Computational study of the optical properties of ZnS nanoparticles
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Computational study of the optical properties of ZnS nanoparticles
M. M. Sigalas1 1
Department of Materials Science, University of Patras, Patra 26500, Greece
ABSTRACT Using the density functional theory (DFT) and time dependent DFT, within the generalized gradient approximation (GGA), the electronic and optical properties of stoichiometric (ZnS)n nanoparticles (NP) were calculated. The dependence of the gap on the size (n) of the nanoparticle will be presented. The effect of replacing S atoms with P, Se or Te atoms in the (ZnS)n nanoparticles and its influence in the gap will be also shown. INTRODUCTION Quantum dots (QD) have a wide range of applications from photovoltaics to bio-labeling. In particular, quantum dot sensitized solar cells (QDSSC) are an attractive alternative to the Dye sensitized solar cells since QD are more stable than dyes. [1] There have been a lot of studies in CdSe nanoparticles (see Ref. [2]). Zinc sulfide (ZnS) nanoparticles (NP) [3] is a similar type system made from more abundant materials and it is a possible alternative to CdSe NP. For practical QDSSC applications, the NPs need to absorb light in the visible spectrum. However, the measured band gap for bulk ZnS is in the ultraviolet (UV) region [4, 5], thus, it does not absorb in the visible region. As it will become clear in the following section, ZnS NPs have similar absorption spectrum. A possible way to move their absorption into the visible spectrum is by substituting some of the atoms (S in the present study) with other atoms (substitutional defects). These types of nanoparticles may be used in QDSSCs and the present computational study can guide further experimental studies to choose the right size and composition of the nanoparticles in order to get the maximum absorption of the solar spectrum.
THEORY AND RESULTS
The Density Functional Theory (DFT) and the time dependent DFT (TDDFT) were used for the calculation of the electronic properties of ZnnSn nanoparticles. The geometry optimization of the NP was done with the PBE gradient corrected functional and the def-SVP basis set. For each n, several initial geometries (resulted from both zinc blende and wurtzite bulk crystalline structures) were checked and fully optimized in order to find the geometry with the lowest energy. Then, the lowest energy structure was re-optimized with the hybrid functional B3LYP. The average size of
ZnnSn NP P were aboutt 0.8, 1.1, an nd 1.4 nm fo or n=15, 33, and 68, resppectively. Alll the calculaations were perrformed witth the GAU USSIAN prrogram (for references and furtheer details of the computattional metho ods see Ref. [2]). Figure 1 show the highest occup pied molecu ular orbital (H HOMO) – llowest unocccupied moleecular LUMO) gap p (HL gap)) and the optical o gap (the first aallowed elecctronic transsition orbital (L calculated with TDD DFT) for ZnnSn NP with h n from 4 tto 68. In all the cases, tthe optical ggap is lower thaan the HL gaap by about 0.2-0.7 eV, but they botth follow thee same trendds. The miniimum and the maximum m vaa
