Subnanometer Resolution and Enhanced Friction Contrast at the

3 days ago - We report high-resolution surface morphology and friction force maps of polycrystalline organic thin films derived by deposition of the n...
0 downloads 6 Views 4MB Size
Download PDF
Article Cite This: Langmuir XXXX, XXX, XXX−XXX

pubs.acs.org/Langmuir

Subnanometer Resolution and Enhanced Friction Contrast at the Surface of Perylene Diimide PDI8-CN2 Thin Films in Ambient Conditions Renato Buzio,*,† Andrea Gerbi,† Mario Barra,‡ Fabio Chiarella,‡ Enrico Gnecco,§ and Antonio Cassinese‡ †

CNR-SPIN Institute for Superconductors, Innovative Materials, and Devices, C.so Perrone 24, 16152 Genova, Italy CNR-SPIN and Physics Department, University of Naples Federico II, Piazzale Tecchio, 80125 Napoli, Italy § Otto Schott Institute of Materials Research, Friedrich Schiller University Jena, Löbdergraben 32, 07743 Jena, Germany ‡

S Supporting Information *

ABSTRACT: We report high-resolution surface morphology and friction force maps of polycrystalline organic thin films derived by deposition of the n-type perylene diimide semiconductor PDI8-CN2. We show that the inplane molecular arrangement into ordered, cofacial slip-stacked rows results in a largely anisotropic surface structure, with a characteristic sawtooth corrugation of a few Ångstroms wavelength and height. Load-controlled experiments reveal different types of friction contrast between the alternating sloped and stepped regions, with transitions from atomicscale dissipative stick−slip to smooth sliding with ultralow friction within the surface unit cell. Notably, such a rich phenomenology is captured under ambient conditions. We demonstrate that friction contrast is well reproduced by numerical simulations assuming a reduced corrugation of the tip−molecule potential nearby the step edges. We propose that the side alkyl chains pack into a compact low-surface-energy overlayer, and friction modulation reflects periodic heterogeneity of chains bending properties and subsurface anchoring to the perylene cores.

■

INTRODUCTION Friction force microscopy (FFM) offers the unique possibility to investigate elementary dissipation mechanisms down to the atomic scale.1 Smooth crystalline surfaces, with well-defined arrangements of atoms and molecules, provide ideal benchmarks for FFM studies. Various experiments and numeric simulations have shown that the sliding motion of the probing tip is in such cases well reproduced by the Prandtl−Tomlison (PT) model.2 This model predicts tip trajectories and energy dissipation starting from the tip−surface interaction potential, which in turn is critically affected by the local crystallography and chemistry,3−5 by elasticity6 and defects,7 or even by subsurface (e.g., charge, deformation, and packing) properties.8−10 It follows that the PT model not only delivers intuitive rationalizations for many fundamental features of dry friction but also makes FFM suitable to explore the spatial symmetry and uniformity of the surface potential energy, thus greatly increasing our capability to characterize novel materials and devices. Perylene diimide derivatives are among the most interesting n-type organic semiconductors because of their relatively strong electron affinities and the tailoring of their charge-transport properties upon changing the substituents on the imide N atoms or on the perylene backbone.11−13 In particular, owing to their chemical and electronic stability and the tendency to form © XXXX American Chemical Society

ordered layers, these molecules represent promising candidates for the development, in particular, of highly performing fieldeffect transistors (OFETs).14 The growth and morphology of thin films, mostly obtained from vapor deposition methods, have been extensively studied at the nanoscale,15−19 and some correlations have been established between morphological properties and electronic transport.12,15−19 At the molecular scale, FFM has shown that friction anisotropy, i.e., the variation of friction when sliding along different surface directions, is sensitive to the local surface crystallography, so that it can be exploited to decipher different azimuthal orientations of the perylene domains.20,21 Notably, such structural details are difficult to detect with other techniques, yet they can greatly contribute to improve general structure−property relationships. Hereafter, we expand the connection between FFM contrast and perylene surface structure beyond friction anisotropy by investigating under ambient conditions thin films of the archetypical N,N′-bis(n-ctyl)-x:y-dicyanoperylene 3,4:9,10-bis(dicarboximide) PDI8-CN2. This is a perylene derivative functionalized with the insertion of two cyano groups CN directly bound to the aromatic core (Figure 1a).11 The Received: December 5, 2017 Revised: January 26, 2018 Published: February 26, 2018 A

DOI: 10.1021/acs.langmuir.7b04149 Langmuir XXXX, XXX, XXX−XXX

Article

Langmuir

modest capillary condensation between the probing tip and the hydrophobic surface, formed by tightly packed alkyl chains, and enables both optimal control of contact forces and routine surface mapping without damage. We show that the largely anisotropic molecular arrangement gives a sawtooth surface corrugation of a few Ångstroms wavelength and height. Additionally, load-controlled experiments reveal sublattice friction contrast consisting in the appearance, for repulsive contact forces, of spatial transitions from dissipative stick−slip motion to smooth sliding with superlow friction. It is worth mentioning that a similar phenomenology has been reported so far only for systems carefully prepared and interrogated under vacuum.8,9 Simulations based on the PT model ascribe the enhanced friction contrast to strong variations in the corrugation of the tip−surface potential, which we attribute to periodic heterogeneity in the lamellar arrangement of the alkyl chains. As further discussed below, this description relies on the general concept that larger compressive loads lead to larger deformation volumes under the tip apex, making FFM progressively more sensitive to the specific interactions coupling the topmost surface layer to the buried subsurface regions.

■

EXPERIMENTAL SECTION

Sample Preparation. Multilayer thin films of PDI8-CN2, with nominal thickness in the range 10−30 nm (∼5−15 molecular layers), were prepared by sublimation in a vacuum14,24 onto H-terminated Si (H−Si) and on SiO2, at the base pressure 0 by considering a surface domain with azimuthal orientation ϕ ≈ 47°. A spatial modulation of wavelength b/sin|ϕ| ≈ 1.1 nm can be clearly distinguished on the lateral force traces acquired at the lowest and intermediate loads (1.8 and 2.7 nN), whereas it is partially disrupted at the highest applied load (3.5 nN). On increasing contact pressure, the tip−surface contact occurs over a region much wider than the unit cell, and the simple PT mechanism introduced to explain the friction contrast is no more appropriate. Indeed, for FN > 3.5 nN surface abrasive wear took place (not shown). We note that the onset of surface wear varied from tip to tip due to unpredictable deviations from the nominal tip curvature radius. This is well-known in AFM/FFM studies at the atomic scale.44 The peculiar friction modulation at the PDI8-CN2 surface was however routinely observed using different tips. Our results attest that careful control of the total normal load enables to obtain true molecular resolution as well as to observe physically relevant transitions between different friction regimes. In the present case, this was made possible by the low and very stable tip−surface adhesion, |Fadh| ∼ 0.5−1.0 nN (from pull-off measurements), that assured a fine-tuning of the total normal force |FN + Fadh| in the low-force range ∼0.5−5

Figure 4. (a) Energy landscape used to simulate the sliding friction of the FFM tip on the corrugated PDI8-CN2 surface (as described in the text). (b, c) Simulated lateral force maps (forth and back). The scan direction is off the [100] direction by −10°. (d) Cross sections corresponding to the straight lines in (b, c). (e) Energy landscape with superimposed the trajectories of the tip apex along the lines in (b) and (c).

sections in Figure 4d. Here, the modulation of friction between stick−slip regime (central region) and steady sliding (side regions) excellently agrees with experimental lateral force profiles of Figure 3b. In order to better understand how the PT model, with the proposed phenomenological potential, is capable of reproducing the lateral force maps we refer to Figure 4e. Here, the tip path corresponding to the straight lines in Figures 4b,c is plotted together with the interaction potential of Figure 4a. The tip tends to follow the pulling force in the dark regions of lower corrugation. However, when the bright spots corresponding to enhanced corrugation of the sloped (lamellar) regions are met, the tip motion is hindered and stick−slip takes place, consistent with the lateral force profiles in Figure 4d. It is now crucial to identify the physical origin of the strongly anisotropic energy landscape of Figure 4a. Evaluation of existing literature shows that transitions from dissipative stick−slip motion to smooth sliding have been reported under vacuum by Fessler et al.,41 for an organic compound having differently orientated benzylammonium cations within the surface unit cell. However, such complex orientational effects are not easily recognized at the PDI8-CN2 surface. As previously mentioned, experimental evidence for PDI8-CN2 indicates that on increasing the normal load FN from ∼0 to a few nN, the corrugation amplitude grows faster at the sloped regions than nearby the lamellar edges (e.g., see Figure 3), thereby leading to a load-driven modulation of both lateral force and friction across the surface unit cell. Accordingly, the energy landscape in Figure 4a holds only in the repulsive regime (FN > 0) and is expected to evolve with FN. Whereas the E

DOI: 10.1021/acs.langmuir.7b04149 Langmuir XXXX, XXX, XXX−XXX

Article

Langmuir

FFM response of PDI8-CN2 under ambient conditions and repulsive loads.

■

CONCLUSIONS We have characterized by FFM, under ambient conditions, multilayer thin films of the core-cyanated n-type perylene diimide semiconductor PDI8-CN2. High-resolution morphologies nicely agree with a bulk structural model, assuming a crystallographically defined arrangement of bent alkyl chains. The surface structure appears stable after prolonged exposure to ambient air and shows very low adhesive forces. This confirms surface hydrophobicity, which is by itself relevant for biosensing applications.51 The evolution of friction contrast on increasing normal load reveals, unexpectedly, friction heterogeneity within the surface unit cell. We suggest a close connection of such phenomenon with experiments on inorganic heterostructures, where FFM actually maps the coupling between an atomically thin overlayer and the underlying substrate. A modulation of the diffusion barrier for adsorbed species might also occur, suggesting possible applications of the PDI8-CN2 thin films as templates for the self-assembly of other molecules and the realization of organic heterostructures.52 From a more general perspective, the viability of high-resolution FFM documented both in the present work and in previous studies on PTCDI-C8 thin films20,21 suggests that FFM might effectively complement other scanning probe microscopy techniques53 to assist the molecular design of novel perylene diimide derivatives. Spatially resolved spectroscopy of shear forces might add unique indications on the structural and functional role played by different core substituents and side chains.

Figure 5. (a) Lateral force map (forward direction) over an organic domain with azimuthal orientation ϕ ≈ 47°. The normal force FN is increased from 1.8 nN (bottom, gray) to 2.7 nN (middle, orange) and 3.5 nN (top, red). (b) Friction loops along the dashed lines in (a). Stick−slip motion at 1.8 and 2.7 nN shows a superimposed modulation of wavelength ∼1.1 nm, roughly consisting of two stick−slip events. The modulation is partially disrupted at the higher load 3.5 nN. Shaded areas correspond to one period of the modulation.

nN.46 Assuming a nominal curvature radius R ∼ 8 nm for the tip apex, one gets a tip−surface interfacial energy |Fadh|/4πR ∼ 5−10 mJ/m2.47,48 Such small values fit the response of typical hydrophobic, methyl-terminated low-energy surfaces.49 It is finally intriguing to discuss the above results in light of recent FFM experiments by Paradinas et al. involving the coreunsubstituted parent material PTCDI-C8.50 In these studies, few-layers-thick PTCDI-C8 thin films appear extremely vulnerable to the sweeping effect of the AFM tip so that the surface molecular packing is irreversibly damaged in the repulsive regime (positive loads, FN > 0). Stable FFM contrast on PTCDI-C8 is claimed only in the attractive regime (FN < 0). Differently, we successfully imaged the PDI8-CN2 multilayers with subnanometer resolution at a repulsive contact force FN in the nN range, which in fact allowed us to report on the enhanced friction contrast. Therefore, albeit our work does not tightly compare the onset of wear for thin films of PTCDI-C8 and PDI8-CN2, it nonetheless gives indications for an effective stability of the PDI8-CN2 surface under the mechanical action of the AFM tip. On one hand, we recognize that the lower coverage of PTCDI-C8 in Paradinas et al.50 might contribute to make such system more vulnerable to mechanical damage compared to the thicker PDI8-CN2 films of the present work. On the other hand, considering that surface wear is assisted by the adsorbed water50 and is usually hindered by small interfacial friction, potential differences between PTCDI-C8 and PDI8CN2 might also arise from the occurrence of systematically smaller friction forces between the tip and the core-cyanated material or from a higher resistance of PDI8-CN2 against moisture adsorption/O2 intrusion. This appears in line with arguments that ascribe to core-cyanated materials a higher resistance against oxidation induced by the ambient gases action.11 In general, bay substitutions can significantly change the physical and chemical properties of perylene derivatives. Besides the energetic stabilization of electrons in a low-lying LUMO, CN groups impose minor distortions to the perylene core that are thought to give weaker but geometrically stiffer intermolecular interactions compared to PTCDI-C8.19 As friction forces are a sensitive gauge of molecular packing and structure, core cyanation might have a specific role in the robust

■

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.langmuir.7b04149. High-resolution topographies of grain boundaries and single-molecule defects, high-resolution friction loops, and topographic profiles of the “slippery” lamellar edges (PDF)

■

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (R.B.). ORCID

Renato Buzio: 0000-0001-5632-5531 Fabio Chiarella: 0000-0003-2537-5282 Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS R.B., A.G., M.B., F.C., and A.C. gratefully acknowledge support by the Italian MIUR through Progetto Premiale 2012 “EOS: organic electronics for advanced research instrumentation”. E.G. and R.B. also acknowledge support from the COST Action MP1303 “Understanding and controlling nano and mesoscale friction”. F

DOI: 10.1021/acs.langmuir.7b04149 Langmuir XXXX, XXX, XXX−XXX

Article

Langmuir

■

(18) Liscio, F.; Albonetti, C.; Broch, K.; Shehu, A.; Quiroga, S. D.; Ferlauto, L.; Frank, C.; Kowarik, S.; Nervo, R.; Gerlach, A.; et al. Molecular Reorganization in Organic Field-Effect Transistors and Its Effect on Two-Dimensional Charge Transport Pathways. ACS Nano 2013, 7 (2), 1257−1264. (19) Liscio, F.; Milita, S.; Albonetti, C.; D’Angelo, P.; Guagliardi, A.; Masciocchi, N.; Della Valle, R. G.; Venuti, E.; Brillante, A.; Biscarini, F. Structure and Morphology of PDI8-CN2 for N-Type Thin-Film Transistors. Adv. Funct. Mater. 2012, 22 (5), 943−953. (20) Kalihari, V.; Tadmor, E. B.; Haugstad, G.; Frisbie, C. D. Grain Orientation Mapping of Polycrystalline Organic Semiconductor Films by Transverse Shear Microscopy. Adv. Mater. 2008, 20 (21), 4033− 4039. (21) Perez-Rodriguez, A.; Barrena, E.; Fernández, A.; Gnecco, E.; Ocal, C. A Molecular−scale Portrait of Domain Imaging in Organic Surfaces. Nanoscale 2017, 9, 5589−5596. (22) Colle, R.; Grosso, G.; Cassinese, A.; Centore, R. Structural, Electronic and Vibrational Properties of N,N ′-1H,1H-Perfluorobutyl Dicyanoperylenecarboxydiimide (PDI-FCN2) Crystal. J. Chem. Phys. 2013, 139 (11), 114507. (23) Rivnay, J.; Jimison, L. H.; Northrup, J. E.; Toney, M. F.; Noriega, R.; Lu, S.; Marks, T. J.; Facchetti, A.; Salleo, A. Large Modulation of Carrier Transport by Grain-Boundary Molecular Packing and Microstructure in Organic Thin Films. Nat. Mater. 2009, 8 (12), 952−958. (24) Buzio, R.; Gerbi, A.; Marrè, D.; Barra, M.; Cassinese, A. Electron Injection Barrier and Energy-Level Alignment at the Au/PDI8-CN2 Interface via Current−voltage Measurements and Ballistic Emission Microscopy. Org. Electron. 2015, 18, 44−52. (25) Chiarella, F.; Barra, M.; Carella, A.; Parlato, L.; Sarnelli, E.; Cassinese, A. Contact-Resistance Effects in PDI8-CN 2 N-Type ThinFilm Transistors Investigated by Kelvin-Probe Potentiometry. Org. Electron. 2016, 28, 299−305. (26) Li, M.; Marszalek, T.; Zheng, Y.; Lieberwirth, I.; Müllen, K.; Pisula, W. Modulation of Domain Size in Polycrystalline N-Type Dicyanoperylene Mono- and Bilayer Transistors. ACS Nano 2016, 10 (4), 4268−4273. (27) Brillante, A.; Salzillo, T.; Della Valle, R. G.; Venuti, E.; Borgatti, F.; Lunedei, E.; Liscio, F.; Milita, S.; Albonetti, C. Photoluminescence as a Probe of Molecular Organization in PDI8-CN2 Ultra-Thin Films. J. Lumin. 2017, 187, 403−409. (28) Oh, J. H.; Liu, S.; Bao, Z.; Schmidt, R.; Würthner, F. Air-Stable N-Channel Organic Thin-Film Transistors with High Field-Effect Mobility Based on N,N?-Bis(heptafluorobutyl)-3,4:9,10-Perylene Diimide. Appl. Phys. Lett. 2007, 91 (21), 212107. (29) Di Girolamo, F. V.; Ciccullo, F.; Barra, M.; Carella, A.; Cassinese, A. Investigation on Bias Stress Effects in N-Type PDI8CN2 Thin-Film Transistors. Org. Electron. 2012, 13 (11), 2281−2289. (30) Youn, J.; Dholakia, G. R.; Huang, H.; Hennek, J. W.; Facchetti, A.; Marks, T. J. Influence of Thiol Self-Assembled Monolayer Processing on Bottom-Contact Thin-Film Transistors Based on NType Organic Semiconductors. Adv. Funct. Mater. 2012, 22 (9), 1856− 1869. (31) Chiarella, F.; Barra, M.; Cassinese, A.; Di Girolamo, F. V.; Maddalena, P.; Santamaria, L.; Lettieri, S. Dicyanoperylene-Diimide Thin-Film Growth: A Combined Optical and Morphological Study. Appl. Phys. A: Mater. Sci. Process. 2011, 104 (1), 39−46. (32) Ferlauto, L.; Liscio, F.; Orgiu, E.; Masciocchi, N.; Guagliardi, A.; Biscarini, F.; Samorì, P.; Milita, S. Enhancing the Charge Transport in Solution-Processed Perylene Di-Imide Transistors via Thermal Annealing of Metastable Disordered Films. Adv. Funct. Mater. 2014, 24 (35), 5503−5510. (33) Macrae, C. F.; Bruno, I. J.; Chisholm, J. A.; Edgington, P. R.; McCabe, P.; Pidcock, E.; Rodriguez-Monge, L.; Taylor, R.; van de Streek, J.; Wood, P. A. Mercury CSD 2.0? New Features for the Visualization and Investigation of Crystal Structures. J. Appl. Crystallogr. 2008, 41 (2), 466−470. (34) Green, C. P.; Lioe, H.; Cleveland, J. P.; Proksch, R.; Mulvaney, P.; Sader, J. E. Normal and Torsional Spring Constants of Atomic

REFERENCES

(1) Park, J. Y.; Salmeron, M. Fundamental Aspects of Energy Dissipation in Friction. Chem. Rev. 2014, 114 (1), 677−711. (2) Schwarz, U. D.; Hölscher, H. Exploring and Explaining Friction with the Prandtl−Tomlinson Model. ACS Nano 2016, 10 (1), 38−41. (3) Pina, C. M.; Miranda, R.; Gnecco, E. Anisotropic Surface Coupling While Sliding on Dolomite and Calcite Crystals. Phys. Rev. B: Condens. Matter Mater. Phys. 2012, 85 (7), 1−4. (4) Schönherr, H.; Kenis, P. J. A.; Engbersen, J. F. J.; Harkema, S.; Hulst, R.; Reinhoudt, D. N.; Vancso, G. J. Scanning Force Microscopy Studies on Molecular Packing and Friction Anisotropy in Thin Films of Tetranitrotetrapropoxycalix[4]arene. Langmuir 1998, 14 (10), 2801−2809. (5) Vancso, G. J.; Förster, S.; Leist, H. Nanometer-Scale Tribological Properties of Highly Oriented Thin Films of Poly(tetrafluoroethylene) Studied by Lateral Force Microscopy. Macromolecules 1996, 29 (6), 2158−2162. (6) Lee, C.; Li, Q.; Kalb, W.; Liu, X. Z.; Berger, H.; Carpick, R. W.; Hone, J. Frictional Characteristics of Atomically Thin Sheets. Science (Washington, DC, U. S.) 2010, 328 (5974), 76−80. (7) Steiner, P.; Gnecco, E.; Krok, F.; Budzioch, J.; Walczak, L.; Konior, J.; Szymonski, M.; Meyer, E. Atomic-Scale Friction on Stepped Surfaces of Ionic Crystals. Phys. Rev. Lett. 2011, 106 (18), 2− 5. (8) Maier, S.; Gnecco, E.; Baratoff, A.; Bennewitz, R.; Meyer, E. Atomic-Scale Friction Modulated by a Buried Interface: Combined Atomic and Friction Force Microscopy Experiments. Phys. Rev. B: Condens. Matter Mater. Phys. 2008, 78 (4), 1−5. (9) Zheng, X.; Gao, L.; Yao, Q.; Li, Q.; Zhang, M.; Xie, X.; Qiao, S.; Wang, G.; Ma, T.; Di, Z.; et al. Robust Ultra-Low-Friction State of Graphene via Moiré Superlattice Confinement. Nat. Commun. 2016, 7, 13204. (10) Shi, R.; Gao, L.; Lu, H.; Li, Q.; Ma, T.-B.; Guo, H.; Du, S.; Feng, X.-Q.; Zhang, S.; Liu, Y.; et al. Moiré Superlattice-Level Stick-Slip Instability Originated from Geometrically Corrugated Graphene on a Strongly Interacting Substrate. 2D Mater. 2017, 4 (2), 025079. (11) Jones, B. A.; Facchetti, A.; Wasielewski, M. R.; Marks, T. J. Tuning Orbital Energetics in Arylene Diimide Semiconductors. Materials Design for Ambient Stability of N-Type Charge Transport. J. Am. Chem. Soc. 2007, 129 (49), 15259−15278. (12) Weitz, R. T.; Amsharov, K.; Zschieschang, U.; Villas, E. B.; Goswami, D. K.; Burghard, M.; Dosch, H.; Jansen, M.; Kern, K.; Klauk, H. Organic N-Channel Transistors Based on Core-Cyanated Perylene Carboxylic Diimide Derivatives. J. Am. Chem. Soc. 2008, 130 (14), 4637−4645. (13) Zschieschang, U.; Amsharov, K.; Jansen, M.; Kern, K.; Klauk, H.; Weitz, R. T. Separating the Impact of Oxygen and Water on the Long-Term Stability of N-Channel Perylene Diimide Thin-Film Transistors. Org. Electron. 2015, 26, 340−344. (14) Barra, M.; Di Girolamo, F. V.; Chiarella, F.; Salluzzo, M.; Chen, Z.; Facchetti, A.; Anderson, L.; Cassinese, A. Transport Property and Charge Trap Comparison for N-Channel Perylene Diimide Transistors with Different Air-Stability. J. Phys. Chem. C 2010, 114 (48), 20387−20393. (15) Petit, M.; Hayakawa, R.; Wakayama, Y.; Chikyow, T. Early Stage of Growth of a Perylene Diimide Derivative Thin Film Growth on Various Si(001) Substrates. J. Phys. Chem. C 2007, 111 (34), 12747− 12751. (16) Krauss, T. N.; Barrena, E.; de Oteyza, D. G.; Zhang, X. N.; Major, J.; Dehm, V.; Würthner, F.; Dosch, H. X-ray/Atomic Force Microscopy Study of the Temperature-Dependent Multilayer Structure of PTCDI-C8 Films on SiO2. J. Phys. Chem. C 2009, 113 (11), 4502−4506. (17) Krauss, T. N.; Barrena, E.; Zhang, X. N.; de Oteyza, D. G.; Major, J.; Dehm, V.; Würthner, F.; Cavalcanti, L. P.; Dosch, H. ThreeDimensional Molecular Packing of Thin Organic Films of PTCDI-C 8 Determined by Surface X-Ray Diffraction. Langmuir 2008, 24 (22), 12742−12744. G

DOI: 10.1021/acs.langmuir.7b04149 Langmuir XXXX, XXX, XXX−XXX

Article

Langmuir Force Microscope Cantilevers. Rev. Sci. Instrum. 2004, 75 (6), 1988− 1996. (35) Schwarz, U. D.; Koster, P.; Wiesendanger, R. Quantitative Analysis of Lateral Force Microscopy Experiments. Rev. Sci. Instrum. 1996, 67 (7), 2560−2567. (36) Horcas, I.; Fernández, R.; Gómez-Rodríguez, J. M.; Colchero, J.; Gómez-Herrero, J.; Baro, A. M. WSXM: A Software for Scanning Probe Microscopy and a Tool for Nanotechnology. Rev. Sci. Instrum. 2007, 78 (1), 013705. (37) Buzio, R.; Gerbi, A.; Marré, D.; Barra, M.; Cassinese, A. Ballistic Electron and Photocurrent Transport in Au/organic/Si(001) Diodes with PDI8-CN2 Interlayers. J. Vac. Sci. Technol., B: Nanotechnol. Microelectron.: Mater., Process., Meas., Phenom. 2016, 34 (4), 041212. (38) Overney, R. M.; Takano, H.; Fujihira, M.; Paulus, W.; Ringsdorf, H. Ansiotropy in Friction and Molecular Stick-Slip Motion. Phys. Rev. Lett. 1994, 72 (22), 3546−3549. (39) Bluhm, H.; Schwarz, U. D.; Meyer, K. P.; Wiesendanger, R. Anisotropy of Sliding Friction on the Triglycine Sulfate (010) Surface. Appl. Phys. A: Mater. Sci. Process. 1995, 61 (5), 525−533. (40) Paradinas, M.; Munuera, C.; Silien, C.; Buck, M.; Ocal, C. Heterogeneous Nanotribological Response of Polymorphic SelfAssembled Monolayers Arising from Domain and Phase Dependent Friction. Phys. Chem. Chem. Phys. 2013, 15 (4), 1302−1309. (41) Fessler, G.; Zimmermann, I.; Glatzel, T.; Gnecco, E.; Steiner, P.; Roth, R.; Keene, T. D.; Liu, S. X.; Decurtins, S.; Meyer, E. Orientation Dependent Molecular Friction on Organic Layer Compound Crystals. Appl. Phys. Lett. 2011, 98 (8), 083119. (42) Buzio, R.; Gerbi, A.; Uttiya, S.; Bernini, C.; Del Rio Castillo, A. E.; Palazon, F.; Siri, A. S.; Pellegrini, V.; Pellegrino, L.; Bonaccorso, F. Ultralow Friction of Ink-Jet Printed Graphene Flakes. Nanoscale 2017, 9 (22), 7612−7624. (43) Hölscher, H.; Ebeling, D.; Schwarz, U. D. Friction at AtomicScale Surface Steps: Experiment and Theory. Phys. Rev. Lett. 2008, 101 (24), 246105. (44) Socoliuc, A.; Bennewitz, R.; Gnecco, E.; Meyer, E. Transition from Stick-Slip to Continuous Sliding in Atomic Friction: Entering a New Regime of Ultralow Friction. Phys. Rev. Lett. 2004, 92 (13), 134301. (45) Buzio, R.; Boragno, C.; Valbusa, U. Friction Laws for Lubricated Nanocontacts. J. Chem. Phys. 2006, 125 (9), 094708. (46) Vilhena, J. G.; Pimentel, C.; Pedraz, P.; Luo, F.; Serena, P. A.; Pina, C. M.; Gnecco, E.; Pérez, R. Atomic-Scale Sliding Friction on Graphene in Water. ACS Nano 2016, 10 (4), 4288−4293. (47) Buzio, R.; Bosca, A.; Krol, S.; Marchetto, D.; Valeri, S.; Valbusa, U. Deformation and Adhesion of Elastomer Poly(dimethylsiloxane) Colloidal AFM Probes. Langmuir 2007, 23 (18), 9293−9302. (48) Buzio, R.; Boragno, C.; Valbusa, U. Contact Mechanics and Friction of Fractal Surfaces Probed by Atomic Force Microscopy. Wear 2003, 254 (9), 917−923. (49) Ecke, S.; Raiteri, R.; Bonaccurso, E.; Reiner, C.; Deiseroth, H. J.; Butt, H. J. Measuring Normal and Friction Forces Acting on Individual Fine Particles. Rev. Sci. Instrum. 2001, 72 (11), 4164. (50) Paradinas, M.; Pérez-Rodríguez, A.; Barrena, E.; Ocal, C. Real Space Demonstration of Induced Crystalline 3D Nanostructuration of Organic Layers. J. Phys. Chem. B 2018, 122, 633. (51) Barra, M.; Viggiano, D.; Di Capua, R.; Di Girolamo, F.; Santoro, F.; Taglialatela, M.; Cassinese, A. Cell Viability Studies and Operation in Cellular Culture Medium of N-Type Organic Field-Effect Transistors. J. Appl. Phys. 2012, 111 (3), 034702. (52) Aversa, L.; Verucchi, R.; Tatti, R.; Di Girolamo, F. V.; Barra, M.; Ciccullo, F.; Cassinese, A.; Iannotta, S. Surface Doping in T6/PDI8CN2 Heterostructures Investigated by Transport and Photoemission Measurements. Appl. Phys. Lett. 2012, 101 (23), 233504. (53) Wei, Q.; Wang, W.; Zhou, E.; Liu, X.-P.; Liang, M.-H.; Deng, K.; Yang, J.-K.; Jiang, P. Positioned Substituent Effect on Self-Assembly Behaviors of Perylene Diimide Derivatives on Graphite. J. Colloid Interface Sci. 2017, 504 (11), 58−67.

H

DOI: 10.1021/acs.langmuir.7b04149 Langmuir XXXX, XXX, XXX−XXX