Molecular Orbital Approach to the Electronic Structure of CdS Ultrafine Particles

Abstract

The electronic structure of CdS ultrafine particles was studied by the semiempirical molecular orbital (MO) method. A practical procedure to perform MO calculations on a very large molecule which contains roughly one thousand atoms was established. The aimed electronic structure was obtained and characterized. A tendency which corresponds to the so-called size quantization effect usually observed in semiconductor ultrafine particles was confirmed. An electronic transition energy, which was almost comparable to experimental data, was obtained. The influence of surface structures on the electronic state was directly calculated by the MO method. Low-coordinated Cd atoms were found to produce surface states. A surface defect site associated with excess Cd was simulated by introducing a sulfur vacancy. A new MO that originated from the defect site, which may act as a trapping site of excited electrons, was identified.

Effect of Phenyl-Capping and Dimer Formation on the Electronic States of CdS Nanoparticles by Means of Semiempirical Molecular Orbital Calculations

“…The contributions of the Cd 5s in LUMO and S 3p in HOMO became dominant with the increase of the cluster size. This observation is consistent with the reported result based on CNDO/2 calculation 1 Density of states for [S 1 Cd 8 (SR) 16 ] 2- clusters with the ligand SR of (a) −SH, (b) −SCH 2 CH 2 OH, and (c) −SPh .…”

“…This observation is consistent with the reported result based on CNDO/2 calculation. 3 By comparing parts a and b of Figure 1, the HOMO state at about -15 eV shifts toward a higher energy, resulting from the coordination of alkanethiolate. Three kinds of density of states in Figure 1b showed similar curves, indicating that the electrons distribute both to CdS and ligand carbon atoms at this energy band.…”

“…Since the chemical structure of polynuclear metal complex is clearly presented, it is a suitable model to investigate the electronic states of semiconductor nanoparticles. Some molecular orbital (MO) approaches to understand the electronic state of inorganic semiconductor nanoparticles have been reported so far, by using extend Hueckel, CNDO/2, CNDO/S, and ab initio methods for CdS nanocrystals. The extend Hueckel approach to Cd x S y clusters of zinc blende structure, where x ≤ 20 and y ≤ 35, has showed the decrease in HOMO−LUMO energy difference .…”

“…The extend Hueckel approach to Cd x S y clusters of zinc blende structure, where x ≤ 20 and y ≤ 35, has showed the decrease in HOMO−LUMO energy difference . The CNDO/2 method is used to investigate surface defects of CdS nanocrystal with 221 Cd atoms . Calculation of the oscillator strength of CdS nanoparticles has been tried by means of the CNDO/S method …”

“…2 The CNDO/2 method is used to investigate surface defects of CdS nanocrystal with 221 Cd atoms. 3 Calculation of the oscillator strength of CdS nanoparticles has been tried by means of the CNDO/S method. 4 Ab initio calculation for a CdS cluster of tetrahedral symmetry up to Cd 13 sulfide clusters allowed us to compare the bond length to the experimental values.…”