J. Phys. Chem. C 2009, 113, 6643–6653
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Reactions of Aromatic Bifunctional Molecules on Silicon Surfaces: Nitrosobenzene and Nitrobenzene Kathryn A. Perrine,† Timothy R. Leftwich,† Conan R. Weiland,‡ Mark R. Madachik,† Robert L. Opila,‡ and Andrew V. Teplyakov*,† Department of Chemistry and Biochemistry and Department of Materials Science and Eigineering, UniVersity of Delaware, Newark, Delaware 19716 ReceiVed: September 17, 2008; ReVised Manuscript ReceiVed: February 18, 2009
Chemical control of interfaces formed on silicon surfaces is important for many practical applications. In this work, the reaction of nitrosobenzene with a clean Si(100)- 2 × 1 surface by [2 + 2] cycloaddition at room temperature is investigated. This reaction is compared to the 1,3-dipolar cycloaddition reaction of nitrobenzene on the same surface and to the cyclocondensation reaction of nitrobenzene with hydrogen-terminated Si(100) surfaces. Fourier transform infrared spectroscopy (FTIR), X-ray photoelectron spectroscopy (XPS), and density functional theory (DFT) methods were used for this study. For both nitrosobenzene and nitrobenzene on Si(100)- 2 × 1, oxygen migrates subsurface, despite substantial kinetic barriers. The effects of oxygen migration are addressed by combining DFT cluster modeling and XPS in the N 1s region. The reaction pathways of these nitrogen-containing bifunctional molecules on a clean Si(100)- 2 × 1 surface lead to the phenylnitrene adduct as the dominant surface species, while the nitrosoadduct is the primary product for the cyclocondensation reaction of nitrobenzene on hydrogen-terminated Si(100). After the formation of nitrosoadducts following adsorption, thermal annealing drives oxygen subsurface leaving the phenylnitrene adduct as the main species. This serves as a solid evidence that a specific surface adduct can be obtained from these bifunctional molecules without decomposition of the phenyl ring. Introduction Surface reactions of hydrocarbon molecules, such as functionalized aromatics, are important for the control and formation of well-ordered surface species. These aromatic molecules are by definition multifunctional, containing an aromatic ring and at least one other functional group. In most cases, specifically on group IV semiconductor surfaces, the other functional group is more reactive than the phenyl ring and can serve to anchor the aromatic entity to the surface (See reviews 1-4 and multiple references therein). In principle, after attachment to a surface, the phenyl rings of the produced surface entities may align in such a way that the lateral conductance of electrons could become possible via their π-systems.2 If the corresponding surface chemistry is well understood, this approach could potentially be utilized in numerous applications such as in the areas of biosensing, surface passivation, microelectronics and nanoelectronics. One of the surfaces that can be used as a starting point for these studies is Si(100)- 2 × 1. This surface can be described as the arrays of surface silicon dimers where each pair of silicon atoms exhibits highly reactive zwitterionic character, with fast inversion of each dimer at room temperature, as observed by STM.4 The two unpaired electrons on every silicon dimer give the Si(100)- 2 × 1 surface its high reactivity.3-10 Nitrobenzene has two possible reacting groups, the phenyl ring and the nitro group, with the nitro group more likely to react on Si(100)- 2 × 1.11,12 This preference in chemical * To whom correspondence should be addressed. Department of Chemistry and Biochemistry, University of Delaware, Newark, DE 19716. Tel.: (302) 831-1969. Fax: (302) 831-6335. E-mail:
[email protected]. † Department of Chemistry and Biochemistry. ‡ Department of Materials Science and Engineering.
reactivity is based on the retention of aromaticity by the final surface adducts, similar to many other reactions on silicon surfaces.1,13 In addition, the effects of π-conjugation of selected aromatic surface species, investigated by scanning tunneling microscopy, suggest that the electronic properties of the aromatic ring can be probed at a molecular level.14 Previous studies of the reaction of nitrobenzene with Si(100)2 × 1 reveal that it undergoes a 1,3-dipolar cycloaddition through the nitro group, forming a five-membered ring with the Si dimer, shown as reaction (b) in Figure 1. This is an energetically downhill reaction where one oxygen atom inserts into the silicon backbond to form a nitrosoadduct on the surface: a four-membered N-O-Si-Si ring. Further reaction can move the remaining oxygen atom subsurface to form a triangular N-Si-Si structure where the nitrogen atom is bound to both silicon atoms of a single surface dimer. This surface species can be described as a phenylnitrene adduct, with the phenyl ring remaining intact.15,16 Similarly, nitromethane and nitroethane will react through the nitro group by 1,3-dipolar cycloaddition on the same silicon surface.11,12,17,18 Additionally, for nitrobenzene on Si(100)- 2 × 1, X-ray photoelectron spectroscopy (XPS) of the N 1s region demonstrates that approximately 85% phenylnitrene adducts, 10% nitrosoadducts and 5% nitroadducts are present on the surface at room temperature.19 In this paper, this reaction is compared to that of cyclocondensation of nitrobenzene on hydrogen-terminated Si(100) surface with subsequent release of a water molecule as shown in Figure 1c. In this case, the phenylnitrene adduct is a minor product of the reaction, at approximately 20%, and the nitrosoadduct is the major product at ∼55%. The nature of the phenylnitrene species produced in the reactions of nitro- and nitrosobenzene with silicon surfaces can also be paralleled with the previous studies
10.1021/jp8082826 CCC: $40.75 2009 American Chemical Society Published on Web 03/31/2009
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Figure 1. Schematic representation of the reactions of nitro- and nitrosobenzene molecules on silicon surfaces. (a) [2 + 2] cycloaddition of nitrosobenzene with Si(100)- 2 × 1. (b) 1,3-Dipolar cycloaddition of nitrobenzene with Si(100)- 2 × 1. (c) Dehydrative cyclocondensation of nitrobenzene with hydrogen-terminated Si(100). Structures A, E, and H show the nitrosoadduct, structures B, C, F, and I exhibit the phenylnitrene adduct, structure D shows the nitroadduct, and structure G is a stable intermediate.
of the products of the cycloaddition reactions of azides followed by the release of N2.11,20-22 This paper presents the surface chemistry of nitrosobenzene with the Si(100)- 2 × 1 surface, shown as reaction (a) in Figure 1, and the resulting surface species as studied by in situ and ex situ analytical methods, such as MIR-FTIR and XPS. Here, the nitrosobenzene reacts with Si(100)- 2 × 1 through a [2 + 2] cycloaddition reaction. This reaction can be paralleled and compared to two other types of reactions previously reported by our group: 1,3-dipolar cycloaddition reaction of nitrobenzene on the same surface (Figure 1b), and dehydrative cyclocondensation of nitrobenzene on hydrogen-terminated Si(100) surface (Figure 1c). The parallels can be drawn among the similar nitrosoadducts (A, E, and H) as well as among the phenylnitrene surface species (C, F, and I). Following the similarities and differences among these surface adducts allows for unambiguous identification of the species present on a surface and also for understanding the effect of kinetic and thermodynamic factors on the distribution of possible products of cycloaddition and cyclocondensation reactions. The distribution of surface adducts is quantified using XPS analysis and density functional theory (DFT) investigation. It is shown that this cycloaddition is followed by oxygen migration and leads to the formation of phenylnitrene adduct as the dominant surface species, leaving the phenyl ring intact verified using FTIR. The importance of oxygen migration is discussed and is shown to affect the stability of predicted surface species using XPS and DFT. Finally we will show that upon thermal annealing of the surface following the [2 + 2] cycloaddition and the cyclocondensation reactions, the remaining nitrogenbound oxygen can be driven subsurface.
Methods A. Experimental. In Situ MIR-FTIR. Multiple internal reflection Fourier transform infrared spectroscopy (MIR-FTIR) was performed in an ultrahigh vacuum (UHV) chamber with a base pressure below 1 × 10-9 torr. A 45 degree beveled edge polished silicon crystal (Harrick Scientific) was cleaned in situ by cycles of argon (Matheson, 99.9999%) sputtering using an ion gun at 1 keV followed by subsequent annealing until a clean silicon surface was obtained as confirmed by Auger electron spectroscopy (AES). After sputtering cycles, the crystal was subsequently annealed for 20 min to temperatures above 1100 K for proper 2 × 1 reconstruction. Surface cleanliness was verified in situ by AES. Nitrobenzene was cleaned using several freeze-pump-thaw cycles and nitrosobenzene (a solid compound) was cleaned using several pump cycles under pressure of ∼10-2 torr of a mechanical pump. The cleanliness of both compounds was verified with an in situ SRS mass spectrometer, and their mass spectra were confirmed by comparison to the mass spectra obtained from NIST database.23 The compound of interest was dosed via the leak valve at exposures reported in Langmuirs (1 L ) 1 × 10-6 torr · s) without correcting for different ion gauge sensitivities when the silicon crystal was held at either cryogenic or room temperatures. Upon dosing, the vial with nitrobenzene compound was held at room temperature, whereas the vial with nitrosobenzene was heated to 323 K, to increase its vapor pressure. A background infrared spectrum was obtained before dosing either compound. For the thermal annealing experiments, the crystal was briefly heated and returned to the initial temperature, at which the background
Aromatic Bifunctional Molecules on Silicon Surfaces spectrum was recorded. All spectra were collected using a resolution of 4.0 cm-1 and 2048 scans per spectrum. Hydrogen-Terminated Silicon Surface Preparation. The modified RCA cleaning and etching procedure was used to obtain a clean hydrogen-terminated silicon surface.24-28 For the first cleaning step, the wafers (Wafer World, Inc.) were cleaned in a fresh 4:1:1 solution of Milli-Q water (resistivity g18 MΩ · cm, Millipore Corporation): hydrogen peroxide (30%, certified ACS grade, Fisher Scientific): ammonium hydroxide (29% certified ACS plus grade, Fisher Scientific) under an 353 K water bath for 10 min. After each step the wafers were rinsed several times with Milli-Q water (resistivity g18 MΩ · cm, Millipore Corporation). The native oxide was removed by a 2 min buffered HF etch using buffered hydrofluoric acid (bufferHF improved from Transene). A protective oxide was grown by submersing the wafers in a 4:1:1 solution of Milli-Q water (resistivity g18 MΩ · cm, Millipore Corporation): hydrogen peroxide (30%, certified ACS grade, Fisher Scientific): hydrochloric acid (37.3% certified ACS grade, Fisher Scientific) at 353 K for 10 min. The final etch was 2 min in 49% HF (J. T. Baker, Inc. - for the ex situ experiments) or 48% HF (Aldrich - for in situ experiments). The wafers were rinsed and dried under nitrogen before further surface modification. This preparation results in predominantly silicon dihydride-covered surface, with the remainder of the surface consisting mostly of monohydride and trihydride minority species.19,25,26,28 After the silicon surfaces were hydrogen-terminated, they were either placed in the XPS instrument described below for in situ XPS studies or placed in a reflux condenser for the condensation reaction. The samples were then removed for further analysis as described below. Ex Situ FTIR. Silicon samples, for the reaction of nitrobenzene with hydrogen-terminated Si(100), were etched using the modified RCA cleaning and HF etching procedure before being placed in a reflux condenser containing neat nitrobenzene as described previously.19 Nitrobenzene was cleaned by bubbling nitrogen through the neat compound before adding it to the reflux condenser. The samples were allowed to react for at least 2 h at 473 K. After the reaction was complete, the sample wafer was removed and dried under nitrogen. Samples were removed for single transmission IR or Ge crystal multiple internal reflection (MIR) analysis using H-terminated Si wafer as background. Transmission FTIR spectra were collected with a resolution of 8.0 cm-1 and 512 scans at an angle of 60° from the surface normal. In Situ XPS. Silicon wafers were etched using a hydrofluoric acid treatment to remove surface oxide, so that when annealed, the surface hydrogen is removed leaving a clean silicon surface for subsequent modification. A polished Si(100) wafer (Wafer World, Inc.) was etched using 5% hydrofluoric acid for 5 min (for the nitrobenzene reaction on Si(100)), or with the abovementioned modified RCA cleaning procedure (for nitrosobenzene reaction on Si(100)) and placed in a transfer chamber, holding pressure in the vacuum level, approximately 1 × 10-6 torr. Note that when these two different etching procedures were utilized for the nitrosobenzene reaction on Si(100), the same results were observed for both etching procedures; therefore, it was decided to use this simpler procedure without repeating the experiments with the RCA cleaning procedure for the reaction of nitrobenzene reaction on Si(100). However, it is possible that the number of defects on this surface would be different compared to the surface prepared using the RCA cleaning procedure. The wafer was transferred to the vacuum chamber with a base pressure of 1 × 10-9 torr and placed on
J. Phys. Chem. C, Vol. 113, No. 16, 2009 6645 the sample stage equipped with resistive heating and a thermocouple to measure the temperature of the stage. The sample was heated to 523 K and annealed for 20 min to remove residual carbon and then flashed to 823 K for 30 s to remove hydrogen. The sample was then cooled to room temperature (about 303 K) via an external liquid nitrogen dewar attached to a copper rod in thermal contact with the heating wire inside the main chamber. All in situ X-ray photoelectron spectroscopic measurements were performed in a PHI-5600 instrument. The main chamber is equipped with both an Al KR monochromatic X-ray source (hν ) 1486.6 electron-volts) and a hemispherical analyzer angled at 45 degrees from the sample surface. Survey XPS spectra were collected before and after flashing to confirm the cleanliness of the silicon surface. Survey spectra were acquired for 10 min in the range of 0-1000 electron-volts (eV), a step size of 1.6 eV/step, 200 ms/step dwell time, and 187.85 eV pass energy. High resolution spectra were collected in a range from 10-20 eV per element using an X-ray voltage of 13.5 keV, at 0.1 eV/step, 200 ms/step dwell time, and 23.5 eV pass energy for a minimum of 3 h total scan time. The compound of interest was attached directly to a leak valve on the main chamber for dosing. These compounds were purified several times by either freeze-pump-thaw cycles or pump cycles. After dosing a 1000 L exposure of nitrobenzene or nitrosobenzene, a survey spectrum and then high resolution spectra were collected until optimal signal-to-noise was obtained. For heating experiments, the number of scans and time for scanning remained constant (about 3-4 h) to compare N 1s spectra before and after heating. Ex Situ XPS. Ex situ XPS analysis was done to test for surface oxidation in the in situ prepared samples and also because the limitations in the heating setup of the instrument for in situ XPS did not allow for annealing at temperatures sufficient for the preparation of smooth and well ordered clean Si(100)- 2 × 1 surface by standard sputtering-annealing cycles. Thus, the results of two sample preparation procedures could be compared. Samples for ex situ XPS analysis were first prepared by Ar cleaning as described above to remove surface contaminants and annealed above 1100 K before nitrobenzene or nitrosobenzene was dosed onto the clean Si(100)- 2 × 1 surface in UHV. After the reaction, the sample was removed from UHV and transferred into the XPS instrument for analysis as described below. Ex situ analysis for X-ray photoelectron spectroscopy was performed in a VG ESCALAB 220i-XL electron spectrometer (VG Scientific, UK) at a pressure around 10-9 mbar in the Surface Analysis Facility in the Department of Chemistry and Biochemistry at the University of Delaware. The monochromatic Al KR X-rays at an energy of 1486.7 eV were used for analysis and operated at 15 kV, 8.9 mA, 120 W, and a nominal spot size of 400 µm. All spectra were collected using a 100 ms per point dwell time, a 100 eV pass energy for survey spectra, and 20 eV pass energy for high resolution spectra. N 1s high resolution in situ and ex situ XPS spectra were calibrated by scanning the in situ prepared nitrobenzene deposited on Si(100)- 2 × 1 sample (from the in situ MIRFTIR experiments) in both XPS chambers until optimal signalto-noise was obtained. To correct for sample charging these XPS spectra were shifted to C ) 285.0 eV. The primary N 1s peak, around 398.0 eV, was shifted by 0.5 eV between the XPS instruments. Thus, all samples presented in this work were also corrected for charging and then only the N 1s regions for in situ XPS were shifted lower by 0.5 eV. These XPS spectra for
6646 J. Phys. Chem. C, Vol. 113, No. 16, 2009 the calibration as well as representative C 1s and Si 2p high resolution spectra are shown in the Supporting Information section. B. Computational Methods. Computational models were investigated using the Gaussian 03 suite of programs using the B3LYP method and 6-311+G(d,p) basis set.29-36 A Si9H12 cluster was used to model the silicon surface, where silicon atoms below the first layer were terminated with hydrogen to maintain silicon tetracoordination. After complete optimization, the molecule of interest was attached to the Si9H12 cluster, atomic positions were fixed for atoms below the second layer of the Si9H12 cluster to properly model the silicon surface. Nitrobenzene or nitrosobenzene was then added to this model Si9H12 cluster for further optimization. Vibrational frequencies were calculated only after a structure was fully optimized at the same level of theory. Transition states were calculated using the same level of theory. To align the predicted DFT frequencies from the optimized structures with the experimental spectra, a scaling factor of 0.9564 was used, similarly to our previous work with nitrobenzene.16 The N 1s binding energies were computed from the core level (Kohn-Sham) eigenvalues produced from the Gaussian optimized calculation, the theoretical (XPS) nitrogen orbital energy. This approximation, based on Koopman’s theorem, had to be calibrated for appropriate comparison with the experimental data. Paranitroaniline was used as the reference to shift the N 1s orbital energies from the optimized DFT structures to the high resolution XPS spectra, approximately 8.76 eV. Details of this shift approximation are found in a separate publication,37 which takes advantage of the previous experimental studies of paranitroaniline.38,39 Results and Discussion I. Reactions on Clean Si(100)- 2 × 1 from Cryogenic to Room Temperatures. The reaction pathway of nitrosobenzene on a clean Si(100)- 2 × 1 surface predicted by density functional theory is shown in Figure 2. Nitrosobenzene initially interacts with the Si(100)- 2 × 1 surface through the formation of a nitrogen dative bond with a stability of -84.2 kJ/mol below the reactants level, and then a small barrier of 16.1 kJ/mol is overcome to form a stable nitrosoadduct, A, at -241.4 kJ/mol. This nitrosoadduct further transitions over a higher barrier of 100.4 kJ/mol, to form a stable intermediate, B, at -324.6 kJ/ mol below the reactant level. From this point, a moderate barrier, of 33.6 kJ/mol, is overcome for oxygen to migrate subsurface and insert into the silicon backbond, leading to the formation of a very stable phenylnitrene adduct, C, at -505.9 kJ/mol. This transformation is similar to the reaction of nitrobenzene and nitromethane with Si(100)- 2 × 1, where both oxygens of the nitro-group insert into the silicon backbond or into the silicon dimer.11,12,15,16,18,20 Nitrobenzene was previously shown computationally to react by 1,3-dipolar cycloaddition on Si(100)- 2 × 1 and have many stable configurations. However, the only structures observed experimentally were the nitroadduct, where both oxygens are bound to the Si dimer, the nitrosoadduct, with the nitroso group bound to the Si dimer and one oxygen migrated subsurface, and the phenylnitrene adduct, with only N bonded to both Si atoms and both oxygen atoms migrated subsurface as shown schematically as (b) in Figure 1. The phenylnitrene adduct is the most thermodynamically stable structure.15,16 We can compare computationally predicted stable intermediates and transition states for the reaction of nitrosobenzene with a Si(100)- 2 × 1 surface, as summarized in Figure 2, with those
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Figure 2. DFT predicted potential energy diagram of the reaction of nitrosobenzene on Si(100): silicon (green), nitrogen (blue), oxygen (red), carbon (gray) and hydrogen (white). After optimization procedure for an empty Si9H12 cluster, the atoms below the second layer were constrained at the same positions for all further studies. Hydrogen atoms used to saturate the subsurface silicon atoms are omitted for clarity. Complete structures and predicted vibrational frequencies are available in the Supporting Information.
for the adsorption and subsequent transformations of nitrobenzene on Si(100)- 2 × 1. The nitrosoadduct, structure A, produced from the [2 + 2] cycloaddition reaction on the Si(100) surface, -241.4 kJ/mol, is comparable in adsorption energy to the nitroadduct from the 1,3-dipolar cycloaddition reaction of nitrobenzene with Si(100)- 2 × 1 with respect to corresponding reactants (nitrobenzene and a cluster representing the Si(100) surface), at -256.3 kJ/mol. This nitroadduct is more stable than structure A by only 15.2 kJ/mol. Once a single oxygen atom inserts into the silicon dimer following either the [2 + 2] cycloaddition reaction, structure C in Figure 1 or 2, or the 1,3dipolar cycloaddition reaction, structure E in Figure 1, there is only a small energy difference for oxygen migration of 30.1 kJ/mol with respect to the reactants for each reaction. If a single oxygen atom is inserted into the silicon dimer to form structure C from Figure 2, this phenylnitrene adduct is more stable than the nitrosoadduct from the 1,3-dipolar cycloaddition reaction (structure E from Figure 1), where there is an additional subsurface oxygen.16 However, when both oxygen atoms from the 1,3-dipolar cycloaddition reaction have migrated subsurface to form the phenylnitrene adduct (structure F from Figure 1), this adduct is more stable than structure C by 230.0 kJ/mol. This computational comparison confirms that subsurface migration of oxygen stabilizes the predicted structures for the adsorption of nitrosobenzene on Si(100)- 2 × 1, similarly to what was shown for nitrobenzene. If we compare the nitrosoadduct (structure A) produced by the [2 + 2] cycloaddition reaction of nitrosobenzene with a clean
Aromatic Bifunctional Molecules on Silicon Surfaces
Figure 3. (a) High resolution N 1s XPS spectra of nitrosobenzene on Si(100) with ex situ and in situ analysis. (b) High resolution N 1s XPS spectra of nitrobenzene on clean Si(100) with ex situ and in situ analysis. The DFT-predicted binding energies are indicated by the sticks below the spectra, as calibrated using paranitroaniline.38,39 The letters given correspond to the structures in Figures 1 and 2.
Si(100) -2 × 1 surface to that of the cyclocondensation reaction of nitrobenzene with hydrogen-terminated Si(100) (structure H from Figure 1), the stabilities computationally predicted for these structures with respect to the corresponding reactants are very similar. Cyclocondensation of nitrobenzene with the monohydride-terminated Si(100) surface is more exothermic. However, since the majority of H-terminated Si(100) is dihydride, as characterized in previous infrared spectroscopy studies,19,25,26,28 it can be assumed that most of the nitrosoadducts from the cyclocondensation reaction (using the dihydride structure model) are comparable in stability to the nitrosoadducts from the [2 + 2] cycloaddition reaction. Although these are all thermodynamic arguments, kinetics could also play a role in the distribution of products as it does for nitrobenzene on Si(100)- 2 × 1 and H-terminated Si(100).16,19 Nevertheless, thermodynamically, structure C, the phenylnitrene adduct, in Figure 2 is predicted to be the most stable structure formed during the nitrosobenzene reaction with Si(100)- 2 × 1. The stable adducts discussed above using DFT are evidenced in high resolution XPS spectra in Figure 3. To compare the different types of nitrogen-containing species formed on the Si(100) surface at room temperature, the high resolution XPS N 1s regions were analyzed. The N 1s region is sensitive to the number of oxygen atoms attached to the nitrogen atom: the more oxygen atoms attached to the nitrogen atom, the higher the binding energy. In situ and ex situ XPS experiments were done to monitor the role of surface oxidation as well as to compare two different surface preparation methods. Prior to dosing nitrosobenzene in situ in an XPS chamber, a silicon surface was cleaned using the modified RCA cleaning procedure, etched using HF, and heated to remove the hydrogen as described in the Methods section to give a clean silicon surface. It should be noted that annealing at temperatures higher than 900 K was not possible in the experimental setup used for in situ XPS experiments and that the formation of the 2 × 1 reconstruction was not confirmed. However, according to our previous studies, the chemical properties of the surface used in these measurements were very similar to the one prepared by annealing in UHV at elevated temperatures. Ex situ XPS experiments were
J. Phys. Chem. C, Vol. 113, No. 16, 2009 6647 performed by cleaning the silicon surface by Ar sputtering and then dosing the aromatic compound in situ, before removing it from UHV for ex situ XPS analysis. These N 1s high resolution spectra as well as the N 1s binding energies predicted by DFT calculations based on Koopman’s theorem,37 indicated by the colored sticks, are all shown in Figure 3. According to the N 1s high resolution XPS studies in Figure 3, the phenylnitrene adduct, (computational structure C from Figure 2) is the dominant surface species produced following the [2 + 2] cycloaddition reaction of nitrosobenzene on Si(100) at room temperature. Both in situ and ex situ XPS spectra show that the phenylnitrene adduct and the nitrosoadduct are present on a surface in roughly the same ratios: 80% phenylnitrene adduct and 20% nitrosoadduct. According to the analysis of our predicted N 1s binding energies, structure A (nitrosoadduct) is likely the minor species present at room temperature. The presence of structure B (stable nitroso intermediate) cannot be ruled out based solely on these studies; however, vibrational spectroscopy studies discussed below will help to distinguish between species A and B unambiguously and rule out the presence of species B in any appreciable amounts. The N 1s region of the nitrosobenzene is compared to that obtained for nitrobenzene adsorbed on Si(100)- 2 × 1, reaction (b) shown in Figure 1, where the phenylnitrene adduct, F, is also the main product observed in both in situ and ex situ XPS spectra, at approximately 85%. The nitrosoadduct, schematically represented as structure E in Figure 1, and the nitroadduct, D, are minor surface species. Both nitrosobenzene and nitrobenzene reactions on Si(100) lead to similar distribution of products where the phenylnitrene adduct (with either one or two oxygen atoms, respectively, migrated subsurface) is the major species. In comparison, the nitrosoadduct, H from Figure 1, was found to be the majority surface species from the cyclocondensation reaction of nitrobenzene with hydrogen-terminated Si(100) at approximately 55%, where the stable intermediate, G from Figure 1, and the phenylnitrene adduct, I from Figure 1, are minor surface products at ∼25 and ∼20%, respectively. These approximate values are consistent with our previous data.19 Clearly, these two surface preparation methods reveal the similar experimental results for nitrobenzene and nitrosobenzene on Si(100), as summarized in Figure 3. Minor differences between in situ and ex situ experiments in absolute binding energies as well as core level shifts are summarized in Table 1 and discussed below. The differences between in situ and ex situ XPS absolute binding energies likely originates from surface oxidation due to exposure to ambient conditions before the (ex situ) XPS analysis. Other differences could be attributed to nearest neighbor effects of oxygen migration subsurface40 or subtle instrumental differences, despite our efforts to carefully calibrate the two instruments used in these studies. Since nitrobenzene has two oxygen atoms migrating subsurface compared to a single oxygen atom migrating subsurface with nitrosobenzene for the phenylnitrene adduct, this could explain the differences in absolute binding energies. Oxygen migration is further addressed in depth in the discussion section. Regardless of the exact absolute binding energies predicted by DFT, it is the energy shifts between the surface adducts predicted by computational methods that are more reliable and relevant for comparison with the experimental data. Table 1 also compares the predicted N 1s binding energy shifts with our own DFT-predicted core level shifts and those of previous studies. For nitrosobenzene there is a shift between the first two observed in situ products, the nitrosoadduct corresponding to the computational structure A from Figure 2 and the phenylni-
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TABLE 1: Experimental and Computationally Predicted N 1s Binding Energies and Corresponding N 1s Core Level Shifts for the Products of Cycloaddition Reactions of Nitro- and Nitroso Compounds on Si(100)-2 × 1
a
These computational studies refer to the DFT investigation of nitromethane on Si(100),18 for comparison to the data presented here. Note that for the nitromethane DFT computations the equivalent core (EC) approximation method was used to calculate N 1s core level shifts in that work.18 All DFT computations for this work were calibrated by comparing with the experimental studies of paranitroaniline.37-39
trene adduct, structure C, of 1.4 eV. A shift of 1.9 eV, is observed between the same products analyzed ex situ. Our computational predictions using the approximation of Koopman’s theorem suggest a shift of 1.3 eV. This predicted shift is in excellent agreement with the in situ XPS nitrosobenzene shift. The larger shift of 1.9 eV for the ex situ XPS binding energy is most likely due to exposure of samples to ambient conditions leading to partial surface oxidation before XPS analysis. This deviation could also be attributed to differences in final state effects that are not taken into account explicitly by Koopman’s theorem.37 Rignanese et al. in a computational study suggest first and second nearest neighbors can affect the N 1s core level shifts by as much as 0.36 eV.40-42 Thus, it is likely that subsurface oxygen produced during surface oxidation in the ex situ experiments reported in this paper has affected the N 1s core level binding energies. In comparison to other theoretical studies, the in situ XPS shift for the nitrosobenzene adducts is in good agreement with previous N 1s core level shift predictions for nitromethane on Si(100).18 Note that the equivalent core (EC) approximation was used to predict the N 1s core level shifts for nitrosoadducts of nitromethane on Si(100). This approach is fully described in ref 18. For nitrobenzene, the experiment and theory agree quite well: 1.5 eV between the first two products, the nitrosoadduct (structure E in Figure 1) and the phenylnitrene adduct (structure F in Figure 1) experimentally and 1.4 eV computationally using our DFT predictions. These N 1s shifts also coincide with similar models for the studies of nitromethane on Si(100).18 The differences between the nitroadduct (D) and the phenylnitrene adduct (F) shifts are 2.9 eV for in situ studies and 3.1 eV for ex situ investigations, respectively, and within 0.1-0.4 eV deviation from our predicted N 1s core level shifts. The small differences in these shifts can be explained by surface oxidation, similarly to the brief discussion given in the previous paragraph. For nitrobenzene, the shift between the phenylnitrene adduct and the nitroadduct is in agreement with nitromethane studies at 3.21 eV. More in depth discussion of predicting N 1s core level shifts and other regions have been addressed and reviewed elsewhere.37 Thus, the DFT predictions can serve as a reliable marker of surface chemistry verifiable with the XPS experiments.
Figure 4. Spectrum (a) in situ MIR-FTIR of 50 L of nitrosobenzene (saturated monolayer) reacted with Si(100)- 2 × 1. Spectrum (b) ex situ FTIR of the cyclocondensation reaction of nitrobenzene with H-terminated Si(100), which was produced by etching in 5% HF for 5 min. The C-H stretch region is a Ge crystal MIR spectrum and the right region is a transmission FTIR spectrum after the reaction. The DFT-predicted frequencies are presented below experimental spectra as follows: gas phase nitrobenzene (PhNO2), gas phase nitrosobenzene (PhNO), nitrosoadduct A from Figure 2, adduct B from Figure 2, and phenylnitrene adduct C from Figure 2. All predicted frequencies were scaled by the same factor as described in the Methods section.
Because we know from DFT predictions and XPS that the nitrosoadduct is a product of the [2 + 2] cycloaddition reaction of nitrosobenzene with Si(100), this nitrosoadduct formed at room temperature can be compared spectroscopically to the nitrosoadduct produced from the cyclocondensation reaction of nitrobenzene with hydrogen-terminated Si(100).19 Figure 4 shows the infrared spectra of nitrosobenzene reacted with Si(100)- 2 × 1 at room temperature (a) and nitrobenzene after the reaction with H-terminated Si(100) at 473 K (b) compared with frequencies computationally predicted for structures A-C in Figure 2. A comparison of the predicted IR spectra with the
Aromatic Bifunctional Molecules on Silicon Surfaces experimental spectra will help determine whether species A or B are present as surface species at room temperature. The C-H stretch region (left panel) in Figure 4 shows that the phenyl ring remains intact after these reactions. The right panel confirms the presence of intact aromatic adducts as the absorption bands corresponding to the CdC stretch, C-H bend, N-O stretch, and N-C stretch regions are clearly observed as indicated above the spectra to help assign the bands in (a) and (b). Further IR assignments can be found in a table in the Supporting Information section. For a more detailed discussion of the cyclocondensation process and its spectral characteristics, the reader is referred to reference 19. Although IR spectra (a) and (b) look remarkably similar, the nitrosoadduct, similar to structure A, was shown to dominate in the cyclocondensation reaction,19 whereas the phenylnitrene adduct is shown from Figure 3 to dominate the [2 + 2] cycloaddition reaction. It is clear that molecular nitrobenzene and nitrosobenzene are not present in either (a) or (b), as expected, due to the absence of the strong NdO stretch that would have been observed for these species around 1500 cm-1. The predicted IR spectra reveal that if the species corresponding to structures A, B, and C are present on a surface, they should all exhibit vibrational characteristics of the CdC stretching and C-H bending of the intact phenyl ring, and the absorption corresponding to the N-C stretch. These predicted vibrational signatures match nicely with the experimental nitrosobenzene spectra in Figure 4 (a). Structure B is expected to have an intense red-shifted absorption band corresponding to the C-H stretch at 2968.5 cm-1, which is not observed experimentally, eliminating structure B as a dominant surface species. This observation confirms that only structure A, the nitrosoadduct, is the minor species present in the XPS studies in Figure 3 on the Si(100) surface at room temperature. In addition, only structures A and C are expected to exhibit strong absorption bands in the N-C stretch region between 1250-1280 cm-1 and the similar C-H stretches as seen in spectra (a). Thus, only the species corresponding to structures A and C are present at room temperature following the reaction of nitrosobenzene with a clean Si(100)2 × 1 surface. The infrared studies of the coverage profile for nitrosobenzene condensed on a clean Si(100) surface at cryogenic temperatures are compared to the room temperature spectrum in Figure 5 to confirm vibrational signatures and indicate the initial stages of the reaction. The C-H stretches at 3060 and 3050 cm-1, the CdC phenyl stretch at 1593 cm-1, and the ring stretching at 1526 cm-1 indicate the presence of the intact phenyl ring. These absorption bands and the NdO stretch at 1501 cm-1 are indicative of a condensed multilayer, showing that nitrosobenzene is condensed on the surface and has not reacted. Below the MIR-FTIR spectra in Figure 5 are shown the predicted frequencies for a gas phase nitrosobenzene and a datively (nitrogen) bonded nitrosobenzene from Figure 2. This datively bonded structure is predicted to be more stable than the unoccupied silicon model cluster and a gas-phase nitrosobenzene molecule by 84.2 kJ/mol. A dative bond formed by an oxygen atom was also analyzed, but the final optimized structure did not result in a dative oxygen-silicon bond. As the exposure of nitrosobenzene is increased at cryogenic temperatures in Figure 5, the absorption bands corresponding to the C-H stretch, CdC stretch, and NdO stretch increase, confirming the formation of the condensed multilayer. A small peak at 1526 cm-1 is indicative of ring stretching of the intact phenyl ring. The predicted frequencies for a single nitrosobenzene molecule line up nicely with the multilayer spectrum, 100
J. Phys. Chem. C, Vol. 113, No. 16, 2009 6649
Figure 5. MIR-FTIR coverage profile of nitrosobenzene reacted on Si(100)- 2 × 1 at cryogenic and room temperatures. (Left) C-H stretch region. (Right) CdC and NdO stretch region. The exposure of the nitrosobenzene on Si(100)- 2 × 1 at 300 K is saturation of surface sites, based on IR signal. Below the experimental spectra are shown the computationally predicted infrared absorption bands corresponding to structures of a single molecule of nitrosobenzene in the gas phase and to the datively bonded nitrosobenzene on a Si(100) -2 × 1 surface. The computationally predicted frequencies are scaled by 0.9564, as described in the Methods section.
L, as expected for the formation of condensed ice layers. In addition, a distinct set of vibrational absorption bands is observed within the 1530-1570 cm-1 spectral region. These vibrations are not consistent with the absorption bands predicted computationally for gas phase nitrosobenzene; however, they are predicted to be observed for a nitrosobenzene molecule datiVely bonded to the Si(100)-2 × 1 surface via the nitrogen atom. The two main absorption bands in the 1530-1570 cm-1 region in the right panel of Figure 5, are separated by approximately 20 cm-1 and match very well with the DFT predictions for the datively N-bonded nitrosobenzene. According to the DFT predictions, these complex modes are suggested to involve the C-H bend and CdC stretch from the phenyl ring with N-O modes coupled to the surface through the nitrogen atom. These vibrational signatures are clearly observed at the lowest exposure studied, 1 L, and do not increase in intensity with increasing nitrosobenzene exposures suggesting that the observed absorption bands are associated with the monolayer properties of nitrosobenzene on Si(100)-2 × 1. This observation is consistent with the presence of datively bonded species within the adsorbed nitrosobenzene monolayer at cryogenic temperatures. For comparison, the room temperature spectrum of a monolayer of nitrosobenzene on Si(100) -2 × 1 is shown below the 1 L cryogenic spectrum in Figure 5, indicating no presence of the datively bonded species in the 1530-1570 cm-1 spectral region. As expected, the weakly bound datively N bond species can only be observed at cryogenic temperatures. In addition, the computationally predicted C-H stretching bands in the left panel from the phenyl ring of the datively bonded nitrosobenzene model match well with the experimental spectra at cryogenic temperatures. The multilayer and predicted frequencies for nitrosobenzene are compared to the set of vibrational absorption bands recorded for the room temperature exposure of this compound to the clean Si(100) -2 × 1 surface, as well as to our previous nitrobenzene on Si(100) studies15 and gas phase nitrosobenzene investigations,43 as summarized in the table in the Supporting Information
6650 J. Phys. Chem. C, Vol. 113, No. 16, 2009
Figure 6. In situ MIR-FTIR of nitrosobenzene on Si(100)- 2 × 1. Spectra are shown as follows from top to bottom: multilayer (100 L) at 141 K; multilayer heated to 293 K; a monolayer of nitrosobenzene, 15 L at 300 K; the same monolayer heated to 700 and 800 K.
section. Approximate assignments are given based on the computational predictions. II. Thermal Stability of Surface Species. After the nitrosobenzene has reacted with the Si(100)- 2 × 1 surface at room temperature, approximately 20% of the nitrosoadduct and 80% of the phenylnitrene adduct are present on the surface as summarized in Figure 3. By heating the surface following the formation of surface adducts, the stability of the surface species can be investigated. The effect of heating nitrosobenzene after reacting with Si(100)- 2 × 1 at both cryogenic and room temperatures is demonstrated in Figure 6. The multilayer is condensed at 141 K and subsequently heated to room temperature. The presence of the monolayer after this brief annealing is indicated by the absorption bands corresponding to the CdC stretch from the phenyl ring, N-O stretches, and a small C-H stretch from the phenyl ring. A 15 L exposure of nitrosobenzene on Si(100)- 2 × 1 at room temperature, shown below the cryogenic spectra in Figure 6, yields a spectrum that is remarkably similar to that of the multilayer heated to 293 K. The C-H, CdC, and N-O stretches are observed in both spectra demonstrating the stability of the nitrosobenzene monolayer under these conditions. When nitrosobenzene adsorbed at room temperature is subsequently heated to 700 K and then 800 K, the intensity of all of the infrared features decreases but some absorption bands are still observed in the C-H stretch and the CdC stretch spectral regions. Further heating to 900 K (not shown) decreases the CdC and N-O stretches from the MIR-FTIR, however Auger electron spectroscopy confirms no noticeable loss of carbon, nitrogen, or oxygen when heated to 830 K from 300 K. This suggests nitrosobenzene is stable and the phenyl ring remains intact when adsorbed on Si(100)- 2 × 1 at cryogenic or room temperatures followed by heating to 830 K. The ultimate result of heating above 900 K is nearly complete decomposition without significant reduction in the concentration of nitrogen, oxygen, or carbon on a silicon sample. Because phenylnitrene species is quite stable upon heating, it seems possible to attain a single type of surface adduct by driving the oxygen atoms from the nitrosoadduct subsurface to
Perrine et al.
Figure 7. (a) In situ high resolution N 1s XPS spectra of nitrosobenzene reacted with Si(100) surface and subsequently heated to 823 K. (b) Cyclocondensation reaction of nitrobenzene on H-terminated Si(100) with in situ heating. * This binding energy corresponds to the stable intermediate of the condensation reaction of nitrobenzene on Hterminated Si(100), structure G in Figure 1.
form the majority phenylnitrene adduct. Similarly, this can be achieved for the nitrosoadduct produced from the cyclocondensation reaction of nitrobenzene with H-terminated Si(100). Figure 7a shows XPS studies of nitrosobenzene reacted with Si(100) and heated to 823 K for 30 s in situ in an attempt to drive the oxygen subsurface, to form more phenylnitrene adduct, corresponding to the computational structure C. For consistency the scan time and the spectral regions were all kept the same. The nitrosobenzene on clean silicon gives the same two products as before, the nitrosoadduct, structure A, and the phenylnitrene adduct, structure C. As the sample is heated, the nitrosoadduct peak disappears so that only phenylnitrene is left on the surface. The in situ heating was repeated for the cyclocondensation reaction of nitrobenzene with H-terminated Si(100) (Figure 7b). Since the cyclocondensation reaction was performed at 473 K, the sample was flashed to 473 K and returned to RT before initial scanning to remove any residual carbon and oxygen from exposure to ambient conditions. As observed in our previous work,19 the cyclocondensation reaction has the nitrosoadduct as the dominant surface species and the phenylnitrene adduct and likely the stable intermediate, denoted by an asterisk, as the minor species (see Figure 1a). After the sample is heated to 823 K for 30 s, the major product is still the nitrosoadduct but the concentration of phenylnitrene adduct has increased noticeably. The concentration of a stable intermediate has also decreased. This suggests that thermal annealing converts species under investigation into the phenylnitrene adducts. Further thermal annealing, for longer periods of time, should drive reaction to the formation of phenylnitrene species. III. Effects of Oxygen Migration. The purpose of this oxygen migration discussion is threefold: (1) to analyze oxygen migration effects on the predicted N 1s core level binding energies with either a fixed or a relaxed subsurface, (2) to confirm that subsurface oxygen migration could pose long-range effects on the adsorption process on the Si(100) -2 × 1 surface as the coverage increases, thus altering the surface energetics, and finally (3) to establish the cause of oxygen migration over a large theoretical barrier from the reaction of N-based aromatic compounds on both clean Si(100) and H-terminated silicon surfaces.
Aromatic Bifunctional Molecules on Silicon Surfaces
J. Phys. Chem. C, Vol. 113, No. 16, 2009 6651 TABLE 2: Comparison of N 1s Core Level Binding Energies Predicted for Structures with Relaxed versus Fixed Positions of Silicon Atoms Representing Subsurface (Beyond Second Layer) adduct
Figure 8. DFT predicted structures: A, the nitrosoadduct, and C, the phenylnitrene adduct, from the [2 + 2] cycloaddition of nitrosobenzene on Si(100). Structure model of phenylnitrene adduct with four oxygen atoms subsurface, 4 Ox. Structure models of nitrobenzene on Si(100): I, the nitroadduct, IIA-C, the nitrosoadduct, and IIIA-D, the phenylnitrene adduct. Color representation: silicon (green), nitrogen (blue), oxygen (red), carbon (gray) and hydrogen (white). Hydrogen atoms terminating silicon model clusters are omitted for clarity.
As expected, oxygen migration has pronounced effects on estimations of N 1s core level shifts using DFT cluster modeling as well as on the thermodynamic stability of investigated structures. Figure 8 and Table 2 summarize the N 1s core level binding energies and energy shifts for nitrosobenzene adducts on Si(100)- 2 × 1, structures A and C from Figure 2, nitrobenzene adducts on Si(100)- 2 × 1, structures I-IIID from our previous studies of nitrobenzene discussed in reference 16 with subsurface oxygen in different positions, as well as an additional phenylnitrene adduct with four subsurface oxygen atoms. (See Supporting Information for complete details.) Structure I corresponds to the nitroadduct, IIA-IIC are nitrosoadducts, and IIIA-IIID and 4 Ox are phenylnitrene adducts. All models were investigated using the B3LYP/6-311+G(d,p) level of theory for both relaxed and fixed subsurface (below the second row of the silicon dimer) and were calibrated using paranitroaniline.37 The table also lists the number of subsurface oxygen atoms and the difference between relaxed and fixed structures predicted N 1s binding energies. The differences between the computational investigations using silicon clusters with fixed vs relaxed silicon and hydrogen atoms representing subsurface (beyond the top two layers) are important to note since it is widely accepted that these two approximations provide the benchmark range in stability investigations. However, in this study, N 1s binding energies in Table 2 from structures A and C, from Figure 2, do not differ by more than 0.1 eV between the relaxed and the fixed subsurface models. Structures I-IIID, the possible structures produced from the reaction of nitrobenzene on Si(100)- 2 × 1,16 with zero or one subsurface oxygen also differ by less than 0.1 eV. If two or more oxygen atoms form Si-O-Si bonds, the difference between relaxing and fixing the subsurface is between 0.1-0.24 eV. Notice that as more oxygen atoms (up to four) are added to the silicon dimer, the difference between the core level N 1s energies does not change linearly with this number, but it does depend on the subsurface oxygen location in the DFT cluster models. Structure IIIC has the largest N 1s shift difference of 0.24 eV due to the placement of both oxygen atoms on opposite sides within the silicon dimer, which is expected to affect the geometry and energetics of the silicon dimer substantially.16 The more the silicon dimer is altered by the placement of the subsurface oxygen atoms, the larger is the difference in N 1s core level energies.
nitroso phenylnitrene nitro nitroso nitroso nitroso phenylnitrene phenylnitrene phenylnitrene phenylnitrene phenylnitrene
structure relaxed Str A Str C Str I Str IIA Str IIB Str IIC Str IIIA Str IIIB Str IIIC Str IIID 4 Ox
399.81 398.52 401.60 400.08 399.86 399.87 398.63 398.56 398.54 398.44 398.49
fixed 399.78 398.46 401.60 400.05 399.81 399.80 398.45 398.44 398.30 398.34 398.31
#O Difference subsurfacea 0.03 0.06 0.00 0.03 0.05 0.07 0.17 0.12 0.24 0.10 0.18
0 1 0 1 1 1 2 2 2 2 4
a Number of subsurface oxygen atoms (atoms in Si-O-Si sequence).
Rignanese et al. have also examined N 1s core level shifts of N insertion into the silicon oxide interface.40 Variation in shifting was attributed to first and second nearest neighbor effects as well as to core level relaxation. Although their method for predicted N 1s core level shifts proposes a difference of ∼0.36 eV, here for a single Si cluster the effects are at most 0.24 eV. Although predicted N 1s binding energies are not greatly affected by fixing the positions of the silicon atoms representing subsurface in a cluster model, structure stability was shown to change substantially when oxygen migrates subsurface for nitrobenzene reaction with Si(100)- 2 × 1, the structures I-IIID in Figure 8.16 In agreement with the N 1s core level binding energies in Table 2, structure IIIC, the phenylnitrene adduct where two subsurface oxygen atoms are on opposite sides of the Si dimer, was found to have the largest stability difference when comparing relaxed subsurface to a fixed subsurface. For nitrosobenzene on Si(100), the difference in stability for structure A between the fixed and relaxed subsurface is 7.7 kJ/mol and for structure C this difference is 30.0 kJ/mol, a more significant change in stability, but not as substantial as for the nitrobenzene structure models IIA-IIID. This shows that subsurface oxygen migration does have an effect on predicted N 1s binding energies; however, it may not be easily interpreted in our experimental XPS spectra. Figure 3 demonstrates that the presence of surface oxygen in ex situ studies affects the distribution of final products in the reaction of nitrosobenzene with Si(100) as compared to the in situ investigation. This observation can be explained by comparison with the previous studies of insertion processes on silicon. For example, in the computational investigation of ammonia reaction with the Si(100)- 2 × 1 surface it has been shown that following ammonia dissociation, the barrier for subsurface nitrogen migration can be reduced substantially if the effects of neighboring silicon surface dimers are considered. The subsurface strain from nitrogen insertion in this case was shown to have long-range and short-range effects on the neighboring silicon dimers.44 Thus, adsorbates, such as ammonia, can cause changes in surface electronic structure of the silicon dimer of Si(100)- 2 × 1 giving a possibility for twodimensional ordering.45,46 Similar behavior is likely observed here for nitrobenzene and nitrosobenzene reactions with silicon, as the adsorption of a single molecule may alter the electronic properties of the neighboring adsorption site (before adsorption of an additional molecule). This makes it possible to observe
6652 J. Phys. Chem. C, Vol. 113, No. 16, 2009 the products that are computationally predicted to be minor, compared to the most stable phenylnitrene-type product. Lastly, above we have experimentally and computationally demonstrated that oxygen migration subsurface is apparent and can effect the distribution of surface species formed in the initial attachment reactions. However, our computational approach does not explain why in our experiments a substantial predicted kinetic barrier is easily overcome for oxygen to migrate subsurface yielding the phenylnitrene adduct as the major surface species of either the reaction of nitro- or nitrosobenzene with Si(100)- 2 × 1. When nitrosobenzene or nitrobenzene react with a clean Si(100)- 2 × 1 surface, in both cases substantial energy is required for the oxygen atoms of the initial adducts to move subsurface. A barrier of 100.4 kJ/mol for nitrosobenzene and 126.0 kJ/mol for the first oxygen atom of the nitrobenzene have to be overcome. It was suggested previously for nitrobenzene on Si(100) that energy could be lost to the surface through vibrational energy modes from the adsorption process,16 as could also be the case for nitrosobenzene. A high barrier was also observed for the remaining oxygen of the nitrosoadduct to migrate subsurface following the cyclocondensation reaction of nitrobenzene with H-terminated Si(100). However, there it was suggested that since the H-terminated silicon surface is reacted with a liquid nitrobenzene, water byproduct or other liquid nitrobenzene molecules could remove thermal energy that could otherwise be used to allow the remaining oxygen to migrate subsurface to form the phenylnitrene adduct.19 In agreement with our study, Barriocanal and Doren have suggested earlier that oxygen migration following nitro group reaction with the Si(100)- 2 × 1 surface affects the energetics of Si dimer, the ring formation with the surface and the nitro group causing the stability changes among the possible structures produced.11,12 Nevertheless, the XPS spectra in Figure 7 prove that the barriers for a single oxygen atom moving subsurface can be overcome to form the phenylnitrene adduct, the dominant surface species, and that kinetics plays a major role in the distribution of products for both nitrobenzene and nitrosobenzene on the Si(100)- 2 × 1 surface. Conclusions Nitrosobenzene was found to react via a [2 + 2] cycloaddition on the Si(100)- 2 × 1 surface at room temperature to leave two primary surface species: the nitrosoadduct and the phenylnitrene adduct. Similarities are apparent between this reaction and the cyclocondensation reaction of nitrobenzene on H-terminated silicon surfaces observed through FTIR, XPS, and confirmed by DFT. Both reactions yield nitrosoadducts and the [2 + 2] cycloaddition of nitrosobenzene on Si(100)- 2 × 1 ultimately leads to the formation of the phenylnitrene adduct as the dominant surface species, as seen for the 1,3-dipolar cycloaddition of nitrobenzene with clean Si(100)- 2 × 1. Heating the surface after these reactions drives oxygen subsurface from the nitrosoadducts for both [2 + 2] cycloaddition and cyclocondensation reactions to allow the phenylnitrene adducts to dominate the silicon surface. The distribution of surface adducts estimated from N 1s high resolution XPS studies suggests that the phenylnitrene adduct dominates both the [2 + 2] cycloaddition reaction of nitrosobenzene on Si(100)- 2 × 1 and the 1,3 dipolar cycloaddition reaction of nitrobenzene on Si(100)- 2 × 1. Here, the N 1s core level shifts predicted using an approximation of Koopman’s theorem are compared to the core level shifts between adducts from N 1s high resolution XPS. In addition, the predicted N 1s core level binding energies were affected by oxygen migration
Perrine et al. into the silicon dimer, but not as significantly as previously observed for nitrobenzene. As shown earlier for other nitrogenbased aromatic molecules, kinetics is a deciding factor in oxygen migration and therefore in the distribution of products on silicon surfaces. Acknowledgment. This work was supported by the National Science Foundation (CHE-0313803, CHE-0650123, and CHE0415979) and the American Chemical Society Petroleum Research Fund (ACS-PRF #44259-AC5). We would like to thank Professor Kate Queeney at Smith College for the modified RCA cleaning and etching procedure. We would also like to thank Mr. Anoop Mathew, Mr. Korhan Demirkan and Mrs. Beverly Wright at the Materials Science Department at the University of Delaware for their help with instrumentation. We would also like to thank Mrs. Mary E. Boggs and Mr. Shawn P. Sullivan for the ex situ XPS instrument time as well as for useful discussions and Professor Thomas P. Beebe, Jr. for the use of the Surface Analysis Facility at the University of Delaware Chemistry and Biochemistry Department. Supporting Information Available: Additional XPS results and the instrument calibration procedure, Cartesian coordinates and IR frequencies, complete ref 31. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) 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. (2) Kirczenow, G.; Piva, P. G.; Wolkow, R. A. Phys. ReV. B 2005, 72. (3) Leftwich, T. R.; Teplyakov, A. V. Surf. Sci. Rep. 2007, 63, 1. (4) Waltenburg, H. N.; Yates, J. T. Chem. ReV. 1995, 95, 1589. (5) Yoshinobu, J.; Tanaka, S.; Nishijima, M. Jpn. J. Appl. Phys., Part 1 1993, 32, 1171. (6) Yoshinobu, J. Prog. Surf. Sci. 2004, 77, 37. (7) Ono, M.; Kamoshida, A.; Matsuura, N.; Ishikawa, E.; Eguchi, T.; Hasegawa, Y. Phys. ReV. B 2003, 67. (8) Jung, Y. S.; Shao, Y. H.; Gordon, M. S.; Doren, D. J.; Head-Gordon, M. J. Chem. Phys. 2003, 119, 10917. (9) Wolkow, R. A. Annu. ReV. Phys. Chem. 1999, 50, 413. (10) Bilic, A.; Reimers, J. R.; Hush, N. S. Functionalization of semiconductor surface by organic layer: Concerted cycloaddition Versus stepwise free-radical reaction mechanisms; Imperial College Press: London, 2006. (11) Barriocanal, J. A.; Doren, D. J. J. Vac. Sci. Technol. A 2000, 18, 1959. (12) Barriocanal, J. A.; Doren, D. J. J. Phys. Chem. B 2000, 104, 12269. (13) Cao, X. P.; Coulter, S. K.; Ellison, M. D.; Liu, H. B.; Liu, J. M.; Hamers, R. J. J. Phys. Chem. B 2001, 105, 3759. (14) Hamers, R. J.; Hovis, J. S.; Coulter, S. K.; Ellison, M. D.; Padowitz, D. F. Jpn. J. Appl. Phys., Part 1 2000, 39, 4366. (15) Bocharov, S.; Teplyakov, A. V. Surf. Sci. 2004, 573, 403. (16) Me´ndez de Leo, L. P.; Teplyakov, A. V. J. Phys. Chem. B 2006, 110, 6899. (17) Bocharov, S.; Mathauser, A. T.; Teplyakov, A. V. J. Phys. Chem. B 2003, 107, 7776. (18) Eng, J.; Hubner, I. A.; Barriocanal, J.; Opila, R. L.; Doren, D. J. J. Appl. Phys. 2004, 95, 1963. (19) Leftwich, T. R.; Madachik, M. R.; Teplyakov, A. V. J. Am. Chem. Soc. 2008, 130, 16216. (20) Bocharov, S.; Dmitrenko, O.; Me´ndez De Leo, L. P.; Teplyakov, A. V. J. Am. Chem. Soc. 2006, 128, 9300. (21) Ellison, M. D.; Hovis, J. S.; Liu, H. B.; Hamers, R. J. J. Phys. Chem. B 1998, 102, 8510. (22) Leftwich, T. R.; Teplyakov, A. V. J. Phys. Chem. C 2008, 112, 4297. (23) The National Institute of Standards and Technology (NIST); NIST Chemistry WebBook, 2005; http:// http://webbook.nist.gov/chemistry/. (24) Wayner, D. D. M.; Wolkow, R. A. J. Chem. Soc. Perkin Trans. 2 2002, 23. (25) Zhang, X.; Garfunkel, E.; Chabal, Y. J.; Christman, S. B.; Chaban, E. E. Appl. Phys. Lett. 2001, 79, 4051. (26) Faggin, M. F.; Green, S. K.; Clark, I. T.; Queeney, K. T.; Hines, M. A. J. Am. Chem. Soc. 2006, 128, 11455.
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J. Phys. Chem. C, Vol. 113, No. 16, 2009 6653 (38) Agren, H.; Roos, B. O.; Bagus, P. S.; Gelius, U.; Malmquist, P. A.; Svensson, S.; Maripuu, R.; Siegbahn, K. J. Chem. Phys. 1982, 77, 3893. (39) Guerra, M.; Jones, D.; Colonna, F. P.; Distefano, G.; Modelli, A. Chem. Phys. Lett. 1983, 98, 522. (40) Rignanese, G. M.; Pasquarello, A.; Charlier, J. C.; Gonze, X.; Car, R. Phys. ReV. Lett. 1997, 79, 5174. (41) Rignanese, G. M.; Pasquarello, A. Surf. Sci. 2001, 490, L614. (42) Rignanese, G. M.; Pasquarello, A. Phys. ReV. B 2001, 6307. (43) Bradley, G. M.; Strauss, H. L. J. Phys. Chem. 1975, 79, 1953. (44) Rodrı´guez-Reyes, J. C. F.; Teplyakov, A. V. Phys. ReV. B. 2008, 78, 165314/1. (45) Queeney, K. T.; Chabal, Y. J.; Raghavachari, K. Phys. ReV. Lett. 2001, 86, 1046. (46) Widjaja, Y.; Musgrave, C. B. J. Chem. Phys. 2004, 120, 1555.
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