Heterogeneous integration of single-crystalline complex-oxide membranes.

Complex-oxide materials exhibit a vast range of functional properties desirable for next-generation electronic, spintronic, magnetoelectric, neuromorphic, and energy conversion storage devices1-4. Their physical functionalities can be coupled by stacking layers of such materials to create heterostructures and can be further boosted by applying strain5-7. The predominant method for heterogeneous integration and application of strain has been through heteroepitaxy, which drastically limits the possible material combinations and the ability to integrate complex oxides with mature semiconductor technologies. Moreover, key physical properties of complex-oxide thin films, such as piezoelectricity and magnetostriction, are severely reduced by the substrate clamping effect. Here we demonstrate a universal mechanical exfoliation method of producing freestanding single-crystalline membranes made from a wide range of complex-oxide materials including perovskite, spinel and garnet crystal structures with varying crystallographic orientations. In addition, we create artificial heterostructures and hybridize their physical properties by directly stacking such freestanding membranes with different crystal structures and orientations, which is not possible using conventional methods. Our results establish a platform for stacking and coupling three-dimensional structures, akin to two-dimensional material-based heterostructures, for enhancing device functionalities8,9.

Germanium p-i-n avalanche photodetector fabricated by point defect healing process.

In this Letter, we report Ge p-i-n avalanche photodetectors (APD) with low dark current (sub 1 µA below V(R)=5 V), low operating voltage (avalanche breakdown voltage=8-13 V), and high multiplication gain (440-680) by exploiting a point defect healing method (between 600°C and 650°C) and optimizing the doping concentration of the intrinsic region (p-type ~10¹7 cm⁻³). In addition, Raman spectroscopy and electrochemical capacitance voltage analyses were performed to investigate the junction interfaces in more detail. This successful demonstration of Ge p-i-n APD with low dark current, low operating voltage, and high gain is promising for low-power and high-sensitivity Ge PD applications.

Predicting adoption of the assistive technology open platform: extended unified theory of acceptance and use of technology.

PURPOSE: The Assistive Technology (AT) Open Platform supports people with disabilities, older people, and developers in co-creating new assistive products outside the business realm. To address dissatisfaction with and non-adoption of commercial assistive products, the National Rehabilitation Centre in South Korea created an AT Open Platform as an open-source AT sharing platform to research and develop appropriate assistive technology suitable for users' needs. The emerging concept of AT Open Platform is new for both assistive product users and developers in South Korea. The Extended Unified Theory of Acceptance and Use of Technology (UTAUT2) was utilised to understand the factors influencing the adoption of the AT Open Platform and to gain further insights on its design and future use. MATERIALS AND METHODS: Interviews were conducted with six potential AT Open Platform users to develop a questionnaire for predicting behavioural intention. Subsequently, we surveyed 175 potential users to validate the UTAUT2. RESULTS: The results revealed that behavioural intention was significantly predicted by social influence, performance expectancy, facilitating conditions, and hedonic motivation. CONCLUSIONS: The AT Open Platform should focus on both online and offline platforms to educate and facilitate the co-creation of ATs for assistive product users and makers. This study, which targeted assistive product users and developers, has significant implications for policymakers and future research in using and adopting the AT Open Platform as it reflects the actual voices of the platform's stakeholders.

To address the issues of dissatisfaction and non-adoption of commercial assistive products, assistive technology platforms are established for the research and development of appropriate assistive technologies suitable for meeting user needs; the results are shared as open-source assistive technology.A survey was conducted with a targeted sample of assistive technology product users and developers. The study results are significant as they represent the perspectives of key stakeholders in the assistive technology platform. The study findings are expected to play an important role in the application and diffusion of the assistive technology platform in South Korea.The survey is the first to illuminate the adoption of an assistive technology platform in South Korea and is an important step towards empowering people with disabilities.

Chinese undergraduate students' motivation to learn Korean as a LOTE.

As rates of multilingualism increase, interest in the field of Languages Other Than English (LOTEs) has been growing over the last few years. This study investigated the motivation held by Chinese undergraduate students for learning Korean as a LOTE using Dörnyei's L2 Motivational Self System (L2MSS). In total, 123 subjects responded to the 6-point Likert scale measuring their Korean learning motivation. The collected data were analyzed using SPSS 22. Logistic regression was applied for identifying variables that distinguished the first-year from the second-year learners of Korean, while canonical correlation analysis was used to examine the correlation between two sets of variables, the first set of dependent variables of the ideal L2 self and the ought-to L2 self, and the second set of independent variables of family influence, instrumentality promotion, instrumentality prevention, attitude to learning Korean, cultural interest, attitude toward community and integrativeness. Results showed that variables of family influence, cultural interest, and attitude to learning Korean were statistically significant in distinguishing the first-year from the second-year learners in terms of affective variables. In addition, canonical analysis showed that the dependent variable set of the ideal L2 self and the ought-to L2 self together shared nearly 69% variance with the independent variable set, indicating that the ideal L2 self and the ought-to L2 self together were highly related with these affective variables in the independent variable set. The findings of the current study suggest that more creative Korean language learning activities be adopted to help sustain the high levels of affect among Korean language learners.

An alternative method to create extramedullary references in total knee arthroplasty.

PURPOSE: Historically, extramedullary techniques have focused on finding the femoral head center and referred mainly to the anterosuperior iliac spine (the FHC method) in total knee arthroplasty (TKA). The purpose of this study was to evaluate a new extramedullary alignment system that uses neutral overall limb mechanical alignment as an extramedullary reference (the MA method). METHODS: A retrospective review of 1018 TKAs (508 FHC group, 510 MA group) was performed to compare the radiographic results obtained using these two methods. There were 75 male patients and 577 female patients with varus osteoarthritis of the knee. Limb and prosthesis alignments in the coronal plane were investigated at 2 months after surgery. RESULTS: The accuracies of postoperative alignments of implanted prostheses were better in the MA group than in the FHC group, as assessed by (1) overall limb alignment (MA group, varus 1.4° ± 2.7°; FHC group, varus 2.2° ± 2.9°), and (2) femoral component coronal alignment (MA group, 89.4° ± 2.3°; FHC group, 88.0° ± 2.7°). Overall limb alignment was within 0° ± 5° of the mechanical axis in 96.1% of the MA group and in 86.6% of the FHC group. Mean femoral component medial angle was within 0° ± 5° in 98.4% of the MA group and in 92.5% of the FHC group. CONCLUSION: The accuracies of the postoperative radiographic alignments of implanted prostheses were found to be improved when the mechanical axis of the overall lower limb was used as an extramedullary reference. This new reference system would help to achieve correct limb and prostheses alignments during TKA. LEVEL OF EVIDENCE: III.

High-Efficiency WSe2 Photovoltaic Devices with Electron-Selective Contacts.

A rapid surge in global energy consumption has led to a greater demand for renewable energy to overcome energy resource limitations and environmental problems. Recently, a number of van der Waals materials have been highlighted as efficient absorbers for very thin and highly efficient photovoltaic (PV) devices. Despite the predicted potential, achieving power conversion efficiencies (PCEs) above 5% in PV devices based on van der Waals materials has been challenging. Here, we demonstrate a vertical WSe2 PV device with a high PCE of 5.44% under one-sun AM1.5G illumination. We reveal the multifunctional nature of a tungsten oxide layer, which promotes a stronger internal electric field by overcoming limitations imposed by the Fermi-level pinning at WSe2 interfaces and acts as an electron-selective contact in combination with monolayer graphene. Together with the developed bottom contact scheme, this simple yet effective contact engineering method improves the PCE by more than five times.

Artificial van der Waals hybrid synapse and its application to acoustic pattern recognition.

Brain-inspired parallel computing, which is typically performed using a hardware neural-network platform consisting of numerous artificial synapses, is a promising technology for effectively handling large amounts of informational data. However, the reported nonlinear and asymmetric conductance-update characteristics of artificial synapses prevent a hardware neural-network from delivering the same high-level training and inference accuracies as those delivered by a software neural-network. Here, we developed an artificial van-der-Waals hybrid synapse that features linear and symmetric conductance-update characteristics. Tungsten diselenide and molybdenum disulfide channels were used selectively to potentiate and depress conductance. Subsequently, via training and inference simulation, we demonstrated the feasibility of our hybrid synapse toward a hardware neural-network and also delivered high recognition rates that were comparable to those delivered using a software neural-network. This simulation involving the use of acoustic patterns was performed with a neural network that was theoretically formed with the characteristics of the hybrid synapses.

A multiple negative differential resistance heterojunction device and its circuit application to ternary static random access memory.

For increasing the restricted bit-density in the conventional binary logic system, extensive research efforts have been directed toward implementing single devices with a two threshold voltage (VTH) characteristic via the single negative differential resistance (NDR) phenomenon. In particular, recent advances in forming van der Waals (vdW) heterostructures with two-dimensional crystals have opened up new possibilities for realizing such NDR-based tunneling devices. However, it has been challenging to exhibit three VTH through the multiple-NDR (m-NDR) phenomenon in a single device even by using vdW heterostructures. Here, we show the m-NDR device formed on a BP/(ReS2 + HfS2) type-III double-heterostructure. This m-NDR device is then integrated with a vdW transistor to demonstrate a ternary vdW latch circuit capable of storing three logic states. Finally, the ternary latch is extended toward ternary SRAM, and its high-speed write and read operations are theoretically verified.

Graphene-assisted spontaneous relaxation towards dislocation-free heteroepitaxy.

Although conventional homoepitaxy forms high-quality epitaxial layers1-5, the limited set of material systems for commercially available wafers restricts the range of materials that can be grown homoepitaxially. At the same time, conventional heteroepitaxy of lattice-mismatched systems produces dislocations above a critical strain energy to release the accumulated strain energy as the film thickness increases. The formation of dislocations, which severely degrade electronic/photonic device performances6-8, is fundamentally unavoidable in highly lattice-mismatched epitaxy9-11. Here, we introduce a unique mechanism of relaxing misfit strain in heteroepitaxial films that can enable effective lattice engineering. We have observed that heteroepitaxy on graphene-coated substrates allows for spontaneous relaxation of misfit strain owing to the slippery graphene surface while achieving single-crystalline films by reading the atomic potential from the substrate. This spontaneous relaxation technique could transform the monolithic integration of largely lattice-mismatched systems by covering a wide range of the misfit spectrum to enhance and broaden the functionality of semiconductor devices for advanced electronics and photonics.

Path towards graphene commercialization from lab to market.

The ground-breaking demonstration of the electric field effect in graphene reported more than a decade ago prompted the strong push towards the commercialization of graphene as evidenced by a wealth of graphene research, patents and applications. Graphene flake production capability has reached thousands of tonnes per year, while continuous graphene sheets of tens of metres in length have become available. Various graphene technologies developed in laboratories have now transformed into commercial products, with the very first demonstrations in sports goods, automotive coatings, conductive inks and touch screens, to name a few. Although challenges related to quality control in graphene materials remain to be addressed, the advancement in the understandings of graphene will propel the commercial success of graphene as a compelling technology. This Review discusses the progress towards commercialization of graphene for the past decade and future perspectives.

Polarity control in a single transition metal dichalcogenide (TMD) transistor for homogeneous complementary logic circuits.

Recently, there have been various attempts to demonstrate the feasibility of transition metal dichalcogenide (TMD) transistors for digital logic circuits. A complementary inverter circuit, which is a basic building block of a logic circuit, was implemented in earlier works by heterogeneously integrating n- and p-channel transistors fabricated on different TMD materials. Subsequently, to simplify the circuit design and fabrication process, complementary inverters were constructed on single-TMD materials using ambipolar transistors. However, continuous transition from the electron-conduction to the hole-conduction state in the ambipolar devices led to the problem of a high leakage current. Here, we report a polarity-controllable TMD transistor that can operate as both an n- and a p-channel transistor with a low leakage current of a few picoamperes. The device polarity can be switched simply by converting the sign of the drain voltage. This is because a metal-like tungsten ditelluride (WTe2) with a low carrier concentration is used as a drain contact, which subsequently allows selective carrier injection at the palladium/tungsten diselenide (WSe2) junction. In addition, by using the operating principle of the polarity-controllable transistor, we demonstrate a complementary inverter circuit on a single TMD channel material (WSe2), which exhibits a very low static power consumption of a few hundred picowatts. Finally, we confirm the expandability of this polarity-controllable transistor toward more complex logic circuits by presenting the proper operation of a three-stage ring oscillator.

Stable and Reversible Triphenylphosphine-Based n-Type Doping Technique for Molybdenum Disulfide (MoS2).

A highly stable and reversible n-type doping technique for molybdenum disulfide (MoS2) transistors and photodetectors is developed in this study. This doping technique is based on triphenylphosphine (PPh3) and significantly improves the performance of MoS2 transistor and photodetector devices in terms of the on/off-current ratio (8.72 × 104 → 8.70 × 105), mobility (12.1 → 241 cm2/V·s), and photoresponsivity ( R) (2.77 × 103 → 3.92 × 105 A/W). The range of doping concentrations is broadly distributed between 1.56 × 1011 and 9.75 × 1012 cm-2 and is easily controlled by adjusting the temperature at which the PPh3 layer is formed. In addition, this doping technique provides two interesting properties that have not been reported for previous molecular doping techniques: (i) high stability leading to small variations in device performance after exposure to air for 14 days (on-current: 1.34% and photoresponsivity: 1.58%) and (ii) reversibility enabling the repetitive formation and removal of PPh3 molecules (doping and dedoping).

Highly Efficient Infrared Photodetection in a Gate-Controllable Van der Waals Heterojunction with Staggered Bandgap Alignment.

In recent years, various van der Waals (vdW) materials have been used in implementing high-performance photodetectors with high photoresponsivity over a wide detection range. However, in most studies reported so far, photodetection in the infrared (IR) region has not been achieved successfully. Although several vdW materials with narrow bandgaps have been proposed for IR detection, the devices based on these materials exhibit notably low photoresponsivity under IR light illumination. Here, highly efficient near-infrared (NIR) photodetection based on the interlayer optical transition phenomenon in a vdW heterojunction structure consisting of ReS2 and ReSe2 is demonstrated. In addition, by applying the gate-control function to the two-terminal vdW heterojunction photodetector, the photoresponsivity is enhanced to 3.64 × 105 A W-1 at λ = 980 nm and 1.58 × 105 A W-1 at λ = 1310 nm. Compared to the values reported for previous vdW photodetectors, these results are the highest levels of photoresponsivity in the NIR range. The study offers a novel device platform for achieving high-performance IR photodetectors.

Artificial optic-neural synapse for colored and color-mixed pattern recognition.

The priority of synaptic device researches has been given to prove the device potential for the emulation of synaptic dynamics and not to functionalize further synaptic devices for more complex learning. Here, we demonstrate an optic-neural synaptic device by implementing synaptic and optical-sensing functions together on h-BN/WSe2 heterostructure. This device mimics the colored and color-mixed pattern recognition capabilities of the human vision system when arranged in an optic-neural network. Our synaptic device demonstrates a close to linear weight update trajectory while providing a large number of stable conduction states with less than 1% variation per state. The device operates with low voltage spikes of 0.3 V and consumes only 66 fJ per spike. This consequently facilitates the demonstration of accurate and energy efficient colored and color-mixed pattern recognition. The work will be an important step toward neural networks that comprise neural sensing and training functions for more complex pattern recognition.

Controlled crack propagation for atomic precision handling of wafer-scale two-dimensional materials.

Although flakes of two-dimensional (2D) heterostructures at the micrometer scale can be formed with adhesive-tape exfoliation methods, isolation of 2D flakes into monolayers is extremely time consuming because it is a trial-and-error process. Controlling the number of 2D layers through direct growth also presents difficulty because of the high nucleation barrier on 2D materials. We demonstrate a layer-resolved 2D material splitting technique that permits high-throughput production of multiple monolayers of wafer-scale (5-centimeter diameter) 2D materials by splitting single stacks of thick 2D materials grown on a single wafer. Wafer-scale uniformity of hexagonal boron nitride, tungsten disulfide, tungsten diselenide, molybdenum disulfide, and molybdenum diselenide monolayers was verified by photoluminescence response and by substantial retention of electronic conductivity. We fabricated wafer-scale van der Waals heterostructures, including field-effect transistors, with single-atom thickness resolution.

Evaluation of laser spallation as a technique for measurement of cell adhesion strength.

Cell adhesion to material surfaces is one of the fundamental phenomena of cellular response to implanted devices. Controlling the strength, dynamics, and mechanics of cell adhesion offer opportunities for designing novel biomaterials for tissue engineering and biotechnology. Many techniques have been developed for the purpose of quantifying various types of cell-biomaterial interaction. One method to evaluate cell affinity for a biomaterial is to measure the stress required to remove adherent cells from the material. This study investigates the possibility of using laser spallation, a technique previously developed for measuring the tensile strength of thin film interfaces, for evaluation of initial cell attachment strength. MC3T3-E1 preosteoblasts were cultured on fibronectin-coated polystyrene, a surface known to engage cells in receptor mediated adhesion, and untreated polystyrene, which elicit nonspecific adhesion mechanisms during early stages of cell attachment. The laser spallation technique effectively detached cells from polymer substrates and also distinguished relative cell adhesion strengths to surfaces with known differences in cell binding affinities. Scanning electron micrographs determined that cell detachment resulting from laser spallation left a cleaner surface than jet impingement, possibly suggesting a more complete detachment mechanism. Absolute values of adhesion strengths determined by laser spallation were significantly higher than those found using jet impingement, a previously reported hydrodynamic technique.

Light-Triggered Ternary Device and Inverter Based on Heterojunction of van der Waals Materials.

Multivalued logic (MVL) devices/circuits have received considerable attention because the binary logic used in current Si complementary metal-oxide-semiconductor (CMOS) technology cannot handle the predicted information throughputs and energy demands of the future. To realize MVL, the conventional transistor platform needs to be redesigned to have two or more distinctive threshold voltages (VTHs). Here, we report a finding: the photoinduced drain current in graphene/WSe2 heterojunction transistors unusually decreases with increasing gate voltage under illumination, which we refer to as the light-induced negative differential transconductance (L-NDT) phenomenon. We also prove that such L-NDT phenomenon in specific bias ranges originates from a variable potential barrier at a graphene/WSe2 junction due to a gate-controllable graphene electrode. This finding allows us to conceive graphene/WSe2-based MVL logic circuits by using the ID-VG characteristics with two distinctive VTHs. Based on this finding, we further demonstrate a light-triggered ternary inverter circuit with three stable logical states (ΔVout of each state <0.05 V). Our study offers the pathway to substantialize MVL systems.

Poly-4-vinylphenol (PVP) and Poly(melamine-co-formaldehyde) (PMF)-Based Atomic Switching Device and Its Application to Logic Gate Circuits with Low Operating Voltage.

In this study, we demonstrate a high-performance solid polymer electrolyte (SPE) atomic switching device with low SET/RESET voltages (0.25 and -0.5 V, respectively), high on/off-current ratio (105), excellent cyclic endurance (>103), and long retention time (>104 s), where poly-4-vinylphenol (PVP)/poly(melamine-co-formaldehyde) (PMF) is used as an SPE layer. To accomplish these excellent device performance parameters, we reduce the off-current level of the PVP/PMF atomic switching device by improving the electrical insulating property of the PVP/PMF electrolyte through adjustment of the number of cross-linked chains. We then apply a titanium buffer layer to the PVP/PMF switching device for further improvement of bipolar switching behavior and device stability. In addition, we first implement SPE atomic switch-based logic AND and OR circuits with low operating voltages below 2 V by integrating 5 × 5 arrays of PVP/PMF switching devices on the flexible substrate. In particular, this low operating voltage of our logic circuits was much lower than that (>5 V) of the circuits configured by polymer resistive random access memory. This research successfully presents the feasibility of PVP/PMF atomic switches for flexible integrated circuits for next-generation electronic applications.

Glycosaminoglycan degradation reduces mineralized tissue-titanium interfacial strength.

Although the localization of the proteoglycan/glycosaminoglycan (GAG) complex at the bone-titanium implant interface has been implied, the role of proteoglycans on the establishment of bone-titanium integration is unknown. The hypothesis to be tested was that proteoglycans play an important role in establishing bone-titanium interfacial adhesion. The objective of this study is to investigate the effect of proteoglycan knockdown by GAG enzymatic degradation on the interfacial strength between mineralized tissue and titanium having different surface topographies. Rat bone marrow-derived osteoblastic cells were cultured on either a machined titanium disk or an acid-etched titanium disk. At day 21 of culture, one of the three following GAG degradation enzymes was added into the culture; chondroitinase AC, chondroitinase B, or keratanase. After 3 days of incubation (at day 24 of culture), the laser spallation technique was applied to the samples in order to assess the tissue-titanium interfacial strength. In this technique, a laser-generated stress wave is used to separate the tissue-titanium interface, and the interfacial strength is determined interferometrically by recording the transient free surface velocity of the tissue. Mineralized tissue cultured on the acid-etched titanium showed 20-30% higher tissue interfacial strength than that cultured on the machined titanium (p < 0.0001). For both the machined and acid-etched surface cultures, administration of the enzyme reduced the interfacial strength by 25-30% compared with the untreated control cultures (p < 0.0001). There were no differences in the effect among the three different enzymes tested. A nanoindentation study revealed that the enzyme treatment did not affect the elastic modulus of the mineralized tissue. Scanning electron microscopic and energy dispersive spectroscopic analyses revealed less post-spallation tissue remnant on the titanium substrates when treated with the enzymes. The tissue remnant was greater in amount on the acid-etched surface than on the machined surface. The results suggest that there exists not only mechanical interlocking but also biological interfacial adhesion between the mineralized tissue and titanium, in which the proteoglycan/GAG complex is involved.

Effects of Electroacupuncture for Knee Osteoarthritis: A Systematic Review and Meta-Analysis.

Purpose. This study aims to verify the effects of electroacupuncture treatment on osteoarthritis of the knee. Methods. MEDLINE/PubMed, EMBASE, CENTRAL, AMED, CNKI, and five Korean databases were searched by predefined search strategies to screen eligible randomized controlled studies meeting established criteria. Any risk of bias in the included studies was assessed with the Cochrane Collaboration's tool. Meta-analysis was conducted using RevMan version 5.3 software. Results. Thirty-one randomized controlled studies of 3,187 participants were included in this systematic review. Meta-analysis was conducted with eight studies including a total of 1,220 participants. The electroacupuncture treatment group showed more significant improvement in pain due to knee osteoarthritis than the control group (SMD -1.86, 95% CI -2.33 to -1.39, I2 75%) and in total WOMAC score than the control group (SMD -1.34, CI 95% -1.85 to -0.83, I2 73%). Compared to the control group, the electroacupuncture treatment group showed more significant improvement on the quality of life scale. Conclusion. Electroacupuncture treatment can relieve the pain of osteoarthritis of the knees and improve comprehensive aspects of knee osteoarthritis and the quality of life of patients with knee osteoarthritis.