where d is the thickness of the film; the subscripts Q and S refer to the quartz or the sample, respectively. By plotting (α hν)1/2 versus hν (i.e. Tauc plot) and using a linear fit, Eg can be easily extracted (Eg is the intercept of the linear fit with the abscissa axis). The Tauc plot (red dashed line in Fig. 4) of the nanofibers annealed at 550 °C gave an energy gap of ∼3.2 eV. Assuming an error of ∼10% in the determination of the band-gap, the value of 3.22 eV is in good agreement with the values reported in the literature for bulk ZnO [1].