Electronic properties of diamond clusters: self-consistent tight binding simulation D.A. Areshkin, O.A. Shenderova, S.P. Adiga, D.W. Brenner * Department of Materials Science and Engineering, North Carolina State University, Raleigh, NC 27695-7907, USA Received 21 August 2003; received in revised form 26 April 2004; accepted 29 April 2004 Available online 28 July 2004 Abstract A self-consistent environment-dependent tight binding method is used to examine electron emission-related properties of hydrogen passivated nano-diamond (ND) particles. For sizes larger than 2.5 nm particle bandgap was found to be equal to the bandgap of bulk diamond. Coulomb potential distributions and electron affinities of clusters were found to be insensitive to the particle size if it exceeds 1.0 nm. Tunneling probabilities for homogeneous and inhomogeneous emission models were estimated. The simulation results indicate that the low emission threshold for hydrogen passivated diamond nano-clusters is due to hydrogen-assisted emission from the edges of small unpassivated islands. Essentially the same mechanism is claimed to be responsible for good emission properties of hydrogen passivated diamond films by Ristein [Diam. Relat. Mater. 9, 1129 (2000)]. D 2004 Elsevier B.V. All rights reserved. Keywords: Diamond clusters; Field emission; Electron affinity 1. Introduction Nano-diamond (ND) clusters, also called ultradispersed diamond (UDD), can be obtained from detonation products in large amounts with high uniformity and reproducibility [1,2]. After removal of an external disordered carbon shell, the sizes of nano-diamond clusters range from 2 to 10 nm, with a narrow peak in size distribution at about 5 nm [3]. Traditionally, UDD is used for galvanic coatings, polymer composites, polishing, and lubricant additives [3]. Attempts to use UDD powder for field emission enhancement [4–7] and suggestions for other electronic applications [8] were also reported. There is experimental evidence [6] that ND coatings deposited on Si needle-shaped field emitters substantially lowers emission threshold voltages relative to that of bare Si needles and needles with micro-diamond powder coatings. ND coatings are created either by bias enhanced microwave plasma chemical vapor deposition [9] or by electrophoresis from a suspension of diamond powder in ethanol [10]. After deposition the coatings are exposed to a hydrogen plasma, which is intended to have a twofold effect. First, it etches much of the loosely bounded carbon atoms, preferentially in the outer regions of the graphene and/or amorphous carbon (a-C) layers that surround ND particles. Second, the hydro- gen treatment is used to saturate free carbon bonds at the surface. TEM images of a Si needle tip with a ND coating, however, show surface roughness and irregularity that likely inhibits perfect hydrogen coverage [7], and the hydrogenat- ed ND surface may therefore contain small, unpassivated islands. Chains of such islands would serve as conducting channels between a graphene/a-C phase backcontact and the cluster’s outer surface from which electrons are emitted. The issue addressed here is a comparison of emission probabil- ities from the surface and through the bulk of ND clusters. These two main field emission mechanisms that are ex- plored are called the Inhomogeneous and Homogeneous Emission Models (IEM and HEM), respectively [11]. At the same time, the field emission threshold from a Mo tip with a single ND particle is higher compared to the threshold of bare Mo [6]. That speaks in favor of the IEM because from the HEM standpoint single particle properties are similar to the properties of ND coating. On the other hand a single particle may not have surface irregularities inherent for a ND coating, which are required for IEM. The goal of 0925-9635/$ - see front matter D 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.diamond.2004.04.012 * Corresponding author. E-mail address: brenner@eos.ncsu.edu (D.W. Brenner). www.elsevier.com/locate/diamond Diamond & Related Materials 13 (2004) 1826 – 1833