Deposition of CdSe quantum dots on graphene sheets Fehmida K. Kanodarwala a , Fan Wang b , Peter J. Reece b , John A. Stride a,c,n a School of Chemistry, University of New South Wales, Sydney, NSW 2052, Australia b School of Physics, University of New South Wales, Sydney, NSW 2052, Australia c Bragg Institute, Australian Nuclear Science and Technology Organisation, PMB 1, Menai, NSW 2234, Australia article info Article history: Received 18 July 2013 Received in revised form 29 August 2013 Accepted 30 August 2013 Available online 11 September 2013 Keywords: Quantum dots Graphene Photoluminescence Exciton abstract High-quality TOP/TOPO capped CdSe quantum dots (QDs) displaying a narrow emission band have been grafted on to graphene nanosheets through a simple wet chemical procedure. A significant red-shift of both the broad absorption and narrow emission spectra of the QDs is induced in the resultant hybrid material, presumably by a strong graphene–QD interaction consistent with a quantum tunnelling phenomenon. The optical properties of monodisperse CdSe QDs of 3–4 nm in diameter have been determined in UV–vis and photoluminescence spectra and HRTEM and XPS data, demonstrating the successful decoration of graphene sheets with CdSe QDs. The CdSe QD absorption was observed to shift to longer wavelengths by up to 30 nm, with a corresponding shift of up to 18 nm observed in the emission band. This effect is equivalent to a narrowing of the band gap by 0.094 70.015 eV in absorption and 0.033 70.013 eV in emission, or an interaction potential of 9.1 71.5 kJ mol 1 . Another is to consider the effective particle growth based upon the narrowed band gap, consistently found to be 28 71%, despite the physical size remaining unchanged. By effectively shifting the absorption and emission of CdSe QDs to longer wavelengths, this type of nanocomposite may have potential applications in the fields of optics, biological imaging and sensing. Crown Copyright & 2013 Published by Elsevier B.V. All rights reserved. 1. Introduction Graphene consists of carbon atoms that are arranged on a honeycomb structure made up of hexagons – a molecular-scale chicken wire [1]. Although the existence of graphene had been hypothesized for a long time, it was only recently (2004) first isolated in its free form by Geim et al. [2]. The structural rigidity of graphene is reflected in its electronic properties. The valence electrons on each carbon centre may be thought of as being sp 2 hybridized, in which the 2s and two 2p orbitals mix, leading to a trigonal planar geometry about the nucleus; as such there exist s bonds between adjacent carbon atoms, separated in graphene by 1.42 Å. According to Pauli‘s exclusion principle, the bands formed by the propagation of s-bonding throughout the 2D lattice are half-filled, forming a deep valence band. The third 2p orbital on each carbon atom lies perpendicular to the planar structure and forms covalent π interactions with neighbouring carbons. As each p orbital is half-filled, this π band also remains half-filled. This unique band structure of graphene grants it unusual semi-metallic behaviour and open to electronic interactions at the surface [3]. Due to its unique electrical [4], mechanical [5], optical and thermal properties [6], graphene can be tailored structurally and chemically in a number of different ways for a variety of applica- tions. It has potential for use as a transparent electrode material [7] in sensors, displays and solar cells, in lithium ion batteries [8] or polymer composites [9]. In order to enhance the properties and widen the scope of applications of graphene, much research is currently being carried out on the surface modification of the material. Metal atoms [10] and molecules [11] have been deposited on the surface and there has been some decoration of the surface with nanoparticles such as metal oxides [12], metals [13] and magnetite [14]. Quantum dots (QD), also known as nanocrystals, qdots or as artificial atoms, are a special class of semiconductor materials, whose excitons are confined in all three spatial dimensions. As a result, they have properties that lie between those of bulk semiconductors and those of discrete molecules [15]. QDs are unique due to their small size, around 2–10 nm, with crystals composed of only a few hundred to thousands of atoms. Materials behave differently at this small length scale, granting them novel electronic and optical properties [16,17] that can be attributed to the phenomenon of quantum confinement [18]. This enables their application in a number of fields such as biological imaging [19], solar cells [20], light emitting diodes [21] and in optoelectronic devices [22]. More recently there has been interest in synthesizing QDs that emit in the infra-red (IR) or near IR region, as this is Contents lists available at ScienceDirect journal homepage: www.elsevier.com/locate/jlumin Journal of Luminescence 0022-2313/$ - see front matter Crown Copyright & 2013 Published by Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jlumin.2013.08.072 n Corresponding author at: School of Chemistry, University of New South Wales, Kensington Sydney, NSW 2052, Australia. Tel.: þ61 2 9385 4672; fax: þ61 2 9385 6141. E-mail address: j.stride@unsw.edu.au (J.A. Stride). Journal of Luminescence 146 (2014) 46–52