Electronic structure of ZnO/ZnS core/shell quantum dots Supriya Saha, Pranab Sarkar ⇑ Department of Chemistry, Visva-Bharati University, Santiniketan 731 235, India article info Article history: Received 16 September 2012 In final form 5 November 2012 Available online 10 November 2012 abstract We report here the results of our theoretical investigation on the electronic structure of ZnO/ZnS core/shell quantum dots (QDs) with special emphasis on the atomic charge distribution, electronic energy levels, HOMO–LUMO gap and their variation with thickness of the ZnS shell. We have also studied the possibility of tuning the electronic energy levels of ZnO/ZnS QDs by passivating the surface of the QDs with different ligands. Ó 2012 Elsevier B.V. All rights reserved. Introduction Studies in colloidal semiconductor nanoparticles have been a subject of extensive research during the last couple of decades. This is primarily because of their diverse application in many fields ranging from optoelectronic devices to the latest third generation solar cells [1–6]. The stability of QDs against chemical degradation and luminescence efficiency from band edge states are greatly im- proved if one epitaxially grows a higher band gap semiconductor material around the QDs, resulting in the so-called core/shell nano- particles [1]. Core/shell semiconductor QDs exhibit many novel properties and have become a subject of current interest from both fundamental and practical points of view. Quantum dots passiv- ated with inorganic shell structures are more robust than QDs pas- sivated with organic molecules as because the inorganic shell protects surface atoms of the core from oxidation and other chem- ical reactions, and thus enhances the photo-stability by several or- ders of magnitude relative to conventional organic molecules. These core/shell type QDs have greater tolerance to processing conditions necessary for incorporation of them into solid state structures. The compounds ZnO and ZnS represent two important members of II–VI semiconductor with wide band gap. They are abundant, stable and environmentally benign and thus deserve special attention. However, the band gaps of ZnO and ZnS are too large to absorb or emit radiation in visible region. This may cause severe limitation to their applicability. The core/shell nanostruc- tures viz. nanowires and nanotubes made of ZnO core and ZnS shell have type II band alignment and both valence and conduction bands of the core are lower in energy than those of the shells. As a result, one of the carriers either the electrons or holes are mostly confined in the core while others are in the shell. For such nano- structures one has the opportunities to tune the optical properties by varying the shell thickness [7–10]. Therefore, the spectral ranges which are otherwise difficult to attain for simple semicon- ductor nanoparticles can be achieved with these core/shell type nanoparticles. Various experimental studies suggest that capping of ZnO QDs with ZnS shell enhances both chemical stability and photo-luminescence quantum efficiency [7,8,11–14]. Because of this enhanced luminescence, these QDs have found application in biological labelling [13,14]. The ZnO/ZnS QDs are also important for transportation of drugs in blood. Xiao, et al. [15,16] have re- cently studied the affinities of flavonoid aglycones for bovine ser- um albumin (BSA) in the presence and absence of ZnO/ZnS QDs. The application of these QDs in various fields requires extensive knowledge of their electronic structure. In view of this, we perform here a theoretical study to explore the electronic structure of –H passivated ZnO/ZnS core/shell QDs. We also try to shed some light on the atomic charge distributions, electronic energy levels, HOMO–LUMO gap etc. of the ZnO/ZnS QDs and how these proper- ties modify as the thickness of the ZnS shell varies. Since the sur- face of the QDs plays a crucial role in dictating the electronic and optical properties, we have investigated the effect of different pas- sivating ligands viz. –OH, –SH and –NH 2 on the electronic energy levels of these QDs. Computational details We have employed the self-consistent charge density-func- tional tight-binding (SCC-DFTB) method for the electronic struc- ture calculation. The details of the method have been described elsewhere [17–20] and here we, therefore, give a brief outline. In this method, the single-particle Kohn–Sham eigen functions w i ðrÞ are expanded in a set of localized atom-centered basis func- tions / m , with m, a compound index that takes care of radial and angular dependence of the function. These functions are determined by self-consistent density-functional calculations on 0009-2614/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.cplett.2012.11.006 ⇑ Corresponding author. Fax: +91 3463 262672. E-mail address: pranab.sarkar@visva-bharati.ac.in (P. Sarkar). Chemical Physics Letters 555 (2013) 191–195 Contents lists available at SciVerse ScienceDirect Chemical Physics Letters journal homepage: www.elsevier.com/locate/cplett