Defect states in ZnO/SnO_2 composite nanostructures (CNs) for possible facilitating role in carrier transport across the junction

摘要

This paper explores the possible role of defect states in charge transport in a type II heterointerface formed by ZnO/SnO_2 composite nanostructures (CNs) grown using VLS technique. XRD and high-resolution TEM analysis revealed the granular growth of SnO_2 in a matrix of ZnO. Raman spectra obtained from CNs were marked by the presence of oxygen vacancies as Raman modes were broadened and shifted. XPS results confirmed the presence of oxygen vacancies in ZnO and SnO_2 and Zn interstitials in ZnO, which showed dependence on growth temperature. Photoluminescence (PL) spectra acquired from CNs were marked by the presence of a very broad PL band in the visible region centered around 2.4 eV, while a very weak near-band-edge emission was observed. The PL band in the visible region showed contributions from Zn interstitials (2.4-2.6 eV) and oxygen vacancies (1.9-2.1 eV) in ZnO, and the PL band of SnO_2 consisted of primarily oxygen vacancies (1.9-2.1 eV). The PL bands obtained from CNs showed the absence of Sn interstitials in SnO_2. The photoluminescence excitation (PLE) spectrum taken from CNs clearly showed absorption in the band gap due to defects. The role of defect states in the charge transfer process in type II heterostructures was further studied by electrical measurements on SnO_2/ZnO bilayer thin films. Temperature-dependent Ⅰ-Ⅴ characterization showed significant accumulation of charges at the interface, which was released by thermal excitation. It is, therefore, concluded that defects can play a positive role in photocatalytic devices, where excess charge is needed at the interface for catalytic reaction.