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A NO2/SbP adsorption system with high adsorption energy (−0.876 eV) and charge transfer value (−0.83 e) is reported.

A CH2O/CsPbBr3 adsorption system with high adsorption energy (−0.69 eV) and charge transfer (0.398 e) is reported.

Conductive hydrogels have attracted tremendous research interest in wearable sensing electronics.

In this work, we predict two novel two-dimensional (2D) auxetic materials, BiSbX3 (X = S, Se) monolayers, through first-principles calculations. Attributed to their special braceletlike structure, the in-plane negative Poisson's ratio (NPR) of BiSbS3 and BiSbSe3 monolayers are as high as -0.25 and -0.26, respectively. The phonon dispersion calculations, ab initio molecular dynamics simulations, and elastic constants calculations demonstrate that these two monolayers possess excellent dynamic, thermal, and mechanical stabilities. The band gap values of BiSbS3 and BiSbSe3 calculated at the HSE level by considering the spin-orbit coupling (SOC) effect are 1.68 and 1.20 eV. The anisotropic carrier mobility and superior optical absorption indicate that they may shine in the next generation of electronic and optoelectronic devices. All of these discoveries not only enrich the types of auxetic materials but also provide a structural reference for designing new auxetic materials on the molecular level. Furthermore, they can provide theoretical guidance for future applications of BiSbX3 (X = S, Se) monolayers in various fields.

Aroused by plant photosynthesis, Z-scheme heterostructures have been considered as a potential photocatalyst for solar-driven water splitting to solve the current energy crisis. Hence, based on first-principles calculations, we predict that Sc2CF2/Janus MoSSe can be used as a Z-scheme heterostructure for efficient photocatalytic water splitting. The research shows that the traditional type-II to direct Z-scheme heterostructure conversion can be realized through different stacking methods of Sc2CF2 and Janus MoSSe. Through the built-in electric field and band bending theory, the transition path of photogenerated carriers is analyzed, which reveals the completely different photocatalytic mechanism of type-II and direct Z-scheme heterostructure. Surprisingly, the direct Z-scheme heterostructure displays a high overpotential of the hydrogen evolution reaction ( χ H 2 =1.01 eV) and the oxygen evolution reaction ( χ O 2 =1.46 eV) compared with the type II heterostructure. More importantly, the direct Z-scheme heterostructure has an ultra-high solar-to-hydrogen (STH) efficiency (36.1%), which breaks through the limitation of traditional theoretical efficiency and reveals a tremendous prospect of commercial application. Furthermore, the calculation of free energy confirms that the water splitting reaction on the direct Z-scheme heterostructure occur spontaneously under the external potential, but not for type-II heterostructure. Finally, introducing the additional electronic conductor (N-doped graphene) can accelerate the electron (hole) transfer and interlayer carrier recombination, which will further improve the photocatalytic performance of Z-scheme heterostructure. These distinctive features make Sc2CF2/Janus MoSSe heterostructure as promising Z-scheme photocatalyst for water splitting.

The gas-sensing performance of PbSnS2 monolayer to sulfur hexafluoride (SF6) decompositions (SO2, H2S, HF, SOF2, and SO2F2) has been systematically studied by density functional theory and nonequilibrium Green’s function calculations. The most stable adsorption configurations, adsorption energies, charge transfer, electron localization function (ELF), density of states (DOS), and ${I}$ – ${V}$ curves of gas molecules on PbSnS2 are calculated and discussed. SO2 exhibits the largest adsorption energy of −0.74 eV with the charge transfer of −0.39 e and HF possesses the biggest charge transfer of −0.66 e with the adsorption energy of −0.46 eV. The results of ELF and DOS show that the interaction between SO2 and PbSnS2 may be stronger than those of other gases. Moreover, ${I}$ – ${V}$ curves show that the responses of PbSnS2 to SO2 is up to 65.5% at the bias of 2.2 V. Therefore, PbSnS2 monolayer may be a qualified material for detecting SF6 decompositions.

Hybrid nanocomposites comprised of metal oxide nanoparticles and three-dimensional (3D) graphene possess the advantages of metal oxide and graphene, which have generated extensive attention. Here, we fabricate flexible micro-supercapacitors (MSCs) based on hybrid materials of single-walled carbon nanotubes (SWCNT)-bridged laser-induced graphene fibers (LIGF) decorated with manganese dioxide (MnO2) nanoparticles. SWCNT is deposited on the LIGF surface and the space between LIGF, which can bridge LIGF to form more conductive paths and provide more active areas to grow with MnO2 nanoparticles. Profiting from the synergistic effect between conductive SWCNT-bridged LIGF network and the MnO2 nanoparticles with high theoretical capacitance, the obtained flexible MSCs based on LIGF-C4/MnO2 hybrid electrodes deliver an outstanding areal capacitance of 156.94 mF cm−2, which is about 8 times higher than that of LIGF-MnO2 based MSC (20 mF cm−2). Additionally, the LIGF-C4/MnO2 MSCs also exhibit considerable areal energy density of 21.8 μWh cm−2, long-term cycling stability, remarkable modular integration capability, and exceptional mechanical flexibility (with 90.5% capacitance retention after 1200 bending cycles). Therefore, the design of hybrid electrode materials proposed in this work offers a facile and novel method to develop flexible energy storage devices with high performance, suggesting great prospects for applications in future various wearable electronics.

In this work, we explore the application potential of AsP/M2CO2 (M = Sc, Zr) van der Waals heterostructures in photocatalytic water splitting through the first-principles calculations. The calculated results show that AsP/Zr2CO2 heterostructure possesses an unfavorable type-Ⅰ band alignment, whereas AsP/Sc2CO2 exhibits a desirable type-Ⅱ band alignment, which is beneficial for separating the photogenerated electron-hole pairs. Also, the band edge positions of AsP/Sc2CO2 heterostructure stride the redox potential of water, ensuring favorable reaction kinetics. Besides, the strong optical absorption of AsP/Sc2CO2 heterostructure in both visible and ultraviolet regions (especially up to 10−6 cm−1 at about 250 nm) makes it possible to utilize solar energy effectively. Meanwhile, AsP/Sc2CO2 heterostructure has an exciton binding energy as low as 0.09 eV, which quantitatively illustrates the high separation efficiency of photogenerated charge carrier. Thus, the type-Ⅱ band alignment, suitable band edge position, strong light absorption, and low exciton binding energy together indicate that AsP/Sc2CO2 heterostructure is a potential photocatalytic material. In addition, the obvious redshift phenomenon in the optical spectrum of AsP/Sc2CO2 heterostructure shows that biaxial strain can improve its light capture capability. Also, the interconversion between type-Ⅱ and type-Ⅰ can be achieved by applying different strains. All these findings suggest that the novel AsP/Sc2CO2 heterostructure has significant application prospects in next-generation photovoltaic and photocatalytic devices.

Heteroatoms modification of carbon electrodes is an effective strategy to boost its electrochemical performance. In this work, we demonstrate a one-step and scalable method to manufacture phosphorus-doped (P-doped) 3D porous graphene electrodes from a flexible H3PO4-incorporated polyimide (PI)/Polyvinyl alcohol (PVA) composite film via laser direct writing. Benefiting from the synchronous P-doping and activation caused by H3PO4, the optimal P-doped porous graphene-based micro-supercapacitors (MSCs) present an impressive areal capacitance of 55.5 mF cm−2, good long-term cyclability (with ∼85% capacitance retention after 10000 cycles), superior mechanism flexibility, and outstanding capability of modular integration in series and parallel. Furthermore, density functional theory (DFT) calculations were also carried out to prove that P-doping could enhance the affinity towards electrolyte ions on the carbon surface to improve the electrochemical performance. In particular, due to the highly hydrophilic nature and good film-forming properties of PVA, other dopants can be also well dissolved in the PI/PVA slurry to synthesize a dopants-incorporated PI/PVA composite membrane. Hence, this method offers a convenient and potential route to prepare free-standing heteroatom-doped porous graphene electrodes, which has promising applications for various flexible/wearable electronics.

With the rapid development of human-machine technology, self-powered pressure sensor integrated systems have been extensively studied. However, there are only a few reports on such multifunctional devices using a single active material. In this work, we report a flexible integrated system, which consists of flexible pressure sensors and supercapacitors. Both of the devices were fabricated based on layered porous Cu@Cu

This study proposes an efficient, facile, and scalable strategy to synthesize in situ heteroatom-doped porous graphene via laser direct writing on the precursor-doped polyimide (PI) film, which is fabricated for the first time through incorporating PI powder and precursors with sodium carboxymethyl cellulose (CMC) binder by a drop-casting and low-temperature drying process. The resulting microsupercapacitors (MSCs) based on the as-prepared heteroatom-doped porous graphene exhibit remarkable capacitive performance. The typical boron-doped MSC prepared on borax-doped polyimide film possesses an ultrahigh areal capacitance of 60.6 mF cm-2 at 0.08 mA cm-2, which is approximately 20 times larger than that of undoped MSC. Furthermore, the boron-doped MSC has impressive cycling stability (with the capacitance retention of 96.3% after 20 000 cycles), exceptional mechanical flexibility, tunable capacitance, and voltage output through arbitrary modular serial and parallel integration. Besides, the nitrogen-doped porous graphene with excellent capacitive performance is also prepared by laser direct scribing on the sulfonated melamine-doped polyimide film, demonstrating excellent scalability and generality of this strategy. Hence, one-step laser direct writing on precursor-doped polyimide films can realize in situ heteroatom doping and generation of hierarchical porous graphene electrodes simultaneously, which opens a new avenue for the facile, cost-effective, and scalable fabrication of heteroatom-doped porous graphene, thus promising for MSCs and various flexible and wearable electronics at large-scale production.

It is found that many two-dimensional materials are easy to obtain out-of-plane piezoelectric properties because of their Janus structure. Here, based on the monolayer GeSe and SnSe, we study the electronic structure and piezoelectricity of the Janus M2SeX (M=Ge, Sn; X=S, Te) monolayers. Due to the lack of inversion symmetry and mirror symmetry, as well as flexible mechanical properties, the 2D Janus M2SeX monolayers have large in-plane piezoelectric coefficients d11 (up to 345.08pm/V) and out-of-plane piezoelectric coefficients d31 (up to 3.83pm/V). All energy band structures of two-dimensional (2D) Janus M2SeX monolayers show indirect bandgap and Zeeman-type spin splitting after considering spin orbit coupling (SOC). The lack of mirror symmetry leads to out-of-plane spin polarization. In addition, the calculation based on the deformation potential theory shows that the 2D Janus M2SeX monolayers have high carrier mobility. The large piezoelectric properties and high carrier mobility show the application potential of 2D Janus M2SeX monolayers in flexible electronic devices and piezoelectric devices.

Real-time monitoring of SF6 status is of exceptional importance to ensure stable operation of the power system, to promptly diagnose whether an insulation failure has occurred and to assess the severity. Considering the excellent physicochemical properties and insulation defects of hexagonal boron nitride, the strategy of surface modification, namely, hydrogenation, fluorination, and combinations of the two ways, is proposed to improve its chemical activity, and the gas-sensing potential of the most effective one is investigated using density functional theory. The typical adsorption models of SF6 decomposition component gases on the surface of F-BN-H are constructed, and the most stable adsorption structures with their density of states, molecular orbitals, and sensing responses are analyzed. The results show that F-BN-H has good sensitivity to decomposition components of SF6. To be specific, SO2 and SOF2 have good adsorption properties, prominent charge transfer, and distinct change in electronic properties. The response of gas adsorption can be detected timely by the change in conductivity, and, additionally, the adsorbent has fast desorption capacity at room temperature. These results indicate that the F-BN-H-based gas sensor is a qualified potential candidate with dependable application and recoverability for detecting SF6 decomposition in high-voltage insulated equipment.

In this study, the adsorption of gas molecules, such as O2, NH3, CO, CO2, H2O, NOx (x = 1, 2) and SO2, on Janus Te2Se monolayer has been investigated by means of density functional theory (DFT) calculations. We show that Janus Te2Se monolayer is preferable for SO2 and NOx molecules with suitable adsorption strength and apparent charge transfers. We further calculated the current-voltage (I-V) curves using the nonequilibrium Green's function (NEGF) method. The transport feature exhibits distinct responses with a dramatic change of I-V curves before and after NOx (SO2) adsorption on Janus Te2Se. Thus, we predict that Janus Te2Se could be a promising candidate for SO2 and NOx sensors with high selectivity and sensitivity. Moreover, the effect of strain on the gas/substrate adsorption systems was also studied, implying that the strained Janus Te2Se monolayer could enhance the sensitivity and selectivity to SO2 and NO2. The adsorbed SO2 and NO2 on Janus Te2Se could escape by releasing the applied strain, which indicates that the capture process is reversible. Our study widens the application of Janus Te2Se not only as piezoelectric materials, but also as a potential gas sensor or capturer of SO2 and NOx with high sensitivity and selectivity.

Two-dimensional Janus nanomaterials, due to their unique electronic, optical, and piezoelectric characteristics resulting from the antisymmetric structures, exhibit great prospects in multifunctional energy application to alleviate the energy crisis. Monolayer Janus Pb2SSe, with a black phosphorus-like structure and an indirect band gap of 1.59 eV as well as high carrier mobility (526-2105 cm2 V-1 s-1), displays outstanding potentials in the energy conversion between nanomechanical energy, solar energy, waste heat, and electricity, which has been comprehensively studied utilizing DFT-based simulations. The research results reveal that monolayer Pb2SSe not only possesses giant in-plane piezoelectricity of d11 = 75.1 pm V-1 but also superhigh out-of-plane piezoelectric coefficients (d31 = 0.5 pm V-1 and d33 = 15.7 pm V-1). Meanwhile, by constructing Pb2SSe bilayers, the out-of-plane piezoelectric coefficients can be significantly enhanced (d31 = 19.2 pm V-1 and d33 = 194.7 pm V-1). In addition, owing to the small conduction band offset, suitable donor band gap and excellent light absorption capability in the Pb2SSe/SnSe (Pb2SSe/GeSe) heterostructure, the power conversion efficiencies were calculated to be up to 20.02% (Pb2SSe/SnSe) and 19.28% (Pb2SSe/GeSe), making it a promising candidate for solar energy collection. Furthermore, from the thermoelectric electron and phonon transport calculations, it can be found that the Pb2SSe monolayer is an n-type thermoelectric material with ultrahigh ZT = 2.19 (1.52) at room temperature, which can be traced back to its ultralow κL = 0.78 (0.99) W m-1 K-1, and superhigh PF = 10.18 (8.25) mW m-1 K-2 along the x(y) direction at the optimal doping concentration at 300 K. The abovementioned versatile characteristics in the Janus Pb2SSe monolayer, along with its comprehensive stabilities (energy, dynamic, thermal, and mechanical stabilities), highlight its potential in clean energy harvesting.

Wearable devices with integrated alarm functions play a vital role in daily life and can help people prevent potential hazards. Although many wearable sensors have been extensively studied and proposed to monitor various physiological signals, most of them are needed to integrate with the external alarm elements to realize warning, such as light-emitting diodes and buzzers, resulting in the system complexity and poor flexibility. In this paper, an integrated sensing and warning multifunctional device based on the mechanical and thermal effect of porous graphene is proposed on a bilayer asymmetrical pattern of laser-induced graphene (LIG). Compared with the strain sensor with nonpatterned LIG, the mechanical performance is greatly improved with the highest gauge factor value of up to 950 for the strain sensor with mesh-patterned LIG. On the contrary, the heating performance of the heater with nonpatterned LIG is better than that with mesh-patterned LIG. Combining the performance differences of different LIG patterns, the integrated wearable device with a bilayer asymmetrical LIG pattern is demonstrated. It can generate enough heating energy to warn the person when the detected signal meets the threshold condition measured in real time by the ultrasensitive strain sensor. This work will provide a new way to construct an integrated wearable device for realizing multifunctional applications. This integrated multifunctional device shows great potential toward the applications in healthcare monitoring and timely warning.

As a novel conductive elastomer, magnetorheological elastomers (MREs) featuring both high sensitivity and wide working range have been employed as a new sensing material for flexible tactile sensors. Their sensing mechanism, that is, the spatial distribution rearrangement of particles under compression, completely differs from their conventional counterparts. The piezo-capacitive effect of MREs resulting from the unique mechanism of particles rearrangement is actually a response to the microscopic mechanical movement of particles. This nature brings a core concern on the intrinsic relationship between their mechanical and electrical properties. This study illuminates them from the perspective of electrical creep and recovery behavior of MREs. We give a meaningful analysis for the capacitance creep-recovery mechanism. The experimental fact strongly demonstrated that the particles rearrangement was the direct cause, while the strain creep was an indirect cause. All the behaviors were well interpreted by an evolution mechanism of the particles rearrangement driven by the mechanical strain creep of the flexible matrix under constant pressure. In simpler terms, the electrical creep was induced by the mechanical creep. We further explored the creep effect in practical applications and found a “self-healing” behavior, which indicated that the MRE sensors could obtain a stable sensing capability after a pre-processing.

Industrial production, environmental monitoring, and clinical medicine put forward urgent demands for high-performance gas sensors. Two-dimensional (2D) materials are regarded as promising gas-sensing materials owing to their large surface-to-volume ratio, high surface activity, and abundant surface-active sites. However, it is still challenging to achieve facilely prepared materials with high sensitivity, fast response, full recovery, and robustness in harsh environments for gas sensing. Here, a combination of experiments and density functional theory (DFT) calculations is performed to explore the application of tellurene in gas sensors. The prepared tellurene nanoflakes via facile liquid-phase exfoliation show an excellent response to NO2 (25 ppb, 201.8% and 150 ppb, 264.3%) and an ultralow theory detection limit (DL) of 0.214 ppb at room temperature, which is excellent compared to that of most reported 2D materials. Furthermore, tellurene sensors present a fast response (25 ppb, 83 s and 100 ppb, 26 s) and recovery (25 ppb, 458 s and 100 ppb, 290 s). The DFT calculations further clarify the reasons for enhanced electrical conductivity after NO2 adsorption because of the interfacial electron transfer from tellurene to NO2, revealing an underlying explanation for tellurene-based gas sensors. These results indicate that tellurene is eminently promising for detecting NO2 with superior sensitivity, favorable selectivity, an ultralow DL, fast response-recovery, and high stability.

Health monitoring and automatic notification system is crucial to the patients who suffer from apnea. However, most of them would not make any alert by themselves. They still need to work together with speakers or vibrators to remind the patients of their health conditions, leading to system complexity, low system integration, and poor reliability. Herein, a one-step health monitoring and automatic notification device (HMAND) based on the electrothermal effect of laser-induced graphene (LIG) is proposed to realize the integrated function of respiration monitoring and self-alarming on a single device at the same time. The periodic temperature change of the device is caused by breathing when it works in the monitoring mode. In the meantime, the HMAND will generate plenty of thermal energy to notify and rouse the patient when it detects abnormal breathing conditions. This study will be helpful for patients who suffer from apnea and open up a new way to realize the innovative applications in multifunctional integrated devices.

The development of two-dimensional materials with gas-sensitive properties has been a research focus in the field of materials science. Considering the excellent properties of graphene and hexagonal boron nitride, a monolayer composite is put forward and its potential for gas sensing has been investigated by density functional theory. Compared to common gases in the atmosphere and three other toxic gases, sulfur dioxide tends to have a strong mutual effect with this composite. Together with electronic analyses, it is proved that the composite presents excellent sensitivity toward sulfur dioxide molecules and the conductivity can be used as a vital parameter to diagnose the status of gas sensor. What’s more, a co-adsorption model demonstrates a favorable performance in high moisture working environment. Thus, we can reasonably draw the conclusion that the presented composite could be an eligible candidate for sulfur dioxide gas detection.

In this work, we predict two novel two-dimensional (2D) auxetic materials, BiSbX

Two-dimensional materials, including transition-metal dichalcogenides (TMDs), have aroused wide interest due to their various applications in gas sensors and photoelectric devices. However, achieving a uniform distribution of materials on the substrate is still a challenging and urgent problem. In this article, the factors influencing the material distribution during the chemical vapor deposition (CVD) process are investigated by a simulation method, and the results show that the unequal velocity distributions caused by the carrier gas flow on the growing surface play an important role in influencing the crystal distribution. Boundary layer theory is used to explain the mechanism of the effect of the carrier gas flow on the growth uniformity. An improved method is proposed to improve the material uniformity by changing the placement of the Si/SiO2 substrate from face down to face up. The simulation and experimental results show that uniformly distributed MoS2 with a large size and good quality can be obta...

Photocatalytic water splitting, as an artificial photosynthesis process for producing clean hydrogen energy, has recently attracted extensive attention due to its bright prospects for solving the shortage of fossil energy and environmental crisis. However, it is still challenging to achieve highly efficient overall water splitting because of the unsuitable electronic structure and the strong dependence on the noble metal co-catalysts to date. The development of photocatalytic water splitting is therefore immensely urgent to enlarge the uptake of photocatalysts by exploring novel compounds. Herein, we propose a novel two-dimensional (2D) pentagonal SiAs2 (penta-SiAs2) photocatalyst suitable for water splitting based on first-principles calculations. Using a hybrid functional, we demonstrate the bandgap of 2D penta-SiAs2 to be 2.35 eV, and its band-edges perfectly stride the redox potential of water. Most importantly, this material possesses both highly desirable carrier mobilities and strong light absorption in the visible and ultraviolet (UV) regions, which are superior to those of most previously reported 2D materials. Furthermore, the free energies suggest that hydrogen and oxygen evolution reactions are feasible with penta-SiAs2. These extraordinary characteristics make penta-SiAs2 a promising 2D material for photocatalytic water splitting and provide a possible route for the future experiment scheme.

The sensing performances of SF6 decompositions, including HF, SOF2, SO2, SO2F2, and H2S, adsorption on monolayer tellurene have been investigated by means of density functional theory (DFT) calculations in this letter. The attained results show that the monolayer tellurene is preferable for SF6 decomposition components, especially for SO2 and SOF2, exhibiting favorable adsorption strength and apparent charge transfers. Besides, the calculated density of states (DOS) further reveals the electronic properties are obviously changed after SO2 and SOF2 adsorption. These results signify that tellurene-based gas sensor is a potential candidate for detecting SF6 decompositions in high-voltage insulation equipment.

Transition metal nanoparticles-graphene nanocomposites incorporate the advantages of graphene and metal nanoparticles, which arouse extensive attention. Here, we design a novel, facile and versatile method for in-situ synthesis of laser-induced porous graphene (LIG) decorated with cobalt particles (Co). The LIG/Co nanocomposites are fabricated through one-step laser direct scribing on a customized film composed of polyimide (PI) powder, polyvinyl alcohol (PVA), and cobalt chloride (CoCl2·6H2O) precursors. Benefiting from the unique properties of Co nanoparticles embedded LIG, the obtained optimal in-plane micro-supercapacitors (IMSC) based on LIG/Co-1.5 possesses an excellent areal capacitance of 110.11 mF cm-2 and a superior energy density of 9.79 μWh cm-2, which are about 79 times that of pure LIG-based IMSCs. Simultaneously, the LIG/Co-1.5 MSCs also present good cycling stability, remarkable modular integration capability, and outstanding mechanical flexibility, showing potential for practical applications. Additionally, the density functional theory (DFT) calculations indicate that the decorating of cobalt particles elevates electron transfer. Moreover, the interaction between electrolyte and electrodes is also improved with the introduction of cobalt particles. Therefore, this strategy offers a new avenue for facile and large-scale manufacturing of various metallic atoms in-situ decorating in porous graphene.

Two-dimensional Janus nanomaterials, due to their unique electronic, optical, and piezoelectric characteristics resulting from the antisymmetric structures, exhibit great prospects in multifunctional energy application to alleviate the energy crisis. Monolayer Janus Pb

Two-dimensional (2D) piezoelectric materials have attracted wide attention because they are of great significance to the composition of piezoelectric nanogenerators. In this work, we have systematically studied the piezoelectric properties of 2D LiAlTe

This study proposes an efficient, facile, and scalable strategy to synthesize

Motivated by the recent progress in synthesizing graphene by using aromatic benzene molecules as precursors, herein, we propose a new two-dimensional (2D) carbon allotrope by using nonaromatic molecule cyclooctatetraene as the precursor from a bottom-up approach. In this structure, all the carbon atoms are threefold coordinated, similar to the sp2-hybridized atoms in graphene; thus, we name the poly-cyclooctatetraene framework as PCF-graphene. First-principles calculations reveal that although PCF-graphene is metastable compared with graphene, it is thermally, mechanically, and dynamically stable. Different from graphene with a single-atomic thickness and zero band gap, PCF-graphene has a finite thickness of 2.45 A and is a semiconductor with a direct band gap of 0.77 eV. It exhibits anisotropies in mechanical properties, carrier mobility, and optical adsorption because of its anisotropic lattice. This study not only sheds insights into the design of 2D carbon materials from some simple organic molecules ...

Motivated by the recent experimental exfoliation of β-Cu2S thin films and the theoretical finding of a new phase labeled the δ-Cu2S monolayer, we carried out extensive studies on thermal conductivity and thermoelectric properties of the new phase using first principles combined with Boltzmann transport theory, focusing on the analysis of group velocities, Gruneisen parameters, three-phonon scattering rates, and the scattering phase space. Our results show that the δ-Cu2S monolayer exhibits an intrinsically ultralow lattice thermal conductivity of 0.10 W m−1 K−1 at 800 K. Such an ultralow lattice thermal conductivity leads to a high thermoelectric figure of merit ZT = 1.33 at 800 K in an optimum p-type doping concentration, which is not only larger than the value of 1.23 in In2S3 doped Cu2S at 850 K but also comparable with the value of 1.7 in Cu1.97S at 1000 K, exhibiting good potential in thermoelectric applications.

Structural symmetry-breaking can lead to novel electronic and piezoelectric properties in two-dimensional (2D) materials. In this paper, we propose a 2D Janus tellurene (Te2Se) monolayer with asymmetric Se/Te surfaces and its derived multilayer structures. The band structure calculations show that the 2D Janus Te2Se monolayer is an indirect gap semiconductor, and the intrinsic mirror asymmetry combined with the spin-orbit coupling induces the Rashba spin splitting and the out-of-plane spin polarization. Moreover, the absence of both the inversion symmetry and out-of-plane mirror symmetry, together with flexible mechanical properties, results in large in-plane and out-of-plane piezoelectric coefficients that are valuable in 2D piezoelectric materials. Furthermore, the out-of-plane piezoelectric effects can exist in multilayer structures under different stacking sequences while the in-plane piezoelectric effect can only exist in some specific stacking patterns. The piezoelectric coefficients of the Janus Te2Se monolayer and multilayers exceed those of many Janus transition metal dichalcogenides and other well-known piezoelectric materials (e.g., α-quartz and wurtzite-AlN). The combination of the SOC-induced spin splitting and large piezoelectricity endows the Janus Te2Se structures with potential for applications in spintronics, flexible electronics and piezoelectric devices.

Motivated by the compatibility with the well-developed Si-based semiconductor technology, research on new Si allotropes is of importance. Here, we study a new three-dimensional (3D) silicon (named as pentagonal-Si), which is entirely composed of pentagons with all atoms in sp3-hybridized states. State-of-the-art theoretical calculations confirm that the new silicon phase is dynamically, thermally and mechanically stable. The calculations of electronic band structure with HSE06 functional show that pentagonal-Si is a semiconductor with an indirect band gap of 2.05 eV that is much larger than that of diamond-Si, leading to distinguished optical absorption. Moreover, pentagonal-Si has a lower mass density (2.09 g/cm3) compared with diamond-Si (2.34 g/cm3), and exhibits a higher carrier mobility (∼3 × 103 cm2/V·s) at ambient temperature, adding new feature for optoelectronic applications.

In this study, the adsorption of gas molecules, such as O2, NH3, CO, CO2, H2O, NOx (x = 1, 2) and SO2, on Janus Te2Se monolayer has been investigated by means of density functional theory (DFT) calculations. We show that Janus Te2Se monolayer is preferable for SO2 and NOx molecules with suitable adsorption strength and apparent charge transfers. We further calculated the current-voltage (I-V) curves using the nonequilibrium Green's function (NEGF) method. The transport feature exhibits distinct responses with a dramatic change of I-V curves before and after NOx (SO2) adsorption on Janus Te2Se. Thus, we predict that Janus Te2Se could be a promising candidate for SO2 and NOx sensors with high selectivity and sensitivity. Moreover, the effect of strain on the gas/substrate adsorption systems was also studied, implying that the strained Janus Te2Se monolayer could enhance the sensitivity and selectivity to SO2 and NO2. The adsorbed SO2 and NO2 on Janus Te2Se could escape by releasing the applied strain, which indicates that the capture process is reversible. Our study widens the application of Janus Te2Se not only as piezoelectric materials, but also as a potential gas sensor or capturer of SO2 and NOx with high sensitivity and selectivity.

Acetone is widely used as an organic solvent or as an intermediate in material preparation. However, acetone is fairly volatile and highly toxic. Long-term exposure to acetone may damage the kidney...