High-κ Wide-Gap Layered Dielectric for Two-Dimensional van der Waals Heterostructures

van der Waals heterostructures of two-dimensional materials have unveiled frontiers in condensed matter physics, unlocking unexplored possibilities in electronic and photonic device applications. However, the investigation of wide-gap, high-κ layered dielectrics for devices based on van der Waals structures has been relatively limited. In this work, we demonstrate an easily reproducible synthesis method for the rare-earth oxyhalide LaOBr, and we exfoliate it as a 2D layered material with a measured static dielectric constant of 9 and a wide bandgap of 5.3 eV. Furthermore, our research demonstrates that LaOBr can be used as a high-κ dielectric in van der Waals field-effect transistors with high performance and low interface defect concentrations. Additionally, it proves to be an attractive choice for electrical gating in excitonic devices based on 2D materials. Our work demonstrates the versatile realization and functionality of 2D systems with wide-gap and high-κ van der Waals dielectric environments.

Effect of Quasi-One-Dimensional Properties on Source/Drain Contacts in Vertical Nanowire Field-Effect Transistors (VNWFETs)

In this study, we investigated the influence of quasi-one-dimensional (Quasi-1D) characteristics on the source and drain contact resistances within vertical nanowire (NW) field-effect transistors (FETs) of diminutive diameter. The top contact of the NW is segregated into two distinct regions: the first encompassing the upper surface, designated as the axial contact, and the second encircling the side surface, known as the radial contact, which is formed during the top-contact metal deposition process. Quantum confinement effects, prominent within Quasi-1D NWs, exert significant constraints on radial transport, consequently inducing a noticeable impact on contact resistance. Notably, in the radial direction, electron tunneling occurs only through quantized, discrete energy levels. Conversely, along the axial direction, electron tunneling freely traverses continuous energy levels. In a meticulous numerical analysis, these disparities in transport mechanisms unveiled that NWs with diameters below 30 nm exhibit a markedly higher radial contact resistance compared to their axial counterparts. Furthermore, an increase in the overlap length (less than 5 nm) contributes to a modest reduction in radial resistance; however, it remains consistently higher than the axial contact resistance.

Synchronous Diagnosis of Respiratory Viruses Variants via Receptonics Based on Modeling Receptor-Ligand Dynamics

The transmission and pathogenesis of highly contagious fatal respiratory viruses are increasing, and the need for an on-site diagnostic platform has arisen as an issue worldwide. Furthermore, as the spread of respiratory viruses continues, different variants have become the dominant circulating strains. To prevent virus transmission, the development of highly sensitive and accurate on-site diagnostic assays is urgently needed. Herein, a facile diagnostic device is presented for multi-detection based on the results of detailed receptor-ligand dynamics simulations for the screening of various viral strains. The novel bioreceptor-treated electronics (receptonics) device consists of a multichannel graphene transistor and cell-entry receptors conjugated to N-heterocyclic carbene (NHC). An ultrasensitive multi-detection performance is achieved without the need for sample pretreatment, which will enable rapid diagnosis and prevent the spread of pathogens. This platform can be applied for the diagnosis of variants of concern in clinical respiratory virus samples and primate models. This multi-screening platform can be used to enhance surveillance and discriminate emerging virus variants before they become a severe threat to public health.

PdSe2/MoSe2: a promising van der Waals heterostructure for field effect transistor application

The field-effect transistor (FET) is a fundamental component of semiconductors and the electronic industry. High on-current and mobility with layer-dependent features are required for outstanding FET channel material. Two-dimensional materials are advantageous over bulk materials owing to their higher mobility, high ON/OFF ratio, low tunneling current, and leakage problems. Moreover, two-dimensional heterostructures provide a better way to tune electrical properties. In this work, the two distinct possibilities of PdSe2/MoSe2heterostructure have been employed through mechanical exfoliation and analyzed their electrical response. These diffe approaches to heterostructure formation serve as crucial components of our investigation, allowing us to explore and evaluate the unique electronic properties arising from each design. This work demonstrates that the heterostructure possesses a better ON/OFF ratio of ∼5.78 × 105, essential in switching characteristics. Moreover, MoSe2provides a defect-free interface to PdSe2, resulting in a higher ON current of ∼10μA and mobility of ∼63.7 cm2V-1s-1, necessary for transistor applications. In addition, comprehending the process of charge transfer occurring at the interface between transition metal dichalcogenides is fundamental for advancing next-generation technologies. This work provides insights into the interface formed between the PdSe2and MoSe2that can be harnessed in transistor applications.

Reversible Molecular Conformation Transitions of Smectic Liquid Crystals for Light/Bias-Gated Transistor Memory

In recent years, organic photonic field-effect transistors have made remarkable progress with the rapid development of conjugated polycrystalline materials. Liquid crystals, with their smooth surface, defined layer thickness, and crystalline structures, are commonly used for these advantages. In this work, a series of smectic liquid crystalline molecules, 2,9-didecyl-dinaphtho-thienothiophene (C10-DNTT), 2,7-didecyl-benzothieno-benzothiopene (C10-BTBT), 3,9-didecyl-dinaphtho-thiophene (C10-DNT), and didecyl-sexithiophene (C10-6T), have been used in photonic transistor memory, functioning as both hole-transport channels and electron traps to investigate systematically the reasons and mechanisms behind the memory behavior of smectic liquid crystals. After thermal annealing, C10-BTBT and C10-6T/C10-DNTT are homeotropically aligned from the smectic A and smectic X phases, respectively. The 3D-ordered structure of these smectic-aligned crystals contributed to efficient photowriting and electrical erasing processes. Among them, the device performance of C10-BTBT was particularly significant, with a memory window of 21 V. The memory ratio could reach 1.5 × 106 and maintain a memory ratio of over 3 orders after 10,000 s, contributing to its smectic A structure. Through the research, we confirmed the memory and light/bias-gated behaviors of these smectic liquid crystalline molecules, attributing them to reversible molecular conformation transitions and the inherent structural inhomogeneity inside the polycrystalline channel layer.

Ferroelectric-Optoelectronic Hybrid System for Photodetection

Photodetectors (PDs), as functional devices based on photon-to-electron conversion, are an indispensable component for the next-generation Internet of Things system. The research of advanced and efficient PDs that meet the diverse demands is becoming a major task. Ferroelectric materials can develop a unique spontaneous polarization due to the symmetry-breaking of the unit cell, which is switchable under an external electric field. Ferroelectric polarization field has the intrinsic characteristics of non-volatilization and rewritability. Introducing ferroelectrics to effectively manipulate the band bending and carrier transport can be non-destructive and controllable in the ferroelectric-optoelectronic hybrid systems. Hence, ferroelectric integration offers a promising strategy for high-performance photoelectric detection. This paper reviews the fundamentals of optoelectronic and ferroelectric materials, and their interactions in hybrid photodetection systems. The first section introduces the characteristics and applications of typical optoelectronic and ferroelectric materials. Then, the interplay mechanisms, modulation effects, and typical device structures of ferroelectric-optoelectronic hybrid systems are discussed. Finally, in summary and perspective section, the progress of ferroelectrics integrated PDs is summed up and the challenges of ferroelectrics in the field of optoelectronics are considered.

Coexisting Phases in Transition Metal Dichalcogenides: Overview, Synthesis, Applications, and Prospects

Over the past decade, significant advancements have been made in phase engineering of two-dimensional transition metal dichalcogenides (TMDCs), thereby allowing controlled synthesis of various phases of TMDCs and facile conversion between them. Recently, there has been emerging interest in TMDC coexisting phases, which contain multiple phases within one nanostructured TMDC. By taking advantage of the merits from the component phases, the coexisting phases offer enhanced performance in many aspects compared with single-phase TMDCs. Herein, this review article thoroughly expounds the latest progress and ongoing efforts on the syntheses, properties, and applications of TMDC coexisting phases. The introduction section overviews the main phases of TMDCs (2H, 3R, 1T, 1T', 1Td), along with the advantages of phase coexistence. The subsequent section focuses on the synthesis methods for coexisting phases of TMDCs, with particular attention to local patterning and random formations. Furthermore, on the basis of the versatile properties of TMDC coexisting phases, their applications in magnetism, valleytronics, field-effect transistors, memristors, and catalysis are discussed. Lastly, a perspective is presented on the future development, challenges, and potential opportunities of TMDC coexisting phases. This review aims to provide insights into the phase engineering of 2D materials for both scientific and engineering communities and contribute to further advancements in this emerging field.

The Integration of Two-Dimensional Materials and Ferroelectrics for Device Applications

In recent years, there has been growing interest in functional devices based on two-dimensional (2D) materials, which possess exotic physical properties. With an ultrathin thickness, the optoelectrical and electrical properties of 2D materials can be effectively tuned by an external field, which has stimulated considerable scientific activities. Ferroelectric fields with a nonvolatile and electrically switchable feature have exhibited enormous potential in controlling the electronic and optoelectronic properties of 2D materials, leading to an extremely fertile area of research. Here, we review the 2D materials and relevant devices integrated with ferroelectricity. This review starts to introduce the background about the concerned themes, namely 2D materials and ferroelectrics, and then presents the fundamental mechanisms, tuning strategies, as well as recent progress of the ferroelectric effect on the optical and electrical properties of 2D materials. Subsequently, the latest developments of 2D material-based electronic and optoelectronic devices integrated with ferroelectricity are summarized. Finally, the future outlook and challenges of this exciting field are suggested.

Power and Sensitivity Management of Carbon Nanotube Transistor Glucose Biosensors

Continuous glucose monitoring (CGM), which is significant for the daily management of diabetes, requires a low-power-consumption sensor system that can track low nanomolar levels of glucose in physiological fluids, such as sweat and tears. However, traditional electrochemical methods are limited to analytes in micromolar to millimolar ranges and entail high power consumption. Carbon nanotube (CNT) film field-effect transistors (FETs) are promising for constructing extremely sensitive biosensors, but their wide applications in CGM are limited by the strong screening effect of physiological fluids and the zero charge of glucose molecules. In this study, we demonstrate a glucose aptamer-modified CNT FET biosensor to realize a highly sensitive CGM system with sub-nW power consumption by applying a suitable gate voltage. A positive gate voltage can enlarge the effective Debye screening length at the double layer to reduce the local ion population nearby and then improve the sensitivity of the FET-based biosensors by 5 times. We construct CNT FET sensors for CGM with a limit of detection of 0.5 fM, a record dynamic range up to 109, and a power consumption down to ∼100 pW. The proposed field-modulated sensing performance scheme is applicable to other aptamer-based FET biosensors for detecting neutral or less charged molecules and opens opportunities to develop facilely modulated, highly sensitive, low-power, and noninvasive CGM systems.

Dramatically Enhanced Valley-Polarized Emission by Alloying and Electrical Tuning of Monolayer WTe2 x S2(1- x ) Alloys at Room Temperature with 1T'-WTe2 -Contact

Monolayer ternary tellurides based on alloying different transition metal dichalcogenides (TMDs) can result in new two-dimensional (2D) materials ranging from semiconductors to metals and superconductors with tunable optical and electrical properties. Semiconducting WTe2 x S2(1- x ) monolayer possesses two inequivalent valleys in the Brillouin zone, each valley coupling selectively with circularly polarized light (CPL). The degree of valley polarization (DVP) under the excitation of CPL represents the purity of valley polarized photoluminescence (PL), a critical parameter for opto-valleytronic applications. Here, new strategies to efficiently tailor the valley-polarized PL from semiconducting monolayer WTe2 x S2(1- x ) at room temperature (RT) through alloying and back-gating are presented. The DVP at RT is found to increase drastically from < 5% in WS2 to 40% in WTe0.12 S1.88 by Te-alloying to enhance the spin-orbit coupling. Further enhancement and control of the DVP from 40% up to 75% is demonstrated by electrostatically doping the monolayer WTe0.12 S1.88 via metallic 1T'-WTe2 electrodes, where the use of 1T'-WTe2 substantially lowers the Schottky barrier height (SBH) and weakens the Fermi-level pinning of the electrical contacts. The demonstration of drastically enhanced DVP and electrical tunability in the valley-polarized emission from 1T'-WTe2 /WTe0.12 S1.88 heterostructures paves new pathways towards harnessing valley excitons in ultrathin valleytronic devices for RT applications.

Investigating the Mobility-Compressibility Properties of Conjugated Polymers by the Contact Film Transfer Method with Prestrain

Up to now, researches on the mobility-stretchability of semiconducting polymers are extensively investigated, but little attention was  paid to their morphology and field-effect transistor characteristics under compressive strains, which is equally crucial in wearable electronic applications. In this work, a contact film transfer method is applied to evaluate the mobility-compressibility properties of conjugated polymers. A series of isoindigo-bithiophene conjugated polymers with symmetric carbosilane side chains (P(SiSi)), siloxane-terminated alkyl side chains (P(SiOSiO)), and combined asymmetric side chains (P(SiOSi)) are investigated. Accordingly, a compressed elastomer slab is used to transfer and compress the polymer films by releasing prestrain, and the morphology and mobility evolutions of these polymers are tracked. It is found that P(SiOSi) outperforms the other symmetric polymers including P(Si─Si) and P(SiO─SiO), having the ability to dissipate strain with its shortened lamellar spacing and orthogonal chain alignment. Notably, the mechanical durability of P(SiOSi) is also enhanced after consecutive compress-release cycles. In addition, the contact film transfer technique is demonstrated to be applicable to investigate the compressibility of different semiconducting polymers. These results demonstrate a comprehensive approach to understand the mobility-compressibility properties of semiconducting polymers under tensile and compressive strains.

Aramid Nanodielectrics for Ultraconformal Transparent Electronic Skins

On-skin electronics require minimal thicknesses and decent transparency for conformal contact, imperceptible wearing, and visual aesthetics. It is challenging to search for advanced ultrathin dielectrics capable of supporting the active components while maintaining bending softness, easy handling, and wafer-scale processability. Here, self-delaminated aramid nanodielectrics (ANDs) are demonstrated, enabling any skin-like electronics easily exfoliated from the processing substrates after complicated nanofabrication. In addition, ANDs are mechanically strong, chemically and thermally stable, transparent and breathable, therefore are ideal substrates for soft electronics. As demonstrated, compliant epidermal electrodes comprising silver nanowires and ANDs can successfully record high-quality electromyogram signals with low motion artifacts and satisfying sweat and water resistance. Furthermore, ANDs can serve as both substrates and dielectrics in single-walled carbon nanotube field-effect transistors (FETs) with a merely 160-nm thickness, which can be operated within 4 V with on/off ratios of 1.4 ± 0.5 × 105 , mobilities of 39.9 ± 2.2 cm2 V-1 s-1 , and negligible hysteresis. The ultraconformal FETs can function properly when wrapped around human hair without any degradation in performance.

Solution-Processed Polymer Dielectric Interlayer for Low-Voltage, Unipolar n-Type Organic Field-Effect Transistors

The integration of organic electronic circuits into real-life applications compels the fulfillment of a range of requirements, among which the ideal operation at a low voltage with reduced power consumption is paramount. Moreover, these performance factors should be achieved via solution-based fabrication schemes in order to comply with the promise of cost- and energy-efficient manufacturing offered by an organic, printed electronic technology. Here, we propose a solution-based route for the fabrication of low-voltage organic transistors, encompassing ideal device operation at voltages below 5 V and exhibiting n-type unipolarization. This process is widely applicable to a variety of semiconducting and dielectric materials. We achieved this through the use of a photo-cross-linked, low-k dielectric interlayer, which is used to fabricate multilayer dielectric stacks with areal capacitances of up to 40 nF/cm2 and leakage currents below 1 nA/cm2. Because of the chosen azide-based cross-linker, the dielectric promotes n-type unipolarization of the transistors and demonstrated to be compatible with different classes of semiconductors, from conjugated polymers to carbon nanotubes and low-temperature metal oxides. Our results demonstrate a general applicability of our unipolarizing dielectric, facilitating the implementation of complementary circuitry of emerging technologies with reduced power consumption.

Landauer-QFLPS Model for Mixed Schottky-Ohmic Contact Two-Dimensional Transistors

Two-dimensional material-based field-effect transistors (2DM-FETs) are playing a revolutionary role in electronic devices. However, before electronic design automation (EDA) for 2DM-FETs can be achieved, it remains necessary to determine how to incorporate contact transports into model. Reported methods compromise between physical intelligibility and model compactness due to the heterojunction nature. To address this, quasi-Fermi-level phase space theory (QFLPS) is generalized to incorporate contact transports using the Landauer formula. It turns out that the Landauer-QFLPS model effectively overcomes the issue of concern. The proposed new formula can describe 2DM-FETs with Schottky or Ohmic contacts with superior accuracy and efficiency over previous methods, especially when describing non-monotonic drain conductance characteristics. A three-bit threshold inverter quantizer (TIQ) circuit is fabricated using ambipolar black phosphorus and it is demonstrated that the model accurately predicts circuit performance. The model could be very effective and valuable in the development of 2DM-FET-based integrated circuits.

Controllable Carrier Doping in Two-Dimensional Materials Using Electron-Beam Irradiation and Scalable Oxide Dielectrics

Two-dimensional (2D) materials, characterized by their atomically thin nature and exceptional properties, hold significant promise for future nano-electronic applications. The precise control of carrier density in these 2D materials is essential for enhancing performance and enabling complex device functionalities. In this study, we present an electron-beam (e-beam) doping approach to achieve controllable carrier doping effects in graphene and MoS2 field-effect transistors (FETs) by leveraging charge-trapping oxide dielectrics. By adding an atomic layer deposition (ALD)-grown Al2O3 dielectric layer on top of the SiO2/Si substrate, we demonstrate that controllable and reversible carrier doping effects can be effectively induced in graphene and MoS2 FETs through e-beam doping. This new device configuration establishes an oxide interface that enhances charge-trapping capabilities, enabling the effective induction of electron and hole doping beyond the SiO2 breakdown limit using high-energy e-beam irradiation. Importantly, these high doping effects exhibit non-volatility and robust stability in both vacuum and air environments for graphene FET devices. This methodology enhances carrier modulation capabilities in 2D materials and holds great potential for advancing the development of scalable 2D nano-devices.

The Role of Carbon in Metal-Organic Chemical Vapor Deposition-Grown MoS2 Films

Acquiring homogeneous and reproducible wafer-scale transition metal dichalcogenide (TMDC) films is crucial for modern electronics. Metal-organic chemical vapor deposition (MOCVD) offers a promising approach for scalable production and large-area integration. However, during MOCVD synthesis, extraneous carbon incorporation due to organosulfur precursor pyrolysis is a persistent concern, and the role of unintentional carbon incorporation remains elusive. Here, we report the large-scale synthesis of molybdenum disulfide (MoS2) thin films, accompanied by the formation of amorphous carbon layers. Using Raman, photoluminescence (PL) spectroscopy, and transmission electron microscopy (TEM), we confirm how polycrystalline MoS2 combines with extraneous amorphous carbon layers. Furthermore, by fabricating field-effect transistors (FETs) using the carbon-incorporated MoS2 films, we find that traditional n-type MoS2 can transform into p-type semiconductors owing to the incorporation of carbon, a rare occurrence among TMDC materials. This unexpected behavior expands our understanding of TMDC properties and opens up new avenues for exploring novel device applications.

Hexakis-TIPS-Alkynylated Nonacenes: Persistent and Processible

Four substituted nonacenes were prepared and characterized by UV-vis and EPR spectroscopy and X-ray crystallography. The compounds are the most stable and soluble nonacenes to date - due to six strategically placed triisopropylsilyl(TIPS)-ethynyl groups. They are stable for several weeks in the solid state. In dilute solution their half-life is 5-9 h. Crystal structure analyses of two nonacenes prove their structures. A nonacene derivative was tested in a solution-processed transistor and exhibits ambipolar charge transport (μe =0.007 cm2 /Vs; μh =0.023 cm2 /Vs).

Ambient-Stable 2D Dion-Jacobson Phase Tin Halide Perovskite Field-Effect Transistors with Mobility over 1.6 Cm2 V-1 s-1

Solution-processed metal halide perovskites hold immense potential for the advancement of next-generation field-effect transistors (FETs). However, the instability of perovskite-based transistors has impeded their progress and practical applications. Here, ambient-stable high-performance FETs based on 2D Dion-Jacobson phase tin halide perovskite BDASnI4 , which has high film quality and excellent electrical properties, are reported. The perovskite channels are established by engineering the film crystallization process via the employment of ammonium salt interlayers and the incorporation of NH4 SCN additives within the precursor solution. The refined FETs demonstrate field-effect hole mobilities up to 1.61 cm2 V-1 s-1 and an on/off ratio surpassing 106 . Moreover, the devices show impressive operational and environmental stability and retain their functional performance even after being exposed to ambient conditions with a temperature of 45 °C and humidity of 45% for over 150 h.

A Novel Biosensing Approach: Improving SnS2 FET Sensitivity with a Tailored Supporter Molecule and Custom Substrate

The exclusive features of two-dimensional (2D) semiconductors, such as high surface-to-volume ratios, tunable electronic properties, and biocompatibility, provide promising opportunities for developing highly sensitive biosensors. However, developing practical biosensors that can promptly detect low concentrations of target analytes remains a challenging task. Here, a field-effect-transistor comprising n-type transition metal dichalcogenide tin disulfide (SnS2 ) is developed over the hexagonal boron nitride (h-BN) for the detection of streptavidin protein (Strep.) as a target analyte. A self-designed receptor based on the pyrene-lysine conjugated with biotin (PLCB) is utilized to maintain the sensitivity of the SnS2 /h-BN FET because of the π-π stacking. The detection capabilities of SnS2 /h-BN FET are investigated using both Raman spectroscopy and electrical characterizations. The real-time electrical measurements exhibit that the SnS2 /h-BN FET is capable of detecting streptavidin at a remarkably low concentration of 0.5 pm, within 13.2 s. Additionally, the selectivity of the device is investigated by measuring its response against a Cow-like serum egg white protein (BSA), having a comparative molecular weight to that of the streptavidin. These results indicate a high sensitivity and rapid response of SnS2 /h-BN biosensor against the selective proteins, which can have significant implications in several fields including point-of-care diagnostics, drug discovery, and environmental monitoring.

Increased Mobility and Reduced Hysteresis of MoS2 Field-Effect Transistors via Direct Surface Precipitation of CsPbBr3-Nanoclusters for Charge Transfer Doping

Transition-metal dichalcogenides (TMDs) and metal halide perovskites (MHPs) have been investigated for various applications, owing to their unique physical properties and excellent optoelectronic functionalities. TMD monolayers synthesized via chemical vapor deposition (CVD), which are advantageous for large-area synthesis, exhibit low mobility and prominent hysteresis in the electrical signals of field-effect transistors (FETs) because of their native defects. In this study, we demonstrate an increase in electrical mobility by ∼170 times and reduced hysteresis in the current-bias curves of MoS2 FETs hybridized with CsPbBr3 for charge transfer doping, which is implemented via solution-based CsPbBr3-nanocluster precipitation on CVD-grown MoS2 monolayer FETs. Electrons injected from CsPbBr3 into MoS2 induce heavy n-doping and heal point defects in the MoS2 channel layer, thus significantly increasing mobility and reducing hysteresis in the hybrid FETs. Our results provide a foundation for improving the reliability and performance of TMD-based FETs by hybridizing them with solution-based perovskites.

Progress, Challenges, and Opportunities in Oxide Semiconductor Devices: A Key Building Block for Applications Ranging from Display Backplanes to 3D Integrated Semiconductor Chips

As Si has faced physical limits on further scaling down, novel semiconducting materials such as 2D transition metal dichalcogenides and oxide semiconductors (OSs) have gained tremendous attention to continue the ever-demanding downscaling represented by Moore's law. Among them, OS is considered to be the most promising alternative material because it has intriguing features such as modest mobility, extremely low off-current, great uniformity, and low-temperature processibility with conventional complementary-metal-oxide-semiconductor-compatible methods. In practice, OS has successfully replaced hydrogenated amorphous Si in high-end liquid crystal display devices and has now become a standard backplane electronic for organic light-emitting diode displays despite the short time since their invention in 2004. For OS to be implemented in next-generation electronics such as back-end-of-line transistor applications in monolithic 3D integration beyond the display applications, however, there is still much room for further study, such as high mobility, immune short-channel effects, low electrical contact properties, etc. This study reviews the brief history of OS and recent progress in device applications from a material science and device physics point of view. Simultaneously, remaining challenges and opportunities in OS for use in next-generation electronics are discussed.

Determining the Number of Graphene Nanoribbons in Dual-Gate Field-Effect Transistors

Bottom-up synthesized graphene nanoribbons (GNRs) are increasingly attracting interest due to their atomically controlled structure and customizable physical properties. In recent years, a range of GNR-based field-effect transistors (FETs) has been fabricated, with several demonstrating quantum-dot (QD) behavior at cryogenic temperatures. However, understanding the relationship between the cryogenic charge-transport characteristics and the number of the GNRs in the device is challenging, as the length and location of the GNRs in the junction are not precisely controlled. Here, we present a methodology based on a dual-gate FET that allows us to identify different scenarios, such as single GNRs, double or multiple GNRs in parallel, and a single GNR interacting with charge traps. Our dual-gate FET architecture therefore offers a quantitative approach for comprehending charge transport in atomically precise GNRs.

Field-effect transistors engineered via solution-based layer-by-layer nanoarchitectonics

The layer-by-layer (LbL) technique has been proven to be one of the most versatile approaches in order to fabricate functional nanofilms. The use of simple and inexpensive procedures as well as the possibility to incorporate a very wide range of materials through different interactions have driven its application in a wide range of fields. On the other hand, field-effect transistors (FETs) are certainly among the most important elements in electronics. The ability to modulate the flowing current between a source and a drain electrode via the voltage applied to the gate electrode endow these devices to switch or amplify electronic signals, being vital in all of our everyday electronic devices. In this topical review, we highlight different research efforts to engineer field-effect transistors using the LbL assembly approach. We firstly discuss on the engineering of the channel material of transistors via the LbL technique. Next, the deposition of dielectric materials through this approach is reviewed, allowing the development of high-performance electronic components. Finally, the application of the LbL approach to fabricate FETs-based biosensing devices is also discussed, as well as the improvement of the transistor's interfacial sensitivity by the engineering of the semiconductor with polyelectrolyte multilayers.

Ambipolar to Unipolar Conversion in C70/Ferrocene Nanosheet Field-Effect Transistors

Organic cocrystals, which are assembled by noncovalent intermolecular interactions, have garnered intense interest due to their remarkable chemicophysical properties and practical applications. One notable feature, namely, the charge transfer (CT) interactions within the cocrystals, not only facilitates the formation of an ordered supramolecular network but also endows them with desirable semiconductor characteristics. Here, we present the intriguing ambipolar CT properties exhibited by nanosheets composed of single cocrystals of C70/ferrocene (C70/Fc). When heated to 150 °C, the initially ambipolar monoclinic C70/Fc nanosheet-based field-effect transistors (FETs) were transformed into n-type face-centered cubic (fcc) C70 nanosheet-based FETs owing to the elimination of Fc. This thermally induced alteration in the crystal structure was accompanied by an irreversible switching of the semiconducting behavior of the device; thus, the device transitions from ambipolar to unipolar. Importantly, the C70/Fc nanosheet-based FETs were also found to be much more thermally stable than the previously reported C60/Fc nanosheet-based FETs. Furthermore, we conducted visible/near-infrared diffuse reflectance and photoemission yield spectroscopies to investigate the crucial role played by Fc in modulating the CT characteristics. This study provides valuable insights into the overall functionality of these nanosheet structures.

Deciphering Vacancy Defect Evolution of 2D MoS2 for Reliable Transistors

Two-dimensional (2D) MoS2 is an excellent candidate channel material for next-generation integrated circuit (IC) transistors. However, the reliability of MoS2 is of great concern due to the serious threat of vacancy defects, such as sulfur vacancies (VS). Evaluating the impact of vacancy defects on the service reliability of MoS2 transistors is crucial, but it has always been limited by the difficulty in systematically tracking and analyzing the changes and effects of vacancy defects in the service environment. Here, a simulated initiator is established for deciphering the evolution of vacancy defects in MoS2 and their influence on the reliability of transistors. The results indicate that VS below 1.3% are isolated by slow enrichment during initiation. Over 1.3% of VS tend to enrich in pairs and over 3.5% of the enriched VS easily evolve into nanopores. The enriched VS with electron doping in the channel cause the threshold voltage (Vth) negative drift approaching 6 V, while the expanded nanopores initiate the Vth roll-off and punch-through of transistors. Finally, sulfur steam deposition has been proposed to constrain VS enrichment, and reliable MoS2 transistors are constructed. Our research provides a new method for deciphering and identifying the impact of defects.

2D Amorphous GaOX Gate Dielectric for β-Ga2O3 Field-Effect Transistors

Appropriate gate dielectrics must be identified to fabricate metal-insulator-semiconductor field-effect transistors (MISFETs); however, this has been challenging for compound semiconductors owing to the absence of high-quality native oxides. This study uses the liquid-gallium squeezing technique to fabricate 2D amorphous gallium oxide (GaOX) with a high dielectric constant, where its thickness is precisely controlled at the atomic scale (monolayer, ∼4.5 nm; bilayer, ∼8.5 nm). Beta-phase gallium oxide (β-Ga2O3) with an ultrawide energy bandgap (4.5-4.9 eV) has emerged as a next-generation power semiconductor material and is presented here as the channel material. The 2D amorphous GaOX dielectric is combined with a β-Ga2O3 conducting nanolayer, and the resulting β-Ga2O3 MISFET is stable up to 250 °C. The 2D amorphous GaOX is oxygen-deficient, and a high-quality interface with excellent uniformity and scalability forms between the 2D amorphous GaOX and β-Ga2O3. The fabricated MISFET exhibits a wide gate-voltage swing of approximately +5 V, a high current on/off ratio, moderate field-effect carrier mobility, and a decent three-terminal breakdown voltage (∼138 V). The carrier transport of the Ni/GaOX/β-Ga2O3 metal-insulator-semiconductor (MIS) structure displays a combination of Schottky emission and Fowler-Nordheim (F-N) tunneling in the high-gate-bias region at 25 °C, whereas at elevated temperatures it shows Schottky emission and F-N tunneling in the low- and high-gate-bias regions, respectively. This study demonstrates that a 2D GaOX gate dielectric layer can be produced and incorporated into an active channel layer to form an MIS structure at room temperature (∼25 °C), which enables the facile fabrication of MISFET devices.

Amplification-Free Detection of SARS-CoV-2 Down to Single Virus Level by Portable Carbon Nanotube Biosensors

The rapid and sensitive detection of trace-level viruses in a simple and reliable way is of great importance for epidemic prevention and control. Here, a multi-functionalized floating gate carbon nanotube field effect transistor (FG-CNT FET) based biosensor is reported for the single virus level detection of SARS-CoV-2 virus antigen and RNA rapidly with a portable sensing platform. The aptamers functionalized sensors can detect SARS-CoV-2 antigens from unprocessed nasopharyngeal swab samples within 1 min. Meanwhile, enhanced by a multi-probe strategy, the FG-CNT FET-based biosensor can detect the long chain RNA directly without amplification down to single virus level within 1 min. The device, constructed with packaged sensor chips and a portable sensing terminal, can distinguish 10 COVID-19 patients from 10 healthy individuals in clinical tests both by the RNAs and antigens by a combination detection strategy with an combined overall percent agreement (OPA) close to 100%. The results provide a general and simple method to enhance the sensitivity of FET-based biochemical sensors for the detection of nucleic acid molecules and demonstrate that the CNT FG FET biosensor is a versatile and reliable integrated platform for ultrasensitive multibiomarker detection without amplification and has great potential for point-of-care (POC) clinical tests.

Observation of Rich Defect Dynamics in Monolayer MoS2

Defects play a pivotal role in limiting the performance and reliability of nanoscale devices. Field-effect transistors (FETs) based on atomically thin two-dimensional (2D) semiconductors such as monolayer MoS₂ are no exception. Probing defect dynamics in 2D FETs is therefore of significant interest. Here, we present a comprehensive insight into various defect dynamics observed in monolayer MoS₂ FETs at varying gate biases and temperatures. The measured source-to-drain currents exhibit random telegraph signals (RTS) owing to the transfer of charges between the semiconducting channel and individual defects. Based on the modeled temperature and gate bias dependence, oxygen vacancies or aluminum interstitials are probable defect candidates. Several types of RTSs are observed including anomalous RTS and giant RTS indicating local current crowding effects and rich defect dynamics in monolayer MoS₂ FETs. This study explores defect dynamics in large area-grown monolayer MoS₂ with ALD-grown Al₂O₃ as the gate dielectric.

Crystal-Structure Simulation of Methylthiolated Peri-Condensed Polycyclic Aromatic Hydrocarbons for Identifying Promising Molecular Semiconductors: Discovery of 1,3,8,10-tetrakis(methylthio)peropyrene Showing Ultrahigh Mobility

The crystal structures of molecular semiconductors critically affect their carrier-transport properties. One of the promising crystal structures that afford high carrier mobility is a brickwork structure recently reported for 1,3,6,8-tetrakis(methylthio)pyrene (MT-pyrene) showing ultrahigh mobility. However, such ultrahigh mobility is not realized in other methylchalocogenolated pyrenes, owing to subtle differences in the molecular positions in their crystal structures. This means that, for developing superior molecular semiconductors, it is desirable to simulate the crystal structure with sufficient quality before time-consuming and labor-intensive synthetic trials. To realize this, a new computational approach is developed to simulate crystal structures of all methylchalocogenolated pyrenes, which is then applied to MT-pyrene-related methylthiolated peri-condensed polycyclic aromatic hydrocarbons including perylene, peropyrene, and terrylene derivatives. Among these, 1,3,8,10-tetrakis(methylthio)peropyrene (MT-peropyrene) is expected to show high mobility based on the simulated crystal structures. Thus, MT-peropyrene is chosen as the synthetic target, and a new peropyrene synthesis method is developed. Thus synthesized MT-peropyrene has virtually the same crystal structure as the simulated one, and its single-crystal field-effect transistors show mobility as high as 30 cm2 V-1 s-1 and band-like transport behaviors. These results indicate that the present crystal-structure simulation is a powerful tool for exploring promising molecular semiconductors.

Antioxidative 2D Bismuth Selenide via Halide Passivation for Enhanced Device Stability

The topological insulator 2D Bi₂Se₃ is promising for electronic devices due to its unique electronic properties; however, it is challenging to prepare antioxidative nanosheets since Bi₂Se₃ is prone to oxidation. Surface passivation using ligand agents after Bi₂Se₃ exfoliation works well to protect the surface, but the process is time-consuming and technically challenging; a passivation agent that is stable under a highly biased potential is significant for in situ passivation of the Bi₂Se₃ surface. In this work, the roles of halide anions (Cl^-, Br^-, and I^-) in respect of the chemical properties of synthetic Bi₂Se₃ nanosheets during electrochemical intercalated exfoliation were investigated to determine the antioxidation capacity. It was found that Bi₂Se₃ nanosheets prepared in a solution of tetrabutylammonium chloride (TBA⁺ and Cl^-) have the best oxidation resistance via the surface bonding of Bi with Cl, which promotes obtaining better device stability. This work paves an avenue for adjusting the components of the electrolyte to further promote the stability of 2D Bi₂Se₃-nanosheet-based electronic devices.

5 nm Ultrathin Crystalline Ferroelectric P(VDF-TrFE)-Brush Tuned for Hysteresis-Free Sub 60 mV dec-1 Negative-Capacitance Transistors

Negative-capacitance field-effect transistors (NC-FETs) have gathered enormous interest as a way to reduce subthreshold swing (SS) and overcome the issue of power dissipation in modern integrated circuits. For stable NC behavior at low operating voltages, the development of ultrathin ferroelectrics (FE), which are compatible with the industrial process, is of great interest. Here, a new scalable ultrathin ferroelectric polymer layer is developed based on trichloromethyl (CCl₃ )-terminated poly(vinylidene difluoride-co-trifloroethylene) (P(VDF-TrFE)) to achieve the state-of-the-art performance of NC-FETs. The crystalline phase of 5-10 nm ultrathin P(VDF-TrFE) is prepared on AlO_X by a newly developed brush method, which enables an FE/dielectric (DE) bilayer. FE/DE thickness ratios are then systematically tuned at ease to achieve ideal capacitance matching. NC-FETs with optimized FE/DE thickness at a thickness limit demonstrate hysteresis-free operation with an SS of 28 mV dec^-1 at ≈1.5 V, which competes with the best reports. This P(VDF-TrFE)-brush layer can be broadly adapted to NC-FETs, opening an exciting avenue for low-power devices.

Metal Halide Perovskite Photo-Field-Effect Transistors with Chiral Selectivity

Organic-inorganic (hybrid) metal halide perovskites (MHPs) incorporating chiral organic ligand molecules are naturally sensitive to left- and right-handed circular polarized light, potentially enabling selective circular polarized photodetection. Here, the photoresponses in chiral MHP polycrystalline thin films made of ((S)-(-)-α-methyl benzylamine)₂PbI₄ and ((R)-(+)-α-methyl benzylamine)₂PbI₄, denoted as (S-MBA)₂ PbI₄ and (R-MBA)₂PbI₄, respectively, are investigated by employing a thin-film field-effect transistor (FET) configuration. The left-hand-sensitive films made of (S-MBA)₂PbI₄ perovskite show higher photocurrent under left-handed circularly polarized (LCP) light than under right-handed circularly polarized (RCP) illumination under otherwise identical conditions. Conversely, the right-hand-sensitive films made of (R-MBA)₂PbI₄ are more sensitive to RCP than LCP illumination over a wide temperature range of 77-300 K. Furthermore, based on FET measurements, we found evidence of two different carrier transport mechanisms with two distinct activation energies in the 77-260 and 280-300 K temperature ranges, respectively. In the former temperature range, shallow traps are dominant in the perovskite film, which are filled by thermally activated carriers with increasing temperature; in the latter temperature range, deep traps with one order of magnitude larger activation energy dominate. Both types of chiral MHPs show intrinsic p-type carrier transport behavior regardless of the handedness (S or R) of these materials. The optimal carrier mobility for both handedness of material is around (2.7 ± 0.2) × 10^-7 cm² V^-1 s^-1 at 270-280 K, which is two magnitudes larger than those reported in nonchiral perovskite MAPbI₃ polycrystalline thin films. These findings suggest that chiral MHPs can be an excellent candidate for selective circular polarized photodetection applications, without additional polarizing optical components, enabling simplified construction of detection systems.

Two-Dimensional Layered Materials Meet Perovskite Oxides: A Combination for High-Performance Electronic Devices

As the Si-based transistors scale down to atomic dimensions, the basic principle of current electronics, which heavily relies on the tunable charge degree of freedom, faces increasing challenges to meet the future requirements of speed, switching energy, heat dissipation, and packing density as well as functionalities. Heterogeneous integration, where dissimilar layers of materials and functionalities are unrestrictedly stacked at an atomic scale, is appealing for next-generation electronics, such as multifunctional, neuromorphic, spintronic, and ultralow-power devices, because it unlocks technologically useful interfaces of distinct functionalities. Recently, the combination of functional perovskite oxides and two-dimensional layered materials (2DLMs) led to unexpected functionalities and enhanced device performance. In this paper, we review the recent progress of the heterogeneous integration of perovskite oxides and 2DLMs from the perspectives of fabrication and interfacial properties, electronic applications, and challenges as well as outlooks. In particular, we focus on three types of attractive applications, namely field-effect transistors, memory, and neuromorphic electronics. The van der Waals integration approach is extendible to other oxides and 2DLMs, leading to almost unlimited combinations of oxides and 2DLMs and contributing to future high-performance electronic and spintronic devices.

Gate Dielectrics Integration for 2D Electronics: Challenges, Advances, and Outlook

2D semiconductors have emerged both as an ideal platform for fundamental studies and as promising channel materials in beyond-silicon field-effect-transistors due to their outstanding electrical properties and exceptional tunability via external field. However, the lack of proper dielectrics for 2D semiconductors has become a major roadblock for their further development toward practical applications. The prominent issues between conventional 3D dielectrics and 2D semiconductors arise from the integration and interface quality, where defect states and imperfections lead to dramatic deterioration of device performance. In this review article, the root causes of such issues are briefly analyzed and recent advances on some possible solutions, including various approaches of adapting conventional dielectrics to 2D semiconductors, and the development of novel dielectrics with van der Waals surface toward high-performance 2D electronics are summarized. Then, in the perspective, the requirements of ideal dielectrics for state-of-the-art 2D devices are outlined and an outlook for their future development is provided.

All-Transfer Electrode Interface Engineering Toward Harsh-Environment-Resistant MoS2 Field-Effect Transistors

Nanoscale electronic devices that can work in harsh environments are in high demand for wearable, automotive, and aerospace electronics. Clean and defect-free interfaces are of vital importance for building nanoscale harsh-environment-resistant devices. However, current nanoscale devices are subject to failure in these environments, especially at defective electrode-channel interfaces. Here, harsh-environment-resistant MoS₂ transistors are developed by engineering electrode-channel interfaces with an all-transfer of van der Waals electrodes. The delivered defect-free, graphene-buffered electrodes keep the electrode-channel interfaces intact and robust. As a result, the as-fabricated MoS₂ devices have reduced Schottky barrier heights, leading to a very large on-state current and high carrier mobility. More importantly, the defect-free, hydrophobic graphene buffer layer prevents metal diffusion from the electrodes to MoS₂ and the intercalation of water molecules at the electrode-MoS₂ interfaces. This enables high resistances of MoS₂ devices with all-transfer electrodes to various harsh environments, including humid, oxidizing, and high-temperature environments, surpassing the devices with other kinds of electrodes. The work deepens the understanding of the roles of electrode-channel interfaces in nanoscale devices and provides a promising interface engineering strategy to build nanoscale harsh-environment-resistant devices.

Gate-Controlled Magnetotransport and Electrostatic Modulation of Magnetism in 2D Magnetic Semiconductor CrPS4

Using field-effect transistors (FETs) to explore atomically thin magnetic semiconductors with transport measurements is difficult, because the very narrow bands of most 2D magnetic semiconductors cause carrier localization, preventing transistor operation. Here, it is shown that exfoliated layers of CrPS₄ -a 2D layered antiferromagnetic semiconductor whose bandwidth approaches 1 eV-allow the realization of FETs that operate properly down to cryogenic temperature. Using these devices, conductance measurements as a function of temperature and magnetic field are performed to determine the full magnetic phase diagram, which includes a spin-flop and a spin-flip phase. The magnetoconductance, which depends strongly on gate voltage, is determined. reaching values as high as 5000% near the threshold for electron conduction. The gate voltage also allows the magnetic states to be tuned, despite the relatively large thickness of the CrPS₄ multilayers employed in the study. The results show the need to employ 2D magnetic semiconductors with sufficiently large bandwidth to realize properly functioning transistors, and identify a candidate material to realize a fully gate-tunable half-metallic conductor.

Ferroelectric Wide-Bandgap Metal Halide Perovskite Field-Effect Transistors: Toward Transparent Electronics

Transparent field-effect transistors (FETs) are attacking intensive interest for constructing fancy "invisible" electronic products. Presently, the main technology for realizing transparent FETs is based on metal oxide semiconductors, which have wide-bandgap but generally demand sputtering technique or high-temperature (>350 °C) solution process for fabrication. Herein, a general device fabrication strategy for metal halide perovskite (MHP) FETs is shown, by which transparent perovskite FETs are successfully obtained using low-temperature (<150 °C) solution process. This strategy involves the employment of ferroelectric copolymer poly(vinylidene fluoride-co-trifluoroethylene) (PVDF-TrFE) as the dielectric, which conquers the challenging issue of gate-electric-field screening effect in MHP FETs. Additionally, an ultra-thin SnO₂ is inserted between the source/drain electrodes and MHPs to facilitate electron injection. Consequently, n-type semi-transparent MAPbBr₃ FETs and fully transparent MAPbCl₃ FETs which can operate well at room temperature with mobility over 10^-3  cm²  V^-1  s^-1 and on/off ratio >10³ are achieved for the first time. The low-temperature solution processability of these FETs makes them particularly attractive for applications in low-cost, large-area transparent electronics.

A Methodology of Fabricating Novel Electrodes for Semiconductor Devices: Doping and Van der Waals Integrating Organic Semiconductor Films

Electrodes are indispensable components in semiconductor devices, and now are mainly made from metals, which are convenient for use but not ideal for emerging technologies such as bioelectronics, flexible electronics, or transparent electronics. Here the methodology of fabricating novel electrodes for semiconductor devices using organic semiconductors (OSCs) is proposed and demonstrated. It is shown that polymer semiconductors can be heavily p- or n-doped to achieve sufficiently high conductivity for electrodes. In contrast with metals, the doped OSC films (DOSCFs) are solution-processable, mechanically flexible, and have interesting optoelectronic properties. By integrating the DOSCFs with semiconductors through van der Waals contacts different kinds of semiconductor devices can be constructed. Importantly, these devices exhibit higher performance than their counterparts with metal electrodes, and/or excellent mechanical or optical properties that are unavailable in metal-electrode devices, suggesting the superiority of DOSCF electrodes. Given the existing large amount of OSCs, the established methodology can provide abundant electrode choices to meet the demand of various emerging devices.

Light-Activated Interface Charge-Alternating Interaction on an Extended Gate Photoelectrode: A New Sensing Strategy for EGFET-Based Photoelectrochemical Sensors

Integration of extended gate field-effect transistors (EGFET) and photoelectrochemical (PEC) measurement to construct highly sensitive sensors is an innovative research field that was proven feasible by our previous work. However, it remains a challenge on how to adjust the interaction between the extended gate and the analyte and study its influence on EGFET-based PEC sensors. Herein, a new sensing strategy was proposed by a mutual electrostatic interaction. Three-dimensional TiO₂ and g-C₃N₄ core-shell heterojunction on flexible carbon cloth (TCN) was designed as the extended sensing gate. Tetracycline (TC) was also used as a model analyte, and it contains electron-donating groups (-NH₂ and -OH) with negative charge. The designed TCN-extended sensing gate was negatively charged in the dark by introducing carbon vacancies with oxygen doping in the g-C₃N₄ shell, while it was positively charged under illustration due to the aggregation of photogenerated holes on the surface. Therefore, a light-activated PEC sensing platform for the sensitive and selective determination of tetracycline (TC) was demonstrated. Such a PEC sensor exhibited wide linear ranges within 100 pM to 1 μM and 1-100 μM with a low detection limit of 0.42 pM. Furthermore, the sensing platform possessed excellent selectivity, good reproducibility, and stability. The proposed sensing strategy in this work can expand the paradigm for developing a light-regulated FET-based PEC sensor by mutual electrostatic interaction, and we believe that this work will offer a new perspective for the design of interface interaction in PEC devices.

Highly Conductive Ultrathin Layers of Conjugated Polymers for Metal-Free Coplanar Transistors with Single-Polymer Transport Layers

Although metal or oxide conductive films are widely used as electrodes of electronic devices, organic electrodes would be more favorable for next-generation organic electronics. Here, using some model conjugated polymers as examples, we report a class of highly conductive and optically transparent polymer ultrathin layers. Vertical phase separation of semiconductor/insulator blends leads to a highly ordered two-dimensional (2D) ultrathin layer of conjugated-polymer chains on the insulator. Afterwards, the thermally evaporated dopants on the ultrathin layer lead to a conductivity of up to 10³ S cm^-1 and a sheet resistance 10³ Ω/square for a model conjugated polymer poly(2,5-bis(3-hexadecylthiophen-2-yl)thieno[3,2-b]thiophenes) (PBTTT). The high conductivity is due to the high hole mobility (∼ 20 cm² V^-1 s^-1), although doping-induced charge density is still in the moderate range of 10²⁰ cm^-3 with a 1 nm thick dopant. Metal-free monolithic coplanar field-effect transistors using the same conjugated-polymer ultrathin layer with alternatively doped regions as electrodes and a semiconductor layer are realized. The field-effect mobility of this monolithic transistor is over 2 cm² V^-1 s^-1 for PBTTT, one order higher than that of the conventional PBTTT transistor using metal electrodes. The optical transparency of the single conjugated-polymer transport layer is over 90%, demonstrating a bright future for all-organic transparent electronics.

Enhancing the Performance of Electret-Free Phototransistor Memory by Using All-Conjugated Block Copolymer

Conjugated polymers are of great interest owing to their potential in stretchable electronics to function under complex deformation conditions. To improve the performance of conjugated polymers, various structural designs have been proposed and these conjugated polymers are specially applied in exotic optoelectronics. In this work, a series of all-conjugated block copolymers (PII2T-b-PNDI2T) comprising poly(isoindigo-bithiophene) (PII2T) and poly(naphthalenediimide-bithiophene) (PNDI2T) are developed with varied compositions and applied to electret-free phototransistor memory. Accordingly, these memory devices present p-type transport capability and electrical-ON/photo-OFF memory behavior. The efficacy of the all-conjugated block copolymer design in improving the memory-photoresponse properties in phototransistor memory is revealed. By optimizing the composition of the block copolymer, the corresponding device achieves a wide memory window of 36 V and a high memory ratio of 7 × 10⁴ . Collectively, the results of this study indicate a new concept for designing electret-free phototransistor memory by using all-conjugated block copolymer heterojunctions to mitigate the phase separation of conjugated polymer blends. Meanwhile, the intrinsic optoelectronic properties of the constituent conjugated polymers can be well-maintained by using an all-conjugated block copolymer design.

Horizontal Arrays of One-Dimensional van der Waals Heterostructures as Transistor Channels

The nanotube/dielectric interface plays an essential role in achieving superb switching characteristics of carbon nanotube-based transistors for energy-efficient computation. Formation of van der Waals heterostructures with hexagonal boron nitride nanotubes could be an effective means to reduce interface state density, but the need for isolating nanotubes during the formation of coaxial outer layers has hindered the fabrication of their horizontal arrays. Here, we develop a strategy to create isolated heterostructure arrays using aligned carbon nanotubes grown on a quartz substrate as starting materials. Air-suspended arrays of carbon nanotubes are prepared by a dry transfer technique and then used as templates for the coaxial wrapping of boron nitride nanotubes. We then fabricate the transistors, where boron nitride serves as interfacial layers between carbon nanotube channels and conventional gate dielectrics, showing hysteresis-free characteristics owing to the improved interfaces. We have also gained a deeper understanding of the strain applied on inner carbon nanotubes, as well as the inhomogeneity of the outer coating, by characterizing individual heterostructures over trenches and on a substrate surface. The device fabrication and characterization presented here essentially do not require elaborate electron microscopy, thus paving the way for the practical use of one-dimensional van der Waals heterostructures for nanoelectronics.

Electrical and Structural Properties of Si1-xGex Nanowires Prepared from a Single-Source Precursor

Si_1-xGe_x nanowires (NWs) were prepared by gold-supported chemical vapor deposition (CVD) using a single-source precursor with preformed Si-Ge bonds. Besides the tamed reactivity of the precursor, the approach reduces the process parameters associated with the control of decomposition characteristics and the dosing of individual precursors. The group IV alloy NWs are single crystalline with a constant diameter along their axis. During the wire growth by low pressure CVD, an Au-containing surface layer on the NWs forms by surface diffusion from the substrate, which can be removed by a combination of oxidation and etching. The electrical properties of the Si_1-xGe_x/Au core-shell NWs are compared to the Si_1-xGe_x NWs after Au removal. Core-shell NWs show signatures of metal-like behavior, while the purely semiconducting NWs reveal typical signatures of intrinsic Si_1-xGe_x. The synthesized materials should be of high interest for applications in nano- and quantum-electronics.

Fully Photoswitchable Phototransistor Memory Comprising Perovskite Quantum Dot-Based Hybrid Nanocomposites as a Photoresponsive Floating Gate

Tremendous research efforts have been dedicated into the field of photoresponsive nonvolatile memory devices owing to their advantages of fast transmitting speed, low latency, and power-saving property that are suitable for replacing current electrical-driven electronics. However, the reported memory devices still rely on the assistance of gate bias to program them, and a real fully photoswitchable transistor memory is still rare. Herein, we report a phototransistor memory device comprising polymer/perovskite quantum dot (QD) hybrid nanocomposites as a photoresponsive floating gate. The perovskite QDs offer an effective discreteness with an excellent photoresponse that are suitable for photogate application. In addition, a series of ultraviolet (UV)-sensitive insulating polymer hosts were designed to investigate the effect of UV light on the memory behavior. We found that a fully photoswitchable memory device was fulfilled by using the independent and sequential photoexcitation between a UV-sensitive polymer host and a visible light-sensitive QD photogates, which produced decent photoresponse, memory switchability, and highly stable memory retention with a memory ratio of 10⁴ over 10⁴ s. This study not only unraveled the mystery in the fully photoswitchable functionality of nonvolatile memory but also enlightened their potential in the next-generation electronics for light-fidelity application.

Ultrasensitive Detection of Interleukin 6 by Using Silicon Nanowire Field-Effect Transistors

Interleukin 6 (IL-6) has been regarded as a biomarker that can be applied as a predictor for the severity of COVID-19-infected patients. The IL-6 level also correlates well with respiratory dysfunction and mortality risk. In this work, three silanization approaches and two types of biorecognition elements were used on the silicon nanowire field-effect transistors (SiNW-FETs) to investigate and compare the sensing performance on the detection of IL-6. Experimental data revealed that the mixed-SAMs-modified silica surface could have superior surface morphology to APTES-modified and APS-modified silica surfaces. According to the data on detecting various concentrations of IL-6, the detection range of the aptamer-functionalized SiNW-FET was broader than that of the antibody-functionalized SiNW-FET. In addition, the lowest concentration of valid detection for the aptamer-functionalized SiNW-FET was 2.1 pg/mL, two orders of magnitude lower than the antibody-functionalized SiNW-FET. The detection range of the aptamer-functionalized SiNW-FET covered the concentration of IL-6, which could be used to predict fatal outcomes of COVID-19. The detection results in the buffer showed that the anti-IL-6 aptamer could produce better detection results on the SiNW-FETs, indicating its great opportunity in applications for sensing clinical samples.

Fabrication and Functionalisation of Nanocarbon-Based Field-Effect Transistor Biosensors

Nanocarbon-based field-effect transistor (NC-FET) biosensors are at the forefront of future diagnostic technology. By integrating biological molecules with electrically conducting carbon-based platforms, high sensitivity real-time multiplexed sensing is possible. Combined with their small footprint, portability, ease of use, and label-free sensing mechanisms, NC-FETs are prime candidates for the rapidly expanding areas of point-of-care testing, environmental monitoring and biosensing as a whole. In this review we provide an overview of the basic operational mechanisms behind NC-FETs, synthesis and fabrication of FET devices, and developments in functionalisation strategies for biosensing applications.

Graphene-Based Microwave Circuits: A Review

Over the past two decades, research on 2D materials has received much interest. Graphene is the most promising candidate regarding high-frequency applications thus far due to is high carrier mobility. Here, the research about the employment of graphene in micro- and millimeter-wave circuits is reviewed. The review starts with the different methodologies to grow and transfer graphene, before discussing the way graphene-based field-effect-transistors (GFETs) and diodes are built. A review on different approaches for realizing these devices is provided before discussing the employment of both GFETs and graphene diodes in different micro- and millimeter-wave circuits, showing the possibilities but also the limitations of this 2D material for high-frequency applications.

2D Materials-Based Static Random-Access Memory

2D transition-metal dichalcogenide semiconductors, such as MoS₂ and WSe₂ , with adequate bandgaps are promising channel materials for ultrascaled logic transistors. This scalability study of 2D material (2DM)-based field-effect transistor (FET) and static random-access memory (SRAM) cells analyzing the impact of layer thickness reveals that the monolayer 2DM FET with superior electrostatics is beneficial for its ability to mitigate the read-write conflict in an SRAM cell at scaled technology nodes (1-2.1 nm). Moreover, the monolayer 2DM SRAM exhibits lower cell read access time and write time than the bilayer and trilayer 2DM SRAM cells at fixed leakage power. This simulation predicts that the optimization of 2DM SRAM designed with state-of-the-art contact resistance, mobility, and equivalent oxide thickness leads to excellent stability and operation speed at the 1-nm node. Applying the nanosheet (NS) gate-all-around (GAA) structure to 2DM further reduces cell read access time and write time and improves the area density of the SRAM cells, demonstrating a feasible scaling path beyond Si technology using 2DM NSFETs. In addition to the device design, the process challenges for 2DM NSFETs, including the cost-effective stacking of 2DM layers, formation of electrical contacts, suspended 2DM channels, and GAA structures, are also discussed.

Enhanced electronic transport properties of Te roll-like nanostructures

In this work, the electronic transport properties of Te roll-like nanostructures were investigated in a broad temperature range by fabricating single-nanostructure back-gated field-effect-transistors via photolithography. These one-dimensional nanostructures, with a unique roll-like morphology, were produced by a facile synthesis and extensively studied by scanning and transmission electron microscopy. The nanostructures are made of pure and crystalline Tellurium with trigonal structure (t-Te), and exhibit p-type conductivity with enhanced field-effect hole mobility between 273 cm²/Vs at 320 K and 881 cm²/Vs at 5 K. The thermal ionization of shallow acceptors, with small ionization energy between 2 and 4 meV, leads to free-hole conduction at high temperatures. The free-hole mobility follows a negative power-law temperature behavior, with an exponent between -1.28 and -1.42, indicating strong phonon scattering in this temperature range. At lower temperatures, the electronic conduction is dominated by nearest-neighbor hopping (NNH) conduction in the acceptor band, with a small activation energy E _NNH ≈ 0.6 meV and an acceptor concentration of N _A ≈ 1 × 10¹⁶ cm^-3. These results demonstrate the enhanced electrical properties of these nanostructures, with a small disorder, and superior quality for nanodevice applications.

Ultra-Narrow Phosphorene Nanoribbons Produced by Facile Electrochemical Process

Phosphorene nanoribbons (PNRs) have inspired strong research interests to explore their exciting properties that are associated with the unique two-dimensional (2D) structure of phosphorene as well as the additional quantum confinement of the nanoribbon morphology, providing new materials strategy for electronic and optoelectronic applications. Despite several important properties of PNRs, the production of these structures with narrow widths is still a great challenge. Here, a facile and straightforward approach to synthesize PNRs via an electrochemical process that utilize the anisotropic Na+ diffusion barrier in black phosphorus (BP) along the [001] zigzag direction against the [100] armchair direction, is reported. The produced PNRs display widths of good uniformity (10.3 ± 3.8 nm) observed by high-resolution transmission electron microscopy, and the suppressed B2g vibrational mode from Raman spectroscopy results. More interestingly, when used in field-effect transistors, synthesized bundles exhibit the n-type behavior, which is dramatically different from bulk BP flakes which are p-type. This work provides insights into a new synthesis approach of PNRs with confined widths, paving the way toward the development of phosphorene and other highly anisotropic nanoribbon materials for high-quality electronic applications.