Asymmetric Transport Research Articles

NbS2/MoSi2P4 van der Waals Heterojunction: Flexibly tunable electrical contact properties and potential applications for Schottky junction devices

To obtain miniaturized and high-performance nanoelectronic devices, it is still crucially technical problem to realize ohmic contact at metal–semiconductor interface. Here, we construct H-, T-NbS2/MoSi2P4 van der Waals heterojunctions (vdWHs) and theoretically explore their mechanic and electronic behaviors, focusing on electrical contact features and tunability as well as Schottky junction device properties. The suitable Poisson’s ratio and sufficient rigidity suggest that such vdWHs are highly favorable to act as electrode or channel materials. The quasi-ohmic contact for H-NbS2/MSP occurs in the intrinsic equilibrium state (ES), and its entire ohmic contact emerges only by a very small interlayer spacing compression. For both H- and T-NbS2/MSP, entire ohmic contacts can be realized by applied appropriate electric field. Based on these studies, we design 5 nm gate-controlled Schottky junction devices. The calculated energy-resolved currents show that the thermal emission transmission plays an important role in rectification, making the rectification ratio exceeding 105, enhanced further by positive gate voltage applied. These behaviors can be identified by the calculated PLDOS, which shows that the highly asymmetric device potential barrier with respect to bias polarity and tuned flexibly by gate voltage results in a highly asymmetric electron transport under different directional biases. Our study provide a new possibility for designing high-performance Schottky junction devices.

Coupled electron transfer reactions in closed bipolar cells: The impact of asymmetric mass transport

Coupled electron transfer reactions in closed bipolar cells: The impact of asymmetric mass transport

Horizontal transport in the bouncing ball system with a sawtooth-shaped table

The system that consists of a ball bouncing off a sawtooth-shaped table vibrating vertically is considered. The horizontal motion in this system is caused by the table shape, and we plotted the mean horizontal velocity as a function of the asymmetry of the table shape. The ball is transported in the direction which the gentler slopes face for the high-bounce parameter region. To give a description to the net asymmetric transport, we derive a simplified model using assumptions of high bounce and probabilistic collision to the left or right slopes. The simplified model exhibits a horizontal transport qualitatively similar to that observed in the original model.

A full-wave time-dependent Schrödinger equation approach for the modeling of asymmetric transport in geometric diodes

Design and modeling of geometric diodes can be really challenging as it requires a rigorous quantum analysis at the nanoscale, accurate enough to capture the asymmetric behavior of charge transport. In this paper, we present a full-wave Time-Dependent Schrödinger Equation (TDSE) approach to model coherent transport in a two-dimensional geometric diode defined by an asymmetric taper of the domain where the charges are confined. The proposed solution clearly shows asymmetric transport, and provides a description of the rectification behavior in response to a parametric change. In general, the above model can account for even more complex geometries in order to optimize diodes performance.

Asymmetric transport computations in Dirac models of topological insulators

This paper presents a fast algorithm for computing transport properties of two-dimensional Dirac operators with linear domain walls, which model the macroscopic behavior of the robust and asymmetric transport observed at an interface separating two two-dimensional topological insulators. Our method is based on reformulating the partial differential equation as a corresponding volume integral equation, which we solve via a spectral discretization scheme.We demonstrate the accuracy of our method by confirming the quantization of an appropriate interface conductivity modeling transport asymmetry along the interface, and moreover, confirm that this quantity is immune to local perturbations. We also compute the far-field scattering matrix generated by such perturbations and verify that while asymmetric transport is topologically protected the absence of back-scattering is not.

Heterostructured MXene/Si Photodiodes With Sub-1-nm h-BN Blocking Layers for Suppressing Dark Current

Integrating two-dimensional materials with three-dimensional photosensitive semiconductors to form Schottky junctions for high-performance photodetection has attracted much research attention due to their excellent optical and electronic properties. However, the large dark current induced by the unexpected interface defects has greatly degraded their optoelectronic performance. Here, we reported a Schottky photodiode based on MXene/h-BN/Si van der Waals heterojunction, where the sub-1-nm h-BN layer significantly suppressed the dark currents by two orders of magnitude and raised the detectivity up to 1013 Jones. Ultrathin h-BN blocking layer free of dangling bonds effectively improved the interface quality due to the decreased interface trap states, resulting in a smaller ideal factor and larger Schottky barrier height. Moreover, asymmetric carrier transport was achieved due to the hindrance of ultrathin h-BN layer on the electron tunneling from n-Si to MXene, weakening the interlayer coupling of photo-generated electron-hole pairs and enhancing the optoelectronic performance. This work demonstrated a feasible strategy to optimize MXene-based photodiodes with sub-1-nm h-BN blocking layer for high-performance photodetection.

Topological charge conservation for continuous insulators

This paper proposes a classification of elliptic (pseudo-)differential Hamiltonians describing topological insulators and superconductors in Euclidean space by means of domain walls. Augmenting a given Hamiltonian by one or several domain walls results in confinement that naturally yields a Fredholm operator, whose index is taken as the topological charge of the system. The index is computed explicitly in terms of the symbol of the Hamiltonian by a Fedosov–Hörmander formula, which implements in Euclidean spaces an Atiyah–Singer index theorem. For Hamiltonians admitting an appropriate decomposition in a Clifford algebra, the index is given by the easily computable topological degree of a naturally associated map. A practically important property of topological insulators is the asymmetric transport observed along one-dimensional lines generated by the domain walls. This asymmetry is captured by the edge conductivity, a physical observable of the system. We prove that the edge conductivity is quantized and given by the index of a second Fredholm operator of the Toeplitz type. We also prove topological charge conservation by stating that the two aforementioned indices agree. This result generalizes to higher dimensions and higher-order topological insulators, the bulk-edge correspondence of two-dimensional materials. We apply this procedure to evaluate the topological charge of several classical examples of (standard and higher-order) topological insulators and superconductors in one, two, and three spatial dimensions.

Dynamic GOLVEN-ROOT GROWTH FACTOR 1 INSENSITIVE signaling in the root cap mediates root gravitropism.

Throughout the exploration of the soil, roots interact with their environment and adapt to different conditions. Directional root growth is guided by asymmetric molecular patterns but how these become established or are dynamically regulated is poorly understood. Asymmetric gradients of the phytohormone auxin are established during root gravitropism, mainly through directional transport mediated by polarized auxin transporters. Upon gravistimulation, PIN-FORMED2 (PIN2) is differentially distributed and accumulates at the lower root side to facilitate asymmetric auxin transport up to the elongation zone where it inhibits cell elongation. GOLVEN (GLV) peptides function in gravitropism by affecting PIN2 abundance in epidermal cells. In addition, GLV signaling through ROOT GROWTH FACTOR 1 INSENSITIVE (RGI) receptors regulates root apical meristem maintenance. Here, we show that GLV-RGI signaling in these 2 processes in Arabidopsis (Arabidopsis thaliana) can be mapped to different cells in the root tip and that, in the case of gravitropism, it operates mainly in the lateral root cap (LRC) to maintain PIN2 levels at the plasma membrane (PM). Furthermore, we found that GLV signaling upregulates the phosphorylation level of PIN2 in an RGI-dependent manner. In addition, we demonstrated that the RGI5 receptor is asymmetrically distributed in the LRC and accumulates in the lower side of the LRC after gravistimulation. Asymmetric GLV-RGI signaling in the root cap likely accounts for differential PIN2 abundance at the PM to temporarily support auxin transport up to the elongation zone, thereby representing an additional level of control on the asymmetrical auxin flux to mediate differential growth of the root.

Do Amino Acid Antiporters Have Asymmetric Substrate Specificity?

Amino acid antiporters mediate the 1:1 exchange of groups of amino acids. Whether substrate specificity can be different for the inward and outward facing conformation has not been investigated systematically, although examples of asymmetric transport have been reported. Here we used LC-MS to detect the movement of 12C- and 13C-labelled amino acid mixtures across the plasma membrane of Xenopus laevis oocytes expressing a variety of amino acid antiporters. Differences of substrate specificity between transporter paralogs were readily observed using this method. Our results suggest that antiporters are largely symmetric, equalizing the pools of their substrate amino acids. Exceptions are the antiporters y+LAT1 and y+LAT2 where neutral amino acids are co-transported with Na+ ions, favouring their import. For the antiporters ASCT1 and ASCT2 glycine acted as a selective influx substrate, while proline was a selective influx substrate of ASCT1. These data show that antiporters can display non-canonical modes of transport.

Regulating Nonlinear Ion Transport through a Solid-State Pore by Partial Surface Coatings

Using functional nanofluidic devices to manipulate ion transport allows us to explore the nanoscale development of blue energy harvesters and iontronic building blocks. Herein, we report on a method to alter the nonlinear ionic current through a pore by partial dielectric coatings. A variety of dielectric materials are examined on both the inner and outer surfaces of the channel with four different patterns of coated or uncoated surfaces. Through controlling the specific part of the surface charge, the pore can behave like a resistor, diode, and bipolar junction transistor. We use numerical simulations to find out the reason for the asymmetric ion transport in the pore and illustrate the relationship between specifically charged surfaces and electroosmotic flow. These findings help understand the role of the corresponding surface composition in ion transport, which provides a direct approach to modify the electroosmotic-flow-driven ionic current rectification in the channel-based device via dielectric coatings.

Formation of local heterogeneity under energy collection in neural networks

Static distribution of intracellular and extracellular ions can induce the spatial distribution of electric field in the cell, while stochastic diffusion and propagation of ions can generate channel current accompanied by the magnetic field. In fact, the field energy (including magnetic and electric fields) in each cell or neuron can be changed under external stimuli or the deformation of shape. Furthermore, energy pumping occurs, and the synaptic connection is created adaptively to keep energy balance when more neurons are clustered in the same region because of electromagnetic field superposition. For a synchronous and homogeneous network, all neurons keep energy balance with the adjacent neurons, and the network becomes uniform and identical. For cardiac tissue and biological media, local heterogeneity such as sinoatrial node can emit continuous wave front, and target waves are propagated to regulate the neural activities and heartbeat rhythm. In this paper, the Hamilton energy in a simple neuron model is calculated according to the Helmholtz theorem, and more neurons are clustered to build a regular network (chain network and square array). The creation and growth of synapse connection are controlled to pump energy, and local heterogeneity is formed by taming one intrinsic parameter when more energy is accumulated in a few neurons in the network. These results indicate that asymmetrical energy transport will induce the occurrence of heterogeneity in the network, and adaptive synaptic regulation on neurons is activated to keep local energy balance before reaching perfect synchronization. When external stimuli with diversity in intensity are applied, neurons are excited with energy diversity, and possible shape deformation is induced to keep local energy balance. As a result, heterogeneity is developed in a local area.

Asymmetrical Transport of Thyroxine Across Human Term Placenta.

Background: Fetal development is crucially dependent on thyroid hormone (TH). Maternal-to-fetal transfer of TH is a prerequisite for fetal TH availability, particularly in the first half of pregnancy. The mechanisms of transplacental transport of TH, however, are yet poorly understood. We, therefore, investigated the TH transport processes across human placentas using an ex vivo perfusion system. Methods: Intact cotyledons from term placentas of uncomplicated pregnancies were cannulated within 30 minutes after delivery and the maternal and fetal circulations were re-established. One hundred nanomolar thyroxine (T4) was added to either the maternal or fetal circulation and perfusions run up to three hours during which samples were taken from both circulations at different time points. Variables included addition of iopanoic acid (IOP) to block activity of the deiodinase type 3 (D3) and bovine serum albumin (BSA) to trap released T4. T4 and 3,3',5'-triiodothyronine concentrations in the perfusates were measured by radioimmunoassays. Results: Maternal-to-fetal transfer was slow, with T4 barely detectable in the fetal circulation unless D3 was blocked by IOP. Fetal T4 was detected after three hours perfusion (10.6 ± 0.6 nM) when BSA (34 g/L) was added in the fetal circulation to trap the released T4. In contrast, fetal-to-maternal transfer of T4 was rapid and maternal T4 increased to 43.6 ± 5.5 nM. Conclusions: Maternal-to-fetal T4 transport is limited, whereas fetal-to-maternal transport is rapid indicating that T4 transport across human term placenta is an asymmetrical process. With the high D3 activity, our observations are compatible with a protective role of the placental barrier. Future studies should reveal how the placenta exerts its gatekeeper function in ensuring optimal TH passage to the fetus.

Giant thermal rectification efficiency by geometrically enhanced asymmetric non-linear radiation.

Thermal rectification is an asymmetric heat transport phenomenon where thermal conductance changes depending on the temperature gradient direction. The experimentally reported efficiency of thermal rectification materials and devices, which are applicable for a wide range of temperatures, is relatively low. Here we report a giant thermal rectification efficiency of 218% by maximizing asymmetry in parameters of the Stefan-Boltzmann law for highly non-linear thermal radiation. The asymmetry in emissivity is realized by sputter-depositing manganese (ε = ∼0.38) on the top right half surface of a polyurethane specimen (ε = ∼0.98). The surface area of the polyurethane side is also dramatically increased (1302%) by 3D printing to realize asymmetry in geometry. There is an excellent agreement between the experimentally measured temperature profiles and finite element simulation results, demonstrating the reliability of the analysis. Machine learning analysis reveals that the surface area is a dominant factor for thermal rectification and suggests novel light-weight designs with high efficiencies. This work may find applications in energy efficient thermal rectification management of electronic devices and housings.

Logic gating of low-abundance molecules using polyelectrolyte-based diodes.

Bioinspired artificial ionic components are extensively utilized to mimic biological systems, as the vast majority of biological signaling is mediated by ions and molecules. Particular attention is given to nanoscale fluidic components where the ion transport can be regulated by the induced ion permselectivity. As a step from fundamentals toward ion-controlled devices, this study presents the use of ionic diodes made of oppositely charged polyelectrolytes, as a gate for low-abundance molecules. The use of ionic diodes that exhibited nonlinear current-voltage responses enabled realization of a basic Boolean operation of an ionic OR logic gate. Aside from the electrical response, the asymmetric ion transport through the diode was shown to affect the transport of low-abundance molecules across the diode, only allowing crossing when the diode was forward-biased. Integration of multiple diodes enabled implementation of an OR logic operation on both the voltage and the molecule transport, while obtaining electrical and optical output readouts that were associated with low and high logic levels. Similarly to electronics, implementation of logic gates opens up new functionalities of on-chip ionic computation via integrated circuits consisting of multiple basic logic gates.

Air-stable ambipolar charge transport behaviors of organic-inorganic hybrid bilayer and application to Au nanoparticle-based floating gate memory

Ambipolar-based devices are considered effective alternatives in various applications, including reconfigurable logic, light-emitting transistors, and neuromorphic devices. On the other hand, the previously reported intrinsic single-layer ambipolar materials have limitations, such as high instability in ambient air and asymmetric transport of electrons and holes. In the present study, high air-stable and symmetrical ambipolar behavior were implemented using an organic-inorganic bilayer. An ambipolar-based memory application was implemented using these characteristics. This fabricated memory device used electrons and holes together with dinaphtho[2,3-b:2′,3′-f]thieno[3,2-b]thiophene organic p-type and zinc tin oxide inorganic n-type semiconductor heterojunction layers, resulting in wide memory window and high retention. In addition, these behaviors were analyzed at the band diagram level through ultraviolet photoelectron spectroscopy and ultraviolet-visible light spectroscopy, and chemical bonding was analyzed using x-ray photoelectron spectroscopy. These reports can provide useful strategies for applying various ambipolar applications in the future.

Asymmetric Thermal and Water Vapor Transport of Polyester Spacer and Cotton Nonwoven Fabric Assembly

ABSTRACT Spacer fabric is often used in cushions, footwear, filter material, and other products because of its high air permeability and three-dimensional (3D) structure. However, for practical applications, it is typically combined with other materials. We investigated the thermal properties and water vapor permeability of assembly-combined thicker spacer (16 mm) and various cotton nonwoven fabrics. The assembly-combined lightest nonwoven (30 g/m2) and spacer fabric exhibited the highest thermal resistance, increasing by 66.62% compared with the bare spacer fabric; however, no significant difference was observed when combined with the heaviest woven fabric (60 g/m2). Furthermore, the fabric arrangement during assembly could affect the heat and moisture-transfer efficiencies. The assembly-combined lightest nonwoven (30 g/m2) and spacer fabric under the upward test condition exhibited the highest thermal resistance, highest Clo, lowest heat transfer coefficient, highest insulation ratio, and lowest evaporation resistance among all assemblies. Higher thermal resistance and lower evaporative resistance could benefit physiological comfort. However, the assembly-combined heaviest nonwoven and spacer fabric under the downward test condition exhibited similar thermal resistance to the spacer fabric and the highest evaporation resistance. The asymmetric heat- and moisture-transfer properties of a porous assembly can contribute toward developing new materials for applications in other engineering fields.

Spin-valley polarized transport in a field-controllable bilayer silicene superlattice

We have theoretically investigated the spin-valley asymmetric transport of massive Dirac fermions in the field-controllable bilayer silicene superlattices. The spin-valley dependent ballistic transmission, conductance, and polarization have been systematically calculated by formulating the scattering matrix method for the completed four-band low-energy effective Hamiltonian. Our results uncover that for a single valley transport, near-perfect spin polarization and its perfect switching could be efficiently modulated by the gate field engineering. Under the one-dimensional periodic field modulation, two types of flat bands with less dispersion and, importantly, the perfect contrast in the spin-dependent subbands are observed for the bilayer silicene superlattice. Together with its larger spin-orbit coupling and better stability, these spin-valley asymmetric characteristics engineered by the gate field indicate that the field-controllable bilayer silicene could be a potential component candidate to achieve a fully spin-valley polarized beam for quantum logic applications.

Linkage between Surface Energy Balance Non-closure and Horizontal Asymmetric Turbulent Transport

Linkage between Surface Energy Balance Non-closure and Horizontal Asymmetric Turbulent Transport

Bioinspired Dual-Driven Binary Heterogeneous Nanofluidic Ionic Diodes.

Recently, bioinspired 2D material-based nanofluidic systems with unique properties and advantages have been receiving considerable research interest and getting rapid development. However, it remains a huge challenge to integrate adaptive responsiveness to external stimuli and asymmetric ion transport characteristics into the 2D nanofluidic systems. Herein, we report a dual-driven switchable asymmetric ionic transport phenomenon through a graphene oxide-based heterogeneous 2D nanofluidic membrane. Taking advantage of the formation of a charge heterojunction induced by the variation of pH or UV irradiation, a maximum ionic current rectification (ICR) ratio of ca. 56 for pH or 140 for light was achieved. Such smart nanofluidic devices with pH and light dual-responsiveness and asymmetric ion transport behaviors provide a universal strategy for potential applications in chemical sensing, water treatment, and energy conversion and establish a promising platform for exploring advanced quantum ionics biodevices with ultrafast signal transmission, nanochannel-structured bioreactors with high efficiency, etc.

Intestinal Permeability of Drugs in Caco-2 Cells Cultured in Microfluidic Devices.

Microfluidic devices are attracting attention for their ability to provide a biomimetic microenvironment wherein cells are arranged in a particular pattern and provided fluidic and mechanical forces. In this study, we evaluated drug transport across Caco-2 cell layers in microfluidic devices and investigated the effects of fluid flow on drug transport and metabolism. We designed a microfluidic device that comprises two blocks of polydimethylsiloxane and a sandwiched polyethylene terephthalate membrane with pores 3.0 µm in diameter. When cultured in a dynamic fluid environment, Caco-2 cells were multilayered and developed microvilli on the surface as compared with a static environment. Drugs with higher lipophilicity exhibited higher permeability across the Caco-2 layers, as well as in the conventional method using Transwells, and the fluidic conditions had little effect on permeability. In the Caco-2 cell layers cultured in Transwells and microfluidic devices, the basal-to-apical transport of rhodamine 123, a substrate of P-glycoprotein, was greater than the apical-to-basal transport, and the presence of tariquidar, an inhibitor of P-glycoprotein, completely diminished asymmetric transport. Furthermore, fluidic conditions promoted the metabolism of temocapril by carboxylesterases. On the other hand, we showed that fluidic conditions have little effect on gene expression of several transporters and metabolic enzymes. These results provide useful information regarding the application of microfluidic devices in drug transport and metabolism studies.