Energetics and stability of diamondlike amorphous carbon

Abstract

Simulations of diamondlike amorphous carbon formed by quenching the liquid under high pressure indicate that two distinctly different dense phases exist. The "as-quenched" phase is metastable and mostly fourfold coordinated. Upon annealing, it converts into a more stable structure with the majority of atoms having threefold coordination. Both structures are consistent with experimental findings. The annealed phase is clearly distinguished from the low-coordinated network of evaporated a-C.PACS numbers: 61.42.+h, 64.70.Pf, 68.60.Dv Much of the current interest in amorphous carbon (a-C) is focused on diamondlike films [1,2] which are highly suitable for hard, transparent coating materials in microelectronics as well as in mechanical applications. Although the macroscopic properties of these films, such as high density, hardness, and optical transparency are well documented, their microscopic nature is under strong debate. The central issue is the extent of fourfold {sp ^) local bonding. The predominance of such atomic coordination could lead, if true, to the appealing picture of an amorphous carbon structure described by a tetrahedral continuous random network (CRN) model, such as that of Polk [3], in analogy with amorphous silicon or germanium.It is well known that the nature of a-C films depends on the preparation conditions and on hydrogen concentration. Evaporated amorphous carbon {e-O tends to be soft and black with a density of ^2.0 gem ~\ somewhat lower than that of graphite (2.27 gem ~^), and to have an optical gap of -^0.5 eV [1]. The essentially graphitic nature of e-C is furthermore established by the domination of sp~ trigonal bonding. Experimental evidence [1,4], confirmed by theoretical calculations [5,6], suggests that the vast majority of atoms, over 80%

Mechanical response of nanocrystalline Cu from atomistic simulations

“…We performed the same analysis to our published data on nanocrystalline diamond [9]. For the cohesive energy, we find that E b =-7.31 eV and E s =-6.73 eV, in good agreement with calculated values for diamond (-7.33 eV) and tetrahedral amorphous C (-6.95 eV) [18]. For the bulk modulus, we find B b =467 GPa, in agreement with calculated value of 443 GPa; moreover, we find the difference between the bulk moduli of crystalline and amorphous phase to be 41 GPa, when detailed calculations yield about 50 GPa [19].…”

Local electric field and enhancement factor around nanographitic structures embedded in amorphous carbon

“…To begin with, all Monte Carlo simulations of the atomic structure of this material fail to observe any significant clustering of sp 2 sites. 8,9 So any such sp 2 structures must be very rare. Furthermore, the probability of these structures making contact with the cathode and extending up to the vacuum surface in a filamentary shape will be even smaller.…”

Electronically designed amorphous carbon and silicon

“…Sputtering and evaporation usually produce disordered phases that are rich in sp 2 bonds, whereas mass-selected ion beam deposition produces a disordered phase with more than 90% sp 3 character [16]. Structural, electronic, and optical properties of these materials are of great technological and scientific importance, which have led to considerable works in this field [14,[16][17][18][19][20][21][22][23][24][25][26].…”