Precise Determination of Optical Band Gap in Cr-Doped Semiconductor Nanowires

Pristine and chromium-doped ZnO nanowires were prepared following the traditional co-precipitation method. X-ray diffraction data identified a pure wurtzite hexagonal crystal structure characteristic for ZnO, irrespective of the doping level. The particle size, as deduced form Williamson–Hall plots, was found to be 45-55 nm for all samples. Scanning electron microscopy revealed a clear nanowires morphology for the pure and doped samples, while elemental analysis ensured the successful Cr-doping. Distinct spectroscopic signatures of Cr-doping were revealed from a detailed deconvolution process applied to optical spectra of doped samples, where Cr 3+ optical transitions were unambiguously identified at ~420 and ~665 nm. Particularly relevant, is the spectral decomposition here performed for the superimposed absorption edge (~385 nm) and Cr 3+ optical resonance at ~420 nm, allowing to claim practically doping-independent optical band gap behavior in the present doping regime. This is further supported by identifying the characteristic ZnO near edge photoluminescence peak (~ 392 nm) which maintains fixed wavelength after Cr-doping. These findings contrast earlier studies on Cr-doped semiconductor nanoparticles and glass systems where the optical band gap has been largely underestimated. We attribute the inconsistence band gap values reported in literature for Cr-doped semiconductors to the proximity of Cr optical transitions to the semiconductor absorption edge.