Ambipolar insulator-to-metal transition in black phosphorus by ionic-liquid gating

Unveiling cutting-edge developments: architectures and nanostructured materials for application in optoelectronic artificial synapses

One possible result of low-level characteristics in the traditional von Neumann formulation system is brain-inspired photonics technology based on human brain idea. Optoelectronic neural devices, which are accustomed to imitating the sensory role of biological synapses by adjusting connection measures, can be used to fabricate highly reliable neurologically calculating devices. In this case, nanosized materials and device designs are attracting attention since they provide numerous potential benefits in terms of limited cool contact, rapid transfer fluidity, and the capture of photocarriers. In addition, the combination of classic nanosized photodetectors with recently generated digital synapses offers promising results in a variety of practical applications, such as data processing and computation. Herein, we present the progress in constructing improved optoelectronic synaptic devices that rely on nanomaterials, for example, 0-dimensional (quantum dots), 1-dimensional, and 2-dimensional composites, besides the continuously developing mixed heterostructures. Furthermore, the challenges and potential prospects linked with this field of study are discussed in this paper.

Electrical Control Grain Dimensionality with Multilevel Magnetic Anisotropy

In alignment with the increasing demand for larger storage capacity and longer data retention, the electrical control of magnetic anisotropy has been a research focus in the realm of spintronics. Typically, magnetic anisotropy is determined by grain dimensionality, which is set during the fabrication of magnetic thin films. Despite the intrinsic correlation between magnetic anisotropy and grain dimensionality, there is a lack of experimental evidence for electrically controlling grain dimensionality, thereby impairing the efficiency of magnetic anisotropy modulation. Here, we demonstrate an electric field control of grain dimensionality and prove it as the active mechanism for tuning interfacial magnetism. The reduction in grain dimensionality is associated with a transition from ferromagnetic to superparamagnetic behavior. We achieve a nonvolatile and reversible modulation of the coercivity in both the ferromagnetic and superparamagnetic regimes. Subsequent electrical and elemental analysis confirms the variation in grain dimensionality upon the application of gate voltages, revealing a transition from a multidomain to a single-domain state, accompanied by a reduction in grain dimensionality. Furthermore, we exploit the influence of grain dimensionality on domain wall motion, extending its applicability to multilevel magnetic memory and synaptic devices. Our results provide a strategy for tuning interfacial magnetism through grain size engineering for advancements in high-performance spintronics.

Phase Engineering of 2D Materials

Polymorphic 2D materials allow structural and electronic phase engineering, which can be used to realize energy-efficient, cost-effective, and scalable device applications. The phase engineering covers not only conventional structural and metal-insulator transitions but also magnetic states, strongly correlated band structures, and topological phases in rich 2D materials. The methods used for the local phase engineering of 2D materials include various optical, geometrical, and chemical processes as well as traditional thermodynamic approaches. In this Review, we survey the precise manipulation of local phases and phase patterning of 2D materials, particularly with ideal and versatile phase interfaces for electronic and energy device applications. Polymorphic 2D materials and diverse quantum materials with their layered, vertical, and lateral geometries are discussed with an emphasis on the role and use of their phase interfaces. Various phase interfaces have demonstrated superior and unique performance in electronic and energy devices. The phase patterning leads to novel homo- and heterojunction structures of 2D materials with low-dimensional phase boundaries, which highlights their potential for technological breakthroughs in future electronic, quantum, and energy devices. Accordingly, we encourage researchers to investigate and exploit phase patterning in emerging 2D materials.

High-performance diodes based on black phosphorus/carbon nanomaterial heterostructures

The performance of diodes, which are the basic building blocks in integrated circuits, highly depends on the materials used. Black phosphorus (BP) and carbon nanomaterials with unique structures and excellent properties can form heterostructures with favorable band matching to fully utilize their respective advantages and thus achieve high diode performance. Here, high-performance Schottky junction diodes based on a two-dimensional (2D) BP/single-walled carbon nanotube (SWCNT) film heterostructure and a BP nanoribbon (PNR) film/graphene heterostructure were investigated for the first time. The fabricated Schottky diode based on the heterostructure with the 10 nm-thick 2D BP stacked on the SWCNT film had a rectification ratio of 2978 and a low ideal factor of 1.5. The Schottky diode based on the heterostructure with the PNR film stacked on the graphene exhibited a high rectification ratio of 4455 and an ideal factor of 1.9. The high rectification ratios for both devices were attributed to the large Schottky barriers formed between the BP and carbon materials, thus leading to a small reverse current. We found that the thickness of the 2D BP in the 2D BP/SWCNT film Schottky diode and the stacking order of the heterostructure in the PNR film/graphene Schottky diode had a significant effect on the rectification ratio. Furthermore, the rectification ratio and breakdown voltage of the resulting PNR film/graphene Schottky diode were larger than those of the 2D BP/SWCNT film Schottky diode, which was attributed to the larger bandgap of the PNRs compared to the 2D BP. This study demonstrates that high-performance diodes can be achieved via the collaborative application of BP and carbon nanomaterials.

Full Control of Solid-State Electrolytes for Electrostatic Gating

Ionic gating is a powerful technique to realize field-effect transistors (FETs) enabling experiments not possible otherwise. So far, ionic gating has relied on the use of top electrolyte gates, which pose experimental constraints and make device fabrication complex. Promising results obtained recently in FETs based on solid-state electrolytes remain plagued by spurious phenomena of unknown origin, preventing proper transistor operation, and causing limited control and reproducibility. Here, a class of solid-state electrolytes for gating (Lithium-ion conducting glass-ceramics, LICGCs) is explored, the processes responsible for the spurious phenomena and irreproducible behavior are identified, and properly functioning transistors exhibiting high density ambipolar operation with gate capacitance of ≈ 20   -   50   µ F c m - 2 \[20{\bm{ - }}50\;\mu F c{m^{{\bm{ - }}2}}\] (depending on the polarity of the accumulated charges) are demonstrated. Using 2D semiconducting transition-metal dichalcogenides, the ability to implement ionic-gate spectroscopy to determine the semiconducting bandgap, and to accumulate electron densities above 10¹⁴ cm^-2 are demostrated, resulting in gate-induced superconductivity in MoS₂ multilayers. As LICGCs are implemented in a back-gate configuration, they leave the surface of the material exposed, enabling the use of surface-sensitive techniques (such as scanning tunneling microscopy and photoemission spectroscopy) impossible so far in ionic-gated devices. They also allow double ionic gated devices providing independent control of charge density and electric field.

A novel CVD graphene-based synaptic transistors with ionic liquid gate

The synaptic devices based on various electronic materials have been widely investigated to realize functions of artificial information processing with low power consumption. In this work, a novel CVD graphene field-effect transistor is fabricated with ionic liquid gate to study the synaptic behaviors based on the electrical-double-layer mechanism. It is found that the excitative current is enhanced with the pulse width, voltage amplitude and frequency. With different situations of the applied pulse voltage, the inhibitory and excitatory behaviors are successfully simulated, at the same time the short-term memory is also realized. The corresponding ions migration and charge density variation are analyzed in the different time segments. This work provides the guidance for the design of artificial synaptic electronics with ionic liquid gate for low-power computing application.

Research trends in biomedical applications of two-dimensional nanomaterials over the last decade - A bibliometric analysis

Two-dimensional (2D) nanomaterials with versatile properties have been widely applied in the field of biomedicine. Despite various studies having reviewed the development of biomedical 2D nanomaterials, there is a lack of a study that objectively summarizes and analyzes the research trend of this important field. Here, we employ a series of bibliometric methods to identify the development of the 2D nanomaterial-related biomedical field during the past 10 years from a holistic point of view. First, the annual publication/citation growth, country/institute/author distribution, referenced sources, and research hotspots are identified. Thereafter, based on the objectively identified research hotspots, the contributions of 2D nanomaterials to the various biomedical subfields, including those of biosensing, imaging/therapy, antibacterial treatment, and tissue engineering are carefully explored, by considering the intrinsic properties of the nanomaterials. Finally, prospects and challenges have been discussed to shed light on the future development and clinical translation of 2D nanomaterials. This review provides a novel perspective to identify and further promote the development of 2D nanomaterials in biomedical research.

Tunable and anisotropic perfect absorber using graphene-black phosphorus nanoblock

Two-dimensional (2D) materials, which have attracted attention due to intriguing optical properties, form a promising building block in optical and photonic devices. This paper numerically investigates a tunable and anisotropic perfect absorber in a graphene-black phosphorus (BP) nanoblock array structure. The suggested structure exhibits polarization-dependent anisotropic absorption in the mid-infrared, with maximum absorption of 99.73% for x-polarization and 53.47% for y-polarization, as determined by finite-difference time-domain FDTD analysis. Moreover, geometrical parameters and graphene and BP doping amounts are possibly employed to tailor the absorption spectra of the structures. Hence, our results have the potential in the design of polarization-selective and tunable high-performance devices in the mid-infrared, such as polarizers, modulators, and photodetectors.

Anomalous Metallic Phase in Molybdenum Disulphide Induced via Gate-Driven Organic Ion Intercalation

Transition metal dichalcogenides exhibit rich phase diagrams dominated by the interplay of superconductivity and charge density waves, which often result in anomalies in the electric transport properties. Here, we employ the ionic gating technique to realize a tunable, non-volatile organic ion intercalation in bulk single crystals of molybdenum disulphide (MoS2). We demonstrate that this gate-driven organic ion intercalation induces a strong electron doping in the system without changing the pristine 2H crystal symmetry and triggers the emergence of a re-entrant insulator-to-metal transition. We show that the gate-induced metallic state exhibits clear anomalies in the temperature dependence of the resistivity with a natural explanation as signatures of the development of a charge-density wave phase which was previously observed in alkali-intercalated MoS2. The relatively large temperature at which the anomalies are observed (∼150 K), combined with the absence of any sign of doping-induced superconductivity down to ∼3 K, suggests that the two phases might be competing with each other to determine the electronic ground state of electron-doped MoS2.

Fabrication Technologies for the On-Chip Integration of 2D Materials

With compact footprint, low energy consumption, high scalability, and mass producibility, chip-scale integrated devices are an indispensable part of modern technological change and development. Recent advances in 2D layered materials with their unique structures and distinctive properties have motivated their on-chip integration, yielding a variety of functional devices with superior performance and new features. To realize integrated devices incorporating 2D materials, it requires a diverse range of device fabrication techniques, which are of fundamental importance to achieve good performance and high reproducibility. This paper reviews the state-of-art fabrication techniques for the on-chip integration of 2D materials. First, an overview of the material properties and on-chip applications of 2D materials is provided. Second, different approaches used for integrating 2D materials on chips are comprehensively reviewed, which are categorized into material synthesis, on-chip transfer, film patterning, and property tuning/modification. Third, the methods for integrating 2D van der Waals heterostructures are also discussed and summarized. Finally, the current challenges and future perspectives are highlighted.

High Photoresponse Black Phosphorus TFTs Capping with Transparent Hexagonal Boron Nitride

Black phosphorus (BP), a single elemental two-dimensional (2D) material with a sizable band gap, meets several critical material requirements in the development of future nanoelectronic applications. This work reports the ambipolar characteristics of few-layer BP, induced using 2D transparent hexagonal boron nitride (h-BN) capping. The 2D h-BN capping have several advantages over conventional Al₂O₃ capping in flexible and transparent 2D device applications. The h-BN capping technique was used to achieve an electron mobility in the BP devices of 73 cm²V⁻¹s⁻¹, thereby demonstrating n-type behavior. The ambipolar BP devices exhibited ultrafast photodetector behavior with a very high photoresponsivity of 1980 mA/W over the ultraviolet (UV), visible, and infrared (IR) spectral ranges. The h-BN capping process offers a feasible approach to fabricating n-type behavior BP semiconductors and high photoresponse BP photodetectors.

Evidence of band filling in PbS colloidal quantum dot square superstructures

PbS square superstructures are formed by the oriented assembly of PbS quantum dots (QDs), reflecting the facet structures of each QD. In the square assembly, the quantum dots are highly oriented, in sharp contrast to the conventional hexagonal QD assemblies, in which the orientation of QDs is highly disordered, and each QD is connected through ligand molecules. Here, we measured the transport properties of the oriented assembly of PbS square superstructures. The combined electrochemical doping studies by electric double layer transistor (EDLT) and spectroelectrochemistry showed that more than fourteen electrons per quantum dot are introduced. Furthermore, we proved that the lowest conduction band is formed by the quasi-fourth degenerate quantized (1S_e) level in the PbS QD square superstructures.

Emerging 2D Memory Devices for In-Memory Computing

It is predicted that the conventional von Neumann computing architecture cannot meet the demands of future data-intensive computing applications due to the bottleneck between the processing and memory units. To try to solve this problem, in-memory computing technology, where calculations are carried out in situ within each nonvolatile memory unit, has been intensively studied. Among various candidate materials, 2D layered materials have recently demonstrated many new features that have been uniquely exploited to build next-generation electronics. Here, the recent progress of 2D memory devices is reviewed for in-memory computing. For each memory configuration, their operation mechanisms and memory characteristics are described, and their pros and cons are weighed. Subsequently, their versatile applications for in-memory computing technology, including logic operations, electronic synapses, and random number generation are presented. Finally, the current challenges and potential strategies for future 2D in-memory computing systems are also discussed at the material, device, circuit, and architecture levels. It is hoped that this manuscript could give a comprehensive review of 2D memory devices and their applications in in-memory computing, and be helpful for this exciting research area.

Ionic gating in metallic superconductors: A brief review

Ionic gating is a very popular tool to investigate and control the electric charge transport and electronic ground state in a wide variety of different materials. This is due to its capability to induce large modulations of the surface charge density by means of the electric-double-layer field-effect transistor (EDL-FET) architecture, and has been proven to be capable of tuning even the properties of metallic systems. In this short review, I summarize the main results which have been achieved so far in controlling the superconducting (SC) properties of thin films of conventional metallic superconductors by means of the ionic gating technique. I discuss how the gate-induced charge doping, despite being confined to a thin surface layer by electrostatic screening, results in a long-range ‘bulk’ modulation of the SC properties by the coherent nature of the SC condensate, as evidenced by the observation of suppressions in the critical temperature of films much thicker than the electrostatic screening length, and by the pronounced thickness-dependence of their magnitude. I review how this behavior can be modelled in terms of proximity effect between the charge-doped surface layer and the unperturbed bulk with different degrees of approximation, and how first-principles calculations have been employed to determine the origin of an anomalous increase in the electrostatic screening length at ultrahigh electric fields, thus fully confirming the validity of the proximity effect model. Finally, I discuss a general framework—based on the combination of ab-initio Density Functional Theory and the Migdal-Eliashberg theory of superconductivity—by which the properties of any gated thin film of a conventional metallic superconductor can be determined purely from first principles.

2D phosphorene nanosheets, quantum dots, nanoribbons: synthesis and biomedical applications

Phosphorene, also known as black phosphorus (BP), is a two-dimensional (2D) material that has gained significant attention in several areas of current research. Its unique properties such as outstanding surface activity, an adjustable bandgap width, favorable on/off current ratios, infrared-light responsiveness, good biocompatibility, and fast biodegradation differentiate this material from other two-dimensional materials. The application of BP in the biomedical field has been rapidly emerging over the past few years. This article aimed to provide a comprehensive review of the recent progress on the unique properties and extensive medical applications for BP in bone, nerve, skin, kidney, cancer, and biosensing related treatment. The details of applications of BP in these fields were summarized and discussed.

Recent advances in black phosphorus-based electrochemical sensors: A review

Since the discovery of liquid-phase-exfoliated black phosphorus (BP) as a field-effect transistor in 2014, BP, with its 2D layered structure, has attracted significant attention, owing to its anisotropic electroconductivity, tunable direct bandgap, extraordinary surface activity, moderate switching ratio, high hole mobility, good biocompatibility, and biodegradability. Several pioneering research efforts have explored the application of BP in different types of electrochemical sensors. This review summarizes the latest synthesis methods, protection strategies, and electrochemical sensing applications of BP and its derivatives. The typical synthesis methods for BP-based crystals, nanosheets, and quantum dots are discussed in detail; the degradation of BP under ambient conditions is introduced; and state-of-the-art protection methodologies for enhancing BP stability are explored. Various electrochemical sensing applications, including chemically modified electrodes, electrochemiluminescence sensors, enzyme electrodes, electrochemical aptasensors, electrochemical immunosensors, and ion-selective electrodes are discussed in detail, along with the mechanisms of BP functionalization, sensing strategies, and sensing properties. Finally, the major challenges in this field are outlined and future research avenues for BP-based electrochemical sensors are highlighted.

Memristors With Controllable Data Volatility by Loading Metal Ion-Added Ionic Liquids

We demonstrate a new memristive device (IL-Memristor), in which an ionic liquid (IL) serve as a material to control the volatility of the resistance. ILs are ultra-low vapor pressure liquids consisting of cations and anions at room temperature, and their introduction into solid-state processes can provide new avenues in electronic device fabrication. Because the device resistance change in IL-Memristor is governed by a Cu filament formation/rupture in IL, we considered that the Cu filament stability affects the data retention characteristics. Therefore, we controlled the data retention time by clarifying the corrosion mechanism and performing the IL material design based on the results. It was found out that the corrosion of Cu filaments in the IL was ruled by the comproportionation reaction, and that the data retention characteristics of the devices varied depending on the valence of Cu ions added to the IL. Actually, IL-Memristors involving Cu(II) and Cu(I) show volatile and non-volatile nature with respect to the programmed resistance value, respectively. Our results showed that data volatility can be controlled through the metal ion species added to the IL. The present work indicates that IL-memristor is suitable for unique applications such as artificial neuron with tunable fading characteristics that is applicable to phenomena with a wide range of timescale.

Slow-light analysis based on tunable plasmon-induced transparency in patterned black phosphorus metamaterial

In this paper, a tunable plasmon-induced transparency (PIT) structure based on a monolayer black phosphorus metamaterial is designed. In the structure, destructive interference between the bright and dark modes produces a significant PIT in the midinfrared band. Numerical simulation and theoretical calculation methods are utilized to analyze the tunable PIT effect of black phosphorus (BP). Finite-difference-time-domain simulations are consistent with theoretical calculations by coupled mode theory in the terahertz frequency band. We explored the anisotropy of a BP-based metasurface structure. By varying the geometrical parameters and carrier concentration of the monolayer BP, the interaction between the bright and dark modes in the structure can be effectively adjusted, and the active adjustment of the PIT effect is achieved. Further, the structure's group index can be as high as 139, which provides excellent slow-light performance. This study offers a new possibility for the practical applications of BP in micro-nano slow-light devices.

Solution-gated transistors of two-dimensional materials for chemical and biological sensors: status and challenges

Two-dimensional (2D) materials have been the focus of materials research for many years due to their unique fascinating properties and large specific surface area (SSA). They are very sensitive to the analytes (ions, glucose, DNA, protein, etc.), resulting in their wide-spread development in the field of sensing. New 2D materials, as the basis of applications, are constantly being fabricated and comprehensively studied. In a variety of sensing applications, the solution-gated transistor (SGT) is a promising biochemical sensing platform because it can work at low voltage in different electrolytes, which is ideal for monitoring body fluids in wearable electronics, e-skin, or implantable devices. However, there are still some key challenges, such as device stability and reproducibility, that must be faced in order to pave the way for the development of cost-effective, flexible, and transparent SGTs with 2D materials. In this review, the device preparation, device physics, and the latest application prospects of 2D materials-based SGTs are systematically presented. Besides, a bold perspective is also provided for the future development of these devices.

Recent advances in two-dimensional ferromagnetism: materials synthesis, physical properties and device applications

Two-dimensional (2D) ferromagnetism is critical for both scientific investigation and technological development owing to its low-dimensionality that brings in quantization of electronic states as well as free axes for device modulation. However, the scarcity of high-temperature 2D ferromagnets has been the obstacle of many research studies, such as the quantum anomalous Hall effect (QAHE) and thin-film spintronics. Indeed, in the case of the isotropic Heisenberg model with finite-range exchange interactions as an example, low-dimensionality is shown to be contraindicated with ferromagnetism. However, the advantages of low-dimensionality for micro-scale patterning could enhance the Curie temperature (T_C) of 2D ferromagnets beyond the T_C of bulk materials, opening the door for designing high-temperature ferromagnets in the 2D limit. In this paper, we review the recent advances in the field of 2D ferromagnets, including their material systems, physical properties, and potential device applications.

Photochemical reaction on graphene surfaces controlled by substrate-surface modification with polar self-assembled monolayers

The unique thinness of two-dimensional materials enables control over chemical phenomena at their surfaces by means of various gating techniques. For example, gating methods based on field-effect-transistor configurations have been achieved. Here, we report a molecular gating approach that employs a local electric field generated by a polar self-assembled monolayer formed on a supporting substrate. By performing Raman scattering spectroscopy analyses with a proper data correction procedure, we found that molecular gating is effective for controlling solid phase photochemical reactions of graphene with benzoyl peroxide. Molecular gating offers a simple method to control chemical reactions on the surfaces of two-dimensional materials because it requires neither the fabrication of a transistor structure nor the application of an external voltage.

Two-Dimensional Pnictogen for Field-Effect Transistors

Two-dimensional (2D) layered materials hold great promise for various future electronic and optoelectronic devices that traditional semiconductors cannot afford. 2D pnictogen, group-VA atomic sheet (including phosphorene, arsenene, antimonene, and bismuthene) is believed to be a competitive candidate for next-generation logic devices. This is due to their intriguing physical and chemical properties, such as tunable midrange bandgap and controllable stability. Since the first black phosphorus field-effect transistor (FET) demo in 2014, there has been abundant exciting research advancement on the fundamental properties, preparation methods, and related electronic applications of 2D pnictogen. Herein, we review the recent progress in both material and device aspects of 2D pnictogen FETs. This includes a brief survey on the crystal structure, electronic properties and synthesis, or growth experiments. With more device orientation, this review emphasizes experimental fabrication, performance enhancing approaches, and configuration engineering of 2D pnictogen FETs. At the end, this review outlines current challenges and prospects for 2D pnictogen FETs as a potential platform for novel nanoelectronics.

Direct Observation of Band Gap Renormalization in Layered Indium Selenide

Manipulation of intrinsic electronic structures by electron or hole doping in a controlled manner in van der Waals layered materials is the key to control their electrical and optical properties. Two-dimensional indium selenide (InSe) semiconductor has attracted attention due to its direct band gap and ultrahigh mobility as a promising material for optoelectronic devices. In this work, we manipulate the electronic structure of InSe by in situ surface electron doping and obtain a significant band gap renormalization of ∼120 meV directly observed by high-resolution angle resolved photoemission spectroscopy. This moderate doping level (carrier concentration of 8.1 × 10¹² cm^-2) can be achieved by electrical gating in field effect transistors, demonstrating the potential to design of broad spectral response devices.

Tunable and Anisotropic Dual-Band Metamaterial Absorber Using Elliptical Graphene-Black Phosphorus Pairs

We numerically propose a dual-band absorber in the infrared region based on periodic elliptical graphene-black phosphorus (BP) pairs. The proposed absorber exhibits near-unity anisotropic absorption for both resonances due to the combination of graphene and BP. Each of the resonances is independently tunable via adjusting the geometric parameters. Besides, doping levels of graphene and BP can also tune resonant properties effectively. By analyzing the electric field distributions, surface plasmon resonances are observed in the graphene-BP ellipses, contributing to the strong and anisotropic plasmonic response. Moreover, the robustness for incident angles and polarization sensitivity are also illustrated.

Ambipolar and n/p-type conduction enhancement of two-dimensional materials by surface charge transfer doping

The controllable and wide-range modulation of the carrier type and mobility in atomically thin two-dimensional (2D) materials is one of the most critical issues to be addressed before 2D materials can be practically used for future electronic and optoelectronic devices. In this work, we propose using a novel surface charge transfer mechanism to accomplish the controllable and wide-range modulation of the carrier type and mobility in 2D materials. Our methodology uses a solution of triphenylboron (TPB) to physically coat 2D materials; the TPB molecule contains positive and negative charge centers that are spatially separable when induced by an electrical field. Consequently, the TPB can transfer either positive or negative charges to 2D materials depending on the direction of the applied electrical field and thus enhance the ambipolar behavior of the 2D-material FET. This method is so versatile that seven types of 2D materials including graphene, black phosphorus and five transition metal dichalcogenides (TMDCs) can be modulated to strong ambipolar behavior with significantly increased conduction. In addition, selectively suppressing or enhancing the negative charge center enables solely p-type and n-type doping. We also accomplish the precise tuning of carrier mobility in TMDCs from ambipolar to p-type by coating a mixture of TPB/BCF in certain concentration ratios.

Gate-Tuned Insulator-Metal Transition in Electrolyte-Gated Transistors Based on Tellurene

Tellurene is a recently discovered 2D material with high hole mobility and air stability, rendering it a good candidate for future applications in electronics, optoelectronics, and energy devices. However, the physical properties of tellurene remain poorly understood. In this paper, we report on the fabrication and characterization of high-performance electrolyte-gated transistors (EGTs) based on solution-grown tellurene flakes <30 nm in thickness. Both Hall measurements and resistance-temperature behavior down to 2 K are recorded at multiple gate voltages, and an electronic phase diagram is generated. The results show that it is possible to cross the insulator-metal transition in tellurene EGTs by tuning gate voltage, achieving mobility up to ∼500 cm² V^-1 s^-1. In particular, a truly metallic 2D state is observed at gate-induced hole densities >1 × 10¹³ cm^-2, as confirmed by the temperature dependence of resistance and magnetoresistance measurements. Wide-range tuning of the electronic ground state of tellurene is thus achievable in EGTs, opening up new opportunities to realize electrical control of its physical properties.

Electric Double Layer Field-Effect Transistors Using Two-Dimensional (2D) Layers of Copper Indium Selenide (CuIn7Se11)

Innovations in the design of field-effect transistor (FET) devices will be the key to future application development related to ultrathin and low-power device technologies. In order to boost the current semiconductor device industry, new device architectures based on novel materials and system need to be envisioned. Here we report the fabrication of electric double layer field-effect transistors (EDL-FET) with two-dimensional (2D) layers of copper indium selenide (CuIn7Se11) as the channel material and an ionic liquid electrolyte (1-Butyl-3-methylimidazolium hexafluorophosphate (BMIM-PF6)) as the gate terminal. We found one order of magnitude improvement in the on-off ratio, a five- to six-times increase in the field-effect mobility, and two orders of magnitude in the improvement in the subthreshold swing for ionic liquid gated devices as compared to silicon dioxide (SiO2) back gates. We also show that the performance of EDL-FETs can be enhanced by operating them under dual (top and back) gate conditions. Our investigations suggest that the performance of CuIn7Se11 FETs can be significantly improved when BMIM-PF6 is used as a top gate material (in both single and dual gate geometry) instead of the conventional dielectric layer of the SiO2 gate. These investigations show the potential of 2D material-based EDL-FETs in developing active components of future electronics needed for low-power applications.

Optically Stimulated Artificial Synapse Based on Layered Black Phosphorus

The translation of biological synapses onto a hardware platform is an important step toward the realization of brain-inspired electronics. However, to mimic biological synapses, devices till-date continue to rely on the need for simultaneously altering the polarity of an applied electric field or the output of these devices is photonic instead of an electrical synapse. As the next big step toward practical realization of optogenetics inspired circuits that exhibit fidelity and flexibility of biological synapses, optically-stimulated synaptic devices without a need to apply polarity-altering electric field are needed. Utilizing a unique photoresponse in black phosphorus (BP), here reported is an all-optical pathway to emulate excitatory and inhibitory action potentials by exploiting oxidation-related defects. These optical synapses are capable of imitating key neural functions such as psychological learning and forgetting, spatiotemporally correlated dynamic logic and Hebbian spike-time dependent plasticity. These functionalities are also demonstrated on a flexible platform suitable for wearable electronics. Such low-power consuming devices are highly attractive for deployment in neuromorphic architectures. The manifestation of cognition and spatiotemporal processing solely through optical stimuli provides an incredibly simple and powerful platform to emulate sophisticated neural functionalities such as associative sensory data processing and decision making.

Mapping multi-valley Lifshitz transitions induced by field-effect doping in strained MoS2 nanolayers

Gate-induced superconductivity at the surface of nanolayers of semiconducting transition metal dichalcogenides (TMDs) has attracted a lot of attention in recent years, thanks to the sizeable transition temperature, robustness against in-plane magnetic fields beyond the Pauli limit, and hints to a non-conventional nature of the pairing. A key information necessary to unveil its microscopic origin is the geometry of the Fermi surface hosting the Cooper pairs as a function of field-effect doping, which is dictated by the filling of the inequivalent valleys at the K/K[Formula: see text] and Q/Q[Formula: see text] points of the Brillouin zone. Here, we achieve this by combining density functional theory calculations of the bandstructure with transport measurements on ion-gated 2H-MoS₂ nanolayers. We show that, when the number of layers and the amount of strain are set to their experimental values, the Fermi level crosses the bottom of the high-energy valleys at Q/Q[Formula: see text] at doping levels where characteristic kinks in the transconductance are experimentally detected. We also develop a simple 2D model which is able to quantitatively describe the broadening of the kinks observed upon increasing temperature. We demonstrate that this combined approach can be employed to map the dependence of the Fermi surface of TMD nanolayers on field-effect doping, detect Lifshitz transitions, and provide a method to determine the amount of strain and spin-orbit splitting between sub-bands from electric transport measurements in real devices.

Anisotropic infrared plasmonic broadband absorber based on graphene-black phosphorus multilayers

Two-dimensional materials (2DMs) such as graphene and black phosphorus (BP) have aroused considerable attentions in the past few years. Engineering and enhancing their light-matter interaction is possible due to their support for localized surface plasmon resonances in the infrared regime. In this paper, we have proposed an infrared broadband absorber consisting of multilayer graphene-BP nanoparticles sandwiched between dielectric layers. Benefiting from the properties of graphene and BP, the absorber exhibits both perfect broadband responses and strong anisotropy beyond individual graphene and BP layers. The absorber is tunable with the variation of geometric parameters as well as the doping levels of graphene and BP. The physical insight is revealed by electric field distributions. Furthermore, the angular robustness for incident wave is investigated. The proposed anisotropic omnidirectional broadband absorber may have promising potential applications in various biosensing, communication and imaging systems.

Electrically-Transduced Chemical Sensors Based on Two-Dimensional Nanomaterials

Electrically-transduced sensors, with their simplicity and compatibility with standard electronic technologies, produce signals that can be efficiently acquired, processed, stored, and analyzed. Two dimensional (2D) nanomaterials, including graphene, phosphorene (BP), transition metal dichalcogenides (TMDCs), and others, have proven to be attractive for the fabrication of high-performance electrically-transduced chemical sensors due to their remarkable electronic and physical properties originating from their 2D structure. This review highlights the advances in electrically-transduced chemical sensing that rely on 2D materials. The structural components of such sensors are described, and the underlying operating principles for different types of architectures are discussed. The structural features, electronic properties, and surface chemistry of 2D nanostructures that dictate their sensing performance are reviewed. Key advances in the application of 2D materials, from both a historical and analytical perspective, are summarized for four different groups of analytes: gases, volatile compounds, ions, and biomolecules. The sensing performance is discussed in the context of the molecular design, structure-property relationships, and device fabrication technology. The outlook of challenges and opportunities for 2D nanomaterials for the future development of electrically-transduced sensors is also presented.

Anomalous Cooling-Rate-Dependent Charge Transport in Electrolyte-Gated Rubrene Crystals

Although electrolyte gating has been demonstrated to enable control of electronic phase transitions in many materials, long sought-after gate-induced insulator-metal transitions in organic semiconductors remain elusive. To better understand limiting factors in this regard, here we report detailed wide-range resistance-temperature ( R- T) measurements at multiple gate voltages on ionic-liquid-gated rubrene single crystals. Focusing on the previously observed high-bias regime where conductance anomalously decreases with increasing bias magnitude, we uncover two surprising (and related) features. First, distinctly cooling-rate-dependent transport is detected for the first time. Second, power law R- T is observed over a significant T window, which is highly unusual in an insulator. These features are discussed in terms of electronic disorder at the rubrene/ionic liquid interface influenced by (i) cooling-rate-dependent structural order in the ionic liquid and (ii) the intriguing possibility of a gate-induced glassy short-range charge-ordered state in rubrene. These results expose new physics at the gated rubrene surface, pointing to exciting new directions in the field.

Monolayer Transition Metal Dichalcogenides as Light Sources

Reducing the dimensions of materials is one of the key approaches to discovering novel optical phenomena. The recent emergence of 2D transition metal dichalcogenides (TMDCs) has provided a promising platform for exploring new optoelectronic device applications, with their tunable electronic properties, structural controllability, and unique spin valley-coupled systems. This progress report provides an overview of recent advances in TMDC-based light-emitting devices discussed from several aspects in terms of device concepts, material designs, device fabrication, and their diverse functionalities. First, the advantages of TMDCs used in light-emitting devices and their possible functionalities are presented. Second, conventional approaches for fabricating TMDC light-emitting devices are emphasized, followed by introducing a newly established, versatile method for generating light emission in TMDCs. Third, current growing technologies for heterostructure fabrication, in which distinct TMDCs are vertically stacked or laterally stitched, are explained as a possible means for designing high-performance light-emitting devices. Finally, utilizing the topological features of TMDCs, the challenges for controlling circularly polarized light emission and its device applications are discussed from both theoretical and experimental points of view.

Ambipolar ferromagnetism by electrostatic doping of a manganite

Complex-oxide materials exhibit physical properties that involve the interplay of charge and spin degrees of freedom. However, an ambipolar oxide that is able to exhibit both electron-doped and hole-doped ferromagnetism in the same material has proved elusive. Here we report ambipolar ferromagnetism in LaMnO₃, with electron-hole asymmetry of the ferromagnetic order. Starting from an undoped atomically thin LaMnO₃ film, we electrostatically dope the material with electrons or holes according to the polarity of a voltage applied across an ionic liquid gate. Magnetotransport characterization reveals that an increase of either electron-doping or hole-doping induced ferromagnetic order in this antiferromagnetic compound, and leads to an insulator-to-metal transition with colossal magnetoresistance showing electron-hole asymmetry. These findings are supported by density functional theory calculations, showing that strengthening of the inter-plane ferromagnetic exchange interaction is the origin of the ambipolar ferromagnetism. The result raises the prospect of exploiting ambipolar magnetic functionality in strongly correlated electron systems.

Electric-field Control of Electronic States in WS2 Nanodevices by Electrolyte Gating

A method of carrier number control by electrolyte gating is demonstrated. We have obtained WS2 thin flakes with atomically flat surface via scotch tape method or individual WS2 nanotubes by dispersing the suspension of WS2 nanotubes. The selected samples have been fabricated into devices by the use of the electron beam lithography and electrolyte is put on the devices. We have characterized the electronic properties of the devices under applying the gate voltage. In the small gate voltage region, ions in the electrolyte are accumulated on the surface of the samples which leads to the large electric potential drop and resultant electrostatic carrier doping at the interface. Ambipolar transfer curve has been observed in this electrostatic doping region. When the gate voltage is further increased, we met another drastic increase of source-drain current which implies that ions are intercalated into layers of WS2 and electrochemical carrier doping is realized. In such electrochemical doping region, superconductivity has been observed. The focused technique provides a powerful strategy for achieving the electric-filed-induced quantum phase transition.

Gate-Induced Metal-Insulator Transition in MoS2 by Solid Superionic Conductor LaF3

Electric-double-layer (EDL) gating with liquid electrolyte has been a powerful tool widely used to explore emerging interfacial electronic phenomena. Due to the large EDL capacitance, a high carrier density up to 10¹⁴ cm^-2 can be induced, directly leading to the realization of field-induced insulator to metal (or superconductor) transition. However, the liquid nature of the electrolyte has created technical issues including possible side electrochemical reactions or intercalation, and the potential for huge strain at the interface during cooling. In addition, the liquid coverage of active devices also makes many surface characterizations and in situ measurements challenging. Here, we demonstrate an all solid-state EDL device based on a solid superionic conductor LaF₃, which can be used as both a substrate and a fluorine ionic gate dielectric to achieve a wide tunability of carrier density without the issues of strain or electrochemical reactions and can expose the active device surface for external access. Based on LaF₃ EDL transistors (EDLTs), we observe the metal-insulator transition in MoS₂. Interestingly, the well-defined crystal lattice provides a more uniform potential distribution in the substrate, resulting in less interface electron scattering and therefore a higher mobility in MoS₂ transistors. This result shows the powerful gating capability of LaF₃ solid electrolyte for new possibilities of novel interfacial electronic phenomena.

Top-gated black phosphorus phototransistor for sensitive broadband detection

The present work reports on a graphene-like material that is promising for photodetection applications due to its high optical absorption and layer-dependent properties. To date, only narrowband photodetectors have been realized; therefore, extending the working wavelength is becoming more imperative for applications such as high-contrast imaging and remote sensing. In this work, we developed a novel detection technique that provides enhanced performance across the infrared and terahertz bands by using an antenna-assisted top-gated black phosphorus phototransistor. By using the proposed sophisticated design, the adverse effect due to the back-gate that is generally employed for a long-wavelength photon coupling can be eliminated. Moreover, the antenna-assisted near-field and dark current can be further tailored electromagnetically and electrostatically by employing a gate finger, thus resulting in improved detection efficiency. Various detection mechanisms such as thermoelectric, bolometric, and electron-hole generation are differentiated on the basis of the device geometry and incident wavelength. The proposed photodetector demonstrated superior performance-excellent sensitivity of more than 10 V W-1, a noise equivalent power value of less than 0.1 nW Hz-0.5, and a fast response time across disparate wavebands. Thus, the photodetector can satisfy diverse application requirements.

Ionic liquid gating control of RKKY interaction in FeCoB/Ru/FeCoB and (Pt/Co)2/Ru/(Co/Pt)2 multilayers

To overcome the fundamental challenge of the weak natural response of antiferromagnetic materials under a magnetic field, voltage manipulation of antiferromagnetic interaction is developed to realize ultrafast, high-density, and power efficient antiferromagnetic spintronics. Here, we report a low voltage modulation of Ruderman-Kittel-Kasuya-Yosida (RKKY) interaction via ionic liquid gating in synthetic antiferromagnetic multilayers of FeCoB/Ru/FeCoB and (Pt/Co)₂/Ru/(Co/Pt)₂. At room temperature, the distinct voltage control of transition between antiferromagnetic and ferromagnetic ordering is realized and up to 80% of perpendicular magnetic moments manage to switch with a small-applied voltage bias of 2.5 V. We related this ionic liquid gating-induced RKKY interaction modification to the disturbance of itinerant electrons inside synthetic antiferromagnetic heterostructure and the corresponding change of its Fermi level. Voltage tuning of RKKY interaction may enable the next generation of switchable spintronics between antiferromagnetic and ferromagnetic modes with both fundamental and practical perspectives.

ON the Nature of Ionic Liquid Gating of La2−xSrxCuO4

Ionic liquids have recently been used as means of modulating the charge carrier properties of cuprates. The mechanism behind it, however, is still a matter of debate. In this paper we report experiments on ionic liquid gated ultrathin La_2−xSr_xCuO₄ films. Our results show that the electrostatic part of gating has limited influence in the conductance of the cuprate in the gate voltage range of 0 to −2 V. A non-electrostatic mechanism takes over for gate voltages below −2 V. This mechanism most likely changes the oxygen concentration of the film. The results presented are in line with previous X-ray based studies on ionic liquid gating induced oxygenation of the cuprate materials YBa₂Cu₃O_7−x and La_2−xSr_xCuO₄.

Oxygen Vacancy Dynamics at Room Temperature in Oxide Heterostructures

Oxygen vacancy dynamic behavior at room temperature in complex oxides was carefully explored by using a combined approach of ion liquid gating technique and resistance measurements. Heterostructures of PrBaCo₂O_5+δ/Gd₂O₃-doped CeO₂ epitaxial thin films were fabricated on (001) Y₂O₃-stabilized ZrO₂ single crystal substrates for systematically investigating the oxygen redox dynamics. The oxygen dynamic changes as response to the gating voltage and duration were precisely detected by in situ resistance measurements. A reversible and nonvolatile resistive switching dynamics was detected at room temperature under the gating voltage >13.5 V with pulse duration >1 s.

Few-Layered Black Phosphorus: From Fabrication and Customization to Biomedical Applications

As a new kind of 2D material, black phosphorus has gained increased attention in the past three years. Although few-layered black phosphorus nanosheets (BPs) degrade quickly under ambient conditions to phosphate anions, which greatly hampers their optical and electronic applications, this property also makes BPs highly biocompatible and biodegradable, and is regarded as an advantage for various biomedical applications. This Concept summarizes the state-of-art progresses of BPs, from fabrication and surface modification to biomedical applications. It is expected that BPs with such fascinating properties will encourage more scientists to engage in expanding its biomedical applications by tackling the scientific challenges involved in their development.

Gate-tunable strong-weak localization transition in few-layer black phosphorus

Atomically-thin black phosphorus (BP) field-effect transistors show strong-weak localization transition, which is tunable through gate voltages. Hopping transports through charge impurity-induced localized states are observed at low carrier density regime. Variable-range hopping model is applied to simulate scattering behaviors of charge carriers. In the high carrier concentration regime, a negative magnetoresistance indicates weak localization effects. The extracted phase coherence length is power-law temperature-dependent [Formula: see text] and demonstrates inelastic electron-electron interactions and the 2D transport features in few-layer BP field-effect devices. The competition between localization and phase coherence lengths is investigated and analyzed based on observed gate-tunable strong-weak localization transition in few-layer BP.

Anomalous Temperature Dependence in Metal-Black Phosphorus Contact

Metal-semiconductor contact has been the performance limiting problem for electronic devices and also dictates the scaling potential for future generation devices based on novel channel materials. Two-dimensional semiconductors beyond graphene, particularly few layer black phosphorus, have attracted much attention due to their exceptional electronic properties such as anisotropy and high mobility. However, due to its ultrathin body nature, few layer black phosphorus-metal contact behaves differently than conventional Schottky barrier (SB) junctions, and the mechanisms of its carrier transport across such a barrier remain elusive. In this work, we examine the transport characteristic of metal-black phosphorus contact under varying temperature. We elucidated the origin of apparent negative SB heights extracted from classical thermionic emission model and also the phenomenon of metal-insulator transition observed in the current-temperature transistor characteristic. In essence, we found that the SB height can be modulated by the back-gate voltage, which beyond a certain critical point becomes so low that the injected carrier can no longer be described by the conventional thermionic emission theory. The transition from transport dominated by a Maxwell-Boltzmann distribution for the high energy tail states, to that of a Fermi distribution by low energy Fermi sea electrons, is the physical origin of the observed metal-insulator transition. We identified two distinctive tunneling limited transport regimes in the contact: vertical and longitudinal tunneling.

Prediction of phonon-mediated superconductivity in hole-doped black phosphorus

We study the conventional electron-phonon mediated superconducting properties of hole-doped black phosphorus by density functional calculations and get quite a large electron-phonon coupling (EPC) constant λ ~ 1.0 with transition temperature T _C ~ 10 K, which is comparable to MgB₂ when holes are doped into the degenerate and nearly flat energy bands around the Fermi level. We predict that the softening of low-frequency [Formula: see text] optical mode and its phonon displacement, which breaks the lattice nonsymmorphic symmetry of gliding plane and lifts the band double degeneracy, lead to a large EPC. These factors are favorable for BCS superconductivity.

Recent progress in 2D group-VA semiconductors: from theory to experiment

This review provides recent theoretical and experimental progress in the fundamental properties, electronic modulations, fabrications and applications of 2D group-VA materials.

Charge-assisted non-volatile magnetoelectric effects in NiFe2O4/PMN-PT heterostructures

The NFO/Pt/PMN-PT heterostructure can suppress the depolarization field, which enhances the polarization-dependent charge effect.