Effect of α-Heteroatoms on the Formation of Alkene ... - ACS Publications

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Effect of α‑Heteroatoms on the Formation of Alkene-Derived Monolayers on H−Si(111): A Combined Experimental and Theoretical Study Satesh Gangarapu,† Sidharam P. Pujari,† Hadas Alon,‡ Bart Rijksen,† Chaim N. Sukenik,*,‡ and Han Zuilhof*,† †

Laboratory of Organic Chemistry, Wageningen University, Dreijenplein 8, 6703 HB Wageningen, The Netherlands Department of Chemistry, Institute for Nanotechnology and Advanced Materials, Bar-Ilan University, Ramat-Gan 52900, Israel

‡

S Supporting Information *

ABSTRACT: We investigate herein whether the reactivity and surface coverage of 1-alkenes toward hydrogen-terminated Si(111) surfaces [H-Si(111)] can be improved by introducing heteroatoms such as oxygen and sulfur at the α-position next to the alkene functional group. To this end, the reactivity of 1pentene, 1-pentyne, vinyl ethyl ether, and vinyl ethyl sulfide toward H−Si(111) and the surface coverage of the resulting monolayers were studied and compared. All modified surfaces were characterized by static water contact angle measurements, ellipsometry, X-ray photoelectron spectroscopy (XPS), and infrared absorption reflection spectroscopy (IRRAS). Quantum chemical calculations were performed to calculate the activation barriers and driving forces for monolayer formation at the M11-L/6-311G(d,p) level of theory. Both experiments and theory indicate that the presence of α-heteroatoms next to the alkene function improved both the reactivity and surface coverage on Hterminated Si(111) surfaces.

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ynes.1 Figure 1 shows the currently proposed mechanism for this radical chain reaction for 1-alkenes12−14

INTRODUCTION Crystalline silicon has a band gap, oxide surface chemistry, and etching properties that have enabled the manufacturing of incredibly powerful integrated circuitry.1,2 The fabrication of organic self-assembled monolayers (SAMs) on semiconductor surfaces has recently become a central issue in surface science and device chemistry because of its ability to install new functionalities on the Si surfaces.3−6 In addition, SAMs on semiconductor surfaces have become attractive model systems for both fundamental scientific research and the development of practical molecular devices because they are easily prepared under mild reaction conditions and without expensive equipment.1,7 The high reactivity of alkenes and alkynes toward silyl radicals has been employed as one of the routes for the formation of covalently bound organic monolayers on silicon.8−10 As known from the literature, radical addition to a double or triple carbon−carbon bond results in the formation of a β-carbon radical.11 Subsequently, on H-terminated Si this radical abstracts a hydrogen atom, either from an olefin in solution or from an adjacent Si−H site on the surface, resulting in a surface-centered radical. In practice, the latter, surfacebound Si radical is preferred owing to the lower bond dissociation energy of 84 kcal mol−1 for the Si−H bond in comparison to the bond dissociation energy of the characteristic C−H bonds in 1-alkenes and 1-alkynes (ca. 101 and 111 kcal mol−1, respectively). This process is also the central propagation step in the attachment of Si−C bound monolayers on H−Si(111) surfaces from 1-alkenes, 1-alkynes, and 3-en-1© 2015 American Chemical Society

Figure 1. Propagation steps in the radical chain mechanism for Si−C bound monolayers on H-terminated Si.13,14

The initiation step in the radical chain reaction can occur in several different ways depending on the reaction conditions (thermal or photochemical, even with visible light).15,16 These reactions are not limited to alkenes or alkynes, as radical attachment and subsequent propagation steps have also been demonstrated for unsaturated heteroatom bonds such as in carbonyls17 and for thiols18,19 and alcohols.20,21 Monolayers formed from 1-alkenes are densely packed, meaning that the experimentally observed chain density is close to the theoretical maximum.22 However, this density (of ∼55% Received: April 14, 2015 Revised: June 27, 2015 Published: July 23, 2015 8318

DOI: 10.1021/acs.langmuir.5b01324 Langmuir 2015, 31, 8318−8327

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Figure 2. Schematic representation of the formation of alkene/alkyne-derived organic monolayers on an H-terminated Si(111) surface, for alkene C5C, vinyl ether C5O, vinyl sulfide C5S, and alkyne C5Y under identical conditions.

Surface modifications were carried out by heating the unsaturated compounds in the presence of H−Si(111) wafers. The resulting monolayers were characterized by XPS, ellipsometry, contact angle, and IRRAS measurements. Furthermore, density functional theoretical studies using the M11-L functional were performed to calculate the driving forces and rates of the addition and hydrogen-transfer reactions. This recently developed functional allows for a combination of high accuracy (compared to other functionals) with high computational throughput, making it possible to obtain a range of quantum chemical data that can be compared directly with experiment.29−31 Through this approach, we aim to delineate the effect of α-heteroatoms on the reactivity and surface coverage of unsaturated compounds on H−Si(111) surfaces.

for long 1-alkenes) is not high enough for certain types of studies, i.e., to study effects of the attached monolayer on the work function of the underlying Si or those related to minimizing the contribution of pinholes in the overall through-monolayer current density.23 For higher densities, monolayers derived from 1-alkynes and trans-3-en-1-ynes have been developed.42 These also yield improved order and monolayer stability,24 which has advantages for a number of applications. Therefore, it is of significant interest to investigate other factors that may yield such improved monolayer properties based on other easily accessible alkene-based systems. Silyl radical reactions have recently been investigated using both experimental and theoretical approaches.8,25,26 These studies revealed important differences in the reactivity of various functional groups. Radical reactions at the H−Si surface have been studied in detail by quantum chemical calculations, and the use of models with a sufficiently large number of Si atoms ensured a good representation of the Si surface.8 However, in view of the computational costs, most studies were limited to small reagent molecules (ethylene and acetylene), which consequently results in a limited view of the reactivity of actual monolayer precursor molecules and precludes insight into the roles of the third and following atoms as seen from the surface. Larger substrate models that would mimic the surface reactivity properly can nowadays be studied by density functional methods. “Classical” DFT functionals such as B3LYP did not always provide accurate estimates, especially for transition-state energies. Therefore, more recently developed, high-quality functionals such as those in the Truhlar series provide a good approach to improving upon previous studies with appropriately sized surface models.27,28 Figure 2 therefore describes our aims in the current study: we aim to investigate the effects of heteroatoms at the α-position with respect to the reactive CC moiety on the formation of Si−C linked alkene-derived monolayers on H−Si(111). To this end, we compare the reactivities of 1-pentene (C5C), 1pentyne (C5Y), ethyl vinyl ether (C5O), and ethyl vinyl sulfide (C5S) in terms of the reaction rate and maximally obtainable monolayer quality. In addition, the study also focuses on the relative reactivity of these alkenes via competition reactions by analyzing the composition of the resulting mixed monolayers. A key point in this study is the use of short alkyl chains so as to maximize the relative effects of the heteroatoms, as the monolayer-stabilizing alkyl chain at the top of the monolayer washes out differences between the various attachment chemistries, i.e., short chains bring out the characteristic differences more clearly. In addition, it allows a clear XPS C 1s-based quantification of the composition of the mixed monolayers.

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MATERIALS AND METHODS

Materials. Acetone (semiconductor-grade VLSI PURANAL Honeywell 17617) and 40% ammonium fluoride solution (40% NH4F) (Sigma/Honeywell, semiconductor grade) were used as received. Hexane (Sigma-Aldrich), dichloromethane (DCM, Fisher), and other solvents used were either analytical grade or distilled prior to use. For rinsing and contact angle measurements, Milli-Q water (≥18.3 MΩ cm resistivity) was used. 1-Pentyne (C5Y, ≥ 99%, boiling point (b.p.) = 40 °C), 1-pentene (C5C, ≥98.5%, bp = 30 °C), ethyl vinyl ether (C5O, ≥ 98%, bp = 33 °C), and ethyl vinyl sulfide (C5S, 96%, bp = 91 °C) were purchased from Sigma-Aldrich, purified by distillation, and after three freeze−thaw cycles stored in a glovebox ([O2] < 0.1 ppm and [H2O] < 0.1 ppm) prior to use. Phosphorusdoped (n-type) Si(111) wafers were of prime grade, single-side polished, 500 ± 25 μm thick, 100-oriented, and had a nominal resistivity of 2−10 Ω cm (Siltronix, Archamps, France). Monolayer Formation on Hydrogen-Terminated Si(111) Surfaces. Silicon wafers were cut into pieces (ca. 10 × 10 mm) and cleaned by rinsing with dichloromethane (DCM), followed by sonication for 10 min in acetone. Subsequently, the samples were further cleaned using air plasma (PDC-002 plasma cleaner, Harrick Scientific Products, Inc., Ossining, NY) for 10 min (0.3 SCFH (standard cubic feet per hour) air flow, 29.6 W power, 300 mTorr pressure) to remove any organic contamination and to obtain a hydrophilic surface. After cleaning and extensive water rinsing, the Si chips were etched for 15 min in argon-saturated 40% aqueous ammonium fluoride (NH4F) solution, followed by rinsing with Milli-Q water and finally being blown dry with a stream of argon. These samples were then immediately transferred to the glovebox and put in a custom-made, screw-capped glass vessel. The reaction mixtures were prepared by adding the desired chemicals to the vessel, after which the screw cap was tightly closed. Then the reaction mixture was held overnight at 80 °C (as shown in Figure 2). Mixed monolayers were prepared by adding equimolar mixtures of the various combinations. After the reaction, the vessel was cooled to room temperature and opened under normal atmospheric conditions. The modified surfaces were rinsed several times with hexane, followed by sonication for 5 min in DCM to remove physisorbed molecules, and blown dry with a stream of dry argon. The modified silicon substrates were directly used 8319

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Table 1. Static, Advancing, and Receding Water Contact Anglesa and Thickness Measurements as Obtained for C5 Monolayers on H−Si(111) Surfaces with Alkene, Vinyl Ether, Vinyl Sulfide, and Alkyne Reactive Moieties monolayers

static (deg)

advancing (deg) θa

receding (deg) θr

hysteresis (deg) θa − θr

C5O C5S C5C C5Y

85 75 95 98

88 83 101 101

68 60 84 86

20 23 17 15

thickness (ellipsometry, nm)

molecular length (theory, nm)b

± ± ± ±

0.60 0.61 0.62 0.62

0.8 0.9 0.9 0.8

0.2 0.2 0.2 0.1

Error ±1° measured for three different consecutive samples. bNominal thicknesses computed by ChemBio3D for “linear”, vertically aligned alkyl chains measured from Si to −CH3. See for more details Figures S1 and S2 and Tables S1a and S1b in the Supporting Information.

a

for surface characterization or stored in the glovebox prior to characterization. Monolayer Characterization. Contact angle measurements were performed with a Krüss DSA 100 contact angle goniometer equipped with an automated drop dispenser and video image capture system. The digital drop images were processed by the image analysis system, which calculated both the left and right contact angles from the drop shape with an accuracy of better than ±1.0°. The overall reproducibility of the contact angles was ±2°. The thickness of the modified silicon surfaces (in the dry state) was measured using a rotating analyzer ellipsometer from Sentech Instruments (type SE400) operating at 632.8 nm (He−Ne laser) and having an angle of incidence of 70°. The optical constants of the substrate were determined with a piece of freshly etched H−Si(111) (n = 3.819 and k = 0.057). Each reported value of the layer thickness is an average of eight measurements taken at different locations on the substrate with an error of C5S > C5C, which is in very good agreement with the trend from

Figure 6. Addition (step 1: top (alkene) and middle (alkyne)) and hydrogen transfer (step 2: bottom) of alkenes with X = CH2, −O−, −S− [1-pentene, C5C; vinyl ethyl ether, C5O; vinyl ethyl sulfide, C5S], and pentyne (C5Y) to the Si4H9 radical and Si7H15 model, respectively.

Figure 7. M11-L-optimized geometries (top) of transition states involved in the addition and H-transfer pathway for selected compounds C5O and C5Y. Profile of the potential energy surface (bottom) for (A) the addition reaction of the unsaturated compounds under study to Si radicals (Si4H9 model) and (B) H- transfer at H− Si(111) (Si7 model) at the M11-L/6-311G(d,p) level of theory (in kcal/mol). 8324

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far apart (−8.4 and −16.7 kcal/mol), in line with the appreciably higher C−H dissociation of sp2-hybridized hydrocarbons than from analogous sp3-hybridized ones.52 This difference is reflected in the barrier for C5Y, which is significantly smaller than for C5C (19.8 vs 23.6 kcal/mol). These trends are consistent with the literature values obtained from calculations on a larger surface slab and with the appreciably faster formation of alkyne-derived monolayers.55 This clearly indicates that the formation of alkyne-derived monolayers (Figure 7 bottom, B) occurs much faster than for alkene-derived monolayers. Upon substitution of heteroatoms O and S (C5O and C5S) next to the radical position, the reaction barriers (24.4 and 25.5 kcal/mol, respectively) are higher than for C5C (23.6 kcal/mol). This observation can be explained by the stabilization effect of the radical upon substitution of either oxygen or sulfur atoms and works as a chain stopper (Figure 1). Rate- and Composition-Determining Steps. From our calculations it follows that the first reaction (addition) in the propagation step of monolayer formation has significantly lower activation energy than the second (H transfer). As a result, it is predominantly the first step that determines which alkene/ alkyne attaches out of a mixture. The degree to which this happens is mimicked by our quantum chemical calculations: the difference in activation energy of 1-pentyne C5Y (5.1 kcal/mol) versus that of 1-pentene C5C (6.6 kcal/mol) would lead to the prediction that at 80 °C alkynes react ∼9 times faster with the surface than alkenes. The experimental observation is a factor of 15.7, which corresponds to a difference in activation energy of 1.9 kcal/mol rather than 1.5 kcal/mol. Replacement of the αmethylene moiety in C5C with a heteroatom such as oxygen (C5O) or sulfur (C5S) lowers the calculated activation energy barrier by ∼1.6 and 0.8 kcal/mol, respectively. This predicts that C5S reacts 4 times faster with the surface than C5C, and this factor of 4 is indeed what was observed experimentally. The activation energy of C5O (5.0 kcal/mol) predicts that C5O reacts ∼10 times faster than C5C, where the experimental ratio is again ∼4. Of course, these comparisons between theory and experiment should not be overinterpreted, given the simplicity of our theoretical model (representing an “empty” surface, without any chain−chain interactions), compared to the experimental data which reflect the effects of the overall concentration of the unsaturated compound in solution but also the increasing interchain interactions during monolayer formation. However, these data give the following picture of the kinetics of monolayer formation in a mixture of alkenes/alkynes: at the concentrations used, the first step (addition of the alkene/ alkyne to the Si radical on the surface) dominates the resulting surface composition. Once the compound is attached via Si−C bond formation, the next step (H transfer) may have a much higher activation energy, but because the C radical has no other place to go, eventually hovering over the right Si−H site will result in H transfer and Si· formation. Which alkene/alkyne will then attach to this new surface site will then be predominantly determined by the activation barriers for the competitive addition reactions. In fact, because the diffusion of unsaturated compounds to the reactive Si· surface sites becomes increasingly difficult as monolayer formation progresses, this composition-determining step will eventually also become the rate-determining one. Our experiments and computations both show that the replacement of a α-CH2 group by an α-O or α-S atom speeds up the first step significantly and therefore also the

the experimental findings reported in the previous section, i.e., C5Y > C5O > C5S > C5C. The decrease in activation energy upon going from alkene to alkyne or upon introduction of an α-heteroatom is in line with the observed decrease in distance between the reactive surface Si atom and the C atom of the reactants in the transition state (data shown in Supporting Information Figure S5). For example, in transition state TS1, the optimized Si−C bond distance is 2.651 Å for C5Y and 2.717 Å for C5C, i.e., for the alkyne the TS is slightly earlier. The Si−C bond distances in C5O and C5S are 2.649 and 2.658 Å, respectively, i.e., rather similar to those of C5Y. We examined the spin densities in the radical intermediates formed by attaching the alkenes and alkyne to the Si4H9 radical. In all cases, the spin density is mainly localized at the C2 carbon in the intermediate. The high-spin density value at C2 that is found for C5C and C5Y (1.02 and 0.99, respectively) compared to those for C5O and C5S (0.87 and 0.83, respectively) indicates that substitution by α-heteroatoms next to the radical center indeed delocalizes the spin density significantly. This will facilitate the first reaction step (addition) but gives a corresponding energy penalty in the second step (H transfer). The reaction energy for the addition of an unsaturated species to the surface, i.e., the formation of the Si−C bond, is exothermic for all molecules (Figure 7 bottom, A). The energy of reaction of C5Y is ∼5 kcal/mol more negative than for alkene C5C, as in C5Y a weaker π−π bond is given up upon addition. Upon substitution of α-heteroatoms next to the radical position, the reaction energies (compared to those of C5C) become more negative by ∼2 kcal/mol for C5O and by ∼4.5 kcal/mol for C5S, indicating the stabilization of such a radical by the adjacent heteroatom. This stabilization is supported by C−H bond dissociation energies determined for analogous structures (97.8 kcal/mol for alkyl-H and 92.3 kcal/mol for an alkoxy-H, respectively).51−53 The radical of C5S is found to be more stable than that of C5O due to the presence of empty d orbitals on the sulfur atom. Hydrogen Atom Transfer at the H-Silicon(111) Surface. The second step in monolayer formation is the abstraction of a neighboring surface hydrogen atom by a carbon radical. To calculate the energies involved in H-atom transfer from the silicon, the surface was mimicked with a Si7model, with five backbonded Si atoms (Figure 6). To preserve the surface geometry of the Si−H sites, the positions of the five back-bonded SiH3 groups were fixed during the optimizations, while the two central Si−H sites were allowed to relax. The reactant and product geometries were also optimized without any set constraints and this did not result in significant energy differences with the partially optimized structure, but geometrical fixation of these five Si atoms is essential to mimicking the TS for chemisorption, as otherwise the ring strain that develops is alleviated by significant repositioning of these backbone Si atoms. The energy barrier for H-transfer and reaction energies of four different molecules (C5C, C5O, C5S, and C5Y) were calculated at the M11-L/6-311G(d,p) level of theory. Transition states (TS2) for the H-transfer exhibit sixmembered ring structures that include the crucial C···H···Si atoms, and such rings are consistent with previous findings (Figure 7 top).54 The hydrogen-transfer reaction energies for the four different molecules are summarized at the bottom of Figure 7. The calculations show that the reaction energies of C5C and C5Y lie 8325

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(3) Aureau, D.; Chabal, Y. J. Formation of Organic Monolayers Through Wet Chemistry. In Functionalization of Semiconductor Surfaces; John Wiley & Sons, Inc., 2012; pp 301−337. (4) Teplyakov, A. V.; Bent, S. F. Semiconductor surface functionalization for advances in electronics, energy conversion, and dynamic systems. J. Vac. Sci. Technol., A 2013, 31 (5), 050810. (5) Hamers, R. J. Formation and Characterization of Organic Monolayers on Semiconductor Surfaces. Annu. Rev. Anal. Chem. 2008, 1 (1), 707−736. (6) Konvalina, G.; Haick, H. Sensors for Breath Testing: From Nanomaterials to Comprehensive Disease Detection. Acc. Chem. Res. 2014, 47 (1), 66−76. (7) Buriak, J. M. Illuminating Silicon Surface Hydrosilylation: An Unexpected Plurality of Mechanisms. Chem. Mater. 2014, 26 (1), 763−772. (8) Rijksen, B.; van Lagen, B.; Zuilhof, H. Mimicking the Silicon Surface: Reactivity of Silyl Radical Cations toward Nucleophiles. J. Am. Chem. Soc. 2011, 133 (13), 4998−5008. (9) Ciampi, S.; Harper, J. B.; Gooding, J. J. Wet chemical routes to the assembly of organic monolayers on silicon surfaces via the formation of Si-C bonds: surface preparation, passivation and functionalization. Chem. Soc. Rev. 2010, 39 (6), 2158−2183. (10) Boukherroub, R. Chemical reactivity of hydrogen-terminated crystalline silicon surfaces. Curr. Opin. Solid State Mater. Sci. 2005, 9 (1−2), 66−72. (11) Chatgilialoglu, C. Structural and Chemical Properties of Silyl Radicals. Chem. Rev. 1995, 95 (5), 1229−1251. (12) Scheres, L.; Giesbers, M.; Zuilhof, H. Organic Monolayers onto Oxide-Free Silicon with Improved Surface Coverage: Alkynes versus Alkenes. Langmuir 2010, 26 (7), 4790−4795. (13) Linford, M. R.; Fenter, P.; Eisenberger, P. M.; Chidsey, C. E. D. Alkyl Monolayers on Silicon Prepared from 1-Alkenes and HydrogenTerminated Silicon. J. Am. Chem. Soc. 1995, 117 (11), 3145−3155. (14) Scheres, L.; Giesbers, M.; Zuilhof, H. Self-Assembly of Organic Monolayers onto Hydrogen-Terminated Silicon: 1-Alkynes Are Better Than 1-Alkenes. Langmuir 2010, 26 (13), 10924−10929. (15) Sun, Q.-Y.; de Smet, L. C. P. M.; van Lagen, B.; Wright, A.; Zuilhof, H.; Sudhölter, E. J. R. Covalently Attached Monolayers on Hydrogen-Terminated Si(100): Extremely Mild Attachment by Visible Light. Angew. Chem., Int. Ed. 2004, 43 (11), 1352−1355. (16) Cicero, R. L.; Linford, M. R.; Chidsey, C. E. D. Photoreactivity of Unsaturated Compounds with Hydrogen-Terminated Silicon(111). Langmuir 2000, 16 (13), 5688−5695. (17) Boukherroub, R.; Morin, S.; Sharpe, P.; Wayner, D. D. M.; Allongue, P. Insights into the Formation Mechanisms of Si−OR Monolayers from the Thermal Reactions of Alcohols and Aldehydes with Si(111)−H. Langmuir 2000, 16 (19), 7429−7434. (18) Huang, Y.-S.; Chen, C.-H.; Chen, C.-H.; Hung, W.-H. Fabrication of Octadecyl and Octadecanethiolate Self-Assembled Monolayers on Oxide-Free Si(111) with a One-Cell Process. ACS Appl. Mater. Interfaces 2013, 5 (12), 5771−5776. (19) Lou, J. L.; Shiu, H. W.; Chang, L. Y.; Wu, C. P.; Soo, Y.-L.; Chen, C.-H. Preparation and Characterization of an Ordered 1Dodecanethiol Monolayer on Bare Si(111) Surface. Langmuir 2011, 27 (7), 3436−3441. (20) Harada, Y.; Koitaya, T.; Mukai, K.; Yoshimoto, S.; Yoshinobu, J. Spectroscopic Characterization and Transport Properties of Aromatic Monolayers Covalently Attached to Si(111) Surfaces. J. Phys. Chem. C 2013, 117 (15), 7497−7505. (21) Yaffe, O.; Ely, T.; Har-Lavan, R.; Egger, D. A.; Johnston, S.; Cohen, H.; Kronik, L.; Vilan, A.; Cahen, D. Effect of Molecule− Surface Reaction Mechanism on the Electronic Characteristics and Photovoltaic Performance of Molecularly Modified Si. J. Phys. Chem. C 2013, 117 (43), 22351−22361. (22) Scheres, L.; Rijksen, B.; Giesbers, M.; Zuilhof, H. Molecular Modeling of Alkyl and Alkenyl Monolayers on Hydrogen-Terminated Si(111). Langmuir 2011, 27 (3), 972−980. (23) Yaffe, O.; Scheres, L.; Segev, L.; Biller, A.; Ron, I.; Salomon, E.; Giesbers, M.; Kahn, A.; Kronik, L.; Zuilhof, H.; Vilan, A.; Cahen, D.

overall monolayer formation. In addition, this higher reaction rate is accompanied by a higher monolayer surface density. Overall this points to the potential for vinyl ethers and vinyl sulfides to rapidly form highly dense monolayers on silicon surfaces.

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CONCLUSIONS Substitution of the α-CH2 moiety in 1-alkenes with an α-O or α-S atom increases the reactivity, surface coverage, and packing density of their derived monolayers on oxide-free, Hterminated Si(111) surfaces. The highest reaction rate and surface coverage were found for the alkyne-derived monolayers. Competition experiments for mixed short-chain monolayer formation show that the reactivity of the Si−H surface toward monolayer formation increases in the order pentene < vinyl sulfide ≤ vinyl ether ≪ pentyne. These experimental findings were confirmed by quantum chemical calculations, which clearly revealed the increased nucleophilicity and the stabilization of the C-centered radical by α-O or α-S atoms. In addition, they indicate that the composition of the resulting monolayer is determined in the first step (addition of alkene/ alkyne to Si radical) in monolayer formation.

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ASSOCIATED CONTENT

S Supporting Information *

Theoretical thickness calculations. Mixed alkene-derived monolayers. DFT-simulated XPS binding energies. The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.langmuir.5b01324.

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AUTHOR INFORMATION

Corresponding Authors

*Tel: +972-3-5318072. E-mail: [email protected]. *Tel: +31-317-482361. E-mail: [email protected]. Author Contributions

S.G. and S.P.P. contributed equally. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank Dr. Luc Scheres (Surfix BV, The Netherlands), Tal Taledano, and Prof. David Cahen (Weizmann Institute of Science) for highly informative discussions. We thank Nguyen Minh Quyen (Wageningen University, The Netherlands) for helping us in thickness calculations. This work was supported by NanoNextNL, a micro and nanotechnology consortium of the government of The Netherlands and 130 partners (program T6-C1.3), and by a 2013−2014 Joseph Meyerhoff Visiting Professorship to H.Z. The work at Bar Ilan was supported by the Israel Science Foundation and by the Edward and Judith Steinberg Chair in Nanotechnology.

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REFERENCES

(1) Li, Y.; Calder, S.; Yaffe, O.; Cahen, D.; Haick, H.; Kronik, L.; Zuilhof, H. Hybrids of Organic Molecules and Flat, Oxide-Free Silicon: High-Density Monolayers, Electronic Properties, and Functionalization. Langmuir 2012, 28 (26), 9920−9929. (2) Halik, M.; Hirsch, A. The Potential of Molecular Self-Assembled Monolayers in Organic Electronic Devices. Adv. Mater. 2011, 23 (22− 23), 2689−2695. 8326

DOI: 10.1021/acs.langmuir.5b01324 Langmuir 2015, 31, 8318−8327

Article

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DOI: 10.1021/acs.langmuir.5b01324 Langmuir 2015, 31, 8318−8327