group-IV Monochalcogenides Research Articles

Controlling the Intrinsic Charge Carrier Properties of Two-Dimensional Monochalcogenides (GeSe).

Strong anisotropy exhibited by materials, particularly in their low-dimensional forms, is a highly intriguing characteristic. In this study, we investigate the effects of geometrical potential and thermodynamics on the electronic properties of monolayer monochalcogenide charge carriers. First, the geometrical potential is introduced in a monolayer structure. We discuss the Fermi surface topology of materials and the effects of the geometrical potential on the low energy bands of 2D group-IV monochalcogenides around the Γ-point. Then, the temperature dependence of the carrier mobility of the monolayer is discussed along with predictions for its potential applications as nanomaterials.

Oxidation-induced graded bandgap narrowing in Two-dimensional tin sulfide for high-sensitivity broadband photodetection

Two-dimensional (2D) layered group-IV monochalcogenides with large surface-to-volume ratio and high surface activity make that their structural and optoelectronic properties are sensitive to air oxidation. Here, we report the utilization of oxidation-induced gradient doping to modulate electronic structures and optoelectronic properties of 2D group-IV monochalcogenides by using SnS nanoplates grown by physical vapor deposition as a model system. By a precise control of oxidation time and temperature, the structural transition from SnS to SnSOx could be driven by the layer-by-layer oxygen doping and intercalation. The resulting SnSOx with a graded narrowing bandgap exhibits the enhanced optical absorption and photocurrent, leading to the fabricated SnSOx photodetector with remarkable photoresponsivity and fast response speed (<64 μs) at a broadband spectrum range of 520–1550 nm. The peak responsivity (7294 A/W) and detectivity (9.54 × 109 Jones) of SnSOx device are at least two orders of magnitude larger than those of SnS photodetector. Moreover, its photodetection performance can be competed with state-of-the-art of 2D materials-based photodetectors. This work suggests that the air oxidation could be utilized as an efficient strategy to engineer the electronic and optical properties of SnS and other 2D group-IV monochalcogenides for the development of high-performance broadband photodetectors.

Unveiling the distinctive mechanical and thermal properties of γ-GeSe

γ-GeSe is a newly identified polymorph among group-IV monochalcogenides, characterized by a distinctive interatomic bonding configuration. Despite its promising applications in electrical and thermal domains, the experimental verification of its mechanical and thermal properties remains unreported. Here, we experimentally characterize the in-plane Young’s modulus (E) and thermal conductivity (\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\:\\kappa\\:$$\\end{document}) of γ-GeSe. The mechanical vibrational modes of freestanding γ-GeSe flakes are measured using optical interferometry. Nano-indentation via atomic force microscopy is also conducted to induce mechanical deformation and to extract the E. Comparison with finite-element simulations reveals that the E is 97.3\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\:\\pm\\:$$\\end{document}7.5 GPa as determined by optical interferometry and 109.4\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\:\\pm\\:$$\\end{document}13.5 GPa as established through the nano-indentation method. Additionally, optothermal Raman spectroscopy reveals that γ-GeSe has a lattice thermal conductivity of 2.3 \\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\:\\pm\\:$$\\end{document} 0.4 Wm−1K−1 and a total thermal conductivity of 7.5 \\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\:\\pm\\:$$\\end{document} 0.4 Wm−1K−1 in the in-plane direction at room temperature. The notably high \\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\:E/\\kappa\\:$$\\end{document} ratio in γ-GeSe, compared to other layered materials, underscores its distinctive structural and dynamic characteristics.

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.

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.

Domain-dependent strain and stacking in two-dimensional van der Waals ferroelectrics

Van der Waals (vdW) ferroelectrics have attracted significant attention for their potential in next-generation nano-electronics. Two-dimensional (2D) group-IV monochalcogenides have emerged as a promising candidate due to their strong room temperature in-plane polarization down to a monolayer limit. However, their polarization is strongly coupled with the lattice strain and stacking orders, which impact their electronic properties. Here, we utilize four-dimensional scanning transmission electron microscopy (4D-STEM) to simultaneously probe the in-plane strain and out-of-plane stacking in vdW SnSe. Specifically, we observe large lattice strain up to 4% with a gradient across ~50 nm to compensate lattice mismatch at domain walls, mitigating defects initiation. Additionally, we discover the unusual ferroelectric-to-antiferroelectric domain walls stabilized by vdW force and may lead to anisotropic nonlinear optical responses. Our findings provide a comprehensive understanding of in-plane and out-of-plane structures affecting domain properties in vdW SnSe, laying the foundation for domain wall engineering in vdW ferroelectrics.

Thermal conductivity of van der Waals heterostructure of 2D GeS and SnS based on machine learning interatomic potential

van der Waals heterostructures have provided an unprecedented platform to tune many physical properties for two-dimensional materials. In this work, thermal transport properties of van der Waals heterostructures formed by vertical stacking of monolayers GeS and SnS have been investigated systematically based on machine learning interatomic potential. The effect of van der Waals interface on the lattice thermal transport of 2D SnS and GeS can be well clarified by introducing various stacking configurations. Our results indicate that the van der Waals interface can strongly suppress the thermal transport capacity for the considered heterostructures, and either the average thermal conductivity per layer or the 2D thermal sheet conductance for the considered heterostructures is lower than that of corresponding monolayers. The suppressed thermal conductivity with tunable in-plane anisotropy in SnS/GeS heterostructures can be ascribed to the enhanced interface anharmonic scattering, and thus exhibits obvious interface-dependent characteristics. Therefore, this work highlights that the van der Waals interface can be employed to effectively modulate thermal transport for the 2D puckered group-IV monochalcogenides, and the suppressed lattice thermal conductivity together with interface-dependent phonon transport properties in the SnS/GeS heterostructure imply the great potential for corresponding thermoelectrical applications.

Tailoring angle dependent ferroelectricity in nanoribbons of group-IV monochalcogenides

Ferroelectricity is significant in low dimensional structures due to the potential applications in multifunctional nanodevices. In this work, the tailoring angle dependent ferroelectricity is systematically investigated for the nanoribbons and nanowires of puckered group-IV monochalcogenides MX (M = ; X = ). Based on first-principles calculations, it is found that the ferroelectricity of nanoribbon and nanowire strongly depends on the tailoring angle. Firstly, the critical width for the bare nanoribbon of group-IV monochalcogenide is obtained and discussed. As the nanowires are concerned, the ferroelectricity will disappear when the tailoring angle becomes small. At last, H-passivation on the edge and the strain engineering are employed to improve the ferroelectricity of nanoribbon, and it is obtained that H-passivation is beneficial to the enhancement of polarization for nanoribbons tailored near the armchair direction, while the polarization of nanoribbons tailored along the diagonal direction will decrease when the edges are passivated with H atoms, and the tensile strain along the length direction always favors the improvement of ferroelectricity of the considered nanoribbons. Therefore, tailoring angle has great influence on the ferroelectricity of nanoribbons and nanowires, which may be used as an effective way to tune the ferroelectricity and further the electronic structures of nanostructures in the field of nanoelectronics.

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.

Shift-Current Photovoltaics Based on a Non-Centrosymmetric Phase in In-Plane Ferroelectric SnS.

The shift-current photovoltaics of group-IV monochalcogenides has been predicted to be comparable to those of state-of-the-art Si-based solar cells. However, its exploration has been prevented from the centrosymmetric layer stacking in the thermodynamically stable bulk crystal. Herein, the non-centrosymmetric layer stacking of tin sulfide (SnS) is stabilized in the bottom regions of SnS crystals grown on a van der Waals substrate by physical vapor deposition and the shift current of SnS, by combining the polarization angle dependence and circular photogalvanic effect, is demonstrated. Furthermore, 180° ferroelectric domains in SnS are verified through both piezoresponse force microscopy and shift-current mapping techniques. Based on these results, an atomic model of the ferroelectric domain boundary is proposed. The direct observation of shift current and ferroelectric domains reported herein paves a new path for future studies on shift-current photovoltaics.

Giant Nonlinear Optical Response via Coherent Stacking ofIn-Plane Ferroelectric Layers.

Thin ferroelectric materials hold great promise for compact nonvolatile memory and nonlinear optical and optoelectronic devices. Herein, an ultrathin in-plane ferroelectric material that exhibits a giant nonlinear optical effect, group-IV monochalcogenide SnSe, is reported. Nanometer-scale ferroelectric domains with ≈90°/270° twin boundaries or ≈180° domain walls are revealed in physical-vapor-deposited SnSe by lateral piezoresponse force microscopy. Atomic structure characterization reveals both parallel and antiparallel stacking of neighboring van der Waals ferroelectric layers, leading to ferroelectric or antiferroelectric ordering. Ferroelectric domains exhibit giant nonlinear optical activity due to coherent enhancement of second-harmonic fields and the as-resulted second-harmonic generationwasobserved to be 100 times more intense than monolayer WS2 . This work demonstratesin-plane ferroelectric ordering and giant nonlinear optical activity in SnSe, which paves the way for applications in on-chip nonlinear optical components and nonvolatile memory devices.

Electrically switchable anisotropic polariton propagation in a ferroelectric van der Waals semiconductor.

Tailoring of the propagation dynamics of exciton-polaritons in two-dimensional quantum materials has shown extraordinary promise to enable nanoscale control of electromagnetic fields. Varying permittivities along crystal directions within layers of material systems, can lead to an in-plane anisotropic dispersion of polaritons. Exploiting this physics as a control strategy for manipulating the directional propagation of the polaritons is desired and remains elusive. Here we explore the in-plane anisotropic exciton-polariton propagation in SnSe, a group-IV monochalcogenide semiconductor that forms ferroelectric domains and shows room-temperature excitonic behaviour. Exciton-polaritons are launched in SnSe multilayer plates, and their propagation dynamics and dispersion are studied. This propagation of exciton-polaritons allows for nanoscale imaging of the in-plane ferroelectric domains. Finally, we demonstrate the electric switching of the exciton-polaritons in the ferroelectric domains of this complex van der Waals system. The study suggests that systems such as group-IV monochalcogenides could serve as excellent ferroic platforms for actively reconfigurable polaritonic optical devices.

Electronic structures and photovoltaic applications of vdW heterostructures based on Janus group-IV monochalcogenides: insights from first-principles calculations.

The van der Waals integration can help 2D materials modulate their properties and provide more opportunities for 2D materials in the next-generation high-performance optoelectronic devices. Using first-principles calculations, we explored the atomic and electronic structures of 2D pristine and Janus group-IV monochalcogenides and found the internal vertical electric field at Janus group-IV monochalcogenides. Then, we constructed vdW heterostructures with pristine and Janus group-IV monochalcogenides monolayers as building blocks and explored their atomic structures and band alignments. Our results demonstrate that these vdW heterostructures can be synthesized experimentally, and the surface termination of the Janus monolayer at the interface can significantly help the heterostructure realize the transition from type I to type II due to the intrinsic electric field. Moreover, we found eight vdW heterostructures with a mismatch of less than 5% exhibiting type II band alignment with charge densities of VBM and CBM mainly localized at different domains of heterostructures, and excellent power conversion efficiency (∼19%) in photovoltaics are also predicted for these heterostructures with type II band alignment. Our results not only give an idea to use the Janus monolayer as building blocks to construct vdW heterostructures and modulate their band alignment but also provide a guide to the experimental researcher to design more efficient photovoltaic devices with Janus group-IV monochalcogenides.

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.

Wavelength dependence of polarization-resolved second harmonic generation from ferroelectric SnS few layers

Two-dimensional group-IV monochalcogenides such as GeS, GeSe, SnS, and SnSe were theoretically predicted as multiferroic materials with two or more ferroic properties. However, most of their bulk crystals are stacked layer by layer with an antiferroelectric manner, which lose the macroscopic in-plane ferroelectricity. In this work, we studied SnS in which the layers are stacked in a ferroelectric manner both experimentally and theoretically. We utilized polarization-resolved second harmonic generation (SHG) microscopy to investigate numerous flakes of ferroelectric SnS few layers on mica substrates. We found the SHG polar patterns dramatically varied in the range of 800 nm and 1000 nm due to the frequency-dependent SHG susceptibilities. First-principles calculations have been performed to study the frequency-dependent and layer-dependent SHG susceptibilities in the ferroelectric SnS with AA and AC stacking orders. The variation trend of calculated SHG polar patterns as a function of frequency agrees well with that of the experimental results. Since polarization-resolved SHG is a noncontact and nondestructive technique to determine the crystal orientation, understanding of its properties is important, especially for monitoring the transition of different ferroic phases.

Density functional theory investigations of PbSnX[formula omitted] (X = S, Se, Te) monolayers: Structural and electronic properties

In asymmetrical Janus materials, the absence of the vertical mirror symmetry results in the extraordinary mechanical and electronic properties of these structures and has been studied by many scientists currently. In this work, based on the group-IV monochalcogenides MX (M = Pb, Sn, X = S, Se, Te), we study the structural, mechanical, and electronic properties of the Janus structures PbSnX2 by using the density functional theory. The obtained results indicate that PbSnX2 is dynamically stable and can be synthesized as free-standing monolayers by experiment. The difference in the calculated work function on the two sides of these monolayers is the result of the difference in vacuum levels between the surfaces originating from the asymmetry structure. It is also found that these three systems behave semiconductor properties with the band gaps can be modified under the effect of strain engineering. Interestingly, the band gap of the Janus PbSnTe2 reduces to zero when 10% compressive strain is applied. Our findings contribute significantly to the understanding of the physical properties of these Janus monolayers and guide material designs, especially, for nanoscale applications.

Tunable polarized terahertz wave generation induced by spontaneous polarization-dependent ultrafast shift current from vertically grown ferroelectric SnS

Ferroelectric group-IV monochalcogenides $MX$ ($M=\mathrm{Sn}/\mathrm{Ge}$ and $X=\mathrm{S}/\mathrm{Se}$) are of particular importance for the new-generation photovoltaic devices due to their special in-plane spontaneous polarization resulting in a bulk photovoltaic effect. However, the spontaneous polarization and its related dynamical photoresponse under an ultrafast laser excitation are rarely studied via a noncontact all-optical method. Herein, the vertically grown SnS nanosheets have been synthesized by a physical vapor deposition (PVD) method and the corresponding spontaneous polarization is characterized via the terahertz (THz) emission spectroscopy. The primary THz emission mechanism is ascribed to the shift current, which is proved by the THz wave dependences on the pump light polarization angle and pump fluence. Interestingly, the shift current depends on the spontaneous polarization along the SnS nanosheet grown direction, according to the reversed polarity of the THz wave under opposite directional excitations. Furthermore, the amplitude and phase of the THz wave are modulated by the pump light polarization angle, resulting in the elliptical THz wave emission with tunable ellipticity and orientation angle. Our study has shed light on the shift current of ferroelectrics and controllable polarized THz wave generation, which provides application prospects for ferroelectric-based photovoltaics, nonlinear photonics, and THz polarized devices.

High-performance thermoelectric monolayer γ-GeSe and its group-IV monochalcogenide isostructural family

Recently synthesized novel phase of germanium selenide (γ-GeSe) adopts a hexagonal lattice and a surprisingly high conductivity than graphite. This triggers great interests in exploring its potential for thermoelectric applications. Herein, we explored the thermoelectric performance of monolayer γ-GeSe and other isostructural γ-phase of group-IV monochalcogenides γ-GeX (X = S, Se and Te) using the density functional theory and the Boltzmann transport theory. A superb thermoelectric performance of monolayer γ-GeSe is revealed with figure of merit ZT value up to 1.13–2.76 for n-type doping at a moderate carrier concentration of 4.73–2.58 × 1012 cm−2 between 300 and 600 K. This superb performance is rooted in its rich pocket states and flat plateau levels around the electronic band edges, leading to promoted concentrations and electronic conductivity, and limited thermal conductivity. Our work suggests that monolayer γ-GeSe is a promising candidate for high performance medium-temperature thermoelectric applications.

Electronic and optical properties of lateral heterostructures within monolayer black phosphorene and group-IV monochalcogenides

Two-dimensional lateral heterostructures (LHSs) exhibit novel electronic and optical properties, which provide new opportunities for optoelectronic devices. In this work, the energetic, electronic, and optical properties of monolayer LHSs of black phosphorene (BP) and group-IV monochalcogenides (GeS, GeSe, SnS, and SnSe) with armchair/zigzag interface are investigated by first-principles calculation. The results show that these heterostructures can exist stably via covalent bonds. These heterostructures show semiconducting properties except for the BP/SnSe heterostructure with zigzag interface, and their bandgaps can be tuned by the width of heterostructures. More importantly, these heterostructures possess wide optical absorption spectra and high adsorption coefficient. Moreover, the band offsets of heterostructures are calculated by the local density of states, which realize type-I, II, and III band alignments. Our results provide a systematical understanding of the properties of LHSs composed of black phosphorene and group-IV monochalcogenides, revealing their potential for photovoltaic applications.