Structure evolution during deposition and thermal annealing of amorphous carbon ultrathin films investigated by molecular dynamics simulations

Abstract

The evolution of the structure of amorphous carbon (a-C) films during deposition and thermal annealing is of significant interest from both the materials science and application perspectives. However, despite the voluminous literature of studies dealing with the deposition and physical properties of a-C films, basic understanding of the structure evolution due to phase change during film growth and heating is fairly sparse and empirical, presumably due to the lack of high-resolution instruments that can probe structural changes at the atomic and molecular levels in real time. Molecular dynamics (MD) is a powerful computational method for studying atomic/molecular-scale movement and interactions. Thus, the objective of this study was to perform MD simulations that provide insight into changes in the structure of ultrathin a-C films during deposition and annealing. Simulation results reveal a multi-layer film structure, even for a-C films as thin as ~20 Å, the existence of a deposition energy that yields a-C films with the highest sp3 content, the transient and steady-state stages of the structure evolution during annealing at different temperatures, and the changes in the hybridization state (mainly in the bulk layer) encountered during annealing at elevated temperatures. The MD results of this study are of particular importance to applications where the deposition conditions and operation temperature affect the structure and, in turn, the physical properties of ultrathin a-C films used as protective overcoats.

Machine learning driven simulated deposition of carbon films: From low-density to diamondlike amorphous carbon

“…Carbon is a prime example: different routes to computationally generate ta-C structures have been explored * mcaroba@gmail.com in detail, the most popular being the "liquid quench" technique [10][11][12][13][14][15][16]. Explicit deposition of carbon atoms [17][18][19][20][21][22][23][24][25][26], mimicking ta-C film growth under experimental conditions, is too computationally costly to be practical at the DFT level. Alternative generation techniques, including quenching from the simulated melt, invariably fall short, each to a different extent, of predicting experimental sp 3 values [27], which can be as high as 90% for "superhard" ta-C [28].…”

A molecular dynamics study of the oxidation mechanism, nanostructure evolution, and friction characteristics of ultrathin amorphous carbon films in vacuum and oxygen atmosphere

“…The overall sp 3 content of the a -C film was varied by changing the kinetic energy of the incident carbon atoms. Additional details about the MD model of ultrathin a -C film growth used in this study can be found elsewhere 22 .…”

“…An elevated temperature may destabilize the microstructure and degrade the properties of a -C films by promoting sp 3 -to- sp 2 rehybridization, consequently lessening the film’s ability to effectively protect the substrate material 18 , 19 . Experimental and computational studies have revealed the existence of a threshold temperature in the range of 200–350 °C above which a -C films undergo significant structural changes 20 – 22 . Increasing the sp 3 fraction and doping with certain elements, such as silicon, have been proven effective methods for enhancing the thermal stability of a -C films by stabilizing the carbon atoms in the sp 3 hybridization state and inhibiting sp 3 -to- sp 2 rehybridization at high temperatures 23 – 25 .…”

“…Therefore, insight into the structural changes and tribological behavior of ultrathin films exposed to different temperatures and environments requires atomic-scale approaches. The problem is further perplexed by the layered structure of a -C films grown by deposition methods that employ energetic particle bombardment 22 . In view of the challenges associated with structural characterization and the quantification of the properties of films at the atomic level, atomistic computational methods have been developed to bridge the gap in fundamental knowledge about the effect of atmospheric conditions on the structural stability and tribological behavior of ultrathin films.…”

“…Subsequently, the excess free oxygen molecules were removed from the simulation box and normal or sliding contact was simulated using the same parameters as in the vacuum simulations. In the foregoing simulations, carbon-carbon interaction was described by the Tersoff potential, whose parametrization is detailed elsewhere 39 and its ability to simulate a-C systems has been previously verified 22,40 . The carbon-oxygen and oxygen-oxygen interactions were described by the Lennard-Jones potential with parameters given elsewhere 41,42 .…”