核壳结构硒化镉/硫化镉/巯基乙酸量子点载流子输运特性

Carrier transport characteristics in CdSe/CdS/Thioglycolic acid ligand quantum dots with a core-shell structure

采用水溶液法合成了巯基乙酸(TGA)包覆的CdSe量子点.通过X射线粉末衍射和高分辨透射显微镜检测结果证实,合成得到闪锌矿结构CdSe量子点.能谱图和傅里叶变换红外光谱图结果说明,在核CdSe纳米粒子表面与配体TGA之间有CdS壳层结构形成.利用样品表面光电压(SPV)谱,指认CdSe量子点精细能带结构以及各自对应的激发态特征:475nm(2.61eV)和400nm(3.1eV)两个波长处的SPV响应峰分别与CdSe核和CdS壳层带-带隙跃迁相对应;370nm(3.35eV)附近SPV响应峰可能与TGA中羰基与巯基或羧基之间发生的n→π*跃迁有关.场诱导表面光电压谱结果证实,合成的CdSe量子点具有明显的N型表面光伏特性,而上述n→π*跃迁则具有P型表面光伏特性.荧光光谱谱线均匀增宽以及SPV响应峰位蓝移,说明样品具有明显的量子尺寸效应.结合不同pH值下合成的CdSe量子点的SPV谱和表面光声谱发现,SPV响应强度与表面光声光谱信号强度变化趋势恰好相反.上述样品表面光伏效应表明,CdSe量子点表面和相界面处的精细电子结构以及光生载流子的输运特性均与量子点的尺寸大小存在某种内在联系.

In the present paper, we synthesize CdSe quantum dots （QDs） that are stabilized by thioglycolic acid according to the water-phase synthesis. The X-ray diffraction and HRTEM results confirm that the samples prepared each possess a sphalerite structure. The EDS and FT-IR spectra of the samples show that a core-shell structure is formed between the CdSe nanoparticles and the ligand. The fine band structures and the characteristics of the surface states in a connection with the structures are identified by the surface photovoltage （SPV） spectrum of the samples. Two SPV response peaks, located at 475 nm （2.61 eV） and 400 nm （3.1 eV）, are closely related to the band-band transitions of the core-CdSe and the shell-CdS, respectively; the SPV response at 370 nm （3.35 eV） is correlated with the n → π＊ transition between the hydroxyl and sulfydryl （or hydroxyl）. It is because of an obvious quantum size effect of the samples that both PL line broadens and SPV response intensity increases with the decrease of the grain size of the sample. The change trend of the surface photoacoustic signal intensity is contrary to that of the SPV response intensity of the samples synthesized at varying pH. Moreover, the fine band structures at surfaces and grain boundaries of CdSe QDs prepared are probed by the SPV spectra of the samples at varying pH values. The relationship between the grain size and the photo-generated carrier transport behavior is discussed according to the detected EFISPV results of the QDs.