Research Article www.acsami.org
Engineering Hybrid Metallic Nanostructures Using a Single Domain of Block Copolymer Templates Zhicheng Liu,†,‡,§ Tongxin Chang,‡,§ Haiying Huang,*,‡ and Tianbai He*,‡ ‡
State Key Laboratory of Polymer Physics and Chemistry, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun 130022, China § University of Chinese Academy of Sciences, Beijing 100049, China S Supporting Information *
ABSTRACT: Building complex nanostructures using a simple patterned template is challenging in material science and nanotechnology. In the present work, three different strategies have been exploited for the successful fabrication of hybrid dots-on-wire metallic nanostructures through combining an insitu method with an ex-situ method. Basically, plasma etching was applied to generate a metallic wire-like nanostructure, and preformed nanoparticles could be placed through multiple means before or after the formation of the wire-like nanostructure. Various monometallic and bimetallic nanostructures have been obtained by utilizing only one functional domain of block copolymer templates. In these cases, full utilization of the functional domain or introduction of the molecular linker is critical to engineering hybrid metallic nanostructures. Other complex and multifunctional hybrid nanostructures can be developed via these strategies similarly, and these nanostructures are promising for useful applications such as optics and surface-enhanced Raman spectroscopy (SERS). KEYWORDS: block copolymers, nanostructures, patterning, templates, SERS, nanoparticle copolymer templates.49−52 Though extensive studies were conducted using one of these methods, only a few reports were found to build metallic nanostructures using a combined strategy of in-situ and ex-situ.53,54 In order to obtain targeted hybrid nanostructures, which are important for the development of complex and multifunctional devices, one must choose diverse nanoscale building blocks. Almost at the same time, the pioneering works of Sohn et al. and Bockstaller et al. presented the first attempt to assemble two different types of nanoparticles in block copolymer thin films.54,55 Later, other binary mixtures of metal nanoparticles were obtained similarly, and used as functional devices.56−61 It is worth noting that such hybrid nanostructures were constructed by utilizing two phases of block copolymer or a mixture of different micelles. Recently, Kim and co-workers showed that plasmonic nanonecklace arrays could be generated by sequential introducing metallic precursors into one phase of block copolymer templates.62 This hybrid linear bimetallic nanostructure is exquisite and easy to process. And yet, it remains a challenge to obtain similar hybrid nanostructures simply by using a single domain of block copolymer templates, which would offer possibilities for the fabrication of more
1. INTRODUCTION Metallic nanostructures are always a hot topic in nanoscience and nanotechnology due to their unique and fascinating electronic, optical, and magnetic properties.1,2 And rational design and fabrication of such functional structures are of particular interest to scientists. Over the past two decades, microphase-separated block copolymer thin films have acted as excellent platforms for the fabrication of well-defined metallic nanostructures.3−6 Block copolymer-templated metallic nanostructures have been wildly investigated both theoretically and experimentally.7−18 Generally, there are two different strategies to obtain metallic arrays: in-situ and ex-situ. For the in-situ method, a range of metallic precursors can be easily incorporated into the microdomains of block copolymer templates, followed by chemical reduction or plasma etching to transform the metallic precursors into desirable nanostructures.19−29 Not only patterned nanodots but also aligned nanowires could be fabricated this way. As for the ex-situ method, numerous preformed nanoparticles were assembled with proper block copolymer templates.30−48 Capillary force, metal coordination, and electrostatic interaction were used to direct the nanoparticle assembly, and these driving forces basically originate from the interactions between nanoparticles and the block copolymer domain. Moreover, nanospheres as well as anisotropic nanoparticles could be placed in or on the block © 2015 American Chemical Society
Received: September 16, 2015 Accepted: October 30, 2015 Published: October 30, 2015 25938
DOI: 10.1021/acsami.5b08751 ACS Appl. Mater. Interfaces 2015, 7, 25938−25945
Research Article
ACS Applied Materials & Interfaces
Figure 1. Schematic representation of three different routes for the fabrication of hybrid dots-on-wire metallic nanostructures using PS-b-P2VP block copolymer templates. applied to generate hybrid dots-on-wire metallic nanostructures. As for Route 2, PS-b-P2VP block copolymer templates were immersed into 10 mg mL−1 HAuCl4 solution for 30 min. Then gold wire-like nanostructures emerged through a short oxygen plasma etching (30 W, 50 mTorr, 45 s). The substrates with wire-like nanostructures were submerged into a gold nanoparticle solution for 30 min. As for Route 3, the treatment of PS-b-P2VP block copolymer templates was the same as Route 2 at first. After that, a longer exposure to oxygen plasma (30 W, 50 mTorr, 55 s) was used to obtain the gold wire-like nanostructure. These wire-like nanostructures on the substrate were dipped into a 1 mM ethanol solution of 4,4′-thiobisbenzenethiol (TBBT) overnight, and rinsed with ethanol to remove unbonded TBBT molecules. Finally, the modified wire-like nanostructures were submerged into a gold nanoparticle solution for 30 min. In order to remove unbonded species, a gentle washing step with water was performed after each immersion of metal precursor or nanoparticle solution. 2.5. Characterizations. The morphology of the PS-b-P2VP block copolymer template was characterized using tapping mode atomic force microscopy (AFM, Bruker). The preformed gold nanoparticles in aqueous solution were observed by TEM (JEOL JEM 1011). The formed metallic nanostructures in this work were characterized using not only a Hitachi S-4800 field emission scanning electron microscope (SEM) operating at 5 kV, but also a THERMO ESCALAB 250 X-ray photoelectron spectrometer (XPS, Al Kα X-rays with photon energy of 1486.6 eV). The UV−vis spectra of the metallic nanostructures were recorded by a Hitachi U-3900 UV−vis spectrophotometer. Ramanscattering spectra were obtained on a LabRam HR 800 confocal Raman microscope (1.8 mW, 633 nm) with a backscattering configuration. Each spectrum was an average of three acquisitions, and the exposure time was 10 s.
complex and multifunctional nanodevices by fully utilizing each phase of the block copolymer template. Here, we present that a novel hybrid dots-on-wire metallic nanostructure could be created using a block copolymer template through three facile routes. From the methodology point of view, these original routes combine not only top-down and bottom-up processings, but also in-situ and ex-situ methods. To the best of our knowledge, this is the first time that a hybrid dots-on-wire nanostructure has been designed and prepared using a block copolymer template. More importantly, only one functional block of the block copolymer template was utilized to achieve the ordered hybrid metallic pattern. Besides, we also demonstrate that the hybrid metallic nanostructures have potential in the optical and SERS fields.
2. EXPERIMENTAL SECTION 2.1. Materials. Cylinder-forming polystyrene-block-poly(2-vinylpyridine) (PS-b-P2VP) block copolymer (62 kg mol−1 for PS, 26 kg mol−1 for P2VP) was purchased from Polymer Source, Inc., and used as received. Other chemicals were provided by Sinopharm Chemical Reagent Co., Ltd. (China). 2.2. Preparation of block copolymer thin film. 0.5 wt % PS-bP2VP chloroform solution was spin-coated onto clean silicon substrates at 3000 rpm for 60 s. Then the block copolymer thin film was annealed for about 1.5 h in a chamber of mixed solvent vapor of toluene and THF, with a volume fraction of 3/7. 2.3. Gold nanoparticle synthesis. The water-soluble citratestabilized gold nanoparticles were synthesized according to the classical Turkevich method.63 In a typical procedure, 1 mL of 1% aqueous hydrogen tetrachloroaurate (HAuCl4) solution was mixed with 100 mL of water in a flask. The solution was heated to boiling under reflux. Then 4 mL of 1% sodium citrate solution was rapidly added into the boiling solution under heating and stirring. The solution was refluxed for an additional 15 min until it turned to a wine red color. The nanoparticles were purified by centrifugation and redispersed in 100 mL of deionized water. 2.4. Fabrication of hybrid dots-on-wire nanostructures. There were three routes for the engineering of hybrid metallic nanostructures. As for Route 1, PS-b-P2VP block copolymer templates were immersed into a gold nanoparticle solution for different periods of time. Then the templates were dipped into various metallic precursor solutions. Oxygen plasma etching using different recipes was
3. RESULTS AND DISCUSSION PS-b-P2VP diblock copolymer is ideal for the formation of metallic nanostructures, because the P2VP phase is able to bind various metallic ions or nanoparticles.2,40,59 Using a solvent annealing process, the asymmetric PS-b-P2VP block copolymer was phase-separated into lying P2VP cylinders embedded in the PS matrix. The domain spacing was about 50 nm, as shown in Figure S1a (Supporting Information). This cylinder-forming block copolymer template is suitable for both the ex-situ deposition of preformed nanoparticles and the in-situ formation of metallic wire-like nanostructures. Moreover, in order to 25939
DOI: 10.1021/acsami.5b08751 ACS Appl. Mater. Interfaces 2015, 7, 25938−25945
Research Article
ACS Applied Materials & Interfaces
Figure 2. SEM images of dots-on-wire nanostructures using Route 1. The assembly time of gold nanoparticles on the P2VP phase was 15 min (a, c, d) and 45 min (b), respectively. The recipes of the oxygen plasma etching: 30 W, 50 mTorr, 55 s (a, b, d); 50 W, 50 mTorr, 30 s (c). Both gold wires (a, b, c) and palladium wires (d) were generated on the substrates. The insets show the corresponding cross-sectional images.
block copolymer template into the nanoparticle solution for different periods of time, one can obtain gold nanoparticle arrays with different particle densities on the template (Figure 2a, b). The longer the assembly time is, the denser the gold nanoparticle array is.49 Following the nanoparticle assembly, metallic wire-like nanostructures were expected to be integrated under the assembled nanoparticles. As shown by the Buriak group, numerous kinds of metal salts could be loaded into the P2VP cylinders through electrostatic attraction.22,64 The polymer is then removed by plasma treatment, resulting in the emergence of metallic wire-like nanostructures. Here, in our cases, we supposed that only a part of the P2VP chains migrated to the template surface and bound with gold nanoparticles, while other parts of the protonated P2VP chain were still able to incorporate metallic precursors through electrostatic attraction and possible chelation. Hence, different metal salts such as HAuCl4 (10 mg mL−1, 30 min) and Na2PdCl4 (10 mM, with 0.9% HCl, 12 h) could be used to efficiently generate the metallic wire-like nanostructures. Using oxygen plasma etching (30 W, 50 mTorr, 55 s), hybrid dots-onwire nanostructures were obtained easily (Figure 2a, b, and d). In this way, monometallic (Au dots on Au wires) as well as bimetallic (Au dots on Pd wires) nanocomposites were accomplished. Clearly, the insets of Figure 2 show that most of the gold nanoparticles were placed exclusively on the metallic wire-like nanostructures, which duplicated the pattern of P2VP cylinders. No obvious change of the deposited gold nanoparticles was noticed before or after the plasma etching process. However, when the immobilized gold nanoparticles were close enough, larger nanoparticles which might consist of two or more gold nanoparticles were formed under the plasma irradiation (Figure 2b). Furthermore, a higher power of oxygen plasma (50 W, 50 mTorr, 30 s) was also applied to acquire the hybrid nanostructure. Interestingly, not continuous wires but
obtain desirable nanostructures, preformed ligand-protected nanoparticles are required as another building block. In our work, a negatively charged citrate-capped gold nanoparticle dispersed in water was prepared according to the classical Turkevich method.63 The as-synthesized gold nanoparticles had a diameter of 16.9 ± 1.7 nm, and their size was comparable with the size of P2VP cylinders (Figure S1b, Supporting Information). Figure 1 schematically illustrates three different ways to fabricate hybrid metallic nanostructures using PS-b-P2VP block copolymer templates (from top to bottom: Route 1, Route 2, and Route 3). Generally, oxygen plasma etching was applied to generate metallic wire-like nanostructures, and gold nanoparticles could be placed through multiple means before (Route 1) or after (Route 2 and 3) the formation of wire-like nanostructures. In order to form metallic wire-like nanostructures, in-situ binding of metallic ions into the P2VP cylindrical phase took place. And the ex-situ preformed gold nanoparticles were immobilized on either the P2VP chains or the formed wire-like nanostructures. By taking advantage of only one functional phase of block copolymer templates, novel hybrid dots-on-wire nanostructures could be achieved. What is more, these three routes are facile and controllable, which will be discussed in detail below. As for Route 1, the placement of preformed gold nanoparticles was completed before the formation of metallic wire-like nanostructures. When PS-b-P2VP block copolymer template was immersed into gold nanoparticle solution, the P2VP phase was swollen and protonated rapidly. Then the positively charged polymer chains penetrated the upper PS layer and made direct contact with the nanoparticle solution. And the negatively charged gold nanoparticles were selectively located on the exposed positively charged protonated P2VP phase mainly through electrostatic interaction.49 By dipping the 25940
DOI: 10.1021/acsami.5b08751 ACS Appl. Mater. Interfaces 2015, 7, 25938−25945
Research Article
ACS Applied Materials & Interfaces
used to anchor two metallic nano-objects.66 The strategy of the utilization of molecular linkers would enable the design and construction of hybrid metallic nanostructures templated by a block copolymer thin film, as shown in Route 3 (Figure 1). At first, the PS-b-P2VP block copolymer template was immersed into aqueous HAuCl4 solution and subsequently treated with a long oxygen plasma etching (30 W, 50 mTorr, 55 s). A typical fingerprint-like continuous gold array was generated without the presence of any residual P2VP polymers (Figure 4a, the
wires with small gold nanoclusters (
