Phonon spectra of clean and Ni-terminated diamond (111) surfaces: An ab-initio study

Abstract

The phonon densities of states (ph-DOSs) of clean and Ni-terminated C(111) surfaces with 1 × 1 and 2 × 1 surface structures were investigated using ab-initio density functional perturbation theory. The ph-DOSs showed vibrational spectra associated with the surface structures of C(111) and Ni/C(111). Further analyses of various surface phonon modes were performed to identify vibrational features involving the surface atoms of C(111) and Ni/C(111). These features provide important information for experimentally verifying the formation of a diamond bulk-like structure at Ni/C(111), as suggested in a previous study.

Introduction

Growth of diamond has attracted a great deal of interest due to its technological importance in a wide range of applications, such as machine tools, optical coatings, high-temperature electronics, and next-generation power devices [[1], [2], [3], [4], [5], [6]]. The C(111) surface in diamond is a growth surface and natural cleavage plane with lower surface formation energy than other surfaces. Its surface structure is known to play an important role in determining the morphology and quality of the growing diamond, as well as many other physical properties [[7], [8], [9]]. Although the detailed atomic geometry of the reconstructed clean C(111) surface is still controversial, most experimental and theoretical studies [10,11] support the Pandey π-bonded 2 × 1 chain model of C(111) [Fig. 1(c)]. The sp² surface bonding character in the π-bonded chain model of 2 × 1 reconstructed C(111) would prevent C adatoms from making sp³ bonds for the growth of diamond, instead promoting the growth of undesired graphite.

To overcome this obstacle to diamond growth, diamond synthesis has typically been conducted using catalytic metals such as Fe, Ni, and Co [12,13]. Until recently, Ni has been the main catalytic metal element used [[14], [15], [16], [17], [18]]. In previous work, we theoretically studied the microscopic role of catalytic metal Ni in the growth of diamond through ab-initio total-energy and electronic-structure calculations [19]. The results showed that surface Ni atoms can enhance the formation of a thermodynamically stable Ni–C interface with a diamond bulk-like 1 × 1 structure, which is essential for the formation of sp³-bonded carbon in diamond. Further investigations showed that this feature originates from (i) strengthening of the C2–C3 bond, (ii) the covalent-like interaction of Ni with the dangling bond of the interfacial C1 atom in 1 × 1 Ni/C(111), and (iii) weakening of the π-bond in the Pandey chain structure of 2 × 1 Ni/C(111) (Fig. 1). These findings provide a good explanation for the efficacy of Ni as a catalyst for diamond growth.

Despite these studies, experimental information on the structures, bonding, and dynamics of Ni/C(111) surfaces remains limited. In particular, surface vibrational measurements are very useful for studying diamond surfaces. There have been no experimental or theoretical works on the vibrational properties of the growth surfaces of diamond in the presence of catalytic metal Ni. In this study, the phonon densities of states (ph-DOSs) of clean and Ni-terminated C(111) surfaces with 1 × 1 and 2 × 1 surface structures were studied by conducting ab-initio calculations based on density functional perturbation theory (DFPT) [20,21]. The calculation revealed discernible features in the vibrational spectra of the C(111) and Ni/C(111) surfaces depending on the surface and interface structures. This could play a significant role in experimentally verifying the atomic structures at Ni/C(111), as well as understanding the microscopic mechanism of the catalytic metal (Ni) in diamond growth.