Quantum confinement effects in semiconductor clusters

Abstract

The band gaps, band structure, and excited-state (exciton) energies of CdS, GaAs, and GaP semiconductor clusters are calculated using pseudopotentials. In addition, the sensitivity of the exciton energies to the size, shape, crystal structure, and lattice constant of the unit cell are investigated. The calculated exciton energies of CdS clusters are in excellent agreement with experiment over a wide range of cluster sizes. Also, the exciton states of small CdS clusters are sensitive to whether their crystal structure is zinc blende or hexagonal. Such a sensitivity is absent in large CdS clusters. Furthermore, small GaAs clusters are shown to exhibit anomalous redshift of their absorption spectra, in sharp contrast to CdS and large GaAs clusters whose spectra always shift to blue with decreasing cluster size. Finally, the lowest-energy non-Franck–Condon transition in GaP clusters always shifts to blue with decreasing cluster size, whereas the higher-energy Franck–Condon transition in small clusters exhibits the anomalous redshift. These novel findings reveal that (1) the optical spectroscopy of semiconductor clusters is strongly material and crystal structure dependent; (2) the spectroscopy of small clusters is dramatically different from those of large clusters and bulk; and (3) these effects cannot be explained, even qualitatively, using the effective-mass approximation.

Laser assisted solid state reaction for the synthesis of ZnS and CdS nanoparticles from metal xanthate

“…The absorption band in CdS nanocrystals is attributed to the first excitonic transition between the ground state and the single electron−hole pair state (1S 3/2 1se) [36] The photoluminescence (PL) spectrum of ZnS nanoparticles after 10 min of irradiation displays a broad emission spectrum between 360 and 610 nm with a peak at approximately 403 nm, which shifted to approximately 426 nm after 30 min irradiation time, as shown in Figure 2(A). This emission band could be ascribed to a recombination of electrons at the sulphur vacancy donor level with holes trapped at the zinc vacancy acceptor level [37,38].…”

Peer Reviewed: Optical Sensing with Quantum Dots

“…The bandgap energy of the semiconductor from such spectra is generally taken as the absorption energy onset. As the particle size decreases, the absorption onset shifts to higher energy (blue shifts), indicating an increase in bandgap energy (3)(4)(5)(6)(7)(8)(9).…”

Electroluminescence and Spectral Shift of CdS Nanoparticles on Si Wafer

“…The emission peak at 526.5nm (2.355eV) is red-shifted from bulk CdS A-exciton transition energy, 2.44 1eV(508nm) at room temperature. [7] The energy with a red-shill of 86 mcV does not come from quantum confinement within the nanoparticles, since such effect increases the exciton energy when the particle size decreases [8,9]. It is due to the capping p-hydroxyl thiophenol group that modifies the energy of emission peak.…”