Abstract
Recently, a monolayer of SnTe was discovered to be a two-dimensional ferroelectric with an in-plane polarization, and, most dramatically, it exhibits a significant enhancement of the ferroelectric phase transition temperature compared to its bulk counterpart. This phenomenon is due to a structural phase transition from bulk-like α/β-SnTe, a topological crystalline insulator, to layered γ-SnTe as the thickness is decreased to a few atomic layers. A detailed understanding of the growth mechanism and phase distribution of ultrathin SnTe films are of great interest for potential applications. Here, we report detailed studies of the molecular beam epitaxial growth and in situ scanning tunneling microscopy characterization of ultrathin SnTe films on graphene substrates. By varying the growth conditions, SnTe can be prepared as either a continuous film or in the form of large rectangular plates. The rate of nucleation of SnTe was found to be highly sensitive to the substrate temperature. The coexistence and competition between the β and γ phases formed at room temperature was studied, and the phase diagram with respect to the average thickness of SnTe and the substrate temperature during growth is drawn.
Highlights
Just as the monolayer transition metal dichalcogenides (TMDCs) can be viewed as graphene-like lattices with a staggered sublattice potential,8 monolayer MX materials are akin to the staggered-lattice form of black phosphorus.9 Inversion symmetry is broken both in the monolayer TMDCs as well as the MXs
As distinct from the TMDCs, the spontaneous polarization in monolayer MX can be tuned by applying an external electric field or stress,7 which makes monolayer MX a promising platform for non-volatile memory devices
In-plane spontaneous polarization emerges within each monolayer, while the coupling between the neighboring monolayers is antipolar, and no net polarization is shown in bulk MX materials
Summary
Just as the monolayer transition metal dichalcogenides (TMDCs) can be viewed as graphene-like lattices with a staggered sublattice potential,8 monolayer MX materials are akin to the staggered-lattice form of black phosphorus.9 Inversion symmetry is broken both in the monolayer TMDCs as well as the MXs. Recent scanning tunneling microscopy (STM) experiments have revealed that monolayer SnTe films grown on graphene substrates show unexpectedly robust in-plane ferroelectricity with a Tc as high as 270 K,11 much higher than that of the bulk material (Tc < 100 K).12 Further studies have revealed that as the thickness decreases, the space group of the SnTe film transitions from Fm3 ̄m (above Tc, paraelectric, β phase) or R3m (below Tc, ferroelectric, α phase) in bulk material to Pmna (4n AL thick) or Pmn21 (4n − 2 AL thick) in nearly strain-free ultrathin films (layered γ phase).13 Since the lattice structure and properties of γ-SnTe agree well with the predictions of ferroelectric MX monolayers, it becomes the first experimentally confirmed 2D ferroelectric material in this family.
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