Monolayer Phase Research Articles

On the excitonic effects of the 1T and 1O[formula omitted] phases of PdS2, PdSe2, and PdSSe monolayers

We report the excitonic effects in the 1T and 1OT phases of PdS2, PdSe2, and PdSSe monolayers. Using fully relativistic norm-conserving non-local PBE pseudopotentials and including the spin–orbit (SOC) coupling, the 1T geometries are indirect band gap semiconductors; conversely, the 1OT structures are semimetallic. The band gaps were corrected using the HSE06 hybrid functional, making all structures semiconductors. Maximally localized Wannier functions were utilized to fit the bands of all six structures. The excitonic band structures were obtained using the Bethe–Salpeter equation (BSE), and the absorption coefficients were estimated at both the BSE and the independent particle approximation (IPA) levels. While the 1OT monolayers are more inclined to absorb light polarized in the y-direction, the 1T structures absorb light equally, independent of the polarization direction. We perceive that the 1T phase is the most promising candidate for photovoltaic devices; in particular, the 1T PdSSe system possesses a maximum power conversion efficiency (PCE) of 29.31% if light trapping techniques are used to enhance absorbance. Amongst the 1OT materials, PdS2 features the highest PCE of 8.38%. Our results constitute the first step to encourage experimental studies towards the fabrication of Pd-based regular and Janus transition metal dichalcogenides photoabsorbers and give hints of how to increase the PCE of such materials.

Strain Engineering of the Electronic and Optical Properties of Predicted Janus CaFBr Monolayer for Potential Use in Optoelectronic Devices: A Density Functional Theory Study

Herein, the biaxial strain effect is investigated on the structural, electronic, and optical properties of 1T and 1H phases of Janus CaFBr monolayer in the framework of density functional theory. It is found that both phases of the Janus CaFBr monolayer are direct semiconductors at equilibrium, with bandgaps of 3.90 and 3.55 eV for 1T and 1H phases, respectively. The thermodynamic stability is examined via cohesive energy and phonon dispersion. The bandgap decreases linearly and is nearly parabolic for 1T and 1H phases, respectively, when switching from the tensile to compressive strain with a drastic shift from direct to indirect bandgap at −10% of compressive strains. The calculated dielectric function and optical properties such as reflectivity, refractive index, extinction, and absorption coefficients enhance under biaxial with an improvement of the absorption coefficient especially in the visible and ultraviolet (UV) regions for 1H and 1T phases, respectively, which is in line with the dielectric constant. The results suggest that the Janus CaFBr monolayer might be a potential candidate for optoelectronic applications in visible/UV detection and absorption.

Controllable topological insulator phases in litharge-phase InBi monolayer

Controllable topological insulator phases in litharge-phase InBi monolayer

Tuning electronic phase of monolayer [formula omitted]-borophene via adsorbed gas molecules: Generation of van Hove singularity

We consider manifolds of the tunable electronic density of states in single-layer β12-borophene, parameterized by a manifold electron hopping amplitude. We study the hybridization properties of orbitals in the presence of gas molecules (GM) with a tight-binding model and Green’s function approach. Independent of how we dope the layer by GM, van Hove singularities are significantly generated and associated with various broadness and flatness features of bands. We demonstrate that β12-borophene with GM-doped {a,e}, {a,c}, {b,d}, and {b,c} atoms is described by a semimetallic phase, where the metallic phase is washed out in the vicinity of Fermi level. We also find a gapless semiconducting phase at the Fermi energy for GM-doped c atoms; however, the most inherent character of the system remains unchanged once all atoms are simultaneously doped with GM. These findings can apply to tuning field-effect transistor-based gas sensors.

Engineering the light absorption spectrum and electronic properties of black and blue phases of a SiSe monolayer via biaxial straining

Engineering the light absorption spectrum and electronic properties of black and blue phases of a SiSe monolayer via biaxial straining

Enhancement of electrochemical performance of monolayer SnS_2 for Li/Na-ion batteries through a sulphur vacancy: a DFT study

Various transition metal dichalcogenides materials have been investigated from bulk to monolayer phases for different advanced technological applications. Tin disulfide monolayer offers advantages as an anode material for Li/Na-ion batteries, although it cannot be considered ideal for direct exploitation. We systematically performed a comparative study of the adsorption and diffusion behaviour of Li/Na on a pristine SnS_2 monolayer and on a SnS_2 monolayer with S-vacancy for enhancement of electrochemical performance, using density functional theory approach. Although all the adsorption sites are exothermic, it was established that Li/Na adatoms mostly prefer to bind strongly on SnS_2 monolayer with S-vacancy but avoiding the S-vacancy site. It was established that avoiding the S-vacancy site along the path, excellent diffusion barriers of 0.19 eV for Li and 0.13 eV for Na were achieved, suggesting possible ultrafast charge/discharge rate. Due to reduced molar mass, the SnS_2 monolayer with S-vacancy has a slightly higher storage capacity than its pristine counterparts for both Li and Na adatoms. The obtained open circuit voltage values are within the range of 0.25–3.00 V assuring that the formation of dendrites can surely be averted for the envisaged battery operation. Understanding the effects of an S-vacancy on the electrochemical properties of Li/Na on the SnS_2 monolayer allows us to consider possible improvements to energy storage devices that can be applied as a result of improved anode material.

Oxygen-terminated Ti3C2 MXene as an excitonic insulator

Excitonic insulators originate from the formation of bound excitons (electron–hole pairs) in semiconductors and provide a solid-state platform for quantum many-boson physics. We determined the excitonic insulator phase of Ti3C2O2 monolayer from its indirect quasiparticle band structure and from the precise evaluation of the relative value of the fundamental bandgap vs the momentum-indirect excitonic binding energy. The excitonic insulator is stable over the ±4% range of compressive and tensile biaxial strain. The energy region relevant for the optical absorption is strongly strain-dependent.

Exotic liquid crystalline phases in monolayers of vertically vibrated granular particles

ABSTRACTVibrated monolayers of granular particles confined into horizontal cavities form a variety of fluid patterns with orientational order that resemble equilibrium liquid-crystal phases. In some cases, one can identify nematic and smectic patterns that can be understood in terms of classical statistical mechanics of hard bodies. Low aspect ratio cylinders project as rectangles and form uniaxial, or 2-atic, and tetratic, or 4-atic, nematic phases. Other polygonal particles may exhibit different liquid-crystal phases, in general p-atic phases, of higher symmetries. We give a brief summary of theoretical work on rectangles and triangles, and provide some experimental results on vibrated monolayers. In the case of equilateral triangles, the theory predicts an exotic triatic phase, or 6-atic phase, with six-fold symmetry and three equivalent directors. The right-angled triangles exhibit a 4-atic phase with strong octatic (8-atic) correlations. Experiments on cylinders show 4-atic textures and, even more remarkable, geometric frustration caused by confinement excites topological defects, which seem to follow the same topological rules as standard liquid crystals. Some of our findings can be understood with the help of simulations of hard particles subject to thermal equilibrium, although standard Density-Functional Theories fail to account for the correct equilibrium phases in some cases.

Critical behavior and exchange splitting in the two-dimensional antiferromagnet Mn on Re(0001)

We investigated the temperature-dependent electronic structure of the antiferromagnetic fcc-monolayer Mn on Re(0001) using vacuum-ultraviolet momentum microscopy. At $T=25$ K the collinear, row-wise antiferromagnetic phase of the Mn monolayer results in a spin splitting of states. Density-functional theory, being in good agreement with the experimental results, reveals the spin and orbital projection of the observed electronic bands. The exchange split bands shift in opposite directions with increasing temperature, decreasing the exchange splitting of a pair of itinerant bands from $280\ifmmode\pm\else\textpm\fi{}10$ meV at 25 K down to $185\ifmmode\pm\else\textpm\fi{}10$ meV at the N\'eel temperature ${T}_{\text{N}}=75\ifmmode\pm\else\textpm\fi{}5$ K. The exchange splitting remains constant for $T>{T}_{\text{N}}$. The persisting exchange splitting is attributed to a remaining short-range, fluctuating antiferromagnetic order far above ${T}_{\text{N}}$.

Highly in-plane anisotropic optical properties of fullerene monolayers

Both the intrinsic anisotropic optical materials and fullerene-assembled 2D materials have attracted much interest in fundamental science and potential applications. The synthesis of a monolayer (ML) fullerene makes the combination of these two features plausible. In this work, using first-principles calculations, we systematically study the electronic structure and optical properties of quasi-hexagonal phase (qHP) ML and quasi-tetragonal phase (qTP) ML fullerenes. The calculations of qHP ML show that it is a semiconductor with small anisotropic optical absorption, which agrees with the recent experimental measurements. However, the results for qTP ML reveal that it is a semimetal with highly in-plane anisotropic absorption. The dichroic ratio, namely the absorption ratio of x- and y-polarized light αxx/αyy, is around 12 at photon energy of 0.29 eV. This anisotropy is much more pronounced when the photon energy is between 0.7 and 1.4 eV, where αxx becomes nearly zero while αyy is more than two orders of magnitude larger. This indicates qTP ML as a candidate for long-pursuit lossless metal and a potential material for atomically thin polarizer. We hope this will stimulate further experimental efforts in the study of qTP ML and other fullerene-assembled 2D materials.

Synthesizability of transition-metal dichalcogenides: a systematic first-principles evaluation

Transition metal dichalcogenides (TMDs) are a class of materials with various useful properties, and it is worthwhile to have a thorough evaluation of the characteristics of the TMDs, most importantly, their structural stability and exfoliability, in a systematic fashion. Here, by employing high-throughput first-principles calculations, we investigate the vast phase space of TMDs, including 16 bulk phases and 6 monolayer phases for all possible TMD combinations [comprising (3d, 4d, 5d) transition-metal cations and (S, Se, Te) anions], totaling 1386 compounds. Through the ‘bird-view’ of the as-large-as-possible configurational and chemical space of TMDs, our work presents comprehensive energy landscapes to elucidate the thermodynamic stability as well as the exfoliability of TMDs, which are of vital importance for future synthesis and exploration towards large-scale industrial applications.

Ni/Al2O3 catalysts for CO2 methanation: Effect of silica and nickel loading

Samples containing from 1 to 33 wt.% of NiO on silica and alumina doped with silica (1 and 20 wt.% silica in the support) have been prepared and characterized by BET, XRD, FT-IR, UV–vis–NIR, FE-SEM, EDXS, and TPR techniques. Catalysts have been pre-reduced in situ before catalytic experiments and data have been compared with Ni/Al2O3 reference sample. Characterization results showed that SiO2 support has a low Ni dispersion ability mainly producing segregated NiO particles and a small amount of dispersed Ni2+ in exchange sites. Instead, for the Si-doped alumina a “surface spinel monolayer phase” is formed by increasing Ni loading and, only when the support surface is completely covered by this layer, NiO is formed. Moreover, H2-TPR results indicated that NiO particles are more easily reduced compared to Ni species. Low loading Ni/SiO2 catalysts show high selectivity and moderate activity for RWGS (reverse Water Gas Shift) reaction, likely mainly due to nickel species dispersed in silica exchange sites, as evidenced by visible spectroscopy. High loading Ni/SiO2 catalysts show both methanation and RWGS but evident short-term deactivation for methanation, attributed to large, segregated Ni metal particles, covered by a carbon veil. Ni on alumina -rich carriers, where nickel disperses forming a surface spinel phase, show high activity and selectivity for methanation, and short-term catalyst stability as well. This activity is attributed to small nickel clusters or metal particles interacting with alumina, formed upon reaction. The addition of SiO2 in Al2O3 support decreases the activity of Ni catalysts in CO2 methanation, because it reduces the ability of the support to disperse nickel in form of the surface spinel phase, thus reducing the amount of Ni clusters in the reduced catalysts.

Influence of InP/ZnS Quantum Dots on Thermodynamic Properties and Morphology of the DPPC/DPPG Monolayers at Different Temperatures.

In this work, the effects of InP/ZnS quantum dots modified with amino or carboxyl group on the characteristic parameters in phase behavior, elastic modulus, relaxation time of the DPPC/DPPG mixed monolayers are studied by the Langmuir technology at the temperature of 37, 40 and 45 °C. Additionally, the information on the morphology and height of monolayers are obtained by the Langmuir-Bloggett technique and atomic force microscope technique. The results suggest that the modification of the groups can reduce the compressibility of monolayers at a higher temperature, and the most significant effect is the role of the amino group. At a high temperature of 45 °C, the penetration ability of InP/ZnS-NH2 quantum dots in the LC phase of the mixed monolayer is stronger. At 37 °C and 40 °C, there is no clear difference between the penetration ability of InP/ZnS-NH2 quantum dots and InP/ZnS-COOH quantum dots. The InP/ZnS-NH2 quantum dots can prolong the recombination of monolayers at 45 °C and accelerate it at 37 °C and 40 °C either in the LE phase or in the LC phase. However, the InP/ZnS-COOH quantum dots can accelerate it in the LE phase at all temperatures involved but only prolong it at 45 °C in the LC phase. This work provides support for understanding the effects of InP/ZnS nanoparticles on the structure and properties of cell membranes, which is useful for understanding the behavior about the ingestion of nanoparticles by cells and the cause of toxicity.

Anisotropic electronic transport properties in two-dimensional ferroelectric In[formula omitted]Se[formula omitted] monolayer

We investigate the electronic structure and electronic transport properties of two-dimensional (2D) In2Se3 monolayer, using density functional theory combined with the nonequilibrium Green’s function (NEGF) method. Two ferroelectric phases of In2Se3 monolayer with zincblende (ZB) and wurtzite (WZ) structures are included, along the zigzag (ZZ) and armchair (AC) directions, respectively. The current–voltage (I–V) characteristics show that the ZB In2Se3 monolayer along the ZZ direction is better for electronic transport. The voltage drop contour reveals that along the ZZ direction, the Se atom has 3 paths to the adjacent Se atoms, while along the AC direction, it reduces to 2 paths. The momentum-dependent transmission reveals that in the process of transformation from the left electrode to the right electrode, there are momentum exchanges among electrons, which is of importance to electronic transport.

Topological phases in nanoparticle monolayers: can crystalline, hexatic, and isotropic-fluid phases coexist in the same monolayer?

Topological phases are stable configurations of matter in 2-dimensions (2D) formed via spontaneous symmetry breaking. These play a crucial role in determining the system properties. Though a number of fundamental studies on topological phase transitions and topological defect dynamics have been conducted with model colloidal systems (typically microns in size), the same is lacking on nanoparticle monolayers (NPMLs, typically made of ligand-coated sub-ten nanometer particles). Here, we show that in an evaporation-driven self-assembly process, the three topological phases, namely crystalline, hexatic, and isotropic-fluid phases, can coexist within the same NPML. We associate this coexistence with the local variation in particle size, which can be described by a unique frequency parameter (p25), quantifying the fraction of NPs that has size deviation greater than or equal to 25% of the mean size (where the deviation,ζ is defined as ζ = ((|Size-mean|)/mean)). The p25-values for the three phases are distinctly different: crystalline arrangement occurs when p25 < ∼0.02, while a hexatic phase exists for 0.02 ≤ p25 ≤ 0.1. For p25 ≥ 0.1, the isotropic-fluid phase occurs. Following KTHNY-theory, we further numerically extrapolate the occurrence of each phase to the accumulated excess planar strain in the NPML due to the presence of various topological defects in the structures.

New insights into band inversion and topological phase of TiNI monolayer.

Two-dimensional (2D) topological insulators (TIs) hold great promise for future quantum information technologies. Among the 2D-TIs, the TiNI monolayer has recently been proposed as an ideal material for achieving the quantum spin Hall effect at room temperature. Theoretical predictions suggest a sizable bandgap due to the spin-orbit coupling (SOC) of the electrons at and near the Fermi level with a nontrivial 2 topology of the electronic states, which is robust under external strain. However, our detailed first-principles calculations reveal that, in contrast to these predictions, the TiNI monolayer has a trivial bandgap in the equilibrium state with no band inversion, despite SOC opening the bandgap. Moreover, we show that electron correlation effects significantly impact the topological and structural stabilities of the system under external strains. We employed a range of density functional theory (DFT) approaches, including HSE06, PBE0, TB-mBJ, and GGA+U, to comprehensively investigate the nontrivial topological properties of this monolayer. Our results demonstrate that using general-purpose functionals such as PBE-GGA for studying TIs can lead to false predictions, potentially misleading experimentalists in their efforts to discover new TIs.

A strain induced polar metal phase in a ferromagnetic Fe3GeTe2 monolayer.

The electronic structure and magnetic properties of the ferromagnetic Fe3GeTe2 monolayer have been extensively studied in recent years. Experimentally, external strain can be produced inevitably during the growth on the substrate. However, the impact of strain on the structural, electronic, and magnetic properties remains largely underexplored. Herein, by using density functional theory, we systematically investigate the crystalline configuration and electronic structure of the Fe3GeTe2 monolayer in the presence of external strain. We find that a moderate compressive strain could break the structural vertical symmetry, leading to a sizable out-of-plane dipole moment, while the ferromagnetism can be retained. Surprisingly, strain-induced polarization in the off-center Fe and Ge atoms barely contributes to the energy states at the Fermi level. The efficient decoupling of the conductivity and polarization in the strained Fe3GeTe2 monolayer results in an extremely rare phase with the coexistence of polarization, metallicity, and ferromagnetism, i.e., magnetic polar metals for potential applications in magnetoelectricity and spintronics.

Review of effective techniques for surface engineering material modification for a variety of applications

&lt;abstract&gt; &lt;p&gt;The status of current advances in modifying surfaces for the protection of materials is reviewed in this research. The main goal of material selection is to improve and reinforce surface functionalities. A few examples of surface modification techniques include sol-gel, cladding, electroplating, plasma and thermal spraying, physical deposition of vapors (PVD), vapor chemical deposition (CVD) and beam electron physical vapor deposition (EB-PVD). Strengthening by flame, induction, laser or electron beam is one type of surface modification procedure. Other types include plasma-immersed ion implantation and ion implantation at high energies, as well as diffusion treatments like carburizing and nitriding. Friction control, improved surface corrosion and wear resistance and changes to a component's mechanical or physical qualities are all possible using surface modification methods. The study also contains contemporary research in laser therapy, PVD, EB-PVD, thermal spraying and ion implantation. Additionally, magnetron sputtering (MS) is a widely used and successful approach for thin film coating in the current study. It is crucial to remember that each approach has a distinct set of restrictions, and the method's parameters might change based on the one that is selected, such as deposition targets, overall vacuum substrate temperature, reactive or mixed gas type, pressure percentage and bias voltage, which all have impacts on the PVD technique's layer qualities. Phase formation, change in phase, hardness and film structure of monolayer and multilayer films formed on the substrate under various circumstances also cause variations in the characteristics. Additionally, ion implantation enhances the surface characteristics of layers by implanting ions such as N&lt;sup&gt;+&lt;/sup&gt;, B&lt;sup&gt;+&lt;/sup&gt;, C&lt;sup&gt;+&lt;/sup&gt;, etc. The study shows that the higher layers of multilayer enhance the degree of hardness and lower friction coefficients. To enhance the protection of thermal resistance, a thermal spraying barrier coating was coated on substrate nickel-base alloys, and the surface materials' texture, hardness and wear rate were altered by laser beam. Additionally, a heat pipe's performance was improved by a factor of 300 by adding a tiny coating of gold.&lt;/p&gt; &lt;/abstract&gt;

Identification of an optimized ratio of amyloid and non-amyloid fractions in engineered fibril solutions from whey protein isolate for improved foaming

Engineered fibril solutions from whey protein beta-lactoglobulin are known for their excellent foaming capacity. These amyloid aggregates solutions (AAS) usually contain a polydisperse mixture of different protein structures. An optimized ratio of amyloid (AF, fibrils) and non-amyloid fractions (n-AF) in AAS may improve foaming, particularly by interactions at the air-water interface. Foamability, surface activity, and monolayer phase behavior at the air-water interface of isolated AF and n-AF as well as AAS with different AF/n-AF-ratios were investigated using drop tensiometry, Langmuir trough and foam analysis. N-AF exhibited faster migration, twice as fast adsorption and thus faster spreading at the air-water interface than the fibrils (AF). N-AF required less energy to assemble in a liquid-expanded phase in a monolayer, i.e., they were more compressible in the monolayer than AF. This resulted in rapid stabilization of lamellae in foam. High surface hydrophobicity of AF results in faster adsorption and formation of capillary forces between adsorbed fibrils, improving the attraction for additional fibrils. Orientation of semi-flexible and larger fibrils in AF consumes high energy. In combination with n-AF, the energy needed for orientation and assembly of fibrils is equivalent, however, the yield of AF in AAS was only 20 %, indicating the interplay of amyloid and non-amyloid proteins at the air-water interface. N-AF can be incorporated into a fibrillar film, which increase the network's density and stiffness and the interfacial film stability. Consequently, in AAS fibrils and non-amyloid material acted synergistically at the air-water interface, whereby only small amounts of amyloid aggregates are required to stabilize foams.

Prediction of van Hove singularities, excellent thermoelectric performance, and non-trivial topology in monolayer rhenium dichalcogenides

Two-dimensional (2D) thermoelectric materials are gaining more intense attention with the ever-increasing global demand for energy. Recently, the search for compounds exhibiting excellent thermoelectric and topological characteristics has also attracted research exploration. In this study, a systematic investigation of Re-based transition metal dichalcogenides (TMDs) (ReX 2 , X = S, Se, and Te) through first-principles calculations identified the stability and electronic properties of the 1T dp (1 T double prime), 1 T’, 1 T, and 2H phases for the pristine bulk and monolayer ReX 2 . Formation energy and phonon dispersion calculations showed that the bulk and monolayer structures are only stable in the 1T dp structure. The calculated bandgaps of bulk under the hybrid functional approach are 1.567 eV, 1.429 eV, and 0.745 eV, while those of the monolayer phases are 1.902 eV, 1.658 eV, and 1.323 eV for ReS 2 , ReSe 2 , and ReTe 2 , respectively. Moreover, van Hove singularities (vHss) are observed in monolayer ReX 2 suggesting possible superconductivity. Remarkably, the calculated figure of merits (ZT) of bulk and monolayer ReX 2 with values up to 2.30 presents them as excellent materials for thermoelectric (TE) applications since good TE materials have ZT > 0.4. In addition, the effect of one- (1 h) and two-sided (2 h) hydrogenation on the structural, electronic, magnetic, and topological properties of monolayer ReX 2 were also investigated. Two-sided hydrogenation caused a stable structural phase transition from 1 T dp to 1T. A ferromagnetic phase transition was also observed upon the two-sided hydrogenation of ReS 2 (2h-ReS 2 ) and the one - sided hydrogenation of ReSe 2 (1h-ReSe 2 ). Finally, one-sided hydrogenation of monolayer ReSe 2 and ReTe 2 (1h-ReSe 2 and 1h-ReTe 2 ) resulted in non-trivial topological phases as confirmed by the calculated Z 2 and Chern topological invariant numbers. Our findings show that the 2D Re-based TMDs exhibit highly tunable properties which have the potential for thermoelectric and spintronics applications. • Monolayer ReX2 is thermodynamically stable in the distorted 1 T phase (1Tdp) • van Hove singularities are present in monolayer ReX2 suggesting superconductivity • Excellent thermoelectric properties are observed in bulk and monolayer ReX2 • Stable structural and magnetic phase transition occurred upon hydrogenation of ReX2 • Hydrogenation of monolayer ReX2 induced non-trivial topological phases