MD simulation of H-terminated diamond surfaces

Surface structure transformations and vibrational dynamics on the H-terminated diamond surfaces

H-covered diamond surface {111}/(2x1) diamond surface

It is generally accepted that at complete or near complete hydrogen coverage diamond {111} surface (C{111}) has a bulk-terminated (1x1) structure, with the dangling bonds or radicals terminated by hydrogen atoms, C{111}/(1x1)H. On the other hand, the clean {111} diamond surface undergo (2x1) reconstruction with a pi-bonded chain structure, C{111}/(2x1). In this case the radicals that would be found on the clean bulk terminated surface are eliminated in favor of chains of sp²-hybridized carbon atoms.

graphitization of {111} diamond surface C{111} surface with low H coverage

Thermal treatment of the hydrogen-free {111} diamond surfaces can lead to graphitization of a surface region. It is known that even a small amount of hydrogen can stabilize the diamond surface against graphitization and the presence of hydrogen plays an important part in CVD diamond growth. The structure of the C{111} surface at intermediate H coverages, however, is largely unknown.

In this project we use molecular dynamics simulation technique with reactive Brenner's potential and Allinger's molecular mechanics force field to investigate the pathway of the {111} diamond surface transformation between the (2x1) pi-bonded chain and the (1x1) bulk terminated structures. Our interest in this project was stimulated by intriguing results from sum-frequency generation spectroscopy experiments of Prof. Shen and co-workers indicating that the C{111}/(2x1) <--> C{111}/(1x1)H phase transformation is asymmetric and is mediated by the a metastable surface structure.

adjacent CH bonds metastable C{111}H structure

We find that at the early stages of hydrogen deposition on a (2x1)-reconstructed surface atomic configurations comprised of adjacent CH bonds can be readily formed. In subsequent surface transformations, these configurations serve as localized structural defects that stabilize a metastable structure, and determine the energy barrier separating the metastable structure reconstruction from the H-terminated (1x1) structure.

vibrational spectrum metastable C{111}H surface

Vibrational spectra calculations confirm that just these strained atomic configurations are responsible for the formation of an additional metastable peak in the high-frequency region. Thermal annealing and further H dosing can cause the irreversible relaxation of these defected configurations, and hence the gradual transfer of the metastable peak strength to the peak corresponding to the (1x1) bulk-terminated surface structure occurs. This picture derived from the computer modeling explains the observations from sum-frequency generation spectroscopy experiments.

The vibrational dynamics of hydrogen on other low-index diamond surfaces was also studied. We found that the CH stretching peak positions in the vibrational spectra of hydrogen are sensitive to the surface structure and can be used for the experimental in situ analysis of the growing CVD diamond phase. The differences in the spectral features corresponding to the CH bending vibrations have been correlated with the results of the vibrational energy relaxation rate estimates. We find that the major contributions to the lifetimes of the excited CH stretching states come from the anharmonic coupling between the CH stretching and CCH bending vibration modes. The latter is highly coupled with the substrate phonons, which leads to the formation of broad spectral regions associated with CCH bending vibrations.

The dynamic analysis of the motion induced by the symmetric and antisymmetric stretching excitation on C{100}/(2x1)H surface shows that the unstability of the pure antisymmetric vibrations is not conne cted with the dephasing of the CH stretching vibrations but rather is a consequence of the inherent nature of the antisymmetric vibrational mode in a system of coupled anharmonic CH oscillators. Click here to see an illustration. vibrational spectra

Publications

L. V. Zhigilei, D. Srivastava, and B. J. Garrison, Intermediate Metastable Structure of the C{111}/(1x1)H - C{111}/(2x1) Surface Phase Transition, Phys. Rev. B 55, 1838-1843, 1997.
Abstract, Full Text: PDF (148 kB), GZipped PS (654 kB)

L. V. Zhigilei, D. Srivastava, and B. J. Garrison, Vibrational Dynamics of the CH Stretching Mode of H-Terminated Diamond Surfaces, Surface Science 374, 333-344, 1997.
Abstract, Full Text: PDF (2.3 MB)

For more information on this project please contact Leonid Zhigilei, lz2n@virginia.edu, Deepak Srivastava, deepak@nas.nasa.gov, or Barbara Garrison, bjg@chem.psu.edu


It is generally accepted that at complete or near complete hydrogen coverage diamond {111} surface (C{111}) has a bulk-terminated (1x1) structure, with the dangling bonds or radicals terminated by hydrogen atoms, C{111}/(1x1)H.	On the other hand, the clean {111} diamond surface undergo (2x1) reconstruction with a pi-bonded chain structure, C{111}/(2x1). In this case the radicals that would be found on the clean bulk terminated surface are eliminated in favor of chains of sp²-hybridized carbon atoms.

Thermal treatment of the hydrogen-free {111} diamond surfaces can lead to graphitization of a surface region. It is known that even a small amount of hydrogen can stabilize the diamond surface against graphitization and the presence of hydrogen plays an important part in CVD diamond growth. The structure of the C{111} surface at intermediate H coverages, however, is largely unknown.
In this project we use molecular dynamics simulation technique with reactive Brenner's potential and Allinger's molecular mechanics force field to investigate the pathway of the {111} diamond surface transformation between the (2x1) pi-bonded chain and the (1x1) bulk terminated structures. Our interest in this project was stimulated by intriguing results from sum-frequency generation spectroscopy experiments of Prof. Shen and co-workers indicating that the C{111}/(2x1) <--> C{111}/(1x1)H phase transformation is asymmetric and is mediated by the a metastable surface structure.

We find that at the early stages of hydrogen deposition on a (2x1)-reconstructed surface atomic configurations comprised of adjacent CH bonds can be readily formed. In subsequent surface transformations, these configurations serve as localized structural defects that stabilize a metastable structure, and determine the energy barrier separating the metastable structure reconstruction from the H-terminated (1x1) structure.

Vibrational spectra calculations confirm that just these strained atomic configurations are responsible for the formation of an additional metastable peak in the high-frequency region. Thermal annealing and further H dosing can cause the irreversible relaxation of these defected configurations, and hence the gradual transfer of the metastable peak strength to the peak corresponding to the (1x1) bulk-terminated surface structure occurs. This picture derived from the computer modeling explains the observations from sum-frequency generation spectroscopy experiments.
The vibrational dynamics of hydrogen on other low-index diamond surfaces was also studied. We found that the CH stretching peak positions in the vibrational spectra of hydrogen are sensitive to the surface structure and can be used for the experimental in situ analysis of the growing CVD diamond phase. The differences in the spectral features corresponding to the CH bending vibrations have been correlated with the results of the vibrational energy relaxation rate estimates. We find that the major contributions to the lifetimes of the excited CH stretching states come from the anharmonic coupling between the CH stretching and CCH bending vibration modes. The latter is highly coupled with the substrate phonons, which leads to the formation of broad spectral regions associated with CCH bending vibrations. The dynamic analysis of the motion induced by the symmetric and antisymmetric stretching excitation on C{100}/(2x1)H surface shows that the unstability of the pure antisymmetric vibrations is not conne cted with the dephasing of the CH stretching vibrations but rather is a consequence of the inherent nature of the antisymmetric vibrational mode in a system of coupled anharmonic CH oscillators. Click here to see an illustration.

Surface structure transformations and vibrational dynamics on the H-terminated diamond surfaces

Publications

Home page of Leonid Zhigilei