Evidence of Self-Assembled Monolayers Preorganization Prior to

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Evidence of Self-Assembled Monolayers Preorganization Prior to Surface Contact: a First Principles Study Jean-Marie Duce´re´,† Alain Este`ve,*,† Ahmed Dkhissi,† Mehdi Djafari Rouhani,†,‡ and Georges Landa† CNRS, LAAS, 7 aVenue du colonel Roche, F-31077 Toulouse, France, and UniVersite´ de Toulouse, UPS, INSA, INP, ISAE, LAAS, F-31077 Toulouse, France ReceiVed: March 9, 2009; ReVised Manuscript ReceiVed: July 6, 2009

We explore the main gas and liquid phase chemistry that governs the typical self assembled monolayers (SAMs) deposition processes before precursors interaction with the surface. A number of pathways are determined (enthalpies and associated activation barriers). These include gas phase hydrolysis, effect of low polar solvent, effect of an excess of water molecules, and gas and liquid phase self-condensation of silane precursors. We demonstrate that partial to full hydrolysis is easily performed in the presence of water molecules. Subsequently, condensation is also shown to occur indicating that preorganization of SAMs is to be expected in practice in the gas and liquid phase, leading to the actual deposition of SAMs aggregates. TABLE 1: Hydrolysis Reaction Energies

Introduction Organosilane self-assembled monolayers (SAMs) grown on silica surfaces have attracted considerable attention due to their importance for surface functionalization in a number of practical micro- and nanodevices. The field of applications is very broad and spans domains such as MEMS (micro electro mechanical system) for the improvement of the tribological properties of micromachines,1 chemical2 and biosensors,3 microelectronics,4 and more generally surface engineering,5 particularly for improving biocompatibility in the field of biomaterials.6,7 Typically, these films are prepared by dipping samples into an organosilane precursor (chloride or alkoxide) solution containing some amount of water. This water can either be added as a reactant or originate from surface moisture extracted by a dry solvent. Water molecules are aimed at promoting the grafting to the surface through hydrolysis of the precursors.8 The dual precursor hydrolysis, either in solution or in contact with the surface hydroxyl groups, already points to the difficulty in understanding and balancing surface versus liquid phase chemical processes. It is to be noted that gas phase deposition of SAMs has also been proposed more recently.9,10 In this context, a wide range of both experimental and theoretical studies have shown the importance of a number of parameters such as the temperature,11 the nature of the solvent12,13 and its water content,12,14-17 the age of the solution,16 the time of reaction,10,11,13,15 or possible post-treatments of layers.18 Indeed, the literature, especially experimental, cannot be fully here referenced. In the last year 2008, a number of studies reported on the optimization of the growth of different organosilane monolayers on several substrates.8,13,19-31 The review by Wen et al.28 on the elucidation of the self-assembly mechanisms, the molecular organization, and the modes and dynamics of intra- and interlayer bonding prevailing in highly ordered organosilane films offers an interesting and helpful reference overview. * Corresponding author. Tel.: 33 5 61 33 63 53. Fax: 33 5 61 33 62 08. E-mail: [email protected]. † CNRS, LAAS. ‡ Universite´ de Toulouse.

reaction

∆E (kJ.mol-1)

PrSiCl3 + H2O f PrSiCl2(OH) + HCl PrSiCl2(OH) + H2O f PrSiCl(OH)2 + HCl PrSiCl(OH)2 + H2O f PrSi(OH)3 + HCl

-11 -4 +4

However, these studies are all dealing with the quality of the final layer, i.e., organosilane SAMs/SiO2 interface, but very few are looking at the chemical reactions occurring in the solution32,33 or in the gas phase, whereas one can expect the final layer to be highly dependent on the nature of the species that actually graft to the surface. For instance, the experimental studies34 showed that, under certain conditions, gas phase reactions can be enhanced in order to better control the growth and to form homogeneous organic films onto microelectronics substrates. In this context, it is surprising to notice the poorness of modeling data to elucidate this specific liquid/gas phase chemistry. Actually, only few investigations do exist on model systems drawing organosilanes grafting onto a silica or hydroxylated silicon substrate. Noticeable exceptions are refs 17 and 35-40 and only ref 35 provides some kinetic data, with the determination of an activated surface mechanism, through DFT-driven molecular dynamics. We wish to point out the fact that this last barrier strongly suggests that, beyond investigated modelsystems, a liquid/gas phase chemistry is to be considered to arrive at a reasonable matching with experimental view points. Even fewer study have concentrated on SAMs chemistry in solution17,38,41 with no kinetic data available. We therefore consider as strategic any information shedding light onto these mechanisms. In this frame, quantum-based modeling is a suitable tool to investigate mechanisms of the chemical reactivity of water on the surface, for the periods of injection of water (or hydrolysis), otherwise technically difficult to study. This is why we perform density functional theory calculations on propylchlorohydroxysilanes in order to gain a better understanding of the reaction paths of the chemical reactions occurring prior to silane grafting. In the first part of this contribution, we study the hydrolysis of propyltrichlorosilane, namely the different steps of the reaction and the effect of an excess of water. In the second part, we deal with the condensation and condensa-

10.1021/jp9021213 CCC: $40.75  2009 American Chemical Society Published on Web 08/06/2009

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Figure 1. (a) Structures of the species involved in the hydrolysis reactions; (b) hydrolysis reaction pathways.

tion reactions that can occur between silane molecules. For this purpose, we consider the reaction of propyltrichlorosilane with propyldichlorohydroxysilane and the dimerization of propyltrihydroxysilane, so that we have information on the reactivity at two levels of the hydrolysis. Computational Details All of the calculations were carried out with the Turbomole 5.9 suite of programs42 within the frame of the density functional theory using the PBE gradient-corrected functional.43 We used the def2-TZVP44 basis set included in the Turbomole package. All of the calculations took advantage of the resolution of the identity (RI-J) and of the multipole accelerated resolution of the identity (MARI-J) approximations45,46 that provide substantial time savings without significant loss of accuracy. These approximations require the use of an auxiliary basis set; we used the corresponding def2-TZVP auxiliary basis set provided by Turbomole.47 The calculations in solvent were performed using the conductor-like screening model (COSMO).48 Successive Hydrolysis Reactions. We consider first the gas phase hydrolysis of propyltrichlorosilane, i.e., the successive

replacement of the three chlorine atoms by three hydroxyl groups mediated by three water molecules (stoichiometric hydrolysis reactions). The corresponding overall reaction energies are gathered in Table 1, and structures and energy profiles are detailed in parts a and b of Figure 1, respectively. The three hydrolysis reactions proceed the same way: an approaching water molecule starts to interact with the chlorine atoms and potentially, after partial hydrolysis, with the existing hydroxyl groups of the silane through hydrogen bonds. In a second step, a bimolecular nucleophilic substitution occurs: H2O binds to the silicon atom, a Si-Cl bond breaks, a proton jumps from H2O to the freed chlorine atom to form HCl. Then HCl leaves to interact with a silane hydroxyl group nearby through hydrogen bonds. This reaction, which is the only one to require an activation energy (78, 103, and 95 kJ mol-1 respectively for first, second, and third hydrolysis), proceeds without intermediates. In all cases, the transition state is located between the Si-Cl cleavage and the proton jump. Finally, the silane and HCl separate to infinity. As can be seen, the trichlorosilane exhibits a behavior different from the partially hydroxylated ones. In the first place,

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TABLE 2: Nucleophilic Substitution Energies and Barriers in Solvents ∆E (kJ mol-1) reaction

gas phase ε ) 1.00

toluene ε ) 2.38

dichloroethane ε ) 10.36

methanol ε ) 32.63

PrSiCl3-H2O f PrSiCl2(OH)-HCl PrSiCl2(OH)-H2O f PrSiCl(OH)2-HCl PrSiCl(OH)2-H2O f PrSi(OH)3-HCl

-22 (Ea ) 78) +9 (Ea ) 103) +11 (Ea ) 95)

-20 (Ea ) 72) +12 (Ea ) 96) +11 (Ea ) 89)

-19 (Ea ) 63) +15 (Ea ) 86) +12 (Ea ) 83)

-19 (Ea ) 60) +14 (Ea ) 83) +12 (Ea ) 81)

the interaction energy with water is much lower than when hydroxyl groups are present. This can be easily understood as Cl has no hydrogen-bonds donating and much weaker accepting capabilities than a hydroxyl group. Second, the substitution of the first chlorine atom is the only one to be exothermic. Also, the activation energy is lower for the first substitution. This behavior can be ascribed to the fact that the presence of hydroxyl groups enables the possibilities to build hydrogen-bonds. As these bonds are stronger with H2O than with HCl, the H2Ocoordinated state is favored with respect to the HCl-coordinated one. The last step is the release of HCl. This is an endothermic reaction for which the energy requirement increases with the number of hydroxyl groups as the possibilities to build hydrogen-bonds increase. We can conclude this part by noticing that if HCl is not eliminated from the medium to displace the substitution reaction equilibrium toward the HCl formation, the stoichiometric hydrolysis is expected to be far from complete, stopping after the first chlorine atom replacement. However, this should not be an issue for reactions in solvent as HCl has

TABLE 3: Hydrolysis Reaction Energies in Presence of Water Excess reaction

∆E (kJ mol-1)

PrSiCl3-H2O + H2O f PrSiCl2(OH)-H2O + HCl PrSiCl2(OH)-H2O + H2O f PrSiCl(OH)2-H2O + HCl PrSiCl(OH)2-H2O + H2O f PrSi(OH)3-H2O + HCl

-41 -4 -2

a low solubility in low polarity solvent and therefore should bubble out the solution, making each substitution an irreversible reaction. The effect of the solvent is considered next. The above three substitution reactions, which are the rate-determining steps, are now calculated in solvent using COSMO in order to study the solvent influence on these reactions. We choose three solvents spanning a wide range of dielectric constant: toluene (ε ) 2.38), dichloroethane (ε ) 10.36), and methanol (ε ) 32.63). In particular, toluene is widely used for experimental deposition of alkylsilane SAMs and has been claimed to be one of the best solvents for this purpose.9 It is important to note that

Figure 2. (a) Structures of the species involved in the “water-assisted” hydrolysis reactions; (b) “water-assisted” hydrolysis reaction pathways.

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Figure 3. (a) Structures of the species involved in the aggregation and condensation reactions; (b) aggregation and condensation reaction pathways.

TABLE 4: Condensation Reaction Energies reaction

∆E (kJ mol-1)

PrSiCl3-(HO)Cl2SiPr-H2O f PrSiCl2OCl2SiPr-H2O + HCl PrSi(OH)3s(HO)3SiPr-H2O f PrSi(OH)2O(OH)2SiPr + H2O

-14 -30

COSMO only allows modeling of the dielectric effect of the solvent but not specific interactions like solute-solvent hydrogen bonds. This last point is particularly relevant concerning methanol. As could be expected for a low polarity solvent such as toluene, the calculated energies (Table 2) are very close to those obtained from gas phase calculations: within 3 kJ mol-1 for the reaction energies and the activation energies 6-7 kJ mol-1 lower. This means that toluene does not play a significant role in these reactions. Increasing the polarity of the solvent does not change the reaction energies but results in a small, yet consistent, lowering of the activation barriers which can be as large as 20 kJ mol-1 in methanol. The main function of the solvent, besides lowering the reaction barriers (in particular for polar solvents), appears to be the dissolution of H2O and HCl, thus inhibiting the complex to go through back reactions and making the reaction irreversible. Therefore, the liquid phase hydrolysis kinetics and potential completion will depend on the exact nature of the solvent. These results also mean that our gas phase calculations can be reliably extended to solvents, as long as specific solvent-solute interactions can be neglected. Beyond pure dielectric effects induced by the nature of the solvent (polar or not), the presence of water excess is of major

importance: it may drastically modify the hydrolysis chemistry seen before. Its presence, either residual in many anhydrous solvent or close to the substrate surface, is to be considered. To investigate its detailed chemistry, we have chosen to model the effect of excess water by first putting a water molecule in interaction with the silane (first step of our hydrolysis study) and then making this complex react with a second water molecule. The chemical mechanism proceeds differently at the molecular level, as we have now a “water-assisted” nucleophilic substitution. However, from a kinetic point of view, the elementary steps previously described remain essentially the same with a slight change in the hydrolysis activation barrier which is lowered in the presence of excess water. The results are presented in Figure 2a,b and Table 3. Following the structure evolution in Figure 2a, the second incoming water molecule interacts with Cl, OH, and H2O. Then, in a single step, it coordinates to Si, a Si-Cl bond breaks, one proton jumps from the coordinated H2O to the other to form H3O+, then one proton jumps from the so-formed H3O+ to Cl and HCl eventually leaves to further interact with H2O. Finally, as previously, HCl is released. As for the stoichiometric reactions, the effect of the presence of this additional water molecule depends on the number of chlorine atoms bonded to Si. For the first chlorine atom substitution, the additional water molecule allows a stronger interaction with the second H2O and makes the substitution itself much easier by increasing the energy gain of the reaction. These contributions make the overall substitution more favorable by 30 kJ mol-1. Moreover, the presence of this water molecule results in a drastic lowering of the barrier by 46 kJ mol-1. For

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Figure 4. (a) Structures of the species involved in the “water-assisted” aggregation and condensation reactions; (b) “water-assisted” aggregation and condensation reaction pathways.

the second and the third substitutions, the presence of the water molecule increases the strength of the interactions with both H2O and HCl by 10-15 kJ mol-1, while not changing the energy of the substitution reaction itself. So in these cases, the overall reaction balance remains mainly unchanged. However, the substitutions proceed faster as the water molecule allows a moderate (2 Cl) or small (1 Cl) lowering of the activation barrier. We believe that the effect of the water molecule lowering the activation barrier in the three cases originates from a lowering of the strain in the transition state: without this molecule, the transition state contains a four-member ring, whereas with this additional water molecule, it is a six-member ring. The additional water molecule allows the entering one to come closer to the silicon atom without increasing the Cl-O repulsion and maintaining the distance Cl-H. For instance, the presence of this additional water molecule enables the Si-O bond to shorten by 0.06 Å in the transition state of the first hydrolysis reaction: 1.83 vs 1.77 Å. Finally, we can point out that, as the additional water molecule does significantly facilitate the monochloro- and dichlorosilane hydrolysis; full silane hydrolysis still relies mainly on the ability to eliminate HCl from the reaction medium. Condensation and Condensation Reactions. We now investigate the possibility of precursor condensation. We are interested in the first stage of the polymerization of the silane, i.e., the formation of the first Si-O-Si sequence. We present the reaction paths for the condensation and condensation reactions of a trichlorosilane with dichlorohydroxysilane and for the dimerization of the trihydroxysilane. We chose these reactions as they are representative of the condensation, respectively at low and high levels of hydrolysis. We first

investigate stoichiometric reactions before considering polymerization reactions in the presence of a water molecule. The results are given in Figure 3a,b and Table 4. The two stoichiometric reactions proceed this way: at first, the silanes start to interact through hydrogen bond(s). Then, one hydroxyl group coordinates to the silicon atom of the other silane while a proton is transferred from this OH group to a chlorine atom (or to the other OH), forming a hydrogenbonded HCl (or H2O) and a Si-O-Si bonds sequence. Finally, the so-formed siloxane-bridged system and HCl (or H2O) separate. The main difference between the two reactions is that the trihydroxysilane dimerization involves much larger energies whatever the step. This is mainly explained by the fact that the hydroxyl groups can build more and stronger hydrogen bonds than the chlorine atoms. Thus, the interaction of two trihydroxysilanes is much stronger than the interaction of trichloro- and dichlorohydroxysilane as in the first case two hydrogen-bonds between hydroxyl groups are built while, in the second, only one is formed between a hydroxyl group and a chlorine atom. The second step is also more favorable in the case of trihydroxysilane dimerization as the so-formed water molecule bridges two hydroxyl groups while HCl only interacts with a chlorine atom through a weak hydrogen-bond. However, the activation barrier is slightly lower in the case of the formation of HCl (105 vs 114 kJ mol-1). Finally, the release of HCl (13 kJ mol-1) is much easier than that of H2O (61 kJ mol-1) due to the hydrogen-bonds that favor H2O formation with respect to HCl formation. From these calculations, we can see that the condensation process can start as soon as hydrolysis has started. Indeed, one

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TABLE 5: Condensation Reactions Energies in Presence of Water Excess reaction

∆E (kJ mol-1)

PrSiCl3-(HO)Cl2SiPr-H2O f PrSiCl2OCl2SiPr-H2O + HCl PrSi(OH)3s(HO)3SiPr-H2O f PrSi(OH)2O(OH)2SiPr + H2O

-14 -30

OH group is necessary to initiate the reaction but as soon as it is present, the condensation reaction can spontaneously proceed, even if it is weakly energetic. A higher hydrolysis level enhances the condensation process as it becomes more favorable: the energy for forming the siloxane sequence goes from -11 to -29 kJ mol-1 and the overall reaction energy (considering HCl and H2O releases) goes from -7 to -21 kJ mol-1. As for the single precursor hydrolysis, we study also the effect of an additional water molecule on the reactions. The results are displayed in Figure 4a,b and Table 5. The additional water molecule seems to play a different role for the condensation and the hydrolysis reactions. While the excess water made the hydrolysis reactions faster, mainly by lowering the strains in the transition states, it did not substantially change the core nucleophilic substitution reaction energy, except in the case of the trichlorosilane. Here, it has a slightly negative influence on the condensation of the chlorosilane, decreasing the energy gain by 6 kJ mol-1 and increasing the barrier height by 14 kJ mol-1. In the trihydroxysilane dimerization, we observe that the excess water molecule slightly increases the reaction energy, by 5 kJ mol-1, and lowers the barrier, by 17 kJ mol-1, making the trihydroxysilane polymerization an autocatalytic reaction as it releases water, which in turn enhances the reaction rate. Conclusion Using DFT calculations, we have studied the hydrolysis and condensation of propyltrichlorosilane. We show that hydrolysis occurs in the presence of water molecules. Its kinetics and completion is essentially monitored by the propensity of the solvent to purge the hydrolysis reaction product (HCl) thus hindering back reactions. A larger water supply does not change this tendency but accelerates the overall hydrolysis. We further demonstrate that the silane condensation can start as soon the hydrolysis has begun. Yet, the condensation is initially a low energy process that becomes more energetic when the hydrolysis level increases. The presence of water tends to make the condensation slower for highly chlorinated species but faster for highly hydroxylated ones. These results indicate that SAM precursors in the presence of water molecules will react and preorganize prior to surface contact, leading to the deposition of aggregates. Acknowledgment. We thank CALMIP supercomputer center for CPU resources. We also thank projects ANR-NANOBIOMOD 05-JC05-58935 for financial support. References and Notes (1) Ashurst, W. R.; Yau, C.; Carraro, C.; Maboudian, R.; Dugger, M. T. J. Microelectromech. Syst. 2001, 10, 41. (2) Price, P. M.; Clark, J. H.; Macquarrie, D. J. J. Chem. Soc., Dalton Trans. 2000, 101. (3) Sassolas, A.; Leca-Bouvier, B. D.; Blum, L. J. Chem. ReV. 2008, 108, 109. (4) Collet, J.; Tharaud, O.; Chapoton, A.; Vuillaume, D. Appl. Phys. Lett. 2000, 76, 1941. (5) Onclin, S.; Ravoo, B. J.; Reinhoudt, D. N. Angew. Chem., Int. Ed. 2005, 44, 6282.

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