group-IV Monochalcogenide Monolayers Research Articles

First-Principles Study of the Schottky Contact, Tunneling Probability, and Optical Properties of MX/TiB4 Heterojunctions (M = Ge, Sn; X = S, Se, Te): Strain Engineering Tunability.

Designing two-dimensional (2D) heterojunctions with rapid response and minimal energy consumption holds immense significance for the advancement of the next generation of electronic devices. Here, we construct a series of Schottky heterojunctions based on TiB4 monolayer and group-IV monochalcogenide monolayers MX (M = Ge, Sn; X = S, Se, Te). Using first-principles calculations, we investigate the structural stability, Schottky contact barrier, tunneling probability, and optical properties of MX/TiB4 heterojunctions. The calculated binding energies reveal that X-type MX/TiB4 heterojunctions exhibit more stable structures than M- and C-type stacking modes. Schottky barrier heights (SBHs) indicate that X-type GeSe/TiB4 and GeTe/TiB4 form n-type Schottky contacts with SBHs of 0.497 and 0.132 eV, respectively, while SnS/TiB4 and SnSe/TiB4 form p-type Schottky contacts with SBHs of 0.557 and 0.418 eV, respectively. Moreover, X-type MX/TiB4 heterojunctions exhibit high susceptibility to interlayer electron tunneling due to their large tunneling probability and strong interlayer interaction. Meanwhile, enhanced optical absorption capacity in MX/TiB4 heterojunctions is also observed compared with individual TiB4 and MX monolayers. By applying in-plane biaxial strain, the transformation of MX/TiB4 heterojunctions from a Schottky contact to an Ohmic contact can also be realized. Our findings could offer valuable candidate materials and guidance for the design of the next generation of nanodevices with high electronic and optical performances.

Near-Field Radiative Heat Transfer between Layered β-GeSe Slabs: First-Principles Approach.

The group-IV monochalcogenide monolayers, GeSe, are interesting and novel two-dimensional (2D) semiconductor materials due to their highly anisotropic physical properties. Monolayers of the different GeSe polymorphs have already had their physical properties and potential applications extensively investigated. However, few-layer homostructures, which can also be approximated as 2D systems in many cases, have not received the same attention. For this reason, in this work, we investigate the optical properties of a free-standing few-layer β-GeSe system and use this information to investigate their performance in the near-field radiative heat transfer (NFRHT). The required optical conductivity of the few-layer 2D material is calculated by using density functional theory (DFT), including spin-orbit coupling. The band structure is investigated for up to five layers, and the effective electron masses are calculated correspondingly. Using this information, both the intraband transitions due to the presence of free electrons introduced by doping and the interband transitions are considered. The contribution of the ionic vibrations is also included in calculating the optical properties because of its relevance to NFRHT through the resulting active optical phonons. With all these contributions included, more realistic predictions of the NFRHT between the layered 2D β-GeSe materials can be obtained. It is found that the heat transfer attainable with the layered system is similar to that of a single layer of β-GeSe we have obtained previously.

Exceptionally high hole mobilities in monolayer group-IV monochalcogenides GeTe and SnTe

Two-dimensional semiconductors are considered as promising channel materials for next-generation nanoelectronics devices, while their practical applications are typically limited by their low mobilities. In this work, using first-principles calculations combined with the Boltzmann transport formalism involving electron–phonon coupling, we study the transport properties of monolayer group-IV monochalcogenides (MX, M = Ge, Sn; X = S, Se, and Te). We find that the GeTe and SnTe possess exceptionally high hole mobilities, which even reach 835 and 1383 cm2/V s, respectively, at room temperature. More interestingly, the hole mobilities increase with the increase in the atomic number of “X” in MXs when “M” remains the same. Such a trend is mainly due to the increased group velocity and decreased density of states, and the latter plays a significant role in determining the carrier scattering space and relaxation time. Meanwhile, different from the acoustic deformation potential theory, we find that the high-energy optical phonons contribute a lot to the scattering. Our work shows that the monolayer GeTe and SnTe are promising p-type semiconductors in nanoelectronics and reveals the intrinsic connection between phonons, charge density of states, and mobility, which would shed light on exploring the two-dimensional materials with high mobility.

Electromechanical response of group-IV monochalcogenide monolayers

The electromechanical response of the group-IV monochalcogenide monolayers upon charge injection was investigated for applications in actuator devices and artificial muscles.

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.

Large piezoelectric response in ferroelectric/multiferroelectric metal oxyhalide MOX2 (M = Ti, V and X = F, Cl and Br) monolayers.

Flexible two-dimensional (2D) piezoelectric materials are promising for applications in wearable electromechanical nano-devices such as sensors, energy harvesters, and actuators. A large piezo-response is required for any practical applications. Based on first-principles calculations, we report that ferroelectric TiOX2 and multiferroelectric VOX2 (X = F, Cl, and Br) monolayers exhibit large in-plane stress (e11) and strain (d11) piezoelectric coefficients. For example, the in-plane piezo-response of TiOBr2 (both e11 = 28.793 × 10-10 C m-1 and d11 = 37.758 pm V-1) is about an order of magnitude larger than that of the widely studied 1H-MoS2 monolayer, and also quite comparable to the giant piezoelectricity of group-IV monochalcogenide monolayers, e.g., SnS. Moreover, the d11 of MOX2 monolayers - ranging from 29.028 pm V-1 to 37.758 pm V-1 - are significantly higher than the d11 or d33 of commonly used 3D piezoelectrics such as w-AlN (d33 = 5.1 pm V-1) and α-quartz (d11 = 2.3 pm V-1). Such a large d11 of MOX2 monolayers originates from low in-plane elastic constants with large e11 due to large Born effective charges (Zij) and atomic sensitivity to an applied strain. Moreover, we show the possibility of opening a new way of controlling piezoelectricity by applying a magnetic field.

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.

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.

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.

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.

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 (

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.

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.

Tunable auxetic properties in group-IV monochalcogenide monolayers

Negative Poisson's ratio (NPR) is a counterintuitive material elastic constant that can lead to many unusual auxetic properties. Here, using first-principles calculations, we report tunable negative Poisson's ratio in the out-of-plane direction in group-IV monochalcogenide monolayers $MX$ ($M=\text{Sn}$, Ge and $X=\text{S}$, Se). SnSe, GeS, and SnS monolayers have intrinsic NPR ${\ensuremath{\nu}}_{zx}$ ranging from $\ensuremath{-}0.004$ to $\ensuremath{-}0.210$ in armchair ($x$) tension, whereas GeSe monolayer possesses a much larger NPR ${\ensuremath{\nu}}_{zy}$ of $\ensuremath{-}0.433$ in zigzag ($y$) tension. Our analysis attributes the NPR effects to the relative position of $M$ and $X$ in the puckered structure and the smaller bending stiffness of $M\text{\ensuremath{-}}X\text{\ensuremath{-}}M$ bond angle. We further established the correlation between electronic structures of materials and their crystal structures. It allows us to fine tune GeSe structure via electron doping, leading to a reversible and continuous change of ${\ensuremath{\nu}}_{zy}$ from $\ensuremath{-}0.821$ up to 0.895. We also demonstrate the concept of strain engineering GeSe monolayer to switch its Poisson's ratio ${\ensuremath{\nu}}_{zx}$ between two different values: 0.583 and $\ensuremath{-}0.433$. Our in-depth study provides not only fundamental knowledge but also practical routes for designing 2D smart materials with tunable negative Poisson's ratio, which are desirable for smart devices at small scale.

Anisotropic thermal expansion of group-IV monochalcogenide monolayers

By employing first-principles calculations, we investigate the thermal expansion behavior of group-IV monochalcogenide monolayers. We find that these two-dimensional (2D) materials exhibit unusually large anisotropic thermal expansion, i.e., the two lattice parameters in the armchair and zigzag directions contract and expand, respectively, with increasing temperature, which is indicative of a second-order phase transition to a square lattice. Such anisotropic thermal expansion gives rise to a secondary pyroelectric response that is effectively 1–2 orders of magnitude larger than that of typical bulk pyroelectrics, suggesting that thermal expansion is essential to properly describe their electronic properties.

First-principles study on the electronic, optical, and transport properties of monolayer α - and β -GeSe

The extraordinary properties and the novel applications of black phosphorene induce the research interest on the monolayer group-IV monochalcogenides. Here using the first-principles calculations, we systematically investigate the electronic, transport and optical properties of monolayer $\alpha-$ and $\beta-$GeSe, the latter of which was recently experimentally realized. We found that, monolayer $\alpha-$GeSe is a semiconductor with direct band gap of 1.6 eV, and $\beta-$GeSe displays indirect semiconductor with the gap of 2.47 eV, respectively. For monolayer $\beta-$GeSe, the electronic/hole transport is anisotropic with an extremely high electron mobility of 7.84 $\times10^4$$cm^2/V\cdot {s}$ along the zigzag direction, comparable to that of black phosphorene. Furthermore, for $\beta-$GeSe, robust band gaps nearly disregarding the applied tensile strain along the zigzag direction is observed. Both monolayer $\alpha-$ and $\beta-$GeSe exhibit anisotropic optical absorption in the visible spectrum.

Electrically tunable polarizer based on 2D orthorhombic ferrovalley materials

The concept of ferrovalley materials has been proposed very recently. The existence of spontaneous valley polarization, resulting from ferromagnetism, in such hexagonal 2D materials makes nonvolatile valleytronic applications realizable. Here, we introduce a new member of ferrovalley family with orthorhombic lattice, i.e. monolayer group-IV monochalcogenides (GIVMs), in which the intrinsic valley polarization originates from ferroelectricity, instead of ferromagnetism. Combining the group theory analysis and first-principles calculations, we demonstrate that, different from the valley-selective circular dichroism in hexagonal lattice, linearly polarized optical selectivity for valleys exists in the new type of ferrovalley materials. On account of the distinctive property, a prototype of electrically tunable polarizer is realized. In the ferrovalley-based polarizer, a laser beam can be optionally polarized in x- or y-direction, depending on the ferrovalley state controlled by external electric fields. Such a device can be further optimized to emit circularly polarized radiation with specific chirality and to realize the tunability for operating wavelength. Therefore, we show that 2D orthorhombic ferrovalley materials are the promising candidates to provide an advantageous platform to realize the polarizer driven by electric means, which is of great importance in extending the practical applications of valleytronics.

Oxidation Resistance of Monolayer Group-IV Monochalcogenides.

Ridged, orthorhombic two-dimensional (2D) group-V elemental and group IV-VI compound analogues of phosphorene provide a versatile platform for nanoelectronics, optoelectronics, and clean energy. However, phosphorene is vulnerable to oxygen in ambient air, which is a major obstacle for its applications. Regarding this issue, here we explore the oxidation behavior of monolayer group-IV monochalcogenides (GeS, GeSe, SnS, and SnSe), in comparison to that of phosphorene and arsenene by first-principles calculations. We find superior oxidation resistance of the monolayer group-IV monochalcogenides, with activation energies for the chemisorption of O2 on the 2D sheets in the range of 1.26-1.60 eV, about twice of the values of phosphorene and arsenene. The distinct oxidation behaviors of monolayer group-IV monochalcogenides and group-V phosphorene analogues originate from their different bond natures. Moreover, the chemisorption of a moderate amount of oxygen atoms does not severely deteriorate the electronic band structures of the monolayer group-IV monochalcogenides. These results shine light on the utilization of the monolayer group-IV monochalcogenides for next-generation 2D electronics and optoelectronics with high performance and stability.