Gas Monolayers Research Articles

Coulomb drag in metal monochalcogenides double-layer structures with Mexican-hat band dispersions

We theoretically study the Coulomb drag resistivity and plasmon modes behavior for a system composed of two parallel p-type doped GaS monolayers with Mexican-hat valence energy band using the Boltzmann transport theory formalism. We investigate the effect of temperature, T, carrier density, p, and layer separation, d, on the plasmon modes and drag resistivity within the energy-independent scattering time approximation. Our results show that the density dependence of plasmon modes can be approximated by p 0.5. Also, the calculations suggest a d 0.2 and a d 0.1 dependencies for the acoustic and optical plasmon energies, respectively. Interestingly, we obtain that the behavior of drag resistivity in the double-layer metal monochalcogenides swings between the behavior of a double-quantum well system with parabolic dispersion and that of a double-quantum wire structure with a large carrier density of states. In particular, the transresistivity value reduces exponentially with increasing the distance between layers. Furthermore, the drag resistivity changes as T 2/p 4 (T 2.8/p 4.5) at low (intermediate) temperatures. Finally, we compare the drag resistivity as a function of temperature for GaS with other Mexican-hat materials including GaSe and InSe and find that it adopts higher values when the metal monochalcogenide has smaller Mexican-hat height.

Band structure engineering in gallium sulfide nanostructures

The gallium sulfide monolayer is demonstrated as a promising two-dimensional semiconductor material with considerable properties. The present work, investigates the bandgap modulation of GaS monolayer under biaxial or uniaxial strain, using density functional theory calculation. We find that GaS monolayer shows an indirect bandgap that limits its optical applications. The uniaxial strain shifts bandgap from indirect to direct in gallium sulfide monolayer at the strain of \(\varepsilon \)= − 10 percent along y direction. Also, by analyzing Poisson’s ratio, we figure out that Gallium Sulfide monolayer has isotropic mechanical nature under pressure. The detailed reasons for the bandgap modulation are discussed by analyzing the projected density of states. Besides, we investigate indirect to direct bandgap shifts on Gallium Sulfide nanoribbons through varying nanoribbon width as well as edge hydrogen passivations. The direct bandgap accompanied by the isotropic mechanical feature in the Gallium Sulfide monolayer, facilitates and moves up its practical application.

Gas-sensing mechanism of Cr doped SnP3 monolayer to SF6 partial discharge decomposition components

In this paper, based on density functional theory (DFT), the possibility of intrinsic and Cr doped SnP3 (Cr-SnP3) monolayer as candidate materials for detecting SF6 decomposed gases SO2, SOF2 and SO2F2 was discussed. The adsorption energy, adsorption distance, charge transfer and density of states of adsorption systems were calculated and analyzed. Results reveal that the Cr-SnP3 monolayer has higher chemical affinity to SO2, SOF2 and SO2F2 molecules than intrinsic SnP3. Larger adsorption energy, obvious charge transfer and significant molecular structural deformation can be observed, indicating the strong interaction between gas molecule and Cr-SnP3 caused by chemisorption, which can effectively excite electrical signals for gas detection. Therefore, Cr-SnP3 monolayer gas sensor may be recommended in the field of SF6 decomposition characteristics products detection.

The interaction between vacancy defects in gallium sulfide monolayer and a new vacancy defect model.

Vacancy defects are inevitable when synthesizing two-dimensional (2D) materials, and vacancy defects greatly affect the physical properties, such as magnetism and electronic properties. Currently, sufficient information is not available on whether and how the interaction of vacancy defects affects material properties and how to control these defects and their associated interaction for the development of new materials. In this study, the interaction between two adjacent vacancy defects of the gallium sulfide (GaS) monolayer is investigated using first-principles calculations based on density functional theory (DFT). The results indicate that the localized size of a Ga vacancy defect is the area within the S atoms second nearest to the neighboring vacancy defect. When the localized sizes of Ga vacancy defects intersect, a non-negligible interaction exists between the Ga vacancy defects. The interaction generally has been ignored by the traditional defect concentrations model but would affect the magnetic and electronic properties of the defective GaS monolayer. A vacancy defect cluster model (VDCM) is developed based on the system clustering method and then used to evaluate the interactions between vacancy defects. In order to check the reliability of the model, this research studies a defective MoS2 monolayer as an example and compares the band gap and density of states (DOS) calculated by using different vacancy defect models, including VDCM. The results indicate that VDCM has good accuracy relative to the traditional vacancy concentration model. This means that with the help of VDCM the properties of the defective system could be calculated more accurately considering some extent of nonuniform distribution of defects based on DFT.

First-principles study on the stability and electronic structure of monolayer GaSe with trigonal-antiprismatic structure

The structural stability and electronic states of GaSe monolayer with trigonal-antiprismatic (AP) structure, which is a recently discovered new polymorph, were studied by first-principles calculations. The AP phase GaSe monolayer was found stable, and the differences in energy and lattice constant were small when compared to those calculated for a GaSe monolayer with conventional trigonal-prismatic (P) structure which was found to be the ground state. Moreover, it was revealed that the relative stability of P phase and AP phase GaSe monolayers reverses under tensile strain. These calculation results provide insight into the formation mechanism of AP phase GaSe monolayers in epitaxially-grown GaSe thin films.

Impact of stacking on the optoelectronic properties of 2D ZrS2/GaS heterostructure

In the present study, we have systematically investigated the structural, electronic and optical properties of 2D ZrS2/GaS (with van der Waals interaction) heterostructure by using first-principles calculations. The pristine ZrS2 and GaS monolayers have semiconducting behavior with band gap of 0.99 eV and 2.39 eV, respectively. It is found that the ZrS2 and GaS monolayers interact with each other through strong van der Waals interactions due to that the band gap significantly reduces 0.88 eV. Our results indicate that the considered three stacking configurations AA, AB and AC shows semiconducting behavior. Promisingly, the optical properties display that the ZrS2/GaS heterostructures strongly absorbs light with energies from visible to ultraviolet (UV) regions. From the result, considered heterostructures may be utilized for optoelectronic devices applications.

Gas sorption-induced pore radius change and their impact on gas apparent permeability

Shale gas reservoirs contain small pore network that typically range from macro to nano meters and abundant gas storage as sorbed gas. In addition, the storage medium in such pore network varies and may be affected by physical parameters of shale matrix. This work presents the influence of gas sorption on pore radius and thickness at low and high pressure and analyzes the gas apparent permeability. Langmuir model is generally used to quantify the adsorbed layer thickness; however, such model undertakes mono-layer gas sorption on pore surfaces and provides poor results especially at high pressure. In this work, supercritical dubinin radushkevich (SDR) model, micropore-filling mechanism based, has been used to quantify the adsorbed layer thickness and their results have been compared with Langmuir model. The impact of adsorbed gas density on gas sorption and the impact of gas sorption crucial factors on pore radius were examined. At the end, gas slippage, geo-mechanical and adsorbed layers impacts were analyzed for evolution of gas apparent permeability. The proposed gas apparent permeability model was also validated with experimental data. The results show that assumed adsorbed gas density in Langmuir model is misleading the accurate measurement of gas sorption. The SDR model does not only provide an accurate adsorbed gas density but also its values are very close to the experimental at both low and high pressure. Gas sorption-induced pore radius based on SDR model was altered when gas sorption behavior alter at same pressure than Langmuir model-based. Gas apparent permeability changes due to gas slippage, geo-mechanical and gas adsorbed layers impacts. High gas adsorbed layer impact was observed when using proposed model as compared with Langmuir model especially at high pressure.

Modulation of 2D GaS/BTe vdW heterostructure as an efficient HER catalyst under external electric field influence

Modeling the 2D Van der Waals (vdW) heterostructure photocatalysts is an effective way to take advantage of solar energy and suppressing the fast recombination rate of photo-generated charge carriers. In the present work, we have systematically investigated the electronic, optical and photocatalytic properties of the GaS/BTe vdW heterostructure under an applied external electric field using first-principles calculations. Our results reveal that the GaS/BTe vdW heterostructure has an indirect band gap of 1.06/1.59 eV with PBE/HSE06 functional without electric field. The results also imply that electrons are likely to transfer from GaS to BTe monolayer due to the deeper potential of BTe monolayer. The GaS/BTe vdW system forms a type-II band alignment and established a large electric field at the interface, controlling to effective separation of the electron–hole pairs. Also, the transverse external electric considerably changes the band gap and transform from type-II to type-I and type-III band alignments. The GaS/BTe vdW heterostructure, also enhanced the optical absorption as compared to pristine GaS and BTe monolayer. Furthermore, the (−)ve electric field significantly increases the optical absorption spectrum in infrared (IR) to visible region, while the (+)ve electric field enhances the optical absorption coefficients in visible to ultraviolet (UV) region. The external transverse electric field enhances the hydrogen evolution reaction (HER) activity on the 2D vdW heterostructure. These obtained results predict that the 2D GaS/BTe vdW heterostructures carry potential applications to enhancing the photocatalytic performance under visible light irradiation.

Type II GaS/AlN van der Waals heterostructure: Vertical strain, in-plane biaxial strain and electric field effect

In this work, the geometrical structure, stability, electronic and optical properties of the GaS/AlN van der Waals heterostructure have been explored based on first principles calculations considering the effect of vertical strain, in-plane biaxial strain and the electric field. It is demonstrated that, the GaS/AlN van der Waals heterostructure possesses an intrinsic typical type-II band alignment with an indirect band gap of 1.65 eV. Due to the difference of work functions between monolayer GaS and AlN, the electrons transfer from AlN layer to GaS layer, causing a build-in electric field which facilitates the separation of free electrons and holes. The band gap value of the heterostructure is insensitive to the vertical strain, while in-plain biaxial strain is an effective way to tune the band gap. The band gap value is in the range of 0.68–1.82 eV under in-plane biaxial strain of −5%–5%. The band offsets of the heterostructure can be increased by positive electric field and decreased by negative electric field. Meanwhile, the type-II band alignment of the heterostructure is retained under vertical strain, in-plane biaxial strain and the electric field. In addition, for the heterostructure, compare to constituent layers, the optical absorption is enhanced in visible region. These results render GaS/AlN heterostructure as a good candidate for photovoltaic devices. • GaS/AlN van der Waals heterostructure possesses a typical type-II band alignment with an indirect band gap of 1.65 eV. • The band gap of the heterostructure can be effectively tuned by strains and electric field. • The type-II band alignment of the heterostructure is retained under strains and electric field. • Compare to constituent layers, the optical absorption coefficient of the heterostructure is enhanced in visible region.

Lattice Thermal Transport in Monolayer Group 13 Monochalcogenides MX (M = Ga, In; X = S, Se, Te): Interplay of Atomic Mass, Harmonicity, and Lone-Pair-Induced Anharmonicity.

We perform a systematic study of the lattice dynamics and the lattice thermal conductivity, κ, of monolayer group 13 monochalcogenides MX (M = Ga, In; X = S, Se, Te) by combining an iterative solution for linearized phonon Boltzmann transport equation and density functional theory. Among the competing factors influencing κ, harmonic parameters along with the atomic masses dominate over anharmonicity. An increase in atomic mass leads to a decrease in phonon frequencies and phonon group velocities and consequently in κ. At T = 300 K, the calculated κ values are 54.9, 48.1, 44.3, 25.0, 22.3, and 17.3 W m-1 K-1 for GaS, InS, GaSe, InSe, GaTe, and InTe monolayers, respectively. Further analysis of anharmonic scattering rates and average scattering matrix elements evidences that the anharmonicity characterized by the third-order IFCs in GaS and InS are the largest among all monolayer group 13 monochalcogenides despite the largest κ values. This is attributed to a strong interaction between nonbonding lone-pair s electrons around the S atom and adjacent bonding electrons. In addition, the κ of these monolayers further reduces to 50% for sample sizes 300-400 nm. Our findings provide fundamental insights into thermal transport in monolayer group 13 monochalcogenides and should stimulate further experimental exploration of thermal transport in these materials for possible theromoelectric and thermal management applications.

Monolayer Gas Adsorption on Graphene-Based Materials: Surface Density of Adsorption Sites and Adsorption Capacity

Surface density of adsorption sites on an adsorbent (including affinity-based sensors) is one of the basic input parameters in modeling of process kinetics in adsorption based devices. Yet, there is no simple expression suitable for fast calculations in current multiscale models. The published experimental data are often application-specific and related to the equilibrium surface density of adsorbate molecules. Based on the known density of adsorbed gas molecules and the surface coverage, both of these in equilibrium, we obtained an equation for the surface density of adsorption sites. We applied our analysis to the case of pristine graphene and thus estimated molecular dynamics of adsorption on it. The monolayer coverage was determined for various pressures and temperatures. The results are verified by comparison with literature data. The results may be applicable to modeling of the surface density of adsorption sites for gas adsorption on other homogeneous crystallographic surfaces. In addition to it, the obtained analytical expressions are suitable for training artificial neural networks determining the surface density of adsorption sites on a graphene surface based on the known binding energy, temperature, mass of adsorbate molecules and their affinity towards graphene. The latter is of interest for multiscale modelling.

Spectral properties of a Co-decorated quasi-two-dimensional GaSe layer

Based on reliable $ab\:initio$ computations and the numerical renormalization group method, systematic studies on a two-dimensional GaSe monolayer with a Co adatom have been carried out. It is shown that the stable lowest-energy configuration of the system involves the Co adatom located over Ga atom. For such configuration, it is demonstrated that the electronic and magnetic properties of the system can be effectively controlled by means of external factors, such as magnetic field, gate voltage or temperature. Moreover, if properly tuned, the GaSe-Co system can also exhibit the Kondo effect. The development of the Kondo phenomenon is revealed in the local density of states of the Co adatom, its magnetic field dependence, which presents the splitting of the Kondo peak, as well as in the temperature dependence of the conductance, which exhibits scaling typical of the spin one-half Kondo effect.

Computational prediction of electronic and optical properties of Janus Ga2SeTe monolayer

Janus group III monochalcogenide structures, which are predicted to have many promising applications in optoelectronics and photocatalytic water splitting, have been of particular interest recently. In this study, electronic properties and optical characteristics of the Janus Ga2SeTe monolayer under a biaxial strain εb and electric field E were considered using the density functional theory. Our calculations demonstrated that the Janus Ga2SeTe monolayer is dynamically and thermally stable and exhibits a semiconducting characteristic with a moderate direct band gap at equilibrium. The band gap of Janus Ga2SeTe monolayer at equilibrium, which is calculated by PBE method and corrected by HSE06 hybrid functional, is smaller than that of both GaSe and GaTe monolayers. Mulliken population analysis shows that there has been a redistribution of charge during the formation of the Janus structure, especially there is a large difference in charge between the two Ga layers in Janus Ga2SeTe monolayer. The biaxial strain has greatly altered the electronic structure of the Janus Ga2SeTe monolayer and direct–indirect band gap transitions were found at appropriate strain εb. While the effect of the E on electronic properties and especially optical properties is weak, the optical absorbance of the Janus Ga2SeTe monolayer can be enhanced by strain engineering, up to 14.42 × 104 cm−1 at εb=−7% in the near-ultraviolet region. The optical absorbance of the Janus Ga2SeTe monolayer is activated in the visible light region that is following its calculated band gap value. This work not only systematically presents the electronic and optical properties of the Janus Ga2SeTe monolayer in the presence of strain engineering and electric field but can also motivate experimental studies for applications in nanoelectromechanical and optoelectronic devices.

Molecular dynamics simulations of interfacial properties of the CO2–water and CO2–CH4–water systems

Molecular dynamics simulations were performed to study the interfacial behavior of the pure carbon dioxide-water system and a binary 40:60 mol. % gas mixture of (carbon dioxide + methane)-water at the temperatures of 275.15 K and 298.15 K and pressures near 4 MPa for CO2 and up to 10 MPa for methane. The simulations are used to study the dynamic equilibrium of the gases at the water-gas interface, to determine the z-density profiles for the gases and water, and calculate the interfacial tension γ under the different temperature/pressure conditions close to those of the formation of clathrate hydrates of these gases. At the same hydrostatic gas phase pressure, the CO2-water interface has a lower interfacial tension than the CH4-water interface. A greater number of CO2 molecules, as much as three times more than methane at the same pressure, were adsorbed at the interfacial layer, which reflects the stronger electrostatic quadrupolar and van der Waals interactions between CO2 and water molecules at the interface. The water surfaces are covered by less than a monolayer of gas even when the pressure of the system goes near the saturation pressure of CO2. The surface adsorbed molecules are in dynamic equilibrium with the bulk gas and with exchange between the gas and interface regions occurring repeatedly within the timescale of the simulations. The effects of the changes in the CO2-water interfacial tension with external temperature and pressure conditions on the formation of the clathrate hydrates and other CO2 capture and sequestration processes are discussed.

Preparation of a bilayer edible film incorporated with lysozyme and its effect on fish spoilage bacteria

AbstractAn edible bilayer film incorporated with lysozyme based on chitosan and sodium alginate was prepared via the layer‐by‐layer method. The film was characterized and its effects against the fish spoilage bacteria Pseudomonas fluorescens and Shewanella putrefaciens were investigated and compared to the monolayer films. The results suggest that electrostatic interactions such as hydrogen bonds occurred between the two layers of the bilayer film. Compared with monolayer films, the mechanical and gas barrier properties of the bilayer film were improved and higher inhibitory activity against both bacteria was exhibited. After treatment with the bilayer film, scanning electron microscopy revealed the serious damage to the bacterial cell surface, and cell membrane permeability and nucleic acid leakage increased. The bilayer film also affected the activity of alkaline phosphatase and adenosine triphosphatase. SDS‐PAGE of bacterial total protein revealed that the protein concentration n decreased, which may be due to the leakage of proteins from the damaged bacterial cell. All the results indicate that the bilayer film possesses good physicochemical properties and can destroy the bacterial cell membrane. Such superior antibacterial properties against fish spoilage bacteria indicate a good potential application of bilayer films in fish preservation.

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

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

Superior and tunable gas sensing properties of Janus PtSSe monolayer

Inspired by superior gas sensing properties of PtSe2 monolayer and tunable gas sensing properties of Janus MoSSe monolayer, we study the gas sensing properties of the Janus PtSSe monolayer for CO, CO2, H2O, NH3, NO and NO2 gas molecules using first-principles density functional calculations. We calculate adsorption height and adsorption energies of the gas molecules to assess the adsorption strength of the gas molecules. Then the charge transfer from PtSSe to gas molecules is evaluated. We also investigate the effects of strain and external electric field on the gas sensing properties of Janus PtSSe monolayer. We finally reveal the origin of the superior gas sensing properties from projected density of states analysis. Our results suggest that the Janus PtSSe monolayer is a promising gas sensor with superior and tunable sensing properties.

Enhancement of hydrogen storage capacity on co-functionalized GaS monolayer under external electric field

Hydrogen storage properties of co-functionalized 2D GaS monolayer have been systematically investigated by first-principles calculations. The strength of the binding energy of hydrogen (H2) molecules to the pristine GaS surface shows the physisorption interactions. Co-functionalized GaS sheet by Li, Na, K and Ca atoms enhanced the capacity of binding energies of hydrogen and strength of hydrogen storage considerably. Besides, DFT calculations show that there is no structural deformation during H2 desorption from co-functionalized GaS surface. The binding energies of per H2 molecules is found to be 0.077 eV for pristine GaS surface and 0.064 eV–0.37 eV with the co-functionalization of GaS surface. Additionally, in the presence of applied external electric field enhanced the strength of binding energies and it is found to be 0.09 eV/H2 for pristine GaS case and 0.19 eV/H2 to 0.38 eV/H2 for co-functionalized GaS surface. Among the studied GaS monolayer is found to be the superior candidate for hydrogen storage purposes. The theoretical studies suggest that the electronic properties of the 2D GaS monolayer show the electrostatic behavior of hydrogen molecules which confirms by the interactions between adatoms and hydrogen molecules before and after hydrogen adsorption.

Exceptionally tunable electronic, optical and transport properties of two dimensional GaS doped with group II and group IVa elements

In this present study, we systematically investigated the structural, electronic, optical and transport properties of pristine and group II, group IVa doped GaS monolayers using density functional theory (DFT). The strong formation energy suggests realization of group II & group IVa doped GaS. A semiconductor to metallic transition occurs in GaS monolayer with doping. Moreover, doped GaS monolayers have shown tunable optical properties. Doped GaS monolayers show optical activity in both ultraviolet (UV) as well as visible region. EEL spectra of GaS shift towards high energy region (red shift) with group II elements doping while no significant shift is observed for group IVa elements doping in both the polarizations. We found quantum conductance of 4G0 for doped GaS monolayer except for Be-doped GaS. The metallic character of doped GaS is clearly captured in tunneling current characteristics showing ohmic-like characteristics for doped GaS monolayer. This is due to finite density of states associated with S atoms of doped GaS. The doping extends the functionalities of pristine GaS with tunable properties suggesting doped GaS as a potential candidate for use in photo-diodes, photo-catalysts, photo-detectors and bio-sensing.

Tuning the electronic transport properties of p-type GaS monolayer by the application of biaxial strain

The first-principles calculations based on Density Functional Theory are performed in order to investigate the effect of strain on the electronic structure and transport properties of the p-type GaS monolayer. Our calculations show that unstrained GaS monolayer is semiconducting in nature having an indirect energy band gap of 2.35 eV. The application of compressive or tensile strain modifies the electronic band structure and energy band gap which further modifies the electronic transport coefficients of GaS monolayers. The energy band gap of GaS monolayer increases (decreases) under the application of compressive (tensile) strain. The Seebeck coefficient of p-type GaS monolayer is increased by applying strain, giving rise to the increase in the power factor.