pubs.acs.org/Langmuir © 2010 American Chemical Society
Extended Surfaces Nanopatterned with Functionalized Cavities for Positioning Nanoparticles Sandrine Dourdain,† Alain Gibaud,‡ Anastasia Delattre,† and Pierre Terech*,† †
CEA, INAC/SPrAM, 17 rue des martyrs, 38054 Grenoble Cedex 9, France, and ‡Laboratoire de Physique de l’Etat Condens e, UMR CNRS 6087, Universit e du Maine, 72085 Le Mans Cedex 09, France Received November 18, 2009. Revised Manuscript Received January 25, 2010
This contribution reports a method delivering inorganic nanopatterned and functional surfaces from the ion beam etching of mesoporous thin films. The nanoscaled patterns are spherical or cylindrical cavities that are arranged on a macroscale. A variety of chemical functions can be grafted selectively in the cavities to add specific interactions with functional nanoparticles to be positioned. The process opens a simple route to localize and organize functional spherical or linear nanoparticles on extended surfaces.
The elaboration of patterned surfaces presenting controlled arrays of functional species at the nanoscale is a very active field of fundamental and applied research. Such interest is driven by multiple potential applications in catalysis, sensing, optoelectronics, magnetism, etc. Various options have been explored to develop such surfaces: electron beam lithography,1 nanoimprint methods,2 focused ion beam techniques,3,4 or nanopatterning based on polymer self-assembly.5,6 These strategies complete the molecular approach consisting of epitaxial and covalent grafting of molecules on crystallographically suitable solid substrates.7 A challenging objective remains the obtaining of nanostructured surfaces with functionalizable nanosites organized over macroscopic distances. Such surfaces could be used to localize and organize functional species on a 2D array suitable for applied devices. Routes that take advantage of lyotropic solutions and their extended organizations with typical micellar sizes in the 2-50 nm range have been explored to pattern surfaces at the nanoscale. Nevertheless, they are restricted to organic surfaces that cannot be selectively functionalized. Lyotropic structures are also commonly used to template inorganic matrices to design large-scale nanostructured and functional materials.8-10 The evaporation-induced self-assembly (EISA) protocol gives rise to solid structured thin films with size-controlled and organized nanopores.11,12 However, the surface of the obtained films does not show systematically the core structuration and does not *Corresponding author. E-mail:
[email protected]. (1) Cui, Y.; Bjork, M. T.; Liddle, J. A.; Sonnichsen, C.; Boussert, B.; Alivisatos, A. P. Nano Lett. 2004, 4, 1093. (2) Guo, L. J. Adv. Mater. 2007, 19, 495. (3) Hannour, A.; Bardotti, L.; Prevel, B.; Bernstein, E.; Melinon, P.; Perez, A.; Gierak, J.; Bourhis, E.; Mailly, D. Surf. Sci. 2005, 594, 1. (4) Prevel, B.; Bardotti, L.; Fanget, S.; Hannour, A.; Melinon, P.; Perez, A.; Gierak, J.; Faini, G.; Bourhis, E.; Mailly, D. Appl. Surf. Sci. 2004, 226, 173. (5) Krishnamoorthy, S.; Pugin, R.; Brugger, J.; Heinzelmann, H.; Hinderling, C. Adv. Mater. 2008, 20, 1962. (6) Glass, R.; Moller, M.; Spatz, J. P. Nanotechology 2003, 14, 1153. (7) Struth, B.; Terech, P.; Rieutord, F. ChemPhysChem 2006, 7, 756. (8) Kresge, C. T.; Leonowicz, M. E.; Roth, W. J.; Vartuli, J. C.; Beck, J. S. Nature 1992, 359, 710. (9) Ozin, G. A.; K. Hou, G. A.; Lotsch, B. V.; Cademartiri, L.; Puzzo, D. P.; Scotognella, F.; Ghadimi, A.; Thomson, J. Mater. Today 2009, 12, 12. (10) Zhao, Y.; Jiang, L. Adv. Mater. 2009, 21, 3621. (11) Lu, Y. F.; Ganguli, R.; Drewien, C. A.; Anderson, M. T.; Brinker, C. J.; Gong, W. L.; Guo, Y. X.; Soyez, H.; Dunn, B.; Huang, M. H.; Zink, J. I. Nature 1997, 389, 364. (12) Brinker, C. J.; Lu, Y. F.; Sellinger, A.; Fan, H. Y. Adv. Mater. 1999, 11, 579.
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provide selectively functionalized nanopatterns. This contribution reports a method using ion beam etching (IBE) of inorganic mesoporous films to deliver surfaces with 2D crystallographic arrays of cavities bearing functional sites. The process produces surfaces with opened spherical or cylindrical pores organized over square centimeters. It can moreover be suitably adapted to graft chemical functionalities in the pores, offering thereby promising applied potentialities taking advantage of a selectivity property between the pores and the nanoparticles (NPs). Silica films can be templated by many surfactants giving rise to organized spherical and cylindrical pores with diameters in the range of 2-10 nm. Scheme 1a,b depicts a cross section of a porous film before and after the removal of the organic template. As illustrated at Scheme 1b,c, the etching of such mesoporous films can provide inorganic nanostructured surfaces of functional opened cavities. Two etching methods have been reported using such inorganic nanostructured films. First, chemical etching using NaOH soaring was employed to reveal aligned cylindrical pores.13,14 Despite simple, this invasive method did not produce constant etching rates and damaged the morphology of the pores. Besides, its extension from silica films to other porous alkoxides has not been demonstrated or is unlikely. Second, the opening of the pores at the silica-air interface was achieved by a thermal treatment by Fisher et al.15 Nevertheless, this method does not ensure the chemical integrity of some functions previously grafted and their selective location in the cavities. The above limitations of the former methods can be avoided with our protocol. Once formed, mesoporous films are mechanically etched by a beam of argon ions whatever the composition of the inorganic replica. The surface of the pores can be chemically functionalized by using the appropriate silica precursor.16,17 During the etching process, the functions grafted in the cavities are preserved under suitable grazing angular conditions of the incident ion beam by a shadowing protection effect. The control of the etched thickness is accurately tuned to deliver etching rates as low as 0.2 nm s-1, (13) Yan, M. H.; Henderson, M. J.; Gibaud, A. Appl. Phys. Lett. 2007, 91. (14) Minhao, Y.; Henderson, M. J.; Gibaud, A. Thin Solid Films 2009, 517, 3028. (15) Fisher, A.; Kuemmel, M.; Jarn, M.; Linden, M.; Boissiere, N. L.; Sanchez, C.; Grosso, D. Small 2006, 2, 569. (16) Stein, A.; Melde, B. J.; Schroden, R. C. Adv. Mater. 2000, 12, 1403. (17) Sayari, A.; Hamoudi, S. Chem. Mater. 2001, 13, 3151.
Published on Web 02/19/2010
DOI: 10.1021/la904342z
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Article Scheme 1. Cross-Sectional View of a Mesoporous Film before (a) and after (b) the Templating Surfactant Removal; (c) Same View after the Opening of the Pores by Etching the Top Surface; (d) Top View of the Opened Pores Illustrating the Organization of Nano-objects at a Surface by the Selective Functionalization of the Poresa
a
For sake of clarity, only one functional graft per pore is presented.
with a horizontality parameter of 4.3% over ca. 100 mm. Such features are unique in the arsenal of etching techniques, to date. To illustrate, silica films were templated by lyotropic solutions of the tribloc copolymers F127 (EO99PO69EO99) and P123 (EO20PO69EO20), providing respectively spherical and cylindrical pores with diameters of 5 and 6 nm. Spherical pores are organized in a cubic Im3m arrangement,18 and cylindrical pores are in a p6m structure.19 Cubic Arrangements of Spherical Pores. The X-ray specular reflectivity (XSR) technique was used to characterize the film thickness on the subnanometer scale. The thickness of the films was decreased by IBE applied with an incident beam perpendicular to the surface.20,21 Figure 1a shows the XSR curves before and after ion beam etching for 240 s. The finite overall thickness of the film produces Kiessig fringes while the periodicity d of the stack of pores generates Bragg reflections (located at qz = 0.067, 0.13, and 0.20 A˚-1 for the nonetched film). The total thickness T of the film determined from the small period of the Kiessig fringes shows that the film thickness decreases from T = 101 nm to T = 65.8 nm after etching, while the Bragg peak spacings are only slightly changed. These two features are consistent with a thickness decrease due to a removal of the top silica material without destroying the underlying periodic porous structure. Nevertheless, the Bragg peaks’ positions indicate that the distance d between two layers of pores (Scheme 1c) is equal to 9.4 and 8.5 nm before and after etching, respectively. The small difference is assigned to a slight anisotropic alteration of the spherical form factor of the pores. The film thickness, originally made up of N = 11 layers of pores, is thereby reduced to N = 8 by the IBE process. Scanning electron microscopy (SEM) provided images of the surface topography of the films. The top view in Figure1b shows the distribution of spherical pores of the film etched for 240 s. The cross-sectional views of the fractured films before and after etching support the reduction of the film thickness, demonstrated by XSR, from 11 to 8 layers ( 1 (Figures 1 c,d). SEM also supports XSR data concerning the slight deformation of the pores (18) Fang, H.; Shi, W. H.; Ma, C. Y. Mater. Lett. 2006, 60, 581. (19) Dourdain, S.; Bardeau, J. F.; Colas, M.; Smarsly, B.; Mehdi, A.; Ocko, B. M.; Gibaud, A. Appl. Phys. Lett. 2005, 86, 113108. (20) Lee, R. E. J. Vac. Sci. Technol. 1979, 16, 164. (21) Uchikoga, S.; Lai, D. F.; Robertson, J.; Milne, W. I.; Hatzopoulos, N.; Yankov, R. A.; Weiler, M. Appl. Phys. Lett. 1999, 75, 725.
7566 DOI: 10.1021/la904342z
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under etching. Elliptical traces of the pores exhibit small axes reduced from 8 to 6 nm along the normal direction to the substrate. Cylindrical Pores Organized in a Hexagonal Symmetry. Figure 2a shows the related XSR of the film at different etching times te. te was finely varied so that the thickness of a monolayer silica film can be reached. The total thickness T of the film, determined from the Kiessig fringes, decreases from T = 87 nm to T = 45 nm after an etching time of 200 s. For longer etching times, the reflectivity curves exhibit smoother profiles with damped oscillations and fringes. The effect is expected for thinner layers as a result of the convolution of larger Kiessig fringes, broader Bragg peaks, and a roughness that becomes significant with respect to the thickness. Films of smaller thicknesses were analyzed with a modeling of the reflectivity curves using the matrix technique.19 The fitting procedure included the number of layers N estimated from the SEM views. Etched thicknesses can be deduced from a linear regression law as a function of te (Figure 2, inset). Consistently, SEM views in Figure 2b,c reveal the presence of aligned grooves at the top surface, organized as a fingerprint-like pattern. As expected for this film, the cross-sectional view (Figure 2b) shows a p6m honeycomb-like organization of pores. Oriented cylindrical traces are also seen on the edge of the film following the curvature of the domain. For longer etching time, Figure 2c shows a nanostructured film as thin as 4 nm, showing that IBE is a gradual and nondestructive method. For spherical or cylindrical pores, the etched surfaces appear as patchworks of nanostructured domains over a few square centimeters. The average size of a domain is presently ca. 0.02 μm2 and could be increased if needed by using prepatterned substrates13,22 or by applying strong magnetic fields during the films deposition.23 The mechanical resistance of the structure made up of spherical pores was found to be larger than with cylindrical pores. With an etching time of 240 s, the etched thickness was 45 nm for cylindrical pores and 35 nm for spherical ones. It is confirmed by the expression of the Young modulus E of a porous material: 24 E ¼ Em ð1 -PÞn where P is the porosity, n is related to the shape of the pores (n = 2.5 and 2.2 for spherical and cylindrical pores, respectively25), and Em is the Young modulus of the matrix between the pores. For identical porosity and Young modulus of the matrix, the Young modulus of a film made up of spherical pores is higher than the one for a film made up of cylindrical pores. The angular incidence of the beam also plays an important role on the etching time. Under grazing incidence, very long etching time had to be applied for etching small thicknesses because of a lower cross section between the incident ion beam and the surface of the porous film. Such inorganic surfaces with well-defined cavities organized according to a specific crystallographic array can be used to position functional NPs. The localization process of the NPs is expected to be amplified if suitable chemical functions are grafted in the opened pores (Scheme 1d). To this purpose, the chemical (22) Miyata, H.; Suzuki, T.; Fukuoka, A.; Sawada, T.; Watanabe, M.; Noma, T.; Takada, K.; Mukaide, T.; Kuroda, K. Nat. Mater. 2004, 3, 651. (23) Tolbert, S. H.; Firouzi, A.; Stucky, G. D.; Chmelka, B. F. Science 1997, 278, 264. (24) Gibson, L. J.; Ashby, M. F. Cellular Solids: Structure and Properties; Cambridge University Press: Cambridge, UK, 1997. (25) Williford, R. E.; Li, X. S.; Addleman, R. S.; Fryxell, G. E.; Baskaran, S.; Birnbaum, J. C.; Coyle, C.; Zemanian, T. S.; Wang, C.; Courtney, A. R. Microporous Mesoporous Mater. 2005, 85, 260.
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Article
Figure 1. (a) Reflectivity curves of the Im3m cubic film before (red) and after etching during 240 s (black). The Bragg reflections (arrows) at 0.067-0.13-0.20 A˚-1 for the etched film are due to the z-periodic stacking of layers. (b) Top SEM view of Im3m mesoporous films after etching for 240 s. (c, d) Cross-sectional views before and after etching. The scale bar is 100 nm.
Figure 3. (a) SEM top view of the distribution of the NPs adsorbed onto a functionalized mesoporous surface. The scale bar is 100 nm. (b) Diagram of the surface number density of the FePt NPs in the opened pores for functionnalized and bare films.
Figure 2. (a) Reflectivity curves of the p6m mesoporous film measured after different etching times. Top inset shows the evolution of the etched thickness as a function of te. Cross-sectional SEM views of p6m mesoporous films after etching for 50 s (b) and 480 s (c). The scale bar is 100 nm.
functionalization of the pores can be envisaged through direct or post synthetic methods.16,17 Subsequently, the IBE step is applied with the incident ion beam under grazing conditions (
