3890
J. Phys. Chem. B 2002, 106, 3890-3895
Binding and Structure of Acetonitrile on Si(111)-7 × 7 Feng Tao, Xian Feng Chen, Zhong Hai Wang, and Guo Qin Xu* Department of Chemistry, National UniVersity of Singapore, 10 Kent Ridge, Singapore, 119260 ReceiVed: July 16, 2001; In Final Form: NoVember 14, 2001
The covalent binding and structure of acetonitrile on Si(111)-7 × 7 have been investigated to provide information on the interaction of π electrons of the CtN group with the dangling bonds of Si(111)-7 × 7. Vibrational features of chemisorbed acetonitrile and acetonitrile-d3 unambiguously show the absence of Ct N stretching mode around 2232 cm-1 coupled with the appearance of CdN stretching mode at 1663 cm-1. Both C 1s and N 1s core levels of chemisorbed acetonitrile display large chemical shifts of 2.2 and 1.3 eV, respectively, compared with those of physisorbed molecules. In addition, the photoemission from πCN was observed to be weakened upon chemisorption. These results clearly suggest that the cyano group directly participates in the interaction with Si surface dangling bonds. Scanning tunneling microscopy images reveal that the cyano groups selectively bind to adatom-rest atom pairs on Si(111)-7 × 7. Density functional theory calculations indicate that the C and N atoms of the cyano group favorably link with the adatom and rest atom, respectively, forming C-Si and N-Si bonds.
I. Introduction The formation of functionalized organic monolayer on silicon surfaces is closely related to the development and fabrication of microelectronic devices and biosensors.1-4 Previous work in this field has been mainly focused on the binding of molecules containing unpolarized CdC or CtC,5-8 five-membered aromatics,9,10 benzene and its derivatives,11-13 and some other unsaturated cyclic hydrocarbons on Si surfaces.14-17 These molecules can be attached to Si(100) through [2 + 2] or/and [4 + 2] cycloaddition reactions. In particular, the adsorption behaviors of acetonitrile18 and tert-butylisonitrile19 on Si(100) were also investigated. For acetonitrile, only weak physisorption was observed;18 but a [1 + 2] cycloaddition between the C atom of isonitrile group and SidSi dimer was proposed for tertbutylisonitrile chemisorbed on Si(100).19 Compared to Si(100), a relatively simpler binding mechanism can be expected for molecules binding on Si(111)-7 × 7, mainly involving the adatom-rest atom pair acting as a diradical which is both electrophilic and nucleophilic.20 For molecules containing unsaturated polarized bonds, interesting attachment chemistry may arise due to the functional polarity and the coexistence of electronically inequivalent surface dangling bonds on adatoms and rest atoms.20,21 To explore the interaction of unsaturated polarized functional group with Si(111)-7 × 7, acetonitrile, the simplest member of nitriles with a large dipole moment of 3.92 D,22 was chosen as a probing molecule. In present work, high-resolution electron energy loss spectroscopy (HREELS) was employed to obtain the vibrational features of adsorbed acetonitrile; the chemical shifts of C 1s and N 1s core levels were measured with X-ray photoelectron spectroscopy (XPS); ultraviolet photoelectron spectroscopy (UPS) was used to investigate the valence band structures of acetonitrile adlayers on Si(111)-7 × 7; scanning tunneling microscopy (STM) images were taken to gain information on the selectivity of surface reaction sites; density functional theory (DFT) calculations were commanded to resolve the binding * To whom correspondence should be addressed.
configurations of acetonitrile on Si(111)-7 × 7. All the experimental evidence and DFT calculation results demonstrate that functionalized organic monolayer of acetonitrile on Si(111)-7 × 7 can be formed through a di-σ binding mechanism involving the coupling of one πCN bond with an adatom-rest atom pair at the interface. II. Experimental Section Experiments were performed in three ultrahigh vacuum (UHV) systems with a base pressure lower than 2 × 10-10 Torr. One of them is equipped with a high-resolution electron energy loss spectrometer (LK-2000-14R) and a quadrupole mass spectrometer (UTI-100). During HREELS measurements, an electron beam with an energy of 5.0 eV impinges on the Si surface at an incident angle of 60° with respect to the surface normal and the EEL spectra were collected at the specular direction. A typical resolution of 55 cm-1 (full width at halfmaximum (fwhm) of the elastic peak) was routinely achieved. Photoelectron spectroscopic studies were carried out on another UHV system mainly equipped with an X-ray source, a differentially pumped helium-resonance lamp, and a concentric hemispherical electron energy analyzer (CLAM2, VG). In the XPS experiments, a Mg X-ray source (hν ) 1253.6 eV) was employed. The binding energy (BE) scale is referenced to the peak maximum of the Si 2p line (BE ) 99.3 eV calibrated to Au 4f7/2)23 of a clean Si(111) substrate with an fwhm of less than 1.2 eV. To obtain a wider flat energy window, He II UV radiation (hν ) 40.80 eV) was employed in our UPS studies. The valence band photoemission spectra are referenced to the Fermi level (EF) of the metallic tantalum sample holder. The STM studies were carried out in the third chamber equipped with Omicron VT STM. Constant current topographs (CCTs) of the clean and acetonitrile-exposed silicon surfaces were collected with a sample bias of Vs ) 1-2 V and a tunneling current of It ) 0.15-0.2 nA. A n-type P-doped Si(111) wafer with a resistivity of 1-30 Ω‚cm (Goodfellow, Cambridge) was cut into samples with a dimension of 8 × 18 × 0.38 mm3 (3 × 8 × 0.38 mm3 for STM
10.1021/jp012710c CCC: $22.00 © 2002 American Chemical Society Published on Web 03/22/2002
Binding and Structure of Acetonitrile on Si(111)-7 × 7
J. Phys. Chem. B, Vol. 106, No. 15, 2002 3891 variable leak valve and was exposed onto the surface at 110 K. The exposures were reported in langmuirs (1 langmuir ) 10-6 Torr‚s) without ion gauge sensitivity calibration. For STM experiments, a direct dosing mechanism was carried out via a stainless steel tube with a diameter of 6 mm. STM images were collected after acetonitrile adsorption at 300 K. III. Results and Discussion III.A. High-Resolution Electron Energy Loss Spectroscopy. Figure 1 presents the HREELS spectra of physisorbed and chemisorbed CH3CN and CD3CN on Si(111)-7 × 7. In Figure 1a, loss features at 391, 931, 1058, 1405, 2232, 2965, and 3020 cm-1 can be readily identified and are consistent with the vibrational signatures of gas-phase acetonitrile (Table 1),24 suggesting the formation of physisorbed multilayer of acetonitrile. Upon annealing the physisorbed multilayer CH3CN/ Si(111)-7 × 7 to 300 K to drive away all physisorbed molecules, loss peaks at 650, 912, 1035, 1372, 1442, 1663, 2950, and 3006 cm-1 are observed. All these vibrational features are consistent with the vibrational signatures of chemisorbed acetonitrile on Pt(111)25 through a side-on di-σ binding mode, indicative of the formation of chemisorbed acetonitrile on Si(111)-7 × 7 (Table 1). Compared to physisorbed molecules, chemisorbed acetonitrile produces significant changes in the HREELS spectrum (Figure 1b). The vibrational feature of νC≡N around 2232 cm-1 is absent in chemisorbed acetonitrile. In addition, a shoulder that appears around 1550-1700 cm-1 is close to the CH3 bending modes and is possibly attributable to the stretching mode of the CdN bond in chemisorbed molecules. To confirm this assignment, a similar HREELS study was carried out using physisorbed (Figure 1c) and chemisorbed (Figure 1d) CD3CN. For multilayer CD3CN, losses at 840, 1092, 2140, 2252, and 2290 cm-1 are in line with the vibrational features of liquid CD3CN.26 The detailed assignments are listed in Table 1. Compared to chemisorbed CH3CN (Figure 1b), changes in the vibrational features of chemisorbed CD3CN can be clearly noticed, as shown in Figure 1d. All the vibrational modes related to C-H bonds display obvious red shifts. Particularly the methyl bending shifts from 1372 and 1442 cm-1 (-CH3) to 1098 cm-1 (-CD3). More importantly, the shoulder around 1550-1700 cm-1 in Figure 1b remains nearly unshifted and appears as an isolated peak at 1658 cm-1 in the spectrum of chemisorbed CD3CN (Figure 1d), confirming our assignment of the shoulder around 1550-1700 cm-1 of chemisorbed CH3CN (Figure 1b) to the stretching mode of the CdN bond. This result also demonstrates the rehybridization of both carbon and nitrogen atoms of the CN group from sp to sp2 after chemisorption. Furthermore, the observation of the CdN stretching mode
Figure 1. HREELS spectra of physisorbed and chemisorbed CH3CN (CD3CN) on Si(111)-7 × 7. Ep ) 5.0 eV. Specular mode. (a) Physisorbed CH3CN; (b) saturated chemisorbed CH3CN; (c) physisorbed CD3CN; (d) saturated chemisorbed CD3CN.
experiments). The Si(111) samples were first cleaned with a hot 5% HF solution and then rinsed with deionized water before mounting on the manipulator. A Ta foil (thickness ∼0.025 mm) was sandwiched between two experimental samples with Ta clips, and in turn was spot-welded to two Ta posts (diameter ∼1.5 mm) at the bottom of a Dewar-type liquid N2 cooled sample holder. The sample can be heated to 1400 K through the resistive heating of Ta foil and cooled to 110 K using liquid N2. Its temperature was measured by a W/Re 5%-W/Re 26% thermocouple (C-type) attached to one of the two samples using a high-temperature ceramic adhesive (Aremco 516). The sample surface was carefully cleaned by several cycles of Ar ion bombardment (30 min at 500 eV and 10 µA‚cm-2) and thermal annealing at 1150 K for 15 min. The surface cleanliness was routinely checked using HREELS and XPS. The formation of 7 × 7 surface structure was verified by STM. Acetonitrile (99%) or Acetonitrile-d3 (99%) purchased from Aldrich Chemical was further purified by several freezepump-thaw cycles before use. In the HREELS, XPS, and UPS studies, acetonitrile was introduced into the chambers via a
TABLE 1: Assignment of Vibrational Frequencies (cm-1) for CH3CN Adsorbed on Si(111)-7 × 7 at 110 K vibrational mode
description
δ(C-C-N) ν(Si-C) ν(Si-N) ν(C-C) Fr(CH3) σs(CH3) σd(CH3) ν(CdN) ν(CtN) νs(CH) νd(CH)
CCN bend SiC stretch SiN stretch CC stretch methyl rock methyl sym bend methyl deg bend CdN stretch CtN stretch CH3 sym stretch CH3 deg stretch
CH3CN gas phase24,25 361
physisorbed CH3CN on Si(111)-7 × 7 391 680
920 1041 1389 1454
931 1058 1405 1405
2268 2954 3009
2232 2965 3020
chemisorbed CH3CN on Si(111)-7 × 7
chemisorbed CH3CN on Pt(111)25
physisorbed CD3CN on Si(111)-7 × 7
chemisorbed CD3CN on Si(111)-7 × 7
840 925 852 1092 1092
668 848 930 848 1098 1098 1658
605 650 850 912 1035 1372 1442 1663
950 1060 1374 1435 1615
2950 3006
2960 3012
chemisorbed CD3CN on Pt(111)25 580
2252 2140 2290
2150 2295
930 850 1100 1100 1625 2120, 2280
3892 J. Phys. Chem. B, Vol. 106, No. 15, 2002
Tao et al. TABLE 2: C 1s and N 1s BEs (eV) of CH3CN Adsorbed on Different Substratesa sample
multilayer C 1s N 1s
monolayer C 1s N 1s
BE shifts binding C 1s N 1s mode
Pt(111) Ni(111) Cu(100) Ni film Pd film Si(111)
286.9 287.0 287.3 287.0 287.0 287.0
284.6 284.5 286.6 284.5 284.5 284.8
2.3 2.5 0.7 2.5 2.5 2.2
a
Figure 2. C 1s and N 1s binding energies (eV) of CH3CN adsorbed on Si(111)-7 × 7 at 110 K as a function of exposure.
around 1658 cm-1 rules out the possibility of the CN group binding to Si(111)-7 × 7 through a side-on tetra-σ bonding mechanism. III.B. X-ray Photoelectron Spectroscopy. Figure 2 presents the C 1s and N 1s BEs as a function of acetonitrile exposure. At very low exposures, the C 1s spectra show a main peak at 284.8 eV with a smaller feature at 287.0 eV. With increasing the exposure from 0.4 to 2.0 langmuirs, the peak at 287.0 eV preferentially grows, suggesting its physisorption nature. N 1s spectra also present similar evolution as a functional of acetonitrile dosage. When the dosage is
