Gold Nanoparticle Patterning on Monomolecular Chemical Templates

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Gold Nanoparticle Patterning on Monomolecular Chemical Templates Fabricated by Irradiation-Promoted Exchange Reaction Jianli Zhao,† Andreas Terfort,‡ and Michael Zharnikov*,† † ‡

Angewandte Physikalische Chemie, Universit€at Heidelberg, Im Neuenheimer Feld 253, 69120 Heidelberg, Germany Institut f€ur Anorganische und Analytische Chemie, Universit€at Frankfurt, Max-von-Laue-Strasse 7, 60438 Frankfurt, Germany

bS Supporting Information ABSTRACT: By use of custom-synthesized, citrate-passivated gold nanoparticles (NPs) as a test system, we have demonstrated a new approach to fabricate high-contrast and high resolution patterns of metal NPs. The patterns were fabricated on monomolecular chemical templates prepared by irradiationpromoted exchange reaction (IPER) lithography. The latter technique allows to introduce molecules bearing NP-binding tail groups into the selected areas of the primary NP-inert matrix, resulting in the desired chemical template. The advantages of the approach are (i) high flexibility in terms of the interfacial chemistry, length scale, and pattern form, (ii) low irradiation dose required for the primary patterning, and (iii) the possibility to use commercial molecules. The suggested strategy relies on strong electrostatic or covalent bonding of the NPs to the preselected functional groups, as was directly demonstrated for some of the target systems. A further important point is that the adsorption of the NPs does not affect the monolayer structure significantly, suggesting the persistence of the electronic structure of the film and NP-bearing molecules in particular, which can be of importance for applications in different areas such as sensor fabrication or nanoengineering.

1. INTRODUCTION The fabrication of nanostructures within both bottom-up and top-down approaches is one of the focus areas of modern science and technology, with a variety of different techniques developed for this purpose. A hot topic in this context is the synthesis and assembly of nanoparticles (NPs), including their deposition onto different substrates, with the ability to arrange NPs into patterns and arrays being one step on the road toward the construction of nanodevices, sensors, and frontier electronics.1
5 In particular, the directed self-assembly of gold NPs has been utilized to prepare electrically conducting nanowires, plasmonic waveguides for electro-optical devices, seeds for the growth of silicon nanorods, and nanostructured catalysts for fuel cell catalytic reactions.6,7 An efficient means for the immobilization and patterning of gold (and other) NPs is provided by self-assembled monolayers (SAMs) which are 2D assemblies of rodlike organic molecules bearing, if necessary, terminal groups with specific functionalities.8
11 The immobilization of NPs is then mediated by the interaction between these functionalities and the surface of NPs or their organic ligands. Possible interaction mechanisms include hydrogen bonds,12 electrostatic attraction,13 metal ion
pyridine complexation,14 ion-induced adsorption,15 covalent bonds,16 and DNA hybridizations.17 In the case of covalent immobilization, covalent bonds can be formed between either the ligands18 or the metal cores19 of NPs and the terminal groups at the substrate surface. Within this strategy, thiol-terminated r 2011 American Chemical Society

SAMs are frequently used to bind coinage metal (in particular gold) NPs,20
29 relying on the high stability of the S
metal bond.9 The formation of this bond in the given case is supposed to result from a partial replacement of organic ligands, which usually cover the metal cores of NPs, by thiol groups. In particular, Morel et al. have observed that the characteristic S
H vibration band at 2552 cm
1 is present in the Raman spectra of dithiol solution but is not perceptible in the case of gold NPs deposited onto dithiol SAMs, which suggests the covalent grafting of these NPs by replacing S
H bonds at the SAM-ambient interface by S
Au ones.30 Note, however, that the existence of covalent bonds, which, as mentioned above, are assumed to be responsible for the immobilization of gold NPs on thiol-terminated SAMs needs probably more unambiguous evidence, because the binding by hydrogen bonds between thiol (from SAMs) and carboxyl (from citric acid) groups is also possible. Apart from the immobilization of NPs onto homogeneous SAMs, these films can be used to fabricate 2D NP patterns on the micro- and nanometer length scale. This can be achieved by controlling the lateral distribution of the terminal functional groups in SAMs with techniques as, e.g., UV photolithography, Received: March 24, 2011 Revised: May 30, 2011 Published: June 16, 2011 14058

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to gold, and used the resulting mixed SAMs or chemical templates for selective immobilization of gold NPs. As the substituents we used both aliphatic and aromatic molecules bearing amino, thiol, and pyridine tail groups (see next section for details) as shown in Figure 1. Herewith we utilized the advantage of IPER to not only permit the mixing of different aliphatic molecules39,40,43,44 but also the mixing of aliphatic and aromatic ones.45 Apart from pursuing the ultimate goal to fabricate welldefined patterns of metal NPs, we monitored the binding of NPs to the terminal groups of the SAMs and possible effects of the NP immobilization on the SAM structure. Note that only few studies so far have been directed to investigate the latter effects which can be important for the fabrication of nanodevices in terms of the structure-dependent electronic properties of the monomolecular films bearing the immobilized NPs. Figure 1. The target molecules of the present study. DDT, a nonsubstituted alkanethiol, served as the primary matrix; AUDT, TPDMT, and PPPT were used as amino-, thiol-, and pyridine-bearing molecular substituents, respectively, to fabricate the mixed SAMs and chemical templates.

electron-beam (e-beam) lithography, scanning probe lithography, focused ion beam lithography, and microcontact printing (μCP).10,31
34 The patterns are then formed by selective adhesion of NPs onto the functional areas. Among the above techniques, e-beam lithography is probably the most flexible one in terms of the length scale (from micrometers to nanometers), lateral resolution (down to few nanometers), and variable form of the patterns as far as a focused electron beam scanning across a SAM-resist-coated substrate is used as the primary patterning tool.35,36 Alternatively, the patterning can be performed in the proximity printing geometry with a stencil mask, which limits the form flexibility but allows fabricating large area patterns in parallel fashion. Since not conventional but chemical templates are necessary, e-beam lithography should be combined with a specific SAM architecture or a postirradiation treatment. The most established approach in this regard is the electron-induced transformation of the terminal tail groups of aromatic SAMs. In particular, nitro (NO2) moieties can be transformed into amino (NH2) ones, whereas the aromatic skeletons carrying these groups remain mostly intact.37,38 An alternative approach, requiring much smaller (by ca. 2 orders of magnitude) irradiation dose and utilizing commercial substances, is the so-called irradiation-promoted exchange reaction (IPER).39
41 The key idea of this technique is a tuning of the extent and rate of the exchange reaction between the primary aliphatic39
41 or aromatic42 SAM and a potential molecular substituent by electron or X-ray irradiation with a variable dose. The irradiation-induced defects promote the exchange, so that, controlling their amount by selection of a proper dose, the extent of the exchange reaction, and following, the composition of the resulting binary mixed SAM can be precisely adjusted.40 By combination of IPER with electron or X-ray lithography, one can fabricate complex chemical lithographic patterns, including gradient ones.41 Here we proved the applicability of the IPER lithography to the fabrication of well-defined patterns of metal NPs, taken customly synthesized gold NPs as a test system. For this purpose, we used SAMs of nonsubstituted alkanethiolates on Au(111) as the primary matrix, exchanged some of the matrix molecules by substituents bearing a terminal functional group with an affinity

2. EXPERIMENTAL SECTION 2.1. Chemicals, Solvents, and Materials. Hydrogen tetrachloroaurate(III) and trisodium citrate (99% purity) were purchased from Sigma Aldrich and used for the synthesis of gold NPs. The solvents, viz., absolute ethanol (purity g99.8%) and tetrahydrofuran (99% purity), were purchased from Sigma-Aldrich and used for the SAM preparation. The SAM constituents were 1-dodecanethiol (DDT), 11-amino-1-undecanethiol (AUDT), [1,1 0 ;4 0 ,1 00 -terphenyl]-4,4 00 -dimethanethiol (TPDMT), and (40 -(pyridin-4-yl)biphenyl-4-yl)methanethiol (PPPT), as shown in Figure 1. DDT and AUDT were purchased from Sigma Aldrich and Asemblon Inc., USA, respectively. TPDMT and PPPT were customly synthesized according to the protocols of earlier works.46,47 The gold substrates were prepared by thermal evaporation of 100 nm of gold (99.99% purity) onto polished single-crystal silicon (100) wafers (Silicon Sense) primed with a 5-nm titanium adhesion layer. The resulting metal films were polycrystalline, with a predominant (111) orientation of the individual grains and a grain size of 20
50 nm. 2.2. Synthesis of Citrate-Stabilized Gold NPs. Gold NPs were synthesized according to the method of Frens.13,48 A 1 mM solution of HAuCl4 3 6H2O (0.17 g) in Milli Q water (18 MΩ) was heated to boiling in a clean conical flask. Sodium citrate (0.29 g) was dissolved in 20 mL of Milli Q water and added to the boiling HAuCl4 solution under stirring and reflux. Then a color change in the above solution from yellow to burgundy over colorless (transparent), gray, and black was observed in 2 min. After 10 min, the reflux was stopped but stirring continued for another 10 min. Finally, the solution was cooled and compensated with Milli Q water for the water loss during the reflux process. 2.3. Preparation of One-Component SAMs. The one-component DDT, AUDT, PPPT, and TPDMT SAMs, which were used as references, were formed by immersion of freshly prepared substrates into 1 mM solutions of the respective compounds in absolute ethanol (DDT, AUDT, and PPPT) or tetrahydrofuran (TPDMT) for 24 h at room temperature. After immersion the samples were thoroughly rinsed with pure ethanol and blown dry carefully with argon. They were either characterized or used immediately or stored under inert gas atmosphere in glass containers until synchrotron-based experiments (see below). No evidence for impurities or oxidative degradation products was found. Note that the properties of TPDMT/Au and PPPT/Au are described in detail in previous works.46,49 14059

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The Journal of Physical Chemistry C 2.4. Fabrication of Mixed SAMs and Chemical Templates by IPER. For the fabrication of large-area, two-component mixed

SAMs, the primary DDT/Au matrices were homogeneously irradiated with 10 eV electrons provided by a flood gun (model 8711, SSI, USA), which was mounted at a distance ∼12 cm from the sample to ensure uniform illumination. The base pressure in the chamber during the irradiation was better than 1  10
8 mbar. For the fabrication of chemical templates, the primary DDT/Au matrices were patterned by a LEO 1530 scanning electron microscope (Zeiss, Germany) with a Raith Elphy Plus pattern generator system. The electron-beam energy was chosen at 1 keV, and the residual gas pressure was about 5  10
6 mbar. The irradiation doses were estimated by multiplication of the exposure time with the current density. Doses of 1 and 2 mC/cm2 were chosen for the homogeneous irradiation and patterning, respectively. These doses are close or exceed slightly the saturation dose for IPER,39
41 so that the maximal possible portion of the NP-binding substituents in the mixed SAMs and SAM-based chemical templates was inserted. Note that this value depends on the character of the substituents and varies from 35 to 70%.40,45 The primary DDT/Au matrices treated by either homogeneous e-beam irradiation or e-beam lithography were immersed in 1 mM solutions of AUDT or PPPT in absolute ethanol or TPDMT in tetrahydrofuran for 2 h at room temperature to perform the exchange reaction. After immersion, the samples were thoroughly rinsed with pure ethanol and blown dry with argon. Note that IPER occurred quite efficiently for all the substituents (see below). In particular, this was the reason for the selection of TPDMT as a thiol-bearing molecule. The use of the respective aliphatic analogue, n-octane-1,8-dithiol, did not result in high quality mixed SAMs. 2.5. Immobilization of Gold NPs on SAMs and SAM Templates. The fabricated homogeneous and mixed SAMs and SAMbased chemical templates were immersed in the solution of gold NPs for 18 h at room temperature. After immersion, the samples were cleaned by ultrasound in Milli Q water for 2 min and rinsed with pure ethanol followed by drying with argon. 2.6. Characterization Techniques for the NPs, SAMs, and NP/SAMs Assemblies. Particle size analyzer, scanning electron microscopy (SEM), atomic force microscopy (AFM), X-ray photoelectron spectroscopy (XPS), synchrotron-based highresolution XPS (HRXPS), and angle-resolved near-edge X-ray absorption fine structure (NEXAFS) spectroscopy were employed for this study. All experiments were performed at room temperature. The XPS, HRXPS, and NEXAFS measurements were carried out under ultrahigh vacuum conditions at a base pressure better than 1  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 size distribution of the gold NPs was analyzed by a NICOMP 380 ZLS particle size analyzer; the error was ∼5%. All the samples after immobilization of gold NPs were studied with a LEO 1530 scanning electron microscope with a field emission gun operating at an accelerating voltage of 3 kV. Some systems were also characterized by AFM using a Digital Instrument 3100 microscope in the tapping mode. The NP density was determined on the basis of several representative SEM images; we estimate an error at (10%. The conventional XPS measurements were performed using a Mg KR X-ray source and a LHS 11 analyzer. The spectra acquisition was carried out in normal emission geometry with an

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energy resolution of ∼0.9 eV. The X-ray source was operated at a power of 260 W and positioned ∼1.5 cm away from the samples. The HRXPS experiments 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 spectra were acquired in normal emission geometry at photon energies of 350 eV for the S 2p region and 580 eV for the C 1s and Au 4f ranges. The energy resolution was 0.2
0.3 eV allowing a clear separation of individual spectral components. 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.50 These spectra were fitted by symmetric Voigt functions and a lineartype background. To fit the S 2p3/2,1/2 doublet we used two peaks with the same full width at half-maximum (fwhm), the standard spin
orbit splitting of ∼1.18 eV (verified by fit), and a branching ratio of 2 (S2p3/2/S2p1/2).50 The fits were performed selfconsistently, i.e., the same fit parameters were used for identical spectral regions. The HRXPS spectra were mostly used for deriving chemical information while the XPS ones were utilized to get the thickness and composition of the target films and NP/SAM assemblies. The NEXAFS measurements were performed at the same beamline as the HRXPS experiments. The spectra acquisition was carried out at the carbon K-edge in the partial electron yield mode with a retarding voltage of
150 V. Linearly polarized synchrotron light with a polarization factor of ∼91% was used. The energy resolution was 0.2
0.3 eV. The incidence angle of the primary X-ray beam 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 molecules in the target 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.51 The raw NEXAFS spectra were normalized to the incident photon flux by division by a spectrum of a clean, freshly sputtered gold sample,51 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.52

3. RESULTS AND DISCUSSION 3.1. Synthesis of Gold NPs. Gold NPs were synthesized through reduction of chloroauric acid by sodium citrate in liquid phase. The NPs were stabilized by citrate ions with negative charges, which lead to repulsive interaction among the particles and prevent the formation of aggregates. Indeed, as shown in the SEM image of the gold NPs on clean silicon (100) surface in Figure 2a, no aggregates were formed. The particles represent highly uniform nanospheres with a narrow size distribution centered at 10 nm, as shown in Figure 2b, where this distribution is presented. Note that the organic shell can be readily modified by exchange with other organic surfactants to produce functionalities on the particle surface. 3.2. Immobilization of Gold NPs on SAMs. To investigate the feasibility of the NPs patterning on SAM-based chemical templates, we first tested and studied their deposition onto the homogeneous and mixed SAMs which were supposed to constitute 14060

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Figure 2. SEM image of gold NPs on a clean silicon (100) wafer surface (a) and the size distribution of these NPs in solution (b).

Figure 3. SEM images taken after the deposition of the Au NPs onto the one-component DDT film (a), one-component AUDT film (b), and mixed DDT-AUDT film (c).

the individual areas of these templates. As shown in parts a and b of Figure 3 and Figure S1 of Supporting Information, the primary

DDT/Au matrix was absolutely inert with respect to the adsorption of the NPs, whereas the reference one-component AUDT, TPDMT, and PPPT films represented suitable templates for their immobilization. For these templates, a formation of dense submonolayers of the NPs with a density of ∼1.7  103 particles/μm2 was observed as shown for AUDT in Figure 3b (for TPDMT and PPPT, see Supporting Information). Significantly, the NPs were adsorbed as isolated particles, rather than aggregates, on all the substrates as a result of the repulsion among the particles mediated by the citrate capping ligands that carry a negative charge. This repulsion however did not result in the formation of an ordered 2D structure of the NPs but just in a certain degree of short-range order around each individual NP with an average interparticle spacing of ∼20 nm. Note that this is a balance between the electrostatic repulsion among the particles and their interaction with the underlying SAM, which is responsible for the packing density of the particles and the character of their arrangement.30 In particular, one cannot increase the packing density of NPs further by grafting more functional groups onto SAM if the repulsion among particles is strong enough to prevent more particles from filling the vacant binding sites on the film. As mentioned in section 2, IPER and IPER lithography do not allow a complete substitution of the DDT molecules in the primary template by the substituent molecules but only a partial substitution, resulting in the mixed DDT+substituent films. The comparison of the XPS spectra of the pristine (DDT) and reference (AUDT, TPDMT, and PPPT) SAMs with the spectra of the respective mixed films prepared by IPER (see Supporting Information) enabled us to determine the portions of the AUDT, TPDMT, and PPPT molecules in the latter films. These portions were found to be 47, 58, and 57% for the DDT+AUDT, DDT +TPDMT, and DDT+PPPT SAMs, respectively. Significantly, all three mixed SAMs have, similar to the reference one-component AUDT, TPDMT, and PPPT films, the ability to serve as templates for the immobilization of the Au NPs. As shown in Figure 3c for DDT+AUDT and in Supporting Information for DDT+TPDMT, and DDT+PPPT (Figure S1 of Supporting Information), the mixing of the NP-neutral DDT and NP-binding substituent (AUDT, TPDMT, and PPPT) molecules results in the similar density and arrangement of the NPs as for the case of the one-component AUDT, TPDMT, and PPPT films. This is understandable since the density of the NPbinding tail groups is significantly larger than the highest possible density of the NPs within the monolayer coverage. Indeed, as shown above, the average diameter of the NPs is ∼10 nm, 14061

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Figure 4. Au 4f7/2, C 1s, and S 2p HRXPS spectra of the pristine (bottom curves) and NP-covered (top curves) TPDMT films. The S 2p spectra are decomposed into the individual contributions related to the thiolate (T1, gray solid line) and thiol (T2, black solid line) species. The intensity ratios of both components are given.

whereas the distance between the potential NP binding sites was ∼0.5 nm (intermolecular spacing)9 on the one-component AUDT, TPDMT, and PPPT templates and did not exceed 1 nm in the respective mixed films at the given mixing ratio (close to 1:1). 3.3. Monitoring NP Immobilization by XPS, HRXPS and NEXAFS Spectroscopy. The binding of the NPs to the AUDT-, TPDMT-, and PPPT-containing SAMs and chemical templates occurred over different mechanisms depending on the character of the terminal (tail) group. In particular, in the case of amino (AUDT) and pyridine (PPPT) termination, the NPs were presumably immobilized via electrostatic interaction.53 Indeed, the pH of the fabricated gold colloid solution was around 5, so that the amine and pyridine tail groups of the respective SAMs were expected to be protonated and thus carrying a positive charge when exposed to the suspension of NPs which in turn, as mentioned above, were charged negatively. In contrast, the SAM with the terminal thiol groups, TPDMT/Au, was expected to attach the NPs covalently through S
Au bonds. The binding of the Au NPs to the AUDT, DDT+AUDT, PPPT, and DDT+PPPT films was monitored by the N 1s XPS spectroscopy characteristic of both amino or pyridine nitrogen. The major effect of the NP adsorption was the intensity reduction (see Supporting Information), which was related to the attenuation of the N 1s signal by the NP overlayer. More effects were observed in the case of the TPDMT and DDT+TPDMT templates. The Au 4f7/2, C 1s, and S 2p HRXPS spectra of the pristine and NP-covered TPDMT films are presented in Figure 4 along with the decomposition of the S 2p spectra into the individual components related to the thiolate (T1) and thiol (T2) species. As compared to the pristine TPDMT film, the intensity of the Au 4f7/2 emission increased significantly after the deposition of the Au NPs (Figure 4a). This increase results from a superposition of the additional signal from the Au NPs and the NP-induced attenuation of the signal from the underlying massive gold substrate.30 Obviously, the effect of the NPs is stronger, resulting in the overall signal increase. In contrast, the intensity of the C 1s emission, which represents a single and symmetric peak at 284.2 eV for TPDMT/Au (as expected for a high quality TPDMT SAM),46 decreased slightly upon the deposition of the NPs. Similar to the Au 4f7/2 case, this is also a complex effect of the citrate shell of the NPs and the NP-induced attenuation of the

signal from the SAM underneath, with the second effect being obviously stronger. Further, the signal at ∼289.0 eV assignable to the C atoms of the
COO(H) moieties, which could be expected in view of the presence of citrate carrying as much as three COO(H) units per molecule in the shell, is practically not perceptible for AuNPs/TPDMT/Au. This suggests that the organic shell of the Au NPs was very thin and provided only a very small contribution to the C 1s HRXPS spectra. Considering the occurrence of three COO(H) units per molecule and the area occupied by the NP (15
20% of the entire area), one can expect that a densely packed monolayer of citrates on the surface of the immobilized NPs will give in the present case a similar 289.0 eV signal as a half monolayer of
COO(H) terminated alkanethiols on flat Au substrate. The latter system provides a clear 289.0 eV signal in the C 1s XPS spectrum,54 which is, however, not the case here. So, we can tentatively estimate the coverage of citrates as significantly less than a densely packed monolayer. In contrast to the Au 4f7/2 and C 1s cases, the changes observed in the S 2p spectra of TPDMT/Au upon the NP adsorption went beyond the intensity change only. As shown in Figure 4c, these spectra can be decomposed into two S 2p3/2,1/2 doublets at BEs of 162.1 and 163.4 eV (S 2p3/2) assigned to the thiolate species bound to gold (peak T1) and to the thiol tail groups (peak T2), respectively.30,46,55,56 As expected,46 the intensity ratio of these doublets, T1/T2 for TPDMT/Au (0.13) deviates from the atomic ratio of sulfur species bound/unbound to gold (1:1) due to the strong attenuation of the thiolate signal by the hydrocarbon matrix of the SAM. The deposition of the NPs would result in additional attenuation of the S 2p signal, which however should be the same for both thiolate and thiol species as far as no chemical changes occurred. This was however not the case, but the T1/T2 ratio increased from 0.13 to 0.16 upon the immobilization of the NPs, indicating an increase in the fraction of the Au-bonded (thiolate) sulfur compared to TPDMT/ Au. This increase can only be associated with the formation of the covalent Au
S bonds between the Au cores of the NPs and the terminal thiol groups of the SAM. This process was possible due to the thin citrate shell which obviously could be penetrated or replaced by the thiol moiety. The relatively small extent of the T1/T2 change reflects the fact that only a small portion of the terminal thiol group participated in the NP bonding as could be 14062

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Figure 5. C K-edge NEXAFS spectra acquired at an X-ray incidence angle of 55° (a) and 90
20° difference curves (b) for the pristine (bottom curves) and NP-covered (top curves) TPDMT films. The characteristic absorption resonances are marked; their positions are indicated by the vertical dashed lines. The horizontal dashed lines in (b) correspond to zero.

expected from comparison of the NP diameter and intermolecular spacing (see above). A simple estimate on the basis of the above T1/T2 ratios suggests that this portion is at least 3%, which, at the given packing density of the SAM (4.63  1014 molecules/cm2),57 corresponds to ∼1.4  105 molecules/μm
2. In consideration of the fact that the NP density is ∼1.7  103 particles/μm2 (see section 3.2), this gives ∼80 binding molecules per NP. Note that the area taken by an NP with a diameter of 10 nm corresponds to the area taken by ∼360 molecules of the underlying SAM. So, ∼22% of these molecules participate in the NP bonding, which is presumably related to the spherical form of the NPs and a somewhat limited permeability of the citrate shell. Apart from the formation of the NP
thiol bonds at the SAMambient interface, the observed change of the T1/T2 ratio could probably originate from a partial SAM disordering upon the deposition of the NPs. This possibility was however ruled out by the NEXAFS experiments as will be shown below. Generally, such experiments provide information about the chemical identity of a target film and average orientation of its constituents.51 A measure of this orientation is the linear dichroism, i.e., the dependence of the absorption resonance intensity on the orientation of the electric field vector of the synchrotron light with respect to the molecular orbital of interest. An efficient way to monitor the linear dichroism is to plot the difference of the NEXAFS spectra acquired at normal (90°) and grazing (20°) angles of X-ray incidence. A disordered film exhibits no dichroism, whereas an ordered one reveals difference peaks at the resonance positions.58 In contrast to the difference curves, a spectrum acquired at the so-called magic angle of X-ray incidence (55°) is not affected by any effects related to molecular

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Figure 6. Schematic representation of NP patterning on SAM-based chemical templates prepared by IPER: (a) e-beam “pre-patterning” of the primary DDT matrix, (b) exchange reaction to introduce the substituent molecules into the irradiated areas to create a chemical template, (c) selective immobilization of the NPs on the areas containing the substituent molecules.

orientation and gives only information on the chemical identity of investigated samples.51 C K-edge NEXAFS spectra of the pristine and NP-covered TPDMT/Au acquired at an X-ray incidence angle of 55° are presented in Figure 5 along with the difference between the spectra acquired at 90° and 20°. The 55° spectrum of TPDMT/ Au exhibits the characteristic absorption resonances of phenyl rings,51,58
61 in accordance with the molecular composition of TPDMT, and is dominated by the intense π1* resonance at 285.0 eV, which is accompanied by the weaker π2* resonance at 288.9 eV, and several broad σ* resonances at higher photon energies. These resonances overlap with characteristic features of the aliphatic linker, most pronounced of which is the so-called Rydberg resonance (R*) at 287.7 eV.58 In accordance with the expectations,46 the NEXAFS spectra of TPDMT/Au exhibit significant linear dichroism as evidenced by the pronounced difference peaks in the respective 90
20° curve in Figure 5b. This curve suggests a high degree of orientational order and dense molecular packing in TPDMT/Au. In accordance with the expected upright orientation of the TPDMT molecules in the respective monolayers, the π*/R* and σ* resonances in the 90
20° curves exhibit positive and negative anisotropy peaks, respectively (the transition dipole moments of these resonances are directed perpendicular to and along the molecular chain, respectively). Both the 55° spectrum and 90
20° curve of TPDMT/Au did not change significantly upon the deposition of the NPs, which suggest that neither the hydrocarbon matrix of the SAM nor its orientational order were affected noticeably by the NP immobilization. Significantly, no peak at ∼288.5 eV corresponding to the π* resonance of the
COO(H) group40,51 appeared for Au NPs/TPDMT/Au, which is in good agreement with the HRXPS 14063

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Figure 7. SEM images of several representative Au NP nanostructures prepared on SAM-based chemical templates using AUDT as the substituent: honeycomb structure (a), letter structure (b), and an enlarged area of the letter structure (c).

results indicating that the organic shell of the NP is too thin to be detected. A tentative analysis referring to ref 40, which contains the C K edge NEXAFS spectra of
COO(H) terminated alkanethiol SAMs, supports our conclusion about a submonolayer coverage of citrates. As for the orientational order, a qualitative statement about the persistence of the difference spectra was complemented by the quantitative analysis of the entire set of the NEXAFS spectra acquired at different angles of X-ray incidence (θ). Within this analysis, the average tilt angles of the π* orbitals, R, was calculated. To avoid any ambiguity related to the decomposition of the spectra and the normalization of the intensities, the most intense and well-separated π1* resonance and the intensity ratios I(R,θ)/I(R,20°) were used for the evaluation according to the formula51 IðR, θÞ ¼ AfP  ð1=3Þ½1 + ð1=2Þð3 cos2 θ
1Þð3 cos2 R
1Þ + ð1
PÞð1=2Þsin2 Rg

ð1Þ

The resulting tilt angles of the π1* orbital for the pristine and NPcovered TPDMT/Au were found to be 68 and 71°, respectively (see Supporting Information), which correspond to molecular tilt angles of 22 and 19° (with respect to the surface normal) as far as the molecular twist is not considered. These values are in good agreement with each other considering the experimental error ((3°). This agreement demonstrates, in accordance with the qualitative analysis of the NEXAFS data, that the orientation order of the TPDMT SAMs was not disturbed significantly by the immobilization of the NPs on top of the film, except probably a slight structural rearrangement induced by the appearing Authiolate bonds at the SAM-ambient interface, resulting in even more upright molecular orientation (as far as we believe that the observed 3° difference is real). Similar results have also been observed by some of the authors before, both with respect to the metal-adsorbate-induced improvement of the orientational order in pristine TPDMT films62 and with respect to the preservation of the SAM structure upon evaporation of nickel layer on crosslinked TPDMT films.63 In all these cases, one can expect that the structure-dependent electronic properties of the SAMs persist upon the deposition event, which makes their application for electronic devices more predictable and reliable. 3.4. Fabrication of NP Patterns. As shown in section 3.2, the gold NPs do not attach to the DDT SAMs but can be immobilized homogeneously on the binary SAMs with the proper functional groups prepared by IPER. This paves the way to fabricate two-dimensional NP nanostructures on SAM-based chemical

templates prepared by IPER lithography. The basic strategy for this purpose is depicted in Figure 6. As the first step, chemical patterns were “prewritten” in an aliphatic resist (DDT/Au) by the focused e-beam (see section 2). Further, the prewritten patterns were “developed” by putting the e-beam treated samples into AUDT solution and performing IPER, which resulted in the formation of mixed DDT-substituent film in the areas exposed to electrons. Finally, the chemical templates were immersed into the suspension of the gold NPs to attach the particles onto the functionalized areas. Several representative examples of nanostructures fabricated in this way using AUDT as substituent are shown in Figure 7 (similar patterns were obtained for TPDMT and PPPT as well); the NPs appear as bright dots with 8
15 nm diameter. Both the honeycomb and letterlike structures exhibit high contrast between the NP (DDT-AUDT) and background (DDT) areas with practically no NPs adsorbed within the latter areas that appear dark in the images, apart from a “fine structure” provided by the individual grains (20
50 nm) of the polycrystalline Au substrate. The line widths in parts a and b of Figure 7 are 400 and 800 nm, respectively, which are consistent with the preprogrammed line widths of the lithographic patterns. Because the edges of the lines in Figure 7 are straight and no significant disorder close to these edges was observed, it should be possible to narrow the line width further and fabricate even finer NP patterns. When the e-beam is focused down to the scale of 10 nm, a single NP-wide string consisting of close-packed gold NPs in one dimension could be in principle fabricated. In particular, such a nanostructure has been reported by Fresco et al. who used AFM-based lithography to electrically cleave thiocarbonate at the resist template to produce thiol docking groups prior to the deposition of gold NPs.7

4. CONCLUSIONS By use of Au NPs as a model system, we have developed a novel and versatile method for the fabrication of NP patterns with high contrast and precision on SAM-based chemical templates. These templates were prepared by IPER lithography using several test molecules. The key idea was to use an NP-inert matrix comprised of nonsubstituted alkanethiols and introduce aliphatic or aromatic molecules with a specific tail group capable of binding metal NPs by either electrostatic interaction or via the formation of covalent bonds between the metal core of NP and the functional groups. The advantages of the approach are the possibility to use commercially available materials (DDT and AUDT in the given case), low irradiation dose, and full flexibility 14064

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The Journal of Physical Chemistry C in terms of both shape and size of the patterns. A further advantage is the option to combine NPs with specific molecules embedded into the matrix comprised by other molecules, which can be probably of interest for the fabrication of sensors and nanodevices. Also, the density of NP can be varied by changing the portion of the NP-binding molecules in the SAM-based chemical templates, resulting, as far as this is necessary, in gradient-like patterns. Beyond of the demonstration of the applicability of the approach, we put some efforts into the understanding of the NP attachment process and clarification of the effect of the NP attachment onto the SAM structure. In particular, the existence of covalent Au
S bonds responsible for the binding of gold NPs on the thiolterminated SAMs was unambiguously proven by HRXPS, which accordingly rules out an alternative binding mechanism mediated by hydrogen bonds between the terminal thiol groups of the SAM template and the carboxyl groups carried by the ligands of the particles. We have also estimated an amount of the thiol groups mediating the attachment of a single NP. Further, we have shown that the structure of SAMs, in terms of orientation, remains fully intact (except probably a minor rearrangement) after the covalent attachment of gold NPs, which suggests the persistence of the structure-dependent electronic properties of the SAMs in the NP/SAM assembly or pattern. This finding can be probably of importance for future NP-based nanofabrication.

’ ASSOCIATED CONTENT

bS

Supporting Information. Supplemental XPS, SEM, and AFM data along with the results of the HRXPS data evaluation and the fits of the NEXAFS data. This material is available free of charge via the Internet at http://pubs.acs.org.

’ AUTHOR INFORMATION Corresponding Author

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

’ ACKNOWLEDGMENT We thank M. Grunze for the support of this work, A. Nefedov and Ch. W€oll (KIT) for the technical cooperation at BESSY II, and the BESSY II staff for the assistance during the synchrotronrelated experiments. This work has been supported by DFG (ZH 63/10-1). ’ REFERENCES (1) Bowen, J.; Manickam, M.; Evans, S. D.; Critchley, K.; Kendall, K.; Preece, J. A. Thin Solid Films 2008, 516, 2987–2999. (2) Li, Y.; Silverton, L. C.; Haasch, R.; Tong, Y. Y. Langmuir 2008, 24, 7048–7053. (3) Xia, Y.; Xiong, Y.; Lim, B.; Skrabalak, S. E. Angew. Chem., Int. Ed. 2009, 48, 60–103. (4) Rezaee, A.; Aliganga, A. K. A.; Mittler, S. J. Phys. Chem. C 2009, 113, 15824–15833. (5) Murray, R. W. Chem. Rev. 2008, 108, 2688–2720. (6) Maye, M. M.; Luo, J.; Han, L.; Kariuki, N. N.; Zhong, C. Gold Bull. 2003, 36/3, 75–82. (7) Fresco, Z. M.; Frechet, J. M. J. J. Am. Chem. Soc. 2005, 127, 8302–8303. (8) Ulman, A. Chem. Rev. 1996, 96, 1533–1554.

ARTICLE

(9) Love, J. C.; Estroff, L. A.; Kriebel, J. K.; Nuzzo, R. G.; Whitesides, G. M. Chem. Rev. 2005, 105, 1103–1169. (10) Gates, B. D.; Xu, Q.; Stewart, M.; Ryan, D.; Willson, C. G.; Whitesides, G. M. Chem. Rev. 2005, 105, 1171–1196. (11) Hartmann, N.; Dahlhaus, D.; Franzka, S. Surf. Sci. 2007, 601, 3916–3920. (12) Hao, E.; Lian, T. Langmuir 2000, 16, 7879–7881. (13) Shein, J. B.; Lai, L. M. H.; Kggers, P. K.; Paddon-Row, M. N.; Gooding, J. J. Langmuir 2009, 25, 11121–11128. (14) Chen, S.; Pei, R.; Zhao, T.; Dyer, D. J. J. Phys. Chem. B 2002, 106, 1903–1908. (15) Sardar, R.; Beasley, C. A.; Murray, R. W. Anal. Chem. 2009, 81, 6960–6965. (16) Yonezawa, T.; Uchida, K.; Yamanoi, Y.; Horinouchi, S.; Terasaki, N.; Nishihara, H. Phys. Chem. Chem. Phys. 2008, 10, 6925–6927. (17) Niemeyer, C. M.; Ceyhan, B.; Gao, S.; Chi, L.; Peschel, S.; Simon, U. Colloid Ploym. Sci. 2001, 279, 68–72. (18) Leem, G.; Zhang, S.; Jamison, A. C.; Galstyan, E.; Rusakova, I.; Lorenz, B.; Litvinov, D.; Lee, T. R. ACS Appl. Mater. Interfaces 2010, 2, 2789–2796. (19) Pichon, B. P.; Demortiere, A.; Pauly, M.; Mougin, K.; Derory, A.; Begin-Colin, S. J. Phys. Chem. C 2010, 114, 9041–9048. (20) Hostetler, M. J.; Green, S. J.; Stokes, J. J.; Murray, R. W. J. Am. Chem. Soc. 1996, 118, 4212–4213. (21) Brust, M.; Bethell, D.; Kiely, C. J.; Schiffrin, D. J. Langmuir 1998, 14, 5425–5429. (22) Chen, S.; Murray, R. W. J. Phys. Chem. B 1999, 103, 9996–10000. (23) Daniel, M.; Astruc, D. Chem. Rev. 2004, 104, 293–346. (24) Morales, G. M.; Jiang, P.; Yuan, S.; Lee, Y.; Sanches, A.; You, W.; Yu, L. J. Am. Chem. Soc. 2005, 127, 10456–10457. (25) Xu, C.; van Zalinge, H.; Pearson, J. L.; Glidle, A.; Cooper, J. M.; Cumming, D. R. S.; Haiss, W.; Yao, J. L.; Schiffrin, D. J.; Proupın-Perez, M.; Cosstick, R.; Nichols, R. J. Nanotechnology 2006, 17, 3333–3339. (26) Weinberger, M.; Rentenberger, S.; Kern, W. Monatsh. Chem. 2007, 138, 309–314. (27) Snow, A. W.; Foos, E. E.; Coble, M. M.; Jernigan, G. G.; Ancona, M. G. Analyst 2009, 134, 1790–1801. (28) Rezaee, A.; Pavelka, L. C.; Mittler, S. Nanoscale Res. Lett. 2009, 4, 1319–1323. (29) Scheres, L.; Klingebiel, B.; ter Maat, J.; Giesbers, M.; de Jong, H.; Hartmann, N.; Zuilhof, H. Small 2010, 6, 1918–1926. (30) Morel, A.; Volmant, R.; Methivier, C.; Krafft, J.; Boujday, S.; Pradier, C. Colloids Surf., B: Biointerfaces 2010, 81, 304–312. (31) Harnett, C. K.; Satyalakshmi, K. M.; Craighead, H. G. Appl. Phys. Lett. 2000, 76, 2466–2468. (32) Garno, J. C.; Yang, Y.; Amro, N. A.; Cruchon-Dupeyrat, S.; Chen, S.; Liu, G.-Yu Nano Lett. 2003, 3, 389–395. (33) Rezaee, A.; Aliganga, A. K. A.; Pavelka, L. C.; Mittler, S. Phys. Chem. Chem. Phys. 2010, 12, 4104–4111. (34) Toikkanen, O.; Doan, N.; Erdmanis, M.; Lipsanen, H.; Kontturi, K.; Parviz, B. J. Micromech. Microeng. 2011, 21, 054025. (35) Zharnikov, M.; Grunze, M. J. Vac. Sci. Technol. B 2002, 20, 1793–1807. (36) Ballav, N.; Chen, C.-H.; Zharnikov, M. J. Photopolymer Sci. Techn. 2008, 21, 511–517. (37) Eck, W.; Stadler, V.; Geyer, W.; Zharnikov, M.; G€olzh€auser, A.; Grunze, M. Adv. Mater. 2000, 12, 805–808. (38) G€olzh€auser, A.; Eck, W.; Geyer, W.; Stadler, V.; Weimann, T.; Hinze, P.; Grunze, M. Adv. Mater. 2001, 13, 806–809. (39) Ballav, N.; Shaporenko, A.; Terfort, A.; Zharnikov, M. Adv. Mater. 2007, 19, 998–1000. (40) Ballav, N.; Shaporenko, A.; Krakert, S.; Terfort, A.; Zharnikov, M. J. Phys. Chem. C 2007, 111, 7772–7782. (41) Ballav, N.; Schilp, S.; Zharnikov, M. Angew. Chem., Int. Ed. 2008, 47, 1421–1424. (42) Ballav, N.; Zharnikov, M. J. Phys. Chem. C 2008, 112, 15037–15044. (43) Ballav, N.; Terfort, A.; Zharnikov, M. J. Phys. Chem. C 2009, 113, 3697–3706. 14065

dx.doi.org/10.1021/jp202758e |J. Phys. Chem. C 2011, 115, 14058–14066

The Journal of Physical Chemistry C

ARTICLE

(44) Ballav, N.; Terfort, A.; Zharnikov, M. Langmuir 2009, 25, 9189–9196. (45) Ballav, N.; Weidner, T.; R€ossler, K.; Lang, H.; Zharnikov, M. ChemPhysChem 2007, 8, 819–822. (46) Tai, Y.; Shaporenko, A.; Rong, H. T.; Buck, M.; Eck, W.; Grunze, M.; Zharnikov, M. J. Phys. Chem. B 2004, 108, 16806–16810. (47) Sch€upbach, B.; Terfort, A. Org. Biomol. Chem. 2010, 8, 3552–3562. (48) Frens, G. Nature 1973, 241, 20–22. (49) Liu, J.; Sch€upbach, B.; Bashir, A.; Shekhah, O.; Nefedov, A.; Kind, M.; Terfort, A.; W€oll, C. Phys. Chem. Chem. Phys. 2010, 12, 4459–4472. (50) Moulder, J. F.; Stickle, W. E.; Sobol, P. E.; Bomben, K. D. In Handbook of X-ray Photoelectron Spectroscopy; Chastian, J., Ed.; Perkin-Elmer Corp.: Eden Prairie, MN, 1992. (51) St€ohr, J. NEXAFS Spectroscopy; Springer Series in Surface Science 25; Springer-Verlag: Berlin, 1992. (52) Batson, P. E. Phys. Rev. B 1993, 48, 2608–2610. (53) Shustak, G.; Shaulov, Y.; Domb, A. J.; Mandler, D. Chem.
Eur. J. 2007, 13, 6402–6407. (54) Ballav, N.; Weidner, T.; Zharnikov, M. J. Phys. Chem. C 2007, 111, 12002–12010. (55) Shen, C.; Haryono, M.; Grohmann, A.; Buck, M.; Weidner, T.; Ballav, N.; Zharnikov, M. Langmuir 2008, 24, 12883–12891. (56) Zharnikov, M. J. Electron Spectrosc. Relat. Phenom. 2010, 178
179, 380–393. (57) Schreiber, F. Prog. Surf. Sci. 2000, 65, 151–256. (58) Zharnikov, M.; Grunze, M. J. Phys.: Condens. Matter 2001, 13, 11333–11365. (59) Horsley, J.; St€ohr, J.; Hitchcock, A. P.; Newbury, D. C.; Johnson, A. L.; Sette, F. J. Chem. Phys. 1985, 83, 6099–6107. (60) Yokoyama, T.; Seki, K.; Morisada, I.; Edamatsu, K.; Ohta, T. Phys. Scr. 1990, 41, 189–192. (61) Solomon, J. L.; Madix, R. J.; St€ohr, J. Surf. Sci. 1991, 255, 12–30. (62) Tai, Y.; Shaporenko, A.; Eck, W.; Grunze, M.; Zharnikov, M. Appl. Phys. Lett. 2004, 85, 6257–6259. (63) Tai, Y.; Shaporenko, A.; Noda, H.; Grunze, M.; Zharnikov, M. Adv. Mater. 2005, 17, 1745–1749.

14066

dx.doi.org/10.1021/jp202758e |J. Phys. Chem. C 2011, 115, 14058–14066