Monolayer State Research Articles

Strong coupling between exciton and quasi-bound state in the continuum with polarization dependence

Exciton–polaritons which are formed by the strong coupling of photon and exciton, have aroused great interest. In this work, we study the strong coupling between the exciton of WS2 monolayer (perovskite) and quasi-bound states in the continuum (QBIC) under y(x) polarization, in the perovskite metasurface with WS2 monolayer. Here, QBIC mode is achieved by introducing the asymmetric parameter. It is found that the energy of Rabi splitting can reach 41.369 meV (235.19 meV) under y-polarization (x-polarization). The dispersion-coupled oscillation model is used to illustrate the strong coupling mechanism. Besides, the influences of the asymmetric parameter and the positions of WS2 monolayer on the Rabi splitting are also discussed, which are verified by finite element method (FEM) simulations. Compared with the previous works, our method provides a way to achieve Rabi splitting with polarization dependence.

Element-specific X-Ray detection of electron paramagnetic resonance in thin films of quantum bits

Element-specific magnetism accessible by synchrotron-based X-ray spectroscopy has proven to be valuable to study spin and orbital moments of transition metals and lanthanides in technologically relevant thin-film and monolayer samples. The access to coherent spin superposition states relevant for emergent quantum technologies remains, however, elusive with ordinary X-ray spectroscopy. Here, we approach the study of such quantum-coherent states via the X-ray detection of microwave-driven electron paramagnetic resonance, which involves much smaller signal levels than X-ray detected ferromagnetic resonance on classical magnets. We demonstrate the feasibility of this approach with thin films of phthalocyanine-based metal complexes containing copper or vanadium centers. We also identify X-ray specific phenomena that we relate to charge trapping of secondary electrons resulting from the decay of the X-ray excited core-hole state. Our findings pave the way toward the element-specific X-ray detection of coherent superposition states in monolayers of atomic and molecular spins on virtually arbitrary surfaces.

Low lattice thermal conductivity induced by antibonding sp-hybridization in Sb2Sn2Te6 monolayer with high thermoelectric performance: A First-principles calculation

Utilizing first-principles calculations and Boltzmann transport theory, the crystal structure, thermal and electronic transport, and thermoelectric (TE) properties of the Sb2Sn2Te6 monolayer are evaluated in the current work. The Sb2Sn2Te6 monolayer is an indirect band gap semiconductor with a band gap of 0.81 eV using the Heyd-Scuseria-Ernzerhof (HSE06) functional in combination with the spin-orbital coupling (SOC) effect. The antibonding states formed by the hybridization of Sb s orbital with Te p orbital near the Fermi level weaken the bonding strength of the Sb-Te bond, leading to significant anharmonicity and low group velocity. Thus, a low lattice thermal conductivity of 2.77 W/m K is achieved for the Sb2Sn2Te6 monolayer at 300 K. The band convergence and multivalley characteristics in the electronic band structure give birth to the enhancements of Seebeck coefficients and carrier mobilities, resulting in a substantial power factor of 76.69 μW/mK2 at 300 K under the carrier concentration of 3.20×1020 cm−3. An optimal figure of merit (ZT) of 2.37 at 900 K for the Sb2Sn2Te6 monolayer is obtained under the carrier concentration of 2.42×1020 cm−3. The present work not only provides fundamental insights into the correlation between the antibonding state, chemical bond and lattice thermal conductivity in Sb2Sn2Te6 monolayer, but also elaborates on the promising prospect of the Sb2Sn2Te6 monolayer in the TE application with high conversion efficiency.

Operando XANES Reveals the Chemical State of Iron-Oxide Monolayers During Low-Temperature CO Oxidation.

We have used grazing incidence X-ray absorption near edge spectroscopy (XANES) to investigate the behavior of monolayer FeO films on Pt(111) under near ambient pressure CO oxidation conditions with a total gas pressure of 1 bar. Spectra indicate reversible changes during oxidation and reduction by O and CO at 150 °C, attributed to a transformation between FeO bilayer and FeO trilayer phases. The trilayer phase is also reduced upon heating in CO+O , consistent with a Mars-van-Krevelen type mechanism for CO oxidation. At higher temperatures, the monolayer film dewets the surface, resulting in a loss of the observed reducibility. A similar iron oxide film prepared on Au(111) shows little sign of reduction or oxidation under the same conditions. The results highlight the unique properties of monolayer FeO and the importance of the Pt support in this reaction. The study furthermore demonstrates the power of grazing-incidence XAFS for in situ studies of these model catalysts under realistic conditions.

Impact of Electric Field and Biaxial Strain on the Structural, Phonon, and Optical Features of Bulk and Monolayer Potassium Nitride

First-principles calculations are used within the framework of density functional theory to investigate the electronic, structural, magnetic and optical properties of Potassium Nitride (KN) in the bulk and monolayer states. This compound is dynamically stable according to phonon calculations. The results show that the energy gap decreases from the bulk to the monolayer. The equilibrium lattice constant increases when changing from bulk to monolayer, and the half-metallic (HM) character remains preserved in that case. According to the Slater–Pauling statute (Zt-4), the total magnetic moment equals 2 µB per unit cell. The electric field and biaxial strain affect the monolayer's electronic and magnetic characteristics were investigated. The magnitude of the spin-up channel concerning the energy gap changes under the biaxial strain. In particular, it decreases under tensile strain and increases under compression strain. Given that the values of magnetic moments remain unchanged, the HM property can be preserved for significant strains. When the electric field reaches -0.6 V/nm, the half-metallic property of this compound will be destroyed. It affects the energy gap and eliminates the HM trait since the magnetic moment of the K grew significantly greater than the moment of the N, and the N played a significant role in the realization of the half-metallic characteristic.

Strain-tunable magnetic and magnonic states in Ni-dihalide monolayers

Strain-tunable magnetic and magnonic states in Ni-dihalide monolayers

A low-cost device for measuring TEER at cultivation of epithelial/endothelial cells on inserts.

Background: The barrier properties of epithelial and endothelial cells are usually studied in vitro when cells are cultured on mesh inserts in culture plates, and it is necessary to assess the state of the cell monolayer on the membrane of the inserts before the experiment. Typically, monolayer density is analyzed by passing a fluorescent label through the insert with cells or by measuring the transendothelial resistance (TEER) of the cell layer. Many studies use both methods of assessing monolayer integrity in parallel, depending on the purpose of the study. The TEER method also allows to record early changes in the monolayer state under the action of various substances. Aim of this work is to demonstrate the possibility of TEER measurements using the proposed device (conductometer) made from freely available components using endothelial/epithelial cells as an example. Materials and Methods. A device (conductometer) for measuring TEER, created on the basis of easily accessible components, is presented. The device was tested by culturing two cell lines – hybridoma of endothelial origin Ea.hy926 and human colon adenocarcinoma Caco-2. Caco-2 cells were cultured for 22 days and Ea.hy926 cells were cultured for 7 days. The integrity of the cell monolayer and the density of intercellular contacts were evaluated by the TEER value determined by the proposed conductometer, as well as by the fluorescein permeability of the cell monolayer. Results: The results of TEER measurements using the proposed device and at the same time, the fluorescein permeability of the cell monolayer during the cultivation of Caco-2 and EA.hy926 cells are presented. For Caco-2 cells from the moment of 100% confluence the TEER value gradually increased reaching maximum values by 20-21 days, after which it decreased slightly. The permeability values decreased as the cells were cultured, indicating the formation of dense contacts. For EA.hy926 cells the rise in TEER values are observed on day 3 and decrease was observed on day 7. The results of TEER and permeability obtained by the proposed conductometer have a strong inverse correlation for both cell lines and are in good agreement with each other. Conclusions. The presented device, made on the basis of simple and affordable components, is similar to commercially available devices and can be used to study the integrity and density of a monolayer in the cultivation of epithelial/endothelial cells, to study the processes of trans/paracellular transport under the action of various substances, as well as in experiments with the co-cultivation of various cell lines.

Pt3 cluster doped SnS2 monolayer as a gas-sensing material to C4F7N decomposition: A DFT study

Perfluoroisobutyronitrile (C4F7N) is a new type of eco-friendly insulating gas widely used in various gas insulation systems. An effective method for diagnosing the operational status of insulation equipment is to design gas sensor materials to detect C4F7N breakdown products. Using density functional theory, we investigated the sensing ability of the Pt3 cluster doped SnS2 monolayer for common C4F7N decomposition gases (CF4, COF2, C2N2, and CF3CN). We explored the adsorption energy, charge transfer, adsorption distance, and density of states of the Pt3-SnS2 monolayer as a C4F7N breakdown gas sensor. The results indicate that the doping of Pt3 clusters has a relatively small improvement in the adsorption capacity of CF4 and COF2 gases, indicating physical adsorption, but a significant improvement in the adsorption capacity of C2N2 and CF3CN gases, indicating chemical adsorption. This work provides additional potential applications for Pt3 cluster doped SnS2 monolayer gas sensing materials.

Effect of alkyl glucoside concentration on functional group structure and adsorption characteristics of anthracite.

To investigate the influence of different concentrations of surfactants on the adsorption of anthracite, the nonionic surfactant alkyl polyglucoside (APG) was selected. The study examined the adsorption characteristics of different concentrations of APG on the surface of anthracite. The results revealed the existence of two modes of APG adsorption on anthracite. Under the action of 0.06 wt% APG, APG was found to adsorb in a monolayer state on the anthracite surface, with a saturation adsorption capacity of 20.06mg/g. When the solution concentration exceeded 0.14 wt%, APG exhibited a double-layer saturation adsorption state on anthracite, with a saturation adsorption capacity of 71.71mg/g. Molecular dynamics simulations complemented these findings, demonstrating that low concentrations of APG could reduce the mobility of water molecules and enhance the hydrophilicity of anthracite. With an increase in the number of APG molecules, multi-layer adsorption occurred on the anthracite surface, making it more hydrophobic. Therefore, the differences in wettability of anthracite induced by different concentrations of APG were primarily attributed to the spatial distribution of the surfactant at the water/coal interface. This study analyzed the adsorption capacity of the surfactant through adsorption experiments and Fourier-transform infrared spectroscopy (FTIR) experiments. Molecular dynamics simulations were conducted to construct six concentration levels of water/APG/anthracite systems. Various aspects, including APG adsorption configurations, interaction energies, relative concentrations of each component, and the diffusion coefficient of water molecules, were discussed to elucidate the reasons for the differential wettability of anthracite induced by different concentrations of APG.

Ab initio calculations on magnetic and optical characteristics of pure ZnO and Mn(II)-doped ZnO monolayers with and without neutral charged vacancies defects

At high Curie temperatures, diluted magnetic semiconductors exhibit induced ferromagnetism and spin-dependent interactions, making them useful in magneto-electronic devices. In spite of the fact that ferromagnetism and optical emission dynamics are characterized by intricate microstructural and compositional features, the origins of these phenomena are not yet completely understood. Here, we used first principles calculations to study how vacancy defects affect the electrical, optical, and magnetic properties of pure ZnO and Mn@ZnO monolayers. In the pristine monolayer, the Zn vacancy produces magnetism with a total magnetic moment of 2 μB, but the oxygen vacancy does not. The magnetic semiconducting characteristic of the Mn substitution at the Zn site is shown by a total magnetization of 5 μB. The magnetic ground state of the ZnO monolayer doped with Mn is significantly influenced by the presence of native defects. More specifically, Zn vacancies are essential for stabilizing the FM states due to the bound magnetic polarons (BMPs) formation, but O vacancies have no influence on the magnetic ground state of the Mn(II)@ZnO monolayer. Based on the optical study, we discovered that Zn and O vacancies in the ZnO ML produce optical absorption peaks at 1.92 eV and 2.90 eV, respectively. The substitution of Mn(II) at Zn site not only increases the band gap of the ZnO monolayer but also produces d-d transition bands at lower energy side of fundamental energy gap peak. The Zn vacancies in the Mn(II)@ZnO ML produce an infrared absorption band around 1.13 eV, which could be due to the formation of the BMP, and enhance the optical absorption efficiency. The ZnO ML’s energy gap and Mn ion’s d-d transition bands exhibit a blueshift in AFM systems and a redshift in the FM systems. In far configuration, the Mn(II)@ZnO monolayer show no shift in the fundamental bandgap and intra-band transition of the Mn(II) spins due to the Mn(II) ion’s paramagnetic nature.

Robust Ferromagnetism in Hexagonal Honeycomb Transition Metal Nitride Monolayer.

Two-dimensional intrinsic magnetic materials with high Curie temperature are promising candidates for next-generation spintronic devices. In this work, we design two kinds of two-dimensional transition metal nitrides, VN2 and FeN2, both with a hexagonal honeycomb lattice. Based on the formation energy, and phonon spectra calculations as well as the molecular dynamics simulations, their structural stability is demonstrated. Then, we determine the ferromagnetic ground states of VN2 and FeN2 monolayers through the energy calculations, and the Curie temperatures of 222 K and 238 K are estimated by solving the Heisenberg model using the Monte Carlo simulation method. Hence, the VN2 and FeN2 monolayers are demonstrated to be new two-dimensional ferromagnetic materials with high temperature ferromagnetism or large-gap half-metallicity.

Synthesis, atomic structure and electronic properties of ferroelectric AgBiP2Se6 ultrathin flakes

Layered transition metal thiophosphate ABP2X6(A = Ag or Cu, B = V, Cr, In or Bi, X = S or Se) as a promising 2D ferroelectric material, has attracted great attention nowadays. In this work, ultrathin AgBiP2Se6 sheets down to 6 nm were synthesized for the first time. Atomic force microscopy, double spherical aberration scanning TEM and Raman spectra were performed to investigate the morphology, atomic structure and vibration modes of the samples. The first-principles calculations utilizing density-functional theory(DFT) were also used to analyze the electronic properties, band structures and projected density of states (PDOS) for both ferroelectric (FE) and paraelectric(PE) states of AgBiP2Se6 monolayer.

Desalination RO reject brine as a novel-based porous geopolymer for phosphorus removal from contaminated media

Desalination reverse osmosis reject brine-based porous geopolymer (RO/GP) was produced and investigated as an improved adsorbent for phosphorus (P) removal from tainted seawater, brackish water, river water, and municipal wastewater effluent. The RO reject brine/geopolymer was produced by reacting metakaolin and fly ash with a Na-alkali activator and anhydrous RO brine as a sacrificial template. The influence of RO reject brine content on water absorption, porosity, mechanical, and structural properties were examined. The developed RO-based geopolymers exhibited the greatest porosity (58.3–84.2 % vol%), a significant ratio of open porosity to total porosity (67.7–92.1 %), and outstanding compression strength (3.6–10.4 MPa). The produced RO/GP structure has an adsorption capacity of 92.4 mg-P/g. The sequestration reaction of phosphorus by RO/GP is of pseudo-second-order kinetic behavior via Chi-squared (χ2), RMSE, and determination coefficient (R2) values. Regarding their agreement with Langmuir behavior, the phosphorus adsorption uptakes occur in homogeneous and monolayer states. The reaction is exothermic, spontaneous, and favorable. The RO/GP exhibits significant affinity for phosphorus co-existing with Cl−, Na+, SO42−, K+, HCO3−, and Ca2+. The RO/GP shows high safety during the adsorption investigation, with a total cost of 0.32 $/kg-P.

Strain-tunable ferromagnetism in 2D non-van-der-Waals CuCr2X4 (X = S, Se)

The magnetic property of non-van-der-Waals CuCr2X4(X = S, Se) monolayer and its strain dependence are investigated systematically from first principles. The CuCr2X4 monolayers are intrinsic ferromagnetic (FM) metals, and the magnetism is mainly contributed by Cr atoms. Considerable magnetic anisotropy energy of 1.28 meV per Cr is found in CuCr2Se4 monolayer, which is one order of magnitude higher than the 0.14 meV per Cr found in CuCr2S4 monolayer. Their easy magnetization axes are along the in-plane directions, which is rare in 2D materials. By performing in-plane biaxial strain, the FM ground states of CuCr2X4 monolayers transform to antiferromagnetic (AFM) states when the compressive strain is larger than 6%, while relatively small tensile strain of 2% would induce the transition between FM and AFM for CuCr2Se4 monolayer. Moreover, we find that room-temperature Curie temperature can be realized in CuCr2X4 monolayer with appropriate tensile strain, indicating that the strained CuCr2X4 monolayer is promising in future 2D spintronic device applications and deserves further experimental exploration.

Electronic structures and Mott state of epitaxial TaS2 monolayers

Electronic structures and Mott state of epitaxial TaS2 monolayers

Temperature effects in topological insulators of transition metal dichalcogenide monolayers

We investigate the role of temperature on the topological insulating state of metal dichalcogenide monolayers, 1T′−MX2 (M=W, Mo and X=S, Se). Using first principles calculations based on density functional theory, we consider three temperature-related contributions to the topological band gap: electrons coupling with short-wavelength phonons, with long-wavelength phonons Fröhlich coupling, and thermal expansion. We find that electron-phonon coupling promotes the topology of the electronic structures of all 1T′−MX2 monolayers, while thermal expansion acts as a counteracting effect. Additionally, we derive the band renormalization from Fröhlich coupling in the two-dimensional context and observe its relatively modest contribution to 1T′−MX2 monolayers. Finally, we present a simplified physical picture to understand the “inverse Varshni” effect driven by band inversion in topological insulators. Our work reveals that, among the four 1T′−MX2 studied monolayers, MoSe2 is a promising candidate for room-temperature applications because its band gap displays remarkable resilience against thermal expansion, while the topological order of WS2 can be tuned under the combined influence of strain and temperature. Both materials represent novel examples of temperature-promoted topological insulators. Published by the American Physical Society 2024

Field induced Chern insulating states in twisted monolayer–bilayer graphene

Unraveling the mechanism underlying topological phases, notably the Chern insulators (ChIs) in strong correlated systems at the microscopy scale, has captivated significant research interest. Nonetheless, ChIs harboring topological information have not always manifested themselves, owing to the constraints imposed by displacement fields in certain experimental configurations. In this study, we employ density-tuned scanning tunneling microscopy (DT-STM) to investigate the ChIs in twisted monolayer–bilayer graphene (tMBG). At zero magnetic field, we observe correlated metallic states. While under a magnetic field, a metal–insulator transition happens and an integer ChI is formed emanating from the filling index s = 3 with a Chern number C = 1. Our results underscore the pivotal role of magnetic fields as a powerful probe for elucidating topological phases in twisted Van der Waals heterostructures.

Wafer-Scale epitaxial molybdenum disulfide ultrathin film on sapphire prepared by low-energy reactive magnetron sputtering

Molybdenum disulfide, a two-dimensional semiconductor, has found broad applications in various functional devices. Currently, developing a reliable method for growing wafer-scale monolayer molybdenum disulfide presents a significant challenge in its integration into the contemporary semiconductor industry. This study employed low-energy reactive magnetron sputtering with hydrogen disulfide gas to deposit wafer-scale epitaxial few-layer and monolayer molybdenum disulfide on c-sapphire substrates. The epitaxial molybdenum disulfide nano-thin film was examined using scanning nano-probe X-ray diffraction technique, revealing a single grain size of no less than 50 × 50 μm2. Furthermore, the growth of monolayer molybdenum disulfide on both c-sapphire and thermal silicon oxide substrates was verified through Raman microscope. Measurements of photoluminescence and X-ray photoelectron spectroscopy also confirmed the monolayer state of the sputtered nano-thin films. This study suggests that reactive magnetron sputtering with hydrogen disulfide gas offers a cost-effective, scalable method for growing large-area molybdenum disulfide nano-thin films, thereby holding significant potential for practical semiconductor industrial applications.

Robust Magnetoelectric Coupling in FeTiO3/Ga2O3 Non-van der Waals Heterostructures.

Magnetoelectric coupling represents a significant breakthrough for next-generation electronics, offering the ability to achieve nonvolatile magnetic control via electrical means. In this comprehensive investigation, leveraging first-principles calculations, we unveil a robust magnetoelectric coupling within multiferroic heterostructures (HSs) by ingeniously integrating a non-van der Waals (non-vdW) magnetic FeTiO3 monolayer with the ferroelectric (FE) Ga2O3. Diverging from conventional van der Waals (vdW) multiferroic HSs, the magnetic states of the FeTiO3 monolayer can be efficiently toggled between ferromagnetic (FM) and antiferromagnetic (AFM) configurations by reversing the polarization of the Ga2O3 monolayer. This intriguing phenomenon arises from polarization-dependent substantial interlayer electron transfers and the interplay between superexchange and direct-exchange magnetic couplings of the iron atoms. The carrier-mediated interfacial interactions induce crucial shifts in Fermi level positions, decisively imparting distinct electronic characteristics near the Fermi level of composite systems. These novel findings offer exciting prospects for the future of magnetoelectric technology.

Strain-engineered magnon states in two-dimensional ferromagnetic monolayers

We systematically investigate the strain-engineered magnon states in two-dimensional (2D) ferromagnetic monolayers. By suitable engineering of an inhomogeneous strain, we demonstrate the emergence of magnon Landau levels and magnon snake states in 2D ferromagnetic monolayers. We show a magnon valley Hall effect and valley filter without relying on any external fields. Our proposal offers us another way to manipulate magnon valley transport and construct different types of flexible spintronic devices, and is experimentally feasible for 2D ferromagnetic materials by using state-of-art techniques. Published by the American Physical Society 2024