Understanding Structural and Optical Properties of Nanoscale CdSe Magic-Size Quantum Dots: Insight from Computational Prediction

Structure and properties of small quantum dots are of fundamental and practical interest due to a broad range of applications that exploit their optical tunability. Although previous experimental and theoretical investigation of very small semiconductor quantum dots have shown relative stability for clusters that deviate from the bulk, these materials have not been systematically characterized. In this work, structures of (CdSe)n (n = 1−37) clusters were studied using a combination of structure enumeration, Monte Carlo search, and local optimization. Binding energy (En and relative binding energy, ΔEn = n(En+1 − En), per CdSe unit) calculations using density functional theory (DFT) were carried out to identify the so-called magic size (MS, ΔEn > 0) quantum dots. (CdSe)n with n = 9, 12, 16, 18, 21, 24, 28, 32, 33, 35, and 36 were found to have high relative stability. MS structural motifs were investigated further for clusters up to (CdSe)54. In addition, time-dependent DFT calculations of one-photon absorption (OPA) spectra for MS clusters were carried out to compare with experimental spectra. Computational prediction provides insight into the cluster formation during early growth and serves as a starting point for further analysis of the nonlinear optical response. The effects of ligands/solvent on the structure, stability, and OPA spectra were also examined for selected clusters.