Performance limit of all-wrapped monolayer MoS2 transistors

All-wrapped transistors consisting of two-dimensional transition-metal dichalcogenide channels are appealing candidates for post-silicon electronics. Based on the Boltzmann transport theory, here we report a comprehensive theoretical survey on the performance limits for monolayer MoS2 transistors with three prototypical gate dielectrics (Al2O3, HfO2 and BN), by including primary extrinsic charge scattering mechanisms present in practical devices. A concept of "dead space" between the dielectrics and channels is proposed and used in calculation to ameliorate the general overestimation in scattering intensity of surface optical phonons, which enables an accurate description of electronic transport behavior. Crucial device indices, including charge mobility and current density, are thoroughly analyzed for transistors at post-silicon technological nodes beyond 1 nm. The on-state current is estimated to be generally greater than 2 mA μm-1 at channel lengths below 10 nm. The results clarify the potential benefits in performance from extremely miniaturized monolayer-channel transistors for More-Moore electronics.

Wafer-Scale Epitaxial Growth of an Atomically Thin Single-Crystal Insulator as a Substrate of Two-Dimensional Material Field-Effect Transistors

As the electron mobility of two-dimensional (2D) materials is dependent on an insulating substrate, the nonuniform surface charge and morphology of silicon dioxide (SiO₂) layers degrade the electron mobility of 2D materials. Here, we demonstrate that an atomically thin single-crystal insulating layer of silicon oxynitride (SiON) can be grown epitaxially on a SiC wafer at a wafer scale and find that the electron mobility of graphene field-effect transistors on the SiON layer is 1.5 times higher than that of graphene field-effect transistors on typical SiO₂ films. Microscale and nanoscale void defects caused by heterostructure growth were eliminated for the wafer-scale growth of the single-crystal SiON layer. The single-crystal SiON layer can be grown on a SiC wafer with a single thermal process. This simple fabrication process, compatible with commercial semiconductor fabrication processes, makes the layer an excellent replacement for the SiO₂/Si wafer.

Towards detection of biomarkers in the eye using an aptamer-based graphene affinity nanobiosensor

We present an approach to enable the sensitive and specific detection of biomarkers in undiluted tears in the eye using an aptamer-based graphene affinity nanosensor. The nanosensor is a graphene field-effect transistor, in which a nucleic acid aptamer and a biomolecule-permeable polyethylene glycol (PEG) nanolayer are immobilized on the graphene surface. The aptamer is capable of specifically recognize the target biomarker and induce a change in the carrier concentration of the graphene, which is measured to determine the biomarker concentration. The PEG nanolayer minimizes nonspecific adsorption of background molecules in the sample that would otherwise interfere with the biomarker detection. Experimental results show that tumor necrosis factor alpha (TNF-alpha), an inflammatory cytokine, can be sensitively and specifically detected in undiluted artificial tears with a limit of detection of 0.34 pM. This ability to detect and measure biomarkers in undiluted physiological fluids allows the nanosensor to be potentially used in applications where sample dilutions are not practical, such as wearable measurements of tear-borne biomarkers in the eye.

Growth of Tellurium Nanobelts on h-BN for p-type Transistors with Ultrahigh Hole Mobility

The lack of stable p-type van der Waals (vdW) semiconductors with high hole mobility severely impedes the step of low-dimensional materials entering the industrial circle. Although p-type black phosphorus (bP) and tellurium (Te) have shown promising hole mobilities, the instability under ambient conditions of bP and relatively low hole mobility of Te remain as daunting issues. Here we report the growth of high-quality Te nanobelts on atomically flat hexagonal boron nitride (h-BN) for high-performance p-type field-effect transistors (FETs). Importantly, the Te-based FET exhibits an ultrahigh hole mobility up to 1370 cm² V^-1 s^-1 at room temperature, that may lay the foundation for the future high-performance p-type 2D FET and metal-oxide-semiconductor (p-MOS) inverter. The vdW h-BN dielectric substrate not only provides an ultra-flat surface without dangling bonds for growth of high-quality Te nanobelts, but also reduces the scattering centers at the interface between the channel material and the dielectric layer, thus resulting in the ultrahigh hole mobility .

Resist-Free Lithography for Monolayer Transition Metal Dichalcogenides

Photolithography and electron-beam lithography are the most common methods for making nanoscale devices from semiconductors. While these methods are robust for bulk materials, they disturb the electrical properties of two-dimensional (2D) materials, which are highly sensitive to chemicals used during lithography processes. Here, we report a resist-free lithography method, based on direct laser patterning and resist-free electrode transfer, which avoids unintentional modification to the 2D materials throughout the process. We successfully fabricate large arrays of field-effect transistors using MoS₂ and WSe₂ monolayers, the performance of which reflects the properties of the pristine materials. Furthermore, using these pristine devices as a reference, we reveal that among the various stages of a conventional lithography process, exposure to a solvent like acetone changes the electrical conductivity of MoS₂ the most. This new approach will enable a rational design of reproducible processes for making large-scale integrated circuits based on 2D materials and other surface-sensitive materials.

Waterproof, flexible field-effect transistors with submicron monocrystalline Si nanomembrane derived encapsulation for continuous pH sensing

To understand the physio-pathological state of patients suffering from chronic diseases, scientists and clinicians need sensors to track chemical signals in real-time. However, the lack of stable, safe, and scalable biochemical sensing platforms capable of continuous operation in liquid environments imposes significant challenges in the timely diagnosis, intervention, and treatment of chronic conditions. This work reports a novel strategy for fabricating waterproof and flexible biochemical sensors with active electronic components, which feature a submicron encapsulation layer derived from monocrystalline Si nanomembranes with a high structural integrity due to the high formation temperature (>1000 °C). The ultrathin, yet dense and low-defect encapsulation enables continuous operation of field-effect transistors in biofluids for chemical sensing. The excellent stability in liquid environment and pH sensing performance of such transistors suggest their great potential as the foundation of waterproof and scalable biochemical sensors with active functionalities in the future. The understandings, knowledge base, and demonstrations for pH sensing reported here set the stage for the next generation long-term biosensing with a broad applicability in biomedical research, food science, and advanced healthcare.

Applications of 2D-Layered Palladium Diselenide and Its van der Waals Heterostructures in Electronics and Optoelectronics

The rapid development of two-dimensional (2D) transition-metal dichalcogenides has been possible owing to their special structures and remarkable properties. In particular, palladium diselenide (PdSe₂) with a novel pentagonal structure and unique physical characteristics have recently attracted extensive research interest. Consequently, tremendous research progress has been achieved regarding the physics, chemistry, and electronics of PdSe₂. Accordingly, in this review, we recapitulate and summarize the most recent research on PdSe₂, including its structure, properties, synthesis, and applications. First, a mechanical exfoliation method to obtain PdSe₂ nanosheets is introduced, and large-area synthesis strategies are explained with respect to chemical vapor deposition and metal selenization. Next, the electronic and optoelectronic properties of PdSe₂ and related heterostructures, such as field-effect transistors, photodetectors, sensors, and thermoelectric devices, are discussed. Subsequently, the integration of systems into infrared image sensors on the basis of PdSe₂ van der Waals heterostructures is explored. Finally, future opportunities are highlighted to serve as a general guide for physicists, chemists, materials scientists, and engineers. Therefore, this comprehensive review may shed light on the research conducted by the 2D material community.

Gamma radiation effects on diamond field-effect biosensors with fibroblasts and extracellular matrix

Due to high biocompatibility, miniaturization, optical transparency and low production cost together with high radiation hardness the diamond-based sensors are considered promising for radiation medicine and biomedicine in general. Here we present detection of fibroblast cell culture properties by nanocrystalline diamond solution-gated field-effect transistors (SG-FET), including effects of gamma irradiation. We show that blank nanocrystalline diamond field-effect biosensors are stable at least up to 300 Gy of γ irradiation. On the other hand, gate current of the diamond SG-FET biosensors with fibroblastic cells increases exponentially over an order of magnitude with increasing radiation dose. Extracellular matrix (ECM) formation is also detected and analyzed by correlation of electronic sensor data with optical, atomic force, fluorescence, and scanning electron microscopies.

An integrated microfluidic system with field-effect-transistor sensor arrays for detecting multiple cardiovascular biomarkers from clinical samples

Certain blood-borne biomarkers offer a potent methodology for understanding the risk of cardiovascular diseases (CVDs) with clinicians generally advocating the use of multiple biomarkers for proper risk assessment of CVDs. Herein four such CVDs biomarkers- C-reactive protein (CRP), N-terminal pro b-type natriuretic peptide (NT-proBNP), cardiac troponin I (cTnI), and fibrinogen- were rapidly (5 min) analyzed from clinical samples (~ 4 µL) on an integrated microfluidic platform equipped with 1) immobilized highly specific aptamer probes and 2) field-effect transistor (FET)-based sensor arrays. The calibration curve from the FET sensor arrays showed good agreement in the physiological concentration ranges for CRP (0.1-50 mg/L), NT-proBNP (50-10,000 pg/mL), cTnI (1-10,000 pg/mL), and fibrinogen (0.1-5 mg/mL). The developed prototype of this fully automated portable device requires minimal reagent and sample inputs and consequently shows great promise for next-generation point-of-care devices assaying multiple CVDs biomarkers in clinical samples.

Recent advances in nanowires-based field-effect transistors for biological sensor applications

Nanowires (NWs)-based field-effect transistors (FETs) have attracted considerable interest to develop innovative biosensors using NWs of different materials (i.e. semiconductors, polymers, etc.). NWs-based FETs provide significant advantages over the other bulk or non-NWs nanomaterials-based FETs. As the building blocks for FET-based biosensors, one-dimensional NWs offer excellent surface-to-volume ratio and are more suitable and sensitive for sensing applications. During the past decade, FET-based biosensors are smartly designed and used due to their great specificity, sensitivity, and high selectivity. Additionally, they have the advantage of low weight, low cost of mass production, small size and compatible with commercial planar processes for large-scale circuitry. In this respect, we summarize the recent advances of NWs-based FET biosensors for different biomolecule detection i.e. glucose, cholesterol, uric acid, urea, hormone, proteins, nucleotide, biomarkers, etc. A comparative sensing performance, present challenges, and future prospects of NWs-based FET biosensors are discussed in detail.

Multi-Layer SnSe Nanoflake Field-Effect Transistors with Low-Resistance Au Ohmic Contacts

We report p-type tin monoselenide (SnSe) single crystals, grown in double-sealed quartz ampoules using a modified Bridgman technique at 920 °C. X-ray powder diffraction (XRD) and energy dispersive X-ray spectroscopy (EDX) measurements clearly confirm that the grown SnSe consists of single-crystal SnSe. Electrical transport of multi-layer SnSe nanoflakes, which were prepared by exfoliation from bulk single crystals, was conducted using back-gated field-effect transistor (FET) structures with Au and Ti contacts on SiO₂/Si substrates, revealing that multi-layer SnSe nanoflakes exhibit p-type semiconductor characteristics owing to the Sn vacancies on the surfaces of SnSe nanoflakes. In addition, a strong carrier screening effect was observed in 70-90-nm-thick SnSe nanoflake FETs. Furthermore, the effect of the metal contacts to multi-layer SnSe nanoflake-based FETs is also discussed with two different metals, such as Ti/Au and Au contacts.

Enzyme-polyelectrolyte multilayer assemblies on reduced graphene oxide field-effect transistors for biosensing applications

We present the construction of layer-by-layer (LbL) assemblies of polyethylenimine and urease onto reduced-graphene-oxide based field-effect transistors (rGO FETs) for the detection of urea. This versatile biosensor platform simultaneously exploits the pH dependency of liquid-gated graphene-based transistors and the change in the local pH produced by the catalyzed hydrolysis of urea. The use of an interdigitated microchannel resulted in transistors displaying low noise, high pH sensitivity (20.3µA/pH) and transconductance values up to 800 µS. The modification of rGO FETs with a weak polyelectrolyte improved the pH response because of its transducing properties by electrostatic gating effects. In the presence of urea, the urease-modified rGO FETs showed a shift in the Dirac point due to the change in the local pH close to the graphene surface. Markedly, these devices operated at very low voltages (less than 500mV) and were able to monitor urea in the range of 1-1000µm, with a limit of detection (LOD) down to 1µm, fast response and good long-term stability. The urea-response of the transistors was enhanced by increasing the number of bilayers due to the increment of the enzyme surface coverage onto the channel. Moreover, quantification of the heavy metal Cu²⁺(with a LOD down to 10nM) was performed in aqueous solution by taking advantage of the urease specific inhibition.

A potentiometric biosensor for rapid on-site disease diagnostics

Quantitative point-of-care (POC) devices are the next generation for serological disease diagnosis. Whilst pathogen serology is typically performed by centralized laboratories using Enzyme-Linked ImmunoSorbent Assay (ELISA), faster on-site diagnosis would infer improved disease management and treatment decisions. Using the model pathogen Bovine Herpes Virus-1 (BHV-1) this study employs an extended-gate field-effect transistor (FET) for direct potentiometric serological diagnosis. BHV-1 is a major viral pathogen of Bovine Respiratory Disease (BRD), the leading cause of economic loss ($2 billion annually in the US only) to the cattle and dairy industry. To demonstrate the sensor capabilities as a diagnostic tool, BHV-1 viral protein gE was expressed and immobilized on the sensor surface to serve as a capture antigen for a BHV-1-specific antibody (anti-gE), produced in cattle in response to viral infection. The gE-coated immunosensor was shown to be highly sensitive and selective to anti-gE present in commercially available anti-BHV-1 antiserum and in real serum samples from cattle with results being in excellent agreement with Surface Plasmon Resonance (SPR) and ELISA. The FET sensor is significantly faster than ELISA (<10 min), a crucial factor for successful disease intervention. This sensor technology is versatile, amenable to multiplexing, easily integrated to POC devices, and has the potential to impact a wide range of human and animal diseases.

Dual aptamer-immobilized surfaces for improved affinity through multiple target binding in potentiometric thrombin biosensing

We developed a label-free and reagent-less potentiometric biosensor with improved affinity for thrombin. Two different oligomeric DNA aptamers that can recognize different epitopes in thrombin were introduced in parallel or serial manners on the sensing surface to capture the target via multiple contacts as found in many biological systems. The spacer and linker in the aptamer probes were optimized for exerting the best performance in molecular recognition. To gain the specificity of the sensor to the target, an antifouling molecule, sulfobeaine-3-undecanethiol (SB), was introduced on the sensor to form a self-assembled monolayer (SAM). Surface characterization revealed that the aptamer probe density was comparable to the distance of the two epitopes in thrombin, while the backfilling SB SAM was tightly aligned on the surface to resist nonspecific adsorption. The apparent binding parameters were obtained by thrombin sensing in potentiometry using the 1:1 Langmuir adsorption model, showing the improved dissociation constants (Kd) with the limit of detection of 5.5 nM on the dual aptamer-immobilized surfaces compared with single aptamer-immobilized ones. A fine control of spacer and linker length in the aptamer ligand was essential to realize the multivalent binding of thrombin on the sensor surface. The findings reported herein are effective for improving the sensitivity of potentiometric biosensor in an affordable way towards detection of tiny amount of biomolecules.

Influence of post-annealing on the off current of MoS2 field-effect transistors

Two-dimensional materials have recently been spotlighted, due to their unique properties in comparison with conventional bulk and thin-film materials. Among those materials, MoS2 is one of the promising candidates for the active layer of electronic devices because it shows high electron mobility and pristine band gap. In this paper, we focus on the evolution of the electrical property of the MoS2 field-effect transistor (FET) as a function of post-annealing temperature. The results indicate that the off current drastically decreased at 200°C and increased at 400°C while other factors, such as the mobility and threshold voltage, show little variation. We consider that the decreasing off current comes from the rearrangement of the MoS2 film and the elimination of the surface residue. Then, the increasing off current was caused by the change of the material's composition and adsorption of H2O and O2.

Human T cells monitored by impedance spectrometry using field-effect transistor arrays: a novel tool for single-cell adhesion and migration studies

Cytotoxic T lymphocytes (CTLs) play an important role in the immune system by recognizing and eliminating pathogen-infected and tumorigenic cells. In order to achieve their function, T cells have to migrate throughout the whole body and identify the respective targets. In conventional immunology studies, interactions between CTLs and targets are usually investigated using tedious and time-consuming immunofluorescence imaging. However, there is currently no straightforward measurement tool available to examine the interaction strengths. In the present study, adhesion strengths and migration of single human CD8(+) T cells on pre-coated field-effect transistor (FET) devices (i.e. fibronectin, anti-CD3 antibody, and anti-LFA-1 antibody) were measured using impedance spectroscopy. Adhesion strengths to different protein and antibody coatings were compared. By fitting the data to an electronically equivalent circuit model, cell-related parameters (cell membrane capacitance referring to cell morphology and seal resistance referring to adhesion strength) were obtained. This electronically-assessed adhesion strength provides a novel, fast, and important index describing the interaction efficiency. Furthermore, the size of our detection transistor gates as well as their sensitivity reaches down to single cell resolution. Real-time motions of individually migrating T cells can be traced using our FET devices. The in-house fabricated FETs used in the present study are providing a novel and very efficient insight to individual cell interactions.