Removal of Micrometer Size Morphological Defects and Enhancement of Ultraviolet Emission by Thermal Treatment of Ga-Doped ZnO Nanostructures

Bhatti AS (2014) Removal of Micrometer Size Morphological Defects and Enhancement of Ultraviolet Emission by Thermal Treatment of Ga-Doped ZnO Nanostructures. PLoS ONE 9(1): e86418. doi:10.1371/journal.pone.0086418 Removal of Micrometer Size Morphological Defects and Enhancement of Ultraviolet Emission by Thermal Treatment of Ga-Doped ZnO Nanostructures Umair Manzoor 0 Do K. Kim 0 Mohammad Islam 0 Arshad S. Bhatti 0 Sangaru Shiv Shankar, King Abdullah University of Science and Technology, Saudi Arabia 0 1 Alamoudi Water Chair, King Saud University, Riyadh. Kingdom of Saudi Arabia, 2 Center for Micro and Nano Devices, Department of Physics, COMSATS Institute of Information Technology, Islamabad. Pakistan, 3 Department of Materials Science and Engineering, Korea Advanced Institute of Science and Technology (KAIST), Daejeon. Republic of Korea, 4 College of Engineering, King Saud University , Riyadh , Saudi Arabia Mixed morphologies of Ga-doped Zinc Oxide (ZnO) nanostructures are synthesized by vapor transport method. Systematic scanning electron microscope (SEM) studies of different morphologies, after periodic heat treatments, gives direct evidence of sublimation. SEM micrographs give direct evidence that morphological defects of nanostructures can be removed by annealing. Ultra Violet (UV) and visible emission depends strongly on the annealing temperatures and luminescent efficiency of UV emission is enhanced significantly with each subsequent heat treatment. X-Ray diffraction (XRD) results suggest that crystal quality improved by annealing and phase separation may occur at high temperatures. - Funding: Authors would like to thank Higher Education Commission (HEC) Pakistan for the financial support through National Research Program for Universities. The authors would also like to extend their sincere appreciation to the Deanship of Scientific Research at King Saud University for its funding of this research through the Research Group Project no. RGP-VPP-283. The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript. Competing Interests: The authors have declared that no competing interests exist. The study of different doped and undoped ZnO nanostructures are of great interest in the scientific community owing to their excellent optical, electrical, gas sensing and piezoelectric properties [14]. Doped ZnO is also one of the most explored materials for practical applications i.e. sensors, transparent conductor oxide (TCO), and photocatalysis etc [5,6]. Some of these applications require high electrical conductivity, which may be achieved by replacing Zn2+ ions with higher valence ions. These ions act as efficient shallow donors and In3+, Al3+ and Ga3+ ions have shown better properties than others [7,8]. In gallium-doped zinc oxide (Ga-doped ZnO), Ga3+ is expected to cause a small lattice distortion (similar radii sizes of Zn and Ga) and at the same time is an efficient shallow donor in ZnO. However, Ga doping may produce ZnGa2O4 phase when the doping exceed a certain limit. Solubility limits are reported to be close to 3 at% of Ga in ZnO [9,10]. Literature is limited and a comprehensive study is required on the effect of annealing on Ga-doped ZnO nanostructures. This will be important for high temperature applications i.e. gas sensors in which usual operating temperatures are more than 400uC [11]. In this report, we demonstrate, for the first time that micrometer size defects in Ga-doped ZnO nanostructures can be removed by carefully tuning the annealing conditions. Phase separation also occurs at 900uC. Direct evidence of sublimation, at temperatures much lower than the synthesis temperatures, is also provided and possible reasons for the improvement in optical and structural properties are discussed. Experimental Procedure Ga-doped ZnO nanostructures were synthesized by vapor transport method. Equal amounts (by weight) of ZnO powder (99.0%, HAYASHI PURE CHEMICALS INDUSTRIES, Osaka, Japan) and carbon black were mixed for 4 hours in a ball mill. 0.15 g Ga2O3 was added in 0.6 g of the mixture, mixed using mortar and pasture (source mixture) and loaded into an alumina boat. A Silicon (Si) substrate was placed on top of alumina boat. The boat was then placed at the center of the tube furnace. Gadoped ZnO nanostructures were synthesized at 950uC with 15 minutes holding time. Ar was used as carrier gas and flow rate of Ar and O2 was 150 and 4 sccm respectively. Morphology was characterized by using scanning electron microscopy (XL30 PHILIPS Netherlands) fitted with Energy Dispersive Spectroscopy (EDX) for elemental analysis. Phase analysis of the deposited nanostructures was done by using x-ray diffraction (XRD RIGAKU Tokyo, Japan). Room temperature photoluminescence of the nanostructures was measured using Xenon lamp with the excitation wavelength of 325 nm. Post-synthesis heat treatment was done by first transferring nanostructures on a Si substrate coated with thin layer of SiO2. Small amount of ethanol was dropped on the nanostructures and dried in air to ensure better dispersion and sticking of nanostructures to the substrate. Figure S1 is the low magnification SEM image of the substrate. The figure clearly suggests that nanostructures are dispersed on the substrate. Post-synthesis heat treatment of these nanostructures was done by heating the same substrate subsequently at 600uC, 700uC, 800uC and 900uC for 1 hour in O2 (99.999% pure, flow rate = 25 sccm). SEM, XRD and photoluminance (PL) were measured after every heat treatment. Results and Discussion Small part of the as-deposited powder was transferred to a substrate and SEM, XRD and PL properties were investigated after each subsequent heat treatment at 600uC, 700uC, 800uC and 900uC for 1 hour. Figure 1(a) is the typical EDX area scan of Gadoped ZnO nanostructures. The results clearly shows primary and secondary peaks of Zn and Ga, suggesting that significant amount of Ga is present. A distinct Si peak is also present which comes from Si substrate. Figure 1(b) is the low magnification image of Ga-doped ZnO nanostructures after transferring on other substrate. The results clearly show different morphologies i.e. comb-shape nanostructures, Nanobelts, nanosheets and nanowires etc. These structures are unique in the sense that there are very few reports on mixed morphologies of Ga-doped ZnO nanostructures [12]. Figure 2(a,f) shows SEM micrographs of ZnO sheet shape structure without any heat treatment and after thermal annealing at 600uC, 700uC, 800uC and 900uC respectively. There is no significant difference in morphology after heating at 600uC and 700uC. SEM micrographs clearly suggest a systematic degradation with temperature after annealing at 800uC and 900uC. Morphological changes are visible and the sheet-shape structure is converted into one sided saw-shape structure at 800uC. The other side only has a rough surface suggesting une (...truncated)