Chemical processing of colloidal cadmium selenide nanoparticles: New approaches to dimensional and morphological control

Abstract

Cadmium selenide (CdSe) quantum dots (QDs) are colloidal semiconductor nanoparticles (NPs) that are nanometer sized fragments of the corresponding bulk crystals. They are being probed as a very interesting system for their applications in LEDs, solar cells and biomedical labeling because of their rich photophysics arising from their size dependent optical and electronic properties and flexible processing chemistry. 1-Octadecene was the only non-coordinating solvent used for the synthesis of CdSe NPs. It was imperative to understand the chemistry of synthesis of CdSe NPs using other non-coordinating solvents. Also, there is a constant search for the greener, cheaper, reproducible and scalable methods for the synthesis of CdSe NPs while not compromising on their quality. Towards the above mentioned goals, the use of heat transfer fluids was successfully demonstrated as cost-effective alternative solvents for quantum dot synthesis. Heat transfer fluids (HTF) are a class of organic liquids commonly used in chemical process industries to transport heat between unit operations. The solvents were found suitable for the hot injection synthesis of QDs while reducing the cost of the raw materials. These solvents were found to slow the growth kinetics of the CdSe NPs, leading to greater control over QD diameter. Although the chemistry of synthesis of CdSe spherical and rod shaped particles were well understood, the synthesis of tetrapod shaped NPs with uniform size and shape is especially difficult to carry out at a large scale. Post-synthesis separation can be applied though this leads to additional processing steps and reduced particle yields. Cationic surfactant ligands were discovered to lead to the successful formation of tetrapod shaped CdSe NPs with highly uniform arm lengths, arm widths and shape. Typical selectivity values for the tetrapod morphology exceeded 90%, much higher than the previously reported values of 40%. The cationic surfactant ligands were found to induce anisotropy during the growth of the CdSe NPs, with cetyltrimethylammonium bromide (CTAB) and didodecyldimethylammonium bromide (DDAB) leading to the specific tetrapod shape. Optimization of the synthesis procedure led to the control over the range of dimensions of the tetrapod shaped particles. The uniformity of size and shape of the CdSe tetrapodal NPs were found to be very sensitive to the ratio of the surfactants and the precursors. It was also observed that the presence of the cationic surfactants led to the exclusive formation of zinc-blende nuclei which is indispensable for the growth of nuclei into tetrapodal NPs. The growth of CdSe tetrapodal NPs from the preformed zinc-blende CdSe NPs, just by the addition of CTAB, demonstrated the crucial role of the cationic surfactants in inducing anisotropy. These findings were helpful in gaining insight in to the nucleation and growth of the anisotropic nanoparticles. These new methods should lead to improvements in current synthesis methods of tetrapodal shaped CdSe NPs, enabling the faster development of polymer/tetrapod photovoltaic devices. These results may be applicable to other compositions, leading to opportunities for large-scale application of shaped NPs.