The binding of multi functional organic molecules on silicon surfaces 5

The Binding of Multi-functional Organic Molecules on Silicon Surfaces Chapter Attachment of Cyanoacetylene and Diacetylene on Si(111)-7×7: Selectivity of Molecular Structures 5.1 Motivation Both experimental and theoretical studies showed that the adjacent adatom-rest atom pair can serve as a diradical to react with unsaturated organic functionalities [1-16] The covalent binding of three typical unsaturated organic molecules, including ethylene, acetylene, and acetonitrile, on Si(111)-7×7 has been extensively investigated Ethylene, acetylene, and acetonitrile are both di-σ-bonded to the neighboring adatom-rest atom pair through the [2+2]-like cycloadditions [3-14] Being a π-conjugated system, acrylonitrile can be bound to Si(111)-7×7 through a [4+2]-like cycloaddition reaction, creating a cyclic, six-membered ketenimine (C=C=N) [15] Recently, the interesting theoretical DFT calculation has demonstrated the feasibility of the formation of [3]-cumulenic adspecies in the reaction of diacetylene on Si(111)-7×7 [16] However, experimental evidence is still to be established Ever since the discovery of several organics in Titan’s atmosphere, the largest satellite of Saturn with a dense atmosphere has become a very interesting subject that is dealt with by extraterrestrial organic chemists and exobiologists [17] Cyanoacetylene (CH≡C-C≡N) and diacetylene (CH≡C-C≡CH) are two conjugated molecules made of a C≡C and C≡N(C) in the atmosphere of Titan Thus, investigating their interactions with Si surfaces will provide the correlation of reaction selectivity and binding configuration 121 The Binding of Multi-functional Organic Molecules on Silicon Surfaces with the functional groups in the molecule, offering the necessary flexibility in the functionalization and modification of silicon surfaces 5.2 Cyanoacetylene binding on Si(111)-7×7 Figure 5.1 shows the high-resolution electron energy loss spectra of Si(111)-7×7 exposed to cyanoacetylene at 110 K as a function of exposure The vibrational frequencies and their assignments for physisorbed and chemisorbed molecules are listed in Table5.1 For physisorbed multilayer, vibrational features at 525, 741, 941, 2100, 2289, 3313cm-1 are unambiguously resolved Table 5.1 clearly shows that the vibrational feature of physisorbed cyanoacetylene (Figure 1d) is in excellent agreement with the IR spectrum of gaseous cyanoacetylene [18, 19] Among these vibrational signatures, the peak at 3313 cm-1 is assigned to the Csp-H stretching mode; The features at 2289, 2100, and 941 cm-1 are related to the C≡N, C≡C and C-C stretching modes, respectively The vibrational features of chemisorbed cyanoacetylene at low exposures (Figure 5.1a) or obtained by annealing the multilayer cyanoacetylene-covered sample to 300 K to drive away all the physisorbed molecules and only retain the chemisorbed molecules (Figure 5.2b), however, are significantly different Losses at 569, 649, 821, 1205, 1570, 2010, and 3035 cm-1 can be readily resolved The disappearance of the C≡N stretching around 2289 cm-1 in chemisorbed molecules strongly suggests the involvement of C≡N group in the surface binding A new feature (at 3035 cm-1) is noticed, which is 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 This is further supported by the absence of the (sp) C-H (at 3313cm-1) and C≡C (at 2100) stretching modes in the vibrational signatures of chemisorbed cyanoacetylene The major 122 The Binding of Multi-functional Organic Molecules on Silicon Surfaces spectroscopic change is the appearance of a new peak at 2010 cm-1, ascribed to the characteristic vibration of a C=C=C=N skeleton (asymmetric stretching mode) [20, 21] This assignment is further supported by the concurrent observation of the symmetric stretching, bending and torsion modes of C=C=C=N at 1570, 1205, 821 cm-1, respectively The formation of a C=C=C=N skeleton is also supported by the absence of the C-C (at 941 cm-1) stretching mode in the vibrational features of chemisorbed molecules Furthermore, the two new peaks at 569 and 649 cm-1 are ascribed to the Si-N and Si-C stretching modes, [12] respectively The detailed assignments are tabulated on Table 5.1 Table 5.1 also lists the main vibrational frequencies of Si-CH=C=C=N-Si structure from our DFT calculations, unambiguously approving our assignment for chemisorbed cyanoacetylene on Si(111)-7×7 The details of DFT theoretical modeling will be given in Section Figure 5.3 presents three possible binding modes of cyanoacetylene on Si(111)7×7 The [2+2]-like cycloaddition through the cyano group forms a surface intermediate of CH≡C-C(Si)=N(Si) (Figure 3a) In this reaction product, the C≡C group is retained However, the disappearance of ≡CH (at 3313 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)CH=C(Si)-C≡N (Figure 5.3b) with a C=C-C≡N conjugated structure In the cycloadduct, two C atoms in C≡C group rehybridize from sp into sp2, together with the retention of the cyano group The disappearance of C≡N (at 2289 cm-1) stretching mode in the chemisorbed molecules also excludes this mode 123 The Binding of Multi-functional Organic Molecules on Silicon Surfaces In fact, experimental results are consistent with the [4+2]-like cycloaddition reaction mechanism, forming a product containing a –CH=C=C=N-skeleton (Figure 5.3c) In this structure, the disappearance of C≡N is expected, together with the conversion of C≡C to C=C upon cycloaddition The characteristic C=C=C=N skeleton of the surface intermediate is further confirmed by the detection of its asymmetric stretching mode at 2010 cm-1, together with its symmetric stretching (at 1570 cm-1), bending ( at 1205 cm-1) and torsion (at 821 cm-1) modes Hence, these results allow us to conclude that cyanoacetylene covalently bonds to the Si surface principally through breaking both πC≡N and πC≡C bonds to react with the dangling bonds located on the adjacent adatom-rest atom pair via the [4+2]-like process 5.3 Diacetylene binding on Si(111)-7×7 Figure 5.4 shows the IR spectra (scanning range 600-4000 cm-1) for gaseous diacetylene (Figure 5.4a) and the high-resolution electron energy loss spectra of Si(111)7×7 exposed to diacetylene at room temperature (Figure 5.4b) The vibrational frequencies and their assignments for gas phase diacetylene and chemisorbed molecules on Si(111)- 7×7 are listed in Table 5.2 Vibrational signatures at 634, 1238, 2021, 2181, 3341 cm-1 can be clearly identified in the IR spectra for gaseous molecules, which are in excellent agreement with the previous studies [22, 23] The features at 3341 and 2021, 2181 cm-1 are related to the ≡C(sp)-H and C≡C stretching modes, respectively The vibrational features of chemisorbed diacetylene are significantly different from the IR spectra of gaseous molecules Losses at 439, 650, 921, 1281, 1626, 2147, 3030, and 3334 cm-1 are readily resolved The coexistence of ≡Csp-H (at 3334 cm-1) and =Csp2H (at 3030 cm-1) stretching modes in the chemisorbed molecules strongly demonstrates 124 The Binding of Multi-functional Organic Molecules on Silicon Surfaces the rehybridization of one of the C≡C groups in the surface binding This conclusion is further supported by the concurrent observation of the C≡C and C=C stretching modes at 2147 and 1626 cm-1, respectively This result shows that diacetylene covalently binds to Si(111)-7×7 through [2+2]-like cycloaddition between one of the C≡C groups and surface reactive sites The formation of two new Si-C linkages is further confirmed by the appearance of a new peak at 650 cm-1 (Si-C stretching mode)[10] In addition, the absence of the Si-H stretching around 2055 cm-1 (Ref 24) suggests the nature of molecular chemisorption for diacetylene on Si(111)-7×7 at room temperature Figure 5.3 also shows two possible routes for the covalent attachment of diacetylene on Si(111)-7×7, namely, a [4+2]-like reaction (Figure 5.3d) and a [2+2]-like pathway (Figure 5.3e), producing a [3]-cumulenic and an enynic-like adspecies, respectively In fact, the major experimental evidence of (1) the coexistence of ≡Csp-H and =Csp2-H stretching modes, (2) the concurrent observation of the C≡C and C=C stretching modes unambiguously excludes the possibilities of the [4+2]-like cycloaddition Thus, vibrational studies of chemisorbed diacetylene strongly suggest the formation of an enynic-like surface intermediate containing a (Si)CH=C(Si)-C≡CH skeleton through the reaction of one of the πC≡C bonds with an adatom-rest-atom pair via a [2+2]-like process 5.4 DFT theoretical Calculations Due to the high reactivity of C≡C and C≡N groups on Si(111)-7×7 [5-9], there exist various competitive reaction pathways over the same surface binding sites (the neighboring adatom-rest atom pair) for both cyanoacetylene and diacetylene Thus, we have theoretically modeled some of the possible configurations to aid the understanding of the reactivity and selectivity of these π-conjugated systems on Si(111)-7 ×7 125 The Binding of Multi-functional Organic Molecules on Silicon Surfaces Figure 5.5 presents the optimized geometries of the local minima for the C3 NH (C4H2) /Si9H12 model systems Table 5.3 reveals that for cyanoacetylene on Si(111)-7×7 the reaction involving both C≡C and C≡N groups is thermodynamically favored compared to other possible reactions This process is exothermic by 64.9 kcal•mol-1 In addition, the calculated vibrational frequencies (Table 5.1) for the cluster corresponding to mode III are very consistent with the experimental observation For diacetylene on Si(111)-7×7, DFT calculation results are shown in Table 5.4 The binding configuration of Mode IV (Figure 5.5) through the [4+2]-like cycloaddition (76.6 kcal•mol-1) is more stable compared to Mode V (Figure 5.5) through the [2+2]-like cycloaddition (63.4 kcal•mol-1) Similar results are presented in the recent theoretical prediction of diacetylene on Si(111)-7×7 by Lu X et al.23 Moreover, The authors also reveal the activation barrier for the formation of Mode V (4.4 Kcal•mol-1) is higher than that of Mode IV (0.6 Kcal•mol-1) Thus, they suggest the formation of [3]-cumulenic adspecies (Mode IV) in the reaction of diacetylene on Si(111)-7×7.23 However, the binding configuration (Mode IV of Figure 5.5) is not observed in our experiment In fact, the barrier energetic difference (about 3.6 kcal/mol) is far too small to direct the process towards one product, especially as no temperature or entropy is taken into account Hence, the absence of [3]-cumulenic adspecies (Mode IV) for diacetylene binding on Si(111)-7×7 at room temperature is reasonable, consistent with our experimental results 5.5 Conclusions Vibrational studies together with DFT calculations have shown the formation of (Si)CH=C=C=N(Si) and (Si)CH=C(Si)-C≡CH surface intermediates in cyanoacetylene and diacetylene chemisorption on Si(111)-7×7, respectively Cyanoacetylene mainly 126 The Binding of Multi-functional Organic Molecules on Silicon Surfaces binds to the surface through a diradical reaction involving both cyano and C≡C groups with an adjacent adatom-rest atom pair For diacetylene, the surface reaction occurs through [2+2]-like cyloaddition between one of C≡C groups and adjacent adatom-rest atom pair The different binding mechanisms of cyanoacetylene and diacetylene on Si(111)-7×7 can be attributed to the effect of the polarities of organic groups and reactive sites ( the adjacent adatom-resr atom) In the two organic molecules, the larger polarity of C≡C-C≡N skeleton makes it both electrophilic and nucleophilic, as compared to C≡CC≡C skeleton On the other hand, the adjacent adatom-rest atom pair can also be considered as a strong dipole Thus, for the chemisorption of cyanoacetylene on Si(111)7×7, lower transition states are expected for the [4+2]-like cycloaddition between polar C≡C-C≡N and the neighboring adatom-rest atom pairs comparing with diacetylene/Si(111)- 7×7 system The resulting chemisorbed species, triply cumulative double bonds (C=C=C=N) or/and C=C-C≡C, can be precursors (or templates) for further construction of bilayer organic films on the semiconductor surfaces 127 The Binding of Multi-functional Organic Molecules on Silicon Surfaces Cyanoacetylene / Si(111)-7x7 110 K 2100 2289 3313 941 525 741 (d) 6.0 L (c) 3.0 L 3035 2010 -1 50 cm 1570 (b) 1.5 L x 1.5 (a) 0.5 L -500 500 1000 1500 2000 2500 3000 3500 4000 4500 -1 Wavenumber (cm ) Figure 5.1 HREELS spectra of cyanoacetylene on Si(111)-7×7 as a function of exposure at 110 K 128 The Binding of Multi-functional Organic Molecules on Silicon Surfaces Cyanoacetylene / Si(111)-7x7 741 ν(C≡C) ν(C≡N) Physisorption (a) 1205 C=C=C=N torison C=C=C=N bending νs (C=C=C=N) νa (C=C=C=N) 1570 ν(Si-N) 649 821 50 cm 3313 x2 ν(Si-C) 569 ν(sp) C-H 2100 2289 941 ν(C-C) 2010 ν(sp ) C-H 3035 525 -1 X3 Chemisorption (b) -500 500 1000 1500 2000 2500 3000 3500 4000 4500 -1 Wavenumber (cm ) Figure 5.2 HREELS spectra of the physisorbed and saturated chemisorption cyanoacetylene on Si(111)-7×7 129 The Binding of Multi-functional Organic Molecules on Silicon Surfaces N≡C C C Si Si Si Si Si Si Si Si =N H =C Si C=C= C N = = = HC≡C H Si Si Si Si Si (b) (a) Cyanoacetylene Mode I H (c) Cyanoacetylene Mode II Cyanoacetylene Mode III HC≡C C=C= C = = = = C Si Si Si Si Si H Si (d) Diacetylene Mode IV C =C Si Si Si Si H Si (e) Diacetylene Mode V Figure 5.3 Schematic diagram of the possible modes for cyanoacetylene and diacetylene covalently bound to Si(111)-7×7 130 The Binding of Multi-functional Organic Molecules on Silicon Surfaces (a) x3 2021 2181 650 439 634 1238 3341 ν(C≡C) 921 sp ν(C -H) 1626 1281 ν(C=C) 2147 sp ν(C -H) 3030 3334 x 1.5 (b) 500 1000 1500 2000 2500 3000 3500 4000 4500 -1 Wavenumber (cm ) Figure 5.4 The IR spectrum (scanning range 600-4000 cm-1) for gaseous diacetylene (Figure 5.4a) and the high-resolution electron energy loss spectrum of Si(111)-7×7 exposed to diacetylene (Figure 5.4b) at room temperature 131 The Binding of Multi-functional Organic Molecules on Silicon Surfaces Cyanoacetylene Mode I Cyanoacetylene Mode III Cyanoacetylene Mode II Diacetylene Mode IV Diacetylene Mode V Figure 5.5 Optimized C3NH / Si9H12 and C4H2 / Si9H12 clusters correspond the five possible attachment configurations through the [4+2]-lke and [2+2]-like cycloadditions 132 The Binding of Multi-functional Organic Molecules on Silicon Surfaces Table 5.1 Vibrational modes assignment for physisorbed and chemisorbed cyanoacetylene (CA) on Si(111)-7×7 IR data (Ref 18) for vapour cyanoacetylene (the vibrational frequencies are given in cm-1) Gas phase Calculation a 499 663 550 634 525 741 C-C stretch 955 912 941 C≡C stretch 2079 2081 2100 C≡N stretch 2274 2273 2289 ≡C-H stretch 3339 a present work 3304 3313 C≡C-C≡N bend C-C≡N bend C≡C-H bend Physisorbed CA on Si(111)-7×7a 247 Vibrational assignments Chemisorbed CA on Si(111)-7×7a Si-N stretch CA Gas Phase IR18 220 Vibrational assignments 569 Calculated vibrational frequencies for CA on Si(111)-7×7a 584 Si-C stretch C=C=C=N torsion C=C=C=N bending C=C=C=N symmetric stretch C=C=C=N asymmetric stretch -CH= stretch 649 821 667 805 1205 1189 1570 1566 2010 2022 3035 3040 133 The Binding of Multi-functional Organic Molecules on Silicon Surfaces Table 5.2 Vibrational modes assignment for gas phase diacetylene and chemisorbed diacetylene on Si(100)-2×1 (The vibrational frequencies are given in cm-1) Vibrational assignments Diacetylene Gas Phase IRa.b C≡C-H bend C≡C-C bend Si-C stretch C≡C-H bend C-C stretch ≡CH bend C=C stretch C≡C stretch =CH stretch ≡C-H stretch Previous IRc 220 483 634 1241 2021 (a) 2181(s) 2019(a) 2189(s) 439 650 626 1238 Chemisorbed Diacetylene on Si(111)-7×7a HREELS 3341 3332 921 1216 1638 2100 3033 3336 a Present work 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surface binding. . .The Binding of Multi- functional Organic Molecules on Silicon Surfaces with the functional groups in the molecule, offering the necessary flexibility in the functionalization and modification... of the possible configurations to aid the understanding of the reactivity and selectivity of these π-conjugated systems on Si(111)-7 ×7 1 25 The Binding of Multi- functional Organic Molecules on