group-III Monochalcogenides Research Articles

Piezoelectricity of Janus group-III monochalcogenide monolayers, multilayers and their vdW heterostructures: Insight from first-principles calculations

Using first-principles calculations, we have explored the piezoelectric properties of Janus multilayers and vdW heterostructures based on Janus group-III monochalcogenides. Our calculation results show that all the in-plane and out-of-plane piezoelectricity exists in Janus group-III monochalcogenide multilayers with the atomic radii difference and within intra-layer dipole moments affecting the e33/d33 value. For the vdW heterostructures, the e33 depends on the stacking configuration and increases with the decreasing interlayer distance. Furthermore, the piezoelectric effect properties of the vdW heterostructures are independent of the biaxial strain. Our understanding of how the layer number and vdW integration affect the piezoelectric effect in 2D materials provides theoretical guidance for the experimental application of 2D Janus monolayer and their vdW heterostructures and will also contribute to the development of robust electrical-mechanical-coupled systems with large power densities and energy harvesting capabilities.

Higher-order anharmonicity and strain impact on the lattice thermal conductivity of monolayer InTe

In this work, we calculated the lattice thermal conductivity of monolayer InTe by means of phonon Boltzmann transport theory with first-principles calculated inter-atomic force constants. The higher-order phonon anharmonicity was found to play a strong impact on thermal transport in InTe. With the involvement of the phonon–phonon scattering process up to the fourth-order, the in-plane lattice thermal conductivity of monolayer InTe is 5.1 W m−1 K−1 at room temperature, which is 35% of that considering only third-order force constants. Furthermore, strain was found to be an effective way to manipulate the thermal transport in InTe, which reduces to one half when applying 5% in-plane tensile strain. The strain adjustment is due to the decreases in the phonon group velocity as well as the increase in the phonon scattering rates. These findings can enrich thermal transport properties of group-III monochalcogenides and benefit the material design of thermoelectrics and thermal management electronic devices.

Surface-functionalization induced spintronic and photocatalytic features in group-III monochalcogenide monolayers: A first-principles study

In this paper, based on first principles calculations, we propose functionalization of group-III monochalcogenides with halogen atoms to form Janus MXY (M = Al, Ga, In; X = Se, Te; Y = Br, I) monolayers with enhanced physical properties. Using phonon dispersion, formation energy and AIMD simulations, nine MXY monolayers are found to be highly stable. The proposed MXY structures are shown to be direct/indirect semiconductors with bandgaps in the range of 1.13 and 3.34 eV. With regard to SOC effect, large Rashba spin splitting is observed at the Γ-point of conduction band in all Janus MXY monolayers with the largest coefficient of 1.021 eVÅ for GaTeBr. Moreover, we apply vertical electric field and biaxial strain as methods to tune the electronic and spintronic properties of MXY monolayers and improve their potential applications in the future devices. Investigating band edge positions, we find that six MXY monolayers fulfil the band alignment requirement for overall water splitting. Finally, Optical absorption, spatial charge separation and Gibbs free energy profile as other factors to assess the efficiency of MXY photocatalysts are studied. We believe that our research can pave a solid way to tune the physical properties of group-III monochalcogenides for a wider range of applications.

The effect of intrinsic electric field on electronic structures: The case of polar Janus group-III monochalcogenides and their van der Waals heterostructures

Electrostatic control and van der Waals integration are useful methods for 2D materials to help modulate their properties. Here, we constructed nine types of monolayer and multilayer Janus structures based on group-III monochalcogenides and investigated their atomic and electronic structures from first-principles calculations. We found the existence of an intrinsic electric field at Janus structures by comparing their average vacuum electron potentials at two surfaces. Careful charge analyses reveal that the electric field at monolayer Janus structures is mainly due to the dipole of the bond that connects group-III elements of weaker negativity and group-VI elements of stronger negativity. The E-filed at multilayer Janus structures maintains owing to the superposition of dipole at each layer when layer numbers (LN) are not very large, and the electric field starts to reduce when LN is large enough and obvious charge transfer happens between two surfaces of Janus multilayers. Electronic structure calculations at Janus structures demonstrate that the electric field can hardly modulate the charge density population in the monolayer case, while the electric field will significantly bend the bands in multilayer cases and make the charge density of the valence band maximum and the conduction band minimum always located at two separating surfaces, which promotes electron–hole to separate. Furthermore, we investigated the band alignment of isolated monolayer pristine group-III monochalcogenides and isolated Janus group-III monochalcogenides of monolayer, bilayer, and trilayer, and constructed van der Waals (vdW) heterostructures with one pristine monolayer and one Janus monolayer. The results show that the surface termination of Janus structures at the interface can significantly influence the band offset and electronic structures of vdW heterostructures. These results can not only provide a new understanding of the intrinsic electric field of monolayer and multilayer 2D Janus structures but also give a guide that uses electrostatic to modulate the properties of group-III monochalcogenides and other 2D materials.

Electronic, spintronic, and piezoelectric properties of new Janus ZnAXY ( A=Si,Ge,Sn , and X,Y=S,Se,Te ) monolayers

Two-dimensional (2D) Janus materials due to their asymmetric structures show fascinating spintronic and piezoelectric properties making them a research hot spot in recent years. In this work, inspired by Janus group-III monochalcogenides, we propose $\mathrm{Zn}AXY$ ($A=\mathrm{Si}$, Ge, Sn, and $X/Y=\mathrm{S}$, Se, Te, $X<Y$) monolayers as a novel structure with broken inversion and mirror symmetry. By calculating cohesive energy and phonon dispersion, eight of nine possible $\mathrm{Zn}AXY$ monolayers are proved to be dynamically stable. In addition, thermal stability of these eight structures is confirmed by ab initio molecular dynamics simulations. The electronic band structures of $\mathrm{Zn}AXY$ monolayers indicate that all of them are indirect semiconductors with strong spin-orbit coupling effects. Lack of inversion symmetry gives rise to Zeeman-type spin splitting at the $K$ point of the conduction band with the highest value of 136 meV for ZnSiSeTe. Furthermore, out-of-plane asymmetry results in Rashba spin splitting (RSS) at the \ensuremath{\Gamma} point of the valence and conduction bands in the most compositions of Janus $\mathrm{Zn}AXY$ monolayers. Among them, ZnGeSTe with ${\ensuremath{\alpha}}_{R}^{{\mathrm{\ensuremath{\Gamma}}}_{V}}$ of $1.79\phantom{\rule{0.28em}{0ex}}\mathrm{eV}\phantom{\rule{0.28em}{0ex}}\AA{}$ and ${\ensuremath{\alpha}}_{R}^{{\mathrm{\ensuremath{\Gamma}}}_{c}}$ of $0.862\phantom{\rule{0.28em}{0ex}}\mathrm{eV}\phantom{\rule{0.28em}{0ex}}\AA{}$ and ZnSiSTe with ${\ensuremath{\alpha}}_{R}^{{\mathrm{\ensuremath{\Gamma}}}_{V}}$ of $1.537\phantom{\rule{0.28em}{0ex}}\mathrm{eV}\phantom{\rule{0.28em}{0ex}}\AA{}$ and ${\ensuremath{\alpha}}_{R}^{{\mathrm{\ensuremath{\Gamma}}}_{c}}$ of $0.756\phantom{\rule{0.28em}{0ex}}\mathrm{eV}\phantom{\rule{0.28em}{0ex}}\AA{}$ are found to be great materials for future spintronic applications. Interestingly, in addition to large RSS, Mexican hat dispersion is observed at the \ensuremath{\Gamma} point of the topmost valence band in these materials. Moreover, the calculated elastic coefficients for the hexagonal $\mathrm{Zn}AXY$ monolayers confirm the mechanical stability of the predicted structures. Finally, Janus $\mathrm{Zn}AXY$ monolayers possess high in-plane (up to 7.46 pm/V) and out-of-plane (up to 0.67 pm/V) piezoelectric coefficients making them appealing alternatives for prevalent piezoelectric materials.

Transfer learning prediction of spin–orbit correction from bond polarizability for electronic properties of group-III monochalcogenides monolayers

Here, it is shown that the spin–orbit correction, Δ SO , for the electronic properties of group-III monochalcogenides is predictable by their corresponding bond polarizabilities. In this regard, the SOC-based correction for the bandgap and Mexican hat have been calculated from the non-relativistic results together with the bond polarizability parameter. The contribution of the spin–orbit interaction in the mentioned parameters of bandgap and Mexican hat is obtained by a normalized root-mean-square error (nRMSE) of 2 . 8 × 1 0 − 4 and 3 . 2 × 1 0 − 4 , respectively. The transfer learning method has been selected to compensate the size of the dataset. The proposed method reduces the high processing cost associated with the SOC correction calculations by the inclusion of the simple parameter of bond polarizability. From the obtained errors, it is found that the case of the deep learning method results in suitable errors. Thus, the underlying elemental characteristics such as the electronegativity of group-III and VI, bond polarizability and the Mexican hat can deliver numerous electronic band properties through the obtained nonlinear deep transfer method. Interestingly, the Mexican hat parameters can solely be described by the corresponding electronegativities in contrast with the SOC bandgap. • It is shown that the spin–orbit correction for the electronic properties of group-III monochalcogenides is predictable by their corresponding bond polarizabilities. • SOC-based correction for the bandgap and Mexican hat have been calculated from the non-relativistic results together with the bond polarizability parameter. • Bandgap and Mexican hat is obtained by a nRMSE of 2.8e−4 and 3.2e−4, respectively. • The transfer learning method has been selected to compensate the size of the dataset. • The proposed method reduces the high processing cost associated with the SOC correction calculations by the inclusion of the simple parameter of bond polarizability. • It is shown that the Mexican hat parameters can solely be described by the corresponding electronegativities in contrast with the SOC bandgap.

Engineering electronic structures of Janus monolayer group-III monochalcogenides via biaxial strain

We investigate the effects of biaxial strain and spin-orbit coupling on the electronic structures of Janus monolayer group-III monochalcogenides based on first-principle calculations. Due to the spin-orbit coupling and mirror symmetry breaking, Rashba band splitting appears at Γ point of the bottom conduction band and the band gap decreases significantly. The spin-orbit coupling strength and electronic structures can be greatly tuned by biaxial strain, which originate from the change of interaction between pz orbitals. As the lattice constant decreases, the interaction between pz orbitals is enhanced, leading to the decrease (increase) of the energy of corresponding bonding (anti-bonding) state, which is the origin of the indirect-direct-indirect transition of the band gap. In addition, when tensile strain is applied, the pz orbital component of Te atoms increases, leading to the enhancement of the Rashba effect. These results are of great significance for the band engineering and spin-orbitronics of two-dimensional materials.

Piezoelectricity in two-dimensional aluminum, boron and Janus aluminum-boron monochalcogenide monolayers

Piezoelectricity is a property of a material that converts mechanical energy into electrical energy or vice versa. It is known that group-III monochalcogenides, including GaS, GaSe, and InSe, show piezoelectricity in their monolayer form. Piezoelectric coefficients of these monolayers are the same order of magnitude as the previously discovered two-dimensional (2D) piezoelectric materials such as boron nitride and molybdenum disulfide (MoS2) monolayers. Considering a series of monolayer monochalcogenide structures including boron and aluminum (MX, M = B, Al, X = O, S, Se, Te), we design a series of derivative Janus structures (AlBX2, X = O, S, Se, Te). Ab-initio density functional theory and density functional perturbation theory calculations are carried out systematically to predict their structural, electronic, electromechanical and phonon dispersion properties. The electronic band structure analysis indicate that all these 2D materials are semiconductors. The absence of imaginary phonon frequencies in phonon dispersion curves demonstrate that the systems are dynamically stable. In addition, this study shows that these materials exhibit outstanding piezoelectric properties. For AlBO2 monolayer with the relaxed-ion piezoelectric coefficients, d 11 = 15.89(15.87) pm V−1 and d 31 = 0.52(0.44) pm V−1, the strongest piezoelectric properties were obtained. It has large in-plane and out-of-plane piezoelectric coefficients that are comparable to or larger than those of previously reported non-Janus monolayer structures such as MoS2 and GaSe, and also Janus monolayer structures including: In2SSe, Te2Se, MoSeTe, InSeO, SbTeI, and ZrSTe. These results, together with the fact that a lot of similar 2D systems have been synthesized so far, demonstrate the great potential of these materials in nanoscale electromechanical applications.

High-temperature ferromagnetism and half-metallicity in hole-doped Janus OM2S(M=Ga,In,andTl) monolayers

Realizing tunable and stable magnetic states in two-dimensional (2D) materials has been an attractive issue in current materials and physics fields. To date, however, the reported magnetic 2D materials mostly have low Curie transition temperature. In this paper, we theoretically propose a class of Janus group-III monochalcogenide $\mathrm{O}{M}_{2}\mathrm{S}(M=\mathrm{Ga},\mathrm{In},\phantom{\rule{0.16em}{0ex}}\text{and}\phantom{\rule{0.16em}{0ex}}\mathrm{Tl})$ monolayers. Among them, ${\mathrm{OTl}}_{2}\mathrm{S}$ possesses the sombrerolike valence band edge (SVBE) and a Van Hove singularity of the density of states, indicating a possible ferromagnetic transition upon hole doping based on the Stoner criteria. In contrast, the SVBE can be formed in ${\mathrm{OGa}}_{2}\mathrm{S}$ and ${\mathrm{OIn}}_{2}\mathrm{S}$ via tensile strain. Also, the SVBE is responsible for the intraband Lifshitz transition in these monolayers. Ferromagnetism and half-metallicity can be introduced in Janus $\mathrm{O}{M}_{2}\mathrm{S}$ monolayers by injecting hole carriers. At the mean field level, the highest Curie transition temperature ${T}_{\mathrm{C}}$ is estimated to be 627.5 K in freestanding ${\mathrm{OGa}}_{2}\mathrm{S}$, which is higher than those of GaSe and $M\mathrm{O}(M=\mathrm{Ga}\phantom{\rule{0.16em}{0ex}}\text{and}\phantom{\rule{0.16em}{0ex}}\mathrm{In})$ monolayers. The origin of the higher ${T}_{\mathrm{C}}$ is attributed to the unique sombrerolike band dispersion. We further show that the magnetic properties of ${\mathrm{OGa}}_{2}\mathrm{S}$ are sensitive to doped hole density and applied strain, implying a potential tunability in experiment. Finally, the feasibility of introducing the magnetic states through $p$-type dopants and monovacancy defect is explored. In this paper, we not only provide a class of nonmagnetic 2D materials for investigating the high-temperature ferromagnetism and half-metallicity but also widen the application potentials of group-III monochalcogenides.

The high piezoelectricity, flexibility and electronic properties of new Janus ZnXY2 (X = Ge, Sn, Si and Y = S, Se, Te) monolayers: A first-principles research

Due to the broken of inversion and mirror symmetry, Janus structures are believed to have abundant properties and can be applied in two-dimensional (2D) materials, but the research of 2D Janus materials is insufficient. In this work, Janus materials ZnXY2 (X = Ge, Sn, Si and Y = S, Se, Te) monolayers are designed referring to group-III monochalcogenides. The electronic, mechanical and piezoelectric properties and stabilities of ZnXY2 monolayers are investigated by the first-principles calculations. The electronic property shows that ZnXY2 monolayers are semiconductors with wide direct band-gaps and large effective masses difference between holes and electrons. The mechanical property shows that ZnXY2 monolayers possess low Young’s modulus, bending modulus and ductile properties, which is beneficial for applications in flexible nanodevices. The piezoelectric property shows that, compared with conventional Janus group-III monochalcogenides and bulk piezoelectric materials, ZnXY2 monolayers exhibit higher in-plane and comparable out-of-plane piezoelectricity. The further analyzations indicate that the distances and electronegativity differences between atoms are two important influence factors to piezoelectricity. In summary, the flexibility, piezoelectricity, direct band-gaps and large effective mass difference make ZnXY2 monolayers promising candidates for optoelectronics, photocatalysis, flexible nanodevices and electromechanical systems.

The Magnetic Proximity Effect Induced Large Valley Splitting in 2D InSe/FeI2 Heterostructures.

The manipulation of valley splitting has potential applications in valleytronics, which lacks in pristine two-dimensional (2D) InSe. Here, we demonstrate that valley physics in InSe can be activated via the magnetic proximity effect exerted by ferromagnetic FeI2 substrate with spin-orbit coupling. The valley splitting energy can reach 48 meV, corresponding to a magnetic exchange field of ~800 T. The system also presents magnetic anisotropy behavior with its easy magnetization axis tunable from in-plane to out-of-plane by the stacking configurations and biaxial tensile strain. The d-orbital-resolved magnetic anisotropic energy contributions indicate that the tensile strain effect arises from the increase of hybridization between minority Fe dxy and states. Our results reveal that the magnetic proximity effect is an effective approach to stimulate the valley properties in InSe to extend its spintronic applications, which is expected to be feasible in other group-III monochalcogenides.

Large-Area Growth and Stability of Monolayer Gallium Monochalcogenides for Optoelectronic Devices

Group III monochalcogenides such as GaSe and GaS have attracted considerable interest as two-dimensional (2D) alternatives to the traditional transition metal dichalcogenides. The production of large-area films as well as the long-term ambient stability remains a challenge for scalable integration of these materials into the next generation of 2D circuitry and optoelectronic devices. In this report, a simple atmospheric-pressure chemical vapor deposition method is proposed to synthesize continuous monolayers of GaSe and GaS. The proposed method utilizes commercially available precursors and does not requires vacuum-sealed ampules or exfoliation. The optimal parameters for continuous monolayer self-limited growth were determined by systematically changing the growth time, the gas flow rate, and the amount of precursors. So far, the study of bare monolayer GaSe by Raman spectroscopy has been difficult due to the very low Raman signal and a rapid laser-induced oxidation of the material. A laser-scanning method that minimizes the cumulative laser damage and allows a reasonable signal-to-noise ratio was utilized to study the time-dependent ambient stability of bare and encapsulated monolayer samples by Raman spectroscopy. Our results reveal that bare GaSe monolayers can stand up to 6 h in air before complete degradation, and encapsulation with transparent polymeric films can delay this process for few days. These results open the door to produce large-area films of monolayer group III monochalcogenides and to consider film encapsulation with different transparent polymers that could further extend the long-term durability of these ambient sensitive 2D materials.

Oxygenation of monolayer gallium monochalcogenides: Design of two-dimensional ternary Ga2XO structures ( X=S,Se,Te )

The possibility of breaking structural symmetry with realization of Janus monolayers offers new possibilities in the field of two-dimensional (2D) materials, and various ternary systems including the class of group-III monochalcogenides have been suggested. However, interaction of oxygen was shown to modify optoelectronic properties of gallium monochalcogenides, and design of ternary systems with oxygen as a third component has not been considered yet. In this paper, we design and investigate 2D ${\mathrm{Ga}}_{2}X\text{O}$ ($X=\mathrm{S},\mathrm{Se},\mathrm{Te}$) systems by using first-principles calculations. Phonon spectra analysis and molecular dynamics simulations indicate that while ${\mathrm{Ga}}_{2}\mathrm{SO}$ and ${\mathrm{Ga}}_{2}\mathrm{SeO}$ are stable even at high temperatures ${\mathrm{Ga}}_{2}\mathrm{TeO}$ is dynamically unstable. Inclusion of oxygen makes ${\mathrm{Ga}}_{2}\mathrm{SO}$ and ${\mathrm{Ga}}_{2}\mathrm{SeO}$ less brittle when compared to their binary constituents. While $\mathrm{Ga}X$ monolayers have indirect band gaps, ${\mathrm{Ga}}_{2}\mathrm{SO}$ and ${\mathrm{Ga}}_{2}\mathrm{SeO}$ become direct band-gap semiconductors and the band gap can be further tuned by tensile/compressive strain. Additionally, depending on the type of the system, strong optical absorption within the infrared, visible, and/or ultraviolet region is also predicted. Finally, structural and electronic properties of bilayers of ${\mathrm{Ga}}_{2}X\text{O}$ are examined and compared with monolayers. Our results not only predict stable 2D ternary ${\mathrm{Ga}}_{2}X\text{O}$ structures but also suggest them as promising materials for optoelectronic applications.

CVD growth of large-area InS atomic layers and device applications.

Group-III monochalcogenides of two-dimensional (2D) layered materials have attracted widespread attention among scientists due to their unique electronic performance and interesting chemical and physical properties. Indium sulfide (InS) is attracting increasing interest from scientists because it has two distinct crystal structures. However, studies on the synthesis of highly crystalline, large-area, and atomically thin-film InS have not been reported thus far. Here, the chemical vapor deposition (CVD) synthesis method of atomic InS crystals has been reported in this paper. The direct chemical vapour phase reaction of metal oxides with chalcogen precursors produces a large-sized hexagonal crystal structure and atomic-thickness InS flakes or films. The InS atomic films are merged with a plurality of triangular InS crystals that are uniform and entire and have surface areas of 1 cm2 and controllable thicknesses in bilayers or trilayers. The properties of the as-grown highly crystalline samples were characterized by spectroscopic and microscopic measurements. The ion-gel gated InS field-effect transistors (FETs) reveal n-type transport behavior, and have an on-off current ratio of >103 and a room-temperature electron mobility of ∼2 cm2 V-1 s-1. Moreover, our CVD InS can be transferred from mica to any substrates, so various 2D materials can be reassembled into vertically stacked heterostructures, thus facilitating the development of heterojunctions and exploration of the properties and applications of their interactions.

Probing Effective Out-of-Plane Piezoelectricity in van der Waals Layered Materials Induced by Flexoelectricity.

Many van der Waals layered 2D materials, such as h-BN, transition metal dichalcogenides (TMDs), and group-III monochalcogenides, have been predicted to possess piezoelectric and mechanically flexible natures, which greatly motivates potential applications in piezotronic devices and nanogenerators. However, only intrinsic in-plane piezoelectricity exists in these 2D materials and the piezoelectric effect is confined in odd-layers of TMDs. The present work is intent on combining the free-standing design and piezoresponse force microscopy techniques to obtain and directly quantify the effective out-of-plane electromechanical coupling induced by strain gradient on atomically thin MoS2 and InSe flakes. Conspicuous piezoresponse and the measured piezoelectric coefficient with respect to the number of layers or thickness are systematically illustrated for both MoS2 and InSe flakes. Note that the promising effective piezoelectric coefficient (deff 33 ) of about 21.9 pm V-1 is observed on few-layered InSe. The out-of-plane piezoresponse arises from the net dipole moment along the normal direction of the curvature membrane induced by strain gradient. This work not only provides a feasible and flexible method to acquire and quantify the out-of-plane electromechanical coupling on van der Waals layered materials, but also paves the way to understand and tune the flexoelectric effect of 2D systems.

Janus single-layer group-III monochalcogenides: a promising visible-light photocatalyst

Single-layer group-III monochalcogenides are promising candidate materials for a great range of applications in both material science and device physics. Herein, we propose a novel family of Janus single-layer group-III monochalcogenides (J-MX, J = Ga, M = In, X = S and Se) using first-principles calculations. We find that these structures exhibit excellent stability as well as high experimental feasibility. Their band gaps can be tuned effectively by external electric field, while the morphologies of band structures changed little. Importantly, their suitable band gaps (2.07 eV–2.60 eV) and band edge positons enable them promising candidates in photocatalytics. Moreover, the compressive strain can further make the band edge positions match with the redox potentials of water splitting better, and hence can increase the efficiency of solar energy conversion. The adsorption of H adatoms on the S surface and decomposition of H2O on the Se surface of InSe–GaS is further investigated to reveal the microscopic mechanism of water splitting. Our works provide a novel family of promising photocatalytic materials.

Commensurate versus incommensurate heterostructures of group-III monochalcogenides

First-principles calculations based on density-functional theory were performed to investigate heterostructures of group-III monochalcogenides (GaS, GaSe, InS, and InSe) and the effects of incommensurability on their electronic structures. We considered two heterostructures: GaS/GaSe, which has a lattice mismatch of 4.7%, and GaSe/InS, with a smaller mismatch of 2.1%. We computed the cost of having commensurate structures, and we also examined the potential energy landscape of both heterostructures in order to simulate the realistic situation of incommensurate systems. We found that a commensurate heterostructure may be realized in GaSe/InS as the interaction energy of this system with the monolayers assuming the average lattice constant is smaller than the interaction energy of an incommensurate system in which each layer keeps its own lattice constant. For GaS/GaSe, on the other hand, we found that the incommensurate heterostructure is energetically more favorable than the commensurate one, even when taking into account the energetic cost due to the lack of proper registry between the layers. Since the commensurate condition requires that one (or both) layer(s) is (are) strained, we systematically investigated the effect of strain on the band gaps and band-edge positions of the monolayer systems. We found that, in all monolayers, the conduction-band minimum is more than two times more sensitive to applied strain than the valence-band maximum; this was observed to strongly affect the band alignment of GaS/GaSe, as it can change from type-I to type-II with a small variation in the lattice constant of GaS. The GaSe/InS heterostructure was found to have a type-II alignment, which is robust with respect to strain in the range of $\ensuremath{-}2%$ to $+2%$.

2D lateral heterostructures of group-III monochalcogenide: Potential photovoltaic applications

Solar photovoltaics provides a practical and sustainable solution to the increasing global energy demand. Using first-principles calculations, we investigate the energetics and electronic properties of two-dimensional lateral heterostructures by group-III monochalcogenides and explore their potential applications in photovoltaics. The band structures and formation energies from supercell calculations demonstrate that these heterostructures retain semiconducting behavior and might be synthesized in laboratory using the chemical vapor deposition technique. According to the computed band offsets, most of the heterojunctions belong to type II band alignment, which can prevent the recombination of electron-hole pairs. Besides, the electronic properties of these lateral heterostructures can be effectively tailored by the number of layers, leading to a high theoretical power conversion efficiency over 20%.

Monolayer group-III monochalcogenides by oxygen functionalization: a promising class of two-dimensional topological insulators

Monolayer group-III monochalcogenides (MX, M = Ga, In; X = S, Se, Te), an emerging category of two-dimensional (2D) semiconductors, hold great promise for electronics, optoelectronics and catalysts. By first-principles calculations, we show that the phonon dispersion and Raman spectra, as well as the electronic and topological properties of monolayer MX can be tuned by oxygen functionalization. Chemisorption of oxygen atoms on one side or both sides of the MX sheet narrows or even closes the band gap, enlarges work function, and significantly reduces the carrier effective mass. More excitingly, InS, InSe, and InTe monolayers with double-side oxygen functionalization are 2D topological insulators with sizeable bulk gap up to 0.21 eV. Their low-energy bands near the Fermi level are dominated by the px and py orbitals of atoms, allowing band engineering via in-plane strains. Our studies provide viable strategy for realizing quantum spin Hall effect in monolayer group-III monochalcogenides at room temperature, and utilizing these novel 2D materials for high-speed and dissipationless transport devices.

Defects and oxidation of group-III monochalcogenide monolayers.

Among various two-dimensional (2D) materials, monolayer group-III monochalcogenides (GaS, GaSe, InS, and InSe) stand out owing to their potential applications in microelectronics and optoelectronics. Devices made of these novel 2D materials are sensitive to environmental gases, especially O2 molecules. To address this critical issue, here we systematically investigate the oxidization behaviors of perfect and defective group-III monochalcogenide monolayers by first-principles calculations. The perfect monolayers show superior oxidation resistance with large barriers of 3.02-3.20 eV for the dissociation and chemisorption of O2 molecules. In contrast, the defective monolayers with single chalcogen vacancy are vulnerable to O2, showing small barriers of only 0.26-0.36 eV for the chemisorption of an O2 molecule. Interestingly, filling an O2 molecule to the chalcogen vacancy of group-III monochalcogenide monolayers could preserve the electronic band structure of the perfect system-the bandgaps are almost intact and the carrier effective masses are only moderately disturbed. On the other hand, the defective monolayers with single vacancies of group-III atoms carry local magnetic moments of 1-2 μB. These results help experimental design and synthesis of group-III monochalcogenides based 2D devices with high performance and stability.