Mohamad Hafiz Mamat

Performance of an Ultraviolet Photoconductive Sensor Using Well-Aligned Aluminium-Doped Zinc-Oxide Nanorod Arrays Annealed in an Air and Oxygen Environment

Ultraviolet (UV) photoconductive sensors were fabricated using an aluminium (Al)-doped zinc-oxide (ZnO) nanorod array with a diameter between 40 and 150 nm and thickness of approximately 1.1 µm. The nanorod arrays were prepared using a sonicated sol–gel immersion and annealed at 500 °C under different ambient conditions of air and oxygen. The annealing process induced the formation of nanoholes on the nanorod surfaces, which increased the nanorod surface area. The nanoholes existed in larger quantities on the nanorod surfaces annealed in air compared with the nanorods annealed in an oxygen environment. This condition reduced the rise and decay time constants of the air-annealed UV sensor. However, the sample annealed in an oxygen ambient shows the highest responsivity of 1.55 A/W for UV light (365 nm, 5 mW/cm2) under a 10 V bias mainly due to defect reduction and improvement in stoichiometric properties. To the best of our knowledge, a UV photoconductive sensor using this ZnO nanostructure has not yet been reported.

Effects of annealing environments on the solution-grown, aligned aluminium-doped zinc oxide nanorod-array-based ultraviolet photoconductive sensor

We have fabricated metal-semiconductor-metal- (MSM-) type ultraviolet (UV) photoconductive sensors using aluminium- (Al-) doped zinc oxide (ZnO) nanorod arrays that were annealed in different environments: air, oxygen, or a vacuum. The Al-doped ZnO nanorods had an average diameter of 60 nm with a thickness of approximately 600nm that included the seed layer (with thickness ∼200 nm). Our results show that the vacuum-annealed nanorod-array-based UV photoconductive sensor has the highest photocurrent value of 2.43 × 10-4 A. The high photocurrent is due to the high concentration of zinc (Zn) interstitials in the vacuum-annealed nanorod arrays. In contrast, the oxygen-annealing process applied to the Al-doped ZnO nanorod arrays produced highly sensitive UV photoconductive sensors, in which the sensitivity reached 55.6, due to the surface properties of the oxygen-annealed nanorods, which have a higher affinity for oxygen adsorption than the other samples and were thereby capable of reducing the sensors dark current. In addition, the sensor fabricated using the oxygen-annealed nanorod arrays had the lowest rise and decay time constants. Our result shows that the annealing environment greatly affects the surface condition and properties of the Al-doped ZnO nanorod arrays, which influences the performance of the UV photoconductive sensors.

High-performance dye-sensitized solar cells based on morphology-controllable synthesis of ZnO-ZnS heterostructure nanocone photoanodes.

High-density and well-aligned ZnO–ZnS core–shell nanocone arrays were synthesized on fluorine-doped tin oxide glass substrate using a facile and cost-effective two-step approach. In this synthetic process, the ZnO nanocones act as the template and provide Zn2+ ions for the ZnS shell formation. The photoluminescence spectrum indicates remarkably enhanced luminescence intensity and a small redshift in the UV region, which can be associated with the strain caused by the lattice mismatch between ZnO and ZnS. The obtained diffuse reflectance spectra show that the nanocone-based heterostructure reduces the light reflection in a broad spectral range and is much more effective than the bare ZnO nanocone and nanorod structures. Dye-sensitized solar cells based on the heterostructure ZnO–ZnS nanocones are assembled, and high conversion efficiency (η) of approximately 4.07% is obtained. The η improvement can be attributed primarily to the morphology effect of ZnO nanocones on light-trapping and effectively passivating the interface surface recombination sites of ZnO nanocones by coating with a ZnS shell layer.

Controllable Growth of Vertically Aligned Aluminum-Doped Zinc Oxide Nanorod Arrays by Sonicated Sol--Gel Immersion Method depending on Precursor Solution Volumes

Aluminium (Al)-doped zinc-oxide (ZnO) nanorod arrays have been successfully prepared using a novel and low-temperature sonicated sol–gel immersion method. The photoluminescence (PL) spectrum reveals the appearance of two emission peaks from the nanorod that are centred at 381 and 590 nm. The nanorod has a hexagonal structure with a flat-end facet, as observed using field-emission electron microscopy (FESEM). Interestingly, all samples have similar surface morphologies and diameter sizes of 40 to 150 nm after immersion in different precursor-solution volumes. The thickness-measurement results show that the thicknesses of the samples increase after immersion in higher precursor-solution volumes. We show for the first time that the growth of nanorod arrays along the c-axis can be controlled using different precursor volumes, and its growth mechanism is discussed. X-ray diffraction (XRD) spectra indicate that the prepared nanorods are ZnO with a hexagonal wurtzite structure that grows preferentially along the c-axis.

Electrical characteristics of sol-gel derived aluminum doped zinc oxide thin films at different annealing temperatures

Aluminum (Al) doped zinc oxide (ZnO) thin films with doping concentration of 1 at.% have been prepared using sol-gel spin-coating method. Annealing process has been applied on the prepared thin films at temperatures between 350 and 500 °C. The thin films were characterized using X-ray diffractometer (XRD), UV-Vis-NIR spectrophotometer and current voltage (I–V) measurement system for structural, optical and electrical properties characterization, respectively. XRD pattern reveals the improvement of c-axis orientation with annealing temperatures. The Urbach energy as calculated from transmittance spectra increased with annealing temperatures. I–V measurement results revealed improvement in electrical properties of the thin films with annealing temperatures.

Enhancement in Dielectric Constant and Structural Properties of Sol-Gel Derived MgO Thin Film using ZnO/MgO Multilayered Structure

High dielectric constant, low porosity and nano-dimension particle of single layer magnesium oxide, MgO and multilayer ZnO/MgO were synthesized at different MgO solution molar concentration by the simple chemical solution technique. The MgO molar concentration was found to alter the properties of both single and multilayer films. Observation reveals the surface morphology change form uniform to agglomerate and porous structure with corresponding increase in molar concentration. The increment in particle size and the formation of agglomerated particle were observed by FESEM (JEOL JSM-J600F). The best prepared dielectric film for both single and multilayer is the film that deposited using 0.4 molar concentration MgO solution due to its high dielectric constant, uniform film, and the particle is in nanometer dimension with nonorod like structure.

Thickness-Dependent Characteristics of Aluminium-Doped Zinc Oxide Nanorod-Array-Based, Ultraviolet Photoconductive Sensors

Aluminium (Al)-doped zinc oxide (ZnO) nanorod arrays were prepared on a seed-layer-coated glass substrate by a sonicated sol–gel immersion method. We have shown, for the first time, that the thickness of the nanorod arrays can be increased incrementally without greatly affecting the diameter of the nanorods, by increasing the number of immersions. The field-emission scanning electron micrographs and thickness measurements revealed that the nanorods had diameters within the range from 40 to 150 nm and thicknesses from 629 to 834 nm with immersion times ranging from 1 to 5 h. The photoluminescence (PL) spectra revealed that the ZnO nanorod quality was enhanced with long immersion times as shown by an improvement in the ratio of the UV peak intensity to the visible emission peak intensity, or IUV/Ivis. The thickness-dependent characteristic of Al-doped ZnO nanorod-array-based, UV photoconductive sensors was studied; minimising the thickness of the nanorod arrays was found to provide high responsivity and good performance. Our experiments showed that a decrease in the thickness of the nanorod arrays improved the responsivity and response time of the UV sensors, with a maximum responsivity of 2.13 A/W observed for a 629-nm-thick nanorod film.

Particles Size and Conductivity Study of P-Type Copper (I) Iodide (CuI) Thin Film for Solid State Dye-Sensitized Solar Cells

Copper Iodide based dye-sensitized solar cells (DSSC) has been reported either deliver small photocurrents or highly unstable. In this research, by added in a small amount of Tetra-methyl-ethylene-diamine (TMED) into CuI sol-gel (CuI in acetonitrile), performance of electrical properties and optical properties of CuI based DSSC have been studied. Particles size and conductivity of CuI solution were measured when addition of TMED to the sol at 0.05M concentrations. Spin-coating technique has been explored to prepare nano-crystalline CuI films at room temperature. The film was examined for their surface morphology, optical and electrical properties by field emission scanning electron microscope (FESEM), ultraviolet visible spectroscopy (UV-Vis), Photoluminescence (PL) and current-voltage (I-V) measurement respectively. The results were then compared with CuI sol-gel which prepared by dissolving CuI powder with acetonitrile only. It showed some improvement to the CuI-based DSSC by incorporation of a small quantity of TMED in the solution of precursor.

Electrical and Structural Properties of ZnO/TiO2 Nanocomposite Thin Films by RF Magnetron Co-Sputtering

In this study, the ZnO/TiO2 nanocomposite thin films were prepared by RF Magnetron co-sputtering ZnO and TiO2 targets at different deposition times from 30-75 minutes. The electrical and structural properties ZnO/TiO2 nanocomposite thin films were characterized by I-V measurement, atomic force microscopy (AFM) and field emission scanning electron microscopy (FE-SEM). The electrical characteristics of nanocomposite films revealed that the conductivity of thin films increases as the thickness increase due to the improvement in surface contact between particles as well as photocatalytic activity. High conductivity at 1.67x10-4 S/cm and lowest resistivity about 5.14x104 Ω/cm were obtained for 75 minutes deposition time. Atomic force microscopy (AFM) showed particle size of ZnO/TiO2 thin films varied from 27nm to 51nm with an increasing in deposition time with granular shapes structures were observed from field emission scanning electron microscopy (FE-SEM).

ZnO Nanorod Arrays Synthesised Using Ultrasonic-Assisted Sol-Gel and Immersion Methods for Ultraviolet Photoconductive Sensor Applications

Zinc oxide (ZnO) nanomaterials have emerged as one of the most promising materials for electronic devices such as solar cells, light-emitting devices, transistors, and sensors. The diverse structures of ZnO nanomaterials produce unique, useful, and novel characteristics that are applicable for high-performance devices. The ZnO nanorod array is a beneficial structure that has become extremely important in many applications due to its porosity, large surface area, high electron mobility, and variety of feasible techniques. The chemistry and physical tuning of its surface state, including processes such as annealing and chemical treatments, enhance its functionality and sensitivity and consequently improve the device performance. These useful characteristics of ZnO nanorod arrays enable the fabrication of ultraviolet (UV) photoconductive sensors with high responsivity and reliability. Although there are many techniques available to synthesise the ZnO nanorod arrays, solution-based methods offer many advantages, including the capacity for low-temperature processing, large-scale deposition, low cost, and excellent ZnO crystalline properties. In this chapter, the synthesis of ZnO nanorod arrays via ultrasonic-assisted sol-gel and immersion methods will be discussed for application to UV photoconductive sensors. The optical, structural, and electrical properties of deposited ZnO nanorod arrays will be reviewed, and the performance of the synthesised ZnO nanorod array-based UV photoconductive sensors will be discussed.