palladium hydrogenation for surface conduction electron emitters fabrication

Nanometer-scale gaps in Pd strips are obtained by hydrogen absorption under high pressure treatment. The resulting lattice constant increase due to the Pd phase transformation after hydrogen uptake results in a large compressive stress on the thin Pd films. Under proper geometric arrangement of the Pd electrode within a surface conduction electron (SCE) emitter structure, a single nanogap per SCE device is obtained. A turn-on voltage of 41V is observed for emitters with a 25nm gap.

Field-emission properties of novel palladium nanogaps for surface conduction electron-emitters

We explore novel nanometer-scale gaps with different widths in palladium (Pd) thin-film strips using hydrogen absorption under high-pressure conditions and different temperatures. Both the experimental measurement and numerical calculation are conducted to examine the electron conduction properties of the newly proposed surface conduction electron-emitters (SCEs). It is shown that this novel structure exhibits a high emission efficiency, so that a low turn-on voltage of 40 V for an SCE with a 30 nm nanogap is obtained. A calibrated model is adopted to predict the effects of the emitter thickness and different material work functions on emission current with different width of nanogaps. It is found that the heightened thickness increases the emission current. However, it tends to saturate for smaller nanogaps. The decrement of work function is proportional to the increase in emission current, which is independent of the width of nanogap.