STM-STS investigation of vacuum annealed ZnO nanoribbons

We present a scanning tunneling microscopy and spectroscopy (STM-STS) investigation of the effects of ultra high vacuum annealing and oxygen exposure ontoZnO nanoribbons synthesised by chemical vapour deposition. STM imaging revealed a width to height ratio for the nanoribbons between 2:1 and 3:1 and average width and height of 90 and 40 nm, respectively. Imaging before annealing showed the presence of surface contaminants, which were removed after annealing at 800 degrees C. Analysis of the STS data before annealing shows n-type behaviour with a band gap of 3.4eV and an upward band bending of 0.9 eV. Annealing up to 700 degrees C induced a reduction in surface band bending, towards a near flat band behaviour. After the 800 degrees C anneal the surface electronic properties Were significantly altered, with a large increase of tunnelin-current at negative sample bias leading to a narrowing of the apparent surface band-gap mid gap Fermi level. This change was attributed to a loss of surface lattice oxygen and was found to be reversible upon O-2 exposure at room temperature. The anneal/O-2 exposure process could lead to a way of cleaning the Zno nanoribbons without using ion bombardment techniques. (C) 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim