Transfer of Electron Density and Formation of Dative Bonds in

Department of Chemistry, Princeton University, Princeton, New Jersey 08544, and Department of Chemistry, National University of Singapore, 10 Kent Rid...
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15474

J. Phys. Chem. C 2008, 112, 15474–15482

Transfer of Electron Density and Formation of Dative Bonds in Chemisorption of Pyrrolidine on Si(111)-7 × 7 Feng Tao,† Yinghui Cai,‡ Yuesheng Ning,‡ Guo-Qin Xu,‡ and Steven L. Bernasek*,† Department of Chemistry, Princeton UniVersity, Princeton, New Jersey 08544, and Department of Chemistry, National UniVersity of Singapore, 10 Kent Ridge, Singapore, 119260 ReceiVed: June 27, 2008; ReVised Manuscript ReceiVed: August 11, 2008

The chemical binding of pyrrolidine on Si(111)-7 × 7 was studied using high-resolution electron energy loss spectroscopy (HREELS), X-ray photoelectron spectroscopy (XPS), and DFT calculation, to obtain a thorough mechanistic understanding of the formation of dative bonds of nitrogen-containing organic molecules on semiconductor surfaces. This study focuses on the electronic and geometric structures of the surface reactive sites. XPS of a chemisorbed monolayer suggests two adsorbates (β1 and β2) associated with the formation of a N · · · Siad dative bond. At low exposure, the adsorbate β1 forms a dative bond as indicated by a higher N1s binding energy compared to the Si-N σ bond in the dissociated product formed at 250 K. At higher exposure, the formation of the second N · · · Siad dative bond in adsorbate β2 was evidenced by an obvious upshift of the N1s core-level in contrast to that observed for physisorbed molecules or Si-N σ-bonded molecules. Vibrational studies show a down-shift of the N-H stretching frequency by ∼95 cm-1 upon the formation of the N · · · Siad dative bond in contrast to the free pyrrolidine molecule, providing new evidence for the dative bonding between amines and silicon surfaces. These studies suggest that the β2 state is an adsorbate directly bonded to the silicon surface adatom via the formation of a N · · · Siad dative bond, and the β1 state is an adsorbate which forms a N · · · Siad dative bond with an adatom and a weak N-H · · · Sire hydrogen bond with an adjacent electron-rich rest atom. In the β1 adsorbate, both electron acceptance by the adatom and electron donation by the rest atom occur simultaneously with one pyrrolidine molecule through dative bonding and hydrogen bonding, respectively. The extra electron-deficient adatom sites on Si(111)-7 × 7 provide an opportunity for forming a second dative bond after the formation of the first in contrast to Ge(100) and Si(100) where the number of electron-deficient and electron-rich sites are equivalent. The difference in electronic and geometric structures of the reactive sites on different semiconductor surfaces offers a useful channel to shape reactivity and selectivity of organic functional groups on silicon surfaces. 1. Introduction The organic modification and functionalization of semiconductor surfaces has attracted much interest recently as the functionalized semiconductor surfaces have promising applications in the design of new-generation microelectronic devices and development of various sensors.1,2 The reaction mechanisms of organic molecules on elemental semiconductor surfaces are diverse and complicated. A thorough understanding of the attachment chemistry of various organic molecules on these semiconductor surfaces is still lacking. The barrier to a thorough understanding mainly results from the diverse reactive sites present on semiconductor surfaces and the multiple-reaction channels available from organic molecules. Based on previous studies of organic reactions on semiconductor surfaces, including Si(100), Ge(100), diamond (100), and Si(111)-7 × 7, these reaction mechanisms can be categorized as four types in general: (1) [2 + 2]-like addition for molecules containing one or more unconjugated CdC, CdN, CdS, CdO, C≡C or C≡N bonds; (2) [4 + 2]-like addition for molecules containing one or more conjugated diene moieties such as CdC-CdC, CdC-C≡C, CdC-C≡N, or C≡C-C≡C; (3) proton transfer reactions of M-H (M ) N, O, S) and photodissociation of C-X (X ) * To whom correspondence should be addressed. E-mail: sberna@ princeton.edu. Fax: (609) 258 1593. † Princeton University. ‡ National University of Singapore.

halogen); and (4) dative bonding of molecules containing an electron-donating group as in trimethylamine or pyridine. These bonding types have been summarized in several reviews.3-8 The formation of a dative bond on elemental semiconductor surfaces was revealed recently in the chemisorption of Ncontaining molecules.9-18 Generally, two kinds of organic molecules are seen to form dative bonds with these elemental semiconductor surfaces. One type is heterocyclic aromatic amines such as pyridine adsorbed on Ge(100),11,12 Si(100),16 and Si(111)-7 × 7,11 or pyrimidine on Ge(100).13 The N · · · Si dative bond forms for pyridine and pyrimidine since the nitrogen atom in these aromatic rings has a localized electron lone pair in one unequivalently hybridized sp2 orbital. In contrast, the lone pair on the unhydridized p orbital of the nitrogen atom of a pyrrole molecule is an integral part of the (4n + 2) π electrons of the aromatic conjugation, and is delocalized on the entire aromatic ring. Thus, pyrrole does not form a dative bond with semiconductor surfaces, but forms a Si-N covalent bond through dissociation of the N-H bond proceeding via an initial attachment at R-C.15,19,20 Also, due to the delocalization of the nitrogen lone pair, N-methylpyrrole cannot form a N · · · Ge dative bond on Ge(100)15 or a N · · · Si dative bond on Si(111)-7 × 7.21 These studies suggest the necessity of a localized lonepair of electrons on the nitrogen atom in aromatic amines available for electron donation in order to form a dative bond.

10.1021/jp8056866 CCC: $40.75  2008 American Chemical Society Published on Web 09/05/2008

Chemisorption of Pyrrolidine on Si(111)-7 × 7 Another category of molecule possibly exhibiting dative bonding is the aliphatic amine, including primary amines such as methylamine, secondary amines such as dimethylamine, and tertiary amines such as trimethylamine. It has been reported by Hamers et al. and Bent et al. that trimethylamine forms a stable dative bond on both Si(100) and Si(111)-7 × 79,10 as does N-methylpyrrolidine on both Si(100) and Ge(100).15 For primary and secondary amines, each has two possible reaction channels, N-H dissociation and N · · · Si dative bonding. Theoretical calculations indicate that both of these pathways are barrierless.9,10 However, the N-H dissociation channel is thermodynamically more favorable in comparison to the formation of the N · · · Si dative bond, even though there is a localized electron pair on the nitrogen atom. Thus, the dissociation product of methylamine and dimethylamine is formed on Si(100) and Si(111)-7 × 7.9,10 Although much attention is paid to the electronic structure of the organic molecules involved in the formation of dative bonds on semiconductor surfaces, much less attention has been paid to the side of the semiconductor surface itself. In fact, electronic and geometric structures of the semiconductor surface also play a major role in the formation of a dative bond, as the organic/semiconductor interfacial reaction is an organic reaction under dry conditions (organic reaction without solvent) and thus the semiconductor surface should be considered as an active reagent. A comparison of the chemisorption of several amines on Si(100) and Ge(100) suggests significant differences in the dative bonding of organic molecules on the two surfaces. For example, 3-pyrroline primarily undergoes dative bonding on Ge(100),15 but a major N-H dissociation and a minor dehydrogenation on Si(100).15,22 A similar difference between Si(100) and Ge(100) is observed for N-methyl-3-pyrroline.15 This molecule chemisorbs on Ge(100) through a dative bond at room temperature, but no dative-bonded product is detected on Si(100). In addition, the saturation coverage of the chemisorbed N-methylpyrrolidine on Ge(100) is significantly larger than that on Si(100) under the same reaction conditions, though it forms dative bonds on both surfaces.15 Pyridine does not form a dative bonded product at room temperature on Si(100),16 though dativebonded pyridine is the major product of pyridine adsorption on Ge(100) at room temperature.11,12 These differences are quite consistent with the intrinsic difference in electronic structure of the SidSi and GedGe dimers of the two (100) surfaces. The (100) surfaces of both Si(100) and Ge(100) have the same surface geometric structures. As is well understood, the two (100) surfaces exhibit parallel packed dimer rows. In each dimer, one atom buckles up and the other buckles down. Therefore, electron density partially transfers from the lower atom to the higher atom, making the buckled-down and buckled-up atoms electron-deficient and electron-rich, respectively (see Figure S1 in the Supporting Information). Therefore, the surface has an equal number of electron-deficient and electron-rich atoms. However, both STM measurement and molecular dynamics simulation consistently show that the SidSi dimer tilt is dynamically changing on the picosecond time scale at room temperature,24,25 but that the GedGe dimer is statically tilted at both low temperature and room temperature.26 As expected, the dynamic nature of the tilting of the SidSi dimer at room temperature definitely impacts the course of the surface reaction, making the formation of a dative bond unfavorable on this surface at room temperature. Alternatively, the static nature of the tilting GedGe dimer even at room temperature favors the formation of the dative bond with organic molecules. The connection between the geometric and electronic structure of reactive sites of (100) surfaces of IVA semiconductor single

J. Phys. Chem. C, Vol. 112, No. 39, 2008 15475 crystals and molecular reactivity on these surfaces suggests the importance of the semiconductor surface itself in the formation of dative bonds between organic molecules and the surface. To more thoroughly understand the mechanism of dative bonding between organic molecules and semiconductor surfaces, the electronic and geometric structure of surface reactive sites must be considered as an important factor shaping the formation of the dative bond. Notably, compared to the equal number of electron-deficient and electron-rich reactive sites on Si(100) and Ge(100), the number of electron-deficient adatoms is twice the number of electron-rich rest atoms on the Si(111)-7 × 7 surface.27 In addition, the adatom and rest atom respectively have a nominal charge of +7/12 and -1 on the Si(111)-7 × 7 surface, showing a much larger charge transfer between electronrich and electron-deficient sites in contrast to the buckled-down and buckled-up atoms on the (100) surfaces of elemental semiconductors. Since Si(111)-7 × 7 has a significantly different surface geometric and electronic structure compared to Si(100) and Ge(100), it was selected here as a test surface to study the impact of the electronic and geometric factors of surface reactive sites in favoring different reaction channels and shaping the formation of an interfacial dative bond. The 7 × 7 reconstructed Si(111) surface has a much more complicated geometric and electronic structure than the (100) surfaces of elemental semiconductor single crystals. Starting with 49 unsaturated “dangling bonds” per bulk-terminated unitcell of (7 × 7) dimension (Figure S2a in the Supporting Information), the Si(111) surface undergoes a complex fourlayer reconstruction (Figure S2a-d in the Supporting Information) which reduces the number of dangling bonds in each unit cell from 49 to 19.28 The structure of the formed Si(111)-7 × 7 can be described with the dimer, adatom, stacking fault (DAS) model.27,28 Figure S3a and b (Supporting Information) presents a top view and a side view of the three-dimensional structure of the Si(111)-7 × 7 surface. The topmost layer is an adatom layer consisting of twelve adatoms in each unit cell with six in each half. The second layer is a rest-atom layer, with six rest atoms in each unit cell and three in each half. The six adatoms and three rest atoms are alternately arranged in each half-unit cell. Both adatoms and rest atoms have a dangling bond. Notably, they are electron-deficient and electron-rich, respectively.28 More importantly, the 7 × 7 unit cell is highly thermodynamically stable in vacuum. An intact 7 × 7 unit cell can be still observed even at a temperature as high as 800 K. Pyrrolidine, a cyclic secondary amine, is chosen as a probing molecule to understand the dative bonding mechanism from the side of surface reactive sites. In the present study, XPS, HREELS, and DFT calculations were used to investigate the chemical binding of pyrrolidine on Si(111)-7 × 7. Two different binding states (β1 and β2) involving the formation of R2NH · · · Siad dative bonds were revealed here. The second dative-bonded product (β2) is formed due to the extra electrondeficient sites made available upon the formation of the first dative-bonded pyrrolidine (β1). In addition, this study revealed the obvious down-shift of the N-H stretching frequency upon the formation of a Si · · · N dative bond, which is new evidence for the formation of the dative bond in addition to the upshift of the N1s core-level in XPS. Compared to the chemical binding of pyrrolidine on Si(100) and Ge(100), this molecule exhibits significant differences in the formation of dative bonds on the Si(111)-7 × 7 surface. This demonstrates the importance of electronic and geometric structures of surface reactive sites for the organic functionalization reaction. While the focus of the present study is on the formation of dative bonds by N-

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containing amines on the semiconductor surface, mechanistic understanding from this work could be relevant for a variety of N-containing compounds important in semiconductor processing, including ammonia and dialkylamino precursors used in deposition. 2. Experimental Section The experiments were performed in two UHV systems. The first one is an XPS/UPS chamber fitted with a mass spectrometer for thermal desorption spectroscopy and low energy electron diffraction (LEED) optics for structure examination. It has a base pressure of 1.0 × 10-10 Torr. A dual-anode X-ray gun, a He UV lamp, and an electron energy analyzer (CLAM 2, VG) were mounted on this chamber for XPS and UPS studies. In the XPS experiments, the Mg X-ray source (hν ) 1253.6 eV) was used. The N 1s and C1s core levels of pyrrolidine on Si(111)-7 × 7 were measured. All of the spectra presented here are referenced to the binding energy (BE) of 99.3 eV30 for the bulk Si2p XPS peak. The second experimental system is a HREELS/UHV system with a base pressure of 6.0 × 10-11 Torr.18 It is equipped with a LEED, an Auger electron spectrometer (AES), and a mass spectrometer as well. This HREELS spectrometer (LK3000-14R) consists of two 127° cylindrical deflector analyzers (CDA) functioning as two monochromators and two 127 °CDAs for energy analysis. For HREELS measurements, an electron beam with a primary energy (Ep) of 5.0 eV collides with the sample surface at an incident angle (θi) of 60° from the surface normal. Studies of both specular and off-specular geometries can be carried out with this HREELS system. The energy resolution of the obtained spectra defined as full width at half-maximum (fwhm) for the elastic peak was determined to be ∼40-50 cm-1. The Si(111) samples (9 mm × 18 mm × 0.38 mm) were cut from n-type (P-doped) silicon wafers with a purity of 99.999% (Goodfellow). A Ta foil with a thickness of 0.025 mm sandwiched between two identical Si samples with a set of Ta clips, was spot-welded to two Ta posts (diameter ∼1.5 mm) at the bottom of a Dewar-type liquid nitrogen-cooled sample holder. This sample assembly and cooling system give a very high cooling efficiency for the sample. A silicon sample can be quickly cooled to 85 K through the liquid nitrogen reservoir. The sample was heated through resistive heating of the sandwiched Ta foil. The sample heating and cooling is precisely controlled by an attached K-type thermocouple, and an associated temperature controller and a DC power supply. After degassing the sample by annealing it at 850 K for 24 h the sample was carefully cleaned by cycles of Ar+ sputtering and annealing to 1200 K for 15 min, producing the 7 × 7 reconstruction. Pyrrolidine (g99.5%, Aldrich) was further purified by several freeze-pump-thaw cycles before being dosed onto the Si(111)-7 × 7 surface through a Varian adjustable leak valve. Nominal exposures were calculated and reported in Langmuir (1 Langmuir ) 10-6 Torr · s) without ion gauge sensitivity calibration. There is an order of magnitude difference in the reported nominal exposure between the XPS/ UHV and HREELS/UHV systems. In the HREELS/UHV system, chemical vapor is introduced to the sample surface using a directed doser positioned in front of the sample. The center of the sample was kept at the same level as the port of the doser. This doser was connected to a vapor introduction manifold through a Varian adjustable leak valve. The dosing tube had an internal diameter of 7 mm and was positioned 2-3 mm from the sample surface. However, in the XPS/UHV system, the sample surface and the port of the doser cannot be kept at the

Figure 1. N1s spectra of pyrrolidine adsorbed on a clean Si(111)-7 × 7 as a function of exposure at 85 K. The exposure listed here was obtained by background dosing as described in the Experimental Section.

same level due to the limitation of the travel distance of the sample manipulator and the geometry of the leak valve of the XPS/UHV system. Thus, the reported nominal exposures from the XPS/UHV system are about 1 order of magnitude larger than those from the HREELS/UHV system. DFT calculations were performed using Gaussian 03 at B3LYP/6-31G(d,p) level. A detailed description of the construction and optimization of the cluster was reported previously.31,32 Briefly, a cluster model, Si30H28 (cluster II) was cut from the central part of a MMFF94 optimized mother cluster I containing three 7 × 7 unit cells (see Figure S4 in the Supporting Information), where the precision of atomic positions suffers the least from boundary effects. Cluster II contains an adatom and an adjacent rest atom from an unfaulted subunit, serving as the reactive site for the attachment of an organic molecule. Capping H atoms at the cluster boundaries are kept frozen. Silicon atoms in the bottom double layers of cluster II are placed at bulk lattice positions prior to the geometry optimization process, with each Si-Si bond length set to 2.3517 Å and all bond angles adjusted to 109.4712°. Cluster III was obtained from further reduction of cluster II. Previous studies show that this model cluster built in three steps can simulate the surface binding of organic molecules on Si(111)-7 × 7 very well.32-34 Two local minima corresponding to stable adsorbed structures were identified, and the transition state linking these minima was located. The transition state was verified by the identification of a single imaginary frequency for this structure. Total energies for all clusters were zero-point corrected. 3. Results and Discussion Figure 1 exhibits the photoemission spectra of the N1s corelevel as a function of pyrrolidine exposure at 85 K. Figure 1a is the N1s spectrum of the clean Si(111)-7 × 7 surface at 85 K before dosing any molecule, which indicates the cleanliness of the sample at this temperature. At exposures lower than 0.26 L, the N1s spectra exhibit a feature at ∼ 399.4 eV. The adsorbate for this range of exposure is called the β1 state. However, a

Chemisorption of Pyrrolidine on Si(111)-7 × 7

Figure 2. N1s spectra of pyrrolidine on Si(111)-7 × 7 at different temperatures. The exposure listed here was obtained by background dosing as described in the Experimental Section.

new N1s photoemission feature appears at higher exposures (g0.26 L). It has a binding energy of ∼401.2 eV at 0.46 L exposure, and is designated the β2 state. At 0.46 L, the photoemission feature at ∼399.4 eV is weak but still identifiable since the peak is largely asymmetric and broad. This suggests the coexistence of β1 and β2. With a further increase of exposure, the N 1s core-level down-shifts back to ∼400.0 eV at 0.91 L. There is no further change for an exposure higher than 0.91 L. As has been seen for other physisorbed nitrogen containing molecules,16 the photoemission feature at ∼400.0 eV can be assigned to physisorbed pyrrolidine. Referring to previous studies of dative bonded nitrogencontaining molecules such as trimethylamine and pyridine on Si(100),9,10,16 the peak at ∼401.2 eV of the β2 state can be attributed to the N1s core-level of dative-bonded pyrrolidine. Notably, the N1s binding energy of the β1 state is higher than the 398.3-398.6 eV of the Si-N σ-bonded molecules such as pyrrolidine and 3-pyrroline on Si(100).22 The difference in the N1s core-level between the β1 state of pyrrolidine on Si(111)-7 × 7 and the chemisorbed pyrrolidine and 3-pyrroline on Si(100) does not result from surface charging or work function variation since the binding energy has been calibrated. More importantly, the N1s binding energy of the β1 state is higher than that of pyrrolidinyl bonded to Si(111)-7 × 7 via a Si-N σ bond formed through N-H dissociation at a temperature of ∼250 K (Figure 2). Thus, the β1 state is not a σ-bonded pyrrolidinyl species on Si(111)-7 × 7. Figure 2 shows the N1s spectra at 150 and 250 K. Figure 2a is a spectrum of a physisorbed multilayer with a peak centered at ∼400.0 eV. Upon annealing the sample exposed to 1.0 L of pyrrolidine to 150 K, the signal is significantly decreased and the N 1s spectrum is also distinctly different from Figure 2a. A peak appears at ∼399.3 eV associated with a large shoulder around ∼401.2 eV. This experimental spectrum (Figure 2b) can be deconvoluted into two peaks with the same full width at half-maximum (fwhm) at 401.2 and 399.3 eV. Thus, definitely two adsorbates β1 and β2 are coexistent in the chemisorbed monolayer. However, when the sample is annealed to 250 K, the peaks assigned to dative-bonded molecules nearly disappear. At 250 K, a symmetric peak at ∼398.6 eV is observed. This peak is similar to what was observed for pyrrolidinyl and

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Figure 3. HREELS spectra of pyrrolidine adsorbed on a clean Si(111)-7 × 7 at 85 K as a function of pyrrolidine exposure. The exposure listed here was obtained by direct dosing as described in the Experimental Section.

TABLE 1: Vibrational frequencies (cm-1) and their assignment for liquid pyrrolidine, physisorbed pyrrolidine and chemisorbed pyrrolidine on Si(111)-7 × 7 vibrational mode

liquid pyrrolidine35

Si · · · · N dative bond ring-deformation ring-breathing CH2 wagging CH2 rocking CH2 wagging CH2 wagging CH2 twisting CH2 bending CH2 stretching N-H stretching

300, 590 902 1025 1082 1108 1221 1247, 1337 1454, 1480 2816, 2868, 2912, 2953 3305

physisorbed chemisorbed pyrrolidine pyrrolidine 630 906

1072 1112 1203 1307 1464 2952 3220

578 N/A 915 1012 1068 1108 1320 1472 1890,2967 3125

3-pyrrolinyl bonded to Si(100) via a Si-N σ bond.22 Thus, the peak at ∼398.6 eV is attributed to the N1s core-level of Si-N σ-bonded pyrrolidine formed through N-H dissociation of β1 and β2 states. The lower intensity of the N1s spectrum observed at 250 K compared to that at 150 K suggests partial molecular desorption of β1 and/or β2 states. Figure 3c and d shows HREELS spectra of the physisorbed multilayers with different exposures. The assignment of these spectra and a comparison with Raman spectra of liquid pyrrolidine35 are listed in Table 1. Among these vibrational features, peaks at ∼3220, ∼2952, and ∼630 cm-1 are assigned to N-H stretching, C-H stretching, and ring deformation modes, respectively. The vibrational features of the physisorbed molecules are consistent with those of liquid pyrrolidine. Figure 3a and b shows the spectra of two kinds of chemisorbed adsorption structures obtained by exposing a clean Si(111)-7 × 7 to a low exposure of pyrrolidine at 85 K. Table 1 lists the vibrational peaks of the chemisorbed molecules and their assignments. Compared to the physisorbed multilayers (Figure 3c and d), several distinct differences can be identified in the spectra of the chemisorbed molecules (Figure 3a and b). For chemisorbed molecules at 85 K, there is no vibrational feature at ∼3220 cm-1, while a new vibrational feature seen as a shoulder at ∼3125 cm-1 is observed in Figure 3a and b. Notably, there is no significant Si-H vibrational peak at ∼2100 cm-1 though a weak and broad vibrational feature is observed in the region of 2000-2200 cm-1 (see Figure S5 in the Supporting Information).

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Figure 4. Schemes showing (a) dative-bonded pyrrolidine on Si(111)-7 × 7, (b) free hexamethylenetetramine hydrohalides, (c) free pyrrolidine, and (d) free hexamethylenetetramine.

This weak Si-H vibrational feature could be due to partial dissociation of the N-H bonds of pyrrolidine molecules at defect sites such as step edges, since the Si-H vibrational peak is strong in HREELS spectra in the specular geometry mode.36 Thus, dissociation of N-H is not a major product of pyrrolidine chemisorbed at 85 K. Notably, a shoulder at ∼ 3125 cm-1 is clearly observed. This feature cannot be assigned to the Csp2-H stretching mode since all the carbon atoms of pyrrolidine are of sp3 hybridization. Dehydrogenation of the CH2 group of pyrrolidine on Si(111)-7 × 7 at 85 K is not expected, though the dehydrogenation of cyclohexane and other hydrocarbon molecules has been observed on catalytic noble-metal singlecrystal surfaces at room temperature and above.37,38 The vibrational feature at ∼3125 cm-1 is possibly related to a weakened N-H stretching mode upon chemisorption. On the other hand, a vibrational feature characteristic of the Si-H σ bond is absent at low exposure, as the interaction between the hydrogen atom of N-H and the rest atom in the β1 state is a weak van der Waals force instead of a Si-H σ bond. Based on the observed N1s photoemission features and vibrational features of the chemisorbed molecules, the chemisorption of pyrrolidine on Si(111)-7 × 7 is proposed to be dative bonding between the electron-rich nitrogen atom and the electron-deficient adatom of Si(111)-7 × 7 (See the schematics in Figure 4a). In this bonding configuration, the hydrogen atom of the N-H bond is still attached to the nitrogen atom upon the formation of a N · · · Siad dative bond. Compared to physisorbed molecules and liquid pyrrolidine, the N-H stretching frequency at low exposure down-shifts by ∼95 and ∼180 cm-1 in the dative-bonded molecules, respectively. Thus, the N-H bond in the dative-bonded pyrrolidine molecule (Figure 4a) could be reasonably considered to be a weakened N-H bond similar to the N-H bond in R3N+-H · · · X- (X ) Cl, Br, or I) salts (Figure 4b).29 The common feature of the two nitrogen atoms in the schematics of Figure 4a and b is their lower electron density compared to nitrogen atoms of free secondary and tertiary amines (Figure 4c and d). This is due to electron donation from the lone pairs of the nitrogen atoms to other atoms. Vibrational studies of the N-H stretching mode in solid hexamethylenetetramine hydrohalides show a significant downshift of 300-700 cm-1 compared to the N-H stretching frequency in solid R2NH,29 consistent with the down-shift of the N-H stretching frequency of pyrrolidine by ∼95 cm-1upon the formation of a dative bond with the Si(111)-7 × 7 surface. The much larger down-shift of the N-H stretching frequency in R3N+-H · · · X- 29 is likely due to the ionic nature of the nitrogen atom, resulting in an even lower electron density on the nitrogen atom in solid hexamethylenetetramine hydrohalide salt, compared to the nitrogen atoms on the dative bonded pyrrolidine on Si(111)-7 × 7. Another difference between the spectra of the chemisorbed and physisorbed molecules is the appearance of a new peak at

∼578 cm-1 for the chemisorbed monolayer. There are two possible assignments for this peak in the spectra of the chemisorbed molecules: (1) the ring-deformation mode which is observed at ∼630 cm-1 in the physisorbed molecules, or (2) the stretching mode of the N · · · Si dative bond. Analysis of the spectra obtained in two off-specular geometries indicates the second possibility based on the relative intensities of this mode in the two off specular geometries (see Supporting Information). Thus, the vibrational feature at ∼578 cm-1 could be assigned as the stretching mode of the N · · · Siad dative bond. As described below, DFT calculation of this vibrational frequency for this structure gave a value of 698 cm-1, in reasonable agreement. As indicated by XPS, the β1 adsorbate is formed at a low exposure and β2 at a relatively higher exposure before the formation of a physisorbed multilayer. The N1s core-levels of the two adsorbates exhibit identifiable differences as shown in Figure 2b. Based on the N1s photoemission features of the two adsorbates, two binding models are proposed in Figure 5. In the β1 state, a N atom bonds to an adatom via a dative bond formed by electron transfer from the N atom to the Si adatom. In addition, a weak hydrogen bond N-H · · · Sire is simultaneously formed between N-H and an electron-rich rest atom adjacent to the dative-bonded adatom. Therefore, each pyrrolidine binds to one adatom and its adjacent rest atom simultaneously. Figure 5b is a schematic of the β2 state (one β1 adsorbate binding to the rest atom adjacent to the adatom bonded to the β2 adsorbate is also shown; β1 is marked with a green dashed line box). In the β2 adsorbate, the N atom significantly donates its electron density to the electron-deficient adatom to form a second N · · · Siad dative bond. There is no hydrogen bonding between the N-H of the β2 state and a rest atom, as all rest atoms are saturated by the β1 adsorbate at low exposure since the number of rest atoms is only half the number of adatoms in each unit cell. The evidence for the dative bonded product β1 includes (1) the observation of a relatively high binding energy for N1s at 399.4 eV at low coverage (Figure 1b and c) in contrast to the low binding energy at 398.6 eV of a dissociated product formed upon annealing dative-bonded products to 250 K (Figure 2c), (2) the observation of the N-H bond at ∼3125 cm-1 (Figure 3), and (3) the observation of the relatively high binding energy for N1s at 399.3 eV (Figure 2b) upon desorbing the physisorbed multilayer and leaving the chemisorbed monolayer after annealing the sample to 150 K. The evidence for the experimental observation of the second dative bonded product β2 includes (1) the observation of a relatively high binding energy photoemission feature for N1s at 401.2 eV for high exposure, before the formation of a physisorbed multilayer (Figure 1e), (2) the observation of the N-H bond at ∼3125 cm-1 (Figure 3), and (3) the observation of a high binding energy peak at 401.2 eV (Figure 2b) upon desorbing the physisorbed multilayer and leaving the chemisorbed monolayer after annealing the sample to 150 K. β1 and

Chemisorption of Pyrrolidine on Si(111)-7 × 7

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Figure 5. Schematics of (a) β1 and (b) β2 adsorbates of pyrrolidine dative-bonded on Si(111)-7 × 7. A green dashed line box marks the β1 state.

β2 coexist in a saturated chemisorbed monolayer as evidenced by the observed N1s photoemission feature at both 401.2 and 399.3 eV in Figure 2b. Although the two adsorbates (β1 and β2) can be differentiated by using their different N1s binding energies (Figure 2b), there is no significant dependence of the vibrational features of chemisorbed pyrrolidine on exposure before the formation of a physisorbed multilayer. At low exposure, only β1 is formed. Both β1 and β2 are coexistent at a relatively high exposure. In principle, there could be some difference in the N-H vibrational frequency at low and high exposures since the electron density of the nitrogen atom in β1 and β2 is different to some extent due to a weak electron donation from a rest atom to the nitrogen atom through the N-H · · · Sire hydrogen bond formed in β1.

However, the vibrational peak corresponding to N-H stretching does not exhibit an obvious shift between 0.020 and 0.040 L exposure in Figure 3a and b due to the limited resolution of the HREELS technique. Particularly, because β1 and β2 coexist at a relatively high exposure, HREELS cannot identify the difference between a relatively high exposure (both β1 and β2 formed) as seen in Figure 3b and a low exposure spectrum (only β1 formed). The proposed binding models of β1 and β2 in Figure 5 are consistent with the observed evolution of the N1s core-levels with the increase of exposure before the formation of a physisorbed multilayer. The N atom in the β1 state has a relatively higher electron density than that in the β2 state as the electron-rich rest atom transfers electron density to the

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Figure 6. HREELS spectra of chemisorbed monolayer of pyrrolidine at 150 K. (a) Entire spectrum. (b) Enlargement of vibrational feature above 2700 cm-1.

nitrogen atom through the N-H · · · Sire hydrogen bond. That is, the formation of a weak hydrogen bond between N-H and the rest atom at low exposure partially compensates the significantly decreased electron density on the nitrogen atom resulting from the formation of a N · · · Siad dative bond. Thus, in the β1 state the nitrogen atom donates significant electron density to the adatom and simultaneously accepts a small amount of electron density from a rest atom through the hydrogen bond. Compared to the β1 state, the N1s core-level in the β2 state thus has a higher binding energy. The observation of two dativebonded surface adsorbates at 85 K is also supported by the XPS spectrum of a chemisorbed monolayer formed by annealing the sample containing both chemisorbed and physisorbed molecules to 150 K. The dative bonding of pyrrolidine in a chemisorbed monolayer at 150 K is further supported by the vibrational spectrum of a chemisorbed monolayer at 150 K (Figure 6). In Figure 6b, an obvious shoulder at ∼3120 cm-1 is clearly identified. It can be assigned to the down-shifted N-H stretching frequency in the dative-bonded β1 and β2 states. The low vibrational frequency of the N-H stretch at ∼3125 cm-1 was also observed with HREELS upon annealing the sample to 250 K and above.39 In addition, there is no significant signal for Si-H stretching at ∼2080 cm-1 for the sample annealed to 250 K, even though the Si-H stretching mode exhibits a large intensity in HREELS.36 This suggests the absence of dissociated product at this temperature. Upon annealing to 250 K, however the N 1s core-level down-shifts to 398.6 eV (Figure 2c), which can be assigned to a nitrogen atom σ-bonded directly to surface silicon atom. Thus, XPS detects the dissociated product formed at 250 K. The difference in the information obtained concerning the product formed at 250 K, using HREELS or XPS, may be due to the interaction of the higher energy X-ray beam with the adsorbate, accelerating the conversion of the dative-bonded molecules to dissociated product, in contrast to the much lower energy electron beam used in HREELS.

Figure 7. Calculated structures of dative-bonded pyrrolidine (β2), σ-bonded dissociated product, and the transition state for the conversion from the dative-bonded pyrrolidine to the dissociated product.

To understand the thermal stability of the dative-bonded pyrrolidine on Si(111)-7 × 7, DFT calculations were performed on this system to simulate the conversion from the dative-bonded β2 adsorption structure to the dissociated product, using Gaussian 03 at the B3LYP/6-31 G(d,p) level. The cluster was built with the same method as in previous studies.31-34 Figure 7a and b are the calculated structures of a dative-bonded molecule (β2 state) and dissociated product of pyrrolidine, respectively. The dative-bonded product β2 is located 20.17 kcal/mol below the separated reactants. The σ-bonded dissociated product is the most stable, and is located 63.47 kcal/mol below the reactants. Figure 7c is the calculated structure of a transition state for the conversion from the dative-bonded product (β2) to the dissociated product. The transition state for the conversion of the dative-bonded product to the dissociated product is located 8.20 kcal/mol below the energy of a free molecule and the free model cluster (Figure 8). Compared to the adsorption structure β1, with a covalent N-H bond, the interaction between the nitrogen atom and the hydrogen atom in this transition state is significantly weaker. Obviously, this conversion is both thermodynamically and kinetically favorable.

Chemisorption of Pyrrolidine on Si(111)-7 × 7

J. Phys. Chem. C, Vol. 112, No. 39, 2008 15481 involves the formation of a weak Siad-N-H · · · Sire hydrogen bond due to the availability of hydrogen-bond acceptor (electron donor), rest atoms on the Si(111)-7 × 7 at low exposures. Both electron acceptance by the adatom and electron donation by the rest atom occur simultaneously for one molecule in the Siad · · · N-H · · · Sire adsorbate (β1). For the β2 state, a dative bond is formed between the nitrogen atom of pyrrolidine and the electron-deficient adatom due to the extra available electrondeficient adatoms upon the passivation of all rest atoms and half of the adatoms by β1 adsorbates. Theoretical calculations show that the conversion from the dative-bonded molecule to the σ-bonded dissociation product of pyrrolidine is thermodynamically and kinetically favorable on the Si(111)-7 × 7 surface. A comparison of the chemical binding of pyrrolidine on Si(100), Ge(100) and Si(111)-7 × 7 demonstrates the importance of electronic and geometric structure of the surface reactive sites in determining reaction channels and shaping the formation of dative bonds of N-containing organic molecules on semiconductor surfaces.

Figure 8. Profile of the energy surface for the formation of dativebonded pyrrolidine (β2) and its conversion into a pyrrolidinyl-like dissociation product on Si(111)-7 × 7.

The calculated vibrational frequency of the N-H stretching mode in the dative-bonded β2 state is ∼3245 cm-1, which is definitely down-shifted in comparison with the N-H stretching frequency (3390 cm-1) in the free reactant molecule calculated using the same method. It is relatively higher than the measured N-H stretching frequency in the dative-bonded adsorbates observed here, however. As noted earlier, the stretching frequency of the dative bonded molecule-surface stretch is calculated as 698 cm-1, in reasonable agreement with the mode observed at 578 cm-1, assigned to this surface-molecule motion. Hamers et al. and Bent et al. studied the adsorption of pyrrolidine on Si(100) and Ge(100). Their studies show pyrrolidine dissociates on Si(100)40-42 through N-H bond scission, and forms a N · · · Ge dative bond at low exposure and dissociated product at high exposure on Ge(100).15 The difference in the reactivity and selectivity of pyrrolidine on Si(100) and Ge(100) is consistent with the difference in the dynamic tilting of the SidSi dimer of Si(100) and GedGe dimer of Ge(100).24-26 Compared to the chemisorption of pyrrolidine on Si(100) and Ge(100), pyrrolidine adsorbed on the Si(111) 7 × 7 surface forms a new complex consisting of a N · · · Siad dative bond and a Sire · · · H-N hydrogen bond, in which both significant electron donation from the nitrogen atom to the adatom and slight electron transfer from a rest atom to the nitrogen atom through a hydrogen bond occurs simultaneously. Another distinct difference between Si(100)/Ge(100) and Si(111)-7 × 7 is the formation of a second dative bond resulting from the different electronic and geometric structure of surface reactive sites. Compared to the equal number of electron-deficient and electron-rich reactive sites on Si(100) and Ge(100), the extra electron-deficient adatoms on Si(111)-7 × 7 allows the formation of the second dative-bonded product. 4. Summary Both electronic and vibrational spectroscopic evidence suggest the formation of dative bonds between pyrrolidine, a cyclic secondary amine, and Si(111)-7 × 7. Two adsorbate states (β1 and β2) were observed. Both are associated with dative bonds formed between ring nitrogen atoms with a localized lone pair and electron-deficient adatoms on the surface. The β1 state

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