3110
J. Phys. Chem. C 2009, 113, 3110–3115
Fabrication and Optical Properties of Gold Semishells Jian Ye,*,†,‡ Pol Van Dorpe,† Willem Van Roy,† Kristof Lodewijks,† Iwijn De Vlaminck,† Guido Maes,‡ and Gustaaf Borghs† IMEC, Kapeldreef 75, LeuVen, Belgium, B-3001, and Chemistry Department, Katholieke UniVersiteit LeuVen, Celestijnenlaan 200F, LeuVen, Belgium, B-3001 ReceiVed: August 13, 2008; ReVised Manuscript ReceiVed: December 16, 2008
Gold (Au) nanoshells are known to exhibit many attractive optical properties caused by the excitation of localized surface plasmon resonances (LSPRs). Reducing the symmetry of these nanoshells has a number of interesting consequences, such as exciting different plasmon modes, making the optical response angledependent, and enhancing the local electric field intensity. In this paper, a versatile procedure involving ion milling has been developed to fabricate reduced-symmetrical Au semishells. This allows us to precisely control the reduced-symmetrical geometry and, particularly, the upward orientation of the created nanoaperture. These features, along with a combination of finite different time domain (FDTD) calculations, suggest Au semishell monolayer structures for a potential application in surface-enhanced Raman spectroscopy (SERS)-based biomolecule detection. Au semishells, additionally, exhibit advantageous features over Au nanoshells, for example, a more pronounced red shift of LSPR bands by tuning the aspect ratio, a larger tuning range of optical properties, increased optical absorption at higher wavelengths, and an enhanced local electromagnetic field. Introduction Gold (Au) nanoshells, a new type of core-shell nanoparticle (NP) composed of a dielectric core, typically silica, coated with an ultrathin Au layer, possess highly favorable optical and chemical properties for applications such as localized surface plasmon resonance (LSPR) sensing, drug delivery, biomedical imaging, and cancer therapeutics.1-3 Recently, several sorts of reduced-symmetrical Au nanoshells have been fabricated, such as nano-half-shells,4 nanocups,5 nanocaps,5-8 nanoeggs,9 and nanocrescents.10-12 Reducing the symmetry of Au nanoshells has a number of interesting consequences. The aggregated Au half-shells offer a superhydrophobic surface when they are immobilized on a substrate using template-assisted self-assembly.4 Nanocup and nanocap arrays show highly tunable optical properties,6,7 and they also render their optical properties dependent on the angle and polarization of the incident light.5,6,8 Nanoeggs composed of a nonconcentric core allow for an excitation mixing of dipolar components in all plasmon modes of the particles.9 Benefiting from both plasmon coupling and sharp features, Au nanocrescent structures have shown the ability to locally enhance the electromagnetic field.10,12 All of these novel properties arising from the reduced symmetry of Au nanoshell geometry have motivated more research efforts in developing new methods of asymmetrical nanoshell fabrication. Raman scattering from molecules adsorbed on metal nanostructures is strongly enhanced due to the excitation of the local electromagnetic field. This gives rise to the well-known surfaceenhanced Raman spectroscopy (SERS), one of the best methods for label-free biomolecular detection and imaging.13 Reducedsymmetrical Au nanocrescents have been proposed and demonstrated for SERS applications in ultrasensitive biomolecular detection with a high local field enhancement factor because of * Corresponding author. Phone: +32-16-288795. Fax: +32-16-281097. E-mail:
[email protected]. † IMEC. ‡ Katholieke Universiteit Leuven.
the enhanced electromagnetic field from the nanoapertures in their structures.10-12 Various techniques including electron beam evaporation4,10 and electroless plating5,9 have been applied to prepare the reduced-symmetrical nanocrescents. However, they are almost inapplicable or inconvenient in the SERS application because of their uncontrollable or downward orientation and random position on the substrate, which limits the molecular binding to the enhanced electromagnetic field regions. The Raman enhancement factors also differ from place to place on a substrate due to the random orientation of reduced-symmetrical structures. Therefore, effective strategies to build stable, controllable, and predictable SERS substrates are needed to meet the ever-increasing demands in biomolecular detection. In this paper, we report a facile method involving ion milling to fabricate Au semishells with a well-controlled orientation, whose dimension is described by inner radius, outer radius, and cutoff height (r/R/H) (Figure 1). Our semishells may include the above-mentioned nanocaps, nano-half-shells, nanocups, or nanocrescents by varying the fractional height (H/2R) of the particles. In contrast to the previous work, the controllable design of the reduced-symmetrical geometry and preferential upward orientation and the stable monolayer configuration and nanoaperture morphologies suggest our Au semishell monolayer structures as potential SERS substrates for biomolecular detection, especially from a practical point of view. Precise control of the cutoff height of Au semishells allows us to explore the optical properties systematically from nanocups or nanocrescents to nano-half-shells and nanocaps with a comparison to complete nanoshells. The aspect ratio (R/(R - r)) and fractional height (H/2R) dependent optical properties of Au semishells are investigated experimentally and theoretically. The Au semishells exhibit enhanced absorption and scattering cross sections at higher wavelengths, compared to the Au nanoshells with the same core size and shell thickness. In particular, enhanced nearinfrared (NIR) absorption of Au semishells indicates a potential application in thermotherapy.
10.1021/jp8072409 CCC: $40.75 2009 American Chemical Society Published on Web 01/30/2009
Fabrication and Properties of Gold Semishells
J. Phys. Chem. C, Vol. 113, No. 8, 2009 3111
Figure 1. Fabrication procedure of Au semishell suspensions (route 1) and monolayer structures (route 2).
Experimental Methods 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 obtained from Sigma Aldrich (Munich, Germany). Tetraethyl orthosilicate (TEOS, 98%) and tetrakis(hydroxymethyl)phosphonium chloride (THPC, 80%) were purchased from Fluka Chemika (Buchs, Germany). 3-Aminopropyltriethoxysilane (APTES, 98%) and 3-mercaptopropyltrimethoxysilane (MPTMS, 95%) were obtained from ABCR GmbH & Co. KG (Karlsruhe, Germany). Potassium carbonate (K2CO3, 95%) was bought from Alfa Aesar (Karlsruhe, Germany). H2O2 (30 wt %), ammonia (30 wt %), and ethanol (absolute) were obtained from Honeywell Specialty Chemicals (Seelze, Germany). H2SO4 (95%) was obtained from Air Products (San Guiliano Milanese, Italy), and formaldehyde (35%) was obtained from Vel (Leuven, Belgium). Characterization. All experimental optical spectra were measured using a Shimadzu UV-1601PC and UV-3100 spectrophotometer with a spectral slit width of 2 nm and a data interval of 0.5 nm. An ITO glass with monolayer structures of Au nanoshells or semishells was oriented perpendicularly to the incident light during the measurement in air. The aqueous suspensions of Au nanoshells and Au semishells were measured in the cuvettes (Eppendorf UVette). All simulated extinction spectra were calculated from FDTD calculations. TEM images were recorded on a 300 kV Philips CM30 instrument equipped with a field emission source. A drop of the aqueous Au nanoshell or semishell suspension was placed onto a carbon-coated copper grid (Holey Carbon, 300 mesh Cu) and left to dry at room temperature for TEM imaging. AFM images were acquired in the tapping mode on a Dimension 3000/Nanoscope IV, VEECO, under ambient conditions with the scan rate between 0.4 and 0.5 Hz. Si cantilevers with a spring constant between 40 and 45 N/m were used at resonance frequencies between 250 and 350 kHz. The free amplitude peak was adjusted to 1 V. All images were flattened with a first-order correction in postprocessing. A drop of the aqueous Au semishell suspension was cast onto a Si substrate and then dried at room temperature for AFM scanning. SEM images were taken using a Philips XL30 FEG instrument operated at an accelerating voltage of 5 kV. Au Nanoshell Preparation. Au nanoshells were synthesized according to a previously described method of Oldenburg et al.14 Briefly, silica particles were synthesized following a Sto¨ber process15 by mixing ammonia and absolute ethanol, followed by adding TEOS dropwise, typically. Then 10 mL of silica colloids was directly functionalized with 50 µL of APTES without purification overnight. Next, these functionalized silica colloids were further decorated with some tiny Au colloids (1-2 nm) that were prepared by the method of Duff et al.16 and then were purified by repeated centrifugations. A subsequent 10 min
reduction in an overnight-aged mixture of HAuCl4 (3 mL, 1%) and K2CO3 (100 mL, 3.6 mM) in the presence of formaldehyde (5 mL) resulted in a continuous Au shell on the silica surface. The resulting Au nanoshells were purified by repeated centrifugations and re-dispersion in deionized water. In this paper, we investigated two sizes of Au nanoshells consisting of a 87 ( 10 nm silica core covered with a 30 ( 6 nm thick Au shell and a 224 ( 24 nm silica core with a 17 ( 2 nm Au shell. Au Semishell Preparation. Au semishell suspensions and monolayer structures were fabricated by using an in-house made ion milling system. Figure 1 outlines the procedure for fabricating Au semishells. Prior to use, indium tin oxide (ITO) glasses or Si substrates were cleaned with a piranha solution (1:3 (v/v) mixture of H2O2 and H2SO4. Caution. Piranha solution reacts Violently with organic materials), rinsed well with deionized water, and dried in a flow of N2. Au semishell suspensions were typically prepared by drop-casting aqueous suspensions of Au nanoshells on an ITO glass or Si substrate and then ion milling for 20 to 140 s. Afterward, the Au semishells were released from the slide into an aqueous suspension by ultrasonication (route 1 in Figure 1). For Au semishell monolayer structures, the clean ITO glass or Si substrates were functionalized by a MPTMS/10% ethanol solution for 3 h and, subsequently, rinsed with ethanol and dried in N2 flow. Then the substrates were placed into a 105 °C oven for 10 min to promote silanization, immediately followed by immersion into the Au nanoshell suspension (≈109 particles/mL) overnight. After the prescribed time period, the substrates were rinsed with deionized water, dried in N2 flow, and applied by an ion milling process for 20 to 140 s (route 2 in Figure 1). The ion milling system is operated with the following parameters: 375 V beam voltage, 400 V accelerator voltage, 2.4 sccm Xe flow rate, 2 sccm Ar flow rate, and below 8.0 × 10-8 mTorr base pressure in the processing chamber (for more details, see the Supporting Information). The calibrated silica milling rate is 8 ( 1 nm/min in our ion milling system, which is much slower than the milling rate of Au (35 ( 1 nm/min). Simulation. Simulations of optical spectra and near-field distribution pictures were obtained on the basis of the finite difference time domain (FDTD) method using the FDTD Solutions (version 5.1) program purchased from Lumerical Solutions, Inc. (Vancouver, Canada). The FDTD method is based on a numerical solution of Maxwell’s equations and can be used to obtain an adequate description of the electromagnetic near-field distribution around the structures with arbitrary shapes. The simulation system consists of a Au nanoshell or semishell. The particle is illuminated with a total-field scattered-field (TFSF) source,17 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 400 to 1700 nm, and the amplitude is set to 1. A perfectly matched layer (PML) is used as the radiation boundary
3112 J. Phys. Chem. C, Vol. 113, No. 8, 2009
Ye et al.
Figure 2. TEM images of different sizes of Au nanoshells (A and B) and semishells (C and D). All scale bars correspond to 100 nm.
Figure 3. SEM images of Au semishells: A-D are different sizes of Au semishells (r/R/H) on an ITO glass made by drop-casting (A and B, 43/73/109 nm; C and D, 115/130/195 nm); E and F are Au semishell suspensions made from B and D, respectively. G and H are Au semishell (115/130/195 nm) monolayer structures on a functionalized Si substrate. The inset is the corresponding AFM image. All scale bars correspond to 200 nm.
condition. The simulation region is 800 × 800 × 800 nm3 with a grid size of 3 nm. The whole simulation region is assumed in air or water depending on the actual media of the particles. We have used the dispersion model for Au derived from the experimental data provided by Johnson and Christy.18 The total complex-valued permittivity of the Au ε(ω) is modeled by the combination of a Drude and a Lorentz model and, hence, results from the sum of three different material modes, εREAL(ω), εL(ω), and εP(ω), where 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, δ0 ) 6.820 216 162 455 338 × 1014 Hz; and εP(ω) is the equation based on the Drude model εP(ω) ) (ωP2)/(iωνC + ω2) with parameters ωP ) 1.353 834 5417 988 594 × 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 400 to 1700 nm. In our experiment of changing the fractional height, we stopped the ion milling every 20 s to check the optical spectrum, in which the silica core would only be milled by 2-3 nm. On the basis of our simulation, we did not see a spectral difference originating from this correction. Therefore, for the sake of simplicity, we took the model without silica core correction for the simulation. Results and Discussion Figure 1 shows the procedure to fabricate Au semishell suspensions and monolayer structures. First, Au nanoshells with different core sizes and shell thicknesses have to be prepared, as the precursors of Au semishells, by the seeding and the electroless plating methods from silica colloids.14 Afterward, by following route 1 (Figure 1), Au semishell suspensions were prepared by drop-casting Au nanoshell suspensions onto an ITO glass or a Si slide, followed by an ion milling process for a
certain amount of time and ultrasonic re-dispersion into water. During the process, ion milling was applied to remove the top part of the Au shells, thus, forming opened nanoaperture structures. In this work, we have synthesized two sizes of Au nanoshells with an 87 ( 10 nm silica core size and a 30 ( 6 nm Au shell thickness (Figure 2A) and a 224 ( 24 nm core size and a 17 ( 2 nm shell thickness (Figure 2B). The aspect ratio (R/(R - r)) of Au semishells is determined by the core sizes and shell thicknesses of Au nanoshells and can be tuned in a broad range. For example, two kinds of Au semishells with aspect ratios of 2.5 (Figure 2C) and 7.6 (Figure 2D) have been fabricated by ion milling 60 s on the above-mentioned Au nanoshell samples in Figure 2A,B, respectively. The TEM images clearly show the nanoaperture structures on the Au semishells. In addition, Figure 3A-D shows SEM images of Au semishells with different sizes on an ITO glass made by drop-casting, confirming the upward orientation of the nanoaperture structures on top of the semishell surfaces as well. Using ultrasonication to re-disperse semishells into water, the opened nanoaperture structures of the semishells would be randomly oriented (Figure 3E,F). Alternatively, by following route 2 (Figure 1), Au semishell monolayer structures were obtained by immobilizing Au nanoshells onto a silanized ITO glass or Si slide and implementing an ion milling process afterward. Figure 3G,H displays the SEM images of the Au semishell monolayer configuration, in which a mercaptosilane functionalization was applied onto the substrate to improve the stability and coverage of semishells on the substrate. The stability of the Au semishell monolayer benefits from the strong chemical affinity between the thiol group and Au surface. Most of the Au semishells were densely packed to form a monolayer with ∼60% coverage on the substrate. Some free spaces between semishells are possibly due to the incomplete functionalization or spatial limit. Au semishell
Fabrication and Properties of Gold Semishells
Figure 4. (A) Normalized simulated (top) and experimental (bottom) optical spectra of Au nanoshell (a and c) and Au semishell (b and d) monolayer structures in air. (B) Simulation extinction peaks of Au nanoshells and semishells with different aspect ratios (R/(R - r)) in air. (C) Electric field profile of a Au semishell (d) in air at resonance λ ) 1070 nm.
monolayer structures possess combined features of the upward orientation of nanoaperture structures and the stable monolayer morphology. The optical properties of the Au semishells and the difference between the semishells and full nanoshells were assessed by full-vectorial, three-dimensional FDTD calculations and spectrally resolved UV-vis absorbance measurements. Figure 4A compares the simulated extinction spectra (top) and the measured spectra (bottom) for two different sizes of Au nanoshells and semishells. The simulated and measured spectra show a reasonable agreement and indicate an interesting trend: removing the top of the nanoshell results in a pronounced red shift of the plasmon resonance. This is corroborated by Figure 4B, which shows the simulated dependence of the extinction peak on the aspect ratio (R/(R - r)) for both Au nanoshells and semishells. The red shift of the resonance is consistent and becomes more pronounced as the aspect ratio increases. The resonance still has a dipolar character but is dominated by the local charge buildup at the edges of the open shell,19 as indicated in Figure 4C, which shows the electric field profile of the nanostructure at resonance. The charge buildup is accompanied by a strong enhancement of the local electric field as compared with the local enhancement of full nanoshells. This last aspect provides interesting prospects for SERS where signals depend strongly on the local electromagnetic enhancement. We also note that the biggest discrepancy between the experimental and calculated spectra exists in the relative ratio of the intensity between the dipolar plasmon band and the quadrupolar band of the Au nanoshell monolayer (sample c in Figure 4A). This can be explained by the reports of Khlebtsov et al.20 and Le et al.,21 where the dipolar resonance can be suppressed and the quadrupolar resonance is enhanced due to the strong interaction of dipolar plasmon resonances if some dimer, trimer, or higherordered clusters of nanoshells exist. Indeed, that is the case in our Au nanoshell monolayer structures in which ∼60% coverage will induce many nanoshell clusters. However, we do not observe the suppressed dipolar phenomenon in Au semishell monolayer structures probably because the dipolar resonance is dominated by the local charge buildup at the nanoapertures. The locality of the excited mode, hence, makes the resonance
J. Phys. Chem. C, Vol. 113, No. 8, 2009 3113 of the semishells more robust toward changes in local packing density. The other discrepancies are most likely attributable to the polydispersed size and inherent rough shell surface of Au semishells.22 For Au semishells dispersed in water, a red shift and broadening of the resonance peaks are observed as a consequence of the higher refractive index and random orientation of the particles, as expected from simulations (see Figure S1 in the Supporting Information). Another advantage of the ion milling technique in the fabrication of Au semishells is that we can prepare semishells with different fractional heights (H/2R) by tuning the ion milling time. This offers us the capability to precisely control the reduced-symmetrical geometry of Au semishells to obtain the above-mentioned nanocups or nanocrescents, nano-half-shells, and nanocaps successively. Therefore, we can systematically investigate the fractional height (H/2R) dependent optical properties of Au semishells. Figure 5A shows the theoretical calculations of the fractional height dependence on the optical properties of Au semishells with an aspect ratio R/(R - r) ) 8.7. A number of interesting results are found when the fractional height is changed. First, the dipolar plasmon band (∼870 nm) of nanoshells shifts to a higher wavelength in an earlier stage for 1 > H/2R > 0.625 and shifts back to a lower wavelength, afterward, for 0.625 > H/2R > 0, generating a “hyperbola-like” trend when H/2R was decreased (from top to bottom in Figure 5A). The maximum red shift from 870 to 1080 nm occurs when H/2R is 0.625. Second, the quadrupolar plasmon band (∼670 nm) of nanoshells generally blue shifts and attenuates gradually (1 > H/2R > 0.5) and completely disappears when H/2R is less than 0.5. We will explain the spectral changes in the extinction in the framework of the plasmon hybridization model, which treats interactions between plasmonic resonances as bonding and antibonding hybridized states.23,24 The red shift of the dipolar band for 1 > H/2R > 0.625 can be attributed to the hybridization of the dipolar resonance of the core-shell structure with the dipolar resonance of the nanoaperture. Once the aperture of the particle closes, the red shift disappears because of the lack of charge buildup in the aperture. The blue shift of the dipolar band (0.625 > H/2R > 0) can be ascribed to a geometrical transition to nanocups, leading to a higher concentration of electromagnetic field lines inside of the metal.25 The quadrupolar resonances also strongly couple to the dipolar aperture resonance, leading to a red shift for small apertures. As the aperture increases, the cavity length of the quadrupolar excitations shrinks, leading to a blue shift and an attenuation.26 Figure 5B displays the experimental fractional height dependent optical properties of Au semishells with an aspect ratio of 7.6. The experimental height of semishells is controlled by the ion milling time from 0 to 140 s and visualized by SEM images. Although it is rather difficult to etch away the bottom part of Au shells due to the intrinsic directional character of the ion milling technique, the experimental results reasonably fit the calculated results, showing a similar dipolar hyperbola shift trend and quadrupolar attenuation. For Au semishells with a smaller aspect ratio of 2.5, only a dipolar hyperbola shift was observed (Figure 5C). In the preceding text, we have demonstrated that both the aspect ratio (R/(R - r)) and fractional height (H/2R) play important roles in tuning the optical properties of Au semishells. We summarize all calculated and experimental results about the plasmon band wavelength of Au semishells with different aspect ratios and fractional heights in Figure 6. The calculations show that dipolar plasmon bands of Au semishells display a hyperbola
3114 J. Phys. Chem. C, Vol. 113, No. 8, 2009
Ye et al.
Figure 5. (A) Simulated and (B and C) experimental fractional height (H/2R) dependent optical properties of Au semishells with different aspect ratios (R/(R - r)) in air. The experimental height (H) of semishells is controlled by the ion milling time from 0 to 140 s and measured by SEM images.
Figure 6. (A) Simulated and experimental relationship between the fractional height (H/2R) and dipolar plasmon band wavelength of Au semishells with different aspect ratios (R/(R - r)) in air (solid lines are visual guides).
shift with a decrease of the fractional height for all kinds of aspect ratios (2.5-24), and the “eccentricity” of the hyperbola becomes smaller when the aspect ratio increases. It also means that the dipolar bands shift in a broader wavelength range when the aspect ratio increases. No matter how the aspect ratio changes, the dipolar plasmon bands always exhibit a red shift for 1 > H/2R > 0.625 (region 1) and a blue shift for 0.625 > H/2R > 0 (region 2). Obviously, 0.625 is the inflection point in the full plasmon bands’ shifting curve. Two experimental results with aspect ratios of 2.5 and 7.6 are also indicated in Figure 4A, which reasonably fit the corresponding calculations. The discrepancies are mainly caused by the polydispersed size, Au shell roughness of the particles, and the difficulties of the fractional height measurement. It is very important and useful to obtain the panorama of optical properties possible with Au semishells with respect to the design and optimization of the particles’ fabrication. Due to the optical absorption in the NIR region, Au nanoshells have been suggested and demonstrated for thermotherapy application.2,3 Rayleigh theory states that, for particles much smaller than the wavelength, the scattering intensity is proportional to R6, where R is the particle radius.27 However, absorption depends on the particle volume, which is proportional to R3 for a sphere. The higher-order dependence on particle size makes
Figure 7. Simulated extinction, absorption, and scattering cross section of a Au nanoshell (100/120/240 nm) and a Au semishell (100/120/180 nm) in air.
scattering more sensitive to size variations than absorption. Therefore, to increase the absorption intensity of Au nanoshells in the NIR region, Au nanoshells are preferred to be grown with a small core size (e.g.,
