Effects of Vacuum Annealing on the Conduction Characteristics of ZnO Nanosheets

Abstract

ZnO nanosheets are a relatively new form of nanostructure and have demonstrated potential as gas-sensing devices and dye sensitised solar cells. For integration into other devices, and when used as gas sensors, the nanosheets are often heated. Here we study the effect of vacuum annealing on the electrical transport properties of ZnO nanosheets in order to understand the role of heating in device fabrication. A low cost, mass production method has been used for synthesis and characterisation is achieved using scanning electron microscopy (SEM), photoluminescence (PL), auger electron spectroscopy (AES) and nanoscale two-point probe. Before annealing, the measured nanosheet resistance displayed a non-linear increase with probe separation, attributed to surface contamination. Annealing to 300 °C removed this contamination giving a resistance drop, linear probe spacing dependence, increased grain size and a reduction in the number of n-type defects. Further annealing to 500 °C caused the n-type defect concentration to reduce further with a corresponding increase in nanosheet resistance not compensated by any further sintering. At 700 °C, the nanosheets partially disintegrated and the resistance increased and became less linear with probe separation. These effects need to be taken into account when using ZnO nanosheets in devices that require an annealing stage during fabrication or heating during use.Electronic supplementary materialThe online version of this article (doi:10.1186/s11671-015-1066-1) contains supplementary material, which is available to authorized users.

Investigation into the effects of surface stripping ZnO nanosheets

“…The synthesised ZnO nanosheets were of rectangular form with lengths typically ranging from 1 to 20 μm and widths of 1 to 15 μm, which is in general agreement with our previous results [7]. Figure 1 shows a SEM image of the grain structure of a typical ZnO nanosheet before argon bombardment, Figure S1 in the supplemental information shows the nanosheet coverage of the sample.…”

“…These devices require surface contamination to be removed in order to create a consistent contact. One method of doing this involves annealing the nanosheets; however, previous work has shown that vacuum annealing causes changes in the polycrystalline structure and to the defect chemistry, which affects the conduction characteristics of the nanosheets [7].…”

Evolution of the Stoichiometry and Electronic Structure of Cobalt Oxide in Thermally Treated Co-Doped ZnO Nanorods for Solar Cells

“…Although there is also a decrease in the oxygen content for the undoped sample, it is attributed to elimination of oxygen impurities on the ZnO surface and the healing of defects. 67 Reduction of ZnO requires high temperatures on the order of 1000 K. 68 Additionally, the oxygen-to-zinc ratio (O:Zn) shows a more pronounced change in samples containing Co relative to the undoped sample, as shown in Figure 8b.…”

Spatial and Contamination-Dependent Electrical Properties of Carbon Nanotubes

“…We proposed that purification or annealing the MWCNTs should improve this as previous work with vacuum annealed ZnO nanosheets showed that annealing to 300 and 500 °C improved the reproducibility of the contacts by removing surface contamination . However, this work also showed that the structure and defect chemistry of the nanosheets changed under vacuum annealing, which alter the conduction mechanism.…”

“…Our previous work has shown that surface contamination, such as amorphous carbon, hydroxyl groups, and adsorbed water, on ZnO nanorods and nanosheets can cause inconsistent and nonreproducible contacts that affect the accuracy of conductivity measurements. , There are a number of methods that are commonly used to remove surface contamination from nanostructures including surface passivation, annealing, and argon bombardment. − Work carried out by Lee et al has shown that rapid annealing to about 800 °C can reduce the contact resistance when measured using Ti/Au deposited electrodes; however, it is not clear whether this is caused by the removal of surface contamination or by an improved contact from chemical changes at the interface as a result of annealing.…”