SnTe Monolayers Research Articles

Giant asymmetric proximity-induced spin–orbit coupling in twisted graphene/SnTe heterostructure

We analyze the spin–orbit coupling effects in a 3∘-degree twisted bilayer heterostructure made of graphene and an in-plane ferroelectric SnTe, with the goal of transferring the spin–orbit coupling from SnTe to graphene, via the proximity effect. Our results indicate that the point-symmetry breaking due to the incompatible mutual symmetry of the twisted monolayers and a strong hybridization has a massive impact on the spin splitting in graphene close to the Dirac point, with the spin splitting values greater than 20 meV. The band structure and spin expectation values of graphene close to the Dirac point can be described using a symmetry-free model, triggering different types of interaction with respect to the threefold symmetric graphene/transition-metal dichalcogenide heterostructure. We show that the strong hybridization of the Dirac cone’s right movers with the SnTe band gives rise to a large asymmetric spin splitting in the momentum space. Furthermore, we discover that the ferroelectricity-induced Rashba spin–orbit coupling in graphene is the dominant contribution to the overall Rashba field, with the effective in-plane electric field that is almost aligned with the (in-plane) ferroelectricity direction of the SnTe monolayer. We also predict an anisotropy of the in-plane spin relaxation rates. Our results demonstrate that the group-IV monochalcogenides MX (M = Sn, Ge; X = S, Se, Te) are a viable alternative to transition-metal dichalcogenides for inducing strong spin–orbit coupling in graphene.

Peculiar band geometry induced giant shift current in ferroelectric SnTe monolayer

The bulk photovoltaic effect (BPVE) occurs when homogeneous noncentrosymmetric materials generate photocurrent or photovoltage under illumination. The intrinsic contribution to this effect is known as the shift current effect. We calculate the shift current conductivities of the ferroelectric SnTe monolayer using first-principles methods. Our results reveal a giant shift-current conductivity near the valley points in the SnTe monolayer. More remarkably, the linear optical absorption coefficient at this energy is very small, resulting in an enormous Glass coefficient that is four orders of magnitude larger than that of BaTiO3. To understand these giant shift-current effects, we employ a three-band model and find that they arise from the nontrivial energy band geometries near the valley points, where the shift-vector diverges. This serves as a prominent example highlighting the crucial role of band geometry in determining the fundamental properties of solids.

First-principles study of wrinkled SnTe monolayer as p-type thermoelectric material

Tin telluride compounds are promising thermoelectric (TE) materials on account of the excellent electronic and thermal transport properties. The present work focuses on examining the TE performance of a two-dimensional wrinkled SnTe monolayer by employing the first-principles calculations and Boltzmann transport theory. The SnTe monolayer demonstrates high mechanical, dynamic and thermal stabilities, which have been thoroughly investigated using the elastic modulus, formation energy and ab initio molecular dynamics simulations. The lattice thermal conductivities of the SnTe monolayer along the zigzag- and armchair-direction are 11.29 and 12.40 W/mK at 300K, respectively. Due to the large Grüneisen parameters, short phonon relaxation time and low phonon group velocity, the p-type SnTe monolayer exhibits high TE performance at 900 K, as characterized by a high ZT of 1.44 along the zigzag-direction. Our present study not only offers valuable insights into the TE properties of the wrinkled SnTe monolayer, but also sheds some light on the theoretical designing of the novel SnTe-based material for TE applications.

Mn-doped SnTe monolayer as toxic gas scavenger or sensor based on first-principles study

Using first-principles theory, we investigated the stability and feasibility on Mn-doped SnTe monolayer and the interactions of Mn-doped SnTe monolayer (Mn-SnTe) for the adsorption of toxic gases H2S, SO2, NO, NO2 and Cl2. The results show that the Mn-SnTe monolayer has a weak interaction for H2S and strong adsorption energies of −1.840, −4.123, −2.954 and −3.578 eV for SO2, NO, NO2 and Cl2, respectively. In addition, due to the high sensitivity of Mn-SnTe monolayer to adsorb SO2 and Cl2, the bandgaps of the developed systems are reduced by about 79.96% and 100%, respectively. The results indicate that the Mn-SnTe monolayer has a high sensitivity for the detection of SO2 and Cl2 gases. Our calculations provide a theoretical basis for the development of Mn-SnTe monolayer for potential applications as sensors or scavengers for toxic gases.

Tuning the stability and optoelectronic properties of SnTe/Sb van der Waals heterostructure by biaxial strain effect

Using density functional theory (DFT), we have investigated the structural, stability, and electronic properties of the isolated antimonene, SnTe monolayers, and the structural, stability, and optoelectronic properties of the SnTe/Sb vdW heterostructure. The results show that the atoms in these monolayers have strong cohesion and that the SnTe/Sb vdW heterostructure is stable. The indirect bandgap energies are calculated to be 1.17[Formula: see text]eV (PBE) (2.28[Formula: see text]eV (HSE)), 1.89[Formula: see text]eV (PBE) (2.93[Formula: see text]eV (HSE)), and 0.32[Formula: see text]eV (PBE) (0.89[Formula: see text]eV (HSE)), respectively. However, these physical properties can be modulated by applying biaxial strain, when the compressive and tensile biaxial strain reached more than 4%, the heterostructure turned into metal, and the electronic bandgap decreased as the tensile and compressive biaxial strain increased from 0 to 8%. The phonon dispersion exhibits imaginary modes, notably above the 6% compressive strain, exhibiting its dynamic instability. The formation energy is negative under all biaxial strain, indicating that the heterostructure is still relatively stable during biaxial strain. An enhancement of optical absorption is observed, especially near the UV-visible regions, when the biaxial strain is incorporated, especially for compressive strains of 4% and 2%, which increases the absorption capacity. Therefore, the application of the biaxial strain can improve the stability, optical, and electronic properties of the SnTe/Sb vdW heterostructure, suggesting its potential for photovoltaic and optoelectronic applications.

Dopability and Magnetic Properties of 3d Transition Metals in an Atomic-Thick SnTe (001) Monolayer

In this paper, we systematically study the dopability and magnetic properties of a 3d TM-doped atomic-thick SnTe(001) monolayer based on first-principles calculations. It is found that, for separately distributed TMs in a SnTe(001) monolayer, all of the TMs, except Sc, Cu, and Zn for the substitutional configuration and Ni, Cu, and Zn for the adsorption and interstitial configuration, could induce local magnetic moments. On the other hand, contradictive to the intuition that TM may adsorb on the SnTe(001) slab, substitution is more favorable. Even though the formation energy of Ni adsorbed and interstitial in a SnTe(001) monolayer is comparably low, its local magnetic moment is 0 as a result of the 3d orbitals fully occupied. Considering both the low formation energy and large magnetic moment, Mn is expected to be a prominent choice to introduce magnetism in atomic-thick SnTe in substitutional configuration. Thus, a pristine flat magnetic SnTe monolayer is probable to be obtained.

First‐principles Study of SO2 and NO Gas Sensors Based on SnTe Monolayer

AbstractThe structural and electronic properties of SnTe monolayer after the adsorption of gas molecules (CH4, CO2, NH3, CO, H2S, NO, SO2, NO2 and Cl2) were investigated by first‐principles calculations based on density functional theory (DFT). The results showed that SO2, NO, NO2 and Cl2 were chemisorbed on SnTe monolayer with strong adsorption energies. In addition, because of the high sensitivity of the adsorption of SO2 and NO gases on SnTe monolayer, the bandgap of the developed system changed significantly, with a reduction of 22.88 % and 71.65 %, respectively. The results indicate that SnTe monolayer has highly sensitive SO2 and NO gas detection capability, especially for NO. And it can act as a disposable gas sensor or metal‐free catalyst for the detection and catalysis of pollution gases, such as NO2 and Cl2.

Anharmonic lattice dynamics and structural phase transition of SnTe monolayer from first principles

In this work we investigate the role of quartic anharmonicity on the lattice- and thermo-dynamic properties of rectangular () and square (β) phases of two-dimensional (2D) SnTe monolayer (ML) by using self-consistent phonon (SCP) theory, based on the first-principles calculations. For both phases, as compared to the usual harmonic approximation (HA), the renormalized phonon frequency at the optical modes (4–10) is found to be increased upon the inclusion of quartic anharmonicity via the SCP method, where the effects of cubic anharmonicity are neglected. At the experimentally observed transition temperature (T c = 270 K), the difference in the vibrational free-energy between the square and rectangular phases of SnTe ML, calculated by using the anharmonic SCP correction is found to be much closer to the structural energy gain as compared to that obtained by using only the quasi-harmonic contribution. This validates the significance of SCP approach over the HA to explain the lattice dynamics properties and predict the T c for SnTe ML and similar 2D compounds. The calculated lattice thermal conductivity of square SnTe ML (e.g. 10.67 W m−1K−1 at 300 K) is higher than that of the rectangular SnTe ML (e.g. 6.72 W m−1K−1 at 300 K) due to the relatively higher corresponding thermodynamic parameters: specific heat capacity, group velocity, and phonon lifetime obtained for the square SnTe ML. Particularly, the low energy phonon modes are found to transport most of the heat in the system and, hence, played major role to the total lattice thermal conductivity.

Strain engineering and hydrogen effect for two-dimensional ferroelectricity in monolayer group-IV monochalcogenides MX (M = Sn, Ge; X = Se, Te, S)

Two-dimensional (2D) ferroelectric compounds are a special class of materials that meet the need for devices miniaturization, which can lead to a wide range of applications. Here, we investigate ferroelectric properties of monolayer group-IV monochalcogenides MX (M = Sn, Ge; X = Se, Te, S) via strain engineering, and their effects with contaminated hydrogen are also discussed. GeSe, GeTe, and GeS do not go through transition up to the compressive strain of –5%, and consequently have good ferroelectric parameters for device applications that can be further improved by applying strain. According to the calculated ferroelectric properties and the band gaps of these materials, we find that their band gap can be adjusted by strain for excellent photovoltaic applications. In addition, we have determined the most stable hydrogen occupancy location in the monolayer SnS and SnTe. It reveals that H prefers to absorb on SnS and SnTe monolayers as molecules rather than atomic H. As a result, hydrogen molecules have little effect on the polarization and electronic structure of monolayer SnTe and SnS.

Systematic Analysis Method for Nonlinear Response Tensors

We propose a systematic analysis method for identifying essential parameters in various linear and nonlinear response tensors without which they vanish. By using the Keldysh formalism and the Chebyshev polynomial expansion method, the response tensors are decomposed into the model-independent and dependent parts, in which the latter is utilized to extract the essential parameters. An application of the method is demonstrated by analyzing the nonlinear Hall effect in the ferroelectric SnTe monolayer for example. It is shown that in this example the second-neighbor hopping is essential for the nonlinear Hall effect whereas the spin–orbit coupling is unnecessary. Moreover, by analyzing terms contributing to the essential parameters in the lowest order, the appearance of the nonlinear Hall effect can be interpreted by the subsequent two processes: the orbital magneto-current effect and the linear anomalous Hall effect by the induced orbital magnetization. In this way, the present method provides a microscopic picture of responses. By combining with computational analysis, it stimulates further discoveries of anomalous responses by filling in a missing link among hidden degrees of freedom in a wide variety of materials.

Vortex-Oriented Ferroelectric Domains in SnTe/PbTe Monolayer Lateral Heterostructures.

Heterostructures formed from interfaces between materials with complementary properties often display unconventional physics. Of especial interest are heterostructures formed with ferroelectric materials. These are mostly formed by combining thin layers in vertical stacks. Here the first in situ molecular beam epitaxial growth and scanning tunneling microscopy characterization of atomically sharp lateral heterostructures between a ferroelectric SnTe monolayer and a paraelectric PbTe monolayer are reported. The bias voltage dependence of the apparent heights of SnTe and PbTe monolayers, which are closely related to the type-II band alignment of the heterostructure, is investigated. Remarkably, it is discovered that the ferroelectric domains in the SnTe surrounding a PbTe core form either clockwise or counterclockwise vortex-oriented quadrant configurations. In addition, when there is a finite angle between the polarization and the interface, the perpendicular component of the polarization always points from SnTe to PbTe. Supported by first-principles calculation, the mechanism of vortex formation and preferred polarization direction is identified in the interaction between the polarization, the space charge, and the strain effect at the horizontal heterointerface. The studies bring the application of 2D group-IV monochalcogenides on in-plane ferroelectric heterostructures a step closer.

Ferroic orders in two-dimensional transition/rare-earth metal halides

Since the discovery of graphene, two-dimensional materials with atomic level thickness have rapidly grown to be a prosperous field of physical science with interdisciplinary interest for their fascinating properties and broad applications. Very recently, the experimental observation of ferromagnetism in a Cr2Ge2Te6 bilayer and a CrI3 monolayer opened a door to pursue long-absent intrinsic magnetic orders in two-dimensional materials. Meanwhile, the ferroelectricity was also experimentally found in a SnTe monolayer and CuInP2S6 few layers. The emergence of these ferroic orders in the two-dimensional limit not only brings new challenges to our physical knowledge but also provides more functionalities for potential applications. Among various two-dimensional ferroic ordered materials, transition/rare-earth metal halides and their derivants are very common. In this Research Update, based on transition/rare-earth metal halides, the physics of various ferroic orders in two-dimensional materials will be illustrated. The potential applications based on their magnetic and polar properties will also be discussed.

Thermoelectric and lattice dynamics properties of layered MX (M = Sn, Pb; X = S, Te) compounds

Lead and tin chalcogenides are extensively studied due to their promising thermoelectric (TE) properties. They show an enhancement in the TE efficiency upon the reduction ofdimension, which is an important feature in device fabrications. Using density functional theory combined with the semi-classical Boltzmann transport formalism, we studied the structural, electronic, and TE properties of two-dimensional (2D) MX (M = Sn, Pb; X = S, Te) monolayers (MLs). Spin-orbit coupling played a significant roleon their electronic structure, particularly for the heavy compounds. Structural optimization followed by phonon transport studies prevailed that the rectangular (γ-) phaseis energetically the most favorable for SnS and SnTe MLs, whereas the square structure is found the most stable for PbS and PbTe MLs.These 2D materials exhibit high Seebeck coefficients (1000–1500 µV/K)and power factors ((33–77.3) × 10−4W m−1 K−2) along with low lattice thermal conductivities (<3Wm−1 K−1)—these are the essential features of good TE materials. The maximum figure of merits (ZT) of 1.04, 1.46, 1.51, and 1.94 are predicted forn-type SnS, SnTe, PbS, andp-type PbTe MLs, respectively at 700 K, which are higher than their bulkZTvalues. Hence, these MLs are promising candidates for TE applications.

Erratum to: 2D Hexagonal SnTe monolayer: a quasi direct band gap semiconductor with strain sensitive electronic and optical properties

Erratum to: 2D Hexagonal SnTe monolayer: a quasi direct band gap semiconductor with strain sensitive electronic and optical properties

Ab Initio Study of Ferroelectric Critical Size of SnTe Low-Dimensional Nanostructures.

Beyond a ferroelectric critical thickness of several nanometers existed in conventional ferroelectric perovskite oxides, ferroelectricity in ultimately thin dimensions was recently discovered in SnTe monolayers. This discovery suggests the possibility that SnTe can sustain ferroelectricity during further low-dimensional miniaturization. Here, we investigate a ferroelectric critical size of low-dimensional SnTe nanostructures such as nanoribbons (1D) and nanoflakes (0D) using first-principle density-functional theory calculations. We demonstrate that the smallest (one-unit-cell width) SnTe nanoribbon can sustain ferroelectricity and there is no ferroelectric critical size in the SnTe nanoribbons. On the other hand, the SnTe nanoflakes form a vortex of polarization and lose their toroidal ferroelectricity below the surface area of 4 × 4 unit cells (about 25 Å on one side). We also reveal the atomic and electronic mechanism of the absence or presence of critical size in SnTe low-dimensional nanostructures. Our result provides an insight into intrinsic ferroelectric critical size for low-dimensional chalcogenide layered materials.

2D Hexagonal SnTe monolayer: a quasi direct band gap semiconductor with strain sensitive electronic and optical properties

The stability and electronic and optical properties of two-dimensional (2D) SnTe monolayer has been systematically studied by using first-principles calculations based on density functional theory. Our computations demonstrate that the predicted 2D SnTe monolayer is a stable quasi-direct semiconductor. Also, analysis of its electronic property shows that the ground state of this monolayer is a quasi-direct semiconductor with a band gap of ~2.00. This band gap can be effectively modulated by external strains. Investigation of optical properties shows that monolayer SnTe exhibits significant absorption and reflectivity in the ultraviolet region of the electromagnetic spectrum.

Coexistence of Magnetism and Ferroelectricity in 3d Transition-Metal-Doped SnTe Monolayer

The realization of multiferroicity in 2D nanomaterials is crucially important for designing advanced nanoelectronic devices such as non-volatile multistate data storage. In this work, the coexistence of ferromagnetism and ferroelectricity is reported in monolayer SnTe system by transition metal (TM) doping. Based on first-principles calculations, the spontaneous spin polarization could be realized by TM doping in ferroelectric SnTe monolayer. In addition to in-plane ferroelectric polarization, the out-of-plane ferroelectric polarization emerges in Mn (Fe)-doped SnTe monolayer due to the internal displacement of TM from the surface. Interestingly, the crystalline field centered on TM and interaction between the dopant and Te gradually enhanced with the increment of atomic number of doping elements, which explains why the formation energy decreases. The realization of multiferroics in SnTe monolayer could provide theoretical guidance for experimental preparation of low-dimensional multiferroic materials.

Electronegativity, phase transition, and ferroelectricity of TeSe2 few-layers

We use first-principles calculations to demonstrate that γ TeSe2 few-layers (FLs) are significantly more stable than α and β FLs due to the difference in the electronegativity of two kinds of atoms, while γ Te FLs are not due to the unfavorable multivalency of Te atoms. The quasiparticle single-shot G0W0 band gaps are 1.13 and 2.30 eV for γ and β monolayers (MLs), respectively. Therefore, they will be useful for optoelectronics operating at room temperature, which is further supported by their dynamic and thermal stability. The γ ML and bilayer (BL) are expected to undergo phase transitions to β ML and α BL under hole doping. Furthermore, the ionicity brings about spontaneous electric polarization in the α BL that is approximately 60% larger than that in the α Te BL. Its ferroelectricity (FE) is comparable to that of SnTe ML, the only 2D FE material experimentally identified up to now. The polarization can be further enhanced by more than 75% under uniaxial tensile strain.

Standing Waves Induced by Valley-Mismatched Domains in Ferroelectric SnTe Monolayers.

Two-dimensional (2D) quasiparticle standing waves originate from the interference of coherent quantum states and are usually created by the scattering off edges, atomic steps, or adatoms that induce large potential barriers. We report standing waves close to the valence band maximum (E_{V}), confined by electrically neutral domain walls of newly discovered ferroelectric SnTe monolayers, as revealed by spatially resolved scanning tunneling spectroscopy. Abinitio calculations show that this novel confinement arises from the polarization lifted hole valley degeneracy and a ∼90° rotation of the Brillouin zones that render holes' momentum mismatched across neighboring domains. These results show a potential for polarization-tuned valleytronics in 2D ferroelectrics.

SnTe monolayer: Tuning its electronic properties with doping

Abstract Two-dimensional (2D) group IV monochalcogenides have attracted extensively attention due to their specific electronic properties and promising applications. The atomic-thick SnTe is an exemplar in 2D group IV monochalcogenides and shows robust ferroelectric properties at room-temperature. Using first principles calculation based on the density functional theory, we investigated the effects of doping on geometric structure, formation energy, band structure and carrier effective mass of SnTe monolayer. The results indicate that the radii and outermost valence electrons of doped atoms significantly affect the geometric structure and electronic structure of the doped compounds. The SnTe monolayer with donor (acceptor) impurities could be transformed into n-type (p-type) degenerate semiconductors with smaller electronics (holes) effective mass. Moreover, the direct bandgap semiconductors were obtained in this study by doping MIV or XVI in SnTe monolayer. The electrical conductivity and photoelectric conversion efficiency can also be improved by doping atoms in SnTe monolayer. These intriguing results make doped SnTe monolayers promising candidate materials for thermoelectric or photoelectric devices.