Bilayer Devices Research Articles

Artificial synapses based on HfOx/TiOy memristor devices for neuromorphic applications

As a result of enormous progress in nanoscale electronics, interest in artificial intelligence (AI) supported systems has also increased greatly. These systems are typically designed to process computationally intensive data. Parallel processing neural network architectures are particularly noteworthy for their ability to process dense data at high speeds, making them suitable candidates for AI algorithms. Due to their ability to combine processing and memory functions in a single device, memristors offer a significant advantage over other electronic platforms in terms of area scaling efficiency and energy savings. In this study, single-layer and bilayer metal-oxide HfOxand TiOymemristor devices inspired by biological synapses were fabricated by pulsed laser and magnetron sputtering deposition techniques in high vacuum with different oxide thicknesses. The structural and electrical properties of the fabricated devices were analysed using x-ray reflectivity, x-ray photoelectron spectroscopy, and standard two-probe electrical characterization measurements. The stoichiometry and degree of oxidation of the elements in the oxide material for each thin film were determined. Moreover, the switching characteristics of the metal oxide upper layer in bilayer devices indicated its potential as a selective layer for synapse. The devices successfully maintained the previous conductivity values, and the conductivity increased after each pulse and reached its maximum value. Furthermore, the study successfully observed synaptic behaviours with long-term potentiation, long-term depression (LTD), paired-pulse facilitation, and spike-timing-dependent plasticity, showcasing potential of the devices for neuromorphic computing applications.

Field-Driven Solid-State Defect Control of Bilayer Switching Devices.

We develop a framework for controlling and investigating reversible ionic transfer between two solid metal oxides layers by examining field-driven changes in electrical properties of the thin film bilayer oxide system Pr0.1Ce0.9O2/La1.85Ce0.15CuO4 (PCO/LCCO). We show that we can reversibly redistribute oxygen ions by applied voltage in a highly controlled and reversible fashion near ambient temperatures over large oxygen ion activity limits, which, for the first time, is directly interpretable by defect chemical models. This allowed us to determine how defect concentrations in each layer systematically varied with voltage and the subsequent impact on each film's conductance. These results showcase the relevance and applicability of defect chemical models, traditionally considered only at elevated temperatures, to the development of bilayer devices of importance to neuromorphic memory applications. This allows for a more systematic approach for studying and understanding the solid-solid exchange process in electrochemically controlled microelectronic devices. Moreover, our work sets the foundation for the development of large-area field-driven defect-controlled bilayer switching devices with potential application to a broad array of functionally modulated devices.

Self-rectifying and forming-free resistive switching with Cu/BN/SiO2/Pt bilayer device

Selector-free 3D vertical crossbar with low power consumption is necessary for developing high-density memory devices. Sneaking current in these devices is the prime factor for their degradation as they lose power, and there are glitches in data retrieval. In this article, we have successfully illustrated a forming free and self-rectifying cell in a chemically synthesized 2D memristive device based on hexagonal Boron Nitride (h-BN) material. The device performs well with a Rectification Ratio (RR) > 102 with SiO2/BN bi-layer structure at ±1.3 V. The device performed over 20000 cycles at a pulse width of 10 μs. Also, the device endurance decays with a different pulse width of 100 μs and 1 ms. Schottky barrier formation and development of conductive filament of oxygen vacancies are the primary causes for the proposed device to show Rectifying Resistive Switching (RRS) and Bipolar Resistive Switching (BRS), respectively. The device also performs good stability with the temperature range from Room Temperature (RT) to 250 °C. The sneak path concern and thermal stability enable potential applications for neuromorphic computing and future memory technology.

Anthracene derivatives with electron-transport units for non-doped blue fluorescent organic light-emitting diodes

Blue organic light-emitting diodes with non-doped emissive layer (EML) structures have been garnered increasing interest due to their convenience of manufacturing process. For this purpose, we designed and synthesized two novel anthracene derivatives, PO2PhAnTz and PO2PhAnBI. Both derivatives exhibited bright blue electroluminescence emission and a relatively low operating voltage with a simple bilayer device structure. The PO2PhAnBI with triphenyl-phosphine oxide moiety as host, anthracene core as dopant and benzimidazole as electron transport group, realized a balanced bipolar transport and attained a maximum external quantum efficiency of 5.74 %. The findings shed light on the molecule design role toward high-efficiency non-doped blue OLEDs.

Simplifying High-Density Memory: Exploiting Self-Rectifying Resistive Memory with TiO2/HfO2 Bilayer Devices

Self-rectifying resistive memory can reduce the complexity of crossbar array architecture for high density memory. It can replace integrated memory and selector with one self-rectifying cell. Such a simple structure can be applied for the vertical resistive memory. Both top and bottom interface between insulating layer and electrodes are crucial to achieve highly self-rectifying memory cell. In this study, bilayer devices composed of HfO2 and TiO2 were fabricated using atomic layer deposition (ALD) for the implementation of self-rectifying memory cells. The physical, chemical, and electrical properties of HfO2/TiO2 and TiO2/HfO2 sandwiched between Pt and TiN electrodes were investigated. By analyzing the conduction mechanism of bilayer devices, the higher rectification ratio of TiO2/HfO2 stack was due to the difference in height and the number of energy barriers.

Production of mono and bilayer devices for wound dressing by coupling of electrospinning and supercritical impregnation techniques

In this paper, electrospinning and supercritical impregnation were coupled to produce polyurethane fibrous membranes loaded with mesoglycan and lactoferrin. The proposed methodology allowed the production of three skin wound healing bilayer systems: a first system containing mesoglycan loaded through electrospinning and lactoferrin loaded by supercritical impregnation, a second system where the use of the two techniques was reversed, and a third sample where the drugs were both encapsulated through a one-step process. SEM analysis demonstrated the formation of microfibers with a homogeneous drug distribution. The highest loadings were 0.062 g/g for mesoglycan and 0.013 g/g for lactoferrin. Then, hydrophilicity and liquid retention analyses were carried out to evaluate the possibility of using the manufacturers as active patches. The kinetic profiles, obtained through in vitro tests conducted using a Franz diffusion cell, proved that the diffusion of the active drugs followed a double-step release before attaining the equilibrium after about 30 h. When the electrospun membranes were placed in contact with HUVEC, HaCaT, and BJ cell lines, as human endothelial cells, keratinocytes, and fibroblasts, respectively, no cytotoxic events were assessed. Finally, the capacity of the most promising system to promote the healing process was performed by carrying out scratch tests on HaCat cells.

Enhancing the Uniformity of a Memristor Using a Bilayer Dielectric Structure.

Resistive random access memory (RRAM) holds great promise for in-memory computing, which is considered the most promising strategy for solving the von Neumann bottleneck. However, there are still significant problems in its application due to the non-uniform performance of RRAM devices. In this work, a bilayer dielectric layer memristor was designed based on the difference in the Gibbs free energy of the oxide. We fabricated Au/Ta2O5/HfO2/Ta/Pt (S3) devices with excellent uniformity. Compared with Au/HfO2/Pt (S1) and Au/Ta2O5/Pt (S2) devices, the S3 device has a low reset voltage fluctuation of 2.44%, and the resistive coefficients of variation are 13.12% and 3.84% in HRS and LRS, respectively, over 200 cycles. Otherwise, the bilayer device has better linearity and more conductance states in multi-state regulation. At the same time, we analyze the physical mechanism of the bilayer device and provide a physical model of ion migration. This work provides a new idea for designing and fabricating resistive devices with stable performance.

TMM−Sim: A versatile tool for optical simulation of thin−film solar cells

The Transfer Matrix Method (TMM) has become a prominent tool for the optical simulation of thin−film solar cells, particularly among researchers specializing in organic semiconductors and perovskite materials. As the commercial viability of these solar cells continues to advance, driven by rapid developments in materials and production processes, the importance of optical simulation has grown significantly. By leveraging optical simulation, researchers can gain profound insights into photovoltaic phenomena, empowering the implementation of device optimization strategies to achieve enhanced performance. However, existing TMM−based packages exhibit limitations, such as requiring programming expertise, licensing fees, or lack of support for bilayer device simulation. In response to these gaps and challenges, we present the TMM Simulator (TMM−Sim), an intuitive and user−friendly tool to calculate essential photovoltaic parameters, including the optical electric field profile, exciton generation profile, fraction of light absorbed per layer, photocurrent, external quantum efficiency, internal quantum efficiency, and parasitic losses. An additional advantage of TMM−Sim lies in its capacity to generate outcomes suitable as input parameters for electro−optical device simulations. In this work, we offer a comprehensive guide, outlining a step−by−step process to use TMM−Sim, and provide a thorough analysis of the results. TMM−Sim is freely available, accessible through our web server (nanocalc.org), or downloadable from the TMM−Sim repository (for Unix, Windows, and macOS) on GitHub. With its user−friendly interface and powerful capabilities, TMM−Sim aims to facilitate and accelerate research in thin−film solar cells, fostering advancements in renewable energy technologies.

One‐Step Photo‐Patterning Process for Multi‐Modal and Transformative Structural Coloration: Toward Anti‐Counterfeiting Applications

AbstractThe selective interactions of light with matter enable the existence of colors in nature. Through the precise modification of material properties, photonic crystals, plasmonic nanoparticles, and dielectric metasurfaces can produce bright and vivid structural colors. However, this approach is rendered inadequate by its complex fabrication processes, limited working area, and lack of inherent material‐property tuning. Here, a novel one‐step fabrication process based on ultraviolet‐curable chitosan (UVCC) for generating structural multicolor patterns in polydimethylsiloxane in a single bending event is presented. UV exposure through a grayscale photomask controls the polymerization of the UVCC, enabling manipulation of its thickness, thereby allowing the production of size‐, wavelength‐, and amplitude‐controllable wrinkles under compressive stress. This enables the exhibition of selective structural coloration in response to incoming incident light. Upon the release of applied stresses, the wrinkles disappear, and the bilayer device becomes transparent again. Notably, repeating the process allows for dynamic covert–overt switching of the structural colors. This firm control over the active‐layer thickness, simple device structure, cost‐effectiveness, facile fabrication process, and vibrant‐color tuning ability make this one‐step photopatterning approach more competitive for structural‐color applications. Finally, the authenticity of a cosmetic cream is evaluated to demonstrate the anti‐counterfeiting effect of the UVCC circumference wrinkles.

Improved resistive switching characteristics of solution processed ZrO2/SnO2 bilayer RRAM via oxygen vacancy differential

In this study, a solution-processed bilayer structure ZrO2/SnO2 resistive switching (RS) random access memory (RRAM) is presented for the first time. The precursors of SnO2 and ZrO2 are Tin(Ⅱ) acetylacetonate (Sn(AcAc)2) and zirconium acetylacetonate (Zr(C5H7O2)4), respectively. The top electrode was deposited with Ti using an E-beam evaporator, and the bottom electrode used an indium–tin–oxide glass wafer. We created three devices: SnO2 single-layer, ZrO2 single-layer, and ZrO2/SnO2 bilayer devices, to compare RS characteristics such as the I–V curve and endurance properties. The SnO2 and ZrO2 single-layer devices showed on/off ratios of approximately 2 and 51, respectively, along with endurance switching cycles exceeding 50 and 100 DC cycles. The bilayer device attained stable RS characteristics over 120 DC endurance switching cycles and increased on/off ratio ∼2.97 × 102. Additionally, the ZrO2/SnO2 bilayer bipolar switching mechanism was explained by considering the Gibbs free energy (ΔG o) difference in the ZrO2 and SnO2 layers, where the formation and rupture of conductive filaments were caused by oxygen vacancies. The disparity in the concentration of oxygen vacancies, as indicated by the Gibbs free energy difference between ZrO2 (ΔG o = −1100 kJ mol−1) and SnO2 (ΔG o = −842.91 kJ mol−1) implied that ZrO2 exhibited a higher abundance of oxygen vacancies compared to SnO2, resulting in improved endurance and on/off ratio. X-ray photoelectron spectroscopy analyzed oxygen vacancies in ZrO2 and SnO2 thin films. The resistance switching characteristics were improved due to the bilayer structure, which combines a higher oxygen vacancy concentration in one layer with a lower oxygen vacancy concentration in the switching layer. This configuration reduces the escape of oxygen vacancies to the electrode during RS.

Room temperature chirality switching and detection in a helimagnetic MnAu2 thin film

Helimagnetic structures, in which the magnetic moments are spirally ordered, host an internal degree of freedom called chirality corresponding to the handedness of the helix. The chirality seems quite robust against disturbances and is therefore promising for next-generation magnetic memory. While the chirality control was recently achieved by the magnetic field sweep with the application of an electric current at low temperature in a conducting helimagnet, problems such as low working temperature and cumbersome control and detection methods have to be solved in practical applications. Here we show chirality switching by electric current pulses at room temperature in a thin-film MnAu2 helimagnetic conductor. Moreover, we have succeeded in detecting the chirality at zero magnetic fields by means of simple transverse resistance measurement utilizing the spin Berry phase in a bilayer device composed of MnAu2 and a spin Hall material Pt. These results may pave the way to helimagnet-based spintronics.

Comparative performance analysis of poly (3-hexylthiophene-2, 5-dial) and [6,6]-phenyl-C61 butyric acid methyl ester-based organic solar cells in bulk-heterojunction and bilayer structure using SCAPS

This work is concerned with the design and photovoltaic performance study of organic semiconducting materials-based solar cells using a SCAPS simulator. Organic solar cells are one form that absorbs light and produces electricity. They exhibit high power conversion efficiency, low manufacturing costs, lightweight, and flexibility which makes them a promising technology for renewable energy. We developed and designed organic solar cell devices with the structure ITO/PEDOT: PSS/P3HT: PCBM/PFN-Br/Al for bulk-heterojunction structure and ITO/PEDOT: PSS/P3HT: PCBM/PFN-Br/Al for bilayer structure where PEDOT: PSS and PFN-Br were used as hole and electron transporting layers (HTL and ETL) respectively. The short circuit current density versus voltage (J-V) characteristics as well as photovoltaic parameters namely open circuit voltage (VOC), short circuit current density (JSC), fill factor (FF), and power conversion efficiency (PCE) have been examined. In addition, the quantum efficiency (QE) of the two structures has been also studied. The effectiveness of the photovoltaic simulated systems has also been attempted to be enhanced by varying the absorber layer's thickness for both architectures. Later, this turned out that variations in the band gap of the absorber layer had an impact on the factors that determined how well solar cells performed. It has been found that the simulated bulk-heterojunction device exhibits 20.43% PCE whereas 3.52% PCE has been calculated from the simulated bilayer device.

Enhanced resistive switching performance and structural evolution of NiO/Nb2O5−x bilayer memristive device

The development of bilayer models is crucial for enhancing the performance and enabling multifunctionality of next-generation resistive random-access memory (RRAM). However, detailed switching information is still insufficient for multilayer RRAM, necessitating direct observation of the microstructural evolution to clarify its switching mechanism. In this study, the electrical properties of Pt/Nb2O5−x/Pt monolayer devices were improved by introducing a polycrystalline NiO layer, and the resulting Pt/NiO/Nb2O5−x/Pt bilayer devices exhibited a doubling of the endurance and a reduction of the set voltage. The bilayer device also exhibited a long retention time (>104 s) and high on/off ratio of 104. Transmission electron microscopy, X-ray photoelectron spectroscopy, and electron energy-loss spectroscopy revealed that the conductive filament (CF) contained oxygen vacancies. In addition, a funnel-shaped CF was observed in the Pt/NiO/Nb2O5−x/Pt system, with the neck at the NiO/Nb2O5−x interface. This study provides a distinctive viewpoint and an innovative strategy for improving RRAM devices and exploring new applications in electronics.

Simplified Bilayer TFETS with Oxide And Group-IV Semiconductor For P-Channel Operation

The paper proposes a p-channel bilayer TFET composed of an n-type oxide semiconductor (n-OS)/p-type group-IV semiconductor (p-IV) heterostructure, allowing for both n- and p-channel TFET operations under the same device structure. The p-IV side in the heterostructure has a metal-oxide-semiconductor (MOS) gate-stack that modulates the surface potential of the p-IV MOS interface and controls the band-to-band tunneling (BTBT) current during p-TFET operation. Technology computer-aided design (TCAD) simulation is used to predict the p-TFET operation with symmetric electrical characteristics to the n-TFET operation. The p-TFET operation of the n-ZnSnO/p-SiGe bilayer device is experimentally demonstrated on a SiGe-on-insulator substrate, under Si back-gate operation. Both n- and p-TFET operations are observed in an identical device by using the top- and back-gate electrodes. Keywords: Bilayer, n-TFET, p-TFET, SiGe, SiGe-on-insulator (SiGeOI), ZnSnO

Transport signatures of plasmon fluctuations in electron hydrodynamics

In two-dimensional electron systems, plasmons are gapless and long-lived collective excitations of propagating charge density oscillations. We study the fluctuation mechanism of plasmon-assisted transport in the regime of electron hydrodynamics. We consider pristine electron liquids where charge fluctuations are thermally induced by viscous stresses and intrinsic currents, while attenuation of plasmons is determined by the Maxwell mechanism of charge relaxation. It is shown that, while the contribution of plasmons to the shear viscosity and thermal conductivity of a Fermi liquid is small, plasmon resonances in the bilayer devices enhance the drag resistance. In systems without Galilean invariance, fluctuation-driven contributions to dissipative coefficients can be described only in terms of hydrodynamic quantities: intrinsic conductivity, viscosity, and plasmon dispersion relation.

Improved Resistive Switching Characteristics and Synaptic Functions of InZnO/SiO2 Bilayer Device

Improved Resistive Switching Characteristics and Synaptic Functions of InZnO/SiO2 Bilayer Device

Improved Uniformity of TaOx-Based Resistive Switching Memory Device by Inserting Thin SiO2 Layer for Neuromorphic System.

RRAM devices operating based on the creation of conductive filaments via the migration of oxygen vacancies are widely studied as promising candidates for next-generation memory devices due to their superior memory characteristics. However, the issues of variation in the resistance state and operating voltage remain key issues that must be addressed. In this study, we propose a TaOx/SiO2 bilayer device, where the inserted SiO2 layer localizes the conductive path, improving uniformity during cycle-to-cycle endurance and retention. Transmission electron microscopy (TEM) and X-ray photoelectron spectroscopy (XPS) confirm the device structure and chemical properties. In addition, various electric pulses are used to investigate the neuromorphic system properties of the device, revealing its good potential for future memory device applications.

Different Energetics at Donor:Acceptor Interfaces in Bilayer and Bulk‐Heterojunction Polymer:Non‐Fullerene Organic Solar Cells

To understand the limitations placed on the open‐circuit voltage of bulk heterojunction (BHJ) organic solar cells, the energy levels of neat donor and acceptor samples are often characterized and applied to study BHJ blends. However, energy levels derived from neat samples may not necessarily reflect those at the donor:acceptor interface in blends. The properties of organic semiconductors are sensitive to microstructural changes, with non‐fullerene acceptors (NFAs) in particular known to exhibit different thin‐film polymorphs. To investigate the influence of differences in molecular packing in neat and blend films, temperature‐dependent current–voltage characteristics are measured for bilayer (BL) and BHJ devices. Herein, the fullerene acceptor PC71BM is compared—whose energy levels are expected to be less sensitive to molecular packing—with the NFA ITIC, paired with the same donor polymer PTB7‐Th. It is found that the interfacial energy levels differ for BL and BHJ devices for the PTB7‐Th:ITIC system but remain the same for the PTB7‐Th:PC71BM system. Furthermore, X‐ray scattering measurements identify that ITIC exhibits a different packing mode in neat films and in BHJ blends. Such microstructure‐dependent differences between neat and blend samples need to be considered when studying energy losses in NFA BHJ solar cells.

Antibacterial and bioactive multilayer electrospun wound dressings based on hyaluronic acid and lactose-modified chitosan

Antibacterial multilayer electrospun matrices based on hyaluronic acid (HA) and a lactose-modified chitosan (CTL) were synthetized (i) by combining electrospun polycaprolactone (PCL) and polysaccharidic matrices in a bilayer device and (ii) by sequentially coating the PCL mat with CTL and HA. In both cases, the antibacterial activity was provided by loading rifampicin within the PCL support. All matrices disclosed suitable morphology and physicochemical properties to be employed as wound dressings. Indeed, both the bilayer and coated fibers showed an optimal swelling capacity (3426 ± 492 % and 1435 ± 251 % after 7 days, respectively) and water vapor permeability (160 ± 0.78 g/m2h and 170 ± 12 g/m2h at 7 days, respectively). On the other hand, the polysaccharidic dressings were completely wettable in the presence of various types of fluids. Depending on the preparation method, a different release of both polysaccharides and rifampicin was detected, and the immediate polysaccharide dissolution from the bilayer structure impacted the antibiotic release (42 ± 4 % from the bilayer structure against 25 ± 2 % from the coated fibers in 4 h). All the multilayer matrices, regardless of their production strategy and composition, revealed optimal biocompatibility and bioactivity with human dermal fibroblasts, as the released bioactive polysaccharides induced a faster wound closure in the cell monolayer (100 % in 24 h) compared to the controls (78 ± 8 % for untreated cells and 89 ± 5 % for cells treated with PCL alone, after 24 h). The inhibitory and bactericidal effects of the rifampicin loaded matrices were assessed on S. aureus, S. epidermidis, E. coli, and P. aeruginosa. The antibacterial matrices were found to be highly effective except for E. coli, which was more resistant even at higher amounts of rifampicin, with a bacterial concentration of 6.4 ± 0.4 log CFU/mL and 6.8 ± 0.3 log CFU/mL after 4 h in the presence of the rifampicin-loaded bilayer and coated matrices, respectively.

Dual-Color Lasers in Interlayer-Free Solution Processed Polymeric Bilayer Devices.

Multiwavelength organic lasers have attracted considerable interest in recent years due to the cost efficiency, wide luminescence coverage, and simple processability of organics. In this work, by simply spin coating immiscible polymeric gain media in sequence, dual-wavelength (blue-green or blue-red) amplified spontaneous emission (ASE) was achieved in bilayer devices. The blue emission, water/alcohol-soluble conjugated polyelectrolyte, poly[(9,9-bis(3'-((N,N-dimethyl)-N-ethylammonium)-propyl)-2,7-fluorene)-alt-2,7-(9,9-dioctylfluorene)]dibromide (PFN-Br), was used as the bottom layer. The commercially available nonpolar solvent soluble polymer poly(9,9-dioctylfluorene-co-benzothiadiazole) (F8BT) and its blend with poly(3-hexylthiophene) (P3HT) were used as the top active layers offering green and red emission, respectively. This novel compact configuration, without interlayers between the two active layers, offers potential for developing various applications. The carefully selected top and bottom layer polymers not only meet the conditions of immiscibility and different emission wavelength range but also have a common absorption band in UV, which allows simultaneous blue-green or blue-red dual-color ASE behaviors observed in the bilayer devices under the same 390 nm laser excitation. By introducing two-dimension (2D) square distributed feedback (DFB) gratings with different periods (300 nm for blue, 330 nm for green, and 390 nm for red) as cavities, single mode blue-green (Eth = 245 μJ cm-2) and blue-red (Eth = 189 μJ cm-2) lasers were achieved by focusing the excitation laser spot on different 2D DFB gratings area. Furthermore, we found it possible to gain sufficient light confinement for red emission along its diagonal direction (Λ ∼424 nm), whereas the 2D DFB gratings offer feedback for blue emission from the 300 nm period along the rectangle direction. Therefore, both blue and red lasers were eventually achieved in the same PFN-Br/F8BT:P3HT bilayer device on the single 2D DFB gratings with a period of 300 nm in this work.