gold nanoparticle patterning on monomolecular chemical templates fabricated by irradiation-promoted...

Published: June 16, 2011

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

ARTICLE

pubs.acs.org/JPCC

Gold Nanoparticle Patterning onMonomolecular Chemical TemplatesFabricated by Irradiation-Promoted Exchange ReactionJianli 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

1. INTRODUCTION

The fabrication of nanostructures within both bottom-up andtop-down approaches is one of the focus areas of modern scienceand technology, with a variety of different techniques developedfor this purpose. A hot topic in this context is the synthesis andassembly of nanoparticles (NPs), including their deposition ontodifferent substrates, with the ability to arrange NPs into patternsand arrays being one step on the road toward the construction ofnanodevices, sensors, and frontier electronics.1�5 In particular,the directed self-assembly of gold NPs has been utilized toprepare electrically conducting nanowires, plasmonic wave-guides for electro-optical devices, seeds for the growth of siliconnanorods, and nanostructured catalysts for fuel cell catalytic reac-tions.6,7 An efficient means for the immobilization and patterningof gold (and other) NPs is provided by self-assembled mono-layers (SAMs) which are 2D assemblies of rodlike organic mole-cules bearing, if necessary, terminal groups with specificfunctionalities.8�11 The immobilization of NPs is then mediatedby the interaction between these functionalities and the surface ofNPs or their organic ligands. Possible interaction mechanismsinclude hydrogen bonds,12 electrostatic attraction,13 metal ion�pyridine complexation,14 ion-induced adsorption,15 covalentbonds,16 and DNA hybridizations.17 In the case of covalentimmobilization, covalent bonds can be formed between eitherthe ligands18 or the metal cores19 of NPs and the terminal groupsat the substrate surface. Within this strategy, thiol-terminated

SAMs are frequently used to bind coinage metal (in particulargold) NPs,20�29 relying on the high stability of the S�metalbond.9 The formation of this bond in the given case is supposedto result from a partial replacement of organic ligands, whichusually cover the metal cores of NPs, by thiol groups. Inparticular, Morel et al. have observed that the characteristicS�H vibration band at 2552 cm�1 is present in the Ramanspectra of dithiol solution but is not perceptible in the case ofgold NPs deposited onto dithiol SAMs, which suggests thecovalent grafting of these NPs by replacing S�H bonds at theSAM-ambient interface by S�Au ones.30 Note, however, thatthe existence of covalent bonds, which, as mentioned above, areassumed to be responsible for the immobilization of gold NPs onthiol-terminated SAMs needs probably more unambiguousevidence, because the binding by hydrogen bonds betweenthiol (from SAMs) and carboxyl (from citric acid) groups isalso possible.

Apart from the immobilization of NPs onto homogeneousSAMs, these films can be used to fabricate 2DNP patterns on themicro- and nanometer length scale. This can be achieved bycontrolling the lateral distribution of the terminal functionalgroups in SAMs with techniques as, e.g., UV photolithography,

Received: March 24, 2011Revised: May 30, 2011

ABSTRACT: By use of custom-synthesized, citrate-passivatedgold nanoparticles (NPs) as a test system, we have demon-strated a new approach to fabricate high-contrast and highresolution patterns of metal NPs. The patterns were fabricatedon monomolecular chemical templates prepared by irradiation-promoted exchange reaction (IPER) lithography. The lattertechnique allows to introduce molecules bearing NP-bindingtail groups into the selected areas of the primary NP-inertmatrix, resulting in the desired chemical template. The advan-tages of the approach are (i) high flexibility in terms of theinterfacial chemistry, length scale, and pattern form, (ii) low irradiation dose required for the primary patterning, and (iii) thepossibility to use commercial molecules. The suggested strategy relies on strong electrostatic or covalent bonding of the NPs to thepreselected functional groups, as was directly demonstrated for some of the target systems. A further important point is that theadsorption of the NPs does not affect the monolayer structure significantly, suggesting the persistence of the electronic structure ofthe film and NP-bearing molecules in particular, which can be of importance for applications in different areas such as sensorfabrication or nanoengineering.

14059 dx.doi.org/10.1021/jp202758e |J. Phys. Chem. C 2011, 115, 14058–14066

The Journal of Physical Chemistry C ARTICLE

electron-beam (e-beam) lithography, scanning probe lithogra-phy, focused ion beam lithography, and microcontact printing(μCP).10,31�34 The patterns are then formed by selective adhe-sion of NPs onto the functional areas. Among the abovetechniques, e-beam lithography is probably the most flexibleone in terms of the length scale (from micrometers to nano-meters), lateral resolution (down to few nanometers), and variableform of the patterns as far as a focused electron beam scanningacross a SAM-resist-coated substrate is used as the primarypatterning tool.35,36 Alternatively, the patterning can be per-formed in the proximity printing geometry with a stencil mask,which limits the form flexibility but allows fabricating large areapatterns in parallel fashion.

Since not conventional but chemical templates are necessary,e-beam lithography should be combined with a specific SAMarchitecture or a postirradiation treatment. The most establishedapproach in this regard is the electron-induced transformation ofthe 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 remainmostly intact.37,38 An alternative approach, requiring much smaller(by ca. 2 orders of magnitude) irradiation dose and utilizingcommercial substances, is the so-called irradiation-promotedexchange reaction (IPER).39�41 The key idea of this techniqueis a tuning of the extent and rate of the exchange reactionbetween the primary aliphatic39�41 or aromatic42 SAM and apotential molecular substituent by electron or X-ray irradiationwith a variable dose. The irradiation-induced defects promote theexchange, so that, controlling their amount by selection of aproper dose, the extent of the exchange reaction, and following,the composition of the resulting binary mixed SAM can beprecisely adjusted.40 By combination of IPER with electron orX-ray lithography, one can fabricate complex chemical litho-graphic patterns, including gradient ones.41

Here we proved the applicability of the IPER lithography tothe fabrication of well-defined patterns of metal NPs, takencustomly synthesized gold NPs as a test system. For this purpose,we used SAMs of nonsubstituted alkanethiolates on Au(111) asthe primary matrix, exchanged some of the matrix molecules bysubstituents bearing a terminal functional group with an affinity

to gold, and used the resulting mixed SAMs or chemical tem-plates for selective immobilization of gold NPs. As the substit-uents we used both aliphatic and aromatic molecules bearingamino, thiol, and pyridine tail groups (see next section fordetails) as shown in Figure 1. Herewith we utilized the advantageof IPER to not only permit the mixing of different aliphaticmolecules39,40,43,44 but also the mixing of aliphatic and aromaticones.45 Apart from pursuing the ultimate goal to fabricate well-defined patterns of metal NPs, we monitored the binding of NPsto the terminal groups of the SAMs and possible effects of the NPimmobilization on the SAM structure. Note that only few studiesso far have been directed to investigate the latter effects which canbe important for the fabrication of nanodevices in terms of thestructure-dependent electronic properties of the monomolecularfilms bearing the immobilized NPs.

2. EXPERIMENTAL SECTION

2.1. Chemicals, Solvents, and Materials. Hydrogen tetra-chloroaurate(III) and trisodium citrate (99% purity) were pur-chased from Sigma Aldrich and used for the synthesis of goldNPs. The solvents, viz., absolute ethanol (purity g99.8%) andtetrahydrofuran (99% purity), were purchased from Sigma-Aldrichand used for the SAM preparation. The SAM constituentswere 1-dodecanethiol (DDT), 11-amino-1-undecanethiol (AUDT),[1,10;40,100-terphenyl]-4,400-dimethanethiol (TPDMT), and(40-(pyridin-4-yl)biphenyl-4-yl)methanethiol (PPPT), as shownin Figure 1. DDT and AUDT were purchased from SigmaAldrich and Asemblon Inc., USA, respectively. TPDMT and PPPTwere customly synthesized according to the protocols of earlierworks.46,47 The gold substrates were prepared by thermalevaporation of 100 nm of gold (99.99% purity) onto polishedsingle-crystal silicon (100) wafers (Silicon Sense) primed witha 5-nm titanium adhesion layer. The resulting metal films werepolycrystalline, with a predominant (111) orientation of theindividual 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 mMsolution 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 tothe boiling HAuCl4 solution under stirring and reflux. Then acolor change in the above solution from yellow to burgundyover colorless (transparent), gray, and black was observed in2 min. After 10 min, the reflux was stopped but stirring con-tinued for another 10 min. Finally, the solution was cooled andcompensated with Milli Q water for the water loss during thereflux process.2.3. Preparation of One-Component SAMs. The one-com-

ponent DDT, AUDT, PPPT, and TPDMT SAMs, which wereused as references, were formed by immersion of freshly preparedsubstrates into 1 mM solutions of the respective compounds inabsolute ethanol (DDT, AUDT, and PPPT) or tetrahydrofuran(TPDMT) for 24 h at room temperature. After immersion thesamples were thoroughly rinsed with pure ethanol and blown drycarefully with argon. They were either characterized or usedimmediately or stored under inert gas atmosphere in glass con-tainers until synchrotron-based experiments (see below). Noevidence for impurities or oxidative degradation products wasfound. Note that the properties of TPDMT/Au and PPPT/Auare described in detail in previous works.46,49

Figure 1. The target molecules of the present study. DDT, a nonsub-stituted alkanethiol, served as the primary matrix; AUDT, TPDMT, andPPPT were used as amino-, thiol-, and pyridine-bearing molecularsubstituents, respectively, to fabricate the mixed SAMs and chemicaltemplates.



2.4. Fabrication of Mixed SAMs and Chemical Templatesby IPER. For the fabrication of large-area, two-component mixedSAMs, the primary DDT/Au matrices were homogeneouslyirradiated 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. Thebase pressure in the chamber during the irradiation was betterthan 1 � 10�8 mbar. For the fabrication of chemical templates,the primary DDT/Au matrices were patterned by a LEO 1530scanning electron microscope (Zeiss, Germany) with a RaithElphy Plus pattern generator system. The electron-beam energywas chosen at 1 keV, and the residual gas pressure was about 5�10�6 mbar. The irradiation doses were estimated by multiplica-tion of the exposure time with the current density. Doses of 1 and2 mC/cm2 were chosen for the homogeneous irradiation andpatterning, respectively. These doses are close or exceed slightlythe saturation dose for IPER,39�41 so that the maximal possibleportion of the NP-binding substituents in the mixed SAMs andSAM-based chemical templates was inserted. Note that this valuedepends on the character of the substituents and varies from35 to 70%.40,45

The primary DDT/Au matrices treated by either homoge-neous e-beam irradiation or e-beam lithography were immersedin 1 mM solutions of AUDT or PPPT in absolute ethanol orTPDMT in tetrahydrofuran for 2 h at room temperature toperform the exchange reaction. After immersion, the sampleswere thoroughly rinsed with pure ethanol and blown dry withargon. Note that IPER occurred quite efficiently for all thesubstituents (see below). In particular, this was the reason forthe selection of TPDMT as a thiol-bearing molecule. The use ofthe respective aliphatic analogue, n-octane-1,8-dithiol, did notresult in high quality mixed SAMs.2.5. Immobilization of Gold NPs on SAMs and SAM Tem-

plates. The fabricated homogeneous and mixed SAMs and SAM-based chemical templates were immersed in the solution of goldNPs for 18 h at room temperature. After immersion, the sampleswere cleaned by ultrasound in Milli Q water for 2 min and rinsedwith pure ethanol followed by drying with argon.2.6. Characterization Techniques for the NPs, SAMs, and

NP/SAMs Assemblies. Particle size analyzer, scanning electronmicroscopy (SEM), atomic force microscopy (AFM), X-rayphotoelectron spectroscopy (XPS), synchrotron-based high-resolution XPS (HRXPS), and angle-resolved near-edge X-rayabsorption fine structure (NEXAFS) spectroscopy were em-ployed for this study. All experiments were performed at roomtemperature. The XPS, HRXPS, and NEXAFS measurementswere carried out under ultrahigh vacuum conditions at a basepressure better than 1� 10�9 mbar. The spectra acquisition timewas selected in such a way that no noticeable damage by theprimary 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%. Allthe samples after immobilization of gold NPs were studied with aLEO 1530 scanning electron microscope with a field emissiongun operating at an accelerating voltage of 3 kV. Some systemswere also characterized by AFM using a Digital Instrument 3100microscope in the tapping mode. The NP density was deter-mined on the basis of several representative SEM images; weestimate an error at (10%.The conventional XPS measurements were performed using

a Mg KR X-ray source and a LHS 11 analyzer. The spectraacquisition was carried out in normal emission geometry with an

energy resolution of∼0.9 eV. The X-ray source was operated at apower of 260W 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 ringBESSY II in Berlin, Germany, using a Scienta R3000 spectro-meter. The spectra were acquired in normal emission geometryat photon energies of 350 eV for the S 2p region and 580 eV forthe C 1s and Au 4f ranges. The energy resolution was 0.2�0.3 eVallowing a clear separation of individual spectral components.The binding energy (BE) scale of the XPS andHRXPS spectra

was referenced to the Au 4f7/2 peak at a BE of 84.0 eV.50 Thesespectra were fitted by symmetric Voigt functions and a linear-type background. To fit the S 2p3/2,1/2 doublet we used two peakswith the same full width at half-maximum (fwhm), the standardspin�orbit splitting of∼1.18 eV (verified by fit), and a branchingratio of 2 (S2p3/2/S2p1/2).

50 The fits were performed self-consistently, i.e., the same fit parameters were used for identicalspectral regions. The HRXPS spectra were mostly used forderiving chemical information while the XPS ones were utilizedto get the thickness and composition of the target films andNP/SAM assemblies.The NEXAFS measurements were performed at the same

beamline as the HRXPS experiments. The spectra acquisitionwas carried out at the carbon K-edge in the partial electron yieldmode with a retarding voltage of �150 V. Linearly polarizedsynchrotron light with a polarization factor of ∼91% was used.The energy resolution was 0.2�0.3 eV. The incidence angle ofthe primary X-ray beam was varied from 90� (E-vector in thesurface plane) to 20� (E-vector nearly normal to the surface) insteps of 10�20� to monitor the orientational order of themolecules in the target films. This approach is based on thelinear dichroism in X-ray absorption, i.e., the strong dependenceof the cross-section of the resonant photoexcitation process onthe orientation of the electric field vector of the linearly polarizedlight with respect to the molecular orbital of interest.51

The raw NEXAFS spectra were normalized to the incidentphoton flux by division by a spectrum of a clean, freshly sputteredgold sample,51 and then the spectra were reduced to the standardform by subtracting a linear pre-edge background and normal-izing to the unity edge jump (determined by a nearly horizontalplateau 40�50 eV above the absorption edge). The energy scalewas referenced to the most intense π* resonance of highlyoriented pyrolytic graphite (HOPG) at 285.38 eV.52

3. RESULTS AND DISCUSSION

3.1. Synthesis of Gold NPs. Gold NPs were synthesizedthrough reduction of chloroauric acid by sodium citrate in liquidphase. The NPs were stabilized by citrate ions with negativecharges, which lead to repulsive interaction among the particlesand prevent the formation of aggregates. Indeed, as shown in theSEM image of the gold NPs on clean silicon (100) surface inFigure 2a, no aggregates were formed. The particles representhighly uniform nanospheres with a narrow size distributioncentered at 10 nm, as shown in Figure 2b, where this distributionis presented. Note that the organic shell can be readily modifiedby exchange with other organic surfactants to produce function-alities on the particle surface.3.2. Immobilization of Gold NPs on SAMs. To investigate

the feasibility of the NPs patterning on SAM-based chemicaltemplates, we first tested and studied their deposition onto thehomogeneous and mixed SAMs which were supposed to constitute



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

DDT/Au matrix was absolutely inert with respect to the adsorp-tion of the NPs, whereas the reference one-component AUDT,TPDMT, and PPPT films represented suitable templatesfor their immobilization. For these templates, a formationof dense submonolayers of the NPs with a density of ∼1.7 �103 particles/μm2was observed as shown for AUDT in Figure 3b(for TPDMT and PPPT, see Supporting Information). Signifi-cantly, the NPs were adsorbed as isolated particles, rather thanaggregates, on all the substrates as a result of the repulsion amongthe particles mediated by the citrate capping ligands that carry anegative charge. This repulsion however did not result in theformation of an ordered 2D structure of the NPs but just in acertain degree of short-range order around each individual NPwith an average interparticle spacing of∼20 nm. Note that this isa balance between the electrostatic repulsion among the particlesand their interaction with the underlying SAM, which is respon-sible for the packing density of the particles and the character oftheir arrangement.30 In particular, one cannot increase thepacking density of NPs further by grafting more functionalgroups onto SAM if the repulsion among particles is strongenough to prevent more particles from filling the vacant bindingsites on the film.As mentioned in section 2, IPER and IPER lithography do not

allow a complete substitution of the DDT molecules in theprimary template by the substituent molecules but only a partialsubstitution, resulting in the mixed DDT+substituent films. Thecomparison of the XPS spectra of the pristine (DDT) andreference (AUDT, TPDMT, and PPPT) SAMs with the spectraof the respective mixed films prepared by IPER (see SupportingInformation) enabled us to determine the portions of the AUDT,TPDMT, and PPPTmolecules in the latter films. These portionswere 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-compo-nent AUDT, TPDMT, and PPPT films, the ability to serve astemplates for the immobilization of the Au NPs. As shown inFigure 3c for DDT+AUDT and in Supporting Informationfor DDT+TPDMT, and DDT+PPPT (Figure S1 of Support-ing Information), the mixing of the NP-neutral DDT andNP-binding substituent (AUDT, TPDMT, and PPPT) mol-ecules results in the similar density and arrangement of theNPs as for the case of the one-component AUDT, TPDMT, andPPPT films. This is understandable since the density of the NP-binding tail groups is significantly larger than the highest possibledensity of the NPs within the monolayer coverage. Indeed, asshown above, the average diameter of the NPs is ∼10 nm,

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 ontothe one-component DDT film (a), one-component AUDT film (b),and mixed DDT-AUDT film (c).



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 therespective 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 templatesoccurred over different mechanisms depending on the characterof the terminal (tail) group. In particular, in the case of amino(AUDT) and pyridine (PPPT) termination, the NPs werepresumably immobilized via electrostatic interaction.53 Indeed,the pH of the fabricated gold colloid solution was around 5, sothat the amine and pyridine tail groups of the respective SAMswere expected to be protonated and thus carrying a positivecharge when exposed to the suspension of NPs which in turn, asmentioned above, were charged negatively. In contrast, the SAMwith the terminal thiol groups, TPDMT/Au, was expected toattach 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 XPSspectroscopy characteristic of both amino or pyridine nitrogen.The major effect of the NP adsorption was the intensity reduc-tion (see Supporting Information), which was related to theattenuation of the N 1s signal by the NP overlayer. More effectswere observed in the case of the TPDMT and DDT+TPDMTtemplates. The Au 4f7/2, C 1s, and S 2p HRXPS spectra of thepristine and NP-covered TPDMT films are presented in Figure 4along with the decomposition of the S 2p spectra into theindividual components related to the thiolate (T1) and thiol(T2) species. As compared to the pristine TPDMT film, theintensity of the Au 4f7/2 emission increased significantly after thedeposition of the Au NPs (Figure 4a). This increase results froma superposition of the additional signal from the Au NPs and theNP-induced attenuation of the signal from the underlying massivegold substrate.30 Obviously, the effect of the NPs is stronger,resulting in the overall signal increase. In contrast, the intensity ofthe C 1s emission, which represents a single and symmetric peakat 284.2 eV for TPDMT/Au (as expected for a high qualityTPDMT SAM),46 decreased slightly upon the deposition of theNPs. Similar to the Au 4f7/2 case, this is also a complex effect ofthe citrate shell of theNPs and theNP-induced attenuation of the

signal from the SAM underneath, with the second effect beingobviously stronger. Further, the signal at ∼289.0 eV assignableto the C atoms of the �COO(H) moieties, which could beexpected in view of the presence of citrate carrying as much asthree COO(H) units per molecule in the shell, is practically notperceptible for AuNPs/TPDMT/Au. This suggests that theorganic shell of the Au NPs was very thin and provided only avery small contribution to the C 1s HRXPS spectra. Consideringthe occurrence of three COO(H) units per molecule and the areaoccupied by the NP (15�20% of the entire area), one can expectthat a densely packed monolayer of citrates on the surface of theimmobilized NPs will give in the present case a similar 289.0 eVsignal as a half monolayer of�COO(H) terminated alkanethiolson flat Au substrate. The latter system provides a clear 289.0 eVsignal in the C 1s XPS spectrum,54 which is, however, not the casehere. So, we can tentatively estimate the coverage of citrates assignificantly 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 NPadsorption went beyond the intensity change only. As shown inFigure 4c, these spectra can be decomposed into two S 2p3/2,1/2doublets at BEs of 162.1 and 163.4 eV (S 2p3/2) assigned to thethiolate species bound to gold (peak T1) and to the thiol tailgroups (peak T2), respectively.30,46,55,56 As expected,46 theintensity ratio of these doublets, T1/T2 for TPDMT/Au (0.13)deviates from the atomic ratio of sulfur species bound/unboundto gold (1:1) due to the strong attenuation of the thiolate signalby the hydrocarbon matrix of the SAM. The deposition of theNPs would result in additional attenuation of the S 2p signal,which however should be the same for both thiolate and thiolspecies as far as no chemical changes occurred. This was howevernot the case, but the T1/T2 ratio increased from 0.13 to 0.16upon the immobilization of the NPs, indicating an increase in thefraction of the Au-bonded (thiolate) sulfur compared to TPDMT/Au. This increase can only be associated with the formation of thecovalent Au�S bonds between the Au cores of the NPs and theterminal thiol groups of the SAM. This process was possible dueto the thin citrate shell which obviously could be penetrated orreplaced by the thiol moiety. The relatively small extent of theT1/T2 change reflects the fact that only a small portion of theterminal thiol group participated in the NP bonding as could be

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 aredecomposed into the individual contributions related to the thiolate (T1, gray solid line) and thiol (T2, black solid line) species. The intensity ratios ofboth components are given.



expected from comparison of the NP diameter and intermole-cular spacing (see above). A simple estimate on the basis ofthe above T1/T2 ratios suggests that this portion is at least3%, which, at the given packing density of the SAM (4.63 �1014molecules/cm2),57 corresponds to∼1.4� 105molecules/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 mol-ecules per NP. Note that the area taken by an NP with a diameterof 10 nm corresponds to the area taken by∼360molecules of theunderlying SAM. So,∼22% of these molecules participate in theNP bonding, which is presumably related to the spherical form ofthe NPs and a somewhat limited permeability of the citrate shell.Apart from the formation of the NP�thiol bonds at the SAM-

ambient interface, the observed change of the T1/T2 ratio couldprobably originate from a partial SAM disordering upon thedeposition of the NPs. This possibility was however ruled out bythe NEXAFS experiments as will be shown below. Generally,such experiments provide information about the chemical iden-tity of a target film and average orientation of its constituents.51

A measure of this orientation is the linear dichroism, i.e., thedependence of the absorption resonance intensity on the ori-entation of the electric field vector of the synchrotron light withrespect to the molecular orbital of interest. An efficient way tomonitor the linear dichroism is to plot the difference of theNEXAFS spectra acquired at normal (90�) and grazing (20�)angles of X-ray incidence. A disordered film exhibits no dichro-ism, whereas an ordered one reveals difference peaks at theresonance positions.58 In contrast to the difference curves, aspectrum acquired at the so-called magic angle of X-ray incidence(55�) is not affected by any effects related to molecular

orientation and gives only information on the chemical identityof investigated samples.51

C K-edge NEXAFS spectra of the pristine and NP-coveredTPDMT/Au acquired at an X-ray incidence angle of 55� arepresented in Figure 5 along with the difference between thespectra acquired at 90� and 20�. The 55� spectrum of TPDMT/Au exhibits the characteristic absorption resonances of phenylrings,51,58�61 in accordance with the molecular composition ofTPDMT, and is dominated by the intenseπ1* resonance at 285.0eV, which is accompanied by the weaker π2* resonance at 288.9eV, and several broad σ* resonances at higher photon energies.These resonances overlap with characteristic features of thealiphatic linker, most pronounced of which is the so-calledRydberg resonance (R*) at 287.7 eV.58 In accordance with theexpectations,46 the NEXAFS spectra of TPDMT/Au exhibitsignificant linear dichroism as evidenced by the pronounceddifference peaks in the respective 90�20� curve in Figure 5b.This curve suggests a high degree of orientational order anddense molecular packing in TPDMT/Au. In accordance withthe expected upright orientation of the TPDMTmolecules in therespective monolayers, the π*/R* and σ* resonances in the90�20� curves exhibit positive and negative anisotropy peaks,respectively (the transition dipole moments of these resonancesare 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, whichsuggest that neither the hydrocarbon matrix of the SAM nor itsorientational order were affected noticeably by the NP immobi-lization. Significantly, no peak at ∼288.5 eV corresponding tothe π* resonance of the �COO(H) group40,51 appeared for AuNPs/TPDMT/Au, which is in good agreement with the HRXPS

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

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



results indicating that the organic shell of the NP is too thin to bedetected. A tentative analysis referring to ref 40, which containsthe CK edge NEXAFS spectra of�COO(H) terminated alkane-thiol SAMs, supports our conclusion about a submonolayercoverage of citrates.As for the orientational order, a qualitative statement about the

persistence of the difference spectra was complemented by thequantitative analysis of the entire set of the NEXAFS spectraacquired at different angles of X-ray incidence (θ). Withinthis analysis, the average tilt angles of the π* orbitals, R, wascalculated. To avoid any ambiguity related to the decompositionof the spectra and the normalization of the intensities, the mostintense and well-separated π1* resonance and the intensity ratiosI(R,θ)/I(R,20�) were used for the evaluation according to theformula51

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 andNP-covered TPDMT/Au were found to be 68 and 71�, respectively(see Supporting Information), which correspond to moleculartilt angles of 22 and 19� (with respect to the surface normal) asfar as the molecular twist is not considered. These values are ingood agreement with each other considering the experimentalerror ((3�). This agreement demonstrates, in accordance withthe qualitative analysis of the NEXAFS data, that the orientationorder of the TPDMT SAMs was not disturbed significantly bythe immobilization of the NPs on top of the film, except probablya slight structural rearrangement induced by the appearing Au-thiolate bonds at the SAM-ambient interface, resulting in evenmore upright molecular orientation (as far as we believe that theobserved 3� difference is real). Similar results have also beenobserved by some of the authors before, both with respect to themetal-adsorbate-induced improvement of the orientational orderin pristine TPDMT films62 and with respect to the preservationof the SAM structure upon evaporation of nickel layer on cross-linked TPDMT films.63 In all these cases, one can expect that thestructure-dependent electronic properties of the SAMs persistupon the deposition event, which makes their application forelectronic 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 immobi-lized homogeneously on the binary SAMs with the proper func-tional groups prepared by IPER. This paves the way to fabricatetwo-dimensional NP nanostructures on SAM-based chemical

templates prepared by IPER lithography. The basic strategy forthis purpose is depicted in Figure 6. As the first step, chemicalpatterns were “prewritten” in an aliphatic resist (DDT/Au) bythe focused e-beam (see section 2). Further, the prewrittenpatterns were “developed” by putting the e-beam treated samplesinto AUDT solution and performing IPER, which resulted in theformation of mixed DDT-substituent film in the areas exposed toelectrons. Finally, the chemical templates were immersed intothe suspension of the gold NPs to attach the particles onto thefunctionalized areas. Several representative examples of nano-structures fabricated in this way using AUDT as substituent areshown in Figure 7 (similar patterns were obtained for TPDMTand PPPT as well); the NPs appear as bright dots with 8�15 nmdiameter. Both the honeycomb and letterlike structures exhibithigh contrast between the NP (DDT-AUDT) and background(DDT) areas with practically no NPs adsorbed within the latterareas that appear dark in the images, apart from a “fine structure”provided by the individual grains (20�50 nm) of the polycrystal-line Au substrate. The line widths in parts a and b of Figure 7 are400 and 800 nm, respectively, which are consistent with thepreprogrammed line widths of the lithographic patterns. Becausethe edges of the lines in Figure 7 are straight and no significantdisorder close to these edges was observed, it should be possibleto narrow the line width further and fabricate even finer NPpatterns.When the e-beam is focused down to the scale of 10 nm,a single NP-wide string consisting of close-packed gold NPs inone dimension could be in principle fabricated. In particular, sucha nanostructure has been reported by Fresco et al. who usedAFM-based lithography to electrically cleave thiocarbonate at theresist template to produce thiol docking groups prior to thedeposition of gold NPs.7

4. CONCLUSIONS

By use of Au NPs as a model system, we have developed anovel and versatile method for the fabrication of NP patternswith high contrast and precision on SAM-based chemical tem-plates. These templates were prepared by IPER lithography usingseveral testmolecules. The key idea was to use anNP-inert matrixcomprised of nonsubstituted alkanethiols and introduce aliphaticor aromatic molecules with a specific tail group capable ofbinding metal NPs by either electrostatic interaction or via theformation of covalent bonds between the metal core of NP andthe functional groups. The advantages of the approach are thepossibility to use commercially available materials (DDT andAUDT in the given case), low irradiation dose, and full flexibility

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).



in terms of both shape and size of the patterns. A furtheradvantage is the option to combine NPs with specific moleculesembedded into the matrix comprised by other molecules, whichcan be probably of interest for the fabrication of sensors andnanodevices. Also, the density of NP can be varied by changingthe portion of the NP-binding molecules in the SAM-basedchemical templates, resulting, as far as this is necessary, ingradient-like patterns.

Beyond of the demonstration of the applicability of the approach,we put some efforts into the understanding of the NP attachmentprocess and clarification of the effect of the NP attachment ontothe SAM structure. In particular, the existence of covalent Au�Sbonds responsible for the binding of gold NPs on the thiol-terminated SAMs was unambiguously proven by HRXPS, whichaccordingly rules out an alternative bindingmechanismmediatedby hydrogen bonds between the terminal thiol groups of theSAM template and the carboxyl groups carried by the ligands ofthe particles. We have also estimated an amount of the thiolgroupsmediating the attachment of a single NP. Further, we haveshown 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 thepersistence of the structure-dependent electronic properties ofthe SAMs in the NP/SAM assembly or pattern. This finding canbe probably of importance for future NP-based nanofabrication.

’ASSOCIATED CONTENT

bS Supporting Information. Supplemental XPS, SEM, andAFM data along with the results of the HRXPS data evaluationand the fits of the NEXAFS data. This material is available free ofcharge 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. Nefedovand Ch. W€oll (KIT) for the technical cooperation at BESSY II,and the BESSY II staff for the assistance during the synchrotron-related experiments. This work has been supported by DFG(ZH 63/10-1).

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