Abstract
Aligned ZnO/ZnSe core/shell nanorods (NRs) with type-II energy band alignment were fabricated by pulsed laser deposition of ZnSe on the surfaces of hydrothermally grown ZnO NRs. The obtained ZnO/ZnSe core/shell NRs are composed of wurtzite ZnO cores and zinc blende ZnSe shells. The bare ZnO NRs are capable of emitting strong ultraviolet (UV) near band edge (NBE) emission at 325-nm light excitation, while the ZnSe shells greatly suppress the emission from the ZnO cores. High-temperature processing results in an improvement in the structures of the ZnO cores and the ZnSe shells and significant changes in the optical properties of ZnO/ZnSe core/shell NRs. The fabricated ZnO/ZnSe core/shell NRs show optical properties corresponding to the two excitonic band gaps of wurtzite ZnO and zinc blende ZnSe and the effective band gap between the conduction band minimum of ZnO and the valence band maximum ZnSe. An extended photoresponse much wider than those of the constituting ZnO and ZnSe and a multi-band photoluminescence including the UV NBE emission of ZnO and the blue NBE emission of ZnSe are observed.
Highlights
Zinc oxide (ZnO), with a wide band gap (3.37 eV) and a large exciton binding energy (60 meV) at room temperature together with its excellent combined properties [1,2], is regarded as a promising material in a variety of applications, especially in photoelectronics
We studied the optical properties corresponding to the respective excitonic band gaps of wurtzite ZnO and zinc blende Zinc selenide (ZnSe) for ZnO/ZnSe heterojunctions in the form of ZnO/ZnSe core/shell NRs
The morphology can be improved to a certain extent by high-temperature annealing, the samples prepared by room temperature (RT) deposition of ZnSe followed by annealing are not as good in morphology as those prepared by depositing ZnSe at 500°C
Summary
Zinc oxide (ZnO), with a wide band gap (3.37 eV) and a large exciton binding energy (60 meV) at room temperature together with its excellent combined properties [1,2], is regarded as a promising material in a variety of applications, especially in photoelectronics. Various ZnO nanostructures, such as nanorods (NRs) and nanowires in particular, are most promising because their properties can be tailored by changing their morphology, structure and size, or modifying their surface with coatings of other materials [5,6]. Zinc selenide (ZnSe), Aligned ZnO nanorods (NRs) or nanowires are superior to the bulk or film materials in both the surface-tovolume ratio for modifying the surface [9] and the lateral size for reducing the nonradiative recombination and carrier scattering loss [11,12]. The surfaces of ZnO NRs can be decorated with ZnSe coatings, constructing the so-called aligned core/shell type-II heterostructures. The band offset between ZnO and ZnSe together with the resulted effective band gap of ZnO/ZnSe core/shell heterojunctions is favorable for improving the transport of both electrons and holes as well as extending the light absorption region to match the solar spectrum. The staggered band alignment in type-II heterojunctions facilitates the separation of photogenerated electrons and holes, which is an essential procedure in a photovoltaic device and quite significant to enhance the conversion efficiency of solar cells
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