Medium-Range Order Structures of Amorphous Diamond-Like Carbon Films
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MEDIUM-RANGE ORDER STRUCTURES DIAMOND-LIKE CARBON FILMS
OF
AMORPHOUS
Xidong Chen *, J. Murray Gibson *, John Sullivan **, and Tom Friedmann** *
Materials Science Division, Argonne National Laboratory, Argonne, IL 60439 Nanostructure and Semiconductor Physics Department, Sandia National Laboratories, Albuquerque, NM 87185-1421
**
Abstract In this paper, we have studied medium-range order structures in amorphous diamond-like carbon films with fluctuation microscopy. Medium-range order structures are quantified by a specific form of a general four-body correlation. We name this function as a pair-pair alignment correlation function. By analyzing speckle dark-field images taken over different areas as a function of momentum transfer in reciprocal space, we measured the pair-pair alignment correlation function for both thermally annealed samples and unannealed samples. We have found that thermal annealing introduces medium-range order in amorphous diamond-like carbon films, causing more pairs of atoms to be aligned. These results agree with density-functional simulations. Larger-scale simulations will be needed to fully understand our experimental results.
Introduction Hydrogen-free amorphous diamond-like carbon films have stimulated great interest because of their useful properties, such as high hardness, chemical inertness, thermal stability, wide optical gap, and negative electron affinity[1]. Consequently, they may have various potential applications in mechanical and optical coatings, MEMS systems, chemical sensors and electronic devices. Amorphous diamond-like carbon films often contains significant amounts of four-fold or sp3 bonded carbon, in contrast to amorphous carbon films prepared by evaporation or sputtering which consist mostly of three-fold or sp2 bonded carbon. The ratio and the structure configurations of these threefold and four-fold carbon atoms certainly decide the properties of these amorphous diamond-carbon films. Although the ratio of three-fold and four-fold carbon has been studied with Raman spectroscopy and electron-loss-energy spectroscopy, very little has been understood regarding key questions such as how the three-fold and the four-fold carbon atoms are integrated in the film, and what structures those three-fold carbon atoms take [2]. These questions cannot be simply answered by normal diffraction technique because they involve structures beyond short-range order.
Theory Through the history of studying amorphous materials, the radial distribution function (RDF) has played an important role. While it is equivalent to an atomic pair correlation function, its importance relies on the fact that this function can be directly measured from diffraction experiments. The RDF gives good quantitative descriptions of short-range order (2 – 5 Å). However, it is almost impossible to interpret the third and the fourth peaks in the RDF. This is because the number of atoms filling a shell of r increases significantly as r increases. At a very large r, the RDF is approaching to the average number de
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