Conductive Buffer Layer Research Articles

Improvement of Memristive Characteristic of In-Ga-Zn-O Thin-Film Memristor by Conductive Buffer Layers

Improvement of Memristive Characteristic of In-Ga-Zn-O Thin-Film Memristor by Conductive Buffer Layers

Fully-vertical GaN-on-SiC Schottky barrier diode with ultrathin AlGaN buffer layer

In this letter, vertically conductive GaN epilayer on SiC substrate was achieved without the typically used conductive buffer layer. Here, in order to reduce the impacts of band offset of different layers on vertical conductivity and improve the vertical carrier transportation, an ultrathin AlGaN buffer layer was employed to replace the thick conductive buffer layer. Fully-vertical Schottky barrier diode (SBD) based on this vertically conductive epi-stack demonstrated a much lower specific on-resistance of 0.84 mΩ·cm2 with superior thermal stability. Moreover, the SBD also exhibited an on/off ratio of ∼5 × 109 and a nearly unity ideality factor of 1.08. This approach lays the foundation for the heterogeneous integration of GaN/SiC based devices.

Interface engineering of Li6.75La3Zr1.75Ta0.25O12via in situ built LiI/ZnLix mixed buffer layer for solid-state lithium metal batteries.

Garnet-type solid-state Li metal batteries (SSLMBs) are viewed as hopeful next-generation batteries due to their high energy density and safety. However, the major obstacle to the development of garnet-type SSLMBs is the lithiophobicity of Li6.75La3Zr1.75Ta0.25O12 (LLZTO), resulting in a large interfacial impedance. Herein, a LiI/ZnLix mixed ion/electron conductive buffer layer is constructed at the interface by an in situ reaction of molten Li metal with ZnI2 film. This mixed buffer layer ensures close contact between the Li metal and garnet, significantly reducing interfacial impedance. As a result, the Li symmetrical cell with the LiI/ZnLix buffer layer shows an interface impedance of 10.3 Ω cm2, much lower than that of the cell with bare LLZTO (1173.4 Ω cm2). The critical current density (CCD) is up to 2.3 mA cm-2, and the symmetric cells present a long cycle life of 2000 h at 0.1 mA cm-2 and 800 h at 1.0 mA cm-2. In addition, the full cells assembled with the LiFePO4 cathode show a capacity of 143.9 mA h g-1 after 200 cycles at 0.5C with a low-capacity decay of 0.021% per cycle. This work reveals a simple, feasible, and practical interface modification strategy for solid-state Li metal batteries.

Boosting hierarchical construction and charge storage capacity of polyaniline arrays grown on the surface of carbon cloth with the aid of graphene interlayer

To satisfy the demands of high areal energy density for wearable devices, a strategy was presented to design hierarchical structured polyaniline (PANI) arrays on the surface of carbon cloth as the composite textile electrodes for lightweight and flexible supercapacitors. Benefiting from the introducing of graphene interlayer as conductive buffer layer and biomass organic acids including succinic acid, citric acid, malic acid, and tartaric acid as the dopants, the oriented growth of dense PANI array with hierarchical structure were greatly promoted especially in the presence of tartaric acid. Unique array structures constructed on the thin graphene layer significantly increase the specific surface area of the electrode, and facilitate the ion transport and dissipate stress from repeated polymeric volume change, resulting in considerably enhanced areal capacitance, rate retention and structural stability for PANI composite textile electrodes. The all–solid–state supercapacitor assembled with redox gel electrolyte possesses a high areal capacitance of up to 2181 mF cm−2 at the current density of 5 mA cm−2, and delivers a maximum energy density of 302.9 μWh cm−2 at the power density of 1752.1 μW cm−2 with excellent cycling stability and bending ability, thus demonstrating its great potential applications for wearable electronics.

Magnetic and electric properties of spinel oxide CoV2O4 (001) films

The epitaxial film growth of CoV2O4 (CVO) on a MgO (001) substrate was successfully achieved using a reactive co-sputtering technique. To explore the conductive CVO films at RT, we grew the films under various conditions, including oxygen-gas flow rates and input radio-frequency power for each target. The Néel temperature of the film was approximately 160 K, which is consistent with previously reported values for bulk CVO. The film was found to be conductive over a wide temperature range and exhibited a negative resistivity temperature coefficient. The mechanism governing the temperature-dependent resistivity of the film can be explained using the one-dimensional variable range-hopping model, at least for temperatures higher than the Néel temperature. The CVO films exhibited a lattice constant close to that of spinel ferrites; therefore, they can be used as conductive buffer layers for electrodes in spintronic applications.

The introduction of carbon nanosheet buffer layer for enhanced hydrogen evolution performance of C3N4/CoP photocatalysts

Various defects of nanocomposites inevitably bring some harmful effects on their photoelectric performance, especially considering that interface defects seriously hinder the carrier interfacial transfer. Here, the interface optimization can be realized between graphitic carbon nitride (C3N4) and CoP co-catalyst by the introduction of good conductive carbon nanosheets (CNs). As a result, the defect density of synthesized C3N4/CNs/CoP is reduced to 1.40 × 1012 cm−3 from 2.55 × 1012 cm−3 of C3N4/CoP, and its interface impedance is correspondingly reduced to 34% of that of C3N4/CoP. Therefore, the optimal H2 evolution rate of 5.26 mmol g−1 h−1 and apparent quantum efficiency of 9.27% at 420 nm are realized, which are 4.8 and 8.1 times that of C3N4/CoP photocatalyst in the absence of CNs, respectively. This work provides a general solution to reduce defect density and carrier transfer resistance in nanocomposites by the introduction of a highly conductive buffer layer.

(Invited) Engineering Interfaces in Solid-State Polymer-Ceramic Composite Electrolytes of Li-Ion Batteries

This talk presents our effort to engineer interfaces in solid-state polymer-ceramic composite electrolytes of lithium-ion batteries. A Grafting agent has been utilized to strengthen the polymer-ceramic interface. Also, a Li-ion conducting buffer layer has been employed to enhance interaction between the ceramic nanofibers and the polymer matrix. In addition, dopants and oxygen vacancies in ceramics can enhance interfacial interaction.

Improvement in interfacial stability of high-rate Ni-rich oxide cathode by multifunctional LiTi0.5Zr1.5(PO4)3 conductive buffer layer

With the advent of the double carbon era, Ni-rich layered oxides with high energy density have become ideal cathode materials to support and secure the energy transition of basic power and transportation. However, with the increase in Ni content, Ni-rich layered oxides suffer from several problems, such as difficult precursor synthesis, structural instability, and high interfacial activity. In this study, a design scheme of uniformly coating LiTi0.5Zr1.5(PO4)3 (LTZP) conductive buffer layer on the outer surface of LiNi0.8Mn0.1Co0.1O2 (NCM811) grains was proposed to overcome the above problems. The results of first-principles calculations and various ex-situ characterizations showed that the NASICON-type LTZP material had excellent compatibility with the NCM811 material. The LTZP surface-modified NCM811cathode material presented outstanding electrochemical performance. The initial discharge specific capacity was 210.16 mAh g−1 at 0.2C, and a remarkably high discharge capacity of 147.68 mAh g−1 was recorded at 10C. The capacity retention rate reached 84.7% after 200 cycles of 1C (25 °C). This work presents a strategy for stabilizing the interface with a conductive buffer layer to enhance the electrochemical performance of Ni-rich cathode materials.

Independent regulation of electrical properties of VO2 for low threshold voltage electro-optic switch applications

Vanadium dioxide has a unique insulator-metal phase transition characteristic near room temperature, meeting optical switches' requirements. However, due to its high resistance in the insulator state, the working voltage threshold of the VO2-based optical switch is also high. It is one of the obstacles for VO2-based optical switch applications. In this work, high-quality VO2 films were prepared by DC pulsed magnetron sputtering. By inserting a conductive ITO buffer layer, VO2 sheet resistance reduced about 2 orders, and the electrical phase change characteristics have almost disappeared. Still, the high optical modulation ability is hardly affected. The interesting results meet the requirements of a low threshold working voltage electro-optic switch. In the case of a 40 µm electrode gap size, the threshold voltage for VO2/ITO is only 1.4 V, which is much lower than 20 V of pure VO2 in the same condition, and the optical switch modulation depth at the wavelength of 1550 nm reaches 35%. For the switch with 5 µm electrode gap size, the threshold voltage is only 1.3 V, and the response times of "ON" and "OFF" are 0.24 ms and 2.1 ms, respectively. This result is valuable for the practical application of VO2-based optical switches.

Tunable surface pseudocapacitance assisted fast and flexible lithium storage of graphene wrapped NiO nano-arrays on nitrogen-doped carbon foams

Advanced materials incorporating battery-like high capacity and capacitor-like high rate capability hold promise for achieving both high energy and power density lithium ion batteries (LIBs). However, capacitive behavior for high capacity NiO anode has seldom been reported because the surface or near-surface fast faradic surface redox reactions highly depends on specific structure design. Herein, a flexible electrode that graphene wrapped NiO nanosheet arrays anchor on three-dimensional porous nitrogen-doped carbon foam (rGO@NiONCF) is fabricated. The rGO@NiONCF composite possesses a unique sandwich structure with 3D porous NCF as the inner conductive scaffold, ultrathin NiO nanosheet arrays as a middle layer, and rGO as the outer conductive buffer layer. The elaborated structure design endows rGO@NiONCF with enhanced electronic conductivity, sufficient electrolyte infiltration and restricted volume variation. As a result, the rGO@NiONCF electrode delivers high reversible capacity (1583 mAh g−1 at 0.1 A g−1) and superior cycling stability (maintains 607 mA h g−1 at 1 A g−1 after 500 cycles). Moreover, the enhanced surface capacitance adroitly facilitates an impressive fast charge and slow discharge performance. These results reveal that rational material design to encourage pseudocapacitance effect is a valuable roadmap for NiO-based battery towards more practical applications.

Achieving an ultra-high capacitive energy density in ferroelectric films consisting of superfine columnar nanograins

Well-crystallized perovskite ferroelectric films usually display a bulk-like polarization response (P) under an external electric field (E), i.e., a large P-E hysteresis loop featuring a sizable remnant polarization and an early polarization saturation. Such characteristics are undesirable for capacitive energy storage applications. In this work, we demonstrate an optimal P-E behavior, i.e., a small remnant polarization and a delayed polarization saturation, in perovskite BaTiO3 films consisting of superfine columnar nanograins. In a low-temperature, nucleation-dominated sputtering deposition, an in-situ grown conductive buffer layer promotes the formation of these nanograins, which display a controllable diameter down to ~10 nm and extend throughout the film thickness. The deterioration of the remnant polarization and its delayed saturation under an electric field, can be attributed to a strong polarization-constraining effect from the densely-packed, non-ferroelectric grain boundaries, which is supported by a phase field modeling simulation. The resulted BaTiO3 film capacitors integrated on Si at 350°C display a high recyclable energy density (Wrec~135±10 J/cm3) and efficiency (η~80%±4%) which are thickness-scalable. An intrinsically high power density, a simple and stable chemical composition, and good thermal (-150°C ~ 170°C) and cycling stabilities (up to ~ 2 × 108 charge-discharge cycles) warrant a broad range of applications for these film capacitors.

Construction of sticky ionic conductive buffer layer for inorganic electrolyte toward stable all-solid-state lithium metal batteries

Inorganic electrolyte based all-solid-state lithium metal batteries (LMBs) are considered the most promising secondary power storage device. However, there still exist significant interfacial issues, which hurdles its advancement in application seriously. Herein, we synthesize a sticky anion-immobilized oligomer ionic conductor (AIOC) to construct a buffer layer for inorganic electrolyte. The AIOC displays high ionic conductivity (2.21 × 10−5 S cm−1), large Li+ transference number (0.65) at 30 °C, which is beneficial to promote homogenous lithium deposition, moreover, it endows a wide electrochemical window of 4.3 V (vs. Li+/Li). Based on these merits, when AIOC serves as buffer layer for Li1.5Al0.5Ge1.5(PO4)3 (LAGP) electrolyte, the interfacial stability and compatibility between LAGP and lithium metal anode can be enhanced remarkably. Upon which, the interfacial resistance of lithium symmetrical cell can be declined sharply (5313 Ω→718 Ω). Enduring 100-h galvanostatic charge-discharge cycling test, the surficial morphologies and elemental constitutions of the lithium metal anode and LAGP electrolyte almost remain unchanged as before. As a result, a LiFePO4/Li all-solid-state cell with AIOC modified Li1.5Al0.5Ge1.5(PO4)3 electrolyte enables to achieve a reversible discharge capacity of 120.1 mAh g−1 at 0.5C with a notable capacity retention of 86.3% in 100 cycles at 50 °C.

Selective area grown AlInGaN nanowire arrays with core-shell structures for photovoltaics on silicon.

To pave the way for InGaN-on-Si integrated photovoltaics, uniform and close-packed n-GaN/(Al)InGaN/p-GaN nanowire (NW) arrays with a ∼0.29 μm thick absorption segment of ∼2.35 eV energy bandgap were fabricated on a Si substrate using Ti-mask selective area growth (SAG) in a molecular beam epitaxy (MBE) chamber. Instead of using thick and insulting buffer layers, this SAG process was realized by employing a 3 nm AlN/GaN: Ge buffer layer to facilitate electrical and thermal conduction between NWs and Si. Scanning transmission electron microscopy and high-resolution electron energy loss spectroscopy mapping revealed the discontinuities of AlN and the embedments of GaN:Ge which contribute to a negligible resistance of the NWs-on-Si interface. AlInGaN active segment exhibits core-shell structures, which suppress nonradiative surface recombination at NW surfaces. Working of AlInGaN core-shell NW solar cells was demonstrated with improved open-circuit voltage (Voc) and higher energy conversion efficiency (η) than those reported for InGaN NW solar cells. Stable output characteristics including the Voc of 1.41 V and η of 2.46% were obtained under 30-Sun illuminations. Such NWs-on-Si devices use Si substrate as the bottom electrode. With a low series resistance of ∼1 Ω, this work paves the way to monolithically integrate MBE-SAG III-nitride devices and Si-based electronics, such as Si solar cells and CMOS devices.

Microstructure of YBa2Cu3Oy coated conductor using {100} ⟨001⟩ textured Cu tape with dual functions of metal substrate and electric stabilizing layer in order to develop low-cost high-TC superconducting wires

A new configuration of coated conductors (CCs), REBa2Cu3Oy (RE: rare earth elements such as Sm, Eu, Gd, and Y)/(Sr, La)TiO3 conductive buffer/Ni-electroplated {100}⟨001⟩ textured Cu and SUS316 lamination tape, has been proposed to reduce material costs. This conductive buffer layer is slightly modified from the previously reported Sr (Ti, Nb)O3 conductive buffer layers by changing the substitution element and site. However, a drastic change is found in the electric resistivity of the conductive buffer layers after oxygen annealing of YBa2Cu3Oy. The resistivity of Sr(Ti, Nb)O3 conductive buffer layers increases by more than three orders of magnitude, while the resistivity of (Sr, La)TiO3 is almost maintained and only minimally increases. Microscopic structural analysis of the CCs using the newly proposed conductive buffer layer is conducted using transmission electron microscopy (TEM). A very clean interface between the metal tape and the conductive buffer layer in the 600 nm thick (Sr, La)TiO3 buffer layers is confirmed by the TEM analysis, which has no reactive layers as well as no inter-diffusion of oxygen and metal elements. This result will soon lead to the development of low-cost and high-performance CCs.

Unraveling the Beneficial Microstructure Evolution in Pyrite for Boosted Lithium Storage Performance.

Pyrite FeS2 as a high-capacity electrode material for lithium-ion batteries (LIBs) is hindered by its unstable cycling performance owing to the large volume change and irreversible phase segregation from coarsening of Fe. Here, the beneficial microstructure evolution in MoS2 -modified FeS2 is unraveled during the cycling process; the microstructure evolution is responsible for its significantly boosted lithium storage performance, making it suitable for use as an anode for LIBs. Specifically, the FeS2 /MoS2 displays a long cycle life with a capacity retention of 116 % after 600 cycles at 0.5 A g-1 , which is the best among the reported FeS2 -based materials so far. A series of electrochemical tests and structural characterizations substantially revealed that the introduced MoS2 in FeS2 experiences an irreversible electrochemical reaction and thus the in situ formed metallic Mo could act as the conductive buffer layer to accelerate the dynamics of Li+ diffusion and electron transport. More importantly, it can guarantee the highly reversible conversion in lithiated FeS2 by preventing Fe coarsening. This work provides a fundamental understanding and an effective strategy towards the microstructure evolution for boosting lithium storage performances for other metal sulfide-based materials.

Boosting Room‐Temperature Magneto‐Ionics in a Non‐Magnetic Oxide Semiconductor

AbstractVoltage control of magnetism through electric field‐induced oxygen motion (magneto‐ionics) could represent a significant breakthrough in the pursuit for new strategies to enhance energy efficiency in magnetically actuated devices. Boosting the induced changes in magnetization, magneto‐ionic rates and cyclability continue to be key challenges to turn magneto‐ionics into real applications. Here, it is demonstrated that room‐temperature magneto‐ionic effects in electrolyte‐gated paramagnetic Co3O4 films can be largely increased both in terms of generated magnetization (6 times larger) and speed (35 times faster) if the electric field is applied using an electrochemical capacitor configuration (utilizing an underlying conducting buffer layer) instead of placing the electric contacts at the side of the semiconductor (electric‐double‐layer transistor‐like configuration). This is due to the greater uniformity and strength of the electric field in the capacitor design. These results are appealing to widen the use of ion migration in technological applications such as neuromorphic computing or iontronics in general.

Integration-Friendly, Chemically Stoichiometric BiFeO3 Films with a Piezoelectric Performance Challenging that of PZT.

As a prototype single-phase multiferroic, BiFeO3 exhibits excellent electrical, magnetic, and magnetoelectric properties, appealing to many modern technological applications. One of its overlooked merits is a high piezoelectric performance originating from its large remnant polarization (Pr) and low dielectric constant (εr). Furthermore, its high Curie temperature and large coercive field ensure good stabilities in device applications. However, to achieve close-to-intrinsic properties, a high processing temperature is usually used for the preparation of highly crystalline (epitaxial or highly oriented) BiFeO3 films. Proliferation of defects due to loss of volatile Bi2O3 in the high-temperature process and its incompatibility with CMOS-Si technologies have hindered the development of BiFeO3 film-based piezoelectric micro-electro-mechanical systems (piezo-MEMS) devices. In this work, we successfully sputter-deposited highly (100) oriented BiFeO3 thick films (∼1 μm) on Si at 350 °C through the use of a conductive perovskite buffer layer of LaNiO3. Formation of bulk and interfacial defects is suppressed by the combination of a low deposition temperature and an oxygen-rich processing atmosphere, resulting in chemically stoichiometric BiFeO3 films. These films displayed a high Pr (∼60 μC·cm-2), a low εr (∼200), and a small dielectric loss (<0.02), as well as large coercive and self-bias voltages in their as-grown and aged states. Together with a large transverse piezoelectric coefficient (e31,f ∼ -2.8 C·m-2), excellent electromechanical performances with outstanding fatigue and aging resistances are demonstrated in patterned BiFeO3-Si cantilever devices. These integration-friendly BiFeO3 films are ideal replacements of PZT films in piezo-MEMS applications.

AlF3-modified anode-electrolyte interface for effective Na dendrites restriction in NASICON-based solid-state electrolyte

Inorganic Na+ conducting solid-state electrolyte (SSE) has received extensive attention in assembling large-scale sodium metal batteries. However, continuous Na dendrites propagation has prevailingly hampered its implementation due to premature cell failure and safety concerns. Herein, a coating layer of AlF3 was decorated between Na anode and NASICON SSE for the first time, which can in situ form a Na+ conducting buffer layer by a conversion reaction with Na during initial cycles. Profiting from the intimate modification of AlF3 interlayer, the Na/SSE interfacial contact is extremely ameliorated. Moreover, owing to the “overpotential triggering” mechanism of the reaction between AlF3 and Na, the locally agminated electron at the anode-electrolyte interface can be released by preferentially driving more Na+ to react with AlF3 instead of forming dendrites at large current density. According to DFT calculations, the critical dendrites length is also enlarged owing to the increased interfacial energy between AlF3-induced interlayer and Na. Therefore, the critical current density of Na/AlF3-NASICON/Na cells has multiplied three-fold to 1.2 ​mA ​cm−2 at 60 ​°C, and the cycling stability of Na metal SSE batteries is also dramatically ameliorated. The rational strategy of modifying AlF3 interlayer is believed to be helpful in resisting dendrites propagation among solid alkalis metal batteries.

Preparation of High-Quality Conductive La0.7Sr0.3MnO3 Buffer Layer Applied to Low-Cost YBCO-Coated Conductors

Through a simple and low-cost way to gain the precursor solution, a conductive and high-quality La0.7Sr0.3MnO3 (LSMO) buffer layer approximately 280 nm thick only by one single coating was successfully deposited on LaAlO3 (LAO) (100) single crystal substrates using an independently developed new method of polymer-assisted chemical solution deposition (PA-CSD). Highly epitaxial YBa2Cu3O7−x (YBCO) superconducting film with thickness of 500 nm managed to be deposited on LSMO/LAO. A study was made on the LSMO buffers and YBCO films with XRD, SEM, and superconducting critical temperature (Tc) analyses. The temperature-dependent resistivities of YBCO on insulating BaZrO3 (BZO) buffer layer and on conductive LSMO buffer layer were also investigated. It was found that the conductive LSMO buffer prepared at 840 °C in argon would possess high-degree out-of-plane and in-plane texturing, good density with a sufficient thickness about 280 nm, and microcrack-free nature. More importantly, the value of the self-field critical current density (Jc) of YBCO superconducting film on LSMO/LAO architecture was 2.8 MA/cm2 at 77 K.

Dealloyed porous gold anchored by in situ generated graphene sheets as high activity catalyst for methanol electro-oxidation reaction.

A novel one-step method to prepare the nanocomposites of reduced graphene oxide (RGO)/nanoporous gold (NPG) is realized by chemically dealloying an Al2Au precursor. The RGO nanosheets anchored on the surface of NPG have a cicada wing like shape and act as both conductive agent and buffer layer to improve the catalytic ability of NPG for methanol electro-oxidation reaction (MOR). This improvement can also be ascribed to the microstructure change of NPG in dealloying with RGO. This work inspires a facile and economic method to prepare the NPG based catalyst for MOR.