Chemical trends in the optical properties of rocksalt nanoparticles.

Milena C C Wobbe,Martijn A Zwijnenburg

doi:10.1039/c5cp04851f

Abstract

The nature and magnitude of the optical gaps of rocksalt alkaline earth (MgO, CaO, SrO, MgS, MgSe) and transition metal chalcogenide (CdO, PbS) nanoparticles are studied using time-dependent density functional theory (TD-DFT) calculations on (MX)32 nanoparticles. We demonstrate, just as we previously showed for MgO, that TD-DFT calculations on rocksalt nanoparticles require the use of hybrid exchange-correlation (XC-)functionals with a high percentage of Hartree-Fock like exchange (e.g. BHLYP) or range-separated XC-functionals to circumvent problems related to the description of charge-transfer excitations. Concentrating on the results obtained with TD-BHLYP we show that the optical gap in rocksalt nanoparticles displays a wide range of behavior; ranging from large optical gaps stemming from a localized excitation involving corner atoms in alkaline earth oxides to a delocalized excitation and small optical gaps in the transition metal chalcogenides. Finally, we rationalize this wide range of behaviour in terms of differences in the degree to which the Coulombic interaction between the excited electron and hole is screened in the different nanoparticles, and relate it to the optical dielectric constants of the bulk materials the nanoparticles are made from.

Highlights

The absorption spectra of nanoparticles of semiconducting materials are generally blue-shifted with respect to that of the infinite bulk due to the quantum confinement.[1,2,3,4,5] The excited state, the exciton, is delocalised over the whole particle and the finite size of the particle means that the exciton is spatially constrained, resulting in an upward shift of the excitation energy, the magnitude of which is inversely proportional to the particle size.Nanoparticles, do not always have blue-shifted absorption spectra
Having successfully studied the effect of the particle size, here we extend our work by focussing on nanoparticles of other materials than magnesium oxide (MgO) that crystallize experimentally in the rocksalt structure; calcium oxide (CaO), strontium oxide (SrO); magnesium sulfide (MgS); magnesium selenide (MgSe), cadmium oxide (CdO) and lead sulphide (PbS)
This all, together with the fact that the excited state localisation predicted by TD-B3LYP had all the hallmarks of a Charge transfer (CT)-state and that predicted by the TD-BHLYP not, made us feel confident that the TD-B3LYP optical gap was a spuriously stabilised CT-state and that TD-B3LYP cannot be properly used to study MgO nanoparticles in the size range relevant to experiment

Summary

Introduction

The absorption spectra of nanoparticles of semiconducting materials are generally blue-shifted with respect to that of the infinite bulk due to the quantum confinement.[1,2,3,4,5] The excited state, the exciton, is delocalised over the whole particle and the finite size of the particle means that the exciton is spatially constrained, resulting in an upward shift of the excitation energy, the magnitude of which is inversely proportional to the particle size. Nanoparticles, do not always have blue-shifted absorption spectra. The pinning of the excited state on corners or edges of a particle, might result in a particle with a red-shifted rather than a blue-shifted optical gap (onset of light absorption).

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Discussion

Conclusion