Letter pubs.acs.org/Langmuir
Site-Selective Assembly and Reorganization of Gold Nanoparticles along Aminosilane-Covered Nanolines Prepared on Indium−Tin Oxide Jeonghyeon Yang, Takashi Ichii, Kuniaki Murase, and Hiroyuki Sugimura* Department of Materials Science and Engineering, Kyoto University, Sakyo, Kyoto, 606-8501, Japan ABSTRACT: We have fabricated gold nanoparticle (AuNP) arrays on indium−tin oxide (ITO) substrates in a nearly one-dimensional fashion. AuNPs were siteselectively immobilized on ITO of which the surface had been patterned by a nanolithography process based on scanning probe microscopy. The fabricated nanoscale lines covered with aminosilane self-assembled monolayer served as chemisorption sites for citrate-stabilized AuNPs of 20 nm in diameter, accordingly, AuNP nanolines with a thickness of single nanoparticle diameter were spontaneously assembled on the lines. In this 1D array, the AuNPs were almost separated from each other due to the electrostatic repulsion between their negatively charged surface layers. Furthermore, a reorganization process of the immobilized AuNP arrays has been successfully demonstrated by replacing each AuNP’s surface layer from citric acid to dodecanethiol. By this process, the AuNPs lost their electrostatic repulsion and became hydrophobic so as to be attracted to each other through hydrophobic interaction, resulting in reorganization of the AuNP array. By repeating the deposition and reorganization cycle, AuNPs were more densely packed. The optical absorption peak of the arrays due to their plasmonic resonance was found to shift from 526 to 590 nm in wavelength with repeating cycles, indicating that the resonance manner was changed from the single nanoparticle mode to the multiple particle mode with interparticle coupling. scanning probe lithography.30−34 Onto such a microfabricated SAM template, AuNPs are automatically aligned along with the minute pattern bearing a particular chemical function.35 Commercially available citrate-stabilized AuNPs are known to show specific chemisorption behaviors to an aminoterminated SAM surface through attractive electrostatic interaction between the AuNPs, which are negatively charged due to the their capping citric acid layer, and the aminoterminated SAM surface, which is positively charged due to the protonation of the amino groups in the weak acid condition of the Au colloid solution.6,8,9,22−25,34,35 In particular, Jiang et al. have fabricated one-dimensional AuNP arrays with a tunable interparticle distance.36,37 They deposited AuNPs in grooves with an aminosilane-covered bottom and showed that the interparticle distance was tunable by controlling the groove width or the nanoparticle size. Although these research results were successful for the fabrication of AuNP arrays with one- or two-dimensional artifical designs, the AuNPs were almost completely separated from each other due to the repulsive interaction between the negatively charged AuNPs. In order to obtain gap-mode plasmonic functions from the arrays, the AuNPs distance should be more closely arranged together. Here, we report on the fabrication of one-dimensional AuNP arrays with single NP thickness. AuNP lines were fabricated on
1. INTRODUCTION Metallic nanoparticles and their assemblies have attracted extensive attention in the past decade.1−11 In particular, gold nanoparticles (AuNPs) are of primary interest in the fields of nanoscale photonics, optical sensing, and other advanced applications due to their unique optical properties related to surface plasmon resonance.7−16 Such plasmonic functions are governed by the size and shape of AuNPs, interparticle distances in the AuNP assembly, and the surrounding media.7−20 The assembly of AuNPs into a desired structure with controlled particle positions is necessary for advanced applications of AuNPs, and many studies have been conducted to this end.5,6,9,21−25 Among various techniques for positioning AuNPs on a solid substrate, the site-selective self-assembly of AuNPs is a powerful method to construct AuNP arrays with a spatial pattern demanded for applications. In this process, each AuNP spontaneously chemisorbs on an area prepared on the substrate with an intentional design having a specific chemical affinity to the AuNP.6,9,21−24 This chemisorption process itself is very simply attained by immersing the sample into a Au colloid solution for a while without any complicated procedures. In order to provide such a chemical affinity to the sample surface, self-assembled monolayers (SAMs), with which the substrate’s surface can be terminated with a particular functional group, are frequently employed.6,26−29 SAMs are known to be patternable into micronanometric designs in a reproducible manner by a variety of lithographic methods including photolithography, electron-beam lithography, and © 2012 American Chemical Society
Received: March 12, 2012 Revised: April 30, 2012 Published: May 7, 2012 7579
dx.doi.org/10.1021/la301042y | Langmuir 2012, 28, 7579−7584
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Letter
an indium−tin oxide (ITO) substrate on which nanoscale lines covered with aminosilane SAM had been prepared by scanning probe lithography (SPL). ITO is well-known to be applicable to observing the surface plasmon resonance.38 In addition, it is compatible with forming a SAM through silane coupling chemistry and is adaptable to the patterning by SPL which requires a conductive substrate.25,34,35 Onto these aminoterminated nanolines, citrate-capped AuNPs were self-aligned in a separated manner. Furthermore, we reorganized the deposited AuNP lines by modifying the NP surfaces with hydrophobic molecules. We have previously reported that citrate-capped AuNPs deposited on an aminosilane SAM surface could be reorganized by changing the surface molecular layer surrounding each of the AuNPs from citric acid to alkanethiol.25 The alkanethiol-modified AuNPs became almost neutral, losing their negative charges provided from the adsorbed citric acid molecules. The repulsive interaction between the nanoparticles disappeared as well. In fact, an attractive interaction, so-called hydrophobic interaction, became apparent due to the hydrophobic nature of the modified AuNPs. Accordingly, most of the separated AuNPs moved to become adjacent, resulting in the formation of a new assembled structure which showed a plasmonic property different from that of separated AuNPs.25 This neutralization process has been applied to AuNP line structures in order to fabricate more closely packed AuNP linear arrays. We have also examined the optical properties arising from interparticle coupling in the AuNP arrays fabricated by the neutralization process.
Scheme 1. Procedure for Fabrication of AuNP Arrays on ITO Substratea
2. EXPERIMENTAL METHODS Glass substrates covered with a film of ITO (thickness = 150 nm, rms roughness = 0.4 nm over 5 × 5 μm2) were purchased from Kuramoto Co., Ltd. Octadecyltrimethoxysilane (ODS) and 3-aminopropyltriethoxysilane (APS) were purchased from Gelest Inc. and Sigma-Aldrich Co., respectively. Trimethoxy(propyl)silane (TPS) and 1-dodecanethiol (DDT) were purchased from Tokyo Chemical Industry Co., Ltd. All the reagents were used as received. A colloidal solution of AuNPs (Φ = 20 ± 3 nm) was purchased from Sigma-Aldrich. Ultrapure water (UPW) with a resistivity of 18.2 MΩ was prepared using a water purification apparatus (RFD 250NB, Toyo Seisakusho Kaisha, Ltd.). The other reagents were of analytical grade and were used as received. Scheme 1 depicts the experimental procedures for the site-selective assembly of AuNPs and reorganization of the assembled AuNPs. First, an ITO substrate surface was covered with an alkylsilane SAM using ODS as a precursor by a vapor phase method.39 Prior to this SAM formation process, the ITO substrate was thoroughly sonicated in ethanol and UPW for 20 min, in that order. Subsequently, the ssubstrate was irradiated with vacuum ultraviolet (VUV) light at a wavelength of 172 nm for 20 min in air in order to remove surface organic contaminants and to introduce hydroxyl groups on it. This hydroxylated ITO substrate was then sealed in a Teflon container with 150 μL ODS under a dry nitrogen atmosphere. The Teflon container was kept at 150 °C for 3 h in an electric oven. Vaporized ODS molecules reacted with the hydroxyl groups on the ITO substrate so as to form a monolayer. The substrate was then sonicated with ethanol for 10 min and blown with a nitrogen gas stream. Next, this ITO substrate covred with the ODS-SAM was treated in another Teflon container together with 150 μL TPS at 110 °C for 1 h. By this TPS treatment, defects in the ODS monolayer were healed to some extent and the methyl-termination of ITO became more complete.34 The sample was sonicated in ethanol for 10 min and blown with dry N2. The methyl-terminated ITO substrate showed a water contact angle of 104 ± 1°. Second, nanoline patterns were fabricated on the methyl-terminated ITO substrate by atomic force microscope (AFM) lithography. An
a
Site-selective self-assembly and re-organization of AuNPs. (The methyl-terminated area consists of ODS and TPS molecules which have different molecule lengths as illustrated in (1) methyl termination step. From (2) to (4), this difference in the alkyl chain length is not depicted. We have highlighted the surface functional groups, that is, CH3, on the region covered with the ODS+TPS monolayer.).
AFM (MFP-3D-SA, Asylum Technology Co., Ltd.) was operated in the contact mode using a rhodium-coated silicon cantilever (tip radius
