Doping-Dependent Energy Transfer from Conjugated Polyelectrolytes to

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C: Physical Processes in Nanomaterials and Nanostructures

Doping-Dependent Energy Transfer from Conjugated Polyelectrolytes to (6,5) Single-Walled Carbon Nanotubes Merve Balci Leinen, Felix J. Berger, Patrick Klein, Markus Mühlinghaus, Nicolas F. Zorn, Simon Settele, Sybille Allard, Ullrich Scherf, and Jana Zaumseil J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.9b07291 • Publication Date (Web): 14 Aug 2019 Downloaded from pubs.acs.org on August 17, 2019

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The Journal of Physical Chemistry

Doping-Dependent Energy Transfer from Conjugated Polyelectrolytes to (6,5) SingleWalled Carbon Nanotubes Merve Balcı Leinen†, Felix J. Berger†, Patrick Klein‡, Markus Mühlinghaus‡, Nicolas F. Zorn†, Simon Settele†, Sybille Allard‡, Ullrich Scherf‡, and Jana Zaumseil†,#*

†

‡

Institute for Physical Chemistry, Universität Heidelberg, D-69120 Heidelberg, Germany Makromolekulare Chemie und Institut für Polymertechnologie, Bergische Universität

Wuppertal, D-42097 Wuppertal, Germany #

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

1 ACS Paragon Plus Environment

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ABSTRACT Conjugated polymers exhibit strong interactions with single-walled carbon nanotubes (SWNTs). These enable the selective dispersion of specific semiconducting SWNTs in organic solvents and polymer-mediated energy transfer to the nanotubes followed by emission in the near-infrared. Conjugated polyelectrolytes with ionic side-chains can add further functionalities to these nanotube/polymer hybrids such as dispersibility in polar solvents (e.g. methanol) and self-doping. Here, we demonstrate and investigate energy transfer from a range of conjugated polymers to pre-selected (6,5) SWNT with varying spectral overlap between the optical transitions of the polymer and nanotube. We find evidence for increased backbone planarization of the polymers wrapped around the nanotubes. Furthermore, ambient p-doping of hybrids of anionic conjugated polyelectrolytes and (6,5) SWNTs blocks energy transfer contrary to cationic polyelectrolytes. By adding a mild reducing agent and thus removing the p-doping, the energy transfer can be fully restored pointing toward an electron exchange mechanism. The p-doping of nanotube/polyelectrolyte hybrids in air and their dopingdependent emission and charge transport properties also become apparent in water-gated fieldeffect transistors based on such networks and might be useful for dual-signal sensing applications.


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INTRODUCTION Purified semiconducting single-walled carbon nanotubes (SWNTs) have emerged as a promising material for thin film optoelectronics1-4 and as optical probes for biomedical applications.5-7 Being able to obtain highly pure and also monochiral dispersions has been crucial for these advances and can be achieved via various separation methods.8-10 The selective dispersion of specific semiconducting nanotubes in organic solvents by wrapping with conjugated polymers (often polyfluorenes or polythiophenes) is especially easy and reproducible.11-13 Even monochiral dispersions can be produced in large amounts.14 Furthermore, the obtained hybrids of conjugated polymers with broad absorption and emission (usually in the visible wavelength range) and single-walled carbon nanotubes with very narrow absorption peaks and photoluminescence in the near-infrared15 have interesting photophysical properties. These might be useful for sensing16 or energy conversion applications17 in particular in combination with the excellent charge transport properties of SWNTs.1,

4

The intimate

interaction of the conjugated backbone of the polymer that is wrapped around the nanotube should facilitate efficient and fast charge and/or energy transfer. Based on the wide range of available conjugated polymers with different ionization potentials/electron affinities and HOMO-LUMO gaps from 3.5 eV to 0.9 eV and the broad distribution of carbon nanotube diameters (0.7 – 2 nm) and thus bandgaps (1.3 eV – 0.5 eV) their interactions in hybrids could be tailored for a number of possible applications. Evidence for energy transfer between common conjugated polymers (e.g., polyfluorenes) and semiconducting carbon nanotubes was reported shortly after the discovery of their disposition for highly selective dispersion18-20 and appeared similar to that of other non-covalent surfactants (e.g., perylene or porphyrin derivatives).21,

22

The polymer-wrapped nanotubes

showed near-infrared photo-luminescence when a dispersion was excited at wavelengths that



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corresponded to the absorption spectrum of the polymer. This observation suggested polymermediated excitation, where the photogenerated exciton is transferred from the polymer to the nanotube,18, 23, 24 without yet making a clear distinction between possible mechanisms (e.g., Förster or Dexter energy transfer). Another interesting class of conjugated polymers for non-covalent interaction with SWNTs are those with ionic side-groups, i.e., conjugated polyelectrolytes (CPEs). They can disperse carbon nanotubes in polar solvents (e.g., ethanol or methanol),25-30 although not very selectively owing to the strong hydrophobic interaction of the nanotubes with the conjugated polymer backbone and the electrostatic repulsion provided by the charged pendant groups. The obtained SWNT/CPE hybrids can show self-doping, due to the stabilization of charges on the nanotubes depending on the type of CPE, and thus could be useful for thermoelectric applications.27, 28, 31 Pure CPEs are also often employed as fluorescence probes where the interaction of an analyte with the ionic side-chains alters or quenches the fluorescence of the CPEs.32 Hybrids of CPEs and carbon nanotubes may enable the combination of their properties and thus their application in sensors. Although commonly SWNT-based (bio)sensors are designed for electrical switching (e.g., in water-gated transistors),33-35 SWNT/CPE hybrids may also introduce near-infrared photoluminescence as a secondary signal for analyte detection. Their spectroscopic and electronic properties in particular in relation to energy transfer from the CPE to the nanotube have not been studied yet. Here we investigate the energy transfer between selected polyfluorene-related polymers and cationic or anionic conjugated polyelectrolytes and a single species of single-walled carbon nanotubes. We chose (6,5) SWNTs due to their availability as a highly pure material in large amounts after selective dispersion with a polyfluorene-bipyridine copolymer (PFO-BPy).14 We create a range of (6,5) SWNT/polymer hybrids and study the effect of different electronic


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properties of the polymer and doping levels on energy transfer to the nanotubes and photoluminescence (PL) yield. We find that under ambient conditions, the anionic side-groups of CPEs can induce and stabilize positive charges on the nanotubes. Emission-excitation maps show that the energy transfer within these hybrids is blocked by this unintentional p-doping. However, it can be fully restored by adding a mild reducing agent. Charge stabilization and destabilization in networks of different CPE-wrapped (6,5) SWNTs was further corroborated with correlated current-voltage and photoluminescence measurements in water-gated fieldeffect transistors.

EXPERIMENTAL SECTION Preparation of (6,5) SWNT dispersions. Nearly monochiral (6,5) SWNT dispersions were prepared as described before14 using CoMoCAT raw material (CHASM Advanced Materials, Lot No. SG65i-L58, average diameter 0.79 nm) and the conjugated polymer poly[(9,9dioctylfluorenyl-2,7-diyl)-alt-(6,6'-[2,2'-bipyridine])] (PFO-BPy, American Dye Source, Mw = 34,000 g/mol). Briefly, 65 mg of PFO-BPy were dissolved in 140 mL toluene. After the addition of 50 mg of CoMoCAT powder shear force mixing was applied using a Silverson L2/Air mixer at maximum speed of 10,230 rpm for 3 days. The dispersion temperature was kept at 20 °C. To separate the non-exfoliated material, the dispersion was centrifuged twice at 60,000 g for 45 minutes (Beckman Coulter Avanti J26XP centrifuge). Excess free polymer was removed by filtration of the supernatant through a polytetrafluoroethylene membrane filter (Merck Omnipore, JVWP, pore size 0.1 μm, diameter 25 mm) and the obtained filter cake was washed with hot toluene (80 °C, 7 times, 5 min each). As shown in Figure 1 the washed filter cake was divided in several parts. One was re-dispersed in toluene by bath sonication for 30 minutes, while the others were used for further processing as described below.



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Polymer exchange. Following the scheme shown in Figure 1 the different portions of the washed (6,5) SWNT filter cake were re-dispersed with either other conjugated polymers in toluene or with conjugated polyelectrolytes in methanol. Conjugated polyelectrolytes were synthesized by nickel-promoted Yamamoto-type coupling and a subsequent polymeranalogous substitution reaction at the bromohexyl side-chains by either trimethylamine (cationic CPE-A) or dimethyl sulfite (anionic CPE-S). A detailed synthesis description and characterization of the obtained polymers is provided in the Supporting Information (Figure S1). Solutions

of

the

conjugated

polyelectrolytes

CPE-A

(poly[9,9-bis(6-

trimethylammoniumhexyl)-9H-fluorene]bromide) (Mn = 7,800 g/mol) and CPE-S (poly[9,9bis(6-sulfonatohexyl)-9H-fluorene]tetrabutylammonium) (Mn = 24,300 g/mol), were prepared at a concentration of 1 mg/mL in methanol. For the preparation of (6,5) SWNT/CPE dispersions, 40 mL of (6,5) SWNT/PFO-BPy dispersion (with an optical density of ~1.0 at the E11 transition, 1 cm path length) were filtered and washed as described above. Additional washing steps with methanol at 50 °C were employed (3 times, 5 min each) to remove traces of toluene. The washed filter cake was cut in half and each half was re-dispersed in 1 mL of either CPE-A or CPE-S solution by bath sonication for 30 minutes, followed by a centrifugation step at 60,000 g for 30 minutes. The resulting polymer-rich dispersions were filtered again through a membrane filter and the filter cakes were washed with 3 mL of methanol to remove free polymer. The washed (6,5) SWNT/CPE filter cakes were re-dispersed in 1 mL of methanol and centrifuged again under the same conditions to remove non-dispersed SWNTs. The supernatant was collected and used for measurements. The polymer exchange of PFO-BPy with various conjugated polymers such as poly[(cyclopentadithiophene-2,7-diyl)-alt-(6,6'-[2,2'-bipyridine])]

(CPDT-BPy,

Mn

=


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11,000 g/mol), poly(9,9-dioctylfluorene-alt-bithiophene) (F8T2, American Dye Source, Mn = 13,000 g/mol) or poly(9,9-dioctylfluorene) (PFO, Sigma Aldrich, Mw ≥ 20,000 g/mol) was carried out in the same way as with the CPEs but with toluene as the solvent for washing and redispersion. For the synthesis of CPDT-BPy, the stannylated CPDT monomer was coupled with commercially available 6,6'-dibromo-2,2'-bipyridine in a Stille-type coupling as shown in the Supporting Information, Figure S1. Optical characterization. Absorbance spectra of all dispersions were obtained with a Cary 6000i spectrometer (Varian Inc.) using cuvettes with 1 cm optical path length. Photoluminescence excitation-emission (PLE) maps were acquired with a Fluorolog 3 spectrometer (Horiba Jobin-Yvon GmbH) with a Xenon lamp (450 W), a double monochromator for excitation and a cooled InGaAs diode array (800-1600 nm) or a PPD-900 photomultiplier (280 - 900 nm) for detection. All measurements were conducted under ambient conditions using 4 × 10 mm quartz cuvettes. The recorded emission intensities were normalized to the power of the excitation light source at the given wavelength and corrected with respect to the wavelength-dependent sensitivity of the detection system. Pure polymer solutions for absorption and excitation-emission spectra were prepared with a concentration of 10 µg/mL. All SWNT dispersions were diluted to ~0.3 absorbance (1 cm path length) at the E11 transition for optical measurements. The dispersions of CPE-S/(6,5) SWNT were also treated with a mild reducing agent ß-mercaptoethanol (BME, Sigma Aldrich, ≥ 99.0%) in order to reduce p-doping of the SWNTs. For this treatment, 20 µL of neat BME were added to a CPE-S/(6,5) SWNT dispersion in methanol to obtain a final volume of 1 mL. Determination of ionization potentials: Photoelectron yield spectroscopy in air (PESA) measurements were carried out with a Riken Keiki AC‐2 on thin films of (6,5) SWNTs, PFO, CPE-A and CPE-S on indium tin oxide (ITO) coated substrates at room temperature.



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Field-effect transistor (FET) fabrication. A side gate electrode as well as interdigitated bottom source/drain electrodes with a channel length of 20 µm and channel width of 10 mm were patterned by standard double-layer resist (MicroChem LOR5B / Microposit S1813) photolithography on glass substrates (SCHOTT AG, AF32 Eco). Chromium (2 nm) and gold (30 nm) were deposited by electron-beam evaporation, followed by lift-off in N-methyl-2pyrrolidone. Before the deposition of SWNTs, the substrates were cleaned again by sonication in acetone and isopropanol, followed by UV-ozone cleaning (UV Ozone Cleaner, Ossila Ltd.). Dispersions of (6,5) SWNTs/CPE were prepared by removing the free polymer as described above. 10 µL of the dispersion were drop-cast onto the source/drain electrode area (substrates heated to 60 °C) to form a dense SWNT network. A polydimethylsiloxane (PDMS, SYLGARD® 184) frame was placed on the substrates enclosing the area of the channels and the side-gate electrode, and filled with de-ionized water as the electrolyte. Transistor measurements. Current-voltage measurements were acquired for water-gated field-effect transistors using a semiconductor parameter analyzer (Agilent 4155C) under ambient conditions. Photoluminescence spectra were recorded using a pulsed supercontinuum laser source (Fianium Ltd., WhiteLase SC400, 20 MHz repetition rate, 6 ps pulse width, wavelength filtered at 575 nm) for excitation and an Acton SpectraPro SP2358 spectrometer with a liquid nitrogen cooled line camera (OMA V:1024, Princeton Instruments) for detection. A x50 NIR-optimized objective (Olympus, N.A. 0.65) was used to focus the laser onto the channel area and collect emission. The scattered excitation light was blocked using a dichroic long-pass filter with an 875 nm cut-off wavelength. During the measurements, a constant drain voltage of -0.1 V was applied while the gate voltage (VG) was swept from -1.0 V to 1.0 V using a Keithley 2612A source meter.


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CoMoCAT SWNTs

N

(6,5) SWNTs

N

-

Shear force mixing

-

Centrifugation

-

Filtration

- Redispersion in toluene

-

n

C8H17 C8H17

PFO-BPy (toluene) CPDT-BPy (toluene) or F8T2 (toluene) or PFO (toluene)

(6,5)/PFO-BPy (toluene)

Wash off free PFO-BPy Redispersion in CP or CPE solution Wash off excess CP or CPE Centrifuge, collect supernatant

C8H17 C8H17 S

N

S

N

S C8H17

n

S

n

CPE-A (methanol)

Br

N

N

-

Br

SO3 NBu4+

-

SO3 NBu4

+

CPE-S (methanol)

C8H17

C8H17

n

C8H17

n

n

(6,5)/CP (toluene)

(6,5)/CPE-A (methanol)

(6,5)/CPE-S (methanol)

Figure 1. Sorting of (6,5) SWNTs using the conjugated polymer PFO-BPy and preparation of (6,5) SWNT/CPE and (6,5) SWNT/CPE dispersions.

RESULTS AND DISCUSSION The goal of this study is to understand the interactions and energy transfer between preselected (6,5) SWNTs wrapped either with conjugated polymers (CPs) or with anionic and cationic conjugated polyelectrolytes (CPEs). The pre-selection of (6,5) SWNTs with PFO-BPy and the subsequent exchange of the wrapping polymer substantially simplifies the analysis and interpretation of the observed spectroscopic features in photoluminescence excitation-emission maps (PLE maps) as only one emission wavelength (the E11 emission form the (6,5) SWNTs) has to be considered. While many polymers disperse a range of semiconducting nanotubes, they rarely produce monochiral dispersions such as the employed PFO-BPy in combination with CoMoCAT nanotubes. In those cases, energy transfer between different nanotube species in the remaining bundles must be considered and the PLE maps are more crowded. Conjugated 9 ACS Paragon Plus Environment


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polyelectrolytes are even less selective towards specific SWNTs due to their ionic sidechains and very strong electrostatic stabilization of nearly all SWNT/CPE hybrids (even those with metallic nanotubes) in polar solvents. A brief comparison of dispersions of CoMoCAT nanotubes with different conjugated polymers and CPEs illustrates this issue in the Supporting Information, Figure S2. All of the semiconducting nanotube species in the CoMoCAT raw material appear in the PLE maps of the CPE dispersions indicating non-selective interactions. Dispersions of CoMoCAT nanotubes with PFO-BPy in toluene on the other hand show almost exclusively (6,5) nanotubes36,14 and hence are used here as the stock material for all experiments. To be able to study the different interactions it is important to ensure that the object of investigation is indeed only the SWNT/polymer hybrid. Hence, as a prerequisite for obtaining meaningful absorption and excitation spectra for all nanotube dispersions we took great care to remove free polymer as much as possible (see Figure 1). This procedure allowed us to resolve the absorption features of the conjugated polymers or CPEs wrapped around the (6,5) nanotubes as opposed to the free polymer in solution. It also enabled the investigation of energy transfer through excitation-emission maps without any distorting effects due to the absorption of incident light by the free polymer. The impact of too much free polymer on the excitationemission spectra and PLE maps is demonstrated in the Supporting Information, Figure S3. It shows dispersions of PFO-BPy-wrapped (6,5) SWNTs before and after removing the free polymer by filtration, washing and redispersion. No E33 and only a weak E44 transition are visible in the PLE map. The expected polymer-mediated excitation only appears as a shoulder that does not coincide with the polymer absorption. Clearly, this experimental issue could lead to misinterpretation of the excitation-emission spectra. The problem is resolved easily by filtration of the dispersion and washing of the filter cake, such that after redispersion the absorbance maximum of the polymer and the E22 transition are comparable. For such polymer10 ACS Paragon Plus Environment

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depleted dispersions all excitations can be observed clearly, while they remain stable enough (i.e., no aggregation) for the duration of the measurement. We made sure that absorption by excess polymer did not affect the PLE maps of any of the SWNT dispersions described in the following. The absorption and excitation spectra of pure PFO-BPy in toluene and those of the (6,5) SWNT/PFO-BPy dispersion are shown in Figure 2. The absorption maximum of the polymer in Figure 2a is only slightly higher than that of the E22 absorption of the nanotubes and hence substantially lower than the corresponding E11 transition. This absorption ratio indicates the absence of any significant amount of free polymer as explained above. The E33 and E44 absorption peaks of the (6,5) nanotubes at 350 and 300 nm are also evident. The excitation spectrum for the E11 emission at 1005 nm of the (6,5) SWNT/PFO-BPy hybrid essentially follows the combined absorption spectra. In particular, excitation of the polymer - although overlapping with the E33 transition - clearly leads to emission from the nanotubes (polymermediated excitation - PME). A slightly red-shifted tail of the hybrid absorption and excitation spectrum compared to the absorption of the pure PFO-BPy may be the result of π-π electron interaction between the nanotube and the wrapping polymer leading to an increased conjugation length.



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Figure 2. (a) Normalized absorbance and excitation spectra of PFO-BPy and (6,5) SWNT/PFO-BPy in toluene. (b-c) PLE maps of (6,5) SWNT/PFO-BPy and pure PFO-BPy, respectively, indicating polymer-mediated energy transfer (PME) to the (6,5) nanotubes.

The HOMO-LUMO gap of PFO-BPy (3.33 eV) is very large in comparison to the optical bandgap (E11 transition) of the (6,5) nanotubes (1.23 eV) and even larger than the energy of the E22 transition (2.15 eV). Hence, we may presume the formation of a type I heterojunction for the obtained nanotube/polymer hybrid.18 Excitation transfer is energetically favorable. The same is the case for all other polymers in this study. The polymer-mediated excitation may occur via dipole-dipole energy transfer as investigated by Eckstein et al.23 or by an electron exchange mechanism due to the strong interaction and minimal distance between the nanotubes and the polymer backbone. In addition, the SWNT/polymer hybrid might be viewed as a single system with shared states as proposed by Kahmann et al.37 To explore energy transfer in various hybrids we first exchanged the PFO-BPy with other polyfluorene-based and related copolymers in toluene. Similar to previous studies38-39 a substitution of a surfactant or a wrapping polymer with another can be achieved by using excess amounts of the latter. First, we demonstrate this exchange with a copolymer that is structurally close to PFO-BPy but contains a cyclopentadithiophene unit (CPDT-BPy, see Figure 1) instead


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of a fluorene unit, which leads to red-shifted absorption and emission (see Figure 3a-c). The well-known polyfluorene-bithiophene copolymer F8T240-41 (see Figure 1) shows an even further red-shifted absorption and emission spectrum (see Figure 3d-f). In both cases the polymer absorption does not overlap with the E22 or the E33 transition of the (6,5) nanotubes. However, in the case of F8T2 the emission spectrum with its broad vibronic progression40 overlaps strongly with the E22 absorption, while the absorption overlap of the nanotubes with the CPDT-BPy emission is minimal.

Figure 3. Normalized absorbance and excitation spectra of the conjugated polymers (a) CPDTBPy, (d) F8T2 and their hybrids with (6,5) SWNTs in toluene. PLE maps of (b) (6,5) SWNT/CPDT-BPy, (c) CPDT-BPy, (e) (6,5) SWNT/F8T2 and (f) F8T2.

For both polymers a strong red-shift of the absorption of the SWNT/polymer hybrid compared to the free polymer in solution occurs; for F8T2 by about 30 nm. This large shift



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indicates a substantial planarization of the F8T2 backbone - in particular of the two thiophene units - and thus more extended conjugation,40 which might be expected from a polymer chain wrapped around a nanotube at a shallow angle.19 For both polymers the excitation spectra clearly show polymer-mediated excitation that coincides with the nanotube/polymer hybrid absorption. The emission wavelengths vary slightly, 1002 nm for (6,5) SWNT/CPDT-BPy and 1007 nm for (6,5) SWNT/F8T2. The E33 and E44 transitions of the (6,5) SWNTs are clearly visible. The PME is only slightly more efficient for the F8T2 hybrid than for the CPDT-BPy hybrid despite its much larger emission overlap with the E22 transition. Next, we used the same polymer exchange protocol for an anionic (CPE-S) and a cationic (CPE-A) conjugated polyelectrolyte (see Figure 1) with a polyfluorene backbone. Both are soluble in methanol. The exchange of the PFO-BPy by CPEs can be assumed to be complete as only (6,5) SWNT/CPE hybrids can be re-dispersed in the highly polar methanol and all other species are separated via centrifugation. Figure 4 shows the absorption and emission spectra of the dispersion of CPE-A wrapped (6,5) SWNTs and pure CPE-A solution. E11 emission occurs at 1001 nm, i.e., slightly blue-shifted from the PFO-BPy dispersions in toluene and thus closer to (6,5) SWNTs dispersed in water by surfactants.42 Very similar to the (6,5) SWNT/PFO-BPy hybrids and also to the more closely related PFO hybrids (see Supporting Information, Figure S4) the polymer-mediated excitation is clearly visible with the excitation spectrum following the absorption of the CPE-A and barely any overlap of the emission spectrum of the CPEs with the absorption peaks of the (6,5) nanotubes.


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Figure 4. (a) Normalized absorbance and excitation spectra of the anionic conjugated polyelectrolyte CPE-A and (6,5) SWNT/CPE-A in methanol. (b-c) PLE maps of (6,5) SWNT/CPE-A and pure CPE-A, respectively.

In stark contrast to that, the hybrids of (6,5) SWNTs wrapped with the anionic CPE-S do not show any PME corresponding to the CPE-S absorption nor E33 and E44 transitions in the emission-excitation map and excitation spectrum as shown in Figure 5. Nevertheless, the (6,5) nanotubes are still present and emit at 1000 nm when excited via the E22 absorption although with a lower efficiency than the CPE-A hybrids (see Supporting information, Table S1). The ratio of the CPE-S and the E22 absorption peaks exclude absorption by excess CPE as a cause for the absence of PME. While the CPE-S is wrapped around the nanotubes and absorbs light, it does not lead to excitation transfer to and emission from the (6,5) nanotubes. Excitation via E33 or E44 absorption is either too low to be detected or also blocked by the CPE-S.



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Figure 5. (a) Normalized absorbance and excitation spectra of the cationic conjugated polyelectrolyte CPE-S and (6,5) SWNT/CPE-S in methanol. (b-c) PLE maps of (6,5) SWNT/CPE-S and CPE-S, respectively.

How can this difference between the two nanotube/CPE hybrids be explained? It is wellknown that nanotubes are slightly p-doped in air while n-doping is not possible under such conditions.43 Hence, nanotubes wrapped with a cationic CPE-A should be essentially neutral, as the positively charged side-groups make p-doping by oxygen energetically unfavorable. The anionic CPE-S on the other hand may induce and actually stabilize a small amount of positive charges on the (6,5) nanotubes.28 Charge accumulation on nanotubes quenches their photoluminescence44, 45 and indeed the observed photoluminescence yield, estimated as the ratio of emission intensity and absorbance for each sample, is lower for the (6,5) SWNT/CPES hybrids than for the CPE-A or PFO hybrids (see Table S1, Supporting Information). However, the doping concentration must be fairly low because no strong trion emission (redshifted from excitonic E11 transition by ~165 nm)46 is observed.


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Figure 6. PLE maps of (6,5) SWNT/CPE-S in methanol (a) before and (b) after the addition of 2% ß-mercaptoethanol as a mild reducing agent. The photoluminescence intensity was normalized to the maximum emission excited at E22 in both PLE maps. (c) Normalized absorbance and scaled excitation spectra before (dashed line) and after (solid line) the addition of BME.

Such mild p-doping of SWNTs in dispersion should be removable by a mild reducing agent such as ß-mercaptoethanol (BME), which was used in previous studies to increase the photoluminescence yield of DNA-wrapped SWNTs.47 Figure 6 shows the PLE maps and excitation spectra of a (6,5) SWNT/CPE-S dispersion in methanol before and after the addition of 2% BME. The overall photoluminescence yield improves significantly (see Table S1, Supporting Information). The emission resulting from E22 excitation, which can be observed for both cases, increases by roughly 50%. However, the polymer-mediated excitation as well as the E33 and E44 transition now become visible again, similar to the PLE maps of the CPE-A hybrid. In addition, after the BME treatment the E11 emission shifts slightly from 1000 nm to 1005 nm. This red-shift of the E11 emission also indicates a transition of the (6,5) nanotubes from hole-doped to neutral.46,

48

The removal of positive charges (holes) from the

nanotube/CPE hybrids not only increases photoluminescence yield but also restores the polymer-mediated excitation. The same overall effect can be observed for dispersions of 17 ACS Paragon Plus Environment


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CoMoCAT raw material with CPE-S directly in methanol followed by BME treatment as shown in the Supporting Information, Figure S5. Clearly, the presence of positive charges (pdoping) or their absence has an effect on the energy transfer in these hybrids even before any severe photoluminescence quenching44 is observed. To understand this behavior better and to be able to draw a rough energy level diagram we measured the ionization potentials of the (6,5) SWNT, the CPEs and PFO as a reference by photoelectron yield spectroscopy in air (see Supporting Information, Figure S6) and used the optical bandgaps to estimate the electron affinities. The resulting energy level diagram is shown in Figure 7. As expected the ionization potential of the cationic CPE-A (-5.78 eV) is more negative than that of PFO (-5.66 eV), while that of the anionic CPE-S is less negative (-5.58 eV). However, overall these differences are small and the HOMO-LUMO gaps are similar. In all cases a type I heterojunction should be formed with the (6,5) nanotubes; energy transfer is energetically favorable and should occur rapidly. The fact that despite the very similar absorption and emission spectra of all three polymers PME does not occur for the (6,5) SWNT/CPE-S hybrid makes a non-radiative dipole-dipole mediated energy transfer (i.e., Förster transfer) unlikely. The restoration of PME in the (6,5) SWNT/CPE-S dispersion after reduction by BME suggests that an electron exchange mechanism (e.g., Dexter transfer) could play a significant role. When the nanotubes are undoped it is energetically favorable for both holes and electrons of the bound exciton to transfer to the valence and conduction band of the (6,5) nanotube, respectively, and recombine there under emission of a near-infrared photon. For the case of CPE-S wrapped nanotubes in methanol positive charges are stabilized on the nanotubes by the anionic sidegroups of the CPE and thus p-doping by oxygen occurs. After excitation of the CPE-S by visible light and formation of the exciton, the presence of positive charges on the nanotube apparently prevents charge exchange and thus also energy transfer from the CPE to the (6,5) SWNTs. When the p-doping is removed by adding the reducing agent 18 ACS Paragon Plus Environment

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BME, charge and energy transfer can take place again as in the case of the PFO or CPE-A hybrids. The unusual lack of excitation via E33 and E44 absorption for mild p-doping might be explained with an inverse type I heterojunction situation. A fast energy transfer from E33 and E44 to CPE-S would lead to the same result as direct excitation of CPE-S. However, given the uncertainty of the exact energy levels, a type II heterojunction and stabilization of a charge transfer complex may also be possible at least with respect to CPE-S and E33. As shown in Figure 6b the addition of BME also restores excitation via E33 and E44.

Figure 7. Energy level diagram of (6,5) SWNTs, PFO, CPE-A and CPE-S based on ionization potentials from PESA measurements and optical bandgaps. The density of states of (6,5) SWNTs were obtained from Ref.

49

and shifted with respect to the measured ionization

potential. The observed absorption and emission transitions are indicated with vertical arrows.



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The impact of doping and charge accumulation on the emission and charge transport properties of (6,5) SWNT/CPE hybrids is investigated further in water-gated field-effect transistors. The basic structure of the employed device is illustrated in Figure 8. A dense network of (6,5) SWNT/CPE hybrids bridging the source/drain electrodes was formed by dropcasting the corresponding dispersions, followed by rinsing with methanol. This semiconducting layer remained spatially separated from the side-gate electrode. De-ionized water served as the electrolyte with sufficiently high ion concentration due to autoprotolysis. Water-gated transistors based on organic semiconductors or carbon nanotubes operate at low voltages and are suitable for a wide range of biosensing applications.33-35, 50 For an applied positive or negative gate voltage the cations (H3O+) or anions (OH-), respectively, move toward the semiconductor and form electric double layers with very high capacitances (few µF/cm2). This leads to the accumulation of electrons or holes in the channel and hence charge transport and current flow. A small constant bias of VDS = -0.1 V is applied throughout the measurements and the drain current (IDS) is recorded as a function of the gate voltage (VG) sweep to obtain transfer characteristics. At the same time the PL spectra of the SWNT/CPE hybrids can be recorded via excitation with a laser and collection of emission through a microscope objective.

Figure 8. Schematic illustration of a water-gated field-effect transistor (FET) with (6,5) SWNT/CPE hybrid network, interdigitated source/drain electrodes and side-gate. PL spectra depending on the applied gate voltage are recorded via excitation with a laser (575 nm) and collection of emitted photons through a microscope objective.


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The ionic nature of the CPEs (anionic or cationic) directly affects the observed charge transport characteristics of the nanotube network in water-gated FET. For the (6,5) SWNT/CPE-S hybrid very clear hole accumulation and good hole transport with almost no hysteresis can be observed (see Figure 9a). Semiconducting SWNT networks should show intrinsically ambipolar behavior, as observed when using water- and oxygen-free ionic liquids as the electrolyte,51 but electron accumulation is suppressed for water-gated FETs under ambient conditions.43 Electron transport should be further impeded by the anionic side-chains of the CPE-S and no electron current is apparent in the transfer characteristics. The turn-on voltage is shifted to positive values (+0.3 V), which indicates mild p-doping of the nanotubes, thus corroborating the PLE data in Figure 6.

Figure 9. Transfer characteristics, gate voltage dependent PL spectra and normalized PL intensities (at E11) for (a-c) (6,5) SWNT/CPE-S and (d-f) (6,5) SWNT/CPE-A water-gated FETs.



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The VG - dependent photoluminescence spectra (Figure 9b) show the expected behavior of SWNTs, quenching and a blue shift of the E11 emission with more negative gate voltages (hole accumulation) and an increase in trion emission at 1169 nm that even exceeds the exciton emission at a gate voltage of -1.0 V (see also Supporting Information, Figure S7). The gatevoltage dependent drop of emission intensity in Figure 9c further supports the notion of partial p-doping of (6,5) SWNT/CPE-S hybrids in air because a positive gate voltage is required to reach the maximum PL intensity that is indicative of undoped nanotubes. At zero gate voltage the PL is already 50% quenched. In contrast to that, the water-gated FETs with (6,5) SWNT/CPE-A hybrids reproducibly show poor hole transport and also no electron transport (Figure 9d). As mentioned before, despite the cationic sidechains of the CPE electron accumulation is not possible in air and water.43 At the same time the positively charged sidegroups should repel positive charge carriers and thus impede hole injection and transport through the nanotube network. The maximum drain currents for water-gated FETs with (6,5) SWNT/CPE-A hybrids were at least two orders of magnitude lower than those with SWNT/CPE-S hybrids at similar network densities. The PL spectra also show that at zero gate voltage the emission was not yet quenched and a significant negative voltage was required to observe charge-induced PL quenching (Figure 9e,f). The water-gated FET data thus fully corroborates the notion that (6,5) SWNT hybrids with cationic CPEs are essentially undoped while those with anionic CPEs are p-doped in air. Either chemical reducing agents or a positive electrochemical potential are required to restore charge neutrality and thus photoluminescence yield and energy transfer.


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CONCLUSION We have investigated the energy transfer between pre-selected (6,5) SWNTs and a range of polyfluorene-related copolymers and polyfluorene-based polyelectrolytes (anionic and cationic). The employed polymer-exchange protocol and removal of excess polymer proved to be highly efficient and simplified the spectral analysis substantially. The red-shifted absorption of the polymers wrapped around the nanotubes indicated backbone planarization and thus increased conjugation. Photoluminescence excitation-emission maps showed clear energy transfer from the polymers to the (6,5) SWNTs for all but one: the anionic conjugated polyelectrolyte CPE-S. The absence of polymer-mediated excitation could be attributed to the partial p-doping of the nanotubes in air that was enhanced by the CPE-S. Charge neutrality and energy transfer could be restored by adding a mild reducing agent. These observations point toward an electron exchange mechanism rather than dipole-dipole interaction between the polymers and the carbon nanotubes for energy transfer, however, further time-resolved studies on these hybrids are needed. The charge transport characteristics and gate-voltage dependent photoluminescence of water-gated field-effect transistors based on anionic and cationic SWNT/CPE hybrids corroborated these findings. The combination of highly doping-dependent energy transfer and photoluminescence with the good charge transport properties of SWNT/CPE hybrids in water-gated transistors could be applied in dual-signal (optical and electrical) sensors.



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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 Deutsche Forschungsgemeinschaft (DFG, ZA 638/7 and SCHE 410/33) and the Struktur- und Innovationsfonds Baden-Württemberg (SI-BW).

ASSOCIATED CONTENT Supporting Information. The Supporting Information is available free of charge on the ACS Publications website at DOI: XXX. Synthesis of conjugated polyelectrolytes and CPDT-BPy, preparation of CoMoCAT dispersions, selectivity of conjugated polymers and polyelectrolytes towards SWNTs, comparison of PLE maps for polymer-rich and polymer-depleted dispersions, absorption and PLE maps of pure PFO and (6,5) SWNT/PFO hybrids, comparison of quantum yields for different (6,5) SWNT hybrids, PLE maps for CoMoCAT dispersions with CPEs, PESA measurements, evolution of PL spectra for (6,5) SWNT/CPE-S water-gated FETs (pdf).


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Doping-Dependent Energy Transfer from Conjugated Polyelectrolytes to

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