Semiconducting Single-walled Carbon Nanotubes Networks Research Articles

Investigating Charge Transport in Semiconducting Single‐Walled Carbon Nanotube Networks by Charge Modulation Microscopy

AbstractSolution‐processed networks of semiconducting single‐walled carbon nanotubes (SWCNTs) hold promise as active layers for large‐scale digital circuits, thermoelectric devices, and healthcare applications. Yet, the combined effects of local network properties and (n,m) species composition need to be addressed to improve the performance and reproducibility of state‐of‐the‐art devices. Charge modulation microscopy (CMM) is used to investigate charge transport in field‐effect transistors (FETs) based on monochiral (6,5) SWCNT networks and multichiral networks containing five semiconducting SWCNT species with different band gaps. By mapping the charge‐modulated signal within the FET channel with sub‐micrometer resolution, the spatial distribution of free carriers and its evolution during the switching of the FET are revealed. The CMM maps provide direct evidence that holes and electrons are transported preferentially through the same percolation paths. A moderate positive correlation between the SWCNT density in the monochiral (6,5) network and the charge density in subthreshold regime is demonstrated. In multichiral networks, the charge transport paths are on average less fragmented when involving low band gap species. CMM emerges as a valuable technique to estimate the size of preferential percolation domains associated with the average distance traversed by charge carriers on SWCNTs of the same chirality before tunneling onto other species.

Comprehensive Model for the Thermoelectric Properties of Two-Dimensional Carbon Nanotube Networks

Networks of semiconducting single-walled carbon nanotubes (SWCNTs) are interesting thermoelectric materials due to the interplay between CNT and network properties. Here, we present a unified model to explain charge and energy transport in SWCNT networks. We use the steady-state master equation for the random resistor network containing both intra- and intertube resistances, as defined through their one-dimensional density of states that are modulated by static Gaussian disorder. The tube-resistance dependence on the carrier density and disorder is described through the Landauer formalism. Electrical and thermoelectric properties of the network are obtained by solving Kirchhoff's laws through a modified nodal analysis, where we use the Boltzmann-transport formalism to obtain the conductivity, Seebeck coefficient, and electronic contribution to the thermal conductivity. The model provides a consistent description of a wide range of previously published experimental data for temperature and charge-carrier-density-dependent conductivities and Seebeck coefficients, with energetic disorder being the main factor to explain the experimentally observed mobility upswing with carrier concentration. Moreover, we show that, for lower disorder energies, the Lorentz factor obtained from the simulation is in accordance with the Wiedemann-Franz law for degenerate-band semiconductors. At higher disorder, deviations from simple band behavior are found. Suppressed disorder energy and lattice thermal conductivity can be the key to higher thermoelectric figures of merit, zT, in SWCNT networks, possibly approaching or even exceeding zT = 1. The general understanding of transport phenomena will help the selection of chirality, composition, and charge-carrier density of SWCNT networks to improve their efficiency of thermoelectric energy conversion.

Probing Charge Transport in Sp 3-Functionalized Single-Walled Carbon Nanotubes with Terahertz Spectroscopy

The covalent functionalization of semiconducting single-walled carbon nanotubes (SWCNTs) with luminescent sp 3-hybridized defects gives rise to tunable, narrowband emission in the near-infrared and increases their photoluminescence quantum yields [ACS Nano 2019, 13, 9259]. To enable their application in electrically driven light sources, a detailed understanding of the impact of sp 3 defects on charge transport within individual nanotubes and nanotube networks is required. Here, we investigate the ultrafast carrier dynamics in sp 3-functionalized, polymer-sorted (6,5) SWCNT dispersions and thin films using optical-pump terahertz-probe (OPTP) spectroscopy. OPTP spectroscopy relies on the photo-generation of charge carriers that are subsequently probed by a single-cycle terahertz pulse, rendering it a highly suitable technique for the determination of intrinsic charge transport properties (e.g., carrier mobility) in a quantitative and contact-free fashion. We observe a large transient photoconductivity of pristine and functionalized nanotubes that is dominated by the contribution of free charge carriers. The photoconductivity intensity and lifetime decrease upon functionalization, corroborating the impact of sp 3 defects on intra-nanotube charge transport. Through combination of (temperature-dependent) OPTP measurements of sp 3-functionalized SWCNT dispersions and films with electrical measurements of ambipolar, light-emitting field-effect transistors based on functionalized SWCNT networks [ACS Nano 2021, 15, 10451], we can separate the contributions of sp 3 defects (intra-nanotube) and nanotube junctions (inter-nanotube) to charge transport in such networks and distinguish the short- and long-range transport characteristics. Fig. 1: Ultrafast optical-pump terahertz-probe spectroscopy is a highly suitable technique to study the intrinsic charge transport properties of sp 3-functionalized (6,5) SWCNTs in dispersions and thin films. Figure 1

Networks of as-dispersed, polymer-wrapped (6,5) single-walled carbon nanotubes for selective Cu2+ and glyphosate sensing.

Networks of semiconducting single-walled carbon nanotubes (SWNTs) can be used as the transducing layer for sensors based on water-gated transistors. To add specific sensing capabilities, SWNTs are often functionalized with additional moieties or selective membranes are applied, thus increasing the complexity of the fabrication process. Here we demonstrate that drop-cast networks of monochiral (6,5) SWNTs, which are commonly dispersed in organic solvents with the polyfluorene-bipyridine copolymer PFO-BPy, can be employed directly and without additional functionalization or ion-selective membranes to detect Cu2+ ions over a wide range of concentrations in aqueous solutions. The observed voltage shifts of water-gated transistors with these (6,5) SWNT networks directly correlate with the cupric ion concentration. They result from induced n-doping due to the complexation of positive copper ions to the bipyridine units of the wrapping polymer. Furthermore, the competitive binding of Cu2+ to the herbicide glyphosate as well as to biologically relevant pyrophosphates can be used for the direct detection and quantification of these molecules at nano- to micromolar concentrations.

Charge Transport in Networks of sp3-Functionalized Single-Walled Carbon Nanotubes

The controlled introduction of luminescent sp3 defects into semiconducting single-walled carbon nanotubes (SWCNTs) leads to narrowband, tunable emission in the near-infrared and increases their photoluminescence (PL) quantum yield for potential optoelectronic applications [ACS Nano 2019, 13, 9259]. While the spectroscopic properties of sp3-functionalized SWCNTs are already well-established, a detailed understanding of the impact of functionalization on their electrical performance especially in a network is still lacking. Here, we investigate charge transport in ambipolar, light-emitting field-effect transistors based on networks of pristine and sp3-functionalized (6,5) SWCNTs. While both hole and electron mobilities decrease with increasing degree of functionalization, the transistors remain fully operational, showing electroluminescence from the defect states. Charge-modulated PL spectroscopy, which exclusively probes the mobile charge carriers in SWCNT networks [ACS Nano 2020, 14, 2412], confirms that the defects are efficiently sampled by mobile carriers and consequently, sp3-functionalized SWCNTs actively participate in the charge transport within the network. Temperature-dependent transport measurements of transistors combined with ultrafast photoconductivity dynamics of individually dispersed SWCNTs measured by optical-pump THz-probe spectroscopy provide further insights into the impact of sp3-functionalization on charge transport within a single nanotube and within a network.

Phenanthroline Additives for Enhanced Semiconducting Carbon Nanotube Dispersion Stability and Transistor Performance

Dispersions of purely semiconducting single-walled carbon nanotubes (SWCNTs) have enabled solution-processed SWCNT networks as active layers in field-effect transistors (FETs) with high carrier mob...

Charge and Thermoelectric Transport in Polymer-Sorted Semiconducting Single-Walled Carbon Nanotube Networks.

Understanding the charge transport mechanisms in chirality-selected single-walled carbon nanotube (SWCNT) networks and the influence of network parameters is essential for further advances of their optoelectronic and thermoelectric applications. Here, we report on charge density and temperature-dependent field-effect mobility and on-chip field-effect-modulated Seebeck coefficient measurements of polymer-sorted monochiral small-diameter (6,5) (0.76 nm) and mixed large-diameter SWCNT (1.17-1.55 nm) networks (plasma torch nanotubes, RN) with different network densities and length distributions. All untreated networks display balanced ambipolar transport and electron-hole symmetric Seebeck coefficients. We show that charge and thermoelectric transport in SWCNT networks can be modeled by the Boltzmann transport formalism, incorporating transport in heterogeneous media and fluctuation-induced tunneling. Considering the diameter-dependent one-dimensional density of states (DoS) of the SWCNTs composing the network, we can simulate the charge density and temperature-dependent Seebeck coefficients. Our simulations suggest that scattering in these networks cannot be described as simple one-dimensional acoustic and optical phonon scattering as for single SWCNTs. Instead the relaxation time is inversely proportional to energy (τ ∝ (E - EC)s, s = -1, EC being the energy of the first van Hove singularity), presumably pointing toward the more two-dimensional character of scattering events and the necessity to include scattering at the SWCNT junctions. Finally, our observation of higher power factors in trap-free, 1,2,4,5-tetrakis(tetramethylguanidino)benzene-treated (6,5) networks than in the RN networks emphasizes the importance of chirality selection to tune the width of the DoS. To benefit from both higher intrinsic mobilities and a large thermally accessible DoS, we propose trap-free, narrow DoS distribution, large-diameter SWCNT networks for both electronic and thermoelectric applications.

Resistive Analysis of Scattering-Dependent Electrical Transport in Single-Wall Carbon-Nanotube Networks

With the development of separation and sorting techniques, highly enriched semiconducting single-walled carbon nanotubes (SWNTs) have become widely accessible, which has led to the rapid growth of high-performance solution-processed SWNT-based thin-film field-effect transistors (TFTs) showing capabilities comparable to the ideal single-SWNT FETs. With such improvements, theoretical studies and detailed analyses of these networks have become necessary. In this work, a model justifying the near-ideal electrical transport in SWNT networks is presented. The field-dependent resistive properties of the networks are calculated using a numerical solver based on the derived individual resistances of SWNTs and the intertube couplings. The model is capable of simulating mixed SWNT networks consisting of both metallic and semiconducting nanotubes of varying chiralities. Our analysis reveals that the high electrical currents in networks could be largely attributed to the suppression of phonon scattering and strong intertube couplings in highly dense SWNT networks (>30-40 SWNTs/μm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> ). Comparisons between the simulated and experimental results indicate good agreement thereby demonstrating the accuracy of the proposed model.

Charge Transport in Mixed Semiconducting Carbon Nanotube Networks with Tailored Mixing Ratios.

The ability to prepare uniform and dense networks of purely semiconducting single-walled carbon nanotubes (SWNTs) has enabled the design of various (opto-)electronic devices, especially field-effect transistors (FETs) with high carrier mobilities. Further optimization of these SWNT networks is desired to surpass established solution-processable semiconductors. The average diameter and diameter distribution of nanotubes in a dense network were found to influence the overall charge carrier mobility; e.g., networks with a broad range of SWNT diameters show inferior transport properties. Here, we investigate charge transport in FETs with nanotube networks comprising polymer-sorted small diameter (6,5) SWNTs (0.76 nm) and large diameter plasma torch SWNTs (1.17-1.55 nm) in defined mixing ratios. All transistors show balanced ambipolar transport with high on/off current ratios and negligible hysteresis. While the range of bandgaps in these networks creates a highly uneven energy landscape for charge carrier hopping, the extracted hole and electron mobilities vary nonlinearly with the network composition from the lowest mobility (15 cm2 V-1 s-1) for only (6,5) SWNT to the highest mobility (30 cm2 V-1 s-1) for only plasma torch SWNTs. A comparison to numerically simulated network mobilities shows that a superposition of thermally activated hopping across SWNT-SWNT junctions and diameter-dependent intratube transport is required to reproduce the experimental data. These results also emphasize the need for monochiral large diameter nanotubes for maximum carrier mobilities in random networks.

Charge Transport in Mixed Semiconducting SWCNT Networks with Tailored Diameter Distributions

The recent availability of purely semiconducting single-walled carbon nanotubes (SWCNTs) and the ability to prepare dense and uniform SWCNT networks from dispersion has led to promising applications in optoelectronics, e.g. in field-effect transistors (FETs), light-emitting diodes, notch filters and solar cells. Further optimization and controlled tuning of nanotube networks is desirable to establish them as competitive semiconducting materials. The impact of the SWCNT network composition on the overall charge transport was demonstrated recently [1]. The uneven energy landscape of networks with a broad tube diameter and thus bandgap distribution results in inferior transport properties. Contrary to common belief, the charge transport in networks is not solely determined by tube-tube junctions but intra-nanotube transport also has a significant impact on the overall network mobility. Here, we examine this hypothesis in a detailed charge transport study of FETs with SWCNT networks consisting of polymer-sorted small diameter (6,5) SWCNTs (0.76 nm) and large diameter plasma torch SWCNTs (1.2-1.5 nm) with different but controlled mixing ratios. All FETs showed balanced ambipolar transport with low off-currents and negligible hysteresis. The extracted mobilities varied characteristically with network composition. The obtained experimental data are compared to results from numerically simulated networks with analogous SWCNT compositions. The employed random resistor network model was recently shown to adequately describe the carrier density-dependent mobility in SWCNT networks [2]. [1] M Brohmann et al., J. Phys Chem. C 122, 19886-19896 (2018) [2] S. P. Schießl et al., Phys. Rev. Mater. 1, 046003 (2017) Figure 1

High-Gain Hybrid CMOS Inverters by Coupling Cosputtered ZnSiSnO and Solution-Processed Semiconducting SWCNT

In this paper, high-gain hybrid complementary inverter was first designed and fabricated by coupling cosputtered ZnSiSnO and solution-processed semiconducting single-walled carbon nanotubes (SWCNTs). Field-effect transistors with ZnSiSnO and SWCNT networks show high electrical performance and acceptable bias stability. ZnSiSnO thin-film transistor shows field-effect mobility of 11.6 cm2/ $\text {V}\cdot \text {s}$ , threshold voltage of 0.98 V, and subthreshold swing of 0.18 V/decade. The corresponding values for the SWCNT transistor are 10.2 cm2/ $\text {V}\cdot \text {s}$ , 0.59 V, and 0.21 V/decade, respectively. The ZnSiSnO/SWCNT inverter shows excellent performance with a voltage gain of 41.5, a high noise margin of 2.62 V, and a low noise margin of 1.86 V at a small ${V}_{\text {DD}}$ of 5 V. The peak consumption is only $3.2\times 10^{8}$ W at ${V}_{\text {DD}} = 5$ V. Our finding underscores the coupling of cosputtered ZnSiSnO and solution-processed semiconducting SWCNT as an alternative strategy to the high-performance inverter development and has the potential for widespread technological applications.

(Invited) Improved Charge and Exciton Transport in Polymer-Removed SWCNT Thin Films: Implications for Photovoltaic and Thermoelectric Energy Harvesting

The chemical and physical structure of semiconducting single-walled carbon nanotubes (SWCNTs) results in optical and electronic properties of promise for a wide variety of applications. Until quite recently the presence of metallic SWCNT impurities has hampered efforts to gain a deeper understanding of their true potential, with the additional complication that most commercially available materials contain tens of different chiral species. Significant effort has been devoted to elegant enrichment strategies aimed at extracting tailored semiconducting SWCNT species from the raw soot, from the use of subtly tunable surfactant interactions to the exploitation of specific DNA sequences. However, conjugated polymers, typically based on the fluorene moiety, appear to show the greatest promise with regards to their high selectivity and viability for scalable manufacturing approaches. Unfortunately, the van de Waals forces between the pi-electron systems of the polymer and SWCNT that enable the selective extraction of semiconducting SWCNTs with high purity also make removal of the polymer difficult. Since these polymers typically have a wide bandgap they act as an insulating coating on the surface of the individual SWCNTs within functional networks, inhibiting the transport of energy in the form of excitons and/or charge carriers. Here we demonstrate approaches aimed at replacing the strongly-bound polymers with variants that can be removed using simple solution-based chemical strategies, resulting in networks with vastly improved energy transport properties. We show that removal of the polymer results in a significant enhancement of the charge carrier mobility and electrical conductivity in the SWCNT networks, allowing optimization of the thermoelectric properties for both p-type and n-type transport. Finally, we extend the approach to samples strongly enriched in a single chiral SWCNT species, which allows us to employ transient spectroscopic techniques to probe enhanced exciton transport through the SWCNT network with high spectral fidelity. Our studies highlight a methodology by which high-performance SWCNT thin films can be prepared that could realize their potential for electronic and optoelectronic applications.

Extracting the field-effect mobilities of random semiconducting single-walled carbon nanotube networks: A critical comparison of methods

The field-effect mobility is an important figure of merit for semiconductors such as random networks of single-walled carbon nanotubes (SWNTs). However, owing to their network properties and quantum capacitance, the standard models for field-effect transistors cannot be applied without modifications. Several different methods are used to determine the mobility with often very different results. We fabricated and characterized field-effect transistors with different polymer-sorted, semiconducting SWNT network densities ranging from low (≈6 μm−1) to densely packed quasi-monolayers (≈26 μm−1) with a maximum on-conductance of 0.24 μS μm−1 and compared four different techniques to evaluate the field-effect mobility. We demonstrate the limits and requirements for each method with regard to device layout and carrier accumulation. We find that techniques that take into account the measured capacitance on the active device give the most reliable mobility values. Finally, we compare our experimental results to a random-resistor-network model.

(Invited) Less Is More: Thermoelectric Performance Enhancements in Polymer-Free Semiconducting Single-Walled Carbon Nanotube Networks

There is a growing interest in the use of carbon nanostructures for a variety of electronic and optoelectronic technologies, including energy harvesting applications such as photovoltaics (PV) and thermoelectrics (TE). We will present a series of studies aimed at improving the TE performance of semiconducting single-walled carbon nanotube (s-SWCNT) thin film networks. The optical and electrical performance of SWCNT ensembles is often limited by the presence of metallic single-walled carbon nanotube (m-SWCNT). We will demonstrate a polymer-based purification strategy that effectively eliminates these, and other impurities, from the raw material, leaving s-SWCNTs in extremely high purity. Modification of this extraction process produces s-SWCNT thin film networks where the polymer can be completely removed, resulting in close tube-tube contacts in a dense s-SWCNT network. Removal of the polymer in the solid state, rather than in solution, also minimizes nanotube bundling during network formation. By controlling the bundle size and extent of polymer remaining in the s-SWCNT network we demonstrate TE power factors that almost double the performance of s-SWCNT networks previously demonstrated. We trace the improved performance to an enhanced electrical conductivity, resulting from improved doping and strongly enhanced charge carrier mobility, and analyze our data within the framework of a recently developed thermoelectric transport model. Finally, we demonstrate that the removal of the polymer from the s-SWCNT network has negligible impact on the thermal conductivity, which appears to be limited by dopant-induced phonon scattering processes. These observations demonstrate the ability to exert exquisite control of the thermoelectric performance by controlling the composition of the s-SWCNT network and tuning the carrier density (i.e., Fermi energy), and touts SWCNTs as an avenue for realizing thermally stable room temperature thermoelectric devices fashioned from inexpensive and abundant organic constituents.

Self‐Assembled Monolayer Dielectrics for Low‐Voltage Carbon Nanotube Transistors with Controlled Network Density

Control of the density and uniformity of semiconducting single‐walled carbon nanotube (SWNT) networks is crucial for their application in field‐effect transistors (FETs) and critically depends on the surface of the substrate. Here, the concept of self‐assembled monolayer (SAM) dielectrics on aluminum/aluminum oxide bottom‐gate electrodes is used to create low‐voltage (&lt;2 V) FETs with SWNT networks. The influence of different phosphonic acid SAMs is investigated on the final network density and charge transport properties of solution‐deposited, polyfluorene‐sorted large‐diameter semiconducting SWNT. Positively charged, imidazolium‐terminated SAMs and those with high dipole moments result in the highest network densities for both spin‐coating and dip‐coating. However, FETs with moderate SWNT coverage on SAMs with long alkyl chains and amine or hydroxyl end‐groups yield the best combination of high on‐conductances and high hole mobilities (up to 20 cm2 V−1 s−1) with low trap densities and short critical channel lengths (&lt;2 μm). These parameters are suitable for future short channel, flexible, and high‐frequency FETs.

(Invited) Thermoelectric Properties of Semiconducting Single-Walled Carbon Nanotube Networks

Nanostructured organic semiconductors (OSCs), including single-walled carbon nanotubes (SWCNTs), offer a number of intriguing technological characteristics for thermoelectric applications, such as earth-abundant raw materials, low-cost deposition, and flexible form factors. We will initially present a series of experiments focused on understanding the thermoelectric performance of enriched semiconducting SWCNT networks dispersed in a wide bandgap semiconducting polymer matrix, followed by more recent work aimed at understanding the role that the semiconducting polymer plays in the observed transport properties. Rational choice of the starting SWCNT material and the semiconducting polymer allows us to sensitively tune the s-SWCNT diameter and band gap distributions within the composites. Consistent with theoretical calculations that consider the density of electronic states in individual s-SWCNTs, we observe a distinct dependence of the thermopower and thermoelectric power factor on the bandgap (or diameter) of the carbon nanotubes. We have measured large thermopower values (as high as ~2,500 µV/K for s-SWCNT networks with very low electrical conductivity) and impressive thermoelectric power factor as large as ~350 µW/m·K2. By varying the carrier density injected into the s-SWCNT networks by a stable charge-transfer dopant, we are able to probe the relationship between the electrical conductivity and Seebeck coefficient (thermopower) in the s-SWCNT networks as a function of the carrier density and position of the Fermi energy. For carbon nanotubes prepared by high-pressure carbon monoxide (HiPCO) conversion, as we tune the carrier density, we are able to maintain a thermopower above 100 µV/K over almost the entire range of hole densities, corresponding to conductivities up to 40,000 S/m, resulting in a thermoelectric power factor of >300 µW/m·K2. These studies suggest that the low dimensionality of the SWCNTs has a stronger impact on the electrical conductivity than the thermopower, implying that they are less strongly coupled in these systems than is observed for compound inorganic semiconductors. By modifying an approach that allows us to strip the dispersing polymer from the s-SWCNTs we were able to demonstrate that the polymer appears to play no role in modifying the barriers to electrical transport present at tube-tube junctions, and simply controls the extent of nanotube bundling and thereby the surface area available to the charge-transfer dopant. This study also indicates that densification of the s-SWCNT network results in a two-fold enhancement of the thermoelectric power factor, suggesting that careful control of the amount and nature of the matrix material is required for high-performance s-SWCNT thermoelectric materials. Finally, we present a data from sensitive transport measurement technique, based on a microfabricated silicon nitride thermal isolation platform, to probe transport in the s-SWCNT networks, showing that the thermoelectric figure of merit (zT) is positively correlated with the measurement temperature, increasing by a factor of ~2.5 from 300 K to 350 K. These observations demonstrate the ability to exert exquisite control of the thermoelectric performance by controlling the composition of the s-SWCNT network and tuning the carrier density (i.e., Fermi energy), and touts SWCNTs as an avenue for realizing thermally stable room temperature thermoelectric devices fashioned from inexpensive and abundant organic constituents.

Understanding Charge Transport in Mixed Networks of Semiconducting Carbon Nanotubes.

The ability to select and enrich semiconducting single-walled carbon nanotubes (SWNT) with high purity has led to a fast rise of solution-processed nanotube network field-effect transistors (FETs) with high carrier mobilities and on/off current ratios. However, it remains an open question whether it is best to use a network of only one nanotube species (monochiral) or whether a mix of purely semiconducting nanotubes but with different bandgaps is sufficient for high performance FETs. For a range of different polymer-sorted semiconducting SWNT networks, we demonstrate that a very small amount of narrow bandgap nanotubes within a dense network of large bandgap nanotubes can dominate the transport and thus severely limit on-currents and effective carrier mobility. Using gate-voltage-dependent electroluminescence, we spatially and spectrally reveal preferential charge transport that does not depend on nominal network density but on the energy level distribution within the network and carrier density. On the basis of these results, we outline rational guidelines for the use of mixed SWNT networks to obtain high performance FETs while reducing the cost for purification.

(Invited) Tunable Thermoelectric Power Factor in Semiconducting Single-Walled Carbon Nanotube Networks

Single-walled carbon nanotubes (SWCNTs) are a versatile electronic material being explored as cost-effective, high-performance active materials in a variety of renewable energy applications such as transparent conducting or light-harvesting layers in photovoltaics and inclusions in thermoelectric composites. We present a series of experiments focused on understanding the thermoelectric performance of enriched semiconducting SWCNT networks dispersed in a semiconducting polymer matrix. Rational choice of the semiconducting polymer allows us to sensitively tune the s-SWCNT diameter and band gap distributions within the composites. We use a stable charge-transfer dopant to control the density of carriers in the s-SWCNT network, as determined by the bleach of the absorption corresponding to the S11 excitonic transition. The performance of these transparent conducting s-SWCNT composite networks is comparable to neat p-type and n-type s-SWCNT networks doped by either nitric acid or hydrazine treatments. By varying the carrier density we are able to probe the relationship between the electrical conductivity and Seebeck coefficient (thermopower) in the s-SWCNT networks as a function of the carrier density and position of the Fermi energy. Although the electrical conductivity of the s-SWCNT networks is poor at very low carrier densities we have measured a colossal thermopower as high as ~2,500 µV/K, which is more than an order of magnitude larger than has been previously reported for SWCNT-based material systems and is consistent with theoretical calculations that consider the density of electronic states in individual s-SWCNTs. As we tune the carrier density, we are able to maintain a thermopower above 200 µV/K over almost the entire range of hole densities, corresponding to conductivities up to 1885 S/m, resulting in a thermoelectric power factor of ~100 µW/m·K2. These studies suggest that the low dimensionality of the SWCNTs has a stronger impact on the electrical conductivity than the thermopower, implying that they are less strongly coupled in these systems than is observed for compound inorganic semiconductors. These observations demonstrate the ability to exert exquisite control of the thermoelectric performance by tuning the carrier density and/or Fermi energy, and touts SWCNTs as an avenue for realizing thermally stable room temperature thermoelectric devices fashioned from inexpensive and abundant organic constituents.

Polymer-sorted semiconducting carbon nanotube networks for high-performance ambipolar field-effect transistors.

Efficient selection of semiconducting single-walled carbon nanotubes (SWNTs) from as-grown nanotube samples is crucial for their application as printable and flexible semiconductors in field-effect transistors (FETs). In this study, we use atactic poly(9-dodecyl-9-methyl-fluorene) (a-PF-1-12), a polyfluorene derivative with asymmetric side-chains, for the selective dispersion of semiconducting SWNTs with large diameters (>1 nm) from plasma torch-grown SWNTs. Lowering the molecular weight of the dispersing polymer leads to a significant improvement of selectivity. Combining dense semiconducting SWNT networks deposited from an enriched SWNT dispersion with a polymer/metal-oxide hybrid dielectric enables transistors with balanced ambipolar, contact resistance-corrected mobilities of up to 50 cm2·V–1·s–1, low ohmic contact resistance, steep subthreshold swings (0.12–0.14 V/dec) and high on/off ratios (106) even for short channel lengths (<10 μm). These FETs operate at low voltages (<3 V) and show almost no current hysteresis. The resulting ambipolar complementary-like inverters exhibit gains up to 61.

Solid organic acid tetrafluorohydroquinone functionalized single-walled carbon nanotube chemiresistive sensors for highly sensitive and selective formaldehyde detection

Abstract Single-walled carbon nanotubes (SWNTs) are ideal candidates as active elements for fabricating gas sensors because their electronic structures are easily disrupted by the binding of analytes, but the utilization of SWNT sensors to detect formaldehyde is limited, mainly because of a weak physical adsorption between SWNTs and formaldehyde. Herein we developed a chemiresistive sensor based on tetrafluorohydroquinone (TFQ) functionalized semiconducting SWNT networks for highly sensitive and rapid detection of formaldehyde vapor. The SWNT network was deposited between interdigitated electrodes and modified by solid organic acid TFQ. The interaction of formaldehyde with TFQ on the surface of SWNTs caused significant increase in the conductance of the SWNT sensors, and enabled the sensors to easily detect formaldehyde at parts-per-billion (ppb) concentration level with the response time of less than 1 min, without the need of pre-concentration of the analytes. The functionalized sensors also show excellent selectivity toward formaldehyde over those interfering organic vapors. This sensor could also be recovered when formaldehyde was replaced with referencing gas. The response mechanism was investigated.