ARTICLE pubs.acs.org/JPCC
Addition of Hydrocarbons to H Si(100) in Extra-Mild Conditions: A Novel Mechanism Valid for Single and Multiple C C Bonds Maurizio Cossi,*,† Alice Boccia,‡ Andrea G. Marrani,‡ and Robertino Zanoni‡ †
Dipartimento di Scienze e Tecnologie Avanzate (DISTA), Centro Interdisciplinare Nano-SiSTeMI, Universita del Piemonte Orientale, via T. Michel 11, I-15100, Alessandria, Italy ‡ Dipartimento di Chimica, Universita degli Studi di Roma “La Sapienza”, piazzale Aldo Moro 5, I-00185, Roma, Italy ABSTRACT: The establishment of strong C Si bonds connecting a molecular moiety to the Si surface has been widely reported with different synthetic recipes, but general and reliable reaction mechanisms have not been described yet for the distinct chemical routes. The coupling of a suitable functional group in the molecule with a reactive termination of the Si surface is a prerequisite for the reaction to happen, and the presence of a C C multiple bond has long been thought to be necessary for an extra-mild attachment, as in the visible light induced organics Si anchoring reaction. In this paper, the addition of saturated and unsaturated hydrocarbons to the hydrogenated Si(100) surface has been modeled by density functional theory calculations. The aim is to describe a mechanism allowing for the addition of single C C bonds to Si(100) and the addition of CdC bonds with preservation of the unsaturation. In fact, both these reactions have been observed recently, but they are not explained by radical-initiated hydrosilylation, the more commonly invoked mechanism for this class of processes. The mechanism proposed here is described by computing the reaction path in the ground state and recomputing the energies in the first excited state. Both for saturated hydrocarbons and for unsaturated hydrocarbons we found that the activation barriers in the excited state reduce to about 60 65% of their ground state value. The barrier lowering is explained in terms of the frontier orbital change along the reaction path. These findings can explain why visible light can activate the formation of a C Si bond, even if it is not energetic enough to break a H Si bond.
1. INTRODUCTION In the past 20 years, the functionalization of oxide-free silicon with organic and organometallic monolayers has been both studied as a basic research issue and applied in different emerging fields such as nanoelectronics, nanosensoring, and biological interfaces at the nanoscale [for a recent general review, see, e.g., ref 1]. These monolayers are characterized by strong covalent Si C bonds, resulting in direct electronic coupling between the molecular functionality and the silicon substrate. Different wet-chemistry procedures have been developed to anchor the organic molecules onto silicon wafers. The first one was simple thermal hydrosylilation,2 involving H-terminated Si surfaces and substituted alkenes (or alkynes): at high temperature radicals are formed on the surface, by abstraction of hydrogen, which initiate chain reactions leading to ordered organic monolayers.3,4 Alternative functionalization methods have been proposed, based on UV irradiation of substituted alkenes and alkynes contacting silicon surfaces,5 or on chemomechanical activation of the surface:6 the proposed mechanisms are initiated by Si radicals, able to start the addition reaction. Moreover, the formation of high-quality, stable monolayers has also been reported under extra-mild conditions, well suitable for building biointerfaces, represented by irradiation with visible light.7,8 The formation of radicals has been proposed even under such conditions, where the energy provided to the Si surfaces is r 2011 American Chemical Society
smaller than the activation barrier for homolytic cleavage of Si H bonds.9 This reaction was first modeled in the case of porous silicon as a nucleophilic attack by the unsaturated hydrocarbon to a localized charge photogenerated on the surface,10 a model which has been extended to flat Si.8 Alternatively, electron photoemission from the Si substrate and their capture by the approaching molecule has been proposed as the initiation step of the addition reaction.11 All the mechanisms cited above are based on the hydrosilylation reaction, namely, the insertion of an unsaturated bond (e.g., from alkene, alkyne, or carbonyl groups) in Si H bonds. This reaction implies the reduction of the unsaturation so that the addition of alkenes results in saturated monolayers, and the addition of alkynes results in monolayers with a single unsaturation. On the other hand, we have recently described the properties of a series of organometallic monolayers on Si(100) and Si(111) surfaces,12 17 studied by a combination of theoretical modeling and X-ray photoelectron spectroscopy (XPS), atomic force microscopy (AFM), and electrochemical characterization. The investigated species were ethyl-, vinyl-, and ethynylferrocene, carrying a C C side chain with an increasing degree of unsaturation, and the monolayers were obtained both with Received: May 19, 2011 Revised: August 23, 2011 Published: August 30, 2011 19210
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Figure 1. Addition of propane (a) and propene without loss of unsaturation (b) to monohydrogenated (100) Si Si dimer.
extra-mild visible light activation and with alternative wetchemistry routes, specifically intended for the preservation of the C C bond order. This study led to two important observations which cannot be explained in terms of the usual hydrosilylation mechanism: (i) The monolayer quality and the electrochemical response were strictly comparable for the hybrids obtained with the two approaches. In fact, the formal redox potentials show an increase at increasing C C unsaturation degree. The same trend is reproduced by density functional theory (DFT) calculations on the Si-molecule clusters.12 (ii) Even a molecule bearing a saturated side chain, ethylferrocene, yields a stable monolayer on the silicon surface.12 14 The above results call for an alternative mechanism for the addition of hydrocarbons to silicon, activated by visible light, suitable for both saturated and unsaturated species, and not involving the reduction of the unsaturation degree. Very recently, a consistent degree of unsaturation in monolayers formed by attaching 1-alkenes on silicon surfaces was detected by NEXAFS (near-edge X-ray adsorption fine structure spectroscopy): this observation was explained in terms of the usual hydrosilylation mechanism, with suitable modifications.18 Such a picture, however, cannot be used to interpret the addition of saturated alkanes, as mentioned above. Previous theoretical analyses concentrated mainly on the study of the radical-inititated hydrosilylation,19 23 also to account for the reactions observed under mild conditions.24,25 Reboredo et al.26 have followed an approach similar to ours, studying the plausibility of optical activation for the hydrosilylation of silicon quantum dots; however, they described the usual process leading to the reduction of the unsaturation degree. In the following we present a computational study of the addition reaction of model hydrocarbons to a small silicon cluster, reproducing a portion of the monohydrogenated Si(100) surface, to investigate the changes in the activation barrier plausibly induced by visible light absorption. Note that on clean (nonhydrogenated) Si(100) surfaces the atoms are arranged to form rows of Si Si dimers,27,28 with a partial π character (analogous to C C double bonds, with the important difference that Si dimers are buckled due to an unbalanced charge distribution).29,30 This feature is lost in monohydrogenated surfaces, where only Si Si single bonds are left and the surface is flat, with no dimer tilting.31,32
2. METHODS The silicon surface was modeled by a Si35H42 cluster, reproducing a portion of the 2 1 hydrogenated (100) surface: in this reconstruction the silicon dimers present on the (100) face are monohydrogenated, leaving Si Si bonds which can be broken to allow the nonreductive addition of alkanes and alkenes. To check the stability of the results with respect to the surface size, some calculations were repeated on a Si55H62 cluster, presenting one more Si Si dimer. Propane and propene were used to model the saturated and unsaturated organic reactants, respectively. All the calculations have been performed with the Gaussian 03 package33 at the DFT level with B3LYP hybrid functional34 and a mixed basis set, comprising 6-31G(d,p) atomic basis35 (a widely used Pople’s double-ζ set plus polarization functions) on carbon and hydrogen atoms, and LANL2DZ36 effective core potentials (ECP) and basis set on silicon. This level is comparable to that used in ref 12 to obtain the redox potentials cited above, with the main difference being the absence of diffuse functions on the light atoms, since no net charge variations are expected in the systems considered here. The basis set convergence was checked by repeating some calculations with the triple-ζ 6-311+G(d,p) set, including diffuse and polarization functions. The first excited state energy was estimated with two methods: (i) as the triplet ground state energy, which is obtained with the same procedure as for the singlet ground state, simply imposing the proper spin multiplicity, and (ii) as the first root of a timedependent DFT (TD-DFT) calculation. Though both these approaches are approximated, they provide useful qualitative information on the change of reaction barriers after absorption of low energy radiation. The ground state reaction paths were determined by finding on the potential energy surfaces the minima and the saddle points corresponding to reactants, products, and transition states, respectively; the nature of the saddle points was confirmed by computing the second derivative matrix and verifying the presence of a single negative eigenvalue. The system was then moved forward and backward along the eigenvector corresponding to the negative eigenvalue, and its geometry was reoptimized, verifying that the same reactant and product minima were found; this allowed the selection of the correct transition states among several others located in the same region of the coordinate space. The reaction paths were then defined as the union of the two optimization paths from the transition state to reactants and 19211
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Figure 2. Reaction path for the addition of propane to Si35H42 cluster in the ground state, S0.
Table 1. Reaction Heat (ΔE, eV) and Activation Barrier (ΔE#, eV) for the Addition of Propane to Si35H42 Cluster (Figure 1a) with Different Basis Setsa,b 6-31G(d,p),
6-31G(d,p),
6-311+G(d,p),
6-311+G(d,p),
LANL2DZa
LANL2DZb
LANL2DZa
LANL2DZb
ΔE
0.18
0.12
0.18
0.13
ΔE#
3.58
3.68
3.61
3.69
a All electron basis set on C and H atoms, ECP on Si atoms. b All electron basis set extended to the two Si atoms involved in the reaction.
products. In the excited state, however, the same procedure could not be applied since it was not possible to identify the correct transition states. The excited state reaction paths were then estimated by running triplet and TD-DFT calculations on the same points along the ground state paths.
3. RESULTS AND DISCUSSION First we considered the addition of a saturated hydrocarbon, propane, to the Si35H42 cluster. This process is not accounted for by any mechanism proposed previously, based on either thermal or light absorption initiation: all these reaction schemes imply unsaturated hydrocarbons, with the unsaturation being usually reduced during the process. The reaction scheme is sketched in Figure 1a; the reaction path computed at the B3LYP/6-31G(dp)/ LANL2DZ level for the singlet ground state (S0) is reported in Figure 2 along with the optimized structures of reactants, product, and transition state. The convergence of the critical point energies with respect to the basis set was checked by performing single point calculations on reactants, transition state, and product, either using a larger basis set, namely 6-311+G(dp), or extending the all electron basis set also to the two Si atoms directly involved in the addition; the geometry of the critical points was not reoptimized. The reaction heat and the activation barrier computed with the different basis sets are reported in Table 1; one can see that expanding the basis set has a negligible effect on the relative energies, while using the all electron basis also for the reacting silicon atoms reduces the reaction heat by
Figure 3. Reaction path (optimized in the ground state) for the addition of propane to Si35H42 cluster: energies computed in S0, in T1, and adding to S0 the first TD-DFT excitation energy. HL, HOMO LUMO regions; MR, multireference region (see text).
0.04 0.05 eV and increases the barrier by 0.08 0.10 eV. However, these variations are too small to alter significantly the picture of the reaction path, so we decided to use the 6-31G(d,p) set for C and H and the LANL2DZ ECP and basis set for Si in all the following calculations. The data collected in Table 1 show that in S0 the addition presents a very small reaction heat and a huge activation barrier, confirming the implausibility of thermal activation for this process. As recalled in the Introduction, some evidence suggests that the reaction can be activated by absorption of visible light; this indicates that the energy barrier should reduce substantially in low-lying excited states. Computing the reaction profile in excited states, however, is much more challenging, since in most cases the variational principle is not applicable, and often the states have a multireference nature with different contributions along the reaction path. As discussed below, this is a key factor also in the present case. If the first excited state is well described by a single highest occupied molecular orbital (HOMO) lowest unoccupied molecular orbital (LUMO) transition, a good compromise is to perform an unrestricted DFT calculation for the triplet ground state; this yields the T1 state rather than S1, but the energies are expected to be very close (the triplet being slightly more stable, according to Fermi's principle). This approach was used also by Reboredo et al.26 to estimate the changes in the activation barrier for hydrosilylation after absorption of visible light. Another approach is to compute the first excitation energy with time-dependent DFT, adding it to the S0 self-consistent field (SCF) energy; with this method the contributions of different transitions to excitation energies can be estimated. In principle, both unrestricted triplet SCF and TD-DFT could be used to search the S1 transition state; in the present case, however, both approaches failed to locate the proper saddle point. This is due partly to the sudden change of the frontier orbitals around the transition state (vide infra), and partly to the presence of many uneffective saddle points in this region. In any case, useful insights about the activation barrier in S1 can be obtained by computing T1 or S0 + TD energies along the ground state reaction path: this provides at least a qualitative estimate of the path in the first excited state. In Figure 3 we 19212
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Figure 4. HOMO and LUMO orbitals for the system propane + cluster in the HL and MR regions (see Figure 3).
Figure 5. Reaction path for the addition of propene to Si35H42 cluster in the ground state, S0.
compare S0, T1, and S0 + TD energies for some points extracted from the S0 reaction path shown above. Figure 3 reveals the existence of different regions in the coordinate space: near reactants and products, T1 and S0 + TD energies are very close to each other, slightly more than 4 eV above the ground state value, while in proximity of the transition state the two approaches differ, and S0 + TD energies are markedly higher. This behavior shows that in the former regions (indicated with “HL” in Figure 3) the first excited state is essentially due to HOMO LUMO transitions and in the latter region (labeled “MR” in Figure 3) it is likely a multireference state, involving other transitions besides HOMO LUMO (accounted for, at least partially, in TD-DFT but not in the unrestricted triplet approach, which appears to be unreliable in this region). In the S0 + TD energy curve the reaction barrier lowers to 2.15 eV, i.e., about 60% of the value in S0; this result is not quantitatively reliable, since it refers to a crude estimate of the actual excited state energy, and has been obtained for geometries optimized in the ground state (the barrier would change, likey lowering, if both reactants and transition state could be optimized in S1). In any case, the curves in Figure 3 show that the addition can be favored in low-lying excited states, in agreement with the observed reactions promoted by visible light absortpion. The change in the activation barrier and the existence of different regions in the coordinate space can be better understood by examining the S0 frontier orbitals of the system, illustrated in Figure 4. The HOMO LUMO gap is 4.83 eV in reactants and products, where the frontier orbitals are concentrated on the silicon cluster: in the HL region the first occupied and virtual orbitals concentrated on propane are found 2.25 and 4.36 eV below HOMO and above LUMO, respectively. The main difference in the MR region is due to two new molecular orbitals, involving both the hydrocarbon moiety and the silicon atoms close to the reaction site, with energies falling inside the reactant HOMO LUMO gap. As a consequence, the gap is reduced to 1.45 eV around the transition state; these orbitals seem to be related to the forming bonds between the hydrocarbon and the reactive silicon atoms. This qualitative picture suggests that the chemical nature of the first excited state changes along the reaction path, so the energy absorbed by the silicon surface in the initiation step
could be transferred to the reactive site, activating the formation of silicon carbon bonds even for saturated hydrocarbons. To check the dependence of the results on the size of the silicon surface model, the calculations were repeated on a larger cluster, namely Si55H62, containing one more Si Si dimer. Reactants, products, and transition state were reoptimized, with very small variations in the internal coordinates involving the alkane silicon bonds, showing that the effect of an additional surface dimer far from the reaction site is almost negligible. This is confirmed by the reaction and activation energies, which vary by less than 0.01 eV with respect to the smaller cluster both in S0 and in S0 + TD curves. On the other hand, the crucial element in our picture is the change of the HOMO/LUMO couple when the transition state is approached: in the MR region the frontier orbitals are localized around the attacking site, thus unlikely affected by the enlarging of the surface. Our results show that also in the HL region the HOMO LUMO gap is stable when passing from Si35H42 to Si55H62 clusters. The same approach was followed to describe the addition of propene to the Si35H42 cluster: as recalled in the Introduction, some mechanisms have been proposed to explain the addition of unsaturated hydrocarbons under mild conditions (visible light absorption, or even in the dark). All these mechanisms, however, imply the loss of the unsaturation on the organic moiety, with the exception of the scheme proposed in ref 18. On the contrary, we computed the reaction path for the addition illustrated in Figure 1b, with the same method as above. In Figure 5 the energy profile for propene addition in S0 is reported: at this level, the computed reaction heat and activation barrier are ΔE = 0.14 eV and ΔE# = 3.66 eV, respectively. Also for this process, then, thermal activation appears to be impossible. The reaction path in the first excited state was approximated with the same method as for propane, with the results illustrated in Figure 6. The addition of the unsaturated hydrocarbon shows the same features as seen above for propane: there are two distinct regions, where the first excited state can be approximated either as a HOMO LUMO (HL) state (so that triplet SCF and timedependent DFT give very similar energies) or as a multireference (MR) state. As in propane addition, the transition state lies in the latter region, where two new frontier orbitals appear, concentrated on the organic moiety and neighboring silicon atoms, with 19213
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unsaturated species, preserving the unsaturation degree, as observed in some recent experiments, not explained by the other mechanisms currently invoked for this kind of reaction. These findings can explain why visible light can activate the formation of a C Si bond, even if it is not energetic enough to break a H Si bond.
’ AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected].
’ REFERENCES
Figure 6. Reaction path (optimized in the ground state) for the addition of propene to Si35H42 cluster: energies computed in S0, in T1, and adding to S0 the first TD-DFT excitation energy. HL, HOMO LUMO regions; MR, multireference region (see text).
energies falling inside the reactant HOMO LUMO gap. The activation barrier in the S0 + TD curve (Figure 6) is 2.37 eV, i.e., around 65% of the barrier in S0; as noted above, this is only a crude estimate of the actual ΔE# in the first excited state, as we used an approximate method to evaluate S1 energies and kept the reaction path optimized in S0. In any case, these results indicate that the barrier reduces substantially in low-lying excited states, due to the change in the frontier orbitals: if the energy absorbed by the silicon substrate during illumination is transferred to the reactive site, crossing HL and MR states in our description, this could explain the observed reactions in which an alkene is bound to the inorganic surface without losing the unsaturation.
4. CONCLUSIONS The addition of propane and propene to a partially hydrogenated silicon cluster has been studied, as a prototypical reaction leading to the establishment of a strong C Si bond. A mechanism has been proposed which allows for the addition of saturated as well as unsaturated hydrocarbons, in the latter case without reducing the carbon carbon double bond. A Si35H42 cluster was used to model the 2 1 monohydrogenated (100) surface. The reaction paths have been computed at the DFT level for the ground state (S0) and approximated in the first excited state (S1), either with triplet unrestricted SCF or by adding to the ground state energy the first time-dependent DFT transition energy. As expected, in S0 the activation barriers are too high to make thermal activation possible for these processes. The reaction paths in S1 could only be approximated, recomputing the energies for the structures optimized in S0: with this approach we found that the activation barrier reduces to 60 65% of the ground state value. Such a reduction arises from the change of HOMO and LUMO orbitals, which are concentrated on the silicon cluster with a large gap in reactants and products, while in proximity of the transition state they are related to the forming bonds between carbon and silicon atoms. The present results indicate that visible light can activate the silicon surface toward the addition of hydrocarbons. A common mechanism can be sketched for the addition of saturated and
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