Electrochemical and Charge Transport Behavior of Molybdenum

Sep 15, 2009 - Sciences Chimiques de Rennes, UMR 6226 CNRS/UniVersité de Rennes 1, Campus de Beaulieu, 35042 Rennes. Cedex, France, and Institut ...
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J. Phys. Chem. C 2009, 113, 17437–17446

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Electrochemical and Charge Transport Behavior of Molybdenum-Based Metallic Cluster Layers Immobilized on Modified n- and p-Type Si(111) Surfaces Bruno Fabre,*,† Ste´phane Cordier,*,† Yann Molard,† Christiane Perrin,† Soraya Ababou-Girard,‡ and Christian Godet*,‡ Sciences Chimiques de Rennes, UMR 6226 CNRS/UniVersite´ de Rennes 1, Campus de Beaulieu, 35042 Rennes Cedex, France, and Institut de Physique de Rennes, CNRS UMR 6251, Equipe de Physique des Surfaces et Interfaces, UniVersite´ de Rennes 1, Campus de Beaulieu, 35042 Rennes Cedex, France ReceiVed: April 7, 2009; ReVised Manuscript ReceiVed: June 19, 2009

Octahedral molybdenum cluster cores [Mo6I8]4+ have been attached in apical positions to p- and n-type Si(111) surfaces through complexation with a pyridine-terminated organic monolayer (2 × 1014 cm-2), which was previously covalently bound to hydrogen-terminated Si(111). This grafting procedure resulted in about a 4 nm thick Mo6-terminated layer. Similar XPS results were found for p- and n-type samples, suggesting that the grafting efficiency and composition of the resulting layers do not depend significantly on the doping type of the surface. The cluster footprint of 10 nm2 indicates a fairly dense molecular packing on the Si(111) surface. The electrochemistry of such Mo6-modified surfaces in acetonitrile was characterized by a single irreversible oxidation peak at 0.92 V versus saturated calomel electrode (SCE) and flat band potential Efb values of -0.55 ( 0.05 V and 0.04 ( 0.05 V for the modified n- and p-type surfaces, respectively. The derivatization of silicon surfaces by Mo6 introduces surface states that are probably due to some unavoidable oxidation of Si(111) and/or the possible presence of interfacial alkoxy species. From capacitance measurements, the total density of the surface states was estimated at 3.7 × 1011 cm-2 and 2.7 × 1011 cm-2 for the modified n-type and p-type Si(111), respectively. Electrical transport measurements through the Mo6-modified monolayer/ Si(111) devices were performed using mercury as a soft top contact. In contrast with the Hg/pyridine-alkyl/ Si(111) junctions, specific features appear in the current density-voltage characteristics of the Hg/Mo6/ pyridine-alkyl/Si(111) junctions. The minima observed in the conductance-voltage plots for p-type and n-type Si(111) are attributed to the loss of one or two electrons from the highest occupied molecular orbital when a negative bias is applied to the substrate; the image charge in the mercury leads to a dipole layer at the Hg/Mo6 interface, which creates an additional barrier as compared with those of the pyridine-modified surfaces. 1. Introduction The structures of Mo6 cluster-based compounds are built up from [Mo6Xi8Xa6]2- building blocks (X ) halogen and/or chalcogen, i ) inner, a ) apical; Figure 1a) wherein the octahedral cluster is face capped by eight covalently bonded inner ligands (Xi) to form a [Mo6Xi8]4+ cluster core that is ionically bonded to six additional terminal ligands (Xa). Their physical properties depend on the number of valence electrons involved in the Mo-Mo bonds (VEC) and on the strength of electronic interactions between the building blocks.1-10 The metallic electrons are localized on a set of 12 metal-metal bonding orbitals, leading to a closed shell configuration with 24 electrons per cluster (Figure 1b).6 A wide variety of interesting physicochemical characteristics have been reported: electrical, superconducting with high critical field, optical, catalytic, and thermoelectric.11-14 For example, the host structure of the AxMo6Q8 series (A ) Cs+, n-C4H9N+, Q ) chalcogen) with a fast reversible intercalation of cations should lead to applications as cathode material for Mg rechargeable batteries.15,16 * Corresponding authors. E-mails: [email protected] (B.F.), [email protected] (S.C.), [email protected] (C.G.). Fax: + 33 (0) 223 236 799. † Sciences Chimiques de Rennes. ‡ Institut de Physique de Rennes.

Figure 1. (a) Representation of the Mo6L8iL6a cluster unit. (b) Schematic representation of the molecular orbital diagram of a M6L8iL6a cluster unit in ideal Oh symmetry.

Interestingly, many routes afford soluble inorganic or hybrid organic/inorganic precursors containing Mo6Xi8Xa6 cluster units.17-22 Their intrinsic physicochemical and structural proper-

10.1021/jp903205a CCC: $40.75  2009 American Chemical Society Published on Web 09/15/2009

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ties (nanosized shape, electronic, and photoluminescence) make them relevant building blocks for the elaboration of supramolecular assemblies and nanostructured materials. For instance, luminescent materials can be obtained either by direct incorporation of cluster units on or in an inorganic matrix23-25 or by copolymerization with monomers after prior cluster functionalization by polymerizable moieties.26 The apical halogen ligands are semilabile and can be replaced in solution by donor ligands to lead to various functional [Mo6Xi8La6]n( cluster units (L ) PR3, pyridine derivatives, etc.), which have been studied over the years.27-34 Today, the knowledge in the field of metal atom cluster chemistry is sufficiently developed to envision the chemical engineering design of nanopatterned systems in which tailored clusters will be used as nanocomponents carrying specific optical, electrical, magnetic, or catalytic properties. However, the integration of functional nanobuilding blocks on conducting surfaces is a preliminary step toward the development of novel electrically addressable and switchable functional devices, e.g., optical or magnetic arrays, biochemical sensors with electrical detection, or hybrid junctions for charge storage components.35 For instance, much research work is actually devoted to the design of molecular junctions or memory cells based on the immobilization of redox-active molecules (e.g., ferrocene, metal-complexed porphyrins, etc.) on conducting surfaces.36-40 Within this frame, we recently proposed to study the physicochemical properties of functional surfaces obtained by the anchoring of cluster building blocks onto a semiconducting surface through an organic linker. Metallic octahedral clusters appear as very attractive versatile building blocks for the design of multifunctional materials. Indeed, their one-electron reversible oxidation process can be interesting for charge retention and because of delocalization of the electrons on all metal centers, the oxidation from 24 to 23 electrons/Mo6 switches the physical properties from phosphorescent and diamagnetic to nonphosphorescent and magnetic behavior. The aim of our program is (i) to define clear chemical processes for the grafting of metallic clusters on semiconducting surfaces and (ii) to study the effect of cluster immobilization on its intrinsic physicochemical properties as well as on the whole cluster/ linker/surface assembly. Recently, we reported preliminary results on the immobilization of [Mo6I8]4+ octahedral clusters on silicon surfaces.41 Thanks to the labile character of the apical ligands, the metallic complexes were attached in apical positions to the semiconductor surface through pyridine groups endcapping an alkyl monolayer covalently bound to a hydrogenterminated silicon surface. Interestingly, the interfacial Si-C bond between the alkyl monolayer and silicon is strong, which confers a better chemical, mechanical, and thermal stability as compared to that with monolayers on silica or gold.42-44 Furthermore, the grafting of these metal clusters onto oxidefree silicon surfaces will enable designing novel well-defined interfaces in particular redox-active molecular junctions, whose electronic properties can be tuned by changing the nature of the metals and ligands, length of the organic linker, and doping type of the underlying silicon substrate.45 In pursuit of our investigations on the metallic cluster-functionalized silicon surfaces, we examine herein the influence of the doping type of the substrate on the grafting efficiency and electrical properties of the resulting assemblies. Various experimental techniques, including spectroscopic ellipsometry, X-ray photoelectron spectroscopy (XPS), and electrochemical impedance spectroscopy (EIS) have been used. Electrical properties of metal insulator semiconductor (MIS) diodes prepared from these functional films are also studied using a mercury drop as the

Fabre et al. soft electrical top contact.45,46 Current transport mechanisms through such films are discussed, and the typical parameters such as rectification factors and barrier heights are estimated from the current-voltage curves. 2. Experimental Section Preparation of Molybdenum Cluster-Terminated Si(111) Surfaces. We reported recently the successful anchoring of Mo6I84+ cluster cores on p-type Si(111) surfaces derivatized by a pyridine-terminated organic monolayer.41 Indeed, the reaction of (n-C4H9N)2[Mo6Xi8(CF3SO3)a6] with pyridine derivatives (PyR) leads easily to the formation of [Mo6Xi8(Py-R)a6]4+ species. Because of the octahedral geometry of the cluster and steric hindrance of the chains on the surface, the six Mo atoms cannot be all bound to pyridine from the surface. In our earlier report,41 XPS analysis revealed a Ia/Ii ratio close to 0.1 on the grafted surface, meaning that apical iodines were not all replaced by triflate groups, leading to a (n-C4H9N)2[Mo6Ii8Ia6-x(CF3SO3)ax] formula for the precursor. Indeed, it turns out that apical iodine ligands Ia are more difficult to replace by CF3SO3 groups on the [Mo6Ii8]4+ cluster core than Xa in [Mo6Xi8]4+ chloride and bromide because of a stronger covalent character of the Mo-Ia bond.47 (n-C4H9N)2[Mo6Ii8Iax(CF3SO3)a6-x] was then prepared from the reaction of (n-C4H9N)2[Mo6Ii8Ia6] and AgCF3SO3 in refluxing THF.48,49 The XPS formula deduced from the Ia/Ii ratio revealed an x value lower than 2, meaning a mixture of [Mo6Ii8Ia(CF3SO3)a5]2- and [Mo6Ii8Ia2(CF3SO3)a4]2-. Note that some loss of triflate moieties has been noticed during XPS measurements due to either X-ray excitation or high vacuum conditions. (n-C4H9N)2[Mo6I8Iax(CF3SO3)a6-x] (x < 2) was crystallized by diffusion of ether in dry dichloromethane. Single side-polished silicon(111) samples (n-type, phosphorus doped or p-type, boron doped, 1-5 Ω cm, thickness ) 525 ( 25 µm, Siltronix) were cut into 1.5 cm × 1.5 cm pieces. Our strategy for anchoring the Mo6I84+ cluster core on n-type Si(111) is identical to that described in our earlier report for p-type substrates41 and is based on the covalent derivatization of Si(111) surfaces by a complexing pyridine-terminated organic monolayer. In the first step, a SiC-linked organic monolayer terminated by carboxyl groups is prepared from the photochemical reaction (λ ) 300 nm) of hydrogen-terminated Si(111) with undecylenic acid. After conversion of the COOH headgroups to N-hydroxysuccinimidyl (NHS) leaving units,50-55 the pyridine moieties were introduced to Si(111) by the amidation reaction with (4-aminomethyl)pyridine. Terminal pyridine moiety is denoted Py-Surf in the following text. The two-step indirect attachment of NHS units has been preferred to the direct attachment of an ω-NHS-functionalized 1-alkene56-58 because it did not require the time-consuming synthesis of the alkene precursor, and it was usually reported that the produced monolayers did not show decreased characteristics, in terms of packing density and ordering, compared with those of the monolayers prepared from the direct attachment procedure. The Mo6I84+ cluster core was then anchored on silicon by immersing overnight the pyridine-terminated surface in a distilled dichloromethane solution (15 mL) containing about 8 × 10-3 M [(n-C4H9)4N]2[Mo6I8Iax(CF3SO3)a6-x]. To obtain derivatized surfaces sufficiently stable for subsequent characterization, we substituted the triflate anions remaining in apical positions of the cluster after the immobilization step by 3-bromopyridine; the modified surface was dipped in neat 3-bromopyridine (99%, Aldrich) for 30-40 min. As a matter of fact, the bromine of 3-bromopyridine acts as a local probe

J. Phys. Chem. C, Vol. 113, No. 40, 2009 17439 SCHEME 1: Immobilization of Mo6 Metallic Cluster on Si(111) Surfaces through Complexation with Pyridine-Terminated Alkyl Monolayers

for XPS studies in order to estimate the number of molybdenum atoms of the Mo6 cluster that are not linked to the surface (Scheme 1). Compared with our earlier report,41 the experimental procedure for anchoring the metallic cluster and subsequent rinsing of the resulting surface have been slightly modified in order to avoid the formation of metallic aggregates as observed by atomic force microscopy (AFM) (Supporting Information) and to improve surface cleanliness. The Mo6 cluster-modified surface was thoroughly rinsed with freshly distilled CH2Cl2 to obtain a contaminant-free perfectly clean surface as observed by AFM. In contrast, when the solvent (HPLC grade CH2Cl2) was not distilled, as in our earlier report,41 the surface showed whitish spots, and its XPS analysis revealed the presence of metallic contaminants such as zinc and calcium. XPS Analysis. After a few minutes exposure to ambient atmosphere, the grafted surfaces were introduced in the UHV chamber and kept at 1 × 10-9 mbar for several hours before XPS analysis. XPS measurements were performed with a Mg KR (hV ) 1254 eV) X-ray source, using a VSW HA100 photoelectron spectrometer with a hemispherical photoelectron analyzer, working at an energy pass of 22 eV. The experimental resolution was then 1.0 eV. The binding energy for the main C-C peak has been taken at 285.0 eV as an internal reference level for all measurements. Spectral analysis included a Shirley background subtraction and peak separation using mixed Gaussian-Lorentzian functions. The thickness, dOML, of the molecular layer immobilized on the Si(111) surface was derived from the attenuation of the Si 2p signal at a normal emission Si2p ), angle, according to [Si 2p]grafted/[Si 2p]bare ) exp(-dOML/λOML with an inelastic mean free path value for Si 2p electrons through Si2p ) 3.5 nm, typical for a dense molecular the organic layer λOML 59,60 Note that the Si 2p signal measured before and after layer. grafting was averaged over several emission angles around a normal emission in order to eliminate photoelectron diffraction effects. The bromine signal was used to discriminate the surfaceimmobilized pyridine from the pyridine units provided after attachment of the metallic cluster. Furthermore, the number of Mo atoms of the Mo6 cluster involved in Mo6-Py-Surf bonds was experimentally determined by XPS. Spectroscopic Ellipsometry. Spectroscopic ellipsometry (SE) experiments were performed in the range 1.0-4.5 eV, at an incidence angle of 70°, using a Horiba (UVISEL) ellipsometer, and analyzed with a two-layer model. For the transparent (k ) 0) organic molecular layers, an energy independent refractive

index, n, was chosen because no improvement in the fitting result was found with a Schott dispersion formula. Electrochemical Characterizations. Cyclic voltammetry and impedance spectroscopy measurements were performed with an Autolab electrochemical analyzer (PGSTAT 30 potentiostat/ galvanostat from Eco Chemie B.V.) equipped with GPES and FRA softwares in a self-designed three-electrode Teflon cell. The working electrode, modified Si(111), was pressed against an opening in the cell bottom using a FETFE (Aldrich) O-ring seal. An ohmic contact was made on the previously polished rear side of the sample by applying a drop of an In-Ga eutectic (Alfa-Aesar, 99.99%). The electrochemically active area of the Si(111) surface (0.14 cm2) was estimated by measuring the charge under the voltammetric peak corresponding to the ferrocene oxidation on Si(111)-H and compared to that obtained with a 1 cm2 Pt electrode under the same conditions. The counter electrode was a platinum grid, and the system 10-2 M Ag+|Ag in acetonitrile was used as the reference electrode (+0.29 V versus aqueous saturated calomel electrode, SCE). All reported potentials are referred to SCE (uncertainty ( 0.01 V). Tetra-n-butylammonium perchlorate Bu4NClO4 was purchased from Fluka (puriss, electrochemical grade) and was used at 0.1 mol L-1 as supporting electrolyte in acetonitrile (anhydrous, analytical grade from SDS). The (CH3CN + 0.1 M Bu4NClO4) electrolytic medium was dried over activated, neutral alumina (Merck) for 30 min, under stirring and under argon. About 20 mL of this solution was transferred with a syringe into the electrochemical cell prior to experiments. All electrochemical measurements were carried out inside a homemade Faraday cage in the dark, at room temperature (20 ( 2 °C) and under a constant flow of argon. For impedance spectroscopy measurements, the amplitude of the alternating current (ac) signal was 10 mV. The differential capacitance C was determined from the imaginary part of the complex impedance Z′′ (C ) -1/2πfZ′′) in the frequency range f (typically from 50 kHz to 500 Hz) in which the phase angle of the complex impedance was greater than 80°, i.e., the range for which the system behaved primarily as a combination of capacitive circuit elements. Electrical Transport Measurements. The mercury drop technique is ideal to circumvent a number of experimental problems in electrical contact with molecular systems.61 A major interest in Hgmoleculesemiconductor junctions is the ease of fabrication, which allows observation of a statistically significant number of junctions. Furthermore, the large surface tension (480 mJ m-2) of mercury limits the occurrence of metal diffusion through pinholes in the molecular layer. XPS measurements after the formation of Hg/Si or Hg/molecular layer junctions do not show persisting traces of mercury on the surface.62 Measurements of current (I)-voltage (V) characteristics were performed with a homemade setup using a Kemula electrode for hanging Hg drop production and a micromanipulator to precisely raise the solid surface in order to adjust the contact area at ∼0.25 mm2 as observed with a magnifying lens (x10) and a CCD video camera (Computar MLH10XC). A new Hg drop (99.999%, Fluka) was used for each measurement. An ohmic contact was made on the previously polished rear side of the sample by applying a silver paste electrode. A Keithley 6487 picoammeter was used to apply a voltage ramp to the Si substrate, with a step of 1 mV (10 mV) at a scan rate of 3 mV/s (30 mV s-1) for the low (high) tension range. The extension of applied ramps was progressively increased in the range of 0/+3 and 0/-3 V, using alternatively positive and negative polarities. The reported transport characteristics were measured in a

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Figure 2. Complex pseudodielectric function of SE data for the molybdenum cluster-modified (a) n-type or (b) p-type Si(111) surfaces. Comparison of the SE data of the pyridine-modified surface before and after immobilization of the molybdenum cluster is shown in panel c. Experimental data are symbols. Continuous lines are fitted curves using a two-phase model (monolayer/silicon).

glovebox with a humidity level of 22-25% of the water saturation pressure, just after the XPS measurements (under UHV) and brief exposure (1 h) to the ambient. Some measurements were repeated after several days in order to check the device stability. Atomic Force Microscopy (AFM). AFM images were recorded in intermittent contact mode with a Nanoscope Dimension 3100 microscope from Digital Instruments using n+type silicon tips (magnetic ac mode, NCLR Reflex, 180-193 kHz resonance frequency) from ScienTec-Nanosensors. 3. Results and Discussion 3.1. Ellipsometry Measurements. Spectroscopic ellipsometry was used to determine the real part of the refractive index n(λ) and monolayer thickness, dOML, for each modified p- or n-type silicon surface. The optical data are presented as plots of real and imaginary parts of the complex pseudodielectric function ε(E). The best fits were obtained using the simplest two-phase model (monolayer/silicon) over the energy range of 1.5-4.5 eV. For the pyridine-modified Si(111) surface, the fitted n and dOML values are 1.67 ( 0.02 and 2.4 ( 0.1 nm, respectively, with no significant effect of the doping type of the underlying substrate (Figure 2). The measured thickness is consistent with the theoretical length of the pyridine-terminated molecular chain estimated at 2.1 nm from calculations of energy minimization using the semiempirical PM3 method. The high value of the refractive index relative to alkane-based monolayers (usually close to 1.40) is ascribed to the presence of the terminal pyridine group; values of 1.54 ( 0.03 have been reported for diversely substituted pyridines in solution63 and for a pyridine

Fabre et al. molecular layer deposited on gold.64 After the immobilization of the molybdenum-based metal cluster (denoted as Mo6) on the pyridine-terminated monolayer, dOML was found to increase to 3.9 and 4.6 ( 0.1 nm for the p- and n-type silicon, respectively, while the refractive index n also increased to 1.79 ( 0.02 irrespective of the doping type of the sample. Therefore, the average thickness increase of about 1.8 nm relative to the pyridine monolayer is in very close agreement with the size of the molybdenum cluster estimated from X-ray diffraction data, namely, 1.6 nm.21 The high refractive index of the molybdenum cluster-modified surface is also consistent with the values previously reported for molybdenum oxide65,66 and mixed molybdenum-tungsten oxide films,67 usually in the range of 1.7-2.3. It must be noticed that the refractive index of any metallic cluster in either liquid or solid phase has been reported so far. Globally, the weak SE differences observed between the p- and n-type samples indicate that the nature of charge carriers in silicon has no significant effect on the structure of the grafted molecular layers. Such a conclusion is adequately supported by X-ray photoelectron spectroscopy data (vide infra). 3.2. X-ray Photoelectron Spectroscopy. Derivatization of Si(111) surfaces by the pyridine-terminated chains and further anchoring of Mo6 cluster units have been controlled by XPS. XPS results obtained from p- and n-type samples are found to be similar, which suggests that the grafting efficiency and composition of the resulting layers do not depend significantly on the doping type of the surface.68 Consequently, only the results obtained from the p-type samples will be discussed in this section, and the same conclusions can be drawn for the n-type samples. Zuilhof and co-workers reported that the efficiency of the photochemical alkyl monolayer formation by visible light on n-type Si(100) was higher than that on p-type Si(100).69 However, no discernible effect was observed after sufficiently long irradiation times (higher than 10 h). First of all, the XPS analysis of the pyridine-modified silicon surface reveals characteristic peaks from the silicon substrate itself and from the C 1s, O 1s, and N 1s core levels of the attached organic molecule. The shape and intensity of these spectra as well as their relative positions in energy are perfectly consistent with the monolayer composition, except for the oxygen which is higher than expected. This is probably due to excess molecular oxygen adsorbed onto the surface, linked neither to silicon nor to carbon. Furthermore, the N 1s spectrum is characterized by a main peak at 400.0 eV corresponding to the pyridine (C5H4N) component and a shoulder at 398.5 eV attributed to the amide (N-CdO) environment (Figure 2b). No signal due to N-O bonds is observed at higher binding energies, which demonstrates the absence of unreacted succinimidyl units. Moreover, the Si 2p spectrum does not show the presence of silicon oxides at a binding energy of 103-104 eV, thus proving that the surface is not or is very weakly oxidized. Considering a monolayer thickness of 2.4 nm as measured by SE, the surface coverage can be estimated at 0.22 pyridine-terminated chain per surface silicon atom, which corresponds to a surface density for the organic chains of ∑OML ) 2 × 1014 cm-2. After attachment of the molybdenum cluster and treatment with 3-bromopyridine, the survey spectrum shows as expected the presence of iodine and molybdenum atoms characterized by their respective d levels (I 3d3/2, I 3d5/2, Mo 3d3/2, Mo 3d5/2, and I 4d5/2 at 632.3, 620.8, 232.5, 229.4, and 50.8 eV, respectively) and 3p components (Mo 3p3/2 at 395.1 eV) (Figure 3a). The experimental ratio between the areas under the Mo and I peaks is estimated at 0.65, which is very close to 6Mo/9I in agreement with one Ia per

J. Phys. Chem. C, Vol. 113, No. 40, 2009 17441 ratio Br/Mo is estimated at 0.09, and this agrees with the presence of approximately one bromopyridine unit per bound metallic cluster. Therefore, it can be concluded that on average the metallic clusters are bound to the silicon surface through four pyridine headgroups. Considering that one apical Ia is attached to the cluster core, a -3 charge is carried by three triflate moieties in order to maintain the total charge balance. The two-step reaction can be written as follows

(I) [(n-C4H9)4N]2[Mo6I8iI1a(CF3SO3)a5] + 4Py-Surf f (CF3SO3)2[Mo6I8iI1a(Py-Surf)a4(CF3SO3)a1] + 2[(n-C4H9)4N)]CF3SO3

(II) (CF3SO3)2[Mo6I8iIa1(Py-Surf)a4(CF3SO3)a1] + Py-Br f (CF3SO3)3[Mo6I8iI1a(Py-Surf)a4(Py-Br)a1]

Figure 3. X-ray photoemission spectra obtained typically on modified n-type and p-type Si(111). (a) Survey spectra of pyridine-modified Si(111) (black line) and Mo6 cluster-modified Si(111) (blue line). (b) N 1s resolved spectrum of pyridine-modified Si(111). Experimental dataaredots.ContinuouslinesarefittedcurvesusingtwoGaussian-Lorentzian mixed peaks, one for the pyridine component (dashed line at 400.0 eV) and the other for the amide nitrogen (dotted line at 398.5 eV). (c) C 1s resolved spectrum of Mo6 cluster-modified Si(111). Experimental dataaredots.ContinuouslinesarefittedcurvesusingfourGaussian-Lorentzian mixed peaks corresponding to different carbon atoms of the film.

Mo6I8i 4+ cluster core. It means that only clusters with one apical iodine ligand were anchored on the surface. The position of the Mo 3d5/2 and 3d3/2 peaks is identical to that observed for the starting product (n-C4H9N)Mo6I14 and is consistent with a MoII oxidation state (Figures S1 and S2 of the Supporting Information).70 Compared with the pyridine-modified surface, the Si 2p signal of the Mo6 cluster-modified surface is as expected strongly attenuated and shows the presence of a very weak peak due to silicon oxides at a binding energy of about 104 eV (Figure S2 of the Supporting Information). Moreover, the C 1s spectrum shows an additional component at higher binding energies that can be assigned to CF3 bonds of triflate anions remaining after the treatment with 3-bromopyridine (Figure 3c). This is corroborated by the presence of F 1s and S 2p signals at 690 and 169 eV, respectively. The ratio between the peak areas of the CF3 component in the C 1s spectrum and total Mo signal is 0.15-0.33, which means that each bound metallic cluster is neutralized by triflate counteranions. The Br 3d signal at 73.0 eV was used as a XPS probe (Figure S3 of the Supporting Information) in order to estimate how many triflate were not exchanged by Py-Surf. The

From XPS data and using the treatment reported by Cicero et al.,71 if one considers a film thickness of about 4 nm as measured by SE and the Si(111) surface atom density of 7.8 × 1014 atoms cm-2,72 the surface coverage can be estimated at 0.012 molybdenum cluster per surface silicon atom, i.e., ∑OML ) 1 × 1013 cm-2. This gives a specific area of approximately Amol ) 10 nm2 per bound cluster. This value is greater than that calculated from crystallographic data of the Mo6 cluster.73 This coverage is more reliable than the larger value reported in our preliminary report41 because previous metallic cluster grafting was characterized by a so-called surface aggregation process as supported by the AFM images showing the presence of homogeneously distributed globular features of 25-40 nm in diameter. Herein, because of the optimization in the preparation conditions of the metallic cluster and the use of previously distilled solvents in the rinsing steps of the silicon surfaces (see Experimental Section), the thickness determined by ellipsomety and morphological characteristics of the Mo6 cluster-modified surface are more consistent with the attachment of a monolayer of metallic clusters. 3.3. AFM. AFM analysis of the Mo6 cluster-modified Si(111) surfaces shows the presence of homogeneously distributed nanoparticles of spherical shape (Figure 4). The observed features can be assigned to the bound metallic cluster units because the topography of the pyridine-modified surface reveals the defectfree atomically flat terraces of underlying Si(111) with step heights of about 3 Å as is commonly observed for ω-functionalized organic monolayers.53,59,74-77 These particles are characterized by a narrow height distribution centered on about 3.5 nm with a measured rootmean-square (rms) roughness of approximately 12 ( 2 Å, averaged on several images. Importantly, the morphological characteristics of these films are independent of the doping type of the underlying silicon surface as supported by similar rms roughness and height distribution. This topography reveals strong improvement compared with nonoptimized surfaces, which show densely packed globular features with heights exceeding several tenths of a nanometer, a broad height distribution centered on about 4-5 nm, and roughness values close to 35 Å (Figure S4 of the Supporting Information). 3.4. Electrochemical Characterization of Mo6 ClusterModified Si(111) Surfaces. Typical cyclic voltammograms of the Mo6 cluster-terminated monolayers in CH3CN + 0.1 M Bu4NClO4 are shown in Figure 5. In the dark, a single irreversible oxidation peak at 0.92 V versus SCE was observed for the modified p-type surface, whereas no anodic current was measured for the modified n-type surface as expected for a semiconductor under depletion conditions, i.e., when few majority charge carriers are available for charge transfer.78 Because the pyridine-terminated

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Figure 4. AFM images (2 µm × 2 µm) of the Mo6 cluster-modified (a) n-type and (b) p-type Si(111) surfaces and their corresponding height distribution profiles.

Figure 5. Cyclic voltammograms at 0.1 V s-1 in the dark of pyridinemodified p-type Si(111) (dashed line) of the Mo6 cluster-modified n-type (dotted line) and p-type (solid line) Si(111) surfaces in CH3CN + 0.1 M Bu4NClO4.

monolayer does not show any oxidation peak within the same potential range, this system can be undoubtedly ascribed to the oxidation of the bound Mo6 clusters, and the measured peak potential is approximately similar to that observed for the metallic cluster in solution. To obtain further insights on the electrical properties of the Mo6 cluster-modified Si(111) surfaces, we performed differential capacitance measurements in the same electrolytic medium. In order to determine the energy levels of the semiconductor bands, it is essential to estimate the flat band potential Efb of the silicon surface, i.e., the electrode potential for which there is no space charge region in the semiconductor. This parameter has been estimated from the commonly used Mott-Schottky plot (Csc-2 versus E, eq 1) that gives the space charge capacitance Csc as a function of the electrode potential E under depletion conditions (i.e., depletion of conduction band electrons or valence band holes, respectively, in the space charge region of the n- or p-type surfaces).79

Csc-2 )

(

k BT 2 E - Efb 2 q qεε0NA

)

(1)

Figure 6. (A,B) C-E curves and (C,D) Mott-Schottky Csc-2 - E plots of the Mo6 cluster-modified (A,C) n-type and (B,D) p-type Si(111) surfaces in CH3CN + 0.1 M Bu4NClO4. Insets in panels A and B are capacitance peaks corresponding to the surface states after background subtraction.

where ε is the relative permittivity of silicon (11.7), ε0 is the permittivity of free space, N is the dopant density of the semiconductor [expressed as Nd, the donor density of n-type Si(111), or Na, the acceptor density of p-type Si(111)], A is the area of the electrode, kB is the Boltzmann constant, T is temperature, and q is the electronic charge. Linear Csc-2-E plots are obtained for potentials above -0.5 V and below 0.0 V for the modified n- and p-type surfaces, the slope and intercept of which enable the flat band potential and dopant density to be determined (Figure 6). The N values obtained for the two surfaces are consistent with the dopant density derived from the four-probe resistivity measurements of these silicon samples (Nd ) (4.6 ( 0.5) × 1014 donor cm-3 and Na ) (1.3 ( 0.2) ×

J. Phys. Chem. C, Vol. 113, No. 40, 2009 17443 1015 acceptor cm-3 for the n- and p-type surfaces, respectively). The calculated values of Efb are -0.55 ( 0.05 V and 0.04 ( 0.05 V for the modified n- and p-type surfaces, respectively. These are in good agreement with other electrochemical data reported for Si-C linked alkyl monolayers on n-type80,81 or hydrogenated p-type82,83 silicon surfaces studied in the same solvent. For n-type substrates, the flat band potential values derived from our electrochemical characterizations are also in agreement with those determined from Hg-based junctions45 within 80 mV. However, a difference of 0.3 V is found for p-type silicon substrates. In the absence of surface states, theoretical values of Efb are only dependent on silicon doping, and values of -0.18 and +0.45 V can be thus calculated for nand p-type silicon, respectively. In our work, the experimental Efb values are shifted in the same magnitude for n- and p-type surfaces, about 300-400 mV, with respect to theoretical Efb. Such discrepancies can be attributed to interface charge trapped in surface states, the density of which calculated from capacitance measurements is noticeably similar for both types of silicon (vide infra). In the case of the junctions studied by Faber et al.,45 the difference between experimental and theoretical Efb was stronger for n-type silicon because the number of fixed charges in the n-type silicon samples was found to be more than four times higher than in the p-type silicon samples. Now, if one considers that the equivalent electrical circuit of the silicon-insulating film-electrolyte interface is similar to that commonly used for the silicon alkyl monolayer-electrolyte interface,72,78,84,85 the equivalent capacitance C of the Mo6 cluster-modified silicon surfaces in accumulation can be described by eq 2, taking into account the presence of surface states in silicon.

C-1)(Csc+ Css)-1+ Cf-1+ CH-1

(2)

where Css, Cf, and CH are the capacitances of the surface states, Mo6 cluster-terminated film, and the Helmholtz layer, respectively. In this expression, Csc is the only potential-dependent capacitance, and CH corresponds to the capacitance of hydrogenterminated Si(111). Consequently, in strong accumulation (E , Efb for n-type silicon or E . Efb for p-type silicon), Csc-1 can be neglected and the C-E curves show a small plateau (Figure 5A,B) that is determined only by the total capacitance of CH and Cf, provided that the density of surface states is low. The capacitance values corresponding to these plateaus are 2.3 and 1.8 µF cm-2 for the modified n- and p-type surfaces, respectively. Assuming that CH is not changed in the presence of the Mo6 cluster-terminated organic monolayer (this assumption has been experimentally verified from capacitance measurements obtained for alkyl monolayers covalently bound to n-type86 and p-type87 Si(111) surfaces), these results indicate a higher capacitance of the film covalently bound to n-type Si(111) and, therefore, lower blocking properties if one considers the film as an ideal capacitor. As mentioned above, the derivatization of silicon surfaces is thought to introduce surface states that are probably due to some unavoidable oxidation of Si(111) and/or the possible presence of interfacial alkoxy species. The presence of these surface states is indicated by the appearance of an extra capacitance peak at electrode potentials near Efb in the C-E curves.78,88 To separate parallel capacitance due to surface states from the underlying Csc, a baseline subtraction was made for each C-E curve. From the obtained capacitance peaks (insets in Figure 6), the total density of the surface states can be calculated (eq 3).

Cp(max) )

1 e2 S 4 kBT tot

(3)

Stot is estimated at 3.7 × 1011 cm-2 and 2.7 × 1011 cm-2 for the modified n-type and p-type Si(111), respectively. These densities correspond to one surface state per about 2000 and 3000 surface silicon atoms, respectively, i.e., less than 0.05% of the total surface. Globally, the electrochemical data show that the Mo6 clustermodified p-type Si(111) surface is characterized by better blocking properties with a smaller surface state density compared with those of the modified n-type surface. Note that this result is consistent with the smaller forward current density JFWD (p-type Si) as compared with JFWD (n-type Si) obtained in Section 3.5 (Table 1). 3.5. Current Transport Measurements through Hg/Mo6 Cluster-Pyridine Monolayer/Si(111) Junctions. Figure 7 shows the current density (J)-voltage (V) characteristics of the pyridine-immobilized surface before and after Mo6 cluster grafting, respectively, for the p-type and n-type silicon substrates. The forward bias is obtained for a positive (negative) voltage applied on the p-type (n-type) silicon substrate. The reverse bias current density is reproducible at several points of the substrate; it is also very stable, except for the Mo6 cluster grafted on n-type silicon, where some aging occurs as evidenced by an increase in JREV by a factor of 50 over 5 days storage in the glovebox. Except for pyridine-modified p-type Si(111), all measured characteristics show a large rectification factor (Table 1), defined as R (V) ) JFWD (V)/JREV (-V). At ( 1.5 V, the rectification is larger for the n-type silicon devices (R > 104) as compared with that for the p-type silicon devices (7 < R < 3 × 102) (Table 1). At (3 V, a quite large rectification R > 105 is observed for n-type silicon. For the p-type Si(111) junctions, a major decrease in the reverse bias current density is observed after immobilization of the Mo6 cluster (Figure 7a). Only small changes are observed in the shape of the J-V characteristic in the forward regime. For n-type Si(111), a different shape of J-V curves is observed at low forward voltage values before and after Mo6 cluster immobilization (Figure 7b). Whereas the behavior of the pyridine-modified n-type Si(111) device is well described by a Schottky barrier limitation of the current density, with a barrier height ΦB ) 0.79 ( 0.01 eV, a 50 times lower current density (at V ) -0.4 V) is observed after cluster immobilization, along with a trend to saturation near 1 × 10-6 A cm-2 (in the range of -0.35 < V < -0.10 V). The lower current density corresponds to an effective barrier increase of 40 meV (ΦB ) 0.83 ( 0.01 eV). At high forward bias (V < -0.6 V), both characteristics recover a similar shape, indicating that the barrier inside the semiconductor becomes negligible and the current becomes limited by a tunneling mechanism as observed previously in alkyl (C10 to C18)/n-type silicon devices.89-92 Furthermore, the magnitude of the forward current density J (-1 V) ) 1 × 10-2 A cm-2 is similar to that obtained with n-octadecyl chains.90 Additional information is revealed from the conductance profiles G(V) ) (dJ/dV), obtained from the derivation of J(V) with respect to the applied voltage. Figure 8 shows the conductance plots of the pyridine-modified surface before and after Mo6 cluster grafting, respectively, for the p-type and n-type silicon substrates. In p-type Si(111) devices, conductance shows a plateau value in the high reverse bias range (V < -0.8 V). This plateau value decreases by two decades, from G ) 2.5 × 10-4 Ω-1 cm-2 for the pyridine/p-type Si(111) device to G ) 1.5 × 10-6 Ω-1 cm-2

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Fabre et al.

TABLE 1: Current Density Values and Rectification Factorsa layer n-type Si(111) p-type Si(111)

Pyr Mo6 + Pyr Pyr Mo6 + Pyr

JREV (A cm-2) -6

1.6 × 10 1.0 × 10-6 3.0 × 10-3 1.1 × 10-5

JFWD (A cm-2) -2

4.4 × 10 3.2 × 10-2 2.3 × 10-2 3 × 10-3

R

VMIN(V)

2.7 × 10 3.1 × 104 7 3 × 102 4

-0.17, -0.32 -0.05

a Values and factors are measured at ( 1.5 V for the pyridine-terminated monolayer-modified p- and n-type Si(111) devices before (Pyr) and after immobilization of the Mo6 cluster (Mo6 + Pyr). A minimum in G(V) is observed at VMIN for the Mo6-modified surfaces.

Figure 7. Current density (J)-voltage (V) characteristics of the freshly prepared pyridine-terminated organic monolayer-modified (a) p-type and (b) n-type Si(111) junctions before (black line) and after immobilization of the Mo6 cluster (blue line).

for the Mo6 cluster/pyridine/p-type Si(111) device. As the applied voltage increases toward the forward regime to +1.5 V, the conductance values are globally increasing to 2 × 10-2 Ω-1 cm-2 at V ) +0.5 V for the pyridine/p-type Si(111) device and 1.5 × 10-3 Ω-1 cm-2 for the Mo6 cluster/pyridine/p-type Si(111) device. However, in the intermediate regime (-0.5 < V < +0.1 V), different features are evidenced before and after immobilization of the metallic cluster. For the pyridine/p-type Si(111) device, a monotonic and nearly exponential increase versus forward bias is observed, whereas for the Mo6 cluster/ pyridine/p-type Si(111) device, an additional dip is found in the G(V) characteristics between -0.2 and +0.1 V as compared with the expected monotonic increase, which is tentatively depicted by the continuous line in Figure 8a. For the n-type Si(111) devices, the conductance plots show a quasi plateau in the high reverse bias regime (V > +0.2 V) with values of 8 × 10-7 and 3 × 10-5 Ω-1 cm-2 at +0.5 V before and after the Mo6 cluster immobilization, respectively (Figure 8b). As the applied voltage is swept toward the forward regime up to -1.0 V, the conductance values are globally increasing to 4 × 10-2 Ω-1 cm-2 for the pyridine/n-type Si(111) and 7 × 10-3 Ω-1 cm-2 for the Mo6 cluster/pyridine/n-type Si(111) device. However, different features are evidenced before and after Mo6 cluster immobilization. For the pyridine-modified surface, G(V) increases monotonically with a nearly exponential

Figure 8. Conductance-voltage characteristics of the freshly prepared pyridine-terminated organic monolayer-modified (a) p-type and (b) n-type Si(111) junctions before (black) and after immobilization (blue) of the Mo6 cluster. Values of VMIN and VFB are indicated by vertical marks. Continuous line is a guide for the eye.

regime between +0.10 and -0.15 V. In contrast, two minima (between 0.00 and -0.50 V) appear in the G(V) plot of the Mo6 cluster/pyridine/n-type Si(111) device. This double minimum feature is still present after a 5 day aging of the sample, although slightly shifted toward more negative voltages. The same order of magnitude of G(V) is retained in the double minimum range, i.e., 3 × 10-6 Ω-1 cm-2 before and after aging, in spite of a large increase in the reverse bias value of G(V). Globally, direct current (dc) transport measurements show an effect of the Mo6 cluster immobilization on p-type and n-type Si(111) devices. The observed changes in the shape of the J(V) characteristics are larger for the n-type Si(111) junctions. For both devices, some dips appear in the G(V) plots, suggesting the appearance of an additional barrier as compared with the pyridine-modified surfaces. A single minimum in the conductance plot is located at VP ) -0.05 V for the p-type Si(111) device, while a double minimum feature is observed in G(V) at VN ) -0.17 and -0.32 V for the n-type Si(111) device. Both minima at VN and VP correspond to low current density regions of the I-V characteristics, where the Schottky barrier in silicon is the limiting transport mechanism. To confirm this observation, we have reported the flat band voltage values derived from electrochemical studies on J-V plots (Figure 8). Because the SCE standard reference potential corresponds to a work function of -4.56 ( 0.10 eV91 and the Hg electrode has a work function of 4.50 ( 0.10 eV, the flat band voltage values VFB on the characteristics of Hg/monolayer/Si(111) junctions

J. Phys. Chem. C, Vol. 113, No. 40, 2009 17445 are taken equal to the electrochemical flat band potentials Efb. In Figure 7, it appears that the voltage range in which the G(V) minima occur corresponds to a weak depletion regime, and the effect vanishes in the accumulation region as soon as the flat band voltage is reached. In principle, we may consider that electrons can be efficiently trapped either at the organic monolayer/Si(111) interface (density of surface states Stot ) 3 × 1011 cm-2) or at electronic states localized inside the molecular layer. Although some SiOx contamination was observed by XPS after the Mo6 cluster immobilization, this surface oxidation is not believed to be responsible for our observations because the aging in the glovebox does considerably increase the reverse bias value of G(V) but leaves unchanged the magnitude of the doubleminimum feature for the n-type Si(111) device. In addition, no minima in G(V) were evidenced for the pyridine-modified surfaces or long alkyl chain-modified n-type Si(111).93 In the following discussion, we thus consider the specific role of bound metallic clusters. With the alkyl chains being characterized by a very high energy gap between their highest occupied molecular orbitals (HOMO) and lowest unoccupied molecular orbitals (LUMO), it is expected that electrical transport at high bias values will be limited by tunneling between the Si semiconductor and the HOMO-LUMO system of the [Mo6I8]4+ clusters through the alkyl chains, while the semiconductor barrier may also be important at low applied bias. Upon bias application between mercury and silicon electrodes, the molecular moieties are no longer under equilibrium, and the electric field within the molecular complex is probably not uniform. In addition, the metal insulator semiconductor (MIS) devices are very asymmetrical because the molecular layer is covalently bonded to the Si substrate through the pyridine-alkyl chain, while it is weakly connected with the mercury drop through van der Waals interactions. In first approximation, it is expected that the HOMO and LUMO levels of the pyridine and [Mo6I8]4+ units will roughly follow the potential at the silicon-alkyl interface.91 Calculations indicate that the Mo6Li8La6 cluster units exhibit a set of 12 metal-metal bonding orbitals, corresponding to the metallic valence electrons. The HOMO level of the Mo6Li8La6 cluster corresponds to the highest level of metal-metal bonding orbitals.2-10 If this HOMO level is not too deep in energy, we assume that the application of a negative potential on the silicon substrate may raise this HOMO level above the Fermi level at the Hg/Mo6 interface and/or provide a negative charge transfer from the Mo6 unit to the mercury. This positive cluster charge leads to a negative image charge at the mercury surface, providing a dipole at the Hg/Mo6 interface, pointing toward the mercury. This effect should increase the effective barrier for electron injection from the Mo6 cluster to the Hg electrode as found experimentally. If a charge q is transferred at a distance δ, the magnitude of the barrier variation ∆E (eV) can be estimated from qe ) (ε0εmol Amol/δ) ∆E, where Amol and εmol are the molecule footprint and dielectric constant, respectively. Using q ) 1e, δ ) 0.5 nm, Amol ) 10 nm2, and εmol ) 3, a typical barrier ∆E ) 0.32 eV is obtained. The observation of two conductance minima (n-type Si device) points toward the existence of two successive oxidation steps for the [Mo6I8]4+ cluster. As far as the p-type Si device is concerned, the strong decrease of JREV after the Mo6 grafting step is also consistent with the existence of a large barrier for electron injection from the cluster to the Hg electrode. Finally, when a sufficiently high positive bias is applied, the HOMO level returns to its neutral (doubly occupied) state and conduc-

tance returns to a higher level, corresponding to depletion (ntype Si device) or accumulation (p-type Si device) regimes. 4. Conclusions In this study, we have demonstrated that the doping type of the Si(111) substrate has no significant effect on the composition and morphology of the overlaying pyridine- and Mo6 clusterterminated organic monolayers. In contrast, the electrochemical characterization of such surfaces reveals that the Mo6 clustermodified monolayer covalently bound to p-type Si(111) shows better blocking properties with a slightly smaller surface state density compared with those of the modified n-type surface. Moreover, the current transport measurements through the Hg/ monolayer/Si(111) junctions show as expected that the current rectification is higher for the n-type silicon devices as compared with that of the p-type silicon devices. Interestingly, we notice that the presence of grafted Mo6 clusters leads to characteristic features in the J (V) and G(V) plots. In particular, two minima are observed in the G(V) plot for the modified n-type surface, which could be caused by the accumulation of a localized charge density in the bound Mo6 clusters. In the near future, the anchoring of other transition metal clusters onto silicon surfaces using the same surface chemistry will be investigated in order to produce functional surfaces showing tunable electronic and current transport properties, for instance, using metallic clusters that can be reversibly oxidized at a lower potential as compared with that of a Mo6-based cluster. Acknowledgment. This work was financially supported by CNRS, UMR 6226 CNRS/Universite´ de Rennes 1, and Agence Nationale de la Recherche (ANR-07-BLAN-0170-02, project “CLUSTSURF”). We thank S. Ollivier for the AFM experiments and A.B. Fadjie-Djomkam for the transport measurements. Supporting Information Available: XPS high-resolution Mo 3d, Si 2p, and Br 3d spectra; AFM images of the Mo6modified surfaces; representation of the Mo6 cluster core, and a schematic representation of the molecular orbital diagram of a M6Li8La6 cluster unit. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Robinson, L. M.; Bain, R. L.; Shriver, D. F.; Ellis, D. E. Inorg. Chem. 1995, 34, 5588. (2) Ramirez-Tagle, R.; Arratia-Perez, R. Chem. Phys. Lett. 2008, 460, 438. (3) Lin, Z. Y.; Williams, I. D. Polyhedron 1996, 15, 3277. (4) Le Beuze, A.; Lamande, P.; Lissillour, R.; Chermette, H. Phys. ReV. B. 1985, 31, 5094. (5) Johnston, R. L.; Mingos, D. M. P. Inorg. Chem. 1986, 25, 1661. (6) Hughbanks, T.; Hoffmann, R. J. Am. Chem. Soc. 1983, 105, 1150. (7) Hughbanks, T. Prog. Solid State Chem. 1989, 19, 329. (8) Gautier, R.; Furet, E.; Halet, J. F.; Lin, Z. Y.; Saillard, J. Y.; Xu, Z. T. Inorg. Chem. 2002, 41, 796. (9) Baranovski, V. I.; Korolkov, D. V. Int. J. Quantum Chem. 2004, 100, 343. (10) Kirakci, K.; Cordier, S.; Shames, A.; Fontaine, B.; Hernandez, O.; Furet, E.; Halet, J. F.; Gautier, R.; Perrin, C. Chem.sEur. J. 2007, 13, 9608. (11) Tarascon, J. M.; Disalvo, F. J.; Murphy, D. W.; Hull, G. W.; Rietman, E. A.; Waszczak, J. V. J. Solid State Chem. 1984, 54, 204. (12) Nocera, D. G.; Gray, H. B. J. Am. Chem. Soc. 1984, 106, 824. (13) Chevrel, R.; Sergent, M. Metal Clusters in Chemistry, Braunstein, P. Oro L. A. Raithby P. R. ed.; Wiley-VCH, 1999; Vol. II. (14) Chevrel, R.; Sergent, M. Superconductivity in Ternary Compounds. In Topics in Current Physics; Fischer, O., Maple, M. P., Eds.; Springer Verlag: Berlin, Heidelberg, NY, 1982. (15) Aurbach, D.; Suresh, G. S.; Levi, E.; Mitelman, A.; Mizrahi, O.; Chusid, O.; Brunelli, M. AdV. Mater. 2007, 19, 4260.

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