Article pubs.acs.org/JPCC
Reaction of Hydroquinone and p-Benzoquinone with the Ge(100)-2 × 1 Surface Bonggeun Shong, Keith T. Wong, and Stacey F. Bent* Department of Chemical Engineering, Stanford University, 381 North-South Mall, Stanford, California 94305, United States S Supporting Information *
ABSTRACT: The adsorption behavior of two bifunctional molecules, hydroquinone (HO−C6H4−OH) and p-benzoquinone (OC6H4O), on the germanium (100) surface is studied. A combination of Fourier transform infrared spectroscopy and X-ray photoelectron spectroscopy experiments together with density functional theory calculations were used to identify the products. The distinct functionalities of the reactants result in different reaction pathways, namely, OH dissociation for hydroquinone and cycloaddition for p-benzoquinone. Yet, the major product formed, a dually tethered surface hydroquinoxy group with an aromatic ring (−OC6H4O−), is the same for both molecules. Dual dissociation of hydroquinone was highly selective in stereochemical configuration. Minor singly tethered products are also found for both precursors. The results suggest that selective attachment of aromatic groups on semiconductor surfaces can be achieved by choosing proper functionalities of the reactants.
1. INTRODUCTION Organic functionalization of semiconductor surfaces has been attracting increasing interest in recent years due to potential applications in various fields such as molecular electronics, sensors, and biocompatible devices.1−9 Reactions of organic molecules with 2 × 1 reconstructed (100) surfaces of Si and Ge mostly occur at the surface dimers, which have double bond and acid−base character.10 Analogies with solution phase organic chemistry of cycloadditions or Lewis acid−base reactions help understand these surface reactions. Attachment of bifunctional molecules is important due to the possibility of having one of the functionalities available for further modification of the surface, for example, by molecular layer deposition.11−13 Recent findings show that the bifunctional nature as well as the surface coverage alters the reactivity of each functional group from that of monofunctional analogues.14−17 Interestingly, some bidentate adsorbates on highly ordered, reconstructed semiconductor surfaces show stereoselectivity over available configurations.18−22 Furthermore, chemical attachment of an aromatic ring on a semiconductor surface is interesting not only in its intriguing chemistry23−25 but also in the possibility of constructing conductive organic nanostructures1 or a semiconducting passivation layer.5 Hydroquinone (HQ) can serve as an exemplar bifunctional aromatic molecule of which two OH moieties are separated by a rigid phenyl spacer. While the OH of monohydric alcohols completely dissociates on Ge(100),26,27 a portion of HQ adsorbates may have unreacted hydroxyl groups due to geometric hindrance. Potential surface products for adsorption of HQ on Ge(100) are shown in Figure 1. p-Benzoquinone (PBQ) provides for interesting comparison to HQ. Possible reaction products for adsorption of PBQ on Ge(100) are shown in Figure 2. Unlike HQ, the nonaromatic nature of the central ring may allow cycloaddition to occur for © 2012 American Chemical Society
Figure 1. Potential surface reactions of HQ with Ge(100).
PBQ on Ge(100). There have been many studies on cycloaddition of organic molecules on Si and Ge surfaces.5,28−31 Because of their similarity with organic reactions, the cycloaddition reactions are named after their organic analogues such as [2 + 2] or [4 + 2]. However, some surface reactions like [4 + 2] trans cycloaddition are specific to group IV semiconductor surfaces and are not observed in traditional organic chemistry.29 [6 + 2] cycloaddition of PBQ32 is an example of a surface-specific reaction. In the study of Ning and co-workers,32 inhomogeneity of the Si(111) surface complicated interpretation of the reaction pathway of PBQ. In contrast, the Ge(100)2 × 1 surface possesses higher controllability over the products9,29,33 and well-ordered reactive sites (surface dimers),10 enabling a clearer description of the reaction. Moreover, although the chemical properties of HQ and PBQ precursors are distinct, both [6 + 2] cycloaddition of PBQ and dual Received: November 9, 2011 Revised: January 15, 2012 Published: January 23, 2012 4705
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The Ge(3d) photoelectron peak was used as an internal standard for calibration of energy scale and peak intensity. Spectra were fit to pure Gaussians with a Shirley baseline, while chemically realistic numbers of components were kept to the same full width at half-maximum (fwhm) within each region. At room temperature and atmospheric pressure, hydroquinone (99.5%, Acros Organics) is a white solid, and p-benzoquinone (98%, Sigma-Aldrich) is a yellow solid. Each compound was further purified by repeated sublimation and pumping cycles before dosing and then exposed to the crystal through a variable leak valve. The molecular identity of each compound was confirmed by an in situ quadrupole mass spectrometer. Density functional theory calculations were carried out employing the Gaussian03 software package.36 The threeparameter hybrid B3LYP exchange-correlation functional was used with a 6-311++G(d,p) basis set for the adsorbate molecules and the Ge dimer atoms and LANL2DZ pseudopotential for subsurface Ge atoms. The terminating H atoms were modeled with a 6-31G(d) basis set. Ge15H16 two-dimer row, Ge23H24 two-dimer trench, and Ge39H32 four-dimer clusters were used to model the Ge(100) surface; usage of the Ge39H32 cluster was restricted to the diagonal-trench configuration. This way, the adsorption energies for different geometries could be calculated under conditions close to those on the 4-dimer cluster while keeping the computational cost manageable. For each cluster, only the top two Ge layers were allowed to relax from the ideal Ge crystal positions. Zero-point energy correction has been made to all reported energies. For unrestricted triplet calculations, ⟨S2⟩ values were checked to ensure a low degree of spin contamination, and all results had values very close to 2.00. In vibrational frequency calculations, these hydrogen atoms were assigned a mass of 74.0 amu to match the atomic mass of Ge. In the simulated FTIR spectra, the frequencies were scaled by a factor of 0.97.37 A Lorenzian line shape with a fwhm of 10 cm−1 and relative calculated intensities were used to represent IR bands.
Figure 2. Potential surface reactions of PBQ with Ge(100).
dissociation of HQ would yield the same hydroquinoxy (−OC6H4O−) moiety on the surface. In this study, we investigate the reaction of HQ and PBQ on the Ge(100)-2 × 1 surface. A combination of X-ray photoelectron spectroscopy (XPS) and Fourier transform infrared (FTIR) spectroscopy experiments with density functional theory (DFT) calculations is used to determine the products and reaction pathways. Our results show that both HQ and PBQ form the same dual bound hydroquinoxy adduct as the major product via completely different reactions: O−H dissociation and [6 + 2] cycloaddition, respectively. In addition, the selectivity among configurations for this product differed significantly due to distinct energetics of the reaction mechanisms.
2. EXPERIMENTAL AND COMPUTATIONAL DETAILS Experiments were conducted under ultrahigh vacuum (UHV) conditions. FTIR spectroscopy experiments were conducted in a previously described reaction chamber34 with a base pressure of ∼1 × 10−10 Torr. Briefly, the surface of a trapezoidal Ge(100) crystal (Harrick Scientific, 19 × 14 × 1 mm, 45° beveled edges) was cleaned via two cycles of Ar+ sputtering and annealing, and the 2 × 1 reconstruction was confirmed by lowenergy electron diffraction (LEED). Cleanliness of the sample was confirmed with Auger electron spectroscopy with levels of carbon, oxygen, and nitrogen below the detection limit. IR spectra were collected in multiple internal reflection geometry by a BioRad FTS-60A spectrometer equipped with a liquid nitrogen cooled mercury−cadmium−telluride detector. All spectra were manually corrected for baseline sloping. Absorption by the CaF2 viewports resulted in a low-frequency cutoff of ∼1050 cm−1. XPS studies of core-level binding energies were performed in a separate chamber that has also been described previously.35 The base pressure of this chamber is less than 2 × 10−10 Torr. The Ge(100) crystal (MTI Corp., 8 × 8 × 0.5 mm) was also cleaned by room temperature Ar+ sputtering followed by annealing. XPS of the clean samples confirmed that carbon, oxygen, and nitrogen levels were below the detection limit, and LEED verified the 2 × 1 reconstruction. Because of interference with the Ge Auger series, C(1s) and O(1s) photoelectron spectra were recorded with the Al and Mg anodes, respectively.
3. RESULTS AND DISCUSSION 3.1. DFT Calculation Results. Figure 3 shows energies of critical points along the pathway of the HQ/Ge(100) reaction. A HQ molecule can be bonded either singly or dually on the surface via successive OH dissociation of its two hydroxyl groups. Direct reaction of the phenyl ring with the surface is not considered because previous studies have shown that phenol exclusively undergoes OH dissociation on the more reactive Si(100),38 and benzene only physisorbs on Ge(100) without formation of covalent bonds to the surface.39 The adsorption energy (Eads) of the single OH dissociated adduct is large enough to persist as a final product. However, if an adjacent surface site is available for further reaction, dissociation of the second hydroxyl group can occur. Because of the ordering of the Ge(100) surface dimers, the dual dissociated product can assume several different geometries including cross-trench (CT), diagonal-trench (DT), end-bridge (EB), cross-bridge (CB), and on-top (OT). Definitions and distances between surface atoms in each configuration are shown in Figure 4. The energies beyond panel c on the Ge23H24 and Ge39H32 clusters were linearly shifted to match the Eads of the single dissociated product calculated with the Ge15H16 cluster. We note that calculations run to cross-check HQ critical point energies using the full Ge39H32 clusters returned values within 4706
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Figure 3. Reaction coordinate diagrams for the reaction of HQ on the Ge(100) surface. (a) Single O-dative bonding; (b) first OH dissociation transition state; (c) single OH dissociation; (d−g) second OH dissociation transition state with (d) CB, (e) DT, (f) EB, and (g) CT configurations; and (h−l) dual OH dissociation with (h) OT, (i) DT, (j) CB, (k) EB, and (l) CT configurations. Energy values are in kcal/mol.
The neighboring Ge dimers across the trench, i.e., the reactive sites for the CT configuration, are aligned in a way to facilitate the second attachment of a single dissociated HQ adduct, having a dimer−dimer distance (5.6 Å between the neighboring down atoms) close to the size of an HQ molecule (O−O distance 5.5 Å) and a favorable angle for the second proton transfer to proceed. As seen in Figure 4, the distances for the other configurations are either too small or too large to be well matched with the O−O distance in HQ. The least stable configuration is OT. Though the OT configuration was found for some other bifunctional aliphatic alcohols on Si(100),20 OT of HQ/ Ge(100) involves severe distortion of the adsorbate as well as the Ge dimer. Moreover, it proceeds via successive interdimer OH dissociation. As a result, OT has a relatively small Eads (−10.4 kcal/mol), and the second transition state could not be located despite multiple attempts. We attempt to quantify the origin of the dissimilarities in the adsorption energies and activation barriers between the different configurations by investigating the amount of distortion in each configuration. First, the adsorption energies (Eads) of dual dissociated configurations are less than twice that of a single dissociation Eads, and this destabilization is defined in eq 1 as
Figure 4. (a) Possible surface configurations of dual dissociation products; (b) O−O distances of reactant molecules (not to scale).
1−2 kcal/mol of those from the corrected calculations on the smaller clusters, supporting the use of this methodology. Since the same GeO and GeH bonds are formed with each configuration of the dual dissociated product, the energetic differences among configurations may originate from the deformation of the phenyl ring and C−O−Ge (or H) angular distortion. Indeed, the distortion of the adsorbate inhibits the second hydroxyl group from dative bonding with the surface when the first is dissociated. For example, the OH dissociated and O dative bonded structure of the CT configuration is endothermic by 10.6 kcal/mol compared to the single OH dissociation product. Considering that the formation of a single dative bond for HQ/Ge(100) is exothermic by 9.2 kcal/mol, there exists destabilization of nearly 20 kcal/mol due to forcing the adsorbate to take a CT configuration. Dissociated and dative bonded structures in the other configurations could not be optimized even after multiple attempts. Therefore, we did not include the second dative bonding in the reaction coordinate diagram. It is apparent in Figure 3 that the activation barriers and Eads depend strongly on the geometrical configuration. The pathway leading to CT (c−g−k) is favored over others both kinetically and thermodynamically. The CT transition state (Figure 3g) lies below the energy of the reactants, unlike for all other configurations. Also, the activation barriers of the second dissociation for all the configurations other than CT (21.5 kcal/mol) are larger than 35 kcal/mol, which is typically high enough to prohibit these pathways at room temperature.34
ΔEads = Eads(dual) − 2Eads(single)
(1)
ΔEads is then compared with distortion of the adsorbate (Edist(HQ)), which is defined as the difference between the electronic energy of a distorted molecule and that of the free molecule. Distorted HQ molecules are obtained by fixing atomic positions of the adsorbed hydroquinoxy moiety and substituting surface Ge atoms with H atoms at fixed directions. Another situation where it is useful to quantify distortion is through the activation barriers. The activation barriers of the second OH dissociations from the single dissociated product are higher than that of the fizrst reaction from the dative bonded state, and these barrier heights (Ea), defined in eq 2, can be correlated with the distortions of the adsorbates at their transition states. Ea = E(TS) − Eads(single) 4707
(2)
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The point with the greatest mismatch is ΔEads for the CB configuration (marked by an arrow). This product has the additional hindrance of having the phenyl ring overlapped with the surface H atom, and thus, the Ge cluster itself is more severely distorted, as in Figure S1, Supporting Information. The correlation between the parameters in Figure 5 implies that the energetic differences between the pathways largely originate from the distortion of the adsorbed HQ. On the basis of the calculation results for the reaction pathways of HQ/Ge(100), we anticipate that HQ will undergo single and dual OH dissociation. The second dissociation will occur mostly in the CT configuration; if no free Ge dimer is available next to a single dissociated species with CT geometry, the adsorbed HQ will remain singly tethered. Figure 6 shows selected possible pathways for the reaction of PBQ on Ge(100). As done above for HQ, the energies of the structures branching from the single O bonded structure (b, 1O) calculated with the Ge23H24 and Ge39H32 clusters were linearly shifted to match the Eads of 1O on the Ge15H16 cluster. Unlike HQ, the six-membered carbon ring of PBQ does not possess aromaticity and may also be involved in the reaction. However, the CH dissociation pathway (a−c−j) may be ruled out by its large barrier above the entrance channel. [2 + 2] cycloaddition with the CC double bond (a−d−i) may be feasible but will not be preferred over other pathways through intermediate (b), as Eads of the CC dative bonded intermediate (a) is small (−0.9 kcal/mol). Thus, CH dissociation (j) can be ruled out, and CC [2 + 2] cycloaddition (i) is deemed less likely to occur than pathways involving the 1O intermediate. We therefore assume that the PBQ/Ge(100) reaction proceeds through a single O bonded intermediate (1O, Figure 6b). This 1O structure is notable in particular. The most stable calculated structure has a triplet spin state with two delocalized unpaired electrons. In organic chemistry literature, quinone radicals (semiquinones) are found to be fairly stable intermediates in many redox reactions.41 Resonance with the dative bonded structure may further improve its stability. In the optimized structure, partial aromaticity is gained as the CO and the CC bonds have some double bond character manifested in vibrational frequencies (described below). The calculated Eads
Since OH dissociation on the (100) surfaces of Ge and Si has proton transfer character,34 the down atom of the surface dimer as well as the transferring H atom gain partial positive charge at the transition state.40 Thus, the transition states were modeled by Oprotonated HQ cations (HQH+), whose atomic positions of the hydroquinoxy moiety, as well as the angles of H atoms being transferred or substituting Ge atoms, were fixed to be those of the calculated transition state geometries. In this way, the difference in electronic energies of the distorted cations compared to those of the fully optimized HQH+ were obtained and referred to as Edist(HQH+). In Figure 5, ΔEads is plotted against Edist(HQ), and Ea is plotted against Edist(HQH+) for each configuration studied.
Figure 5. Blue dots: destabilizations of dual adsorption (ΔEads) versus distortion of adsorbed HQ molecules (Edist(HQ)). Red crosses: activation barriers of second OH dissociation (Ea) versus distortion of protonated HQ cation at transition states (Edist(HQH+)).
A strong correlation is observed for both Edist(HQ) vs ΔEads and Edist(HQH+) vs Ea, as all points are close to the y = x line.
Figure 6. Reaction coordinate diagrams for the reaction of PBQ on the Ge(100) surface. Energies (in kcal/mol) of (a) CC dative bonding and (b) single O bonding; (c−g) transition states for (c) CH dissociation, (d) CC [2 + 2] cycloaddition, (e) [6 + 2] cycloaddition (CT), (f) [4 + 2] cycloaddition (EB), and (g) CO [2 + 2] cycloaddition; and (h−l) products of (h) CO [2 + 2] cycloaddition, (i) CC [2 + 2] cycloaddition, (j) CH dissociation, (k) [4 + 2] cycloaddition (EB), and (l) [6 + 2] cycloaddition (CT). 4708
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of 1O is 19.4 kcal/mol, much larger than typical calculated dative bond strengths of carbonyl oxygen on the Ge(100) surface of 8−13 kcal/mol.29,33,42 Though these carbonyl dative bonded species are not observed at room temperature, other dative bonded adsorbates on the Ge(100) with calculated Eads of 18−20 kcal/mol have been detected at room temperature.24,27,43 Therefore, 1O may remain as a product, especially if the surface becomes crowded, thereby inhibiting further reaction. Multiple pathways branch from the 1O state, with small activation barriers ranging from 7.1 kcal/mol for [6 + 2] cycloaddition with EB configuration to 15.0 kcal/mol for [4 + 2] trans heterocycloaddition of the OC−CC moiety29 (abbreviated as [4 + 2]) with OT configuration. The energies of these critical points are summarized in Table 1; only a subset Table 1. Energies of Critical Points along Reaction Pathways Branching from 1O of PBQ/Ge(100)a kcal/mol [2 [4 [4 [6 [6 [6 [6 [6
+ + + + + + + +
2] 2] 2] 2] 2] 2] 2] 2]
CO OT EB OT DT CB EB CT
Ea
Eads
7.2 15.0 10.6 not determined 13.5 12.7 7.1 7.4
−15.2 −15.9 −29.0 −5.4 −21.6 −27.3 −28.1 −39.6
Figure 7. FTIR spectra of hydroquinone (HQ) on the Ge(100) surface; (a) dual dissociation (CT) calculation, (b) single dissociation calculation, (c) chemisorption saturation at 310 K, and (d) physisorbed multilayer.
Figure 7. The spectrum of unreacted physisorbed multilayers (d) corresponds well with IR spectra of solid HQ in the literature.44 Upon chemisorption (c), the modes involving motion of the OH group (i.e., 1378 and 3252 cm−1) disappear along with the appearance of a GeH stretch at 1989 cm−1 indicating that an OH dissociation reaction occurred. In the 1100−1600 cm−1 region of the chemisorbed spectra, only 4 modes comprised of two major peaks, each accompanied by blue-shifted minor peaks, are discerned. From XPS analysis in the next section, we determine that there is more dual dissociation than single dissociation of HQ at saturation. Thus, we assign the two major peaks at 1190 and 1485 cm−1 to the asymmetric ν(CO) + βH + α and βH + ν(CC) + ν(CO) combination modes of dual dissociated HQ calculated at 1186 and 1467 cm−1, respectively. Agreement with the calculated spectrum in Figure 7a is good with the exception of additional peaks at 1230 and 1589 cm−1, which are symmetric ν(CO) + ν(CC) and ν(CC) + βH + α modes. These features correspond to the 1261 and 1612 cm−1 Raman active modes and IR inactive modes of pure HQ,44 respectively. Although our cluster model predicted nonzero IR absorbances, these peaks should have low IR intensity because the adsorbed molecule is only slightly distorted from centrosymmetric, yielding a small net change in dipole moment for these symmetric modes.45 The minor peaks at 1209 and 1499 cm−1 in the chemisorption spectrum are blue-shifted 19 and 14 cm−1 from the respective major peaks. In the calculated single dissociated HQ spectrum (Figure 7b), the asymmetric ring stretch modes are blue-shifted 23 and 16 cm−1 from those in the dual dissociated HQ spectrum. These shifts can be explained by substitution of a surface Ge atom in the dual dissociated structure with a much smaller H atom in the single bound adsorbate. Since the DFT calculations suggest the single dissociated product also as a stable product, we assign these peaks to single dissociated HQ. Full assignment of the IR spectra can be found in Table S1, Supporting Information. It was suggested by the calculated reaction pathways that the second OH dissociation of HQ would proceed mostly with CT configuration. Calculated IR spectra for dual dissociation with
a
Ea is the energetic difference between 1O and each transition state. The transition state leading to [6 + 2] OT could not be determined.
are shown in Figure 6 for clarity. Energies of all these transition states are below that of the reactants and may be accessible if thermal accommodation at 1O is incomplete. The [6 + 2] reaction barriers toward the EB and CT configurations are the lowest among the pathways, and thus, the [6 + 2] product with EB and CT configurations are kinetically favored. However, surface reactions on Ge(100) tend to be under thermodynamic control,29,33 and CT will be preferred due to its higher adsorption energy. The structures less stable than 1O (CO [2 + 2], [4 + 2] OT, and [6 + 2] OT) will have negligible probability of existence when equilibrated. However, the [6 + 2] cycloaddition product with other configurations (EB, CB, and DT) and the [4 + 2] EB product may coexist with [6 + 2] CT, as their adsorption energies are sufficiently large. Ning and coworkers32 also found that the [6 + 2] product is most exothermic for PBQ/Si(111), followed by the [4 + 2] product. The transition state energies of these cycloaddition reactions are less affected by the configurations compared to the dual OH dissociation of HQ. However, the trend observed for Eads of the different [6 + 2] configurations (CT > EB > CB > DT ≫ OT) resembles that of HQ adsorption. We expect that the energetic differences among [6 + 2] configurations are again caused by distortion of the hydroquinoxy adsorbate. In summary, on the basis of DFT calculation results, we conclude that PBQ/Ge(100) will form [6 + 2] CT as a major product. However, unlike the high selectivity expected for HQ/ Ge(100), some minor products such as [6 + 2] EB, CB, and DT; [4 + 2] EB; and CC [2 + 2] may form as well. Also, 1O may be present at higher coverages when the surface becomes crowded. 3.2. IR Spectroscopy Results. Experimental and calculated FTIR spectra of HQ on the Ge(100) surface are shown in 4709
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the same surface hydroquinoxy group as dual dissociated HQ/ Ge(100), gaining aromaticity that does not exist in the PBQ molecule. Magnified regions of the chemisorption spectra where the major peaks are located are shown in Figure 8g, in comparison with HQ/Ge(100). The integrated PBQ spectral features are larger than those of HQ, suggesting higher saturation coverage for PBQ. The peaks assigned to dual dissociated HQ (1485 and 1190 cm−1) are also present in the PBQ/Ge(100) spectrum at nearly the same frequencies (1487 and 1189 cm−1). These major IR modes were also present in vibrational analysis of PBQ/Si(111) at similar frequencies.32 The calculated IR spectrum of the [6 + 2] product (Figure 8d) is also very similar to the calculated HQ dual dissociation spectrum, except for 1 cm−1 shifts and the absence of the ν(GeH). Thus, it is apparent that [6 + 2] cycloaddition with CT configuration is the major pathway for PBQ/Ge(100). We note that the [6 + 2] product is found at a temperature as low as 170 K; IR spectra showing successive chemisorption followed by multilayer growth at 170 K are shown in Figure S2, Supporting Information. This result confirms the low activation barrier for the [6 + 2] cycloaddition reaction suggested from DFT. However, multiple smaller peaks are also present in the PBQ/Ge(100) chemisorption spectra. First, there are several red-shifted shoulders beside the major 1487 and 1189 cm−1 peaks, causing significantly broader peak shapes than seen for HQ. These red-shifts are found in the calculated spectra of [6 + 2] products in configurations other than CT. The βH + ν(CO) + α mode calculated at 1187 cm−1 for the CT configuration is shifted to 1165, 1161, and 1159 cm−1 in EB, CB, and DT configurations, respectively. Also, the βH + ν(CC) + ν(CO) mode at 1468 cm−1 for CT appears at 1457, 1452, and 1463 cm−1 for EB, CB, and DT. Since the calculated reaction pathways toward [6 + 2] products with multiple configurations were energetically viable, we assign these red-shifted peaks to other configurations of [6 + 2] cycloaddition. Though it is difficult to quantify each configuration from IR data, at least two configurations other than CT must have been formed to explain the two red-shifted features found next to the 1189 cm−1 peak. Small features at 1592 and 1651 cm−1 are also found at wavenumbers above the phenyl ring stretching region. Other possible adsorption structures are CC [2 + 2] (Figure 8a) and [4 + 2] EB (Figure 8b), both possessing conjugated CC and CO moieties. Calculated carbonyl stretches of these structures are found at 1659 and 1641 cm−1, respectively, and both are possible explanations for the 1651 cm−1 peak. However, the ν(CC) of the CC [2 + 2] product calculated at 1605 cm−1 has a very low IR absorption crosssection. Only the [4 + 2] EB product has a ν(CC) mode with appreciable intensity at 1599 cm−1, near the experimentally observed peak at 1592 cm−1. Thus, we assign these peaks to conjugated vibrational modes of the [4 + 2] product. We can infer that the fraction of the [4 + 2] product is minor since the intensity of the 1651 cm−1 peak is low. A carbonyl is a strong IR absorbing group; for example, in the chemisorption spectrum of phenyl isocyanate/Ge(100), ν(CO) had nearly the same intensity as the ring stretching modes.31 Therefore, the [4 + 2] product cannot explain several other minor peaks with intensities greater than the 1651 cm−1 carbonyl peak. The calculated peaks for the [6 + 2] product at 1230 and 1587 cm−1 are symmetric stretches and are not expected experimentally in the IR spectrum, as described above for dual dissociated HQ/Ge(100). Instead, as-yet unassigned peaks on
configurations other than CT presented red-shifts of the two ring stretch modes ranging from 4 to 29 cm−1; if minor configurations exist, they will cause broadening or shouldering at lower wavenumbers. However, the peaks found in the HQ chemisorption IR spectrum are sharp with narrow fwhm and no appreciable red-shifted features. This indicates that the adsorption of HQ indeed prefers only a certain configuration, possibly dominated by CT. The peak broadening effect of multiple configurations is discussed more in detail with comparison to the PBQ/Ge(100) IR spectra below. Figure 8 shows the FTIR spectra of PBQ on Ge(100). The multilayer spectrum of PBQ (Figure 8f) corresponds well with
Figure 8. FTIR spectra of PBQ on Ge(100); (a) CC [2 + 2] calculation, (b) [4 + 2] EB calculation, (c) 1O calculation, (d) [6 + 2] CT calculation, (e) chemisorption saturation at 310 K, and (f) physisorbed multilayers; (g) magnified spectra near 1485 cm−1 and 1190 cm−1 of PBQ and HQ on Ge(100).
solid-phase PBQ IR spectra in the literature.46,47 Importantly, the chemisorption spectrum (Figure 8e) closely resembles the spectrum of chemisorbed HQ/Ge(100) and is very different from the multilayer spectrum. This result indicates fundamental modification of the molecular structure upon chemisorption. Full assignments of the IR spectra are found in Table S2, Supporting Information. The major peaks are explained by formation of the [6 + 2] cycloaddition product, which creates 4710
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Figure 9. C(1s) and O(1s) core-level photoelectron spectra of (a) HQ/Ge(100) and (b) PBQ/Ge(100).
oxygen.50 To further clarify the assignment of the minor peaks, we compared O(1s) BEs of 32 chemically different oxygen atoms directly bonded to conjugated rings of 29 gaseous molecules in the literature (Table S3, Supporting Information).49,51−53 The average O(1s) BE of 17 sp3 oxygen atoms was 539.3 ± 0.3 eV, while that of 15 carbonyl (sp2) oxygen atoms was 536.9 ± 0.4 eV, 2.4 eV below sp3 oxygen. For HQ, the single dissociated structure is the sole assigned minor product, and the minor peak therefore corresponds to free (sp3) oxygen of the single dissociated adduct. However, in the IR analysis of PBQ/Ge(100), a mixture of minor [4 + 2] and 1O products with a major [6 + 2] product was suggested. Each of these minor products has one unbound CO moiety. While the [4 + 2] cycloaddition product is a conjugated ketone, 1O was suggested to possess semiquinone-like partial aromaticity and a CO bond order close to 1. Therefore, CO of the [4 + 2] product has carbonyl character, while the free oxygen of 1O is sp3 hybridized and would behave closer to those of alcohols or ethers. The O(1s) at 532.3 eV of PBQ/ Ge(100) is thus assigned to the free oxygen in 1O that lacks bonding with Ge. O(1s) of chemisorbed PBQ has yet another peak at 529.1 eV, shifted −1.5 eV from the major peak. We assign this feature to the sp2 carbonyl oxygen of the [4 + 2] product. The fractions of higher BE O(1s), assigned to single-bound species, relative to the entire O(1s) signal, are 0.21 for PBQ and 0.24 for HQ at saturation. Since one of two oxygen atoms in the single bonded species is free and the other one is incorporated in the surface bound oxygen, the fraction of the single bonded species is simply twice the fraction of free oxygen. Therefore, 48% of adsorbed HQ molecules were single dissociated at saturation, while 52% were dual dissociated. In other words, the ratio of single dissociated species to dual dissociated species (nsingle/ndual) of HQ is 0.94. However, the carbonyl oxygen of PBQ accounted for 6% of the total area, which corresponds to 12% of the adsorbates. Therefore, the [6 + 2] product comprised 46%, and 1O comprised 42%. Thus, nsingle/ndual of PBQ considering the [6 + 2] and 1O products is 0.93. Total coverages of the adsorbates were calculated by comparing the integrated area of the C(1s) peak to that of saturated pyridine/Ge(100), which is known to occupy 0.25 ML at saturation (1 ML = one adsorbate molecule per surface atom).54 The saturation coverages of HQ and PBQ were determined to be 0.25 and 0.43 ML, respectively. The larger saturation coverage of PBQ is consistent with the higher IR absorbance noted in Figure 8. It can be explained by recognizing that HQ adsorption also deposits H atoms at the surface, using up more surface sites per adsorption event. In summary, analysis of XPS of HQ and PBQ confirmed the formation of the products suggested by DFT calculations and
the chemisorbed PBQ spectrum at 1212, 1242, 1368, 1443, 1501, and 1572 cm−1 all match well with the calculated spectrum of 1O, and hence, we assign them to the 1O product. The weaker intensities of the 1O peaks compared to those of [6 + 2] suggest that this product only accounts for a small fraction of adsorbates. Summarizing the IR analysis, we find that dual dissociation of HQ/Ge(100) occurs and that its adsorption is dominated by the CT configuration. Single dissociated HQ also coexists as a minor product. For PBQ/Ge(100), the [6 + 2] cycloaddition product was determined as the major product but with multiple configurations, possibly including CT, EB, CB, and DT. 1O and [4 + 2] EB are present as minor products. 3.3. XPS Results. Figure 9 shows high resolution XPS spectra of HQ/Ge(100) and PBQ/Ge(100) at saturation coverages. The spectra are similar for the two adsorbates, suggesting similar chemical environments of the adsorbate atoms after reaction. Both C(1s) regions showed two peaks with relative intensity of 2:1. The relative chemical shift of 1.5 eV between the two peaks is close to the 1.6 eV split found in photoelectron spectra of pure HQ, while clearly distinguishable from the 2.6 eV split of pure PBQ.48,49 Thus, it suggests formation of a surface hydroquinoxy moiety for adsorption of both HQ and PBQ. The higher binding energy (BE) peak at 285.1 eV is assigned to C bonded to O and the lower BE (283.8 eV) peak to C bonded only to C. We could not further distinguish the configurations from the C(1s) XPS data. Evidence of GeC bond formation is not explicitly found in the C(1s) spectra. However, the C(1s) peak of hydrocarbons chemisorbed on the Ge(100) surface is found at 283.5−285.0 eV,28 which overlaps with the PBQ/Ge(100) C(1s) peaks. If only a small amount of the [4 + 2] product is present as suggested by the IR analysis, its C(1s) signal would not be easily distinguished from those of [6 + 2] and 1O. However, the O(1s) spectra show different behavior between the reactants. There are peaks at 530.8 eV for HQ and 530.6 eV for PBQ as a major component. These BEs match those of Ge surface-bonded oxygen atoms found in the literature.15−17 Furthermore, adsorbate oxygen atoms with quite different chemical environments give almost equal BEs if they are covalently bonded to a semiconductor surface.30 Therefore, we assign these major peaks to oxygen bonded to the Ge surface, regardless of the product structure and configuration. For PBQ, the surface oxygen includes two oxygen atoms per [6 + 2] adsorbate, and one oxygen atom per molecule in 1O and [4 + 2] structures. A second component at higher BE (shifts from major peaks of 1.9 eV for HQ, 1.7 eV for PBQ) exist for both adsorbates. It is known that 1s electrons of sp3 hybridized oxygen with a similar chemical environment show a higher BE than that of sp2 4711
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(5) Avasthi, S.; Qi, Y.; Vertelov, G. K.; Schwartz, J.; Kahn, A.; Sturm, J. C. Appl. Phys. Lett. 2010, 96, 222109. (6) Bent, S. F.; Kachian, J. S.; Rodríguez-Reyes, J. C. F.; Teplyakov, A. V. Proc. Natl. Acad. Sci. U.S.A. 2011, 108, 956. (7) Bent, S. F. Surf. Sci. 2002, 500, 879. (8) Hamers, R. J. Annu. Rev. Anal. Chem. 2008, 1, 707. (9) Kachian, J. S.; Wong, K. T.; Bent, S. F. Acc. Chem. Res. 2010, 43, 346. (10) Zandvliet, H. J. W. Phys. Rep. 2003, 388, 1. (11) Kim, A.; Filler, M. A.; Kim, S.; Bent, S. F. J. Am. Chem. Soc. 2005, 127, 6123. (12) Loscutoff, P. W.; Lee, H.-B.-R.; Bent, S. F. Chem. Mater. 2010, 22, 5563. (13) George, S. M.; Yoon, B.; Dameron, A. A. Acc. Chem. Res. 2009, 42, 498. (14) Kim, A.; Filler, M. A.; Kim, S.; Bent, S. F. J. Phys. Chem. B 2005, 109, 19817. (15) Lee, H.; Youn, Y.-S.; Kim, S. Langmuir 2009, 25, 12574. (16) Kachian, J. S.; Bent, S. F. J. Phys. Chem. C 2010, 114, 22230. (17) Yang, S.; Kim, Y.; Park, S.; Lim, H.; Lee, H. J. Phys. Chem. C 2011, 115, 9131. (18) Shachal, D.; Manassen, Y.; Ter-Ovanesyan, E. Phys. Rev. B 1997, 55, 9367. (19) Harikumar, K. R.; Lim, T.; McNab, I. R.; Polanyi, J. C.; Zotti, L.; Ayissi, S.; Hofer, W. A. Nat. Nanotechnol. 2008, 3, 222. (20) Bae, S.-S.; Kim, K.-J.; Lee, H.-K.; Lee, H.; Kang, T.-H.; Kim, B.; Kim, S. Langmuir 2009, 26, 1019. (21) Mathieu, C.; Bai, X.; Gallet, J.-J.; Bournel, F.; Carniato, S.; Rochet, F.; Magnano, E.; Bondino, F.; Funke, R.; Köhler, U.; Kubsky, S. J. Phys. Chem. C 2009, 113, 11336. (22) Hahn, J. R.; Bharath, S. C.; Jeong, S.; Pearl, T. P. J. Chem. Phys. 2011, 134, 044704. (23) Tao, F.; Bernasek, S. L.; Xu, G.-Q. Chem. Rev. 2009, 109, 3991. (24) Wang, G. T.; Mui, C.; Tannaci, J. F.; Filler, M. A.; Musgrave, C. B.; Bent, S. F. J. Phys. Chem. B 2003, 107, 4982. (25) Coustel, R.; Carniato, S.; Boureau, G. J. Chem. Phys. 2011, 134, 234708. (26) Bae, S.-S.; Kim, D. H.; Kim, A.; Jung, S. J.; Hong, S.; Kim, S. J. Phys. Chem. C 2007, 111, 15013. (27) Kachian, J. S.; Bent, S. F. J. Am. Chem. Soc. 2009, 131, 7005. (28) Lee, S. W.; Hovis, J. S.; Coulter, S. K.; Hamers, R. J.; Greenlief, C. M. Surf. Sci. 2000, 462, 6. (29) Wang, G. T.; Mui, C.; Musgrave, C. B.; Bent, S. F. J. Am. Chem. Soc. 2002, 124, 8990. (30) Huang, J. Y.; Shao, Y. X.; Huang, H. G.; Cai, Y. H.; Ning, Y. S.; Tang, H. H.; Liu, Q. P.; Alshahateet, S. F.; Sun, Y. M.; Xu, G. Q. J. Phys. Chem. B 2005, 109, 19831. (31) Loscutoff, P. W.; Wong, K. T.; Bent, S. F. J. Phys. Chem. C 2010, 114, 14193. (32) Ning, Y. S.; Shao, Y. X.; Xu, G. Q. J. Phys. Chem. C 2010, 114, 10455. (33) Wang, G. T.; Mui, C.; Musgrave, C. B.; Bent, S. F. J. Phys. Chem. B 2001, 105, 12559. (34) Mui, C.; Han, J. H.; Wang, G. T.; Musgrave, C. B.; Bent, S. F. J. Am. Chem. Soc. 2002, 124, 4027. (35) Filler, M. A.; Van Deventer, J. A.; Keung, A. J.; Bent, S. F. J. Am. Chem. Soc. 2005, 128, 770. (36) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Montgomery, J. A., Jr.; Vreven, T.; Kudin, K. N.; Burant, J. C.; Millam, J. M.; Iyengar, S. S.; Tomasi, J.; Barone, V.; Mennucci, B.; Cossi, M.; Scalmani, G.; Rega, N.; Petersson, G. A.; Nakatsuji, H.; Hada, M.; Ehara, M.; Toyota, K.; Fukuda, R.; Hasegawa, J.; Ishida, M.; Nakajima, T.; Honda, Y.; Kitao, O.; Nakai, H.; Klene, M.; Li, X.; Knox, J. E.; Hratchian, H. P.; Cross, J. B.; Bakken, V.; Adamo, C.; Jaramillo, J.; Gomperts, R.; Stratmann, R. E.; Yazyev, O.; Austin, A. J.; Cammi, R.; Pomelli, C.; Ochterski, J. W.; Ayala, P. Y.; Morokuma, K.; Voth, G. A.; Salvador, P.; Dannenberg, J. J.; Zakrzewski, V. G.; Dapprich, S.; Daniels, A. D.; Strain, M. C.; Farkas, O.; Malick, D. K.; Rabuck, A. D.; Raghavachari,
FTIR spectroscopy. We were able to quantify the fractions of each surface product. HQ/Ge(100) forms single and dual OH dissociation products at a saturation ratio of nearly 1:1, whereas for PBQ/Ge(100), [6 + 2], 1O, and [4 + 2] products are found, with a saturation ratio of approximately 1:1:0.3. For both reactions, the major product was the hydroquinoxy moiety on the surface. Singly tethered species were found with surface densities close to those of the hydroquinoxy adsorbates for both molecules, although the calculation results predicted that the dual bonded hydroquinoxy structures are energetically more favorable. We propose that crowding of the surface would inhibit the second attachment of the bifunctional precursor, especially when the adjacent reactive sites (Ge dimers) with a preferable configuration are blocked by already adsorbed molecules.
4. CONCLUSIONS We have performed infrared spectroscopy, X-ray photoelectron spectroscopy, and density functional theory calculation studies of the reactive adsorption of HQ and PBQ on the Ge(100) surface. HQ undergoes OH dissociation, while PBQ reacts via cycloaddition. Despite the different reaction pathways, the same surface hydroquinoxy adduct is formed in reactions of both HQ and PBQ. Fundamental insights into selectivity and mechanisms of surface reactions can be drawn from our findings: (1) dual dissociation of HQ and [6 + 2] cycloaddition of PBQ occur with a dominant configuration (CT) for which alignment and distance of surface sites are suitable for the second reaction to occur; (2) [6 + 2] and [4 + 2] heterocycloadditions of PBQ proceed through an intermediate involving oxygen bonded to the down Ge atom; and (3) for both molecules, the ratio of dually tethered to singly tethered products at saturation is approximately 1:1.
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ASSOCIATED CONTENT
S Supporting Information *
Calculated structures of HQ adsorbates (Figure S1), complete assignment of the IR spectra (Tables S1 and S2), PBQ IR spectra at 170 K (Figure S2), and gas-phase O(1s) BEs of aromatic molecules (Table S3). This material is available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*Phone: 650-723-0385. Fax: 650-723-9780. E-mail: sbent@ stanford.edu. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was supported by the National Science Foundation (CHE 0910717). B.S. acknowledges fellowship support from the Samsung Scholarship.
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REFERENCES
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