The Absorption of Diamondoids from Time-dependent Density Functional Calculations
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The Absorption of Diamondoids from Time-dependent Density Functional Calculations M´arton V¨or¨os1 , Tam´as Demj´en2 , Adam Gali1,2 1 Department of Atomic Physics, Budapest University of Technology and Economics, Budafoki u´ t 8., H-1111, Budapest, Hungary 2 Hungarian Academy of Sciences, Research Institute for Solid State Physics and Optics, Budapest, POB 49, H-1525, Hungary ABSTRACT Diamondoids are small diamond nanocrystals with perfect hydrogenated surfaces. Recent absorption measurements showed that the spectrum of diamondoids exhibit features that are not understood from the theoretical point of view, e.g. optical gaps are only slightly larger than the gap of bulk diamond which runs against the quantum confinement effect. Previous calculations, even beyond standard density functional theory (DFT), failed to obtain the experimental optical gaps (Eg ) of diamondoids. We show that all-electron time-dependent DFT (TD-DFT) calculations including the PBE0 hybrid functional in the TD-DFT kernel are able to provide quantitatively accurate results. Our calculations demonstrate that Rydberg transitions govern the low energy part of the absorption spectrum, even for relatively large nanodiamonds resulting in low Eg . Since the optical gap of these diamondoids lies in the ultraviolet spectral region, we investigated whether simple adsorbates of the surface are able to shift the gap towards the infrared region. We found that a double bonded sulfur atom at the surface results in a substantial gap reduction. INTRODUCTION The optical gap of semiconductor nanoparticles (NP) can be tuned by reducing their size that is called the quantum confinement (QC) effect [1]. This is now a major focus of nanoscale research in the applications of biomarkers, photovoltaics and photocatalysis, therefore, understanding the optical properties of small semiconductor NPs is of high interest. Due to computational limitations accurate ab initio methods for calculating optical response of NPs are restricted to single, perfect and isolated structures[2, 3, 4, 5] with relatively small particle size which cannot be directly compared to experimental data taken usually from larger NPs with unknown surface structure and various size distributions. Recently, ultrasmall diamond nanoparticles (DNP), also called diamondoids, containing 10-26 carbon atoms have been isolated and purified from petroleum [6]. The resulted DNPs were selected by different size and shape via high performance chromatography [6]. The optical spectrum of the 99% purified DNPs have been very recently detected by ultraviolet absorption spectroscopy at room temperature and above [7]. The obtained optical spectrum of DNPs measured by Landt et al.[7] is rather surprising because, i) the optical gaps are relatively close to the indirect band gap of bulk diamond, ii) none of recent theoretical studies could predict quantitatively the optical gaps of nanodiamonds. The DNPs fabricated by Landt et al. [7] have a fundamental importance: i) their chemical structure is exactly known, ii) they build pe
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