Multifunctional In-Memory Logics Based on a Dual-Gate Antiambipolar Transistor toward Non-von Neumann Computing Architecture.

In-memory computing may make it possible to realize non-von Neumann computing because the logic circuits are unified in the memory units. We investigated two types of in-memory logic operations, namely, two-input logic circuits and multifunctional artificial synapses. These were realized in a dual-gate antiambipolar transistor (AAT) with a ReS2/WSe2 heterojunction, in which polystyrene with a zinc phthalocyanine core (ZnPc-PS4) was incorporated as a memory layer. Here, reconfigurability is a key concept for both types of device operations and was achieved by merging the Λ-shaped transfer curve of the AAT and the nonvolatile memory effect of ZnPc-PS4. First, we achieved electrically reconfigurable two-input logic circuits. Versatile logic circuits such as AND, OR, NAND, NOR, and XOR circuits were demonstrated by taking advantage of the Λ-shaped transfer curve of the dual-gate AAT. Importantly, the nonvolatile memory function provided the electrical switching of the individual circuits between AND/OR, NAND/NOR, and XOR/NAND circuits with constant input signals. Second, the memory effect was applied to multifunctional artificial synapses. The inhibitory/excitatory and long-term potentiation/depression synaptic operations were electrically reconfigured simply by controlling one parameter (readout voltage), making three distinct responses possible even with the same presynaptic signals. These findings provide hints that may lead to the realization of new in-memory computing architectures beyond the current von Neumann computers.

Vertical molecular transistors: a new strategy towards practical quantum devices.

Considerable effort has been dedicated to improving molecular devices since they were initially proposed by Aviram and Ratner in 1974. Organic molecules are small and have discrete molecular orbitals. These features can facilitate fascinating quantum transport phenomena, such as single-carrier tunneling, resonant tunneling, and quantum interference. The effective gate modulation of these quantum transport phenomena holds the promise of realizing a new computing architecture that differs from that of current Si electronics. In this article, we review the recent research progress on molecular transistors, specifically vertical molecular transistors (VMTs). First, we discuss the benefits of VMTs for future molecular-scale transistors compared with the currently dominant lateral molecular transistors. Subsequently, we describe representative examples of VMTs, where single molecules, self-assembled monolayers, and isolated molecules are used as transistor channels. Finally, we present our conclusions and perspectives about the use of VMTs for attractive quantum devices.

Reconfigurable Logic-in-Memory Constructed Using an Organic Antiambipolar Transistor.

We demonstrate an electrically reconfigurable two-input logic-in-memory (LIM) using a dual-gate-type organic antiambipolar transistor (DG-OAAT). The attractive feature of this device is that a phthalocyanine-cored star-shaped polystyrene is used as a nano-floating gate, which enables the electrical switching of individual logic circuits and stores the circuit information by the nonvolatile memory effect. First, the DG-OAAT exhibited Λ-shaped transfer curves with hysteresis by sweeping the bottom-gate voltage. Programming and erasing operations enabled the reversible shift of the Λ-shaped transfer curves. Furthermore, the top-gate voltage effectively tuned the peak voltages of the transfer curves. Consequently, the combination of dual-gate and memory effects achieved electrically reconfigurable two-input LIM operations. Individual logic circuits (e.g., OR/NAND, XOR/NOR, and AND/XOR) were reconfigured by the corresponding programming and erasing operations without any variations in the input signals. Our device concept has the potential to fulfill an epoch-making organic integration circuit with a simple device configuration.

Optically Controllable Organic Logic-in-Memory: An Innovative Approach toward Ternary Data Processing and Storage.

Logic-in-memory (LIM) has emerged as an energy-efficient computing technology, as it integrates logic and memory operations in a single device architecture. Herein, a concept of ternary LIM is established. First, a p-type 2,7-dioctyl[1]benzothieno[3,2-b][1]benzothiophene (C8-BTBT) transistor is combined with an n-type PhC2H4-benzo[de]isoquinolino[1,8-gh]quinolone diimide (PhC2-BQQDI) transistor to obtain a binary memory inverter, in which a zinc phthalocyanine-cored polystyrene (ZnPc-PS4) layer serves as a floating gate. The contrasting photoresponse of the transistors toward visible and ultraviolet light and the efficient hole-trapping ability of ZnPc-PS4 enable us to achieve an optically controllable memory operation with a high memory window of 18 V. Then, a ternary memory inverter is developed using an anti-ambipolar transistor to achieve a three-level data processing and storage system for more advanced LIM applications. Finally, low-voltage operation of the devices is achieved by employing a high-k dielectric layer, which highlights the potential of the developed LIM units for next-generation low-power electronics.

Carrier-Transport Mechanism in Organic Antiambipolar Transistors Unveiled by Operando Photoemission Electron Microscopy.

Organic antiambipolar transistors (AATs) have partially overlapped p-n junctions. At room temperature, this p-n junction induces a negative differential transconductance in an AAT. However, the detailed carrier-transport mechanism remains unclear. Herein, an operando photoemission electron microscopy is used to tackle this issue owing to the technique's ability to visualize conductive electrons in real time during transistor operation. Notably, it is observed that when the AAT is on, a depletion layer forms at the lateral p-n junction. The visualized depletion layer shows that both p- and n-type channels have pinch-off states in the gate voltage range when the AAT is in on state. The steep potential gradient at the lateral p-n interface enhances the electron conduction from n-type to p-type semiconductor. Another significant finding is that most electrons are considered to recombine with the accumulated holes in the p-type semiconductor, affording the reduction of photoemission intensity by ≈80%. This technique provides a thorough understanding of carrier transport in AATs, further improving the device performance.

Electrically Reconfigurable Organic Logic Gates: A Promising Perspective on a Dual-Gate Antiambipolar Transistor.

Electrically reconfigurable organic logic circuits are promising candidates for realizing new computation architectures, such as artificial intelligence and neuromorphic devices. In this study, multiple logic gate operations are attained based on a dual-gate organic antiambipolar transistor (DG-OAAT). The transistor exhibits a Λ-shaped transfer curve, namely, a negative differential transconductance at room temperature. It is important to note that the peak voltage of the drain current is precisely tuned by three input signals: bottom-gate, top-gate, and drain voltages. This distinctive feature enables multiple logic gate operations with "only a single DG-OAAT," which are not obtainable in conventional transistors. Five logic gate operations, which correspond to AND, OR, NAND, NOR, and XOR, are demonstrated by adjusting the bottom-gate and top-gate voltages. Moreover, varying the drain voltage makes it possible to reversibly switch two logic gates, e.g., NAND/NOR and OR/XOR. In addition, the DG-OAATs show a high degree of stability and reliability. The logic gate operations are observed even months later. The hysteresis in the transfer curves is also negligible. Thus, the device concept is promising for realizing multifunctional logic circuits with a simple transistor configuration. Hence, these findings are expected to surpass the current limitations in complementary metal-oxide-semiconductor devices.

Enhanced Selectivity in Volatile Organic Compound Gas Sensors Based on ReS2-FETs under Light-Assisted and Gate-Bias Tunable Operation.

Using a single-device two-dimensional (2D) rhenium disulfide (ReS2) field-effect transistor (FET) with enhanced gas species selectivity by light illumination, we reported a selective and sensitive detection of volatile organic compound (VOC) gases. 2D materials have the advantage of a high surface-area-to-volume ratio for high sensitivity to molecules attached to the surface and tunable carrier concentration through field-effect control from the back-gate of the channel, while keeping the top surface open to the air for chemical sensing. In addition to these advantages, ReS2 has a direct band gap also in multilayer cases, which sets it apart from other transition-metal dichalcogenides (TMDCs). We take advantage of the effective response of ReS2 to light illumination to improve the selectivity and gas-sensing efficiency of a ReS2-FET device. We found that light illumination modulates the drain current response in a ReS2-FET to adsorbed molecules, and the sensing activity differs depending on the gas species used, such as acetone, ethanol, and methanol. Furthermore, wavelength and carrier density rely on certain variations in light-modulated sensing behaviors for each chemical. The device will distinguish the gas concentration in a mixture of VOCs using the differences induced by light illumination, enhancing the selectivity of the sensor device. Our results shed new light on the sensing technologies for realizing a large-scale sensor network in the Internet-of-Things era.

Single-charge transport through hybrid core-shell Au-ZnS quantum dots: a comprehensive analysis from a modified energy structure.

We examined the modified electronic structure and single-carrier transport of individual hybrid core-shell metal-semiconductor Au-ZnS quantum dots (QDs) using a scanning tunnelling microscope. Nearly monodisperse ultra-small QDs are achieved by a facile wet chemical route. The exact energy structures are evaluated by scanning tunnelling spectroscopy (STS) measurements at 300 mK for the individual nanoobjects starting from the main building block Au nanocrystals (NCs) to the final Au-ZnS QDs. The study divulges the evolution of the energy structure and the charge transport from the single metallic building block core to the core-shell metal-semiconductor QDs. Furthermore, we successfully determined the contributions related to the quantum-confinement-induced excitonic band structure of the ZnS nano-shell and the charging energy of the system by applying a semi-empirical approach considering a double barrier tunnel junction (DBTJ) arrangement. We detect strong conductance peaks in Au-ZnS QDs due to the overlapping of the energy structure of the Au nano-core and the discrete energy states of the semiconductor ZnS nano-shell. Our findings will help in understanding the electronic properties of metal-semiconductor QDs. The outcomes therefore have the potential to fabricate tailored metal-semiconductor QDs for single-electron devices.

Multi-Valued Logic Circuits Based on Organic Anti-ambipolar Transistors.

Multivalued logic circuits, which can handle more information than conventional binary logic circuits, have attracted much attention as a promising way to improve the data-processing capabilities of integrated circuits. In this study, we developed a ternary inverter based on organic field-effect transistors (OFET) as a potential component of high-performance and flexible integrated circuits. Key elements are anti-ambipolar and n-type OFETs connected in series. First, we demonstrate an organic ternary inverter that exhibits three distinct logic states. Second, the operating voltage was greatly reduced by taking advantage of an Al2O3 gate dielectric. Finally, the operating voltage was finely tuned by the designing of the device geometry. These results are achievable owing to the flexible controllability of the device configuration, suggesting that the organic ternary inverter plays an important role with regard to high-performance organic integrated circuits.

Interface Engineering for Controlling Device Properties of Organic Antiambipolar Transistors.

The main purpose of this study is to establish a guideline for controlling the device properties of organic antiambipolar transistors. Our key strategy is to use interface engineering to promote carrier injection at channel/electrode interfaces and carrier accumulation at a channel/dielectric interface. The effective use of carrier injection interlayers and an insulator layer with a high dielectric constant (high-k) enabled the fine tuning of device parameters and, in particular, the onset (Von) and offset (Voff) voltages. A well-matched combination of the interlayers and a high-k dielectric layer achieved a low peak voltage (0.25 V) and a narrow on-state bias range (2.2 V), indicating that organic antiambipolar transistors have high potential as negative differential resistance devices for multivalued logic circuits.

Vertical resonant tunneling transistors with molecular quantum dots for large-scale integration.

Quantum molecular devices have a potential for the construction of new data processing architectures that cannot be achieved using current complementary metal-oxide-semiconductor (CMOS) technology. The relevant basic quantum transport properties have been examined by specific methods such as scanning probe and break-junction techniques. However, these methodologies are not compatible with current CMOS applications, and the development of practical molecular devices remains a persistent challenge. Here, we demonstrate a new vertical resonant tunneling transistor for large-scale integration. The transistor channel is comprised of a MOS structure with C60 molecules as quantum dots, and the structure behaves like a double tunnel junction. Notably, the transistors enabled the observation of stepwise drain currents, which originated from resonant tunneling via the discrete molecular orbitals. Applying side-gate voltages produced depletion layers in Si substrates, to achieve effective modulation of the drain currents and obvious peak shifts in the differential conductance curves. Our device configuration thus provides a promising means of integrating molecular functions into future CMOS applications.

Laser Patterning of Optically Reconfigurable Transistor Channels in a Photochromic Diarylethene Layer.

Optical switching organic field-effect transistors (OFETs) provide a new direction for optoelectronics based on photochromic molecules. However, the patterning of OFETs is difficult because conventional fabrication processes, including lithography and ion etching, inevitably cause severe damage to organic molecules. Here, we demonstrate laser patterning of one-dimensional (1D) channels on an OFET with a photochromic diarylethene (DAE) layer. The main findings are (i) a number of 1D channels can be repeatedly written and erased in the DAE layer by scanning focused ultraviolet and visible light laser beams alternately between the source and drain electrodes, (ii) the conductivity (or resistivity) of the 1D channel can be controlled by the illumination conditions, such as the laser power density and the scan speed, and (iii) it is possible to draw an analogue adder circuit by optically writing 1D channels so that a portion of the channels overlaps and to perform optical summing operations by local laser illumination of the respective channels. These findings will open new possibilities for realizing various optically reconfigurable, low-dimensional organic transistor circuits, which are not possible with conventional thin film OFETs.

Large Magnetoresistance in Single-Radical Molecular Junctions.

Organic radicals are promising building blocks for molecular spintronics. Little is known about the role of unpaired electrons for electron transport at the single-molecule level. Here, we examine the impact of magnetic fields on electron transport in single oligo(p-phenyleneethynylene) (OPE)-based radical molecular junctions, which are formed with a mechanically controllable break-junction technique at a low temperature of 4.2 K. Surprisingly huge positive magnetoresistances (MRs) of 16 to 287% are visible for a magnetic field of 4 T, and the values are at least 1 order of magnitude larger than those of the analogous pristine OPE (2-4%). Rigorous analysis of the MR and of current-voltage and inelastic electron-tunneling spectroscopy measurements reveal an effective reduction of the electronic coupling between the current-carrying molecular orbital and the electrodes with increasing magnetic field. We suggest that the large MR for the single-radical molecular junctions might be ascribed to a loss of phase coherence of the charge carriers induced by the magnetic field. Although further investigations are required to reveal the mechanism underlying the strong MR, our findings provide a potential approach for tuning charge transport in metal-molecule junctions with organic radicals.

Optically controllable dual-gate organic transistor produced via phase separation between polymer semiconductor and photochromic spiropyran molecules.

We produced an optically controllable dual-gate organic field-effect transistor by a simple one-step spin-coating of a mixed solution of photochromic spiropyran (SP) and poly(3-hexylthiophene) (P3HT). Postannealing enhanced polymer chain ordering of P3HT to induce phase separation into an SP-rich lower layer and an SP-free upper layer. These layers worked independently as transistor channels with distinct optical responsivity. The top channel was optically inactive, but the bottom channel was optically active, because of the photoisomerization of SP. These results demonstrate the potential of our technique to produce a multifunctional photoactive organic transistor by a simple process.

Recent progress in photoactive organic field-effect transistors.

Recent progress in photoactive organic field-effect transistors (OFETs) is reviewed. Photoactive OFETs are divided into light-emitting (LE) and light-receiving (LR) OFETs. In the first part, LE-OFETs are reviewed from the viewpoint of the evolution of device structures. Device performances have improved in the last decade with the evolution of device structures from single-layer unipolar to multi-layer ambipolar transistors. In the second part, various kinds of LR-OFETs are featured. These are categorized according to their functionalities: phototransistors, non-volatile optical memories, and photochromism-based transistors. For both, various device configurations are introduced: thin-film based transistors for practical applications, single-crystalline transistors to investigate fundamental physics, nanowires, multi-layers, and vertical transistors based on new concepts.

Photoisomerization-induced manipulation of single-electron tunneling for novel Si-based optical memory.

We demonstrated optical manipulation of single-electron tunneling (SET) by photoisomerization of diarylethene molecules in a metal-insulator-semiconductor (MIS) structure. Stress is placed on the fact that device operation is realized in the practical device configuration of MIS structure and that it is not achieved in structures based on nanogap electrodes and scanning probe techniques. Namely, this is a basic memory device configuration that has the potential for large-scale integration. In our device, the threshold voltage of SET was clearly modulated as a reversible change in the molecular orbital induced by photoisomerization, indicating that diarylethene molecules worked as optically controllable quantum dots. These findings will allow the integration of photonic functionality into current Si-based memory devices, which is a unique feature of organic molecules that is unobtainable with inorganic materials. Our proposed device therefore has enormous potential for providing a breakthrough in Si technology.

Unique device operations by combining optical-memory effect and electrical-gate modulation in a photochromism-based dual-gate transistor.

We demonstrate a new device that combines a light-field effect and an electrical-gate effect to control the drain current in a dual-gate transistor. We used two organic layers, photochromic spiropyran (SP)-doped poly(triarylamine) (PTAA) and pristine PTAA, as top and bottom channels, respectively, connected to common source and drain electrodes. The application of voltage to the top and bottom gates modulated the drain current through each layer independently. UV irradiation suppressed the drain current through the top channel. The suppressed current was then maintained even after the UV light was turned off because of an optical memory effect induced by photoisomerization of SP. In contrast, UV irradiation did not change the drain current in the bottom channel. Our dual-gate transistor thus has two organic channels with distinct photosensitivities: an optically active SP-PTAA film and an optically inactive PTAA film. This device configuration allows multi-level switching via top- and bottom-gate electrical fields with an optical-memory effect.

Optically and electrically driven organic thin film transistors with diarylethene photochromic channel layers.

We achieved drain-current switching of diarylethene-channel field-effect transistors with light- and electric-field effects. The drain current was reversibly changed by alternating ultraviolet and visible light irradiation. Stress is placed on the fact that the on/off ratio realized by light irradiation was 1 × 10(2) (1 × 10(4)%) and this value is much larger than those in other photochromism-based transistors. These results indicate that the drain current was effectively controlled by light irradiation. Furthermore, the on and off states modulated by light were maintained without light irradiation even after 1 week, exhibiting that our transistor works as an optical memory. We clarified that the light-driven modulation can be attributed to the transformation in the π-conjugation system accompanied by photoisomerization. These findings have the potential to attain high-performance optoelectrical organic devices including optical sensors, optical memory, and photoswitching transistors.

Enhanced electrical conductivity in poly(3-hexylthiophene)/fluorinated tetracyanoquinodimethane nanowires grown with a porous alumina template.

We report on improved electrical conductivity in poly(3-hexylthiophene) (P3HT)/2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F4-TCNQ) composite nanowires grown using an anodized aluminum oxide (AAO) template. The electrical conductivity of individual nanowire measured by four-probe scanning tunneling microscopy shows that F4-TCNQ molecules are effectively doped into P3HT by capillary force. The resistivity is tuned in the 0.1-10 Ω cm range by changing the F4-TCNQ concentration from 10 to 0.1 wt % and is 2-4 orders of magnitude smaller than that of the corresponding P3HT/F4-TNCQ thin film composites. The AAO template-assisted synthesis approach thus appears to be effective for high chemical doping and for improving the electrical conductivity of the molecular wires.

Molecular alignment and energy-level diagram at heteromolecular interface of quaterrylene and terrylene-3,4,11,12-tetracarboximide.

Heteromolecular layers consisting of quaterrylene (QT) and terrylene-3,4,11,12-tetracarboximide (TTCDI-C12) were prepared on SiO2 surfaces and the electronic energy level alignment at TTCDI-C12/QT interface was examined. TTCDI-C12 layers were grown in nearly perpendicular orientation on QT layers by an ultraslow deposition technique, thereby achieving formation of a well-defined TTCDI-C12/QT interface. Atomic force microscopy (AFM) measurements ensured excellent surface flatness of each layer, surface roughnesses of which were comparable to that of pristine SiO2. Energy level alignment at the heteromolecular interface was evaluated by using ultraviolet photoelectron spectroscopy (UPS) and optical absorption measurements. No shift in energy level was served at the heteromolecular interface, indicating that charge transfer does not occur and a dipole moment is not formed at the well-defined TTCDI-C12/QT interface.