AB initio study of the structural, electronic, and optical properties of the (CdSe)n; n=1-3 clusters and its isomers

Abstract

Cadmium selenide compound (CdSe) in nano-sized form is extensively used in many optoelectronic applications. In nano-particle dimension, the characteristics of CdSe strongly depend on its stoichiometric clusters. (CdSe)n with n = 1-3 stoichiometric clusters have many possible isomers. Unfortunately, it is difficult and laborious to deduce the intrinsic features of the isomers experimentally. Hence, the aim of this research is to ascertain the preferred stable structures and respective electronic and optical properties through theoretical calculations. To achieve this goal, a computational study was carried out. The geometrical optimization calculations to reach full geometry relaxation were implemented using ABINIT package version 7.8.2, which adopted the density functional theory (DFT). To determine the stable structure, ABINIT code conduct self-consistent field calculations of forces repeatedly to reach the threshold values of the convergence criteria. Octopus package version 7.1. was then used to determine the absorption spectra of the most low-laying stable isomers. Casida's formulation to find out the excitation energies of linear-response was implemented. The optimized isomers were placed in a sufficient spaced simulation-box to enable Octopus to calculate the oscillator strengths and strengths function. The band structure of zinc blende CdSe semiconductor was calculated using four different pseudopotentials. The results revealed that placing the cadmium atom d-shell as core electrons enhanced the band gap value by more than 50%, compared to other pseudopotentials. (CdSe)2 clusters’ results showed six possible stable isomers, three of them are detected for the first time. Rhombus structure is the most stable (CdSe)2 isomer, with bond length of 2.50-2.74 Å and average Cd-Se-Cd, Se-Cd-Se angles were 64.5o-123o and 56.3o -114.2o respectively. The total ground-state energy differences were 0-1.82 eV. The calculated absorption spectra of (CdSe)2 stable isomers indicated a strong optical response in the 4.87-70 eV region, with 2-3 major peaks. The number of stable isomers of (CdSe)3 was 22. Planar ring structure was the most stable isomer of (CdSe)3. The average Cd-Se bond length is 2.66 Å while average Cd-Se-Cd and Se-Cd-Se angles were 85.5o and 131.1o, respectively with energy differences 0.0-3.44 eV and HOMO-LUMO gap of 0.64-2.45 eV. From the calculated absorption spectra of (CdSe)3 stable isomers, a strong response region was in a very narrow domain; i.e. 6.20-7.75 eV. The maximum strength function was more dispersed between 1.57 and 6.00. The number of main peaks was up to 3, with increasing number of minor peaks. Matching the calculated absorption peaks with experimental peaks in the literature showed that matching for (CdSe)2 is weak, on the other hand, matching (CdSe)3 showed satisfactory. The stable isomers of (CdSe)n n=1-3 have high probability to exist in any experiment including cadmium selenide. Each isomer exhibits its own specific absorption spectrum that can be used as its fingerprint. The optical absorption of these bare clusters lies in the ultraviolet region, doping these clusters with other atoms to tune its optical properties is advised. The results obtained in this work were comparable to other computational and experimental results. In brief, the goals of this work were accomplished totally. A significant portion of this study results were strongly supported by available previous computational and experimental works. More advanced experimental methods at the clusters size limit should be explored.