Charge transfer at organic–inorganic interface of surface-activated PbS by DFT method Nguyen Thuy Trang a, ⁎, Luu Manh Quynh a , Tran Van Nam a , Hoang Nam Nhat b a Faculty of Physics, Hanoi University of Science, 334 Nguyen Trai, Thanh Xuan, Hanoi, Viet Nam b Faculty of Engineering Physics and Nanotechnology, University of Engineering and Technology, 144 Xuan Thuy, Cau Giay, Hanoi, Viet Nam abstract article info Article history: Received 10 July 2012 Accepted 24 September 2012 Available online 2 October 2012 Keywords: Organic–inorganic interface PbS 4-Aminothiophenol Charge transfer DFT Electronic structure of a surface-activated PbS by a bio-active molecule 4-aminothiophenol (4-ATP) was in- vestigated using density functional theory (DFT). The obtained results demonstrated the importance of charge transfer which accounted for the flipping of surface rumpling and the nature of the binding between the activated surface and the capping agent. The influence of 4-ATP–PbS topology on bonding nature between surface atoms was discussed. The capping-induced bonding nature shift was interpreted as surface core level shifts (SCLSs) of PbS. © 2012 Elsevier B.V. All rights reserved. 1. Introduction Hybrid nanocomposites which are composed of nanoparticles (NPs) with inorganic nanocrystalline cores and organic shells have been strongly attracting researches because of their wide range of applications from optoelectronics to biology [1–5]. One of the most brilliant candi- dates for fabricating such kind of composites is nanocrystalline lead sul- fide PbS. The narrow bulk band gap E g ~ 0.29 eV (at low temperature) [6] and strong quantum confinement effect with Bohr radius ~ 18 nm [7] allow to easily optimize the optical band gap as well as the absorption and emission bands of the material by controlling the particle size [8]. As a result, optical properties of PbS nanocomposites have been inten- sively studied for photoemission elements in organic light emitting diodes (OLEDs) and photovoltaic devices. The size-induced widening of absorption and emission ranges from near infrared (NIR) to visible (VIS) region of hybrid PbS NPs makes it easier to change the optical- active region of the hybrid devices than the devices using organic mole- cules alone [9–11]. Hence PbS-based devices are more efficient than the organic one. Moreover, recent electrochemical investigations have suggested that PbS nanocomposites can act as electrochemical biosensor [12–16]. Under applied voltages ~1.1 V, Pb 2+ ions in PbS NCs can be oxidized to neutral Pb atoms, which are recognizable with a high peak in the cycle voltammogram [12]. This observation promised a high sensi- tivity of PbS-based bio-sensors. The development of hybrid nanocomposite devices usually faces com- plicated interactions between different fragments including interparticle interaction, NPs–solvent interaction, intraparticle interaction and interac- tion between NPs and organic electron-acceptor or donor agents in solu- tions. It has been shown via optical observations that those interactions were in close relations with each other. The interparticle interaction was shown to occur between NPs via long-range Forster resonant energy transfer (FRET) which results in an enhancement of the low energy emis- sion [17,18]. In solution, it was dominated by the interaction between NPs and solvent [19]. Besides, the interactions between NPs and solvent di- poles tend to increase the intra-NP charge transfer (CT) rate via a statistical mechanism which was attributed to the long time scales of CT processes relative to the time scale of molecular motion [20]. Optical mea- surements on mixtures of PbS NPs and exposed the fact that due to the energy level alignment, the charge exchanges of PbS NPs and electron donating molecules only occur in excited state while that of PbS NPs and electron accepting molecules can occur at ground state [19]. It was shown that all of the CT processes strongly depend on NP size [20,21]. To clarify such complicated interactions, electronic structure aspects should be involved. In this work, in the framework of density functional theory (DFT), we investigated electronic structure of a PbS — organic molecule junction at which a clear chemical bonding should occur so that the charge can be directly transferred. The selected organic mole- cule is 4-aminothiophenol (4-ATP) (Fig. 1a) because of two reasons. Firstly, it has been frequently used for nanoparticle coating as efficient surface stabilizer to prevent particle aggregations and especially as bio-activator owing to its free amino group (\NH2) which is highly Surface Science 608 (2013) 67–73 ⁎ Corresponding author. E-mail address: trangnguyenphys@gmail.com (N.T. Trang). 0039-6028/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.susc.2012.09.014 Contents lists available at SciVerse ScienceDirect Surface Science journal homepage: www.elsevier.com/locate/susc