Thermally and photoinduced polymerization of ultrathin sexithiophene films Anke Sander, Rene Hammer, Klaus Duncker, Stefan Förster, and Wolf Widdra Citation: The Journal of Chemical Physics 141, 104704 (2014); doi: 10.1063/1.4894437 View online: http://dx.doi.org/10.1063/1.4894437 View Table of Contents: http://scitation.aip.org/content/aip/journal/jcp/141/10?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Effect of UV irradiation during synthesis of polypyrrole by a one-step deposition/polymerization process J. Vac. Sci. Technol. B 25, 670 (2007); 10.1116/1.2436480 Photooxidation of plasma polymerized polydimethylsiloxane film by 172 nm vacuum ultraviolet light irradiation in dilute oxygen J. Appl. Phys. 100, 033510 (2006); 10.1063/1.2227275 Surface stress control using ultraviolet light irradiation of plasma-polymerized thin films Appl. Phys. Lett. 88, 143119 (2006); 10.1063/1.2183807 A new lithography of functional plasma polymerized thin films AIP Conf. Proc. 550, 548 (2001); 10.1063/1.1354454 Pulsed plasma polymerization of an aromatic perfluorocarbon monomer: Formation of low dielectric constant, high thermal stability films J. Vac. Sci. Technol. B 18, 799 (2000); 10.1116/1.591279
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THE JOURNAL OF CHEMICAL PHYSICS 141, 104704 (2014)
Thermally and photoinduced polymerization of ultrathin sexithiophene films Anke Sander,1 Rene Hammer,1 Klaus Duncker,1 Stefan Förster,1 and Wolf Widdra1,2,a) 1
Institute of Physics, Martin-Luther-Universität Halle-Wittenberg, Von-Danckelmann-Platz 3, 06120 Halle(Saale), Germany 2 Max Planck Institute of Microstructure Physics, Weinberg 2, 06120 Halle(Saale), Germany
(Received 17 June 2014; accepted 14 August 2014; published online 12 September 2014) The thermally-induced polymerization of α-sexithiophene (6T) molecules on Ag(001) and Au(001) gives rise to long unbranched polymer chains or branched polymer networks depending on the annealing parameters. There, the onset temperature for polymerization depends on the strength of interaction with the underlying substrate. Similar polymerization processes are also induced by ultraviolet radiation with photon energies between 3.0 and 4.2 eV. Radical formation by an electronic excitation in the 6T molecule is proposed as the driving mechanism that necessitates the interplay with the metallic substrate. © 2014 AIP Publishing LLC. [http://dx.doi.org/10.1063/1.4894437] I. INTRODUCTION
At room temperature, organic α-sexithiophene (6T) molecules arrange flat-lying in highly ordered structures on many metal substrates in the monolayer regime.1–8 There, the structure-driving interactions between the molecules are mainly of repulsive and of van-der-Waals type.6 For the application in organic electronic devices polythiophene films have several advantages over the shorter oligomers. On the one hand polymers are easier to process and a higher thermal and chemical stability is expected due to the covalent bonding. Apart from the already applied approach to deposit directly polythiophene there is also the possibility to deposit thiophenes or thiophene compounds, respectively, on a substrate and to form covalent bonds between them by further processing. For example, Cai et al. showed the thermallyinduced production of graphene nanoribbons from precursor monomers.9 Similar surface-assisted reactions are also imaginable for thiophenes. There are a few reports on polymerization of thiophene films after exposure to X-ray photons, ultraviolet (UV) radiation, or electrons.10–12 Further studies addressed the influence of thermal annealing on thiophenes. In a first work Destri et al. showed that heating of crystalline 6T powder to 580 K in N2 leads to the formation of polythiophene.13 To our knowledge this finding was not followed up. Other studies report on the changes in the morphology of 6T films and hence the transport properties due to thermal annealing without considering changes in the molecular structure.14, 15 In this paper, a systematic study of the polymerization of ultrathin 6T films on Ag(001) and Au(001) is presented. Differences in the behaviour on both substrates allow conclusions on the influence of the different “vertical” molecule-substrate interactions as well as of the different surface reconstructions. In general, on both substrates the polymerization occurs at temperatures significant lower than found for crystalline powder. Apart from this thermally-induced reaction we also report a) [email protected]
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on a light-induced polymerization and its dependence on the photon energy in the range between 1.7 and 4.2 eV. The comparison of these two different polymerization paths allows for a more general assessment of the reaction mechanism. II. EXPERIMENTAL DETAILS
The experiments have been performed in three different ultrahigh vacuum (UHV) chambers. All chambers have a base pressure of about 10−10 mbar and are equipped with Ar+ ion sputtering, heating facilities, and home-built Knudsen cells for 6T molecular beam deposition. The first chamber is provided with a high-temperature scanning tunneling microscope (STM). A thermocouple welded to the crystals allows for precise temperature monitoring during the STM measurements. The other UHV systems have separate preparation and analysis chambers. The second is equipped with a roomtemperature STM (Omicron Nanotechnology) and provides ultraviolet photoelectron spectroscopy (UPS) and photoemission electron microscopy (PEEM), while the third chamber provides a low-temperature STM operating at temperatures between 25 and 300 K. In the latter the thermocouple can be contacted to the sample holder by a translation stage during preparation. Furthermore, in this chamber a Kelvin probe allows for precise determination of work function changes. For all STM measurements the tips were etched from 0.2 mm tungsten wires. The light source for the threshold PEEM experiments consists of an all-fiber-based femtosecond laser and a non-collinear optical parametric amplifier (NOPA).16 Details of the setup are reported in Refs. 17 and 18. The NOPA output is frequency doubled enabling photoexcitations in a range between 1.3 and 5 eV. The photon exposure of the sample during the PEEM measurement is calibrated with a laser powermeter. The Ag and Au substrates were cleaned by several cycles of Ar+ ion sputtering at 1 kV and subsequent annealing to ∼650 K. The 6T molecules were evaporated onto the samples at room temperature by sublimation from a Knudsen cell hold at a temperature of about 500 K.
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III. RESULTS A. Thermally-induced polymerization
In Fig. 1 several room-temperature STM images of a monolayer 6T adsorbed on Ag(001) are shown upon annealing to different temperatures as indicated in the images. Figure 1(a) depicts the typical molecular arrangement after room temperature deposition. The rod-like 6T molecules are arranged in different domains of the dominating diagonal row structure which is described in detail by Duncker et al.7 After annealing to 420 K the highly ordered monolayer structures cannot be observed any longer, as shown in Fig. 1(b). Instead, most molecules show more or less random positions and orientations. The observed stripy appearance of some molecules is caused by diffusion and indicates a higher mobility there. These observations indicate an annealing-induced decrease in the molecule density on the surface compared to the wellordered monolayer structures. Only few of the molecules imaged in Fig. 1(b) keep their rod-like all-trans conformation and preferably arrange parallel to each other as marked by circle 1. In contrast, the majority of the molecules exists as isomers with several inter-ring bondings of the cis type. Obviously, some of the bent molecules appear significantly longer than the rod-like 6T and also show intersections as highlighted by circle 2. After heating to 480 K (Figs. 1(c) and 1(d)) the number of long chains is significantly increased and a more complex network of intersecting structures (knots) is formed. The comparison of Figs. 1(c) and 1(d) which are STM images of the same sample position but with a time lag of several minutes in between allows to derive information about the room temperature dynamics of the network.19 Prominent changes in the image are marked by the arrows. It becomes evident that only the molecular chains between two knots or the loose chain ends are able to diffuse and the knots
(b)
(a)
serve as stationary points for the whole network. The horizontal and mainly one STM scanline wide stripes observed solely in the open areas in between the molecular chains are interpreted as fast diffusing atoms or molecules. On closer inspection of Figs. 1(c) and 1(d) one additionally recognizes that the chains consist of uniformly sized subunits which look similar to the submolecular structure of 6T molecules (e.g., Fig. 2 in Ref. 7). Based on their regular spacing of (0.38 ± 0.05) nm, we assign them to individual thiophene rings. The STM image in Fig. 1(e) after heating to 530 K reveals no dramatic changes compared to Fig. 1(c). The molecular chains are longer and the number of knots between the chains is increased. A quantitative comparison reveals that in the temperature range between 480 K and 530 K the surface fraction which is covered by molecular material remains constant at (44 ± 2)%. At the same time the number of separate polymer chains reduces by one-third from 0.09 to 0.06 chains/nm2 . Hence, the average length of the polymer chains increases approximately by a factor of 2. Furthermore, the number of branching points almost doubles to 0.14/nm2 . Both observations indicate that the degree of cross-linking increases with higher temperatures without a significant desorption of the polymerized material. We note in Fig. 1(e) the absence of diffusion stripes in the intermolecular areas. Instead, some small round particles can be observed there for which the exact chemical nature is not known yet. In Fig. 1(f) the STM image for 1 ML 6T/Ag(001) after annealing to the intermediate temperature 460 K is shown. Note that the STM image in Fig. 1(f) has been recorded by a low-temperature STM at 80 K to avoid surface diffusion. All other images in Fig. 1 have been recorded at room temperature. Upon annealing to 460 K, the polymerization process has led to long unbranched polymer chains with up to
(c)
1
2 300 K
420 K
(e)
(d)
480 K
(f )
530 K
460 K, @ 80 K
FIG. 1. (a)–(e) Room-temperature STM measurements of 1 ML 6T/Ag(001) (a) without annealing and (b)–(f) after annealing. Annealing to (b) 420 K, (c), (d) 480 K, and (e) 530 K. (f) STM measurement at 80 K of 1 ML 6T/Ag(001) after annealing to 460 K. (a) (25 × 25) nm2 , −0.3 V, 2.2 nA; (b) (15 × 15) nm2 , −0.8 V, 3.1 nA; (c), (d) (20 × 20) nm2 , −0.3 V, 0.8 nA; (e) (20 × 20) nm2 , −0.4 V, 1.4 nA; (f) (60 × 60) nm2 , U = −0.65 V, I = 30 pA. The images of (a) and (e) are partly differentiated for higher contrast.
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FIG. 2. (a)–(c) STM measurements of 1 ML 6T/Au(001). (a) After deposition and without annealing, measured at room temperature, (b) after annealing to 450 K, and (c) after annealing to 470 K, measured at 80 K. (b) and (c) depict sample areas with completely quenched substrate reconstruction. (d) STM image at room temperature of P3HT/Au(001) prepared by electro spray deposition (ESD). (a) (25 × 25) nm2 , −0.2 V, 0.7 nA; (b) (20 × 20) nm2 , −0.3 V, 50 pA; (c) (30 × 30) nm2 , −0.7 V, 50 pA; (d) (50 × 50) nm2 , −2.2 V, 0.1 nA.
120 thiophene segments. The polymers are perfectly arranged in a monomolecular 2D layer with preferential orientation along the substrate high-symmetry directions. Note that occasionally also polymer loops are formed. In contrast to Ag(001), the Au(001) surface is intrinsically reconstructed20 which results in two different highlyordered 6T structures in the monolayer depending whether the underlying reconstruction is quenched or preserved.8 In Fig. 2(a) a room-temperature STM measurement depicts the coexistence of both structures, marked as DR and RR in Fig. 2(a), on the same substrate terrace. Upon heating, the 6T structures vanish in a temperature range between 387 and 402 K and fast molecule diffusion sets in Ref. 8. Figures 2(b) and 2(c) show STM images after annealing to 450 and 470 K, respectively, which have been recorded at 80 K. In both images the reconstruction of the Au(001) substrate is completely lifted. The observation of short molecules of various shapes and without any distinct order in Fig. 2(b) is similar to Fig. 1(b) depicting 6T/Ag(001) after annealing to slightly lower temperatures. Due to their length and the observation of six subunits these are 6T molecules in different isomeric forms. Chains with significantly more than 6 subunits are also observed in
few cases. These chains already show a few branches (B) or cross-linkings (CL). From subsequent STM measurements at room temperature, it is known that the cross-linkings do not change their positions and only the chains in between are able to move. Upon annealing to 470 K a complex network is formed as can be seen in the STM image of Fig. 2(c). In contrast to Fig. 2(b) very few single 6T molecules are observable and the network consists of numerous cross-linkings and long molecular chains. Compared to the temperature dependent behaviour of 1 ML 6T/Ag(001) the network on the unreconstructed parts of Au(001) is of similar complexity after heating to similar temperatures. For comparison, thin films of poly(3-hexylthiophene) (P3HT) have been prepared onto Au(001) under UHV conditions by electro spray deposition (ESD).21 A STM image of the resulting polymer network is depicted in Fig. 2(d). The observed structures, like coils, folded, and crossing chains, are strikingly similar to the thermally-induced changes of the 6T molecules. However, the deposited polymer chains exhibit strict linear configurations without branching. So far, the molecular and network structures on unreconstructed Au(001) have been presented. In the following
FIG. 3. (a)–(f) STM measurements of 1 ML 6T/Au(001) at various temperatures. (a) In situ measurement at 450 K and (b) after cooling to room temperature and to 80 K (inset). (c)–(f) In situ measurements at (c) 470 K, (d) 540 K, (e) 560 K, and (f) 570 K. (a) (30 × 30) nm2 , −0.3 V, 0.6 nA; (b) (30 × 30) nm2 , −0.6 V, 0.9 nA (inset: (25 × 25) nm2 , −1.5 V, 3 pA); (c) (30 × 30) nm2 , −0.6 V, 0.4 nA; (d) (30 × 30) nm2 , −0.3 V, 0.6 nA; (e) (55 × 55) nm2 , −0.5 V, 1.5 nA; (f) (45 × 45) nm2 , −1.4 V, 2.1 nA.
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the behaviour of the 6T monolayer on reconstructed Au(001) upon annealing will be discussed. In Fig. 3(a) an STM measurement at an elevated surface temperature of 450 K is shown. One observes on both Au(001) terraces areas without substrate reconstruction around the step edge as well as reconstructed areas aside. While the reconstructed parts look mainly homogeneous, clear but partly smeared features are visible in the unreconstructed parts. Complex structures are especially observable in the very proximity of the step edge and extend even over both terraces. The stripes within the image and the smeared step edges point to a fast diffusion on the surface which can be suppressed by lowering the sample temperature. Figure 3(b) shows an STM measurement after cooling down to room temperature. On the reconstructed area in the upper left corner no molecular structures are visible. In contrast, rod-like 6T molecules, isomers, and a couple of intersecting molecular chains can be found on the unreconstructed parts of the two terraces. The appearance of the molecular structures is comparable to Fig. 2(b). Here, one additionally observes 6T molecules in all-trans conformation which are, comparable to the findings for Ag(001) (see Fig. 1(b)), arranged parallel to each other. They are mainly oriented along a high-symmetry direction. Areas of high mobility can also be found between the molecular structures and chains. In these areas reproducible STM imaging is not possible. To obtain detailed information about the molecular order on the reconstructed areas the sample is cooled down to 80 K and the result is shown in the inset of Fig. 3(b). The rodlike 6T molecules are very similarly arranged as in the ML structure of the reconstructed areas. So indeed, also on top of the reconstruction rows there is still a 6T structure present but with less order as compared to the as-deposited case and probably with a reduced molecular density due to desorption processes upon heating. This explains the high mobility of the molecules at room and higher temperatures which hampers imaging by STM. The in situ STM measurement at 470 K in Fig. 3(c) reveals network structures close to the step edges. These networks are more complex than those found at 20 K lower temperature (Fig. 3(a)). They extend on the terraces of the unreconstructed areas. The increasing complexity of the network comes along with a higher density of immobile knots which suppresses diffusion even at elevated temperatures. This becomes even more evident in the in situ measurement at 540 K depicted in Fig. 3(d). On the two unreconstructed terraces in the middle of the image an almost static, very complex network can be observed. In this temperature range first molecular chains are observed on top of the substrate reconstruction. The imaging is quite blurry since these structures are still small enough to diffuse. Comparable to the polymerization onset on the unreconstructed areas (see Fig. 3(a)) most of the imaged structures can be found close to the step edge since this is an intrinsic diffusion barrier. To conclude, network formation processes also take place on top of the reconstruction rows but at about 120 K higher temperatures compared to the unreconstructed areas. From our data the onset of the polymerization processes is determined to about 530 K, where individual network-like structures are formed mainly at the step edges.
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The in situ measurement at 560 K in Fig. 3(e) images the progression of network complexity on the reconstructed areas while it only marginally increases on the unreconstructed areas. As guide for the eyes a line is drawn between the unreconstructed (top right) and reconstructed (bottom left) areas. It partly seems that the network formation comes along with an altering or even destruction of the reconstruction rows. While at 560 K the network on both substrate areas can still be discriminated this is not longer possible at higher temperatures. The in situ STM measurement at 570 K in Fig. 3(f) reveals an almost homogeneous sample morphology. The observed network is of high complexity and only very few loose ends of the chains can be observed which indicates a significant increase of the interconnections. Furthermore, due to the high complexity and the immobile cross-linkings even at these high temperatures no noteworthy diffusion is observable and a quite static structure is imaged. A quantitative analysis of the network formation with increasing temperatures on the unreconstructed areas of Au(001) confirms the impression from the STM images. The relative surface coverage of molecular chains increases from 0.15 at 450 K over 0.5 at 540 K to 0.65 at 580 K. The surface coverage is given relative to the monolayer coverage of the dense diagonal row structure.8 B. Light-induced polymerization
Besides thermally-induced polymerization as discussed above, also photon-stimulated polymerization of oligomer condensates has been reported previously.10–12, 22–25 Here we address the light-induced polymerization of ultrathin 6T films on Ag(001) under conditions where pure electron-induced processes, e.g., by secondary electrons or fast photoelectrons can be neglected. Photoemission spectra for a pristine 6T monolayer on Ag(001) and upon exposure to UV radiation are depicted in Fig. 4(a). For the experiments UV photons of 3.8 eV were used with a controlled exposure of 3.0 × 106 nm−2 . Additionally the spectrum of the clean substrate as measured under identical conditions is shown. The characteristic features of the 6T film (A, B) and of the substrate silver d-band (C, D) are marked accordingly. No significant shifts of the features are observable for the different spectra. Both spectra of the 6T monolayer are scaled such that the substrate related peaks C and D have the same intensity. Obviously, the intensity of the characteristic 6T features decreases after UV exposure. Furthermore, the work function increases by ∼130 meV, i.e., changes towards the value of bare silver or 6T films with lower coverages. For reference, Fig. 4(b) displays the coverage-dependent work function of 6T on Ag(001) as measured by the Kelvin probe technique.26 The absolute work function value of the bare Ag(001) was derived from UPS measurements with an error less than 100 meV. The coverage dependent decrease of the work function has been determined with a relative error of less than 5 meV. It is worth mentioning that the observed work function decrease with 6T coverage is in good agreement with our UPS data (not discussed here) and also with previous literature reports.27 The decrease of the photoemission intensity of the 6T features in
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FIG. 4. (a) Electronic structure of the valence band of bare Ag(001) (black curve), of 1 ML 6T/Ag(001) before (blue curve) and after (red dotted curve) UV exposure (hν = 3.8 eV, = 3.0 × 106 nm−2 ) revealed by UPS. (b) Work function of 6T/Ag(001) in dependence on the 6T coverage measured by a Kelvin probe. The absolute work function value for bare Ag(001) was derived from UPS measurements. (c) The STM measurement depicts the surface morphology of 1 ML 6T/Ag(001) after UV illumination (hν = 3.8 eV, = 3.0 × 106 nm−2 , (55 × 55) nm2 , U = −0.3 V, I = 1.0 nA).
UPS and the change of the work function upon UV irradiation indicate a rearrangement of the molecular structures that might be accompanied by a desorption or dewetting of the organic molecules. Despite the signal decrease of features A and B, the intensities of these two 6T features relative to each other are similar before and after UV exposure which indicates a preservation of thiophene compounds. In Fig. 4(c) an STM image of the 6T film after UV exposure is depicted. In contrast to the pristine 6T monolayer no well-ordered molecular structures are found. Instead, the molecules are mainly isomerized and in disordered arrangement. Furthermore, some of the observed structures are longer than a single 6T molecule. Accordingly, the STM image reveals a surface morphology similar to that after thermal treatment of a 6T film. These STM observations are in agreement with the increase of the substrate signal in UPS and the increase of the work function. The existence of oligothiophenes longer than 6T is not in contradiction to the unchanged posi-
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tions and constant relative intensities of the characteristic 6T features in UPS. Binding energies and photoemission intensities are expected to remain unchanged under formation of longer or slightly shorter oligothiophenes based on the systematic studies of Fujimoto et al.28 In XPS a shift of the C 1s- and S 2p 3 -peaks to lower bind2 ing energies by 0.5 and 0.7 eV, respectively, is observed after UV exposure (not shown here). This is consistent with the results of Salaneck et al. after X-ray exposure of thiophene on gold, where a shift of 0.7 and 1.1 eV, respectively, is observed and is explained by the formation of polythiophene.10 Note that this shift is in line with the modification of the metalorganic interface dipole that is responsible for the work function increase. In order to reveal the processes which take place during UV exposure of thin 6T films, PEEM as a spatially resolved method is applied. The locally-restricted illumination with UV laser light in PEEM enables several measurements with, e.g., different photon energies or photon fluences on the same sample solely by changing the sample position. In the experiments the 6T films have been exposed to UV laser light and the corresponding PE signal has been measured spatially resolved in PEEM. In Fig. 5(a) the PEEM intensity, normalized to the respective laser power, is shown for 4 ML 6T/Ag(001) as a function of the exposure time to UV radiation (hν = 3.5 eV) for different laser powers. Each curve was measured at a pristine sample position. At all positions, a similar starting PE intensity is found. Obviously, the decrease of the PE intensity is faster for higher UV laser powers. Note that the instantaneous decay in Fig. 5(a) rules out a photoinduced desorption as explanation for the initial decrease of the photoemission intensity for 4 ML. Since the work function (Fig. 4(b)) is almost constant between 2 and 4 ML, desorption from a 4 ML thick film does not alter the work function significantly. Instead, a modified interface dipole due to polymerization at the interface can change the work function. After normalization to the respective initial intensities, the data sets of Fig. 5(a) are depicted double logarithmically in Fig. 5(b).29 Within the experimental error bars, the data points for different UV intensities fall entirely on the same curve which can be described by an exponential function I () = I0 · e− F , with I0 the initial PE intensity, the photon fluence, and F the fluence at which the relative photoemission is reduced to 1/e. Analogous measurements on 1 ML 6T/Ag(001) also show a single-exponential decay of the PE intensity with photon fluence. The results for 1 ML as well as for 4 ML 6T/Ag(001) are depicted in Fig. 5(c). The comparison between both data sets reveals that the decay is much faster for the thinner film and, accordingly, F is reduced by nearly one order of magnitude from 3 × 106 nm−2 for 4 ML to 4 × 105 nm−2 at 1 ML. Based on the single exponential decay in both cases, we expect similar processes induced by UV exposure for 1 ML and 4 ML thick 6T films. However, the 1 ML thick film is much more sensitive or the UV-induced processes are much more effective, respectively, which suggests an influence of the substrate interface. Additionally, photo-induced desorption might also lead to an increase of the work function for 1 ML since the coverage dependence of the work function is large in the monolayer regime (see Fig. 4(b)). Therefore,
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FIG. 5. (a) Temporal dependence of the normalized PE intensity in PEEM during UV exposure (hν = 3.5 eV) for different laser powers. (b) Double-logarithmic plot of the data sets of (a) as a function of the UV photon fluence (lower abscissa). The red curve represents the exponential function fitted to the data points. Additionally, the dependence of the PEEM intensity on the IR fluence is shown (black markers, upper abscissa, hν = 1.7 eV). (c) Normalized PE intensity in dependence on the photon fluence for 1 ML (hν = 3.8 eV) and 4 ML 6T/Ag(001) (hν = 3.5 eV).
the photoemission intensity is not sufficient to quantify the polymerization rate in the monolayer. In general, the decay of the PE intensity correlates with the fluence, i.e., the number and not the rate of the incoming photons. The PE signal is locally reduced by single photons and not by, for example, two-photon-processes. In addition to the data set shown, a similar drop of the PE signal is also observed for photon energies of 3.0 eV, 3.9 eV, and 4.2 eV and a similar behaviour is also expected for photon energies between these values. On the upper abscissa in Fig. 5(b), additionally the PE intensity in dependence on the photon fluence at a significantly lower photon energy of 1.7 eV is shown. Since no direct PEEM imaging is possible at 1.7 and 3.0 eV, the sample was additionally illuminated by 3.9 eV probing photons to determine the PE intensity. Whereas a low and constant photon intensity was used for probing to avoid significant reaction by 3.9 eV photons, the intensity of the 1.7 or 3.0 eV laser light for polymerization was varied. We find a similar drop in PE intensity for exposure to 3.0 eV photons as discussed above. In contrast, the PE intensity for exposure to 1.7 eV photons stays constant even at high photon fluences (Fig. 5(b)). This observation implies the existence of a threshold photon energy for the photon-induced polymerization. IV. DISCUSSION
Upon annealing of 6T monolayers on Ag(001) and Au(001), the molecules change their conformation and react with each other. Reactions at the end of single 6T molecules lead to the formation of long, ideally unbranched polymers. There, the length of the polymer chains depends on the thermal treatment and the structure of the underlying substrate. The onset temperature for polymerization is lower for substrates with a stronger molecule-substrate interaction. Besides the formation of long polymer chains also reactions at the central units of molecules and/or polymers take place. This results in knots and branches bound tightly to the substrate. They are not able to move even at elevated temperatures. With
increasing temperature the number of knots increases yielding a more complex and more rigid network. Also after UV exposure, thin 6T films on Ag(001) show a lowered coverage and polymerization structures are observed. The unchanged photoemission spectrum proves the existence of thiophene compounds which are only slightly shorter or even longer than 6T.28 Laser-PEEM experiments revealed an exponential decay of the PE signal during UV exposure that is identified as a one-photon process. The decay is faster for 1 ML than for 4 ML. However, a desorption-induced component to the decrease of the PE signal can be ruled out only for the 4 ML data. The dependence of the processes on the photon energy implies the necessity of an excitation into the lowest unoccupied molecular orbitals. Note that the photoexcitation for photon energies between 3.0 and 3.8 eV might populate molecular orbitals between the lowest unoccupied molecular orbital and the vacuum level or just 100 meV above it. These excitations do not produce large amounts of photoelectrons or secondaries. Therefore, we disregard any low-energy electron-driven chemistry in our study that might be present for significantly higher photon energies. In fact, Modelli and Burrow showed that relevant electron attachment levels are located between 1.2 and 3.0 eV above the vacuum level.30 To populate these states, photon energies of at least 4.8–6.6 eV are necessary. Hedhili et al. also find an energetic threshold for ring-rupture appearing at about 5 eV above the vacuum level.31 Again, this threshold is not reached in our present study. We propose that upon annealing as well as upon UV excitation (3.0 to 4.2 eV), a polymerization of the 6T takes place. It has been argued that in general the oxidation of the monomer to a cation radical is an important step in the (electro) polymerization of thiophenes.32 The radical can then take part in further reactions. Also chemical polymerizations based on thiophene radicals formed by electron transfer have been reported for photon energies in the 3–5 eV range, as, e.g., for illumination of a solution of thiophene derivatives in the presence of a suited electron acceptor.33 However, the
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formation of radical cations is not restricted to solution chemistry but is also reported for metallic surfaces. Lipton-Duffin et al. observed radical formation for diiodobenzene on a copper surface.34 The interaction between the organic molecules and the metal surface is of importance also in the present study since in the absence of the metal substrate no polymerization of 6T takes place in the equivalent temperature range as used here. First, the normally low catalytic effect of gold and silver single crystal surfaces might be enhanced by the high number of free noble metal adatoms which are present already at room temperature. They might act as nanoparticles with high catalytic activity.35, 36 Furthermore, the interaction between the molecular and the substrate electronic states is essential for radical formation which in turn is the basis for the polymerization. This agrees well with the observation of polymerization only in the presence of the metallic substrate. The radicals formed by electronic excitations are the starting point for subsequent reactions like the bonding between 6T molecules or also to the decomposition of single molecules to smaller parts. The latter reaction seems indeed to be necessary since the lengths of the observed polymers are not strictly integer multiples of the length of a single 6T molecule. The thermal energy at about 450 K is not sufficient to break up C–H, C–S-, or C–C bonds in the thiophene in the absence of an electronic modification of the molecular orbitals. In the polymerization process two radical cations (or one radical and a neutral monomer) are connected.37 When the bonding is formed between two Cα atoms a straight elongation of the chain takes place whereas a bonding to an innermolecular Cβ atom leads to the formation of a branching. After the reaction of two radicals two protons are released, i.e., two hydrogen atoms desorb from the surface, and a rearomatization leads to a new thiophene bonding. Depending on the local position where the reaction takes place within the organic molecule, also small thiophene compounds like 1T or 2T might split from the formed polymer. In the case of UV exposure of thin 6T films, the photoexcitation plays an essential role for possible reactions between the molecules. The activation barrier for the radical formation can be overcome by a photon-induced excitation. Afterwards the formed radicals might react in a similar manner as after thermal treatment. V. SUMMARY
The annealing of ultrathin 6T films on Ag(001) and Au(001) leads to local reactions between neighbouring 6T molecules. Depending on the surface temperature and the annealing time, long unbranched polythiophene polymers or branched polymer networks are formed. There, the complexity and the onset temperature depend on the interaction with the underlying substrate. Similar molecule-substrate interactions on Ag(001) and on unreconstructed Au(001) lead to comparable polymer structures upon annealing. In contrast, the polymerization process takes place at about 120 K higher temperatures for 6T adsorption on the reconstructed Au(001)“(5 × 20)” where a lower molecule-substrate interaction is
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predicted. Similar structural changes and polymerization are found for exposure of the 6T film to UV radiation in the range from 3.0 to 4.2 eV. The observed reactions in both cases, by thermal annealing as well as by UV excitation, are explained by a polymerization via formation of cation radicals which necessitates an interplay with metallic substrate states. ACKNOWLEDGMENTS
This work was supported by the DFG through the Sonderforschungsbereich SFB/TRR-102 “Polymers under multiple constraints: restricted and controlled molecular order and mobility.” The authors thank Wolfgang Binder and Mario Kiel for fruitful discussions as well as Ralf Kulla for technical support. 1 A.
Soukopp, K. Glockler, P. Kraft, S. Schmitt, M. Sokolowski, E. Umbach, E. Mena-Osteritz, P. Bäuerle, and E. Hadicke, Phys. Rev. B 58, 13882 (1998). 2 S. Prato, L. Floreano, D. Cvetko, V. De Renzi, A. Morgante, S. Modesti, F. Biscarini, R. Zamboni, and C. Taliani, J. Phys. Chem. B 103, 7788 (1999). 3 M. Kiguchi, G. Yoshikawa, and K. Saiki, J. Appl. Phys. 94, 4866 (2003). 4 A. Mäkinen, J. Long, N. Watkins, and Z. Kafafi, J. Phys. Chem. B 109(12), 5790 (2005). 5 H. Inoue, G. Yoshikawa, and K. Saiki, Jpn. J. Appl. Phys. 45, 1794 (2006). 6 M. Kiel, K. Duncker, C. Hagendorf, and W. Widdra, Phys. Rev. B 75, 195439 (2007). 7 K. Duncker, M. Kiel, A. Höfer, and W. Widdra, Phys. Rev. B 77, 155423 (2008). 8 A. Höfer, K. Duncker, M. Kiel, S. Wedekind, and W. Widdra, Phys. Rev. B 83, 075414 (2011). 9 J. Cai, P. Ruffieux, R. Jaafar, M. Bieri, T. Braun, S. Blankenburg, M. Muoth, A. P. Seitsonen, M. Saleh, X. Feng, K. Mullen, and R. Fasel, Nature (London) 466, 470 (2010). 10 W. Salaneck, C. Wu, J. Nilsson, and J. Bredas, Synth. Met. 21, 57 (1987). 11 K. M. Baumgärtner, M. Volmer-Uebing, J. Taborski, P. Bäuerle, and E. Umbach, Ber. Bunsen-Ges. Phys. Chem. 95, 1488 (1991). 12 H. Raza, P. Wincott, G. Thornton, R. Casanova, and A. Rodriguez, Surf. Sci. 390, 256 (1997). 13 S. Destri, M. Mascherpa, and W. Porzio, Adv. Mater. 5, 43 (1993). 14 P. Viville, R. Lazzaroni, J. L. Bredas, P. Moretti, P. Samori, and F. Biscarini, Adv. Mater. 10, 57 (1998). 15 S. Cho, K. Lee, J. Yuen, G. Wang, D. Moses, A. J. Heeger, M. Surin, and R. Lazzaroni, J. Appl. Phys. 100, 114503 (2006). 16 C. Homann, C. Schriever, P. Baum, and E. Riedle, Opt. Express 16, 5746 (2008). 17 K. Duncker, M. Kiel, and W. Widdra, Surf. Sci. 606, L87 (2012). 18 A. Höfer, K. Duncker, M. Kiel, S. Förster, and W. Widdra, IBM J. Res. Dev. 55, 4:1 (2011). 19 See supplementary material at http://dx.doi.org/10.1063/1.4894437 for a STM video of the chain movement at room temperature of this sample position. 20 R. Hammer, A. Sander, S. Förster, M. Kiel, K. Meinel, and W. Widdra, Phys. Rev. B 90, 035446 (2014). 21 S. Förster and W. Widdra, J. Chem. Phys. 141, 054713 (2014). 22 F. P. Netzer, Chem. Phys. Lett. 146, 566 (1988). 23 J. E. Hernandez, H. Ahn, and J. E. Whitten, J. Phys. Chem. B 105, 8339 (2001). 24 A. Nambu, H. Kondoh, I. Nakai, K. Amemiya, and T. Ohta, Surf. Sci. 530, 101 (2003). 25 C. L. Huisman, A. Huijser, H. Donker, J. Schoonman, and A. Goossens, Macromolecules 37, 5557 (2004). 26 H. A. Engelhardt, P. Feulner, H. Pfnür, and D. Menzel, J. Phys. E 10, 1133 (1977). 27 T. Schwieger, X. Liu, H. Peisert, B. Adolphi, N. Kiriy, and M. Knupfer, J. Appl. Phys. 97, 123712 (2005).
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Fujimoto, U. Nagashima, H. Inokuchi, K. Seki, Y. Cao, H. Nakahara, J. Nakayama, M. Hoshino, and K. Fukuda, J. Chem. Phys. 92, 4077 (1990). 29 The unit of the fluence is chosen to nm−2 , since the area of a single 6 T molecule is approx. 1 nm2 . Hence, the fluence can directly be related to the number of photons striking each molecule. 30 A. Modelli and P. D. Burrow, J. Phys. Chem. A 108, 5721 (2004). 31 M. N. Hedhili, P. Cloutier, A. D. Bass, T. E. Madey, and L. Sanche, J. Chem. Phys. 125, 094704 (2006).
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Genies, G. Bidan, and A. Diaz, J. Electroanal. Chem. Interfacial Electrochem. 149, 101 (1983). 33 M. Fujitsuka, T. Sato, H. Segawa, and T. Shimidzu, Synth. Met. 69, 309 (1995). 34 J. A. Lipton-Duffin, O. Ivasenko, D. F. Perepichka, and F. Rosei, Small 5, 592 (2009). 35 M. Haruta, Catal. Today 36, 153 (1997). 36 M. Haruta, Chem. Rec. 3, 75 (2003). 37 J. Roncali, Chem. Rev. 92, 711 (1992).
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THE JOURNAL OF CHEMICAL PHYSICS 141, 104704 (2014)
Thermally and photoinduced polymerization of ultrathin sexithiophene films Anke Sander,1 Rene Hammer,1 Klaus Duncker,1 Stefan Förster,1 and Wolf Widdra1,2,a) 1
Institute of Physics, Martin-Luther-Universität Halle-Wittenberg, Von-Danckelmann-Platz 3, 06120 Halle(Saale), Germany 2 Max Planck Institute of Microstructure Physics, Weinberg 2, 06120 Halle(Saale), Germany
(Received 17 June 2014; accepted 14 August 2014; published online 12 September 2014) The thermally-induced polymerization of α-sexithiophene (6T) molecules on Ag(001) and Au(001) gives rise to long unbranched polymer chains or branched polymer networks depending on the annealing parameters. There, the onset temperature for polymerization depends on the strength of interaction with the underlying substrate. Similar polymerization processes are also induced by ultraviolet radiation with photon energies between 3.0 and 4.2 eV. Radical formation by an electronic excitation in the 6T molecule is proposed as the driving mechanism that necessitates the interplay with the metallic substrate. © 2014 AIP Publishing LLC. [http://dx.doi.org/10.1063/1.4894437] I. INTRODUCTION
At room temperature, organic α-sexithiophene (6T) molecules arrange flat-lying in highly ordered structures on many metal substrates in the monolayer regime.1–8 There, the structure-driving interactions between the molecules are mainly of repulsive and of van-der-Waals type.6 For the application in organic electronic devices polythiophene films have several advantages over the shorter oligomers. On the one hand polymers are easier to process and a higher thermal and chemical stability is expected due to the covalent bonding. Apart from the already applied approach to deposit directly polythiophene there is also the possibility to deposit thiophenes or thiophene compounds, respectively, on a substrate and to form covalent bonds between them by further processing. For example, Cai et al. showed the thermallyinduced production of graphene nanoribbons from precursor monomers.9 Similar surface-assisted reactions are also imaginable for thiophenes. There are a few reports on polymerization of thiophene films after exposure to X-ray photons, ultraviolet (UV) radiation, or electrons.10–12 Further studies addressed the influence of thermal annealing on thiophenes. In a first work Destri et al. showed that heating of crystalline 6T powder to 580 K in N2 leads to the formation of polythiophene.13 To our knowledge this finding was not followed up. Other studies report on the changes in the morphology of 6T films and hence the transport properties due to thermal annealing without considering changes in the molecular structure.14, 15 In this paper, a systematic study of the polymerization of ultrathin 6T films on Ag(001) and Au(001) is presented. Differences in the behaviour on both substrates allow conclusions on the influence of the different “vertical” molecule-substrate interactions as well as of the different surface reconstructions. In general, on both substrates the polymerization occurs at temperatures significant lower than found for crystalline powder. Apart from this thermally-induced reaction we also report a) [email protected]
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on a light-induced polymerization and its dependence on the photon energy in the range between 1.7 and 4.2 eV. The comparison of these two different polymerization paths allows for a more general assessment of the reaction mechanism. II. EXPERIMENTAL DETAILS
The experiments have been performed in three different ultrahigh vacuum (UHV) chambers. All chambers have a base pressure of about 10−10 mbar and are equipped with Ar+ ion sputtering, heating facilities, and home-built Knudsen cells for 6T molecular beam deposition. The first chamber is provided with a high-temperature scanning tunneling microscope (STM). A thermocouple welded to the crystals allows for precise temperature monitoring during the STM measurements. The other UHV systems have separate preparation and analysis chambers. The second is equipped with a roomtemperature STM (Omicron Nanotechnology) and provides ultraviolet photoelectron spectroscopy (UPS) and photoemission electron microscopy (PEEM), while the third chamber provides a low-temperature STM operating at temperatures between 25 and 300 K. In the latter the thermocouple can be contacted to the sample holder by a translation stage during preparation. Furthermore, in this chamber a Kelvin probe allows for precise determination of work function changes. For all STM measurements the tips were etched from 0.2 mm tungsten wires. The light source for the threshold PEEM experiments consists of an all-fiber-based femtosecond laser and a non-collinear optical parametric amplifier (NOPA).16 Details of the setup are reported in Refs. 17 and 18. The NOPA output is frequency doubled enabling photoexcitations in a range between 1.3 and 5 eV. The photon exposure of the sample during the PEEM measurement is calibrated with a laser powermeter. The Ag and Au substrates were cleaned by several cycles of Ar+ ion sputtering at 1 kV and subsequent annealing to ∼650 K. The 6T molecules were evaporated onto the samples at room temperature by sublimation from a Knudsen cell hold at a temperature of about 500 K.
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III. RESULTS A. Thermally-induced polymerization
In Fig. 1 several room-temperature STM images of a monolayer 6T adsorbed on Ag(001) are shown upon annealing to different temperatures as indicated in the images. Figure 1(a) depicts the typical molecular arrangement after room temperature deposition. The rod-like 6T molecules are arranged in different domains of the dominating diagonal row structure which is described in detail by Duncker et al.7 After annealing to 420 K the highly ordered monolayer structures cannot be observed any longer, as shown in Fig. 1(b). Instead, most molecules show more or less random positions and orientations. The observed stripy appearance of some molecules is caused by diffusion and indicates a higher mobility there. These observations indicate an annealing-induced decrease in the molecule density on the surface compared to the wellordered monolayer structures. Only few of the molecules imaged in Fig. 1(b) keep their rod-like all-trans conformation and preferably arrange parallel to each other as marked by circle 1. In contrast, the majority of the molecules exists as isomers with several inter-ring bondings of the cis type. Obviously, some of the bent molecules appear significantly longer than the rod-like 6T and also show intersections as highlighted by circle 2. After heating to 480 K (Figs. 1(c) and 1(d)) the number of long chains is significantly increased and a more complex network of intersecting structures (knots) is formed. The comparison of Figs. 1(c) and 1(d) which are STM images of the same sample position but with a time lag of several minutes in between allows to derive information about the room temperature dynamics of the network.19 Prominent changes in the image are marked by the arrows. It becomes evident that only the molecular chains between two knots or the loose chain ends are able to diffuse and the knots
(b)
(a)
serve as stationary points for the whole network. The horizontal and mainly one STM scanline wide stripes observed solely in the open areas in between the molecular chains are interpreted as fast diffusing atoms or molecules. On closer inspection of Figs. 1(c) and 1(d) one additionally recognizes that the chains consist of uniformly sized subunits which look similar to the submolecular structure of 6T molecules (e.g., Fig. 2 in Ref. 7). Based on their regular spacing of (0.38 ± 0.05) nm, we assign them to individual thiophene rings. The STM image in Fig. 1(e) after heating to 530 K reveals no dramatic changes compared to Fig. 1(c). The molecular chains are longer and the number of knots between the chains is increased. A quantitative comparison reveals that in the temperature range between 480 K and 530 K the surface fraction which is covered by molecular material remains constant at (44 ± 2)%. At the same time the number of separate polymer chains reduces by one-third from 0.09 to 0.06 chains/nm2 . Hence, the average length of the polymer chains increases approximately by a factor of 2. Furthermore, the number of branching points almost doubles to 0.14/nm2 . Both observations indicate that the degree of cross-linking increases with higher temperatures without a significant desorption of the polymerized material. We note in Fig. 1(e) the absence of diffusion stripes in the intermolecular areas. Instead, some small round particles can be observed there for which the exact chemical nature is not known yet. In Fig. 1(f) the STM image for 1 ML 6T/Ag(001) after annealing to the intermediate temperature 460 K is shown. Note that the STM image in Fig. 1(f) has been recorded by a low-temperature STM at 80 K to avoid surface diffusion. All other images in Fig. 1 have been recorded at room temperature. Upon annealing to 460 K, the polymerization process has led to long unbranched polymer chains with up to
(c)
1
2 300 K
420 K
(e)
(d)
480 K
(f )
530 K
460 K, @ 80 K
FIG. 1. (a)–(e) Room-temperature STM measurements of 1 ML 6T/Ag(001) (a) without annealing and (b)–(f) after annealing. Annealing to (b) 420 K, (c), (d) 480 K, and (e) 530 K. (f) STM measurement at 80 K of 1 ML 6T/Ag(001) after annealing to 460 K. (a) (25 × 25) nm2 , −0.3 V, 2.2 nA; (b) (15 × 15) nm2 , −0.8 V, 3.1 nA; (c), (d) (20 × 20) nm2 , −0.3 V, 0.8 nA; (e) (20 × 20) nm2 , −0.4 V, 1.4 nA; (f) (60 × 60) nm2 , U = −0.65 V, I = 30 pA. The images of (a) and (e) are partly differentiated for higher contrast.
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FIG. 2. (a)–(c) STM measurements of 1 ML 6T/Au(001). (a) After deposition and without annealing, measured at room temperature, (b) after annealing to 450 K, and (c) after annealing to 470 K, measured at 80 K. (b) and (c) depict sample areas with completely quenched substrate reconstruction. (d) STM image at room temperature of P3HT/Au(001) prepared by electro spray deposition (ESD). (a) (25 × 25) nm2 , −0.2 V, 0.7 nA; (b) (20 × 20) nm2 , −0.3 V, 50 pA; (c) (30 × 30) nm2 , −0.7 V, 50 pA; (d) (50 × 50) nm2 , −2.2 V, 0.1 nA.
120 thiophene segments. The polymers are perfectly arranged in a monomolecular 2D layer with preferential orientation along the substrate high-symmetry directions. Note that occasionally also polymer loops are formed. In contrast to Ag(001), the Au(001) surface is intrinsically reconstructed20 which results in two different highlyordered 6T structures in the monolayer depending whether the underlying reconstruction is quenched or preserved.8 In Fig. 2(a) a room-temperature STM measurement depicts the coexistence of both structures, marked as DR and RR in Fig. 2(a), on the same substrate terrace. Upon heating, the 6T structures vanish in a temperature range between 387 and 402 K and fast molecule diffusion sets in Ref. 8. Figures 2(b) and 2(c) show STM images after annealing to 450 and 470 K, respectively, which have been recorded at 80 K. In both images the reconstruction of the Au(001) substrate is completely lifted. The observation of short molecules of various shapes and without any distinct order in Fig. 2(b) is similar to Fig. 1(b) depicting 6T/Ag(001) after annealing to slightly lower temperatures. Due to their length and the observation of six subunits these are 6T molecules in different isomeric forms. Chains with significantly more than 6 subunits are also observed in
few cases. These chains already show a few branches (B) or cross-linkings (CL). From subsequent STM measurements at room temperature, it is known that the cross-linkings do not change their positions and only the chains in between are able to move. Upon annealing to 470 K a complex network is formed as can be seen in the STM image of Fig. 2(c). In contrast to Fig. 2(b) very few single 6T molecules are observable and the network consists of numerous cross-linkings and long molecular chains. Compared to the temperature dependent behaviour of 1 ML 6T/Ag(001) the network on the unreconstructed parts of Au(001) is of similar complexity after heating to similar temperatures. For comparison, thin films of poly(3-hexylthiophene) (P3HT) have been prepared onto Au(001) under UHV conditions by electro spray deposition (ESD).21 A STM image of the resulting polymer network is depicted in Fig. 2(d). The observed structures, like coils, folded, and crossing chains, are strikingly similar to the thermally-induced changes of the 6T molecules. However, the deposited polymer chains exhibit strict linear configurations without branching. So far, the molecular and network structures on unreconstructed Au(001) have been presented. In the following
FIG. 3. (a)–(f) STM measurements of 1 ML 6T/Au(001) at various temperatures. (a) In situ measurement at 450 K and (b) after cooling to room temperature and to 80 K (inset). (c)–(f) In situ measurements at (c) 470 K, (d) 540 K, (e) 560 K, and (f) 570 K. (a) (30 × 30) nm2 , −0.3 V, 0.6 nA; (b) (30 × 30) nm2 , −0.6 V, 0.9 nA (inset: (25 × 25) nm2 , −1.5 V, 3 pA); (c) (30 × 30) nm2 , −0.6 V, 0.4 nA; (d) (30 × 30) nm2 , −0.3 V, 0.6 nA; (e) (55 × 55) nm2 , −0.5 V, 1.5 nA; (f) (45 × 45) nm2 , −1.4 V, 2.1 nA.
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the behaviour of the 6T monolayer on reconstructed Au(001) upon annealing will be discussed. In Fig. 3(a) an STM measurement at an elevated surface temperature of 450 K is shown. One observes on both Au(001) terraces areas without substrate reconstruction around the step edge as well as reconstructed areas aside. While the reconstructed parts look mainly homogeneous, clear but partly smeared features are visible in the unreconstructed parts. Complex structures are especially observable in the very proximity of the step edge and extend even over both terraces. The stripes within the image and the smeared step edges point to a fast diffusion on the surface which can be suppressed by lowering the sample temperature. Figure 3(b) shows an STM measurement after cooling down to room temperature. On the reconstructed area in the upper left corner no molecular structures are visible. In contrast, rod-like 6T molecules, isomers, and a couple of intersecting molecular chains can be found on the unreconstructed parts of the two terraces. The appearance of the molecular structures is comparable to Fig. 2(b). Here, one additionally observes 6T molecules in all-trans conformation which are, comparable to the findings for Ag(001) (see Fig. 1(b)), arranged parallel to each other. They are mainly oriented along a high-symmetry direction. Areas of high mobility can also be found between the molecular structures and chains. In these areas reproducible STM imaging is not possible. To obtain detailed information about the molecular order on the reconstructed areas the sample is cooled down to 80 K and the result is shown in the inset of Fig. 3(b). The rodlike 6T molecules are very similarly arranged as in the ML structure of the reconstructed areas. So indeed, also on top of the reconstruction rows there is still a 6T structure present but with less order as compared to the as-deposited case and probably with a reduced molecular density due to desorption processes upon heating. This explains the high mobility of the molecules at room and higher temperatures which hampers imaging by STM. The in situ STM measurement at 470 K in Fig. 3(c) reveals network structures close to the step edges. These networks are more complex than those found at 20 K lower temperature (Fig. 3(a)). They extend on the terraces of the unreconstructed areas. The increasing complexity of the network comes along with a higher density of immobile knots which suppresses diffusion even at elevated temperatures. This becomes even more evident in the in situ measurement at 540 K depicted in Fig. 3(d). On the two unreconstructed terraces in the middle of the image an almost static, very complex network can be observed. In this temperature range first molecular chains are observed on top of the substrate reconstruction. The imaging is quite blurry since these structures are still small enough to diffuse. Comparable to the polymerization onset on the unreconstructed areas (see Fig. 3(a)) most of the imaged structures can be found close to the step edge since this is an intrinsic diffusion barrier. To conclude, network formation processes also take place on top of the reconstruction rows but at about 120 K higher temperatures compared to the unreconstructed areas. From our data the onset of the polymerization processes is determined to about 530 K, where individual network-like structures are formed mainly at the step edges.
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The in situ measurement at 560 K in Fig. 3(e) images the progression of network complexity on the reconstructed areas while it only marginally increases on the unreconstructed areas. As guide for the eyes a line is drawn between the unreconstructed (top right) and reconstructed (bottom left) areas. It partly seems that the network formation comes along with an altering or even destruction of the reconstruction rows. While at 560 K the network on both substrate areas can still be discriminated this is not longer possible at higher temperatures. The in situ STM measurement at 570 K in Fig. 3(f) reveals an almost homogeneous sample morphology. The observed network is of high complexity and only very few loose ends of the chains can be observed which indicates a significant increase of the interconnections. Furthermore, due to the high complexity and the immobile cross-linkings even at these high temperatures no noteworthy diffusion is observable and a quite static structure is imaged. A quantitative analysis of the network formation with increasing temperatures on the unreconstructed areas of Au(001) confirms the impression from the STM images. The relative surface coverage of molecular chains increases from 0.15 at 450 K over 0.5 at 540 K to 0.65 at 580 K. The surface coverage is given relative to the monolayer coverage of the dense diagonal row structure.8 B. Light-induced polymerization
Besides thermally-induced polymerization as discussed above, also photon-stimulated polymerization of oligomer condensates has been reported previously.10–12, 22–25 Here we address the light-induced polymerization of ultrathin 6T films on Ag(001) under conditions where pure electron-induced processes, e.g., by secondary electrons or fast photoelectrons can be neglected. Photoemission spectra for a pristine 6T monolayer on Ag(001) and upon exposure to UV radiation are depicted in Fig. 4(a). For the experiments UV photons of 3.8 eV were used with a controlled exposure of 3.0 × 106 nm−2 . Additionally the spectrum of the clean substrate as measured under identical conditions is shown. The characteristic features of the 6T film (A, B) and of the substrate silver d-band (C, D) are marked accordingly. No significant shifts of the features are observable for the different spectra. Both spectra of the 6T monolayer are scaled such that the substrate related peaks C and D have the same intensity. Obviously, the intensity of the characteristic 6T features decreases after UV exposure. Furthermore, the work function increases by ∼130 meV, i.e., changes towards the value of bare silver or 6T films with lower coverages. For reference, Fig. 4(b) displays the coverage-dependent work function of 6T on Ag(001) as measured by the Kelvin probe technique.26 The absolute work function value of the bare Ag(001) was derived from UPS measurements with an error less than 100 meV. The coverage dependent decrease of the work function has been determined with a relative error of less than 5 meV. It is worth mentioning that the observed work function decrease with 6T coverage is in good agreement with our UPS data (not discussed here) and also with previous literature reports.27 The decrease of the photoemission intensity of the 6T features in
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FIG. 4. (a) Electronic structure of the valence band of bare Ag(001) (black curve), of 1 ML 6T/Ag(001) before (blue curve) and after (red dotted curve) UV exposure (hν = 3.8 eV, = 3.0 × 106 nm−2 ) revealed by UPS. (b) Work function of 6T/Ag(001) in dependence on the 6T coverage measured by a Kelvin probe. The absolute work function value for bare Ag(001) was derived from UPS measurements. (c) The STM measurement depicts the surface morphology of 1 ML 6T/Ag(001) after UV illumination (hν = 3.8 eV, = 3.0 × 106 nm−2 , (55 × 55) nm2 , U = −0.3 V, I = 1.0 nA).
UPS and the change of the work function upon UV irradiation indicate a rearrangement of the molecular structures that might be accompanied by a desorption or dewetting of the organic molecules. Despite the signal decrease of features A and B, the intensities of these two 6T features relative to each other are similar before and after UV exposure which indicates a preservation of thiophene compounds. In Fig. 4(c) an STM image of the 6T film after UV exposure is depicted. In contrast to the pristine 6T monolayer no well-ordered molecular structures are found. Instead, the molecules are mainly isomerized and in disordered arrangement. Furthermore, some of the observed structures are longer than a single 6T molecule. Accordingly, the STM image reveals a surface morphology similar to that after thermal treatment of a 6T film. These STM observations are in agreement with the increase of the substrate signal in UPS and the increase of the work function. The existence of oligothiophenes longer than 6T is not in contradiction to the unchanged posi-
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tions and constant relative intensities of the characteristic 6T features in UPS. Binding energies and photoemission intensities are expected to remain unchanged under formation of longer or slightly shorter oligothiophenes based on the systematic studies of Fujimoto et al.28 In XPS a shift of the C 1s- and S 2p 3 -peaks to lower bind2 ing energies by 0.5 and 0.7 eV, respectively, is observed after UV exposure (not shown here). This is consistent with the results of Salaneck et al. after X-ray exposure of thiophene on gold, where a shift of 0.7 and 1.1 eV, respectively, is observed and is explained by the formation of polythiophene.10 Note that this shift is in line with the modification of the metalorganic interface dipole that is responsible for the work function increase. In order to reveal the processes which take place during UV exposure of thin 6T films, PEEM as a spatially resolved method is applied. The locally-restricted illumination with UV laser light in PEEM enables several measurements with, e.g., different photon energies or photon fluences on the same sample solely by changing the sample position. In the experiments the 6T films have been exposed to UV laser light and the corresponding PE signal has been measured spatially resolved in PEEM. In Fig. 5(a) the PEEM intensity, normalized to the respective laser power, is shown for 4 ML 6T/Ag(001) as a function of the exposure time to UV radiation (hν = 3.5 eV) for different laser powers. Each curve was measured at a pristine sample position. At all positions, a similar starting PE intensity is found. Obviously, the decrease of the PE intensity is faster for higher UV laser powers. Note that the instantaneous decay in Fig. 5(a) rules out a photoinduced desorption as explanation for the initial decrease of the photoemission intensity for 4 ML. Since the work function (Fig. 4(b)) is almost constant between 2 and 4 ML, desorption from a 4 ML thick film does not alter the work function significantly. Instead, a modified interface dipole due to polymerization at the interface can change the work function. After normalization to the respective initial intensities, the data sets of Fig. 5(a) are depicted double logarithmically in Fig. 5(b).29 Within the experimental error bars, the data points for different UV intensities fall entirely on the same curve which can be described by an exponential function I () = I0 · e− F , with I0 the initial PE intensity, the photon fluence, and F the fluence at which the relative photoemission is reduced to 1/e. Analogous measurements on 1 ML 6T/Ag(001) also show a single-exponential decay of the PE intensity with photon fluence. The results for 1 ML as well as for 4 ML 6T/Ag(001) are depicted in Fig. 5(c). The comparison between both data sets reveals that the decay is much faster for the thinner film and, accordingly, F is reduced by nearly one order of magnitude from 3 × 106 nm−2 for 4 ML to 4 × 105 nm−2 at 1 ML. Based on the single exponential decay in both cases, we expect similar processes induced by UV exposure for 1 ML and 4 ML thick 6T films. However, the 1 ML thick film is much more sensitive or the UV-induced processes are much more effective, respectively, which suggests an influence of the substrate interface. Additionally, photo-induced desorption might also lead to an increase of the work function for 1 ML since the coverage dependence of the work function is large in the monolayer regime (see Fig. 4(b)). Therefore,
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FIG. 5. (a) Temporal dependence of the normalized PE intensity in PEEM during UV exposure (hν = 3.5 eV) for different laser powers. (b) Double-logarithmic plot of the data sets of (a) as a function of the UV photon fluence (lower abscissa). The red curve represents the exponential function fitted to the data points. Additionally, the dependence of the PEEM intensity on the IR fluence is shown (black markers, upper abscissa, hν = 1.7 eV). (c) Normalized PE intensity in dependence on the photon fluence for 1 ML (hν = 3.8 eV) and 4 ML 6T/Ag(001) (hν = 3.5 eV).
the photoemission intensity is not sufficient to quantify the polymerization rate in the monolayer. In general, the decay of the PE intensity correlates with the fluence, i.e., the number and not the rate of the incoming photons. The PE signal is locally reduced by single photons and not by, for example, two-photon-processes. In addition to the data set shown, a similar drop of the PE signal is also observed for photon energies of 3.0 eV, 3.9 eV, and 4.2 eV and a similar behaviour is also expected for photon energies between these values. On the upper abscissa in Fig. 5(b), additionally the PE intensity in dependence on the photon fluence at a significantly lower photon energy of 1.7 eV is shown. Since no direct PEEM imaging is possible at 1.7 and 3.0 eV, the sample was additionally illuminated by 3.9 eV probing photons to determine the PE intensity. Whereas a low and constant photon intensity was used for probing to avoid significant reaction by 3.9 eV photons, the intensity of the 1.7 or 3.0 eV laser light for polymerization was varied. We find a similar drop in PE intensity for exposure to 3.0 eV photons as discussed above. In contrast, the PE intensity for exposure to 1.7 eV photons stays constant even at high photon fluences (Fig. 5(b)). This observation implies the existence of a threshold photon energy for the photon-induced polymerization. IV. DISCUSSION
Upon annealing of 6T monolayers on Ag(001) and Au(001), the molecules change their conformation and react with each other. Reactions at the end of single 6T molecules lead to the formation of long, ideally unbranched polymers. There, the length of the polymer chains depends on the thermal treatment and the structure of the underlying substrate. The onset temperature for polymerization is lower for substrates with a stronger molecule-substrate interaction. Besides the formation of long polymer chains also reactions at the central units of molecules and/or polymers take place. This results in knots and branches bound tightly to the substrate. They are not able to move even at elevated temperatures. With
increasing temperature the number of knots increases yielding a more complex and more rigid network. Also after UV exposure, thin 6T films on Ag(001) show a lowered coverage and polymerization structures are observed. The unchanged photoemission spectrum proves the existence of thiophene compounds which are only slightly shorter or even longer than 6T.28 Laser-PEEM experiments revealed an exponential decay of the PE signal during UV exposure that is identified as a one-photon process. The decay is faster for 1 ML than for 4 ML. However, a desorption-induced component to the decrease of the PE signal can be ruled out only for the 4 ML data. The dependence of the processes on the photon energy implies the necessity of an excitation into the lowest unoccupied molecular orbitals. Note that the photoexcitation for photon energies between 3.0 and 3.8 eV might populate molecular orbitals between the lowest unoccupied molecular orbital and the vacuum level or just 100 meV above it. These excitations do not produce large amounts of photoelectrons or secondaries. Therefore, we disregard any low-energy electron-driven chemistry in our study that might be present for significantly higher photon energies. In fact, Modelli and Burrow showed that relevant electron attachment levels are located between 1.2 and 3.0 eV above the vacuum level.30 To populate these states, photon energies of at least 4.8–6.6 eV are necessary. Hedhili et al. also find an energetic threshold for ring-rupture appearing at about 5 eV above the vacuum level.31 Again, this threshold is not reached in our present study. We propose that upon annealing as well as upon UV excitation (3.0 to 4.2 eV), a polymerization of the 6T takes place. It has been argued that in general the oxidation of the monomer to a cation radical is an important step in the (electro) polymerization of thiophenes.32 The radical can then take part in further reactions. Also chemical polymerizations based on thiophene radicals formed by electron transfer have been reported for photon energies in the 3–5 eV range, as, e.g., for illumination of a solution of thiophene derivatives in the presence of a suited electron acceptor.33 However, the
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formation of radical cations is not restricted to solution chemistry but is also reported for metallic surfaces. Lipton-Duffin et al. observed radical formation for diiodobenzene on a copper surface.34 The interaction between the organic molecules and the metal surface is of importance also in the present study since in the absence of the metal substrate no polymerization of 6T takes place in the equivalent temperature range as used here. First, the normally low catalytic effect of gold and silver single crystal surfaces might be enhanced by the high number of free noble metal adatoms which are present already at room temperature. They might act as nanoparticles with high catalytic activity.35, 36 Furthermore, the interaction between the molecular and the substrate electronic states is essential for radical formation which in turn is the basis for the polymerization. This agrees well with the observation of polymerization only in the presence of the metallic substrate. The radicals formed by electronic excitations are the starting point for subsequent reactions like the bonding between 6T molecules or also to the decomposition of single molecules to smaller parts. The latter reaction seems indeed to be necessary since the lengths of the observed polymers are not strictly integer multiples of the length of a single 6T molecule. The thermal energy at about 450 K is not sufficient to break up C–H, C–S-, or C–C bonds in the thiophene in the absence of an electronic modification of the molecular orbitals. In the polymerization process two radical cations (or one radical and a neutral monomer) are connected.37 When the bonding is formed between two Cα atoms a straight elongation of the chain takes place whereas a bonding to an innermolecular Cβ atom leads to the formation of a branching. After the reaction of two radicals two protons are released, i.e., two hydrogen atoms desorb from the surface, and a rearomatization leads to a new thiophene bonding. Depending on the local position where the reaction takes place within the organic molecule, also small thiophene compounds like 1T or 2T might split from the formed polymer. In the case of UV exposure of thin 6T films, the photoexcitation plays an essential role for possible reactions between the molecules. The activation barrier for the radical formation can be overcome by a photon-induced excitation. Afterwards the formed radicals might react in a similar manner as after thermal treatment. V. SUMMARY
The annealing of ultrathin 6T films on Ag(001) and Au(001) leads to local reactions between neighbouring 6T molecules. Depending on the surface temperature and the annealing time, long unbranched polythiophene polymers or branched polymer networks are formed. There, the complexity and the onset temperature depend on the interaction with the underlying substrate. Similar molecule-substrate interactions on Ag(001) and on unreconstructed Au(001) lead to comparable polymer structures upon annealing. In contrast, the polymerization process takes place at about 120 K higher temperatures for 6T adsorption on the reconstructed Au(001)“(5 × 20)” where a lower molecule-substrate interaction is
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predicted. Similar structural changes and polymerization are found for exposure of the 6T film to UV radiation in the range from 3.0 to 4.2 eV. The observed reactions in both cases, by thermal annealing as well as by UV excitation, are explained by a polymerization via formation of cation radicals which necessitates an interplay with metallic substrate states. ACKNOWLEDGMENTS
This work was supported by the DFG through the Sonderforschungsbereich SFB/TRR-102 “Polymers under multiple constraints: restricted and controlled molecular order and mobility.” The authors thank Wolfgang Binder and Mario Kiel for fruitful discussions as well as Ralf Kulla for technical support. 1 A.
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