The binding of multi functional organic molecules on silicon surfaces 3

The Binding of Multi-functional Organic Molecules on Silicon Surfaces Chapter Adsorption of Nitrogen-containing Aromatic Molecules on Si(111)-7×7 3.1 Motivation The reactivity and reaction mechanisms of aromatic molecules with the Si-dangling bonds of Si(111)-7×7 have been explored [1-6] For five-membered heterocyclic molecules, thiophene [1] and furan [2] react with the adjacent adatom-rest atom pair through their two α-carbon atoms to form C-Si σ-linkages, denoted as a [4+2]-like cycloaddition In the case of pyrrole, the dissociative adsorption via the breakage of its NH bond was observed to be the dominant reaction channel [3] The resulting fragments of pyrrolyl and H-atom are bonded to adjacent adatom and rest atom, respectively Among the six-membered aromatic systems, benzene readily reacts with the surface, forming a di-σ bonded 1,4-cyclohexadiene-like surface intermediate through the [4+2] addition reaction [4, 5] Replacing one of the C-atoms in benzene ring by N-atom results in pyridine with a non-even electronic density distribution A dative bonding between the electron-rich N-atom of pyridine and electron-deficient Si-adatoms was detected in addition to the di-σ binding configuration through the N-atom and its opposite C-atom [6] To further understand the effect of constituent ring atoms on the reactivity, nitrogencontaining aromatic molecules (pyrazine, pyrimidine, and s-triazine) are interesting systems for investigation 50 The Binding of Multi-functional Organic Molecules on Silicon Surfaces 3.2 Pyrazine adsorption 3.2.1 High-resolution electron energy loss spectroscopy Figure 3.1 shows the high-resolution electron energy loss spectra of Si(111)-7×7 exposed to pyrazine at 110 K The vibrational frequencies and their assignments for physisorbed and chemisorbed molecules on a Si(111)-7×7 surface are listed in the Table 3.1 This table clearly shows that the vibrational features of physisorbed pyrazine (Figures 3.1c,d, e) are in excellent agreement with the IR spectrum of liquid pyrazine [7, 8] Among these vibrational signatures, the peak at 3071 cm-1 is assigned to the (sp2) C-H stretching modes of the molecule The feature at 1556 cm-1 is related to the conjugated C=C (N) stretching vibration The vibrational features of chemisorbed pyrazine taken at low exposures (Figure 3.1a) or obtained after annealing the multilayer pyrazine-covered sample to 300 K to drive away all the physisorbed molecules and retain only the chemisorbed molecules (Figure 3.1f), however, are significantly different Losses at 497, 712, 905, 1065, 1230, 1615, and 3071 cm-1 can be readily resolved Compared to physisorbed molecules, chemisorbed pyrazine does not lead to obvious variations in the stretching frequency of C (sp2)-H This result indicates that the C atoms of the molecule are not involved in the chemical binding with the surface (A C (sp3)-H vibrational peak, red-shifted by 80-100 cm-1 from the C (sp2)-H stretching mode, would be expected if the C atoms were involved in binding with Silicon surface dangling bonds) The 1615 cm-1 loss observed is attributed to the non-conjugated C=C stretching vibration in the chemisorbed pyrazine This assignment is consistent with the results obtained for liquid 1,4-cyclohexadiene (at 1639cm-1) [9], and chemisorbed benzene (at 1635 cm-1) [5] and chlorobenzene (at 51 The Binding of Multi-functional Organic Molecules on Silicon Surfaces 1628cm–1) [10] on Si(111)-7×7 with a 1,4-hexadiene-like surface intermediate On the other hand, the loss feature at 497 cm-1 is associated with the Si-N stretching mode, consistent with previous studies on the binding of unsaturated N-containing organic molecules on Si surfaces through the N-Si linkages [11-13] Furthermore, we did not observe any peak at ~2055 cm -1 associated with the Si-H stretching mode [14], indicating the molecular nature of chemisorbed pyrazine on the Si(111)-7×7 surface The detailed assignments are tabulated in Table 3.1 The main vibrational features of chemisorbed pyrazine correlates well with the calculated vibrational frequencies for chemisorbed pyrazine on Si(111)-7×7 The details of DFT theoretical modeling will be given in Section To further confirm our assignments, pyrazine-d4 adsorption was also studied in our HREELS experiments Figure 3.2 presents the vibrational features of physisorbed pyrazine-d4 and the saturated chemisorption molecules on Si(111)-7×7 A new peak at 2291 cm-1 is identified and attributed to the C (sp2)-D stretching in the vibrational spectrum of physisorbed pyrazine-d4 (Figure 3.2a), replacing the C (sp2)-H stretching mode at 3071 cm-1 for physisorbed molecules (Figure 3.1e) Upon chemisorption (Figure 3.2b), the =C-D stretching mode remains at the almost same vibrational frequency, retaining sp2 hybridization for all four carbon atoms of the chemisorbed pyrazine on Si(111)-7×7 This result clearly illustrates that the C-atoms not directly bind to the silicon surface dangling bonds In addition, the spectrum of chemisorbed pyrazine-d4 (Figure 3.2b) also shows the characteristic vibrational modes of the unconjugated C=C at 1595 cm-1 and Si-N at 512 cm-1, further confirming our vibrational assignment for chemisorbed pyrazine on Si(111)-7×7 surfaces The absence of C (sp3)-H (D) stretching 52 The Binding of Multi-functional Organic Molecules on Silicon Surfaces mode, together with the formation of C-N bond and unconjugated C=C bond, suggests that pyrazine is directly bonded to the adjacent adatom and rest-atom on Si(111)-7×7 through the two para-nitrogen atoms, forming the two new Si-N sigma linkage 3.2.2 X-ray photoelectron spectroscopy The N (1s) and C (1s) spectra of pyrazine following a sequence of exposure at 110 K are shown in Figure 3.3 and Figure 3.4, respectively At very low exposures, the N 1s spectra show a main peak centered at 399.0 eV With increasing the exposure from 3.0 to 8.0L, the peak at 400.6 eV preferentially grows, suggesting its physisorption nature C 1s spectra also present a similar evolution as a function of pyrazine dosage When the dosage is below 0.5L, the main feature is the peak at 285.8 eV At a high exposure of 8.0L, the 286.6 eV peak dominates the spectra, attributable to physisorbed pyrazine The binding energy of N 1s and C 1s core level for physisorbed pyrazine on Si(111)-7×7 is in excellent with the results of condensed pyrazine on Au.[15] Compared to the value (284.7 eV) of physisorbed benzene [16], the higher C 1s BE of 286.6 eV observed for physisorbed pyrazine is due to the effect of the more electronegative N-atoms incorporated in the aromatic ring To unambiguously assign these peaks, chemisorbed monolayer was obtained by annealing the multilayer pyrazine-covered surface (8.0L) to 300 K to drive away all physisorbed molecules Chemisorbed pyrazine gives a single nearly symmetrical peak for the N 1s core level (Figure 3.3h) centered at 399.0 eV Its narrow FWHM of ~ 1.4 eV, close to the overall resolution of our XPS spectrometer, suggests that the two N-atoms in chemisorbed pyrazine are chemically indistinguishable Compared with the value of 400.6 eV observed for physisorbed pyrazine, the chemisorption on Si(111)-7×7 results in 53 The Binding of Multi-functional Organic Molecules on Silicon Surfaces a significant down-shift of 1.6 eV in the binding energy of N (1s) The large magnitude of this shift strongly demonstrates that the nitrogen atoms are directly bonded to the surface reactive sites Similar trend was previously observed for (CH3)N=N(CH3) on Si surfaces, N 1s binding energy changing from 400.8 eV for physisorbed molecules to a value of 399.0 eV for chemisorbed state with direct Si-N bond formation.[17] Figure 3.4h shows the corresponding C 1s spectrum for pyrazine chemisorbed on Si(111)-7×7 A single peak at 285.8 eV can be readily resolved, suggesting the existence of only one chemically distinguishable form of carbon in chemisorbed species This C 1s peak (285.8 eV) is noticed to be ~ 1.2 eV higher than the value of 284.6 eV observed for the C-atom directly linked to surfaces through the Si-CH2- group [18, 19] Its down-shift of 0.8 eV from the value (286.6 eV) of physisorbed molecules can be attributed to the increase of electron density on these carbon atoms upon chemisorption Pyrazine binds to an adjacent pair of adatom-rest atom through its two para-nitrogen atoms, forming N-Si linkages The chemisorption process is expected to destroy the ring π-bond or aromaticity This blocks the electron-withdrawing effect of nitrogen atoms through π-bonding, subsequently enhancing the electron density of carbon atoms and down-shift in C 1s in chemisorbed pyrazine compared to physisorbed molecules The inset of Figure 3.4 presents the ratio of AC1s / ASi2p as a function of pyrazine exposure at room temperature The value for each point was obtained by averaging three separate measurements to reduce possible errors AC1s is the C 1s peak area of chemisorbed pyrazine at 300 K The saturation of the AC1s / ASi2p ratio indicates the completion of chemisorption To estimate its absolute saturation coverage, XPS measurements for pyrazine-saturated Si(111)-7×7 are compared to those of chemisorbed 54 The Binding of Multi-functional Organic Molecules on Silicon Surfaces benzene The saturation coverage of benzene, θ benzene, was known to be ~ 0.10, defined as the ratio of the reacted adatoms to the 49 silicon atoms in a unit cell [20] This value becomes ~0.42 if the coverage is defined as the ratio between the reacted adatoms and the total adatoms on Si(111) )-7×7 The peak-area ratio, (AC1s / ASi2p) for pyrazine-saturated Si(111)-7×7 is 0.301, whereas a saturation ratio of 0.488 was also found for chemisorbed benzene, which corresponds to an absolute coverage of 0.42 [20] After considering the numbers of carbon atoms in these two molecules, the saturation coverage, θpyrazine is estimated to be ~ 0.39 [= 0.42 × 0.301 / 0.488 × / 4] This value approximately corresponds to pyrazine molecules / unit cell.” 3.3 Pyrimidine adsorption 3.3.1 High-resolution electron energy loss spectroscopy Figure 3.5 shows the high resolution electron energy loss spectra of pyrimidine exposed Si(111)-7×7 at 110 K as a function of exposure The vibrational frequencies and their assignments for physisorbed and chemisorbed molecules are summarized in Table 3.2 Vibrational signatures at 384, 712, 1021, 1220, 1402, 1547, 3074 cm-1 can be clearly identified in the spectrum of physisorbed molecules (Figures 3.5c, d, e) These vibrational features of physisorbed pyrimidine are in excellent agreement with the IR and Raman vibrational energies of liquid-phase pyrimidine with in ~20 cm-1 (Table 3.2)[21-24] Among them, the peak at 3074 cm-1 is assigned to the C (sp2)-H stretching mode; the feature at 1547 cm-1 is related to the conjugated C=C (N) stretching vibration The vibrational features of chemisorbed pyrimidine at low exposures or obtained by annealing the multilayer pyrimidine exposed sample to 300 K to drive away all the physisorbed molecules and only retain the chemisorbed molecules are shown in Figure 55 The Binding of Multi-functional Organic Molecules on Silicon Surfaces 3.5a and Figure 3.6b, respectively Losses at 530, 620, 748, 893, 998, 1202, 1621, 2891, 3074 cm-1 can be readily resolved The absence of the Si-H stretching around 2000-2100 cm-1 [14] suggests the nature of molecular chemisorption for pyrimidine on Si(111)-7×7 Compared to the physisorbed molecules, interesting changes are noticed The C-H stretching of chemisorbed molecules presents as an obvious doublet at 3074 and 2891 cm-1 Previous studies showed that benzene can be covalently attached to Si(111)-7×7 through [4+2]-like cycloaddition to form two new Si-C σ-bond linkages [5], evidenced by the doublet at ~3050 and ~2908cm-1 assigned to C(sp2)-H and C(sp3)-H stretching modes, respectively Based on a similar argument, the two ν(C-H) peaks at 3074 and 2891cm-1 observed in Figure 3.6b suggest the occurrence of one or more C-atoms rehybridizing from sp2 to sp3 The formation of Si-C linkage is further supported by the appearance of a vibrational feature at 530 cm-1 [25] In addition, a new peak at 1621 cm-1 is resolved, attributable to non-conjugated C=C(N) stretching vibration in chemisorbed pyrimidine This assignment is consistent with the results obtained for liquid 1,4cyclohexadiene (at 1639cm-1) [9], and chemisorbed benzene (at 1635 cm-1) [5] and chlorobenzene (at 1628cm–1) [10] on Si(111)-7×7 with a 1,4-cyclohexadiene-like surface intermediate The fact of forming non-conjugated surface species rules out the possibility of [2+2]-like addition mechanisms Furthermore, new intensities appearing 620 cm-1 (Figure 3.6b), can be assigned to the stretching modes of the Si-N bonds in the chemisorbed molecules This result clearly reveals that pyrimidine binding to the surfaces directly involves the nitrogen atoms as well The detailed assignments are tabulated in Table 3.2 The main vibrational features of chemisorbed pyrimidine correlate well with the calculated vibrational frequencies of the 1,4-N, C-dihydropyrimidine-like structure on 56 The Binding of Multi-functional Organic Molecules on Silicon Surfaces Si(111)-7×7 from our DFT calculations, confirming our assignments for chemisorbed pyrimidine on Si(111)- 7×7 Our HREELS results show that upon chemisorption, the coexistence of C(sp2)-H and C(sp3)-H stretching modes is observed together with the appearance of the nonconjugated C=C(N) vibrational feature(at 1621 cm-1) as well as the formation of the Si-N and Si-C bonds Thus, our vibrational observation strongly suggests that pyrimidine covalently bonds to the adjacent adatom and rest-atom on Si(111)-7×7 through a nitrogen atom and its para-carbon atom to form Si-N and Si-C di-σ via the [4+2]-like cycloaddition mechanism This is also further supported by the XPS results 3.3.2 X-ray photoelectron spectroscopy Figure 3.7b shows the N 1s photoemission spectra of chemisorbed pyrimidine obtained by annealing the sample with multilayer pyrimidine (Figure 3.7a) prepared at 110K to 300K N 1s photoemission spectrum of physisorbed molecules (Figure 3.7a) contains a symmetric peak at 400.4 eV with a typical FWHM (~1.2 eV) under our XPS resolution This binding energy is consistent with that of condensed pyrimidine on Au.[15] Compared to physisorption, however, significant changes can be found in the N 1s spectrum of chemisorbed molecules, broadened with a nearly symmetric flat-top shape This strongly suggests the significant modification on the electronic properties of nitrogen atoms after chemisorption The broad N 1s photoemission band of chemisorbed molecules can be fitted into two peaks centered at 400.4 and 399.0 eV, demonstrating the existence of two chemically different nitrogen atoms The ratio of integrated peak areas for these two peaks is 49.8%: 51.2%, approximately 1:1 Thus, the large down-shift of 57 The Binding of Multi-functional Organic Molecules on Silicon Surfaces 1.4eV in the core level for one of the nitrogen atoms confirms its participation in the cycloaddition with reactive sites on Si(111)-7×7, consistent with our vibrational analyses Figure 3.8 shows the fitted C 1s XPS spectra for physisorbed and chemisorbed pyrimidine on Si(111)-7×7 The C 1s spectrum of physisorbed molecules is deconvoluted into three peaks centered at 287.9, 287.1 and 285.7 eV with equal FWHM and an area ratio of 24%: 52%: 24%, approximately 1:2:1 (Figure 3.8a), in excellent accordance with the results of condensed pyrimidine on Au [15] Our results showed that the BEs of C1 and C2 separated by 0.8 eV are ~2.2 eV higher than that of the C3 atom The detailed assignment is listed in Table 3.3 Due to the higher electron negativity of nitrogen atoms, the neighboring C-atoms are expected to have a lower electron density, subsequently a higher BE of the C 1s core level Thus, the peaks at 287.9 and 285.7eV can be attributed to C1 and C3, respectively, whereas the intensity at 287.1 eV is contributed by the two C2atoms [15] Similarly, the C1s spectrum of chemisorbed molecules is be fitted into three peaks at 285.3, 286.5, and 287.5 eV with the same FWHM and an area ratio of 50%: 24%: 26%, approximately 2:1:1 According to the HREELS results, chemisorbed pyrimidine has 1,4-N, C-dihydropyrimidine-like structure In this configuration, one of the C2-atoms and its para-nitrogen atom are covalently linked to surface silicon atoms Thus the peak at 285.3eV can be reasonably assigned to the C2 atom directly bonded to silicon surface and C3-atom The large chemical shift of 1.8 eV in the core-level of this C2-atom, compared to these in physisorbed molecules, is attributable to its enhanced electron density after bonding with the Si-atom The peaks at 287.5eV and 286.5eV are associated with C1 and unreacted C2-atom, respectively, slightly down-shifted from their values of physisorbed molecules Their down-shift can be attributed to the increase of electron density on these 58 The Binding of Multi-functional Organic Molecules on Silicon Surfaces carbon atoms upon chemisorption The chemisorption process is expected to destroy the ring π-bond or aromaticity This blocks the electron-withdrawing effect of nitrogen atoms through π-bonding, subsequently enhancing the electron density of carbon atoms and down-shift in C 1s in chemisorbed pyrimidine compared to physisorbed molecules 3.3.3 Ultraviolet photoelectron spectroscopy He II valence band spectra following a sequence of pyrimidine exposures at 110 K are shown in Figure 3.9 The orbital constitution of physisorbed pyrimidine is shown in the form of the bar graph below Curve g, shifted to account for the work function and final state relaxation effects when condensed on solid-state surfaces For a clean Si(111)7×7 surface [26, 27], the existence of two peaks at 0.3 (S1) and 0.7 (S2) (Figure 3.9) below EF is due to the dangling bond surface states at adatoms and rest atoms, respectively, according to the STM studies of a clean Si(111)-7×7 surface [26, 27] Increasing pyrimidine exposure leads to the gradual attenuation of the surface states and total quenching around 2.4 L exposures The disappearance of the surface states may be caused by the consumption of surface dangling bonds and redistribution of their electron density at the pyrimidine / Si(111)-7×7 interface Upon 3.2 L exposure at 110 K, a physisorbed multilayer results in four dominant features at 12.12(D), 9.37(C), 6.62(B), and 5.15(A) eV The energy separations between two successive levels agree well with those gas-phase values [28, 29] A large body of evidence concerning the electronic structures of pyrimidine has been accumulated [3033] The four bands of A, B, C, and D observed for physisorbed molecules in our experiment can be assigned to πc=c(N) (2b1), n (7b2), πc=c(N) + n (11a1), and πc=c(N) (1b1), respectively 59 The Binding of Multi-functional Organic Molecules on Silicon Surfaces Pyrimidine/Si(111)-7x7 399.0 N 1s 110 K 400.4 XPS Intensity (a.u) (b) 400.4 Chemisorption (a) Physisorption 390 392 394 396 x 1/4 398 400 402 404 406 408 Binding Energy (eV) Figure 3.7 Fitting N 1s XPS spectra for physisorbed and chemisorbed pyrimidine on Si(111)-7×7 75 The Binding of Multi-functional Organic Molecules on Silicon Surfaces N N a 287.9 287.1 285.7 Physisorption x 1/2 Intensity(a.u) 285.3 286.5 287.5 Chemisorption b and 2 280 282 284 286 288 x2 290 292 Binding Energy(eV) Figure 3.8 Fitting C 1s XPS spectra for physisorbed and chemisorbed pyrimidine on Si(111)-7×7 76 The Binding of Multi-functional Organic Molecules on Silicon Surfaces UPS He II ' Pyrimidine/Si(111)-7x7 ' C B 110 K ' D ' A C i 300 K B A D 2b1 7b2 1a2 11a1 1b1 h 6.4 L g 3.2 L f 2.4 L e 2.0 L d 1.6 L c 0.8 L b 0.2 L a L S2 S1 16 14 12 10 -2 -4 -6 EB-EF (eV) Figure 3.9 He II valence band spectra of pyrimidine on Si(111)-7×7 as a function of pyrimidine exposure at 110 K Figure 3.9i is the difference spectrum of saturated chemisorption monolayer 77 The Binding of Multi-functional Organic Molecules on Silicon Surfaces 3053 345 730 993 1161 1396 1550 S-triazine / Si(111) 110 K Intensity (a.u) x 0.5 e 1.6 L d 0.8 L c 0.4 L b 0.2 L -1 x 1.5 a 0.1 L 1635 2909 3036 503 670 55 cm -500 500 1000 1500 2000 2500 3000 3500 4000 -1 Wavenumber (cm ) Figure 3.10 HREELS spectra of s-triazine on Si(111)-7×7 as a function of exposure at 110 K Ep=5.0eV, specular geometry 78 The Binding of Multi-functional Organic Molecules on Silicon Surfaces S-triazine / Si(111) 55 cm 2909 3036 Chemisorption 3053 993 1161 1396 1550 731 345 Intensity(a.u) 1204 1359 1635 503 670 940 110 K -1 Physisorption -500 500 1000 1500 2000 2500 3000 3500 -1 Wavenumber (cm ) Figure 3.11 HREELS spectra of the physisorbed and saturated chemisorption s-triazine on Si(111)-7×7 79 The Binding of Multi-functional Organic Molecules on Silicon Surfaces XPS C 1s 287.4 (66.4 %) (33.6 %) 286.0 287.9 (b) Chemisorption (a) Physisorption 280 282 284 x 0.5 286 288 290 292 Binding Energy (eV) Figure 3.12 Fitting C 1s XPS spectra for physisorbed and chemisorbed s-triazine on Si(111)-7×7 80 The Binding of Multi-functional Organic Molecules on Silicon Surfaces XPS N 1s (30.5 %) 399.0 400.4 (69.5 %) Intensity (a.u) (b) Chemisorption 400.6 x 0.5 (a) Physisorption 392 394 396 398 400 402 404 406 408 Binding Energy (eV) Figure 3.13 Fitting N 1s XPS spectra for physisorbed and chemisorbed s-triazine on Si(111)-7×7 81 The Binding of Multi-functional Organic Molecules on Silicon Surfaces N N N Si Si Si N Si Si Si Si Pyrazine Mode I Si Si Si Si Si Pyrazine Mode II Si N Si Si Si Pyrazine Mode V Si Si N Si Si Pyrazine Mode IV N N N N Si Si Si N N Si Si Si Pyrazine Mode III N N N Si Si N N N Si Si Si Si Si Si Si Si Pyrimidine Mode II Pyrimidine Mode I Si Si Si Si Pyrimidine Mode III N N Si Si Si N N Si Si Si Pyrimidine Mode IV Si Si N Si Pyrimidine Mode V N Si Si Pyrimidine Mode VI Si Si Si S-Triazine Mode I Si Si N N Si Si Si N Si Si N N Si N Si N N Si Si Si S-Triazine Mode II Si Si Si N Si S-Triazine Mode III Figure 3.14 Scheme of possible binding modes of pyrazine, pyrimidine and s-tirazine on Si(111)- 7×7, respectively 82 The Binding of Multi-functional Organic Molecules on Silicon Surfaces Table 3.1 Vibrational modes assignment for physisorbed and chemisorbed pyrazine (PZ) on Si(111)-7×7 IR data (Ref.7) for liquid pyrazine is included for comparison (the vibrational frequencies are given in cm-1) description IR liquid Oop ring deformation Oop skeletal bend Si-N stretch Ring deformation Skeletal deformation Oop skeletal bend Oop C-H bend Oop C-H bend 350 756 785 927 Oop C-H bend Oop C-H bend Ring breathing Ring deformation 960 983 1016 1018 C-H bend 1063 C-H bend 1130 Ring deformation 1149 C-H bend 1233 In plane (ring mode) In plane (ring mode) In plane (ring mode) In plane (ring mode) C=C stretch 1346 1483 C-H stretch 3012 C-H stretch 3040 3043 C-H stretch 3055 3071 C-H stretch 3069 1411 1525 Si-N stretch 497 503 712 728 Oop C-H bend Ring deformation 905 834 932 Asym Ring deformation Sym ring deformation 769 Calculated vibrational frequencies for PZ/Si(111) Oop CH bend 402 … chemisorbed PZ/Si(111) 1065 1015 In-plane C-H bend 1230 1267 Sym C=C stretch 418 … 602 704 physisorbed PZ/Si(111) description 1615 1601 1030 1215 1144 1408 1556 3071 C-H stretch 3071 3073 3095 83 The Binding of Multi-functional Organic Molecules on Silicon Surfaces Table 3.2 Vibrational modes assignment for physisorbed and chemisorbed pyrimidine (PM) on Si(111)-7×7 IR (Ref 21) and Raman (Ref 23) data for liquid pyrimidine are included for comparison (the vibrational frequencies are given in cm-1) Description IR Liquid21 Raman Liquid23 Physisorbed PM/Si(111) γ(CC) γ(CC) … … 347 398 384 β(CC) β(CC) ν(CH) 621 679 720 γ(CH) β(CH) γ(CH) γ(CH) γ(CH) ν(CC) 1071 1075 β(CH) 1160 1226 1228 1220 … 1356 1571 612 Oop C-H bend 748 731 Ring deform Ring deform Ring deform 893 998 891 893 960 1001 1469 1569 620 1398 1466 514 1402 1399 530 1158 In plane (ring mode) In plane (ring mode) In plane (ring mode) In plane (ring mode) In plane (ring mode) Si-C stretching Si-N stretching 1139 β(CH) Calculation vibrational for PM/Si(111) 810 960 980 1026 Chemisorbed PM/Si(111) Description 624 679 712 960 980 1021 Ring deform 1079 ν(CC) ν(−C−H) ν(=CH) 1202 1208 ν C=C(N) 1621 1612 ν (-C-H) 1547 Ring deform 2891 2912 3040 ν(=CH) 3049 3054 ν(=CH) 3053 3053 ν(=CH) 3086 3087 3017 3074 ν(=CH) 3074 3085 3102 84 The Binding of Multi-functional Organic Molecules on Silicon Surfaces Table 3.3 Fitted results of XPS spectra for chemisorbed and physisorbed pyrimidine on Si(111)-7×7 N Liquid on Au physisorption chemisorption down shift N N N C1 C2 C2 C3 400.5 400.5 287.4 286.7 286.7 285.6 400.4 400.4 287.9 287.1 287.1 285.7 400.4 399.0 287.5 286.5 285.3 285.3 1.4 0.2 0.6 1.8 0.4 Table 3.4 Vibrational modes assignment for physisorbed and chemisorbed s-triazine (TZ) on Si(111)-7×7 IR (gas) and Raman (liquid) data (Refs.35, 36) s-triazine are included for comparison (the vibrational frequencies are given in cm-1) description IR (gas) Oop skeletal bend Ring deformation Oop skeletal bend Oop C-H bend 676 737 927 Oop C-H bend Raman (liquid) physisorbed TZ/Si(111) 340 676 345 938 730 Ring deformation 992 C-H bend 1132 Si-N stretch Si-C stretch Oop C-H bend Ring deformation chemisorbed TZ/Si(111) 503 670 940 1031 C-H bend description 993 1172 In plane (ring mode) 1367 In plane (ring mode) 1409 1410 1557 1555 1635 1396 In plane (ring mode) 1359 Sym C=C stretch In plane (ring mode) 1204 In-plane C-H bend 1176 In-plane C-H bend 1161 1550 C=N stretch -CH2 stretch =CH stretch 2909 3057 3042 3053 3036 85 The Binding of Multi-functional Organic Molecules on Silicon Surfaces Table 3.5 The adsorption energies corrsponding to possible binding modes (shown in Figure 3.14) of pyrazine, pyrimidine and s-triazine on Si(111)-7×7 from DFT (pPB/DN**) Calculations, respectively.( All energies are in kcal·mol-1) Binding Mode Reaction Mechanism Pyrazine Mode I Pyrazine Mode II Pyrazine Mode III Pyrazine Mode IV Pyrazine Mode V Pyrmidine Mode I Pyrimidine Mode II Pyrimidine Mode III Pyrimidine Mode IV PyrimidineMode V Pyrimidine Mode VI S-triazine Mode I S-triazine Mode II S-triazine Mode III Dative bond [2+2]-like cycloaddition [2+2]-like cycloaddition [4+2]-like cycloaddition [4+2]-like cycloaddition Dative bond [2+2]-like cycloaddition [2+2]-like cycloaddition [2+2]-like cycloaddition [4+2]-like cycloaddition [4+2]-like cycloaddition Dative bond [2+2]-like cycloaddition [4+2]-like cycloaddition Adsorption Energy a 14.2 23.46 0.14 29.46 33.29 22.39 0.36 6.74 18.46 29.40 36.98 4.84 7.42 30.07 a Adsorption energy is obtained by substracting the total energy of pyrazine, pyrimidine, and s-triazine-bonded cluster from the sum of energies of substrate cluster and gaseous pyrazine, pyrimidine, and s-triazine molecules, respectively 86 The Binding of Multi-functional Organic Molecules on Silicon Surfaces Reference: Y Cao, K S Yong, Z Q Wang, W S 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