Tuning the HOMO–LUMO gap of SiC quantum dots by surface functionalization Supriya Saha, Pranab Sarkar ⇑ Department of Chemistry, Visva-Bharati University, Santiniketan 731235, India article info Article history: Received 22 November 2011 In final form 28 March 2012 Available online 5 April 2012 abstract We present density-functional theoretical results of structural, electronic and optical properties of bare and surface passivated (–H, –OH, –NH 2 ) stoichiometric Si n C n (n = 10, 16, 28, 37, 43, 55, 68) quantum dots (QDs) as a function of the size of the dots. We have studied both the effect of size and surface passivation on the band edge electronic and optical properties of SiC nanoparticles. We show that the HOMO–LUMO gap of SiC nanoparticles can be tuned by using different functionalizing agents. Thus –H passivated QDs have the largest HOMO–LUMO gap followed by –OH and –NH 2 passivated QDs. We also present few results on non-stoichiometric Si m C n clusters. Ó 2012 Elsevier B.V. All rights reserved. In the recent past, the research in semiconductor nanocrystals have got tremendous attention because of their unique size and shape dependent optoelectronic properties. Experimentalist have developed many methods for the design and growth of nanocrys- tals of different size and shape and have striking properties con- trolled by quantum confinement. These nanoparticles may serve as building blocks for the new generation opto-electronic devices as transistors, light-emitting diodes, lasers or solar cells and also in spintronics. Silicon carbide (SiC), a wide band gap semiconductor having excellent thermal conductivity, high breakdown electric field, high saturated electron drift velocity and high chemical stability offers attractive applications in microelectronics and opto-electronics [1–4]. The device made of SiC has the advantages of operating at high temperature, high frequency and chemically hostile environ- ment. Very recently, Botsoa et al. have shown the applicability of SiC QDs as fluorescence imaging of biological living cells [5]. SiC, in general exists in many different polytypes but the only cubic polytype, called b-SiC, is of the zinc-blende structure. Because of its many characteristics common to diamond and also higher elec- tron mobility b-SiC is of special interest. There are extensive exper- imental research exploring the synthesis, characterization and application of SiC QDs. However, theoretical research exploring the electronic structure of SiC QDs are scarce. Olander et al. [6] have studied the ab initio absorption spectra of small SiC clusters. Arulsamy et al. [7] have studied the effect of elemental size and composition on the electronic properties of SiC nanoparticles. Reb- oredo et al. [8] have recently studied SiC nanoparticle up to diam- eter 3 nm and have showed that surface composition and termination play a dominant role in determining the optical gaps and thermodynamic stability. Li et al. [9] have investigated the effect of surface charge and optical characteristics of colloidal cubic SiC. In experimental realizations, nanocrystals are formed by kinet- ically controlled precipitation, and are terminated with capping ligands which provide stabilization of the otherwise reactive dan- gling orbitals of surface atoms. The role of surface-related states is a likely possibility since a large number of atoms composing the nanocrystallite reside on the surface. So, the surface passivation and also the nature of the surface passivating agent in addition to size and shape plays an important role in tuning the electronic and optical properties. In this Letter we will present our results of the effect of both size and surface passivation on the electronic and optical proper- ties of SiC clusters. In experimental studies one often uses large or- ganic molecules such as cystine or trioctylphosphineoxide (TOPO), but for computational reason we have chosen to work with smaller ones. In this Letter we considered –H, –OH and –NH 2 atoms/groups as the passivating agents. We show how the HOMO–LUMO gap, density of states (DOSs), HOMO, LUMO densities modifies because of surface passivation. We also explore the possibility of tuning the HOMO–LUMO gap by using different kind of passivating agents. The electronic structure calculation method, the self-consistent charge density-functional tight-binding (SCC-DFTB) method used in this Letter has been described in detail elsewhere [10–13]. The DFTB method is a parametrized DFT scheme based on expanding the solutions to the Kohn–Sham (KS) equations in a basis set of Linear combination of atomic orbitals (LCAO). The LCAOs were obtained from self-consistent calculations on the isolated neutral atoms using a large set of Slater-type orbitals. For the system of interest, the effective one electron potential in the KS Hamiltonian is approximated by a superposition of the atomic potentials of the corresponding neutral, non-interacting atoms [10]. Further- more, we make use of a tight-binding approximation so that only 0009-2614/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.cplett.2012.03.107 ⇑ Corresponding author. E-mail addresses: pranab.sarkar@visva-bharati.ac.in, pranab_69@yahoo.co.in (P. Sarkar). Chemical Physics Letters 536 (2012) 118–122 Contents lists available at SciVerse ScienceDirect Chemical Physics Letters journal homepage: www.elsevier.com/locate/cplett