Preparation Of CdTe Semiconductor QDs In Water And Investigation On Their Optics Properties

Recently, semiconductor of Quantum dots (QDs) exibits uniqe photo- luminescence properties, having wide applications in light-emitting devices, sensor, photoelectrochemical cell, optical display, and especially biological imaging. Hence, they are of great interest by many researchers for both fundamental research and technical application. Luminescent semiconductor QDs have several inherent functional advantages compared to traditional organic dyes. The photoluminescence (PL) emissions of high-quality semiconductor QDs are strongly size-dependent, which can cover an entire optical window only by change the size of the as-prepared QDs. They are narrow, symmetric, quite stable against photo-oxidation and photo-bleaching. Furthermore, QDs having different sizes can be exicited by only one exiciting laser. These spectral features make QDs possible to carry out multiple labeling with a single-laser setup. QDs with core/shell structure can be formed by epitaxially growing a lattice-matched, higher band gap material on the top of the core, which is of high stability. With the recent developments in synthetic chemistry, nearly mondisperse, highly luminescent semiconductor QDs covering an entire optical window could be prepared at low cost with so-called greener approaches, which is obtained by decompositing lipophilic organometallic compounds at hightemperature in organic media. However, it should be treated by phase transition to make the as-prepared QDs soluble in water. Thus the study of synthesizing semiconductor QDs directly in water is attracting more and more attention. Why CdTe is chosen is that it is a representative example of the important luminescent semiconductor QDs that has been used in numerous applications. In this work, we will focus on the preparation of CdTe and their growth kinetic, structure, luminescent properties and the control of the size of the as-prepared QDs. The main results we obtained are outlined as follows:(1) Preparation of luminescent CdTe semiconductor QDsFirstly, we prepare the solution of NaHTe by using the material of NaBH4 and Te powder in a little air-free bottle. Then we injected the as-prepared NaHTe into an air-free three-neckbottle, which contained Cd(ClO4)2.6H2O and MPA at the proper pH. The two solution react and CdTe semiconductor is obtained. In this way, we can obtain QDs having different sizes only by prolonging refluxing time.(2) The relationship between the growth speed of CdTe semiconductor QDs and the experimental parameters.We study the influence factors on the growth speed of the as-prepared CdTe semiconductor QDs using the photoluminescence spectra and UV-vis spectra, whose peak will shift toward longer wavelength as CdTe semiconductor QDs grow larger. It has been found that higher molor ratio of Cd2+:Te-, fewer amount of 3-mercaptopropionic acid(MPA) and higher reacting temperature all will accerlate the growth rate of CdTe semiconductor QDs. Both the amount of ligands and temperature have significant influence on the growth rate of CdTe QDs.(3) The luminescent properties and distribution of the as-prepared CdTe semiconductor QDs particles.The molor ratio, the amount of MPA, reacting temperature, the value of pHvalue and the time for preserving the samples at room-temperature are all important factors to influence the luminescent properties and the size distribution. Having calculated the photoluminescence quantum yield (PLQY) and the full width at half maximum (FWHM), we find that only under the condition that the molor rate of Cd2+:Te is at least larger than 1, CdTe semiconductor QDs with higher PLQY and narrower FWHM can be obtianed. When molor ratio of Cd2+:Te-:MPA is 1.0:0.25:1.8, we obtain the highest quality sample Traditionally, the reacting temperature can reach 110℃, which reflects that the higher the temperature is, the higher quality CdTe semiconductor QDs are. We synthesize a high quality of CdTe semiconductor QDs using the hydrothermal method at the temperature of 180℃. As the pH value of the as-prepared CdTe semiconductor QDs is changed, it is found that when pH value is 4.6, the PL intensity of the luminescent CdTe semiconductor QDs is 12.85 times of the original sample, exhibiting a very high quality. For some samples, the time for preserving samples changes the luminescent properties. It is found that, PLQY can increase from 2.99% to 36.95%, and the size of the particles become more uniform, which is the result of the restructure at the surface of the CdTe semiconductor QDs. This decreases the defects on the surface of the CdTe semiconductor QDs, and the PL intensiy of the luminescent CdTe semiconductor QDs increases and the size of the particles become more uniform.(4) The structure and stability of CdTe semiconductor QDsWe characterize the structure of the as-prepared CdTe semiconductor QDs by X-ray powder Diffraction (XRD) and Selected Area Electron Diffraction (SAED), which show that the as-prepared CdTe semiconductor QDs have the structure of cubic zinc blende, in which the diffraction peaks shift toward higher angles. This is due to the formation of the structure of core/shell. We also characterize the distribution of the as-prepared CdTe semiconductor QDs by transmission electronmicroscopy (TEM), and we find that the sample is monodipersive. Gel-electrophoresis is carried out to testify that the electric charges at the surface of the CdTe semiconductor QDs is negative. It can be also found that the particles with different sizes move to the positive electrode nearly at the same speed, which means that the different amount of ligands have little influence on the moving speed of the CdTe semiconductor QDs. Using a 488nm laser to excite the CdTe semiconductor QDs, we find that as the amount of MPA is 85.5μl , the stablest CdTe semiconductor QDs can be obtained. This means that under such condition CdTe semiconductor QDs with the best quality can be aquired at the molor ratio of Cd2+:HTe-:MPA=1.0:0.25:1.8.