(201l) Crystal Structure of Coalescing CdSe Nanoparticles By Molecular Dynamics Simulations

Cadmium selenide (CdSe) nanoparticles are widely used for CdSe/CdTe heterojunction photovoltaic devices due to their intermediate energy band gap that allows reasonable conversion efficiency, stability and low cost. Such nanocrystals find a score of applications including tunable light emitting diodes, photovoltaics and single electron transistors. Despite their extensive investigation, atomic level description of the entire nanocrystal (both core and surface) is not trivial to the difficulty of probing the nanocrystal surface. Surface composition and stoichiometry, however, is important since it affects the presence of ligands. Thus, it is essential to characterize nanocrystal surface during sintering to facilitate implementation of these nanocrystals in electronic, opto-electronic, and electro-optic devices.

Here, the evolution of surface composition of free-standing, coalescing CdSe nanoparticles is investigated for different particle sizes during sintering at various temperatures by atomistic molecular dynamics (MD) simulations. Sintering of Cd and Se nanoparticles with atomic ratio of Cd:Se 1.2:1 results in the formation of alloyed CdSe nanoparticles, consistent with the literature. Cadmium atoms exhibit increased mobility upon coalescence and substitute interstitially Se atoms in the Se lattice. The initial particle size affects the sintering rate. Furthermore, the X-ray diffraction patterns (XRD) of CdSe nanoparticles are calculated during sintering, revealing the formation of zinc blende and cubic structures for initial CdSe particle diameters of about 4 nm. The sintering and surface composition of these CdSe nanocrystals is compared with MD-obtained coalescence rate and surface stoichiometry of CdTe particles. The MD-derived XRD patterns of CdSe and CdTe nanoparticles are compared to experimental data.