group-IV Monochalcogenides Research Articles

Colloquium : Physical properties of group-IV monochalcogenide monolayers

We survey the state-of-the-art knowledge of ferroelectric and ferroelastic group-IV monochalcogenide monolayers. These semiconductors feature remarkable structural and mechanical properties, such as a switchable in-plane spontaneous polarization, soft elastic constants, structural degeneracies, and thermally-driven two-dimensional structural transformations. Additionally, these 2D materials also display selective valley excitations, valley Hall effects, and persistent spin helix behavior. After a description of their Raman spectra, a discussion of optical properties arising from their lack of centrosymmetry---such as an unusually strong second-harmonic intensity, large bulk photovoltaic effects, photostriction, and tunable exciton binding energies---is provided as well. The physical properties observed in these materials originate from (correlate with) their intrinsic and switchable electric polarization, and the physical behavior hereby reviewed could be of use in non-volatile memory, valleytronic, spintronic, and optoelectronic devices: these 2D multiferroics enrich and diversify the 2D materials toolbox.

Metastable piezoelectric group-IV monochalcogenide monolayers with a buckled honeycomb structure

Twelve two-dimensional group-IV monochalcogenide monolayers (SiS, SiSe, SiTe, GeS, GeSe, GeTe, SnS, SnSe, SnTe, PbS, PbSe, and PbTe) with a buckled honeycomb atomistic structure--belonging to symmetry group P3m1--and an out-of-plane intrinsic electric polarization are shown to be metastable by three independendent methods. First, we uncover a coordination-preserving structural transformation from the low-buckled honeycomb structure onto the lower-energy Pnm2$_1$ (or Pmmn for PbS, PbSe, and PbTe) phase to estimate {\em energy barriers} $E_B$ that must be overcome during such structural transformation. Using the curvature of the local minima and $E_B$ as inputs to Kramers escape formula, large escape times are found, implying the structural metastability of the buckled honeycomb phase (nevertheless, and with the exception of PbS and PbSe, these phases display escape times ranging from 700 years to multiple times the age of the universe, and can be considered "stable" for practical purposes only in that relative sense). The second demonstration is provided by phonon dispersion relations that include the effect of long-range Coulomb forces and display no negative vibrational modes. The third and final demonstration of structural metastability is furnished by room-temperature {\em ab initio} molecular dynamics for selected compounds. The magnitude of the electronic band gap evolves with chemical composition. Different from other binary two-dimensional compounds such as transition metal dichalcogenide monolayers and hexagonal boron nitride monolayers which only develop an in-plane piezoelectric response, the twelve group-IV monochalcogenide monolayers with a buckled honeycomb structure also display out-of-plane piezoelectric properties.

Alkalized SnS Nanoflakes with Enhanced Sensing Properties towards Methanol Vapor

Two-dimensional (2D) group-Ⅳ monochalcogenides MX (M = Ge, Sn; X = S, Se) are isoelectronic and isostructural analogues to phosphorene. Due to superior oxidation resistance relative to black phosphorus, MX are recently explored to be potentialsubstitute of phosphorene for highly sensitive gas sensor applications. In this work, nanoflakes of tin monosulfide (SnS) was synthesized by a one-step solvothermal method and then employed for methanol-sensing at room temperature. The as-synthesized SnS shows nanoflower-like hierarchical architecture assembled by 2D nanoflakes. To achieve satisfactory gas response magnitude, a KOH alkalization process for post-treatment of the synthesized SnS was developed further. It is proved that appropriate alkalization could induce the ultrathin nanoflakes and introduce additional –O function terminals on the surface, which are favorable for methanol adsorption and response. The resultant alkalized SnS sensor displays a ∼3.2-fold enhancement in gas response to 5 ppm methanol. And it is capable of sensing rarefied methanol vapor as low as 100 ppb. The response enhancement could attribute to the alkalization-induced ultrathin geometry of SnS nanoflakes, introduction of lattice defects and –O functional terminals available for active adsorption of gas molecules. Further first-principles calculations clarify the crucial contribution and underlying mechanism of attached –O terminal on methanol adsorption and reaction clearly.

Beyond Graphene: Low-Symmetry and Anisotropic 2D Materials

Low-symmetry 2D materials—such as ReS2 and ReSe2 monolayers, black phosphorus monolayers, group-IV monochalcogenide monolayers, borophene, among others—have more complex atomistic structures than the honeycomb lattices of graphene, hexagonal boron nitride, and transition metal dichalcogenides. The reduced symmetries of these emerging materials give rise to inhomogeneous electron, optical, valley, and spin responses, as well as entirely new properties such as ferroelasticity, ferroelectricity, magnetism, spin-wave phenomena, large nonlinear optical properties, photogalvanic effects, and superconductivity. Novel electronic topological properties, nonlinear elastic properties, and structural phase transformations can also take place due to low symmetry. The “Beyond Graphene: Low-Symmetry and Anisotropic 2D Materials” Special Topic was assembled to highlight recent experimental and theoretical research on these emerging materials.

Electronic and optical properties of low-dimensional group-IV monochalcogenides

The group-IV monochalcogenides are a set of binary materials MX composed by a group-IV element (M=Sn or Ge) and a chalcogen (X=S or Se). Strong anisotropy is one of the most interesting facets of these materials, especially in their low-dimensional form. In this Tutorial, we present some of the recent studies on the electronic and optical properties of 2D group-IV monochalcogenides, including predictions from first-principles density functional theory calculations and available experimental observations. We discuss the variation of the bandgap from bulk down to monolayer and the respective band structures, which are characterized by multiple valence and conduction band valleys, making these materials suitable for a variety of applications, including valleytronics. We also discuss the emergence of spin–orbit splitting, piezoelectricity, and ferroelectricity as a result of the polar character of the monolayers. Current predictions of carrier mobilities in monolayers and their potential application as thermoelectric materials are also presented.

Tuning energy barriers by doping 2D group-IV monochalcogenides

Structural degeneracies underpin the ferroic behavior of anisotropic next-generation two-dimensional materials and lead to peculiar two-dimensional structural transformations under external fields, charge doping, and/or temperature. The most direct indicator of the ease of these transformations is an elastic energy barrier, defined as the energy difference between the (degenerate) structural ground state unit cell and a unit cell with an increased structural symmetry. Proximity of a two-dimensional material to a bulk substrate can affect the magnitude of the critical fields and/or temperature at which these transformations occur, with the first effect being a relative charge transfer, which could trigger a structural quantum phase transition. With this physical picture in mind, we report the effect of modest charge doping (within −0.2 and +0.2 electrons per unit cell) on the elastic energy barrier Js of ferroelastic black phosphorene and nine ferroelectric/ferroelastic monochalcogenide monolayers. Js is the energy needed to create a Pnm21→P4/nmm two-dimensional structural transformation, and it is sensitive to the orbital character of the electronic charge added or removed. Similar to the effect on the elastic energy barrier of ferroelastic SnO monolayers, group-IV monochalcogenide monolayers show a tunable elastic energy barrier for similar amounts of doping, and a decrease (increase) of Js can be engineered under a modest hole (electron) doping of not more than one-tenth of an electron or a hole per atom.

Experimental formation of monolayer group-IV monochalcogenides

Monolayer group-IV monochalcogenides (MX, M=Ge, Sn, Pb; X=S, Se, Te) are a family of novel two-dimensional (2D) materials that have atomic structures closely related to that of the staggered black phosphorus lattice. The structure of most monolayer MX materials exhibits a broken inversion symmetry and many of them exhibit ferroelectricity with a reversible in-plane electric polarization. A further consequence of the noncentrosymmetric structure is that when coupled with strong spin–orbit coupling, many MX materials are promising for the future applications in non-linear optics, photovoltaics, spintronics, and valleytronics. Nevertheless, because of the relatively large exfoliation energy, the creation of monolayer MX materials is not easy, which hinders the integration of these materials into the fast-developing field of 2D material heterostructures. In this Perspective, we review recent developments in experimental routes to the creation of the monolayer MX, including molecular beam epitaxy and two-step etching methods. Other approaches that could be used to prepare the monolayer MX are also discussed, such as liquid phase exfoliation and solution-phase synthesis. A quantitative comparison between these different methods is also presented.

Normal-to-topological insulator martensitic phase transition in group-IV monochalcogenides driven by light

A material potentially exhibiting multiple crystalline phases with distinct optoelectronic properties can serve as a phase-change memory material. The sensitivity and kinetics can be enhanced when the two competing phases have large electronic structure contrast and the phase change process is diffusionless and martensitic. In this work, we theoretically and computationally illustrate that such a phase transition could occur in the group-IV monochalcogenide SnSe compound, which can exist in the quantum topologically trivial Pnma-SnSe and nontrivial Fmbar 3m-SnSe phases. Furthermore, owing to the electronic band structure differences of these phases, a large contrast in the optical responses in the THz region is revealed. According to the thermodynamic theory for a driven dielectric medium, optomechanical control to trigger a topological phase transition using a linearly polarized laser with selected frequency, power and pulse duration is proposed. We further estimate the critical optical electric field to drive a barrierless transition that can occur on the picosecond timescale. This light actuation strategy does not require fabrication of mechanical contacts or electrical leads and only requires transparency. We predict that an optically driven phase transition accompanied by a large entropy difference can be used in an “optocaloric” cooling device.

First-principles calculations combined with experiments to study the gas-sensing performance of Zn-substituted SnS.

Two-dimensional group-IV monochalcogenides MX (M = Ge, and Sn; X = S, and Se) are explored for their potential in gas-sensing applications. In this work, a combined theoretical and experimental study on pure SnS and Zn-substituted SnS for promising methanol sensors is performed. The adsorption characteristics of methanol on pure and Zn-substituted SnS were first calculated using first-principles based on density functional theory (DFT). It is clarified theoretically that the incorporation of Zn dopant could enhance the adsorption capability of the SnS surface to methanol molecules, thus achieving obvious response enhancement and selectivity improvement. Further experimental investigation is carried out based on the successful synthesis of pure and Zn-substituted SnS with hierarchical architecture via a one-step solvothermal process. Gas-sensing measurement indicates that the Zn-substituted SnS sensor is promising for selective detection of rarefied methanol. At room temperature, the as-synthesized hierarchical SnS with an appropriate amount of Zn-doping can sense methanol vapor of as low as 100 ppb. In particular, Zn-doping can enhance the sensing response of methanol significantly, with a 32.8-fold increase in response value achieved in comparison to that of the pure SnS. The underlying mechanism for the response enhancement of Zn-substituted SnS is also analyzed and demonstrated in detail. The present work demonstrates that Zn-doping is highly effective for improving the response and selectivity of SnS towards methanol vapor, and the Zn-substituted SnS is promising for highly sensitive methanol sensors with low consumption.

Group-IV monochalcogenide monolayers: Two-dimensional ferroelectrics with weak intralayer bonds and a phosphorenelike monolayer dissociation energy

We performed density functional theory calculations with self-consistent van der Waals corrected exchange-correlation (XC) functionals to capture the structure of black phosphorus and twelve monochalcogenide monolayers and find the following results: (a) The in-plane unit cell changes its area in going from the bulk to a monolayer. The change of in-plane distances implies that bonds weaker than covalent or ionic ones are at work within the monolayers themselves. This observation is relevant for the prediction of the critical temperature $T_c$. (b) There is a hierarchy of independent parameters that uniquely define a ground state ferroelectric unit cell (and square and rectangular paraelectric unit cells as well): only 5 optimizable parameters are needed to establish the unit cell vectors and the four basis vectors of the ferroelectric ground state unit cell, while square and rectangular paraelectric structures are defined by only 3 or 2 independent optimizable variables, respectively. (c) The reduced number of independent structural variables correlates with larger elastic energy barriers on a rectangular paraelectric unit cell when compared to the elastic energy barrier of a square paraelectric structure. This implies that $T_c$ obtained on a structure that keeps the lattice parameters fixed (for example, using an NVT ensemble) should be larger than the transition temperature on a structure that is allowed to change in-plane lattice vectors (for example, using the NPT ensemble). (d) The dissociation energy (bulk cleavage energy) of these materials is similar to the energy required to exfoliate graphite and MoS$_2$. (e) There exists a linear relation among the square paraelectric unit cell lattice parameter and the lattice parameters of the rectangular ferroelectric ground state unit cell. These results highlight the subtle atomistic structure of these novel 2D ferroelectrics.

The phonon-related effective Hamiltonian method for displacive ferroelectric materials

In this manuscript, we explain the theoretical principles and some technical details of a first-principles derived effective Hamiltonian method, which is used to calculate the ferroelectric (FE) and structural properties in displacive FE materials. Exploiting that the key instabilities responsible for this phase transition are governed by soft phonons, this Hamiltonian is phonon-related, i.e. using phonon modes as variables and describing their intra- and interactions. Upon retaining the predictive power of first-principles calculations, this method is computationally cheap, which enables large supercell being used so that the thermodynamic limit can be reached easily in the simulations at finite temperatures. Besides the known success in perovskites, it also qualifies for characterizing FE phase transitions in the rocksalt type group-IV monochalcogenides, and shows potential applications in more complicated systems, e.g. M-type hexaferrite.

Two-dimensional M2SD (M = Ge, Sn; D = Se, Te) monolayers with puckered structure: Electronic structure and optical properties

Abstract The anion substitution is an effectively method to modulate the physical properties of the two-dimensional (2D) semiconductors, which can facilitate the applications of 2D materials in the optoelectronic devices. Recently, the monolayer group-IV monochalcogenides have been paid much attention due to its unique electronic structure. However, few results on anion substitution in monolayer group-IV monochalcogenides are reported. Here, the electronic structure and optical properties of 2D puckered M2SD (M = Ge, Sn; D = Se, Te) monolayers are investigated by first-principles calculations. The phonon spectra and binding energy of 2D M2SD monolayers confirm its structure stability. The lattice constant, bond length and bond angle of Ge2SD and Sn2SD monolayers are related to the electronegativity and radius of Se and Te atoms. The 2D M2SD monolayers have an indirect band gap ranging from 0.70 to 1.23 eV, which can turn into direct band gap by applying the in-plane strains. Besides, a transition from semiconductor to metal in 2D Ge2STe and Sn2STe monolayers appears at a large compressive biaxial strain. Based on the dielectric functions, all of the 2D M2SD monolayers show a strong anisotropy in the low energy range (

Intrinsic persistent spin helix state in two-dimensional group-IV monochalcogenide MX monolayers ( M=Sn or Ge and X=S , Se, or Te)

Energy-saving spintronics are believed to be implementable on systems hosting the persistent spin helix (PSH) since they support an extraordinarily long spin lifetime of carriers. However, achieving the PSH requires a unidirectional spin configuration in the momentum space, which is practically nontrivial due to the stringent conditions for fine-tuning the Rashba and Dresselhaus spin-orbit couplings. Here, we predict that the PSH can be intrinsically achieved on a two-dimensional (2D) group-IV monochalcogenide $MX$ monolayer, a new class of the noncentrosymmetric 2D materials having in-plane ferroelectricity. Due to the ${C}_{2v}$ point-group symmetry in the $MX$ monolayer, a unidirectional spin configuration is preserved in the out-of-plane direction and thus maintains the PSH that is similar to the [110] Dresselhaus model in the [110]-oriented quantum well. Our first-principle calculations on various $MX$ ($M=$ Sn, Ge; $X=$ S, Se, Te) monolayers confirmed that such typical spin configuration is observed, in particular, at near the valence-band maximum where a sizable spin splitting and a substantially small wavelength of the spin polarization are achieved. Importantly, we observe reversible out-of-plane spin orientation under opposite in-plane ferroelectric polarization, indicating that an electrically controllable PSH for spintronic applications is plausible.

Prediction of ferroelectricity-driven Berry curvature enabling charge- and spin-controllable photocurrent in tin telluride monolayers

In symmetry-broken crystalline solids, pole structures of Berry curvature (BC) can emerge, and they have been utilized as a versatile tool for controlling transport properties. For example, the monopole component of the BC is induced by the time-reversal symmetry breaking, and the BC dipole arises from a lack of inversion symmetry, leading to the anomalous Hall and nonlinear Hall effects, respectively. Based on first-principles calculations, we show that the ferroelectricity in a tin telluride monolayer produces a unique BC distribution, which offers charge- and spin-controllable photocurrents. Even with the sizable band gap, the ferroelectrically driven BC dipole is comparable to those of small-gap topological materials. By manipulating the photon handedness and the ferroelectric polarization, charge and spin circular photogalvanic currents are generated in a controllable manner. The ferroelectricity in group-IV monochalcogenide monolayers can be a useful tool to control the BC dipole and the nonlinear optoelectronic responses.

Theoretical prediction of germanium selenium nanosheet as a potential anode material for high-performance alkali-metal based battery

Alkali-metal (AM) ion batteries have been attracting great attention due to the high specific capacity, fast charging rate, and good reversibility. In this work, the adsorption and diffusion of Li, Na, and K atoms on a two-dimensional (2D) group-IV monochalcogenide, germanium selenium nanosheet (GeSeNS), have been systematically investigated by first-principles calculations. The considered AM atoms could form strong binding and exist in stable configuration with GeSeNS. A semiconducting to metallic transition induces an excellent electrical conductivity after AM intercalation on GeSeNS, that is essential for an optimal anode material. Our results show that the low energy barriers of AM atoms diffusion along zigzag direction are 0.329 (Li), 0.175 (Na) and 0.132 (K) eV, respectively, which are lower than values of the commercial graphite anode material and corresponding to ultrafast charge/discharge capability. The average voltages are about 1.82, 1.45 and 1.28 V for the LixGeSe, NaxGeSe and KxGeSe systems, respectively. Hence, our results suggest that GeSeNS could be a potential electrode material for application in low-cost AM ion batteries with a high rate capability and high charging voltage, thus encouraging further experimental work.

First-principles prediction of a new ground state for surface-oxidized phosphorene with remarkable piezoelectricity

In this paper, a new ground state (Pca21-PO) of surface-oxidized phosphorene is identified from a series of possible crystalline configurations. The crystalline structure of Pca21-PO is based on the previously proposed θ-P with further distortion. It is predicted to be more favorable than the previously proposed Pmn21-PO and confirmed to be dynamically stable. The calculated band structures show that Pca21-PO is an indirect band gap semiconductor with a gap of 0.607 eV and 1.396 eV based on DFT-PBE and HSE06 methods, respectively. It is a potential piezoelectric material with coefficients of −9.38 pm V−1, 50.32 pm V−1, 0.83 pm V−1 and –0.09 pm V−1 for d11, d12, d21 and d22, respectively. These coefficients are comparable to those of group-IV monochalcogenides and more remarkable than that of the experimental MoS2. In particular, Pca21-PO possesses piezoelectric effect in arbitrary direction in view of the absence of both inverse and mirror symmetries, which can be considered as a potential material for applications in nano-scale devices, such as sensors, actuators, electric field generators and any other applications requiring electrical and mechanical energy conversion.

Unique mechanical responses of layered phosphorus-like group-IV monochalcogenides

As interesting analog of black phosphorus, group-IV monochalcogenides MX (M = Ge, Sn; X = S, Se) are greatly expected to have potential applications in photodetectors, photovoltaics, thermoelectric, and piezoelectric devices. In this paper, we conduct a thorough investigation on the mechanical properties of four MX monochalcogenides, i.e., single- and multi-layered GeS, GeSe, SnS, and SnSe, including their ideal tensile strength, evolution of monolayer bulking height, and phase transition under uniaxial strain within the framework of density functional theory. The ultimate tensile strengths of the four monochalcogenides are found rather low, while the failure strains are remarkably high, showing ultrahigh mechanical flexibility. Meanwhile, the ideal strength demonstrates strong anisotropy, which may induce the exfoliated MX thin flakes showing an obvious in-plane cleavage tendency of much easier to crack along the zigzag orientation. In addition, it is found that all of them are potential auxetic materials with large negative Poisson's ratios, just like single-layer black phosphorus. Furthermore, we find all the monolayer monochalcogenides will demonstrate geometrical phase transition when uniaxial compressive strains are applied. It is expected that these newly found unique features of monolayer group-IV monochalcogenides and their derivatives make them attractive candidates for their incorporation in future mechanical, nanoelectronics, and thermal applications.

Giant spin Hall effect in two-dimensional monochalcogenides

One of the most exciting properties of two dimensional materials is their sensitivity to external tuning of the electronic properties, for example via electric field or strain. Recently discovered analogues of phosphorene, group-IV monochalcogenides (MX with M = Ge, Sn and X = S, Se, Te), display several interesting phenomena intimately related to the in-plane strain, such as giant piezoelectricity and multiferroicity, which combine ferroelastic and ferroelectric properties. Here, using calculations from first principles, we reveal for the first time giant intrinsic spin Hall conductivities (SHC) in these materials. In particular, we show that the SHC resonances can be easily tuned by combination of strain and doping and, in some cases, strain can be used to induce semiconductor to metal transition that makes a giant spin Hall effect possible even in absence of doping. Our results indicate a new route for the design of highly tunable spintronics devices based on two-dimensional materials.

Stacking induced indirect-to-direct bandgap transition in layered group-IV monochalcogenides for ideal optoelectronics

Ideal optoelectronic semiconductors, with suitable direct bandgaps, high optical absorptions, low carrier effective masses, high mobility, low cost, and high stability, are predicted in AB-stacking group-IV monochalcogenides.

Electronic structures and transport properties of SnS-SnSe nanoribbon lateral heterostructures.

The electronic structures of phosphorene-like SnS/SnSe nanoribbons and the transport properties of a SnS-SnSe nanoribbon lateral heterostructure are investigated by using first-principles calculations combined with nonequilibrium Green's function (NEGF) theory. It is demonstrated that SnS and SnSe nanoribbons with armchair edges (A-SnSNRs and A-SnSeNRs) are semiconductors, independent of the width of the ribbon. Their bandgaps have an indirect-to-direct transition, which varies with the ribbon width. In contrast, Z-SnSNRs and Z-SnSeNRs are metals. The transmission gap of armchair SnSNR-SnSeNR is different from the potential barrier of SnSNR and SnSeNR. The I-V curves of zigzag SnSNR-SnSeNR exhibit a negative differential resistive (NDR) effect due to the bias-dependent transmission in the voltage window and are independent of the ribbon width. However, for armchair SnSNR-SnSeNR, which has a low current under low biases, it is only about 10-6 μA. All the results suggest that phosphorene-like MX (M = Sn/Ge, X = S/Se) materials are promising candidates for next-generation nanodevices.