Electrostatic induced molecular tilting in self-assembled monolayers of n -octadecylamine on mica

Subscriber access provided by Red de Bibliotecas del CSIC The Journal of Physical Chemistry C is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties. Article Electrostatic Induced Molecular Tilting in SelfAssembled Monolayers of n-Octadecylamine on Mica Jaime Oviedo, Miguel A. San-Miguel, Jose A. Heredia-Guerrero, and Jose Jesus Benitez J. Phys. Chem. C, Just Accepted Manuscript • Publication Date (Web): 06 Mar 2012 Downloaded from http://pubs.acs.org on March 7, 2012 Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. 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Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts. 1 Electrostatic Induced Molecular Tilting in Self-Assembled Monolayers of n-Octadecylamine on Mica Jaime Oviedo,1 Miguel A. San-Miguel,1 José A. Heredia-Guerrero,2 José J. Beníte.2* 1Physical Chemistry Department, University of Seville, Seville (Spain) 2Materials Science Institute of Seville (ICMS), Spanish Research Council (CSIC)- University of Seville Americo Vespuccio 49. Isla de la Cartuja, 41092. Seville (Spain) *Corresponding author: [email protected]s, phone: 34 954489551, fax: 34 954460665. Page 1 of 27 ACS Paragon Plus Environment The Journal of Physical Chemistry 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 2 ABSTRACT Self-assembled monolayers of n-octadecylamine on mica (ODA/mica SAMs) have been investigated by atomic force microscopy (AFM) and by attenuated total reflectance infrared (ATR-FTIR) and X-ray photoelectron (XPS) spectroscopies. Topographic data characterizes a stable configuration with the alkyl skeleton tilted ≈ 46° from the surface normal that is rationalized according to a well established structural alkyl chain packing model. Extended contact with air increases molecular tilting up to ≈ 58°. ATR-FTIR and XPS reveals the presence of protonated amino groups within the monolayer and its increment upon exposure to air. The transition between both tilted states is explained assuming the protonation reaction as the driving force and introducing a model to evaluate an electrostatic repulsions term in the overall cohesive energy balance of the system. ODA molecules in the self-assembled monolayer respond to their spontaneous protonation by atmospheric water by tilting as a mechanism to relax the repulsions between –NH3+ heads. Keywords: self-assembled monolayer, octadecylamine, SAM ripening, amine protonation. Page 2 of 27 ACS Paragon Plus Environment The Journal of Physical Chemistry 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 3 INTRODUCTION A monolayer of self-assembled molecules (SAM) on a flat surface is a typical example of a well ordered 2D system. The most widely studied are those constituted by linear long chain functionalized alkyl molecules.1 The technological interest of these systems is based in its consideration as basic steps in a more complex and hierarchical process of nanostructures fabrication.2 Most of these self-assembled systems are designed by the formation of a strong bond between the molecule and the support to ensure its stability. This is the case of alkythiols on gold and, in a lesser extent, alkylsilanes on silicon. In addition to this, the presence of a metallic or semiconductor support facilitates their integration in conventional lithographic procedures. For these very same reasons, much less attention has been paid to amphiphilic molecules weakly bonded to a non-metallic support. However, from a fundamental point of view, these are very adequate systems to investigate intermolecular interactions. In the absence of a support driving the structure of molecular packing, 2D ordering is ruled by molecule to molecule interactions, among them van der Waals, electrostatic forces and hydrogen bonding. Direct structural determination of self-assembled monolayers is commonly attained by high resolution scanning probe microscopies such as scanning tunneling (STM) and atomic force (AFM) microscopies.3,4,5 In the particular case of long chain alkyl SAMs weakly adsorbed on a non-conducting support, STM cannot be used. Furthermore, the mechanical effect caused by the scanning tip when operating an AFM microscope in lateral force mode, the usual conditions to obtain atomic resolution, favors the disruption and erosion of the SAMs. Few years ago, a model describing the molecular packing of linear long chain alkyl molecules in SAMs was proposed.6,7,8 It is based on the maximum interlocking of the zig-zag arrays of methylene groups between adjacent alkyl chains; and it has Page 3 of 27 ACS Paragon Plus Environment The Journal of Physical Chemistry 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 4 explained very successfully the experimentally observed step-wise topographic and frictional changes of alkanethiols on gold and alkylsilanes on Si and mica SAMs under the pressure exerted by the scanning probe. The model predicts the existence of specific tilting angles and monolayer heights and, based exclusively on van der Waals interactions, it estimates the activation energy required for the transition between tilted states. Due to the above mentioned difficulties in the structural characterization of weakly adsorbed SAMs, this research takes advantage of the model to study the molecular packing of delicate octadecylamine (ODA) SAMs on mica using the topographic data obtained by AFM. In the particular case of ODA/mica SAMs there are some experimental observations to be explained. For instance, the monolayer height is significantly shorter if compared with analogous SAMs.6,8 This suggests a more tilted configuration despite the associated cohesive energy loss of the packed layer.9 Second, a characteristic and spontaneous height reduction is systematically observed when ODA/mica SAMs are left in contact with air (ripening) for an extended period of time.10,11 Progressive amino group (-NH2) protonation and the derived electrostatic repulsion between resulting (-NH3+) groups was early proposed to explain such height reduction, however no additional data supporting this hypothesis has yet been provided. To investigate these findings, in this article we have used ATR-FTIR and XPS to detect and quantify amino protonation and we have introduced an electrostatic repulsive term arising from the presence of protonated amino (-NH3+) groups in the energy balance of ODA/mica SAMs. To our knowledge, it is the first time that such study is reported in the literature. Page 4 of 27 ACS Paragon Plus Environment The Journal of Physical Chemistry 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 5 EXPERIMENTAL n-Octadecylamine (ODA, C18H37NH2, Fluka >99%) was used as received with no further purification. ODA/mica SAM islands have been prepared by immersing the freshly cleaved mica (muscovite, New York Corp.) pieces in the 15 mM in chloroform (Panreac > 99.8% stabilized with 50 ppm of amylene) solution for 1 minute. They are removed with no rinsing and dried under dry N2 for 2-3 minutes. After preparation, samples are allowed to ripen in contact with air for controlled periods of time (from 2 hours to 7 days) inside a Petri dish. No specific room humidity control is performed, but it ranges from 45% to 55% RH (temperature calibrated relative humidity HIH Series Honeywell sensor). This article is mainly focused on the study of islands topographic changes upon contact with air, thus, though tracking analysis are done along the process, fresh samples, i.e. those analyzed approximately 2 hours after preparation, and those extensively ripened (5-7 days) showing the already mentioned height reduction are the most significant. For this reason both samples are continously contrasted along the article. The AFM microscope is a Nanotec “Cervantes” with a 10 µm x 10 µm scanner and connected to a “Dulcinea” electronics. A rectangular Olympus RC800PSA lever with k = 0.1 N/m was employed. For distance calibration we used a NT-MDT TGT01 silicon grating (2.12 µm pitch) for the X and Y directions, and a Nanosensor H8 certified grating with 7.0 nm step height for the Z direction. Images were processed and analyzed using the WSxM software.12 AFM images were acquired using the so called “jumping mode” in air at 20-25 °C and 45-55% RH. In this mode, a full approachretract cycle is completed at every point of the matrix defining an image. Feedback enabling at the contact force set point provides the topographic data while pull-off forces define the adhesion map. Standard contact mode AFM has been unsuitable for Page 5 of 27 ACS Paragon Plus Environment The Journal of Physical Chemistry 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 6 the study of non-ripened ODA/mica SAMs, in particular when the scanning size and speed are reduced. ODA/mica SAMS islands do need some time, typically several hours, to gain cohesion and resist the mechanical erosion and compression from the tip working in permanent contact. With jumping mode, dragging forces associated to contact and tip displacement in the (X,Y) plane are avoided because rastering is done out of contact.13 To further prevent mechanical damage, the set point (i.e. the vertical load exerted by the tip on the SAMs islands when re-establishing contact) was kept as low as possible. With this type of lever the force applied to the islands when reestablishing contact is calculated to be around 4.5 nN. 256 lines are acquired in every image at 1 line/s rate. A more detailed description of the AFM acquisition conditions is given elsewhere.14 Stable and reproducible approach/retract cycling is considered as a reference for the absence of material picking by the probe. ATR-FTIR spectra have been collected using a PIKE MIRacleTM ATR accessory (ZnSe crystal plate) attached to a Jasco FT/IR 6200 spectrometer equipped with a MCT detector operating at 4 cm-1 resolution. 250 scans have been accumulated in each run using non-polarized radiation. To prevent interferences arising from mica support, small pieces were repetitively cleaved to the minimum thickness to allow handling and deposition of n-octadecylamine by dipping.15 XPS spectra were obtained with a Physical Electronics PHI 5700 spectrometer using non-monochromatic Mg Kα radiation (300 W, 15 kV, 1253.6 eV) and a multichannel detector. Spectra were recorded in the constant pass energy mode at 29.35 eV with a 720 µm diameter analysis area. Binding energy (B.E.) values were referred to the methylene C1s peak at 285 eV. C1s, O1s, K2p and N1s regions have been collected. Particularly, the N1s signal has been analyzed in detail and experimental data have been fitted to Gauss–Lorentz curves with fixed 1.8 eV FWHM using the resident software Page 6 of 27 ACS Paragon Plus Environment The Journal of Physical Chemistry 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 7 (PHI Multipack v.8.2B). This band width is extracted from the XPS of bulk ODA containing only the (–NH2) contribution. The relative concentration of N species is calculated from the area of the fitting components. In some cases and to improve the fitting, virtual weak contributions were added to account for some asymmetry on the low and high B.E. sides of the N1s peak. EXPERIMENTAL RESULTS Topographic characterization of ODA/mica SAMs In our experimental conditions, ODA forms rounded, 500-750 nm in diameter and one monolayer thick islands on mica. In such islands ODA molecules are attached to the surface of mica by the amino group and expose the terminal methyl group, as deduced from friction and adhesion maps.9,14 When freshly prepared (i.e. ≈ 2 hours old) isolated ODA/mica SAM islands with relatively sharp edges are detected, Figure 1a. Ripening in air causes their progressive contour rounding and aggregation, Figure 1b. However the most striking observation is the spontaneous island height reduction from an initial 17Å value to about 13Å after exposition to the atmosphere for about a week. No such height reduction is observed at periods of aging below 5-7 days. Island flattening is a sudden event and only in very rare occasions both heights can be observed in the same analysis area (see Figure S1 in supporting information). In both cases height values are well below the full molecular length (24.6Å) and, therefore, a configuration in which the alkyl axis is tilted from the surface normal direction is proposed (theoretically 46° and 58°, respectively). Structural and chemical characterization of ODA/mica SAMs Page 7 of 27 ACS Paragon Plus Environment The Journal of Physical Chemistry 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 8 Use of contact mode AFM has been unsuccessful to obtain molecular resolution images of ODA/mica SAMs.9 The pressure and dragging exerted by the scanning probe is too high to prevent the displacement of the weakly bonded ODA molecules. This disadvantage has hindered the structural resolution of ODA/mica SAMs packing. We also lack direct structural information of ODA/mica SAMs from diffraction techniques. Consequently, we have to rely on indirect structural data obtained from ATR-FTIR spectroscopy, mostly from the analysis of vibrational modes of methylene (-CH2-) groups. The frequencies of methylene C-H stretching bands of ODA/mica SAMs (νa 2917 cm-1 and νs 2850 cm-1), Figure 2a, are much closer to those reported for crystalline than to the ones corresponding to liquid n-alkanes.16 The peak positions are also very similar to those of solid octadecylamine and well-packed alkyl monolayers.17,18 Another indication of an ordered, crystal-like structure of ODA/mica SAMs is the splitting of the (–CH2–) scissoring deflection around 1470 cm-1, Figure 2b.19 The magnitude of the experimental splitting (6 cm-1) is also consistent with a tilted configuration of the alkyl molecules within the crystalline subcell.18 Finally the series of methylene twistingrocking (T) and /or wagging (W) progression bands in the 1350-1250 cm-1, Figure 2b are an additional indication of a high degree of chain ordering within the ODA/mica SAMs structure.20,21 The methylene C-H stretching region displays very slight modifications upon ripening and tilting (i to ii in Figure 2a). No νa and νs frequency modification is observed, however a mild peak sharpening (FWHM from 20 to 16 cm-1 for νa and 12 to 10 cm-1 for νs) upon aging can be detected. The bandwidth has been related to the molecule mobility within the packed layer22 and therefore, a slight improvement of the conformational ordering in the ODA/mica SAMs can be deduced. The same conclusion Page 8 of 27 ACS Paragon Plus Environment The Journal of Physical Chemistry 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 15 increment of charge density in the confined structure of the ODA/mica SAM is accompanied by an increase of the electrostatic repulsions and, to accommodate the charge increment, the system reacts by separating them and reaching the Θ = 1/3 state, Figure 6. Separation is achieved by a step-wise alkyl chain to chain sliding and the transition to the next allowed tilted configuration (n = 2 to n = 3). In these conditions, the new cohesive-repulsive energy balance is represented by the Θ = 1/3 curve in Figure 5b and predicts a head to head separation (d) of ≈ 9Å, in good agreement with the one calculated from experimental AFM data. From the curves in Figure 5b, the n = 2 to n = 3 transition needs ∆E ≈ 12 kJ on going from a protonation percentage of 25% to 33.3%. This would require a process supplying about 140 kJ/mol, which is a higher value but in the order of magnitude of the amine protonation enthalpy reported. In addition to this, it should take into account that mica surfaces carries a negative charge and the incorporation of positively charged groups provides an additional electrostatic energy that may contribute to the n = 2 to n = 3 transition. CONCLUSIONS Extended contact with air (ripening) is an important parameter influencing the topography of weakly adsorbed self-assembled monolayers of n-octadecylamine on mica. The spontaneous protonation of weak basic amino groups induced by atmospheric water introduces significant electrostatic repulsions between molecules in such a confined structure. Starting from a well established model describing the close packing of alkyl chains in SAMs, the contribution of the electrostatic repulsions have been evaluated using dipole to dipole interactions. The balance between cohesive and repulsion energy as a function of the head to head separation in ODA/mica SAMs show Page 15 of 27 ACS Paragon Plus Environment The Journal of Physical Chemistry 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 16 a series of minima that can be assigned to experimentally observed tilted configurations. Tilting, and the consequent head to head separation, is the response of the packed system to relax such electrostatic repulsions. The model proposed predicts an energy gap for the transition between tilted configurations in the order of magnitude of the driving process: the amine protonation by water. ACKNOWLEDGMENT Funding is provided by the Spanish Ministerio de Ciencia e Innovación under project CTQ2008-00188. SUPPORTING INFORMATION Supporting information contains AFM data showing the coexistence of the h ≈ 17Å and the h ≈ 13Å phases in a n-octadecylamine self-assembled monolayer on mica; XPS data on the progressive protonation of ODA molecules along ripening; the algorithm to calculate the convergence constant (C) in the electrostatic repulsion energy calculation. This information is available free of charge via the internet at http://pubs.acs.org. Page 16 of 27 ACS Paragon Plus Environment The Journal of Physical Chemistry 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 17 TABLES Table 1 Molecular Tilting Angle from the Surface normal (θ) Predicted from the Structural Model Proposed by Salmeron et al. cos θ n 0 ½ 1 1 ½ 2 2 ½ 3 3 ½ 4 m = 0 1 0.966 0.883 0.782 0.685 0.601 0.531 0.475 0.425 m = 1 0.956 0.923 0.844 0.748 0.655 0.575 0.508 0.454 0.406 m = 2 0.852 0.823 0.752 0.666 0.584 0.512 0.452 0.405 0.362 m = 3 0.735 0.710 0.649 0.575 0.503 0.442 0.390 0.349 0.312 Page 17 of 27 ACS Paragon Plus Environment The Journal of Physical Chemistry 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 18 REFERENCES 1 Ulman, A. Chem. Rev. 1996, 96, 1533-1554. 2 Whitesides, G. M; Grzybowski, B. Science 2002, 295, 2418-2421. 3 Widrig, C. A.; Alves, C.; Porter, M. D. J. Am. Chem. Soc. 1991, 113, 2805-2810. 4 Alves, C. A.; Smith, E. L.; Porter, M. D. J. Am. Chem. Soc. 1992, 114, 1222-1227. 5 Liu, G-y.; Fenter, P.; Chidsey, C. E. D., Ogletree, D. F.; Eisenberger, P.; Salmeron, M. J. Chem. Phys. 1994, 101, 4301-4306. 6 Barrena, E.; Kopta, S.; Ogletree, D. F.; Charych, D. H.; Salmeron, M. Phys. Rev. Lett. 1999, 82, 2880-2883. 7 Würger, A. Phys. Rev. Lett. 1999, 83, 1696. 8 Barrena, E.; Ocal, C.; Salmeron, M. J. Chem. Phys. 2000, 113, 2413-2418. 9 Benítez, J. J.; Kopta, S.; Ogletree, D. F.; Salmeron, M. Langmuir 2002, 18, 60966100. 10 Benítez, J. J.; Kopta, S.; Díez-Pérez, I.; Sanz, F.; Ogletree, D. F.; Salmeron, M. Langmuir 2003, 19, 762-765. 11 Benítez, J. J.; Salmeron, M. J. Chem. Phys. 2006, 125, 044708. 12 Horcas, I.; Fernandez, R.; Gomez-Rodriguez, J. M; Colchero, J.; Gomez-Herrero, J.; Baro, A. Rev. Sci. Instrum. 2007, 78, 013705. 13 de Pablo, P. J.; Colchero, J.; Gómez-Herrero, J.; Baró, A. M. Appl. Phys. Lett. 1998, 73, 3300-3302. 14 Benítez, J. J.; Heredia-Guerrero, J. A.; Salmeron, M. J. Phys. Chem. C 2010, 114, 12630-12634. 15 Guzonas, D. A.; Hair, M. L.; Tripp, C. P. Appl. Spectrosc. 1990, 44, 290-293. 16 Snyder, R. G.; Strauss, H. L.; Elliger, C. A. J. Phys. Chem. 1982, 86, 5145-5150. 17 Hostetler, M. J.; Stokes, J. J.; Murray, R. W. Langmuir 1996, 12, 3604-3612. Page 18 of 27 ACS Paragon Plus Environment The Journal of Physical Chemistry 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 19 18 Chernyshova, I. V.; Hanumantha Rao, K.; Vidyadhar, A.; Shchukarev, A. V. Langmuir 2000, 16, 8071-8084. 19 Snyder, R. G. J. Mol. Spectrosc. 1961, 7, 116-144. 20 Smith, E. L.; Porter, M. D. J. Phys. Chem. 1993, 97, 8032-8038. 21 Snyder, R. G. J. Chem. 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R.; Pasricha, R.; Mandale, A. B.; Sastry, M. Langmuir 2003, 19, 6277-6282. 33 Matsuo, Y.; Miyabe, T.; Fukutsuka, T.; Sugie, Y. Carbon 2007, 45, 1005-1012. 34 Chmielewska, E.; Tylus, W. In Combined and Hybrid Adsorbents; Loureiro, J. M.; Kartel, M. T., Eds.; Springer, 2006; pp 63-70. Page 19 of 27 ACS Paragon Plus Environment The Journal of Physical Chemistry 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 20 35 Israelachvili, J. Intermolecular and Surface Forces; Academic Press: New York, 1991; p 89. 36 Wohlfarth, C. In Handbook of Chemistry and Physics 87th Edition; Lide, D. R., Ed.; CRC Press: Boca Raton FL, 2006; secc. 6, pp 132-153. 37 Didymus, J. M.; Mann, S.; Benton, W. B.; Collins, I. R. Langmuir 1995, 11, 31303136. 38 Paoletti, P.; Barbucci, R.; Vacca, A. J. Chem. Soc. Dalton Trans. 1972, 18, 20102013. TABLE OF CONTENT Page 20 of 27 ACS Paragon Plus Environment The Journal of Physical Chemistry 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 Figure 1. Topographic AFM images of ODA/mica SAMs islands prepared from a 15 mM chloroform solution: (a) imaged 2 hours after preparation, (b) after 7 days in contact with air. Height histograms (right) show an island height reduction from 17Å to 13Å after such ripening time. 138x175mm (300 x 300 DPI) Page 21 of 27 ACS Paragon Plus Environment The Journal of Physical Chemistry 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 Figure 2. ATR-FTIR spectra of ODA/mica SAMs: (i) 2 hours after preparation and (ii) after 7 days ripening in air in (a) the 3000-2800 cm-1 and (b) the 1700-1200 cm-1 regions. Spectroscopic features indicate a closed packing of alkyl molecules and a slight improvement of ordering with ripening time. In addition to this, the presence of a fraction of octadecylamonium octadecylcarbamate (-NH-COO-) and additional protonated amino groups (-NH3+) are detected. 125x111mm (600 x 600 DPI) Page 22 of 27 ACS Paragon Plus Environment The Journal of Physical Chemistry 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 Figure 3. N1s XPS spectra of ODA/mica SAMs exposed to air for (i) 2 hours and (ii) 7 days. Contributions of –NH3+ (401.7 eV), -NH-COO- (400.4 eV) and –NH2 (399.2 eV) have been extracted. Extended contact with air causes further amino groups protonation but no additional carbamation. 137x193mm (600 x 600 DPI) Page 23 of 27 ACS Paragon Plus Environment The Journal of Physical Chemistry 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 Figure 4. Schematic drawing illustrating the reduction of lateral molecule to molecule interactions in a self-assembled monolyer upon tilting (θ). Asterisk indicates the methylene (–CH2-) groups having no counterparts on one side. 113x94mm (300 x 300 DPI) Page 24 of 27 ACS Paragon Plus Environment The Journal of Physical Chemistry 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60