Modification of Alkaneselenolate Monolayers by Low-Energy

University, Ramat Gan 52900, Israel 2. J. Phys. Chem. C , 2008, 112 (4), pp 1191–1198. DOI: 10.1021/jp077234q. Publication Date (Web): January 4...
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J. Phys. Chem. C 2008, 112, 1191-1198

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Modification of Alkaneselenolate Monolayers by Low-Energy Electrons T. Weidner,‡.| A. Shaporenko,‡ N. Ballav,‡ A. Ulman,§,† and M. Zharnikov*,‡ Angewandte Physikalische Chemie, UniVersita¨t Heidelberg, Im Neuenheimer Feld 253, 69120 Heidelberg, Germany, Department of Chemical and Biological Sciences and Engineering, Polytechnic UniVersity, Six Metrotech Center, Brooklyn, New York, and Department of Chemistry, Bar-Ilan UniVersity, Ramat Gan 52900, Israel 2 ReceiVed: September 9, 2007; In Final Form: October 25, 2007

The effect of low-energy (50 eV) electron irradiation on alkaneselenolate (AS) self-assembled monolayers (SAMs) was studied by synchrotron-based X-ray photoelectron spectroscopy and near-edge X-ray absorption fine structure spectroscopy. As a test system, SAMs of dodecaneselenolate (C12Se) were used, and an analogous dodecanethiolate (C12S) SAM was taken as a reference. Both the alkyl matrix and headgroup-substrate interface in AS SAMs were found to be affected by a variety of closely interrelated irradiation-induced processes, which mostly follow a pseudo first-order kinetics and level off at high irradiation doses. The crosssections of the most prominent processes were obtained and found to be in a range of 2-3 × 10-16 cm2. The values are very close to the parameters for analogous alkanethiolate (AT) SAMs, which exhibit similar behavior upon exposure to low-energy electron irradiation. At the same time, the saturation values of the fingerprint parameters for some irradiation-induced processes in AS SAMs appeared to be slightly smaller than the values for analogous AT films. This is explained by a stronger headgroup-substrate bond in the case of selenium.

1. Introduction Self-assembled monolayers (SAMs) became increasingly popular in view of their ability to tailor surface properties such as wetting, adhesion, lubrication, corrosion, and biocompatibility, as well as other potential applications, ranging from nanofabrication to molecular electronics.1-5 An important property of these systems is their reaction to ionizing radiation. On one hand, most of the standard techniques for SAM characterization, such as X-ray photoelectron spectroscopy (XPS),6 X-ray diffraction,7,8 and X-ray absorption spectroscopy (XAS)6,9-12 involve their exposure to electrons or X-rays. On the other hand, SAMs can be used as ultrathin resists or templates within the framework of conventional and chemical electron and X-ray lithography.13-18 Further, irradiation of SAMs can serve as a versatile tool to adjust their composition and fabricate molecular blends.19-21 The exposure of SAMs to electron and X-ray irradiation has been studied by different experimental techniques, including infrared-reflection absorption spectroscopy,17,22,23 XPS,17,24-30 XAS,17,27,29,30 mass spectroscopy,29-32 ellipsometry,23 and more. It has been found that the irradiation results in a variety of complex, closely interrelated processes, including partial decomposition of the SAM constituents, desorption of hydrogen and molecular fragments, orientational and conformational disordering, damage of the headgroup-substrate interface, and cross-linking within the residual film. The exact course, kinetics, and balance of these processes were found to depend on * Author to whom correspondence should be addressed. Phone: +496221-54 4921; fax: +49-6221-54 6199; e-mail: [email protected]. ‡ Universita ¨ t Heidelberg. § Polytechnic University. † Bar-Ilan University. | Present address: Department of Bioengineering, University of Washington, Seattle, WA 98195-1750.

molecular architecture of the SAM substituents,13,33 packing density in the SAM,34 and substrate.33,35 Most of studies have been performed on alkanethiolate (AT) SAMs on Au(111),17,24,26-29,31-33,35 which are probably the best studied SAMs, and are often considered as the model systems for SAM studies.2,5,7,36 In the present work we study the effect of low-energy electron irradiation on the alkaneselenolate (AS) SAMs on Au(111), which differ from alkanethiolates by the headgroup. Selenium is a homologue of sulfur; both elements have the same valence electron configurations and are neighbors in the VIB column of the periodic table. The chemical properties of sulfur and selenium are quite similar, but the reactivity of selenium toward coinage metals, gold in particular, is considered to be slightly higher than that of sulfur, which results in the stronger headgroup-substrate bond on the same substrate,37-39 although an opposite opinion can be found in literature.40,41 Seleniumderived SAMs are considered as alternatives to thiol-derived SAMs, in particular, in view of molecular electronics applications, since selenium offers a better electronic match for the metal surface than the usually used sulfur.42-45 As was shown recently, contact resistance of the anchor group is of crucial importance for the electronic transport in molecular junctions.46 So far there has not been much work on selenium-based films, but the available results show that high-quality SAMs with different organic moieties (aliphatic, aromatic, and semifluorinated) can be formed on both (111) Au and Ag substrates.37,38,40,41,47-56 Neither electron nor X-ray modification of selenium-derived SAMs have been studied so far. Note that the results obtained for low-energy electrons are valid for X-rays as well, since the major impact of X-ray irradiation results from photoelectrons and inelastic and secondary electrons.17,24 In the following section we describe the experimental procedure and techniques. The results are presented and briefly

10.1021/jp077234q CCC: $40.75 © 2008 American Chemical Society Published on Web 01/04/2008

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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 AS SAMs were prepared from didodecyl diselelenide (CH3(CH2)11Se-Se(CH2)11CH3: C12Se-SeC12), which was synthesized according to the protocol of ref 38. The gold substrates were prepared by thermal evaporation of 200 nm of gold (99.99% purity) onto polished single-crystal silicon (100) wafers (Silicon Sense) primed with a 5 nm titanium adhesion layer. Such evaporated films are standard substrates for thiolderived SAMs. They are polycrystalline, with a grain size of 20-50 nm as observed by atomic force microscopy. The grains predominantly exhibit a (111) orientation.57,58 The SAMs were formed according to the procedure of ref 38, i.e., by immersion of freshly prepared substrates into a 5 µM solution of the precursor in absolute ethanol (Riedel-de-Hae¨n, Germany) at room temperature for 24 h. After immersion, the samples were carefully rinsed with pure ethanol, blown dry with argon, and kept for several days in argon-filled glass containers until the characterization. No evidence for impurities or oxidative degradation products was found. As a direct reference to the C12Se films (Se-Se bond cleaves upon the adsorption),37,38 we used SAMs formed from dodecanthiol (CH3(CH2)11SH, C12SH) The C12SH substance was purchased from Sigma-Aldrich-Chemie-GmbH (Germany) and used without further purification. The C12S SAMs were prepared by immersion of freshly prepared Au substrates into a 1 mM C12SH solution in absolute ethanol at room temperature for 24 h (this concentration is typical for the fabrication of highquality AT SAMs).2,8 The same cleaning and sample storage procedure as for the selenium-based films was used. Note that the difference between the concentrations of the target compounds for the preparation of C12Se and C12S films (5 µM vs 1 mM) is related to a low quality of C12Se SAMs in the case of mM concentration.38 In contrast, for µmol solution, C12Se SAMs have similar packing density, orientational order, and molecular inclination as high-quality films formed from C12SH.38 The conformational and orientational order in the above C12Se SAMs is even somewhat higher than that in their thiolate analogous prepared at the optimal conditions, as demonstrated by a smaller molecular inclination (≈28° in C12Se films vs 30-32° in AT SAMs) and slightly lower energy of the characteristic stretching modes of methylene (νa(CH2): 2919.7 cm-1 for C12Se films vs 2920.3 cm-1 for C12S SAMs).38 The C12Se and C12S SAMs were irradiated with a Leybold flood gun FG-10/35, which was mounted at a distance of ≈15 cm from the sample to ensure its uniform illumination. The energy of the primary electron beam was set to 50 eV. Several selected doses up to 4 mC/cm2 were used; most of irradiationinduced effects level off at the latter dose.17 The doses were estimated by multiplication of the exposure time with the current density, which was monitored by a Faraday cup. Both original and irradiated films were characterized by synchrotron-based XPS and angle-resolved NEXAFS spectroscopy. The measurements were performed at the HE-SGM beam line of the synchrotron storage ring BESSY II in Berlin, Germany. The characterization was carried out in situ, 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.17,26-28 The XPS measurements were performed using the synchrotron light as a primary X-ray source and a VG CLAM 2

Figure 1. C1s XPS spectra of pristine and irradiated C12Se films (open circles) acquired at a photon energy of 400 eV, along with the corresponding fits by a single peak (solid lines). A background is shown by dotted line. The irradiation doses are indicated at the respective curves. The BE position of the emission for the pristine film is marked by the dashed line.

analyzer. The spectra acquisition was carried out in normal emission geometry with an energy resolution of ≈0.4 eV. Most spectra were collected with a photon energy of 400 eV. The energy scale was referenced to the Au 4f7/2 peak at a binding energy (BE) of 83.95 eV.59 The spectra were fitted by symmetric Voigt functions and either a Shirley-type or linear background. To fit the Se 3d5/2,3/2 and S 2p3/2,1/2 doublets we used a pair of such peaks with the same fwhms, the branching ratios of 3:2 (3d5/2/3d3/2) and 2 (2p3/2/2p1/2), and spin-orbit splitting values (verified by fit) of ≈0.86 eV (3d5/2/3d3/2) and ≈1.18 eV (2p3/2/ 2p1/2).60 The fits were carried out self-consistently, i.e., the same peak parameters were used for identical spectral regions. The film thickness was determined on the basis of the intensity ratios of the C1s and Au4f emissions,61 assuming a standard exponential attenuation of the photoelectron signal and using the attenuation lengths reported in ref 62 (the procedure has been verified for several reference samples). NEXAFS spectra were acquired at the C K-edge in the partial electron yield mode with a retarding voltage of -150 V. Linear polarized synchrotron light with a polarization factor of ≈82% was used. The energy resolution was ≈0.40 eV. The incidence angle of the light was varied from 90° (E-vector in surface plane) to 20° (E-vector near surface normal) in steps of 10° to 20° to monitor the orientational order in the SAMs. Raw NEXAFS spectra were normalized to the incident photon flux by division through a spectrum of a clean, freshly sputtered gold sample. Further, the spectra were reduced to the standard form by subtracting linear preedge background and normalizing to the unity edge jump (determined by a nearly horizontal plateau 40-50 eV above the absorption edge).63 The photon energy (PE) scale was referenced to the pronounced π1* resonance of highly oriented pyrolytic graphite at 285.38 eV.64 Both XPS and NEXAFS spectra are only presented for the C12Se films, since such spectra for the C12S SAMs have been published before.17,26-28 For the latter systems, we only present the characteristic parameters derived from the spectra. 3. Results 3.1. XPS. C1s XPS spectra of pristine and irradiated C12Se films are presented in Figure 1. The spectrum of the pristine

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Figure 2. Top panel: Thicknesses of the C12Se (full up triangle) and C12S (open down triangle) SAMs as functions of the irradiation dose. Bottom panel: fwhm of the C1s emission for the C12Se (full up triangle) and C12S (open down triangle) SAMs as functions of the irradiation dose.

film exhibited a single emission with a BE of 284.8 eV and a full width at half-maximum (fwhm) of 1.29 eV. This emission is assigned to the aliphatic chain. In the course of irradiation, the emission shifted to lower BE and broadened, and its intensity decreased. In accordance with previous results for AT SAM, for which a similar behavior was observed,17,28,33 the downward shift is assigned to progressing dehydrogenation of the film, the broadening to the chemical and structural inhomogeneity, and the intensity decrease to the desorption of the SAM constituents and their fragments. The dose dependencies of the thickness of the C12Se film and fwhm of the C1s emission are presented in Figure 2, where the analogous data for the C12S SAM are given for comparison. Following the formalism of refs 31 and 33, we described the above dose dependencies by a standard saturation function within a pseudo first-order kinetics formalism,

I ) Isat + (Ipris - Isat) × exp(-σQ/eSirrad)

(1)

where I is the value of a characteristic film parameter in a course of irradiation, Ipris and Isat are the parameter values for the pristine and strongly irradiated film (a leveling off behavior), respectively, Q is the cumulative charge delivered to the surface in Coulombs, e is the electron charge, Sirrad is the area irradiated by the electron beam, and the cross-section σ (expressed here in cm2) is a measure of a rate at which the saturation behavior is achieved. The respective fitting curves are shown in Figure

Figure 3. Se 3d XPS spectra of pristine and irradiated C12Se films (open circles) acquired at a photon energy of 400 eV. The spectra are decomposed by the individual doublets (solid lines). A background is shown by dotted line. The irradiation doses are indicated at the respective curves.

2, and the derived cross-sections are compiled in Table 1. These cross-sections are quite similar for the C12Se and C12S SAMs, even though the values for the former system are slightly larger. The Se 3d XPS spectra of pristine and irradiated C12Se films are presented in Figure 3. The spectrum of the pristine film exhibited a single Se 3d5/2,3/2 doublet with a BE of ∼54.15 eV (Se 3d5/2), accompanied by a weak Au 5p3/2 emission. This doublet is assigned to the selenolate headgroup formed after the cleavage of Se-Se bond in the adsorbate.38,54 In the course of irradiation, the intensity of the selenolate-related doublet decreased, and a new doublet at a BE of 55.3 eV (Se 3d5/2) appeared and continuously increased in intensity. In accordance with previous results for AT SAM, for which a similar behavior of the S 2p XPS spectra (thiolate headgroup) was recorded,17,28,33 the observed change of the of the Se 3d spectra was attributed to the damage of the primary selenolate-gold bonds and appearance of new, irradiation-induced, selenolate-derived species. The latter species can be either diselenides (Se 3d5/2 BE for C12SeSeC12 is 55.3 eV)37 or dialkyl selenides; considering the analogous results for AT SAMs,28 we favor the latter assignment (see also below). The dose dependencies for the intensities of the pristine selenolate doublet, irradiation-induced doublet, and total Se 3d intensity for the C12Se film are presented in Figure 4, in which the analogous data for the C12S SAM are shown for comparison. All the values are normalized to either the Se 3d (C12Se) or S 2p (C12S) intensity for the pristine films. However, in spite of the normalization, the intensities in Figure 4 are not fully representative for the relative

TABLE 1: Cross-Sections (σ) of the Most Prominent Processes Induced by Electron Irradiation (50 eV) in the C12Se and C12S Filmsa desorption (thickness) chemical and structural disordering (C1s fwhm) damage of the pristine headgroups (Se3d/S2p) formation of irradiation-induced headgroup-derived species (Se3d/ S2p) desorption of H/fragmentation (R*: 55°) cross-linking (π*: 55°) structural disordering (R*: 90° to 20°) structural disordering (C-C σ*: 90° to 20°)

C12Se (10-16 cm2)

C12S (10-16 cm2)

1.75 2.75 1.7 3.2

1.50 2.25 1.6 3.3

2.4 1.9 2.9 2.5

2.7 2.3 3.15 1.9

a The cross-sections were derived in accordance with eq 1. The values for the C12S films are given for comparison. The parameter or data set, which was the basis for the corresponding cross-section, are given in the parentheses. The accuracy of the σ values is about (0.2 × 10-16 cm2.

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Figure 4. Dose dependencies of the intensities related to the headgroupderived species in the C12Se (top panel) and C12S (bottom panel) SAMs. The values are normalized to either the Se 3d (C12Se) or S2p (C12S) intensity for the pristine films. Top panel: pristine selenolate moieties (up triangles), irradiation-induced selenium species (circles), and the total Se 3d intensity (diamonds). Bottom panel: pristine thiolate moieties (up triangles), irradiation-induced sulfur species (circles), and the total S 2p intensity (diamonds).

amount of the headgroup-derived species in the SAMs, since the attenuation of the respective signals decreases in the course of the electron treatment because of the thickness reduction of the hydrocarbon overlayer (see Figure 2) and upward diffusion of the released alkylselenolate (alkylthiolate) species into the aliphatic matrix.28 In particular, these are the reasons for the observed increase in the total Se 3d (S 2p) intensity with increasing dose. Whereas the exact depth distribution of the irradiation-induced species in the aliphatic matrix is unknown, the extent of the thickness reduction is known (Figure 2) and could be easily corrected for. The results of the correction procedure are presented in Figure 5. The most striking difference as compared to Figure 4 is a continuous decrease of the total Se3d/S2p intensity in the course of irradiation, except for a small “bump” at low doses, related presumably to the diffusion of the released alkylselenolate (alkyltheolate) species into the aliphatic matrix.28 The curves for the pristine and irradiationinduced species in Figure 4 were fitted according to eq 1; the respective cross-section values are compiled in Table 1. Whereas these cross-sections are quite similar for the C12Se and C12S SAMs, the values for the creation of the irradiation-induced species are almost twice that high as those for the damage of the pristine species. This difference can only be explained by the above-mentioned upward diffusion of the former species into the aliphatic matrix, which results in a smaller attenuation of the respective signal, i.e., in a faster rise of this signal in the course of irradiation. Along with the intensities of the selenide-stemming emissions, the FWHMs of the Se 3d5/2,3/2 components related to both pristine selenolate species and irradiation-induced moieties increase in the course of irradiation, representing the growing inhomogeneity of the SAM-gold interface and residual hydrocarbon matrix. The respective increase is, however, small, being only about 5% for the strongly irradiated films. The irradiation-induced desorption involved mostly a fragmentation of the aliphatic chains and not the takeoff of the entire SAM constituents. This is shown in Figure 6, in which the dose dependencies of the XPS C1s/Se3d and C1s/S2p total intensity

Weidner et al.

Figure 5. Dose dependencies of the intensities related to the headgroupderived species in the C12Se (top panel) and C12S (bottom panel) SAMs. The values are normalized to either the Se 3d (C12Se) or S2p (C12S) intensity for the pristine films and corrected for the thickness reduction induced by electron irradiation. Top panel: pristine selenolate moieties (up triangles), irradiation-induced selenium species (circles), and the total Se 3d intensity (diamonds). Bottom panel: pristine thiolate moieties (up triangles), irradiation-induced sulfur species (circles), and the total S 2p intensity (diamonds).

Figure 6. Dose dependencies of the XPS C1s/Se3d and C1s/S2p total intensity ratios for the C12Se (full up triangle) and C12S (open down triangle) SAMs, respectively.

ratios for the C12Se and C12S SAMs, respectively, are presented. According to this figure, these ratios decreased significantly in the course of irradiation, suggesting that desorption of hydrocarbon species (i.e., molecular fragments) prevailed over the desorption of headgroup-containing species, including the entire molecules. 3.2. NEXAFS. Carbon K-edge NEXAFS spectra of pristine and irradiated C12Se SAMs acquired at an X-ray incident angle of 55° are presented in Figure 7. Because of the magic angle (55°) geometry, these spectra are not affected by the molecular orientation, but are exclusively representative of the electronic structure of unoccupied molecular orbitals of the target moieties.63 The spectrum of pristine C12Se SAM exhibited a C1s absorption edge related to C 1s f continuum excitations and all characteristic absorption resonances of all trans alkyl chains, namely a sharp resonance at ≈287.7 eV, and two broader resonances at ≈293.4 eV and ≈301.6 eV. The two latter features are commonly related to valence, antibonding C-C σ* and C-C′ σ* orbitals9,63 while the resonance at 287.7 eV is alternatively attributed to the excitations into pure valence orbitals,63,65 predominantly Rydberg states66,67 and mixed valence/Rydberg states.68 This resonance consists of several

Modification of Alkaneselenolate Monolayers

Figure 7. Carbon K-edge NEXAFS spectra of pristine and irradiated C12Se SAMs acquired at an X-ray incident angle of 55°. The irradiation doses are indicated at the respective curves. The characteristic absorption resonances are indicated.

individual resonances, which are merged together.65-67 We will denote it as an R* resonance but take into account a possible admixture of antibonding C-H* orbitals. In the course of irradiation, the intensities of all characteristic absorption resonances of the alkyl chains decreased, and a new resonance at a photon energy of ≈285.0 eV appeared and increased in intensity. In accordance with previous results for AT SAM, for which a similar behavior was observed,17,33 we attribute the intensity decrease of the R* resonance to the desorption of hydrogen, fragmentation of the alkyl chains, and conformational disordering; the intensity decrease of the σ* resonances to the two latter processes; and the appearance of the new resonance to cross-linking between the residuals of the SAM constituents, involving, among other bonds, CdC linkages. The new resonance can be identified as π1* resonance characteristic of CdC bonds (this resonance is also observed for phenyl rings).63 The appearance of these bonds follows hydrogen desorption (dominant effect) and molecular fragmentation, resulting in the creation of nonsaturated bonds and radicals in the aliphatic matrix, which interact with each other and build new bonds. As for the R* resonance, its intensity is intrinsically related to the presence of intact methylene groups, but, because of delocalization of the respective molecular orbitals,10 depends also on the length of the alkyl chain as long as the all trans conformation is preserved.11 Therefore, creation of the conformational defects should affect the intensity of this resonance as well, as we mentioned above. Dose dependencies of the intensities of the π1* and R* resonances for the C12Se and C12S SAMs are presented in Figure 8. These dependencies were fitted by eq 1; the respective cross-section values are compiled in Table 1. These cross-sections are quite similar for the C12Se and C12S SAMs, with slightly larger values for the latter system. Except for the electronic structure, NEXAFS spectra provide information on the molecular orientation in the target films. The films with a high orientational order exhibit pronounced linear dichroism, i.e., a dependence of the absorption resonance intensity on the incidence angle of X-rays. A fingerprint of the linear dichroism is a difference between the spectra acquired at the normal and grazing incidence of the primary X-rays. Such difference (90° to 20°) spectra of pristine and irradiated C12Se

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Figure 8. Dose dependencies of the intensities of the π1* (full up triangle) and R* (full circles) resonances for the C12Se (top panel) and C12S (bottom panel) SAMs.

Figure 9. Difference between the C K-edge NEXAFS spectra acquired at X-ray incident angles of 90° and 20° for pristine and irradiated C12Se SAMs. The irradiation doses are indicated at the respective curves. The characteristic absorption resonances are indicated. The dashed lines correspond to zero.

SAMs are presented in Figure 9. The spectrum of the pristine film exhibited pronounced difference peaks related to the R* and σ* resonances. The molecular orbitals related to the R* resonance are supposed to be orientated perpendicular to the alkyl chains,9-11 whereas the transition dipole moments (TDMs) of the orbitals corresponding to the C-C σ* and C-C′ σ* resonances are believed to be directed along the chain axis.10 Considering the TDM orientations, observed signs of the difference peaks for the R* and σ* resonances suggest the expected upright orientation of the alkyl chains in C12Se/Au. In the course of irradiation, the intensities of all difference peaks decreased, until these peaks disappeared completely at high doses. In accordance with previous results for AT SAM, for which a similar behavior was observed,17,33 we attribute the respective changes to progressive disordering of the aliphatic matrix. The dose dependencies of the intensities of the R* and C-C σ* difference peaks in the 90° to 20° difference spectra of the C12Se and C12S SAMs are presented in Figure 10. These

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Figure 10. Dose dependencies of the intensities of the C-C σ* (full up triangle) and R* (full circles) peaks in the 90° to 20° difference spectra of the C12Se (top panel) and C12S (bottom panel) SAMs.

dependencies were fitted by eq 1; the respective cross-sections are compiled in Table 1. These parameters are quite similar for the C12Se and C12S SAMs, with slightly larger values for the latter system. As for the π1* resonance, no pronounced difference peak was observed at the respective photon energy in the difference spectra in Figure 9. This suggests the expected stochastical orientation of cross-linking bonds within the irradiated C12Se films. Similar situation occurs in AT SAMs.17,33 4. Discussion A combination of XPS and NEXAFS spectroscopy gives a consistent and clear picture of the processes occurring upon the irradiation of AS SAMs by low-energy electrons. Both the alkyl matrix and headgroup-substrate interface are affected by parallel and mutually interrelated irradiation-induced processes, which mostly follow a pseudo first-order kinetics, since the respective dose dependencies can be reasonably fitted by an exponential function (see eq 1). Another characteristic feature of these processes is their leveling off at high irradiation doses. In the aliphatic matrix, the dominating processes are progressive cleavage of C-H and C-C bonds, resulting in desorption of hydrogen and hydrocarbon fragments, so that the thickness of the film reduces by about 30% at high irradiation doses. These processes are accompanied by conformational and orientational disordering of the initially well-ordered C12Se film. The bond cleavage leads to the appearance of noticeable amount of nonsaturated bonds and free radicals in the residual film, which, because of the disordering, appear to come close to each other, building intermolecular cross-linking bonds. At the headgroup-substrate interface, a progressive cleavage of the pristine selenolate bonds occurs, followed by “take off” of the released species with their subsequent upward diffusion into the chemically reactive (under electron irradiation) aliphatic matrix and bonding to unsaturated bonds or free radicals, appearing there during irradiation (see above). Some of the released species, including entire molecules, desorb from the surface, but the extent of this process is relatively small, according to the data presented in Figure 5 (a small decrease of the total Se 3d intensity) and in Figure 6 (a noticeable decrease of the C1s/Se3d intensity ratio). At high doses, only

Weidner et al. about 45% of the pristine selenolate species remain intact, and most of the irradiation-induced selinide-containing moieties are integrated into the residual hydrocarbon matrix as dialkyl selenides. Thus, the electron irradiation progressively transforms the densely packed and well ordered AS SAM into a disordered film of unsaturated hydrocarbons, which, however, is still bonded to the substrate by selenolate linkages, even though their amount reduces significantly as compared to the pristine film. The profound cross-linking occurring in this disordered film upon extensive irradiation results in the leveling off of the most of the irradiation-induced processes. In particular, a further desorption of hydrocarbon moieties is not possible anymore (or possible to a small extent only), since, for their release, one should cleave not just one C-C bond as in the pristine film, but several bonds, including probably some CdC linkages as well. Also the cleavage of the headgroup-substrate bond is hardly possible, since it should involve movement a significant amount of cross-linked chains attached to this group. Even if such a cleavage occurs, the respective bond can recombine again as far as the respective fragments stay in place.29,30 The only processes, which probably progress even at high irradiation doses (even though with a smaller rate as compared to the low dose regime) are cleavage of C-H bonds, desorption of hydrogen, and cross-linking. The character and exact course of the processes occurring in AS SAMs upon low-energy electron irradiation are similar to those observed in AT SAMs, which follows both from literature data17,24,26-29,31-33,35 and comparative measurements on the C12S films performed in this study. Also, the cross-sections of the major irradiation-induced processes in the AS and AT SAMs compiled in Table 1 are quite similar, even though some differences are observed for several individual processes. It is not clear, however, whether these minor differences are significant or mostly related to the limited accuracy of the experiments and the data evaluation procedure. An important point in this context is the fact that the cross-section of an irradiation-induced process is not the only characteristic parameter of this process. The second important parameter is the saturation value of the respective fingerprint feature. In other words, it is not only the rate but also the extent of the change that is of importance. Considering from this viewpoint the thickness reduction (Figure 2, top panel) and destruction of the pristine headgroups (Figure 5), one can say that AS films are somewhat more stable toward electron irradiation than their AT counterparts. In fact, the extent of the thickness reduction in the latter SAMs is slightly larger than in the former films. Also, the extent of the reduction of the pristine headgroup in AS SAMs is somewhat lower than that in the AT films. Considering that the structure and the packing density of the AS and AT SAMs on Au(111) are quite similar,38 the higher stability of the former films can be exclusively related to the stronger headgroupsubstrate bond in the case of selenium.37-39 Generally, selenium exhibits a more metallic character and is characterized by a stronger adsorption energy than sulfur.69 Of course, this situation cannot be simply generalized to metal-organic compounds, but there is a sound experimental evidence that organoselenols bind stronger than organothiols to coinage metals.37-39 In particular, this follows from the displacement and competitive adsorption experiments involving diphenyl diselenide and diphenyl disulfide,49 analysis of the XPS data for C12Se and C12S SAMs,37 and monitoring of the adsorbate-induced shift of the substrate emission (HRXPS) for different thiolate- and selenolate-derived SAMs.38,39 Moreover, selenium is slightly less electronegative

Modification of Alkaneselenolate Monolayers than sulfur and larger in size (see ref 37 for details), which suggests a stronger selenolate (soft base)-gold (soft acid) bonding than in the case of thiolate.37 Whereas there are no data on the modification of AS SAM by electron irradiation in the literature, there are a variety of publications devoted to AT SAMs as was mentioned above.17,24,26-29,31-33,35 In particular, the cross-sections of some electron-irradiation processes are available for 10 eV electron energy.31,33 The respective values for the damage of the pristine headgroups and thickness reduction in C12S/Au are (1.8 ( 0.2) × 10-16 cm2 and (1.92 ( 0.2) × 10-16 cm2, respectively,33 which are quite close to the corresponding values presented in Table 1. An unknown factor is the energy dependence of the cross-sections. There are reliable data only for the energies below 20 eV,31,32 which show a resonant behavior at 10 eV,31,32 and tentative data for higher energies (up to 100 eV), which exhibit only a weak effect of the electron energy.22 5. Conclusion We studied the effect of low-energy electron irradiation on AS SAMs on Au(111), taking C12Se films as a test system and an analogous thiol-derived SAM, C12S, as a reference. For this purpose, two complementary experimental techniques, viz. synchrotron-based XPS and NEXAFS spectroscopy, were used. Both the alkyl matrix and headgroup-substrate interface in AS SAMs were found to be affected by a variety of closely interrelated irradiation-induced processes, which mostly follow a pseudo first-order kinetics and level off at high irradiation doses. The most prominent processes in alkyl matrix are progressive conformational and orientational disordering and cleavage of C-H and C-C bonds, followed by desorption of hydrogen and hydrocarbon fragments, and appearance of crosslinking bonds between the molecular residuals. These changes are accompanied by progressive cleavage of the pristine selenolate-substrate bonds at the headgroup-substrate interface, followed by upward diffusion of the released species into the aliphatic matrix, terminated by the building of dialkyl selenide complexes. The cross-sections of the most prominent irradiation-induced processes in AS SAMs, within pseudo first-order kinetics behavior, were obtained; most of the values are in the range of 2-3 × 10-16 cm2. These values were found to be quite similar to the analogous parameters for AT SAMs, which exhibit similar behavior upon exposure to low-energy electron irradiation. At the same time, the saturation values of the fingerprint parameters for some irradiation-induced processes in AS SAMs, i.e., the extent of the changes, were found to be slightly smaller than the analogous values for AT films, which can be explained by a stronger headgroup-substrate bond in the case of AS SAMs, since the structure and packing density of AS and AT films are quite similar. Note that the results obtained in this study are relevant for X-ray damage as well, since, as mentioned above, the major effect of X-rays is mediated by secondary electrons, especially in the case of a metal substrate. Acknowledgment. We thank M. Grunze for the support of this work, Ch. Wo¨ll (Universita¨t Bochum) for providing us with the experimental equipment and technical cooperation at the synchrotron, and the BESSY II staff for their assistance. This work has been supported by DFG (ZH 63/9-2) and the German BMBF (05KS4VHA/4). References and Notes (1) Ulman, A. An Introduction to Ultrathin Organic Films: LangmuirBlodgett to Self-Assembly; Academic Press: New York, 1991.

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