Unipolar Behavior Research Articles

High-yield and sustainable synthesis of quinoidal compounds assisted by keto-enol tautomerism.

The classical synthesis of quinoids, which involves Takahashi coupling and subsequent oxidation, often gives only low to medium yields. Herein, we disclose the keto–enol-tautomerism-assisted spontaneous air oxidation of the coupling products to quinoids. This allows for the synthesis of various indandione-terminated quinoids in high isolated yields (85–95%). The origin of the high yield and the mechanism of the spontaneous air oxidation were ascertained by experiments and theoretical calculations. All the quinoidal compounds displayed unipolar n-type transport behavior, and single crystal field-effect transistors based on the micro-wires of a representative quinoid delivered an electron mobility of up to 0.53 cm2 V−1 s−1, showing the potential of this type of quinoid as an organic semiconductor.

Observation of different transport behaviors in a two-dimensional MoTe2 field-effect transistor with engineered gate stack

Back-gate field-effect transistor (FET) fabricated from exfoliated two-dimensional (2D) materials have been widely applied on the electronic device applications of the novel transition metal dichalcogenides (TMDs) since their experimental discovery. Si substrate covered with thick SiO2 dielectric layer is commonly used due to the natural back-gate geometry and good optical contrast which, however, lacks sufficient gate control over the ultra-thin channel because of the thick dielectric. Here, in this work, we report a high-performance back-gate MoTe2 FET fabricated on a novel Al2O3/ITO (indium tin oxide)/SiO2/Si stack substrate. The stack thickness has been carefully designed and engineered to enhance the gate control of the MoTe2 channel while enabling sufficient optical contrast between the substrate and the exfoliated 2D flakes. Unipolar and ambipolar transport behaviors on different substrates have been observed and analyzed. As compared to the conventional 300 nm SiO2/Si substrate, the MoTe2 FET on the engineered gate stack has exhibited higher on/off current ratio with smaller cut-off current and smaller subthreshold slope of 126.57 mV/dec. These results are attractive in building novel logic devices based on the 2D MoTe2 with unipolar and ambipolar behavior broadening the applications in advanced nanoelectronics and optoelectronics systems

Carbonyl-Terminated Quinoidal Oligothiophenes as p-Type Organic Semiconductors.

A series of quinoidal oligothiophenes terminated with carbonyl groups (nTDs, n = 2–4) are studied as p-type organic semiconductors for the active materials in organic field-effect transistors (OFETs) both by the theoretical and experimental approaches. The theoretical calculations clearly show their high-lying highest occupied molecular orbital (HOMO) energy levels (EHOMOs), small reorganization energies for hole transport (λholes), and large contribution of sulfur atoms to HOMOs, all of which are desirable for p-type organic semiconductors. Thus, we synthesized nTDs from the corresponding aromatic oligothiophene precursors and then evaluated their physicochemical properties and structural properties. These experimental evaluations of nTDs nicely proved the theoretical predictions, and the largest 4TDs in the series (4,4′′′-dihexyl- and 3′,4,4″,4′′′-tetrahexyl-5H,5′′′H-[2,2′:5′,2″:5″,2′′′-quaterthiophene]-5,5′′′-dione) can afford solution-processed OFETs showing unipolar p-type behaviors and hole mobility as high as 0.026 cm2 V−1 s−1.

Two Isomeric Azulene-Decorated Naphthodithiophene Diimide-based Triads: Molecular Orbital Distribution Controls Polarity Change of OFETs Through Connection Position.

Altering the charge carrier transport polarities of organic semiconductors by molecular orbital distribution has gained great interest. Herein, we report two isomeric azulene-decorated naphthodithiophene diimide (NDTI)-based triads (e.g., NDTI-B2Az and NDTI-B6Az), in which two azulene units were connected with NDTI at the 2-position of the azulene ring in NDTI-B2Az, whereas two azulene units were incorporated with NDTI at the 6-position of the azulene ring in NDTI-B6Az. The two isomeric triads were excellently soluble in common organic solvents. Density functional theory calculations on the molecular orbital distributions of the triads reveal that the lowest unoccupied molecular orbitals are completely delocalized over the entire molecule for both NDTI-B2Az and NDTI-B6Az, indicating great potential for n-type transport behavior, whereas the highest occupied molecular orbitals are mainly delocalized over the entire molecule for NDTI-B2Az or only localized at the two terminal azulene units for NDTI-B6Az, implying great potential for p-type transport behavior for the former and a disadvantage of hole carrier transport for the latter. Under ambient conditions, solution-processed bottom-gate top-contact transistors based on NDTI-B2Az showed ambipolar field-effect transistor (FET) characteristics with high electron and hole mobilities of 0.32 (effective electron mobility ≈0.14 cm2 V-1 s-1 according to a reliability factor of 43%) and 0.03 cm2 V-1 s-1 (effective hole mobility ≈0.01 cm2 V-1 s-1 according to a reliability factor of 33%), respectively, whereas a typically unipolar n-channel behavior is found for a film of NDTI-B6Az with a high electron mobility up to 0.13 cm2 V-1 s-1 (effective electron mobility ≈0.06 cm2 V-1 s-1 according to a reliability factor of 43%). The results indicate that the polarity change of organic FETs based on the two isomeric triads could be controlled by the molecular orbital distributions through the connection position between the azulene unit and NDTI.

Polygenic risk score models based on anxiety-related traits as endophenotypes to predict unipolar depression and suicidal behavior

The present study included the construction of polygenic risk score (PRS) models of individual differences in anxiety level, which increased values represent the endophenotype of depression and suicidal behavior, to predict these psychopathologies based on genotyping of 63 SNPs of serotoninergic, hypothalamicpituitary-adrenal and neurotrophic factors systems together with the relative leukocyte telomere length. According to logistic regression analysis, PRS models of individual differences in Neuroticism (EPI) level (P<0,2) predicted decreased risk of unipolar depression (P0.2=0,033, β=-5,24, r2=0,78) in women, which points to the necessity to consider sex during polygenic risk score analysis.

Diffusive and Anti-Diffusive Behavior for Kinetic Models of Opinion Dynamics

In the present paper, we study a class of nonlinear integro-differential equations of a kinetic type describing the dynamics of opinion for two types of societies: conformist ( σ = 1 ) and anti-conformist ( σ = − 1 ). The essential role is played by the symmetric nature of interactions. The class may be related to the mesoscopic scale of description. This means that we are going to statistically describe an individual state of an agent of the system. We show that the corresponding equations result at the macroscopic scale in two different pictures: anti-diffusive ( σ = 1 ) and diffusive ( σ = − 1 ). We provide a rigorous result on the convergence. The result captures the macroscopic behavior resulting from the mesoscopic one. In numerical examples, we observe both unipolar and bipolar behavior known in political sciences.

Resistive switching memory effects in chalcogenide semiconductor ZnGa2Se4 thin films

ABSTRACTZnGa2Se4 thin films have been deposited on pyrographite substrate using flash evaporation technique. Platinum electrodes were used to establish electrical contacts. The electrical characteristics of Pt/ZnGa2Se4/Pyrographite/Pt device studied under sweep and pulse voltage mode. The device shows good memory window (LRS/HRS ≈ 104). The scanning electron micrographs (SEM) are used to illustrate the formation of local conducting paths at grain boundaries (GBs) under applied electric field. Remarkable resistive switching results were observed, such as unipolar resistive behaviour, a high switching ratio (≈104), a long retention time (102 s) and high endurance at ambient conditions. Our experimental findings indicate that the kinetics of filament formation is highly dependent on the microstructure of ZnGa2Se4 thin film. The implications are discussed.

An efficient Verilog-A memristor model implementation: simulation and application

Complementary metal–oxide–semiconductor (CMOS) technology is reaching its limits due to the continuous shrinking process, which has an impact on various aspects including device size, performance, and power consumption. The memristor is one of the promising devices under investigation for use with deep-nanometer CMOS, having applicability in several fields due to its nonlinear behavior, nonvolatility, low power consumption, high density, and CMOS compatibility. Several models for memristors have been developed to date, but there is a requirement for compact models that are both flexible and sufficiently accurate. A general memristor model generated in Verilog-A is discussed herein to confirm its behavior in the one-transistor one-resistor (1T1R) oxide-based random-access memory (OxRAM) configuration, and validated at circuit level. The results of the model correlate well with experimental characterization data for the HfO2-based OxRAM memristor device, describing the characteristics of both its bipolar and unipolar memristor behaviors. The 1T1R structure is analyzed using the Spectre circuit simulator. Two cases are considered, using the cell as either programmable read-only memory (PROM) or electrically erasable programmable read-only memory (EEPROM). The simulation results confirm the desired nonlinear memristor characteristic, and the applicability of the model to fit and simulate different switching behaviors. The results are verified against both electrical and experimental characterization data, suggesting that the Verilog-A model is suitable for low-power and high-density logic circuit applications at the industrial level.

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.

Large strain under low driving field in lead‐free relaxor/ferroelectric composite ceramics

Abstract0.75(Na0.5Bi0.5)TiO3–0.25SrTiO3 lead‐free incipient piezoceramic is a promising candidate for actuator applications due to their large reversible electromechanical strains at the relatively low driving field of 40 kV/cm. In order to further reduce the driving field of 0.75(Na0.5Bi0.5)TiO–0.25SrTiO3 relaxor ceramic to meet the requirements for real actuators application, the relaxor/ferroelectric (RE/FE) 0‐3 composite ceramics method was employed. The polarization and strain behaviors were examined as a function of the weight ratio of the relaxor/ferroelectric phases. It was found that 90 wt% 0.75(Na0.5Bi0.5)TiO3–0.25SrTiO3/10 wt% 0.96(Na0.84K0.16)1/2Bi1/2TiO3–0.04SrTiO3 RE/FE 0‐3 type composite samples provided a high unipolar strain of 0.25% and the corresponding large‐signal piezoelectric coefficient, d*33 of 833 pm/V at 30 kV/cm, which are 32% higher than the values of the pure 0.75(Na0.5Bi0.5)TiO3–0.25SrTiO3. The enhanced electric‐field‐induced strain at relatively lower field was attributed primarily to the reduction in the RE‐FE phase transition electric field. It was also found that the RE/FE composite ceramics exhibited significantly reduced frequency dependence in the unipolar strain behavior at room temperature.

A facile synthetic route to tungsten diselenide using a new precursor containing a long alkyl chain cation for multifunctional electronic and optoelectronic applications.

Single source precursors for coating and subsequent thermal decomposition processes enable a large-scale, low-cost synthesis of two-dimensional transition metal dichalcogenides (TMDs). However, practical applications based on two-dimensional TMDs have been limited by the lack of applicable single source precursors for the synthesis of p-type TMDs including layered tungsten diselenide (WSe2). We firstly demonstrate the simple and facile synthesis of WSe2 layers using a newly developed precursor that allows improved dispersibility and lower decomposition temperature. We study the thermal decomposition mechanism of three types of (Cat+)2[WSe4] precursors to assess the most suitable precursor for the synthesis of WSe2 layers. The resulting chemical and structural exploration of solution-processed WSe2 layers suggests that the (CTA)2[WSe4] may be a promising precursor because it resulted in the formation of high-crystalline WSe2. In addition, this study verifies the capability of WSe2 layers for multifunctional applications in optoelectronic and electronic devices. The photocurrent of WSe2-based photodetectors shows an abrupt switching behavior under periodic illumination of visible or IR light. The extracted photoresponsivity values for WSe2-based photodetectors recorded at 0.5 V correspond to 26.3 mA W−1 for visible light and 5.4 mA W−1 for IR light. The WSe2-based field effect transistors exhibit unipolar p-channel transistor behavior with a carrier mobility of 0.45 cm2 V−1 s−1 and an on-off ratio of ∼10.

Ambipolar transport in tin dioxide thin film transistors promoted by PCBM fullerene

In this article, the effect of phenyl-C61-butyric acid methyl ester (PCBM) layer on the electrical performance of field-effect transistors (FETs) based on antimony-doped tin dioxide (Sb:SnO2) is reported. PCBM is a soluble variety of fullerene, n-type organic semiconductor, known to promote the p-type doping of semiconducting materials such as diamond and graphene, via charge transfer. Sb:SnO2 is an emerging low-cost transparent oxide semiconductor material that exhibits strong unipolar behavior (n-type). Ambipolar character in tin dioxide normally is not observed, however in this study we find that the deposition of PCBM on top of Sb:SnO2 promotes ambipolar behavior in Sb:SnO2 FETs. At negative gate bias (VG 0, and enhances the current in inversion mode. Besides, PCBM deposition also results in an increase of hole mobility at VG < 0.

Polyoxometalates‐Modulated Reduced Graphene Oxide Flash Memory with Ambipolar Trapping as Bidirectional Artificial Synapse

AbstractEmploying flash memories that function as the analogue synapse cleft for implementation of neural systems is a popular approach. For practical application in synapse mimicking, ambipolar trapping of both holes and electrons enables the weight updated linearity, symmetric conductivity variation, and desired variation margin, which are consistent with biological synapse. Nevertheless, most of the flash memory exhibits unipolar charge trapping behavior. Here, the first polyoxometalates (POMs)‐modulated reduced graphene oxide (RGO)‐based flash memory with ambipolar charge trapping characteristics is reported. Keggin type [PW12O40]3− (PW) is selected to function as both catalyst for in situ photoreduction of the graphene oxide (GO) into RGO under UV illumination and electron trapping sites. The electrons accepting property of PW induces energy level realignment between the PW/RGO interface and the subsequent ambipolar charge trapping. Furthermore, the selected features of biological synaptic behavior, including short‐term plasticity and long‐term plasticity as well as paired pulse facilitation and depression, are mimicked. It is worth noting that almost symmetrical potentiation and depression synaptic behavior is obtained. This work provides new opportunities to realize reconfigurable characteristics of electronic devices via engineering of POMs, which is significantly meaningful to emerging artificial intelligence.

Al2O3 thin film multilayer structure for application in RRAM devices

This study investigated the effect of resistive switching on Pt/Al2O3/Cu/Al2O3/ITO multilayer structures grown by RF/DC magnetron sputtering. A reproducible unipolar switching free forming was found only for the negative voltage. The SET and RESET processes occurred at very low voltage values, which may be advantageous for practical applications. The ON/OFF ratio was approximately five orders of magnitude for more than 103 s, which is desirable for nonvolatile memories such as resistive random-access memories (RRAMs). We explain the unipolar behavior of the characteristic (I–V) curves in terms of the formation and rupture of conductive filaments, connected via a Cu metal layer.

MoS2 nanosheets via electrochemical lithium-ion intercalation under ambient conditions

Two-dimensional (2D) transition metal dichalcogenides (TMDs) are continuously attracting attention for both fundamental studies and technological applications. The physical and chemical properties of ultrathin TMD sheets are extraordinarily different from those of the corresponding bulk materials and for this reason their production is a stimulating topic, especially when the preparation method enables to obtain a remarkable yield of nanosheets with large area and high quality. Herein, we present a fast (<1 h) electrochemical exfoliation of molybdenum disulfide (MoS2) via lithium-ion intercalation, by using a solution of lithium chloride in dimethyl sulfoxide (DMSO). Unlike the conventional intercalation methods based on dangerous organolithium compounds, our approach leads to the possibility to obtain mono-, bi- and tri-layer thick MoS2 nanosheets with a large fraction of the semiconducting 2H phase (∼60%), as estimated by X-ray photoelectron spectroscopy (XPS). The electrical properties of the exfoliated material were investigated through the fabrication and characterization of back-gated field-effect transistors (FETs) based on individual MoS2 nanosheets. As-fabricated devices displayed unipolar semiconducting behavior (n-type) with field-effect mobility µFE ≤ 10−3 cm2 V−1 s−1 and switching ratio Ion/Ioff ≤ 10, likely limited by 1T/2H polymorphism and defects (e.g. sulfur vacancies) induced during the intercalation/exfoliation process. A significant enhancement of the electrical performances could be achieved through a combination of vacuum annealing (150 °C) and sulfur-vacancy healing with vapors of short-chain alkanethiols, resulting in µFE up to 2 × 10−2 cm2 V−1 s−1 and Ion/Ioff ≈ 100. Our results pave the way towards the fast preparation – under ambient conditions – of semiconducting MoS2 nanosheets, suitable for application in low cost (opto-)electronic devices.

Effect of the Channel Length on the Transport Characteristics of Transistors Based on Boron-Doped Graphene Ribbons.

Substitutional boron doping of devices based on graphene ribbons gives rise to a unipolar behavior, a mobility gap, and an increase of the ratio of the transistor. Here we study how this effect depends on the length of the doped channel. By means of self-consistent simulations based on a tight-binding description and a non-equilibrium Green’s function approach, we demonstrate a promising increase of the ratio with the length of the channel, as a consequence of the different transport regimes in the ON and OFF states. Therefore, the adoption of doped ribbons with longer aspect ratios could represent a significant step toward graphene-based transistors with an improved switching behavior.

Unipolar n-Type Black Phosphorus Transistors with Low Work Function Contacts.

Black phosphorus (BP) is a promising two-dimensional (2D) material for nanoscale transistors, due to its expected higher mobility than other 2D semiconductors. While most studies have reported ambipolar BP with a stronger p-type transport, it is important to fabricate both unipolar p- and n-type transistors for low-power digital circuits. Here, we report unipolar n-type BP transistors with low work function Sc and Er contacts, demonstrating a record high n-type current of 200 μA/μm in 6.5 nm thick BP. Intriguingly, the electrical transport of the as-fabricated, capped devices changes from ambipolar to n-type unipolar behavior after a month at room temperature. Transmission electron microscopy analysis of the contact cross-section reveals an intermixing layer consisting of partly oxidized metal at the interface. This intermixing layer results in a low n-type Schottky barrier between Sc and BP, leading to the unipolar behavior of the BP transistor. This unipolar transport with a suppressed p-type current is favorable for digital logic circuits to ensure a lower off-power consumption.

Intrinsic Point Defects in Inorganic Cesium Lead Iodide Perovskite CsPbI3

Cesium lead iodide (CsPbI3) has recently emerged as a promising solar photovoltaic absorber. However, the cubic perovskite (α-phase) remains stable only at high temperature and reverts to a photoinactive nonperovskite (δ-phase) CsPbI3 at room temperature. In this work, the formation energies and transition energy levels of intrinsic point defects in γ- (more stable than α-phase) and δ-phases have been studied systematically by first-principles calculations. It is found that CsPbI3 exhibits a unipolar self-doping behavior (p-type conductivity), which is in contrast to CH3NH3PbI3. Most of the intrinsic defects induce deeper transition energy levels in δ-phase than in γ-phase. This is due to the small Pb–I–Pb bond angles in δ-phase that results in the weak antibonding character of valence band maximum (VBM). However, the strong antibonding character of VBM plays a critical role in keeping defect tolerance in semiconductors. Therefore, these results indicate the importance of the large metal–halide–metal bond...

Carrier polarity engineering in carbon nanotube field-effect transistors by induced charges in polymer insulator

We present that the electrical conduction type in carbon nanotube field-effect transistors (CNT-FETs) can be converted by induced charges in a polyvinyl alcohol (PVA) insulator. When the CNT channels are covered with pure PVA, the FET characteristics clearly change from unipolar p-type to ambipolar. The addition of ammonium ions (NH4+) in the PVA leads to further conversion to unipolar n-type conduction. The capacitance − voltage characteristics indicate that a high density of positive charges is induced at the PVA/SiO2 interface and within the bulk PVA. Electrons are electrostatically accumulated in the CNT channels due to the presence of the positive charges, and thus, stable n-type conduction of PVA-coated CNT-FETs is observed, even under ambient conditions. The mechanism for conversion of the conduction type is considered to be electrostatic doping due to the large amount of positive charges in the PVA. A blue-shift of the Raman G-band peak was observed for CNTs coated with NH4+-doped PVA, which corresponds to unipolar n-type CNT-FET behavior. These results confirm that carrier polarity engineering in CNT-FETs can be achieved with a charged PVA passivation layer.

Roll-to-Roll Production of Layer-Controlled Molybdenum Disulfide: A Platform for 2D Semiconductor-Based Industrial Applications.

A facile methodology for the large-scale production of layer-controlled MoS2 layers on an inexpensive substrate involving a simple coating of single source precursor with subsequent roll-to-roll-based thermal decomposition is developed. The resulting 50 cm long MoS2 layers synthesized on Ni foils possess excellent long-range uniformity and optimum stoichiometry. Moreover, this methodology is promising because it enables simple control of the number of MoS2 layers by simply adjusting the concentration of (NH4 )2 MoS4 . Additionally, the capability of the MoS2 for practical applications in electronic/optoelectronic devices and catalysts for hydrogen evolution reaction is verified. The MoS2 -based field effect transistors exhibit unipolar n-channel transistor behavior with electron mobility of 0.6 cm2 V-1 s-1 and an on-off ratio of ≈10³. The MoS2 -based visible-light photodetectors are fabricated in order to evaluate their photoelectrical properties, obtaining an 100% yield for active devices with significant photocurrents and extracted photoresponsivity of ≈22 mA W-1 . Moreover, the MoS2 layers on Ni foils exhibit applicable catalytic activity with observed overpotential of ≈165 mV and a Tafel slope of 133 mV dec-1 . Based on these results, it is envisaged that the cost-effective methodology will trigger actual industrial applications, as well as novel research related to 2D semiconductor-based multifaceted applications.