Label-free single-molecule detection of DNA-hybridization kinetics with a carbon nanotube field-effect transistor

Single-molecule measurements of biomolecules can provide information about the molecular interactions and kinetics that are hidden in ensemble measurements. However, there is a requirement for techniques with improved sensitivity and time resolution for use in exploring biomolecular systems with fast dynamics. Here, we report the detection of DNA hybridization at the single-molecule level using a carbon nanotube field-effect transistor. By covalently attaching a single-stranded probe DNA sequence to a point defect in a carbon nanotube, we are able to measure two-level fluctuations in the conductance of the nanotube in the presence of a complementary DNA target. The kinetics of the system are studied as a function of temperature, allowing the measurement of rate constants, melting curves and activation energies for different sequences and target concentrations. The kinetics demonstrate non-Arrhenius behaviour, in agreement with DNA hybridization experiments using fluorescence correlation spectroscopy. This technique is label-free and could be used to probe single-molecule dynamics at microsecond timescales.     

Monitoring single-molecule reactivity on a carbon nanotube

Using point-functionalized carbon nanotube devices, we demonstrate continuous, multihour monitoring of a single carboxylate group interacting with its immediate environment. The conductance of the nanotube device directly transduces single-molecule attachments and detachments in the presence of the carboxylate-selective reagent 1-ethyl-3-(3-dimethylaminopropyl) carbodiimide hydrochloride (EDC). Because only one carboxylate is present in the device, it can be studied through hundreds of reactions, providing the statistical accuracy to directly determine a 12 s lifetime of the carboxy-EDC complex. An additional instability of the complex is transduced in real time and observed to have a median lifetime of 2 ms. By determining a turnover time in good agreement with bulk measurements and simultaneously illuminating additional dynamics, these results demonstrate this platform's potential for complementing optical methods in single-molecule research.     

Suppression of leakage current in carbon nanotube field-effect transistors

Carbon nanotube field-effect transistor(CNT FET)has been considered as a promising candidate for future high-performance and low-power integrated circuits(ICs)applications owing to its ballistic transport and excellent immunity to short channel effects(SCEs).Still,it easily suffers from the ambipolar property,and severe leakage current at off-state originated from gate-induced drain leakage(GIDL)in CNT FETs with small bandgap.Although some modifications on device structure have been experimentally demonstrated to suppress the leakage current in CNT FETs,there is still a lack of the structure with excellent scalability,which will hamper the development of CNT FETs toward a competitive technology node.Here,we explore how the device geometry design affects the leakage current in CNT FETs,and then propose the possible device structures to suppress off-state current and check their availability through the two-dimensional(2D)TCAD simulations.Among all the proposed structures,the L-shaped-spacer CNT FET exhibits significantly suppressed leakage current and excellent scalability down to sub-50 nm with a simple self-aligned gate process.According to the simulation results,the 50 nm gate-length L-shaped-spacer CNT FET exhibits an off-state current as low as approximately 1 nA/μm and an on-current as high as about 2.1 mA/μm at a supply voltage of-1 V and then can be extended as a universal device structure to suppress leakage current for all the narrow-bandgap semiconductors based FETs.     

Frequency response of top-gated carbon nanotube field-effect transistors

The ac performance of carbon nanotube field-effect transistors (CNFETs) has been characterized using two approaches involving: 1) time- and 2) frequency-domain measurements. A high input impedance measurement system was used to demonstrate time-domain switching of CNFETs at frequencies up to 100 kHz. The low level of signal crosstalk in CNFETs fabricated on quartz substrates enabled frequency-domain measurements of the ac response of CNFETs in the megahertz range, over five orders of magnitude higher in frequency than previously reported ac measurements of CNFET devices.     

High-performance carbon nanotube field-effect transistor with tunable polarities

State-of-the-art carbon nanotube field-effect transistors (CNFETs) behave as Schottky-barrier-modulated transistors. It is known that vertical scaling of the gate oxide significantly improves the performance of these devices. However, decreasing the oxide thickness also results in pronounced ambipolar transistor characteristics and increased drain leakage currents. Using a novel device concept, we have fabricated high-performance enhancement-mode CNFETs exhibiting n- or p-type unipolar behavior, tunable by electrostatic and/or chemical doping, with excellent OFF-state performance and a steep subthreshold swing (S=63 mV/dec). The device design allows for aggressive oxide thickness and gate-length scaling while maintaining the desired device characteristics.     

Global and local charge trapping in carbon nanotube field-effect transistors

The influences of trapped charges on carrier transport in carbon nanotubes (CNTs) are studied using CNT field-effect transistors with a partial top-gate and a global back-gate. Trapped charges induced by the global back-gate voltage sweeping (± 20?V) promote the device from 'ON' states to near 'OFF' states at zero gate voltage. The channel conductance and field-effect mobility of the device are significantly affected by the pre-trapped charges induced by global back-gate voltage pulses. When the partial top-gate is swept, the pre-trapped charges induced by the global back-gate voltage pulses change the conduction type of the device. In contrast, the pre-trapped charges induced by the partial top-gate voltage pulses could force the device to the 'ON' or 'OFF' state during the top-gate sweeping (± 4?V).     

Calibration method for a carbon nanotube field-effect transistor biosensor

An easy calibration method based on the Langmuir adsorption theory is proposed for a carbon nanotube field-effect transistor (NTFET) biosensor. This method was applied to three NTFET biosensors that had approximately the same structure but exhibited different characteristics. After calibration, their experimentally determined characteristics exhibited a good agreement with the calibration curve. The reason why the observed characteristics of these NTFET biosensors differed among the devices was that the carbon nanotube (CNT) that formed the channel was not uniform. Although the controlled growth of a CNT is difficult, it is shown that an NTFET biosensor can be easy calibrated using the proposed calibration method, regardless of the CNT channel structures.     

Role of defects in single-walled carbon nanotube chemical sensors

We explore the electronic response of single-walled carbon nanotubes (SWNT) to trace levels of chemical vapors. We find adsorption at defect sites produces a large electronic response that dominates the SWNT capacitance and conductance sensitivity. This large response results from increased adsorbate binding energy and charge transfer at defect sites. Finally, we demonstrate controlled introduction of oxidation defects can be used to enhance sensitivity of a SWNT network sensor to a variety of chemical vapors.     

Importance of the Debye screening length on nanowire field effect transistor sensors

Nanowire field effect transistors (NW-FETs) can serve as ultrasensitive detectors for label-free reagents. The NW-FET sensing mechanism assumes a controlled modification in the local channel electric field created by the binding of charged molecules to the nanowire surface. Careful control of the solution Debye length is critical for unambiguous selective detection of macromolecules. Here we show the appropriate conditions under which the selective binding of macromolecules is accurately sensed with NW-FET sensors.     

Label-free protein biosensor based on aptamer-modified carbon nanotube field-effect transistors

We have fabricated label-free protein biosensors based on aptamer-modified carbon nanotube field-effect transistors (CNT-FETs) for the detection of immunoglobulin E (IgE). After the covalent immobilization of 5'-amino-modified 45-mer aptamers on the CNT channels, the electrical properties of the CNT-FETs were monitored in real time. The introduction of target IgE at various concentrations caused a sharp decrease in the source-drain current, and a gradual saturation was observed at lower concentrations. The amount of the net source-drain current before and after IgE introduction on the aptamer-modified CNT-FETs increased as a function of IgE concentration. The detection limit for IgE was determined as 250 pM. We have also prepared CNT-FET biosensors using a monoclonal antibody against IgE (IgE-mAb). The electrical properties of the aptamer- and antibody-modified CNT-FETs were compared. The performance of aptamer-modified CNT-FETs provided better results than the ones obtained using IgE-mAb-modified CNT-FETs under similar conditions. Thus, we suggest that the aptamer-modified CNT-FETs are promising candidates for the development of label-free protein biosensors.     

Flexible carbon nanotube sensors for nerve agent simulants

Chemiresistor-based vapour sensors made from network films of single-walled carbon nanotube (SWNT) bundles on flexible plastic substrates (polyethylene terephthalate, PET) can be used to detect chemical warfare agent simulants for the nerve agents Sarin (diisopropyl methylphosphonate, DIMP) and Soman (dimethyl methylphosphonate, DMMP). Large, reproducible resistance changes (75-150%), are observed upon exposure to DIMP or DMMP vapours, and concentrations as low as 25?ppm can be detected. Robust sensor response to simulant vapours is observed even in the presence of large equilibrium concentrations of interferent vapours commonly found in battle-space environments, such as hexane, xylene and water (10?000?ppm each), suggesting that both DIMP and DMMP vapours are capable of selectively displacing other vapours from the walls of the SWNTs. Response to these interferent vapours can be effectively filtered out by using a 2?μm thick barrier film of the chemoselective polymer polyisobutylene (PIB) on the SWNT surface. These network films are composed of a 1-2?μm thick non-woven mesh of SWNT bundles (15-30?nm diameter), whose sensor response is qualitatively and quantitatively different from previous studies on individual SWNTs, or a network of individual SWNTs, suggesting that vapour sorption at interbundle sites could be playing an important role. This study also shows that the line patterning method used in device fabrication to obtain any desired pattern of films of SWNTs on flexible substrates can be used to rapidly screen simulants at high concentrations before developing more complicated sensor systems.     

High performance n-type carbon nanotube field-effect transistors with chemically doped contacts

Short channel ( approximately 80 nm) n-type single-walled carbon nanotube (SWNT) field-effect transistors (FETs) with potassium (K) doped source and drain regions and high-kappa gate dielectrics (ALD HfO(2)) are obtained. For nanotubes with diameter approximately 1.6 nm and band gap approximately 0.55 eV, we obtain n-MOSFET-like devices exhibiting high on-currents due to chemically suppressed Schottky barriers at the contacts, subthreshold swing of 70 mV/decade, negligible ambipolar conduction, and high on/off ratios up to 10(6) at a bias voltage of 0.5 V. The results compare favorably with the state-of-the-art silicon n-MOSFETs and demonstrate the potential of SWNTs for future complementary electronics. The effects of doping level on the electrical characteristics of the nanotube devices are discussed.     

Dielectrophoretic batch fabrication of bundled carbon nanotube thermal sensors

We present a feasible technology for batch assembly of carbon nanotube (CNT) devices by utilizing ac electrophoretic technique to manipulate multiwalled bundles on an Si/SiO/sub 2/ substrate. Based on this technique, CNTs were successfully and repeatably manipulated between microfabricated electrodes. By using this parallel assembly process, we have investigated the possibility of batch fabricating functional CNT devices when an ac electrical field is applied to an array of microelectrodes that are electrically connected together. Preliminary experimental results showed that over 70% of CNT functional devices can be assembled successfully using our technique, which is considered to be a good yield for nanodevices manufacturing. Besides, the devices were demonstrated to potentially serve as novel thermal sensors with low power consumption (/spl sim/microwatts) with electronic circuit response of approximately 100 kHz in constant current mode operation. In this paper, we will present the fabrication process of this feasible batch manufacturable method for functional CNT-based thermal sensors, which will dramatically reduce production costs and production time of nanosensing devices and potentially enable fully automated assembly of CNT-based devices. Experimental results from the thermal sensors fabricated by this batch process will also be discussed.     

The role of metal-nanotube contact in the performance of carbon nanotube field-effect transistors

Single-wall carbon nanotube field-effect transistors (CNFETs) have been shown to behave as Schottky barrier (SB) devices. It is not clear, however, what factors control the SB size. Here we present the first statistical analysis of this issue. We show that a large data set of more than 100 devices can be consistently accounted by a model that relates the on-current of a CNFET to a tunneling barrier whose height is determined by the nanotube diameter and the nature of the source/drain metal contacts. Our study permits identification of the desired combination of tube diameter and type of metal that provides the optimum performance of a CNFET.     

Fabrication of nanometer-scale carbon nanotube field-effect transistors on flexible and transparent substrate

We have successfully fabricated nanometer-scale carbon nanotube field effect transistors (CNT FETs) on a flexible and transparent substrate by electron-beam lithography. The measured current-voltage data show good hole conduction FET characteristics, and the on/off ratio of the current is more than 10(2). The conductance (as well as current) systematically decreases with the increase of the strain, suggesting that the bending of the substrate still affects the deformation condition of the short channel CNT FETs.     

Self-aligned ballistic n-type single-walled carbon nanotube field-effect transistors with adjustable threshold voltage

Near ballistic n-type single-walled carbon nanotube field-effect transistors (SWCNT FETs) have been fabricated with a novel self-aligned gate structure and a channel length of about 120 nm on a SWCNT with a diameter of 1.5 nm. The device shows excellent on- and off-state performance, including high transconductance of up to 25 microS, small subthreshold swing of 100 mV/dec, and gate delay time of 0.86 ps, suggesting that the device can potentially work at THz regime. Quantitative analysis on the electrical characteristics of a long channel device fabricated on the same SWCNT reveals that the SWCNT has a mean-free-path of 191 nm, and the electron mobility of the device reaches 4650 cm(2)/Vs. When benchmarked by the metric CV/ I vs Ion/Ioff, the n-type SWCNT FETs show significantly better off-state leakage than that of the Si-based n-type FETs with similar channel length. An important advantage of this self-aligned gate structure is that any suitable gate materials can be used, and in particular it is shown that the threshold voltage of the self-aligned n-type FETs can be adjusted by selecting gate metals with different work functions.     

Carbon nanotube multi-channeled field-effect transistors

Field-effect transistors (FETs) with multiple channels of single-wall carbon nanotubes (SWCNTs) have been constructed. SWCNT channels of the FETs are dispersedly aligned between the source and the drain by electric-field manipulation of surface decorated SWCNTs. The obtained multichanneled FETs not only can meet the requirement of large output current and high transconductance, but also manifested good reliability and applicability. It is found that the transconductance of the multi-channel FET has an almost linear dependency on the SWCNT channel number, which opens up a promising way to tune the transconductance of FETs by controlling the channel number.     

Active properties of carbon nanotube field-effect transistors deduced from S parameters measurements

AC performances of carbon nanotube field-effect transistors (CNT-FETs) are analyzed by means of scattering parameters measurements. The active ac properties of CNT-FETs are clearly demonstrated up to 80 MHz and indications of active behavior are obtained up to 1 GHz. From these measurements, a small signal equivalent circuit is proposed and validated up to 10 MHz. The extraction procedure and the determination of the intrinsic ac elements of CNT-FETs are pointed out.     

Single-walled carbon nanotube thin film gas sensors controlled by diffusion

The electrical and gas sensing properties of mat-type thin films fabricated by the packing of single-walled carbon nanotubes are investigated. The responses of the sensors with various thicknesses or resistances are measured, and the behavior is explained by a diffusion model. The successful explanation of the sensor response using the diffusion model suggests that the mat-type structure of compacted carbon nanotubes can be treated as a porous medium for gas sensor applications.     

基于层层自组装纳米碳管薄膜的应变传感器

以层层自组装单壁碳纳米管网状薄膜为敏感材料,在柔性基底上制作了应变传感器,并对薄膜和器件性能进行了测试。测试结果表明:薄膜组装均匀,电导性良好,电阻随组装层数增加呈指数态下降;器件对应变呈现较好的敏感特性、线性响应和可恢复性,灵敏度为4.25;通过添加防护层屏蔽外界湿度及光照影响,传感器稳定性显著提高。该传感器可用于弯曲表面的应力应变检测。