Swee Yong Pung

New Insights on the Burstein-Moss Shift and Band Gap Narrowing in Indium-Doped Zinc Oxide Thin Films

The Burstein-Moss shift and band gap narrowing of sputtered indium-doped zinc oxide (IZO) thin films are investigated as a function of carrier concentrations. The optical band gap shifts below the carrier concentration of 5.61 × 1019 cm-3 are well-described by the Burstein-Moss model. For carrier concentrations higher than 8.71 × 1019 cm-3 the shift decreases, indicating that band gap narrowing mechanisms are increasingly significant and are competing with the Burstein-Moss effect. The incorporation of In causes the resistivity to decrease three orders of magnitude. As the mean-free path of carriers is less than the crystallite size, the resistivity is probably affected by ionized impurities as well as defect scattering mechanisms, but not grain boundary scattering. The c lattice constant as well as film stress is observed to increase in stages with increasing carrier concentration. The asymmetric XPS Zn 2p3/2 peak in the film with the highest carrier concentration of 7.02 × 1020 cm-3 suggests the presence of stacking defects in the ZnO lattice. The Raman peak at 274 cm-1 is attributed to lattice defects introduced by In dopants.

Photocatalytic Degradation of Rhodamine B Using MnO2 and ZnO Nanoparticles

The oxide semiconductor with different band gap energy i.e. manganese dioxide (MnO2) (Eg: 1.30eV) and zinc oxide (ZnO) (Eg: 3.37 eV) nanoparticles were used to degrade RhodamineB (RhB) under irradiation of UV light (254 nm).The MnO2 nanoparticles were synthesized by hydrothermal method (160 °C, 4 h) using analytical grade manganese sulfate hydrate and potassium permanganate (VII) as precursors. The XRD analysis reveals that the nanoparticles were d-MnO2.The photocatalytic study shows that degradation of RhB solution by ZnO nanoparticles (rate constant: 0.02749 min-1) was approximately four times faster than the d-MnO2 nanoparticles (rate constant:0.0067 min-1). This observation could be attributed to the higher reducing and oxidizing power of ZnO in producing free radicals for photodegradation of RhB solution.

Insights on semiconductor-metal transition in indium-doped zinc oxide from x-ray photoelectron spectroscopy, time-of-flight secondary ion mass spectrometry and x-ray diffraction

ZnO thin films doped with various amounts of In impurities were prepared by magnetron sputtering at a substrate temperature of 150°C. The shift in optical bandgap of the In-doped ZnO films is studied as a function of carrier concentration. Nominally doped ZnO films exhibit an increase in the measured optical band gap known as the Burstein-Moss effect. Dominant band gap narrowing is observed with increased doping. XPS and TOFSIMS analyses confirm that In is incorporated in the ZnO material. The In 3d peaks show that no metallic In is present as a result of heavy doping. The XRD phase analysis shows a preferential c-axis growth but a shift of the ZnO (002) peak to lower 2-theta values with increasing FWHM as the carrier concentration increases indicates the decline in the quality of crystallinity. An elongation of the c lattice constant is also observed and is likely to be caused by intersitital In as the amount of In dopants increases. The incorporation of In induces a semiconductor-metal transition betwee...

Effect of CVD Synthesis Parameters on the Growth of Catalyst-Free ZnO NRs

The main development of ZnO nanorods (NRs) is focused on the gold catalyst and heteroepitaxial approach.However, the presence of Au may generate undesired deep level traps in the ZnO bandgap, which could be very harmful to the performance of transistors. The objective of this study is to synthesize ZnO NRs via homoepitaxial growth without using foreign catalyst by Chemical Vapour Deposition (CVD) technique. The growth of catalyst-free ZnO NWs at different CVD synthesis parameters such as amount of Zn powder, substrate location and synthesis duration on the catalyst-free ZnO NRs were studied systematically. The effect of these parameters on the size and areal density of ZnO NRs provided a better understanding on the growth mechanism of NRs via the Vapour-Solid (VS) mechanism.

Growth Mechanism of Au-Catalyzed Zno Nanowires: VLS or VS-VLS?

A systematic study was carried out to study the effect of process parameters on the growth of Au-catalyzed ZnO nanowires (NWs). Growth of Au-catalyzed ZnO NWs could be mainly occurred at the tip or at the base of NWs. This study provided useful information in determining the process window for the tip-growth Au-catalyzed ZnO NWs. Besides, a generic growth mechanism, i.e. a combination of Vapor-Liquid-Solid and Vapor-Solid (VLS and VS) mechanism is proposed to explain the tip-growth and base-growth Au-catalyzed ZnO NWs.

Ex Situ Doping of ZnO Nanorods by Spray Pyrolysis Technique

An ultraviolet (UV) shielding agent based on Fe-doped zinc oxide nanorods (ZnONRs) was synthesized by ex-situdoping using spray pyrolysis technique. These Fe-doped ZnO NRs could reduce the inherent photocatalytic activity of zinc oxide while still maintaining their ultraviolet filtering capability. In this work, the effect of doping duration by spray pyrolysis technique on the optical property and photocatalytic efficiency of ZnO NRs was studied. The room temperature photoluminescence (PL) analysis on the Fe-doped ZnO NRs indicates the red-shift of violet emission peak, i.e. from 378.97 nm (undoped) to 381.86 nm (60 mins.doping). Besides, the reduction of IUV/Vis ratio of PL reveals that the ex-situ Fe doping deteriorated the crystal quality of ZnO NRs. The photocatalytic study shows that the rate constant of Fe-doped ZnO NRs was smaller than the undoped ZnO NRs. It means that the Fe-doped ZnO NRs were less effective in degrading the RhB solution.

Growth of (002)–oriented ZnO thin films on largely lattice–mismatched substrates using atomic layer deposition

ZnO thin films have been successfully deposited by Atomic Layer Deposition (ALD) using Diethylzinc (DEZn) and water (H2O) as precursors. The preferred orientations of the thin films were found to be strongly dependent on the deposition temperature. (100)–oriented ZnO thin films were grown in the temperature range of 155 to 220°C, whereas (002)–oriented ZnO thin films were formed between 220 to 300°C. It is worth mentioning that ALD technique allowed ZnO thin films with preferred orientation, i.e. (002), to be deposited on both Si and glass substrates which have a large lattice mismatch to ZnO. This process capability could be attributed to the unique characteristics of a slow growth rate due to the self–limiting growth and a relatively high deposition temperature (220–300°C) which provided sufficient energy for Zn and O atoms to migrate towards in the ALD process. Besides, the (002)–oriented ZnO thin films have the best crystal quality and lowest resistivity. The thickness of as–deposited films could be controlled at nanoscale as the growth rate was proportional to the ALD process cycles.

Effect of Co-Dopant on the Phase Structure and Photoluminescence Properties of Sr2mgsi2O7: En2+ Phosphors

The role of co-dopants, i.e. Dy, Ce, Tb or Y in SMS : Eu, RE3+ phosphor sintered using solid state reaction method at 1250°C under forming gas was studied. All SMS phosphors exhibited tetragonal Sr2MgSi2O7 as the domain phase and monoclinic SrSiO3 as the minor phase based on XRD analysis. It is noted that only single blue emission (469nm) contributed by Eu2+ ions was detected in RTPL measurement regardless types of co-dopants introduced during the sintering process. This result indicates that the co-dopants did not act as luminance centres in the SMS phosphor. The time-decay PL and temperature dependent PL analysis suggest that Ce3+ ions acted as sensitizer which improved the luminescence intensity and afterglow property of SMS phosphor. The present of Dy3+ ions pro-longed the afterglow property by introducing more traps density in the SMS phosphor structure. The co-doping of Tb or Y into the SMS phosphor generated many non-radiative recombination centres which deteriorated the optical performance of SMS: Eu2+, RE3+ phosphor.

Photocatalytic activity of ZnO nanodisks in degradation of Rhodamine B and Bromocresol Green under UV light exposure

Semiconductor-based photocatalyst is an alternative approach for wastewater treatment. The effectiveness of semiconductor nanoparticles to eliminate textile dye substances in the contaminant water has been widely studied in the last decade. This work discusses the photocatalytic activities of ZnO nanodisks by using different dye substance (rhodamine B, RhB and bromocresol green, BCG) under UV exposure. ZnO nanodisks were successfully synthesized through solution precipitation method. The formation of ZnO in hexagonal nanodisks structure was confirmed by XRD, FESEM and EDX analysis. The result showed that the photodegradation efficiency of BCG dye is higher than RhB dye under 75 min of UV irradiation. This could be explained by the dye molecular structure as BCG dyes containing more hydroxyl groups (-OH) than RhB dyes.

A Comparison Study between ZnO Nanorods and WO3/ZnO Nanorods in Bromocresol Green Dye Removal

This work studied the photocatalytic performance of ZnO nanorods and WO3/ZnO nanorods in bromocresol green (BCG). The ZnO nanorods were pre-synthesized via solution precipitation method. Subsequently, the nanorods were kept in sodium tungstate solution for the deposition of WO3. The present of WO3 was confirmed by XRD and EDX analysis. ZnO nanorods (64.34%) showed a higher photodegradation efficiency of BCG removal than WO3/ZnO nanorods (60.03%) under 75 minutes of UV irradiation. This could be attributed to the formation of WO3/ZnO shell-core nanostructure which limited the generation of holes and hydroxyl free radicals that needed for the photodegradation of BCG dyes.