Grafting of Cobaltic Protoporphyrin IX on Semiconductors toward

Article pubs.acs.org/JPCC

Grafting of Cobaltic Protoporphyrin IX on Semiconductors toward Sensing Devices: Vibrational and Electronic High-Resolution Electron Energy Loss Spectroscopy and X‑ray Photoelectron Spectroscopy Study A. M. Botelho do Rego,*,† A. M. Ferraria,† and M. Rei Vilar‡ †

Centro de Química-Física Molecular and IN, Instituto Superior Técnico, Universidade Técnica de Lisboa, P-1049-001 Lisboa, Portugal ‡ Université Paris Diderot, Sorbonne Paris Cité, ITODYS, UMR7086 CNRS, F-75205 Paris Cedex 13, France S Supporting Information *

ABSTRACT: Cobaltic protoporphyrin IX (CoPP) was grafted on GaAs(100), Si(111), and highly oriented pyrolytic graphite, HOPG(0001), surfaces using different preparation conditions. Modified surfaces were characterized by X-ray photoelectron spectroscopy (XPS) and high-resolution electron energy loss spectroscopy (HREELS) recorded in vibrational and electronic domains. XPS confirms the adsorption of the porphyrin on all of the substrates through its main fingerprint, the photoelectron Co 2p region. HREELS spectra also attest to the presence of CoPP on the substrates. The presence of oxides is also clearly detected. Analysis of the corresponding cross sections confirms that vibration excitations are mostly produced via an electron impact mechanism. Analysis of the electronic domain allowed observation of the Q, Soret, N, and C bands present in UV/vis spectra. Quantitative results for relative differential cross sections of vibrational and electronic excitations suggest that negative-ion resonances may play a role in the excitation.

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INTRODUCTION Porphyrins regulate many energy processes such as the cellular metabolism, the oxygenation of the human body, or even the photosynthesis. This family of molecules and their derivatives has strongly drawn the attention of chemists, physicists, and biologists during the second part of 21st century.1 Their application in different domains such as catalysis and photovoltaic processes and more recently as chemical recognizers is, from a technological point of view, very motivating. Recently, porphyrins were applied in chemical sensors based on FET-like transistors,2 where the gate is replaced by a porphyrin layer for chemical recognition. For this purpose, porphyrins were assembled on GaAs(100) to couple molecular and semiconductor properties, protecting, concomitantly, the semiconductor surface against oxidation or contamination.3 Control of the adsorption process and its characterization remains then a challenge, especially in the case of III−V semiconductors, such as GaAs, whose surfaces are chemically unstable.3,4 Chemical recognition operates when the surface interacts with the analyte. The target capture changes the electron distribution in the molecules, inducing a variation in the dipole moment and a consequent change in the electric field inside the underneath semiconductor surface. As a result, a variation in the electronic current across the device can be detected. Metal porphyrins have high affinity toward different gases such as nitric oxide, carbon monoxide, and oxygen or even water5 and can be used, as sensors, to detect them. To provide more © 2013 American Chemical Society

stability to the layer, porphyrins should have reactive functions, enabling their chemisorption to the surfaces. Carboxylic groups showed to be very efficient for this purpose, provided that the substrate surface is slightly oxidized by an oxygen sub- or monolayer).6,7 To bring together both specifications of containing a sensitive metallic center and groups to chemically bind to GaAs(100) without the need for intricate synthesis, cobaltic protoporphyrin IX chloride (CoPP) was chosen. Scheme 1 presents the molecular structure of CoPP. Dimensions of the CoPP molecule were obtained with the software HyperChem 6.0 using the molecular mechanics method MM+. Interfacial bonding investigated by IR spectroscopy showed that CoPP adsorbs on GaAs(100) through a chemical reaction between the carboxylic groups and terminal hydroxyl groups remaining in the oxides present at the semiconductor surface.5 The absence of bands corresponding to the stretching modes characteristic of carboxylic groups in the IR spectra after adsorption proves the existence of a chemisorbed monolayer. For comparison purposes, also a freshly peeled graphite (0001) surface was used because carboxylic groups do not react with it. It is expected that molecules adsorb through π−π interactions, Special Issue: Ron Naaman Festschrift Received: February 27, 2013 Revised: July 19, 2013 Published: July 20, 2013 22298

dx.doi.org/10.1021/jp402032x | J. Phys. Chem. C 2013, 117, 22298−22306

The Journal of Physical Chemistry C

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neither HREELS nor UV/vis absorption spectra of CoPP were ever published. Some important information has been found for metallic porphyrins in solution. Their optical electronic absorption spectra consist of a weak transition to the first excited state (S0 → S1) at about 550 nm (∼2 eV), the Q band, and a stronger transition to the second excited state (S0 → S2) at about 400 nm (∼3 eV), denoted as the Soret or B band. Both bands arise from π−π* transitions and can be explained by considering the “Gouterman four-orbital model”: two orbitals (a1u and a2u) and a degenerate pair of π* orbitals (egx and egy). The two highest occupied π orbitals happen to have about the same energy. One might imagine that this would lead to two almost coincident absorption bands because of a1u → eg and a2u → eg transitions, but, in fact, these two transitions mix together by a process known as configurational interaction, resulting in two bands with very different intensities and wavelengths: constructive interference leads to the intense short wavelength, the B or Soret band, while the weak long-wavelength Q band results from destructive combinations.15 Near the Soret band, another band at higher energy, called the N or variable band, usually assigned to charge transfer from the porphyrin ligand to the central metal atom upon excitation, appears.16 The change of the central atom oxidation state does not seem to substantially affect the general shape of the spectrum, but bands are shifted. As a matter of fact, many references concerning porphyrin electronic excited states exist mainly based on UV/vis absorption and emission spectroscopy. For instance, for cobalt complexes of meso-tetraphenylporphyrinates, a red shift occurs in all of the bands (B and Q) when CoII is replaced by CoIII and the Q band presents two sub-bands instead of the single one exhibited by the cobalt(II) complex.17 Electronic HREELS spectra of CoPP on GaAs(100) and on the other two substrates were interpreted based on the latter data.

Scheme 1. Molecular Structure of CoPP

between the molecule and highly oriented pyrolytic graphite, lying parallel to the graphite surface.8 Knowing the presence of CoPP on the surface, its amount and the molecule orientation on the surface is crucial for the preparation of sensitive surfaces. Reproducible and sensitive surface methods of analysis are required because the modified semiconductor surfaces only contain tiny amounts of molecular material. However, the study of molecular porphyrin monolayers or submonolayers adsorbed on substrates such as GaAs(100) cannot be performed with the traditional analytical tools used for solutions or thick films deposited on UV/vis transparent substrates. First, semiconductors are not normally transparent in the UV/vis and, second, because the amount of material contained in a monolayer or submonolayer is under the limits of detection of absorption UV/vis spectroscopy. Electrons are then excellent candidates to probe these layers (monolayers or even fractions) because they strongly interact with CoPP covalent electrons. Hence, X-ray photoelectron spectroscopy (XPS) and high-resolution electron energy loss spectroscopy (HREELS) are the most adequate techniques for characterization of the CoPP monolayers deposited on different substrates. Essentially, XPS can confirm the presence of porphyrin on the different substrates, and through its angleresolved mode, it can also allow estimation of the parameters of the layered structure substrate/oxide/CoPP. On the other hand, HREELS enables the study of vibrations and electronic excitations in the chemisorbed CoPP molecules on the GaAs(100) surfaces. It is here worth emphasizing that HREELS uses very low intensity electron beams, which allied to the low energy of the probes avoid degradation of the molecular system. Another important point to underline is the capability of HREELS to measure excitations in large ranges of energy from vibrations to the UV range without changing the source of excitation and/or the detector, making HREELS a technique of analysis unique for organic monolayers. Besides vibrations induced by electrons corresponding to either IR- or Ramanactive modes,9,10 other interesting features of HREELS related to electronic excitations of the layer and their thresholds could be detected, giving additional information.11 In this work, a systematic study of CoPP films grafted on different substrates, with an emphasis on GaAs(100), and prepared following different protocols is presented. Few studies exist in the literature using this same spectroscopy on related molecules physically adsorbed in a mono- or multilayer: zinc phthalocyanine (ZnPc) on silver,12 copper phthalocyanine (CuPc) on Au(100),13 and tin phthalocyanine (SnPc) on InSb.14 However, to our knowledge,

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EXPERIMENTAL SECTION Wafers of semi-insulating single-crystal GaAs undoped with orientation (100) ± 0.5° were acquired from different sources. Silicon (111) wafers, type p doped with bore, 525 ± 25 μm thick, and one face polished, were purchased from Siltronix (France). Highly oriented pyrolytic graphite (HOPG) ZYA grade was obtained from UCAR-Union Carbide Corp. Cobaltic protoporphyrin IX chloride (CoPP) was purchased from Sigma-Aldrich, acetone extra pure (puriss. >99%) and acetonitrile (ACN) for spectroscopy (purity >99.9%) were purchased from Merck, ethanol absolute (>99.8%) and hydrogen peroxide (>30%) were purchased from Riedel-de Haën, anhydrous N,N′-dimethylformamide (DMF; purity >99.8%) and sulfuric acid (99.999%) were purchased from Aldrich, and hydrofluoric acid (>40%, puriss. p.a.) was obtained from Fluka. All chemicals were used without further purification. Deionized water (DIW) with 18.2 MΩ·cm of resistivity was obtained from a Millipore system fed with distilled water. GaAs(100) substrates were degreased with acetone and ethanol, then etched with a 1% solution of hydrofluoric acid for 5 s, then rapidly rinsed, and dried under an argon flux. Silicon substrates were pretreated using the following protocol: they were first etched in an acidic peroxide solution of H2O2/H2SO4 (1:1) for 10 min followed by 10% HF for 30 s, more H2SO4 for 10 min, and finally again with 10% HF for 30 s. The substrates were rinsed with DIW for 5 s between each step and at the end. An additional rinsing in the CoPP solvent 22299



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Table 1. Samples Used with Different Preparation Parameters (the Main Changes Are Indicated in Boldface) A B C D E F G H I J

reference sample time effect stirring effect time and stirring effect solvent effect light effect concentration effect substrate and stirring effect substrate and stirring effect other conditions

time of interaction (h)

dynamic conditions

solvent

luminosity

concentration

substrate

18 3 18 0.3 18 18 18 18 18 4

stirred stirred static static stirred stirred stirred static static static

DMF DMF DMF DMF ACN + 2% DMF DMF DMF DMF DMF ACN + 10% H2O

dark dark dark dark dark daylight dark dark dark daylight

1 mM 1 mM 1 mM 1 mM 1 mM 1 mM 10 mM 1 mM 1 mM 0.08 M

GaAs GaAs GaAs GaAs GaAs GaAs GaAs HOPG Si GaAs

The UV/vis spectrum of CoPP was recorded from a 2 × 10−5 M CoPP solution in ACN with 10% water in a Hitachi U-2000 spectrophotometer at a scan speed of 400 nm min−1. All spectraXPS, HREELS, IR, and UV/viswere recorded at room temperature.

(DMF) for 5 s and drying under an argon flux preceded the interaction with the CoPP solution. The HOPG was peeled with an adhesive tape just before immersion in the CoPP solution. The preparation of samples of CoPP adsorbed on different substrates, using different conditions, is gathered in Table 1. Substrates and samples were always stocked and carried out under an argon atmosphere. The XPS spectrometer used was an XSAM800 (Kratos) operated in fixed analyzer transmission mode. Both Al Kα (hν = 1486.6 eV) and Mg Kα (hν = 1253.6 eV) radiation were used. Further details can be found elsewhere.3 In the conditions reported there, the full width at half-maximum (fwhm) for a reference peak used as calibration (Ag 3d5/2) is ∼1.0 eV. HREELS vibrational spectra were obtained using a LK Technologies ELS3000 described before.18 This new generation spectrometer can achieve a nominal resolution of 0.5 meV, which corresponds to the fwhm of the energy distribution of the electron beam directly transmitted from the monochromator to the analyzer. For a detector current of ≥10 pA, a resolution of 1.0 meV is guaranteed by the constructor. However, through the interaction of incident electrons with the sample surface, the elastically backscattered electron distribution is broadened and typical spectra present an elastic peak fwhm of around 7.5 meV. For the vibrational energy loss range, a specular geometry (equal incidence and analysis angles) was used because this configuration is fixed by the spectrometer. The nominal energy, En, is an experimental parameter given by eΔV, where e is the electron charge and ΔV the difference between the voltages applied to the sample holder and electronemitting filament, respectively. For the electronic domain recording, a LK Technologies 2000R with a nominal resolution of 5 meV was used with a typical fwhm of the backscattered elastic peak of 20−25 meV.19 Ei, the real incident energy of the incident electrons, can be measured by the energy extent of the global spectrum from the elastic peak maximum, where the loss is zero up to the cutoff of the spectrum, where the loss is maximal and, consequently, equal to Ei.20 Another method to determine Ei consists of lowering the voltage V of the sample to determine the value of V at which the electron intensity, measured on the sample holder, vanishes. This value corresponds to the difference between the real and nominal energies. Incidence and analysis angles for spectra in the electronic energy loss range were in off-specular geometry, enabling analysis of the impact interactions and, consequently, revealing the composition of the layer surface region at a depth of, approximately, 1 nm. Spectra were recorded under ∼10−8 Pa.

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RESULTS AND DISCUSSION XPS Characterization. The survey spectrum of an etched GaAs surface grafted with CoPP (C34H32CoN4O4Cl), as a function of the binding energy (BE), presents a multitude of peaks assigned to both the porphyrin and substrate. In Figure 1a, only the more relevant features are indicated. Features that are not identified in Figure 1a are Auger structures and/or other less intense photoelectron peaks. The presence of CoPP molecules adsorbed on the different substrates is undoubtedly attested by the XPS Co 2p and N 1s regions. Besides these heteroatoms, the free porphyrin used in

Figure 1. (a) XPS survey spectrum and XPS regions (b) Co 2p and (c) N 1s of, from bottom to top, CoPP/Si (black thin line), CoPP/ HOPG (black thick line), and CoPP/GaAs (gray thick line). Spectra were obtained with Al Kα and/or Mg Kα and were smoothed for the sake of clarity. 22300



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global description of the extreme surface of a sample with a depth of analysis inferior to the nanometer. To distinguish the information contained in the HREELS spectra and, consequently, the depth from where it comes from, the nature of the interactions leading to the different characteristic peaks of the spectrum needs to be established. For conducting or semiconducting crystalline surfaces, atomically flat, this information can be extracted from geometrical considerations because electrons suffering dipolar interaction are backscattered in a narrow lobe of a few degrees centered in the specular direction (incident and backscattering angles are equal). However, for surfaces with some roughness or in molecular films, these lobes, containing backscattered electrons, become too wide to apply this geometrical analysis. In this case, an analytical analysis of cross sections as a function of the incident electron energy, Ei, allows distinction of the nature of the excitations involved. Actually, for a dipolar mechanism, differential cross sections are a function of 1/Ei or 1/√Ei, whereas for an impact mechanism, cross sections increase linearly with Ei and, for a resonance mechanism, they are bellshaped for a relatively narrow energy range (around 2 eV wide). This analysis is necessary here and will be performed both in the vibrational and in the electronic excitation energy range. In Figure 2, different vibrational spectra of CoPP chemisorbed on GaAs(100) prepared from a 10−3 M solution

this study also contains chloride as a counterion, but no measurable amount of chloride was detected with XPS. Parts b and c of Figure 1 show the Co 2p doublet and N 1s of CoPP adsorbed on different substrates: GaAs, HOPG, and Si. The doublet has a spin−orbit separation of ∼15.3 eV, with Co 2p3/2 centered at 780.2 ± 0.2 eV. The N is centered at 398.7 ± 0.2 eV. Semiconductors, and particularly GaAs, have very reactive surfaces that oxidize very rapidly in contact with water and air, and even light exposure has a role on the modification of the surface state.4 XPS was used not only to attest the presence of the porphyrin adsorbed on the surface but also to characterize the effect of porphyrin solutions on the very unstable GaAs substrate. XPS shows that the experimental conditions of interaction affect the degree of oxidation of the substrate. The estimated values, presented in Table 2, were obtained by fitting Table 2. Parameters of the CoPP Layer on GaAs for Several Preparation Conditions Extracted from ARXPS Assuming a Schematic Model Described in the Supporting Information

A C D G

18 h, 1 mM, stirred 18 h, 1 mM, static 17 min, 1 mM, static 18 h, 10 mM, stirred

CoPP thickness (nm)

oxide thickness (nm)

covered fraction

nC/ nCo

1.6

1.4

0.8

34

1.5

1.0

0.7

34

1.3

0.4

1

40

1.4

1.5

0.8

37

a very schematic island-like model to experimental atomic ratios C/Ganonoxidized, Co/Ganonoxidized, and Gaoxidized/Ganonoxidized of a molecular layer of CoPP covering partially (or totally) a flat oxidized substrate, similar to the ones presented elsewhere,21 but with an additional oxide interlayer between the porphyrins and substrate. Shadow effects were neglected. Values in Table 2 should be taken as trends more than as absolute values, given the simplicity of the model (for details, please read the Supporting Information). Anyway, several conclusions can be extracted: (i) The extent of oxidation was larger in sample A, but none of the samples is exempt of oxide (immediately after etching and before any surface interaction, the thickness of the oxide layer is still 0.1−0.2 nm). (ii) Every substrate surface is covered by a monolayer of CoPP because the thickness is around 1.6 nm, the linear dimensions of the CoPP molecule (see Scheme 1). (iii) In the samples prepared with longer interaction times (samples A, C, and G), the layer does not completely cover the substrate. (iv) An excess of carbon, most probably due to some remaining solvent trapped in the organic layer, is present in the sample prepared with a short interaction time (sample D) and in the one prepared with a more concentrated solution (sample G) (the GaAs carbonaceous contamination, before interaction, is substantially removed by degreasing and etching pretreatments). HREELS Studies. Vibrational Domain. In HREELS, the depth of analysis depends on the mechanism of the incident electron interaction with the surface.22 Actually, dipole interactions can be held at distances higher than 10 nm. In contrast, an impact mechanism (including resonance via negative ions) always occurs at distances lower than 1 nm, involving electron penetration and escape in the surface region of the film. This means that impact interactions can give a

Figure 2. HREELS spectra of a sample of CoPP chemisorbed on GaAs(100) prepared from a 10−3 M solution in pure DMF for 18 h under stirring (sample A), recorded in the vibrational domain at different electron energies: from bottom to top, 2, 2.5, 3, 4, 5, 6, and 7 eV in specular conditions (θi = θr = 60°). All spectra are normalized to the PE intensity, and vertical offsets were included for better visualization. For comparison, the FTIRS spectrum of CoPP in a KBr pellet is presented on the top.

in pure DMF for 18 h of interaction under stirring (sample A) recorded at different incident electron energies are shown with the main features labeled. For comparison purposes, an IR spectrum of CoPP in a KBr pellet was added to the set. The different features appearing in the spectra of Figure 2 can be assigned by comparing their positions with those found in the IR spectra.23−26 The well-known GaAs(100) surface phonon is clearly observed, located at 282 cm−1 with a double scattering at around 560 cm−1. The corresponding gain of energy due to the existence of excited phonons at the surface of 22301



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the GaAs at −282 cm−1 is also noticeable.4 Different losses due to vibrational modes of CoPP, also existing in the IR spectrum, are identified in Table 3. More specifically, those corresponding

incident electron energy of excitation between 2 and 7 eV. The ratio Iloss/IPE is proportional to the differential cross sections of the electron-induced excitations. Iloss was evaluated by fitting the HREELS spectra with a sum of Gaussian curves with constant fwhm, equivalent to that of the elastic peak and positions given by the IR spectra of CoPP (see Table 3) and the study mentioned above on the GaAs oxides.4 Correlation factors were better than 0.99. Contrary to all other losses, the GaAs(100) surface phonon at 282 cm−1 is clearly excited through a dipolar interaction because the cross section, after a rapid initial increase, decreases with energy. All other cross sections, corresponding essentially to the different molecular vibrations of the porphyrin, increase with the incident electron energy, revealing a significant impact character of their excitation mechanism. This being so, electrons are mostly backscattered from the first angstroms of the extreme surface. From the figure, one can conclude that CH stretching vibrations exhibit an unambiguous impact mechanism. The remaining losses, corresponding to the porphyrin ring vibrations, show hybrid mechanisms where the impact interaction is not so evident, as in the case of the CH stretching modes. This behavior has been reported already in previous HREELS studies, namely, that referred to above, on polystyrene ultrathin films.11 Furthermore, these cross sections seem also to be modulated by two Gaussian-like features centered around 3 and 5 eV, suggesting additional excitations via negative-ion creation. These effects appear to be less important in the cross sections associated with the CH stretching losses. HREELS analysis, combined with XPS results, shows a real surface mostly covered by the porphyrin, the oxides of As and Ga, and some residual contamination. This characterization is therefore the way to optimize the functionalization of the GaAs surface by selecting appropriate parameters for the sample preparation. A set of HREELS spectra of samples of CoPP on GaAs prepared under different conditions is presented in Figure 4. All spectra were recorded in specular geometry (θi = θr = 60°) using 5 eV incident electrons. For assignment purposes, an IR spectrum of CoPP in a KBr pellet was added to the set. The different preparation parameters are the time of interaction (3 and 18 h), the stirring conditions, the concentration of the CoPP solution, the nature of the solvent used (ACN or DMF), and the exposure to daylight (see Table 1). The GaAs surface is systematically perceived as all spectra exhibit characteristic losses and gains of energy corresponding to the surface phonon at +282 and −282 cm−1, respectively.28 Most peaks present in the IR spectrum of CoPP in KBr are present in the HREELS spectra of all samples. However, in some of them, the presence of oxides is confirmed by two features at 805 and 1080 cm−1, which are particularly intense for samples A and C prepared with a 1 mM solution in DMF interacting with the substrate for 18 h. The figure also shows that in HREELS spectra the deformation of the pyrrolic cycle is intense. However, no band at 1713 cm−1 corresponding to CO is present, whatever the preparation method. This means that in all cases molecules are grafted onto the GaAs(100) surface, which prevents the CoPP molecules from laying parallel to the substrate, tending toward a perpendicular orientation. For the other preparations, these peaks seem to be much less important, showing that substrate oxides are preferentially covered by the porphyrin. The effect of the light was tested (sample F) and makes CoPP chemisorption easier. To test the effect of the

Table 3. Assignments of Features Appearing in the HREELS Spectrum of CoPP Chemisorbed on GaAs(100)23−26 wavenumber (cm−1) 282 694 805 1080 1244 1429 2950 3056

assignments surface phonon of GaAs(100) pyrrolic cycle in-plane bending + GaO stretching pyrrolic cycle in-plane bending + As−O bending in As2O3 ring bending (1087 cm−1) + As−O stretching in As2O3 (at 1060 cm−1) ring “breathing” CH3 bending asym CH3 stretching aromatic and vinyl C−H stretching

to the porphyrin ring located between 600 and 1600 cm−1 and those of the stretching modes of the aliphatic and aromatic CH around 3000 cm−1 are given.27 Spectra also show that the IRactive band located at 1713 cm−1 is not present in the HREELS spectra. This corroborates the fact that molecules chemisorb through the carboxylic groups, staying with their plane nearly perpendicular to the substrate. However, some contribution corresponding to oxides, namely, around 805 and 1060 cm−1 (broadening the porphyrin band located at 1080 cm−1), can also be observed in these spectra. Their relative importance changes with the incident energy because of different penetrations of the electron beam in the sample. Additionally, one can detect a slight aliphatic contamination at 1465 and 2850 cm−1. Figure 3 shows a log−log representation of the intensity of several electron energy losses normalized to the PE intensty (Iloss/IPE) corresponding to typical vibrations appearing in the HREELS spectra of CoPP/GaAs(100) (sample A) versus the

Figure 3. Electron energy loss intensity normalized to the elastic peak of different vibrations appearing in the HREELS spectra of CoPP/ GaAs(100), in sample A, versus the incident electron energy of excitation between 2 and 7 eV. Values in the legend are given in cm−1. 22302



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Figure 4. Comparison of HREELS spectra in the vibrational domain of samples of CoPP adsorbed on GaAs prepared under different conditions recorded with 5 eV incident electrons in specular geometry (θi = θr = 60°). Sample preparation details are presented in Table 1. A vertical setoff is used for clarity sake. All spectra are normalized to the elastic peak intensity, and vertical offsets were included for better visualization. For comparison, the FTIRS spectrum of CoPP in a KBr pellet is presented at the bottom. The HREELS spectrum of sample J was recorded at the same geometry and nominal energy as the rest of them but in a different spectrometer with lower resolution and was, therefore, normalized to the background.

Figure 5. HREELS electronic spectra of CoPP/GaAs (sample J) recorded at different incident electron energies (from bottom to top: from 2.5 to 5.5 by steps of 0.5 eV and from 6.5 to 9.5 by steps of 1 eV) in a nonspecular geometry (incident angle = 60° and analysis angle = 30°) compared with a UV/vis absorption spectrum of a CoPP diluted solution (in red). All spectra are normalized to the elastic peak.

background intensity set to 1. For comparison and assignment purposes, an UV/vis absorption spectrum of a diluted solution is included in the figure. The UV/vis absorption spectrum of CoPP shows that the excitation at lower energy is a Q band displaying two sub-bands at 2.19 and 2.33 eV, confirming that the central cobalt is present as CoIII. The Soret or B band appears at 2.95 eV, the N band at 3.51 eV, and the M band doublet at 4.5 eV, as a shoulder. At higher energy, another band arises at 5.39 eV. Former studies on cobalt phthalocyanine (CoPc) under the form of evaporated films29 also revealed in the visible absorption spectra a Q band located at 1.8 eV, showing a characteristic splitting of 0.23 eV. In the UV region, at least three absorption bands were identified: the Soret band and variable N and C bands, centered at 3.85, 4.39, and 5.92 eV, respectively. The strong absorption N peak is assigned by the authors to π → d transitions because the central atom has partially occupied d orbitals, whereas the C band is assigned to d → π* transitions. The electronic part of the spectrum unravels irrefutably the presence of the porphyrin on the surface. Actually, in the HREELS spectra presented in Figure 5, a band is observed at the same position where the Q bands appear in the UV/vis spectrum. One can find first a narrow peak located at 2.19 eV, revealing that incident electrons with 2.6 eV can relax by transferring almost all of the energy through the excitation of the electronic state at 2.19 eV. This is corroborated by Fink et al.,13 who observed no variation in the energy position and splitting in the electronic excitations of the Q bands for CuPc/ Au(100) relative to the solution. For higher incident energies, the whole structure is clearly present in all of the HREELS spectra, diminishing its intensity as the incident energy increases and other relaxation channels are opened. The Soret and N bands seem to have relative intensities much closer than those found in the UV/vis spectrum. This leads to a unique band located between both and centered at 3.3 eV. The excitation in the optical spectrum at 5.39 eV is also undoubtedly present in HREELS spectra. Nevertheless, in the

concentration, a stirred interaction for 18 h was used with 10−3 and 10−2 M porphyrin concentrations: samples A and G, respectively. For the most concentrated sample, a HREELS spectrum typical of porphyrin is obtained, whereas for the less concentrated sample, a lot of oxide (especially arsenic oxide) was present, as discussed above. Also, studying the effect of the interaction time, with samples A and B having interacted respectively for 18 and 3 h with the porphyrin solution, one can notice that sample B exhibits no oxides or, at least, much fewer oxides than sample A. In summary, only two of the samples, A and C, do not present a total coverage of the whole GaAs substrate surface with CoPP molecules. One can also conclude that CoPP layers are resistant to light exposure. However, for long times of interaction and low porphyrin concentration (∼10−3 M), a higher oxidation of the substrate is noticed, independently of the solution being stirred or not. These observations are compatible with XPS data in Table 2. There also samples A and C show a thicker oxide layer and lower substrate coverage, justifying the large contribution of the oxide modes for the vibrational spectrum. Electronic Domain. In the vibrational domain, the presence of CoPP in the HREELS spectra is always partly masked by the oxide contribution, the modes of which strongly appear at 800 and 1080 cm−1. In contrast, in the electronic domain investigated here between 1 and 9 eV, no contribution of oxides exists and only losses due to electronic excitations of the porphyrin can be observed. To attest to this sentence, a spectrum of uncovered GaAs with its native oxides was recorded to compare with spectra for the CoPP-covered samples. This enabled us to check the presence of grafted molecules of CoPP even in samples prepared at concentrations lower than 10−3 M. Figure 5 shows a set of HREELS spectra obtained with incident electron energies ranging from 1 to 9 eV on sample J. All spectra are here normalized to the average 22303



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interval between 3.3 and 5.39 eV, where a big valley appears in the optical spectrum, having a minimum around 4 eV, a continuum energy loss spectrum is obtained for all of the incident energies. This continuum may be due to one (or all) of several reasons: (i) the existence of a quasi-continuum of delocalized π states (given the large number of delocalized orbitals in the porphyrin molecule) not excitable by optical radiation but excitable by electrons because of the breaking of the symmetry and/or spin selection rules in the electron energy loss spectroscopy; (ii) a solid state effect; (iii) multiple losses; combinations and losses due to molecular complexes; (iv) secondary emission. Relative intensity for HREELS Q and Soret + N bands appearing in spectra of Figure 5 are presented, as a function of the real energy Er of the incident electrons, in Figure 6. Given the complexity of the spectra, only maxima of the main spectral bands were taken into account without any fitting procedure.

tance compared to the error bars associated with the data, we cannot be very assertive about that. To better understand the origin of the intensity of the spectrum for energy losses higher than 3.5 eV, some of the samples characterized by vibrational HREELS and XPS were compared through the electronic HREELS spectra. All spectra were recorded at the same nominal energy (5 eV) for the same geometry with angles of incidence and analysis relative to the normal to the surface of 60 and 30°, respectively. The effect of the sample preparation parameters, such as the porphyrin concentration, the role of stirring, the interaction time, and the substrate nature, on the surface final state is shown in Figure 7.

Figure 6. Relative intensity (normalized to the elastic peak) for electronic excitations at 2.18 (full squares), 2.31 (empty circles), and 3.25 eV (gray triangles) as a function of the incident electron energy, Ei.

Figure 7. HREELS spectra in the energy loss region corresponding to electronic excitations for the GaAs substrate, HOPG, samples A, C, D, and G−J, and the zoom of sample H. All of the spectra were normalized to the same elastic peak area. The nominal energy was the same for all of the spectra.

There is a common characteristic to all of the spectra: all of them present the same threshold for the Q and Soret + N bands, attesting to the existence at the extreme surface of the porphyrin molecules. The exception is sample H, where CoPP was deposited on a freshly peeled HOPG(0001). Regarding the previous assignment, Figure 7 is very elucidating. Actually, because of the π−π*interaction, CoPP molecules are expected to lie horizontally on graphite and have a huge disturbance of their π system largely affecting the Q and Soret bands. The same effect, but even stronger, was verified for a ZnPc monolayer on a Ag(110) surface, where the Q and Soret bands completely disappear.12 However, it was concluded later32 that the effect was due to the following: (i) The lowest unoccupied molecular orbital was partially filled because of molecule− substrate interaction, an effect that was later demonstrated by photoemission. (ii) There are not enough molecules on the surface, i.e., monolayer coverage. On HOPG, the Q bands are hardly perceived and the Soret + N band has a very low

Figure 6 shows that both Q bands display very similar dependences on Ei, with the first having a threshold energy at 1.9 eV, differing from the second one of ca. 0.1 eV, as expected. The Soret + N band presents a threshold at about 3 eV, which is in good agreement with the value in the optical spectrum. Until Ei = 4.5 eV, the trend of the dependence of all of the curves is typical of a resonant mechanism via a negative-ion formation, as was found in former studies, namely, in polystyrene.30 Relative intensities corresponding to the Q band (2.18 and 2.31 eV) have a maximum at around 3 eV, which also may correspond to a resonant structure already found in the vibrational spectra. Interactions via negative-ion resonances,31 being a special case of impact, also mean that electrons do penetrate some angstroms into the surface region of the sample. For incident energies higher than 4.5 eV, relative intensities versus Ei are compatible with the existence of other resonant-like structures. However, given their relative impor22304



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the extreme surface with a depth inferior to the nanometer. The interaction between the solution and substrate was performed using different conditions of concentration, time, stirring, and light. All of the preparation protocols led to grafting of the porphyrin onto the substrates, as was detected either by XPS or by HREELS. Using a simple model to describe the coverage of the substrate, analysis of XPS data shows a thickness of the CoPP layer on the order of the porphyrin dimensions, which corresponds to the formation of a submonolayer or, at most, one monolayer. In the case of the semiconductors, the oxide layer is variable and its thickness can be kept lower when adsorption times are shorter, stirring is not used, or the CoPP concentration is increased. This also leads to higher covered fractions. In good agreement with these latter results, vibrational HREELS showed that all surfaces studied here contain CoPP molecules and oxides in different proportions, depending on the method of preparation. Electron energy losses corresponding to the vibrational modes of the porphyrin were detected, and the corresponding differential cross sections were analyzed. By energy analysis, one could conclude that impact is the major mechanism. Therefore, the depth of HREELS analysis was concluded to be less than the nanometer. Strong vibrational modes of CoPP existing in the IR spectrum could be found in the HREELS spectra, except those corresponding to the carboxylic vibrations. This conclusion corroborates a previous study of the same systems by FTIRS in the ATR/MIR mode, proving grafting of the molecules through a chemical reaction between the carboxylic acids and semiconductor surfaces. HREELS spectra of CoPP on HOPG are not enough resolved to attest to the grafting mode of the molecules, but they should be mostly aggregated to the surface through π−π interactions between the porphyrin and graphite. On the other side, the electronic domain of HREELS spectra shows undoubtedly excitation of the different bands observed in the UV/vis absorption spectrum of CoPP: the Q, Soret, N, and C bands. One should emphasize that using both vibrational and electronic domains of HREELS was very helpful in this study. Actually, even for samples where the presence of porphyrin was not very clear in the vibrational domain, because CoPP features were veiled by the oxide ones (for instance, samples A and C), the electronic range undoubtedly unraveled the presence of the porphyrin at the extreme surface of the sample. XPS and HREELS clearly show that the porphyrin molecules cover the semiconductor surface and are more evenly distributed when the interaction is static and more aggregated when the interaction is kept under stirring.

intensity (its clear identification required a zoom with a factor of around 10). On the contrary, the large band that follows that threshold, not observed in the optical spectrum, is clearly favored when the substrate is HOPG, suggesting that the interactions between the CoPP and graphite π systems and between the Co 3d electrons and the graphite π system lead to a split of energies being responsible for enlargement of the bands related to π → d and d → π* transitions, giving rise to a single large band. The presence of the porphyrin on the HOPG substrate was well attested by Co 2p and the N 1s XPS spectra presented above. Also spectra for the naked substrates GaAs and HOPG are shown, and they exhibit very different shapes and intensities compared to the same substrates covered with porphyrin molecules. The large band following the Soret + N threshold is also present in other samples prepared on other substrates: silicon and GaAs presenting different oxidation degrees. This fact excludes the assignment of this band to a possible secondary emission from the GaAs or GaAs oxides. Therefore, in the samples where the CoPP molecules aggregate, a strong interaction between the π systems of different molecules should exist, having the same effect as the interaction between the molecule and graphite π system. The presence of this band in the electronic spectra of CoPP on semiconductor substrates is, therefore, related to molecular aggregation. When the results obtained with vibrational HREELS are crossed with the ones obtained in XPS, the following interpretations may be drawn: samples A and C, very similar in both vibrational HREELS and XPS, display, in the electronic HREELS domain, a very different behavior. This is here attributed to the fact that molecules in sample A are aggregated under the form of dense islands on a thick, oxide layer. Such a surface is necessarily rough, which decreases a lot the elastic peak, enhancing the normalized intensity of losses. In contrast, sample C shows by XPS that the CoPP layer has an average density similar to that of sample A. However, molecules seem to be more evenly distributed on an equally oxidized substrate. Therefore, the effect of lateral interactions among molecules is much smaller, and after the Soret + N threshold, a clear decrease of the intensity for larger energy losses is observed. Accompanying this large band, also other effects on Q bands exist: for lower and, especially, for higher energy losses, replicas of Q bands, due to Davydov splitting,33 appear. This is clear proof of strong aggregation. An orientation effect may also be responsible, at least partly, for the shape of this band. However, in light of the existing electron−surface interaction theories, it is not easy to rationalize in detail all of the spectral characteristics. Samples D and G seem to have an intermediate behavior between A and C. Sample I behavior, which was prepared in the same conditions as those of A, was similar, showing that the nature of the semiconducting substrate does not play a major role in the structure of the CoPP layer.

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ASSOCIATED CONTENT

S Supporting Information *

Description of the schematic model used to extract parameters of the CoPP layer on GaAs from angle-resolved XPS. This material is available free of charge via the Internet at http:// pubs.acs.org.

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CONCLUSIONS Monolayers of CoPP have been created on GaAs(100), Si(111), and HOPG(0001) after interaction of the substrate with a liquid solution, using different solvents containing the porphyrin molecules. These modified surfaces were studied by XPS and HREELS, with both techniques having different analysis depths: XPS probes about 10 nm, whereas HREELS in the impact regime, which is the case, is much more sensitive to

AUTHOR INFORMATION

Corresponding Author

*Tel.: 351 21 8419255. Fax: 351 21 8464455. E-mail: amrego@ ist.utl.pt. Notes

The authors declare no competing financial interest. 22305



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ACKNOWLEDGMENTS This work was part of the Growth Project GRD2-2000-30012 SENTIMATS financed by EU. We also acknowledge the “Financiamento Plurianual” from FCT (Portugal) and the project GRICES/Ambassade Française in Portugal. We thank Dr. Yvette Jugnet from the Institut de Recherches sur la Catalyse (IRC-CNRS), Villeurbanne, France, for recording HREELS vibrational spectra. Finally, we thank Prof. Ron Naaman for inspiring this work.

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