Papers by Keyword: Photoconductivity

Abstract: The electrical characterization of high-purity semi-insulating 4H-SiC is carried out by means of current deep level transient spectroscopy (I-DLTS). Measurements are performed by employing either an electrical or optical pulse (below/above bandgap). The study performed on as-grown material, either annealed or oxidized, reveals the presence of six levels with ionization energies in the 0.4-1.3 eV range.

Abstract: In this study, we investigated the photoconductivity of pharma-grade zinc oxide (ZnO) which was excited under ultraviolet - long wavelength (UVA) and white light illumination. The morphological structures of ZnO were studied using field-emission scanning electron microscopy (FESEM) and transmission electron microscopy (TEM). The ZnO sample was composed of micro/nanorods, slabs, tripods, tetrapods and irregular-shaped particles. The energy-dispersive X-ray spectroscopy (EDS) result revealed the high purity of ZnO, with the relative O/Zn atomic ratio lower than 1. The absorption spectrum of ZnO suspension was performed using UV-Visible spectroscopy. ZnO sample exhibited strong absorption at 387 nm, corresponding to optical bandgap of 3.23 eV. The ZnO powder was converted into pellets to study its photoconductivity under different intensity of UVA (0.5-2.0 mW/cm^-2) and white light exposure (200-1000 lux) from conventional fluorescent lamp. The significant increase in surface conductivity of the ZnO pellets could be due to UVA illumination that produces photo-generated charge carriers on the surface of ZnO pellet. There was a small persistency of current flow after the UVA exposure was terminated. Under the white light exposure, the surface conductivity was slightly higher than that of dark condition. This is probably due to the small amount of defects, particularly oxygen vacancies and zinc interstitials that produce some light-generated charges on the ZnO crystal structures. The white light exposure may cause the excitation of electrons from the defects energy level to the conduction band, this being the underlying cause for the increase in the conductivity. ZnO pellet exhibited a dramatic enhancement of photoconductivity under UVA illumination if compared with small improvement of photoconductivity under white light exposure.

Abstract: Ultraviolet-A (UVA) radiation is present in sunlight and have been associated with various types of human skin cancers. In this study, rubber-grade zinc oxide (ZnO) powder was used as the targeted materials to study its UVA photoresponse as well as its antibacterial function. ZnO powder was synthesized using French process. The morphological structures of the samples were investigated using field emission scanning electron microscopy and transmission electron microscopy. The dominant morphology of the sample was micro/nanoplate. The optical bandgap of the ZnO sample is 3.19 eV based on the UV-Visible measurement. Current-voltage measurement was conducted to study the effect of UVA (390 nm) illumination on the photoconductivity of the ZnO pellet. Photoconductivity was observed to increase significantly under UVA exposure due to light absorption on the surface of ZnO to raise the electrons across the bandgap. The current response of the UVA-induced also revealed the small persistent photoconductivity after the UVA light was turned off. Besides, higher voltage bias would lead to higher current flow under the same intensity of UVA exposure. According to the antibacterial test towards Staphylococcus aureus, the percentage inhibition of the bacterial after 24 h incubation increase when the concentration of ZnO suspension increases. The UVA illumination had improved the inhibition of the bacterial growth. This is due to the excitation of ZnO and increasing of free charge carriers in the solution, leading to potential distortion to the membrane surface of the bacteria. ZnO powder performed high absorption of UVA and they are not only can be used to block the UVA sunlight, but also have higher antibacterial capability under UVA excitation.

Abstract: Crystalline germanium nanoclusters (NCs) are grown by a molecular-beam epitaxy technique on chemically oxidized Si (100) surface at 700oC. Deposition of silicon on the surface with Ge nanoclusters leads to surface reconstruction and formation of polycrystalline diamond-like Si coverage, while nanoclusters core becomes tetragonal SiGe alloy. Possible mechanisms for nanoclusters growth are discussed. Selective photoexcitation of Ge or SiGe nanoclusters or space-charge layer of underlying Si allows to observe two non-equilibrium steady-states with higher and lower conductivity values as compared to the equilibrium one. The persistent photoconductivity (PPC) behaviour was observed after excitation of electron-hole pairs in Si (001) substrate. This effect may be attributed to spatial carrier separation by macroscopic fields in the depletion layer of the near-surface Si. Decreasing of surface conductivity, driven by optical recharging of NCs and Si/SiO₂ interface states, is observed in the spectral range from 0.6 to 1.0 eV. Conductivity drop is discussed in the terms of hole accumulation by Ge-NC states enhancing the local-potential variations and, therefore, decreasing the surface conductivity of p-Si.

Abstract: Pure and Aluminium-doped ZnO (Zn_1-xAl_xO) x = 0 to 5 wt% thin films were deposited onto glass substrate by sol-gel spin coating method. The influence of various aluminium concentration in ZnO thin films on the structural, surface, optical and photoconducting properties were investigated. The GIXRD studies confirmed the polycrystalline nature with wurtzite structure of pure and Al doped ZnO films. Films with 1.5 % concentration of aluminium showed maximum absorption and transmission in the UV and visible regions respectively. The FESEM images showed crack free films with increasing grain boundaries upon doping. The average grain size is found to decrease due to aluminium doping. AFM images showed doped films with 1.5 % have better smoothness than other films. The photoconductivity measurements reveal that there is increase in the photocurrent compared to dark current for the Al doped ZnO. The photocurrent reaches its maximum value for ZnO: Al-1.5 % and then decreases upon increase in Al concentration. The photoresponse has slightly degraded upon aluminium doping onto ZnO. Temperature dependent conductivity shows that the thermal activation energy for the film decreases up to 1.5 % aluminium concentration and then increases for other concentrations in the temperature range 308 to 375K.

Abstract: Beta-carotene dye which is extracted from daucus carota material was used as sensitizer to fabricate dye-sensitized solar cell (DSSC). DSSCs were arranged in a sandwich structure consisting of fluorine-doped tin oxide (FTO) as a transparent conducting oxide (TCO), titanium dioxide (TiO₂) layer, beta-carotene dye, iodide/tri-iodide redox electrolyte, and carbon layer as a counter electrode. Beta-carotene dye has an absorbance in wavelength zones from 415 to 508 nm. Meanwhile, it has the largest photoconductivity of 28.3×10-4 and 8.2×10^-4 (Ω.m)-1 in dark and bright conditions, respectively. Moreover, the photoelectrochemical performance of the DSSC based on beta-carotene dye showed that the maximum voltage of 23.9×10^-2 V and the maximum current of 3.3×10^-5 A. However, the photo-to-electric conversion efficiency of this DSSC was very low i.e. 12.5×10^-4 %.

Abstract: InP nanocrystals were prepared by an economic chemical reaction. A very high density of surface states is found at 0.5 eV below the intrinsic conductive band edge. Mixing these InP nanocrystals with MEH-PPV, obtained the composite. Devices with structure ITO/composite(MEH-PPV)/Al were fabricated and investigated. Photocurrent spectra showed that the interface between a conjugated polymer and a semiconductor nanocrystal can be used to provide efficient charge separation for neutral excitons on both the ground states and excited ones. Incorporation of nanocrystals, the conductivity of diode shows large improvements.

Abstract: The substrate temperature in depositions of thin films plays a vital role in the characteristics of deposited films. We studied few characteristics of cadmium sulphide thin film deposited at different temperature (150°C-300°C) on corning 7059 glass substrate. We measured transmittance, absorbance, band gap and reflectance via UV spectroscopy. It was found that the transmittance for 300nm to 1100nm was greater than 80%. The resistivity and mobility was calculated by Vander Pauw method which were 10-80 cm and 2-60 cm²V^-1S^-1 respectively. The thermoelectric properties of the film were measured by hot and cold probe method which shows the N-type nature of the film.

Abstract: Cadmium sulfide (CdS) film by evaporation deposition and CdS/Cuprous sulfide (Cu₂S) complex film by dipping were obtained on glass substrates. The influence of different annealing temperatures was investigated. Scanning electron microscopy (SEM) demonstrates CdS films are continuous, homogeneous. X-ray diffraction (XRD), photoluminescence (PL) and Raman spectra reveal the CdS films were hexagonal structure and 400°C is favor of the crystallization and aggregation. The conductivity of CdS/Cu₂S complex film is better than that of CdS film.

Abstract: The photoconductivity of a nanocomposite MEH-PPV:TiO₂ thin film is investigated. The nanocomposite MEH-PPV:TiO₂ thin film was deposited on a glass substrate by spin coating technique. The composition of the TiO₂ powder was varied from 5 wt% to 20 wt% (with 5 wt% interval). The concentration of the MEH-PPV is given by 1 mg/1 ml. The current voltage characteristics were measured in dark and under illumination. The photoconductivity showed increment in value as the composition of the TiO₂ is raised in the polymer based solution. The absorption showed augmentation as the amount of TiO₂ is increased. The escalation of the current voltage is then supported by the results of surface morphology.