Fig.1 a) The scheme of 2D ReS2 phase transition under STEM. a,b and a + b are the three low index directions of ReS2. e– beam exposure creates a new T phase embedded in the pristine T′ phase. b–d) Atomic structures and electronic structures of T’’ (tetramerization in two directions) phase, T’ (dimerization in one direction) phase, and T (no dimerization) phase from DFT calculation, respectively.
Advanced science
Sub-Nanometer Electron Beam Phase Patterning in 2D Materials
Fig.2 STEM HAADF images of atomic-scale phase transition from pristine T’’ phase into 1D T’ or T phases, via 1D e– beam exposure direction along a, b and a+b crystal directions (scheme on the right), respectively. False color is applied to STEM images. e– beam scanning areas are marked by green and red boxes. Scale bars =1 nm.
Fig.3 c) Energy-Surface Area (E-S) relations of T, T’ and T’’ phase under the uniaxial strain along a crystal direction. Different phases are shown by different symbols: T phase, green squares; T’ phase, orange dots; T’’ phase, blue triangles. d) E-S relations of T, T’, and T’’ phase under biaxial strain. Tangent lines are presented by the gray dotted lines.
Our DFT calculations reveal the energy-surface (E-S) relations of the three phases in 1L ReS2 under strain (Fig.3 c,d). In terms of the uniaxial case, the stability superiority of the T’’ phase reduces upon a compressive strain along lattice direction a and a crossover of the total energies of T’’and T’ phases is found. Yet the T phase remains very unstable under uniaxial compressive strain, and it becomes the most stable phase when a biaxial strain is applied. The transition lattice constants are comparable with the experimentally derived lattice constants measured. Formation energies of S vacancies (single and bi- vacancies) and their associated displacement threshold energies (Td) of 1L ReS2 were revealed by DFT calculations. It indicates S-3 vacancy is the easiest one to be created, which is consistent with the experimental observation.
This work demonstrates that down to atomic precision, the focused e– beam patterning technique is capable of engineering the metallic T or T’ phase from 1D line to 2D surface at both grain domains and boundaries on the semiconducting T’’ phased ReS2 and ReSe2 monolayers. It provides an ideal patterning precision up to the sub-Å scale after aberration correction and results in phase patterning areas from several to ≈100 nm2, which is orders of magnitude greater than any conventional lithography techniques.