3D-Tailored Gold Nanoparticles for Light Field Enhancement and

Mar 4, 2011 - Centre for Micro-Photonics, Faculty of Engineering and Industrial Sciences, Swinburne University of Technology, Hawthorn, VIC,. 3122 ...
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3D-Tailored Gold Nanoparticles for Light Field Enhancement and Harvesting over Visible-IR Spectral Range Lorenzo Rosa,† Kai Sun,‡ Vygantas Mizeikis,§ Sven Bauerdick,|| Lloyd Peto,|| and Saulius Juodkazis*,† †

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Centre for Micro-Photonics, Faculty of Engineering and Industrial Sciences, Swinburne University of Technology, Hawthorn, VIC, 3122, Australia ‡ Research Institute for Electronic Science, Hokkaido University, N21-W10, CRIS Bldg., Kita-ku, Sapporo 001-0021, Japan § Division of Global Research Leaders, Research Institute for Electronics, Shizuoka University, 3-5-1 Johoku, Naka-ku, Hamamatsu 432-8561, Japan Raith GmbH, Konrad-Adenauer-Allee, 8-PHOENIX West 44263 Dortmund, Germany ABSTRACT: A method for practical area upscaling of nanopatterning for light-harvesting and photocatalytic applications is presented. Large area electron beam lithography is used to design patterns of simple-shape nanoparticles. After evaporation of gold, ion beam lithography is used to slice nanoparticles with grooves as narrow as 17 ( 3 nm in width for the required spectral performance and light field enhancement. It is demonstrated by systematic numerical simulations that cutting grooves into the Si and SiO2 substrates up to a ∼10 nm depth augments the volume where the light-field enhancement occurs. The dominant component of the field enhancement in the groove is |Ez|2, perpendicular to the substrate's surface. The application potential of 3D-tailored nanoparticles in light harvesting applications is discussed.

’ INTRODUCTION For wider use of nanotechnology, efficiency and throughput are key parameters, as defined by Tennant's law1 which depict an empirical trade-off between the resolution or the feature size, d [nm], and the final throughput, T [μm2/h], achievable in planar lithography: pffiffiffiffi ð1Þ d½nm = 23 5 T i.e., an efficient fabrication is not compatible with small feature size. For the electron beam lithography (EBL) task to reach the required d = 10 nm resolution, 1 h would be consumed for a 1 μm2 area on the final product,1 which makes EBL an expensive and resource-consuming technology, however, providing high resolution and on-chip integration. When wet bath2 or sputtering3 is used for deposition of photocatalytic or plasmonic nanoparticles, the most limiting step is due to EBL. For an increasing number of applications, fabrication of structures with the ability to localize light within ∼10 nm gaps and slots is highly required. In fact, metallic nanogaps have shown the ability of trapping conducting molecules by electrostatic means4 and, when coupled to metallic nanoantennas, they can produce strong light intensity gradients with comparatively low incident power,5 enabling accurate trapping and positioning of living cells without damage.6 Moreover, arrays of parallel slots, ion-milled in optically thick gold films, have shown interesting properties, achieving light focusing with physical sizes down to the single-wavelength range, impossible with refractive lensing.7 The milling of nanogaps in optically small metal nanoparticles on dielectric substrates is a little explored but promising avenue r 2011 American Chemical Society

for nanofocusing and nonlinear plasmonic applications.8-11 In fact, nanoparticles exhibit localized surface plasmon resonance (LSPR), which can be harnessed to produce field enhancement in biosensors with sensitivities enabling single-molecule detection12 and tweezing.13,14 Nanosensors based on refractive index induced plasmon shift have been studied on coated gold nanoparticles for local analysis of protein interaction with biological membranes15 and for label-free molecular binding detection.16 The plasmonic resonances associated with nanoparticle dimers can be tuned and broadened by introducing a conductive overlap between the particles, even switching on and off specific resonances.17,18 Moreover, sensors based on plasmonic nanoparticles interacting with textured surfaces can have their properties easily modified by optical tweezing of the nanoparticles themselves, leading to all-optical tunability.19 In this work, we explore numerically for the first time the field enhancement properties of gold plasmonic nanoparticles on semiconducting (Si) and dielectric (SiO2) substrates, where nanometric-size cross-shaped slots and corresponding grooves have been cut in the substrate. The technical ability to fabricate such nanoparticles is also demonstrated. For a nanoparticle resting on a dielectric substrate, we show separate contributions of the different electric field components, in particular the Ez component normal to the surface, which controls the charge transport through the interface in photocatalytic experiments. We demonstrate a processing sequence consisting of two EBL Received: September 23, 2010 Revised: January 14, 2011 Published: March 04, 2011 5251

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Figure 1. Processing sequence: (a) lithographic definition of elementary nanoparticles, development of resist, sputtering for a better filling of the openings in resist or evaporation of metal for more vertical side walls, lift off and Gaþ-ion slicing with IBL. See text for discussion of different IBL cutting patterns 1-4 shown in (a); w is the width of the groove and d is the depth of the cut into the substrate. The evaporated 40 nm high Au nanoparticles of (b) 200 and (c) 500 nm (inset) diameter on Si substrate after lift-off.

Figure 2. SEM images of the through-cuts of Au-nanoparticles by lines w = 17 ( 3 nm wide with IBL: (a) a slanted view, (b) a close up of a centered cut with a ∼10-nm-deep trench, (c) the cuts made with ∼20 nm vertical and horizontal offsets from the center (the cross-hair marks the center from which the offset, o, is calculated).

Figure 3. (a) The absorption, σabs, and scattering, σscat, cross sections plotted as average of four single nanoparticles (Figure 2(b)) and the entire assemble of four particles. Calculations were carried out by importing SEM images and extruding them to a height of 40 nm. Gray regions depict error range. Numerical simulations of the ideal structures with cuts (b) just reaching Si substrate and (c) depth d, into Si substrate for different width, w, of the groove. Geometrical cross section is given by dashed line; the extinction cross section is σext = σabs þ σscat. The “boxed” regions in (c) depict the notch feature in the scattering and absorption, which depends on the depth, d. The bandgap wavelength of Si is λSi[μm] = ((1.23975)/(1.11[eV])) = 1.117 μm. The F- and H-modes are marked (see text for discussion).

and ion beam lithography (IBL) steps which are facilitated by a shared sample handling approach delivering high repeatability in sample positioning and processing. This approach enables us to pattern a large 1 cm2 area of a sensor or solar cell by ∼100 nm

nanoparticles by slicing the larger prefabricated ones. By adding the IBL slicing step, it is possible to create large volumes with light field enhancement favorable for surface-enhanced Raman scattering (SERS) and is considerably simpler to implement as 5252

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compared to dry-etching of surface-structured patterns.20 Threedimensional (3D) slicing with resolution down to 10-20 nm opens new avenues in creation of hot-spots for SERS, allows us to freely tailor shape and size of nanoparticles, and allows control polarization at the interface/groove.

’ RESULTS AND DISCUSSION Attempts to measure fields in a nanogap or at sharp corners and quantify enhancement factors have only been performed through complicated indirect means.20-24 Any external probe coupled to a narrow gap perturbs the field destroying consistency. We present here a numerical study of the field enhancement by nanoparticles 3D-tailored using tightly focused ionbeams and present a proof-of-principle demonstration of 3D texturing of nanoparticles by IBL. Gold Nanoparticles on Si Substrate. Figure 1 shows the implemented sample preparation sequence and the patterns of deposited nanoparticles. The EBL-defined and electron-beam evaporated Au nanoparticles are then sliced by ion-beam normal to the Si substrate (Figure 2). The most narrow cut through the nanoparticles of 36 ( 3 nm in height is 16 ( 3 nm with 15-nmwide cuts achieved on smaller 200-nm-diameter nanoparticles (not shown). The time required for two cuts through the particle was ∼1 s. Positioning of the entire nanoparticle array is made first and then cutting is carried out on the entire array without addressing position of each single nanoparticle. This makes such fabrication comparatively fast. The possibility of positioning the cutting line with a ∼15 nm precision is shown in Figure 2(c). Such off-centering of the cuts brings about means to control the spectral response and light enhancement as we demonstrated numerically for Si solar cell.25 It is noteworthy that the comparatively low electron acceleration voltages of 30 keV used for EBL are favorable for significant reduction of electron damage of substrate and generation of secondary electrons which limits resolution due to resist backexposure.1 High energy electrons are responsible for creation of defects in the substrate when very high resolution patterns with only ∼2 nm separation of Au nanoparticles are fabricated with 100 keV electrons3 since optical extinction spectra can be explained by electrical transport of electrons via defects in substrate for apparently separated nanoparticles.17 Defects induced in substrate by EBL can limit applicability of fabricated patterns in solar cells. The demonstrated capability to slice nanoparticles by IBL is promising for the light field enhancement and is systematically investigated numerically for the patterns shown in Figure 2(a). Scattering and Absorption Cross Sections. Figure 3 shows results of numerical simulations for the absorption and scattering cross sections. The actual SEM images (inset of Figure 3(a)) were imported for simulations and extruded to the known height of 40 nm. Since the cutting precision was approximately (10 nm, we calculated the cross sections for the each of four particles separately and then averaged them and compared with the calculations made for the all four particles together (a). It can be seen that the cross sections were not very different due to the difference of the groove position within the experimental (10 nm range. The cross sections of all four particles differ from the single one due to interaction between them especially at the longer wavelengths. The resonant wavelengths of the two main particle resonant modes, identified as higher-wavelength or fundamental

Figure 4. (a) Resonant wavelength vs groove depth d for the fundamental (black) and the higher-order (red) modes of the cut nanoparticles on Si substrate for different groove width w. (b) Normalized light field intensity |E|2 and z-component |Ez|2 maximum enhancement for the linear 45° polarized illumination and w = 20 nm on interface and (c) at the bottom of the groove. (d) The E-field intensity distributions of the modes: |Ez|2 top-view on the Au-substrate interface and the central-sideview of |E|2 (below) cross sections in log-scale (see, text for details).

(F-mode) and lower-wavelength or higher-order (H-mode), and are studied for response to linearly polarized excitation (see, Figure 3(b,c)). Spectral locations of the maximum light field enhancement for the F- and H-modes are marked in Figure 3(b,c). The panel (b) shows cross sections of the ideal structure and it is close to those shown in (a) for single particles. The effect of groove cut into the substrate is illustrated in panel (c). The depth, d, controls spectral position of a recognizable notchfeature in the scattering cross-section which has the corresponding increase of the absorption cross section, σabs. Field Enhancement for Total Field and Ez Intensity. Figure 4 shows the light field enhancement as a function of the groove width, w, and depth, d. The maximum field enhancement is at the Au-Si interface, while its evolution at the Au-Si interface and at the groove's bottom for increasing depth is shown in (b) and (c), respectively. The field configuration of the two modes is shown in (d), where the overall |E|2 intensity on the transverse plane is in the bottom panel, and the top one contains the intensity profile 5253

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Figure 5. The absorption, σabs, and scattering, σscat, cross sections of a Au nanoparticle on SiO2, (a) with groove and (b) without. The “boxed” regions in (b) depict the notch feature in the scattering and absorption, which depends on the depth, d.

Figure 6. (a) Resonant wavelength vs groove depth, d, for the fundamental (black) and the higher-order (red) modes of the cut nanoparticles on SiO2 substrate for different groove width, w. (b) Normalized light field intensity |E|2 and z-component |Ez|2 maximum enhancement for 45° polarized illumination and w = 15 nm on interface and at the groove bottom (c). (d) The E-field intensity distributions of the modes: |Ez|2 top-view on the Au-substrate interface and the central-side-view of |E|2 (below) cross sections in log-scale (same as in Figure 4).

|Ez|2 on the groove's bottom of the field component normal to the surface. The relative contribution of |Ez|2 is important as it governs the extraction/injection of electrons/holes from the surface in applications where charge transport through the interface is taking place.26,27 Both the F-mode and the H-mode have a smooth field configuration, characterized by clearly defined hot-spots at the interface, on opposite center corners parallel to the incoming light polarization, showing dipole-like resonance, the F-mode also showing more widely-distributed hot-spots at the outer groove corners, but with overall smaller peak intensity. In both cases, the depth, d = 5 nm, shows the strongest peaks at the trench bottom for both components, with enhancement reaching a peak value of 45 for the F-mode and 79 for the H-mode. The enhancement on the |Ez|2 component alone at the interface varies, though stabilization around 30 and 10% for the F- and H-mode, respectively, while it raises to around 50 and 80% for the F- and H-mode, respectively, at the groove bottom. We notice the resonant wavelengths in Figure 4(a) to be lower for larger w, and to constantly decrease as d is increased, more sharply as the groove width is enlarged, closely following the slopes on the two sides of the scattering cross-section notch in Figure 3. Gold Nanoparticles on SiO2 Substrate. Results of a systematic theoretical study of the nanoparticle behavior on dielectric SiO2 substrates are presented below. It shows the potential to increase the field enhancement by 3 orders of magnitude as compared with Si substrate for similar geometry. Cross Sections. Figure 5 shows results of numerical simulations for the absorption and scattering cross sections grooves of different depth and width. The maximum cross-section is increased almost 3-fold as compared with Si substrate. The cross sections feature now two clear dips, a major one around 1200 nm wavelength, and a minor one around 750 nm, which correspond to the two resonance modes, whose sensitivity to d and w appears lower than for the Si-substrate. The two dips correspond to the F- and H-modes of the nanoparticle, the F-mode having a 60% larger cross-section. This is due to the contribution of the two quarters of the particle that do not interact with the central nanogap, which turn on separate monopole-like resonances that add hot-spots to their corners. 5254

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The Journal of Physical Chemistry C Field Enhancement for Total Field and Ez Intensity. The field configuration of the F-mode and the H-mode is similar to the previous case of Au-on-Si, though it becomes more complex. The F-mode hot-spots become more prominent and in the H-mode, faint interface standing waves appear at the edges of the gold surface. The |Ez|2 values remain remarkably stable at the trench bottom for varying d, while the depth d = 5 nm still shows the strongest peaks for |Ez|2 at the interface. The |Ez|2-to- |E|2 ratio remains around 30% for the F-mode and 10% for the H-mode at the trench bottom, while it is 80 to 95% on the interface, where relative enhancement reaches values of 620 for the F-mode and 1350 for the H-mode. The resonant wavelengths are correspondingly shorter than for silicon substrate, showing a very small dependence on d and w for the both modes. The field enhancement is larger in the case of SiO2 as compared with Si due to a splitting of an inherent nanoparticle resonance when substrate is present. The plasmon resonance that the particle would support inside a homogeneous medium is split due to the mismatched refractive index at the two interfaces, as has been shown experimentally for silver nanocubes,22 which can also apply to the present case.28 For a given nanoparticle structure, the field enhancement is maximum at an intrinsic wavelength, and by changing the substrate refractive index, the resonant wavelengths of the modes can be tuned or detuned with respect to the intrinsic maximum (Figure 6(b)). The refractive index of silicon varies strongly within the operation bandwidth, which interacts strongly with gold and contributes the complex resonances around 1 μm wavelength.25

’ CONCLUSIONS EBL patterning can be made quickly by implementing patchexposure rather than a raster scan, making large area (up to wafer size) fabrication practical (eq 1), since the feature size of nanoparticles is comparatively large ∼200-500 nm. The functional nanogaps, holes, and grooves for light field enhancement are subsequently fabricated by 3D patterning using IBL. Hence, by adding the IBL step of 3D cutting and trimming of nanoparticles, it is possible to make nanofabrication more efficient and more functional, since features down to ∼15 ( 5 nm size can be added by postprocessing of larger nanoparticles. Since the IBL is inherently resist-free processing, it expands the possibilities of nanofabrication. It is also worth noting that the IBL can also perform the EBL function. The presented systematic numerical study of spectral and light field enhancement dependencies on the width and depth of the cuts show promising application potential in sensor (Au-onSiO2) and solar cell (Au-on-Si) fields. Significant light enhancement is obtained for the 5-10 nm deep grooves with the E-field component perpendicular to the Au-substrate interface. This could find application for the charge transport enhancement through the interface. Other plasmonic structures29,30 can deliver the same or better field enhancement performance, only at the expense of a much more complicated and difficult to control fabrication process. ’ EXPERIMENTAL SECTION Fabrication and Postprocessing of Plasmonic Nanoparticles. We used EBL for the pattern definition of nanoparticles

of circular circumference, followed by development for removal of exposed regions in a positive type of resist (polymethyl

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methacrylate), then electron-beam evaporation of gold and the final lift-off. This sequence is schematically shown in Figure 1. It is important to tailor the exposure (current and time) of resist in such a way that the deposited dose would result in undercut grooves (pits or holes) after development. This settle tuning is essential for a good lift off, which is also critically dependent on the normal incidence direction during metal deposition (c). In order to avoid resist adhesion to the nanoparticles, the resist film should be a few times thicker than the height of the metal film, which is about 40 nm. For a better substrate to plasmonic metal adhesion, a few nanometers of Cr or Ti are typically deposited first. For the EBL, we used a sub-10 nm resolution Raith 150TWO equipped with a 30 keV electron gun. Au evaporation rather than sputtering is used since it delivers better quality samples with narrower extinction peaks. Samples were characterized by scanning electron microscopy (SEM). It is noteworthy that sputtering can deliver better filling of the edges inside the resist openings (see Figure 1(a)). After the sputtering and lift-off, a Gaþ ion beam patterning (Raith ionLiNE) is performed. This IBL postprocessing step is used for cutting through the nanoparticles and the substrate (see Figure 1(a)). The groove width down to d = 15 ( 3 nm is achievable for the nanoparticles up to 50 nm height. The used IBL 3D patterning equipment allows to trim, cut, and drill with a resolution of ∼15 nm freely in 3D space. This is realized by the sample's nanopositioning along three space coordinates and additionally by rotation around two complementary axes. This is demonstrated by off-center cuts positioned with a precision of o ≈ 20 nm. For the ion beam patterning, we used a 1.4 pA beam current at 40 kV, step size 4 nm, dwell time per pixel 1 μs, and 3000 loops resulting in a total dose of 104 pC/cm (line dose, equivalent to 104 μC/cm2 area dose). The total exposure time of a cross pattern on a single nanoparticles is about 1-1.5 s with 600-800 nm long cuts through 40-nm thick gold. Since the EBL and IBL share the same sample positioning hardware and software, it is possible to perform dicing and cutting of nanoparticles on the entire area with approximately 15 nm repeatability from particle to particle. This high precision is adequate for most applications.

’ NUMERICAL SIMULATIONS We used finite-differences time domain (FDTD) numerical simulations (Lumerical) to systematically investigate spectral properties of the light field enhancement for different size and shape nanoparticles cut by IBL. Also, light field intensity components vertical to the substrate, |Ez|2, and parallel |Ex,y|2 were examined at different locations near the Au-nanoparticle on Sisubstrate or SiO2-substrate. The actual materials' (Au, Si, and SiO2) properties were taken into account by using experimentally defined dielectric functions available in the Lumerical database. In order to reflect the actual shape and size effects, the SEM images were imported and extruded to the used the 36 nm height of evaporated Au-nanoparticles. Numerical error is reduced by aligning the slots to the FDTD mesh and made finer around slot center and Au-substrate interface to capture maximum enhanced field features, leaving maximum approximation to the rounded outside boundary of the nanoparticle, where the field is least impaired. In order to estimate the staircase effect approximation impairment in this case, we searched for modes having hot spots along the upper nanoparticle interface far from the substrate, when the free outer corners are rounded due to 5255

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The Journal of Physical Chemistry C surface tension effects. The field divergence effect as rounding radius is reduced to zero (sharp corners) has been correctly modeled for radii equal to or greater than 2 nm. In simulation, a 0.5 ps propagation of a broadband pulse spanning the 800 to 2300 nm bandwidth is used.

’ AUTHOR INFORMATION Corresponding Author

*Phone: þ61 (0)3 9214 8718. Fax: þ61 (0)3 9214 5435. E-mail: [email protected] .

’ ACKNOWLEDGMENT Support via a Discovery ARC Grant DP0988054 is gratefully acknowledged. Samples for the IBL processing were prepared at the NSW Node of the Australian National Fabrication Facility.

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