Current Rectification Behavior Research Articles

Effects of Coulomb Blockade on the Charge Transport through the Topological States of Finite Armchair Graphene Nanoribbons and Heterostructures.

In this study, we investigate the charge transport properties of semiconducting armchair graphene nanoribbons (AGNRs) and heterostructures through their topological states (TSs), with a specific focus on the Coulomb blockade region. Our approach employs a two-site Hubbard model that takes into account both intra- and inter-site Coulomb interactions. Using this model, we calculate the electron thermoelectric coefficients and tunneling currents of serially coupled TSs (SCTSs). In the linear response regime, we analyze the electrical conductance (Ge), Seebeck coefficient (S), and electron thermal conductance (κe) of finite AGNRs. Our results reveal that at low temperatures, the Seebeck coefficient is more sensitive to many-body spectra than electrical conductance. Furthermore, we observe that the optimized S at high temperatures is less sensitive to electron Coulomb interactions than Ge and κe. In the nonlinear response regime, we observe a tunneling current with negative differential conductance through the SCTSs of finite AGNRs. This current is generated by electron inter-site Coulomb interactions rather than intra-site Coulomb interactions. Additionally, we observe current rectification behavior in asymmetrical junction systems of SCTSs of AGNRs. Notably, we also uncover the remarkable current rectification behavior of SCTSs of 9-7-9 AGNR heterostructure in the Pauli spin blockade configuration. Overall, our study provides valuable insights into the charge transport properties of TSs in finite AGNRs and heterostructures. We emphasize the importance of considering electron-electron interactions in understanding the behavior of these materials.

Light-Regulated Nanofluidic Ionic Diodes with Heterogeneous Channels Stemming from Asymmetric Growth of Metal–Organic Frameworks

Nanofluidic ionic diodes have attracted much attention, because of the unique property of asymmetric ion transport and promising applications in molecular sensing and biosensing. However, it remains a challenge to fabricate diode-like nanofluidic system with molecular-size pores. Herein, we report a new and facile approach to construct nanofluidic ionic diode by in situ asymmetric growth of metal-organic frameworks (MOFs) in nanochannels. We implement microwave-assisted strategy to obtain asymmetric distribution of MOFs in porous anodic aluminum oxide with barrier layer on one side. After etching the barrier layer and modifying with positively charged molecules, the nanofluidic device possesses asymmetric geometry and surface charge, performing the ionic current rectification (ICR) behavior in different electrolyte concentrations. Moreover, the ICR ratio is readily regulated with visible light illumination mainly due to the enhancement of surface charge of MOFs, which is further confirmed by finite element simulation. This study provides a reliable way to build the nanofluidic platform for investigating the asymmetric ion transport through the molecular-size pores, which is envisaged to be important for molecular sensing based on ICR with molecular-size pores.

Large-Area Synthesis and Patterning of All-Inorganic Lead Halide Perovskite Thin Films and Heterostructures.

All-inorganic lead halide perovskites have attracted tremendous interest for their excellent stability when compared with hybrid perovskites. Here we report a large-area growth of monocrystalline all-inorganic perovskite thin films and further patterning them into heterostructure arrays. We show that highly oriented CsPbBr3 microcrystal domains can be readily grown on muscovite mica substrates with a well-defined epitaxial relationship, which can further expand and eventually merge into large-area monocrystalline CsPbBr3 thin films with an excellent optical quality. Taking a step further, we show the large-area CsPbBr3 thin film can be further patterned and selectively transformed into CsPbI3 using a selective anion-exchange process to produce CsPbBr3-CsPbI3 lateral heterostructure arrays with spatially modulated photoluminescence emission and an apparent current rectification behavior. The capability to grow large-area CsPbBr3 monocrystalline thin films and heterostructure arrays defines a robust material platform for both the fundamental investigations and potential applications in optoelectronics.

Tuning the current rectification behavior of Rh2-based molecular junctions by varying their supramolecular structures.

Molecular junctions with similar backbones, tunable chemical structures and controllable length are critical for the systematic study of the structure-functionality relationships of their charge transport behavior. Taking advantage of the feasibility and tunability of stepwise fabrication, we built series of asymmetric supramolecular SAMs on gold using Rh2(O2CCR3)4 (Rh2, R = CH3, H, and F) as the building blocks and conjugated N,N'-bidentate ligands (pyrazine (LS), 4,4'-bipyridine (LM) and 1,2-bis(4-pyridyl)ethene (LL)) as the bridges. By varying the Rh2 units and bridging ligands, series of supramolecules with similar backbone and tunable chemical structures were assembled on gold. Their charge transport behavior was examined using conductive-probe atomic force microscopy. Notably, current rectification diminishes gradually as the degree of conjugation of the bridging ligands gets larger from LS to LL due to the decrease in the energy gap between the donor and the acceptor in π(Rh2)-π(L) conjugated MO arrays. Additionally, current rectification can be enhanced when the charge transport mechanistic transits from tunneling in dimers to hopping in tetramers. Unlike charges hopping along the MO arrays in tetramers, charges tunnel through the frontier MOs in dimers. The occupied frontier MOs of dimers localize near the center of the supramolecules or delocalize on the donor and acceptor, which contributes to the weakening of the asymmetric charge tunneling. This work reveals that the frontier MO configurations of these supramolecules could be adjusted by varying their chemical structures, and consequently realize tuning of their charge transport behavior, which deepens the understanding of the charge transport behavior and benefits the establishment of the structure-functionality relationship of Rh2-based molecular junctions.

Scalable lateral heterojunction by chemical doping of 2D TMD thin films

Scalable heterojunctions based on two-dimensional transitional metal dichalcogenides are of great importance for their applications in the next generation of electronic and optoelectronic devices. However, reliable techniques for the fabrication of such heterojunctions are still at its infancy. Here we demonstrate a simple technique for the scalable fabrication of lateral heterojunctions via selective chemical doping of TMD thin films. We demonstrate that the resistance of large area MoS2 and MoSe2 thin film, prepared via low pressure chalcogenation of molybdenum film, decreases by up to two orders of magnitude upon doping using benzyl viologen (BV) molecule. X-ray photoelectron spectroscopy (XPS) measurements confirms n-doping of the films by BV molecules. Since thin films of MoS2 and MoSe2 are typically more resistive than their exfoliated and co-evaporation based CVD counterparts, the decrease in resistance by BV doping represents a significant step in the utilization of these samples in electronic devices. Using selective BV doping, we simultaneously fabricated many lateral heterojunctions in 1 cm2 MoS2 and 1 cm2 MoSe2 films. The electrical transport measurements performed across the heterojunctions exhibit current rectification behavior due to a band offset created between the doped and undoped regions of the material. Almost 84% of the fabricated devices showed rectification behavior demonstrating the scalability of this technique.

Solution‐Grown Large‐Sized Single‐Crystalline 2D/3D Perovskite Heterostructure for Self‐Powered Photodetection

AbstractOrganic–inorganic hybrid perovskite heterostructures, particularly 2D/3D perovskite heterostructures, have enjoyed great success in optoelectronic field. However, the photodetection performance of those heterostructures is obscured by extensive disorder in their polycrystalline films. Here, a delicate solution method is demonstrated for creating a sizable crystal of vertical 2D/3D perovskite heterostructure featuring a well‐defined interface and high single‐crystalline quality, namely (4‐AMP)(MA)2Pb3Br10/MAPbBr3 (MA, methylammonium; 4‐AMP, 4‐(aminomethyl)piperidinium). These high‐quality crystals are ideal mediums for charge transport and exploiting their optoelectronic properties. Electrical transport measurements demonstrate that the (4‐AMP)(MA)2Pb3Br10/MAPbBr3 heterostructure can form vertical diode with obvious current rectification behavior and excellent photocurrent generation characteristics. Strikingly, benefitting from the built‐in electrical potential at the junction, photodetectors based on those millimeter‐thickness heterostructure crystals exhibit high performance in self‐driven operation mode, including large on/off switching radio (≈105), fast response time (600/600 µs), and high detectivity (≈1012). This work enables an important advance in the development of single‐crystalline perovskite heterostructures for both fundamental demonstrations and high‐performance optoelectronic devices.

Detection of the trace level of heavy metal ions by pH-regulated conical nanochannels

The exceeding of the heavy metal ions concentration in human body to a certain critical level can lead to serious diseases. Since this critical level is usually of trace value, developing an efficient yet economic method to estimate that value is both desirable and necessary. Based on the ion current rectification behavior of a conical, polyethylene terephthalate nanochannel surface modified by 18-crown-6, we propose a method to estimate the concentration of Pb2+ in an aqueous medium. Numerical simulation is conducted to examine the performance of this nanochannel under various conditions by varying the background LiCl concentration, the concentration of Pb2+, and the degree of surface modification. Regression analysis is also conducted to obtain the range of Pb2+ concentration suitable for detection and its lower limit. We show that the performance of the nanochannel proposed is satisfactory. The possibility of detecting the concentration of Cu2+ by the nanochannel proposed is also examined.

Ion current rectification behavior of a nanochannel having nonuniform cross-section.

Due to its versatile applications in biotechnology, ion current rectification (ICR), which arises from the asymmetric nature of the ion transport in a nanochannel, has drawn much attention, recently. Here, the ICR behavior of a pH-regulated nanochannel comprising two series connected cylindrical nanochannels of different radii is examined theoretically, focusing on the influences of the radii ratio, the length ratio, the bulk concentration, and the solution pH. The results of numerical simulation reveal that the rectification factor exhibits a local maximum with respect to both the radii ratio and the length ratio. The values of the radii ratio and the length ratio at which the local maximum in the rectification factor occur depend upon the level of the bulk salt concentration. The rectification factor also shows a local maximum as the solution pH varies. Among the factors examined, the solution pH influences the ICR behavior of the nanochannel most significantly.

Regulating the ionic current rectification behavior of branched nanochannels by filling polyelectrolytes

The overlapping of the electric double layer (EDL) in a nanochannel yields many interesting and significant electrokinetic phenomena such as ionic current rectification (ICR), which occurs only at a relatively low bulk salt concentration (∼1 mM) where the EDL thickness is comparable to the nanochannel size. In an attempt to raise this concentration to higher levels and the ICR performance improved appreciably, a branched nanochannel filled with polyelectrolytes (PEs) is proposed in this study. We show that these objectives can be achieved by choosing appropriate PE. For example, if the stem side of an anodic aluminun oxide nanochannel is filled with polystyrene sulfonate (PSS) an ICR ratio up to 850 can be obtained at 1 mM, which was not reported in previous studies. Taking account of the effect of electroosmotic flow, the underlying mechanisms of the ICR phenomena observed are discussed and the influences of the solution pH, the bulk salt concentration, and how the region(s) of a nanochannel is filled with PE examined. We show that the ICR behavior of a branched nanochannel can be modulated satisfactorily by filling highly charged PE and the solution pH.

Ion Current Rectification Behavior of Conical Nanopores Filled with Spatially Distributed Fixed Charges

Ion current rectification (ICR) phenomena in conical nanopores have been extensively investigated since their early discovery. Several theoretical explanations were proposed to quantitatively descr...

Direct van der Waals epitaxial growth of 1D/2D Sb2Se3/WS2 mixed-dimensional p-n heterojunctions

The mixed-dimensional integration of two-dimensional (2D) materials with non-2D materials can give rise to prominent advances in performance or function. To date, the mixed-dimensional one-dimensional (1D)/2D heterostructures have been fabricated using various physical assembly approaches. However, direct epitaxial growth method which has notable advantages in preparing large-scale products and obtaining perfect interfaces is rarely investigated. Herein, we demonstrate for the first time the direct synthesis of the 1D/2D mixed-dimensional heterostructures by sequential vapor-phase growth of Sb2Se3 nanowires on WS2 monolayers. X-ray diffraction (XRD) pattern and Raman spectrum confirm the composition of the Sb2Se3/WS2 heterostructures. Transmission electron microscope (TEM) measurement demonstrates high quality of the heterostructures. Electrical transport characterization reveals that Sb2Se3 nanowire exhibits p-type characteristic and that WS2 monolayer exhibits n-type behavior, and that the p-n diode from 1D/2D mixed-dimensional Sb2Se3/WS2 heterostructure possesses obvious current rectification behavior. Optoelectronic measurements of the heterostructures show apparent photovoltaic response with an open-circuit voltage of 0.19 V, photoresponsivity of 1.51 A/W (Vds = 5 V) and fast response time of less than 8 ms. The van der Waals epitaxial growth mode of Sb2Se3 nanowires on WS2 monolayers is verified by stripping the Sb2Se3 nanowire from the heterostructures using tape. Together, the direct van der Waals epitaxy opens a facile pathway to 1D/2D mixed-dimensional heterostructures for functional electronic and optoelectronic devices.

Bioinspired fractal nanochannels for high-performance salinity gradient energy conversion

Bioinspired nanofluidic systems have shown a promising application in ion transport tuning and energy conversion. Here, we have numerically investigated the ionic current rectification behavior and power generation via bioinspired fractal nanochannels. Fractal nanochannels can present a strong ionic current rectification effect due to the asymmetric distribution of the electrical double layer overlapping induced by the fractal geometry. At a concentration of 200 mM, the ionic current rectification factor in the trifurcated nanochannel is almost improved by 100%, compared to that in the bifurcated nanochannel. Moreover, ionic current can rectify at much higher salt concentrations in trifurcated nanochannel. Fractal nanochannels can considerably improve the power density and energy conversion efficiency for enhanced osmotic current, diffusive voltage and ion selectivity originating from considerably improved electrical double layer overlapping degree. At a concentration ratio of 1000-fold, compared to the straight nanochannel, the power densities of the bifurcated and trifurcated nanochannels are, respectively, improved by 153% and 357% while the energy conversion efficiency are augmented by 147% and 336%. Furthermore, the salt dependent ion transportation and power generation via the furcated nanochannels are also discussed. This paper may contribute to designing appropriate artificial nanofluidic systems for efficient ion transport tuning and energy conversion.

Piezo‐Phototronic Effect for Enhanced Flexible MoS2/WSe2 van der Waals Photodiodes

AbstractThe recent discoveries of transition‐metal dichalcogenides (TMDs) as novel 2D electronic materials hold great promise to a rich variety of artificial van der Waals (vdWs) heterojunctions and superlattices. Moreover, most of the monolayer TMDs become intrinsically piezoelectric due to the lack of structural centrosymmetry, which offers them a new degree of freedom to interact with external mechanical stimuli. Here, fabrication of flexible vdWs p–n diode by vertically stacking monolayer n‐MoS2 and a few‐layer p‐WSe2 is achieved. Electrical measurement of the junction reveals excellent current rectification behavior with an ideality factor of 1.68 and photovoltaic response is realized. Performance modulation of the photodiode via piezo‐phototronic effect is also demonstrated. The optimized photoresponsivity increases by 86% when introducing a −0.62% compressive strain along MoS2 armchair direction, which originates from realigned energy‐band profile at MoS2/WSe2 interface under strain‐induced piezoelectric polarization charges. This new coupling mode among piezoelectricity, semiconducting, and optical properties in 2D materials provides a new route to strain‐tunable vdWs heterojunctions and may enable the development of novel ultrathin optoelectronics.

Pseudo-diode based on protonic/electronic hybrid oxide transistor

Current rectification behavior has been proved to be essential in modern electronics. Here, a pseudo-diode is proposed based on protonic/electronic hybrid indium-gallium-zinc oxide electric-double-layer (EDL) transistor. The oxide EDL transistors are fabricated by using phosphorous silicate glass (PSG) based proton conducting electrolyte as gate dielectric. A diode operation mode is established on the transistor, originating from field configurable proton fluxes within the PSG electrolyte. Current rectification ratios have been modulated to values ranged between ∼4 and ∼50 000 with gate electrode biased at voltages ranged between −0.7 V and 0.1 V. Interestingly, the proposed pseudo-diode also exhibits field reconfigurable threshold voltages. When the gate is biased at −0.5 V and 0.3 V, threshold voltages are set to ∼−1.3 V and −0.55 V, respectively. The proposed pseudo-diode may find potential applications in brain-inspired platforms and low-power portable systems.

Current rectification and asymmetric photoresponse in MoS2 stacking-induced homojunctions**J.X. and Q.S.Z. performed the experiments. J.X.Y. provided band structure simulation. J.D.Z. and Z.L. helped to prepare the samples. W.Z. provided the STEM images. All authors discussed the results. J.X, J.X.Y. and Z.X.S. conceived the study. J.X. and Q.S.Z. wrote the manuscript with contributions from co-authors.

The layer-dependent electronic and optoelectronic properties in transition metal dichalcogenides (TMDs) have been investigated extensively. Current rectification behavior has been demonstrated using heterojunctions constructed from different TMDs materials, as well as single layer-few layer homojunctions utilizing the layer-dependent behavior of TMDs. MoS2 is the best known TMDs and high quality few-layer samples with different stacking sequences can be facilely obtained by the chemical vapor deposition (CVD) method. In this paper, we firstly report the stacking-dependent junctions formed by the interface of AA-AB stacked bilayer MoS2 samples, together with AA bilayer-single layer (AA-1L) and AB bilayer-single layer (AB-1L) junctions. Current rectification and asymmetric photoresponse are observed for the unique AA-AB junctions, similar to that of bilayer-monolayer (2L-1L) junctions. Current mappings clearly show that the photocurrents are mainly localized along the interfaces, confirming that intrinsic junctions are responsible to their electronic/optoelectronic performances, rather than Schottky barrier between electrode and sample. Our finding demonstrates the promise of using stacking-modulated 2D materials for future electronics and optoelectronics.

Two Step Chemical Vapor Deposition of In2Se3/MoSe2 van der Waals Heterostructures

Two-dimensional transition metal dichalcogenides heterostructures have stimulated wide interest not only for the fundamental research, but also for the application of next generation electronic and optoelectronic devices. Herein, we report a successful two-step chemical vapor deposition strategy to construct vertically stacked van der Waals epitaxial In2Se3/MoSe2 heterostructures. Transmission electron microscopy characterization reveals clearly that the In2Se3 has well-aligned lattice orientation with the substrate of monolayer MoSe2. Due to the interaction between the In2Se3 and MoSe2 layers, the heterostructure shows the quenching and red-shift of photoluminescence. Moreover, the current rectification behavior and photovoltaic effect can be observed from the heterostructure, which is attributed to the unique band structure alignment of the heterostructure, and is further confirmed by Kevin probe force microscopy measurement. The synthesis approach via van der Waals epitaxy in this work can expand the way to fabricate a variety of two-dimensional heterostructures for potential applications in electronic and optoelectronic devices.

Ion Current Rectification Behavior of Bioinspired Nanopores Having a pH-Tunable Zwitterionic Surface.

The ion current rectification behavior of bioinspired nanopores is modeled by adopting a bullet-shaped nanopore having a pH-tunable zwitterionic surface, focusing on discussing the underlying mechanisms. We show that with its specific geometry, such nanopore is capable of exhibiting several interesting behaviors, including ion concentration polarization and ion current rectification. The influences of the nanopore shape, solution pH, and bulk salt concentration on the associated ion current rectification behavior are examined. We found that if pH exceeds the isoelectric point, the rectification factor has a local maximum as the curvature of the nanopore surface varies, and if it is lower than the isoelectric point, that factor increases (rectification effect decreases) monotonically with increasing surface curvature. In addition to capable of interpreting relevant electrokinetic phenomena, the results gathered also provide necessary information for a sophisticated design of relevant devices.

Influences of Cone Angle and Surface Charge Density on the Ion Current Rectification Behavior of a Conical Nanopore

Due to its potential applications in biotechnology, ion current rectification (ICR) arising from the asymmetric nature of ion transport in a nanochannel has drawn the attention of researchers in various fields. Previous studies usually neglect the effects of osmotic and electroosmotic flows. In this study, a more general model taking account of these effects is adopted to describe the ICR behavior of a conical nanopore. The influences of the cone angle, surface charge density, and bulk salt concentration on this behavior are investigated, and mechanisms proposed to explain the results are obtained. We show that if the cone angle is enlarged by fixing the nanopore tip radius and raising its base radius, the ICR ratio has a local maximum. This behavior may not present if the cone angle is enlarged by fixing the nanopore base radius and raising its tip radius. The local maximum in the ICR ratio does not exist if the bulk salt concentration is sufficiently low or sufficiently high. This ratio also has a local...

Towards Understanding the Ion Transport in Polyelectrolyte-Modified Nanopores with Bipolar Charges

Fundamental understanding of ion transport is crucial for designing nanofluidic/nanopore devices. In particular, nanopores modified with synthetic or biological polyelectrolyte (PE) brushes have emerged as a potential tool for rectifying ionic current and characterizing individual biomolecules. Due to its asymmetric charge density distribution, bipolar nanopore exhibits strong ionic current rectification (ICR) behavior, which is observed as a diode-like current-voltage characteristic. In this work, the ion transport is theoretically investigated by using a continuum-based model, including Poisson−Nernst−Planck (PNP) equations and Stokes and Brinkman equations. In this study, we examine the influences of the bulk salt concentration and the charge density on ICR behavior. The simulated results suggest that the degree of ICR decreases with increasing bulk salt concentration. Besides, the ionic enrichment/depletion is more significant when the charge density becomes larger, resulting in a larger degree of ICR. However, if the positive and negative charge densities are asymmetrically distributed on the either side of the nanopore, the degree of ICR displays a local maximum. The results obtained also imply that the ICR behavior in a PE-modified nanopore with bipolar charges can be regulated by its surface modification.

Two-dimensional lateral heterojunction through bandgap engineering of MoS2 via oxygen plasma

The present study explores the structural, optical (photoluminescence (PL)), and electrical properties of lateral heterojunctions fabricated by selective exposure of mechanically exfoliated few layer two-dimensional (2D) molybdenum disulfide (MoS2) flakes under oxygen (O2)-plasma. Raman spectra of the plasma exposed MoS2 flakes show a significant loss in the structural quality due to lattice distortion and creation of oxygen-containing domains in comparison to the pristine part of the same flake. The PL mapping evidences the complete quenching of peak A and B consistent with a change in the exciton states of MoS2 after the plasma treatment, indicating a significant change in its band gap properties. The electrical transport measurements performed across the pristine and the plasma-exposed MoS2 flake exhibit a gate tunable current rectification behavior with a rectification ratio up to 1.3 × 103 due to the band-offset at the pristine and plasma-exposed MoS2 interface. Our Raman, PL, and electrical transport data confirm the formation of an excellent lateral heterojunction in 2D MoS2 through its bandgap modulation via oxygen plasma.