Structure-Building Forces in Biphenyl-Substituted Alkanethiolate Self

Article pubs.acs.org/JPCC

Structure-Building Forces in Biphenyl-Substituted Alkanethiolate Self-Assembled Monolayers on GaAs(001): The Effect of the Bending Potential Hao Lu*,† and Michael Zharnikov* Applied Physical Chemistry, Heidelberg University, Im Neuenheimer Feld 253, 69120 Heidelberg, Germany ABSTRACT: Molecular assembly on a technologically relevant GaAs substrate is an important and application-related issue. In this context, self-assembled monolayers (SAMs) formed from a series of ω-(4′-methylbiphenyl-4-yl)alkanethiols, CH3(C6H4)2(CH2)nSH (BPn, n = 1−6), were prepared on GaAs(001) and characterized in detail by highresolution X-ray photoelectron spectroscopy and near-edge X-ray absorption fine structure spectroscopy. The resulting films exhibited pronounced, “odd−even” variation in molecular orientation and packing density with the number (n) of methylene groups in the alkyl linker; viz., smaller molecular inclination, associated with a higher packing density, was observed for an odd n, while the opposite was the case for an even n. Such an odd−even behavior confirms once again the existence of a bending potential for GaAs(001), which is equivalent to the analogous potential for the Au(111) substrate, where similar odd−even behavior has been observed. This potential plays an important role in the balance of the structure-building interactions, predefining the orientation of the alkyl linker, transferred in an odd−even fashion, depending on the parity of n, to the adjacent biphenyl spacer. The above effects were found to be superimposed onto pronounced dependence of the film quality on the length of the BPn precursor. This occurred due to proneness of the GaAs substrate to oxidation, hindering an efficient self-assembly, as well as due to a limited ability of the shortchain monolayers to protect this sensitive substrate from the postpreparation oxide regrowth. A proper selection of the parameters is, thus, very important for the design of functional monomolecular films on GaAs.

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INTRODUCTION Self-assembled monolayers (SAMs) of organic molecules on solid substrates attract significant attention due to their technological relevance and as important test systems for fundamental research.1 The lateral density and structure of these films stem from a complex interplay of several factors, viz., intermolecular interaction, the anchor group−substrate interaction, corrugation of the binding energy hypersurface, and mismatch between the optimal molecular lattice and the structural template provided by the substrate.2 At a given mismatch and the corrugation of the binding energy hypersurface, the intermolecular interaction is frequently believed to play a dominant role as compared to the anchor group− substrate interaction, as was in particular assumed for SAMs of nonsubstituted alkanethiolates (ATs) on coinage metal substrates. These films exhibit intermolecular lattice spacing of ∼5 and ∼4.67−4.77 Å on Au(111) and Ag(111), respectively, accompanied by a molecular tilt of ∼27−35° on Au(111) and ∼10−12° on Ag(111).1−9 The tilt can then be considered as the only means to achieve the optimal interchain spacing (∼4.4 Å), characteristic of the respective bulk materials, whereas the substrate−S−C moiety is assumed to behave as a “free joint”, adapting to an optimal geometrical configuration and enabling the necessary molecular tilt.2,10 Even though the above model provides a simple and straightforward explanation of the experimental data, it contradicts the results of the recent studies which give © 2015 American Chemical Society

unequivocal evidence for the preferable binding geometry of the substrate−S−C joint, favoring bond angles of ∼104° and ∼180° for Au(111) and Ag(111) substrates, respectively.11−19 This geometry was assumed to be governed by the substratedependent bending potential, entering the balance of the structure-building interactions and determining, to a significant extent, the different tilt angles of the alkyl chains on Au and Ag, in both nonsubstituted and oligophenyl-substituted AT SAMs.11−19 Unambiguous proof for the existence of such a potential is so-called odd−even effects occurring in phenylsubstituted SAMs,12 methyl-terminated biphenyl-substituted SAMs,13−17 and terphenyl-substituted AT SAMs18,19 as well as in analogous alkaneselenolate monolayers which, in this regard, behave similarly to the AT films.20 In all these systems one observes a dependence of the molecular inclination and, consequently, the packing density of the SAM constituents on the parity of the number of methylene units in the alkyl linker. In particular, on Au, a higher packing density, associated with smaller molecular inclination, was obtained for an odd number of methylene moieties in the linker, while the opposite was the case for an even number. Such a behavior, confronting the thermodynamically favorable maximization of the packing density, could only be explained assuming a predefined, Received: July 21, 2015 Revised: November 11, 2015 Published: November 12, 2015 27401

DOI: 10.1021/acs.jpcc.5b07067 J. Phys. Chem. C 2015, 119, 27401−27409

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The Journal of Physical Chemistry C

observed.44 The direction (phase) of these effects was similar to that on Au(111) rather than on Ag(111), which was surprising in view of the difference in molecular inclination in the nonsubstituted AT SAMs (∼14°) as compared to Au (∼30°) and its similarity with respect to Ag (∼10−12°).1,9 One has however to consider that the molecular organization on the latter two substrates occurs differently from that on GaAs (see above), so that one cannot make a conclusion on the character of the bending potential on the basis of the molecular inclination only. In any case, additional proof for the existence and the direction (phase) of the bending potential for the GaAs substrate is highly desirable. A suitable reference in this regard is biphenyl-substituted AT SAMs, which are archetypical and the most studied systems in the context of the odd−even effects on coinage metal substrates.11,13−17 Along these lines, we prepared a series of biphenyl-substituted AT SAMs on GaAs(001) and characterized these films in detail by advanced spectroscopic techniques such as high-resolution X-ray photoelectron spectroscopy (HRXPS) and near-edge X-ray absorption fine structure (NEXAFS) spectroscopy. The major goal, as mentioned above, was to examine the universality of the Au-like bending potential for GaAs. In addition, compared to the previously studied terphenyl-substituted AT SAMs,44 we wanted to look at the effect of the shorter molecular backbone (biphenyl plus a short alkyl linker) on the molecular organization of the monolayers.

preferential orientation of the alkyl linker, associated with a certain, preferable substrate−S(Se)−C angle. The presence of such a preferable angle, which was assumed to be ∼104° for the Au substrate, suggested the existence of a certain bending potential at the substrate−S(Se)−C joint, as mentioned above. Interestingly, the relation between the molecular orientation and packing density on one side and the parity of the alkyl linker length on the other side is the opposite on Ag as compared to Au, suggesting a different value of the preferable substrate−S(Se)−C angle, viz., ∼180°. In contrast to the SAMs on coinage metal supports, there is a limited amount of studies concerning molecular assembly on semiconductor substrates (except for Si), in particular in terms of the structure-building interactions. A particular interesting system in this regard is GaAs, representative of the technologically important column III−V semiconductors. GaAs is known to enable flexible band gap engineering and is also well suitable for heteroepitaxy, thus providing a versatile platform for nanofabrication as well as for the design of chemical sensors.21−25 An essential constraint to realize these promising applications is chemical instability of the GaAs surface, which can be, in particular, overcome by its functionalization with thiolate SAMs; such films can not only passivate the underlying GaAs and protect it from oxidation, but also modify its surface properties in a desirable fashion. In this context, a variety of thiolate SAMs have been prepared on GaAs and characterized in terms of the structure and properties, including nonsubstituted26−33 and substituted34−38 AT monolayers, as well as nonsubstituted34,39,40 and substituted41−43 aromatic thiolate SAMs. Nevertheless, very few studies among those mentioned above dealt with the mechanism behind the molecular assembly on this quite specific substrate. In particular, recently, Dubowski et al. used density functional theory to study the structure and bonding of AT SAMs on GaAs.32,33 At the same time, Allara et al. addressed the assembly mechanism on GaAs in great detail by the example of nonsubstituted AT SAMs with variable chain length, putting a special focus on an octadecanethiol (ODT) monolayer on GaAs(001).28−31 According to the experimental data, this film represents a pseudohexagonal overlayer with alkyl chains tilted at ∼14° with respect to the surface normal; the adopted molecular arrangement is, however, incommensurate with the underlying GaAs(001) substrate in both the symmetry (hexagonal versus square) and the lateral dimensions (5.02 Å versus 3.995 and 5.65 Å).28,30 Driven by the intermolecular interaction, the above mismatch can be overcome by the surface reconstruction during the molecular assembly, but only within a relatively small domain range (∼10 nm). Accordingly, nonsubstituted AT SAMs with a shorter (as compared to ODT) chain length, associated with the smaller intermolecular interaction, exhibit a lower packing density and reduced conformational order, along with a partial oxidation of the underlying GaAs substrate.29,30 The above mechanism assumes that the intermolecular interaction drives the molecular assembly, disregarding a possible existence of the bending potential, which is, as discussed above, of importance in the case of the coinage metal substrates. However, recently, the existence of such a potential has also been evidenced for thiolate SAMs on GaAs(001), taking a series of terphenyl-substituted AT monolayers, (C6H5)(C6H4)2(CH2)nSH (TPn) with n = 1−6, as a test example.44 For these films, pronounced odd−even effects affecting the molecular inclination and packing density were

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EXPERIMENTAL SECTION Chemicals and Substrates. ω-(4′-methylbiphenyl-4-yl)alkanethiols, CH3(C6H4)2(CH2)nSH (BPn, n = 1−6), were custom synthesized according to the literature procedure;14 the molecular structure of these precursors is depicted in Scheme 1. Scheme 1. Molecule Structure of the BPn (n = 1−6) Precursors

In addition to the above compounds, we also used ODT, CH3(CH2)17SH, as a reference system. ODT was purchased from Fluka Chemicals and used as received. Single side polished n+-type doped GaAs(001) wafers were used as substrates. They were purchased from American Xtal Technologies, Fremont, CA, and delivered from the manufacturing center in Guangzhou, China. According to the specification, the Si dopant density (Ne) in these wafers was (1.5−1.6) × 1018 cm−3. SAM Preparation. Since the GaAs surface, including GaAs(001) in particular, is prone to rapid and extensive oxidation as well as to reconstruction toward more stable surface termination,31 preparation of high-quality SAMs on this particular substrate requires careful selection of the experimental conditions. On the basis of previous experience38,44 and test experiments, involving optimization of individual factors such as the etching agent, washing step, and solvent, the most suitable preparation procedure was selected. Within this procedure, GaAs substrates were first sonicated in absolute ethanol and then etched in concentrated HCl (37%) for 1 min to remove the native oxide layer, with subsequent rinsing in degassed absolute ethanol to remove residual acid. The resulting substrates exhibited a surface As/Ga ratio of 86/100 27402


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results in a characteristic dependence of the absorption resonance intensity on the incidence angle of X-rays if there is a predominant molecular orientation or distinct orientational order in the system studied.53 Note that a tilt angle of ∼14° observed for the ODT SAMs on a high-quality GaAs(001) substrate31 was well reproduced in our NEXAFS experiments, implying the validity of the preparation procedure and a comparable quality of the substrates.

as measured by XPS, which agrees well with the results of previous studies.37 A root-mean-square roughness was estimated at 0.35 nm for a scan area of 1 × 1 μm2, which was only slightly higher than the analogous value for the pristine GaAs (0.21 nm). The freshly etched substrates were immediately immersed into a 0.1 mM solution of BPn (3 mM for ODT)31 in degassed anhydrous ethanol and incubated there for 24 h at room temperature. All preparation steps were carried out under an inert gas (Ar) atmosphere to reduce surface oxidation. After the SAM formation, the samples were taken out of the reactor, sonicated briefly in absolute ethanol to remove physisorbed molecules, dried in an argon flow, and preserved in inert-gas-filled containers until the characterization. SAM Characterization. The BPn and ODT SAMs on GaAs, prepared by the above procedure, were characterized by HRXPS and NEXAFS spectroscopy. All experiments were performed at room temperature and under ultrahigh vacuum conditions at a base pressure better than 1.5 × 10−9 mbar. The spectral acquisition time was optimized and kept sufficiently short to avoid irradiation-induced damage, possible during the experiments involving X-rays.45,46 HRXPS measurements were carried out at the HE-SGM beamline (bending magnet) of the synchrotron storage ring BESSY II in Berlin. The spectra were acquired in normal emission geometry using a Scienta R3000 spectrometer attached to a custom-designed experimental station.47 A photon energy (PE) of either 350 or 580 eV was selected, depending on the binding energy (BE) range, characteristic energy resolution, and desirable sampling depth.48−50 The spectral data acquired at higher PE (580 eV) provide information within a larger sampling depth, and were accordingly used to estimate the effective thickness of the SAMs. The energy resolution was ∼0.3 eV at a PE of 350 eV, and somewhat lower at a PE of 580 eV. The BE scale was referenced to the As 3d5/2 emission of the bulk GaAs at a BE of 41.1 eV.29,40,41 The spectra were decomposed into individual emissions using Voigt peak profiles and a Shirley background. The decomposition was performed self-consistently over the entire data set. As 3d, Ga 3d, and S 2p emissions were fitted by two peaks with the same full width at half-maximum (fwhm), a suitable spin−orbit splitting (0.69, 0.43, and 1.18 eV, respectively),51 and standard branching ratios, viz., 2/1 for 2p3/2/2p1/2 and 3/2 for 3d5/2/3d3/2. NEXAFS spectroscopy measurements were also performed at the HE-SGM beamline of the BESSY II facility. This beamline provides linearly polarized synchrotron light with a polarization factor of ∼91%. The spectra were measured at the carbon K-edge in the partial electron yield (PEY) mode. A retarding voltage was set to −150 V. The PE scale was referenced to the pronounced π* resonance of highly oriented pyrolytic graphite at 285.38 eV.52 The energy resolution was ∼0.3 eV. The incidence angle of the synchrotron light was varied from 90° (normal incidence geometry) to 20° (grazing incidence geometry) in steps of 10−20°. In the former case, the E vector was parallel to the surface plane, whereas, in the latter case, this vector was almost perpendicular to this plane. This procedure was used to monitor the orientational order within the molecular films. It relied on so-called linear dichroism in Xray absorption, which is generally defined as a strong dependence of the cross-section of the resonant photoexcitation process on the orientation of the transition dipole moment (TDM) of the molecular orbital with respect to the electric field vector of the linearly polarized light.53 This effect

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RESULTS HRXPS. The formation of the BPn SAMs was first monitored by their As 3d and Ga 3d HRXP spectra taken at a PE of 350 eV, as shown in Figure 1 for n = 2, 3, 5, and 6. At

Figure 1. As 3d (a) and Ga 3d (b) HRXP spectra of the BPn SAMs on GaAs(001) (n = 2, 3, 5, 6). The spectra were measured at a PE of 350 eV. The spectra are decomposed in several doublets associated with individual chemical species in the samples studied. Stoichiometric GaAs, elementary As, and S−As components are marked by light gray, dark gray, and black shading, respectively, in the As 3d spectra. Stoichiometric GaAs and gallium oxide or surface Ga 3d are marked by light gray and dark gray shading, respectively, in the Ga 3d spectra. The nonshaded shoulders at the higher BE side of the major features are assigned to arsenic (a) and gallium (b) oxides. The assignments were made in accordance with refs 29, 38, 40, 41, and 44.

this PE, the attenuation length of the photoelectrons (λ) is quite small,54 so that the effective sampling depth (∼3λ) is small as well and the acquired spectra have a larger spectral contribution from the topmost part of the samples. The spectra in Figure 1 are decomposed into individual components, with the parameters listed in Table 1. These spectra are dominated by an intense doublet related to the stoichiometric GaAs. It is accompanied by less intense features associated with the elemental As in the As 3d spectra (Figure 1a) and with a gallium oxide or surface Ga 3d component in the Ga 3d case (Figure 1b). Most importantly, in the As 3d case, an additional doublet at ∼42.3 eV, assigned to the S−As bond,29,38,40,41,44 should be introduced to reproduce the shape of the experimental spectra. By contrast, an analogous feature related to the possible S−Ga bond is not perceptible in the Ga 3d spectra. However, this type of bond has been observed by timeof-flight secondary ion mass spectrometry (ToF-SIMS) for an ODT SAM on GaAs(001)26,27 and, therefore, cannot be completely excluded. One can also mention in this context that a well-ordered and densely packed ODT SAM could be formed 27403


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The Journal of Physical Chemistry C Table 1. Parameters of the Individual Spectral Components in the As 3d and Ga 3d HRXP Spectra in Figure 1a core level As 3d5/2

Ga 3d5/2

binding energy (eV) 41.1 41.82 ± 42.31 ± >43 19.20 ± 19.80 ± >19.8

0.08 0.1 0.02 0.02

assignment

fwhm (eV) (PE = 350 eV)

spin−orbit splitting (eV)

branching ratio

GaAs elementary As As−S arsenic oxides GaAs Ga2O3 or surface Ga 3d component gallium oxides

0.69 ± 0.02

0.69

3/2

0.57 ± 0.01

0.43

3/2

a The parameters were derived from a fitting procedure performed in a self-consistent fashion (see the Experimental Section). The error bars reflect the scattering of the fitting parameters upon the decomposition of the experimental spectra for different samples. The assignments were made in accordance with refs 29, 38, 40, 41, and 44.

on the Ga-terminated GaAs(111) substrate, even though their quality was inferior to that of the analogous monolayer prepared on the As-terminated GaAs(111) surface.28 In addition to the characteristic features discussed above, broad shoulders related to oxide species were always perceptible at the high BE side of the major emissions; they could be related to oxide residuals surviving the etching procedure and subsequent self-assembly,29 but, most likely, resulted from postoxidation during the storage and handling of the samples before the spectroscopic experiments. Despite the passivation by the BPn SAMs, penetration of airborne oxidative species into the underlying GaAs was still possible, mediated, in particular, by defects in the monomolecular films. Significantly, the relative intensity of the oxide-stemming shoulder decreased with increasing n, which, as expected, suggests that the thicker films provide a better protection against the postpreparation oxidation. An additional factor affecting the extent of the surface oxidation is the so-called self-cleaning process taking place upon the molecular self-assembly.29 This process, occurring presumably at both As and Ga binding sites, can result not only in the removal of surface contamination but, likely, also in a partial reduction of residual (after the etching procedure) oxide, with its efficiency increasing with increasing molecular length, due to the stronger thermodynamical drive for the self-assembly. Even though the underlying GaAs substrates were partly oxidized, the BPn SAMs were generally contamination-free and well-defined, as evidenced by the S 2p and C 1s HRXP spectra in parts a and b, respectively, of Figure 2. In particular, the S 2p spectra of all these films exhibit a single S 2p3/2,1/2 doublet at a characteristic BE position of ∼162.4 eV (S 2p3/2), associated with the thiolate-type sulfur bonded to the GaAs substrate in the SAM fashion, while no unbound, disulfide, or oxidized sulfur species were perceptible. The BE value of ∼162.4 eV is close to the analogous values observed for the thiolate SAMs on GaAs(001),29,40−42,44 but is noticeably higher than those for the thiolate monolayers on noble metal substrates (∼162.0 eV).55 This difference can be explained by the final state effects in photoemission, viz., efficient screening of the photoemission hole by conduction electrons of a metal substrate (such as Au or Ag), resulting in the lowering of the effective BE as compared to that of a less conductive semiconductor such as GaAs.29,38,44 The fwhm value of the S 2p3/2 and S 2p1/2 components is ∼0.9 ± 0.05 eV. This value is close to the reported ones (0.9−1.1 eV) for thiolate SAMs on GaAs,29,38 but is noticeably higher than the analogous values (0.55−0.6 eV) for AT SAMs on coinage metal substrates.55,56 This can be explained by a structural and chemical heterogeneity of the GaAs(001) surface, resulting in superposition of several

Figure 2. S 2p (a) and C 1s (b) HRXP spectra of the BPn SAMs on GaAs(001) (n = 1−6). The spectra were measured at a PE of 350 eV. The S 2p spectra are fitted by a doublet with the standard intensity ratio and spin−orbit splitting (see the Experimental Section). The C 1s spectra are decomposed into the main emission and a high BE shoulder (see the text for details).

different bonding configurations associated with specific, slightly different binding energies.29 The C 1s XP spectra of the BPn SAMs in Figure 2b are dominated by a strong emission at a BE of 284.5−284.6 eV assigned to the biphenyl moiety,40,41 accompanied by a weak shoulder at a higher BE (∼0.6 eV shift). Such a shoulder is typical for SAMs with an oligophenyl backbone18,55 and BPn monolayers in particular.57 It can be either assigned to the αcarbon of the alkyl linker (i.e., the carbon atom bonded to the sulfur headgroup) or associated with the shakeup process in the aromatic matrix;55,57 the latter explanation seems to be more likely taking into account the small sampling depth in our experiments at the given PE (350 eV).54 Significantly, no signature of carbon-related contamination including CO, COOH, etc. could be detected. Consequently, the partial oxidation only involved the GaAs substrate, prone to this process. In addition to the above analysis of the spectra, the effective thickness of the BPn SAMs was estimated. For this purpose, spectroscopic data acquired at PE of 580 eV (not shown) were used. The thickness values (dsample) were calculated on the basis of the C 1s/As 3d intensity ratios,38,56,58 according to the standard expression 27404


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Figure 3. HRXPS-derived effective thicknesses (a) and normalized intensities of the S 2p signal (b) for the BPn SAMs on GaAs(001) (n = 1−6). IC 1s (sample) IAs 3d IC 1s (ref) IAs 3d

1 − exp = exp

−dsample

λC 1s(EC) −dsample

λAs 3d(EAs)

exp

explained by a progressive increase in the packing density with increasing length of the SAM precursor. This assumption relies on, potentially, better the self-assembly ability of the longer molecules, which becomes especially crucial for such sensitive substrates as GaAs. NEXAFS Spectroscopy. A NEXAFS experiment involves excitation of core-level electrons into unoccupied molecular orbitals, which are characteristic of specific bonds, functional groups, or the entire molecule in a particular system. The respective absorption resonances offer then a distinct spectroscopic fingerprint of these entities, while the linear dichroism effects, monitored upon the variation of the incidence angle of the primary X-ray beam (see the Experimental Section), provide information on the orientational order and molecular orientation in the system, averaged over the entire ensemble.53 Figure 4a shows carbon K-edge NEXAFS spectra of the BPn SAMs acquired at an X-ray incidence angle of 55°. At this specific“magic angle”geometry, the acquired spectra are not affected by the molecular orientation but are only

−dref λAs 3d(EAs)

1 − exp

−dref λC 1s(EC)

(1)

An ODT SAM with a well-known thickness (drefer) of 23.8 Å was used as a reference.30,31 The attenuation lengths of the C 1s (λC 1s) and As 3d (λAs 3d) photoelectrons were taken at 11.3 and 16.7 Å, respectively, in agreement with the respective kinetic energies, viz., 290 eV (EC) for C 1s and 534 eV (EAs) for As 3d.59 The derived values of the effective thickness are presented in Figure 3a as a function of n. Following the increase in the length of the molecular backbone, the effective thickness increases with increasing n, exhibiting, at the same time, pronounced and systematic zigzag deviations from the general trend. Such a systematic variation is obviously associated with the parity of n, manifesting the existence of the odd−even effects in the BPn SAM on GaAs(001). Larger thickness values (as compared to the general trend), associated with a higher packing density, are observed for the monolayers with the odd n, whereas the opposite is the case for the monolayer with the even n. Note that this behavior correlates well with the smaller molecular inclination in the former case as will be shown below, upon consideration of the NEXAFS data. The odd−even variation of the effective thickness and, consequently, of the effective packing density is supported further by the data in Figure 3b, which show the normalized intensities of the S 2p emission in the BPn SAMs. These intensities exhibit clear odd−even variation, superimposed on a continuous decrease with increasing n (e.g., from n = 1 to n = 3, and further to n = 5). The direction of the odd−even variation in Figure 3b is, however, opposite that observed in Figure 3a, which can appear strange at first glance. It should be noted, however, that this intensity is affected by two factors, viz., the packing density of the thiolate headgroups and the attenuation of the S 2p signal by the hydrocarbon overlayer. These factors work in opposite directions since a higher packing density is associated with a higher effective thickness and, consequently, with a stronger attenuation of the photoemission signal. Therefore, obviously, the effect of the attenuation is stronger than the factor of the packing density, resulting in the inversion of the odd−even behavior. This assumption agrees with the literature data for analogous SAMs on noble metal substrates,57,60 including the BPn monolayers,57 and is also logical in view of a particularly strong attenuation at the relevant kinetic energy. The continuous decrease of the normalized S 2p intensity with increasing n can be tentatively

Figure 4. C K-edge NEXAFS spectra of the BPn SAMs on GaAs(001) measured at an X-ray incidence angle of 55° (a) as well as at the normal (90°; solid line) and grazing (20°; shaded) incidence of X-rays (b). The most intense resonance (π1*) is marked. 27405


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The Journal of Physical Chemistry C representative of the electronic structure of the studied films.53 As seen in the figure, the spectra are typical of high-quality SAMs with an oligophenyl backbone or substitution;18,44 they are dominated by the strong π1* resonance of the phenyl rings at a PE of ∼285.1 eV, accompanied by the respective π2* resonance at ∼288.8 eV, the R*/CS* resonance at about 287.8 eV, and several broad σ* resonances at higher PEs (the assignments were performed in accordance with the literature data).61 Significantly, there are no features related to contamination, including a sharp and distinct resonance at a PE of ∼288.5 eV characteristic of COOH (the most typical contamination).40 Note that the resonances associated with the alkyl linker in the BPn molecules, including the most intense R* feature at ∼287.6 eV,53,62 cannot be clearly distinguished since they are comparably weak due to the attenuation of the PEY signal by the topmost parts of the SAMs, as well as its overlapping with the absorption edge and overshadowing by the stronger resonances from the biphenyl moiety. The above analysis of the magic angle data was performed in parallel with the qualitative analysis of the angular dependence of the NEXAFS spectra, looking specifically for the linear dichroism effects, to study the molecular orientation and orientational order in the BPn SAMs. This dependence can be in particular emphasized by plotting the difference between the spectra acquired at normal and grazing incidence of X-rays, corresponding to incidence angles of 90° and 20°, respectively. Such difference spectra are shown in Figure 4b; they exhibit significant linear dichroism with higher intensity of the π* resonances at 90° compared to 20° and the opposite behavior of the σ* resonance intensity, better visible at large n. Considering that the TDMs of the π* resonances are perpendicular to the plane of the phenyl rings while the TDMs of the σ* resonances are parallel to this plane, this behavior suggests the expected, upright orientation of the biphenyl moieties in the BPn films. By close comparison, it is noted that the dichroic intensity difference is larger for the SAMs with an odd n, implying an odd−even variation in the orientation of the biphenyl moieties. These qualitative considerations were complemented by numerical analysis of the entire set of the NEXAFS spectra, performed in accordance with the standard theoretical framework.44,53 Within this procedure, the most prominent absorption resonance (π1*) was selected and its intensity I was monitored as a function of the X-ray incidence angle θ. The respective dependence was analyzed on the basis of the standard theoretical expression for a vector-type orbital:

where γ is the so-called twist angle of the biphenyl backbones, describing rotation of these moieties around the 4,4′-axis.63 This angle was assumed to be 32° on the basis of the value for the respective bulk material64 as well as other relevant literature data.63 The derived values of β for the BPn SAMs are presented in Figure 5. They exhibit pronounced odd−even behavior,

Figure 5. Average tilt angles of the biphenyl moieties in the BPn SAMs on GaAs(001). The values were derived from the NEXAFS data, assuming a twist angle of 32°. The error bars correspond to the accuracy of the theoretical fits. The accuracy of the NEXAFS experiment is generally lower and is usually estimated at ±3−5°.

suggesting smaller molecular inclination for the odd n and larger inclination for the even n, which agrees well with the odd−even trend in the film thickness in Figure 3a; viz., a smaller molecular inclination corresponds to a thicker and more densely packed film and vice versa. In addition, as a general trend overimposed on the odd−even variation, molecular inclination becomes smaller with increasing n, which is especially obvious for n = 5 and 6. This suggests, in accordance with the HRXPS data (see the previous section), a comparably higher quality of the monolayers formed by the longer molecular precursors.

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DISCUSSION Similar to the terphenyl-substituted AT (TPn) SAMs on GaAs(001),44 systematic odd−even effects with the analogous dependence on the parity of n were observed in the archetypical BPn (n = 1−6) monolayers on the same substrate; viz., a smaller molecular inclination along with a higher packing density occurs in the films with an odd n, whereas the opposite is the case for an even n. The above relation to the parity of n, which can be described as a direction or a “phase” of the odd− even effects, is the same as that reported for the biphenyl- and terphenyl-substituted AT SAMs on Au(111) but is opposite that for Ag(111), where a smaller molecular inclination and higher packing density are obtained for even n.14,18 The observed odd−even behavior is most likely caused by the existing, Au-like bending potential affecting the geometry of the substrate−S−C anchoring group in the thiolate SAMs on GaAs. This potential favors a preferable GaAs−S−C bond angle of ∼104°,44,56 and predefines, thus, the orientation of the alkyl linker, which, as far as the all-trans conformation is maintained, “transfers” this preferable orientation to the adjacent biphenyl moiety. Accordingly, a small inclination of these moieties is favored at an odd n, as illustrated in Figure 6, allowing the energetically preferred high packing density, so that the bending potential works cooperatively with the other

⎧ 1⎡ ⎤ 1 I(α , θ ) = A ⎨P ⎢1 + (3 cos2 θ − 1)(3 cos2 α − 1)⎥ ⎦ ⎩ 3⎣ 2 ⎫ 1 + (1 − P) (sin 2 α)⎬ ⎭ 2 (2)

where A is a constant, P is a polarization factor of the primary X-ray beam, and α is the average tilt angle of the molecular orbital. To avoid a normalization problem and to simplify the procedure, not the absolute intensities of the π1* resonance but the intensity ratios I(θ)/I(90°) were analyzed, where I(θ) and I(90°) are the intensities of this resonance at X-ray incidence angles of θ and 90°, respectively. The derived average tilt angles of the π1* orbitals could be easily converted to the average tilt angle (β) of the biphenyl moieties using the equation63 cos α = sin β cos γ

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of the GaAs surface to oxidation. The SAMs prepared from longer constituents are presumably capable of removing a part of the surface-bound oxygen upon molecular assembly (selfcleaning) as well as to provide a better protection against the postpreparation oxidation (oxide regrowth). This effect can be clearly seen by the given example of the BPn series and is even perceptible in the case of terphenyl-substituted ATs on GaAs, even though to a lesser extent.44 Note that the analogous effect was also observed for the nonsubstituted AT SAMs on GaAs(001): the densely packed and thick ODT SAMs were found to be significantly more effective in protecting the interface from the postoxidation compared to the thinner and less dense dodecanethiolate SAMs.29 Finally, as mentioned above, the bending potential that plays an important role in both BPn and TPn SAMs on GaAs is overcompensated by other factors in the case of the nonsubstituted AT monolayers on this substrate. Indeed, the latter systems exhibit a tilt angle of ∼14°,36 which is closer to that for the nonsubstituted AT SAMs on Ag(111) (∼10− 12°)1,9,11 than on Au(111) (∼30°),7,8 implying rather an Aglike bending potential. The latter is certainly not the case, so that the effect of the Au-like bending potential, well visible for both BPn and TPn SAMs, is obviously overcompensated by other factors in the case of the nonsubstituted AT monolayers on GaAs(001).44

Figure 6. Schematic cartoon illustrating the molecular orientation and packing in the BPn SAMs on GaAs(001).

structure-building interactions. However, the opposite is the case at an even n: the preferable binding geometry favors a larger inclination of the biphenyl moieties, hindering the energetically preferred dense molecular packing, so that the structure-building interactions work competitively, resulting in a compromise molecular arrangement with a lower packing density. The respective stress is then partly released by either deviations from the optimal GaAs−S−C bond angle or a disturbance of the all-trans conformation of the alkyl linkers. Despite the similarity of the odd−even effects for GaAs(001) and Au(111), there are some principal differences. First, these effects are weaker in the case of GaAs, as revealed by the smaller odd−even changes in both molecular tilt (6−9° versus 12−17° for Au and Ag) and packing density (7−10% versus 10−15% for Au and Ag).13,14 These differences are presumably related to a larger structural and chemical heterogeneity of the GaAs surface, which cannot be as well controlled and cleaned as the surface of the coinage metals. Consequently, there is most likely a certain distribution of bonding configurations in the case of GaAs, diminishing the effect of the bending potential. As mentioned in the section “HRXPS”, the existence of such a distribution, favored also by the noncommensurate character of the molecular lattice,28 is reflected by the comparably large fwhm of the S 2p3/2,1/2 emissions (∼0.9 eV versus 0.55−0.6 eV for Au and Ag55). Note that a possible relation between the bending potential and specific adsorption geometry (bonding configuration) is still not clear at the moment since there are no dedicated studies of this issue and only a limited theory work on odd−even effects in SAMs in general (see, e.g., ref 65). The situation is additionally complicated by the fact that the exact adsorption geometry is known neither for organothiols on GaAs(001) nor even for such well-studied systems as nonsubstituted AT SAMs on Au(111).66 The binding energy values are, however, comparable for these two types of substrates, viz., ∼2.1 eV for GaAs33 and 1.6−2.3 eV for coinage metals,5,67 so that a similar behavior, such as the existence of the bending potential, can be expected. Second, along with the pronounced odd−even behavior, a continuous deterioration in the SAM quality, in terms of the packing density and molecular inclination, was observed on going from the BP6 to BP1 film in the case of GaAs (as illustrated in Figure 6), whereas such a tendency was not perceptible in the BPn monolayers on Au(111). The major reason for this behavior is, as mentioned above, the proneness

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CONCLUSIONS Archetypical BPn (n = 1−6) SAMs were prepared on a GaAs(001) substrate and characterized in detail by complementary spectroscopy techniques, viz., synchrotron-based HRXPS and NEXAFS spectroscopy. With varying n, systematic odd−even variations in the molecular arrangement were observed; viz., a smaller molecular inclination and higher packing density occurred at an odd parity of n, while the opposite was the case at an even n. This behavior mimics that of the terphenyl-substituted AT (TPn) SAMs on GaAs(001), underlining the generality of the odd−even effects in the substituted AT monolayers on this technologically important substrate. The occurrence of these effects represents evidence for the existence of a bending potential at the SAM/GaAs interface, favoring a certain orientation of the alkyl linker, transferred to the attached oligophenyl moiety. Significantly, the relation between the molecular structure and the parity of n in the BPn and TPn SAMs on GaAs(001) corresponds to that for the analogous systems on Au(111), with a favored GaAs− S−C angle of ∼104°, even though the orientation of the nonsubstituted AT monolayers on GaAs(001) is similar to that on Ag(111), where the opposite phase of the odd−even effects takes place. Along with the pronounced odd−even behavior, the quality of the BPn SAMs exhibits a well-perceptible dependence on the length of the SAM precursors, with a higher packing density, a smaller molecular inclination, and less oxidation of the underlying GaAs substrate for the larger n as compared to the shorter molecules. This effect is typical of GaAs and is explained by the superior molecular assembly and self-cleaning ability for the longer molecules as well as by a better protection of the underlying substrate from postpreparation oxide regrowth provided by such monolayers. We believe that the above findings give valuable insight into molecular assembly on GaAs, relevant for the design of functional monomolecular films on this scientifically interesting and technologically important substrate. 27407


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AUTHOR INFORMATION

Corresponding Authors

*Phone: +49-6131-379-365. E-mail: [email protected]. *Phone: +49-6221-54 4921. Fax: +49-6221-54 6199. E-mail: [email protected]. Present Address †

H.L.: Max Planck Institute for Polymer Research, Ackermannweg 10, 55128 Mainz, Germany. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank Manfred Buck and Piotr Cyganik for providing us with the synthesized BPn precursor molecules. We are obliged to A. Nefedov and Ch. Wö ll (Karlsruhe Institute of Technology, KIT) for the technical cooperation at BESSY II and recognize the assistance of the BESSY II staff during the synchrotron-related experiments. This work has been supported financially by the Deutsche Forschungsgemeinschaft (German Research Foundation, DFG), Grant ZH 63/17-1, and a China Scholarship Council (CSC) scholarship to H.L.

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