Fabrication, Characterization, and Optical Properties of Gold

1822

Langmuir 2009, 25, 1822-1827

Fabrication, Characterization, and Optical Properties of Gold Nanobowl Submonolayer Structures Jian Ye,*,†,‡ Pol Van Dorpe,† Willem Van Roy,† Gustaaf Borghs,† and Guido Maes‡ IMEC, Kapeldreef 75, LeuVen, B-3001, Belgium, and Chemistry Department, Katholieke UniVersiteit LeuVen, LeuVen, B-3001, Belgium ReceiVed NoVember 13, 2008. ReVised Manuscript ReceiVed NoVember 26, 2008 We report on a versatile method to fabricate hollow gold nanobowls and complex gold nanobowls (with a core) based on an ion milling and a vapor HF etching technique. Two different sized hollow gold nanobowls are fabricated by milling and etching submonolayers of gold nanoshells deposited on a substrate, and their sizes and morphologies are characterized by scanning electron microscopy (SEM) and atomic force microscopy (AFM). Optical properties of hollow gold nanobowls with different sizes are investigated experimentally and theoretically, showing highly tunable plasmon resonance ranging from the visible to the near-infrared region. Additionally, finite difference time domain (FDTD) calculations show an enhanced localized electromagnetic field around hollow gold nanobowl structures, which indicates a potential application in surface-enhanced Raman scattering (SERS) spectroscopy for biomolecular detection. Finally, we demonstrate the fabrication of complex gold nanobowls with a gold nanoparticle core which offers the capability to create plasmon hybridized nanostructures.

Introduction Various nanostructuressstructures with at least one dimension between 1 and 100 nmsincluding nanospheres, nanowires, nanotubes, nanoholes, nanoshells, and nanorods have been fabricated for different applications due to their unique optical, electrical, mechanical, catalytic, and magnetic properties.1,2 It is well-known that the properties of such nanostructures are strongly dependent on their size, shape, and composition. Effective strategies to control the structural parameters are extremely required in order to meet the ever-increasing demands placed on new nanostructures preparation and applications development. As an example, a number of approaches to fabricate nanobowl structures with controlled morphology and different compositions have been reported.3-11 Two-dimensional patterned conducting polymer-nanobowl sheets have been synthesized by Chen et al. via a chemical polymerization and have been suggested as potential hypersensitive sensors.3 Wang et al. have shown ordered arrays of TiO2 nanobowls from polystyrene sphere templating and atomic layer deposition, and they can be useful for the size selection of submicron spheres and the fabrication of nanodot patterns as reusable masks.4,5 Peng et al. have also reported TiO2 * To whom correspondence should be addressed. Phone: +32-16-288795. Fax: +32-16-281097. E-mail: [email protected]. † IMEC. ‡ Katholieke Universiteit Leuven. (1) Prasad, P. N. Nanophotonics; John Wiley & Sons, Inc.: Hoboken, NJ, 2004. (2) Wolf, E. L. Nanophyscis and Nanotechnology: an Introduction to Modern Concepts in Nanoscience; Wiley-VCH Verlag GmbH & Co. KGaA: Weinheim, 2004. (3) Chen, J.; Chao, D.; Lu, X.; Zhang, W.; Manohar, S. K. Macromol. Rapid Commun. 2006, 27, 771. (4) Wang, X. D.; Graugnard, E.; King, J. S.; Wang, Z. L.; Summers, C. J. Nano Lett. 2004, 4, 2223. (5) Wang, X. D.; Lao, C.; Graugnard, E.; Summers, C. J.; Wang, Z. L. Nano Lett. 2005, 5, 1784. (6) Peng, J.; Li, X.; Kim, D. H.; Knoll, W. Macromol. Rapid Commun. 2007, 28, 2055. (7) Liu, J.; Zhu, M.; Zhan, P.; Dong, H.; Dong, Y.; Qu, X.; Nie, Y.; Wang, Z. Nanotechnology 2006, 17, 4191. (8) Chen, X.; Wei, X.; Jiang, K. Opt. Express 2008, 16, 11888. (9) Srivastava, A. K.; Madhavi, S.; White, T. J.; Ramanujan, R. V. J. Mater. Chem. 2005, 15, 4424. (10) Xia, Y.; Halas, N. J. MRS Bull. 2005, 30, 338. (11) Liu, J.; Dong, H.; Li, Y.; Zhan, P.; Zhu, M.; Wang, Z. Jpn. J. Appl. Phys 2006, 45, L582.

arrays prepared from a block copolymer templating with sol-gel chemistry and their photocatalytic properties.6 Additionally, some metallic nanobowl structures such as Pt,7 Ni,8 and Co9 have been fabricated. Nevertheless, it is still a significant challenge to develop versatile methods to create nanobowl structures with designable and predictable properties. Due to the collective excitation of surface free electrons by incident light, Au and Ag nanostructures may exhibit surface plasmon resonances (SPRs) or localized surface plasmon resonances (LSPRs) if the oscillation of free electrons is confined to a finite volume.10 Therefore, different Au and Ag nanostructures have been prepared with controllable shapes and dimensions to tailor their LSPRs for applications including electromagnetic field enhancement, optical imaging, light transmission enhancement, colorimetric sensing, and nanoscale waveguiding.10 To the best of our knowledge, very limited studies on Au or Ag nanobowl structures have been reported. One example from Liu et al.11 showed that ordered arrays of Au nanobowls prepared from a combination of nanosphere lithography and sputtering technique possessed some features such as a reduced symmetry geometry and a high ratio of surface area to volume. However, their work mainly focuses on the characterization of the morphology and dimensions; the investigation of optical property and applications of Au nanobowls is still limited. Some other similar Au nanostructures with a reduced symmetry, such as nano half-shells,12 nanocups,13,14 nanocaps,13,14 and nanocrescents,15 have been published very recently and have displayed a number of interesting properties. The patterned Au half-shells offered a superhydrophobic surface when they were immobilized on a substrate using template-assisted self-assembly.12 Au nanocup and nanocap arrays not only showed highly tunable optical properties,13 but also rendered their optical properties dependent on the angle and polarization of the incident light.14 Au nanocrescents exhibited a capability to locally enhance the electromagnetic field, which is useful for surface-enhanced Raman (12) Love, J. C.; Gates, B. D.; Wolfe, D. B.; Paul, K. E.; Whitesides, G. M. Nano Lett. 2002, 2, 891. (13) Charnay, C.; Lee, A.; Man, S.; Moran, C. E.; Radloff, C.; Bradley, R. K.; Halas, N. J. Phys. Chem. B 2003, 107, 7327. (14) Cortie, M.; Ford, M. Nanotechnology 2007, 18, 235704. (15) Lu, Y.; Liu, G. L.; Kim, J.; Mejia, Y. X.; Lee, L. P. Nano Lett. 2005, 5, 119.

10.1021/la803768y CCC: $40.75  2009 American Chemical Society Published on Web 01/06/2009

Gold Nanobowl Submonolayer Structures

scattering (SERS) spectroscopy in ultrasensitive biomolecular detection.15 In this paper, we introduce a new method involving an ion milling technique and a vapor HF etching process to fabricate submonolayers of hollow Au nanobowls and complex Au nanobowls. The methodology is based on the fabrication of a submonolayer of Au open nanoshells with a well-controlled orientation produced by ion milling upon a submonolayer of Au nanoshells. Vapor HF is further applied to remove the silica cores while keeping the outer Au shells intact. Scanning electron microscopy (SEM), transmission electron microscopy (TEM), and atomic force microscopy (AFM) have been applied for characterization. Optical properties of Au nanobowls with different dimensions are investigated experimentally and theoretically. Additionally, finite difference time domain (FDTD) calculation is performed to visualize the electromagnetic field distribution of a Au nanobowl structure. Finally, the fabrication of complex nanobowls with a Au nanoparticle (NP) core inside is also demonstrated, which provides a further possibility to prepare more complex nanobowl structures.

Experimental Section Materials. Hydrogen tetrachloroaurate trihydrate (HAuCl4 · 3H2O, reagent ACS) and sodium citrate (99.8%) were received from Acros Organics (Geel, Belgium). Sodium hydroxide (NaOH, 98%) was from Sigma Aldrich (Munich, Germany). Tetraethyl orthosilicate (TEOS, 98%) was purchased from Fluka Chemika (Buchs, Germany). 3-Aminopropyltriethoxysilane (APTES, 98%) and 3-mercaptopropyltrimethoxysilane (MPTMS, 95%) were from ABCR GmbH & Co. KG (Karlsruhe, Germany). Potassium carbonate (K2CO3, 95%) was from Alfa Aesar (Karlsruhe, Germany) and formaldehyde (35%) was from Vel (Leuven, Belgium). H2O2 (30 wt %), ammonia (30 wt %), and ethanol (absolute) were from Honeywell Specialty Chemicals (Seelze, Germany). HCl (37%), HNO3 (69.5%), H2SO4 (95%), and 2-propanol were obtained from Air Products (San Guiliano Milanese, Italy). Characterization Techniques. SEM images were taken using a Philips XL30 FEG instrument operated at an accelerating voltage of 5 kV. TEM images were recorded on a 300 kV Philips CM30 instrument equipped with a field emission source. UV-vis spectroscopy was performed using a Shimadzu UV-1601PC spectrophotometer with a spectral slit width of 0.8 nm and a data interval of 0.2 nm. AFM images were acquired in the tapping mode on a Dimension 3100/Nanoscope IV, VEECO, under ambient conditions with the scan rate between 0.4 and 0.5 Hz. Si cantilevers with a force constant between 12 and 103 N/m were used at resonance frequencies between 200 and 400 kHz. PPP-RT-NCHR probes from NANOSENSORS were applied in order to observe the cavity of nanobowls more clearly. Preparation of Au Nanoshell Submonolayer. Au nanoshells were synthesized according to a modified method of Oldenburg et al.16 Briefly, silica colloids were synthesized following a Sto¨ber procedure,17 by mixing ammonia and ethanol followed by dropwise adding TEOS typically. Then 10 mL silica colloids were directly functionalized with 50 µL of APTES overnight without purification. Next, functionalized silica colloids were decorated with some tiny Au colloids (1-2 nm) that were prepared by a method of Duff et al.18 and were purified by repeated centrifugations. A subsequent 10 min reduction in an overnight aged solution of HAuCl4 (3 mL, 1%) and K2CO3 (100 mL, 3.6 mM) in the presence of formaldehyde (0.5 mL) resulted in a continuous Au shell on the silica surface. The resulting Au nanoshells were purified by repeated centrifugations and washing with deionized H2O and were finally redispersed into deionized H2O. The Au nanoshell submonolayer was fabricated according to a procedure we described before.19,20 Prior to use Si or quartz substrates were submerged in a piranha solution for 30 min, rinsed well with deionized water and ethanol, and dried in a flow of nitrogen. Piranha solution consists of a 1:3 (v/v) mixture of H2O2 (30%) and H2SO4 (16) Oldenburg, S. J.; Averitt, S. L.; Westcott, S. L.; Halas, N. Chem. Phys. Lett. 1998, 288, 243. (17) Sto¨ber, W.; Fink, A.; Bohn, E. J. Colloid Interface Sci. 1968, 26, 62. (18) Duff, D. G.; Baiker, A. Langmuir 1993, 9, 2301.

Langmuir, Vol. 25, No. 3, 2009 1823 (96%). (CAUTION: Piranha solution reacts Violently with organic materials.) Next, the clean substrates were immersed into a 10% (v/v) solution of MPTMS in a 95:5 (v/v) ethanol/H2O mixture for 3 h. Subsequently, the substrates were removed from the silane solution and extensively rinsed with ethanol and then dried in nitrogen flow. The silanization of the substrates was promoted in a 105 °C oven for 10 min. The silane-functionalized substrates were placed into the Au nanoshell suspension (≈108 particles/mL) overnight. After the prescribed time, the substrates were removed from the nanoshell suspension and rinsed copiously with deionized water. Preparation of Au Nanobowls. Au nanobowl submonolayers were fabricated by applying an ion milling process and a vapor HF etching process on Au nanoshell submonolayer structures. Ion milling was performed in an in-house-made ion miller system. Ion milling is a process in which accelerated Xe atoms are used to bombard the sample surface, thereby etching away the target sample. In our experiments, ion milling was used with the following parameters: 400 V accelerator voltage, 2.4 sccm Xe flow rate, and below 8.0 × 10-8 Torr base pressure in the processing chamber. The vapor HF etching was done in an adapted commercially available system for wafer cleaning.21 Nitrogen gas was bubbled through a 49% HF solution in order to get a vapor of HF and water into the reaction hood. The nitrogen flow was adjusted at ∼0.5 L/min, and the temperature in the hood was controlled at ∼35 °C. In order to get reproducible etching results, the chamber was usually purged by nitrogen gas for 5 min before and after the etching process. Preparation of Complex Au Nanobowls. Complex Au nanobowls (structures with a Au core inside each Au nanobowl) submonolayer structures were fabricated from the same fabrication procedure of Au nanobowls, except by using silica-coated Au (Au@SiO2) colloids instead of silica colloids as the precursors. The synthesis of Au@SiO2 colloids has been described before.22 First, 55 ( 7 nm Au NPs were prepared by adding 0.8 mL of 1% (w/v) sodium citrate into a 100 mL boiling aqueous solution containing 0.01% (w/v) HAuCl4. Next, 4 mL Au NP suspension (∼0.5 mg/mL) was added into 20 mL of 2-propanol under continuous stirring followed by 0.5 mL of ammonia and 54 µL of TEOS. The reaction was allowed to proceed for 1 h at room temperature. The obtained silica shell thickness was 98 ( 12 nm, confirmed by TEM images. Following the same procedure of making Au nanoshells, submonolayer preparation, ion milling, and vapor HF etching process, a submonolayer of complex Au nanobowl structures was obtained.

Simulation Simulations of optical spectra and electromagnetic field profile were obtained based on the FDTD method using the program FDTD Solutions (version 5.1) purchased from Lumerical Solutions, Inc. (Vancouver, Canada). The simulations were performed with the parallel FDTD option on a HP ProLiant DL145 G3 Server with two dual-core AMD Opteron 2000 processors at 2.8 GHz with 16 GB of RAM. The FDTD method is based on a numerical solution of the Maxwell equations. The particle is illuminated with a total-field scattered-field (TFSF) source23 which propagates in the k ) -Z direction. The direction of the electric field E is perpendicular to k and parallel to the X direction. The wavelength of incident light is varied from 500 to 1100 nm, and the amplitude is set as 1. A perfectly matched layer (PML) is used as radiation boundary condition. The simulation region is 800 × 800 × 800 nm3 with a grid size of 3 nm. The whole simulation region is assumed in air. We have used the dispersion model for Au derived from the experimental data provided by Johnson and Christy.24 The total complex(19) Ye, J.; Bonroy, K.; Nelis, D.; Frederix, F.; D’Haen, J.; Maes, G.; Borghs, G. Colloids Surf., A 2008, 321, 313. (20) Ye, J.; Chen, C.; Van Roy, W.; Van Dorpe, P.; Maes, G.; Borghs, G. Nanotechnology 2008, 19, 325702. (21) Witvrouw, A.; Du Bois, B.; De Moor, P.; Verbist, A.; Van Hoof, C.; Bender, H.; Bae, K. Proc. SPIE 2000, 130, 4174. (22) Ye, J.; Van de Broek, B.; De Palma, R.; Libaers, W.; Clays, K.; Van Roy, W.; Borghs, G.; Maes, G. Colloids Surf., A 2008, 322, 225. (23) Tanev, S.; Tuchin, V. V.; Paddon, P. Laser Phys. Lett. 2006, 3, 594. (24) Johnson, P. B.; Christy, R. W. Phys. ReV. B 1972, 6, 4370.

1824 Langmuir, Vol. 25, No. 3, 2009

Ye et al. Table 1. Dimension and Fabrication Parameters of Au Nanobowl Submonolayer Structure inner shell ion milling HF etching sample no. diameter (nm) thickness (nm) time (s) time (min) 1 2 3a a

Figure 1. Experimental procedure for fabricating Au nanobowls.

valued permittivity of the Au ε(ω) is modeled by the combination of a Drude model and a Lorentz model, and hence results from the sum of three different material modes: εREAL(ω), εL(ω), and εP(ω). εREAL(ω) ) 6.8065 is the basic background permittivity. εL(ω) is given by a Lorentz model

εL(ω) ) εLorentz

ω02 ω02 - 2iδ0ω - ω2

with parameters εLorentz ) 1.6748, ω0 ) 4.506 608 080 759 082 × 1015 Hz, and δ0 ) 6.820 216 162 455 338 × 1014 Hz. εP(ω) is the equation based on the Drude model

εP(ω) )

ωP2 iωνC + ω2

with parameters ωP ) 1.353 834 541 798 859 4 × 1016 Hz and νC ) 1.068 689 183 387 936 × 1014 Hz. This fit provides an accurate description of the dielectric data of Au in the wavelength range from 500 to 1100 nm.

Results and Discussion Figure 1 outlines the procedure to make Au nanobowl submonolayer structures. First, as the precursors of nanobowls, Au nanoshells with different core sizes and shell thicknesses can be synthesized by the seeding and the electroless plating methods from silica colloids.16 In this work, we synthesized two sized Au nanoshells with an 87 ( 10 nm silica core size and a 30 ( 6 nm Au shell thickness, and a 224 ( 24 nm core size and a 17 ( 2 nm shell thickness, respectively. Benefitting from the strong affinity between the thiol group and Au surface, Au nanoshells were subsequently immobilized on the substrates that were functionalized by an organic MPTMS silane layer. Next, ion milling was applied to remove the top part of Au shells, thus forming open-nanoshell structures. The dimension of the opennanoshell structure can be tailored by controlling the ion milling time. Finally, vapor HF was used to etch away the silica core without destroying the outer Au shells. Fabrication parameters of Au nanobowls such as the time of ion milling and vapor HF etching are mainly dependent on the silica core size and Au shell thickness. The general strategy is to apply longer ion milling time for a thicker Au shell and longer vapor HF etching time for a larger silica core size. For example, Au nanobowl sample 1 was fabricated by applying 60 s of ion milling and 3 min of vapor HF etching on a submonolayer of Au nanoshells with an 87 nm core and a 30 nm shell, and sample 2 was fabricated by applying 40 s of ion milling and 45 min of vapor HF etching on a submonolayer of Au nanoshells with a 224 nm core size and a 17 nm shell thickness (Table 1). Figure 2 shows the SEM images of the two different sized Au nanobowl structures at each step of the fabrication process. In Figure 2a, a submonolayer of Au complete nanoshells with an 87 nm core size and a 30 nm shell thickness was formed on the substrate before the ion milling; nanoshells were randomly

87 ( 10 224 ( 24 251 ( 12

30 ( 6 17 ( 2 25 ( 3

60 40 60

3 45 45

Complex structure with a 55 nm Au NP core inside.

distributed on the substrate and some nanoshell clusters were also observed during the fabrication process. In Figure 2b, the top part of Au nanoshells was cut away and a submonolayer of Au open nanoshells with an aperture of ∼100 nm was generated after 60 s of ion milling. Figure 2c shows the resulting Au nanobowls (sample 1) with a sharp aperture edge after removing the silica core by 3 min of vapor HF etching. Figure 2d-f indicates a similar morphology change during the fabrication process of Au nanobowls (sample 2) with a 224 nm inner diameter and a 17 nm shell thickness: the removing of the top part of Au nanoshell and the etching of silica core, respectively. In contrast, a larger aperture of ∼180 nm on the top part of the particles was produced by 40 s of ion milling. In addition, a much rougher Au nanobowl surface was obtained, which was determined by the applied electroless plating during the Au nanoshell preparation process. We also observed that single particles or particle clusters remain at the same location during the ion milling and HF etching process. Moreover, we measured side-view SEM images by tilting our samples to 89° in order to observe the morphology change more clearly during the fabrication process of Au nanobowls of sample 2 (Figure 3). The height of complete Au nanoshells was the same as their diameter, ∼250 nm before the ion milling process (Figure 3a). After 40 s of ion milling, the top part of the Au shell was removed and the silica core was exposed; meanwhile, the height of the Au shells was decreased to ∼190 nm (Figure 3b). A 45 min vapor HF etching kept the Au shell intact and etched away the silica core (Figure 3c). Figure 3 also indicates a rather rough Au shell surface, which is consistent with the observation in Figure 2. AFM measurements were used to investigate the fabrication of Au nanobowl structures in further detail. Figure 4 shows the AFM images of Au nanobowls (sample 2) at each fabrication step. The line scan and illuminated-top-view images are shown to provide a clearer view of the morphology change. We can find a complete but rough Au shell surface around the particles before the ion milling process (Figure 4a). Next, the Au shell was opened by the ion milling and an aperture structure of ∼200 nm was clearly shown on the top part the particles (Figure 4b,d), which is close to the observation from SEM images in Figure 2. Additionally, the top surface of the particles becomes smoother due to the exposed silica core surface. Figure 4c indicates the cavity structures in Au nanobowls after the vapor HF etching. The depth of the cavity is estimated at ∼160 nm, and the height of the nanobowl is ∼210 nm. The difference between them is ∼50 nm, which is much larger than the thickness (17 nm) of the nanoshell. Furthermore, the size of the aperture structures shrinks largely. We attribute these dimensional deviations of the cavity and the aperture structures to the geometry limit and easy breaking of AFM probes. The calculated and experimental optical spectra of two different sized Au nanobowl structures during the fabrication process are investigated in Figure 5. The fabrication process starts from Au nanoshells (S1 and S4 in Figure 5a,b) to Au open nanoshells (S2 and S5 in Figure 5a,b) and finally reaches Au nanobowls (S3 and S6 in Figure 5a,b). The calculated spectra show an interesting trend: removing the top of the nanoshells results in a pronounced red shift of the plasmon resonance and a blue shift occurs by removing the silica core afterward (Figure 5a). For example, a

Gold Nanobowl Submonolayer Structures

Langmuir, Vol. 25, No. 3, 2009 1825

Figure 2. SEM images of different sized (top, sample 1; bottom, sample 2) Au nanobowl submonolayers at each step of the fabrication process. (a, d) Self-assembled Au nanoshells. (b, e) Ion milled Au open nanoshells. (c, f) Vapor HF etched Au nanobowls. All insets are the corresponding highly magnified SEM/TEM images.

Figure 3. Side-view SEM images of the Au nanobowl submonolayer structures (sample 2) at each step of the fabrication process. (a) Selfassembled nanoshells, (b) open nanoshells after ion milling 40 s, and (c) nanobowls after vapor HF etching 45 min.

Au nanoshell exhibits a dipolar plasmon band at 578 nm before the ion milling (S1 in Figure 5a); its resulting open nanoshell displays a red shift with a plasmon band at 643 nm (S2 in Figure 5b) while its resulting nanobowl presents a blue shift with a plasmon band at 620 nm (S3 in Figure 5c). The red shift of the plasmon band is attributed to the hybridization of the dipolar resonance of the core-shell structure with the dipolar resonance of the nanoaperture.25,26 Once an aperture structure is opened on the nanoshell, the red shift occurs because of the charge buildup in the aperture. However, the plasmon blue shift from the open (25) Prodan, E.; Radloff, C.; Halas, N. J.; Nordlander, P. Science 2003, 302, 419. (26) Cole, R. M.; Baumberg, J. J.; Garcia de Abajo, F. J.; Mahajan, S.; Abdelsalam, M.; Bartlett, P. N. Nano Lett. 2007, 7, 2094.

nanoshell to the nanobowl is a consequence of the decrease of the environmental refractive index from the silica core (n ) 1.45) to air (n ) 1).22,27,28 The plasmon blue shift occurs due to the decreased polarizability of the environmental media, increasing the restoring force for the electron oscillation in the metal, thereby promoting the plasmon oscillations to a higher energy level.27 The calculated spectra furthermore indicate that a similar shift trend happens on larger particles and the shifts become more pronounced when the aspect ratio between the core size and the shell thickness increases (S4-S6 in Figure 5a). Figure 5b presents the corresponding experimental spectra at each step of Au nanobowl preparation. It is shown that the number and relative positions of plasmon bands from Au nanoshells, open nanoshells, and nanobowls are rather consistent with the calculation. The discrepancies between the experimental and calculated spectra are most likely attributed to the polydispersed size and rough Au shell surface. FDTD calculations not only provide us the optical spectra of Au nanobowls but also give us electromagnetic near-field distribution pictures. Figure 6 indicates the electromagnetic field profiles of a Au nanobowl (sample 2) excited at the wavelengths of 620 and 968 nm. The resonance at 620 nm exhibits a quadrupolar feature in the broken-symmetrical nanobowl geometry (Figure 6a). The resonance at 968 nm displays a dipolar character and is dominated by the local charge buildup at the edges of the open shell (see “hot spots” in Figure 6b). This charge buildup is accompanied by a strong enhancement of the local electromagnetic field. The last aspect provides Au nanobowls with interesting prospects for SERS application of biomolecular detection where signals strongly depend on the local electromagnetic field enhancement. Recently some complex and hybrid metallic nanostructures such as Au nanoshell dimers,29 Au nanorice,30 and Ag ring-disk nanocavities31 have been suggested or demonstrated to tune the (27) Prodan, E.; Lee, A.; Nordlander, P. Chem. Phys. Lett. 2002, 360, 325. (28) Graf, C.; Van Blaaderen, A. Langmuir 2002, 18, 524. (29) Talley, C. E.; Jackson, J. B.; Oubre, C.; Grady, N. K.; Hollars, C. W.; Lane, S. M.; Huser, T. R.; Nordlander, P.; Halas, N. J. Nano Lett. 2005, 5, 1569. (30) Wang, H.; Brandl, D. W.; Le, F.; Nordlander, P.; Halas, N. J. Nano Lett. 2006, 6, 827.

1826 Langmuir, Vol. 25, No. 3, 2009

Ye et al.

Figure 4. AFM topographic (1 µm × 1 µm) and representative line scan images of the Au nanobowl submonolayer structures (sample 2) at each step of the fabrication process. (a) Nanoshells, (b) open nanoshells after ion milling 40 s, and (c) nanobowls after vapor HF etching 45 min. (d) Illuminated-top-view mode of (b) for clearer observation. All arrows marked in the topographic images correspond to the arrows in the line scan images.

Figure 5. Normalized (a) calculated and (b) experimental optical properties of two different sized Au nanobowl submonolayer structures at each step of the fabrication process. S1-S6 correspond to the samples of a-f in Figure 2, respectively.

electromagnetic field enhancement in the visible and near-infrared regions for SERS application. By extending the above fabrication procedure of Au nanobowls, complex Au nanobowl (structures with a Au core inside each Au nanobowl) structures were fabricated by using Au@SiO2 colloids instead of silica colloids as the precursors. Figure 7 indicates the TEM images of Au, Au@SiO2, and

Au@SiO2@Au colloids we used to make complex Au nanobowls. The Au NPs used as cores have an average size of 55 nm (Figure 7a). An ∼100 nm silica layer was coated on the Au NP surface using the Sto¨ber method (Figure 7b). Au@SiO2@Au colloids with a 25 ( 3 nm outer Au shell layer were obtained by using the same electroless plating procedure of Au nanoshells (Figure

Gold Nanobowl Submonolayer Structures

Langmuir, Vol. 25, No. 3, 2009 1827

Figure 6. Electromagnetic field profileS of a Au nanobowl (sample 2) in air at wavelengths of (a) 620 and (b) 968 nm.

Figure 7. TEM images of (a) Au, (b) Au@SiO2, and (c) Au@SiO2@Au colloids.

versatile method presented here could be further extended to a wide range of core materials with various sizes, shapes, and compositions, which offers us a rather effective strategy to control the plasmonic hybridization in nanostructures.

Conclusion

Figure 8. (a) Low and (b, c) high magnification SEM images of complex Au nanobowls (sample 3). (a, b) 40° tilt; (c) 89° tilt.

7c). Next, particle immobilization, ion milling, and vapor HF etching were subsequently applied to the Au@SiO2@Au particles on the substrate. The resulting complex Au nanobowls are indicated in Figure 8. It is shown that there is one Au NP core inside each nanobowl structure and the locations of the cores are random. Au cores may sit at the middle of nanobowl bottoms or attach to the inner sidewalls of nanobowls. The interactions between the Au cores and nanobowls might give rise to more enhanced local fields as ideal SERS substrates. Their detailed optical properties and local field studies are underway. This

In summary, we have successfully fabricated two different sized hollow Au nanobowl submonolayer structures using ion milling and the vapor HF etching technique. SEM and AFM measurement results display the morphology change during the fabrication process and the obtained cavity structures. UV-vis spectra and calculations indicate that the plasmon resonance of Au nanobowls can be easily tuned by changing the ratio of inner diameter to the shell thickness. A largely enhanced localized electromagnetic field at the edges of the nanobowl cavity is produced due to the broken-symmetry geometry, which shows a promising application in SERS-based biomolecular detection. Furthermore, we demonstrate that this method can be extended to fabricate complex Au nanobowl structures with a Au NP core. Acknowledgment. P.V.D. acknowledges financial support from the FWO of Flanders. LA803768Y (31) Hao, F.; Nordlander, P.; Burnett, M. T.; Maier, S. A. Phys. ReV. B 2007, 76, 245417.