Maximum Rectification Ratio Research Articles

Spin multiple functional devices in zigzag-edged graphyne nanoribbons based molecular nanojunctions

Graphyne, a two-dimensional carbon allotrope being proven to possess amazing electronic properties as graphene, have been successively proposed in theory and experiment. Here, the electronic band structures of zigzag-edged δ graphyne nanoribbons (ZδGYNR) and zigzag-edged γ graphyne nanoribbons (ZγGYNR) have been revealed with a metallic behavior in non-magnetic state, a metallic spin splitting in ferromagnetic state and a spin degeneracy with a direct band gap in antiferromagnetic state, which is much similar to zigzag-edged graphene case. We also report a comprehensive study of the intrinsic spin-resolved transport properties for the three junctions with a carbon chain sandwiching between ZδGYNR/ZγGYNR leads by using density functional theory coupled with the non-equilibrium Green’s function method. The results reveal that the designed junctions perform multiple functions with wonderful spin filtering, rectification, and a spin negative differential resistance(SNDR) effect, and so on. Specifically, the spin filtering efficiency approximate 100% within a large bias range, the maximum rectification ratio can be up to 1.5×104 and an obvious SNDR with the maximum peak to valley ratio up to 9.10×103 can also be found. The GYNR-based multiple functional device is demonstrated, and mechanisms are proposed for the above phenomena.

Effect of (Ce, Al) co-doped ZnO thin films on the Schottky diode properties fabricated using the sol-gel spin coating

In this work, Schottky diodes based on undoped and (Ce, Al) co-doped ZnO thin films have been fabricated using the sol-gel spin coating. The effect of (Ce, Al) co-doped ZnO films on the structural and optical properties has been studied, and the electrical characteristic of the fabricated Schottky diode devices are investigated. X-ray diffraction and field emission scanning electron microscopy are used to study the crystalline structure and surface morphology, respectively. Room temperature Raman spectroscopy showed that the intensity of the dominant E2 high peak decreased after doping. The photoluminescence studies at room temperature showed that the intensity of the UV and the visible emission peak decreased after doping. UV-vis spectroscopy measurements at room temperature showed a decrease in the optical band gap after doping compared to undoped ZnO thin films. The I–V characteristics of fabricated Schottky diodes manifest good device behaviour at higher levels of doping (7.0 at.%) with an ideality factor of 2.40, the barrier height of 0.77 eV and series resistance of 262 Ω. The maximum rectification ratio was found to be nearly 105 at 7.0 at.%. Moreover, the electrical properties of the Schottky diode devices analyzed with Cheung-Cheung function revealed that the Schottky diode parameters were higher compared to those obtained from the conventional thermionic emission theory. Furthermore, the free electron carrier concentration for the doped samples was found to be lower compared with undoped ZnO films.

Gate-Tunable and Programmable n-InGaAs/Black Phosphorus Heterojunction Diodes.

Semiconductor heterostructures have enabled numerous applications in diodes, photodetectors, junction field-effect transistors, and memory devices. Two-dimensional (2D) materials and III-V compound semiconductors are two representative materials providing excellent heterojunction platforms for the fabrication of heterostructure devices. The marriage between these semiconductors with completely different crystal structures may enable a new heterojunction with unprecedented physical properties. In this study, we demonstrate a multifunctional heterostructure device based on 2D black phosphorus and n-InGaAs nanomembrane semiconductors that exhibit gate-tunable, photoresponsive, and programmable diode characteristics. The device exhibits clear rectification with a large gate-tunable forward current, which displays rectification and switching with a maximum rectification ratio of 4600 and an on/off ratio exceeding 105, respectively. The device also offers nonvolatile memory properties, including large hysteresis and stable retention of storage charges. By combining the memory and gate-tunable rectifying properties, the rectification ratio of the device can be controlled and memorized from 0.06 to 400. Moreover, the device can generate three different electrical signals by combining a photoresponsivity of 0.704 A/W with the gate-tunable property, offering potential applications, for example, multiple logic operator. This work presents a heterostructure design based on 2D and III-V compound semiconductors, showing unique physical properties for the development of multifunctional heterostructure devices.

Molecular Rectifiers on Silicon: High Performance by Enhancing Top-Electrode/Molecule Coupling.

One of the simplest molecular-scale electronic devices is the molecular rectifier. In spite of considerable efforts aimed at understanding structure-property relationships in these systems, devices with predictable and stable electronic properties are yet to be developed. Here, we demonstrate highly efficient current rectification in a new class of compounds that form self-assembled monolayers on silicon. We achieve this by exploiting the coupling of the molecules with the top electrode which, in turn, controls the position of the relevant molecular orbitals. The molecules consist of a silane anchoring group and a nitrogen-substituted benzene ring, separated by a propyl group and imine linkage, and result from a simple, robust, and high-yield synthetic procedure. We find that when incorporated in molecular diodes, these compounds can rectify current by as much as 3 orders of magnitude, depending on their structure, with a maximum rectification ratio of 2635 being obtained in ( E)-1-(4-cyanophenyl)- N-(3-(triethoxysilyl) propyl)methanimine (average Ravg = 1683 ± 458, at an applied voltage of 2 V). This performance is on par with that of the best molecular rectifiers obtained on metallic electrodes, but it has the advantage of lower cost and more efficient integration with current silicon technologies. The development of molecular rectifiers on silicon may yield hybrid systems that can expand the use of silicon toward novel functionalities governed by the molecular species grafted onto its surface.

The maximum rectification ratio of pyrene-based molecular devices: a systematic study

We apply the NEGF + DFT technique to study the effect of anchoring groups on the electronic transport properties of a single pyrene molecule attached to two Au electrodes via three different anchoring groups (namely NO2, NH2 and CN). More specifically, we investigate the effect of asymmetric electrode coupling together with B and N doping on rectification ratio of a pyrene-based molecular device. The results indicate that the rectification ratio can be tuned by selecting configurations of maximum difference in the coupling parameters in the two sides of the gold electrodes, and its magnitude depends on the strength of the electronic coupling of the pyrene molecule to the gold electrodes. In addition, we observe that doping the molecule with B and N atoms decreases the coupling parameters by creating a resonant peak close to the Fermi level.

Edge modification induced giant rectification effect in armchair C2N-h2D nanoribbons

The recently synthesized two-dimensional C2N-h2D material has received extensive attention due to its similar honeycomb structure with graphene as well as uniform distribution of nitrogen atoms and holes. We perform first-principles calculations and find that armchair C2N-h2D nanoribbon (AC2NNR) is quite sensitive to edge modification. Edge unmodified and H-modified AC2NNRs are direct band gap semiconductors, while O-modified becomes an indirect band gap semiconductors. Interestingly, when both edge and sub-edge are modified with H or O atoms, it turns into a metal. Metal-semiconductor heterojunctions constructed from property-distinctive AC2NNRs all exhibit forward-blocking and reverse-conducting rectifying diode behaviors, and the maximum rectification ratio reaches an incredible value of 1010, showing good application prospects.

Effects of different tailoring graphene electrodes on the rectification and negative differential resistance of molecular devices

The electronic transport properties of oligo p-phenylenevinylene (OPV) molecule sandwiched with symmetrical or asymmetric tailoring graphene nanoribbons (GNRs) electrodes are investigated by nonequilibrium Green’s function in combination with density functional theory. The results show that different tailored GNRs electrodes can modulate the current–voltage characteristic of molecular devices. The rectifying behavior can be observed with respect to electrodes, and the maximum rectification ratio can reach to 14.2 in the asymmetric AC–ZZ GNRs and ZZ–AC–ZZ GNRs electrodes system. In addition, the obvious negative differential resistance can be observed in the symmetrical AC-ZZ GNRs system.

Improving the rectifying performance of the pyrene-benzene system by optimizing its hydrocarbon bridge: A first-principles investigation

We report a computational study on enhancing the rectification ratio (RR) of the pyrene-benzene junction by modifying the structure of its acyclic aliphatic hydrocarbon bridge. The RR values for a series of pyrene-benzene systems with different types of bridges are determined. Notably, the maximum RR of the pyrene-(CH)3-(CH2)3-(CH)3-benzene system is 93.8, and the same hydrocarbon bridge increases the RR of the organic photovoltaic materials pyrene-DMA (N,N-dimethylaniline) and pyrene-DCNB (p-dicyanobenzene) by 10 times and limits reverse current during photo-induced electron transfer. These findings are particularly important for the design of molecular devices where both a high RR and conductivity are desirable. Moreover, they provide new insight into phenomena that indirectly affects the photoelectric conversion efficiency viz. enhancing the RR of the materials.

Thermal diode using controlled capillary in heterogeneous nanopores

The development of a compact, efficient, reliable thermal diode is crucial to improve advanced thermal management efficiency and controllability, and to enable brand new applications such as thermal logic gates and computers. In this study, we examine a nanoscale and efficient capillary-controlled thermal diode mechanism in Ar-filled Pt-based heterogeneous nanoporous structures, using Grand Canonical Monte Carlo (GCMC) simulation combined with Non-Equilibrium Molecular Dynamics (NEMD) simulation at the temperature range of 70–150 K and the pressure of 1.66 atm. Results show that the large thermal conductivity contrast between the controlled adsorption and capillary states using the structural heterogeneity (nanopillars on only one surface) and/or material heterogeneity (two different materials for nanogap surfaces) allows for the maximum thermal rectification ratio, Rmax ∼ 140 with minimal hysteresis under the cyclic operating temperatures −40 K < ΔT < +40 K. It is also found that the material heterogeneity is equivalent to the structural heterogeneity for minimizing the hysteresis in adsorption-capillary transition, but the heat flux across the nanogap with the material heterogeneity reduces due to weaker Ar-solid interaction. The coupled structural-material heterogeneity for the capillary-driven thermal diode is also discussed. The obtained results pave pathways for advanced thermal management systems such as thermal transistors, thermal logic gates, and computers.

Rectifying performances of oligo phenylene ethynylene molecular devices based on graphene electrodes

With the experimental advances in microscale fabrication technology, the designing of functional devices by using single molecules has become one of the most promising methods for the next generation of electronic devices. Molecular rectifier, as a basic component almost for any electronic device, has become a research hotspot in molecular electronics. Recently, one-dimensional graphene nanoribbons (GNRs) which cut off from the novel two-dimensional material-graphene were used as the electrodes for several molecular devices due to their unique electronic structures and transport characteristics. The GNRs have less serious contact problems than metallic electrode materials like gold. In this paper, we investigate the rectifying performances of oligo phenylene ethynylene molecular devices based on graphene electrodes by using the density-functional theory and the non-equilibrium Green's function method. The effect of functional group on the rectifying performances of molecular device is discussed. The results show that the functional group plays a significant role in determining the rectifying performances of oligo phenylene ethynylene molecular device. The rectifying ratio can be effectively tuned by the functional group: adding the donor group (NH2) can lead to the positive rectifying phenomenon, adding the acceptor group (NO2) can trigger the negative rectifying phenomenon, and simultaneously adding NH2 and NO2 groups can bring about an alternate phenomenon between positive and reverse rectifying . The physical mechanism of the rectifying behavior is explained based on the transmission spectra and molecular projected self-consistent Hamiltonian. The transmission spectra of four models (M1-M4) bias voltages in range from-1.0 V to 1.0 V are given. The main transmission peak of M1 for positive bias is similar to that for negative bias, resulting in a weak rectification ratio. However, for M2 and M3, the main transmission peaks for positive and negative bias are significantly different from each other, which shows obviously a rectifying behavior. For M4, the main transmission peak is higher for the bias of (0.44-0.83 V) and also for the bias (0.95-1.00 V), showing an alternate phenomenon between positive and reverse rectifying. The maximum rectification ratio reaches 2.71 by adding an acceptor group (NO2), which suggests that this system has attractive potential applications in future molecular circuit.

Effect of edge modification on the rectification in graphene ribbons device

We perform first-principles calculations based on density functional theory and non-equilibrium Green's function to investigate the electronic transport properties of the 12-ZGNRs devices with edge modification of OH/NH2, OH/NO2 and OH/SO2. The device with modified edge by OH/SO2 shows the maximum (reverse) rectification ratio of 2076.33(1937.33). We discuss the effect of edge modification on rectifying phenomenon by calculating the transmission spectra and the energy band structures of the related electrodes as well as the PDOS at different bias. And the observed negative differential resistance effect is explained by the transmission spectra for device with modified edge by OH/NH2 and OH/NO2. The results indicate that the edge modification of the OH/SO2 that causes a asymmetric energy band improves the electron transport of the device, suggesting a method to design graphene rectifier with good performance.

Molecular diodes with rectification ratios exceeding 105 driven by electrostatic interactions.

Molecular diodes operating in the tunnelling regime are intrinsically limited to a maximum rectification ratio R of ∼103. To enhance this rectification ratio to values comparable to those of conventional diodes (R ≥ 105) an alternative mechanism of rectification is therefore required. Here, we report a molecular diode with R = 6.3 × 105 based on self-assembled monolayers with Fc-C≡C-Fc (Fc, ferrocenyl) termini. The number of molecules (n(V)) involved in the charge transport changes with the polarity of the applied bias. More specifically, n(V) increases at forward bias because of an attractive electrostatic force between the positively charged Fc units and the negatively charged top electrode, but remains constant at reverse bias when the Fc units are neutral and interact weakly with the positively charged electrode. We successfully model this mechanism using molecular dynamics calculations.

Electric and photovoltaic characteristics of a multi-layer ReS2/ReSe2 heterostructure

Among the various unique properties of two-dimensional materials, the ability to form a van der Waals (vdW) heterojunction between them is very valuable, as it offers a superior interface quality without the lattice mismatch problem. In this work, a ReS2/ReSe2 vdW heterostructure was fabricated, and its electrical and photovoltaic behaviors were discovered. The heterojunction showed a gate-tunable diode property with the maximum rectification ratio of 3150. Under illumination, it exhibited a photovoltaic effect with an efficiency of ∼0.5%. This study outlines the potential of Re-based 2D semiconductors and their integration by forming a vdW heterojunction for use in optoelectronic devices.

Chemical Vapor Deposition Growth of Degenerate p-Type Mo-Doped ReS2 Films and Their Homojunction

Substitutional doping of transition metal dichalcogenide two-dimensional materials has proven to be effective in tuning their intrinsic properties, such as band gap, transport characteristics, and magnetism. In this study, we realized substitutional doping of monolayer rhenium disulfide (ReS2) with Mo via chemical vapor deposition. Scanning transmission electron microscopy demonstrated that Mo atoms are successfully doped into ReS2 by substitutionally replacing Re atoms in the lattice. Electrical measurements revealed the degenerate p-type semiconductor behavior of Mo-doped ReS2 field effect transistors, in agreement with density functional theory calculations. The p-n diode device based on a doped ReS2 and ReS2 homojunction exhibited gate-tunable current rectification behaviors, and the maximum rectification ratio could reach up to 150 at Vd = -2/+2 V. The successful synthesis of p-type ReS2 in this study could largely promote its application in novel electronic and optoelectronic devices.

Tuning the properties of a self-powered UV photodetector based on ZnO and poly(3,4-ethylenedioxythiophene): Poly(styrenesulfonate) by hydrogen annealing of ZnO nanorod arrays

Exploring low cost and high performance self-powered UV photodetectors is very important for applications. In the present work, highly ordered and vertically oriented ZnO nanorod arrays on a glass substrate with an Al doped ZnO seed layer were synthesized by a hydrothermal method. It is shown that the point defects and Fermi level of the ZnO NRs can be effectively tuned by hydrogen annealing. The electrical properties of the ZnO/PSS:PEDOT heterojunctions using the hydrogen annealed ZnO nanorod arrays are improved significantly. A minimum ideality factor of 1.2 and a maximum rectification ratio of 255 at ±2V are achieved for the heterojunction with ZnO nanorod arrays annealed at 300°C under hydrogen atmosphere. It is demonstrated that all the UV photodetectors based on the ZnO/PSS:PEDOT heterojunction can be operated at a self-powered mode with a response time <1s. The photodetector with the 300°C hydrogen annealed ZnO nanorod arrays shows the highest photocurrent of 82nA and the highest photosensitivity of ~1000 at zero bias under 365nm UV light with an intensity of 2.0mW/cm2. The improvement in photoelectrical properties of the photodetector is ascribed to the reduced point defects in the ZnO nanorod arrays and increased barrier height of the heterojunction, which is effectively tuned by hydrogen annealing.

Photo-switchable two-dimensional nanofluidic ionic diodes.

The bottom-up assembly of ion-channel-mimetic nanofluidic devices and materials with two-dimensional (2D) nano-building blocks paves a straightforward way towards the real-world applications of the novel transport phenomena on a nano- or sub-nanoscale. One immediate challenge is to provide the 2D nanofluidic systems with adaptive responsibilities and asymmetric ion transport characteristics. Herein, we introduce a facile and general strategy to provide a graphene-oxide-based 2D nanofluidic system with photo-switchable ionic current rectification (ICR). The degree of ICR can be prominently enhanced upon UV irradiation and it can be perfectly retrieved under irradiation with visible light. A maximum ICR ratio of about 48 was achieved. The smart and functional nanofluidic devices have applications in energy conversion, chemical sensing, water treatment, etc.

Ion current rectification in a confined conical nanopore with high solution concentrations

Ion transport in a confined conical nanochannel with high solution concentrations was studied using molecular dynamics simulation. The simulation results indicated that the ion current rectification appeared at high solution concentrations, even without electrical double layer (EDL) overlapping, which was still influenced by both the solution concentration and surface charge properties as it would be in a low solution concentration. With solution concentrations increasing from 0.41 to 2.08 M, a maximum rectification ratio was obtained. This phenomenon was attributed to the competition between the axial binding energy gradient arising from the confined conical geometry intensifying the ion axial asymmetric concentration polarisation and the decreasing thickness of the EDL weakening the concentration polarisation.

Physical properties of annealed ZnO nanowire/CuSCN heterojunctions for self-powered UV photodetectors.

The low-cost fabrication of ZnO nanowire/CuSCN heterojunctions is demonstrated by combining chemical bath deposition with impregnation techniques. The ZnO nanowire arrays are completely filled by the CuSCN layer from their bottoms to their tops. The CuSCN layer is formed of columnar grains that are strongly oriented along the [003] direction owing to the polymeric form of the β-rhombohedral crystalline phase. Importantly, an annealing step is found essential in a fairly narrow range of low temperatures, not only for outgassing the solvent from the CuSCN layer, but also for reducing the density of interfacial defects. The resulting electrical properties of annealed ZnO nanowire/CuSCN heterojunctions are strongly improved: a maximum rectification ratio of 2644 at ±2 V is achieved following annealing at 150 °C under air atmosphere, which is related to a strong decrease in the reverse current density. Interestingly, the corresponding self-powered UV photodetectors exhibit a responsivity of 0.02 A/W at zero bias and at 370 nm with a UV-to-visible (370-500 nm) rejection ratio of 100 under an irradiance of 100 mW/cm(2). The UV selectivity at 370 nm can also be readily modulated by tuning the length of ZnO nanowires. Eventually, a significant photovoltaic effect is revealed for this type of heterojunctions, leading to an open circuit voltage of 37 mV and a short circuit current density of 51 μA/cm(2), which may be useful for the self-powering of the complete device. These findings show the underlying physical mechanisms at work in ZnO nanowire/CuSCN heterojunctions and reveal their high potential as self-powered UV photodetectors.

Forming-free and self-rectifying resistive switching of the simple Pt/TaOx/n-Si structure for access device-free high-density memory application.

The search for self-rectifying resistive memories has aroused great attention due to their potential in high-density memory applications without additional access devices. Here we report the forming-free and self-rectifying bipolar resistive switching behavior of a simple Pt/TaOx/n-Si tri-layer structure. The forming-free phenomenon is attributed to the generation of a large amount of oxygen vacancies, in a TaOx region that is in close proximity to the TaOx/n-Si interface, via out-diffusion of oxygen ions from TaOx to n-Si. A maximum rectification ratio of ∼6 × 10(2) is obtained when the Pt/TaOx/n-Si devices stay in a low resistance state, which originates from the existence of a Schottky barrier between the formed oxygen vacancy filament and the n-Si electrode. More importantly, numerical simulation reveals that the self-rectifying behavior itself can guarantee a maximum crossbar size of 212 × 212 (∼44 kbit) on the premise of 10% read margin. Moreover, satisfactory switching uniformity and retention performance are observed based on this simple tri-layer structure. All of these results demonstrate the great potential of this simple Pt/TaOx/n-Si tri-layer structure for access device-free high-density memory applications.

Junction Characteristics System Based on Composite Organic Semiconductors: Polystyrene/Polyaniline Doped by [BMIM] [BF4] Ionic Liquid

Abstract A junction of emeraldine salt (ES [BF4 −]) doped by [BMIM] [BF4] may be preferred over the other junctions due to its low ideality factor and maximum rectification ratio. Schottky barrier diode based on composite of polyaniline (ES [BF4 −]) with polystyrene has been created and characterized using aluminium as Schottky contact and argent as an ohmic contact. The observed current–voltage characteristics can be satisfactorily fitted using the modified Schottky equation. Capacitance–voltage (C–V) in reverse bias and current–voltage (I–V) plots indicates a rectification. Various junction parameters were calculated from the temperature dependent I–V and C–V data and discussed. These results indicate that the composite materials have better mechanical strength and diode quality compared to the pure semiconducting polymer.