Franklin Chau-Nan Hong

Enhanced hole injections in organic light-emitting devices by depositing nickel oxide on indium tin oxide anode

An ultrathin layer of nickel oxide (NiO) was deposited on the indium tin oxide (ITO) anode to enhance the hole injections in organic light-emitting diode (OLED) devices. A very low turn-on voltage (3 V) was actually observed for the device with NiO on ITO. The enhancement of hole injections by depositing NiO on the ITO anode was further verified by the hole-only devices. The excellent hole-injection ability of NiO was also demonstrated by devising a device with patterned NiO on the ITO anode. Our results suggest that the NiO/ITO anode is an excellent choice to enhance hole injections of OLED devices.

Hydrogen-doped high conductivity ZnO films deposited by radio-frequency magnetron sputtering

Hydrogen-doped zinc oxide (ZnO:H) films were deposited by rf magnetron sputtering as transparent conductive films. The resistivity of ZnO:H film was significantly reduced by the addition of H2 in Ar during rf sputtering. The electrical resistivity of ZnO:H films reached 2×10−4Ωcm. The carrier concentration increased with increasing H2 concentration during deposition. X-ray diffraction results showed that the d0002 interplanar spacing increased with increasing H2 concentrations. The carrier concentration was significantly reduced in two orders of magnitude by increasing the substrate temperature from 150 to 250 °C during deposition. Both results suggested that the increase of carrier concentration by adding H2 during sputtering was due to the hydrogen donor rather than the oxygen vacancies in ZnO films, consistent with the theoretical predictions by Van de Walle. UV–visible spectroscopy further showed that the transmittance is high up to 100% in the visible range. The band gap determined by optical absorpt...

Enhanced performance of organic light-emitting devices by atmospheric plasma treatment of indium tin oxide surfaces

Atmospheric plasma treatment of indium tin oxide (ITO) surfaces has been studied and demonstrated to be the most efficient method in improving the performance of vacuum-deposited double-layer organic light-emitting diode devices, among various plasma treatment methods including low-pressure Ar plasma and low-pressure O2 plasma treatment. Although with a current–voltage characteristic close to low-pressure O2 plasma treatment, the atmospheric plasma treatment exhibits a 40% increase of electroluminescence efficiency. X-ray photoelectron spectroscopy results show that the atmospheric plasma treatment increases the work function and reduces the carbon contamination of ITO surfaces. Our results suggest that atmospheric plasma treatment is a cheaper, more convenient, and more efficient method than low-pressure O2 plasma treatment for improving device performance.

Visible electroluminescence from silicon nanocrystals embedded in amorphous silicon nitride matrix

Visible electroluminescence from silicon nanocrystals (Si-NCs) embedded in amorphous silicon nitride (a‐SiNx) films has been observed. The Si‐NC∕a‐SiNx films were deposited by evaporating silicon from electron gun into the inductively coupled plasma of nitrogen. The density of Si-NCs in the a‐SiNx matrix was around 1012cm−2. Strong room temperature photoluminescence was observed in 2.8 and 3.0eV, different from literature values. The electroluminescence (EL) devices were fabricated with Si‐NCs∕a‐SiNx film as the active layer using the Al or Ca∕Ag cathode and the indium tin oxide anode. Through tunneling, the electrons and holes were respectively injected from the cathode and anode into Si-NCs and confined within Si-NCs for light emission by the high band gap a‐SiNx matrix. For the device with Ca∕Ag cathode, the turn-on voltage was as low as 10V and the EL efficiency was about 1.6×10−1 Cd∕A. The EL spectra consisted of two broad peaks centered around 2.5 and 2.8eV. Our results demonstrate that Si‐NCs∕a‐SiN...

Diamond-like carbon nanocomposite films

Diamond-like carbon (DLC) nanocomposite films were deposited at room temperature by inductively coupled plasma chemical vapor deposition using hexamethyldisilane (HMDS), hexamethyldisilazane (HMDSN), and hexamethyldisiloxane (HMDSO) precursors. High-resolution transmission electron microscopy showed that all the films contained nanoparticles. The DLC nanocomposite films deposited by HMDS contained hollow spherical nanocrystallites, called nanoballs, of hexagonal silicon carbide. The nanocomposite films deposited by HMDSN contained crystalline Si3N4 nanoparticles. The nanocomposite films deposited by HMDSO contained amorphous SiOx nanoparticles. Although both types of films had similar hardness, the DLC nanocomposite films exhibited much lower compressive stresses than the DLC films deposited by methane, i.e., 1.5 vs 11 GPa, respectively. Through the enhancement of gas phase reactions, the inductively coupled plasma should be responsible for the formation of nanoparticles in the nanocomposite films.

The effects of pre-treatment and catalyst composition on growth of carbon nanofibers at low temperature

Abstract Carbon nanofiber films have been grown on Fe–Ni alloy catalysts at 400°C by a thermal chemical vapor deposition (CVD) method using acetylene as a carbon source. Fe–Ni–C alloy catalysts were found to be more suitable for carbon nanofiber growth at 400°C than pure Ni or Fe–Ni catalysts. When Fe–Ni catalysts were pre-treated in hydrogen at a low temperature (400°C), the catalyst lost its activity in growing carbon nanofibers at 400°C by forming an amorphous carbon layer covering catalytic sites on the surface. The growth rate of carbon nanofibers was improved significantly by pre-treating the catalyst with a low concentration of carbon source (e.g. acetylene, carbon monoxide) forming Fe–Ni–C alloy during the temperature-rising step before growth. Carbon nanofiber walls consisted of imperfect graphite structure as characterized by transmission electron microscopy.

Adhesion improvements for diamond-like carbon films on polycarbonate and polymethylmethacrylate substrates by ion plating with inductively coupled plasma

Abstract An inductively coupled plasma (ICP) ion plating method using hexamethyldisiloxane (HMDSO) and oxygen reactants was employed to deposit hard diamond-like carbon (DLC) coatings on polycarbonate (PC) and polymethylmethacrylate (PMMA) substrates. O2 plasma was the most effective pretreatment method for improving the adhesion of the DLC on the polymer by removing the contaminants and forming the polar functional groups on the surfaces. The adhesion of the DLC film on the PMMA substrate was always much worse than that on the PC substrate. To improve the DLC film adhesion on the PMMA substrate, interfacial layers were deposited by remote plasma polymerization in order to avoid the destruction of the functional groups in the monomer precursors. Interfacial layers deposited using HMDSO/C2H5OH mixture, methylmethacrylate and propylene oxide monomers were successful to improve the adhesion of the DLC films on the PMMA from 1A (with no interfacial layer) to 4A–5A, as measured by the X-cut method. The adhesion abilities of the interfacial layers were strongly dependent on the gas phase reactions during depositions. To understand the critical roles of the interfacial layers, the effects of the deposition conditions as well as the structures of the interfacial layers were studied.

The effect of the structure of molybdenum oxides on the selective oxidation of methanol

Abstract Selective oxidation of methanol to formaldehyde has been studied over a series of molybdates, molybdenum oxides, mixed tungsten - molybdenum oxides, and supported molybdenum oxide. Reactor studies, Temperature Programmed Reaction (TPR) and in-situ FTIR have shown that water and methanol adsorb dissociatively and reversibly on the same catalyst sites and that the rate determining step is the breaking of a carbon-hydrogen bond. The reaction is of a redox type and a nearly fully oxidized catalyst is most active and selective. The difference in activity between molybdenum trioxide and ferric molybdate can be explained by a mechanism in which formation of adsorbed methoxys does not occur on the predominant (010) face of MoO 3 . Oxide-support interactions are responsible for the lower selectivity of silica supported catalysts.

The fabrication of ZnO nanowire field-effect transistors by roll-transfer printing

A method with the potential to fabricate large-area nanowire field-effect transistors (NW-FETs) was demonstrated in this study. Using a high-speed roller (20-80 cm min(-1)), transfer printing was successfully employed to transfer vertically aligned zinc oxide (ZnO) nanowires grown on a donor substrate to a polydimethylsiloxane (PDMS) stamp and then print the ordered ZnO nanowire arrays on the received substrate for the fabrication of NW-FETs. ZnO NW-FETs fabricated by this method exhibit high performances with a threshold voltage of around 0.25 V, a current on/off ratio as high as 10(5), a subthreshold slope of 360 mV/dec, and a field-effect mobility of around 90 cm(2) V(-1) s(-1). The excellent device characteristics suggest that the roll-transfer printing technique, which is compatible with the roll-to-roll (R2R) process and operated in atmosphere, has a good potential for the high-speed fabrication of large-area nanowire transistors for flexible devices and flat panel displays.

Radio-frequency microdischarge arrays for large-area cold atmospheric plasma generation

By flowing gases through arrays of microhollow cathode holes, large area (12 mm in diameter) uniform and stable discharges could be generated by a rf power supply. Both the rf power and the gas flow through the cathode holes played key roles in maintaining uniform and stable discharges. The discharges could be stable for a period longer than one hour in pure helium (He) and in He containing 1% hexamethyldisiloxane (HMDSO). By using a third steel electrode biased with a pulse power supply (100 kHz, 50% duty cycle), the plasma from arrays of cathode holes could be extended to 20 mm in length. Amorphous carbon films deposited by the extended atmospheric plasma using 1% HMDSO/He reactants exhibited the same structure as those by low pressure plasma chemical vapor deposition.