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Mixed Aliphatic Self-Assembled Monolayers with Embedded Polar Group Eric Sauter, Charles-Olivier Gilbert, Joël Boismenu-Lavoie, Jean-Francois Morin, and Michael Zharnikov J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b08671 • Publication Date (Web): 25 Sep 2017 Downloaded from http://pubs.acs.org on September 26, 2017

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The Journal of Physical Chemistry

Mixed Aliphatic Self-Assembled Monolayers with Embedded Polar Group

Eric Sauter,1 Charles-Olivier Gilbert,2 Joël Boismenu-Lavoie,2 Jean-François Morin,2 and Michael Zharnikov1*

1

Applied Physical Chemistry, Heidelberg University, Im Neuenheimer Feld 253, 69120 Heidelberg, Germany

2

Département de Chimie and Centre de Recherche sur les Matériaux Avancés (CERMA), Université Laval, 1045 av. de la Médecine, Québec QC Canada G1V 0A6

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Abstract We studied structure, molecular orientation, morphology, and electrostatic properties of mixed self-assembled monolayers (SAMs) comprised of alkanethiolates (ATs) which are modified by a dipolar ester group embedded into the alkyl backbone at two different orientations, viz. with the dipole directed upwards and downwards from the substrate. The packing density and molecular orientation in these SAMs was found to be similar to those of the "parent" single-component monolayers. Applying X-ray photoelectron spectroscopy (XPS) as a morphology tool, we could estimate that the mixed SAMs represent homogeneous intermolecular mixtures of both components, down to the molecular level, excluding existence of "hot spots" for charge injection. The analysis of the C 1s XPS spectra and the work function data suggests that the composition of the mixed SAMs fully mimicked the mixing ratio of both components in solutions from which these SAMs were prepared, which suggests a minor role of the dipole-dipole interaction in the overall balance of the structurebuilding forces. Varying this composition, work function of the gold substrate could be tuned linearly, in controlled fashion within a ~1.1 eV range, between the ultimate values for the single-component monolayers with a fixed orientation of the embedded ester group, viz. 3.83 eV and 4.92 eV, respectively. This adjustment could be performed keeping the chemical composition at the SAM-ambient interface unchanged, which, along with tunability and homogeneity of these films, is a great advantage of the mid-chain substituted monolayers, qualifying them for energy level alignment in model systems and organic electronics devices.


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1. Introduction The energy level alignment between the metal electrodes and adjacent organic semiconductor as well as between the buffer layers and adjacent electrode/active layer is a key issue in organic electronics and photovoltaics. A frequently used approach to address this issue is the introduction of an interfacial dipole, by modification of the electrodes/buffer layers with selfassembled monolayers (SAMs), resulting in a desired change of their work function.1-12 SAMs, which are densely packed 2D assemblies of rod-like amphiphilic molecules covalently linked to the substrate by a suitable docking group,13-15 are ideally suitable for this purpose. All three basic blocks of SAMs, viz. the tail group comprising the SAM-ambient interface, molecular backbone, responsible for self-assembly, and the docking group, providing the anchoring to the substrate, can be flexibly designed in context of the interfacial dipole. The most popular approach is the introduction of a dipolar tail group, with such moieties as −CF3, −C6F5 or CN increasing the work function and such groups as −CH3 or −NH2 decreasing the work function.2,3,5,6,8,11 A modification of the docking group is less frequently used, but can potentially lead to good results, especially when combined with a suitable choice of the tail group.12 Finally, the use of a dipolar backbone16 or dipolar functional group embedded in the backbone17-19 became recently a prospective issue in context of electrostatic interfacial engineering. The latter approach is particularly useful since, in contrast to the tail group modification, it allows to entangle the adjustment of the interfacial dipole and the chemical composition of the SAM-ambient interface, which is of primary importance since (i) this composition affects the growth of organic semiconductors and (ii) is prone to chemical changes upon this growth, affecting the terminal dipolar group (if this was the way to control the work function) and distorting, thus, the desirable energy level alignment. The chemical composition of the SAM-ambient interface can also be kept unchanged or only slightly altered if the molecules with the differently oriented, embedded dipolar groups are combined together as a mixed SAM. By varying the portions of both components, the work function can then be flexibly varied between the ultimate values corresponding to the onecomponent SAMs with a certain orientation of the dipolar backbone or a mid-chain dipolar group. For the dipolar backbone, this concept has been successively demonstrated by mixed monolayers of carboranethiol isomers on gold and silver: the work function of the functionalized substrates was continuously varied within a ~0.8 eV range with minimal alterations in surface energy.16 For the mid-chain group, this concept has been successively realized for aromatic, terphenyl-methanethiol derived SAMs on gold in which the central ring 3 ACS Paragon Plus Environment


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has been substituted by a 2,5-pyrimidine unit bearing a sizable dipole moment of ~2.3 Debye.18 The work function of the mixed SAMs comprising the molecules with the opposite orientations of the embedded pyrimidine unit, abbreviated as TP1-up and TP1-down, according to the direction of dipole, could be continuously varied within a ~1.0 eV range at a persistent composition of the SAM-ambient interface (terminal phenyl ring). Interestingly, this variation did not occur as a linear but as an S-shape curve as a function of the composition of both components in the primary mixed solutions. This was explained by a different composition of the films on the surface compared to the situation in solution with a 50:50 mixture being energetically favorable due to the stabilizing head to tail arrangement of the neighboring dipoles,18 i.e. due to the active participation of the dipole-dipole interaction in the balance of the SAM building forces. Another interesting aspect was the possibility to use high-resolution photoelectron spectroscopy (XPS) as a tool to monitor the electrostatic properties of the mixed SAMs and their morphology down to the molecular level.18 This approach was based on the distinctly different C 1s XPS spectra of the TP1-up and TP1-down monolayers17 in combination with state-of-the-art theoretical simulations.18 This allowed to prove the “electrostatic” homogeneity of these layers (in contrast to the phase separation), which makes possible to avoid injection hot-spots when using the SAM-modified electrodes in devices. Here, in contrast to aromatic monolayers studied in the given context so far,18 we apply the above approach to a highly promising alternative class of SAMs, viz. alkanethiolate (AT) films with a dipolar group embedded into the alkyl backbone.19-22 As such group we choose carboxylic acid ester, which can be embedded into alkyl backbone in different orientations.19,20

The

selected

molecules,

SH(CH2)10COO(CH2)9CH3

and

SH(CH2)10OOC(CH2)9CH3 (Figure 1), abbreviated as C10EC10 and C10rEC10 ("r" for reverse), respectively, form well-defined SAMs on gold with the work function differing by ~1.1 eV,19 providing, thus, an ideal platform for studying their mixture in context of electrostatic interfacial engineering, even though the respective applications can be affected to some extent by insulating properties of these, comparably thick aliphatic monolayers. The particular questions are (i) the structure of the mixed SAMs, (ii) the relation between the solution and in-SAM compositions of both constituents; (iii) tunability and exact behavior of the work function upon the variation of the SAM composition, and (iv) morphology of the mixed monolayers. Note that the single-component C10EC10 and C10rEC10 SAMs have been extensively 4 ACS Paragon Plus Environment

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characterized by infrared reflection absorption spectroscopy (IRRAS), high-resolution XPS, and scanning tunnelling spectroscopy.19,20 Accordingly, the packing and orientation of the molecules in these SAMs on Au(111) is very similar to non-substituted ATs. They exhibit (√3×√3)R30° arrangement,19 corresponding to a packing density of ~4.63×1014 cm-2.23 The overall average alkyl chain tilt from the surface normal and twist around the long axis were estimated at 31° (±4°) and 60° (±5°), respectively, with a higher conformational (all- trans) ordering for the bottom segments (between the docking and ester group) of C10EC10 and C10rEC10 and a lower ordering, with some gauche defects, for the top segments (above the ester group when going from the substrate). The alignment of the top segment is also further perturbed by the presence of the ester group itself, which introduces an additional negative or positive tilt of 7-9° to the top segment depending on the orientation of the ester group.19,20 The COO group is roughly coplanar with the C−C−C plane of the bottom and top alkyl segments.20 The dipole moment of this group is strongly tilted with respect to the molecular backbone (see Figure 1) and, consequently, with respect to the substrate. There is, however, a noticeable projection of this moment onto the surface normal, resulting in specific electrostatic properties of the mid-chain ester substituted SAMs.19,20

Figure 1. The structures of the SAM precursors used in this study, along with their abbreviations. The directions of the dipole moments of the embedded ester groups with respect to the molecular backbone are marked by blue arrows.

2. Experimental Part The SAM precursors were custom-synthesized according to the literature recipes;19,20 the exact description of the respective procedures is provided in the Supporting Information. The Au 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 Ti 5 ACS Paragon Plus Environment


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adhesion layer. The Au films were polycrystalline, exposing preferably (111) orientated surfaces of individual crystallites. The single-component C10EC10 and C10rEC10 SAMs were formed by immersion of the substrates into 1 mM solutions of the SAM precursors in ethanol (Sigma-Aldrich) under nitrogen at ambient temperature for 24 h. The mixed SAMs were prepared by coadsorption of both precursors; with their relative portions in solution being varied. After immersion, the samples were carefully rinsed with pure solvent (ethanol) and blown dry with a stream of nitrogen or Ar. In addition, reference SAMs of hexadecanethiolate (C16; SH(CH2)15CH3) and icosanethiolate (C20; SH(CH2)19CH3) were prepared on the same substrates using the same procedure as for the single-component mid-ester monolayers. The SAMs were characterized by synchrotron-based X-ray photoelectron spectroscopy (XPS) and near-edge X-ray absorption fine structure (NEXAFS) spectroscopy. The measurements were performed at the HE-SGM beamline (bending magnet) of the synchrotron storage ring BESSY II in Berlin, using a custom-made experimental station equipped with a Scienta R3000 electron energy analyzer (for XPS) and partial electron yield (PEY) detector (for NEXAFS spectroscopy).24 The experiments were performed in UHV (a base pressure of 1×10-9 mbar), at room temperature. The acquisition of the XPS spectra 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.25 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 half-maximum (fwhm), a standard25 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 SAMs and their packing densities were calculated using standard procedures,26,27 based on the C1s/Au4f and S2p/Au4f intensity ratios, respectively. For the thickness evaluation, a standard expression for the attenuation of the photoemission signal was assumed28 and the literature values for attenuation lengths were used.29 The spectrometer-specific coefficients were determined by using C16 SAM as a reference, relying on the well-known thickness of this monolayer (18.9±0.1 Å ). In a similar fashion, this SAM (4.63×1014 molecules/cm2)14 served as a reference for the packing densities evaluation. 6 ACS Paragon Plus Environment

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The acquisition of the NEXAFS spectra was carried out at the carbon and oxygen K-edges in the PEY mode with retarding voltages of −150 V and −350 V, respectively. As the primary Xray source, linearly polarized synchrotron light with a polarization factor of ~91% was used. The incidence angle of the X-rays, θ, was varied between the normal (θ =90°) and grazing (θ =20°) incidence geometry, to monitor the linear dichroism effects reflecting molecular orientation in the SAMs.30 The energy resolution was ~0.3 eV at the C K-edge and ~0.6 eV at the O K-edge. The photon energy (PE) scale was referenced to the pronounced π* resonance of highly oriented pyrolytic graphite at 285.38 eV.31 Raw NEXAFS spectra were normalized to the incident photon flux by division through a spectrum of a clean, freshly sputtered gold sample. Subsequently, these spectra were reduced to the standard form by subtracting a linear pre-edge background and normalizing to the unity edge jump (determined by a nearly horizontal plateau 40-50 eV above the respective absorption edges). The morphology and electrostatic properties of the SAMs were studied by the C 1s XPS experiments (see above for the technical details) and the work function measurements. The XPS approach was based on the established procedures.18,22 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 C16 SAM on Au were used as references.

3. Results and Discussion 3.1. Basic Characterization The XPS spectra of the single-component and mixed C10EC10-C10rEC10 films suggest the formation of the well-defined and contamination-free SAMs. The S 2p spectra of these monolayers are presented in Figure 2a; they are somewhat noisy because of the strong attenuation of the respective signal by the quite thick hydrocarbon overlayer. Nevertheless, it can be clearly distinguished that these spectra exhibit a single doublet at ~162.0 eV (S 2p3/2), representative of the thiolate species bound to noble metal substrates,32 underlying the SAMlike character of both single-component and mixed films. The intensity of this doublet does not change noticeably over the series, which suggests that the packing density of the mixed SAMs is similar to that of the single-component ones. This tentative conclusion was supported by numerical evaluation of the XPS data (see Section 2), which gave an analogous result - a packing density of (4.5 ± 0.3)×1014 cm-2 and a similar thickness of 22.8 ± 0.5 Å in all monolayers studied. 7 ACS Paragon Plus Environment


The C 1s XPS spectra of the single-component and mixed C10EC10-C10rEC10 SAMs will be discussed in next section, in context of electrostatic properties and morphology of these films. The O 1s spectra are presented in Figure 2b. The spectra, which look similar for the entire series, exhibit two emissions at 532.4 eV (1) and 533.7 (2), characteristic of the ether and carbonyl oxygen, respectively. The positions and intensities of these emissions do not change noticeably over the series, which suggest (i) small electrostatic effects on these atoms and (ii) similar packing densities of all SAMs studied. XPS: S 2p

a

hν ν = 350 eV

b

O 1s hν ν = 580 eV 2 1

C10rEC10

Intensity (arb. units)

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C10rEC10 25%-75% 25%-75% 50%-50% 50%-50% 75%-25%

75%-25%

C10EC10 C10EC10 168 166 164 162 160

535

530

Binding energy (eV)

Figure 2. S 2p (a) and O 1s (b) XPS spectra of the single-component and mixed C10EC10 and C10rEC10 SAMs (open circles). The compositions of the mixed SAMs reflect the relative portions of C10EC10 (first number) and C10rEC10 (second number) in the solutions, from which the SAMs were grown. The spectra in panel (a) are fitted by a single doublet at ~162.0 eV (S 2p3/2) (orange solid lines) and a background (blue solid lines). The spectra are shifted vertically for comparison. The positions of the S 2p doublet (S 2p3/2) and the component O 1s peaks are tracked by the red vertical dashed lines. Component peaks in the O 1s spectra are marked by numbers (see text for details). C K-edge and O K-edge NEXAFS data for the single-component and mixed C10EC10C10rEC10 SAMs (and also for the reference C20 monolayer) are presented in Figures 3 and 4, respectively. The data combine the 55° spectra, which are exclusively characteristic of the electronic structure of the films,30 and the difference (90°-20°) spectra, which are representative of orientational order and molecular orientation in the films.30 The spectra of 8 ACS Paragon Plus Environment

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the single-component and mixed C10EC10-C10rEC10 SAMs look very similarly, suggesting, in accordance with the XPS data, similar structure of these films.

a1

NEXAFS: C K-edge

2

b

1

90°-20°

55°

C20

3

4

C20


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3 2

C10rEC10

C10rEC10

25%-75% 25%-75%

50%-50% 50%-50%

75%-25% 75%-25%

C10EC10 C10EC10

290

300

310

290

300

310

Photon energy (eV)

Figure 3. C K-edge NEXAFS data for the single-component and mixed C10EC10 and C10rEC10 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 angle of 90° and 20° (b). The compositions of the mixed SAMs reflect the relative portions of C10EC10 (first number) and C10rEC10 (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. The 55° C K-edge spectra of the single-component and mixed C10EC10-C10rEC10 SAMs in Figure 3a are dominated by a double peak feature comprised by a resonance at ∼287.7 eV (1) frequently associated with predominantly Rydberg states of the alkyl segments33 (see ref 34 for a discussion regarding the alternative assignments) and the π(C*=O) resonance at 288.5 eV (4) related to the ester group.20,35 The former resonance is also well visible in the spectrum of the reference C20 SAM, along with the characteristic resonances at ∼293.4 eV (2) and ∼301.6 eV (3) related to valence, antibonding C−C σ* and C−C' σ* orbitals of the alkyl chain, 9 ACS Paragon Plus Environment


respectively.34,36 The latter resonances are also present in the spectra of single-component and mixed C10EC10-C10rEC10 SAMs, as can be expected since they also contain alkyl segments. The 55° O K-edge spectra of the single-component and mixed C10EC10C10rEC10 SAMs in Figure 4a are dominated by a π(C=O*) resonance at 531.9 eV (1) related to the embedded ester group,37,38 accompanied by several weaker resonances with π* (2) and σ* (3) character at higher photon energy (see ref 37 for the assignments). NEXAFS: O K-edge 55°

b

a 90°-20°

C20 C10rEC10 1


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3 2

C10rEC10 25%-75% 25%-75% 50%-50% 50%-50% 75%-25% 75%-25%

C10EC10

C10EC10 520 530 540 550 560

520 530 540 550 560

Photon energy (eV)

Figure 4. O K-edge NEXAFS data for the single-component and mixed C10EC10 and C10rEC10 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 angle of 90° and 20° (b). The compositions of the mixed SAMs reflect the relative portions of C10EC10 (first number) and C10rEC10 (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. These resonances in both C an O K-edge spectra exhibit pronounced linear dichroism, i.e. intensity dependence on X-ray incidence angle, as evidenced by the appearance of the intense peaks at the positions of these resonances in the difference spectra in Figures 3b and 4b. This suggests a high orientational order both in the single-component C10EC10 and C10rEC10 10 ACS Paragon Plus Environment

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SAMs (in agreement with the previous work)19,20 and in the respective mixed SAMs, with an upright orientation of the molecular chains, as follows from the specific signs of the difference peaks in the 90°-20° spectra.20 Quantitative evaluation of the NEXAFS data in context of molecular orientation was performed with the difference method for the C K-edge and the standard intensity-vs-angle approach for the O K-edge (see ref 30 for the general description of these approaches). The difference method allowed to avoid spectra decomposition, non-trivial for the C K-edge spectra in the given case. The most prominent R* and π(C=O*) resonances with the transition dipole moments (TDMs) perpendicular to the alkyl chain and the COO plane of the embedded ester group, respectively, were considered. Within the difference methods,30,39 upon subtracting two NEXAFS spectra recorded at different x-ray incidence angles θ and θ1 one gets for a plane orbital, such as the R* one, Ip(θ) - Ip(θ1) = C (1 - 3 cos2γ)(cos2θ - cos2θ1),

(1)

where θ is the incidence angle of X-rays, γ is the angle between the sample normal and the normal to the R* plane (i.e. the molecular tilt angle), Ip(θ) and Ip(θ1) are the resonance intensities, and C is a normalization constant which depends on the excitation probability from the C 1s core level into a given molecular orbital. Even though the latter parameter is unknown, expression (1) can be used for a comparison of similar molecular orbitals in different systems or even for the quantitative evaluation of the NEXAFS spectra, if one has a reference sample with the same molecular orbitals and a known molecular structure. Ip(θ )-Ip(θ 1) (arb. units)

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4.0

C10EC10 75%-25% 50%-50% 25%-75% C10rEC10

3.5 3.0 2.5 2.0 1.5 1.0 0.5 0.0 -0.8

-0.6

-0.4

-0.2

cos2(θ)-cos2(θ1)

Figure 5. Intensity differences for the R* resonance in the C K-edge NEXAFS spectra of the single-component and mixed C10EC10 and C10rEC10 SAMs versus cos2(θ)-cos2(θ1). 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 11 ACS Paragon Plus Environment


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C10EC10 (first number) and C10rEC10 (second number) in the solutions, from which the SAMs were grown. Regretfully, such a reference sample is not available in the given case since the intensity of the R* resonance depends on the alkyl chain length due to delocalization of the respective molecular orbital over the entire chain,34,36 but the orbitals of both segments in the mid-ester molecules can be somehow coupled resulting in an intensity value somewhere between those for C10 and C20. At the same time, we can compare the single-component and mixed films and calculate the tilt angles of the mixed SAMs using the known values of the tilt angles for the single-component films as a reference. Accordingly, Ip(θ) - Ip(θ1) for all these films are plotted in Figure 5 as functions of cos2θ - cos2θ1, with the slopes of the respective straight lines representative of the values of γ (see Eq 1). As seen in this plot, the lines for the singlecomponent C10EC10 and C10rEC10 SAMs have similar slopes, suggesting, in accordance with the literature data,19,20 similar molecular inclination (31°±4°, averaged over the both segments). The lines for the mixed SAMs exhibit similar slope as well, which is even slightly steeper as compared to that for the single-component films and composition independent. Thus, the molecular inclination and orientational order in the mixed SAMs are similar to those in the single-component films and are probably even slightly better. The average molecular tilt angle in the mixed SAMs was estimated at ~27°±4°, same for all compositions. Within the evaluation of the O K-edge data, the intensity of the π(C=O*) resonance was plotted as a function of θ and compared to a theoretical dependence for a vector-type orbital followed the standard approach.30 This procedure resulted in the similar average tilt angles of the π(C=O*) orbital in the single-component and mixed mid-ester SAMs, viz. 27°±4°, in good agreement with the previous data for the C10EC10 monolayer.20 But the major conclusion of this evaluation, along with that for the C K-edge NEXAFS data (see above), is that the mixed mid-ester films have similar orientational order and molecular inclination as the singlecomponent ones.

3.2. Morphology and Electrostatic Properties As shown recently,22,18 XPS can be used as a sensitive tool to study polar SAMs, including mixed ones. The approach is based on electrostatic effects in photoemission, exhibited in the C 1s spectra.22 These spectra for the single-component mid-ester SAMs and reference C20 monolayer are presented in Figure 6. The spectrum of the latter film is typical of non12 ACS Paragon Plus Environment

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substituted AT SAMs, exhibiting a single peak stemming from the alkyl backbone at a BE of ~285.0 eV. This peak has a certain "fine structure" but it is hardly perceptible and can only be recognized within a very detailed analysis.32,40 In contrast to C20, the C 1s spectrum of the C10EC10 SAM exhibits not one but two major peaks at ~284.6 eV (1) and ~285.6 eV (2), related to the bottom and top alkyl segments, respectively, and also the comparably weak peaks at 286.8 eV (3) and 289.2 eV (4) associated with the ether and carbonyl carbon, respectively. The "splitting" of the single peak into two upon the mid-chain ester substitution stems from the potential discontinuity (step) inside the monolayer associated with the embedded dipolar ester group.19,20,22 This step occurs in the opposite direction in the case of the C10rEC10 SAM, leading to the shift of the top segment peak toward that of the bottom segment. As a result, both these peaks overlap in the single one (1+2), shifted to a lower BE (~284.65) as compared to the reference C20 case. The minor peaks related to the ether (3) and carbonyl (4) carbon are visible as well, with only slightly different BEs as compared to the spectrum of the C10EC10 monolayer.

Figure 6. C 1s XPS spectra of the C10EC10, C10rEC10, and C20 (reference) SAMs acquired at a photon energy of 580 eV. The spectra of the ester substituted monolayers are decomposed in component peaks, highlighted by different colors and marked by numbers. See text for details. The spectra presented in Figure 6 were acquired at a photon energy of 580 eV, allowing to see the peak stemming from the bottom segment of the C10EC10 and C10rEC10 SAMs due to a moderate attenuation of the respective signal at the given kinetic energy of the photoelectrons. 13 ACS Paragon Plus Environment


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The electrostatic effects in the mixed SAMs can, however, be better monitored using the spectra acquired at a lower photon energy since, because of the strong attenuation of the photoelectron signal, they are almost exclusively representative of the top alkyl segment of the SAMs, reflecting the electrostatic effect of the embedded ester groups most clearly. Such spectra, collected at a photon energy of 350 eV for the single-component and mixed C10EC10 and C10rEC10 SAMs, are presented in Figure 7. The spectra of the single-component films are indeed dominated by the peak associated with the top alkyl segment (with similar widths for the C10EC10 and C10rEC10 films) and all other features being comparably weak. The spectra of the mixed monolayers should represent a superposition of those of the singlecomponent films, with a proper impact of the electrostatic effects. The character of the superposition depends, however, on the film morphology, viz. the kind of intermolecular mixing.18 A homogeneous mixing will result in a homogeneous potential step within the monolayer, with the amplitude defined by the weighted sum of the individual dipoles, depending on the mixing ratio. Accordingly, the BE shift and consequently the BE position of the peak related to the top alkyl segment should change continuously with the mixing ratio, between the ultimate values for the C10EC10 and C10rEC10 SAMs. In contrast, in the case of the phase separation and sufficiently large C10EC10 and C10rEC10 domains, they will contribute individually into the spectra and they will represent a linear combination of the single-component spectra, with the weights corresponding to the composition of the mixed films. In the case of the phase separation but small domains, down to the molecular level, the spectra will generally behave similarly to the case of the homogeneous intermolecular mixture (see above), but will be broadened as compared to the single component case.18 The experimental spectra of the mixed SAMs in Figure 7 are clearly dominated by a single peak (accompanied by minor features) with a continuous shift in the peak position with the varied mixing ratio. These spectra differ distinctly from the simulated spectra for the phase separation in large domains, shown as well in Figure 7. Moreover, as demonstrated in Figure 8, the fwhms of the dominant peak in the spectra of the mixed films are, within the experimental error, identical to those for the single-component films. This suggests that the scenario of the phase separation can be excluded and the mixed SAMs represent homogeneous mixtures of the molecules.


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25%-75%

50%-50%

75%-25%

C10EC10 290

288

286

284

Binding energy (eV)

Figure 7. C 1s XPS spectra of the single-component and mixed C10EC10 and C10rEC10 SAMs monolayers of different compositions (open circles), along with simulations of the latter spectra for the case of the phase separation (solid red lines). The compositions of the mixed SAMs reflect the relative portions of C10EC10 (first number) and C10rEC10 (second number) in the solutions, from which the SAMs were grown. The positions of the dominant component peak, representative of the top segment of the assembled C10EC10 and C10rEC10 molecules, are tracked by a vertical dashed line. In this situation, the BE shift of the dominant peak in the C 1s spectra should represent a fingerprint of the mixing ratio in the monolayer, which can be different from that in solution. To monitor this effect, the BE position is plotted as a function of the SAM composition in Figure 8. The observed dependence has a linear character (within the error of the experiment), which suggests that the composition in solution is mimicked in the SAMs. Interestingly, this situation is distinctly different from the aromatic SAMs with the embedded dipolar pyrimidine group, where a 50%-50% composition is preferable for the mixed SAMs, due to the stabilizing head to tail arrangement of the neighboring dipoles (see Section 1).18 Obviously, for the C10EC10 and C10rEC10 mixtures, the impact of the dipole-dipole interaction on the SAM composition is much lower, presumably, in view of much stronger contribution associated with the intermolecular interaction of the comparably long alkyl segments. 15 ACS Paragon Plus Environment

285.8

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Peak width (eV)

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C 1s binding energy (eV)


0.0

284.6 0

20

40

60

80

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Figure 8. Dependence of the BE position (red circles, left axis) and FWHM (blue diamonds, right axis) of the dominant component peak in the C 1s XPS spectra of the single-component and mixed C10EC10 and C10rEC10 SAMs on the portion of C10EC10 in the solutions, from which the SAMs were grown. The linear dependence of the BE position and the constancy of the fwhm are highlighted by the color-coded dashed lines. The continuous change in the mid-SAM potential step, traced indirectly by the C 1s spectra, could be directly monitored by the measurement of the work function of the single-component and mixed C10EC10 and C10rEC10 SAMs. The respective results are presented in Figure 9, along with the analogous data for the pyrimidine-substituted monolayers18 given for comparison. In full agreement with the C 1s XPS data, the work function of the mixed SAMs varies continuously and in a linear fashion between the ultimate values for the C10EC10 monolayer (3.83 eV) and C10rEC10 film (4.92 eV). This behavior is in a striking contrast to the S-like curve for the pyrimidine-substituted monolayers, reflecting the predominance of the 50%-50% composition (see above). Thus, the work function data support and evidence the conclusion that the solution composition is fully mimicked in the SAMs at the coadsorption of the C10EC10 and C10rEC10 precursors. On the other hand, from the practical viewpoint, these data suggest that the work function of gold can be varied continuously and in controlled fashion in the ~1.1 eV range by mixed C10EC10-C10rEC10 monolayers.


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Work function (meV)

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5000 4800 4600 4400 4200 4000 3800 3600 0

20

40

60

80

100

C10rEC10 (%)

Figure 9. Dependence of the work function of the single-component and mixed C10EC10 and C10rEC10 SAMs (blue circles) on the portion of C10rEC10 in the solutions, from which the SAMs were grown. The linear character of this dependence is highlighted by the blue dashed line. For comparison, an analogous curve for the aromatic SAMs with embedded pyrimidine group is given (red dashed line);18 this curve is shifted by +0.15 eV to make the different behavior better pronounced.

5. Conclusions We studied structure, molecular orientation, morphology, and electrostatic properties of the mixed C10EC10-C10rEC10 SAMs on gold by using several complementary spectroscopic techniques and work function measurements. According to the XPS and NEXAFS spectroscopy data, the mixed SAMs are well defined and contamination-free. The packing density and molecular orientation in these SAMs is similar to those of the "parent" singlecomponent monolayers. Applying XPS as a morphology tool, we could estimate that the mixed C10EC10-C10rEC10 SAMs represent homogeneous intermolecular mixtures of both components, down to the molecular level. The analysis of the C 1s XPS spectra and the work function data suggests that the compositions of the mixed SAMs fully mimicked the mixing ratio of both components in solutions from which these SAMs were prepared. This behavior is associated with comparably small contribution of the dipole-dipole interaction between the embedded ester groups into the entire balance of the structure-building forces in the C10EC10-C10rEC10 SAMs comprised of the quite long molecules. From the practical viewpoint, the data presented in this work suggest that the mixed C10EC10-C10rEC10 SAMs allow to tune the work function of the gold substrate in controlled fashion within a ~1.1 eV range, between the ultimate values for the C10EC10 and C10rEC10 monolayers, viz. 3.83 eV and 4.92 eV, respectively, and with persistent chemical 17 ACS Paragon Plus Environment


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composition at the SAM-ambient interface. Due to the intermolecular mixing, the SAMinduced work function change is absolutely homogeneous, down to the molecular level, excluding formation of hot or cold spots for injection of the charge carriers as far as these SAMs are used for the energy level alignment at the interfaces between electrodes, buffer layers, and organic semiconductor in organic solar cells, organic light emission diodes, and organic transistors. Of course, aliphatic SAMs have certain limitations in terms of inferior electric transport properties as compared to aromatic ones,41-43 but they probably can be used in certain cases, when these properties are not of primary importance or when a methyl termination and persistent interfacial chemistry are of particular significance. From the principal viewpoint, this work demonstrates that not only aromatic but also aliphatic mixed SAMs with mid-chain substituted dipolar groups can be successively used for the work function control and tuning at surfaces and interfaces.

Associated content Supporting Information The Supporting Information is available free of charge on the ACS Publication website at DOI: Exact description of the procedures for the synthesis of the SAM precursors. (PDF).

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 experiments there. The work was financially supported by 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. 18 ACS Paragon Plus Environment

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