Monolayer State Research Articles

Coupling between clay swelling/collapse and cationic partition

The semipermeability of clay aggregates in mudstones, shales, and marine sediments gives rise to geological ultrafiltration of ions and water. This behavior has been numerically analyzed based on the theory of the electric double layer (EDL). The model of EDL is suited for describing ionic partitions in clay pore with a width of a few nanometers. However, the interlayer space of smectite might be hardly described by the EDL model. An explicit atomic-scale description is more appropriate for the interlayer space. Besides, clay swelling or collapse corresponding to a variation of environmental conditions, and its impact on partitions of water and ions between interlayers and environment, have not been taken into account numerically. With atomic-scale molecular dynamics simulations and thermodynamic integrations based on them, the coupling between montmorillonite swelling/collapse and partitions of water and cations under different aqueous activity conditions is systematically disclosed here. With the combination of the Horinek’s force field to describe cations and ClayFF force field for atoms in solid layers, consistent swelling/collapse of Na- and K-montmorillonite with experimental observations as a function of water activity is shown. The swelling/collapse is connected to the transition between monolayer and bilayer hydration states. Decomposition of the swelling/collapse free energy shows both energy and entropy contributions are important in determining the thermodynamically stable hydration state. Taking environmental water activity and Na+/K+ ionic activity ratio as variables, a picture of cationic partition in response to those variables is established. It shows, under all circumstances, K+ ions are enriched in montmorillonite, which might explain K+ depletion in pore fluids of sediments. A low water activity improves K+ enrichment in montmorillonite. In addition, cationic partition accompanies variations of hydration states and alters the ionic concentration of the aqueous environment. These results can be applied to explain water and ionic partitions during subsurface water flow and the diagenesis processes of sediments.

Coexistence of valley polarization and Chern insulating states in MoS2 monolayers with n-p codoping

The electronic and topological properties of MoS2 monolayers with n-p codoping effect are investigated by using first-principles calculations. Two types of the doped Nb atoms play the roles of the p-type and n-type dopants, respectively. The n-p codoping is found inducing a large valley polarization, associated with the strong magnetization induced by the Nb dopants. Interestingly, the system simultaneously owns a perfect Chern insulating band gap opened exactly at the Fermi level. The nontrivial band gap comes from the lifting of the degeneracy of the dxz and dyz orbitals of Nb2 atoms after the spin-orbit coupling is considered. Our work inspires exciting prospects to tune the novel properties of materials with n-p codoping effects.

Two-Dimensional Nature and the Meaning of the Density of States in Redox Monolayers

It has been suggested that the transport of charge in molecular electronics and redox junctions are, at the sub-nanoscale, i.e. for dimensions lower than 5 nm, fundamentally related [\textit{Phys. ...

Observation of excitonic series in monolayer and few-layer black phosphorus

We report the full sequence of excitonic states in monolayer and few-layer black phosphorus (BP). High-quality BP samples are acquired by treating the silicon oxide substrate with a self-assembled monolayer hydrophobic molecule, which facilitates the access to their ground and higher-lying states of intrinsic excitons through the reflection and photoluminescence excitation spectra. Combined with first-principles and model calculations, we further determine that the exciton binding energy shrinks by \ensuremath{\sim}6-fold from 940 meV in monolayer to 160 meV in bilayer BP, yet it only decreases slightly to 108 meV in trilayer. This could be attributed to the strong interlayer coupling and lattice shrinking in multilayer BP samples. Our results shed light on the unique excitonic optical properties of two-dimensional BP.

Gate-Defined Accumulation-Mode Quantum Dots in Monolayer and Bilayer WSe2

We report the fabrication and characterization of gate-defined hole quantum dots in monolayer and bilayer WSe$_2$. The devices were operated with gates above and below the WSe$_2$ layer to accumulate a hole gas, which for some devices was then selectively depleted to define the dot. Temperature dependence of conductance in the Coulomb blockade regime is consistent with transport through a single level, and excited state transport through the dots was observed at temperatures up to 10 K. For adjacent charge states of a bilayer WSe$_2$ dot, magnetic field dependence of excited state energies was used to estimate $g$-factors between 0.8 and 2.4 for different states. These devices provide a platform to evaluate valley-spin states in monolayer and bilayer WSe$_2$ for application as qubits.

Quantum spin Hall effect, thermoelectric performance, and optical properties of XBi (X = Sc, Y) monolayers

Abstract The investigations of quantum spin Hall effect and the edge states manipulation of two dimensional topological insulators are very salient for the practical applications and fundamental sciences. The high thermoelectric efficiency of these materials has also been confirmed, recently. So, in this study the first-principles calculations are implemented based on density functional theory in the presence of spin orbit interaction, to evaluate the topological phase transition and thermoelectric performance of XBi (X = Sc, Y) monolayers under in-plane strains. It is found that the compressive in-plane strains and spin orbit interaction, which host considerable effects on the electronic structure, can cause the topologically nontrivial phase and high thermoelectric performance. The topological state of XBi monolayers is confirmed with the Z2 topological invariant calculation. These results provide these monolayers for the novel thermoelectric and nanoelectronics quantum devices and topological phenomena. Also some optical properties of XBi monolayers are calculated and investigated under in-plane strains. The results show the considerable transparency and reflectivity of these monolayers near zero energy.

Identification of lipid biomarkers of metastatic potential and gene expression (HER2/p53) in human breast cancer cell cultures using ambient mass spectrometry.

In breast cancer, overexpression of human epidermal growth factor receptor 2 (HER2) correlates with overactivation of lipogenesis, mutation of tumor suppressor p53, and increased metastatic potential. The mechanisms through which lipids mediate p53, HER2, and metastatic potential are largely unknown. We havedeveloped a desorption electrospray ionization mass spectrometry (DESI-MS) method to identify lipid biomarkers of HER2/p53 expression, metastatic potential, and disease state (viz. cancer vs. non-cancerous) in monolayer and suspension breast cancer cell cultures (metastatic potential: MCF-7, T-47D, MDA-MB-231; HER2/p53: HCC2218 (HER2+++/p53+), HCC1599 (HER2-/p53-), HCC202 (HER2++/p53-), HCC1419 (HER2+++/p53-) HCC70 (HER2-/p53+++); non-cancerous: MCF-10A). Unsupervised principal component analysis(PCA) of DESI-MS spectra enabled identification of twelve lipid biomarkers of metastatic potential and disease state, as well as ten lipids that distinguish cell lines based on HER2/p53 expression levels (> 200 lipids were identified per cell line). In addition, we developed a DESI-MS imaging (DESI-MSI) method for mapping the spatial distribution of lipids in metastatic spheroids (MDA-MB-231). Of the twelve lipids that correlate with changes in themetastatic potentialof monolayercell cultures, three were localized to the necrotic coreof spheroids, indicating a potential role in promoting cancer cell survival in nutrient-deficient environments. One lipid species, which was not detected in monolayer MDA-MB-231 cultures, was spatially localized to the periphery of the spheroid, suggesting a potential role in invasion and/or proliferation. These results demonstrate thatcombiningDESI-MS/PCA of monolayer and suspension cell cultures with DESI-MSI of spheroids isa promising approach for identifying lipid biomarkers of specific genotypes and phenotypes, as well as elucidating the potential functionof these biomarkers in breast cancer. Graphical Absract.

Strain-tunable out-of-plane polarization in two-dimensional materials

Two-dimensional (2D) ferroelectrics are a special class of materials that fit the need of miniaturization of devices, which can lead to a plethora of applications. Up to now, albeit the breakthrough in the discovery of a handful of 2D ferroelectrics, the mechanism of which still requires better understanding. In this work, we clarify that the ferroelectric ground state in monolayer (ML) ${\mathrm{In}}_{2}{\mathrm{Se}}_{3}$ is strongly affected by in-plane strain, and the strain-induced phase transition is of a first-order transition. We further propose that strain is an effective approach to tune the ferroelectric ground state in 2D materials, which can even intrinsically introduce out-of-plane polarizations. By performing first-principles calculations, we show that the epitaxial strain is able to tune the structural transition between polar and nonpolar layer groups. According to this, the epitaxial strain can not only fully suppress the polarization in ML-${\mathrm{In}}_{2}{\mathrm{Se}}_{3}$, but also cause out-of-plane polarizations in a group of 2D nonpolar materials, i.e., ML-${\mathrm{Bi}}_{2}{\mathrm{Se}}_{3}$, ML-${\mathrm{Bi}}_{2}{\mathrm{Te}}_{3}$, ML-${\mathrm{Sb}}_{2}{\mathrm{Te}}_{3}$, and ML-${\mathrm{Sb}}_{2}{\mathrm{Se}}_{3}$. The coupling mechanism between strain and polarization in these compounds is further illustrated.

Quantum valley Hall effect in wide-gap semiconductor SiC monolayer

We have investigated the valley Chern number and gapless edge states in wide-gap semiconductor SiC and BN monolayers by using the density functional theory calculations. We found that while SiC monolayer has a non-quantized valley Chern number due to a partial mixing of the Berry curvature peaks pertaining to the opposite valleys, there exist topologically protected gapless edge states within the bulk gap, leading to a quantum valley Hall effect. Doping of the opposite charge carriers causes a backscattering-free valley current flowing on the opposite edge, which can be used for experimental confirmation and application at room temperature. BN monolayer, on the other hand, was found to have gapped edge states due to the too large staggered AB-sublattice potentials.

Prototyping a Versatile Two-Layer Multi-Channel Microfluidic Device for Direct-Contact Cell-Vessel Co-Culture.

Microfluidic devices are gaining increasing popularity due to their wide applications in various research areas. Herein, we propose a two-layer multi-channel microfluidic device allowing for direct-contact cell-vessel co-culture. Using the device, we built a co-culture model of the outer blood-retina barrier (oBRB), mimicking the in vivo retinal pigment epithelial cells-Bruch membrane-fenestrated choroids. To demonstrate the versatility of the design, we further modified the device by inserting platinum electrodes for trans-epithelial electrical resistance (TEER) measurement, demonstrating the feasibility of on-chip assessment of the epithelial barrier integrity. Our proposed design allows for direct-contact co-culture of cell–cell or cell–vessel, modifiable for real-time evaluation of the state of the epithelial monolayers.

Real-space investigation of the charge density wave in VTe2 monolayer with broken rotational and mirror symmetries

Recently the charge density wave (CDW) in vanadium dichalcogenides have attracted increasing research interests, but a real-space investigation on the symmetry breaking of the CDW state in VTe2 monolayer is still lacking. We have investigated the CDW of VTe2 monolayer by low energy electron diffraction (LEED) and scanning tunneling microscope (STM). While the LEED experiments revealed a (4X4) CDW transition at 192+-2 K, our low-temperature STM experiments resolved the (4X4) lattice distortions and charge-density modulation in real space, and further unveiled a 1D modulation that breaks the three-fold rotational and mirror symmetries in the CDW state. In accordance with the CDW state at low temperature, a CDW gap of 12 meV was detected by scanning tunneling spectroscopy (STS) at 4.9 K. Our work provides real-space evidence on the symmetry breaking of the (4X4) CDW state in VTe2 monolayer, and implies there is a certain mechanism, beyond the conventional Fermi surface nesting or the q-dependent electron-phonon coupling, is responsible for the formation of CDW state in VTe2 monolayer.

Experimental Observations Indicating the Topological Nature of the Edge States on HfTe5**Supported by the Chinese Academy of Sciences, the National Natural Science Foundation of China under Grant No 11227903, the BM-STC under Grant No Z181100004218007, the National Basic Research Program of China under Grant Nos 2015CB921300 and 2015CB921304, National Key R&D Program of China under Grant No 2017YFA0302903, the Strategic

The topological edge states of two-dimensional topological insulators with large energy gaps furnish ideal conduction channels for dissipationless current transport. Transition metal tellurides XTe5 (X=Zr, Hf) are theoretically predicted to be large-gap two-dimensional topological insulators, and the experimental observations of their bulk insulating gap and in-gap edge states have been reported, but the topological nature of these edge states still remains to be further elucidated. Here, we report our low-temperature scanning tunneling microscopy/spectroscopy study on single crystals of HfTe5. We demonstrate a full energy gap of ∼80 meV near the Fermi level on the surface monolayer of HfTe5 and that such an insulating energy gap gets filled with finite energy states when measured at the monolayer step edges. Remarkably, such states are absent at the edges of a narrow monolayer strip of one-unit-cell in width but persist at both step edges of a unit-cell wide monolayer groove. These experimental observations strongly indicate that the edge states of HfTe5 monolayers are not trivially caused by translational symmetry breaking, instead they are topological in nature protected by the 2D nontrivial bulk properties.

Electric-Field-Controllable Conductance Switching of an Overcrowded Ethylene Self-Assembled Monolayer.

Molecular isomerism has been discussed from the viewpoint of the tiniest switch and memory elements in electronics. Here, we report an overcrowded ethylene-based molecular conductance switch, which fulfills all the essential requirements for implementation into electronic devices, namely, electric-field-controllable reversible conductance change with a molecular-level spatial resolution, robust conformational bistability under ambient conditions, and ordered monolayer formation on electrode surfaces. The conformational state of this overcrowded ethylene, represented by a folded or twisted conformer, is susceptible to external environments. Nanoscopic measurements using scanning tunneling microscopy techniques, together with theoretical simulations, revealed the electronic properties of each conformer adsorbed on Au(111). While the twisted conformer prevails in the molecularly dispersed state, upon self-assembly into a monolayer, a two-dimensional network structure of the folded conformer is preferentially formed due to particular intermolecular interaction. In the monolayer state, folded-to-twisted and its reverse isomerization can be controlled by the modulation of electric fields.

Observation of 2D semiconductor P-type dark-exciton lifetime using two-photon ultrafast spectroscopy.

We report direct measurements of intrinsic lifetimes of P-type dark-excitons in MoS2 monolayers. Using sub-gap excitation, we demonstrate two-photon excited direct population of P-type dark excitons, observe their scattering to bright states and decay with femtosecond resolution. In contrast to one-photon excitation schemes, non-monotonic density variation in bright exciton population observed under two-photon excitation shows the indirect nature of its population and competing decay pathways. Detailed modeling of different recombination pathways of bright and dark excitons allows experimental measurement of 2P dark → 1S bright exciton scattering rates. These insights into the dark states in a MoS2 monolayer pave the way for novel devices such as quantum memories and computing.

Spin-spiral formalism based on the multiple-scattering Green's function technique with applications to ultrathin magnetic films and multilayers

Based on the Korringa-Kohn-Rostoker Green's function technique we present a computational scheme for calculating the electronic structure of layered systems with homogeneous spin-spiral magnetic state. From the self-consistent non-relativistic calculations the total energy of the spin-spiral states is determined as a function of the wave vector, while a relativistic extension of the formalism in first order of the spin-orbit coupling gives an access to the effect of the Dzyaloshinskii-Moriya interactions. We demonstrate that the newly developed method properly describes the magnetic ground state of a Mn monolayer on W(001) and that of a Co monolayer on Pt(111). The obtained spin-spiral energies are mapped to a classical spin model, the parameters of which are compared to those calculated directly from the relativistic torque method. In case of the Co/Pt(111) system we find that the isotropic interaction between the Co atoms is reduced and the Dzyaloshinskii-Moriya interaction is increased when capped by a Ru layer. In addition, we perform spin-spiral calculations on Ir/Fe/Co/Pt and Ir/Co/Fe/Pt multilayer systems and find a spin-spiral ground state with very long wavelength due to the frustrated isotropic couplings between the Fe atoms, whereas the Dzyaloshinskii-Moriya interaction strongly depends on the sequence of the Fe and Co layers.

Studies of crab digestive phospholipase acting on phospholipid monolayers: Activation by temperature

Secreted phospholipases A2 (sPLA2) are water-soluble lipolytic enzymes that act at the interface of organized lipid substrates, where the catalytic step is coupled to various interfacial phenomena as enzyme penetration, solubilisation of reaction products, lateral packing and loss of mechanical stability of organized assemblies of phospholipid molecule, among others. Using the monomolecular film technique, we compared the interfacial properties of crab digestive sPLA2 (CDPL) with those of the porcine pancreatic one (PPPL). A kinetic study on the surface pressure dependency of the two sPLA2 was performed using monomolecular films of three different substrates: di C12-PC (1.2-dilauroyl-sn-glycerol-3-phosphocholine); di C12-PG (1.2-dilauroyl-sn-glycerol-3-phosphoglycerol) and di C12-PE (1.2-dilauroyl-sn-glycerol-3-phosphoethanolamine). The use of a substrate in monolayer state, during the catalytic reactions, allows us to monitor the effect of several physicochemical parameters by altering the “quality of interface”. The effect of temperature on the hydrolysis rate of these substrates was also checked. Our results show that activities of both phospholipases were affected by the variation of the subphase temperature. CDPL was irreversibly inactivated by p-bromo-phenacyl bromide, the specific inhibitor of sPLA2. The hyperbolic catalytic behaviour observed was coherent with hopping mode of action, one of the two characteristic mechanisms of interfacial catalysis of sPLA2.

Intense-field interaction regime with weak laser pulses and localized plasmonic enhancement: Reference-free demonstration by 3rd- and 5th-order infrared spectroscopies.

In bulk materials, intense field interaction is accompanied by undesired nonresonant processes. Plasmonic nanostructures localize enhanced fields exclusively in their vicinity. We report a 4-fold vibrational population inversion between all the excited and the ground states in the molecular monolayer on the surface of gold nanoantennas. Excited population assessment relies on a novel reference-sample-free evaluation of the field enhancement with 5th- and 3rd-order nonlinear infrared spectroscopies and on quantitative modeling of coherent excitation dynamics. This study opens opportunities for precise population control utilizing population inversion for vibrational transitions using weak fields.

Charge Density Wave Transition in Monolayer CuTe driven by Coulomb Correlation

Dimensionality control provides a route to adjust physical properties in van der Waals layered materials. Bulk CuTe forming a layered structure exhibits a charge density wave (CDW) phase in Te chains with a periodicity of 5×1×2 at temperature below 335 K. The stability of the CDW state in a CuTe monolayer, however, has been investigated neither experimentally nor theoretically. Here, we report the theoretical prediction for the CDW phase in a CuTe monolayer using the first principles calculations. Similarly to its bulk structure, we find the phonon soft mode at q = (0.4, 0.0), indicating structural instability, which only appears with the correlation effect on Cu. The role of Coulomb correlations in driving the CDW transition suggests an electron-electron correlation and an electron-phonon interaction as the origin of the CDW instability. We expect the periodicity of the CDW phase in the CuTe monolayer to be 5×1. We show, by reducing the interlayer interactions, that tuning of the CDW modulation may be possible, as demonstrated by the modulation pattern in quasi-one-dimensional Te chains being different from that in the bulk counterpart.

The electronic properties of hydrogenated Janus MoSSe monolayer: a first principles investigation

The geometry, stability and electronic properties of hydrogenated Janus MoSSe monolayer (MoSSe-Hx) (x = 0–16) were investigated by first-principles calculations. The most stable position for H adsorption on the surface of Janus MoSSe monolayer was studied. Energetically, Sets is the most favorable and stable site for H adsorption. Janus MoSSe-Hx is predicted to be stable because its formation energy is lower than graphane and H2S, and the adsorption energy is lower than H atoms adsorbed on graphene and MoS2. The analysis of the electronic density distribution and bond population also show that Janus MoSSe-Hx forms stable structure. Then, the variations of electronic structure of Janus MoSSe-Hx were investigated. With the increase of the adsorbed numbers of H atoms, the density of state (DOS) of Janus MoSSe monolayer changed. It can be found that the DOS of Janus MoSSe-H2 has additional peaks in the band gap, which are introduced from H 1s doping states. With the increase of H atoms adsorbed, there are more H 1s doping states in the band gap. Correspondingly, the band gap of the Janus MoSSe-Hx monolayer gradually decreases. The band gap of pristine Janus MoSSe is 1.47 eV, and the band gap of 2 H adsorbed on Janus MoSSe is 0.31 eV. When 16 H atoms are adsorbed on Se side of Janus MoSSe, the band gap is 0.23 eV. Also, the work function of the Janus MoSSe-Hx monolayer was observed to decrease as the number of adsorbed H atoms increased. After hydrogenation process, the Se atoms transfer electrons to H atoms and Mo atoms, Mo atoms neutralize part of electrons, which leads to the weakening of the electric field pointing outward from Mo atoms to vacuum in the Janus MoSSe. As a result, the work function of the pristine Janus MoSSe is 5.24 eV, and the work function of Janus MoSSe-Hx(x = 16) decreases to 3.70 eV. Therefore, our results provide a new method to realize band gap and work function modulation of Janus MoSSe by simple hydrogenation method. Our results suggest that the hydrogenated Janus MoSSe monolayer has potential applications in designing novel electronic and optical device, or photocatalyst such as water splitting with the intrinsic built-in electronic field.

Dependence of the magnetic interactions in MoS2 monolayer on Mn-doping configurations

Understanding the magnetic properties of the various Mn doping configurations that can be encountered in 2H-MoS2 monolayer could be beneficial for its use in spintronics. Using density functional theory plus Hubbard term (DFT + U) approach, we study how a single isolated, double- and triple-substitution configurations of Mn atoms within a MoS2 monolayer could contribute to its total magnetization. We find that the doping-configuration plays a critical role in stabilizing a ferromagnetic state in a Mn-doped MoS2 monolayer. Indeed, the Mn–Mn magnetic interaction is found to be ferromagnetic and strong for Mn in equidistant substitution positions where the separation average range of 6–11 . The strongest ferromagnetic interaction is found when substitutions are in second nearest neighbor Mo-sites of the armchair chain. Clustering is energetically favorable but it strongly reduces the ferromagnetic exchange energies. Furthermore, in term of electronic properties, we show that the Mn-doped MoS2 monolayer can change its electronic behavior from semiconductor to half-metallic depending on the doping configuration. Our results suggest that ordering the Mn dopants on MoS2 monolayer is needed to increase its potential ferromagnetism.