Behavior Of Monolayers Research Articles

Controllable Doping Characteristics for WSxSey Monolayers Based on the Tunable S/Se Ratio.

Transition metal dichalcogenides (TMDs) have attracted much attention because of their unique characteristics and potential applications in electronic devices. Recent reports have successfully demonstrated the growth of 2-dimensional MoSxSey, MoxWyS2, MoxWySe2, and WSxSey monolayers that exhibit tunable band gap energies. However, few works have examined the doping behavior of those 2D monolayers. This study synthesizes WSxSey monolayers using the CVD process, in which different heating temperatures are applied to sulfur powders to control the ratio of S to Se in WSxSey. Increasing the Se component in WSxSey monolayers produced an apparent electronic state transformation from p-type to n-type, recorded through energy band diagrams. Simultaneously, p-type characteristics gradually became clear as the S component was enhanced in WSxSey monolayers. In addition, Raman spectra showed a red shift of the WS2-related peaks, indicating n-doping behavior in the WSxSey monolayers. In contrast, with the increase of the sulfur component, the blue shift of the WSe2-related peaks in the Raman spectra involved the p-doping behavior of WSxSey monolayers. In addition, the optical band gap of the as-grown WSxSey monolayers from 1.97 eV to 1.61 eV is precisely tunable via the different chalcogenide heating temperatures. The results regarding the doping characteristics of WSxSey monolayers provide more options in electronic and optical design.

Stability and mixing behavior of vanadium-iron oxide monolayers on Pt(111) and Ru(0001) substrates

Cation mixing is a well-recognized means to obtain oxides of desired functionality with predetermined structure and stoichiometry, which yet has been only little analyzed at the nanoscale. In this context, we present a comparative analysis of the stability and mixing properties of O-poor and O-rich two-dimensional V–Fe oxides grown on Pt(111) and Ru(0001) surfaces, with the aim of gaining an insight into the role of substrate and oxygen conditions on the accessible Fe contents. We find that due to the high oxygen affinity of the Ru substrate, the mixed O-rich layers are highly stable while the stability of O-poor layers is limited to inaccessibly oxygen-poor environments. In contrast, on the Pt surface, O-poor and O-rich layers coexist with, however, a much lower Fe content in the O-rich phase. We show that cationic mixing (formation of mixed V–Fe pairs) is favored in all considered systems. It results from local cation–cation interactions, reinforced by a site effect in O-rich layers on the Ru substrate. In O-rich layers on Pt, Fe–Fe repulsion is so large that it precludes the possibility of substantial Fe content. These findings highlight the subtle interplay between structural effects, oxygen chemical potential, and substrate characteristics (work function and affinity towards oxygen), which governs the mixing of complex 2D oxide phases on metallic substrates.

Dynamical spin structure factors of GeCH3 monolayer due to spin-orbit coupling, strain, and external magnetic field

In this study, we introduced the Kane-Mele model in the presence of an external magnetic field and used a tight-binding approach to examine the dynamical spin susceptibility of the GeCH3 monolayer. We employed Green's function technique to determine the dynamical spin susceptibility behavior of the GeCH3 monolayer in the presence of an external magnetic field, strain, spin-orbit coupling (SOC), electron, and hole doping at room temperature. Our results show that the transverse excitation modes tend to move to lower (higher) frequencies with tensile (compressive) biaxial strain.In addition, the frequency position of the sharp peak in the transversedynamical spin susceptibility is not affected by changes in the external magnetic field, electron doping, or SOC. Also, the influences of the wave vector and doping on the frequency dependence of the transversedynamical spin susceptibility are studied in detail. These results offered here exhibit that the GeCH3 monolayer is appropriate for use in detectors.

A FEM Free Vibration Analysis of Variable Stiffness Composite Plates through Hierarchical Modeling

Variable Angle Tow (VAT) laminates offer a promising alternative to classical straight-fiber composites in terms of design and performance. However, analyzing these structures can be more complex due to the introduction of new design variables. Carrera’s unified formulation (CUF) has been successful in previous works for buckling, vibrational, and stress analysis of VAT plates. Typically, one-dimensional (1D) and two-dimensional (2D) CUF models are used, with a linear law describing the fiber orientation variation in the main plane of the structure. The objective of this article is to expand the CUF 2D plate finite elements family to perform free vibration analysis of composite laminated plate structures with curvilinear fibers. The primary contribution is the application of Reissner’s mixed variational theorem (RMVT) to a CUF finite element model. The principle of virtual displacements (PVD) and RMVT are both used as variational statements for the study of monolayer and multilayer VAT plate dynamic behavior. The proposed approach is compared to Abaqus three-dimensional (3D) reference solutions, classical theories and literature results to investigate the effectiveness of the developed models. The results demonstrate that mixed theories provide the best approximation of the reference solution in all cases.

Unravelling the regulating role of strain engineering on the phonon dispersion, mechanical behavior, and electronic transport properties of pentagonal PtTe2 monolayer

Inspired by the experimental synthesis of PdSe2 monolayer with a novel buckling configuration, the mechanical property, phonon dispersion curve, electronic structure, and carrier mobility of anisotropic pentagonal PtTe2 monolayer under strain engineering are systematically investigated by first-principles calculations and deformation potential (DP) theory. Ab initio molecular dynamics (AIMD) simulations show that the pentagonal PtTe2 monolayer is thermally stable at 300 and 600 K. Further phonon dispersion investigation verifies the dynamic stability of pentagonal PtTe2 monolayer under tensile strain, as evidenced by the absence of imaginary frequency. When subjected to tensile strains ranging from 2% to 4% in uniaxial (a- and b-direction) and biaxial directions, the mechanical properties and electronic band structures of pentagonal PtTe2 monolayer exhibit significant changes, leading to the electronic band convergence and decreasing bandgap. The effective mass, elastic constant, deformation potential, and carrier mobility also vary considerably under tensile strains, and a high hole mobility of 3618.51 cm2/V·s is discovered along the a-direction with 3% strain applied in the y-axis. Specifically, the hole mobility of pentagonal PtTe2 monolayer shows large anisotropy ratio, reaching up to 13.99 under 2% tensile strain in the y-axis. Our current work not only unravels the fundamental understanding of the anisotropic mechanical properties, phonon dispersion and electronic transport properties of pentagonal PtTe2 monolayer under strain engineering, but also provides theoretical guidance for applying strain engineering to tune the unique physical properties of two-dimensional materials with pentagonal configurations.

Local spectroscopy of a gate-switchable moiré quantum anomalous Hall insulator

In recent years, correlated insulating states, unconventional superconductivity, and topologically non-trivial phases have all been observed in several moiré heterostructures. However, understanding of the physical mechanisms behind these phenomena is hampered by the lack of local electronic structure data. Here, we use scanning tunnelling microscopy and spectroscopy to demonstrate how the interplay between correlation, topology, and local atomic structure determines the behaviour of electron-doped twisted monolayer–bilayer graphene. Through gate- and magnetic field-dependent measurements, we observe local spectroscopic signatures indicating a quantum anomalous Hall insulating state with a total Chern number of ±2 at a doping level of three electrons per moiré unit cell. We show that the sign of the Chern number and associated magnetism can be electrostatically switched only over a limited range of twist angle and sample hetero-strain values. This results from a competition between the orbital magnetization of filled bulk bands and chiral edge states, which is sensitive to strain-induced distortions in the moiré superlattice.

Tailoring the Adsorption and Antibacterial Bifunctions of Newly Constructed Porous Alginate–Chitosan–Graphene Oxide Nanosheets Gel Beads

Organic dye molecules and pathogenic microorganisms exist widely in printing and dyeing wastewater, causing persisting concerns about water environment safety. Herein, newly developed alginate–chitosan–graphene oxide nanosheets gel beads with both exceptional adsorption and antibacterial functions are reported. NaHCO3 is selected as porogen and Ag+ is used as antibacterial enhancer. The adsorption properties of the beads fit well with the Langmuir isotherm and pseudo‐second‐order model, suggesting a monolayer behavior of the adsorption and their reaction rate is controlled by chemisorption. The theoretical liminal value of the adsorption of methylene blue by the beads reaches 847.46 mg g−1. After five adsorption–desorption cycles, the beads retain almost intact adsorption activity, and the sterilization rates of Escherichia coli and Staphylococcus aureus remain at 99.99%. The results give insight into developing new processing routes for constructing gel beads for versatile applications.

Interfacial Behavior of Cubosomes: Combined Langmuir-Blodgett/Langmuir-Schaefer and AFM Investigations.

The Langmuir technique was applied for the first time to compare the layers obtained by spreading lipid liquid-crystalline nanoparticles monoolein 1-oleoyl-rac-glycerol (GMO)/Pluronic F108 cubosomes with the monolayers obtained by mixing the same components in chloroform at the air-water interface. The differences in the monolayer behavior and in the acting intermolecular forces were examined. The similarity of the isotherms obtained for the mixed components system and the cubosome-derived layer proved the disintegration of cubosomes into a single monolayer upon contact with the air-water interface. Despite the low Pluronic F108 content in both types of layers, a strong structural role of this stabilizer was also demonstrated. Cubosome-derived systems supported on hydrophilic mica substrates were prepared either using the combined Langmuir-Blodgett and Langmuir-Schaefer technique or via direct adsorption from the solution. The topographies of the obtained layers were studied by atomic force microscopy (AFM). Images obtained in the air mode revealed the disintegration of cubosomes and the formation of large crystallized structures of the polymer, while AFM imaging performed in water confirmed the presence of intact cubosomes on the surface of mica. We proved that the original structure of cubosomes remains on one condition: the films must not dry out; therefore, the aqueous environment must be preserved. This new approach provides an explanation in the ongoing discussion of what happens to lipid nanoparticles with or without cargo when they come into contact with an interface.

Theoretical study on the negative differential resistance behavior of N-doped GeSe monolayer

Using a combination of non-equilibrium Green’s function (NEGF) and density functional theory (DFT), the electronic structures and transport characteristics of N-doped GeSe monolayer were investigated systematically. The calculated band structures illustrated that the doping of N atoms changed GeSe monolayer from a semiconductor to a metal. Further, a clear negative differential resistance behavior (NDR) was found in the voltage-current curves of the N-doped GeSe based devices. Moreover, the current peak-to-valley ratio (PVR) varied with the doping concentration of N atoms. As the doping concentration decreased, the peak of NDR shifted toward the millivolt region. Especially, the NDR behavior with a PVR up to 103 entered the mV bias region when the doping concentration was reduced to 0.69%. These results provide a novel approach to design the low power nanodevice based on the GeSe monolayer.

Understanding the Two-Dimensional Mixing Behavior of 1-Naphthalenethiol and Octanethiol.

A two-dimensional (2D) mixture in the form of a self-assembled monolayer composed of two distinct organothiol compounds was created by sequentially depositing 1-naphthalenethiol (1NT) and octanethiol (OT) on a gold surface. By varying the sequence of deposition, two mixed surface systems were created. The surface structure of the resulting mixed monolayer was characterized with Scanning Tunneling Microscopy (STM) and showed surface disorder across all investigated domains. Elemental analysis was carried out with X-ray Photoelectron Spectroscopy (XPS) and indicated that the 1NT monolayer was prone to significant oxidation. Reductive desorption (RD) was used to characterize the binding strength and electrochemical environments of the molecular components in the mixture, and confirmed disordered molecular layers. Due to the presence of oxidized species in the 1NT monolayer, 1NT was displaced by OT resulting in a novel surface structure composed of either OT or 1NT. Monolayers of OT that were exposed to a solution of 1NT resulted in disordered surface structures with a significant amount of gold vacancy islands. To date, there is no experimental phase diagram explaining the chemical behavior of two-dimensional mixtures. This study addresses the need for an experimental understanding of the phase behavior of mixed organothiol self-assembled monolayers (SAMs).

The Electroless Monolayer and Duplex Ni–B and Ni–P Coatings for 316L Stainless Steel in Synergistic Combination of Mechanical (Wear) and Chemical (Corrosion) Processes

Conventional processes are performed to improve the low hardness and low wear resistance properties of 316L steel, but these processes generally decrease the corrosion resistance. Electroless nickel alloy coatings provide a hard, wear‐resistant, and corrosion‐resistant surface. Thus, the present study aims to investigate the wear, corrosion, and tribocorrosion behaviors of monolayer and duplex coatings with nickel–boron (Ni–B) and nickel–phospore (Ni–P) on 316L steel in comparison with 316L steel in dry sliding and 0.9 wt% NaCl solution environments. It is determined that the coatings have a mixture of crystal and amorphous structures, the interfaces on the 316L are uniform, and the compatibility between the layers is good. The Ni–B, Ni–P/Ni–B, and Ni–B/Ni–P coatings are 2.3, 2.06, and 1.6 times as hard as the 316L, respectively. The wear rates of Ni–B, Ni–P/Ni–B, and Ni–B/Ni–P coatings show decrease by 99.3%, 92.5%, and 99.1% in the dry‐sliding condition and by 98.5%, 30.1%, and 19.1% in the tribocorrosion condition compared with that of 316L, respectively. It is observed that the monolayer Ni–B coating exhibits superior hardness, a higher contact angle, low electrical conductivity, and better tribological performance in both dry sliding and tribocorrosion conditions compared to the 316L and duplex coatings.

Noise Energy Adopted to Understand Nickel Electroplating Process

Electrochemical noise (EN) coupled with chronoamperometry and cyclic voltammetry techniques were adopted to investigate the influences of substrate, temperature (T), current density (D K) and Ni2+ concentration (c i) on nickel electroplating process. The results show that the noise energy (E D) is weakly affected by c i as far as the concentration range concerned, and is limitedly influenced by the substrate, D K and T. The influence of D K on E D is achieved by its effect on the nucleation behavior of the initial deposit monolayer (the D K-depended nucleation behavior). E D mainly reflects the foreign work consumed in the procedure of formation of the initial deposit monolayer but not that in its subsequent growth process, and E D may be used as an index of EN to characterize the difficulty level of localized attacks (such as the formation of initial monolayer of crystallites and pits) for a defined area.

Towards predicting shear-banding instabilities in lipid monolayers

Langmuir monolayers are advantageous systems used to investigate how lipid membranes get involved in the physiology of many living structures, such as collapse phenomena in alveolar structures. Much work focuses on characterizing the pressure-bearing capacity of Langmuir films, expressed in the form of isotherm curves. These show that monolayers experience different phases during compression with an according evolution of their mechanical response, incurring into instability events when a critical stress threshold is overcome. Although well-known state equations, which establish an inverse relationship between surface pressure and area change, are able to properly describe monolayer behaviour during liquid expanded phase, the modelling of their nonlinear behaviour in the subsequent condensed region is still an open issue. In this regard, most efforts are addressed to explain out-of-plane collapse by modelling buckling and wrinkling mainly resorting to linearly elastic plate theory. However, some experiments on Langmuir monolayers also show in-plane instability phenomena leading to the formation of the so-called shear bands and, to date, no theoretical description of the onset of shear banding bifurcation in monolayers has been yet provided. For this reason, by adopting a macroscopic description, we here study material stability of the lipid monolayers and exploit an incremental approach to find the conditions that kindle shear bands. In particular, by starting from the widely assumed hypothesis that monolayers behave elastically in the solid-like region, in this work a hyperfoam hyperelastic potential is introduced as a new constitutive strategy to trace back the nonlinear response of monolayer response during densification. In this way, the obtained mechanical properties together with the adopted strain energy are successfully employed to reproduce the onset of shear banding exhibited by some lipid systems under different chemical and thermal conditions.

First-principles insight into adsorption behavior of a Pd-doped PtTe2 monolayer for CO and C2H2 and the effect of an applied electric field

Using first-principles theory, we propose a Pd-doped PtTe2 (Pd–PtTe2) monolayer as an outstanding sensing material for CO and C2H2 to estimate the operational condition of oil-immersed transformers. A Te atom of the pristine PtTe2 monolayer is substituted by a Pd atom to model the Pd–PtTe2 structure and the calculated formation energy is −0.013 eV. The Pd–PtTe2 monolayer conducts chemisorption of CO but physisorption of C2H2 with adsorption energies of −1.121 and −0.740 eV, respectively. The electronic properties show the sensing mechanism and bonding behavior in the gas adsorptions, while the analyses of band structure and recovery property show the potential of the Pd–PtTe2 monolayer as a room-temperature C2H2 sensor, and a reusable CO sensor following a heating process of 34 min. Moreover, the applied electric fields can effectively modulate the bandgap of the gas systems, uncovering the promoted sensing response under positive electric fields. This work explores the PtTe2-based nano-surfaces for gas-sensing application in electrical power systems, which paves the way for further investigation on the PtTe2 monolayer for applications in the fields of electrical engineering and gas sensing generally.

Formation behavior of monolayers on the water surface of water-soluble thermoplastic and insoluble-thermosetting copolymers with hyperbranched units containing s-benzenetricarbamide cores

The formation and structure of Langmuir monolayers and Langmuir-Blodgett films were studied using water-soluble polymers, which have difficulty forming monolayers on the water surface, and polymers that are insoluble in all solvents. The network copolymers consisting of hyperbranched units with an s-benzenetricarbamide core used in this study can be freely controlled by azidation/non-azidation of both ends of the hard segment. The water-soluble copolymer, which exhibits the properties of a thermoplastic resin, can form a monolayer when spread on the water surface at a low temperature at which the solubility decreases. Even when the subphase temperature increased, no significant change in the monolayer behavior of the thermoplastic copolymer was observed. This behavior was attributed to the formation of an insoluble single-particle layer with hydrophilic groups facing inward on the water surface. The insoluble copolymer, which exhibits thermosetting properties, forms a tablet that floats on the water surface for an extended period. The surface pressure-time isotherms showed the formation of a monolayer from the tablet over time, and it was possible to measure the surface pressure-area isotherms by two-dimensional compression after a certain period of time.

Formation and Thermal Stability of Ordered Self-Assembled Monolayers by the Adsorption of Amide-Containing Alkanethiols on Au(111).

We examined the surface structure, binding conditions, electrochemical behavior, and thermal stability of self-assembled monolayers (SAMs) on Au(111) formed by N-(2-mercaptoethyl)heptanamide (MEHA) containing an amide group in an inner alkyl chain using scanning tunneling microscopy (STM), X-ray photoelectron spectroscopy (XPS), and cyclic voltammetry (CV) to understand the effects of an internal amide group as a function of deposition time. The STM study clearly showed that the structural transitions of MEHA SAMs on Au(111) occurred from the liquid phase to the formation of a closely packed and well-ordered β-phase via a loosely packed α-phase as an intermediate phase, depending on the deposition time. XPS measurements showed that the relative peak intensities of chemisorbed sulfur against Au 4f for MEHA SAMs formed after deposition for 1 min, 10 min, and 1 h were calculated to be 0.0022, 0.0068, and 0.0070, respectively. Based on the STM and XPS results, it is expected that the formation of a well-ordered β-phase is due to an increased adsorption of chemisorbed sulfur and the structural rearrangement of molecular backbones to maximize lateral interactions resulting from a longer deposition period of 1 h. CV measurements showed a significant difference in the electrochemical behavior of MEHA and decanethiol (DT) SAMs as a result of the presence of an internal amide group in the MEHA SAMs. Herein, we report the first high-resolution STM image of well-ordered MEHA SAMs on Au(111) with a (3 × 2√3) superlattice (β-phase). We also found that amide-containing MEHA SAMs were thermally much more stable than DT SAMs due to the formation of internal hydrogen networks in MEHA SAMs. Our molecular-scale STM results provide new insight into the growth process, surface structure, and thermal stability of amide-containing alkanethiols on Au(111).

Thermodynamic behaviors and hysteresis loops of an edge-modified Kekulene monolayer: A Monte Carlo study

We studied the magnetic and thermodynamic behaviors of an edge-modified Kekulene monolayer using the Monte Carlo method and established the Ising model with the mixed spins of 2 and 5/2. The function curves of temperature on magnetization, susceptibility, internal energy, and specific heat induced by various physical parameters are discussed respectively. The relationship between blocking temperature and various physical parameters is obtained. We also clarify the role of crystal field, exchange coupling and temperature on the hysteresis loops.

Biaxial strain-modulated electronic and optical properties of transition metals doped-WSe2 monolayer

We report the effect of biaxial strain (εxy) on the electronic and optical properties of the WSe2 monolayer, doped with transition metal (TM) atoms using density functional theory (DFT) calculations. The TM dopants mainly include Niobium (Nb), Tantalum (Ta), and Rhenium (Re), which are popular choices for p and n-type doping in the WSe2 monolayer. We determine the modulation of electronic bandgap with strain, i. e. ΔEg/Δε, have the following order: pristine WSe2 > Nb-doped WSe2 > Ta-doped WSe2 > Re-doped WSe2. All the TM dopants modified the optical responses of the WSe2 monolayer, with the absorption edge shifting towards lower energies (redshift phenomenon). By applying strain, the main changes in the near-infrared (NIR) region to red-visible regions are observed. We believe that knowledge of the behavior of TM-doped WSe2 monolayer with strain is crucial for their further use in emerging optoelectronic device applications.

Comparative Study on Wear Behaviors of Monolayer and Heterogeneous Multilayer Ta Coatings in Atmospheric and SBF Environments

Monolayer Ta and multilayer Ti/Zr/Ta and Zr/Ti/Ta coatings were prepared by magnetron sputtering on TC4 substrates to improve the surface friction and wear properties in a simulated body fluid (SBF) environment and an atmospheric environment. Optical microscopy, scanning electron microscopy, laser scanning confocal microscopy and nano scratch testing were employed to establish the structure-property-environment relationships. By controlling the preparation parameters, the outermost layer of all three samples was Ta coating, and the total coating thickness of each sample was about 3 μm. Friction and wear testing revealed that, compared to bare TC4 substrate, and multilayer Ti/Zr/Ta and Zr/Ti/Ta coatings, the monolayer Ta possessed the lowest friction coefficient as well as the minimum wear rate (i.e., calculation result of the wear track width and wear depth). This was mainly attributed to excellent adhesion strength, a particular structure and solid lubrication of the monolayer coating. The same coating sample exhibited a stronger wear resistance in the SBF environment than in the atmospheric environment. Furthermore, the wear behaviors and mechanisms of various coatings under different experimental environments are also discussed.

Investigation the sensing behavior of pristine and Ti-doped C2N monolayer toward H2S gas

As a group of chemicals and air contaminants, volatile organic compounds or VOCs have a threatening impact upon the ecological environment and the human well-being. One of the encouraging technologies to enrich, separate and use VOCs is their adsorption. Within this study, density functional theory (DFT) computations were undertaken for inspecting how the molecules of VOCs were adsorbed on the pure C2N monolayer (P-C2NML) and the Ti-doped C2N monolayer (T- C2NML) since its specific surface is large and it has a porous structure. The results revealed that the hydrogen sulfide (H2S) molecule could be adsorbed on the P-C2NML with a weak energy of adsorption. However, this adsorption ability could be enhanced by forming a chemical bond which is strong. The best adsorption capacity (AC) for the H2S molecule on the T-C2NML surface with adsorption energy of − 1.987 eV was larger compared with that of the P-C2NML. Moreover, the results of electronic distribution and DOS analysis revealed that doped Ti acted as a bridge between the H2S molecule and the C2NML and strengthen the interaction between them, which can substantially improve the AC. Hence, the T-C2NML can be considered an encouraging adsorbent to enrich and use VOCs.