Photophysics of nanomaterials for opto-electronic applications
Kahmann, Simon
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6 Hybrid Excited States in Polymer
Wrapped CNTs
The insights into the photoinduced polaron signatures of conjugated polymers acquired in Chap-ters 4 and 5 are used in this investigation to study the physical properties of a hybrid nanosystem comprising semiconducting single walled carbon nanotubes with conjugated polymers wrapped around them. Mid infrared fingerprints indicate that charge carriers form on the wrapping poly-mer chains, even for excitation energy below their band gap. This feature cannot easily be explai-ned, given the known position of the energy levels of the hybrid components. Quantum chemical calculations consequently reveal a mixing of states between them, which promotes the observed charge formation on the polymer.
6.1 Introduction
Polymer wrapped semiconducting single walled carbon nanotubes (CNTs) attract considerable research interest, since they allow for solution deposition, possibly on flexible substrates whilst providing a high stability towards environmental degradation. Crucially, the wrapping polymer not only promotes stability of the suspension, but also serves as a sensitive tool to select se-miconducting tubes, which are generally mixed with metallic species after synthesis.[1,2]This selectivity is a key reason for an improved performance of CNT electronic devices, e.g. field ef-fect transistors or photodetectors.[3–6]
While charge conduction in these systems is commonly discussed as if the polymer chains were absent, the close proximity of the two components and theirπ-electron systems suggest this approximation to be invalid. For films of polymer wrapped CNTs, photoluminescence (PL) spectroscopy often reveals a shortening of polymer PL lifetime when compared to the neat po-lymer. This effect was previously attributed to a rapid energy or an electron transfer towards the CNTs.[7–9]It is of utmost importance to understand the components’ interaction in greater
S. Kahmann, J. M. Salazar-Rios, M. Zink, S. Allard, U. Scherf, M. C. dos Santos, C. J. Brabec, M. A. Loi, Excited-State Interaction of Semiconducting Single-Walled Carbon Nanotubes with Their Wrapping Polymers, J. Phys. Chem. Lett., just accepted, doi:10.1021/acs.jpclett.7b02553.
Calculations were carried out by Maria Cristina dos Santos.
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PF12 PCBM (8,7) CNT
Ener
gy / eV
P3DDTFigure 6.1: Energy levels of neat polymers, PCBM and a representative CNT alongside an illustration of a
polymer chain wrapped around a nanotube (a). Absorption spectra of P3DDT wrapped CNTs in solution and film along a film spectrum of neat P3DDT.
detail, especially when developing electro-optical applications that involve charge carrier trans-port through networks of polymer wrapped CNTs.
6.2 Results and Discussion
The spectroscopic properties of polymer wrapped CNTs change greatly with the sample quality (e.g. amount of residual metallic tubes or bundles) and preparation technique (film vs. solu-tion, casting method). The general behaviour of cast films is thus examined before focusing on the interaction mechanism of two different polymers, P3DDT and PF12, with CNTs, for which the relevant energy levels are depicted in Figure 6.1 (a). The absorption spectra in Figure 6.1 (b) include neat P3DDT as well as the CNT sample (in solution and as a drop-cast film). The polythi-ophene, P3DDT, exhibits a comparatively narrow band gap and was previously shown to exhibit a high dispersion yield for HiPCO CNTs with a diameter of approximately 1 nm.[10] The poly-mer absorbs most strongly from 1.9 to 3.5 eV, a spectral region with few nanotube transitions. Its spectrum, when wrapping CNTs, exhibits distinct peaks at 2.3 and 2.05 eV, which are indicative for a strongπ-π interaction between wrapping polymer chains and the CNTs.[11]Interestingly, whether the tubes are in solution or in a film has a negligible impact on the position of their ground state transition (S11).
From the ratio of the polymer absorption to the S11transitions of the tubes, the amount of
un-bound polymer in this purified sample can be inferred to be negligible, similar as previously demonstrated.[5]The same is true for the second investigated sample containing CNTs wrapped with the wide band gap polymer PF12 (Figure A3).
In contrast to highly mono-chiral samples, charge or energy transfer processes from small to
P HO T OP H YS IC S OF NAN OM A TE R IALS F OR OPT O-EL
Figure 6.2: Steady state (a) and time resolved (b)-(d) PL spectra of polymer wrapped CNTs. (b) displays the
time resolved PL of the CNTs, (c) and (d) of the polymers P3DDT and PF12 in neat films and for polymer wrapped CNTs. The polymer PL lifetimes are significantly reduced when wrapped around CNTs.
large diameter tubes could dominate in the polychiral samples considered here.[12,13]The steady-state PL intensity in Figure 6.2 (a), however, generally follows the absorption profile of the re-spective sample in solution and thereby shows that on-site radiative recombination competes favourably with inter-tube exciton migration. The PL intensity, nonetheless, is distinctively low, which is in agreement with the determined PL lifetimes of approximately 30 ps (Figure 6.2 (b)) and previous reports.[9,14]The emission profile of the polymers in Figure 6.2 (a) displays more pronounced vibrational peaks due to changes in chain conformation, analogous to the absorp-tion spectra noted above.[11]Upon wrapping, the PL lifetime of both, P3DDT (c) and PF12 (d), decreases by a factor of ten for both decay components, which could be due to both an energy or a charge transfer towards the wrapped tubes, indicating their strong interaction.
In order to understand the nature of the transfer process, pump-probe techniques were em-ployed. As discussed in previous chapters, charge carriers generally lead to the formation of ad-ditional absorption bands in polymers. One of these at an energy slightly smaller than the band gap (P2) and a second one in the MIR spectral region (P1). Upon excitation with 2.3 eV (532 nm),
the formation of the P2absorption for P3DDT can be observed around 1.23 eV in blend with
the electron acceptor PCBM (quasi-steady-state, Figure 6.3 (a)). When wrapped around CNTs,
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Figure 6.3: NIR steady-state PIA of neat P3DDT and when wrapped around CNTs (a). The latter exhibits
narrow signals attributable to the CNTs, but the characteristic P2absorption of the polymer cannot be
observed. Transient spectra at three selected energies upon CNT- (b) and polymer excitation (c) include long-lasting signals. Steady state photoinduced absorption spectrum of P3DDT wrapped tubes along with calculated spectra (d). The latter were generated for a red-shifted absorption as outlined in the main text.
however, no such distinct feature is observable. In contrast, a rich pattern of narrow absorp-tion/bleach transitions emerges. The presence of these features points to long-lived species for-med on the nanotubes. Examining their transient behaviour at three selected probe energies (Figure 6.3 (b), (c)), a strongly negative response for all chiralities is found at short delay times, when directly pumping the tubes at 1.6 eV (780 nm, (b)). A constant signal corresponding to the steady-state sign is reached after approximately 150 ps. This long lasting component is assigned to charges that can readily form upon tube excitation (see discussion below).[15,16]The ultrafast response is complex and may involve several phenomena, including singlet-, triplet-, biexciton, and trion transitions (section 2.3.1),[17–19]but will not be discussed further in this investigation. For an excitation energy of 2.3 eV (532 nm), i.e. in the absorption region of the polymer, the same trends at long delay time is observed (Figure 6.3 (c)). Crucially, though, no comparable dip mani-fests at early times. This is suggestive of a charge-, rather than energy transfer from the polymer towards the tubes, for which a similar behaviour at early delays is expected.
The formation of charge carriers has been greatly investigated for mono-chiral CNT samples and
P HO T OP H YS IC S OF NAN OM A TE R IALS F OR OPT O-EL
form upon photoexcitation. The most prominent of the latter is the trion peak, which involves the generation of a three-particle state (2 holes and 1 electron or vice versa, section 2.3.1) at an energy lower than the S11transition. This effect was considered to explain the PIA spectrum
through a combination of ground state bleach (GSB), and the additional trion absorption in such mono-chiral samples. Good agreement with the experimental data here could, however, not be found (also consider discussion in the appendix).
In contrast, the experimental spectrum can be mimicked by assuming a small red-shift of the CNT ground state absorption under illumination, similarly to previously proposed.[16,22]To this end, the transmission spectrum T (E ) in darkness is used to calculate a spectrum T∗(E ) = T (E +
d E ) as an artificial (shifted) transmission spectrum under illumination. The corresponding
arti-ficial PIA spectrum is then calculated as: −∆T /T = c1·(T∗−T )/T +c2. Results for several negative
d E (i.e. red-shifts) are displayed in Figure 6.3 (b). Although the magnitude cannot be followed
entirely, a remarkable agreement between the simulated PIA spectrum and the experimental curve is achieved with this crude model.
Instead of trion absorption, a small reduction in ground state energy is thus predominantly re-sponsible for the steady-state PIA signal of the CNTs. This red-shift was previously assigned to a Stark effect.[16,22]This being a likely explanation, it should be noted that a change in charge carrier concentration itself can be responsible for such a red-shift of the ground state transition, as this leads to a drastic reduction of the fundamental band gap energy.[23,24]Concluding above measurements in the NIR spectral region, it can be stated that charges are generated on the CNTs. For polymer excitation, there is furthermore evidence for an electron transfer towards the nanotubes. Since this process would also lead to a so far elusive hole remaining on the polymer, the following investigations turn to the MIR spectral region, where CNT signals are expected to not disturb the analysis.
Figure6.4 (a) depicts the MIR photoinduced absorption of neat P3DDT (top) and when wrapped around CNTs (bottom) (overview spectra are attached in the appendix section 8.4). Both sam-ples display a broad absorption band peaking around 0.1 eV, when exciting above the polymer band gap energy (2.3 eV), which is attributed to the formation of polarons on the polymer.[25–27] The narrow features superimposed are infrared active vibrations (IRAVs), as discussed in Chap-ter 4.[26,28]Notably, the signals’ position is independent of the excitation energy and only varies in strength.
The black lines in Figure 6.4 (a) indicate the spectra obtained for excitation at 1.6 eV (780 nm), i.e. below the polymer band gap energy. As expected, it remains flat for the neat polymer, while for the CNT sample the same polaron signature can be observed as for above gap excitation. Pho-ton absorption of the CNTs thus leads to the formation of charges on the polymer, as uniquely evidenced by the spectral signature of their polarons.
As the position of the absorption band in the MIR works as a highly sensitive probe for the
envi-HYBR ID E X CITED S T A TE S IN POL YMER WR
Figure 6.4: Below polymer band gap excitation generates the same polaron signature as above band gap
excitation for the P3DDT wrapped CNTs (a). No such signature is observable in the neat case (numbers denote the pump energy). The maximum of the P1absorption shifts towards higher energy for polymer
chains with larger disorder (b). The circles indicate the interaction of the polaron absorption band with the CNT optical phonon, which is only observable for the purified sample. PF12 wrapped CNTs also give rise to the polaron fingerprint upon below band gap excitation (c). The signal strength is significantly lower than for the sample containing P3DDT.
ronment of the charge carrier, the maximum of the P1signal, is analysed in Figure 6.4 (b). The
position shifts to lower energy from the blend with the electron acceptor PCBM (which simulta-neously confirms the assignment to positive polarons) over the neat case to the wrapped tubes. This trend underlines the strongπ-π interaction between the wrapping polymer and the CNTs, as discussed above for the absorption and PL spectra (Figure 6.1 (b) and 6.2 (a)). Although the employed samples were highly purified, the presence of unattached polymer chains cannot per
se be excluded and free excess polymer could take up charge carriers from the wrapping chains
to stabilise them. Therefore, to further clarify the location of these carriers, a sample was added to this study, for which the purified CNTs were mixed with excess P3DDT prior to deposition. The presence of unbound chains between the tubes can be assured in this case. Indeed, the cor-responding PIA spectrum in Figure 6.4 (b) displays a shift of the envelope towards higher energy. For the purified samples, it can therefore be concluded that charges reside on the chains wrap-ped around the CNTs.
Supporting this claim, distinct differences in the vibrational signals can be observed in the CNT
P HO T OP H YS IC S OF NAN OM A TE R IALS F OR OPT O-EL
(1594 cm−1) can be observed, which is not present in the sample where extra polymer has been
added. This spectral feature (indicated by a circle) coincides with the CNT G-band (or optical phonons) (section 2.3.2).[29]Given its asymmetric shape, it is attributed to a Fano resonance[30] between the energetically broad electronic background of the polaron and the narrow vibration (also consider discussion in Chapter 8 for this phenomenon). This feature is thus only observa-ble if the CNTs are in proximity of the charge.
As the formation of a polymer polaron signature upon below band gap excitation is a surpri-sing observation and to verify that this is not an exception for a particular material, the same investigations were carried out with CNTs wrapped with the wider band gap polymer PF12. This polymer exhibits a lower lying HOMO level, than P3DDT (Figure 6.1). This discussion hence helps to rule out that few CNTs of extremely narrow diameter (and wider band gap) are respon-sible for the hole transfer observed above.
Again, every pump energy (3, 2.3, 1.6 eV), above and below the polymer band gap, generates a distinct polaron band in the MIR for PF12 wrapped CNTs, which is similar to the one observed for the blend with PCBM (Figure 6.4 (c), additional data in the appendix). It is thus concluded that even for below polymer gap excitation, polarons are formed on the chains. Accordingly, also the narrow feature at 0.197 eV linked to CNT phonons can be observed. The signal strength for these samples, however, is markedly lower than for the P3DDT investigation.
At this point, it could be argued that holes, generated by CNT excitation into a higher manifold (S22), could transfer towards the polymer prior to thermalisation. Thermalisation into the S11
manifold, however, was reported to occur on an ultrafast timescale shorter than 100 fs,[31]thus making a transfer from S22unlikely. In contrast, these findings show that the common way of
depicting the energy levels, as depicted in Figure 6.1 (a), cannot be invoked to explain the charge carrier formation on the polymer. A hole transfer from the CNTs towards either polymer would be energetically forbidden. The close proximity of the two entities and the interaction of their respectiveπ-systems is hence assumed to create a new hybrid system with states shared bet-ween the two components. This was similarly observed for the "pea in a pod" systems of small molecules filled into CNTs.[32]
Such a hybridisation of states also helps to explain why energy barriers in transport measure-ments using field effect transistors cannot be observed[3,5,33]or why different threshold voltages manifest in these devices, when using different wrapping polymers.[5]
To prove the proposed hybrid nature of these systems, the low-lying excited states of P3DDT:CNT and PF12:CNT were investigated by means of the quantum chemical method ZINDO/S-CI. In this method, the wavefunction of the molecular excited state is written as a linear combination of Slater determinants representing all of the singly excited states that can be built from the refe-rence ground state determinant. The hybrid systems were modelled as a molecule comprising a
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Figure 6.5: Geometric optimisation of oligomer chains wrapped around a nanotube leads to the
arrange-ments shown in (a). The calculated contributions to the first excitation of naked CNT and when wrapped by the respective oligomers is shown in (b).
finite (8,6) nanotube and an oligomer wrapping the nanotube in a helical conformation (shown in Figure 6.5 (a)).
Figure 6.5 (b) depicts the calculated absorption spectra alongside the absorption of a naked (8,6) nanotube for comparison. Due to the finite size of the system, the main S11transition peak
of the naked nanotube is split into two main contributions. Additionally, in the P3DDT:CNT system, the peaks are split farther apart due to a strong polymer-nanotube interaction, which also shifts the transitions to lower energy. For comparison, the CNT absorption region for both samples is shown in Figure A3 (b), where the expected red-shifted peaks for the P3DDT sample can be observed. It is to note, however, that besides selecting different chiralities, the changed permittivity of the tube environment due to the different polymers will also affect the peak posi-tion. Table A3 displays the most important contributions forming these excited states with their corresponding coefficients associated with the probability of a particular excitation.
The leading excitations are H-8→L and H-1→L+1 (H-i denotes i-th level below the HOMO and L+i the i-th level above the LUMO) and most molecular orbitals involved are mainly localised on the nanotube. Nonetheless, in good agreement with the proposition of a hybrid nature, there is a considerable contribution of orbitals presenting a large degree of mixing between the poly-mer and the CNT. Some examples are depicted in Figure 6.6. Especially H-13 displays an orbital spread over the entire system. Apart from the transition H-7→L+10 (HOMO-LUMO polymer transition) appearing in the 1.07 eV excited state, all transitions involving hybridised orbitals, display a large contribution from the polymer in the starting orbital and the final orbital is lo-calised on the nanotube. These excitations can thus, in effect, lead to an electron transfer from the polymer to the nanotube at an excitation energy commonly assumed to be purely due to the CNT transitions.
The right panel of Figure 6.5 (b) shows the spectrum of the PF12:CNT system, which is similar to the spectrum of the naked nanotube shown in the left panel. There is some degree of
hybridi-P HO T OP H YS IC S OF NAN OM A TE R IALS F OR OPT O-EL
Figure 6.6: Representative molecular orbitals of the P3DDT:CNT system. Amongst the occupied states,
there is an increasing hybridisation of the polymer-nanotube wavefunctions in going from H-17 to H-1; for H-7 it is basically localised on the polymer and corresponds to the HOMO of the unperturbed chain. From H-6 to H (not shown) the wavefunctions are localised on the nanotube, similarly to the L state and the subsequent states until L+9. L+10 is localised on the polymer and is its LUMO state. The different colours indicate opposite signs of the wavefunction.
sation of the molecular states, but to a significantly smaller proportion than for the P3DDT:CNT system. This is in good agreement with the experimentally determined smaller polaron signal as shown in Figure 6.4 (c). The reason for a weaker interaction lies in the geometry of the polymer wrapped nanotube. As the fluorene units are longer and stiffer than the thiophene ring, besides the geometrical constraints imposed by the dodecyl chains, the fluorene rings do not adsorb to the nanotube surface as close as the thiophenes, leading to a weaker interaction.
6.3 Conclusion
In conclusion, the excited states interaction of films of polymer wrapped single walled carbon nanotubes with a polythiophene and a polyfluorene as wrapping agent were investigated. Pho-ton absorption leads to the formation of long-lived charge carriers located on the nanotubes, which can be observed by a rich absorption-bleach pattern in steady-state photoinduced ab-sorption spectra. Furthermore, polaron formation on the polymer is observed, independent of whether the excitation energy lies above or below the polymer band gap. This process cannot be explained by considering the classical energy level scheme for two separated systems, but is a strong proof for the hybridised nature of the excited states of these nanoobjects. This
as-HYBR ID E X CITED S T A TE S IN POL YMER WR
observations in charge transport related applications of these hybrid systems. P HO T OP H YS IC S OF NAN OM A TE R IALS F OR OPT O-EL
Preparation of semiconducting SWNT dispersions: Both polymers were synthesised in the group
of Prof. U. Scherf at the University of Wuppertal. Poly(9,9-di-n-dodecylfluorenyl-2,7-diyl) (PF12, Mn=280000 g mol−1, Mw=589000 g mol−1) was synthesised in a Yamamoto-type homocoupling reaction. Poly(3-dodecylthiophene-2,5-diyl) (P3DDT, Mn=29.200 g mol−1, Mw=35.900 g mol−1) was synthesised via the GRIM method.
HiPCO SWNTs (d=0.8-1.2 nm), purchased from Unidym Inc., were used as received. The poly-mers were solubilised in toluene using a high power ultrasonicator (Misonix 3000) with cup horn bath (output power 69 W) for 20 min. Subsequently, SWNTs were added to form the SWNT:po-lymer dispersions with weight ratio of 1:2 (3 mg of SWNTs, 6 mg of poSWNT:po-lymer, 15 mL of toluene). These solutions were then sonicated for 2 h at 69 W and 16◦C . After ultrasonication, the
disper-sions were centrifuged at 30000 rpm (109000g) for 1 h in an ultracentrifuge (Beckman Coulter Optima XE-90) to remove all remaining bundles and heavy-weight impurities. During the cen-trifugation, these components precipitate at the bottom of the centrifugation tube, while the individualised SWNTs wrapped by the polymer, and free polymer chains remain in the upper part as supernatant. One extra step of ultracentrifugation was implemented to decrease the amount of free polymer in solution (enrichment).[3]For this purpose, the supernatant obtained after the first ultracentrifugation is centrifuged for 5 h, 55000 rpm (367000g), the individualised semiconducting CNTs are then precipitated to form a pellet and the free polymer is kept in the supernatant. Finally, the pellet is re-dispersed by sonication in toluene.
UV/vis: Absorption spectra were recorded with a Shimadzu 3600 UV-vis-NIR spectrometer. Films
were cast on glass substrates and solutions measured at low concentration in quartz cuvettes.
Photoluminescence: Films were deposited on quartz substrates, sealed, using epoxy glue, and
excited at 400 nm using the second harmonic of a mode-locked Ti:sapphire laser at a repetition rate of 76 MHz. Steady-state spectra were recorded with an InGaAs detector from Andor. Time-resolved traces were taken with a Hamamatsu streak camera working either in the synchroscan or single sweep mode. An optical pulse selector was used to vary the repetition rate of the excit-ing pulses where necessary.
Transient absorption: Transient absorption spectra were recorded using a super continuum
la-ser source (SuperK Extreme, NKT Photonics) that provided both the pump and probe at a MHz repetition rate. The pulse duration was approximately 1 ps and the pump power was always kept below 4µJ cm−2in order to avoid thermal effects, sample degradation as well as non-linear higher order processes. The signals were measured with an auto-balanced photoreceiver (Nir-vana 2017, New Focus) and amplified with a Lock-In amplifier (SR 830 DSP, Stanford Research Systems). All measurements were carried out at room temperature.
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the sample with a 532 nm laser, chopped at 141 Hz, and probing with the continuous spectrum of a Xe lamp. The transmitted light is dispersed by a 1200 lines mm−1grating monochromator (iHR320, Horiba) and detected by a Si detector down to an energy of 1.1 eV and an InGaAs detec-tor for lower values. Additional measurements with a blocked Xe lamp account for the sample PL.
FTIR-PIA: Thin films were drop-cast from solution onto ZnSe platelets. The sample, cooled
down to 77 K, was excited via different high power LEDs or a laser emitting at 532 nm. To avoid perturbations from the pump light, a GaAs filter was installed in front of the detector. Measure-ments were carried out as discussed before.
Computational details: The geometry of a finite (8,6) nanotube 52 Å length and with edge bonds
saturated by hydrogen was optimised by the Density Functional derived method Third order Self-Consistent Charge Density Functional Tight-Binding with empirical dispersion (DFTB3-D3) and 3OB parameter set.[35–37]PF12 and P3DDT were modelled as oligomers containing 8 and 16 units respectively. These oligomers were arranged in a helical conformation around the na-notube and the geometry of the hybrid systems was optimised within the DFTB3-D3 method, keeping the nanotube conformation frozen in its ground state geometry. The optimised geome-tries of the naked nanotube and the hybrids formed with PF12 and P3DDT wrapping the nano-tube were used to calculate the energies, oscillator strengths and wavefunctions of the low lying excited states using the method ZINDO/S-CI.[38]In the presented calculation, all possible singly excited determinants were used. Calculations were carried out using the softwares DFTB+ and GAUSSIAN 09. Due to finite size effects, the electronic level spacing of these systems is larger than it would be calculated for an infinite nanotube-polymer hybrid. Consequently, the excited states have higher energy. This behaviour has been corrected by a contraction of the electronic spectra, using the S11transition of the air suspended (8,6) nanotube, that peaks at 1.114 eV, to
correct the spectrum of the naked finite nanotube. The same contraction factor was used for all spectra. P HO T OP H YS IC S OF NAN OM A TE R IALS F OR OPT O-EL
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