4.3 Results
4.3.3 CH 3 -Terminated Si(111) Surfaces
also evident. This mode occurred at a much lower frequency (∼100 cm−1), which was beyond the range for which experimental data could be collected.
The emergence of two strong surface modes, a perpendicular mode at 552 cm−1 and a parallel mode at 494 cm−1, is in excellent agreement with the experimental observations in the present work. This supports the assignment of the mode at 583 cm−1 to a Si–Cl stretching motion and the mode at 528 cm−1 to a bending motion. The calculated value of 552 cm−1 for the Si–Cl stretching mode is in good agreement with previous results (538 cm−1)26 at the B3-LYP/6-31G∗ level using a two-layer cluster model. The slightly low values of the computed frequencies compared to the experimental values could have been due to the use of the B-LYP density functional or to the relatively small size of the clusters used to describe such collective vibrations.
Similar models were attempted in the periodic unit cell calculations for surface SiCl3 groups, but this surface configuration produced very large steric repulsions. The van der Waals radius of chlorine is 1.8 ˚A,30 indicating that optimal distance between the chlorine atoms should be ≥3.6 ˚A. Since the Si–Si distance on the Si(111) surface is only 3.8 ˚A,31 even with best possible dihedral rotations, no reasonable configurations containing SiCl3 groups on all surface silicon atoms could be generated. In an additional attempt to per- form calculations on this surface, SiCl3 groups were attached to the seven surface silicon atoms in the cluster shown in Figure 4.5. The resulting optimized structure (not shown) was highly distorted and was not a realistic surface configuration. The presence of a uniform trichlorinated Si(111) surface can therefore be ruled out based solely on steric considera- tions.
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2000 2400
2800 3200
Absorbance Absorbance
wavenumber (cm-1) 30˚
74˚
ν δ Si-Cl δs (C-H) ρ (C-H) ν (C-H)
Figure 4.6: TIRS and proposed peak assignments of the CH3-terminated Si(111) surface collected at an incident beam angle of 74◦ (bottom) and 30◦ (top) off surface normal. The spectra are on the same scale of absorbance (absorbance units), although they are offset for easier viewing. Data are shown after subtraction of H2O and flattening of the baseline.
TIRSpeakarea -Rθν(Si–Cl)δ(Si–Cl)ν:δ(Si–Cl)aδs(C–H)ρ(C–H) -Cl74◦(1.0±0.2)x10−2(3.7±0.9)x10−33±1–– 30◦–(3.8±0.6)x10−3––– -CH374◦(-7±2)x10−3,b(-2.3±0.01)x10−33±1(2.2±0.2)x10−3(1.8±0.2)x10−2 30◦–(-2.8±1.6)x10−3––(1.7±0.3)x10−2 -C2H574◦(-5±1)x10−3(-3±1)x10−31.9±0.6–– 30◦–(-3.6±0.5)x10−3––– Table4.2:IntegratedareasofTIRSpeaksonCl-andCH2-terminatedSi(111)surfacesassignedtoSi–ClandC–Hmodes.a Ratioof theintegratedareaofthestretchingmodeνtothebendingmodeδ.b NegativeareasidentifyfeaturesontheH-terminatedsurfacethat appearasnegativepeaksinthedifferencespectrumoftheCl-terminatedsurface.
in the absorption spectrum represent features of the Cl-terminated surface that disappeared during the alkylation reaction. On the Cl-terminated Si surfaces shown in Figures 4.3 and 4.4, the integrated areas of the Si–Cl stretching and bending modes at 583 and 528 cm−1, respectively, when examined at an IR incident angle of 74◦, were found to be 1.0 x 10−2 ± 0.2 x 10−2 and 3.7 x 10−3 ± 0.9 x 10−3, respectively (Table 4.2). Similar areas were found for the corresponding negative Si–Cl features when the CH3-terminated surface was examined under identical scan conditions, shown in Table 4.2. Measurement of the areas of the Si–Cl bending mode when observed at an IR incident angle of 30◦ gave similar results. This indicates that all Cl initially present on the surface is removed during the alkylation reaction, in agreement with previous XPS observations described in Chapter 3. On the CH3-terminated Si surface, a small, broad peak centered at∼2068 cm−1 was observed, possibly indicating that a small amount of surface Si–H contamination was present. Unfortunately, the region near the Si–H bending mode at 626 cm−1 was obscured by a large Si–Si phonon mode, so it was not possible to determine if the peak at 2068 cm−1 did indeed represent surface Si–H groups.
New distinct peaks of the C–H stretching modes appeared around 2900 cm−1 on the CH3-terminated surface, shown in Figure 4.7. Vibrational absorption features at 2856, 2909, 2928, and 2965 cm−1were measured on the surface when the spectra were collected at an incident IR beam angle of both 74◦ and 30◦, although all observed features were re- duced in intensity when the incident IR beam angle was moved to 30◦. From a simple group theory argument, a CH3- group should have two IR-active vibrational modes, ana1 sym- metric stretching mode and aneasymmetric stretching mode at slightly higher wavenum- ber.32 Previous investigations of CH3-terminated porous Si surfaces, with an amorphous surface structure but high surface IR signal, have observed broad C–H symmetric and asym- metric stretches centered at∼2900 and 2970 cm−1, respectively.33 FTIR studies of silicon oxide surfaces terminated with trimethylsilane have also reported methyl C–H stretches at 2904 and 2963 cm−1.34 Other investigations of alkylated porous Si have observed C–H stretching at 2860 and 2936 cm−1 from a terminating 6-trifluoroacetamindohexyl group, in which all C–H bonds are on methylene (CH2) groups.35 With this information,36 the peaks at 2909 and 2965 cm−1 likely represent the methyl C–H symmetric and asymmetric
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2900 2950
3000 3050
3100
Absorbance Absorbance
Wavenumber (cm-1) 30˚
74˚
2856
2909
2928
2965
ν (CH3) ν (CH2)
Figure 4.7: TIRS and proposed peak assignments of the C–H stretching region of the CH3- terminated Si(111) surface collected at an incident beam angle of 74◦ (bottom) and 30◦ (top) off surface normal. Positions of relevant peaks are noted above the spectra. Spectra are shown without alteration such as background smoothing on the same scale, but are offset for clarity.
stretching vibrations, respectively, while the peaks at 2856 and 2928 cm−1 are possibly from methylene C–H stretching motions. These methylene signals could originate from hydrocarbon contamination that is known to be present on the CH3-terminated Si(111) sur- face prepared through the two-step chlorination/alkylation route.
The low wavenumber region of vibrational absorptions on the CH3-Si(111) surface col- lected from two different incident IR beam angles, along with proposed peak assignments, are shown in Figure 4.8. When the surface was examined with the IR beam incident on the surface at the Brewster angle, a sharp feature at 1257 cm−1was clearly visible, but was not seen when the IR beam was moved towards surface normal. This peak was assigned as the C–H symmetric bending, or “umbrella,” mode of the methyl group.33, 37 The polarization- type experiments revealed that it is a motion that is perpendicular to the Si(111) surface.
This demonstrates that the methyl group introduced to the surface through the two-step chlorination/alkylation route is oriented perpendicular to the surface, consistent with the model of surface alkyl bonding proposed in Figure 1.4. A second sharp peak at 757 cm−1 appeared at both data collection angles. This absorption energy is expected for a methyl C–H rocking mode,37 which would be parallel to the surface if it originates on a methyl group normal to the Si(111) surface, and therefore is expected to be observed even when the incident IR beam is itself near to surface normal. The integrated areas for both of these peaks at both data collection angles are given in Table 4.2.
Another feature present in the spectrum collected at an IR incident angle of 74◦ off normal was a small, broad absorption near 1100 cm−1, corresponding possibly to the TO mode of Si–O–Si. Interestingly, this feature was not observed when the TIR spectrum of the CH3-terminated surface was collected at an incident angle of 30◦off surface normal. It was already demonstrated (Figure 4.2) that because the TO mode is parallel to the Si(111) surface, it should appear with equal intensity no matter what angle the surface is illumi- nated with the IR beam. Because the spectra in Figure 4.8 were taken from two different samples, inclusion of the TO peak on one sample could possibly have been caused by small variations in the amount of O incorporated in the near-surface Si lattice that cannot be pre- vented by the alkylation technique used here. The amount of oxide on the surface was estimated by assuming that the native silicon oxide was 15 ˚A thick, and by determining the
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1100 1200
1300
Absorbance Absorbance
wavenumber (cm-1) 30˚
74˚
ν δ Si-Cl
δs (C-H) TO ρ (C-H)
Figure 4.8: TIRS and proposed peak assignments of the CH3-terminated Si(111) surface collected at an incident beam angle of 74◦(bottom) and 30◦(top) off surface normal shown in the low wavenumber region. The spectra are on the same scale of absorbance (ab- sorbance units), although they are offset for easier viewing. Data are shown after subtrac- tion of H2O and flattening of the baseline.
ratio of the integrated areas of the TO peak on the CH3-terminated surface to that on the native oxide surface (negative peak in Figure 4.2). The TO peak on the CH3-terminated surface corresponded to a possible Si–O–Si coverage of 0.03 ML, or approximately 0.5 ˚A.
In the low energy region of the TIR spectrum, shown in Figure 4.8, a feature at≈730 cm−1was observed as a large shoulder on the methyl C–H rocking motion peak when data was collected at the Brewster angle that disappeared when the IR beam was moved towards surface normal. This polarization-type behavior indicated it could be a surface motion positioned perpendicular to the Si(111) surface plane, although this was higher in energy than most proposed Si–C stretches previously identified by HREELS investigations.10–15 Finally, Figure 4.8 shows a broad feature at ∼620 cm−1 at both data collection angles.
This corresponded to a Si–Si lattice phonon mode that was not perfectly subtracted from the CH3-terminated surface by referencing with the Cl-terminated surface, possibly due to a slight difference in placement of the two surfaces in the IR sample holder or a small temperature variation during data collection.38 A sharp absorption at 667 cm−1 was also observed from atmospheric CO2(g) contamination in the IR sample compartment.