Chain-Length-Dependent Branching of Irradiation-Induced Processes

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Chain-Length-Dependent Branching of Irradiation-Induced Processes in Alkanethiolate Self-Assembled Monolayers H. Hamoudi,† F. Chesneau,† C. Patze, and M. Zharnikov* Angewandte Physikalische Chemie, UniVersita¨t Heidelberg, Im Neuenheimer Feld 253, 69120 Heidelberg, Germany ReceiVed: October 1, 2010; ReVised Manuscript ReceiVed: NoVember 22, 2010

The effect of X-ray and low-energy (50 eV) electron irradiation on short-chain alkanethiolate (AT) selfassembled monolayers (SAMs) on Au(111) was studied by synchrotron-based high resolution X-ray photoelectron spectroscopy and infrared reflection absorption spectroscopy. As a test system, a SAM of hexanethiolate (C6) was used, whereas an analogous long-chain film, dodecanethiolate (C12) SAM, was taken as a reference. While both the C6 and C12 films exhibited a full range of irradiation-induced reactions characteristic of AT SAMs on coinage metal substrates, the branching of the reactions in these two systems was distinctly different. Whereas in the case of C12/Au, the dominant processes were decomposition of the alkyl chains and capture of the released alkylsulfide moieties in the aliphatic matrix, desorption of the complete molecular species emerging after the cleavage of the thiolate-gold bond prevailed in the case of C6/Au. This behavior was explained by higher volatility of the released C6 species and the reduced chemical activity of the alkyl matrix in C6/Au as compared to C12/Au. This matrix contained far less active sites than C12/Au due to enhanced quenching of the primary dissociative excitations by the dipole-image dipole interaction with the substrate. The efficiency of this distance-dependent process is especially high for short-chain AT SAMs. 1. Introduction Self-assembled monolayers (SAMs) have long been important components of interfacial engineering and modern nanotechnology owing to their ability to tailor surface properties such as wetting, adhesion, lubrication, corrosion, and biocompatibility.1-5 An important characteristic of these systems is their reaction to ionizing radiation. First, SAMs can be used as ultrathin resists or templates within the framework of conventional, as well as chemical, electron and X-ray lithography.6-16 Second, irradiation of SAMs can serve as a versatile tool to adjust their composition and fabricate molecular blends.17-20 Third, SAMs can be transformed into ultrathin, free-standing sheets with the help of ionizing radiation.21,22 Fourth, SAMs represent important model systems for the evaluation and monitoring of beam damage effects in complex macromolecular and biological systems.23,24 Fifth, most of the standard techniques for SAM characterization,25 above all X-ray photoelectron spectroscopy (XPS),26,27 X-ray diffraction,28,29 and X-ray absorption spectroscopy (XAS)26,30-33 involve their exposure to electrons or X-rays, so that an estimate of the possible unintentional radiation damage is necessary. It is usually performed with the help of reference samples.27 Note that the major impact of X-ray irradiation, used as the primary tool in the above techniques, is not provided by X-rays themselves but by the emitted photoelectrons and the related inelastic and secondary electrons.34,10 The irradiation-induced modification of SAMs has been studied by different experimental techniques, in particular by XPS,10,23,34-43 infrared-reflection absorption spectroscopy,10,44,45 * To whom correspondence should be addressed. Phone: +49-6221-54 4921. Fax: +49-6221-54 6199. E-mail: Michael.Zharnikov@ urz.uni-heidelberg.de. † These authors contributed equally to this work.

XAS,10,23,37,41-43 and mass spectrometry.23,43,44,46,47 The exposure of SAMs to electrons or X-rays was found to result 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 branching of these processes were found to depend on the molecular architecture of the SAM substituents,6,48 packing density of the SAM,49 and the nature of the substrate.48,50 Note that all of these processes typically evolve at a high rate during the initial stage of the irradiation treatment and exhibit a leveling off behavior at high doses.10 A variety of different SAM systems have been investigated so far with respect to their reaction to ionizing radiation but most studies have been performed on alkanethiolate (AT) SAMs on Au(111),10,23,34,36,37,40,44,47,48,50 which are generally considered as the archetypical SAM system and are frequently used as references and in model experiments.2,5,28,51 In the present work, we study the effect of low-energy electron and X-ray irradiation on short-chain AT SAMs on Au(111). Whereas most of the previous experiments dealing with the effects of ionizing radiation have been performed on long-chain AT SAMs,10,23,34,36,37,40,44,48,47,50 studies of their shortchain counterparts are rare.44,47,52 Even though some differences in cross sections of the individual irradiation processes between the short- and long-chain AT SAMs have been reported,44 it is commonly believed that the general course of these processes is quite similar.10 It is, however, not the case, as will be demonstrated below. 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.

10.1021/jp109434k  2011 American Chemical Society Published on Web 12/09/2010

Branching of Irradiation-Induced Processes in SAMs

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2. Experimental Section The target compounds for SAM preparation were hexanethiol (C6SH) and dodecanethiol (C12SH), the latter system being used as reference. These compounds were purchased from Sigma Aldrich and used as received. All solvents were of analytical purity. The substrates were prepared by deposition of 100 nm of gold onto polished single crystal silicon (100) wafers primed with a 5-nm Ti interlayer for adhesion promotion. The resulting metal films were polycrystalline, with a grain size of 20-50 nm; such films are most frequently used for SAM preparation. In addition, Au substrates with a large terrace size (100-200 nm) were prepared by thermal evaporation of 150 or 300 nm of Au on freshly cleaved mica (rate 2 nm/s, temp. 340 °C).53 The grains and terraces of the gold substrates on both silicon and mica exhibited predominantly the (111) orientation, which was corroborated by the spectroscopic54,55 and STM measurements, respectively. The SAMs were formed by immersion of freshly prepared substrates into 1 mM solution of the target compounds in ethanol for 24 h at room temperature After immersion, the samples were carefully rinsed with pure ethanol and blown dry with argon. The C6 and C12 films were irradiated with electrons or X-rays. Electron irradiation was performed with a Leybold flood gun FG-10/35 mounted at a distance of ∼15 cm from the sample to ensure uniform illumination. The energy of the primary electron beam was set to 50 eV. The doses were estimated by multiplication of the exposure time by the current density, which was monitored with a Faraday cup. X-ray irradiation was performed using the primary photon beam of the undulator beamline I311 at the MAX-lab synchrotron radiation facility in Lund, Sweden. The photon energy was 150 eV. Both original and irradiated films were characterized by synchrotron-based high-resolution XPS spectroscopy (HRXPS) and infrared reflection absorption spectroscopy (IRRAS). All experiments were performed at room temperature. The HRXPS characterization was carried out in situ, under UHV conditions at a base pressure greater than 1.5 × 10-9 mbar. The spectra acquisition time was selected in such a way that no noticeable damage by the primary X-rays occurred during the measurements. Monitoring of X-ray-induced damage by HRXPS was performed at the undulator beamline I311 at the MAX-lab synchrotron radiation facility in Lund, Sweden. We used synchrotron light as the primary X-ray source and a SCIENTA SES200 electron energy analyzer for spectra acquisition. The measurements were carried out in normal emission geometry with an energy resolution of ∼50 meV. Monitoring of electron-induced damage by HRXPS was performed at the HE-SGM beamline of the synchrotron storage ring BESSY II in Berlin, Germany. We used the synchrotron light as the primary X-ray source and a VG Scienta R3000 analyzer for spectra acquisition. The measurements were carried out in normal emission geometry with an energy resolution of 0.1-0.2 eV. The energy scale of the HRXPS spectra was referenced to the Au 4f7/2 peak at a binding energy (BE) of 84.0 eV.56 The spectra were fitted by symmetric Voigt functions and either a Shirley-type or linear background. To fit the S 2p3/2,1/2 doublets, we used a pair of such peaks with the same fwhms, a branching ratios of 2 (2p3/2/2p1/2), and a spin-orbit splitting (verified by fit) of ∼1.18 eV.56 The fits were carried out self-consistently, i.e., the similar 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,57 assuming a

Figure 1. S 2p HRXPS spectra of C6 (upper panel) and C12 (bottom panel) SAMs acquired in the course of exposure of these films to intense X-ray (150 eV) radiation provided by the I311 undulator beamline at the Max-lab synchrotron radiation facility. The exposure time is indicated at the respective curves. Also presented are fits by the doublets related to the pristine thiolate (S2, solid blue line), pristine “different thiolate” (S1, solid dark yellow line), and irradiation-induced dialkylsulfide (S3, solid red line) moieties.

standard exponential attenuation of the photoelectron signal and using the attenuation lengths reported in ref 58. A pristine C12 SAM was used as reference to determine the spectrometerspecific constants necessary for the evaluation. Monitoring of electron-induced damage by IRRAS was performed ex situ, i.e., after exposure of the irradiated samples to ambient which was necessary for their transfer to the spectrometer. The spectra were measured using a Vertex 70 Fourier transform spectrometer (Bruker) equipped with a liquidnitrogen-cooled mercury-cadmium-telluride detector. The spectra were recorded using p-polarized light at a fixed incidence angle of 80° with respect to the surface normal. The spectra were acquired at 2 cm-1 resolution with the accumulation of 1024 scans over 4000-660 cm-1 spectral range. The spectra are reported in absorbance units A ) -log R/R0, where R is the reflectivity of the substrate with the monolayer and R0 is the reflectivity of the reference. Substrates covered with a perdeuterated octadecanethiolate SAM were used as reference. 3. Results 3.1. X-ray-Induced DamagesMonitoring by HRXPS. S 2p HRXPS spectra of C6 and C12 SAMs acquired after the exposure of these films to intense X-ray radiation (150 eV) are presented in Figure 1. The spectra of the pristine films are dominated by a characteristic doublet at a BE of ∼162.0 eV (S 2p3/2). This doublet can be assigned to the thiolate moiety bonded to the surface of gold.27,55,59 For C12/Au, this doublet is practically the only perceptible spectral feature, which suggests that all of the constituents of this SAM are bound to the substrate in the same manner. For C6/Au, this doublet is accompanied by an additional one at a BE of ∼161.0 eV (S 2p3/2), with the intensity of this feature being ∼20% of the total S 2p intensity. This feature can be ascribed to either atomic sulfur60 or a thiolate-type bound sulfur with a different binding chemistry

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and/or geometry as compared to the “conventional” thiolatetype bond observed in thiol-derived SAMs on coinage metal substrates61,62 (the exact geometry of the latter bond is still under discussion).63-67 Both assignments are discussed in detail in refs 68 and 69, but in most situations, including the present case, the “different thiolate” assignment can be favored. Note that the doublet at ∼161.0 eV is frequently present in the HRXPS spectra of thiolate SAMs.27,68,70,71 In particular, it has been observed for C6/Au before.66 The spectrum of C12/Au exhibits significant changes after just a few minutes of irradiation. In addition to the original thiolate-related doublet, a new doublet appears at a BE of 163.4 eV. This doublet can be associated with the cleavage of the original S-Au bonds and trapping of the released fragments in the aliphatic matrix, presumably, in the form of dialkylsulfides which are depth-distributed within this matrix.40 The trapping is mediated by the high chemical reactivity of the aliphatic matrix due to the irradiation-induced cleavage of C-H and C-C bonds.10,40,44,47 In contrast to C12/Au, the changes occurring in the S 2p spectra of C6/Au are noticeably less pronounced. Even after 18 min of irradiation which is by a factor of ∼2.5 longer than for C12/Au, the doublet related to the dialkylsulfide species is hardly perceptible. This suggests, at least at first sight, an abnormally high stability of C6/Au with respect to ionizing radiation, which contradicts the general view about the sensitivity of AT SAMs to that external stimulus. Such an unusual behavior had to be clarified in detail, to which purpose we have performed the electron-irradiation experiments described in the next two sections. The advantages of using electrons instead of X-rays are the easier adjustment and exact knowledge of the irradiation dose. The physics and chemistry of irradiationinduced processes should be almost identical since, as mentioned in Section 1, the major effect in the case of X-rays is not provided by the X-rays themselves but by the photo-, inelastic, and secondary electrons.10,34 Note that for the systems in this study, these electrons originated mostly from the substrate owing to the noticeably higher photoionization cross sections of heavy atoms (Au) as compared to light ones (C).72,73 3.2. Electron-Induced DamagesMonitoring by HRXPS. S 2p HRXPS spectra of pristine and irradiated C6 and C12 SAMs are presented in Figures 2 and 3, respectively, along with the corresponding fits by the doublets related to the pristine and irradiation induced sulfur-derived species. Similar to the data in Figure 1, the spectrum of pristine C12/Au exhibits only the thiolate-related doublet (162.0 eV for S 2p3/2), whereas in the case of C6/Au, this doublet is accompanied by the additional feature (161.0 eV for S 2p3/2) associated with the “different thiolate” (∼ 20% of the total intensity). The spectra of C6/Au and C12/Au exhibit typical changes in the course of electron irradiation, analogous to the changes observed in Figure 1. The intensity of the doublets (162.0 and 161.0 eV for S 2p3/2) related to the pristine thiolate species decreases whereas the new doublet (163.4 eV) associated with the irradiation-induced dialkylsulfide species appears and increases in intensity. Similar to the data in Figure 1, the rates and extents of the changes observed for C6/Au and C12/Au are noticeably different. In particular, whereas the intensity of the dialkylsulfide doublet for C6/Au does not exceed that of the thiolate doublet even at a maximum dose of 15 mC/cm2, this happens at a dose of 2 mC/cm2 in the case of C12/Au. More detailed information can be obtained from the dose dependencies of the intensities related to the headgroup-derived species in C6/Au and C12/Au, which are presented in Figures

Hamoudi et al.

Figure 2. S 2p HRXPS spectra of pristine and irradiated C6 SAMs (open circles) acquired at a photon energy of 350 eV, along with the corresponding fits by the doublets related to the pristine thiolate (S2, solid blue line), pristine “different thiolate” (S1, solid dark yellow line), and irradiation-induced dialkylsulfide (S3, solid red line) moieties. The BE positions of the doublets (S 2p3/2) are highlighted by the vertical dashed lines. The background of the individual spectra is shown by the horizontal dashed lines. The irradiation doses are indicated at the respective curves.

Figure 3. S 2p HRXPS spectra of pristine and irradiated C12 SAMs (open circles) acquired at a photon energy of 350 eV, along with the corresponding fits by the doublets related to the pristine thiolate (S2, solid blue line) and irradiation-induced dialkylsulfide (S3, solid red line) moieties. The BE positions of the doublets (S 2p3/2) are highlighted by the vertical dashed lines. The background of the individual spectra is shown by the horizontal dashed lines. The irradiation doses are indicated at the respective curves.

4 and 5, respectively. In the case of C12/Au (Figure 5), the observed behavior is in agreement with previous experiments.41,48 In particular, the crossing of the curves describing the thiolate and dialkylsulfide species occurs at ∼1.5 mC/cm2 as previously observed.41,48 Another characteristic feature is the initial increase and subsequent decrease of the total S 2p intensity. First, this behavior is related to the different location of the emerging dialkylsulfide species as compared to the original thiolate ones. Photoemission signal from the species trapped in the alkyl matrix is less attenuated than the signal from those located at the SAMsubstrate interface.55 Thus, following the emerging dialkylsulfide moieties, the total S 2p signal increases. Second, the S 2p signals


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Figure 4. Dose dependencies of the intensities related to the headgroup-derived species in C6/Au. The values are normalized to the total S 2p intensity for the pristine film: pristine thiolate moieties (S1 + S2, green diamonds and solid line), pristine “conventional” thiolate moieties (S2, blue up triangles and solid line), pristine “different thiolate” moieties (S1, dark yellow down triangles and solid line), irradiationinduced dialkylsulfide species (S3, red circles and solid line), and the total S2p intensity (S1 + S2 + S3, black squares and line). Figure 6. C 1s XPS spectra of pristine and irradiated C6 (upper panel) and C12 (bottom panel) SAMs. The irradiation doses are indicated at the respective curves.

Figure 5. Dose dependencies of the intensities related to the headgroup-derived species in C12/Au. The values are normalized to the total S 2p intensity for the pristine film: pristine thiolate moieties (S2, blue up triangles and solid line), irradiation-induced dialkylsulfide species (S3, red circles and solid line), and the total S2p intensity (S2 + S3, black squares and line).

related to both thiolate and dialkylsulfide species increase due to the reduction of the film thickness (less attenuation) associated with the irradiation-induced desorption of chain fragments (see below). Along with the hydrocarbon pieces, sulfur-containing fragments desorb to some extent resulting, at higher doses, in the observed decrease in the total S 2p intensity. The behavior of C6/Au is radically different (Figure 4). First of all, the formation of the dialkylsulfide species occurs to a noticeably lower extent as compared to C12/Au. Second, the total S 2p intensity does not exhibit a bump at low irradiation doses but decreases continuously and to a higher extent in the course of the irradiation treatment as compared to C12/Au. Finally, the reduction of the thiolate species occurs at a lower rate relative to C12/Au as far as one considers both types of thiolates (161 and 162 eV) together. At the same time, the reduction rate of the conventional thiolate in C6/Au is quite similar to that of C12/Au, at least during the initial stage of irradiation. Complementary information about the irradiation-induced modification of C6 and C12 films is provided by the C 1s HRXPS spectra presented in Figure 6. The spectra of pristine C6 and C12 films exhibit a single emission at 284.35 and 284.9

eV, respectively, in accordance with literature data.66 This emission, which is broader in the case of C6/Au, is associated with the alkyl backbone of C6 and C12 SAMs. The difference in the BE position is related to the different screening of the photoemission hole by the conduction electrons of the substrate; the extent of this effect decreases with increasing distance between the excitation site and the substrate.74 The difference in the fwhm is related to a stronger screening gradient in the vicinity of the substrate, where not only the image charge but also the direct electron transfer screening may occur to some extent.75,76 During irradiation, the C 1s emission shifted to lower BE, broadened, and decreased in intensity, which is a typical behavior of AT SAMs.10,40,48 The downward shift is assigned to the progressive 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.10,40,48 The dose dependency of the effective thickness of the C6 and C12 films, derived on the basis of the C1s and Au4f spectra (see Section 2), is presented in Figure 7. In contrast to the differences observed in the S 2p spectra (see Figures 2 and 3), the behavior of the relative thickness in these films is almost identical, even though the absolute values are noticeably higher for the thicker C12 SAM. 3.3. Electron-Induced DamagesMonitoring by IRRAS. Infrared reflection absorption spectra were recorded in the C-H stretching region. The spectra for both pristine and irradiated C6 and C12 SAMs are shown in Figure 8. The characteristic absorption maxima associated with the symmetric (s) and asymmetric (as) stretching modes of methyl and methylene are marked. All bands are well pronounced in the case of pristine C12/Au, whereas the methylene bands in the spectrum of pristine C6/Au are comparably weak. This is presumably related to a lower degree of orientational and conformational order in the latter film as compared to C12/Au. The positions of the symmetric (s) and asymmetric (as) stretching modes of methylene in the latter film are ∼2919 and ∼2850 cm-1, respectively. Such band positions are characteristic for crystalline-like packing

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Figure 7. Relative thicknesses of C6 (blue up triangles and solid line) and C12 (red down triangles and solid line) SAMs as functions of irradiation dose. The values are normalized to the thickness of the respective pristine film.

Hamoudi et al. et al.59 gave the same development of the intensities of the two characteristic methyl modes for the presence of gauche conformation at the topmost CH2 group. The appearance of these gauche defects, associated with reduced molecular order in the film, is also evidenced by the blue shift of the methylene-related absorption peaks observed for C12/Au. For comparison, a liquid alkane at room temperature has ∼40% C-C gauche conformations and peak maxima at 2928 and 2856 cm-1.77 Finally, the observed broadening of the absorption peaks is a fingerprint of chemical and structural inhomogeneity.10 In contrast to C12/Au, no significant reduction in the intensity of the methyl stretching bands upon exposure to electrons is observed in the IRRAS spectra of C6/Au, which suggests that the methyl moieties in this particular system survive the irradiation treatment, at least during its initial stage (up to 1.5 mC/cm2). Similar behavior has also previously been observed for C4/Au and C8/Au,44 which, on one hand, underlines the reliability of our results, and, on the other hand, implies that the observed reaction to ionizing radiation is typical for all shortchain AT SAMs. In contrast to the intensity of the methyl stretching bands, their renormalization behavior is similar for C6/Au and C12/Au. In the case of C6/Au, this intensity renormalization appears as an increase in the intensity of the asymmetric methyl stretching band and a simultaneous decrease in the intensity of the symmetric band. This suggests, similar to the case of C12/Au, a partial reorientation of the methyl groups in C6/Au, associated with a partial replacement of the all-trans conformation of the upper CH2 entities by a gauche conformation. This process seems however to occur to a smaller extent in C6/Au as compared to C12/Au. 4. Discussion

Figure 8. IRRAS spectra of pristine and irradiated (1.5 mC/cm2) C6 and C12 SAMs acquired in the region of the characteristic stretching modes of methylene and methyl groups. The positions of these modes for pristine C12/Au are highlighted by the vertical dashed lines; the modes are marked (see text for details).

of the alkyl chains. This suggests the expected all-trans conformation of these chains with a very low amount of gauche defects in C12/Au.77,78 The exposure of C12/Au to electrons resulted in a shift of the peak positions and in changes in their widths and heights, with the methyl-related bands being affected the most. This suggests, in accordance with literature data,10,44,45,47 that the methyl groups in C12/Au are affected more strongly by the irradiation treatment than the methylene units. Apart from dissociation, a reorientation of the methyl groups occurs as follows from the exact behavior of the respective bands, i.e., a much stronger decrease of the symmetric methyl stretching band (almost disappeared) as compared to the asymmetric one. Indeed, the pronounced symmetric CH3 mode is associated with a strong component of the corresponding transition dipole moment in the normal direction typical of well-ordered AT films. A relative change in the intensity of this mode with respect to the antisymmetric CH3 mode implies a reorientation or a loss of the orientational order of the methyl groups, i.e., a partial replacement of the all-trans conformation of the upper CH2 entities by a gauche conformation. Note that IR spectra simulations for octadecanethiolate SAMs on gold by Laibinis

The different behavior of C6/Au and C12/Au with respect to ionizing radiation is unexpected and not easy to explain, at least at first sight. Indeed, the chemical compositions of the C6 and C12 moieties are almost identical, with the only difference being the length of the alkyl chain. Also, the crystallographic structures of these films are quite similar, even though the short chain C6 film is presumably characterized by a larger amount of Gauche defects and larger orientational disorder as compared to C12/Au.28 According to STM data,66 the dominant structural motif in both C6/Au and C12/Au is the (23 × 3)rect lattice (the latter structure is also frequently denoted as c(4 × 2) superlattice).28,79,80 In C12/Au, this structure, however, coexists with the (3 × 3)R30° arrangement, with the relative weights of both structures being elusive and presumably dependent upon the identity of the substrate and the exact parameters of the preparation procedure.66 Note that such a coexistence is typical for long-chain AT SAMs on Au(111),81 even though the (23 × 3)rect lattice is dominating. However, it would probably be too much speculation to relate the observed differences in the behavior of C6/Au and C12/Au solely to the presence of a certain fraction of the (3 × 3)R30° motif in the latter SAM. Similarly, it appears unreasonable to us to associate this behavior with the occurrence of differently bound thiolate species in C6/Au. First, the portion of these species is low (∼20%). Second, not only these species, but also conventional thiolate moieties in C6/Au behave differently under ionizing radiation as compared to C12/Au. A tentative explanation of the observed behavior can be provided by the exact analysis of the experimental data in view of the specific course of the individual irradiation-induced processes in C6/Au and C12/Au. At first sight, the S 2p and C


Figure 9. Schematic representation of the modification of C6/Au (on the left) and C12/Au (on the right) by ionizing radiation. The branching of the individual irradiation-induced processes is different for these two systems (see text for details).

1s HRXPS data seem to contradict one another. According to the former data (Figures 1-3), the C6 SAM is considerably more stable toward ionizing radiation as compared to C12. However, as shown in Figure 7, the relative extents of irradiation-induced desorption in C6/Au and C12/Au are quite similar, implying comparable sensitivities of these films to irradiation treatment. This seeming contradiction can be partly resolved in view of the IRRAS data in Figure 8. In the case of C12/Au, these data suggest significant disordering and fragmentation of the film, accompanied by almost complete damage of the SAM-ambient interface. In contrast, the spectra of irradiated C6/Au are characteristic of the almost intact molecular species, with the terminal methyl groups being only slightly affected by the irradiation treatment. In view of these observations, it is reasonable to assume that the major difference between the behavior of C6/Au and C12/ Au with respect to ionizing radiation is different branching of the irradiation-induced reactions, above all fragmentation and desorption (Figure 9). In C12/Au, fragmentation of the alkyl backbone and almost complete damage of the SAM-ambient interface (methyl moieties), followed by desorption of a significant portion of the released hydrogen and hydrocarbon fragments occur. These processes are accompanied by the damage of the SAM-substrate interface associated with the cleavage of the pristine thiolate-gold bonds with most of the released alkylsulfide fragments being captured in the chemically active hydrocarbon matrix in the form of dialkylsulfides. The capture is mostly mediated by the broken bonds and radicals appearing after the cleavage of C-H bonds throughout the alkyl matrix. It should be noted that the analogous active sites appearing after the scission of C-C bonds likely contribute to some extent as well. Such an active matrix and the comparatively large length of the C12 moieties lead us to believe that the desorption of the complete molecules is unlikely for C12/Au. Note that the above behavior is typical for longchain AT SAMs on Au(111), including films of hexadecanethiolates82 and octadecanethiolates.45,48 In contrast to C12/Au, not fragmentation but release and desorption of the complete and almost complete C6 moieties appearing after the cleavage of the thiolate-Au bonds seem to be the dominating irradiation-induced processes for C6/Au. The thinner and less active (see below) alkyl matrix has a reduced ability to capture the released C6 moieties which are smaller and more volatile than the C12 ones. Since most of the released sulfur-containing moieties desorb, the dialkylsulfide species are not observed in the S 2p HRXPS spectra (Figures 1 and 2) of C6/Au thus giving the impression that the C6 SAMs are stable

J. Phys. Chem. C, Vol. 115, No. 2, 2011 539 toward ionizing radiation. There is, however, a noticeable thickness reduction associated with this desorption, as demonstrated in Figure 7. Apart from the comparatively thin hydrocarbon matrix and enhanced (as compared to C12) volatility of the C6 species, there is another phenomenon which, in our opinion, is mostly responsible for the observed branching of the irradiation-induced processes in C6/Au. This phenomenon is the quenching of the excited dissociative states in the alkyl matrix due to the coupling of the respective dipole with the image dipole induced in the substrate.44 If such a quenching occurs fast enough, then bond dissociation will not take place, leaving intact molecular fragments or entire molecules. The rate (or probability) of quenching strongly depends upon the distance between the excited site in the SAM and the substrate as the strength of the dipole-image dipole coupling is proportional to the third power of the separation between the dipoles involved. An important consequence of this is the enhanced probability of C-C and C-H bond scission in the regions adjacent to the SAM-ambient interface as compared to those close to the SAM-substrate one. This results in a depth distribution of the irradiation-induced bond scission events in the SAM83 and predominant damage at the SAM-ambient interface compared to the interior of the SAM.10,44,45,47 Apart from the above effects, the enhanced quenching of the excited states in the vicinity of the substrate should also result in a comparatively small rate and extent of irradiation-induced scission of C-C and C-H bonds in shortchain AT SAMs on metal substrates, as was reported in the literature on the basis of electron-stimulated desorption data (hydrogen yield was monitored).44 Presumably, this is what we observe here. Due to extensive quenching of the excited states in C6/Au, damage to the methyl groups at the SAM-ambient interface occurs at a considerably lower rate as compared to C12/Au. Consequently, the cleavage of the C-C and C-H bonds in the interior of the C6 SAM is reduced, resulting in a less active matrix (as compared to C12/Au) thus diminishing the probability of capturing the alkylsulfide fragments released after cleavage of the thiolate-Au bond. However, despite a low rate, C-C and especially C-H bond scission events occur in C6/Au to some extent leading, after extensive irradiation, to cross-linking between the residual C6 species in the SAM. This cross-linking “seals” the film, preventing further changes (except, probably, for progressive C-H bond scissions) and resulting in a leveling off behavior for the majority of the irradiation-induced processes. Above all, further degradation of the thiolate-gold interface does not occur in the course of the prolonged irradiation treatment since cross-linking bonds between the SAM constituents prevent the release of individual molecular species. A similar situation takes place in aromatic thiol-derived SAMs where extensive cross-linking transforms these films in quasi-polymer sheets preventing the scission of the thiolate-gold bonds.6,10,21 Even if the latter bond is cleaved, it can recombine as long as the released fragments stay in place,23,24 kept there by cross-linking bonds with their neighbors. Note that in contrast to the alkyl matrix, the cleavage of the pristine thiolate-gold bonds should occur at similar rates and to a similar extent in C6/Au and C12/Au since the locations of the headgroup-substrate interface are identical for both films. Indeed, this seems to be the case, as follows from Figures 2 and 3. Therefore, it is not the release of the molecular fragments themselves but their subsequent destiny which is important for the behavior of these systems. The further relevant difference is the extent of fragmentation of the alkyl chains as mentioned above.

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5. Conclusions We studied the effect of X-ray and low-energy electron irradiation on short-chain AT SAMs on Au(111), taking C6 films as test systems and long-chain AT SAMs, C12, as reference. For this purpose, two complementary experimental techniques, viz., synchrotron-based HRXPS and IRRAS were used. Both the alkyl matrix and headgroup-substrate interface in C6 and C12 SAMs were found to be affected by a variety of closely interrelated irradiation-induced processes. The branching of these processes was however distinctly different in these two systems. In C12/Au, which exhibited the typical behavior of long-chain AT SAMs, the dominant irradiation-induced processes are decomposition of the alkyl chains with subsequent desorption of the released fragments and damage of the SAM-ambient interface. The alkylsulfide species appearing after cleavage of the thiolate-gold bond are mostly captured in the alkyl matrix, which becomes chemically active upon progressive cleavage of C-H and C-C bonds. In contrast, in C6/Au, the dominant irradiation-induced process is the desorption of the entire C6 moieties released after cleavage of the thiolate-substrate bonds. This occurs due to the comparatively high volatility of these species as well as small depth and low chemical activity of the alkyl matrix in the case of C6/Au. The latter was explained by the quenching of the primary dissociative excitation due to the interaction of the respective dipole with the induced dipole in the metal substrate. Since this interaction depends strongly upon the spacing between the dipoles, i.e., between the excited site and the metal substrate, such a quenching is much more efficient in short-chain AT SAMs as compared to long-chain ones. There are several practical implications of the above findings. First, short-chain AT SAMs are not well-suited as reference systems to estimate the extent of possible irradiation-induced damage in experiments involving ionizing radiation. Usually, for this purpose, one monitors the S 2p XPS spectra of a reference sample,27 which is suitable as long as this sample is a long-chain AT SAM on Au but, as shown in this work, no longer reliable in the case of analogous short-chain films. Second, short-chain AT SAMs on Au can be good candidates as primary templates for irradiation-induced exchange reactions.17,18 The specific behavior of these systems can affect both the course and kinetics of this reaction, giving new opportunities for the fabrication of mixed SAMs and chemical lithography.13,20 Acknowledgment. We thank M. Grunze for support of this work, Ch. Wo¨ll and A. Nefedov (KIT) for the technical cooperation at BESSY II, E. Moons and L. S. O. Johansson (Karlstad University) for the technical cooperation at Max-lab, and BESSY II and Max-lab staff for the assistance during the experiments. This work has been supported by DFG (ZH 63/ 9-3, ZH 63/10-1, and ZH 63/14-1). References and Notes (1) Ulman, A. An Introduction to Ultrathin Organic Films: LangmuirBlodgett to Self-Assembly; Academic Press: New York, 1991. (2) Ulman, A. Chem. ReV. 1996, 96, 1533–1554. (3) Tour, J. M. Molecular Electronics; World Scientific: Singapore, 2003. (4) Smith, R. K.; Lewis, P. A.; Weiss, P. S. Prog. Surf. Sci. 2004, 75, 1–68. (5) Love, J. C.; Estroff, L. A.; Kriebel, J. K.; Nuzzo, R. G.; Whitesides, G. M. Chem. ReV. 2005, 105, 1103–1169. (6) Geyer, W.; Stadler, V.; Eck, W.; Zharnikov, M.; Go¨lzhaüser, A.; Grunze, M. Appl. Phys. Lett. 1999, 75, 2401–2403. (7) Eck, W.; Stadler, V.; Geyer, W.; Zharnikov, M.; Go¨lzhaüser, A.; Grunze, M. AdV. Mater. 2000, 12, 805–808.

Hamoudi et al. (8) Go¨lzhaüser, A.; Geyer, W.; Stadler, V.; Eck, W.; Grunze, M.; Edinger, K.; Weimann, Th.; Hinze, P. J. Vac. Sci. Technol., B 2000, 18, 3414. (9) Yang, X. M.; Peters, R. D.; Kim, T. K.; Nealey, P. F.; Brandow, S. L.; Chen, M.-S.; Shirey, L.-M.; Dressick, W. J. Langmuir 2001, 17, 228– 233. (10) Zharnikov, M.; Grunze, M. J. Vac. Sci. Technol., B 2002, 20, 1793– 1807. (11) La, Y.-H.; Jung, Y. J.; Kim, H. J.; Kang, T.-H.; Ihm, K.; Kim, K.-J.; Kim, B.; Park, J. W. Langmuir 2003, 19, 4390–4395. (12) Kim, S. O.; Solak, H. H.; Stoykovich, M. P.; Ferrier, N. J.; de Pablo, J. J.; Nealey, P. F. Nature 2003, 424, 411–414. (13) Ballav, N.; Schlip, S.; Zharnikov, M. Ang. Chem. Int. Ed. 2008, 47, 1421–1424. (14) Ballav, N.; Chen, C.-H.; Zharnikov, M. J. Photopolym. Sci. Technol. 2008, 21, 511–517. (15) Schilp, S.; Ballav, N.; Zharnikov, M. Ang. Chem. Int. Ed. 2008, 47, 6786–6789. (16) Ballav, N.; Thomas, H.; Winkler, T.; Terfort, A.; Zharnikov, M. Ang. Chem. Int. Ed. 2009, 48, 5833–5836. (17) Ballav, N.; Shaporenko, A.; Terfort, A.; Zharnikov, M. AdV. Mater. 2007, 19, 998–1000. (18) Ballav, N.; Shaporenko, A.; Krakert, S.; Terfort, A.; Zharnikov, M. J. Phys. Chem. C 2007, 111, 7772–7782. (19) Ballav, N.; Weidner, T.; Ro¨βler, K.; Lang, H.; Zharnikov, M. ChemPhysChem 2007, 8, 819–822. (20) Ballav, N.; Terfort, A.; Zharnikov, M. Langmuir 2009, 25, 9189– 9196. (21) Eck, W.; Ku¨ller, A.; Grunze, M.; Vo¨lkel, B.; Go¨lzhaüser, A. AdV. Mater. 2005, 17, 2583–2587. (22) Turchanin, A.; Beyer, A.; Nottbohm, C.; T.; Zhang, X.; Stosch, R.; Sologubenko, A.; Mayer, J.; Hinze, P.; Weimann, T.; Go¨lzhaüser, A. AdV. Mater. 2009, 21, 1233–1237. (23) Feulner, P.; Niedermayer, T.; Eberle, K.; Schneider, R.; Menzel, D.; Baumer, A.; Schmich, E.; Shaporenko, A.; Tai, Y.; Zharnikov, M. Phys. ReV. Lett. 2004, 93, 1783021–1783024. (24) Shaporenko, A.; Zharnikov, M.; Feulner, P.; Menzel, D. J. Phys.: Condens. Matter. 2006, 18, S1677–S1689. (25) Kind, M.; Wo¨ll, C. Prog. Surf. Sci. 2009, 84, 230–278. (26) Zharnikov, M.; Grunze, M. J. Phys.: Condens. Matter 2001, 13, 11333–11365. (27) Zharnikov, M. J. Electron Spectrosc. Relat. Phenom. 2010, 178179, 380–393. (28) Schreiber, F. Prog. Surf. Sci. 2000, 65, 151–256. (29) Schreiber, F. J. Phys.: Condens. Matter. 2004, 16, R881–R900. (30) Outka, D. A.; Sto¨hr, J.; Rabe, J. P.; Swalen, J. D. J. Chem. Phys. 1988, 88, 4076–4087. (31) (a) Ha¨hner, G.; Kinzler, M.; Wo¨ll, Ch.; Grunze, M.; Scheller, M.; Cederbaum, L. S. Phys. ReV. Lett. 1991, 67, 851–854. (b) Ha¨hner, G.; Kinzler, M.; Wo¨ll, Ch.; Grunze, M.; Scheller, M.; Cederbaum, L. S. Phys. ReV. Lett. 1992, 69, 694–694 (erratum). (32) Ha¨hner, G.; Kinzler, M.; Thu¨mmler, C.; Wo¨ll, Ch.; Grunze, M. J. Vac. Sci. Technol. 1992, 10, 2758–2763. (33) Willey, T. M.; Vance, A. L.; van Buuren, T.; Bostedt, C.; Nelson, A. J.; Terminello, L. J.; Fadley, C. S. Langmuir 2004, 20, 2746–2752. (34) Laibinis, P. E.; Graham, R. L.; Biebuyck, H. A.; Whitesides, G. M. Science 1991, 254, 981–983. (35) Sesnardi, K.; Froyd, K.; Parikh, A. N.; Allara, D. L.; Lercel, M. J.; Graighead, H. G. J. Phys. Chem. 1996, 100, 15900–15909. (36) Wirde, M.; Gelius, U.; Dunbar, T.; Allara, D. L. Nucl. Instrum. Methods Phys. Res., Sect. B 1997, 131, 245–251. (37) Ja¨ger, B.; Schu¨rmann, H.; Mu¨ller, H. U.; Himmel, H. J.; Neumann, M.; Grunze, M.; Wo¨ll, Ch. Z. Phys. Chem. 1997, 202, 263–272. (38) Frydman, E.; Cohen, H.; Maoz, R.; Sagiv, J. Langmuir 1997, 13, 5089–5106. (39) Zerulla, D.; Chasse, T. Langmuir 1999, 15, 5285–5294. (40) Heister, K.; Zharnikov, M.; Grunze, M.; Johansson, L. S. O.; Ulman, A. Langmuir 2001, 17, 8–11. (41) Weidner, T.; Shaporenko, A.; Ballav, N.; Ulman, A.; Zharnikov, M. J. Phys. Chem. C 2008, 112, 1191–1198. (42) Weidner, T.; Ballav, N.; Grunze, M.; Terfort, A.; Zharnikov, M. Phys. Status Solidi 2009, 246, 1519–1528. (43) Turchanin, A.; Ka¨fer, D.; El-Desawy, M.; Wo¨ll, C.; Witte, G.; Go¨lzhaüser, A. Langmuir 2009, 25, 7342–7352. (44) Olsen, C.; Rowntree, P. A. J. Chem. Phys. 1998, 108, 3750–3764. (45) Mu¨ller, H. U.; Zharnikov, M.; Vo¨lkel, B.; Schertel, A.; Harder, P.; Grunze, M. J. Phys. Chem. B 1998, 102, 7949–7959. (46) Rowntree, P.; Dugal, P.-C.; Hunting, D.; Sanche, L. J. Phys. Chem. 1996, 100, 4546–4550. (47) Huels, M. A.; Dugal, P. C.; Sanche, L. J. Chem. Phys. 2003, 118, 11168–11178. (48) Zharnikov, M.; Frey, S.; Heister, K.; Grunze, M. Langmuir 2000, 16, 2697–2705.

Branching of Irradiation-Induced Processes in SAMs (49) Frey, S.; Rong, H.-T.; Heister, K.; Yang, Y.-J.; Buck, M.; Zharnikov, M. Langmuir 2002, 18, 3142–3150. (50) Chen, C.-H.; Huang, M.-L.; Wang, S.-C.; Klauser, R.; Shaporenko, A.; Zharnikov, M. J. Phys. Chem. B 2006, 110, 17878–17883. (51) Thin Films: Self-Assembled Monolayers of Thiols; Ulman, A., Ed.; Academic Press: San Diego, CA, 1998. (52) Kondoh, H.; Nozoye, H. J. Phys. Chem. B 1998, 102, 2367–2372. (53) Satterley, C. J.; Lovelock, K. R. J.; Thom, I.; Dhanak, V. R.; Buck, M.; Jones, R. G. Surf. Sci. 2006, 600, 4825–4828. (54) Ko¨hn, F. Angle-Dependent X-Ray Photoelectron Spectroscopy of Organic Monolayers on Gold Surfaces, Diploma Thesis; Universita¨t Heidelberg: Heidelberg, November 1998. (55) Heister, K.; Zharnikov, M.; Grunze, M.; Johansson, L. S. O. J. Phys. Chem. B 2001, 105, 4058–4061. (56) Moulder, J. F.; Stickle, W. E.; Sobol, P. E.; Bomben, K. D. Handbook of X-ray Photoelectron Spectroscopy; Chastian, J., Ed.; PerkinElmer Corp.: Eden Prairie, MN, 1992. (57) Thome, J.; Himmelhaus, M.; Zharnikov, M.; Grunze, M. Langmuir 1998, 14, 7435–7449. (58) Lamont, C. L. A.; Wilkes, J. Langmuir 1999, 15, 2037–2042. (59) Laibinis, P. E.; Whitesides, G. M.; Allara, D. L.; Tao, Y.-T.; Parikh, A. N.; Nuzzo, R. G. J. Am. Chem. Soc. 1991, 113, 7152–7167. (60) Yang, Y. W.; Fan, L. J. Langmuir 2002, 18, 1157–1164. (61) Himmelhaus, M.; Gauss, I.; Buck, M.; Eisert, F.; Grunze, M. J. Electron Spectrosc. Relat. Phenom. 1998, 92, 139–149. (62) Ishida, T.; Mizutani, W.; Akiba, U.; Umemura, K.; Inoue, A.; Choi, N.; Fujihira, M.; Tokumoto, H. J. Phys. Chem. B 1999, 103, 1686–1690. (63) Cossaro, A.; Mazzarello, R.; Rousseau, R.; Casalis, L.; Verdini, A.; Kohlmeyer, A.; Floreano, L.; Scandolo, S.; Morgante, A.; Klein, M. L.; Scoles, G. Science 2008, 321, 943–946. (64) Chaudhuri, A.; Lerotholi, T. J.; Jackson, D. C.; Woodruff, D. P.; Dhanak, V. Phys. ReV. Lett. 2009, 102, 126101 1–4. (65) Chaudhuri, A.; Lerotholi, T. J.; Jackson, D. C.; Woodruff, D. P.; Dhanak, V. R. Surf. Sci. 2010, 604, 227–234. (66) Chesneau, F.; Zhao, J.; Shen, C.; Buck, M.; Zharnikov, M. J. Phys. Chem. C 2010, 114, 7112–7119.

J. Phys. Chem. C, Vol. 115, No. 2, 2011 541 (67) Maksymovych, P.; Voznyy, O.; Dougherty, D. B.; Sorescu, D. C.; Yates, J. T., Jr. Prog. Surf. Sci. 2010, 85, 206–240. (68) Shaporenko, A.; Terfort, A.; Grunze, M.; Zharnikov, M. J. Electron Spectrosc. Relat. Phenom. 2006, 151, 45–51. (69) Weidner, T.; Kra¨mer, A.; Bruhn, C.; Zharnikov, M.; Shaporenko, A.; Siemeling, U.; Tra¨ger, F. Dalton Trans. 2006, 2767–2777. (70) Heister, K.; Rong, H.-T.; Buck, M.; Zharnikov, M.; Grunze, M.; Johansson, L. S. O. J. Phys. Chem. B 2001, 105, 6888–6894. (71) Shaporenko, A.; Heister, K.; Ulman, A.; Grunze, M.; Zharnikov, M. J. Phys. Chem. B 2005, 109, 4096–4103. (72) Band, I. M.; Kharitonov, Yu. I.; Trzhaskovskaya, M. B. At. Data Nuc. Data Tables 1979, 23, 443–505. (73) Yeh, J. J.; Lindau, I. At. Data Nucl. Data Tables 1985, 32, 1–155. (74) Heister, K.; Johansson, L. S. O.; Grunze, M.; Zharnikov, M. Surf. Sci. 2003, 529, 36–46. (75) Neppl, S.; Bauer, U.; Menzel, D.; Feulner, P.; Shaporenko, A.; Zharnikov, M.; Kao, P.; Allara, D. L. Chem. Phys. Lett. 2007, 447, 227– 231. (76) Kao, P.; Neppl, S.; Feulner, P.; Allara, D. L.; Zharnikov, M. J. Phys. Chem. C 2010, 114, 13766–13773. (77) Porter, M. D.; Bright, T. B.; Allara, D. L.; Chidsey, C. E. D. J. Am. Chem. Soc. 1987, 109, 3559–3568. (78) Nuzzo, R. G.; Dubois, L. H.; Allara, D. L. J. Am. Chem. Soc. 1990, 112, 558–569. (79) Fenter, P.; Eisenberger, P.; Liang, K. S. Phys. ReV. Lett. 1993, 70, 2447–2450. (80) Fenter, P.; Eberhardt, A.; Eisenberger, P. Science 1994, 266, 1216– 1218. (81) Noh, J.; Nakajima, K.; Hara, M.; Sasabe, H.; Knoll, W. Korea Polym. J. 1998, 6, 307–311. (82) Zharnikov, M.; Frey, S.; Go¨lzhaüser, A.; Geyer, W.; Grunze, M. Phys. Chem. Chem. Phys. 1999, 1, 3163–3171. (83) Tai, Y.; Shaporenko, A.; Eck, W.; Grunze, M.; Zharnikov, M. Langmuir 2004, 20, 7166–7170.

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