pubs.acs.org/Langmuir © 2009 American Chemical Society
Soft Lithographic Patterning of Spin Crossover Nanoparticles Christophe Thibault,†,‡ Gabor Molnar,‡,§ Lionel Salmon,‡,§ Azzedine Bousseksou,‡,§ and Christophe Vieu*,†,‡ †
CNRS, LAAS, 7 Avenue du Colonel Roche, F-31077 Toulouse, France, ‡Universit e de Toulouse, UPS, INSA, INP, ISAE, F-31077 Toulouse, France, and §Laboratoire de Chimie de Coordination, CNRS, 205 Route de Narbonne, 31077 Toulouse, France Received November 2, 2009. Revised Manuscript Received November 24, 2009
Microtransfer molding has been used to fabricate homogeneous micropatterns and nanopatterns of spin crossover nanoparticles of [Fe(NH2trz)](tos)2 over a large area. We show that the use of an aprotic solvent (n-octane) may lead to successful results. Very well organized micropatterns are obtained, showing spin crossover phenomenon. Dark field optical and AFM images and Raman microspectrometry results are reported.
1. Introduction Certain transition-metal complexes are known to display a molecular bistability in their high-spin (HS) and low-spin (LS) electron configurations. These so-called spin crossover (SCO) complexes can change their optical, magnetic, mechanical, and electrical properties with various external stimuli such as temperature, pressure, magnetic field, or light irradiation. During the past few decades, this phenomenon has been extensively studied and well understood on the molecular level as well as in the solid state.1 Recently, a great deal of attention has been focused on the synthesis and properties of SCO materials on the nanometric *Corresponding author. E-mail:
[email protected]. (1) G€utlich, P., Goodwin, H. A., Eds. Spin Crossover in Transition Metal Compounds; Topics in Current Chemistry; Springer: Berlin, 2004; Vols. I-III, p 233. (2) Soyer, H.; Mingotaud, C.; Boillot, M.-L.; Delhaes, P. Langmuir 1998, 14, 5890. (3) Soyer, H.; Dupart, E.; Gomez-Garcı´ a, C. J.; Mingotaud, C.; Delhaes, P. Adv. Mater. 1999, 11, 382. (4) Letard, J.-F.; Nguyen, O.; Soyer, H.; Mingotaud, C.; Delhaes, P.; Kahn, O. Inorg. Chem. 1999, 38, 3020. (5) Jaiswal, A.; Floquet, S.; Boillot, M.-L.; Delhaes, P. Chem. Phys. Chem. 2002, 12, 1045. (6) Nakamoto, A.; Ono, Y.; Kojima, N.; Matsumura, D.; Yokoyama, T. Chem. Lett. 2003, 32, 336. (7) Lee, S.-W.; Lee, J.-W.; Jeong, S.-H.; Park, I.-W.; Kim, Y.-M.; Jin, J.-I. Synth. Met. 2004, 142, 243. (8) Ruben, M; Rojo, J.; Romero-Salguero, F. J.; Uppadine, L. H.; Lehn, J.-M. Angew. Chem., Int. Ed. 2004, 43, 3644. (9) Cobo, S.; Molnar, G.; Real, J. A.; Bousseksou, A. Angew. Chem., Int. Ed. 2006, 45, 5786. (10) Seredyuk, M.; Gaspar, A. B.; Ksenofontov, V.; Reiman, S.; Galyametdinov, Y.; Haase, E.; Rentschler, W.; G€utlich, P. Chem. Mater. 2006, 18, 2513. (11) Kuroiwa, K.; Shibata, T.; Sasaki, S.; Ohba, M.; Takahara, A.; Kunitake, T.; Kimizuka, N. J. Polym. Sci., Part A: Polym. Chem. 2006, 44, 5192. (12) Molnar, G.; Cobo, S.; Real, J. A.; Carcenac, F.; Daran, E.; Vieu, C.; Bousseksou, A. Adv. Mater. 2007, 19, 2163. (13) Coronado, E.; Galan-Mascaros, J. R.; Monrabal-Capilla, M.; Garciı´ aMartı´ nez, J.; Pardo-Ibanez, P. Adv. Mater. 2007, 19, 1359. (14) Matsuda, M.; Tajima, H. Chem. Lett. 2007, 36, 700. (15) Agustı´ , G.; Cobo, S.; Gaspar, A. B.; Molnar, G.; Ould Moussa, N.; Szilagyi, P.; Palfi, V.; Vieu, C.; Mu~noz, M. C.; Real, J. A.; Bousseksou, A. Chem. Mater. 2008, 20, 6721. (16) Boldog, I; Gaspar, A. B.; Martı´ nez, V.; Pardo-Iba~nez, P.; Ksenofontov, V.; Bhattacharjee, A.; G€utlich, P.; Real, J. A. Angew. Chem., Int. Ed. 2008, 47, 6433. (17) Volatron, F.; Catala, L.; Riviere, E.; Gloter, A.; Stephan, O.; Mallah, T. Inorg. Chem. 2008, 47, 6584. (18) Larionova, J.; Salmon, L.; Guari, Y.; Tokarev, A.; Molvinger, K.; Molnar, G.; Bousseksou, A. Angew. Chem., Int. Ed. 2008, 47, 8236. (19) Cavallini, M.; Bergenti, I.; Milita, S.; Ruani, G.; Salitros, I.; Qu, Z.; Chandrasekar, R.; Ruben, M. Angew. Chem., Int. Ed. 2008, 47, 8596. (20) Forestier, T.; Mornet, S.; Daro, N.; Nishihara, T.; Mouri, S.; Tanaka, K.; Fouche, O.; Freysz, E.; Letard, J.-F. Chem. Commun. 2008, 4327.
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Figure 1. Thermal variation of the absorbance of an octane suspension of Fe(NH2Trz)3(tos)2 nanoparticles measured at 540 nm in the heating (0) and cooling (b) modes. The inset shows the nanoparticle size distribution and the chain structure of the [Fe(NH2trz)3]2þ cation.
scale, which connects the molecular and bulk properties.2-26 These efforts have been primarily stimulated by fundamental questions such as the size reduction effect on the dynamics and cooperativity of the SCO phenomenon. In addition, potential applications of SCO materials in memory, switching, and sensor devices necessitate methods of processing them on the micrometric and nanometric scales. In this respect, the interfacing of the molecular switching units with a microscaled device environment is of crucial importance. Only recently, a breakthrough in this direction has been achieved as a result of the combination of sequential assembly and electron beam lithography techniques.9,12,15 This powerful type of nanopatterning method is very expensive and slow in obtaining the nanopatterned surface. For this reason, a soft lithography (21) Matsuda, M.; Isozaki, H.; Tajima, H. Chem. Lett. 2008, 37, 374. (22) Forestier, T.; Kaiba, A.; Pechev, S.; Denux, D.; Guionneau, P.; Etrillard, C.; Daro, N.; Freysz, E.; Letard, J.-F. Chem.;Eur. J. 2009, 15, 6122. (23) Arnaud, C.; Forestier, T.; Daro, N.; Freysz, E.; Letard, J.-F.; Pauliat, G.; Roosen, G. Chem. Phys. Lett. 2009, 470, 131. (24) Catala, L.; Volatron, F.; Brinzei, D.; Mallah, T. Inorg. Chem. 2009, 48, 3360. (25) Boillot, M.-L.; Pillet, S.; Tissot, A.; Rivire, E.; Claiser, N.; Lecomte, C. Inorg. Chem. 2009, 48, 4729. (26) Shi, S.; Schmerber, G.; Arabski, J.; Beaufrand, J.-B.; Kim, D. J.; Boukari, S.; Bowen, M.; Kemp, N. T.; Viart, N.; Rogez, G.; Beaurepaire, E.; Aubriet, H.; Petersen, J.; Becker, C.; Ruch, D. Appl. Phys. Lett. 2009, 95, 3303.
Published on Web 12/02/2009
DOI: 10.1021/la904162m
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Figure 2. (A, B) Dark field optical images and (C) optical profilometry of microstructures obtained by μTM. Scheme 1. Procedure Used for μTMa
a
See the text for further explanations.
method for the fabrication of SCO units in ordered nanopatterns would be very desirable. Because it is known that the SCO properties depend critically on the local molecular environment (e.g., packing, solvent molecules, anions, etc.),1 the development of methods giving access to nanostructures that preserve the integrity of the material is technologically of the utmost importance to the application of SCO compounds. Cavallini et al.19 have successfully demonstrated that microinjected molding in capillaries (MIMIC) and lithographically controlled wetting (LCW) can be used to fabricate micronic and nanometric stripes of SCO materials. However, these techniques suffer from several drawbacks, such as thickness inhomogeneity between the stripes and, of course, the fact that only stripes can be achieved. They are also very dependent on the nature of the solvent, and only protic solvents (water, ethanol, etc.) can be used. 1558 DOI: 10.1021/la904162m
Herein we report the successful patterning of [Fe(NH2trz)](tos)2 (tos = tosylate, NH2trz = 4-amino-1,2,4-triazole) SCO nanoparticles (Figure 1) at the frontier of micrometric (i.e., optically accessible) and nanometric scales with different types of features using more versatile soft lithographic techniques. This SCO compound exhibits a relatively abrupt thermal transition between the HS and LS forms around room temperature (Figure 1), and we thus consider it to be one of the most useful compounds in this class of switchable metal complexes. We use microtransfer molding (μTM)27,28 (Scheme 1) to fabricate micrometric and nanometric structures. Until now, only protic solvents have been used in soft lithography because of the swelling of PDMS in aprotic solvents.29,30 Here we demonstrate that this is possible to use aprotic solvents (n-octane) and to follow a different procedure than the one typically used for μTM. The obtained patterns were characterized by atomic force microscopy (AFM), dark-field optical microscopy, and Raman microscopy.
2. Experimental Section [Fe(NH2trz)](tos)2 nanoparticles were synthesized using the reverse micelle technique.4 Sodium bis(2-ethylhexyl) sulphosuccinate (NaAOT) was used as a surfactant. The micellar polar core was a water-ethanol mixture, and the apolar solvent consisted (27) Zhao, X. M.; Xia, Y. N.; Whitesides, G. M. Adv. Mater. 1996, 8, 837. (28) Thibault, C.; Severac, C.; Trevisiol, E.; Vieu, C. Microelectron. Eng. 2006, 83, 1513. (29) Ng Lee, J.; Park, C.; Whitesides, G. M. Anal. Chem. 2003, 75, 6544. (30) Favre, E. Eur. Polym. J. 1996, 32, 1183.
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Figure 3. (A) AFM image and (C) the corresponding cross section of nanostructures obtained by μTM. (B) Dark field optical image of a large area of the same sample of nanostructures. of n-octane. The size distribution of the nanoparticle suspensions in n-octane was investigated by dynamic light scattering by means of a Malvern Zetasizer instrument. Both a freshly prepared and a 1-month-old suspension had a similar average diameter of ca. 4 nm (inset of Figure 1). Variable-temperature UV-vis absorption spectra of the nanoparticle suspensions in n-octane were recorded by using a Cary-50 spectrophotometer and a thermostatted cell holder. The thermal variation of the absorbance at 540 nm revealed a complete LS to HS transition when heating the suspension to above ca. 310 K (Figure 1). The first step in the fabrication of a stamp for μTM consists of generating a silicon master. This was achieved by photolithography on AZ1529 or by electron beam lithography on polymethylmetacrylate (PMMA) depending on the feature size (respectively, micrometer or nanometer) followed by pattern transfer using deep reactive ion etching (RIE). The etch depth varies from 7 μm (micrometer features) to 200 nm (nanometer features). To enable simple demolding of this master, an antiadhesive treatment is carried out by a well-established process using octadecyltrichlorosilane (OTS) in the liquid phase. The final step consists of curing the PDMS prepolymer solution (containing a mixture in a 10:1 mass ratio of PDMS oligomers and a reticular agent from Sylgard 184 Kit (Dow Corning)) on the silicon master. A silicon master can be reused more than 100 times, and each stamp can be used for several prints. Several patterns were tested: 300 and 500 nm width lines with a pitch of 1 μm as well as a mesh of 2 μm lines with a 10 μm pitch. Initially, the nanoparticle suspension (30 μL) was poured onto the stamp, making sure that the molded PDMS stamp was wetted by the liquid. After 1 min, the stamps were dried under nitrogen flow for 1 min and the excess material was scraped away using a piece of flat PDMS. We then printed the stamp manually with pressure onto glass substrates for 30 s. Atomic force microscopy (AFM), dark field optical microscopy, and optical profilometry have been used to characterize the stamps and the resulting prints on the glass substrates under Langmuir 2010, 26(3), 1557–1560
ambient conditions. Raman spectra of the patterns were recorded using a Labram-HR (Jobin Yvon) microspectrometer. The 514.5 nm line of an Arþ laser was used as the excitation source. The sample temperature was controlled using a THMS600 variable-temperature stage (Linkam).
3. Results and Discussion Scheme 1 presents the schematic procedure used for μTM. (A) A drop of the nanoparticle suspension was placed on the patterned surface of the PDMS stamp. The PDMS stamp started to swell because of the octane solution, and all of the features were completely deformed. (B) The excess solution was removed by blowing it off of the surface of the stamp with a brisk stream of nitrogen and drying it under this nitrogen flow until the PDMS stamp recovered its initial form (between 1 and 2 min depending on the temperature of the room). There is still some material on the surface of the stamp, and the general procedure for removing this excess was to scrape it away using a piece of flat PDMS. (C) The filled stamp was placed in contact with a substrate with a glass backplane on top of it. (D) We then applied a pressure of 100 g for 15 s and peeled away the stamp. The whole process took less than 5 min. The key point of this process is the drying step: one has to minimize the deformation of the stamp, which occurs because of swelling. Therefore, the octane must be removed from the PDMS stamp as much as possible before the stamp comes into contact with the substrate. We first produced micrometric features with this technique to control deposition parameter such as inking and contact time. Figure 2A shows 5 μm meshes perfectly ordered with no defects. This is revealed by the use of dark field optical microscopy. These features were obtained over large areas (Figure 2B) of up to several square centimeters with good thickness homogeneity. DOI: 10.1021/la904162m
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Figure 4. (A)Raman spectra (excited at 514.5 nm) acquired at 225 and 325 K of the microstructures fabricated by mTM. (B) For comparison, the spectra of the bulk material are also shown.
Indeed, optical profilometry reveals a thickness of ca. 4 μm (Figure 2C). It is important to note that our process exhibits quite a large latitude, which enables control of the thickness in the range of 100 nm-10 μm depending of the feature depth of the stamp and the solution concentration. Initially, we observed that the produced patterns were not stable over long periods of time. To conserve the patterns for several months, we found that it is necessary to dry the material for 30 min at 60 °C in an oven. The resolution limit of the patterned stripes was further moved into the nanometer regime by using μTM. Structures with a resolution of approximately 300 nm could be achieved by an estimation of the fwhm. Figure 3A-C shows an AFM image of the nanostructures obtained by μTM. These lines cover, such as the micrometric features, a surface of several square centimeters (Figure 3B) with good thickness homogeneity and very few defects. Raman microspectroscopy is a useful technique for probing the spin state of transition-metal complexes with a high spatial resolution, and it was therefore used to investigate the patterns obtained by μTM. This is particularly important for small
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features and thin films that contain only a few nanoparticles because the laser spot size (ca. 1.5 μm) in the Raman experiment allows spin-state detection at a resolution of a single mesh in the case of the microstructures. With this aim, we performed Raman characterization on a freshly prepared structure as a function of temperature. Figure 4 shows selected Raman spectra of the microstructures as well as that of the microcrystalline bulk sample. In both cases, the spectra were acquired at 225 and 325 K (i.e., in the LS and HS states, respectively). The spectrum of the microstructures is dominated by the strong Raman scattering of the NaAOT surfactant, which makes the comparison with spectra of the bulk SCO sample difficult. However, in the spectral window between 1300 and 1600 cm-1 we could repetitively observe the appearance of two vibrational modes at around 1369 and 1547 cm-1 when cooling the sample below ca. 295 K. These frequencies, tentatively assigned to triazole ring-stretching modes, display comparable behavior in the bulk material and can therefore be identified as markers of the low spin state of the compound. These spectral changes were also confirmed in a concentrated slurry of nanoparticles.
4. Conclusions Microtransfer molding has been used to fabricate homogeneous micropatterns and nanopatterns of spin crossover nanoparticles over a large area. We demonstrate for the first time the possibility of using an aprotic solvent to pattern nanoparticles by soft lithography from the micrometer to the nanometer scale. Furthermore, we show that the nanoparticles keep their spin crossover properties (with a transition at around room temperature) after the soft lithography step. We believe, therefore, that this low-cost patterning technique provides interesting perspectives for the integration of spin crossover materials in microscaled and nanoscaled devices. Acknowledgment. This work was financially supported by the ANR NANOMOL project. We are grateful to Franck Carcenac (LAAS) for his help with e-beam lithography.
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