Ordered magnetic multilayer nanobowl array by nanosphere template method

Solid State Communications 150 (2010) 2357–2361

Contents lists available at ScienceDirect

Solid State Communications journal homepage: www.elsevier.com/locate/ssc

Ordered magnetic multilayer nanobowl array by nanosphere template method Y.J. Zhang a,b , Y.X. Wang a , W.E. Billups b , H.B. Liu a , J.H. Yang a,∗ a

Physics College of JiLin Normal University, SiPing JiLin 136000, PR China

b

The Richard E. Smalley Institute for Nanoscale Science and Technology, Rice University, Houston, TX 77005, USA

article

info

Article history: Received 10 March 2010 Received in revised form 5 September 2010 Accepted 27 September 2010 by R. Haug Available online 7 October 2010 Keywords: A. Magnetic films and multilayers A. Nanostructures B. Nanofabrications

abstract Ordered magnetic multilayer [Co/Pt]n nanobowls have been fabricated over a silicon substrate based on a polystyrene (PS) monolayer film. The ordered PS monolayer was first prepared by the self-assembly technique, which was used as the template for the multilayer film [Co/Pt]n deposition. The ordered magnetic multilayer [Co/Pt]n nanobowl array was obtained after the transferring and the selective etching process. The nanobowls show a uniform size and smooth surfaces. The nanobowls stuck to the neighbors and notches were observed in the bowl brims because of the contact points between the closed-packed PS beads. The nanobowls could be separated from their neighbors by thinning the PS beads before the film deposition and no notches were observed anymore. Compared to the chemical method, this method showed more flexible choices of the material to fabricate the nanobowls, which extended the application scope of the nanobowls greatly. © 2010 Elsevier Ltd. All rights reserved.

1. Introduction Ordered nanostructure arrays have found applications recently in microelectronics [1], magnetism [2,3], biology [4] and optics [5]. In most cases the ordered nanostructure array is fabricated by using either lithography, imprint lithography or the nanosphere template method, which is inexpensive and convenient for fabrication over a large area. In addition to nanodots [6,7], nanoholes [8], nanocolumns [9] and nanotubes [10,11], other nanostructures that can be fabricated by the nanosphere-assisted method include hollow spheres [12,13] and nanobowls [14–16]. These are beyond the capabilities of conventional e-beam lithography and imprint lithography. The new nanostructures have potential applications in the optical, catalytic, biosensing, and surface-enhanced Raman scattering areas [17–20]. The nanobowl array is usually obtained by a chemical process under the nanosphere-assisted method [21–24]. The dramatic challenge for the chemical method is how to fabricate nanobowls composed of magnetic multilayers, such as Co/Pt multilayers, which are used widely in magnetic memory techniques. In this letter, we report a method to fabricate [Co/Pt]n magnetic multilayer nanobowls that are separated from each other. The method allows the multilayer to be deposited onto the separated ordered colloidal monolayer (substrate A) allowing the magnetic film to be transferred to another substrate (substrate B). After the selective etching of the colloids, the ordered nanobowl array is

∗

Corresponding author. Tel.: +86 434 3294566; fax: +86 434 3294566. E-mail address: [email protected] (J.H. Yang).

0038-1098/$ – see front matter © 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.ssc.2010.09.047

formed on substrate B. The highly ordered magnetic nanobowl [Co/Pt]n multilayer exhibits a perfect structure, allowing insight into the micro-mechanism for the magnetic properties. It is also promising theoretically and practically for the design and fabrication of high-density magnetic memory devices. 2. Experiment For all experiments, (100)-oriented Si wafers were cut into 1 × 1 cm2 pieces. The substrates were cleaned at 80 °C for 15 min prior to use by a conventional cleaning process using a 1:1:5 solution of NH4 OH(25%), H2 O2 (30%), and water. The substrates were then washed thoroughly and kept in 10% sodium dodecyl sulfate solution to modify the surface. This makes the substrate hydrophilic after 24 h. The monodisperse polystyrene particles (purchased from the Duke Scientific Corporation) with diameters of 200 nm that were received as a 10 wt% suspension in water were further diluted with an equal amount of ethanol. Seven microlitres of the diluted polystyrene solution were applied onto the modified substrate and allowed to spread over the entire substrate. The wafer was then immersed slowly into the glass vessel filled with water. The polystyrene particles then formed an unordered monolayer on the water surface. Some sodium dodecyl sulfate solution (2 wt%) was dipped onto the water surface leading to a large monolayer with highly ordered areas. The monolayers were then lifted off using the modified silicon wafer described above. The monolayer of the polystyrene spheres was arranged in a two-dimensional hexagonal pattern. After the formation of the single layer of polystyrene on the Si substrate, the multilayer [Co10 Å/Pt10 Å]15 was deposited on

2358

Y.J. Zhang et al. / Solid State Communications 150 (2010) 2357–2361

a

300nm

b

c

300nm

d 300nm

Fig. 1. Steps for the fabrication of the multilayer film nanobowl. The ordered PS monolayer was deposited onto the silicon wafer (substrate A) by the self-assembly process (a); The multilayer film deposition leads to ordered bottom-up nanobowls on the colloids and nanodots between the colloids (b); The colloids with the nanobowls were transferred to another wafer (substrate B) and the nanodots were left on the original substrate A (c); Selective etching of the colloids results in the formation of bottom-down nanobowls on substrate B. The left column shows the section view, the middle column shows the tilted view and the right column shows the corresponding SEM images.

the substrate (including the sphere and the pores between the neighbor spheres) in the magnetron sputtering system. The film was subsequently covered with an additional 20 Å—thick Pt layer to prevent further oxidation. The Co and Pt films were deposited by dc magnetron sputtering in a 4 × 10−6 Pa Ar atmosphere at room temperature, and the deposition rates were 0.01 nm/s and 0.015 nm/s, respectively. The scanning electron microscopy (SEM) images were recorded on a JEOL 6500 F, a high resolution (1.5 nm) thermal field emission electron microscope, operating at 5.0 kV. The atomic force microscopy (AFM) measurement was performed on a Nanoscope III from Digital Instruments, in tapping mode. The transmission electron microscopy (TEM) was performed on JEM-2100HR, operating at 200 keV. The selective etching was performed in a plasma cleaner (Model 1020, E.A. Fischione Instruments Inc.), and the working gas was a mixture of 80% O2 and 20% Ar. 3. Results and discussion The steps required to fabricate the nanobowl when a polystyrene monolayer is used as the template are illustrated in Fig. 1. The ordered PS monolayer is deposited onto the silicon wafer by the self-assembly method (Fig. 1(a)). The [Co/Pt]n magnetic multilayer is then deposited onto the ordered array. When the

multilayer is deposited onto the PS array, two different kinds of nanostructure arrays form, the ‘‘cap’’ array on the spheres and the nanodot array in the pores between the neighboring spheres (Fig. 1(b)). In order to obtain the nanobowl array, the colloidal array with the magnetic film is transferred onto substrate B. The nanodots between the neighboring spheres are left on the original substrate (Fig. 1(c)). The nanobowl array is obtained on the new substrate after the removal of the PS beads by the selective etching process (Fig. 1(d)). The corresponding scanning electron microscopy image is shown on the right. Fig. 2 shows the SEM images of the ordered Co/Pt multilayer magnetic nanobowl array viewed from different angles. The ordered nanobowl array can be seen over a very large area. The top view of the SEM image shows that the Co/Pt multilayer nanobowls are still in a close-compact arrangement and the diameters are around 200 nm, in agreement with the size of the PS beads (Fig. 2(a)). This indicates that the pattern and the shape of the Co/Pt multilayer nanobowls are not changed after the PS particles are removed by etching for a very short time. The bowl size and density can be easily controlled by changing the size of the starting PS particles. The ordered area covers hundreds of square micrometers. An area density of 3 × 109 bowl cm−2 is determined from the image in Fig. 2(a). To see the bowl shape clearly, Fig. 2(b) shows the SEM image of a side-view of the multilayer nanostructure tilted 45°. When the multilayer is deposited on the spheres, the thickness

Y.J. Zhang et al. / Solid State Communications 150 (2010) 2357–2361

2359

a

A

B

500nm

200nm

b

A B

500nm 0.2 0.4

c

X 0.200 µm/div Z 500.000 µm/div

0.6 0.8

µm

Fig. 3. When the multilayer is deposited onto the close-compact colloids, the notches will be observed owing to the connecting points between the neighboring colloids marked by arrow A in the SEM (a) and AFM (b) images, and the protruding parts between the notches are labeled by arrow B. When the neighboring protruding parts stick to one another, they look like hollow pillars together under SEM and AFM measurement.

200nm

Fig. 2. The top view shows the perfect ordered nanobowl array in agreement with the PS pattern after the removal of the PS beads by selective etching (a); The tilted view shows the bowl-shape nanostructures over a large area (b); The disordered area near the crack shows the shapes from different view-angles, confirming the formation of the nanobowls (c).

decreases gradually along the surface from the top, which leads to the thinner brim of the nanobowl as confirmed by the gradual change of the transparent effect of the bowl brim under SEM in Fig. 2(b). Fig. 2(c) shows the nanobowls near a crack where the shapes and the surfaces of the bowls can be clearly viewed from all directions. The Co/Pt nanobowl exhibits a smooth surface and uniform size. The SEM image also shows that the nanobowls are connected to their neighbors. Several notches can be seen in each nanobowl brim as a result of the contacting points between the closed-packed PS beads. When the magnetic film is deposited onto the closed-packed PS array the thickness decreases along the surface of the sphere. The connection between the neighbors cannot be avoided unless the thickness is very small. Fig. 3(a) gives the clear image for the notches in the bowl brims when the bowls are connected to their neighbors. The arrow A points to the notches and the arrow B points to the protruding parts next to the notches. When the protruding parts stick together, they look

like hollow pillars as shown under SEM. The AFM measurement in Fig. 3(b) gives a clear image for the ordered nanobowl array, and the notches can be observed clearly in the image because of the lateral enlargement effect during the AFM measurement process. The white circles show the bowl shape for the eye guideline. The nanobowl can be separated cleanly from its neighbors by etching the PS beads before the multilayer deposition. The separated PS array was obtained after etching for 1 min (Fig. 4(a)). The detailed etching experimental process was reported in our previous work [25]. The separated nanobowl array is obtained after film deposition, pattern transfer and selective etching as mentioned above. The top view of the SEM image shows the perfectly ordered [Co/Pt]n nanobowl patterns clearly (Fig. 4(b)). Although the distances between the beads are enlarged, the areal density is maintained, 3 × 109 bowl cm−2 for the PS beads of diameter 200 nm. The outside and inside surfaces of the bowls are both smooth, and no notches of the bowls are found under SEM. These nanobowls are highly ordered and separated from each other, forming a two-dimensional nanobowl array. The tilted view shows the ordered bowl-like structures; the thickness of the bowl brim decreases from the center to the brim gradually and no notches of the bowls are observed again (Fig. 4(c)). Fig. 4(d) shows the image of different view angles and the bowl shape can be observed from all directions, confirming the formation of perfect nanobowl structures. Fig. 4(e) shows the AFM image of the 3D view, which confirms the perfect bowl array.

2360

Y.J. Zhang et al. / Solid State Communications 150 (2010) 2357–2361

b

a

200nm

c

200nm

d

200nm

200nm

e

µm 1.5 1.0 0.5

X 0.500 µm/div Z 150.000 nm/div

Fig. 4. After etching by oxygen plasma, the colloids are separated from their neighbors (a); Based on the separated colloids, perfect nanobowls are fabricated and no notches are observed (b); The tilted view reveals the nanobowl structures (c); When the nanobowls are collected together, we can get the images from different view-angle patterns, confirming the formation of the nanobowls (d); the AFM image shows the ordered array of the separated nanobowls.

TEM measurement shows that the nanobowl array still maintains the ordered pattern in some places after transferral to the carbon film on the copper grid which indicates that the action force between the magnetic film and PS sphere is larger than that between the PS sphere and the substrate - the very weak Van der Waals action (Fig. 5). This fact also indicates why it is possible to transfer the nanobowl from substrate A to substrate B while keeping the multilayer perfect. The thickness of the multilayer wall measured at the bottom region is about 30 nm, and the thickness relation is t = t0 sin θ , where t0 is the thickness of the bottom of the bowl, and θ is the angle to the normal direction of the sphere surface, that is, the film deposition direction. In some places

the Co/Pt multilayer nanobowls are broken for the TEM specimen preparation, while the feature spherical shape can still be seen. In 2005 Albrecht Manfred and his coworkers developed a method to prepare high-density recording media by fabricating a Co/Pt multilayer magnetic cap array on a nanosphere array [26]. However, the multilayer stays on the polystyrene colloids, which then becomes unstable and damaged above 200 °C. Thus, the temperature of the multilayer will be limited to a very small range. In addition, the magnetic nanodot array between the spheres cannot be avoided, and will deteriorate the memory properties significantly, especially when the sphere diameter is under 100 nm. Thirdly, the thickness of the multilayer is strictly

Y.J. Zhang et al. / Solid State Communications 150 (2010) 2357–2361

2361

Acknowledgements This work is supported partly by National Programs for High Technology Research and Development of China (863) (No. 2009AA03Z303), the National Youth Program Foundation of China (No. 10804036 and 10904050), and Program for New Century Excellent Talents in University (No. NCET-09-0156). θ

References

50nm

Fig. 5. TEM measurement shows the nanobowl thickness decreases from the bottom to the bowl brim.

limited to avoid connection with the neighboring magnetic cap due to the closed-stacked PS array. The approach presented here avoids these undesirable effects, making it more suitable for the application of magnetic recording media of a high density. The thickness of the bowl can be tuned precisely by varying the number of multilayer cycles. The size and the area density of the nanobowl can be adjusted by using different sized PS spheres during the formation of the templates. The nanobowl size and the distance between the neighbors can be corrected by thinning the beads. In conclusion, we have prepared highly ordered magnetic multilayer nanobowls based on a Co/Pt multilayer and polystyrene latex particles. The thickness of the Co/Pt walls can be controlled precisely by coating the Co/Pt walls onto the sphere surface by magnetron sputtering. The final nanobowl arrays are achieved after sputtering, transferring and etching. In addition to their inherited well-ordered arrangement over a large area, the nanobowls also exhibit smooth interior and exterior surfaces and uniform size. We expect that these magnetic multilayer nanobowls can be used for high density magnetic recording. The approach presented here could also be extended to a wide range of coating materials allowing controlled wall thickness and size.

[1] W.B. Choi, B.H. Cheong, J.J. Kim, J. Chu, Adv. Funct. Mater. 13 (2003) 80. [2] Y. Takamura, R.V. Chopdekar, A. Scholl, A. Doran, J.A. Liddle, B. Harteneck, Y. Suzuki, Nano Lett. 6 (2006) 1287. [3] S.M. Weekes, F.Y. Ogrin, W.A. Murray, Langmuir 20 (2004) 11208. [4] O.D. Velev, E.W. Kaler, Langmuir 15 (1999) 3693. [5] G. Ctistis, E. Papaioannou, P. Patoka, J. Gutek, P. Fumagalli, M. Giersig, Nano Lett. 9 (2009) 1. [6] J. Rybczynski, U. Ebels, M. Giersig, Colloids Surf. A 2219 (2003) 1. [7] D.G. Choi, H.K. Yu, S.G. Jang, S.M. Yang, J. Am. Chem. Soc. 126 (2004) 7019. [8] C. Haginoya, M. Ishibashi, K. Koike, Appl. Phys. Lett. 71 (1997) 2934. [9] Y. Li, T. Sasaki, Y. Shimizu, N. Koshizaki, J. Am. Chem. Soc. 130 (2008) 14755. [10] K. Kempa, B. Kimball, J. Rybczynski, Z.P. Huang, P.F. Wu, D. Steeves, M. Sennett, M. Giersig, L.N. Rao, D.L. Carnahan, D.Z. Wang, J.Y. Lao, W.Z. Li, Z.F. Ren, Nano Lett. 3 (2003) 13. [11] Z.P. Huang, D.L. Carnahan, J. Rybczynski, M. Giersig, M. Sennett, D.Z. Wang, J.G. Wen, K. Kempa, Z.F. Ren, Appl. Phys. Lett. 82 (2003) 460. [12] V.G. Pol, D.N. Srivastava, O. Palchik, V. Palchik, M.A. Slifkin, A.M. Weiss, A. Gedanken, Langmuir 18 (2002) 3352. [13] J.B. Liu, W. Dong, P. Zhan, S.Z. Wang, J.H. Zhang, Z.L. Wang, Langmuir 21 (2005) 1683. [14] X.D. Wang, E. Graugnard, J.S. King, Z.L. Wang, C.J. Summers, Nano Lett. 4 (2004) 2223. [15] A.K. Srivastava, S. Madhavi, T.J. White, R.V. Ramanujan, J. Mater. Chem. 15 (2005) 4424. [16] Y. Li, W.P. Cai, G.T. Duan, Chem. Mater. 20 (2008) 615. [17] D. Jagadeesan, U. Mansoori, P. Mandal, A. Sundaresan, M. Eswaramoorthy, Angew. Chem. Int. Ed. 47 (2008) 7685. [18] X. Li, J. Peng, J.H. Kang, J.H. Choy, M. Steinhart, W. Knoll, D.H. Kim, Soft Mater. 4 (2008) 515. [19] P. Kumnorkaew, Y.K. Ee, N. Tansu, J.F. Gilchrist, Langmuir 24 (2008) 12150. [20] X. Wang, C. Lao, E. Graugnard, C.J. Summers, Z.L. Wang, Nano Lett. 5 (2005) 1784. [21] X. Chen, X. Wei, K. Jiang, Opt. Express 16 (2008) 11888. [22] J. Chen, D. Chao, X. Lu, W. Zhang, S.K. Manohar, Macromol. Rapid Commun. 27 (2006) 771. [23] T. Chen, T. Tsai, K. Hsieh, S. Chang, N. Tai, C.L. Hsuen, Nanotechnology 19 (2008) 465303. [24] T. Yu, B. Varghese, Z. Shen, C. Lim, C. Sow, Mater. Lett. 62 (2008) 389. [25] Y.J. Zhang, X.H. Wang, Y.X. Wang, H.L. Liu, J.H. Yang, J. Alloys Compd. 452 (2008) 473. [26] M. Albrecht, G. Hu, L.I. Guhr, C.T. Ulbrich, J. Bonerberg, P. Leiderer, G. Schatz, Nat. Mater. 4 (2005) 203.