Full orientation control of epitaxial MoS2 on hBN assisted by substrate defects

Abstract

Inversion asymmetry in two-dimensional materials grants them fascinating properties such as spin-coupled valley degrees of freedom and piezoelectricity, but at the cost of inversion domain boundaries if the epitaxy of the grown 2D layer -on a polar substrate -cannot adequately distinguish what are often near-degenerate 0° and 180° orientations. We employ first-principles calculations to identify a method to lift this near-degeneracy: the energetic distinction between eclipsed and staggered configurations during nucleation at a point defect in the substrate. For monolayer MoS2 grown on hexagonal boron nitride, the predicted defect complex can be more stable than common MoS2 point defects because it is both a donor-acceptor pair and a Frenkel pair shared between adjacent layers of a 2D heterostack. Orientation control is verified in experiments that achieve ~90% consistency in the orientation of as-grown triangular MoS2 flakes on hBN, as confirmed by aberration-corrected scanning/transmission electron microscopy. This defect-enhanced orientational epitaxy could provide a general mechanism to break the near-degeneracy of 0/180° orientations of polar 2D materials on polar substrates, overcoming a long-standing impediment to scalable synthesis of single-crystal 2D semiconductors. † yow5110@psu.edu ‡ vhc2@psu.edu § nua10@psu.edu * These authors contributed equally to this work.* These authors contributed equally to this work. First-principles calculationDensity functional theory calculations were performed using the Perdew-Burke-Ernzerhof parametrization of the generalized gradient approximation (GGA-PBE) exchange-correlation functional [1,2] and pseudopotentials constructed from the projector augmented wave (PAW) method [3,4], as implemented in the Vienna Ab initio Simulation Package (VASP) [5]. Van der Waals corrections were included using the DFT-D3 [6], DFT-TS [7], and vdW-DF2 [8] methods. Both DFT-D3 and DFT-TS show excellent agreement with random phase approximation treatments of the van der Waals interaction in the interlayer binding energy of bulk MoS2 (<10% error) [9-11], but overbind hBN layers by 80-100% [10][11][12]. vdW-DF yields a similar binding energy for MoS2 and better binding energy for hBN [10,11]. All corrections yield excellent results for corrugation, i.e. the energy variation upon sliding adjacent layers relative to each other. Ionic relaxations were all performed at the PBE level with vdW corrections using the DFT-D3 method (unless otherwise noted, e.g. for calculations using DFT-TS and vdW-DF2) until forces were smaller than 0.01 eV/Å. Hybrid functional eigenvalues and total energies were calculated using the range-separated form of Heyd, Scuseria, and Ernzerhof (HSE06) [13,14] and using structures relaxed at the HSE06 level until forces were smaller than 0.02 eV/Å.

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