Immobilization of diamond nanocrystals on graphene
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Immobilization of diamond nanocrystals on graphene Sven Lange, Annika Pille, Valter Kiisk, Tauno Kahro, Harry Alles, Ilmo Sildos Institute of Physics, University of Tartu, Riia Str. 142, 51014 Tartu, Estonia ABSTRACT Immobilization of fluorescent nanoparticles on graphene is an important step in the assembly of certain graphene-based photonic devices as well as in optical visualization of graphene and its defects. Hereby we report a viable approach to deposit diamond nanoparticles on a wide-area graphene substrate. It is demonstrated that a suitable plasma treatment leads to a selective immobilization of deposited nanodiamond on graphene with practically no agglomeration. Absence of photoemission from the individual adsorbed diamond nanoparticles suggests an energy transfer from the excited N-V centers to graphene. INTRODUCTION Graphene, discovered just a decade ago, is a monolayer of graphite with excellent electrical, optical, thermal and mechanical properties suggesting several perspective applications, such as transparent electrodes, chemical sensors, supercapacitors and optoelectronic devices [1,2]. Yet, many challenges exist in the preparation of graphene (especially high-quality largearea graphene) as well as in the assembling of graphene-based structures (e.g. decoration of graphene with functional nanoparticles). Diamond is one of the few inorganic materials exhibiting intense photostable fluorescence in nanosecond timescale due to several well-known impurity centers, such as nitrogen-vacancy (N-V) defects. Such fluorescent nanodiamond (ND) could be an interesting option as a nanolocal light source or as a fluorescent marker on graphene, since both graphene and diamond are allotropes of carbon. However, the chemical compatibility, physical interactions and proper deposition methods of NDs on graphene are nearly unexplored to date. There is only one study of NDs on graphene which reports fluorescence resonant energy transfer (FRET) from N-V centers to graphene [3]. There are also several promising studies of the reversed scenario (i.e. graphene deposited on diamond substrate), which have shown that diamond is rather inert against graphene and maintains its intrinsic electronic structure and current-carrying capacity much better than the conventional SiO2/Si substrate [4,5]. Dropwise application of untreated ND colloidal dispersion onto a pristine graphene sheet is problematic due to a non-selective deposition and agglomeration of NDs during the drying process. However, it is known that the adsorption and agglomeration are influenced by the electrokinetic (zeta) potentials of adsorbant and substrate [6]. In particular, it has been shown that plasma-hydrogenated NDs display a positive zeta potential which can be further influenced by the pH of the solution [7]. Therefore one can expect that an adjustment of the zeta potentials of NDs and graphene in opposite directions results in an attraction of NDs towards graphene while at the same time also increasing the repulsion between NDs to avoid clustering. The main
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