First-Principles Study of Water Ice Adsorption on the Methyl

ARTICLE pubs.acs.org/JPCC

First-Principles Study of Water Ice Adsorption on the Methyl-Terminated Si(111) Surface A. Ambrosetti,* F. Costanzo, and P. L. Silvestrelli Dipartimento di Fisica, Universita di Padova, via Marzolo 8, I-35131, Padova, Italy DEMOCRITOS National Simulation Center, Trieste, Italy ABSTRACT: The properties of water adsorbed on the methyl-terminated Si(111) surface, CH3:Si(111), are investigated using a first principles approach to complement a recent experimental study. We confirm the existence of weak hydrogen bonds between water and the substrate; however we show that the bonding configuration is different from that assumed in the analysis of the experimental data. In fact, a water molecule prefers to interact with the substrate by adopting a “face” approach direction in which a H atom points toward the C atom of the methyl group; instead the suggested “vertex” configuration, characterized by a weak C
H 3 3 3 OH2 hydrogen bond, turns out to be clearly unfavored. This behavior resembles that observed in the study of hydrogen bonding in the methane
water complex. Our energetic analysis indicates that water
water interactions are much stronger than those between water and the substrate, thus confirming the hydrophobic behavior of the CH3:Si(111) surface, although the contact-angle estimate (73°), in good agreement with the reported experimental value, is also compatible with a partially wettable character of the substrate.

I. INTRODUCTION The study of the interaction of water with solid surfaces represents a very active field of research1 due to its relevance both for understanding the basic mechanisms of adsorption and interfacial phenomena and for countless practical applications, ranging in the most diverse fields from mineral separation to electrodes, sensors, etc. However, despite the extensive experimental and theoretical investigations of the past years, the understanding of hydration processes still appears incomplete. From the experimental side, a detailed microscopic description of the structures adopted by water at interfaces often represents a challenging problem. Theoretical investigations, particularly those based on first-principles simulations, can provide a detailed microscopic description of both structural and electronic properties, thus leading to a more complete picture of the complex phenomena underlying water adsorption and, at the same time, providing useful indications for further experimental measurements. The adsorption process mainly depends on the relative strengths of water
water and water
surface interactions1
3 but also on crucial geometrical factors, like the particular surface periodicity of the substrate. In fact, when adsorbed at the interface, water tends to minimize its energy, leading to structures which might differ from that of bulk water ice. In particular, large deviations from the bulk ice structure are certainly expected if the interactions of water with the given surface are dominant with respect to the water
water attraction; in this case interfacial interactions may induce significant distortions in the bulk ice lattice. Among the possible modifications of the Si(111) substrate, the hydrogen-terminated Si(111) surface represents a wellstudied system, also because of its low number of defect sites;4 r 2011 American Chemical Society

this surface, however, degrades rapidly in air. Good resistance to oxidation can instead be obtained5 by saturating every Si atop site of the unreconstructed Si(111) surface by methyl (CH3) groups with the C atoms directly bonded to the surface Si atoms and their three H atoms per molecule pointing upward. The interest for the CH3:Si(111) surface has been boosted by a recent X-ray experimental study6 of ice adsorption on this substrate at low temperature (100 K), which reports a dramatic enhancement of the LUMO peak intensity, indicating a perturbation of the LUMO orbital directly induced by the ice adsorption. The X-ray absorption process is controlled by dipole selection rules, so that a core electron could only be excited from a 1s orbital into a state with p symmetry. Since the LUMO orbital has a totally symmetric character, the detection of a significant transition probability to the LUMO state can only be explained by assuming a modification of the LUMO orbital itself, in particular an enhancement of its p character. In ref 6 this peculiar effect has been attributed to some sort of orbital interaction between water and the CH3 groups, despite the assumed hydrophobicity of the CH3:Si(111) surface, with the formation of weak hydrogen bonds (H-bonds) of the kind C
H 3 3 3 OH2. In order to complement the experimental analysis, we have investigated the process of water adsorption on CH3:Si(111), using a theoretical first-principles approach to evaluate the relevant bonding and structural properties. In particular, our energetic analysis confirms the presence of a weak interaction Received: March 23, 2011 Revised: May 12, 2011 Published: May 13, 2011 12121

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(much weaker than the water
water interaction) which can be classified as a kind of weak H-bond, that is however characterized by a geometry different from that proposed in ref 6.

II. METHOD Calculations were carried out within the density functional theory (DFT) approach, using the CPMD first-principles package.7 A generalized gradient approximation (GGA) was adopted with the PBE exchange-correlation functional.8 This functional has been chosen because it is characterized by a deep physical foundation and a good reliability; furthermore it was proved9 to account for a certain amount of binding in several weakly interacting systems, in such a way to partially mimic even van der Waals effects, although, due to its local character, it cannot reproduce genuine nonlocal correlations. For these reasons PBE has also been used in our previous firstprinciples simulations of related systems, namely, interfacial water on the Cl- and H-terminated Si(111) surfaces,2,10 Cl:Si(111) and H:Si(111). In particular, in ref 10 we have shown that by including explicit van der Waals (vdW) corrections, using a postprocessing approach,11,12 does not lead to qualitative changes in the description obtained at the pure PBE level (see below for a further discussion about the vdW effects in the present system). Similar conclusions have been recently drawn by Hmiel and Xue13 who applied semiempirical vdW corrections to PBE simulations of water adsorption on H-passivated Si nanowires. The CH3:Si(111) surface is assumed to be in the 1  1 (unreconstructed) structure and is modeled with a periodically repeated slab with a (2  2) unit cell and including six Si doublelayers (with eight Si atoms per layer) plus a vacuum region, 16 Å wide, separating the repeated images of the slab. The bottom layer dangling bonds are saturated with H atoms; the lowest-layer Si atoms and the saturation hydrogens are kept fixed to the bulk crystallographic positions, while the five top Si double layers are allowed to fully relax during structural optimizations. Valence electron wave functions are expanded in plane waves, with an energy cutoff of 70 Ry; moreover, the sampling of the Brillouin zone is limited to the Γ-point only: in similar systems10 we have explicitly checked that this represents a good approximation. III. RESULTS In order to verify the capability of the present approach, especially of the chosen PBE functional, of adequately reproducing the weak interactions that characterize our system, we have first carried out preliminary tests considering a model system, which is expected 6 to share the same bonding properties of water on CH 3:Si(111), namely, a methane molecule interacting with water (CH 4
H2O). Among the possible weakly bonded configurations, we find a clear preference for the system to adopt the so-called “face” (F) structure (we here adopt the same nomenclature used in ref 14), in which a H atom of water points toward a tetrahedron face of methane (see Figure 1, left panel, and Table 1), in line with the results of previous theoretical14,15 and experimental16 studies. Instead, another configuration that could also be relevant for describing the interaction of water with the CH3: Si(111) surface, namely, the so-called “vertex” (V) one, characterized by a weak H-bond, in which one of the methane H atoms points toward the water O atom, turns out to be clearly unfavored (see Figure 1, right panel, and Table 1). Interestingly, a previous detailed theoretical analysis,14 showed that

Figure 1. Equilibrium structures for the methane
water complex, CH4
H2O: “face” (F, left panel) and “vertex” (V, right panel) configurations; dashed lines represent H bonds.

Table 1. Binding Energy (in meV), Eb, and Equilibrium Distances (in Å), for the Methane
Water Complex, CH4
H2O, in the Face (F) and Vertex (V) Configurations, Compared with Reference Theoretical Data14,15 (in parentheses, see text)a configuration F V

Eb b

dC 3 3 3 O c

35 (36, 45 ) b

14 (24 )

dC 3 3 3 H b

3.41 (3.60 )

2.46 (2.54c)

b

3.46 (3.86 )

a

dC 3 3 3 O and dC 3 3 3 H denote the distances from the methane C atom and the water O atom and from the methane C atom and the water H atom involved in the formation of the H bond in the F configuration, respectively. b Reference 13. c Reference 14.

the V structure maximizes the electrostatic and polarization interaction but also the Heitler
London exchange repulsion, while the F configuration appears to be the least electrostatically favorable but is the optimal one for reducing the Heitler
London exchange repulsion. It should be stressed that both the F and V bonding structures have to be classified as H-bonds, although a precise characterization of them is nontrivial (see ref 15): in particular, in the F configuration, the H bond is formed because of the attraction between an electron-deficient H atom of water and the center of one of the four tetrahedron faces of methane, which is a region of high electron density acting as a H-bond acceptor.15 As can be seen looking at Table 1, the performances of our approach, with the PBE functional, are satisfactory: in fact, even at a quantitative level, our results agree reasonably well with those obtained by Szczesniak et al.14 and by Raghavendra and Arunan,15 who used a more expensive (and presumably more accurate) quantum-chemistry method based on the Møller
Plesset perturbation theory. As far as the adsorption of water ice on the CH3:Si(111) surface is concerned, following the same scheme adopted in refs 2 and 10 before considering the deposition of ice layers on the substrate, we have studied the interaction of a single water molecule (monomer) with the CH3:Si(111) surface, since a detailed understanding of this system (concerning the nature of the bonding, the bonding sites, and the orientation of the water molecule) represents an essential step for elucidating the more complex process of the adsorption of an extended water film. As expected (see Figure 2 and Table 2), the bonding properties of the water monomer on the CH3:Si(111) surface resemble those previously observed in the CH4
H2O model system: in fact, the preferred geometry is again the F one, although the distance (2.63 Å) between the water H atom and the C atom of the underlying methyl group is slightly larger than that in CH4
H2O (the distance between the water O atom and C is instead almost identical), and the same is true for the binding energy (49 meV), 12122

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Figure 3. V (left panel) and I (right panel) configurations for the water monomer adsorbed on CH3:Si(111).

Figure 2. F configuration for the water monomer adsorbed on CH3: Si(111). In this and in the following figures yellow, white, red, and black spheres indicate Si, H, O, and C atoms, respectively; dashed lines represent H bonds. For clarity, only a few Si atoms belonging to the upper part of the substrate are shown.

Table 2. Binding Energy, Eb, Adsorption Energy, Eads, H-Bond energy, EHB (in meV, see text for the definitions), and Selected Distances (in Å) Relative to the Adsorption on the CH3:Si(111) Surface of the Water Monomer (F configuration) and the Extended Water Structures (BL and TF)a system monomer BL TF

Eb

Eads

49

49

110 161

435 588

EHB

317 312

dC 3 3 3 O

dC 3 3 3 H

3.42

2.63

3.16 3.19

2.18 2.21

a dC 3 3 3 O and dC 3 3 3 H denote the distances from the C atom of the methyl group forming the H bond with water, and the water O atom and H atom (that involved in the H bond), respectively.

which is defined as Eb ¼ Ewater þ Esub
Etot

ð1Þ

where Ewater and Esub, respectively, are the energies of the isolated water molecule and substrate and Etot is the energy of the whole system (water þ substrate). Interestingly, in ref 6 the predicted range for the C 3 3 3 O distance (between 2.7 and 3.8 Å) is compatible with our estimate obtained for the favored F structure (3.42 Å). The expected binding energy (0.3 kcal/mol = 13 meV) reported in ref 6 is much smaller than our computed value (49 meV), however that prediction was actually referred to the alternative V configuration, which is characterized by a C
H 3 3 3 OH2 H bond (in general, the strength of the C
H 3 3 3 O H-bond is in the range 0.3
3.5 kcal/ mol, that is 13
152 meV, depending on the hybridization of the C atom17). In any case, our computed binding energy of the water monomer with the CH3:Si(111) surface, within the physisorption energy range, is much smaller than that relative to typical H bonds between water molecules (which is of the order of 200
300 meV), thus suggesting, from an energetic point of view, a clear hydrophobic character of the substrate. Interestingly, the binding energy of the water monomer in the favored F structure is intermediate between the values relative to the optimal adsorption configuration in the case2 of H:Si(111), 41 meV, and of Cl:Si(111), 62 meV. Note that

the F configuration appears to be electrostatically less favored than the V one, in which the dipole moment of the water molecule is essentially parallel to that of the substrate, a behavior in line with that observed (see above discussion) in the methane
water complex. Probably, a key role in making the F configuration optimal is played by the planar structure formed above the C atom by the three H atoms, since this feature is common both to the CH3 group on the Si(111) surface and to the CH4 molecule. Since the potential-energy surface of a molecule interacting with a surface is more complex than that relative to bonding between two simple molecules, it is crucial to check whether other possible adsorption configurations could be characterized by a binding energy comparable to that computed for the optimal F structure, in such a way to represent alternative initial configurations for depositing extended water layers. In particular, we calculated the binding energy for two configurations where the water molecule has both the H atoms pointing upward: in the first one (see Figure 3, left panel) the water O atom is placed on top of the H atom of a CH3 group, being essentially the V structure discussed above, while in the second one, I (see Figure 3, right panel), the O atom is in a sort of interstitial site among four CH3 groups. However, both these configurations are clearly unfavored, with respect to the most stable F arrangement described above, since the V one is even unbound (by about 1 meV), while the I configuration has a binding energy of only 6 meV; thus both these possible adsorption structures can be ruled out. As already mentioned in section II, the present results were not explicitly corrected for vdW effects since these are not expected to qualitatively affect the basic properties of the investigated processes. However, for the specific case of the interaction of the water monomer with the CH3:Si(111) surface, we made some test calculations to assess the influence of vdW interactions, by applying the recently developed approach in which DFT results are corrected for vdW effects by exploiting the maximally localized Wannier function formalism; this technique has been already successfully applied to several systems, including small molecules, bulk solids, and surfaces.10
12,18,19 Basically, we find that, taking the vdW corrections into account in the evaluation of the binding energy, the preference for the F configuration is even increased, since in this case the additional vdW energetic contribution is larger (of the order of 20%) than in the other structures considered,24 thus giving further support to our previous conclusion. According to Waluyo et al.6 the experimental evidence of an enhancement of the LUMO peak intensity upon ice adsorption is likely due to some weak H bonding of water with the methyl group. As shown above, our results confirm the existence of such a weak bonding, although the preferred adsorption configuration 12123

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Figure 4. Plot of the LUMO orbital density relative to the water monomer adsorbed on CH3:Si(111) in the F configuration. The orange region corresponds to the density isosurface value of 4  10
5 e au
3.

is not that predicted in ref 6. In any case also the F configuration could be well compatible with the induction of a p character enhancement in the LUMO orbital suggested in ref 6 because it will certainly introduce some modifications in the electron density distribution of the CH3 and water monomer fragments, although the weak binding strength indicates that the overlap between water and substrate electronic orbitals at the interface is negligible. In order to verify the hypothesis of an electronic state symmetry modification induced by the water
methyl interaction we have explicitly calculated the density profile corresponding to the LUMO orbital in the case of the water monomer adsorption in the F configuration (see Figure 4). As suggested by Waluyo et al. the water molecule does indeed induce a clearly observable anisotropy in the density of the LUMO state: in fact the LUMO orbital density exhibits an appreciable contribution located just below the C atom of the methyl group involved in the hydrogen bonding with the water molecule, a feature which is absent below the other CH3 groups, thus confirming the appearance of a symmetry breaking upon water adsorption. Actually, our analysis indicates that the bonding interaction is due to the formation of a H bond rather than to the existence of a genuine orbital interaction. In fact, looking at the electron density difference, between the system made of the monomer adsorbed on CH3:Si(111) and the separated components (see Figure 5), one can see that, upon water adsorption, there is an electron charge transfer from the substrate to the planar region formed by the three H atoms of the CH3 group facing the water molecule and from one of the water H atoms toward the O atom, thus supporting the mechanism detailed above for the formation of an H bond, similarly to the methane
water complex; interestingly, there is also a small transfer to the O atom from the closest H atom of an adjacent CH3 termination, which probably corresponds to a weak tendency to form a H bond compatible with the V structure, although it cannot be considered as a true H bond due also to the fact that the H 3 3 3 OH2 distance, 2.96 Å, is larger than the sum of the vdW radii of the O and H atoms, that is, 2.72 Å (see also the unbound character of the V configuration reported above).

Figure 5. Plot of the electron density difference between the full system (water monomer adsorbed on CH3:Si(111) in the F configuration) and the sum of separated fragments (water and substrate). Blue and orange regions correspond to density isosurface values of 9  10
4 and
6  10
4 e au
3, respectively.

As a next step toward a realistic description of water ice interacting with CH3:Si(111), we considered the adsorption of a periodically repeated ice bilayer (BL), consisting of six water molecules per supercell, and of a thin film ice slab (TF), composed of four ice bilayers, following again the same procedure applied in refs 2 and 10. The crystalline structure of both the BL and the TF were obtained from the cubic structure (Ic) √ of ice, whose √ twodimensional surface can be described by a (2/3 3  2/3 3)R30 superlattice. This represents a metastable phase which, for instance, has been experimentally20 observed on Cl:Si(111) surfaces in vapor deposition processes. In actual experiments6 the precise morphology of ice adsorbed on CH3:Si(111) is not easy to characterize since probably three-dimensional amorphous ice clusters are formed; therefore, in our approach, based on first-principles simulations performed using a relatively small reference supercell, we decided to adopt the Ic crystalline phase for the water film. Similarly to the monomer adsorption case, the binding energy of a periodic water structure adsorbed on the CH3:Si(111) surface can be defined as Eb ¼ Eice þ Esub
Etot

ð2Þ

where Eice represents the energy of the isolated water structure (BL or TF) and Etot is the energy of the whole system (water structure þ substrate). As can be seen in Table 2, for the water BL and TF adsorbed in their most stable configurations (shown in Figure 6 and Figure 7), the binding energies of 110 and 161 meV, respectively, in the case of the water BL are larger than that previously computed2 considering both the H:Si(111) and Cl:Si(111) substrate (83 and 90 meV, respectively), while intermediate in the case of the TF (we found 140 and 283 meV, respectively). Also the preferred adsorption configurations for the BL and the TF are characterized by the presence of one water molecule (per supercell) directly interacting with the CH3: 12124

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Figure 6. Water bilayer (BL) adsorbed on CH3:Si(111).

Si(111) substrate in the F arrangement (see Figure 6 and Figure 7), with a H atom pointing downward toward the surface, similar to that found in the study of the water monomer adsorption, which, as expected, determines the basic bonding properties in extended water structures too. Compared to the adsorbed water monomer, both the BL and the TF structures appear slightly closer to the methyl surface (in the F structure the C 3 3 3 O distance is 3.16 and 3.19 Å, respectively); the C 3 3 3 H distances are also reduced with respect to the single water molecule (2.18 Å for the BL and 2.21 Å for the TF). When considering the interaction of our water TF with the CH3:Si(111) surface, one must point out that two different orientations of the TF are possible, corresponding to a different number of H atoms (either 1 or 2, hence we refer to them as TF1 or TF2 structure, respectively) per supercell pointing toward the surface: in Figure 7 we show the TF1 configuration; the other one can (TF2) be obtained simply by turning the first one upside down. This difference is expected to be important on the basis of the findings reported above: in fact, these H atoms exposed to the surface can be considered as the main source of attraction between water and the underlying substrate, similarly to that observed in our previous study2 on water on the Cl:Si(111) surface. The same behavior was also reported in ref 3. However, given the particular conformation of the methyl-group termination, and considering the mismatch between the periodicity of the CH3:Si(111) surface and that of the water TF, one expects that geometrical and electrostatic effects could also play a decisive role in determining the favored adsorption configuration. In order to elucidate these important effects, we have therefore considered the interaction with the substrate of the TF obtained by inverting the orientation of the reference (TF1 structure, see Figure 7) in such a way that two H atoms are now exposed to the surface (TF2 structure). Our calculations show that this alternative TF2 configuration is energetically very unfavorable (it is even unbound by about 20 meV); this behavior can be at least partially explained by the fact that the mentioned mismatch between ice and the substrate forces at least one of the water bottom H atoms to be located in a region far from the optimal site. Note that, in the case of the Cl:Si(111)substrate, one found a completely different behavior:2 in fact, the TF1

Figure 7. Thin-film ice slab (TF) adsorbed on CH3:Si(111).

structure with only one H atom per supercell exposed to the surface was very unfavored (by about 260 meV) with respect to the TF2 one. Therefore, with respect to the previously studied simpler systems (H:Si(111) and Cl:Si(111)), in this case the bonding is somehow more selective and oriented, which could explain the smaller binding energy compared to the case of the water TF on the Cl:Si(111) surface (see above); the water BL exhibits instead a different behavior (the binding energy is larger) probably because of its higher structural flexibility. By analyzing the distribution of the total charge density, we find that the slab used for reproducing the CH3:Si(111) substrate is characterized by a value of the positive z component (the one orthogonal to the surface) of the dipole moment of 1.73 D. Interestingly, in the case of the Cl:Si(111) substrate,2 the corresponding value was
3.35 D, that is a dipole moment about twice in magnitude and with opposite orientation. By considering that the z component of the dipole moment relative to the TF1 and TF2 structure is about 12125

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þ5 and
5 D, respectively, as already found in ref 2, clearly it turns out that TF1 is favored with respect to TF2 also because it represents an optimal electrostatic configuration having its dipole aligned with that of the underlying CH3:Si(111) substrate (in the case of Cl:Si(111) the TF2 structure was instead the favored one due to the electrostatic attraction). This electrostatic contribution is certainly less important in the case of adsorption of the water monomer since the dipole moment of the single water molecule of about 1.8 D is much smaller than the z component of the TF, while instead the number of H bonds per supercell is just one for both the adsorbed monomer and the TF1 structure. In order to definitively rule out the relevance of V-like configurations in the adsorption process of water ice on CH3: Si(111), we have also forced the water BL to assume a configuration where no bottom H atoms point toward the substrate, in such a way to expose only water O atoms to the surface, which could allow the formation of C
H 3 3 3 OH2 H bonds. However, this structure turns out to be unstable (it is unbound by more than 100 meV), so that a subsequent geometry optimization procedure unavoidably leads to recover a configuration with one bottom water H atom pointing downward. Hence, also in the case of extended water deposition the occurrence of V-like bonding configurations appears to be clearly inhibited. As done in refs 2 and 10 we have further characterized the water
CH3:Si(111) interface, by evaluating the so-called adsorption energy and H-bond energy. The adsorption energy, Eads, can be defined as Eads ¼ ðEsub þ NEH2 O
Etot Þ=N

ð3Þ

where Etot is the total energy of the TF (or the water BL or the water monomer) adsorbed on the surface, Esub is the energy of the isolated, clean CH3:Si(111) substrate, and EH2O that of the free water molecule; N is the total number of water molecules contained in the supercell. Therefore Eads corresponds to the average binding energy, per water molecule, for the system under consideration and takes into account both the gain in energy due to the formation of the water
water H-bond network and that due to the interaction with the substrate. The Eads values are reported in Table 2 (clearly, for the monomer Eads, by definition, coincides with the binding energy). The larger value of Eads for the TF structure than for the water BL can be easily attributed to the higher average coordination of water molecules due to the presence of multiple BLs. An estimate of the average H-bond strength (EHB energy) can be obtained (see also refs 2 and 10) by computing EHB ¼ ðEads N
E1ads NWS Þ=NHB

ð4Þ

where Eads has been defined above, Eads1 is the adsorption energy of the water monomer, and NWS is the number of water
surface H bonds per supercell (in our case NWS = 1). Note that our computed EHB in the water BL and TF, as found in the case of water on H:Si(111) and Cl:Si(111),2 have comparable values and, more importantly, they are much larger than the adsorption energy of the water monomer, showing again the dominant effect of water
water H-bond interactions in water ice adsorbed on CH3:Si(111). Clearly the above energetic analysis leads to attribute a pronounced hydrophobic character to the CH3:Si(111) substrate. However, an alternative evaluation of the degree of hydrophobicity (more directly related to the experimental investigation) is represented by the estimate of the contact angle at the water
surface

interface. The contact angle notion is clearly strictly applicable only to macroscopic samples; however, an indirect estimate can be obtained also from our calculations, as done in ref 10 by assuming21 a linear correlation between the contact angle and the water monomer binding energy. In particular, by considering that the measured22 contact angle in the case of water ice on the H:Si(111) surface is 91°, and our computed binding energy for a water monomer on that substrate was 41 meV,2 one can easily get an extrapolated value of 73°, which is in remarkable agreement with the experimental6 value of 78 ( 2°. The comparison of this estimate with the corresponding value obtained for water on the Cl:Si(111) surface (45°), suggests that the hydrophobic character of the CH3:Si(111) substrate is intermediate between that of the H: Si(111) and the Cl:Si(111) surface, in line with the energetic analysis reported above. Actually, since it is assumed21 that the wetting transition occurs at a contact angle of 90°, our estimated angle would be also compatible with a partially wettable character of the CH3:Si(111) surface. As already noted elsewhere,10 we can thus again conclude that a contact angle smaller than 90° does not necessarily imply strong water
surface interactions. In order to get a more complete picture of the interface properties, we have also computed the energy barriers for sliding the water TF on the surface: this has been accomplished by evaluating the changes in the binding energy of TF over CH3: Si(111) observed by displacing the water structure along different directions in the x,y plane (the x,y coordinates of the water molecules were constrained to assigned values, while instead the z coordinates were fully relaxed). As expected, the weak water
surface interactions reflect in small energy barriers for the sliding motion, which, however, due to the peculiar geometry of the CH3: Si(111) substrate, possess a certain degree of anisotropy (they assume different values for different sliding directions), particularly because of the presence of the three H atoms in the CH3 terminations. We remind that one of the water molecules closest to the surface has, in the most stable configuration, one H atom just above a CH3 group and pointing down toward the C atom. Sliding the TF, this H atom will be displaced from a position corresponding to the center of the CH3 group to a more unfavorable location; basically, two distinct paths are of primary interest: the first one passes exactly above the position of one of the underlying CH3 H atoms, while the second one follows a direction equidistant from the positions of two adjacent H atoms of the CH3 termination. The first sliding path appears unfavored with respect to the second one: in fact the estimated values of the two energy barriers are 102 and 85 meV, respectively; clearly such a behavior can be easily explained in terms of steric arguments. These estimates are substantially larger than the corresponding values computed in the water adsorption on the H:Si(111) and Cl:Si(111) surfaces (about 50 meV),2 an effect that cannot be attributed to a substantial degree of hydrophilicity but instead to the specific (somehow more selective) geometric arrangements characterizing the CH3:Si(111) substrate (see above conclusion about the preferred adsorption sites).

IV. CONCLUSIONS In conclusion, our detailed first-principles analysis of water adsorbed on the CH3:Si(111) surface confirms the existence of weak, well-oriented H bonds between water and the substrate suggested by the experimental findings;6 however we have shown that the favored bonding configuration is not the proposed V one (C
H 3 3 3 OH2 H-bond) but instead the F one, where a H atom points toward the C atom of the methyl group in line with what 12126

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water complex. Our energetic analysis indicates that water
water interactions are stronger than those between water and the substrate, thus suggesting a hydrophobic character of the CH3:Si(111) surface, although our macroscopic contact-angle estimate (73°), in good agreement with the reported experimental value, is also compatible with a partially wettable character of the substrate. It would be extremely interesting if further experiments could validate our conclusions. In particular, the structures and dynamics of interfacial water on CH3:Si(111) could be directly observed, with high spatial resolution (as low as 0.01 Å), using the ultrafast electron crystallography;20 moreover, the sliding of ice layers on the CH3:Si(111) substrate could be measured by means, e.g., of the quartz microbalance technique.23

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that they did not result from a full vdW-corrected geometry optimization procedure but just represent postprocessing evaluations simply considering the equilibrium structures obtained at the PBE level. Therefore they should only be regarded as qualitative estimates. Moreover, we point out that adding these computed vdW energy corrections to the PBE binding energy would likely result in a substantial overestimate of the binding. A more consistent procedure would involve the use of alternative functionals, such as revPBE or BLYP.

’ ACKNOWLEDGMENT We acknowledge financial support by MIUR within the PRIN 2008 project,and by the Universita di Padova within the “Progetto di ricerca di Ateneo 2009”. We also acknowledge allocation of computer resources from the project “Calcolo per la Fisica della Materia” at CINECA. ’ REFERENCES (1) Thiel, P. A.; Madey, T. E. Surf. Sci. Rep. 1987, 7, 211. (2) Silvestrelli, P. L.; Toigo, F.; Ancilotto, F. J. Phys. Chem. B 2006, 110, 12022. (3) Biering, S.; Hermann, A.; Schmidt, W. G. Phys. Rev. B 2006, 73, 235429. (4) Yablonovitch, E.; Allara, D. L.; Chang, C. C.; Gmitter, T.; Bright, T. B. Phys. Rev. Lett. 1986, 57, 249. (5) Yu, H. B.; Webb, L. J.; Ries, R. S.; Solares, S. D.; Goddard, W. A.; Heath, J. R.; Lewis, N. S. J. Phys. Chem. B 2005, 109, 671. (6) Waluyo, I.; Ogasawara, H.; Kawai, M.; Nilsson, A.; Yamada, T. J. Phys. Chem. C 2010, 114, 19004. (7) Car, R.; Parrinello, M. Phys. Rev. Lett. 1985, 55, 2471. We have used the code CPMD developed by H. Hutter et al. (1990
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90 meV for the F structure, and
69 and
73 meV for the V and I configurations, respectively. In reporting these data, however, we stress 12127

dx.doi.org/10.1021/jp202704c |J. Phys. Chem. C 2011, 115, 12121–12127