Morphological Investigation of Mn12 Single-Molecule Magnets

Jul 27, 2009 - We report on the adsorption of Mn12 single-molecule magnets bearing external biphenyl groups on Au(111) surfaces after a simple dipping...
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Morphological Investigation of Mn12 Single-Molecule Magnets Adsorbed on Au(111) Gonzalo Otero,§ Emi Evangelio,† Celia Rogero,‡ Luis Vazquez,§ Jordi Gomez-Segura,† Jose Angel Martı´ n Gago,‡,§ and Daniel Ruiz-Molina*,† † Centro de Investigaci on en Nanociencia y Nanotecnologı´a (CIN2, CSIC-ICNC), Esfera UAB, Campus UAB, Edifici CM7, 08193 Cerdanyola del Vall es, Spain, ‡Centro de Astrobiologı´a (INTA-CSIC), Carretera Torrej onAjalvir, Km. 4,2. E-28850 Torrej on de Ardoz, Madrid, Spain, and §Instituto de Ciencia de Materiales de Madrid (CSIC), C\ Sor Juana In es de la Cruz n° 3, 28049, Madrid, Spain

Received February 27, 2009. Revised Manuscript Received July 9, 2009 We report on the adsorption of Mn12 single-molecule magnets bearing external biphenyl groups on Au(111) surfaces after a simple dipping procedure. Topographic AFM images confirm that the biphenyl groups favor the adsorption of the molecules without the need of functionalization with thiols or thioether groups. The first formed molecular layer covers homogenously the whole surface, whereas further growth takes place mostly in the form of molecular wires (or aggregates) and, occasionally, as molecular islands. Interestingly, the Mn12 core is preserved for all the cases, although its aggregation state appears to influence significantly the rigidity of the molecular aggregates. Force-volume imaging experiments have demonstrated that molecules at the second layer are stiffer, that is, more rigid, than the molecules lying at the background layer. This fact clearly reveals that the interplay of attractive and repulsive forces between molecules and the molecule-surface interaction modulate the mechanical properties of the Mn12 single-molecule magnets upon grafting. These results are very important to understand how surface-induced morphological deformations can modify the magnetic properties of these molecular systems on the translation from the macroscopic to a surface.

Introduction Single-molecule magnets (SMMs) combine large-spin ground states and high axial magnetic anisotropy, resulting in a barrier for the spin reversal.1 As a consequence, they exhibit slow magnetization relaxation rates characteristic of nanodomain particles, such as out-of-phase ac magnetic susceptibility signals2 and hysteresis magnetization loops,3 whose origin is based on individual molecules. Accordingly, one of the challenges attract*To whom correspondence should be addressed. E-mail: [email protected]. (1) (a) Christou, G.; Gatteschi, D.; Hendrickson, D. N.; Sessoli, R. MRS Bull. 2000, 25, 66. (b) Gatteschi, D.; Sessoli, R. Angew. Chem., Int. Ed. 2003, 42, 268. (c) Gatteschi, D.; Sessoli, R.; Villain, J. Molecular Nanomagnets; Oxford University Press: Oxford, UK, 2006. (d) Cornia, A.; Fabretti, A. C.; Zobbi, L.; Caneschi, A.; Gatteschi, D.; Mannini, M.; Sessoli, R. In Single-Molecule Magnets and Related Phenomena; Winpenny, R., Ed.; Structure and Bonding Book Series; Springer: Berlin, 2006; Vol. 122, p 133. (2) (a) Ruiz, D.; Sun, Z. S.; Albela, B.; Folting, K.; Ribas, J.; Christou, G.; Hendrickson, D. N. Angew. Chem., Int. Ed. 1998, 37, 300. (b) Sun, Z. M.; Ruiz, D.; Rumberger, E.; Incarvito, C. D.; Folting, K.; Rheingold, A. L.; Christou, G.; Hendrickson, D. N. Inorg. Chem. 1998, 37, 4758. (3) (a) Sessoli, R.; Gatteschi, D.; Caneschi, A.; Novak, M. A. Nature 1993, 365, 141. (b) Sessoli, R.; Tsai, H.-K.; Schake, A. R.; Wang, S.; Vincent, J. B.; Folting, K.; Gatteschi, D.; Christou, G.; Hendrickson, D. N. J. Am. Chem. Soc. 1993, 115, 1804. (4) (a) Gomez-Segura, J.; Veciana, J.; Ruiz-Molina, D. Chem. Commun. 2007, 3699. (b) Gatteschi, D.; Bogani, L.; Cornia, A.; Mannini, M.; Sorace, L.; Sessoli, R. Solid State Sci. 2008, 10, 1701. (5) (a) Kim, G. H.; Kim, T. S. Phys. Rev. Lett. 2004, 92, 137203. (b) Heersche, H. B.; de Groot, Z.; Folk, J. A.; van der Zant, H. S. J.; Romeike, C.; Wegewijs, M. R.; Zobbi, L.; Barreca, D.; Tondello, E.; Cornia, A. Phys. Rev. Lett. 2006, 96, 206801. (c) Jo, M.-H.; Grose, J. E.; Baheti, K.; Deshmukh, M.; Sokol, J. J.; Rumberger, E. M.; Hendrickson, D. N.; Long, J. R.; Park, H.; Ralph, D. C. Nano Lett. 2006, 6, 2014. (d) Bogani, L.; Danieli, C.; Biavardi, E.; Bendiab, N.; Barra, A.-L.; Dalcanale, E.; Wernsdorfer, W.; Cornia, A. Angew. Chem., Int. Ed. 2009, 48, 746. (6) Bogan, L.; Wernsdorfer, W. Nat. Mater. 2008, 7, 179. (7) (a) Chudnovsky, E. M.; Tejada, J. Macroscopic Quantum Tunneling of the Magnetic Moment; Cambrigde University Press: Cambridge, UK, 1998. (b) Del Barco, E.; Vernier, N.; Hernandez, J. M.; Tejada, J.; Chudnovsky, E. M.; Molins, E.; Bellesa, G. Europhys. Lett. 1999, 47, 722. (c) Leuenberger, M. N.; Loss, D. Nature 2001, 40, 789. (d) Affronte, M.; Troiani, F.; Ghirri, A.; Candini, A.; Evangelisti, M.; Corradini, V.; Carretta, S.; Santini, P.; Amoretti, G.; Tuna, F.; Timco, G.; Winpenny, R. E. P. J. Phys. D: Appl. Phys. 2007, 40, 2999.

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ing more attention over the past few years has been the development of new techniques that allow their nanostructuration and positioning on surfaces4 or nanoscale junctions5 to promote their potential use for high-density information storage devices6 and quantum computing applications.7 Most of the work so far reported in this direction8 has been focused on the gold deposition of large metal complexes comprising a dodecamanganese core, the well-known Mn12O12(O2CR)16(H2O)4 family (referred to from now on as Mn12).9 Advantages of this family of clusters are multifold,10 including a rich synthetic behavior11 that allows the functionalization of the peripheral organic ligands with surface-binding functionalities. Such groups enhance the molecular (8) Deposition of other SMMs has already been successfully attempted: (a) Gomez-Segura, J.; Dı´ ez-Perez, I.; Ishikawa, N.; Nakano, M.; Veciana, J.; RuizMolina, D. Chem. Commun. 2006, 2866. (b) Vitali, L.; Fabris, S.; Mosca Conte, A.; Brink, S.; Ruben, M.; Baroni, S.; Kern, K. Nano Lett. 2008, 8, 3364. (c) Affronte, M.; Troiani, F.; Ghirri, A.; Candini, A.; Evangelisti, M.; Corradini, V.; Carretta, S.; Santini, P; Amoretti, G.; Tuna, F.; Timco, G.; Winpenny, R. E. P. J. Phys. D: Appl. Phys. 2007, 40, 2999. (d) Corradini, V.; Biagi, R.; del Pennino, U.; De Renzi, V.; Gambardella, A.; Affronte, M.; Muryn, C. A.; Timco, G. A.; Winpenny, R. E. P. Inorg. Chem. 2007, 46, 4937. (e) Moro, F.; Corradini, V.; Evangelisti, M.; De Renzi, V.; Biagi, R.; del Pennino, U.; Milios, C. J.; Jones, L. F.; Brechin, E. K. J. Phys. Chem. B. 2008, 112, 9729. (9) Other nanostructuration methodologies of SMMs on surfaces based on noncovalent interactions have been reported: (a) Ruiz-Molina, D.; Mas-Torrent, M.; Gomez, J.; Balana, A. I.; Domingo, N.; Tejada, J.; Martı´ nez, M. T.; Rovira, C.; Veciana, J. Adv. Mater. 2003, 15, 42. (b) Cavallini, M.; Gomez-Segura, J.; RuizMolina, D.; Massi, M.; Albonetti, C.; Rovira, C.; Veciana, J. Angew. Chem., Int. Ed. 2005, 44, 888. (c) Cavallini, M.; Gomez-Segura, J.; Albonetti, C.; Ruiz-Molina, D.; Veciana, J.; Biscarini, F. J. Phys. Chem. B 2006, 110, 11607. (d) Martínez, R. V.; García, F.; García, R.; Coronado, E.; Forment-Aliaga, A.; Romero, F. M.; Tatay, S. Adv. Mater. 2007, 19, 291. (e) Kim, K.; Seo, D. M.; Means, J.; Meenakshi, V.; Teizer, W.; Zhao, H.; Dunbar, K. R. Appl. Phys. Lett. 2004, 85, 3872. (f) Means, J.; Meenakshi, V.; Srivastava, R. V. A.; Teizer, W.; Kolomenskii, Al. A.; Schuessler, H. A.; Zhao, H.; Dunbar, K. R. J. Magn. Magn. Mater. 2004, 284, 215. (g) Meenakshi, V.; Teizer, W.; Naugle, D. G.; Zhao, H.; Dunbar, K. R. Solid State Commun. 2004, 132, 471. (h) Gomez-Segura, J.; Kazakova, O.; Davies, J.; Josephs-Franks, P.; Veciana, J.; Ruiz-Molina, D. Chem. Commun. 2005, 5615. (i) Cavallini, M.; Biscarini, F.; Gomez-Segura, J.; Ruiz-Molina, D.; Veciana, J. Nano Lett. 2003, 3, 1527. (j) Condorelli, G. G.; Motta, A.; Pellegrino, G.; Cornia, A.; Gorini, L.; Fragala, L. L.; Sangregorio, C.; Sorace, L. Chem. Mater. 2008, 20, 2405. (10) Ruiz, D.; Christou, G.; Hendrickson, D. N. Mol. Cryst. Liq. Cryst. 2000, 343, 17.

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stability of the resulting monolayers on gold, both with12 and without13 the assistance of a stamp. As an alternative approach, the use of prefunctionalized gold14 or silicon15 surfaces with a monolayer of ligands bearing ending carboxylate groups has also been successful. Along with the development of new nanostructuration methods, different studies have also been developed to asses the magnetic and electronic properties of the resulting nanostructures on a surface. This information is crucial because the deposition on conducting surfaces can be accompanied by a significant modification of their electronic and structural uniqueness and, therefore, of the SMM behavior. The chemical composition of the adsorbates can be obtained by X-ray photoelectron spectroscopy (XPS),16 although an unambiguous determination of manganese oxidation states is difficult using this technique alone.17 Other approaches to gain information on the electronic structure of the adsorbates have been suggested, including scanning tunneling spectroscopy (STS)18 and resonant photoelectron spectroscopy (RPES).19 More recently, X-ray absorption spectroscopy (XAS) and X-ray magnetic circular dichroism (XMCD) methods at low temperature have also been successfully used to provide fundamental evidence for the loss of the peculiar magnetic properties of Mn12 due to both molecular deformations and/or a systematic partial reduction of the peripheral MnIII ions to MnII, as confirmed by XMCD.20 These results were, therefore, compatible with the hypothesis that the possible reduction of deposited clusters modifies their magnetic properties, though does not dramatically alter their chemical composition and size.21 In addition to electronic changes upon grafting on a given surface, structural modifications are also expected to modify, as well, their (11) (a) Aubin, S. M. J.; Sun, Z.; Eppley, H. J.; Rumberger, E. M.; Guzei, I. A.; Folting, K.; Gantzel, P. K.; Rheingold, A. L.; Christou, G.; Hendrickson, D. N. Polyhedron 2001, 20, 1139 and references cited within . (b) Chakov, N. E.; Lee, S. C.; Harter, A. G.; Kuhns, P. L.; Reyes, A. P.; Hill, S. O.; Dalal, N. S.; Wernsdorfer, W.; Abboud, K. A.; Christou, G. J. Am. Chem. Soc. 2006, 128, 6975. (c) Rumberger, E. M.; del Barco, E.; Lawrence, J.; Hill, S.; Kent, A. D.; Zakharov, L. N.; Rheingold, A. L.; Hendrickson, D. N. Polyhedron 2005, 24, 2557. (d) Gomez-Segura, J.; Campo, J.; Imaz, I.; Wurst, K.; Veciana, J.; Gerbier, P.; Ruiz-Molina, D. Dalton Trans. 2007, 2450. (e) Ruiz-Molina, D.; Gerbier, P.; Rumberger, E.; Amabilino, D. B.; Guzei, I. A.; Folting, K.; Huffman, J. C.; Rheingold, A. L.; Christou, G.; Veciana, J.; Hendrickson, D. N. J. Mater. Chem. 2002, 12, 1152. (12) Mannini, M.; Bonacchi, D.; Zobbi, L.; Piras, F. M.; Speets, E. A.; Caneschi, A.; Cornia, A.; Magnani, A.; Ravoo, B. J.; Reinhoudt, D. N.; Sessoli, R.; Gatteschi, D. Nano Lett. 2005, 5, 1435. (13) (a) Cornia, A.; Fabretti, A. C.; Pacchioni, M.; Zobbi, L.; Bonacchi, D.; Caneschi, A.; Biagi, R.; Del Pennino, U.; De Renzi, V.; Gurevich, L.; Van der Zant, H. S. J. Angew. Chem., Int. Ed. 2003, 42, 1645. (b) Zobbi, L.; Mannini, M.; Pacchioni, M.; Chastanet, G.; Bonacchi, D.; Zanardi, C.; Biagi, R.; del Pennino, U.; Gatteschi, D.; Cornia, A.; Sessoli, R. Chem. Commun. 2005, 12, 1640. (14) Naitabdi, A.; Bucher, J.-P.; Gerbier, P.; Rabu, P.; Drillon, M. Adv. Mater. 2005, 17, 1612. (15) (a) Condorelli, G. G.; Motta, A.; Fragala, I. L.; Giannazzo, F.; Raineri, C.; Caneschi, A.; Gatteschi, D. Angew. Chem., Int. Ed. 2004, 43, 4081. (b) Condorelli, G. G.; Motta, A.; Favazza, M.; Nativo, P.; Fragala, I. L.; Gatteschi, D. Chem.;Eur. J. 2006, 12, 3558. (16) (a) Zobbi, L.; Mannini, M.; Pacchioni, M.; Chastanet, G.; Bonacchi, D.; Zanardi, C.; Biagi, R.; Del Pennino, U.; Gatteschi, D.; Cornia, A.; Sessoli, R. Chem. Commun. 2005, 1640. (b) Pineider, F.; Mannini, M.; Sessoli, R.; Caneschi, A.; Barreca, D.; Armelao, L.; Cornia, A.; Tondello, E.; Gatteschi, D. Langmuir 2007, 23, 11836. (17) Voss, S.; Fonin, M.; RSdiger, U.; Burgert, M.; Groth, U.; Dedkov, Y. S. Phys. Rev. B 2007, 75, 045102. (18) (a) Voss, S.; Fonin, M.; RSdiger, U.; Burgert, M.; Groth, U. Appl. Phys. Lett. 2007, 90, 133104. (b) Burgert, M.; Voss, S.; Herr, S.; Fonin, M.; Groth, U.; R€udiger, U. J. Am. Chem. Soc. 2007, 129, 14362. (19) del Pennino, U.; De Renzi, V.; Biagi, R.; Corradini, V.; Zobbi, L.; Cornia, A.; Gatteschi, D.; Bondino, F.; Magnano, E.; Zangrando, M.; Zacchigna, M.; Lichtenstein, A.; Boukhvalov, D. W. Surf. Sci. 2006, 600, 4185. (20) Mannini, M.; Sainctavit, Ph.; Sessoli, R.; Cartier dit Moulin, Ch.; Pineider, F.; Arrio, M.-A.; Cornia, A.; Gatteschi, D. Chem.;Eur. J. 2008, 14, 7530. (21) (a) Bogani, L.; Cavigli, L.; Gurioli, M.; Novak, R. L.; Mannini, M.; Caneschi, A.; Pineider, F.; Sessoli, R.; Clemente-Leon, M.; Coronado, E.; Cornia, A.; Gatteschi, D. Adv. Mater. 2007, 19, 3906. (b) Barraza-Lopez, S.; Avery, M. C.; Park, K. Phys. Rev. B 2007, 76, 224413.

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SMM character most likely due to deformation-induced JahnTeller isomerism.22 This was not the case for Fe4 clusters that are likely to impose a more rigid structure, maintaining their SMM behavior.23 However, despite the numerous and well-conducted studies previously described, the extent to which structural modifications materialize upon surface adsorption has not been revealed so far mainly because of the difficulties to obtain information by standard surface analysis techniques, such as XPS or XAS. In this sense, molecular size and bidimensional arrangement have been estimated by scanning tunneling microscopy (STM), though information on structural Mn12 deformation and/or the sample stiffness remains elusive.24 The identification of structural modifications after absorption of SMMs on a given surface is not only important but also a very hot topic in the field of molecular magnetism nowadays. If we really want to understand the magnetization relaxation of these molecules on surfaces, that is, before any application becomes a reality, it is strictly necessary to understand the morphological variations of these molecules on the translation from the macroscopic to the nanoscopic world. Surface-induced morphological deformations may clearly influence the Jahn-Teller anisotropy axis and, consequently, their anomalous magnetization relaxation. Therefore, only by having a clear picture on how the morphology along their deposition process is modified will we be able to understand their magnetic properties on a surface. To gain further insight into this central issue, herein we report a detailed and systematic AFM study of the structure of Mn12 clusters physiadsorbed on Au(111). For this, the Mn12O12(O2CC6H4C6H5)16(H2O)4 (1) cluster was the molecule of choice. Figure 1 shows a schematic representation of the molecule in two orthogonal views, including the molecular dimensions. The interest is twofold. First, complex 1 has already been successfully nanostructured on different surfaces, thanks to its relatively large dimensions, which makes it a better candidate to be analyzed by SPM techniques. Second, the 16 biphenyl groups are prone to enhance adsorbate-surface interactions, as previously reported on Au colloids investigated by surface-enhanced Raman scattering (SERS).25

Experimental Section Sample Preparation. Complex 1 was obtained as previously

described26 and purified by crystallization techniques. Gold wafers were purchased from Arrandee, and the surface was annealed at about 600 °C in air by means of a butane flame to promote the formation of atomically flat Au(111) terraces. Each gold wafer was dipped separately into the corresponding Mn12 solution at room temperature following a methodology already described and widely used.27 After standing undisturbed for 1 h, the samples were left to dry over silica. Subsequently, AFM images were acquired on different parts of the sample to check the cleanliness of the surface as well as the formation of relatively large Au(111) terraces (see the Supporting Information). Moreover, to assess the reproducibility of the results, two distinct (22) Sun, Z.; Ruiz, D.; Dilley, N. R.; Soler, M.; Ribas, J.; Folting, K.; Maple, M. B.; Christou, G.; Hendrickson, D. N. Chem. Commun. 1999, 1973. (23) Mannini, M.; Pineider, F.; Sainctavit, Ph.; Joly, L.; Fraile-Rodrı´ guez, A.; Arrio, M.-A.; Cartier dit Moulin, C.; Wernsdorfer, W.; Cornia, A.; Gatteschi, D.; Sessoli, R. Adv. Mater. 2009, 21, 167. (24) Naitabdi, A.; Bucher, J.-P.; Gerbier, P.; Rabu, P.; Drillon, M. Adv. Mater. 2005, 17, 1612. (25) Yu, K. H.; Rhee, J. M.; Lee, Y.; Lee, K.; Yu, S.-C. Langmuir 2001, 17, 52– 55. (26) Ruiz-Molina, D.; Gerbier, P.; Rumberger, E.; Amabilino, D. B.; Guzei, I. A.; Folting, K.; Huffman, J. C.; Rheingold, A.; Christou, G.; Veciana, J.; Hendrickson, D. N. J. Mater. Chem. 2002, 12, 1152. (27) (a) Baselta, D. R.; Baldeschwieler, J. D. J. Appl. Phys. 1994, 76, 33. (b) Scheider, F. Prog. Surf. Sci. 2001, 65, 151.

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Figure 1. Molecular structure of complex 1. Top (left) and side (right) views of the molecular structure of complex 1 obtained by molecular modeling.

replicates were studied. Satisfactory control experiments (XPS and AFM) in the absence of Mn12 molecules were also carried out to assess the lack of impurities, in both the solvent and/or the substrate, which may mask the experimental results. Atomic Force Microscopy (AFM). AFM images were acquired with NanoScope IIIa equipment (Veeco) with a NanoScope Extender electronic circuit for measurement of the phase shift.28 The samples were studied by AFM immediately after drying the sample in tapping mode and in air conditions with silicon TESP cantilevers from Veeco with a nominal radius of curvature of ∼10 nm and constant force of ∼42 N nm-1. The cantilevers were excited at their resonance frequency in the 270-330 kHz range. The topography and the corresponding phase-contrast images were recorded simultaneously along each scan line. Set point/free amplitude ratios of ∼0.87 were employed for the morphological characterizations. The NanoScope Extender does not report the phase angle data in real units because it measures the cosine of the phase lag.22,29 In addition, we also performed force-volume imaging.27a Whereas a simple force curve records the deflection undergone by the cantilever as it approaches and retracts from a point on the sample surface, a force-volume image contains an array of force curves over the entire sample area. Each force curve is measured at a unique x-y position in the imaged area, and force curves from an array of x-y points are combined into a three-dimensional array, or “volume,” of force data. For our measurements, amplitude and phase shift curves were acquired by approaching the tip toward the sample from a distance with negligible tip-sample interaction. Then the amplitude and phase shift were recorded for the different tip-sample distances (the curves, consisting of 512 data points, were acquired at a frequency close to 3 Hz). The force-volume imaging in this work was performed over 368  368 nm2 areas with 1024 data points. These parameters imply that each data point was obtained over 11.5  11.5 nm2 sample areas. Under our operation conditions, the lateral drift during the measurements of each data point (i.e., force curve) was less than 0.05 nm. The normalized amplitude versus distance and phase shift versus distance curves shown below are the average of at least 30 individual curves taken on either MW or BL structures. The energy dissipated by the tip-sample interactions (Edis) can be obtained from such curves because it is related to the phase shift angle (j) through the expression30 E dis ¼ ðπkA0 2 =QÞrsp ðsin j - rsp f =f 0 Þ ¼ F E rsp ðsin j - rsp f =f 0 Þ ¼ F E εdis

ð1Þ

(28) Babcock, K. L.; Prater, C. B. Phase Imaging: Beyond Topography; Digital Instruments: Santa Barbara, CA, 1995; www.veeco.com. (29) Bhushan, B.; Qi, J. Nanotechnology 2003, 14, 886. (30) Tamayo, J.; Garcı´ a, R. Appl. Phys. Lett. 1998, 73, 2926.

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where f and f0 are the excitation and natural frequencies of the cantilever, respectively; A and A0 are the tapping and the free amplitude, respectively; rsp is the A/A0 ratio; Q and k are the quality factor and spring constant of the cantilever; and FE = πkA02/Q. Therefore, for a given cantilever, with fixed Q, k, and f0 values and a fixed free amplitude and excitation frequency, Edis is just proportional to εdis, which, in turn, will depend on rsp and sin j. The data pair (rsp, j) can be obtained from the amplitude and phase curves described above. Thus, using the data obtained through force-volume imaging, we can obtain the εdis versus rsp plots on the different Mn12 structures. X-ray Photoelectron Spectroscopy (XPS). XPS spectra were recorded in an ultra-high-vacuum system equipped with a hemispherical electron analyzer and nonmonochromatic Mg KR (1253.6 eV) X-ray source. The contribution of the Mg KR satellite line was subtracted and the spectra background corrected by a Shirley routine. Peak fit procedures were done according to standard methods. The fitting of the Mn 2p peaks was performed using Gaussian doublets with a spin-orbit splitting of 11.7 eV and intensity ratio of approximately 0.5, in agreement with the literature. To fit the experimental data points, we used a trialand-error procedure. The fit with Gaussian functions is particularly justified, in our case, because of the limited resolution (1 eV) corresponding to the energy analyzer working conditions.

Results and Discussion 1. Sample Preparation and XPS Studies. We have prepared different samples by dipping Au(111) substrates for 1 h in CH2Cl2 solutions of complex 1 with different concentrations. The concentrations were 10-12 M, 10-10 M, 10-8 M, and 10-4 M. Core level peaks of Au, C, O, and Mn were exclusively detected. The presence of other elements from the solvent in the sample surface was disregarded. Under these experimental conditions, the Mn12 coverage, confirmed by XPS spectra, ranges from a molecular submonolayer, ∼0.3 ML, in the case of the smallest concentration, up to ∼1.5 ML, in the case of the largest concentration. Figure 2 shows the Mn 2p core level peak of the molecular layer for the two extreme cases, high and low coverage (10-4 M and 10-12 M, respectively). In both spectra, the line shape can be decomposed in four different peaks, labeled A-D. The best fit is shown in Figure 2 as a continuous line with a very satisfactory overlap of the experimental data points. The main experimental peak, centered at 641.5 eV of binding energy, includes the contribution from the core levels Mn 2p3/2 of the molecule and the Au 4p1/2 of the surface.31 In the central part (31) (a) Coronado, E.; Forment-Aliaga, A.; Romero, F. M.; Corradini, V.; Biagi, R.; De Renzi, V.; Gambardella, A.; del Pennino, U. Inorg. Chem. 2005, 44, 7693. (b) Voss, S.; Fonin, M.; R€udiger, U.; Burgert, M.; Groth, U.; Dedkov, Y. S. Phys. Rev. B 2007, 75, 045102.

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Figure 3. 815  815 nm2 AFM topographical (a) and phase shift (b) images taken on the same area of a Au(111) surface dipped for 1 h into a 10-4 M solution of complex 1. In the phase image, the most representative surface features, namely, background layer, molecular wires, and molecular islands, are indicated. The vertical scales correspond to 14 nm (a) and 5° (b). Both horizontal bars correspond to 102 nm.

Figure 2. XPS spectra recorded for two samples with coverage larger than 1 ML (b) and submonolayer coverage (a). Experimental data are represented by points. The line overlapping the experimental data points corresponds to the best fit in curve components, which are represented underneath.

of the molecule (core), there are eight Mn atoms in a oxidation state of Mn3þ and four Mn atoms in Mn4þ (see Figure 1), all of them contributing to the Mn 2p3/2 core level peak line shape.32 A detailed deconvolution of the spectrum reveals that the overall peak shape, for both cases, can be fitted by using several components, with binding energies of 641.0, 642.15, and 643.3 eV, which we have assigned to Mn3þ (peak A), Mn4þ (peak B), and Au 4p1/2 (peak D), respectively.33 The intensity ratio between the Mn3þ and Mn4þ components (A/B) is approximately 2, which is in quite good agreement with the ratio Mn3þ/ Mn4þ in the molecular core. Peak C, centered at 647.5 eV of binding energy, has been previously observed in this family of molecular layers.31 Such a peak could be tentatively related to the presence of Mn2þ ions, particularly, assigned to manganese carbonate,34 and/or to the interaction of several groups with fragments of the molecule.32 In any case, the area of this peak is small and does not alter significantly the intensity ratio between Mn4þ and Mn3þ. Finally, the peaks that appear in the spectrum after 650 eV correspond to the spin-orbit splitting (ΔE = 11.7 eV, Mn 2p1/2,) contribution of peaks A, B, and C, in good agreement with previously reported spectra.31 In the spectrum corresponding to the low-concentration sample (upper curve in Figure 2), the coverage is smaller than one single layer and, therefore, the signal from the Au 4p1/2 core level peak (D) dominates the spectrum. In addition, all the Mn-related core level peaks have been found to appear at similar binding energies in the spectra. The absence of a binding energy shift for the low- and high-coverage cases, with just a change in their (32) (a) Garcı´ a, R.; Perez, R. Surf. Sci. Rep. 2002, 47, 197. (b) Nait Abdi, A.; Bucher, J. P.; Rabu, P.; Toulemonde, O.; Drillon, M.; Gerbier, P. J. Appl. Phys. 2004, 95, 7345. (33) di Castro, V.; Polzonetti, G. J. Electron Spectrosc. Relat. Phenom. 1989, 48, 117. (34) (a) Round1, A. N.; Miles, M. J. Nanotechnology 2004, 15, S176. (b) Argaman, M.; Golan, R.; Thomson, N. H.; Hansma, H. G. Nucleic Acids Res. 1997, 25, 4379. (c) Lee, S.-J.; Gavriilidis, A.; Pankhurst, Q. A.; Kyek, A.; Wagner, F. E.; Wong, P. C. L.; Yeung, K. L. J. Catal. 2001, 200, 298.

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Figure 4. 445  445 nm2 AFM topographical images of Au(111) dipped for 1 h into solutions of complex 1 with different concentrations: (a) 10-12 M, (b) 10-8 M, and (c) 10-4 M. High-magnification image (150  150 nm2) taken on (c), in which the structure of the background layer can be clearly distinguished (d). The vertical scale corresponds to 14 nm (a-c) and 5 nm (d). The horizontal bars indicate 74 nm (a-c) and 25 nm (d).

relative intensity, indicates the presence of similar chemical species for both coverage situations, discarding any chemical process of the Mn12 clusters on the Au(111) surface. 2. AFM Topographic Characterization. Surface topography for all four concentrations (from low to high coverage) shows clear differences with respect to that of the clean surface, confirming the adsorption of the Mn12 clusters. In general, we can distinguish three different types of features on the surface morphology, which will be denominated from now as background layer (BL), molecular wires (MW), and molecular islands (MI). Representative AFM images, topographical and phase contrast, exhibiting the three previously described motives are shown in Figure 3. For all the concentrations, the BL is formed by a random distribution of quasi-spherical features, lacking any ordering, whose number increases on increasing the concentration of the solution. As an example, the formation of the BL at three different concentrations (10-12, 10-8, and 10-4 M) is shown in Figure 4. Langmuir 2009, 25(17), 10107–10115

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Figure 5. Three-dimensional topographic images showing the molecular wires after dipping Au(111) substrates for 1 h into solutions of complex 1 with different concentrations: (a) 10-10 M (685  685 nm2), (b) 10-4 M (452  452 nm2), and (c) 10-4 M (500  500 nm2). The vertical bars correspond to 100, 40, and 120 nm, respectively. (d) Surface profile taken on the stepped micrograin surface of figure a, showing preferential formation of MW on the upper part of the steps.

A detail of the BL in the image with larger coverage is shown in Figure 4d. The apparent size of these features in the AFM images is compatible, taking into account tip size convolution effects, with that of individual molecules. After measuring several height profiles, the obtained averaged value for the molecular height oscillates in the 1-2 nm range, which is lower than that derived from the theoretical dimensions expected for a single molecule (2.5 nm, see Figure 1). This divergence between the experimental and the expected height can arise either from a remaining tip load effect, despite setting conveniently the imaging conditions to minimize this deformation, or, most likely, from a surfaceinduced molecular deformation. Regarding the molecular width, considered as the full width at half-maximum (fwhm), its average value is ∼12 nm. This value, although clearly larger than the expected one, is also realistic, taking into account tip convolution effects. However, in any case, not even a partial order of the molecules on the surface due to a template effect of the gold herringbone reconstruction was detected.35 The formation of the second molecular layer takes place sequentially in the form of molecular wires (MWs) or aggregates, which, in fact, can already be observed at very low concentrations, even when the background layer is not completed. In the case of the smallest concentration (10-12 M), the MWs have a maximum length of 80 nm and an average height and width of 2-3 nm and ∼12 nm, respectively. Whereas the average width remains similar to the topographic dimensions of the molecules in the background layer (considering tip convolution effects), the average height is higher and in better agreement with the theoretical dimensions of a molecule, giving first evidence of the surface effect on possible molecular deformations. These values suggest that the MWs are most likely monomolecular, although 2-3 molecules are compatible with the experimental values. Interestingly, the higher the concentration of complex 1 in the solution, the longer is the wire length (see Figure 5). In the case of the Au(111) substrates dipped into the 10-10 M concentration, wire lengths of 150 nm are found. A detailed inspection of images recorded at lower concentrations indicates that the formation of (35) (a) Martı´ nez, R. V.; Garcı´ a, F.; Garcı´ a, R.; Coronado, E.; Forment-Aliaga, A.; Romero, F. M.; Tatay, S. Adv. Mater. 2007, 19, 29. (b) Pineider, F.; Mannini, M.; Sessoli, R.; Caneschi, A.; Barreca, D.; Armelao, L.; Cornia, A.; Tondello, E.; Gatteschi, D. Langmuir 2007, 23, 11836.

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the MW takes place preferentially at the upper part of the step edges of the Au(111) surface, but on the top of the molecular layer rather than on the bare gold substrate surface. A surface profile taken along one of the stepped large micrograins of the gold surface after dipping for 1 h into a solution of 10-10 M concentration of complex 1 is shown in Figure 5d. On the terrace, close to the step edges, protrusions are clearly observed, which correspond to the initial stages of formation of the molecular wires. Moreover, branched or looped morphologies are found for the 10-1210-8 M concentration range, whereas for a concentration of 10-4 M, an intricate MW network is formed, from which the total length is difficult to be estimated (see Figure 5c). Such a feature is evidence that the MWs tend to aggregate both longitudinally and laterally instead of stacking one on top of each other to lead to three-dimensional structures. Detailed images of NW structures are shown in Figures 6 and 7. Moreover, a qualitative inspection of these images indicates that the branches of the molecular wires tend to adopt a preferential angle. The quantitative study of several angles, measured on different images at the position where three or more branches meet together, confirms the existence of a preferential angle of 106° ( 10°, as can be seen in the histogram shown in Figure 6d. It is worth mentioning that both the drift of the AFM and the eventual influence of the underlying Au morphology could increase the error bar. The existence of this preferential angle, which differs from the relative orientation of the molecules in the crystalline network (90°), is an indication of the nature of the NW molecular assembly. The value of 106° approximately corresponds to the opening of the biphenyl arms in the molecular structure (see Figure 1). Likely, the MW could be formed by an intermixing of the peripheral organic branches of neighboring molecules. Such a different molecular packing may also explain the different stiffness properties found for the BL and the MW by AFM force-volume imaging studies, as it will be discussed in the next section. Finally, we have performed a time-dependence study of the molecular diffusion and aggregation on the surface of the sample prepared from the 10-8 M solution. Two topographic images, taken 1 and 5 h after removal of the sample from the solution, are shown in Figure 7. Clearly, after 5 h at room temperature and atmospheric pressure, the MW became longer and wider. This result indicates that some of the molecules at the background layer after longer experimental times diffuse on the surface, DOI: 10.1021/la900710c

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Figure 6. AFM topographical images of molecular wires with different forms and orientations: 125  170 nm2 (a), 130  195 nm2 (b), and 32  35 nm2 (c). The vertical scale corresponds to 6.5, 5.5, and 6.5 nm, (a-c), respectively. The horizontal bars indicate 26, 30, and 5.1 nm, (a-c), respectively. (d) Histogram of the measured angles between crossing wires.

Figure 7. AFM topographical images (163  163 nm2) of a sample prepared after dipping for 1 h into a 10-8 M solution of complex 2 in CH2Cl2 measured 1 h (a) and 5 h (b) after being removed from the solution. The vertical scale corresponds to 4 nm, whereas the horizontal bars indicate 41 nm.

attaching to the existing wires. Such an aggregation process eventually may take place in the form of molecular islands (MI) (see Figure 2), with an average height of 2-3 nm. Figure 3 shows that the phase-contrast signal at MWs and MIs is clearly different than that on BL features. One source of such contrast could be the relatively rapidly varying topography at MW features.32a However, we should note that a similar phase contrast has been routinely obtained when imaging low-dimensional structures, such as DNA.33a,34a Moreover, phase-contrast imaging has been previously applied to distinguish Mn12 singlemolecule magnets from substrate features due to its sensitivity to changes in energy dissipation processes.35a To prove that the phase contrast observed at MW structures is not due to its morphology, we show in Figure 8 a cross section (top curve) and its corresponding phase-contrast profile (bottom curve) of a surface where five MW structures are found. Clearly, the changes in phase contrast are found at MW structures, whereas the contrast at surface spots where the topography changes abruptly 10112 DOI: 10.1021/la900710c

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is plainly smaller. Finally, we should note that the phase contrast at MI (Figure 3) is also quite different from that measured at BL features (and similar to that obtained on MWs). MIs are quite smoother than MWs, which supports that the phase contrast at MWs is truly a material property and does not have a strong topographic contribution. 3. Force-Volume Imaging. Initially, the differential deformation of MW and BL structures by the tip load was studied. Surface profiles of two MWs and BL structures (left and right sides of the profile, respectively) imaged at relative low (solid line) and high (dashed lines) loads in tapping mode are shown in Figure 9. Clearly, all structures are deformed under higher loads (smaller feature heights). However, the deformation was not permanent (i.e., plastic) because subsequent scans at low loads revealed the successive recovery of the initial topographic dimensions for both BL and MW motives. What is more important to emphasize is the fact that BL structures tend to be deformed to a larger extent, when the deformation is compared to the corresponding feature height, than MW structures under the same tip load, as qualitatively deduced from Figure 9. To gain further insight into these effects, force-volume imaging experiments have been used to sample local variations in surface stiffness between BL and MW motives. This is done by recording the cantilever amplitude of oscillation as the tip approaches a surface and then plotting it as a function of distance between the tip and the surface for each one of the cases (for more details, see the Experimental Section). Panels a (normalized amplitude vs distance) and b (phase shift vs distance) in Figure 10 show the averaged curves measured on both types of structures. In the case of the amplitude curves, both exhibit a similar behavior, although, for that measured on the Mn12 wires, the amplitude seems to decrease for rsp < 0.9 more sharply than for that obtained on the soft layer (BL). This behavior could indicate that, for the same rsp, the tip indentation on the sample is larger on the Mn12 soft layer than on the Mn12 wire, in qualitative agreement with Figure 9. The corresponding phase shift versus distance curves are displayed in Figure 10b. In such curves, the peak observed for phase shift angles larger than 90° corresponds to the region in which tip-sample attractive interactions operate, whereas repulsive interactions dominate when phase shift angles are smaller than 90°.36a Following this criterion, the repulsive regime for MW structures starts at higher rsp values (rsp ∼ 0.95) than those for BL features (rsp ∼ 0.91). As seen in Figure 10, both curves almost overlap for the initial stages of the attractive region but they clearly diverge from the condition in which the phase lag has attained its maximum value within the attractive regime to the repulsive regime where the difference between both curves becomes more evident. Because of the relative low resolution of force-volume imaging (one pixel corresponds to a 1.5 x 1.5 nm2 area), it is likely that some contribution coming from the surrounding BL region is present in the MW data. However, the observed differences in both curves support our statement that both structures display different behaviors. This is particularly evident for the phase-contrast data, which are very sensitive to changes in energy dissipation processes.35a It is worth noting that, in both cases, the amplitude and phase curves do not show a bistability in response at the crossover distance between the attractive and repulsive regimes. However, it has been reported that its apparition may depend on the specific tip-sample system, the tip’s free oscillation amplitude and cantilever’s stiffness.37a In contrast, the amplitude and phaselag curves obtained on the bare gold substrate (Supporting Information) did show bistability. In addition, the amplitude Langmuir 2009, 25(17), 10107–10115

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Figure 8. AFM surface (a) and phase-contrast (b) profiles measured simultaneously of a region comprising five MW structures. The vertical bars correspond to 4 nm (a) and 5° (b).

Figure 9. AFM surface profiles of a region exhibiting MW and BL structures. The scans were recorded sequentially at low (solid line) and high (dashed line) tip loads. Both types of structures are indicated.

decreased, as the tip approached the surface, with a higher slope than that for the BL and MW structures. The values of εdis as a function of the amplitude ratio, rsp, on both Mn12 wire and the background layer can be obtained from the curves displayed in Figure 11. Because these data have been obtained with the same cantilever (i.e., same k, Q, and f0) and under the same operating conditions (i.e., same f and A0), the differences in εdis will reflect the differences in energy dissipation. Thus, using the data obtained through force-volume imaging and considering that f = f0, we can obtain the εdis versus rsp plots on both structures (Figure 11a). For large rsp ratios, both curves overlap, indicating that the energy dissipation is similar on both structures. This zone corresponds mainly to the attractive interaction regime. However, for rsp < 0.93, that is, clearly in the repulsive interaction regime, the energy dissipated on the layer structure becomes larger than that dissipated on the molecular wires. This difference increases as the amplitude ratio decreases. In the repulsive regime, the tip samples, the contact region, and sample viscoelastic deformation become more important, leading to higher energy dissipation. This fact agrees with the phase images shown in Figure 3, where the phase shift was smaller for MW structures than for the background layer.29,30 It should be noted that the εdis versus rsp curve obtained on the gold surface (Supporting Information) does show a sharp change due to the measured bistability (see above). Nevertheless, the curve is similar to those obtained on Langmuir 2009, 25(17), 10107–10115

Figure 10. Amplitude ratio (a) and phase shift (b) versus z piezo displacement curves obtained with the same cantilever under the same experimental conditions obtained on the background layer (solid line) and on the MW (dotted line) structures.

both MW and BL structures at low rsp values. This fact is not striking because the gold surface is not totally rigid (in many cases, we did observe permanent indentations on it after performing amplitude and phase vs distance curves). Therefore, it is possible that some contribution from the underlying gold substrate could be present in the data of the energy dissipation obtained on MS and mainly BL structures.38a DOI: 10.1021/la900710c

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decreases with the sample stiffness (i.e., with Youngs modulus). Experimentally, this has also been found on cross-linked silane layers.29 Thus, these results show that the Mn12 molecular wires are stiffer than the surrounding background layer. Finally, although we have not performed force-volume imaging measurements on MI structures, the evident phase contrast obtained on them (Figure 3) suggests that, provided that MI presents the same main energy dissipation mechanism than that on BL and MW features, MI domains would be even stiffer than MW structures. This fact would be consistent with the ability of MWs to form branched and looped MW networks, which implies that, likely, MWs are not such ordered and compact.

Figure 11. εdis versus rsp plot obtained on the background layer (solid line) and on the MW (dotted line) structures (a). Plot of the derivative with respect to rsp of the normalized E*dis curves for the background layer (solid line) and MW (dotted line) structures (b).

Further information regarding the energy dissipation processes can be obtained from the shape of the derivative with respect to rsp of E*dis, which is defined as the normalization of the Edis curve with respect to its maximum value (i.e., E*dis = Edis/Edis(max)).36 In the present experiment, we have derived the normalized εdis with respect to rsp, because it is equivalent to the normalized Edis. The result is displayed in Figure 11b. Despite some fluctuations, both curves are rather similar, exhibiting a minimum value for rsp of about 0.96. The shape of these derivative curves resembles that obtained from simulations and experiments for a tip-sample interaction governed by viscoelastic processes. The viscous force depends on both the indentation and how the indentation changes with time. Because we may observe deformation on the Mn12 structures induced by the tip, this assignment is consistent. We can gain an insight on the source of this dissipation difference between the background layer and the molecular wires from the studies on phase contrast and energy dissipation processes. Tamayo and Garcı´ a37 proved that, for a tip-sample interaction with viscous damping with/without adhesion hysteresis, the phase shift (36) (a) Garcı´ a, R.; San Paulo, A. Phys. Rev. B 1999, 60, 4961. (b) García, R.; Gomez, C. J.; Martínez, N. F.; Patil, S.; Dietz, C.; Margele, R. Phys. Rev. Lett. 2006, 97, 016103. (37) (a) Chen, X.; Davies, M. C.; Roberts, C. J.; Tendler, S. J. B.; Williams, P. M.; Burnham, N. A. Surf. Sci. 2000, 460, 292. (b) Tamayo, J.; García, R. Appl. Phys. Lett. 1997, 71, 2394.

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Conclusions We have studied the adsorption of Mn12 single-molecule magnets on Au(111) surfaces without the need of any specific functionalization neither of the cluster terminated by thiol/ tioether groups nor of the surface. This was simply achieved by dipping Au(111) substrates into four different CH2Cl2 solutions of complex 1 at different concentrations, which leads to the adsorption of the molecules with different surface coverage, ranging from a submonolayer up to about 1.5 ML for the highest concentration. This fact has allowed us to study the differential surface effect on Mn12 molecules directly in contact with the surface versus those separated by a molecular layer. The combination of both XPS and AFM studies shows the integrity of the molecular entities of complex 1 on Au(111) in all the cases, even though a small partial reduction takes place, as previously shown.21 As expected, the main motive observed in the AFM images for all the concentrations is the random distribution of the molecules on the surface, from a partial to a full surface coverage, depending on the concentration of the solution. However, in any case, not even a partial order of the molecules on the surface due to a template effect of the gold substrate was detected. For higher concentrations, the coverage slightly exceeds a monolayer even though the concentration increases up to 8 orders of magnitude. This fact suggests that, whereas there is a clear tendency of the molecules to adsorb on the gold bare surface through interactions with the biphenyl groups, π-π stacking driving forces between molecules are not strong enough to promote a multilayer growth. Moreover, the formation of the second molecular layer takes place sequentially in the form of molecular wires (MW) likely formed by individually aligned Mn12 molecules. XPS experiments have confirmed that neither the binding energies nor the chemical state of the Mn atoms in the magnetic core are altered with the molecular coverage. Moreover, force-volume imaging experiments have demonstrated that molecules at the MW are stiffer, that is, more rigid, than the molecules lying at the background layer. This fact clearly reveals that the interplay of attractive and repulsive forces between molecules and the molecule-surface interaction modulate the mechanical properties of the Mn12 single-molecule magnets upon grafting. This fact is expected to modify the local molecular anisotropy of the molecule and, therefore, their SMM behavior, together with variations of the electronic structure. Accordingly, such deformation has to be taken into account in future studies that involve the deposition and nanostructuration of SMMs being required, in some cases, the use of SMMs with the highest structural robustness, as very recently shown.38 (38) (a) Berquand, A.; Mazeran, P. E.; Laval, J. M. Surf. Sci. 2003, 523, 125. (b) Mannini, M.; Pineider, F.; Sainctavit, Ph.; Danieli, C.; Otero, E.; Sciancalepore, C.; Talarico, A. M.; Arrio, M.-A.; Cornia, A.; Gatteschi, D.; Sessoli, R. Nat. Mater. 2009, 8, 194.

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Acknowledgment. This work was supported by projects MAT2006-13765-C02, MAT2008-1497nan, FIS2006-12253C06-03, Consolider CSD2007-00010, and CSD2007-41 and the EU through the network of excellence MAGMANet, Contract No. NMP3-CT-2005-515767. E.E. thanks the Spanish Government for a predoctoral grant.

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Supporting Information Available: Flame annealing procedure, characterization of the clean gold surface, contaminants of the process, XPS spectrum of the sample after the dipping procedure. This material is available free of charge via the Internet at http://pubs.acs. org.

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