Langmuir 2000, 16, 10369-10375
10369
Soft Lithographic Approach to the Fabrication of Highly Ordered 2D Arrays of Magnetic Nanoparticles on the Surfaces of Silicon Substrates Ziyi Zhong, Byron Gates, and Younan Xia* Department of Chemistry, University of Washington, Seattle, Washington 98195-1700
Dong Qin Center for Nanotechnology, University of Washington, Seattle, Washington 98195-2140 Received August 21, 2000 This paper describes a simple and convenient method that uses patterned monolayers as templates to fabricate highly ordered 2D arrays of magnetic particles (Co, Ni, or R-Fe, and ferrites such as MgFe2O4 or NiFe2O4) with lateral dimensions in the range of 70-460 nm. In this method, the hydrophilic, hydroxylterminated surface of a Si/SiO2 wafer was patterned with a hydrophobic monolayer of octadecyltrichlorosilane using microcontact printing with an elastomeric stamp and subsequently used as template to define and deposit a regular 2D array of 2-propanol droplets that contained inorganic salts such as Co(NO3)2, Ni(NO3)2, and Fe(NO3)3, or a combination of these compounds. Evaporation of the solvent led to the formation of a 2D array of nitrate nanoparticles on the hydrophilic, bare regions of Si/SiO2. Each nanoparticle could be well-positioned within the hydrophilic region by withdrawing the substrate from the nitrate solution and by letting the solvent evaporate with the wafer being held at a specified orientation relative to the gravitational field. The nitrate was subsequently converted into metal oxide (Co3O4, NiO, and R-Fe2O3) by thermal decomposition in air at 600 °C, and finally into a magnetic substance (that is, Co, Ni, and R-Fe) through the reduction by hydrogen gas at 400 °C. The dimensions of these particles could be controlled by changing the concentration of the nitrate solution and/or the area of the hydrophilic region. We have also shown that coprecipitation of two (or more) different nitrates within the liquid droplets could lead to the formation of highly ordered 2D arrays of magnetic ferrites such as MgFe2O4 or NiFe2O4. The magnetic properties of these 2D arrays of nanoparticles supported on silicon substrates were studied using magnetic force microscopy.
Introduction Nanostructures with at least one dimension in the range of 1-100 nm have attracted steadily growing interest owing to their fascinating properties and unique applications that cannot be offered by their micro- or macroscopic counterparts.1 As one of the simplest forms of nanostructures, nanocrystallites have been the subject of intensive research for many years.2 The ability to obtain nanocrystallites with well-controlled sizes has played an important role in many areas of modern science and technology. Using nanocrystallites as model systems, for example, a wealth of interesting physics and chemistry has been learned by studying the evolution of fundamental (electronic, electrical, optical, mechanical, and magnetic) properties with size.3 With nanocrystallites as active components, a wide range of new types of devices are also appearing rapidly in the prototype forms in many research areas. Examples include quantum dot lasers,4 single* Corresponding author. E-mail:
[email protected]. (1) See, for example: (a) Ozin, G. A. Adv. Mater. 1992, 4, 612. (b) Engineering a Small Word: From Atomic Manipulation to Microfabrication. A special issue in Science 1991, 254, 1277. (2) Reviews: (a) Fendler, J. H. Chem. Rev. 1987, 87, 877. (b) Steigerwald, M. L.; Brus, L. E. Acc. Chem. Res. 1990, 23, 183. (c) Bawendi, M. G.; Steigerwald, M. L.; Brus, L. E. Annu. Rev. Phys. Chem. 1990, 41, 477. (d) Wang, Y. Acc. Chem. Res. 1991, 24, 133. (e) Weller, H. Angew. Chem., Int. Ed. Engl. 1993, 32, 41. (3) See, for example: (a) Alivisatos, A. P. Science 1996, 271, 933. (b) Suslick, K. S.; Fang, M.; Hyeon, T. J. Am. Chem. Soc. 1996, 118, 11960. (c) Ingert, D.; Pileni, M. P. J. Phys. Chem. B 1999, 103, 4. (d) Gibson, C. P.; Putzer, K. J. Science 1995, 267, 1338. (4) Fafard, S.; Hinzer, K.; Raymond, S.; Dion, M.; McCaffrey, J.; Feng, Y.; Charbonneau, S. Science 1996, 274, 1350.
electron transistors,5 and light-emitting diodes (LEDs).6 For most of these applications, the lateral dimensions of the nanocrystallite may represent the ultimate limit to size in miniaturizing currently existing functional devices. A variety of methods have already been demonstrated for generating nanocrystallites with well-defined properties from an extremely broad range of materials.1-3 Nanocrystallites of semiconductors, for example, have been successfully prepared using approaches such as “arrested precipitation” in the cavities of inverse micelles, vesicles, or zeolites; homogeneous nucleation and crystallization in gaseous, liquid, sol-gel, or polymeric matrices; and template-directed synthesis in biological macromolecules or components.7 Most of these methods are very effective in producing stable dispersions of well-controlled nanocrystallites in solvents or matrix materials. Although these dispersions are useful in a lot of applications (such as in imaging systems or drug formulation, and as paints, coatings, or abrasives), their existence as colloidal dispersions poses a challenge for the fabrication of nanoscale devices. In most cases, the integration of nanocrystallites into a functional device requires the availability of a highly ordered 2D array of nanocrystallites supported on the surface of an appropriate solid substrate. In the past, such a highly ordered array of nanocrystallites was most (5) Klein, D. L.; Roth, R.; Lim, A. K. L.; Alivisatos, A. P.; McEuen, P. L. Nature 1997, 389, 699. (6) Empedocles, S. A.; Norris, D. J.; Bawendi, M. G. Phys. Rev. Lett. 1996, 77, 3873. (7) See, for example: Hadjipanayis, G. C., Siegel, R. W., Eds. Nanophase Materials: Synthesis, Properties, Applications; Kluwer Academic Publishers: Norwell, MA, 1994.
10.1021/la001211k CCC: $19.00 © 2000 American Chemical Society Published on Web 12/02/2000
10370
Langmuir, Vol. 16, No. 26, 2000
Zhong et al.
Scheme 1
commonly fabricated on the surface of a solid support using e-beam lithography, in combination with a selective deposition or etching process.8 More recently, several in situ deposition approaches have also been explored for fabricating regular arrays of this type: notable examples include the techniques based on molecular beam epitaxy (MBE),9 chemical vapor deposition (CVD),10 precipitation in the Langmuir-Blodgett (LB) films,11 and pulsed electrochemical deposition.12 Although these methods are flexible and capable of producing supported regular arrays of a rich variety of nanocrystallites (and some of them have also been used to fabricate functional devices), their accuracy in controlling the size distribution and spatial arrangement still needs to be demonstrated or greatly improved. Here we describe a simple and convenient method that allowed for the production of highly ordered 2D arrays of magnetic nanoparticlessmetals such as Co, Ni, and R-Fe that are ferromagnetic, or ferrites such as MFe2O4 (M ) Mg, Ni) that are ferrimagneticson the surfaces of Si/SiO2 substrates rapidly and in a controllable fashion (Scheme 1 and Figure 1). The key component of this method was a solid substrate whose surface had been patterned as an array of hydrophilic grids separated by hydrophobic selfassembled monolayers (SAMs). A similar approach has recently been demonstrated by Whitesides et al. for gold and silver substrates, with the crystallization of CuSO4 and CdS particles as two examples.13 Previous methods for fabricating such an ordered array involved the use of scanning probe nanolithography14 and e-beam writing,15 both of which are sequential processes that generate each patterned 2D array of particles in a serial fashion. Owing to the lengthy time required for patterning each surface, it seems to be highly impractical to use these serial techniques for mass production.16 We believe that the procedure described here should be able to provide an alternative (and probably cost-effective) (8) Smith, H. I.; Craighead, H. G. Phys. Today 1990, February, 57. (9) Notel, R.; Niu, Z.; Ramsteiner, M.; Schonherr, H.-P.; Tranpert, A.; Daweritz, L.; Ploog, K. H. Nature 1998, 392, 56. (10) Liu, D. C.; Lee, C. P. Appl. Phys. Lett. 1993, 63, 3503. (11) (a) Zhao, X. K.; Fendler, J. H. J. Phys. Chem. 1991, 95, 3716. (b) Iakovenko, S. A.; Trifonov, A. S.; Giesig, M.; Mamedov, A.; Nagesha, D. K.; Hanin, V. V.; Soldatov, E. C.; Kotov, N. A. Adv. Mater. 1999, 11, 388. (12) Hsiao, G. S.; Anderson, M. G.; Gorer, S.; Harris, D.; Penner, R. M. J. Am. Chem. Soc. 1997, 119, 1439. (13) Qin, D.; Xia, Y.; Xu, B.; Yang, H.; Zhu, C.; Whitesides, G. M. Adv. Mater. 1999, 11, 1433. (14) (a) Kent, A. D.; Shaw, T. M.; von Molnar, S.; Awschalom, D. D. Science 1993, 262, 1249. (b) Shi, J.; Gider, S.; Babcock, K.; (b) Awschalom, D. D. Science 1996, 271, 937. (c) Kolb, D. M.; Will, R. U. Science 1997, 275, 1097. (15) (a) Lee, K. L.; Hatzakis, M. J. Vac. Sci. Technol. 1989, B7, 1941. (b) Heath, J. R.; Williams, R. S.; Shiang, J. J.; Wind, S. J.; Chu, J.; D’Emic, C.; Chen. W.; Stanis, C. L.; Bucchiganano, J. J. J. Phys. Chem. 1996, 100, 3144. (16) Xia, Y.; Rogers, J. A.; Paul, K.; Whitesides, G. M. Chem. Rev. 1999, 99, 1823.
Figure 1. Schematic outline of the procedure that was used to fabricate ordered 2D arrays of magnetic nanoparticles of metals supported on Si/SiO2 substrates. Note that the thickness of the siloxane monolayer has been exaggerated in this drawing; the actual value should be in the range of 2-3 nm.
approach to the large-scale production of ordered 2D arrays of nanoparticles (made of a rich variety of inorganic or organic materials). The ordered arrays of magnetic nanoparticles fabricated using this method are immediately useful in a number of technologically important areassfor example, as magnetic recording media for ultrahigh-density information storage17 or as superparamagnetic materials for use in biotechnology.18 Recent demonstrations by several groups have also suggested that 2D arrays of these metal nanoparticles could be employed as catalysts and templates to synthesize carbon nanotubes as patterned arrays through processes such as chemical vapor deposition.19 These arrays of nanoparticles can also serve as 2D periodic structures to diffract light, or as masks in reactive ion etching (RIE) or metastable atom lithography to transfer patterned nanostructures into the underlying substrates. Experimental Section Chemicals, Materials, and Substrates. Octadecyltrichlorosilane (OTS), hexadecanethiol (HDT), toluene, ethanol, and hydrated nitrate salts such as Co(NO3)2‚6H2O, Ni(NO3)2‚6H2O, and Fe(NO3)2‚9H2O were all obtained from Aldrich. Poly(dimethylsiloxane) (PDMS) elastomer kits (Sylgard 184) were purchased from Dow Corning (Midland, MI). Polished Si(100) wafers (Cz, phosphorus-doped, ∼10 Ω cm, test grade, and SEMI standard flatness) were supplied by Silicon Sense (Nashua, NH). Thin films of gold or silver (∼50 nm thick) were prepared using thermal evaporation (Varian 3118) onto the silicon wafers whose (17) A recent study: Sun, S.; Murray, C. B.; Weller, D.; Folks, L.; Moser, A. Science 2000, 287, 1989. (18) Whitesides, G. M.; Kazlauskas, R.; Josephon, L. TIBTECH 1983, 144.
Fabrication of 2D Arrays of Magnetic Particles surfaces had been primed with thin layers (2-3 nm thick) of titanium or chromium. Microcontact Printing (µCP). The elastomeric stamps were fabricated according to the published procedure.20A solution of HDT in ethanol (∼5 mM) was used as the “ink” for printing SAMs of HDT on the surfaces of gold or silver thin films. After being applied with the ink solution (by a cotton Q-tip), the stamp was dried in a stream of nitrogen gas for a few minutes and finally brought into contact with the surface of a gold (or silver) substrate for 5-10 s. The PDMS stamp was then separated carefully from the surface of the gold film. A PDMS stamp whose surface had been patterned with an array of parallel lines was used to generate the regular 2D array of square grids by printing twice on the same substrate, with the orientation of the lines rotated by ∼90° between the two impressions. For µCP on the surfaces of silicon substrates, a freshly prepared 0.2% solution of OTS in toluene was used as the ink.21 Although the as-received silicon wafers are covered with thin layers of native oxide, their surfaces are usually hydrophobic owing to contamination by organic chemicals. These surfaces became hydrophilic again after they had been treated with a piranha solutionsa 7:3 (v/v) mixture of 98% H2SO4 and 30% H2O2sat 80-90 °C for 30-40 min. After cleaning, these silicon wafers were thoroughly rinsed with deionized water and used immediately. CAUTION: the piranha solution is an extremely strong oxidant and should be handled very carefully! Formation of 2D Arrays of Nitrate Nanoparticles. Figure 1 and Scheme 1 outline the basic procedure that we have used to generate an ordered 2D array of nitrate nanoparticles on the surface of a silicon substrate. In this procedure, the hydrophilic, hydroxyl-terminated surface of a Si/SiO2 wafer was first patterned with a 2D array of square grids of hydrophobic featuressthat is, a siloxane SAM terminated in the -CH3 group. When this SAMpatterned wafer was withdrawn (with a rate of approximately 2 mm/s) from an 2-propanol solution that contained a nitrate salt, an ordered array of uniform, hemispherical droplets was preferentially deposited on the hydrophilic, bare regions because only these regions could be wetted by the solution.22 The volume of solution in each droplet retained on the surface was determined by the feature size or shape of the test pattern, as well as the contact angle of the solution. The patterned monolayer thus provides a very convenient way of dispensing the solution into a highly ordered 2D array of liquid droplets (or microreactors) that can be easily controlled in the range of ∼100 aL (corresponding to a 4.0 µm2 grid) and ∼1 nL (corresponding to a 1600 µm2 grid). After the solvent had completely evaporated, an ordered 2D array of nitrate nanoparticles was left behind on the surface of this silicon substrate. The lateral position of the nanoparticle within each hydrophilic region could be controlled by changing the orientation (with respect to the gravitational field) along which the surface was withdrawn from the solution and held while letting the solvent evaporate. It also seemed to be possible to use some other influence (e.g., flowing of gas) to position the evaporating droplet within each hydrophilic region. The sizes of the particles could be varied (in a controllable fashion) over a broad range by changing the feature size of the test pattern (or other parameters that may influence the volume of liquid droplets trapped on the hydrophilic grids) and the concentration of the solution. The success of this process relies on the selective dewetting of the solution from those hydrophobic regions and formation of isolated structures as a patterned 2D array on the hydrophilic regions. The interfacial energy (or contact angle) of the solvent seemed to play an important role in determining the uniformity of the resulting 2D arrays of liquid droplets. We tried a number of common solvents, including water (γ ) 73.8 dyn/cm), ethanol (19) See, for example: Kong, J.; Soh, H,. T.; Cassell, A. M.; Quate, G. F.; Dai, H. Nature 1998, 395, 878. (20) See, for example: (a) Kumar, A.; Biebuyck, H.; Whitesides, G. M. Langmuir 1994, 8, 2672. (b) Xia, Y.; Whitesides, G. M. Annu. Rev. Mater. Sci. 1998, 28, 153. (c) Xia, Y.; Whitesides, G. M. Angew. Chem., Int. Ed. Engl.1998, 37, 551. (21) See, for example: (a) Xia, Y.; Mrksich, M.; Kim, E.; Whitesides G. M. J. Am. Chem. Soc. 1995, 117, 9576. (b) Jeon, N. L.; Clem, P. G.; Payne, D. A.; Nuzzo, R. G. Langmuir 1996, 12, 5350. (22) Biebuyck, H. A.; Whitesides, G. M. Langmuir 1994, 10, 2790.
Langmuir, Vol. 16, No. 26, 2000 10371 (24.0 dyn/cm), and 2-propanol (21.7 dyn/cm). We found that 2-propanol worked best for the present system shydrophilic Si/ SiO2 wafers whose surfaces had been patterned with hydrophobic siloxane monolayers of OTS. The use of an aqueous solution of nitrate salt often led to the formation of irregular or inhomogeneous patterns because of the high surface energy of water and the relatively poor quality of the siloxane monolayer. Formation of 2D Arrays of Magnetic Nanoparticles. The silicon substrates that had been patterned with regular 2D arrays of nitrate particles were heated in air at 600 °C for 3 h and then treated at 400 °C for 2 h in a flow of hydrogen gas to obtain magnetic nanoparticles of metals shown in Scheme 1. We chose nitrates as the starting materials because they have relatively low decomposition temperatures, as well as high solubilities in a number of polar solvents. Other inorganic substances such as oxalates and citrates can also be used as starting materials. To determine the phase and composition of the final product, the corresponding nitrate salt was also treated under the same conditions and subsequently characterized using X-ray diffraction (XRD). Figure 2 shows XRD patterns of the samples obtained from cobalt, nickel, and iron nitrate, respectively. By comparing with the ASTM cards, we could conclude that the reactions followed the paths outlined in Scheme 1. All the XRD peaks of each sample could be assigned to a single, pure phase as indicated on each spectrum. Instrumentation. The X-ray diffraction measurement was carried out on a Philips PW1710 diffractometer (Cu KR). The patterned arrays of nanoparticles were characterized using a Leica DMLM optical microscope, a JEOL field-emission scanning electron microscope (6300F, Peabody, MA), a scanning probe microscope (including AFM and MFM) by Digital Instruments (Nanoscope III, Santa Barbara, CA), and an AFM by Thermomicroscope (Explorer, Mountain View, CA). In all SEM measurements, the accelerating voltage was 5 kV, and the samples were not coated with any additional conductive layers.
Results and Discussion Silicon versus Gold (or Silver) Substrate. Our initial effort was concentrated on gold and silver substrates (polycrystalline thin films evaporated on silicon wafers) whose surfaces had been patterned with the SAMs of alkanethiols by µCP.20 We selected these two systems because they had been shown to work best for selective wetting or dewetting studies owing to the high quality associated with these monolayers. These surfaces were previously explored by Whitesides et al. to fabricate regular arrays of magnetic microstructures by using selective wetting (or dewetting) and deposition (at room temperature) with a colloidal solution that contained magnetite nanoparticles.23 These surfaces were also recently used as templates to generate patterned 2D arrays of nanoparticles (CuSO4 and CdS) by selective dewetting and crystallization at room temperature.13 These two metal surfaces were expected to be able to sustain the moderately high temperatures (
