Thick Junction Research Articles

Putting Order into PM6:Y6 Solar Cells to Reduce the Langevin Recombination in 400 nm Thick Junction

Increasing the active layer thickness without sacrificing the power conversion efficiency (PCE) is one of the great challenges faced by organic solar cells (OSCs) for commercialization. Recently, PM6:Y6 as an OSC based on a non‐fullerene acceptor (NFA) has excited the community because of its PCE reaching as high as 15.9%; however, by increasing the thickness, the PCE drops due to the reduction of the fill factor (FF). This drop is attributed to change in mobility ratio with increasing thickness. Furthermore, this work demonstrates that by regulating the packing and the crystallinity of the donor and the acceptor, through volumetric content of chloronaphthalene (CN) as a solvent additive, one can improve the FF of a thick PM6:Y6 device (≈400 nm) from 58% to 68% (PCE enhances from 12.2% to 14.4%). The data indicate that the origin of this enhancement is the reduction of the structural and energetic disorders in the thick device with 1.5% CN compared with 0.5% CN. This correlates with improved electron and hole mobilities and a 50% suppressed bimolecular recombination, such that the non‐Langevin reduction factor is 180 times. This work reveals the role of disorder on the charge extraction and bimolecular recombination of NFA‐based OSCs.

Visible-to-near-infrared organic photodiodes with performance comparable to commercial silicon-based detectors

Photodetection in the visible and near-infrared (NIR) spectral regions offers a wide range of applications, such as image arrays of high pixel density and artificial intelligence. In this work, broadband organic photodiodes (OPDs) are developed with performance comparable to that of crystalline silicon-based commercial devices. Through the strategy of combining an NIR-absorbing non-fullerene acceptor and a thick junction, the resulting devices show significantly improved performance parameters, with a suppressed dark current density of 0.35 nA/cm2, an enhanced spectral response covering 300–1000 nm, and external quantum efficiency over 60% el/ph. Owing to the low dark current noise and high responsivity to NIR wavelengths, an unexpectedly high specific detectivity of 5.1 × 1013 Jones at 930 nm is obtained together with a linear dynamic range of 157 dB and a −3 dB cutoff frequency of 4.5 kHz. These results reveal that the NIR OPD has great potential for 2D and 3D imaging applications with a high frame rate and multiple band selection.

First principle studies of rubidium lead halides towards photovoltaic application

All inorganic perovskites have attracted enormous attention of late owing to their outstanding applications in photovoltaic area including highly stable perovskite solar cells. In-depth understanding of the optoelectronic and transport properties of such materials are vital for practical implementation of the same. The carrier transport properties of the electronic devices based on perovskite materials significantly depend on the effective mass of the respective charge carriers. Here, we have performed first principle calculations with FP-LAPW method for the orthorhombic rubidium lead halides (RbPbX3, where X = I, Br, Cl) to study the optoelectronic and transport properties. The reduced effective mass is found to be minimum for RbPbI3, suggesting an efficient charge carrier transport in the corresponding material. Our calculated values such as the dielectric constants, refractive indices, absorption coefficients and reflectivities show good agreement with reported experimental data. Excitons for RbPbI3 and RbPbBr3 are of Mott–Wannier type while it is of Frenkel type for RbPbCl3. Moreover, estimated band edges of RbPbI3 show a favorable charge carrier transport when it is used with TiO2. Spectroscopic limited maximum efficiency (SLME) of 11.59% can be achieved for 500 nm thick single junction solar cell of RbPbI3.

(Invited) Nanostructures and Crystal Defects in Thick GaN and SiC Epitaxial Layers for Power Electronic Switches

The state-of-the-art power switching devices made from SiC and GaN semiconductors contain a high density of crystal defects. Most of these defects are present in starting wafers and some are generated during device processing. There is little conclusive evidence so far on the exact role that the crystal defects play on device performance, manufacturing yield, and more importantly, long-term field-reliability especially when devices are operating under extreme stressful environments. This paper provides a review of the current state-of-the-art of SiC and GaN power semiconductor material technology, and the potential impact crystal defects may have on high-density power switching electronics. Group III-Nitride materials suffer from both extended and point defects, each of which will challenge the material’s application in both vertical and lateral power devices. The extended defects include vertical threading dislocations of both edge and screw type. The latter defects have been shown to be correlated to leakage in vertical two terminal device structures while the influence of the former is still undetermined and remains a critical research issue. Channel surfaces in vertical three terminal devices will also degrade due to vertical threading dislocations. These extended defects occur in all epitaxial layers grown on c-plane substrates (the predominant and largest area substrate type) and are the result of the lack of a high quality substrate bulk material as well as substrate surface. The present substrate technology after multiple efforts (either GaN, SiC or Si) still have a high defect density indicating that low-defect seeds may be non-existing. Point defects influence the background carrier concentration in low-doped layers and various recombination processes. The intrinsic nature of III-N materials makes nitrogen-vacancies a predominant point defect that automatically dopes the crystal n-type, but there are impurities (oxygen and carbon) from most growth environments that also contribute to conductivity. These defects are not well understood and not well controlled. Such defects must be minimized in order to realize thick epitaxial layers with drift regions that will support both high blocking voltages and low on-state resistances. A wide range of materials characterization methods have been applied to further understand these impurities. The semiconductor and dielectric materials used in the construction of power electronics switching devices employed for energy efficiency applications experience extreme electrical and thermal stresses during the on-state as well as when switching high voltages and high currents. For example, in a bipolar type power semiconductor switching device in order to achieve significant conductivity modulation and reduce the drift-region resistance, high-level injection of minority carriers is typical. If the semiconductor contains a high density of crystal defects, as is the case with WBG semiconductors, minority carrier recombination phenomenon can be adversely affected. For example, it has been shown more than a decade ago [3, 4] that basal plane dislocations (BPDs) present in SiC are excited by the energy released from minority carrier recombination process; the result is excess BPD generation and its glide within the semiconductor material that leads to an increase in the drift-region resistance and thermal run-away in a forward biased bipolar junction diode The reported experimental data for high-voltage SiC semiconductor strongly suggests that material defects have profound impact on its characteristics under extreme environments. Since GaN semiconductor has even higher density of material defects, this problem is further exacerbated especially in thick epitaxial GaN layers which are required for power switches beyond 5kV. The integration of the pillar junction scheme with the trench-gate GaN MOSFET processes developed at the University of Maryland will be reported. The thick pillar junction epitaxial layers have been grown by a commercial vendor, using a windowed GaN growth process and/or selective deep etching, followed by GaN regrowth to fabricate columns with opposite carrier polarities. The doping density of the n--pillars under the gate has been specified with background carrier concentration n ≤ 1x1016 cm-3, while the p-pillars will have a higher doping density (p ≥ 1x1018 cm-3) to maximize depletion in the n--pillar. The p-body and n+-source layers are grown by subsequent epitaxial growth across the whole structure. Our current research into optimized trench gate fabrication and gate dielectric processing has been used to fabricate gate trenches into the structure. These devices can be compared to traditional trench MOSFETs with extremely thick blocking layers of comparable thickness to the pillar junction layer in order to determine the role of defects in thick GaN epi layers in terms of limiting device performance and reliability.

On the Electro‐Optics of Carbon Stack Perovskite Solar Cells

Mesoporous carbon stack architecture is attracting considerable interest as a candidate for scalable, low‐cost perovskite solar cells amenable to high‐throughput manufacturing. These cells are characterized by microns‐thick mesoporous titania and zirconia layers capped by a nonselective carbon electrode with the whole stack being infused with a perovskite semiconductor. Although the architecture does not deliver the >20% power conversion efficiencies characteristic of perovskite planar and mesoporous geometries, it does produce cells with respectable efficiencies >16%, which is unexpected due to the carbon electrode being a nonideal anode and the active layers being so thick. Optimization of these cells requires an understanding of the coupled efficiencies of light absorption, charge generation, and extraction which is currently unavailable. Herein, a combined experimental‐simulation study that elucidates photogeneration and extraction is reported. By determining the optical constants of the individual components and using effective‐medium approximations, the internal quantum efficiencies (IQE) in both the titania and zirconia layers are determined to be ≈85%. Numerical drift‐diffusion simulations indicate that this high IQE is a consequence of the thick junctions reducing minority carrier concentrations at the electrodes, thereby decreasing surface recombination. This insight can now be used to tune the carbon stack for efficiency and simplicity.

Effects of hot pressing temperature on the alignment and phase composition of hot-deformed nanocrystalline Nd-Fe-B magnets

Abstract The magnetic properties, alignment and phase composition of nanocrystalline Nd-Fe-B magnets hot deformed (HD) at 830 °C employing different hot-pressed precursors fabricated at various hot pressing (HP) temperatures were investigated. Both the remanence and coercivity simultaneously increased first and then decreased with the HP temperature from 450 °C to 650 °C. The optimum magnetic properties of HD magnets were obtained at HP temperature of 550 °C with remanence and coercivity of 1.40 T and 924 kA/m, respectively. The usage of the HP temperatures lower than 500 °C was resulted in high porosity and low crystallization degree for hot pressed magnets, causing aggregation of the RE-rich phase and small amount of residual amorphous after hot deformation. This procedure increased the volume fraction of nonmagnetic phase and reduced magnetic properties. Furthermore, the platelet-shaped grains were separated by thick triple junctions, which deteriorated the alignment of grains. When HP temperature was 550 °C, the hot-deformed magnet gained the optimum alignment, and the stacked platelet-shaped grains were separated by smooth thin grain boundary. However, at HP temperature of 650 °C equiaxed coarse grains were dominant instead of platelet-shaped grains in the hot-deformed magnet. The high HP temperature led to apparent grain growth, and then the nucleated grains further grew and were hard to be deformed, developing a poor alignment. A schematic model of the microstructure evolution for hot-deformed magnet is proposed.

Graphene‐Based Transparent Conducting Electrodes for High Efficiency Flexible Organic Photovoltaics: Elucidating the Source of the Power Losses

Solution processed flexible organic solar cells (OSCs) are of interest due to their potential use as environmentally friendly, shapeable, or wearable energy. Such flexible devices require compatible transparent conducting electrodes (TCEs). The use of three‐layer graphene as a useful TCE for flexible OSCs is reported. The conformal coating of the graphene‐based TCE with good retention of performance was achieved using a bulk heterojunction (BHJ) active layer comprised of the non‐polymeric molecular (5Z,5′Z)‐5,5′‐[(5‴,5‴‴′‐{4,8‐bis[5‐(2‐ethylhexyl)‐4‐n‐hexylthiophen‐2‐yl]benzo[1,2‐b:4,5‐b′]dithiophene‐2,6‐diyl}bis{3′,3″,3‴‐tri‐n‐hexyl‐[2,2′:5′,2″:5″,2‴‐quaterthiophene]‐5‴,5‐diyl})bis(methanylylidene)]bis[3‐n‐hexyl‐2‐thioxothiazolidin‐4‐one] (BQR) donor and [6,6]‐phenyl‐C71‐butyric acid methyl ester (PC71BM) as the acceptor. This material combination enables thick BHJ junctions to be used so that the roughness of the graphene surface did not lead to shorted devices. The best graphene/poly(ethylene terephthalate) (PET) devices (PET/graphene/molybdenum oxide/BHJ/calcium/aluminum) show a photoconversion efficiency (PCE) of 5.8%, which while excellent was lower than that of a similar device architecture that used ITO/glass as the anode. The power losses of the graphene/PET‐based cells mainly resulted from absorption losses caused by the optical profile distribution in the device and the relatively high sheet resistance of the anode, leading to an 18% decrease in the short‐circuit current and lower fill factor, respectively.

Thickness Limitation of Band-to-Band Tunneling Process in GaAsSb/InGaAs Type-II Tunnel Junctions Designed for Multi-Junction Solar Cells

This article reports on the impact of the thickness and/or the composition on the performance of type-II n+ InGaAs/p+ GaAsSb tunnel junctions. The InGaAs/GaAsSb staggered band-offset heterojunction is expected to improve tunneling properties. Devices have been grown by molecular beam epitaxy with various thicknesses and/or Sb and In concentrations. For thin elastically strained type-II tunnel junctions, the electrical characteristics exhibit degraded transport performances compared to the reference p+ GaAs/n+ GaAs tunnel junction structures, while much better tunneling peak currents are achieved with strain-relaxed thick type-II tunnel junctions. Based on a theoretical analysis of the local density of states and the band-edges profiles of the type-II tunnel junctions, we propose a suitable design for type-II tunnel junctions with high tunneling current density toward their use in multi-junction solar cells.

Thick junction photodiodes based on crushed perovskite crystal/polymer composite films

Thick junction perovskite photodiodes are achieved with decent broadband photoresponse, enhanced device stability and compelling fast response of 800 ns.

Multimodal quantitative magnetic resonance imaging analysis with individualized postprocessing in patients with drug-resistant focal epilepsy and conventional visual inspection negative for epileptogenic lesions

OBJECTIVES:Approximately one-third of candidates for epilepsy surgery have no visible abnormalities on conventional magnetic resonance imaging. This is extremely discouraging, as these patients have a less favorable prognosis. We aimed to evaluate the utility of quantitative magnetic resonance imaging in patients with drug-resistant neocortical focal epilepsy and negative imaging.METHODS:A prospective study including 46 patients evaluated through individualized postprocessing of five quantitative measures: cortical thickness, white and gray matter junction signal, relaxation rate, magnetization transfer ratio, and mean diffusivity. Scalp video-electroencephalography was used to suggest the epileptogenic zone. A volumetric fluid-attenuated inversion recovery sequence was performed to aid visual inspection. A critical assessment of follow-up was also conducted throughout the study.RESULTS:In the subgroup classified as having an epileptogenic zone, individualized postprocessing detected abnormalities within the region of electroclinical origin in 9.7% to 31.0% of patients. Abnormalities outside the epileptogenic zone were more frequent, up to 51.7%. In five patients initially included with negative imaging, an epileptogenic structural abnormality was identified when a new visual magnetic resonance imaging inspection was guided by information gleaned from postprocessing. In three patients, epileptogenic lesions were detected after visual evaluation with volumetric fluid-attenuated sequence guided by video electroencephalography.CONCLUSION:Although quantitative magnetic resonance imaging analyses may suggest hidden structural lesions, caution is warranted because of the apparent low specificity of these findings for the epileptogenic zone. Conversely, these methods can be used to prevent visible lesions from being ignored, even in referral centers. In parallel, we need to highlight the positive contribution of the volumetric fluid-attenuated sequence.

Rotator Cuff Tear Morphologic Parameters at Magnetic Resonance Imaging: Relationship With Muscle Atrophy and Fatty Infiltration and Patient-Reported Function and Health-Related Quality of Life.

The aim of this study was to determine the relationship between rotator cuff tear (RCT) morphologic parameters and muscle atrophy and fatty infiltration, and patient-reported outcome measures, in patients with symptomatic full-thickness RCT. Rotator cuff tear location, length, width, thickness, and musculotendinous junction position were assessed in 57 magnetic resonance imaging scans and correlated to the outcome measures using multivariate regression analysis. Supraspinatus tendon tear length (odds ratio [OR], 2.218; 95% confidence interval [CI], 1.460-3.370), supraspinatus musculotendinous junction position (OR, 2.037; 95% CI, 1.322-3.137), and infraspinatus tendon tear width (OR, 2.371; 95% CI, 1.218-4.615) were identified as the strongest determinants of supraspinatus muscle atrophy, supraspinatus muscle fatty infiltration, and infraspinatus muscle fatty infiltration, respectively. The extent of supraspinatus tendon and musculotendinous junction retraction influences the development of supraspinatus muscle atrophy and fatty infiltration, whereas the extent of infraspinatus tendon tear width influences the development of infraspinatus muscle fatty infiltration. Morphologic parameters defining RCT at magnetic resonance imaging did not correlate with clinical shoulder function scores.

Current phase relation from graphs and diagrams and application to thick ferromagnetic Josephson junctions

In this work we present a method of representing terms in the current-phase-relation of a ballistic Josephson junction by combinations of diagrams, used in previous work to represent an equivalent of the matching condition determinant of the junction. This is accomplished by the expansion of the logarithm of this determinant in Taylor series and keeping track of surviving terms, i.e. terms that do not annihilate each other. The types of the surviving terms are represented by connected graphs, whose points represent diagrammatic terms of the determinant expansion. Then the theory is applied to obtain approximations of the current-phase relation of relatively thick ballistic ferromagnetic Josephson junctions with non-collinear magnetizations. This demonstrates the versatility of the method in developing approximations schemes and providing physical insight into the nature of contributions to the supercurrent from the available particle excitations in the junction. We also discuss the strong second harmonic contribution to the supercurrent in junctions with three mutually orthogonal magnetization vectors and a weak intermediate ferromagnet.

Room temperature current modulation in large area electronic junctions of spin crossover thin films

We report large-area (∼3 mm2), pinhole free crossbar junctions of thin films of the molecular complex [Fe(HB(tz)3)2] displaying spin transition around 336 K. The charge transport in the thinner junctions (10 and 30 nm) occurs by a tunneling mechanism, which is not affected substantially by the spin transition. The thicker junctions (100 and 200 nm) exhibit rectifying behavior and a reproducible drop of their electrical resistance by ca. 65–80% when switching the molecules from the high-spin to the low-spin state. This current modulation is ascribed to a bulk-limited charge transport mechanism via a thermally activated hopping process. The demonstrated possibility of resistance switching in ambient conditions provides appealing prospects for the implementation of molecular spin crossover materials in electronic and spintronic devices.

Sexual dimorphism of enamel area, coronal dentin area, bicervical diameter and dentinoenamel junction scallop area in longitudinal ground section.

Introduction:Sex determination plays an important role in forensics; several studies done using radiographs, model cast and animal teeth have shown that males possess larger tooth crown and more dentin than that of the females. As physical sectioning of tooth provides more accurate measurement of enamel thickness when compared with other methods, the present study was done to evaluate and compare enamel area (EA), coronal dentin area (CDA), bi-cervical diameter (BCD), average enamel thickness (AET) and dentinoenamel junction scallop area (DEJ-SA) in longitudinal ground sections of first premolars between males and females.Materials and Methods:A total of 60 extracted first premolar teeth were used for the study, of which 30 were from male and 30 were from female. A longitudinal ground section of 15 maxillary and 15 mandibular premolars of approximately 50 μm was prepared buccolingually at the center of each tooth and mounted on the slide. Multiple photomicrographs were captured with the help of Image Analysis System Software-Progres, Speed XT core 3. EA, CDA, length of the dentinoenamel junction and BCD were measured. Average DEJ-SA and AET were also calculated.Results:Mann–Whitney U- test was used for statistical analysis. It was found that EA and AET were significantly greater in females than in males. On the contrary, it was found that CDA was significantly greater in males than in females. However, no significant difference was found in BCD and DEJ-SA between males and females.Conclusion:Permanent first premolar can be reliably used in the field of forensic in establishing gender of individuals by measuring its EA, CDA and AET.

Thermo-mechanical behaviour of primary system components of future FBR during crash Cooling: A numerical simulation

In a pool-type fast breeder reactor, the shutdown by SCRAM involves crash cooling of primary sodium at the rate of 480 K/h. This high cooling rate can develop high thermal stresses at the thicker junctions that are immersed in sodium, due to a lower Fourier number and a higher Biot number, which results in higher Fitz number. Thermo-mechanical analysis of various critical locations within sodium with the transient loading, and parametric studies with varying cooling rates and thicknesses has been carried out. Due to the drop in sodium temperature during crash cooling, there is a fall in the sodium level, resulting from the volumetric contraction of sodium. Thus the locations of the hot pool components that are at the free level interface of sodium and argon experience an abrupt shift in the ambience, i.e., from sodium to argon cover gas. A numerical model has been developed to simulate the complex loading condition using CAST3M code. The model has been validated with theoretical equations and the overall heat balance equations. Transient analysis of all the hot pool components has been performed to estimate the structural damage due to thermo-mechanical behavior during crash cooling. Based on this study, it is concluded that for the future design of fast reactors with 60 years of design life, controlled cooling is not essential from the structural damage point of view. The recommendation simplifies the design of decay heat removal system.

Research on Crack Formation in Gypsum Partitions with Doorway by Means of FEM and Fracture Mechanics

Cracking damage in non-loadbearing internal partition walls is a serious problem that frequently occurs in new buildings within the short term after putting them into service or even before completion of construction. Damage in partition walls is sometimes so great that they cannot be accepted by their occupiers. This problem was illustrated by the example of damage in a gypsum partition wall with doorway attributed to deflection of the slabs beneath and above it. In searching for the deflection which causes damage in masonry walls, fracture mechanics applied to the Finite Element Method (FEM) have been used. For a description of gypsum behaviour, the smeared cracking material model has been selected, where stresses are transferred across the narrowly opened crack until its width reaches the ultimate value. Cracks in the Finite Element models overlapped the real damage observed in the buildings. In order to avoid cracks under the deflection of large floor slabs, the model of a wall with reinforcement in the doorstep zone and a 40 mm thick elastic junction between the partition and ceiling has been analysed.

Low roughness silver nanowire flexible transparent electrode by low temperature solution-processing for organic light emitting diodes

Highly smooth, conductive, and uniform flexible transparent composite film is produced by embedding silver nanowires (AgNWs) into polymer without high-temperature annealing. The insert of poly (vinyl alcohol) (PVA) layer enhances the hydrophilicity of the substrate surface and avoids the agglomeration of AgNWs. The thick junctions between nanowires are welded to the same height as a single wire by mechanical pressing, resulting in a great increase of the electrical conductivity and low surface roughness. The sheet resistance of the PVA/AgNW composite film reaches 3 Ω/sq with a transmittance of 83%, and does not show obvious change after tape test and substrate bending, suggesting good adhesion of AgNWs to the substrate and excellent flexibility. The composite film possesses better stability of resisting the chemical corrosion than conventional annealed-AgNW film. The scattering of AgNWs can break the waveguide mode and the total internal reflection, which consequently enhances the out-coupling efficiency in an organic light emitting diode (OLED). Flexible OLED employing the PVA/AgNW composite film as anode on polyethylene naphthalate (PEN) substrate presents a current efficiency improvement of 27% compared to controlled ITO-anode device with same structure, demonstrating its exciting prospect for applications in flexible optoelectronics.

Scalable GaSb/InAs Tunnel FETs With Nonuniform Body Thickness

GaSb/InAs heterojunction tunnel field-effect transistors are strong candidates in building future low-power integrated circuits, as they could provide both steep subthreshold swing and large ON-state current ($I_{\rm{ON}}$). However, at short channel lengths they suffer from large tunneling leakage originating from the small band gap and small effective masses of the InAs channel. As proposed in this article, this problem can be significantly mitigated by reducing the channel thickness meanwhile retaining a thick source-channel tunnel junction, thus forming a design with a non-uniform body thickness. Because of the quantum confinement, the thin InAs channel offers a large band gap and large effective masses, reducing the ambipolar and source-to-drain tunneling leakage at OFF state. The thick GaSb/InAs tunnel junction, instead, offers a low tunnel barrier and small effective masses, allowing a large tunnel probability at ON state. In addition, the confinement induced band discontinuity enhances the tunnel electric field and creates a resonant state, further improving $I_{\rm{ON}}$. Atomistic quantum transport simulations show that ballistic $I_{\rm{ON}}=284$A/m is obtained at 15nm channel length, $I_{\rm{OFF}}=1\times10^{-3}$A/m, and $V_{\rm{DD}}=0.3$V. While with uniform body thickness, the largest achievable $I_{\rm{ON}}$ is only 25A/m. Simulations also indicate that this design is scalable to sub-10nm channel length.

Optimal control problem in a domain with branched structure and homogenization

We consider a linear parabolic problem in a thick junction domain which is the union of a fixed domain and a collection of periodic branched trees of height of order 1 and small width connected on a part of the boundary. We consider a three‐branched structure, but the analysis can be extended to n‐branched structures. We use unfolding operator to study the asymptotic behavior of the solution of the problem. In the limit problem, we get a multi‐sheeted function in which each sheet is the limit of restriction of the solution to various branches of the domain. Homogenization of an optimal control problem posed on the above setting is also investigated. One of the novelty of the paper is the characterization of the optimal control via the appropriately defined unfolding operators. Finally, we obtain the limit of the optimal control problem. Copyright © 2016 John Wiley & Sons, Ltd.

Subthreshold Continuum Conductance Change in NbO Pt Memristor Interfaces

Bioinspired semiconductor-based devices with adaptive and dynamic properties will have many advantages over conventional static digital silicon-based technologies. The ability to compute, process, and retain information in parallel, without referencing other circuit elements, offers enhanced speed, storage density, energy efficiency, and functionality benefits. A novel crossbar microwire-based device consisting of Nb/NbO/Pt structure that exhibits neural synapse-like adaptive conductivity (i.e., synaptic plasticity) is presented. The neuromorphic memristive junction, formed at the interface between the Pt metal wire and the thermally annealed core–shell Nb–NbO wire, demonstrates 1000 times conductivity change with an effective continuum of resistance levels. The device can also be fully activated to display standard resistance switching between two states. In the subthreshold regime, the voltage flux applied through the ∼400 nm thick NbO junction is shown to have a linear relationship to the charge produc...