Non-equilibrium Technique Research Articles

Enhanced CO2 hydrogenation to light hydrocarbons on Ni-based catalyst by DBD plasma

The high thermal stability of CO2 makes its conversion low in conventional thermal catalysis. Comparatively, non-equilibrium plasma technique provides an efficient way for CO2 hydrogenation under mild reaction conditions. Introducing effective catalysts to the dielectric barrier discharge (DBD) plasma reactor may further improve CO2 hydrogenation performance. In this way, the interaction between plasma and catalysts is important to contribute CO2 hydrogenation performance. Considering Ni-based catalyst is active for CO2 hydrogenation, we used three different kinds of carrier materials including CeO2, γ-Al2O3 and ZSM-5 as supports to prepare nickel-based catalysts by incipient impregnation in this paper. The supported Ni catalysts were tested for the plasma-induced CO2 hydrogenation to CH4 and C2+ hydrocarbons in the DBD-plasma reactor. It was found that the 15Ni/CeO2 catalyst achieved the best CO2 conversion of 85.7% and nearly 100% CH4 selectivity at 300 °C due to its high reducibility and CO2 chemisorption capacity. In addition, a significant synergistic effect was found between DBD plasma and catalyst. The performance of plasma-catalysis was considerably better than that of thermal catalysis plus pure plasma reaction, and the synergistic effects were enhanced with increase of reaction temperature. Moreover, the mechanism of the synergistic effect was proposed by analyzing the optical emission spectroscopy (OES) of DBD plasma, which provides a theoretical basis for further optimizing and application of plasma-catalysis for CO2 hydrogenation reaction.

Effect of space UV irradiation on the mechanical and tribological properties of Cr and B doped hydrogen-containing diamond-like films

To promote the wide application of hydrogen-containing diamond-like films (a-C:H) as solid lubricant materials in aerospace environment, a series of a-C:H, a-C:H doped with Cr elements (Cr/a-C:H) and a-C:H doped with B elements (B/a-C:H) were prepared on the GCr15 steel by non-equilibrium magnetron sputtering technique. The effects of ultraviolet (UV) irradiation on the mechanical and tribological properties of the three DLC films were investigated. UV irradiation was found to cause a decline in hardness and modulus of elasticity, and increased residual stress and surface roughness in all three films. UV irradiation has a minor impact to the coefficient of friction of the three films, but leads to a decrease in wear life. The wear life of Cr/a-C:H films was least affected by UV irradiation. UV irradiation can lead to the breakage of sp 3C-H bonds in a-C:H film and B/a-C:H film, resulting in the increase of dangling bonds in the films and the decrease of wear life. In the Cr/a-C:H film, the broken sp 3 C-H bond is partially reorganized into sp 2 C-H bond, which further exerts the passivation effect of C-H bond, resulting in a smaller decrease in the wear life of the Cr/a-C:H film.

Inducing concentration gradients across the membrane in molecular dynamics simulations to investigate membrane permeability.

Membranes compartmentalize cells and their intracellular components, endowing cells with control over the local concentrations of metabolites. As a matter of fact, most ions and substrates maintain an electrochemical gradient across the membranes. Replicating such gradients in molecular dynamic (MD) simulations, which very often use periodic boundary conditions (PBC), is challenging. Here we introduce a novel, non-equilibrium technique for MD simulations which enables one to generate a sustained concentration gradient across the membrane and directly calculate permeability using the relation derived from Fick's first law, J=P(Δ;C). We generate concentration gradients by unidirectionally biasing targeted permeants near the periodic boundaries forcing them into the next periodic image without directly influencing permeants in the vicinity of the membrane. We used O2 to evaluate the method with a thorough statistical treatment through block averaging. We validated our technique on an exemplary POPC membrane, with calculated O2 permeabilities in agreement with published experimental and computational values. Our method yielded a permeability of P = 13.32 ± 2.22 cm/s for a system with a solution-phase concentration of [O2] = 19.2 mM using the CSVR thermostat over the course of a microsecond simulation. We found that the permeant concentration significantly alters the calculated permeability (P = 6.35 ± 4.86 cm/s for [O2] = 174 µM versus P = 23.8 ± 2.71 cm/s for [O2] = 153 mM). The thermostat choice also affected the permeability with the Langevin thermostat reducing permeability (P = 11.38 ± 2.78 cm/s; [O2] = 20.0 mM) as it fails to conserve momentum. By using its fundamental definition, we can robustly determine bulk permeability of membranes using our concentration gradient method, even those containing proteins which we intend to study in future work.

Photogalvanic effects in Janus monolayer In2SSe with vacancy defects

Linear photogalvanic effects (PGEs) in Janus monolayer In 2 SSe with five different central regions, which are pure, S-vacancy, In-vacancy, SSe-vacancy and InIn-vacancy are investigated by non-equilibrium Green's function technique combined with density functional theory. Janus monolayer In 2 SSe is a kind of new two-dimensional (2D) indirect bandgap semiconductor that generates photoresponse in the absence of external bias voltage, showing potential applications in low power consumption photoelectronic devices. It is found that the photoresponse is related to the polarization angle in the form of cosine function. All the introduction of defects has enhanced the photocurrent, but the degree is different. The concentration and the position of vacancy defects also affect the photoresponse. Compared with other vacancy types, single In vacancy has the best performance because it has the highest photocurrent (29.80) and the largest ER (183.33) simultaneously. Our results indicate that appropriate vacancy defects can significantly enhance the photoresponse and ER. Janus monolayer In 2 SSe is promising in photoelectric detection. • The photoresponse is related to the polarization angle in the form of cosine function. • Indium vacancy defects can significantly enhance the photoresponse and ER. • The concentration of vacancy defects also affects the photoresponse.

Nanointerconnect design based on edge fluorinated/hydrogenated zigzag borophene nanoribbons: an ab initio analysis.

Using an ab initio framework and non-equilibrium Green's function technique, the effect of hydrogen and fluorine atom passivation on the electronic and transport properties of borophene nanoribbons (BNRs) are explored. For zigzag edge states, we have explored all potentially stable combinations of hydrogen and fluorine passivation. Fluorine passivation leads to thermodynamically stable structures with improved stability for the increased concentration of F atoms, according to our binding energy (Eb) calculations. Furthermore, density-of-states and dispersion relation (E-k structures) computations indicate that fluorine-passivated BNRs are primarily metallic in nature. We proposed these nanostructures for their use in metal interconnects because of their increased metallicity. We have used the typical two-probe setup to calculate the critical parameters like quantum resistance (RQ), kinetic inductance (LK), and quantum capacitance (CQ) to evaluate their performance as metal interconnects. Because they have the lowest estimated values of LK = 26.1 nH μm-1, and CQ = 399 pF cm-1, the zigzag BNRs (ZBNRs) with two edge fluorinated (F-BNR-F) nanostructures may be considered as a promising candidate for nanoscale interconnect applications.

Deep Learning-Enhanced Potentiometric Aptasensing with Magneto-Controlled Sensors.

Bioelectronic sensors that report charge changes of a biomolecule upon target binding enable direct and sensitive analyte detection but remain a major challenge for potentiometric measurement, mainly due to Debye Length limitations and the need for molecular-level platforms. Here, we report on a magneto-controlled potentiometric method to directly and sensitively measure the target-binding induced charge change of DNA aptamers assembled on magnetic beads using a polymeric membrane potentiometric ion sensor. The potentiometric responses of the negatively charged aptamer, serving as a receptor and reporter, were dynamically controlled and modulated by applying a magnetic field. Based on a potentiometric array, this non-equilibrium measurement technique combined with deep learning algorithms allows for rapidly and reliably classifying and quantifying diverse small molecules using antibiotics as models. This potentiometric strategy opens new modalities for sensing applications.

Overcoming the doping limit in semiconductors via illumination

It has been shown that illumination could have significant effects on the dopability in semiconductors. However, a general understanding on these effects is still lacking. In this paper, we present a self-consistent scheme to study the doping properties in semiconductors under illumination, and unravel a general picture: the excess carriers induced by illumination can substantially suppress the formation of compensating defects and enhance the carrier mobility as well as the density of the majority carrier, because the illumination leads to an asymmetric Fermi level shift. This is exemplified by a prototype Mg doped GaN system upon exposure to light and is consistent with available experimental observations. Our work provides a fundamental understanding of the physical process occurring in semiconductors upon illumination, and paves the way to overcome the doping bottleneck via nonequilibrium techniques in semiconductors.

Phononic heat conductance of gold atomic contacts: Coherent versus incoherent transport

We present here a theoretical method to determine the phononic contribution to the thermal conductance of nanoscale systems in the phase-coherent regime. Our approach makes use of classical molecular dynamics (MD) simulations to calculate the temperature-dependent dynamical matrix, and the phononic heat conductance is subsequently computed within the Landauer-B\"uttiker formalism with the help of nonequilibrium Green's function techniques. Tailored to nanostructures, crucial steps of force constant and heat transport calculations are performed directly in real space. As compared to conventional density functional theory (DFT) approaches, the advantage of our method is two-fold. First, interatomic interactions can be described with the method of choice. Semiempirical potentials may lead to large computational speedups, enabling the study of much larger systems. Second, the method naturally takes into account the temperature dependence of atomic force constants, an aspect that is ignored in typical static DFT-based calculations. We illustrate our method by analyzing the temperature dependence of the phononic thermal conductance of gold (Au) chains with lengths ranging from 1 to 12 atoms. Moreover, in order to evaluate the importance of anharmonic effects in these atomic-scale wires, we compare the phase-coherent approach with nonequilibrium MD (NEMD) simulations. We find that the predictions of the phase-coherent method and the classical NEMD approach largely agree above the Debye temperature for all studied chain lengths, which shows that heat transport is coherent and that our phase-coherent approach is well suited for such nanostructures.

Passive Film Structure and Corrosion Initiation in Al Alloys with Far-from-Equilibrium Compositions and Microstructures

Al alloys with far-from-equilibrium compositions and microstructures are reported to possess high corrosion resistance and strength. These alloys produced are by non-equilibrium processing techniques such as high-energy ball milling, sputtering, and ion implantation. Depending on the processing techniques and conditions, the microstructure of the alloys could be heterogeneous. For example, alloys produced by high-energy ball milling and subsequent consolidation comprise a matrix with high solid solubility of the solute and uniformly distributed secondary phases. Such a heterogeneous microstructure is expected to influence the passive film composition and localized corrosion mechanisms. In the present study, passive film composition on the ball-milled alloys was studied using a combination of scanning transmission electron microscope, X-ray photoelectron spectroscopy, and secondary ion mass spectrometry (SIMS). Additionally, pits and regions near pits were investigated which helped in understanding factors causing pit initiation and repassivation.

Thermal drag effect in quantum Hall circuits

We study the thermal drag between two mesoscopic quantum Hall (QH) circuits. Each circuit consists of Ohmic contact perfectly coupled to quantum Hall edge states. The drag is caused by strong capacitive coupling between Ohmic contacts. The non-equilibirum conditions and the electron-electron interaction are taken into account by using the non-equilibrium bosonization technique. The thermal drag current in the passive circuit, the noise power of the corresponding heat current, and the Fano factor are calculated and analyzed.

Microscopic investigation for the evaluation of atmospheric pressure non-equilibrium plasma technique used to remove contaminants from ancient Chinese murals

• Dust and soot are the common deterioration aspects of ancient Chinese murals. • Atmospheric pressure non-equilibrium plasma has the efficiency in removing dust and soot. • The used cleaning parameters of plasma lead to the safe removal of contaminants. • Plasma cleaning causes inevitably damages the black pigment on the mural surface. • The cleaning mechanism of contaminants by the plasma technique is explained. Ancient Chinese murals suffered from long-time contamination, which caused the content of the painting to be unrecognized and the surface quality degradation. However, it is difficult to remove these contaminants on the mural surfaces without any damage to the substrate. Herein, atmospheric pressure non-equilibrium plasma (APNP) cleaning, as a potential tool to conserve and restore cultural heritage, was proposed to eliminate the contaminants on the surface of ancient Chinese murals. The experiments demonstrated that APNP was more suitable for cleaning hydrocarbon contaminants such as soot than dust. The soot containing carbonaceous particles on the mural surface could be removed entirely by APNP and did not cause significant changes to physicochemical properties on the mural interface. We found that critical plasma cleaning time and working distance were crucial for eliminating contaminants and protecting the surface of murals in an actual situation. Meanwhile, reactive molecular dynamics (RMD) simulations were performed to simulate the complex reaction processes of removing soot contaminants by APNP. During RMD simulation, the possible reaction pathway and evolution behavior of chemical bonds of reactive oxygen species dissociating carbonaceous molecules were analyzed. `The increased mobility capacity of atomic oxygen in the system facilitated the cleavage of carbonaceous molecular structures. .

An investigation of the YidC-mediated membrane insertion of Pf3 coat protein using molecular dynamics simulations.

YidC is a membrane protein that facilitates the insertion of newly synthesized proteins into lipid membranes. Through YidC, proteins are inserted into the lipid bilayer via the SecYEG-dependent complex. Additionally, YidC functions as a chaperone in protein folding processes. Several studies have provided evidence of its independent insertion mechanism. However, the mechanistic details of the YidC SecY-independent protein insertion mechanism remain elusive at the molecular level. This study elucidates the insertion mechanism of YidC at an atomic level through a combination of equilibrium and non-equilibrium molecular dynamics (MD) simulations. Different docking models of YidC-Pf3 in the lipid bilayer were built in this study to better understand the insertion mechanism. To conduct a complete investigation of the conformational difference between the two docking models developed, we used classical molecular dynamics simulations supplemented with a non-equilibrium technique. Our findings indicate that the YidC transmembrane (TM) groove is essential for this high-affinity interaction and that the hydrophilic nature of the YidC groove plays an important role in protein transport across the cytoplasmic membrane bilayer to the periplasmic side. At different stages of the insertion process, conformational changes in YidC’s TM domain and membrane core have a mechanistic effect on the Pf3 coat protein. Furthermore, during the insertion phase, the hydration and dehydration of the YidC’s hydrophilic groove are critical. These results demonstrate that Pf3 coat protein interactions with the membrane and YidC vary in different conformational states during the insertion process. Finally, this extensive study directly confirms that YidC functions as an independent insertase.

Current cross correlations in a quantum Hall collider at filling factor two

We use the nonequilibrium bosonization technique to study the effects of Coulomb interactions in mesoscopic electron colliders based on quantum Hall edge states at filling factor $\ensuremath{\nu}=2$. The current cross correlations and Fano factor, which carry the information about the exclusion statistics, are calculated. It is shown that both these quantities have a nonanalytical dependence on the source transparency, which scales as $log(1/{T}_{s})$ at small ${T}_{s}\ensuremath{\ll}1$. This is a consequence of electron-electron interactions in the outgoing nonequilibrium states of the collider.

Phase Control of Multivalent Vanadium Oxides VOx by Ion‐Beam Sputter‐Deposition

Vanadium–oxygen materials are of interest for various applications and fields of solid‐state physics owing to the unequaled plethora of different phases. The wealth of phases and complexity of its phase diagram infer a strong sensitivity on the growth parameters for each phase. Thus, the reproducible growth of vanadium‐oxide thin‐films of defined phases by nonequilibrium techniques is challenging. Here, it is shown that ion‐beam sputter‐deposition (IBSD) is a powerful tool to reproducibly deposit defined polycrystalline vanadium oxide films by precisely controlling oxygen flux and substrate temperature in the growth process. Hence, it is demonstrated that IBSD has the potential to reliably produce binary phases (including unstable phases) from the vanadium–oxygen phase space. X‐ray diffraction (XRD) and Raman spectroscopy are used to establish a map of the different crystalline phases dependent on the growth parameters. In particular, it is proved that thin‐film V3O7 can be realized by IBSD and its Raman fingerprint is unambiguously identified.

Corrosion behavior of an in situ consolidated nanocrystalline Al-V alloy

Supersaturated solid solutions of Al and corrosion-resistant alloying elements (M: V, Mo, Cr, Ti, Nb), produced by non-equilibrium processing techniques, have been reported to exhibit high corrosion resistance and strength. The corrosion mechanism for such improved corrosion performance has not been well understood. We present a fundamental understanding of the role of V in corrosion of an Al-V alloy, which will provide a theoretical background for developing corrosion-resistant Al alloys. High-energy ball milling of the elemental powder of Al and V produced an in situ consolidated Al-V alloy, which exhibited high solid solubility of V. The corrosion resistance of Al-V alloy was significantly higher than that of pure Al, which was attributed to the (1) enrichment of V at the passive film/substrate interface, (2) incorporation of V into the passive film, and (3) deposition of V on the iron-containing cathodic particles and therefore, retardation of cathodic reaction.

Effect of interfacial intermixing on spin-orbit torque in Co/Pt bilayers

Using the first-principles non-equilibrium Green's function technique with supercell disorder averaging, we study the influence of interfacial intermixing on the spin-orbit torque in Co$\mid$Pt bilayers. Intermixing is modeled by inserting one or more monolayers of a disordered CoPt alloy between Co and Pt. Dampinglike torque is moderately enhanced by interfacial intermixing, while the fieldlike torque, which is small for abrupt interfaces, is strongly enhanced and becomes comparable to the dampinglike torque. The enhancement of the fieldlike torque is attributed to the interface between Co and the intermixed region. The planar Hall-like torque increases with intermixing but remains relatively small. The behavior of the torques is similar for bilayers with (111) and (001)-oriented interfaces. Strong dependence of the fieldlike torque on intermixing could provide a way to tune the fieldlike-to-dampinglike torque ratio by interface engineering.

Electron passivation in CaF2 on calcium metal anodes.

Current electrolytes in calcium-ion batteries suffer from a lack of stability and degradation caused by reduction from the anode. The solid-electrolyte interphase (SEI) that forms on the anodes during operation stems the flow of electrons from the anode to the electrolyte. CaF2 is a common inorganic compound found in the SEI, and is derived from electrolyte salts such as Ca(PF6)2. CaF2 can exist in crystalline, polycrystalline, and amorphous phases in the SEI, and as recent work has shown, different phases of the same compound can have vastly different electronic conductivities. Using the non-equilibrium Green's function technique with density functional theory (NEGF-DFT), we find that amorphous phase systems enhance electron tunneling in thin CaF2 films by 1-2 orders of magnitude when compared to crystalline and polycrystalline CaF2 systems. Transport through several amorphous structures was considered showing that, despite their random structures, their conductance properties are similar. Through analysis of the decay constant β and the low-bias conductance of each system, we show that crystalline and polycrystalline CaF2 offer greater protection of the electrolyte than amorphous CaF2.

Semiflexible oligomers crystallize via a cooperative phase transition.

Semicrystalline polymers are ubiquitous, yet despite their fundamental and industrial importance, the theory of homogeneous nucleation from a melt remains a subject of debate. A key component of the controversy is that polymer crystallization is a non-equilibrium process, making it difficult to distinguish between effects that are purely kinetic and those that arise from the underlying thermodynamics. Due to computational cost constraints, simulations of polymer crystallization typically employ non-equilibrium molecular dynamics techniques with large degrees of undercooling that further exacerbate the coupling between thermodynamics and kinetics. In a departure from this approach, in this study, we isolate the near-equilibrium nucleation behavior of a simple model of a melt of short, semiflexible oligomers. We employ several Monte Carlo methods and compute a phase diagram in the temperature-density plane along with two-dimensional free energy landscapes (FELs) that characterize the nucleation behavior. The phase diagram shows the existence of ordered nematic and crystalline phases in addition to the disordered melt phase. The minimum free energy path in the FEL for the melt-crystal transition shows a cooperative transition, where nematic order and monomer positional order move in tandem as the system crystallizes. This near-equilibrium phase transition mechanism broadly agrees with recent evidence that polymer stiffness plays an important role in crystallization but differs in the specifics of the mechanism from several recent theories. We conclude that the computation of multidimensional FELs for models that are larger and more fine-grained will be important for evaluating and refining theories of homogeneous nucleation for polymer crystallization.

Heat transport in a two-level system driven by a time-dependent temperature

The field of thermotronics aims to develop thermal circuits that operate with temperature biases and heat currents just as how electronic circuits are based on voltages and electric currents. Here, we investigate a thermal half-wave rectifier based on a quantum two-level system (a qubit) that is driven by a periodically modulated temperature difference across it. To this end, we present a non-equilibrium Green's function technique, which we extend to the time domain to account for the time-dependent temperature in one of two thermal reservoirs connected to the qubit. We find that the qubit acts a thermal diode in parallel with a thermal capacitor, whose capacitance is controlled by the coupling to the reservoirs. These findings are important for the efforts to design non-linear thermal components such as heat rectifiers and multipliers that operate with more than one diode.

Time-resolved photoemission spectroscopy on correlated electrons: Insights from dynamical mean-field theory

Time- and angular-resolved photoemission spectroscopy (trARPES) can directly probe the electronic structure of quantum materials out of equilibrium. This can shed light on the interaction of the electrons with spin, lattice, and orbital degrees of freedom, and help to unravel pathways towards novel out-of-equilibrium phases. Dynamical mean-field theory (DMFT) and its extensions provide a versatile toolbox to interpret such experiments through a theoretical simulation of the underlying microscopic processes. The approach can be applied both to Mott insulators and correlated metals, and it is formulated in terms of non-equilibrium Green's functions, which directly relate to the photoemission spectrum. This article reviews the theoretical description of trARPES within DMFT and related diagrammatic non-equilibrium Green's function techniques. Several applications are discussed, including the photo-induced melting of excitonic order, femtosecond relaxation processes in Mott insulators, and the manipulation of the electronic structure of Mott and charge transfer insulators using photo-doping and strong THz fields.