Enhancement of multi-photon Raman scattering and photoluminescence emission from Li-doped ZnO nanowires

Uniform Li-doped ZnO nanowires have been fabricated by thermal evaporation method. The ZnO formation and the Li incorporation have been discussed and the formation path of preparation products has also been explained in terms of the stability of Zn and Li vapor in an aerobic atmosphere. The strongest vibration mode at 437.2 cm(-1) in the multi-photon Raman scattering of the as-prepared products at room temperature indicates that the Li-doped ZnO nanowires still keep the hexagonal (wurtzite) structure. Raman spectra of Li-doped ZnO with different doping concentrations confirm that the variation of Raman peaks in the range of 550 cm(-1) similar to 650 cm(-1) is caused by the doping of lithium. The photoluminescence property at room temperature shows a blue-shift compared with undoped ZnO in the ultraviolet region. The Gauss fitting analyzed results indicate that the broad-band is composed of one green luminescence (GL) band, one yellow luminescence (YL) band, and one red luminescence (RL) band, respectively. The GL (2.447 eV) could be attributed to the singly ionized oxygen vacancies. The origin of the YL (2.245 eV) could be assigned to the transition of an electron from the conduction band (or a shallow donor) to the Li acceptor level. The RL (1.977 eV) could be assigned to the transitions between a shallow donor (at low temperature) or the conduction band (at higher temperature). Furthermore, the luminescence spectra at low temperature confirm the increase of defect levels like oxygen vacancies.