Thickness-dependent interfacial Coulomb scattering in atomically thin field-effect transistors

Study of MoS2 as an Electric Field Sensor and the Role of Layer Thickness on the Sensitivity

In this study, we investigate the scope of molybdenum disulfide (MoS2) as an electric field sensor. We show that MoS2 sensors can be used to identify the polarity as well as to detect the magnitude of the electric field. The response of the sensor is recorded as the change in the drain current when the electric field is applied. The sensitivity, defined as the percentage change in the drain current, reveals that it has a linear relation with the magnitude of the electric field. Furthermore, the sensitivity is highly dependent on the layer thickness, with the single-layer device being highly sensitive and the sensitivity decreasing with the thickness. We have also compared the electric field sensitivity of MoS2 devices to that of previously studied graphene devices and found the former to be exceptionally sensitive than the latter for a given electric field magnitude.

Interfacial epitaxy of multilayer rhombohedral transition-metal dichalcogenide single crystals

Rhombohedral-stacked transition-metal dichalcogenides (3R-TMDs), which are distinct from their hexagonal counterparts, exhibit higher carrier mobility, sliding ferroelectricity, and coherently enhanced nonlinear optical responses. However, surface epitaxial growth of large multilayer 3R-TMD single crystals is difficult. We report an interfacial epitaxy methodology for their growth of several compositions, including molybdenum disulfide (MoS2), molybdenum diselenide, tungsten disulfide, tungsten diselenide, niobium disulfide, niobium diselenide, and molybdenum sulfoselenide. Feeding of metals and chalcogens continuously to the interface between a single-crystal Ni substrate and grown layers ensured consistent 3R stacking sequence and controlled thickness from a few to 15,000 layers. Comprehensive characterizations confirmed the large-scale uniformity, high crystallinity, and phase purity of these films. The as-grown 3R-MoS2 exhibited room-temperature mobilities up to 155 and 190 square centimeters per volt second for bi- and trilayers, respectively. Optical difference frequency generation with thick 3R-MoS2 showed markedly enhanced nonlinear response under a quasi-phase matching condition (five orders of magnitude greater than monolayers).

Determining the Electron Scattering from Interfacial Coulomb Scatterers in Two-Dimensional Transistors

Two-dimensional (2D) transistors are promising for potential applications in next-generation semiconductor chips. Owing to the atomically thin thickness of 2D materials, the carrier scattering from interfacial Coulomb scatterers greatly suppresses the carrier mobility and hampers transistor performance. However, a feasible method to quantitatively determine relevant Coulomb scattering parameters from interfacial long-range scatterers is largely lacking. Here, we demonstrate a method to determine the Coulomb scattering strength and the density of Coulomb scattering centers in InSe transistors by comprehensively analyzing the low-frequency noise and transport characteristics. Moreover, the relative contributions from long-range and short-range scattering in the InSe transistors can be distinguished. This method is employed to make InSe transistors consisting of various interfaces a model system, revealing the profound effects of different scattering sources on transport characteristics and low-frequency noise. Quantitatively accessing the scattering parameters of 2D transistors provides valuable insight into engineering the interfaces of a wide spectrum of ultrathin-body transistors for high-performance electronics.

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.

Performance limit of all-wrapped monolayer MoS2 transistors

All-wrapped transistors consisting of two-dimensional transition-metal dichalcogenide channels are appealing candidates for post-silicon electronics. Based on the Boltzmann transport theory, here we report a comprehensive theoretical survey on the performance limits for monolayer MoS2 transistors with three prototypical gate dielectrics (Al2O3, HfO2 and BN), by including primary extrinsic charge scattering mechanisms present in practical devices. A concept of "dead space" between the dielectrics and channels is proposed and used in calculation to ameliorate the general overestimation in scattering intensity of surface optical phonons, which enables an accurate description of electronic transport behavior. Crucial device indices, including charge mobility and current density, are thoroughly analyzed for transistors at post-silicon technological nodes beyond 1 nm. The on-state current is estimated to be generally greater than 2 mA μm-1 at channel lengths below 10 nm. The results clarify the potential benefits in performance from extremely miniaturized monolayer-channel transistors for More-Moore electronics.

Lattice Vacancy Induced Energy Renormalization of Photonic Quasiparticles in Two-Dimensional Semiconductors

Coulomb interactions among dense charges and quasiparticle energy renormalization are at the center of quantum science because they significantly reshape the fundamental electronic and photonic properties of materials. While lattice vacancies are ubiquitous in solid materials, their physical effect on the Coulomb interaction among quasiparticles is normally weak and negligible. Here we show that in atomically thin semiconductors the presence of lattice vacancies emerges as an important but unexplored origin for the nontrivial renormalization of quasiparticle binding energies, due to the subtle modification of overall dielectric functions at low dimensionality. Such a renormalization effect leads to unusual reduction in the energy scales of photonic quasiparticles and red shifts of photoluminescence as the density of lattice vacancies increases. With strict configurative form factors derived, a dielectric screening model is also established for the generalized trilayer systems to capture the fine modification in the energy scales of quasiparticles and to elucidate the dielectric functions versus realistic Bohr lengths. This finding highlights the essential but commonly neglected role of lattice vacancies and deciphers the longstanding enigma of unpredictable photoluminescent line shifts in low-dimensional systems.

Mobility Enhancement of Strained MoS2 Transistor on Flat Substrate

Strain engineering has been proposed as a promising method to boost the carrier mobility of two-dimensional (2D) semiconductors. However, state-of-the-art straining approaches are largely based on putting 2D semiconductors on flexible substrates or rough substrate with nanostructures (e.g., nanoparticles, nanorods, ripples), where the observed mobility change is not only dependent on channel strain but could be impacted by the change of dielectric environment as well as rough interface scattering. Therefore, it remains an open question whether the pure lattice strain could improve the carrier mobilities of 2D semiconductors, limiting the achievement of high-performance 2D transistors. Here, we report a strain engineering approach to fabricate highly strained MoS2 transistors on a flat substrate. By mechanically laminating a prefabricated MoS2 transistor onto a custom-designed trench structure on flat substrate, well-controlled strain can be uniformly generated across the 2D channel. In the meantime, the substrate and the back-gate dielectric layer remain flat without any roughness-induced scattering effect or variation of the dielectric environment. Based on this technique, we demonstrate the MoS2 electron mobility could be enhanced by tension strain and decreased by compression strain, consistent with theoretical predictions. The highest mobility enhancement is 152% for monolayer MoS2 and 64% for bilayer MoS2 transistors, comparable to that of a silicon device. Our method not only provides a compatible approach to uniformly strain the layered semiconductors on flat and solid substrate but also demonstrates an effective method to boost the carrier mobilities of 2D transistors.

Two-dimensional MoSi2As4-based field-effect transistors integrating switching and gas-sensing functions

Multifunctional nanoscale devices integrating multiple functions are of great importance for meeting the requirements of next-generation electronics. Herein, using first-principles calculations, we propose multifunctional devices based on the two-dimensional monolayer MoSi₂As₄, where a single-gate field-effect transistor (FET) and FET-type gas sensor are integrated. After introducing the optimizing strategies, such as underlap structures and dielectrics with a high dielectric constant (κ), we designed a 5 nm gate-length MoSi₂As₄ FET, whose performance fulfilled the key criteria of the International Technology Roadmap for Semiconductors (ITRS) for high-performance semiconductors. Under the joint adjustment of the underlap structure and high-κ dielectric material, the on/off ratio of the 5 nm gate-length FET reached up to 1.38 × 10⁴. In addition, driven by the high-performance FET, the MoSi₂As₄-based FET-type gas sensor showed a sensitivity of 38% for NH₃ and 46% for NO_2. Moreover, the weak interaction between NH₃ (NO₂) and MoSi₂As₄ favored the recycling of the sensor. Furthermore, the sensitivity of the sensor could be effectively improved by the gate voltage, and was increased up to 67% (74%) for NH₃ (NO₂). Our work provides theoretical guidance for the fabrication of multifunctional devices combining a high-performance FET and sensitive gas sensor.

Ultrabroadband Imaging Based on Wafer-Scale Tellurene

High-resolution imaging is at the heart of the revolutionary breakthroughs of intelligent technologies, and it is established as an important approach toward high-sensitivity information extraction/storage. However, due to the incompatibility between non-silicon optoelectronic materials and traditional integrated circuits as well as the lack of competent photosensitive semiconductors in the infrared region, the development of ultrabroadband imaging is severely impeded. Herein, the monolithic integration of wafer-scale tellurene photoelectric functional units by exploiting room-temperature pulsed-laser deposition is realized. Taking advantage of the surface plasmon polaritons of tellurene, which results in the thermal perturbation promoted exciton separation, in situ formation of out-of-plane homojunction and negative expansion promoted carrier transport, as well as the band bending promoted electron-hole pair separation enabled by the unique interconnected nanostrip morphology, the tellurene photodetectors demonstrate wide-spectrum photoresponse from 370.6 to 2240 nm and unprecedented photosensitivity with the optimized responsivity, external quantum efficiency and detectivity of 2.7 × 10⁷  A W^-1 , 8.2 × 10⁹ % and 4.5 × 10¹⁵  Jones. An ultrabroadband imager is demonstrated and high-resolution photoelectric imaging is realized. The proof-of-concept wafer-scale tellurene-based ultrabroadband photoelectric imaging system depicts a fascinating paradigm for the development of an advanced 2D imaging platform toward next-generation intelligent equipment.

Multilayer In-Plane Heterostructures Based on Transition Metal Dichalcogenides for Advanced Electronics

In-plane heterostructures of transition metal dichalcogenides (TMDCs) have attracted much attention for high-performance electronic and optoelectronic devices. To date, mainly monolayer-based in-plane heterostructures have been prepared by chemical vapor deposition (CVD), and their optical and electrical properties have been investigated. However, the low dielectric properties of monolayers prevent the generation of high concentrations of thermally excited carriers from doped impurities. To solve this issue, multilayer TMDCs are a promising component for various electronic devices due to the availability of degenerate semiconductors. Here, we report the fabrication and transport properties of multilayer TMDC-based in-plane heterostructures. The multilayer in-plane heterostructures are formed through CVD growth of multilayer MoS₂ from the edges of mechanically exfoliated multilayer flakes of WSe₂ or Nb_xMo_1-xS₂. In addition to the in-plane heterostructures, we also confirmed the vertical growth of MoS₂ on the exfoliated flakes. For the WSe₂/MoS₂ sample, an abrupt composition change is confirmed by cross-sectional high-angle annular dark-field scanning transmission electron microscopy. Electrical transport measurements reveal that a tunneling current flows at the Nb_xMo_1-xS₂/MoS₂ in-plane heterointerface, and the band alignment is changed from a staggered gap to a broken gap by electrostatic electron doping of MoS₂. The formation of a staggered gap band alignment of Nb_xMo_1-xS₂/MoS₂ is also supported by first-principles calculations.

Noninvasive Photodelamination of van der Waals Semiconductors for High-Performance Electronics

Atomically thin 2D van der Waals semiconductors are promising candidate materials for post-silicon electronics. However, it remains challenging to attain completely uniform monolayer semiconductor wafers free of over-grown islands. Here, the observation of the energy-funneling effect and ambient photodelamination phenomenon in inhomogeneous few-layer WS₂ flakes under low-illumination fluencies down to several nW µm^-2 and its potential as a noninvasive atomic-layer etching strategy for selectively stripping the local excessive overlying islands are reported. Photoluminescent tracking on the photoetching traces reveals relatively fast etching rates of around 0.3-0.8 µm min^-1 at varied temperatures and an activation energy of 1.7 eV. By using crystallographic and electronic characterization, the noninvasive nature of the low-power photodelamination and the highly preserved lattice quality are also confirmed in the as-etched monolayer products, featuring a comparable density of atomic defects (≈4.2 × 10¹³ cm^-2 ) to pristine flakes and a high electron mobility of up to 80 cm² V^-1 s^-1 at room temperature. This approach opens a noninvasive postetching route for thickness uniformity management in 2D van der Waals semiconductor wafers for electronic applications.

Effect of Surface Modification on the Fundamental Electrical Characteristics of Solution-Gated Indium Tin Oxide-Based Thin-Film Transistor Fabricated by One-Step Sputtering

Our solution-gated indium tin oxide (ITO)-based thin-film transistor (TFT) produced by single-step sputtering has great future potential in bioelectronics. In particular, chemical modifications of the ITO channel surface are expected to contribute to biomolecular recognition with ultrahigh sensitivity owing to a remarkably steep subthreshold slope (SS). In this study, we investigate the effect of a chemical modification of an aptamer as a receptor molecule at the ITO channel surface on the electrical characteristics of the solution-gated TFT. In this case, a SARS-CoV-2 aptamer is immobilized using a spacer molecule on an aryl diazonium monolayer that is electrochemically deposited with a radical scavenger. The monolayer is expected to not only passivate the ITO channel surface but also change the electron density in the ITO channel owing to the reduction reaction of aryl diazonium salts. Indeed, the electrochemical deposition of aryl diazonium salts decreases the leakage current through the ITO channel surface and provides a steep SS, which is near the thermal limit at 300 K, owing to the decrease in depletion layer capacitance. After the aptamer immobilization, the leakage current and SS unexpectedly return close to their original values before the surface modifications. This finding indicates that aptamer molecules should be carefully used because their negative charges would attract cations around the detection interface. Eventually, the solution-gated ITO-based TFT with the SARS-CoV-2 aptamer clearly responds to inactivated SARS-CoV-2 particles owing to the successful surface modification.

Layer-Structured Anisotropic Metal Chalcogenides: Recent Advances in Synthesis, Modulation, and Applications

The unique electronic and catalytic properties emerging from low symmetry anisotropic (1D and 2D) metal chalcogenides (MCs) have generated tremendous interest for use in next generation electronics, optoelectronics, electrochemical energy storage devices, and chemical sensing devices. Despite many proof-of-concept demonstrations so far, the full potential of anisotropic chalcogenides has yet to be investigated. This article provides a comprehensive overview of the recent progress made in the synthesis, mechanistic understanding, property modulation strategies, and applications of the anisotropic chalcogenides. It begins with an introduction to the basic crystal structures, and then the unique physical and chemical properties of 1D and 2D MCs. Controlled synthetic routes for anisotropic MC crystals are summarized with example advances in the solution-phase synthesis, vapor-phase synthesis, and exfoliation. Several important approaches to modulate dimensions, phases, compositions, defects, and heterostructures of anisotropic MCs are discussed. Recent significant advances in applications are highlighted for electronics, optoelectronic devices, catalysts, batteries, supercapacitors, sensing platforms, and thermoelectric devices. The article ends with prospects for future opportunities and challenges to be addressed in the academic research and practical engineering of anisotropic MCs.

Reduced dopant-induced scattering in remote charge-transfer-doped MoS2 field-effect transistors

Efficient doping for modulating electrical properties of two-dimensional (2D) transition metal dichalcogenide (TMDC) semiconductors is essential for meeting the versatile requirements for future electronic and optoelectronic devices. Because doping of semiconductors, including TMDCs, typically involves generation of charged dopants that hinder charge transport, tackling Coulomb scattering induced by the externally introduced dopants remains a key challenge in achieving ultrahigh mobility 2D semiconductor systems. In this study, we demonstrated remote charge transfer doping by simply inserting a hexagonal boron nitride layer between MoS₂ and solution-deposited n-type dopants, benzyl viologen. A quantitative analysis of temperature-dependent charge transport in remotely doped devices supports an effective suppression of the dopant-induced scattering relative to the conventional direct doping method. Our mechanistic investigation of the remote doping method promotes the charge transfer strategy as a promising method for material-level tailoring of electrical and optoelectronic devices based on TMDCs.

Abnormally weak intervalley electron scattering in MoS2 monolayer: insights from the matching between electron and phonon bands

It is known that carrier mobility in layered semiconductors generally increases from two-dimensions (2D) to three-dimensions due to fewer scattering channels resulting from decreased densities of electron and phonon states. In this work, we find an abnormal decrease of electron mobility from monolayer to bulk MoS₂. By carefully analyzing the scattering mechanisms, we can attribute such abnormality to the stronger intravalley scattering in the monolayer but weaker intervalley scattering caused by few intervalley scattering channels and weaker corresponding electron-phonon couplings compared to the bulk case. We show that it is the matching between the electronic band structure and phonon spectrum rather than their densities of electronic and phonon states that determines scattering channels. We propose, for the first time, the phonon-energy-resolved matching function to identify the intra- and inter-valley scattering channels. Furthermore, we show that multiple valleys do not necessarily lead to strong intervalley scattering if: (1) the scattering channels, which can be explicitly captured by the distribution of the matching function, are few due to the small matching between the corresponding electron and phonon bands; and/or (2) the multiple valleys are far apart in the reciprocal space and composed of out-of-plane orbitals so that the corresponding electron-phonon coupling strengths are weak. Consequently, the searching scope of high-mobility 2D materials can be reasonably enlarged using the matching function as useful guidance with the help of band edge orbital analysis.

Coulomb Screening and Scattering in Atomically Thin Transistors across Dimensional Crossover

Layered two-dimensional dichalcogenides are potential candidates for post-silicon electronics. Here, we report insightfully experimental and theoretical studies on the fundamental Coulomb screening and scattering effects in these correlated systems, in response to the changes of three crucial Coulomb factors, including electric permittivity, interaction distance, and density of Coulomb impurities. We systematically collect and analyze the trends of electron mobility with respect to the above factors, realized by synergic modulations on channel thicknesses and gating modes in dual-gated MoS₂ transistors with asymmetric dielectric cleanliness. Strict configurative form factors are developed to capture the subtle parametric changes across dimensional crossover. A full diagram of the carrier scattering mechanisms, in particular on the pronounced Coulomb scattering, is unfolded. Moreover, we clarify the presence of up to 40% discrepancy in mobility by considering the permittivity modification across dimensional crossover. The understanding is useful for exploiting atomically thin body transistors for advanced electronics.

Nondestructive and local mapping photoresponse of WSe2 by electrostatic force microscopy

We report a local mapping photoresponse of WSe₂ using a second-harmonic (2w) channel based on nondestructive electrostatic force microscopy (EFM). The 2w signals resulting from interaction between WSe₂ and EFM tip are intrinsically related to the electrical conductivity of WSe₂. The photoresponse images and rise/decay time constants of WSe₂ are obtained by local mapping 2w signals under illumination. We observe that the local photoresponse signals of WSe₂ increase with the positive tip gate voltage while the WSe₂ shows a p-type behavior in dark conditions We find that the reduced mobility of the photogenerated charge carriers resulting from the enhanced carrier scattering in the accumulation regime of WSe₂ is responsible for the gate-dependent photoresponse behavior. Our results provide a deep understanding the intrinsic optoelectrical properties of WSe₂ and contribute to the developments in the optoelectronic devices based on van der Waals layered materials.

Impact of the Channel Thickness on the Photoresponse of Black Arsenic Mid-Infrared Photodetectors

Recently explored black arsenic is a layered two-dimensional low-symmetry semiconducting material that, owing to its inherent narrow bandgap (∼0.31 eV) in its bulk form, is attractive for mid-infrared optoelectronics. Several studies have been conducted on its structural, charge-transport, and thermal properties for implementation in nanoelectronics. Herein, the thickness-dependent optoelectronic performance of black arsenic devices for mid-infrared wavelengths (2.0-4.0 μm) is investigated. The device was fabricated over an hBN/SiO2/Si substrate using mechanical exfoliation of black arsenic. It is observed that the optoelectronic properties of the devices vary significantly with the thickness of the black arsenic channel of the devices. A peak photoresponsivity of 244 A/W was achieved at 3.00 μm for a 60 nm-thick black arsenic channel. However, the maximum detectivity of 6.14 × 109 Jones was found for a lower thickness (∼25 nm) of black arsenic, along with an excellent (i.e., the least) noise-equivalent power of ∼89 fW/Hz1/2. Our findings reveal that the optoelectronic properties of black arsenic are excellent and can be tuned through thickness control. The promising results suggest the considerable potential of black arsenic in future opto- and nanoelectronic devices.

Layer-by-layer epitaxy of multi-layer MoS2 wafers

The 2D semiconductor of MoS₂ has great potential for advanced electronics technologies beyond silicon. So far, high-quality monolayer MoS₂ wafers have been available and various demonstrations from individual transistors to integrated circuits have also been shown. In addition to the monolayer, multilayers have narrower band gaps but improved carrier mobilities and current capacities over the monolayer. However, achieving high-quality multi-layer MoS₂ wafers remains a challenge. Here we report the growth of high-quality multi-layer MoS₂ 4-inch wafers via the layer-by-layer epitaxy process. The epitaxy leads to well-defined stacking orders between adjacent epitaxial layers and offers a delicate control of layer numbers up to six. Systematic evaluations on the atomic structures and electronic properties were carried out for achieved wafers with different layer numbers. Significant improvements in device performances were found in thicker-layer field-effect transistors (FETs), as expected. For example, the average field-effect mobility (μ_FE) at room temperature (RT) can increase from ∼80 cm²·V^–1·s^–1 for monolayers to ∼110/145 cm²·V^–1·s^–1 for bilayer/trilayer devices. The highest RT μ_FE of 234.7 cm²·V^–1·s^–1 and record-high on-current densities of 1.70 mA·μm^–1 at V_ds = 2 V were also achieved in trilayer MoS₂ FETs with a high on/off ratio of >10⁷. Our work hence moves a step closer to practical applications of 2D MoS₂ in electronics.

Clean BN-Encapsulated 2D FETs with Lithography-Compatible Contacts

Device passivation through ultraclean hexagonal BN encapsulation has proven to be one of the most effective ways of constructing high-quality devices with atomically thin semiconductors that preserve the ultraclean interface quality and intrinsic charge transport behavior. However, it remains challenging to integrate lithography-compatible contact electrodes with flexible distributions and patterns. Here, we report the feasibility of a straightforward integration of lithography-defined contacts into BN-encapsulated two-dimensional field-effect transistors (2D FETs), giving rise to overall device quality comparable to the state-of-the-art results from the painstaking pure dry transfer processing. The electronic characterization of FETs consisting of WSe₂ and MoS₂ channels reveals an extremely low scanning hysteresis of ∼2 mV on average, a low density of interfacial charged impurities of ∼10¹¹ cm^-2, and generally high charge mobilities over 1000 cm² V^-1 s^-1 at low temperatures. The overall high device qualities verify the viability of directly integrating lithography-defined contacts into BN-encapsulated devices to exploit their intrinsic charge transport properties for advanced electronics.

Anisotropic Carrier Mobility from 2H WSe2

Transition metal dichalcogenides (TMDCs) with 2H phase are expected to be building blocks in next-generation electronics; however, they suffer from electrical anisotropy, which is the basics for multi-terminal artificial synaptic devices, digital inverters, and anisotropic memtransistors, which are highly desired in neuromorphic computing. Herein, the anisotropic carrier mobility from 2H WSe₂ is reported, where the anisotropic degree of carrier mobility spans from 0.16 to 0.95 for various WSe₂ field-effect transistors under a gate voltage of -60 V. Phonon scattering, impurity ions scattering, and defect scattering are excluded for anisotropic mobility. An intrinsic screening layer is proposed and confirmed by Z-contrast scanning transmission electron microscopy (STEM) imaging to respond to the electrical anisotropy. Seven types of intrinsic screening layers are created and calculated by density functional theory to evaluate the modulated electronic structures, effective masses, and scattering intensities, resulting in anisotropic mobility. The discovery of anisotropic carrier mobility from 2H WSe₂ provides a degree of freedom for adjusting the physical properties of 2H TMDCs and fertile ground for exploring and integrating TMDC electronic transistors with better performance along the direction of high mobility.

Atomic Layer MoTe2 Field-Effect Transistors and Monolithic Logic Circuits Configured by Scanning Laser Annealing

Atomically thin semiconductors such as transition metal dichalcogenides have recently enabled diverse devices in the emerging two-dimensional (2D) electronics. While scalable 2D electronics demand monolithic integrated circuits consisting of complementary p-type and n-type transistors, conventional p-type and n-type doping in desired regions, monolithically in the same semiconducting atomic layers, remains elusive or impractical. Here, we report on an agile, high-precision scanning laser annealing approach to realizing 2D monolithic complementary logic circuits on atomically thin MoTe₂, by reliably designating p-type and n-type transport polarity in the constituent transistors via localized laser annealing and modification of their Schottky contacts. Pristine p-type field-effect transistors (FETs) transform into n-type ones upon controlled laser annealing on their source/drain gold electrodes, exhibiting a mobility of 96.5 cm² V^-1 s^-1 (the highest known to date) and an On/Off ratio of 10⁶. Elucidation and validation of such an on-demand configuration of polarity in MoTe₂ FETs further enable the construction and demonstration of essential logic circuits, including both inverter and NOR gates. This dopant-free, spatially precise scanning laser annealing approach to configuring monolithic complementary logic integrated circuits may enable programmable functions in 2D semiconductors, exhibiting potential for additively manufactured, scalable 2D electronics.

Synthesis and Characterization of Metallic Janus MoSH Monolayer

Janus transition-metal dichalcogenides (TMDCs) are emerging as special 2D materials with different chalcogen atoms covalently bonded on each side of the unit cell, resulting in interesting properties. To date, several synthetic strategies have been developed to realize Janus TMDCs, which first involves stripping the top-layer S of MoS₂ with H atoms. However, there has been little discussion on the intermediate Janus MoSH. It is critical to find the appropriate plasma treatment time to avoid sample damage. A thorough understanding of the formation and properties of MoSH is highly desirable. In this work, a controlled H₂-plasma treatment has been developed to gradually synthesize a Janus MoSH monolayer, which was confirmed by the TOF-SIMS analysis as well as the subsequent fabrication of MoSSe. The electronic properties of MoSH, including the high intrinsic carrier concentration (∼2 × 10¹³ cm^-2) and the Fermi level (∼ - 4.11 eV), have been systematically investigated by the combination of FET device study, KPFM, and DFT calculations. The results demonstrate a method for the creation of Janus MoSH and present the essential electronic parameters which have great significance for device applications. Furthermore, owing to the metallicity, 2D Janus MoSH might be a potential platform to observe the SPR behavior in the mid-infrared region.

Solution-Gated Ultrathin Channel Indium Tin Oxide-Based Field-Effect Transistor Fabricated by a One-Step Procedure that Enables High-Performance Ion Sensing and Biosensing

In this paper, we propose a one-step procedure for fabricating a solution-gated ultrathin channel indium tin oxide (ITO)-based field-effect transistor (FET) biosensor, thus providing an ″all-by-ITO″ technology. A thin-film sheet was placed on both ends of a metal shadow mask, which were contacted with a glass substrate. That is, the bottom of the metal shadow mask corresponding to the channel was slightly raised from the substrate, resulting in the creeping of some particles into the gap during sputtering. Owing to this modified metal shadow mask, a thin ITO channel (<30-40 nm) and thick ITO source/drain electrodes (ca. 100 nm) were simultaneously fabricated during the one-step sputtering. The thickness of ITO films was critical for them to be semiconductive, depending on the maximum depletion width (∼30-40 nm for the ITO channel), similarly to 2D materials. The ultrathin ITO channel worked as an ion-sensitive membrane as well owing to the intrinsic oxidated surface directly contacting with an electrolyte solution. The solution-gated 20-nm-thick channel ITO-based FET, with a steep subthreshold slope (SS) of 55 mV/dec (pH 7.41) attributable to the electric double-layer capacitance at the electrolyte solution/channel interface and the absence of interfacial traps among electrodes formed in one step, demonstrated an ideal pH responsivity (∼56 mV/pH), resulting in the real-time monitoring of cellular respiration and the long-term stability of electrical properties for 1 month. Moreover, the chemical modification of the ITO channel surface is expected to contribute to biomolecular recognition with ultrahigh sensitivity owing to the remarkably steep SS, which provided the exponential pH sensitivity in the subthreshold regime. Our new device produced in this one-step manner has a great future potential in bioelectronics.

Enhanced performance of multilayer MoS2transistors encapsulated with a photoresist

We report the enhanced performance of multilayer MoS₂transistors by AZ^®5214E photoresist encapsulation. The MoS₂transistors with SiO₂bottom-gate dielectrics exhibited an average increase of 4× in the on/off-current ratio and a 50% increase in the field-effect mobility after photoresist encapsulation. The Y-function method further showed a decrease of 87% in the contact resistance and a 30% increase in the intrinsic carrier mobility, suggesting that photoresist encapsulation provides not only n-type doping but also dielectric screening. The Raman spectra of the photoresist-encapsulated MoS₂also suggest n-type doping, which may be due to the electropositive hydroxyl groups in the photoresist. The operation of the photoresist-encapsulated MoS₂transistors remained stable in ambient air for at least one month. These results demonstrate that simple photoresist encapsulation can be an effective performance booster of MoS₂and other transition metal dichalcogenides transistors.

Restricted Channel Migration in 2D Multilayer ReS2

When thickness-dependent carrier mobility is coupled with Thomas-Fermi screening and interlayer resistance effects in two-dimensional (2D) multilayer materials, a conducting channel migrates from the bottom surface to the top surface under electrostatic bias conditions. However, various factors including (i) insufficient carrier density, (ii) atomically thin material thickness, and (iii) numerous oxide traps/defects considerably limit our deep understanding of the carrier transport mechanism in 2D multilayer materials. Herein, we report the restricted conducting channel migration in 2D multilayer ReS₂ after a constant voltage stress of gate dielectrics is applied. At a given gate bias condition, a gradual increase in the drain bias enables a sensitive change in the interlayer resistance of ReS₂, leading to a modification of the shape of the transconductance curves, and consequently, demonstrates the conducting channel migration along the thickness of ReS₂ before the stress. Meanwhile, this distinct conduction feature disappears after stress, indicating the formation of additional oxide trap sites inside the gate dielectrics that degrade the carrier mobility and eventually restrict the channel migration. Our theoretical and experimental study based on the resistor network model and Thomas-Fermi charge screening theory provides further insights into the origins of channel migration and restriction in 2D multilayer devices.

In Situ Study of Molecular Doping of Chlorine on MoS2 Field Effect Transistor Device in Ultrahigh Vacuum Conditions

We report a precise measurement of the sensor behavior of the field effect transistor (FET) formed with the MoS₂ channel when the channel part is exposed to Cl₂ gas. The gas exposure and the electrical measurement of the MoS₂ FET were executed with in situ ultrahigh-vacuum (UHV) conditions in which the surface analysis techniques were equipped. This makes it possible to detect how much sensitivity the MoS₂ FET can provide and understand the surface properties. With the Cl₂ gas exposure to the channel, the plot of the drain current versus the gate voltage (I _d-V _g curve) shifts monotonically toward the positive direction of V _g, suggesting that the adsorbate acts as an electron acceptor. The I _d-V _g shifts are numerically estimated by measuring the onset of I _d (threshold voltage, V _th) and the mobility as a function of the dosing amounts of the Cl₂ gas. The behaviors of both the V _th shift and the mobility with the Cl₂ dosing amount can be fitted with the Langmuir adsorption kinetics, which is typically seen in the uptake curve of molecule adsorption onto well-defined surfaces. This can be accounted for by a model where an impinging molecule occupies an empty site with a certain probability, and each adsorbate receives a certain amount of negative charge from the MoS₂ surface up to the monolayer coverage. The charge transfer makes the V _th shifts. In addition, the mobility is reduced by the enhancement of the Coulomb scattering for the electron flow in the MoS₂ channel by the accumulated charge. From the thermal desorption spectroscopy (TDS) measurement and density functional theory (DFT) calculations, we concluded that the adsorbate that is responsible for the change of the FET property is the Cl atom that is dissociated from the Cl₂ molecule. The monotonic shift of V _th with the coverage suggests that the MoS₂ device sensor has a good sensitivity to detect 10^-3 monolayers (ML) of adsorption corresponding to the ppb level sensor with an activation time of 1 s.

In Situ Oxygen Doping of Monolayer MoS2 for Novel Electronics

In 2D semiconductors, doping offers an effective approach to modulate their optical and electronic properties. Here, an in situ doping of oxygen atoms in monolayer molybdenum disulfide (MoS₂ ) is reported during the chemical vapor deposition process. Oxygen concentrations up to 20-25% can be reliable achieved in these doped monolayers, MoS_{2- x} O_x . These oxygen dopants are in a form of substitution of sulfur atoms in the MoS₂ lattice and can reduce the bandgap of intrinsic MoS₂ without introducing in-gap states as confirmed by photoluminescence spectroscopy and scanning tunneling spectroscopy. Field effect transistors made of monolayer MoS_{2- x} O_x show enhanced electrical performances, such as high field-effect mobility (≈100 cm² V^-1 s^-1 ) and inverter gain, ultrahigh devices' on/off ratio (>10⁹ ) and small subthreshold swing value (≈80 mV dec^-1 ). This in situ oxygen doping technique holds great promise on developing advanced electronics based on 2D semiconductors.

TFT Channel Materials for Display Applications: From Amorphous Silicon to Transition Metal Dichalcogenides

As the need for super-high-resolution displays with various form factors has increased, it has become necessary to produce high-performance thin-film transistors (TFTs) that enable faster switching and higher current driving of each pixel in the display. Over the past few decades, hydrogenated amorphous silicon (a-Si:H) has been widely utilized as a TFT channel material. More recently, to meet the requirement of new types of displays such as organic light-emitting diode displays, and also to overcome the performance and reliability issues of a-Si:H, low-temperature polycrystalline silicon and amorphous oxide semiconductors have partly replaced a-Si:H channel materials. Basic material properties and device structures of TFTs in commercial displays are explored, and then the potential of atomically thin layered transition metal dichalcogenides as next-generation channel materials is discussed.

Two-Dimensional Platinum Diselenide: Synthesis, Emerging Applications, and Future Challenges

In recent years, emerging two-dimensional (2D) platinum diselenide (PtSe2) has quickly attracted the attention of the research community due to its novel physical and chemical properties. For the past few years, increasing research achievements on 2D PtSe2 have been reported toward the fundamental science and various potential applications of PtSe2. In this review, the properties and structure characteristics of 2D PtSe2 are discussed at first. Then, the recent advances in synthesis of PtSe2 as well as their applications are reviewed. At last, potential perspectives in exploring the application of 2D PtSe2 are reviewed.

Observing the Layer-Number-Dependent Local Dielectric Response of WSe2 by Electrostatic Force Microscopy

We investigate the layer-number-dependent dielectric response of WSe₂ by measuring the phase shift (Φ) through an electrostatic force microscopy (EFM). The measured Φ results stem mainly from the capacitive coupling between the tip and WSe₂ based on the plane capacitor model, leading to changes in the second derivative of the capacitance (C'') values, which increase in a few layers and saturate to the bulk value under an applied EFM tip bias. The C'' value is related to the dielectric polarization, reflecting the charge carrier concentration and mobility of WSe₂ flakes with different numbers of layers. This implies that the dielectric constant of WSe₂ shows layer-number-dependent behavior which increases with the number of layers, approaching the bulk value. Furthermore, we also construct a spatially resolved C'' map to observe the local dielectric response of WSe₂ flakes. Our work could be significant in that it can improve the performance of novel electronic devices based on the controllable dielectric properties of 2D vdW semiconductor materials.

Site-specific electrical contacts with the two-dimensional materials

Electrical contact is an essential issue for all devices. Although the contacts of the emergent two-dimensional materials have been extensively investigated, it is still challenging to produce excellent contacts. The face and edge type contacts have been applied previously, however a comparative study on the site-specific contact performances is lacking. Here we report an in situ transmission electron microscopy study on the contact properties with a series of 2D materials. By manipulating the contact configurations in real time, it is confirmed that, for 2D semiconductors the vdW type face contacts exhibit superior conductivity compared with the non-vdW type contacts. The direct quantum tunneling across the vdW bonded interfaces are virtually more favorable than the Fowler–Nordheim tunneling across chemically bonded interfaces for contacts. Meanwhile, remarkable area, thickness, geometry, and defect site dependences are revealed. Our work sheds light on the significance of contact engineering for 2D materials in future applications.

Here, the authors use in situ transmission electron microscopy to measure the interface properties of electrical contacts with MoS₂, ReS₂, and graphene, and find that direct quantum tunnelling across van-der-Waals-bonded interfaces is more favourable than Fowler–Nordheim tunnelling across chemically bonded interfaces.

Layer-Selective Synthesis of MoS2 and WS2 Structures under Ambient Conditions for Customized Electronics

Transition metal dichalcogenides (TMDs) have attracted significant interest as one of the key materials in future electronics such as logic devices, optoelectrical devices, and wearable electronics. However, a complicated synthesis method and multistep processes for device fabrication pose major hurdles for their practical applications. Here, we introduce a direct and rapid method for layer-selective synthesis of MoS₂ and WS₂ structures in wafer-scale using a pulsed laser annealing system (λ = 1.06 μm, pulse duration ∼100 ps) in ambient conditions. The precursor layer of each TMD, which has at least 3 orders of magnitude higher absorption coefficient than those of neighboring layers, rigorously absorbed the incoming energy of the laser pulse and rapidly pyrolyzed in a few nanoseconds, enabling the generation of a MoS₂ or WS₂ layer without damaging the adjacent layers of SiO₂ or polymer substrate. Through experimental and theoretical studies, we establish the underlying principles of selective synthesis and optimize the laser annealing conditions, such as laser wavelength, output power, and scribing speed, under ambient condition. As a result, individual homostructures of patterned MoS₂ and WS₂ layers were directly synthesized on a 4 in. wafer. Moreover, a consecutive synthesis of the second layer on top of the first synthesized layer realized a vertically stacked WS₂/MoS₂ heterojunction structure, which can be treated as a cornerstone of electronic devices. As a proof of concept, we demonstrated the behavior of a MoS₂-based field-effect transistor, a skin-attachable motion sensor, and a MoS₂/WS₂-based heterojunction diode in this study. The ultrafast and selective synthesis of the TMDs suggests an approach to the large-area/mass production of functional heterostructure-based electronics.

Molecular Adsorption of NH3 and NO2 on Zr and Hf Dichalcogenides (S, Se, Te) Monolayers: A Density Functional Theory Study

Due to their atomic thicknesses and semiconducting properties, two-dimensional transition metal dichalcogenides (TMDCs) are gaining increasing research interest. Among them, Hf- and Zr-based TMDCs demonstrate the unique advantage that their oxides (HfO₂ and ZrO₂) are excellent dielectric materials. One possible method to precisely tune the material properties of two-dimensional atomically thin nanomaterials is to adsorb molecules on their surfaces as non-bonded dopants. In the present work, the molecular adsorption of NO₂ and NH₃ on the two-dimensional trigonal prismatic (1H) and octahedral (1T) phases of Hf and Zr dichalcogenides (S, Se, Te) is studied using dispersion-corrected periodic density functional theory (DFT) calculations. The adsorption configuration, energy, and charge-transfer properties during molecular adsorption are investigated. In addition, the effects of the molecular dopants (NH₃ and NO₂) on the electronic structure of the materials are studied. It was observed that the adsorbed NH₃ donates electrons to the conduction band of the Hf (Zr) dichalcogenides, while NO₂ receives electrons from the valance band. Furthermore, the NO₂ dopant affects than NH₃ significantly. The resulting band structure of the molecularly doped Zr and Hf dichalcogenides are modulated by the molecular adsorbates. This study explores, not only the properties of the two-dimensional 1H and 1T phases of Hf and Zr dichalcogenides (S, Se, Te), but also tunes their electronic properties by adsorbing non-bonded dopants.

Thickness effect on low-power driving of MoS2 transistors in balanced double-gate fields

In field-effect transistors (FETs), when the thickness of the semiconducting transition metal dichalcogenides (TMDs) channel exceeds the maximum depletion depth, the entire region cannot be completely controlled by a single-gate electric field. The layer-to-layer carrier transitions between the van der Waals interacted TMD layers result in the extraordinary anisotropic carrier transport in the in-plane and out-of-plane directions. The performance of the TMD FETs can be largely enhanced by optimizing the thickness of the TMD channel as well as increasing the effective channel area through which the gate field is delivered. In this study, we investigated the carrier behavior and device performance in double-gate FETs fabricated using a 57 nm thick MoS₂, which is thicker than the maximum depletion depth of about 50 nm, and a much thinner 4 nm thick MoS₂. The results showed that in the thick MoS₂, the gate voltages at both ends formed two independent channels which had no synergistic effect on the device performance owing to the inefficient delivery of the vertical electric field. On the other hand, in the thin MoS₂ channel, the double-gate voltages effectively controlled one channel, resulting in twice the carrier mobility and operation in a low electric field region, i.e. below 0.2 MV cm^-1.

Nucleation promoted synthesis of large-area ReS2 film for high-speed photodetectors

Rhenium disulfide (ReS₂) is a transition metal dichalcogenide with a layer-independent direct bandgap. Notably, the weak interlayer coupling owing to its T-phase structure enables multi-layer ReS₂ to behave similarly to decoupled monolayers. This inherent characteristic makes continuous multilayer ReS₂ film a unique platform for large-area electronic applications. To date, the bulk of work on ReS₂ has been conducted using mechanically exfoliated samples or small size flakes (<1 mm²) with no potential for large-scale electronics. A chemical vapor deposition (CVD) synthesis of a large area, continuous ReS₂ film directly on a SiO₂ substrate is also known to be more challenging compared with that of other 2D materials, such as MoS₂ and WS₂. This is partly due to its tendency to grow into discrete dendritic structures. In this study, a large-area (>1 cm²), continuous multilayer ReS₂ film is directly synthesized on a SiO₂ substrate without any transfer process. The polycrystalline ReS₂ film synthesized by this method exhibits one of the fastest photoresponse speeds (0.03 s rise time and 0.025 s decay time) among the reported CVD films. The photoresponsivity R _λ was also the highest among large-area CVD films. The synthesis method for a continuous multilayer ReS₂ film is amenable to large-scale integration and will pave the way for practical optoelectronic applications based on 2D layered materials.

Direct Visualization of Electric-Field-Induced Structural Dynamics in Monolayer Transition Metal Dichalcogenides

Layered transition metal dichalcogenides offer many attractive features for next-generation low-dimensional device geometries. Due to the practical and fabrication challenges related to in situ methods, the atomistic dynamics that give rise to realizable macroscopic device properties are often unclear. In this study, in situ transmission electron microscopy techniques are utilized in order to understand the structural dynamics at play, especially at interfaces and defects, in the prototypical film of monolayer MoS₂ under electrical bias. Through our sample fabrication process, we clearly identify the presence of mass transport in the presence of a lateral electric field. In particular, we observe that the voids present at grain boundaries combine to induce structural deformation. The electric field mediates a net vacancy flux from the grain boundary interior to the exposed surface edge sites that leaves molybdenum clusters in its wake. Following the initial biasing cycles, however, the mass flow is largely diminished and the resultant structure remains stable over repeated biasing. We believe insights from this work can help explain observations of nonuniform heating and preferential oxidation at grain boundary sites in these materials.

2D Perovskite Sr2 Nb3 O10 for High-Performance UV Photodetectors

2D perovskites, due to their unique properties and reduced dimension, are promising candidates for future optoelectronic devices. However, the development of stable and nontoxic 2D wide-bandgap perovskites remains a challenge. 2D all-inorganic perovskite Sr₂ Nb₃ O₁₀ (SNO) nanosheets with thicknesses down to 1.8 nm are synthesized by liquid exfoliation, and for the first time, UV photodetectors (PDs) based on individual few-layer SNO sheets are investigated. The SNO sheet-based PDs exhibit excellent UV detecting performance (narrowband responsivity = 1214 A W^-1 , external quantum efficiency = 5.6 × 10⁵ %, detectivity = 1.4 × 10¹⁴ Jones @270 nm, 1 V bias), and fast response speed (t_rise ≈ 0.4 ms, t_decay ≈ 40 ms), outperforming most reported individual 2D sheet-based UV PDs. Furthermore, the carrier transport properties of SNO and the performance of SNO-based phototransistors are successfully controlled by gate voltage. More intriguingly, the photodetecting performance and carrier transport properties of SNO sheets are dependent on their thickness. In addition, flexible and transparent PDs with high mechanical stability are easily fabricated based on SNO nanosheet film. This work sheds light on the development of high-performance optoelectronics based on low-dimensional wide-bandgap perovskites in the future.

Two-dimensional CaFCl: ultra-wide bandgap, strong interlayer quantum confinement, and n-type doping

Two-dimensional (2D) ultra-wide bandgap (UWBG) semiconductors have attracted tremendous attention because of their unique electronic properties and promising applications. Using first-principles calculations, monolayer (bilayer) CaFCl has a cleavage energy of 0.93 J m-2 (0.72 J m-2), suggesting that the exfoliation of monolayer and few-layer materials from the bulk phase could be feasible. The CaFCl monolayer is an UWBG semiconductor with a direct bandgap of 6.62 eV. In addition to the dynamic and thermodynamic stability, it can remain thermally stable at 2200 K, suitable for operation in high-temperature environments. The bandgap of monolayer CaFCl can be tuned by external strain and layer thickness. The decrease of the layer thickness leads to not only a bandgap increase but also an indirect-to-direct bandgap transition, suggesting a strong interlayer quantum confinement effect. Under biaxial strain, the direct bandgap can also be turned into an indirect one. The adsorption of a tetrathiafulvalene (TTF) molecule introduces deep donor states in the gap of CaFCl. Under an external electric field with direction from CaFCl to TTF, the TTF-derived donor states move closer to the conduction band edge of CaFCl and then the adsorption complex becomes effectively n-doped. Furthermore, monolayer CaFCl exhibits pronounced optical absorption in the ultraviolet range of the solar spectrum. These results render CaFCl an attractive 2D material for applications in flexible nanoelectronic and optoelectronic devices.

Accurate Method To Determine the Mobility of Transition-Metal Dichalcogenides with Incomplete Gate Screening

van der Waals layered transition-metal dichalcogenides usually exhibit high contact resistance because of the induced Schottky barriers, which occur at nonideal metal-semiconductor contacts. These barriers usually contribute to an underestimation in the determination of mobility, when extracted by standard, two-terminal methods. Furthermore, in devices based on atomically thin materials, channels with thicknesses of up to a few layers cannot completely screen the applied gate bias, resulting in an incomplete potential drop over the channel; the resulting decreased field effect causes further underestimation of the mobility. We demonstrate a method based on Kelvin probe force microscopy, which allows us to extract the accurate semiconductor mobility and eliminates the effects of contact quality and/or screening ability. Our results reveal up to a sevenfold increase in mobility in a monolayer device.

Edge Contact for Carrier Injection and Transport in MoS2 Field-Effect Transistors

The contact properties of van der Waals layered semiconducting materials are not adequately understood, particularly for edge contact. Edge contact is extremely helpful in the case of graphene, for producing efficient contacts to vertical heterostructures, and for improving the contact resistance through strong covalent bonding. Herein, we report on edge contacts to MoS₂ of various thicknesses. The carrier-type conversion is robustly controlled by changing the flake thickness and metal work functions. Regarding the ambipolar behavior, we suggest that the carrier injection is segregated in a relatively thick MoS₂ channel; that is, electrons are in the uppermost layers, and holes are in the inner layers. Calculations reveal that the strength of the Fermi-level pinning (FLP) varies layer-by-layer, owing to the inhomogeneous carrier concentration, and particularly, there is negligible FLP in the inner layer, supporting the hole injection. The contact resistance is large despite the significantly reduced contact resistivity normalized by the contact area, which is attributed to the current-crowding effect arising from the narrow contact area.

Thickness-dependent photoelectric properties of MoS2/Si heterostructure solar cells

In order to obtain the optimal photoelectric properties of vertical stacked MoS₂/Si heterostructure solar cells, we propose a theoretical model to address the relationship among film thickness, atomic bond identities and related physical quantities in terms of bond relaxation mechanism and detailed balance principle. We find that the vertical stacked MoS₂/Si can form type II band alignment, and its photoelectric conversion efficiency (PCE) enhances with increasing MoS₂ thickness. Moreover, the optimal PCE in MoS₂/Si can reach 24.76%, inferring that a possible design way can be achieved based on the layered transition metal dichalcogenides and silicon.

Reconfigurable Local Photoluminescence of Atomically-Thin Semiconductors via Ferroelectric-Assisted Effects

Combining a pair of materials of different structural dimensions and functional properties into a hybrid material system may realize unprecedented multi-functional device applications. Especially, two-dimensional (2D) materials are suitable for being incorporated into the heterostructures due to their colossal area-to-volume ratio, excellent flexibility, and high sensitivity to interfacial and surface interactions. Semiconducting molybdenum disulfide (MoS₂), one of the well-studied layered materials, has a direct band gap as one molecular layer and hence, is expected to be one of the promising key materials for next-generation optoelectronics. Here, using lateral 2D/3D heterostructures composed of MoS₂ monolayers and nanoscale inorganic ferroelectric thin films, reversibly tunable photoluminescence has been demonstrated at the microscale to be over 200% upon ferroelectric polarization reversal by using nanoscale conductive atomic force microscopy tips. Also, significant ferroelectric-assisted modulation in electrical properties has been achieved from field-effect transistor devices based on the 2D/3D heterostructrues. Moreover, it was also shown that the MoS₂ monolayer can be an effective electric field barrier in spite of its sub-nanometer thickness. These results would be of close relevance to exploring novel applications in the fields of optoelectronics and sensor technology.

Structures and characteristics of atomically thin ZrO2 from monolayer to bilayer and two-dimensional ZrO2-MoS2 heterojunction

The understanding of the structural stability and properties of dielectric materials at the ultrathin level is becoming increasingly important as the size of microelectronic devices decreases. The structures and properties of ultrathin ZrO₂ (monolayer and bilayer) have been investigated by ab initio calculations. The calculation of enthalpies of formation and phonon dispersion demonstrates the stability of both monolayer and bilayer ZrO₂ adopting a honeycomb-like structure similar to 1T-MoS₂. Moreover, the 1T-ZrO₂ monolayer or bilayer may be fabricated by the cleavage from the (111) facet of non-layered cubic ZrO₂. Moreover, the contraction of in-plane lattice constants in monolayer and bilayer ZrO₂ as compared to the corresponding slab in cubic ZrO₂ is consistent with the reported experimental observation. The electronic band gaps calculated from the GW method show that both the monolayer and bilayer ZrO₂ have large band gaps, reaching 7.51 and 6.82 eV, respectively, which are larger than those of all the bulk phases of ZrO₂. The static dielectric constants of both monolayer ZrO₂ (ε _‖ = 33.34, ε _⊥ = 5.58) and bilayer ZrO₂ (ε _‖ = 33.86, ε _⊥ = 8.93) are larger than those of monolayer h-BN (ε _‖ = 6.82, ε _⊥ = 3.29) and a strong correlation between the out-of-plane dielectric constant and the layer thickness in ultrathin ZrO₂ can be observed. Hence, 1T-ZrO₂ is a promising candidate in 2D FETs and heterojunctions due to the high dielectric constant, good thermodynamic stability, and large band gap for applications. The interfacial properties and band edge offset of the ZrO₂-MoS₂ heterojunction are investigated herein, and we show that the electronic states near the VBM and CBM are dominated by the contributions from monolayer MoS₂, and the interface with monolayer ZrO₂ will significantly decrease the band gap of the monolayer MoS₂.

Field-Effect Transistors Based on 2D Organic Semiconductors Developed by a Hybrid Deposition Method

Solution-processed 2D organic semiconductors (OSCs) have drawn considerable attention because of their novel applications from flexible optoelectronics to biosensors. However, obtaining well-oriented sheets of 2D organic materials with low defect density still poses a challenge. Here, a highly crystallized 2,9-didecyldinaphtho[2,3-b:2',3'-f]thieno[3,2-b]thiophene (C10-DNTT) monolayer crystal with large-area uniformity is obtained by an ultraslow shearing (USS) method and its growth pattern shows a kinetic Wulff's construction supported by theoretical calculations of surface energies. The resulting seamless and highly crystalline monolayers are then used as templates for thermally depositing another C10-DNTT ultrathin top-up film. The organic thin films deposited by this hybrid approach show an interesting coherence structure with a copied molecular orientation of the templating crystal. The organic field-effect transistors developed by these hybrid C10-DNTT films exhibit improved carrier mobility of 14.7 cm2 V-1 s-1 as compared with 7.3 cm2 V-1 s-1 achieved by pure thermal evaporation (100% improvement) and 2.8 cm2 V-1 s-1 achieved by solution sheared monolayer C10-DNTT. This work establishes a simple yet effective approach for fabricating high-performance and low-cost electronics on a large scale.

A field-effect approach to directly profiling the localized states in monolayer MoS2

A fundamental understanding of the charge transport mechanism in two-dimensional semiconductors (e.g., MoS₂) is crucial for fully exploring their potential in electronic and optoelectronic devices. By using monolayer graphene as the barrier-free contact to MoS₂, we show that the field-modulated conductivity can be used to probe the electronic structure of the localized states. A series of regularly distributed plateaus were observed in the gate-dependent transfer curves. Calculations based on the variable-range hopping theory indicate that such plateaus can be attributed to the discrete localized states near mobility edge. This method provides an effective approach to directly profiling the localized states in conduction channel with an ultrahigh resolution up to 1 meV.

Temperature-Dependent Opacity of the Gate Field Inside MoS2 Field-Effect Transistors

The transport behaviors of MoS₂ field-effect transistors (FETs) with various channel thicknesses are studied. In a 12 nm thick MoS₂ FET, a typical switching behavior is observed with an I_on/I_off ratio of 10⁶. However, in 70 nm thick MoS₂ FETs, the gating effect weakens with a large off-current, resulting from the screening of the gate field by the carriers formed through the ionization of S vacancies at 300 K. Hence, when the latter is dual-gated, two independent conductions develop with different threshold voltage (V_TH) and field-effect mobility (μ_FE) values. When the temperature is lowered for the latter, both the ionization of S vacancies and the gate-field screening reduce, which revives the strong I_on/I_off ratio and merges the two separate channels into one. Thus, only one each of V_TH and μ_FE are seen from the thick MoS₂ FET when the temperature is less than 80 K. The change of the number of conduction channels is attributed to the ionization of S vacancies, which leads to a temperature-dependent intra- and interlayer conductance and the attenuation of the electrostatic gate field. The defect-related transport behavior of thick MoS₂ enables us to propose a new device structure that can be further developed to a vertical inverter inside a single MoS₂ flake.

2D Molecular Crystal Bilayer p-n Junctions: A General Route toward High-Performance and Well-Balanced Ambipolar Organic Field-Effect Transistors

Ambipolar organic field-effect transistors (OFETs) are vital for the construction of high-performance all-organic digital circuits. The bilayer p-n junction structure, which is composed of separate layers of p- and n-type organic semiconductors, is considered a promising way to realize well-balanced ambipolar charge transport. However, this approach suffers from severely reduced mobility due to the rough interface between the polycrystalline thin films of p- and n-type organic semiconductors. Herein, 2D molecular crystal (2DMC) bilayer p-n junctions are proposed to construct high-performance and well-balanced ambipolar OFETs. The molecular-scale thickness of the 2DMC ensures high injection efficiency and the atomically flat surface of the 2DMC leads to high-quality p- and n-layer interfaces. Moreover, by controlling the layer numbers of the p- and n-type 2DMCs, the electron and hole mobilities are tuned and well-balanced ambipolar transport is accomplished. The hole and electron mobilities reach up to 0.87 and 0.82 cm² V^-1 s^-1 , respectively, which are the highest values among organic single-crystalline double-channel OFETs measured in ambient air. This work provides a general route to construct high-performance and well-balanced ambipolar OFETs based on available unipolar materials.

Direct Mapping of the Gate Response of a Multilayer WSe2/MoS2 Heterostructure with Locally Different Degrees of Charge Depletion

Understanding the interlayer charge coupling mechanism in a two-dimensional van der Waals (vdW) heterojunction is crucial for optimizing the performance of heterostructure-based (opto)electronic devices. Here, we report mapping the gate response of a multilayer WSe₂/MoS₂ heterostructure with locally different degrees of charge depletion through mobile carrier measurements based on electrostatic force microscopy. We observed ambipolar or unipolar behavior depending on the degree of charge depletion in the heterojunction under tip gating. Interestingly, the WSe₂ on MoS₂ shows gating behavior that is more efficient than that on the SiO₂/Si substrate, which can be explained by the high dielectric environment and screening of impurities on the SiO₂ surface by the MoS₂. Furthermore, we found that the gate-induced majority carriers in the heterojunction reduce the carrier lifetime, leading to the enhanced interlayer recombination of the photogenerated carriers under illumination. Our work provides a comprehensive understanding of the interfacial phenomena at the vdW heterointerface with charge depletion.

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.