Inversion Domain Boundary Induced Stacking and Bandstructure Diversity in Bilayer MoSe₂

Abstract

Interlayer rotation and stacking were recently demonstrated as effective strategies for tuning physical properties of various two-dimensional materials. The latter strategy was mostly realized in heterostructures with continuously varied stacking orders, which obscure the revelation of the intrinsic role of a certain stacking order in its physical properties. Here, we introduce inversion-domain-boundaries into molecular-beam-epitaxy grown MoSe2 homobilayers, which induce uncommon fractional lattice translations to their surrounding domains, accounting for the observed diversity of large-area and uniform stacking sequences. Low-symmetry stacking orders were observed using scanning transmission electron microscopy and detailed geometries were identified by density functional theory. A linear relation was also revealed between interlayer distance and stacking energy. These stacking sequences yield various energy alignments between the valence states at the Γ and K points of the Brillouin zone, showing stacking-dependent bandgaps and valence band tail states in the measured scanning tunneling spectroscopy. These results may benefit the design of two-dimensional multilayers with manipulable stacking orders

Inversion domain boundaries in MoSe2 layers

“…The rst grain boundaries (denoted as yellow lines) are composed of the chains of 4-fold rings (4|4) with point-sharing at a common Se site. This type of phase boundary has been previously observed in grain boundary of CVD-grown MoSe 2 , 32,33 and molecular beam grown MoSe 2 , [35][36][37] or in grain boundary of 2H phase, 21 or boundary of 2H and 1T phase, 28 in MoS 2 . In contrast, the second grain boundaries (denoted as green lines) comprise of chains of 8-fold rings.…”

“…This result is consistent with the fact that 4|4 IDBs is oen observed in previous reports. 37,38,45 Recently, Zande et al showed that the measured in-plane electrical conductivity of the monolayer MoS 2 is slightly increased by the mid-gap states of Mo atoms at grain boundaries. 20 Therefore, it is important to understand how the 4|4and 8|8-fold rings contribute to electrical transport in the present IDBs model.…”

Healing of Planar Defects in 2D Materials via Grain Boundary Sliding

“…In bilayer MoSe 2 and other group VIB TMDC films, the most thermodynamically stable stacking order is 2H which takes the AA' interlayer stacking sequence. Other stacking registries, such as 3R (AB), 2Ha (AB'), and fractional translated states, can be regarded as stacking faults in 2H‐based MoSe 2 crystals . Density functional theory (DFT) calculations suggest that the formation energy of bilayer 2H‐stacked MoSe 2 is 33 and 216 meV nm −2 lower than those of the 3R‐stacked and N‐stacked crystals (Figure S1, Supporting Information), respectively .…”

“…Other stacking registries, such as 3R (AB), 2Ha (AB'), and fractional translated states, can be regarded as stacking faults in 2H‐based MoSe 2 crystals . Density functional theory (DFT) calculations suggest that the formation energy of bilayer 2H‐stacked MoSe 2 is 33 and 216 meV nm −2 lower than those of the 3R‐stacked and N‐stacked crystals (Figure S1, Supporting Information), respectively . The N stacking order can be obtained from the 3R phase by gliding the top layer relative to the bottom layer for 95 pm along the armchair direction .…”

“…Second, there is an increase in interlayer vdW binding energy when the high‐energy stacking order, i.e., 3R and intermediate states, transfers into the low‐energy 2H stacking . Hence, from a thermodynamics point of view, the MTB has a tendency to migrate toward the high‐energy domain, leading to the healing of stacking faults.…”