ZnSe量子点敏化纳米TiO2薄膜光电子特性研究

Photoelectron characteristics of ZnSe quantum dots-sensitized mesoporous La-doped nano-TiO_2 film

利用改进的直接吸附法制备核-壳ZnSe量子点敏化介孔掺镧nano-TiO_2复合薄膜.通过瞬态光伏和稳态表面光伏技术以及相应的检测手段,探测复合薄膜的微结构、光电子特性以及光生载流子在异质结薄膜中的传输机制.研究证实,包覆在Zn Se量子点外层的配体L-Cys主要通过其羧基与介孔nano-TiO_2表面未饱和的Ti原子键合,并完成量子点敏化复合薄膜的制备,由此实现对量子点在薄膜上沉积量的有效控制.实验结果表明:ZnSe量子点敏化nano-TiO_2薄膜的表面光伏响应出现在300—800 nm(紫外-可见-近红外)波长范围内,敏化后nano-TiO_2薄膜的光学带隙远小于敏化前薄膜以及ZnSe量子点的光学带隙;与具有p-型光伏特性的ZnSe量子点不同,敏化后薄膜显示出明显的n-型光伏特性,这将有利于光生电子由薄膜的外表面向光阳极基底方向迁移和注入;敏化后薄膜中光生载流子寿命、电子-空穴对分离速度和扩散长度的提高导致了瞬态光伏响应强度的增加和响应范围的扩大.

In the paper,the core-shell Zn Se quantum dots（QDs）-sensitized mesoporous La-doped nano-TiO2 thin film is prepared by a direct adsorption method.Photoelectron characteristics,photogenerated carriers transport mechanism,and microstructure of the QDs-sensitized nano-TiO2 thin film are probed via the stationary surface photovoltaic（SPV）and the transient photovoltaic technologies,supplemented by the Brunauer-Emmet-Teller adsorption isotherm technique,scanning electron microscope,Fourier transform infrared（FT-IR） absorption spectrum,and ultraviolet-visible（UV-VIS）absorption spectrum.The experimental results confirm that the surface of the nano-TiO2 film is covered with the Zn Se QDs with smaller particles by a chemical absorbing way,resulting in denser composite film of the QDs and the mesoporous nano-TiO2 than the nano-TiO2 film.In our experiment,the adsorption quantity of Zn Se QDs on nano-TiO2 film can be controlled effectively.The results show that ligand L-Cys capped at the outer layer of Zn Se QDs plays an important role in the sensitization process.Specifically,the peak of νS—Hin the ligand disappears at 2552 cm^-1in the FT-IR spectrum of the Zn Se QDs capped by the ligand as a stabilizer.This indicates that the S—H bond is broken.In the meantime,the peak of the C—S stretching vibration in the ligand shifts from 638 cm^-1to 663 cm^-1due to the formation of Zn—S bond.These imply that the core-shell Zn Se/Zn S/L-Cys QDs are obtained.On the other hand,according to the peak of —COOH stretching vibration disappearing at 1600 cm^-1in the FT-IR spectrum of the core-shell QDs-sensitized mesoporous nano-TiO2 film,the unsaturated Ti atoms on the surface of the TiO2 film are bonded to carboxy groups from the ligand capped at the QDs.That is,the ligand acts as a bridge between the QDs and the nano-TiO2 film for achieving the sensitization.Some excellent photovoltaic characteristics of the composite film are found as follows.1） The SPV responses of the QDs-sensitized film appear in a wavelength region of 300 nm to 800 nm（UV-VIS-Near-IR）,causing the region of SPV response to enlarge about 200 nm over that of the Zn Se QDs,and 400 nm over that of the nano-TiO2 thin film.2） The QDs-sensitized film displays an n-type photovoltaic characteristic that is different from that of the QDs.This may be more favorable for transferring those carriers from the film surface to the photo-anode material.3） Both the separation rate and the diffusion length of photogenerated electron-hole pairs are obviously increased,and the lifetime of free charge carriers in the Zn Se QDs-sensitized film prolongs about an order of magnitude over that of the nano-TiO2 film and Zn Se QDs.