“…ZnO has a wide direct band gap, which coupled with the large exciton binding energy (60 meV), makes it a candidate for a variety of optoelectronic device applications such as light emitting diodes, lasers and UV photo-detectors [1,2]. ZnO gas sensors have also been demonstrated, with sensitivity to a range of vapours including alcohols, water vapour, ammonia and hydrogen [3,4,5,6,7,8].…”