Unique Device Architecture Research Articles

Graphene/aluminum oxide interfaces for nanoelectronic devices

In this work, we study theoretically and experimentally graphene/aluminum oxide interfaces as 0D/2D interfaces for quantum electronics as the nature of the interface is of paramount importance to understand the quantum transport mechanism. Indeed, the electronic transport is driven either by a channel arising from a strong hybridization at the interface, or by tunneling across a van der Waals interface, with very different electric characteristics. By combining electronic spectroscopy and scanning microscopy with density functional theory calculations, we show that the interface is of weak and van der Waals nature. Quantum transport measurements in a single electron transistor confirm this result. Our results provide a first insight into the interfacial properties van der Waals materials based single electron device, and the key role played by the control of the interface states. The weak van der Waals coupling reported is promising for single electron device, where the control of the environmental charges is known to be a key challenge towards applications. Moreover, the unique vertical device architecture, enabled by the dual role of graphene including its vertical electric field transparency, opens the doors for a new class of single electron devices with higher scaling capability and functionalities. This work paves the way to new atomic environment control in single electron device.

3D to 2D perspectives - Traditional and new doping and metrology challenges at the nanoscale

In this perspectives paper we will explore the doping state-of-the-art as it evolves for 3D to 2D structures and materials, and the following impact on the metrology methods needed to characterise them. Historically, silicon devices for logic complementary metal-oxide-semiconductor (MOS) are built in bulk monocrystalline substrates. In this context bulk silicon substrates are 3D, as there is not the electrostatic confinement associated with more technologically advanced device structures. The determination of technology figures-of-merit such as sheet resistance, impurity concentration and activation profiles versus depth, and reverse bias leakage current to track crystal quality post-doping process, have been the typical methods for evaluation. Since the emergence of FinFET, gate-all-around (GAA) for logic, and layered 2D materials such as transition-metal-dichalcogenides (TMDs) for MOS channel active areas, these figures-of-merit are more awkward to extract, or simply do not apply in the usual sense. Logic devices, like MOS transistors, depend on doping processes to lower access resistance and thus drive performance, traditionally relied on ion implantation to introduce impurities to the substrate, which is a physical bombardment that damages the target material. Gentler doping processes that rely on in-diffusion are an emerging category, and most recently the concept of terminating or functionalising the (near-) surface of the semiconductor is growing in popularity, at least in research labs. Likewise, methods to characterise these require evolution to adapt to the times, from the conventional high-throughput techniques to the more exploratory, albeit lower throughput, methods. Metrology relies upon high-sensitivity, large dynamic range of detection, spatial resolution, as well as time- and cost-efficient sample preparation and post-analysis data interpretation. Driven primarily by the continued development of novel, scaled-down, truly unique device architectures and the introduction of novel materials in the nano-fabrication technology, the portfolio of dopant characterisation methods is unremittingly enlarging its numbers. Herein, we focus our discussion onto dopant profiling for fin and nanowire FETs, and relate these to analysis of test vehicles such as arrays of fin structures with varying dimensions. Although reports detailing correlative use of the dopant characterisation methods are only a few, we present examples that provided unique new information, in absence of which the overall description of the dopant process is inconclusive and, in some cases, misleading.

Demonstration of Photoelectrochemical‐Type Photodetectors Using Seawater as Electrolyte for Portable and Wireless Optical Communication

AbstractThe emerging photoelectrochemical‐type photodetector (PEC‐PD), because of its unique device architecture by using an aqueous electrolyte, is naturally applicable in the pursuit of underwater optical communication without sophisticated device packaging and assembling. Unfortunately, the traditional PEC detecting process typically involves a large electrochemical workstation with long cables and wire connections for photo signal acquisition, which hinders its applications in wireless optical communication. In this work, an AlGaN nanowire‐based self‐powered PEC‐PD is fabricated that uses seawater as an electrolyte, and then an optical communication system including the PEC‐PD and a portable electrochemical workstation as the signal receiver for data communication is built. Essentially, the self‐power PEC‐PD shows a high responsivity of −15.5 mA W–1, a fast response/recovery time of ≈100 ms, and high stability operation in the seawater electrolyte. Foremost, the optical communication system shows high accuracy in wireless transmission of ASCII code signals in a seawater environment. Such a demonstration offers a new device architecture for possible under‐seawater communication systems by leveraging the unique operation principle of photoelectrochemical devices in the future.

Ambipolar Gas Sensing Behaviors for Collection of Two-Dimensional Data from a Single Device

An electronic nose (e-nose), an array of different individual chemiresistive sensors, has been proposed to mimic olfaction system of mammals. The e-nose provides a unique pattern for each gas species, which enables one to distinguish gases and their concentrations. Therefore, it demands as much reliable data as possible to improve gas selectivity. Increasing the number of constituting sensors allows to collect massive data, thus gives more chances to reduce errors, but this suffers from the complex and high-cost fabrication. More economical solution is collecting highly-distinguishable data from each sensor, which can produce more reliable results with reduced errors. In this presentation, we propose a new concept of collecting two-dimensional response data by making a single device show ambipolar sensing behaviors, thus, both p-type (the resistance increases in oxidative gases) and n-type (the resistance decreases in oxidative gases) sensing behaviors. The proposed concept utilizes the response direction (sign of the gas response) as well as the response amplitude, differentiated from other methods that produces monotonous data from each sensor by reading only the response amplitude.Firstly, we have demonstrated the ambipolar gas sensing behaviors in diode-type device; p-type NiO/n-type SnO2 diode, and NiO/Ti/Au Schottky diode. The ambipolar behavior is attributed to the predominant modulation of heterojunction barrier. Our unique device architecture has a single well-defined heterojunction formed by vertically aligned nanostructures and electrodes in a top-bottom configuration, facilitating the heterojunction barrier modulation. We also show the ambipolar sensing behavior in an organic thin-film transistor (OTFT)-based gas sensor that can operate in both n-channel and p-channel modes depending on the gate bias. Indeed the ambipolar OTFT-based gas sensor produces two-dimensional data depending on the gate biases. Our result reveal that the proposed ambipolar sensing concept allows the collection of two-dimensional data from a single device in a reliable manner, which can significantly contribute to enhance gas selectivity of e-nose.

Microcavity polymer electroluminescent devices with solution-processed dielectric distributed Bragg reflectors utilizing inorganic copper (I) thiocyanate and insulating polymers

Abstract The concept of using printed inorganic/organic hybrid distributed Bragg reflectors (DBRs) utilizing inorganic semiconductor and insulating polymers in microcavity polymer electroluminescent devices is introduced to provide an approach to achieve the spectral narrowing and the strong forward directionality. The large refractive index contrast of approximately 0.5 (0.44) between inorganic copper(I) thiocyanate, CuSCN, and insulating polymer of poly(vinylidene fluoride-trifluoroethylene), P(VDF-TrFE) (cellulose acetate, CA) results in the fabrication of solution-processed inorganic-organic hybrid dielectric DBRs with high reflectivity (>90%) from nanostructures consisting of only four (five) bilayers. For DBRs composed of CuSCN/CA alternative dielectric layers, all-solution processed microcavity polymer light-emitting diode based on highly conductive poly(ethylenedioxythiophene):poly(styrenesulfonate) anode except for Ag cathode exhibits the narrowing of EL spectrum with a full width at half maximum of approximately 25 nm and the maximum luminance of above 10,000 cd/m2. From the viewpoint of dielectric DBRs based on ferroelectric polymer P(VDF-TrFE) with both low refractivity and high permittivity, we demonstrate a microcavity AC voltage-driven polymer electroluminescent device (μcACEL) which exhibits the spectral narrowing and the strong forward directionality. This work is anticipated to be useful for the development of solution-processed μcACEL with unique device architecture.

Unraveling the Morphology in Solution-Processed Pseudo-Bilayer Planar Heterojunction Organic Solar Cells.

The conventional bulk heterojunction (BHJ) structure is widely used for fabricating high-performance organic solar cells (OSCs) due to the nanometer-scale phase separation of the donor/acceptor component. However, the elaborate control of the BHJ morphology is difficult to carry out because the morphology evolution is such a complicated process. The compatibility requirement of materials in the same solvent restricts the structural diversity of the molecules to some extent. Meanwhile, the nanoscopic interpenetrating donor/acceptor domains reduce their crystallinity. The bilayer planar heterojunction (PHJ), by contrast, possesses complementary advantages that can make it an alternative candidate to achieve device fabrication and produce different vertical stratification in heterojunction films. However, the flat contact area limits the charge separation and transmission efficiency. The sequential solution processed approach was used to facilitate material diffusion in layers. Also, solvent additives were employed to further enhance the diffusion and thus the device performance. Nevertheless, the morphology of the formed pseudo-bilayer planar heterojunction (PPHJ) has not been fully revealed yet. Here, we carefully study the morphology of the nonfullerene-based PPHJ device in three dimensions. High hole mobility of 2.09 × 10-4 cm2 V-1 s-1 and electron mobility of 7.91 × 10-5 cm2 V-1 s-1 were obtained in the solution-processed PPHJ device. Meanwhile, a distinct phase separation size with a vertical rearrangement of donor and acceptor was observed, which enable the pseudo-bilayer devices to be equipped with a comparable spectral response to the BHJ devices. We demonstrate that a unique device architecture (ITO/ZnO/PBDB-T/ITIC/MoO3/Ag) with a power conversion efficiency of 7% can be obtained from a larger molecular weight of PBDB-T without using extra additives. The solution-processed PPHJ films have much in common with the BHJ films. The results proposed that with appropriate molecular design and vertical phase separation optimization, the performance of the solution-processed PPHJ-based OSCs can be further improved.

A New Architecture for Fibrous Organic Transistors Based on a Double-Stranded Assembly of Electrode Microfibers for Electronic Textile Applications.

Herein, a unique device architecture is proposed for fibrous organic transistors based on a double-stranded assembly of electrode microfibers for electronic textile applications. A key feature of this work is that the semiconductor channel of the fiber transistor comprises a twist assembly of the source and drain electrode microfibers that are coated by an organic semiconductor. This architecture not only allows the channel dimension of the device to be readily controlled by varying the thickness of the semiconductor layer and the twisted length of the two electrode microfibers, but also passivates the device without affecting interconnections with other electrical components. It is found that the control of crystalline nanostructure of the semiconductor layer is critical for improving both the production yield of the device and the charge-carrier transport in the device. The resulting fibrous organic transistors show a high output current of over -5 mA at a low operation voltage of -1.3 V and a good on/off current ratio of 105 . The device performance is maintained after repeated bending deformation and washing with a strong detergent solution. Application of the fibrous organic transistors to switch current-driven LED devices and detection of electrocardiography signals from a human body are demonstrated.

Organic Light-Emitting Field-Effect Transistors: Device Geometries and Fabrication Techniques.

Organic light-emitting transistors (OLETs), as novel and attractive kinds of organic electronic devices, have gained extensive attention from both academia and industry. The unique device architectures can simultaneously combine the electrical switching functionality of organic field-effect transistors and the light generation capability of organic light-emitting diodes in a single device, thereby holding great promise for reducing the complicated processes of next-generation pixel circuitry. This review involves the design, fabrication, and applications of OLETs with a comprehensive coverage of this field with the aim to give a deep insight into the intrinsic mechanisms of devices. Challenges and future prospects of OLETs are also discussed.

(Invited) Alternating Current Electroluminescence for Stimuli-Interactive Sensing Display

Development of stimuli-interactive display capable of spontaneously visualizing various external human sensible inputs has been of great interest and tremendous efforts are devoted to the visualization of nonvisible human senses such as touch, smell, taste, and sound. Field induced electroluminescence of either organic or inorganic fluorescent materials under alternating current (AC) has been extensively studied and its unique device architecture in which an emitting layer is separated with an insulator from electrode offers a new platform for designing and developing emerging stimuli-interactive displays. In this presentation, high-performance field-induced AC polymer electroluminescence (AC-PEL) devices are demonstrated with high brightness, high efficiency and color and intensity-tunability. We also present a pressure interactive AC display sensor that allows for both sensing and visualisation of pressure. Light emission upon exposure to an AC field between two electrodes is controlled by the capacitance change of the insulator arising from the pressure applied on top. Besides capacitive pressure sensing, our EL sensor allows for direct visualisation of the static and dynamic information of position, shape, and size of a pressurising object on a non-pixelated single device platform. Finally, the presentation shows that simultaneous sensing and visualization of the conductive substance is achieved when the conductive object is coupled with the light emissive material layer on our novel parallel-type AC-PEL device. A variety of conductive materials can be detected regardless of their work functions, and thus information written by a conductive pen is clearly visualized, as is a human fingerprint with natural conductivity.

Wavelength-dependent visible light response in vertically aligned nanohelical TiO2-based Schottky diodes

Enhancements in photocatalytic performance under visible light have been reported by noble metal functionalization on nanostructured TiO2; however, the non-uniform and discrete distribution of metal nanoparticles on the TiO2 surface makes it difficult to directly clarify the optical and electrical mechanisms. Here, we investigate the light absorption and the charge separation at the metal/TiO2 Schottky junctions by using a unique device architecture with an array of TiO2 nanohelixes (NHs) forming Schottky junctions both with Au-top and Pt-bottom electrodes. Wavelength-dependent photocurrent measurements through the Pt/TiO2 NHs/Au structures revealed that the origin of the visible light absorption and the separation of photogenerated carriers is the internal photoemission at the metal/nanostructured TiO2 Schottky junctions. In addition, a huge persistent photoconductivity was observed by the time-dependent photocurrent measurement, implying a long lifetime of the photogenerated carriers before recombination. We believe that the results help one to understand the role of metal functionalization on TiO2 and hence to enhance the photocatalytic efficiency by utilizing appropriately designed Schottky junctions.

Multi-bit dark state memory: Double quantum dot as an electronic quantum memory

Quantum dot clusters enable the creation of dark states which preserve electrons or holes in a coherent superposition of dot states for a long time. Various quantum logic devices can be envisioned to arise from the possibility of storing such trapped particles for future release on demand. In this work, we consider a double quantum dot memory device, which enables the preservation of a coherent state to be released as multiple classical bits. Our unique device architecture uses an external gating for storing (writing) the coherent state and for retrieving (reading) the classical bits, in addition to exploiting an internal gating effect for the preservation of the coherent state.

Highly-sensitive H2 sensor operating at room temperature using Pt/TiO2 nanoscale Schottky contacts

Despite growing demands for high performance hydrogen (H2) sensors operating at low temperature, metal oxides-based H2 sensors intrinsically require an elevated operating temperature due to a high activation energy for gas adsorption on metal oxides surface. Here, we present a highly-sensitive H2 sensor operating even at room temperature, enabled by the Pt/TiO2 nanoscale Schottky contacts which utilizes sensitive modulation of the Schottky barrier height by dissociative H2 adsorption on the catalytic Pt layer, followed by the dipole layer formation at the junction. Our device showed very high H2 response and short response and recovery times at low temperatures which are attributed to the unique device architecture having the highly-porous nanoscale Schottky contacts facilitating the gas diffusion and dissociation, and the top-and-bottom electrodes configuration making effective current modulation. Based on our results, we propose Schottky contact-assisted H2 sensing mechanisms and a promising approach to further improve H2 sensing performance at room temperature.

Modeling, Simulation, and Implementation of Solar‐Driven Water‐Splitting Devices

An integrated cell for the solar-driven splitting of water consists of multiple functional components and couples various photoelectrochemical (PEC) processes at different length and time scales. The overall solar-to-hydrogen (STH) conversion efficiency of such a system depends on the performance and materials properties of the individual components as well as on the component integration, overall device architecture, and system operating conditions. This Review focuses on the modeling- and simulation-guided development and implementation of solar-driven water-splitting prototypes from a holistic viewpoint that explores the various interplays between the components. The underlying physics and interactions at the cell level is are reviewed and discussed, followed by an overview of the use of the cell model to provide target properties of materials and guide the design of a range of traditional and unique device architectures.

Deairing Techniques for Double‐Ended Centrifugal Total Artificial Heart Implantation

The unique device architecture of the Cleveland Clinic continuous-flow total artificial heart (CFTAH) requires dedicated and specific air-removal techniques during device implantation in vivo. These procedures comprise special surgical techniques and intraoperative manipulations, as well as engineering design changes and optimizations to the device itself. The current study evaluated the optimal air-removal techniques during the Cleveland Clinic double-endedcentrifugal CFTAH in vivo implants (n = 17). Techniques and pump design iterations consisted of developing a priming method for the device and the use of built-in deairing ports in the early cases (n = 5). In the remaining cases (n = 12), deairing ports were not used. Dedicated air-removal ports were not considered an essential design requirement, and such ports may represent an additional risk for pump thrombosis. Careful passive deairing was found to be an effective measure with a centrifugal pump of this design. In this report, the techniques and design changes that were made during this CFTAH development program to enable effective residual air removal and prevention of air embolism during in vivo device implantation are explained.

Graphene-multiferroic interfaces for spintronics applications

Graphene and magnetoelectric multiferroics are promising materials for spintronic devices with high performance and low energy consumption. A very long spin diffusion length and high carrier mobility make graphene attractive for spintronics. The coupling between ferroelectricity and magnetism, which characterises magnetoelectrics, opens the way towards unique device architectures. In this work, we combine the features of both materials by investigating the interface between graphene and BaMnO3, a magnetoelectric multiferroic. We show that electron charge is transferred across the interface and magnetization is induced in the graphene sheet due to the strong interaction between C and Mn. Depending on the relative orientation of graphene and BaMnO3, a quasi-half-metal or a magnetic semiconductor can be obtained. A remarkably large proximity induced spin splitting of the Dirac cones (~300 meV) is achieved. We also show how doping with acceptors can make the high-mobility region of the electronic bands experimentally accessible. This suggests a series of possible applications in spintronics (e.g. spin filters, spin injectors) for hybrid organic-multiferroic materials and reveals hybrid organic-multiferroics as a new class of materials that may exhibit exotic phenomena such as the quantum anomalous Hall effect and a Rashba spin-orbit induced topological gap.

Ultrahigh Photogain Nanoscale Hybrid Photodetectors.

A class of ultrasensitive nanoscale hybrid photodetectors formed from carbon electrode-molecule junctions using P3HT:PCBM as photoresponsive semiconductors are demonstrated. The unique device architecture, tunability of nanoscale channel lengths and the optimized contact nature of semiconductor/electrode interfaces led to ultrahigh photogains of 1000 with graphene nanoelectrodes and 1 000 000 with single-walled carbon nanotube nanoelectrodes.

Significant enhancement of infrared photodetector sensitivity using a semiconducting single-walled carbon nanotube/C60 phototransistor.

A highly sensitive single-walled carbon nanotube/C60 -based infrared photo-transistor is fabricated with a responsivity of 97.5 A W(-1) and detectivity of 1.17 × 10(9) Jones at 1 kHz under a source/drain bias of -0.5 V. The much improved performance is enabled by this unique device architecture that enables a high photoconductive gain of ≈10(4) with a response time of several milliseconds.

InGaAs (110) Surface Cleaning Using Atomic Hydrogen

A major obstacle facing III-V semiconductor based metal oxide semiconductor field effect transistors (MOSFETs) is the large density of trap states that exist at the semiconductor/oxide interface.[1] These trap states can pin the Fermi level preventing the MOSFET from acting as a switch in logic devices. Several sources of Fermi level pinning have been proposed including oxidation of the III-V substrate.[2, 3] In order to minimize the presence of III-V oxides it is crucial to employ either an ex-situ etch or to use an in-situ method such as atomic hydrogen cleaning.[4, 5] Although atomic H cleaning of III-V surfaces is well known, it has never been demonstrated on InGaAs (110) crystallographic faces. Furthermore, tri-gate field effect transistors (finFETs) have recently been employed in commercially available logic chips.[6] This unique device architecture allows for a reduction in short channel effects, minimization of the subthreshold swing, and a higher transconductance.[7] The InGaAs (110) surface would be the sidewalls of a vertically aligned (001) based finFETs.[8] Therefore, it is essential to find an in-situ method to efficiently remove any oxides or contamination from the (110) surfaces that is also compatible with the (001) surface. In this study, STM was employed to determine if atomic hydrogen can be used to remove the native oxide from air exposed InGaAs (110) samples. A post clean anneal was used to restore the surface to molecular beam epitaxy (MBE) levels of cleanliness.

Field-effect transistors built from all two-dimensional material components.

We demonstrate field-effect transistors using heterogeneously stacked two-dimensional materials for all of the components, including the semiconductor, insulator, and metal layers. Specifically, MoS2 is used as the active channel material, hexagonal-BN as the top-gate dielectric, and graphene as the source/drain and the top-gate contacts. This transistor exhibits n-type behavior with an ON/OFF current ratio of >10(6), and an electron mobility of ∼33 cm(2)/V·s. Uniquely, the mobility does not degrade at high gate voltages, presenting an important advantage over conventional Si transistors where enhanced surface roughness scattering severely reduces carrier mobilities at high gate-fields. A WSe2-MoS2 diode with graphene contacts is also demonstrated. The diode exhibits excellent rectification behavior and a low reverse bias current, suggesting high quality interfaces between the stacked layers. In this work, all interfaces are based on van der Waals bonding, presenting a unique device architecture where crystalline, layered materials with atomically uniform thicknesses are stacked on demand, without the lattice parameter constraints. The results demonstrate the promise of using an all-layered material system for future electronic applications.

A yeast‐based rapid prototype platform for gene control elements in mammalian cells

Programming genetic circuits in mammalian cells requires flexible, tunable, and user-tailored gene-control systems. However, most existing control systems are either mechanistically specific for microbial organisms or must be laboriously re-engineered to function in mammalian cells. Here, we demonstrate a ribozyme-based device platform that can be directly transported from yeast to mammalian cells in a "plug-and-play" manner. Ribozyme switches previously prototyped in yeast are shown to regulate gene expression in a predictable, ligand-responsive manner in human HEK 293, HeLa, and U2OS cell lines without any change to device sequence nor further optimization. The ribozyme-based devices, which exhibit activation ratios comparable to the best RNA-based regulatory devices demonstrated in mammalian cells to-date, retain their prescribed functions (ON or OFF switch), tunability of regulatory stringency, and responsiveness to different small-molecule inputs in mammalian hosts. Furthermore, we observe strong correlations of device performance between yeast and all mammalian cell lines tested (R(2) = 0.63-0.97). Our unique device architecture can therefore act as a rapid prototyping platform (RPP) based on a yeast chassis, providing a well-developed and genetically tractable system that supports rapid and high-throughput screens for generating gene-controllers with a broad range of functions in mammalian cells. This platform will accelerate development of mammalian gene-controllers for diverse applications, including cell-based therapeutics and cell-fate reprogramming.