Nanodiamond has a rich diversity of forms ranging from particles of a few nanometres to films constituting crystallites of hundreds of nanometres and microns thick. The allotropic behaviour of carbon leads to complicated structures at the nanoscale comprised of both sp3 and sp2 bonding. Unfortunately, the variety of such forms results in an inevitable layer of jargon, which can be unhelpful for the uninitiated. One of the easiest ways to diffuse such confusion is to divide the field with clear definitions, something that has been especially opaque with nanodiamond films. The images below are an effort in this direction, subdividing the fields of nanodiamond films and particles.
The smallest known diamond-structured particles are in fact not strictly speaking diamonds, but diamondoids. These particles are generally isolated from petroleum, being a source of many problems in natural gas, gas condensates and light crude oil flow systems, where they can act as flocculation sources, blocking flow paths. A single adamantane molecule weighs 10−21 carats, i.e., 2×10−22 g. Adamantane is not truly a diamond as every carbon atom is at the surface and thus is bonded to at least one hydrogen atom. To date, there are few applications of adamantane and the higher diamondoids and, as they are not true diamonds, they will not be discussed in detail in this book.
The next biggest diamond particles, and thus the smallest true diamonds, are made from detonation synthesis, such as with trinitrotoluene (TNT) and hexogen (see Chapters 1 to 5). During the detonation shockwave, the pressure and temperature reach the stability region of diamond in the phase diagram. Thus, for a brief moment of typically around 1 µs, diamond particles can be grown. These particles are usually called ultra-dispersed diamond (UDD) and their mean size is quoted as around 4 nm. This size results in a specific surface area greater than 400 m2 g−1, with more than 15% of UDD particle carbon atoms located at the surface. This has profound implications on the surface chemistry and stability of such particles. It has been shown that diamond is actually energetically favoured over polycyclic aromatics for diameters of less than 3 nm with hydrogen termination. Thus, at this length scale, diamond cannot truly be said to be meta-stable. The reactivity of such fine particles can also differ substantially from bulk diamond surfaces.
Finally, as one gets to sizes of greater than 20 nm, nanodiamond particles behave like bulk diamond. This is predominantly due to the far reduced concentration of atoms at the surface with regards to the bulk. These diamonds are usually produced from top down methods, such as jet milling, or the abrasion of larger diamonds, which is an expensive and relatively time-consuming process. However, their quality generally exceeds that of smaller diamonds due to their reduced surface-to-volume fraction. Their Raman and X-ray diffraction spectra are far more reminiscent of bulk diamond than there smaller counterparts. These types of nanodiamond have been exploited in the abrasives industry for decades.
For films, the difference is very much complicated by mostly historical terminology. In the earlier days of diamond growth, when chemical vapour deposition (CVD) reactor design was in its infancy and nucleation densities were very low, nanocrystalline diamond was a name given to thin diamond films, which generally had low quality. Diamond growth evolved and high-quality single crystal and microcrystalline films dominated research, leaving nanocrystalline diamond very much in the background. Recently, nanocrystalline diamond has developed into a sophisticated material with a wide variety of applications and associated terminology (see Chapters 10 to 20). Figure 2 attempts to clarify some of this terminology.
The smallest grain size diamond films are called ultrananocrystalline diamond (UNCD), a term originating from Argonne National Laboratory. These films have grain sizes around 5 nm, with a considerable amount of amorphous grain boundaries, which are very similar to diamond-like carbon (DLC). DLC is, strictly speaking, not diamond but is included in the table for clarity, and because it has many similarities with UNCD. DLC has no crystalline structure, as seen in the TEM image (Figure 2), whereas UNCD clearly exhibits the lattice planes of diamond, with amorphous DLC-like regions between grains. UNCD is very much a special case, with all other types of nanodiamond film being termed nanocrystalline diamond (NCD). NCD films have grain sizes generally below 100 nm, but sometimes films with grains up to 500 nm are also labelled NCD. Generally speaking, NCD films contain less sp2 hybridisation and are thus more transparent than UNCD films; this is particularly acute when the films are grown without re-nucleation, i.e., with a low methane concentration and high power density.
Perhaps the most convincing definition between the various forms of diamond is in their resulting properties. In this way it is easy to distinguish between nano-carbons, as measurements such as Young's modulus, optical transparency, thermal conductivity etc. are objective real world properties that can be quantified and exploited in real world applications. Ultimately the real world application arena is the true measure of a useful material and relegates all ambiguity and argument about material classification to semantics.
This book aims to clarify some of the idiosyncrasies of the field of nanodiamond, and highlight the wide application space within which it is exploited. The majority of this field is covered within these pages, with self-contained chapters authored by known experts. These chapters overlap in places, due to the encouraging merging of fundamental and applied science. The interdisciplinary nature of materials science blurs this boundary even further, and for this reason chapters are not grouped into distinct sub sections.
The first few chapters focus on the production, purification, and fundamental properties of nanodiamond particles, with routes to applications (see Chapters 1 to 6). The chapters that follow detail the broad horizon space of nanodiamonds from electrochemistry to cell labelling and drug delivery (see Chapters 6 to 10). The diversity of the applications space of nanodiamond particles is startling, and impossible to cover exhaustively, not least as new ideas surface frequently. However, only a detailed understanding of the surface properties can lead to the full exploitation of this unique material, which is still work in progress.
The nanodiamond film section begins with nucleation, a section related to both films and particles and a key area where diamond nanoparticles have made a major impact. The chapters that then follow deal with the growth and doping of films, with some attention to new low-temperature growth capabilities (Chapters 10 to 14). The applications of nanodiamond films are probably even more diverse than nanodiamond particles and certainly well established. The chapters in this book deal almost entirely with doped films with electrochemistry, superconductivity, and field emission (Chapters 15 to 20).
It is hoped that the chapters within this book, coupled with their citations, will help in some way to clarify the field of nanodiamond. At the very least, it is envisioned that this text will serve as a reliable entry point to the various disciplines of the field.
Oliver A. Williams, Cardiff
