Immobilization of [60]fullerene on silicon surfaces through a calix[8]arene layerFilippo Busolo, Simone Silvestrini, Lidia Armelao, and Michele Maggini

Citation: The Journal of Chemical Physics 139, 164715 (2013); doi: 10.1063/1.4827114 View online: http://dx.doi.org/10.1063/1.4827114 View Table of Contents: http://scitation.aip.org/content/aip/journal/jcp/139/16?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Surface functionalization of 6H-SiC using organophosphonate monolayers Appl. Phys. Lett. 100, 101601 (2012); 10.1063/1.3691919 Fabrication of a highly oriented line structure on an aluminum surface and the nanoscale patterning on thenanoscale structure using highly functional molecules J. Vac. Sci. Technol. A 27, 793 (2009); 10.1116/1.3125264 Wettability control of a polymer surface through 126 nm vacuum ultraviolet light irradiation J. Vac. Sci. Technol. A 22, 1309 (2004); 10.1116/1.1701867 Modification of polycarbonate and polypropylene surfaces by argon ion cluster beams J. Vac. Sci. Technol. B 19, 2050 (2001); 10.1116/1.1410944 Effect of hydrogen termination on the work of adhesion between rough polycrystalline silicon surfaces J. Appl. Phys. 81, 3474 (1997); 10.1063/1.365045

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THE JOURNAL OF CHEMICAL PHYSICS 139, 164715 (2013)

Immobilization of [60]fullerene on silicon surfaces througha calix[8]arene layer

Filippo Busolo,1 Simone Silvestrini,1 Lidia Armelao,2,a) and Michele Maggini1,a)

1Department of Chemical Sciences, ITM-CNR University of Padova, Via F. Marzolo 1, 35131 Padova, Italy2Department of Chemical Sciences, IENI-CNR and INSTM, University of Padova, Via F. Marzolo 1,35131 Padova, Italy

(Received 2 July 2013; accepted 13 October 2013; published online 31 October 2013)

In this work, we report the functionalization of flat Si(100) surfaces with a calix[8]arene deriva-tive through a thermal hydrosilylation process, followed by docking with [60]fullerene. Chemicalgrafting of calix[8]arene on silicon substrates was evaluated by X-ray photoelectron spectroscopy,whereas host-guest immobilization of fullerene was demonstrated by atomic force microscopy andsessile drop water contact angle measurements. Surface topographical variations, modelled on thebasis of calix[8]arene and [60]fullerene geometrical parameters, are consistent with the observedmorphological features relative to surface functionalization and to non-covalent immobilization of[60]fullerene. © 2013 AIP Publishing LLC. [http://dx.doi.org/10.1063/1.4827114]

I. INTRODUCTION

The formation of Si-C bonds on oriented silicon surfaces,such as Si(100) and Si(111), has been extensively used forthe immobilization of molecular structures,1–4 with the objec-tive of enhancing the functionality of existing and emergingsilicon-based materials and devices.5–8 In particular, molec-ular host-guest recognition at silicon surfaces was used tofabricate novel sensing platforms based on electrical, optical,or mechanical transduction of chemical binding events.9–12 Ithas been demonstrated that the transduction itself is depen-dent on the host-guest interactions that may, in principle, leadto the formation of supramolecular structures with interest-ing properties of their own and new interactions with the sil-icon substrate. In this connection, host-guest recognition isbeing thoroughly explored as a primary approach to surfacefunctionalization.13–16

[60]fullerene and its derivatives have been widely used todevelop hybrid materials where the peculiar properties of thisfascinating spherical molecule are transferred onto varioussurfaces.17, 18 Some noticeable examples are the wettabilitymodulation of porous silicon19 and the modification of elec-tronic properties of semiconductors thanks to the electron-accepting characteristics of the fullerene sphere.20–23 In thementioned cases, the fullerene cage was functionalized withappropriate grafting groups for the bonding to the surface.Fullerene-based supramolecular systems,24 on the other hand,do not require the functionalization of the carbon allotropeguest, which fully retains its electronic properties and, at leastin principle, is less prone to aggregation.25–28

Since Atwood’s29 and Shinkai’s30 discovery, that atoluene solution of [60]fullerene and p-tert-butyl-calix[8]arene31 (a synthetic macrocycle based on phenol) forms asparingly soluble precipitate identified as a 1:1 host-guestcomplex, the attention has moved towards the preparation

a)Authors to whom correspondence should be addressed. Electronicaddresses: lidia.armelao@unipd.it and michele.maggini@unipd.it

and 2D confinement of this supramolecular system onto sur-faces as a milestone towards the development of moleculardevices.32 So far, few examples have been reported, wherethe calix[8]arene-fullerene inclusion complex was exploitedto control the deposition of the fullerene on gold.33, 34

This work explores the functionalization of flat Si(100)surfaces with calix[8]arene, followed by docking with[60]fullerene. The functional layers were prepared by thermalhydrosilylation of a hydrogen-terminated silicon substrate35

with 2,11,17,23,29,35,41-octa-tert-butyl-calix[8]arene-49,50,51,52,53,54,55,56-octa-undec-10-enoate (2) that was em-ployed either neat or as a mixture with the diluting additive1-octene. Sessile drop water contact angle (WCA), X-rayphotoelectron spectroscopy (XPS), and atomic force mi-croscopy (AFM) were used to provide chemical and topo-graphical information on the surfaces at every step of thefunctionalization process. Computer simulations were alsoemployed to confirm the structure of the host-guest layer onthe semiconductor surface.

II. EXPERIMENTAL

The detailed procedure for the synthesis of calix[8]arene2, according to Scheme 1, is reported in the supplementarymaterial,36 together with pertinent characterization data (seeFigures S1–S4 for 1H- and 13C-NMR, mass and thermogravi-metric analysis, respectively). In short, 2 was synthesizedby a two-step procedure starting from p-tert-butylphenol andparaformaldehyde, which were reacted to give p-tert-butyl-calix[8]arene (1). The reaction of 1 with 10-undecenoyl chlo-ride gave derivative 2 in 80% overall isolated yield after col-umn chromatography purification.

A. Chemical etching of silicon substrates

Hydrogen-terminated crystalline silicon layers (Si-Hsamples) were prepared following a reported procedure.37

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164715-2 Busolo et al. J. Chem. Phys. 139, 164715 (2013)

SCHEME 1. Synthesis of calix[8]arene 2.

Si(100) slabs (p-type, 1 × 1 cm 500 μm thickness) werefirst cleaned by sonication (10 s) in acetone, toluene, ace-tone again, and ethanol, then treated with a 3:1 (v/v) conc.H2SO4/30% H2O2 at 100 ◦C for 30 min, followed by rins-ing with Milli-Q water. The etching was carried out with a2% (w/w) solution of aqueous, argon-deaerated hydrofluoricacid, for 2 min at room temperature, followed by rinsing withargon-deaerated Milli-Q water for a few seconds and dryingunder a nitrogen stream. Freshly etched slabs are very reac-tive, therefore the functionalization step was performed im-mediately after their preparation.

B. Preparation of calix[8]arene monolayers

The grafting of compound 2 on flat silicon surfaces wascarried out following a previously reported thermal hydrosily-lation procedure.35 A freshly etched Si-H sample was placedin a three-necked, flat-bottomed glass vial. Neat derivative 2(2 ml) was degassed with argon for 1 h at 140 ◦C, then cooledto 60 ◦C and transferred into the flask by means of a pipette.The vial, with the silicon slab covered by 2, was immersed inan oil bath at 140 ◦C and kept at that temperature for 5 h underargon atmosphere. The functionalized slab (Si-2) was washedthoroughly with ethanol, dichloromethane and dried under anitrogen flow.

C. Preparation of mixed calix[8]arene/1-octenemonolayers

The grafting procedure is the same as reported inSec. II B, but 2 ml of an equimolar mixture of compound 2and 1-octene were used instead of neat 2. The degasing stepin this case was carried out at 110 ◦C rather than 140 ◦C. Thesamples will be referred to as Si-2:Oct.

D. Immobilization of [60]fullerene on calix[8]arenemonolayers

Silicon substrates Si-2 and Si-2:Oct were immersedinto a 10−3 M solution of [60]fullerene in toluene for 3 h

at room temperature. Excess fullerene was removed byrinsing thoroughly with 1,2-dichlorobenzene, toluene, anddichloromethane. Si-2-C60 and Si-2:Oct-C60 were dried un-der a stream of nitrogen.

E. Sessile drop water contact angle measurements

WCAs were measured with a custom-made apparatusconsisting of a motorized syringe pump, a video camera, andcomputer-controlled motorized sample and camera stages.38

Small drops (volume ≈ 1 μl) of deionized water were de-posited on the substrate with the syringe pump, and their equi-librium profile acquired with the camera. At least five dropswere observed for each sample to get statistically sound re-sults. The representative contact angle � was then measuredas the mean of all measurements, carried out on both sidesof each drop. The corresponding error was estimated to betypically ±2◦.

F. Chemical characterization

XPS was used as the main tool to assess the chemi-cal composition of the surfaces. Analyses were performedon a Perkin-Elmer � 5600-ci spectrometer using non-monochromatized Al Kα radiation (1486.6 eV). The sampleanalysis area was 800 μm in diameter and the working pres-sure was lower than 10−9 mbar. The spectrometer was cali-brated by assuming the binding energy (BE) of the Au 4f7/2

line at 83.9 eV with respect to the Fermi level. The stan-dard deviation for the BEs values was ±0.2 eV. Survey scanswere obtained in the 0−1300 eV range. Detailed scans wererecorded for the C1s, O1s, and Si2p regions. No further el-ement was detected. The samples were sufficiently electri-cally conductive that no compensation for charging effectswas required. The residual BE shifts (on the order of ∼0.5eV) were corrected by assigning to the C1s peak associatedwith adventitious hydrocarbons a value of 284.8 eV.39 Sam-ples were mounted on steel holders and introduced directly inthe fast-entry lock system of the XPS analytical chamber. The

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164715-3 Busolo et al. J. Chem. Phys. 139, 164715 (2013)

analysis involved Shirley-type background subtraction,40

nonlinear least-squares curve fitting adopting Gaussian-Lorentzian peak shapes, and peak area determination by in-tegration. The atomic compositions were evaluated from peakareas using sensitivity factors supplied by Perkin-Elmer, tak-ing into account the geometric configuration of the apparatus.The experimental uncertainty on the reported atomic compo-sition values does not exceed ±5%.

G. Morphological characterization

The surface features of the samples were observed byAFM. Images were taken using a Park Autoprobe CP instru-ment operating in contact mode and topographies were ac-quired under environment conditions. The background wassubtracted from the images using the ProScan 1.3 softwarefrom Park Scientific. The scan rate was 1.0 Hz and the forceset point was the lowest available, to improve resolution whileminimizing sample damages. Surface morphological investi-gations were performed on as-prepared samples (1 × 1 cm2)and each was observed in different areas to check for homo-geneity. Roughness of the samples is reported as the standarddeviation of the height over the sample surface.

H. Surface modelling

To support the experimental AFM data, Hyperchem,41

AMBER force-field calculations of single calixarene 2 andthe host-guest 2-[60]fullerene complex with respect to the1-octene layer were performed. Optimization was carriedout by application of the steepest-descent method, followedby conjugate gradient method, Fletcher-Rieves. This wasdone with the purpose of evaluating the theoretical lengthsof the molecules protruding from the silicon surface uponcalix[8]arene grafting and supramolecular immobilization of[60]fullerene cages.

III. RESULTS AND DISCUSSION

Functionalized calix[8]arene 2 was synthesized by atwo-step procedure starting from p-tert-butyl-calix[8]arene1 which, in turn, was prepared from p-tert-butylphenol andparaformaldehyde in 82% isolated yield.42 The reaction of1 with 10-undecenoyl chloride in dichloromethane at roomtemperature afforded 2 in 80% isolated yield after SiO2 flashcolumn chromatography (Scheme 1). NMR spectroscopyand mass determination were used to validate the proposedmolecular structure (see Figures S1–S3 in the supplementarymaterial36). Calix[8]arene 2 is made by eight aromatic sub-structures, linked by methylene bridges, that form a macrocy-cle and may rotate to give, at least in principle, 17 possibleconformations because each p-tert-butyl group may face up-or downward. For smaller calix[n]arenes (up to n = 6), thesteric hindrance of the substituents on the phenyl rings limitsthe conformational freedom of the macrocyclic structure that,for this reason, can be studied by NMR.43 For n > 6, how-ever, the macrocycle is large enough to allow for free rotationof the aromatic rings that translates into a general broaden-

TABLE I. Observed water contact angles.

Sample name WCA (deg)a

Si-H 80Si-2 94Si-2-C60 88Si-2:Oct 90Si-2:Oct-C60 84

aError associated with measurements ± 2◦.

ing of all proton resonances in the NMR spectrum. In ourcase (see Figure S136) the singlet at 1.08 ppm accounts forthe hydrogen atoms on the tert-butyl groups. All the otherresonances in the aliphatic region were assigned to the pro-tons of the alkyl chains with reasonable integration values.In particular, the broadening of the resonances at 2.33 and1.64 ppm (assigned to methylene protons in α- and β-positionto the carbonyl group, respectively) and that at 1.30 ppm (rel-ative to the other methylene protons) indicates different tiltangles for the alkyl chains with respect to the macrocycle.This is also corroborated by the broad singlet at 3.61 ppm(methylene units on the annulus) and 6.91 ppm (protons onthe phenyl rings). On the basis of the 1H-NMR data, it is rea-sonable to assume that the alkenyl termini may rotate freelyand be therefore available for grafting to the silicon surface,albeit with different tilt angles.

WCA determinations were used either as an indirectproof that functionalization of the silicon substrates by deriva-tive 2 (either neat or diluted with octyl chains) took place orto evaluate fullerene host-guest immobilization. As reportedin Table I, freshly etched silicon samples (Si-H), showed amean WCA of 80◦, which increased to 94◦ and 90◦ for thefunctionalized counterparts, Si-2 and Si-2:Oct, respectively.This increase in the WCA hints at the presence of a hydropho-bic layer over the silicon surface, which is to be expected uponfunctionalization.

For both Si-2 and Si-2:Oct, immersion into a[60]fullerene solution led to a decrease of the contactangles by 6◦: to 88◦ for Si-2-C60 and 84◦ for Si-2:Oct-C60.This reduction is compatible with the immobilization of[60]fullerene by the calix[8]arene host. It has been reportedthat [60]fullerene shows a contact angle of 77◦ on flat sur-faces. Roughening a flat surface with a relatively hydrophiliccoating is indeed bound to decrease its WCA.44

A closer inspection of the morphological and chemicalcharacteristics of the functionalized silicon surfaces was car-ried out by XPS and AFM. XPS on Si-H samples showedonly adventitious carbon and oxygen contaminants on the Sisurface, as reported in literature for similar Si(100) etchingprocedures.19 AFM images of Si-H are reported in Figure 1,together with the surface profile recorded along the path high-lighted in blue. The Si-H sample appears flat, with a rough-ness of 0.1 nm and no particular morphological feature. Themeasured values are in good agreement with the data reportedin literature for analogous silicon hydride surfaces.9 After thethermal hydrosilylation procedure in the presence of 2 (sam-ple Si-2) the XPS survey spectrum (Figure 2) showed an in-crease of carbon and oxygen lines with respect to the silicon

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164715-4 Busolo et al. J. Chem. Phys. 139, 164715 (2013)

FIG. 1. AFM image of Si-H sample and profile collected along the blue line.

FIG. 2. Wide XPS scan spectra (survey) of Si-H sample, compared to thesame substrate after grafting of calix[8]arene derivative (Si-2).

related peaks, corresponding to C, O, and Si amounts of 50,16, and 34 at.%, respectively. Such a variation of chemicalcomposition between a freshly etched and a functionalizedsurface is in good agreement with the grafting of molecularstructures on silicon slabs.9

A careful analysis of the high resolution spectra (mul-tiplex, Figure 3) reveals a more complex band-shape forthe XPS peaks recorded on Si-2, if compared to its un-treated counterpart Si-H. Detailed analysis of the carbon peak(Figure 3(c)) gives clear evidence that calix[8]arene 2 hasbeen bound to the surface. The C1s line shows a broad-band profile with a distinct component on the high energyside. Spectral deconvolution of the region reveals three com-ponents, centered at binding energy (BE) of 284.8 eV (CI),286.3 eV (CII), and 289.0 eV (CIII). The main peak at lowerBE corresponds to the aliphatic and aromatic hydrocarbonbackbone,45 whereas the middle band is associated to C–Osingle bonds. Lastly, the peak at higher BE (CIII), that is typ-ical for carboxylic carbon atoms,45 is assigned to the estergroups of 2. This latter component represents ∼4% of the totalcarbon amount, in agreement with that expected for 2 (<5%).

The O1s region displays a symmetric peak-shape centredat 532.6 eV (Figure 3(b)). This signal encompasses carbonylgroups, C–O–C moieties as well as the contribution of a par-tial oxidation of the silicon substrate. Indeed, the Si2p peak(Figure 3(a)) shows two distinct components: a main bandat 99.2 eV, associated to elemental Si and a broad band at∼103 eV due to SiO× species. Peak integration shows thatnearly 90% of the surface sites correspond to elemental Si,whereas the residual 10% represents oxidized Si, in line withliterature data.9

AFM images of Si-2 (Figure 4(a)) reveal a slight in-crease in the surface roughness, now 0.2 nm compared to the0.1 nm of Si-H. The valley-to-valley distances are alsonoticeably larger, hinting at the presence of aggregatedcalix[8]arene structures, or isles, on the silicon substrate, thatpreclude sharp protrusions. Even though AMBER force-fieldcalculations of the structure of a single calixarene moleculeyield an estimated 1.5–2.0 nm height, the recorded peak-to-valley height for the Si-2 sample is in the order of a fewÅ (a 0.4 nm example is reported in Figure 4(b)). This canbe expected, since the AFM tip can never penetrate betweenthe calixarene moieties down to the silicon surface and there-fore it cannot record the full height of the grafted molecule.

FIG. 3. Detailed XPS spectra (multiplex) of the Si-2 sample. The Si2p, O1s, and C1s regions are reported, the latter together with the fitting components (redlines).

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164715-5 Busolo et al. J. Chem. Phys. 139, 164715 (2013)

FIG. 4. AFM image of samples (a) Si-2 and (b) Si-2-C60. Profiles collected along the blue lines are reported at the bottom.

Still, there is some space available between calix[8]arenemolecules, that present an umbrella-like shape with an esti-mated radius of 1.4 nm and preclude the formation of close-packed layers due to steric reasons. In the areas shaded by thissteric hindrance, unreacted silicon hydride moieties are leftamenable to oxidation, hence the remarkable oxygen contentrecorded in the XPS analysis. This phenomenon has been ob-served previously for different molecules bearing bulky head-groups.9, 46

After treating Si-2 with [60]fullerene, the topography ofthe surface showed an overall variation, that can be appreci-ated in Figure 4(b). Sample Si-2-C60 comprised [60]fullerenemolecules docked on calix[8]arene baskets and the observedprofiles reveal a roughness of 1.1 nm. Simulating the struc-ture of the calix-docked [60]fullerene yields a 0.26 nm heightincrease in comparison to neat calix[8]arene, so that such aroughness increase, from 0.2 (Si-2 sample) to 1.1 nm, is largerthan expected. Isolated peaks whose heights vary from 5 to20 nm can also be observed in Figure 4(b) (black line). Wehypothesize that the arrangement of calix[8]arene moleculeson the silicon surface may bring docked fullerenes very closeto each other, thus promoting their well-known tendency toaggregate.47, 48 This would account for the increase in the

overall roughness of the sample and, locally, for the forma-tion of taller structures (physisorption of fullerenes on the sil-icon surface can be ruled out since the samples were exten-sively washed and sonicated before collecting AFM images).Fullerenes may also deposit in between calixarene isles, in-teraction with the grafted molecules being strong enough toprevent their removal in the washing procedure. These sur-faces might then act as seeds for further fullerene stacking.

To back this hypothesis we consider the “diluted”calix[8]arene layers of Si-2:Oct samples, which were pre-pared by reacting Si-H with a 1:1 molar solution of 2 and1-octene. Surfaces with diluted calix[8]arene layers were pre-pared in pursuit of a more uniform distribution of the fullerenemolecules upon docking, in order to attain homogeneoussupramolecular host-guest systems.

A comparison between the Si2p line of the XPS spec-tra of Si-2 and Si-2:Oct layers49 is reported in Figure 5and shows a decrease in the oxide component on the lat-ter. This finding suggests that the octyl chains fill spaces be-tween the calix[8]arene baskets, thus preventing substrate ox-idation. Spectral deconvolution of the Si2p line followed byquantitative analysis reveals that only 2% of the surface sitesof Si-2:Oct consisted of oxidized silicon atoms. The atomic

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FIG. 5. Comparison between Si2p lines of samples Si-2 and Si-2:Oct.

composition of this sample differs accordingly, from that ofthe undiluted Si-2 sample, with a decrease in the oxygen con-tent. The observed atomic compositions were 51% for C, 12%for O, and 37% for Si.

Likewise, the dilution of the calix[8]arene layer in the Si-2:Oct sample can be expected to have an effect on the shapeof the C1s line. Due to the presence of hydrocarbon chainson the silicon surface, the contribution of ester carbons (CIII)

decreases with respect to the total carbon amount, giving ameasured value of ∼2% (versus 4% in Si-2).

The surface morphology of the sample Si-2:Oct and theeffect of fullerene docking (sample Si-2:Oct-C60) can be ap-preciated in the AFM images reported in Figure 6 that showa micrograph of the silicon surfaces grafted with the di-luted calixarene monolayer. With respect to the correspond-ing dense monolayers in Figure 4(a), the surface shows higherroughness (0.75 nm instead of 0.2 nm), with reduced valley-to-valley distances and peak-to-valley heights up to 1.2 nm(Figure 6(a)). This is in good agreement with the estimateddifference in height between the calix[8]arene and the octylmoiety (1.16 ÷ 1.34 nm), indicating that the taller structuresare indeed diluted by shorter ones.

The last sample to be imaged, Si-2:Oct-C60, inFigure 6(b), displayed a 0.37 nm roughness and a general in-crease of the peak-to-valley heights, typically in the 1.5 nmrange. According to our molecular modelling, the insertion of[60]fullerene into the host increases its height (1.2–1.3 nm)by about 0.2 nm, in agreement with the experimental observa-tions. A comparison of the morphology of Si-2:Oct-C60 withits non-diluted analogue, Si-2-C60, reveals a more homoge-neous surface for the former, only slightly rougher due to the

FIG. 6. AFM image of samples (a) Si-2:Oct and (b) Si-2:Oct-C60. Profiles collected along the blue lines are reported at the bottom.

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164715-7 Busolo et al. J. Chem. Phys. 139, 164715 (2013)

presence of octyl chains spacing the calix[8]arene molecules,but lacking the pillar-like structures that were ascribed to ex-tensive aggregation of fullerene spheroids in the latter. It isimportant to notice that the relatively high, inherent roughnessof Si(100) limits the possibility to obtain molecular resolutiontopography of vicinal objects.50 As a consequence, while wecannot rule out limited aggregation of fullerenes on the sur-face of the Si-2:Oct-C60 sample, we can state that the pres-ence of the octyl spacers between the calix[8]arene moietiescan at least hamper extensive aggregation effects.

IV. CONCLUSIONS

This work reports the covalent linking of calix[8]arene 2,onto Si(100), as a first step in the fabrication of supramolec-ular structures on a substrate suited for integration into well-established silicon technologies. The grafting was achievedby hydrosilylation chemistry and samples characterized viaXPS, AFM, and water contact angle measurements. Exper-imental data confirm that calix[8]arene moieties were cova-lently bound to the silicon surface and were able to act as hostsfor fullerene molecules. The marked tendency of fullereneto aggregate on the functionalized surface, forming tall pil-lars in some areas, led us to explore the possibility to di-lute the calix[8]arene layer with alkyl chains in order to keepthe docked fullerenes away from each other and discourageclustering. From a chemical point of view, this approach re-duced the amount of oxidized areas on the surface, indicat-ing that aliphatic chains were taking up spaces between thecalix[8]arene hosts. Upon docking fullerenes to the dilutedcalix[8]arene layer, the clusterization was indeed stronglyreduced, with flatter morphologies and no observed pillarstructures.

As a result, calix[8]arene-functionalized silicon chips canbe used as a host surface, toward electronically and struc-turally unique molecules such as [60]fullerene. The long-termobjective of this research is the controlled formation of arraysof the nitrogen endofullerene N@C60 (which would offer longspin coherence times) on a surface, towards electron spin en-semble quantum computing.51, 52

ACKNOWLEDGMENTS

We thank R. Riccò, D. Dattilo, and G. Fois fortheir help in working out detailed experimental proce-dures. MIUR (FIRB projects RINAME RBAP114AMK, Ital-NanoNet RBPR05JH2P and NANOSOLAR RBAP11C58Y)and University of Padova (Progetto Strategico HELIOSSTPD08RCX5) are gratefully acknowledged for financialsupport.

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