Two-Dimensional Electronics - Prospects and Challenges

During the past 10 years, two-dimensional materials have found incredible attention in the scientific community. The first two-dimensional material studied in detail was graphene, and many groups explored its potential for electronic applications. Meanwhile, researchers have extended their work to two-dimensional materials beyond graphene. At present, several hundred of these materials are known and part of them is considered to be useful for electronic applications. Rapid progress has been made in research concerning two-dimensional electronics, and a variety of transistors of different two-dimensional materials, including graphene, transition metal dichalcogenides, e.g., MoS2 and WS2, and phosphorene, have been reported. Other areas where two-dimensional materials are considered promising are sensors, transparent electrodes, or displays, to name just a few. This Special Issue of Electronics is devoted to all aspects of two-dimensional materials for electronic applications, including material preparation and analysis, device fabrication and characterization, device physics, modeling and simulation, and circuits. The devices of interest include, but are not limited to transistors (both field-effect transistors and alternative transistor concepts), sensors, optoelectronics devices, MEMS and NEMS, and displays

Inorganic micro/nanostructures-based high-performance flexible electronics for electronic skin application

Electronics in the future will be printed on diverse substrates, benefiting several emerging applications such as electronic skin (e-skin) for robotics/prosthetics, flexible displays, flexible/conformable biosensors, large area electronics, and implantable devices. For such applications, electronics based on inorganic micro/nanostructures (IMNSs) from high mobility materials such as single crystal silicon and compound semiconductors in the form of ultrathin chips, membranes, nanoribbons (NRs), nanowires (NWs) etc., offer promising high-performance solutions compared to conventional organic materials. This thesis presents an investigation of the various forms of IMNSs for high-performance electronics. Active components (from Silicon) and sensor components (from indium tin oxide (ITO), vanadium pentaoxide (V2O5), and zinc oxide (ZnO)) were realised based on the IMNS for application in artificial tactile skin for prosthetics/robotics. Inspired by human tactile sensing, a capacitive-piezoelectric tandem architecture was realised with indium tin oxide (ITO) on a flexible polymer sheet for achieving static (upto 0.25 kPa-1 sensitivity) and dynamic (2.28 kPa-1 sensitivity) tactile sensing. These passive tactile sensors were interfaced in extended gate mode with flexible high-performance metal oxide semiconductor field effect transistors (MOSFETs) fabricated through a scalable process. The developed process enabled wafer scale transfer of ultrathin chips (UTCs) of silicon with various devices (ultrathin chip resistive samples, metal oxide semiconductor (MOS) capacitors and n‐channel MOSFETs) on flexible substrates up to 4″ diameter. The devices were capable of bending upto 1.437 mm radius of curvature and exhibited surface mobility above 330 cm2/V-s, on-to-off current ratios above 4.32 decades, and a subthreshold slope above 0.98 V/decade, under various bending conditions. While UTCs are useful for realizing high-density high-performance micro-electronics on small areas, high-performance electronics on large area flexible substrates along with low-cost fabrication techniques are also important for realizing e-skin. In this regard, two other IMNS forms are investigated in this thesis, namely, NWs and NRs. The controlled selective source/drain doping needed to obtain transistors from such structure remains a bottleneck during post transfer printing. An attractive solution to address this challenge based on junctionless FETs (JLFETs), is investigated in this thesis via technology computer-aided design (TCAD) simulation and practical fabrication. The TCAD optimization implies a current of 3.36 mA for a 15 μm channel length, 40 μm channel width with an on-to-off ratio of 4.02x 107. Similar to the NRs, NWs are also suitable for realizing high performance e-skin. NWs of various sizes, distribution and length have been fabricated using various nano-patterning methods followed by metal assisted chemical etching (MACE). Synthesis of Si NWs of diameter as low as 10 nm and of aspect ratio more than 200:1 was achieved. Apart from Si NWs, V2O5 and ZnO NWs were also explored for sensor applications. Two approaches were investigated for printing NWs on flexible substrates namely (i) contact printing and (ii) large-area dielectrophoresis (DEP) assisted transfer printing. Both approaches were used to realize electronic layers with high NW density. The former approach resulted in 7 NWs/μm for bottom-up ZnO and 3 NWs/μm for top-down Si NWs while the latter approach resulted in 7 NWs/μm with simultaneous assembly on 30x30 electrode patterns in a 3 cm x 3 cm area. The contact-printing system was used to fabricate ZnO and Si NW-based ultraviolet (UV) photodetectors (PDs) with a Wheatstone bridge (WB) configuration. The assembled V2O5 NWs were used to realize temperature sensors with sensitivity of 0.03% /K. The sensor arrays are suitable for tactile e-skin application. While the above focuses on realizing conventional sensing and addressing elements for e-skin, processing of a large amount of data from e-skin has remained a challenge, especially in the case of large area skin. A Neural NW Field Effect Transistors (υ-NWFETs) based hardware-implementable neural network (HNN) approach for tactile data processing in e-skin is presented in the final part of this thesis. The concept is evaluated by interfacing with a fabricated kirigami-inspired e-skin. Apart from e-skin for prosthetics and robotics, the presented research will also be useful for obtaining high performance flexible circuits needed in many futuristic flexible electronics applications such as smart surgical tools, biosensors, implantable electronics/electroceuticals and flexible mobile phones

VLSI Design

This book provides some recent advances in design nanometer VLSI chips. The selected topics try to present some open problems and challenges with important topics ranging from design tools, new post-silicon devices, GPU-based parallel computing, emerging 3D integration, and antenna design. The book consists of two parts, with chapters such as: VLSI design for multi-sensor smart systems on a chip, Three-dimensional integrated circuits design for thousand-core processors, Parallel symbolic analysis of large analog circuits on GPU platforms, Algorithms for CAD tools VLSI design, A multilevel memetic algorithm for large SAT-encoded problems, etc

Carbon-Based Nanomaterials for (Bio)Sensors Development

Carbon-based nanomaterials have been increasingly used in sensors and biosensors design due to their advantageous intrinsic properties, which include, but are not limited to, high electrical and thermal conductivity, chemical stability, optical properties, large specific surface, biocompatibility, and easy functionalization. The most commonly applied carbonaceous nanomaterials are carbon nanotubes (single- or multi-walled nanotubes) and graphene, but promising data have been also reported for (bio)sensors based on carbon quantum dots and nanocomposites, among others. The incorporation of carbon-based nanomaterials, independent of the detection scheme and developed platform type (optical, chemical, and biological, etc.), has a major beneficial effect on the (bio)sensor sensitivity, specificity, and overall performance. As a consequence, carbon-based nanomaterials have been promoting a revolution in the field of (bio)sensors with the development of increasingly sensitive devices. This Special Issue presents original research data and review articles that focus on (experimental or theoretical) advances, challenges, and outlooks concerning the preparation, characterization, and application of carbon-based nanomaterials for (bio)sensor development

Graphene inspired sensing devices

Graphene’s exciting characteristics such as high mechanical strength, tuneable electrical prop- erties, high thermal conductivity, elasticity, large surface-to-volume ratio, make it unique and attractive for a plethora of applications including gas and liquid sensing. Adsorption, the phys- ical bonding of molecules on solid surfaces, has huge impact on the electronic properties of graphene. We use this to develop gas sensing devices with faster response time by suspending graphene over large area (cm^2) on silicon nanowire arrays (SiNWAs). These are fabricated by two-step metal-assisted chemical etching (MACE) and using a home-developed polymer-assisted graphene transfer (PAGT) process. The advantage of suspending graphene is the removal of diffusion-limited access to the adsorption sites at the interface between graphene and its support. By modifying the Langmuir adsorption model and fitting the experimental response curves, we find faster response times for both ammonia and acetone vapours. The use of suspended graphene improved the overall response, based on speed and amplitude of response, by up to 750% on average. This device could find applications in biomedical breath analysis for diseases such lung cancer, asthma, kidney failure and more. Taking advantage of the mechanical strength of graphene and using the developed PAGT process, we transfer it on commercial (CMOS) Ion-Sensitive Field-Effect Transistor (ISFET) arrays. The deposition of graphene on the top sensing layer reduces drift that results from the surface modification during exposure to electrolyte while improving the overall performance by up to about 10^13 % and indicates that the ISFET can operate with metallic sensing membrane and not only with insulating materials as confirmed by depositing Au on the gate surface. Post- processing of the ISFET top surface by reactive ion plasma etching, proved that the physical location of trapped charge lies within the device structure. The process improved its overall performance by about 105 %. The post-processing of the ISFET could be applied for sensor performance in any of its applications including pH sensing for DNA sequencing and glucose monitoring.Open Acces

Graphene field-effect devices at high frequencies

In this work, the high frequency response of several types of graphene field-effect transistors (GFET) is analyzed. In the first part, insulating substrates such as sapphire and hexagonal boron nitride are used to optimize device performance. In the second part, few-layer graphene is used as gate material to obtain ultra-thin GFET. Using large area CVD-grown graphene, an array of similar GFETs for improved device comparability and reproducibility is presented in the last part

Novel Two-dimensional Nanomaterials and Their Gas Sensing Properties

Graphene, an atomic thin two-dimensional (2D) material with C atoms arranged in a honeycomb lattice, has sparked an unprecedented research interest across various scientific communities since its initial mechanical isolation in 2004. The linear energy dispersion with respect to the momentum within 1 eV around the Fermi level at the high symmetric K (Dirac) points in the Brillouin zone renders graphene a wonder material for scientists. However, graphene’s semimetallic nature significantly limits its high-end applications, e.g., in digital logic circuits. Therefore, continued efforts in opening the band gap for graphene and in searching for novel 2D semiconducting materials are rewarding. Various methods have been proposed for generating band gaps in graphene and other related 2D nanomaterials; however, few can be utilized to tune the band gap over a wide range on the same device and many are realized at the cost of severe degradation of carrier mobility. Recently, a new graphene-based crystalline structure, graphene monoxide (GMO), has been discovered based on electron diffraction observations during in situ thermal reduction of multilayer graphene oxide (GO) under vacuum in a transmission electron microscope (TEM) chamber. Supported by infrared spectroscopy and first-principles calculations, the new 2D material was identified as a two-phase hybrid containing GMO domains that evolve in the graphene matrix. GMO extends the electronic property of a graphene derivative into the semiconductor world, enabling potential applications for nanoelectronics. Another route to address the graphene band gap bottleneck is to search for new 2D nanomaterial candidates, among which 2D transition metal dichalcogenides (e.g., MoS2) and black phosphorus (BP) are attracting significant attention. Although both are layered structures and have a tunable band gap, a higher carrier mobility and a wider band gap ranging from 0.3 eV for bulk-like BP to 1.8 eV of monolayer BP make BP an outstanding candidate for future electronic applications. Conductance-based nanoscale gas sensors based on these 2D nanomaterials are attractive due to their superior sensitivity/selectivity and relatively low cost. Experimental studies have shown that in general semiconducting materials exhibit better sensitivity than insulating/metallic materials. Thus, it is crucial to understand the gas sensing mechanism of semiconducting materials and to gain better insights into the performance enhancement. This thesis aims to explore the fundamental properties of novel 2D nanomaterials and to understand their gas sensing performance. Various GMO properties were calculated using density functional theory (DFT)-based techniques. Infrared (IR) spectra of GMO were calculated for both pure GMO and GMO domains embedded into the graphene matrix to facilitate its identification during formation. GMO has three IR active modes that are distinctive from those of graphene and GO. The electronic and mechanical properties of GMO were predicted to illuminate its potential applications in semiconductor devices. The band gap of GMO can be tuned over a wide range from 0 to 1.35 eV. The capability of heat removal in intrinsic GMO was also simulated with and without planar lattice strains and compared with that of graphene and silicon. GMO exhibits a superior thermal conductivity (\u3e3,000 Wm-1K-1), 80% of that of graphene along the armchair direction for large lateral sample sizes (\u3e5 µm). The magnetic properties of zigzag graphene nanoribbons (ZGNRs) induced by GMO domains (or epoxy pair chains) were investigated. The epoxy pair chains can generate finite spin moments in ZGNRs irrespective of the spin coupling between ribbon edges. The gas sensing properties of selected 2D nanomaterials were characterized both theoretically and experimentally. First, we developed statistical thermodynamics models with the gas binding energy from DFT calculations as the only input to characterize the monolayer gas adsorption density on graphene and BP thin films. Our statistical thermodynamics models can successfully predict the gas adsorption density with high accuracy compared with experimental data. Second, an analytical model was established to interpret why semiconducting materials are preferred for gas sensing applications using a BP thin film-based gas sensors as an example. The sensitivity model suggests that the optimum thickness of BP thin film is from several to 10+ nm, corresponding with a band gap of 0.3 to 0.6 eV. Third, van der Pauw and Hall measurements were performed to obtain the sheet resistance, the carrier concentration, and the carrier mobility for thermally-reduced GO (TRGO) at various temperatures to illuminate relative contributions from the carrier concentration and the carrier mobility to the sheet resistance change upon gas adsorption, which suggests that the conductance change upon gas adsorption mainly results from the carrier concentration change. Finally, the sensitivity enhancement from the nanocrystalline particles deposited on the surface of graphene-base materials was also investigated