We report the observation of the enhancement of a transverse optical (TO) phonon mode of e 1 symmetry and a normally forbidden surface optical (SO) phonon mode of b 1 symmetry in CdSe quantum dots (QDs) due to adsorption of 4mercaptopyridine molecules. The former is observed in 3, 4, and 5 nm diameter particles, while the latter is observed only in the 2 nm particles. Maximum enhancement of the phonon modes is obtained through a coupling of the charge-transfer transition and the exciton transition which are in resonance with the laser energy. Selection rules using Herzberg−Teller (vibronic) coupling are invoked to explain the observed enhancements.
We report a simple method to synthesize monodisperse hydrophobic PbTe quantum dots followed by a stability study of the as-synthesized quantum dots in air. We provide evidence that air-stable PbTe quantum dots can be synthesized using this method. PbCl 2 complexed with oleylamine was used as the Pb precursor. Tellurium powder dissolved in tri-n-octylphosphine was used as the Te precursor. Oleylamine was used as the only capping ligand. The size and shape of the PbTe quantum dots were controlled by changing variables such as injection temperature, growth temperature, and growth time. Both Pb to oleylamine and Pb to Te feed mole ratios have been examined to obtain the optimal synthetic conditions. The PbTe quantum dots can be changed from hydrophobic to hydrophilic through ligand exchange using 4-mercaptopyridine to replace oleylamine as the capping ligand. The colloidal PbTe quantum dots were characterized by transmission electron microscopy, high resolution transmission electron microscopy, selected area X-ray diffraction, energy-dispersive X-ray spectroscopy, FT-IR spectrometer, 31 PNMR spectrometer, and powder X-ray diffraction. The sizes of the PbTe quantum dots synthesized ranged from 2.6 nm to 14.
Self-assembled quantum dots of CdSe with ZnCdMgSe barriers have been grown by molecular beam epitaxy on InP substrates. The optical and microstructural properties were investigated using photoluminescence (PL) and atomic force microscopy (AFM) measurements. Control and reproducibility of the quantum dot (QD) size leading to light emission throughout the entire visible spectrum range has been obtained by varying the CdSe deposition time. Longer CdSe deposition times result in a redshift of the PL peaks as a consequence of an increase of QD size. AFM studies demonstrate the presence of QDs in uncapped structures. A comparison of this QD system with CdSe∕ZnSe shows that not only the strain but also the chemical properties of the system play an important role in QD formation.
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