Formation of Stable Nitrene Surface Species by the Reaction of

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Formation of Stable Nitrene Surface Species by the Reaction of Adsorbed Phenyl Isocyanate at the Ge(100)‑2 × 1 Surface Keith T. Wong,† Jukka T. Tanskanen,†,‡ and Stacey F. Bent*,† †

Department of Chemical Engineering, Stanford University, 381 North-South Mall, Stanford, California 94305, United States Department of Chemistry, University of Eastern Finland, P.O. Box 111, FI-80101 Joensuu, Finland

‡

S Supporting Information *

ABSTRACT: The reaction of phenyl isocyanate (PIC) following adsorption at the Ge(100)-2 × 1 surface has been investigated both experimentally and theoretically by Fourier transform infrared (FTIR) spectroscopy, X-ray photoelectron spectroscopy, temperature-programmed desorption, quantum chemical calculations, and molecular dynamics simulations. PIC initially adsorbs by [2 + 2] cycloaddition across the CN bond of the isocyanate, as previously reported, but this initial product converts to a second product on the time scale of minutes at room temperature. The experimental and theoretical results show that the second product formed is phenylnitrene (C6H5N) covalently bonded to the germanium surface via a single Ge−N bond. This conclusion is further supported by FTIR spectroscopy experiments and density functional theory calculations using phenyl isocyanate-15N and phenyl-d5 isocyanate.

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INTRODUCTION By combining established knowledge of inorganic semiconductor processing techniques with the tailorability of organic molecules, organic functionalization of semiconductors offers the ability to tune the properties of semiconductor surfaces and inorganic−organic interfaces. Possible applications of organic functionalization include molecular electronics, nanoscale patterning, surface passivation, chemical and biological sensors, organic thin film deposition, and organic electronics.1−10 Most work on dry functionalization of semiconductors has focused on the (100)-2 × 1 reconstructed surfaces of group 14 elements silicon and germanium (also referred to as group IV elements according to CAS or old IUPAC naming conventions).11−19 The Ge(100)-2 × 1 surface has recently received increasing attention because there is the potential to take advantage of its favorable properties such as higher electron and hole mobilities and a lower dopant activation temperature than for silicon.16,20,21 Organic functionalization provides the fine control over interfacial properties that is necessary in using germanium in many applications, and the current study combines experimental measurements under ultrahigh vacuum (UHV) with quantum chemical calculations to enhance our fundamental understanding of the reactions of organic molecules at the Ge(100)-2 × 1 surface. The Ge(100)-2 × 1 reconstructed surface consists of ordered rows of dimers, and a detailed description of the surface properties that are important in organic functionalization can be found in several reviews.11−17,19,22 Briefly, the reactivity of Ge(100)-2 × 1 surface dimers is often understood by analogy to organic chemistry. The dimer atoms are joined by a strong σ and a weak π bond. This partial double bond imparts olefinic character and enables many pericyclic-like reactions such as the © 2013 American Chemical Society

cycloaddition of unsaturated organic molecules at the surface. In addition, the surface dimers are tilted out of the surface plane, and this asymmetry is associated with an uneven distribution of charge between dimer atoms, with the tilted-up and tilted-down atoms being electron-rich and electrondeficient, respectively. The uneven charge distribution gives the surface dimers zwitterionic character and allows for Lewis acid/base or nucleophilic/electrophilic reactions. The adsorption of phenyl isocyanate (PIC; structure shown in Figure 1) at the Ge(100)-2 × 1 surface has previously been investigated.23 Isocyanates and related isothiocyanates offer the chance to investigate the reaction of a cumulated double bond containing multiple heteroatoms and the effects of exchanging oxygen with sulfur. In fact, reaction energetics differed significantly between PIC and phenyl isothiocyanate, leading to the formation of distinct products at the Ge(100)-2 × 1 surface. The strength of the Ge−S bond favored the [2 + 2] cycloaddition of phenyl isothiocyanate across the CS bond. In contrast, phenyl isocyanate initially reacted by [2 + 2] cycloaddition across the CN bond to form the CN [2 + 2] product (shown in Figure 1) instead of the cycloaddition of the CO bond that is analogous to the CS bond of phenyl isothiocyanate. CN [2 + 2] cycloaddition is also the primary adsorption pathway observed for several other isocyanates and diisocyanates at the Ge(100)-2 × 1 surface and is likely favored by the stability of the carbonyl group that is formed.24−26 However, it was noted in the previous study of PIC adsorption that infrared spectral changes were observed on a time scale of minutes after adsorption,23 and this postReceived: September 19, 2013 Revised: November 26, 2013 Published: December 10, 2013 15842

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for IR and TPD experiments, and approximately 8 × 8 × 1 mm3 Ge crystals (MTI Corp.) were used for XPS experiments. A Bio-Rad FTS-60A spectrometer in multiple internal reflection (MIR) geometry was used in conjunction with a liquid-nitrogencooled mercury−cadmium−telluride (MCT) detector to collect FTIR spectra. The beam path outside of the UHV chamber was sealed and purged with air treated with a purge gas generator (Parker Filtration) to eliminate spectral features from H2O and CO2. Absorption by CaF2 windows below ∼1050 cm−1 limits the spectral range of our system. IR spectra were corrected for baseline fluctuation by manually subtracting spline functions fit to points devoid of spectral features. For TPD experiments, a digital controller (Eurotherm) was used to ramp the temperature linearly at a rate of 1 K/s, and the sample surface was positioned approximately 10 mm from the 3-mm-diameter inlet to a stainless steel shroud enclosing the quadrupole mass spectrometer (Vacuum Generators). The shroud helped minimize the signal from molecules desorbing from the sample holder or other parts of the chamber, and additional holes in the shroud located 20 mm behind the filament facilitated pumping inside the shroud. Up to five masses were recorded simultaneously during TPD experiments. C 1s photoelectron spectra were collected using Al Kα radiation, and N 1s and O 1s spectra were collected using Mg Kα radiation due to interference from the Ge Auger series. All spectra were collected with a 25 eV pass energy using a five-channel hemispherical analyzer (SPECS Surface Nano Analysis GmBH). Before photoelectron spectra were fit, background curvature was removed by subtracting a spectrum of the clean Ge(100)-2 × 1 surface. Spectra were then fit with a Shirley baseline and a chemically realistic number of pure Gaussian components constrained to the same full width at half-maximum (fwhm) within a given spectrum. The Ge 3d5/2 photoelectron peak was used as an internal standard to calibrate the energy scale and peak intensity. Relative sensitivity factors determined for our system were used in quantitative comparisons. Intensities in all photoelectron spectra shown have been scaled by the transmission function of our analyzer. PIC (≥99%, Sigma-Aldrich), PIC−15N (98 atom % 15N, SigmaAldrich), and PIC-d5 (98 atom % D, Cambridge Isotope Laboratories) are clear liquids at room temperature. After several freeze−pump− thaw cycles, PIC, PIC−15N, and PIC-d5 were dosed by backfilling the UHV chamber through a variable leak valve except in TPD experiments when PIC was dosed through a directed doser. The precursor identity was verified by quadrupole mass spectrometry. Exposures are reported in units of langmuir (L; 1 L = 10−6 Torr·s) and are not corrected for ion gauge sensitivity. Quantum chemical calculations were carried out using the Gaussian 0329 and Gaussian 0930 software suites. The Becke3 Lee−Yang−Parr (B3LYP) three-parameter DFT method was employed for geometry optimization and frequency calculations. The B3LYP method has been previously shown to provide predictive results for similar systems.17,22,27,31,32 Single-point Hartree−Fock (HF) energy calculations were also performed at the B3LYP-optimized geometries to provide additional data for comparison using an ab initio method. The Ge(100)-2 × 1 surface was modeled by a Ge9H12 one-dimer cluster except where otherwise noted. For all calculations, Ge dimer atoms and all adsorbate atoms were modeled using the triple-ζ 6-311+ +G(d,p) basis set. Subsurface Ge atoms were modeled using the LANL2DZ pseudopotential in order to reduce the computational cost of calculations and were terminated with hydrogens, modeled using the 6-31G(d) basis set, in order to fill their valence and approximate neighboring Ge atoms. Terminating hydrogens were assigned a mass of 74 amu in frequency calculations to eliminate artificial Ge−H vibrations in the calculated IR spectra, and all calculated frequencies have been scaled by a factor of 0.97.33 Lorentzian line shapes (4 cm−1 fwhm) with the calculated intensities have been used to represent IR bands in calculated spectra. To investigate the dynamic stabilities of the PIC-derived surface species at room temperature, phenylnitrene and aziridine-like structures on the Ge(100)-2 × 1 surface were simulated by DFTMD calculations at 298 K using the Vienna Ab initio Simulation Program (VASP).34,35 A periodic slab model with four surface dimers

Figure 1. Schematic drawings of PIC, PIC−15N, PIC-d5, and three surface species on Ge(100)-2 × 1 that are discussed: CN [2 + 2] cycloaddition product of PIC, phenylnitrene product, and aziridinelike product.

adsorption time-dependent behavior is the topic of this study. Fourier transform infrared (FTIR) spectroscopy, X-ray photoelectron spectroscopy (XPS), and temperature-programmed desorption (TPD) experiments are combined with ab initio calculations and density functional theory-molecular dynamics (DFT-MD) simulations to show that the reaction of PIC after adsorption leads to the loss of CO, leaving a C6H5N species bonded to the surface. This C6H5N species may form either one or two covalent Ge−N bonds to the Ge(100)-2 × 1 surface to form the products shown in Figure 1 and referred to here as the phenylnitrene product and the aziridine-like product, respectively. Experimental and theoretical results for PIC indicate that the phenylnitrene product forms at the surface, and this conclusion is further supported by FTIR experiments and quantum chemical calculations for the two isotopically labeled forms of PIC shown in Figure 1 (i.e., phenyl isocyanate-15N (PIC−15N) and phenyl-d5 isocyanate (PICd5)). The formation of a long-lived, surface-bound nitrene may enable new functionalization strategies for the Ge(100)-2 × 1 surface or the analogous Si(100)-2 × 1 surface. The adsorbed phenylnitrene product is found to be most stable in a triplet state, and the unpaired electrons, although likely stabilized by delocalization in the phenyl ring or surface, may allow radical reactions on the surface. The ability to perform radical chemistry at the surface would complement the well-known Lewis acid/base and olefinic character of the clean group 14 (100)-2 × 1 surface.

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EXPERIMENTAL AND COMPUTATIONAL METHODS

FTIR spectroscopy and TPD studies were performed in a UHV chamber described previously,27 and XPS experiments were performed in a separate UHV chamber also described previously.28 Briefly, both chambers had base pressures of less than 2 × 10−10 Torr, and the Ge substrates were prepared by cycles of argon ion sputtering (1 kV accelerating voltage, 20 mA emission current) followed by annealing to 800−880 K. Temperature was monitored with a type-K thermocouple either directly in contact with the surface of the crystal (IR/TPD chamber) or attached to the sample holder near the crystal (XPS chamber). Low-energy electron diffraction (LEED) was used to verify the 2 × 1 reconstruction after annealing. Trapezoidal Ge crystals (19 × 14 × 1 mm, 45° bevels, Harrick Scientific) were used as the substrate 15843

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and a unit cell composition of Ge32H8 was generated from the Ge9H12 cluster and fully optimized to represent the Ge(100)-2 × 1 surface. The DFT-MD calculations were carried out by using the PBE functional36 and standard versions of the projector augmented wave (PAW) potentials37 for all atoms, as implemented in VASP. A time step of 1.0 fs, Γ-point k sampling, an energy cutoff value of 420 eV, a Nosé thermostat, and default accuracy parameters for the fast Fourier transform (FFT) grid and real space projectors (PREC=NORMAL in VASP input) were adopted in the calculations, and this computational approach enabled well-behaved simulations to be performed. Total energies averaged over the DFT-MD simulation length, without taking into account the nonconverged energies of the first 3 ps, were utilized in the comparison of the energetics of the phenylnitrene and aziridinelike surface structures.

surface to form a new product (hereafter referred to as the long-time product). The peaks of the long-time product do not match the calculated spectra for any of the other expected products for molecular PIC adsorption on Ge(100)-2 × 1.23 To search for other products that may account for the IR spectrum at longer times following PIC adsorption on Ge(100)-2 × 1, DFT calculations were performed. To facilitate this process and increase confidence in the conclusions, we also investigated two isotopically labeled forms of PIC experimentally and theoretically: PIC−15N and PIC-d5. The left panel of Figure 3 shows the experimental spectra of PIC and these two isotopologues scanned immediately after dosing (black spectra) and the calculated spectra for the CN [2 + 2] cycloaddition products (blue spectra). There is good agreement between the experimental and calculated spectra, and the shifting of peaks for the isotopologues is accurately predicted. The experimental spectra (shown in black) in the right panel of Figure 3 show the spectral changes after at least 13 min at room temperature for the three forms of PIC. As expected, negative peaks match those in the initial spectra because of disappearance of the CN [2 + 2] product. Positive peaks are characteristic of the long-time product and may be used to identify this product. The peak at 1335 cm−1 is red shifted 8 cm−1 from PIC to PIC−15N, suggesting that this mode is associated with a vibration involving nitrogen. The ν(Car−N) mode for the initial CN [2 + 2] product exhibits a similar red shift (11 cm−1) upon substitution of 15N; therefore, we speculate that the peak at 1335 cm−1 also corresponds to a ν(Car−N) vibration. However, this mode is located 95 cm−1 higher in frequency for the long-time product than the initial CN [2 + 2] product, indicating a significantly stronger Car− N bond in the long-time product. Only one surface species was found by DFT calculations that yields a ν(Car−N) mode with frequency ∼95 cm−1 higher than that of the ν(Car−N) mode of CN [2 + 2] PIC: phenylnitrene covalently bonded to a single surface Ge atom (shown schematically in Figure 1 as the phenylnitrene product). Phenylnitrene surface species may form from phenyl isocyanate by the loss of CO to vacuum. The calculated spectra (shown in blue) in the right panel of Figure 3 correspond to this covalently bonded phenylnitrene product. Again, calculations were performed for PIC, PIC−15N, and PIC-d5. There is reasonably good agreement between the positions of peaks in the calculated spectra and the positive peaks corresponding to the long-time product in the experimental spectra. Most importantly, the calculated spectra for the phenylnitrene product predict peaks near 1330 cm−1 associated with C ar−N stretching and near 1430 cm −1 associated with C−H bending for PIC and PIC−15N. These peaks are characteristic of the long-time product. Moreover, DFT calculations predict a 10 cm−1 red shift of the ν(Car−N) from PIC to PIC−15N, which matches well with the experimental red shift of 8 cm−1. The calculated spectrum of PIC-d5 also corresponds well with the experimental spectrum of this isotopologue. Strong peaks are observed at 1433 cm−1 for PIC and PIC−15N but not for PIC-d5, suggesting that the peaks at 1433 cm−1 result from C−H deformation. The PIC-d5 experimental spectrum instead contains a peak near 1220 cm−1 that matches well with the calculated frequency of a C−D deformation mode red shifted by the substitution of deuterium for hydrogen. All three calculated spectra contain several additional C−H (or C−D) bending features, which are labeled in blue in Figure 3, in the region ranging from about 1100 to 1350 cm−1, in general agreement with the weak features in this

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RESULTS AND DISCUSSION FTIR spectra of Ge(100)-2 × 1 exposed to PIC are shown in Figure 2 along with the DFT-predicted spectrum for PIC

Figure 2. IR spectra of PIC adsorbed on Ge(100)-2 × 1: DFTcalculated spectrum of the CN [2 + 2] product of PIC on a Ge23H24 cluster (blue spectrum), saturation coverage spectrum of PIC immediately after adsorption, and spectrum 20 min after exposure. The spectrum after 20 min is shown both as an incremental spectrum (ratioed to the initial spectrum) and an absolute spectrum (ratioed to the clean surface before PIC exposure).

adsorbed by [2 + 2] cycloaddition across the CN bond. The initial spectrum scanned immediately after dosing PIC is consistent with the CN [2 + 2] calculated spectrum (shown in blue), as reported previously.23 This is most apparent by the presence of a CO stretching mode at 1686 cm−1 that is characteristic of adsorption by CN [2 + 2] cycloaddition. Additionally, a strong peak at 1240 cm−1 is attributed to Car−N stretching between the isocyanate nitrogen and aromatic carbon in the phenyl ring; this mode has consistently been observed for isocyanates adsorbed by CN [2 + 2] cycloaddition.23,24,32 Twenty minutes after exposure at room temperature, the incremental spectrum shows a decrease in many of the features present in the initial spectrum, including the CO and Car−N stretching modes, which indicates the elimination of some CN [2 + 2] product by desorption or reaction. Concurrent with the decrease in these modes is the growth of peaks at 1335 and 1433 cm−1 (marked in red in Figure 2), suggesting that some adsorbed PIC reacts at the 15844

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Figure 3. Experimental (black) and calculated (blue) IR spectra of PIC (bottom), PIC−15N (middle), and PIC-d5 (top) adsorbed on Ge(100)-2 × 1. The left panel shows the initial experimental spectra after exposure at room temperature and the DFT-calculated spectra of the CN [2 + 2] product on a Ge23H24 cluster. The right panel shows the experimental incremental changes after 13−20 min at room temperature (ratioed to the initial spectra in the left panel) and calculated spectra of the phenylnitrene product on a Ge23H24 cluster. Experimental frequencies of the Car−N stretching mode are labeled in cm−1 for PIC and PIC−15N to show the red shift upon substitution of 15N.

The phenylnitrene product that gives rise to the long-time IR spectra exhibits two key features according to DFT calculations: shortening of the Car−N bond compared to PIC and other possible products and the distortion of the phenyl ring. These features lead to the unusual spectral peak positions observed in Figures 2 and 3. First, the calculated Car−N bond length is 1.34 Å for the phenylnitrene product. This is shorter than the bond lengths calculated for the aziridine-like product (1.41 Å) or the initial CN [2 + 2] product (1.42 Å). Shortening of the Car− N bond is indicative of a higher bond order, which implies a stronger Car−N bond and explains the 95 cm−1 blue shift of the ν(Car−N) vibrational mode for the long-time product compared to the CN [2 + 2] product. Second, the sharp mode at 1433 cm−1 in the long-time spectrum of PIC can be attributed to the shifting of δ(C−H) modes (which also involve some C−C stretching motion) resulting from the distortion of the phenyl ring. DFT calculations for a free PIC molecule, the CN [2 + 2] product, and the aziridine-like product show that the C−C bond lengths differ very little (1.39−1.41 Å), as expected for a phenyl ring. However, for the phenylnitrene product, the C1−Cortho bond lengths are elongated slightly to 1.43 Å, whereas the Cortho−Cmeta bond lengths are reduced to 1.38 Å. As evidenced by the calculated vibrational frequencies, this small change in geometry is enough to red shift a δ(C−H) mode to 1433 from 1491 cm−1. To understand the energetics of the PIC reaction at the Ge(100)-2 × 1 surface, HF- and B3LYP-calculated energies are shown in Table 1. The discrepancies between the HF and B3LYP energies show an uncertainty of at least ∼5 kcal/mol in the calculated energies. Nonetheless, the results provide for useful comparison to the experimental data, and a reaction coordinate diagram using the HF electronic energies is shown in Figure 4. (For simplicity, only HF energies are discussed in the following text except where otherwise noted.) PIC initially

region in the experimental spectra. Overall, DFT-calculated spectra for the phenylnitrene product reproduce the key IR spectral features of the long-time product observed experimentally for all three isotopologues with a level of accuracy similar to that in previous studies.23,25,38,39 In addition, the phenylnitrene surface species, which contains an unpaired electron formally located on nitrogen, may be compared to an anilino radical (C6H5NH·). Raman spectra of the anilino radical and its deuterated isotopologue (C6D5ND·)40 share similar features with the IR spectra of the long-time products of PIC and PIC-d5, respectively, further supporting the assignment. Phenylnitrene adsorbed by a single Ge−N covalent bond is an unusual surface species that would be deemed unlikely if it were not for the agreement among experimental, calculated, and literature spectra. For example, previous theoretical studies have predicted that nitrenes, which are typically highly reactive intermediates, will react with Ge(100)-2 × 1 or Si(100)-2 × 1 to form aziridine-like adducts containing a Ge−N−Ge or Si− N−Si ring,41 such as that shown in Figure 1. Such a reaction is analogous to the cycloaddition of a nitrene with an alkene, which is well known to occur in the organic chemistry literature.42 Moreover, aziridine-like surface species have been observed for the adsorption of nitrobenzene and nitrosobenzene on Si(100)-2 × 1 by oxygen migration subsurface and for benzyl azide adsorption on Si(100)-2 × 1 by nitrogen elimination.43−45 However, the DFT-calculated spectra for phenylnitrene adsorbed on a Ge(100)-2 × 1 surface dimer in an aziridine-like geometry do not match well with the experimental spectra (Supporting Information Figure S1). Most notably, the ν(Car−N) mode has a predicted vibrational frequency (1215 cm−1) similar to that of PIC adsorbed by C N [2 + 2] cycloaddition (1221 cm−1), which cannot explain the experimental peak at 1335 cm−1. 15845

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Table 1. Calculated HF and B3LYP Energiesa

PIC N dative bonded/Ge(100)-2 × 1 PIC N dative-CN [2 + 2] TS/ Ge(100)-2 × 1 PIC CN [2 + 2] /Ge(100)-2 × 1 phenylnitrene/Ge(100)-2 × 1 + CO(g) aziridine-like/Ge(100)-2 × 1 + CO(g) phenylnitrene(g) + CO(g)

HF electronic energy

B3LYP free energy

19.8 26.4

17.0 21.2

0 23.2 27.3 64.6

0 28.7 22.3 61.2

phenylnitrene product is most stable in a triplet state with two unpaired electrons. This product is likely stabilized by the delocalization of unpaired electrons in the phenyl ring, which is known to stabilize radical species based on the organic chemistry literature,46−48 or at the Ge surface (Supporting Information Figure S2). Attempts were made to calculate the transition-state energies for the conversion of PIC from the CN [2 + 2] product to the phenylnitrene or aziridine-like products and for the desorption of these species. A very high singlet transitionstate energy (55.2 kcal/mol) was found for the conversion of CN [2 + 2] PIC to phenylnitrene, but no triplet transition state could be located. Likewise, transition states could not be located for the other reactions. These processes are shown by dotted lines in Figure 4 to note that activated transition states may exist, but we speculate that these processes may have little or no activation barrier, thus making it difficult to locate the correct transition states in DFT calculations. Additionally, coverage-dependent FTIR spectroscopy results suggest that interadsorbate interactions may facilitate the surface reaction of PIC (Supporting Information Figure S3). Such interactions may not be accurately captured by the small cluster calculations used here and are a topic for future investigations. TPD spectra, shown in Figure 5, provide additional evidence for the formation of phenylnitrene. Desorption peaks are

a

Reported HF energies are pure electronic energies. Reported B3LYP energies are free energies from frequency calculations with rotational and translational entropy components removed for structures representing a surface or species adsorbed on a surface. In both cases, PIC adsorbed by CN [2 + 2] cycloaddition is taken to be the zero of energy.

Figure 5. TPD spectra of Ge(100)-2 × 1 exposed to 2.7 L PIC at room temperature. A linear temperature ramp rate of 1 K/s was used. All four mass traces shown were recorded during a single TPD run.

Figure 4. Reaction coordinate diagram for the reaction of PIC adsorbed on a Ge9H12 cluster by CN [2 + 2] cycloaddition. The CN [2 + 2] product is taken to be the zero of energy, and the reaction may proceed left toward the molecular desorption of PIC or right toward the reactive desorption of CO and phenylnitrene. Dotted lines connect points that may be separated by transition states. HF electronic energies are shown.

observed at two temperatures, ∼365 and ∼600 K, indicating the desorption of two distinct surface species upon heating. Desorption peaks at ∼365 K are observed for mass to charge ratios, m/z, of 119 and 92. These m/z values correspond to the parent mass of PIC and one of the major fragments in its cracking pattern, respectively.49 Using a simple Redhead analysis50 and assuming a typical pre-exponential factor of 1013 s−1, the activation energy for desorption is estimated to be ∼23 kcal/mol. The desorption energy estimated by TPD agrees well with the 26.4 kcal/mol calculated activation energy for PIC desorption (pink pathway in Figure 4, mediated by N dativebonded PIC). Higher-temperature desorption peaks at ∼600 K are observed for m/z ratios of 66, 92, and 93 in Figure 5. The m/z = 66 fragment does not correspond to a significant peak in the PIC cracking pattern; it instead matches the second most abundant species in the cracking pattern of aniline (m/z = 93 is the most abundant mass in the aniline cracking pattern and is also observed).49 Although a mass spectrum of phenylnitrene is not available because this species typically exists only as a shortlived intermediate, its mass spectrum may be expected to

adsorbed by CN [2 + 2] cycloaddition may pass over a transition state (26.4 kcal/mol activation energy, pink pathway) back to N dative-bonded PIC, which may subsequently desorb (5.7 kcal/mol activation energy, not shown in Figure 4). Alternatively, adsorbed PIC may release CO into the gas phase and convert to the phenylnitrene product (23.2 kcal/mol endergonic, blue pathway) or the aziridine-like product (27.3 kcal/mol endergonic, green pathway). The phenylnitrene and aziridine-like products may subsequently desorb as phenylnitrene. Although the formation of the phenylnitrene or aziridine-like product is energetically uphill, under UHV conditions CO will be quickly pumped away once desorbed from the surface, making the reactions effectively irreversible. HF and B3LYP results differ in whether the phenylnitrene or aziridine-like product is more stable (see Table 1), but both show that the energies of these two products are within the error of the calculations. It should be noted that the 15846

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converts to phenylnitrene after a 5 min, 450 K annealing (Figure 6); therefore, the XPS data here should reflect only the long-time product, phenylnitrene. The C 1s spectrum is fit by two peaks at energies similar to what has been observed for other phenyl- or phenylene-containing compounds.23,26,38 The N 1s spectrum is also fit to two features with a major peak at 397.8 eV that is consistent with nitrogen covalently bonded to germanium.51 The ratio of the integrated area of the N 1s peak at 397.8 eV to the total integrated area of both C 1s peaks is 1:5.7, respectively. This ratio is close to the 1:6 ratio of phenylnitrene. Molecular PIC has an N/C ratio of 1:7, suggesting that the surface species is, in fact, slightly carbondeficient compared to PIC, as expected from the loss of CO. A second N 1s peak accounting for 10% of the total N 1s area is fit at 400.0 eV. Given the small size of this peak, we attribute it to minor side products that may form upon annealing or error in the baselining and fitting processes. The O 1s peak at 530.8 eV is small relative to the carbon and nitrogen peaks (∼20% as large as the major N 1s peak after accounting for relative sensitivity factors), which indicates that most oxygen from PIC desorbs from the surface in forming the long-time product. The remaining oxygen may be atomic oxygen at the surface or incorporated into the Ge−Ge bonds; the O 1s binding energy is consistent with such species.32,52−55 Thus, the XP spectra are consistent with the formation of phenylnitrene after annealing, with only small amounts of atomic oxygen or other side products. To verify the stability of the phenylnitrene product on Ge(100)-2 × 1 at room temperature, we performed 5 ps DFTMD simulations (PBE functional and PAW basis) on a periodic slab with a phenylnitrene surface coverage of 0.125 molecule per surface atom (0.8 molecule/nm2). The arizidine-like structure was investigated by the same computational approach for comparison. During the simulations, both species remained stable on the surface, and the arizidine-like structure remained in an upright orientation during the simulation, whereas the phenylnitrene structure underwent distortion from the orientation corresponding to the optimized structure toward a more tilted orientation as a result of an interaction between the phenyl ring and a neighboring dimer (videos of the simulations are provided in the Supporting Information). This interaction introduced by temperature may affect the vibrational features of the phenyl group, potentially explaining the small differences between the B3LYP-calculated and the experimental IR spectra (Figure 3). Total energies averaged over the simulation length suggest the aziridine-like structure to be favored over the phenylnitrene structure by 14 kcal/mol, but

contain features similar to those of aniline. Additionally, desorbed phenylnitrene is expected to be highly reactive because of its unpaired electrons and may react with molecular hydrogen or other sources of hydrogen, consistent with the m/ z = 93 and 92 peaks (C6H5NH2 and C6H5NH, respectively) also observed at ∼600 K. Using a Redhead analysis again with the same pre-exponential factor as above, a peak desorption temperature of 600 K corresponds to a desorption energy of ∼39 kcal/mol. This is similar to the calculated change in energy for the desorption of C6H5N from the phenylnitrene (41.4 kcal/mol) or aziridine-like (37.3 kcal/mol) products, as shown in Figure 4. Although the TPD data cannot differentiate between desorption from phenylnitrene and aziridine-like products on the basis of the desorption temperature, the observed masses and peak desorption temperature are consistent with the desorption of C6H5N from one of these surface species. Peak desorption temperatures of 365 and 600 K correlate well with the IR spectrum collected after annealing to an intermediate temperature, 450 K, shown in Figure 6. The

Figure 6. Experimental FTIR spectrum of Ge(100)-2 × 1 exposed to 5 L of PIC followed by annealing to 450 K for 5 min.

spectrum contains only peaks associated with the phenylnitrene surface product. Peaks characteristic of the initial CN [2 + 2] cycloaddition product of PIC are absent after annealing, indicating that all adsorbed PIC molecules either desorb or convert to phenylnitrene. Conversely, PIC adsorbed at low temperature (e.g., ∼230 K) shows a spectrum consistent with only the CN [2 + 2] product, and no changes in the FTIR spectrum are observed over 30 min. XPS data are also consistent with the formation of phenylnitrene at the surface. Figure 7 shows XP spectra of the O 1s, N 1s, and C 1s photoelectron regions for Ge(100)2 × 1 exposed to a saturation dose of PIC and annealed to 450 K for 5 min. As discussed, IR spectra show that PIC desorbs or

Figure 7. O 1s, N 1s, and C 1s XP spectra of Ge(100)-2 × 1 exposed to 10 L of PIC at room temperature and subsequently annealed to 450 K for 5 min. The raw data and peak fits are shown, and the peak centers for fitted peaks are labeled in eV. 15847

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the anilino radical. The formation of phenylnitrene on the surface is further supported by XPS results showing negligible oxygen left on the surface and a carbon to nitrogen ratio close to that expected for phenylnitrene. TPD experiments show desorption at two temperatures that are attributable to PIC molecular desorption and phenylnitrene desorption. HF and B3LYP calculations show that the energies of the phenylnitrene and aziridine-like products are very similar, and the calculated energetics are consistent with the desorption temperatures in TPD experiments. Finally, DFT-MD simulations provide evidence for the dynamic stability of the phenylnitrene product. The mechanism of and driving forces for isocyanate decomposition at the Ge(100)-2 × 1 surface remain topics for future studies, but the interaction between adsorbed PIC molecules at the surface may be important in driving the conversion of CN [2 + 2] species to the phenylnitrene product. This work serves as a step in developing our understanding of the complicated chemistry of functional groups containing multiple heteroatoms and multiple unsaturated bonds with semiconductor surfaces. It also highlights the importance of understanding the time-dependent behavior of organic molecules on semiconductor surfaces, which is vital for application of organic functionalization to devices. Moreover, the formation of a surface-bound radical species may open the door to new functionalization chemistries.

this energy difference is small with respect to the total energy fluctuation of about ±30 kcal/mol during the simulations (Supporting Information Figure S4). Also, we note that the conversion of one product to the other is not observed during the simulations. Thus, although the DFT-MD results are not conclusive regarding which product will form or be more stable, they do provide evidence that the phenylnitrene product, previously thought to be an unlikely surface species, is a stable species on the Ge(100)-2 × 1 surface. It is an important finding that the phenylnitrene product is stable on the surface, and in combination with the other experimental and theoretical results presented, we conclude that PIC initially adsorbed by CN [2 + 2] cycloaddition converts to phenylnitrene bonded to the surface by a single Ge−N bond at room temperature (concurrent molecular desorption cannot be ruled out based on the results presented here). The behavior reported here for PIC after adsorption on Ge(100)-2 × 1 is unique among isocyanates that have been studied at this surface to date, but the decomposition of other isocyanates and related isothiocyanates upon adsorption on semiconductor surfaces has been observed.14,23,25 Hamers et al. noted the decomposition of PIC on Si(100)-2 × 1,14 whereas phenyl isothiocyanate and t-butyl isothiocyanate underwent reactive desorption, leaving atomic sulfur on the surface,23,25 and t-butyl isocyanate dissociated the Cal−N bond to form germyl isocyanate. 25 Isothiocyanate decomposition on Ge(100)-2 × 1 and isocyanate decomposition on Si(100)2 × 1 have been attributed to the strength of Ge−S and Si−O bonds, respectively,14,23,25 but it is so far unclear what drives the decomposition of PIC or t-butyl isocyanate on Ge(100)-2 × 1. The fact that a nitrene product was not observed for t-butyl isocyanate suggests that the delocalization of an unpaired electron in the phenyl ring of PIC may be crucial to stabilizing the nitrene surface species; it is well known in the organic chemistry literature that a phenyl ring can stabilize radical species.46−48 Such a delocalization may contribute to the shortening of the Car−N bond and the distortion of the phenyl ring noted in DFT calculations for the phenylnitrene product. There was also no experimental evidence for the formation of analogous nitrene products in studies of phenylene diisocyanates adsorbed on Ge(100)- 2 × 1.24,26 The different reactivity of diisocyanates may result from electronic effects due to the presence of a second isocyanate on the phenyl ring or steric effects, which may hinder interadsorbate interactions (discussed above and in the Supporting Information). The possible role of interadsorbate interactions in the reaction of PIC at the Ge(100)-2 × 1 surface remains an interesting topic for future investigations. The results presented here and comparison to related adsorption systems suggest that PIC is particularly suited to formation of a stable nitrene surface species.

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ASSOCIATED CONTENT

S Supporting Information *

DFT-calculated IR spectra of the aziridine-like product, DFT calculated isosurfaces for molecular orbitals containing unpaired electrons, experimental FTIR spectra comparing the timedependent behavior at low and high coverages of PIC, total energies from DFT-MD simulations of phenylnitrene and aziridine-like structures on periodic Ge(100)-2 × 1 slab, and videos of simulations. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Tel: 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 and 1213879). J.T.T. gratefully acknowledges the Academy of Finland (grant 256800/2012) and the Finnish Cultural Foundation for financial support. We gratefully acknowledge Paul W. Loscutoff for scientific discussions and initial work regarding PIC adsorption.

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CONCLUSIONS The reaction of PIC after adsorption on the Ge(100)-2 × 1 surface has been investigated by FTIR spectroscopy, XPS, TPD, quantum chemical calculations, and molecular dynamics simulations. PIC initially adsorbs by cycloaddition across the CN bond of the isocyanate,23 but changes in the IR spectrum that occur on a time scale of minutes at room temperature or with annealing indicate that this initial product converts to phenylnitrene covalently bonded to Ge via a single Ge−N bond. The IR spectra of the long-time product for PIC, PIC−15N, and PIC-d5 match well with DFT-calculated spectra for the phenylnitrene product as well as literature spectra for

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REFERENCES

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