Benzaldehyde on Water-Saturated Si(001): Reaction with Isolated

Article pubs.acs.org/JPCC

Benzaldehyde on Water-Saturated Si(001): Reaction with Isolated Silicon Dangling Bonds versus Concerted Hydrosilylation D. Pierucci,†,‡,∥ A. Naitabdi,†,‡,⊥ F. Bournel,†,‡,⊥ J.-J. Gallet,†,‡,⊥ H. Tissot,†,‡,∥ S. Carniato,†,‡ and F. Rochet*,†,‡,⊥ †

Sorbonne Universités, UPMC Univ. Paris 06, UMR 7614, Laboratoire de Chimie Physique Matière et Rayonnement, 11 rue P. et M. Curie, 75005 Paris, France ‡ CNRS, UMR 7614, Laboratoire de Chimie Physique Matière et Rayonnement, 75005 Paris, France

U. Köhler§ and D. Laumann§ §

Fakultät für Physik und Astronomie, Institut für Experimentalphysik IV AG Oberflächenphysik, Ruhr-Universität Bochum, D-44780 Bochum, Germany

S. Kubsky,∥ M. G. Silly,∥ and F. Sirotti∥ ∥

Synchrotron SOLEIL, L’Orme des Merisiers, Saint-Aubin, 91192 Gif sur Yvette, France S Supporting Information *

ABSTRACT: Despite strong similarities due to the common presence of silicon monohydrides and isolated silicon dangling bonds (silicon radicals), the water-saturated Si(001)-2 × 1 surface and the hydrogenterminated Si(001)-2 × 1 surface show very different reactivities with respect to benzaldehyde. By using real-time scanning tunneling microscopy, synchrotron radiation photoemission, X-ray absorption, and high-resolution electron energy loss spectroscopies in combination, we demonstrated that benzaldehyde reacts with the silicon dangling bonds of water-saturated Si (001). As we found no evidence for the abstraction of a nearby H leading to the formation of a new dangling bond, the formation of a stable radical adduct is a plausible explanation. This observation contrasts with the H-terminated case for which benzaldehyde grafting occurs via a radical chain reaction that can propagate after abstraction of a nearby H by the radical adduct. Also at odds with the H-terminated case, a second chemisorption channel is observed [i.e., a concerted hydrosilylation reaction between a surface monohydride (SiH) and the carbonyl moiety] without any participation of the silicon dangling bond. We discuss how the presence of hydroxyls on water-saturated Si(001)-2 × 1 could make its reactivity markedly different from that of H-terminated Si(001)-2 × 1.

1. INTRODUCTION The water-saturated Si(001)-2 × 1 surface [denoted in the following (H,OH)-Si(001)-2 × 1] has been extensively studied1−7 to determine its chemistry and the spatial arrangement of silicon monohydrides (SiH) and hydroxyls (SiOH) resulting from the dissociative reaction of water with the silicon dimers. A “perfect” (H,OH)-Si(001)-2 × 1 surface would consist in silicon dimers whose dangling bonds are all capped by an H and an OH moiety. However, scanning tunneling microscopy (STM) showed that the surface is not fully passivated, as about 1/(100) of a monolayer (ML) of tricoordinated silicon atoms (termed isolated dangling bonds, IDB) are left,2 even after prolonged exposure to water at room temperature. Because of obvious similarities with hydroxylated silica surfaces,8−10 and the H-terminated Si(001)-2 × 1 surface,11−14 © 2014 American Chemical Society

the reactivity of (H,OH)-Si(001)-2 × 1 begins to stimulate interest within the surface science community. Most of the experimental and theoretical works concern the reactivity of (H,OH)-Si(001)-2 × 1 with organometallics,15−17 as this surface can be the starting substrate in the atomic layer deposition of high κ dielectrics. On the other hand, little is known about its interaction with π-bonded organic molecules. To date, only the adsorption of 4-nitrobenzoic acid18 and styrene19 on (H,OH)-Si(001)-2 × 1 are reported. Both hydroxyls and monohydrides are reactive, depending on the functionality of the molecule. 4-Nitrobenzoic acid is thought to be grafted to the surface via a water-elimination reaction Received: August 3, 2013 Revised: April 6, 2014 Published: April 15, 2014 10005

dx.doi.org/10.1021/jp4077678 | J. Phys. Chem. C 2014, 118, 10005−10016

The Journal of Physical Chemistry C

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nearby SiH or SiOH, we discuss the possibility of a radical adduct. Moreover, we observe another reaction channel, giving a Si−O−CH2−C6H5 product via a concerted hydrosilylation process between the molecule and a surface SiH.

involving the carboxylic moiety and a surface hydroxyl. On the other hand, styrene bonds to the surface via the formation of a Si−C bond, after abstraction of a surface hydrogen by the molecule. However, STM data are still needed, to state whether the bonding of styrene occurs via a direct hydrosilylation reaction20 or via a silicon radical mediated process, as observed for H-terminated Si(001)-2 × 1.11 In this latter case, the πbonded molecule attaches to the silicon dangling bond to give a radical adduct that then captures a H atom from a nearby SiH (the so-called Chidsey mechanism21). In the wake of our preceding work on styrene adsorption,19 we present here a study of the reaction of another π-bonded molecule, benzaldehyde (C6H5CHO), with (lightly doped ptype) (H,OH)-Si(001)-2 × 1. Indeed, the hydrosylilation of aldehydes (acetaldehyde and benzaldehyde) on H-terminated Si(001)-2 × 1 via a radical chain reaction was observed by STM.12 The limiting step in the radical chain reaction being the H abstraction from a nearby hydride, the permanence time of the radical adduct on the surface must be sufficiently long. Density functional theory (DFT) slab calculations by Kanai et al.22 for the H-terminated surface show indeed that the bonding energy of the radical benzaldehyde adduct is 0.94 eV. This value is greater than that of formaldehyde, 0.78 eV, due to charge delocalization in the benzene ring. The calculated H-abstraction barriers are 0.79 eV (0.35 eV) for benzaldehyde (formaldehyde). Kanai et al. do not describe the formation of the radical adduct, that should be barrierless, according to the DFT slab calculation by Tan and Pei (on the water-terminated surface).23 Indeed the experimental measurements of rate constants and activation energies of an aldehyde (propionaldehyde) reacting with silyl radicals in solution point to a very low action barrier energy [i.e., 0.99 ± 0.41 kcal mol−1 (0.043 ± 0.018 eV)].24 Besides hydrosilylation reactions involving the surface SiH, competing reactions with the surface SiOH may be a distinctive feature of (H,OH)-Si(001)-2 × 1. A direct reaction of the aldehyde with a SiOH to give a silyl-hemiacetal seems unlikely, as on silica the interaction of aldehydes with surface OH groups is limited to hydrogen bonding.25,26 The capture of a nearby OH by the radical molecular adduct can be envisaged as a possibility, but investigations on such a mechanism were not observed experimentally for styrene.19 Indeed the barrier is prohibitive for benzaldehyde (in the 1.5−2 eV range), as shown by Tan and Pei23 in their recent density functional theory (DFT) periodic calculation of the reactivity of (H,OH)Si(001)-2 × 1 with alkenes and aldehydes. A third possibility, not envisaged in our preceding work on styrene,19 but worth considering, is the capture, by the radical adduct, of a hydrogen provided by a nearby hydroxyl. Tan and Pei show that the activation barrier of this process (in the 0.9−1.1 eV range for benzaldehyde) is comparable, or even lower, than the hydrogen capture from a silicon monohydride. As the oxygen adatom then inserts into a Si−Si bond, a new Si dangling bond is created, and the chain reaction can propagate.23 To draw a complete picture of the molecule/surface interactions and, specifically, to examine whether the dangling bonds play a role in the adsorption process, we used real-time STM imagery in combination with space-averaging spectroscopies, that is, synchrotron radiation X-ray photoelectron spectroscopy (XPS), near-edge X-ray absorption fine structure (NEXAFS) spectroscopy, and high-resolution energy electron loss spectroscopy (HREELS) supported by vibrational mode DFT simulations. We find that the molecule does react with the silicon dangling bond. As we do not observe H abstraction from

2. EXPERIMENTS AND SIMULATIONS The experiments were carried out in three independent ultrahigh vacuum (UHV) chambers, dedicated to STM, synchrotron radiation XPS/NEXAFS, and HREELS experiments. For STM and synchrotron radiation experiments, the Si (001) samples were cut from the same wafer, a lightly p-doped (boron) Si (001) wafer with a resistivity ρ = 18 Ω × cm, corresponding to boron concentration NBoron ∼ 7.5 × 1014 atoms/cm3. For HREELS intrinsic wafers of resistivity ρ = 2000 Ω × cm were used to optimize the spectral resolution.27 Lightly p-doped samples were also used for the sake of comparison with the STM and XPS/NEXAFS experiments. Identical cleaning procedures of the Si (001) samples were used in all cases. The Si (001) samples were cleaned from their native oxide by one flash annealing (Joule effect) at 1150 °C (measured by an infrared pyrometer) under ultrahigh vacuum (UHV) after carefully degassing the sample at 650° for 20 h. (H,OH)-Si(001)-2 × 1 surfaces were prepared by exposing the clean surface to H2O at room temperature to 4.5 L [1 langmuir (L) = 10−6 Torr × s] of water (under a nominal pressure of 0.75 × 10−8 Torr). This exposure was sufficient to saturate the surface. Then the water-covered surface was exposed to benzaldehyde at room temperature. The pressure gauges in the different experimental setups were not calibrated against one another. Therefore, the respective nominal working pressures may differ (they are noted PSTM, PXPS, and PHREELS respectively). The “dosing-while-scanning” approach we used in the STM experiments creates further uncertainty about the “real” pressure and benzaldehyde flux, due to surface shadowing by the tip.28 STM images were obtained at room temperature using a VT XA STM setup (Omicron NanoTechnology). The filled-states (empty-states) STM images were obtained in constant current mode (I = 100 pA) at a sample bias of Vbias = −2 V (+1.5 V). The chemically etched W tip was cleaned by heating in UHV. A scanning while dosing approach was used to monitor in the real-time the adsorption of molecules on the surface. XPS/NEXAFS experiments were carried out at TEMPO beamline29 (SOLEIL French synchrotron facility). The photon source is a HU80 Apple II undulator set to deliver linearly polarized light. The end-station is fitted with a modified 200 nm hemispheric electron analyzer (Scienta 200) equipped with a delay line detector.30 The X-ray spot (normally focused in a spot 100 μm long in the horizontal direction by 80 μm wide in the vertical dimension) was deliberately defocused to a spot of dimensions 390 × 200 μm2, without losing photoelectron count rate, to reduce beam damage. Moreover, the sample was moved by 500 μm after each acquisition in order to collect subsequent data on a fresh area. During the XPS measurements, the photoelectrons were detected at 0° from the sample surface normal n⃗ and at 46° from the polarization vector E⃗ . The Si 2p spectra were measured at a photon energy hν = 150 eV (overall resolution ∼70 meV), the C 1s spectra at hν = 350 eV (overall resolution ∼100 meV), and the O 1s spectra at hν = 640 eV (overall resolution ∼140 meV). The zero binding energy (BE) (i.e., the Fermi level) was taken at the leading edge of a clean molybdenum foil in electrical contact with the silicon sample. 10006



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Details on the fitting procedure are reported in Figure S1 of the Supporting Information. The Si 2p3/2 binding energy was found at 99.26 eV for the pristine (H,OH)-Si(001)-2 × 1 surface and at 99.29 eV after reaction with benzaldehyde. The C 1s NEXAFS spectra of the adsorbed layer were recorded in the Auger yield mode31 using an energy window of 13 eV, centered at 260 eV. The measurement of the C 1s edge due to about a monolayer of carbon was made feasible thanks to a procedure allowing the cleaning of the mirrors and leading to a minimization of the absorption dips in the photon flux around 285 eV.32 The measurements were done at three different angles θ between the normal to the sample surface n⃗ and the polarization vector E⃗ : θ = 10° (grazing incidence, E⃗ nearly perpendicular to the surface), θ = 44°, and θ = 90° (normal incidence, E⃗ parallel to the surface). The spectra were normalized by dividing the “surface plus adsorbate” spectrum by that of the pristine (H,OH)-Si(001)-2 × 1 surface. The normalized Auger yield is set to zero before the C 1s (O 1s) edge and to one at hν = 320 eV (hν = 580 eV). The HREELS-experiments were performed in a separate chamber equipped with a toroid-spectrometer (Gossmann electron optics; design H. Froitzheim) capable of achieving an energy resolution of 1 meV. In the present experimental conditions, a resolution of the spectrometer of 4−5 meV was used, resulting in a full width at half-maximum ( f whm) of the elastically reflected peak around 10−12 meV. All vibrational spectra are taken at a sample temperature of 300 K in the specular configuration (θi = θf = 60° with respect to the surface normal) with a primary beam energy Ep of 5 eV. Low-doped samples (ρ = 2000 Ω × cm) were used to avoid plasmon excitations which result in a broadening of the quasielastic peak.33,34 In all HREEL-spectra shown in this work, the loss intensities are normalized with respect to the intensity of the elastically reflected intensity. The geometries of the H−Si−Si−OH decorated dimer and of the envisioned benzaldehyde configurations were optimized at a DFT level of theory, using a silicon cluster (single dimer Si9H12) that mimics the surface and three subsurface layers. The latter ones were hydrogen-terminated to preserve the sp3 hybridization of the bulk diamond lattice. Optimizations were performed with GAMESS35, using the Becke three-parameter exchange functional,36 along with the Lee−Yang−Parr37 gradient-corrected correlation functional [the so-called Becke three-parameter Lee−Yang−Parr (B3LYP) functional]. The following basis sets were used: (1) oxygens: a 6-311G* basis set (plus diffuse s and p orbitals); (2) carbons: a 6-31G* basis set; (3) hydrogens (except OH): a 631G basis set plus polarization p orbital (exponent =1); (4) hydrogen (OH): 631G+3 polarization p orbitals whose exponents are 1, 0.275, and and 0.075 respectively; and (5) silicons: effective core potentials (SBJK).4 Then we used GAMESS-US to calculate the vibrational spectra of the H−Si−SiOH unit and of the adduct. The frequencies were determined from the Hessian matrix (the hydrogen atoms bonded to the Si clusters were frozen out). As HREELS allows one to probe only vibration modes implying dynamic dipole moments normal to the Si surface,27 we calculated the vibrational spectra corresponding to such a polarization. A Lorentzian broadening of 5 meV f whm (about half the value of the experimental broadening) enables a better distinction of the various components in the plotted spectrum.

3. RESULTS AND DISCUSSION We show in Figure 1 (panels a and b) the dual bias (Vbias = −2 V and +1.5 V) STM images of a lightly p-doped (Nboron = 7.5 ×

Figure 1. STM images of the water-saturated (4.5 L) p-doped (Nboron = 7.5 × 1014atoms/cm3, ρ = 18 Ω × cm)-Si(001)-2 × 1 surface. (a and b) were acquired on the same area at filled (Vbias = −2 V) and emptystate (Vbias = +1.5 V) conditions, respectively. Set point current I was 100 pA. Neutral isolated dangling bond (IDB) appears bright in both conditions. Their areal density is 0.020 ± 0.005 defects/Si atoms. (c) High-contrast filled-state image [same imaging conditions as (a)], illustrating the surface distribution of hydroxyls and monohydrides.

1014 atoms/cm3, ρ = 18 Ω × cm) Si(001)-2 × 1 surface passivated with 4.5 L of water at room temperature (PSTM = 0.75 × 10−8 Torr). Isolated dangling bonds (IDBs) appear as bright features, both in occupied and unoccupied state images, with an areal density of 0.020 ± 0.005 defects/Si atoms Their 10007



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Figure 2. Real-time (scanning-while-dosing) filled-state STM images of (H,OH)-Si(001)-2 × 1 (lightly p doped ρ = 18 Ω × cm) exposed to benzaldehyde at nominal PSTM = 3.75 × 10−8 Torr: (a) pristine surface; (b) after 7 min dosing; and (c) after 14 min dosing. (a′, b′, and c′) are duplicated from (a, b, and c), respectively, but with a grid to facilitate the localization of the adsorption events. The white circles indicate isolated IDB, before (dashed circle) and after (solid circle) reaction with the molecule. Arrow indicates the transformation of an IDB into a double-dotlike feature (two events). Zoomed region, centered on a reacted IDB, is shown in Figure 3. Vbias = −2 V and I = 100 pA.

apparent height is ∼0.8 Å in the filled-state image and ∼1.6 Å (Å) in the empty-state images. The dual-bias images presented here are identical to previously published dual-bias images of IDB on low-doped n type38,39 and p-type14 high-doped Hterminated Si(001)-2 × 1. A downward band bending of −0.27 eV is measured via Si 2p XPS (Table S2 of the Supporting Information). Knowing the defect density, the positive average charge per IDB is a 4 × 10−3 unit charge. The IDB is therefore quasi-neutral (i.e., singly occupied). However, during STM measurements, the IDB occupation can be modified by tip induced band bending effects,40,41 especially at low doping,42 and nonequilibrium defect charging due to the different tunneling rates between tip and defect, on the one hand, and defect and bulk, on the other.43−45 In atomically resolved images (Figure 1, panels a and b), the spatial arrangement of H and OH terminations can also be determined. In filled state images (Figure 1a), medium bright elongated features placed every five dimers in a row (on average), are due to SiOH pairs on the same dimer (likely stabilized by hydrogen bonding).6,7 SiH and SiOH can also be distinguished, due to a greater apparent height of the latter.6,7 Their arrangement is better seen in the high contrast occupied state image of Figure 1c, which shows clearly that the OH species tend to be aligned on the same side of the dimer row (as a consequence, the H are also aligned but on the opposite side). The (H,OH)-Si(001)-2 × 1 surface was exposed to benzaldehyde under a nominal pressure PSTM = 3.75 × 10−8 Torr. The occupied-state images of Figure 2 (panels a−c) were obtained using the “scanning-while-dosing” procedure. The pristine water-covered surface (Figure 2a) was exposed to a maximum benzaldehyde nominal dose of 31.5 L for 14 min (Figure 2c). The real-time STM images show that the preferential reaction sites are the IDBs (Figure 3, panels a and b). In filled state images, the IDB changes from a bright protrusion to a circular dark halo after reaction with the molecule. This depression has a radius (∼2.5 nm) greater than the width (∼1

nm) of the intact IDB, as can be seen in the profile given in Figure 3g [see height width profiles (ii and i), respectively]. The center of the halo is not exactly confounded with the location of the reacted dangling bond. As shown in Figure 3, the halo extends over the trench, separating two dimer rows, nearest to the reacted IDB. The latter is still seen as a faint protrusion (indicated by the arrow in Figure 3, panels b and g). Therefore, the molecular footprint could well extend over the trench. Note that no IDB identified as a bright protrusion is recreated in the vicinity of a reacted IDB. After the 14 min time span of the scanning, 16 IDBs over a total of 272 IDBs have reacted [statistics made on the whole 50 × 50 nm2 images from which the images (zoom) shown here are adapted], that is ∼6% of the initial value. There are rare events (2 in the whole 50 × 50 nm2 image) corresponding to the transformation of an IDB into a double-dot feature (the arrows in Figure 2, panels b and c). The latter is an O-bridge dimer •Si−O−Si• (where • is an unpaired electron) that likely results from the reaction HO− Si−Si• → •Si−O−Si• + H (the loss of a H from a surface silanol could lead to an instable nonbonding O, that then inserts into the silicon dimer bond23). The advantage of the scanning-while-dosing method is that one can follow in real-time the chemical evolution of a given dimer/dangling bond interacting with benzaldehyde. On the other hand, the presence of the tip may diminish the effective molecular flux. Due to shadowing effects, only a fraction fsh of the molecular flux reaches the scanned region. fsh is estimated to be about 0.14 for a tip radius of 50 nm and a tip−surface distance of 0.5 nm.28 It is therefore crucial to image areas positioned at macroscopic distances from the “scanning-whiledosing areas”. The dual-bias images of an unshadowed area presented in Figure 4 were obtained after an overall dose of 58.7 L (PSTM = 3.75 × 10−8 Torr, duration 26 min). First let us focus on the reacted IDB for which we have now empty-state images. In Figure 4, they inscribed within a pink ○ to distinguish them from the unreacted IDB (inscribed in green ◊). The faint protrusion (apparent height ∼0.8 Å) associated with an off-centered wide dark depression encompassing an 10008



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Figure 3. Zoomed region (taken from the area enclosed in the green square in the STM images shown in Figure 2 (panels b and c) featuring an IDB (a) before and (b) after the reaction. (c and d) are duplicated from (a and b) for clarity. The solid circles indicate the area of the isolated dangling bond (IDB), while the dotted circles show the region of the halo resulting from the adsorption of the benzaldehyde molecule. Solid lines (i and ii) are cross-sectional profiles which are plotted in (g). (e and f) are schematics shown to illustrate the position of the IDB and its disappearance after the adsorption of a benzaldehyde molecule. Small gray circles indicate reacted Si atoms: Si−H or Si−OH. Note that the halo is not centered on the IDB; the asymmetry is discussed in the text. Scanning conditions are Vbias = −2 V and I = 100 pA.

adjacent dimer row seen in filled-state images turns into a bright protrusion of apparent height 1.3 Å in the empty-state image (see Figure 4, panels c and d for a zoomed-in image and height profile). Its width is ∼5.5 Å, smaller than that of the unreacted IDB (i.e., ∼8.7 Å). The positions of the faint (filled state) and bright protrusion (empty state) coincide and are found at the position of the unreacted dangling bond. The position of the reacted IDB is well-identified, and its biasdependent features are distinctively different from that of the unreacted one. There is no hint for the reformation of an IDB (a bright protrusion in dual-bias images) in the vicinity of the reacted IDB that would result from the abstraction of nearby H, in contrast to the case of H-terminated Si(001)-2 × 1.12 The dark depression off-centered with respect to the position of the reacted IDB seen in filled-state images suggests that the

Figure 4. STM images acquired on the same area of (H,OH)-Si(001)2 × 1 (lightly p doped ρ = 18 Ω × cm) exposed to 58.7 L of benzaldehyde under a pressure PSTM = 3.75 × 10−8 Torr. (a) Filledstate (Vbias = −2 V) and (b) empty-state (Vbias = +1.5 V) images at a current set point I = 100 pA. Green ◊: bright (in filled state)/bright (in empty state) corresponding to unreacted dangling bonds; Pink ○: dark (in filled-state image)/bright (in empty-state image) adsorption site corresponding to reacted dangling bonds; green □: bright (in filled state)/dark (in empty state) adsorption site of a closed-shell adduct, resulting from a concerted hydrosilylation. (c) Zoomed part showing the empty-state image of the reacted dangling bond; (d) height profiles for the filled-state and empty-state image of the reacted IDB.

molecule (and particularly its bulky phenyl ring) extend over the trench separating two dimer rows. The last piece of 10009



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when it is not aligned with valence band states. This is likely the case of the single occupied molecular orbital of the radical, that should lie at about midgap, given the surface Fermi level position measured by Si 2p XPS (Supporting Information). Defect imaging can be considered a double barrier junction with different tunneling rates between the bulk and the defect, on the one hand, and the defect and the tip, on the other hand.43−45 With a negative Vbias, the band bending is further increased42 with respect to the zero-bias situation, and a wide space layer develops. While this decouples the reacted IDB level from the bulk and strongly decreases the tunneling bulk/defect tunneling rate, the defect/tip tunneling rate is high. As a consequence, the defect becomes positively charged. The dark depression in the filled state−state is off-centered with respect to the unreacted IDB position. This suggests that the phenyl ring lies flat over the trench. We note that a bent geometry is energetically favored, according to Kanai et al.22 Due to steric hindrance with the H/OH termination of the second silicon atom of the reacted dimer, the rotation around the Si−O axis should be limited, explaining why the filled-state depression is displaced over the trench. In the unshadowed zone, most of the IDBs have reacted after a dose of 58.7 L: we count 35 reacted IDBs over a total of 45 IDB (in an area of 2040 nm2), that is about 80% of the initial amount. Therefore, the maximum surface density of benzaldehyde having reacted with the IDB is equal to the initial density of these defects, that is, 2 × 10−2 of a ML. This low maximum surface density makes its characterization by XPS and HREELS difficult, if not impossible (see below). Most interestingly, the dual-bias analysis enables the detection of a second reaction channel that would have gone unnoticed without the empty-state image. The defect inscribed in a light green square in Figure 4, appears, like the IDB, as a bright protrusion in the filled-state image, but in the emptystate image, it shows up as a depression, while the IDB remains bright. This defect was absent on pristine (H,OH)-Si(001)-2 × 1, for which we have obtained extended scanned areas. It is not correlated to the presence of dangling bonds, therefore its surface concentration should be limited only by the number of available reaction sites (i.e., the silicon dimers) corresponding to 0.5 ML, making it characterizable by our spectroscopic techniques. In the XPS setup, we have exposed the surface to 67.5 L of benzaldehyde, under a nominal pressure PXPS = 7.5 × 10−8 Torr. The measurement of the normalized O 1s area before and after exposure to benzaldehyde indicates an increase in the oxygen coverage (see Figure S3 of the Supporting Information) of 0.070 ± 0.015 ML with respect to the initial 0.5 ML of the pristine surface. The O coverage increase corresponds exactly to the amount of adsorbed benzaldehyde molecules, if no oxygenated species are eliminated during the reaction (it is naturally the case for a hydrosilylation reaction). The C 1s XPS and NEXAFS spectra given in Figures 5 and 6, respectively, should be essentially representative of the bright/dark feature observed by STM. The C 1s XPS spectrum (Figure 5) exhibits two peaks that can be fitted with Voigt components (full width at halfmaximum, fwhm = 0.86 eV), one at binding energy BE ∼ 284.83 eV and the other at BE ∼ 286.30 eV (93% and 7% of spectral weight, respectively). The low-binding energy (BE) peak at 284.83 eV is attributed to the aromatic carbons.49−51,19 The high BE peak at 286.30 eV is distant from the phenyl component by 1.47 eV, a value much smaller than that

information comes from the band bending measured by Si 2p XPS (see Figure S2 and Table S2 of the Supporting Information). After exposure to the molecule gives complementary information on the reacted IDB, the observed downward band bending (−0.30 eV) is in fact practically equal to that of the pristine (H,OH)-Si(001) surface. This is indicative that the reacted IDB remains electrically active (with states within the surface gap). The formation of closed-shell adduct via, for example, formation a Si−O bond should produce bonding and antibonding states out of the gap46 and therefore the bands should go flat, which is not observed. What bonding geometries could explain the STM images of the reacted IDB? First, the IDB are sufficiently far apart (see Figure 2) to make the grafting of a closed-shell adduct on a pair of dangling bonds via its oxygen and (phenyl) carbon ends impossible. A closed-shell Si−O−CH2−C6H5 adduct (on an adjacent dimer in the same row or on an nearby silicon of the adjacent row) is also excluded because in occupied state images of the H-terminated surface the adduct appears as a protrusion, and because the new IDB formed in its vicinity, the IDB should also appear as a bright protrusion,12 which is not observed. One may speculate that the new IDB is not seen because it makes a bond with the phenyl ring. It is, however, well-known that aromatic molecules interact strongly with two dangling bonds but not with one.47,48 Dissociative adsorption (via C−H or C− C bond breaking) could also be considered, but it should also require at least two silicon dangling bonds. In any case, we find no indication for molecular fragmentation on several sites from the dual-bias images that shows essentially one protrusion at the position of the unreacted IDB. Let us now consider the hypothesis of an open-shell radical molecular species, formed after the attack of HSi-Si• or HOSiSi• moieties (• denotes the unpaired electron) by the carbonyl and formation of a Si−O−CH•−C6H5 species (the CαH bears formally an unpaired electron). This is indeed the simplest bonding geometry one can imagine, given that the pristine IDB is surrounded only by silicon atoms capped either by H or by OH. The absence of IDB reformation in the vicinity of the reacted IDB is an argument in favor of a radical adduct. The remaining band bending observed by XPS also point to the formation of an electrically active defect (a donor), explainable by an open-shell adduct. Recent DFT calculations by Tan and Pei23 are in support of this hypothesis, as they point to the stability of the radical adduct. Tan and Pei find a large adsorption energy of 1.175 eV (1.57 eV, when dispersion corrections are included) for the radical adduct on (H,OH)Si(001)-2 × 1. Therefore, it is expected that the molecule does not return to the gas phase. Moreover an activation barrier of ∼1 eV must be surmounted for a H transfer from an OH (at intradimer, intrarow, or cross-trench positions) to produce a closed-shell Si−O−CH2−C6H5 (the latter is only 0.1−0.2 eV lower in energy than the radical species). Taking a preexponential frequency of 1013 Hz and a barrier of 1 eV (DFT calculation including dispersion), a lifetime of 6.5 h for the radical species is predicted, to be compared with the observed time stability of the reacted IDB (at least greater than 8 h). While the preceding arguments can call for the radical adduct, the fact remains that the dark halo observed at Vbias < 0 needs further explanations. The most pressing question is why a wide depression is seen. A depression in constant current image means that the density of filed states is locally diminished (e.g., by a local downward band bending around a positively charged defect). The defect may reach a nonequilibrium charge state 10010



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π1*(e2u) levels of benzene into levels 18a′ and 19a′. The component at 286 eV is therefore ascribed to a transition from C 1s (C-R) (where C-R denotes the ring carbon atom next to the carbonyl) to the 19 a′ level.54 The peak at ∼287.5 eV corresponds to excitations from C 1s (CO) to orbitals of primarily the π*CO character. For benzaldehyde adsorbed on (H,OH)-Si(001)-2 × 1, XPS suggests the breaking of the CO double bond. As a consequence, the π1* splitting due to conjugation in gas phase benzaldehyde should be eliminated, and the C 1s (C O) →π*CO transition should be quenched. This is indeed confirmed by the NEXAFS spectrum, where we see a single narrow component ( f whm = 0.6) at hν = 285 eV corresponding to the C 1s (ring) → π*ring transition. Consequently, the NEXAFS spectrum of adsorbed benzaldehyde presents the same components as that of gas-phase benzene55 (see Figure 6). By varying θ from 10° (grazing light incidence) to 90° (normal incidence), we have observed no dichroic effect in the absorption intensities (see Figure S4 of the Supporting Information). Given that the (two-domain) surface has a 4-fold rotation symmetry along the surface normal, the π system may be either randomly oriented or the C 2p component makes an angle of 54.7° (magic angle) with respect to the surface normal.31 Thus, XPS and NEXAFS prove that the carbonyl bond opens and that Si−O−C units are formed. A closed shell adduct Si− O−CH2−C6H5 can be formed by direct hydrosilylation20 of the molecule. This adduct is depicted in Figure 5b, showing that the HOMO is localized on the molecule. However, in contrast to the SOMO of the radical, the HOMO is positioned below the valence band maximum (the carbon π level are found at about 3.5 eV below the valence band maximum)56. Therefore, in filled state images, electrons picked up by the tip from the molecule should be easily replenished by electrons coming from silicon, in contrast to the radical adduct case, whose localized level is in the surface gap. A steady-state charging of the molecule is not expected for the closed-shell adduct, and hence, the molecule appears as a protrusion. Illustrative HREELS spectra (measured in specular geometry) of the (H,OH)-Si(001)-2 × 1 surface exposed to increasing doses of benzaldehyde are presented in Figure 7. They are obtained using an intrinsic silicon wafer (ρ = 2000 Ω × cm) to optimize the resolution. Complementary measurements were also carried out on the substrates used for STM and XPS/ NEXAFS experiments (p doped, ρ = 18 Ω × cm), with qualitatively identical results (see Figure S4 of the Supporting Information). Theoretical vibration energies are calculated for the H−Si− Si−OH unit, for the hydrosilylation product HOSi−Si− OCHH2−C6H5. The vibration modes of the radical adduct HOSi−Si−OCHH•−C6H5 is also calculated to look for specific features that could permit its identification at low coverage. No scaling of the theoretical energy values is made. In Figure 8, we plot the spectral intensities (calculated with the dipolar moment perpendicular to the surface) of the various modes. We start with the experimental spectrum of the pristine (H,OH)-Si(001)-2 × 1 surface, we interpret thanks to the DFT cluster calculation. The component at 258 meV is attributed to the Si↔H stretching mode (calculated at 268 meV) and that found at 451 meV to the SiO↔H stretching mode (calculated at 489 meV). According to the calculation (Figure 9), the experimental components at 101 meV correspond in fact to

Figure 5. High-resolution C 1s spectrum (hv = 350 eV) of lightly pdoped (NBoron = 7.5 × 1014 atoms/cm3, ρ = 18 Ω × cm) Si(001)-2 × 1 surface passivated with 4.5 L of water at room temperature and then exposed to 67.5 L of benzaldehyde (PXPS = 7.5 × 10−8 Torr, 900 s).

Figure 6. Bottom curve: C 1s edge NEXAFS spectrum (Auger Yield Mode) of benzaldehyde on (H,OH)-Si(001)-2 × 1 (θ = 44°), see caption of Figure 6. Middle curve: the inner-shell electron energy loss spectrum (ISEELS) of gas-phase benzaldehyde (from ref 54). Top curve: ISEELS spectrum of gas-phase benzene (from ref 55).

separating the phenyl and carbonyl C 1s in the intact molecule (2.7 eV).52 Therefore, the conservation of the carbonyl is excluded. As the XPS literature53 indicates that (in polymers) the BE shift between CHx and COC is 1.44 eV, the component at 286.3 eV can be attributed to the α C in a Si−O−C linkage. A Si−O−C−O linkage is excluded, as (in polymers) the OCO moiety component is 2.9 eV higher in BE than the CHx one. Therefore, there is no indication of benzaldehyde oligomers (starting from the radical adduct) nor of hemiacetal-like adducts, resulting from the capture of a nearby OH by the radical adduct, in agreement with the calculations of Tan and Pei.23 In Figure 6 (bottom curve), we show the C 1s NEXAFS spectrum (θ = 44°), corresponding to the XPS spectrum of Figure 5. For the sake of comparison, we show in the same figure the inner-shell electron energy loss spectra (ISEELS) of gas phase benzaldehyde (middle curve) and that of gas phase benzene (top curve). In gas phase benzaldehyde, the main absorption line is found at an excitation energy of 285.1 eV, with a shoulder at ∼286 eV.54 Conjugation between the π*ring and π*CO orbitals causes the splitting of the two degenerate 10011



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Figure 9. Plot of Si↔H stretching mode intensity (258 meV) versus C↔H (ring) intensity (380 meV) for the intrinsic substrate (blue ●). Black line is a linear fit of the experimental data. Figure 7. HREELS spectra of (H,OH)-Si(001)-2 × 1 exposed to increasing doses of benzaldehyde. Assignment of the different loss intensities is indicated by the blue dotted lines. We use an intrinsic substrate for better spectral resolution. Quasielastic peak shows a width of 12 meV.

meV), a Si↔OH stretching mode (calculated at 89 meV), and a swinging mode SiOH↓↑ (calculated at 108 meV). The component at 203 meV is a double loss.57 HREELS (Figure 7) gives evidence also for the adsorption of the benzaldehyde molecule on the surface. With increasing doses, one observes the growth of an intense peak at 128 meV. This loss is attributed to a stretching C↔O mode. Indeed the

three unresolved modes of the hydroxyl, a SiH↓↑ bending mode coupled with a Si↔OH stretching mode (calculated at 85

Figure 8. Calculated infrared spectra intensities (arbitrary units) of the H−Si−Si−OH unit (bottom), the radical adduct sitting next to a hydroxyl (middle), and the silylation product sitting next to a hydroxyl (top). No scaling of the DFT energy values is made. The corresponding ball-and-stick models are given. To ease comparison with the HREELS spectra of Figure 7 measured in specular geometry, the dipolar moment is placed perpendicularly to the surface. Lorentzian broadening of 5 meV is used to plot the intensities. 10012



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Figure 10. (a) Chidsey mechanism:21 formation of a radical intermediate, followed by H abstraction of a nearby SiH, the latter reaction being blocked on (H,OH)-Si(001)-2 × 1. (b) Tan and Pei mechanism:23 formation of a radical intermediate, followed by H abstraction from a nearby hydroxyl, the latter reaction being not observed on (H,OH)-Si(001)-2 × 1. (c) Four-center concerted mechanism (direct hydrosilylation):20 the transition state may be stabilized by H-bonding on (H,OH)-Si(001)-2 × 1.

calculation points to one C↔O (+bending SiOH↓↑) mode at 133 meV for the hydrosilylation (closed-shell) product. For the radical adduct two C↔O modes are calculated at somewhat higher energy, 147 and 150 meV, respectively. This observation confirms the opening of the carbonyl π bond, for which XPS and NEXAFS also gave evidence. Upon benzaldehyde exposure, we see in Figure 8 a peak at 380 meV, with a shoulder at ∼360 meV. The former is attributed to the phenyl ring (sp2) C↔H stretching mode as it is calculated at 399 and 397 meV for the radical adduct and the silylation product, respectively. Thus, this mode cannot help distinguish between the radical and the hydrosilylation product. On the other hand, the shoulder at 365 meV is a distinctive mark of the closed-shell adduct, as the calculated CαH2 stretching mode is red-shifted from the ring mode by 27 meV. Unfortunately, for the very initial exposures, the identification of the radical species is made impossible due to a C contamination visible on the pristine (H,OH)-Si(001) precisely at ∼365 eV. For the high doses, the asymmetric shape of the C↔H peak points to the formation of the hydrosilylation closed-shell model. DFT cluster calculations also provide interesting qualitative information on the intensity and energy change of the hydroxyl modes when a hydrogen bonding is established with the hydrosilylation product. As it can be seen in the ball and stick models of Figure 8, the proton of the hydroxyl is attracted by the π system of the ring and the oxygen of the molecule. Calculations show that this dative H-bonding causes a red shift of the SiO↔H stretching mode from 489 meV (no H bond) to 475 meV (−14 meV). As a matter of fact, the HREELS mode observed at 451 meV (Figure 8) of the pristine surface strongly

decreases in intensity after prolonged dosing (670 L), while a new component shows up at ∼434 eV, red shifted by −17 eV, in excellent accord with the theoretical red-shift value. In essence, HREELS indicates that a H bond is established between the surface OH and the benzaldehyde molecule. Now can we use the HREELS mode intensities to monitor the molecular attachment? If we consider first the SiO↔H stretching mode, its intensity depends on the orientation of the O−H bond axis with respect to the surface normal. For the HO−Si−Si−H unit, the calculation is made with a single-dimer cluster that may not be representative of the average O−H orientation on the real surface, as interactions with nearby hydroxyls are not taken into account. Therefore, the observed decrease of the SiO↔H HREELS intensity does not necessarily mean that OH moieties are consumed by the reaction. Neither is the decrease in intensity of the 101 meV HREELS peak evidence of OH reacting with the aldehyde. Indeed, according to theory, the SiOH↓↑ bending mode, at 108 meV in the absence of H bonding, shifts to 127 meV when the H bond is established, merging with the coupled C↔O (+SiOH↓↑) mode calculated at 133 meV. Therefore, the intensity is simply transferred to higher energy. On the other hand, the intensity of the monohydride SiH neither presents this orientational issue nor is sensitive to the presence of the aldehyde. Therefore, the plot of the Si↔H mode intensity (at 258 meV) against the C↔H (ring) intensity (380 meV) can be a valuable indicator of a hydrosylilation reaction involving the monohydride. The plot is given in Figure 9. We see a clear correlation between the decrease of the Si↔H intensity and the increase of the C↔H (ring) intensity. This 10013



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10a).21 In their DFT calculation of the direct reaction of an alkene with an SiH, Coletti et al.20 considered a direct concerted pathway that passes through a four-center transition state (Figure 10c). For ethylene, they calculated a very large barrier of ∼2.6 eV. However, benzaldehyde is a much bigger molecule than ethylene (London interactions are expected to be more intense) and, above all, it can make a hydrogen bond with an OH of the same dimer. Therefore, we propose that, when the reactivity of benzaldehyde with (H,OH)-Si(001)-2 × 1 is compared to that with H−Si(001)-2 × 1, it is the hydrogen bond that makes the difference. Indeed H-bonding could lower the transition state energy of the direct hydrosilylation process and the activation barrier, while it could stabilize the radical intermediate state (the adduct) in the reaction induced by the silicon dangling bond to such an extent that the latter species cannot further abstract a nearby H.

gives strong evidence that H from SiH units are consumed in a hydrosilylation reaction.

4. CONCLUSIONS Although the water-covered Si (001) surface, (H,OH)-Si(001)2 × 1, presents strong analogies with the H-terminated Si(001)2 × 1 surface due to the existence of monohydrides and of isolated dangling bonds, their reactivity with a carbonyl-bearing molecule, benzaldehyde, shows noticeable differences. As shown by STM, benzaldehyde reacts with the quasi-neutral isolated silicon dangling bond of (H,OH)-Si(001)-2 × 1 formed on a lightly p-doped sample. However, in contrast to the H−Si(001)-2 × 1 case where the Chidsey mechanism21,12 is operative, we do not see the reformation of an isolated dangling bond via the abstraction of a nearby H atom from a silicon monohydride (see Figure 10a). Nor is the Tan and Pei mechanism23 observed, where the H atom is abstracted from an OH neighbor. The nature of the product formed by the reaction of the molecule with the isolated dangling bond is discussed. We tentatively propose that a radical molecular adduct is formed. It is clear that further studies are necessary to confirm this hypothesis, for instance the use of ultrahigh vacuum surface electron spin resonance.58 STM also shows the existence of a second type of molecular species located on the silicon dimers, with dual-bias footprints markedly different from that of the preceding species. Apparently, its bonding needs not be mediated by a silicon dangling bond. The chemical nature of this second species, whose maximum areal density should be limited only by the density of dimers (0.5 monolayer), is revealed by electron (XPS/NEXAFS) and vibrational (HREELS) surface spectroscopies. Surface spectroscopies concur to show that the π-bond of the carbonyl unit opens to form a Si−O−C unit. XPS indicates that the α carbon has only one oxygen neighbor, excluding oligomerization or capture of surface hydroxyls. The attachment via the benzene ring is also excluded. HREELS, in combination with vibrational mode calculations, indicates that the energies of the C−H stretching and bending modes of the α carbon are well-accounted for, if we consider the hydrosilylation product. A strong piece of evidence for a concerted hydrosilylation reaction is also provided by the correlation between the decrease of the Si−H stretching intensity and the increase of the (ring) C−H stretching intensities. Finally, the evolution of the hydroxyl modes (energies and intensities) with increasing benzaldehyde coverage are interpreted, through the DFT simulation, in terms of a hydrogen bonding established between the reaction product and a nearby OH. The absence of radical chain reaction and the observation of concerted hydrosilylation reaction are the two main features opposing (H,OH)-Si(001)-2 × 1 and H−Si(001)-2 × 1. With concern for the nonpropagation of the radical reaction (Figure 10 a, 10b), the chemical disorder of (H,OH)-Si(001)-2 × 1 could be invoked. However, Figure 1c shows that relatively long sections of aligned SiH are present on the surface, similar to H−Si (001)-2 × 1. If we take into consideration the possible formation of a radical adduct, the latter should be trapped in a minimum of energy, with a high barrier to surmount to capture a nearby hydrogen, from an OH or a SiH, as suggested by recent calculations.23 H bonding with a nearby OH could provide an extra stabilization energy and increase the barrier. On H-terminated surfaces, the concerted hydrosilylation reaction is generally considered as less likely than the Chidsey radical reaction, due to kinetic barrier considerations (Figure

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ASSOCIATED CONTENT

S Supporting Information *

The XPS spectra fitting procedure, the Si 2p spectra for (H,OH)- Si(001)-2 × 1 before and after exposure to 67.5 L benzaldehyde dose at room temperature with the fit parameters used, the binding energies (BE) (Si 2p3/2), Fermi level position at the surface EF − ESVBM, surface band bending qVbb, surface charge density, the determination of the molecular coverage from O 1s XPS normalized intensities, the angular dependent C 1s NEXAFS spectra, and the HREELS spectra of the lightly doped surface. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest. ⊥ Associated with Synchrotron SOLEIL.

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REFERENCES

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Benzaldehyde on Water-Saturated Si(001): Reaction with Isolated

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