Carbon nanotube field effect transistors with suspended graphene gates

Novel field effect transistors with suspended graphene gates are demonstrated. By incorporating mechanical motion of the gate electrode, it is possible to improve the switching characteristics compared to a static gate, as shown by a combination of experimental measurements and numerical simulations. The mechanical motion of the graphene gate is confirmed by using atomic force microscopy to directly measure the electrostatic deflection. The device geometry investigated here can also provide a sensitive measurement technique for detecting high-frequency motion of suspended membranes as required, e.g., for mass sensing.     

Current saturation and voltage gain in bilayer graphene field effect transistors

The emergence of graphene with its unique electrical properties has triggered hopes in the electronic devices community regarding its exploitation as a channel material in field effect transistors. Graphene is especially promising for devices working at frequencies in the 100 GHz range. So far, graphene field effect transistors (GFETs) have shown cutoff frequencies up to 300 GHz, while exhibiting poor voltage gains, another important figure of merit for analog high frequency applications. In the present work, we show that the voltage gain of GFETs can be improved significantly by using bilayer graphene, where a band gap is introduced through a vertical electric displacement field. At a displacement field of -1.7 V/nm the bilayer GFETs exhibit an intrinsic voltage gain up to 35, a factor of 6 higher than the voltage gain in corresponding monolayer GFETs. The transconductance, which limits the cutoff frequency of a transistor, is not degraded by the displacement field and is similar in both monolayer and bilayer GFETs. Using numerical simulations based on an atomistic p(z) tight-binding Hamiltonian we demonstrate that this approach can be extended to sub-100 nm gate lengths.     

Performance of an integrated microoptical system for fluorescence detection in microfluidic systems

This article presents a new integrated microfluidic/microoptic device designed for basic biochemical analysis. The microfluidic network is wet-etched in a Borofloat 33 (Pyrex) glass wafer and sealed by means of a second wafer. Unlike other similar microfluidic systems, elements of the detection system are realized with the help of microfabrication techniques and directly deposited on both sides of the microchemical chip. The detection system is composed of the combination of refractive circular or elliptical microlens arrays and chromium aperture arrays. The microfluidic channels are 60 microm wide and 25 microm deep. The elliptical microlenses have a major axis of 400 microm and a minor axis of 350 microm. The circular microlens diameters range from 280 microm to 350 microm. The apertures deposited on the outer chip surfaces are etched in a 3000-A-thick chromium layer. The overall thickness of this microchemical system is < 1.6 mm. A limit of detection of 3.3 nM for a Cy5 solution in phosphate buffer (pH 7.4) was demonstrated. The cross-talk signal measured between two adjacent microchannels with 1 mm pitch was < 1:5600, meaning that < or = 1.8 x 10(-4)% of the fluorescence light power emitted from one microchannel filled with a 50 microM Cy5 solution reaches the photodetector at the adjacent microchannel. This performance compares very well with that obtainable in microchemical chips using confocal fluorescence systems, taking differences in parameters, such as excitation power into microchannels, data acquisition rates, and signal filtering into account.     

High-throughput microfluidic systems for disease detection

Accuracy and rapid response are critical to the detection of an acute infectious disease, not only because the detection results can affect the medical treatment, but also can prevent disease outbreaks. Since the current culture-based technology is time consuming and experience dependent, academia and industrial researchers are using microfluidics and nucleic acids as the fundamental ideas to build pioneering tools against infectious disease. While many point-of-care microfluidic systems have been realized to execute nucleic acid applications, high-throughput microfluidic systems are under development for various nucleic acid applications because of high efficiency and demand from the market. Building a high-throughput system is an interdisciplinary challenge because of the design concerns from science and the manufacturing concerns from engineering, but its realization will be a milestone. This article is aimed to review three essential steps of the nucleic acid-based detection realized in high-throughput formats, including polymerase chain reaction, capillary electrophoresis, and nucleic acid purification.     

High on/off ratios in bilayer graphene field effect transistors realized by surface dopants

The unique property of bilayer graphene to show a band gap tunable by external electrical fields enables a variety of different device concepts with novel functionalities for electronic, optoelectronic, and sensor applications. So far the operation of bilayer graphene-based field effect transistors requires two individual gates to vary the channel's conductance and to create a band gap. In this paper, we report on a method to increase the on/off ratio in single gated bilayer graphene field effect transistors by adsorbate doping. The adsorbate dopants on the upper side of the graphene establish a displacement field perpendicular to the graphene surface breaking the inversion symmetry of the two graphene layers. Low-temperature measurements indicate that the increased on/off ratio is caused by the opening of a mobility gap.     

Combinatorial detection of volatile organic compounds using metal-phthalocyanine field effect transistors

We apply percolation theory to explain the operation of multiple-use gas sensors based on organic field effect transistors (OFETs). For reversible operation, we predict that energetic disorder in the channel can obscure interactions with the analyte, because chemically induced traps are overwhelmed by the natural disorder. Consequently, the sensitivity of an energetically disordered OFET-based chemical sensor is significantly inferior to the ideal disorder-free case. Current modulation in disordered OFETs is predicted to rely on morphological alteration of percolation paths. The theory is compared to results from an array of metal phthalocyanine (MPC) transistors exposed to low concentrations of solvents. Despite the presence of very large adsorption fractions of solvent on the channel, the current modulation is small, consistent with theory. Chemical selectivity is possible, however, because the central metal atom of the MPC determines the strength of the solvent-MPC interaction, which in turn determines the amount of solvent adsorbed on the OFET channel. This work suggests that OFET-based sensors may be better suited to applications where the analyte binding energy exceeds the intrinsic energetic disorder of the organic semiconductor.     

Mechanical strain and electric-field modulation of graphene transistors integrated on MEMS cantilevers

Journal of Materials Science - This work proposes a structure which allows characterization of graphene monolayers under combined electric field and mechanical strain modulation. Our approach is...     

Multiplexed spectral signature detection for microfluidic color-coded bioparticle flow

Here, we report a high-speed photospectral detection technique capable of discriminating subtle variations of spectral signature among fluorescently labeled cells and microspheres flowing in a microfluidic channel. The key component used in our study is a strain-tunable nanoimprinted grating microdevice coupled with a photomultiplier tube (PMT). The microdevice permits acquisition of the continuous spectral profiles of multiple fluorescent emission sources at 1 kHz. Optically connected to a microfluidic flow chamber via a multimode optical fiber, our multiwavelength detection platform allows for cytometric measurement of cell groups emitting nearly identical fluorescence signals with a maximum emission wavelength difference as small as 5 nm. The same platform also allows us to demonstrate microfluidic flow cytometry of four different microsphere types in a wavelength bandwidth as narrow as 40 nm at a high (>85%) confidence level. Our study shows that detection of fluorescent spectral signatures at high speed and high resolution can expand specificity of multicolor flow cytometry. The enhanced capability enables multiplexed analysis of color-coded bioparticles based on single-laser excitation and single-detector spectroscopy in a microfluidic setting. The fluorescence signal discrimination power achieved by the optofluidic technology holds great promise to enable quantification of cellular parameters with higher accuracy as well as enumeration of a larger number of cell types than conventional flow cytometric methods.     

Geometrical effect in submicrometer channel organic field effect transistors

The electrical behaviors of submicrometer bottom-gate bottom-contact organic field effect transistors (OFETs) with submicrometer channel lengths and channel widths were investigated. Short-channel effects (SCEs) were observed for devices with shorter channel lengths and wider channel widths. The SCEs were effectively suppressed by reducing the channel width to 50 nm. The relationship between the drain current density and the drain voltage normalized by their respective channel lengths revealed that the drain current characteristics of shorter length channels fall into two types: parasitic contact resistances at lower drain voltage and SCEs caused by the space charge limiting current at higher drain voltages. The carrier mobility was also investigated, and found to be enhanced in the narrower channel width.     

Thiophene-based molecular and polymeric semiconductors for organic field effect transistors and organic thin film transistors

Organic electronics has been a popular field for the last two decades, due to its potential to commercialize cheap-price and large-area flexible electronics. The devices based on organic compounds heavily rely on organic semiconductors (OSs). Primary challenge for materials chemist is the new OSs construction that has ameliorated attainment in organic thin film transistors (OTFTs) and organic field effect transistors (OFETs). The construction of air-stable (stable in air) n-channel OSs (electron-conducting materials) is particularly needed with capability comparable to that of p-channel materials (hole-conducting materials). In the last 10 years, there have been significant advancements in thiophene-based OSs. Thiophene-mediated molecules have a prominent role in the advancement of OSs. The main significance in thiophene-based molecules is their cheap-price (in comparison to silicon), processability at low temperature, structural flexibility, ability to be applied on flexible substrates, and high charge transport characteristics. In this paper, we review the progress in the performance of thiophene-based OSs that has been reported in the last 18 years, with a major emphasis on the last 10 years. This approach provides a crisp introduction to organic devices and catalogs progress toward the fabrication of thiophene containing p, n and ambipolar channel OSs, and discusses their characteristics. Finally, review discusses current challenges and future research directions for thiophene based OSs. This review would be beneficial for further developments in the technological performance. Moreover, this review will serve to accelerate knowledge and lays the foundation for improved applications. Hopefully, this struggle pushes the reader’s mind to consider new perspectives, think differently and forge new connections.     

An integrated fluorescence detection system in poly(dimethylsiloxane) for microfluidic applications

This paper describes a prototype of an integrated fluorescence detection system that uses a microavalanche photodiode (microAPD) as the photodetector for microfluidic devices fabricated in poly(dimethylsiloxane) (PDMS). The prototype device consisted of a reusable detection system and a disposable microfluidic system that was fabricated using rapid prototyping. The first step of the procedure was the fabrication of microfluidic channels in PDMS and the encapsulation of a multimode optical fiber (100-microm core diameter) in the PDMS; the tip of the fiber was placed next to the side wall of one of the channels. The optical fiber was used to couple light into the microchannel for the excitation of fluorescent analytes. The photodetector, a prototype solid-state microAPD array, was embedded in a thick slab (1 cm) of PDMS. A thin (80 microm) colored polycarbonate filter was placed on the top of the embedded microAPD to absorb scattered excitation light before it reached the detector. The microAPD was placed below the microchannel and orthogonal to the axis of the optical fiber. The close proximity (approximately 200 microm) of the microAPD to the microchannel made it unnecessary to incorporate transfer optics; the pixel size of the microAPD (30 microm) matched the dimensions of the channels (50 microm). A blue light-emitting diode was used for fluorescence excitation. The microAPD was operated in Geiger mode to detect the fluorescence. The detection limit of the prototype (approximately 25 nM) was determined by finding the minimum detectable concentration of a solution of fluorescein. The device was used to detect the separation of a mixture of proteins and small molecules by capillary electrophoresis; the separation illustrated the suitability of this integrated fluorescence detection system for bioanalytical applications.     

Apta-biosensors for nonlabeled real time detection of human IgE based on carbon nanotube field effect transistors

In this study, we demonstrated the aptamer-based biosensor (apta-biosensor) using CNT-FET devices for label free detection of allergy diagnosis by IgE detection. In order to detect the IgE, two kinds of receptor (monoclonal IgE antibody and anti-IgE aptamer)-modified CNT-FET devices were fabricated. The binding event of the target IgE onto receptors was detected by monitoring the gating effect caused by the charges of the target proteins. Since the CNT-FET biosensors were used in buffer solution, it was crucial to use small-size receptors like aptamers than whole antibodies so that the charged target IgE could approach the CNT surface within the Debye length distance to give a large gating effect. The results show that CNT-FET biosensors using monoclonal IgE antibody had very low sensitivity (minimum detectable level 1000 ng/mL), while those based on anti-IgE aptamer could detect 50 ng/mL. Moreover, the aptamer-modified CNT-FET herein could successfully block non-target proteins and could selectively detect the target protein in an environment similar to human serum electrolyte. Therefore, aptamer-based CNT-FET devices enable the production of label-free ultrasensitive electronic biosensors to detect clinically important biomarkers for disease diagnosis.     

Gate coupling and charge distribution in nanowire field effect transistors

We have modeled the field and space charge distributions in back-gate and top-gate nanowire field effect transistors by solving the three-dimensional Poisson's equation numerically. It is found that the geometry of the gate oxide, the semiconductivity of the nanowire, and the finite length of the device profoundly affect both the total amount and the spatial distribution of induced charges in the nanowire, in stark contrast to the commonly accepted picture where metallic dielectric properties and infinite length are assumed for the nanowire and the specific geometry of the gate oxide is neglected. We provide a comprehensive set of numerical correction factors to the analytical capacitance formulas, as well as to numerical calculations that neglect the semiconductivity and finite length of the nanowire, that are frequently used for quantifying carrier transport in nanowire field effect transistors.     

Decorrelation-based blood flow velocity estimation: effect of spread of flow velocity,linear flow velocity gradients,and parabolic flow

In recent years, a new method to measure transverse blood flow, based on the decorrelation of the radio frequency (RF) signals has been developed. In this paper, we investigated the influence of nonuniform flow on the velocity estimation. The decorrelation characteristics of transverse blood flow using an intravascular ultrasound (IVUS) array catheter are studied by means of computer modeling. Blood was simulated as a collection of randomly located point scatterers; moving this scattering medium transversally across the acoustical beam represented flow. First-order statistics were evaluated, and the signal-to-noise ratio from the signals were measured. The correlation coefficient method was used to present the results. Three velocity profiles were simulated: random spread of blood-flow velocity, linear blood-flow velocity gradient, and parabolic blood-flow. Radio frequency and envelope signals were used to calculate the decorrelation pattern. The results were compared to the mean decorrelation pattern for plug blood-flow. The RF signals decorrelation patterns were in good agreement with those obtained for plug blood flow. Envelope decorrelation patterns show a close agreement with the one for plug blood flow. For axial blood flow, there is a discrepancy between decorrelation patterns. The results presented here suggest that the decorrelation properties of an IVUS array catheter for measuring quantitative transverse blood flow probably will not be affected by different transverse blood-flow conditions     

Insulating behavior in ultrathin bismuth selenide field effect transistors

Ultrathin (approximately three quintuple layer) field-effect transistors (FETs) of topological insulator Bi(2)Se(3) are prepared by mechanical exfoliation on 300 nm SiO(2)/Si susbtrates. Temperature- and gate-voltage-dependent conductance measurements show that ultrathin Bi(2)Se(3) FETs are n-type and have a clear OFF state at negative gate voltage, with activated temperature-dependent conductance and energy barriers up to 250 meV.     

Intrinsic current-voltage characteristics of graphene nanoribbon transistors and effect of edge doping

We demonstrate that the electronic devices built on patterned graphene nanoribbons (GNRs) can be made with atomic-perfect-interface junctions and controlled doping via manipulation of edge terminations. Using first-principles transport calculations, we show that the GNR field effect transistors can achieve high performance levels similar to those made from single-walled carbon nanotubes, with ON/OFF ratios on the order of 10(3)-10(4), subthreshold swing of 60 meV per decade, and transconductance of 9.5 x 10(3) Sm-1.     

Tandem isotachophoresis-zone electrophoresis via base-mediated destacking for increased detection sensitivity in microfluidic systems

Electrophoresis in microfluidic devices is becoming a useful analytical platform for a variety of biological assays. In this report, we present a method that allows for an increased sensitivity of detection of fluorescent molecules in microfluidic electrophoresis devices. This capability is provided by the implementation of a particular buffer system that is designed to initially function in an isotachophoretic (ITP) mode and, then after a controlled amount of electric current has been applied to the system, to transition to a zone electrophoretic mode. In the initial ITP mode, analytes dissolved in a large volume of injected sample are concentrated into a single narrow zone. After application of a sufficient and adjustable amount of electric current, the system switches into a zone electrophoretic mode, where the concentrated analytes are separated according to their electrophoretic mobilities. Application of this tandem ITP-zone electrophoretic strategy to the concentration, separation, and detection of fluorescent reporter molecules in a standard microfluidic device results in an approximately 50-fold increase in detection sensitivity relative to equivalent separations that are obtained with zone electrophoresis alone. Even with very long initial sample plugs (up to 3000 microm), this strategy produces electrophoretic separations with high resolutions and peak efficiencies. This strategy can be implemented to increase detection sensitivity in any standard microfluidic electrophoresis platform and does not require any specialized hardware or microchannel configurations.     

Conditions for similitude between the fluid velocity and electric field in electroosmotic flow

Electroosmotic flow is fluid motion driven by an electric field acting on the net fluid charge produced by charge separation at a fluid-solid interface. Under many conditions of practical interest, the resulting fluid velocity is proportional to the local electric field, and the constant of proportionality is everywhere the same. Here we show that the main conditions necessary for this similitude are a steady electric field, uniform fluid and electric properties, an electric Debye layer that is thin compared to any physical dimension, and fluid velocities on all inlet and outlet boundaries that satisfy the Helmholtz-Smoluchowski relation normally applicable to fluid-solid boundaries. Under these conditions, the velocity field can be determined directly from the Laplace equation governing the electric potential, without solving either the continuity or momentum equations. Three important consequences of these conditions are that the fluid motion is everywhere irrotational, that fluid velocities in two-dimensional channels bounded by parallel planes are independent of the channel depth, and that such flows exhibit no dependence on the Reynolds number. Similitude is demonstrated by comparing measured and computed fluid streamlines with computed electric flux lines.