Adjustment of the Work Function of Pyridine and Pyrimidine

Article

Adjustment of the Work Function of Pyridine and Pyrimidine Substituted Aromatic Self-Assembled Monolayers by Electron Irradiation Eric Sauter, Can Yildirim, Andreas Terfort, and Michael Zharnikov J. Phys. Chem. C, Just Accepted Manuscript • Publication Date (Web): 26 May 2017 Downloaded from http://pubs.acs.org on May 27, 2017

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The Journal of Physical Chemistry

Adjustment of the Work Function of Pyridine and Pyrimidine Substituted Aromatic Self-Assembled Monolayers by Electron Irradiation

Eric Sauter,1 Can Yildirim,1 Andreas Terfort,2 and Michael Zharnikov1* 1 2

Applied Physical Chemistry, Heidelberg University, 69120 Heidelberg, Germany

Institut für Anorganische und Analytische Chemie, Universität Frankfurt, Max-von-LaueStraße 7, 60438 Frankfurt, Germany

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Abstract

Self-assembled monolayers (SAMs) are frequently used to manipulate injection barriers in organic electronics by introduction of a specific dipole moment at the interfaces between the electrodes and adjacent organic layers. This is usually achieved by the selection of a proper dipolar terminal tail group comprising the SAM-ambient interface, which was recently complemented by embedding such a group into the molecular backbone. Here we show that the work function of SAMs can also be adjusted by electron irradiation in a quite broad range and in controlled fashion as far as these films contain pyridine or pyrimidine group. This effect is demonstrated by the example of several representative aromatic SAMs with either terminal pyridine group or embedded pyrimidine group. The observed behavior is presumably related to specific chemical transformations involving the nitrogen atom in these moieties. The SAMs with the embedded pyrimidine group are then especially attractive since this moiety is decoupled from the SAM-ambient interface. The extent of the effect is very large (a work function change of up to ~0.8 eV) as far as it is monitored in situ but is diminished upon the exposure of the irradiated films to ambient. Practical implications of this effect are discussed, including work function lithography, which is demonstrated by representative patterns.


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1. Introduction Flexible control of properties of surfaces and interfaces is one of the key issues of modern physical chemistry and nanotechnology. A useful approach in this regard is fabrication of self-assembled monolayers (SAMs) comprised of densely packed, rod-like amphiphilic molecules covalently linked to the substrate by a suitable docking (head) group and redefining its physical and chemical identity. Due to the flexible molecular architecture of the SAM constituents, almost any surface property can be adjusted and different types of the substrates can be adapted. A particular useful parameter in this context is the work function (WF) of the substrate, which is of primary importance for a proper energy level alignment at the interfaces as far as this substrate is electronically linked with another layer within a complex assembly. Consequently, SAMs are frequently used in organic electronics as intermediate layers at the interfaces between the electrodes, buffer layers, and organic semiconductors, improving energy level alignment and charge carrier injection.1-8 A change in the WF is mostly achieved by the use of dipolar moieties, attached usually to the molecular backbone of the SAM constituents as terminal tail groups2,3,5,8,9 or, in some cases, embedded into the backbone, decoupling control over the charge injection from control of the chemistry at the SAMambient interface.10,11 Along with the chemical design, the WF of SAMs can be potentially varied by physical means such as electron irradiation which, in combination with e-beam lithography, can open a way for WF patterning, useful in context of organic electronics.12 In connection with this, aromatic SAMs can be of particular interest, since, in contrast to aliphatic monolayers, which become severely damaged upon exposure to electrons,13 they persist as well-defined organic films upon such a treatment, apart from extensive intermolecular cross-linking and chemical modification of the terminal groups.13-20 In this context, in the given work, we study the effect of electron irradiation on the WF of several representative aromatic SAMs on Au(111) substrate. As test systems, shown in Figure 1, we selected a non-substituted aromatic SAM (TP1)21-26 as well as monolayers with the terminal (PyPP1)27 and embedded (TP1-down and TP1-up)10,11 dipolar groups. A special feature of these groups is their cyclic aromatic character, so that they are not a standard substitution but a part of the aromatic backbone, contributing to irradiation-induced cross-linking as was recently demonstrated for the PyPP1 film.28 An additional attribute of all SAM precursors in Figure 1 is a methylene linker between the aromatic backbone and the thiolate head group. This linker decouples the aromatic matrix from the substrate, reducing the structural stress and resulting in the improving quality of the 3 ACS Paragon Plus Environment


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monolayers in terms of the packing density, molecular orientation, and long-range structural order.25-27,29,30 This guaranties well-defined character of these films, qualifying them as suitable test systems for the given experiments. Note that the TP1 and PyPP1 films have already been studied with respect to their reaction to electron irradiation,28 but not in context of WF modification.

2. Experimental part All solvents and chemicals were purchased from Sigma-Aldrich. The SAM precursors used in this study are shown in Figure 1, along with their abbreviations. They were customsynthesized according to the literature recipes, viz. TP1 according to ref 21, PyPP1 according to ref 31, and TP1-up and TP1-down according to ref 10. The gold substrates were purchased from Georg Albert PVD- Beschichtungen (Silz, Germany). They were prepared by thermal evaporation of 30 nm of gold (99.99% purity) onto polished single-crystal silicon (100) wafers (Silicon Sense) that had been precoated with a 9 nm titanium adhesion layer. The films were polycrystalline, exposing preferably (111) orientated surfaces of individual crystallites. The SAMs were formed by immersion of the substrates into solutions of the SAM precursors in ethanol (concentration range 10×10−6 –1000×10−6 M) under nitrogen at ambient temperature for 24 h. After immersion, the samples were carefully rinsed with pure solvent and blown dry with a stream of nitrogen or Ar. In addition, a reference SAM of hexadecanethiolate (HDT) was prepared on the same substrates using a standard procedure.32 All experiments were performed at room temperature. The electron irradiation was carried out homogeneously over the entire sample area. A flood gun (FG20, Specs, Germany) was used. The energy of electrons was set to 50 eV. The flux was calibrated by a Faraday cup. A base pressure during the irradiation was better than 1×10-8 mbar. Work function measurements were carried out using a UHV Kelvin Probe 2001 system (KP technology Ltd., UK). The pressure in the UHV chamber was ~10-10 mbar. Freshly sputtered gold and HDT SAM on Au were used as references. The work function measurements were performed both in situ, immediately after the irradiation treatment, and, for selected samples, ex situ, after the intentional exposure of the irradiated samples to ambient and their storage under these conditions (in dark) for 24 h. The chemical changes in the SAMs upon exposure to electrons were monitored by laboratory X-ray photoelectron spectroscopy (XPS) and, for selected samples, by synchrotron-based XPS. The XPS characterization was performed both in situ, immediately after the irradiation 4 ACS Paragon Plus Environment

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treatment, and, for selected samples, ex situ, after the intentional exposure of the irradiated samples to ambient and their storage under these conditions (in dark) for 24 h. The laboratory XPS measurements were carried out with a MAX200 (Leybold-Heraeus) spectrometer equipped with an Mg Kα X-ray source (200 W) and a hemispherical analyzer. Normal emission geometry was used. The recorded spectra were corrected for the spectrometer transmission. The synchrotron-based XPS measurements were performed at the HE-SGM beamline (bending magnet) of the synchrotron storage ring BESSY II in Berlin, Germany, using a custom-made experimental station equipped with a Scienta R3000 electron energy analyzer.33 The synchrotron light served as the primary X-ray source. The spectra acquisition was carried out in normal emission geometry with an energy resolution of ∼0.3 eV at an excitation energy of 350 eV and somewhat lower resolution at higher excitation energies. The binding energy (BE) scale of the XPS spectra was referenced to the Au 4f7/2 emission at 84.0 eV.34 The spectra were fitted by symmetric Voigt functions and a Shirley-type background. To fit the S 2p3/2,1/2 doublets we used two peaks with the same full width at halfmaximum (fwhm), a standard34 spin-orbit splitting of ~1.2 eV (verified by fit), and a branching ratio of 2 (S 2p3/2/S 2p1/2). The fits were performed self-consistently: the same fit parameters were used for identical spectral regions. The effective thickness of the monolayers was calculated using a standard procedure,35 based on the C 1s/Au 4f intensity ratio. A standard expression for the attenuation of the photoemission signal was assumed36 and the literature values for attenuation lengths were used.37 The spectrometer-specific coefficients were determined by using HDT/Au as a reference, relying on the well-known thickness of this monolayer. Along with the homogeneous electron irradiation, WF patterns were fabricated by proximity printing lithography. These patterns were written by homogeneous electron irradiation through a mask (Quantifoil, R 1/4, Plano, representing holey (1 µm) carbon film over Cu 400 mesh) placed on the SAM surface. The same setup as for the homogeneous irradiation was used. The fabricated WF patterns as well as respective morphology patterns were imaged by a Solver Next microscope (NT-MDT). The atomic force microscopy (AFM) measurements were carried out under ambient conditions, either in the surface potential or tapping mode, respectively. 5 ACS Paragon Plus Environment


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3. Results 3.1. in situ Measurements Dependence of the WF of the TP1, PyPP1, TP1-up and TP1-down SAMs on irradiation dose is shown in Figure 2a. The measurements were performed in situ, immediately after the irradiation treatment. The WFs of the pristine TP1, TP1-up and TP1-down SAMs was found to be 4.16 eV, 3.77 eV, and 4.86 eV, respectively, in good agreement with the experimental values from the previous work10,11 and in reasonable correlation with the results of the DFT calculations for these molecular films10. The embedded pyrimidine groups shift the WF downwards (TP1-up) or upwards (TP1-down) as compared to the non-substituted film (TP1), dependent of the direction of the respective dipole moment, in full agreement with the results of the DFT calculations10. The terminal pyridine group, with the dipole moment directed in the same way as for TP1-down, provides even a larger shift as compared to the TP1-down case, resulting in a WF of ~5.20 eV for PyPP1/Au, in a reasonable correlation with the literature data.38 The dipole moments of the pyridine and pyrimidine moieties are similar (2.2 D and 2.3 D, respectively)39,40 but the electrostatic screening of the dipole imbedded into the molecular chain (TP1-down) within a densely packed SAM should be stronger than that at the SAM-ambient interface (PyPP1), which explains a larger WF shift in the latter case (with respect to the non-substituted film, TP1). Electron irradiation of the TP1 SAM results in a slow and continuous increase of WF, in agreement with the literature data for analogous non-substituted aromatic monolayers.41 The increase in WF, associated with combined effect of irradiation-induced cross-linking and a partial damage of the SAM-substrate interface,41 is +0.03 eV at 10 mC/cm2 and +0.19 eV at 60 mC/cm2. In contrast, both pyridine and pyrimidine substituted SAMs exhibit not an increase but a drop in the WF, which occurs rapidly at low doses (up to 10 mC/cm2) and is much more extensive, viz. −0.79 eV, −0.76 eV, and −0.41 eV for PyPP1/Au, TP1-down/Au, and TP1-up/Au, respectively. Significantly, the irradiation-induced changes in WF are persistent as far as these samples are kept in vacuum, as demonstrated in Figure 2b where dependence of the WFs of the irradiated (10 mC/cm2) PyPP1 and TP1-up SAMs on time is presented. The observed strong changes in the WF of the PyPP1, TP1-up and TP1-down SAMs the can only stem from irradiation-induced modification of pyridine and pyrimidine units. As shown in our previous publication dealing with several pyridine-terminated SAMs,28 the pyridine moieties participate in the formation of the cross-linking network, exhibiting a simultaneous 6 ACS Paragon Plus Environment

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chemical modification and partial protonation. The analogous situation occurs for TP1down/Au and TP1-up/Au as well. The laboratory C 1s, N 1s, and S 2p XPS spectra of the pristine and irradiated TP1-up/Au SAMs are presented in Figure 3, representative of TP1down/Au as well. The C 1s and N 1s spectra of the pristine TP1-up film exhibit peaks characteristic of the molecular backbone,10 whereas the S 2p spectrum shows a characteristic S 2p3/2,1/2 doublet (1) at a BE position of ~162.0 eV (S 2p3/2) corresponding to the thiolate species bound to noble metal substrates.42 Specifically, the C 1s spectrum exhibits the characteristic component peaks associated with the bottom (1) and top (2) phenyl rings of the TP1-down backbone as well as an additional peak (3) assigned to the carbon atoms in the pyrimidine rings.10 The difference in the binding energies of the peaks 1 and 2 stems from the electrostatic shift provided by the embedded pyrimidine group.10 The intensity of the C 1s signal in Figure 3a did not change noticeably upon irradiation, suggesting an extensive cross-linking in the SAM matrix,20,28 but its character changes significantly already at low irradiation doses. The splitting of the peaks 1 and 2 becomes less pronounced at already 5 mC/cm2 and is hardly perceptible at 10 mC/cm2, suggesting a decrease or even elimination of the electrostatic shift associated with the embedded pyrimidine group (analogous effect for the inverse electrostatic shift was observed for TP1down/Au). The positions of the peaks 1 and 2 shift first to higher BE in the course of irradiation and then, after the merging of both peaks, the joint feature progressively shifts to lower BE. The N 1s spectra of TP1-up/Au in Figure 3b exhibit several changes upon the irradiation. First, the intensity of the N 1s signal decreases to some extent suggesting a partial release of nitrogen-containing species. Second, the N 1s peak shifts and broadens, suggesting a modification and progressive heterogeneity of the pyrimidine-derived moieties in the monolayer. Third, a weak additional "shoulder" appears at the high BE side of the main peak; it can be assigned to a partial protonation of the pyrimidine-derived moieties, similar to the pyridine case.28,43 The S 2p spectra of TP1-up/Au in Figure 3c exhibit the behavior typical of the homogeneous and heterogeneous aromatic SAMs,20,28 viz. appearance and progressive increase in intensity of a new S 2p3/2,1/2 doublet (2) at ~163.4 eV (S 2p3/2) upon the irradiation, on the expense of the doublet stemming from the thiolate group. This suggests a partial damage of the SAM/substrate interface, which is typical irradiation-induced process.20,28 This damage occurs however much slower (with respect to the dose) and to a lesser extent as compared to 7 ACS Paragon Plus Environment


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alkanethiolate SAMs on the same substrate (see e.g. ref 13), suggesting an efficient crosslinking in the aromatic matrix. The most important processes discussed above are additionally highlighted by the curves presented in Figures 4 and 5. Figure 4a shows the thickness of the TP1-up SAM, which, similar to the intensity of the C 1s signal, did not change noticeably in the course of irradiation, apart from a very small decrease (~5%) at low irradiation doses. Figure 4b presents the total intensity of the N 1s signal, which decreased slowly in the course of irradiation, achieving a loss of 20-23% at high doses. Figure 4c presents the total intensity of the S 2p signal as well as intensities of the both component doublets as functions of irradiation dose. The total intensity did not change noticeably in the course of irradiation, whereas the intensities of both component doublets show the opposite behavior, with progressive increase in the intensity of the irradiation-induced doublet on the expense of the thiolate one. Significantly, all changes highlighted in Figure 4 occur rather slow in the course of irradiation, exhibiting no correlation with the change in the WF (Figure 2a). The changes of the N 1s spectra were additionally monitored by the synchrotron-based XPS, taken the TP1-up SAM as a representative test system. The derived dependences of the BE position and fwhm of the N 1s peak for the TP1-up SAM on irradiation dose are presented in Figure 5. Both parameters exhibit progressive increase with the dose, occurring in an exponential-like fashion, with a similar rate (with respect to the dose) as the WF change.

3.2. ex situ Measurements The changes in the WF observed under the in situ conditions (Figure 2a) did not persist upon exposure of the irradiated SAMs to ambient. This effect is illustrated by Figure 6 where the WFs of the pristine SAMs of this study are compared with those of the irradiated (10 mC/cm2) films for both in situ case and after the exposure of the irradiated SAMs to ambient. Whereas the WF of the reference, non-substituted SAM (TP1) exhibited only a slight increase (~0.09 eV) upon the exposure to ambient, those of the pyridine-terminated (PyPP1) and pyrimidine-substituted (TP1-up and TP1-down) monolayers showed a much noticeable increase, partly compensating (PyPP1 and TP1-down) or even overcompensating (TP1-up) the effect of irradiation. The resulting changes in the WF as compared to the pristine SAMs were estimated at +0.17 eV, −0.48 eV, −0.47 eV, and +0.25 eV for the TP1, PyPP1, TP1down, and TP1-up films, respectively.


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These changes could be directly visualized by the WF patterns prepared in proximity printing geometry. Surface potential images of such patterns are presented in Figure 7. In accordance with the WF measurements on the homogeneously irradiated samples, the TP1 and TP1-up patterns exhibit a higher WF within the irradiated areas, whereas the inverse situation occurs for the PyPP1 and TP1-down patterns. Possible chemical changes of the irradiated SAMs upon their exposure to ambient were monitored by XPS and AFM. As a representative example, synchrotron based C 1s, N 1s, and O 1s spectra of the pristine and irradiated (10 mC/cm2) TP1-up SAMs before and after exposure to ambient are shown in Figure 8. The C 1s and N 1s peaks, shifted to the higher BEs upon irradiation with the given dose, exhibited pronounced downward shifts upon exposure to ambient (Figures 8a and 8b, respectively), suggesting certain chemical changes or/and electrostatic "recharging" induced by this exposure. In addition, the intensity of the C 1s signal increased slightly upon the exposure, accompanied by the appearance of a pronounced O 1s signal (Figure 8c), which was completely absent in the spectra of the pristine and irradiated films, as far as they were kept in vacuum. This behavior suggests adsorption of carbon- and oxygen-containing airborne species, in accordance with previous observation.13,41 This effect is illustrated by Figure 9 where a representative morphology image of a TP1-up SAM pattern along with the height profile across the "written" dot-like features are presented. The reason for this effect is presumably chemical activation of the SAM-ambient interface, which is a side effect in the given case but can be useful as well, e.g. for in situ electron-beam induced deposition of a predefined material.44,45

4. Discussion Upon electron irradiation, the PyPP1, TP1-up, and TP1-down SAMs exhibit behavior similar to that of oligophenyl-based films, such as the TP1 monolayer, viz. a progressive and extensive cross-linking, involving also the pyridine and pyrimidine moieties; this process prevents release of individual molecules and their fragments and slows down the damage of the SAM/substrate interface. At the same time, the PyPP1, TP1-up, and TP1-down SAMs exhibit distinctly different changes in the WF upon electron irradiation as compared to the reference TP1 film. These changes can only be related to the specific behavior of the pyridine and pyrimidine moieties, superimposing onto the change in WF associated with the oligophenyl backbone, which is represented by the example of the reference TP1 film.



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The WF changes associated with the pyridine and pyrimidine moieties can be related to an interplay of several different effects. First, as shown by the XPS data, the electrostatic shifts associated with the terminal pyridine and mid-chain pyrimidine groups are partly eliminated due to their modification and intermolecular cross-linking. Theoretically, complete elimination of the electrostatic shifts should result in the WF values similar to that of the nonsubstituted SAM and be persistent upon the exposure to ambient, which both are not the case (see Figures 2 and 6). In particular, the WF of TP1-up/Au did not increase, toward the TP1 values but decrease upon electron irradiation. This suggests that there is another contribution, general for the PyPP1, TP1-up, and TP1-down SAMs and lowering the WF of these systems upon electron irradiation. This contribution works in the same direction as the elimination of the electrostatic shift (see above) for PyPP1/Au and TP1-down/Au, but in the opposite direction for TP1-up/Au. Consequently, the ultimate change in the WF for the latter system is noticeably smaller than that for the former films, viz. −0.41 eV versus −(0.76-0.79) eV; see Figure 2a. Significantly, the WF-lowering contribution is mostly eliminated upon the exposure of the PyPP1, TP1-up, and TP1-down SAMs to ambient, manifested by the similar increase in the WF for all these films (Figure 6). This effect cannot be solely related to the adsorption of the airborne species since the increase in the WF is much smaller for TP1/Au, which has the same chemical composition of the SAM/ambient interface as the TP1-up and TP1-down monolayers. Regretfully, the XPS data do not give any other hint except for clear signatures of air exposure-induced chemical changes, electrostatic "recharging", and adsorption of airborne species (Figures 8 and 9), so that the only possibility is a reasonable assumption. One possible mechanism, which might be responsible for the observed WF behavior is illustrated in Figure 10a. The "driving force" of this mechanism are hydrogen atoms releases upon irradiation-induced cleavage of C−H bonds in the heteroaromatic backbones, which is the primary process triggering the cross-linking.13,14,16,17,19,20 The released hydrogen atoms are chemically active, in particular, with respect to nitrogen, inducing e.g. nitro-to-amino and nitrile-to-amino transformations in the nitro- and nitrile-terminated aromatic SAMs.15,18,46 Consequently, they can also trigger specific chemical reactions involving the pyridine and pyrimidine groups in the PyPP1, TP1-up, and TP1-down SAMs, resulting, in particular, in formation of positively charged pyridinium or pyrimidinium moieties capable to decrease the WF of these films. These moieties remain stable as long as the film is kept in vacuum but undergone further 10 ACS Paragon Plus Environment

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chemical reactions, resulting in disappearance of the positive charge, upon the exposure of the irradiated films to ambient. An alternative mechanism, involving the released hydrogen atoms as well, is presented in Figure 10b. Accordingly, the reactive hydrogen atoms reduce (some of) the heterocycles to the respective amines, with the nitrogen atoms becoming less electron-withdrawing, so that the WF is lowered. Further, upon the exposure to ambient, these amines react with airborne CO2 and form carbamates, which results in the WF increase due to the electron-withdrawing character of the carbonyl group. Regretfully, the available spectroscopic data do not provide a clear evidence in favor of one of the above mechanisms, letting other possibilities open as well. At the same time, these data suggest chemical transformation of the pyridine and pyrimidine moieties upon electron irradiation, with certain heterogeneity of the resulting chemical structures. A pronounced positive BE shift of both C 1s and N 1s XPS peaks occurring at low irradiation doses (Figure 8) correlates with our assumption about the appearance of positively charged (Figure 10a) or WF lowering (Figure 10b) species in the irradiated SAMs. Significantly, this shift is reversed upon the exposure of the films to ambient, manifesting elimination of the positive charge (Figure 10a) or a transformation of the WF lowering moieties (Figure 10b), in full agreement with the WF behavior (Figure 6). Apart from the pure scientific issues discussed above, practical implications of the results should be considered. The major advantages of the presented electron irradiation approach are (i) the possibility of precise tuning of the WF within the given dynamical range by selection of a proper dose and, as mentioned in Section 1, (ii) the possibility to apply e-beam lithography for WF patterning. The latter is demonstrated in Figure 7 where representative examples of such patterns are presented. Consequently, e-beam lithography, which also has the advantages of a flexible form of the written features as well as dose variation across the pattern,47 can be a valuable alternative to other approaches to WF patterning such as microcontact printing.12,48,49 Significantly, the dynamic range of the in situ variation of the WF covered by the PyPP1, TP1-up, and TP1-down SAMs is exceptionally large, viz. from +5.2 eV to +3.4 eV, with a small gap around +4.0 eV, between the minimal value for TP1up/Au and the maximal value for TP1-down/Au (Figure 2a), which, however, can be covered by a proper mixed SAM of both these constituents.11 Consequently, a suitable SAM can be easily selected for a specific application and its WF subsequently fine-tuned or patterned within its specific dynamical range (Figure 2a). Regretfully, the SAM-specific dynamical 11 ACS Paragon Plus Environment


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ranges as well as the joint dynamical range of the entire SAM series decrease noticeably upon the exposure of the monolayers to ambient (Figure 6), necessary, in most cases, for their integration into the devices. One cannot exclude, however, that the in situ WF values of the pyridine- and pyrimidine-substituted SAMs remain persistent and valid if the subsequent fabrication steps, such as deposition of an organic semiconductor, are performed without the exposure to ambient, e.g. in the same vacuum chamber or under inert atmosphere, as it e.g. occurred for UV/ozone treated Au.50 This can be a subject of further research in context of the results presented in the given work.

5. Conclusions Here we show that the WF of SAMs can be adjusted by electron irradiation in a quite broad energy range and in controlled fashion as far as these films contain pyridine or pyrimidine group. This effect is demonstrated by the example of several representative heteroaromatic monolayers with either terminal pyridine group or embedded pyrimidine group, with either upward or downward orientation, and compared to the behavior of the reference, nonsubstituted oligophenyl-based SAM. The observed behavior of the pyridine and pyrimidine substituted films is presumably related to specific chemical transformations involving the nitrogen atom in these moieties. The SAMs with the embedded pyrimidine group are then especially attractive since this moiety is decoupled from the SAM-ambient interface, detangling the dipole control at a particular interface and the nucleation and growth mode of the adjacent organic semiconductor. The extent of the irradiation-induced WF changes is exceptionally large, varying from 0.4 to 0.8 eV depending on the specific system and covering a WF range of +(3.4-5.2) eV for the entire series as far as WF is monitored in situ, but the effect is diminished upon the exposure of the irradiated films to ambient. Two possible mechanisms behind this behavior are presented. Practical implications of the approach are discussed in context of WF engineering in organic electronics. Particular strengths of the approach are continuous tunability of the WF by selection of irradiation dose as well as WF patterning. The latter was demonstrated by representative examples in framework of proximity printing lithography.

Acknowledgement We thank the Helmholtz Zentrum Berlin for the allocation of synchrotron radiation beamtime at BESSY II and A. Nefedov and Ch. Wöll for the technical cooperation during the 12 ACS Paragon Plus Environment

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experiments there. We appreciate a financial support of the German Research Society (Deutsche Forschungsgemeinschaft; DFG) within the grant ZH 63/22-1.

Author information Corresponding Author *Phone: +49 6221 54 4921. Fax: +49 6221 54 6199. E-mail: [email protected]

Notes The authors declare no competing financial interest.



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Figure Captions Figure 1. The structures of the SAM precursors of this study, along with their abbreviations: [1,1´:4´,1´´-terphenyl]-4-methanethiol

(TP1),

(4-(2-phenylpyrimidin-5-yl)phenyl)

methanethiol (TP1-down), (4-(5-phenyl pyrimidin-2-yl)phenyl)methanethiol (TP1-up), and (4-(4-(4-pyridyl)phenyl)phenyl)methanethiol (PyPP1). The directions of the dipole moments of the pyridine and pyrimidine moieties are marked by blue arrows. Figure 2. (a) Dependence of the WF of the TP1, PyPP1, TP1-up, and TP1-down SAMs on irradiation dose. The measurements were performed in situ, directly after the irradiation treatment. (b) Dependence of the WF of the irradiated (10 mC/cm2) PyPP1 and TP1-up SAMs on time. The samples were kept in vacuum, without exposure to ambient. The legends are given in the panels. Figure 3. Laboratory C 1s (a), N 1s (b), and S 2p (c) XPS spectra of the pristine and irradiated TP1-up SAMs. The doses are marked at the respective spectra. The spectra are decomposed into the component peaks (a and b) or doublets (c) shown in different colors and marked by numbers. The sums of the individual components are drawn by the wine solid lines. See text for details. Figure 4. Dependence of the effective thickness (a) and XPS intensities (b, c) for the TP1-up SAM on irradiation dose. (b) total intensity of the N 1s signal as well as intensities of the pristine and irradiation-induced component peaks. (c) the total intensity of the S 2p signal as well as intensities of the S 2p component doublets corresponding to the pristine thiolate and irradiation-induced species. The experimental dependences are tentatively traced by solid curves shown in different colors, as guides for the eyes. The legends are given in the panels. Figure 5. Dependence of the BE position (a) and fwhm (b) of the N 1s peak for the TP1-up SAM on irradiation dose. The data were derived from the synchrotron-based XPS spectra (not shown). The experimental dependences are tentatively tracked by exponential functions (solid curves). Figure 6. WF of the pristine and irradiated (10 mC/cm2) TP1, PyPP1, TP1-up and TP1-down SAMs. The WF was either measured in situ, immediately after the irradiation treatment, or after exposure to ambient. The legend is given in the figure. Figure 7. Surface potential images (3D representation) of e-beam patterned TP1, PyPP1, TP1-up, and TP1-down SAMs. The patterning was performed in proximity printing geometry (see text for details). The dose was set to 10 mC/cm2. The imaging was performed ex situ, 19 ACS Paragon Plus Environment


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after exposure to ambient; the relations between the WFs of the pristine and irradiated areas for the SAMs under consideration are shown in Figure 6 (pristine vs. 24h/air). Figure 8. Synchrotron-based C 1s (a), N 1s (b), and O 1s (c) XPS spectra of the pristine and irradiated (10 mC/cm2) TP1-up SAMs before and after exposure to ambient. The legends are marked at the respective spectra. The positions of the main peaks are highlighted by vertical red lines (a and b); the O 1s peak (c), appearing after the exposure to ambient, is highlighted by the red arrow. See text for details. Figure 9. A representative morphology image of a TP1-up SAM pattern (a) along with the height profile across the "written" dot-like features (b). The patterning was performed in proximity printing geometry (see text for details). The dose was set to 10 mC/cm2. Figure 10. Proposed models of irradiation-induced modification of the pyridine group in the PyPP1 SAM, representative of the embedded pyrimidine groups of the TP1-up and TP1-down monolayers as well: (a) formation of positively charged pyridinium or pyrimidinium moieties (in situ) and their subsequent "recharging" (ex situ); (b) formation of amine (in situ) and carbomate (ex situ) groups.


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Figure 1



TP1 PyPP1 TP1-up TP1-down

5.0

a

4.5

Work function (eV)

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4.0 3.5 0

10

20

30

40

50

60

Irradiation dose (mC/cm2)

b

5.0 4.5 TP1up PyPP1

4.0 3.5 0

6

12

18

24

Time (hours)

Figure 2


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a

XPS: C 1s

b

N 1s

c

S 2p

40 mC/cm2

40 mC/cm2

40 mC/cm2

Intensity (arb. units)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


20 mC/cm2 2

20 mC/cm

20 mC/cm2

10 mC/cm2 10 mC/cm2

10 mC/cm2

5 mC/cm2 5 mC/cm2

pristine

295

290

2 3

285

5 mC/cm2

2

pristine

1

pristine

1

280

410

405

400

395

390

165

160

Photon energy (eV)

Figure 3



Thickness (Å)

20

a

18 16 14 12


10 1.0


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1.0

b

0.8 0.6 N 1s content

0.4 0.2 0.0

c

0.8 0.6 0.4 0.2

S 2p thiolate S 2p damaged S 2p total

0.0 0

10

20

30

40

50

60

Dose (mC/cm²) Figure 4


N 1s fwhm (eV)

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N 1s position (eV)

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399.0

a

398.9 398.8 398.7 398.6 398.5 1.20 1.15

b

1.10 1.05 1.00 0.95 0

2

4

6

8

10

2

Dose (mC/cm )

Figure 5


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Work function (eV)


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TP1 PyPP1 TP1-up TP1-down

5.0 4.5 4.0 3.5 pristine

10 mC/cm2 24h/air

Figure 6


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Figure 7



HRXPS: C 1s


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a N 1s

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b O 1s

hν ν = 350 eV

hν ν = 580 eV

10mC/cm2 +24h/air

10mC/cm2 +24h/air

c

hν ν = 580 eV

10mC/cm2 +24h/air

10mC/cm2 10mC/cm2

10mC/cm2

pristine

pristine

290

285

405

400

pristine

395

540

535

530

Photon energy (eV)

Figure 8


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Figure 9



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Figure 10


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TOC graphic


Adjustment of the Work Function of Pyridine and Pyrimidine

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