ARTICLE pubs.acs.org/JPCC
Modification of Self-Assembled Monolayers of Perfluoroterphenyl-Substituted Alkanethiols by Low-Energy Electrons Frederick Chesneau,† Hicham Hamoudi,† Bj€orn Sch€upbach,‡ Andreas Terfort,‡,* and Michael Zharnikov†,* † ‡
Angewandte Physikalische Chemie, Universit€at Heidelberg, 69120 Heidelberg, Germany Institut f€ur Anorganische und Analytische Chemie, Universit€at Frankfurt, Max-von-Laue-Strasse 7, 60438 Frankfurt, Germany ABSTRACT: Using self-assembled monolayers (SAMs) of perfluoroterphenyl-substituted alkanethiols (FTPn) on Au(111) as test systems, we studied the effect of low-energy (10 eV) electron irradiation on fluorinated aromatic SAMs. FTPn films were found to mimic the typical behavior of aromatic hydrocarbon SAMs under ionizing radiation. The dominant process is the cleavage of C-F bonds in the FTP moieties followed by desorption of the released fluorine atoms, rapid conformational and orientational disordering, and cross-linking between the residual skeletons of the FTP backbones. The stability of these skeletons and the development of a cross-linked network hinder other typical irradiation-induced processes such as desorption of molecular fragments and damage of the headgroupsubstrate interface. Along with these findings, FTPn SAMs gave a unique possibility to monitor the destiny of the fluorine atoms, which are analogues of the hydrogen atoms in the respective hydrocarbon systems. The extent of fluorine release upon irradiation was estimated and the cleavage of C-F bonds was found to be the main channel of this release, which is in striking contrast to fluorocarbon aliphatic monolayers for which such a release is dominated by the desorption of fluorocarbon fragments. Finally, irradiation-induced reorientation of FTPn films at very small irradiation doses was observed suggesting that their structure is kinetically trapped and can be relaxed toward the thermodynamic minimum by physical means.
1. INTRODUCTION Molecular self-assembly has become a standard and widely spread tool in macromolecular chemistry, surface engineering as well as micro- and nanofabrication. Among other approaches, 2D assembly of amphiphilic molecules on solid substrates, denoted usually as formation of a self-assembled monolayer (SAM), became increasingly popular because it allows tailoring of the surface and interfacial properties and serves as the basis for numerous applications like for example control of biofouling, sensor fabrication, and molecular electronics.1-5 Further options, such as SAM-based lithography,6-17 fabrication of metalorganic multilayer systems,18 or preparation of free-standing nanosheets19,20 are provided by physical modification of SAMs by different means, including, above all, ionizing radiation and UV light. This requires a precise knowledge of the SAM response, which can be quite complex, to the primary physical tool. In particular, exposure of SAMs to ionizing radiation results in a variety of closely interrelated processes occurring at different rates, namely damage to the tail groups, partial decomposition of the SAM constituents, desorption of hydrogen and molecular fragments, orientational and conformational disordering, damage to the headgroup-substrate interface, and cross-linking within the residual film.10,21-23 The exact course and kinetics of these processes as well as the efficiency of the irradiation treatment depend upon the specific molecular architecture of the SAM constituents, their packing density, and the identity of r 2011 American Chemical Society
the substrate.6,7,10,24,25 Among these parameters, the packing density and - closely related - the perfection of the crystallographic structure directly influence the quenching probability of the primary excitation, diminishing the effect of radiation for densely packed films and SAMs of better crystallinity.25 The substrate material determines the strength of the headgroupsubstrate bond and the secondary electron yield,26 which adds to the primary X-ray or electron flux giving the total irradiation load.27 However, the molecular architecture has the largest effect; above all the identity of the molecular spacer is of importance because it primarily determines the balance between the decomposition and disordering on one side and cross-linking on the other side.10,15 In the case of aliphatic spacers, the decomposition and disordering processes prevail resulting, even at moderate irradiation doses, in the progressive loss of the film identity.21,22,24 In contrast, in the case of aromatic spacers, the cross-linking processes are dominant, transforming the primary molecular film into a 2D polymer-like layer thus enabling the selective modification of the tail group without disintegration of the monomolecular film.6,7,25 The dominance of the cross-linking processes in the given case is due to the stability of the aromatic skeleton. Whereas C-H bonds in the phenyl rings can be easily Received: December 9, 2010 Revised: January 24, 2011 Published: February 24, 2011 4773
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The Journal of Physical Chemistry C cleaved by electrons or X-rays, the skeletons themselves remain intact and cross-link with the neighboring moieties.6,10,23,28 The above results have been obtained for aromatic hydrocarbons, whereas, to the best of our knowledge, no data on irradiation-induced modification of analogous fluorocarbon systems are available. In the present work, we aimed to eliminate this drawback and studied the effect of low-energy electron irradiation (10 eV) on SAMs of perfluoroterphenyl-substituted alkanethiols, C6F5-C6F4-C6F4-(CH2)n-SH, n = 2, 3 (FTP2 and FTP3, respectively) on Au(111). Note that the results obtained for low-energy electrons are valid for X-rays and deep/extreme UV as well because the major impact of X-ray or deep/extreme UV light results from photoelectrons as well as inelastic and secondary electrons.10,26 As for the systems of this study, apart from the higher reactivity and electronegativity of fluorine relative to hydrogen (e.g. ref 29) and higher bond energy of C-F as compared to C-H,29-32 these films differ from the respective hydrocarbon monolayers by the conformation of the oligophenyl moiety (helical in fluorocarbons and presumably planar in hydrocarbons) and larger spacing between the SAM constituents (e.g., by ∼10% for FTP3/Au relative to the analogous hydrocarbon system).33 Note that the above differences are mostly related to the fact that the van der Waals radius of the fluorine atom is significantly larger than that of hydrogen. Along with the above-mentioned common features with the hydrocarbons, FTP2/Au and FTP3/Au also have some differences between one another. Most importantly, FTP3/Au is characterized by a denser (by ∼10%) molecular packing and smaller (by ∼12°) molecular inclination as compared to FTP2/Au.33 Both of these differences are related to the different number of CH2 groups in the alkyl linker of the respective molecules, which result in the so-called odd-even effects.34-44 In the following section, we describe the experimental procedure and techniques. The results are presented and briefly discussed in section 3. An extended analysis of the data is given in section 4, followed by a summary in section 5.
2. EXPERIMENTAL SECTION FTP2 and FTP3 were synthesized according to the protocol given in ref 45. The gold substrates were prepared by thermal evaporation of 30-100 nm of gold (99.99% purity) onto polished single-crystal silicon (100) wafers (Silicon Sense) primed with a 5 nm titanium adhesion layer (rate 2 nm/s, temp. 340 °C).46 The resulting metal substrates were polycrystalline, with predominant (111) orientation and a grain size of 20-50 nm. The SAMs were prepared according to the protocol in ref 33, that is by immersion of the freshly prepared substrates into a 1 mM solution of either FTP2 or FTP3 in THF at room temperature for 24 h. After immersion, the samples were carefully rinsed with pure solvent and blown dry with argon. In addition to the FTPn films, we prepared several reference systems, namely, SAMs formed from dodecanethiol (C12), partially fluorinated alkanethiol (CF3(CH2)9(CH2)2SH, abbreviated below as F10H2), and terphenyl-substituted alkanethiol (C6H5-C6H4-C6H4-(CH2)2-SH, abbreviated below as TP2). The same substrates as for the FTPn SAMs were used. The preparation was performed according to the protocols of refs 47 (C12), 48 (F10H2), and 37 (TP2). Modification of C12/Au,24 F10H2/Au and analogous films,49-51 and systems close to TP2/Au52,53 by electrons has already been studied. We preferred, however, to repeat some of the respective measurements under the
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exact conditions of our experiments to have direct comparison to the FTPn SAMs. The electron irradiation was carried out in UHV with a flood gun, which was mounted at a distance of ∼15 cm from the samples to ensure a uniform illumination over the whole sample area. The energy of the primary electron beam was set to 10 eV and the current density was kept at ∼3.5 μA cm-2 and was monitored with a Faraday cup. Several selected doses up to 31 mC/cm2 were used at which point most of the irradiationinduced effects level off.10 The doses were estimated by multiplication of the exposure time by the current density. The pristine and irradiated films were characterized by laboratory-based X-ray photoelectron spectroscopy (XPS), synchrotron-based XPS, and near-edge X-ray absorption fine structure (NEXAFS) spectroscopy, and water contact angle goniometry allowing us to monitor the irradiation-induced effects both in the fluorocarbon/hydrocarbon matrix and at the SAM-substrate interface. The laboratory-based XPS characterization was carried out in situ, that is without exposure of the irradiated samples to ambient; the synchrotron-based XPS and NEXAFS experiments were conducted ex situ after transportation of the preliminary irradiated samples to the synchrotron radiation facility, where these experiments were performed (below). The measurements were carried out under UHV conditions at a base pressure better than 1.5 10-9 mbar. All experiments were performed at room temperature. The spectra acquisition time was selected in such a way that no noticeable damage by the primary X-rays occurred during the measurements.10,27,54,55 Laboratory-based XPS measurements were performed using a Mg KR X-ray source and an LHS 11 analyzer. The spectra acquisition was carried out in normal emission geometry with an energy resolution of ∼0.9 eV. The X-ray source was operated at a power of 240 W and positioned ∼1.5 cm away from the samples. The synchrotron-based XPS measurements were performed at the HE-SGM beamline (bending magnet) of the synchrotron storage ring BESSY II in Berlin, Germany, using a Scienta R3000 spectrometer. The synchrotron light served as the X-ray primary source. The spectra acquisition was carried out in normal emission geometry with an energy resolution of ∼0.3 eV. The energy scale of the XPS spectra was referenced to the Au 4f7/2 peak at a binding energy (BE) of 84.0 eV.56 The spectra were fitted by symmetric Voigt functions and a Shirley-type background. To fit the S 2p3/2,1/2 doublet, we used two peaks with the same full width at half-maximum (fwhm), the standard56 spin-orbit splitting of ∼1.18 eV (verified by fit), and a branching ratio of 2 (S2p3/2/S2p1/2). The fits were performed self-consistently: the same fit parameters were used for identical spectral regions. In addition, XPS data were used to determine the effective thickness of the target SAMs. This was done on the basis of the Au 4f intensity, assuming a standard exponential attenuation of the photoelectron signal and using the attenuation lengths reported in ref 57. The latter should be valid in the case of FTPn SAMs because the introduction of fluorine does not significantly affect the attenuation length of the Au4f electrons through the SAM.58 The NEXAFS measurements were performed at the same beamline as the synchrotron-based XPS experiments. The spectral acquisition was carried out both at the carbon and the fluorine K-edge (the latter data not shown) in the partial electron yield (PEY) mode with retarding voltages of -150 and -450 V, respectively. Linear polarized synchrotron light with a polarization factor of ∼91% was used. The energy resolution was ∼0.30 eV. 4774
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The Journal of Physical Chemistry C The incidence angle of the light was varied from 90° (E-vector in the surface plane) to 20° (E-vector nearly normal to the surface) in steps of 10°-20° to monitor the orientational order of the FTPn molecules within the films. This approach is based on the linear dichroism in X-ray absorption, that is, the strong dependence of the cross-section of the resonant photoexcitation process on the orientation of the electric field vector of the linearly polarized light with respect to the molecular orbital of interest.59 The raw NEXAFS spectra were normalized to the incident photon flux by division by a spectrum of a clean, freshly sputtered gold sample. Further, the 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 energy scale of the C K-edge spectra was referenced to the most intense π* resonance of highly oriented pyrolytic graphite (HOPG) at 285.38 eV.60 Advancing and receding contact angles of Millipore water were measured with a Kr€uss goniometer Model G1. The measurements were performed under ambient conditions. At least three measurements at different locations on each sample were made. The averaged values are reported. Deviations from the average were less than (1°. In addition to the spectroscopic experiments and contact angle measurements, resist performance of the FTPn SAMs was tested. For this purpose, the SAMs were first patterned through a mesh in proximity printing geometry and then etched for 10 min in a ferrocyanide-based solution.61 The imaging of the fabricated patterns was conducted using a LEO 1530 scanning electron microscope. The e-beam energy was 5 keV, and the residual gas pressure was ∼5 10-6 mbar.
3. RESULTS FTP2/Au and FTP3/Au exhibited similar behaviors in the majority of the irradiation-induced processes, which is presumably related to the similar chemical compositions and only moderate differences in the packing density and molecular inclination in these systems.33 Therefore, in some cases, we present the results for one of these systems only, representative of both FTP2/Au and FTP3/Au. 3.1. XPS. C 1s and F 1s XPS spectra of FTP3/Au acquired in the course of exposure of this film to electrons are shown in the left and right panels of Figure 1. The C 1s spectrum of the pristine film exhibits three emissions at ∼284.2, ∼285.3, and ∼287.2 eV assigned to the carbon atoms in the aliphatic linker (284.2 eV) and FTP moiety (285.3 and 287.2 eV), respectively. The ∼285.3 eV emission is associated with the carbon atoms in the para positions (except for the terminal carbon), whereas the ∼287.2 eV emission is related to the carbon atoms in the ortho and meta positions and the terminal carbon atom.33 The F 1s XPS spectrum of pristine FTP3/Au exhibits a single emission at a BE of ∼687.3 eV. This emission can be clearly assigned to the fluorine atoms of the perfluoroterphenyl (FTP) moiety. The individual components of the C 1s spectrum behave differently in the course of the irradiation treatment. The emission related to the carbon atoms directly bonded to fluorine (C-F) decreases in intensity, whereas the intensities of the emissions assigned to the residual carbon atoms in the FTP moiety (C-C) and aliphatic linker (C-H) increase. This suggests a partial cleavage of the C-F bonds in the FTP moiety with subsequent
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Figure 1. C 1s (left panel) and F 1s (right panel) XPS spectra of FTP3/ Au acquired in the course of exposure of this film to electrons (open circles) along with the corresponding fits by several components (solid lines, see the text for details). The spectra were acquired at the synchrotron. The individual components are assigned. The irradiation doses are indicated at the respective curves.
desorption of the released fluorine, presumably in atomic or ionic form. 50,62 The additional peaks observed at ∼288.2 eV and ∼290.1 eV at higher doses (>10 mC/cm2) can be associated with the reattachment of released fluorine atoms to the rings under the formation of CF2 groups. The 290.1 eV peak can be assigned to CF2 species themselves, whereas the 288.2 eV peak can be ascribed to the C-F carbon adjacent to the CF2 moiety. The above assignments are supported by the fact that these peaks were also observed in in situ XPS spectra of the extensively irradiated FTPn films (data not shown). The positions of these features agree well with those of the analogous species produced during the electron irradiation and plasma deposition of hexafluorobenzene films.63-65 Note that the CF2 related peaks are weak (
