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J. Phys. Chem. C 2008, 112, 16622–16628
Microwave Effects on Chemical Functionalization of Hydrogen-Terminated Porous Silicon Nanostructures Alain Petit,† Michel Delmotte,‡ Andre´ Loupy,† Jean-Noe¨l Chazalviel,§ Franc¸ois Ozanam,§ and Rabah Boukherroub*,| Laboratoire de Chimie des Proce´de´s et Substances Naturelles, UMR 8182, ICMMO Baˆtiment 410, UniVersite´ Paris-Sud, 91405 Orsay Ce´dex, France, Laboratoire d’Inge´nierie des Mate´riaux, UMR 8006, ENSAM, 151, bd de l’hoˆpital, 75013 Paris, France, Physique de la Matie`re Condense´e, Ecole Polytechnique, CNRS, Route de Saclay, 91128 Palaiseau, France, and Institut de Recherche Interdisciplinaire (IRI), USR 3078, and Institut d’Electronique de Microe´lectronique et de Nanotechnologie (IEMN), UMR 8520, Cite´ Scientifique, AVenue Poincare´, BP 60069, VilleneuVe d’Ascq, France ReceiVed: July 30, 2008
This article reports on the chemical functionalization of hydrogen-terminated porous silicon (PSi-H) nanostructures with alkenes, aldehydes, and alkyl halides under microwave irradiation. The technique allows for the introduction of different functional groups on the surface. The use of microwave irradiation has a net effect on the hydrosilylation reaction rate. However, no specific microwave effect was observed when the freshly prepared PSi-H surface was modified with an alkene bearing an aldehyde function. The reaction takes place at both the carbon-carbon and the carbon-oxygen double bonds. The polar aldehyde functional group did not show a dominant effect in directing the reaction selectively at the carbonyl group. Introduction The preparation and patterning of organic monolayers on semiconductor surfaces is currently an active research field from fundamental aspects and expected potential applications of the resulting structures.1-3 The use of organic monolayers covalently attached to semiconductor surfaces allows for bringing diverse chemical and biochemical functionalities onto microelectronic platforms and integrating these structures with existing microelectronics technologies. In the past decade, there has been a huge body of work devoted to the functionalization of crystalline silicon (c-Si) and porous silicon (PSi) surfaces. Porous silicon, obtained by electrochemical dissolution of crystalline silicon in HF-based solutions, displays a high surface area and interesting optical properties.4,5 These properties were used for sensing both chemical and biological species on the surface.6,7 Moreover, when compared to c-Si, PSi was found to be biocompatible.8 This property offers new opportunities for exploring PSi-based structures for in vivo and in vitro studies, drug delivery, tissue engineering, and biomaterials. Bayliss et al. have investigated the adherence and viability of rat neuronal (B50)9 and mammalian10 cells cultured directly on the PSi structure. Bhatia et al. have studied the attachment, viability, and function of rat hepatocytes on oxidized nanoporous silicon.11 Freshly prepared porous silicon surfaces are terminated with silicon-hydrogen bonds, which have the tendency to oxidize when exposed to ambient air. The oxidation reaction depends strongly on the environment (humidity and temperature). This reaction is accelerated by UV irradiation to give an oxide monolayer with a high density of active electronic defects. This * To whom correspondence should be addressed. Tel: +33 (0)3 20 19 79 87. Fax: +33 (0)3 20 19 78 84. E-mail: rabah.boukherroub@ iemn.univ-lille1.fr. † Universite ´ Paris-Sud. ‡ Laboratoire d’Inge ´ nierie des Mate´riaux. § Ecole Polytechnique. | Institut de Recherche Interdisciplinaire (IRI), and Institut d’Electronique de Microe´lectronique et de Nanotechnologie (IEMN).
structural evolution induces remarkable changes in the electronic and optical properties of the material.5 Moreover, for many applications in chemical and biosensors, stable surfaces are required. To overcome this evolution, several methods are proposed in the literature.12 They are based on controlled modification of the surface with different means. The thermal oxidation of the surface is one of the most studied reactions reported so far. Recently, organic functionalization of hydrogenterminated c-Si and PSi surfaces has been successfully used to efficiently passivate the surface and to introduce organic or biological functionality on the surface.1-3 In our laboratory, we have used thermal activation to covalently graft organic monolayers on PSi-H through Si-C and Si-O-C bonds.13-19 Simple and functional alkenes, and aldehydes, react with the freshly prepared PSi surface to yield stable organic monolayers covalently bound to the surface. This way, one can tailor the wetting properties of the surface by introducing hydrophobic or hydrophilic functional end groups. Moreover, the optical properties of the PSi surface have been preserved by the chemical treatment. The photo- and electroluminescent characteristics of the material are stabilized over time.20-22 We have found that the chemically modified PSibased electroluminescent diodes are stable for several hours under continuous operation, whereas the freshly prepared diode degraded after only 20 minutes.21,22 In a recent report, we have used microwave irradiation to functionalize PSi-H surfaces with alkenes.23 We have shown that different organic groups such as acid and ester functionalities can be introduced on the surface. There was a remarkable increase in the rate of the hydrosilylation reaction and surface coverage when using microwave activation. The PSi surfaces chemically functionalized under microwave irradiation have shown a very good stability in different aqueous and organic media. For example, the PSi surface derivatized with ethyl undecylenate did not show any structural degradation or surface oxidation when subjected to 2.4 M HCl at 100 °C for 9 h. Only
10.1021/jp806786s CCC: $40.75 2008 American Chemical Society Published on Web 10/01/2008
Hydrogen-Terminated Porous Silicon Nanostructures ester hydrolysis to the corresponding acid was observed. Xiao et al. have exploited microwave irradiation to introduce acid functionality on hydrogen-terminated PSi-H by the reaction with R-bromo-ω-carboxyalkanes,24 diazirine,25 and azide26 compounds. This article reports on the chemical derivatization of freshly prepared PSi-H with bifunctional alkenes, aldehydes, and alkyl halides under microwave irradiation. The work is a continuation of our previous one on the same system extended to other organic reagents. We show that microwave irradiation is an effective technique for thermal activation of the reaction of hydrogen-terminated PSi surfaces with various alkenes, aldehydes, and alkyl halides. The resulting PSi surfaces were characterized by transmission FTIR. The method allows for improving the reaction rate and surface coverage. A mechanistic approach is proposed for alkyl halide reaction with PSi-H surfaces even though it is hard to draw any final conclusion about the reaction pathway without any further investigation. Experimental Section Materials. All cleaning and etching reagents were clean-room grade. All chemicals were reagent grade or higher and were used as received unless otherwise specified. 1-tetradecene, 1-hexadecene, 1-octadecene, decanal, undecylenic aldehyde, bromodecane, and dichloromethane (CH2Cl2) were all available from Aldrich. Sample Preparation. Double-side polished Si(100) oriented p-type silicon wafers (boron-doped, 1-15 ohm cm resistivity) were first cleaned in 3:1 concentrated H2SO4:30% H2O2 for 15 min at 80 °C and then rinsed copiously with Milli-Q water. The clean wafers were immersed in 5% aqueous HF solution for 1 min at room temperature to remove the native oxide. The hydrogen-terminated surfaces were electrochemically etched in a 1:1 (v/v) solution of pure ethanol and 48% aqueous HF for 2 min at a current density of 40 mA/cm2. After etching, the samples were rinsed with pure ethanol and dried under a stream of dry nitrogen prior to use. Microwave experiments were performed in a Synthewave 402 monomode reactor from Prolabo Company operating at 2450 MHz.27 All reactions were conducted in a specially adapted open cylindrical Pyrex vessel containing the organic reagent and the PSi sample under continuous argon bubbling and mechanical stirring to ensure oxygen elimination and temperature homogeneity in the solution. The mixtures were heated in the monomode reactor at temperatures and for given reaction times as indicated in the text. The reactor was cooled to room temperature. The excess of unreacted and physisorbed reagent was removed by rinsing, at room temperature, with dichloromethane, and then the sample was dried under a stream of nitrogen. The temperature was controlled throughout the reaction and evaluated by an infrared detector that indicated the surface temperature (IR detector was calibrated according to the emissivity factor using an optical fiber inside the reaction mixture). A computer system was used for automatic control of the irradiation (power and temperature) as well as data processing. FTIR Spectroscopy. Transmission FTIR spectra were recorded using a Bomem MB100 spectrometer at 4 cm-1 resolution. The samples were mounted in a purged sample chamber. To avoid significant errors in the quantitative estimation of the grafting efficiency, we took care to adjust the sample in the same position before and after chemical modification. Background spectra were obtained using a flat untreated Si(100)
J. Phys. Chem. C, Vol. 112, No. 42, 2008 16623 wafer that had been dipped in 5% HF aqueous solution to remove the native oxide layer. Thermal Behavior of Reagents under Microwave Irradiation. A study of the thermal effects induced by the microwavematerial interactions was carried out using the Synthewave 402 reactor to appreciate the changes in polarity of the substrates.28 It is to be expected that an increase in the material polarity results in a stronger microwave absorption and consequently leads to an increase in the raised temperature according to irradiation time. Measurements of Dielectric Characteristics (E′ and E′′). The dielectric measurements were performed using the technique known as “small perturbation of a resonant cavity”, which is described in details elsewhere.29-31 In this method, the perturbations caused by a sample to the resonance frequency and amplitude of a microwave cavity are compared with standards. A small quantity of solvent is placed in the center of a resonant cavity, which is a TE013 single-mode rectangular cavity. The dielectric characteristics of permittivity (′) and absorption (′′) are deduced from the cavity frequency and the transmission at resonance by means of the measurements of the perturbation due to the sample at frequency of 2450 ( 14 MHz. An accurate calibration is performed with a precise volume of a standard material. In all experiments, 1-decanol was used as a standard material (′ ) 2.67, ′′ ) 0.4135 at 25 °C). Eqs 1 and 2 give the dielectric constants as functions of the resonance frequency FM and the transmission τM:
ε ′ ) 1 + KC ε ″ ) K′C
V 2ν
V FC - FM ν FC
(√
1
τM
-
1
√τC
(1)
)
(2)
where ν and V are the material and the cavity volume (ν ) 1.5 × 10-7 m3 and V ) 10-3 m3), with ν , V; FC and τC are the resonance frequency (Hz) and the transmission of the empty cavity; KC and K′C are constants depending on the cavity. Results and discussion The field of microwave-assisted organic32-34 and solid-phase organic chemistry35 is growing rapidly because of the great impact of microwave activation on reaction times, conversion, and specificity. Doped silicon is known to absorb microwave energy efficiently.36 The heat transfer to the near surface of the material will lead to a temperature gradient, which is expected to influence the reaction rate during the chemical treatment of hydrogen-terminated PSi-H surfaces with organic reagents. The silicon substrates used in this work have a resistivity of 1-15 ohm cm, which is in the range of a maximum absorption of microwave radiation.36 We have first examined the thermal behavior of the reactants under microwave irradiation, which is believed to be dominated by the polarity of the molecules.37,38 When a simple alkene such as 1-dodecene was subjected to microwave irradiation in the absence of a PSi substrate in the mixture, a slow increase of the temperature was observed. This behavior is related to the weak polarity of the compound, which is quasi-transparent to microwave irradiation. For a restituted microwave power of 300 W, the maximum value available from the equipment, the temperature rises slowly to reach a plateau at 90 °C after 15 min irradiation (part a of Figure 1). When functional alkenes bearing a polar group at one end such as organic acids or esters are treated under the same conditions, the rise in the temperature is more important. This is due to the presence of polar groups
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Figure 1. Profiles of temperature rise under microwave irradiation at a restituted fixed emitted power of 300 W: 1-dodecene (a), undecylenic acid (b), 1-decanal (c), and the reaction mixture of 1-dodecene and porous silicon at an optimal temperature of 180 °C by modulating the emitted power (d).
in these compounds, which makes the molecule absorb the microwave energy more efficiently. For instance, the temperature increase is more pronounced for undecylenic acid (part b of Figure 1) and ethyl undecylenate (not shown) to reach a plateau at ∼150 °C. More surprising is the thermal behavior of decanal (part c of Figure 1). A huge increase of the temperature was observed when this compound was exposed to microwave irradiation. The temperature reaches 200 °C in 5 min and keeps increasing. This behavior is somehow surprising if one compares the chemical structure of this compound to organic esters and acids described above. In fact, these compounds bear a carbonyl group as well but display a different thermal behavior. This may be due to the difference in local viscosity of the different chemical reagents. However, this result is very interesting to investigate specific microwave effect of the carbonyl group in the aldehyde functional compounds. When a PSi substrate is introduced in neat 1-dodecene, the mixture shows a very rapid increase in the temperature and reaches 180 °C in less than 3 min (part d of Figure 1). This means that the PSi substrate absorbs the microwave irradiation and transfers efficiently the energy to the reactant. It is important to notice that this temperature was measured in the liquid phase (by infrared detection or via an optical fiber) because we were not able to evaluate directly the temperature of the silicon substrate. Scanning electron microscopy analysis showed that the PSi films investigated in this work are 3.8 µm thick with an average pore diameter of ca. 10 nm. The freshly prepared PSi surface is covered with a monolayer of silicon-hydrogen bonds. IR analysis of PSi-H shows peaks due to stretching modes νSi-Hx centered at 2115 cm-1, Si-H2 scissor mode δSi-H2 at 912 cm-1, FSi-Hx at 669 and 629 cm-1 and Si-O-Si stretching mode at 1037 cm-1 (part a of Figure 2). The latter peak is present in all PSi samples and probably indicates a small oxidation of the reactive surface upon exposure to ambient air. After reaction of the hydrogen-terminated PSi with 1-tetradecene at 180 °C for 30 min under microwave irradiation, additional peaks characteristic of the alkyl chain appear at 2924 cm-1 (C-H stretching modes) and at 1465 cm-1 (methylene bending modes) (part b of Figure 2). A large decrease of the νSi-Hx (2115 cm-1), δSi-H2 (912 cm-1), and FSi-Hx (669 and 629 cm-1) peaks was observed, indicating that the reaction was very efficient and took
Figure 2. Transmission-mode FTIR spectra of freshly prepared porous silicon before (a), and after chemical functionalization under microwave irradiation with 1-tetradecene for 30 min at 180 °C (b), with 1-hexadecene for 30 min at 180 °C (c), and with 1-octadecene for 15 min at 180 °C (d).
place with silicon hydride consumption. Moreover, there was no increase of the νSi-O-Si band, in agreement with a thermal process occurring without any apparent oxidation of the surface. This result is consistent with a hydrosilylation process leading to the formation of an organic monolayer covalently attached to the PSi surface through Si-C bonds. As shown in a previous report, classical thermal heating of freshly prepared PSi with alkenes without exposing the mixture to microwave irradiation at 180 °C for 30 min led to a less efficient chemical grafting of alkyl chains on the surface.23 Under microwave irradiation, the reaction of freshly prepared PSi-H surface with 1-hexadecene at 180 °C for 30 min leads to the formation of an organic monolayer covalently attached to the surface (part c of Figure 2). The same features as for tetradecene are observed in the transmission FTIR spectrum with an additional small peak at 1050 cm-1. The latter peak is assigned to Si-O-Si vibration modes due to a partial oxidation of the surface as a result of thermal reaction of residual water or peroxides present in the chemical reactant. In fact, hexadecene
Hydrogen-Terminated Porous Silicon Nanostructures
Figure 3. Transmission-mode FTIR spectra of hydrogen-terminated porous silicon surfaces before (a) and after microwave irradiation in decanal for 15 min at 180 °C (b), and in undecylenic aldehyde for 30 min at 100 °C (c), 30 min at 150 °C (d), 30 min at 180 °C (e).
was used without any further purification, so it is not excluded that it contains some impurities. Comparable results were obtained when the PSi-H surface was allowed to react with 1-octadecene under microwave irradiation for 15 min at 180 °C (part d of Figure 2). Analysis of the FTIR spectra can be used to monitor the hydrosilylation reaction efficiency of 1-hexadecene by evaluation of the ratio of the integrated area of the Si-H intensity before and after chemical functionalization. The PSi surface modified with 1-hexadecene for 30 min at 180 °C under microwave irradiation gave an efficiency of 35%, comparable to the one reported for 1-dodecene (38%) under similar conditions.23 In a similar way, the reaction of a PSi-H surface with decanal under microwave irradiation at 180 °C for 15 min yields an organic monolayer covalently attached to the surface through Si-O-C bonds. The reaction takes place even at temperatures below 100 °C, but the surface coverage is lower. Part b of Figure 3 displays the IR spectrum of the decyloxy-terminated PSi surface. It exhibits an asymmetric C-H stretching mode at 2925 cm-1 and a methylene bending mode at 1470 cm-1 characteristic of the alkyl chain, and a broad peak in the region of 1000-1100 cm-1 assigned to Si-O-C stretching modes. A contribution from partial oxidation of the surface during the chemical process will lead to vibration modes of Si-O-Si in the same region. The reaction takes place with Si-H consumption as evidenced by a net decrease of the Si-H intensity after the chemical treatment. Ketones have shown similar behavior as aldehydes. Indeed, the reaction of PSi-H with 2-decanone at 180 °C for 15 min led to the formation of an organic monolayer covalently attached to the PSi surface (not shown in the figure). FTIR analysis shows a similar spectrum with a higher degree of oxidation. In contrast to the case of simple alkenes, aldehydes and ketones bear a polar end group that absorbs microwave energy. Moreover, the pronounced nucleophilic character of aldehydes compared to simple alkenes would make aldehydes more reactive. To distinguish between pure thermal and microwave specific effects, we have used an alkene bearing a polar end group (carbonyl functional group): undecylenic aldehyde. If a pure microwave effect dominates the process, one would expect the reaction to occur at the polar carbon-oxygen (CdO) double bond. Activation by polar molecules is a specific effect observed in microwave activation of organic processes in molecular
J. Phys. Chem. C, Vol. 112, No. 42, 2008 16625 solutions. For nonpolar media, addition of active species is required to enhance the reaction rate when microwave irradiation is used as a source of energy. These activators can be either polar organic molecules (solvents) or polar mineral oxides (solid supports) that absorb efficiently microwave energy, transfer it to the homogeneous/heterogeneous medium, and allow the reaction to occur with increased rate and high yields. The knowledge of the evolution of the dielectric characteristics (′ and ′′) of the products during their transformation or according to temperature constitutes an excellent indicator in the understanding of microwave effects. It allows us to link electromagnetic conditions with the heating provided within materials. The dielectric permittivity coefficient ′ expresses mainly the change in distribution of the electric field; it represents the ability of a material to be polarized by an external electric field. The dielectric absorption coefficient ′′ evaluates the volumic density of electric power converted into heat within the product. They were measured by the small perturbation method of a resonant cavity and summarized in Table 1.39 Figure 4 displays the temperature increase profiles of different molecules investigated in this study under microwave activation for a restituted microwave power of 180 W. It clearly shows a difference in the behavior of the reactants. Whereas there is only a slight difference in the temperature rise for the aldehydes (99 °C for undecanal and 95 °C for undecylenic aldehyde), there is a significant difference between the aldehydes and the other reactants (dodecene, undecylenic acid, and ethyl undecylenate). Dodecene showed the weakest increase over time (51 °C). The thermal behavior of the reactant is in agreement with the dielectric characteristics of permittivity (Figure 5) and absorption (Figure 6) at 2.45 GHz measured as a function of temperature. The dielectric absorption coefficients of undecanal and undecylenic aldehyde showed maxima of same values at a temperature ∼30 °C and slightly higher values for undecanal at temperatures higher than 50 °C. This indicates that the relaxation frequencies for both reactants are equal at 2.45 GHz for temperatures ∼30 °C, and the intermolecular interactions are slightly more intense for undecanal at temperatures higher than 50 °C (0.04 unit variation of the dielectric characteristic of absorption). The results are in agreement with the generation of more intense heating using undecanal even we suppose that the wave propagation within the Prolabo oven is not sensitive to a difference in the permittivity of 0.05 unit. On the other hand, the dielectric characteristics of dodecene and undecylenic acid remained almost constant in the investigated temperature range. First, we have carried out the reaction of freshly prepared PSi-H with undecylenic aldehyde at 100 °C for 30 min under microwave irradiation. The FTIR spectrum of the resulting PSi surface displays C-H stretching modes at 2925 cm-1, vibrations due to CdC and CdO double bonds at 1641 and 1731 cm-1, respectively, and weak signals at 3062 and 3079 cm-1 assigned to the vibrations of the vinyl C-Hx of the carbon-carbon double bond (part c of Figure 3). This is consistent with a nonselective chemical process taking place at both CdC and CdO double bonds. We did not estimate the amount of the CdC and the CdO attached to the surface. The reaction may also take place simultaneously at both functional groups to yield loops. The grafting of the bifunctional species by both or either end may be explained by the fact that silicon is absorbing the microwave irradiation efficiently, and thus the high temperature generated at the close proximity of the surface will lead to the unspecific activation of the reaction of unsaturated CdC or CdO bonds with the surface (an order of magnitude estimate of the
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TABLE 1: Dielectric Characteristics of Permittivity (E′) and Absorption (E′′) at Room Temperature, and Maximum Temperatures Reached after 15 Min Microwave Irradiation at 180 W t)0 reactant dodecene undecylenic acid ethyl undecylenate undecanal undecenal
′ (24 °C) 2.45 2.47 3.54 4.71 5.35
Tmax after 15 min, microwave 180 W ′′ (24 °C) 0.03 0.08 0.28 0.38 0.39
temperature rise of silicon relative to the liquid may be as follows: limiting layer δ ) ∼1 mm, absorbed power P ) ∼30 W, surface area S ) ∼3 cm2, thermal conductivity K ) ∼10-2 W/cm °C; hence ∆T ) Pδ/SK ) ∼100 °C, a huge overheating as compared to the liquid indeed). Put another way, the strong overheating of the liquid near the surface is so high that it activates as efficiently the CdC reaction as it does the CdO reaction and that it blurs any specific microwave activation effect on the CdO site. The surface will then react independently with the unsaturated CdC or CdO bonds present close to the reactive
′ (Tmax°C) 2.46 (51 °C) 2.56 (62 °C) 3.44 (76 °C) 4.58 (99 °C) 5.02 (95 °C)
′′ (Tmax°C) 0.02 (51 °C) 0.08 (62 °C) 0.14 (76 °C) 0.24 (99 °C) 0.20 (95 °C)
sites. The probability of the reaction with one or the other of the two functional groups is determined by the orientation of the molecules at the vicinity of the surface during the chemical treatment. We have found that the temperature does not have a pronounced effect on the reaction mechanism. There was no difference either in the monolayer composition nor in the selectivity, when the reaction was conducted at 150 °C (part d of Figure 3) or 180 °C (part e of Figure 3). The major variation is only the higher surface coverage obtained upon increasing the temperature. A similar behavior was observed for simple
Figure 4. Temperature increase profiles of the different reactants under microwave irradiation (power ) 180 W) for 15 min.
Figure 5. Permittivity (′) of the different reactants versus temperature.
Hydrogen-Terminated Porous Silicon Nanostructures
J. Phys. Chem. C, Vol. 112, No. 42, 2008 16627
Figure 6. Characteristic of absorption (′′) of the different reactants versus temperature.
and surface oxidation. Different reaction schemes can account for the simultaneous grafting and oxidation of the surface. A first process would lead to grafting of the alkyl part of the reactant according to:
≡Si-H + R-X f ≡ Si-R + H-X The second one would lead to the formation of Si-X bonds, which are prone to hydrolysis and responsible for the surface oxidation: H2O
≡Si-H + R-X f ≡ Si-X + R-H 98 SiO2 Figure 7. Transmission-mode FTIR spectra of freshly prepared porous silicon surfaces before (a) and after chemical derivatization with bromodecane under microwave irradiation for 30 min at 120 °C (b) and 30 min at 150 °C (c).
alkenes and aldehydes as well. It is well documented in molecular chemistry that the rate constants for addition of silicon centered radicals to carbonyls are about the same as those for alkenes, at room temperature. The small variations in the rate constant become insignificant at higher temperatures like those used in this study, which supports a reaction occurring through a radical mechanism. Finally, we have studied the reaction of PSi-H surfaces with alkyl halides using microwave irradiation as an activation mode. Reaction of freshly prepared PSi sample (IR spectrum shown in part a of Figure 7) with bromodecane at 120 °C for 30 min under microwave irradiation led to the chemical grafting of the decyl group on the surface accompanied with surface oxidation. IR analysis clearly shows vibrations due to C-H stretching of the alkyl chain around 2900 cm-1 and peaks due to Si-O-Si stretching at 1100 cm-1 (part b of Figure 7). Peaks associated with chemical oxidation of the PSi surface are also observed at 2200 and 2250 cm-1 and unambiguously assigned to stretching of Si-H bonds bearing oxygen in their backbonds with different oxidation states. Increasing the temperature to 150 °C and keeping the time constant yielded a comparable surface with a higher concentration of grafted alkyl chains and a significant surface oxidation. One can notice the complete disappearance of the SiH2 scissor mode at 910 cm-1 (part c of Figure 7). A similar behavior has been observed during the thermal treatment of p-type hydrogen-terminated crystalline silicon Si(111) with tetradecyl bromide.40 Alkyl iodides gave comparable results. The reaction of freshly prepared PSi-H with alkyl halides under microwave irradiation takes place with Si-H consumption
Two distinct initiation schemes could be thought of: the microwave irradiation could either yield Si-H bond cleavage through heating in silicon and formation of silyl surface radicals, or result in the homolytic decomposition of the alkyl halide. Xiao et al. have proposed a radical-based mechanism for the reaction of alkyl bromides with PSi-H surface under microwave irradiation.24 They assumed that microwave heating is responsible for the Si-H bond scission and formation of surface silicon radical. The latter reacts with R-Br to yield both Si-Br and Si-R terminations. The reaction of free silyl radicals with organic halides in solution is well documented in the literature.41 Silanes are used as effective reagents to reduce alkyl halides to the corresponding alkanes in a free radical process. In other words, reduction of alkyl halides with silanes leads to the formation of silyl halides in a free radical process41 or under metal catalysis.42 If a parallel exists between molecular solution and the surface-related chemistry, one would expect to obtain a fully halogenated PSi surface when the PSi-H surface is reacted with alkyl halides. In contrast, formation of Si-R surface species and release of a halide radical appears energetically unfavorable. Therefore, the presence of both alkyl chains and oxidized silicon-silicon bonds (resulting most likely from hydrolysis of Si-X bonds) on the PSi surface excludes the reaction pathway proposed for free radical reduction of alkyl halides with silanes. This is in agreement with the conclusions of our previous report.40 On the other hand, grafting and oxidation may take place simultaneously as a result of a homolytic decomposition of the alkyl halide to yield both alkyl and halide radicals. These radicals are able to react with the silicon-hydrogen bonds terminating the PSi surface by hydrogen abstraction followed by chemical bond formation of Si-R or/and Si-X (according
16628 J. Phys. Chem. C, Vol. 112, No. 42, 2008 to the equations above). Because of the high polarity of the silicon-halide bond, simple exposure to ambient air or solvent rinsing will lead to surface oxidation. It is well-known that silicon surfaces terminated with halogens react almost spontaneously with moisture present in air to yield an oxidized surface. Conclusions In conclusion, we have successfully used microwave irradiation to derivatize hydrogen-terminated PSi surfaces with various chemical reagents such as functional alkenes, aldehydes, ketones, and alkyl halides to form organic monolayers covalently attached to the surface. Different functional groups can be introduced on the surface in a straightforward manner. On one hand, the microwave activation has a pronounced effect on the reaction rate and chemical yield. The resulting surfaces are stable in different organic and aqueous media. On the other hand, we were not able to demonstrate microwave specific effects when using a bifunctional alkene. In fact, the reaction of hydrogenterminated PSi surface with undecylenic aldehyde led to the formation of an organic monolayer terminated with both CdC and CdO double bonds as evidenced by the presence of stretching modes of both double bonds in the FTIR spectrum. This means that the reaction took place at both ends separately or/and simultaneously, in a similar way as in a classical thermal process. The lack of selectivity was attributed to the temperature gradient in the surface vicinity. A large increase of the temperature in the close proximity of the active sites suppresses the competitive pathways between the two functional groups. Future work will focus on optimization of the reaction conditions and understanding the reaction mechanism. We will study the different parameters influencing the reaction selectivity by adding polar solvents known for their potential microwave energy absorption. Acknowledgment. The Centre National de la Recherche Scientifique (CNRS) and the Nord-Pas-de-Calais region are gratefully acknowledged for financial support. References and Notes (1) Buriak, J. M. Chem. ReV. 2002, 102, 1272–1308. (2) Wayner, D. D. M.; Wolkow, R. A. J. Chem. Soc., Perkin Trans. 2 2002, 2, 23–34. (3) Boukherroub, R. Curr. Opin. Solid-State Mater. Sci. 2005, 9, 66– 72. (4) Canham, L. T. Appl. Phys. Lett. 1990, 57, 1046. (5) Cullis, A. J.; Canham, L. T.; Calcott, P. D. J. J. Appl. Phys. 1997, 82, 909. (6) Janshoff, A.; Dancil, K. P. S.; Steinem, C.; Greiner, D. P.; Lin, V. S. Y.; Gurtner, C.; Motesharei, K.; Sailor, M. J.; Ghadiri, M. R. J. Am. Chem. Soc. 1998, 120, 12108. (7) Chan, S.; Horner, S. R.; Fauchet, P. M.; Miller, B. L. J. Am. Chem. Soc. 2001, 123, 11797. (8) Canham, L. T. AdV. Mater. 1995, 7, 1033.
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