Polarity-Reversed Robust Carrier Mobility in Monolayer MoS2 Nanoribbons

Emerging pnictogen-based 2D semiconductors: sensing and electronic devices

Pnictogens are an intensively studied group of monoelemental two-dimensional materials. This group of elements consists of phosphorus, arsenic, antimony, and bismuth. In this group, the elements adopt two different layered structural allotropes, orthorhombic structure with true van der Waals layered interactions and rhombohedral structure, where covalent interactions between layers are also present. The orthorhombic structure is well known for phosphorus and arsenic, and the rhombohedral structure is the most thermodynamically stable allotropic modification of arsenic, antimony, and bismuth. Due to the electronic structure of pnictogen layers and their semiconducting character, these materials have huge application potential for electronic devices such as transistors and sensors including photosensitive devices as well as gas and electrochemical sensors. While photodetection and gas sensing applications are often related to lithography processed materials, chemical sensing proceeds in a liquid environment (either aqueous or non-aqueous) and can be influenced by surface oxidation of these materials. In this review, we explore the current state of pnictogen applications in sensing and electronic devices including transistors, photodetectors, gas sensors, and chemical/electrochemical sensors.

Strong anisotropy and layer-dependent carrier mobility of two-dimensional semiconductor ZrGeTe4

Layered ZrGeTe4is a new type of ternary anisotropic semiconductor. The strong in-plane anisotropy may give us another degree of freedom for controlling electrical and optical properties, and designing advanced nanodevices. Using first-principles calculations, physical properties such as band structure, phonon vibration, and carrier mobility of layered ZrGeTe4from bulk to monolayer were investigated. The bulk and few-layer ZrGeTe4are predicted as indirect bandgap semiconductors, but the monolayer ZrGeTe4turns out to be a direct band gap semiconductor with moderate value of 1.08 eV. Electronic structure calculations reveal that the van der Waals interaction is the main reason of causing the transition from indirect band gap to direct one. Phonon calculations demonstrate that the layered ZrGeTe4is mechanically stable and anisotropic. In orders of magnitude, the predicted average carrier mobility of ZrGeTe4(∼103cm2V-1s-1) is between that of graphene (∼105) and MoS2(∼102), and the anisotropy of electronic mobility is similar to that of black phosphorus, while hole mobility varies with the numbers of layers.

DFT coupled with NEGF study of the electronic properties and ballistic transport performances of 2D SbSiTe3

Identifying novel 2D semiconductors with promising electronic properties and transport performances for the development of electronic and optoelectronic applications is of utmost importance. Here, we show a detailed study of the electronic properties and ballistic quantum transport performance of a new 2D semiconductor, SbSiTe3, based on density functional theory (DFT) and non-equilibrium Green's function (NEGF) formalism. Promisingly, monolayer SbSiTe3 owns an indirect band gap of 1.61 eV with a light electron effective mass (0.13m0) and an anisotropic hole effective mass (0.49m0 and 1.34m0). The ballistic performance simulations indicate that the 10 nm monolayer SbSiTe3 n- and p-MOSFETs display a steep subthreshold swing of about 80 mV dec-1 and a high on/off ratio (106), which indicate a good gate-controlling capability. As the channel length of SbSiTe3 decreases to 5 nm, its p-MOSFET also effectively suppresses the intra-band tunneling. Therefore, 2D SbSiTe3 is a potential semiconductor for future nanoelectronics.

Monolayer Bi2Se3-xTex: novel two-dimensional semiconductors with excellent stability and high electron mobility

Two-dimensional materials play a vital role in next-generation microelectronics, optoelectronics and flexible electronics due to their novel physical properties caused by quantum-confinement effects. In this work, we investigate the stability and the possibility of exfoliation of monolayer Bi₂Se_3-xTe_x (x = 0, 1, 2) using first-principles calculations. Our calculations show that these materials are indirect bandgap semiconductors, and the elastic modulus is smaller than other conventional materials, which indicates better flexibility. We find that the electron mobility of monolayer Bi₂SeTe₂ along the armchair direction is higher than that of black phosphorene, reaching 2708 cm² V^-1 s^-1, and the electron mobility of monolayer Bi₂Se₃ along the zigzag direction is about 24 times larger than the hole mobility. The remarkable electron mobilities and highly anisotropic properties of these new monolayers pave the way for future applications in high-speed (opto)electronic devices.

Strain engineering and lattice vibration manipulation of atomically thin TaS2 films

Beside the extraordinary structural, mechanical and physical properties of two-dimensional (2D) materials, the capability to tune properties via strain engineering has shown great potential for nano-electromechanical systems. External strain, in a controlled manner, can manipulate the optical and electronic properties of the 2D materials. We observed the lattice vibration modulation in strained mono- and few-layer tantalum sulfide (TaS₂). Two Raman modes, E_1g and E¹_2g, exhibit sensitive strain dependence, with the frequency of the former intensity increasing and the latter decreasing under a compressive strain. The opposite direction of the intensity shifts, which cannot be explained solely by van der Waals interlayer coupling, is attributed to strain-induced competition between the electron–phonon interlayer coupling and possible stacking-induced changes of the intralayer transport. Our results enrich the understanding of the lattice vibration of TaS₂ and point to strain engineering as a powerful tool for tuning the electron–phonon coupling of 2D materials.

We observed lattice vibration modulation in strained mono- and few-layer tantalum sulfide. E_1g and E_2g exhibit sensitive strain dependence with the frequency of the former intensity increasing and the latter decreasing under a compressive strain.

Room Temperature Bound Excitons and Strain-Tunable Carrier Mobilities in Janus Monolayer Transition-Metal Dichalcogenides

The successful synthesis of Janus transition metal dichalcogenides offers new opportunities in two-dimensional materials due to its novel properties induced by structural mirror asymmetry. Herein, by using the first-principle calculations, the thermodynamical stability for monolayers MoSSe and WSSe is demonstrated by phonon dispersion with no imaginary frequencies. No longitudinal optical-transverse optical (LO-TO) splitting exists at the Γ point and phonon frequencies are red-shifted, since the 2D Coulomb screening effect is introduced to eliminate the spurious interaction between adjacent layers. An indirect-direct-indirect transition in band gaps for both MoSSe and WSSe is observed, and tunable mobilities can be realized by uniaxial strain, reaching up to 10⁶ cm² V^-1 s^-1 when applying 2% tensile strain along the zigzag direction to monolayer MoSSe, which provides a good platform for flexible electronic devices. Large band gaps of 2.569 and 2.666 eV are predicted for monolayers MoSSe and WSSe when considering many-body quasiparticle corrections. The enhanced electron-hole interaction caused by a weak screening effect leads to considerable binding energies for both MoSSe and WSSe, and such tightly binding excitons with large oscillator strengths generate strong absorption peaks in visible region. The remarkable properties of Janus monolayers MoSSe and WSSe make them promising in next-generation microelectronic, optoelectronic, and valleytronic devices.

Stability, electronic and mechanical properties of chalcogen (Se and Te) monolayers

The successful experimental fabrication of 2D tellurium (Te) has resulted in growing interest in the monolayers of group VI elements. By employing density functional theory, we have explored the stability and electronic and mechanical properties of 1T-MoS₂-like chalcogen (α-Se and α-Te) monolayers. Phonon spectra are free from imaginary modes suggesting these monolayers to be dynamically stable. The stability of these monolayers is further confirmed by room temperature AIMD simulations. Both α-Se and α-Te are indirect gap semiconductors with a band gap (calculated using the hybrid HSE06 functional) of 1.16 eV and 1.11 eV, respectively, and these gaps are further tunable with mechanical strains. Both monolayers possess strong absorption spectra in the visible region. The ideal strengths of these monolayers are comparable with those of many existing 2D materials. Significantly, these monolayers possess ultrahigh carrier mobilities of the order of 10³ cm² V^-1 s^-1. Combining the semiconducting nature, visible light absorption and superior carrier mobilities, these monolayers can be promising candidates for the superior performance of next-generation nanoscale devices.

Thermoelectric Properties of NiCl3 Monolayer: A First-Principles-Based Transport Study

By employing the first-principles-based transport theory, we investigate the thermoelectric performance based on the structural and electronic properties of NiCl 3 monolayer. The NiCl 3 monolayer is confirmed to be a stable Dirac spin gapless semiconductor with the linear energy dispersion having almost massless carrier, high carrier mobility and fully spin-polarization. Further, NiCl 3 monolayer processes the optimum power factor of 4.97 mWm - 1 K - 2 , the lattice thermal conductivity of 1.89 Wm - 1 K - 1 , and the dimensionless figure of merit of 0.44 at room temperature under reasonable carrier concentration, indicating that NiCl 3 monolayer may be a potential matrix for promising thermoelectrics.

Prediction of staggered stacking 2D BeP semiconductor with unique anisotropic electronic properties

By comprehensive structure design and first-principles calculations, we report a novel two-dimensional (2D) BeP nanomaterial with exotic structural and properties. This BeP 2D material is formed by a couple honeycomb sheets by slab staggered stacking and strong interlayer bondings. It behaves as a natural 2D semiconductor with several notable properties: a modest bandgap (~1.34 eV), high room-temperature electron mobility (~10⁴ cm² V^-1 s^-1) and high visible-light absorption coefficient (~10⁵ cm^-1); Moreover, due to the unique stacking topology, BeP may display distinctive direction-dependent electric transport by the anisotropic polarity of electron and hole mobilities, that is, it exhibits n-type (electron mobility  >  hole mobility) along the armchair direction while acts as p -type (hole mobility  >  electron mobility) in the zigzag direction, thus promising for applications in nanoelectronics. The BeP has good dynamic and thermal stabilities and is also the lowest-energy structure of 2D space indicated by particle swarm search, implying the high feasibility of experimental synthesis.

Electronegativity, phase transition, and ferroelectricity of TeSe2 few-layers

We use first-principles calculations to demonstrate that γ TeSe₂ few-layers (FLs) are significantly more stable than α and β FLs due to the difference in the electronegativity of two kinds of atoms, while γ Te FLs are not due to the unfavorable multivalency of Te atoms. The quasiparticle single-shot G₀W₀ band gaps are 1.13 and 2.30 eV for γ and β monolayers (MLs), respectively. Therefore, they will be useful for optoelectronics operating at room temperature, which is further supported by their dynamic and thermal stability. The γ ML and bilayer (BL) are expected to undergo phase transitions to β ML and α BL under hole doping. Furthermore, the ionicity brings about spontaneous electric polarization in the α BL that is approximately 60% larger than that in the α Te BL. Its ferroelectricity (FE) is comparable to that of SnTe ML, the only 2D FE material experimentally identified up to now. The polarization can be further enhanced by more than 75% under uniaxial tensile strain.

Interfacing Boron Monophosphide with Molybdenum Disulfide for an Ultrahigh Performance in Thermoelectrics, Two-Dimensional Excitonic Solar Cells, and Nanopiezotronics

A stable ultrathin 2D van der Waals (vdW) heterobilayer, based on the recently synthesized boron monophosphide (BP) and the widely studied molybdenum disulfide (MoS₂), has been systematically explored for the conversion of waste heat, solar energy, and nanomechanical energy into electricity. It shows a gigantic figure of merit (ZT) > 12 (4) for p (n)-type doping at 800 K, which is the highest ever reported till date. At room temperature (300 K), ZT reaches 1.1 (0.3) for p (n)-type doping, which is comparable to experimentally measured ZT = 1.1 on the PbTe-PbSnS₂ nanocomposite at 300 K, while it outweighs the Cu₂Se-CuInSe₂ nanocomposite (ZT = 2.6 at 850 K) and the theoretically calculated ZT = 7 at 600 K on silver halides. Lattice thermal conductivity (κ_l ≈ 49 W m^-1 K^-1) calculated at room temperature is lesser than those of black phosphorene (78 W m^-1 K^-1) and arsenene (61 W m^-1 K^-1). The nearly matched lattice constants in the commensurate lattices of the constituent monolayers help to preserve the direct band gap at the K point in the type II vdW heterobilayer of MoS₂/BP, where BP and MoS₂ serve as donor and acceptor materials, respectively. An ultrahigh carrier mobility of ∼20 × 10³ cm² V^-1 s^-1 is found, which exceeds those of previously reported transition metal dichalcogenide-based vdW heterostructures. The exciton binding energy (0.5 eV) is close to those of MoS₂ (0.54 eV) and C₃N₄ (0.33 eV) single layers. The calculated power conversion efficiency (PCE) in the monolayer MoS₂/BP heterobilayer exceeds 20%. It surpasses the efficiency in MoS₂/p-Si heterojunction solar cells (5.23%) and competes with the theoretically calculated ones, as listed in the manuscript. Furthermore, a high optical absorbance (∼10⁵ cm^-1) of visible light and a small conduction band offset (0.13 eV) make MoS₂/BP very promising in 2D excitonic solar cells. The out-of-plane piezoelectric strain coefficient, d₃₃ ≈ 3.16 pm/V, is found to be enhanced 4-fold (∼14.3 pm/V) upon applying 7% vertical compressive strain on the heterobilayer, which corresponds to ∼1 kbar of hydrostatic pressure. Such a high out-of-plane piezoelectric coefficient, which can tune top-gating effects in ultrathin 2D nanopiezotronics, is a relatively new finding. As BP has been synthesized recently, experimental realization of the multifunctional, versatile MoS₂/BP heterostructure would be highly feasible.

Semiconducting SN2 monolayer with three-dimensional auxetic properties: a global minimum with tetracoordinated sulfurs

Designing new two-dimensional (2D) materials, exploring their unique properties and diverse potential applications are of paramount importance to condensed matter physics and materials science. In this work, we predicted a novel 2D SN2 monolayer (S-SN2) by means of density functional theory (DFT) computations. In the S-SN2 monolayer, each S atom is tetracoordinated with four N atoms, and each N atom bridges two S atoms, thus forming a tri-sublayer structure with square lattice. The monolayer exhibits good stability, as demonstrated by the moderate cohesive energy, all positive phonon modes, and the structural integrity maintained through 10 ps molecular dynamics simulations up to 1000 K. It is an indirect-bandgap semiconductor with high hole mobility, and the bandgap can be tuned by changing the thickness and external strains (the indirect-bandgap to direct-bandgap transition occurs when the biaxial tensile strain reaches 4%). Significantly, it has large Young's modulus and three-dimensional auxetic properties (both in-plane and out-of-plane negative Poisson's ratios). Therefore, the S-SN2 monolayer holds great potential applications in electronics, photoelectronics and mechanics.

Strain-engineering the in-plane electrical anisotropy of GeSe monolayers

The anisotropic ratio of the effective mass and mobility of charge carriers of GeSe monolayer along two principle axes can be controlled by using simple strain conditions.

First-principles simulation of monolayer hydrogen passivated Bi2O2S2–metal interfaces

Lateral SBH and Fermi level change in the hydrogen-passivated Bi₂O₂S₂ FET.

Strain-enhanced power conversion efficiency of a BP/SnSe van der Waals heterostructure

The BP/SnSe vdW heterostructure is a promising photovoltaic materials and the power conversion efficiency can reach to 17.24%.

Electronic and photocatalytic properties of two-dimensional boron phosphide/SiC van der Waals heterostructure with direct type-II band alignment: a first principles study

Designing van der Waals (vdW) heterostructures of two-dimensional materials is an efficient way to realize amazing properties as well as opening opportunities for applications in solar energy conversion and nanoelectronic and optoelectronic devices. In this work, we investigate the electronic, optical, and photocatalytic properties of a boron phosphide–SiC (BP–SiC) vdW heterostructure using first-principles calculations. The relaxed configuration is obtained from the binding energies, inter-layer distance, and thermal stability. We show that the BP–SiC vdW heterostructure has a direct band gap with type-II band alignment, which separates the free electrons and holes at the interface. Furthermore, the calculated absorption spectra demonstrate that the optical properties of the BP–SiC heterostructure are enhanced compared with those of the constituent monolayers. The intensity of optical absorption can reach up to about 10⁵ cm⁻¹. The band edges of the BP–SiC heterostructure are located at energetically favourable positions, indicating that the BP–SiC heterostructure is able to split water under working conditions of pH = 0–3. Our theoretical results provide not only a fascinating insight into the essential properties of the BP–SiC vdW heterostructure, but also helpful information for the experimental design of new vdW heterostructures.

We investigate the structural, electronic, optical and photocatalytic properties of boron phosphide and SiC monolayers and their corresponding van der Waals heterostructure by density functional theory.

Copper halide diselenium: predicted two-dimensional materials with ultrahigh anisotropic carrier mobilities

On the basis of first-principles calculations, we discuss a new class of two-dimensional materials—CuXSe₂ (X = Cl, Br) nanocomposite monolayers and bilayers—whose bulk parent was experimentally reported in 1969. We show the monolayers are dynamically, mechanically and thermodynamically stable and have very small cleavage energies of ∼0.26 J m⁻², suggesting their exfoliation is experimentally feasible. The monolayers are indirect-gap semiconductors with practically the same moderate band gaps of 1.74 eV and possess extremely anisotropic and very high carrier mobilities (e.g., their electron mobilities are 21 263.45 and 10 274.83 cm² V⁻¹ s⁻¹ along the Y direction for CuClSe₂ and CuBrSe₂, respectively, while hole mobilities reach 2054.21 and 892.61 cm² V⁻¹ s⁻¹ along the X direction). CuXSe₂ bilayers are also indirect band gap semiconductors with slightly smaller band gaps of 1.54 and 1.59 eV, suggesting weak interlayer quantum confinement effects. Moreover, the monolayers exhibit high absorption coefficients (>10⁵ cm⁻¹) over a wide range of the visible light spectra. Their moderate band gaps, very high unidirectional electron and hole mobilities, and pronounced absorption coefficients indicate the proposed CuXSe₂ (X = Cl, Br) nanocomposite monolayers hold significant promise for application in optoelectronic devices.

The physical and bonding properties of a new class of two-dimensional materials – CuXSe₂ (X = Cl, Br) – are investigated using first-principles methods. 2D CuXSe₂ are indirect band gap and possess extremely anisotropic and very high carrier mobilities.

A van der Waals Heterostructure Based on Graphene-like Gallium Nitride and Boron Selenide: A High-Efficiency Photocatalyst for Water Splitting

Hydrogen generation by photocatalytic water splitting has attained more and more research interests in the recent years since the solar energy can be directly transferred and stored as hydrogen. However, the search for a high-efficiency photocatalyst for water splitting is a really challenge. In this paper, we designed a novel 2D material-based van der Waals heterostructure (vdWH) composed by g-GaN and BSe, which is thermally stable at room temperature. The g-GaN/BSe vdWH has suitable band-edge positions for the oxidation and reduction reactions of water splitting at pH 0 and 7. The carrier mobility of this heterostructure is high, indicating the effective occurrence of reactions for water splitting. The g-GaN/BSe vdWH also possesses a type-II band alignment, which can promote the separation of the photogenerated electron-hole pairs constantly. Moreover, a large built-in electric field can be established at the interface, which will further prevent the recombination of photogenerated charges. In addition, the g-GaN/BSe vdWH also exhibits outstanding sunlight-absorption ability, and the biaxial strain can further enhance this ability. Thus, we conclude that the g-GaN/BSe vdWH can act as a high-efficiency photocatalyst for water splitting.

2D Ca3 Sn2 S7 Chalcogenide Perovskite: A Graphene-Like Semiconductor with Direct Bandgap 0.5 eV and Ultrahigh Carrier Mobility 6.7 × 104 cm2 V-1 s-1

Graphene, a star 2D material, has attracted much attention because of its unique properties including linear electronic dispersion, massless carriers, and ultrahigh carrier mobility (10⁴ -10⁵ cm² V^-1 s^-1 ). However, its zero bandgap greatly impedes its application in the semiconductor industry. Opening the zero bandgap has become an unresolved worldwide problem. Here, a novel and stable 2D Ruddlesden-Popper-type layered chalcogenide perovskite semiconductor Ca₃ Sn₂ S₇ is found based on first-principles GW calculations, which exhibits excellent electronic, optical, and transport properties, as well as soft and isotropic mechanical characteristics. Surprisingly, it has a graphene-like linear electronic dispersion, small carrier effective mass (0.04 m₀ ), ultrahigh room-temperature carrier mobility (6.7 × 10⁴ cm² V^-1 s^-1 ), Fermi velocity (3 × 10⁵ m s^-1 ), and optical absorption coefficient (10⁵ cm^-1 ). Particularly, it has a direct quasi-particle bandgap of 0.5 eV, which realizes the dream of opening the graphene bandgap in a new way. These results guarantee its application in infrared optoelectronic and high-speed electronic devices.

Electronic and transport properties and physical field coupling effects for net-Y nanoribbons

Recently, a new type of quasi-1D graphene-like nanoribbons, periodically embedded with four- and eight- membered rings, has been successfully fabricated, and based on this structure, a novel planar 2D carbon allotrope, the so-called the net-Y, has been proposed. Here, we study various nanoribbons derived from such a 2D monolayer focusing on the structure stability, electronic, and transport properties, especially on the physical field coupling effects of electronic behaviors. Very high stability is predicted for various types of nanoribbons by the calculated binding energy and molecular dynamics simulation. Different edge shapes and widths have a significant influence on their electronic properties. Armchair nanoribbons are always semiconductors, and possess a high carrier mobility. After hydrogen termination, some metallic nanoribbons can become semiconductors or quasi-metals with massless Dirac-fermion behavior. In particular, the electronic properties of ribbons can be effectively modulated by applying strain and electric field. The band gap size and the transition from indirect to direct band gap can be realized upon strain or electric field. These flexibly tunable electronic properties for nanoribbons expand their applications in nanoelectronics and optoelectronics.

Control of highly anisotropic electrical conductance of tellurene by strain-engineering

Tailoring the electronic anisotropy of two-dimensional (2D) semiconductors with strain-engineering is critical in nanoelectronics. Recently, 2D tellurene has been predicted theoretically and fabricated experimentally. It has potential applications in nanoelectronics, in particular, β-phase tellurene (β-Te) shows a desirable direct band gap (1.47 eV), high carrier mobility (2.58 × 103 cm2 V-1 s-1) and high stability under ambient conditions. In this work, we demonstrated, with first-principles density functional theory calculations, that the highly anisotropic electron mobility and electrical conductance of β-Te can be controlled by strain-engineering. The direction of electrical conductance of β-Te can be changed from the armchair to the zigzag direction at the strain between -1% and 0%. Meanwhile, we found that the bandgap of β-Te under strain experiences an indirect-direct transition with a conduction band minimum (CBM) shift from the X to Γ point. The significant dispersion of the bottom of the conduction bands along the Γ-Y direction switches to the X-Γ direction under uniaxial or biaxial strain which makes the rotation of the effective masses tensor. The qualitative rotation of the spatial anisotropic electron effective masses tensor by 90° also rotates the direction of the electrical conduction as the carrier mobility is inversely dependent on the effective masses. On the another hand, we also found that the deformation potential constant also plays an important role in the rotation of electrical conductance anisotropy. While anisotropic conductance of hole is impregnable under strain. In order to verify that β-Te can sustain large strain, we studied its stability and mechanical properties and found that β-Te shows superior mechanical flexibility with a small Young's modulus (27.46 GPa (armchair)-61.99 GPa (zigzag)) and large anisotropic strain-stress (12.89 N m-1 at the strain of 38% along armchair direction and 25.72 N m-1 at the strain of 26% along zigzag direction). The high anisotropic carrier mobility and superior mechanical flexibility of β-Te make it a promising candidate for flexible nanoelectronics.

Dependence of the magnetic interactions in MoS2 monolayer on Mn-doping configurations

Understanding the magnetic properties of the various Mn doping configurations that can be encountered in 2H-MoS₂ monolayer could be beneficial for its use in spintronics. Using density functional theory plus Hubbard term (DFT  +  U) approach, we study how a single isolated, double- and triple-substitution configurations of Mn atoms within a MoS₂ monolayer could contribute to its total magnetization. We find that the doping-configuration plays a critical role in stabilizing a ferromagnetic state in a Mn-doped MoS₂ monolayer. Indeed, the Mn-Mn magnetic interaction is found to be ferromagnetic and strong for Mn in equidistant substitution positions where the separation average range of 6-11 [Formula: see text]. The strongest ferromagnetic interaction is found when substitutions are in second nearest neighbor Mo-sites of the armchair chain. Clustering is energetically favorable but it strongly reduces the ferromagnetic exchange energies. Furthermore, in term of electronic properties, we show that the Mn-doped MoS₂ monolayer can change its electronic behavior from semiconductor to half-metallic depending on the doping configuration. Our results suggest that ordering the Mn dopants on MoS₂ monolayer is needed to increase its potential ferromagnetism.

Electronic properties of polymorphic two-dimensional layered chromium disulphide

Two-dimensional (2D) Cr-based layered and non-layered materials such as CrI₃, Cr₂Ge₂Te₆, Cr₂S₃, CrSe, and CrOX (X = Cl and Br) have attracted considerable attention due to their potential application in spintronics. Despite few experimental studies, theoretical studies reported that 2D chromium dichalcogenide (CrS₂) materials show unique properties such as valley polarization, piezoelectric coupling, and phase dependent intrinsic magnetic properties. Here, we report for the first time the synthesis of 2D layered CrS₂ flakes down to the monolayer via the chemical vapor deposition (CVD) method, its phase structures and electronic properties. We observed the 2H, 1T, and 1T' phases coexisting in CVD grown monolayer CrS₂. The formation of 1T' phases from 1T phases is described by dimerization of metal atoms at room temperature according to our molecular dynamics studies. The coexistence of 1T and 1T' phases with 2H phases is referred to as the 1T and 1T' puddling phenomenon. We theoretically showed that the monolayer 2H-CrS₂ is a direct bandgap semiconductor with a gap of approximately 0.95 eV predicted by the PBE functional, while the 1T- and 1T'-CrS₂ are metallic and semi-metallic with approximately 10 meV gap, respectively. Furthermore, 2H CrS₂ exhibits nonmagnetic semiconducting properties while for ferromagnetic spin configuration, the 1T and 1T' CrS₂ show magnetic characteristics with 0.531μ_B and 2.206μ_B magnetic moment per Cr atom respectively, for ferromagnetic spin configuration as predicted from DFT+U calculation. Importantly, CrS₂-based field-effect transistors exhibit a p-type behavior. Our study would stimulate further exploration of 2D layered CrS₂ with astonishing properties and open up a whole new avenue for the urgent need for developing multifunctional 2D materials for nanoelectronics, valleytronics, and spintronics.

Monolayer SnP3: an excellent p-type thermoelectric material

Monolayer SnP₃ is a novel two-dimensional (2D) semiconductor material with high carrier mobility and large optical absorption coefficient, implying its potential applications in the photovoltaic and thermoelectric (TE) fields. Herein, we report on the TE properties of monolayer SnP₃ utilizing first principles density functional theory (DFT) together with semiclassical Boltzmann transport theory. Results indicate that it exhibits a low lattice thermal conductivity of ∼4.97 W m^-1 K^-1 at room temperature, mainly originating from its small average acoustic group velocity (∼1.18 km s^-1), large Grüneisen parameters (∼7.09), strong dipole-dipole interactions, and strong phonon-phonon scattering. A large in-plane charge transfer is observed, which results in a non-ignorable bipolar effect on the lattice thermal conductivity. The exhibited mixed mode between in-plane and out-of-plane vibrations enhances the complexity of the phonon phase space, which enhances the possibility of phonon scattering processes and results in suppression of thermal conductivity. A highly twofold degeneracy appearing at the K point gives a high Seebeck coefficient. Our calculated figure of merit (ZT) for optimal p-type doping at 500 K can approach 3.46 along the armchair direction, which is better than the theoretical value of 1.94 reported in the well-known TE material SnSe. Our studies here shed light on monolayer SnP₃ in use as a TE material and supply insights to further optimize the TE properties in similar systems.

Two-dimensional MgX2Se4 (X = Al, Ga) monolayers with tunable electronic properties for optoelectronic and photocatalytic applications

Two new two-dimensional (2D) layered materials, namely, MgX₂Se₄ (X = Al, Ga) monolayers, are predicted to possess novel electronic properties. Ab initio electronic structure calculations show that both MgAl₂Se₄ and MgGa₂Se₄ monolayers are direct-gap semiconductors with bandgaps of 3.14 eV and 2.34 eV, respectively. The bandgap of both 2D materials is very sensitive to the in-plane biaxial strain, while the strain induced bandgap changes allow the tuning of optical absorption from the violet to green-light region. Also importantly, the in-plane electron mobility of both 2D materials is predicted to be as high as ∼0.7-1.0 × 10³ cm² V^-1 s^-1, notably higher than that of the MoS₂ sheet (∼200 cm² V^-1 s^-1), while it is comparable to that of black phosphorene (∼1000 cm² V^-1 s^-1), suggesting their potential application in n-type field-effect transistors. Moreover, suitable bandgap and band-edge alignment make the monolayer MgX₂Se₄ a potential photocatalyst for water splitting. Lastly, we show that MgX₂Se₄ possesses a lower monolayer cleavage energy than that of graphite, indicating easy exfoliation of MgX₂Se₄ layers from their bulk.

Structural engineering of bilayer PtSe2 thin films: a first-principles study

PtSe₂ is an emerging layered two-dimensional material of applied interest. Its monolayer shows promising properties for applications in electronic devices, while the bandgap of a multilayer PtSe₂ film can be tuned via changing its thickness. In this work the bilayer PtSe₂ thin films are investigated as an example of structural engineering with first-principles calculations. Various van der Waals corrections schemes are firstly discussed, and the optB86b scheme shows a better description of the semiconductor-metal transition for PtSe₂ films. Six bilayer PtSe₂ thin films in different stacking modes are constructed in order to structurally tune the electronic and transport properties. The bandgap can be effectively broadened with the structural engineering for wider potential applications. The carrier mobility, dynamical stability and Raman spectra are also calculated and discussed.

Solvent-Exchange Strategy toward Aqueous Dispersible MoS2 Nanosheets and Their Nitrogen-Rich Carbon Sphere Nanocomposites for Efficient Lithium/Sodium Ion Storage

Major challenges in developing 2D transition-metal disulfides (TMDs) as anode materials for lithium/sodium ion batteries (LIBs/SIBs) lie in rational design and targeted synthesis of TMD-based nanocomposite structures with precisely controlled ion and electron transport. Herein, a general and scalable solvent-exchange strategy is presented for uniform dispersion of few-layer MoS₂ (f-MoS₂ ) from high-boiling-point solvents (N-methyl-2-pyrrolidone (NMP), N,N-dimethyl formaldehyde (DMF), etc.) into low-boiling-point solvents (water, ethanol, etc.). The solvent-exchange strategy dramatically simplifies high-yield production of dispersible MoS₂ nanosheets as well as facilitates subsequent decoration of MoS₂ for various applications. As a demonstration, MoS₂ -decorated nitrogen-rich carbon spheres (MoS₂ -NCS) are prepared via in situ growth of polypyrrole and subsequent pyrolysis. Benefiting from its ultrathin feature, largely exposed active surface, highly conductive framework and excellent structural integrity, the 2D core-shell architecture of MoS₂ -NCS exhibits an outstanding reversible capacity and excellent cycling performance, achieving high initial discharge capacity of 1087.5 and 508.6 mA h g^-1 at 0.1 A g^-1 , capacity retentions of 95.6% and 94.2% after 500 and 250 charge/discharge cycles at 1 A g^-1 , for lithium/sodium ion storages, respectively.

Thermoelectric Performance of Two-Dimensional AlX (X = S, Se, Te): A First-Principles-Based Transport Study

By using the first-principles calculations in combination with the Boltzmann transport theory, we systematically study the thermoelectric properties of AlX (X = S, Se, Te) monolayers as indirect gap semiconductors. The unique electronic density of states, which consists of a rather sharp peak at the valence band maxima and an almost constant band at the conduction band minima, makes AlX (X = S, Se, Te) monolayers excellent thermoelectric materials. The optimized power factors at room temperature are 22.59, 62.59, and 6.79 mW m-1 K-2 under reasonable electronic concentration for AlS, AlSe, and AlTe monolayers, respectively. The figure of merit (zT) increases with temperature and the optimized zT values of 0.52, 0.59, and 0.26 at room temperature are achieved under moderate electronic concentration for AlS, AlSe, and AlTe monolayers, respectively, indicating that two-dimensional layered AlX (X = S, Se, Te) semiconductors, especially AlSe, can be potential candidate matrices for high-performance thermoelectric nanocomposites.

Strain-engineered indirect-direct band-gap transitions of PbPdO2 slab with preferred (0 0 2) orientation

Layered transition metal oxide PbPdO₂ has great potential application in electronic devices because of its unique electronic structure and large thermoelectric power at room temperature. In this work, strain effect on the electronic structure of PbPdO₂ slab with preferred (0 0 2) orientation was systematically investigated using first-principles calculation. The calculated results indicate that PbPdO₂ ultrathin slab possesses a small indirect gap while an indirect-direct band gap transition occurs when a moderate 2% compression or tensile strain is applied on the slab. Moreover, this strain induced indirect-direct band gap transition was analyzed in detail using the charge density difference at different point of valence band. The charge transfer and energy barrier with charge polarization resulting from the changes of bond length and angle for Pd-O bonding under the strain, have been accounted for this transition. Remarkablely, for the (0 0 2) preferred orientation PbPdO₂ slab, the predicted carrier mobilities of electrons and holes are 11 645.31 and 694.60 cm² V^-1 s^-1 along the x-axis direction, 935.05 and 16.05 cm² V^-1 s^-1 along the y -axis direction, respectively. These calculated mobilities of electrons along the x-axis direction are larger than those for 2D MoS₂ (~400 cm² V^-1 s^-1), and being comparable to those for InSe (10³ cm² V^-1 s^-1) and black phosphorene (10³-10⁴ cm² V^-1 s^-1). It is strong suggested that the (0 0 2) orientated PbPdO₂ slab with high mobility should be an ideal candidate material for the application of electronics devices.

The sp2 character of new two-dimensional AsB with tunable electronic properties predicted by theoretical studies

As a competitive candidate for replacing graphene that possesses an appropriate fundamental bandgap, structural stability and tunable electronic properties, the recently synthesized honeycomb arsenene has rekindled much enthusiasm in the area of two-dimensional materials. By using first-principles calculations and acoustic phonon limited deformation potential theory, we identify a compelling two-dimensional electronic material, single-layer AsB, which is a direct-gap semiconductor with a bandgap (E_g) of 1.18 eV, almost the same as that of bulk silicon. The orbital projected band structure and electron density as well as partial density of states demonstrate that the frontier state of the planar atomic structural AsB is sp² orbital hybridization, which is distinct from that of buckled arsenene monolayers. Layer thickness, stacking order and strain are effective ways to tune the frontier states, and thus the band structure and bandgap of AsB. Moreover, thicker AsB exhibits one-layer localized states in the AB-stacking structure, which is in sharp contrast to other layered materials such as MoS₂ and phosphorene. Benefiting from the non-localized p_z orbital and larger elastic modulus, the carrier mobility of AsB is in the range of 10³-10⁴ cm² V^-1 s^-1, which is much higher than that of pristine arsenene and some other analogues. Our work provides an effective way to tailor the electronic properties of 2D arsenene, which may open up new avenues for applying it in future nano-optoelectronics and electronics.

Superior Thermoelectric Performance of Ordered Double Transition Metal MXenes: Cr2TiC2T2 (T = -OH or -F)

Using SCAN-rVV10+U, we show Cr₂TiC₂ and Cr₂TiC₂T₂ (T = -F and -OH) MXenes are moderate band gap semiconductors mostly in the antiferromagnetic state. All investigated MXene structures show large Seebeck coefficients (>400 μV/K), especially Cr₂TiC₂ (>800 μV/K) and Cr₂TiC₂F₂ (>700 μV/K). The hole relaxation time of p-type Cr₂TiC₂(OH)₂ is found to be ∼8 ps, ensuring its superior electron transport properties in comparison to other investigated MXenes. It is also discovered that the surface functionalization could decrease the phonon thermal conduction and that Cr₂TiC₂(OH)₂ has the smallest lattice thermal conductivity (∼6.5 W/m·K) and the largest electron thermal conduction (>50 W/m·K with n = 10¹⁹ cm^-3). We predict the ZT value of p-type Cr₂TiC₂(OH)₂ can reach 3.0 at 600 K with the maximum thermoelectric conversion efficiency of 20%. Overall, the thermoelectric property of Cr-based ordered double transition metal MXenes is far superior to that of any known two-dimensional structures in the MXene family.

Highly in-plane anisotropic 2D semiconductors β-AuSe with multiple superior properties: a first-principles investigation

Discovering highly in-plane anisotropic two-dimensional (2D) semiconductors with multiple superior properties (good stability, widely tunable bandgap and high mobility) are of great interest for fundamental studies and for developments of novel (opto)electronic devices. By means of state-of-the-art first-principles calculations, herein we present a thorough investigation on the stability, electronic properties and promising applications of previously unexplored 2D semiconductors-gold-selenium (β-AuSe) with strong in-plane anisotropy, whose layered bulk counterpart was synthesized fifty years ago. We show that they have stable structures, widely tunable bandgap varying from 1.66 eV in monolayer to 0.70 eV in five-layer, strong light absorption coefficient (~10⁵ cm^-1) within the whole visible light range, and high/ultrahigh carrier mobility (10³-10⁵ cm² V ^-1 s ^-1). More importantly, they show highly in-pane anisotropic behaviors in absorption coefficients, photoconductance and carrier mobility. Especially, the anisotropic ratio of carrier mobility is much higher than the literature reported ones. The above findings show that the in-plane anisotropic 2D β-AuSe are promising candidates for developing polarization-sensitive photodetectors, synaptic devices and micro digital inverters based on multiple superior properties and highly anisotropic behaviors. Besides, few-layer β-AuSe systems can serve as channel materials in field-effect transistors with high mobility or be applied in solar cells with strong light absorption. Our findings demonstrate that few-layer 2D β-AuSe have great potential for multifunctional applications and thus stimulate immediately experimental interests.

Two-Dimensional T-NiSe2 as a Promising Anode Material for Potassium-Ion Batteries with Low Average Voltage, High Ionic Conductivity, and Superior Carrier Mobility

Potassium-ion batteries (KIBs) have attracted great attention due to their unique advantages including abundant resources, low redox potential of K, and feasible usage of cheap aluminum current collector in battery assembly. In the present work, through first-principles calculations, we find that the recently synthesized two-dimensional T-NiSe₂ is a promising anode material of KIBs. It possesses a large capacity (247 mAh/g), small diffusion barrier (0.05 eV), and low average voltage (0.49 V), rendering T-NiSe₂ a high-performance KIB anode candidate. In addition, we analyze the carrier mobility of T-NiSe₂, and the results demonstrate that it possesses a superior carrier mobility of 1685 cm²/(V s), showing the potential for applications in nanoelectronic devices.

Halogenation of SiGe monolayers: robust changes in electronic and thermal transport

Phonon and electronic transport of buckled structured SiGe monolayer and halogenated SiGe monolayers (X₂-SiGe, X = F, Cl, and Br) are investigated for the first-time using ab initio density functional theory (DFT). The phonon calculations reveal complete dynamical stability of SiGe and fluorinated (F₂-SiGe) monolayers in contrast to earlier reported works, where a small magnitude of imaginary frequency in SiGe monolayer near the zone centre of the Brillouin zone (BZ) is observed. The phonon calculations of chlorinated and brominated SiGe reveal no dynamical stability even with very high convergence parameters and better computational accuracy. The lower value of lattice thermal conductivity in the case of F₂-SiGe is attributed to the strong phonon anharmonic scattering and larger contribution of the three phonon process to anharmonic scattering. The semimetallic nature of the SiGe monolayer turns to semiconducting after halogenation. We have also calculated the electron relaxation time to study their precise thermoelectric parameters. The enhancement of the Seebeck coefficient and reduction in lattice thermal conductivity in the SiGe monolayer is observed after halogenation which results in the improvement of the thermoelectric figure of merit (ZT). The room temperature figure of merit, ZT, which is 0.112 for the SiGe monolayer, enhances significantly to 0.737 after addition of fluorine atoms. Our study suggests that the halogenation of two-dimensional materials can improve their thermoelectric properties.

Unravelling the physisorption characteristics of H2S molecule on biaxially strained single-layer MoS2

Sensing ultra-low levels of toxic chemicals such as H₂S is crucial for many technological applications. In this report, employing density functional theory (DFT) calculations, we shed light on the underlying physical phenomena involved in the adsorption and sensing of the H₂S molecule on both pristine and strained single-layer molybdenum disulfide (SL-MoS₂) substrates. We demonstrate that the H₂S molecule is physisorbed on SL-MoS₂ for all values of strain, i.e. from -8% to +8%, with a modest electron transfer, ranging from 0.023e^- to 0.062e^-, from the molecule to the SL-MoS₂. According to our calculations, the electron-donating behaviour of the H₂S molecule is halved under compressive strains. Moreover, we calculate the optical properties upon H₂S adsorption and reveal the electron energy loss (EEL) spectra for various concentrations of the H₂S molecule which may serve as potential probes for detecting H₂S molecules in prospective sensing applications based on SL-MoS₂.

Gallium Thiophosphate: An Emerging Bidirectional Auxetic Two-Dimensional Crystal with Wide Direct Band Gap

Two-dimensional (2D) materials with negative Poisson's ratio (NPR) attract considerable attention because of their exotic mechanical properties. We propose a new 2D material, monolayer GaPS₄, which shows NPR for both in-plane (-0.033) and out-of-plane (-0.62) directions. Such coexistence of NPR in two distinct directions could be explained by its corner- and edge-shared tetrahedra pucker structure. GaPS₄ has an ultralow cleavage energy of 0.23 J m^-2 according to our calculation, such that exfoliation of the bulk material is feasible for the preparation of mono- and few-layer GaPS₄. Direct wide band gap of 3.55 eV and moderate electron mobility have been revealed in monolayer GaPS₄, while the direct gap feature is robust within a strain range of -6% to 6%. These findings render 2D GaPS₄ a promising candidate for applications in nanoelectronics and low-dimensional electromechanical devices.

Edge-modulated dual spin-filter effect in zigzag-shaped buckling Ag2S nanoribbons

Unlike MoS2, single-layered Ag2S nanoribbons (Ag2SNRs) exhibit a nonmetal-shrouded and a zigzag-shaped buckling structure and possess two distinct edges, S- or Ag-terminated ones. By performing first principle calculations, the spin-dependent electron transport of Ag2SNRs in a ferromagnetic state has been investigated. It is found that the SS- and AgAg-terminated Ag2SNRs exhibit semi-metallic characteristics, but with opposite spin-polarized directions. And AgS-terminated ones show metallic characteristics, but with completely spin-unpolarized transmission. That is to say, all three states, i.e., spin up polarized, spin down polarized and spin unpolarized ones, could be achieved by modulating the edge geometry. Further analysis shows that, the spatial separation on edges of the energy states with different spins around EF is responsible for the switch in the three states. The system could operate as a dual spin-filter, and the direction of the spin polarization can be switched by the edge morphology. Furthermore, calculations show that such a phenomenon is robust to the width of the ribbon and strain, showing great application potential.

Electronic Doping Controlled Migration of Dislocations in Polycrystalline 2D WS2

Migration of dislocations not only determines the durability of large-scale nanoelectronic and opto-electronic devices based on polycrystalline 2D transition-metal dichalcogenides (TMDCs), but also plays an important role in enhancing the performance of novel memristors. However, a fundamental question of the migration dependence on the electronic effects, which are inevitable in practical field-effect transistors based on 2D TMDCs, and its interplay with different dislocations, remains unexplored. Here, taking WS₂ as an example, first-principle calculations are used to show that the electronic contributions arising from defect states can greatly influence the migration barriers of dislocations. The barrier height can be reduced by as much as 50%, which is mainly attributed to the change in electronic occupation and the band energy of defect levels controlled by electronic chemical potential (Fermi level). The reduced barriers in turn lead to significantly enhanced migration, and thus the plasticity. Since defect levels from dislocations locate deep inside the bandgap, the doping-induced tuning of barrier height can be achieved at relatively low doping concentration through either chemical doping or electrode gating. The effective electromechanical coupling in 2D TMDCs can provide new opportunities in material engineering for various potential applications.

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

Coordination-related, 2D structural phase transitions are a fascinating facet of 2D materials with structural degeneracy. Phosphorene and its new phases, exhibiting unique electronic properties, have received considerable attention. The 2D group IV-IV monochalcogenides (i.e. GeS, GeSe, SnS and SnSe) like black phosphorous possess puckered layered orthorhombic structure. The 2D group IV-IV monochalcogenides with advantages of earth-abundance, less toxicity, environmental compatibility and chemical stability, can be widely used in optoelectronics, piezoelectrics, photodetectors, sensors, Li-batteries and thermoelectrics. In this review, we summarized recent research progress in theory and experiment, which studies the fundamental properties, applications and fabrication of 2D group IV-IV monochalcogenides and their new phases, and brings new perspectives and challenges for the future of this emerging field.

Deriving MoS2 nanoribbons from their flakes by chemical vapor deposition

Two-dimensional (2D) materials have attracted great interest due to their unique structures and exotic properties related to promising applications and fundamental research. Reducing the dimensionality of 2D materials into their 1D nanostructure is also highly desirable for the exploitation of novel properties and offers new research opportunities. In this work, we demonstrate a bottom-up synthesis of molybdenum disulfides (MoS₂) nanoribbons on graphene substrate via chemical vapor deposition (CVD) by precisely tuning the growth parameters into a sulfur-enriched condition. MoS₂ nanoribbons are mainly formed from the CVD grown MoS₂ flakes along the armchair (AC) direction. Atomic resolution ADF-STEM imaging characterizations show an alternating presence of molybdenum and sulfur zigzag edge terminations at the edges of MoS₂ nanoribbons. While at the apex of the nanoribbon, sulfur terminated zigzag edges become dominant. Taking these results together, we revealed the underlying growth mechanism of MoS₂ nanoribbons. Electronic transport properties of the MoS₂ nanoribbons were also measured by fabricating back-gate-effect transistors (FETs). The nanoribbon FETs present n-type behavior with a current on/off ratio higher than 10⁴ at V _DS = 1 and a carrier mobility of 1.39 cm² V^-1 s^-1. This work offers a new route to synthesize 1D MoS₂ nanoribbons, which has great potential in fabricating other 2D materials-derived 1D nanostructures.

Lead-free double perovskites Cs2InCuCl6 and (CH3NH3)2InCuCl6: electronic, optical, and electrical properties

Searching for alternatives to lead-containing metal halide perovskites, we explored the properties of indium-based inorganic double perovskites Cs₂InMX₆ with M = Cu, Ag, Au and X = Cl, Br, I, and of its organic-inorganic hybrid derivative MA₂InCuCl₆ (MA = CH₃NH₃⁺) using computation within Kohn-Sham density functional theory. Among these compounds, Cs₂InCuCl₆ and MA₂InCuCl₆ were found to be potentially promising candidates for solar cells. Calculations with different functionals provided the direct band gap of Cs₂InCuCl₆ between 1.05 and 1.73 eV. In contrast, MA₂InCuCl₆ exhibits an indirect band gap between 1.31 and 2.09 eV depending on the choice of exchange-correlation functional. Cs₂InCuCl₆ exhibits a much higher absorption coefficient than that calculated for c-Si and CdTe, common semiconductors for solar cells. Even MA₂InCuCl₆ is predicted to have a higher absorption coefficient than c-Si and CdTe across the visible spectrum despite the fact that it is an indirect band gap material. The intrinsic charge carrier mobilities for Cs₂InCuCl₆ along the L-Γ path are predicted to be comparable to those for MAPbI₃. Finally, we carried out calculations of the band edge positions for MA₂InCuCl₆ and Cs₂InCuCl₆ to offer guidance for solar cell heterojunction design and optimization. We conclude that Cs₂InCuCl₆ and MA₂InCuCl₆ are promising semiconductors for photovoltaic and optoelectronic applications.

Enhanced Valley Splitting of Transition-Metal Dichalcogenide by Vacancies in Robust Ferromagnetic Insulating Chromium Trihalides

Recently, single-layer CrI₃, a member of the chromium trihalides CrX₃ (where X = Cl, Br, or I), has been exfoliated and experimentally demonstrated as an atomically thin material suitable for two-dimensional spintronics. Valley splitting due to the magnetic proximity effect has been demonstrated in a WSe₂/CrI₃ van der Waals heterojunction. However, the understanding of the mechanisms behind the favorable performance of CrI₃ is still limited. Here, we systematically study the carrier mobility and the intrinsic point defects in CrX₃ and assess their influence on valley splitting in WSe₂/CrI₃ by first-principles calculations. The flat-band nature induces extremely large carrier mass and ultralow carrier mobility. In addition, intrinsic point defects-localized states in the middle of the band gap-show deep transition energy levels and act as carrier recombination centers, further lowering the carrier mobility. Moreover, vacancies in CrI₃ can enhance ferromagnetism and valley splitting in a WSe₂/CrI₃ heterojunction, proving that chromium trihalides are excellent ferromagnetic insulators for spintronic and valleytronic applications.

Atomic-Scale Dynamics and Storage Performance of Na/K on FeF3 Nanosheet

Developing highly efficient FeF₃-based cathode materials for Na/K-ion batteries is greatly needed, which needs long cycling life and rate performance besides large voltage and capacity. Accordingly, we designed a two-dimensional (2D) FeF₃ nanosheet to obtain highly efficient Na/K-ion batteries. Moreover, first-principles calculations were implemented to discuss systematically the Na and K storage mechanism on the FeF₃(012) nanosheet. The adsorption energies of Na and K are -3.55 and -3.98 eV, respectively, which can guarantee the Na/K loading process. Interestingly, Na and K adatoms on FeF₃(012) prefer to get together in the form of the Na dimer and K tetramer, respectively. Energy barrier of the K tetramer is lower than that of the Na dimer (0.43 eV vs 0.45 eV). As a result, the K tetramer possesses a larger diffusion coefficient than the Na dimer (4.22 × 10^-10 cm²·s^-1 vs 3.32 × 10^-10 cm²·s^-1). That is to say, good Na/K-ion mobility can be achieved. Also, the FeF₃(012) nanosheet exhibits high initial discharge voltage (4.10 V for K and 3.74 V for Na). Moreover, it has a stable discharge voltage curve in Na/K-ion batteries. Besides, the FeF₃(012) nanosheet is more favorable to be fabricated as a flexible cathode material for potassium batteries. Therefore, the 2D FeF₃ nanosheet belongs to a promising cathode material in Na/K-ion batteries.

Electronic structure, carrier mobility and strain modulation of CH (SiH, GeH) nanoribbons

Using first-principles calculations coupled with deformation potential (DP) theory, we have systematically studied the band structure, carrier mobility and strain modulation of monolayer graphane (CH), silicane (SiH) and germanane (GeH) nanoribbons. It is found that all the CH (SiH, GeH) nanoribbons are semiconductor with a wide range of band gap. The carrier mobility results show that the armchair germanane nanoribbon (AGeNR) has the characteristic of p -type semiconductor in electrical conduction because its hole mobility is larger than the electron mobility. While the graphane nanoribbon (CNR) behaves as n-type semiconductor in electrical conduction. Compared to AGeNR and CNR, the mobilities of other nanoribbons are much smaller. Moreover, the band structure and carrier mobility of AGeNR and CNR can be effectively tuned by strain. There are different state competing for the valence band maximum (VBM). When the strain exceeds certain value, the VBM is transited to a new band-edge state accompanied with a large increase of hole mobility. The new band-edge state has smaller DP constant because its bond character makes it less sensitive to strain, and thus resulting in higher hole mobility.

Flexible Molybdenum Disulfide (MoS2) Atomic Layers for Wearable Electronics and Optoelectronics

Flexible, stretchable, and bendable materials, including inorganic semiconductors, organic polymers, graphene, and transition metal dichalcogenides (TMDs), are attracting great attention in such areas as wearable electronics, biomedical technologies, foldable displays, and wearable point-of-care biosensors for healthcare. Among a broad range of layered TMDs, atomically thin layered molybdenum disulfide (MoS₂) has been of particular interest, due to its exceptional electronic properties, including tunable bandgap and charge carrier mobility. MoS₂ atomic layers can be used as a channel or a gate dielectric for fabricating atomically thin field-effect transistors (FETs) for electronic and optoelectronic devices. This review briefly introduces the processing and spectroscopic characterization of large-area MoS₂ atomically thin layers. The review summarizes the different strategies in enhancing the charge carrier mobility and switching speed of MoS₂ FETs by integrating high-κ dielectrics, encapsulating layers, and other 2D van der Waals layered materials into flexible MoS₂ device structures. The photoluminescence (PL) of MoS₂ atomic layers has, after chemical treatment, been dramatically improved to near-unity quantum yield. Ultraflexible and wearable active-matrix organic light-emitting diode (AM-OLED) displays and wafer-scale flexible resistive random-access memory (RRAM) arrays have been assembled using flexible MoS₂ transistors. The review discusses the overall recent progress made in developing MoS₂ based flexible FETs, OLED displays, nonvolatile memory (NVM) devices, piezoelectric nanogenerators (PNGs), and sensors for wearable electronic and optoelectronic devices. Finally, it outlines the perspectives and tremendous opportunities offered by a large family of atomically thin-layered TMDs.

Potential Applications of Halide Double Perovskite Cs2AgInX6 (X = Cl, Br) in Flexible Optoelectronics: Unusual Effects of Uniaxial Strains

The discovery of halide double perovskite Cs₂AgInX₆ (X = Cl, Br) has provided an efficient way to search promising solar cell absorbers. Here, theoretical calculations on strained Cs₂AgInX₆ (X = Cl, Br) not only comprehensively and firstly help understand their physical properties but also provide a guideline to extend their potential applications. Although Cs₂AgInX₆ possesses a similar structure, the variations of physical properties of strained Cs₂AgInX₆ are different. Only compressive Cs₂AgInBr₆ undergoes a direct-to-indirect transition, which enables it to be a good radiation detection material. Moreover, the mobility of Cs₂AgInCl₆ is reduced by strains, while that of Cs₂AgInBr₆ is enhanced (reduced) by compression (tension). That is because the contribution degrees of Ag-d _z², d _{x²- y²} and In-d _z², d _{x²- y²} on the band edges of Cs₂AgInX₆ (X = Cl, Br) are inconsistent. In addition, the absorption coefficients of Cs₂AgInX₆ (X = Cl, Br) are deteriorated negligibly by strain, making it a potential material for further applications of photovoltaics and flexible optoelectronics.