Semiconducting Carbon Nanotube Films Research Articles

(Invited) Globally Aligned, Wafer-Scale Deposition of Carbon Nanotubes Via Interfacial Assembly

Floating evaporative self-assembly (FESA) is a solution-based technique for fabricating highly-aligned semiconducting carbon nanotube films.1,2 Field-effect transistors (FETs) based on carbon nanotubes aligned via FESA have exhibited current densities exceeding that of GaAs and Si, demonstrating the promise of this technique.3 However, the global uniformity of FESA films is poor, as the films are composed of stripes of aligned and randomly oriented carbon nanotubes. Additionally, this technique is inherently difficult to scale. In the work presented here, we demonstrate a scalable technique to overcome the shortcomings of FESA and to achieve globally uniform films of aligned carbon nanotubes at desirable packing densities for FETs.In our technique, carbon nanotubes are continuously aligned across target substrates by lifting the substrates through a flowing nanotube ink/water interface. The ink/water interface collects and confines the nanotubes, which is vital to achieving a high degree of alignment. Polarized Raman spectroscopy mapping is used to quantify the degree of alignment in the resulting aligned nanotube films. The full width at half maximum (FWHM) of the alignment distribution within the films is 6.0° measured across a 1 mm2 area. We control the aligned carbon nanotube packing density from 0.2 to 59 µm-1 by varying the ink concentration from 0.1 to 100 µg mL-1. Using optimized conditions, we demonstrate the scalability of this technique by aligning carbon nanotubes across a 100 mm wide wafer. Charge transport measurements are performed using FETs with the aligned carbon nanotube films as the active channels, and the performance is compared to FETs based on FESA films.1. Jinkins, K., R. et al. Nanotube Alignment Mechanism in Floating Evaporative Self-Assembly. Langmuir 33, 13407–13414 (2017).2. Joo, Y., Brady, G. J., Arnold, M. S. & Gopalan, P. Dose-Controlled, Floating Evaporative Self-assembly and Alignment of Semiconducting Carbon Nanotubes from Organic Solvents. Langmuir 30, 3460–3466 (2014).3. Brady, G. J. et al. Quasi-ballistic carbon nanotube array transistors with current density exceeding Si and GaAs. Sci. Adv. 2, (2016).

Wafer-Scale Uniform Carbon Nanotube Transistors for Ultrasensitive and Label-Free Detection of Disease Biomarkers.

Carbon nanotube (CNT) field-effect transistor (FET)-based biosensors have shown great potential for ultrasensitive biomarker detection, but challenges remain, which include unsatisfactory sensitivity, difficulty in stable functionalization, incompatibility with scalable fabrication, and nonuniform performance. Here, we describe ultrasensitive, label-free, and stable FET biosensors built on polymer-sorted high-purity semiconducting CNT films with wafer-scale fabrication and high uniformity. With a floating gate (FG) structure using an ultrathin Y2O3 high-κ dielectric layer, the CNT FET biosensors show amplified response and improved sensitivity compared with those sensors without Y2O3, which is attributed to the chemical gate-coupling effect dominating the sensor response. The CNT FG-FETs are modified to selectively detect specific disease biomarkers, namely, DNA sequences and microvesicles, with theoretical record detection limits as low as 60 aM and 6 particles/mL, respectively. Furthermore, the biosensors exhibit highly uniform performance over the 4 in. wafer as well as superior bias stress stability. The FG CNT FET biosensors could be extended as a universal biosensor platform for the ultrasensitive detection of multiple biological molecules and applied in highly integrated and multiplexed all CNT-FET-based sensor architectures.

Globally Aligned, Wafer-Scale Deposition of Carbon Nanotubes Via Interfacial Assembly

Floating evaporative self-assembly (FESA) is a solution-based technique for fabricating highly-aligned semiconducting carbon nanotube films.1,2 Field-effect transistors (FETs) based on carbon nanotubes aligned via FESA have exhibited current densities exceeding that of GaAs and Si, demonstrating the promise of this technique.3 However, the global uniformity of FESA films is poor, as the films are composed of stripes of aligned and randomly oriented carbon nanotubes. Additionally, this technique is inherently difficult to scale. In the work presented here, we demonstrate a scalable technique to overcome the shortcomings of FESA and to achieve globally uniform films of aligned carbon nanotubes at desirable packing densities for FETs.In our technique, carbon nanotubes are continuously aligned across target substrates by lifting the substrates through a flowing nanotube ink/water interface. The ink/water interface collects and confines the nanotubes, which is vital to achieving a high degree of alignment. Polarized Raman spectroscopy mapping is used to quantify the degree of alignment in the resulting aligned nanotube films. The full width at half maximum (FWHM) of the alignment distribution within the films is 6.0° measured across a 1 mm2 area. We control the aligned carbon nanotube packing density from 0.2 to 59 µm-1 by varying the ink concentration from 0.1 to 100 µg mL-1. Using optimized conditions, we demonstrate the scalability of this technique by aligning carbon nanotubes across a 100 mm wide wafer. Charge transport measurements are performed using FETs with the aligned carbon nanotube films as the active channels, and the performance is compared to FETs based on FESA films. 1. Jinkins, K., R. et al. Nanotube Alignment Mechanism in Floating Evaporative Self-Assembly. Langmuir 33, 13407–13414 (2017).2. Joo, Y., Brady, G. J., Arnold, M. S. & Gopalan, P. Dose-Controlled, Floating Evaporative Self-assembly and Alignment of Semiconducting Carbon Nanotubes from Organic Solvents. Langmuir 30, 3460–3466 (2014).3. Brady, G. J. et al. Quasi-ballistic carbon nanotube array transistors with current density exceeding Si and GaAs. Sci. Adv. 2, (2016).

High-speed electroluminescence from semiconducting carbon nanotube films

High-speed light emitters integrated on silicon chips can enable novel architectures for silicon-based optoelectronics, such as on-chip optical interconnects and silicon photonics. However, conventional light sources based on compound semiconductors face major challenges for their integration with the silicon-based platforms because of the difficulty of their direct growth on a silicon substrate. Here, we report high-speed, ultra-small-size on-chip electroluminescence (EL) emitters based on semiconducting single-walled carbon nanotube (SWNT) thin films. The peaks of the EL emission spectra are about 0.15-eV redshifted from the peaks of the absorption and photoluminescence emission spectra, which probably suggest emission from trions. High-speed responses of ∼100 ps were experimentally observed from the EL emitters, which indicate the possibility of several-GHz modulation. The pulsed light generation was also obtained by applying the pulse voltage. These high-speed and ultra-small-size EL emitters can enable novel on-chip optoelectronic devices for highly integrated optoelectronics and silicon photonics.

Ultrafast wafer-scale assembly of uniform and highly dense semiconducting carbon nanotube films for optoelectronics

We report an innovative method for the fast preparation of wafer-scale uniform single-wall carbon nanotubes (SWCNT) films with controllable density by NaHCO3 tuning the interaction between amine-functionalized substrates and surfactant-dispersing SWCNTs in aqueous solution. With this technique, a 4-inch SWCNT film with a linear density of ∼30 tubes/μm can be achieved in 1 s, and over 60 tubes/μm within 30 s using a high-concentration SWCNT solution. The SWCNT density changes by less than 8% over a 4-inch area. The electrical uniformity of the as-prepared wafer-scale single-chirality SWCNT film was demonstrated to be within 10.8% for subthreshold swing and 12% for carrier mobility and 13.5% for their on-current, which is the most uniform SWCNT films reported so far fabricated from the surfactant-dispersed SWCNT solution. Further, the photodetectors made by the uniform semiconducting SWCNT films exhibit excellent cascading ability, which linearly amplify the output photovoltages and could produce a photovoltage responsivity of 2.5 × 106 V/W. When three-stage cascading photodetectors are used as optical gate to establish an integrated photoelectric system, a 20-mW/cm2 light illumination produces the current responsivity of 147.3 A/W under a source/drain voltage of −0.1 V. Our present technique lays an important foundation for their applications in integrated circuits.

Enhanced thermoelectric properties of semiconducting carbon nanotube films by UV/ozone treatment

We examine the effect of UV/O3 oxidation on the thermoelectric properties of semiconducting carbon nanotube films. The oxidative UV/O3 treatment leads to the introduction of epoxy and carbonyl groups and a significant increase in the thermoelectric power factor up to 140 μW m−1 K−2. This power factor is three times larger than that of chemically-doped films because of the enhanced Seebeck coefficient. Characterization with Raman and mid-IR absorption/extinction spectroscopy reveals that the UV/ozone treatment results in simultaneous charge carrier doping and defect formation. This simple way of enhancing thermoelectric properties is suitable for the production of large-area, flexible thermoelectric devices based on semiconducting carbon nanotubes.

Ammonia Plasma-Induced n-Type Doping of Semiconducting Carbon Nanotube Films: Thermoelectric Properties and Ambient Effects.

We explore an n-type doping strategy of semiconducting single-walled carbon nanotubes (sc-SWCNTs) by a covalent functionalization in ammonia plasma and elucidate the effect of air exposure on thermoelectric properties of the sc-SWCNTs before and after doping. Without doping, the sc-SWCNT films have a Seebeck coefficient of 125 μV/K and a power factor (PF) of 95 μW/m K2 in ambient conditions. Heating of such films in air up to 100 °C and above is not changing their thermoelectric properties noticeably; however, the films can be converted to an n-type material simply by gas desorption at low pressure and room temperature, showing an outstanding negative Seebeck coefficient of -133 μV/K and a PF of 55 μW/m K2. Doping of the sc-SWCNT films with ammonia plasma leads to the reduction of the Seebeck coefficient down to 40 μV/K in ambient conditions, which is the result of two competing effects: attachment of electron-donating functional groups during plasma treatment and adsorption of water molecules when exposing films to air. At temperatures slightly higher than the boiling point of water, the doped films of sc-SWCNTs show the lowest Seebeck coefficient of -80 μV/K in air. A similar value of the Seebeck coefficient is obtained for the same films at low pressures and room temperature. To our knowledge, this is one of the best values ever reported for n-type pure carbon nanotube films.

Performance improvement induced by asymmetric Y2O3-coated device structure to carbon-nanotube-film based photodetectors

Film-based semiconducting carbon nanotube (CNT) photodetectors are promising candidates for industrial applications. However, unintentional doping from the environment such as water/oxygen (H2O/O2) redox, polymers, etc. changes the doping level of the CNT film. Here, we evaluate the performance of film-based barrier-free bipolar diodes (BFBDs), which are basically semiconducting CNT films asymmetrically contacted by perfect n-type ohmic contact (scandium, Sc) and p-type ohmic contact (palladium, Pd) at the two ends of the diode. We show that normal BFBD devices have large variances of forward current, reverse current, and photocurrent for different doping levels of the channel. We propose an asymmetric Y2O3-coated BFBD device in which the channel is covered by a layer of an Y2O3 film and an overlap between the Sc electrode and the Y2O3 film is designed. The Y2O3 film provides p-type doping to the channel. The overlap section increases the length of the base of the pn junction, and the diffusion current of holes is suppressed. In this way, the rectifier factors (current ratio when voltages are at +0.5 V and −0.5 V) of the asymmetric Y2O3-coated BFBD devices are around two orders of magnitude larger and the photocurrent generation is more stable compared to that of normal devices. Our results provide a way to conquer the influence of unintentional doping from the environment and suppress reverse current in pn diodes. This is beneficial to applications of CNT-based photodetectors and of importance for inspiring methods to improve the performances of devices based on other low dimensional materials.

Room temperature infrared imaging sensors based on highly purified semiconducting carbon nanotubes.

High performance infrared (IR) imaging systems usually require expensive cooling systems, which are highly undesirable. Here we report the fabrication and performance characteristics of room temperature carbon nanotube (CNT) IR imaging sensors. The CNT IR imaging sensor is based on aligned semiconducting CNT films with 99% purity, and each pixel or device of the imaging sensor consists of aligned strips of CNT asymmetrically contacted by Sc and Pd. We found that the performance of the device is dependent on the CNT channel length. While short channel devices provide a large photocurrent and a rapid response of about 110 μs, long channel length devices exhibit a low dark current and a high signal-to-noise ratio which are critical for obtaining high detectivity. In total, 36 CNT IR imagers are constructed on a single chip, each consists of 3 × 3 pixel arrays. The demonstrated advantages of constructing a high performance IR system using purified semiconducting CNT aligned films include, among other things, fast response, excellent stability and uniformity, ideal linear photocurrent response, high imaging polarization sensitivity and low power consumption.

Free Carrier Generation and Recombination in Polymer-Wrapped Semiconducting Carbon Nanotube Films and Heterojunctions

Semiconducting single-walled carbon nanotubes (s-SWCNTs) are promising for solution-processed, thin film photovoltaics due to their strong near-infrared absorptivity and excellent transport properties. We report on the generation yield and recombination kinetics of free charge carriers in photoexcited thin films of polymer-wrapped s-SWCNTs with and without an overlying electron-accepting C60 layer, using time-resolved microwave photoconductivity (TRMC). Free carriers are generated in neat s-SWCNT films, even without an obvious driving force for exciton dissociation. However, most carriers recombine in <10 ns. Adding C60 increases the yield and extends the lifetime of a significant fraction of free carriers to ≫100 ns via interfacial charge separation. Spectral dependencies indicate that the driving force for interfacial electron transfer vanishes for large-diameter SWCNTs, from which we approximate (9,7) s-SWCNT energetics. We estimate a free carrier generation yield of ∼6% in neat s-SWCNT films and 9 GHz...

Efficient Exciton Relaxation and Charge Generation in Nearly Monochiral (7,5) Carbon Nanotube/C60 Thin-Film Photovoltaics

We report on photovoltaic diodes based on bilayer heterojunctions between nearly monochiral, polymer wrapped (7,5) semiconducting carbon nanotube photoabsorbing films and C60. The internal quantum efficiencies (IQEs) for exciton dissociation and subsequent charge collection at the nanotubes’ visible E22 and near-infrared E11 and E11 + X resonances are 84% ± 7%, 85% ± 5%, and 84% ± 14%, respectively. The high IQE at each transition shows that recombination losses during relaxation and/or direct dissociation of “hot” E11 + X and E22 excitons are negligible. A peak external quantum efficiency (EQE) of 34% is achieved at the E11 transition. Zero-bias photoresponsivity is invariant up to short-circuit current densities of at least 23 mA cm–2, indicating negligible losses via trion, charge-exciton, and charge–charge recombination relaxation pathways. An open circuit voltage of 0.49 V and power conversion efficiency of 7.1% are achieved in response to monochromatic excitation of the diodes at the E11 transition. The high IQE across multiple spectral windows, invariant photoresponsivity, and attractive open circuit voltage relative to the 1.18 eV optical bandgap demonstrate the future promise of using monochiral and multichiral semiconducting carbon nanotube films for broadband solar photovoltaic applications.

Enhancing extraction of photogenerated excitons from semiconducting carbon nanotube films as photocurrent

We study the effect of residual polymer on exciton transport and the external quantum efficiency (EQE) of photocurrent generation in thin film semiconducting single walled carbon nanotube (s-SWCNT)/C60 heterojunction diodes. Specifically, increasing the s-SWCNT film content from 22% to 43% increases peak EQE from absorption by s-SWCNTs from 15% to 23%. We monitor intertube exciton energy transfer via steady state photoluminescence spectroscopy and determine the length scale for exciton migration via s-SWCNT film thickness dependence of EQE. We observe increased intertube exciton transfer in photoluminescence spectra with increased polymer removal, and EQE-thickness dependence suggests increased intratube exciton transport along isolated pathways. Our results extend the state of the art with respect to the use of s-SWCNT thin films as photoabsorbers in photovoltaics, describe exciton migration in s-SWCNT films, and provide a framework for the design of high efficiency s-SWCNT photovoltaic and photodetector devices.

Fundamental Performance Limits of Carbon Nanotube Thin-Film Transistors Achieved Using Hybrid Molecular Dielectrics

In the past decade, semiconducting carbon nanotube thin films have been recognized as contending materials for wide-ranging applications in electronics, energy, and sensing. In particular, improvements in large-area flexible electronics have been achieved through independent advances in postgrowth processing to resolve metallic versus semiconducting carbon nanotube heterogeneity, in improved gate dielectrics, and in self-assembly processes. Moreover, controlled tuning of specific device components has afforded fundamental probes of the trade-offs between materials properties and device performance metrics. Nevertheless, carbon nanotube transistor performance suitable for real-world applications awaits understanding-based progress in the integration of independently pioneered device components. We achieve this here by integrating high-purity semiconducting carbon nanotube films with a custom-designed hybrid inorganic-organic gate dielectric. This synergistic combination of materials circumvents conventional design trade-offs, resulting in concurrent advances in several transistor performance metrics such as transconductance (6.5 μS/μm), intrinsic field-effect mobility (147 cm(2)/(V s)), subthreshold swing (150 mV/decade), and on/off ratio (5 × 10(5)), while also achieving hysteresis-free operation in ambient conditions.

Thickness dependent sensing mechanism in sorted semi-conducting single walled nanotube based sensors

Single walled carbon nanotube (SWCNT) networks present outstanding potential for the development of SWCNT-based gas sensors. Due to the complexity of the transport properties of this material, the physical mechanisms at stake during exposure to gas are still under debate. Previously suggested mechanisms are charge transfer between gas molecules and SWCNT and Schottky barrier modulation. By comparing electrical measurements with an analytical model based on Schottky barrier modulation, we demonstrate that one mechanism or the other is predominant depending on the percolation of metallic carbon nanotubes. Below the metallic SWCNT percolation threshold, sensing is dominated by the modulation of the Schottky barrier, while above this threshold, it is only attributed to a charge transfer between SWCNT and gas molecules. Both mechanisms are discussed in terms of sensitivity and resolution leading to routes for the optimization of a gas sensor architecture based on highly enriched semiconducting carbon nanotube films.

Efficiently Harvesting Excitons from Electronic Type-Controlled Semiconducting Carbon Nanotube Films

We have employed thin films of highly purified semiconducting carbon nanotubes as near-infrared optical absorbers in heterojunction photovoltaic and photodetector devices with the electron acceptor C(60). In comparison with previous implementations of more electrically heterogeneous carbon nanotube/C(60) devices, we have realized a 10× gain in zero-bias quantum efficiency (QE) and even more substantial gains in power conversion efficiency (η(p)). The semiconducting nanotube/C(60) heterojunctions are highly rectifying with a peak external QE, internal QE, and η(p) of 12.9 ± 1.3, 91 ± 22, and 0.6%, respectively, in the near-infrared. We show that the device efficiency is determined by the effective length scale for exciton migration in the nanotube films, confirm the high internal QE via photoluminescence quenching, and demonstrate that the driving force for exciton dissociation at the fullerene-fullerene heterointerface is optimized for diameters <1.0 nm. These results will guide the development of next-generation high-performance carbon nanotube-based solar cells and photosensitive devices.

Evaluation of sorted semi-conducting carbon nanotube films for gas sensing applications

Abstract This work focuses on the elaboration of sorted semi-conducting single walled carbon nanotube films (SC-SWCNT films), and the evaluation of a gas microsensor based on these films. First, we show that semi-conducting carbon nanotubes could be sorted from solutions containing both metallic and semi-conducting nanotubes by ultracentrifugation in a density gradient. SC-SWCNT films were then obtained by filtration through a nitrocellulose membrane, then transferred on TLM (transmission line method) electrodes to separately measure the resistivity and the contact resistance. We finally show characterization results obtained using NO2 and NH3 as a target gas.