Lewis Acid Induced Functionalization of Photoluminescent 2D Silicon Nanosheets for the Fabrication of Functional Hybrid Films

Various Lewis acids are found to efficiently catalyze hydrosilylation reactions of hydride‐terminated 2D silicon nanosheets at room temperature. The hydride‐terminated nanosheets can be functionalized with a variety of unsaturated functional substrates and still possess their unique characteristic (opto)electronic properties (e.g., photoluminescence). This is demonstrated by successfully implementing the readily functionalized materials into new silicon/semiconducting polymer‐based field‐effect transistors (FETs). Surface modification of the freestanding silicon nanosheets opens new possibilities to form highly homogeneous blends with the already broadly used conventional polymers poly(3‐hexylthiophene‐2,5‐diyl). The consequential combination of the different properties of the materials enables the enhancement of the sensitivity of the solution‐gated FETs and increases the transconductance of the operating device.     

2D TMD Channel Transistors with ZnO Nanowire Gate for Extended Nonvolatile Memory Applications

2D transition metal dichalcogenides (TMDs) have been extensively studied due to their excellent physical properties. Mixed dimensional devices including 2D materials have also been studied, motivated by the possibility of any synergy effect from unique structures. However, only few such studies have been conducted. Here, semiconducting 1D ZnO nanowires are used as thin gate material to support 2D TMD field effect transistors (FETs) and 2D stack‐based interface trap nonvolatile memory. For the trap memory, deep level electron traps formed at the first MoS₂/second MoS₂ stack interface are exploited, since the first MoS₂ is treated in an atomic layer deposition chamber for a short while. On the one hand, a complementary inverter type memory device can also be achieved using a long single ZnO wire as a common gate to simultaneously support both n‐ and p‐channel TMD FETs. In addition, it is found that the semiconducting ZnO nanowire itself operates as an n‐type channel when the TMD materials can become a top‐gate to charge the ZnO channel. It means that 2D (bottom gated) and 1D channel (top gated) FETs are respectively operational in a single device structure. The 1D–2D mixed devices seem deserving broad attention in both aspects of novelty and functionality.     

High‐Performance Field Effect Transistors Using Electronic Inks of 2D Molybdenum Oxide Nanoflakes

Planar 2D materials are possibly the ideal channel candidates for future field effect transistors (FETs), due to their unique electronic properties. However, the performance of FETs based on 2D materials is yet to exceed those of conventional silicon based devices. Here, a 2D channel thin film made from liquid phase exfoliated molybdenum oxide nanoflake inks with highly controllable substoichiometric levels is presented. The ability to induce oxygen vacancies by solar light irradiation in an aqueous environment allows the tuning of electronic properties in 2D substoichiometric molybdenum oxides (MoO_3?x). The highest mobility is found to be ≈600 cm² V^?1 s^?1 with an estimated free electron concentration of ≈1.6 × 10²¹ cm^?3 and an optimal I_On/I_Off ratio of >10⁵ for the FETs made of 2D flakes irradiated for 30 min (x = 0.042). These values are significant and represent a real opportunity to realize the next generation of tunable electronic devices using electronic inks.     

Controlled Vapor–Solid Deposition of Millimeter‐Size Single Crystal 2D Bi2O2Se for High‐Performance Phototransistors

Atomically thin 2D materials have received intense interest both scientifically and technologically. Bismuth oxyselenide (Bi₂O₂Se) is a semiconducting 2D material with high electron mobility and good stability, making it promising for high‐performance electronics and optoelectronics. Here, an ambient‐pressure vapor–solid (VS) deposition approach for the growth of millimeter‐size 2D Bi₂O₂Se single crystal domains with thicknesses down to one monolayer is reported. The VS‐grown 2D Bi₂O₂Se has good crystalline quality, chemical uniformity, and stoichiometry. Field‐effect transistors (FETs) are fabricated using this material and they show a small contact resistivity of 55.2 Ω cm measured by a transfer line method. Upon light irradiation, a phototransistor based on the Bi₂O₂Se FETs exhibits a maximum responsivity of 22 100 AW^?1, which is a record among currently reported 2D semiconductors and approximately two orders of magnitude higher than monolayer MoS₂. The Bi₂O₂Se phototransistor shows a gate tunable photodetectivity up to 3.4 × 10¹⁵ Jones and an on/off ratio up to ≈10⁹, both of which are much higher than phototransistors based on other 2D materials reported so far. The results of this study indicate a method to grow large 2D Bi₂O₂Se single crystals that have great potential for use in optoelectronic applications.     

Electron Field Emission of Geometrically Modulated Monolayer Semiconductors

Electron field emission, electrons emitted from solid surfaces under high electric field, offers significant scientific interests in materials sciences and potential optoelectronics applications. 2D atomic layers, such as MoS₂, exhibit fascinating properties for diverse applications in next‐generation nanodevices and rich physical phenomena for fundamental research. However, the study on field emission of semiconducting monolayers is lacking owing to its low efficiency and stability of electron emission. Here, electron field emission of the geometrically modulated monolayer semiconductors suspended with 1D nanoarrays is demonstrated. Chemical vapor deposition synthesis of two prototype monolayers of transition metal dichalcogenides (TMD), MoS₂ and MoSe₂, is presented and their diverse band structures offer an ideal platform to explore the fundamental process of the electron emission in the TMD. Geometrical modulation and charge transfer of the semiconducting monolayers can be significantly tuned with the structural suspension with the 1D ZnO nanoarrays. Possible mechanisms on the enhanced electron emission of the 2D monolayers are discussed. With geometrical control of the monolayers, a highly efficient and stable electron emission of TMD monolayers is achieved in low turn‐on electric fields, enabling applications on electrons sources and opening a new avenue toward geometrically tuned atomic layers.     

Graphene-Enhanced Metal Transfer Printing for Strong van der Waals Contacts between 3D Metals and 2D Semiconductors

2D semiconductors have shown great potentials for ultra-short channel field-effect transistors (FETs) in next-generation electronics. However, because of intractable surface states and interface barriers, it is challenging to realize high-quality contacts with low contact resistances for both p- and n- 2D FETs. Here, a graphene-enhanced van der Waals (vdWs) integration approach is demonstrated, which is a multi-scale (nanometer to centimeter scale) and reliable (≈100% yield) metal transfer strategy applicable to various metals and 2D semiconductors. Scanning transmission electron microscopy imaging shows that 2D/2D/3D semiconductor/graphene/metal interfaces are atomically flat, ultraclean, and defect-free. First principles calculations indicate that the sandwiched graphene monolayer can eliminate gap states induced by 3D metals in 2D semiconductors. Through this approach, Schottky barrier-free contacts are realized on both p- and n-type 2D FETs, achieving p-type MoTe₂, p-type black phosphorus and n-type MoS₂ FETs with on-state current densities of 404, 1520, and 761 µA µm⁻¹, respectively, which are among the highest values reported in literature.     

2D Ruddlesden–Popper Perovskite Single Crystal Field-Effect Transistors

2D Ruddlesden–Popper perovskites (2D PVKs) have attracted huge interest because of their excellent optoelectronic properties, yet the understanding of their electrical properties is inadequate due to the difficulties in obtaining 2D PVK field-effect transistors (FETs) with decent performance. Herein, the fabrication and characterization of 2D PVK ((BA)₂(MA)_n−1Pb_nI_3n+1) single crystal FETs are reported, which exhibit reliable field effect electrical characteristics at low temperatures. Kelvin probe force microscopy (KPFM) results reveal that both ion migration and contact resistance seriously degrade device performance. While ion migration can be suppressed at low temperatures, contact resistance seems to fundamentally determine device performance. On one hand, Schottky contacts are observed to form at the metal/2D PVK interface because of Fermi level pinning, resulting in significant charge injection resistance, although this can be remarkably improved by replacing Au electrodes with Ca. On the other hand, the out-of-plane mobility is found to be three orders of magnitude lower than the in-plane mobility in 2D PVKs, causing large interlayer transport resistance. Thus, a low work-function metal and a thin crystal are important for achieving high device performance. This work provides important experimental insights into fabrication and electrical properties of 2D PVK FETs.     

Charge Carrier Mobility and Series Resistance Extraction in 2D Field-Effect Transistors: Toward the Universal Technique

2D semiconductor field-effect transistors (2D FETs) have emerged as a promising candidate for beyond-silicon electronics applications. However, its device performance has often been limited by the metal-2D semiconductor contact, and the non-negligible contact resistance (R_SD) not only deteriorates the on-state current but also hinders the direct characterization of the intrinsic properties of 2D semiconductors (e.g., intrinsic charge carrier mobility, μ_int). Therefore, a proper extraction technique that can independently characterize the metal-2D semiconductor contact behavior and the intrinsic properties of a 2D semiconducting layer is highly desired. In this study, a universal yet simple method is developed to accurately extract the critical parameters in 2D FETs, including characteristic temperature (T_o), threshold voltage (V_T), R_SD, and μ_int. The practicability of this method is extensively explored by characterizing the temperature-dependent carrier transport behavior and the strain-induced band structure modification in 2D semiconductors. Technology computer aided design simulation is subsequently employed to verify the precision of R_SD extraction. Furthermore, the universality of the proposed method is validated by successfully implementing the extraction to various 2D semiconductors, including black phosphorus, indium selenide, molybdenum disulfide, rhenium disulfide, and tungsten disulfide with top- and bottom-gated configurations.     

A General Approach to Probe Dynamic Operation and Carrier Mobility in Field‐Effect Transistors with Nonuniform Accumulation

Revealing the intrinsic electrical properties is the basis of understanding new functional materials and developing their applications. However, in nonideal field‐effect transistors (FETs), conventional current–voltage characterizations do not accurately probe charge transport, particularly for newly developed semiconductors. Here, a generalized gated four‐probe (G‐GFP) technique is developed, which detects dynamic changes in carrier accumulation and transport. The technique is suitable for exploring the intrinsic properties of semiconductors in FETs with arbitrary contacts and in any operational regimes above the threshold. Application to simulated transistors confirms its accuracy in probing the evolution of channel potential, drift field, and gate‐dependent carrier mobility for devices with a contact‐limited operation and disordered semiconductors. Comparative experiments are performed based on FETs with various materials, device structures, and operational temperatures. The G‐GFP technique proves to exclude the various injection properties, to detect in situ how carriers are accumulated, and to clarify carrier mobility of the semiconductors. In particular, the well‐known “double‐slope” features in the current–voltage relations are controllably generated and their origins are identified. The approach could be used to explore electronic properties of newly developed materials such as organic, oxide, or 2D semiconductors.     

Carrier Type Control of WSe2 Field‐Effect Transistors by Thickness Modulation and MoO3 Layer Doping

Control of the carrier type in 2D materials is critical for realizing complementary logic computation. Carrier type control in WSe₂ field‐effect transistors (FETs) is presented via thickness engineering and solid‐state oxide doping, which are compatible with state‐of‐the‐art integrated circuit (IC) processing. It is found that the carrier type of WSe₂ FETs evolves with its thickness, namely, p‐type (<4 nm), ambipolar (≈6 nm), and n‐type (>15 nm). This layer‐dependent carrier type can be understood as a result of drastic change of the band edge of WSe₂ as a function of the thickness and their band offsets to the metal contacts. The strong carrier type tuning by solid‐state oxide doping is also demonstrated, in which ambipolar characteristics of WSe₂ FETs are converted into pure p‐type, and the field‐effect hole mobility is enhanced by two orders of magnitude. The studies not only provide IC‐compatible processing method to control the carrier type in 2D semiconductor, but also enable to build functional devices, such as, a tunable diode formed with an asymmetrical‐thick WSe₂ flake for fast photodetectors.     

Fully Transparent p‐MoTe2 2D Transistors Using Ultrathin MoOx/Pt Contact Media for Indium‐Tin‐Oxide Source/Drain

Since transition metal dichalcogenide (TMD) semiconductors are found as 2D van der Waals materials with a discrete energy bandgap, many 2D‐like thin field effect transistors (FETs) and PN diodes are reported as prototype electrical and optoelectronic devices. As a potential application of display electronics, transparent 2D FET devices are also reported recently. Such transparent 2D FETs are very few in report, yet no p‐type channel 2D‐like FETs are seen. Here, 2D‐like thin transparent p‐channel MoTe₂ FETs with oxygen (O₂) plasma‐induced MoO_x/Pt/indium‐tin‐oxide (ITO) contact are reported for the first time. For source/drain contact, 60 s short O₂ plasma and ultrathin Pt‐deposition processes on MoTe₂ surface are sequentially introduced before ITO thin film deposition and patterning. As a result, almost transparent 2D FETs are obtained with a decent mobility of ≈5 cm² V^?1 s^?1, a high ON/OFF current ratio of ≈10⁵, and 70% transmittance. In particular, for normal MoTe₂ FETs without ITO, O₂ plasma process greatly improves the hole injection efficiency and device mobility (≈60 cm² V^?1 s^?1), introducing ultrathin MoO_x between Pt source/drain and MoTe₂. As a final device application, a photovoltaic current modulator, where the transparent FET stably operates as gated by photovoltaic effects, is integrated.     

Progress,Challenges, and Opportunities for 2D Material Based Photodetectors

2D material based photodetectors have attracted many research projects due to their unique structures and excellent electronic and optoelectronic properties. These 2D materials, including semimetallic graphene, semiconducting black phosphorus, transition metal dichalcogenides, insulating hexagonal boron nitride, and their various heterostructures, show a wide distribution in bandgap values. To date, hundreds of photodetectors based on 2D materials have been reported. Here, a review of photodetectors based on 2D materials covering the detection spectrum from ultraviolet to infrared is presented. First, a brief insight into the detection mechanisms of 2D material photodetectors as well as introducing the figure‐of‐merits which are key factors for a reasonable comparison between different photodetectors is provided. Then, the recent progress on 2D material based photodetectors is reviewed. Particularly, the excellent performances such as broadband spectrum detection, ultrahigh photoresponsivity and sensitivity, fast response speed and high bandwidth, polarization‐sensitive detection are pointed out on the basis of the state‐of‐the‐art 2D photodetectors. Initial applications based on 2D material photodetectors are mentioned. Finally, an outlook is delivered, the challenges and future directions are discussed, and general advice for designing and realizing novel high‐performance photodetectors is given to provide a guideline for the future development of this fast‐developing field.     

Star‐Shaped Oligothiophenes for Solution‐Processible Organic Field‐Effect Transistors

We have designed and successfully synthesized star‐shaped oligothiophenes, which could be used as semiconducting materials for solution‐processible organic field‐effect transistors (FETs). By systematically changing the chemical structure of the star‐shaped oligothiophenes we obtained the structural requirements needed for making working FETs from them. UV‐vis fluorescence measurements showed that a molecule of the star‐shaped compounds under consideration is not a fully conjugated molecule, but it has three independently conjugated oligothienyl‐phenylene blocks. A possible scheme of molecular packing of the star‐shaped oligothiophenes in a lamellar structure was proposed and confirmed by atomic force microscopy (AFM) and X‐ray diffraction (XRD) measurements. Although the star‐shaped semiconductors show a somewhat lower mobility than their linear analogs, they possess better solubility and film‐forming properties, leading to improved spin‐coating processing. The best FETs were made by spin‐coating 1,3,5‐tris(5″‐decyl‐2,2′:5′,2″‐terthien‐5‐yl)benzene from a chloroform solution, which resulted in a mobility of 2 × 10^–4 cm² V ^–1s^–1, a 10² on/off ratio at gate voltages of 0 V and –20 V, and a threshold voltage close to 0 V.     

Implementation of Low‐Power Electronic Devices Using Solution‐Processed Tantalum Pentoxide Dielectric

The development of solution‐processed field effect transistors (FETs) based on organic and hybrid materials over the past two decades has demonstrated the incredible potential in these technologies. However, solution processed FETs generally require impracticably high voltages to switch on and off, which precludes their application in low‐power devices and prevent their integration with standard logic circuitry. Here, a universal and environmentally benign solution‐processing method for the preparation of Ta₂O₅, HfO₂ and ZrO₂ amorphous dielectric thin films is demonstrated. High mobility CdS FETs are fabricated on such high‐κ dielectric substrates entirely via solution‐processing. The highest mobility, 2.97 cm² V^?1 s^?1 is achieved in the device with Ta₂O₅ dielectric with a low threshold voltage of 1.00 V, which is higher than the mobility of the reference CdS FET with SiO₂ dielectric with an order of magnitude decrease in threshold voltage as well. Because these FETs can be operated at less than 5 V, they may potentially be integrated with existing logic and display circuitry without significant signal amplification. This report demonstrates high‐mobility FETs using solution‐processed Ta₂O₅ dielectrics with drastically reduced power consumption; ≈95% reduction compared to that of the device with a conventional SiO₂ gate dielectric.     

Conjugated‐Polymer‐Based Lateral Heterostructures Defined by High‐Resolution Photolithography

Solution processing of polymer semiconductors provides a new paradigm for large‐area electronics manufacturing on flexible substrates, but it also severely restricts the realization of interesting advanced device architectures, such as lateral heterostructures with defined interfaces, which are easily accessible with inorganic materials using photolithography. This is because polymer semiconductors degrade, swell, or dissolve during conventional photoresist processing. Here a versatile, high‐resolution photolithographic method is demonstrated for patterning of polymer semiconductors and exemplify this with high‐performance p‐type and n‐type field‐effect transistors (FETs) in both bottom‐ and top‐gate architectures, as well as ambipolar light‐emitting field‐effect transistors (LEFETs), in which the recombination zone can be pinned at a photolithographically defined lateral heterojunction between two semiconducting polymers. The technique therefore enables the realization of a broad range of novel device architectures while retaining optimum materials performance.     

Controlled Doping of Wafer‐Scale PtSe2 Films for Device Application

Semiconductive transition metal dichalcogenides (TMDs) have been considered as next generation semiconductors, but to date most device investigations are still based on microscale exfoliation with a low yield. Wafer scale growth of TMDs has been reported but effective doping approaches remain challenging due to their atomically thick nature. This work reports the synthesis of wafer‐scale continuous few‐layer PtSe₂ films with effective doping in a controllable manner. Chemical component analyses confirm that both n‐doping and p‐doping can be effectively modulated through a controlled selenization process. The electrical properties of PtSe₂ films have been systematically studied by fabricating top‐gated field effect transistors (FETs). The device current on/off ratio is optimized in two‐layer PtSe₂ FETs, and four‐terminal configuration displays a reasonably high effective field effect mobility (14 and 15 cm² V^?1 s^?1 for p‐type and n‐type FETs, respectively) with a nearly symmetric p‐type and n‐type performance. Temperature dependent measurement reveals that the variable range hopping is dominant at low temperatures. To further establish feasible application based on controllable doping of PtSe₂, a logic inverter and vertically stacked p–n junction arrays are demonstrated. These results validate that PtSe₂ is a promising candidate among the family of TMDs for future functional electronic applications.     

Selective Electrochemical Etching of Single‐Walled Carbon Nanotubes

Single‐walled carbon nanotubes (SWNTs) are a promising material for future nanotechnology. However, their applications are still limited in success because of the co‐existence of metallic SWNTs and semiconducting SWNTs produced samples. Here, electrochemical etching, which shows both diameter and electrical selectivity, is demonstrated to remove SWNTs. With the aid of a back‐gate electric field, selective removal of metallic SWNTs is realized, resulting in high‐performance SWNT field‐effect transistors with pure semiconducting SWNT channels. Moreover, electrochemical etching is realized on a selective area. These findings would be valuable for research and the application of SWNTs in electrochemistry and in electronic devices.     

Understanding Solvent-Induced Delamination and Intense Water Adsorption in Janus Transition Metal Dichalcogenides for Enhanced Device Performance

Recently, there has been considerable interest in 2D Janus transition metal dichalcogenides owing to their unique structure that exhibits broken mirror symmetry along the out-of-plane direction, which offers fascinating properties that are applicable in various fields. This study investigates the issue of process instability in Janus MoSSe, which is mainly caused by its nonzero net dipole moments. It systematically investigates whether the built-in dipole moments in Janus MoSSe make it susceptible to delamination by most polar solvents and increase its vulnerability to intense moisture adsorption, which leads to the deterioration of its semiconducting properties. To address these issues, as an example of device applications, field-effect transistors (FETs) based on a van der Waals heterostructure are devised, where the bottom h-BN (top h-BN) insulating material is employed to prevent delamination (adsorption of moisture). The fabricated FETs exhibit improved electron mobility and excellent stability under ambient conditions.     

High-Purity Monochiral Carbon Nanotubes with a 1.2 nm Diameter for High-Performance Field-Effect Transistors

Monochiral single-walled carbon nanotubes (SWCNTs) are promising materials with potential applications in 3D integrated circuits and optoelectronic hybrid circuits. However, the purity and device performance of monochiral SWCNTs are still far lower than expected. Here, the authors demonstrate that specific monochiral SWCNTs can be wrapped by conjugated polymers containing pyridine units, and the supramolecular assemblies show surprising suspension stability even after high-intensity ultracentrifugation. Additionally, two novel methods are developed, namely, enhanced ultracentrifugation (E-UCG) and stepwise extraction processing (STEP), which successfully achieve isolation of (10,8) and (12,5) SWCNTs with respective diameters of 1.24 and 1.2 nm at high monochiral purity (92.3% and 95.6%). Their S₁₁ absorption and fluorescence emission peaks are both at ≈1.5 µm (optical telecommunications C-band). Both micro- and nanoscale field-effect transistor (FET) devices can be fabricated from the as-isolated (10,8) SWCNTs, and these FETs exhibit excellent electrical performance and a high semiconducting purity of up to 99.94%.     

Degradable Conjugated Polymers: Synthesis and Applications in Enrichment of Semiconducting Single‐Walled Carbon Nanotubes

Polymers which enrich semiconducting single‐walled carbon nanotubes (SWNTs) and are also removable after enrichment are highly desirable for achieving high‐performance field‐effect transistors (FETs). We have designed and synthesized a new class of alternating copolymers containing main‐chain fluorene and hydrofluoric acid (HF) degradable disilane for sorting and preferentially suspending semiconducting nanotube species. The results of optical absorbance, photoluminescence emission, and resonant Raman scattering show that poly[(9,9‐dioctylfluorenyl‐2,7‐diyl)‐alt‐co‐1,1,2,2‐tetramethyl‐disilane] preferentially suspends semiconducting nanotubes with larger chiral angle (25°–28°) and larger diameter (1.03 nm–1.17 nm) (specifically (8,7), (9,7) and (9,8) species) present in HiPCO nanotube samples. Computer simulation shows that P1 preferentially interacts with (8,7) (semiconducting) over (7,7) (metallic) species, confirming that P1 selects larger diameter, larger chiral angle semiconducting tubes. P1 wrapped on the surface of SWNTs is easily washed off through degradation of the disilane bond of the alternating polymer main chain in HF, yielding “clean” purified SWNTs. We have applied the semiconducting species enriched SWNTs to prepare solution‐processed FET devices with random nanotube network active channels. The devices exhibit stable p‐type semiconductor behavior in air with very promising characteristics. The on/off current ratio reaches up to 15 000, with on‐current level of around 10 μA and estimated hole mobility of 5.2 cm² V^?1 s^?1.