Building Organic Monolayers Based on Fluorinated Amines on the Si

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Building Organic Monolayers Based on Fluorinated Amines on the Si(111) Surface Yuexing Cui, Fangyuan Tian, Fei Gao, and Andrew V. Teplyakov J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/jp507158x • Publication Date (Web): 27 Oct 2014 Downloaded from http://pubs.acs.org on November 2, 2014

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The Journal of Physical Chemistry

Building Organic Monolayers Based on Fluorinated Amines on The Si(111) Surface Yuexing Cui,1 Fangyuan Tian,2 Fei Gao,1 Andrew V. Teplyakov1*

1. Department of Chemistry & Biochemistry, University of Delaware, Newark, DE 19716 2. Department of Chemistry & Biochemistry, University of San Diego, San Diego, CA 92110

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* Corresponding author. Department of Chemistry & Biochemistry, University of Delaware, Newark, DE 19716. Tel.: (302) 831-1969; Fax: (302) 831-6335; E-mail: [email protected]

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ABSTRACT Functionalized silicon surfaces can serve as starting points for a wide variety of structures. Controlled introduction of fluorine-containing monolayers on silicon may affect a number of chemical and physical properties of silicon substrates. This approach becomes especially interesting when these monolayers are built based on the interaction of amino-functionalized fluoroorganics with chlorinated silicon single crystals. In this work, a carefully prepared H-terminated Si(111) surface is converted into Cl-Si(111) by mild chlorination with PCl5 and then reacted with trifluoroethylamine (TFEA) and pfluoroaniline (pFA) using a wet-chemistry procedure in an oxygen-free environment. The surface species formed and the efficiency of the reactions are monitored by infrared spectroscopy, X-ray photoelectron spectroscopy, and complemented with density functional theory (DFT) studies. Although the reaction of TFEA can be optimized to form a nearly complete monolayer, the similar procedure with pFA results primarily in surface oxidation, despite similar reaction energy landscapes predicted by DFT. This difference is discussed based on the differences of adsorption geometries of the two amines on Cl-terminated Si(111) surfaces.

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I. INTRODUCTION Studies of organic monolayers on semiconductor materials have drawn substantial attention over the last three decades, especially due to the applications of these substrates in microelectronics, photovoltaics, and biosensing.1 On silicon-based substrates, the formation of organic monolayers by wet chemistry methods is commonly achieved via hydrosilylation reactions between alkenes or alkynes and H-terminated silicon leading to the formation of Si-C bonds.2-3 Although alternative approaches using coupling chemistry have also been proposed,4-5 for the most part they lead to the formation of monolayers with substantial oxygen incorporation at the interface, which may be detrimental to the electronic properties of the resulting substrates. That is why in a pursuit of alternative functionalization chemistry, recent efforts focused on silicon surface modification leading to Si-N linkers.6-7 Such an interface may be substantially more stable than that formed by Si-C bonds based on a comparison of the average bond dissociation enthalpy. Several monolayer formation processes have been developed for N-containing compounds on silicon surfaces leading to the formation of Si-N linkages. For example, ammonia reacts with Si(100) surface in ultra-high vacuum (UHV) conditions8-9 by forming Si-NH2 and Si-H at 300 K, followed by dissociation to Si-NH-Si as the temperature increases.10-12 A number of studies focus on the surface reactions of clean silicon surfaces with amines, including dimethylamine (DMA), trimethylamine (TMA),13-16 triethylamine(TEA),17 aniline,18 and aryl amines19-20, performed in UHV conditions. However, UHV surface preparation and interface processing are time-consuming and expensive, and for most chemistries described to date, the formation of Si-N bonds is accompanied by the processes leading to other surface species (such as Si-H). Thus, further development of

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wet-chemistry modification procedures that are more convenient and accessible is required. Bergerson et al.21-22 described the utility of using Cl-terminated porous silicon (PSi) in a reaction with amines (butylamine, octylamine, and aniline) by a wet-chemistry approach. The formation of Si-NX-Si surface species was proposed; however, substantial oxidation prevented full characterization of the underlying surface chemistry. Ten years later, Heath group functionalized Si(111) surfaces directly with azide groups by reacting Cl-terminated silicon with NaN2/HMPA or methanol solution.23 Despite this success, relatively low coverage, additional surface species, and fast oxidation limited the practical application of this functionalized surface. More recently, a novel approach to obtain Si-N linkages on single crystalline silicon surface by reacting a well-defined Clterminated Si(111) surface with ammonia saturated THF solution at room temperature was reported.24 Following this reaction, a silicon single crystalline surface functionalized predominantly with secondary amino-groups (-NH-) was formed and remained stable with respect to oxidation if kept under THF. All this work is taking advantage of the decreased energy barrier for a reaction of amines with Cl-terminated silicon compared to that for H-terminated silicon25 and may lead to tunability of surface electronic properties as previously reported following computational investigations.26 The work reported in this manuscript utilizes a wet-chemistry approach to generate Cl-terminated Si(111) surface using PCl5/chlorobenzene solution to react with H-terminated Si surface previously described by Bansal et al.27 A 99% surface Cl coverage was reported previously, with every single silicon reactive site terminated with one chlorine atom. This surface was reacted with trifluoroethylamine (TFEA) and pfluoroaniline (pFA), respectively. The presence of fluorine introduces an additional

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spectroscopic tracking label28 that can be easily followed by X-ray photoelectron spectroscopy (XPS) and infrared spectroscopy, and the results of these studies were interpreted with the help of density functional theory (DFT) computational investigation. In the future, it is expected that combining the utility of a Si-N-based interface with the electronic properties of fluorine-substituted alkyl groups in an oxygen-free environment will allow for tunability of electrical, mechanical, and chemical properties of the surfaces produced.

II. EXPERIMENTAL SECTION II.1. Materials. The n-type double-side polished Si(111) wafers (>0.1 ohm-cm resistance, 500 µm thickness) were obtained from Virginia Semiconductor. All chemicals were reagent grade or better and used as received: hydrogen peroxide (Fisher, 30% certified ACS grade), ammonium hydroxide (Fisher, 29% certified ACS plus grade), buffer-HF improved (Transene Company, INC.),ammonium fluoride (Fluka, Sigma Aldrich, 40% in H2O), hydrochloric acid (Fisher, 37.3% certified ACS grade),chlorobenzene (Acros), PCl5 (Aldrich), benzoyl peroxide (Acros), 2,2,2-trifluoroethylamine (Aldrich, 99.5%), pfluoroaniline (Aldrich, 99%), methanol (Fisher, 99.9%),and tetrahydrofuran (THF) (Fisher, distilled from Na/benzophenone). The deionized water used to rinse the surfaces and containers was from a first generation Milli-Q water system (Millipore) with 18 MΩ.cm resistivity.

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II.2. Surface Preparation Procedure. II.2.1. Preparation of Hydrogen-Terminated Si(111) Surface. The hydrogen-modified Si(111) surface was prepared by a modified RCA cleaning procedure.29 The Teflon beakers were cleaned in a SC1 solution containing 4:1:1 Milli-Q water, hydrogen peroxide, and ammonium hydroxide for 30 minutes with N2 bubbling through the solution in an 80 ºC water bath. Then, the Si(111) wafer was placed into precleaned beakers and covered with SC1 solution for10 minutes in the same water bath. After rinsing with Milli-Q water, the clean wafer was etched in HF buffer solution for 2 minutes and rinsed again with Milli-Q water. After that, the wafer was placed in a freshly prepared SC2 solution of 4:1:1 Milli-Q water, hydrogen peroxide, and hydrochloric acid in the same 80 oC water bath for 10 minutes to grow a silicon oxide layer. Then, the rinsed wafer was etched in HF buffer solution again for 1 minute, followed by 6 minutes in 40% ammonium fluoride solution to form a well-ordered hydrogen-terminated Si(111) surface, as confirmed by the sharp absorption band corresponding to monohydride Si-H at 2083 cm-1 observed in infrared studies.30 II.2.2. Preparation of Chlorine-Terminated Si (111) Surface. The chlorine-terminated Si(111) surface was prepared by a previously described procedure.27 The solution of PCl5 in chlorobenzene solvent with trace amount of benzoyl peroxide as reaction initiator was under N2 atmosphere for at least 30 min to remove gaseous impurities. This procedure was performed in a Schlenk line system. A hydrogenterminated Si(111) wafer was placed into this solution immediately after being dried with N2. The entire setup was placed in an oil bath at 110 ºC for 1 h to prepare the chlorineterminated Si(111) sample.

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II.2.3. Preparation of Amine-Functionalized Si(111) Surface. For trifluoroethylamine reactions, the freshly prepared chlorine-terminated Si(111) sample was immediately transferred into a round flask with 5ml solution of 2% trifluoroethylamine in dry THF as solvent or neat TFEA in a Schlenk line system, as indicated below. After that, the entire setup was either placed in a water bath at room temperature for reaction times from 1 hour to 24 hours, or placed in an oil bath at 60 oC for 1, 2, 4 and 7.5 hours to determine optimal reaction conditions. The room temperature studies suggested that this thermal regime is sufficient and the corresponding results are presented below. For the p-fluoroaniline reaction, different reaction conditions were attempted, including placing the Cl-terminated Si(111) wafer into neat p-fluoroaniline at elevated temperature (from room temperature to 180 oC which is the boiling point for pfluoroaniline) for different reaction times ranging from 1 to 4 hours. In addition, THF was used as a solvent to dilute p-fluoroaniline. Then, the Cl-terminated silicon wafer was placed into this solution at room temperature for 1, 2, 4, 7.5, 10 and 24 hours. All attempts to optimize this procedure mostly resulted in silicon surface oxidation as indicated below. II.3. Characterization Techniques. II.3.1. Fourier-Transform Infrared Spectroscopy (FTIR). Single beam spectra were collected using a Nicolet Magna-IR 560 spectrometer with a liquid-nitrogen-cooled MCT detector. The FT-IR spectra were collected in the range of 4000-650 cm-1 with a 60° angle with respect to the incoming infrared beam with a splitter aperture of 100. 512 scans per spectrum and a resolution of 8 cm-1 were used to

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collect all the spectra. The native-oxide-covered and hydrogen-terminated Si(111) wafers were used as background, as indicated below. II.3.2. X-ray Photoelectron Spectroscopy (XPS). The XPS spectra were collected on a PHI-5600 instrument with a monochromatic Al Kα anode at an energy of 1486.6 eV. The measurements were performed in a vacuum chamber with a base pressure of 1 × 10−9 Torr. The takeoff angle was 45° with respect to the analyzer. The survey spectra were collected over the energy range of 0-1000 eV. The high-resolution spectra for each element, Si 2p, O 1s, C 1s, Cl 2p, and N 1s were collected over the range of 20 eV at 0.1 eV/step. The data analysis was performed using Casa software. All peak positions and relative sensitivity factors were calibrated to the C 1s peak, which was taken to have the binding energy of 284.6eV. II.3.3. Computational Details. Density functional theory calculations were performed using Gaussian 09 suite of programs31 with the B3LYP functional and 6-311G+(d,p) basis set.32-35 A Si9H12 cluster with a single Si-H reactive site and a Si17H24 model including two surface reactive sites were used to compare with XPS observations and to explore the possible surface reaction mechanisms. N 1s core-level energy in the models were predicted using Koopmans' theorem, and the correction factor to the predicted core-level energy for N 1s was found to be 8.06eV based on our previous investigations.36 All of the predicted N 1s energies from calculations were corrected by this factor and then compared with the experimental results.

III. RESULTS AND DISCUSSION

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III.1. DFT Computational Studies of Possible Reactions of pFA and TFEA on ClSi(111). Before exploring and comparing the reactions of two fluorinated amines, TFEA and pFA, with Cl-Si(111) surfaces, it is worth comparing the electronic properties of these molecules. Figure 1 compares the highest occupied molecular orbitals (HOMOs) of these fluorinated amines, which are expected to define the rate of nucleophilic attack onto a surface Si-Cl bond. This brief comparison suggests that the dynamics of their interaction with the surface may be different. The HOMO of TFEA is very well defined, centered on a nitrogen atom, and stabilized by the presence of a C-H bond in anticoplanar position with respect to the lone pair on a nitrogen atom, resulting in Bohlmann bands in vibrational spectra.37-39 On the other hand, the HOMO of pFA is delocalized over the entire molecular system, and has no additional stabilization because no C-H bonds at the α-carbon position are available.

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Figure 1. DFT computationally-predicted HOMOs of A) TFEA and B) pFA. Greycarbon, white-hydrogen, blue-nitrogen.

The first question that may be answered by a computational investigation is whether these differences result in a pronounced change of the respective energy landscapes for interaction of these amines with a Cl-Si(111) surface. The simplest initial reaction of an amine with a Si-Cl bond of such a surface can be mimicked using a cluster model representing a single surface Si-Cl reactive site and leading to the formation of silicon-bound secondary amine and HCl. Because of the excess of amines in a solution, the resulting HCl molecule is likely immediately reacted with another amine, as summarized in Figures 2 and 3.

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Figure 2. DFT predicted reaction pathway for the TFEA reaction with a chlorineterminated Si(111) surface: silicon (gray), chlorine (green), nitrogen (blue), fluorine (purple), and hydrogen (white).

Figure 3. DFT predicted reaction pathway for the pFA reaction with a chlorineterminated Si(111) surface: silicon (gray), chlorine (green), nitrogen (blue), fluorine (purple), and hydrogen (white).

Surprisingly, despite the projected differences in chemical reactivity between the two amines, the DFT studies of the initial reaction of both molecules suggest that the energy landscape should be very similar and, if anything, pFA may be expected to be more efficient in a reaction with chlorinated silicon surfaces. In order to test the

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efficiency of the surface modification, the experimental studies were performed, as outlined below. III.2. Experimental Confirmation of the Proposed Mechanisms of the Reaction of TFEA and pFA with Cl-Si(111).

Figure 4. The high resolution F 1s XPS spectra of Si(111) surface following the reaction of Cl-Si(111) with 2% v/v TFEA in THF solution for a) 10 hours, b) 6.5 hours, c) 4 hours, and d) 1.5 hours. The F 1s spectrum of a fluorine-free Cl-Si(111) surface is shown as spectrum (e).

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Figure 4 summarizes the experimental investigation of the F 1s spectral region collected following the reaction between TFEA and Cl-Si(111) as a function of the reaction time. As opposed to the sample that has no surface fluorine at the start of the reaction shown in Figure 4e, the reaction clearly introduces this element to the surface. The peak that grows in intensity at 688 eV and reaches its maximum intensity for a reaction that is performed for 4 hours is consistent with the presence of a single fluorinecontaining species on the surface. This observation is also consistent with the results of the DFT study, which predicts that the C-F species in the surface-reacted TFEA should exhibit very similar binding energies within the F 1s spectral region. The position of the peak is also consistent with the previously reported spectra of fluoroalkyl groups40 and with the energy predicted by DFT for the key possible surface species described below. Upon further increase of the reaction time, the overall intensity of the F 1s peak actually starts to decrease, its width increases (suggesting the presence of multiple types of species) and in some cases for long reaction times, the presence of an additional feature was observed at higher energies. It should be pointed out that all the features are observed above 685 eV, which is where the signature of Si-F is expected.40 Thus, based on these studies, the 4 hour reaction time yields the optimal protocol for surface reaction, and a longer reaction period results in fluorine removal and possibly additional surface reactions detrimental to the formation of the chemisorbed monolayer.

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Figure 5. The XPS N 1s spectra for the product of the reaction of the Cl-Si(111) surface with 2% v/v TFEA in THF solution for a) 10 hours, b) 6.5 hours, c) 4 hours, d) 1.5 hours, e) before the reaction has started. f) and g) are the DFT computational predictions of N 1s core level energy in secondary and tertiary amine structures depicted in the figure.

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Although the F 1s spectral region provided an excellent starting point for understanding the TEFA reaction with the Cl-Si(111) surface and yielded the optimum reaction time, this part of the XPS investigation did not provide any insight into the nature of the chemical bonds formed between the amino-group and the surface as a result of this reaction. Based on numerous previous studies, the computationally predicted N 1s spectra can serve as a reliable marker for comparison with the experimental spectra to deduce the structure of the surface linkers formed.7, 24 The computationally predicted and calibrated41 features describing the N 1s spectral region are shown together with the experimental data in Figure 5. The predicted and calibrated XPS features for secondary and tertiary amines, at 398.78 eV and 398.53 eV, respectively, are in very good agreement with the observed spectrum for the 4 hour reaction time in Figure 5c. For amines on the silicon surface, the corresponding peaks are expected around 399-400 eV,7, 24

which is also fully consistent with the recorded spectral data. Longer reaction times

result in spectra similar to spectra shown in Figure 5a and 5b for a 10 hour and 6.5 hour process, respectively. The intensity of the N 1s feature decreases for reaction times above 4 hours, suggesting nitrogen removal. The width of the peak increases and in some cases an additional feature becomes noticeable at approximately 402 eV. This is illustrated in Figure 5b for the 6.5 hours reaction time. Most likely, this feature corresponds to surface oxidation forming Si-O-N-like species based on a comparison with numerous previous studies of oxidation of amino linkers on silicon surfaces.24 Thus, following the evolution of the N 1s spectral region as a function of the reaction time, it can be concluded that the reaction that ran for 4 hours results in a single

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type of surface species without noticeable oxidation, however, further reaction leads to nitrogen removal and possible oxidation processes incorporating oxygen into the Si-N linkers. Unfortunately, since the possible nitrogen-containing surface species resulting from this reaction have very similar predicted core-level nitrogen energies, it is not feasible to distinguish these species based solely on the results presented in Figure 5.

Figure 6. The XPS Cl 2p spectra following the reaction of Cl-Si(111) with 2% v/v TFEA in THF solution for a) 10 hours, b) 6.5 hours, and c) 4 hours, d) 1.5 hours. Spectrum e) shows the same spectral range for Cl-terminated Si(111) before the reaction. 16


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In order to further understand the reaction of TFEA with the Cl-Si(111) surface, additional quantitative information based on the XPS investigation may be required. Figure 6 follows chlorine removal on the Cl-Si(111) surface as a function of its reaction time with TFEA in solution. Compared to the initial surface presented in Figure 6e, substantial, albeit not complete, chlorine removal is recorded for a reaction time of 1.5 hours. As would be expected based on the studies of the N 1s and F 1s spectral regions presented above, the 4 hour contact time results in a complete chlorine removal and further reaction does not affect this spectral window. Summarized in Figure 7 are the studies of the Si 2p spectral range as a function of the reaction time. Again, fully consistent with the results presented above, the chlorination of the Si(111) surface and its reaction with the TFEA solution leads to a minimal surface oxidation of up to 4% if the reaction is run for 4 hours or less. Further reaction leads to the formation of oxidized surface species, which is manifested in a clear and prominent feature observed at approximately 103 eV and corresponds to 34% of the spectral signature of silicon for the 10 hour reaction. The feature centered around 103 eV corresponds to the formation of species labeled as SiOx in Figure 7 and is similar to the previously observed features of oxidized species on Si(111).42 In this case, however, SiOx notation encompasses a number of mixed species that also include nitrogen and fluorine, not only the surface oxides or hydroxides. It should also be pointed out that parts of the observed feature are consistent with the presence of N-containing species likely formed on defect sites, similarly to the previously discussed reaction of chlorinated Si(111) surface with NH3.24

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Figure 7. The XPS Si 2p spectra for the reaction of the Cl-Si(111) surface with 2% v/v TFEA in THF solution for a) 10 hours, b) 6.5 hours, c) 4 hours, and d) 1.5 hours. The Si 2p spectra of the Cl-Si(111) surface before the reaction is shown as (e).

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Figure 8. The summary of the XPS studies of Si 2p, N 1s, Cl 2p, and F 1s spectral regions for the reaction of the Cl-Si(111) surface with 2% v/v pFA in THF solution for 2.5 hours, 4 hours, and 12 hours.

In order to compare the reactivity of TFEA and pFA, a set of experiments similar to the one described above in detail for TFEA was also performed for pFA. The summary of these studies is presented in Figure 8. Unfortunately, the reaction of pFA does not lead to a clean incorporation of amine observed for TFEA but rather yields immediate surface 19



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oxidation. Despite some fluorine incorporation and substantial chlorine removal by 4 hours reaction time, the clear result of this reaction is surface oxidation, as confirmed by the results for Si 2p, where surface oxidation starts immediately, even for the reaction times shorter than the ones reported in this figure. The changes in the N 1s spectra as a function of the reaction time indicate that a small amount of nitrogen is incorporated into the sample; however, the observed features suggest that most of the surface species produced by this reaction are very different from the target secondary amines and are produced only at very long reaction times, when the silicon surface has substantial oxygen incorporation. In addition, F 1s spectra are indicative of multiple very different Fcontaining species present on a surface, as opposed to the intact p-fluoro-aromatic species. Based on our XPS data for each spectral region, surface adsorbate coverage was quantified with the overlayer-substrate model first proposed by Briggs and Seah and studied in detail on silicon substrates.43 The coverage of surface nitrogen, fluorine, and chlorine species was calculated by the following equation:  λ sin Θ  SFSi  Φ Ov =   aOv  SFOv

 ρ Si   ρ Ov

 I Ov   I Si

  (1) 

in which Φ Ov is the coverage of specific surface species, penetration depth λ = 2.0 nm at take-off angle of 45o ( Θ ), aOv is the atomic diameter of the overlayer element, which could be nitrogen, fluorine, or chlorine, SF is the sensitivity factor, 0.90 for Si 2p and 1.69 for N 1s, and atomic density ρ equals to 5.41×1022 and 5.0×1022 atom/cm3 for N and Si, respectively. I is the peak intensity obtained from XPS experimental results analyzed by CASA XPS software.44

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In the reaction of TFEA with the Cl-terminated Si(111) surface, the Cl 2p region spectra in Figure 3 shows that surface chlorine atoms are gradually removed from the surface as a function of the reaction time. Based on the peak area of the Cl 2p signal and the Si 2p signal, surface Cl coverage was determined to be 24.2% of the original 100% after 1.5 hours reaction, and zero within the signal-to-noise ratio in the setup for the 4 hour reaction, which indicates that surface chlorine is completely removed. Surface fluorine coverage was determined according to the F 1s and Si 2p signals. The apparent fluorine coverage increases with increasing reaction time and reaches its highest value after 4 hours reaction, then drops to 47.0% after 7.5 hours reaction time. This result is consistent with the nitrogen coverage calculation as the highest value for the nitrogen coverage was found to be 41 % for the 4 hour experiment. Thus, XPS studies indicated that the formation of TFEA and pFA layers on Si(111) surface is dependent on the reaction time and that pFA reaction leads to surface oxidation while TFEA reaction produces surface amine species. With TFEA, 4 hours reaction period results in the removal of all surface chlorine and does not lead to substantial surface oxidation. However, this does not help one to identify the surface species produced as a result of this surface modification process. That is why the FTIR measurements were performed to examine the nature of the surface species formed. Figure 9 shows the most informative spectral regions of the infrared spectra of TFEA reacted with chlorine-terminated Si(111) and compares the experimental results with the computationally predicted spectra of possible surface species formed, specifically secondary and tertiary amines.

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Figure 9: (a) FTIR spectrum of the Si (111) surface following modification with2% TFEA in THF at 60 °C for 4 hours; (b) DFT-predicted infrared spectrum of TFEA; DFTpredicted infrared spectra of TFEA-Si(111) following formation of (c) secondary amine and (d) tertiary amine scaled by a common scaling factor of 0.95 for comparison.

The N-H spectral region shown in the left panel of Figure 9 is notoriously difficult to follow on the silicon samples by vibrational spectroscopy because of the relatively low intensity of the N-H stretching vibrations and also because of the possibility of overlapping O-H vibrations. The experimental spectrum recorded for the reaction of TFEA with the Cl-Si(111) surface exhibits no noticeable signal. Obviously, for molecularly-trapped TFEA or for a secondary amine, the appearance of an absorption band in this spectral region would be expected. For comparison, this panel also provides a spectrum recorded at the same exact experimental setup with the same conditions for ammonia reacted with the same surface,24 that has been shown to form a high coverage, with nearly every two silicon surface atoms connected to a single nitrogen of a secondary amine species. Thus, despite an increased absorbance axis scale, no vibrational signatures 22


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of the primary or secondary amines are recorded following TFEA reaction with a ClSi(111) surface. The next panel in Figure 9 shows the experimental studies of the C-H stretching region. It exhibits two sharp and well-defined peaks at 2850 and 2915 cm-1, corresponding to the asymmetric and symmetric vibrations of the CH2 group in TFEA. This observation confirms that the CH2 part of the modifier molecule remains intact but based on the computational prediction for possible surface species shown in Figure 9b-d, it is nearly impossible to distinguish these signatures in the species examined with the computational models. The next panel examines the spectral region, where an intense and prominent transition corresponding to the –NH2 group should be observed. Consistent with the rest of the results, no absorption signal is recorded in this spectral region, confirming that no primary amines are adsorbed on a chemically modified surface following the reaction. Finally, a portion of the fingerprint region is presented in the left panel and also helps to confirm that mostly secondary or tertiary amine species but no primary amines would be present on a surface following its modification with TFEA.

IV. Summary and Mechanistic Explanation of the Reaction Mechanism. To sum up the experimental studies, an efficient reaction of TFEA with a Clterminated Si(111) surface can be achieved at room temperature with best nitrogen surface coverage of approximately 41% compared to the absolute maximum coverage of 50% if every surface nitrogen atom would be connected to two silicon atoms following the formation of tertiary amine. Despite the fact that the DFT studies predicted very similar surface reaction pathways for TFEA and pFA, the latter does not react with the same surface nearly as efficiently as TFEA. The reaction does occur for pFA, however,

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as the main outcome of the process is surface oxidation rather than selective reaction leading to the formation of tertiary amine. Since variations in reaction time, process temperature, and use of a solvent, did not yield any noticeable improvements within the range of conditions tested, it is reasonable to assume that the reaction proceeds differently for TFEA and pFA with the Cl-Si(111) surface. Two key contributions may shed light on these differences. First, as was pointed out in section III.1, the different geometric properties of HOMOs and LUMOs for TFEA and pFA likely influence the dynamics, and thus the efficiency, of surface processes. The second contribution may be rationalized if a larger computational model is considered for a reaction of pFA with the Cl-Si(111) surface. If a cluster model representing three neighboring silicon surface atoms is reacted with TFEA or pFA, as summarized in Figure S1 in the supporting information section, it becomes clear that the initial formation of primary amines, the first step of the process, creates a geometry that seems to shield neighboring Cl-Si sites in the case of pFA, but the access would be rather simple in the case of TFEA. This observation suggests that the surface is indeed reactive; however, the pFA modification process is limited by molecular steric hindrance effect and that is what likely causes oxidation to occur. The process is much more facile in the case of TFEA. Interestingly, the simple consideration of a single surface Si-Cl site in a computational study does not produce any substantial thermodynamic or kinetic differences for these reactions. This observation implies that other aromatic amines, including nearly any substituted anilines will likely have very similar factors issuing their reactivity with the Cl-Si(111) surface. The supporting information section also provides a summary of experimental and computational studies of the aniline reaction with Cl-Si(111). The computational investigation in Figure S2 in

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the supporting information section suggests that the first step in a reaction with a chlorinated silicon surface is not substantially different for all the amines studied. The experimental XPS investigation summarized in Figure S3 in the supporting information section suggests that very similar reactions and surface coverages are observed for pFA and aniline. In other words, the electronic differences between fluorinated and nonfluorinated amines have a much lower effect on the efficiency of surface reactions with the Cl-Si(111) surface, compared to whether the amines are aromatic or not. Again, the steric effects of hindering the reaction following the first step according to the proposed mechanism seem to dominate for aromatic amines, and thus other reactions become available, that include surface and hydrocarbon fragment oxidation processes. Perhaps in an ideal oxygen-free environment, better coverages could be reached, especially considering that the reaction of the Cl-Si(111) surface can be improved by using neat aniline compared to the solution that had to be used in the case of pFA that is solid at room temperature.

V. CONCLUSIONS Both trifluoroethylamine and p-fluoroaniline were found to react with the Clterminated Si(111) surface. The reaction products were examined using XPS and FT-IR, and the experimental results were compared with those provided by the computational DFT studies. An efficient reaction for TFEA was found to yield the highest surface coverage without substantial surface oxidation for a 4 hour contact time. The primary product of this functionalization is a tertiary amine with two Si-N bonds and an intact CH2CF3 group. The reaction of pFA, though possible, leads predominantly to surface

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oxidation within the range of experimental conditions tested. Despite apparent differences in electronic structure of the amino-groups in TFEA and pFA, the energy landscapes characterizing the initial reaction of both amines with a cluster modeling the Cl-Si(111) surface were very similar, suggesting that the dynamics of surface reactions and possibly the shielding of neighboring Si-Cl surface sites by an aromatic primary amine formed by pFA, rather than stability of surface species and kinetic factors, may determine the efficiency of amination processes. The last assumption was elaborated by a set of computational and experimental studies of aniline that showed high similarity with pFA reactivity on Cl-Si(111).

Supporting Information Available: DFT analysis of pFA and aniline interaction with cluster models representing Cl-Si(111) surface, summary of the XPS investigation of aniline and pFA reactions with a Cl-Si(111) surface, and complete reference 31. This material is available free of charge via the Internet at http://pubs.acs.org.

Acknowledgements This work was supported by the National Science Foundation (CHE 1057374). Acknowledgment is also made to the donors of the Petroleum Research Fund administered by the American Chemical Society for partial support of this research. We would like to thank Prof. Robert L. Opila and his group in the Department of Materials Science, University of Delaware for the ex situ XPS data collection and useful discussions. We also thank Prof. Douglass F. Taber in University of Delaware for helpful

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discussions. YC acknowledges Mr. David Plastino and Chemistry Alumni Science Scholar Award.

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Building Organic Monolayers Based on Fluorinated Amines on the Si

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