Spectroscopic Characterization of Fluorinated Benzylphosphonic

Abraham , F.; Ford , W. E.; Scholz , F.; Nelles , G.; Sandford , G.; von Wrochem , F. Surface Energy and Work Function Control of AlOx/Al Surfaces by ...
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Spectroscopic Characterization of Fluorinated Benzylphosphonic Acid Monolayers on AlOx/Al Surfaces William E. Ford,*,† Ffion Abraham,§ Frank Scholz,† Gabriele Nelles,† Graham Sandford,§ and Florian von Wrochem*,† †

Sony Europe Limited, Materials Science Laboratory, Hedelfinger Strasse 61, Stuttgart 70327, Germany Department of Chemistry, Durham University, South Road, Durham DH1 3LE, United Kingdom

§

S Supporting Information *

ABSTRACT: We recently reported on how the surface energy and work function of AlOx/Al substrates can be tuned by selfassembled monolayers of fluorinated and nonfluorinated benzylphosphonic acid derivatives in view of organic electronic applications. In this contribution, we present a thorough investigation of these monolayers by photoemission (XPS) and infrared (PM-IRRAS) spectroscopies, to provide a quantitative understanding of their structural properties (packing density and orientation) and chemical composition. A detailed analysis of XPS chemical shifts makes an assignment of the carbon species present in the SAMs feasible, from the low-binding-energy aromatic carbon (∼284.5 eV) to the highly electronegative fluorine-substituted carbon (∼287.5 eV), whereby an upper limit for the fraction of nonspecific hydrocarbons (1050 cm−1 due to the PMIRRAS background absorption band associated with vibrations of Al−O in the AlOx layer at 955 cm−1, where P−O−H and P− O−Al also occur.71,100 The symmetric and asymmetric stretching bands of the PO32− group are found in the 1200− 1000 cm−1 range, but these tend to be quite broad.100 Therefore, the νPO band is left as the primary vibration within the PO3 moiety to provide information about bonding within the series of SAMs investigated herein. The PO stretching vibration of PA derivatives is found in the 1300−1200 cm−1 range. This band appears in the powder (KBr) spectra of 9, 10, 2, and 11 at 1263, 1253, 1285, and 1267 cm−1, respectively (Table 1). The only SAM spectrum in which the νPO band clearly appears is that of SAM 6 (at 1276 cm−1, Figure S2). This information alone does not distinguish

between the possible binding modes, but it is useful when considered together with the scissoring vibration of the CH2 group. The band for the scissoring vibration, which is found in the powder spectra of all 11 PAs at 1407 ± 7 cm−1 (Figure S3), is absent in all 11 SAM spectra (Figure S2). Ideally, the TDM for this vibration should be parallel to both the methylene H− C−H plane and the P−C−C plane, but there are deviations due to vibronic couplings with adjacent groups. Nearly ideal is the mode at 1401 cm−1 of 9 (Figure S4), where there is only a minor deviation because of contributions from the ring. A larger deviation is seen in the modes at 1413 and 1417 cm−1 for 11 (Figure S7), where the TDM is nearly perpendicular to the plane of the P−C−C group due to strong coupling of the CH2 scissor with the ν15 ring mode. In the tridentate binding mode, the P−C bond is expected to be normal to the AlOx/Al surface, so that the TDM of the scissor vibration is ∼35° from the surface normal, whereas in the bidentate mode the TDM is ∼90° from the surface normal (i.e., parallel to the surface).32 Our IR data thus support the bidentate over the tridentate mode. This conclusion applies to the monodentate configuration as well, where the P−C bond is also expected to be normal to the surface (Figure 9), but the single P−O−Al bond connection makes orientation of the P−C bond subject to alteration by intermolecular interactions, such as hydrogen bonding of the free P−OH or PO groups.85,95,99 Preferential binding through bidentate mode as suggested above is consistent with the absence of the νPO band in the SAMs (except for SAM 6), because the TDMs of the PO and CH2 vibrations have similar directions (Figures S4−S7). However, we note that the protonation of the bonded PO3 group (Figure 9c) or hydrogen bonding of PO with a neighboring Al−OH (Figure 9d) or P−OH (Figure 9e) group might also result in attenuation of the νPO band.85,95,99

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CONCLUSIONS We have presented a detailed XPS and PM-IRRAS survey of monolayers of 10 benzylphosphonic acid derivatives on AlOx/ Al. These PAs differ in the number of phenyl rings (1 or 2), the fluorine substitution pattern (2, 4, or 5), and/or in terminal methoxy ring substitution. Dodecylphosphonic acid was included in this study as a reference. From XPS, we conclude that well-defined SAMs were formed with the expected chemical composition. A careful analysis of C 1s chemical shifts enabled us to draw conclusions on the molecular structure within the SAMs as well as on the upper limits to the amount of nonspecific contaminants. IR characterization of all PAs was carried out, both as monolayers and in the bulk, where DFT calculations of four representative PAs made it possible to identify several IR bands that are characteristic for benzylphosphonic acids. In particular, a distinct three-band pattern due to the benzyl CH2 group has been identified involving a Fermi resonance interaction between the symmetric J

DOI: 10.1021/acs.jpcc.6b11089 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C stretching vibration and the first overtone of the CH2 scissoring vibration. We are aware of only one previous example in which the IR signature of the benzyl CH2 group has been unambiguously identified in a SAM.92 Furthermore, absence of the CH2 scissoring and PO stretching bands in the PMIRRAS spectra indicate that the PO3 moiety chemisorbs to the AlOx/Al substrate via a bidentate rather than tridentate or monodentate mode.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.6b11089. Additional XPS and IR data, as well as transition dipole moments and vibrational modes from DFT simulations (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

Graham Sandford: 0000-0002-3266-2039 Florian von Wrochem: 0000-0003-2298-9270 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank Josef Michl and Michael Zharnikov for helpful discussions and Felix Hanke from Biovia for his support in the graphical representation of transition dipole moments from DFT calculations.

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DOI: 10.1021/acs.jpcc.6b11089 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.jpcc.6b11089 J. Phys. Chem. C XXXX, XXX, XXX−XXX