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Direct Fabrication of Monodisperse Silica Nanorings from Hollow Spheres − A Template for Core−Shell Nanorings Kuo Zhong,† Jiaqi Li,‡,§ Liwang Liu,⊥ Ward Brullot,† Maarten Bloemen,† Alexander Volodin,§ Kai Song,*,∥ Pol Van Dorpe,‡,§ Niels Verellen,*,‡,§ and Koen Clays*,† †
Department of Chemistry, KU Leuven, Celestijnenlaan 200D, B-3001 Leuven, Belgium IMEC, Kapeldreef 75, B-3001 Leuven, Belgium § Laboratory of Solid-State Physics and Magnetism, Department of Physics and Astronomy, KU Leuven, Celestijnenlaan 200D, B-3001 Leuven, Belgium ⊥ Laboratory of Soft Matter and Biophysics, Department of Physics and Astronomy, KU Leuven, Celestijnenlaan 200D, B-3001 Leuven, Belgium ∥ Laboratory of Bio-inspired Smart Interface Science, Technical Institute of Physics and Chemistry, Chinese Academy of Sciences, Beijing 100190, China ‡
S Supporting Information *
ABSTRACT: We report a new type of nanosphere colloidal lithography to directly fabricate monodisperse silica (SiO2) nanorings by means of reactive ion etching of hollow SiO2 spheres. Detailed TEM, SEM, and AFM structural analysis is complemented by a model describing the geometrical transition from hollow sphere to ring during the etching process. The resulting silica nanorings can be readily redispersed in solution and subsequently serve as universal templates for the synthesis of ring-shaped core−shell nanostructures. As an example we used silica nanorings (with diameter of ∼200 nm) to create a novel plasmonic nanoparticle topology, a silica-Au core−shell nanoring, by self-assembly of Au nanoparticles ( L/v, the hole on the top surface increases in diameter d, and the bottom surface starts to be etched from the inside of the hollow sphere because the reactive species can penetrate through the hole in the top. It also takes t = L/v time to penetrate through the bottom part of the shell, so t = 2L/v is a critical value for a completely etched bottom surface. Once there is a hole on the bottom surface, the rate at which the height H changes is doubled (equal to 2v) since the etching occurs at both the top and bottom surfaces simultaneously. This is also observed in the experimental data in Figure 3B as a change in the slope of H, indicated by two arrows. Based on this discussion the evolution of the height of the hollow spheres can be expressed as ⎧ 2R − νt , ⎪ ⎪ H (t ) = ⎨ ⎪ 2R + 2L − 2νt , ⎪ ⎩
Next, we construct a simple model, illustrated in Figure 3A, in order to explain the formation mechanism of the nanorings. The model is placed in a Cartesian coordinate system with the center of the hollow sphere as the origin and the y-axis as the etch direction. In the plasma environment, the SF6 reactive species are accelerated perpendicularly to the sample in a unidirectional electric field.48 As a result, for the initial etching stage, the top surface of the hollow spheres is preferentially etched, leading to the formation of a hole in the top surface. The diameter of the hole, defined as the opening size d, is a function of the etching time t and can be described by the following equation L ν L R ≤t< ν ν
(2)
where R is the outer radius of the hollow spheres (R = r + L). Fits of eqs 1 and 2 to the experimental data points in Figure 3B show an excellent agreement (red and blue curves). Moreover, these two fitting curves now allow us to determine the etch rate v, which is found to be approximately 16 nm/min. It is evident that the change of H presents two different slopes. This indicates that the rate at which the height decreases doubles once the bottom surface completely opened. This inflection point is found at t = 3 min. As mentioned in the Introduction, one of the main benefits of our method is that the obtained nanorings can be readily redispersed in solution for further functionalization. To illustrate the possibilities this offers, we assemble Au nanoparticles (NPs) onto the silica template using an APTMS molecular linker layer.47 This way, we obtain a unique composite nanostructure consisting of a ring-shaped dielectric core with a metallic shell which also supports strong collective near-infrared plasmonic resonances. To investigate the morphology-dependent properties of the silica nanoring template-assisted assembled Au NPs, three types of samples were prepared (details can be found in the Experimental Section) and characterized by TEM and visibleNIR spectroscopy. Sample A contains only the isolated Au NPs dispersed in an aqueous solution, shown in Figure 4A (left). The TEM image shows their narrow size distribution with diameter determined ∼10 nm. The typical extinction band around 533 nm for Au NPs dispersed in water is shown in panel A (right).47 The other two samples consist of silica nanorings coated with Au NPs on their surface (self-assembly). Sample B has a partially coated surface, while for sample C the surface is completely covered. The TEM images in Figures 4B and 4C compare the surface morphology of the obtained composites. The corresponding optical spectra show that a low surface coverage broadens the single Au nanoparticle extinction band of panel A and induces a red-shift. This is a consequence of small particle aggregates forming on the ring’s surface.23−25 An important additional observation is that no distinct spectral features are induced by the bare silica nanorings. For the dense coverage of sample C, the TEM data show that a continuous Au coating is reached. This now has a profound impact on the optical properties: three pronounced extinction bands indicated by colored dotsappear in the near-infrared spectral window (panel C). Note that for the lowest energy band (pink
Figure 3. A) Schematic of the geometrical parameters used to describe the changes of the hollow spheres during the etching process. The equator section of the sphere model highlighted in gray represents the resulting ring-shaped structures. B) Diagram showing the evolution of the height H (blue dots) and opening size d (red squares) of nanosphere/nanoring structures for increasing etch time. Data points are determined from the SEM and AFM analysis shown in Figure 2. Blue and red curves are fits for H and d according to the eqs 1 and 2, respectively.
⎧ 0, ⎪ ⎪ d (t ) = ⎨ ⎪ 2 r 2 − (r − νt + L)2 , ⎪ ⎩
2L ν R 2L ≤t< ν ν
0
