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Magnetic Nanoparticle Assembly Arrays Prepared by Hierarchical Self-Assembly on Patterned Surface Tianlong Wena*, Dainan Zhanga,b, Qiye Wena, Huaiwu Zhanga†, Yulong Liaoa, Qiang Lia, Qinghui Yanga, Feiming Baia , Zhiyong Zhonga
DOI: 10.1039/x0xx00000x www.rsc.org/
Inverted pyramid hole arrays were fabricated by photolithography and used as templates to direct the growth of colloidal nanoparticle assemblies. Cobalt ferrite nanoparticles deposit in the holes to yield high quality pyramid magnetic nanoparticle assembly arrays by carefully controlling the evaporation of carrier fluid. Magnetic measurements indicate that the pyramid magnetic nanoparticle assembly arrays preferentially magnetize perpendicular to the substrate. Chemically synthesized colloidal nanoparticles with size uniformity and novel properties1-5 are ideal building blocks to make nanoparticle based artificial materials, where they mimic the atomic crystals by building solid matters with colloidal nanoparticles instead of atoms.6 These nanoparticle based artificial materials are promising for a broad spectrum of device applications such as flexible flash memories,7 displays,8 and field effect transistors9 and many other applications in such as lithography10 and data storage.11 One of the technical hurdles deterring device fabrication is the difficulties to organize colloidal nanoparticles at designated locations in large area and high quality.12 It is shown that large area monolayer of colloidal nanoparticles with long range order can be obtained by carefully controlling the evaporation direction of carrier fluid and the mass flow direction of nanoparticles during selfassembly.13 Besides 2 D monolayers, it is also required to organize colloidal nanoparticles into complex hierarchical structures such as nanoparticle assembly arrays (NAAs) where assemblies of nanoparticles are spatially organized in 2 D patterns. Small nanoparticle clusters in arrays have been fabricated by guiding the deposition of nanoparticles during evaporation on pre-patterned substrate. 14-17Random colloidal nanoparticle assemblies on substrate have been prepared by several methods, showing novel magnetic properties.18, 19 However, ordered nanoparticle assembly arrays are often needed for device fabrication and to have tunable properties. To fabricate NAAs where each assembly of nanoparticles is composed of a tremendous amount of nanoparticles (~ millions), evaporation of nanoparticle solution should be cautiously controlled to allow them to reach their equilibrium positions during deposition.20 Here we use ~ 8 nm cobalt ferrite (CoFe2O4)
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nanoparticles as the fundamental building blocks to make magnetic NAAs (MNAAs) by guiding the deposition of nanoparticles in the pyramid pits.20 Unlike the NAAs prepared in reference 20 for Raman scattering enhancement, we removed the colloidal nanoparticles bridging nanoparticle assemblies, which is critical to decouple the magnetic interactions between nanoparticle assemblies. Compared with the self-assembly of gold nanoparticles in reference 20, the hierarchical self-assembly of cobalt ferrite nanoparticle is much difficult due to (1) the greatly reduced inter-particle interactions, (2) largely enhanced Brownian motion and (3) significantly increased number of nanoparticles in each assembly unit. As a result, self-assembly of cobalt ferrite nanoparticles by simply reducing evaporation rate of carrier fluid does not yield large area uniform NAAs. In this work, we explored to design a apparatus that fabricate high quality NAAs by controlling both evaporation rate and directions of a stable nanoparticle solution. Colloidal nanoparticle cores are coated with surfactants to prevent agglomeration and make them dispersible in organic solvents such as toluene and hexane. Self-assembly of nanoparticles often occurs at the drying front when evaporating the organic carrier solvents on a surface.21 Lithographically fabricated features on surfaces can significantly affect the movement of the drying front and thus be used to direct the self-assembly of colloidal nanoparticles.16, 22, 23 Here photo-lithography or e-beam lithography is used to pre-pattern the surface and generate the first layer of the hierarchical structures. Typically, when drying a droplet of colloidal nanoparticle solution on a lithographically patterned hole arrays, the moving contact lines of the nanoparticle solution are locally pinned at holes, and the meniscus due to the bending of the moving contact line will drag nanoparticles towards and then preferentially fill these holes22. Depending on the size and shape of the filling nanoparticles and the pre-patterned holes, organization of the nanoparticle clusters can be precisely controlled in the holes. 16, 23 If the size of nanoparticles is comparable with the holes, nanoparticle arrays are generated on the surface. If the size of nanoparticles is smaller than half of the holes, multiple nanoparticles can be contained in each hole to yield nanoparticle cluster arrays on the surface.23 Here we show that when the dimension of the pre-patterned holes are much greater than the diameter of the filling colloidal nanoparticles, a large number of nanoparticles (~millions) can be deposited in these ‘big’ holes by
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controlling the evaporation of carrier fluid to form NAAs. Further, the shape and size of these assemblies are determined by the prepatterned holes. These NAAs allow us to study the collective behaviours of colloidal nanoparticle assemblies with determined shape and size. Unlike the NAAs that are prepared without template and have random shape, size and directions24-26, the NAAs prepared in this work have approximately the same size, shape and aligned directions, which allow us to study the anisotropy of individual assembly by measuring the total response of the array. This is critically important because the physical response of an individual nanoparticle assembly of this dimension is often too weak to measure by a conventional instrument. And the measurement of an ensemble of nanoparticle assemblies with random orientations will eliminate the directionality associated properties. Inverted pyramid hole arrays (IPHAs) were used as the templates to direct the growth of NAAs. The base width and height of the pyramid are ~ 3 µm and ~2 µm respectively, which is much larger than the diameter of colloidal nanoparticles (~ 8 nm). The pitch is ~ 5 µm, and edge distance is ~2 µm. The scanning electron microscopy (SEM) image of an IPHA is shown in large area in Figure 1(a) and in high magnification in Figure 1 (b) respectively. In each pyramid, several millions of colloidal nanoparticles can be contained in contrast to the small number of nanoparticles for the nanoparticle cluster arrays. Large-area IPHAs were fabricated on thermally oxidized (100) silicon substrate by photolithography, followed by
Journal Name DOI: 10.1039/C4NR07489K
Figure 1 SEM images of an IPHA (a) in large area and (b) in high magnification. The inset in (a) is a picture of the patterned area on a silicon substrate. Cobalt ferrite nanoparticles were synthesized by simultaneous thermal decomposition of iron (III) and cobalt (II) acetylacetonate at high temperature in organic solvent (see supplemental information for synthesis details).1 Oleic acid and oleylamine were used as surfactants to coat the nanoparticle cores during synthesis, which prevent nanoparticle from agglomeration. Figure 1S (see supplemental information) shows a transmission electron microscopy (TEM) image of cobalt ferrite nanoparticles. A spacing of ~ 2-3 nm between nanoparticle cores is induced by the steric
Figure 2. Procedure for making MNAA on an IPHA. Step 1: A drop of cobalt ferrite nanoparticles was put on top of an IPHA on silicon substrate that was placed in a petri-dish; step 2: a fluorinated polyether plate was put on top of the nanoparticle solution, and the petri-dish was covered by a glass slides to allow carrier fluid to slowly evaporate; step 3: after carrier fluid was completely evaporated, the polymer plate was removed from the IPHA; and step 4: the excess nanoparticles on silicon wall was polished away to make MNAA, and one of the nanoparticle crystal in inverted pyramid hole was magnified. fluorinated etching, wet chemical anisotropy etching27 and removal of silicon oxide hard mask by buffered oxide etch. The fabricated IPHAs were finally cleaned by D.I. water in ultrasonic bath. The fabrication process is given in supplemental information. A picture of an IPHA on silicon substrate is shown in the inset of Figure 1(a), which can act as grating to divide the white light into a full spectrum of colours.
2 | J. Name., 2012, 00, 1-3
repulsion due to surfactant coating. As shown in Figure 1S, uncontrolled evaporation of nanoparticle solution on a flat surface yields a mixture of random monolayer, multilayer and gaps, which is in contrast to the uniform nanoparticle assembly over large area obtained by controlled evaporation.13
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Figure 3 (a) An SEM image of cobalt ferrite nanoparticle filled IPHA, and (b) the element concentration profile along the dotted green line in (a), which is obtained by EDX line scan. (c), (d) and (e) show the SEM image, iron mapping and cobalt mapping in a typical area of the cobalt ferrite filled IPHA.
Lithographically patterned hole arrays can direct nanoparticles to preferentially grow in the holes. However, uncontrolled or improperly controlled evaporation on the IPHAs would result in unevenly filling of the hole arrays and inhomogeneous nanoparticle growth. As shown in Figure 2S (a) in supplemental information, natural evaporation of colloidal solution would not give nanoparticles sufficient time to grow bigger during deposition. In this dynamically controlled growth, small nanoparticle assemblies with grain size < 1 µm form both in holes and on the wall, and each hole is filled with several nanoparticle assemblies with voids between them. The small grain size induced by rapid evaporation is similar to the atomic crystal fractionalization during rapid cooling,28 where fast crystallization does not give enough time for atoms to diffuse. To obtain large nanoparticle assemblies, both evaporation rate and direction should be properly controlled. Figure 2S (b) in supplemental information shows nanoparticle assemblies in SEM formed by slowly (~ 5 µm/min) pulling the IPHAs out from colloidal solution with a dip coater. The assemblies appear darker than the surroundings in the SEM image. The growth of nanoparticle assemblies is regulated by the shape of the inverted pyramid holes. In contrast to fast evaporation, only one large nanoparticle assemblies is contained in each hole for slow pulling, and
nanoparticles are mainly deposited in the holes rather than on the silicon wall. As shown in Figure 3S (see supplemental information), nanoparticles mainly diffused from solution along the edge of the inverted pyramid holes during growth. The growth plane is flat and perpendicular to the substrates. By slow pulling, moving speed of contact line is small, allowing nanoparticles to diffuse from solution to the hole and find their equilibrium position during deposition. However, by slow pulling, nanoparticles are not actively driven from the solution to the drying front. Complete filling of holes are not achieved even though very small moving speed is used and a day is taken to make a half centimetre square’s sample. Learning from the condensation of atoms, evaporation of nanoparticle solution should be controlled such that colloidal nanoparticles have sufficient time to diffuse to the holes and locate their equilibrium positions during growth. Further, the evaporation direction should also be controlled so that nanoparticles in solution will be continuously driven toward the drying front to feed the growth of nanoparticle assemblies.13 To satisfy above conditions, we modified the method for microsphere cluster growth on patterned surface that is used in reference 23 to fabricate NAAs on IPHAs. The procedure of our method is schematically depicted in Figure 2. Firstly, a droplet of nanoparticle solution is spread on the top of an
Figure 4. The HRSEM images of the cobalt ferrite nanoparticle filled IPHA. The area enclosed by the red and green squares in (b) are magnified in (a) and (c) respectively.
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IPHA by a micro-syringe, which is placed in a petri-dish. A fluorinated polyether plate with very smooth surface and low surface energy is then placed on top of the nanoparticle solution. The petridish is then partially covered by a glass slides to reduce the evaporation rate. Nanoparticle solution is trapped between the IPHA substrate and the polymer plate, only allowing evaporation to occur in the lateral directions. The evaporation is slow and steady due to the small gap between the polymer plate and the substrate as well as the partial covering of the petri-dish, giving sufficient time for nanoparticle movement in solution and growth. Evaporation through the small gap between the polymer plate and the substrate will continuously drive nanoparticles in solution to move in the lateral direction towards the drying front, which is locally pinned at the holes. At the inverted pyramid holes, nanoparticles are pulled by capillary force along the edge and side of the pyramid, and deposited at a steady rate to form MNAAs on IPHAs. After carrier fluid is completely evaporated, sporadic nanoparticle clusters on silicon wall were finally removed by polish.
Journal Name DOI: 10.1039/C4NR07489K nanoparticle assemblies would occur due to the ~2-3 µm separation. Cobalt ferrite nanoparticles of ~8 nm are single domain.29 Each one behaves like a small magnet, whose magnetization directions can be coherently switched by thermal agitation. The switching probability is determined by the volume of the nanoparticle and their magnetic anisotropy.30 The dipolar interactions between magnetic nanoparticles also affect the switch of their magnetization directions, making them behave collectively in the nanoparticle crystal.31 As a result, the properties of magnetic nanoparticle crystals are determined by the composing nanoparticles and the interactions between them. Here we also introduced a shape asymmetry of the magnetic nanoparticle assemblies in the out-of-plane direction, which might also affect the magnetic properties of the magnetic nanoparticle assemblies.
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SEM image in Figure 3(a) shows the top view of a MNAA prepared by above method. Compared with natural evaporation and dip coating, the controllability of nanoparticle growth is greatly improved. MNAAs obtained by this method have greatly improved the uniformity over large area. Each inverted pyramid hole is almost fully filled by a pyramid nanoparticle assembly in contrast to the many small nanoparticle assemblies in a hole prepared by rapid evaporation. The MNAA is uniform across the whole patterned areas with determined shapes and aligned orientations. Energy dispersive x-ray spectroscopy (EDX) was performed along the green line in Figure 3(a). The element concentration profiles along the green line are shown in Figure 3(b). Here the elements in nanoparticle cores (cobalt, iron and oxygen) and surfactant (carbon and oxygen) are analysed. The periodic variation of concentration along the green line is corresponding to the periodic structure of the MNAAs. Cobalt, iron, oxygen and carbon were observed at the location of nanoparticle crystals, proving successful deposition of nanoparticles in the holes rather than on the walls. EDX was also performed in an area enclosing a 3×4 matrix of MNAA, whose SEM image is shown in Figure 3(c). The iron and cobalt mapping in this area is shown in Figure 3(d) and (e) respectively, which is consistent with the SEM observations. High resolution SEM (HRSEM) was performed on a small area, as shown in Figure 4. Figure 4(a) shows that cobalt ferrite nanoparticles are only deposited in the pyramid holes, and almost no nanoparticle is observed on the silicon wall. To see more details of the MNAA, the area enclosed by red and green squares in Figure 4 (a) is further magnified in Figure 4(b) and 4 (c) respectively. Individual nanoparticles on the surface of the nanoparticle crystal can be resolved in Figure 4 (b) and (c). The nanoparticle assemblies are very dense, and the surface is very smooth due to controlled and slow evaporation of carrier fluid. Some vacancies are discernible on the surface as well. The boundary between the nanoparticle crystal and silicon wall is shown in Figure 4(c). No excessive nanoparticles are observed on the silicon walls, proving successful removal of nanoparticles on silicon wall by polish.
Figure 5. (a) out-of-plane and (b) in-plane M-H curves of cobalt ferrite MNAAs. (c) compares the in-plane and outof-plane M-H curves of cobalt ferrite nanoparticles deposited on a flat silicon substrate.
We have shown that high quality and large area pyramid MNAAs can be successfully fabricated by controlling evaporation of colloidal nanoparticle solution on the IPHAs. These magnetic nanoparticle assemblies in MNAAs have approximately the same size, shape and orientations, allowing us to detect the approximate magnetic properties of individual magnetic nanoparticle assemblies by measuring the whole MNAAs. Here no interaction between
Figure 5 (a) and (b) shows the magnetic hysteresis loops of the MNAA measured by applying the external magnetic field perpendicular and parallel to the substrate respectively. A small coercivity of ~ 200 Oe is observed for both measurements. It is clear
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Journal Name that MNAA can be easily magnetized along the out-of-plane direction, and it is difficult to be magnetized along the in-plane direction. This observation is opposite to the 2 D magnetic nanoparticle thin films, where they behave like a continuous ferromagnetic thin film and can be easily magnetized along the inplane direction to minimize electrostatic energy.32 A drop of cobalt ferrite nanoparticle solution was evaporated on the smooth surface of a silicon substrate to make a reference sample, where the crystals are not patterned by lithography. The magnetic properties were then measured under the same condition by a vibrating sample magnetometry (VSM), as shown in Figure 5(c). The in-plane and out-of-plane hysteresis loop of the unpatterned cobalt ferrite magnetic nanoparticle crystals are almost superimposed on each other, as shown in Figure 5(c). No preferential magnetization direction was observed for this unpatterned sample. Here we attribute the magnetic anisotropy of the pyramid magnetic assemblies to their asymmetric shape along the out-of-plane directions, which within our knowledge have not been observed in previous studies. Further work needs to be done to clearly understand the magnetization mechanism of these magnetic nanoparticle assemblies and the shape induced anisotropy.
Conclusions We have successfully fabricated large-area MNAAs by controlling the evaporation rate and direction of carrier fluid on IPHAs. Magnetic measurement indicates that these magnetic nanoparticle assemblies with pyramid shape can be easily magnetized along the axis of symmetry. Our study shows that the size, shape and orientation of nanoparticle assemblies can be engineered by the prepatterned surface. Due to the aligned orientation of nanoparticle assemblies, it is possible to detect the anisotropic physical properties of individual nanoparticle assembly. Further work is being carried out to improve nanoparticle crystal growth and to understand the magnetization mechanism of MNAAs.
Acknowledgement This work is financially supported by National Natural Science Foundation of China (No. 51401046, No. 61131005), Keygrant Project of Chinese Ministry of Education (No. 313013), New Century Excellent Talent Foundation (No. NCET-11-0068), and start-up research fund from the University of Electronic Science and Technology of China.
Notes and references a
State Key Laboratory of Electronic Thin Films and Integrated Devices, University of Electronic Science and Technology of China, Chengdu 610054, China b Department of Electrical and Computer Engineering, University of Delaware, Newark, Delaware 19716, USA *E-mail: [email protected] †E-mail: [email protected] Electronic Supplementary Information (ESI) available: Experimental methods and characterizations, a TEM image of cobalt ferrite nanoparticles, SEM images of cobalt ferrite nanoparticle crystals on IPHA formed by fast evaporation and slow pulling out from nanoparticle solution by a dip coater, a SEM image of a partially filled inverted pyramid hole. See DOI: 10.1039/c000000x/
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1. S. H. Sun, H. Zeng, D. B. Robinson, S. Raoux, P. M. Rice, S. X. Wang and G. X. Li, J Am Chem Soc, 2004, 126, 273-279. 2. J. Park, K. J. An, Y. S. Hwang, J. G. Park, H. J. Noh, J. Y. Kim, J. H. Park, N. M. Hwang and T. Hyeon, Nat Mater, 2004, 3, 891-895. 3. T. L. Wen, L. N. Brush and K. M. Krishnan, J Colloid Interf Sci, 2014, 419, 79-85. 4. V. F. Puntes, K. M. Krishnan and A. P. Alivisatos, Science, 2001, 291, 2115-2117. 5. W. W. Yu, J. C. Falkner, C. T. Yavuz and V. L. Colvin, Chem Commun, 2004, 2306-2307. 6. E. V. Shevchenko, D. V. Talapin, N. A. Kotov, S. O'Brien and C. B. Murray, Nature, 2006, 439, 55-59. 7. S. T. Han, Y. Zhou, Z. X. Xu, L. B. Huang, X. B. Yang and V. A. L. Roy, Adv Mater, 2012, 24, 3556-3561. 8. T. H. Kim, K. S. Cho, E. K. Lee, S. J. Lee, J. Chae, J. W. Kim, D. H. Kim, J. Y. Kwon, G. Amaratunga, S. Y. Lee, B. L. Choi, Y. Kuk, J. M. Kim and K. Kim, Nat Photonics, 2011, 5, 176-182. 9. D. V. Talapin and C. B. Murray, Science, 2005, 310, 86-89. 10. T. L. Wen, R. A. Booth and S. A. Majetich, Nano Lett, 2012, 12, 5873-5878. 11. L. H. Wu, P. O. Jubert, D. Berman, W. Imaino, A. Nelson, H. Y. Zhu, S. Zhang and S. H. Sun, Nano Lett, 2014, 14, 3395-3399. 12. S. A. Majetich, T. Wen and R. A. Booth, Acs Nano, 2011, 5, 60816084. 13. T. L. Wen and S. A. Majetich, Acs Nano, 2011, 5, 8868-8876. 14. M. S. Onses, C. C. Liu, C. J. Thode and P. F. Nealey, Langmuir, 2012, 28, 7299-7307. 15. Q. Dai, J. Frommer, D. Berman, K. Virwani, B. Davis, J. Y. Cheng and A. Nelson, Langmuir, 2013, 29, 7472-7477. 16. J. Henzie, S. C. Andrews, X. Y. Ling, Z. Y. Li and P. D. Yang, P Natl Acad Sci USA, 2013, 110, 6640-6645. 17. S. Palacin, P. C. Hidber, J. P. Bourgoin, C. Miramond, C. Fermon and G. M. Whitesides, Chem Mater, 1996, 8, 1316-1325. 18. T. Ozdemir, D. Sandal, M. Culha, A. Sanyal, N. Z. Atay and S. Bucak, Nanotechnology, 2010, 21. 19. D. Lisjak, P. Jenus and A. Mertelj, Langmuir, 2014, 30, 6588-6595. 20. M. Alba, N. Pazos-Perez, B. Vaz, P. Formentin, M. Tebbe, M. A. Correa-Duarte, P. Granero, J. Ferre-Borrull, R. Alvarez, J. Pallares, A. Fery, A. R. de Lera, L. F. Marsal and R. A. Alvarez-Puebla, Angew Chem Int Edit, 2013, 52, 6459-6463. 21. X. M. Lin, H. M. Jaeger, C. M. Sorensen and K. J. Klabunde, J Phys Chem B, 2001, 105, 3353-3357. 22. J. A. Liddle, Y. Cui and P. Alivisatos, J Vac Sci Technol B, 2004, 22, 3409-3414. 23. Y. D. Yin, Y. Lu, B. Gates and Y. N. Xia, J Am Chem Soc, 2001, 123, 8718-8729. 24. O. Kasyutich, R. D. Desautels, B. W. Southern and J. van Lierop, Phys Rev Lett, 2010, 104. 25. B. Kowalczyk, K. J. M. Bishop, I. Lagzi, D. W. Wang, Y. H. Wei, S. B. Han and B. A. Grzybowski, Nat Mater, 2012, 11, 227-232. 26. A. M. Kalsin, M. Fialkowski, M. Paszewski, S. K. Smoukov, K. J. M. Bishop and B. A. Grzybowski, Science, 2006, 312, 420-424. 27. M. J. Madou, Fundamentals of microfabrication : the science of miniaturization, 2nd edn., CRC Press, Boca Raton, 2002.
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28. W. D. Callister and D. G. Rethwisch, Materials science and engineering : an introduction, 8th edn., John Wiley & Sons, Hoboken, NJ, 2010. 29. S. A. Majetich, T. Wen and O. T. Mefford, Mrs Bull, 2013, 38, 899903. 30. T. L. Wen and K. M. Krishnan, J Phys D Appl Phys, 2011, 44. 31. S. A. Majetich and M. Sachan, J Phys D Appl Phys, 2006, 39, R407R422. 32. V. F. Puntes, P. Gorostiza, D. M. Aruguete, N. G. Bastus and A. P. Alivisatos, Nat Mater, 2004, 3, 263-268.
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Nanoscale Accepted Manuscript
This article can be cited before page numbers have been issued, to do this please use: T. Wen, D. Zhang, Q. Wen, H. Zhang, Y. Liao, Q. Li, Q. Yang, F. Bai and Z. Zhong, Nanoscale, 2015, DOI:
This is an Accepted Manuscript, which has been through the Royal Society of Chemistry peer review process and has been accepted for publication. Accepted Manuscripts are published online shortly after acceptance, before technical editing, formatting and proof reading. Using this free service, authors can make their results available to the community, in citable form, before we publish the edited article. We will replace this Accepted Manuscript with the edited and formatted Advance Article as soon as it is available. You can find more information about Accepted Manuscripts in the Information for Authors. Please note that technical editing may introduce minor changes to the text and/or graphics, which may alter content. The journal’s standard Terms & Conditions and the Ethical guidelines still apply. In no event shall the Royal Society of Chemistry be held responsible for any errors or omissions in this Accepted Manuscript or any consequences arising from the use of any information it contains.
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Cite this: DOI: 10.1039/x0xx00000x
Received 00th January 2012, Accepted 00th January 2012
Magnetic Nanoparticle Assembly Arrays Prepared by Hierarchical Self-Assembly on Patterned Surface Tianlong Wena*, Dainan Zhanga,b, Qiye Wena, Huaiwu Zhanga†, Yulong Liaoa, Qiang Lia, Qinghui Yanga, Feiming Baia , Zhiyong Zhonga
DOI: 10.1039/x0xx00000x www.rsc.org/
Inverted pyramid hole arrays were fabricated by photolithography and used as templates to direct the growth of colloidal nanoparticle assemblies. Cobalt ferrite nanoparticles deposit in the holes to yield high quality pyramid magnetic nanoparticle assembly arrays by carefully controlling the evaporation of carrier fluid. Magnetic measurements indicate that the pyramid magnetic nanoparticle assembly arrays preferentially magnetize perpendicular to the substrate. Chemically synthesized colloidal nanoparticles with size uniformity and novel properties1-5 are ideal building blocks to make nanoparticle based artificial materials, where they mimic the atomic crystals by building solid matters with colloidal nanoparticles instead of atoms.6 These nanoparticle based artificial materials are promising for a broad spectrum of device applications such as flexible flash memories,7 displays,8 and field effect transistors9 and many other applications in such as lithography10 and data storage.11 One of the technical hurdles deterring device fabrication is the difficulties to organize colloidal nanoparticles at designated locations in large area and high quality.12 It is shown that large area monolayer of colloidal nanoparticles with long range order can be obtained by carefully controlling the evaporation direction of carrier fluid and the mass flow direction of nanoparticles during selfassembly.13 Besides 2 D monolayers, it is also required to organize colloidal nanoparticles into complex hierarchical structures such as nanoparticle assembly arrays (NAAs) where assemblies of nanoparticles are spatially organized in 2 D patterns. Small nanoparticle clusters in arrays have been fabricated by guiding the deposition of nanoparticles during evaporation on pre-patterned substrate. 14-17Random colloidal nanoparticle assemblies on substrate have been prepared by several methods, showing novel magnetic properties.18, 19 However, ordered nanoparticle assembly arrays are often needed for device fabrication and to have tunable properties. To fabricate NAAs where each assembly of nanoparticles is composed of a tremendous amount of nanoparticles (~ millions), evaporation of nanoparticle solution should be cautiously controlled to allow them to reach their equilibrium positions during deposition.20 Here we use ~ 8 nm cobalt ferrite (CoFe2O4)
This journal is © The Royal Society of Chemistry 2012
nanoparticles as the fundamental building blocks to make magnetic NAAs (MNAAs) by guiding the deposition of nanoparticles in the pyramid pits.20 Unlike the NAAs prepared in reference 20 for Raman scattering enhancement, we removed the colloidal nanoparticles bridging nanoparticle assemblies, which is critical to decouple the magnetic interactions between nanoparticle assemblies. Compared with the self-assembly of gold nanoparticles in reference 20, the hierarchical self-assembly of cobalt ferrite nanoparticle is much difficult due to (1) the greatly reduced inter-particle interactions, (2) largely enhanced Brownian motion and (3) significantly increased number of nanoparticles in each assembly unit. As a result, self-assembly of cobalt ferrite nanoparticles by simply reducing evaporation rate of carrier fluid does not yield large area uniform NAAs. In this work, we explored to design a apparatus that fabricate high quality NAAs by controlling both evaporation rate and directions of a stable nanoparticle solution. Colloidal nanoparticle cores are coated with surfactants to prevent agglomeration and make them dispersible in organic solvents such as toluene and hexane. Self-assembly of nanoparticles often occurs at the drying front when evaporating the organic carrier solvents on a surface.21 Lithographically fabricated features on surfaces can significantly affect the movement of the drying front and thus be used to direct the self-assembly of colloidal nanoparticles.16, 22, 23 Here photo-lithography or e-beam lithography is used to pre-pattern the surface and generate the first layer of the hierarchical structures. Typically, when drying a droplet of colloidal nanoparticle solution on a lithographically patterned hole arrays, the moving contact lines of the nanoparticle solution are locally pinned at holes, and the meniscus due to the bending of the moving contact line will drag nanoparticles towards and then preferentially fill these holes22. Depending on the size and shape of the filling nanoparticles and the pre-patterned holes, organization of the nanoparticle clusters can be precisely controlled in the holes. 16, 23 If the size of nanoparticles is comparable with the holes, nanoparticle arrays are generated on the surface. If the size of nanoparticles is smaller than half of the holes, multiple nanoparticles can be contained in each hole to yield nanoparticle cluster arrays on the surface.23 Here we show that when the dimension of the pre-patterned holes are much greater than the diameter of the filling colloidal nanoparticles, a large number of nanoparticles (~millions) can be deposited in these ‘big’ holes by
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controlling the evaporation of carrier fluid to form NAAs. Further, the shape and size of these assemblies are determined by the prepatterned holes. These NAAs allow us to study the collective behaviours of colloidal nanoparticle assemblies with determined shape and size. Unlike the NAAs that are prepared without template and have random shape, size and directions24-26, the NAAs prepared in this work have approximately the same size, shape and aligned directions, which allow us to study the anisotropy of individual assembly by measuring the total response of the array. This is critically important because the physical response of an individual nanoparticle assembly of this dimension is often too weak to measure by a conventional instrument. And the measurement of an ensemble of nanoparticle assemblies with random orientations will eliminate the directionality associated properties. Inverted pyramid hole arrays (IPHAs) were used as the templates to direct the growth of NAAs. The base width and height of the pyramid are ~ 3 µm and ~2 µm respectively, which is much larger than the diameter of colloidal nanoparticles (~ 8 nm). The pitch is ~ 5 µm, and edge distance is ~2 µm. The scanning electron microscopy (SEM) image of an IPHA is shown in large area in Figure 1(a) and in high magnification in Figure 1 (b) respectively. In each pyramid, several millions of colloidal nanoparticles can be contained in contrast to the small number of nanoparticles for the nanoparticle cluster arrays. Large-area IPHAs were fabricated on thermally oxidized (100) silicon substrate by photolithography, followed by
Journal Name DOI: 10.1039/C4NR07489K
Figure 1 SEM images of an IPHA (a) in large area and (b) in high magnification. The inset in (a) is a picture of the patterned area on a silicon substrate. Cobalt ferrite nanoparticles were synthesized by simultaneous thermal decomposition of iron (III) and cobalt (II) acetylacetonate at high temperature in organic solvent (see supplemental information for synthesis details).1 Oleic acid and oleylamine were used as surfactants to coat the nanoparticle cores during synthesis, which prevent nanoparticle from agglomeration. Figure 1S (see supplemental information) shows a transmission electron microscopy (TEM) image of cobalt ferrite nanoparticles. A spacing of ~ 2-3 nm between nanoparticle cores is induced by the steric
Figure 2. Procedure for making MNAA on an IPHA. Step 1: A drop of cobalt ferrite nanoparticles was put on top of an IPHA on silicon substrate that was placed in a petri-dish; step 2: a fluorinated polyether plate was put on top of the nanoparticle solution, and the petri-dish was covered by a glass slides to allow carrier fluid to slowly evaporate; step 3: after carrier fluid was completely evaporated, the polymer plate was removed from the IPHA; and step 4: the excess nanoparticles on silicon wall was polished away to make MNAA, and one of the nanoparticle crystal in inverted pyramid hole was magnified. fluorinated etching, wet chemical anisotropy etching27 and removal of silicon oxide hard mask by buffered oxide etch. The fabricated IPHAs were finally cleaned by D.I. water in ultrasonic bath. The fabrication process is given in supplemental information. A picture of an IPHA on silicon substrate is shown in the inset of Figure 1(a), which can act as grating to divide the white light into a full spectrum of colours.
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repulsion due to surfactant coating. As shown in Figure 1S, uncontrolled evaporation of nanoparticle solution on a flat surface yields a mixture of random monolayer, multilayer and gaps, which is in contrast to the uniform nanoparticle assembly over large area obtained by controlled evaporation.13
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Figure 3 (a) An SEM image of cobalt ferrite nanoparticle filled IPHA, and (b) the element concentration profile along the dotted green line in (a), which is obtained by EDX line scan. (c), (d) and (e) show the SEM image, iron mapping and cobalt mapping in a typical area of the cobalt ferrite filled IPHA.
Lithographically patterned hole arrays can direct nanoparticles to preferentially grow in the holes. However, uncontrolled or improperly controlled evaporation on the IPHAs would result in unevenly filling of the hole arrays and inhomogeneous nanoparticle growth. As shown in Figure 2S (a) in supplemental information, natural evaporation of colloidal solution would not give nanoparticles sufficient time to grow bigger during deposition. In this dynamically controlled growth, small nanoparticle assemblies with grain size < 1 µm form both in holes and on the wall, and each hole is filled with several nanoparticle assemblies with voids between them. The small grain size induced by rapid evaporation is similar to the atomic crystal fractionalization during rapid cooling,28 where fast crystallization does not give enough time for atoms to diffuse. To obtain large nanoparticle assemblies, both evaporation rate and direction should be properly controlled. Figure 2S (b) in supplemental information shows nanoparticle assemblies in SEM formed by slowly (~ 5 µm/min) pulling the IPHAs out from colloidal solution with a dip coater. The assemblies appear darker than the surroundings in the SEM image. The growth of nanoparticle assemblies is regulated by the shape of the inverted pyramid holes. In contrast to fast evaporation, only one large nanoparticle assemblies is contained in each hole for slow pulling, and
nanoparticles are mainly deposited in the holes rather than on the silicon wall. As shown in Figure 3S (see supplemental information), nanoparticles mainly diffused from solution along the edge of the inverted pyramid holes during growth. The growth plane is flat and perpendicular to the substrates. By slow pulling, moving speed of contact line is small, allowing nanoparticles to diffuse from solution to the hole and find their equilibrium position during deposition. However, by slow pulling, nanoparticles are not actively driven from the solution to the drying front. Complete filling of holes are not achieved even though very small moving speed is used and a day is taken to make a half centimetre square’s sample. Learning from the condensation of atoms, evaporation of nanoparticle solution should be controlled such that colloidal nanoparticles have sufficient time to diffuse to the holes and locate their equilibrium positions during growth. Further, the evaporation direction should also be controlled so that nanoparticles in solution will be continuously driven toward the drying front to feed the growth of nanoparticle assemblies.13 To satisfy above conditions, we modified the method for microsphere cluster growth on patterned surface that is used in reference 23 to fabricate NAAs on IPHAs. The procedure of our method is schematically depicted in Figure 2. Firstly, a droplet of nanoparticle solution is spread on the top of an
Figure 4. The HRSEM images of the cobalt ferrite nanoparticle filled IPHA. The area enclosed by the red and green squares in (b) are magnified in (a) and (c) respectively.
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IPHA by a micro-syringe, which is placed in a petri-dish. A fluorinated polyether plate with very smooth surface and low surface energy is then placed on top of the nanoparticle solution. The petridish is then partially covered by a glass slides to reduce the evaporation rate. Nanoparticle solution is trapped between the IPHA substrate and the polymer plate, only allowing evaporation to occur in the lateral directions. The evaporation is slow and steady due to the small gap between the polymer plate and the substrate as well as the partial covering of the petri-dish, giving sufficient time for nanoparticle movement in solution and growth. Evaporation through the small gap between the polymer plate and the substrate will continuously drive nanoparticles in solution to move in the lateral direction towards the drying front, which is locally pinned at the holes. At the inverted pyramid holes, nanoparticles are pulled by capillary force along the edge and side of the pyramid, and deposited at a steady rate to form MNAAs on IPHAs. After carrier fluid is completely evaporated, sporadic nanoparticle clusters on silicon wall were finally removed by polish.
Journal Name DOI: 10.1039/C4NR07489K nanoparticle assemblies would occur due to the ~2-3 µm separation. Cobalt ferrite nanoparticles of ~8 nm are single domain.29 Each one behaves like a small magnet, whose magnetization directions can be coherently switched by thermal agitation. The switching probability is determined by the volume of the nanoparticle and their magnetic anisotropy.30 The dipolar interactions between magnetic nanoparticles also affect the switch of their magnetization directions, making them behave collectively in the nanoparticle crystal.31 As a result, the properties of magnetic nanoparticle crystals are determined by the composing nanoparticles and the interactions between them. Here we also introduced a shape asymmetry of the magnetic nanoparticle assemblies in the out-of-plane direction, which might also affect the magnetic properties of the magnetic nanoparticle assemblies.
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SEM image in Figure 3(a) shows the top view of a MNAA prepared by above method. Compared with natural evaporation and dip coating, the controllability of nanoparticle growth is greatly improved. MNAAs obtained by this method have greatly improved the uniformity over large area. Each inverted pyramid hole is almost fully filled by a pyramid nanoparticle assembly in contrast to the many small nanoparticle assemblies in a hole prepared by rapid evaporation. The MNAA is uniform across the whole patterned areas with determined shapes and aligned orientations. Energy dispersive x-ray spectroscopy (EDX) was performed along the green line in Figure 3(a). The element concentration profiles along the green line are shown in Figure 3(b). Here the elements in nanoparticle cores (cobalt, iron and oxygen) and surfactant (carbon and oxygen) are analysed. The periodic variation of concentration along the green line is corresponding to the periodic structure of the MNAAs. Cobalt, iron, oxygen and carbon were observed at the location of nanoparticle crystals, proving successful deposition of nanoparticles in the holes rather than on the walls. EDX was also performed in an area enclosing a 3×4 matrix of MNAA, whose SEM image is shown in Figure 3(c). The iron and cobalt mapping in this area is shown in Figure 3(d) and (e) respectively, which is consistent with the SEM observations. High resolution SEM (HRSEM) was performed on a small area, as shown in Figure 4. Figure 4(a) shows that cobalt ferrite nanoparticles are only deposited in the pyramid holes, and almost no nanoparticle is observed on the silicon wall. To see more details of the MNAA, the area enclosed by red and green squares in Figure 4 (a) is further magnified in Figure 4(b) and 4 (c) respectively. Individual nanoparticles on the surface of the nanoparticle crystal can be resolved in Figure 4 (b) and (c). The nanoparticle assemblies are very dense, and the surface is very smooth due to controlled and slow evaporation of carrier fluid. Some vacancies are discernible on the surface as well. The boundary between the nanoparticle crystal and silicon wall is shown in Figure 4(c). No excessive nanoparticles are observed on the silicon walls, proving successful removal of nanoparticles on silicon wall by polish.
Figure 5. (a) out-of-plane and (b) in-plane M-H curves of cobalt ferrite MNAAs. (c) compares the in-plane and outof-plane M-H curves of cobalt ferrite nanoparticles deposited on a flat silicon substrate.
We have shown that high quality and large area pyramid MNAAs can be successfully fabricated by controlling evaporation of colloidal nanoparticle solution on the IPHAs. These magnetic nanoparticle assemblies in MNAAs have approximately the same size, shape and orientations, allowing us to detect the approximate magnetic properties of individual magnetic nanoparticle assemblies by measuring the whole MNAAs. Here no interaction between
Figure 5 (a) and (b) shows the magnetic hysteresis loops of the MNAA measured by applying the external magnetic field perpendicular and parallel to the substrate respectively. A small coercivity of ~ 200 Oe is observed for both measurements. It is clear
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Journal Name that MNAA can be easily magnetized along the out-of-plane direction, and it is difficult to be magnetized along the in-plane direction. This observation is opposite to the 2 D magnetic nanoparticle thin films, where they behave like a continuous ferromagnetic thin film and can be easily magnetized along the inplane direction to minimize electrostatic energy.32 A drop of cobalt ferrite nanoparticle solution was evaporated on the smooth surface of a silicon substrate to make a reference sample, where the crystals are not patterned by lithography. The magnetic properties were then measured under the same condition by a vibrating sample magnetometry (VSM), as shown in Figure 5(c). The in-plane and out-of-plane hysteresis loop of the unpatterned cobalt ferrite magnetic nanoparticle crystals are almost superimposed on each other, as shown in Figure 5(c). No preferential magnetization direction was observed for this unpatterned sample. Here we attribute the magnetic anisotropy of the pyramid magnetic assemblies to their asymmetric shape along the out-of-plane directions, which within our knowledge have not been observed in previous studies. Further work needs to be done to clearly understand the magnetization mechanism of these magnetic nanoparticle assemblies and the shape induced anisotropy.
Conclusions We have successfully fabricated large-area MNAAs by controlling the evaporation rate and direction of carrier fluid on IPHAs. Magnetic measurement indicates that these magnetic nanoparticle assemblies with pyramid shape can be easily magnetized along the axis of symmetry. Our study shows that the size, shape and orientation of nanoparticle assemblies can be engineered by the prepatterned surface. Due to the aligned orientation of nanoparticle assemblies, it is possible to detect the anisotropic physical properties of individual nanoparticle assembly. Further work is being carried out to improve nanoparticle crystal growth and to understand the magnetization mechanism of MNAAs.
Acknowledgement This work is financially supported by National Natural Science Foundation of China (No. 51401046, No. 61131005), Keygrant Project of Chinese Ministry of Education (No. 313013), New Century Excellent Talent Foundation (No. NCET-11-0068), and start-up research fund from the University of Electronic Science and Technology of China.
Notes and references a
State Key Laboratory of Electronic Thin Films and Integrated Devices, University of Electronic Science and Technology of China, Chengdu 610054, China b Department of Electrical and Computer Engineering, University of Delaware, Newark, Delaware 19716, USA *E-mail: [email protected] †E-mail: [email protected] Electronic Supplementary Information (ESI) available: Experimental methods and characterizations, a TEM image of cobalt ferrite nanoparticles, SEM images of cobalt ferrite nanoparticle crystals on IPHA formed by fast evaporation and slow pulling out from nanoparticle solution by a dip coater, a SEM image of a partially filled inverted pyramid hole. See DOI: 10.1039/c000000x/
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