Stabilisation of hBN/SiC Heterostructures with Vacancies and Transition-Metal Atoms

TL;DR

We address the stabilization and functionalization of the hBN/SiC heterostructure by defect engineering and single-atom catalysis. Using DFT and machine-learning molecular dynamics, we map how boron monovacancies nucleate interlayer Si–N bonds and how transition-metal adatoms can be anchored at these sites, enabling isolated single-atom centers within a high-bandgap vdW platform with a lattice mismatch of $18.77\%$ and a bandgap around $3.26$ eV. The results show that vacancy configurations and $d$-electron count govern whether interlayer bonding is favored and quantify barriers up to $3.76$ eV for bond rupture, while Cu illustrates barrierless diffusion on pristine surfaces but robust trapping at vacancy sites; two fabrication routes are proposed to control bonding and reactivity. Overall, the hBN/SiC system emerges as a defect-programmable platform for atomically precise transition-metal function, with potential impact on catalysis and quantum technologies, and provides design rules for stabilizing single-atom centers at vdW interfaces.

Abstract

When two-dimensional atomic layers of different materials are brought into close proximity to form van der Waals (vdW) heterostructures, interactions between adjacent layers significantly influence their physicochemical properties. These effects seem particularly pronounced when the interface exhibits local order and near-perfect structural alignment, leading to the emergence of Moiré patterns. Using quantum mechanical density functional theory calculations, we propose a prototypical bilayer heterostructure composed of hexagonal boron nitride (hBN) and silicon carbide (SiC), characterized by a lattice mismatch of 18.77\% between their primitive unit cells. We find that the removal of boron atoms from specific lattice sites can convert the interlayer interaction from weak vdW coupling to robust localized silicon-nitrogen covalent bonding. Motivated by this, we study the binding of transition-metal adatoms and formulate design guidelines to enhance surface reactivity, thereby enabling the controlled isolation of single-metal atoms. Our machine-learning-assisted molecular dynamics simulations confirm both dynamical stability and metal anchoring feasibility at finite temperatures. Our results suggest the hBN/SiC heterostructure as a versatile platform for atomically precise transition-metal functionalization, having potential for next-generation catalytic energy-conversion technologies.

Figures (4)

• Figure 1: (a) Heterostructure supercell model. Boron, nitrogen, silicon, and carbon are represented in green, gray, blue, and brown, respectively. (b) Out-of-plane displacements of hBN layer in response to the lattice mismatch of $1.48$%. The size of each symbol represents the deviation of an atom's $z$-coordinate from the average value. Green symbols indicate atoms with $z$-coordinates below the average ($\Delta z^{\rm min} = -0.17$ Å), while gray symbols represent atoms with $z$-coordinates above the average ($\Delta z^{\rm max} = 0.23$ Å). For the SiC layer $\Delta z^{\rm min}$ and $\Delta z^{\rm max}$ are about $-0.05$ and $0.05$ Å, which were not depicted. (c) Phonon density of states (DOS) of the hBN/SiC system, shown in units of states/meV. (d) Energy levels and bandgap for hBN, SiC, and the hBN/SiC heterostructure. Energy levels are shifted to valence band maximum (VBM) of the heterostructure system and set as zero.
• Figure 2: Local defect structures of the (a) three-bonded $V_{\mathrm{B}}$, (b) four-bonded $V_{\mathrm{B}}$, and (c) $V_{\mathrm{N}}$ are presented along with detailed structural information. (d) Formation energy of the defects as a function of the position of the Fermi-level under N-rich and N-poor conditions: The VBM is aligned to zero for the sake of simplicity. The conduction band minimum (CBM) is at $3.26$ eV. For each defect the ($+$), ($0$), and ($-$) charge states may appear. The N-rich and N-poor conditions are defined in section II of the SM. (e-g) Electronic structure for the ground states of the defects. Kohn-Sham (KS) levels are represented in spin-up ($\uparrow$) and spin-down ($\downarrow$) channels. The occupied and unoccupied levels are represented in black and blue, respectively, for each charge state ($q$). The number of unpaired electrons can be determined from the spin state ($S$). For example, $S = 1$ indicates two unpaired electrons. (h-j) Energy profile for interlayer distance (bond length) scan of the $V_{\mathrm{B}}$ defect in different systems: the hBN/SiC heterostructure, AA$^{\prime}$ stacking, and the $14^{\circ}$-twisted bilayer. Total energies are shifted for simplicity. Both low and high spin states are investigated, and a four-bonded system is considered for the heterostructure.
• Figure 3: (a) Activation barrier ($\Delta E^{*}$) for converting the bonded hBN/SiC configuration to its non-bonded counterpart. (b) Total-energy difference between the bonded and non-bonded minima ($\Delta E^{0}$). (c) Binding energy of a single-atom transition metal (TM) to the chemically bonded hBN/SiC interface, $E_{\mathrm{bind}}^{\mathrm{b}}$. (d) Net Bader charge transferred from the TM atom to the interface in the bonded configuration, $q_{\mathrm{TM}}^{\mathrm{b}}$.
• Figure 4: (a) Binding energies of Cu, Pd, and Pt as a function of their distance from the $V_{\mathrm{B}}$ center along the zigzag (zz) and armchair (ac) directions of the hBN layer. During the geometry optimizations, the SiC substrate is fixed, while atoms in the hBN layer and the transition metal are allowed to relax along the $z$-direction (perpendicular to the surface). (b) Time evolution of the Cu$-$V$_{\mathrm{B}}$ distance extracted from ten independent MLMD simulations at 300 K. Each trace represents one run, and a separation of zero denotes the Cu atom occupying the vacancy site.