Temperature-Dependent Charge Transport in Polymer-Sorted

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C: Physical Processes in Nanomaterials and Nanostructures

Temperature-Dependent Charge Transport in Polymer-Sorted Semiconducting Carbon Nanotube Networks with Different Diameter Distributions Maximilian Brohmann, Marcel Rother, Stefan P. Schiessl, Eduard Preis, Sybille Allard, Ullrich Scherf, and Jana Zaumseil J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b04302 • Publication Date (Web): 30 Jul 2018 Downloaded from http://pubs.acs.org on August 5, 2018

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The Journal of Physical Chemistry

Temperature-Dependent Charge Transport in Polymer-Sorted Semiconducting Carbon Nanotube Networks with Different Diameter Distributions

Maximilian Brohmann1, Marcel Rother1, Stefan P. Schießl1, Eduard Preis2, Sybille Allard2, Ullrich Scherf2, and Jana Zaumseil1,3* 1

Institute for Physical Chemistry, Universität Heidelberg, D-69120 Heidelberg, Germany

2

Macromolecular Chemistry and Institute for Polymer Technology, Wuppertal University, D-

42097 Wuppertal, Germany 3

Centre for Advanced Materials, Universität Heidelberg, D-69120 Heidelberg, Germany

1 ACS Paragon Plus Environment

The Journal of Physical Chemistry 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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ABSTRACT

The availability of purely semiconducting single-walled carbon nanotube (s-SWCNT) dispersions has prompted their widespread application in solution-processed thin film transistors with excellent device performance but has also raised the question, how their precise composition influences charge transport properties in random networks. Here, we compare hole and electron transport in three different polymer-sorted s-SWCNT networks from nearly monochiral (6,5) nanotubes (diameter 0.76 nm) to mixed networks of s-SWCNTs with medium (0.8-1.3 nm) and large (1.2-1.6 nm) diameters. Temperature-dependent fieldeffect mobilities are extracted from gated four-point probe measurements that exclude any contributions by contact resistance and indicate thermally activated transport. The mobility data can be fitted to the fluctuation induced tunneling (FIT) model, although with significant differences between the network compositions. The network with the broadest diameter and thus bandgap range results in the strongest temperature dependence in agreement with numerical simulations based on a random resistor model of nanotube junctions. However, the experimental data for mixed networks of large diameter nanotubes and their deviation from the simple junction model implies a significant contribution of intra-nanotube transport with its specific diameter and temperature dependence to the overall charge transport properties of the network.


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INTRODUCTION Over the last decade, semiconducting single-walled carbon nanotubes (s-SWCNTs) have emerged as a promising material for thin film electronics due to their outstanding charge transport, optical and mechanical properties.1-2 For the application of s-SWCNTs on a largescale, solution-processed random networks are better suited than single nanotubes or aligned arrays.3-5 These networks can be created by spin-coating or printing from purified dispersions. Since all bulk nanotube growth processes produce mixtures of metallic and semiconducting nanotubes with different diameter ranges, highly selective separation methods must be applied to achieve the necessary semiconducting purity for reproducible devices. Selective dispersion by wrapping with conjugated polymers has become one of the most effective and popular methods to produce semiconducting nanotube dispersions with high selectivity (>99.8 % s-SWCNTs) and high yield.6-9 Depending on the chosen nanotube raw material and wrapping polymer the obtained dispersions contain mixtures of certain semiconducting SWCNT species with different diameters and thus different bandgaps. The availability of such highly purified s-SWCNT dispersions has enabled a broad range of applications in flexible and stretchable optoelectronics over the past few years.3,

10-16

In

particular, field-effect transistors (FETs) and electronic circuits based on s-SWCNT networks and thin films have shown excellent device performance.5,

17

However, highly purified

semiconducting carbon nanotubes remain a precious resource and thus using the optimum network density and composition for maximum charge carrier mobility and on/off ratio in FETs is important and requires a fundamental understanding of all network parameters and their impact. Apart from obvious parameters, such as density and nanotube length, the composition of a given network in terms of the present s-SWCNT species is expected to influence the overall



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charge transport properties. Current flow within a single s-SWCNT is already well understood and can be described as ballistic transport for short distances18 or as diffusive transport for nanotube lengths above the scattering mean free path with field-effect mobilities of about 103-104 cm2V-1s-1 at room temperature.19-21 However, in random s-SWCNT networks charges (holes or electrons) have to hop from one nanotube to another, which is generally seen as the bottleneck for transport and hence as the limitation of the effective carrier mobility in nanotube networks to typically less than 100 cm2V-1s-1.22 Conductive atomic force microscopy measurements indicate that the SWCNT-SWCNT junction resistances are orders of magnitude higher than the intra-nanotube resistance and support this notion.23-24 However, very little is known about the factors governing the hopping process (e.g. doping, contact angle, residual surfactant etc.) or the effect of the surrounding nanotubes on transport along a single nanotube in a network. One of the standard approaches to investigate charge transport in semiconductors of all types is to perform temperature-dependent measurements of conductivity or carrier mobility. Such measurements were previously carried out for a number of dense and purely semiconducting SWCNT networks and all revealed thermally activated transport suggesting charge hopping at the inter-nanotube junctions.25-28 Various classical transport models were employed to try to fit the mobility data. The fluctuation induced tunneling (FIT) model29 was generally found to describe the temperature dependence of the experimental mobility data much better than the variable range hopping (VRH) model, which is well established for the description of other disordered semiconductors.30 As the FIT model was not developed for s-SWCNTs but for materials with disordered conductive segments, it does not take into account the different densities of states or intrinsic mobilities of different nanotubes. Hence, it remains unclear, what conclusions about the transport can be drawn from the extracted fit parameters. Furthermore, a recent study found that the dipolar disorder induced by the gate dielectric also 4 ACS Paragon Plus Environment

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had some influence on the temperature dependence of the carrier mobility in a given s-SWCNT network.28 All of these previous studies investigated only one network composition each, despite the observed influence of the nanotube diameter distribution on the transport characteristics in network FETs.31 Except one, they all investigated only hole transport and the FETs showed some non-idealities such as hysteresis. A comprehensive understanding of the various contributions to charge transport in random semiconducting nanotube networks as well as the role of residual polymer is still missing. In this work, we specifically investigate the impact of the s-SWCNT network composition on the temperature-dependent charge transport properties in nanotube network FETs. We compare three different polymer-sorted and aerosol-jet printed s-SWCNT networks with different nanotube diameter and thus bandgap distributions. For all SWCNT network compositions - with and without residual wrapping polymer - we observe balanced ambipolar transport, high on/off ratios of 106 and no hysteresis, which indicates intrinsic transport properties without metallic nanotubes or other significant extrinsic effects that could distort the obtained temperature-dependent mobilities. A gated four-point probe (gFPP) transistor geometry allows us to separate and determine contact resistance and intrinsic field-effect mobilities of holes and electrons independently at temperatures from 300 K to 100 K. Based on these temperature-dependent data, that can be fitted well with the FIT model, we show the impact of both the SWCNT diameter distribution and the mean diameter on charge transport and charge injection in random nanotube networks. We compare the experimental data to a recently developed random-resistor network model32 to evaluate and separate the contributions of inter-nanotube hopping and intra-nanotube transport to the network mobility.



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EXPERIMENTAL SECTION Preparation of SWCNT Dispersions. CoMoCAT™ single-walled carbon nanotubes (CHASM Advanced Materials Inc., SG65i-L58, diameter 0.7-1.0 nm), HiPco™ single-walled carbon nanotubes (Unidym Inc., batch 2172, diameter 0.8-1.3 nm), plasma torch singlewalled carbon nanotubes (Raymor Industries Inc., RN-220, diameter 0.9-1.5 nm, batch RN23-118) and poly[(9,9-di-n-octylfluorene-2,7-diyl)-alt-(2,2’-bipyridine-6,6’-diyl)] (PFOBPy, American Dye Source Inc., ADS153UV, Mw = 34 kg·mol−1) were used as purchased. Poly(3-dodecylthiophene-2,5-diyl) (P3DDT, Mw = 9.7 kg·mol−1) was synthesized via Grignard metathesis reaction. Monochiral dispersions of (6,5) nanotubes were obtained as described recently.8 Briefly, 0.5 g·L-1 PFO-BPy were dissolved in toluene (anhydrous, Sigma), 0.38 g·L-1 of CoMoCAT™ raw material was added, and mixed (10230 rpm) in a shear force mixer (Silverson L2/Air) for 72 h at 20 °C. After centrifugation at 60 000 g (Beckman Coulter Avanti J-26XP) for 60 min the supernatant was collected and centrifuged again at 284 600 g (Beckman Coulter Optima XP) for 13 h to pelletize the wrapped SWCNTs. HiPco™ and plasma torch nanotubes were selectively dispersed by adding 1.5 g·L1

of SWCNT raw material to 2 g·L-1 polymer solution (P3DDT or PFO-BPy, respectively) in

toluene followed by bath sonication for 50 min at 20 °C, centrifugation and pelletization as described above. All SWCNT pellets were washed three times with tetrahydrofuran (THF) to remove residual unwrapped polymer and dried and stored in air. Pellets were redispersed in toluene using bath sonication immediately before further usage. Device Fabrication. Bottom electrodes in a four-point probe layout (L = 40 µm; W = 1 mm; pair of voltage probes of width WP1 = WP2 = 4 µm, separated by a distance D = 24 µm and each 6 µm from the respective electrode) were patterned on thin glass substrates (SCHOTT AG, AF 32® eco) by photolithography and electron beam evaporation of chromium (2 nm) and gold (30 nm) followed by lift-off in N-methyl-2-pyrrolidone (NMP). The SWCNTs were 6 ACS Paragon Plus Environment

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deposited via aerosol-jet printing (Optomec Inc., AJ200) of the redispersed nanotube pellets as described previously.11 The E11 peak absorbance for 1 cm path length of the s-SWCNT dispersions was adjusted to 1.0 (CoMoCAT/PFO-BPy at 996 nm and plasma torch/PFO-BPy at 1637 nm) or to 0.5 (HiPco/P3DDT at 1141 nm) by dilution with toluene. The ink was prepared by adding 5 vol% terpineol to the respective SWCNT dispersion to ensure a dense aerosol. To achieve isotropic SWCNT networks the nanotube ink was printed in horizontal and vertical lines over the channel area with a 25 µm pitch. The deposited SWCNT films were washed with THF and isopropanol to remove residual terpineol and wrapping polymer. For polymer-stripping of the deposited CoMoCAT/PFO-BPy networks33 the respective samples were annealed at 400 °C for 60 min in vacuum (5·10-3 mbar). Subsequently the nanotube films were immersed in 100 mL toluene with 7 mg rhenium(I) pentacarbonyl chloride (Re(CO)5Cl, Sigma) and stirred for 2.5 h at 110 °C. Two washing steps in toluene and THF at 110 °C and 60 °C for 1 h each were performed to remove precipitated rhenium salt and polymer. All nanotube networks were patterned using standard photolithography and oxygen plasma treatment (Nordson MARCH, AP-600/300™; 100 W for 2 min) to remove all SWCNTs outside the channel area. Prior to the deposition of the hybrid dielectric all samples were annealed at 300 °C for 45 min in dry nitrogen atmosphere to remove residual moisture and solvents. Spin coating of 6 g·L-1 poly(methyl methacrylate) (PMMA, Polymer Source Inc., syndiotactic, Mw = 350 kg·mol−1) in n-butyl acetate at 6000 rpm for 60 s followed by atomic layer

deposition

(Ultratech

Inc.,

Savannah

S100)

at

100 °C

using

tetrakis(dimethylamino)hafnium (Strem Chemicals Inc.) and water as precursors resulted in 61 nm HfOx on top of a 11 nm layer of PMMA. Thermal evaporation of 30 nm silver top gate electrodes using shadow masks completed the devices.



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Characterization. UV-vis-nIR absorption spectra were recorded using a Cary 6000i spectrometer (Varian Inc.). Atomic force microscope images were obtained using a Dimension Icon (Bruker Corp.) atomic force microscope in ScanAsyst® mode. Currentvoltage measurements were performed under vacuum (≤ 10-6 mbar) in a closed-cycle cryogenic probe station (Lake Shore Cryotronics Inc., CRX-6.5K) with an Agilent 4155C semiconductor parameter analyzer. Temperature-dependent measurements always started at the base temperature of the system (10 K). After each temperature step of 20 K a hold time of 20 min allowed the system to thermally equilibrate before measurements. Device capacitances were measured directly on each transistor using an impedance spectrometer (Ametek Inc., Modulab XM MTS). The effective areal capacitance was extracted from the on-state at a frequency of 1 kHz.

RESULTS AND DISCUSSION To investigate semiconducting SWCNT networks with very different diameter and thus bandgap distributions and to correlate those with their charge transport properties in fieldeffect transistors we selectively dispersed semiconducting nanotubes from different growth processes in toluene via polymer-wrapping with two different polymers.34-36 Figure 1 shows the absorption spectra of the resulting s-SWCNT dispersions (D1-D3) after removal of unwrapped excess polymer. The combination of CoMoCAT raw nanotube material with the polyfluorene derivative PFO-BPy (Figure 1a) yielded an essentially monochiral dispersion of (6,5) SWCNTs (D1) with a diameter of 0.76 nm and a large bandgap of 1.27 eV as previously demonstrated.8 The expected sharp E11 and E22 transitions are indicated in Figure 1b. HiPco nanotubes dispersed with the polythiophene derivative P3DDT resulted in a range of different semiconducting SWCNTs (D2) with a wide diameter distribution from 0.76 to 1.31 nm and a 8 ACS Paragon Plus Environment

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correspondingly broad bandgap range from 0.80 to 1.27 eV (see Figure 1c). Using PFO-BPy again but as a wrapping polymer for plasma torch grown SWCNTs gave a dispersion (D3) of semiconducting nanotubes with larger diameters (1.17 – 1.55 nm) and small bandgaps (0.70 – 0.88 eV), although with a narrower distribution compared to the HiPco nanotube dispersion. Note, the estimated diameter and bandgap ranges were based on the E11 and E22 transitions in UV-vis-nIR absorption data. All absorption spectra indicated a high purity of the s-SWCNT dispersions with a negligible amount of residual wrapping polymer, which was achieved by pelletizing the s-SWCNT dispersions, repeated washing with THF and redispersing them in toluene (see experimental section for details). They were also essentially free of any metallic nanotubes as confirmed by Raman measurements (see Supporting Information, Figure S1). A comparison of the extracted SWCNT diameter and bandgap distributions based on the Raman radial breathing mode (RBM) data with the UV-vis-nIR absorption-based ranges is shown in Supporting Information, Table S1.



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Figure 1. (a) Molecular structures of the employed wrapping polymers PFO-BPy and P3DDT, (b) UV-vis-nIR absorption spectra of the SWCNT dispersions in toluene (normalized to the largest s-SWCNT E11 absorption peak), and (c) bandgap ranges of the respective s-SWCNT dispersions (D1-D3) based on UV-vis-nIR absorption data. To prepare dense and comparable randomly oriented s-SWCNT networks from all nanotube dispersions for field-effect transistors (FETs) in a bottom-contact/top-gate geometry as shown in Figure 2a, aerosol-jet (AJ) printing was chosen as the deposition technique due to its high 10 ACS Paragon Plus Environment

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reproducibility and low material consumption.11 The different s-SWCNT dispersions were printed directly onto photolithographically structured gold source (S) and drain (D) electrodes and two pairs of voltage probes (VP1, VP2) within the channel that enabled gated four-pointprobe (gFPP) measurements. A hybrid dielectric consisting of 11 nm PMMA and 61 nm HfOx and a silver top gate electrode completed the devices. As shown before, the PMMA facilitated a smooth interface with the s-SWCNTs with a low trap-density and dipolar disorder, while HfOx with a high dielectric constant ensured low-voltage device operation and encapsulation.37 The gFPP geometry (Figure 2b) permitted direct determination of the gate-voltage dependent contact resistance (RC) in all devices and extraction of the intrinsic and temperature-dependent charge carrier mobilities within the networks without any aberration due to contact resistance changes. The SWCNT network density was adjusted in the printing process to be high (≥40 SWCNT/µm, see Figure 2c) to reduce interaction of the active channel with the polar glass substrate and thus to suppress current hysteresis to a minimum.11,

38

The high nanotube density also allowed for a more robust comparison

between the different nanotube networks despite some expected variations in nanotube length (~1 µm) as the observed carrier mobility in network transistors typically saturates above a linear network density of ~15 SWCNT/µm.38

Figure 2. (a) Schematic device structure of the fabricated SWCNT FETs with Cr/Au bottom electrodes in a four-point-probe layout, printed nanotube network, hybrid dielectric (PMMA/HfOx) and silver top gate electrode. (b) Optical micrograph of a typical device



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showing two pairs of voltage probes (VP1, VP2) in the channel. (c) Atomic force microscopy image of a representative printed nanotube film with high network density.

Transfer characteristics of the fabricated transistors at small source-drain bias (VDS = -0.1 V) showed operation at very low gate voltages, hysteresis-free ambipolar transport and a high on/off ratio of about 106 for all three s-SWCNT network compositions, measured at room temperature (see Supporting Information, Figure S2). For a meaningful investigation of charge transport in these networks it is crucial to reduce unintentional side effects stemming from the substrate (e.g. electron trapping), the contacts (contact resistance) or impurities (traps) to a minimum, i.e., the intrinsic properties should be observed rather than extrinsic effects. This requirement was apparently fulfilled for all presented s-SWCNT network transistors as indicated by the balanced hole and electron transport, lack of hysteresis and low turn-on voltages. The high on/off ratios further corroborated the absence of noticeable amounts of metallic nanotubes in these very dense networks. The off-currents were limited by the gate leakage (~ few pA). Furthermore, all output characteristics show perfectly linear current increases in the low drain voltage region, which indicates ohmic injection of both electrons and holes for all network compositions (see Supporting Information, Figure S3). To ensure accurate mobility calculations the gate capacitances where measured directly on each transistor while the devices were in the on-state.38 SWCNT networks based on dispersions D1 and D3 exhibited contact-resistance corrected carrier mobilities between 4 and 9 cm2 V-1 s-1. The HiPco/P3DDT networks (D2) showed 5-20 times lower on-currents and correspondingly lower mobilities (0.2 - 1 cm2 V-1 s-1), especially for electrons. All extracted device parameters are listed in Table S2, Supporting Information. The significantly lower carrier mobility in FETs based on the HiPco/P3DDT networks might be a result of the broad diameter and thus bandgap distribution of the network. As proposed


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recently, the charge transport through a network of different semiconducting SWCNTs might be limited by a small fraction of small bandgap nanotubes acting as charge traps if the energy difference is large compared to kT (k – Boltzmann constant, T - temperature) and the carrier density is low.31 In the worst case only a fraction of the present SWCNTs will contribute to the overall current and thus limit the device performance. The effective network density for charge transport and the impact of small bandgap nanotubes as traps is expected to depend both on charge carrier density and temperature. With this hypothesis in mind one might expect that the D2 networks with a broad bandgap distribution should also show a stronger dependence of the effective carrier mobility on temperature compared to the other networks. Hence, we recorded temperature-dependent transfer characteristics to gain further insight into charge transport and charge injection in these different s-SWCNT networks. Although reproducible transfer curves could be measured down to the base temperature of the cryostat (10 K), we limited the investigated temperature range to 100 – 300 K (see Figure 3). At low temperatures and for high current densities the actual temperature in the transistor channel can differ substantially from the set temperature in the cryogenic sample chamber due to current-induced Joule heating as described by Nikoforov et al.39 The resulting error is assumed to be fairly high (>10 %) below 100 K and can lead to distorted temperature-dependent mobility values unless the temperature is measured directly in the channel. Hence, 100 K was chosen here as the lowest temperature for analysis and low drain voltages were used to reduce the overall current density and Joule heating. All samples were first cooled down in vacuum and then heated up stepwise. After each temperature step of 20 K a hold time of 20 min allowed the system to fully equilibrate before each measurement.



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Figure 3. Temperature-dependent transfer characteristics (forward and reverse gate voltage sweeps at VDS = -0.1 V) at temperatures from 100 K to 300 K (measured with steps of 20 K, only every other transfer curve shown here) for all s-SWCNT networks.

All s-SWCNT network FETs (D1-D3) show monotonically decreasing on-currents with decreasing temperature as expected for thermally activated charge transport. However, there is a clear difference between the HiPco (D2, broad diameter distribution) network and the other two, i.e., D1 (monochiral (6,5) SWCNTs) and D3 (narrow distribution of large diameter nanotubes). While the on-currents decrease by less than one order of magnitude from 300 K to 100 K for the D3 network FETs and by a factor of 30 for the monochiral D1 network transistors, the drain current drops by more than two orders of magnitude for the HiPco network (D2). In addition, the onset-voltages shift to higher absolute values for both electrons and holes at lower temperatures for all network compositions as shown in Figure 4. Thus, a simple comparison of on-currents or on-conductance is inadequate to describe the temperature-dependent charge transport in these three networks and their differences.


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Figure 4. Temperature-dependent onset-voltages for electron and hole transport for all sSWCNT network compositions.

The onset-voltages and the gap between electron and hole transport onset at room temperature appear to correlate with the mean s-SWCNT bandgap of the respective nanotube film. The smallest gap between onset-voltages (see Table S2, Supporting Information) was observed for the narrow bandgap network D3 and the largest for the monochiral (6,5) network (D1) with the biggest bandgap. However, upon cooling to 100 K the absolute onsetvoltages increased by about 1 V for both D1 and D3, whereas for the HiPco network (D2) they shifted by up to 3 V. Assuming the absence of significant extrinsic trap states, this shift might be interpreted as the result of a number of small bandgap nanotubes (below the percolation limit) acting as trap states within the network that have to be filled before the network becomes conducting but do not contribute to the current at low temperatures. The energy barrier between those and the surrounding nanotubes might be shallow enough to be overcome at higher temperatures and charge carrier densities but at low temperatures they become effective deep trap states, leading to an onset voltage shift. This intrinsic trapping would affect holes and electrons equally while extrinsic traps (e.g., by residual polymer or water) should be polarity specific.



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The observed onset voltage shifts also raise the question how to extract temperaturedependent carrier mobilities and contact resistances (RC) in a reproducible and comparable way for all networks, given that both depend on the mobile carrier density.7, 21, 40 Extracting these values at the maximum gate voltage (VG = ± 5 V) for all devices and temperatures is questionable because the effective mobile charge carrier concentrations will differ significantly depending on the onset-voltage shift, possibly leading to the observation of ambiguous transport behavior. Hence, the most reliable way is to extract mobilities and RC at a constant gate overdrive, i.e., the difference between gate voltage and onset-voltage. Here we chose to extract the carrier mobilities and contact resistances for all network FETs and temperatures at a gate overdrive of ±2 V, which should correspond to a mobile carrier concentration of approximately 1·1012 cm-2. The output characteristics for all SWCNT network compositions indicate ohmic contacts (see Figure S3, Supporting Information). The distorting influence of contact resistance and its modulation by the gate voltage on the extracted carrier mobilities of high mobility semiconductors were recently highlighted.41-43 In particular for temperature-dependent studies any effect of the contact resistance on the mobility should be excluded. Hence, a thorough analysis of RC was performed using the gFPP transistor layout. Contact resistances were determined directly by estimating the voltage drop at the electrodes by extrapolation of the potential gradient inside the transistor channel measured via the two voltage probes. The contact resistance-corrected charge carrier mobilities (µRC) were calculated likewise.44-46 To avoid any detrimental effects by stray fields the SWCNT films were patterned such that there was no overlap with the voltage probes outside the gated channel area (see experimental section).47 The width-normalized contact resistances (hole or electron injection) for FETs with Cr/Au electrodes and for all three network compositions (D1-D3) showed a pronounced gate voltage


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dependence, i.e., lower RC for higher VG (see Supporting Information, Figure S4) as well as the expected increase of RC upon cooling to 100 K. The lowest RC values for the monochiral (6,5) network (D1) measured at room temperature were 2.9 kΩ·cm for holes and 6.6 kΩ·cm for electrons. These values are fairly similar and indicate an electrode work function centered at the nanotube bandgap. For the HiPco networks (D2) the contact resistances were somewhat imbalanced indicating a higher injection barrier for electrons (RC = 41 kΩ·cm) than for holes (RC = 4.7 kΩ·cm), which is in agreement with previous literature values for this SWCNT network composition.28 The network with the largest diameters and smallest bandgaps (D3) gave very low and more balanced contact resistances for holes (RC = 0.07 kΩ·cm) and electrons (RC = 0.48 kΩ·cm), again similar to previously reported values.7, 48 A more detailed analysis of the temperature dependence of RC is shown in Figure 5 with the width-normalized contact resistances extracted at 2 V gate overdrive. Cooling from 300 K to 100 K led to a substantial increase of RC for electrons and holes for all network compositions, thus indicating thermally activated carrier injection. Typically, the temperature dependence of charge injection in inorganic or organic semiconductors can be described by an Arrheniuslike behavior, i.e., RC ∝ exp(EA/kT), with EA as the injection barrier.42,

49-50

However, the

extracted RC values for SWCNT networks did not follow such a simple Arrhenius-like trend. Two different gradients for ln(RC) vs. 1/T were found above and below 200 K. For temperatures above 200 K the contact resistance roughly followed the expected Arrhenius behavior and activation energies of 40 – 70 meV could be extracted. These very similar values for all networks indicate that the contact resistance is not just a simple function of the difference between the work function of the gold electrodes and the conduction or valence band of the nanotubes. Energetic disorder, the gate field and the carrier mobility (in vertical



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direction) within the nanotube layer appear to play a significant role as in disordered organic semiconductors.42

Figure 5. Width-normalized temperature-dependent contact resistances for electrons and holes for FETs with all s-SWCNT network compositions extracted at a gate voltage overdrive of 2 V. The solid lines represent Arrhenius fits between 300 K and 200 K to extract activation energies (EA) for the injection of electrons and holes.

Despite the ohmic and relatively low contact resistances, a direct comparison of the linear apparent (µeff) and contact resistance corrected (µRC) carrier mobilities for transistors measured at room temperature shows a significant underestimation of the field-effect mobilities for all network compositions (D1-D3). Even for the relatively long channels with L = 40 µm we found that the corrected carrier mobilities were up to 50% higher than those extracted directly from the transfer curves (see Figure S5, Supporting Information). The relative differences between the apparent and corrected mobilities correlated with the contact resistance values for the respective network. The smallest effect was found for the D3 network transistors. Given the observed gate voltage and temperature dependence of the contact resistance, any distorting influence on the extracted carrier mobilities at different


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temperatures must be excluded to ensure meaningful data for fitting to any transport models. Thus, all reported mobilities in this study are contact resistance corrected. Figure 6 shows the corrected field-effect mobilities (linear regime) for all three network compositions (D1-D3) from 100 K to 300 K. Analogous to the contact resistance all mobility values were extracted at a gate voltage overdrive of 2 V. The overall trends are the same for all networks and similar to previous reports on other polymer-sorted nanotube networks.25, 2728

The carrier mobility decreases with decreasing temperature but levels off at lower

temperatures. We do not observe any anomalous or non-monotonic variations of the hole or electron mobility with temperature. In contrast to charge transport in conventional disordered organic semiconductors, which can be described well with the variable range hopping (VRH) model, a plot of ln(µRC) versus 1/T (or 1/T2) does not yield a linear fit. In agreement with previous studies25-26, 28 we found that the temperature-dependent carrier mobilities in these s-SWCNT networks were fitted best with the fluctuation induced tunneling (FIT) model.29 According to this model, the nanotube junctions can be viewed as tunneling barriers between longer conducting segments. The carrier mobility is then described by

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்ೄ ା்

ቁ

(1)

where A is a temperature-independent prefactor, T is the sample temperature, TS is the transition temperature between thermally independent and thermally activated transport, and TB is the temperature required to overcome the tunneling barrier.29 Hence an activation energy or barrier height EB = kTB can be extracted from the respective fits of the mobility data to the FIT model as represented by the solid lines in Figure 6.



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Figure 6. Temperature-dependent carrier mobilities extracted at a 2 V gate voltage overdrive for FETs with all s-SWCNT networks. Solid lines represent fits corresponding to the fluctuation induced tunneling (FIT) model with EB as the tunneling barrier height.

A comparison of the different SWCNT networks indicates a similar temperature dependence for electron and hole transport in the monochiral (6,5) nanotube (D1) and the larger diameter s-SWCNT (D3) networks with somewhat higher tunneling barriers for the small diameter nanotubes (EB = 164 - 186 meV) than for the narrow bandgap plasma torch SWCNTs (EB = 119 - 156 meV). These values are consistent with the observed balanced ambipolar transfer characteristics. The broad diameter range HiPco nanotube network (D2) exhibited a more pronounced temperature dependence over the whole temperature range from 100 K to 300 K and a higher energy barrier (EB = 290 meV) for holes. For electron transport this value was even larger with EB = 680 meV, although the fit was less reliable here. Note, the nonlinear nature of the fluctuation induced tunneling model in combination with least-square fitting algorithms leads to a lower reliability of the extracted parameters compared to a linearized Arrhenius fit and the exact values may depend on the chosen temperature range and starting parameters (e.g., prefactor A). The extracted energy barriers should thus be used and interpreted with caution and serve here as relative rather than absolute values. Also, the


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model does not take into account the transport within the nanotubes. Nevertheless, higher tunneling barriers are reproducibly observed for the broad diameter range HiPco network (D2) compared to the other two networks (D1, D3). While the FIT model has been used to model transport in a wide variety of nanotube networks, including those with metallic nanotubes and without any surfactant,26 the impact of the residual wrapping polymer on charge transport remains an open question. For the two networks with lower barrier heights the wrapping polymer PFO-BPy was used, while P3DDT was used for the HiPco network. Both polymers are essentially insulating and could be seen as additional barriers for charge transfer between nanotubes. The HOMO-LUMO gap of PFO-BPy (3.2 eV) is very large compared to the bandgap of the nanotubes and thus charge transfer between polymer and nanotubes can be excluded. For P3DDT the HOMO-LUMO gap is smaller (1.9 eV) but still large compared even to the nanotubes with the smallest diameter in this distribution. To investigate the role of the wrapping polymer and to corroborate that the differences in temperature dependence indeed originate from the different nanotube compositions we fabricated transistors with monochiral (6,5) SWCNT networks (equivalent to D1) and removed all residual PFO-BPy. The successful polymer removal was confirmed by UV-visnIR absorption measurements (see Supporting Information, Figure S6). We used the polymer-stripping method introduced by Joo et al. with Re(CO)5Cl as the stripping agent.33, 51 This procedure consisted of an initial vacuum annealing step of the as-printed SWCNT network (400 °C) followed by immersion in a toluene solution with an excess of Re(CO)5Cl (see experimental section). After this treatment the SWCNT network apparently remained intact with a high network density similar to the other nanotube networks (see Figure 7a). However, the treated SWCNTs appeared to be more bundled than those in untreated networks. The SWCNTs at the top of the network are able to move during the polymer



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stripping process because they are not attached to the substrate and strong capillary forces during drying will cause further bundling. This notion was corroborated by the broadening and slight red shift of the (6,5) SWCNT E11, E22 and E33 transitions52-53 in the UV-vis-nIR absorption spectrum (see Supporting Information, Figure S6a). The observed bundling is in agreement with the reported effect that Re(CO)5Cl treatment of SWCNT/PFO-BPy dispersions results in aggregation of the bare nanotubes.33, 51 The final devices neither showed improved charge transport nor higher drain currents (see Figure 7b). Instead, the hole mobility dropped slightly (to 3.5 cm2 V-1 s-1) compared to the untreated (6,5) SWCNT network (D1). The onset-voltage for electron transport increased by 0.5 V and the electron mobility also decreased significantly (to 2.3 cm2 V-1 s-1). Overall, the removal of the wrapping polymer did not improve the device characteristics probably due to the increased SWCNT bundling, which is considered to lower the effective conductivity within a nanotube via scattering and ineffective gating.24 Based on the absorption spectra in Figure 1b and a recent study on the binding configuration of the related poly(9,9dioctylfluorene-2,7-diyl) on s-SWCNTs54 the surface coverage of the wrapping polymer in untreated nanotube networks should not exceed 20%, which would allow for enough direct SWCNT-SWCNT junctions to be available even without removing all polymer while still preventing undesired bundling. The lower carrier mobilities for the rhenium salt treated (6,5) SWCNT network in comparison to the non-treated reference (D1) persisted at low temperatures (see Supporting Information, Figure S7). However, the relative decrease of the carrier mobility upon cooling to 100 K as well as the extracted barrier energies according to the FIT model (EB = 159 meV for holes and EB = 181 meV for electrons) were very similar to those of the untreated (6,5) SWCNT network (D1). This observation corroborates the apparently negligible impact of the


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residual wrapping polymer on the primary charge transport processes and indicates that networks produced from nanotube dispersions sorted by different polymers can be compared.

Figure 7. (a) Atomic force micrograph of a Re(CO)5Cl treated (6,5) SWCNT network. (b) Transfer characteristics (forward and reverse gate voltage sweeps at VDS = -0.1 V) of a Re(CO)5Cl treated (6,5) SWCNT network compared to an untreated (6,5) nanotube network (D1).

To further evaluate the effective temperature dependence, i.e., the relative decrease of the carrier mobilities from 300 K to 100 K, for all network compositions, the respective hole mobilities were normalized to their maximum values at 300 K. This composite plot in Figure 8 shows clearly that the broad diameter range HiPco network (D2) exhibits the most pronounced temperature dependence of the hole mobility. Both the untreated and the



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Re(CO)5Cl-treated (6,5) SWCNT networks show a lesser mobility decrease with temperature. This disparity could be explained by the different SWCNT network compositions (broad diameter range vs. monochiral).

Figure 8. Normalized temperature-dependent hole mobilities extracted at a 2 V gate overdrive for all untreated s-SWNT networks (D1-D3) and the Re(CO)5Cl treated (6,5) nanotube network FETs. Solid lines represent fits corresponding to the fluctuation induced tunneling (FIT) model with EB as the tunneling barrier height.

Typically, the channel resistance in SWCNT networks is considered to be dominated by the SWCNT-SWCNT junctions as their resistance is substantially higher than that along a nanotube.24, 26,

28

In addition, for the HiPco network the energy levels (conduction/valence

band) of two adjacent SWCNTs are likely to differ significantly due to the broad distribution of bandgaps. Hence, aside from overcoming the activation barrier for charge transfer between two equal nanotubes charges may have to hop up or down in energy to move through the network, which creates an energy surface with valleys and hills in the range of 50 to 24 ACS Paragon Plus Environment

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400 meV.30 For the monochiral (6,5) SWCNT network (D1) energetic differences between hopping sites should only arise from dipolar disorder in the film (probably about 40 – 60 meV as determined from the inhomogeneous broadening of the absorption) and thus charges will experience a more even energy surface. This should lead to higher and less temperature-dependent carrier mobilities, as indeed observed experimentally. However, this notion does not explain the even smaller mobility reduction upon cooling and the lower extracted barrier height for the large diameter plasma torch SWCNT network (D3). The diameter range of these larger SWCNTs sorted with the same polymer as the (6,5) SWNTs gives a bandgap spread of approximately 180 meV, which is much smaller compared to the 470 meV of the HiPco network (D2) but certainly much larger than the modest energetic disorder in the nearly monochiral (6,5) network (D1). The combination of the higher absolute mobility values and lower temperature dependence of the mobility for the large diameter nanotubes highlights the shortcomings of the assumption that only the nanotube junctions determine charge transport in a network. Evidently, the nanotubenanotube junctions are only one part of the overall picture and the diameter and temperature dependence of transport within the individual nanotubes should not be completely neglected. Semiconducting SWCNTs exhibit ballistic charge transport if the nanotube length is shorter than the phonon scattering mean free path. If the SWCNT is longer than that, transport can be described as diffusive.19-20 For a given SWCNT species the intra-tube carrier mobility was found to scale with 1/T and to be proportional to d2, where d is the SWCNT diameter.21 This temperature dependence of the intra-nanotube carrier mobility is opposed to the thermally activated charge transport across the SWCNT-SWCNT junctions (inter-nanotube transport), although not as steep. Thus, overall a decrease of mobility at lower temperatures is still observed for networks. Due to the diameter dependence of the intra-nanotube carrier mobility, this counter-acting temperature dependence should be more pronounced for larger



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SWCNT diameters and for sparse or (semi-)aligned networks. A convolution of the temperature and diameter dependence of the intra-nanotube transport with the temperature and composition (bandgap spread) dependence of the inter-nanotube hopping in a network could explain the observed experimental trends. The interplay of both effects might be further clarified by investigating simulated networks that completely disregard the transport along the nanotubes as previously shown for mixed sSWCNT networks using an adapted random resistor model.32 Here, the different nanotube networks were modeled as random resistor networks of SWCNT-SWCNT junctions, while the resistance of each SWCNT segment and its carrier density or temperature dependence were neglected. The conductances for the resulting resistor networks were solved numerically based on Kirchhoff’s current law using a conventional approach for disordered systems and taking into account the one-dimensional density of states of each SWCNT and the overall charge carrier density. The hopping prefactor for the calculation of the bond conductance across each nanotube-nanotube-junction was the same for all SWCNT chiralities and an energetic disorder of 45 meV was imposed to mimic dipolar disorder in a thin film with a surrounding dielectric. This approach enabled the extraction of charge carrier density dependent mobilities for each SWCNT network composition. Figure 9 shows the extracted normalized carrier mobilities of three different network compositions (corresponding to the examined experimental SWCNT networks D1-D3, see Supporting Information, Table S3 for simulation details) in a temperature range from 200 K to 400 K. For all SWCNT network compositions, the charge carrier density dependent mobility (averaged over five randomly created networks) reaches a maximum value for carrier densities between

1·1011 cm-2 and

5·1011 cm-2 (see Supporting Information, Figure S8) and the peak mobility increases with temperature. This dependence can be explained directly with the Miller-Abrahams approach employed in the simulation to describe the charge transfer between two adjacent SWCNTs.32


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The peak carrier mobilities of the HiPco network (D2) are the lowest and the relative mobility reduction with decreasing temperature is also the largest. This again reveals the impact of the spread of different energy levels across the network on charge transport. Applying the FIT model to these data gives an effective barrier height of 42 meV. The extracted effective barrier energies for the other two simulated SWCNT networks (corresponding to D1 and D3) are significantly lower (EB = 20 - 24 meV) indicating a less pronounced temperature dependence of the carrier mobility in agreement with the experimental observations. However, in contrast to the experimental data the simulated peak mobility values of the monochiral (6,5) SWCNT network (D1) are higher than those of the larger diameter plasma torch SWCNTs (D3) over the whole temperature range. The randomresistor network model can predict the impact of a broad SWCNT versus narrow diameter distribution on the carrier mobility reasonably well but fails for the comparison of networks with vastly different nanotube diameters. This highlights the fact that the model only takes into account the disordered energetic landscape produced by the junctions between different nanotubes but not the diameter and temperature dependence of the transport along the nanotubes. Ideally, these hypotheses should be tested with tailored networks of similar average diameters and with different bandgap distributions or vice versa, but the range of available highly purified semiconducting nanotube dispersions is still limited. Mixing of chromato-graphically sorted s-SWCNTs might be an option to achieve such dispersions and networks.55



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Figure 9. Normalized temperature-dependent carrier mobilities for three different simulated SWCNT network compositions analogous to the experimental SWCNT networks D1-D3 obtained from a random-resistor-model.32 Solid lines represent fits corresponding to the fluctuation induced tunneling (FIT) model to extract the respective effective tunneling barrier heights EB.

CONCLUSION We have investigated the impact of the composition of dense, polymer-sorted and purely semiconducting SWCNT networks with different diameter and bandgap distributions on charge transport in field-effect transistors that showed intrinsic transport behavior without extrinsic trap states and corrected for contact resistance. Temperature-dependent measurements revealed thermally activated transport for holes and electrons that could be fitted best with the fluctuation induced tunneling model for all network compositions. We found the strongest temperature dependence for the network with the broadest diameter distribution indicating that variations in the energetic landscape of the network influence the


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effective carrier mobility as corroborated by random-resistor network simulations. However, the smallest temperature dependence and highest mobilities were found for large diameter networks with a narrow bandgap distribution. They even outperformed nearly monochiral but small-diameter (6,5) SWNT networks, which should be ideal according to transport simulations that take only junctions and energetic disorder into account. The observed experimental data suggest that unlike often assumed the nanotube-nanotube junctions are not the only limiting factor for the network mobility but the transport within and along the individual nanotubes and its dependence on diameter and temperature contribute significantly as well. The highest mobilities, lowest contact resistance and least temperature dependence are hence expected for SWCNT networks with only one nanotube species with a large diameter. While there is currently no polymer-wrapping or chromatography process that leads to monochiral, large diameter nanotube dispersions the presently available dispersions with a relatively narrow bandgap spread seem ideal for applications that require high carrier mobility. The only drawback of such nanotube networks for field-effect transistors is the intrinsic limitation of the achievable on/off ratio due to the small bandgap and strong ambipolarity. These issues could partially be overcome by suitable p- and n-doping schemes.56



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AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] ORCID Jana Zaumseil: 0000-0002-2048-217X

ACKNOWLEDGMENT This research was supported by the Struktur- und Innovationsfonds Baden-Württemberg (SIBW) and the Deutsche Forschungsgemeinschaft (DFG ZA 638/7 and SCHE 410/33).

ASSOCIATED CONTENT Supporting Information. Supporting Information available: Raman spectra of SWCNT dispersions (radial breathing mode), comparison of extracted SWCNT diameters/bandgaps from Raman and UV-vis-nIR absorption measurements, electrical characteristics of SWCNT network FETs at room temperature, temperature- and gate voltage-dependent contact resistances, comparison of contact resistance corrected and uncorrected carrier mobilities, UV-vis-nIR-absorption analysis and temperature-dependent carrier mobilities for Re(CO)5Cl treated and untreated (6,5) SWCNTs, random-resistor SWCNT network model: input parameters and charge carrier density dependence of carrier mobilities (PDF). This material is available free of charge via the Internet at http://pubs.acs.org.


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REFERENCES 1. Cao, Y.; Cong, S.; Cao, X.; Wu, F.; Liu, Q.; Amer, M. R.; Zhou, C., Review of Electronics Based on Single-Walled Carbon Nanotubes. Top. Curr. Chem. 2017, 375, 75. 2. Sun, D.-M.; Liu, C.; Ren, W.-C.; Cheng, H.-M., All-Carbon Thin-Film Transistors as a Step Towards Flexible and Transparent Electronics. Adv. Electron. Mater. 2016, 2, 1600229. 3. Zhang, H.; Xiang, L.; Yang, Y.; Xiao, M.; Han, J.; Ding, L.; Zhang, Z.; Hu, Y.; Peng, L.-M., High-Performance Carbon Nanotube Complementary Electronics and Integrated Sensor Systems on Ultrathin Plastic Foil. ACS Nano 2018, 12, 2773-2779. 4. Geier, M. L.; McMorrow, J. J.; Xu, W.; Zhu, J.; Kim, C. H.; Marks, T. J.; Hersam, M. C., Solution-Processed Carbon Nanotube Thin-Film Complementary Static Random Access Memory. Nat. Nanotechnol. 2015, 10, 944-948. 5. Tang, J.; Cao, Q.; Tulevski, G.; Jenkins, K. A.; Nela, L.; Farmer, D. B.; Han, S.-J., Flexible CMOS Integrated Circuits Based on Carbon Nanotubes with sub-10 ns Stage Delays. Nat. Electron. 2018, 1, 191-196. 6. Nish, A.; Hwang, J.-Y.; Doig, J.; Nicholas, R. J., Highly Selective Dispersion of Single-Walled Carbon Nanotubes Using Aromatic Polymers. Nat. Nanotechnol. 2007, 2, 640646. 7. Schießl, S. P.; Fröhlich, N.; Held, M.; Gannott, F.; Schweiger, M.; Forster, M.; Scherf, U.; Zaumseil, J., Polymer-Sorted Semiconducting Carbon Nanotube Networks for High-Performance Ambipolar Field-Effect Transistors. ACS Appl. Mater. Interfaces 2015, 7, 682-689. 8. Graf, A.; Zakharko, Y.; Schießl, S. P.; Backes, C.; Pfohl, M.; Flavel, B. S.; Zaumseil, J., Large Scale, Selective Dispersion of Long Single-Walled Carbon Nanotubes with High Photoluminescence Quantum Yield by Shear Force Mixing. Carbon 2016, 105, 593-599. 9. Samanta, S. K.; Fritsch, M.; Scherf, U.; Gomulya, W.; Bisri, S. Z.; Loi, M. A., Conjugated Polymer-Assisted Dispersion of Single-Wall Carbon Nanotubes: The Power of Polymer Wrapping. Acc. Chem. Res. 2014, 47, 2446-2456. 10. Lau, P. H.; Takei, K.; Wang, C.; Ju, Y.; Kim, J.; Yu, Z.; Takahashi, T.; Cho, G.; Javey, A., Fully Printed, High Performance Carbon Nanotube Thin-Film Transistors on Flexible Substrates. Nano Lett. 2013, 13, 3864-3869. 11. Rother, M.; Brohmann, M.; Yang, S. Y.; Grimm, S. B.; Schiessl, S. P.; Graf, A.; Zaumseil, J., Aerosol-Jet Printing of Polymer-Sorted (6,5) Carbon Nanotubes for Field-Effect Transistors with High Reproducibility. Adv. Electron. Mater. 2017, 3, 1700080. 12. Cong, S.; Cao, Y.; Fang, X.; Wang, Y.; Liu, Q.; Gui, H.; Shen, C.; Cao, X.; Kim, E. S.; Zhou, C., Carbon Nanotube Macroelectronics for Active Matrix Polymer-Dispersed Liquid Crystal Displays. ACS Nano 2016, 10, 10068-10074.



Page 32 of 36

13. Gaviria Rojas, W. A.; McMorrow, J. J.; Geier, M. L.; Tang, Q.; Kim, C. H.; Marks, T. J.; Hersam, M. C., Solution-Processed Carbon Nanotube True Random Number Generator. Nano Lett. 2017, 17, 4976-4981. 14. Berger, F. J.; Higgins, T. M.; Rother, M.; Graf, A.; Zakharko, Y.; Allard, S.; Matthiesen, M.; Gotthardt, J. M.; Scherf, U.; Zaumseil, J., From Broadband to Electrochromic Notch Filters with Printed Monochiral Carbon Nanotubes. ACS Appl. Mater. Interfaces 2018, 10, 11135-11142. 15. Graf, A.; Held, M.; Zakharko, Y.; Tropf, L.; Gather, M. C.; Zaumseil, J., Electrical Pumping and Tuning of Exciton-Polaritons in Carbon Nanotube Microcavities. Nat. Mater. 2017, 16, 911-917. 16. Ye, Y.; Bindl, D. J.; Jacobberger, R. M.; Wu, M.-Y.; Roy, S. S.; Arnold, M. S., Semiconducting Carbon Nanotube Aerogel Bulk Heterojunction Solar Cells. Small 2014, 10, 3299-3306. 17. Yang, Y.; Ding, L.; Han, J.; Zhang, Z.; Peng, L.-M., High-Performance Complementary Transistors and Medium-Scale Integrated Circuits Based on Carbon Nanotube Thin Films. ACS Nano 2017, 11, 4124-4132. 18. Javey, A.; Guo, J.; Wang, Q.; Lundstrom, M.; Dai, H., Ballistic Carbon Nanotube Field-Effect Transistors. Nature 2003, 424, 654-657. 19. Charlier, J.-C.; Blase, X.; Roche, S., Electronic and Transport Properties of Nanotubes. Rev. Mod. Phys. 2007, 79, 677-732. 20. Perebeinos, V.; Tersoff, J.; Avouris, P., Electron-Phonon Interaction and Transport in Semiconducting Carbon Nanotubes. Phys. Rev. Lett. 2005, 94, 086802. 21. Zhou, X.; Park, J.-Y.; Huang, S.; Liu, J.; McEuen, P. L., Band Structure, Phonon Scattering, and the Performance Limit of Single-Walled Carbon Nanotube Transistors. Phys. Rev. Lett. 2005, 95, 146805. 22. Liang, X.; Xia, J.; Dong, G.; Tian, B.; Peng, l., Carbon Nanotube Thin Film Transistors for Flat Panel Display Application. Top. Curr. Chem. 2016, 374, 80. 23. Topinka, M. A.; Rowell, M. W.; Goldhaber-Gordon, D.; McGehee, M. D.; Hecht, D. S.; Gruner, G., Charge Transport in Interpenetrating Networks of Semiconducting and Metallic Carbon Nanotubes. Nano Lett. 2009, 9, 1866-1871. 24. Nirmalraj, P. N.; Lyons, P. E.; De, S.; Coleman, J. N.; Boland, J. J., Electrical Connectivity in Single-Walled Carbon Nanotube Networks. Nano Lett. 2009, 9, 3890-3895. 25. Gao, J.; Loo, Y.-L. L., Temperature-Dependent Electrical Transport in PolymerSorted Semiconducting Carbon Nanotube Networks. Adv. Funct. Mater. 2015, 25, 105-110. 26. Itkis, M. E.; Pekker, A.; Tian, X.; Bekyarova, E.; Haddon, R. C., Networks of Semiconducting SWNTs: Contribution of Midgap Electronic States to the Electrical Transport. Acc. Chem. Res. 2015, 48, 2270-2279.


Page 33 of 36 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


27. Bisri, S. Z.; Derenskyi, V.; Gomulya, W.; Salazar-Rios, J. M.; Fritsch, M.; Fröhlich, N.; Jung, S.; Allard, S.; Scherf, U.; Loi, M. A., Anomalous Carrier Transport in Ambipolar Field-Effect Transistor of Large Diameter Single-Walled Carbon Nanotube Network. Adv. Electron. Mater. 2016, 2, 1500222. 28. Lee, S.-H.; Xu, Y.; Khim, D.; Park, W.-T.; Kim, D.-Y.; Noh, Y.-Y., Effect of Polymer Gate Dielectrics on Charge Transport in Carbon Nanotube Network Transistors: Low-k Insulator for Favorable Active Interface. ACS Appl. Mater. Interfaces 2016, 8, 3242132431. 29. Sheng, P., Fluctuation-Induced Tunneling Conduction in Disordered Materials. Phys. Rev. B 1980, 21, 2180-2195. 30. Tessler, N.; Preezant, Y.; Rappaport, N.; Roichman, Y., Charge Transport in Disordered Organic Materials and Its Relevance to Thin-Film Devices: A Tutorial Review. Adv. Mater. 2009, 21, 2741-2761. 31. Rother, M.; Schiessl, S. P.; Zakharko, Y.; Gannott, F.; Zaumseil, J., Understanding Charge Transport in Mixed Networks of Semiconducting Carbon Nanotubes. ACS Appl. Mater. Interfaces 2016, 8, 5571-5579. 32. Schießl, S. P.; de Vries, X.; Rother, M.; Massé, A.; Brohmann, M.; Bobbert, P. A.; Zaumseil, J., Modeling Carrier Density Dependent Charge Transport in Semiconducting Carbon Nanotube Networks. Phys. Rev. Mater. 2017, 1, 046003. 33. Joo, Y.; Brady, G. J.; Kanimozhi, C.; Ko, J.; Shea, M. J.; Strand, M. T.; Arnold, M. S.; Gopalan, P., Polymer-Free Electronic-Grade Aligned Semiconducting Carbon Nanotube Array. ACS Appl. Mater. Interfaces 2017, 9, 28859-28867. 34. Ozawa, H.; Ide, N.; Fujigaya, T.; Niidome, Y.; Nakashima, N., One-Pot Separation of Highly Enriched (6,5)-Single-Walled Carbon Nanotubes Using a Fluorene-Based Copolymer. Chem. Lett. 2011, 40, 239-241. 35. Mistry, K. S.; Larsen, B. A.; Blackburn, J. L., High-Yield Dispersions of LargeDiameter Semiconducting Single-Walled Carbon Nanotubes with Tunable Narrow Chirality Distributions. ACS Nano 2013, 7, 2231-2239. 36. Lee, H. W.; Yoon, Y.; Park, S.; Oh, J. H.; Hong, S.; Liyanage, L. S.; Wang, H.; Morishita, S.; Patil, N.; Park, Y. J., et al., Selective Dispersion of High Purity Semiconducting Single-Walled Carbon Nanotubes with Regioregular Poly(3alkylthiophene)s. Nat. Commun. 2011, 2, 541. 37. Held, M.; Schießl, S. P.; Miehler, D.; Gannott, F.; Zaumseil, J., Polymer/Metal Oxide Hybrid Dielectrics for Low Voltage Field-Effect Transistors with Solution-Processed, HighMobility Semiconductors. Appl. Phys. Lett. 2015, 107, 083301. 38. Schießl, S. P.; Rother, M.; Lüttgens, J.; Zaumseil, J., Extracting the Field-Effect Mobilities of Random Semiconducting Single-Walled Carbon Nanotube Networks: A Critical Comparison of Methods. Appl. Phys. Lett. 2017, 111, 193301. 39. Nikiforov, G. O.; Venkateshvaran, D.; Mooser, S.; Meneau, A.; Strobel, T.; Kronemeijer, A.; Jiang, L.; Lee, M. J.; Sirringhaus, H., Current-Induced Joule Heating and 33 ACS Paragon Plus Environment


Page 34 of 36

Electrical Field Effects in Low Temperature Measurements on TIPS Pentacene Thin Film Transistors. Adv. Electron. Mater. 2016, 2, 1600163. 40. Shimotani, H.; Tsuda, S.; Yuan, H.; Yomogida, Y.; Moriya, R.; Takenobu, T.; Yanagi, K.; Iwasa, Y., Continuous Band‐Filling Control and One‐Dimensional Transport in Metallic and Semiconducting Carbon Nanotube Tangled Films. Adv. Funct. Mater. 2014, 24, 3305-3311. 41. Bittle, E. G.; Basham, J. I.; Jackson, T. N.; Jurchescu, O. D.; Gundlach, D. J., Mobility Overestimation Due to Gated Contacts in Organic Field-Effect Transistors. Nat. Commun. 2016, 7, 10908. 42. Natali, D.; Caironi, M., Charge Injection in Solution-Processed Organic Field-Effect Transistors: Physics, Models and Characterization Methods. Adv. Mater. 2012, 24, 13571387. 43. Choi, H. H.; Cho, K.; Frisbie, C. D.; Sirringhaus, H.; Podzorov, V., Critical Assessment of Charge Mobility Extraction in FETs. Nat. Mater. 2017, 17, 2-7. 44. Pesavento, P. V.; Chesterfield, R. J.; Newman, C. R.; Frisbie, C. D., Gated FourProbe Measurements on Pentacene Thin-Film Transistors: Contact Resistance as a Function of Gate Voltage and Temperature. J. Appl. Phys. 2004, 96, 7312-7324. 45. Podzorov, V.; Sysoev, S. E.; Loginova, E.; Pudalov, V. M.; Gershenson, M. E., Single-Crystal Organic Field Effect Transistors with the Hole Mobility ∼8 cm2/V s. Appl. Phys. Lett. 2003, 83, 3504-3506. 46. Richards, T. J.; Sirringhaus, H., Analysis of the Contact Resistance in Staggered, TopGate Organic Field-Effect Transistors. J. Appl. Phys. 2007, 102, 094510-094510. 47. Pesavento, P. V.; Puntambekar, K. P.; Frisbie, C. D.; McKeen, J. C.; Ruden, P. P., Film and Contact Resistance in Pentacene Thin-Film Transistors: Dependence on Film Thickness, Electrode Geometry, and Correlation with Hole Mobility. J. Appl. Phys. 2006, 99, 094504. 48. Schießl, S. P.; Gannott, F.; Etschel, S. H.; Schweiger, M.; Grünler, S.; Halik, M.; Zaumseil, J., Self-Assembled Monolayer Dielectrics for Low-Voltage Carbon Nanotube Transistors with Controlled Network Density. Adv. Mater. Interfaces 2016, 3, 1600215. 49. Hamadani, B. H.; Natelson, D., Temperature-Dependent Contact Resistances in HighQuality Polymer Field-Effect Transistors. Appl. Phys. Lett. 2004, 84, 443-445. 50. Bürgi, L.; Richards, T. J.; Friend, R. H.; Sirringhaus, H., Close Look at Charge Carrier Injection in Polymer Field-Effect Transistors. J. Appl. Phys. 2003, 94, 6129-6137. 51. Joo, Y.; Brady, G. J.; Shea, M. J.; Oviedo, M. B.; Kanimozhi, C.; Schmitt, S. K.; Wong, B. M.; Arnold, M. S.; Gopalan, P., Isolation of Pristine Electronics Grade Semiconducting Carbon Nanotubes by Switching the Rigidity of the Wrapping Polymer Backbone on Demand. ACS Nano 2015, 9, 10203-10213.


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52. Naumov, A. V.; Ghosh, S.; Tsyboulski, D. A.; Bachilo, S. M.; Weisman, R. B., Analyzing Absorption Backgrounds in Single-Walled Carbon Nanotube Spectra. ACS Nano 2011, 5, 1639-1648. 53. O'Connell, M. J.; Bachilo, S. M.; Huffman, C. B.; Moore, V. C.; Strano, M. S.; Haroz, E. H.; Rialon, K. L.; Boul, P. J.; Noon, W. H.; Kittrell, C., et al., Band Gap Fluorescence from Individual Single-Walled Carbon Nanotubes. Science 2002, 297, 593-596. 54. Shea, M. J.; Mehlenbacher, R. D.; Zanni, M. T.; Arnold, M. S., Experimental Measurement of the Binding Configuration and Coverage of Chirality-Sorting Polyfluorenes on Carbon Nanotubes. J. Phys. Chem. Lett. 2014, 5, 3742-3749. 55. Liu, H.; Tanaka, T.; Urabe, Y.; Kataura, H., High-Efficiency Single-Chirality Separation of Carbon Nanotubes Using Temperature-Controlled Gel Chromatography. Nano Lett. 2013, 13, 1996-2003. 56. Xu, Q.; Zhao, J.; Pecunia, V.; Xu, W.; Zhou, C.; Dou, J.; Gu, W.; Lin, J.; Mo, L.; Zhao, Y., et al., Selective Conversion from P-Type to N-Type of Printed Bottom-Gate Carbon Nanotube Thin-Film Transistors and Application in Complementary Metal–Oxide– Semiconductor Inverters. ACS Appl. Mater. Interfaces 2017, 9, 12750–12758.



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Temperature-Dependent Charge Transport in Polymer-Sorted

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