As the most common impurity in natural diamond, nitrogen could form a series of color centers that are of optical and spin properties suitable for applications in quantum computing, magnetometry, and biological sensing.[1] To populate nitrogen centers in the near-surface region of diamond, there has been recent effort [2-12] of using activated dinitrogen to impact fully hydrogenated diamond surfaces as synthesized by chemical vapor deposition. For example, Hoffman and co-workers [5] reported on the formation of surface nitrogen dimers on diamond(001)-2×1-H by microwave N2 plasma. By contrast, there is implication [7-8] of nitrogen into the subsurface region (depth: 6 ± 5 Å) of the same diamond(001)-2×1-H by low energy N2+ ions. It remains, however, unclear about the atomistic understanding into the configuration and bonding of the nitrogen species in the subsurface region of diamond(001)-2×1-H.

Here, I present studies into subsurface nitrogen in diamond (001)-2×1-H by density functional theory simulations, revealing in each case information regarding structure, energy and vibration that depend on the atomistic coordination as detailed below.

  • Interstitial Ni

We have studied 10 configurations of a single interstitial nitrogen in the topmost 3 layers of diamond (001)-2×1-H. In all configurations, the carbon network is strongly distorted by an interstitial nitrogen atom, in which 2 carbon-carbon bonds are cleaved. The interstitial nitrogen is bound to 3 neighboring carbon atoms, of which one carbon becomes sp2 hybridized. The stability of the interstitial configuration depends on the location of nitrogen and the direction of the formed C(sp2)-N bond, exhibiting formation energies of +4.83 to +11.04 eV. The most characteristic mode of vibration arises from the C(sp2)-N bond, i. e., stretch (1530 to 1901 cm⁻¹/ 189.7 to 235.8 meV) and swing (1101 to 1389 cm⁻¹/ 136.6 to 172.2 meV). Migration of nitrogen interstitial is also evaluated, involving a series of steps and intermediates; the rate determining step is of a barrier of 6.02 eV.

  • Substitutional Ns

We have also evaluated 3 configurations of a single substitutional nitrogen in the topmost 3 layers of diamond(001)-2×1-H. In all configurations, the carbon network is largely preserved, giving formation energy of +2.79 to +3.41 eV. The substitutional nitrogen is bound to four neighboring carbons, and the formed C-N bond is larger than a typical C-N single bond, giving the characteristic C-N swing vibration of 844 to 979 cm⁻¹ (105 to 121 meV).

  • Dinitrogen N2i

We have finally evaluated 6 configurations of a pair of nitrogen atoms in the topmost 3 layers of diamond(001)-2×1-H. Our computations were restricted to the N2i species that mimics the encounter of an interstitial Ni and a substitutional Ns species. The formation energy depends on the location of N2i and the direction of the formed Ni-Ns bond, ranging from +4.12 to +9.71 eV. The most characteristic motions of vibration arise from N-N stretch mode of 1419 cm⁻¹ (176.0 meV), C-N swing mode of 968 cm⁻¹ (120 meV) and N-N swing mode of 471 cm⁻¹ (58.5 meV).

Reference

[1] Ashfold, M. N., Goss, J. P., Green, B. L., May, P. W., Newton, M. E., & Peaker, C. V. (2020). Nitrogen in diamond. Chemical reviews, 120(12), 5745-5794.

[2] Attrash, M., Kuntumalla, M. K., Michaelson, S., & Hoffman, A. (2020). Nitrogen-terminated polycrystalline diamond surfaces by microwave chemical vapor deposition: Thermal stability, chemical states, and electronic structure. The Journal of Physical Chemistry C, 124(10), 5657-5664.

[3] Attrash, M., Kuntumalla, M. K., Michaelson, S., & Hoffman, A. (2021). Nitrogen terminated diamond (111) by RF (N2) plasma–chemical states, thermal stability and structural properties. Surface Science, 703, 121741.

[4] Zheng, Y., Hoffman, A., & Huang, K. (2021). Atomistic insight into nitrogen-terminated diamond (001) surfaces by the adsorption of N, NH, and NH2: A density functional theory study. Langmuir, 37(20), 6248-6256.

[5] Zheng, Y., Kuntumalla, M. K., Attrash, M., Hoffman, A., & Huang, K. (2021). Effect of surface hydrogenation on the adsorption and thermal evolution of nitrogen species on diamond (001) by microwave N2 plasma. The Journal of Physical Chemistry C, 125(51), 28157-28161.

[6] Kuntumalla, M. K., Zheng, Y., Attrash, M., Gani, G., Michaelson, S., Huang, K., & Hoffman, A. (2022). Microwave N2 plasma nitridation of H-diamond (111) surface studied by ex situ XPS, HREELS, UPS, TPD, LEED and DFT. Applied Surface Science, 600, 154085.

[7] Kuntumalla, M. K., Gani, G., Fischer, M., & Hoffman, A. (2023). Bonding, retention and thermal stability of shallow nitrogen in diamond (100) by low-energy nitrogen implantation. Surfaces and Interfaces, 37, 102649.

[8] Kuntumalla, M. K., Fischer, M., & Hoffman, A. (2024). Subsurface nitrogen bonding and thermal stability of low-energy nitrogen implanted H-Diamond (100) surfaces studied by XPS and HREELS. Surface Science, 739, 122399.

[9] Kuntumalla, M. K., Fischer, M., Gani, G., & Hoffman, A. (2024). Influence of 1 keV N2+ Implantation on Nitrogen Bonding, Defect Formation, and Thermal Stability of the Polycrystalline Diamond Near-Surface Region Studied by XPS, TPD, and HREELS. The Journal of Physical Chemistry C, 128(6), 2588-2603.

[10] Kuntumalla, Mohan Kumar, et al. “Bonding, Thermal and Ambient Stability of Nitrogen-Terminated Diamond (100) Surfaces by Plasma Exposure Studied by Ex-Situ XPS, HREELS, and DFT Modeling.” Novel Aspects of Diamond II: Science and Technology. Cham: Springer Nature Switzerland, 2024. 175-210.

[11] Fischer, M., Maity, S., Kuntumalla, M. K., Gani, G., & Hoffman, A. (2024). Subsurface nitrogen bonding, thermal stability, and retention of 200 eV N2+ implanted polycrystalline diamond studied by in situ X-ray photoelectron spectroscopy. Applied Surface Science, 657, 159740.

[12] Zheng, Y., Hoffman, A., & Huang, K. (2024). Nitridation of diamond (111) surface by density functional theory. The Journal of Chemical Physics, 160(21), 214713.