Luminescence And Vibrating Properties Of Zn-based GroupⅡ-Ⅵ Nanostructures

The luminescence and vibrating properties are one of the most important physical characterizations in semiconductor materials. To investigate these properties can provide the most foundational properties or information of the band structure, defects, electrons and phonons in the relative semiconductor materials. Zn-based group II-VI nanostructure compounds, as a direct wide band gap semiconductor materials, have got more and more attention for their promising application in electronic and optoelectronic, especially the luminescence devices. In this dissertation, ZnS and ZnSe nanostructures with different morphologies have been successfully synthesized by the thermal evaporation method, what's more, their structure, luminescence and vibrating properties have been systematically investigated. These significant results are introduced as following:1. Both excitonic and defect-related information of ZnS nanobelts and nanowires have been investigated by a temperature-dependent photoluminescence (PL) spectrum. PL spectra of ZnS nanobelts and nanowires differ significantly in the ultra-violet (UV) and visible emission regions. In UV emission regions, due to high quality crystals, free exciton B (FXB), free exciton A (FXA), FXA-one longitudinal optical (LO) phonon replica are observed in ZnS nanobelts, as well as free-to-bound (e, A) with its one LO phonon replica, while neutral-donor bound exciton (D°, X) and free-to-bound (e, A) are observed in ZnS nanowires at 10K. The peak and relative intensity of the FX and (D°, X) versus temperature follow well with conventional empirical relations. In the visible emission regions, it is commonly observed weak donor-acceptor pair (DAP) and self-activated (SA) emission from ZnS nanowires, but the lattice imperfection known as Y band emission is only observed at 10K in ZnS nanobelts. The Y band emission disappears at some temperature lower than 50K. The peak position and full width at half maximum (FWHM) of DAP and SA emission bands display different temperature dependence. Detailed study on temperature-dependent PL spectra of ZnS nanobelts and nanowires provides crucial information on the nature of the electronic states and recombination mechanisms in these nanostructures.2. By changing the deposition zone of the products, it is feasible to modulate the structural phase of ZnSe nanostructure from the cubic to the hexagonal phase. The as-synthesized ZnSe nanostructures collected from silicon substrates in downstream 31,27 and 22cm from the center of the furnace are denoted as S1, S2 and S3 respectively. From their respective x-ray powder diffraction (XRD) patterns, it is observed that all the diffraction peaks of S1 and S3 can be indexed to the cubic and hexagonal phase respectively, while S2 belongs to the phase between the cubic and hexagonal, indicating that as the deposition changing, a structural phase evolution take place from cubic to hexagonal among these three samples. Similar phenomenon is also found in the relevant Roman scattering spectrum. From the room-temperature Raman scattering spectra of these three samples, it is observed the transverse optic (TO) and the longitudinal optic (LO) modes, as well as other modes. While the E1(TO) which can merely exist in the hexagonal phase is only observed in S3, that is to say, S3 is of the hexagonal phase, which is in accordance with the former deduction. The PL spectra of the three samples further demonstrate the structural phase evolution among them. Compared with the emission peak of S1, it is observed that the peak of S3 have a obvious blue-shift. The reason for the blue-shift can be the band gap of ZnSe with a hexagonal phase (S3) is larger than that of ZnSe with a cubic phase (S1).