Article pubs.acs.org/Langmuir
Electron Processing at 50 eV of Terphenylthiol Self-Assembled Monolayers: Contributions of Primary and Secondary Electrons Justine Houplin, Céline Dablemont, Leo Sala, Anne Lafosse, and Lionel Amiaud* Institut des Sciences Moléculaires d’Orsay (ISMO), CNRS, Université Paris-Sud, Université Paris-Saclay, F-91405 Orsay, France ABSTRACT: Aromatic self-assembled monolayers (SAMs) can serve as platforms for development of supramolecular assemblies driven by surface templates. For many applications, electron processing is used to locally reinforce the layer. To achieve better control of the irradiation step, chemical transformations induced by electron impact at 50 eV of terphenylthiol SAMs are studied, with these SAMs serving as model aromatic SAMs. Highresolution electron energy loss spectroscopy (HREELS) and electron-stimulated desorption (ESD) of neutral fragment measurements are combined to investigate electron-induced chemical transformation of the layer. The decrease of the CH stretching HREELS signature is mainly attributed to dehydrogenation, without a noticeable hybridization change of the hydrogenated carbon centers. Its evolution as a function of the irradiation dose gives an estimate of the effective hydrogen content loss cross-section, σ = 2.7−4.7 × 10−17 cm2. Electron impact ionization is the major primary mechanism involved, with the impact electronic excitation contributing only marginally. Therefore, special attention is given to the contribution of the low-energy secondary electrons to the induced chemistry. The effective cross-section related to dissociative secondary electron attachment at 6 eV is estimated to be 1 order of magnitude smaller. The 1 eV electrons do not induce significant chemical modification for a 2.5 mC cm−2 dose, excluding their contribution.
■
INTRODUCTION In recent years, growing interest has been paid to the usage of self-assembled monolayers (SAMs)1 in the development of molecular platforms for building chemical or biological sensors2,3 or, more generally, supramolecular assembly driven by surface templates. SAMs are ordered and densely packed molecules, chemisorbed on substrates. Molecules present three essential parts: a head group chosen for its binding properties to the desired substrate, a tail group that will constitute the surface of the film, and a spacer group that will bring organization and mechanical properties through lateral interaction. However, SAMs would need a patterning step, allowing for spatially resolved chemical transformation for the applications cited above.4 Chemical contrast can be induced in aromatic SAMs under irradiation.5 Patterning has been obtained at different scales under electron irradiation depending upon irradiation techniques that are used: high-energetic focused beam or low-energy beam through mask. Energies vary from 10 eV to 20 keV, and typical doses extend from 10 to 80 mC/cm2.6−11 In the case of SAMs composed of an aromatic spacer and a nitro or amino tail group, chemical contrast is visible with atomic force microscopy (AFM) and X-ray microscopy. Modifications in the aromatic spacer are visible in AFM as changes in height and elastic film properties.6 Disorganization also affects linear dichroism.12 The hydrogen content evolution under irradiation cannot be extracted from near-edge X-ray absorption fine structure (NEXAFS) and X-ray photoelectron spectroscopy (XPS) measurements.8 For SAMs with a tail group limited to a hydrogen atom, only one observation of dehydrogenation has been reported, to our knowledge, with infrared (IR) spectroscopy.11 This dehydro© 2015 American Chemical Society
genation is nevertheless an important question to investigate. NEXAFS measurements of electron-irradiated aromatic SAMs indicate that the aromatic character is maintained or converted to aliphatic CC, because these two contributions cannot be separated,8,11 and at the same time, new C−C bonds are created. The main chemical transformation induced is indeed cross-linking between adjacent molecules that locally reinforces the layer stability. Irradiated aromatic SAMs become also resistant to material diffusion and can be used as templates for further surface functionalization13 or in hybrid organic/metal layer surfaces.14 As an interesting application, the cross-linking ability of aromatic SAMs is also exploited in building templates for carbon nanosheets.15,16 In all of these studies, despite different irradiation doses and incident electron energies, the aromatic layer behaves as a negative resist with cross-linking abilities. Nevertheless, the irradiation procedure could be optimized to improve the quality of the patterning.7 The irradiation conditions can be adjusted in terms of doses and incident electron energy. The energy can be tuned to adjust initial reactive center creation. These centers can be generated either directly by primary electron impact or indirectly by secondary low-energy electron interaction through ionization, electronic excitation, or electronic attachment.17−19 In this paper, we present a detailed study of low-energy electron irradiation of p-terphenylthiol [TPT, HS−(C6H4)2− C6H5] SAMs using the high-resolution electron energy loss spectroscopy (HREELS) technique, sensitive to the hydrogen Received: June 9, 2015 Revised: November 23, 2015 Published: November 25, 2015 13528
DOI: 10.1021/acs.langmuir.5b02109 Langmuir 2015, 31, 13528−13534
Article
Langmuir
incident current on the SAMs (uncertainty estimated to 5%). The current was rather constant during irradiation (50 eV, 0.7−0.9 μA; 1 eV, 0.1−0.2 μA). In organic compounds, the mean free path of 50 eV electrons is in the range of 5−7.5 Å.31 Beam penetration in the film is estimated to 1−3 times the mean free path of 50 eV electrons.26 Thus, the whole layer depth may be exposed to 50 eV electron irradiations. The conclusion is the same for irradiation at 1 eV, because the mean free path of 1 eV electron is expected to be much longer than the mean free path of 50 eV electron. Irradiation experiments presented here have been run on three samples. On each sample, irradiations have been performed successively, and doses presented here are cumulated doses. Electron-Stimulated Desorption (ESD) Experiments. ESD experiments were performed under the same conditions as electron processing of SAMs, with the difference that the electron beam was chopped by changing the Wehnelt potential every 5 min to periodically cancel the electron emission and thereby be able to analyze the background contribution. The neutral fragments desorbing under electron irradiation were analyzed using an electron ionization quadrupole mass spectrometer (QMS). The instrument is optimized for low partial pressure detection of neutral species, which eventually fragment in the ionization head of the instrument (Hiden Analytical, Eionization = 70 eV, and mass range = 1.5−300 amu). The species are analyzed according to m/z, the mass/charge ratio.
content, aiming to provide a better understanding of 50 eV electron-induced chemical modification. The evolution of energy loss spectra along the 50 eV irradiation process shows surface modification toward marginal changes of the carbon backbone and strong depletion of the CH stretching signature at 379 meV associated with hydrogen content loss. This loss is quantitatively analyzed to extract the effective cross-section for hydrogen content loss. The role of secondary electrons is also discussed. Their implication in observed transformations at 50 eV is shown to be negligible by comparison to 1 and 6 eV irradiation experiments and by considering the overall efficiency of the hydrogen content loss.
■
EXPERIMENTAL SECTION
Chemicals. 1,1,4′,1″-Terphenyl-4-thiol (TPT) was purchased from Sigma-Aldrich (97%, Saint-Quentin Fallavier, France). All solvents were reagent-grade. Reagents were used without any further purification. Formation of Thiol SAMs. Glass substrates (11 × 11 mm) coated successively with a 2.5 nm thick layer of chromium and a 250 nm thick layer of gold were purchased from Arrandee (Werther, Germany). The gold-coated substrates were annealed by a brief passage in a flame to ensure good crystallinity of the topmost layers by micrometer to submicrometer domains.4,20,21 The gold samples were cleaned by 30 min of ultraviolet (UV)−ozone. The substrates were immersed in freshly prepared saturated solutions of thiol in absolute ethanol at a thiol concentration of ∼1 mM for 2 h under stirring, following a procedure applied to a series of thiol SAMs.22−24 After 5 min of sonication in ethanol and rinsing in ethanol (15 min) to evacuate all non-covalently bound molecules, the surfaces were rinsed in Milli-Q water for 15 min and then dried under a flow of dry nitrogen. The presence of a layer of TPT in the standing-up phase was checked by ex situ XPS analysis, and thickness was estimated to ∼15 Å.22 HREELS Characterization and Electron Processing of SAMs. Characterization by HREEL spectroscopy and low-energy electron processing of the TPT SAMs were performed in an ultrahigh vacuum (UHV) setup (base pressure below 2 × 10−10 Torr), equipped with a load−lock system. The HREEL spectrometer consists of a double monochromator and a single analyzer (model IB 500 by Omicron). Energy loss spectra were obtained at room temperature and recorded in either the specular geometry (θi = θf = 55° with respect to the normal to the sample surface) or off-specular (θi = 55°, and θf = 75°). HREELS probing energy was set at EHREELS = 6 eV. The overall resolution, measured as full width at half maximum (fwhm) of the elastic peak, was ΔEfwhm ≈ 6−8 meV. Incident probing current on the sample was 30 pA. Comparable backgrounds were observed, so that the spectra can be directly compared without normalization. They are shifted for clarity reasons. Although HREELS probing depth is hard to determine precisely,25 the typical probing depth in the case of organic molecular layers is in the 24−36 Å range at 6 eV, which is sufficient to probe the full sample thickness.26 To remove some sample−sample variability, a selection was performed on the following two criteria. Samples with well-defined peaks in their spectra were kept. Samples with clear rehybridization of CH centers that point out a lying-down phase were excluded.27−29 Nevertheless, some differences in peak intensities may remain in the very low energy loss range (0−90 meV), corresponding to the domain of coupled angular deformation modes.30 Samples were irradiated with a commercial electron gun (model EGL-2 Kimball physics) at room temperature. The beam profile was characterized with a Faraday cup, and the sample was set at a distance from the electron gun aperture, assuring a relatively uniform irradiation of its accessible surface. The total irradiated area was Airrad = 1.00 ± 0.05 cm2. During irradiation, the current I conducted through the SAM was measured with a Keithley ammeter in an independent circuit and was found to be comparable to the current measured directly with the Faraday cup for the same irradiation conditions. Thus, the current measurement provided a good estimate of the
■
RESULTS Irradiation of TPT SAMs at 50 eV Incident Energy. Figure 1 presents the HREEL spectrum of a freshly prepared
Figure 1. Selected TPT HREEL spectra (EHREELS = 6 eV, and ΔEfwhm = 7 meV) for different irradiation doses at 50 eV: (a, in red) pristine SAM, (b) 0.1, (c) 0.3, (d) 0.7, (e) 3.0, (f) 3.6, and (g, in green) 9.5 mC cm−2. The curves have been arbitrarily shifted vertically for clarity reasons. The arrows indicate the peak intensity evolutions with irradiation dose.
TPT SAM (curve a). Attributions based on the literature have been first discussed by Amiaud et al.,22 and we proposed reassignments based on the simulated IR spectrum of the isolated TPT molecule by Houplin et al.30 The spectrum consists in a low-energy part (0−210 meV), composed of mixed and overlapping contributions of ring deformations and CH bending modes, and a clearly separated signature peaking 13529
DOI: 10.1021/acs.langmuir.5b02109 Langmuir 2015, 31, 13528−13534
Article
Langmuir at 379 meV. The main resolved structures are observed at 56, 87, 94, and 101 meV (out-of-plane bending γCH), 124 meV (inplane bending δph), 146 meV (in-plane bending δCH, stretching νCC, and stretching νCS), 160 meV (δCH), 184 meV (νCC and δCH), 196 meV (νCC), and 379 meV (νCH). Combinations of losses are seen in the 260−330 meV range.30 A shoulder visible at 364 meV can be attributed to partial carbon rehybridization, associated with a reduced number of flat-lying molecules.27−29 A combination of losses from lower energy modes and partial solvent pollution cannot be completely excluded. Samples are then exposed to successive irradiations by 50 eV electrons, with the final cumulated dose being 9.5 mC cm−2 (curves b−g in Figure 1). The initial spectrum undergoes noticeable changes after irradiation. In the low-energy loss part, modifications are comparable to those observed under irradiation at 6 eV.22 The 124 meV (δph) and 184 meV (νCC and δCH) signals decrease in the early stages of irradiation, followed by the mixed mode at 146 meV (δCH, νCC, and νCS). In the broad feature with signatures identified at 56, 87, 94, and 101 meV (γCH), relative intensities are changed along the irradiation process. For the higher irradiation doses (curves f and g), the reduced peak at 184 meV reveals a weak signature at 177 meV and a new contribution at 137 meV. Considering νCH at 379 meV, the electron processing mainly affects νCH intensity with a clear reduction of the peak area, without any significant shape change. Spectrum evolution with irradiation can be discussed in terms of (i) layer disorganization, (ii) carbon content evolution, and (iii) hydrogen content evolution. Disorganization of the Layer. The comparison of signal intensities for different scattering geometries, specular and offspecular, is a way to probe the global film order using dipolar modes. In the case of well-ordered films, dipole moments in the molecular layer are aligned. The dipolar excitation mechanism in HREELS is then strongly sensitive to the interaction geometry and reduced significantly in off-specular geometry. The other excitation mechanism, the impact excitation, is less sensitive to interaction geometry and gives comparable contributions to the signal in specular and off-specular geometries. Figure 2 compares two HREEL spectra recorded in specular and off-specular geometries (Δθ = 20°). It is noticeable that, although the off-specular spectrum has a much smaller elastic peak, it exhibits remarkably similar vibrational signatures with a resolution comparable to the resolution of the specular spectrum (fwhm = 8.2 meV off-specular and 6.5 meV specular). It means that the pristine SAMs already suffer some disorder. This observation is in agreement with other studies on various alkanethiol SAMs.32,33 The interpretation is the following: considering that the dipolar excitation actually contributes to the signal, as shown by the recorded excitation functions,29,34 the weak sensitivity to interaction geometry can be attributed to short-range order by domains but mismatching in orientation at long range. The domains, whose typical size is a few 10 nm, all contribute within the probed area of millimeter size. Monomers are chemisorbed, with a tilt angle of ∼30° with respect to the normal of the surface,35 in a direction that is supposed to change from one domain to the other. We can then understand that an assembly of molecules randomly tilted at ±30° with respect to the normal of the surface is probed. Thus, eventual moderate molecular orientation changes induced by irradiation may not significantly affect the HREEL signal. Losses at 56 meV (γCH) and 184 meV (νCC and δCH) show a signal decrease in off-specular geometry and thereby testify to partial order.
Figure 2. TPT HREEL spectra in the specular geometry (red curve, θi = θf = 55°, and ΔEfwhm = 6.5 meV) and off-specular geometry (black curve, θi = 55°, θf = 75°, and ΔEfwhm = 8.2 meV). The curves have been shifted vertically, without normalization, until the background at 300 meV is matching. Both elastic peaks are shown divided by 200. The dashed area represents the signal fraction Γ associated with the hydrogen content and used for cross-section extraction in the Hydrogen Content Evolution section. Γspecular ≈ Γoff‑specular within a 6% relative error.
Their evolution during the irradiation process shows that they are sensitive to layer disorganization to some extent. Further interpretation based on this evolution is not straightforward as a result of the overlapping with the zero energy loss peak and neighboring peaks. Conversely, CH stretching loss at 379 meV is a broad and well-separated signature. It is composed of the vibrations of the 13 CH bonds in different orientations, which result in 13 vibration modes weakly dispersed in energy around 379 meV. The associated dynamic dipole moments have different orientations with respect to the surface, reducing its sensitivity to interaction geometry. Its integrated intensity Γ is preserved in off-specular geometry within a 6% relative error. As a result, the CH stretching is a signature robust to irradiation-induced disorganization. Carbon Content Evolution. The carbon content is known to be mostly quantitatively preserved by irradiation of aromatic SAMs, because XPS signal intensities for C 1s and S 2p were shown to be conserved.8,11,36,37 Irradiation at 100 eV and 60 mC cm−2 of TPT SAMS leads to a carbon content reduction estimated to 4%.16 The present HREELS measurements are in agreement with XPS analysis conclusions. The conservation of the 196 meV signature (aromatic νCC) all along the irradiation process shows that the carbon content is quantitatively preserved, within the restriction that the signature is too small to account for variation of a few percent. Aliphatic CC stretching gives a signature at 201 meV38,39 that may contribute in the signal at 196 meV. CC stretching, expected as a weak contribution at 245 meV,39 is not observed. In conclusion, the carbon content can be considered as conserved in quantity, but aromatic carbon and aliphatic CC that could result from ring opening cannot be distinguished. An appropriate schematic representation of the carbon skeleton of the irradiated SAM was published in Figure 1 of ref 12 by Meyerbroeker et al. Evolution of CH Bonds. The preservation of aromatic carbon or conversion to aliphatic CC keep the carbon centers sp2-hybridized. This is validated by the fact that the νCH feature remains peaked at 379 meV, without any growing contribution of sp3-hybridized CHx groups expected at 364 meV.22 Moreover, this peak intensity clearly decreases during the irradiation. This is a major difference from irradiation at 6 13530
DOI: 10.1021/acs.langmuir.5b02109 Langmuir 2015, 31, 13528−13534
Article
Langmuir
carbon content is denatured at the end of the irradiation procedure and is partially aliphatic. Hydrogen Content Evolution. The advantage of the HREELS approach is the exploitation of the strong surface sensitivity of its signal that can be used to give quantitative insights into the chemical evolution under irradiation. The HREELS literature and considerations on electron mean free path allow for the conclusion that the probing depth with 6 eV electrons in HREELS is sufficient to probe the whole irradiated layer (see the Experimental Section). The hydrogen content evolution is then accessible by following the νCH signature at 379 meV. This signature is well-separated from other vibrations and not perturbed by other spectrum changes. In addition, this signature is robust with regard to changes of the interaction angle and, therefore, with regard to moderate irradiationinduced disorganization. Moreover, the contribution of resonant impact interaction at the probing energy for TPT,22 other aromatic compounds,44 and aliphatic double bond C C45 enhances the sensitivity of HREELS to CH stretching. We can then consider that HREELS νCH intensity is proportional to the hydrogen content in the film, and we can follow its evolution with the irradiation dose. To remove any minor deformation of the νCH peak, the signal intensity is evaluated by integrating the band over the range of 340−410 meV. Furthermore, the contribution of the background is subtracted (taken as a straight baseline from 340 to 410 meV). In Figure 4,
eV, where the peak area was conserved but CH centers were rehybridized from sp2 to sp3.22 The decrease of the CH content is also affecting the intensities of the losses at 184 meV (νCC and δCH) and 146 meV (δCH, νCC, and νCS). When the conservation of the carbon content mentioned above is taken into account, the decrease of the CH related signatures can be attributed to CH bond breaking. This is in agreement with IR observations of irradiated biphenyl SAMs showing a decrease of hydrogen-stretching signatures.11 As a consequence, the release of hydrogen is expected to take place. Therefore, we have performed ESD experiments to estimate film damage under irradiation and check for hydrogen release. The production of neutral fragments remains small, with a poor signal/noise ratio, as seen in Figure 3. The signal is only 1−3
Figure 3. ESD rates measured for TPT SAMs under 50 eV electrons irradiation for selected fragments: (left) CH2 (m/z 12 D), (middle) CH3 (m/z 15 D), and (right) C6H5 (m/z 77 D). The periodic measurement sequences are clearly visible.
times bigger than the background as a result of residual composition of the vacuum. The desorbing neutral species detected during irradiation at 50 eV are H2, CH, CH2, and CH3 in comparable intensities and a smaller contribution from C6H5 (m/z 77 D), attributed to the entire phenyl ring. CHy desorption, with y = 1, 2, and 3, can be considered as a production of highly hydrogenated species compared to pristine TPT. It attests for the breaking of many CH bonds in TPT and confirms that atomic hydrogen is the major primary product of the 50 eV irradiation. The moderate loss of carbon atoms and entire phenyl rings indicates some abrasion of the film but to a quite marginal extent, as already discussed above. In conclusion, electron irradiation at 50 eV mainly results in the breaking of CH bonds. This leads to the observed decrease of the hydrogen content spectral signature. This evolution is compatible with cross-linking between adjacent monomers, in agreement with previous studies on aromatic SAM irradiation (e.g., review refs 7−12, 15, and 16). Besides the dehydrogenation, no noticeable rehybridization of carbon centers is observed. Ring opening, leaving aliphatic CC, is possible and would not be distinguished from intact TPT. Note that, for the higher irradiation doses, the signature of the irradiated sample is much less structured than the pristine sample, attesting to a chemical composition that tends to evolve to amorphized hydrogenated carbon matrixes.40 The weak signature appearing at 177 meV and a contribution at 137 meV (curves f and g), tentatively ascribed to deformation δs(sp3CHx) and aliphatic C−C stretching,38,41−43 show that the
Figure 4. Depletion of the hydrogen content under 50 eV electron irradiation. The relative hydrogen quantity γH = Γ/ Γ0 is represented as a function of the fluence ϕ (cm−2, lower axis) and the cumulated doses (mC cm−2, upper axis). In addition, the solid square represents a signal depletion extracted from IR spectroscopy measurements performed on 50 eV electron-irradiated biphenyl SAM.11 Hydrogen content loss is fitted by a standard depletion model, with the formula shown on graph (γH lim = 28 ± 13%, and σ = 2.7−4.7 × 10−17 cm2).
the evolution of the hydrogen content is followed as the relative quantity γH(ϕ) = Γ/Γ0, where Γ0 and Γ are the νCH peak areas of the pristine and irradiated SAMs, respectively. The fluence ϕ (electrons cm−2) for the irradiation period t is determined by ϕ(t ) =
∫0
t
I(t ′) dt ′ eA irrad
(1)
where Airrad is the irradiation area and I is the current transmitted through the sample (see Experimental Section). Error bars on γH take into account the uncertainties on background subtractions, area calculations, and repeatability of experiments. The eventual error contribution of irradiation13531
DOI: 10.1021/acs.langmuir.5b02109 Langmuir 2015, 31, 13528−13534
Article
Langmuir induced disorganization to νCH depletion is rounded up to 6%. This estimation is based on the off-specular results (Figure 2). Error bars on ϕ result from the irradiation area, current measurement, and cumulated irradiation time uncertainties (estimated to 10%). Our choice for the hydrogen content quantification is supported by an additional point derived from one IR published measurement11 (solid square in Figure 4). The depletion of the νCH band (3010−3090 cm−1) after irradiation at 50 eV of biphenylthiol SAMs was estimated for a fluence of 3 mC cm−2, following the same procedure. This additional point nicely integrates the CH depletion trend measured by HREELS. Figure 4 shows an exponential decay of the hydrogen content, which can be fitted by a standard depletion model46,47 given by eq 2. γH(ϕ) =
Γ = (1 − γH lim)e(−σϕ) + γH lim Γ0
Figure 5. Effective cross-section (given in cm2) for hydrogen content loss in TPT SAMs induced by 50 eV electron irradiation (blue) and estimated cross-section for secondary electron-induced processes (red; see the text) compared to data available in the literature for 50 eV electron-induced processes on gaseous benzene.49−53
section measurements for electronic excitation are available for some given final excited states, for example, 3.0 × 10−17 cm2 for 1 B1u excitation and 2.2 × 10−16 cm2 for 1E1u excitation.52 Keeping in mind that we are extrapolating from gaseous benzene to TPT SAM, we conclude that impact ionization (M + e− → M+ + 2e−) is the major primary mechanism leading to hydrogen content loss under 50 eV electron impact. As a matter of fact, impact electronic excitation (M + e− → M* + e−) is marginally contributing. The contribution of low-energy secondary electrons to the induced chemistry in aromatic SAMs has been discussed in the literature.54 On gold, secondary electron energy distribution strongly peaks at ≈1 eV (asymmetric peak having a fwhm ≈ 5 eV) and spreads up to 50 eV.55,56 At such low energies, below ionization and excitation thresholds, these secondary electrons are able to induce chemical transformation only through dissociative electron attachment (DEA). In the range of 1−17 eV, DEA processes on gaseous benzene have been identified by the production of C6H5− (peak at 8 eV; fwhm ≤ 1 eV) and C2H2− (peak at 9 eV; fwhm ∼ 3 eV).57 Bigger aromatic compounds exhibit DEA resonance for (M − H)− production in the same energy range,44 but the energy decreases to 7 eV with the increasing π system extension. For example, compounds made of three phenyl rings, phenanthrene and anthracene, exhibit DEA resonances at 7−8 eV, with fwhm ≈ 3 eV and cross-section for electronic attachment typically between 1 and 8 × 10−17 cm2. On TPT SAMs, an effective cross-section for chemical modification induced by irradiation at 6 eV was estimated to 1.2 × 10−16 cm2.22 The proposed selfpropagating radical reactions contribute to the overall efficiency of the CH content transformation reaction, proceeding without hydrogen loss. Nevertheless, the contribution of secondary electrons to the chemical transformations proceeding with H loss can be estimated by multiplying the DEA cross-section(s) by the production yield of secondary electrons formed within the window of the resonance.58 The yield of secondary electron production by electron impact at 50 eV on gold is typically 0.6.59,60 The resonance mentioned just above for TPT SAMs has a width of ≈3 eV.34 We have graphically estimated the fraction of electrons, produced with an energy of 6.0 ± 1.5 eV. They represent 5−10% of the overall distribution. Thus, the associated maximum cross-section can crudely be estimated to 3−8 × 10−18 cm2 (also represented in Figure 5). To extend the study, we performed an irradiation experiment with electrons at 1 eV (the most probable energy of secondary electrons). In this case, there is no apparent change in the HREEL spectra after irradiation up to 2.5 mC cm−2 (Figure 6), excluding the 1 eV secondary electron contribution to the SAM chemical modification under 50 eV electron irradiation. To conclude, the contribution of secondary electrons to 50 eV electron
(2)
γH lim = 28 ± 13% is a parameter adjusted to take into account the high-dose regime where the CH content is resistant to further irradiation. σ = 2.7−4.7 × 10−17 cm2 is the effective cross-section for hydrogen content loss under 50 eV electron impact. Significant modifications are visible for doses as low as 0.3 mC cm−2, which would correspond to 4.3 electrons per molecule, considering a typical SAM packing density of 21.6 Å2/molecule.1
■
DISCUSSION The cross-section extracted from the CH signal depletion is an effective measure of the hydrogen content loss under electron irradiation at 50 eV. It takes into account all processes that can be induced by 50 eV electron impact on the sample and that lead, directly or indirectly, to hydrogen loss. It might be a chain of reactions starting with the production of neutral and/or ionic radicals. Different electron−molecule primary interactions are able to produce radicals that will evolve with the release of hydrogen: dissociative ionization, dissociative electronic excitation, or dissociative electronic attachment.17−19,48 To estimate which processes are contributing, one has to consider primary electrons with kinetic energy of 50 eV and secondary electrons of lower energy produced by ionization of the sample. Their relative contributions can be estimated considering the cross-sections available in the literature for electron/molecular system interactions at 50 eV. A consistent set of data was gathered concerning electron interactions with gaseous benzene. Meaningful comparisons between C6H6(g) and TPT SAMs were already performed and shown to bring interesting insights into the electronic structure and electron attachment resonance.30,34 Figure 5 is a graphical view of these data, where the crosssection determined for hydrogen content loss of TPT SAMs under 50 eV irradiation is also represented. The total scattering cross-section of 50 eV electrons on gaseous benzene is 3.8 × 10−15 cm2.49 This overall cross-section is 2 orders of magnitude above the cross-section measured here for chemical modification and concerns both elastic and inelastic impacts on benzene. The total inelastic cross-section, 1.8 × 10−15 cm2,50 includes in particular impact ionization, electron impact excitation, and electron attachment (EA). EA is unlikely for energies high above ionization potential and can be omitted at 50 eV.18 The cross-section for electron impact ionization on benzene at 50 eV is measured at 1.2 × 10−15 cm2.51 Cross13532
DOI: 10.1021/acs.langmuir.5b02109 Langmuir 2015, 31, 13528−13534
Langmuir
■
Article
AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
■
ACKNOWLEDGMENTS The authors thank Dr. Christophe Poulard for his help for the UV−ozone treatment. The authors acknowledge the financial support by C’Nano, Ile-de-France, Programme Francilien de Recherche en Nanosciences, through the 2009 SAMTOX-NC granted project. This work was conducted within the framework of the COST action CM1301 (CELINA) and the ANR-DFG 2014 HREELM project. The acquisition of the HREELS equipment at the LCAM−ISMO was financially supported by Conseil Général de l’Essonne and the LabEx PALM “ProDaC” project. The stay in France of Leo Sala was supported by Erasmus Mundus Master Course SERP-Chem.
■
Figure 6. TPT HREEL spectra (EHREELS = 6 eV, and ΔEfwhm = 7 meV) for different irradiation doses at 1 eV: (a) pristine SAM (red), (b) 0.35 mC cm−2 (blue), and (c) 2.5 mC cm−2 (purple). Curves have been arbitrarily shifted for clarity reasons.
ABBREVIATIONS USED SAM, self-assembled monolayer; TPT, p-terphenylthiol; HREELS, high-resolution electron energy loss spectroscopy; ESD, electron-stimulated desorption; UHV, ultrahigh vacuum; fwhm, full width at half maximum; QMS, quadrupole mass spectrometer; DEA, dissociative electron attachment
■
irradiation-induced chemistry appears to be much weaker than direct impact ionization and electronic excitation. Finally, we can compare the H content loss cross-section on aromatic SAMs to the depletion cross-section for the CH stretch IR signature of alkyl SAM on gold under electron irradiation at different energies.61 The highest energy probed in this last study is 20 eV, which is out of the resonance observed at 10 eV. The measured cross-section is 1.5 × 10−16 cm2, which is 10 times more efficient than the electron-induced H content loss observed in the present study at 50 eV. It is indeed known that aliphatic layers are less resistant to electron irradiation than the aromatic layers.62 Moreover, in the case of the aliphatic layer, a single CC bond cleavage induces the loss of several CHx centers and strong depletion of the CH signature, whereas in the aromatic layer, CC bonds are more difficult to cleave. This may even have only indirect consequences on the number of CH centers, unless an entire aromatic ring would be lost.
■
REFERENCES
(1) Schreiber, F. Prog. Surf. Sci. 2000, 65 (5−8), 151−256. (2) Härtl, A.; Schmich, E.; Garrido, J. A.; Hernando, J.; Catharino, S. C. R.; Walter, S.; Feulner, P.; Kromka, A.; Steinmüller, D.; Stutzmann, M. Nat. Mater. 2004, 3 (10), 736−742. (3) Yang, W.; Auciello, O.; Butler, J. E.; Cai, W.; Carlisle, J. A.; Gerbi, J. E.; Gruen, D. M.; Knickerbocker, T.; Lasseter, T. L.; Russell, J. N.; Smith, L. M.; Hamers, R. J. Nat. Mater. 2002, 1 (4), 253−257. (4) Love, J. C.; Estroff, L. A.; Kriebel, J. K.; Nuzzo, R. G.; Whitesides, G. M. Chem. Rev. 2005, 105 (4), 1103−1170. (5) Gölzhäuser, A.; Geyer, W.; Stadler, V.; Eck, W.; Grunze, M.; Edinger, K.; Weimann, T.; Hinze, P. J. Vac. Sci. Technol., B: Microelectron. Process. Phenom. 2000, 18 (6), 3414−3418. (6) Gölzhäuser, A.; Eck, W.; Geyer, W.; Stadler, V.; Weimann, T.; Hinze, P.; Grunze, M. Adv. Mater. 2001, 13 (11), 803−806. (7) Küller, A.; Eck, W.; Stadler, V.; Geyer, W.; Gölzhäuser, A. Appl. Phys. Lett. 2003, 82 (21), 3776−3778. (8) Zharnikov, M.; Shaporenko, A.; Paul, A.; Gölzhäuser, A.; Scholl, A. J. Phys. Chem. B 2005, 109 (11), 5168−5174. (9) Turchanin, A.; El-Desawy, M.; Gölzhäuser, A. Appl. Phys. Lett. 2007, 90 (5), 053102. (10) Turchanin, A.; Tinazli, A.; El-Desawy, M.; Großmann, H.; Schnietz, M.; Solak, H. H.; Tampé, R.; Gölzhäuser, A. Adv. Mater. 2008, 20 (3), 471−477. (11) Geyer, W.; Stadler, V.; Eck, W.; Zharnikov, M.; Gölzhäuser, A.; Grunze, M. Appl. Phys. Lett. 1999, 75 (16), 2401−2403. (12) Meyerbroeker, N.; Waske, P.; Zharnikov, M. J. Chem. Phys. 2015, 142 (10), 101919. (13) She, Z.; DiFalco, A.; Hähner, G.; Buck, M. Beilstein J. Nanotechnol. 2012, 3, 101−113. (14) Tai, Y.; Shaporenko, A.; Noda, H.; Grunze, M.; Zharnikov, M. Adv. Mater. 2005, 17 (14), 1745−1749. (15) Turchanin, A.; Gölzhäuser, A. Prog. Surf. Sci. 2012, 87 (5−8), 108−162. (16) Angelova, P.; Vieker, H.; Weber, N.-E.; Matei, D.; Reimer, O.; Meier, I.; Kurasch, S.; Biskupek, J.; Lorbach, D.; Wunderlich, K.; Chen, L.; Terfort, A.; Klapper, M.; Müllen, K.; Kaiser, U.; Gölzhäuser, A.; Turchanin, A. ACS Nano 2013, 7 (8), 6489−6497.
CONCLUSION
HREEL spectra of a model aromatic SAM of TPT [HS− (C6H4)2−C6H5] have been recorded along the irradiation process by 50 eV electrons under UHV conditions. The overall evolution of the spectrum shows loss of the hydrogen content. The evolution of νCH intensity was used to extract the effective cross-section for hydrogen content loss, σ = 2.7−4.7 × 10−17 cm2. By comparison to cross-sections available in the literature, mostly for gaseous benzene, we conclude that (i) impact ionization is the major primary mechanism involved, with impact electronic excitation contributing only marginally, and (ii) reactive processes induced by low-energy secondary electrons contribute with a cross-section estimated as 1 order of magnitude smaller. In particular, 1 eV electron irradiation does not induce significant chemical modification at 2.5 mC cm−2. 13533
DOI: 10.1021/acs.langmuir.5b02109 Langmuir 2015, 31, 13528−13534
Article
Langmuir (17) Lafosse, A.; Bertin, M.; Hoffman, A.; Azria, R. Surf. Sci. 2009, 603 (10−12), 1873−1877. (18) Arumainayagam, C. R.; Lee, H.-L.; Nelson, R. B.; Haines, D. R.; Gunawardane, R. P. Surf. Sci. Rep. 2010, 65 (1), 1−44. (19) Böhler, E.; Warneke, J.; Swiderek, P. Chem. Soc. Rev. 2013, 42 (24), 9219. (20) Dishner, M. H.; Ivey, M. M.; Gorer, S.; Hemminger, J. C.; Feher, F. J. J. Vac. Sci. Technol., A 1998, 16 (6), 3295−3300. (21) Arrandee metal GmbH. http://www.arrandee.com/ (accessed Sept 18, 2015). (22) Amiaud, L.; Houplin, J.; Bourdier, M.; Humblot, V.; Azria, R.; Pradier, C.-M.; Lafosse, A. Phys. Chem. Chem. Phys. 2014, 16 (3), 1050−1059. (23) Briand, E.; Salmain, M.; Herry, J.-M.; Perrot, H.; Compère, C.; Pradier, C.-M. Biosens. Bioelectron. 2006, 22 (3), 440−448. (24) Tielens, F.; Costa, D.; Humblot, V.; Pradier, C.-M. J. Phys. Chem. C 2008, 112 (1), 182−190. (25) Pireaux, J. J.; Gregoire, C.; Vermeersch, M.; Thiry, P. A.; Caudano, R. Surf. Sci. 1987, 189−190, 903−912. (26) Pireaux, J. J.; Gregoire, C.; Caudano, R.; Vilar, M. R.; Brinkhuis, R.; Schouten, A. J. Langmuir 1991, 7 (11), 2433−2437. (27) Rufael, T. S.; Huntley, D. R.; Mullins, D. R.; Gland, J. L. J. Phys. Chem. 1994, 98 (49), 13022−13027. (28) Barnes, C. J.; Whelan, C. M.; Gregoire, C.; Pireaux, J.-J. Surf. Rev. Lett. 1999, 06 (02), 193−203. (29) Houplin, J. Structuration chimique induite et contrôlée par impact d’électrons lents sur films moléculaires supportés. Ph.D. Thesis, Université Paris Sud (Paris XI), Orsay, France, 2015. (30) Houplin, J.; Amiaud, L.; Sedzik, T.; Dablemont, C.; Teillet-Billy, D.; Rougeau, N.; Lafosse, A. Eur. Phys. J. D 2015, 69 (9), 217. (31) Tanuma, S.; Powell, C. J.; Penn, D. R. Surf. Interface Anal. 1994, 21, 165−176. (32) Duwez, A.-S.; Yu, L. M.; Riga, J.; Pireaux, J.-J.; Delhalle, J. Thin Solid Films 1998, 327−329, 156−160. (33) Duwez, A.-S.; Yu, L.-M.; Riga, J.; Delhalle, J.; Pireaux, J.-J. Langmuir 2000, 16 (16), 6569−6576. (34) Houplin, J.; Amiaud, L.; Dablemont, C.; Lafosse, A. Phys. Chem. Chem. Phys. 2015, 17 (45), 30721−30728. (35) Fuxen, C.; Azzam, W.; Arnold, R.; Witte, G.; Terfort, A.; Wöll, C. Langmuir 2001, 17 (12), 3689−3695. (36) Eck, W.; Stadler, V.; Geyer, W.; Zharnikov, M.; Gölzhäuser, A.; Grunze, M. Adv. Mater. 2000, 12 (11), 805−808. (37) Beyer, A.; Godt, A.; Amin, I.; Nottbohm, C. T.; Schmidt, C.; Zhao, J.; Gölzhäuser, A. Phys. Chem. Chem. Phys. 2008, 10 (48), 7233− 7238. (38) Hamann, T.; Kankate, L.; Böhler, E.; Bredehöft, J. H.; Zhang, F. M.; Gölzhäuser, A.; Swiderek, P. Langmuir 2012, 28 (1), 367−376. (39) National Institute of Standards and Technology (NIST). NIST Chemistry WebBook; NIST: Gaithersburg, MD, 2015; http://webbook. nist.gov/chemistry/ (accessed Sept 18, 2015). (40) Küppers, J. Surf. Sci. Rep. 1995, 22 (7−8), 249−321. (41) Kato, H. S.; Noh, J.; Hara, M.; Kawai, M. J. Phys. Chem. B 2002, 106 (37), 9655−9658. (42) Duwez, A.-S.; Yu, L.-M.; Riga, J.; Delhalle, J.; Pireaux, J.-J. J. Phys. Chem. B 2000, 104 (37), 8830−8835. (43) Duwez, A.-S. J. Electron Spectrosc. Relat. Phenom. 2004, 134 (2− 3), 97−138. (44) Tobita, S.; Meinke, M.; Illenberger, E.; Christophorou, L. G.; Baumgärtel, H.; Leach, S. Chem. Phys. 1992, 161 (3), 501−508. (45) Walker, I. C.; Stamatovic, A.; Wong, S. F. J. Chem. Phys. 1978, 69 (12), 5532−5537. (46) Olsen, C.; Rowntree, P. A. J. Chem. Phys. 1998, 108 (9), 3750− 3764. (47) Raut, U.; Fulvio, D.; Loeffler, M. J.; Baragiola, R. A. Astrophys. J. 2012, 752 (2), 159. (48) Bald, I.; Langer, J.; Tegeder, P.; Ingólfsson, O. Int. J. Mass Spectrom. 2008, 277 (1−3), 4−25. (49) Moiejko, P.; Kasperski, G.; Szmytkowski, C.; Karwasz, G. P.; Brusa, R. S.; Zecca, A. Chem. Phys. Lett. 1996, 257 (3−4), 309−313.
(50) Kato, H.; Garcia, M. C.; Asahina, T.; Hoshino, M.; Makochekanwa, C.; Tanaka, H.; Blanco, F.; García, G. Phys. Rev. A: At., Mol., Opt. Phys. 2009, 79 (6), 062703. (51) Hwang, W.; Kim, Y.-K.; Rudd, M. E. J. Chem. Phys. 1996, 104 (8), 2956−2966. (52) Kato, H.; Hoshino, M.; Tanaka, H.; Limão-Vieira, P.; Ingólfsson, O.; Campbell, L.; Brunger, M. J. J. Chem. Phys. 2011, 134 (13), 134308. (53) Beenakker, C. I. M.; de Heer, F. J. Chem. Phys. Lett. 1974, 29 (1), 89−92. (54) Turchanin, A.; Käfer, D.; El-Desawy, M.; Wöll, C.; Witte, G.; Gölzhäuser, A. Langmuir 2009, 25 (13), 7342−7352. (55) Palmberg, P. W.; Rhodin, T. N. J. Appl. Phys. 1968, 39 (5), 2425−2432. (56) Seah, M. P. Surf. Sci. 1969, 17 (1), 132−160. (57) Fenzlaff, H.-P.; Illenberger, E. Int. J. Mass Spectrom. Ion Processes 1984, 59 (2), 185−202. (58) Mason, N. J. AIP Conference Proceedings; AIP Publishing: Melville, NY, 2003; Vol. 680, pp 885−890. (59) Petry, R. L. Phys. Rev. 1926, 28 (2), 362−366. (60) Völkel, B.; Gölzhäuser, A.; Müller, H. U.; David, C.; Grunze, M. J. Vac. Sci. Technol., B: Microelectron. Process. Phenom. 1997, 15 (6), 2877−2881. (61) Rowntree, P. A. Surf. Sci. 1997, 390 (1−3), 70−78. (62) Zharnikov, M.; Grunze, M. J. Vac. Sci. Technol., B: Microelectron. Process. Phenom. 2002, 20 (5), 1793−1807.
13534
DOI: 10.1021/acs.langmuir.5b02109 Langmuir 2015, 31, 13528−13534
