Bilayer Research Articles

An Oxide-Based Bilayer ZrO₂/IGZO Memristor for Synaptic Plasticity and Artificial Nociceptor

As artificial intelligence technology advances, memristors are promising to be used as artificial synapses to mimic various biological learning behaviors. In this work, all the memristors are fabricated using 45-nm complementary metal–oxide–semiconductor (CMOS) technology on Si/SiO2 substrate. By inserting a 3-nm indium gallium zinc oxide (IGZO) layer into the Pt/ZrO2/TiN single-layer (SL) memristor, the storage window of the bilayer (BL) memristor is enhanced to 11 times that of the single one. It demonstrates the multilevel conductivity modulating properties of the BL memristors, which can be used to mimic the learning behavior of biological synapses. The Pt/ZrO2/IGZO/TiN memristor has achieved synaptic plasticity, including long-term potentiation/depression (LTP/LTD), spike-timing-dependent plasticity (STDP), and paired-pulse facilitation (PPF). Moreover, the inadaptation, threshold, hyperalgesia, and allodynia characteristics of a nociceptor based on Pt/ZrO2/IGZO/TiN memristor have been accomplished. The BL oxide-based memristor with outstanding synaptic properties and the simulation as an artificial nociceptor provides a feasible application in the future neuromorphic systems for bionic robots.

ZTO/MgO-Based Optoelectronic Synaptic Memristor for Neuromorphic Computing

Synapse having good linearity plays a vital role in the memory and computing of human brain. Therefore, the achievement of efficient learning process in neuromorphic computing by the implementation of the synaptic functions, such as long-term potentiation/ depression (LTP/LTD) and spike time-dependent plasticity (STDP) of two-terminal optoelectronic memristor device, is critical for the next-generation artificial intelligence. In this work, we improve the resistive switching and synaptic characteristics of a Zn2SnO4 (ZTO)-based optoelectronic synaptic memristor (OSM) by the insertion of an ultrathin MgO layer. For this bilayer (BL) structured OSM, the nonlinearities of LTP and LTD curves are improved to 1.96 and 0.33, respectively. Asymmetrical STDP response demonstrates the suitability of device toward the Hebbian learning. In addition, a Hopfield neural network (HNN) is successfully trained to recognize a 10 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\times10$ </tex-math></inline-formula> pixel input image with an accuracy of <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\sim 100$ </tex-math></inline-formula> % after 15 iterations. Under blue light (405 nm) illumination, OSM emulates the synaptic functions, such as paired pulse facilitation, learning experience behavior, and short- to long-term memory transition. The photoresponse and relaxation characteristics of the device depend on the ionization and neutralization of oxygen vacancies. This highly transparent ZTO/MgO-based OSM with the convergence of “nonvolatile electronic memory and visible light sensor” is suitable as an artificial synapse for neuromorphic computing applications.

Bilayer MSe2 (M = Zr, Hf, Mo, W) performance as a hopeful thermoelectric materials

Significant advancements in nanoscale material efficiency optimization have made it feasible to substantially adjust the thermoelectric transport characteristics of materials. Motivated by the prediction and enhanced understanding of the behavior of two-dimensional (2D) bilayers (BL) of zirconium diselenide (ZrSe2), hafnium diselenide (HfSe2), molybdenum diselenide (MoSe2), and tungsten diselenide (WSe2), we investigated the thermoelectric transport properties using information generated from experimental measurements to provide inputs to work with the functions of these materials and to determine the critical factor in the trade-off between thermoelectric materials. Based on the Boltzmann transport equation (BTE) and Barden-Shockley deformation potential (DP) theory, we carried out a series of investigative calculations related to the thermoelectric properties and characterization of these materials. The calculated dimensionless figure of merit (ZT) values of 2DBL-MSe2 (M = Zr, Hf, Mo, W) at room temperature were 3.007, 3.611, 1.287, and 1.353, respectively, with convenient electronic densities. In addition, the power factor is not critical in the trade-off between thermoelectric materials but it can indicate a good thermoelectric performance. Thus, the overall thermal conductivity and power factor must be considered to determine the preference of thermoelectric materials.

Analogy and dissimilarity of excitons in monolayer and bilayer of MoSe2

Excitons in thin layers of semiconducting transition metal dichalcogenides are highly subject to the strongly modified Coulomb electron–hole interaction in these materials. Therefore, they do not follow the model system of a two-dimensional hydrogen atom. We investigate experimentally and theoretically excitonic properties in both the monolayer (ML) and the bilayer (BL) of MoSe2 encapsulated in hexagonal BN. The measured magnetic field evolutions of the reflectance contrast spectra of the MoSe2 ML and BL allow us to determine g-factors of intralayer A and B excitons, as well as the g-factor of the interlayer exciton. We explain the dependence of g-factors on the number of layers and excitation state using first principles calculations. Furthermore, we demonstrate that the experimentally measured ladder of excitonic s states in the ML can be reproduced using the approach with the Rytova–Keldysh potential that describes the electron–hole interaction. In contrast, the analogous calculation for the BL case requires taking into account the out-of-plane dielectric response of the MoSe2 BL.

Effects of alloying elements on the interfacial segregation of bismuth in tin-based solders

Sn-Bi solders are potential candidates for replacing traditional lead-based solders in electronic packaging due to their low melting points, superior wettability, and mechanical properties. The grain coarsening due to the interfacial Bi segregation is a vital issue that affects the reliability of Sn-Bi solder joints. Through first-principles calculations, this study systematically revealed the effects of different alloying elements on the interfacial segregation of Bi towards Sn(100) surface. Bi tends to segregate to the top-surface of Sn due to the formation of vacancies on Sn surface. However, doping alloying elements into the Sn-Bi solders could effectively suppress the interfacial Bi segregation. The presence of elements such as Pd, Pt, and Au located at the adjacent surface layer of Bi could inhibit the Bi precipitation, by apparently increasing the segregation energies of Bi to the Sn surface. The results can be interpreted by the enhanced bond orders and charge transfer between Bi, alloying elements, and their neighboring Sn atoms. This study could provide valuable insights into the design of multi-component Sn-Bi solders with minor alloying elements doping in the future.

Electronic Properties of Single‐Layer and Bilayer Graphene Nanoribbons

Herein, a tight‐binding description is utilized to investigate electronic structures and density of states of single‐layer (SL) and bilayer (BL) graphene ribbons with and without the influence of external electric fields. Analyses are implemented to reveal the similarity and difference among electronic properties of three types of structures (specifically SL and BL ribbons with AA and AB stackings). Moreover, both armchair and zigzag edge orientations are considered. It is indicated in the results that variation in electronic properties of these structures in the presence of external electric fields depends on both structural form and edge orientation. The following two points are demonstrated: 1) a transverse field has a significant effect on adjusting bandgap of the zigzag configurations, whereas a vertical electric field has a distinct impact on energy bands of armchair edge structures, and 2) among considered structures, AB‐stacking BL ribbons are invariably the structures that are most strongly influenced by external fields. Interestingly, AB‐stacking BL ribbons are also capable of enlarging bandgap with both edge terminations. Herein, an insight into how the electronic structure and charge distribution of both SL and BL graphene nanoribbons can be controlled not only in armchair but also in zigzag edge terminations using electric stimulants is provided by the results.

Geometry-Dependent Magnetoelectric and Exchange Bias Effects of the Nano L-T Mode Bar Structure Magnetoelectric Sensor.

The geometry-dependent magnetoelectric (ME) and exchange bias (EB) effects of the nano ME sensor were investigated. The sensor consisted of the Longitudinal-Transverse (L-T) mode bi-layer bar structure comprising the ferromagnetic (FM) and ferroelectric (FE) materials and the anti-ferromagnetic (AFM) material. The bi-layer ME coefficient was derived from constitutive equations and Newton's second law. The trade-off between peak ME coefficient and optimal thickness ratio was realized. At the frequency × structure length = 0.1 and 1200, minimum and maximum peak ME coefficients of the Terfenol-D/PZT bi-layer were around 1756 and 5617 mV/Oe·cm, respectively, with 0.43 and 0.19 optimal thickness ratios, respectively. Unfortunately, the bi-layer could not distinguish the opposite magnetic field directions due to their similar output voltages. PtMn and Cr2O3, the AFM, were introduced to produce the EB effect. The simulation results showed the exchange field starting at a minimum PtMn thickness of 6 nm. Nevertheless, Cr2O3 did not induce the exchange field due to its low anisotropy constant. The tri-layer ME sensor consisting of PZT (4.22 nm)/Terfenol-D (18 nm)/PtMn (6 nm) was demonstrated in sensing 2 Tbit/in2 magnetic bits. The average exchange field of 5100 Oe produced the output voltage difference of 12.96 mV, sufficient for most nanoscale magnetic sensing applications.

Migration-Enhanced Metal–Organic Chemical Vapor Deposition of Wafer-Scale Fully Coalesced WS2 and WSe2 Monolayers

Metal–organic chemical vapor deposition (MOCVD) is widely employed for the wafer-scale synthesis of transition metal dichalcogenide (TMDC) monolayers (MLs). Despite large efforts devoted to understanding the intricate nucleation and lateral growth mechanisms of TMDCs, little attention has been paid to the migration of adatoms on the top of an ML and its influence on parasitic/premature bilayer (BL) nucleation. In this work, using a commercial multi-wafer MOCVD platform, a novel two-stage migration-enhanced MOCVD process is introduced to realize the deposition of wafer-scale fully coalesced tungsten disulfide (WS2) and tungsten diselenide (WSe2) MLs with only sparse BL nucleation in a reasonable deposition time. With the WS2 ML coverage exceeding 99% on 2 in. sapphire substrates within 3 h, BL coverage is suppressed to ∼15%. Following the same migration enhancement approach, WSe2 MLs are synthesized in 90 min with <20% BL coverage. The migration of W adatoms on the already formed stable WS2 (or WSe2) ML domains is promoted by ramping down the delivery of the tungsten precursor. From the qualitative analysis of the nanomorphology, the migration length of W adatoms is estimated to be ≤100 nm. This approach can be seen as a reliable and solid basis for the development of future large-scale TMDC ML deposition techniques.

Unveiling Electronic Behaviors in Heterochiral Charge-Density-Wave Twisted Stacking Materials with 1.25 nm Unit Dependence.

Layered charge-density-wave (CDW) materials have gained increasing interest due to their CDW stacking-dependent electronic properties for practical applications. Among the large family of CDW materials, those with star of David (SOD) patterns are very important due to the potentials for quantum spin liquid and related device applications. However, the spatial extension and the spin coupling information down to the nanoscale remain elusive. Here, we report the study of heterochiral CDW stackings in bilayer (BL) NbSe2 with high spatial resolution. We reveal that there exist well-defined heterochiral stackings, which have inhomogeneous electronic states among neighboring CDW units (star of David, SOD), significantly different from the homogeneous electronic states in the homochiral stackings. Intriguingly, the different electronic behaviors are spatially localized within each SOD with a unit size of 1.25 nm, and the gap sizes are determined by the different types of SOD stackings. Density functional theory (DFT) calculations match the experimental measurements well and reveal the SOD-stacking-dependent correlated electronic states and antiferromagnetic/ferromagnetic couplings. Our findings give a deep understanding of the spatial distribution of interlayer stacking and the delicate modulation of the spintronic states, which is very helpful for CDW-based nanoelectronic devices.

Reaction-Diffusion-Driven Stoichiometric Gradient in Coevaporated Superconducting NiBi3 Thin Films

The intermetallic compound NiBi3 belongs to the interesting class of superconductors that are based on strong ferromagnetic elements. Although NiBi3 has been grown in single-crystal form and has also been demonstrated to form at the interface of Ni and Bi layers, uniform NiBi3 thin films have yet to appear in the literature. In this work, we have explored the codeposition method, wherein high-purity Ni and Bi metals are evaporated simultaneously at controlled rates, to study the dynamics involved in the formation of NiBi3 films. We have used the Rutherford backscattering spectrometry (RBS) technique to make a quantitative stoichiometric comparison of codeposited films grown at two different evaporation rates of Bi, while keeping the evaporation rate of Ni fixed. We find that, due to the highly diffusive nature of the Bi atoms in this system, the growth dynamics favors the formation of almost stoichiometric NiBi3 only at the top surface (few tens of nanometers) of the films, whereas there is a nonlinear stoichiometric variation toward the substrate. The average local bonding environment around Ni, studied by synchrotron-based X-ray absorption spectroscopy, agreed with the overall findings of RBS. Since the physical properties of NiBi3 are sensitive to the ratio of Ni and Bi, we find the superconducting transition and magnetic coercivity to be consistent with the above observations.

The electronic structure of a strongly bound sandwich MoS2-WS2 heterobilayer.

First of all, we show that two kinds of sandwich bilayers (BLs) are dynamically, thermally, and mechanically stable, which are degenerate p-type materials with intercalated Ca atoms, i.e., Nb-doped MoS2 homobilayers (HoBLs) and Nb-doped WS2-MoS2 heterobilayers (HtBLs) with 25% Nb content. Specifically, their interlayer bindings are five times stronger than van der Waals interactions in their pristine counterparts. Both of them are semiconductors with indirect band gaps in the visible region within the HSE06 exchange-correlation functional. Depending upon the presence and absence of centrosymmetry, they display interesting spin-valley coupling effects in such a way that opposite hidden spin polarization or opposite spin splitting is observed at opposite k-points. They can be easily engineered into direct gap materials under compressive (>2%) strain along the zigzag direction even with an explicit consideration of giant spin splitting. Under strain, they satisfy thermodynamic conditions for bifunctional catalysis in photocatalytic water splitting. In addition, the photoholes of the BLs can be subjected to lower overpotentials than those of pristine BLs for the oxygen evolution reaction. Electrons and holes in the sandwich HtBL can be separated into different layers under photon irradiation, allowing it to be more efficient than the corresponding HoBL in solar energy harvesting.

Self-cleaning and fully polymer-based super-moisture-resistant gas barrier coating films with 2D polymers for flexible electronic devices and packaging applications

Through alternate stacking of spray-coated 2DP and spin-coated PDMS, bi-layers (BL)n assemblies have been designed. The WVTR value was improved from 1.4 g m−2 day−1 for PET to the optimum value of 9 × 10−4 g m−2 day−1 for PET/(BL)4.

Band-gap engineering and optoelectronic properties of 2D WSi2N4 nanosheets: A first principle calculations

The successful growth of centimeter-scale monolayer films of MoSi 2 N 4 and WSi 2 N 4 has attracted great interest (Science 369, 670, 2020). Here, we systematically investigate the effects of biaxial and uniaxial strain on the electronic structures of WSi 2 N 4 nanosheets with different layers of monolayer(ML), bilayer(BL) and trilayer(TL), respectively, by using first-principles calculations. In addition, combined with the non-equilibrium Green’s function methods, the photoelectronic response of WSi 2 N 4 ML is also explored. The results indicate that: 1) The biaxial and uniaxial strain can flexibly increase or decrease the band gap of WSi 2 N 4 nanosheets. Particularly, tensile strain realizes the transition of WSi 2 N 4 nanosheets from semiconductor to metal. 2) The tensile strain effectively improves the optical properties of WSi 2 N 4 nanosheets in the visible light region, and the red/blue shift of the absorption peaks is observed in the optical spectrum by tensile/compressive strain. 3) The pin-junction photodiode of WSi 2 N 4 ML generates sizable photocurrents under illumination, showing a strong photoelectronic response to purple light. Furthermore, the biaxial tensile strain significantly enhances the photoelectric performance of the WSi 2 N 4 photodiode and effectively modulates its detection range in the visible region. Hence, our findings suggest that WSi 2 N 4 nanosheets are a promising multifunctional material for nanoelectronic and optoelectronic applications. • Electronic structure and optoelectronic properties studies of the 2D WSi 2 N 4 nanosheets. • Strain engineering can effectively modulate the band gap and optical properties of WSi 2 N 4 nanosheets. • The pristine WSi 2 N 4 monolayer is predicted to be a good photoelectric sensor, showing a strong photoelectronic response to purple light. • The tensile strain significantly enhance the photoelectric performance of the WSi 2 N 4 photodiode and effectively modulates its detection range in the visible region.

High Dielectric Breakdown Strength Nanoplatelet‐Based Multilayer Thin Films

AbstractDielectric materials that can withstand high voltages are of great interest due to the growing need for high‐performance insulation systems in electronics. Polymer nanocomposites have gained popularity as electrical insulators due to their processability, high operating voltage, and tortuous paths for current flow created by the nanoparticles in the polymer matrix. The dielectric breakdown strength of a relatively thick multilayer thin film containing polyethylenimine (PEI) and vermiculite clay (VMT), thickened with tris(hydroxymethyl)aminomethane (tris), is evaluated as a function of bilayers (BL) deposited. The resulting nanobrick wall structure of this clay‐based assembly is ideal for protective insulation. An 8 BL PEI+tris/VMT film achieves a dielectric breakdown strength of 245 kV mm−1, with a thickness of 5 µm. With increasing bilayers, the breakdown strength gradually decreases, but 20 BL of PEI+tris/VMT achieves a breakdown voltage of 2.36 kV. This nanoplatelet‐based system is the first “thick growing” layer‐by‐layer deposited film to be used as an insulating layer. Its unusually high breakdown strength can be useful for the protection of various high voltage electronics.

Unraveling the spin current flow in Bi layers

Although heavy metal materials, due to their large spin-orbit interaction, would be expected to act as an efficient converter of spin current to charge current, Bismuth does not obey this rule. Heterostructures of yttrium iron garnet (YIG)/Bi are investigated by means of the spin pumping technique and the results show that Bi exhibits negligible spin-charge conversion. In addition to not converting spin to charge, Bi exhibits an exotic spin relaxation mechanism, which is driven by its large diamagnetic response. By considering a nonlinear stochastic model of diffusing spin particles, the results show that the Bi layer acts as a binary classifier device that categorizes the pumped spins into relaxed spins and those that flow upwards and eventually reach the top layer of Pt. This shows that the investigation reported here opens different perspectives involving fundamental aspects in the fields of machine learning and condensed matter physics.

Kinetic and structural insights into the grain boundary phase transitions in Ni-Bi alloys

Kinetic and structural changes associated with grain boundary phase transition induced by Bi alloying in dilute Ni–Bi alloys (0–0.3 at.% of Bi) are investigated in a concerted manner. Grain boundary diffusion of Ni in the Ni-Bi alloys is measured in Harrison's B- and C-type kinetic regimes applying the radiotracer technique and utilizing the 63Ni radioisotope. The measurements are performed across the single phase towards two-phase regions of the bulk phase diagram. In case of the single phase region it is the Ni(Bi) solid solution. In case of two phase areas these are Ni(Bi) solid solution + liquid Bi above 927 K or Ni(Bi) solid solution + NiBi below 927 K. Three distinct regions of the dependence of the Ni grain boundary diffusion rates on the Bi concentration are distinguished. A slight increment in Ni grain boundary diffusion is observed at lower Bi contents which is driven by Bi grain boundary segregation. Beyond a critical Bi concentration and still within the single-phase solid solution region, the Ni diffusivity starts to enhance dramatically with increasing Bi content. This is correlated with the appearance of multi-layer Bi segregation along the grain boundaries. It is substantiated by high-angle annular dark-field scanning transmission electron microscopy combined with energy dispersive X-ray spectroscopy measurements. At higher Bi concentrations, which corresponds to the appearance of a liquid layer of Bi at the grain boundaries, the Ni diffusivity attains the highest values. A serial of structural transitions between the different grain boundary phases is for the first time demonstrated to be accompanied by remarkable changes of the grain boundary diffusion rates. The present results highlight a close correlation between the atomic transport and structure modifications due to grain boundary phase transitions in the Ni–Bi alloys. Utilizing the present experimental results, a Ni–Bi grain boundary diffusion map is proposed.

Current-induced magnetization reversal in (Ga,Mn)(Bi,As) epitaxial layer with perpendicular magnetic anisotropy

Pulsed current-induced magnetization reversal is investigated in the layer of (Ga,Mn)(Bi,As) dilute ferromagnetic semiconductor (DFS) epitaxially grown under tensile misfit strain causing perpendicular magnetic anisotropy in the layer. The magnetization reversal, recorded through measurements of the anomalous Hall effect, appearing under assistance of a static magnetic field parallel to the current, is interpreted in terms of the spin–orbit torque mechanism. Our results demonstrate that an addition of a small fraction of heavy Bi atoms, substituting As atoms in the prototype DFS (Ga,Mn)As and increasing the strength of spin–orbit coupling in the DFS valence band, significantly enhances the efficiency of current-induced magnetization reversal, thus reducing considerably the threshold current density necessary for the reversal. Our findings are of technological importance for applications to spin–orbit torque-driven nonvolatile memory and logic elements.

Stacking pattern induced high ZTs in monolayer SnSSe and bilayer SnXY (X/Y = S, Se) materials with strong anharmonic phonon scattering

Two-dimensional (2D) Sn-based Janus materials have attracted enormous attention on account of the inexpensive, earth-abundant, and eco-friendly merits. Whereas, few works give explicit analysis about the underlying mechanism of stacking pattern from monolayer (ML) to bilayer (BL) configuration on the thermoelectric properties of Sn-based Janus materials. In our present work, the elastic modulus, phonon dispersion and ab initio molecular dynamics (AIMD) simulations are performed, which firstly verifies the mechanical, dynamic and thermal stabilities of ML SnSSe and BL SnXY (X/Y = S, Se) materials. Notably, the lowest lattice thermal conductivity (κl∼1.85 W/m K@300 K) is observed in BL SnSe2. Consequently, the optimal values of figure-of-merit (ZT) are observed for n-type (∼4.57@900 K) or p-type (∼3.71@900 K) BL SnSe2. Additionally, the optimal ZT can be improved from ML to BL configuration because of the weaker phonon transport caused by structure engineering, as evidenced by the maximal ZTs of n-type and p-type ML SnSSe (∼1.11 and ∼1.20@900 K) and BL SeSnS/SSnSe (∼2.55 and ∼1.71@900 K). Our present work not only provides deep insight into electronic and phonon transport properties in ML SnSSe and BL SnXY, but also paves the way for the theoretical investigation of high thermoelectric performance for Sn-based Janus materials.

Wrinkling and crumpling in twisted few and multilayer CVD graphene: High density of edge modes influencing Raman spectra

Richness and complexity of Raman spectra related to graphene materials is established from years to decades, with, among others: the well-known G, D, 2D, … bands plus a plethora of weaker bands related to disorder behavior, doping, stress, crystal orientation or stacking information. Herein, we report on how to detect crumpling effects in Raman spectra, using a large variety of few and multilayer graphene. The main finding is that these crumples enhance the G band intensity like it does with twisted bi layer graphene. We updated the D over G band intensity ratio versus G bandwidth plot, which is generally used to disentangle point and linear defects origin, by reporting surface defects created by crumples. Moreover, we report for the first time on the existence 23 resonant additional bands at 633 nm. We attribute them to edge modes formed by high density of crumples. We use Raman plots (2D bands versus G band positions and widths) to gain qualitative information about the way layers are stacked.

Preparation of Bi-2212/YBCO heterostructure and their interfacial characters

YBa 2 Cu 3 O 7-X (YBCO) thin films were prepared on LaAlO 3 (LAO) substrates by a sol-gel method, and the epitaxial growth of Bi-2212 thin films on YBCO thin films was investigated. Both YBCO and Bi 2 Sr 2 Ca 1 Cu 2 O 8+σ (Bi-2212) bilayer films exhibit good biaxial texture and superconducting properties. Afterward, a cross-shaped Bi-2212/YBCO heterostructure was fabricated, and its interfacial atomic arrangement and I - V characteristics were analyzed. Atomic-resolution STEM images obtained from a spherical-aberration-corrected transmission electron microscope show that two superconducting films exhibit a layered structure and the atoms inside the films are artfully arranged. Moreover, the order of the seven atomic layers between Bi-2212 and YBCO layers with a thickness of about 1.32 nm is misarranged. Among them, the Y-O layer of YBCO and the Sr-O layer of Bi-2212 share a CuO 2 layer. The I-V curves of Bi-2212/YBCO bilayer films show that the seven misarranged atomic layers at Bi-2212/YBCO interface acts as a barrier layer, which means that a Josephson junction can be fabricated using this interface characters.