Nanocrystalline diamond

Diamond properties are significantly affected by crystallite size. High surface to volume fractions result in enhanced disorder, sp2 bonding, hydrogen content and scattering of electrons and phonons. Most of these properties are common to all low dimensional materials, but the addition of carbon allotropes introduces sp2 bonding, a significant disadvantage over systems such as amorphous silicon. Increased sp2 bonding results in enhanced disorder, a significantly more complex density of states within the bandgap, reduction of Young's modulus, increased optical absorption etc. At sizes below 10 nm, many diamond particle and film properties deviate substantially from that of bulk diamond, mostly due not only to the contribution of sp2 bonding, but also at the extreme low dimensions due to size effects. Despite these drawbacks, nano-diamond films and particles are powerful systems for a variety of applications and the study of fundamental science. Knowledge of the fundamental properties of these materials allows a far greater exploitation of their attributes for specific applications. This review attempts to guide the reader between the various nanocrystalline diamond forms and applications, with a particular focus on thin films grown by chemical vapour deposition.