Letter pubs.acs.org/NanoLett
Plasmonic Manipulation of Color and Morphology of Single Silver Nanospheres Ichiro Tanabe and Tetsu Tatsuma* Institute of Industrial Science, University of Tokyo, Komaba, Meguro-ku, Tokyo, 153-8505, Japan S Supporting Information *
ABSTRACT: Optical control of size, shape, or orientation of a metal nanoparticle is important for development of nanoscale optical devices and elements of photonic circuits. Thus far, however, independent control of two or more parameters has not yet been achieved. Here we place a simple spherical Ag nanoparticle on TiO2 with high refractive index and separate a plasmon mode localized at the Ag-TiO2 interface from the other mode distributed over the nanoparticle. Selective excitation of each mode gives rise to a corresponding morphological change and selective suppression of the plasmon mode, resulting in multicolor changes of scattering light from orange to red, green, or a dark color.
KEYWORDS: Localized surface plasmon resonance, silver nanoparticle, photoelectrochemistry, titanium dioxide, nanoparticle manipulation
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mode causes photo-oxidative dissolution of the whole particle surface and size reduction, resulting in a redshift of the scattering color from orange to red. In addition, simultaneous excitation of both modes leads to a dark color (i.e., scattering is suppressed). Thus, we achieved the multicolor changes, which have been possible only for particle ensembles with broadly distributed size and shape30,31 for a single Ag nanosphere. Anatase TiO2 films (∼150 nm thick) were prepared by a spray pyrolysis technique32 on Pyrex or ITO (indium tin oxide)-coated glass plates. Commercially available monodisperse spherical Ag nanoparticles protected with citric acid (diameter = 100 nm, pH ∼ 7.0, Sigma-Aldrich) were cast on the films (50 μL cm−2) and left for 3−6 h in dark. The Ag nanospheres on TiO2 (∼105 particles cm−2) were observed by dark-field microspectroscopy (C10027, Hamamatsu Photonics) combined with optical microscopy (BX-51, Olympus) with a 100× field lens, atomic force microscopy (AFM) (NanoNavi Station/SPA400, SII Nanotechnology) and scanning electron microscopy (SEM). The protecting agent on Ag nanoparticle was not removed, because the removal resulted in photoetching of the Ag nanoparticles during the dark-field observation. Figure 1a shows a typical dark-field image and corresponding scattering spectrum and a SEM image of the same single Ag nanoparticle on TiO2. The scattering spectrum is characterized by two plasmon bands in the visible region and the dark-field
etal nanoparticles and semiconductor nanoparticles (quantum dots) have attracted much attention for their characteristic optical, magnetic, and chemical properties different from bulk metal.1−4 In particular, their optical properties have been exploited for sensitizers and antennae for solar cells,5−8 light-emitting devices, 9,10 photocatalysts,6,11−13 chemical analyses,14 and biosensors.15,16 The use of single nanoparticles would allow development of nanoscale optical devices and elements of photonic circuits. For development of sophisticated devices, it is important to manipulate optical properties of nanoparticles. The properties can be controlled by regulating local dielectric environment14 and by taking advantage of electromagnetic coupling between two nanoparticles17,18 or a nanoparticle and a metal film.19 For manipulation of a single nanoparticle without modification of its environment, it is necessary to control the particle size,20,21 shape,20,22,23 or orientation.24,25 Remote control of those parameters by light is most desirable. In recent years, optical manipulation of diameter,22,23,26 aspect ratio,27 and tilt angle28,29 of plasmonic nanoparticles have been studied for tuning of their optical properties. Thus far, however, independent control of two or more parameters has not yet been achieved to the best of our knowledge. In the present work, we place a simple spherical silver (Ag) nanoparticle on titanium dioxide (TiO2) with high refractive index, and separate a plasmon mode localized at the Ag−TiO2 interface from the other mode distributed over the nanoparticle. Those two plasmon modes can therefore be excited independently. Selective excitation of the former mode gives rise to photooxidative dissolution at the interface and a blueshift of the scattering color from orange to green. Excitation of the latter © 2012 American Chemical Society
Received: August 6, 2012 Revised: August 31, 2012 Published: September 4, 2012 5418
dx.doi.org/10.1021/nl302919n | Nano Lett. 2012, 12, 5418−5421
Nano Letters
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In order to support the analysis, we also simulated scattering spectra of Ag nanoparticles on TiO2 and spatial distribution of electric fields around the nanoparticles by a FDTD (finitedifference time-domain) method via FDTD Solutions (Lumercial Solutions). The simulation domain (400 × 400 × 400 nm) consisted of 4 nm cubic cells and the central region (140 × 140 × 140 nm) was further meshed with a three-dimensional grid of 1 nm spacing. The dielectric functions of Ag and TiO2 were extracted from the data of Palik33 and Jellison,34 respectively. Profiles of electron oscillation at each plasmon band were simulated on the basis of the particle model shown in Figure 1b (inset). The nanoparticles used in the experiments are imperfect spheres with facets, and the TiO2 film is not perfectly smooth also. Therefore, the contact of Ag nanoparticles with the TiO2 film is more like a plane contact rather than a point contact. Thus we cut the bottom of a 100 nm diameter sphere and placed it on TiO2 for simulation. We changed the degree of cutting from 0 to 15 nm (Supporting Information Figure S1) and adopted the model cut by 8 nm. The simulated scattering spectrum has two resonance bands (Figure 1b), as does the experimentally obtained one. At the shorter peak wavelength (405 nm), the electric field is distributed over the nanoparticle (Figure 1c). On the other hand, at the longer peak wavelength (615 nm), the electric field is localized at the interface between Ag nanoparticle and TiO2 (Figure 1d). Hereafter, we call those two plasmon modes “full-surface mode” and “interface mode”, respectively. We also examined another model in which a Ag nanosphere dents into TiO2 and similar two plasmon bands were obtained as well (Supporting Information Figure S2). The real system should be in between those two models for limiting cases, which both show similar behavior. Schatz, Van Duyne, and co-workers35 have found that a Ag nanocube on a glass substrate exhibits two plasmon bands (distal and proximal plane modes), while its calculated spectrum shows only one peak without the glass plate. However, the band separation is not observed in the case of
Figure 1. (a) Typical dark-field image and corresponding scattering spectrum and a SEM image of the same single Ag nanoparticle on TiO2. (b) Scattering spectrum and (c,d) spatial distributions of electric field intensity for (c) the full-surface and (d) interface modes of a Ag nanoparticle on TiO2 (inset in b) simulated by a FDTD method.
image exhibits orange scattering light, which coincides with the spectrum.
Figure 2. Typical (a−c) scattering spectra and (d−f) dark-field images before and after (a,d) 460−500 nm, (b,e) 600−700 nm, or (c,f) 460−700 nm light irradiation (10 mW cm−2 for 5 min). 5419
dx.doi.org/10.1021/nl302919n | Nano Lett. 2012, 12, 5418−5421
Nano Letters
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spherical nanoparticles placed on a glass substrate both in an experiment (Supporting Information Figure S3a) and in simulation with the bottom-cut model (Supporting Information Figure S3b). This is due to low refractive index of glass (1.46) and poor contact with it. On the other hand, the nanocube allowed the band separation due to a large contact area with glass. In this study, the TiO2 substrate with high refractive index (∼2.5) caused the band separation even for the spherical nanoparticles. The band separation allows selective optical excitation of each plasmon mode. On this basis, we tried selective photoetching of the excited sites and color manipulation of single Ag nanoparticles. The Ag nanoparticles on TiO2 were irradiated with 460−500, 600−700, or 460−700 nm light (10 mW cm−2, 5 min) at 30−40% or
