Nickel Deposition on Fluorinated, Aromatic Self-Assembled Monolayers

Article pubs.acs.org/JPCC

Nickel Deposition on Fluorinated, Aromatic Self-Assembled Monolayers: Chemically Induced Cross-Linking as a Tool for the Preparation of Well-Defined Top Metal Films Frederick Chesneau,† Andreas Terfort,‡ and Michael Zharnikov†,* †

Angewandte Physikalische Chemie, Universität Heidelberg, 69120 Heidelberg, Germany Institute of Inorganic and Analytical Chemistry, University of Frankfurt, Max-von-Laue- Str. 7, 60438 Frankfurt/Main, Germany

‡

ABSTRACT: We studied the deposition of metal atoms (Ni as a test adsorbate) on fluorinated self-assembled monolayers (SAMs) using films of perfluoroterphenyl-substituted alkanethiols, C6F5(C6F4)2(CH2)3SH (FTP3), and partly fluorinated alkanethiols, F(CF2)10(CH2)2SH (F10H2), on Au(111) as representative test systems. Unlike the F10H2 films, their FTP3 counterparts were found to stop efficiently the penetration of nickel atoms into the SAM. The primary process is the Ni-mediated loss of fluorine atoms followed by extensive cross-linking between the partly defluorinated FTP backbones. The stability of these backbones and the rapid development of the intermolecular cross-linking, affecting predominantly the topmost part of the FTP3 SAM, are the key components to hinder the metal penetration. The chemically induced cross-linking in combination with the entirely reactive SAM represents a new concept to prepare a well-defined metal film at the SAM−ambience interface. This can be useful in context of novel metal/SAM-insulator/metal assemblies that are of potential interest for electronic and spintronic applications.

1. INTRODUCTION The fabrication of nanoelectronic components has been the subject of intense research in the past 20 years.1−4 Metal− insulator−metal (MIM) assemblies are a promising class of such items which, in addition to being of academic interest, have found applications as building blocks of displays,5 antenna-coupled detectors,5 and spintronic devices.6 The latter devices, which use not only the electron’s charge but also its spin, bring the promise of nonvolatile memory, enhanced processing speed, and lower power consumption for a variety of applications.6,7 Generally, a key element of a MIM device is the insulating dielectric layer (1−3 nm) prepared e.g. by plasma oxidation of crystalline metal films.8 The thickness and quality of this layer control, to a large extent, the electrical and magnetic properties as well as the performance of the entire assembly.5 Along with inorganic materials, organic ones can be used for the preparation of the dielectric layer. In particular, fabrication of MIM devices for spintronic applications with a 26−30 nm thick organic insulating layer has been recently reported.9−11 Importantly, the spin relaxation time through the organic layers was found to be in excess of 1 s,9 suggesting that organic materials can be used for spin transport in MIM assemblies. Along with MIM assemblies exploiting bulk organic materials, those based on monomolecular dielectric layers are of interest since such layers can then be specifically designed and precisely scaled in the technologically relevant nanometer thickness range (1−5 nm). Relevant systems in this context are self-assembled monolayers (SAMs) which are 2D polycrystalline films of semirigid, rodlike molecules that are chemically anchored to a substrate.12−17 Their constituents can be flexibly © 2014 American Chemical Society

designed by a suitable combination of the individual building blocks, which are (1) a headgroup, that mediates the anchoring to the substrate, (2) a tail group, that is exposed to the ambience, and (3) a spacer, that separates the head and tail groups and provides structural stability to the monolayer, typically by van der Waals interactions. A SAM can be easily prepared on a metal substrate serving as the bottom electrode in a MIM assembly, using e.g. the most popular thiol headgroup.13,14,16 In contrast, the preparation of a suitable top contact, i.e., a well-defined metal layer at the SAM− ambience interface, represents a complex problem that has not been solved so far, even though a variety of promising reports can be found in the literature.4,18−30 Generally, studies of metal adsorption onto the SAM-modified substrates show that metal atoms eventually penetrate into the SAM layer and diffuse to the SAM−substrate interface.29−36 The diffusion occurs via both static and dynamic channels, viz. structural defects in the film (e.g., domain boundaries, pinholes, etc.) and openings appearing due to the dynamical movement of the SAM constituents, respectively.29,30,37−39 The diffusion can be partly suppressed for reactive metal adsorbates on the SAMs bearing chemically reactive tail groups that provide nucleation centers for the metal film formation at the SAM−ambience interface.31 Following this approach, partial or temporary stabilization of some (mostly reactive) metal adsorbates (e.g., Ti, Cr, Pd, and Al) at the SAM−ambience interface has been achieved using Received: March 13, 2014 Revised: April 30, 2014 Published: May 14, 2014 11763

dx.doi.org/10.1021/jp5025334 | J. Phys. Chem. C 2014, 118, 11763−11773

The Journal of Physical Chemistry C

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HOOC−, CH3CO2−, HOCH2−, thiophene, pyridine, and HSterminated monolayers.29−31,37,38,40−43 A different approach, reported by us earlier,21−23 utilizes aromatic SAMs and combines the functionalization of the SAM constituents (by HS headgroups) with the elimination of diffusion channels by a 2D polymerization of the monomolecular film. The polymerization is mediated by extensive electron irradiation, resulting in breaking of C−H bonds and subsequent cross-linking of the residual aromatic backbones.44−47 Consequently, the diffusion of metals into the SAM could be suppressed almost completely.21−23 However, as was found later and will be published elsewhere, the efficiency of the irradiation-induced cross-linking to suppress the metal interdiffusion depends strongly on the initial quality of the monolayers and subtle parameters of the experiments. In this context, we looked for an alternative to the irradiation induced cross-linking of the aromatic SAMs and came to the idea of a chemically induced cross-linking. The key principle is to use not just reactive tail groups but reactive molecular backbones and to mediate cross-linking of these backbones by the metal adsorbates themselves, relying on their chemical reactivity. As a suitable molecular system, fluorinated aromatic SAMs were used. It is indeed well-known that the interaction of a variety of metals with fluorocarbons, also arranged as a polymer film48 or a SAM,49 results in their extensive defluorination, chemically triggered by the metal adsorbate. Among the relevant metals are Fe, Ni, and Cr, which can be of particular interest in context of potential spintronic MIM devices. However, this is of no use in the case of partly fluorinated aliphatic SAMs, since the defluorination is accompanied by the almost complete destruction of the SAM matrix as well as by the penetration of the adsorbate metal into the SAM and to the SAM−substrate interface, so that a welldefined top metal layer cannot be formed.49 This can, however, be different in the case of fluorinated aromatic SAMs, as we demonstrate in this work by the example of a representative system, monolayers of perfluoroterphenyl-substituted alkanethiols, C6F5(C6F4)2(CH2)3SH (FTP3; see Figure 1), on

reactivity and possesses distinct ferromagnetic properties as a bulk material. 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 FTP3 was synthesized according to the literature.51 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, temperature 340 °C).52 The resulting metal substrates were polycrystalline, with predominant (111) orientation and a grain or terrace size of 20−50 nm. The SAMs were prepared according to the literature protocol,50 i.e., by immersion of the freshly prepared substrates into a 1 mM solution of 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 FTP3 films, two reference systems were prepared, viz. SAMs of partly fluorinated alkanethiols, F(CF2)10(CH2)2SH (F10H2), and SAMs of terphenyl-substituted alkanethiols, C6H5(C6H4)2(CH2)3SH (TP3), on Au(111) (see Figure 1). F10H2 and TP3 were synthesized according to the literature.53,54 The same substrates as for the FTP3 SAMs were used. The preparation of the SAMs was performed according to literature protocols, i.e., by immersion of the freshly prepared substrates into a 1 mM solution of F10H2 in ethanol53 or TP3 in THF54,55 at room temperature for 24 h. The metal deposition was carried out in UHV with an ebeam evaporator (Omicron) which was mounted at a distance of ∼15 cm from the substrates to ensure a uniform metal deposition over the whole sample area. The deposition was carried out at an angle of 20° or 45° with respect to the sample surface. The deposition rate was set to 0.2−0.3 nm/min. It was calibrated with a quartz crystal microbalance (QCM) or X-ray photoelectron spectroscopy (XPS), in the case of the 20° geometry. According to the reference measurements on metal substrates, the Ni coverages determined by XPS were very close to the QCM values (within 0.1−0.2 nm). Ni evaporation was monitored in situ by laboratory-based XPS, synchrotron-based XPS, near-edge X-ray absorption fine structure (NEXAFS) spectroscopy, ion-scattering spectroscopy (ISS), and work function measurements. The experiments 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.45,56−58 Laboratory-based XPS measurements were performed using a Mg Kα X-ray source (1253.6 eV) 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.59 The synchrotron light served as the Xray primary source. The spectra acquisition was carried out in normal emission geometry with an energy resolution of ∼0.3

Figure 1. Schematic representation of the molecules used in this study along with their abbreviations.

Au(111). As shown recently, these films exhibit very high structural quality, including dense molecular packing and a long-range order.50 This can be certainly of advantage for reduction of potential defects in the monolayer and efficient cross-linking upon the metal deposition. As a test adsorbate metal, we selected Ni that has potentially the required chemical 11764



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3. RESULTS 3.1. XPS and ISS: Ni. Figure 2a presents Ni 2p XP spectra of Ni/FTP3/Au at several selected Ni coverages. The spectra

eV at a photon energy of 350 eV and a somewhat lower resolution at higher photon energies. The energy scale of the XP spectra was referenced to the Au 4f7/2 peak at a binding energy (BE) of 84.0 eV.60 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 standard60 spin−orbit splitting of ∼1.18 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 NEXAFS measurements were performed at the same beamline as the synchrotron-based XPS experiments. The spectral acquisition was carried out at the carbon K-edge in the partial electron yield (PEY) mode with a retarding voltage of −150 V. Linearly polarized synchrotron light with a polarization factor of ∼91% was used. The energy resolution was ∼0.30 eV. 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 in the SAMs. This approach is based on the linear dichroism in X-ray absorption, i.e., 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.61 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.62 ISS measurements were performed with a focused ion gun (Leybold-Heraeus) and a LHS 11 analyzer. The primary ion beam (He+, MESSER 99.999%) was accelerated to 1 keV and operated at a current of about 100 nA/cm2. The beam was focused to a spot with a diameter of ca. 500 μm by two focal lenses. The angle between the primary beam and sample surface was 35°. During data acquisition, the beam was rastered over a 2 × 2 mm2 area, which was gated with a 70% aperture. 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. Absolute work function values were obtained by sputtering each sample with 3 keV argon ions and measuring the work function of the clean gold substrate. The value of the work function of gold was taken to be 5.1 eV. Using this approach, work function measurements were found to be reproducible within 2 mV. To interpret the NEXAFS spectra of Ni/FTP3/Au, model quantum mechanical calculations of the single molecule and Nimolecule complexes were performed using the semiempirical ZINDO/S method. All calculations were performed with the Orca 2.8 software package.63 Model densities of states were obtained by applying equal Gaussian envelopes of 0.5 eV width to each molecular orbital at the ground state binding energies to account for the solid state broadening and then summing, together with a rigid energy shift of a typical value of 285.6 eV applied to the calculated electronic structure by ZINDO/S.

Figure 2. (a) Ni 2p XP spectra of Ni/FTP3/Au acquired with a Mg Kα source (open circles), along with tentative decomposition by several individual peaks (solid lines) which are assigned (see text for details); the Ni coverages are indicated at the respective curves. (b) XPS intensity ratio of the Ni 2p signal to the total C 1s signal as a function of the takeoff angle for Ni coverages of 0.1 nm (filled circles, black line) and 0.7 nm (open circles, gray line). The vertical gray solid lines in (a) are guides for the eyes.

are dominated by the strong emission of metallic Ni at 853.2− 853.6 eV which increases in intensity and shifts to lower binding energies, toward the ultimate, literature value of 852.7 eV for metallic nickel,60 with increasing Ni coverage. This is accompanied by a reduction in fwhm of this emission from 2.1 eV at 0.1 nm Ni coverage to 1.6 eV at 0.6 nm coverage, suggesting the improving chemical homogeneity of the nickel film. Along with the above emission, weak signals associated with nickel fluoride and C−Ni moieties can be traced, which can only result from the interaction of Ni atoms with the FTP3 moieties. Significantly, as shown in Figure 2b, the ratio of the Ni 2p intensity to the total intensity of the C 1s signal from the FTP3 monolayer increases at the change from normal to grazing emission. This suggests that the Ni film is predominantly located at the SAM−ambience interface, building, as was intended, a top metal overlayer. The extent of the ratio increase is significantly larger for the 0.7 nm Ni film as compared to the 0.1 nm case, which suggests that the penetration and diffusion of Ni into the FTP3 monolayer gets more inhibited with increasing coverage, so that the Ni overlayer becomes better defined. The on-top character of the Ni film is also supported by the ISS spectrum of Ni/FTP3/Au shown in Figure 3. This spectrum was acquired immediately af ter the Ni deposition and 11765



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the rings and to the aliphatic linker), whereas the emission at ∼287.2 eV is related to the carbon atoms bonded to fluorine atoms.50 Apart from the overall intensity decrease due to the growing Ni overlayer, the spectral envelope changes significantly in the course of the Ni evaporation, suggesting extensive modification of the fluorocarbon matrix. The peak associated with C−F bonds (∼287.5 eV) decreases in intensity with increasing nickel coverage, implying extensive cleavage of these bonds upon the metal deposition. Accordingly, the peak associated with C−C bonds in the FTP moiety (∼285.3 eV), which is indirectly affected by the presence of the fluorine atoms as well, also decreases in intensity to a similar extent. Simultaneously, a variety of new emissions, marked as 1−4 in Figure 4, appear at lower binding energies. Some of these features point toward extensive polymerization66 (1 and 2) and conjugation67,68 (3) of the FTP3 film during the metal deposition, suggesting that the nickel atoms cause intermolecular carbon−carbon bond formation after the abstraction of fluorine.69 In parallel, imbedding of Ni in the intermolecular bonds is possible. Accordingly, the peak at ∼283.3 eV (4), which increases in intensity with increasing Ni coverage, can be ascribed to C−Ni bonds.70 In addition, the peak at 284.0 eV, which exhibits a similar behavior, can also contain contributions from C−Ni bonds in view of the fact that a similar peak was observed at the deposition of nickel on graphite.67 But, presumably, apart from the pristine sample, this peak contains mostly contributions from C−C moieties in the modified FTP3 matrix. The total C 1s intensity decreases with increasing Ni coverage as mentioned above. This, however, does not necessary mean that there is extensive desorption of the FTP3 molecules or their fragments (apart from fluorine) but is presumably related to the attenuation of the photoemission signal by the growing Ni overlayer at the SAM−ambience interface. Indeed, the ratio of the total C 1s intensity to that of the Au 4f signal, which, due to the similar attenuation of both signals, is not affected by the appearance of the Ni overlayer, exhibits only small decrease at the initial stages of the Ni evaporation and is almost constant at the later stages (see Figure 5a). So, there is almost no loss of carbon, in contrast to F10H2/Au that exhibited noticeably larger decrease of the C 1s/Au 4f intensity ratio in the course of the Ni evaporation (see Figure 5a). At the same time, there is significant loss of fluorine as mentioned above. To avoid problems related to the changing FTP matrix and growing Ni overlayer throughout the experiment, we chose to use the intensity ratio of the C 1s (C−F) peak to the total carbon signal as a measure of fluorine loss in the films. The respective data are shown in Figure 5b for two different photon energies. As seen in this figure, the relative C−F signal decreases significantly with increasing Ni coverage. This decrease is more pronounced at 350 eV (55%) than at 1254 eV (33%), indicatingin view of the smaller sampling depth in the 350 eV casethat most of the fluorine abstraction and subsequent chemical reactions (cross-linking, etc.) occur at the topmost part of the film. Consequently, we expect nickel to be found mostly in this area, in agreement with the conclusions of section 3.1. The curve for 1254 eV exhibits a saturation behavior at high Ni coverage at the level of ∼33% decrease as compared to the intensity ratio value for the pristine film. This corresponds coarsely to defluorination of one of three rings of the FTP3 moiety, with, presumably, the topmost ring affected mostly by the defluorination process. Note that, in contrast to FTP3/Au, the decrease of the analogous intensity ratio at 1254

Figure 3. ISS spectra of Ni/FTP3/Au (top curve) and Ni/TP3/Au (bottom curve) acquired at an incident helium ion energy of 1000 eV. The spectra were acquired immediately after the Ni deposition.

is therefore, in view of the high surface sensitivity of ISS,21,64 representative of the topmost area of the Ni/FTP3/Au sample. The spectrum exhibits a pronounced and strong peak at ∼800 eV assigned to Ni,21 which is characteristic of the deposition of this metal on top of the monolayer and suggests that the metal penetration into the SAM occurs to a limited extent only, if at all. As a negative control, an analogous spectrum of a similar nonfluorinated system, Ni/TP3/Au, is presented in Figure 3. This spectrum shows almost no signal of Ni, which corresponds, in agreement with the literature data,65 to extensive penetration of Ni atoms into the monolayer and their subsequent diffusion to the SAM−substrate interface. This underlines the effect of fluorine in preventing such a penetration in the case of the FTP3 monolayer. 3.2. XPS: FTP3 Matrix. To understand the above observations, one has to monitor the changes occurring in the FTP3 monolayer upon the Ni deposition. Figure 4 shows

Figure 4. C 1s XP spectra of FTP3/Au and Ni/FTP3/Au (open circles), along with tentative decomposition by several individual peaks (solid lines) which are marked (see text for details). The features related to the pristine film are drawn with black solid lines. The spectra were acquired at the synchrotron, at a photon energy of 350 eV. The Ni coverages are indicated at the respective curves. The vertical gray solid lines are guides for the eyes.

the C 1s XP spectra of FTP3/Au and Ni/FTP3/Au at two different nickel coverages. The spectrum of the pristine SAM exhibits three emissions at ∼284.0, ∼285.3, and ∼287.2 eV assigned to the carbon atoms in the aliphatic linker (284.0 eV) and the FTP moiety (285.3 and 287.2 eV), respectively.50 The emission at ∼285.3 eV is associated with the carbon atoms bonded exclusively to other carbon atoms (bonding between 11766



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Figure 6. F 1s XP spectra (open circles) of FTP3/Au and Ni/FTP3/ Au (a) as well as F10H2/Au and Ni/F10H2/Au (b), along with tentative decomposition by several individual peaks (solid lines) which are marked (see text for details). The spectra were acquired at the synchrotron, at a photon energy of 750 eV. The Ni coverages are indicated at the respective curves. The vertical gray solid lines are guides for the eyes.

Figure 5. XPS intensity ratios for Ni/FTP3/Au and Ni/F10H2/Au as functions of the Ni coverage. (a) Intensity ratio of the total C 1s signal to the Au 4f signal from the substrate for Ni/FTP3/Au (black circles, black solid line) and Ni/F10H2/Au (gray down triangles, gray solid line). (b) Intensity ratio of the C 1s (C−F) signal to the total C 1s signal measured at photon energy of 350 eV for Ni/FTP3/Au (black diamonds, black dashed line) and 1254 eV for Ni/FTP3/Au (black circles, black solid line) and Ni/F10H2/Au (gray down triangles, gray solid line). The intensity ratios are normalized to the respective values for the pristine films.

differences in its presence in Ni/FTP3/Au and Ni/F10H2/Au are, most probably, related to the specific reaction pathways in both cases. The difference between the behavior of Ni/FTP3/Au and Ni/F10H2/Au is further underlined by the S 2p XP spectra shown in Figure 7. The spectra of pristine FTP3/Au and F10H2/Au are dominated by a characteristic S 2p3/2,1/2 doublet (1) at a BE of 161.9−162.0 eV (S 2p3/2), which can be clearly assigned to thiolate species bonded to the surface of gold.72 This doublet is accompanied by a frequently observed,72−77 less intense doublet (2) at a BE of ∼161.0 eV (S 2p3/2), ascribed to either atomic sulfur78 or a thiolate-type bound sulfur with a specific binding chemistry and/or geometry.79,80 As seen in Figure 7a, the intensity of the dominant S 2p doublet for Ni/ FTP3/Au decreases with increasing nickel coverage, which can be explained by the increasing attenuation of the respective photoelectron signal by the growing Ni overlayer. The intensity decrease is accompanied by a noticeable broadening of the doublet, which stems presumably from progressing disordering of the FTP3 monolayer resulting in significant distribution of the local binding configurations for the thiolate headgroups, accompanied by the respective distribution of the binding energies. The situation is distinctly different in the case of Ni/F10H2/ Au as seen in Figure 7b. First, the total S 2p intensity does not decrease but increases in the course of the Ni evaporation, which can be explained by combined effect of extensive desorption of the hydrocarbon fragments of the F10H2 matrix and diffusion of Ni to the SAM−substrate interface, similar to the case of Fe.49 Indeed, the dominant S 2p doublet (1) does not only broaden but also shifts to lower binding energies by ∼0.25 eV, indicating a significant change in the environment of the thiolate headgroups. Further, two new doublets at 162.8 eV

eV for F10H2/Au was found to be almost 64% at a Ni coverage of 1.4 nm (as shown in Figure 5b). This is almost double amount as compared to FTP3/Au. So, the loss of fluorine is much more extensive in the case of partly fluorinated alkanethiolate (PFAT) SAMs. This is also supported by the behavior of the ratio of the total F 1s intensity to that of the Au 4f signal (not shown). Whereas the decrease of this intensity ratio was ∼72% for F10H2/Au at a Ni coverage of 1.4 nm, it was only ∼40% for FTP3/Au. The Ni-induced cleavage of C−F bonds can be followed not only by desorption of fluorine but also by its trapping in the FTP3 matrix in form of either F2, NiF2, or CFx moieties. The formation of the F2 species is thought rather unlikely since their bond energy is low and their reactivity high.71 In contrast to this, nickel fluoride is a very stable material and therefore could be expected to be the main fluorine-containing byproduct. However, as shown in Figure 6a, the F 1s XP spectra of Ni/ FTP3/Au exhibit only a small signal associated with nickel fluoride. This is in striking contrast to the corresponding fluoroalkane system, F10H2/Au, for which the nickel fluoride signal increases strongly in intensity with increasing Ni coverage (Figure 6b), consistent with the behavior observed for the deposition of iron on similar, partly fluorinated alkanethiol SAMs.49 This difference can be tentatively explained by the trapping of nickel fluoride in the heavily damaged F10H2 film, following the extensive penetration of Ni into the monolayer (similar to the case of Fe),49 whereas in the FTP3 case penetration of Ni occurs to a small extent only. However, in view of the nonvolatile character of nickel fluoride, the 11767



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Figure 7. S 2p XP spectra (open circles) of FTP3/Au and Ni/FTP3/ Au (a) as well as F10H2/Au and Ni/F10H2/Au (b), along with tentative decomposition by several individual doublets (solid lines) which are marked (see text for details). The spectra were acquired at the synchrotron, at a photon energy of 750 eV. The Ni coverages are indicated at the respective curves. The vertical gray solid lines are guides for the eyes.

Figure 8. (a) C K-edge NEXAFS spectra of FTP3/Au and Ni/FTP3/ Au acquired at an X-ray incident angle of 55°. (b) Difference between the spectra acquired at X-ray incident angles of 90° and 20°. The Ni coverages are given at the respective curves. The characteristic absorption resonances are indicated (see text for details). The horizontal dotted lines in the bottom panel correspond to zero. The vertical gray solid lines are guides for the eyes.

(3) and 163.6 eV (4) appear in the spectra, manifesting significant damage and modification of the SAM−substrate interface. The former doublet can be ascribed to various Ni−S species,81−83 while the latter feature is associated with the formation of dialkyl sulfide species as observed at electron or Xray irradiation of alkanethiolate SAMs on gold.58 3.2. NEXAFS Spectroscopy: FTP3 Matrix. The modification of the FTP3 monolayer in the course of the Ni evaporation was also monitored by NEXAFS spectroscopy. C K-edge NEXAFS spectra of FTP3/Au and Ni/FTP3/Au acquired at an X-ray incident angle of 55° are presented in Figure 8a. These spectra are only characteristic of the electronic structure of the systems under consideration and are not affected by any effects related to molecular orientation.61 The spectrum of the pristine film is dominated by two sharp resonances at ∼285.7 eV (1) and ∼287.6 eV (2) corresponding to the C1s → π1* transitions from the FTP carbon atoms which are not bonded (C−C) and directly bonded (C−F) to fluorine atoms, respectively.50,84,85 Further, there are several less intense π*- and σ*-like resonances, including the most pronounced π2* and σ* features 3 and 4.50,84,85 The spectrum of FTP3/Au changes significantly in the course of Ni evaporation. The resonances associated with the FTP moiety decrease in intensity, which can be associated with extensive defluorination of the FTP3 monolayer. This supports our conclusions about the C−F bond cleavage from the HRXPS data. Furthermore, consistent with the HRXPS results, several resonances (5, 6) appear at the lower photon energy side of the C 1sC−C → 1π* peak (1), viz. at 284.1 eV (5) and 283.4 eV (6), and grow in intensity with increasing Ni coverage. On one hand, the appearance of these resonances can

be associated with intermolecular conjugation throughout the SAM. Indeed, NEXAFS data of oligoacene molecules show a clear shift of the C 1sC−C → π* resonance to lower photon energies with increasing conjugation, e.g. from 285.0 eV for benzene to 284.1 eV for pentacene.86 In addition, many common conjugated polymers exhibit C 1s → π* transitions in the 284.4−284.9 eV region.87 On the other hand, on the basis of our quantum mechanical calculations (not shown) and C 1s XPS data as well as published Ti−C NEXAFS data,88 the low photon energy resonances (5, 6) can be alternatively assigned to the C 1sC−Ni → 1π* transitions. Thus, presumably, these resonances stem both from the conjugation effects and C−Ni species. Along with the access to the electronic structure, NEXAFS data provide information about molecular orientation and orientational order in molecular films.61 A fingerprint of molecular orientation is the linear dichroism, which is practically exhibited as dependence of the absorption resonance intensity on the incidence angle of the X-ray beam. A convenient way to monitor the respective effects is to plot differences between the spectra acquired at the normal and grazing incidence of the primary X-rays. Such difference spectra for FTP3/Au and Ni/FTP3/Au are presented in Figure 8b. The spectrum of the pristine film exhibits pronounced peaks at the positions of the characteristic C 1s → π* resonances, which is a clear signature of high orientational order. In addition, the signs of the observed difference peaks, viz. the positive sign for the π*-like resonances and the negative sign for the σ*-like 11768



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signal at 1346 eV (Figure 5b). This suggests that the defluorination takes place mostly at the initial stages of the Ni evaporation, progressing much slower after the formation of the continuous Ni film at the SAM−ambience interface.

ones, suggest an upright molecular orientation in all the FTP3 films, in accordance with the literature data.50 The intensities of the difference peaks decrease strongly with increasing Ni coverage, to even larger extent than the intensity of the corresponding absorption resonances. Nevertheless, the dichroism is still perceptible even at large Ni coverages, suggesting that the orientational order in the FTP3 matrix does not suffer a complete breakdown upon Ni evaporation but just a noticeable deterioration, following the partial defluorination of the FTP moieties. This is understandable since the orientational order is mediated by the interaction between the intact FTP backbones. The removal of a part of the fluorine atoms will thus disrupt this interaction and, subsequently, the orientational order. 3.4. Kelvin Probe Measurements. The deposition of metal on SAMs can result in a change in their work function, ϕ.34,35 We expect the extent of change to depend on the difference between the work function of the SAM covered gold and that of the deposited metal. For instance, potassium has a low work function (∼2 eV) and therefore induces dramatic changes in the work function of alkanethiolates on gold.34,35 By contrast, nickel, the metal used in this study, has a work function close to that of gold (4.9−5.0 vs 5.1−5.2 eV for gold). This should result in a lower rate of change compared to potassium upon metal deposition. Figure 9 shows the change in the work functions of FTP3/ Au and F10H2/Au as functions of the nickel coverage. Since

Figure 10. Schematic representation of the Ni deposition on FTP3/ Au. Ni atoms interact with the topmost part of the FTP moieties of the pristine SAM (a), resulting in activation and cleavage of C−F bonds, release of fluorine, and formation of NiF2 followed by the extensive cross-linking of the modified FTP skeletons preventing the penetration of Ni into the SAM and to the SAM−substrate interface (b). As a result, a closed Ni film can be formed at the SAM−ambience interface (c).

It contrast to FTP3/Au, the work function of F10H2/Au exhibits a slow, monotonous decay with increasing nickel coverage, with a value of ∼5.2 eV for ∼1.2 nm of Ni. This behavior can be attributed to the progressing penetration of nickel through the SAM accompanied by defluorination of the molecules.

4. DISCUSSION The presented experimental results provide a clear evidence for the Ni-induced defluorination and cross-linking in the FTP3 SAMs. These processes involve activation and cleavage of C−F bonds and occur predominately in the topmost part of the monolayer, upon the reaction of the Ni atoms with the rings comprising the SAM−ambience interface. The extensive and rapid cross-linking processes “seal” the modified FTP3 matrix and prevent the penetration of the deposited Ni atoms into the SAM in the course of further Ni evaporation. As mentioned above, the sealing is mostly mediated by the top part of the FTP backbone; in view of the 33% fluorine loss at the saturation of the respective curve (Figure 5b), one can coarsely assume that only the topmost of the three rings is involved. Note that the specific reactivity of the Ni atoms and the stability of the aromatic backbones are presumably the key factors behind the specific behavior of the fluorinated aromatic monolayers during the Ni deposition. Accordingly, other typical processes induced by metal deposition such as extended molecular decomposition, desorption of the entire molecules, and damage to the headgroup−substrate interface were not perceptible or occurred to a very limited extent only. Note that this would be probably different in the case of more reactive and less selective adsorbates such as Ti, which is potentially capable to destroy the entire molecular structure.33

Figure 9. Work functions of Ni/FTP3/Au (black circles, black line) and Ni/F10H2/Au (black down triangles, gray line) as functions of the Ni coverage.

both films contain fluorine, their molecular dipoles point away from the surface; therefore, substrates modified by such SAMs exhibit higher work function than the substrate itself. We therefore expect that the work function of the total systems (metal−SAM−metal) will decrease upon nickel deposition. If nickel fully covers the surface of the SAM, a value of ϕ close to that of nickel is expected. Indeed, the work function of FTP3/ Au exhibits a distinct decrease upon Ni deposition dropping by ∼0.5 eV for just ∼0.1 nm of Ni. After the deposition of ∼0.4 nm of Ni, it stabilizes at ∼5 eV, which is the value for a clean nickel surface. This result can be interpreted as the formation of a continuous Ni layer on top of the modified FTP3 monolayer starting from ∼0.4 nm of Ni. Note that this amount of Ni corresponds approximately to two atomic layers, so that the formation of a continuous film can be indeed expected. Remarkably, the onset of the leveling-off behavior of the work function correlates coarsely with the slower decrease and subsequent saturation of the relative intensity of the fluorine 11769



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metallization, resulting in a poorly defined Ni “overlayer” and a complete loss of the identity of the F10H2 film. A higher amount of fluorine atoms per molecule and the involvement of the entire F10 segment in the defluorination process explain the much higher extent of the NiF2 formation as compared to the FTP3 case. In addition to the above factors, the exact pathways of the metal mediated reactions can be different for the PFAT and FTP3 monolayers, as we mentioned above. Whereas a detailed description of this reaction for the PFAT films and poly(tetrafluoroethylene) can be found in the literature,48,49 an analogous description for fluorinated aromatic layers does not exist yet, and one can only rely on the literature describing general chemical reactions of different Ni complexes with hexafluorobenzene and related compounds.11,48,89−92 Note that the intermolecular cross-linking of aromatic SAMs has already been used by us to prevent the penetration of metal atoms into the monolayers, as mentioned in section 1.21,22 However, in contrast to the present work, the cross-linking was mediated by electron irradiation before the metal deposition. The respective studies were quite successful, but as we found later, the efficiency of the irradiation-induced cross-linking depends strongly on the quality of the monolayers and subtle parameters of the experiments. In this context, we expect that the results demonstrated in the present study provide a more robust strategy of SAM metallization, apart from the useful information about the processes occurring in fluorinated aromatic monolayers upon their interaction with moderately reactive metal atoms. It should, however, be mentioned that the experimental methods used in the given study have their limitations (as any experimental tools), so that a small amount of metal penetration at defects/grain boundaries, below the sensitivity of XPS, cannot be excluded completely. Additional experiments using techniques such as STM and/or SIMS are therefore highly desirable.

The exact mechanisms of the Ni-induced defluorination and subsequent cross-linking are unclear, also in view of the partly conflicting literature data for the analogous reactions.11,48,89−93 Generally, it is well-known that reaction of Ni with fluorinated aryls leads to the activation and cleavage of C−F bonds. The most likely primary step is the oxidative addition of Ni into one of the aryl−F bonds to produce the reactive arylnickel species (aryl−Ni−F).91 Subsequently, two such species can react with one another producing a diarylnickel intermediate (aryl−Ni− aryl), followed by a reductive elimination of biaryl and the regeneration of Ni.91 The application of this process to the topmost part of the FTP matrix will certainly result in its rapid and extensive cross-linking, in full agreement with the experimental data presented in section 3. A byproduct of this process is NiF2 that is formed during the elimination step. A comparably small amount of NiF2 observed in our case can be tentatively explained by the limited character of the defluorination reaction, affecting predominantly the topmost part of the FTP3 monolayer. Accordingly, just two to three Ni atoms pro FTPn molecule are sufficient to seal the FTPn monolayer completely, accompanied by the formation of just 2−3 equivalents of NiF2. A partial desorption of these species as well as formation of other fluorine-derived moieties, with their subsequent desorption, cannot be excluded as well, along with alternative passways of Ni-induced reactions such as creation of highly reactive aryl−Ni0 complexes89 or formation of reactive −C− radicals and fluoride ions after an electron transfer from the incoming metal atoms onto the FTP moieties.49 The latter scenario is particular interesting since −C− radicals can trigger the extensive cross-linking in the monolayer, also with a further involvement of Ni and partial formation of aryl−Ni−aryl bonds. Note that the formation of radicals after the abstraction of fluorine is believed to be the primary defluorination mechanism at the reaction of Fe with the PFAT SAMs.49 The character of the radicals (−CF−) is however different in this case as compared to the FTP3 situation (−C−) which can result in different passways of the subsequent reactions. The abstracted fluorine ion can desorb or be trapped as salt. The probability of trapping is lower for the ions created at the vicinity of the SAM−ambience interface, which can explain the comparably low content of the NiF2 species in the FTP3 case. Among other possibilities, formation of CFx fragments with x = 1−4 cannot be completely excluded, even though we do not have any evidence for this process, involving a partial destruction of the aromatic rings (as a source of carbon). These fragments are thermodynamically very stable due to the strong C−F bond and are even quite stable at n < 4 due to the special electronic situation of the fluorine substituents (high electronegativity and optimal π-back-donation by the free electron pairs). These species can quickly desorb in UHV and thus become undetectable by surface spectroscopic techniques. The extensive, Ni-induced cross-linking in the topmost part of the FTP3 monolayer is presumably the major factor responsible for the distinctly different behavior of this system as compared to the PFAT films such as F10H2/Au. In contrast to FTP3, the F10H2 backbone is prone to the decomposition via the cleavage of C−C bonds,48,49,94 accompanied by a rapid and extensive disordering. The above processes are dominant at the initial stages of induced damage while the cross-linking starts only at the later stages.45,94 Accordingly, the penetration of Ni into the F10H2 monolayer and its diffusion to the SAM/ Au interface are not suppressed at the initial stages of the

5. CONCLUSIONS In context of metallization of SAMs and preparation of welldefined ferromagnetic films at the SAM−ambience interface, we studied the deposition of nickel on fluorinated aromatic (FTP3) and aliphatic (F10H2) monolayers on gold, with the latter film serving as a reference only. The effect and outcome of the metal deposition were monitored in detail by several complementary experimental techniques, viz. laboratory- and synchrotron-based XPS, NEXAFS spectroscopy, ISS, and Kelvin probe measurements. The primary process for both FTPn and F10H2 monolayers is the Ni-induced defluorination of the molecular backbones. In the case of FTPn/Au, this occurs predominately in the topmost part of the monolayer over the activation and cleavage of C−F bonds and is accompanied by a rapid cross-linking reaction between the partly or completely defluorinated aromatic rings. The latter process polymerizes the topmost part of the monolayer and makes it practically impermeable for the Ni atoms. Consequently, well-defined nickel metal films can be formed at the SAM−ambience interface. In contrast to the FTPn system, the Ni-induced cleavage of C−F bonds in the F10H2 monolayer occurs more extensively and is accompanied by the cleavage of C−C bonds. The crosslinking takes place less efficiently as compared to the above processes, so that Ni atoms penetrate into the monolayer and to the SAM−substrate interface, destroying the film and 11770



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building, among other species, a noticeable amount of nickel fluoride trapped in the matrix. Note that the above results are not only specific of Ni but, presumably, applicable to other metals having a reactivity toward fluorocarbons, such as Fe and Cr. This can be of importance for potential MIM applications of aromatic fluorocarbon SAMs.

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AUTHOR INFORMATION

Corresponding Author

*Phone +49-6221-54 4921, fax +49-6221-54 6199, e-mail [email protected]. Present Address

F.C.: BASF SE, Ludwigshafen, Germany. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank Ch. Wöll and A. Nefedov (KIT) for the technical cooperation at BESSY II and BESSY II staff for the assistance during the synchrotron-related experiments. This work has been supported by DFG (ZH 63/10-1).

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REFERENCES

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Nickel Deposition on Fluorinated, Aromatic Self-Assembled Monolayers

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