X-ray Photoelectron Spectroscopy: A Powerful Tool for Electronic and

Jul 1, 2009 - Dipartimento di Scienze Chimiche, Università di Catania and I.N.S.T.M. UdR of Catania, Viale Andrea Doria 6, 95125 Catania, Italy, and ...
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J. Phys. Chem. C 2009, 113, 13558–13564

X-ray Photoelectron Spectroscopy: A Powerful Tool for Electronic and Structural Investigations of Covalently Assembled Monolayers. A Representative Case Study Antonino Gulino,*,† Fabio Lupo,† Maria E. Fragala`,† and Sandra Lo Schiavo‡ Dipartimento di Scienze Chimiche, UniVersita` di Catania and I.N.S.T.M. UdR of Catania, Viale Andrea Doria 6, 95125 Catania, Italy, and Dipartimento di Chimica Inorganica, Chimica Analitica e Chimica Fisica, UniVersita` di Messina, 98166, Messina, Italy ReceiVed: March 26, 2009; ReVised Manuscript ReceiVed: May 25, 2009

In this investigation, we report on a representative study that stands for an example of electronic and structural analyses by X-ray photoelectron spectroscopy of covalently assembled molecular monolayers. This technique was revealed as a potent apparatus to get information not available with many other instruments. Two covalently assembled monolayers of Rh2(form)2(O2C-C6H4-OH)2 and Rh2(form)2(O2C-C4H8-OH)2 (form ) N,N′di-p-tolyl-formamidinate anion) molecules were fabricated on both Si(100) and silica substrates, already functionalized with a covalent 4-ClCH2C6H4SiCl3 monolayer. Both structural and chemical characterizations were carried out by angle-resolved X-ray photoelectron measurements. Surface morphological characterization was performed by atomic force microscopy measurements. The present results provide information on the grafting of the dirhodium complexes and highlight the reliability of the photoelectronic technique. Introduction 1

Thin films are generally obtained by sublimation, spin coating,2 or the Langmuir-Blodgett technique.3 With these techniques, the molecule organization within the film is difficult to control. Moreover, the resulting films can be soluble in organic solvents. Monolayer chemistry is an emerging and growing field.4-9 Molecular monolayers are molecules bound to a surface. They are not free gaseous molecules nor bulk or thin films. They are not molecules in solution where the solvent mediates many effects. Using a metaphor, molecules covalently assembled on surfaces are similar to helium-filled balloons bound to a child’s wrist. Advantages of these monolayers, with regard to thin films, include that intrinsic molecular properties are emphasized even though molecules are bound to a solid surface. Many surfaces can be used as substrates to fabricate monolayers. Nevertheless, atomically flat and chemically well-defined silicon surfaces remain the most important technological substrates, in the perspective of fabrication of electronic devices that can be easily integrated within electronic circuits.10-13 For example, in the case of redox-active molecules attached to an electroactive surface, information can be stored in the discrete redox states of the molecules to build molecular-based information storage materials.14 Silicon is not transparent; therefore, no UV-vis measurements in the transmission mode can be afforded, and reflectance measurements on silicon-supported monolayers are not informative since scattering effects dominate. In addition, it is not possible to use solid state NMR because of the low signals of monolayers. In contrast, some XPS15 (Xray photoelectron spectroscopy), SIMS16 (secondary ion mass spectroscopy), and ATR-FTIR-13a,17 (attenuated total reflectanceFTIR) measurements have been reported. * To whom correspondence should be addressed. E-mail: agulino@ dipchi.unict.it. † Universita` di Catania and I.N.S.T.M. UdR of Catania. ‡ Universita` di Messina.

In this context, XPS was revealed as a unique technique for an in-depth chemical and structural monolayer characterization.15 In fact, this is a surface technique suitable to probe thicknesses of the same order of the photoelectron inelastic mean free paths (a few tens of Ångstroms).18 This technique allows one to immediately obtain qualitative and quantitative analyses as well as oxidation states. Moreover, analysis of particular spectral features such as spin-orbit coupling,18a shakeup phenomena,18d Auger parameters,18a and the presence of screened and unscreened states18e,f etc. can be used for more elegant studies. Instruments equipped with a monochromatized X-ray radiation, the use of low pass energy values, and conducting samples under investigation represent a combination of optimal factors to increase the spectral resolution and avoid charging effects that can cause band broadening.18a If, in addition, the instrument allows a good control of the acceptance angle of the analyzer and good precision of the sample holder, one can afford reliable angle-resolved18d measurements that are unique to obtain film thickness and surface coverage.19 The synthesis and characterization of bimetallic compounds having a lantern structure represent a fertile area of research.20,21 In this context, our attention has been focused on dirhodium(II,II) molecular complexes, a flexible class of compounds characterized by a rich chemistry that ranges from axial and/or equatorial reactivity, redox, catalytic, sensing, and biological activity to the formation of supramolecular species.20,21 In particular, their redox activity is connected to the reversible oneelectron oxidation dirhodium(II,II) f dirhodium(II,III) process and may be controlled by varying, to some extent, the bridging ligands around the dirhodium core.20,21 In addition, in the past decade, their axial reactivity toward small molecules (CO, NOx, Lewis bases, water, etc.) was successfully exploited for sensing purposes.21 In the perspective to fabricate dirhodium-bound monolayers for gas-sensing applications, we planned the synthesis of two well-defined dirhodium(II,II) complexes (Figure 1), appropriately functionalized with OH-ancillary groups, namely, [Rh2(form)2(O2C-C6H4-OH)2], 1, and [Rh2(form)2(O2C-C4H8-OH)2], 2, where NCN represents the p-tolylfor-

10.1021/jp9027436 CCC: $40.75  2009 American Chemical Society Published on Web 07/01/2009

Investigations of Covalently Assembled Monolayers

Figure 1. Schematic representation of 1 and 2.

Figure 2. Representative 1-based monolayer on Si(100).

mamidinate anions and the arrows indicate the free axial binding sites.15b In this context, we reported on the synthesis of 1 and fabricated its covalently bound monolayer (Rh-SAM) using siloxane-based coupling layers on both quartz and silicon substrates (Figure 2).15b This monolayer bound on quartz proved to be useful for the selective optical sensing of traces of CO in air, due to the CO affinity for the free Rh axial ligation sites.15b The system was read-out optically and demonstrated a very fast response time to very small amounts of CO in air.15b Moreover, it is robust and stable up to 200 °C and can be regenerated by a thermal treatment under air or nitrogen stream for a few minutes.15b In the present investigation, apart from the synthesis of 2, we fabricated monolayers of both complexes for a full XPS chemical and structural characterization, to rationalize the relationship between microscopic structure and macroscopic properties, and to highlight the remarkable suitability of the XPS technique as a tool for monolayer studies. Experimental Details Aldrich-grade reagents and solvents, some of them packed under nitrogen, were used throughout all present syntheses. Complex 1 was synthesized and characterized as already reported.15b Complex 2 was similarly synthesized. In particular, an acetone solution (5 mL) of [Rh2(form)2(CF3COO)2(H2O)2] (0.300 g, 0.328 mmol) was added to a water solution (50 mL) of 5-hydroxypentanoic acid sodium salt (NaO2C-C4H8-OH),

J. Phys. Chem. C, Vol. 113, No. 31, 2009 13559 and the resulting mixture was left to stir for 2 h. The resulting green solid was filtered off, washed with warm water, and dried at 50 °C (24 h). The crude solid was then dissolved in chloroform (40 mL), and the resulting solution was filtered through Celite. By removal of the solvent under reduced pressure, 2 was obtained as a green light solid. Yield, 80%. Rh2(C15H15N2)2(O2C-C4H8-OH)2: calcd C, 54.19; H, 5.46; N, 6.32. Found: C, 54.60; H, 5.51; N, 6.72. IR (KBr pellet, cm-1): 1620 (CO2asym), 1590 (N-C-N). 1H NMR (pyridine-d5): δ ) 1.81 (m, 2H), 1.92 (m, 2H), 2.12 (s, 6H), 2.57 (t, J ) 7 Hz, 2H), 3.83 (t, J ) 7 Hz, 2H), 4.9 (s, br, 1H), 7.01(dd, J ) 64 and 8.1 Hz, 8H), 7.72 (t, 8.3 Hz, 1H). Fused silica (quartz) substrates were cleaned by immersion in “piranha” solution (c H2SO4:30% H2O2 70:30 v/v) at 80 °C for 1 h and then left to cool to room temperature. Subsequently, substrates were repeatedly rinsed with double-distilled water and immersed in a H2O:30% H2O2:NH3 5:1:1 v/v/v mixture at room temperature for 40 min.9h,15i Then, they were washed with double-distilled water and dried under vacuum immediately before coupling agent (CA) deposition. Si(100) substrates, obtained from STMicroelectronics (Catania, Italy), were first cleaned with “piranha” solution (c H2SO4:35% H2O2 70:30 v/v) at room temperature for 10 min, rinsed in double-distilled water for 2 min, etched in 2.5% hydrofluoric acid for 90 s, washed with double-distilled water, and accurately dried with prepurified N2. Subsequently, they were treated for 10 min with UV and ozone using the Ozon-Generator (Fisher 500) system to obtain a SiO2 thin (21 Å, vide infra) layer. All of the successive sample manipulations were performed in a glovebox under N2 atmosphere. In particular, all freshly cleaned substrates were immersed, at room temperature for 20 min, in a 0.5:100 (v/v) n-pentane solution of the chemisorptive siloxane, trichloro[4(chloromethyl)phenyl]silane, to afford a monolayer of the CA.9h,15i Then, the chlorobenzyl-functionalized substrates were washed with copious amounts of pentane, sonicated in acetone for 2 min to remove any physisorbed CA, loaded into glass pressure vessels under N2, immersed in a dry CH2Cl2/toluene (1:1 v/v) solution of complex 1 or 2 (0.5 × 10-3 M), and heated for 48 h at 90 °C while excluding light. Finally, the functionalized substrates bearing the covalently self-assembled dirhodium complexes (Figure 2) were cooled to room temperature and repeatedly washed and sonicated with dichloromethane and toluene to remove any residual physisorbed material. The films strongly adhere to the substrates and, when stored in a desiccator with the exclusion of light, were stable for months, as evidenced by UV/vis spectroscopy. Neither washing nor sonication with common organic solvents removed the films from the surface. Angle-resolved X-ray photoelectron spectra (AR-XPS) were measured at 90, 45, 30, 15, and 5° relative to the surface plane with a PHI 5600 Multi Technique System, which offers a good control of the photoelectron takeoff angle (base pressure of the main chamber 2 × 10-10 Torr).18d The acceptance angle of the analyzer and the precision of the sample holder concerning the angle were (3 and (1°, respectively. The spectrometer was equipped with a dual anode X-ray source, a spherical capacitor analyzer (SCA) with a mean diameter of 279.4 mm, and an electrostatic lens system Omni Focus III. Samples were mounted on Mo stubs. Spectra were excited with monochromatized Al KR X-ray radiation. The XPS peak intensities were obtained after Shirley background removal.22 No relevant charging effect was observed on the Si(100)-supported monolayers. The fwhm of the Ag 3d5/2 peak of a clean sample at the lowest pass energy (2.95 eV) was 0.47 eV. Nevertheless, this value was mainly due to the natural width of the Ag 3d5/2 and did not give the

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real resolution of our instrument. In fact, using the Si 2p3/2 peak in the same instrumental conditions, we obtained the fwhm value of 0.36 eV. Therefore, experimental uncertainties in binding energies were e0.3 eV.18b Freshly prepared samples were quickly transferred from the glovebox under N2 atmosphere to the XPS main chamber. XPS spectra proved to be higly reproducible, thus ruling out significant degradation of these monolayers under X-ray. Some spectra were deconvoluted by fitting the spectral profiles with a series of symmetrical Gaussian envelopes after subtraction of the background. This process involved data refining, based on the method of least-squares fitting, that was carried out until there was the highest possible correlation between the experimental spectrum and the theoretical profile. The R-factor (residual or agreement factor), R ) [Σ(Fo - Fc)2/Σ(Fo)2]1/2, after minimization of the function Σ(Fo - Fc)2, converged to R values e0.03.23 UV-visible measurements were performed using a UV-vis V-650 Jasco spectrophotometer. Experimental uncertainty was within (0.2 nm. Atomic force microscopy (AFM) measurements were performed with a Solver P47 NTD-MDT instrument in semicontact mode (resonance frequency 150 Hz). The monolayer surface was flat and homogeneous with no pinholes. Infrared spectra were recorded on KBr pellets with a PerkinElmer FT 1720X spectrometer. The NMR measurements were performed with a Bruker AMX 300 spectrometer.

Gulino et al.

Figure 3. AFM image of a monolayer of complex 1 on a Si(100) substrate.

Results and Discussion The Rh-SAMs have been synthesized by covalent grafting of the appropriate dirhodium complex to silica or silicon substrates that were previously cleaned, hydroxylated, and silylated.9h,15b In particular, the silylation was performed under rigorously inert atmosphere, with the trichloro[4-(chloromethyl)phenyl]silane, 4-ClCH2C6H4SiCl3, a bifunctional CA that bonds both to the substrate and to the dirhodium molecule.9h,15i These dirhodium siloxane-based Rh-SAMs strongly adhere to the substrate, are robust and insoluble in common organic solvents, and cannot be removed by the “Scotch-tape decohesion” test6a,15f as evidenced by UV-vis and XPS measurements, performed on silica- and Si(100)-supported monolayers, respectively. No relevant differences have been found in the AFM images or XPS binding energies, depending on the particular dirhodium complex 1 or 2 grafted on the SAM. Therefore, hereafter, we will preferentially discuss about the Rh-SAM obtained using complex 1 on Si(100). Nevertheless, XPS atomic concentration analysis evidence a higher molecular density of 2 with respect to 1 on their respective Rh-SAMs, and these data will be taken into account later. The surface morphologies of the Rh-SAMs were obtained by AFM measurements. Figure 3 shows a representative AFM image of the Si-supported Rh-SAM, indicating a relatively smooth film surface with no apparent features. The obtained average peak-to-peak Rmax is equal to 0.782 nm with a 0.706 Å mean roughness. The molecular monolayer characterization of the Rh-SAMs was carried out with XPS. This technique is ideal since it allows high vertical resolution, gives information on the bonding states of the grafted molecules, and allows one to estimate the surface elemental composition, making due allowance for the relevant atomic sensitivity factors.18a AR-XPS of the Rh-SAM in the Rh 3d energy region unambiguously confirmed the presence of complex 1 on the substrate surface.24 In particular, the Rh 3d5/2 and 3d3/2 signals

Figure 4. Monochromatized Al KR-excited XPS of the Rh-SAM of complex 1 on Si(100) substrate, in the Rh 3d energy region, at a 45° electron takeoff angle.

were clearly observed at 308.7 and 313.5 eV, respectively, with a 4.8 eV spin-orbit splitting (Figure 4). These overall values are in agreement with previously reported XPS data for similar systems.24 Moreover, the absence of any further splitting is a clear indication of two equivalent Rh(II) ions, thus confirming the Rh(II)-Rh(II) complex formulation. Table 1 collects data of XPS atomic concentration analysis for the Rh-SAM at different photoelectron angles (PTA), once the relevant atomic sensitivity factors have been accounted for.18a It emerges that on going to low photoemission angles, the Rh atomic concentration shows a moderate increase only, the Rh being 0.3% at 45° and 0.4% at 5°. Moreover, there is evidence of a decrease of both Si and O peaks, while the C signal significantly increases (from 41.8% at 45° to 64.3% at 5°) on decreasing of the PTA. Two features are always evident in the XPS spectrum of the Rh-SAM in the Si energy region (Figure 5). The 2p doublet at 99.0 (2p3/2) and 99.6 (2p1/2) eV,15c consistent with the presence of Si, whose atomic concentration ranges from 13.9% at 45° to 2.6% at 5°, and the broadband at 102.8 eV,15c,25 consistent with the presence of some SiO2, whose atomic concentration ranges from 11.0% at 45° to 9.8% at 5°. Moreover, the 2:1 spin-orbit doublet, at lower binding energy, shows a separation of 0.6 eV, well-tuned with that expected for a Si(100) substrate.15c,25e The whole Si 2p signal varies from 24.9% at 45° to 12.4% at 5° photoelectron takeoff angles (Table 1), and the decreasing intensity behavior of the Si 2p signal, on decreasing the photoelectron takeoff angle, is mainly due to the falloff of the Si(100) component. Analogously, the O 1s signal suffers an intensity decrease on going from 45° (29.0%) to 5° (20.0%) (Table 1). It therefore

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TABLE 1: XPS-Derived Atomic Concentration Analysis for 1 Rh-SAM and 2 Rh-SAM PTAa (deg)

C

Si

Si(SiO2)

O

Rh

N

Cl

5 15 30 45 90

64.3 (65.0) 57.0 (55.8) 47.6 (48.1) 41.8 (39.4) 31.1 (30.8)

2.6 (1.4) 4.9 (4.3) 10.7 (8.7) 13.9 (15.7) 24.2 (25.1)

9.8 (7.8) 11.0 (11.0) 11.0 (11.6) 11.0 (11.0) 10 (10.1)

20.0 (20.3) 23.9 (23.7) 27.5 (27.5) 29.0 (29.9) 30.3 (30.8)

0.4 (1.1) 0.4 (0.9) 0.3 (0.7) 0.3 (0.6) 0.3 (0.6)

1.0 (2.5) 0.9 (2.4) 0.8 (1.4) 0.7 (1.3) 0.8 (1.2)

1.9 (1.9) 1.9 (1.9) 2.1 (2.0) 3.2 (2.2) 3.3 (1.4)

a

PTA, photoelectron takeoff angle.

Figure 5. Monochromatized Al KR-excited XPS of the Rh-SAM of complex 1 on Si(100) substrate, in the Si 2p energy region, at a 45° electron takeoff angle.

Figure 6. Monochromatized Al KR-excited XPS of the Rh-SAM of complex 1 on Si(100) substrate, in the O 1s energy region, at a 45° electron takeoff angle.

transpires that this signal is largely due to the SiO2 substrate phase. Figure 6 shows a peak at 532.9 eV, typical of SiO2, and a shoulder at 531.4 eV that accounts for the oxygen of the carboxylic functionalities.18a The oxygen atomic percentage due to this shoulder nicely fit the nitrogen atomic concentration, as expected for the present dirhodium complex. Figure 7 shows the C 1s photoelectron spectrum of the Rh-SAM. A careful deconvolution of the band envelope reveals three components: The first centered at 285.0 eV is due to both aliphatic and aromatic backbones;18a,26 the band at 286.5 eV, in tune with literature data,18a,27 is due to the unreacted benzylic chloride moiety of the Cl-CH2C6H4- fragment and to the CdN groups; finally, the band at 288.4 eV accounts for the carbon centers of the complex, bonded to two oxygen atoms.18a Aromatic carbon usually shows XPS shakeup satellites at 6-7 eV to higher binding energy with respect to the main peak.18a In Figure 7, there is evidence of

Figure 7. Monochromatized Al KR-excited XPS of the Rh-SAM of complex 1 on Si(100) substrate, in the C 1s energy region, at a 45° electron takeoff angle.

Figure 8. Monochromatized Al KR-excited XPS of the Rh-SAM of complex 1 on Si(100) substrate, in the Cl 2p energy region, at a 45° electron takeoff angle.

a very weak πfπ* shakeup feature, centered at 291.6 eV, whose intensity is of 2.3%, with respect to the main peak. Figure 8 shows the Cl 2p3/2,1/2 spin-orbit doublet at 199.0 and 200.9 eV of the unreacted benzylic chloride moiety, of the functionalized Si(100) surface.15i The reaction between complex 1 and the chlorobenzyl-terminated monolayer cannot be quantitative due to the large size of the dirhodium complex. The observed atomic concentration Cl/Rh ratios (45° takeoff angle), taking into account that there are two Rh ions per molecule and each dirhodium molecule should substitute for two -CH2Cl moieties, indicate a yield of ∼9%. In the case of the complex 2 Rh-SAM, a 28% yield has been obtained, thus inferring a higher reaction yield, as expected on the basis of the different size of the anchoring groups (phenol vs aliphatic alcohol). Similar values have already been obtained for other similar monolayers.15a,d,e,g Finally, the nitrogen atomic concentration shows a nonnegligible increase upon decreasing the photoelectron takeoff angle from 0.7% (at 45°) to 1.0% (at 5°) (Table 1). The N 1s spectrum (Figure 9) shows a peak at 399.0 eV. This peak

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Figure 9. Monochromatized Al KR-excited XPS of the Rh-SAM of complex 1 on Si(100) substrate, in the N 1s energy region, at a 45° electron takeoff angle.

Figure 10. XPS atomic concentrations (IC/ISi) vs the electron angle of the Rh-SAM of complex 1 on Si(100).

accounts for the four nitrogen atoms of the p-tolylformamidinate anions.15c,18a,28 AR-XPS-derived atomic concentration analysis showed the expected atomic ratios (e.g., N/Rh ) 2.3 ( 0.3). Figure 10 shows the AR-XPS angular dependence of the IC/ISi intensity ratios (IC and ISi are the total intensities of carbon and silicon, respectively) vs the photoelectron angle for the complex 1 monolayer. The ratios exponentially decrease with the photoelectron angle θ, consistently with the presence of a carbonaceous overlayer on Si. The intensity of AR-XPS signals due to the silicon substrate covered by an overlayer of thickness d and that due to the overlayer itself are, respectively,18a

Figure 11. XPS atomic concentrations (ISiO2/ISi) vs the electron angle of the Si(100)/SiO2 cleaned substrate; R2 value ) 0.998. C C and λC1s are the mean free paths of Si 2p and C 1s where λSi2p photoelectrons in a carbonaceous overlayer. In principle, there could be an effect on the mean free paths depending on the density of the carbonaceous layer, even though such an effect should not be so large to affect the results. Therefore, we have C C and 3.6 nm for λC1s ) chosen to use values (4.15 nm for λSi2p already reported for a carbonaceous monolayer on silicon; a system very similar to our monolayer.25e,29 I∞Si and I∞C are the intensities of pure Si and C elements on the same instrument and setting. However, it is convenient to take the I∞C/I∞Si ratio as the ratio of the adopted Wagner sensitivity factors.18a Given this, eq 3 can be adopted to fit experimental data and provides an estimation of the thickness of the carbonaceous overlayer.18a,25e,29 The obtained d values of 18.7 and 14.8 ( 3 Å, for complex 1 or 2, respectively, are strongly consistent with the presence of a monolayer of complex 1 or 2 on the silicon surface. The R2 values of the fits are 0.995 and 0.994, respectively. On the basis of AR-XPS results, using eq 4 where nRh represents the number of Rh molecules/cm3 in the monolayer, σ is the photoelectron cross-section, λ is the inelastic mean free path, T(E) is the analyzer transmission function of the instrument, d is the monolayer thickness, and θ is the photoelectron takeoff angle, it is possible to estimate the nRh surface coverage with dirhodium molecules.19 By considering all of the analyzed takeoff angles, mean values of 3.2 × 1013 and 6.0 × 1013 molecules/cm2 have been obtained for complex 1 and 2, respectively.

IRh ISi

) nRh(atom/cm3) σRh λRh/monolayer T(ERh)(1 - edRh/monolayer/λRh/monolayer cos θ)

C

ISi ) I∞Si e-d/λSi2psenθ

(1)

nSi(atom/cm3) σSi λSi/Si T(ESi)(1 - edRh/monolayer/λSi/monolayer cos θ)

(4)

and C -d/λC1s senθ

IC ) I∞C (1 - e

)

(2)

Combining eqs 1 and 2, the IC/ISi intensity ratio is: C

I∞C (1 - e-d/λC1ssenθ) IC ) C ISi I∞Si e-d/λSi2psenθ

(3)

These values reproduce the trend of the Cl/Rh XPS ratios for monolayers of 1 and 2 and are really good once the absolute accuracy of the method ((30%)19a,b has been taken into account. Moreover, these surface coverage values for a monolayer of 1 or 2 are in tune with UV-vis results of 4.2 × 1013 and 7.7 × 1013 molecules/cm2 estimated using the Lambert-Beer model applied to monolayers.9i,30 The thickness of the SiO2 layer obtained on Si(100), after 10 min with UV and ozone (see the Experimental Details), estimated using eq 3, is 21 Å (Figure 11).

Investigations of Covalently Assembled Monolayers Conclusion The high resolution of the XPS instrument that we adopted for the present investigation is stated by the possibility to solve very well the Si 2p spin-orbit doublet. We obtained evidence of two equivalent Rh(II) centers, thus discriminating a mixedvalence dirhodium core. The N 1s binding energy univocally stated the presence of a form- anion, being the XPS sharp peak at 399.0 eV, 1 eV below the amine ionization energy. Moreover, the equivalence of the nitrogen atoms indicates that the negative charge of the form- anion is shared (resonates) between the two N of the N-C-N group. The presence of a band at 531.4 eV in the O 1s energy range evidenced the carboxylic -COOgroup. The deconvoluted C 1s envelope confirmed the -COOgroup (band at 288.4 eV) and the unreacted -CH2Cl moieties (band at 286.5 eV) of the functionalized substrate. The overall AR-XPS atomic concentration analysis allowed both thickness and molecule surface density estimation. Finally, the observed Cl/Rh ratio, which indicates a yield of ∼9% (surface coverage ) 3.2 × 1013 molecules/cm2) for the reaction between complex 1 and the CL-functionalized Si(100) substrate, points toward a “porous” monolayer that is reversibly permeable to gaseous molecules, thus rendering the system suited for a very fast response time to very small amounts of CO and that can be easily restored with N2. In summary, the XPS technique is very useful to study monolayers since it provides chemical information that is unique and not available with many other spectroscopic techniques. Acknowledgment. We thank NATO (SfP Project 981964) and the MUR, Roma, for financial support (PRIN 2005 and FIRB 2003). References and Notes (1) (a) Brusso, J. L.; Hirst, O. D.; Dadvand, A.; Ganesan, S.; Cicoira, F.; Robertson, C. M.; Oakley, R. T.; Rosei, F.; Perepichka, D. F. Chem. Mater. 2008, 20, 2484. (b) Arfaoui, I.; Bermu´dez, V.; Bottari, G.; De Nadai, C.; Jalkanen, J.-P.; Kajzar, F.; Leigh, D. A.; Lubomska, M.; Mendoza, S. M.; Niziol, J.; Rudolf, P.; Zerbetto, F. J. Phys. Chem. B 2006, 110, 7648. (c) Kurtikyan, T. S.; Ford, P. C. Angew. Chem., Int. Ed. 2006, 45, 492. (2) (a) Chen, J.; Frisbie, C. D.; Bates, F. S. J. Phys. Chem. C 2009, 113, 3903. (b) Marin, V.; Holder, E; Hoogenboom, R.; Schubert, U. S. Chem. Soc. ReV 2007, 36, 618. (c) Mitzi, D. B. J. Mater. Chem. 2004, 14, 2355. (d) Yamaguchi, T.; Kimura, T.; Matsuda, H.; Aida, T. Angew. Chem., Int. Ed. 2004, 43, 6350. (e) Kroeze, J. E.; Savenije, T. J.; Warman, J. M. AdV. Mater. 2002, 14, 1760. (3) (a) Mitsuishi, M.; Matsuiab, J.; Miyashita, T. J. Mater. Chem. 2009, 19, 325. (b) Talham, D. R.; Yamamoto, T.; Meisel, M. W. J. Phys.: Condens. Matter 2008, 20, 184006. (c) Qian, D.-J.; Nakamura, C.; Miyake, J. Chem. Commun. 2001, 2312. (4) (a) Shi, F. N.; Cunha-Silva, L.; Sa´ Ferreira, R. A.; Mafra, L.; Trindade, T.; Carlos, L. D.; Almeida Paz, F. A.; Rocha, J. J. Am. Chem. Soc. 2008, 130, 150. (b) Massue, J.; Quinn, S. J.; Gunnlaugsson, T. J. Am. Chem. Soc. 2008, 130, 6900. (c) Li, Y.; Yan, B. J. Solid State Chem. 2008, 181, 1032. (d) Yan, B.; Lu, H.-F. Inorg. Chem. 2008, 47, 5601. (e) Su, Y.; Li, L.; Li, G. Chem. Commun. 2008, 4004. (f) Qiao, X.; Yan, B. J. Phys. Chem. B 2008, 112, 14742. (g) Altman, M.; Zenkina, O.; Evmenenko, G.; Dutta, P.; van der Boom, M. E. J. Am. Chem. Soc. 2008, 130, 5040. (5) (a) Crivillers, N.; Mas-Torrent, M.; Perruchas, S.; Roques, N.; VidalGancedo, J.; Veciana, J.; Rovira, C.; Basabe-Desmonts, L.; Ravoo, B. J.; Crego-Calama, M.; Reinhoudt, D. N. Angew. Chem., Int. Ed. 2007, 46, 2215. (b) Masuda, Y.; Yamagishi, M.; Koumoto, K. Chem. Mater. 2007, 19, 1002. (c) Chaker, J. A.; Santilli, C. V.; Pulcinelli, S. H.; Dahmouche, K.; Briois, V.; Judeinstein, P. J. Mater. Chem. 2007, 17, 744. (d) Moloney, M. P.; Gun’ko, Y.; Kelly, J. M. Chem. Commun. 2007, 3900. (e) Ipe, B. I.; Yoosaf, K.; Thomas, K. G. J. Am. Chem. Soc. 2006, 128, 1907. (f) Altman, M.; Shukla, A. D.; Zubkov, T.; Evmenenko, G.; Dutta, P.; van der Boom, M. E. J. Am. Chem. Soc. 2006, 128, 7374. (g) Wang, L.; Yoon, M.-H.; Lu, G.; Yang, Y.; Facchetti, A.; Marks, T. J. Nat. Mater. 2006, 5, 893. (h) Chen, M.-S.; Dulcey, C. S.; Chrisey, L. A.; Dressick, W. J. AdV. Funct. Mater. 2006, 16, 774. (i) Ramos, G.; Belenguer, T.; Levy, D. J. Phys. Chem. 2006, 110, 24780. (j) Zhao, L.; Loy, D. A.; Shea, K. J. J. Am. Chem. Soc. 2006, 128, 14250.

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