Quantized growth of CdTe quantum dots; Observation of magic-sized CdTe quantum dots

This work presents experimental observation of the quantized growth of CdTe quantum dots (QD) in the presence of hexadecylamine (HDA), hexylphosphonic acid (HPA), and trioctylphosphine oxide (TOPO) above 200 degrees C. The crystal growth of CdTe QDs is monitored by in situ UV-vis absorption spectroscopy. The high-temperature absorption spectra indicate the evolution of multiple peaks corresponding to various sizes of QDs. Analysis of the growth kinetics suggests quantized growth of the CdTe QDs in the coordinating solvent mixture. The high-resolution transmission electron microscopy (HRTEM) images and electron diffraction pattern show that most of the QDs have the zinc blende crystal structure. The HRTEM images indicate nanotwinning and stacking faults in larger CdTe QDs. Domain sizes in the HRTEM images correlate well with the smallest observed magic-sized CdTe QDs, in agreement with the proposed aggregation growth mechanism under the experimental conditions. The smallest observed zinc blende CdTe QDs with the diameter of 1.9 +/- 0.3 nm are isolated by quenching the reaction mixture during the initial phase of the QD synthesis. The experimental observation suggests that the surprising stability of the magic-sized CdTe QDs is the result of the surface stabilization of the QDs of the HDA and/or HPA. As previously suggested, the aggregation is driven by dipole-dipole interaction between CdTe nanoparticles. The results show that the aggregation of quantum dots could be very important at the early stage of the growth. The magic-sized QDs can be dissolved in either methanol or toluene, which suggests heterogeneity of their surface chemistry. The QDs dissolved in the methanol phase exhibit relatively strong white light emission from 400 to 650 nm with an emission quantum yield of approximately 4%. The QDs dissolved in the toluene phase exhibit very weak emission.