NANO LETTERS
Monolayer and Multilayer Films of [Mn12O12(O2CMe)16]
2004 Vol. 4, No. 3 399-402
Jonathan S. Steckel,† Nicole S. Persky,† Chelsea R. Martinez,† Calvin L. Barnes,† Elizabeth A. Fry,† Jaideep Kulkarni,‡ James D. Burgess,‡ Rachel B. Pacheco,§ and Sarah L. Stoll*,§ Department of Chemistry, Oberlin College, Oberlin, Ohio 44074, Department of Chemistry, Case Western ReserVe UniVersity, CleVeland, Ohio 44122, and Department of Chemistry, Georgetown UniVersity, Washington, D.C. 20057 Received May 29, 2003; Revised Manuscript Received January 14, 2004
ABSTRACT The manganese oxo-cluster, [Mn12O12(O2CMe)16(H2O)4]‚2(HO2CMe)‚4(H2O), (Mn-12), has been the subject of intense interest for its novel magnetic properties. Although it is a model system for nanoscale magnetic data storage, no methods for forming films or attaching the cluster to surfaces have been reported. Here we demonstrate a simple approach using functionalized self-assembled monolayers and short-chain polyelectrolytes to make monolayer and nanometer-scale multilayer films of Mn-12.
The discovery of unusual magnetic behavior in the manganese oxo-cluster, [Mn12O12(O2CMe)16(H2O)4]‚2(HO2CMe)‚ 4(H2O) (Mn-12), was significant in that it allowed for the first observation of quantum tunneling of magnetization.1 The evidence for tunneling is discrete steps in magnetization as a function of the field in the hysteresis loop. The magnetic properties also provide support for the fact that a single molecule has the potential to act like a nanoscale magnet. For example, the orientation of the magnetic moment of an individual Mn-12 molecule has an extremely slow relaxation time (on the order of months at 2 K) after the magnetic field has been removed.2 Although the Mn12O12 core mimics a tiny bar magnet, investigations have shown that the nature of the ligand, cocrystallized solvent, and cluster isomerization can effect the rate of resonant magnetization tunneling.3,4 Studies in which the acetate group has been exchanged for a variety of encapsulating carboxylic acids have concluded that the ligand can both control the solubility of the cluster and tune the redox potential of the metal centers. As a model system for nanostructured data storage, one of the goals yet to be realized is a method for patterning Mn-12 clusters on a surface in a controlled fashion. Recent work has been directed toward the discovery of new clusters with enhanced properties.5 Before practical devices can be fabricated, however, it will be important to identify means for structuring these clusters into nanoscale architectures. Although elegant synthetic work has shown that it is possible * Corresponding author. E-mail:
[email protected]. Phone: 202687-5839. Fax: 202-687-6209. † Oberlin College. ‡ Case Western Reserve University. § Georgetown University. 10.1021/nl0343553 CCC: $27.50 Published on Web 02/12/2004
© 2004 American Chemical Society
to functionalize Mn-12 selectively (with the potential for attaching a tether to the cluster),6 binding the complex to surfaces has not yet been demonstrated. Clearly, synthetic methodologies to form nanostructured, functional magnetic and electronic materials are an important goal not only for data storage but for a variety of practical applications.7 Previous approaches to patterning a surface with nanoparticles have relied on hydrophilic and hydrophobic interactions. For example, a self-assembled monolayer (SAM) film formed by alkanethiolates with different terminal chemical functionalities was used to wet the substrate surface selectively, controlling the areal deposition of iron oxides from colloidal solutions. The size of the patterned features depended on whether microcontact printing or dip-pen nanolithography was used.8 By contrast, we have prepared monolayer films of structurally well-defined, discrete clusters using simple coordination chemistry to control film formation. Here we demonstrate that Mn-12 can be attached to potentially any surface covered with carboxylate groups. Initially, we have used carboxylate-terminated SAMs to bind Mn-12 clusters to a flat surface. Subsequently, we have adapted layer-by-layer assembly methods to form nanometerthick films of Mn-12 layers held together using poly(acrylic acid) (PAA). The PAA forms an organic sheet functionalized with carboxylate groups to which the Mn-12 layer is attached. Multilayers of nanoparticles with short-chain polymers (held together by Coulombic or van der Waals interactions) have exhibited a variety of interesting optical9 and electronic properties,10 but as yet there are no reports of discrete molecular clusters in multilayer films. Because the properties
Figure 1. QCM data of flowing Mn-12 solution over a carboxylate-terminated SAM. The arrows indicate changes in the flowing solution.
of the films depend on the nanoparticle size, this system has the advantage that the cluster size is uniquely defined. To form monolayers composed of Mn-12 clusters, a substrate prepatterned with carboxylate-terminated SAMs was soaked in an ethanolic solution of Mn-12.11 The acetate groups undergo ligand exchange with carboxylate SAMs attaching the cluster to the surface, according to equation 1 below: Ausur-S-(CH2)10-CO2H + Mn12O12(O2CMe)16 f Ausur-S-(CH2)10-CO2-Mn12O12(O2CMe)15 + MeCO2H (1) This equation describes the exchange of one acetate group; however, it is likely that several acetate groups could exchange for entropic reasons (e.g., a surface chelate effect).12 We have used ellipsometry to determine the film thickness and found the SAM to be 18((3) Å (close to the reported value for the carboxylate-terminated SAM)13 and the SAMMn-12 film to be 26((5) Å. Thus, the average thickness of the Mn-12 monolayer based on ellipsometry is approximately 8 Å, between the diameter (10 Å) and thickness (5 Å) of the cluster. One might predict a face-down orientation, given that ligand-exchange studies show that the four acetate groups on the face of the cluster are most easily lost.14 However, the ellipsometric data suggest that the orientation (and thus the binding site) for the cluster is both face down and side on. To determine the mass of the films, we have also tracked changes in the quartz crystal microbalance (QCM) resonance frequency using acoustic impedance.15 Using the Sauerbrey equation,16 the frequency change (-40 Hz) reflected an increase in mass of 177((1) ng/cm2. One can make assumptions about either the mass or the area covered by an individual cluster. It is reasonable to limit the variations in mass to the stoichiometric range of Mn12O12Acx with x ) 12-15. Starting with the model that the clusters are closepacked, the area covered by a single cluster depends on the van der Waals radii, which we assume to be between 10 400
and 12 Å. Assuming that the four most loosely bound acetate groups were lost, the stoichiometry would be Mn12O12Ac12. For this molecular weight, the diameter of the close-packed cluster including the van der Waals radii was calculated to be approximately 12 Å. In addition to the measurements carried out under a nitrogen atmosphere with dry films, solution-phase, passivemode QCM was used to characterize the dynamics of film formation. A wall-jet flow cell was used to provide a controlled flux of Mn-12 to the QCM electrode surface such that mass transfer of Mn-12 was not rate-limiting for the surface-attachment reaction.17 After a stable baseline frequency for ethanol was obtained, the solution was switched to an ethanolic solution of Mn-12 (1 mM). Within approximately 10 min, a stable resonance frequency plateau was observed, indicating a -42-Hz frequency decrease consistent with the QCM acoustic impedance measurements conducted under nitrogen (Figure 1). Both the solution-phase, passive-mode QCM measurements and the acoustic impedance QCM measurements were remarkably reproducible between electrodes (within 5%). The mass changes agree with the self-limiting mechanism of surface binding through carboxylate exchange. After reverting the flow from Mn-12 back to pure ethanol, the resonance frequency remained unchanged, suggesting that the clusters were not removed with fresh (noncoordinating) solvent. In addition, the stability of the resonance frequency on changing the flow from Mn12 back to ethanol clearly indicates that the frequency decrease due to surface cluster attachment is not affected by changes in solution viscosity, density, or conductivity. Band-pass measurements indicate no detectable change in energy loss from the QCM upon cluster deposition, which strongly suggests that viscoelastic effects do not contribute to the mass increase measured for the cluster-immobilization reaction. The rate of cluster attachment to the SAM interface is within the same order of time as for ligand exchange observed for Mn-12 with carboxylic acids.18 Multilayer films were prepared using ethanol solutions of PAA and Mn-12. A derivatized glass slide was first coated Nano Lett., Vol. 4, No. 3, 2004
Figure 2. Fluorescein absorbance as a function of the bilayer for Mn-12/PAA multilayered films.
Figure 3. Ellipsometry data showing thickness vs bilayer for Mn-12/PAA mulilayered films.
with the PAA dye tagged with fluorescein at a loading of 1 dye molecule per monomer unit. A nanometer-thick film was formed through an alternating sequence of soaking the substrate in a solution of PAA (resulting in a carboxylatecoated surface) and then immersing it in a solution of Mn-12 (to form a monolayer of clusters). The growth of the multilayer was monitored by an increase in the absorption of the fluorescein in UV-visible spectra (Figure 2). Using Beer’s law and the fluorescein absorbance values at 493 nm for each layer, the thickness of the first layer was calculated to be 8 Å, and the films were found to grow with a linear slope of 7 Å/layer.19 In addition, we also followed the growth of multilayer films on gold-coated silicon substrates using ellipsometry. The increase in film thickness with the bilayer was monotonic (Figure 3), with a slope of 10 Å/layer that is consistent with the UV-visible spectroscopy data. Using a 16-bilayer-thick film, we were able to perform grazing incidence infrared spectroscopy to characterize further the structure of the cluster upon attachment to the Nano Lett., Vol. 4, No. 3, 2004
surface. The metal-oxygen stretches are quite distinct for the Mn-12 cluster; in fact, the derivatives of [Mn12O12 (O2CR)16] all have closely related peaks between 550 and 700 cm-1.20 The broad stretch centered at 658 cm-1 in the multilayer film corresponds to the Mn-O stretches observed for Mn-12 (Supporting Information). The thermal decomposition of Mn-12 results in three very intense peaks at 615, 497, and 415 cm-1, whereas decomposition by hydrolysis results in the formation of a strong peak above 900 cm-1 (most likely due to a Mn3+-OH stretch).21 The absence of these peaks indicates that there are no decomposed clusters on the surface. We believe that this is strong evidence that ligand exchange results in intact, surface-bound clusters. We were also interested in determining the mode of binding via shifts in the FTIR of the -COOH terminus of the SAMs upon coordination to the cluster. Because of the overlap in carboxylate stretches, definitive characterization was precluded. Given the presence of cluster-bound acetate, carboxylate-terminated SAMs, and metal-coordinated SAMs, this is not surprising. To confirm that the layers are held by coordinative covalent bonds rather than Coulombic interactions, we attempted to grow multilayered films using equivalent concentrations of an alternate polyanion, parastyrene sulfonate, with noncoordinating -SO32- groups. Using ellipsometry, no film growth was observed, suggesting that the layers are not held by charge interactions. This is consistent with the fact that the layers are formed by structurally intact, neutral clusters, with attachment to the film achieved through ligand exchange only. In conclusion, we have shown that we can attach Mn-12 molecules to form a monolayer of clusters on surfaces covered with carboxylate groups. The cluster layer formation is self-limiting and remains structurally intact as additional layers are added. The multilayers composed of Mn-12 clusters grow in a monotonic and even fashion, resulting in nanometer-thick films. Because we believe that we have accomplished the attachment of individual Mn-12 molecules to the substrate, it will be of interest to determine the effect of binding on the magnetic properties. Although the spin state of Mn-12 has been found to be independent of the 401
carboxylate group ligated to the clusters, the rate of resonant magnetic tunneling is often affected.22 Our future work will involve the magnetic characterization of these surface-bound clusters. We are also currently investigating the insertion of Mn-12 into MCM-41 functionalized with carboxylate groups and are comparing the magnetic characteristics of these surface-bound clusters. Acknowledgment. This work was funded by ACS-PRF GB-31564, an NSF Research Opportunity Award. S.L.S. thanks Professor Tom Mallouk for the use of the ellipsometer and for helpful discussions. Supporting Information Available: FTIR spectra of the 16-bilayer film of Mn-12 and PAA. This information is available free of charge via the Internet at http://pubs.acs.org. References (1) Thomas, L.; Lionti, F.; Ballou, R.; Gateschi, D.; Sessoli, R.; Barbara, B. Nature 1996, 383, 145-147. (2) Christou, G.; Gatteschi, D.; Hendrickson, D. N.; Sessoli, R. MRS Bull. 2000, 25, 66-71. (3) (a) Sun, Z.; Ruiz, D.; Dilley, N. R.; Soler, M.; Ribas, J.; Folting, K.; Maple, M. B.; Christou, G.; Hendrickson, D. N. Chem. Commun. 1999, 1973-1974. (b) Aubin, S. M. J.; Sun, Eppley, H. J.; Rumberger, E. M.; Guzei, I. A.; Folting, K.; Gantzel, P. K.; Rheingold, A. L.; Christou, G.; Hendrickson, D. N. Inorg. Chem. 2001, 40, 2127-2146. (4) (a) Sessoli, R.; Tsai, H. L.; Schake, A. R.; Wang, S.; Vincent, J. B.; Folting, K.; Gatteschi, D.; Christou, G.; Hendrickson, D. N. J. Am. Chem. Soc. 1993, 115, 1804-1816. (b) Sessoli, R.; Gatteschi, D.; Caneschi, A.; Novak, M. A. Nature 1993, 365, 141-143. (5) Brechin, E. K.; Boskovic, C.; Wernsdorfer, W.; Yoo, J.; Yamaguchi, A.; Sanudo, E. C.; Concolino, T. R.; Rheingold, A. L.; Ishimoto, H.; Hendrickson, Christou, G. J. Am. Chem. Soc. 2002, 124, 97109711. (6) (a) Solar, M.; Artus, P.; Folting, K.; Huffman, J. C.; Hendrickson, D. N.; Christou, G. Inorg. Chem. 2001, 40, 4902-4912. (b) Artus, P.; Boskovic, C.; Yoo, J.; Streib, W. E.; Brunel, L. C.; Hendrickson, D. N.; Christou, G. Inorg. Chem. 2001, 40, 4199-4210. (7) Mucic, R. C.; Storhoff, J. J.; Mirkin, C. A.; Letsinger, R. L. J. Am. Chem. Soc. 1998, 120, 12674-12675. (8) (a) Palacin, S.; Hidber, P. C.; Bourgoin, J. P.; Miramond, C.; Fermon, C.; Whitesides, G. M. Chem. Mater. 1996, 8, 1316-1325. (b) Qin, D.; Xia, Y.; Xu, B.; Yang, H.; Zhu, C.; Whitesides, G. M. AdV. Mater.
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(11)
(12) (13) (14) (15) (16) (17) (18) (19)
(20)
(21) (22)
1999, 11, 1433-1437. (c) Zhong, Z.; Gates, B.; Xia, Y. Langmuir 2000, 16, 10369-10375. (d) Liu, X.; Fu, L.; Hong, S.; Dravid, V.; Mirkin, C. A. AdV. Mater. 2002, 14, 231. (e) Hwang, K. Y.; Woo, S. Y.; Lee, J. H.; Jung, D.-Y.; Kwon, Y.-U. Chem. Mater. 2000, 12, 2059-2064. Colvin, V. L.; Schlamp, M. C.; Alivisatos, A. P. Nature 1994, 370, 354-357. (a) Feldheim, D. L.; Grabar, K. C.; Natan, M. J.; Mallouk, T. E. J. Am. Chem. Soc. 1996, 118, 7640-7641. (b) Cassagneau, T.; Mallouk, T. E.; Fendler, J. H. J. Am. Chem. Soc. 1998, 120, 7848-7859. The substrate was prepared by soaking a silicon substrate with 1000-Å gold sputtered on the surface, cleaned in piranha solution (H2SO4/ H2O2, 70:30 (v/v). Note that piranha solution is a Very strong oxidant and is dangerous to handle without gloVes, goggles, and a face shield. The SAMs were formed by soaking the substrates in 0.1 M ethanolic solutions of 11-mercaptodecanoic acid for 4 h. These were dried under nitrogen. The monolayer of Mn-12 was formed by soaking the SAMs substrate in a 1 mM solution of Mn-12 in ethanol for 15 min. Major, R. C.; Zhu, X.-Y. J. Am. Chem. Soc. 2003, 125, 8454-8455. Nuzzo, R. G.; Dubois, L. H.; Allara, D. L. J. Am. Chem. Soc. 1990, 112, 558-569. Artus, P.; Boskovic, C.; Yoo, J.; Streib, W.; Brunnel, L. C.; Hendrickson, D.; Christou, G. Inorg. Chem. 2001, 40, 4199-4210. The electrode area was 0.2 cm2 for 10-MHz quartz. Sauerbrey, G. Z. Phys. (Munich) 1959, 155, 206. Wang, Y.; Farrell, N.; Burgess, J. D. J. Am. Chem. Soc. 2001, 123, 5576-5577. Soler, M.; Artus, P.; Folting, K.; Huffman, J. C.; Hendrickson, D. N.; Christou, G. Inorg. Chem. 2001, 40, 4902-4912. Using Beer’s law, A ) cl, we can calculate γ, the coverage in units of moles of polymer/cm2. Assuming that the density of PAA is approximately 0.8 g/cm3 and knowing the molecular weight of the polymer, we found the thickness of the first layer to be 8 Å. See, for example, (a) Ruis, D.; Sun, Z.; Albela, Folting, K.; Ribas, R.; Christou, G.; Hendrickson, D. Angew. Chem., Int. Ed. 1998, 37, 300. (b) An, J.; Chen, Z.-D.; Bian, J.; Chen, J.-T.; Wang, S. X.; Gao, S.; Xu, G. X. Inorg. Chim. Acta 2000, 299, 28. (c) Sessoli, R.; Tsai, H. L.; Schake, A. R.; Wang, S.; Vincent, J. B.; Folting, K.; Gatteschi, D.; Christou, G.; Hendrickson, D. N. J. Am. Chem. Soc. 1993, 115, 1804. Yang, D. S.; Wang, M. K. Chem. Mater. 2001, 13, 2589. Sessoli, R.; Tsai, H. L.; Schake, A. R.; Wang, S.; Vincent, J. B.; Folting, K.; Gatteschi, D.; Christou, G.; Hendrickson, D. N. J. Am. Chem. Soc. 1993, 115, 1804-1816.
NL0343553
Nano Lett., Vol. 4, No. 3, 2004
