Kiel, Germany--Scientists at Kiel University have created nanostructured zinc oxide (ZnO) UV photodetectors using a new single-step flame-transport-synthesis process.1 The result is a network of interconnected ZnO nanotetrapods forming a bridge between electrodes on a chip.
Silicon or gallium nitride based UV detectors are already available in the market; however they lack a certain level of selectivity and also cannot function in harsh environments. High production costs, multistep processes, and the requirement of specific operating conditions limit the field of application for these sensors.
Making ZnO nanotetrapods
A simple oven or airbrush gun-type burner is used to create the high temperatures needed to convert zinc microparticles into nano-tetrapods. The process takes place in a normal air environment, with the necessary amount of oxygen regulated by the flame itself. "This burner-flame transport synthesis method allows us to grow the zinc oxide nano-microstructures directly on the chip," says Yogendra Kumar Mishra, the main author of the study. "And that only takes a few seconds; it is just a matter of driving the chip through the flame while the nanotetrapods assemble themselves onto it."
When building a sensor device from nanostructures, one of the biggest challenges is interconnecting them to electrical contacts on chips, says Dawit Gedamu, the first author of the paper. Most of the existing synthesis methods, such as chemical vapor deposition (CVD) or vapor-liquid-solid (VLS) growth, only allow synthesis of different nanostructures under specific conditions. For instance, the presence of catalytic particles, particular substrates, complex temperature requirements and atmospheric conditions, and other factors must be met.
The ZnO sensors are "extremely promising," says Mishra. "Nanostructures made from zinc oxide are highly interesting for multifunctional applications, due to their sensibility to UV light and their electrical and mechanical properties." In addition, ZnO is relatively inexpensive and easy to synthesize. The ZnO sensor reacts to UV light within milliseconds of its exposure; in addition, it works in rather rough environments. The next logical step for the Kiel University scientists is therefore to find the ways to produce these nano-tetrapods on a larger scale.
1. Dawit Gedamu et al., Advanced Materials (2013); doi: 10.1002/adma.201304363