Alfred Iing Yoong Tok

Fabrication of Single‐ and Multilayer MoS2 Film‐Based Field‐Effect Transistors for Sensing NO at Room Temperature

Single- and multilayer MoS(2) films are deposited onto Si/SiO(2) using the mechanical exfoliation technique. The films were then used for the fabrication of field-effect transistors (FETs). These FET devices can be used as gas sensors to detect nitrous oxide (NO). Although the single-layer MoS(2) device shows a rapid response after exposure to NO, the current was found to be unstable. The two-, three-, and four-layer MoS(2) devices show both stable and sensitive responses to NO down to a concentration of 0.8 ppm.

Lanthanide-doped NaxScF3+ x nanocrystals : crystal structure evolution and multicolor tuning

Rare-earth-based nanomaterials have recently drawn considerable attention because of their unique energy upconversion (UC) capabilities. However, studies of Sc(3+)-based nanomaterials are still absent. Herein we report the synthesis and fine control of Na(x)ScF(3+x) nanocrystals by tuning of the ratio of oleic acid (OA, polar surfactant) to 1-octadecene (OD, nonpolar solvent). When the OA:OD ratio was increased from low (3:17) to high (3:7), the nanocrystals changed from pure monoclinic phase (Na(3)ScF(6)) to pure hexagonal phase (NaScF(4)) via a transition stage at an intermediate OA:OD ratio (3:9) where a mixture of nanocrystals in monoclinic and hexagonal phases was obtained and the coexistence of the two phases inside individual nanocrystals was also observed. More significantly, because of the small radius of Sc(3+), Na(x)ScF(3+x):Yb/Er nanocrystals show different UC emission from that of NaYF(4):Yb/Er nanocrystals, which broadens the applications of rare-earth-based nanomaterials ranging from optical communications to disease diagnosis.

Quantum‐Dot‐Sensitized TiO2 Inverse Opals for Photoelectrochemical Hydrogen Generation

A new nanoarchitecture photoelectrode design comprising CdS quantum-dot-sensitized, optically and electrically active TiO(2) inverse opals is developed for photoelectrochemical water splitting. The photoelectrochemical performance shows high photocurrent density (4.84 mA cm(-2) at 0 V vs. Ag/AgCl) under simulated solar-light illumination.

Surface modifications of ZnO quantum dots for bio-imaging

The wide bandgap and large exciton binding energy of ZnO may generate new applications in bio-imaging after careful surface modifications. Formation of chemically pure ZnO colloidal nanocrystals with controlled size without unwanted by-products or agglomeration has been the major challenge to fully utilize ZnOs unique properties. In this research, colloidal ZnO nanocrystals were synthesized by a soft chemical method. Particle size and colloidal stability were controlled by capping agents. Influences of the surface modifications on particle size, size distribution and photoluminescence properties were investigated. Pure ZnO showed high intensity UV emission and very low intensity in the visible range, indicating good surface morphology of the ZnO nanoparticles with little surface defects. Transmission electron microscopy (TEM) analysis revealed that the capped crystals were close to spherical shape with single-crystal size about 6 nm. X-ray diffraction (XRD) analyses revealed single-phase ZnO nanoparticles. For bio-imaging, emission in visible wavelength range is preferred. Both TiO2 and SiO2 were effective in shifting the emission peak to the visible range with high intensity. Optimum capping thickness is 0.5 nm. ZnO–TiO2 quantum dots (QDs) showed good bio-imaging capability on plant cells. Quantum yields of the pure ZnO and TiO2 capped ZnO were measured and compared to commercial fluorescence materials.

Photon Upconversion in Hetero‐nanostructured Photoanodes for Enhanced Near‐Infrared Light Harvesting

A hetero-nanostructured photoanode with enhanced near-infrared light harvesting is developed for photo-electrochemical cells. By spatially coating upconversion nanoparticles and quantum dot photosensitizers onto TiO2 inverse opal, this architecture allows direct irradiation of upconversion nanoparticles to emit visible light that excites quantum dots for charge separation. Electrons are injected into TiO2 with minimal carrier losses due to continuous electron conducting interface.

TiO2 inverse-opal electrode fabricated by atomic layer deposition for dye-sensitized solar cell applications

TiO2 inverse opals (TIO) fabricated by the atomic layer deposition (ALD) technique showed a superior infiltration result when compared to those fabricated by the conventional nanoparticles-infiltration method reported in previous studies. The ALD can achieve high filling fractions of more than ca. 96% of the maximum possible infiltration by conformal filling of 288, 390 and 510 nm opals, giving rise to high quality TIO. The photoelectrochemical performances of the ALD-fabricated TIO photoanodes of different sizes are investigated systematically for the first time in dye-sensitized solar cells (DSCs). When the TIO with a size of 288 nm was used as photoanode and indoline dye as a sensitizer in DSCs, the power conversion efficiency of the cell could attain 2.22% (Air Mass 1.5). It is found that the efficiency increases with decreasing lattice size of TIO electrode due to the larger surface area for dye loading. Owing to the selective reflectivity of the inverse opal, IPCE spectra of TIO electrodes revealed a strong wavelength dependence. Strategies relating to the characteristics of selective reflection and the design of composite photoanodes to enhance the efficiency of DSCs are discussed.

Homogeneous Photosensitization of Complex TiO2 Nanostructures for Efficient Solar Energy Conversion

TiO2 nanostructures-based photoelectrochemical (PEC) cells are under worldwide attentions as the method to generate clean energy. For these devices, narrow-bandgap semiconductor photosensitizers such as CdS and CdSe are commonly used to couple with TiO2 in order to harvest the visible sunlight and to enhance the conversion efficiency. Conventional methods for depositing the photosensitizers on TiO2 such as dip coating, electrochemical deposition and chemical-vapor-deposition suffer from poor control in thickness and uniformity, and correspond to low photocurrent levels. Here we demonstrate a new method based on atomic layer deposition and ion exchange reaction (ALDIER) to achieve a highly controllable and homogeneous coating of sensitizer particles on arbitrary TiO2 substrates. PEC tests made to CdSe-sensitized TiO2 inverse opal photoanodes result in a drastically improved photocurrent level, up to ~15.7 mA/cm2 at zero bias (vs Ag/AgCl), more than double that by conventional techniques such as successive ionic layer adsorption and reaction.

A Novel Photoanode with Three‐Dimensionally, Hierarchically Ordered Nanobushes for Highly Efficient Photoelectrochemical Cells

In recent years, photoelectrochemical (PEC) cells have attracted worldwide attention as cheap alternatives to conventional devices for solar energy conversion. Crucial to the light harvesting and conversion effi ciency of a PEC cell is a nanostructured photoanode, in which the incident photons are captured, electron–hole pairs are generated, and the subsequent electron transfer takes place. [ 1 , 2 ] To realize highly effi cient PEC cells, a nanostructured photoanode should possess several favorable intrinsic characteristics, such as adequate specifi c surface area to permit high photosensitizer loading (in the case of TiO 2 ), direct electron transport pathways for long electron diffusion length, and strong light scattering to promote the light harvesting ability by confi ning the light within the cell. [ 3–6 ] It is thus highly desirable to develop a photoanode that meets all the above requirements. Towards this goal, immense efforts have been concentrated on tailoring the nanometer-scale features of photoanode materials. [ 7 ] Nanoparticle fi lms provide very high surface areas to increase the amount of sensitizer loading, but they lack direct electrical contacts and light-scattering ability. [ 8 , 9 ]

Applications of atomic layer deposition in solar cells

Atomic layer deposition (ALD) provides a unique tool for the growth of thin films with excellent conformity and thickness control down to atomic levels. The application of ALD in energy research has received increasing attention in recent years. In this review, the versatility of ALD in solar cells will be discussed. This is specifically focused on the fabrication of nanostructured photoelectrodes, surface passivation, surface sensitization, and band-structure engineering of solar cell materials. Challenges and future directions of ALD in the applications of solar cells are also discussed.

Photoluminescence phenomena of Ce3+-doped Y3Al5O12 nanophosphors

The photoluminescence phenomena of Ce3+-doped Y3Al5O12 nanophosphors synthesized by the chemical gelation were investigated and compared with bulk phosphors. The oxidation state of the cerium ions in the nanophosphors was determined to be trivalent. This is an essential characteristic of phosphor’s emission. The Stokes shift for the nanophosphors was less than the bulk phosphors, which indicates that the nanophosphors had restricted surroundings. The photoionization effect was observed for the nanophosphors where the 5d electrons were lost to the conduction band and trapped at the surface-defective sites. Another observation was the high concentration of surface defects on the nanophosphors. These surface defects were investigated using the high resolution transmission electron microscope.