Photonic active diamond nanoparticles attract increasing attention from a wide community for applications in drug delivery and monitoring experiments as they do not bleach or blink over extended periods of time. To be utilized, the size of these diamond nanoparticles needs to be around 4 nm. Cluster formation is therefore the major problem. In this paper we introduce a new technique to modify the surface of particles with hydrogen, which prevents cluster formation in buffer solution and which is a perfect starting condition for chemical surface modifications. By annealing aggregated nanodiamond powder in hydrogen gas, the large (>100 nm) aggregates are broken down into their core ( approximately 4 nm) particles. Dispersion of these particles into water via high power ultrasound and high speed centrifugation, results in a monodisperse nanodiamond colloid, with exceptional long time stability in a wide range of pH, and with high positive zeta potential (>60 mV). The large change in zeta potential resulting from this gas treatment demonstrates that nanodiamond particle surfaces are able to react with molecular hydrogen at relatively low temperatures, a phenomenon not witnessed with larger (20 nm) diamond particles or bulk diamond surfaces.

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 fundament al 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

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We report on the electronic and optical properties of boron-doped nanocrystalline diamond (NCD) thin films grown on quartz substrates by CH 4 /H 2 plasma chemical vapor deposition.Diamond thin films with a thickness below 350 nm and with boron concentration ranging from 10 17 cm -3 to 10 21 cm -3 have been investigated. UV Raman spectroscopy and AFM have been used to assess the quality and morphology of the diamond films. Hall effect measurements confirmed the expected p-type conductivity. At room temperature, the conductivity varies from 1.5x10 -8 Ω -1 cm -1 for a non-intentionally doped film up to 76 Ω -1 cm -1 for a heavily B-doped film. Increasing the doping level results in a higher carrier concentration while the mobility decreases from 1.8 cm 2 V -1 s -1 down to 0.2 cm 2 V -1 s -1 . For NCD films with low boron concentration, the conductivity strongly depends on temperature. However, the conductivity and the carrier concentration are no longer temperature-dependent for films with the highest boron doping, and the NCD films exhibit metallic properties. Highly doped films show superconducting properties with critical temperatures up to 2K. The critical boron concentration for the metal-insulator transition is in the range from 2x10 20 cm -3 up to 3x10 20 cm -3 . We discuss different transport mechanisms to explain the influence of the grain boundaries and boron doping on the electronic properties of NCD films. Valence band transport dominates at low boron concentration and high temperatures, 2 whereas hopping between boron acceptors is the dominant transport mechanism for boron doping concentration close to the Mott transition. Grain boundaries strongly reduce the mobility for low and very high doping levels. However, at intermediate doping levels where hopping transport is important, grain boundaries have a less pronounced effect on the mobility. The influence of boron and the effect of grain boundaries on the optoelectronic properties of the NCD films are examined using spectrally resolved photocurrent measurements and photothermal deflection spectroscopy. Major differences occur in the low energy range, between 0.5 -1.0 eV, where both Boron impurities and the sp 2 carbon phase in the grain boundaries govern the optical absorption.

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