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.     

Nonvolatile Floating‐Gate Memories Based on Stacked Black Phosphorus–Boron Nitride–MoS2 Heterostructures

Research on van der Waals heterostructures based on stacked 2D atomic crystals is intense due to their prominent properties and potential applications for flexible transparent electronics and optoelectronics. Here, nonvolatile memory devices based on floating‐gate field‐effect transistors that are stacked with 2D materials are reported, where few‐layer black phosphorus acts as channel layer, hexagonal boron nitride as tunnel barrier layer, and MoS₂ as charge trapping layer. Because of the ambipolar behavior of black phosphorus, electrons and holes can be stored in the MoS₂ charge trapping layer. The heterostructures exhibit remarkable erase/program ratio and endurance performance, and can be developed for high‐performance type‐switching memories and reconfigurable inverter logic circuits, indicating that it is promising for application in memory devices completely based on 2D atomic crystals.     

Carrier Modulation of Ambipolar Few‐Layer MoTe2 Transistors by MgO Surface Charge Transfer Doping

Semiconducting molybdenum ditelluride (2H‐MoTe₂), a fast‐emerging 2D material with an appropriate band gap and decent carrier mobility, is configured as field‐effect transistors and is the focus of substantial research interest, showing hole‐dominated ambipolar characteristics. Here, carrier modulation of ambipolar few‐layer MoTe₂ transistors is demonstrated utilizing magnesium oxide (MgO) surface charge transfer doping. By carefully adjusting the thickness of MgO film and the number of MoTe₂ layers, the carrier polarity of MoTe₂ transistors from p‐type to n‐type can be reversely controlled. The electron mobility of MoTe₂ is significantly enhanced from 0.1 to 20 cm² V^?1 s^?1 after 37 nm MgO film doping, indicating a greatly improved electron transport. The effective carrier modulation enables to achieve high‐performance complementary inverters with high DC gain of >25 and photodetectors based on few‐layer MoTe₂ flakes. The results present an important advance toward the realization of electronic and optoelectronic devices based on 2D transition‐metal dichalcogenide 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.     

High‐Sensitivity Floating‐Gate Phototransistors Based on WS2 and MoS2

In recent years, 2D layered materials have been considered as promising photon absorption channel media for next‐generation phototransistors due to their atomic thickness, easily tailored single‐crystal van der Waals heterostructures, ultrafast optoelectronic characteristics, and broadband photon absorption. However, the photosensitivity obtained from such devices, even under a large bias voltage, is still unsatisfactory until now. In this paper, high‐sensitivity phototransistors based on WS₂ and MoS₂ are proposed, designed, and fabricated with gold nanoparticles (AuNPs) embedded in the gate dielectric. These AuNPs, located between the tunneling and blocking dielectric, are found to enable efficient electron trapping in order to strongly suppress dark current. Ultralow dark current (10^?11 A), high photoresponsivity (1090 A W^?1), and high detectivity (3.5 × 10¹¹ Jones) are obtained for the WS₂ devices under a low source/drain and a zero gate voltage at a wavelength of 520 nm. These results demonstrate that the floating‐gate memory structure is an effective configuration to achieve high‐performance 2D electronic/optoelectronic devices.     

Variable Temperature Mobility Analysis of n‐Channel,p‐Channel,and Ambipolar Organic Field‐Effect Transistors

The temperature dependence of field‐effect transistor (FET) mobility is analyzed for a series of n‐channel, p‐channel, and ambipolar organic semiconductor‐based FETs selected for varied semiconductor structural and device characteristics. The materials (and dominant carrier type) studied are 5,5′′′‐bis(perfluorophenacyl)‐2,2′:5′,2″:5″,2′′′‐quaterthiophene ( 1 , n‐channel), 5,5′′′‐bis(perfluorohexyl carbonyl)‐2,2′:5′,2″:5″,2′′′‐quaterthiophene ( 2 , n‐channel), pentacene ( 3 , p‐channel); 5,5′′′‐bis(hexylcarbonyl)‐2,2′:5′,2″:5″,2′′′‐quaterthiophene ( 4 , ambipolar), 5,5′′′‐bis‐(phenacyl)‐2,2′: 5′,2″:5″,2′′′‐quaterthiophene ( 5 , p‐channel), 2,7‐bis((5‐perfluorophenacyl)thiophen‐2‐yl)‐9,10‐phenanthrenequinone ( 6 , n‐channel), and poly(N‐(2‐octyldodecyl)‐2,2′‐bithiophene‐3,3′‐dicarboximide) ( 7 , n‐channel). Fits of the effective field‐effect mobility (µ_eff) data assuming a discrete trap energy within a multiple trapping and release (MTR) model reveal low activation energies (E_As) for high‐mobility semiconductors 1 – 3 of 21, 22, and 30 meV, respectively. Higher E_A values of 40–70 meV are exhibited by 4 – 7 ‐derived FETs having lower mobilities (µ_eff). Analysis of these data reveals little correlation between the conduction state energy level and E_A, while there is an inverse relationship between E_A and µ_eff. The first variable‐temperature study of an ambipolar organic FET reveals that although n‐channel behavior exhibits E_A = 27 meV, the p‐channel regime exhibits significantly more trapping with E_A = 250 meV. Interestingly, calculated free carrier mobilities (µ₀) are in the range of ～0.2–0.8 cm² V^?1 s^?1 in this materials set, largely independent of µ_eff. This indicates that in the absence of charge traps, the inherent magnitude of carrier mobility is comparable for each of these materials. Finally, the effect of temperature on threshold voltage (V_T) reveals two distinct trapping regimes, with the change in trapped charge exhibiting a striking correlation with room temperature µ_eff. The observation that E_A is independent of conduction state energy, and that changes in trapped charge with temperature correlate with room temperature µ_eff, support the applicability of trap‐limited mobility models such as a MTR mechanism to this materials set.     

Solution‐Deposited Zinc Oxide and Zinc Oxide/Pentacene Bilayer Transistors: High Mobility n‐Channel,Ambipolar, and Nonvolatile Devices

A solution processed n‐channel zinc oxide (ZnO) field effect transistor (FET) was fabricated by simple dip coating and subsequent heat treatment of a zinc acetate film. The field effect mobility of electrons depends on ZnO grain size, controlled by changing the number of coatings and zinc acetate solution concentration. The highest electron mobility achieved by this method is 7.2 cm² V^?1 s^?1 with On/Off ratio of 70. This electron mobility is higher than for the most recently reported solution processed ZnO transistor. We also fabricated bilayer transistors where the first layer is ZnO, and the second layer is pentacene, a p‐channel organic which is deposited by thermal evaporation. By changing the ZnO grain size (or thickness) this type of bilayer transistor shows p‐channel, ambipolar and n‐channel behavior. For the ambipolar transistor, well balanced electron and hole mobilities are 7.6 × 10^?3 and 6.3 × 10^?3 cm² V^?1 s^?1 respectively. When the ZnO layer is very thin, the transistor shows p‐channel behavior with very high reversible hysteresis. The nonvolatile tuning function of this transistor was investigated.     

Carbon Nanotube Based Multifunctional Ambipolar Transistors for AC Applications

Field‐effect transistors (FETs) fabricated on large diameter carbon nanotubes (CNTs) present typical ambipolar transfer characteristics owing to the small band‐gap of CNTs. Depending on the DC biasing condition, the ambipolar FET can work in three different regions, and then can be used as the core to realize multifunctional AC circuits. The CNT FET based circuits can work as a high‐efficiency ambipolar frequency doubler in the ambipolar transfer region, and also can function as in‐phase amplifier and inverted amplifier in the linear transfer region. Due to current saturation of the CNT FET, an AC amplifier with a voltage gain of 2 is realized when the device works in the linear transfer region. Achieving an actual amplification and frequency doubling functions indicates that complicated radio frequency circuits or systems can be constructed based on just one kind of device: ambipolar CNT FETs.     

High‐Performance,Air‐Stable,Top‐Gate,p‐Channel WSe2 Field‐Effect Transistor with Fluoropolymer Buffer Layer

High‐performance, air‐stable, p‐channel WSe₂ top‐gate field‐effect transistors (FETs) using a bilayer gate dielectric composed of high‐ and low‐k dielectrics are reported. Using only a high‐k Al₂O₃ as the top‐gate dielectric generally degrades the electrical properties of p‐channel WSe₂, therefore, a thin fluoropolymer (Cytop) as a buffer layer to protect the 2D channel from high‐k oxide forming is deposited. As a result, a top‐gate‐patterned 2D WSe₂ FET is realized. The top‐gate p‐channel WSe₂ FET demonstrates a high hole mobility of 100 cm²_­ V^?1 s^?1 and a I_ON/I_OFF ratio > 10⁷ at low gate voltages (V_GS ca. ?4 V) and a drain voltage (V_DS) of ?1 V on a glass substrate. Furthermore, the top‐gate FET shows a very good stability in ambient air with a relative humidity of 45% for 7 days after device fabrication. Our approach of creating a high‐k oxide/low‐k organic bilayer dielectric is advantageous over single‐layer high‐k dielectrics for top‐gate p‐channel WSe₂ FETs, which will lead the way toward future electronic nanodevices and their integration.     

All‐Layered 2D Optoelectronics: A High‐Performance UV–vis–NIR Broadband SnSe Photodetector with Bi2Te3 Topological Insulator Electrodes

Nanoelectronics is in urgent demand of exceptional device architecture with ultrathin thickness below 10 nm and dangling‐bond‐free surface to break through current physical bottleneck and achieve new record of integration level. The advance in 2D van der Waals materials endows scientists with new accessibility. This study reports an all‐layered 2D Bi₂Te₃‐SnSe‐Bi₂Te₃ photodetector, and the broadband photoresponse of the device from ultraviolet (370 nm) to near‐infrared (808 nm) is demonstrated. In addition, the optimized responsivity reaches 5.5 A W^?1, with the corresponding eternal quantum efficiency of 1833% and detectivity of 6 × 10¹⁰ cm Hz^1/2 W^?1. These figures‐of‐merits are among the best values of the reported all‐layered 2D photodetectors, which are several orders of magnitude higher than those of the previous SnSe photodetectors. The superior device performance is attributed to the synergy of highly conductive surface state of Bi₂Te₃ topological insulator, perfect band alignment between Bi₂Te₃ and SnSe as well as small interface potential fluctuation. Meanwhile, the all‐layered 2D device is further constructed onto flexible mica substrate and its photoresponse is maintained roughly unchanged upon 60 bending cycles. The findings represent a fundamental scenario for advancement of the next generation high performance and high integration level flexible optoelectronics.     

Salt‐Assisted Growth of P‐type Cu9S5 Nanoflakes for P‐N Heterojunction Photodetectors with High Responsivity

P‐n junctions based on two dimensional (2D) van der Waals (vdW) heterostructure are one of the most promising alternatives in next‐generation electronics and optoelectronics. By choosing different 2D transition metal dichalcogenides (TMDCs), the p‐n junctions have tailored energy band alignments and exhibit superior performance as photodetectors. The p‐n diodes working at reverse bias commonly have high detectivity due to suppressed dark current but suffer from low responsivity resulting from small quantum efficiency. Greater build‐in electric field in the depletion layer can improve the quantum efficiency by reducing recombination of charge carriers. Herein, Cu₉S₅, a novel p‐type semiconductor with direct bandgap and high optical absorption coefficient, is synthesized by salt‐assisted chemical vapor deposition (CVD) method. The high density of holes in Cu₉S₅ endows the constructed p‐n junction, Cu₉S₅/MoS₂, with strong build‐in electric field according to Anderson heterojunction model. Consequently, the Cu₉S₅/MoS₂ p‐n heterojunction has low dark current at reverse bias and high photoresponse under illumination due to the efficient charge separation. The Cu₉S₅/MoS₂ photodetector exhibits good photodetectivity of 1.6 × 10¹² Jones and photoresponsivity of 76 A W^?1 under illumination. This study demonstrates Cu₉S₅ as a promising p‐type semiconductor for high‐performance p‐n heterojunction diodes.     

Large‐Scale Growth of Two‐Dimensional SnS2 Crystals Driven by Screw Dislocations and Application to Photodetectors

2D SnS₂ crystals are attracting increasing attention owning to the huge potential for electronic and optoelectronic applications. However, batch production of 2D SnS₂ crystals via a simple vapor process remains challenging by far. Moreover, the growth mechanism for vapor growth of 2D SnS₂ is not well documented as well. Herein, a simple approach is presented for preparation of large‐scale 2D SnS₂ crystals on mica sheets and it is demonstrated that these 2D crystals follow a screw‐dislocation‐driven (SDD) spiral growth process. The synthesized 2D crystals show hexagonal and truncated triangular shapes with the lateral size ranging from a few micrometers to dozens of micrometers. Observations of key features for screw dislocations, such as helical fringes, dislocation hillocks, and herringbone contours, solidly confirm the SDD spiral growth behavior of the SnS₂. Possible mechanism is proposed in this work to show the generation and propagation of screw dislocations. Furthermore, in order to explore the optoelectronic property of the SnS₂, photodetectors based on 2D SnS₂ crystals are fabricated. The resulting device shows excellent operating characteristics, including good photo‐stability and reproducibility as well as a fast photoresponse time (≈42 ms), which enable the SnS₂ a promising candidate for photodetectors.     

Poly(diketopyrrolopyrrole‐benzothiadiazole) with Ambipolarity Approaching 100% Equivalency

As a characteristic feature of conventional conjugated polymers, it has been generally agreed that conjugated polymers exhibit either high hole transport property (p‐type) or high electron transport property (n‐type). Although ambipolar properties have been demonstrated from specially designed conjugated polymers, only a few examples have exhibited ambipolar transport properties under limited conditions. Furthermore, there is, as yet, no example with ‘equivalent’ hole and electron transport properties. We describe the realization of an equivalent ambipolar organic field‐effect transistor (FET) by using a single‐component visible–near infrared absorbing diketopyrrolopyrrole (DPP)‐benzothiadiazole (BTZ) copolymer, namely poly[3,6‐dithiene‐2‐yl‐2,5‐di(2‐decyltetradecyl)‐pyrrolo[3,4‐c]pyrrole‐1,4‐dione‐5’,5’’‐diyl‐alt‐benzo‐2,1, 3‐thiadiazol‐4,7‐diyl] ( PDTDPP‐alt‐BTZ ). PDTDPP‐alt‐BTZ shows not only ideally balanced charge carrier mobilities for both electrons (?_e = 0.09 cm²V^?1s^?1) and holes (?_h = 0.1 cm²V^?1s^?1) but also its inverter constructed with the combination of two identical ambipolar FETs exhibits a gain of ～35 that is much higher than usually obtained values for unipolar logic.     

N‐Type Conjugated Polymer‐Enabled Selective Dispersion of Semiconducting Carbon Nanotubes for Flexible CMOS‐Like Circuits

Sorting of semiconducting single‐walled carbon nanotubes (SWNTs) by conjugated polymers has attracted considerable attention recently because of its simplicity, high selectivity, and high yield. However, up to now, all the conjugated polymers used for SWNT sorting are electron‐donating (p‐type). Here, a high‐mobility electron‐accepting (n‐type) polymer poly([N,N′‐bis(2‐octyldodecyl)‐naphthalene‐1,4,5,8‐bis(dicarboximide)‐2,6‐diyl]‐alt‐5,5′‐(2,2′‐bithiophene)) (P(NDI2OD‐T2)) is utilized for the sorting of high‐purity semiconducting SWNTs, as characterized by Raman spectroscopy, dielectric force spectroscopy and transistor measurements. In addition, the SWNTs sorted by P(NDI2OD‐T2) have larger diameters than poly(3‐dodecylthiophene) (P3DDT)‐sorted SWNTs. Molecular dynamics simulations in explicit toluene demonstrate distinct linear or helical wrapping geometry between P(NDI2OD‐T2) and different types of SWNTs, likely as a result of the strong interactions between the large aromatic core of the P(NDI2OD‐T2) backbone and the hexagon path of SWNTs. By using high‐mobility n‐type P(NDI2OD‐T2) as the sorting polymer, ambipolar SWNT transistors with better electron transport than that attained by P3DDT‐sorted SWNTs are achieved. As a result, flexible negated AND and negated OR logic circuits from the same set of ambipolar transistors are fabricated, without the need for doping. The use of n‐type polymers for sorting semiconducting SWNTs and achieving ambipolar SWNT transistor characteristics greatly simplifies the fabrication of flexible complementary metal‐oxide‐semiconductor‐like SWNT logic circuits.     

Sub‐Millimeter‐Scale Monolayer p‐Type H‐Phase VS2

2D H‐phase vanadium disulfide (VS₂) is expected to exhibit tunable semiconductor properties as compared with its metallic T‐phase structure, and thus is of promise for future electronic applications. However, to date such 2D H‐phase VS₂ nanostructures have not been realized in experiment likely due to the polymorphs of vanadium sulfides and thermodynamic instability of H‐phase VS₂. Preparation of H‐phase VS₂ monolayer with lateral size up to 250 µm, as a new member in the 2D transition metal dichalcogenides (TMDs) family, is reported. A unique growth environment is built by introducing the molten salt‐mediated precursor system as well as the epitaxial mica growth platform, which successfully overcomes the aforementioned growth challenges and enables the evolution of 2D H‐phase structure of VS₂. The honeycomb‐like structure of H‐phase VS₂ with broken inversion symmetry is confirmed by spherical aberration‐corrected scanning transmission electron microscopy and second harmonic generation characterization. The phase structure is found to be ultra‐stable up to 500 K. The field‐effect device study further demonstrates the p‐type semiconducting nature of the 2D H‐phase VS₂. The study introduces a new phase‐stable 2D TMDs materials with potential features for future electronic devices.     

MoS2 Van der Waals p–n Junctions Enabling Highly Selective Room‐Temperature NO2 Sensor

Van der Waals p–n junctions of 2D materials present great potential for electronic devices due to the fascinating properties at the junction interface. In this work, an efficient gas sensor based on planar 2D van der Waals junctions is reported by stacking n‐type and p‐type atomically thin MoS₂ films, which are synthesized by chemical vapor deposition (CVD) and soft‐chemistry route, respectively. The electrical conductivity of the van der Waals p–n junctions is found to be strongly affected by the exposure to NO₂ at room temperature (RT). The MoS₂ p–n junction sensor exhibits an outstanding sensitivity and selectivity to NO₂ at RT, which are unavailable in sensors based on individual n‐type or p‐type MoS₂. The sensitivity of 20 ppm NO₂ is improved by 60 times compared to a p‐type MoS₂ sensor, and an extremely low limit of detection of 8 ppb is obtained under ultraviolet irradiation. Complete and very fast sensor recovery is achieved within 30 s. These results are superior to most of the previous reports related to NO₂ detection. This work establishes an entirely new sensing platform and proves the feasibility of using such materials for the high‐performance detection of gaseous molecules at RT.     

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.     

Pressure‐Tunable Ambipolar Conduction and Hysteresis in Thin Palladium Diselenide Field Effect Transistors

Few‐layer palladium diselenide (PdSe₂) field effect transistors are studied under external stimuli such as electrical and optical fields, electron irradiation, and gas pressure. The ambipolar conduction and hysteresis are observed in the transfer curves of the as‐exfoliated and unprotected PdSe₂ material. The ambipolar conduction and its hysteretic behavior in the air and pure nitrogen environments are tuned. The prevailing p‐type transport observed at atmospheric pressure is reversibly turned into a dominant n‐type conduction by reducing the pressure, which can simultaneously suppress the hysteresis. The pressure control can be exploited to symmetrize and stabilize the transfer characteristics of the device as required in high‐performance logic circuits. The transistors are affected by trap states with characteristic times in the order of minutes. The channel conductance, dramatically reduced by the electron irradiation during scanning electron microscope imaging, is restored after an annealing of several minutes at room temperature. The work paves the way toward the exploitation of PdSe₂ in electronic devices by providing an experiment‐based and deep understanding of charge transport in PdSe₂ transistors subjected to electrical stress and other external agents.