title: Three-dimensional spirals of atomic layered MoS2.
abstract: Atomically thin two-dimensional (2D) layered materials, including graphene, boron nitride, and transition metal dichalcogenides (TMDs), can exhibit novel phenomena distinct from their bulk counterparts and hold great promise for novel electronic and optoelectronic applications. Controlled growth of such 2D materials with different thickness, composition, and symmetry are of central importance to realize their potential. In particular, the ability to control the symmetry of TMD layers is highly desirable because breaking the inversion symmetry can lead to intriguing valley physics, nonlinear optical properties, and piezoelectric responses. Here we report the first chemical vapor deposition (CVD) growth of spirals of layered MoS2 with atomically thin helical periodicity, which exhibits a chiral structure and breaks the three-dimensional (3D) inversion symmetry explicitly. The spirals composed of tens of connected MoS2 layers with decreasing areas: each basal plane has a triangular shape and shrinks gradually to the summit when spiraling up. All the layers in the spiral assume an AA lattice stacking, which is in contrast to the centrosymmetric AB stacking in natural MoS2 crystals. We show that the noncentrosymmetric MoS2 spiral leads to a strong bulk second-order optical nonlinearity. In addition, we found that the growth of spirals involves a dislocation mechanism, which can be generally applicable to other 2D TMD materials.
authors.affiliation: Department of Chemistry and ‡Department of Physics, University of California at Berkeley , Berkeley, California 94720, United States.
authors.forename: Liming
authors.initials: L
authors.lastname: Zhang
authors.forename: Kaihui
authors.initials: K
authors.lastname: Liu
authors.forename: Andrew Barnabas
authors.initials: AB
authors.lastname: Wong
authors.forename: Jonghwan
authors.initials: J
authors.lastname: Kim
authors.forename: Xiaoping
authors.initials: X
authors.lastname: Hong
authors.forename: Chong
authors.initials: C
authors.lastname: Liu
authors.forename: Ting
authors.initials: T
authors.lastname: Cao
authors.forename: Steven G
authors.initials: SG
authors.lastname: Louie
authors.forename: Feng
authors.initials: F
authors.lastname: Wang
authors.forename: Peidong
authors.initials: P
authors.lastname: Yang
mesh_terms: D003460:Crystallization; D004220:Disulfides / Q000737:chemistry*; D008958:Models, Molecular; D008982:Molybdenum / Q000737:chemistry*; D049329:Nanostructures / Q000737:chemistry* / Q000648:ultrastructure*; D036103:Nanotechnology; D014835:Volatilization
chemical_list: D004220:Disulfides; D008982:Molybdenum; C082964:molybdenum disulfide
keywords: Molybdenum disulfide (MoS2); chemical vapor deposition; screw dislocation; spirals; symmetry breaking