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Communication: A two-armed polymer with a crown ether core self-assembles to produce macroporous films with pores perpendicularly reaching through the film down to the substrate. A possible assembling mechanism is discussed. The pore size can be conveniently adjusted by changing the solution concentration. These through-hole macroporous films provide a template for fabricating an array of Cu nanoparticle aggregates.
Fabrication of a Metal Particle Array Based on a Self-Assembled Template from a Two-Armed Polymera Jun Fu,1 Xiaoshuang Feng,2 Yanchun Han,*1 Caiyuan Pan,2 Yuming Yang,1 Binyao Li1 1
State Key Laboratory of Polymer Physics and Chemistry, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun 130022, P. R. China Fax: þ86-431-5262126; E-mail:
[email protected] 2 Department of Polymer Science & Engineering, University of Science & Technology of China, Hefei 230026, P. R. China
Keywords: macroporous films; nanoparticles; self-assembly; templates; two-armed polymers
Introduction [1–3]
Nanoparticles of metals, semiconductors and magnets usually show extraordinary electronic, catalytic, optical and magnetic properties due to the quantum size effect and, thus, have great significance for a wide range of applications. Therefore, a lot of methods have been established to produce ordered nanostructures of these functional inorganic materials.[1–11] Block copolymers[3–11] have recently emerged to the most active candidates due to their ability to form highly ordered structures by means of self-assembly from macro- to meso- and microscopic scales. For example, block copolymer lithography[4–8] utilizes the phase separation pattern with one block selectively removed as a mask for etching the exposed inorganic substrate via a series of lithographic procedures to produce ordered quantum dots a
: Supporting information for this article is available on the journal’s homepage at http://www.mrc-journal.de or from the author.
Macromol. Rapid Commun. 2003, 24, No. 8
or cylinder arrays. This method, although successful in creating high-density ordered array over large areas, is expensive and complicated for the involvement of the lithographic technique. Alternatively, amphiphilic block copolymers (ABC)[2,3,9–11] can self-assemble in selective solvents to produce core-shell micelles in which the cations or anions preferentially segregate in the core or shell and react to create inorganic nanoparticles directly, depending on the specific affinity between the polymer block and the ions. These inorganic/micelle nanohybrids can self-assemble to form highly ordered arrays on a substrate and, after removing the polymers, ordered nanoparticles are produced that can either show some special properties themselves or act as a mask for further lithographic application to pattern other materials. However, this ABC-based strategy is primarily determined by the block copolymer component and appropriately selected solvent in which the copolymer can produce micelles suitable for application as nanoreactors. Therefore, only a few ABC/selective solvent
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systems have been developed that are suitable for creating highly ordered nanostructures. In addition, other highly aligned microphase-separated periodic structures of block copolymers (e.g., hexagonal cylinders)[12,13] can – after removal of the dispersed phase – serve as template within which the growth of high-density and high-order nanowires and/or nanocylinders of metals and semiconductors can occur. Undoubtedly, these methods are highly successful in creating ordered nanostructures with specific arrangements and properties, and sizes and structures can be manipulated within a fairly wide range.[10] However, most of the current approaches toward the involvement of lithographic technique or selective degradation of one block are expensive and complicated. Therefore, it is an urgent task to develop convenient and cheap strategies to produce nanostructured materials. The present work introduces a novel convenient strategy to fabricate a metal particle array in a macroporous template, self-assembled from a functional two-armed polymer with crown ether core. This molecular structure is designed to combine the assembling ability of crown ether[14] and the flexibility of polymer chains. When spin coated onto the substrate from solutions, macroporous films are obtained with pores perpendicular to the substrate that are subsequently utilized as template for the deposition of Cu particles by rapid reduction of Cu2þ in acidic solution.
Experimental Part Sample Preparation Two model polymers, [poly(methyl methacrylate)]-dibenzo18-crown-6-[poly(methyl methacrylate)] (1), with different molecular weights (1a: M n ¼ 7 200, M w/M n ¼ 1.18, and 1b: M n ¼ 36 700, M w/M n ¼ 1.25) were synthesized by atomtransfer radical polymerization (ATRP).[15] Powdered polymers 1a and 1b were dissolved in purified tetrahydrofuran (THF), toluene, chloroform and 1,4-dioxane, respectively, to form solutions with different concentrations. The solutions were spin coated onto freshly cleaned silicon wafers with native oxide cover to produce thin films. The films were dried in vacuum at 50 8C for more than 24 h to remove residual solvent. Film thickness was determined by ellipsometry (AUEL-3, Xi’an Jiaotong University, Xi’an, China). Fabrication of Cu Particles and XPS Characterization The silicon wafer covered with self-assembled membrane (SAM) was adhered to a copper sheet with CuO on the surface. The other side of the Cu sheet was adhered to an Al sheet. This sandwiched setup was then immersed in 10 mol/L aqueous hydrochloric (HCl) acid for 3–10 s, and subsequently rinsed with deionized water. Finally, the polymer SAM on the silicon wafer was removed by ice acetic acid, and then rinsed with deionized water. The wafer was blown dry in an N2 flow. The resulting wafer surface was then characterized by means of Xray photoelectron spectroscopy (XPS; VG ESCALAB MK, VG Company, UK).
Atomic Force Microscopy (AFM) Characterization All AFM images were obtained by scanning the samples under a commercial scanning probe microscope (SPA300HV with a SPI3800N Probe Station, Seiko Instruments Inc., Japan) in dynamic force mode (DFM; tapping mode as referred in this paper) by using a silicon cantilever with an integrated 49 nm tip characterized by scanning over very sharp needle array (< 10 nm, NT-MDT, Russia) before use. All analyses of the images were conducted in the integrated software.
Results and Discussion It is well-known[14] that the polar crown ether rings with crown-like stereo configuration have good affinity to each other so that crown-ether-containing compounds can form regular structures. In particular, the crown ether core in functional two-armed polymer 1 may assemble to form supramolecules (2 in Figure 1), in which the crown ether rings overlap to form a channel surrounded by flexible polymer corona. Normally, such supramolecules tend to self-assemble spontaneously or self-organize to form larger structures[16] and lead to entanglements of the hairy polymer corona which may stabilize the secondary supra-structure to some extent. In solution, however, the self-assembly of supramolecule 2 has to compete with the solvent effect, i.e. the polymer tends to dissolve in solution. Under proper conditions, these competing effects may compromise to form a hollow cylindrical structure (3 in Figure 1). To demonstrate the solvent effect relative to the proposed assembling mechanism, different solvents including THF, CHCl3, toluene, and 1,4-dioxane were used to produce solutions of polymer 1a for creating the proposed selfassembled structures. THF solutions were suitable to produce macroporous structures (Figure 1) while solutions in other solvents resulted in flat thin films only. In addition, polymer 1b of higher molecular weight (MW) as compared with 1a did not give assembled formations, no matter which of the solvents mentioned above was used. Such dependence on solvents and MW clearly demonstrate the effect of the solvent on the assembling mechanism. Good solvent or large MW usually mean strong polymer/solvent interaction that can decompose supramolecule 2 so that the assembling process is prohibited. However, this solvent dependence indicates the possibility of changing the assembly formation by varying the concentration of the solution. Furthermore, complexing with potassium cations prevents the crown ether cores from forming regular structures.[17] This phenomenon indirectly supports the rationality of supramolecule 2 and corresponding structure 3 in the proposed assembling process (Figure 1). Figure 1 also displays a representative morphology of the pore arrays of a thin film spin coated from an 0.5 wt.-% solution of polymer 1a in THF. On average, these holes are about 80 nm in diameter and 30 nm in depth. As discussed above, the polymer-solvent effect is a key factor in
Fabrication of a Metal Particle Array Based on a Self-Assembled Template . . .
Figure 1. Schematic representation of possible assembling process and typical surface morphology of a self-assembled film from a 0.5 wt.-% THF solution of polymer 1.
influencing the assembly formation. By changing concentration of the polymer/THF solutions, the pores vary in diameter and arrangement (Figure 2). As the concentration is decreased from 0.5 wt.-% to 0.25 wt.-% to 0.1 wt.-%, the average diameter of the pores increases from about 80 nm to 300 nm and 450 nm, respectively. A more diluted solution means a larger solvent effect that can swell supramolecule 3 and results in an expansion of pore diameters. When the solution concentration is further decreased (e.g., 0.08 wt.-%), only isolated island structures with many tiny holes are obtained. In contrast, films spin cast from more concentrated solutions (e.g., 1.0 wt.-%) show no apparent porous structure according to AFM. Therefore, these two cases will not be considered for the following applications.
On the other hand, the depths of these pores are about 30, 25 and 14 nm, respectively, according to the corresponding AFM images (Figure 2). These depth values are consistently a bit higher than the corresponding film thicknesses as determined by ellipsometry, i.e., 26.1 0.4, 16.9 0.3 and 7.2 0.3 nm, respectively. This difference between AFM and ellipsometry values originates from the macroporous structure where the vertical fluctuation in thickness between the film and the pores cannot be neglected. Therefore, the occupancy of the pores on the films should be discounted to unify these data of film thickness and pore depth. From Figure 2a to 2c, the substrates are actually covered with the films by fractions of 0.855, 0.662 and 0.560, respectively. These fractions combined with the
Figure 2. Morphology evolution with decreasing solution concentration: (a) 0.5, (b) 0.25, and (c) 0.1 wt.-%.
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Figure 3. Schematic illustration of the deposition of nanoparticles into self-assembled templates.
corresponding ellipsometry film thickness and pore depth values immediately lead to the conclusion that there should be no film at the bottom of the holes. Namely, the pores are through the film so that the substrate is exposed to the air wherever the holes exist. This point could also be confirmed by means of XPS. The through-hole structure is the most distinguished advantage of the self-assembled macroporous films because it can be utilized for the fabrication of inorganic arrays on the substrate (in fact, highly ordered macroporous films[18] are obtained by evaporating the solvents slowly but the resulting pores have bottoms that make them unsuitable for this application). An experiment depositing elementary Cu particles was performed as schematically represented in Figure 3. During this process, the CuO on the copper sheet surface rapidly reacts with the acid to produce Cu2þ in acidic solution. At the same time, Al vigorously reacts with the acid to produce active hydrogen atoms, which can reduce Cu2þ to elementary Cu. The metallic Cu precipitates onto the self-assembled film, both on the surface and into the holes. After removing the polymer films by ice acetic acid, isolated Cu particles are observed by means of tapping
mode AFM (Figure 4b). XPS data demonstrate the presence of elementary Cu on the silicon wafer surface. Figure 4a and 4b show the template and the corresponding Cu particles. The holes have average diameters of about 300 nm, while the average diameter of the Cu particles is about 290 nm and the height (about 19 nm) is lower than the corresponding pore depth (ca. 25 nm). An AFM scan over 1.5 1.5 mm2 area reveals that the Cu particles are composed of nanoparticles, which are apparently about 60 nm in diameter and 13 nm in relative height (inset in Figure 4b). This abnormal difference between apparent particle diameter and height should be assigned to the tip geometry effect[19] that is prevalent when the particle size is close to that of the tip. According to Zenhausern et al.,[19] the corrected value for the nanoparticle diameter should be 9 nm, which is smaller than the relative particle height, indicating that the particles are accumulated from nanoparticles. As the Cu nanoparticles are deposited from Cu2þ which in turn was generated in a diffusion-controlled rapid reaction in solution with very low concentration of Cu2þ, the Cu particles hardly grow before being deposited onto the solid surface with uniform size. However, small particles are also evidenced as the
Figure 4. Typical morphologies and cross profiles of (a) the self-assembled macroporous template and (b) the corresponding array of Cu particles composed of nanoparticles (inset in (b)).
Fabrication of a Metal Particle Array Based on a Self-Assembled Template . . .
background in the inset of Figure 4b reveals. This might be due to the penetration of Cu particles through very small, invisible holes (Figure 4a).
Conclusions A two-armed polymer with crown ether core is demonstrated to self-assemble to cylindrical pores on silicon wafers. This assembling process only occurs in THF solution while good solvents and large MW do not favor such assembling behavior. In general, the pore diameters increase with decreasing solution concentration, indicating that the solvent effect plays an important role. The cylindrical pores are exclusively perpendicular to the substrate and reach through the film down to the substrate. This structure provides a template for the fabrication of arrayed Cu nanoparticle aggregates. This new approach to fabricate inorganic cylinder arrays is convenient and cheap because it is free of any complicated or expensive process.[4–7] However, better control of both the template structure and the nanoparticles is being pursued. Acknowledgement: This work was subsidized by the National Natural Science Foundation of China (general program: 20023003, 20274050 and major program: 50290090), the National Science Fund for Distinguished Young Scholars of China (50125311), and the Ministry of Science and Technology of China for Special Pro-funds for Major State Basic Research Projects (2002CCAD4000). The authors also thank the Chinese Academy of Sciences for funding through the Distinguished Talents Program and Intellectual Innovations Project (KGCX2205-03), and Jilin Province for Distinguished Young Scholars Fund (20010101).
Received: March 19, 2003 Revised: April 11, 2003 Accepted: April 11, 2003
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