1310
J. Phys. Chem. B 2007, 111, 1310-1317
Grafting and Polymer Formation on Silicon from Unsaturated Grignards: II. Aliphatic Precursors S. Fellah,† A. Amiar,†,‡ F. Ozanam,† J.-N. Chazalviel,*,† J. Vigneron,§ A. Etcheberry,§ and M. Stchakovsky| Physique de la Matie` re Condense´ e, EÄ cole Polytechnique, CNRS, 91128 Palaiseau, France, Laboratoire d’EÄ lectrochimie et de Traitements de Surface, UniVersite´ Ibn-Tofail, Ke´ nitra, Morocco, Institut LaVoisier, UniVersite´ de Versailles St-Quentin-en-YVelines, 45 aV. des EÄ tats-Unis, 78000 Versailles, France, and Horiba Jobin-YVon, Z.I. de la Vigne aux Loups, 5 aV. Arago, 91380 Chilly-Mazarin, France ReceiVed: May 29, 2006; In Final Form: October 27, 2006
Anodic decomposition of a vinylmagnesium halide or an ethynylmagnesium halide at a surface-hydrogenated silicon electrode leads to the formation of polymeric layers covalently anchored to the silicon surface. These layers have been characterized using spectroellipsometry and photoluminescence, infrared, and X-ray photoelectron spectroscopy. In the case of vinyl precursors, it appears that the multiple bonds are largely broken in the process. In the case of ethynyl, the layer formation rate is much higher for the chloride than for the bromide. The obtained polymer appears as a saturated skeleton bearing halide and unsaturated ethynyl groups. Furthermore, it appears that the solvent may be attacked by the ethynyl radicals leading to contamination of the polymer by solvent fragments, an effect that can largely be avoided by using appropriate solvents. The reaction pathways are discussed.
Introduction
Experimental Section
Organic functionalization of the silicon surface has been a very active field in recent years.1-16 Covalent anchoring of organic species to the silicon surface through a direct Si-C bond has been especially looked for because the obtained surfaces exhibit superior properties in terms of electronic passivation and chemical stability in a wet environment.3-16 Among other methods leading to such anchoring, hydrosilylation of unsaturated compounds on a hydrogenated silicon surface has been especially popular.2-11 However, anodic decomposition of a Grignard reagent at a surface-hydrogenated silicon electrode has been shown to provide a faster route to obtaining similarly bound alkyl layers at a Si surface.12-16 The mechanism involves the formation of organic radicals, the abstraction of hydrogen atoms from the Si surface, and the reaction of additional organic species with the dangling Si bonds formed.14 Upon pursuing the anodization, in the case of alkyl Grignards the grafting is self-limited to a monolayer because the newly generated alkyl radicals are unable to abstract a hydrogen atom from other alkyl chains. However, in the case of unsaturated Grignards it has been found that no such self-limitation is present because the first grafted monolayer may be attacked by new radicals, generally leading to the formation of a polymeric layer.15 In a previous article, we have examined the case of aryl Grignards in some detail.16 We report here on a similar study performed for unsaturated aliphatic Grignards, namely, vinylmagnesium and ethynylmagnesium halides. Our primary focus will be on the characterization of the formed polymers and the understanding of the reaction mechanisms.
The (111), p-type, FZ silicon samples were prepared as described in our previous article,16 either in the shape of simple platelets for the ellipsometric, XPS, and photoluminescence investigations or in the shape of 45°-bevelled ATR prisms for the IR investigations. The surfaces were prepared atomically flat and hydrogenated through classical NH4F treatment.17 In some cases, the oxide was simply removed in HF instead of NH4F, which leads to hydrogenated surfaces with some atomic roughness. The Grignard reagents were purchased from Aldrich and used as supplied (1 M vinylmagnesium bromide in tetrahydrofuran (THF), 1.6 M vinylmagnesium chloride in THF, 0.5 M ethynylmagnesium bromide in THF, 0.5 M ethynylmagnesium chloride in THF). In some cases, a solvent substitution from THF to benzene or dichlorobenzene (DCB) was made. This substitution was carried out by adding benzene or DCB to the Grignard solution and then reconcentrating it by evaporating THF using a vacuum pump. This procedure was repeated twice in order to ensure a good substitution yield. The surface modification was carried out in a nitrogen-purged glove box. All of the experimental details together with information on the spectroellipsometer, the IR, photoluminescence, and XPS spectrometers can be found in our previous article.16
* Corresponding author. Tel: +33-1-69 33 46 63. Fax: +33-1-69 33 30 04. E-mail:
[email protected]. † CNRS-Ecole Polytechnique. ‡ Universite ´ Ibn-Tofail. § Universite ´ de Versailles Saint-Quentin-en-Yvelines. | Horiba Jobin-Yvon.
Results Spectroellipsometry and Photoluminescence. Silicon surfaces modified by anodic treatment in a vinylmagnesium halide or ethynylmagnesium chloride electrolyte for a few minutes at current densities on the order of 1 mA/cm2 come out covered with a layer visible to the naked eye (from light brown to blue or green). However, similar treatments in ethynylmagnesium bromide electrolyte do not lead to a significant change in the aspect of the surface. A more quantitative characterization of
10.1021/jp063291s CCC: $37.00 © 2007 American Chemical Society Published on Web 01/25/2007
Grafting and Polymer Formation on Silicon
J. Phys. Chem. B, Vol. 111, No. 6, 2007 1311
Figure 1. Analysis of a (111) Si surface modified with ethynylmagnesium chloride in DCB (0.5 mA/cm2, 15 min) using spectroellipsometry. (a) Raw data (lines) with fits (points). (b) Model refractive index as a function of photon energy.
the optical properties of these films in the visible range was obtained through spectroscopic ellipsometry and photoluminescence. Figure 1 shows typical spectroellipsometric data obtained on a layer made from ethynylmagnesium chloride in DCB. This layer is especially thick, which has allowed us to perform a reliable determination of the dielectric function using a TaucLorenz fit (Figure 1b).18 The fit was somewhat improved by adding an outer layer with a porosity gradient, mimicking surface roughness. The thickness of this extra layer is remarkably small, an indication of the good homogeneity and overall quality of the layer. Comparable dielectric functions were obtained for the various layers obtained from the different precursors. However, no meaningful data could be obtained from surfaces treated with ethynylmagnesium bromide. Table 1 summarizes the results obtained for some representative samples. From these data, one can conclude that the thickness of the layers increases with the Faradaic charge, an expected result indeed, but for the case of ethynyl, it also is affected by the type of halide (chloride > bromide) and by the type of solvent (DCB > THF). In our previous work,16 we have reported that polymers formed from aryl precursors exhibit strong photoluminescence. In contrast to this behavior, those formed from vinyl precursors are weakly photoluminescent. Their luminescence extends through the visible range, with a maximum in the blue region (Figure 2a). The polymers formed from ethynyl precursors in DCB are significantly more luminescent. The spectrum is again quite broad but exhibits a maximum at around 650 nm (Figure
Figure 2. Typical photoluminescence spectra of Si surfaces modified (a) with vinylmagnesium bromide in THF and (b) with ethynylmagnesium chloride in THF and in DCB. (c) Same as b for thicker layers (prepared in DCB). The inset schematizes the interference effects responsible for the modulation superimposed on the PL curves. Excitation light 350 nm, ∼0.1 mW/mm2.
2b). For the layers having a thickness greater than ∼2 000 Å, a small sine wave modulation appears on the spectrum, which can be understood as an effect of multiple reflections and interferences within the layer (Figure 2c; the modulation is actually periodic in 1/λ, with a period of n/2d, where λ is the vacuum wavelength, d is the layer thickness, and n is the refractive index of the material). Finally, the spectrum appears to be sensitive to the solvent used for polymer formation, with the materials obtained in THF exhibiting a luminescence much weaker and somewhat blue-shifted compared to those obtained in DCB (Figure 2b). In either case, we have verified that the photoluminescence from the solvent is weak under the conditions of excitation used here so that the major features in these spectra cannot be attributed to residual amounts of solvent in the dried polymer films.19
TABLE 1: Thickness of the Polymeric Layer Obtained as Deduced from Spectroellipsometry According to the Grignard Used, the Current Density J, and the Duration of Applied Polarization Grignard used
J (mA/cm2)
duration of polarization, min
thickness of the polymeric layer, Å
color of the polymeric layer
vinylmagnesium bromide in THF ethynylmagnesium bromide in THF ethynylmagnesium chloride in THF ethynylmagnesium chloride in dichlorobenzene ethynylmagnesium chloride in dichlorobenzene
0.5 0.5 0.1 0.1 0.5
23 15 15 15 15
600 not measurable 240 1500 8700
brown no very light brown blue green
1312 J. Phys. Chem. B, Vol. 111, No. 6, 2007
Figure 3. Photoluminescence of a Si surface modified with ethynylmagnesium chloride in DCB (polymer layer is 0.6 µm thick). (a) Emission spectra for different excitation wavelengths (the various visible lines of a Kr+ laser, indicated in nanometers on each curve) and the same incident photon flux of 5 × 1016 cm-2 s-1. (b) Variation of the emission intensity at 750 and 650 nm as a function of incident photon energy. Note the semiquantitative agreement with the fraction of absorbed photons where the absorption coefficient R has been derived from the ellipsometric measurements of Figure 1.
The observed photoluminescence was not sufficiently intense to allow for obtaining reliable excitation spectra. However, for the case of the polymer obtained from ethynylmagnesium chloride in DCB, we were able to record emission spectra for various excitation wavelengths up to the dark-red range (Figure 3). The emission intensity at two wavelengths (750 and 650 nm) was then plotted as a function of the photon energy of the excitation light. The fraction of the absorbed light was also plotted as 1 - exp(-Rd), where d is the layer thickness and R is the absorption coefficient, as deduced from the ellipsometric data of Figure 1b (R ) 4πk/λ). The emission intensities at 750 and 650 nm exhibit similar variations and are nearly proportional to the fraction of absorbed light, an agreement that we regard as satisfactory because the expression used for the absorbed light does not take into account the interference effects of the incident beam. In particular, the mild decrease in the emission intensity upon increasing the excitation wavelength appears to be compatible with the tailing of the absorption at low photon energies. XPS. X-ray photoemission is useful in monitoring the chemical composition of the material, especially in extracting the amount of halogen and oxygen. In the case of thin layers, the attenuation of the Si 2p signal provides a qualitative indication of the polymer thickness. However, this approach is limited to the case of thin layers: for thick films, it is common to observe small Si 2p and Si 2s contributions associated with residual uncoated regions of the silicon substrate. Examples of such small signals can be seen in Figures 4 and 5. Finally, mild ionic abrasion allows for investigating XPS as a function of depth in the layer, providing access to composition profiles
Fellah et al.
Figure 4. XPS profiles of a (111) SiH surface modified with vinylmagnesium bromide (J ) 0.5 mA/cm2 during 300 s). The abraded thickness between two spectra is estimated to be on the order of 10 Å.
(main carbon component, halogen and oxygen impurities) including in the region of the buried Si/polymer interface. Figure 4 shows representative XPS profiles for a layer obtained from vinylmagnesium bromide in THF. Table 2 summarizes the results for as-grown layers obtained from vinyl precursors. Carbon appears to be the dominant element present in the layers by far. Oxygen is present at the level of a few percent. However, profiling shows that this oxygen contribution is largely due to surface contamination. Some halogen is present through the layer at a rather constant atomic concentration on the order of 1%. The unexpected presence of some bromine in the layers made from vinylmagnesium chloride is plausibly due to bromine present as an impurity either in the Grignard or in our cell. Finally, the profiles exhibit specific features at the Si/ polymer interface: the Si 2p spectrum clearly involves highbinding-energy components suggestive of the presence of electron-withdrawing substituents on silicon, and the C 1s line appears to be shifted to lower binding energy. Figure 5 shows XPS profiles for layers obtained from ethynyl precursors. Table 3 gathers the atomic compositions obtained for such as-grown layers. Halogen is present in the layers at a significant level comparable to that of oxygen. These concentrations are generally on the order of 5-10%. The profiles indicate that oxygen concentration abruptly decreases as soon as ionic abrasion is performed. Hence, this oxygen is located mostly at the outer surface and is a result of atmospheric contamination. The halogen is present at a constant level through the layer. Its atomic concentration is lower than at the surface but is still in the 5-10% range. The occasional presence of a detectable amount of bromine in the layers made from ethynylmagnesium chloride may be attributed to bromine being present as an
Grafting and Polymer Formation on Silicon
J. Phys. Chem. B, Vol. 111, No. 6, 2007 1313
Figure 6. XPS C 1s spectra taken from the profile in Figure 5. Curve a corresponds to curve 7 in Figure 5 (bulk polymer), curve b to curve 16, and curve c to curve 17 (last spectrum before almost complete disappearance of the polymer). Note the low-energy shift of the peak for curve c. The inset shows that curve b can be described as a linear combination of curves a and c.
Figure 5. XPS profiles of a (111) Si surface modified with ethynylmagnesium chloride in THF (J ) 0.5 mA/cm2, during 1000 s). The abraded thickness between two spectra is estimated to be on the order of 50 Å.
TABLE 2: Values of the Atomic Percentages Obtained from XPS for the Modification of the Silicon Surface in the Presence of Vinyl Precursors in THF (J ) 0.5 mA/cm2) vinylmagnesium bromide 300 s vinylmagnesium bromide 1000 s vinylmagnesium chloride 300 s vinylmagnesium chloride 1000 s
%Si
%C
%O
64.4 0.4 66 10.9
30 90 24.6 78.1
4.4 3.9 3.1 4.6
%Cl
%Br
1.9 2.1
0.3 1.1 0.4 0.4
TABLE 3: Values of the Atomic Percentages Obtained from XPS for the Modification of the Silicon Surface in the Presence of Ethynyl Precursors (J ) 0.5 mA/cm2, 1000 s) %Si %C %O %Cl %Br ethynylmagnesium bromide in THF 4.4 71.1 12 ethynylmagnesium bromide in benzene 3.7 68.1 11.9 ethynylmagnesium chloride in THF 0.2 86.2 6.2 6.7
10 7.6
impurity either in the Grignard or in our cell. In agreement with the optical data, the layers are thicker for the chloride precursor than for the bromide, and the solvent appears to have little effect on the layer thickness in the case of the bromide but has a strong effect in the case of the chloride (DCB > THF). The buried silicon/polymer interface exhibits features similar to those found above for the vinyl precursors: a high-energy component for Si 2p, a low-energy shift of C 1s, and an increase in the oxygen and halogen concentrations. The common features of the silicon/polymer interface for the various polymers will now be examined in detail. A striking observation is the low-energy shift of the C 1s peak down to 283 eV, which appears only in the buried interfacial zone. Figure 6 shows high-resolution C 1s spectra that summarize this behavior. Curve a is characteristic of the bulk response of the
Figure 7. Analysis of the XPS Si 2p spectra taken from the profile in Figure 5. (a) Fit of curve 17 from Figure 5 (last spectrum before almost complete disappearance of the polymer) as a Shirley background plus two spin-orbit doublets (bulk Si and Si bound to C). (b) Fit of curve 16 from Figure 5. Note the two extra lines at ca. 100.8 and 101.8 eV corresponding to Si atoms with O and Cl substituents, respectively.
polymer film with a C 1s partial asymmetric shape. Curve b represents a typical intermediate configuration. Curve c is obtained when the profile reaches the ultimate layers of the polymer film just before the carbon signal vanishes. The C 1s peak now appears centered close to 283 eV. This signal is probably typical of the anchoring part. Such a low-energy position of C 1s is very specific and considered as an unambiguous indication of Si-C bonding.20-23 Similar lowenergy positions for the C 1s peak are reported on alkylterminated silicon surfaces.24 Therefore, our XPS results demonstrate that Si-C bonds are present at the buried interface. When the interfacial zone is reached, we also observe that the Si 2p spectrum exhibits a rather complex structure involving several components. This suggests that Si-C, Si-O, and Si-X bonds are present at the Si/polymer interface. Gradual modifications of the Si 2p and Si 2s spectra are observed as etching progresses through the buried interface. At the last stages of layer abrasion corresponding to the unique and specific lowbinding-energy C 1s position (Figure 6, curve c), the Si 2p spectrum can be fitted with only two spin-orbit doublets (Figure 7a); one centered at 99.3 eV associated with bulk Si and the other one at 99.8-99.9 eV related to the Si/polymer interface (here and below the energies given are those of the 2p3/2 line maximum). At this ultimate stage of polymer profiling, O and X signals are missing, which is plausibly due to higher sputtering yields for O and X as compared to those for C. The Si 2p contribution associated with a bulk SiC phase would appear at 100.4 eV,20-23 as compared to 99.3 eV for bulk Si. However,
1314 J. Phys. Chem. B, Vol. 111, No. 6, 2007
Figure 8. IR spectra of the (111) Si surface after anodic treatment in vinylmagnesium bromide (a) in THF and (b) in DCB (J ) 0.5 mA/ cm2, during 5 min).
for alkyl-terminated Si the chemical shift of Si 2p may be as low as 0.2-0.3 eV.24 In our case, because the chemical shift of the second Si 2p component is only 0.5-0.6 eV, local formation of an SiC compound can be excluded, and this shift can be attributed to the Si-C bonds associated with polymer anchoring. Quantitative analysis yields for the local chemical composition an atomic ratio of Si(99.8 eV)/C 1s(283 eV) = 2, which appears surprisingly large. This value, however, represents an overestimate because partial removal of the last CHx monolayer is expected to leave silicon atoms with a different surface termination (e.g., SiH species unduly fitted as part of the 99.8 eV doublet). When we consider abrasion times just before the previous one, the Si 2p spectrum is more structured. At this stage, the C 1s spectrum exhibits polymer and interfacial contributions (Figure 6, inset). Moreover, specific oxygen and halogen concentrations are present. The Si 2p spectrum still involves the two previously mentioned contributions (bulk silicon at 99.3 eV and Si-C anchoring sites at 100.0 eV rather than 99.8 eV at these profile levels), but new specific Si contributions, typically centered at 100.8 and 101.8 eV, have to be added (Figure 7b). These additional contributions imply that substituents more electronegative than carbon are also bound to silicon, in agreement with the maxima in oxygen and halogen concentrations in the same region. This points to the presence of Si-O and Si-X bonds at the Si/polymer interface, in addition to Si-C bonds. At this stage, an atomic composition determination shows that the local atomic ratio Si(99.8-100.0 eV)/C 1s(283 eV) is now close to 1.5 and that (O + X)/C(99.8-100.0 eV) is always lower than 0.2. Despite the plausible biases introduced by the different sputtering yields of C, O, and X, these numerical values strongly suggest dominant SiC bonding, with SiO and SiX as minor components. To summarize, the XPS profiles establish that very reproducible and homogeneous polymer layers are made from vinyl and ethynyl magnesium halides. For both precursors, the interfacial Si/polymer regions are similar and exhibit specific features, suggesting the presence of predominant Si-C bonds mixed with some minor Si-O and Si-X contributions. IR Spectroscopy. IR spectroscopy has been used as a systematic tool to characterize the various types of bonds in the films formed, providing clear-cut information on their chemical nature. Figure 8 shows spectra recorded for layers obtained from vinylmagnesium bromide. The dominant features are the νCH2 bands at 2860 and 2930 cm-1 and the δCH2 band at 1450 cm-1. These features are similar to those found in polyethylene or paraffin, though they are much broader here, which suggests a random distribution of neighborhoods. One can also notice weaker peaks. Among them are the negative SiH peak at 2083 cm-1 (loss of surface SiHs associated with
Fellah et al.
Figure 9. IR spectra of the (111) Si surface after anodic treatment in ethynylmagnesium chloride (a) in THF and (b) in DCB (J ) 0.1 mA/ cm2, during 15 min).
Figure 10. IR spectra of the (111) Si surface after 15 min of anodic treatment in ethynylmagnesium chloride in DCB at various current densities (indicated on the curves). For current densities higher than 0.2 mA/cm2 (thick layers), some DCB is incorporated into the films (dashed line representing the spectrum of DCB recorded in transmission).
grafting of the polymer) and positive peaks at 1375 cm-1 (methyl end groups), 1640 cm-1 (CdC double bonds), and 3010 and 3070 cm-1 (vinylic νCH’s). A weak peak around 1100 cm-1 is occasionally observed, which may indicate some oxidation of the Si surface. There is little difference, either in the shape of the spectrum or in its absolute magnitude, whether the solvent used is THF or DCB, which indicates the near absence of solvent effects for vinylic precursors. Figures 9 and 10 show spectra obtained for layers formed from ethynylmagnesium chloride. As previously reported, modification with ethynylmagnesium bromide essentially leads to layers so thin that the associated IR signals are hardly reliable. (In previous work,15 the weakness of these signals had led us to the erroneous conclusion that grafting in that case is selflimited to a monolayer.) The magnitude of the signals shown in Figures 9 and 10 clearly confirms the increase in thickness with increasing Faradaic charge or when THF is replaced by DCB. (Note that for thicknesses above ∼λ/25 the evanescent wave does not reach the outer part of the polymer layer and the signal levels off. This limit is clearly reached here for our thickest layers.) The shape of the spectra is rather complex. Here again, we notice the negative νSiH band associated with the grafting of the polymer. The νCH and δCH regions indicate the presence of saturated (2870, 2950, 1450, and 1370 cm-1), olefinic (3000 and 3070 cm-1), and acetylenic CHs (3220 and 3300 cm-1). The bands at 1650 and 2100 cm-1 can be attributed
Grafting and Polymer Formation on Silicon to the stretching modes of double and triple C-C bonds, respectively. In particular, the two narrow peaks at 1640 and 1660 cm-1, superimposed on a broader band, indicate two welldefined types of neighborhoods around the double bonds. (Plausible attributions are 1640 cm-1 ) monosubstituted or 1,1disubstituted ethylene, 1660 cm-1 ) cis 1,2-disubstituted ethylene,25 though halogen substitution on ethylenic groups may lead to alternate attributions.) The set of bands at 950 and 10701100 cm-1, together with a broad νOH structure at 3000-3700 cm-1, is plausibly associated with some oxidation of silicon. The origin of the bands around 1250 and 1940 cm-1, and of the tailing absorption from 1650-1800 cm-1, is less clear. Interestingly, a more distinct band at 1725 cm-1 present when the solvent is THF disappears when THF is replaced by DCB. Finally, one notices the appearance of new sharp bands for layers thicker than ∼0.3 µm (current densities larger than 0.2 mA/ cm2 for a 15 min anodization). The position of these bands, at 1035, 1130, 1435, and 1455 cm-1, exactly matches those of liquid DCB (Figure 10), which indicates that some solvent incorporation takes place in the spongy polymer layer. These bands decrease somewhat when the sample is left in air for a few days, which indicates that DCB may not be chemically bound to the polymer structure. The observation of these bands for the thickest layers is clearly due to the longer time for outdiffusion and evaporation of the solvent. Finally, the baseline observed for films obtained at high current densities (Figure 10) is quite reproducible and is typical of electronic absorption in the polymer layer (excitons, polarons, or bipolarons26-30). After aging for a few days, together with the above-mentioned decrease of the solvent bands, the spectra exhibit a significant decrease of the acetylenic νCH band at 3300 cm-1 and an associated growth of a broad band centered around 1700 cm-1. If the latter band is attributed to the stretching mode of >CdO groups, then the observed evolution may be traced back to the atmospheric oxidation of some ethynyl groups (-CtCH) to R-keto aldehydes (-(CO)-CHO), a reaction indeed reported in the literature in different contexts.31 Discussion All of the observations are consistent with the formation of a polymer where the multiple bonds of the precursor have been partially broken, though some of them are still surviving. XPS profiling indicates a material highly homogeneous in density and composition in which deviations in oxygen and halogen concentrations occur at the surfaces only. Interestingly, the spectroellipsometric data point to a material with a pseudogap on the order of 2 to 3 eV,32 which is consistent with the observation of visible photoluminescence. The energy of the photoluminescence is lower for the polymers formed from ethynyl precursors, which is plausibly associated with the presence of triple bonds and possibly some conjugation. The photoluminescence data, as analyzed in Figure 3, are quite similar to those of an amorphous semiconductor. Namely, the emission consists of a homogeneous broad band centered at an energy somewhat below the optical gap, a well-known effect of disorder leading to radiative recombination from “band tails”.33,34 As compared to vinyl, ethynyl precursors exhibit higher halogen incorporation into the polymer and more sensitivity to the solvent, in terms of both the growth rate and the incorporation of solvent fragments into the polymer. To determine whether we can account for these observations, we are now going to address the reaction mechanisms leading to polymer formation. The anodic decomposition of a Grignard RMgX is known to lead to the generation of the active radicals R• and X•,35,36 which
J. Phys. Chem. B, Vol. 111, No. 6, 2007 1315 may abstract hydrogen from the silicon surface, ultimately leading to the formation of a surface Si-R bond.14 The presence of Si-C bonding in the present case is further confirmed by our XPS measurements (low-energy shift of C 1s in the interface region). This early step is mandatory: we have found that anodic treatment of the hydrogenated silicon surface in allylmagnesium bromide electrolyte does not lead to any surface modification. This is clearly due to the fact that the allyl radical is not sufficiently reactive to break a surface Si-H bond (CH2CHCH2-H bond energy ) 83 kcal/mol, Si-H ≈ 90 kcal/ mol),37,14 and no surface modification can occur. A similar result has been reported for benzyl magnesium chloride.16 This absence of grafting for weakly reactive radicals stands as a proof that a polymer layer is formed only if covalent grafting to silicon can take place. Finally, as shown elsewhere, steric hindrance among the grafted organic groups limits the fraction of substituted Si-H sites to at most 50%, and some of the remaining sites are actually left substituted by the halogen.14 This fact accounts for the above XPS result that besides Si-C bonding the buried interface invariably exhibits an excess concentration of halogen and also of oxygen after some Si-X groups have undergone hydrolysis by reaction with atmospheric moisture. Note also that our IR data are not incompatible with the presence of Si-O-C groups, but this hypothesis, implausible in view of the reaction mechanisms as determined for alkyl grafting,14 can clearly be rejected on the basis of our C 1s XPS spectra. Our main concern here will be what occurs once the first monomeric layer has been grafted onto the Si surface. Let us first consider the case of vinyl precursors. If new unsaturated CH2CH• species impinge onto the surface, then two kinds of reaction may be envisioned. The first kind is as follows:
tSi-CHdCH2 + •HCdCH2 f tSi-CHdCH• + H2CdCH2 (hydrogen abstraction) (1a) tSi-CHdCH• + •HCdCH2 f tSi-CHdCH-CHdCH2 (1b) the latter should probably be written as an electrochemical step:
tSi-CHdCH• + XMgCHdCH2 + h+ f tSi-CHdCH-CHdCH2 + XMg+, which amounts to the substitution of a hydrogen with a new radical in the same vein as for aryl precursors.16 An alternate pathway is the addition of the new radical to the double bond of the grafted monomer:37
tSi-CHdCH2 + •HCdCH2 f tSi-CH•-CH2-CHdCH2 (addition) (2a) Note that addition may actually take place on either of the two ends of the double bond (yielding tSi-CH•-CH2-CHdCH2 or tSi-CH(CHdCH2)-CH2•). Though at this early growth stage of the polymer layer steric hindrance may favor addition to the outer end, this preference will probably disappear at a later growth stage. The obtained surface radical may rearrange in various ways: reaction with a halogen or solvent radical, hydrogen abstraction from the solvent, or reaction with another vinyl radical, typically
1316 J. Phys. Chem. B, Vol. 111, No. 6, 2007
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tSi-CH•-CH2-CHdCH2 + X• f tSi-CHX-CH2-CHdCH2 (2b)
mol.38 Rearrangement can first proceed according to the following paths:
tSi-CH•-CH2-CHdCH2 + Solv• f tSi-CHSolv-CH2-CHdCH2 (2c)
tSi-C•dCH-CtCH + X• f tSi-CXdCH2-CtCH (4b)
tSi-CH•-CH2-CHdCH2 + SolvH f tSi-CH2-CH2CHdCH2 + Solv• (2d) tSi-CH•-CH2-CHdCH2 + •HCdCH2 f tSi-CH-CH2-CHdCH2 | (2e) CHdCH2 tSi-CH•-CH2-CHdCH2 + •HCdCH2 f tSi-CH2-CH2-CHdCH2 + HCtCH (2f) where eqs 2b, 2c, and 2f can be written in electrochemical form, in the same way as eq 1b. Note that Scheme 1 would lead to a conjugated polymer. In contrast, in Scheme 2, part of the double bonds are broken, and the obtained polymer appears as a saturated skeleton, possibly decorated with unsaturated groups. In contrast to the results on aryl precursors,16 the experimental data here tend to indicate that the multiple bonds are largely broken in the process (more saturated CHs than olefinic CHs), and Scheme 2 is then dominant. In fact, the driving force for step 1a is very small, which makes process 1 far less probable than 2a (thermodynamically favorable, with an estimated enthalpy change on the order of -25 kcal/mol).38 Note that the disordered character of the polymer expected from Scheme 2 with a randomly branched skeleton and various decorating groups (vinyls or halogens) is fully consistent with the large width of the νCH bands. Among the various final steps 2b-2f, the absence of significant solvent effects would tend to point to paths 2c and 2d as being negligible. Path 2b, associated with halogen incorporation into the polymer, would contribute to a minor extent (ca. 1% halogen in the polymer according to XPS; if care is taken in the possible incorporation of some Grignard precursor into the polymer, then this figure may still represent an overestimate of the amount of halogen actually bound to the polymer skeleton). Paths 2e and 2f would then represent the major pathway. This is consistent with the number of CdC double bonds found from IR and with thermodynamic estimates for the enthalpy changes for reactions 2d-2f (-7, -80, and -57 kcal/mol, respectively38). The case of ethynyl precursors is similar though more complex because the triple bonds initially present may be transformed into double or single bonds. Here again, we think that the following substitution route has a low probability:
tSi-CtCH + •CtCH f tSi-CtC• + HCtCH (hydrogen abstraction) (3a) tSi-CtC• + •CtCH f tSi-CtC-CtCH (3b) It is more probable that the addition-rearrangement route dominates, as sketched hereafter:
tSi-CtCH + •CtCH f tSi-C•dCH-CtCH (addition) (4a) [or tSi-C(CtCH)dCH•] The estimated enthalpy change for eq 4a is now ∼-55 kcal/
tSi-C•dCH-CtCH + Solv• f tSi-CSolvdCH-CtCH (4c) tSi-C•dCH-CtCH + SolvH f tSi-CHdCH-CtCH + Solv• (4d) tSi-C•dCH-CtCH + •CtCH f tSi-CdCH-CtCH | (4e) CtCH We can now discuss the differences between vinyl and ethynyl. Two obvious differences are that in the case of ethynyl there is no analogue of path 2f but further bond breaking may occur after reaction 4 because the double bonds that are left will also represent target sites for the addition of newly formed radicals. We do not write the associated equations explicitly, but they can readily be transposed from 2b-2f, which accounts for the observation of saturated CHs in the IR spectra. Another important difference between vinyl and ethynyl is that the energy of the HC2-H bond is stronger than that of the H2CCH-H bond (132 vs 104 kcal/mol37,39) which makes hydrogen abstraction from the solvent a more probable process in the case of ethynyl. Namely, using a Solv-H bond energy of 92 kcal/mol (THF37) we get an enthalpy change of -40 kcal/ mol for R• + SolvH f RH + Solv• as compared to -12 kcal/ mol for the case of vinyl and an enthalpy change of -12 kcal/ mol for 4d as compared to -7 kcal/mol for 2d.38 These numbers account for the stronger solvent effects observed for ethynyl: hydrogen abstraction from the solvent acts as an efficient radical recombination mechanism in solvents prone to radical attack such as THF, resulting in a lower polymer growth rate and possible incorporation of solvent fragments into the polymer. Because the radicals formed from THF plausibly rearrange by β cleavage and ring opening,37 this incorporation may occur in the form of •CH2-CH2-CH2-CHdO radicals, which accounts for the observation of a larger proportion of saturated CHs and the presence of a νCO band at 1725 cm-1 in the polymer formed from ethynylmagnesium chloride in THF (Figure 9). Also, because the generation of R• radicals is in competition with that of X• radicals,35,36 the latter will be favored in the case of ethynyl. This accounts for the higher incorporation of chlorine in polymers formed from ethynyl (∼10%) as compared to those formed from vinyl (∼1%) (pathway 4b is more important than 2b). The observed higher chlorine concentration at the outer surface (Figure 5) is plausibly due to a higher lifetime of Cl• as compared to that of R•, resulting in a dominant role of 4b following current switch-off. (In contrast, the lower Br concentration at the outer surface in Figure 4 may result from some hydrolysis of R-Br surface groups.) Finally, for the case of ethynylmagnesium bromide it is plausible that Br• radicals are generated more efficiently than HCtC•, which provides a simple explanation for the low polymer growth rate from ethynylmagnesium bromide as compared to that from ethynylmagnesium chloride. (Polymer growth requires R• radicals.) We expect that if we used ethynylmagnesium iodide then the generation of ethynyl radicals would be almost completely quenched and no polymer should be obtained. Such an experiment is presently being planned.
Grafting and Polymer Formation on Silicon Conclusions Anodic decomposition of vinyl and ethynyl Grignards at a hydrogenated silicon electrode has been shown to result in the formation of polymers where the multiple bonds of the precursors are largely broken. The mechanisms can be discussed on the basis of a substitution or an addition-rearrangement route. From the experimental data and from thermodynamic considerations, the addition-rearrangement route appears to be strongly favored. The proposed reactions allow one to account for the main experimental findings regarding polymer composition and the effects of changing the solvent and/or the halogen in the Grignard precursor. The obtained polymers are covalently anchored to the silicon surface. This feature, together with the flexibility of varying polymer thickness and composition, may open the way to interesting applications, for example, in the use of such polymer films in field-effect sensors. Acknowledgment. We are indebted to P.-C. Lacaze for a useful suggestion. Supporting Information Available: Thermodynamic estimates of the reaction enthalpies for the various mechanisms discussed in the text. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Ulman, A. AdV. Mater. (Weinheim, Ger.) 1990, 2, 573. (2) Buriak, J. M. Chem. ReV. 2002, 102, 1271. (3) Wayner, D. D. M.; Wolkow, R. A. J. Chem. Soc., Perkin Trans. 2 2002, 2, 23. (4) Boukherroub, R. Curr. Opin. Solid State Mater. Sci. 2005, 9, 66. (5) Scho¨ning, M. J.; Lu¨th, H. Phys. Status Solidi A 2001, 185, 65. (6) Linford, M. R.; Fenter, P.; Eisenberger, P. M.; Chidsey, C. E. D. J. Am. Chem. Soc. 1995, 117, 3145. (7) Buriak, J. M.; Stewart, M. P.; Geders, T. W.; Allen, M. J.; Choi, H. C.; Smith, J.; Raftery, D.; Canham, L. T. J. Am. Chem. Soc. 1999, 121, 11 491. (8) Sieval, A. B.; Demirel, A. L.; Nissink, J. W. M.; Linford, M. R.; van der Maas, J. H.; de Jeu, W. H.; Zuilhof, H.; Sudho¨lter, E. J. R. Langmuir 1998, 14, 1759. (9) Boukherroub, R.; Morin, S.; Bensebaa, F.; Wayner, D. D. M. Langmuir 1999, 15, 3831. (10) Cicero, R. L.; Linford, M. R.; Chidsey, C. E. D. Langmuir 2000, 16, 5688. (11) Jin, H.; Kinser, C. R.; Bertin, P. A.; Kramer, D. E.; Libera, J. A.; Hersam, M. C.; Nguyen, S. T.; Bedzyk, M. J. Langmuir 2004, 20, 6252. (12) Dubois, T.; Ozanam, F.; Chazalviel, J.-N. Proc. Electrochem. Soc. 1997, 97-7, 296. (13) Fide´lis, A.; Ozanam, F.; Chazalviel, J.-N. Surf. Sci. Lett. 2000, 444, L7.
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