Letter pubs.acs.org/NanoLett
From Rings to Crescents: A Novel Fabrication Technique Uncovers the Transition Details Vladimir E. Bochenkov*,‡,† and Duncan S. Sutherland*,‡ ‡
Interdisciplinary Nanoscience Center (iNANO), Aarhus University, Aarhus, Denmark Chemistry Department, Lomonosov Moscow State University, Moscow, Russia
†
S Supporting Information *
ABSTRACT: A novel fabrication route is reported for the generation of substrate-supported symmetric and asymmetric metal nanostructures. We combine a colloidal template and angled evaporation to deposit in situ mask materials for subsequent lithographic pattern transfer. The technique is demonstrated for the fabrication of concentric and nonconcentric gold rings and crescents. Optical properties of localized plasmon resonances in such structures are studied by UV−vis−NIR spectroscopy and finite-difference time domain simulations during the transition from rings to crescents revealing the development of strong quadrupolar modes KEYWORDS: Fabrication, nanorings, asymmetric rings, nanocrescents, colloidal lithography, plasmonics
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introduced, uses a sacrificial layer in combination with a colloidal templated perforated thin film as a pattern transfer mask.5 Masks are produced by the evaporation of the material through the nanosphere mask assembled on the sacrificial polymer layer followed by particle removal. Holes etched in the sacrificial layer are used for example as lift off evaporation masks. This method allows fabrication of the new nanostructure types, such as cones, as well as more complex structures, such as disc pairs of the same or different material.14 Very recently, it has been used for the fabrication of split rings.15 A current fast developing area in plasmonics exploits symmetry-broken structures, since the reduction of symmetry can lead to a range of interesting phenomena. For example the appearance of otherwise dark modes, which can be used for metamaterials engineering,16,17 the generation of significant near field enhancements relevant for surface enhanced spectroscopies,18,19 the improvement of high refractive index sensitivity,20 or demonstration of plasmonic interference effects, such as through a Fano-type resonance.21 Plasmonic structures with reduced symmetry have been produced using top-down methods, such as electron beam over small areas.22−24 While being very accurate and controllable in small numbers, these methods lack the simplicity and high throughput capability of
lasmonic nanostructures are a topic of strong current interest due to their capability to confine and manipulate light at the nanoscale and the potential for broad application, such as for surface-enhanced spectroscopies, for biosensors and as metamaterials.1−3 The recent progress in the field of plasmonics is to a large extent determined by the development of efficient and reliable experimental methods for their fabrication and characterization. Among them, there is a subfamily of colloidal lithography techniques, which allow fabrication of nanostructures of different geometry and size over large area surfaces (several cm2).4−7 Such self-assembly based approaches are attractive due to their time- and costefficiency compared to the top-down methods. These techniques are based on the self-organization of colloidal particles on surfaces, which form the nanofabrication template used for further surface patterning via the combination of deposition and/or etching steps. The systematic variation of process parameters (deposition angle, thickness, evaporation technique) and particular steps sequence has allowed the production of diverse nanostructures. Two main approaches can be identified among the reported methods. The first one utilizes the colloidal particles directly as an etch and/or deposition mask. This approach has been successfully applied to produce arrays of various metallic nanostructures8 such as triangular pyramids,9 disks,10 crescents,11 rings,12 and holes in optically thin metal films.13 The other general approach, hole mask colloidal lithography, which has more recently been © 2013 American Chemical Society
Received: December 19, 2012 Revised: February 20, 2013 Published: February 25, 2013 1216
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self-assembly approaches. A number of symmetry broken structures have been prepared on surfaces utilizing colloidal templates, e.g., nanocups,25 nanoeggs,26 and single and overlapped nanocrescents.11,27 Here we report on a novel fabrication method based on selfassembled colloidal masks and apply it to a systematic study of the intrinsic changes in the plasmonic nanosystem, induced by the gradually increasing symmetry breaking during the transition from concentric ring (CR) to nonconcentric ring (NCR) to crescent. The experimental data are well supported by the results of finite-difference time-domain (FDTD) simulations, which provide detailed information about the nature of the underlying resonances. We show that a transition from NCR to a crescent structure is accompanied by abrupt changes in the spectra, resulting in a large splitting between plasmon peaks in orthogonally polarized light for both dipolar and quadrupolar resonances. The proposed colloidal lithography based technique comprises of four major steps: (i) deposition of colloidal particles, (ii) modification of the colloidal mask by in situ angled evaporation deposition, (iii) pattern transfer by vacuum deposition, and (iv) removal of the particles and the sacrificial metal layer. At first, the nanospheres are adsorbed at the substrate surface to form the mask. The colloidal mask selfassembly technique has been described elsewhere,4 so the procedure will be presented here only briefly. Monodispersed polymeric colloidal particles of size range 60−3000 nm are deposited on substrates by electrostatic self-assembly. Subsequently, each of the colloidal particles is used as a template to generate an in situ lift off mask in the local vicinity of the particle by angled physical vapor deposition. Figure 1 shows schematically the approach applied to the fabrication of symmetric and asymmetric rings making use of in situ deposition of a resist mask. The concept is to use the adsorbed colloidal nanoparticle to deposit a structured resist layer (SiO2 in this case seen in step 2) around the nanoparticle to define an adhesive region (based on a titanium adhesive layer) on the surface and then to use angled evaporation to deposit a structure (Au in this case seen in step 3) on to the adhesive domain with shape defined by the gap between the colloidal particle and the resist layer. Subsequent removal of the nanoparticle and the residual material leaves gold nanorings (a, b) or crescents (c). The process step (2) of defining the resist layer gives the opportunity to systematically control the shape of the resist pattern and to define systematically varied shape of nanostructure by changing the silica deposition angle αSiO2. The particles and the gold not attached directly to the substrate material are removed by tape striping (particles) and lift off in aqueous buffer (ultrasonic). Here the resist layer patterns the titanium adhesion layer and allows the selective removal of the gold from unwanted areas to form the nanostructures. In the present work, we used colloidal particles of 270 nm in diameter. Scanning electron microscopy (SEM) (FEI Magellan FEG XHR-SEM) images of nanostructures produced on silicon substrates are presented in Figure 1d−f. The metal particles, evenly distributed across the surface, are located at the bottom of conical wells in the silica resist layer. The resist layer provides a shadow mask during the deposition of the metal, it in addition can provide an efficient scratch and wear protection later giving robust material interfaces. The average outer ring radius estimated from SEM images is 129 ± 3.8 nm for CR and NCR, while the inner radius is slightly different: 84 ± 3 nm and 91 ± 2 nm, respectively
Figure 1. Top panel: particle−hole−mask lithography procedure for the fabrication of (a) concentric rings, (b) nonconcentric rings and (c) crescents: (1) deposition of Ti adhesion layer at an angle αdep to substrate normal with sample rotation, (2) deposition of silica layer (a) at normal incidence with sample rotation (b, c) and at an angle αSiO2 to substrate normal without rotation, (3) gold layer deposition at an angle αdep to substrate normal accompanied by sample rotation, and (4) top and cross-sectional view of the nanostructures obtained after particle and sacrificial layer removal. Straight arrows indicate the vapor incidence direction. Bottom panel: SEM images of corresponding structures obtained with 270 nm particles and silica resist layer deposited at (d) 0°, (e) 10°, and (f) 30° to substrate normal.
(Figure 1, parts d and e). Angled evaporation of the SiO2 resist layer at 10° to substrate normal results in asymmetric rings which are characterized by the nonconcentric inner and outer walls with the center offset of 24 ± 3 nm. Deposition of the SiO2 resist layer at 30° results in the formation of nanocresents with the width of the thickest part 104 ± 3 nm and sharp tips with the curvature radius of 6 ± 1 nm, as shown in Figure 1f. Extinction spectra of the nanostructures fabricated on glass substrates, presented in Figure 2a, were obtained by UV−vis− NIR spectroscopy (Shimadzu 3600 UV/vis-NIR) with glass substrate as a reference. Extinction data is calculated as (1 − T), where T is total transmission. The spectrum of symmetric rings (Figure 2a, red) contains two major peaks: a low energy (LE) peak at 1750 nm, and a low-intensity high energy (HE) peak near 600 nm. These peaks are attributed to the symmetric bonding and asymmetric antibonding dipole modes in nanorings.12 When the rings become nonconcentric, the new resonance peak near 1200 nm appears. Besides, symmetry breaking is accompanied by a pronounced red-shift of the LE resonance to 1900 nm, and slight shift of the HE peak to 615 1217
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Figure 2. Experimental (a) and simulated (b) extinction spectra of symmetric rings (red), asymmetric rings (blue) and crescents (green). The insets illustrate the actual shape of the structures: (a) SEM images, the length of the scale bars is 100 nm; (b) simulated structure, (substrate and silica layers are not shown for clarity). Electric field polarization is indicated by the double-headed arrow on the simulated spectra. (c) Charge distribution for asymmetric dipolar (red), quadrupolar (yellow), and symmetric dipolar (white) localized plasmonic modes in asymmetric ring and a crescent for two polarizations of incident light.
results in larger dipole moment and therefore stronger multipolar peaks. There is a slight shift of the LE peak to longer wavelength arising likely from the relative difference in the inner and outer wall radius. The relatively small change in the spectral peak position of this mode at the orthogonal polarizations likely arises from the concentration of charge at the thin part of the ring dominating the resonance in both polarizations. The plasmon resonances at the crescents can be understood in terms of the asymmetric rings resonances with a superimposed electrostatic component arising from the split. The LE peak can be thought of as a standing wave (here reflecting from the two ends of the ring) similar to that occurring in the ring, but with either an electrostatic stabilization (Figure 2c, left panel) or destabilization (Figure 2c, right panel) giving energy splitting. The polarization dependent peak splitting in the HE peak should be thought of as relating to changes in the location of the charge concentrations and the width of the wall of the corresponding NCR mode making the comparison to a ring less clear. In the same way as for the dipolar modes the quadrupolar modes can be thought of as standing waves of nanowire plasmon modes of higher order. The crescent structure similarly shows substantial polarization dependent energy splitting compared to the nonconcentric ring quadrupole mode arising from the electrostatic stabilization/ destabilization from the charge concentration at the sharp ends. The steady-state dynamics of the local electric field at different resonances for an asymmetric ring and a crescent
nm (Figure 2a, blue). The extinction spectrum of the crescents obtained in nonpolarized light is shown in Figure 2a (green) and has four resonances, in an agreement with previously reported results.27 The peaks are centered at λ=690, 1120, 1430, and 1980 nm (Figure 2a, green). We have carried out FDTD simulations of the optical response of the structures matching those used in the experiments (see Figure S1 in the Supporting Information for details) and extinction spectra are shown in Figure 2b. The simulated spectra reveal a good agreement with the experimental data, including the peak positions and relative intensities ensuring the correct attribution of the plasmon resonances. The spectra of the low-symmetry structures are simulated for two orthogonal light polarization, which are indicated by arrows. The corresponding spectral curves are presented by continuous and dash lines. For the NCR the difference between the two polarized-light spectra is small (Figure 2b, blue), while the spectra of the crescent obtained for orthogonal light polarization differ significantly (Figure 2b, green). The superposition of these curves gives the experimentally observed four-peaks shape (Figure 2a, green). Charge density plots at the different resonances are shown for the NCR and crescent for the two polarizations confirming that the HE and LE peaks are dipolar in character. The symmetry breaking introduced by the shift of center of the inner and outer ring walls for the nonconcentric ring allows the excitation of multipolar modes also from normal illumination due to the appearance of the nonzero projection of the dipole moment on the axis of light polarization. Increasing an offset 1218
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ization which is decreased when the distance between tips is increased due to lowering of the electrostatic effects. The observed spectral changes are in a good agreement with the results predicted theoretically for the transition from nanoshell to nanoegg to nanocup induced by increased nonconcentricity of the metal shell and dielectric core,29 and then observed experimentally for asymmetric structures by gradually thinning one wall of a concentric nanoshell by an electron-beam-induced ablation process.30 Spatial distribution of the plasmon-induced near-field has been investigated for the three types of obtained structures. It was found that the highest values of the local field was observed at the LE plasmon mode. The data is presented in Figure 4.
support our mode assignment (see animations in the Supporting Information). To follow the plasmonic modes development during the transition from CR to crescent, the FDTD calculations of structures with varied center offset between inner and outer circular surfaces, but keeping all the other parameters constant, have been carried out. The extinction spectra together with the simulated structures are depicted in Figure 3. The development
Figure 3. Calculated extinction spectra of gold structures with varied offset across the range from concentric (a) to nonconcentric (b, c) ring, to a crescent (d−f) for light polarization parallel (red) and perpendicular (blue) to the offset axis. Corresponding structures are shown on the right. Dash lines follow the evolution of the particular resonances.
Figure 4. Calculated relative local electromagnetic field (E2/E02) for the lowest energy (symmetric dipole) resonance. (a) Field intensity versus the distance from the metal surface for concentric ring (red squares), nonconcentric ring (blue circles) and crescent (green triangles). The field is estimated between metal and silica wall at 2 nm above the glass substrate. (b) Corresponding near-field plots at the metal/glass interface (top) and close up cross-sectional view (bottom). The location of the cross-section is marked by dash line on the top images. Glass/metal and glass/vacuum interfaces are marked by white lines in the lower images.
of the quadrupole mode (m = 2) as the asymmetry in the nonconcentric rings and thus the symmetry breaking is increased can be clearly seen (Figure 3a−c). This is different from the recently reported elliptical ring system,28 which exhibited only the multipolar modes with odd angular momentum (m = 1, 3, 5) since the ones with even m are dark in this symmetry (point group D2h). In the less symmetric NCR system studied here (point group C2v) all resonant modes are allowed, but only the quadrupole is clearly visible since higher multipolar modes have too low effective dipole moment. Symmetry selection rules can be understood using group theory (see Symmetry Considerations section in the Supporting Information). The polarization-dependent peak splitting of the dipolar LE peak and the quadrupolar peak in the crescents are increased for smaller gap sizes which implies an abrupt onset of electrostatic effects when the ring wall is broken. At this onset the structure transforms to a crescent, and the spectra undergo an abrupt qualitative change comprising a large splitting between plasmon peaks in orthogonal light polar-
The field intensity is estimated at 2 nm above the glass surface. The high near-field is localized between nanostructure and the wall of the silica layer cavity. For the crescent structure the estimated near field enhancement factor (Elocal/E0) reaches 80 near the tips, close to the values obtained in the previous studies.18 Asymmetric and symmetric gold nanorings exhibit near-field enhancement factor of around 30 and 20, respectively. The near-field decays linearly on the logarithmic scale and the slope is steeper for the crescent than for nanorings. Estimated decay length equals δ = 9.5 ± 0.4 nm for nanorings and 4.8 ± 0.2 nm for crescent structure. Thus, the asymmetric nanostructures produced by the proposed technique exhibit 1219
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large near-field enhancements, that can be used for localizing refractive index sensing or surface-enhanced spectroscopy. In summary, we have developed a novel patterning technique for the fabrication of short-range ordered arrays of concentric and nonconcentric nanorings and nanocrescents with controlled asymmetry. We demonstrate the approach to follow the change of the localized plasmon resonances during the transition from rings to crescents. We have shown that symmetry breaking enables appearance of the multipolar resonant plasmon modes which undergo polarization dependent splitting when the structure transforms into a crescent. According to the modeling, the symmetry breaking results from an offset between the inner and outer ring surface which leads to the concentration of charges on one side of the structure and a highly enhanced electric field, with the field enhancement factor reaching 80 for the crescent. The fabrication technique allows control over size of the structures by change of the nanosphere size and metal deposition angle. Since our approach does not require an ion milling step, it has a great potential for the broad application for fabrication of various plasmonic nanostructures of different shape and material.
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ASSOCIATED CONTENT
S Supporting Information *
Experimental and simulation details, discussion about symmetry selection rules for different plasmonic modes of CR and NCR, and animation of the electric field dynamics at dipole and quadrupole resonances for CR, NCR, and crescent. This material is available free of charge via the Internet at http:// pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*E-mail: (V.E.B.)
[email protected]; (D.S.S.) duncan@inano. au.dk. Author Contributions
The manuscript was written through contributions of both authors. Both authors have given approval to the final version of the manuscript. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The work was funded through the Danish research council FNU Grant (Sags No. 09-065929), the innovation consortium GENIUS, and the EU FP7 Project Grant INGENIOUS (Grant Agreement No. 248 236), V.E.B. acknowledges the support from an RFBR grant (13-07-01026).
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REFERENCES
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