Properties Of Monolayers Research Articles

Spectroscopic Ellipsometry Study of the Temperature Dependences of the Optical and Exciton Properties of MoS2 and WS2 Monolayers

The optical properties of MoS2 and WS2 monolayers are significantly influenced by fabrication methods, especially with respect to the behavior of excitons at the K−point of the Brillouin zone. Using spectroscopic ellipsometry, we obtain the complex dielectric functions of monolayers of these materials from cryogenic to room temperatures over the energy range 1.5 to 6.0 eV. The excitonic structure of each sample is analyzed meticulously by fitting the data to a standard analytical function to extract the energy positions of the excitons at each temperature. At low temperatures, excitonic structures are blue-shifted and sharpened due to the reduction in phonon noise and lattice distance. The excitons of monolayers fabricated by MOCVD separate into sub-structures at low temperatures, while monolayers grown by LPCVD and APCVD remain a single peak. The origin of these peaks as charged or neutral excitons follows from their temperature dependences.

Materials beyond monolayers: The magnetic quasi-1D semiconductor CrSBr

AbstractThe all-surface nature of atomically thin van der Waals materials can present challenges for practical applications. Fortunately, new layered materials are on the horizon that preserve their useful properties even when thicker than a monolayer. Here, we summarize our interest in one of these emergent materials, the magnetic semiconductor CrSBr. We describe monolayer properties exhibited by this material in its bulk form, discussing how the quasi-1D electronic structure of CrSBr allows mono- or bilayer physics to be displayed even in thick crystals. Long-range magnetic order offers additional tuning with the coupled lattice, spin, orbit, and charge degrees of freedom enabling magneto-correlated phenomena. We discuss the stability of CrSBr in air and show atomic scale structural manipulation through electron beam-driven transformations. We conclude that the stability and structural amenability of CrSBr provide opportunities for imagining devices that use bulk crystals yet exploit unique magnetic and quantum confinement effects. Graphical abstract

Improved gas sensing properties of copper-doped MoSe2 monolayers: a first-principles study on CO2 and NO2 adsorption

Abstract This paper investigates the impact of copper (Cu)-doped MoSe2 monolayer (ML) on gas sensing properties. The adsorption behaviour of CO2 and NO2 gas molecules on Cu-doped MoSe2 ML is reported and various sensing and electronic parameters such as adsorption energy, charge transfer and recovery time, are computed to delineate the adsorption characteristics and gas sensitivity. In our study, the doped Cu-atom in Se vacant MoSe2 ML shows the adsorption energies to be −1.32 eV (O-orient) and −1.72 eV (C-orient), for CO2, while for NO2, they are −0.879 eV (O-orient) and −1.06 eV (N-orient), respectively. The negative formation energy of −6.62 eV shows the electronic stability of Cu-doped MoSe2 ML and thermal stability, demonstrated by the molecular dynamics study through the Nose-Vervlet Thermostat (NVT) algorithm. Also, significant changes are observed in electronic conductivity and work function upon adsorbed Cu-MoSe2 ML. Lastly, these outcomes illustrate that Cu doping amplifies the capture ability of MoSe2 ML towards gas molecules, fostering improved electron interaction between the substrate surface and gas molecules, consequently enhancing its gas sensing ability.

Searching for the role of membrane lipids in the mechanism of antibacterial effect of hinokitiol

The aim of this work was to investigate the effect of monoterpenoid hinokitiol (β-thujaplicin) on the monolayers and bilayers composed of lipids typical for bacteria membranes and gain insight into the potential role of the lipids in antibacterial activity and selectivity of this compound. To explore this issue, the in vitro studies were performed on different bacterial strains to verify antibacterial potency of hinokitiol. Then, the experiments on E. coli and S. aureus bacteria membrane models (i.e. model multicomponent monolayers and bilayers) were done. Finally, the effect of hinokitiol on one component lipid monolayers was investigated. The lipids used in the experiments included Phosphatidylethanolamines (PEs), Phosphatidylglycerols (PGs) and Cardiolipins differing in the structure of the polar head and/or the hydrophobic chains. This choice allowed the analysis of correlations between the lipid structure and the effect of hinokitiol.In vitro tests confirmed the antimicrobial activity of hinokitiol against most of the strains tested. In addition, the in vitro tests showed that E. coli bacteria were more sensitive to hinokitiol than S. aureus bacteria. Interestingly, the studies on model systems evidenced that hinokitiol molecules are of stronger effect on E.coli film and they are able to insert into these systems even at membrane-related surface pressures. Moreover, the structure of the lipid and its content in the model system correlated with the effect exerted by hinokitiol on the monolayer properties. It was found that hinokitiol differs in the affinity to particular lipids and additionally hinokitiol/lipid interactions may occur according to different mechanisms. Namely, depending on the lipid structure, hinokitiol may incorporate into the lipid film (Cardiolipins and PEs) or interact preferentially with the lipid polar head (PGs) and form hydrogen bonds. The effect of hinokitiol on the lipids was determined by the charge and size of the polar head as well as by the spatial size of the lipid molecule. Moreover, comparing the lipids of the same polar heads, hinokitiol caused stronger expansion of the film formed from the lipid having unsaturated chains. The results obtained may explain the difference in the effect of hinokitiol on particular bacterial strains. In conclusions, it can be suggested that the lipids should be considered as the bacteria membrane structural elements of a possible role in the mechanism of action of hinokitiol.

Buckling-induced variations in electronic, thermal, and optical properties of B[formula omitted]C[formula omitted]N[formula omitted] monolayer: DFT and AIMD computational approaches

Density functional theory is employed to study the novel properties of B3C2N3 monolayer to gain a deeper understanding of variation of electronic, thermal, and optical characteristics arising due to buckling effects. The band structure analysis reveals an energy gap reduction of the buckled B3C2N3 monolayer, causing a displacement of the band gap from the visible to the infrared range. Moreover, the buckling controls the location of the initially, and finally, direct band gap moving it from the K to the Γ point in the B3C2N3 monolayer. The phonon band structure calculations indicate that buckled B3C2N3 monolayers are dynamically stable, while ab-initio molecular dynamics simulations, AIMD, evaluate and confirm the thermal stability of both flat and buckled B3C2N3 monolayers. The buckling phenomenon at low temperatures has no a significant impact on the heat capacity contrary to what happens in the high temperature limit. The optical characteristics of the B3C2N3 monolayer, including refractive index, optical conductivity, static dielectric function, and plasmon frequency, are evaluated at different levels of the buckling parameter. The static dielectric function and plasmon frequency are enhanced with the buckling due to the screening of the electron–electron interactions, affecting the collective oscillations. Enhanced screening gives rise higher plasmon frequencies. Tuning the buckling parameter illustrates the significance of buckling as an alternative mechanism for adjusting the performance of B3C2N3 two-dimensional materials for different technological applications such as solar and optoelectronic systems.

Strain- and Carrier-Engineered Electromagnetic Properties in Transition-Metal Chalcogenide Monolayers: Implications for Nanoscale Spintronic Devices

Strain- and Carrier-Engineered Electromagnetic Properties in Transition-Metal Chalcogenide Monolayers: Implications for Nanoscale Spintronic Devices

Mechanical Properties of Two-Dimensional Metal Nitrides: Numerical Simulation Study

It is expected that two-dimensional (2D) metal nitrides (MNs) consisting of the 13th group elements of the periodic table and nitrogen, namely aluminium nitride (AlN), gallium nitride (GaN), indium nitride (InN) and thallium nitride (TlN), have enhanced physical and mechanical properties due to the honeycomb, graphene-like atomic arrangement characteristic of these compounds. The basis for the correct design and improved performance of nanodevices and complex structures based on 2D MNs from the 13th group is an understanding of the mechanical response of their components. In this context, a comparative study to determine the elastic properties of metal nitride nanosheets was carried out making use of the nanoscale continuum modelling (or molecular structural mechanics) method. The differences in the elastic properties (surface shear and Young’s moduli and Poisson’s ratio) found for the 2D 13th group MNs are attributed to the bond length of the respective hexagonal lattice of their diatomic nanostructure. The outcomes obtained contribute to a benchmark in the evaluation of the mechanical properties of AlN, GaN, InN and TlN monolayers using analytical and numerical approaches.

Importance of magnetic shape anisotropy in determining triaxial magnetic anisotropy of ferromagnetic semiconductor CrSCl monolayer

Abstract Magnetic anisotropy (MA) is pivotal for stabilizing long-range magnetic order in two-dimensional (2D) systems against thermal fluctuations. Here, we conduct a comprehensive investigation of the electronic and magnetic properties of CrSCl monolayer using first-principles methods and Monte Carlo (MC) simulations. Our results reveal that CrSCl monolayer exhibit a direct band gap ferromagnetic semiconductor (FMS) with a high Curie temperature (TC, 143 K). Notably, we identify triaxial magnetic anisotropy in this monolayer, characterized by the easy magnetization axis along the y-axis, intermediate axis along the x-axis, and hard axis along the z-axis. This anisotropy arises from a combination of magnetocrystalline anisotropy and shape anisotropy, in which shape anisotropy dominating over weak magnetocrystalline anisotropy. Orbital projection analysis shows that the major contribution of magnetic anisotropy energy comes from the d orbital of Cr atom. These findings provide some insights into the strain response of MA and suggest that studies of other FM monolayers may uncover future contenders for strain-switchable and ultra-compact spintronics devices.

Magnetic ordering and dynamics in monolayers and bilayers of chromium trihalides: atomistic simulations approach

We analyze magnetic properties of monolayers and bilayers of chromium iodide, \\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\hbox {CrI}_3$$\\end{document}, in two different stacking configurations: AA and rhombohedral ones. Our main focus is on the corresponding Curie temperatures, hysteresis curves, equilibrium spin structures, and spin wave excitations. To obtain all these magnetic characteristic, we employ the atomistic spin dynamics and Monte Carlo simulation techniques. The model Hamiltonian includes isotropic exchange coupling, magnetic anisotropy, and Dzyaloshinskii-Moriya interaction. As the latter interaction is relatively weak in pristine \\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\hbox {CrI}_3$$\\end{document}, we consider a more general case, when the Dzyaloshinskii-Moriya interaction is enhanced externally (e.g. due to gate voltage, mechanical strain, or proximity effects). An important issue of the analysis is the correlation between hysteresis curves and spin configurations in the system, as well as formation of the skyrmion textures.

Tunable electronic and optoelectronic characteristics of two-dimensional g-GeC monolayer: a first-principles study

Two-dimensional (2D) semiconductor materials have emerged as one of the hotspots in recent years due to their potential applications in beyond-Moore technologies. In this work, we systematically investigate the electronic and optoelectronic properties of the g-GeC monolayer combined with strain engineering using first-principles calculations. The results show that g-GeC monolayer possesses a suitable direct bandgap and a strain-tunable electronic structure. On this basis, the designed g-GeC monolayer-based two-probe photodetector exhibits a robust broadband optical response in the visible and near-ultraviolet ranges, along with significant polarization sensitivity and high extinction ratio. In addition, strain engineering can greatly improve the optoelectronic performance of the g-GeC-based photodetector and effectively tune its detection range in the visible and near-ultraviolet regions. These findings not only deepen the comprehension of g-GeC nanosheet but also provide the possibility of its application in nano-optoelectronic devices.

The electronic and magnetic properties of Cr-doped ZnO monolayer at higher percentage by first principles calculations

The crystal structure, structural stability, electronic band structures and magnetization of Cr-doped ZnO monolayer (ML) have been investigated by GGA+U method using first principles calculations in this work. The stability of these systems is confirmed by the negative formation energies at different Cr concentrations. The calculated band gaps with and without the Hubbard U correction aligned well with other computational results. With the increase of Cr concentration, the band gap of Cr-doped ZnO monolayer decreases for the spin-up configuration. However, in the spin-down configuration, the band gap increases first and then decreases and the maximum band gap is obtained at 22.22% Cr concentrations. At 33.33% Cr concentration, the ZnO ML exhibits metallic behavior. The density of states (DOS) analysis reveals an asymmetric nature, indicating ferromagnetism in Cr-doped ZnO MLs at various concentrations, with a magnetic moment per Cr atom of approximately 3.99 μB. The total magnetic moment rises with increasing Cr concentration. Charge density difference analysis shows increased magnetization with higher Cr concentration. At 22.22% Cr concentration, the ZnO ML exhibits ferromagnetism and semiconducting properties, suggesting significant potential for spintronic applications as a diluted magnetic semiconductor.

Multifaceted insights into pentagonal BeP2 monolayer: From electrochemical to mechanical resilience for alkali-ion batteries using DFT

Herein, we elucidated the properties of a pentagonal BeP2 monolayer, emphasizing its robust dynamic, thermal, and mechanical stabilities, as well as its promising electrochemical characteristics. Combining density functional theory (DFT) with a thermodynamic energy decomposition scheme, we identified BeP2 as a potentially excellent 2D material for energy storage devices. It is characterized by large lattice parameters and semi-metallic behavior, exhibiting a zero density of states (DOS) at the Fermi level. It demonstrates favorable open-circuit voltages, low metal ion migration barriers, and negligible structural modifications for Na and K ions. The specific capacities of Li/Na/K reached remarkable values, surpassing those of most previously reported monolayers. Emerging ion battery technology necessitates solid, directly comparable data to establish an optimized standard organic solvent electrolyte. With this objective in mind, we systematically investigated systems combining two Li/Na/K salts with three different alkyl carbonate solvents. Furthermore, we proposed a defect in BeP2 to explore all possible avenues for sustaining the pentagonal BeP2 monolayer (supplementary Fig. S5 and S6). In summary, our findings strongly indicate that the pentagonal BeP2 monolayer holds considerable promise as an anode material for sodium-ion batteries (NIBs) and potassium-ion batteries (KIBs) owing to its high electrochemical and mechanical sustainability and excellent compatibility with electrolytes.

Calculation of structural, electronic, optical, dynamic and thermoelectric properties of monolayer made of carbon and nitrogen

Calculation of structural, electronic, optical, dynamic and thermoelectric properties of monolayer made of carbon and nitrogen

Unveiling the optoelectronic properties of bulk, monolayer, and bilayer TiS2: A DFT approach

Over the years, there has been significant research aimed at improving the performance of titanium disulfides (TiS2) in a wide range of applications, including lubricants, batteries, thermoelectric and electronic devices, catalysts, superconductors, photovoltaic devices, and more. This work investigates the optoelectronic properties of TiS2 using Density Functional Theory (DFT) calculations. The slab models were constructed for bulk, bilayer, and monolayer TiS2 (001) planes based on the 1T-TiS2 hexagonal phase. The GGA-PBE functional yielded the most accurate bandgaps: 0.178 eV (bulk), 0.047 eV (bilayer), and 0.112 eV (monolayer). The calculated lattice constants for bulk TiS2 with GGA-PBE were a = b = 3.407 Å and c = 5.697 Å, with an equilibrium volume of 57.24 ų. Electronic density of states (DOS) analysis revealed semiconducting behavior for both bulk and monolayer TiS2, with dominant peaks at the valence band. Bilayer TiS2 exhibited a higher DOS in the conduction band, indicating a more conductor-like character. Light absorption calculations showed the strongest peak for bilayer TiS2 (∼610,000 cm⁻¹ at 14.8 eV), followed by monolayer (∼13 eV) and bulk (∼12 eV). These results suggest that bulk TiS2 is preferable for applications requiring superior electrical properties, while bilayer TiS2 is more advantageous for applications focusing on light capture.

A Janus WSi2P2As2 monolayer: A promising material for detecting NO2 with excellent sensitivity, selectivity, reversibility, and humidity resistance

Realizing highly sensitive, selective, humidity-resistant, and reversible detection of hazardous gas molecules with low detection limit in the air environment at room temperature is essential. Herein, we predict a novel two-dimensional Janus WSi2P2As2 material and propose a NO2 gas sensor with excellent response, selectivity, humidity resistance, and reversibility based on WSi2P2As2, by employing density functional theory, statistical thermodynamic modeling, and the non-equilibrium Green’s function approach. The Janus WSi2P2As2 monolayer is a stable semiconductor with a direct band gap of 0.85 eV and an intrinsic out-of-plane dipole moment. We study the sensing properties of the Janus WSi2P2As2 monolayer towards several typical toxic gases (NO, NO2, NH3, SO2, and H2S). All the gas molecules are physically adsorbed on the surface. Different adsorption energies and electronic structures are found for the top- and bottom-surface adsorptions due to the intrinsic dipole of the Janus WSi2P2As2. The experimental viability is characterized by the adsorption density and the recovery time. The statistical estimation of the adsorption density shows that the oxide gases can be adsorbed practically even at ultra-low concentration of part per billion (ppb) at room temperature. The rapid natural desorption of these toxic gases at 300 K indicates the recyclability of the WSi2P2As2 material. Furthermore, we evaluate the sensing performance of the WSi2P2As2-based gas sensor by the conductance change. The sensor exhibits ultra-response of 374 % and excellent selectivity towards NO2 molecules. Importantly, the sensor remains operating with high performance in the humid environment and air environment. These results demonstrate that the Janus WSi2P2As2 is a promising gas-sensing material for the NO2 detection.

Theoretical Study of the Adsorption and Sensing Properties of Cr-Doped SnP3 Monolayer for Dissolved Characteristic Gases in Oil

Dissolved gas analysis (DGA) is a vital method for the online detection of transformer operation state. The adsorption performance of a SnP3 monolayer modified by transition metal Cr regarding six characteristic gases (CO, C2H4, C2H2, CH4, H2, C2H6) dissolved in oil was studied. The study reveals the relevant adsorption and gas-sensing response mechanisms through calculations of the adsorption energy, density of states, differential charge density, energy gap, and recovery time. The results display a considerable increase in the adsorption effect of the Cr-SnP3 monolayer on six gases. The CO, C2H2, and C2H4 gases lead to chemical adsorption, and the CH4, H2, and C2H6 gases lead to physical adsorption. Combined with the recovery time, the Cr-SnP3 monolayer has a strong adsorption effect on CO and C2H2 gases at normal temperatures and even high temperatures, and the adsorption is stable. C2H4 gas can be rapidly desorbed from the Cr-SnP3 monolayer at 398 K. Therefore, the Cr-SnP3 monolayer can be expected to serve as a CO and C2H2 gas adsorbent and a resistive gas sensor for C2H4 gas. This research offers a theoretical foundation for the development of the Cr-SnP3 monolayer in gas-sensitive materials.

A DFT investigation of monolayer InP: Effective toxic gas sensor with adsorption and optical response

An examination was conducted on the electronic and optical characteristics of gas (H2S, CO2, CO, SO2, and SO) adsorption on a monolayer of InP using first-principles calculations based on DFT. To identify the optimal and most sensitive adsorption site for the adsorbed gases, four initial adsorption sites were selected. Various aspects such as adsorption distance, charge densities, and adsorption energy were analyzed across different types of adsorption to determine the most favorable adsorption configurations. Our research indicates that InP monolayers can chemically adsorb CO2, CO, SO2, and SO, forming new bonds with these gas molecules. Furthermore, H2S can be physically absorbed onto InP with a high level of adsorption energy. The optical findings reveal that the presence of gas molecules alters the conductivity and optical properties of the InP monolayer, especially noticeable in the UV range. InP emerges as a suitable material for detecting CO2, CO, SO2, and SO due to its distinct characteristics.

Structural and electronic properties of amorphous silicon and germanium monolayers and nanotubes: A DFT investigation

A recent breakthrough has been achieved by synthesizing monolayer amorphous carbon (MAC). Here, we used ab initio (DFT) molecular dynamics simulations to study silicon and germanium MAC analogs. Typical unit cells contain more than 600 atoms. We also considered their corresponding nanotube structures. The cohesion energy values for MASi and MAGe are approximately 3.0 eV/atom lower than the energy ordering of silicene and germanene, respectively. Their electronic behavior varies from metallic to small band gap semiconductors. Since silicene, germanene, and MAC have already been experimentally realized, the corresponding MAC-like versions we propose are within our present synthetic capabilities.

Monolayer instability of ionic surfactants at the water/air interface due to biosurfactant molecules: A molecular dynamic approach

The decomposition of monolayers prepared with anionic surfactants, sodium dodecyl sulfate (SDS), or cationic ones, dodecyltrimethylammonium bromide (DTAB), at the water/air interface was studied when biosurfactants (surfactin) are added to the films. Molecular dynamics simulations were carried out for monolayers with a fixed number of SDS or DTAB molecules and a different amount of biosurfactants to form SDS/surfactin and DTAB/surfactin mixtures. It is found that the presence of the biosurfactant modifies interfacial and structural properties of the ionic monolayers at the interface; the DTAB monolayer is distorted even for a few surfactin molecules, whereas the SDS monolayer remains steady up to a given amount of biosurfactant. In both monolayers the surface tension decreases with the number of biosurfactants, however while for the DTAB/surfactin the values seem to fluctuate, for the SDS/surfactin mixture the values decay linearly, with a discontinuity for a given surfactin concentration. The linear behaviour of the surface tension for the SDS/surfactin allows us to evaluate free energies for the monolayers and a two-dimensional equation of state. The structure of the monolayers was analyzed in terms of density profiles, order parameters, thickness of the interfaces and the data show that the DTAB/surfactin is more distorted than the SDS/surfactin by the presence of few surfactin molecules. The results indicate that SDS surfactants are more likely to form longer lasting films with fewer surfactin molecules than DTAB.

Unveiling the tunable electronic, optoelectronic, and strain-sensitive gas sensing properties of Janus ZrBrCl: Insights from DFT study

Janus monolayers materials exhibit extraordinary physical and chemical properties because of crystal field changes caused by their asymmetric structures. Here, we investigate the electronic, switching, optoelectronic, and strain-sensitive gas-sensing properties of a Janus ZrBrCl monolayer via density-functional theory and the nonequilibrium Green’s function methods. The electronic properties can be tuned by applying biaxial strain and electric fields. In particular, the band gap can be changed from indirect to direct via applied a + 8 % biaxial tensile strain. It can be transformed from a semiconductor to a metal with an applied −14 % biaxial compressive strain, with a 1011 switching ratio. In addition, a biaxial strain and an electric field can modulate visible light absorption and photocurrents. Under a −12 % biaxial compressive strain, the absorption coefficient increased to 5.26 × 105 cm−1 from the intrinsic 4.08 × 105 cm−1. Across biaxial strains from −4% to + 4 %, the photocurrent density peak displayed red and blue shifts, respectively, with increasing tensile and compressive strains. NH3, NO2, and NO physically adsorb on the Br side of the ZrBrCl monolayer. Maximum gas sensitivity values of a ZrBrCl sensor are 52 % in the zigzag direction and 21 % in the armchair direction. After application of the −14 % biaxial strain, the maximum values respectively increased to 76 % and 179 %. The biaxial compressive strain-sensitive gas-sensing properties are driven by changes in the electrostatic potential difference between the Br and Cl surfaces. These findings indicate promising candidate materials for high-performance switching and optoelectronic devices, and also provide a strategy for improved gas sensors.