Insight Into The Extinction Of Of Colloidal Semiconductor Nanocrystals

Colloidal semiconductor nanocrystals,i.e. quantum dots have been extensively explored due to their size dependent optical and opto-electronic properties, and are potential in some fields such as quantum dots LED, solar cell, and biomedical labeling. For anyone who is working on the applications or basic research of these colloidal quantum dots, this has always been a source of distress, since some works requires the accurate estimate for the concentration of a given quantum dots sample. Therefore, it is essential to determine actual concentration of these quantum dots. A crucial problem how to determine the concentration or mass of the used quantum dots, which is prerequisite for use in real life. However, the characteristics of the semiconductor quantum dots may led it difficult to be accurately quantified by the traditional gravimetric analysis. Recently, determining the concentration of colloidal quantum dots has been reported by use of Lambert–Beer’s law, which provide the extinction coefficient of quantum dots with various size. Currently, the report on the extinction coefficient was focused on the mainly II-VI quantum dots such as Cd S, Cd Se and Cd Te. The determination of the extinction coefficient of quantum dots makes it possible to calculate the quantum dots concentration in solutions by simply taking UV-vis-NIR spectrometer of samples for further study, thereby making its applications in bio-labeling and photovoltaic cells feasible. In this theses, the study on the extinction coefficient were carried our on various quantum dots including compositions, sizes and crystal structures, which has been well characterized by this optical technique. Accordingly, some work in this thesis has been made as following:1. The nearly monodisperse Cu In S2, Ag In S2 and Cu In Zn S3 quantum dots have been successfully prepared by use of hot injection method. The molar extinction coefficients of ternary Cu In S2 and quaternary Cu In Zn S3 with emission covering the whole visible to NIR have been obtained. At the first absorption peak position, the molar extinction coefficient values are fitted, and found to obey a power function. By comparable study, the exponent of the Cu In S2 quantum dots is slightly smaller than those reported for other NC materials. For the Ag In S2 quantum dots, at the absorption position of 350 nm, the molar extinction coefficient values are fitted, and found to obey a power function.2. To insight into the relation between crystal structure and molar extinction coefficient, Cu In S2, Cu In Zn S3 and Cd Se quantum dots with both wurtzite structure and zinc-blende was synthesized by different ways, respectively. The experimental result show that the extinction coefficient of these quantum dots with different particle size have minor difference only at the region of large size particles, which also obey a power function.3. Cd Se quantum dots considered to be a typical sample, the molar extinction coefficient of core/shell quantum dots such as Cd Se/Cd S and Cd Se/Zn S quantum dots was qualitative studied by preparation of these core/shell structure nano particles with various shell thickness and core size. By Lambert–Beer’s law, at the first absorption peak position of core/shell quantum dots, the molar extinction coefficient values decreased with the increasing of shell thickness(Cd S and Zn S shell), and are fitted, and found to obey a power function.4. In principal, quantunary quantum dots was found to be a multinary compound consisting of binary and ternary quantum dots. Based on this fact, the studies on the relationship of extinction coefficient were carried on for the fist time between binary(Zn S and Cd S), ternary(Cu In S2) and quantunary(Cu In Zn S3 and Cu In Cd S3) quantum dots. Experimental result indicate that the exponent of the Cu In S2 quantum dots is slightly smaller than those reported for that of Cu In Zn S3 materials, which means that the molar extinction coefficient of quaternary Cu In Zn S3 quantum dots is larger than that of ternary Cu In S2 quantum dots for a given size. This phenomenon that a much larger extinction coefficient is observed for these three materials could be explained by quantitatively determining the extinction coefficients of Zn S quantum dots. The relation on molar extinction coefficient relation among quantunary, binary and ternary was fit, and obey a function function. Also this result has been found in Cu In Cd S3 quantum dots system consisting Cu In S2 and Cd S quantum dots for a given particle size.