Properties Of 2D Materials Research Articles

Theoretical investigation of the electronic structure and thermoelectric performance of 2D GeSb2Te4 and GeBi2Te4

Thermoelectric technology is attractive for waste heat recovery and clean energy; it is highly desired as a potential power supply. In this paper, we investigated the stability, elastic, electronic structures and thermoelectric properties of unexplored 2D materials, GeSb2Te4 and GeBi2Te4, which deliver high stability, acceptable cleavage energies (0.32–0.33 J/m2), and narrow band gaps of 0.80 and 0.69 eV. We also revealed that the septuple layers possess anisotropic electron and hole mobilities of about 1593.71/791.50 and 522.04/240.52 cm2/Vs, which result in high conductivities of 107–108 S/m, as well as desirable thermoelectric power factors of 28.08–70.382 mW/Km2. Besides, owing to the low group velocities and strong dissipative scattering for low-lying phonons, the materials inherit the low lattice thermal conductivities of 1.51 and 0.41 W/mK. As a result, their thermoelectric figure of merit reaches 1.60 and 1.70 at 300 K, and rises further to 3.80 and 4.16 at 700 K. Together, 2D GeSb2Te4 and GeBi2Te4 are suitable candidates for low-medium temperature thermoelectric application.

Hybrid organic-inorganic two-dimensional metal carbide MXenes with amido- and imido-terminated surfaces.

Two-dimensional (2D) transition-metal carbides and nitrides (MXenes) combine the electronic and mechanical properties of 2D inorganic crystals with chemically modifiable surfaces, which provides an ideal platform for both fundamental and applied studies of interfaces. Good progress has been achieved in the functionalization of MXenes with small inorganic ligands, but relatively little work has been reported on the covalent bonding of various organic groups to MXene surfaces. Here we synthesize a family of hybrid MXenes (h-MXenes) that incorporate amido- and imido-bonding between organic and inorganic parts by reacting halogen-terminated MXenes with deprotonated organic amines. The resulting hybrid structures unite tailorability of organic molecules with electronic connectivity and other properties of inorganic 2D materials. Describing the structure of h-MXene necessitates the integration of concepts from coordination chemistry, self-assembled monolayers and surface science. The optical properties of h-MXenes reveal coherent coupling between the organic and inorganic constituents. h-MXenes also exhibit superior stability against hydrolysis.

Compositional and Structural Disorder in Two-Dimensional AIIIBVI Materials

Two-dimensional (2D) van der Waals (vdW) AIIIBVI semiconductor materials, such as InSe and GaSe, are of considerable interest due to their potential use in various microelectronics applications. The range of properties of materials of this class can be extended further through the use of quasi-binary alloys of the InSe(Te)-GaSe(Te) type. In this work, we study the effect of compositional and structural disorder in 2D In(Ga)Se(Te) on the band structure and electronic properties using first principles modeling. The results for In(Ga)Se demonstrate a noticeable decrease in the band gap for structures with a random distribution of indium and gallium cations, while for In(Ga)Te with a random cation distribution, metallization occurs. Changes in the compositional arrangement of chalcogens (there can be either the same or different atoms on each side of the vdW gap) lead to pronounced changes in the band gap, but no significant changes in topology are observed. In addition, a significant effect of the distance between the layers on the band gap under compression along the c axis was found for both alloys under study. An important point of our study is that van der Waals gap engineering is a very powerful tool to control the properties of 2D materials and its alloys.

Theoretical designs of low‐barrier ferroelectricity

AbstractFerroelectrics with electrically switchable spontaneous polarizations can be used for information storage, where a low switching barrier is favorable to reduce the energy cost and enhance the speed for data writing. Meanwhile their robustness at working temperature should be ensured, which is a challenge for the designs of low‐barrier ferroelectrics. Here we review several types of ferroelectric mechanisms that may render both low switching barriers and room‐temperature robustness, which have been theoretically proposed in previous studies. (1) The prediction of sliding ferroelectricity with ultralow switching barriers has been experimentally confirmed in a series of van der Waals layers, which may enable convenient electrical control of various physical properties in 2D materials, like magnetic, photovoltaic, valleytronic and topological properties. (2) Hydrogen‐bonded ferroelectricity spontaneously formed by head‐to‐tail chains can be switched by proton‐transfer crossing a low barrier, and a mechanism of ultra‐high piezoelectricity utilizing the specific features of hydrogen bonding has been proposed. (3) High‐ionicity ferroelectricity induced by covalent‐like ionic bondings may entail high polarizations and low barriers during switching, which is attributed to the features of long‐range Coulomb interaction, and the long ion‐displacements crossing unitcell may give rise to unconventional ferroelectricity with quantized polarizations even in crystals of non‐ferroelectric point groups. Those low‐barrier ferroelectric mechanisms may bring in both new physics and technological advances, which are to be further explored.This article is categorized under: Structure and Mechanism > Computational Materials Science

High Carrier Mobility and Controllable Electronic Property of the h-BN/SnSe2 Heterostructure.

Building two-dimensional (2D) vertical van der Waals heterostructures (vdWHs) is one of the effective methods to regulate the properties of single 2D materials. In this paper, we stack the hexagonal boron nitride (h-BN) monolayer (ML) on the SnSe2 ML to construct the stable h-BN/SnSe2 vdWH, of which the crystal and electronic structures, together with the optical properties, are also analyzed by the first-principles calculations. The results show that the h-BN/SnSe2 vdWH belongs to a type-I heterostructure with an indirect bandgap of 1.33 eV, in which the valence band maximum and conduction band minimum are both determined by the component SnSe2 ML. Interestingly, the h-BN/SnSe2 vdWH under the tensile strain or electric field undergoes the transitions both from type-I to type-II heterostructure and from the indirect to direct bandgap semiconductor. In addition, the carrier mobility of the h-BN/SnSe2 heterostructure has a significant enhancement relative to that of the SnSe2 ML, up to 104 cm2 V-1 s-1. Meanwhile, the h-BN/SnSe2 heterostructure presents the superb optical absorption and unique type-II hyperbolic property. Our findings will broaden the potential applications of SnSe2 ML and provide theoretical guidance for the related experimental studies.

Natural overlaying behaviors push the limit of planar cyclic deformation performance in few‐layer MoS2 nanosheets

AbstractAs a typical two‐dimensional (2D) transition metal dichalcogenides (TMDCs) material with nonzero band gap, MoS2 has a wide range of potential applications as building blocks in the field of nanoelectronics. The stability and reliability of the corresponding nanoelectronic devices depend critically on the mechanical performance and cyclic reliability of 2D MoS2. Although an in situ technique has been used to analyze the mechanical properties of 2D materials, the cyclic mechanical behavior, that is, fatigue, remains a major challenge in the practical application of the devices. This study was aimed at analyzing the planar cyclic performance and deformation behavior of three‐layer MoS2 nanosheets (NSs) using an in situ transmission electron microscopy (TEM) variable‐amplitude uniaxial low‐frequency and cyclic loading–unloading tensile acceleration test. We also elucidated the strengthening effect of the natural overlaying affix fragments (other external NSs) or wrinkle folds (internal folds from the NS itself) on cycling performances and service life of MoS2 NSs by delaying the whole process of fatigue crack initiation, propagation, and fracture. The results have been confirmed by molecular dynamics (MDs) simulations. The overlaying enhancement effect effectively ensures the long‐term reliability and stability of nanoelectronic devices made of few‐layer 2D materials.image

Temperature-dependent photoconductivity in two-dimensional MoS2 transistors

The photoconductivity in monolayer MoS2 back-gate transistors is studied as a function of temperature and pressure. The photocurrent increases linearly with the light intensity up to a maximum responsivity of ∼30 A/W in air. Time-resolved photocurrent measurements confirm that the photoresponse is dominated by the photogating effect. The device shows slow photoresponse with two-time constants that are attributed to the photobolometric effect and the desorption of adsorbates, respectively. An enhancement of the photocurrent is observed above room temperature and below the atmospheric pressure, that is, when the photoinduced desorption of adsorbates such as O2 and H2O molecules is facilitated. Indeed, the light-induced removal of adsorbates from the surface of MoS2 enhances the n-doping level and the current of the channel. Moreover, at lower pressures, the reverse mechanism of re-adsorption in dark conditions is suppressed and results in a persistent photocurrent. The study clarifies the photocurrent relaxation dynamics and unveils the key role of surface adsorbates in the optoelectronic properties of monolayer MoS2 and other similar 2D materials.

Structural, electronic, optical, and thermoelectric properties of 2D honeycomb-like 1T-Rb2S monolayer: A DFT study

The unique and alluring properties of 2D materials in contrast to the bulk counterparts have fascinated many researchers worldwide. They show excellent tunable properties due to the quantum confinement effect. In this study, ab initio simulations for the 1T-Rb2S monolayer have been performed based on the Density Functional Theory (DFT). DFT calculations are carried out by the PBE-GGA exchange-correlation functional via Wien2K software. On performing electronic and structural studies, the Rb2S monolayer shows an indirect band gap and is a semiconductor that has a bandgap value of about 1.64 eV. The dynamical and structural stability of the Rb2S monolayer is validated via phonon calculations. The optical properties were studied to determine the photovoltaic applications of the material in the nano regime. The optical properties of the Rb2S were carried out using the OPTIC package in Wien2k. The optical properties such as dielectric function (ε), absorption coefficient (α), Energy Loss (L), Reflectivity (R), and the refractive index (n), as a function of energy, have been computed. Rb2S monolayers display more prominent results in the ultra-violet region than the visible region ensuring that Rb2S monolayers are exceptional candidates for photovoltaic applications. The thermal properties of the 2D 1T-Rb2S have been computed by employing BoltzTraP interface, the thermoelectric analysis such as electrical conductivity, electronic thermal conductivity, Seebeck coefficient (S), the figure of merit (ZT) and the power factor (PF) has been computed and it rigidly ensures that the Rb2S monolayer is a potential candidate for thermoelectric device applications. The charge transfer mechanism is analysed by computing the mobility of charge carriers (ϑ), effective mass (meff) and relative ratio of the effective mass (D).

Unveiling localized electronic properties of ReS2 thin layers at nanoscale using Kelvin force probe microscopy combined with tip-enhanced Raman spectroscopy

Electronic properties of two-dimensional (2D) materials can be strongly modulated by localized strain. The typical spatial resolution of conventional Kelvin probe force microscopy (KPFM) is usually limited in a few hundreds of nanometers, and it is difficult to characterize localized electronic properties of 2D materials at nanoscales. Herein, tip-enhanced Raman spectroscopy (TERS) is proposed to combine with KPFM to break this restriction. TERS scan is conducted on ReS2 bubbles deposited on a rough Au thin film to obtain strain distribution by using the Raman peak shift. The localized contact potential difference (CPD) is inversely calculated with a higher spatial resolution by using strain measured by TERS and CPD-strain working curve obtained using conventional KPFM and atomic force microscopy. This method enhances the spatial resolution of CPD measurements and can be potentially used to characterize localized electronic properties of 2D materials.

A Hybrid‐Density Functional Theory Study of Intrinsic Point Defects in MX2 (M = Mo, W; X = S, Se) Monolayers

Defects can strongly influence the electronic, optical, and mechanical properties of 2D materials, making defect stability under different thermodynamic conditions crucial for material–property engineering. Herein, an account of the structural and electronic characteristics of point defects in monolayer transition metal dichalcogenides MX2 with M = Mo/W and X = S/Se is investigated through density functional theory using the hybrid HSE06 exchange–correlation functional including many‐body dispersion corrections. For the simulation of charged defects, a charge compensation scheme based on the virtual crystal approximation (VCA) is employed. The study relates the stability and the electronic structure of charged vacancy defects in monolayer MoS2 to an explicit calculation of the S monovacancy in MoS2 supported on Au(111), and finds convincing indication that the defect is negatively charged. Moreover, it is shown that the finite‐temperature vibrational contributions to the free energy of defect formation can change the stability transition between adatoms and monovacancies by 300–400 K. Finally, defect vibrational properties are probed by calculating a tip‐enhanced Raman scattering image of a vibrational mode of a MoS2 cluster with and without an S monovacancy.

Photoluminescence of Two-Dimensional MoS2 Nanosheets Produced by Liquid Exfoliation

Extraordinary properties of two-dimensional materials make them attractive for applications in different fields. One of the prospective niches is optical applications, where such types of materials demonstrate extremely sensitive performance and can be used for labeling. However, the optical properties of liquid-exfoliated 2D materials need to be analyzed. The purpose of this work is to study the absorption and luminescent properties of MoS2 exfoliated in the presence of sodium cholate, which is the most often used surfactant. Ultrasound bath and mixer-assisted exfoliation in water and dimethyl sulfoxide were used. The best quality of MoS2 nanosheets was achieved using shear-assisted liquid-phase exfoliation as a production method and sodium cholate (SC) as a surfactant. The photoluminescent properties of MoS2 nanosheets varied slightly when changing the surfactant concentrations in the range C(SC) = 0.5–2.5 mg/mL. This work is of high practical importance for further enhancement of MoS2 photoluminescent properties via chemical functionalization.

Revisiting the Hetero-Interface of Electrolyte/2D Materials in an Electric Double Layer Device.

Electric double layer (EDL) devices based on 2D materials have made great achievements for versatile electronic and opto-electronic applications; however, the ion dynamics and electric field distribution of theEDL at the electrolyte/2D material interface and their influence on the physical properties of 2D materials have not been clearly clarified. In this work, by using Kelvin probe force microscope and steady/transient optical techniques, the character of the EDL and its influence on the optical properties of monolayer transition metal dichalcogenides (TMDs) are probed. The potential drop, unscreened EDL potential distribution, and accumulated carriers at the electrolyte/TMD interface arerevealed, which can be explained by nonlinear Thomas-Fermi theory. By monitoring the potential distribution along the channel, the evolution of the electric field-induced lateral junction in the TMD EDL transistor isaccessed, giving rise to the better exploration of EDL device physics. More importantly, EDL gate-dependent carrier recombination and exciton-exciton annihilation in monolayer TMDs on lithium-ion solid state electrolyte (Li2 Al2 SiP2 TiO13 ) areevaluated for the first time, benefiting from the understanding of the interaction between ions, carriers, and excitons. Thework will deepen the understanding of the EDL for the exploitation of functional device applications.

Open-ended exploration of ultrashort pulse lasers: an innovative design strategy for devices based on 2D materials

Ultrashort pulse lasers have vital significance in the field of ultrafast photonics. A saturable absorber (SA) as the core device to generate ultrashort pulses has innovative design strategies; the most interesting of which is the integration strategy based on 2D materials. This review presents recent advances in the optoelectronic properties of 2D materials and in the way the materials are prepared, characterized, and integrated into devices. We have done a comprehensive review of the optical properties of materials and material-based devices and their current development in the field of fiber lasers and solid-state lasers. Finally, we offer a look at future applications for 2D materials in ultrafast lasers and their prospects.

Engineering the Strain and Interlayer Excitons of 2D Materials via Lithographically Engraved Hexagonal Boron Nitride.

Strain engineering has quickly emerged as a viable option to modify the electronic, optical, and magnetic properties of 2D materials. However, it remains challenging to arbitrarily control the strain. Here we show that, by creating atomically flat surface nanostructures in hexagonal boron nitride, we achieve an arbitrary on-chip control of both the strain distribution and magnitude on high-quality molybdenum disulfide. The phonon and exciton emissions are shown to vary in accordance with our strain field designs, enabling us to write and draw any photoluminescence color image in a single chip. Moreover, our strain engineering offers a powerful means to significantly and controllably alter the strengths and energies of interlayer excitons at room temperature. This method can be easily extended to other material systems and offers promise for functional excitonic devices.

Hybrid Metasurfaces of Plasmonic Lattices and 2D Materials

AbstractPlasmonic metasurfaces can significantly enhance the interaction between light and 2D materials. Hybrid structures of plasmonic lattices and 2D materials show great promise for both fundamental and practical studies because of their unprecedented ability for precise manipulation of light at the nanoscale. This review starts with an overview of the basic concepts of plasmonic lattices and optical properties of 2D materials, as well as fabrication strategies for hybrid metasurfaces. Then, the enhanced photoluminescence, quantum emission, optoelectronic detection, nonlinear process, and valleytronics in hybrid metasurfaces are summarized, and their development for nanophotonic functional devices are reviewed. Further, several compelling topics are also outlined that provide outlooks for future directions of hybrid metasurfaces such as novel structural design and high‐quality fabrication, all‐dielectric metasurfaces, dynamic metasurfaces, and plasmonic mediation of chemical reactions and physical processes. It is believed that hybrid metasurfaces of plasmonic lattices and 2D materials can open prospects for versatile platforms for light‐matter interactions and contribute to the revolutions on nanophotonic devices.

Extra-High Mechanical and Phononic Anisotropy in Black Phosphorus Blisters.

Strain is an effective strategy to modulate the electrical, optical, and optoelectronic properties of 2D materials. Conventional circular blisters could generate a biaxial stretching of 2D membranes with notable strain gradients along the hoop direction. However, such a deformation mode cannot be utilized to investigate mechanical responses of in-plane anisotropic 2D materials, for example, black phosphorus (BP), due to its crystallographic orientation dependence. Here, a novel rectangular-shaped bulge device is developed to uniaxially stretch the membrane, and further provide a promising platform to detect orientation-dependent mechanical and optical properties of anisotropic 2D materials. Impressively, the derived anisotropic ratio of Young's modulus of BP flakes is much higher than the values obtained via the nanoindentation method. The extra-high strain-dependent phononic anisotropy in Raman modes along different crystalline orientations is also observed. The designed rectangular budge device expands the uniaxial deformation methods available, allowing to explore the mechanical, and strain-dependent physical properties of other anisotropic 2D materials more broadly.

The effect of different strain on the structural and optical properties of multilayer γ-InSe

Strain engineering is one promising approach to tune electronic and optical properties of 2D materials. But which type mechanical strain is most efficient is unclear. Here we systematically investigate and compare the tuning effect from four different type strains for multilayer γ-InSe, including in-plane uniaxial tensile strain, in-plane biaxial tensile strain, hydrostatic compressive strain and out-of-plane axial compressive strain. The evolution of its phonon modes and luminescent properties upon strain were investigated using in situ Raman and photoluminescence (PL) spectroscopy. All phonon modes exhibit blue shift upon compression strain, while red shift upon tensile strain. Band gap reduction was observed for both compression strain and tensile strain. Strain type has great effect on the shift rate of Raman mode and band gap. Among four types strain, biaxial tensile strain exhibits the largest shift rates, being 8.49, 9.24 and 8.65 cm-1/% for three phonon modes and 85 meV/% for band gap. It suggests that biaxial tensile strain is more efficient for γ-InSe. This result is helpful for practical strain-based manipulation for 2D materials.

Measurement of the conduction band spin-orbit splitting in WSe2 and WS2 monolayers

The conduction band spin-orbit splitting ∆${}_{c}$ has a strong impact on optical, transport, and spin-valley properties of 2D materials based on transition metal dichalcogenide monolayers. Here, the authors study charge tunable devices based on WSe${}_{2}$ and WS${}_{2}$ monolayers. In addition to the well-known radiative recombination of neutral and charged excitons, photoluminescence measurements highlight a weaker intensity optical transition. This line is interpreted as an impurity-assisted recombination of the negatively charged exciton (triplet trion), whose radiative recombination energy differs by ∆${}_{c}$ from that of the trion's direct optically active recombination.

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.

Mech2d: An Efficient Tool for High-Throughput Calculation of Mechanical Properties for Two-Dimensional Materials.

Two-dimensional (2D) materials have been a research hot topic in the passed decades due to their unique and fascinating properties. Among them, mechanical properties play an important role in their application. However, there lacks an effective tool for high-throughput calculating, analyzing and visualizing the mechanical properties of 2D materials. In this work, we present the mech2d package, a highly automated toolkit for calculating and analyzing the second-order elastic constants (SOECs) tensor and relevant properties of 2D materials by considering their symmetry. In the mech2d, the SOECs can be fitted by both the strain-energy and stress-strain approaches, where the energy or strain can be calculated by a first-principles engine, such as VASP. As a key feature, the mech2d package can automatically submit and collect the tasks from a local or remote machine with robust fault-tolerant ability, making it suitable for high-throughput calculation. The present code has been validated by several common 2D materials, including graphene, black phosphorene, GeSe2 and so on.