Femtosecond Electron Delocalization in Poly(thiophene) Probed by

Article pubs.acs.org/JPCC

Femtosecond Electron Delocalization in Poly(thiophene) Probed by Resonant Auger Spectroscopy C. Arantes,†,§ B. G. A. L. Borges,† B. Beck,† G. Araújo,† L. S. Roman,‡ and M. L. M. Rocco*,† †

Instituto de Química, Universidade Federal do Rio de Janeiro, 21941-909, Rio de Janeiro, RJ, Brazil Departamento de Física, Universidade Federal do Paraná, 81531-990, Curitiba, PR, Brazil

‡

ABSTRACT: Ultrafast electron dynamics in the low-femtosecond regime was evaluated for poly(thiophene) by resonant Auger spectroscopy using the corehole clock method. Sulfur KL2,3L2,3 Auger decay spectra were measured as a function of the photon energy. Remarkable changes developed by tuning the photon energy along the sulfur 1s absorption edge, depending on the nature of the intermediate core excited states. Features characteristics of the Auger Resonant Raman effect were observed. Competition between core hole decay and delocalization of the photoexcited electron was monitored. Branching ratios of Raman (spectator) and normal Auger channels were calculated and electron delocalization times derived.

1. INTRODUCTION Ultrafast electron dynamics is a relevant topic for fundamental and many applied areas like femtochemistry, surface photochemistry, molecular electronics, solar energy, and so on.1−5 It is possible to apply two different spectroscopic approaches to pursue it. Ultrashort pulse laser pump−probe spectroscopies have been used widely, where delocalization of excited electrons in the presence of delocalized valence hole states are probed.6,7 Besides laser-based pump−probe methods, resonant Auger spectroscopy following core excitation emerges as an alternative with major advantages. First, the core hole lifetime probed by core level spectroscopy can be used as a fast internal clock and insofar very low time scales in the range of femtoseconds (10−15 s) down to hundred attoseconds (10−18 s) can be achieved,8−10 the so-called core-hole clock (CHC) method.1−3 To access dynamic processes in the attosecond range reliably, shorter core-hole lifetimes are required, as probing Coster−Kronig autoionization channels with attosecond core-hole lifetimes, like processes at sulfur 2s (S 2s core-hole lifetime is 0.5 fs). The probability for these transitions is higher and the corresponding core-hole lifetimes shorter than in the case of decay processes involving different values of n (principal quantum number).8 Second, the inherent atomic specificity of core levels, which deals with localized hole states, can be explored. It is worth mentioning that pump−probe experiments employing XUV photons produced by intense pulsed X-ray sources opened the possibility to access core levels,11 but there are only few sources offering this special experimental setup and only one reaching the upper half of the soft X-ray energies.10 For some applications, the surface sensitivity of low energy electrons adds further up. Unprecedented results, ranging from attosecond charge transfer dynamics from sulfur atoms evaluated by ultrafast core-hole decay via Coster−Kronig processes8,12,13 to © 2013 American Chemical Society

orbital-dependent charge transfer dynamics in conjugated selfassembled monolayers assessed by resonant Auger spectroscopy following nitrogen 1s core excitation14 and electron delocalization through the phosphate backbone of genomic DNA, which demonstrated other charge/electron transport pathway in DNA other than through the bases stacks,15 were already mediated by the CHC method. An extensive description of the use of core spectroscopies to study charge transfer dynamics is presented and discussed in refs 1−3. After X-ray absorption, a core hole can be created either by excitation or ionization and two main decay processes compete: X-ray emission (fluorescence) and Auger decay. The nonradiative process dominates for light elements, and the Auger process is discussed in terms of autoionization (participator and spectator channels) and normal Auger decay. After ionization, Auger emission leads the system in a 2 h (two holes) final state (normal Auger). Core excited systems will give rise to two different final states. If the excited electron remains as a spectator for the Auger process, the system will end up in a 2h1e (two holes−one electron) final state. Because of the screening of the spectator electron on the two holes, the spectator signal will appear at higher kinetic energy as compared to the normal Auger, the so-called spectator shift. The participator (or participant) decay leads to 1 h (one hole) states since the participator electron recombines with the core hole, corresponding to the well-known 1 h energies measured by valence photoemission, and therefore, it is also termed resonant photoemission. Received: December 21, 2012 Revised: March 20, 2013 Published: March 27, 2013 8208

dx.doi.org/10.1021/jp312660d | J. Phys. Chem. C 2013, 117, 8208−8213

The Journal of Physical Chemistry C

Article

Figure 1. (a) XAS spectrum of PT at the sulfur K-edge. Inset: molecular structure of PT. (b) Sulfur KL2,3L2,3 Auger decay spectra obtained for photon energies labeled as A−F in the XAS spectrum of PT. Results of curve fitting for spectator Auger are as follows: π* in red, σ*(S−C) in blue, and Rydberg in green. Normal Auger peaks are shadowed gray.

compilation of some representative results for atoms and molecules can be found in ref 19. In this work, we applied the CHC method to better understand electron dynamics in poly(thiophene) (PT) thin films. Poly(thiophene) has been a promising and well studied semiconducting polymer for organic electronic devices due to the possibility to achieve strong interchain interaction and higher degree of crystallinity leading to higher charge carrier mobility.21 It has been potentially applied mainly to optoelectronic devices22,23 and field effect transistors.24 Electron delocalization times in the range of femtoseconds were determined for PT demonstrating the important contribution of the CHC method to achieve electron dynamics in complex materials.

On this basis, the CHC method can be addressed arriving to the desired information, which is the electron delocalization time (τED).15 If the excited electron remains localized in the course of the decay process, the spectator Auger intensity will dominate the decay spectra (2h1e final state). If otherwise the electron can delocalize to the substrate or to the surroundings, the final state will be a 2h (two holes) final state (normal Auger). These two channels compete, and by measuring the intensity of both channels, it is possible to gain information about the delocalization time, using the well-establish relationship τED = (I2h1e/I2h)τCH, where τCH is the core-hole lifetime and I2h1e and I2h the intensity of spectator and normal Auger components, respectively.2,15−17 Here, we considered the spectator Auger decay to be the dominant resonant decay process. If the photon bandwidth is narrower than the core-hole lifetime broadening of the neutral core-excited state, it is possible to achieve ARR (Auger Resonance Raman) conditions since the excitation-decay sequence can no longer be considered separately but as a one-step process (coherent process). One characteristic feature of ARR is the observation of liner dispersion of the spectator (and participator) electron with photon energy as a consequence of energy conservation.2,18−20 Another outstanding consequence of ARR is the narrowing of the lines. The different behavior with photon energy probed by resonant and nonresonant Auger electrons can be used to separate both contributions in the decay spectra. A comprehensive discussion of the ARR conditions and also a

2. EXPERIMENTAL SECTION PT films were potenciostatically deposited onto ITO substrates (Delta Technologies, Rs 5−15 Ω) in a glovebox using an AUTOLAB 3530 potentiostat controlled through GPES computer interface. The counter electrode was a Pt plate, and the pseudoreference electrode was an Ag wire. The oxidation potential for PT monomer is 2.06 V; in this way, the films must be deposited under higher potential. We have used 3 V, after checking for polymer over oxidation carrying out cyclic voltammetry experiments for films synthesized on different potentials. The cyclic voltammograms were similar and reversible, indicating that no irreversible damage occurred in the conjugated system. The thickness has being controlled by 8209



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time deposition. The electrolyte was a 0.1 mol L−1 of Me4BF4 in acetonitrile (high performance liquid chromatography), which contained thiophene monomer (Aldrich) in a concentration of 0.01 mol L−1. PT film was deposited at room temperature, and its thickness measured using a surface profiler Detak3 (Veeco/Sloan). The thickness of the polymer film used for the experiments was 100 nm. X-ray absorption (XAS) and Auger decay measurements on PT films were performed at the soft X-ray spectroscopy (SXS) beamline at the Brazilian Synchrotron Light Source (LNLS), mounted with a double-crystal type monochromator using the Si(111) plane with an energy resolution of 0.38 eV at the sulfur K-edge. The experimental setup includes a sample manipulator and a concentric hemispherical electron energy analyzer housed in an UHV chamber with a base pressure of 10−8 mbar. The photoabsorption spectrum was recorded by measuring the total electron yield (electron current at the sample) simultaneously with a photon flux monitor (Au grid). The final data was normalized by this flux spectrum to correct for fluctuations in beam intensity. The energy calibration was performed by taking the well-known value for the LIII transition (2p3/2 → 4d) of metallic molybdenum. Auger decay spectra were measured by the hemispherical electron energy analyzer employing a pass energy of 20 eV. No beam damage effects were observed in the photoabsorption and photoemission spectra.

apparent at the lower energy side of the main peaks, with a value of 2112.2 eV kinetic energy, obtained by fitting the data using Gaussian functions.15 This feature is the predominant peak at higher photon energies above sulfur 1s ionization potential, which is characteristic of a normal Auger peak. The spectrum in Figure 1b-F is representative of the normal Auger spectrum. This is an important finding not reported before showing delocalization of the resonantly excited electron for the [S1s] π* state. Line narrowing of the spectator peaks was clearly observed as well, a direct consequence of the ARR effect. By increasing the photon energy, the intensity of the π* contribution to the pair decreases, while the σ* counterpart remains (Figure 1b-C). Additionally, the feature peaked around 2112 eV kinetic energy gains major intensity upon increasing the photon energy (Figure 1b-D). This broad peak remains with almost constant kinetic energy for all nonresonant spectra as well, as expected for normal Auger peaks. On the basis of the spectra of sulfur containing molecules,32 this peak is assigned to the 1D2 final state. Two equally intense peaks now dominate the decay spectra shown in Figure 1b-D. This represents the case in which the time scale of delocalization is comparable to the core-hole decay, and consequently, both the spectator (also called Raman) and normal Auger channels are visible. Besides the σ*(S−C) spectator and normal Auger peaks, another feature clearly emerges for higher photon energies and with ∼2117 eV kinetic energy (Figure 1b-E). This feature disperses in energy and vanishes completely above IP. This peak, which may be due to Rydberg excitation, is located at ∼1.5 eV away from the σ*(S−C) spectator Auger peak. The importance of core-to-Rydberg transitions to enhance the production of atomic fragments (neutral and ionic) of molecules was discussed for condensed thiophene,27 suggesting that singly charged intermediates (S 2p−2 Rydberg+1) via the KL2,3L2,3 spectator decay are more efficient in generating S+ fragment desorption than doubly charged intermediates (S 2p−2). The possibility of identifying Rydberg-like states by RAS in organic thin films is of relevance because it gives an estimate of the vacuum level (Ev). As mentioned, at greater photon energies (Figure 1b-F) Auger decay spectra are dominated by the normal Auger peak. It is worth mentioning that previously reported results for PT did not indicate any ultrafast electronic dynamics.31 This may be due to different preparation protocols, leading to possible structural differences, i.e., powder samples in ref 31 and thin films here. If we consider that intrachain coupling should only weakly depend on the structure of the sample, by comparing both results it could be inferred that interchain electron transfer dominates the delocalization dynamics for PT films over intrachain dynamics. This reinforces the importance of the use of well prepared thin films for electron delocalization investigations in poly(thiophene) and related polymeric materials. The interplay between intrachain and interchain interactions in semiconducting polymers is an important topic. 33 As mentioned in ref 34, apart from charge delocalization, interchain interactions is an important intrinsic factor responsible for charge transport in conjugated polymers as well as π−π stacking and the presence of side groups. Alone the delocalization of π-electrons along the polymer backbone is not sufficient for conduction because of inherent localization in a one-dimensional system. It is in this context that the possibility of interchain transfer gathers importance. This is well illustrated with the case of regioregularity in alkyl-thiophene polymers.34 For example, in the case of P3HT (poly-3-

3. RESULTS AND DISCUSSION In order to gain insight into the dynamics of electron delocalization in poly(thiophene), excitation and Auger decay spectra were measured as a function of the photon energy around the sulfur K-edge. Photoabsorption (XAS) spectrum of PT recorded at the sulfur K-edge is depicted in Figure 1a, which presents as an inset the polymer structure. The spectrum shows a sharp peak followed by broad bands, which corresponds to electronic transitions from the sulfur 1s electron.25 Following the well-known assignment for gas-phase thiophene,26 the first intense peak, labeled as (B) in Figure 1a, is due to the overlapping of the S 1s → π* and S 1s → σ*(S−C) transitions, which lie very close in energy. The other minor features correspond to higher energy excitations probably containing Rydberg character and above sulfur 1s ionization potential σ*(C−C) shape resonances are expected.26,27 Similar features were observed in XAS spectra of poly(3-methylthiophene)28 and poly(3-hexylthiophene).29 No molecular orientation was measured here. Clear angular dependence of NEXAFS spectra was demonstrated for thiophene-based polymers prepared by doctor blade casting.30 Sulfur KL2,3L2,3 Auger decay spectra were acquired by tuning the excitation energy. Figure 1b plots Auger decay spectra at photon energies labeled from A−F in Figure 1a. Noticeable changes are apparent. The first important remark is the appearance of two narrow and well-separated peaks at the maximum resonance energy at 2472.6 eV (Figure 1b-B). These peaks are related to the S 1s → π* and S 1s → σ* (S−C) transitions, appearing at 2113.7 and 2115.4 eV kinetic energies, respectively. Their assignments are consist with the expected different screening of the core-hole due to π* and σ* electrons, giving rise to different Auger spectator shifts. Similar splitting was reported previously for powder samples of PT,31 showing the importance of the resonance Auger spectroscopy (RAS) for identifying close lying states, not resolved by XAS. The spectator nature of both signals will be explored below. It is worthwhile to notice that at this photon energy a shoulder is 8210



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hexylthiophene), self-organization results in a lamella structure with two-dimensional conjugated sheets formed by interchain stacking, whose mobilities can reach values as high as 0.1 cm2 V−1 s−1 due to efficient interchain transport.35 This discussion was also applied to other polymeric systems, like MDMOPPV:PCBM.36 As already mentioned, it is possible to differentiate the degree of delocalization of the excited electron by plotting the spectator (localization) and normal (delocalization) Auger electron kinetic energies obtained by RAS as a function of the photon energy. This is plotted in Figure 2, which shows

Figure 3. Electron delocalization times as a function of photon energy. They were derived using a S 1s core-hole lifetime of τCH = 1.27 fs.

unoccupied states into which the electron tunnels. One would then expect that the energy dependence of the tunneling probability is governed by the change in the energetic position of the electron and by the change of the density of unoccupied states. If the density of states is almost constant over the energy range in question, one would predict that the tunneling probability should strongly increase (exponentially with decreasing Evac − Eres) with increasing energy above EF.1,16 Such energy dependence should be even more pronounced because also the width of the barrier will decrease with increasing energy. This would explain the behavior observed in Figure 3 for the energy dependence of the electron delocalization time. Electron delocalization times for PT are in the low fs regime, with a value of 9.9 fs on resonance and ∼1 fs at the photon energy corresponding to similar σ*(S−C) spectator and normal Auger signals. The delocalization of σ*(S−C) electron may be due to efficient polarization screening of the core hole, which cancels the excitonic energy of the localized state.39

Figure 2. Kinetic energies of the S KL2,3L2,3 Auger electrons as a function of photon energy.

accordingly different behavior for spectator and normal Auger electrons. π*, σ*, and Rydberg states (spectator and 2h1e states) show kinetic energy shift with photon energy while normal Auger (2h state) presents almost constant kinetic energy. Close inspection of the σ*(S−C) spectator line shows, however, a complex two-step profile. Somewhat similar behavior was reported before for RAS studies on condensed molecules containing third-row elements, such as Si, S, P, and Cl.32 For sulfur containing molecules, it was explained as due to spectator transitions other than that for the σ* resonance.32,37 Although there might be other possible resonant Auger decay pathways such as participant Auger, the spectator Auger can be considered the dominant decay pathway as demonstrated before for condensed thiophene27 and powdered PT.31 The low contribution of the participant Auger decay after sulfur 1s photoexcitation was also discussed for sulfur containing molecules and other molecules containing third-row elements.32 In order to derive electron delocalization times, branching ratios of spectator (summation over π*, σ*(S−C), and Rydberg contributions) and normal Auger signals were calculated, which are plotted in Figure 3 as a function of the photon energy. They were derived using the simple rate approach equation mentioned above and S 1s core-hole lifetime of τCH = 1.27 fs corresponding to 0.52 eV lifetime width.38 The electron delocalization time shortens exponentially with increasing excitation energy. Similar behavior for the energy dependence of the electron delocalization time was reported for dry and wet DNA through P KL2,3L2,3 resonant Auger decay,15 which was explained as a result of tunneling barrier and density of states in the conduction band. One possible mechanism, which can contribute to electron transfer processes, is due to resonant tunneling.16 In the resonant tunneling model, the tunneling probability depends on the shape of the tunneling barrier, the energetic position of the resonance level, and the density of

4. CONCLUSIONS Resonant Auger spectroscopy (RAS) using the core-hole clock (CHC) method was applied to poly(thiophene) (PT) following S K-edge photoexcitation. It was possible to disentangle transitions to π*, σ*, and Rydberg states by RAS not resolved by XAS. Measurement of Rydberg-like states led to the estimate of the vacuum level for PT. Dispersion and peak narrowing, which well characterizes the so-called Auger resonant Raman effect, could be clearly observed. Electron delocalization times in the femtosecond domain were measured for PT for the first time, with a value of 9.9 fs on resonance, demonstrating delocalization of the excited electron. By comparison to previous study on powdered PT, an interchain delocalization pathway was suggested. Another conduction pathway through the σ*(S−C) electrons other than π−π coupling would be possible as demonstrated by RAS. Finally, we emphasize the importance of the CHC method to probe femtosecond electron delocalization dynamics in polymeric thin films.

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*(M.L.M.R.) Tel: +55-21-2562-7786. Fax: +55-21-2562-7265. E-mail: [email protected]. 8211



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Divisão de Metrologia de Materiais (Dimat), Inmetro, Duque de Caxias, RJ, CEP 25250-020, Brazil. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Research partially supported by LNLS, National Synchrotron Light Laboratory, Brazil. M.L.M.R. and L.S.R. would like to thank CNPq for financial support. We would also like to acknowledge CAPES, CNPq, the technical assistance of the soft X-ray group from LNLS, and D. C. da Silva, N. D. Yamamoto, and L. Micaroni for the ITO/PT samples preparation.

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REFERENCES

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Femtosecond Electron Delocalization in Poly(thiophene) Probed by

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