Gregory K. L. Goh

Influence of Channel Layer Thickness on the Electrical Performances of Inkjet-Printed In-Ga-Zn Oxide Thin-Film Transistors

Inkjet-printed In-Ga-Zn oxide (IGZO) thin-film transistors (TFTs) with bottom-gate bottom-contact device architecture are studied in this paper. The impact of the IGZO film thickness on the performance of TFTs is investigated. The threshold voltage, field-effect mobility, on and off drain current, and subthreshold swing are strongly affected by the thickness of the IGZO film. With the increase in film thickness, the threshold voltage shifted from positive to negative, which is related to the depletion layer formed by the oxygen absorbed on the surface. The field-effect mobility is affected by the film surface roughness, which is thickness dependent. Our results show that there is an optimum IGZO thickness, which ensures the best TFT electrical performance. The best result is from a 55-nm-thick IGZO TFT, which showed a field-effect mobility in the saturation region of 1.41 cm2/V·s, a threshold voltage of 1 V, a drain current on/off ratio of approximately 4.3 × 107, a subthreshold swing of 384 mV/dec, and an off-current level lower than 1 pA.

Synthesis and characterization of AgInS2–ZnS heterodimers with tunable photoluminescence

AgInS2–ZnS nanoparticles with well-defined heterodimer structures were synthesized using a hot injection method. The emission spectra of the obtained heterodimers under UV excitation could be tuned by controlling the diffusion of Zn in AgInS2. The ZnS component in the heterodimers played an important role in improving quantum yield. The obtained heterodimers demonstrated two photon fluorescence properties. The cell imaging applications of the obtained heterodimers excited by either UV or infrared were demonstrated.

Growth Kinetics and Cracking of Liquid-Phase-Deposited Anatase Films

Anatase TiO 2 films were deposited on glass slides by liquid phase deposition at temperatures of 50-200°C. All films displayed preferential c-axis orientation, and morphological examinations revealed that the films underwent a transition from continuous nucleation to grain growth. Kinetic analysis by the Hancock and Sharp method revealed that film growth was controlled by a phase boundary process before transitioning to a diffusion-controlled process. This transition was responsible for the change to a more columnar film morphology. It was also observed that the films cracked during drying when the films were above the critical thickness. Nitrogen adsorption-desorption isotherms revealed that the films contained pores of 4 nm and smaller and support a capillary-stress-induced film-cracking mechanism.

CuInZnS-decorated graphene nanosheets for highly efficient visible-light-driven photocatalytic hydrogen production

Photocatalytic H2 production from water splitting using semiconductor photocatalysts has attracted much attention due to the increasing global energy crisis. In the past few decades, numerous photocatalysts have been proposed, however, it is still a challenge to develop highly active photocatalysts for water splitting under visible light. Here we report a new composite material consisting of Cu0.02In0.3ZnS1.47 (CIZS) nanospheres and reduced graphene oxide (rGO) nanosheets as a highly active photocatalyst for hydrogen evolution under visible light. These composites were prepared through a solvothermal method in which rGO nanosheets served as a supporting material to load CIZS nanospheres. The nanocomposites demonstrated a high H2 production rate of 3.8 mmol h−1 g−1, which is about 1.84 times that of pure CIZS nanospheres under visible-light irradiation. The high H2 production rate arose from the presence of graphene, which served as an electron collector and transporter to efficiently lengthen the lifetime of photogenerated charge carriers from CIZS nanospheres. This study presents an effective approach to synthesize graphene-based nanocomposites in the field of energy conversion.

Highly efficient rutile TiO2 photocatalysts with single Cu(II) and Fe(III) surface catalytic sites

Highly active photocatalysts were obtained by impregnation of nanocrystalline rutile TiO2 powders with small amounts of Cu(II) and Fe(III) ions, resulting in the enhancement of initial rates of photocatalytic degradation of 4-chlorophenol in water by factors of 7 and 4, compared to pristine rutile, respectively. Detailed structural analysis by EPR and X-ray absorption spectroscopy (EXAFS) revealed that Cu(II) and Fe(III) are present as single species on the rutile surface. The mechanism of the photoactivity enhancement was elucidated by a combination of DFT calculations and detailed experimental mechanistic studies including photoluminescence measurements, photocatalytic experiments using scavengers, OH radical detection, and photopotential transient measurements. The results demonstrate that the single Cu(II) and Fe(III) ions act as effective cocatalytic sites, enhancing the charge separation, catalyzing “dark” redox reactions at the interface, thus improving the normally very low quantum yields of UV light-activated TiO2 photocatalysts. The exact mechanism of the photoactivity enhancement differs depending on the nature of the cocatalyst. Cu(II)-decorated samples exhibit fast transfer of photogenerated electrons to Cu(II/I) sites, followed by enhanced catalysis of dioxygen reduction, resulting in improved charge separation and higher photocatalytic degradation rates. At Fe(III)-modified rutile the rate of dioxygen reduction is not improved and the photocatalytic enhancement is attributed to higher production of highly oxidizing hydroxyl radicals produced by alternative oxygen reduction pathways opened by the presence of catalytic Fe(III/II) sites. Importantly, it was demonstrated that excessive heat treatment (at 450 °C) of photocatalysts leads to loss of activity due to migration of Cu(II) and Fe(III) ions from TiO2 surface to the bulk, accompanied by formation of oxygen vacancies. The demonstrated variety of mechanisms of photoactivity enhancement at single site catalyst-modified photocatalysts holds promise for developing further tailored photocatalysts for various applications.

Hydrothermal synthesis of sodium potassium niobate solid solutions at 200 °C

For the first time, a series of sodium potassium niobate solid solutions with compositions around the morphotropic phase boundary (MPB) are hydrothermally synthesized at 200 °C using a simple KOH and NaOH mixture with Nb2O5 as a precursor powder. Rietveld refinement of X-ray diffraction data indicated the presence of a second sodium niobate perovskite phase when the concentration of NaOH (compared to the total hydroxyl ion concentration) is above 11.7%. The presence of the second phase is attributed to the different solubilities of the intermediate potassium and sodium hexaniobate species. It is also found that heat-treating the mixed-phase powders for two hours at a temperature of 800 °C is effective in obtaining the desired single-phase solid solution with compositions near the MPB, thereby opening the way to using hydrothermal synthesis in simplifying the laborious solid-state process.

Crystallinity and Orientation of Solution Deposited Anatase TiO2 Films

The crystallinity of anatase TiO 2 material synthesized from an acidic fluoride-based solution at 60 to 200°C was raised by increasing the growth temperature by an amount that was significantly lower than that required bypostdeposition heat-treatment in air. As higher temperatures lead to more complete dehydration reactions and lower proton incorporation, the long-range attractive forces necessary for crystallinity are disrupted to a lesser degree. In addition, films grown on glass slides at 60 and 100°C exhibited preferential c-axis orientation due to restricted lateral and off-normal growth of crystallites elongated along the c-axis. As the island nucleation density was higher at 100°C, this resulted in even less lateral and off-normal crystallite growth compared to the film grown at 60°C, giving rise to a greater degree of c-axis preferred orientation for the 100°C film.

Electrical characteristics of zinc oxide-organic semiconductor lateral heterostructure based hybrid field-effect bipolar transistors

Zinc oxide-organic semiconductor lateral heterostructure based field-effect bipolar transistors (FEBTs) having heterointerfaces approximately midway between the source and drain electrodes are fabricated and characterized. These hybrid FEBTs comprise zinc oxide (ZnO) and p-channel organic semiconductors [Pentacene and α-sexithiophene (6T)] supporting electron transport and hole transport on either side of the heterojunction, respectively. Current flow in the transistor channel is established as a result of carrier injection across the heterointerface followed by recombination. In steady state, such devices possess significant populations of holes and electrons in the transistor channel and operate in bipolar mode.

Optical and Electrical Properties of Ga-Doped ZnO Single Crystalline Films Grown on MgAl2O4(111) by Low Temperature Hydrothermal Synthesis

Single crystalline gallium-doped ZnO films have been hydrothermally grown at 90°C using a ZnO seed layer on MgAl 2 O 4 (111) substrates. High resolution X-ray diffraction showed an epitaxial relationship between the ZnO single crystalline film and the spinel substrate with an out-of-plane orientation of Ga:ZnO〈001〉∥MgAl 2 O 4 〈111〉 and an in-plane orientation of Ga:ZnO[110]∥MgAl 2 O 4 [112] and Ga:ZnO[110]∥MgAl 2 O 4 [110]. The effects of the doping concentration as well as the postannealing treatments on the electrical and optical properties were examined. After thermal treatment, the carrier concentration in the 2.48% Ga-doped ZnO films was 2 orders of magnitude higher at 3.1 × 10 20 cm ―3 with a carrier mobility of 28 cm 2 /V s. The blueshift of the optical bandgap was also observed from the photoluminescence and optical absorption measurements as the doping concentration increased. This can be explained in the framework of the induced stress and Burstein―Moss effects at high carrier concentration.

High thermal stability PS-b-PEO templated mesoporous titania film

For many advanced applications, high thermal stability above 400 °C remains as a challenge for the ordered mesoporous titania films. In this work, we attempt to increase the thermal stability of mesoporous structure in titania film crystallization via PS-b-PEO block copolymer templating route. This paper reports the highly crystallized mesoporous titania film on silicon substrate thermally stable at 600 °C. The photocatalytic activity of the titania mesoporous film was also shown to be twice of that templated by F127 for degradation of methylene blue (MB). The present results also indicate that at low crystallinity, photocatalytic activity is controlled primarily by crystal perfection rather that surface area.