Adsorption and migration behaviours of Nb–C atoms on clean diamond (0 0 1) surface: A first principles study
Graphical abstract
Introduction
Recent studies on fundamental and applied science behind the synthesis and applications of ultrananocrystalline diamond (UNCD) film technology demonstrated that UNCD exhibits a synergistic combination of exceptional mechanical, chemical, electrical, and electron emission properties, which may be suitable for coating mechanical pumps [1], field-emission cathodes [2], [3], micro-electromechanical systems devices [4], [5], and biomedical devices [6], [7]. UNCD film is normally deposited by using Ar, C60, or CH4 in hydrogen-poor or non-hydrogen environment [8], [9]. In such environment, the growth of columnar crystal structures can be limited [10], [11] and the renucleation of diamond can be enhanced with the increase in C2 [8], [10], [11], thereby maintaining nano-sized grains (approximately 2–10 nm) [9], [11] in UNCD films, even when the film thickness is over micrometers. Nevertheless, without hydrogen-atom etching, the grain boundary of UNCD films is filled with amorphous phase consisting of many defects and sp2 carbon bonds. This composite structure of nanodiamond grains and amorphous boundary influences the coefficient of thermal conductivity, as well as the optical and mechanical properties [12], [13], [14], [15], thereby restricting the application of UNCD films. Our recent studies show that energy barriers for the carbon atoms migrating on a clean diamond (0 0 1) surface in non-hydrogen or hydrogen-poor environment are higher than those under hydrogen-rich conditions [16], [17], [18]. The high energy barrier could result in insufficient migration of carbon atoms during deposition of diamond films by CVD, leading to growth of an amorphous structure. Thus, developing another method to build nanostructures during diamond film deposition is necessary.
The current proposed method is similar to the deposition of Ti–Si–N films. By adding a small amount of Si in the TiN deposition process, the diffusion of Ti and N atoms is promoted [19], which can significantly improve the morphology and structure of the film [20]. A smooth, uniform, and nanostructured Ti–Si–N film is fabricated. The two-phase nanostructure of the film consists of nanocrystalline TiN surrounded by monolayer interface Si–N. As a result, Ti–Si–N films have relatively high hardness (>40 GPa) and high thermal stability [21].
In the present study, Nb is selected as the non-carbon addition because of the following reasons. Firstly, Petrikowski et al. reported that stable NbC interlayers enhance the adhesion of diamond films, because the hardness and thermal expansion coefficient of NbC are similar to those of diamond films [22]. Thus, Nb addition to diamond deposition can facilitate the formation of strong interfaces between diamond grains. Secondly, adding Nb metal powder during the ultrasonication of silicon substrate can increase the nucleation density of UNCD to 33 times than that of the normal pretreatment [23]. Thirdly, as mentioned above, carbon atom migration has a high energy barrier. Energy lug boss in potential energy surface (PES) is due to short carbon covalent radius (∼0.077 nm) and long distance along the diffusion path (>0.252 nm); the sum of the covalent radii of carbon and niobium is approximately 0.211 nm. Thus, Nb addition may reduce the energy barrier needed for carbon atom migration.
In this study, first principles calculations were conducted to investigate the Nb atom migration on a clean diamond (0 0 1) surface and the influence of Nb addition on the carbon atom migration.
Section snippets
Calculation details
Calculations were performed using Vienna Ab-initio Simulation Package (VASP) code [24], [25], [26], which is based on density functional theory. In specific calculations, a plane-wave basis was set and periodic boundary conditions were used to determine the Kohn–Sham ground state. The projector-augmented wave (PAW) method [27], [28] was used to describe electronic and ionic interactions. Local density was described with generalized gradient approximation (GGA) based on the
Adsorption and migration behaviours of a single Nb atom on clean diamond (0 0 1) surface
Adsorption and migration behaviours of Nb atoms on clean diamond (0 0 1) surface were investigated. On the basis of the special structure of the diamond, six highly symmetrical positions existed on the diamond (0 0 1) reconstruction surface (Fig. 1): (1) on the bridge site of dimer ring-closing bond (P1); (2) on the top of the C atom in the carbon dimer (P2); (3) at the bridge site of dimer ring-opening bond (P3); (4) between the carbon dimer row located on top of the atom in the third layer (P4);
Conclusion
In this study, the adsorption and migration activation energies of a single Nb atom, two atoms of 1C1Nb, and three atoms of 2C1Nb on clean diamond (0 0 1) surface were systematically investigated using first principles methods. The following conclusions can be drawn from the results:
- (1)
The activation energies of a single Nb atom migrating on the diamond (0 0 1) surface along the dimer row and the dimer chain are 2.008 and 2.099 eV, respectively, which indicate that migration along two directions is the
Acknowledgements
The authors acknowledge the financial support provided by the National Natural Science Foundation of China (Grant Nos. 50845065 and 51562031), the Natural Science Foundation of Inner Mongolia Autonomous Region (Grant Nos. 2014MS0516 and 2015MS0550), the Science and Technology Foundation of Baotou (Grant No. 2013J2001-1), and the Inner Mongolia College Scientific Research Project (Grant No. NJZY153).
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