J. Phys. Chem. C 2010, 114, 17591–17596
17591
Interaction of Liquids with Nanoporous Cluster Assembled Au Films F. Bisio,*,† M. Prato,‡ O. Cavalleri,§ E. Barborini,| L. Mattera,§ and M. Canepa§ CNR-SPIN, C.so Perrone 24, I-16152 GenoVa, Italy, Istituto Nazionale di Fisica Nucleare, Sezione di GenoVa, Via Dodecaneso 33, 16146 GenoVa, Italy, CNISM, Sede Consorziata di GenoVa and Dipartimento di Fisica, UniVersita` di GenoVa, Via Dodecaneso 33, I-16146 GenoVa, Italy, and Tethis S. r. l. Via Franco Russoli 3, 20143 Milan, Italy ReceiVed: August 6, 2010
We report a spectroscopic ellipsometry investigation of the interaction of nanoporous cluster-assembled Au films with pure ethanol in a liquid environment. The Au films were first characterized in atmosphere in terms of the mean size of the constituent clusters and their porosity. Upon immersion in liquid, a clear correlation between the relative density of the films and the corresponding fraction of pores that could be filled by ethanol molecules was observed. Low-density porous Au structures exhibit abundant open pores, accessible by molecules from the ambient, whereas for higher-density structures the fraction of accessible pores sharply drops. We found evidence that the pore accessibility in cluster-assembled films with pore dimension in the sub-10 nm range is strongly dependent on fine morphological details of the solid scaffold. Introduction The interface between solids and liquids is home to a huge variety of physicochemical phenomena, having far-reaching scientific and technological fallouts in fields as diverse as biology, chemistry, and physics. The mechanism of interaction between solid surfaces and molecules in the liquid phase has accordingly been the subject of the scientists’ attention for a very long time.1 Most of the surfaces commonly encountered in nature, from cell membranes to catalysts’ surfaces, are far from ideal and exhibit complex morphological, structural, and chemical features that can be tailored for activating specific molecular functionalities. Nanoporous surfaces2 are a class of systems having a great potential in this respect. Their high specific area makes them appealing as catalysts, chemical actuators,3 and high-efficiency detectors,4 whereas the control over the pore size provides a mechanism for tuning their degree of interaction with molecules in heterogeneous phase.5 In the liquid environment, where the majority of chemical and biological processes occur, the presence of nanoscale solid structures of sizes comparable with the molecular dimension strongly affects the solid/liquid interface structure.6,7 The nanoscale morphology of a porous material has therefore profound implications in determining the macroscopic properties of these systems in various applications contexts (nanofluidics, drug delivery, catalysis, sensors, etc.). The correct assessment of the molecular configuration in contact with the surface is therefore a key issue for exploiting their potential with respect to molecular processes.6 In this work we report a spectroscopic ellipsometry (SE) investigation of the interaction of nanoporous cluster-assembled Au films with pure ethanol (etOH) performed under in situ liquid conditions. Comparing SE measurements recorded with samples in atmosphere and in liquid, we could unveil a clear correlation * To whom correspondence should be addressed. E-mail: bisio@ fisica.unige.it. † CNR-SPIN. ‡ INFN. § Universita` di Genova. | Tethis Srl.
between the relative density of the material and the fraction of open pores accessible by etOH, thereby also highlighting the possibility of nonobvious correlations between overall porosity of a material and effectively accessible surface area. Cluster-assembled films are a class of systems fabricated via the stacking and assembly of preformed clusters onto a suitable substrate.8,9 Upon a cluster deposition energy sufficiently small (few tenths of eV/atom), cluster-assembled materials exhibit a porous structure characterized by high specific area and lower relative density with respect to bulk counterparts.9,10 These systems are endowed with an intrinsic granular structure and their physical properties result from the interplay of singlecluster functionalities and collective behavior.11 Cluster films have been long regarded as prototypical systems for moleculesurface interaction, from high-efficiency gas sensors12 to substrates for protein docking.13 Their surface morphology, easily accessible by surface-specific probes, has been investigated in a number of works,14 whereas comparatively little is experimentally known about their inner structure. In particular, the extent to which inner pores are accessible by foreign molecules, a key issue in several applications, is a topic so far largely unexplored. Within this framework, we chose to address Au as the cluster material and etOH as the liquid since this combination can be considered a benchmark solid/liquid system. Au is a material mostly renown for being highly chemically inert and biocompatible, and therefore of broad interest in the fields of chemistry, physics and material science. EtOH, on the other hand, is one of the most popular and broadly employed solvents: it is transparent in a relatively large wavelength range, substantially harmless, easy to handle and almost apolar (as compared to e.g. water). EtOH is therefore widely employed in conveying the deposition of more complex organic molecules from solution, particularly alkanethiols, whose interaction with Au substrates has been the subject of many investigations in recent years.15-17 Experimentally, SE was chosen for its capability to access both surface and bulk features of our samples, with a probing depth of the order of the radiation skin depth (tens of nm for bulk Au), thereby proving effective in assessing the
10.1021/jp107420c 2010 American Chemical Society Published on Web 09/10/2010
17592
J. Phys. Chem. C, Vol. 114, No. 41, 2010
Bisio et al.
response of nanoporous materials and giving the possibility of directly working in liquid environment.5,10 Materials and Methods Deposition of Cluster-Assembled Films. Cluster-assembled Au films few-tens-of-nm thick were deposited onto 15 × 15 mm2 silicon wafers by means of a supersonic cluster beam deposition apparatus equipped with a pulsed microplasma cluster source (PMCS) by the Tethis Srl company. The principle of operation of a PMCS has been thoroughly described in previous publications.18 The Au clusters produced in the PMCS, with typical diameter in the few-nanometer range are transported by a supersonic gas jet from the PMCS to the deposition chamber, where they are deposited on a clean Si wafer. The Au films exhibit a marked nanoscale porosity and surface roughness, as thoroughly described in recently published work.10 Spectroscopic Ellipsometry. The SE characterization of the samples was performed by a J. A. Woollam M-2000S rotatingcompensator spectroscopic ellipsometer, capable of measuring at 225 wavelengths in the 245-725 nm spectral range, at angles of incidence adjustable between 40° and 90°. The output of the standard SE measurements is the ratio F of the p-polarized and s-polarized complex Fresnel reflection coefficients rpp and rss, routinely expressed in terms of the two angles Ψ and ∆ as19,20
F)
rpp ) tan Ψei∆ rss
(1)
The samples can be modeled as a stack of layers characterized by their complex dielectric function ε ) (ε1, ε2) and their effective thickness d. Information about the system properties can be obtained comparing the experimental Ψ(Λ) and ∆(Λ) spectra with the data calculated for the layer stack until the appropriate layer parameters that minimize the deviation with respect to experimental data (mean square error, MSE) are found.10,21 Optical simulations and fitting were performed with the aid of the WVASE32 program, supplied by the manifacturer. Measurements were performed at 65° angle of incidence. In Situ SE Characterization. The SE measurements with the sample kept in pure ethanol (EtOH, Fluka, purity >99.8%) were performed in a homemade Teflon cell, designed to allow optical access to the sample while immersed in liquid.22,23 The sample was first characterized by SE in air, then inserted in the empty deposition cell and remeasured, in order to quantify the effect of the cell viewports on the SE spectra. Pure ethanol was subsequently admitted in the cell, and the Ψ, ∆ spectra were recorded as a function of immersion time. Starting the acquisition of the spectra a few minutes after immersion, we did not notice any variations over a time scale of a few hours. The system dynamics occurs therefore on a time scale shorter than needed to start our acquisition process. Accordingly, we show spectra that have been recorded a few minutes after immersion in etOH. Results and Discussion Porous Cluster-Assembled Film. In Figure 1a we report the ∆ and Ψ spectra of a cluster-assembled Au film in air (red and blue symbols, respectively), recorded for a freshly deposited film (we will refer to this as the “fresh” sample). The morphology of this system was deduced from the SE data by the application of an optical-layer model, thoroughly described in previous work.10 The model, sketched in Figure 1b, treats
Figure 1. Panel a: Experimental (symbols) and simulated (solid black lines) ∆ and Ψ spectra for a cluster-assembled Au film in atmosphere. Panel b: layer model for the cluster-assembled Au film in atmosphere. The best-fit thickness and density parameters of the Au layers are also reported. Panel c: imaginary part of the dielectric constant of bulk Au (symbols) and of Au in the cluster-assembled film (solid line). From the magnitude of ε2 in the long-wavelength spectral range, a mean cluster size of 6-7 nm was deduced.
the Au film as a stack of two layers, each characterized by a different degree of porosity, sitting atop a SiO2 /Si substrate. The Au layers are modeled as Bruggemann effective media24 consisting of a mixture of Au nanoclusters and voids. The degree of porosity is pictorially represented in the figure by the relative width of the “void” section in each layer, whereas the layer height in the drawings is proportional to the layer thickness deduced in the model (in general, an uncertainty of the order of (5% on the porosity and (3 nm on the layer thickness was estimated). The optical constants of the Au fraction εNP are modeled, keeping into account the granular nature of the films, as the sum of the bulk Au dielectric function (obtained from SE measurements of a thick, flat Au film25), and a Drude-like contribution that effectively accounts for the excess scattering of the Au free electrons at the cluster surface. Though crude, this two-layer model proves effective in providing a reasonable picture of the cluster-assembled film morphology with a limited set of physically meaningful parameters, as demonstrated by appropriate comparison with literature data.10 A critical point that is worth stressing concerns the modeling of heterogeneous film properties like cluster-size distribution within the film. The Au clusters produced in the PMCS exhibit in fact a log-normal size distribution, as characterized by atomic force microscopy (AFM) measurements,10 that is expected to be mostly preserved upon the cluster stacking process. The optical model, through the magnitude of the Drude-like contribution due to surface scattering of free electrons, allows us to obtain an effective measure of the Au cluster size within the whole film10,26-28 that effectively represents the mean particle size. The cluster size dispersion, instead, cannot be deduced from this model. However, we point out that the mean cluster size that we deduce closely matches
Interaction of Liquids with Au Films
J. Phys. Chem. C, Vol. 114, No. 41, 2010 17593
Figure 2. Top panels: Experimental ∆ and Ψ spectra for a fresh cluster-assembled film in pure ethanol (symbols) and results of layer-model simulations (solid black lines). In panel a, the spectra were calculated assuming no penetration of the solvent within the film. In panel b, a partial filling of the top-layer voids by ethanol is allowed. In panel c, a partial filling of top-layer and inner-layer voids is allowed. Bottom panels: layer models for the cluster-assembled Au film in pure ethanol relative to the simulated spectra in the corresponding top panels.
the value derived from AFM data,10 thereby suggesting that this physically relevant parameter can be successfully extracted from optical modeling. The best-fit calculations for ∆ and Ψ (black lines in Figure 1a) yield the layer-resolved morphological parameters (thickness and void fractions) reported in Figure 1b. The deviation between data and model (mean-square error, MSE ) 19.5) is fairly low, suggesting a good adherence of the model to the physical structure of the film, especially considering the complex nature of the system. In detail, on top of an inner porous layer, 30 nm thick and characterized by a relative bulk density F ≈ 80%, sits a “surface” layer approximately 20 nm-thick mostly consisting of voids, that occupy a relative fraction F > 80%. The best-fit imaginary part of εNP, ε2NP, is reported in Figure 1c as the solid black line, along with its flat, bulk counterpart (red symbols). The increase of ε2NP at the red end of the spectrum is the fingerprint of the extra Drude scattering in the film, from which a mean cluster diameter of 6-7 nm can be inferred.29 Having characterized the system in atmosphere, we moved on to probe its response in contact with the liquid medium. In Figure 2, top panels, we report the ∆ and Ψ experimental spectra (symbols) recorded in the presence of etOH. The spectra exhibited significant changes with respect to the atmosphere case, whose fitting requires an appropriate adaptation of the optical-layer models to account for the presence of the liquid. In the bottom panels of Figure 2, we present various models, in order of increasing degree of interdiffusion between etOH and the Au film. In all models, the morphological parameters deduced from the SE measurements in atmosphere (layer thickness, porosity and optical constants of the Au clusters) have been kept strictly fixed, thereby neglecting any possible
deformation of the pores by the molecules.30 In addition we assume no change in etOH’s optical constants with respect to reference values,31 neglecting any molecular confinement effects. The simplest model for the Au/etOH interaction, sketched on the left side of the figure, is reported mainly as a term of comparison with respect to more sophisticated approaches. In this model, no penetration whatsoever of the solvent within the pores is allowed, and the only change made with respect to the atmospheric case is the replacement of the optical constants of the ambient from air (ε ) 1) to ethanol.31 The comparison of the ∆ and Ψ spectra calculated under this drastic approximation (black lines in Figure 2a) with the experimental spectra yields a deviations as high as MSE ) 532. This value, necessarily high due to the unphysical model assumptions, can be therefore viewed as a reference value for quantifying the degree of improvement of more refined models. A step forward, in terms of the etOH/Au interaction, allows for the partial filling of the film voids by the solvent. This situation can be modeled by replacing the void fraction of the film by an effective medium whose dielectric function is a linear combination of the dielectric functions of atmosphere and ethanol (more complex effective-medium approximations lead to negligible variations with respect to the linear approximation). In the hypothesis only the topmost layer can be filled by the solvent (middle panels in Figure 2), the best fit (black lines in Figure 2b) is found assuming (85 ( 5)% of the voids in the top layer are filled with ethanol (MSE ) 57). This picture can be further refined (rightmost panels in Figure 2) allowing the solvent to partially fill both the outer and the inner layer voids. In such a case, the best fit (black lines in Figure 2c) is found when (85 ( 3)% of the voids in the top layer and 25 ( 15% of
17594
J. Phys. Chem. C, Vol. 114, No. 41, 2010
Figure 3. Panel a: Experimental ∆ and Ψ spectra for an aged clusterassembled film in atmosphere (symbols) and results of layer-model simulations (solid black lines). Panel b: layer model for an aged clusterassembled film in atmosphere. Panel c: imaginary part of the dielectric constant of flat Au (symbols) and of Au clusters in the aged film (solid line). From the magnitude of εNP 2 in the long-wavelength range, a mean cluster size of 8-10 nm was deduced.
the voids of the inner layer are filled with etOH, yielding an appreciable variation of the error, that decreaes by ∼8% to MSE ) 53. In this analysis, the uncertainties in the EtOH filling fractions have been estimated by a systematic evaluation of the dependence of the MSE upon the free fitting parameters. A graph reporting the MSE dependence upon the void fraction in inner and outer layer is reported in the Supporting Information. Aged Cluster-Assembled Film. The influence of the clusterfilm morphology on its response to a liquid can be further assessed taking advantage of the naturally occurring aging of the Au films. Upon weeks of permanence at room temperature, in fact, nanostructured Au films typically exhibit aging effects, driven by substrate dewetting32 or by a relaxation of the metastable Au nanostructures via surface diffusion33 that lead to coarsening or densification of the material.34 While these effect might be not desirable for applications, aging provides an intriguing possibility of directly testing the correlation between surface/pore morphology and penetration depth of molecules within the film. In Figure 3 we report the SE characterization in air of a cluster-assembled Au film left aging at room temperature for 80 days after its fabrication (henceforth referred to as the “aged” sample). In Figure 3a its experimental ∆, Ψ spectra are shown (symbols), whereas in panel b, the layer model corresponding to the best-fit ∆, Ψ (black lines in Figure 3a) is reported (MSE ) 21.4). The best-fit optical constant of the Au clusters ε2NP is displayed as the black line in Figure 3c, along with its flat-Au counterpart. Some differences appear for this sample compared to the fresh case. The morphological model suggests in fact, in the aged case, a considerably thinner surface layer (
