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.     

Impact of Thickness on Contact Issues for Pinning Effect in Black Phosphorus Field‐Effect Transistors

Metal/semiconductor contact is a significant constraint in short‐channel field effect transistors (FETs) comprising black phosphorus (BP) and other 2D semiconductors. Due to the pinning effect at metal/2D semiconductor interface, the Schottky barrier usually does not follow the Schottky–Mott rule, resulting in thickness‐dependent FET performance. In this work, the Schottky barrier in BP FETs is investigated via theory calculation and electrical measurement. A simple metal/BP contact model is presented based upon thickness‐dependent electrical characteristics of BP FETs. The model considers the Schottky barrier as a combined effect of the Schottky–Mott rule and the pinning effect and provides a feasibility to track the conducting behavior of other 2D semiconductor FETs.     

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.     

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.     

Selective Dispersion of Large‐Diameter Semiconducting Carbon Nanotubes by Functionalized Conjugated Dendritic Oligothiophenes for Use in Printed Thin Film Transistors

Selective dispersion of semiconducting single walled carbon nanotubes (s‐SWCNTs) by conjugated polymer wrapping is recognized as the most promising scalable method for s‐SWCNT separation. Despite a number of linear conjugated polymers being reported for use in s‐SWCNT separation, these linear polymers suffer batch‐to‐batch variation for their undefined molecular structure. Here, it is reported that conjugated dendritic oligothiophenes with multiple diketopyrrolopyrrole groups at the periphery have the capability of selectively dispersing large diameter s‐SWCNTs with high dispersion efficiency and certain chiral selectivity. Printed top‐gated thin film transistors using the dendrimer sorted s‐SWCNTs show high charge carrier mobility of up to 57 cm² V^?1 s^?1 and on/off ratios of ≈10⁶ with high reproducibility, which is ascribed to the defined and monodispersed molecular structure of dendrimers. Moreover, owing to the multiple peripheral anchoring groups of these dendritic molecules, these dendrimer‐s‐SWCNT dispersions display excellent stability. The current work proves that dendritic molecules are excellent dispersion reagents for s‐SWCNT separation.     

STM碳纳米管针尖的研究

合成和提纯了单壁碳纳米管（SWCNTs)。使用水溶胶体,SWCNTs被组装在钨（W）针尖上,它可以用作扫描隧道显微镜（STM）的针尖,得到高定向石墨（HOPG）和金（Au)膜表面的原子分辨像,用场发射显微镜（FEM）研究了SWCNTs的场发射特性,得到了原子分辨的场发射图,与计算的（9,9）扶手椅型的SWCNT结构相一致,研究了SWCNTs的电学特性,观察到了负阻特性。     

High‐Strength Laminated Copper Matrix Nanocomposites Developed from a Single‐Walled Carbon Nanotube Film with Continuous Reticulate Architecture

A critical challenge in nanocomposite fabrication by adding SWCNTs as reinforcement is to realize an effective transfer of the excellent mechanical properties of the SWCNTs to the macroscale mechanical properties of the matrix. Using directly grown SWCNT films with continuous reticulate structure as the template, Cu/SWCNTs/Cu laminated nanocomposites are fabricated by an electrodepositing process. The resulting Cu/SWCNTs/Cu laminated nanocomposites exhibit extremely high strength and Young's modulus. The estimated Young's modulus of the SWCNT bundles in the composite are between 860 and 960 GPa. Such a high strength and an effective load‐transfer capacity are ascribed to the unique continuous reticulate architecture of SWCNT films and the strong interfacial strength between the SWCNTs and Cu matrix. Raman spectroscopy is used to characterize the loading status of the SWCNTs in the strained composite. It provides a route to investigate the load transfer of SWCNTs in the metal matrix composites.     

Phototransistors with Negative or Ambipolar Photoresponse Based on As‐Grown Heterostructures of Single‐Walled Carbon Nanotube and MoS2

A facile synthesis method for the heterostructures of single‐walled carbon nanotubes (SWCNTs) and few‐layer MoS₂ is reported. The heterostructures are realized by in situ chemical vapor deposition of MoS₂ on individual SWCNTs. Field effect transistors based on the heterostructures display different transfer characteristics depending on the formation of MoS₂ conduction channels along SWCNTs. Under light illumination, negative photoresponse originating from charge transfer from MoS₂ to SWCNT is observed while positive photoresponse is observed in MoS₂ conduction channels, leading to ambipolar photoresponse in devices with both SWCNT and MoS₂ channels. The heterostructure phototransistor, for negative photoresponse, exhibits high responsivity (100–1000 AW^?1) at low bias voltages (0.1 V) in the visible spectrum (500–700 nm) by combining high mobility conduction channel (SWCNT) with efficient light absorber (MoS₂).     

Facile Fabrication of SWCNT/SnO2 Nanowire Heterojunction Devices on Flexible Polyimide Substrate

We report on the fabrication and electronic properties of single‐walled carbon nanotube (SWCNT)/tin oxide nanowire (SnO₂ NW) heterojunction device arrays on flexible polyimide (PI) substrates. Hetero‐NW junctions consisting of crossed SnO₂ NWs and SWCNTs were fabricated by sliding transfer of SnO₂ NWs onto the SWCNT channels on PI substrate. Individual SWCNTs and SnO₂ NWs field effect transistors showed p‐ and n‐type transfer properties with current on/off ratios of 7.0 × 10⁵ and 2.7 × 10⁶, respectively. The heterojunction diode showed a rectifying behavior with a rectification ratio of higher than 10³ at ±1 V and the analysis with an equivalent circuit model of serially connected diode and resistor estimated an ideality factor of 1.5 and the resistance of 20 MΩ. The rectification of AC input signal was clearly demonstrated by fabricating a full‐wave bridge circuit of heterojunctions. In addition, the heterojunctions showed a high UV photosensitivity of ～10⁴ under reverse bias, suggesting their implicit applications in UV sensors.     

Radiation-hardened property of single-walled carbon nanotube film-based field-effect transistors under low-energy proton irradiation

Strong C-C bonds,nanoscale cross-section and low atomic number make single-walled carbon nanotubes(SWCNTs)a potential candidate material for integrated circuits(ICs)applied in outer space.However,very little work combines the simula-tion calculations with the electrical measurements of SWCNT field-effect transistors(FETs),which limits further understanding on the mechanisms of radiation effects.Here,SWCNT film-based FETs were fabricated to explore the total ionizing dose(TID)and displacement damage effect on the electrical performance under low-energy proton irradiation with different fluences up to 1 x 1015 p/cm2.Large negative shift of the threshold voltage and obvious decrease of the on-state current verified the TID ef-fect caused in the oxide layer.The stability of the subthreshold swing and the off-state current reveals that the displacement damage caused in the CNT layer is not serious,which proves that the CNT film is radiation-hardened.Specially,according to the simulation,we found the displacement damage caused by protons is different in the source/drain contact area and chan-nel area,leading to varying degrees of change for the contact resistance and sheet resistance.Having analyzed the simulation results and electrical measurements,we explained the low-energy proton irradiation mechanism of the CNT FETs,which is essen-tial for the construction of radiation-hardened CNT film-based ICs for aircrafts.     

High‐Speed,Low‐Voltage,and Environmentally Stable Operation of Electrochemically Gated Zinc Oxide Nanowire Field‐Effect Transistors

Single‐crystal, 1D nanostructures are well known for their high mobility electronic transport properties. Oxide‐nanowire field‐effect transistors (FETs) offer both high optical transparency and large mechanical conformability which are essential for flexible and transparent display applications. Whereas the “on‐currents” achieved with nanowire channel transistors are already sufficient to drive active matrix organic light emitting diode (AMOLED) displays; it is shown here that incorporation of electrochemical‐gating (EG) to nanowire electronics reduces the operation voltage to ≤2 V. This opens up new possibilities of realizing flexible, portable, transparent displays that are powered by thin film batteries. A composite solid polymer electrolyte (CSPE) is used to obtain all‐solid‐state FETs with outstanding performance; the field‐effect mobility, on/off current ratio, transconductance, and subthreshold slope of a typical ZnO single‐nanowire transistor are 62 cm²/Vs, 10⁷, 155 μS/μm and 115 mV/dec, respectively. Practical use of such electrochemically‐gated field‐effect transistor (EG FET) devices is supported by their long‐term stability in air. Moreover, due to the good conductivity (≈10^?2 S/cm) of the CSPE, sufficiently high switching speed of such EG FETs is attainable; a cut‐off frequency in excess of 100 kHz is measured for in‐plane FETs with large gate‐channel distance of >10 μm. Consequently, operation speeds above MHz can be envisaged for top‐gate transistor geometries with insulator thicknesses of a few hundreds of nanometers. The solid polymer electrolyte developed in this study has great potential in future device fabrication using all‐solution processed and high throughput techniques.     

Room‐Temperature Self‐Organizing Characteristics of Soluble Acene Field‐Effect Transistors

We report on the room‐temperature self‐organizing characteristics of thin films of the organic small‐molecule semiconductor triethylsilylethynyl‐anthradithiophene (TES‐ADT) and its effect on the electrical properties of TES‐ADT‐based field‐effect transistors (FETs). The morphology of TES‐ADT films changed dramatically with time, and the field‐effect mobility of FETs based on these films increased about 100‐fold after seven days as a result of the change in molecular orientation from a tilted structure in the as‐prepared film to a well‐oriented structure in the final film. We found that the molecular movement is large enough to induce a conformational change to an energetically stable state in spin‐coated TES‐ADT films, because TES‐ADT has a low glass‐transition temperature (around room temperature). Our findings demonstrate that organic small‐molecule semiconductors that exhibit a low crystallinity immediately after spin‐coating can be changed into highly crystalline structures by spontaneous self‐organization of the molecules at room temperature, which results in improved electrical properties of FETs based on these semiconductors.     

Surface‐Dominated Transport Properties of Silicon Nanowires

Surface effects are widely recognized to significantly influence the properties of nanostructures, although the detailed mechanisms are rarely studied and unclear. Herein we report for the first time a quantitative evaluation of the surface‐related contributions to transport properties in nanostructures by using Si nanowires (NWs) as a paradigm. Critical to this study is the capability of synthesizing SiNWs with predetermined conduction type and carrier concentration from Si wafer of known properties using the recently developed metal‐catalyzed chemical etching method. Strikingly, the conductance of p‐type SiNWs is substantively larger in air than that of the original wafer, is sensitive to humidity and volatile gases, and thinner wires show higher conductivity. Further, SiNW‐based field‐effect transistors (FETs) show NWs to have a hole concentration two orders of magnitude higher than the original wafer. In vacuum, the conductivity of SiNWs dramatically decreases, whereas hole mobility increases. The device performances are further improved by embedding SiNW FETs in 250 nm SiO₂, which insulates the devices from atmosphere and passivates the surface defects of NWs. Owing to the strong surface effects, n‐type SiNWs even change to exhibit p‐type characteristics. The totality of the results provides definitive confirmation that the electrical characteristics of SiNWs are dominated by surface states. A model based on surface band bending and carrier scattering caused by surface states is proposed to interpret experimental results. The phenomenon of surface‐dependent transport properties should be generic to all nanoscale structures, and is significant for nanodevice design for sensor and electronic applications.     

Non‐Lithographic Fabrication of All‐2D α‐MoTe2 Dual Gate Transistors

As one of the emerging new transition‐metal dichalcogenides materials, molybdenum ditelluride (α‐MoTe₂) is attracting much attention due to its optical and electrical properties. This study fabricates all‐2D MoTe₂‐based field effect transistors (FETs) on glass, using thin hexagonal boron nitride and thin graphene in consideration of good dielectric/channel interface and source/drain contacts, respectively. Distinguished from previous works, in this study, all 2D FETs with α‐MoTe₂ nanoflakes are dual‐gated for driving higher current. Moreover, for the present 2D dual gate FET fabrications on glass, all thermal annealing and lithography processes are intentionally exempted for fully non‐lithographic method using only van der Waal's forces. The dual‐gate MoTe₂ FET displays quite a high hole and electron mobility over ≈20 cm² V^?1 s^?1 along with ON/OFF ratio of ≈10⁵ in maximum as an ambipolar FET and also demonstrates high drain current of a few tens‐to‐hundred μA at a low operation voltage. It appears promising enough to drive organic light emitting diode pixels and NOR logic functions on glass.     

Carbon nanotube electronics

We evaluate the potential of carbon nanotubes (CNTs) as the basis for a new nanoelectronic technology. After briefly reviewing the electronic structure and transport properties of CNTs, we discuss the fabrication of CNT field-effect transistors (CNTFETs) formed from individual single-walled nanotubes (SWCNTs), SWCNT bundles, or multiwalled (MW) CNTs. The performance characteristics of the CNTFETs are discussed and compared to those of corresponding silicon devices. We show that CNTFETs are very competitive with state-of-the-art conventional devices. We also discuss the switching mechanism of CNTFETs and show that it involves the modulation by the gate field of Schottky barriers at the metal-CNT junctions. This switching mechanism can account for the observed subthreshold and vertical scaling behavior of CNTFETs, as well as their sensitivity to atmospheric oxygen. The potential for integration of CNT devices is demonstrated by fabricating a logic gate along a single nanotube molecule. Finally, we discuss our efforts to grow CNTs locally and selectively, and a method is presented for growing oriented SWCNTs without the involvement of a metal catalyst.     

Catalytic and Electron Conducting Carbon Nanotube–Reinforced Lysozyme Crystals

Novel reinforced cross‐linked lysozyme crystals containing homogeneous dispersions of single‐walled carbon nanotubes bundles (SWCNTs) are produced and characterized. The incorporation of SWCNTs inside lysozyme crystals gives rise to reinforced composite materials with tunable mechanical strength and electronic conductivity, while preserving the crystal quality and morphology. These reinforced crystals show increased catalytic activity at higher temperatures, being active even above the denaturation temperature. The electron transport through the crystals is linked to the content and distribution of SWCNT bundles inside the crystals. The electron conduction through the crystals is isotropic and very efficient, presenting high conductivity values up to 600 nS at very low (0.05 wt%) SWCNT concentration. To obtain these crystals, a new protocol based on the in situ crystallization of lysozyme in composite SWCNT–peptide hydrogels is developed. These peptide hydrogels are able to homogeneously disperse bundles of hydrophobic SWCNTs allowing first, the crystallization of the enzyme lysozyme and second, transferring the new properties of the inorganic component to the crystals. Taken together, these composite crystals represent an example of the versatility of proteins as biological substrates in the generation of novel functional materials, opening the door to use them in catalysis and bioelectronics at macroscale.     

The Effect of Gate‐Dielectric Surface Energy on Pentacene Morphology and Organic Field‐Effect Transistor Characteristics

The effects of the surface energy of polymer gate dielectrics on pentacene morphology and the electrical properties of pentacene field‐effect transistors (FETs) are reported, using surface‐energy‐controllable poly(imide‐siloxane)s as gate‐dielectric layers. The surface energy of gate dielectrics strongly influences the pentacene film morphology and growth mode, producing Stranski–Krastanov growth with large and dendritic grains at high surface energy and three‐dimensional island growth with small grains at low surface energy. In spite of the small grain size (≈ 300 nm) and decreased ordering of pentacene molecules vertical to the gate dielectric with low surface energy, the mobility of FETs with a low‐surface‐energy gate dielectric is larger by a factor of about five, compared to their high‐surface‐energy counterparts. In pentacene growth on the low‐surface‐energy gate dielectric, interconnection between grains is observed and gradual lateral growth of grains causes the vacant space between grains to be filled. Hence, the higher mobility of the FETs with low‐surface‐energy gate dielectrics can be achieved by interconnection and tight packing between pentacene grains. On the other hand, the high‐surface‐energy dielectric forms the first pentacene layer with some voids and then successive, incomplete layers over the first, which can limit the transport of charge carriers and cause lower carrier mobility, in spite of the formation of large grains (≈ 1.3 μm) in a thicker pentacene film.     

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.     

Nitrogen‐Doped Single‐Walled Carbon Nanotubes Grown on Substrates: Evidence for Framework Doping and Their Enhanced Properties

Nitrogen‐doped single‐walled carbon nanotubes (SWCNTs) are synthesized directly on silicon and quartz substrates through a normal chemical vapor deposition (CVD) method. Thermogravimetry mass spectrometry measurements and Raman spectroscopy give firm evidence for framework nitrogen doping. X‐ray‐photoelectron‐spectroscopy analysis further obtains the bonding style of the nitrogen atoms in the carbon framework. The nitrogen doping significantly changes the properties of the SWCNTs. All of the tubes behave like metallic tubes in field‐effect transistors. The doped nitrogen atoms introduce a stronger affinity for the SWCNTs to metal nanoparticles. Compared with pristine SWCNTs, the nitrogen‐doped tubes show enhanced sensitivity and selectivity for electrochemical detection of some electrophilic species including O₂, H₂O₂, and Fe³⁺. They also present improved electrocatalytic activity for oxygen reduction. These unique properties of the nitrogen‐doped SWCNTs endow them with important potential applications in various fields.     

Impact of Energy Relaxation of Channel Electrons on Drain‐Induced Barrier Lowering in Nano‐Scale Si‐Based MOSFETs

Drain‐induced barrier lowering (DIBL) is one of the main parameters employed to indicate the short‐channel effect for nano metal‐oxide semiconductor field‐effect transistors (MOSFETs). We propose a new physical model of the DIBL effect under two‐dimensional approximations based on the energy‐conservation equation for channel electrons in FETs, which is different from the former field‐penetration model. The DIBL is caused by lowering of the effective potential barrier height seen by the channel electrons because a lateral channel electric field results in an increase in the average kinetic energy of the channel electrons. The channel length, temperature, and doping concentration‐dependent DIBL effects predicted by the proposed physical model agree well with the experimental data and simulation results reported in Nature and other journals.