Carbon Nanotube Network Field-effect Transistors Research Articles

(Invited) Alteration of the Electrical Transport in Carbon Nanotube Network Field-Effect Transistors Using Polymer Encapsulants and Gate Dielectrics

Enriched semiconducting single-walled carbon nanotubes (SWCNTs) inks of high purity opens a wealth of possibilities for device fabrication using printing techniques and solution processes. [1] Logics, displays, sensors and large-scale integrated circuits [2] appear to be within reach. For those promises to concretize, a better control over parameters such as carrier type and mobility, operation power and device-to-device variability is needed. Beside the ambient effect (notably water and oxygen) on the transport of carbon nanotube network field-effect transistors (CNN-FETs), the nature of the surrounding materials (as a passivation layer or as a gate dielectric) appears to play a crucial role in determining the device's characteristics.Firstly, we have tested a large number of polymeric materials as the bottom gate in three-terminal CNN-FETs. We have found that the transport characteristics parameters such as the threshold voltage and the hysteresis are affected by the chemical nature of the polymer. [3] Subjecting those CNN-FETs with various gate dielectrics to a series of volatile analytes, we observed drastically different responses which suggests the usefulness of polymer gate dielectric variations in cross-reactive sensors arrays.Secondly, polymeric materials were used as an encapsulant over bottom gate CNN-FETs. [4] A smooth and continuous variation of the threshold voltage has been achieved by using a series of poly(styrene–co–2-vinyl pyridine) copolymers with different monomer ratios. A surface charge density measurement methodology has been developed to rationalize the threshold voltage variations upon encapsulation with various polymers. Finally, we explored the use of n-doping molecules blended with polymer encapsulants as an efficient way to obtain n-type transport characteristics. This strategy is further used in the implementation of simple p-n junctions showing a modest rectification.

(Invited) Alteration of the Electrical Transport in Carbon Nanotube Network Field-Effect Transistors Using Polymer Encapsulants and Gate Dielectrics

Enriched semiconducting single-walled carbon nanotubes (SWCNTs) inks of high purity opens a wealth of possibilities for device fabrication using printing techniques and solution processes. [1] Logics, displays, sensors and large-scale integrated circuits [2] appear to be within reach. For those promises to concretize, a better control over parameters such as carrier type and mobility, operation power and device-to-device variability is needed. Beside the ambient effect (notably water and oxygen) on the transport of carbon nanotube network field-effect transistors (CNN-FETs), the nature of the surrounding materials (as a passivation layer or as a gate dielectric) appears to play a crucial role in determining the device’s characteristics.Firstly, we have tested a large number of polymeric materials as the bottom gate in three-terminal CNN-FETs. We have found that the transport characteristics parameters such as the threshold voltage and the hysteresis are affected by the chemical nature of the polymer. [3] Subjecting those CNN-FETs with various gate dielectrics to a series of volatile analytes, we observed drastically different responses which suggests the usefulness of polymer gate dielectric variations in cross-reactive sensors arrays.Secondly, polymeric materials were used as an encapsulant over bottom gate CNN-FETs. [4] A smooth and continuous variation of the threshold voltage has been achieved by using a series of poly(styrene–co–2-vinyl pyridine) copolymers with different monomer ratios. A surface charge density measurement methodology has been developed to rationalize the threshold voltage variations upon encapsulation with various polymers. Finally, we explored the use of n-doping molecules blended with polymer encapsulants as an efficient way to obtain n-type transport characteristics. This strategy is further used in the implementation of simple p-n junctions showing a modest rectification.

Carbon Nanotube Transistors as Gas Sensors: Response Differentiation Using Polymer Gate Dielectrics

The high sensitivity of carbon nanotube devices as gas sensors is generally attributed to the large surface area of the material, whereas selectivity is often imparted by the “lock-and-key” mechanism. In contrast with this picture, where little function is conferred to the underlying substrate, we demonstrate that bottom gate carbon nanotube network field-effect transistors (CNN-FETs) fabricated on a variety of polymer dielectrics, but with the same pristine carbon nanotubes, show differentiated responses to analytes. Interaction with the environment thus occurs not only directly with carbon nanotubes but is often mediated by the substrate. In a broad survey of polymers, we identify several materials suitable as the gate dielectric of printed devices and that yield a vanishing gate hysteresis in ambient air. The other two classes of polymers that induce a lagging or advancing hysteresis in CNN-FETs are found desirable for gas sensing applications. Moreover, we have tested CNN-FETs with different polymer g...

Ferroelectric polarization effect on hysteresis behaviors of single-walled carbon nanotube network field-effect transistors with lead zirconate-titanate gating

We report the fabrication of single-walled carbon nanotube (SWCNT) network transistors by ferroelectric Pb(Zr0.4Ti0.6)O3 (PZT) bottom-gating and investigate the polarization effects of PZT on the transport properties of the transistor device. Our devices exhibit typical p-channel transistor characteristics and a large hysteresis loop with high ON/OFF current ratio and large ON current as well as memory window (MW) measured up to 5.2 V. The origin of clockwise hysteresis is attributed to ferroelectric polarization modulated charge trapping/de-trapping process in the interface states between SWCNT networks and PZT. The retention time about 104s with two high stable current states preliminarily demonstrates great potential for future non-volatile memory applications based on such SWCNT/PZT hybrid systems.

In Situ Raman Mapping of Charge Carrier Distribution in Electrolyte-Gated Carbon Nanotube Network Field-Effect Transistors

Solution-processed networks of purely semiconducting single-walled carbon nanotubes (s-SWNTs) can be used to create high-mobility field-effect transistors (FETs) for flexible electronics. In order to optimize network alignment and density, an understanding of carrier distribution within the FET channel is necessary. Here, we used confocal Raman microscopy to investigate charge accumulation and doping in electrolyte-gated FETs with asymmetric layers of only (7,5) nanotubes, that were selected by dispersion in poly(9,9-dioctylfluorene). The nanotube FETs exhibited hole mobilities of up to 7.5 cm2 V–1 s–1 and on/off ratios of 105. All Raman modes decreased in intensity with hole accumulation. The G′-peak and D-peak shifted linearly with negative gate voltages to higher wavenumbers. Using the G′-peak shift, the charge carrier distribution in an operating FET was mapped at different gate and source-drain voltages with high spatial resolution (∼300 nm) and over large areas. With this simple technique, we were a...

Physical and Electrical Characteristics of Carbon Nanotube Network Field-Effect Transistors Synthesized by Alcohol Catalytic Chemical Vapor Deposition

Carbon nanotubes (CNTs) have been explored in nanoelectronics to realize desirable device performances. Thus, carbon nanotube network field-effect transistors (CNTNFETs) have been developed directly by means of alcohol catalytic chemical vapor deposition (ACCVD) method using Co-Mo catalysts in this work. Various treated temperatures, growth time, and Co/Mo catalysts were employed to explore various surface morphologies of carbon nanotube networks (CNTNs) formed on the SiO2/n-type Si(100) stacked substrate. Experimental results show that most semiconducting single-walled carbon nanotube networks with 5–7 nm in diameter and low disorder-induced mode (D-band) were grown. A bipolar property of CNTNFETs synthesized by ACCVD and using HfO2as top-gate dielectric was demonstrated. Various electrical characteristics, including drain current versus drain voltage(Id-Vd), drain current versus gate voltage(Id-Vg), mobility, subthreshold slope (SS), and transconductance(Gm), were obtained.

Understanding doping effects in biosensing using carbon nanotube network field-effect transistors

Systematic theoretical studies based on a comprehensive heterogeneous stick percolation model are performed to gain insights into the essence of doping effects in electrical sensing of biomolecules, such as proteins and DNA fragments, using carbon nanotube network field-effect transistors (CNNFETs). The present work demonstrates that the electrical response to doping of CNNFETs is primarily caused by conductance change at the electrode-nanotube contacts, in contrast to that in the channel as assumed previously. However, the presence of intertube junctions in the channel could reduce the sensitivity of CNNFET-based biosensors and is partially responsible for the experimentally observed channel-length dependent sensitivity.

Charge Transport in Interpenetrating Networks of Semiconducting and Metallic Carbon Nanotubes

Carbon nanotube network field effect transistors (CNTN-FETs) are promising candidates for low cost macroelectronics. We investigate the microscopic transport in these devices using electric force microscopy and simulations. We find that in many CNTN-FETs the voltage drops abruptly at a point in the channel where the current is constricted to just one tube. We also model the effect of varying the semiconducting/metallic tube ratio. The effect of Schottky barriers on both conductance within semiconducting tubes and conductance between semiconducting and metallic tubes results in three possible types of CNTN-FETs with fundamentally different gating mechanisms. We describe this with an electronic phase diagram.

P-Channel, n-Channel and ambipolar field-effect transistors based on functionalized carbon nanotube networks

We report on the transport properties of carbon nanotube network field-effect transistors (CNNFETs) produced from size-selected and functionalized single-walled carbon nanotubes (SWNTs). The SWNTs were functionalized by grafting octadecylamine chains to the tube ends and spin casting onto prefabricated bottom gated silicon field-effect transistor substrates. Acid-oxidative cutting and centrifuge fractionation were employed to select the mean diameter and length of the SWNT bundles deposited within the active area of the CNNFETs. By comparing CNNFETs with different SWNT bundle thickness, we demonstrated that thicker-bundle samples exhibited low on/off ratio but comparatively higher field-effect mobility than small-bundle samples, which yielded devices with higher on/off ratio but lower field-effect mobility. Electronic transfer characteristics of the CNNFETs were dominated by the channel rather than contact resistance. These results demonstrate a potential new route for fabricating p - and n -type CNNFET devices.

Electroluminescence from Single-Wall Carbon Nanotube Network Transistors

The electroluminescence (EL) properties from single-wall carbon nanotube network field-effect transistors (NNFETs) and small bundle carbon nanotube field effect transistors (CNFETs) are studied using spectroscopy and imaging in the near-infrared (NIR). At room temperature, NNFETs produce broad (approximately 180 meV) and structured NIR spectra, while they are narrower (approximately 80 meV) for CNFETs. EL emission from NNFETs is located in the vicinity of the minority carrier injecting contact (drain) and the spectrum of the emission is red shifted with respect to the corresponding absorption spectrum. A phenomenological model based on a Fermi-Dirac distribution of carriers in the nanotube network reproduces the spectral features observed. This work supports bipolar (electron-hole) current recombination as the main mechanism of emission and highlights the drastic influence of carrier distribution on the optoelectronic properties of carbon nanotube films.

Electrical detection of hybridization and threading intercalation of deoxyribonucleic acid using carbon nanotube network field-effect transistors

The authors study deoxyribonucleic acid (DNA) sensing characteristics of carbon nanotube network field-effect transistors (CNNFETs) by monitoring their electrical responses upon immobilization with a DNA probe, hybridization with DNA analytes, and intercalation with a N,N′-bis(3-propylimidazole)-1,4,5,8-naphthalene diimide modified with Os(2,2′-bipyridine)2Cl+ pendants. The CNNFETs immobilized by single-stranded DNA molecules demonstrate the selective sensing of its complementary and single-base mismatched DNA (difference of ∼16% in reduction of normalized drain current Id). Subsequent intercalation demonstrates a further sensitivity enhancement (difference of ∼13% in Id reduction) due to specific binding between hybridized DNA and intercalators, corroborated by the x-ray photoelectron spectroscopy study.

Radiation hardness of the electrical properties of carbon nanotube network field effecttransistors under high-energy proton irradiation

The effect of high-energy proton irradiation on the physical properties of carbon nanotubes(CNTs) was investigated. The focus of the study was on the electrical properties ofsingle-walled carbon nanotube (SWNT) network devices exposed to proton beams.Field-effect transistors (FETs) of network type were fabricated using SWNTs andwere then irradiated by high-energy proton beams of 10–35 MeV with a fluence of4 × 1010–4 × 1012 cm−2 that are comparable to the aerospace radiation environment. The electrical properties ofboth metallic and semiconducting CNT network FET devices underwent no significantchange after the high-energy proton irradiation, indicating that the CNT network devicesare very tolerant in proton beams. Raman spectra confirm the proton-radiation hardness ofCNT network FET devices. The radiation hardness of CNT network FET devices promisestherefore the potential usefulness of CNT-based electronics for future space application.

Label-free detection of DNA hybridization using carbon nanotube network field-effect transistors.

We report carbon nanotube network field-effect transistors (NTNFETs) that function as selective detectors of DNA immobilization and hybridization. NTNFETs with immobilized synthetic oligonucleotides have been shown to specifically recognize target DNA sequences, including H63D single-nucleotide polymorphism (SNP) discrimination in the HFE gene, responsible for hereditary hemochromatosis. The electronic responses of NTNFETs upon single-stranded DNA immobilization and subsequent DNA hybridization events were confirmed by using fluorescence-labeled oligonucleotides and then were further explored for label-free DNA detection at picomolar to micromolar concentrations. We have also observed a strong effect of DNA counterions on the electronic response, thus suggesting a charge-based mechanism of DNA detection using NTNFET devices. Implementation of label-free electronic detection assays using NTNFETs constitutes an important step toward low-cost, low-complexity, highly sensitive and accurate molecular diagnostics.