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J. Phys. Chem. B 2006, 110, 18154-18157
ARTICLES Hierarchical Self-assembly of Silver Nanocluster Arrays on Triblock Copolymer Templates Zhongtao Shi,†,‡ Min Han,§ Fengqi Song,† Jianfeng Zhou,§ Jianguo Wan,† and Guanghou Wang*,† National Laboratory of Solid State Microstructures and Department of Physics, Nanjing UniVersity, Nanjing, 210093, China, College of Sciences, Ningbo UniVersity of Technology, Ningbo, 315016, China, and National Laboratory of Solid State Microstructures and Department of Materials Science and Engineering, Nanjing UniVersity, Nanjing, 210093, China ReceiVed: May 27, 2006; In Final Form: August 1, 2006
Poly(styrene-b-butadiene-b-styrene) (SBS) triblock copolymer templates which present in-plane cylinders of polystyrene (PS) aligned parallel to the plane of the substrate have been prepared by a solvent-induced orderdisorder phase transition method. Silver nanoclusters have been obliquely deposited onto the SBS copolymer templates at low coverage, utilizing the directed low-energy cluster beam deposition (LECBD) method. The morphology of the samples has been characterized by a tapping-mode AFM. It is shown that the silver nanoclusters form ordered linear arrays and the intercluster distance within each individual linear array is comparable to the cluster size. Optical absorption spectra indicate that the surface plasmon resonance (SPR) of the silver nanocluster linear arrays occurs at about 444.5 nm, manifesting a red shift of ∼21.4 nm compared to the SPR absorption of silver nanoclusters deposited on a fused quartz substrate. This is attributed mainly to the near-field electrodynamic interactions between the silver nanoclusters. This hierarchical approach to create ordered nanostructures transcends the spatial limits of lithography and provides a promising route to achieve well-ordered cluster-based nanostructures.
Introduction Nanofabrication is central to modern technology and is key to producing powerful electronic/optical devices, miniaturized sensors, and other advanced technological devices.1 There are some well-established methods for preparing nanoscale patterns from top-down, for example, electron beam lithography,2 optical lithography,3 and focused ion beam lithography.4 A different alternative that is not limited by the resolution of fabrication equipment or resist materials is utilizing the bottom-up selfassembling process. It has been known for decades that block copolymers can self-assemble into patterns with length scales on the order of several tens to hundreds of nanometers.5,6 The pattern can be controlled by manipulating the chain length (molecule weight), chemical functionality, volume fraction of each block, etc.6,7 These ordered patterns can be further used as suitable scaffolds for nanoscale organization of inorganic materials.8,9 In this paper we report on a simple approach, based on the low-energy nanoclusters with controlled size obliquely deposited onto the block copolymer templates at low coverage, for obtaining nanocluster arrays. The hierarchical approach for * Corresponding author. E-mail:
[email protected]. Postal address: Department of Physics, Nanjing University, 22 Hankou Road, Nanjing, China, 210093. Phone: +86-25-83595082. Fax: +86-25-83595535. † National Laboratory of Solid State Microstructures and Department of Physics, Nanjing University. ‡ Ningbo University of Technology. § National Laboratory of Solid State Microstructures and Department of Materials Science and Engineering, Nanjing University.
creating ordered nanostructure transcends the spatial limits of lithography and provides a promising route for achieving wellordered cluster-based nanostructures. Experimental Methods The triblock copolymer poly(styrene-b-butadiene-b-styrene) (SBS) with a weight-averaged molecular weight of 140000 Da, a polydispersity index of 1.2, and PS weight fraction of 30% was obtained from Aldrich Chemical Inc. The copolymer was dissolved in toluene to produce a 1 wt % solution. The solution, about 30 L, was spin-coated onto a fused quartz substrate at about 2500 rpm for 1 min to form a film. Then the samples were put inside a glass vessel in which some drops of volatile toluene were added and the vessel was completely covered with aluminum foil to generate a very slow evaporation rate (∼0.001 mL/h). A magnetron plasma gas-aggregation cluster source was used to produce a well-directed beam of silver clusters for softlanding cluster deposition.10 During the experiment, the background pressure of the deposition chamber was about 5 × 10-5 Pa. At room temperature the silver clusters were obliquely deposited onto the SBS template by rotating the substrate holder. The incident angle of the cluster beam was kept 45° relative to the normal of the substrate surface. The deposition rate was measured in situ with a quartz crystal film thickness monitor and controlled in the range of 5-10 Å/min, with a dc magnetron sputtering power of about 20 W. A tapping mode AFM (NanoScope IIIa, Digital Instrument, Inc.) was utilized to observe the morphology of the samples.
10.1021/jp063271q CCC: $33.50 © 2006 American Chemical Society Published on Web 08/26/2006
Hierarchical Self-assembly of Ag Nanocluster Arrays
J. Phys. Chem. B, Vol. 110, No. 37, 2006 18155
Figure 1. Typical 500 × 500 nm phase image of the SBS template by the tapping mode AFM micrograph. The inset shows the corresponding fast Fourier transform (FFT) (a); schematic of the SBS triblock copolymer monolayer with cylinders of polystyrene aligned parallel to the plane of the substrate (b).
The optical properties of the samples were measured by a UVvis spectrophotometer (UV-1100, BRAIC). Results and Discussion A tapping mode AFM phase image of the SBS copolymer template is shown in Figure 1a. A stripe pattern can be clearly observed. Since the glass transform temperature, Tg, of PB is -90 °C and the Tg of PS is 100 °C, the PB block is softer than the PS block at room temperature. It has been demonstrated that the tip indentation of the AFM is higher on softer materials; larger (smaller) indentations are related to soft, PB-rich (glassy, PSrich) areas.11 Consequently, the brighter domains in Figure 1a can be assigned to the harder materials, i.e., to PS-rich areas. Therefore, Figure 1a shows that the in-plane cylinders of PS are aligned parallel to the plane of the film. In the fast Fourier transform (FFT) image shown in the inset of Figure 1, the number of pairs of spots on the FFT pattern corresponds to the number of “grains”; the arcing of the spot in the polar direction is due to the orientation distribution of the domains. A schematic view of the measured morphology is shown in Figure 1b. The topographic parameters measured by AFM are the periodicity of the ordered structure, L. Since the surface energy of PB is lower than that of PS, the film surface will be enriched with a very thin layer of PB shown as the surface layer with thickness t.11 The mean width of the in-plane cylinder is about 25 nm, while the space between the adjacent PS domains is around 15 nm. The unique pattern of the template should be attributed to
Figure 2. Typical 500 × 500 nm phase image of the silver nanoclusters deposited on the fused quartz substrate by the tapping mode AFM micrograph (a) and typical 500 × 500 nm phase image of silver nanoclusters linear arrays on the SBS template by the tapping mode AFM micrograph. The inset shows the corresponding fast Fourier transform (FFT) (b).
the balance between minimizing the interaction energy between unlike blocks and maximizing the conformation entropy.12 We have selected silver cluster for deposition because welldefined noble metal nanoparticle arrays have attracted much attention due to the wide technological potential of their application, such as optical filters,13 plasmonic waveguides,14 bio/chemsensors,15,16 and substrates for surface-enhanced spectroscopies.17 At room temperature, silver clusters were obliquely deposited onto the SBS template at a low coverage of
