CdTe, CdTe/CdS core/shell, and CdTe/CdS/ZnS core/shell/shell quantum dots (QDs) are potential candidates for bio-imaging and solar cell applications because of some special physical properties in these nano materials. For example, the band gap energy of the bulk CdTe is about 1.5 eV, so that principally they can emit 790 nm light, which is in the near-infrared range (also called biological window). Moreover, theoretically hot exciton generated by QDs is possible to be caught since the exciton relaxation process in QDs is slower than in bulk materials due to the large intraband energy gap in QDs. In this dissertation, we have synthesized the CdTe and CdTe/CdS core/shell QDs, characterized their structure, and analyzed their photophysical properties.
We used organometallic methods to synthesize the CdTe QDs in a noncoordinating solvent. To avoid being quenched by air, ligands, solvent, or other compounds, CdS shell was successfully deposited on the CdTe QDs by different methods, including the slow injection method, the successive ion layer adsorption and reaction (SILAR) method, and thermal-cycling coupled single precursor method (TC-SP). Our final product, quasi-type- II CdTe/CdS core/shell QDs were able to emit at 770 nm with a fluorescence quantum yield as high as 70%. We also tried to deposit a second shell ZnS on CdTe/CdS core/shell QDs since some compounds can quench CdTe/CdS core/shell QDs. Even though different methods were used to deposit ZnS shell on the CdTe/CdS core/shell QDs, CdTe/CdS/ZnS core/shell/shell QDs still can be quenched.
Furthermore, the CdTe/CdS core/shell and CdTe/CdS/ZnS core/shell/shell QDs were transferred into aqueous phase, phosphate buffered saline or deionized water, by switching the hydrophilic ligands (thiol or PEG ligands). The thioglycolic acid (TGA)-capped CdTe/CdS core/shell QDs can be kept in aqueous phase with high fluorescence quantum yield (60% - 70%) for more than two months. However, some other compounds in organic or aqueous phase can quench CdTe/CdS QDs. Additionally, the stability of the different ligands capped CdTe/CdS QDs was tested by dialysis measurement, the hydrodynamic diameters of CdTe and CdTe/CdS core/shell QDs were measured by dynamic light scattering, and dissolving issue was found when CdTe and CdTe/CdS core/shell QDs were diluted in CHCl3.
We have characterized the CdTe core and the CdTe/CdS core/shell QDs by powder X-ray diffraction (XRD), transmission electron microscopy (TEM), small angle X-ray scattering (SAXS), and ICP-OES measurements. We have found that the CdTe core was of a zincblende structure, and the shell was a wurtzite structure. And the CdTe/CdS QDs were core/shell QDs instead of alloying QDs.
We have also analyzed the photophysical properties of CdTe and CdTe/CdS core/shell QDs. Time-resolved photoluminescence (PL) measurements showed the emission decay lifetimes in the tens of nanoseconds. Additionally, ultrafast charge carrier relaxation dynamics of the CdTe core and CdTe/CdS core/shell QDs were studied by the femtosecond transient absorption (TA) spectroscopy. The transient absorption spectra of CdTe and CdTe/CdS core/shell QDs showed multiple bleaches, which have been assigned to the 1S3/2(h)-1S(e), 2S3/2(h)-1S(e), and 1P3/2(h)-1P(e) transitions. The spectral shifts of these bleaches after shell deposition have been analyzed in the context of a quasi-type-II carrier distribution in the core/shell samples, and interestingly the red shift was only contributed from the conduction band energy levels shifting to lower energy. In addition, the ultrafast evolution of these bleach features has been examined to extract electron cooling rates in these samples. A fast decay component in the 1S3/2(h)-1S(e) transition of the small CdTe QDs was discovered due to the hole being trapped by the defects on the surface of QD.
Further, we have studied the PL quenching process of the air exposed CdTe QDs via the PL decay and transient absorption measurements. Oxygen was shown to cause strong PL quenching of the CdTe QDs. There was no significant difference of the PL decay lifetimes between the CdTe QDs under argon and air, but a fast decay lifetime of 2.6 ps was observed in transient absorption data, indicating that the quenching process happened in a very short time scale (~ 2.6 ps).