Mixed Monomolecular Films with Embedded Dipolar Groups on Ag

Article Cite This: J. Phys. Chem. C XXXX, XXX, XXX−XXX

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Mixed Monomolecular Films with Embedded Dipolar Groups on Ag(111) Eric Sauter,† Charles-Olivier Gilbert,‡ Jean-François Morin,‡ Andreas Terfort,§ and Michael Zharnikov*,† †

Applied Physical Chemistry, Heidelberg University, Im Neuenheimer Feld 253, 69120 Heidelberg, Germany Département de Chimie and Centre de Recherche sur les Matériaux Avancés (CERMA), Université Laval, 1045 avenue de la Médecine, Québec, Quebec, Canada G1V 0A6 § Institut für Anorganische und Analytische Chemie, Universität Frankfurt, Max-von-Laue-Straße 7, 60438 Frankfurt, Germany

J. Phys. Chem. C Downloaded from pubs.acs.org by KAOHSIUNG MEDICAL UNIV on 08/17/18. For personal use only.

‡

ABSTRACT: We studied the application of the concept of embedded dipoles in monomolecular self-assembly to the Ag(111) substrate (evaporated film), using two different types of molecules with either pyrimidine groups embedded into aromatic backbones or ester groups embedded into aliphatic backbones as test systems. For both types of self-assembled monolayers (SAMs), the orientation of the embedded dipolar group was varied and the molecules with the oppositely oriented dipoles were combined together as mixed monolayers. In all cases, pronounced electrostatic effects of the embedded dipolar groups were observed, reflected, in a fully consistent manner, by the electrostatic shift in photoemission and by work function variation. The character and extent of these effects were, however, distinctly different for both types of SAMs, which was explained in context of structure−building interactions, specific orientation of the dipole moment of the embedded group with respect to the molecular backbone, and molecular orientation in general. The SAMs with the embedded pyrimidine group were found to be especially useful in context of the electrostatic interface engineering, allowing, in the case of Ag, a flexible tuning of the work function in the ∼0.85 eV range without changing the character of the SAM−substrate and SAM−ambient interfaces. An analogous behavior can also be expected for other substrates.

1. INTRODUCTION Performance of organic electronics and photovoltaics devices relies to a large extent on the energy level alignment between the adjacent electrode, buffer, and active layers. This can be achieved by selection of suitable materials and tuning of their properties (e.g., by exact chemical composition, doping, or morphology), but also by the introduction of intermediate monomolecular films, self-assembled monolayers (SAMs), providing suitable electrostatic shift at the relevant interfaces.1−14 Such films usually consist of three building blocks, namely, docking group, molecular backbone, and terminal tail group, that mediate bonding to a particular substrate, promote efficient molecular assembly, and define specific physicochemical identity of the SAM−ambient interface, respectively.15−17 Adjustment of the electrostatic shift provided by a particular SAM is usually achieved by selection of a suitable dipolar tail group, which either increases (−CF3, −C6F5, or CN) or decreases (−CH3 or −NH2) the work function of the substrate.2,3,5,6,8,11 This approach, however, has several drawbacks, namely, possible influence of the selected tail group on the morphology of the adjacent layer (e.g., organic semiconductor) and, vice versa, possible reorientation or even chemical modification of this group induced by the adjacent layer, which can distort the desired energy level alignment. © XXXX American Chemical Society

Alternative strategies, suggested recently by us and others, involve the use of a dipolar molecular backbone or embedding of a dipolar group into the backbone, untangling thus, completely or at least to a significant extent, dipole engineering and interfacial chemistry and protecting the dipolar group from possible modification or reorientation upon deposition of a buffer or organic semiconductor layer during the assembly of an organic solar cell or an organic transitor.18−22 In particular, the concept of embedded dipoles was successfully demonstrated for both aliphatic and aromatic SAMs using the dipolar ester and pyrimidine groups, respectively.19,22 These groups were introduced in two opposite orientations with respect to the backbone, with the dipole moment directed either from or to the substrate, resulting in a work function difference of 1− 1.1 eV.19,22 By combining the molecules with the oppositely oriented dipolar groups in a joint mixed SAM, flexible variation of the work function within the above energy window could be achieved, which is potentially useful for precise tuning of the energy level alignment in organic electronics and photovoltaics devices.20,23 As an additional “bonus”, the electrostatic Received: May 14, 2018 Revised: July 31, 2018 Published: August 1, 2018 A

DOI: 10.1021/acs.jpcc.8b04540 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C

adhesion layer. The Ag films were polycrystalline, exposing preferably (111) orientated surfaces of individual crystallites. Consequently, we describe these substrates as Ag(111) in the manuscript but keep in mind their exact nature. The quality of the substrates was verified by atomic force microscopy. The size of individual crystallites was 50−100 nm; the root-meansquare value was estimated at ∼1.5 nm (a scan area of 500 × 500 nm2). The preparation of the SAMs on Ag(111) followed an established procedure allowing to avoid the oxidation of the substrate and to remove possible contaminants relying on the self-cleaning process upon the self-assembly.17,28,29 Freshly prepared substrates kept under argon before SAM fabrication were used. Possible oxidation of these substrates before SAM fabrication was not specifically monitored but tested by XPS after the fabrication, with no traces of oxidation recorded. The single-component C10EC10-up/down SAMs were formed by immersion of the substrates into 1 mM solutions of the SAM precursors in ethanol under nitrogen at ambient temperature for 24 h. The single-component PPmP1-up/down SAMs were prepared by immersion of the substrates into 0.5 mM solutions of the SAM precursors in tetrahydrofuran (THF) under nitrogen at ambient temperature for 24 h. The mixed SAMs were prepared by coadsorption of both precursors, either C10EC10-up/down or PPmP1-up/down, with their relative portions in solution, either ethanol or THF, being varied. After immersion, the samples were carefully rinsed with pure solvent and blow-dried with a stream of nitrogen or Ar. In addition, the reference samples of hexadecanethiol (C16) and C10EC10-up SAM on Au(111) were prepared according to the literature procedures.22,27 The samples were either characterized immediately after the preparation or put under an inert gas in plastic containers for the transportation to the synchrotron. The SAMs were characterized by synchrotron-based XPS, near-edge X-ray absorption fine structure (NEXAFS) spectroscopy, and work function measurements. The spectroscopic experiments were carried at the HE-SGM beamline (bending magnet) of the synchrotron storage ring BESSY II in Berlin, using a custom-designed experimental station.30 The measurements were conducted at a base pressure of 1 × 10−9 mbar and room temperature; special care was taken to avoid X-rayinduced damage during the spectra acquisition. For this purpose, the spectra acquisition time was kept reasonably short and X-ray spot position on a particular sample was varied. The XPS spectra were measured with a Scienta R3000 electron energy analyzer in normal emission geometry. The primary photon energy (PE) was varied. The energy resolution was ∼0.3 eV at a PE of 350 eV and somewhat lower at higher PEs. The binding energy (BE) scale was referenced to the Au 4f7/2 emission at 84.0 eV.31 The NEXAFS spectra were measured at the carbon, nitrogen, and oxygen K edges in the partial electron yield acquisition mode with retarding voltages of −150, −300, and −350 V, respectively. Polarization factor of the linearly polarized synchrotron light was estimated at ∼88%. The incidence angle of the light was varied in steps between the normal and grazing geometry to monitor the linear dichroism effects associated with the molecular orientation and orientational order in the SAMs.32 The energy resolution was ∼0.3 eV at the C K edge, ∼0.45 eV at the N K edge, and ∼0.6 eV at the O K edge. The PE scale was referenced to the pronounced π* resonance of highly oriented pyrolytic graphite at 285.38 eV.33

homogeneity of these mixed SAMs, down to the molecular level, could be precisely monitored by X-ray photoelectron spectroscopy (XPS),20 relying on the electrostatic effects in photoemission.24 All activities described in the above paragraph were, however, related to the gold substrate, with only one exception dealing with mixed monolayers of carboranethiol isomers (dipolar molecular backbone) on a silver surface.21 In the present work, applying a similar approach to the SAMs with embedded dipolar groups, we wanted to test the extent to which the concept of embedded dipole, working excellently in the case of Au, is applicable to an alternative support, taking Ag(111) as a representative example (see the next section for detailed information about the substrates). Along these lines, we studied both aliphatic and aromatic SAMs with suitable embedded polar groups directed electrostatically either to or from the substrate (Figure 1), as well as mixed monolayers

Figure 1. Structures of the aromatic and aliphatic SAM precursors used in this study, along with their acronyms. For the aromatic SAMs, Pm, P, and “1” mean pyrimidine, phenyl, and number of the methylene groups in the aliphatic linker, respectively. For the aliphatic SAMs, Cn refers to the number of the methylene and methyl moieties in the segments above (“top”) and below (“bottom”) of the ester (E) group. The directions of the dipole moments of the embedded dipolar pyrimidine and ester groups with respect to the molecular backbone (upright molecular orientation with the thiolate anchor to the substrate) are marked by blue arrows. They are included as “up” and “down” in the acronyms.

comprising the molecules with the opposite orientations of the embedded dipoles. Additional goals of this work were to prove the validity of the electrostatic effects in photoemission for an arbitrary substrate and to test the correlation between these effects and work function. The choice of Ag(111) for this study was related to its general suitability for the formation of thiolate SAMs but distinctly different properties in terms of molecular self-assembly compared to Au(111),14,17,25,26 which potentially allows to achieve different adsorption geometries and, consequently, different electrostatic properties of the monolayers.

2. EXPERIMENTAL SECTION The precursors for the aliphatic and aromatic SAMs with embedded dipolar groups (Figure 1) were custom-synthesized according to the reported procedures.19,22,23,27 All other chemicals and solvents were purchased from Sigma-Aldrich and used as received. The Ag substrates were purchased from Georg Albert PVD-Beschichtungen (Silz, Germany). They were prepared by thermal evaporation of 30 nm of silver (99.99% purity) onto polished single-crystal silicon (100) wafers (Silicon Sense) that had been precoated with a 9 nm Ti B


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The Journal of Physical Chemistry C Raw NEXAFS spectra were normalized to the incident photon flux by division through a spectrum of a clean, freshly sputtered gold sample and, subsequently, reduced to the standard form.32 Work function measurements were carried out using a ultrahigh vacuum Kelvin Probe 2001 system (KP Technology Ltd., U.K.). The pressure in the vacuum chamber was ∼10−9 mbar. Freshly sputtered gold and C16 SAM on Au were used as references. The reference work function values were set to 4.322 and 5.2 eV,12 respectively.

3. RESULTS AND DISCUSSION 3.1. XPS. The XPS data of both single-component and mixed C10EC10-up/down and PPmP1-up/down films imply the formation of well-defined and contamination-free SAMs. In particular, the S 2p spectra of all these monolayers (not shown) exhibit a single doublet at ∼162.0 eV (S 2p3/2), representative of the thiolate species bound to noble metal substrates,34 underlying the SAM character of both singlecomponent and mixed films. The intensity of this doublet, as well as the intensities of the Ag 3d, O 1s, and N 1s signals (not shown), are nearly persistent over the entire C10EC10-up/ down and PPmP1-up/down series, suggesting that the molecular packing density does not change noticeably upon the change in the orientation of the embedded dipole and mixing the molecules with the opposite dipole orientations. At the same time, the exact composition of the mixed SAMs could not be determined straightforwardly from the XPS data because the spectroscopic signatures of the differently oriented dipolar groups in the O 1s spectra of the C10EC10-up/down SAMs23 and in the N 1s spectra of the PPmP1-up/down monolayers19 are nearly identical. This information can, however, be obtained from the C 1s data presented in Figure 2, relying on the electrostatic effects in photoemission. Accordingly, the introduction of the embedded dipole into the molecular backbone in a SAM results in an electrostatic shift of the C 1s peak associated with the backbone segment located above the group (top segment) as compared to that located below (bottom segment) even if both segments are chemically identical.19,22,24 The value of this shift is proportional to the electrostatic field provided by the embedded dipolar groups and is determined by their density, dipole moment, and orientation.20,23 The spectra in Figure 2 were acquired at a photon energy of 350 eV to attain an especially strong attenuation of the C 1s signal.35 Consequently, the dominant component peak in these spectra is mostly representative of the top segment of the respective backbone, namely, the terminal phenyl ring for the PPmP1-up/down SAMs and the top alkyl segment for the C10EC10-up/down monolayers. The C 1s spectra of the single-component PPmP1-down and PPmP1-up SAMs in Figure 2a are similar to the analogous spectra of these monolayers on Au,19,20 exhibiting characteristic peaks related to the top and bottom phenyl rings as well as to the carbon atoms in the pyrimidine ring (see ref 19 for the exact assignments). These spectra show distinctly different positions of the dominant peak (∼284.1 and ∼284.9 eV, respectively), reflecting the effect of the embedded dipolar pyrimidine groups. The spectra of the mixed SAMs show a continuous shift of this peak between the ultimate values for the single-component monolayers, with the respective BE positions given in Figure 3a. Such a continuous shift, along with the nearly constant fwhm of the dominant peak (see Figure 2a), is in distinct contrast to the weighted superposition

Figure 2. C 1s XPS spectra of the single-component and mixed PPmP1-up/down (a) and C10EC10-up/down (b) monolayers of different compositions. The spectra are shifted vertically for comparison. The compositions of the mixed SAMs reflect the relative portions of the up (first number) and down (second number) molecules in the solutions, from which the SAMs were grown. The positions of the dominant component peak, representative of either the terminal phenyl ring (a) or top alkyl segment (b) of the assembled molecules, are tracked by the red dashed lines. The full widths at halfmaximum (fwhm) of the dominant component peak are given at the respective spectra.

Figure 3. Dependence of the BE position (black squares) of the dominant component peak in the C 1s XPS spectra of the singlecomponent and mixed PPmP1-up/down (a) and C10EC10-up/down (b) SAMs on Ag(111) on the portion of PPmP1-up (a) or C10EC10up (b) in the solutions from which the SAMs were grown. The character of the dependence is highlighted by red lines.

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additional factor can probably be a dipole paring, following a twist of 180° of the chain plane in one of the neighboring molecules. 3.2. Work Function. An alternative way to monitor electrostatic effects of the embedded dipolar groups is the work function, which also has a practical importance in context of SAM-mediated energy level alignment at the electrode− organic semiconductor and electrode−buffer layer interfaces in organic electronics and photovoltaics (see references in Section 1). The work function of the single-component and mixed PPmP1-up/down and C10EC10-up/down SAMs is presented in Figure 4. Apart from the fact that the shift in the

of the spectra of the single-component PPmP1-up and PPmP1down SAMs, as could be expected in the case of phase separation in the mixed monolayers.20 Consequently, the spectra in Figure 2a suggest a homogeneous mixture of both components in the mixed SAMs. The C 1s spectra of the single-component C10EC10-down and C10EC10-up SAMs in Figure 2b are similar to the analogous spectra of these monolayers on Au,22,23 exhibiting characteristic peaks related to the top and bottom alkyl segment as well as to the carbonyl and ether carbon atoms associated with the embedded ester group (see ref 22 for the exact assignments). These spectra show somewhat different positions of the dominant peak (284.9 and 285.2 eV, respectively), reflecting the effect of the embedded dipolar ester groups. The spectra of the mixed SAMs reveal a continuous shift of this peak between the ultimate values for the single-component monolayers, with the respective BE positions given in Figure 3b. Similar to the PPmP1-up/down case and according to the literature,20 such a continuous shift, along with the persistent fwhm of the dominant peak (see Figure 2b), suggests a homogeneous mixture of both components in the mixed SAMs. The difference between the ultimate values of the C 1s binding energy in Figure 3a,b reflects the electrostatic effect of the embedded dipole in the case of the Ag(111) substrate. In addition, the exact shape of the respective curves, which are distinctly different for the PPmP1-up/down and C10EC10up/down systems, is representative of the difference between the compositions of the parent solutions and the resulting mixed SAMs. Whereas the difference between the ultimate values (single-component monolayers) is comparable for the PPmP1-up/down SAMs on Ag (∼0.85 eV) with that for the Au-based system (∼1.05 eV),19,20 the difference for the C10EC10-up/down monolayers on Ag (∼0.3 eV) is much smaller than that for Au (∼1.1 eV).22,23 Because the density of the SAMs on Ag(111) is similar to that on Au(111) (and, most likely, even somewhat higher, at least for the C10EC10-up/ down films),36 the above behavior can only be explained by orientation effects, as will be discussed in Section 3.3. As to the exact shape of the curves in Figure 3, they reflect the variation in the “internal” electrostatic shift associated with the embedded dipolar groups and, in this sense, are representative of the composition of the mixed SAMs because the value of the electrostatic shift is determined by the portions of the molecules with oppositely directed dipoles. In accordance with such an interpretation, the curves in Figure 3 show significant deviation of the composition of the mixed SAM from that in the solution for the PPmP1-up/down system (Figure 3a; a S-like curve) and an almost linear relation between the solution/SAM compositions for the C10EC10up/down system (Figure 3b). Interestingly, this behavior mimics that of the Au substrate20,23 and can be tentatively explained by the same line of arguments. Accordingly,20 for the aromatic SAMs with an embedded dipolar pyrimidine group, building a considerable part of the molecular backbone, a 50− 50% composition is preferred for the mixed SAMs because of the stabilizing head-to-tail arrangement of the neighboring dipoles (in the sense of their interaction), giving the observed S-like deviation. In contrast, the impact of the dipole−dipole interaction on the SAM composition is much lower in the C10EC10-up/down case, most likely, in view of a much stronger contribution associated with the intermolecular interaction of the comparably long alkyl segments.23 An

Figure 4. Dependence of the work function (black squares) of the single-component and mixed PPmP1-up/down (a) and C10EC10up/down (b) SAMs on Ag(111) on the portion of PPmP1-up (a) or C10EC10-up (b) in the solutions from which these monolayers were grown. The character of the dependence is highlighted by red lines.

C 1s XPS peak of the top segment and the work function variation occurs in opposite directions, the curves in Figure 4 mimic those in Figure 3, which is understandable in view of the same underlying factor (electrostatic effect of the embedded dipolar groups). For the PPmP1-up/down SAMs, the work function is varied between 5.0 and 4.15 eV (∼0.85 eV range), again exhibiting the preference of the 50−50% composition for the mixed SAMs as follows from the observed shape of the curve in Figure 4a. This is comparable with the case of Au, where the work function of these SAMs is varied between 4.8 and 3.8 eV (∼1 eV range),20,37 exhibiting the preference of the 50−50% composition as well.20 Note that the work functions of clean, single crystalline Ag(111) and Au(111) substrates are 4.53 and 5.33 eV, respectively,38 but the work function of the respective evaporated films, such as in our work, can be slightly different. Nevertheless, taking 4.53 eV as the reference, one sees that the deposition of the C10EC10-up/down SAMs results mostly in the lowering of the work function, whereas D


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The Journal of Physical Chemistry C the assembly of the PPmP1-up/down monolayers allows for both decrease and increase in the work function of Ag in quite a broad range. This situation is even more favorable as compared to the Au substrate, where, in spite of the broader range, only decrease in the work function is possible with the PPmP1-up/down SAMs.19,20 A further interesting aspect is the difference between the absolute work function variation ranges on Ag(111) and Au(111) with respect to the values for the clean substrates. This difference suggests different interfacial dipole for nonsubstituted thiolate SAMs on these two substrates. For the C10EC10-up/down SAMs (Figure 4b), the work function is varied between 4.6 and 4.15 eV (∼0.45 eV range), exhibiting a nearly linear behavior as a function of the solution composition, suggesting negligible deviations in the composition of the mixed SAMs as compared to the one in the deposition solution. Whereas the same linear behavior was observed for the Au substrate as well,23 the range of the work function variation there was noticeably larger, namely, from 4.8 to 3.8 eV (∼1 eV interval).23 3.3. NEXAFS Spectroscopy. As mentioned above, the difference in the electrostatic effect of the embedded dipolar groups for the SAMs on Ag and Au substrates is most likely related to the specific molecular orientation on these two supports. A suitable approach to monitor this parameter is NEXAFS spectroscopy, which also provides information about the chemical composition of the samples studied. As an example of such information, relevant in the context of the present study, we show the N K edge and O K edge NEXAFS spectra of the single-component and mixed PPmP1-up/down and C10EC10-up/down SAMs in Figure 5. The 55° N K edge NEXAFS spectra of the singlecomponent and mixed PPmP1-up/down SAMs in Figure 5a exhibit characteristic features of pyrimidine,39,40 namely, a strong π* resonance at ∼398.5 eV (1), a mixed π*−Rydberg (R*) feature at 402.7 eV (2), and further Rydberg and σ*-like resonances at higher excitation energies (3). The spectra of the PPmP1-up and PPmP1-down SAMs look almost identical, which is understandable in the context of the identical chemical composition of the precursors (Figure 1). Of primary importance is, however, the nearly same intensity of the absorption resonances for the single-component and mixed monolayers (6.4; 6.3; 6.6; 6.1; and 6.2, respectively, normalized to the height of the absorption edge), suggesting a nearly constant packing density over the series. A similar situation occurs for the single-component and mixed C10EC10-up/down SAMs, whose spectra in Figure 5b exhibit characteristic features of the ester group,41,42 namely, a π(C O*) resonance at 531.9 eV (1), as well as several weaker resonances with the π* (2) and σ* (3) character at higher photon energies (see ref 41 for the exact assignments). The above data were complemented by the C K edge spectra, that were measured under varying angles (see Section 2), providing information on both chemical composition and molecular orientation. Representative NEXAFS data for the single-component and mixed PPmP1-up and PPmP1-down SAMs are shown in Figure 6. Both magic angle (Figure 6a) and difference (Figure 6b) spectra are presented, with the latter ones reflecting linear dichroism effects (see Section 2). Apart from few minor differences, the 55° spectra of the PPmP1-up and PPmP1-down SAMs, as well as those of the mixed SAMs, are very similar to each other and agree well with the literature data for the single-component films on Au(111).19 They are

Figure 5. N K edge (a) and O K edge (b) NEXAFS spectra of the single-component and mixed PPmP1-up/down (a) and C10EC10up/down (b) SAMs on Ag. The spectra were acquired at an X-ray incidence angle of 55° (magic angle); at this particular orientation, the spectra are not affected by molecular orientation effects and are entirely representative of the electronic structure.32 The compositions of the mixed SAMs reflect the relative portions of the ‘up’ (first number) and ‘down’ (second number) molecules in the solutions from which the SAMs were grown. The spectra are shifted vertically for comparison. Characteristic absorption resonances are marked by numbers (see text for details).

dominated by the characteristic π1* resonance (1), consisting of three features at 284.85/285.0, 285.3, and 286.0 eV (shoulder-like) and accompanied by a variety of π*- and σ*like resonances at higher excitation energies (see refs 19 and 39 for the exact assignments). The difference spectra of the single-component and mixed PPmP1-up and PPmP1-down SAMs in Figure 6 exhibit distinct peaks at the positions of the characteristic absorption resonances, suggesting a certain orientational order in these monolayers. The amplitudes of these difference peaks do not vary much over the series, except for the spectrum of the single-component PPmP1-down SAM, where the amplitude is somewhat smaller. The signs of the difference peaks imply an upright molecular orientation.19 Numerical evaluation of the entire set of the NEXAFS data within the standard formalism for the vector-type orbital,32 which was the π1* one in the given case, gave average tilt angles of 56−59° for the entire PPmP1-up/down series, corresponding to the average molecular tilt angles of 31−34° (with ±3° error). The latter angles are larger as compared to those for the singlecomponent PPmP1-up and PPmP1-down SAMs on Au, namely, 18 and 17°, respectively.19 The respective difference explains the difference between the work function ranges achieved with the PPmP1-up/down SAMs on Ag and Au (∼0.85 vs ∼1.05 eV19) because a larger molecular inclination means a smaller vertical component of the embedded dipole E


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Figure 7. C K edge NEXAFS data for the single-component and mixed C10EC10-up and C10EC10-down SAMs, including the spectra acquired at an X-ray incidence angle of 55° (a) and the difference between the spectra acquired at X-ray incidence angles of 90 and 20° (b). The compositions of the mixed SAMs reflect the relative portions of C10EC10-up (first number) and C10EC10-down (second number) in the solutions from which the SAMs were grown. The spectra are shifted vertically for comparison; the same intensity scale is applied in both panels. Characteristic absorption resonances are marked by numbers (see text for details). Dotted lines in panel (b) correspond to zero.

Figure 6. C K edge NEXAFS data for the single-component and mixed PPmP1-up and PPmP1-down SAMs, including the spectra acquired at an X-ray incidence angle of 55° (a) and the difference between the spectra acquired at X-ray incidence angles of 90 and 20° (b). The compositions of the mixed SAMs reflect the relative portions of PPmP1-up (first number) and PPmP1-down (second number) in the solutions from which the SAMs were grown. The spectra are shifted vertically for comparison; the same intensity scale is applied in both panels. The dominant absorption resonance is marked (see text for details). Dotted lines in panel (b) correspond to zero.

relying on the intensity-versus-angle dependence for selected absorption resonances,32 is not as straightforward as in the case of PPmP1-up/down, we applied an alternative, “difference” method.32,47 Within this method, one monitors the difference in the intensity of a selected absorption resonance upon X-ray incidence angle variation and fits it to the theoretical expression (for a plane orbital)

and, consequently, a smaller electrostatic effect for the entire SAM. The C K edge NEXAFS data for the single-component and mixed C10EC10-up/down SAMs are presented in Figure 7. Similar to the PPmP1-up/down case (Figure 6), the data combine the 55° spectra (Figure 7a) and the difference 90− 20° curves (Figure 7b). The spectra of the single-component and mixed C10EC10-up/down SAMs look very similar to each other, suggesting, in accordance with the XPS data, similar structure of these films. The 55° spectra in Figure 7a are dominated by the merged Rydberg (R*) resonance of the alkyl segments at ∼287.7 eV43 (1; there are also alternative assignments)44 and the π(C*O) resonance of the ester group at 288.5 eV27,45 (2). This double feature is accompanied by the characteristic resonances at ∼293.4 eV (3) and ∼301.6 eV (4) associated with the valence, antibonding C−C σ* and C−C′ σ* orbitals of the alkyl chain, respectively.44,46 The difference spectra of the single-component and mixed C10EC10-up and C10EC10-down SAMs in Figure 7b are almost identical, exhibiting pronounced peaks at the positions of the characteristic absorption resonances of the alkyl chains. This suggests similar, quite high orientational order in all these SAMs, with the molecular chains having an upright orientation, as follows from the specific signs of the difference peaks taking into account the orientation of the respective molecular orbitals.27 Because the evaluation of the NEXAFS data for the C10EC10-up/down SAMs within the standard approach,

Ip(θ1) − Ip(θ2) = C(1 − 3 cos 2 γ )(cos 2 θ1 − cos 2 θ2) (1)

where θ1 and θ2 are two different incidence angles of X-rays, γ is the angle between the sample normal and the normal to the orbital plane, Ip(θ1) and Ip(θ2) are the resonance intensities, and C is a normalization constant, which depends on the excitation probability from the selected core level into a given molecular orbital.32 The latter parameter is generally unknown but can be derived from the measurements on a reference sample, which has the same molecular orbital and a known molecular structure. As a suitable orbital, the R*, with a plane-like character, was selected. The normal to the orbital plane is directed along the alkyl backbone, so that γ in eq 1 will be the molecular tilt angle, averaged over both individual molecules and their top/bottom segments (if the planes of both segments and embedded ester group are coplanar, the top segment is canted over ∼9° with respect to the long axis of the bottom segment).27 NEXAFS spectroscopy cannot distinguish between these segments because they are chemically identical and the electrostatic effects (Section 3.1) do not affect the NEXAFS spectra. As a F


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nonsubstituted alkanethiolate SAMs on this substrate (45°),48 as this is the case for Au(111).22,27 3.4. Additional Aspects. In addition to the structural and electrostatic properties of the embedded dipole SAMs, there are several other aspects, that are of importance in context of their technological applications, independent of the substrate. The first aspect concerns their electric transport properties, which, according to the general knowledge on this subject,49−51 should be superior for the PPmP1-up/down SAMs as compared to the C10EC10-up/down monolayers, making the former films better suited for the interface engineering in organic electronics and photovoltaics, unless it is a gate engineering where one would rather like a higher resistance to decrease the gate leakage current. Note that the major factor is not the molecular length, which is larger in the C10EC10-up/ down case, but the character of the molecular backbone,49−51 which is mostly aromatic for PPmP1-up/down and aliphatic for C10EC10-up/down. Of course, the introduction of the methylene linker decouples to some extent the electronic subsystems of the metal substrate and conjugated segment,52−54 but this effect is small as compared to the low electric conductance of the long, pure aliphatic C10EC10-up/ down chains. Further, according to the available experimental data, the introduction of the embedded dipolar group and its orientation do not influence the electric transport properties significantly, apart from variation in the transition voltage and a slight asymmetry in conductance, which are special issues.55,56 This was directly demonstrated for the PPmP1-up and PPmP1-down SAMs on Au(111)55,56 and can be assumed for the C10EC10-up/down monolayers as well, based on the results of the experiments for a variety of alkanethiolate SAMs with terminal and embedded polar groups.57 Finally, in view of the results for the PPmP1-up and PPmP1-down SAMs on Au(111),55 we also do not expect noticeable variation in the electric transport properties between the single-component and mixed SAMs for both PPmP1-up/down and C10EC10up/down series. The second aspect is SAM stability, which can be considered as especially critical for substrates prone to oxidation, such as Ag(111). The major factors here are exposure to ambient oxygen and light,16,17 which are of importance if one deals with a SAM as such, but of no importance if a SAM becomes a part of an organic electronics device. Whereas we have not specifically addressed the stability of the PPmP1-up/down and C10EC10-up/down SAMs on Ag(111), we could monitor this property on a time scale of a week during the experiments at the synchrotron, requiring a prolonged storage of the samples and their handling under ambient conditions. No traces of degradation, including oxidation of the substrate, oxidation of the docking group, or the breakage of the S−C bonds at the SAM−substrate interface were observed. Such a behavior corresponds to our experience with an even more reactive substrate, GaAs, which could be effectively protected from oxidation and degradation by the formation of a high quality SAM.58−60 Such a high-quality and dense molecular packing is also characteristic of the PPmP1-up/down and C10EC10-up/down SAMs, so that a reasonable stability can be expected.

suitable reference sample, the single-component C10EC10-up SAM on Au was chosen. The average tilt angle of the alkyl chains in this sample was estimated at ∼30° by dedicated infrared spectroscopy experiments.22,27 The R* resonance intensity data for the single-component and mixed C10EC10-up and C10EC10-down SAMs are presented in Figure 8 in the fashion corresponding to eq 1,

Figure 8. Intensity differences for the R* resonance in the C K edge NEXAFS spectra of the single-component and mixed C10EC10-up and C10EC10-down SAMs on Ag(111) vs cos2(θ1) − cos2(θ2). θ2 was fixed at 55°; θ1 was varied. The legend is given in the plot. The color-coded dashed lines represent the best linear fits to the experimental data. The compositions of the mixed SAMs reflect the relative portions of C10EC10-up (first number) and C10EC10-down (second number) in the solutions from which the SAMs were grown. The data for the single-component C10EC10-up SAMs on Au(111) are used as a reference.

namely, Ip(θ1) − Ip(θ2) versus cos2 θ1 − cos2 θ2. The observed dependences could be indeed adequately fitted by straight lines. The slopes of these lines, which appear noticeably steeper in the case of Ag as compared to Au, give then the values of γ (see eq 1), which were estimated at 17.5−18.5° (with ±3° error) for both single-component and mixed C10EC10-up and C10EC10-down SAMs. This average molecular tilt angle is somewhat higher than the literature value for nonsubstituted alkanethiolate SAMs on Ag (∼12°; but higher values are also frequently reported)36 but distinctly smaller than that for the C10EC10-up/down monolayers on Au (∼30°).22,27 Such a smaller molecular inclination in the case of Ag means a noticeably larger angle between the embedded dipole, strongly tilted with respect to the molecular chain (Figure 1), and the surface normal. Consequently, the vertical component of the dipole moment, determining the electrostatic effect of the embedded dipoles, should be significantly smaller on Ag than on Au, which explains the comparably small shifts of the dominant component peak in the C 1s XPS spectra (Section 3.1) and the comparably small range of the work function variation (Section 3.2) for the C10EC10-up/down SAMs on Ag. Generally, the specific orientation of the dipole moment of the embedded ester group with respect to the alkyl backbone makes the electrostatic effect of this group quite prone to the molecular orientation, which is presumably valid not only for the Ag(111) substrate but for other potential substrates as well. Of course, the twist of the alkyl segments, along the molecular axis, affects the orientation of the embedded dipole as well (see discussion in ref 22), but the tilt is of major importance. As to the twist of the top and bottom alkyl segments in the C10EC10-up/down SAMs on Ag(111), it is most likely similar to that of the

4. CONCLUSIONS In the present work, we studied the application of the concept of embedded dipoles in monomolecular self-assembly to the Ag(111) substrate using two different types of SAMs, namely, G


Article

The Journal of Physical Chemistry C Notes

PPmP1-up/down and C10EC10-up/down, as test systems. Apart from either aromatic or aliphatic character of these films, the major differences between them were (i) the “contribution” of the embedded dipolar group in context of the structure−building interactions and (ii) the orientation of the dipole moment associated with the embedded group with respect to the molecular backbone. For both types of SAMs, the orientation of the embedded dipolar group was varied and the molecules with oppositely oriented dipoles were combined together as mixed monolayers, which, according to the XPS data, represented homogeneous mixtures of the components. In all cases, pronounced electrostatic effects of the embedded dipolar group were observed, reflected, in a fully consistent fashion, by the electrostatic shift in photoemission and by the work function variation. The character and extent of these effects were, however, distinctly different for both types of the SAMs studied. In the PPmP1-up/down case, the composition of the mixed monolayers was affected by the dipole−dipole interaction between the embedded groups, with a preference of the 50−50% ratio. In contrast, for the C10EC10-up/down system, the composition of the mixed SAMs mimicked that of the parent solution because of a comparably weak contribution of the embedded dipolar group to the entire balance of the structure−building interactions. As to the work function, important in context of the energy level alignment, the PPmP1-up/down SAMs allowed to vary it gradually in the 0.85 eV range, making possible both its increase and decrease with respect to the value for clean Ag. In contrast, the work function range accessible with the C10EC10-up/down monolayers is much smaller (∼0.45 eV), which was explained by the strong inclination of the dipole moment of the embedded ester group with respect to the molecular backbone. The projection of this moment on the surface normal, determining the strength of the electrostatic effects, is then especially sensitive to the molecular orientation, in contrast to the PPmP1-up/down SAMs, where the dipole moment of the embedded pyrimidine group is directed along the molecular chain. Consequently, these films, along with other aromatic SAMs with embedded pyrimidine groups, are well suitable for electrostatic engineering of different interfaces in organic electronics and photovoltaics, which will only require the adaptation of the docking group to the material on which these SAMs will be assembled. Apart from the above conclusions related to the possible applications of the embedded dipole SAMs, the results of the present study, namely, the observation of distinct electrostatic effects in photoemission and their good correlation with the work function behavior, suggest the general character of these effects. Their understanding is an important prerequisite for correct interpretation of the XPS spectra of monomolecular films. At the same time, they represent a valuable tool for characterization of such systems.

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The authors declare no competing financial interest.

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ACKNOWLEDGMENTS 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 experiments there. The work was financially supported by the German Research Society (Deutsche Forschungsgemeinschaft; DFG) within the grant ZH 63/22-1.

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Jean-François Morin: 0000-0002-9259-9051 Andreas Terfort: 0000-0003-2369-5151 Michael Zharnikov: 0000-0002-3708-7571 H


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Mixed Monomolecular Films with Embedded Dipolar Groups on Ag

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