The binding of multi functional organic molecules on silicon surfaces 4

Chapter 4 Acetylethyne Binding on Si111-7×7 and Si00-2×1 Surfaces: Selectivity of Surface Structures 4.1 Motivation The covalent binding of organic molecules on semiconductor surfaces

Chapter 4 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

7×7 and Si(100)-2×1, the different binding configuration for acetylethyne bonding on two surfaces would be expected

4 2 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

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]

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

(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 sphybridization

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

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 do 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 adatom [25] Furthermore, it was found that the maximum number of adatoms involved

center-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

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] 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] 2 (Figure 4.5f), are well consistent with our experimental vibrational spectrum

1) stretching mode in the chemisorbed molecules also excludes these modes

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 O-atoms 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

6 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

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-1are 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

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 tetra-

σ bonded surface intermediate through the reaction of both πC=O and πC≡C bonds with adjacent two dimers

4.3.2 X-ray photoelectron spectroscopy

Figure 4.8 presents the O 1s XPS spectra for physisorbed and chemisorbed acetylethyne on Si(100)-2×1 O 1s photoemission spectrum of physisorbed molecules (Figure 4.8a) shows a symmetric peak at 533.7 eV with a typical FWHM (~1.2 eV) under our XPS resolution, which is close to the value observed for oxygen atoms in molecules containing intact carbonyl groups [10, 34] Compared to the physisorption spectrum, the

O 1s (532.5 eV) core level of chemisorbed molecules (Figure 4.8b) displays a downshift

of 1.2 eV, implying the direct involvement of the O-atom in acetylethyne binding on Si(100)-2×1 This value is very close to that observed for 9,10-phenanthrenequinone (532.4 eV) [35] and for 1,2-cyclohexanedione (532.4eV) [36] chemisorbed on the Si(100) surface through oxygen atoms Thus, the O 1s XPS results support the formation of a Si–

O linkage in chemically bonded acetylethyne on the Si(100) surfaces

Figure 4.9 shows the fitted C 1s XPS spectra for physisorbed and chemisorbed

C1H≡C2-C3 (C4H3)=O on Si(100)-2×1 The C 1s spectrum of physisorbed molecules is deconvoluted into three peaks centered at 289.0(24%), 285.6(53%) and 284.3 (23%) eV with an area ratio of 1:2:1 (Figure 4.9a) The peak at 289.0 eV can be assigned to the C atom of carbonyl, similar to the value (288.7-289.3eV) obtained in molecules containing intact carbonyl groups [34] The photoemission feature at 286.0 and 284.7eV 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 4.9b), the C 1s spectrum is significantly different, which implies large changes in electronic structures upon chemisorption It can

This result is indeed consistent with the formation of a tetra-σ linkage through two like cycloaddition reactions These constituent C 1s peaks can be attributed to the C3 (287.0 eV) and C1/C2/C4 (284.5 eV) of the reaction adduct (Si)C1H=C2(Si)-C3(Si)(C4H3)-O(Si) Table 4.5 lists the detailed assignment of C 1s and O 1s core levels for physisorbed and chemisorbed acetylethyne on Si(100)-2×1 Since the C4 of C4H3 is not involved in the surface reaction, it is very close to its C 1s value of 284.3 eV The C1 and C2 atoms in chemisorbed acetylethyne is chemically analogous to the C atoms of chemisorbed acetylene on Si(100)-2×1, giving a similar binding energy of 284.3 eV for C 1s

[2+2]-photoemission [36] Compared to physisorbed molecules, the C 1s core level of C≡C

group displays obvious down shift by 1.1 eV, which suggests that the C≡C group is directly involved in the binding with silicon surface The C3 atom is related to C 1s peak

at 287.0 eV Its BE downshifts by ~2.0 eV referenced to the value of physisorbed acetylethyne This result shows that the carbonyl group is strongly modified as a result of bonding to the Si(100) surface The rehybridization of the O and C3 atoms of carbonyl groups and their 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

4.3.3 Possible binding configurations

HREELS studies of chemisorbed acetylethyne show that the vibrational modes related to C≡C and C=O disappear simultaneously, thereby excluding the possibilities of the [2+2]-like cycloaddition occurring only through the C=O or the C≡C group Moreover, the absence of the characteristic feature of the C=C=C asymmetric stretching

mode (around 1950 cm-1) [17, 18] rules out the possibility of the [4+2]-like cycloaddition reaction Thus, our results show that acetylethyne is mainly covalently attached to Si(100)-2×1 with a tetra-σ binding mechanism through two [2+2]-like cycloaddition reactions of the C=O and C≡C groups with the silicon surfaces This conclusion is further supported by XPS studies On the other hand, considering the feasible [2+2]-like cyloaddition between the C≡C of acetylene [4, 5] or the C=O group in acetone [11] and surface reactive sites, it is reasonably deduced that acetylethyne is bonded to two neighboring dimers of the Si surface with four newly formed σ bonds Figure 4.10 shows the buckling dimers and two possible tetra-σ binding configurations of acetylethyne on Si(100)-2×1 For configurations A (cross-row bridge) and B (in-row bridge), the separations between the two middle dangling bonds are 5.55 and 3.85 Å, respectively [37] Considering the molecular dimension of acetylethyne, configuration B is expected

to be thermodynamically preferred compared to A

4.4 Conclusions

The formation of (Si)CH=C=C(CH3)-O(Si) and (Si)CH=C(Si)-C(Si)(CH3)-O(Si) surface intermediates in acetylethyne adsorption on Si(111)-7×7 and Si(100)-2×1, respectively, is clearly demonstrated in this chapter On Si(111)-7×7, the surface reaction occurs mainly through a [4+2]-like cycloaddition pathway between the adjacent adatom-rest atom pair and the two terminal atoms (O and C1) of the molecule The attachment of acetylethyne on Si(100)-2×1 mainly involves two [2+2]-like cylcoaddition of the C=O and C≡C groups to two adjacent dimers, forming a tetra-σ bonded intermediate The different reaction mechanisms for acetylethyne binding on Si(111)-7×7 and Si(100)-2×1

between the two terminal C and O-atoms in CH≡C-C(CH3)=O matches well with the separation of 4.5 Å between the adatom and its adjacent rest atom on Si(111)-7×7 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 On Si(100)-2×1, the dimensions of the -(Si)C-O(Si) or –(Si)C=CH(Si) formed via the tetra-σ bonds are close

to the separation (2.3 Å ) of a Si-dimer Thus, the different binding configurations for acetylethyne bonding on two surfaces show the correlation of reaction mechanism with surface structures

Figure 4.1 HREELS spectra of the physisorbed and saturated chemisorption acetylethyne

ν(Si-O) 714

ν(Si-C) 626 361