Hydrogen Interactions with Diamond

Diamond is a material of extreme properties in many respects. The discovery, several decades ago, that polycrystalline diamond films may be grown on different substrates by chemical vapour deposition (CVD) using a hydrocarbon–hydrogen rich gas mixture under mild conditions has resulted in an intense research activity in both the science and technology of diamond. Both the quality and crystallite size of the diamond films are largely determined by the concentration of hydrogen in the precursor gas mixture from which growth occurs. Today diamond growth can be achieved on a variety of substrates and it is possible to control the diamond crystallite grain size within the nano to micron range, as well as incorporating different dopants in the films during the deposition process.

A diverse range of technological applications are envisaged for the diamond films which are formed using this CVD approach. This in turn has focused attention on understanding the basic physical, chemical, mechanical and interfacial properties of diamond materials, in a research field that spans physics, chemistry, materials science, electronics and biology.

One of the central themes in this basic research is the interaction of hydrogen with diamond. This is crucial since the CVD diamond is always grown in a hydrogen containing environment so it is present within the material. Also hydrogen has a major role in controlling the interfacial, electronic and optical properties of the as-deposited films, on which many of the applications depend. Hydrogen is a main player in the “magic” of diamond formation by CVD and its role has been investigated both experimentally and theoretically.

It has been determined that hydrogen is retained in the deposited polycrystalline diamond bulk during growth, predominantly decorating grain boundaries and internal or bulk defects. The investigation of the hydrogen in these regions has been the subject of many research studies that have attempted to clarify its bonding configuration and concentration and its influence on the electronic, chemical and physical properties of the host diamond matrix. As an example and of particular present interest is the bonding of hydrogen with a NV centre resulting in the formation of the so-called NVH- complex, which is optically active. Single photon emission is readily detected from such isolated centers by conventional optical methods and have the potential to be applied as a candidate for the building blocks of quantum computers and quantum cryptography. Another example is that hydrogen seems to associate also with substitutional boron atoms resulting in the formation of BH complexes, apparently resulting in n-type conductivity. The advancement of our understanding in these questions is based on complementary theoretical models combined with experimental studies.

Hydrogen bonding on diamond surfaces has pronounced effects on its electronic properties. It has been found, for example, that whilst free diamond surfaces display positive electron affinity, the hydrogen-terminated interface may display negative electron affinity (NEA). The NEA property of diamond is stable in air up to several hundred °C and results in a high electron emission yield for excited electrons (this yield exceeds that of a surface with positive EA by several orders of magnitude!) The other fascinating phenomenon that is closely related to hydrogen surface termination is a p-type diamond surface conductivity. It arises on the hydrogenated diamond surface exposed to ambient conditions, resulting in electron transfer from the diamond matrix to electrochemical couples in a wetting layer at the interface. This synergy of NEA and surface conductivity makes diamond an exceptionally good candidate for electron emission devices: it displays an outstandingly high electron emission yield coefficient. Hydrogenated diamond surfaces may be chemically functionalized to form particular bonding with biological systems opening the path for many biological applications such as bio-sensors. The electro-chemical properties of hydrogenated diamond surfaces are also found to be advantageous.

Whilst it is thus well-established that hydrogen bonding has pronounced effects on the chemical, physical and electronic properties of diamond surfaces and the bulk, the details of such effects still need to be evaluated and are the subject of intensive research. It is clear that in order to fully realize the expected potential of diamond CVD, the influence of hydrogen on its properties must be well understood and controlled.

The purpose of this special PCCP issue is to bring together some of the research related to different aspects of the interaction of hydrogen with diamond. The papers published in this special issue represent only a small portion of the large body of research being carried out on the subject presently and which has been published in recent decades.

The papers of J.-C. Arnault et al. and H. Girard et al. (DOI: 10.1039/c1cp20109c and DOI: 10.1039/c1cp20424f) in this special issue deal with the surface chemical properties of hydrogenated nano-diamond films. Here questions linked to controlled surface preparation and characterization are most challenging considering the heterogeneous nature of the nano-diamond particles. In particular, the study of J.-C. Arnault et al. (DOI: 10.1039/c1cp20109c) is focused on the chemical properties of hydrogenated nano-diamond films investigated by surface electron spectroscopies, and is complemented by the study of H. Girard et al. (DOI: 10.1039/c1cp20424f) on functionalization of hydrogenated nano-diamond particles.

The paper of Sh. Michaelson et al. (DOI: 10.1039/c1cp00019e) discusses the bonding, chemical reactivity and thermal stability of hydrogenated polycrystalline diamond surfaces exposed to different environments. These are important chemical properties of the hydrogenated surfaces that must be understood and controlled for any application where the hydrogen termination is of relevance.

The interaction of hydrogen plasmas with polycrystalline diamond films exhibiting the (100) preferred orientation is discussed in the paper of P. John et al. (DOI: 10.1039/c1cp20099b). In this paper some basic fundamental questions are considered, especially relating to diamond etching rates, and associated chemical and morphological effects. Understanding these effects is necessary for the reproducible preparation of hydrogenated poly-crystalline films surfaces.

A central tool for the investigation of hydrogen bonding with diamond surfaces is high resolution electron energy loss spectroscopy (HR-EELS) using low energy electrons. A. Lafosse et al. (DOI: 10.1039/c1cp20219g) present a fundamental study using HR-EELS of poly-crystalline and nano-crystalline hydrogenated (deuterated) diamond films surfaces combined with excitation function measurements. The aim of these studies is a more profound understanding of the nature of hydrogen bonding and electronic properties of the surfaces.

Model calculations using advanced computational methods is a very active area, which aids and complements experimental studies. In this issue J. Goss et al. (DOI: 10.1039/c1cp00038a) present results of first principle density functional modeling of selected H-containing point defects. These calculations render the vibrational frequencies of hydrogen bonded to different defects as well as the influence of strain on these values.

Finally, the last paper of this special issue, by J. Barjon et al. (DOI: 10.1039/c1cp20303g), discusses the interaction and influence of hydrogen with boron impurities in mono-crystalline and poly-crystalline diamond. The formation and properties of (B,H) complexes are discussed and it is suggested that it may provide a route for the patterning of diamond conductive structures for nano-technological applications.

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