- (a) et (b) : Modèle (marron = C ; rosé = H) d’une portion d’un film hybride diamanoïde/graphène à 6 couches (deux couches L2 puis L1 sont à rajouter sous L4). (c) La structure de L1 + L2 est ici celle du diamant vue selon [111]. L3 et L4 sont graphéniques, mais L2 présente des liaisons pz pendantes (en jaune dans (b)) qui interagissent fortement avec L3.
The diamond form of carbon (sp3 hybridised) is an insulating material because its gap is high ( 5 eV), which does not allow the material electrons to contribute to the overall conductivity. On the other hand, the graphenic form of carbon (sp2 hybridised) has no gap, making it a conducting material. Therefore, none of those two forms are suitable for electronic applications, in spite of the high expectations put in graphene in the field.
In 2007 was predicted the existence of a stable form of hydrogenated graphene, so-called graphane, which consists in a single-atom layer of sp3 carbon atoms, each of them bonded to a hydrogen atom alternatively displayed on either layer sides. In 2009 was also predicted that the hydrogenation of a bilayer graphene could induce the sp3 to sp2 conversion of the carbon atoms, resulting in a stable bilayered material in which half of the carbon atoms from each layer are bonded to one outer hydrogen atom, the other half being bonded to carbon atoms from the other layer. This material was named diamane, and exhibits either the diamond (face-centred cubic) or lonsdaleite (hexagonal) structure, depending on whether the initial graphene stacking sequence is AB (as in graphite), or AA respectively. If the initial graphene is made of N layers (N > 2), it was also predicted that the sp2àsp3 conversion induced by the hydrogenation of both surface layers generates enough stress for the conversion to propagate within inner layers, resulting in a film-like material which structure is either diamond or lonsdaleite of nanosized thickness with both surfaces hydrogenated. Such materials, named diamanoïds, are of great interest for electronics as they exhibit a tailorable gap in function of N (e.g., 3 eV for N = 2, 2.5 eV for N = 5). However, no attempt to prepare such materials had been convincingly successful so far.
The Laboratorio Nanociencias from the Pontificia Universidad Catolica Madre y Maestra (Dominican Republic), CEMES and LPCNO (Toulouse), and the University of Manitoba (Canada) have combined their research efforts to demonstrate, for the first time, the successful synthesis of diamanoïds, as well as diamanoïd/graphene hybrids, using a low temperature, low pressure hydrogenation process (PUCMM patent). The hydrogenated character of the surfaces as well as the complex inner structure of the materials obtained were demonstrated by comparing DFT-based modelling to experimental data obtained from Raman spectroscopy (visible and UV), infrared microscopy, and low energy (5 keV) electron diffraction. The next step will aim at demonstrating the successful synthesis of diamane starting from bilayer graphene, as soon as such a material with suitable quality is available.
References
Piazza F., Gough K., Monthioux M., Puech P., Gerber I., Wiens R., Paredes G., Ozoria C. (2019) Low temperature, pressureless sp2 to sp3 transformation of ultrathin, crystalline carbon films. Carbon 145, 10-22.
Piazza F., Monthioux M., Puech P., Gerber I. (2019) Towards a better understanding of the structure of diamanoïds and diamanoïd/graphene hybrids. Carbon (under review) – arXiv #1907.09033.
Contacts
Dr. Pascal PUECH, Dr. Marc MONTHIOUX