Bias Window Research Articles

Zero‐Bias Anti‐Ohmic Behaviour in Diradicaloid Molecular Wires

AbstractOpen‐shell materials bearing multiple spin centres provide a key route to efficient charge transport in single‐molecule electronic devices. They have narrow energy gaps, and their molecular orbitals align closely to the Fermi level of the metallic electrodes, thus allowing efficient electronic transport and higher conductance. Maintaining and stabilising multiple open‐shell states—especially in contact with metallic electrodes—is however very challenging, generally requiring a continuous chemical or electrochemical potential to avoid self‐immolation of the open‐shell character. To overcome this issue, we designed, synthesised, and measured the conductance of a series of bis(indeno) fused acenes, where stability is imparted by a close‐shell quinoidal conformation in resonance with the diradical electronic configuration. We show here that these compounds have anti‐ohmic behaviour, with conductance increasing with increasing molecular length, at an unprecedented rate and across the entire bias window ( ). Density Functional Theory (DFT) calculations support our findings, showing the rapidly narrowing HOMO–LUMO gap, unique to these diradicaloid structures, is responsible for the observed behaviour. Our results provide a framework for achieving efficient transport in neutral compounds and demonstrate the promise that diradicaloid materials have in single‐molecule electronics, owing to their great stability and unique electronic structure.

High-temperature quantum valley Hall effect with quantized resistance and a topological switch.

Edge states of a topological insulator can be used to explore fundamental science emerging at the interface of low dimensionality and topology. Achieving a robust conductance quantization, however, has proven challenging for helical edge states. In this work, we show wide resistance plateaus in kink states-a manifestation of the quantum valley Hall effect in Bernal bilayer graphene-quantized to the predicted value at zero magnetic field. The plateau resistance has a very weak temperature dependence up to 50 kelvin and is flat within a dc bias window of tens of millivolts. We demonstrate the electrical operation of a topology-controlled switch with an on/off ratio of 200. These results demonstrate the robustness and tunability of the kink states and its promise in constructing electron quantum optics devices.

Zero-Bias Anti-Ohmic Behaviour in Diradicaloid Molecular Wires.

Open-shell materials bearing multiple spin centres provide a key route to efficient charge transport in single-molecule electronic devices. They have narrow energy gaps, and their molecular orbitals align closely to the Fermi level of the metallic electrodes, thus allowing efficient electronic transport and higher conductance. Maintaining and stabilising multiple open-shell states-especially in contact with metallic electrodes-is however very challenging, generally requiring a continuous chemical or electrochemical potential to avoid self-immolation of the open-shell character. To overcome this issue, we designed, synthesised, and measured the conductance of a series of bis(indeno) fused acenes, where stability is imparted by a close-shell quinoidal conformation in resonance with the diradical electronic configuration. We show here that these compounds have anti-ohmic behaviour, with conductance increasing with increasing molecular length, at an unprecedented rate and across the entire bias window ( ). Density Functional Theory (DFT) calculations support our findings, showing the rapidly narrowing HOMO-LUMO gap, unique to these diradicaloid structures, is responsible for the observed behaviour. Our results provide a framework for achieving efficient transport in neutral compounds and demonstrate the promise that diradicaloid materials have in single-molecule electronics, owing to their great stability and unique electronic structure.

FeN4-Embedded Graphene as a Highly Sensitive and Selective Single-Atom Sensor for Reaction Intermediates of Electrochemical CO2 Reduction.

Exploring effective ways to detect intermediates during the electrochemical CO2 reduction reaction (CO2RR) process is pivotal for understanding reaction pathways and underlying mechanisms. Recently, two-dimensional FeN4-embedded graphene has received increasing attention as a promising catalyst for CO2RR. Here, by means of density functional theory computations combined with the non-equilibrium Green's function (NEGF) method, we proposed a detection device to evaluate the performance of FeN4-embedded graphene in intermediates detection during the CO2RR process. Our results reveal that the four key intermediates, including *COOH, *OCHO, *CHO, and *COH, can be chemisorbed on FeN4-embedded graphene with high adsorption energies and appropriate charge transfer. The computed current-voltage (I-V) characteristics and transmission spectra suggest that the adsorption of these intermediates induces significant type-dependent changes in currents and transmission coefficients of FeN4-embedded graphene. Remarkably, the FeN4-embedded graphene is more sensitive to *COOH and *COH than to *OCHO and *CHO within the entire bias window. Consequently, our theoretical study indicates that the FeN4-embedded graphene can effectively detect the key intermediates during the CO2RR process, providing a practical scheme for identifying catalytic reaction pathways and elucidating underlying reaction mechanisms.

Supramolecular tunnelling junctions with robust high rectification based on assembly effects.

The performance of large-area molecular diodes can in rare cases approach the lower limit of commercial semiconductor devices but predictive structure-property design remains difficult as the rectification ratio (R) achieved by self-assembled monolayer (SAM) based diodes depends on several intertwined parameters. This paper describes a systematic approach to achieve high rectification in bisferrocenyl-based molecular diodes, HSCnFc-CC-Fc (n = 9-15) immobilised on metal surfaces (Ag, Au and Pt). Experiments supported by molecular dynamics simulations show that the molecular length and bottom electrode influence the SAM packing, which affects the breakdown voltage (VBD), the associated maximum R (Rmax), and the bias at which the Rmax is achieved (Vsat,R). From the electrical characterisation of the most stable Pt-SCnFc-CC-Fc//GaOx/EGaIn junctions, we found that VBD, Vsat,R, and Rmax all scale linearly with the spacer length of Cn, and that Rmax for all the SAMs consistently exceeds the "Landauer limit" of 103. Our data shows that the robust switching of M-SCnFc-CC-Fc//GaOx/EGaIn junctions is the result of the combined optimisation of parameters involving the molecular structure, the type of metal substrate, and the applied operating conditions (bias window), to create stable and high-performance junctions.

Monte Carlo global variance reduction method combining source bias and weight window

The global tally problem has a wide range of applications in major research fields such as Monte Carlo simulations of pin-by-pin reactor models and time-dependent particle transport problems in multi-physics coupling calculations. Due to the uneven power distribution of the simulated system, the statistical errors of all tallies are unevenly distributed, resulting in some low global efficiency. For this kind of problem with global characteristics, it is necessary to develop global variance reduction techniques to obtain the accurate distribution of target tallies in the entire space. A large number of global variance reduction algorithms have been studied based on the consideration of flattening global tally error distribution, so as to improve global efficiency. This work focuses on the combination of two efficient global variance reduction algorithms, namely, the uniform tally density algorithm and the weight window algorithm, which belong to source bias and transport process bias, respectively. In tally, a method is proposed to adjust the weight window parameters by using the bias factor of the uniform tally density algorithm. Then, the weight window method will be used to reduce the weight fluctuation caused by the uniform tally density method. In this way, an organic combination of these two algorithms can be realized. A series of comparative tests are carried out based on the Hoogenboom-Martin pressurized water reactor benchmark, and it is verified that the hybrid global variance reduction algorithm proposed in this work is better than the single weight window algorithm or the uniform tally density algorithm. In terms of reducing the maximum error, the global efficiency of the hybrid algorithm is 2.6 times and 3 times that of the weight window algorithm and the uniform tally density algorithm, respectively. In addition, through the comparative analysis of computational asymmetry degree and computational efficiency, it is verified that the uniform tally density algorithm has better performance than the classical uniform fission site algorithm, and the performance advantages of the uniform tally density algorithm are quantitatively evaluated based on some new indicators. The results show that the hybrid global variance reduction algorithm proposed in this work can solve the global tally problem efficiently, thereby further promoting research in related fields.

Anisotropic half-metallicity in zigzag edge SiP3 nanoribbons.

We investigate magnetic and spin-transport properties of the zigzag-edge nanoribbon structures of two-dimensional (2D) SiP3 system within first-principle calculations. Based on the edge terminations, the nanoribbons can be categorized in three subclasses, two of them with even number of silicon (Si) atoms and one of them with odd number of Si atoms in the one-dimensional unitcell with antiferromagnetic bulk spin-ordering, analogous to their 2D counterpart. All the ribbons exhibit indirect band-gap semiconducting behavior and the gap decreases with increase in width of the ribbons. The nanoribbon with only phosphorus (P) atoms at least along one of the edges show half-metallic behavior over a small bias window, when probed with cross-ribbon external electric field. Especially, the nanoribbon with odd number of Si atoms stabilizes in a ground state with net non-zero magnetization and exhibits half-metallicity only under forward bias. Such anisotropic half-metallicity along with the precise control of the electronic properties in terms of edge and external bias manipulation hold immense potential for spintronic and switching device applications.

90% Yield Production of Spiropyran Covalently Grafted MXene‐Based RRAM Devices for Optoelectronic Dual‐Response Switching

AbstractOver the past decade, great efforts have been devoted to both intellectual property protection and information security protection. As a promising electroactive material, MXene is covalently or non‐covalently functionalized with organic species/polymers to produce a large amount of novel functional materials for biomedicine, optoelectronics, and energy storage. By using spiropyran covalently functionalized MXene (MXene‐SP) as an active material, an optoelectronic dual‐response resistive random access memory (RRAM) device is successfully fabricated. Upon UV illumination, the active layer is changed from MXene‐SP (SP: ring‐closed spiropyran form) observed under the illumination of blue light to MXene‐MC (MC: ring‐opened merocyanine form) due to photo‐induced “close‐to‐open” isomerization. The as‐fabricated ITO/MXene‐SP/ITO device exhibits typical nonvolatile optoelectronic dual‐response RRAM performance, with a production yield exceeding 90%. The achieved switch‐on/off voltages are −1.25/2.07 V under the illumination of blue light and −0.58/0.91 V under UV illumination. The switching bias window (Δ|VON − VOFF|) and the ON/OFF current ratio of MXene‐MC are 44.9% and 14.7% of these of MXene‐SP, respectively. By utilizing the window difference between the RRAM performance achieved under the illumination of different lights, one can easily encrypt the quick response code.

Research on the source-detector variance reduction method based on the AIS adjoint Monte Carlo method

The coupled variance reduction method based on adjoint calculation has a good variance reduction effect when solving the source-detector problem. This paper incorporates the Automatic Importance Sampling (AIS) method into the adjoint Monte Carlo method and then proposes a coupled variance reduction method based on the AIS adjoint Monte Carlo method. The result shows that the total flux deviation between the AIS adjoint Monte Carlo method and the original adjoint method is 0.13%, indicating good agreement and verifying the accuracy of the AIS adjoint method. In this paper, the AIS adjoint Monte Carlo method is applied to the calculation of a commercial reactor shielding example, and four sets of source bias and weight window parameters of the AIS adjoint calculation are used as calculation examples. The maximum statistical error is less than 5.00%, and the relative deviation from the measured value is within 20.00%. Generally, the calculation efficiency of AIS-CADIS is slightly better than that of CADIS.

Investigation on the non-linear behaviour of silicon nanowires and assessment of the biosensing potential

We perform a rigorous study on the conduction characteristics of long, low doped and relatively thick p-type silicon nanowire (SiNW) with different bias polarities/strengths to find out the origin of the non-linear electrical characteristics and to find out the applicable bias window for ensuring the gate responsive operation as biosensor. Depending on the polarity and strength of the drain bias (VDS)/liquid gate voltage (VGS), conduction is found to be confined in the bulk, in the surface accumulation region or in a path that comprises both accumulation region and the bulk thereby results in the diode or transistor like non-linear output characteristics. Although the SiNW is long without any junctions in the source/drain, the transfer characteristics exhibits a distinct shift with the increase of VDS when VDS is positive. The non-linear characteristics of SiNW is analysed to extract the gate responsive biasing window and it is found that the positive VDS application provides the flexibility of choosing diverse range of VDS and VGS voltages. As such, the shift in the transfer characteristic which is not appreciated for logic operation appears to be beneficiary for nanowire biosensors. This study sheds light on the variable electrical characteristics of SiNW reported in the literature and shows the way to choose appropriate bias conditions for biosensing applications.

Band Structure and Quantum Conductance of Surface-unsaturated and Hydrogenated Sb and Bi Monolayer Nanoribbons

Electronic structure and quantum conductance of surface-unsaturated and hydrogenated Sb and Bi monolayer nanoribbons are theoretically investigated by first-principles calculations combined with non-equilibrium Green’s function method. Band structures, electronic transmission spectra and current-voltage curves of these Sb and Bi monolayer derived nanoribbons along zigzag crystallographic orientations are calculated to explore their potential applications in topological nanoelectronics. It is verified that extremely high conductivity under low bias voltage is acquired from the scattering-forbidden topological edge-states of these nanoribbons, as indicated by Dirac-point-like energy dispersion of band-edges near Fermi level, which also provides an evident negative differential conductance under 0.2 ∼ 0.3 V voltage bias when the ballistic conductance peak at Fermi level shifting out of bias window. The present study suggests Sb and Bi monolayers after acquiring chemical stability by hydrogenation are prospective candidates to be applied for ultrahigh power and zero-loss nanotransistors.

Trans-polyacetylene based organic spin valve for a multifunctional spin-based device: A first principle analysis

Carbon-based materials are the promising candidates for the design of multifunctional spintronic devices, owing to their fascinating inherent attributes such as the ballistic transport nature, long spin coherence time, etc. Here, we report the spin-dependent electron transport properties of a trans-polyacetylene channel sandwiched between two semi-infinite hydrogen-passivated zigzag graphene nanoribbon (ZGNR) electrodes within the framework of spin-polarized density functional theory (DFT) and non-equilibrium Green's function (NEGF) formalisms. The proposed device exhibits an excellent spin-dependent transport behavior within the bias window of [-0.5 V, 0.5 V]. In parallel configuration (PC), the device offers an ideal spin filtration efficiency of almost 100% within the bias range of [-0.4 V, 0.4 V], and in antiparallel configuration (APC), it exhibits negative differential resistance (NDR), dual-spin rectification, and dual-spin filtration effects. Also, a high tunnel magnetoresistance (TMR) of 1.2× 10 5 % is achieved. To validate the aforementioned effects, the transmission spectra along with the area enclosed by the curve, eigenvalues, eigenstates, and quantum conductance have also been analyzed. In addition, the impact of hybridization and torsion defects on the spin-dependent transport phenomenon through trans-polyacetylene has been investigated. Furthermore, the modelled device is observed to exhibit the spin Seebeck effect in PC under the influence of temperature gradient. The proposed device may stand as a good contender for various spintronic and spin caloritronic applications. • First principle based analysis of a trans-polyacetylene based multifunctional spin-based device. • The device displays a perfect spin filtration efficiency and dual rectification effects. • A high tunnel magnetoresistance (TMR) of 1.2× 10 5 % is achieved. • Thermal Spin Seebeck and spin filtering effects are observed in the proposed model. • A potential candidate in the field of spintronic and spin caloritronic applications.

The tunnel-current and photon-induced current in nanoscale photovoltaics

In this work, the photo-induced current in a nanoscale photovoltaic device consisting of a quatum dot (QD) that is coupled to two metallic electrodes is derived by using the nonequilibrium Green function (NEGF) technique. The QD is assumed to have two levels and only one of them is within the bias window. The dependence of photocurrent on the photon’s energy and the level widths is analysed. This study shows the photocurrent could be maximized for a given range of photons’ energy which relates to the gap between levels and the level widths.

Applying bias voltage to improve rectification performance of molecular junction through side gate-group

Molecular diode is one of the most significant molecular devices for it being promised as electronic component in the molecular logic circuit. Controlling electronic transport to improve the rectification ratio of molecular diode is a crucial target for researchers. Here, based on the density functional theory and non-equilibrium Green's function method, the rectification performances of diblock co-oligomer molecules are investigated systematically. The numerical results show that, side gate-group can slightly enhance rectification ratios of diblock-molecule diodes, but it produces evident hindering effects on the electronic transports of the molecular junctions. Using electrode protrusion to contact with the side gate-group, the bias voltage is introduced to modulate the electronic transports of molecular junctions. By this strategy, the voltage at which the lowest unoccupied molecular orbital (LUMO) enters bias window is lowered evidently in the positive bias regime, which further significantly enhances the rectification ratios of the molecular diodes. The calculations additionally show that the LUMO is less delocalized under positive bias than it under negative bias, which is disadvantageous for enhancing the rectification performances of diblock co-oligomer molecular diode, thus very large rectification ratios are difficult to be achieved when one fabricates diblock co-oligomer molecular diodes in experimental fabrications.

Achieving High-Performance Molecular Rectification through Fast Screening Alkanethiol Carboxylate-Metal Complexes Electroactive Units

Achieving High-Performance Molecular Rectification through Fast Screening Alkanethiol Carboxylate-Metal Complexes Electroactive Units

Spin-polarized transport properties of the FeCl2/WSe2/FeCl2 van der Waals heterostructure

The discovery of the giant magnetoresistance effect has led to the rapid development of spintronics. Although the half-metals can provide a 100% spin polarization rate and significantly improved giant magnetoresistance, the materials with low Curie temperatures present challenges for their use at room temperature. In an attempt to identify the half-metallic material with high Curie temperatures for spintronics, this study investigates a van der Waals heterostructure with vertically integrated FeCl2/WSe2/FeCl2. The spin-polarized transport properties of the device based on the heterostructure studied by the density function theory combined with nonequilibrium Green's function reveal comprehensive spintronics functions, including giant magnetoresistance, spin filtering, and negative differential resistance effect. The mechanism of the negative differential resistance effect has further been elucidated by the band alignment of the heterostructure under different biases within the bias window.

Regional and Single Nucleotide Correction of Sequence Bias in Chromatin Accessibility Data

Chromatin accessibility assays directly measure which regions of DNA are actively regulated in the genome. Genomic chromatin accessibility data provide the gold standard of spatial and temporal resolution of regulatory landscapes in various cell types and conditions. Accessibility assays depend on enzymatic digestion of DNA and these enzymes are biased with respect to the sequences they recognize and cut. Sequence biases can lead to incorrect conclusions about the nature of the chromatin landscape within an experiment. Previous work from our group has shown that we can use observed k‐mer cut frequencies and k‐mer representation in the genome to correct the majority of this bias. However, these k‐mer scaling approaches did not effectively correct ATAC‐seq data, which assesses chromatin accessibility using the Tn5 transposase enzyme. Here, we find that the oligomeric state of Tn5 and wide sequence bias window precludes effective k‐mer scaling to account for Tn5 sequence bias. Tn5 sequence specificity is influenced by sequence content up to ten bases away from the Tn5 insertion site in both directions. In comparison, Benzonase, Cyanase, MNase, and DNase are primarily influenced by the sequence within 3 bases of the cleavage site. We could not effectively apply k‐mer scaling to ATAC‐seq data because the majority of insertion sites happen at unique 20‐mers in the genome and the method requires many genomic instances per k‐mer. To overcome this limitation, we applied an interpretable machine learning approach, prediction rules ensemble, to correct Tn5‐digested naked DNA. Application of a prediction rules ensemble effectively corrects local (single nucleotide resolution) sequence bias as well as sequence biases influenced by regional GC content. Correction of Tn5 sequence biases is important as the field continues to generate ATAC‐seq data at accelerating rates and use these data to accurately infer regulatory element activity.

Sensitivity implications for programmable transistor based 1T-DRAM

In this letter, we report an insightful evaluation of the base performance and sensitivity of 1T-DRAM implemented through a programmable or reconfigurable transistor (RFET). Although RFET based 1T-DRAM exhibits enhanced retention time (>64 ms at 85 °C and 125 °C) with an appreciable sense margin (∼16 µA/µm) and total length scalability down to 50 nm, metrics are sensitive to biases, parameter variations and traps. RFET exhibiting highest retention is expected to offer a relatively narrow operating window (~75 mV) for read bias. This stringent criterion can be relaxed with a wider read bias window (~150 mV) in RFET architectures exhibiting retention lower than maximum but still higher than >64 ms.

Length dependence of magnetoresistance in organic spin valves

With the Su–Schrieffer–Heeger model and Green's function method, the length dependence of magnetoresistance in organic spin valves is calculated in the frame of tunneling transport. Based on different energy level alignments between the molecule and the electrodes, the length effect on the magnetoresistance is investigated in three transport schemes, barrier tunneling, resonant tunneling, and transition between them. In the first scheme, a length-induced exponential or linear decline of the magnetoresistance is obtained. An oscillation and nonlinear dependence are observed in the second and third schemes. The mechanism is explained by investigating the transmission spectra at different lengths, where the evolution of the efficient transmission in the bias window with length differs in different schemes. The results agree with many experimental measurements qualitatively.

Effect of Impurity Adsorption on the Electronic and Transport Properties of Graphene Nanogaps.

Graphene stands out as a versatile material with several uses in fields that range from electronics to biology. In particular, graphene has been proposed as an electrode in molecular electronics devices that are expected to be more stable and reproducible than typical ones based on metallic electrodes. In this work, we study by means of first principles, simulations and a tight-binding model the electronic and transport properties of graphene nanogaps with straight edges and different passivating atoms: Hydrogen or elements of the second row of the periodic table (boron, carbon, nitrogen, oxygen, and fluoride). We use the tight-binding model to reproduce the main ab-initio results and elucidate the physics behind the transport properties. We observe clear patterns that emerge in the conductance and the current as one moves from boron to fluoride. In particular, we find that the conductance decreases and the tunneling decaying factor increases from the former to the latter. We explain these trends in terms of the size of the atom and its onsite energy. We also find a similar pattern for the current, which is ohmic and smooth in general. However, when the size of the simulation cell is the smallest one along the direction perpendicular to the transport direction, we obtain highly non-linear behavior with negative differential resistance. This interesting and surprising behavior can be explained by taking into account the presence of Fano resonances and other interference effects, which emerge due to couplings to side atoms at the edges and other couplings across the gap. Such features enter the bias window as the bias increases and strongly affect the current, giving rise to the non-linear evolution. As a whole, these results can be used as a template to understand the transport properties of straight graphene nanogaps and similar systems and distinguish the presence of different elements in the junction.