Formation of Organic Monolayers on Silicon via Gas-Phase

Nov 3, 2005 - a radical chain reaction mechanism or through direct radical attachment to the silicon dangling bonds. Introduction. The controlled form...
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Langmuir 2006, 22, 3180-3185

Formation of Organic Monolayers on Silicon via Gas-Phase Photochemical Reactions Brian J. Eves† and Gregory P. Lopinski* Steacie Institute for Molecular Sciences, National Research Council, Ottawa, Ontario, Canada ReceiVed NoVember 3, 2005. In Final Form: January 6, 2006 A new method for the formation of molecular monolayers on silicon surfaces utilizing gas-phase photochemical reactions is reported. Hydrogen-terminated Si(111) surfaces were exposed to various gas-phase molecules (hexene, benzaldehyde, and allylamine) and irradiated with ultraviolet light from a mercury lamp. The surfaces were studied with in situ Fourier transform infrared spectroscopy, high-resolution electron energy loss spectroscopy, and scanning tunneling microscopy. The generation of gas-phase radicals was found to be the initiator for organic monolayer formation via the abstraction of hydrogen from the H/Si(111) surface. Monolayer growth can occur through either a radical chain reaction mechanism or through direct radical attachment to the silicon dangling bonds.

Introduction The controlled formation of organic monolayers on silicon surfaces offers the promise of enhancing the functionality of structures and devices based on this material. These monolayers can simply act as passivating layers, stabilizing the properties of the underlying substrate, or can be used to tailor the physical and chemical properties of the surface. Covalent attachment of a wide range of organic functional groups to the silicon surface is expected to enable the fabrication of novel hybrid molecule/ silicon sensors and devices. Methods for the direct covalent attachment of molecules to silicon (with no intervening oxide layer) usually begin with hydrogen-terminated silicon surfaces. These surfaces are reasonably stable and can be handled in various solvents, facilitating the attachment of a variety of molecules via wet chemical processes. In particular, the reaction of alkenes with H-terminated silicon surfaces has been shown to occur under a range of experimental conditions. Linford and Chidsey first reported the formation of organic monolayers on H/Si(111) via pyrolysis of diacyl peroxides.1 Subsequently, they used these peroxides to initiate the reaction of terminal alkenes (and alkynes) with the surface.2 Since these pioneering studies, it has been found that photon irradiation (with wavelengths between 254 and 650 nm) of H-terminated silicon immersed in alkenecontaining solutions leads to monolayer growth.3-6 Alkenes have also been grafted to the H/Si(111) surface via heating the silicon/ alkene system to temperatures greater than 140 °C.2,7,8 These thermal attachment reactions have recently been shown to work in the gas phase as well as in solution.9 The growth of partial alkyl monolayers on H/Si(111) under ultrahigh vacuum (UHV) * Corresponding author. E-mail: [email protected]. † Current address: Institute for National Measurement Standards, National Research Council, Ottawa, Ontario, Canada. (1) Linford, M. R.; Chidsey, C. E. D. J. Am. Chem. Soc. 1993, 115, 12631. (2) Linford, M. R.; Fenter, P.; Eisenberger, P. M.; Chidsey, C. E. D. J. Am. Chem. Soc. 1995, 117, 3145. (3) Terry, J.; Linford, M. R.; Wigren, C.; Cao, R.; Pianetta, P.; Chidsey, C. E. D. Appl. Phys. Lett. 1997, 71, 1056. (4) Cicero, R. L.; Linford, M. R.; Chidsey, C. E. D. Langmuir 2000, 16, 5688. (5) Sun, Q. Y.; de Smet, L. C. P. M.; van Lagen, B.; Wright, A.; Zuilhof, H.; Sudholter, E. J. R. Angew. Chem., Int. Ed. 2004, 43, 1352. (6) Sun, Q. Y.; de Smet, L. C. P. M.; van Lagen, B.; Giesbers, M.; Thune, P. C.; van Engelenburg, J.; de Wolf, F. A.; Zuilhof, H.; Sudholter, E. J. R. J. Am. Chem. Soc. 2005, 127, 2514. (7) Boukherroub, R.; Bensebaa, F.; Morin, S.; Wayner, D. D. M. Langmuir 1999, 15, 3831. (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.

conditions has also been demonstrated if initial reactive sites (i.e., Si dangling bonds) are created.10,11 Although the mechanism of these reactions is still a matter of some debate, one aspect of monolayer formation in the photochemical systems is well understood. A radical chain reaction mechanism, first proposed by Chidsey,2 has been found to be responsible for the monolayer growth.12 In this mechanism, an alkene initially reacts with a dangling bond (Si radical) on the surface, breaking the C-C double bond and forming a covalent Si-C link and a secondary carbon radical. This radical can then abstract hydrogen from an adjacent site on the surface, creating a new reactive site for the next incoming alkene. The mechanism for the generation of this initial Si dangling bond, however, is not yet resolved (apart from the UHV studies where the dangling bonds were produced by electrons from the STM tip10,11). Whereas the radical initiators likely create dangling bonds via H abstraction, the proposal that the thermal and photochemical processes also break Si-H bonds is less plausible. On the basis of energetic considerations alone, photons of wavelengths of less than 365 nm are required to create a dangling bond whereas previous studies have suggested that the photochemical breaking of the Si-H bond requires 157 nm photons.13 Thus, although it is possible (although somewhat unlikely) that UV-induced reactions involve Si-H bond breaking, the more recently reported visiblelight-induced attachment reactions most certainly do not.6 The temperature of 140 °C used in the thermal reactions is also insufficient to break the Si-H bond. Because presumably only a small concentration of initiators is required to trigger the reaction, it is difficult to rule out the possible role of impurities, such as oxygen, in initiating these reactions. This work was motivated by the desire to limit the role of impurities that are invariably present during the formation of alkyl monolayers via solution-phase reactions. To accomplish this, we investigated reactions analogous to those carried out in solution in the well-controlled environment of a high-vacuum system. It was found that monolayer formation could indeed be (9) Kosuri, M.; Gerung, H.; Li, Q.; Han, S. M.; Bunker, B. C.; Mayer, T. M. Langmuir 2003, 19, 9315. (10) Lopinski, G. P.; Wayner, D. D. M.; Wolkow, R. A. Nature 2000, 406, 48. (11) Cicero, R. L.; Chidsey, C. E. D.; Lopinski, G. P.; Wayner, D. D. M.; Wolkow, R. A. Langmuir 2002, 18, 305. (12) Eves, B. J.; Sun, Q.-Y.; Lopinski, G. P.; Zuilhof, H. J. Am. Chem. Soc. 2004, 126, 14318. (13) Zhu, L.; Cronin, T. J. Chem. Phys. Lett. 2000, 317, 227.

10.1021/la052960a CCC: $33.50 Published 2006 by the American Chemical Society Published on Web 02/25/2006

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achieved via photochemical reactions in the gas phase although the wavelength dependence is quite different from that reported in solution. The gas-phase approach reported here offers a route to organic monolayer formation that is fully compatible with existing semiconductor processing methods such as chemical vapor deposition. We show that the initiation mechanism in these reactions involves the production of gas-phase radicals that abstract hydrogen atoms from the silicon surface, leading to the formation of reactive sites. Monolayer growth then proceeds via the propagating radical chain reaction discussed above or by direct reaction of gas-phase radicals with silicon dangling bonds. The later route provides a mechanism for monolayer formation of molecules incapable of undergoing the chain reaction. Implications of these results for the initiation mechanism operable during the wet chemical functionalization schemes are also discussed. Experimental Section Two types of n-type Si(111) wafers (double-side polished 120 Ω‚cm and single-side polished 1-5 Ω‚cm wafers) from Virginia Semiconductor were used in the present experiments. The wafers were cleaned in a 3:1 sulfuric acid/hydrogen peroxide solution (semiconductor grade) and subsequently rinsed in deionized water (Milli-Q). The cleaned wafers were hydrogen terminated in a degassed (15 min of bubbling with argon) solution of 40% NH4F (semiconductor grade) for 15 min and quickly rinsed in degassed deionized water before being placed directly into a vacuum chamber. All other chemicals were bought from Aldrich and degassed via freeze/pump/ thaw cycles using liquid N2. The surfaces were irradiated with a mercury pen lamp supplied by Oriel (model 6035). An irradiance of ∼2 × 10-4 W/cm2 has been measured15 for the pen lamp at a distance of 10 cm. The ultraviolet radiation produced by the lamp was dominated by the 254 nm line (∼95%) but was also found to contain a weaker 185 nm line (∼5%). A small turbopumped high-vacuum system and FTIR spectrometer were coupled together to perform transmission FTIR on double-side polished H-Si(111) wafers in a controlled environment. Zinc selenide windows were used to transmit the IR beam into the chamber, and a sapphire window (mounted orthogonal to the axis of the IR beam) was used to transmit the UV radiation. Care was taken to limit indirect illumination of the sample by the UV radiation through the use of apertures. The sample was mounted with the surface normal at approximately 45° with respect to the IR beam so as to enable the detection of the Si-H stretching intensity. The base pressure of the vacuum system was approximately 1 × 10-7 Torr. These experiments were conducted at reactant gas pressures of 2-30 Torr. Spectra were obtained at a resolution of 4 cm-1, and a spectrum measured from the clean H-Si(111) surface prior to exposure to the reactant gas was used as a reference. Scanning tunneling microscopy (STM) and high-resolution electron energy loss spectroscopy (HREELS) were carried out on samples in a separate ultrahigh vacuum (UHV) chamber. The STM used for the experiments was a UHV1 from Omicron. Samples were gently heated to less than 200 °C prior to imaging to remove any physisorbed contaminants. An LK3000 spectrometer (LK Technologies, Bloomington, IN) was employed for the HREELS measurements. Spectra were acquired in the specular geometry (60° with respect to the surface normal) at an incident beam energy of 6 eV and a nominal resolution of 4 meV. Samples could be freely moved between the STM and the HREELS without removing the sample from the UHV chamber. Gas-phase reactions were also performed in the load lock of the UHV system, employing a gas pressure of 1 Torr. The load lock was fitted with a sapphire window for transmission of the UV radiation. (14) Pussel, A.; Wetterauer, U.; Hess, P. Phys. ReV. Lett. 1998, 81, 645. (15) Mitchell, S. A. J. Phys. Chem. B 2003, 107, 9388.

Figure 1. Transmission FTIR of the loss in the Si-H bend (b) and the Si-H stretch (2) intensity as a function of the total reaction time for (a) a H/Si(111) surface exposed to 2 Torr of hexene and irradiated with a mercury pen lamp. The rate of reaction is severely inhibited when an optical filter blocks the 185 nm line of the mercury pen lamp for (b) a H/Si(111) surface reacting with 2 Torr of hexene. After 88 min, the filter is removed.

Results and Discussion To investigate the photochemical reactivity under gas exposure, a H/Si(111) surface (made from the low-doped, double-side polished wafers) was placed into the high-vacuum system coupled to the FTIR spectrometer. 1-Hexene was chosen as the initial reactant molecule because of its large vapor pressure and on the basis of the observation that alkenes react readily with Hterminated silicon in solution. Exposing a H/Si(111) surface to 4 Torr of hexene gas and irradiating with a white-light source for more than 3 h failed to alter the transmission IR spectrum of the H-terminated surface significantly. However, exposure of the H/Si(111) sample to hexene under the UV illumination of a mercury pen lamp resulted in a decrease in the intensity of the characteristic Si-H modes found at 636 cm-1 (Si-H bend) and 2083 cm-1 (Si-H stretch).16 This intensity loss was monitored while irradiating the sample and is plotted in Figure 1a. The intensity of the two Si-H modes track each other within experimental error, and the reaction is found to be rather rapid, appearing to reach completion in ∼8 min. This loss of Si-H is correlated with an increase in the intensity of the C-H stretching modes, although these modes were very weak (∼0.2 mA units) and their observation required pumping out the hexene gas, making them unsuitable for monitoring the reaction rate. The observation of these C-H modes, however, allows the loss of Si-H to be attributed to the growth of an alkyl monolayer rather than just hydrogen desorption. It is important to note that the loss of Si-H intensity is an indirect measure of the reaction and can be used only as a semiquantitative probe of the reaction kinetics. It is known from previous FTIR measurements of alkyl monolayers on Si(111) that the Si-H stretching mode appears to disappear completely (16) Stuhlmann, Ch.; Bogda´nyi, G.; Ibach, H. Phys. ReV. B 1992, 45, 6786.

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even though steric considerations limit the reaction to only 3050% of the available Si-H sites.17 This apparent disappearance of the Si-H stretch can be attributed to a broadening of this sharp mode as has been observed for hydrocarbon physisorption on this surface.18 Thus, the intensity loss plotted in Figure 1a reflects both the actual loss of surface hydrogen resulting from the addition reaction as well as the broadening effect of the alkyl monolayer on the residual Si-H species. The wavelength-dependent nature of the reaction is evident if a low-pass filter, which removes the 185 nm line, is placed between the sample and the source (Figure 1b). Exposures of up to 88 min did not result in a significant loss of the Si-H mode intensities. Upon removal of the filter, the reaction once again proceeds rapidly as before. This demonstrates conclusively that it is the weak 185 nm line produced by the Hg pen lamp that is responsible for inducing the reaction of hexene with the surface. This observation suggests that the adsorption of light by the reactant molecules is necessary for the reaction to proceed because alkenes typically have negligible absorption at wavelengths greater than 200 nm.19,20 Furthermore, benzaldehyde, which has a significantly lower threshold energy for light absorbance13 than hexene, reacted with the H/Si(111) surface when exposed to the 254 nm filtered mercury vapor pen lamp. Thus, the wavelength dependence of the reaction appears to be determined by the absorption properties of the gas-phase molecule and not the hydrogen-terminated silicon surface. Upon completion of the FTIR studies, the samples were transferred to the UHV system where HREELS and STM were used to characterize both the irradiated and nonirradiated sides of the samples. Figure 2a shows HREELS spectra for the sample used to collect the data previously shown in Figure 1. The spectrum of the irradiated side is characteristic of a surface that has reacted to completion and is similar to that of alkyl monolayers prepared via solution-phase methods.12 The small peak at 2095 cm-1 is attributed to the Si-H stretch from the residual hydrogens. (The lower intrinsic resolution of the HREELS experiments means that the broadening effect discussed above is not significant.) The characteristic peaks of an alkyl-terminated surface are found in the spectrum at 2950, 1450, 1360, and 1060 cm-1, which correspond to the C-H stretch, the CH2 scissor, the CH3 symmetric bend, and the C-C stretching modes, respectively.21,22 A close examination of the spectrum also reveals a peak at 670 cm-1 that can be attributed to the Si-C stretch,23-25 confirming that the alkyl chains are attached to the surface via a covalent link. The HREELS spectrum of the nonirradiated side of the sample (also shown in Figure 2a) also shows significant growth of an alkyl monolayer, though in this case the reaction has not gone to completion as evidenced by the relatively large peak at the Si-H stretching frequency and the noticeable Si-H bend at 630 cm-1. The HREELS results show that, surprisingly, monolayer growth occurs on both the irradiated and nonirradiated silicon surfaces. (17) Boukherroub, R.; Morin, S.; Sharpe, P.; Wayner, D. D. M. Langmuir 2000, 16, 7429. (18) Ye, S.; Ichihara, T.; Uosaki, K. Appl. Phys. Lett. 1999, 75, 1562. (19) Samson, J. A. R.; Marmo, F. F.; Watanabe, K. J. Chem. Phys. 1962, 36, 783. (20) Lee, S.-H.; Lee, Y.-Y.; Lee, Y. T.; Yang, X. J. Chem. Phys. 2003, 119, 827. (21) Sung, M. M.; Kluth, G. J.; Yauw, O. W.; Maboudian, R. Langmuir 1997, 13, 6164. (22) Kluth, G. J.; Carraro, C.; Maboudian, R. Phys. ReV. B 1999, 59, R10449. (23) Yoshinobu, J.; Tsuda, H.; Onchi, M.; Nishijima, M. Solid State Commun. 1986, 60, 801. (24) Huang, C.; Widdra, W.; Wang, X. S.; Weinberg, W. H. J. Vac. Sci. Technol., A 1993, 11, 801. (25) Yamada, T.; Inoue, T.; Yamada, K.; Takano, N.; Osaka, T.; Harada, H.; Nishiyama, K.; Taniguchi, I. J. Am. Chem. Soc. 2003, 125, 8039.

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Figure 2. (a) HREELS spectra of the irradiated (i) and nonirradiated (ii) sides of a double-side polished H/Si(111) wafer exposed to 2 Torr of hexene and illuminated for 8 min with a mercury pen lamp. (b) STM image (34 × 34 nm) of the irradiated side of a H/Si(111) sample that has been exposed to 30 Torr of hexene and illuminated for 3 min. The sample bias was -2.1 V, and the tunneling current was 20 pA.

To rule out the possibility that hot carriers transmitted through the sample were responsible for the observed reaction on the back side, two single-side polished wafers were placed back to back (but only in casual contact) and irradiated from the front side as before. The nonirradiated sample was still found to exhibit a partial monolayer as above. Further experiments in which the sample was turned away from the UV source, ensuring that only the gas-phase molecules were irradiated, showed that it is actually not necessary to irradiate the silicon for monolayer growth to occur. As a result, direct photochemical and hot-carrierinduced desorption of H atoms from the H/Si(111) surface (or any other mechanism involving light absorption in the sample) can be ruled out as the initiation mechanism for these gas-phase reactions. An STM image of a H/Si(111) surface irradiated for 3 min in the presence of 30 Torr of hexene is shown in Figure 2b. The image reveals a network of irregularly shaped islands strikingly similar to those observed in the growth of alkyl monolayers formed by solution-phase photochemical (447 nm) reactions.12 The meandering shape of these extended islands suggests that they have formed via a random walk process and that the monolayer growth proceeds by the propagating radical chain reaction discussed in the Introduction. It is interesting to compare the rate of reaction in the present gas-phase experiments (using 185 nm irradiation) with that observed previously in solution using 447 nm photons. For the solution-phase reactions, 3 min of irradiation led to the nucleation of a smaller number of isolated islands and a much lower coverage of attached molecules.12 Normalizing this rate to the photon fluence in the two cases, we

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Figure 4. HREELS spectra of the initial H/Si(111) surface (a) and nonirradiated side (b) of the sample after reacting with allylamine at 1 Torr for 5 min with the UV mercury lamp.

Figure 3. Reaction mechanism for the gas-phase photochemical attachment of hexene to the H/Si(111) surface. Gas-phase radicals are generated by UV irradiation. The generated radicals abstract H from the H/Si(111) surface to create Si dangling bonds. There are two possible mechanisms for monolayer growth. Hexene reacts with the dangling bonds via the radical chain reaction mechanism (path A) or radicals directly attach to the dangling bonds (path B).

can estimate that the initial island nucleation rate is approximately 4 orders of magnitude higher in the present case. The photodissociation of propene, the closest available analogue of hexene, has been studied in the literature.20 The absorption spectrum of gaseous propene cuts off around 192 nm. When irradiated with 157.6 nm photons, propene photodissociates into 11 distinct photofragments via 13 different dissociation pathways. For photon wavelengths greater than 185 nm, corresponding to the present experiments, 11 energetically feasible dissociation pathways are still available. The pathway with the highest branching ratio (60%) for 157.6 nm photons results in the formation of C2H2, CH3, and H, the latter two of which are radicals. Including the other dissociation pathways, 6 out of the available 11 photofragments are radicals. On the basis of our experimental observations and the known photodissociation channels of propene, a mechanism for monolayer formation via gas-phase photochemical processes can be proposed. This mechanism is depicted schematically in Figure 3. The 185 nm photons photodissociate a fraction of the hexene into multiple fragments, some of which are gas-phase radicals. These radicals can abstract hydrogen from the H-terminated surface and create Si dangling bonds, as has been observed for atomic H.26 These dangling bonds are then available to react with hexene, forming islands of covalently attached molecules via the propagating radical chain reaction mechanism (Figure 3, path A). Because this mechanism requires irradiation of just the gas-phase molecules and not the surface, it explains the observation of monolayer growth on the nonirradiated side of the sample. The simple model described thus far is complicated by the large number and type of radicals produced under UV irradiation, some of which could react with the surface, resulting in the grafting of multiple species and the formation of a heterogeneous monolayer. A competing route for monolayer formation occurs if the density of gas-phase radicals is sufficiently high that such a radical has a comparable probability of reacting with a silicon dangling bond as with an intact reactant molecule (Figure 3, path B). Because a radical reacting directly with a Si dangling bond would lead to no further reactions, the observation of extended island structures for hexene suggests that this is not the case (26) Koleske, D. D.; Gates, S. M.; Jackson, B. J. Chem. Phys. 1994, 101, 3301.

under the reaction conditions used here. However, we note that the size of the radical chain reaction islands can be suppressed by premature capping of the dangling bonds by gas-phase radicals. For molecules incapable of undergoing a radical chain reaction, monolayer formation can still occur via direct attachment of radical fragments with the generated dangling bonds. In contrast to the extended islands observed for the chain reaction, this process will result in the formation of isolated single molecules grafted to the surface. In an attempt to use the gas-phase approach to form a monolayer with a reactive terminal group that could be used for coupling complex molecules (such as biomolecules or molecular switches) to the surface, the reaction of allylamine was investigated. It was hoped that this bifunctional molecule would react with the surface through its alkene end, leaving a terminal amine group for further reaction. Irradiating allylamine with a mercury lamp in the presence of the H/Si(111) surface resulted in the growth of monolayers on both the irradiated and nonirradiated sides of the sample. The HREELS spectrum for the nonirradiated side is shown in Figure 4. The small intensity of the Si-H stretching mode indicates that the reaction has gone to completion on the nonirradiated side after only 5 min of irradiation (significantly quicker than hexene). However, an examination of the spectra indicates that the molecules have primarily reacted with the amine end of the molecule, forming Si-N bonds. The primary evidence for this is the presence of the mode at 812 cm-1 that can be assigned to the Si-N stretch.27 The peak at 510 cm-1 is attributed to the Si-N bending mode (presumably coupled to bulk Si phonon modes observed at 505 cm-1 on H/Si(111)). In addition, modes at 924 cm-1 (sp2-bonded CH2 wag), 1670 cm-1 (CdC stretch), and the shoulder at 3050 cm-1 (sp2-bonded C-H stretch) are indicative of a nonreacted terminal alkene group.28 The other peaks at 1094, 1420, and 2950 cm-1 can be assigned to the C-C stretch, CH2 scissor mode, and the C-H stretch for sp3-bonded carbon.21,22 Last, a small N-H stretch is visible as a shoulder at 3335 cm-1 and is attributed to the Si-bound amine (although a small number of terminal amines cannot be ruled out). No evidence of a Si-C bond at 670 cm-1 is observed. Although the reaction of allylamine with the surface did not result in the desired amine-terminated surface, the terminal alkenes produced by this reaction could still prove useful for coupling complex molecules to the surface via subsequent reactions. The photodissociation pathways of allylamine have not been reported in the literature. Methylamine, the closest analogue, has four main dissociation pathways when irradiated with 194-244 (27) Colaianni, M. L.; Chen, P. J.; Yates, Y. T., Jr. J. Chem. Phys. 1992, 96, 7826. (28) Pouchert, C. J. The Aldrich Library of Infrared Spectra, 2nd ed.; Aldrich Chemical Company: Milwaukee, WI, 1975; pp 1, 2.

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Figure 5. HREELS spectra of benzaldehyde-functionalized Si(111) surfaces prepared by two different methods. First, a H/Si(111) sample was exposed to atomic hydrogen to generate Si dangling bonds and subsequently dosed with 1 × 10-6 Torr of benzaldehyde for 10 min (a). A second H/Si(111) sample was irradiated with UV photons in 1 Torr of benzaldehyde for 3 min (b).

nm photons:29 H + CH3NH, H + CH2NH2, CH3 + NH2, H2 + CH3N. The majority (75%) dissociates into CH3NH and H radicals. The cleavage of the N-H bond has also been reported as the main dissociation route for n-butylamine, n-amylamine, and n-hexylamine irradiated by a mercury arc lamp.30 It is reasonable, therefore, to suggest that the main dissociation pathway for allylamine is also via the loss of a H off of the terminal amine group to form C3H5NH and H. The reaction of the former could account for the observation of species grafted through a Si-N bond and having a terminal alkene functionality as seen in the HREELS spectrum. We have separately confirmed that allylamine does not grow islands via the radical chain reaction mechanism at pressures up to 7 Torr. This was accomplished by generating dangling bonds on the H/Si(111) surface via exposure to H atoms generated by a hot filament in UHV. When a surface with a substantial number of dangling bonds, as confirmed by STM measurements, was exposed to allylamine at 7.2 Torr for 16 min no island formation was observed (not shown). The absence of a chain reaction for allylamine is consistent with previous studies that have reported the absence of this type of reaction for shorter-chain alkenes,10,31 presumably because of a shorter residence time on the surface. Thus, monolayer growth in this case does not appear to occur via the radical chain reaction process but rather via the reaction of gas-phase radicals directly with the surface, as discussed earlier. Although the initial stages of the gas-phase photochemical reaction with allylamine were not probed by STM, such studies were carried out for the reaction of 1,3-diaminopropane.32 For this system, STM studies confirmed the direct attachment mechanism, resulting in the attachment of isolated molecules in the initial stages rather than the islands observed when the chain reaction scheme is operable. Some potential difficulties of this gas-phase approach for functionalizing silicon are illustrated by the case of UV-stimulated reactions of benzaldehyde with hydrogen-terminated silicon. Figure 5 shows HREELS spectra for two H/Si(111) surfaces that were reacted with benzaldehyde via different processes. To create the first surface, isolated silicon dangling bonds were formed by exposure to H atoms in UHV. The surface was subsequently exposed to 1 × 10-6 Torr of benzaldehyde for 10 min. Benzaldehyde reacts with single dangling bonds to create surface (29) Waschewsky, G. C. G.; Kitchen, D. C.; Browning, P. W.; Butler, L. J. J. Phys. Chem. 1995, 99, 2635. (30) Calvert, J. G.; Pitts, J. N., Jr. Photochemistry; Wiley: New York, 1966; pp 455-460. (31) DiLabio, G. A.; Piva, P. G.; Kruse, P.; Wolkow, R. A. J. Am. Chem. Soc. 2004, 126, 16048. (32) Eves, B. J.; Lopinski, G. P., in preparation.

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species bound via a Si-O bond. The reaction proceeds via the radical chain mechanism as confirmed by STM images (not shown). The HREELS spectrum (Figure 5a) shows characteristic peaks at 800 and 1070 cm-1, corresponding to the Si-O and C-O stretches, respectively.33,34 The C-H stretch for sp2-bonded carbon is clearly visible at 3065 cm-1, whereas the C-H stretch for carbon in sp3 geometry is seen as a weaker shoulder at 2935 cm-1. Additional weak peaks at 1180, 1490, and 1600 cm-1 can be assigned to phenyl ring modes.35 The characteristic mode for monosubstituted phenyl rings, expected in the range of 700750 cm-1, is obscured by the tails of the more intense 630 and 800 cm-1 modes. The presence of the intense Si-H modes at 630 cm-1 (bend) and 2088 cm-1 (stretch) show that only a partial monolayer has been formed. The second surface was created by irradiating a H/Si(111) surface with the Hg vapor lamp in the presence of 1 Torr of benzaldehyde for 3 min. The spectra of the two surfaces are clearly different, indicating that the UV photochemical reaction has produced a different surface than that obtained for benzaldehyde simply reacting with Si dangling bonds. The weak intensity of the residual Si-H modes in Figure 5b suggests that the reaction has gone to completion. The phenyl ring modes at 710, 1490, and 1600 cm-1 along with the sp2 C-H stretch at 3065 cm-1 are observed more prominently than in Figure 5a, consistent with a higher coverage of aromatic species on the surface. The characteristic modes of the Si-O-C link at 800 and 1070 cm-1 are observed (as in Figure 5a); however, the relative intensity of these peaks has changed significantly. In addition, small upshifts of ∼20 cm-1 in these modes are observed. An additional new mode is also observed at 418 cm-1. For H/Si(111) surfaces with oxygen inserted into most of the available Si-Si back bonds, characteristic Si-O-Si modes are observed at ∼410, ∼830, and 1095 cm-1 with the last of these being the most prominent.36 The spectral changes observed for the photochemically reacted surface can therefore be explained by the incorporation of oxygen into the Si-Si back bonds. The apparent upshift of the 800 and 1070 cm-1 features can be accounted for by the growth of the Si-O-Si modes on the high-frequency side of both of these peaks. It is interesting that although the frequency of the Si-O-Si modes indicate a fairly high oxygen coverage the main residual Si-H stretching peak is not shifted, indicating that the majority of these Si sites have no or at most one O atom inserted into the three available back bonds. The high-frequency tail on the Si-H stretch is indicative of sites that have two or three back bonds oxidized because in these cases the Si-H stretch shifts to 2150 and 2250 cm-1, respectively.36 The fact that the Si-O-Si and Si-H modes indicate different extents of oxidation suggests that the oxidation is not uniform and is likely concentrated at silicon sites to which hydrocarbons have bonded rather than those that have remained unreacted. The change in relative intensity of the 800 and 1070 cm-1 features can also be partially explained by this oxidation although it is also possible that not all of the phenyl rings are bound to the surface via a Si-O-C link. It is evident that the photochemical reaction has produced a complex surface accompanied by a significant amount of oxidation. To understand these observations, the photodissociation products of benzaldehyde must be considered. Three photodissociation pathways have been observed:13 C6H5 + HCO, (33) Edamoto, K.; Kubota, Y.; Onchi, M.; Nishijima, M. Surf. Sci. 1984, 146, L533. (34) Stroscio, J. A.; Bare, S. R.; Ho, W. Surf. Sci. 1985, 154, 35. (35) Pouchert, C. J. The Aldrich Library of Infrared Spectra, 2nd ed.; Aldrich Chemical Co.: USA, 1975; p 497-498. (36) Ikeda, H.; Nakagawa, Y.; Toshima, M.; Furuta, S.; Zaima, S.; Yasuda, Y. Appl. Surf. Sci. 1997, 117/118, 109.

Formation of Organic Monolayers on Silicon

C6H5CO + H, and C6H6 + CO. The first two pathways generate a number of radical species that are expected to abstract H and create dangling bonds but will also likely react with the surface, leading to the incorporation of oxygen into the Si-Si back bonds. The C6H5 and C6H5CO radicals are likely to react with Si dangling bonds leading to phenyl rings bound to the surface by Si-C rather than Si-O-C bonds. Thus, the example of benzaldehyde indicates that not all molecules are suitable for the formation of high-quality monolayers via the gas-phase route. The formation of multiple photodissociation products is likely to lead to the incorporation of multiple species on the surface. Oxygencontaining radicals should probably be avoided because they could lead to unwanted oxidation of the surface. We note that recently the photochemical (254 nm) reaction of aliphatic alcohols and aldehydes with the Si(111) surface under solution-phase conditions has been reported to lead to monolayers attached via Si-O-C linkages.37 The present observations with benzaldehyde suggests that the possibility of unwanted oxidation reactions should probably also be considered in those systems. In addition to offering a new approach to the formation of molecular monolayers on silicon, the observations reported here have some mechanistic implications for the commonly used solution-phase functionalization methods involving photochemical reactions of alkenes. The wavelength dependence of the gasphase reactions is determined by the absorbance spectrum of the molecule, requiring wavelengths shorter than 200 nm for alkenes. In contrast, the solution-phase reactions with 1-alkenes occur for wavelengths ranging from 254 to 650 nm and are obviously not limited by the absorbance spectrum of the reactant molecules. Discoloration of the reactant solution (1-alkene in mesitylene) has been reported for prolonged irradiation using 254 nm photons,6 indicating the formation of additional species, likely including radicals. Clearly, photochemical processes are occurring in solution and could be responsible for initiating monolayer formation. However, because alkenes should not absorb significantly at 254 nm it is not clear what is responsible for the UV-induced reactions. Because Hg pen lamps are typically used in these experiments, it is possible that the weak 185 nm spectral line is responsible; however, it is unlikely that this line would penetrate the glassware commonly used in these experiments. Impurities in the solution are a more likely culprit for the absorption at 254 nm and hence could be the initiators of monolayer growth at this wavelength. For example, molecular oxygen is known to form charge-transfer complexes with alkenes that extend their absorption out to 300 nm and lead to photochemical reactions at 254 nm.38 In this regard, it is worth noting that alkyl monolayers made via solution-phase photochemical reactions typically exhibit a significant coverage of oxygen3,4,7,39 that has likely incorporated into the Si-Si back bonds. Using the gas-phase approach reported here, it might be possible to lower the degree of oxidation accompanying the formation of the molecular monolayers. It is harder to account for the observed reactions at wavelengths longer than 350 nm because it is difficult even to identify a plausible impurity to absorb at this wavelength. However, silicon does absorb appreciably at these wavelengths, which has led to suggestions of a mechanism involving exciton formation followed by hole transfer to the alkenes.6,40,41 Although no evidence for such a route was found in the present gas-phase experiments (no (37) Hacker, C. A.; Anderson, K. A.; Richter, L. J.; Richter, C. A. Langmuir 2005, 21, 882. (38) Hashimoto, S.; Akimoto, H. J. Phys. Chem. 1987, 91, 1347. (39) Miramond, C.; Vuillaume, D. J. Appl. Phys. 2004, 96, 1521. (40) Stewart, M. P.; Buriak, J. M. J. Am. Chem. Soc. 2001, 123, 7821. (41) Langner, A.; Panarello, A.; Rivillon, S.; Vassylyev, O.; Khinast, J. G.; Chabal, Y. J. J. Am. Chem. Soc. 2005, 127, 12798.

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reaction was observed for prolonged irradiation with white light), it is important to note that the flux of molecules impinging on the surface at 1 Torr is still several orders of magnitude lower than in solution. The reduced flux of molecules at the sample in the case of the gas-phase reactions is expected to reduce the probability of possible reactions involving hole transfer to the alkene. Conversely, the higher mean free path in the gas phase will increase the probability of reactive radicals impinging on the surface. The fact that the solution-phase reaction of alkenes at 447 nm is approximately 4 orders of magnitude less efficient (on a per photon basis) than the UV gas-phase reaction is further evidence that different mechanisms are likely operable in the two cases. Detailed kinetic studies of the solution-phase photochemical reactions, particularly further measurements of the wavelength dependence, are required to answer the remaining mechanistic questions definitively. Finally, we note certain similarities between the mechanism proposed here and that reported very recently for solution-phase photochemical (254 nm) reactions of alkenes with H-terminated diamond surfaces.42 In that study, the attachment reactions are also demonstrated to involve the formation of radical species that lead to the formation of reactive sites on the surface. However, for these reactions the radicals are not produced by direct photodissociation of the reactant molecules but rather arise from light absorption in the substrate and photoeletron ejection from the diamond substrate (facilitated by the negative electron affinity of the H-terminated diamond surface). Gas-phase functionalization reactions of the type demonstrated in the current work are also likely to proceed on these diamond surfaces.

Conclusions The exposure of H/Si(111) surfaces to gas-phase molecules irradiated with UV photons has been shown to lead to the growth of molecular monolayers. The monolayer growth was demonstrated to be initiated via the abstraction of H from the surface via gas-phase radicals generated by the photodissociation of the reactant molecules. The growth of the monolayers has been shown to proceed either by the radical chain reaction mechanism or by direct attachment of gas-phase radicals to silicon dangling bonds. In these gas-phase reactions, the interaction of UV photons with the silicon surface is not necessary for monolayer growth to occur, ruling out mechanisms involving direct photolysis of the Si-H bond or electron-hole pair generation in the silicon substrate. The wavelength dependence and rate of reaction are found to be clearly different from those observed in the case of the solution-phase reactions, suggesting that different mechanisms are likely operating in the two cases. The gas-phase methods reported here have potential advantages over solution-phase approaches because they are not limited to molecules capable of undergoing the radical chain reaction and are likely to result in higher-quality surfaces with lower levels of impurity (particularly oxygen) and can be implemented in a chemical vapor deposition system. The usefulness of this method is limited, however, to molecules with a significant vapor pressure that photodissociate into a limited number of reactive species. Acknowledgment. We thank D. Moffatt for technical assistance and T. Mischki, S. Mitchell, and D.D.M. Wayner for helpful discussions. LA052960A (42) Nichols, B. M.; Butler, J. E.; Russell, J. N.; Hamers, R. J. J. Phys. Chem. B 2005, 109, 20938.