Modification of Nitrile-Terminated Biphenylthiol Self-Assembled

Article pubs.acs.org/Langmuir

Modification of Nitrile-Terminated Biphenylthiol Self-Assembled Monolayers by Electron Irradiation and Related Applications Nikolaus Meyerbröker and Michael Zharnikov* Angewandte Physikalische Chemie, Universität Heidelberg, Im Neuenheimer Feld 253, D-69120 Heidelberg, Germany ABSTRACT: Here we describe the behavior of self-assembled monolayers (SAMs) of 4′-cyanobiphenyl-4-thiol (CBPT) on Au(111) upon electron irradiation. Under such a treatment, the aromatic framework of CBPT SAMs is laterally cross-linked while the nitrile groups, located at the SAM−ambience interface, are reduced to active amine moieties which can be used as docking sites for the coupling of other species. This makes CBPT monolayers as a promising system for conventional and chemical lithography as well as for nanofabrication. Along these lines, we demonstrate the preparation of complex polymer brushes, patterning of the underlying substrate, and fabrication of molecule-thin, free-standing membranes on the basis of CBPT SAMs. The balance between the application-favorable processes and defragmentation in these films is studied in detail, and comparison to the well-established (for the relevant applications) system of 4′-nitrobiphenyl-4-thiols is performed. Taking CBPT SAMs as a model system, the effect of the energy of the primary electrons on the extent of the chemical transformation and cross-linking in substituted aromatic SAMs is investigated.

1. INTRODUCTION Self-assembled monolayers (SAMs) represent a useful tool to prepare well-defined and chemically uniform surfaces and interfaces.1−4 In addition, due to a broad variety of SAMbuilding monomers, they enable tuning the surfaces properties with regard to chemical reactivity, wettability, or biocompatibility. The related applications range from chemical sensors over nanofabrication and molecular electronics to biological and medical issues.1,3 Some of these applications do not only rely upon the specific design of the SAM-building monomers but on modification of SAMs by ionizing radiations such as electrons,5,6 X-rays,7,8 or deep/extreme UV light,9,10 with the primary role of electrons in the underlying processes.11,12 Significantly, the character of the modification depends on the identity of the SAM constituents, above all on the molecular chain.6,8 In particular, monolayers comprising aliphatic moieties are predominately degenerated under electron irradiation.13 Therefore, aliphatic SAMs are generally considered as a positive resist in electron beam lithography,14 although it was reported that, under the circumstances, they can also act as a negative resist.15 In contrast, if a SAM consists of an aromatic framework,16,17 electron irradiation induces a lateral crosslinking within the film, preventing molecular decomposition and desorption and resulting in quasi-polymer, supported film.18,19 Therefore, aromatic SAMs can act as negative resists in standard lithographic applications.20 Furthermore, it is possible to prepare ultrathin, free-standing membranes based on these SAM,21,22 which can be converted to graphenoid sheets upon extensive annealing at elevated temperatures.23,24 Further possibilities arise from the use of aromatic monomers with functional tail groups.25 The most prominent example in this regards is SAM of 4′-nitrobiphenyl-4-thiol © 2012 American Chemical Society

(NBPT) on gold substrate. Upon extensive electron irradiation, the terminal nitro groups of this SAM are converted to chemically reactive amino moieties, which occurs parallel to the cross-linked process in the aromatic matrix.26 The extent of the conversion, i.e., the surface density of the chemically reactive species depends on the irradiation dose, which should be in a range of 5−40 mC/cm2 at an electron energy of 50 eV, and can be precisely controlled.26−28 These species can be used as specific sites for the coupling of further chemical compounds with the density given by that of the amino groups. Prominent examples in this regard are the formation of a multifunctional chemical pattern,29 preparation of supported protein arrays,10 and fabrication of polymer brushes by surface-initiated polymerization (SIP).27,28,30 In the latter case, the reaction is started by the coupling of an initiator to the amino tail groups of the irradiated NBPT SAM, followed by SIP in the presence of a monomer and a catalyst. This system translates the irradiation-controlled surface density of the amino groups via the initiator density into the height of the polymer brushes, relying on the progressive alignment of the growing polymer chains in the areas where they are confined, following the higher initiator density. Thus, it is possible to create gradient patterns applying different doses over the primary aromatic SAM. 27,28 In addition, beyond the above lithographic applications, NBPT SAMs were used for the fabrication of free-standing nanosheets which can be further coupled to other chemical species, resulting, for example, in the preparation of Janus-like membranes31 and polymer carpets.32,33 Received: April 5, 2012 Revised: May 30, 2012 Published: May 31, 2012 9583

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Figure 1. Schematic of the reaction of NBPT (a) and CBPT (b) SAMs toward electron irradiation. The aromatic frameworks of the monolayers are cross-linked by the irradiation, so that the molecular skeletons stay in place. In parallel, the NO2 tail groups of NBPT monolayer are reduced to amines whereas the nitrile tail groups of CBPT film are transformed to CH2NH2 moieties. Preparation of SAMs. Two types of gold substrates were used (both from Georg Albert PVD, Germany). By default, we used 100 nm polycrystalline gold films deposited by physical vapor deposition on polished silicon (100) wafers primed with a 5 nm titanium layer as adhesion promoter. For membrane preparation, we used gold substrates prepared by epitaxial growth of 300 nm gold on fresh cleaved mica.44 Both types of substrates were cleaned by ozonation with a low pressure Hg lamp; both CBPT and reference NBPT SAMs were prepared according to well-established protocols as reported, for example, in ref 45, viz., gold substrates were immersed into a 10 mM solution of the respective thiol in degassed DMF under argon atmosphere and absence of light. After 72 h, the substrates were removed from the flask, rinsed with ethanol, and dried in a nitrogen stream. All glass equipment (flasks, pipettes) used for the SAM preparation was cleaned with piranha solution, rinsed with Millipore water, and stored in a drying cabinet. Electron Irradiation and Patterning. The SAMs were irradiated both homogeneously and in a lithographic fashion, depending on the particular purpose. Homogeneous irradiation was performed with a flood gun (FG20, Specs Germany) under UHV conditions. The electron energy was set to 50 eV, and the dose was calibrated by an array of Faraday cups. For simple, single-dose lithographic patterns, a transmission electron microscopy (TEM) grid (Quantifoil, R2/4) serving as a stencil mask was placed on the SAMs and connected electrically to the sample holder by conductive carbon tape. The patterning was then performed in the proximity printing geometry. For multiple-dose irradiation and complex lithographic patterns, we used an electron beam writer consisting of a scanning electron microscope (LEO 1530) and a special lithographic unit (Raith Elphy Quantum). The energy of the electron beam could be set to values from 0.5 to 20 keV, and the dose was calibrated by a Faraday cup. For controlling irradiation over a large dose scale, dwell time and area step size of the writing electron beam was set manually to avoid an overexposure at low or an underexposure at high doses, respectively.27 SAM Characterization. Both pristine and homogeneously irradiated SAMs were characterized by X-ray photoelectron spectroscopy (XPS), infrared reflection adsorption spectroscopy (IRRAS), and near-edge X-ray adsorption fine structure (NEXAFS) spectroscopy. XPS measurements were carried out under UHV conditions (residual pressure less than 2 × 10−9 mbar) with a MAX200 (Leybold-Heraeus) spectrometer equipped with a Mg Kα X-ray source (200 W) and a hemispherical analyzer. The recorded spectra were divided by a device specific transmissions function, and the binding energy (BE) scale was referenced to the Au 4f7/2 peak of the gold substrate at 84.0 eV.46 To avoid any kind of contamination, the XPS measurements were carried out in the same UHV system where the electron irradiation was performed. In addition, on the basis of the XPS data, the effective thickness was evaluated using the attenuation of the Au4f signal;47 the required attenuation length and the spectrometer-specific constant were obtained from the specially performed calibration measurements on a series of freshly prepared alkanethiolate SAMs with different chain lengths. The accuracy of the absolute thickness values is ca. 5%: the relative accuracy is even higher. In addition to the laboratory XPS experiments, synchrotron-based measurements were performed for the pristine SAMs to ensure their quality. The experiments were carried out at the HE-SGM beamline (bending magnet) of the synchrotron storage ring BESSY II in Berlin,

So far, only the nitro group has been exploited for the above applications, in the framework of electron beam chemical lithography (EBCL) and nanofabrication on the basis of aromatic SAMs. In a quest for suitable alternatives, we came along the nitrile group, taking 4′-cyanobiphenyl-4-thiol(CBPT) SAM on Au(111) as a representative test system. First, the biphenyl framework should ensure cross-linking upon electron irradiation, which is a prerequisite for both lithographic applications and membrane fabrication. Second, like the nitro group in the case of NBPT SAMs, the nitrile group of CBPT monolayers can be considered as an oxidized amine which is inert to a variety of compounds, providing an inert primary template for EBCL. Third, similar to the nitro group, it should be possible to reduce the nitrile moiety by electron irradiation. However, in contrast to nitro, the reduction of a nitrile results in a methylene amine. The additional CH2 group can possibly improve the accessibility of the amine moieties, leading to an increased coupling density (Figure 1). Note that nitrile-substituted, thiol-derived SAMs on gold are not unknown in the literature. In particular, NC-terminated alkanethiols were used to investigate the effect of strong dipole−dipole interaction at the SAM−ambience interface on the structure of SAMs.34 Further, NC-terminated aliphatic and aromatic SAM were used to study femtosecond dynamics of the charge transfer in potential molecular wires.35−37 In parallel, significant efforts were made to gain information on the molecular orientation and organization in these systems.38−42 In particular, the properties of CBPT SAMs on Au(111) were studied in detail.38,41 It was found that these SAMs are welldefined and densely packed, with the molecules being both tilted and twisted within the proposed herringbone arrangement.38,41

2. EXPERIMENTAL SECTION Synthesis of 4′-Cyanobiphenyl-4-thiol (CBPT). 4′-Cyanobiphenyl-4-thiol (CBPT; the correct denomination according to IUPAC is 4′-mercaptobiphenyl-4-carbonitrile) was synthesized based on commercially available 4′-hydroxy-4-biphenylcarbonitrile (NC-bphOH; TCI Europe) according to a procedure similar described in ref 43. NC-bph-OH was dissolved in dry DMF, deprotonated with 1.2 equiv of NaH, and kept at 60 °C for 4 h after addition of 1.5 equiv of dimethylthiocarbamoyl chloride (ClC(S)NMe2). After cooling to room temperature, water was added dropwise and the crude Othiocarbamate (NC-bph-OC(S)NMe2) was extracted by dichloromethane and recrystallized in a 1:1 mixture of ethanol and ethyl acetate. The resulting cottonlike crystals were rearranged to the corresponding S-thiolcarbamate (NC-bph-SC(O)NMe2) by melting under nitrogen atmosphere for 10 h at 210 °C. The resulting raw product was recrystallized with ethanol. Finally, the CBPT compound was obtained after refluxing for 4 h in a solution of 0.2 M KOH in MeOH. The crude product was precipitated by successive addition of concd hydrochloric acid and purified by vacuum sublimation (p < 0.05 mbar) at 100 °C, obtaining white crystals with an overall yield of 52%. 9584

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sheets were transferred to an alternative support and the PMMA layer was removed by immersion in acetone. The free-standing membranes were characterized by scanning electron microscopy (Leo 1530, Zeiss) using an in-lens detector and by fluorescence microscopy. For these purposes, the membranes were transferred to a TEM grid (1500 mesh). For the characterization by fluorescence microscopy, the membranes were additionally immersed into a filtered (0.47 μm pore size) PBS-buffered solution of AlexaFluor647-labeled fibrinogen (Invitrogen, Germany; 0.25 mg of protein per mL of buffer solution) for 60 min. To remove excess of unattached protein, the samples were immersed into Millipore water and dried in air.

Germany, using a Scienta R3000 spectrometer. The spectra were acquired in normal emission geometry at photon energies ranged from 350 to 580 eV depending on the BE region. The energy resolution was 0.2−0.3 eV, allowing a clear separation of individual spectral components. IRRAS measurements were performed under rough vacuum conditions (p < 1 mbar) using a Fourier transform spectrometer (Bruker IFS66v) equipped with a liquid nitrogen-cooled MCT detector. The angle of incidence was fixed at 82°, and the spectra were recorded at a resolution of 2 cm−1 with an accumulation of 1024 scans. As a reference, we used hexadecanethiol SAM on gold. Perdeuterated alkanethiols could not be used, because the C−D vibrations appear in the same region as the nitrile band, hindering the numerical analysis of the spectra. NEXAFS measurements were performed at the same beamline as the synchrotron-based XPS experiments. The acquisition of the spectra was carried out at the carbon and nitrogen K-edges in the partial electron yield mode with retarding voltages of −150 and −300 V, respectively. Linearly polarized synchrotron light with a polarization factor of ∼85% was used. The energy resolution was 0.2−0.3 eV. The incidence angle of the primary X-ray beam was varied from 90° (Evector 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 molecules in the target films. This approach is based on the linear dichroism in X-ray absorption, i.e., the strong dependence of the crosssection 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.48 The raw NEXAFS spectra were normalized to the incident photon flux by division by a spectrum of a clean, freshly sputtered gold sample,48 and then the spectra were reduced to the standard form by subtracting a linear pre-edge background and normalizing to the unity edge jump (determined by a nearly horizontal plateau 40−50 eV above the absorption edge). The energy scale was referenced to the most intense π* resonance of highly oriented pyrolytic graphite (HOPG) at 285.38 eV.49 Fabrication of Polymer Brushes. Polymer brushes of poly-Nisopropylacrylamide (PNIPAAM) were fabricated on patterned CBPT and NBPT SAMs by surface-initiated atom transfer radical polymerization (SI-ATRP) as described below. In brief, electron-beampatterned SAMs were immersed into a 10% solution of the polymerization initiator, bromoisobutyryl bromide (BIBB), in dichloromethane (plus 2% triethylamine) under strictly anhydrous conditions. The polymerization was carried out by immersing the BIBB-activated templates into a 25% solution of the inhibitor-free Nisopropylacrylamide (NIPAAM) monomer in a 4:1 mixture of water and methanol in the absence of oxygen. As catalyst, a Cu(I)−amine complex was added to start the polymerization. The polymer brushes were allowed to grow overnight at room temperature. Fabrication of Gold Patterns. Patterned CBPT SAMs on Au/Si substrates were immersed into a thiosulfate-based etching solution for 30−40 min at room temperature.50 The etching bath consisted of a 1 M KOH solution containing 0.1 M K2S2O3, 10 mM K4Fe(CN)6, and 1 mM K3Fe(CN)6. Subsequently, the samples were washed copiously with Millipore water and dried with nitrogen. Characterization of the Polymer Brushes and Gold Patterns. Fabricated polymer brushes and gold patterns were characterized by atomic force microscopy (AFM). The measurements were carried out with a Dimension 3100 AFM with Nanoscope IIIa controller in tapping mode under ambient conditions. Preparation and Characterization of Free-Standing Membranes. The fabrication of SAM-based membranes was performed according to well-established protocols of refs 51 and 52. In brief, CBPT SAMs were homogeneously irradiated by electrons (50 eV) with a dose of 40 mC/cm2. The resulting, cross-linked monolayers were spin-coated with a poly(methyl methacrylate) (PMMA) layer (950 kDa; 5% in chlorobenzene) at 4000 rpm, followed by floating on hydrofluoric acid to cleave the gold from mica. The gold was dissolved in Lugol’s solution (aqueous KI/I2, 2%). Finally, the SAM/PMMA

3. RESULTS AND DISCUSSION 3.1. Characterization of Pristine CBPT SAMs. Purity and quality of CBPT monolayers were checked by synchrotronbased XPS. The obtained spectra, which are shown in Figure 2a, are in a good agreement with literature data.41 Figure 2a

Figure 2. C 1s (a), S 2p (b), N 1s (c), and O 1s (d) synchrotron-based XPS spectra of pristine CBPT SAMs. The C 1s and S 2p spectra were recorded at a photon energy of 350 eV; the N 1s and O 1s spectra at 580 eV.

exhibits the C 1s spectrum which can be fitted by two components: a more intense peak at 284.4 eV originates from the aromatic backbone while the second signal at 285.8 eV can be assigned to the nitrile carbon. The sulfur S 2p spectrum in Figure 2b shows a single doublet at 162.1 eV (S 2p3/2) which is typical for a thiolate species bonded to the gold surface.41,53 No signals related to further sulfur-containing species could be found, indicating the SAM character and high purity of the films. By comparing the intensity ratio of the S 2p signal and Au 4f emission, the package density of CBPT SAMs could be estimated at ∼86% of the respective value obtained for hexadecanethiolate SAMs on Au(111). The N 1s spectrum exhibits, as expected,41 a single emission from the nitrile nitrogen at 398.6 eV (Figure 2c). No oxygen contamination could be traced (Figure 2d). 3.2. Electron Beam Irradiation of CBPT SAMs. CBPT SAMs were successively irradiated with low-energy electrons (50 eV) under UHV conditions and characterized in situ by XPS and ex situ by IRRAS and NEXAFS spectroscopy. The results, which are presented in Figure 3, are compared with the analogous data obtained for the reference NBPT monolayers. As mentioned in section 2, in situ XPS measurements were performed to avoid adsorption of the airborne species, which 9585

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In contrast to the C1s and N1s emissions, the total intensity of the S 2p signal increased slightly upon electron irradiation (Figure 3d only the data for CBPT SAMs are shown; the results for NBPT films are similar). This behavior is partly related to the thickness reduction (see above) and, partly, to the breaking of the S−Au bonds resulting in appearance of an addition doublet at 163.4 eV in the S 2p XPS spectra (not shown).19,55 The thickness reduction leads to a lesser attenuation of the S 2p signal for all sulfur-containing species, whereas progressive propagation and capture of the released sulfur in the aromatic matrix results in a closer placement of such species with respect to the SAM-ambience interface, giving a stronger S 2p signal. Note that both cleavage of the S−Au bonds and the propagation of the released species are noticeably slowed by the cross-linking in the biphenyl matrix as shown by the curves in Figure 3d. In contrast, 50% of all S−Au bonds are cleaved at a dose of only 2 mC/cm2 in the case of aliphatic SAMs in which molecular defragmentation prevails over cross-linking.56 The above analysis shows that CBPT and NBPT SAMs behave similarly under electron irradiation, even though there are some minor differences. In view of the lithographic applications, it is, however, necessary to have a closer look at the SAM−ambience interface as well as to consider in detail the irradiation-induced desorption of the nitrogen-containing species (given by of the total N1s intensity; see above) and the conversion of the nitrile or nitro tail groups to amino moieties. As for the total amount of nitrogen, it was reduced by 36% (Figure 3e) and 22% (Figure 3f), respectively, after irradiation of CBPT and NBPT SAMs with a dose of 40 mC/ cm2. The higher extent of the nitrogen desorption in CBPT films is presumably related to the presence of the additional single bonds between the nitrogen atom and the biphenyl moiety (Figure 1). These bonds can be cleaved by electrons, resulting in the defragmentation of the tail group manifested by the loss of the nitrogen atom. As for the conversion of the tail groups, it can be easily monitored by XPS in the case of NBPT SAMs because the N 1s emission of the nitro moiety at 405.6 eV is clearly distinguishable from the signal of the amine group at 399.2 eV.26,57 Moreover, the vanishing of the nitro O 1s signal at 532.1 eV is a measure for the reduction as well.17 The situation is, however, more complex in the case of CBPT films. Unfortunately, the N 1s emission of the nitrile group has almost the same binding energy as the N 1s signal of the amine moiety, so that the reduction of a nitrile to an amine cannot be followed by XPS but only the total amount of nitrogen can be derived (see above). There are, however, two complementary options. First, the irradiation-induced conversion of the nitrile group in CBPT films can be monitored by IRRAS following the intensity of the characteristic C−N stretching band at 2233 cm−1. The respective spectra are shown in Figure 4a while the intensity of the band is plotted as a function of irradiation dose in Figure 4b. At a dose of 40 mC/cm2 the intensity of the nitrile peak decreased to ca. 10% of the original value. This result can be, however, affected by orientational effects because the intensity of a signal in IRRAS depends also on the orientation of a functional group with respect to the substrate. In particular, an increase in the tilt angle of the nitrile group by irradiation would result in an additional decrease in the C−N band intensity. Hence, the results by IRRAS should be verified by another technique. As such a technique we chose NEXAFS spectroscopy as described below. Carbon and nitrogen K-edge NEXAFS spectra of CBPT SAMs acquired at an X-ray incident angle of 51° with

Figure 3. Dependence of the characteristic parameters of NBPT and CBPT SAMs on irradiation dose: absolute (a) and relative (b) film thickness as well normalized intensities of the individual contributions in the XPS spectra (c−f). (c) C 1s total; (d) S 2p total (filled circles), thiolate (down triangles), and irradiation-induced species (up triangles); (e) N 1s total (filled circles), NC (filled down triangles), and NH2 (dashed green lines) for CBPT SAMs; (f) N 1s total (open circles), NO2 (open down triangles), and NH2 (open up triangles) for NBPT SAMs. The observed intensities can be associated with the relative content of the respective species. In contrast to all other values, the CN content was estimated on the basis of the NEXAFS data and, along with the N 1s total intensity, used to calculate the NH2 content.

can occur during the exposure of the irradiated films to ambience and affect the derived parameters, above all the effective film thickness.6,54 Both CBPT and NBPT SAMs exhibited a similar behavior under electron irradiation. The intensities of the C 1s (Figure 3c) and N 1s (Figure 3e and 3f) signals decreased continuously upon electron irradiation, manifesting progressive defragmentation and desorption events. The extent of these processes was, however, quite low, especially for the C 1s signal, which exhibited a decrease by only 8% at a dose of 40 mC/cm2 (Figure 3c). Such a small decrease can be explained by extensive cross-linking of the biphenyl framework, preventing the extensive defragmentation of the aromatic matrix with subsequent desorption of the molecular fragments. In agreement with the behavior of the C 1s and N 1s signals, the thickness of CBPT and NBPT SAMs (calculated on the basis of the Au 4f signal; see section 2) decreased only slightly upon electron irradiation, viz., from 14.1 to 13.6 Å for CBPT films and from 14.6 to 13.1 Å for NBPT monolayers (Figure 3a). Note that the higher thickness of the NBPT films in the pristine state can be associated with their bulkier tail group (nitro). The higher decrease in thickness in the case of these films (Figure 3b) can be explained by comparing the tail groups before and after irradiation. In NBPT films, the nitro group is reduced to a distinctly smaller amine group, whereas in CBPT SAMs the nitrile group is converted to a methylene amine which has a comparable size. 9586

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energies (the assignment is performed in accordance with refs 41 and 48). The N K-edge spectrum of pristine CBPT SAM is dominated by a double resonance at ∼398.8 and ∼399.7 eV (CN, Figure 5b).37,41 This spectrum resembles that of benzonitrile.58,59 The appearance of the strong double resonance is related to the conjugation between the π* orbitals of the nitrile group and the adjacent phenyl ring. As a result, the degeneracy of the former orbital is lifted and it splits into two states with different energies, polarized either perpendicular (π1*) or parallel (π3*) to the ring.37,41 Note that such a splitting is not perceptible in the C K-edge spectrum due to the overlap of the weaker resonance (π1*) with the other features.41,42 All the characteristic resonances of the nitrile group can be used to evaluate its degradation and transformation in the course of irradiation, along with the π1* resonance of the phenyl ring, which can provide useful information on the irradiation-induced processes in the aromatic matrix. The respective intensities are presented in Figure 5c and Figure 5d as functions of the irradiation dose for the C and N K-edge, respectively. All intensities exhibit a nearly exponential decrease in the course of irradiation. At 40 mC/ cm2, the π1*(CN) and π3*(CN) resonances fall to about 20% of their origin intensity (Figure 5d), whereas the π*(CN) resonance decreases to a somewhat lower intensity value of about 10% (Figure 5c). Within the experimental error, these values are in a good agreement with those obtained by infrared spectroscopy (Figure 4), which also means that the inclination of the nitrile groups was not changed significantly by the irradiation treatment. Through combination of the change in the total nitrogen content determined by XPS (Figure 3e) and the respective changes in the intensity of the characteristic fingerprints of the nitrile group in the IRRAS (Figure 4) and NEXAFS spectra (Figure 5c and 5d), it is now possible to calculate the amount of the amine nitrogen in CBPT monolayers in the course of irradiation. These data are presented in Figure 3e along with the analogous values for the primary nitrile species. Comparison of Figure 3e and Figure 3f suggests that, along with the higher loss of the nitrogen-containing species in the case of CBPT films (see above), the irradiation-induced reduction of the nitrile groups is less effective than in the nitro case (compare the curves for the CN and NO2 species). As a result, only 50% of the pristine nitrile groups in the CBPT SAMs are reduced to amines at 40 mC/cm2 (Figure 3e), whereas in NBPT SAMs almost 70% of the amino groups are converted (Figure 3f). With respect to the total amount of the residual tail groups, we have 70% of amines in the case of CBPT SAMs and almost 90% for NBPT films at the given dose. Parallel to the destiny of the tail groups, processes in the aromatic matrix could be monitored. In particular, the NEXAFS spectra were used to investigate the molecular orientation in the irradiated CBPT films. A suitable way to monitor such effects is to plot the difference between the spectra recorded at 90° (normal) and 20° (grazing) incident angles of X-rays.48 The maxima observed at the positions of characteristic absorption resonances in these spectra can then be considered as fingerprints of the molecular orientation in the assembly. In accordance with the expectations and literature data,41 the C and N K-edge difference spectra in Figure 5e and 5f exhibit well-pronounced positive peaks at the positions of the π* resonances and negative peaks at the positions of the σ* resonances. However, after an electron exposure of 15 mC/ cm2, these peaks vanished nearly completely at the C K-edge

Figure 4. (a) IRRAS spectra of pristine and irradiated CBPT films collected in the region of the stretching band of nitrile at 2233 cm−1; (b) intensity of this band as a function of irradiation dose.

respect to the surface plane (magic angle for an X-ray beam with a polarization factor of 0.85) are presented in Figure 5. At

Figure 5. C K-edge (a) and N K-edge (b) NEXAFS spectra of pristine and irradiated CBPT SAMs acquired at an X-ray incident angle of 55°; dose dependence on the most prominent absorption resonances at the C K-edge (c) and N K-edge (d); difference between the spectra acquired at X-ray incident angles of 90° and 20° for the C K-edge (e) and N K-edge (f). The most prominent absorption resonances are marked. The spectra of the pristine films are shadowed.

this particular geometry, the spectra are not affected by orientational effects and are only representative of the electronic structure of a sample.48 The C K-edge spectrum of pristine CBPT SAM in Figure 5a exhibits a variety of the characteristic absorption resonances. The spectrum is dominated by the π1* resonance of the phenyl ring (CC) at 284.9 eV which is accompanied by the respective π2* resonance at 288.9 eV (CC), the π* resonance of the nitrile group at 286.4 eV (CN), and several broad σ*-like features at higher photon 9587

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Figure 6. (a) AFM images and corresponding height profiles of PNIPAAM brushes grown on CBPT and NBPT SAMs irradiated in a dose gradient fashion (0−100 mC/cm2; linear increase). The numbers in the images were “written” simultaneously; they indicate the irradiation dose in mC/cm2. (b) AFM image and height profiles for the polymer brushes grown on gradient square patterns written in CBPT SAM by electrons of different energies; the image corresponds to an energy of 0.5 keV (as a representative example); the profiles correspond to energies of 0.5, 1, 3, 5, 10, and 20 keV; (c) joint graphical representation of the representative (0.5, 3, 10, and 20 keV) height profiles; (d) brush height versus electron energy for doses of 10, 40, and 150 mC/cm2.

the more extensive defragmentation of the tail groups in the latter case (see previous section). Further, we investigated the influence of the primary energy of electrons on the dose-dependent height of the polymer brush on the CBPT templates. For this purpose, we wrote gradient square patterns in these templates, continuously increasing the dose from square to square, and performed SIATRP with NIPAAM. The respective results are presented in Figure 6b. For all energies, the brush height increased with increasing dose, following the increasing density of the NH2 groups, achieved a maximum, and subsequently decreased with progressing exposure due to the decomposition of the tail groups in this dose range. For an energy of 0.5 keV, the maximum was achieved at 15 mC/cm2. At higher energies, the maximum shifts to higher doses. Also the lowest dose for detecting polymer brushes moves to higher values with the increasing energy. Such an onset is presumably related to a critical density of the NH2 groups which is necessary for the brush nucleation. Some of the above results are additionally summarized in Figure 6c where the brush height is plotted as a function of irradiation dose for different energies of electrons. Interestingly the maximum height (∼53 nm) is almost independent of the energy while the respective dose shifts to higher values with increasing energy of the primary electrons, as mentioned above. An alternative way to monitor the effect of the energy is to plot the brush height versus this energy when keeping the dose constant (Figure 6d). A progressive height decrease is observed at a low irradiation dose (10 mC/cm2), whereas the opposite behavior occurs at a high dose (150 mC/cm2) following the shift of the bell-like curves in Figure 6c. A more complex behavior is observed at an intermediate dose (40 mC/cm2), which can also be explained by the curves in Figure 6c. The observed effect of electron energy can be explained by a joint impact of the primary and secondary electrons that mediate all the irradiation-induced processes in the SAMs. The most important of these processes in terms of EBCL are the cleavage of C−H bonds within the aromatic matrix and the

and strongly decreased in intensity at the N K-edge. Thus, irradiated CBPT monolayers are mostly disordered, with a limited extent of the orientational order remaining at the SAM−ambience interface. These results differ from those obtained for NBPT or BPT SAMs, which maintain a certain degree of the orientational order even at extensive electron irradiation.20,60 3.3. CBPT SAMs as Templates for Electron Beam Chemical Lithography (EBCL). The lower density of the amino groups in electron beam-modified CBPT SAMs as compared to NBPT films can be partly compensated by the better accessibility and, probably, somewhat higher chemical activity of these moieties as mentioned in section 1. To prove this, we performed SI-ATRP experiments with the CBPT and NBPT templates patterned by e-beam lithography. By applying different doses, we were able to control the surface density of the amine groups, writing in particular a linear gradient pattern with a dose varying from 0 to 100 mC/cm2 (3 keV). The AFM images of the respective polymer (PNIPAAM) brushes are shown in Figure 6a; as mentioned in section 1, the density of the amine groups was “translated” into the thickness of the brushes due to the chain confinement effect. Accordingly, we observe a linear increase in the brush height with increasing dose, starting from ∼5 mC/cm2 and following the increasing density of the amine groups, which goes over to a slow decrease at high doses, reflecting the irradiation-induced degeneration of these groups. The initial linear increase in the brush height with increasing dose means that the density of the amino groups can be well controlled by the applied dose in a certain dose range, which makes possible the fabrication of complex, gradient-like chemical and topographical patterns. As for the comparison between CBPT and NBPT, both AFM images and height profiles of the brushes are almost identical for the CBPT and NBPT templates in the range of 0−25 mC/cm2, which suggests that our hypothesis about the compensation of the lower density of the active sites by their better accessibility is correct. At higher doses, the brush on the NBPT template is somewhat higher as compared to CBPT, which is presumably related to 9588

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Figure 7. (a) AFM image and the corresponding height profile (along the white dashed line) of the etched Au(100 nm)/Si substrate. The substrate was covered by CBPT SAM which was patterned by electron irradiation (50 eV, 40 mC/cm2) through a stencil mask (Quantifoil TEM grid with 2 μm hole size) before the etching; (b) a selected, enlarged area of the AFM image in panel a (shown by the white rectangle) together with the corresponding depth profile along the white dashed line; (c) SEM image of the CBPT covered Au(100 nm)/Si substrate after electron beam patterning (square pattern) and subsequent etching. The energy and dose were varied from square to square. The remaining gold substrate looks bright, whereas the silicon support appears dark. Dose−energy combinations with sufficient resist capability are labeled green, and those with poor and no resist capability are marked yellow and red, respectively.

Figure 8. Free-standing nanomembranes based on cross-linked CBPT films spanned over a TEM grid (1000 meshes corresponding to a hole width of 19 μm and to a bar width of 6 μm). (a) SEM images recorded at 0.5 kV (a) and 3 keV (b, c) at the focus either on the membrane’s plane (b) or on the plane of the underlying sample holder (c); (d) a fluorescence microscopy image after immersion of the membrane-covered TEM grid into a solution of fluophore-labeled fibrinogen. The borders of the broken membrane in one of the openings are marked by the black arrows in panels b and c; they are not visible in panel c, manifesting the transparency of the membrane.

and 7b. The height profiles in these figures correspond to the thickness of the gold layer deposited on the silicon substrate by PVD (100 nm), so that the entire gold film was etched away within the nonirradiated areas, whereas it was efficiently protected by the cross-linked CBPT film in the areas treated by the electron beam. Further, we studied the effect of dose and energy of the primary electrons on the resist performance of the CBPT films. This performance was defined as the ability to resist the etching agents after the irradiation-induced cross-linking. For this purpose, we “wrote” a square pattern in the SAM resist, varying the energy of the primary electrons and the dose from square to square, and subsequently exposing this pattern as a whole to the etching solution. AFM image of the resulting pattern is presented in Figure 7c; the residual gold, protected by the properly cross-linked CBPT resist, appears bright while the areas where gold was etched appear dark. Dose-energy combinations with high, medium, and poor resist ability are marked by the green, yellow, and red rectangles, respectively. Significantly, for every primary energy, the resist ability of CBPT SAMs correlates well with the EBCL performance of these films (Figure 6b and 6c), which is understandable because the underlying irradiation-induced processes are closely related. In particular, the doses associated with the optimal resist ability

modification of the tail groups, which is presumably assisted by the liberated hydrogen atoms. In the given energy range, both the cross-section for the interaction of the primary electrons with the film and the total yield of the secondary electrons decrease with increasing energy of the primary electrons,61,62 in agreement with the results presented in Figure 6. Note that the major effect is presumably provided by the secondary electrons, the energy of which is fully sufficient for the mediation of the major irradiation processes as far as it exceeds a certain threshold value (∼7 eV).63,64 3.4. CBPT SAMs as Resist Materials for Conventional Lithography. Along with the performance of CBPT SAMs in the framework of EBCL, we tested their usefulness as resist materials for conventional lithography. As mentioned in section 1, aromatic SAMs become cross-linked under electron irradiation, following the cleavage of the C−H bonds in the molecular backbones. The resulting cross-linked films can act as a negative resist, protecting underlying substrate from etching agents.18 CBPT SAMs exhibit this typical behavior as shown in Figure 7 where the results of the respective experiments are presented. A homogeneous irradiation of a CBPT film on Au/ Si through a TEM grid as a stencil mask and its subsequent immersion into a gold etching solution results in a well-defined gold pattern as demonstrated by the AFM images in Figure 7a 9589

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4. CONCLUSIONS Using a combination of several complementary experimental techniques, we investigated the reaction of CBPT SAMs on Au(111) substrate toward electron irradiation in view of their possible applications in conventional and chemical lithography as well as in nanofabrication. This work was partly motivated by the assumption that, in the framework of EBCL, these SAMs can be considered as an alternative to the well-established NBPT system that exhibits a useful transformation of the nitro tail groups to amino moieties under electron irradiation. It occurred that CBPT SAMs are indeed well suitable for EBCL, relying on the irradiation-induced transformation of the nitrile tail groups to amino moieties. This transformation is, however, accompanied by a partial defragmentation of the tail groups, which occurs to a somewhat higher extent as compared to NBPT films. As a result, both absolute and relative densities of the chemically active amino groups in the irradiated CBPT films are slightly lower than the respective values for the NBPT films exposed to the same dose. However, as far as the amino groups are used as the docking sites for coupling of other moieties, their lower density in the irradiated CBPT films is partly compensated by their better accessibility and, presumably, higher chemical activity. Accordingly, CBPT SAMs exhibit a similar EBCL performance as the NBPT films in the wide range of doses and are only slightly inferior to them at high irradiation load. Parallel to EBCL, we demonstrated that CBPT SAMs can be used as positive resists in a framework of conventional lithography and as starting materials for the fabrication of molecule-thin, free-standing membranes. In addition, we studied the effects of the energy and dose of the primary electrons on the extent of the nitrile−amino conversion (EBCL) and cross-linking (conventional lithography). It was demonstrated that the density of the amino groups can be well controlled by the applied dose, which enables the fabrication of complex, gradient-like chemical and topographical patterns. This is, however, possible only in a limited dose range because of the progressive degradation of the tail groups and hydrocarbon matrix at high irradiation load. The optimal doses for the nitrile−amino conversion and the cross-linking were found to be quite similar. As for the energy effects, which are presumably typical of the entire family of substituted aromatic SAMs, it was shown that both extent of the nitrile− amino conversion and that of cross-linking depend strongly on the energy of the primary electrons. In particular, the optimal dose for both processes was found to increase with increasing electron energy in the 0.5−20 keV range and should therefore be selected accordingly. A similar behavior was also observed for the onset of the film degradation, which clearly defines the suitable dose range for selected energy of the primary electrons.

of CBPT SAMs in Figure 7c practically coincide with the doses corresponding to the maxima of the bell-like curves in Figure 6c. Accordingly, an optimal dose, to achieve the best performance of the CBPT resist, should be selected depending on the energy of the primary electrons. A closely related finding is the fact that higher irradiation load does not necessarily mean a higher extent of cross-linking, which is especially obvious for the 0.5 keV case. Presumably, such a load results in an extensive degradation and defragmentation of the film, which outweighs the positive (in the given case) effect of the cross-linking. 3.5. Fabrication of Free-Standing Membranes on CBPT Basis. In addition to the lithographic applications of CBPT films, we proved their usefulness for the fabrication of free-standing membranes. As mentioned in section 1, aromatic SAMs are generally well suitable for this purpose, providing membranes with a thickness of only 1 nm but with sufficient mechanical stability to be free-standing over micrometer scale openings. To fabricate such membranes, CBPT SAMs were homogeneously irradiated with 40 mC/cm2 (50 eV), removed from their substrate following the established procedure,23,24 and transferred to TEM grids. Figure 8a shows a SEM image of CBPT membrane stretched over a square-shaped TEM grid (1000 mesh) with a 19 μm opening width. Assuming that the membrane has the same thickness as the precursor monolayer, the aspect ratio of the lateral size and thickness exceeds 104. Because the membrane is ultimately thin, the energy of the SEM electron beam had to be adjusted to values about 500 eV for getting a sufficient contrast to see the membrane. Under this condition, the areas where the membrane covers the grid openings are clearly visible (gray squares) and well distinguishable from the areas where it is broken (black square). Note that the metal bars of the copper grid appear only slightly brighter than the membrane itself at the given energy of the primary electrons. When this energy was increased to 3 keV, the membrane became gradually transparent and the image contrast had to be set to high values to make it visible, at least in the openings in which it is partly broken (Figure 8b). Alternatively, to prove the transparency of the CBPT-based membrane at the given conditions, it was possible to focus the SEM image through the membrane and image the surface of the underlying sample holder (Figure 8c). Note that commercially available carbon supports (thickness ∼10 nm) are not fully transparent even if the electron beam energy is 20 keV. Fluorescence microscopy is another way to make the membrane visible. For this purpose, we stretched a CBPTbased membrane over a TEM grid and subsequently deposited a droplet (20 μL) of fluorescently labeled fibrinogen (AlexaFluor 647 fibrinogen conjugate) dissolved in PBSbuffered solution on its surface. As a result, fibrinogen, which is a highly sticky protein, was attached to the membrane. Figure 8d shows a fluorescence microscopy image of this membrane. The openings covered by the free-standing membrane look red in this image whereas the openings where the membrane was broken appear dark. The metal bars, most of which are covered by CBPT film, appear dark as well, due to the metal-mediated quenching of the fluorescence. This confirms (additionally) the ultimate thinness of the membrane because the efficiency of the quenching depends strongly on the distance between the fluorescence source and the metal.

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AUTHOR INFORMATION

Corresponding Author

*Tel: +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 Dr. W. Eck for the initiation of this project, Prof. M. Grunze for the support, Ch. Wöll and A. Nefedov (KIT) for the technical cooperation at BESSY II, and BESSY II staff for the 9590

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(19) Turchanin, A.; Kafer, D.; El-Desawy, M.; Woll, C.; Witte, G.; Gölzhäuser, A. Molecular mechanisms of electron-induced crosslinking in aromatic SAMs. Langmuir 2009, 25, 7342−7352. (20) Gölzhäuser, A.; Geyer, W.; Stadler, V.; Eck, W.; Zharnikov, M.; Grunze, M. Nanoscale patterning of self-assembled monolayers with electrons. J. Vac. Sci. Technol., B: Microelectron. Nanometer Struct.-Process., Meas., Phenom. 2000, 18, 3414−3418. (21) Eck, W.; Küller, A.; Grunze, M.; Volkel, B.; Gölzhäuser, A. Freestanding nanosheets from crosslinked biphenyl self-assembled monolayers. Adv. Mater. 2005, 17, 2583−2857. (22) Beyer, A.; Nottbohm, C. T.; Sologubenko, A. S.; Ennen, I.; Hutten, A.; Rosner, H.; Eck, W.; Mayer, J.; Gölzhäuser, A. Novel carbon nanosheets as support for ultrahigh-resolution structural analysis of nanoparticles. Ultramicroscopy 2008, 108, 885−892. (23) Turchanin, A.; Beyer, A.; Nottbohm, C. T.; Zhang, X. H.; Stosch, R.; Sologubenko, A.; Mayer, J.; Hinze, P.; Weimann, T.; Gölzhäuser, A. One nanometer thin carbon nanosheets with tunable conductivity and stiffness. Adv. Mater. 2009, 21, 1233−1237. (24) Turchanin, A.; Weber, D.; Buenfeld, M.; Kisielowski, C.; Fistul, M. V.; Efetov, K. B.; Weimann, T.; Stosch, R.; Mayer, J.; Gölzhäuser, A. Conversion of self-assembled monolayers into nanocrystalline graphene: Structure and electric transport. ACS Nano 2011, 5, 3896− 3904. (25) Ulman, A. Self-assembled monolayers of 4-mercaptobiphenyls. Acc. Chem. Res. 2001, 34, 855−863. (26) Eck, W.; Stadler, V.; Geyer, W.; Zharnikov, M.; Gölzhäuser, A.; Grunze, M. Generation of surface amino groups on aromatic selfassembled monolayers by low energy electron beams - A first step towards chemical lithography. Adv. Mater. 2000, 12, 805−808. (27) Steenackers, M.; Küller, A.; Ballav, N.; Zharnikov, M.; Grunze, M.; Jordan, R. Morphology control of structured polymer brushes. Small 2007, 3, 1764−1773. (28) He, Q.; Küller, A.; Grunze, M.; Li, J. B. Fabrication of thermosensitive polymer nanopatterns through chemical lithography and atom transfer radical polymerization. Langmuir 2007, 23, 3981− 3987. (29) Gölzhäuser, A.; Eck, W.; Geyer, W.; Stadler, V.; Weimann, T.; Hinze, P.; Grunze, M. Chemical nanolithography with electron beams. Adv. Mater. 2001, 13, 803−806. (30) Schmelmer, U.; Jordan, R.; Geyer, W.; Eck, W.; Gölzhäuser, A.; Grunze, M.; Ulman, A. Surface-initiated polymerization on selfassembled monolayers: Amplification of patterns on the micrometer and nanometer scale. Angew. Chem., Int. Ed. 2003, 42, 559−563. (31) Zheng, Z.; Nottbohm, C. T.; Turchanin, A.; Muzik, H.; Beyer, A.; Heilemann, M.; Sauer, M.; Gölzhäuser, A. Janus nanomembranes: A generic platform for chemistry in two dimensions. Angew. Chem., Int. Ed . 2010, 49, 8493−8497. (32) Amin, I.; Steenackers, M.; Zhang, N.; Beyer, A.; Zhang, X. H.; Pirzer, T.; Hugel, T.; Jordan, R.; Gölzhäuser, A. Polymer Carpets. Small 2010, 6, 1623−1630. (33) Amin, I.; Steenackers, M.; Zhang, N.; Schubel, R.; Beyer, A.; Gölzhäuser, A.; Jordan, R. Patterned Polymer Carpets. Small 2011, 7, 683−687. (34) Zharnikov, M.; Frey, S.; Shaporenko, A.; Harder, P.; Allara, D. L. Self-assembled monolayers of nitrile-functionalized alkanethiols on gold and silver substrates. J. Phys. Chem. B 2003, 107, 7716−7725. (35) Neppl, S.; Bauer, U.; Menzel, D.; Feulner, P.; Shaporenko, A.; Zharnikov, M.; Kao, P.; Allara, D. L. Charge transfer dynamics in selfassembled monomolecular films. Chem. Phys. Lett. 2007, 447, 227− 231. (36) Kao, P.; Neppl, S.; Feulner, P.; Allara, D. L.; Zharnikov, M. Charge transfer time in alkanethiolate self-assembled mono layers via resonant Auger electron spectroscopy. J. Phys. Chem. C 2010, 114, 13766−13773. (37) Hamoudi, H.; Neppl, S.; Kao, P.; Schupbach, B.; Feulner, P.; Terfort, A.; Allara, D.; Zharnikov, M. Orbital-dependent charge transfer dynamics in conjugated self-assembled monolayers. Phys. Rev. Lett. 2011, 107, 027801.

assistance during the synchrotron-based experiments. This work has been supported by the Volkswagen Stiftung (83227) and DFG (Ec 152/4-1).

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dx.doi.org/10.1021/la301399a | Langmuir 2012, 28, 9583−9592