ZnSe/ZnS/L-Cys核壳结构量子点光声与表面光伏特性

ZnSe量子点光电子特性的研究对于其微观电子结构探测和应用领域的扩展具有重要的意义.本文结合表面光伏与光声技术以及激光Raman研究了不同回流温度下制备L-半胱氨酸(L-Cys)为配体核壳结构ZnSe量子点的微结构和光声与表面光伏特性.结果发现,具有n-型光伏特性的ZnSe量子点在近紫外到可见光范围内展示出优良的表面光伏性质.尤其在波长为350-550 nm范围内光子能量绝大部分用于产生表面光伏效应,而不是用于无辐射跃迁导致的晶格热振动,同时证实了光声与表面光伏效应之间的能量互补关系.实验指认ZnSe量子点在300-350 nm短波区域出现的光声信号和在1120,1340和1455 cm~(-1)高频区域出现的Raman峰与配体L-Cys的多声子振动模式密切相关.实验结果表明,随着回流温度的降低,ZnSe量子点的平均粒径有减小趋势,这在改善样品的表面效应和小尺寸效应的同时,有利于提高核壳结构ZnSe量子点的光伏转换效率.

Photoacoustic and surface photovoltaic characteristics of L-Cysteine-capped ZnSe quantum dots with a core-shell structure

The study on photoelectronic characteristics of ZnSe quantum dots (QDs) is of significance for investigating its microelectronic structure and expanding its potential applications because ZnSe QD has low biologic toxicity. In the present paper, the surface photovoltaic and photoacoustic technologies, and laser Raman, X-ray diffraction, transmission electron microscopy and Foureier transform infrared spectroscopy spectrum are jointly used to probe the microstructures, the photoacoustic and surface photovoltaic characteristics of L-Cysteine-capped ZnSe QDs prepared by water-phase synthesis at different reflux temperatures. The results indicate that the ZnSe QDs with a mean grain size of about 3 nm has a core-shell ZnSe/ZnS/L-Cys structure, in which the sulfhydryl groups in ligand prefer reacting with Zn atom at the (220) face to form the ZnS shell layer between the core-ZnSe and ligand L-Cys. The results show that the QDs with n-type photovoltaic property display a wide range of surface photovoltaic response and weak photoacoustic signal upon the illumination of near ultraviolet to visible light as compared with others QDs with similar core-shell structures in II-VI group. Especially, the strong SPV response and the weak PA signal in a wavelength region of 350-550 nm imply that the photon energies in the range are almost all used to produce the surface photovoltaic (SPV) phenomenon instead of the thermal lattice vibration caused by non-radiative de-excitation process. This reveals the energy complementary relationship between the photoacoustic and the surface photovoltaic phenomena of the QDs. The PA signals appearing in a short wavelength range of 300-350 nm and the Raman peaks located in a high frequency ranges of 1120 cm^-1, 1340 cm^-1 and 1455 cm^-1 are identified as relating closely to the multi-phonon vibration modes of ligand L-Cys. At low reflux temperature, the photoelectric threshold of the SPV response that relates to the core-ZnSe displays a red shift to a certain extent as compared with the bulk ZnSe. The narrowed bandgap may be attributed to quantum confinement effect of the QDs. In addition, the intensity of the SPV response that relates to the core-ZnSe gradually increases with the decrease of the reflux temperature. The results show that the above improved surface photovoltaic characteristics of the QDs may benefit from the reduced average grain size of the ZnSe QDs, thus causing its surface and small-size effects.