Growth, electronic properties and applications of nanodiamond O.A. Williams a,b, ⁎ , M. Nesladek a,b,c , M. Daenen a , S. Michaelson d , A. Hoffman d , E. Osawa e , K. Haenen a,b , R.B. Jackman f a Institute for Materials Research, Hasselt University, Wetenschapspark 1, B-3590 Diepenbeek, Belgium b Division IMOMEC, IMEC vzw, Division IMOMEC, Wetenshapspark 1, B-3590 Diepenbeek, Belgium c CEA-LIST (Recherche Technologique), CEA-Saclay, 91191 Gif sur Yvette, France d Schulich Faculty of Chemistry, Technion, Israeli Inst. of Technology, Haifa 32000, Israel e NanoCarbon Research Institute, AREC, Faculty of Textile Science and Technology, Shinshu University, 3-15-1 Tokita, Ueda, Nagano 386-8567, Japan f London Centre for Nanotechnology, University College London, 17-19 Gordon Street, London WC1H 0AH, UK Available online 13 February 2008 Abstract Nanodiamond or nanocrystalline diamond is a broad term used to describe a plethora of materials. It is generally accepted that nanocrystalline diamond (NCD) consists of facets less than 100 nm in size, whereas a second term “ultrananocrystalline diamond” (UNCD) has been coined to describe material with grain sizes less than 10 nm. These differences in morphology originate in the growth process. Conventional hydrogen rich gas phases produce facetted diamond with grain size proportional to film thickness and low sp 2 content. If these films are thin the grains can be less than 100 nm and hence NCD. By starving the plasma of hydrogen, the reduction in etching of sp 2 can lead to re-nucleation. At the extreme this results in very small grain sizes of around 3–5 nm, UNCD. The electronic properties of these two materials are vastly different. NCD is basically very thin microcrystalline diamond and thus can be doped with boron. It is intrinsically transparent, with absorption increasing with doping level. UNCD is highly absorbing due to its higher sp 2 content, and exhibits a reduced bandgap due to disorder. By adding nitrogen to the gas phase, the density of states within the bandgap increases and ultimately metallic conductivity can be achieved. This conductivity is n-type but not doping. © 2008 Elsevier B.V. All rights reserved. Keywords: Nanocrystalline; Nanoparticles; Chemical vapour deposition; Optoelectronic properties 1. Introduction Nanodiamond is a continuum of materials. From the smallest diamondoids (adamantane and the higher diamondoids [1]) to the larger grain size films and particles there exists a plethora of sizes of particles and films with different morphology, sp 2 content and hydrogen incorporation. In order to reduce this complexity, this review will deal with diamond films with nanocrystalline grain sizes, with a short mention of nano-particles with regards to seeding substrates, but not their electro-optical properties. Indeed, it is difficult to define even particular forms of nanocrystalline diamond films due to the variation between lab- oratories and even reactors. However, there is one fundamental criterion that seems to divide the majority of research in the field of films into two distinct categories. That criterion is simply whether the films are grown with the suppression or enhancement of re- nucleation processes. In conventional diamond growth, one uses a highly dilute concentration of methane in hydrogen, the so-called hydrogen rich gas phase chemistry known to reduce re-nucleation by the significantly higher etch rate of graphite over diamond in such a plasma [2]. Thus the suppression of re-nucleation results in an evolution of grain size from the small crystals at the seeded substrate to the film surface as seen in Fig. 1 (top). This means that after a given thickness the grains will no longer be of nanometre scale, and the film will be what is conventionally referred to as microcrystalline diamond (μCD). If this material is grown very thin and with a very high nucleation density, then coalesced films with grain sizes less than 100 nm can be grown and this material is termed nanocrystalline diamond or NCD [3]. The initial nucleation Available online at www.sciencedirect.com Diamond & Related Materials 17 (2008) 1080 – 1088 www.elsevier.com/locate/diamond ⁎ Corresponding author. Institute for Materials Research, Hasselt University, Wetenschapspark 1, B-3590, Diepenbeek, Belgium. E-mail address: oliverwilliams@mac.com (O.A. Williams). 0925-9635/$ - see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.diamond.2008.01.103