Optical properties of near band-edge transitions in well-aligned and tilted ZnO nanostructures

Optical properties of the below and above band-edge transitions in vertically aligned and tilted ZnO nanostructures have been characterized using photoluminescence (PL) and thermoreflectance (TR) measurements in the temperature range between 30 and 300 K. The PL peaks of the well-aligned nanorods with the largely {0 0 0 1 } planes show considerable difference in energy value with respect to those of the other sample with the largely side planes of {1 0 (1) over bar 0}. Two emission peaks, denoted as E-A,E-B and E-D2, can be detected in the PL spectra of the tilted ZnO nanorods, while two emission bands of E-A,E-B and E-D1 are detected in the vertically aligned ZnO sample. Transition origins of the PL peaks in the vertically aligned and tilted ZnO nanorods are evaluated. The E-A,E-B feature is the band-edge luminescence and the weak E-D1 peak is the green emission caused by a defect of the oxygen-zinc antisite or oxygen vacancy. The E-D2 feature is also a defect luminescence coming from the conduction-band bottom to an imperfection level caused by oxygen interstitials where the PL light emits only from the side planes of {1 0 (1) over bar 0} in the hexagonal ZnO nanorods. Band-edge free excitons of the vertically aligned and tilted ZnO nanostructures have been characterized using TR experiments. The excitonic transitions of the tilted ZnO nanorods show energy blueshift behaviour with respect to those of the other vertically aligned sample due to the surface effect. Temperature dependences of the transition energies of the PL peaks and excitonic transitions in the ZnO nanorods are analysed. The optical-anisotropic effect in between the two different types of ZnO nanostructures is discussed.