Journal of Electron Spectroscopy and Related Phenomena
Photoabsorption and desorption studies on thiophene-based polymers following sulphur K-shell excitation
Research highlights
▶ Photon stimulated ion desorption (PSID) measurements at the S K-shell excitation energies were performed on three thiophene-based polymer films, following their NEXAFS spectra. ▶ For poly(thiophene) (PT) and poly(3-methylthiophene) (P3MT) it was found that the S 1s → π*, σ* (S–C) excitation produces S+ desorption efficiently, showing the importance of the resonant Auger process for breaking the C-S bond. ▶ In the case of poly(3-hexylthiophene) (P3HT) S+ desorption seems to be suppressed due to the orientation of 3-hexyl side-chains on the surface. ▶ Desorption ion yield curves for molecular fragments reproduce the photoabsorption spectrum, indicating that the indirect process is here predominant.
Introduction
Poly(thiophene) (PT) and poly(3-alkylthiophene) (P3AT) comprise an important class of semiconducting polymers that have been potentially applied mainly to electroluminescent and photovoltaic devices [1], [2] and field effect transistors [3]. The P3AT family has been the most promising and 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 mobility.
Nanocomposites made of carbon nanotubes (CNT) and thiophene-based semiconducting polymers, especially poly(3-hexylthiophene), have also been widely investigated using different approaches [4], [5], [6]. These nanocomposites have presented interaction between the highly delocalized electrons of carbon nanotubes and electrons correlated with the lattice of the polymer backbone that can improve the exciton dissociation efficiency due to electron transfer.
Among the thiophene-based polymers studied in this work, the PT and poly(3-methylthiophene) (P3MT) films are insoluble and the poly(3-hexylthiophene) (P3HT) is environmentally stable and soluble. So, the first two films have been prepared mainly via electropolymerization, while the third is usually prepared via other solution based techniques, such as spin coating [7], [8].
Information regarding to both unsubstituted and 3-alkyl substituted thiophene-based polymer films, as well as, their nanocomposites are helpful to a better understanding of the mechanism of polymer formation and their properties explored for the devices fabrication. A clear comprehension of their properties with respect to the structure modification and preparation are very important, not only to establish a general concept about the behavior of these semiconducting polymers, but also to optimize their optical and electronic properties.
Their optical and electrical properties, i.e., the mobility of the hole and electron are strongly dependent on regioregularity, molecular weight, homogeneity and morphology of the film [12]. Theoretical studies have shown that in non-crystalline polymers the charge mobility is mainly determined by the local molecular packing and is independent of global morphology [9]. Systematically studies supported by atomic force microscopy (AFM) and X-ray diffraction (XRD) measurements on pristine regioregular P3AT were also reported in the literature [10], showing that the length of the side-chain leads to an increase in the interspacing between the rigid conjugated backbones, and the melting temperature decreases for increasing alkyl side-chain lengths.
Photoexcitation processes on polymer films are of great interest due to the rapid development of light emitting diodes and solar cells based on organic semiconductors. These processes have being investigated using a variety of pump–probe optical techniques [11], [12]. Studies using different polarizing spectroscopies have been carried out to measure the molecular order in poly(3-alkylthiophene) thin films [13]. Some of the results indicated that the alkyl side-chains exhibit significant disorder for all films regardless of the thermal treatment.
Despite the importance of these thiophene-based polymer films for the understanding of the processes involved in electronic devices based on organic thin films, photofragmentation studies are still rare. Photon stimulated ion desorption (PSID) measurements using synchrotron radiation (SR) have been applied to investigate polymer surfaces [14]. Recently, in our group it was reported PSID results for P3MT [15], and P3HT and P3HT/MWCNT [16] that showed different behavior of ion desorption around the S K-edge. These results have been discussed in terms of Auger stimulated ion desorption (ASID) and X-ray induced electron stimulated desorption (XESD) processes. Both processes compete and their contribution to ion desorption is still an issue [17]. P3HT films were also investigated using different techniques, including electron stimulated desorption (ESD) [18].
The PSID technique has shown a great potential to give electronic and structural information with chemical selectivity and surface sensitivity, being an interesting technique to investigate photochemical reactions. Tunable-synchrotron radiation may be applied to control these types of reactions, since it can be applied as a scissor or scalpel breaking specific bonds after photoexcitation [19]. Ionic desorption following deep core-level photoexcitation from polymer films has been understood taking into account direct (ASID) and indirect (XESD) processes. In the Auger induced process, which dominates for light elements, the relaxation process following excitation (resonant Auger) or ionization (normal Auger) of a core electron may produce localized positive holes in valence orbitals, promoting a strong coulombic repulsion, and as a consequence the desorption of ionic species. In the indirect process, ion desorption is induced by inelastically scattered low-energy secondary electrons. Thus, the investigation of the surface and interface using not only microscopic but also spectroscopic techniques may contribute enormously for the understanding of different processes and properties.
In this work we present a comparative PSID study of three different thiophene-based polymer films at the sulphur K-edge, using synchrotron radiation operating in a single-bunch mode. PSID spectra were measured using the time-of-flight (ToF-MS) technique for mass analysis at different photon energies, following the corresponding photoabsorption (NEXAFS) spectra. The influence of the size of 3-alkyl substituent groups is discussed following the PSID results. Relative desorption ion yield (RDIY) curves as a function of photon energy for some selected ion fragments were presented and interpreted in terms of the direct and indirect processes leading to ion desorption.
Section snippets
Experimental
Photoabsorption and ion desorption measurements on PT and P3AT films were performed at the Brazilian Synchrotron Light Source (LNLS). These measurements were carried out during single-bunch operation mode of the storage ring, which presents a period of 311 ns and width of 60 ps. The soft X-ray spectroscopy (SXS) beamline (790–4000 eV) mounted with a double-crystal type monochromator and using the InSb(1 1 1) plane gives an energy resolution of ∼2.8 eV at 4 keV [20]. The experimental set-up for the
Results and discussion
PSID measurements were performed on three different thiophene-based polymer films in order to investigate the photofragmentation and ion desorption processes on their surfaces. Fig. 1 displays the PSID spectra from (a) PT, (b) P3MT and (c) P3HT films. These spectra were acquired using the photon energies A, B and E selected from the NEXAFS spectra obtained for the respective films at the sulphur K-edge (e.g.: PT spectrum in Fig. 2(a)).
The intense peaks at the extremes of all PSID spectra
Conclusions
PSID measurements at the S K-shell excitation energies were performed on three thiophene-based polymer films, the energies selected by their NEXAFS spectra. Although these three systems show very similar photoabsorption profile at the S K-edge, the PSID spectra present different positive ion desorption profiles. PT and P3MT assignments of the positive ions indicate that the 3-methyl side-chains do not cause an obstacle for desorption of S+ and S2+ species. However, for the P3HT film these
Acknowledgments
Research partially supported by LNLS – National Synchrotron Light Laboratory, Brazil. The authors would like to thank CNPq, FAPERJ and CAPES for financial support. The authors would also like to strongly acknowledge the technical assistance of the Accelerator and the Soft X-ray Groups of LNLS. The authors acknowledge Dr. Rogério Valaski and Natasha Yamamoto for the polymer film deposition.
References (30)
- et al.
Thin Solid Films
(2010) - et al.
Polymer
(2007) - et al.
Surf. Sci.
(2004) - et al.
Surf. Sci.
(2009) - et al.
Surf. Sci.
(2005) - et al.
Rev. Sci. Instrum.
(1992) Simion 3D Version 6.0 User's Manual
(1995)- et al.
Sol. Energy Mater. Sol. Cells
(2007) - et al.
Surf. Sci.
(2002) - et al.
J. Vac. Sci. Technol. A
(2006)
J. Phys.: Conf. Ser.
Organic Electroluminescent Materials and Devices
Rep. Prog. Phys.
Macromolecules
Nanotechnology
Cited by (7)
Investigation of carbon nanotube/polyaniline nanocomposite thin films produced by interfacial polymerization through electron desorption
2013, Journal of Molecular StructureCitation Excerpt :ESID is a well known technique based on the production of fragments (positive and negative), due to the impact of electrons of known energy on a solid surface, in which the energy transfer process occurs solely through electronic transitions, like those induced by photons (PSID). Although the PSID technique was extensively employed in desorption studies of polymers [18,21,23–26] results using electron sources are still scarce. Unlike photons, cross-sections for Auger decay processes due to electron impact present a maximum that varies between 2 and 4 times the inner-shell ionization energy [19,20].
The interplay of electronic structure, molecular orientation and charge transport in organic semiconductors: Poly(thiophene) and poly(bithiophene)
2013, Organic ElectronicsCitation Excerpt :They are probably due to Rydberg features and σ*(C–C) shape resonances, appearing above sulfur 1s ionization potential. Similar features were observed in XAS spectra of thiophene-based polymers [29–31]. We will concentrate our analysis on the two main transitions (S 1s π* and S 1s σ*(S–C)).
Molecular orientation and femtosecond charge transfer dynamics in transparent and conductive electrodes based on graphene oxide and PEDOT:PSS composites
2019, Physical Chemistry Chemical PhysicsFemtosecond electron delocalization in poly(thiophene) probed by resonant Auger spectroscopy
2013, Journal of Physical Chemistry C