Gas Monolayers Research Articles

HyperSolar reaches new milestones and reveals next steps for its renewable hydrogen technology

Two-dimensional (2D) materials have been paid great attention due to their unique properties and promising applications in photochemistry and photoelectricity. In this theoretical study, a systematic investigation has been carried out to explore the effect of several isovalent ionic dopants (O, Se, Te, Al, In, Sc and Y) on the electronic structure of GaS monolayer using the HSE06 functionals. Although monodoping with O reduces the band gap of GaS monolayer, its conduction band minimum (CBM) shifts downward below the hydrogen reduction potential, while the band gap remains almost unchanged for monodoping with Se, Te, Al and In dopants. However, the scenario changes on monodoping with Sc and Y dopants, which can effectively narrow the band gap because of shifting up for valence band maximum (VBM) and shifting down for CBM, respectively. In addition, (Sc, Te) and (Y, Te)-codoped GaS monolayers are also investigated. The calculation reveals that band gap is further narrowed for both codoped cases. The extent of band gap narrowing in the case of codoping with (Y, Te) is quite significant (almost 1 eV) to effectively improve the visible light activity of GaS monolayer. Moreover, both codoped systems are more suitable for overall water splitting than that of monodoping systems, which has been confirmed though band edge alignment with respect to water redox levels. However, upon (Sc, Te) codoping system, the CBM mainly stems from 3d orbital of Sc, which is extremely detrimental for improving solar-to-hydrogen conversion efficiency. Fortunately, (Y, Te)-codoped GaS monolayer shows better for water splitting than that of (Sc, Te)-codoping. The calculated optical absorption spectra confirm that (Y, Te)-codoped system has significantly improved visible light absorption. According to the defect formation energy calculation, the formation energies are all negative values except monodoping with In and Te systems, indicating dopants are possible introduced in the experiments. Thus, these findings imply that (Y, Te) is a suitable dopant pair for the controlled band gap engineering of GaS monolayer and (Y, Te)-codoped GaS monolayer should be a promising visible light photocatalyst for water splitting.

Optical properties of GaS-Ca(OH)2bilayer heterostructure

Finding novel atomically thin heterostructures and understanding their characteristic properties are critical for developing better nanoscale optoelectronic devices. In this study, we investigate the electronic and optical properties of a GaS-Ca(OH)${}_{2}$ heterostructure using first-principle calculations. The band gap of the GaS-Ca(OH)${}_{2}$ heterostructure is significantly reduced when compared to those of the isolated constituent layers. Our calculations show that the GaS-Ca(OH)${}_{2}$ heterostructure is a type-II heterojunction which can be used to separate photoinduced charge carriers where electrons are localized in GaS and holes in the Ca(OH)${}_{2}$ layer. This leads to spatially indirect excitons which are important for solar energy and optoelectronic applications due to their long lifetime. By solving the Bethe-Salpeter equation on top of a single shot GW calculation $({G}_{0}{W}_{0})$, the dielectric function and optical oscillator strength of the constituent monolayers and the heterostructure are obtained. The oscillator strength of the optical transition for the GaS monolayer is an order of magnitude larger than the Ca(OH)${}_{2}$ monolayer. We also found that the calculated optical spectra of different stacking types of the heterostructure show dissimilarities, although their electronic structures are rather similar. This prediction can be used to determine the stacking type of ultrathin heterostructures.

Thickness-dependent nanofriction of a rare gas monolayer sliding on Pb(111) ultrathin films

The friction can be affected dramatically by quantum size effects (QSEs) and edge effects at nanoscale. The modulations of QSEs on nanofriction of a rare gas (RG) monolayer sliding on Pb(111) ultrathin films were investigated by using the first-principles approach within density functional theory (DFT) with van der Waals (vdW) interaction correction. Our findings revealed that there exist even-odd oscillations in the friction with the thickness of Pb(111) substrate and the friction can be tuned up to 30% by the different thicknesses of Pb(111) films. Moreover, such modulation is more obvious for the RG adatoms with larger radius. The underlying physics is that the oscillations of the electronic density of states at Fermi level induce different interactions and energy barriers between RG and Pb(111) films with different thicknesses. Overall, we here propose an approach to tune friction and a way to identify the electronic contribution to friction via the different thicknesses of substrates at nanoscale.

Novel hetero-bilayered materials for photovoltaics

The recently synthesized GaS and MoSe2 nanosheets have been used as appropriate substrates for other layered materials, e.g. silicene/GaS heterosheets akin to graphene/BN systems. Here, we have performed a comprehensive first-principles study of the electronic and optical properties of two-dimensional (2D) GaS/MoSe2 hetero-bilayers based on density functional theory (DFT). We found almost all proposed GaS/MoSe2 hetero-bilayers in the current study have an indirect band gap from the Γ point to the K point, except for one with a direct band gap at the K point. Tunable band gaps GaS/MoSe2 hetero-bilayers can be controlled by strain modulation. State-of-the-art GW-Bethe–Salpeter method, accounting for electron-electron and electron-hole interactions, has been employed to compute accurate absorbance spectra for layered materials. Compared with its composing GaS and MoSe2 monolayers, GaS/MoSe2 hetero-bilayers show superior behavior on optical absorbance, indicating a stronger visible-light absorption and applications in solar energy harvesting. We foresee that the novel GaS/MoSe2 hetero-bilayers would stimulate the fabrication of materials with unprecedented optical and physico-chemical properties that may apply in nanodevices and photovoltaic cells.

Influential Electronic and Magnetic Properties of the Gallium Sulfide Monolayer by Substitutional Doping

The structural, electronic, and magnetic properties of the GaS monolayer doped by 12 different kinds of atoms were investigated systemically using first-principles calculations. N is found to be the most promising candidate for p-type doping among dopants at the S site, including nonmetal atoms H, B, C, N, O, and F and transition metal atoms V, Cr, Mn, Fe, Co, and Ni. Transition metal atoms appear to be hardly incorporated in the GaS monolayer under either S- or Ga-rich conditions. While the net magnetic moments of doped GaS by nonmetal atoms are either 0 or 1 μB, the value of transition metal dopants decreases from 5 to 0 μB by adding the number of valence electrons from V to Ni. In the case of transition metal dopants at the Ga site, the majority spin states of Cr and Co are located closest to the conduction band minimum and valence band maximum, respectively. Magnetic ground states exist in all of the monolayers doped by these impurities. Indirect band gap of the pristine GaS monolayer is regulated to ...

Piezoelectricity in two-dimensional group-III monochalcogenides

We find that several layer-phase group-III monochalcogenides, including GaS, GaSe and InSe, are piezoelectric in the monolayer form. First-principles calculations reveal that the piezoelectric coefficients of monolayer GaS, GaSe and InSe are on the same order of magnitude as the earlier discovered two-dimensional piezoelectric materials, such as BN and MoS2 monolayers. Our study expands the family of two dimensional piezoelectric materials, suggesting that strong piezoelectric response can occur in a wide range of two dimensional materials with broken inversion symmetry. The co-existence of piezoelectricity and superior photo-sensitivity in these two-dimensional semiconductors enables the integration of electromechanical and optical sensors on the same material platform.

Red-to-Ultraviolet Emission Tuning of Two-Dimensional Gallium Sulfide/Selenide.

Graphene-like two-dimensional (2D) nanostructures have attracted significant attention because of their unique quantum confinement effect at the 2D limit. Multilayer nanosheets of GaS-GaSe alloy are found to have a band gap (Eg) of 2.0-2.5 eV that linearly tunes the emission in red-to-green. However, the epitaxial growth of monolayers produces a drastic increase in this Eg to 3.3-3.4 eV, which blue-shifts the emission to the UV region. First-principles calculations predict that the Eg of these GaS and GaSe monolayers should be 3.325 and 3.001 eV, respectively. As the number of layers is increased to three, both the direct/indirect Eg decrease significantly; the indirect Eg approaches that of the multilayers. Oxygen adsorption can cause the direct/indirect Eg of GaS to converge, resulting in monolayers with a strong emission. This wide Eg tuning over the visible-to-UV range could provide an insight for the realization of full-colored flexible and transparent light emitters and displays.

Detection of Volatile Organic Compounds Using Microfabricated Resonator Array Functionalized with Supramolecular Monolayers.

This paper describes the detection of volatile organic compounds (VOCs) using an e-nose type integrated microfabricated sensor array, in which each resonator is coated with different supramolecular monolayers: p-tert-butyl calix[8]arene (Calix[8]arene), 2,3,7,8,12,13,17,18-octaethyl-21H,23H-porphine (Porphyrin), β-cyclodextrin (β-CD), and cucurbit[8]uril (CB[8]). Supramolecular monolayers fabricated by Langmuir-Blodgett techniques work as specific sensing interface for different VOCs recognition which increase the sensor selectivity. Microfabricated ultrahigh working frequency film bulk acoustic resonator (FBAR) transducers (4.4 GHz) enable their high sensitivity toward monolayer gas sensing which facilitate the analyses of VOCs adsorption isotherms and kinetics. Two affinity constants (K1, K2) are obtained for each VOC, which indicate the gas molecule adsorption happen inside and outside of the supramolecular cavities. Additional kinetic information on adsorption and desorption rate constants (ka, kd) are obtained as well from exponential fitting results. The five parameters, one from the conventional frequency shift signals of mass transducers and the other four from the indirect analyses of monolayer adsorption behaviors, thus enrich the sensing matrix (Δf, K1, K2, ka, kd) which can be used as multiparameter fingerprint patterns for highly selective detection and discrimination of VOCs.

A first-principles investigation of various gas (CO, H2O, NO, and O2) absorptions on a WS2 monolayer: stability and electronic properties

Using first-principles calculations, we investigate the interactions between a WS2 monolayer and several gas molecules (CO, H2O, NO, and O2). Different sets of calculations are performed based on generalized-gradient approximations (GGAs) and GGA + U ( eV) calculations with D2 dispersion corrections. In general, GGA and GGA + U establish good consistency with each other in terms of absorption stability and band gap estimations. Van der Waals density functional (vdW-DF) calculations are also performed to validate long-range gas molecule–WS2 monolayer interactions, and the resultant absorption energies of four gas-absorption cases (from 0.21 to 0.25 eV) are significantly larger than those obtained from calculations using empirical D2 corrections (from 0.11 to 0.19 eV). The reported absorption energies clearly indicate van der Waals interactions between the WS2 monolayer and gas molecules. The NO and O2 absorptions are shown to narrow the band gaps of the WS2 material to 0.75–0.95 eV and produce small magnetic moments (0.71 μB and 1.62 μB, respectively). Moreover, these two gas molecules also possess good charge transferability to WS2. This observation is important for NO- and O2-sensing applications on the WS2 surface. Interestingly, WS2 can also activate the dissociation of O2 with an estimated barrier of 2.23 eV.

The electronic structure of quasi-free-standing germanene on monolayer MX (M = Ga, In; X = S, Se, Te).

For the first time by using the ab initio density functional theory, the stability and electronic structures of germanene on monolayer GaS, GaSe, GaTe and InSe have been investigated. Germanene preserves its buckled-honeycomb structure on all the studied substrates similar to the free-standing case. Moreover, germanene stays neutral and preserves its Dirac-cone-like band structure on monolayer GaTe and InSe. In these two cases, a bandgap of 0.14-0.16 eV opens at the Dirac point of germanene, while the effective masses remain as small as 0.05-0.06 times the free-electron mass. The estimated carrier mobility is up to 2.2 × 10(5) cm(2) V(-1) s(-1). These features show that monolayer GaTe and InSe are promising as substrates for germanene devices.

Intrinsic defects in gallium sulfide monolayer: a first-principles study

The electronic and magnetic properties of native point defects, including vacancies (VGa and VS), antisites (GaS and SGa) and interstitials (Gai and Si) in monolayer and bulk GaS, were systemically studied using the density functional theory method.

Functionalization of a GaSe monolayer by vacancy and chemical element doping.

Based on first-principles plane-wave calculations, functionalization of the two-dimensional single-layered GaSe structure through vacancy and chemical element doping has been investigated. Our calculations show that the pristine GaSe monolayer, which is normally a non-magnetic, indirect-band-gap semiconductor, can induce net magnetic moments by introduction of Ga mono-vacancy, Ga di-vacancy, and GaSe3 and Ga2Se6 vacancy complexes. Magnetic moments can also be induced by selectively doping specific transition-metal atoms as well as A group atoms. The introduced donor or acceptor states are localized in the band gap, which expands the utilization of the single-layered GaSe in nanoelectronics and spintronics. In spite of the intrinsic p-type character of the two-dimensional GaSe material, substitution of Si for Ga and substitution of Cl for Se exhibit n-type character at relatively low dopant concentrations. These findings will provide useful supplements to the experimental studies on the newly synthesized two-dimensional layered metal monochalcogenides, which allows us to go beyond the current scope that is limited to applications within graphene, BN, and transition-metal dichalcogenide-based nanostructures.

A statistical thermodynamics model for monolayer gas adsorption on graphene-based materials: implications for gas sensing applications

Field-effect transistor-based conductance gas sensors are attracting considerable research interest because of their miniaturized size, high sensitivity, and portability. Here, we propose statistical thermodynamics models to analytically characterize monolayer gas molecule adsorption on crystalline and amorphous solid surfaces in terms of adsorption density and coverage, respectively, which are critical for gas sensing applications. By testing the monolayer adsorption of gas molecules, such as CO, NO, NH3 and NO2, on graphene and reduced graphene oxide (rGO), we found that adsorption density and coverage can be estimated from only the binding energy of gas molecules with a high accuracy compared with the exact values obtained by solving the Schrödinger equation. Moreover, the experimentally observed (e.g., NO2) density on graphene is close to our theoretical prediction. The proposed approaches to quantitatively characterize gas adsorption on solid surfaces are of great significance to help understand and optimize the performance of gas sensors.

Effect of Gas Species on Gas–Monolayer Interactions: Tangential Momentum Accommodation

The tangential momentum accommodation coefficient (TMAC) of five gas species—He, N2, Ar, CO2, and SF6—was measured on an octadecyltrichlorosilane-coated glass surfaces at 1.0 atm and 24 ± 2 °C. The TMAC was determined from measurement of the damping on a glass sphere near a glass plate when the separation between the sphere and plate was less than 1500 nm so that the flow was at or near the slip-flow regime. The results show that the damping depends on the gas species and that the fitted accommodation coefficient for these gases decreases as the molar mass increases. This trend can be explained using a simple physical model of the collision between two spheres, consisting of an incoming sphere to model the gas and an initially stationary sphere to model part of the monolayer adsorbed on the solid. In this simple model, heavier incoming molecules conserve more average tangential momentum than do lighter molecules. We also consider the possibility of separating a mixture of gases based on differences in TMACs.

Thermolubricity of gas monolayers on graphene.

Nanofriction of Xe, Kr and N₂ monolayers deposited on graphene was explored with a quartz crystal microbalance (QCM) at temperatures between 25 and 50 K. Graphene was grown by chemical vapour deposition and transferred to the QCM electrodes with a polymer stamp. It was found to strongly adhere to the gold electrodes at temperatures as low as 5 K and at frequencies up to 5 MHz. At low temperatures, the Xe monolayers are fully pinned to the graphene surface. Above 30 K, the Xe film slides and the depinning onset coverage beyond which the film starts sliding decreases with temperature. Similar measurements repeated on bare gold show an enhanced slippage of the Xe films and a decrease of the depinning temperature below 25 K. Nanofriction measurements of Kr and N₂ confirm this scenario. This thermolubric behaviour is explained in terms of a recent theory of the size dependence of static friction between adsorbed islands and crystalline substrates.

Monolayer gas adsorption in plasmonic sensors: Comparative analysis of kinetic models

The analysis of adsorption kinetics is given, assuming no dissociative adsorption takes place and no multiple binding sites exist. The phenomenological nonlinear model and the simple linear model are considered from the point of view of surface plasmon resonance (SPR) chemical sensors. A simple method for analytical calculation of the necessary rate constants is proposed. A comparative analysis of the described models is performed. Two conditions that affect the validity of the linear model are found. First one, referring to the ratio between the overall number of particles and the number of binding sites, and the second one, that guarantees the validity of linear model in much broader spectrum of practical situations, referring to the ratio of rate constants. A method for avoiding the accumulation of numerical errors in calculus is also proposed. The presented approach is general and can be used in various applications, including other types of chemical sensors, membranes for filtering, different catalytic systems, resonant micro or nano-electro-mechanical structures.

Tunable electronic and dielectric behavior of GaS and GaSe monolayers

Here we present first-principles calculations to investigate systematically the electronic behavior and the electron energy low-loss spectra (EELS) of monolayer, bilayer, four-layer, and bulk configurations of periodic GaX (X = S, Se), as well as the effect of mechanical strain on the electronic properties of the GaX monolayer. We predicate that the GaX monolayer is a semiconductor with an indirect band gap, however, the difference between the direct and indirect gaps is so small that electrons can transfer easily between this minimum with a small amount of thermal energy. Owning to strong surface effects, the electronic and dielectric properties of GaX vary drastically with number of layers in a sheet. In detail, the band gap increases from multilayer-to-single layer and EELS shifts towards larger wavelengths with a decrease in the layer thickness. Moreover, we demonstrate that the band gaps of GaX monolayers can be widely tuned by mechanical deformation, making them potential candidates for tunable nanodevices. The present study provides theoretical insight leading to a better understanding of these novel 2D structures.

Imbibition Transition: Gas Intercalation Between Graphene and a Solid Surface

Arguments are presented to the effect that a first-order transition can occur in which a gas monolayer imbibes within a slit pore, forming an intercalated layer. The calculation is carried out at T=0 for simplicity. The concept of this transition is potentially relevant to structural transitions observed when gas absorbs within metal-organic-framework materials and the response of graphene/silica electronic devices.

Excitation and detection of surface-plasmon polariton waves on Cu(111) with gratings of rare gas monolayers

We excited surface-plasmon polariton waves (SPPWs) on Cu(111) by coupling optical beams with adsorbed xenon gratings. The SPPWs's excitation causes a resonancelike dip in the angle-resolved reflectivity difference measurement. From the resonance we determined optical constants epsilonCu(633 nm)=-9.53+i0.142 and epsilonCu(780 nm)=-13.44+i0.18. The grating-coupled SPPWs can be used to study mass transport on thin films.

Vaporization and layering of alkanols at the oil/water interface

This study of adsorption of normal alkanols at the oil/water interface with x-ray reflectivityand tensiometry demonstrates that the liquid to gas monolayer phase transition at thehexane/water interface is thermodynamically favourable only for long-chain alkanols. Asthe alkanol chain length is decreased, the change in excess interfacial entropy per areaΔSaσ decreases to zero. Systems with small values ofΔSaσ formmulti-molecular layers at the interface instead of the monolayer formed by systems with much largerΔSaσ. Substitution of n-hexane by n-hexadecane significantly alters the interfacial structure for agiven alkanol surfactant, but this substitution does not fundamentally change the phasetransition behaviour of the monolayers. These data show that the critical alkanol carbonnumber, at which the change in excess interfacial entropy per area decreases to zero, isapproximately six carbons larger than the number of carbons in the alkane solventmolecules.