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Strong multiphoton-absorption-induced UV luminescence from ZnO nanorod arrays grown by vapour-liquid-solid mechanism

ZnO is one of the most promising materials for optoelectronic devices for UV spectral range because of its wide and direct bandgap (3.37 eV) and high exciton binding energy (60 meV) [1]. The generation of multiphoton absorption (MPA) induced luminescence in ZnO by highly intense fs-lasers in near infrared offers one effective route for optical excitation. Therefore, much attention has been paid recently to investigate this specific material behaviour [2]. Here we report on the observation of strong bandedge emission with little or no deep level emission in c-axis oriented ZnO nanorod arrays grown by vapour-liquid-solid (VLS) mechanism (Fig. 1) after illuminating with focused fs-pulses. UV-VIS Luminescence Spectra were measured with an Ocean Optics spectrometer and compared with data from a reference single crystal and nanorods fabricated by low temperature chemical procedure [3] (Fig. 2). VLS-type ZnO nanorods were grown via vapour phase transport on a-plane (11–20) sapphire substrates with Au as catalyst. The SEM image in Fig 1 shows hexagonal nanorods which are relatively well aligned perpendicular to the substrate and have an average (diagonal) diameter of about 90 nm.