Self-Assembled Monolayers of Cyclic Aliphatic Thiols and Their

Article pubs.acs.org/JPCC

Self-Assembled Monolayers of Cyclic Aliphatic Thiols and Their Reaction toward Electron Irradiation Prashant A. Waske, Nikolaus Meyerbröker, Wolfgang Eck, and Michael Zharnikov* Angewandte Physikalische Chemie, Universität Heidelberg, Im Neuenheimer Feld 253, 69120 Heidelberg, Germany S Supporting Information *

ABSTRACT: We investigated an old dogma, namely, that aromatic self-assembled monolayers (SAMs) are cross-linked and stabilized by irradiation with electrons, whereas aliphatic SAMs on the same substrate are degraded under the same conditions. To this end, we prepared SAMs of novel cycloaliphatic molecules (4-cyclohexylcyclohexanethiols, CCHT) on Au(111) surfaces and studied their response to electron irradiation. The CCHT monolayers demonstrated a comparably low extent of molecular defragmentation and damage of the headgroup−substrate interface. This was explained by the dominance of irradiation-induced cross-linking over defragmentation and desorption events, which was additionally supported by the negative resist performance of the CCHT films in the framework of electron beam lithography. This behavior is distinctly different from SAMs of linear aliphatic molecules, which become severely damaged at comparably low irradiation doses and exhibit the positive resist performance. It is, however, similar to the behavior of aromatic monolayers, which underlines the role of the cyclic structure in the balance of cross-linking and defragmentation in monomolecular films.

1. INTRODUCTION Self-assembled monolayers (SAMs) became increasingly popular in view of their ability to tailor surface properties and a variety of different applications, ranging from nanofabrication to molecular electronics and bioanalytics.1−5 An important property of these systems is their reaction to ionizing radiation. On one hand, many standard characterization techniques applied to SAMs, such as X-ray photoelectron spectroscopy (XPS),6−10 X-ray absorption spectroscopy (XAS),6−9,11 X-ray diffraction,12−15 and X-ray standing-wave approach,16 involve their exposure to X-rays or secondary electrons, with the major impact provided by the latter factor.17 On the other hand, SAMs can be used as ultrathin resists or templates within the framework of conventional and chemical lithography performed with electrons, X-rays, and extreme ultraviolet light, with the primary role of the secondary electrons in two latter cases.18−28 Finally, exposure of SAMs to electrons can serve as a nanofabrication tool, e.g., to prepare complex molecular blends29−32 or to transform SAMs into ultrathin, free-standing sheets.33−36 SAMs are usually comprised of semirigid molecules that are chemically anchored to a suitable substrate.1,2,5 Generally, these molecules consist of three essential building blocks, viz., the headgroup that binds strongly to the substrate, the tail group that constitutes the outer surface of the film, and a spacer that connects the head and tail groups and is responsible for selfassembly. Exposure of the SAM to ionizing radiation results in a variety of complex, closely interrelated processes affecting all building blocks, including damage of the tail groups, partial decomposition of the SAM constituents with subsequent desorption of hydrogen and molecular fragments, orientational © 2012 American Chemical Society

and conformational disordering, damage of the headgroup− substrate interface, and cross-linking within the residual film.22,25,27 The exact course, relative weight, and kinetics of these processes depend on the molecular architecture of the SAM constituents, their packing density, and the substrate, along with thermodynamic parameters such as temperature.22,38−40 In particular, the packing density and, presumably, also the perfection of the crystallographic structure directly influence the quenching probability of the primary excitation, diminishing the effective radiation load for densely packed films and SAMs with better crystallinity.38 The substrate material determines the strength of the headgroup−substrate bond41 and the secondary electron yield,17 which adds to the primary X-ray, extreme UV, or electron flux, giving the total irradiation load.42 However, the biggest influence on the extent and course of irradiation-induced reactions is the molecular architecture, viz., the identity of the molecular backbone. This identity determines the balance between decomposition, disordering, and cross-linking.18,22,27,41 In the case of an aliphatic spacer, the decomposition and disordering processes prevail, resulting in progressive loss of the film identity.41,43−45 In contrast, in the case of aromatic spacers the cross-linking processes are dominant, transforming the primary molecular film into a 2D polymer-like layer37 and enabling selective modification of the tail groups18,19,38 or fabrication of free-standing nanosheets.33−36 This requires, however, huge irradiation doses (30−50 mC/cm2)18,37,46 which are far beyond the irradiation Received: November 9, 2011 Revised: May 7, 2012 Published: June 4, 2012 13559

dx.doi.org/10.1021/jp210768y | J. Phys. Chem. C 2012, 116, 13559−13568

The Journal of Physical Chemistry C

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molecules was calculated to be about 34.6 Å2/molecule, which is still much lower than that for alkanethiols on Au but significantly higher (by 2.6 times) than the areal density in CHT/Au. The latter difference is presumably related to a higher rigidity of CPT as compared to CHT.49 In view of the observed differences in the packing density of the single-ring systems, we decided in favor of two-ring systems in the present work. First, a longer molecular chain favors the formation of densely packed SAMs, which can be crucial in terms of irradiation-induced cross-linking. Second, the number of the carbon atoms in the CCHT precursors is identical to those in dodecanethiol (DDT) and biphenylthiol (BPT), which are well-suited reference systems to investigate the behavior of linear aliphatic and aromatic SAMs, respectively, under electron irradiation.

load necessary to destroy aliphatic SAMs completely (5−8 mC/cm2).40,41 By considering this behavior, one obvious question is, what makes aromatic SAMs so stable under ionizing irradiation as opposed to linear aliphatic SAMs? Is it the aromatic character of the molecule and chemical stabilization of radicals formed after H desorption via resonating structures? Or is it just the stability of a cyclic framework, in which most fragments arising from initial C−C bond cleavage are still connected to the rest of the molecule and therefore cannot desorb during electron irradiation but stabilize themselves again? If it is the cyclic structure (excluding aromaticity) which gives the stability to the film during the irradiation process, in that scenario cycloaliphatic SAMs should also behave in a manner similar to aromatic monolayers. Unfortunately, in the literature there are no reports available describing the electron irradiation behavior of cycloaliphatic SAMs. To fill this gap, we have synthesized two novel cyclic aliphatic SAM precursors, viz., two isomers of 4-cyclohexylsubstituted cyclohexanethiols (CCHT; see Scheme 1),

2. EXPERIMENTAL SECTION 2.1. Synthesis of cis- and trans-CCHT. Synthesis of the trans-CCHT (4a) and cis-CCHT (4b) was carried out using commercially available 4-cyclohexylcyclohexanol 1 (ABCR), which contains a 60:40 ratio of trans- and cis-isomers. The isomers were separated by repeated recrystallization of 1 in cyclohexane at room temperature.50 The trans-isomer 1b crystallized first (in the form of white needles), whereas the cisisomer remained still in solution. The residual solvent was concentrated under reduced pressure to obtain a white solid of cis-1a isomer. The synthesis route is illustrated in Scheme 2. The alcohols 1a and 1b were treated with mesylsulfonyl chloride in the presence of Et3N to afford mesylated products which were further purified by crystallization. The crude cis-product 2a was recrystallized from cyclohexane at room temperature (RT) to

Scheme 1. Two Different Isomeric Forms of CCHT, viz., the Cis- and Trans-Conformersa

Scheme 2. Synthesis of 4a and 4ba

a

The conformers differ in the position of the cyclohexyl ring relative to the thiol group.

prepared the respective SAMs on Au(111) substrate, and studied their behavior under irradiation with low-energy electrons. Note that apart from the latter property, the parameters of the CCHT SAMs are of interest on their own. In the literature, there are only few reports on cycloaliphatic SAMs and they all dealt with single-ring systems.47−49 In particular, Kwon et al.47 studied Raman spectroscopy of cyclohexanethiol (CHT) on silver sol. They found that the molecules adsorb on silver with a coverage-dependent conformation. In particular, both the equatorial and axial chair conformers exist on the surface at low coverage. In contrast, at higher coverage, only the thermodynamically favored equatorial chair conformers were present.47 Further, Noh and Hara48 prepared CHT SAMs on Au(111) and studied their structure by scanning tunneling microscopy (STM). They observed the formation of a long-range (5 × 2√10)R48° superstructure containing two different molecular features which, according to the authors’ opinion, can be associated with the different conformers of CHT. The respective areal density was found to be 88.3 Å2/molecule, which is noticeably lower than the analogous value for alkanethiols (AT) on Au(111) (21.6 Å2/molecule).12,14 Finally, Noh49 studied the formation and structure of cyclopentanethiol (CPT) SAMs on Au(111) and observed an ordered, long-range (√3.5 × √5) R25° superstructure. The average areal density for the adsorbed

a

(i) Methanesulfonyl chloride, Et3N, CH2Cl2, 0°C to RT, 2 h; (ii) KSAc, DMF, reflux, 16 h; (iii) LiAlH4, dry Et2O, 0°C to RT, 2 h. The conformation of the cis- and trans-isomers 1a and 1b is shown as well. Note that there is inversion of the conformation (cis to trans and trans to cis) in step ii.

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afford colorless needles, and recrystallization of crude transisomer 2b in n-pentane at RT afforded colorless pellets of pure 2b. We could differentiate cis- and trans-isomers using 1H NMR spectroscopy; additionally, the structures of 2a and 2b were confirmed by single crystal X-ray analysis (see section 3.1). The reaction of potassium thioaceate (KSAc) with cismesylate 2a yielded trans-thioacetate 3a in an inversion in configuration51 (cf. Scheme 2). The reduction of the thioacetate with LiAlH4 under inert atmosphere afforded trans-CCHT 4a. By using same procedure trans-mesylate 2b afforded cis-CCHT 4b. Analysis of the SAM precursors was performed by NMR, mass spectrometry measurements, and X-ray crystallography. The NMR analysis was carried out on a Bruker Avance 300 or Bruker Avance 600 spectrometer. X-ray crystallography measurements were performed on Bruker Apex instrument. Mass spectrometry analysis was conducted within the EI/CI mode on a JEOL JMS-700 magnetic sector instrument. In particular, 1H and 13C NMR analysis showed that the compounds 4a and 4b are pure and free from the other isomer. Detailed information about the experimental conditions and complete analytical and spectroscopic data are provided in the Supporting Information. 2.2. Preparation of SAMs. The substrate used was a 30 nm thick gold layer evaporated on titanium primed (5 nm) polished silicon (100) wafer, purchased from Georg Albert PVD. This substrate predominantly exhibit (111) orientation, as reported in the literature.52 The SAM preparation conditions such as the solvent, precursor concentration, and temperature were optimized to achieve an optimum quality of the SAMs. The most reliable results were obtained by immersing gold substrate in an n-pentane solution at 2 mM concentration of the precursors under an argon atmosphere for 72 h. After immersion, SAMs were carefully rinsed with pure ethanol and blown dry with argon. In addition to the CCHT monolayers, we also prepared reference DDT and BPT SAMs on Au(111) substrates. The preparation was performed according to standard protocols.40,53 2.3. SAM Characterization and e-Beam Damage Experiments. Both pristine and irradiated films were characterized by conventional X-ray photoelectron spectroscopy (XPS) and synchrotron-based high-resolution XPS spectroscopy (HRXPS). In addition, the pristine films were characterized by infrared reflection absorption spectroscopy (IRRAS) and near-edge X-ray absorption fine structure (NEXAFS) spectroscopy. All experiments were performed at room temperature. The XPS, HRXPS, and NEXAFS measurements were carried out 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. The XPS measurements were performed under UHV conditions with a Max200 (Leybold-Heraeus) spectrometer equipped with Mg Kα X-ray source (250 W) and a hemispherical analyzer. The recorded spectra were divided by a transmission function of the spectrometer. The synchrotron-based HRXPS measurements were performed at the HE-SGM beamline (bending magnet) of the synchrotron storage ring BESSY II in Berlin, Germany, using a Scienta R3000 spectrometer. The synchrotron light served as the X-ray primary source. The spectra acquisition was carried

out in normal emission geometry with an energy resolution of ∼0.3 eV. Along with the characterization of the pristine films, XPS and HRXPS measurements were used to monitor the electroninduced damage. Apart from a few control experiments, the measurements were performed in situ, without the exposure of the irradiated samples to ambience. The electron irradiation was carried out in the adjacent (XPS) or same (HRXPS) vacuum chamber with a FG-10/35 (Leybold) flood gun 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 with the current density, which was monitored with a Faraday cup. Since the energy resolution of the synchrotron-based HRXPS spectra was noticeably better than that of their XPS counterparts, we only present the HRXPS spectra. The XPS data were mainly used for an estimate of the film thickness and packing density (see below). In this regard, they are more reliable than the synchrotronbased HRXPS data (due to the lower energy of the primary Xrays and a stronger effect of the sample positioning on the intensity of the photoemission signal). The binding energy (BE) scale of the XPS and HRXPS spectra was referenced to the Au 4f7/2 peak at a BE of 84.0 eV.54 The spectra were fitted by symmetric Voigt functions and a Shirley-type background. To fit the S 2p3/2,1/2 doublet, we used a pair of such peaks with the same full width at half-maximum (fwhm), a branching ratio of 2 (2p3/2/2p1/2), and a spin−orbit splitting (verified by fit) of ∼1.18 eV.54 The peak fits were carried out self-consistently, i.e., the similar peak parameters were used for identical spectral regions. The effective thickness of the monolayers was calculated using the intensity ratios of the C 1s and Au 4f emissions and assuming a standard exponential attenuation of the photoelectron signal. 55 Attenuation lengths obtained by us experimentally on the basis of the calibration measurements for a series of alkanethiolate SAMs with different thickness on gold were used, viz., 22.5 and 26.1 Å for the C 1s and Au 4f photoelectrons, respectively (Mg Kα excitation); these values are quite close to the literature ones.56 The pre-exponential factors necessary for the evaluation57 were determined along with the attenuation lengths. The thickness of alkanethiolate SAMs was calculated using a height equivalent to one C−C bond perpendicular to the surface of 1.1 Å56 and a length of the S−Au bond of 1.9 Å.58 The NEXAFS measurements were performed at the same beamline as the HRXPS experiments. The acquisition of the spectra was carried out at the carbon K-edge in the partial electron yield (PEY) mode with a retarding voltage of −150 V. Linear polarized synchrotron light with a polarization factor of ∼91% was used. The energy resolution was ∼0.30 eV. The incidence angle of the light was varied from 90° (E-vector in the surface plane) to 20° (E-vector nearly normal to the surface) in steps of 10°−20° to monitor the orientational order of the CCHT molecules within the films. This approach is based on the linear dichroism in X-ray absorption, i.e., the strong dependence of the cross-section of the resonant photoexcitation process on the orientation of the electric field vector of the linearly polarized light with respect to the molecular orbital of interest.59 The raw NEXAFS spectra were normalized to the incident photon flux by division by a spectrum of a clean, freshly sputtered gold sample. Further, the spectra were reduced to the 13561



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standard form by subtracting a linear pre-edge background and normalizing to the unity edge jump (determined by a nearly horizontal plateau 40−50 eV above the absorption edge). The energy scale of the C K-edge spectra was referenced to the most intense π* resonance of highly oriented pyrolytic graphite (HOPG) at 285.38 eV.60 The IRRAS measurements were performed using a Vertex 70 Fourier transform spectrometer (Bruker) equipped with a liquid-nitrogen-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 the 4000−660 cm−1 spectral region. 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. Gold substrates covered with a perdeuterated DDT SAM were used as the reference. In addition to the spectroscopic experiments, resist performance of the CCHT films was tested. For this purpose, the films were first prepatterned through a mesh in proximity printing geometry by 50 eV electrons and then etched for 20 min in a thiosulfate-based solution.61 Imaging of the fabricated patterns was conducted with a LEO 1530 scanning electron microscope. The e-beam energy was 7 keV; the residual gas pressure was ∼5 × 10−6 mbar.

molecules (cf. Figure S3, upper left, Supporting Information). Two molecules are parallel to one another and the mesylate groups are pointing toward the edge of the unit cell. A packing diagram shows a layered two-dimensional structure which consists of eight molecules (cf. Figure S3, upper right, Supporting Information). In case of the trans-isomer 2b, the mesyl and cyclohexyl groups are at equatorial positions (cf. Figure 1). Similar to the case of 2a, the unit cell of 2b consists of four molecules (cf. Figure S3, lower left, Supporting Information). These are arranged in a zigzag fashion in which two molecules are in close vicinity via a mesylate group. A packing diagram of 2b consists of 16 molecules arranged in 2D layers; it looks more crowded than the cis-mesylate packing diagram (cf. Figure S3, lower right, Supporting Information). As seen in Figure 1, both isomers exhibit the energetically more stable chair conformation in the solid state rather than the boat conformation. 3.2. Characterization of the Pristine SAMs by XPS and HRXPS. The S 2p and C 1s HRXPS spectra of the pristine CCHT films are shown in Figures 2 and 3, respectively, along

3. RESULTS AND DISCUSSION 3.1. X-ray Crystallography. Since the SAM precursors 4a (trans-CCHT) and 4b (cis-CCHT) are liquids, crystallographic analysis could only be performed for the intermediate substances 2b and 2a (there is conformation inversion in step ii; see Scheme 2). The results of this analysis are, however, presumably valid for the densely packed assemblies of cisCCHT and trans-CCHT as well. The crystal structure of 2a shows the exact conformation of the cis-isomer (cf. Figure 1) in which the mesyl group (OMs) is in an axial position while the cyclohexyl ring lies at an equatorial position. The respective unit cell consists of four

Figure 2. Normalized S 2p HRXPS spectra of the pristine CCHT films acquired at a photon energy of 350 eV, along with the spectra of the reference DDT and BPT SAMs. The BEs of the S 2p3/2 emission (thiolate) are shown at the respective curves. The vertical dashed lines are guides for the eye. The spectra of the CCHT SAMs are decomposed into the doublets related to the thiolate (S2, blue solid line) and “different thiolate” (S1, red solid line). The background of the spectra is shown as a dotted line.

with the spectra of the reference DDT and BPT SAMs. The S 2p spectra of the CCHT films are dominated by a characteristic doublet at a BE of ≈161.85 eV (S 2p3/2), which is slightly lower than the analogous value for the reference films (162.0 eV). This doublet can be assigned to the thiolate moiety bonded to the surface of gold,10,62,63 which qualifies the CCHT films as well-defined SAMs. The above main feature is accompanied by an additional doublet at a BE of ≈161.0 eV (S 2p3/2), which appears as a weak shoulder at the low BE flank of the main S 2p3/2 emission. The intensity of this additional doublet is 15− 20% of the total S 2p intensity. This feature is observed

Figure 1. Structure and conformation of 2a (cis) and 2b (trans) according to single crystal X-ray analysis. 13562



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to the above thickness values, the packing density in the CCHT films can be estimated on the basis of the S 2p/Au 4f intensity ratio, using a SAM with a known packing density (DDT/Au in our case) as a reference.72 Due to the quite close binding energies of the Au 4f and S 2p emissions, both signals are attenuated similarly as far as the primary excitation is performed at high photon energy, so that their ratio is a measure of the coverage.73 The S 2p/Au 4f intensity ratios for DDT/Au, cisCCHT/Au, and trans-CCHT/Au are 0.078, 0.051 and 0.04, respectively. Considering that the areal density in DDT/Au is 21.6 Å2/molecule,12 the densities in cis-CCHT/Au and transCCHT/Au are 33 and 42 Å2/molecule, respectively, corresponding to the higher effective thickness in the former case. These values are much closer to the average areal density for the SAMs of rigid aliphatic cyclic thiols (CPT) (34.6 Å2/ molecule)49 than that for the monolayers comprised of the less rigid molecules (CHT, 88.3 Å2/molecule).48 3.3. Characterization of the Pristine SAMs by IRRAS. The IRRAS spectra of the CCHT films are shown in Figure 4.

Figure 3. C 1s HRXPS spectra of the pristine CCHT films acquired at a photon energy of 350 eV, along with the spectra of the reference DDT and BPT SAMs. The BEs of the C 1s emission are shown at the respective curves. The vertical dashed lines are guides for the eye.

frequently in the HRXPS spectra of thiolate SAMs10,64−66 and can be ascribed to either atomic sulfur appearing after the cleavage of the S−C bond67 or a thiolate-type bound sulfur with a different binding chemistry and/or geometry as compared to the “conventional” thiolate-type bond observed in thiol-derived SAMs on coinage metal substrates68,69 Both assignments are discussed in detail in refs 66 and 70. We favor the “different thiolate” assignment, but have to say that the “bond breaking” description cannot be excluded completely. Note, however, that the breaking of the S−C bond occurs presumably after the SAM formation, so that the released molecular backbone is captured in the SAM matrix and does not destroy the molecular order (as long as the extent of bond breaking events is low). The C 1s HRXPS spectra of the pristine CCHT films in Figure 3 exhibit single emission with no additional features related to oxidative products or contamination, which, along with the S 2p data, qualify the CCHT films as well-defined and contamination-free SAMs. The binding energy of the emission is between the values for the DDT and BPT SAMs. This relation appears quite logical considering the identity of the CCHT precursors which are, similar to DDT, aliphatic but, similar to BPT, cyclic. Interestingly, the BE of the main emission for cis-CCHT/Au is slightly higher than that for transCCHT/Au. Considering that spectra acquired at a photon energy of 350 eV can be predominantly associated with the SAM−ambience interface, this difference might imply the different extent of the substrate-related screening of the photoemission hole,71 assuming a larger separation between the SAM−ambience interface and the substrate for cis-CCHT/ Au. These considerations are tentative only but agree well with the effective thicknesses of the CCHT films derived on the basis of the C1s/Au4f intensity ratio (see section 2). Indeed, the effective thickness of the cis-CCHT film (9.8 Å) is ∼22% higher than that of the trans-CCHT SAM (7.6 Å). In addition

Figure 4. IRRAS spectra of the pristine CCHT films (C−H stretching region). The most prominent vibration modes are marked.

The spectra exhibit two sharp peaks at 2935 and 2857 cm−1 which can be assigned to the CH2 asymmetric stretching (νas) and symmetric stretching (νs) modes. The positions of these modes are almost the same for cis-CCHT/Au and transCCHT/Au, which indicates that these films are quite similar. The intensity of the modes is slightly lower in the case of transCCHT/Au than for its cis-counterpart, which can be explained by the lower packing density in the former case (see the previous section). For bulk cyclohexanethiol, the reported values for νas(CH2) and νs(CH2) are 2941 and 2874 cm−1, respectively.74 These values are slightly higher than those for the CCHT films, which is presumably related to the differences in the exact molecular packing between the substrate-confined monolayers and bulk materials. Note that the reference measurements on DDT/Au resulted in the values reported in the literature, viz., 2919 and 2850 cm−1 for νas(CH2) and νs(CH2), respectively.40,75 These values are lower than the band energies in the CCHT films, due to the dominance of the alltrans conformation of the linear aliphatic chains in DDT/Au. It 13563



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is well-known that each gauche bond in an all-trans alkyl chain shifts the vibrational frequencies of the adjoining methylene groups by 10−20 wavenumbers.75 More complex conformational distortions will result in an even larger shift, as happens in the CCHT monolayers. 3.4. Characterization of the Pristine SAMs by NEXAFS Spectroscopy. Sampling the electronic structure of the unoccupied molecular orbitals, NEXAFS spectroscopy provides information about the chemical identity of the adsorbed film. Further, using the angular dependence of the transition matrix elements for resonant excitations,59 the average orientation of the constituents can be probed. A fingerprint of such an orientation is the linear dichroism (see section 2) which can be efficiently monitored by plotting the difference of the NEXAFS spectra acquired at normal (90°) and grazing (20°) angles of Xray incidence. In contrast, a spectrum acquired at the so-called magic angle of X-ray incidence (55°) is not affected by any effects related to molecular orientation and gives only information on the chemical identity of investigated samples.59 The magic angle NEXAFS spectra of the CCHT films are presented in Figure 5, along with the respective difference

truly perceptible. At the same time, the spectra are very similar, both regarding the overall shape and the position of the individual resonances, to the spectra of diamondoid thiol monolayers on gold.11,77 Similar to our case, in all diamondoids, the C−H σ*/R* resonances are present as a relatively sharp peak at about 287−289 eV, whereas the most pronounced C− C σ* resonance is centered around 292−293 eV.11,77 This similarity is probably not accidental considering that the small diamondoids are nonaromatic cyclic structures as well. Note that the resonance assignments of refs 11 and 77 look quite reasonable to us and we will use them for further discussion. The spectra of both cis-CCHT/Au and trans-CCHT/Au exhibit pronounced linear dichroism as evidenced by the respective 90°−20° difference curves. The extent of the dichroism is higher for cis-CCHT/Au, which implies a more upright molecular orientation as compared to trans-CCHT/Au. This corresponds to a higher packing density in the former film, which agrees well with the results of the XPS and IR experiments. As for the possible reason, for the higher packing density and smaller molecular inclination in the case of cisCCHT/Au, it can be the bending potential at the headgroup, favoring an substrate−S−Au angle of 120°.78−80 Such a bonding geometry favors a smaller molecular inclination in the case of cis-CCHT/Au, which is exactly what we observe in the experiment. 3.5. Reaction of CCHT films to Electron Irradiation. As mentioned in section 2, the CCHT films were successively irradiated and characterized in situ in the same vacuum chamber to avoid airborne contamination after exposure of irradiated samples to ambient air. It is well-known that such an adsorption occurs to some extent since the surface of the irradiated samples is reactive.22,81 As for the CCHT films, we performed ex situ characterization experiments that revealed that the intensity of the carbon signal doubled and an additional oxygen (O 1s) peak appeared upon exposure of the irradiated films to ambient air. This contamination was not observed under in situ conditions (there was only very weak signal; see Figure S12 in Supporting Information). The reaction of the cis-CCHT and trans-CCHT films to electron irradiation was found to be very similar. Therefore, we present here the data for cis-CCHT/Au only; the data for transCCHT/Au are compiled in the Supporting Information. The S 2p HRXPS spectra of the pristine and irradiated cis-CCHT films are shown in Figure 6, along with the corresponding fits with the doublets related to the pristine and irradiation-induced sulfur-derived species. As mentioned in section 3.2, the spectrum of the pristine film consists of two doublets (S1 and S2) that correspond to the different thiolate species attached to the substrate. The spectrum of cis-CCHT/Au exhibits typical22 changes in the course of electron irradiation. The intensity of the doublets (162.0 and 161.0 eV for S 2p3/2) related to the pristine thiolate species decreases, whereas a new doublet (S3; 163.4 eV for S 2p3/2) appears and increases in intensity. This doublet can be associated with the cleavage of the original S−Au bonds and trapping of the released fragments in the hydrocarbon matrix, presumably in the form of dialkyl sulfides that are depth-distributed within this matrix.42 The trapping is mediated by the high chemical reactivity of the aliphatic matrix due to irradiation-induced cleavage of C−H and, probably, C−C bonds.22,42−45 The intensities of the individual contributions in the S 2p HRXPS spectra of cis-CCHT/Au are presented in Figure 7 as

Figure 5. Carbon K-edge NEXAFS spectra of the pristine CCHT films acquired at an X-ray incident angle of 55°, along with the respective difference between the spectra acquired at X-ray incident angles of 90° and 20°. The most prominent absorption resonances are marked. Dashed lines correspond to zero.

curves (90°−20°). The 55° spectra of cis-CCHT/Au and transCCHT/Au look very similar. In agreement with the expectations, these spectra do not exhibit pronounced pre-edge π*like resonances typical of the aromatic SAMs6,18,53 but are dominated by several σ* resonances at 287.6, 291.7, 302.2, and 312 eV. Significantly, the signature of contamination is very weak and includes only a low-intensity π*(CC) feature at 285.0 eV while no signatures of the CO or COOH species (quite pronounced if these species are available) are observed. The spectra of the CCHT films differ from that of molecular cyclohexane, which exhibits σ*(C−C), σ*(C−C)″, and σ*(C− H)″ resonances (according to the assignment by the authors) at 286.8, 287.4, and 287.8 eV,76 as the σ*(C−C) resonance is not 13564



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the damage of the pristine headgroups (∼0.21 × 10−16 cm−2) and creation of the irradiation-induced species (∼0.21 × 10−16 cm−2) are significantly lower than those for alkanethiol SAMs (∼1.6 × 10−16 and ∼3.3 × 10−16 cm−2, respectively, for DDT/ Au:)82 but comparable to those for the aromatic ones (∼0.2 × 10−16 and ∼0.15 × 10−16 cm−2, respectively, for BPT/Au:).83 Also, the saturation values for the damage of the pristine headgroups (∼65%) and creation of the irradiation-induced species (∼35%) are close to the analogous values for the aromatic SAMs (55−60% and 30%, respectively, for BPT/ Au)83 but differ significantly from the values for the aliphatic monolayers (60% and 70%, respectively, for DDT/Au).82 Note that due to the trapping of the released fragments in the hydrocarbon matrix, the sum intensity for DDT/Au prevails 100%.82 Thus, the behavior of the headgroup−substrate interface in the CCHT SAMs exposed to electrons is similar to that in the aromatic monolayers. Similar conclusion follows also from consideration of the C 1s HRXPS data. The C 1s HRXPS spectra of the pristine and irradiated cis-CCHT films are presented in Figure 8. As

Figure 6. S 2p HRXPS spectra of the pristine and irradiated cis-CCHT films acquired at a photon energy of 350 eV. The spectra are decomposed into the doublets related to the pristine thiolate (S2, blue solid line), pristine “different thiolate” (S1, red solid line), and irradiation-induced dialkyl sulfide (S3, green solid line). The background of the individual spectra is shown as a dotted line.

Figure 8. C 1s HRXPS spectra of the pristine and irradiated cis-CCHT films acquired at a photon energy of 350 eV. The vertical dashed line is a guide for the eye. Figure 7. Intensities of the individual S 2p contributions for cisCCHT/Au as functions of the irradiation dose: S1 + S2 (blue squares and solid line) and S3 (green circles and solid line). The values are normalized to the total S 2p intensity of the pristine film.

mentioned in section 2, the spectrum of the pristine cis-CCHT film exhibits only one emission. During irradiation, the C 1s emission shifted to higher BE, broadened, and decreased in intensity. The upward 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.22 The dose dependency of the effective thickness of the cis-CCHT film, derived on the basis of the C 1s and Au 4f spectra (see section 2), is presented in Figure 9. Similar to the parameters related to the headgroup−substrate interface (see above), the thickness follows an exponential law with a typical saturationlike behavior at high doses. Both the cross-section of the irradiation-induced desorption (∼0.12 × 10−16 cm−2) and the saturation value for the effective thickness (∼80%) are close to the analogous values for the aromatic SAMs (∼0.2 × 10−16

functions of the irradiation dose. The observed behavior can be well-described with a pseudo-first-order kinetics equation41,43 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 the process rate. Significantly, the cross sections for 13565



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Note that during the above analysis of the S 2p HRXPS spectra, we considered the relative intensities of the S1 and S2 species together. As far as we treated them individually (see Figures S8−S11 in the Supporting Information), we found that the cross sections for the damage of the conventional thiolate headgroups (S2) are even less than those for S1 and S2 together, whereas those for the S1 species are noticeably higher. On one hand, this finding means that our above conclusion about the radiation stability of the thiolate headgroups in the CCHT SAMs is valid. On the other hand, this finding indicates that the natures of the S1 and S2 species are probably different, with the latter species being less stable toward electron irradiation. This rather favors the S−C bond breaking model for the S1 species as compared to “different thiolate” assignment (see section 3.2). 3.6. Lithography Experiments. An ultimate proof of the above scenario can be provided by lithography experiments. It is well-known that linear (i.e., noncyclic) aliphatic SAMs, in which irradiation-induced damage and fragmentation prevail over cross-linking, serve as positive resists, allowing efficient substrate etching over the irradiated areas and protecting the nonirradiated areas.22 In contrast, aromatic SAMs, in which irradiation-induced cross-linking prevails over damage and fragmentation, serve as negative resists, protecting the irradiated areas and allowing efficient substrate etching over the nonirradiated areas.18,22 SEM images of Au patterns fabricated by proximity printing lithography with the cis-CCHT (a), trans-CCHT (b), and DDT (c) SAMs as resists are shown in Figure 10, with the areas exposed and nonexposed to electrons being marked. In the case of the reference DDT resist, irradiated areas were efficiently etched, so that the pattern appears as a gold mesh. This corresponds to the expected positive resist performance. In contrast, for both cis-CCHT and trans-CCHT resists, nonirradiated areas were preferably etched, so that the pattern appears as an array of gold squares. This corresponds to the negative resist performance and can only be explained by the dominance of cross-linking over the fragmentation and desorption processes in these systems. This is the final evidence that the CCHT SAMs behave similar to aromatic monolayers under ionizing radiation.

Figure 9. Relative thickness of cis-CCHT/Au as a function of the irradiation dose. The values are normalized to the thickness of the pristine film.

cm−2 and ∼78%, respectively, for BPT/Au)77 but differ significantly from the values for the aliphatic monolayers (∼1.5 × 10−16 cm−2 and ∼55%, respectively, for DDT/Au).82 Thus, the behavior of the hydrocarbon matrix in the CCHT SAMs exposed to electrons is similar to that in aromatic monolayers. Considering the results obtained on the basis of the S 2p and C 1s data together, we can conclude that the CCHT SAMs behave similarly to aromatic monolayers under electron irradiation, which can only be ascribed to the stability of the cyclic skeleton. Whereas C−H bonds can be easily cleaved by electrons, cyclic skeletons of the individual SAM constituents remain stable to some extent and cross-link to one another over the radicals and “free” bonds appearing after the cleavage of C− H moieties. Even if a C−C bond within a cyclic ring is broken, the fragments are still connected to the matrix, so that no disorption of hydrocarbon fragments occurs and the loose ends of the ring become quickly integrated into the cross-linking network. The overall extensive cross-linking largely prevents desorption of the individual molecules and their fragments (if such fragments are created) and hinders the damage of the headgroup−substrate interface, keeping the individual molecules in their place. In contrast, in linear aliphatic monolayers such as of DDT on gold, molecular fragments appearing after cleavage of C−C bonds can easily desorb as they are no longer chemically bonded to the monolayer.

4. CONCLUSIONS We have successfully synthesized CCHT compounds with either trans- or cis-conformations and used them as the precursors to prepare SAMs on polycrystalline Au(111) substrates. The SAMs were characterized by a variety of spectroscopic techniques, including XPS, HRXPS, IRRAS, and

Figure 10. SEM images of Au patterns fabricated by proximity printing lithography with the (a) cis-CCHT, (b) trans-CCHT, (c) and DDT SAMs as resists. The areas exposed and nonexposed to electrons are marked. 13566



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(5) Love, J. C.; Estroff, L. A.; Kriebel, J. K.; Nuzzo, R. G.; Whitesides, G. M. Chem. Rev. 2005, 105, 1103−1169. (6) Zharnikov, M.; Grunze, M. J. Phys.: Condens. Matter. 2001, 13, 11333−11365. (7) Cheng, F.; Gamble, L. J.; Castner, D. G. Anal. Chem. 2008, 80, 2564−2573. (8) Kind, M.; Wöll, Ch. Prog. Surf. Sci. 2009, 84, 230−278. (9) Dubey, M.; Weidner, T.; Gamble, L. J.; Castner, D. G. Langmuir 2010, 26, 14747−14754. (10) Zharnikov, M. J. Electron Spectrosc. Relat. Phenom. 2010, 178− 179, 380−393. (11) Willey, T. M.; Fabbri, J. D.; Lee, J. R. I.; Schreiner, P. R.; Fokin, A. A.; Tkachenko, B. A.; Fokina, N. A.; Dahl, J. E. P.; Carlson, R. M. K.; Vance, A. L.; et al. J. Am. Chem. Soc. 2008, 130, 10536−10544. (12) Schreiber, F. Prog. Surf. Sci. 2000, 65, 151−256. (13) Kondoh, H.; Iwasaki, M.; Shimada, T.; Amemiya, K.; Yokohama, T.; Ohta, T.; Shimomura, M.; Kono, K. Phys. Rev. Lett. 2003, 90, 066102. (14) Schreiber, F. J. Phys.: Condens. Matter. 2004, 16, R881−R900. (15) McGuiness, C. L.; Diehl, G. A.; Blasini, D.; Smilgies, D.-M.; Zhu, M.; Samarath, N.; Weidner, T.; Ballav, N.; Zharnikov, M.; Allara, D. L. ACS Nano 2010, 4, 3447−3465. (16) Roper, M. G.; Skegg, M. P.; Fisher, C. J.; Lee, J. J.; Dhanak, V. R.; Woodruff, D. P.; Jones, R. G. Chem. Phys. Lett. 2004, 389, 87−91. (17) Laibinis, P. E.; Graham, R. L.; Biebuyck, H. A.; Whitesides, G. M. Science 1991, 254, 981−983. (18) Geyer, W.; Stadler, V.; Eck, W.; Zharnikov, M.; Gölzhäuser, A.; Grunze, M. Appl. Phys. Lett. 1999, 75, 2401−2403. (19) Eck, W.; Stadler, V.; Geyer, W.; Zharnikov, M.; Gölzhäuser, A.; Grunze, M. Adv. Mater. 2000, 12, 805−808. (20) Gölzhäuser, A.; Geyer, W.; Stadler, V.; Eck, W.; Grunze, M.; Edinger, K.; Weimann, Th.; Hinze, P. J. Vac. Sci. Technol. B 2000, 18, 3414−3418. (21) 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. (22) Zharnikov, M.; Grunze, M. J. Vac. Sci. Technol. B 2002, 20, 1793−1807 and references therein. (23) 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. (24) Kim, S. O.; Solak, H. H.; Stykovich, M. P.; Ferrer, N. J.; de Pablo, J. J.; Nealey, P. F. Nature 2003, 424, 411−414. (25) Turchanin, A.; Schnietz, M.; El-Desawy, M.; Solak, H. H.; David, C.; Gölzhäuser, A. Small 2007, 3, 2114−2119. (26) Turchanin, A.; Tinazli, A.; El-Desawy, M.; Großmann, H.; Schnietz, M.; Solak, H. H.; Tampé, R.; Gölzhäuser, A. Adv. Mater. 2008, 20, 471−477. (27) Ballav, N.; Chen, C.-H.; Zharnikov, M. J. Photopolym. Sci. Technol. 2008, 21, 511−517. (28) Ballav, N.; Thomas, H.; Winkler, T.; Terfort, A.; Zharnikov, M. Angew. Chem. Int. Ed. 2009, 48, 5833−5836. (29) Ballav, N.; Shaporenko, A.; Terfort, A.; Zharnikov, M. Adv. Mater. 2007, 19, 998−1000. (30) Ballav, N.; Shaporenko, A.; Krakert, S.; Terfort, A.; Zharnikov, M. J. Phys. Chem. C 2007, 111, 7772−7782. (31) Ballav, N.; Weidner, T.; Rößler, K.; Lang, H.; Zharnikov, M. Chem. Phys.Chem. 2007, 8, 819−822. (32) Ballav, N.; Terfort, A.; Zharnikov, M. J. Phys. Chem. C 2009, 113, 3697−3706. (33) Eck, W.; Küller, A.; Grunze, M.; Völkel, B.; Gölzhäuser, A. Adv. Mater. 2005, 17, 2583−2587. (34) Turchanin, A.; Beyer, A.; Nottbohm, C.; T. Zhang, X.; Stosch, R.; Sologubenko, A.; Mayer, J.; Hinze, P.; Weimann, T.; Gölzhäuser, A. Adv. Mater. 2009, 21, 1233−1237. (35) Schnietz, M.; Turchanin, A.; Nottbohm, C. T.; Beyer, A.; Solak, H. H.; Hinze, P.; Weimann, T.; Gölzhäuser, A. Small 2009, 5, 2651− 2655.

NEXAFS spectroscopy. The SAMs were found to be welldefined and contamination-free. The cis-CCHT SAMs exhibited a higher packing density (33 Å2/molecule) and a smaller molecular inclination as compared to the trans-CCHT films (42 Å2/molecule). This difference was tentatively explained by the effect of the bending potential at the thiolate headgroup, favoring a more upright molecular orientation in the case of cis-CCHT/Au. However, the packing density in both cis-CCHT and trans-CCHT monolayers is significantly lower than that in SAMs of noncyclic aliphatic thiols (21.6 Å2/ molecule), which is presumably related to the nonplanar conformation of the CCHT backbone, preventing a dense molecular arrangement. The reaction of the CCHT SAMs toward electron irradiation was studied by in situ HRXPS and e-beam lithography experiments. The behavior of these SAMs was found to be somewhat similar to that of aromatic monolayers but distinctly different from that of aliphatic films with a linear (noncyclic) backbone. This underlines the role of the cyclic structure in the balance of cross-linking and defragmentation in monomolecular films. Due to the stability and connectivity of the cyclic skeletons, these moieties stay in place under electron irradiation and cross-link to one another preventing irradiation-induced desorption of the molecules and their fragments as well as extensive damage to the headgroup−substrate interface. It is unclear, however, to what extent the individual rings of the CCHT molecules remain intact and what are the exact structural motifs in the irradiated CCHT films. We plan to address these questions in the future.

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ASSOCIATED CONTENT

S Supporting Information *

Supplemental X-ray crystallography and spectroscopic data as well as the data related to the modification of the trans-CCHT SAMs by electrons and to the behavior of the S1 and S2 species separately. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Phone: +49-6221-54 4921. Fax: +49-6221-54 6199. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank M. Grunze for support of this work, F. Rominger for the single crystal X-ray analysis, N. Nieth for the mass spectrometry measurements, J. Graf for the NMR analysis, and A. Nefedov and Ch. Wöll (KIT) for the technical cooperation at BESSY II, as well as H. Hamoudi, F. Chesneau and BESSY II staff for the assistance during the synchrotron-based experiments. This work has been supported by the Volkswagen Stiftung (I/83 227) and DFG (EC 152/4-1 and ZH 63/9-3).

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REFERENCES

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Self-Assembled Monolayers of Cyclic Aliphatic Thiols and Their

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