Ultra-low-energy nitrogen ion interactions with diamond surfaces: Impact of crystal orientation on bonding and thermal stability

This study investigates the influence of crystal orientation on nitrogen incorporation, bonding configurations, and thermal stability in (100)- and (111)-oriented single-crystal diamond (SCD) surfaces implanted with ultra-low-energy N2+ ions (100 and 200 eV; dose: 1 × 1015 ions/cm2). Upon impact, N2+ ions dissociate, and each interacting nitrogen atom receives half the kinetic energy (50 and 100 eV), making the process ideal for studying near-threshold defect formation. Low-energy electron diffraction revealed a reconstructed surface with coexisting (2 × 1) and (1 × 2) domains on (100) SCD, while the (111) surface maintained a well-ordered (1 × 1) structure. X-ray photoelectron spectroscopy showed that (100) SCD incorporates and retains more nitrogen than (111), with superior thermal stability up to 1000 °C. Despite the low implantation energy, both surfaces exhibit minimal structural defect (Cdef), although (100) SCD is slightly more susceptible to defect formation. Implanted nitrogen forms primarily C–N/C=N and N–(sp2 C/Cdef) bonds, with the former dominating. These bonding configurations evolve with temperature and crystal orientation. The N–(sp2 C/Cdef) component remains stable between 300 and 700 °C and decreases at higher temperatures. At lower temperatures, nitrogen remains in interstitial form (Ni), forming Ni–Ci complexes with mobile carbon interstitials (Ci), which stabilize N–(sp2 C/Cdef) bonding. At elevated temperatures (800–1000 °C), Ni converts to substitutional nitrogen (Ns), reducing the defect-associated bonding signal. Overall, (100) SCD shows a stronger tendency for defect-mediated nitrogen stabilization than (111). These insights into ultra-low-energy nitrogen–diamond interactions may guide future strategies for surface engineering and stabilization of shallow nitrogen-vacancy centers in diamond-based quantum technologies.