Monolayer Transition Research Articles

Physicochemical Studies on Amino Acid Based Metallosurfactants in Combination with Phospholipid.

Dicarboxylate metallosurfactants (AASM), synthesized by mixing N-dodecyl aminomalonate, -aspartate and -glutamate with CaCl2, MnCl2 and CdCl2, were characterized by XRD, FTIR, and NMR spectroscopy. Layered structures, formed by metallosurfactants, were evidenced from differential scanning calorimetry and thermogravimetric analyses. Solvent-spread monolayer of AASM in combination with soyphosphatidylcholine (SPC) and cholesterol (CHOL) were studied using Langmuir surface balance. With increasing mole fraction of AASM mean molecular area increased and passed through maxima at ~60 mol% of AASMs, indicating molecular packing reorganization. Systems with 20 and 60 mol% AASM exhibited positive deviations from ideal behavior signifying repulsive interaction between the AASM and SPC, while synergistic interactions were established from the negative deviation at other combinations. Dynamic surface elasticity increased with increasing surface pressure signifying formation of rigid monolayer. Transition of monolayer from gaseous to liquid expanded to liquid condensed state was established by Brewster angle microscopic studies. Stability of the hybrid vesicles, formed by AASM+SPC+CHOL, were established by monitoring their size, zeta potential and polydispersity index values over 100 days. Size and spherical morphology of hybrid vesicles were confirmed by transmission electron microscopic studies. Biocompatibility of the hybrid vesicles were established by cytotoxicity studies revealing their possible applications in drug delivery and imaging.

Short-range correlation of stress chains near solid-to-liquid transition in active monolayers.

Using a three-dimensional model of cell monolayers, we study the spatial organization of active stress chains as the monolayer transitions from a solid to a liquid state. The critical exponents that characterize this transition map the isotropic stress percolation onto the two-dimensional random percolation universality class, suggesting short-range stress correlations near this transition. This mapping is achieved via two distinct, independent pathways: (i) cell-cell adhesion and (ii) active traction forces. We unify our findings by linking the nature of this transition to high-stress fluctuations, distinctly linked to each pathway. The results elevate the importance of the transmission of mechanical information in dense active matter and provide a new context for understanding the non-equilibrium statistical physics of phase transition in active systems.

Anisotropic Phonon Behavior and Phase Transition in Monolayer ReSe2 Discovered by High Pressure Raman Scattering.

Re-based transition metal dichalcogenides have attracted extensive attention owing to their anisotropic structure and excellent properties in applications such as optoelectronic devices and electrocatalysis. The present study methodically investigated the evolution of specific Raman phonon mode behaviors and phase transitions in monolayer and bulk ReSe2 under high pressure. Considering the distinctive anisotropic characteristics and the vibration vectors of Re and Se atoms exhibited by monolayer ReSe2, we perform phonon dispersion calculations and propose a methodology utilizing pressure-dependent polarized Raman measurements to explore the precise structural evolution of monolayer ReSe2 under the stress fields. Varied behaviors of the Eg-like and Ag-like modes, along with their specific vector transformations, have been identified in the pressure range 0-14.59 GPa. The present study aims to offer original perspectives on the physical evolution of Re-based transition metal dichalcogenides, elucidating their fundamental anisotropic properties and exploring potential applicability in diverse devices.

Orientational phase transition in monolayers of multipolar straight rigid rods: The case of 2-thiophene molecule adsorption on the Au(111) surface.

Monte Carlo simulations and finite-size scaling theory have been carried out to study the critical behavior and universality for the isotropic-nematic (IN) phase transition in a system of straight rigid pentamers adsorbed on a triangular lattice with polarized nonhomogeneous intermolecular interactions. The model was inspired by the deposition of 2-thiophene molecules over the Au(111) surface, which was previously characterized by experimental techniques and density functional theory. A nematic phase, observed experimentally by the formation of a self-assembled monolayer of parallel molecules, is separated from the isotropic state by a continuous transition occurring at a finite density. The precise determination of the critical exponents indicates that the transition belongs to the three-state Potts universality class. The finite-size scaling analysis includes the study of mutability and diversity. These two quantities are derived from information theory and they have not previously been considered as part of the conventional treatment of critical phenomena for the determination of critical exponents. The results obtained here contribute to the understanding of formation processes of self-assembled monolayers, phase transitions, and critical phenomena from novel compression algorithms for studying mutual information in sequences of data.

Room-temperature antiferromagnetic order in monolayer Fe2C

We have studied the room-temperature long-range antiferromagnetic order in the ${\mathrm{Fe}}_{2}\mathrm{C}$ monolayer using the combination of first-principles calculations and the Green's function analysis within the anisotropic Heisenberg model. The ${\mathrm{Fe}}_{2}\mathrm{C}$ monolayer is a semimetal with the out-of-plane antiferromagnetic order between two $\mathrm{Fe}$ planes. The Green's function approach within the random phase approximation is formulated to calculate the temperature-dependent antiferromagnetic magnon energy and the spin correlation function in a two-dimensional antiferromagnetic monolayer. The correlation function is used to evaluate the sublattice magnetization and study the magnetic phase transition in monolayer ${\mathrm{Fe}}_{2}\mathrm{C}$. Moreover, the spin Hamiltonian and the Green's function formalism are developed to investigate the antiferromagnetic ${\mathrm{Fe}}_{2}\mathrm{C}$ monolayer in the presence of an external magnetic field applied along the easy axis. The H-T phase diagram shows that the antiferromagnetic to spin-flop and the spin-flop to paramagnetic phase transitions occur in low temperatures. Finally, we estimate the N\'eel temperature and the critical values of the magnetic field strength for these two phase transitions.

Characterization of the ferroelectric phase transition in monolayer In[formula omitted]Se[formula omitted] grown on bilayer graphene

Ferroelectric transitions in monolayer β–In2Se3 grown on bilayer graphene were studied using low energy electron diffraction and scanning tunneling microscopy methods. Violation of the electron dispersion during transitions was studied using angle-resolved photoelectron spectroscopy. In contrast to the bulk case, we observed coexistence of stripe (β′) and zigzag (α′ ) phases in the low temperature limit. Experimental results were confirmed by total energy calculations of the corresponded structures that were found using ab initio random structure searching technique. As total energies of antiferroelectric β′ phase and ferroelectric α′ phase are almost the same, monolayer In2Se3 appears to be promising material for the ferroelectric switching devices.

Magnetic order transition in monolayerMoS2induced by strong intervalley correlation

In this work, we study a model for monolayer molybdenum disulfide with including the intravalley and intervalley electron-electron interaction. We solve the model at a self-consistent mean-field level and get three solutions $L_{0}$, $L_{+}$ and $L_{-}$. As for $L_{0}$, the spin polarizations are opposite at $\textbf{K}$ and $\textbf{K}^{\prime}$ valley and the total magnetization is zero. $L_{\pm}$ describe two degenerate spin-polarized states, and the directions of polarization are opposite for the states of $L_{+}$ and $L_{-}$. Based on these results, the ground state can be deduced to be spin polarized in domains in which their particular states can be randomly described by $L_{+}$ or $L_{-}$. Therefore, a zero net magnetization is induced for zero external magnetic field $\mathbf{B}$, but a global ferromagnetic ground state for a nonzero $\mathbf{B}$. We estimate the size of domains as several nanometers. As the increase of the chemical potential, the ground state changes between $L_{0}$ and $L_{\pm}$, indicating first order phase transitions at the borders, which is coincident with the observation of photoluminescence experiments in the absence of the external magnetic field [J. G. Roch $\it{et}$ $\it{al.}$, Phys. Rev. Lett. ${\bf 124}$, 187602 (2020)].

Exciton Transitions in Monolayer WS2 Activated by Swift Heavy Ion Irradiation

The emergence of monolayer transition metal dichalcogenides (TMDCs) has provided a favorable platform for exploring a broad range of novel optoelectronic device applications and quantum phenomena due to their remarkable physical and chemical characteristics. Understanding the transition and motion of excitons in TMDCs is of fundamental interest for their optoelectronic devices. Here, we demonstrate the exciton transition in monolayer WS2 that can be fine-tuned by swift heavy ion (SHI) irradiation. The dependence of trion and exciton emissions on SHI irradiation in the as-transferred and as-grown monolayer WS2 was investigated by micro photoluminescence (PL) spectra where different PL responses were monitored. The trion-to-exciton transition in the as-transferred WS2 monolayer was ascribed to SHI irradiation that resulted in charge localization and pre-existing defect annealing, whereas the exciton-to-trion transition in the as-grown WS2 monolayer was attributed to the competition of charge localization and charge transfer caused by SHI irradiation. Femtosecond transient absorption measurements were performed to study the exciton relaxation dynamics in SHI-irradiated WS2, and it was shown that defect-assisted recombination dominates the exciton relaxation process for SHI-irradiated WS2. These results open up new opportunities to manipulate and tune the optical properties of two-dimensional TMDCs, which are of special importance for the development of optoelectronic and valleytronic devices.

Adsorption model and phase transitions of diblock perfluoroalkylated surfactants at the water∣alkane interface

We present a theory of the adsorption behaviour and phase transitions in monolayers of perfluoroalkylated alcohols, n-CnF2n+1CmH2mOH, at the water∣oil interface, and validate it for a range of temperatures and surfactant structures. The reason for the observed cohesive behaviour is identified as dispersion attraction between the fluorocarbon blocks. The London constant is determined from the increment of the lateral attraction parameter with the size of the fluorocarbon chain. The monolayers exhibit phase transition from liquid expanded state to van der Waals crystal. However, they are supercritical with respect to the gas–liquid transition. For the description of the liquid phase, we use the sticky disc model – fluid monolayer made of hard discs interacting with a short-ranged sticky potential. For the crystalline phase, a two-dimensional cell model is developed using the same interaction potential. This new model coincides with the empirical equation of state of Jura and Harkins, and ascribes physical meaning to its parameters. We extend the theory of Ivanov et al. for the adsorption constant Ka to diblock molecules; it predicts accurately the dependence of Ka and the adsorption heat on the surfactant structure. An invariant phase diagram of the monolayers is constructed.

Uniaxially Incommensurate Structure and Metal-insulator Transition of Metallic Indium Monolayer on Si(111)

Uniaxially Incommensurate Structure and Metal-insulator Transition of Metallic Indium Monolayer on Si(111)

The Role of Tricellulin in Epithelial Jamming and Unjamming via Segmentation of Tricellular Junctions.

Collective cellular behavior in confluent monolayers supports physiological and pathological processes of epithelial development, regeneration, and carcinogenesis. Here, the attainment of a mature and static tissue configuration or the local reactivation of cell motility involve a dynamic regulation of the junctions established between neighboring cells. Tricellular junctions (tTJs), established at vertexes where three cells meet, are ideally located to control cellular shape and coordinate multicellular movements. However, their function in epithelial tissue dynamic remains poorly defined. To investigate the role of tTJs establishment and maturation in the jamming and unjamming transitions of epithelial monolayers, a semi‐automatic image‐processing pipeline is developed and validated enabling the unbiased and spatially resolved determination of the tTJ maturity state based on the localization of fluorescent reporters. The software resolves the variation of tTJ maturity accompanying collective transitions during tissue maturation, wound healing, and upon the adaptation to osmolarity changes. Altogether, this work establishes junctional maturity at tricellular contacts as a novel biological descriptor of collective responses in epithelial monolayers.

Thickness dependence of hydrogen-induced phase transition in MoTe2

Two-dimensional transition metal dichalcogenides (TMDs) usually exist in two or more structural phases with different physical properties, and can be repeatedly switched between these phases via different stimuli, making them potentially useful for memory devices. An understanding of the physics of interfaces between the TMDs and conventional semiconductors, or other 2D-crystals forming heterogenous or homogeneous assemblies is central to their successful application in technologies. However, to date, most theoretical works have explored phase-change properties of isolated TMD monolayers in vacuum. Using \textit{ab-initio} calculations, we show how interfacial effects modify the thermodynamics and kinetics of the phase transition by studying hydrogen-induced transitions in monolayers and bilayers of MoTe$_{2}$. The phase-change properties of MoTe$_{2}$ show substantial thickness-dependence, with the timescale for a transition in the hydrogenated bilayer being about $10^7$-times longer than that in a monolayer at room temperature. Our study highlights the importance of taking effects of immediate environment into account when predicting properties of 2D crystals.

Modified tailoring the electronic phase and emergence of midstates in impurity-imbrued armchair graphene nanoribbons

We theoretically address the electronic structure of mono- and simple bi-layer armchair graphene nanoribbons (AGNRs) when they are infected by extrinsic charged dilute impurity. This is done with the aid of the modified tight-binding method considering the edge effects and the Green’s function approach. Also, the interplay of host and guest electrons are studied within the full self-consistent Born approximation. Given that the main basic electronic features can be captured from the electronic density of states (DOS), we focus on the perturbed DOS of lattices corresponding to the different widths. The modified model says that there is no metallic phase due to the edge states. We found that the impurity effects lead to the emergence of midgap states in DOS of both systems so that a semiconductor-to-semimetal phase transition occurs at strong enough impurity concentrations and/or impurity scattering potentials. The intensity of semiconductor-to-semimetal phase transition in monolayer (bilayer) ultra-narrow (realistic) ribbons is sharper than bilayers (monolayers). In both lattices, electron-hole symmetry breaks down as a result of induced-impurity states. The findings of this research would provide a base for future experimental studies and improve the applications of AGNRs in logic semiconductor devices in industry.

Soft hydrogen plasma induced phase transition in monolayer and few-layer MoTe2

Phase transition from the semiconducting hexagonal (2H) phase to the metallic monoclinic (1T′) phase in two-dimensional (2D) transition metal dichalcogenides like MoTe2 is not only of great importance in fundamental study but also of technological significance for broad device applications. Here we report a universal, facile, scalable and reversible phase engineering technique (between 2H and 1T′ phases) for both monolayer and few-layer MoTe2 based on a soft hydrogen plasma treatment. The 2H → 1T′ transition was confirmed by a series of characterizations including Raman spectra and mapping studies, XPS analysis and FET device measurements at varying temperatures. We attribute the phase transition to the warping of Te–Mo bonds and the lateral sliding of the top Te-layer induced by the soft hydrogen ion bombardment according to both the structural and electronic characterizations as well as the horizontal comparison with the cases of Ar or O2 plasma treatment. We have also prepared a 2D heterostructure containing periodical 2H and 1T′ MoTe2 and showed that such phase transition can be readily reversed by post annealing. These results thus provide a robust and efficient approach for the phase engineering of monolayer and few-layer MoTe2 and could aid the development of 2D optoelectronic, memory and reconfigurable devices.

Reversible Phase Transitions in the Phospholipid Monolayer.

The polymorphism of phospholipid monolayers has been extensively studied because of its importance in surface thermodynamics, soft matter physics, and biomembranes. To date, the phase behavior of phospholipid monolayers has been nearly exclusively studied with the classical Langmuir-type film balance. However, because of experimental artifacts caused by film leakage, the Langmuir balance fails to study the reversibility of two-dimensional surface phase transitions. We have developed a novel experimental methodology called the constrained drop surfactometry capable of providing a leakage-proof environment, thus allowing reversibility studies of two-dimensional surface phase transitions. Using dipalmitoylphosphatidylcholine (DPPC) as a model system, we have studied the reversibility of isothermal and isobaric phase transitions in the monolayer. It is found that not only the compression and expansion isotherms but also the heating and cooling isobars, completely superimpose with each other without hysteresis. Microscopic lateral structures of the DPPC monolayer also show reversibility not only during the isothermal compression and expansion processes but also during the isobaric heating and cooling processes. It is concluded that the two-dimensional surface phase transitions in phospholipid monolayers are reversible, which is consistent with the reversibility of phase transitions in bulk pure substances. Our results provide a better understanding of surface thermodynamics, phase change materials, and biophysical studies of membranes and pulmonary surfactants.

Strongly Interaction-Enhanced Valley Magnetic Response in Monolayer WSe_{2}.

We measure the doping dependence of the valley Zeeman splitting of the fundamental optical transitions in monolayer WSe_{2} under an out-of-plane magnetic field by optical reflection contrast and photoluminescence spectroscopy. A nonlinear valley Zeeman effect, correlated with an over fourfold enhancement in the g factor, is observed. The effect occurs when the Fermi level crosses the spin-split upper conduction band, corresponding to a change of the spin-valley degeneracy from two to four. The enhancement increases and shows no sign of saturation as the sample temperature decreases. Our result demonstrates the importance of the Coulomb interactions in the valley magnetic response of two-dimensional transition metal dichalcogenide semiconductors.

Optical spectroscopy of excited exciton states in MoS2 monolayers in van der Waals heterostructures

The optical properties of MoS2 monolayers are dominated by excitons, but for spectrally broad optical transitions in monolayers exfoliated directly onto SiO2 substrates detailed information on excited exciton states is inaccessible. Encapsulation in hexagonal boron nitride (hBN) allows approaching the homogenous exciton linewidth, but interferences in the van der Waals heterostructures make direct comparison between transitions in optical spectra with different oscillator strength more challenging. Here we reveal in reflectivity and in photoluminescence excitation spectroscopy the presence of excited states of the A-exciton in MoS2 monolayers encapsulated in hBN layers of calibrated thickness, allowing to extrapolate an exciton binding energy of about 220 meV. We theoretically reproduce the energy separations and oscillator strengths measured in reflectivity by combining the exciton resonances calculated for a screened two-dimensional Coulomb potential with transfer matrix calculations of the reflectivity for the van der Waals structure. Our analysis shows a very different evolution of the exciton oscillator strength with principal quantum number for the screened Coulomb potential as compared to the ideal two-dimensional hydrogen model.

Structural and electronic phase transitions in ferromagnetic monolayer VS2 induced by charge doping

Among the known transition metal dichalcogenides, monolayer ${\mathrm{VS}}_{2}$ has attracted particular attention because of its intrinsic ferromagnetism and great application potential as a high-performance functional nanomaterial. Here, using first-principles calculations, we study the structural and electronic phase transitions in monolayer ${\mathrm{VS}}_{2}$ induced by charge doping. We show that without electron or hole doping, ${\mathrm{VS}}_{2}$ stabilizes in the 2H phase and is a bipolar magnetic semiconductor (BMS) whose valence and conduction states near the Fermi level carry opposite spin polarization. With the increase of hole doping concentration, ${\mathrm{VS}}_{2}$ will first experience an electronic phase transition from a BMS to a half metal, followed by a 2H-to-1T structural phase transition (SPT) which concomitantly results in another electronic phase transition from the half metal to a normal metal. Moreover, the reduced reaction barrier from the 2H to 1T phases by hole doping can make the occurrence of the SPT easier. However, electron doping can only induce the BMS-to-half metal electronic phase transition but will not trigger the SPT within the experimentally accessible doping regime. The different effects of hole and electron dopings on the SPT are further explained by the energy band diagram of ${\mathrm{VS}}_{2}$. These results establish the potential for ${\mathrm{VS}}_{2}$ utilization in innovative phase-change electronic and spintronic devices.

Multi-artery heat-pipe spreader: monolayer-wick receding meniscus transitions and optimal performance

For high heat flux and low thermal resistance, the multiple-artery heat-pipe spreader uses distributed high-permeability arteries (posts) for liquid supply and high-capillary pressure monolayer wick for liquid spreading and evaporation. Experiments indicate the receding meniscus transitions in monolayer play a role in sudden drop in thermal resistance prior to dryout. Using monolayer SEM images and the minimum surface energy principles, the meniscus dynamics up to dryout is analyzed, and the meniscus location, capillary pressure, effective thermal conductivity, and permeability are also predicted for heterogeneous, periodic sintered copper-particle (including bimodal particle size) unit cells. The liquid thickness is nonuniform within the heterogeneous unit cell, and with increase in the wick superheat local dryout occurs (meniscus snaps) in the loose-packed region influencing the wet-wick properties and the occurrence of the minimum thermal resistance. The monolayer wick continues to function under local dryout (away from post) until a receding dry front is formed followed by complete dryout. These predictions are in good agreement with experiments. The optimal wick thermal-hydraulic performance, i.e., dimensionless ratio of heat flux to thermal resistance (wick figure of merit Zm) is sought through analysis. The uniform, sintered, close-packed 30–50μm particles give the highest Zm over a range of superheat in the wet regime, and 30μm particles give a record low resistance near 2.5μK/(W/m2)

Adsorption Properties from Pressure-Varying Langmuir Parameters: n-Butane and Isobutane on Activated Carbon

Isothermal adsorption data for n-butane and isobutane on BAX 1500 activated carbon reported by Whittaker et al. [Whittaker, P. B.; Wang, X.; Zimmermann, W.; Regenauer-Lieb, K.; Chua, H. T. Predicting the Integral Heat of Adsorption for Gas Physisorption on Microporous and Mesoporous Adsorbents. J. Phys. Chem. C 2014, 118 (16), 8350−8358, DOI: 10.1021/jp410873v] were modeled with pressure-varying Langmuir adsorption parameters using flexible least squares for pressure-varying linear regression. Coverage varies with pressure and at distinct transitions; when the ratio of uptake to capacity is 0.69 ± 0.04, monolayer coverage is achieved or micropore volume is filled. Monolayer transitions are observed for the 298, 323, and 348 K isotherms, while micropore volume transitions are only apparent for the 298 K isotherms. The resultant adsorbent surface area is 1335 ± 25 cm2 g–1, and the micropore volume is 0.48 ± 0.03 cm3 g–1. Molecular areas, corresponding to excluded adsorbate areas, are dependent upon the temp...