Abstract
Carbon exhibits a remarkable range of structural forms, due to the availability of sp3, sp2 and sp1 chemical bonds. Contrarily to other group IV elements such as silicon and germanium, the formation of an amorphous phase based exclusively on sp3 bonds is extremely challenging due to the strongly favored formation of graphitic-like structures at room temperature and pressure. As such, the formation of a fully sp3-bonded carbon phase requires an extremely careful (and largely unexplored) definition of the pressure and temperature across the phase diagram. Here, we report on the possibility of creating full-sp3 amorphous nanostructures within the bulk crystal of diamond with room-temperature ion-beam irradiation, followed by an annealing process that does not involve the application of any external mechanical pressure. As confirmed by numerical simulations, the (previously unreported) radiation-damage-induced formation of an amorphous sp2-free phase in diamond is determined by the buildup of extremely high internal stresses from the surrounding lattice, which (in the case of nanometer-scale regions) fully prevent the graphitization process. Besides the relevance of understanding the formation of exotic carbon phases, the use of focused/collimated ion beams discloses appealing perspectives for the direct fabrication of such nanostructures in complex three-dimensional geometries.
Highlights
IntroductionCarbon is an extremely “versatile” chemical element due to the availability of different types of hybridized chemical bonds (sp[1], sp and sp3), that determine a remarkable range of possible allotropic forms, both in bulk form and as nanostructures.[1] In many respects, diamond lies at the very extreme of such a range, as far as bulk structures are concerned: due to is strong covalent sp[3] bond, the diamond crystal is characterized by extreme mechanical (high hardness, low friction coefficient), optical (broad transparency from the near UV to the far IR), thermal (large thermal conductivity, low thermal expansion coefficient) and electrical (extreme dielectric strength, high carrier mobility) properties.[2] These unique characteristics have motivated a remarkable body of scienti c work aimed at better understanding its fundamental properties, as well as its synthesis and Moving towards more “defective” and technologically viable forms of sp3-bonded carbon, different forms of polycrystalline diamond,[9] ultra-nanocrystalline diamond,[10] nano-twinned diamond[11] and amorphous diamond-like carbon[12,13] have been widely investigated for several decades, with the promise of further expanding the applicability of extreme physical properties into technological landscapes in which synthesis and fabrication techniques can be realistically scaled to large production volumes
Carbon is an extremely “versatile” chemical element due to the availability of different types of hybridized chemical bonds, that determine a remarkable range of possible allotropic forms, both in bulk form and as nanostructures.[1]
We demonstrate by means of high-resolution transmission electron microscopy (HRTEM) and energy loss spectroscopy (EELS) that these structures are lacking any measurable fraction of sp[2] bonds, speci cally because their size (i.e. $100–200 nm, depending upon fabrication parameters) and depth below the crystal surface (i.e. $1.6 mm) is such to inhibit any form of graphitization by the development of strong (i.e. >40 GPa) internal pressures
Summary
Carbon is an extremely “versatile” chemical element due to the availability of different types of hybridized chemical bonds (sp[1], sp and sp3), that determine a remarkable range of possible allotropic forms, both in bulk form and as nanostructures.[1] In many respects, diamond lies at the very extreme of such a range, as far as bulk structures are concerned: due to is strong covalent sp[3] bond, the diamond crystal is characterized by extreme mechanical (high hardness, low friction coefficient), optical (broad transparency from the near UV to the far IR), thermal (large thermal conductivity, low thermal expansion coefficient) and electrical (extreme dielectric strength, high carrier mobility) properties.[2] These unique characteristics have motivated a remarkable body of scienti c work aimed at better understanding its fundamental properties, as well as its synthesis and Moving towards more “defective” and technologically viable forms of sp3-bonded carbon, different forms of polycrystalline diamond,[9] ultra-nanocrystalline diamond,[10] nano-twinned diamond[11] and amorphous diamond-like carbon[12,13] have been widely investigated for several decades, with the promise of further expanding the applicability of extreme physical properties into technological landscapes in which synthesis and fabrication techniques can be realistically scaled to large production volumes In this context, the higher thermodynamical stability of sp2-bonded carbon at room pressure and temperature conditions represents a fundamental limitation: in these conditions, graphite and graphite-like phases constitute the ultimate “ground state” for carbon structures when a critical amount of structural disorder is introduced. A careful control of environmental parameters (pressure in particular) allows the engineering of novel forms of carbon, as demonstrated by the fact that exerting high (i.e. $102 GPa) pressures on glassy carbon (i.e. an amorphous sp[2] phase) yields the formation of phases characterized by high sp[3] content with no long-range ordering, whose structural stability can to some extent be tuned if an careful control of temperature variable can be achieved.[17,18,19] In this context, a powerful and versatile tool is represented by the local laser heating of different types of carbon structures under different mechanical stress conditions, either exerted from external pressure sources[20] or established within the sample by the coexistence of carbon phases characterized by different densities and mechanical properties.[21]
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