Oliver A. Williams received his PhD on the electronic properties of diamond from the University College London, UK. He then moved to the Argonne National Laboratory, USA to work on nanocrystalline diamond. Following this he worked for the Institute for Materials Research in Belgium, an affiliated lab of IMEC vzw and then received the Fraunhofer Attract award at the Fraunhofer Institute for Applied Solid State Research in Freiburg to develop MEMS structures from nanocrystalline diamond before moving to Cardiff University as a Reader in Experimental Physics focusing on the growth and applications of nanodiamond.

Nanodiamond

By Oliver Williams

The Royal Society of Chemistry

Copyright © 2014 The Royal Society of Chemistry
All rights reserved.
ISBN: 978-1-84973-639-8
CHAPTER 1

Distribution, Diffusion and Concentration of Defects in Colloidal Diamond

AMANDA S. BARNARD

CSIRO Materials Science and Engineering, 343 Royal Parade, Parkville, Victoria, 3052, Australia

Email: amanda.barnard@csiro.au

1.1 Introduction

It is often convenient to think of nanodiamond as pure, and free of defects, but this is not necessarily realistic. Nanodiamonds can (and do) contain a variety of defects, whether we want them there or not. These include intrinsic point defects, such as lattice vacancies, and incidental impurities, such as nitrogen, which are a result of the synthesis and/or purification processes. In general, defects are always thermodynamically unstable, but the relative (in)stability of these defects, and hence the probability that they can be removed from the particle, can depend on the location of the defect within the particle. This is quite different to the case of bulk diamond, where all lattice sites are geometrically (and, therefore, energetically) equivalent.

There are of course, types of defects that are very useful, and are therefore introduced deliberately. Well known examples are the p-type or n-type dopants used in electronic applications, but there are other types of useful point defects that are not dopants. Collectively these are often referred to as “functional defects” (as they provide some functionality), and include the range of optically active defects and colour centres. The most simple defect in diamond is a single, neutral lattice vacancy, which is commonly referred to as a GR1 defect (where GR stands for general radiation). Vacancies are omnipresent in diamond, and so this defect has been extensively studied in various states.

Since nitrogen is also widespread in diamond, numerous studies have also focused on characterising and understanding the properties of different types of N-related defects, including the single substitutional nitrogen impurity, known as the C-centre. However, arguably, the most widely studied defect in nanocrystalline diamond is the paramagnetic nitrogen–vacancy complex (N–V), which forms when a vacancy (GR1) migrates to bind with a C-centre. The energy-level structure of the negatively charged N–V defect results in emissions characterised by a narrow zero-phonon line (ZPL) at 637 nm (the neutral N–V centre has a zero-phonon line at 575 nm) accompanied by a wide-structured side band of lower energy due to transition from the same excited state, but with formation of phonons localised on the defect. The optical emission from N–V centres in diamond nanocrystals has been shown to strongly depend on the crystal size, and the charge state is related to the temperature.

Defects, such as GR1 centres and N–V centres, are mobile within diamond, and may migrate if a driving force is sufficient to overcome the kinetic energy barriers associated with diffusion. The diffusion of an N–V centre is vacancy assisted, and the rate-limiting step is the C–N exchange energy. During this migration, if an N–V centre interacts with another single nitrogen atom (or a migrating vacancy interacts with a nitrogen dimer, known as an A-centre), then an H3 centre is formed. The H3 centre consists of two N atoms surrounding a vacancy. It is one of the most studied in diamond, and may be formed abundantly by irradiation with 1 to 2 MeV electrons to doses of 1018–1020 electrons cm-2, and annealing at 1200 K for 20 h in a vacuum. If we continue this logica