The binding of multi functional organic molecules on silicon surfaces 4

The Binding of Multi-functional Organic Molecules on Silicon Surfaces Chapter Acetylethyne Binding on Si(111)-7×7 and Si(00)-2×1 Surfaces: Selectivity of Surface Structures 4.1 Motivation The covalent binding of organic molecules on semiconductor surfaces has recently become an increasingly important aspect of surface modification and functionalization for potential application in molecular electronics [1-3].Recently, the attention is being directed towards the reactivity and selectivity of multifunctional molecule on Si(111)-7×7 In order to construct organic monolayer with available functionalities for further reaction to form multilayer organic film, an understanding of the attachment mechanism of multifunctional molecules becomes essential For multifunctional molecules, the reaction may be complex Different functional groups in the molecule may compete for active sites on the surface It is also possible that more than one functional groups in the molecule simultaneously bind to the surface Acetylethyne (CH≡C-C(CH3)=O) contains conjugated C≡C and C=O groups Previous studies showed that both acetylene [4-9] and acetone [10-13] can covalently bind to the reactive sites of Si(111)-7×7 and Si(100)-2×1 through a [2+2]-like addition mechanism Due to acetylethyne displaying a combined chemical structure of acetylene and acetone, it can be chosen as a template to demonstrate the selectivity and reactivity of functional groups coexisting in a multifunctional molecules on Si(111)-7×7 and Si(100)-2×1 In addition, considering the great difference of the electronic density distribution and the spatial separations among surface Si atoms containing dangling bonds between Si(111)- 90 The Binding of Multi-functional Organic Molecules on Silicon Surfaces 7×7 and Si(100)-2×1, the different binding configuration for acetylethyne bonding on two surfaces would be expected Formation of cumulative double bonds (C=C=C) in acetylethyne binding on Si(111)-7×7 4.2.1 High resolution electron energy loss spectroscopy Figure 4.1 shows the high-resolution electron energy loss spectra of the physisorbed acetylethyne (C1H≡C2-C3 (C4H3)=O) and the saturated chemisorption monolayer on Si(111)-7×7 The vibrational frequencies and their assignments for physisorbed and chemisorbed acetylethyne are listed in Table 4.1 A condensed multilayer acetylethyne is formed after exposing 8.0 L onto the Si(111)-7×7 at 110 K For this surface (Figure 4.1a), the energy-loss peaks at 580, 708, 979, 1195, 1412, 1684, 2100, 2931, and 3240 cm-1 are readily resolved, which are in good agreement with the IR and Raman spectroscopic data of liquid-phase acetylethyne within ~15cm-1[14] Among these vibrational signatures, the two peaks at 3240 and 2931 cm-1 are assigned to the C-H stretching modes of the ≡CH and –CH3 groups, respectively The features at 1684 and 2100 cm-1 are related to the C=O and C≡C stretching modes, respectively The vibrational features of chemisorbed acetylethyne were obtained by annealing the multilayer acetylethyne-covered sample to 300 K to drive away all physisorbed molecules and only retain chemisorbed monolayer (Figure 4.1b) Losses at 361, 626, 714, 846, 985, 1181, 1398, 1921, 2904, and 3016 cm-1 can be identified The absence of the Si-H stretching around 2050 cm-1 [15] suggests the nature of molecular chemisorption for acetylethyne on Si(111)-7×7 The disappearance of C=O (at 1684 cm-1) and C≡C (at 2100 cm-1) stretching modes in the chemisorbed molecules strongly demonstrates their 91 The Binding of Multi-functional Organic Molecules on Silicon Surfaces simultaneous involvement in the surface binding, clearly ruling out the possibility of the [2+2]-like cycloaddition occurring only through the C=O group or the C≡C group The major spectroscopic change is the appearance of a new peak at 1921 cm-1, ascribed to the characteristic vibration of a C=C=C skeleton (asymmetric stretching mode)[16-18] This assignment is further supported by the concurrent observation of its torsion (846 cm-1) and bending (361 cm-1) modes Furthermore, the two new peaks at 626 and 714 cm-1 are ascribed to the Si-C and Si-O stretching modes [19, 20], respectively Additional new feature is noticed at 3016 cm-1, assigned to (sp2) C-H stretching vibration This result shows the rehybridization of C1-atom of the C2≡C1H group from sp to sp2 due to its binding with the Si-dangling bond The main vibrational results of chemisorbed acetylethyne strongly suggest the formation of an allenic-like surface intermediate containing a –CH=C=C(CH3)-O- skeleton through the reaction of both πC=O and πC≡C bonds with an adjacent adatom-rest atom pair via the [4+2]-like process Table 4.1 also lists the main vibrational frequencies of the Si-CH=C=C(CH3)-O-Si structure from our DFT calculations, further confirming our assignments for chemisorbed acetylethyne on Si(111)-7×7 The details of DFT theoretical modeling will be given in Section 4.2.4 4.2.2 X-ray photoelectron spectroscopy Figure 4.2 presents the O 1s XPS spectra for physisorbed and chemisorbed acetylethyne on Si(111)-7×7 O 1s photoemission spectrum of physisorbed molecules (Figure 4.2a) shows a symmetric peak at 533.4 eV with a typical FWHM (~1.2 eV) under our XPS resolution The 533.4 eV binding energy observed here is close to the value observed for oxygen atoms in molecules containing intact carbonyl groups [20-22] Compared to the physisorption spectrum, the O 1s (532.2 eV) core level of chemisorbed 92 The Binding of Multi-functional Organic Molecules on Silicon Surfaces molecules (Figure 4.2b) displays a downshift of 1.2 eV, implying the direct involvement of the O-atom in acetylethyne binding on Si(111)-7×7 This value is in good agreement with results obtained for other molecules covalently attached to the surface through the Si-O bond [21, 22] Figure 4.3 shows the fitted C 1s XPS spectra for physisorbed and chemisorbed C1H≡C2-C3 (C4H3)=O on Si(111)-7×7 The C 1s spectrum of physisorbed molecules is deconvoluted into three peaks centered at 289.0, 285.4, and 284.1 eV with an area ratio of 1:2:1 (Figure 4.3a) The peak at 289.0 eV can be assigned to the C atom of carbonyl, similar to the value obtained in molecules containing intact carbonyl groups on Si(111) surface [20, 21] The photoemission feature at 285.4 and 284.1 eV are associated with C1H≡C2- and –C4H3, respectively, in good agreement with the C 1s BEs determined for the C atoms with sp and sp3 hydridizations [11, 23] For chemisorbed acetylethyne (Figure 4.3b), the C 1s spectrum is significantly different, which implies large changes in electronic structures upon chemisorption It can be fitted into three peaks at 286.0, 285.2, and 284.1 eV with an area ratio of 1:1:2, respectively The deconvoluted C 1s XPS data obtained from chemisorbed acetylethyne can be reasonably explained by the [4+2]-like cycloaddition These constituent C 1s peaks can be attributed to the C3(286.0 eV), C2(285.2 eV), and C1/C4(284.1 eV) of the reaction adduct (Si)C1H=C2=C3(C4H3)-O(Si) Table 4.2 lists the detailed assignment of C 1s and O 1s core levels for physisorbed and chemisorbed acetylethyne on Si(111)-7×7 This assignment is justified and consistent if comparison with previous studies is made The C1 atom with sp2 hybridization in chemisorbed acetylethyne is chemically analogous to the C atoms of chemisorbed acetylene on Si(111)-7×7, giving similar binding energies 93 The Binding of Multi-functional Organic Molecules on Silicon Surfaces (284.0 eV) for C 1s photoemission [24] Since the C4 of C4H3 is not involved in the surface reaction, it retains its C 1s value of 284.1eV The C2 remains the sp hybridization in chemisorbed state Thus, its C 1s binding energy (285.2 eV) is not expected to shift significantly from the value (285.4 eV) of physisorbed molecules The C3 atom is related to C 1s peak at 286.0 eV Although the C3 atom retains the same sp2 hydridization upon chemisorption, its C 1s BE downshifts by ~3.0 eV referenced to the value of physisorbed acetylethyne The rehybridization of the oxygen atom and its bonding to a Si atom with a much lower electronegativity (Pauling electronegativity =1.90) reduce the electronic polarization in the C3-O Thus, compared to physisorbed acetylethyne, a much higher electron density is expected at the C3 atom, leading to a lower C 1s BE of the C3 atom 4.2.3 Scanning tunneling microscopy In order to further elucidate site-selectivity of acetylethyne binding on Si(111)-7×7, STM was used to investigate the extent and spatial distribution of the present surface reaction system at atomic resolution Figure 4.4a shows STM constant current topograghs (CCTs) of a clean Si(111)-7×7surface at room temperature with a defect density of < 0.5%, estimated by counting an area containing about 1500 adatoms Figure 4.4b is the typical STM topograph of Si(111)-7×7 exposed to 0.4 L (direct dosing) acetylethyne at room temperature Comparison with the clean and acetylethyne-covered surfaces reveals that the 7×7 reconstruction is preserved after acetylethyne adsorption reaction However, some adatoms become invisible as a result of reaction, increasing in number with the acetylethyne exposure The apparent formation of darkened sites was previously observed in the adsorption of other small molecules, such as NH3 [25], H2O [26], C2 H2 [27], C2H4 [28], C6H6 [29], C6H5Cl [30], and C4H4S [31] on Si(111)-7×7 In all these 94 The Binding of Multi-functional Organic Molecules on Silicon Surfaces cases, the darkening of adatoms in STM images was attributed to the consumption of the adatom dangling bonds due to the surface-adsorbates bond formation We found no bias dependence for the intensity at the reacted adatoms (darkened sites), suggesting that the adsorbed acetylethyne and reacted adatoms not have orbitals close to the Fermi level EF A statistical counting of darkened dangling bond sites can provide information on the spatial selectivity for acetylethyne chemisorption Careful analysis of STM images (not shown) obtained after several different exposures of acetylethyne manifests the preferential adsorption on the center adatom sites of faulted halves The results also show that the reactivity of center-adatoms is about twice of corner adatoms At saturated chemisorption (Figure 4.4b), substantial adsorption also occurs on unfaulted halves However, the preference of center adatoms over the corner adatom sites is still evident The higher selectivity of acetylethyne binding to the faulted half and center adatom sites of a Si(111)-7×7 unit cell can be understood when considering the higher electrophilicity of the faulted subunits [32] and a smaller strain for molecules binding on the centeradatom [25] Furthermore, it was found that the maximum number of adatoms involved in acetylethyne chemisorption for every faulted or unfaulted half unit cell is three, equal to the number of the rest atoms Thus, it is reasonable to deduce that every acetylethyne molecule binds with the neighboring adatom-rest atom pair 4.2.4 DFT Theoretical Calculations In general, there are three possible ways for acetylethyne binding on Si(111)-7×7: (a) [2+2]-like cycloaddition through the C=O group; or (b) the C≡C group; (c) [4+2]-like cycloaddition through the terminal C and O-atoms of CH≡C-C(CH3)=O DFT theoretical 95 The Binding of Multi-functional Organic Molecules on Silicon Surfaces calculations were carried out to obtain the optimized geometric structures and energies for these possible adsorption configurations Figure 4.5 presents the six optimized geometries of the local minima for acetylethyne/Si9H12 model system Their adsorption energies are given in Table 4.3 The calculation result reveals that the [4+2]-like cycloadditions are thermodynamically favored compared to the [2+2]-like cycloadditions Cluster [4+2] (Figure 4.5f) is seen to be most stable, where the O and C1 atoms are linked to the adatom and rest atom to form a structure containing cumulative double bonds (C=C=C) This process is exothermic by 82.6 kcal•mol-1 In addition, the calculated vibrational frequencies (Table 4.1) of the most stable intermediate, Cluster [4+2] (Figure 4.5f), are well consistent with our experimental vibrational spectrum 4.2.5 Adsorption geometry Figure 4.5 presents six possible binding modes of acetylethyne on Si(111)-7×7 The [2+2]-like cycloaddition through the carbonyl group forms a surface intermediate of C1H≡C2-C3(Si)(C4H3)-O(Si) (Figures 4.5a, b) In this reaction product, the C≡C group is retained However, the disappearance of ≡CH (at 3240 cm-1) and C≡C (at 2100 cm-1) stretching modes in our HREELS results rules out this possibility The [2+2]-like reaction through the C≡C group to a neighboring adatom-rest atom pair gives a chemisorbed species of (Si)C1H=C2(Si)-C3(C4H3)=O (Figures 4.5c, d) with a C1=C2-C3=O conjugated structure In these cycloadducts, both C1 and C2 atoms rehybridize from sp into sp2, together with the retention of the carbonyl group The disappearance of C=O (at 1684 cm1 ) stretching mode in the chemisorbed molecules also excludes these modes 96 The Binding of Multi-functional Organic Molecules on Silicon Surfaces In fact, the experimental results are consistent with the [4+2]-like cycloaddition reaction mechanism, forming a product containing a –C1H=C2=C3(C4H3)-O-skeleton (Figures 4.5e, f) In this structure, the disappearance of C3=O is expected, together with the conversion of C1≡C2 to C1=C2 upon cycloaddition The characteristic C=C=C skeleton of the surface intermediate is further confirmed by the detection of its asymmetric stretching mode at 1921 cm-1, together with its torsion (at 846 cm-1) and bending (361 cm-1) modes Hence, the vibrational characteristics allow us to conclude that acetylethyne covalently bonds to the Si surface principally through breaking both πC=O and πC≡C bonds to react with the dangling bonds located on the adjacent adatom-rest atom pair via the [4+2]-like process The reactivity of acetylethyne on Si(111)-7×7 can also be reasonably explained considering the spatial arrangements of the acetylethyne molecule and the neighboring adatom-rest atom pair on the surface The distance between the two terminal C and Oatoms in CH≡C-C(CH3)=O matches well with the separation of 4.5 Å between the adatom and its adjacent rest atom However, great structural strains may exist in the (Si)C-O(Si) or –(Si)C=CH(Si) formed through the [2+2]-like addition of C=O or C≡C groups, respectively, implying the instability of [2+2]-like cycloadducts For acetylethyne binding to a pair of adatom and rest-atom through its C1 and O atoms, there are two types of configuration, that is, O-atom binding to the rest-atom (Figure 4.5e) or the adatom (Figure 4.5f) According to our calculation results, the binding state with O-atom linking to an adatom is significantly more stable (by more than kcal/mol) than the alternative configuration with O-atom binding to a rest-atom On the other hand, the selective attachment of oxygen atom to the adatom over the rest atom is 97 The Binding of Multi-functional Organic Molecules on Silicon Surfaces possibly attributable to a barrierless pathway passing through a dative-bonded precursor [12] The oxygen atom has a couple of lone-pair electrons Thus, it can possibly act as a donor to provide electrons to form a dative-bonded precursor with electron-deficient Si dangling bonds on adatoms, lowers the energy barrier of the surface reaction This possibly explains the selectivity from the kinetic point of view Thus, the formation of cumulative double bonds (C=C=C) through the [4+2]-like cycloaddition of acetylethyne with Si-dangling bonds on Si(111)-7×7 is thermodynamically and kinetically preferred 4.3 Formation of a tetra-σ bonded intermediate in acetylethyne binding on Si(100)2×1 4.3.1 High resolution electron energy loss spectroscopy Figure 4.6 shows the high-resolution electron energy loss spectra recorded as a function of acetylethyne exposure on Si(100)-2×1 Figures 4.6d and 4.6e display the vibrational features for physisorbed multilayer acetylethyne after exposing 2.4 and 3.6 L onto the Si(100)-2×1 surface at 110K, respectively The loss features at 585, 713, 997, 1189, 1408, 1695, 2100, 2938, and 3271 cm-1 are readily resolved, which are in good agreement with the IR and Raman spectroscopic data of liquid-phase acetylethyne within ~15cm-1.[14] Among these vibrational signatures, the two peaks at 3271 and 2938 cm-1 are assigned to the C-H stretching modes of the ≡CH and –CH3 groups, respectively The features at 1695 and 2100 cm-1 are related to the C=O and C≡C stretching modes, respectively The detailed assignments for physisorbed and chemisorbed acetylethyne together with the IR and Raman spectroscopic data of liquid-phase acetylethyne [14] are listed in Table 4.4 98 The Binding of Multi-functional Organic Molecules on Silicon Surfaces The vibrational features of chemisorbed acetylethyne at low exposures (Figure 4.6a) or prepared after annealing the multilayer acetylethyne-covered sample to 300 K (Figure 4.7b) are significantly different Losses at 567, 688, 825, 979, 1171, 1405, 1580, 2920, and 3067 cm-1 can be identified The absence of the Si-H stretching around 2050 cm-1 [15] suggests the nature of molecular chemisorption for acetylethyne on Si(100)-2×1 The disappearance of C=O (at 1695 cm-1) and C≡C (at 2100 cm-1) stretching modes in the chemisorbed molecules strongly demonstrates their simultaneous involvement in the surface binding, clearly ruling out the possibility of the [2+2]-like cycloaddition occurring only through the C=O group or the C≡C group This result implies two possible binding schemes: (1) formation of cumulative double bonds (C=C=C) involving conjugated C=O and C≡C bonds of acetylethyne through the [4+2]-like reaction or (2) forming a tetra-σ linkage via two [2+2]-like addition reactions The new peak at 1580 cm-1 observed for chemisorbed acetylethyne can be ascribed to the C=C stretching mode This assignment is consistent with the results obtained for chemisorbed phenylacetylene [23] and diacetylene [33] on Si(100) and further supported by the observation of the feature at 3067 cm-1 for the =C-H stretching vibration together with the disappearance of the ≡C-H stretching mode ( 3271 cm-1) However, the characteristic feature of the C=C=C asymmetric stretching mode (around 1950 cm-1) [17, 18] was not evidenced, ruling out the possibility of the [4+2]-like cycloadditon reaction Thus, our main vibrational results of chemisorbed acetylethyne strongly suggest the formation of a tetraσ bonded surface intermediate through the reaction of both πC=O and πC≡C bonds with adjacent two dimers 99 The Binding of Multi-functional Organic Molecules on Silicon Surfaces Physisorbed HC ≡C -C (C H3)=O 1,2 289.0 (24.5 %) 285.4 (51.5 %) 284.1 (24.0 %) Intensity (a.u) x 0.5 (a) Chemisorbed HC ≡C -C (C H3)=O 284.1 (50.8 %) 285.2 (24.6 %) 286.0 (24.6 %) 1,4 (b) 295 290 285 280 275 Binding Energy (eV) Figure 4.3 Fitted C 1s XPS spectra for physisorbed and saturated chemisorbed acetylethyne on Si(111)-7×7 106 The Binding of Multi-functional Organic Molecules on Silicon Surfaces (b) Figure 4.4 Constant-current-topograph (CCT) (~200Å×200Å, Vs=+1.0V, I T=0.15nA) images of clean (a) and saturated chemisorption acetylethyne (b) on Si(111)-7×7 at 300 K 107 The Binding of Multi-functional Organic Molecules on Silicon Surfaces C4 C4 C2 C3 O Si2 Ad Re Si1 Si2 Ad Re Si1 Si4 Si3 Si4 Si3 C1 C2 C3 O C1 (b) [2+2]2 (a) [2+2]1 C4 C4 C3 O O Si2 Ad Re Si1 C3 C2 C2 C1 Re Si1 Si3 Si3 C1 Si2 Ad Si4 Si4 (d) [2+2]4 (c) [2+2]3 C4 C2 C3 C4 C3 C2 O C1 Re Si1 Si2 Ad C1 Si2 Ad O Re Si1 Si3 Si4 (e) [4+2]1 Si3 Si4 (f) [4+2]2 Re: rest atom Ad: adatom Figure 4.5 Optimized C4H4O/Si9H12 clusters corresponding to the six possible attachment modes through [2+2]-like cycloadditions via a carbonyl or C≡C group and [4+2]-like addition reactions via the two terminal atoms of acetylethyne 108 The Binding of Multi-functional Organic Molecules on Silicon Surfaces 2938 2100 1695 3271 110 K 997 1189 1408 585 713 Acetylethyne/Si(100)-2x1 (e) 3.6 L (d) 2.4 L 3052 -1 55 cm 1589 (c) 1.6 L (b) 0.8 L (a) 0.2 L -500 500 1000 1500 2000 2500 3000 3500 4000 4500 -1 Wavenumber (cm ) Figure 4.6 HREELS spectra of acetylethyne on Si(100)-2×1 as a function of exposure at 110 K 109 The Binding of Multi-functional Organic Molecules on Silicon Surfaces 110 K 3271 2938 2100 979 1171 1405 567 688 825 1695 997 1189 1408 585 713 Acetylethyne / Si(100)-2x1 1580 2920 3067 (a) Physisorption -1 55 cm x 1.5 (b) chemisorption -500 500 1000 1500 2000 2500 3000 3500 4000 -1 Wavenumber (cm ) Figure 4.7 HREELS spectra of the physisorbed and saturated chemisorption acetylethyne on Si(100)- 2×1 110 The Binding of Multi-functional Organic Molecules on Silicon Surfaces XPS Acetylethyne / Si(100) O 1s 532.5 (a) Chemisorption X2 533.7 (b) Physisortion X 1/2 526 528 530 532 534 536 538 540 Binding Energy (eV) Figure 4.8 O 1s XPS for physisorbed and chemisorbed acetylethyne on Si(100)-2×1 111 The Binding of Multi-functional Organic Molecules on Silicon Surfaces 284.3(23%) 285.6(53%) Physisorption HC ≡C -C (C H3)=O 289.0(24%) (a) 284.5(75.5%) Chemisorption HC ≡C -C (C H3)=O 287.0(24.5%) (b) 294 292 290 288 286 284 282 280 278 276 Binding Energy (ev) Figure 4.9 Fitted C 1s XPS spectra for physisorbed and saturated chemisorbed acetylethyne on Si(100)-2×1 112 The Binding of Multi-functional Organic Molecules on Silicon Surfaces 5.55 Å CH3 H 3.85 Å + C C C O Cross-row bridging CH≡C-C(CH3)=O HC C O C CH3 2×1 (A) (B) In row bridging Figure 4.10 The buckling dimers of the Si(100) at a cryogenic temperature and two possible tetra-σ binding configurations of acetylethyne on Si(100)-2×1 113 The Binding of Multi-functional Organic Molecules on Silicon Surfaces Table 4.1 Vibrational modes assignment for physisorbed and chemisorbed acetylethyne on Si(111)-7×7 IR and Raman data (Ref.14) for liquid acetylethyne are included for comparison (the vibrational frequencies are given in cm-1) a: present work Physisorbeda Vibrational Assignments Chemisorbed a Calculated a 3240 -CH= stretching 3016 3025 2978(a), 2926(s) 2931 -CH3 stretching 2904 2940(a), 2894(s) 2099 2092 2100 C=C=C asymmetric stretch 1921 1935 C=O stretching 1690 1685 1684 -CH3 deformation 1398 1412(a),1364 (s) -CH3 deformation 1426(a), 1364(s) 1429(a), 1368(s) 1412 C-C stretching 1181 1139 C-C stretching 1198 1198 1195 CH3 rock 985 1020, 967 CH3 rock 1022, 981 1027, 983 979 C=C=C torsion 846 830 ≡C-C stretching 741 743 - Si-O stretching 714 701 ≡CH bending 700 700 708 Si-C stretching 626 624 C=O rock 587 590 580 C=C=C bending 361 371 CH3 torsion 435 436 - C-C≡C bending 228 230, 183 - Vibrational Assignments Infrared liquid14 ≡CH stretching 3359, 3262 -CH3 stretching 3011(a), 2972(a), 2921(s) 3017(a), C≡C stretching Raman liquid14 114 The Binding of Multi-functional Organic Molecules on Silicon Surfaces Table 4.2 Fitted results of XPS spectra from chemisorbed and physisorbed acetylethyne on Si(111)-7×7.a HC≡C2-C3(C4H3)=O Physisorption Chemisorption O 533.4 532.2 1.2 sp2 – sp3 C1 285.4 284.1 1.3 sp – sp2 C2 285.4 285.2 0.2 sp – sp C3 289.0 286.0 3.0 sp2 – sp2 C4 284.1 284.1 sp3 –sp3 a Down-shift Rehybridization All energies are in eV Table 4.3 Adsorption energies of the local minima in the C4H4O/Si9H12 model system from pBP/DN Functional group C3=O C3=O C1≡C2 C1≡C2 C1≡C2-C3=O C1≡C2-C3=O Reaction model [2+2]1 [2+2]2 [2+2]3 [2+2]4 [4+2]1 [4+2]2 25.5 26.4 56.0 54.6 76.3 82.6 Adsorption energya a Adsorption energy: ∆E = [E(Si9H12) + E(C4H4O)] − E(C4H4O/Si9H12) All energies are in kcal mol-1 115 The Binding of Multi-functional Organic Molecules on Silicon Surfaces Table 4.4 Vibrational modes assignment for physisorbed and chemisorbed acetylethyne on Si(100)-2×1 IR and Raman dara (Ref.14) for liquid acetylethyne are included for comparison (the vibrational frequencies are given in cm-1) Vibrational Assignments ≡CH stretching Infrared liquid14 Raman liquid14 3359, 3262 Physisorbeda 3271 =CH stretching 3067 -CH3 stretching 3011(a), 2972(a), 2921(s) 3017(a), 2978(a), 2926(s) 2938 C≡C stretching 2099 2092 2100 C=O stretching 1700 1694 1695 C=C stretching -CH3 deformation C-C stretching CH3 rock 2920 1580 1426(a), 1364(s) 1429(a), 1368(s) 1408 1405 1198 1198 1189 1171 1022, 981 1027, 983 997 979 =CH bending 825 ≡C-C stretching 741 743 - ≡CH bending 700 700 713 Si-O stretching C=O rock 688 587 590 583 Si-C stretching 567 CH3 torsion 435 436 - C-C≡C bending a Chemisorbed a 228 230, 183 - present work 116 The Binding of Multi-functional Organic Molecules on Silicon Surfaces Table 4.5 Fitted result of XPS spectra for chemisorbed and physisorbed acetylethyne on Si(100)-2×1.a HC1≡C2-C3(C4H3)=O physisorption chemisorption down-shift rehybridization O C1 C2 C3 C4 533.7 285.6 285.6 289.0 284.3 532.5 284.5 284.5 287.0 284.5 1.2 1.1 1.1 2.0 -0.2 sp2 – sp3 sp – sp2 sp – sp2 sp2 – sp3 sp3 –sp3 a All energies are 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Organic Molecules on Silicon Surfaces (2 84. 0 eV) for C 1s photoemission [ 24] Since the C4 of C4H3 is not involved in the surface reaction, it retains its C 1s value of 2 84. 1eV... H2 [27], C2H4 [28], C6H6 [29], C6H5Cl [30], and C4H4S [31] on Si(111)-7×7 In all these 94 The Binding of Multi- functional Organic Molecules on Silicon Surfaces cases, the darkening of adatoms.. .The Binding of Multi- functional Organic Molecules on Silicon Surfaces 7×7 and Si(100)-2×1, the different binding configuration for acetylethyne bonding on two surfaces would be