Reaction of Phenyl Isocyanate and Phenyl Isothiocyanate with the Ge

Aug 2, 2010 - The adsorption of phenyl isocyanate and phenyl isothiocyanate at the Ge(100)-2 × 1 surface was probed using multiple internal reflectio...
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J. Phys. Chem. C 2010, 114, 14193–14201

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Reaction of Phenyl Isocyanate and Phenyl Isothiocyanate with the Ge(100)-2 × 1 Surface Paul W. Loscutoff, Keith T. Wong, and Stacey F. Bent* Department of Chemical Engineering, Stanford UniVersity, 381 North-South Mall, Stanford, CA 94305 ReceiVed: May 14, 2010; ReVised Manuscript ReceiVed: July 9, 2010

The adsorption of phenyl isocyanate and phenyl isothiocyanate at the Ge(100)-2 × 1 surface was probed using multiple internal reflection Fourier transform infrared (FTIR) spectroscopy, density functional theory (DFT), and X-ray photoelectron spectroscopy (XPS). Results for phenyl isocyanate indicate that the saturation product is a [2 + 2] cycloaddition reaction product across the CdN bond, forming a surface-bound carbonyl. DFT reveals that this is the most stable of the expected products, and its calculated vibrational spectrum agrees closely with experimental spectra. FTIR spectroscopy studies of phenyl isothiocyanate reveal multiple surface adducts, with the product of the [2 + 2] cycloaddition reaction across the CdS bond as the major species and the product of the [2 + 2] cycloaddition reaction across the CdN bond as a minor product. In addition to these products, an excess of sulfur measured by XPS indicates the presence of a dissociative desorption product. The DFT calculated pathways and vibrational spectra for phenyl isothiocyanate confirm the FTIR and XPS assignments, and suggest the 1,3-dipolar cycloaddition product as a potential pathway for dissociative desorption. I. Introduction Organic functionalization of semiconductors is a field that has observed continued growth over the past decade. Creating interfaces between inorganic and organic functionalities is a vital step for numerous applications, from organic semiconductors to biological and chemical sensors, to molecular patterning for electronics. The direct attachment of organic molecules to semiconductors has been studied on a variety of surfaces, with specific attention to the (100)-2 × 1 reconstructed surfaces of silicon and germanium.1–7 These surfaces are highly reactive, with a dimer bond that displays both double bond and zwitterion properties. This surface characteristic leads to interesting analogues with organic chemistry, in that the dimer bond can show similar reactivity to an olefin or a carbonyl, depending on the specific surface reaction. Germanium has gained increased interest due to its high electron mobility, and it is currently under investigation for use as a channel material in transistors.8 In organic functionalization, the Ge(100)-2 × 1 surface often demonstrates a higher degree of selectivity of products than does the Si(100)-2 × 1 surface, allowing for increased control with the germanium surface.9 This selectivity stems from the weaker bonding observed for most organic molecules to the Ge(100)-2 × 1 surface compared to Si(100)-2 × 1. As an example, Wang calculates that the Ge-C bond is 7-9 kcal/mol weaker than the Si-C bond.10 This weaker binding of products allows for thermodynamic control of the product distribution, with the most energetically favorable product presenting as the majority product. Recent studies have revealed interesting reactivity trends on the Ge(100)-2 × 1 surface when comparing molecules containing oxygen with those containing sulfur.7,11 Specifically, dissociative adsorption of ethanethiol and dative bonding of diethyl sulfide on the Ge(100)-2 × 1 surface are kinetically and thermodynamically favored over the same reactions of their oxygen analogues, demonstrating strong bonding between the S atoms and Ge surface.11 To further probe the differences * Corresponding author. E-mail: [email protected].

between sulfur and oxygen reactivity at the Ge(100)-2 × 1 surface, we have studied the reactions of another set of the congeners in the isocyanate and isothiocyanate functional groups. Isocyanates and isothiocyanates are highly reactive organic moieties which include a cumulated double bond. This cumulated double bond allows for several possible reactions with the Ge(100)-2 × 1 surface, and the high reactivity of the organic moieties will likely lead to facile reactions. Figure 1 shows possible reaction products for the isocyanate and isothiocyanate groups with the Ge dimer. These include [2 + 2] cycloaddition products by reaction across the CdX double bond or the CdN double bond, a 1,3-dipolar cycloaddition product across the XdCdN cumulated double bond, and dative-bonded products through both the N and X atoms, where X represents O or S in the isocyanate and isothiocyanate group, respectively. Here, we investigate the product distribution of phenyl isocyanate and phenyl isothiocyanate on the Ge(100)-2 × 1 surface. Among the possible reaction products shown in Figure 1, the cycloaddition products are of interest because they would allow for a conjugated bonding system attached directly to the surface, if R were chosen as a conjugated moiety. Direct attachment of a conjugated system, such as the phenyl group used in this study, to the surface may allow for transfer of electrons across the organic/inorganic interface. In addition, the possibility exists to extend the conjugation beyond a simple phenyl ring to much more elaborate systems, by using molecular layer deposition (MLD) chemistry to build the desired functionality from the substrate.12–14 Thus, the isocyanate and isothiocyanate molecules contain the potential for a rich surface chemistry with prospective applications as inorganic/organic interfaces for molecular electronics. Interestingly, despite the many possible products, studies on the Si(100)-2 × 1 surface have shown that phenyl isothiocyanate undergoes a highly selective reaction leading to the formation of a [2 + 2] cycloaddition product across the CdN double bond,15 while Hamers et al. note that the reactions of isocyanates are difficult to analyze due to decomposition at the surface.6 The Ge(100)-2 × 1 surface dimer is a favorable inorganic

10.1021/jp104388a  2010 American Chemical Society Published on Web 08/02/2010

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Figure 1. Potential surface products for the isocyanate and isothiocyanate groups reacting with the Ge(100)-2 × 1 surface dimer.

reactant, due to the high degree of surface ordering and product selectivity. We show in the present study that the reactivity trends are reversed between phenyl isocyanate and phenyl isothiocyanate at the Ge(100)-2 × 1 surface; namely, the major product for phenyl isocyanate is a [2 + 2] cycloaddition product across the CdN bond, whereas the major product for phenyl isothiocyanate is a [2 + 2] cycloaddition product across the CdS bond, and the [2 + 2] cycloaddition adduct across the CdN bond is only a minor product. We propose that the greater strength of the GesS bond than the GesO bond causes these differences in reactivity. Additionally, the strength of GesS bonds leads to a dissociative desorption product for phenyl isothiocyanate in which atomic sulfur is left at the surface. These results have implications for future studies of organosulfur compounds adsorbed on germanium surfaces. II. Experimental Section Experiments were performed in ultrahigh vacuum on the clean, reconstructed Ge(100)-2 × 1 surface. Fourier transform infrared (FTIR) spectroscopy studies were carried out in a chamber that has been described in detail previously.16 The base pressure in the chamber was less than 1 × 10-10 Torr. The reconstructed surface was prepared by argon ion sputtering (20 mA emission current at 0.5 keV accelerating voltage with 7-8 µA sample current) for 20 min at room temperature followed by annealing to 900 K for 5 min. The cleaned surface had undetectable levels of carbon, oxygen, and nitrogen by Auger electron spectroscopy, and the 2 × 1 reconstruction was confirmed by low-energy electron diffraction (LEED). IR spectra were collected on a BioRad FTS-60A spectrometer, in a multiple internal reflection (MIR) geometry using a narrow band mercury-cadmium-telluride (MCT) detector. The Ge crystal substrate was a trapezoid with dimensions of 19 mm × 14 mm × 1 mm and 45° beveled edges (Harrick Scientific). Spectra were baseline corrected using linear functions derived between points lacking spectral features. Multilayer, physisorbed spectra are collected with the substrate held at low temperatures to allow for condensation of the molecules at the surface, while chemisorbed spectra are collected with the substrate at room temperature to prevent condensation. In some cases, spectra were collected at 310 K to allow for a tighter control over substrate temperature, which resulted in an increase of the signal-to-noise ratio without changing the adsorption behavior from that at room temperature.

X-ray photoelectron spectroscopy (XPS) studies were performed in a separate chamber, which has also been described previously.17 The base pressure of this chamber was less than 2 × 10-10 Torr. Samples were approximately 8 mm × 8 mm × 0.5 mm (MTI Corp.) in size and were heated radiatively by a tungsten-rhenium filament heater, which was coated with sapphire beads and positioned behind the sample plate. The samples were cleaned by argon ion sputtering for 30 min (20 mA emission current at 1 keV accelerating voltage for 12 µA sample current, with an incident ion beam at 12° from the surface normal) at room temperature, followed by annealing to 900 K for 15 min. The difference in sputtering parameters between the XPS and FTIR chambers was due to the difference in chamber geometries and ion beam focus. XPS of the clean samples confirmed undetectable levels of carbon, nitrogen, and oxygen, and LEED verified the 2 × 1 reconstruction. Spectra were collected with a pass energy of 25 eV, with the energy scale and peak intensity calibrated using the Ge(3d) photoelectron peak. Spectra were fit to pure Gaussians with a Shirley baseline and realistic number of components held to the same fwhm for each spectrum. Phenyl isocyanate (g99% Sigma-Aldrich) and phenyl isothiocyanate (g99% Sigma-Aldrich), both clear liquids at room temperature and atmospheric pressure, were transferred to sample vials under dry air. Compounds were purified by several freeze-pump-thaw cycles and then exposed to the crystal through a variable leak valve. Exposures are reported in Langmuir (1 L ) 10-6 Torr · s), and pressures were not corrected for ionization gauge sensitivity. The molecular identity of each compound was confirmed by an in situ quadrupole mass spectrometer. Quantum chemical calculations were carried out using the Gaussian 03 software suite,18 using Becke3 Lee-Yang-Parr (B3LYP) three-parameter density functional theory (DFT). Previous studies of B3LYP indicate that it provides predictive results for similar systems, and that the results agree well with experimental results that are available.9,16,17,19 A single-dimer Ge9H12 cluster was used to model the surface in most cases, allowing for intradimer reactions to occur. In several calculations, Ge23H24 two-dimer trench clusters or Ge15H16 two-dimer row clusters were used to model interdimer reactions. The Ge dimer atoms and adsorbate atoms were modeled using the triple-ζ 6-311++G(d,p) basis set. Subsurface Ge atoms were modeled using the LANL2DZ pseudopotential, which keeps calculations computationally practical and provides sufficiently

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Figure 2. FTIR spectra of phenyl isocyanate (PIC) on the Ge(100)-2 × 1 surface, showing a chemisorbed monolayer and physisorbed multilayer. DFT calculated vibrational spectra for the expected reaction products are given for comparison.

accurate results. The subsurface Ge-cluster atoms were terminated with hydrogens to fill their valence and approximate neighboring Ge atoms, with the hydrogens modeled using the 6-31G(d) basis set. In all frequency calculations, the hydrogens were assigned a mass of 74.0 amu, to match the mass of Ge atoms and prevent false Ge-H vibrations. All single-dimer calculations were carried out without geometric constraints, while the two-dimer calculations have all atoms except the top two planes of atoms (including the dimers) frozen to prevent nonphysical dimer distortion. Local minima and transition states were verified using frequency calculations, and internal reaction coordinate calculations were used with geometry optimizations to ensure that the transition states connected the intermediates and products. Reported energies were not zero-point corrected. In the simulated FTIR spectra, the frequencies have been scaled by 0.961420 and Lorentzian lineshapes with a fwhm of 4 cm-1 and the calculated intensities have been used to represent FTIR bands. III. Results and Discussion Phenyl Isocyanate. FTIR spectra of phenyl isocyanate both chemisorbed and physisorbed on Ge(100)-2 × 1 are shown in Figure 2. The multilayer physisorbed spectrum, collected at 132 K, agrees well with the known spectrum of phenyl isocyanate from the literature21 with an intense isocyanate asymmetric stretching mode from 2250 to 2300 cm-1 and ring modes from the phenyl group at 1599 and 1512 cm-1. Such agreement indicates that phenyl isocyanate does not decompose while dosing. The spectrum of phenyl isocyanate adsorbed on the surface at room temperature (302 K) differs from the phys-

isorbed spectrum, indicating that reaction with the surface has taken place. The spectrum of the chemisorbed layer was collected at a saturation dose of phenyl isocyanate; no further increase is observed in the intensity of the peaks at doses above 10 L, indicating that saturation has been reached. Subsaturation spectra contained the same peaks as the saturation spectrum, with no spectral shifts observed. The features of the monolayer spectrum that differ from the multilayer spectrum are important in assigning the structure of the chemisorption product(s). A major difference between the multilayer and monolayer spectra is the lack of an isocyanate mode in the chemisorbed films. The absence of this mode indicates that reaction with the surface involves the isocyanate group, as is expected due to the high reactivity of this group. Additionally, it rules out the presence of either of the dativebonded products shown in Figure 1. Although the dative-bonded state interacts through the isocyanate group by donating electron density from either the N or O atom, the resulting electron donation is expected to lead to a shift of the mode, not complete elimination of the mode. Indeed, both the O-dative bond and N-dative bond calculated spectra in Figure 2 include large peaks due to the isocyanate stretching mode. The chemisorption spectrum is composed of four features at frequencies of 1686, 1589, 1491, and 1240 cm-1. By comparison to the multilayer spectrum, the peaks at 1589 and 1491 cm-1 can be attributed to phenyl ring modes. Thus, the peaks at 1686 and 1240 cm-1 are characteristic features of phenyl isocyanate bonding to the Ge surface and are assigned, respectively, to ν(CdO) and ν(CarsN). These peaks are present in the product of [2 + 2] cycloaddition across the CdN bond of the isocyanate,

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Figure 3. Reaction coordinate diagrams for the CN [2 + 2], CO [2 + 2], and 1,3-dipolar cycloaddition products of phenyl isocyanate with the Ge(100)-2 × 1 surface.

as has been previously observed for the reaction of 1,4phenylene diisocyanate on the Ge(100)-2 × 1 surface.12 The ν(CdO) mode results from breaking the cumulated double bond of the isocyanate upon reaction of the CdN portion across the dimer. This leaves a four-member ring consisting of the two Ge dimer atoms and the C and N atoms of the isocyanate group. Also in this product is an unusually strong ν(CarsN) mode near 1240 cm-1, resulting from the isocyanate nitrogen and aromatic carbon of the phenyl ring.12 For further confirmation, the infrared vibrational spectra were calculated using DFT for each of the expected products in Figure 1. These calculated spectra are shown in Figure 2. From the spectra, it is clear that neither of the dative-bonded states is present as a final product on the surface, as each of these contains very strong features for the isocyanate stretching mode. The best agreement between the experimental and calculated spectra is for the CN [2 + 2] cycloaddition product, with the frequencies of the calculated ν(CdO) and ν(CarsN), which were discussed already, matching well to those features in the saturation spectrum. The spectrum for this product contains four clear features, and allowing for the error associated with DFT calculated spectra, the frequencies and intensities of the peaks are in good agreement with the experimental saturation spectrum. Comparisons between the other calculated spectra and the experimental spectrum yield little agreement. The experimental spectrum lacks the strong ν(CdN) peak at 1616 cm-1 for the CO [2 + 2] cycloaddition product, and the characteristic doublet at 1306 and 1290 cm-1 for the 1,3-dipolar cycloaddition product is not observed. To gain further insight into the reaction, the pathways were modeled using DFT. Figure 3 gives the reaction coordinate diagram for the expected products shown in Figure 1. Stable dative-bonded states for both N and O function as intermediates for the different pathways, and there is no barrier to the dative bonded states. The reaction coordinate diagram reveals that the CN [2 + 2] cycloaddition product is the most stable product, and the reaction is exoenergetic by 22.2 kcal/mol. It proceeds through the N-dative-bonded state, which is located 2.7 kcal/mol below the entrance channel. From this intermediate state, there exists a slight barrier at the transition state, which corresponds to a bending of the cumulated double bond, as the electrophilic

carbon coordinates with the nucleophilic up atom of the dimer. In the final product, the CdN double bond is reduced to a single bond, and a four-member ring is formed containing the surface dimer atoms and the C and N of the isocyanate. With an insignificant barrier and a highly stable product, it is clear that the CN [2 + 2] cycloaddition product should be observed at room temperature, as confirmed by FTIR spectroscopy studies. The pathways to other expected products are not nearly as favorable as that of the CN [2 + 2] cycloaddition product. Interestingly, the 1,3-dipolar cycloaddition product has the highest activation barrier and is the least stable product, with a final energy less than 1 kcal/mol below the entrance channel. The poor energetics of this reaction can be explained by the forced coordination of the electron-rich oxygen of the isocyanate group to the electron-rich up atom of the Ge dimer. Calculations for interdimer reaction across a two-dimer cluster, with both the oxygen and nitrogen atoms coordinating to the electronpoor down atoms of different dimers, were unable to isolate a stable species. The CO [2 + 2] cycloaddition product is not as energetically unfavorable as the 1,3-dipolar cycloaddition product but also has a higher barrier and lower product stability than the CN [2 + 2] cycloaddition product. The reaction proceeds through the O-dative-bonded state, which lies 4.8 kcal/ mol beneath the entrance channel. As the reaction continues, the carbon atom coordinates with the up atom of the dimer, leading to a transition state 3.8 kcal/mol above the entrance channel. The carbon atom moves closer to the Ge atom to complete a four-member ring in the final product, which is stable by only 8.8 kcal/mol, an energy for which products would not be observed at room temperature over the time scale of our measurement. Thus, both the reaction coordinates and vibrational spectra calculated by DFT confirm the CN [2 + 2] cycloaddition product as the observed chemisorbed species for phenyl isocyanate at the Ge(100)-2 × 1 surface. Although it is not included here, FTIR studies have shown a time-dependent progression of the surface adduct from the CN [2 + 2] cycloaddition product formed initially and discussed above to a second unidentified product. Initial studies indicate that the majority of the CN [2 + 2] product converts to this second product on a time scale of 20-30 min at room temperature, and further studies are being carried out to identify this product. Results pertaining to the time-dependent behavior

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Figure 4. FTIR spectra of phenyl isothiocyanate (PITC) on the Ge(100)-2 × 1 surface showing coverage dependence and multilayer.

of phenyl isocyanate will be presented in a forthcoming article. Due to the measurement time of our XPS system, which is on the order of hours, XPS spectra are expected to contain features from both the time-dependent product and any remaining CN [2 + 2] product, and, thus, will be discussed in the future article focused on the time dependence of phenyl isocyanate. Phenyl Isothiocyanate. Infrared spectra collected after room temperature adsorption of phenyl isothiocyanate at the Ge(100)-2 × 1 surface are shown in Figure 4. The spectra show fewer features than those of phenyl isocyanate, but coverage-dependent changes are readily observed. Comparison with the multilayer spectrum taken at 146 K, also shown in Figure 4, reveals significant differences. The most apparent difference in the monolayer spectra is the absence of the isothiocyanate stretching mode, which is a very strong feature between 2000 and 2200 cm-1 in the multilayer spectrum, indicating that the isothiocyanate moiety is not present in the reaction product of phenyl isothiocyanate with the surface. We therefore conclude that reaction of phenyl isothiocyanate with the surface has occurred through the isothiocyanate group. Using the same argument as that for phenyl isocyanate adsorption, the absence of any peak in this spectral region rules out the presence of the dative-bonded species of Figure 1, since any such products should exhibit isothiocyanate modes, albeit shifted slightly from its position in the multilayer spectrum. Thus, we expect the reaction products to be one or more of the other three potential reaction products: CS [2 + 2] cycloaddition, CN [2 + 2] cycloaddition, or 1,3-dipolar cycloaddition. Comparing the multilayer spectrum and saturation chemisorption spectrum (20 L), the only peaks that match are those from the aromatic ring breathing modes at 1585 and 1483 cm-1, suggesting that the peaks at 1545, 1214, and 1165 cm-1 arise from bonds affected by the surface reaction. The peak at 1545 cm-1 is attributed to a ν(CdN) mode,22 indicating the presence of a CS [2 + 2] cycloaddition product. The ν(CsS) and ν(GesS) modes, which are also unique to the CS [2 + 2] cycloaddition product, are not observed, as they occur below the cutoff frequency for the experimental setup. The presence of a secondary CN [2 + 2] cycloaddition product at the surface is indicated by features at 1214 and 1165 cm-1, arising from the ν(CarsN) and ν(CdS) modes, respectively.12,22 Both of these features are much less intense than the feature at 1545 cm-1, indicating that the CN [2 + 2] cycloaddition product is likely a minor product relative to the major CS [2 + 2] cycloaddition product in the reaction of phenyl isothiocyanate with the

Ge(100)-2 × 1 surface. These two product assignments account for all of the features in the IR spectra; the 1,3-dipolar product is not observed. Unlike phenyl isocyanate, phenyl isothiocyanate exhibits no time dependence. However, a coverage-dependent change is observed in the FTIR spectra. Namely, the two features at 1585 and 1545 cm-1, which were assigned to an aromatic ring breathing mode and the ν(CdN) mode, respectively, appear as a single broad peak at 1568 cm-1 at low coverages. We do not know the origin of this spectral change, but we speculate that it may be a result of overlap between these two modes at lower coverages. For example, it is possible that, at lower coverage, the phenyl ring is free to tilt with respect to the surface and can interact with adjacent unoccupied germanium dimers, causing a red-shift of the ring mode at 1585 cm-1, such that it overlaps with the ν(CdN) mode. This tilting of the phenyl ring can affect the conjugation of the CdN bond to the phenyl ring, inhibiting electron delocalization into the ring and resulting in a slight blue-shift of the ν(CdN) mode from 1545 cm-1, increasing the overlap of the two vibrational features. If at higher coverages there are no longer adjacent empty dimers available for interaction with the phenyl ring or the tilt angle of the phenyl ring is affected by adjacent adsorbates, the ring mode may blueshift to the observed frequency of 1585 cm-1 while the ν(CdN) mode red-shifts to the observed frequency of 1545 cm-1. To confirm the FTIR spectroscopy assignments, DFT was used to calculate the IR spectra of the expected products, and the results are given in Figure 5. The experimental spectra at 1 and 10 L are also included to allow for comparison of the features that change with exposure. Comparing the spectra, the absence of a dative-bonded product is confirmed, as the calculations for both the S-dative-bonded and N-dative-bonded products contain very strong modes from the isothiocyanate. Of the cycloaddition products, the CS [2 + 2] calculated spectrum is the only one to agree well with the strong experimental peaks from 1550 to 1600 cm-1, confirming the CS [2 + 2] cycloaddition adduct as the major product. While the calculated spectrum for the CS [2 + 2] cycloaddition product agrees well with the experimental spectra over the 1500-1600 cm-1 range, the calculated spectrum for this product does not contain the features observed experimentally in the 1150-1250 cm-1 region. The CN [2 + 2] cycloaddition spectrum has two peaks in this range that are of similar intensity and agree well with the experimental spectra. These peaks, at 1214 and 1165 cm-1, are assigned to ν(CarsN) involving the

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Figure 5. IR spectra calculated by DFT for phenyl isothiocyanate (PITC) reaction products, with experimental spectra of 1 and 10 L exposure given for comparison.

Figure 6. Reaction coordinate diagrams for the CN [2 + 2], CS [2 + 2], and 1,3-dipolar cycloaddition products of phenyl isothiocyanate with the Ge(100)-2 × 1 surface.

aromatic carbon and ν(CdS), respectively. On the basis of the relative intensities of the experimental and calculated modes, the assignment of a CS [2 + 2] cycloaddition major product with a CN [2 + 2] cycloaddition minor product is confirmed. We note that, although the calculated spectra give insight to the surface products, they do not reveal information about the coverage dependence of the experimental spectra. As suggested above, these spectral changes are likely due to molecular interactions and packing at the surface, and the single dimer calculations do not capture such effects. To further investigate the adsorption energetics of phenyl isothiocyanate, reaction pathways were calculated by DFT for the expected products. The reaction coordinate diagrams for these products are given in Figure 6. As was the case for phenyl isocyanate, the dative-bonded states function as reaction intermediates for the final products. Whereas the N-dative bond strength for phenyl isothiocyanate is 2.7 kcal/mol, the S-dative bond is much stronger at 9.5 kcal/mol, a value similar to previous reports of S-dative bonds on germanium.7,11

The reaction coordinate diagram reveals that all three phenyl isothiocyanate products are stable relative to the reactants. Even the least stable of the products, the 1,3-dipolar cycloaddition, is 16.1 kcal/mol exoenergetic, considerably more stable than the analogous phenyl isocyanate product, which is only 0.5 kcal/ mol exoenergetic. The CN [2 + 2] cycloaddition product is slightly more stable at -17.7 kcal/mol. The most stable phenyl isothiocyanate product is the CS [2 + 2] cycloaddition product, at an energy 28.7 kcal/mol below the entrance channel. Of the pathways, only the CN [2 + 2] cycloaddition product is activated relative to the energy of the reactants. However, its activation barrier of 3.8 kcal/mol is easily overcome at room temperature. Notably, for each of the phenyl isothiocyanate adsorption products that involve Ge-S bonding (i.e., the CS [2 + 2] and 1,3dipolar cycloaddition products), the activation barriers and binding energies are significantly more favorable than those for the analogous Ge-O bonded products formed by adsorption of phenyl isocyanate. This effect is due to the high stability of both dative and covalent Ge-S bonds.7,11 As can be seen in Figure 6, the

Phenyl Isocyanate and Phenyl Isothiocyanate stability of the S-dative bond lowers the energy of the entire CS [2 + 2] cycloaddition reaction pathway below the entrance channel. A similar stabilization is observed for the 1,3-dipolar cycloaddition, where the transition state from the N-dative bond has the smallest barrier of 0.1 kcal/mol, as the sulfur atom coordinates to the Ge dimer atom. The pathways shown in Figure 6 are consistent with the formation of the two products determined by FTIR spectroscopy. According to kinetic and thermodynamic arguments, the CS [2 + 2] cycloaddition adduct is expected as the major product, as observed experimentally. The pathway to form the CN [2 + 2] cycloaddition product, although weakly activated, is also expected to occur, consistent with the observation by FTIR spectroscopy. Similarly, the energetics shown in Figure 6 would point to the formation of the 1,3-dipolar cycloaddition product at room temperature; however, features resulting from this product are not observed in the FTIR spectra. As discussed in the following paragraphs, XPS indicates that the 1,3-dipolar product may react further to leave atomic S at the surface, which lacks features visible with our FTIR spectroscopy apparatus. In addition to FTIR and DFT, XPS spectra were taken of the phenyl isothiocyanate surface at saturation. Due to limitations of sensitivity and scan time, we were unable to probe subsaturation coverages with XPS. The phenyl isothiocyanate saturation spectra for the N(1s), S(2p), and C(1s) regions are given in Figure 7. Although the C(1s) and S(2p) regions contain large peaks, the N(1s) region shows a feature that is barely discernible above the noise. Fits in this region were unreliable due to the low signal-to-noise ratio (SNR), but there appears to be at least one peak at 397.7 eV, and the possibility of smaller peaks at higher binding energies. The S(2p) spectrum contains a strong peak centered at 161.9 eV, with a shoulder on the high binding energy side, confirming the presence of multiple products. The C(1s) spectrum also contains two features, but in this case, the peaks represent the aromatic and isothiocyanate carbons. A comparison of the S(2p) and N(1s) XPS high resolution scans indicates that the sulfur peak is much larger than that of nitrogen, with a higher SNR, even though these elements are present in a 1:1 ratio in the parent molecule. Given the XPS sensitivity factors for S(2p) and N(1s) of 2.16 and 1.68, respectively,23 for an equal amount of N and S on the surface, the S(2p) peak area should only be 28% larger than the N(1s) peak area. Hence, the XPS results indicate that there is excess sulfur at the surface. Using estimates of the N(1s) peak area, we find that the relative ratio of sulfur to nitrogen at the surface is close to 6:1, with an associated error of approximately 30%, from the instrument and peak fitting. A similar analysis with the C(1s) region reveals that the carbon to nitrogen ratio is close to the value of 7:1 expected for the phenyl isothiocyanate molecule, indicating that excess sulfur is present at the surface. The divergence from the expected surface composition suggests that some of the phenyl isothiocyanate undergoes reaction at the Ge(100)-2 × 1 surface, leading to desorption of a new product that leaves sulfur on the surface. We propose a possible reactive desorption pathway that is consistent with the excess sulfur and may also explain the lack of features from the 1,3-dipolar cycloaddition product in the FTIR spectra. Recall that, although the 1,3-dipolar cycloaddition product is kinetically accessible and energetically stable (Figure 6), this product was not observed at the surface. Due to the formation of the favorable Ge-S bond, the 1,3-dipolar cycloaddition product contains a highly reactive carbanion species which we speculate may undergo further reaction. One possible reaction is that to form phenyl isonitrile and a bridge-bonded sulfur atom at the surface,

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Figure 7. XPS high resolution scans of (a) N(1s), (b) S(2p), and (c) C(1s) electron energies for saturation coverage of phenyl isothiocyanate.

as shown in Figure 8, which leads to a stable gaseous product and surface-bound sulfur. The vibrational modes for the resulting bridge-bonded sulfur atom fall below the cutoff of our IR spectroscopy system and would not be detectable. DFT calculations indicate that the reaction in Figure 8 is stable by only 3 kcal/mol, but it is possible that the sulfur atom migrates to a more stable position at the Ge surface, similar to the migration of nitrogen atoms into the Si(100)-2 × 1 surface.24 It should also be noted that the decomposition leading to non-stoichiometric surface composition may be caused by the X-ray irradiation. Future experiments such as time-dependent XPS, temperature programmed desorption, STM, and synchrotron studies are suggested to further probe the nature of this reactive

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Figure 8. Potential mechanism for further reaction of the phenyl isothiocyanate 1,3-dipolar cycloaddition product to form a bridgebonded sulfur with desorption of phenyl isonitrile.

desorption product and the bonding configurations of sulfur atoms remaining at the surface. XPS also allows for quantification of the amount of adsorbates on the surface. By comparison to XPS data for pyridine taken on the same system, which is known to adsorb on Ge(100)-2 × 1 with a saturation coverage of 0.25 monolayers (ML; 1 ML ) 1 adsorbate per surface atom),25,26 we estimate that PITC saturates with a coverage of approximately 0.19 ML based on the C(1s) XPS data. Due to the excess sulfur already discussed, comparison of the PITC S(2p) data to the N(1s) data of thiophene scaled by the relative sensitivity factors of the N(1s) and S(2p) photoelectron peaks indicates a higher sulfur coverage of approximately 0.61 ML at saturation. Thus, we estimate that molecular adsorbates account for approximately 0.19 ML and surfacebound sulfur accounts for 0.42 ML. Comparison of Reactivity. Despite the similarities between phenyl isocyanate and phenyl isothiocyanate, these two molecules vary significantly in their reactivity with the Ge(100)-2 × 1 surface. The FTIR spectra after chemisorption show clear differences, with ν(CdO) and ν(CarsN) peaks observed for phenyl isocyanate, indicating the CN [2 + 2] cycloaddition adduct as the major product, and a ν(CdN) feature for phenyl isothiocyanate, indicating the CS [2 + 2] cycloaddition adduct as the major product. There is also spectral evidence for the CN [2 + 2] cycloaddition adduct for phenyl isothiocyanate, as a minor product. Thus, the IR data indicates that the reactivity for these two molecules is reversed; i.e., the major product for the isocyanate is the minor product for the isothiocyanate. A second difference in reactivity is that phenyl isocyanate demonstrates time dependence, with the CN [2 + 2] cycloaddition reaction product continuing to react at room temperature, while the reaction of phenyl isothiocyanate shows no such transient behavior. Comparison of the DFT results for phenyl isocyanate and phenyl isothiocyanate yields insight to the reactivity differences observed in the IR spectra. While the N-dative bonds have the same energy, the S-dative bond of phenyl isothiocyanate is twice as stable as the O-dative bond of phenyl isocyanate. This has been previously reported for other systems, and is attributed to the lower electronegativity of sulfur.7,11 The calculations indicate that reactions of phenyl isothiocyanate forming GesS bonds are more exoenergetic than the analogous reactions of phenyl isocyanate forming GesO bonds. Since previous studies have shown that the GesS covalent bond is stronger than the GesO covalent bond,7,11 the calculations suggest that the bonds broken within the isothiocyanate or isocyanate also play a significant role in determining the product stability. For the CN [2 + 2] cycloaddition reaction of phenyl isocyanate, the product contains a carbonyl group, which is much more stable than the thiocar-

Loscutoff et al. bonyl group formed for phenyl isothiocyanate. The difference in reactivity, with the isocyanate group forming a [2 + 2] cycloaddition product across the CdN bond and the isothiocyanate group forming a [2 + 2] cycloaddition product across the CdS bond has also been observed in organic chemistry.27 Since these cases do not involve the Ge surface, the selectivity arises primarily from the difference in carbonyl and thiocarbonyl stability. Interestingly, reports on the reaction of phenyl isothiocyanate with the Si(100)-2 × 1 surface indicate the formation of the CN [2 + 2] cycloaddition product.15 This is not altogether unexpected, because the interactions of sulfur with Si are not as favorable as those with Ge, and these energetic differences may be sufficient to change the reaction pathway to favor the CN [2 + 2] cycloaddition product over the CS [2 + 2] cycloaddition product on the Si(100)-2 × 1 surface. IV. Conclusions We have shown that the reactions of phenyl isocyanate and phenyl isothiocyanate with the Ge(100)-2 × 1 surface lead to formation of different major products. Although the molecules are similar, the change from oxygen in isocyanate to sulfur in isothiocyanate causes a change in product distribution from a [2 + 2] cycloaddition product across the CdN bond in phenyl isocyanate to primarily a [2 + 2] cycloaddition product across the CdS bond in phenyl isothiocyanate. The energetics of these reactions show that the GesS bond is more stable than the GesO bond, while the CdO bond is more stable than the CdS bond, leading to the observed reactivity, which is confirmed by FTIR spectroscopy and XPS. The experimental spectra also reveal the presence of multiple products for the reaction of phenyl isothiocyanate. In this case, the [2 + 2] cycloaddition across the CdS is the major product, but the [2 + 2] cycloaddition across the CdN is a minor product. Additionally, the reaction of phenyl isothiocyanate with the surface led to an excess of sulfur, indicating a desorption product in which the molecular adsorbate dissociates and a fragment desorbs to leave S at the surface. Overall, the reactions of phenyl isocyanate and phenyl isothiocyanate yield different major products with no activation barrier. The difference in reactivity for phenyl isocyanate and phenyl isothiocyanate can be used to obtain desired attachment to the Ge(100)-2 × 1 surface. The CN [2 + 2] cycloaddition product of phenyl isocyanate forms a shorter tether of the phenyl ring to the surface, but the remaining carbonyl could interact with adsorbate molecules at adjacent sites. The CS [2 + 2] cycloaddition product of phenyl isothiocyanate has a longer tether to the surface, which has the potential to allow for a reordering of the adsorbate rings to lower the surface energy through π-stacking and fill in gaps that could occur at defect sites and edges. Both molecules form products that contain conjugated chains attaching the phenyl ring to the surface, making the product useful for organic electronics applications. It is possible that, by adding functional groups to the phenyl ring, the distribution of products can be changed, leading to enhanced selectivity of the major products, or a greater relative amount of the minor products at the surface. Adding functionality to the ring also allows for further reaction on the organic, to form organic oligomers in a molecular layer deposition scheme. In this way, the organic layer could be built up as desired, to create structures that could impart order to an organic electronic interface, conduct charge as molecule wires, or even function as single molecule transistors. Acknowledgment. We would like to acknowledge support of this work from the National Science Foundation (CHE

Phenyl Isocyanate and Phenyl Isothiocyanate 0615087 and CHE 0910717) and the Semiconductor Research Corporation via the Center for Advanced Interconnect Science and Technology (SRC task 1292.040). We would also like to thank J. Kachian for helpful discussions regarding FTIR and XPS studies and J. Tanskanen for discussions and help regarding the quantum chemical calculations. Supporting Information Available: Complete ref 15. This material is available free of charge via the Internet at http:// pubs.acs.org. References and Notes (1) Bent, S. F. Surf. Sci. 2002, 500, 879. (2) Bent, S. F. J. Phys. Chem. B 2002, 106, 2830. (3) Filler, M. A.; Bent, S. F. Prog. Surf. Sci. 2003, 73, 1. (4) Loscutoff, P. W.; Bent, S. F. Annu. ReV. Phys. Chem. 2006, 57, 467. (5) Hamers, R. J. Annu. ReV. Anal. Chem. 2008, 1, 707. (6) Hamers, R. J.; Coulter, S. K.; Ellison, M. D.; Hovis, J. S.; Padowitz, D. F.; Schwartz, M. P.; Greenlief, C. M.; Russell, J. N. Acc. Chem. Res. 2000, 33, 617. (7) Kachian, J. S.; Wong, K. T.; Bent, S. F. Acc. Chem. Res. 2010, 43, 346. (8) Takagi, S.; Tezuka, T.; Irisawa, T.; Nakaharai, S.; Maeda, T.; Numata, T.; Ikeda, K.; Sugiyama, N. Mater. Sci. Eng., B 2006, 135, 250. (9) Wang, G. T.; Mui, C.; Musgrave, C. B.; Bent, S. F. J. Am. Chem. Soc. 2002, 124, 8990. (10) Wang, G. T.; Mui, C.; Musgrave, C. B.; Bent, S. F. J. Phys. Chem. B 2001, 105, 12559. (11) Kachian, J. S.; Bent, S. F. J. Am. Chem. Soc. 2009, 131, 7005. (12) Kim, A.; Filler, M. A.; Kim, S.; Bent, S. F. J. Am. Chem. Soc. 2005, 127, 6123.

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