Radical Polymers Alter the Carrier Properties of Semiconducting

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Radical Polymers Alter the Carrier Properties of Semiconducting Carbon Nanotubes Yongho Joo, Sanjoy Mukherjee, and Bryan W. Boudouris ACS Appl. Polym. Mater., Just Accepted Manuscript • DOI: 10.1021/acsapm.8b00080 • Publication Date (Web): 03 Jan 2019 Downloaded from http://pubs.acs.org on January 10, 2019

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ACS Applied Polymer Materials

Radical Polymers Alter the Carrier Properties of Semiconducting Carbon Nanotubes Yongho Joo,†,¶ Sanjoy Mukherjee,†,§ Bryan W. Boudouris†,‡,* † Charles D. Davidson School of Chemical Engineering, Purdue University, West Lafayette, IN 47907, USA ‡ Department of Chemistry, Purdue University, West Lafayette, IN 47907, USA ¶ Current Address: Institute of Advanced Composite Materials, Korea Institute of Science and Technology, 92 Chudong-ro, Bongdong-eup, Wanju-gun, Jeonbuk, 55324, Republic of Korea § Current Address: Materials Research Laboratory, University of California, Santa Barbara, CA 93106, USA. * To whom correspondence should be addressed, [email protected]. Abstract. Radical polymers are composed of non-conjugated macromolecular backbones and pendant groups that bear stable open-shell sites; these functional macromolecules have been utilized in myriad electrochemical, optoelectronic, and thermoelectric devices to date. Here, we combine radical polymers with semiconducting single-walled carbon nanotubes (SWCNTs) to form composite materials using a scalable fabrication process. Importantly, we are able to tune the charge carrier characteristics of the SWCNTs by controlling the macromolecular architecture of the radical polymer used to form the composite. This is a critical handle as creating stable, electrontransporting (i.e., n-type) SWCNT thin films using scalable processes is a long-sought goal in the field. Specifically, hole-transporting (i.e., p-type) SWCNT thin films were deposited using a simple drop-casting methodology. Then, a radical polymer thin film was coated on top of the SWCNT thin film in order to create a composite system. Three different radical polymer chemistries were evaluated in terms of their ability to manipulate the optoelectronic properties of the composite systems relative to the pristine SWCNT thin films. Two of the three radical polymer chemistries, poly(2,2,6,6-tetramethylpiperidinyloxy methacrylate) (PTMA) and poly(2,3bis(2',2',6',6'-tetramethylpiperidinyl-N-oxyl-4'-oxycarbonyl)-5-norbornene) (PTNB), contained different macromolecular backbones, but shared the same nitroxide radical open-shell chemistry. Instead, the third radical polymer, poly(2,6-di-tert-butyl-4-((3,5-di-tert-butyl-4-phenoxyl)(4vinylphenyl)methylene)cyclohexa-2,5-dienone) (PGSt), was based upon the galvinoxyl styrene radical. Neither of the nitroxide-based radical polymers demonstrated any significant electrochemical interaction with the SWCNT thin films. Conversely, the addition of the galvinoxyl styrene-based radical polymer converted the SWCNT thin films from ones that were unipolar ptype materials to a system that allowed for both hole and electron transport (i.e., ambipolar behavior was observed) when the thin films were incorporated into organic field-effect transistor (OFET) test bed devices. In these ambipolar systems, the hole mobility and electron mobility values were determined to be 0.07 cm2 V-1 s-1 and 0.24 cm2 V-1 s-1, respectively. A combination of ultraviolet-visible-near infrared (UV-Vis-NIR) light absorption and Raman spectroscopy revealed the means by which this doping occurred. This provides for a foundational underpinning for the observed behavior, and this work also demonstrates a straightforward and scalable means by which to apply these functional open-shell macromolecules in flexible and printed electronic devices. Keywords. Radical polymers; single-walled carbon nanotube (SWCNTs); Radical polymercarbon nanotube composite materials; Carbon nanotube doping; Ambipolar organic field-effect transistors (OFETs)

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Introduction Single-walled carbon nanotubes (SWCNTs) are of immense interest as next-generation organic electronic materials due to their exceptional properties, including their: ability to be processed from solution, high carrier mobility, robust thermal stability, relatively high thermal conductivity, and stability to myriad chemical environments.1-3 In particular, high-performance ptype (i.e., hole-transporting) unipolar devices are easily attained from solution deposition processes using purified semiconducting SWCNT under ambient conditions.4-7 Often in these ptype systems, oxygen molecules interact with the SWCNT surfaces, and this gas-surface interaction allows the oxygen molecules to act as electron-withdrawing groups, which imparts pdoped characteristics to the SWCNTs.8,9, 10 Therefore, extensive optimization has occurred with respect to the chemical and nanostructural manipulation of, and device physics associated with, ptype unipolar devices.11-14 However, it is advantageous for SWCNTs to operate as both p-type and n-type (i.e., electron-transporting) semiconductors for complementary circuitry.15 Despite this need, n-type doping in SWCNTs has proven to be significantly more challenging to achieve than p-type doping, which is a common theme across many organic electronic materials platforms. This opening has led to widespread n-doping strategies that include using low work function contact metals7,

16-20

and different types of small molecule and polymeric dopants (e.g., doping with

potassium metal,21, 22 polyethyleneimine,23 polyaniline,24 nitrogen,25, 26 organorhodium dimers,27 and small molecule reducing agents).28-30 While each of these methods has its strengths, establishing alternative mechanisms to induce the n-type doping is critical for the long-term interests of SWCNT electronics. Here, we address this opportunity by combining a new class of redox-active macromolecules, radical polymers (i.e., macromolecules with non-conjugated macromolecular backbones and with stable open-shell units present on their pendant groups), with


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SWCNTs in order to achieve high-performance n-type SWCNTs transistors using a straightforward processing strategy that is compatibile with low-cost, relatively high-throughput manufacturing techniques (e.g., inkjet printing). Radical-containing small molecules and polymers have been utilized in myriad electrochemical and electrical applications due to their charge storage and charge transporting abilities.31-34 In fact, in energy storage applications, conducting carbon fibers are often blended with radical polymers. In these systems, the redox-active nature of radical polymers allows for the open-shell macromolecules to serve as the charge storage phase. That is, charge can be stored at or transferred between stable pendant radical groups through oxidation-reduction (redox) reactions. Then, the conducting carbon phase is added in order to inject and extract charge from the radical polymers to the external circuit. Here, we utilize a different type of carbon nanotubes (i.e., not carbon fiber) that are semiconducting (i.e., not metallic) in nature; thus, the blending of carbon nanotubes and radical polymers offers the potential to change the charge carrier density in the semiconducting carbon nanotube phase. In turn, this provides for a different operating mechanism than what is oft-observed in radical polymer-based batteries. Due to the singularly occupied molecular orbital (SOMO) energy levels, the open-shell groups of the radical polymers accept or donate electrons to form corresponding anions or cations. In turn, this redox mechanism also can be an effective doping mechanism with high-performance semiconducting materials, if the appropriate redox couple is found.37,

38, 39

In the current effort, we have evaluated three distinct

radical polymers. In particular, we assess two materials that have the same pendant group chemistry (i.e., the nitroxide radical), but with different macromolecular backbones. We establish that the macromolecular backbone has little impact on the observed electronic behavior of the SWCNT-radical polymer composites. Then, we demonstrate how moving from the preferentially-


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oxidized nitroxide radical family to the preferentially-reduced galvinoxyl radical significantly alters the charge transport ability of SWCNTs. In particular, we are able to observe a transition from unipolar, p-type behavior in the pristine SWCNTs to one that shows ambipolar behavior in the galvinoxyl radical-doped SWCNTs. In fact, the doping provides for an electron mobility value of 0.24 cm2 V-1 s-1 in the radical polymer-doped materials system. In this way, we demonstrate how proper control of the open-shell chemistry of the macromolecular system allows for ready manipulation of their end-use application in SWCNT-based organic electronic devices, and this highlights how radical polymers can be applied as doping in composite polymer applications. Materials and Experimental Methods SWCNT and Polymer-SWCNT Thin Film Preparation Silicon wafers with 300 nm SiO2 layers were cleaned using a piranha solution (1:3, H2O2 and H2SO4) for 1 hour at 90 °C. These substrates were thoroughly washed with DI water and dried using compressed nitrogen. In the following step, a solution of hexamethyldisilazane (HMDS) was spun-coat onto the substrates, and the HMDS-coated substrates were baked at 110 °C. This process helps in the formation of a hydrophobic surface, which improves the film quality associated with the deposition of the SWCNTs. These modified substrates were thoroughly washed with 2propanol and dried using compressed nitrogen. A commercially-available, semi-enriched carbon nanotube solution (IsoNanotubes-98%, NanoIntegris, Inc.) was deposited onto these HMDS modified substrates. For each active area, 0.1 mL of the SWCNT solution, where dichlorobenzene acted as the solvent (20 µg mL-1), were drop-cast and left for an hour. The films were further cleaned using DI water to remove excess material and dried using compressed air.40 This lead to SWCNT thin films with average thickness values of 24 nm. After the deposition of the SWCNT films, the radical polymers were spun-coat onto the films. Here, the radical polymers used were


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poly(2,2,6,6-tetramethylpiperidinyloxy

methacrylate)

(PTMA),

poly(2,3-bis(2',2',6',6'-

tetramethylpiperidinyl-N-oxyl-4'-oxycarbonyl)-5-norbornene) (PTNB), and poly(2,6-di-tertbutyl-4-((3,5-di-tert-butyl-4-phenoxyl)(4-vinylphenyl)methylene)cyclohexa-2,5-dienone) (PGSt). The PTMA and PGSt macromolecules were polymerized using radical-mediated polymerization schemes, as reported previously,37, 41 and the PTNB radical polymer was synthesized using a ringopening metathesis route using conditions that we have detailed earlier.35 In all cases, 0.1 mg mL1

solutions of the radical polymers were used, and the casting solvent was chloroform. In these

cases the radical polymer solutions were spun-coat atop the existing SWCNT thin films to form the composite layer, and the average thickness of the coated radical polymer films was 5 nm. This led to the composite films being ~80% SWCNTs, by weight. Thin Film Imaging and Spectroscopic Characterization The surface morphology was probed using an atomic force microscope (AFM) operating in tapping mode (Digital Instruments, Nanoscope III Multimode). A triangular cantilever tip with the integral pyramidal Si3N4 was utilized for acquisition of the images. Raman spectra were collected in the back-scattering configuration using a He-Ne LASER (i.e., at a wavelength of 633 nm) as the excitation source. The Raman spectroscopy data were collected using a LabRAM HR Evolution Raman microscope (Horiba, Ltd.). The signal was calibrated with reference to the 520.7 cm-1 peak position of the silicon wafer. Ultraviolet-visible-near infrared (UV-Vis-NIR) absorption spectra were acquired using a Cary 6000i spectrophotometer over a wavelength range of 300 nm ≤ λ ≤ 1800 nm, and a glass substrate (Delta Technologies, Loveland, CO) was used as the blank.


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Field-effect Transistor (FET) Fabrication, Characterization, and Evaluation SWCNT organic field-effect transistors (OFETs) were fabricated in bottom contact-bottom gate geometries. Thermal evaporation of Ti (10 nm)/Au (50 nm) was used to create source and drain contacts with a channel width (Wch) and channel length (Lch) of 10 mm and 100 µm, respectively. All current-voltage characteristics of the SWCNT OFETs were acquired using two Keithley 2400 source meters, and the data were acquired while the devices were housed under vacuum conditions in a Lakeshore probe station at room temperature. The hole and electron mobility values reported were calculated according to Equation 1. 𝜇𝜇 =

𝐿𝐿𝑐𝑐ℎ 1

1 𝑑𝑑𝑑𝑑𝑑𝑑𝑑𝑑

(Equation 1)

𝑊𝑊𝑐𝑐ℎ 𝐶𝐶𝑖𝑖 𝑉𝑉𝑑𝑑𝑑𝑑 𝑑𝑑𝑑𝑑𝑔𝑔𝑔𝑔

Here, the channel length is Lch = 100 µm, the channel width is Wch = 1 cm, VDS is the source-drain voltage, and Ci is the intrinsic capacitance for the SWCNT OFETs. In the case of SWCNT OFETs with thin dielectrics, the quantum capacitance model is used instead of the parallel plate capacitance model in order to avoid an overestimation of the capacitance value.42, 43 The intrinsic capacitance, Ci, is calculated from the following equation. 𝐶𝐶𝑖𝑖 = �2𝜋𝜋𝜀𝜀

1

0 𝜀𝜀𝑆𝑆𝑆𝑆𝑆𝑆2

𝜌𝜌 sinh(2𝜋𝜋𝑡𝑡𝑜𝑜𝑜𝑜 ⁄𝜌𝜌)

ln �𝑅𝑅

𝜋𝜋

� + 𝐶𝐶𝑄𝑄

−1

�

−1

𝜌𝜌−1

(Equation 2)

Here, 𝜌𝜌 = 10 tubes µm-2 is the density of SWCNTs (see below), ɛ0 is the permittivity of free space,

𝜀𝜀𝑆𝑆𝑆𝑆𝑂𝑂2 = 2.95 is the dielectric constant of the SiO2 layer, 𝑡𝑡𝑆𝑆𝑆𝑆𝑂𝑂2 = 300 𝑛𝑛𝑛𝑛 is the thickness of the SiO2 dielectric, Cq= 4.0×10-10 F m-1 is the quantum capacitance of SWCNTs, and R = 0.7 nm is

the average radius of the semiconducting SWCNTs. Given these parameters, the calculated intrinsic capacitance of Ci = 22.7 nF cm-2 was used for the mobility calculations.


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Results and Discussion In order to dope the SWCNTs, radical polymers with different oxidation-reduction (redox) properties were coated to the top of deposited SWCNT films. Specifically, the aliphatic backbonebased radical polymers, PTMA, PTNB, and PGSt (Figure 1a), were combined with SWCNTs to establish their doping impact on this emerging class of next-generation semiconducting materials. First, the pristine SWCNT films were deposited by simple drop casting, and the resultant thin film structure was one that showed a randomly organized SWCNT network (Figure 1b). We note that the average bundle diameter of the SWCNTs was 9 µm, indicating a tube areal density of ~10 tubes µm-2. Next, the radical polymer solutions, at a concentration of 0.1 mg of polymer per 1 mL of chloroform, were spun-coat atop the SWCNT films to yield the composite thin films (Figure 1c). The composite polymer-SWCNT thin films were subsequently dried in vacuum to remove any residual solvent; however, no heat was applied to the films in order to prevent any degradation of the open-shell species within the radical polymers. Using this straightforward sequential coating process resulted in effective doping of the SWCNTs when the galvinoxyl radical was present.

Figure 1. (a) Chemical structures of the radical polymers used in this effort; they are PTMA, PTNB, and PGSt. The PTMA and PTNB macromolecules are based on the nitroxide radical while the PGSt radical polymer is based on the galvinoxyl radical. (b) AFM image of a SWCNT thin film that was deposited by drop-casting the materials onto a silicon dioxide substrate. The inset shows the thickness profile indicating the SWCNT density of ~10 nanotubes µm-1. (c) AFM image of a PGSt-SWCNT composite film where the PGSt was deposited by spin-coating on the predeposited SWCNT film. The inset shows the thickness profile. The AFM images in panels (b) and (c) both represent scanning over an area of 10 μm × 10 μm in size.


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In absorption spectroscopy, the optical transitions of SWCNT are characterized by van Hove singularities that arise in the absorption spectra. That is, the one-dimensional (1D) carbon nanotubes share specific optoelectronic properties because of the 1D quantum confinement effect of electrons in the orbitals of the sp2 carbon atoms of which the nanotubes are comprised.44, 45 With the absorption of light, the electron in the van Hove singularities of the valence band are excited to the corresponding conduction band. As shown in the ultraviolet-visible-near infrared (UV-VisNIR) optical absorbance spectra, the first and second optical transitions of the van Hove singularities (labeled as S11 and S22 in Figure 2) associated with SWCNTs decreased slightly in intensity when the SWCNTs were doped with the nitroxide radical-containing polymers (i.e., PTMA and PTNB), and these signals were significantly reduced for the galvinoxyl radicalcontaining polymer (i.e., PGSt). The large reduction in the absorption intensity of the S11 and S22 peaks of the PGST-SWCNT samples is mainly due to charge transfer between the SWCNTs and the open-shell groups of the PGSt, although higher order effects are at play that cause the slight reduction in the intensity associated with the nitroxide-containing polymers. This charge transfer results in doping of the SWCNTs, and this is then reflected by the suppression of their interbond optical absorption shown in Figure 2. These data support the idea that there is a strong redox interaction between the PGSt radical polymer and the SWCNTs, and this interaction was established to a greater extent through probing the sample using Raman spectroscopy as well.


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Figure 2. UV-Vis-NIR light absorption spectra of a pristine SWCNT thin film and composite thin films composed of PTMA-SWCNT, PTNB-SWCNT, and PGSt-SWCNT. The van Hove singularity at the positions labeled S11 (λ ~1780 nm-1) and S22 (λ ~1050 nm-1) are due to the presence of the SWCNTs in the sample. These peaks were significantly quenched after deposition of the PGSt radical polymer to the top of the SWCNT thin film. This indicates that there was an electron transfer between the SWCNTs and the PGSt thin film. In addition to the clear signature of doping shown by the light absorption data, the results associated with Raman spectroscopy further corroborated the doping effect of PGSt on the SWCNTs.45 The Raman spectra (with an excitation wavelength of 633 nm) of the SWCNT films before and after coating with PTMA, PTNB, and PGSt (Figure 3a and Figure 3b) highlight the clear differences in spectroscopic signatures between the galvinoxyl radical-based macromolecule and the nitroxide-based radical polymers. That is, the pristine SWCNTs show the characteristic signals in the Raman spectra, namely the G-band and 2D-band modes, and these modes appear at 1591.4 cm-1 and 2637 cm-1, respectively. After doping the SWCNTs with PGSt, a significant upshift in the G-band (~0.5 cm-1) and a clear downshift in 2D band (~6 cm-1) were observed, and these shifts are consistent with n-type doping in carbonacious materials.46 Conversely, the PTMA, and PTNB coating alter the peak positions within the range of standard deviation observed in the Raman spectra, as highlighted by the shift in the trends of the processed data that are shown in Figure 3c.


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Figure 3. Raman spectra of the (a) G-band (λ ~ 1591.4 nm-1) and (b) 2D-band (λ ~2637 nm-1) of pristine the pristine SWCNT thin film and the radical polymer-containing composite SWCNT thin films. In these two panels, dashed vertical lines serve as guides to the eye in order to demonstrate the shifts in the G-band and 2D-band signals upon coating with the PGSt radical polymer. (c) Gband and 2D-band peaks shift summary upon deposition of the radical polymer thin films upon the SWCNTs thin films. The upshift in the G-band and downshift in the 2D-band are consistent with n-type doping in the PGSt-SWCNT composite system. In order to evaluate how these doping processes impact the solid-state charge transport properties of the composite materials, SWCNT organic field-effect transistors were fabricated in a bottom contact-bottom gate geometry with a channel width and channel length of 10 mm and 100 µm, respectively. Figure 4a and Figure 4b show the transfer characteristics of the SWCNT FETs operating in the p-type regime and n-type regime, respectively. The unipolar, hole-only operation for the pristine SWCNT, PTMA-SWCNT, and PTNB-SWCNT systems is apparent due to the fact that observed drain current increases as the devices are swept into the negative gate voltage regime (Figure 4a), and the drain current in the far positive gate voltage regime is quite low (Figure 4b). These data are consistent with the current state of the art, which demonstrates that network devices fabricated from purified SWCNT (semiconducting-enriched) films have hole mobility values that range between 0.01 cm2 V-1 s-1 ≤ μh ≤ 40 cm2 V-1 s-1 and ON/OFF ratios of ~105.39, 42, 47 Importantly, the fact that the PTMA-based and PTNB-based composites demonstrate remarkably similar (and inert, relatively to the pristine SWCNT thin film) behavior highlights the point that the electrochemical properties of non-conjugated radical polymers are decoupled from


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the chemistry associated with the macromolecular backbone of the polymeric system. Moreover, these data stress the fact that the nitroxide radical serves as a relatively inert player in terms of being able to dope the SWCNTs in a manner that would alter their charge carrier properties (Figure 4c).

Figure 4. Representative transfer (Id-Vg) curves of SWCNT OFETs in the range of (a) –20 ≤ Vg ≤ +20 V and (b) +20 ≤ Vg ≤ +50 V at an applied source-drain voltage magnitude of |𝑉𝑉𝑑𝑑 | = 20 𝑉𝑉. (c) Summary of the device performance of polymer-SWCNT composite OFETs. Only the PGStcoated SWCNT thin film exhibited any kind of n-type behavior, and the electron mobility was roughly a factor of 35 times higher than the hole mobility in the PGSt-SWCNT OFETs. While the nitroxide-based radical polymers (i.e., PTMA and PTNB) do not significantly alter the performance characteristics of the SWCNT FETs, PGSt alters the electronic peroformance of the SWCNTs in a rather striking manner. This is consistent with the doping changes observed in the spectroscopic data shown above. That is, ambipolar charge transport behavior was observed. Specifically, the hole mobility was determined to be 0.7×10-2 cm2 V-1s-1, the electron mobility was 0.24 cm2 V-1s-1, the p-type ON/OFF ratio remained at ~103, and the n-type ON/OFF ratio was ~3×104 when the SWCNT thin films were doped with PGSt (Figure 4c). All of the performance metrics extracted from this transfer cuves are summarzied in Table 1. These data reiteratre the fact that doping only occurs in this applied SWCNT-radical polymer systems if the appropriate macromolecuar chemistry is employed (i.e., in this case with the galvionxyl radical), and again,


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the macromolecular backbone has little impact on the observed transport behavior of the composite systems. Table 1. SWCNT OFET performance with and without the addition of radical polymers Active Material SWCNT PTMA-SWCNT PTNB-SWCNT PGSt-SWCNT

µ𝒉𝒉+ (cm2 V-1s-1)

0.010 (±0.003) 0.013 (±0.002) 0.011 (±0.002) 0.007 (±0.001)

µ𝒆𝒆− (cm2 V-1s-1) 0.24 (±0.052)

Ion/Ioff (h+) 1.5×103 2.4×103 3.1×103 0.7×103

Ion/Ioff (e-)

3×104

Figure 5 shows the ambipolar output characteristics of the PGSt-SWCNT OFETs acquired at room temperature. The linear and saturation regimes were characterized for both hole and electron charge transport at low (Vd < ±10V) magnitudes of the drain voltage values. As expected for ambipolar behavior in OFETs, standard saturation behavior is observed at higher values of the gate voltage, and a superlinear current increases at lower applied gate voltages because opposite charge carriers (i.e., electrons and holes) are being injected into the semiconducting medium.48-51 Notably, the electron mobility was ~35 time higher than that of hole mobility, and the ON/OFF ratio was ~40 times higher than that of hole ON/OFF ratio. While selecting open-shell with properly-aligned SOMO energy levels is critical to the success of this doping procedure, we also note that the strong π-π interaction between the SWCNT and the conjugated side chains of PGSt enables the observed chemical doping effect. The combination of these two effects speaks to why this strong change in electronic behavior is only observed in the PGSt-based composite thin film. While no spectroscopic signature of a charge transfer complex state was observed for the composite thin films, we hypothesize that the increased π-π interactions possible in PGSt allow for the redox-active sites within the radical polymer to be in closer spatial proximity than what is achievable in either of the nitroxide-based radical polymers. In turn, this aids in increasing the electrochemical doping between the PGSt and the SWCNTs. That is, the strong chemical


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interaction is likely the primary factor for the doping, driving the ambipolar behavior, and this provides a clear design handle that can be utilized in the creation of next-generation radicalcontaining dopants for SWCNT systems and their application to organic electronic circuitry.

Figure 5. Representative output characteristic (Id-Vd) of a PGSt-SWCNT OFET in the range of – 60 V ≤ Vd ≤ +50 V, and with a variation in the value of the gate voltage in ranges from –50 V ≤ Vg ≤ +50 V. In each case the gate voltage has been stepped by 10 V between each of the subsequent curves. Conclusions The results presented here emphasize the importance of proper design strategies to dope the SWCNT with open-shell radical polymers as the electrochemical and long-range interactions between the two species significantly impact the optoelectronic properties of the composite materials. In particular, when radical polymers that contain nitroxide functionalities are used as dopants for SWCNTs there is little change observed in the optical or electronic properties of the SWCNTs relative to the corresponding pristine SWCNT. Conversely, when carbon nanotube thin films are coated with a galvinoxyl radical-based polymer, PGSt, significant changes are observed in terms of the optoelectronic properties. In both of these instances, the change in the doping ability of the radical polymers is independent of the chemistry of the macromolecular backbone and focuses heavily on the electrochemical properties of the open-shell groups of the radical polymers. Moreover, this first-of-its-kind doping strategy is relatively straightforward to employ as it only


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requires the sequential solution deposition of the two materials without the utilization of any type of high energy (e.g., probe tip sonication) processing. Therefore, this effort pinpoints the underlying physical phenomena that allow for radical polymers to be applied in a new manner to SWCNT optoelectronic devices. AUTHOR INFORMATION Corresponding Author * Bryan W. Boudouris Present Address ¶

Current Address: Institute of Advanced Composite Materials, Korea Institute of Science and

Technology (KIST), 92 Chudong-ro, Bongdong-eup, Wanju-gun, Jeonbuk, 55324, Republic of Korea §

Current Address: Materials Research Laboratory, University of California, Santa Barbara, CA

93106, USA. Author Contributions Y. J. fabricated the SWCNT-radical polymer composite thin films and evaluate the optoelectronic properties of the pristine and composite SWCNT systems. S. M. synthesized the radical polymers utilized in this work. All of the authors contributed to the formulation of the project, the design of experiments, and the writing and editing of the manuscript. Notes. The authors note no competing interests. ACKNOWLEDGMENT This work was supported by the Air Force Office of Scientific Research (AFOSR) through the Organic Materials Chemistry Program (Grant Number: FA9550-15-1-0449, Program Manager: Dr. Kenneth Caster), and we thank the AFOSR for their gracious support.


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References 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12.

13. 14. 15.

16.

De Volder, M. F.; Tawfick, S. H.; Baughman, R. H.; Hart, A. J. Carbon Nanotubes: Present and Future Commercial Applications. Science 2013, 339, 535-539. Wang, C.; Takei, K.; Takahashi, T.; Javey, A. Carbon Nanotube Electronics – Moving Forward. Chemical Society Reviews 2013, 42, 2592-2609. Hersam, M. C. Progress Towards Monodisperse Single-walled Carbon Nanotubes. Nat. Nanotechnol. 2008, 3, 387-394. Bachtold, A.; Hadley, P.; Nakanishi, T.; Dekker, C. Logic Circuits with Carbon Nanotube Transistors. Science 2001, 294, 1317-1320. Javey, A.; Guo, J.; Wang, Q.; Lundstrom, M.; Dai, H. Ballistic Carbon Nanotube Fieldeffect Transistors. Nature 2003, 424, 654. LeMieux, M. C.; Roberts, M.; Barman, S.; Jin, Y. W.; Kim, J. M.; Bao, Z. Self-sorted, Aligned Nanotube Networks for Thin-film Transistors. Science 2008, 321, 101-104. Chen, Z.; Appenzeller, J.; Lin, Y.-M.; Sippel-Oakley, J.; Rinzler, A. G.; Tang, J.; Wind, S. J.; Solomon, P. M.; Avouris, P. An Integrated Logic Circuit Assembled on a Single Carbon Nanotube. Science 2006, 311, 1735-1735. Jhi, S.-H.; Louie, S. G.; Cohen, M. L. Electronic Properties of Oxidized Carbon Nanotubes. Phys. Rev. Lett. 2000, 85, 1710. Collins, P. G.; Bradley, K.; Ishigami, M.; Zettl, d. A. Extreme Oxygen Sensitivity of Electronic Properties of Carbon Nanotubes. Science 2000, 287, 1801-1804. Sumanasekera, G.; Adu, C.; Fang, S.; Eklund, P. Effects of Gas Adsorption and Collisions on Electrical Transport in Single-walled Carbon Nanotubes. Phy. Rev. Lett. 2000, 85, 1096. Joo, Y.; Brady, G. J.; Arnold, M. S.; Gopalan, P. Dose-Controlled, Floating Evaporative Self-assembly and Alignment of Semiconducting Carbon Nanotubes from Organic Solvents. Langmuir 2014, 30, 3460-3466. Sangwan, V. K.; Ortiz, R. P.; Alaboson, J. M. P.; Emery, J. D.; Bedzyk, M. J.; Lauhon, L. J.; Marks, T. J.; Hersam, M. C. Fundamental Performance Limits of Carbon Nanotube Thin-Film Transistors Achieved Using Hybrid Molecular Dielectrics. ACS Nano 2012, 6, 7480-7488. Shulaker, M. M.; Hills, G.; Patil, N.; Wei, H.; Chen, H.-Y.; Wong, H.-S. P.; Mitra, S. Carbon Nanotube Computer. Nature 2013, 501, 526. Kim, B.; Jang, S.; Geier, M. L.; Prabhumirashi, P. L.; Hersam, M. C.; Dodabalapur, A. High-speed, Inkjet-printed Carbon Nanotube/Zinc Tin Oxide Hybrid Complementary Ring Oscillators. Nano Lett. 2014, 14, 3683-3687. Lee, J. S.; Son, S. K.; Song, S.; Kim, H.; Lee, D. R.; Kim, K.; Ko, M. J.; Choi, D. H.; Kim, B.; Cho, J. H. Importance of Solubilizing Group and Backbone Planarity in Low Band Gap Polymers for High Performance Ambipolar Field-effect Transistors. Chem. Mater. 2012, 24, 1316-1323. Shahrjerdi, D.; Franklin, A. D.; Oida, S.; Ott, J. A.; Tulevski, G. S.; Haensch, W. Highperformance Air-stable n-Type Carbon Nanotube Transistors with Erbium Contacts. ACS Nano 2013, 7, 8303-8308.


15


17. 18. 19. 20.

21. 22. 23. 24. 25. 26. 27. 28. 29.

30. 31.

Page 16 of 19

Wang, Z.; Xu, H.; Zhang, Z.; Wang, S.; Ding, L.; Zeng, Q.; Yang, L.; Pei, T.; Liang, X.; Gao, M. Growth and Performance of Yttrium Oxide as an Ideal High-κ Gate Dielectric for Carbon-based Electronics. Nano Lett. 2010, 10, 2024-2030. Zhang, Z.; Wang, S.; Ding, L.; Liang, X.; Pei, T.; Shen, J.; Xu, H.; Chen, Q.; Cui, R.; Li, Y. Self-aligned Ballistic n-Type Single-walled Carbon Nanotube Field-effect Transistors with Adjustable Threshold Voltage. Nano Lett. 2008, 8, 3696-3701. Kim, H.-S.; Jeon, E.-K.; Kim, J.-J.; So, H.-M.; Chang, H.; Lee, J.-O.; Park, N. Air-stable n-Type Operation of Gd-contacted Carbon Nanotube Field Effect Transistors. Appl. Phys. Lett. 2008, 93, 123106. Ding, L.; Wang, S.; Zhang, Z.; Zeng, Q.; Wang, Z.; Pei, T.; Yang, L.; Liang, X.; Shen, J.; Chen, Q. Y. High-performance n-Type Single-walled Carbon Nanotube Field-effect Transistors: Scaling and Comparison with Sc-contacted Devices. Nano Lett. 2009, 9, 42094214. Derycke, V.; Martel, R.; Appenzeller, J.; Avouris, P. Controlling Doping and Carrier Injection in Carbon Nanotube Transistors. Appl. Phys. Lett. 2002, 80, 2773-2775. Zhou, C.; Kong, J.; Yenilmez, E.; Dai, H. Modulated Chemical Doping of Individual Carbon Nanotubes. Science 2000, 290, 1552-1555. Shim, M.; Javey, A.; Shi Kam, N. W.; Dai, H. Polymer Functionalization for Air-stable nType Carbon Nanotube Field-effect Transistors. J. Am. Chem. Soc. 2001, 123, 1151211513. Zujovic, Z. D.; Zhang, L.; Bowmaker, G. A.; Kilmartin, P. A.; Travas-Sejdic, J. Selfassembled, Nanostructured Aniline Oxidation Products: A Structural Investigation. Macromolecules 2008, 41, 3125-3135. Xiao, K.; Liu, Y.; Hu, P. a.; Yu, G.; Sun, Y.; Zhu, D. n-Type Field-effect Transistors Made of an Individual Nitrogen-doped Multiwalled Carbon Nanotube. J. Am. Chem. Soc. 2005, 127, 8614-8617. Dong, H.; Zhao, Y.; Tang, Y.; Burkert, S. C.; Star, A. Oxidative Unzipping of Stacked Nitrogen-doped Carbon Nanotube Cups. ACS Appl. Mater. Inter. 2015, 7, 10734-10741. Geier, M. L.; Moudgil, K.; Barlow, S.; Marder, S. R.; Hersam, M. C. Controlled n-Type Doping of Carbon Nanotube Transistors by an Organorhodium Dimer. Nano Lett. 2016, 16, 4329-4334. Kim, S. M.; Jang, J. H.; Kim, K. K.; Park, H. K.; Bae, J. J.; Yu, W. J.; Lee, I. H.; Kim, G.; Loc, D. D.; Kim, U. J. Reduction-controlled Viologen in Bisolvent as an Environmentally Stable n-Type Dopant for Carbon Nanotubes. J. Am. Chem. Soc. 2008, 131, 327-331. Wang, H.; Wei, P.; Li, Y.; Han, J.; Lee, H. R.; Naab, B. D.; Liu, N.; Wang, C.; Adijanto, E.; Tee, B. C.-K. Tuning the Threshold Voltage of Carbon Nanotube Transistors by n-Type Molecular Doping for Robust and Flexible Complementary Circuits. Proc. Natl. Acad. Sci. 2014, 111, 4776-4781. Ogunro, O. O.; Nicolas, C. I.; Mintz, E. A.; Wang, X.-Q. Band Gap Opening in the Cycloaddition Functionalization of Carbon Nanotubes. ACS Macro Lett. 2012, 1, 524-528. Oyaizu, K.; Nishide, H. Radical Polymers for Organic Electronic Devices: A Radical Departure from Conjugated Polymers? Adv. Mater. 2009, 21, 2339-2344.


16

Page 17 of 19 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


32. 33. 34. 35. 36. 37. 38. 39.

40. 41. 42. 43. 44. 45. 46. 47. 48.

Wilcox, D. A.; Agarkar, V.; Mukherjee, S.; Boudouris, B. W. Stable Radical Materials for Energy Applications. Annu. Rev. Chem. Biomol. Eng. 2018, 9, 83-103. Tomlinson, E. P.; Hay, M. E.; Boudouris, B. W. Radical Polymers and Their Application to Organic Electronic Devices. Macromolecules 2014, 47, 6145-6158. Wingate, A. J.; Boudouris, B. W. Recent Advances in the Syntheses of Radical‐containing Macromolecules. J. Polym. Sci. A 2016, 54, 1875-1894. Hay, M. E.; Hui Wong, S.; Mukherjee, S.; Boudouris, B. W. Controlling Open‐shell Loading in Norbornene‐based Radical Polymers Modulates the Solid‐state Charge Transport Exponentially. J. Polym. Sci. B 2017, 55, 1516-1525. Joo, Y.; Agarkar, V.; Sung, S. H.; Savoie, B. M.; Boudouris, B. W. A Nonconjugated Radical Polymer Glass with High Electrical Conductivity. Science 2018, 359, 1391-1395. Mukherjee, S.; Boudouris, B. W. Design of a Three-state Switchable Chromogenic Radical-based Moiety and Its Translation to Molecular Logic Systems. Mol. Sys. Des. Eng. 2017, 2, 159-164. Schwarze, M.; Naab, B. D.; Tietze, M. L.; Scholz, R.; Pahner, P.; Bussolotti, F.; Kera, S.; Kasemann, D.; Bao, Z.; Leo, K. Analyzing the n-Doping Mechanism of an Air-Stable Small-Molecule Precursor. ACS Appl. Mater. Interfaces 2017, 10, 1340-1346. Wang, H.; Wei, P.; Li, Y.; Han, J.; Lee, H. R.; Naab, B. D.; Liu, N.; Wang, C.; Adijanto, E.; Tee, B. C.-K. Tuning the Threshold Voltage of Carbon Nanotube Transistors by n-Type Molecular Doping for Robust and Flexible Complementary Circuits. Proc. Natl. Acad. Sci. 2014, 111, 4776-4781. Rouhi, S.; Alizadeh, Y.; Ansari, R. On the Wrapping of Polyglycolide, Poly(ethylene oxide), and Polyketone Polymer Chains around Single-walled Carbon Nanotubes using Molecular Dynamics Simulations. Braz. J. Phys. 2015, 45, 10-18. Rostro, L.; Baradwaj, A. G.; Boudouris, B. W. Controlled Radical Polymerization and Quantification of Solid State Electrical Conductivities of Macromolecules Bearing Pendant Stable Radical Groups. ACS Appl. Mater. Interfaces 2013, 5, 9896-9901. Rouhi, N.; Jain, D.; Zand, K.; Burke, P. J. Fundamental Limits on the Mobility of Nanotube‐Based Semiconducting Inks. Adv. Mater. 2011, 23, 94-99. Zhou, W.; Wang, Y. Y.; Lim, T.-S.; Pham, T.; Jain, D.; Burke, P. J. Detection of Single Ion Channel Activity with Carbon Nanotubes. Sci. Rep. 2015, 5, 9208. Dresselhaus, M. S.; Dresselhaus, G.; Eklund, P. C. (Eds.), Science of Fullerenes and Carbon Nanotubes: Their Properties and Applications. Elsevier: 1996, Orlando, FL. pp. 413-463. Dresselhaus, M.; Jorio, A.; Saito, R. Characterizing Graphene, Graphite, and Carbon Nanotubes by Raman spectroscopy. Annu. Rev. Condens. Matter Phys. 2010, 1, 89-108. Das, A.; Pisana, S.; Chakraborty, B.; Piscanec, S.; Saha, S.; Waghmare, U.; Novoselov, K.; Krishnamurthy, H.; Geim, A.; Ferrari, A. Monitoring Dopants by Raman Scattering in an Electrochemically Top-gated Graphene Transistor. Nat. Nanotechnol. 2008, 3, 210-215. Biswas, C.; Lee, Y. H. Graphene versus Carbon Nanotubes in Electronic Devices. Adv. Func. Mat. 2011, 21, 3806-3826. Schmechel, R.; Ahles, M.; von Seggern, H. A Pentacene Ambipolar Transistor: Experiment and Theory. J. Appl. Phys. 2005, 98, 084511.


17


49. 50. 51.

Page 18 of 19

Zaumseil, J.; Sirringhaus, H. Electron and Ambipolar Transport in Organic Field-effect Transistors. Chem. Rev. 2007, 107, 1296-1323. Das, S.; Demarteau, M.; Roelofs, A. Ambipolar Phosphorene Field Effect Transistor. ACS Nano 2014, 8, 11730-11738. 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, 1500222-1500228.


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Radical Polymers Alter the Carrier Properties of Semiconducting

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