Seong Shan Yap

Low-temperature synthesis of indium tin oxide nanowires as the transparent electrodes for organic light emitting devices

Low-temperature growth of indium tin oxide (ITO) nanowires (NWs) was obtained on catalyst-free amorphous glass substrates at 250 °C by Nd:YAG pulsed-laser deposition. These ITO NWs have branching morphology as grown in Ar ambient. As suggested by scanning electron microscopy (SEM) and high-resolution transmission electron microscopy (HRTEM), our ITO NWs have the tendency to grow vertically outward from the substrate surface, with the (400) plane parallel to the longitudinal axis of the nanowires. These NWs are low in electrical resistivity (1.6×10⁻⁴ Ω cm) and high in visible transmittance (~90–96%), and were tested as the electrode for organic light emitting devices (OLEDs). An enhanced current density of ~30 mA cm⁻² was detected at bias voltages of ~19–21 V with uniform and bright emission. We found that the Hall mobility of these NWs is 2.2–2.7 times higher than that of ITO film, which can be explained by the reduction of Coulomb scattering loss. These results suggested that ITO nanowires are promising for applications in optoelectronic devices including OLED, touch screen displays, and photovoltaic solar cells.

Direct synthesis of nanodiamonds by femtosecond laser irradiation of ethanol

Carbon nanomaterials exhibit novel characteristics including enhanced thermal, electrical, mechanical, and biological properties. Nanodiamonds; first discovered in meteorites are found to be biocompatible, non-toxic and have distinct optical properties. Here we show that nanodiamonds with the size of <5 nm are formed directly from ethanol via 1025 nm femtosecond laser irradiation. The absorption of laser energy by ethanol increased non-linearly above 100 μJ accompanied by a white light continuum arises from fs laser filamentation. At laser energy higher than 300 μJ, emission spectra of C, O and H in the plasma were detected, indicating the dissociation of C2H5OH. Nucleation of the carbon species in the confined plasma within the laser filaments leads to the formation of nanodiamonds. The energy dependence and the roles of the nonlinear phenomenon to the formation of homogeneous nanodiamonds are discussed. This work brings new possibility for bottom-up nanomaterials synthesis based on nano and ultrafast laser physics.

Pulsed laser deposition of Si nanodots for photonic applications

Several growths of Si nanodots on Si and GaAs substrates were conducted by pulsed laser deposition (PLD) using a KrF laser of 248nm, 15ns, 12Hz and a Ti-sapphire laser of 800nm, 130fs, 1kHz at 1x10-5mbar vacuum. The laser fluencies on a Si target were varied from 3 to 32J/cm2 for the nanosecond (ns) PLD growths and 1-2.75J/cm2 for the femtosecond (fs) PLD. Wide range of nanodots from 20nm to a few micron size droplets were observed from both the ns and fs PLD. Auger electron spectroscopy of the nanodots was conducted and which indicated that the nanodots were without contamination. A technique using a mask consisting of an array of small holes was used to obtain high density nanodots with uniform size. The array of 100nm diameter holes was created by E-beam lithography. With this technique we have achieved 100nm Si dots with 300nm spacing between them, with few defects. We have observed that laser fluences closer to the ablation threshold work better for deposition using the EBL mask. In summary, we have demonstrated the growth of 100nm Si nanodots in an array with very few defects using the EBL masking technique.

Pulsed laser deposition of Al-doped ZnO films on glass and polycarbonate

Abstract. Al-doped ZnO (AZO) films were deposited on glass and polycarbonate (PC) at room temperature by using pulsed Nd:YAG laser at 355 nm. AZO thin films were obtained for both substrates at laser fluences from 2 to 5  J/cm2 in O2 partial pressure of 2.1 Pa. The effects of laser fluence on the structural, electrical, and optical properties of the films were investigated. The films with lowest resistivity and highest transmittance have been obtained at 2  J/cm2. The resistivities were 2.29×10−3  Ω cm for AZO on glass and 1.49×10−3  Ω cm for AZO on PC. With increasing laser fluence, the deposited films have lower crystallinity, higher resistivity, and smaller optical bandgap.

Comparison of measured and computed radial trajectories of plasma focus devices UMDPF1 and UMDPF0

In published literature, there has been scant data on radial trajectory of the plasma focus and no comparison of computed with measured radial trajectory. This paper provides the first such comparative study. We compute the trajectories of the inward-moving radial shock and magnetic piston of UMDPF1 plasma focus and compare these with measured data taken from a streak photograph. The comparison shows agreement with the measured radial trajectory in terms of average speeds and general shape of trajectory. This paper also presents the measured trajectory of the radially compressing piston in another machine, the UMDPF0 plasma focus, confirming that the computed radial trajectory also shows similar general agreement. Features of divergence between the computed and measured trajectories, towards the end of the radial compression, are discussed. From the measured radial trajectories, an inference is made that the neutron yield mechanism could not be thermonuclear. A second inference is made regarding the speeds of axial post-pinch shocks, which are recently considered as a useful tool for damage testing of fusion-related wall materials.

Nanostructured Diamond-Like Carbon Films Grown by Off-Axis Pulsed Laser Deposition

Nanostructured diamond-like carbon (DLC) films instead of the ultrasmooth film were obtained by pulsed laser ablation of pyrolytic graphite. Deposition was performed at room temperature in vacuum with substrates placed at off-axis position. The configuration utilized high density plasma plume arriving at low effective angle for the formation of nanostructured DLC. Nanostructures with maximum size of 50 nm were deposited as compared to the ultrasmooth DLC films obtained in a conventional deposition. The Raman spectra of the films confirmed that the films were diamond-like/amorphous in nature. Although grown at an angle, ion energy of >35 eV was obtained at the off-axis position. This was proposed to be responsible for subplantation growth of sp3 hybridized carbon. The condensation of energetic clusters and oblique angle deposition correspondingly gave rise to the formation of nanostructured DLC in this study.

Pulsed laser interference patterning of polyimide grating for dye-doped polymer laser

Abstract. Direct laser interference patterning of polyimide (PI) films was performed by using a pulsed 355-nm laser. At laser fluence of 0.4  J/cm2, gratings with spatial periods of 3.8  μm to 344 nm were created. The highest aspect ratio of the grating structure (0.8) was obtained for the 344-nm grating. An all-polymer dye laser was then fabricated by spin-coating a layer of disodium fluorescein (DF)-doped polyvinyl alcohol (PVA) film on bare and patterned PI substrate. Green laser emission was obtained when transversely pumped by a 355-nm laser. The lasing threshold reduced by ∼10 times for the sample with 344-nm grating while the laser intensity was ∼18 times higher. The enhancements are ascribed to the 344-nm grating structures, which act as an efficient distributed feedback resonator and distributed Bragg reflector grating for DF-doped PVA emitting at ∼563  nm, on top of being a passive light-trapping structures.

Effects of Laser Wavelength and Fluence in Pulsed Laser Deposition of Ge Films

Nanosecond lasers with ultra‐violet, visible and infrared wavelengths: KrF (248 nm, 25 ns) and Nd:YAG (1064 nm, 532 nm, 355 nm, 5 ns) were used to ablate polycrystalline Ge target and deposit Ge films in vacuum (<10‐6 Torr). Time‐integrated optical emission spectra were obtained for laser fluence from 0.5–10 J/cm2. Neutrals and ionized Ge species in the plasma plume were detected by optical emission spectroscopy. Ge neutrals dominated the plasma plume at low laser fluence while Ge+ ions above some threshold fluence. The deposited amorphous thin‐film samples consisted of particulates of size from nano to micron. The relation of the film properties and plume species at different laser fluence and wavelengths were discussed.

Pulsed laser produced plasma for self-assembled growth of Al-doped ZnO nanostructures at room temperature

Summary form only given. ZnO based nanomaterials attract tremendous interest for applications in optoelectronics and sensors. Among the various chemical and physical deposition methods, laser produced plasma enable the deposition at relatively low or room temperature because of the capability to produce vapor/species with a wide range velocity and energy. The active plasma constituent may also be beneficial in promoting the formation of nucleation site for nanomaterials growth. In this work, a 355 nm and 532 nm laser is used for plasma plume generation from an Al-doped ZnO (AZO) target in different background gas while Si and glass substrates were held at room temperature to ensure that no external thermal energy was involved in materials growth. The presence of ambient gas condition during deposition affect the plasma generation and expansion mechanisms and can results in distinctly different growth mode. O2 background gas at optimized pressure preserved the stoichiometry of the final AZO films, while heavier or large gas molecules such as Ar effectively confined the plasma plume and increase probability of collisions. In addition, Ar gas may also enhance the generation of zinc suboxide species, which is important to the formation of ZnO nanostructure. The effects of different background gas on the self-assembled growth of AZO nanostructure in laser ablated plasma are discussed.

Ion jet produced by anomalous resistance in plasma focus discharge

Summary form only given. The dense plasma focus (DPF) device belongs to the category of noncylindrical Z-pinch facility that produced pinched plasma of very high temperature and high density by electromagnetic acceleration and compression. The plasma pinch produced in few mbar of gas filling exhibited a large range of complex physical phenomena which is also an abundant source of multiple pulsed radiations such as neutrons, ion beam and electron beam, soft and hard X-ray. In this work, ion beam is generated by a 3.3 kJ Mather type plasma focus operated in less than 1 mbar of deuterium filling. The electrode geometry of the plasma focus tube has been optimized to allow the matching of the electrical characteristic time and current sheath dynamic time. Reproducible pinch is observed in low pressure discharge where the current drops for about 26% from the peak value during pinch and last for more than 350 ns. This indicates a large portion of energy has been drawn by the pinch action. This phenomenon is less significant in normal plasma focus discharge operated at higher operation pressures. The ion beam energy measured is found correlated closely to the magnitude of current drop. The instabilities onset during the pinch resulting anomalous resistance that accelerates ions and electrons beams to intense high energy. This effect was found to be enhanced in lower pressure operation with prolong current drop. At discharge voltage of 15 kV, average ion beam of 20-50 keV were measured. The ion beam was determined by using two biased ion collectors for time-offlight measurement, solid state nuclear track detectors for angular distribution measurement. These results confirmed the presence of a large accelerating electric field that could be due to the intensified instabilities at low pressure regime.