Synthesis And Characterization Of CdSe:Ag And ZnSe:Cu Quantum Dots In Aqueous Phase

Semiconductor quantum dots (QDs) have tremendous prospect in the field ofsolar cells, light-emitting diodes, biomedical, due to their excellent optical andelectrical properties, such as absorption and emission wavelengthtunable. Atpresent, most of the QDs are synthesized in the organic phase with highly toxic andexpensive synthetic material, and which require a higher temperature. The surfaceof the obtained quantum dots is often capped with hydrophobic organic ligand thatmust be removed by ligand exchange while they were applied in the field ofbiomedicine or used for fabricating devices. As we know, changing the size andcomposition of the quantum dots is the basic means for controlling their properties.However, in order to broaden the fluorescence emission range of QDs, it is difficultto achieve in the actual operation by precisely controlling of the composition orreducing the particle size. While the QDs are extremely small, the performancewould be dropped rapidly that resulting from the defects of the structure andsurface. Although it can be obtained over a wide range of emission wavelength withdifferent sizes of quantum dots, a serious re-absorption phenomenon certainlyappears since the quantum dots are combined together, which will directly lead tothe conversion efficiency of the device decreases. In this dissertation, we haveobtained the results as listed as following:Firstly, CdSe quantum dots capped with MPA were simply synthesized viaaqueous phase reaction. We took advantage of a simple and feasibleelectrochemical method to prepare H2Se as the Se source. Both the reflux heatingtime and the electrolytic time for preparing H2Se gas were discussed. Throughprolonging the reflux heating time, the emission wavelength of the obtained CdSequantum dots would move from456nm to502nm. And by adjusting the electrolytictime, the photoluminescence wavelength covered from494to520nm, thecorresponding PL quantum yields were about18%. XRD pattern revealed that thestructure of as-synthesized CdSe QDs was zinc blende structure. In addition, TheCdSe QDs that exhibited uniform size distribution and good crystallnity wereobserved by HRTEM, with average diameter of2.7nm. Secondly, Ag dopants were added into the solution when the CdSe QDs weregrown to a certain size. The Ag doped CdSe quantum dots (d-dots) weresuccessfully synthesized via ion diffusion mechanism for the first time, whichcould be confirmed by the results of the XPS analysis. The optical test resultsshowed that doping Ag inhibited the intrinsic emission of CdSe QDs, and anewemission peak in the long side of the wavelength. The dopant emissionexhibited larger Stockes shift of~0.51eV than that of the band-gap emission. Theas-prepared of CdSe:Ag d-dots showed a high quantum yield (QY). The QY was upto28%in doping30min. In addition, by changing the size of the matrix, theemission wavelength of the obtained CdSe:Ag d-dots moved from546nm to583nm.In order to improve the photoluminescence performance, CdSe:Ag/CdS core-shellquantum dots were prepared by epitaxial growing CdS shells outer of the CdSe:Agd-dots. It could be found that the PL intensity significantly enhanced, thecorresponding QY was increased from28%to37%.Finally, for taking into account the high toxicity of the heavy metal Cd, wetook advantage of ZnSe QDs as the matrix, and prepared ZnSe:Cu d-dots that thephotoluminescence emission was from blue to green. XPS spectra demonstratedthat Cu dopant was successfully incorporatedinto the matrix ZnSe QDs. The XRDpatterns showed the d-dots still maintained the zinc blende structure. HRTEMimages indicated that the d-dots had good dispersion, and was nearly spherical, thesize was~4nm. The PL spectra results of ZnSe:Cu d-dots were similar with theCdSe:Ag d-dots. While the Cu-related emission emerged, both the intrinsicemission of ZnSe QDs and the trap emission were suppressed. Through thecombination of the energy level schematic and optical spectra, Cu impurity createda new impurity level between the conduction and valence band. Cu-doped emissionwas generated by the radiative recombination of the electron-hole pair in theimpurity level. In addition, we investigated the impact of reflux heating time andthe ratio for the d-dots fluorescence properties. While the ratio of Zn/Se is1.2:1,the intensity of photoluminescence emission reached the strongest after refluxing60min. tuning the ratio of Zn/Se could be obtained ZnSe:Cu d-dots that the sizecontinuously changed, and then fluorescence emission wavelength covered therange from463nm to518nm, corresponding to the QY could be up to27%.