2D Liquids Research Articles

Wafer-Scale Transfer of MXene Films with Enhanced Device Performance via 2D Liquid Intercalation.

Wafer-scale transfer processes of 2D materials significantly expand their application space in scalable microelectronic devices with excellent and tunable properties through van der Waals (vdW) stacking. Unlike many 2D materials, wafer-scale transfer of MXene films for vdW contact engineering has not yet been reported. With their rich surface chemistry and tunable properties, the transfer of MXenes can enable enormous possibilities in electronic devices using interface engineering. Taking advantage of the MXene hydrophilic surface, a straightforward, green, and fast process for the transfer of MXene films at the wafer scale (4-inch) is developed. Uniform vdW stacking of several types of large-area heterojunctions including MXene/MXene (Ti3C2Tx, Nb2CTx, and V2CTx), MXene/MoS2, and MXene/Au is further demonstrated. Multilayer support is applied to minimize damage or deformation in the transfer process of patterned Ti3C2Tx film. It allows us to fabricate thin film transistors and manipulate the MXene/MoS2 interface through the intercalation of various 2D liquids. Particularly noteworthy is the significant enhancement of the interfacial carrier transfer efficiency by ≈2 orders of magnitude using hydrogen iodide (HI) intercalation. This finding indicates a wide range of possibilities for interface engineering by transferring MXene films and employing liquid-assisted interfacial intercalation.

Atomic‐Scale Observations of the Immiscible Melted Metals Confined in a Single‐Atom Layer

Elucidating how changing of the dimensionality affects the property of the matter is the key challenge problem of the nanoscience. In particular, it has recently been reported that 2D liquids have fundamentally different dynamic properties to 3D liquids. Herein, the ultimate physical limit is addressed, when the liquid is confined to a single‐atom layer. The case has been realized with the Tl–Pb alloy layer grown on Si(111) substrate terminated by a single‐layer . At room temperature, the alloy behaves as a system of the immiscible melted metals confined in a single‐atom layer. In the scanning tunneling microscopy observation, Tl and Pb arrays have different contrasts that allows a direct visualization of evolution of the forming structures within atomic layer, as a function of the time and layer composition. In particular, it has been found that the structures look much like the Turing patterns. The obtained experimental dataset, especially the recorded STM videos, is believed to provide a hint for the prospective theoretical understanding of the dynamics of the single‐atom‐thick liquids.

Classification and structural characteristics of amorphous materials based on interpretable deep learning

Defining the structure characteristics of amorphous materials is one of the fundamental problems that need to be solved urgently in complex materials because of their complex structure and long-range disorder. In this study, we develop an interpretable deep learning model capable of accurately classifying amorphous configurations and characterizing their structural properties. The results demonstrate that the multi-dimensional hybrid convolutional neural network can classify the two-dimensional (2D) liquids and amorphous solids of molecular dynamics simulation. The classification process does not make a priori assumptions on the amorphous particle environment, and the accuracy is 92.75%, which is better than other convolutional neural networks. Moreover, our model utilizes the gradient-weighted activation-like mapping method, which generates activation-like heat maps that can precisely identify important structures in the amorphous configuration maps. We obtain an order parameter from the heatmap and conduct finite scale analysis of this parameter. Our findings demonstrate that the order parameter effectively captures the amorphous phase transition process across various systems. These results hold significant scientific implications for the study of amorphous structural characteristics via deep learning.

Bulk moduli of two-dimensional Yukawa solids and liquids obtained from periodic compressions

Langevin dynamical simulations are performed to determine the bulk modulus in two-dimensional (2D) dusty plasmas from uniform periodic radial compressions. The bulk modulus is calculated directly from its physical definition of the ratio of the internal pressure/stress to the volume strain. Under various conditions, the bulk moduli obtained agree with the previous theoretical derivations from completely different approaches. It is found that the bulk moduli of 2D Yukawa solids and liquids are almost independent of the system temperature and the external compressional frequency.

Direct Measurement of Adhesions of Liquids on Graphite

Two-dimensional (2D) materials have a great potential to both enhance existing technologies and create a range of new applications. Precise knowledge of interfacial energy in contacts between 2D materials and liquids is a key step toward understanding their interfacial properties and functioning. Here, we propose a direct method to measure the graphite–liquid interfacial energies. The method is based on the use of superlubricity phenomenon that is a state of ultralow friction and wear, which has been achieved for a number of layered materials. This new methodology may be extended to other 2D materials and liquids and is expected to have an impact on both the fundamental understanding of the solid–liquid interactions and the design and fabrication of devices including 2D materials.

Micro-structure and motion of two-dimensional dense short spherocylinder liquids

We numerically investigate the micro-structure and motion of 2D liquids composed of dense short spherocylinders, by reducing the shape aspect ratio from 3. It is found that reducing shape aspect ratio from 3 causes a smooth transition from heterogeneous structures composed of crystalline ordered domains with good tetratic alignment order to those with good hexagonal bond-orientational order at an aspect ratio equaling 1.35. In the intermediate regime, both structural orders are strongly deteriorated, and the translational hopping rate reaches a maximum due to the poor particle interlocking of the disordered structure. Shortening rod length allows easier rotation, induces monotonic increase of rotational hopping rates, and resumes the separation of rotational and translational hopping time scales at the small aspect ratio end, after the crossover of their rates in the intermediate regime. At the large shape aspect ratio end, the poor local tetratic order has the same positive effects on facilitating local rotational and translational hopping. In contrast, at the small shape aspect ratio end, the poor local bond orientational order has the opposite effects on facilitating local rotational and translational hopping.

Viscosity of two-dimensional strongly coupled dusty plasma modified by a perpendicular magnetic field.

Transport properties of two-dimensional (2D) strongly coupled dusty plasmas have been investigated in detail, but never for viscosity with a strong perpendicular magnetic field; here, we examine this scenario using Langevin dynamics simulations of 2D liquids with a binary Yukawa interparticle interaction. The shear viscosity η of 2D liquid dusty plasma is estimated from the simulation data using the Green-Kubo relation, which is the integration of the shear stress autocorrelation function. It is found that, when a perpendicular magnetic field is applied, the shear viscosity of 2D liquid dusty plasma is modified substantially. When the magnetic field is increased, its viscosity increases at low temperatures, while at high temperatures its viscosity diminishes. It is determined that these different variational trends of η arise from the different behaviors of the kinetic and potential parts of the shear stress under external magnetic fields.

Standard States for Adsorption on Solid Surfaces: 2D Gases, Surface Liquids, and Langmuir Adsorbates

Standard states are utilized to compare thermodynamic data obtained from different experiments and calculations, and this ability to compare thermodynamic data plays an important role in science and society. For molecules adsorbed on surfaces, there are currently no universally accepted standard states. Here, standard states are proposed for the different types of molecular adsorbate phases, with the intent to enable physical insight to be gained by tabulating experimental/calculated values, such that comparison between different systems and existing societal tabulations of chemical standard state tabulated values can be done directly. A “density based” standard state is proposed for 2D gases, and a “relative coverage based” standard state is proposed for immobile adsorbates and nonislanding 2D liquids. These units are chosen based upon the units which the activity depends on. The standard states recommended here are chosen due to the entropies associated with them, such that physical insight can be gaine...

Microphase separation as the cause of structural complexity in 2D liquids

Under certain thermodynamic conditions, a two-dimensional liquid becomes a statistically stable mosaic of small differently-ordered clusters. We apply to this mosaic a special coarsening procedure that accounts for short-time average and topologic features of a particle near environments. We then show that the coarsened mosaic consists of two different components separated at the length-scale of few inter-particle distances. Using bond order parameters and bond lengths as instant local characteristics, we show that these components have internal properties of spatially heterogeneous crystalline or amorphous phases, so the coarsened mosaic can be seen as a microphase-separated state. We discuss general conditions favouring stability of the mosaic state, and suggest some systems for searching for this special state of matter.

The Roton Minimum: Is it a General Feature of Strongly Correlated Liquids?

AbstractThe roton minimum is a deep minimum in the collective excitation spectrum of the liquid, forming around fairly high k ‐values. We have discovered, through MD simulations, that this appears to be a general feature of strongly coupled liquids and is ubiquitous in 2D and 3D Yukawa liquids. We suggest that the physical origin of the roton minimum has to be sought in the quasi‐localization of particles in a strongly correlated liquid and in the ensuing formation of local microcrystals whose averaged frequency dispersion would show roton minimum‐like feature (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Viscoelasticity of 2D Liquids Quantified in a Dusty Plasma Experiment

The viscoelasticity of two-dimensional liquids is quantified in an experiment using a dusty plasma. An experimental method is demonstrated for measuring the wave-number-dependent viscosity η(k), which is a quantitative indicator of viscoelasticity. Using an expression generalized here to include friction, η(k) is computed from the transverse current autocorrelation function, which is found by tracking random particle motion. The transverse current autocorrelation function exhibits an oscillation that is a signature of elastic contributions to viscoelasticity. Simulations of a Yukawa liquid are consistent with the experiment.

Time-correlation functions and transport coefficients of two-dimensional Yukawa liquids

The existence of coefficients for diffusion, viscosity, and thermal conductivity is examined for two-dimensional (2D) liquids. Equilibrium molecular dynamics simulations are performed using a Yukawa potential and the long-time behavior of autocorrelation functions is tested. Advances reported here as compared to previous 2D Yukawa liquid simulations include an assessment of the thermal conductivity, using a larger system size to allow meaningful examination of longer times, and development of improved analysis methods. We find that the transport coefficient exists for diffusion at high temperature and viscosity at low temperature, but not in the opposite limits. The thermal conductivity coefficient does not appear to exist at high temperature. Further advances in computing power could improve these assessments by allowing even larger system sizes and longer time series.

Superdiffusion in two-dimensional Yukawa liquids

Superdiffusion of two-dimensional (2D) liquids was studied using an equilibrium molecular dynamics simulation. At intermediate temperatures, the mean-squared displacement, probability distribution function (PDF), and velocity autocorrelation function (VACF) all indicate superdiffusion; the VACF has a long-time tail; and the PDF indicates no Lévy flights. These effects are predicted to occur in 2D dusty plasmas and other 2D liquids that can be modeled with a long-range repulsive potential.

Phonons in Yukawa lattices and liquids

The understanding of the theoretical structure of phonon dispersion in Yukawa lattices and the relationship between these perfect lattice phonons on the one hand, and the excitations in the disordered and liquid states on the other, is an important issue in analysing experimental and simulation results on plasma crystals. As the first step in this programme, we have numerically calculated the full phonon spectrum for 2D triangular Yukawa lattices, for a wide range of κ (screening parameter) values and along different propagation angles. Earlier calculations of the excitation spectra of the 2D and 3D Yukawa liquids were based on the quasilocalized charge approximation (QLCA), whose implicit premise is that the spectrum of an average distribution (governed by the isotropic liquid pair correlation function) is a good representation of the actual spectrum. To see the implications of this model more clearly, we compare the high r (near crystallization) QLCA phonon spectra with the angle-averaged phonon spectra of the lattice phonons.

Alignment and rheo-oscillator criteria for sheared nematic polymer films in the monolayer limit

Monolayer films of liquid crystalline polymers (LCPs) are modelled with a mesoscopic two-dimensional (2D) analogue of the Doi-Hess (1981, 1976) rigid rod model. One aim is to establish a more complete solution to the classical problem of how orientational degeneracy of quiescent nematic equilibria breaks in weak shear. We exploit the simplicity of 2D liquids to extend results of Kuzuu and Doi (1983,1984), Marrucci and Maffetone (1989-1991), See, Doi and Larson (1990), Forest <em> et al.</em> (2003-2004). We recall the distinction between two versus three dimensional quiescent phase diagrams and the isotropic-nematic phase transition, then analyze the deformation of these respective bifurcation diagrams in shear flow. We give a simple proof that limit cycles, known as tumbling orbits, must arise beyond the parameter boundary for the steady-unsteady transition. Finally, we show the shear-perturbed 2D phase diagram is significantly more robust to closure approximations than the 3D system.

Fluctuating topological defects in 2D liquids: heterogeneous motion and noise.

We measure the defect density as a function of time at different temperatures in simulations of a two-dimensional system of interacting particles. Just above the solid to liquid transition temperature, the power spectrum of the defect fluctuations shows a 1/f signature, which crosses over to a white noise signature at higher temperatures. When 1/f noise is present, the 5-7 defects predominantly form stringlike structures, and the particle trajectories show a 1D correlated motion that follows the defect strings. At higher temperatures this heterogeneous motion is lost.

Local structure, fluctuations, and freezing in two dimensions

Starting from the concept of local solidlike order in two-dimensional (2D) liquids we introduce and quantify the corresponding ensembles of fluctuations, using a probabilistic-based method of local-structure analysis (LSA). A systematic LSA (including size dependence) was performed for a hard disk and 2D Lennard-Jones systems, simulated using Monte Carlo and molecular-dynamics methods. We find that the onset of freezing is accompanied by a dramatic crossover between ensembles of fluctuations. Some universal features related to the onset of freezing in two dimensions are found and corresponding freezing criteria are formulated: (i) the liquid starts to freeze when the concentration of solidlike atoms constitutes 0.50\char21{}0.56, and (ii) a Lindemann-like freezing criterion: the rms fluctuation constitutes, at the onset, 0.12\char21{}0.13. Those criteria offer an effective method for a localization of the onset of freezing in computer simulations. We point out, in this context, that in computer simulations there is a possibility that all quantitative characterizations of the onset of freezing are related to a metastable range. This important methodological topic is discussed briefly in the light of recent results both for 2D and 3D systems.

Phase diagram of a two-dimensional liquid inGaAs/AlxGa1−xAsbiased double quantum wells

Photoluminescence (PL) and PL excitation (PLE) measurements have been performed in ${\mathrm{G}\mathrm{a}\mathrm{A}\mathrm{s}/\mathrm{A}\mathrm{l}}_{x}{\mathrm{Ga}}_{1\ensuremath{-}x}\mathrm{As}$ biased double quantum well heterostructures. The recombination of electrons, e, with holes, h, located in the same or in two adjacent wells, has been investigated for different exciting power densities, P, and temperatures, T. For increasing P or decreasing T, a sharp transition from two gases of photoexcited electrons and holes, spatially separated and confined in the two wells, to two two-dimensional (2D) liquids has been observed. The gas-to-2D-liquid transition is evidenced by a strong screening of applied biases and by major changes in the optical spectra. The phase diagram in the $(P,T)$ plane of the $e\ensuremath{-}h$ system has been determined. Time-resolved PL, cw PL, and PLE in the presence of a magnetic field normal to the quantum wells support the presence of e- and h-liquid phases in the two wells with a critical density equal to $8.8\ifmmode\times\else\texttimes\fi{}{10}^{10} {\mathrm{cm}}^{\ensuremath{-}2}$ and a binding energy of 2.5 meV.

LOCALIZATION-FERMI LIQUID TRANSITION OF 3HE ADSORBED ON 4HE-COATED HECTORITE

To create a phase diagram of two-dimensional (2D)3He fluids, we measured heat capacity of the3He adatoms down to 20mK as a function of the density. The3He adatoms are deposited on hectorite precoated with4He in the amount of 24.70μmol/m2, just above the onset density for the4He super-fluid. Isotherms of the heat capacity steeply increase with the3He density up to nc=2.4μmol/m2. Above nc, the heat capacity shows properties of the 2D Fermi liquids. We pointed out that the density dependence of the heat capacity is similar to those of the insulator-metal (I-M) transitions of some electron systems, e.g. Si:P. Assuming the transition from a localized state to the Fermi liquid at the density nc, the diameter of a localization area aH* was obtained as about 3.8A using the Mott relation that the mean distance equals 2.2 aH*at the transition. Effective mass and 2D spin fluctuations of the Fermi liquids are enhanced with the density by increasing correlations in the 2D liquids.

Interplay between fermion condensation and density-wave instability

It is shown that the phase transition of density-wave origin in homogeneous liquids is preceded by fermion condensation. Thus fermion condensation may be observed in low-density electron liquids, neutron stars, and liquid He3. Three-dimensional (3D) and two-dimensional (2D) liquids are considered.