Recent progress in 2D group IV-IV monochalcogenides: synthesis, properties and applications

Phase transition and metallization of semiconductor GeSe at high pressure

Over the past few decades, semiconductor materials of the group IV-VI monochalcogenides have attracted considerable interest from researchers due to their rich structural characteristics and excellent physical properties. Among them, GeS, GeSe, SnS, and SnSe crystallize in an orthorhombic structure (Pbnm) at ambient conditions. It has been reported that GeS, SnS, and SnSe transform into a higher symmetry orthorhombic structure (Cmcm) at high pressure, while the phase transformation route of GeSe at high pressure remains controversial. As an IV-VI monochalcogenide, GeSe possesses excellent application prospects and has been extensively studied in the fields of optoelectronic and thermoelectric. Here we systematically investigate the structural behavior, optical and electrical properties of GeSe at high pressure. GeSe undergoes a phase transition from thePbnmtoCmcmphase at 33.5 GPa, like isostructural GeS, SnS, and SnSe. The optical bandgap of GeSe decreases gradually as pressure increases and undergoes a semiconducting to metallic transition above 12 GPa. This study exhibits a high-pressure strategy for modulating structural behavior, optical and electrical properties of the group IV-VI monochalcogenides to expand its prospects in optoelectronic and thermoelectric properties.

GeSe/WSe2 mixed dimensional p-n junction photoelectric properties

Heterojunctions prepared utilizing diverse 2D materials enhance a variety of optoelectronic devices. Here, we present GeSe/WSe2 mixed-dimensional p-n heterojunctions, which broaden the possibility of material combination and selection in 2D/layered heterojunction devices, while also providing material parameters to facilitate the development of optoelectronic devices based on 2D/layered semiconductor materials.

Copper Functionalized SnSe Nanoflakes Enabling Nonlinear Optical Features for Ultrafast Photonics

This study enhances the ultrafast photonics application of tin selenide (SnSe) nanoflakes via copper (Cu) functionalization to overcome challenges such as low conductivity and weak near-infrared (NIR) absorption. Cu functionalization enhances concentration, induces strain, and reduces the bandgap through Sn substitution and Sn vacancy filling with Cu ions. Demonstrated by density functional theory calculations and experimental analyses, Cu-functionalized SnSe exhibits improved NIR optical absorption and superior third-order nonlinear optical properties. Z-scan measurements and femtosecond transient absorption spectroscopy reveal better performance of Cu-functionalized SnSe in terms of nonlinear optical properties and shorter carrier relaxation times compared to pristine SnSe. Furthermore, saturable absorbers based on both SnSe types, when integrated into an erbium-doped fiber laser, show that Cu functionalization leads to a decrease in pulse duration to 798 fs and an increase in 3 dB spectral bandwidth to 3.44 nm. Additionally, it enables stable harmonic mode-locking of bound-state solitons. This work suggests a new direction for improving wide bandgap 2D materials by highlighting the enhanced nonlinear optical properties and potential of Cu-functionalized SnSe in ultrafast photonics.

Recent Progress on Layered Sn and Pb-Based Mono Chalcogenides: Synthesis, Structure, Optical, and Thermoelectric Properties and Related Applications

The research on two-dimensional materials has gained significant traction due to their potential for thermoelectric, optical, and other properties. The development of two-dimensional (2D) nanostructured-based TE generators and photodetectors has shown promising results. Over the years, researchers have played a crucial role in advancing this field, enhancing the properties of 2D materials through techniques such as doping, alloying, and various growth methods. Among these materials, black phosphorus, transition metal dichalcogenides, graphene, and IVA-VIA compounds stand out for their remarkable electronic, mechanical, and optical properties. This study presents a comprehensive review of the progress in the field, focusing on IVA-VIA compounds and their applications in TE and photodetector technologies. We summarize recent advancements in enhancing these materials' TE and optical properties and provide an overview of various synthesis techniques for their fabrication. Additionally, we highlight their potential applications as photodetectors in the infrared spectrum. This comprehensive review aims to equip researchers with a deep understanding of the TE and optical properties of 2DMs and their potential applications and to inspire further advancements in this field of research.

High-throughput screening on optoelectronic properties of two-dimensional InN/GaN heterostructure from first principles

CONTEXT

A novel 2D InN/GaN lateral heterostructure (LHT) was simulated by stitching monolayer of 2D InN and monolayer of 2D GaN. The structural stability, electronic structure, and optical properties were systematically investigated using first-principle calculations and by considering the effects of strain. The results indicated that the designed heterostructure has a direct bandgap of 2.26 eV which is further affected by applied biaxial strain. The bandgap of 2D InN/GaN lateral heterostructure decreases with the increase in biaxial strain, and tensile strain triggers a direct-to-indirect energy gap changeover at + 6%. Additionally, under compressive strain, heterostructure remains a direct bandgap semiconductor. Furthermore, the strain significantly affects the optical characteristics of lateral heterostructure. It has been noticed that the first optical absorption peak moves from 2.51 eV (ɛ =  - 4%) to 1.40 eV (ɛ = 10%). Therefore, 2D InN/GaN lateral heterostructure provides an approachable way for utilizing in optoelectronic devices through the creation of in-plane lateral heterostructures.

METHODS

We performed all the computations using a self-consistent method based upon density functional theory. We used the PBEsol functional in the GGA to account for the exchange-correlation effects. We introduced a 10-Å vacuum region in the z-direction to avoid interaction between periodic images. We considered non-negligible weak dispersion correction in the lateral heterostructure using Grimme's DFT-D3 approach. In this study, we also computed the electrical and optical properties employing the local modified Becke-Johnson (lmBJ) exchange potential under meta-GGA functional to obtain more precise results.

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 ([Formula: see text]) 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[Formula: see text]7.5 GPa as determined by optical interferometry and 109.4[Formula: see text]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 [Formula: see text] 0.4 Wm-1K-1 and a total thermal conductivity of 7.5 [Formula: see text] 0.4 Wm-1K-1 in the in-plane direction at room temperature. The notably high [Formula: see text] ratio in γ-GeSe, compared to other layered materials, underscores its distinctive structural and dynamic characteristics.

Two-Dimensional GeS/SnSe2 Tunneling Photodiode with Bidirectional Photoresponse and High Polarization Sensitivity

A two-dimensional (2D) broken-gap (type-III) p-n heterojunction has a unique charge transport mechanism because of nonoverlapping energy bands. In light of this, type-III band alignment can be used in tunneling field-effect transistors (TFETs) and Esaki diodes with tunable operation and low consumption by highlighting the advantages of tunneling mechanisms. In recent years, 2D tunneling photodiodes have gradually attracted attention for novel optoelectronic performance with a combination of strong light-matter interaction and tunable band alignment. However, an in-depth understanding of the tunneling mechanisms should be further investigated, especially for developing electronic and optoelectronic applications. Here, we report a type-III tunneling photodiode based on a 2D multilayered p-GeS/n+-SnSe2 heterostructure, which is first fabricated by the mechanical exfoliation and dry transfer method. Through the Simmons approximation, its various tunneling transport mechanisms dependent on bias and light are demonstrated as the origin of excellent bidirectional photoresponse performance. Moreover, compared to the traditional p-n photodiode, the device enables bidirectional photoresponse capability, including maximum responsivity values of 43 and 8.7 A/W at Vds = 1 and -1 V, respectively, with distinctive photoactive regions from the scanning photocurrent mapping. Noticeably, benefiting from the in-plane anisotropic structure of GeS, the device exhibits an enhanced photocurrent anisotropic ratio of 9, driven by the broader depletion region at Vds = -3 V under 635 nm irradiation. Above all, the results suggest that our designed architecture can be potentially applied to CMOS imaging sensors and polarization-sensitive photodetectors.

2D Janus Polarization Functioned by Mechanical Force

2D polarization materials have emerged as promising candidates for meeting the demands of device miniaturization, attributed to their unique electronic configurations and transport characteristics. Although the existing inherent and sliding mechanisms are increasingly investigated in recent years, strategies for inducing 2D polarization with innovative mechanisms remain rare. This study introduces a novel 2D Janus state by modulating the puckered structure. Combining scanning probe microscopy, transmission electron microscopy, and density functional theory calculations, this work realizes force-triggered out-of-plane and in-plane dipoles with distorted smaller warping in GeSe. The Janus state is preserved after removing the external mechanical perturbation, which could be switched by modulating the sliding direction. This work offers a versatile method to break the space inversion symmetry in a 2D system to trigger polarization in the atomic scale, which may open an innovative insight into configuring novel 2D polarization materials.

Optical and thermoelectric properties of new Janus ZnMN2 (M = Ge, Sn, Si and N = S, Se, Te) monolayers: a first-principles study

Thermoelectric materials have received great interest because they directly tap into the vast reserves of currently underused thermal energy, in an environmentally friendly manner. In this work, we investigated the electronic, optical and thermoelectric properties of novel ZnMN2 (M = Ge, Sn, Si and N = S, Se, Te) monolayers by performing density functional theory calculations. The dynamic and thermal stabilities of ZnMN2 (M = Ge, Sn, Si and N = S, Se, Te) monolayers were confirmed by their phonon band structures and ab initio molecular dynamics (AIMD) simulations, which showed that all the studied monolayers are stable. Calculated electronic band structures showed that ZnSiTe2, ZnGeSe2, and ZnSnTe2 have a direct band gap, while the remaining monolayers have an indirect band gap. Optical properties in terms of the imaginary part of the dielectric function have also been investigated, which showed that all the first excitonic peaks lie in the visible region. Transport coefficients, such as the Seebeck coefficient (S), electrical conductivity (σ) and power factor (PF) were calculated using the Boltzmann theory and plotted against chemical potential. The results demonstrated that the peak values of the p-type region for the PF are greater than those of the n-type region. Notably, ZnSiTe2 exhibits a large PF due to its smaller Seebeck coefficient and higher electrical conductivity compared to ZnSnS2, indicating that it is a promising candidate for thermoelectric applications. Our findings reveal that ZnMN2 (M = Ge, Sn, Si and N = S, Se, Te) monolayers open up new possibilities for optoelectronics and thermoelectric device applications.

Gas Sensing Properties of Pd-Decorated GeSe Monolayer toward Formaldehyde and Benzene Molecules: A First-Principles Study

In this study, the gas sensing properties of formaldehyde (HCHO) and benzene (C6H6) adsorbed on two-dimensional (2D) pristine GeSe and Pd-decorated GeSe (Pd-GeSe) monolayers are studied by using first-principles calculations. The adsorption energies, electronic properties, optical properties, sensitivity, and recovery time of the gas adsorption systems have been thoroughly investigated. It is found that the adsorption of C6H6 on two substrate surfaces and the adsorption of HCHO on pristine GeSe are examples of physical adsorption. However, after HCHO adsorption on the Pd-GeSe monolayer, the adsorption system exhibits an increased adsorption energy of -1.21 eV, which is more favorable compared with the other adsorption systems studied. Moreover, the electron localization function and charge transfer from Pd-GeSe to HCHO are significantly enhanced, indicating distinct chemical adsorption behavior. Furthermore, it demonstrates a larger band gap change rate of 18.8% and a significant enhancement of optical absorption upon the adsorption of HCHO on the Pd-GeSe monolayer. Additionally, the appropriate sensitivity and moderate recovery time for the adsorption of HCHO on the Pd-GeSe surface indicate that the Pd-GeSe monolayer possesses an outstanding sensing capability for HCHO gas.

Stability of mechanically exfoliated layered monochalcogenides under ambient conditions

Monochalcogenides of groups III (GaS, GaSe) and VI (GeS, GeSe, SnS, and SnSe) are materials with interesting thickness-dependent characteristics, which have been applied in many areas. However, the stability of layered monochalcogenides (LMs) is a real problem in semiconductor devices that contain these materials. Therefore, it is an important issue that needs to be explored. This article presents a comprehensive study of the degradation mechanism in mechanically exfoliated monochalcogenides in ambient conditions using Raman and photoluminescence spectroscopy supported by structural methods. A higher stability (up to three weeks) was observed for GaS. The most reactive were Se-containing monochalcogenides. Surface protrusions appeared after the ambient exposure of GeSe was detected by scanning electron microscopy. In addition, the degradation of GeS and GeSe flakes was observed in the operando experiment in transmission electron microscopy. Additionally, the amorphization of the material progressed from the flake edges. The reported results and conclusions on the degradation of LMs are useful to understand surface oxidation, air stability, and to fabricate stable devices with monochalcogenides. The results indicate that LMs are more challenging for exfoliation and optical studies than transition metal dichalcogenides such as MoS2, MoSe2, WS2, or WSe2.

Intrinsic Atomic-Scale Antiferroelectric VOF3 Nanowire with Ultrahigh-Energy Storage Properties

Antiferroelectrics with antiparallel dipoles are receiving tremendous attention for their technological importance and fundamental interest. However, intrinsic one-dimensional (1D) materials harboring antiferroelectric ordering have rarely been reported despite the promise of novel paradigms for miniaturized and high-density electronics. Herein, based on first- and second-principles calculations, we demonstrate the VOF3 atomic wire, exfoliated from an experimentally synthesized yet underexplored 1D van der Waals (vdW) bulk, as a new 1D antiferroelectric material. The energetic, thermal, and dynamic stabilities of the nanowire are confirmed theoretically. Moreover, the temperature-dependent phase transitions and double-hysteresis polarization-field loops are computed for the VOF3 nanowire by constructing the second-principles model. According to the hysteresis loops, high energy densities and efficiencies can be obtained simultaneously at room temperature in the VOF3 nanowire under moderate applied fields. Our identified 1D atomic wire not only expands the family of antiferroelectricity but also holds potential for novel high-power energy storage nanodevices.

Broadband SnS/Te photodetector to long-wavelength infrared: breaking the spectrum limit through alloy engineering

Materials based on group IV chalcogenides, are considered to be one of the most promising materials for high-performance, broadband photodetectors due to their wide bandgap coverage, intriguing chemical bonding and excellent physical properties. However, the reported photodetectors based on SnS are still worked at relatively narrow near-infrared band (as far as 1550 nm) hampered by the nonnegligible bandgap of 1.1-1.5 eV. Here, a novel photodetector based on Te alloyed SnS thin film was demonstrated with an ultra-broadband response up to 10.6 µm. By controlling the Te alloyed concentration in SnS increasing to 37.64%, the bandgap narrows to 0.23 eV, exhibiting a photoresponse potential at long-wavelength infrared excitation. Our results show Te-alloying can remarkably enhance the detection properties of SnS/Te photodetectors. The photoresponsivity and detectivity of 1.59 mA/W and 2.3 × 108 Jones were realized at 10.6 µm at room temperature. Moreover, the nonzero photogain was observed generated by nonlinearly increased photocurrent density, resulting in a superlinear dependency between photoresponsivity and light intensity. Our studies successfully broaden photoresponse spectrum of SnS toward the mid-infrared range for the first time. It also suggests that alloying is an effective technique for tuning the band edges of group IV chalcogenides, contributing deep implications for developing future optoelectronic applications.

Monolayer Ge2Te2P4 as a promising photocatalyst for solar driven water-splitting: a DFT study

The buckling hexagonal structure of Ge2Te2P4 was studied by first-principles calculations. The newly proposed structure was proven to be stable by analyzing its cohesive energy, phonon dispersion, elastic constants and AIMD results. Poisson's ratio of the Ge2Te2P4 monolayer is in the range 0.16-0.18, and Young's modulus is in the range 40.16-43.74 N m-1. The substituted Te atoms enhance the sp2 orbitals which strengthen the σ-bonds and therefore the thickness of the Ge2Te2P4 monolayer is smaller than that of monolayer GeP3. The Ge2Te2P4 monolayer has an indirect band gap of 1.85 eV, which can be narrowed by strains. The compressive band gaps from -2% to -4% change the electronic structure from the indirect band gap into the direct band gap. Strains can also increase the light absorption rate α(ω) in the visible region, which is 2-3 × 105 cm-1 at equilibrium. The Ge2Te2P4 monolayer has a suitable band gap and an appropriate VBM and CBM position for hydrogen generation. Under strain rate of 4% and higher, the VBM and CBM remain at suitable positions for hydrogen production. Another advantage of the Ge2Te2P4 monolayer is that its charge carrier mobilities are really high. The highest electron mobility is 1301.47 cm2 V-1 s-1, and the highest hole mobility is 28627.24 cm2 V-1 s-1, which are much higher than the mobility in monolayer GeP3. The Ge2Te2P4 monolayer has advantages for photocatalytic applications and it is necessary to perform further study on the material.

Optical excitations of graphene-like materials: group III-nitrides

By using first-principles calculations, we have studied the electronic and optical characteristics of group III-nitrides, such as BN, AlN, GaN, and InN monolayers. The optimized geometry, quasi-particle energy spectra, charge density distributions, band-decomposed charge densities, and Van Hove singularities in density of states are described in the work using physical and chemical pictures and orbital hybridizations found in B-N, Al-N, Ga-N, and In-N chemical bonds. Moreover, the dielectric functions, energy loss functions, absorption coefficients, and reflectance spectra with electron-hole interactions of optical properties are successfully achieved. More importantly, the close relations between electronic and optical properties are successfully demonstrated. The theoretical framework will be useful to research other graphene-like materials.

Self-limiting stoichiometry in SnSe thin films

Unique functionalities can arise when 2D materials are scaled down near the monolayer limit. However, in 2D materials with strong van der Waals bonds between layers, such as SnSe, maintaining stoichiometry while limiting vertical growth is difficult. Here, we describe how self-limiting stoichiometry can promote the growth of SnSe thin films deposited by molecular beam epitaxy. The Pnma phase of SnSe was stabilized over a broad range of Sn : Se flux ratios from 1 : 1 to 1 : 5. Changing the flux ratio does not affect the film stoichiometry, but influences the predominant crystallographic orientation. ReaxFF molecular dynamics (MD) simulation demonstrates that, while a mixture of Sn/Se stoichiometries forms initially, SnSe stabilizes as the cluster size evolves. The MD results further show that the excess selenium coalesces into Se clusters that weakly interact with the surface of the SnSe particles, leading to the limited stoichiometric change. Raman spectroscopy corroborates this model showing the initial formation of SnSe2 transitioning into SnSe as experimental film growth progresses. Transmission electron microscopy measurements taken on films deposited with growth rates above 0.25 Å s-1 show a thin layer of SnSe2 that disrupts the crystallographic orientation of the SnSe films. Therefore, using the conditions for self-limiting SnSe growth while avoiding the formation of SnSe2 was found to increase the lateral scale of the SnSe layers. Overall, self-limiting stoichiometry provides a promising avenue for maintaining growth of large lateral-scale SnSe for device fabrication.

Spin-orbit splitting and piezoelectric properties of Janus Ge2XY (X ≠ Y = P, As, Sb and Bi)

The coexistence of spin-orbit coupling and piezoelectricity in a single material may have potential application in multifunctional devices, including spintronics, nanorobotics and piezotronics. Spin-orbit coupling provides a new means to manipulate electron's spin without an additional external magnetic field, while piezoelectricity refers to the interplay between mechanical stresses and electric polarization. Using first-principles calculations, the structural, electronic, optical, spin, and piezoelectric properties of the Janus Ge₂XY (X ≠ Y = P, As, Sb, and Bi) monolayers were systematically investigated. All the Ge₂XY are energetically and dynamically stable in the α phase. At the GW level, Ge₂AsSb, Ge₂AsBi, and Ge₂SbBi have direct fundamental band gaps of 0.65, 0.64, and 0.91 eV. At the GW + BSE level, their optical gaps are 0.42, 0.45, and 0.63 eV, and the optical absorption coefficients can reach about 10^-5 cm^-1 in the infrared light region, which reveals that they have potential for application in infrared photodetectors. For Ge₂PBi, Ge₂AsBi, and Ge₂SbBi containing the heavy Bi element, the lowermost conduction band and uppermost valence band have large spin splitting along the M-K and K-Γ lines, and the bands near the Fermi level possess Rashba spin splitting at the Γ point. Ge₂PBi and Ge₂SbBi have both large in-plane piezoelectric coefficients d₁₁ (-0.75 and -3.18 pm V^-1) and out-of-plane piezoelectric coefficients d₃₁ (0.37 and 0.30 pm V^-1). Our findings are helpful to understand the mechanism of the spin-orbit physics and piezoelectricity of Janus Ge₂XY monolayers and guide experiments in exploring novel multifunctional materials.

Ultrahigh Carrier Mobility in Two-Dimensional IV-VI Semiconductors for Photocatalytic Water Splitting

Two-dimensional materials have been developed as novel photovoltaic and photocatalytic devices because of their excellent properties. In this work, four δ-IV-VI monolayers, GeS, GeSe, SiS and SiSe, are investigated as semiconductors with desirable bandgaps using the first-principles method. These δ-IV-VI monolayers exhibit exceptional toughness; in particular, the yield strength of the GeSe monolayer has no obvious deterioration at 30% strain. Interestingly, the GeSe monolayer also possesses ultrahigh electron mobility along the x direction of approximately 32,507 cm²·V^-1·s^-1, which is much higher than that of the other δ-IV-VI monolayers. Moreover, the calculated capacity for hydrogen evolution reaction of these δ-IV-VI monolayers further implies their potential for applications in photovoltaic and nano-devices.

Electrical Transport Properties Driven by Unique Bonding Configuration in γ-GeSe

Group IV monochalcogenides have recently shown great potential for their thermoelectric, ferroelectric, and other intriguing properties. The electrical properties of group IV monochalcogenides exhibit a strong dependence on the chalcogen type. For example, GeTe exhibits high doping concentration, whereas S/Se-based chalcogenides are semiconductors with sizable bandgaps. Here, we investigate the electrical and thermoelectric properties of γ-GeSe, a recently identified polymorph of GeSe. γ-GeSe exhibits high electrical conductivity (∼10⁶ S/m) and a relatively low Seebeck coefficient (9.4 μV/K at room temperature) owing to its high p-doping level (5 × 10²¹ cm^-3), which is in stark contrast to other known GeSe polymorphs. Elemental analysis and first-principles calculations confirm that the abundant formation of Ge vacancies leads to the high p-doping concentration. The magnetoresistance measurements also reveal weak antilocalization because of spin-orbit coupling in the crystal. Our results demonstrate that γ-GeSe is a unique polymorph in which the modified local bonding configuration leads to substantially different physical properties.

Theoretical prediction of a type-II BP/SiH heterostructure for high-efficiency electronic devices

The generation of layered heterostructures from a combination of two or more different two-dimensional (2D) materials is considered as a powerful strategy to modify the electronic properties of 2D materials and enhance their performance in devices. Herein, using first-principles calculations, we systematically study the electronic properties and the band alignment in a heterostructure formed from 2D boron phosphide (BP) and silicane (SiH) monolayers. The BP/SiH heterostructure is structurally and mechanically stable in the ground state. The generation of the BP/SiH heterostructure leads to a reduction in the band gap, thus enhancing the optical absorption coefficient compared to the constituent BP and SiH monolayers. In addition, the BP/SiH heterostructure has a high carrier mobility of 3.2 × 10⁴ cm² V^-1 s^-1. Furthermore, the combined BP/SiH heterostructure gives rise to the formation of a type-II band alignment, inhibiting the recombination of the photogenerated carriers. The electronic properties and band alignment of the BP/SiH heterostructure can be tuned by an applied external electric field, which causes a reduction in the band gap and leads to the transition of the band alignment from type-II to type-I. Our findings could act as theoretical guidance for the use of the BP/SiH heterostructure in the design of high-efficiency nanodevices.

Fundamental Properties of Hydrogen-Functionalized GaSe Monolayer

Functionalization reveals potential opportunities for modifying essential properties and designing materials due to the strong interaction between functionalized atoms and the surface. Among them, hydrogenation possesses such a way to control electronic and optical characteristics. In this paper, the stability and transformed electronic, optical properties of H-functionalized GaSe in two cases (single and double sites) were reported that exhibit the effects of hydrogen functionalization via first-principles calculations. Formation energies suggest that H-functionalized GaSe systems are stable for construction. H-GaSe and 2H-GaSe display distinct properties based on the functionalized way (single- or double-site functionalization). Accordingly, H-GaSe is metallic, while 2H-GaSe belongs to a semiconductor. The magnetic configuration with ferro- and anti-ferromagnetic could be found in H- and 2H-functionalized cases through spin distribution, respectively. Especially, the chemical hybridized bonds of Se-H, Ga-Se, and Ga-Ga corresponding to s-sp³ and sp³-sp³ bondings, respectively, are clearly verified in the orbital-projected density of states and charge density. The optical properties of 2H-GaSe could provide the main characteristics of a semiconductor, which is the limited range of transparency by electronic absorption at short and long wavelengths. Moreover, increasing the number of GaSe segments (L) could change the band gap leading to application in the band gap engineering of the 2H-GaSe systems. Thus, hydrogen functionalization could provide the possible manner for adjusting and controlling features of GaSe, promising for the development of electronic devices and applications.

Gas adsorption effects of monolayer GeSe in terms of anisotropic transport properties

Two-dimensional layered semiconducting material germanium selenide (GeSe) has attracted significant attention due to its environmental friendship, anisotropic electronic structures, and strong air-stability. To evaluate the candidacy of monolayer GeSe as a potential gas sensing material, the adsorption characteristics of various small gas molecules on monolayer GeSe are comprehensively studied combining density functional theory calculations and non-equilibrium Green's function formalism. The charge transfer reaction between gas molecules and monolayer GeSe leads to the marked change of the carrier density, which further affects the anisotropic transport characteristics of monolayer GeSe. The calculated band structures andI-Vcurves reveal distinctive responses of monolayer GeSe to the different gas molecules, and higher sensitivity of the monolayer GeSe in presence of SO₂, NH₃, NO₂gas molecules along the zigzag direction is obtained. These results suggest that monolayer GeSe along the zigzag direction has promising application in gas detector.

Prediction of 2D IV-VI semiconductors: auxetic materials with direct bandgap and strong optical absorption

Auxetic materials are highly desirable for advanced applications because of their negative Poisson's ratios, which are rather scarce in two-dimensional materials. Motivated by the elemental mutation method, we predict a new class of monolayer IV-VI semiconductors, namely, δ-IV-VI monolayers (GeS, GeSe, SiS and SiSe). Distinctly different from the previously predicted IV-VI monolayers, the newly predicted δ-MX (X = Ge and Si; M = S and Se) monolayers exhibit a puckered unit cell with a space group of Pca2₁. Their stabilities were confirmed by first-principles lattice dynamics and molecular dynamics calculations. In particular, all these MX monolayers possess a large bandgap in the range of 2.08-2.65 eV and pronounced anisotropic mechanical properties, which are demonstrated by direction-dependent in-plane Young's moduli and Poisson's ratios. Furthermore, all these 2D MX monolayers possess negative Poisson's ratios (even up to about -0.3 for SiSe). Strong optical absorption is observed in these δ-IV-VI monolayers. These interesting physical properties will stimulate the development of 2D flexible devices based on IV-VI semiconductor monolayers.

2D Layered Material Alloys: Synthesis and Application in Electronic and Optoelectronic Devices

2D layered materials (2DLMs) have come under the limelight of scientific and engineering research and broke new ground across a broad range of disciplines in the past decade. Nevertheless, the members of stoichiometric 2DLMs are relatively limited. This renders them incompetent to fulfill the multitudinous scenarios across the breadth of electronic and optoelectronic applications since the characteristics exhibited by a specific material are relatively monotonous and limited. Inspiringly, alloying of 2DLMs can markedly broaden the 2D family through composition modulation and it has ushered a whole new research domain: 2DLM alloy nano-electronics and nano-optoelectronics. This review begins with a comprehensive survey on synthetic technologies for the production of 2DLM alloys, which include chemical vapor transport, chemical vapor deposition, pulsed-laser deposition, and molecular beam epitaxy, spanning their development, as well as, advantages and disadvantages. Then, the up-to-date advances of 2DLM alloys in electronic devices are summarized. Subsequently, the up-to-date advances of 2DLM alloys in optoelectronic devices are summarized. In the end, the ongoing challenges of this emerging field are highlighted and the future opportunities are envisioned, which aim to navigate the coming exploration and fully exert the pivotal role of 2DLMs toward the next generation of electronic and optoelectronic devices.

Bonding, structure, and mechanical stability of 2D materials: the predictive power of the periodic table

This tutorial review describes the ongoing effort to convert main-group elements of the periodic table and their combinations into stable 2D materials, which is sometimes called modern 'alchemy'. Theory is successfully approaching this goal, whereas experimental verification is lagging far behind in the synergistic interplay between theory and experiment. The data collected here gives a clear picture of the bonding, structure, and mechanical performance of the main-group elements and their binary compounds. This ranges from group II elements, with two valence electrons, to group VI elements with six valence electrons, which form not only 1D structures but also, owing to their variable oxidation states, low-symmetry 2D networks. Outside of these main groups reviewed here, predominantly ionic bonding may be observed, for example in group II-VII compounds. Besides high-symmetry graphene with its shortest and strongest bonds and outstanding mechanical properties, low-symmetry 2D structures such as various borophene and tellurene phases with intriguing properties are receiving increasing attention. The comprehensive discussion of data also includes bonding and structure of few-layer assemblies, because the electronic properties, e.g., the band gap, of these heterostructures vary with interlayer layer separation and interaction energy. The available data allows the identification of general relationships between bonding, structure, and mechanical stability. This enables the extraction of periodic trends and fundamental rules governing the 2D world, which help to clear up deviating results and to estimate unknown properties. For example, the observed change of the bond length by a factor of two alters the cohesive energy by a factor of four and the extremely sensitive Young's modulus and ultimate strength by more than a factor of 60. Since the stiffness and strength decrease with increasing atom size on going down the columns of the periodic table, it is important to look for suitable allotropes of elements and binaries in the upper rows of the periodic table when mechanical stability and robustness are issues. On the other hand, the heavy compounds are of particular interest because of their low-symmetry structures with exotic electronic properties.

Universal Zigzag Edge Reconstruction of an α-Phase Puckered Monolayer and Its Resulting Robust Spatial Charge Separation

Edges are important, because they dictate the stability and properties of nanoribbons. Here, we reveal a universal reconstruction of the ZZ edge into a (2 × 1) tubed [ZZ(Tube)] edge, enabling an ultimate narrow nanotube to terminate nanoribbons for α-puckered group-V elemental and compound monolayers (GeS/Se and SnS/Se). The reconstructed edge formations are confirmed by CALYPSO. The ZZ(Tube) edge forms easily, is highly stable, and is semiconducting. Remarkably, the ZZ(Tube) edge always exhibits a type-II band structure and robust spatial charge separation. For a compound monolayer monochalcogenide, mild (2 × 1) ZZ(S-R) occurs at the chalcogenide-terminated edge. TDDFT simulations indicate that charge separation occurs only at 672 fs, while the lifetime is over 5 ns, thus facilitating robust spatial charge accumulation. These remarkable features of ZZ(Tube) edge-terminated α-puckered nanoribbons are ideal for optoelectronic and photocatalytic applications.

Multielement 2D layered material photodetectors

The pronounced quantum confinement effects, outstanding mechanical strength, strong light-matter interactions and reasonably high electric transport properties under atomically thin limit have conjointly established 2D layered materials (2DLMs) as compelling building blocks towards the next generation optoelectronic devices. By virtue of the diverse compositions and crystal structures which bring about abundant physical properties, multielement 2DLMs (ME2DLMs) have become a bran-new research focus of tremendous scientific enthusiasm. Herein, for the first time, this review provides a comprehensive overview on the latest evolution of ME2DLM photodetectors. The crystal structures, synthesis, and physical properties of various experimentally realized ME2DLMs as well as the development in metal-semiconductor-metal photodetectors are comprehensively summarized by dividing them into narrow-bandgap ME2DLMs (including Bi₂O₂X (X = S, Se, Te), EuMTe₃(M = Bi, Sb), Nb₂XTe₄(X = Si, Ge), Ta₂NiX₅(X = S, Se), M₂PdX₆(M = Ta, Nb; X = S, Se), PbSnS₂), moderate-bandgap ME2DLMs (including CuIn₇Se₁₁, CuTaS₃, GaGeTe, TlMX₂(M = Ga, In; X = S, Se)), wide-bandgap ME2DLMs (including BiOX (X = F, Cl, Br, I), MPX₃(M = Fe, Ni, Mn, Cd, Zn; X = S, Se), ABP₂X₆(A = Cu, Ag; B = In, Bi; X = S, Se), Ga₂In₄S₉), as well as topological ME2DLMs (MIrTe₄(M = Ta, Nb)). In the last section, the ongoing challenges standing in the way of further development are underscored and the potential strategies settling them are proposed, which is aimed at navigating the future advancement of this fascinating domain.

γ-GeSe: A New Hexagonal Polymorph from Group IV-VI Monochalcogenides

The family of group IV-VI monochalcogenides has an atomically puckered layered structure, and their atomic bond configuration suggests the possibility for the realization of various polymorphs. Here, we report the synthesis of the first hexagonal polymorph from the family of group IV-VI monochalcogenides, which is conventionally orthorhombic. Recently predicted four-atomic-thick hexagonal GeSe, so-called γ-GeSe, is synthesized and clearly identified by complementary structural characterizations, including elemental analysis, electron diffraction, high-resolution transmission electron microscopy imaging, and polarized Raman spectroscopy. The electrical and optical measurements indicate that synthesized γ-GeSe exhibits high electrical conductivity of 3 × 10⁵ S/m, which is comparable to those of other two-dimensional layered semimetallic crystals. Moreover, γ-GeSe can be directly grown on h-BN substrates, demonstrating a bottom-up approach for constructing vertical van der Waals heterostructures incorporating γ-GeSe. The newly identified crystal symmetry of γ-GeSe warrants further studies on various physical properties of γ-GeSe.

Electronic, optical, and thermoelectric properties of Janus In-based monochalcogenides

Inspired by the successfully experimental synthesis of Janus structures recently, we systematically study the electronic, optical, and electronic transport properties of Janus monolayers In₂XY(X/Y= S, Se, Te withX≠Y) in the presence of a biaxial strain and electric field using density functional theory. Monolayers In₂XYare dynamically and thermally stable at room temperature. At equilibrium, both In₂STe and In₂SeTe are direct semiconductors while In₂SSe exhibits an indirect semiconducting behavior. The strain significantly alters the electronic structure of In₂XYand their photocatalytic activity. Besides, the indirect-direct gap transitions can be found due to applied strain. The effect of the electric field on optical properties of In₂XYis negligible. Meanwhile, the optical absorbance intensity of the Janus In₂XYmonolayers is remarkably increased by compressive strain. Also, In₂XYmonolayers exhibit very low lattice thermal conductivities resulting in a high figure of meritZT, which makes them potential candidates for room-temperature thermoelectric materials.

Functionalization of electronic, spin and optical properties of GeSe monolayer by substitutional doping: a first-principles study

Substitutional doping has traditionally been used to modulate the existing properties of semiconductors and introduce new exciting properties, especially in two-dimensional materials. In this work, we have investigated the impact of substitutional doping (using group III, IV, V, and VI dopants) on the structural, electronic, spin, and optical properties of GeSe monolayer by using first-principles calculations based on density functional theory. Our calculated binding energies, formation energies and phonon dispersion curves of the doped systems support their stability and hence the feasibility of physical realization. Our results further suggest that switching between metallic and semiconducting states of GeSe monolayer can be controlled by dopant atoms with a different number of valence electrons. The band gap of the semiconducting structures can be tuned within a range of 0.2864 eV to 1.17 eV by substituting with different dopants. In addition, most of the doped structures maintain the low effective mass, 0.20m0to 0.59m0for electron and 0.21m0to 0.52m0for hole, which ensures the enhanced transport properties of GeSe based electronic devices. Moreover, when Ge is substituted with group V dopants, a magnetic moment is introduced in an otherwise non-magnetic GeSe monolayer. The optical absorption coefficient of the doped structures can be significantly improved (>2×) in the visible and infrared regions. These intriguing results would encourage the applications of doped GeSe monolayer in next-generation electronic, optoelectronic and spintronic devices.

Novel Two-Dimensional Layered MoSi2Z4 (Z = P, As): New Promising Optoelectronic Materials

Very recently, two new two-dimensional (2D) layered semi-conducting materials MoSi2N4 and WSi2N4 were successfully synthesized in experiments, and a large family of these two 2D materials, namely MA2Z4, was also predicted theoretically (Science, 369, 670 (2020)). Motivated by this exciting family, in this work, we systematically investigate the mechanical, electronic and optical properties of monolayer and bilayer MoSi2P4 and MoSi2As4 by using the first-principles calculation method. Numerical results indicate that both monolayer and bilayer MoSi2Z4 (Z = P, As) present good structural stability, isotropic mechanical parameters, moderate bandgap, favorable carrier mobilities, remarkable optical absorption, superior photon responsivity and external quantum efficiency. Especially, due to the wave-functions of band edges dominated by d orbital of the middle-layer Mo atoms are screened effectively, the bandgap and optical absorption hardly depend on the number of layers, providing an added convenience in the experimental fabrication of few-layer MoSi2Z4-based electronic and optoelectronic devices. We also build a monolayer MoSi2Z4-based 2D optoelectronic device, and quantitatively evaluate the photocurrent as a function of energy and polarization angle of the incident light. Our investigation verifies the excellent performance of a few-layer MoSi2Z4 and expands their potential application in nanoscale electronic and optoelectronic devices.

Polymorphism in Post-Dichalcogenide Two-Dimensional Materials

Two-dimensional (2D) materials exhibit a wide range of atomic structures, compositions, and associated versatility of properties. Furthermore, for a given composition, a variety of different crystal structures (i.e., polymorphs) can be observed. Polymorphism in 2D materials presents a fertile landscape for designing novel architectures and imparting new functionalities. The objective of this Review is to identify the polymorphs of emerging 2D materials, describe their polymorph-dependent properties, and outline methods used for polymorph control. Since traditional 2D materials (e.g., graphene, hexagonal boron nitride, and transition metal dichalcogenides) have already been studied extensively, the focus here is on polymorphism in post-dichalcogenide 2D materials including group III, IV, and V elemental 2D materials, layered group III, IV, and V metal chalcogenides, and 2D transition metal halides. In addition to providing a comprehensive survey of recent experimental and theoretical literature, this Review identifies the most promising opportunities for future research including how 2D polymorph engineering can provide a pathway to materials by design.

Flexoelectricity in atomic monolayers from first principles

We study the flexoelectric effect in fifty-four select atomic monolayers using ab initio Density Functional Theory (DFT). Specifically, considering representative materials from each of the Group III monochalcogenides, transition metal dichalcogenides (TMDs), Groups IV, III-V, and V monolayers, Group IV dichalcogenides, Group IV monochalcogenides, transition metal trichalcogenides (TMTs), and Group V chalcogenides, we perform symmetry-adapted DFT simulations to calculate transversal flexoelectric coefficients along the principal directions at practically relevant bending curvatures. We find that the materials demonstrate linear behavior and have similar coefficients along both principal directions, with values for TMTs being up to a factor of five larger than those of graphene. In addition, we find electronic origins for the flexoelectric effect, which increases with monolayer thickness, elastic modulus along the bending direction, and sum of polarizability of constituent atoms.

Theoretical prediction of structural, mechanical, and electronic properties of Janus GeSnX2 (X = S, Se, Te) single-layers

The breaking of the vertical mirror symmetry in two-dimensional Janus structures has given rise to many outstanding features that do not exist in the original materials. In this work, we study the structural, mechanical, and electronic properties of Janus GeSnX₂ (X = S, Se, Te) single-layers using density functional theory. The stability of the investigated Janus structures has been tested through the analysis of their phonon dispersions and elastic parameters. It is found that, with low in-plane stiffness, Janus GeSnX₂ single-layers are more mechanically flexible than other two-dimensional materials and their mechanical properties exhibit very high anisotropy. All three single-layers are semiconductors and their bandgap can be altered easily by strain engineering. Due to the asymmetric structure, a vacuum level difference between the two sides is observed, leading to the difference in work function on the two sides of single-layers. Our findings not only provide necessary information about the physical properties of Janus GeSnX₂ single-layers but also provide the impetus for further studies on these interesting materials both theoretically and experimentally.

The breaking of the vertical mirror symmetry in two-dimensional Janus structures has given rise to many outstanding features that do not exist in the original materials.

Structural and electronic properties of a novel two-dimensional Janus Pd4S3Se3 monolayer controllable by electric field and strain engineering

In this work, we investigate the structural and electronic properties of a newly-discovered two-dimensional Janus Pd4S3Se3 monolayer, as well as its controllable structural and electronic properties under an electric field and strain engineering using first-principles calculations.

Liquid-Metal Synthesized Ultrathin SnS Layers for High-Performance Broadband Photodetectors

Atomically thin materials face an ongoing challenge of scalability, hampering practical deployment despite their fascinating properties. Tin monosulfide (SnS), a low-cost, naturally abundant layered material with a tunable bandgap, displays properties of superior carrier mobility and large absorption coefficient at atomic thicknesses, making it attractive for electronics and optoelectronics. However, the lack of successful synthesis techniques to prepare large-area and stoichiometric atomically thin SnS layers (mainly due to the strong interlayer interactions) has prevented exploration of these properties for versatile applications. Here, SnS layers are printed with thicknesses varying from a single unit cell (0.8 nm) to multiple stacked unit cells (≈1.8 nm) synthesized from metallic liquid tin, with lateral dimensions on the millimeter scale. It is reveal that these large-area SnS layers exhibit a broadband spectral response ranging from deep-ultraviolet (UV) to near-infrared (NIR) wavelengths (i.e., 280-850 nm) with fast photodetection capabilities. For single-unit-cell-thick layered SnS, the photodetectors show upto three orders of magnitude higher responsivity (927 A W^-1 ) than commercial photodetectors at a room-temperature operating wavelength of 660 nm. This study opens a new pathway to synthesize reproduceable nanosheets of large lateral sizes for broadband, high-performance photodetectors. It also provides important technological implications for scalable applications in integrated optoelectronic circuits, sensing, and biomedical imaging.

Liquid-Phase Exfoliated GeSe Nanoflakes for Photoelectrochemical-Type Photodetectors and Photoelectrochemical Water Splitting

Photoelectrochemical (PEC) systems represent powerful tools to convert electromagnetic radiation into chemical fuels and electricity. In this context, two-dimensional (2D) materials are attracting enormous interest as potential advanced photo(electro)catalysts and, recently, 2D group-IVA metal monochalcogenides have been theoretically predicted to be water splitting photocatalysts. In this work, we use density functional theory calculations to theoretically investigate the photocatalytic activity of single-/few-layer GeSe nanoflakes for both the hydrogen evolution reaction (HER) and the oxygen evolution reaction (OER) in pH conditions ranging from 0 to 14. Our simulations show that GeSe nanoflakes with different thickness can be mixed in the form of nanoporous films to act as nanoscale tandem systems, in which the flakes, depending on their thickness, can operate as HER- and/or OER photocatalysts. On the basis of theoretical predictions, we report the first experimental characterization of the photo(electro)catalytic activity of single-/few-layer GeSe flakes in different aqueous media, ranging from acidic to alkaline solutions: 0.5 M H2SO4 (pH 0.3), 1 M KCl (pH 6.5), and 1 M KOH (pH 14). The films of the GeSe nanoflakes are fabricated by spray coating GeSe nanoflakes dispersion in 2-propanol obtained through liquid-phase exfoliation of synthesized orthorhombic (Pnma) GeSe bulk crystals. The PEC properties of the GeSe nanoflakes are used to design PEC-type photodetectors, reaching a responsivity of up to 0.32 AW-1 (external quantum efficiency of 86.3%) under 455 nm excitation wavelength in acidic electrolyte. The obtained performances are superior to those of several self-powered and low-voltage solution-processed photodetectors, approaching that of self-powered commercial UV-Vis photodetectors. The obtained results inspire the use of 2D GeSe in proof-of-concept water photoelectrolysis cells.

Tunable anisotropic plasmon response of monolayer GeSe nanoribbon arrays

Recently, emerging two-dimensional (2D) germanium selenide (GeSe) has drawn lots of attention due to its in-plane anisotropic properties and great potential for optoelectronic applications such as in solar cells. However, methods are still sought to enhance its interaction with light to enable practical applications. Herein, we numerically investigate the localized plasmon response of monolayer GeSe nanoribbon arrays systematically, and the results show that localized surface plasmon polaritons in the far-infrared range with anisotropic behavior can be efficiently excited to enhance the light-matter interaction. We further show that the plasmon response of monolayer GeSe nanoribbons could be tuned effectively through the nanoribbon width, local refractive index, substrate thickness and carrier concentration, pointing out the ways for controlling the localized plasmon response. In the case of monolayer GeSe nanoribbons on a substrate of finite thickness, a Fabry-Pérot-like (FP-like) quantitative model has been proposed to explain the overall spectral response originating from overlapped FP and plasmon modes, and it matches well with the simulation results. All in all, we investigate the plasmon response of the novel 2D GeSe nanoribbons thoroughly for the first time, bringing opportunities for potential applications of novel polarization-dependent optoelectronic devices.

Sub-5 nm monolayer germanium selenide (GeSe) MOSFETs: towards a high performance and stable device

Two-dimensional (2D) black phosphorene (BP) field-effect transistors (FETs) show excellent device performance but suffer from serious instability under ambient conditions. Isoelectronic 2D germanium selenide (GeSe) shares many similar properties with 2D BP, such as high carrier mobility and anisotropy, but is stable under ambient conditions. Herein, we explore the quantum transport properties of sub-5 nm ML GeSe MOSFETs using first-principles quantum transport simulation. A p-type (zigzag-directed) device is superior to other types (n- and p-type armchair-directed and n-type zigzag-directed). The on-state current of p-type devices (zigzag-directed), even at a 1 nm gate-length, can fulfill the requirements of high-performance applications for the next decade in the International Technology Roadmap for Semiconductors (ITRS, 2013 version). To the best of our knowledge, these ML GeSe MOSFETs have the smallest gate-length that can fulfill the ITRS HP on-state current requirements among reported 2D material FETs.

Enhanced Shift Currents in Monolayer 2D GeS and SnS by Strain-Induced Band Gap Engineering

Group IV monochalcogenides exhibit spontaneous polarization and ferroelectricity, which are important in photovoltaic materials. Since strain engineering plays an important role in ferroelectricity, in the present work, the effect of equibiaxial strain on the band structure and shift currents in monolayer two-dimensional (2D) GeS and SnS has systematically been investigated using the first-principles calculations. The conduction bands of those materials are more responsive to strain than the valence bands. Increased equibiaxial compressive strain leads to a drastic reduction in the band gap and finally the occurrence of phase transition from semiconductor to metal at strains of -15 and -14% for GeS and SnS, respectively. On the other hand, tensile equibiaxial strain increases the band gap slightly. Similarly, increased equibiaxial compressive strain leads to a steady almost four times increase in the shift currents at a strain of -12% with direction change occurring at -8% strain. However, at phase transition from semiconductor to metal, the shift currents of the two materials completely vanish. Equibiaxial tensile strain also leads to increased shift currents. For SnS, shift currents do not change direction, just as the case of GeS at low strain; however, at a strain of +8% and beyond, direction reversal of shift currents beyond the band gap in GeS occur.

Two-dimensional few-layered PC3 as a promising photocatalyst for overall water splitting

Recently, 2D carbon phosphides (PCs) have attracted much attention due to their superior electronic and photovoltaic properties suitable for potential applications in field effect transistors and photodetectors. In this work, we systematically investigate the stability, electronic properties, optical absorption and photocatalytic water splitting performance of few-layered PC3 by using the first principles calculation method. Numerical results indicate that both monolayered and bilayered PC3 can serve as efficient photocatalysts for overall water splitting due to their high stability, moderate band gaps, suitable band edge positions, anisotropic high carrier mobilities and strong capacity of solar absorption. Compared with monolayered PC3, bilayered PC3 displays higher carrier mobilities (2500-23 000 cm2 V-1 s-1) and a wider optical absorption spectrum. Moreover, by applying an in-plane biaxial strain, the utilization of solar energy and the pH range suitable for overall water splitting can be improved effectively for both monolayered and bilayered PC3. Our work reliably expands the potential application of 2D few-layered PC3 in the field of nano-electronics and nano-optoelectronics.

One-dimensional flat bands in twisted bilayer germanium selenide

Experimental advances in the fabrication and characterization of few-layer materials stacked at a relative twist of small angle have recently shown the emergence of flat energy bands. As a consequence electron interactions become relevant, providing inroads into the physics of strongly correlated two-dimensional systems. Here, we demonstrate by combining large scale ab initio simulations with numerically exact strong correlation approaches that an effective one-dimensional system emerges upon stacking two twisted sheets of GeSe, in marked contrast to all moiré systems studied so far. This not only allows to study the necessarily collective nature of excitations in one dimension, but can also serve as a promising platform to scrutinize the crossover from two to one dimension in a controlled setup by varying the twist angle, which provides an intriguing benchmark with respect to theory. We thus establish twisted bilayer GeSe as an intriguing inroad into the strongly correlated physics of lowdimensional systems.

Two-dimensional materials with intrinsic auxeticity: progress and perspectives

Materials with a negative Poisson's ratio, termed auxetic materials, will expand (contract) in the transverse direction when they are stretched (compressed) longitudinally. They provide exciting prospects for enhancing mechanical properties and a wide range of promising applications. Driven by the current demand for nano-devices with specific functionalities, research studies on two-dimensional auxetic materials with an intrinsic negative Poisson's ratio are intensively simulated. Herein, we summarize the recent efforts and progress in material discovery and property characterization on two-dimensional auxetic materials. We categorize the rich variety of the proposed two-dimensional auxetic materials based on their structures and whether they have been synthesized experimentally. The underlying mechanisms for forming a negative Poisson's ratio as well as the modulation of negative Poisson's ratio are discussed. Finally, we close by assessing the challenges and perspectives faced by future research and development of this compelling class of materials.