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Controlling Interparticle Gaps in Self-Organizing Gold Nanostructures on Templates Made by a Modified Hard Anodization Technique Peter Nielsen,*,†,‡ Ole Albrektsen,† Søren Hassing,† and Per Morgen‡ Institute of Sensors, Signals and Electrotechnics (SENSE), The Faculty of Engineering, UniVersity of Southern Denmark, CampusVej 55, DK-5230 Odense M, Denmark, Department of Physics and Chemistry, The Faculty of Science, UniVersity of Southern Denmark, CampusVej 55, DK-5230 Odense M, Denmark ReceiVed: December 15, 2009; ReVised Manuscript ReceiVed: January 14, 2010
In this article, we demonstrate how the formation of large-area self-organizing gold nanostructures formed on porous alumina templates can be grown with interparticle gaps that can be tuned both by appropriate choice of anodization technique and by the amount of deposited gold. The gold nanostructures reported in this work are formed by sputter-coating the porous alumina templates made with a hard anodization technique in oxalic acid, and the interparticle gap size is reproducibly controlled simply by adjusting the amount of sputter-coated gold. To make a change from mild and into hard anodization regimes in oxalic acid, several stages are used in the anodization procedure, including the use of a protective porous oxide layer initially created under mild anodization conditions. This simple stepwise anodization technique, which only uses electrolyte cooling, can facilitate burn-free anodization at even higher voltages during hard anodization processing in oxalic acid. The formation of the gold nanostructures is studied with scanning electron microscopy, and the influence of the morphology on the optical properties of the nanostructures is investigated by optical reflectance spectroscopy. Tunability of the localized surface plasmon resonances is demonstrated, and it can be optimized and exploited with a special view to surface enhanced molecular sensing techniques. Introduction Optical properties of metal nanoparticles have in recent decades attracted increasing attention due to the particles’ capability of assisting in the creation of immense electromagnetic fields in their vicinity and in the gap between particles, often figuratively termed “hot-spots”.1 The huge local enhancement of the exciting field is commonly interpreted as resonant electron oscillations in nanostructures typically made from silver or gold. Known as localized surface plasmon resonances, the wavelengths of these optical resonances can be deduced from reflection and extinction spectra, where the resonances will show their presence in the form of characteristic peaks and dips with spectral positions depending on the size, shape, material, surrounding medium and degree of aggregation of the nanoparticles. Raman spectroscopy is in many applications limited by the relatively weak Raman signal from molecules, as compared to, for example, fluorescence. By utilizing the strong enhancement of the electromagnetic field in the vicinity of the mentioned metallic nanostructures, surface enhanced Raman scattering2-5 (SERS) can overcome some of the limitations, and sensitivities down to Raman signals from a few or even single molecules have been reported. Such enhancements have in particular been associated with the earlier mentioned “hot-spots” in the junctions between almost touching metal nanoparticles.1 With the purpose of studying the enhancement mechanism and reproducibility of “hot-spots” on a fabricated substrate, it is important to develop a method capable of controlling the dimensions of the involved nanoparticles, such as the size, shape, periodicity, degree and type of aggregation, as well as the size of the interparticle gaps between the individual nanoparticles. Gaining control of these * Corresponding author. E-mail:
[email protected]. † SENSE. ‡ Department of Physics and Chemistry.
parameters on the nanoscale and utilizing the particles’ unique qualities within diverse fields of science, such as in sensors, solar cells, microelectronics, hydrogen storage and surfaceenhanced spectroscopy, is all encompassed by the field of nanotechnology. Approaches within nanofabrication are manifold and typically classified as either “top-down” or “bottom-up” techniques. Common serial top-down techniques, such as electron beam lithography (EBL) and focused ion beam (FIB) methods for pattern control, are often far superior to other nanofabrication techniques in that they can define the size, shape, and pattern of the nanoparticles almost arbitrarily, though they are troubled by technical challenges when fabricating sub-10 nm gaps.6,7 Furthermore, the extensive processing time needed in EBL and FIB for the fabrication of metal nanostructures over large areas makes these techniques expensive, time-consuming, and in this respect inferior to parallel techniques. Less expensive but still time-consuming alternative parallel top-down techniques include colloidal lithography and roughened metal surfaces.2 Although proven successful within surface-enhanced spectroscopy, the critical dimensions of the obtained nanostructures obtained with the latter technique are of a fluctuating and irreproducible nature. Consequently, “bottom-up” techniques with comparably very low fabrication and time expenses per area have advanced as an elegant and prevalent alternative to meet the demand for control of the dimensions of nanostructures on a large scale. Self-organizing, porous, anodic alumina with hexagonally ordered nanopores has in recent years attracted a great deal of attention for technologic and scientific applications. By appropriate choices of electrolyte, temperature, anodization time, and applied voltage during a mild anodization (MA) process, the dimensions of the hexagonally ordered porous oxide can be controlled over a wide range by a two-step method.8-14 The window of possible dimensions can be further widened by the
10.1021/jp9118452 2010 American Chemical Society Published on Web 02/04/2010
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use of hard anodization (HA) techniques,15,16 in which the anodization voltage is increased to 3-4 times the level employed during the mild anodization, resulting in a two orders of magnitude higher current density and, thus, a much higher alumina growth rate. The huge amount of heat generated by such a HA process necessitates an efficient cooling of the substrate,16 whereas for the MA processes, cooling by a lowered temperature of the electrolyte by itself is sufficient. Especially at anodization at higher voltages, the formation of protective oxide layers and ramping of the voltage might be necessary to avoid breakdown effects.13,15 Although the past 15 years of research within anodization has been focused mainly on aluminum and its applicability in templates for nanostructures, anodization of other materials is gathering interest as, for example, anodization of Ti for solar cells.17,18 Whereas our earlier work has focused on the fabrication of large-area self-organizing gold nanostructures with sub-10 nm gaps on mildly anodized porous alumina templates,19 we here present a new technique based on HA for fabrication of selforganizing gold nanostructures with size-controllable interparticle gaps over large areas. A hexagonally ordered porous alumina template with an interpore distance of 270 nm is fabricated by the use of a novel stepwise HA technique utilizing protective oxide layers created by MA prior to entering the HA regime. The developed technique facilitates HA without the need for direct cooling of the substrate and can be used to tailor the alumina template dimensions continuously. The porous alumina is afterward removed by selective etching, and the underlying imprinted aluminum is subsequently sputter-coated with gold, forming a new type of tunable, self-organizing structures. The dependence of the shape of the structures and the size of the interparticle gaps on the amount of sputter-coated gold is monitored systematically. The morphology of the fabricated structures is investigated by scanning electron microscopy (SEM), and the optical properties of the structures are examined by UV/vis reflection measurements. Experimental Section Synthesis of hexagonally ordered porous templates by anodization in various electrolytes8-15 and of different materials17,18 is a well-established method in bottom-up nanotechnology. The overall fabrication procedure presented in this article is illustrated in Figure 1. It combines the known MA technique in step 1 with a new stepwise ramping procedure in step 2 for making HA in oxalic acid by cooling the substrate with only the used electrolyte. A selective etching (step 3) removes the porous alumina entirely, and by adjusting the amount of sputtercoated gold in step 4, the size of the formed gold nanostructures and the interparticle gaps are demonstrated tunable over large areas. Furthermore, the continuous self-organization voltage regime (110-160 V) for the HA in oxalic acid in step 2 can be used to tailor the interpore distances in the range of 220-300 nm,16 in addition to the resulting gap sizes after sputter-coating. Hexagonally ordered porous alumina samples were prepared using high-purity (99.999%) aluminum foils from Goodfellow cut into the dimensions 1 × 25 × 40 mm. The foils were degreased in acetone with ultrasound for 10 min and subsequently annealed for 5 h at 500 °C under a low flow rate of argon. The annealing was performed in a tube furnace from Lenton Thermal Designs type LTF 16/50/180 fitted with a Eurotherm temperature controller, model 2408 CP. Following this, the lower half of the annealed substrate, corresponding to an area of approximately 5 cm2 (25 × 20 mm) was electropolished to a mirror-like surface with a Struers Lectropol-5,
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Figure 1. Schematic representation of the fabrication process developed for making self-organizing hexagonally ordered gold nanostructures. (1) An annealed and electropolished aluminum sheet is anodized shortly in oxalic acid (MA conditions), resulting in a protective porous oxide layer with an average interpore distance ∆1 ) 103 nm. (2) The anodization voltage is ramped to 120 V (HA conditions), resulting in a thick porous layer with an average interpore distance ∆2 ) 270 nm. (3) The oxide layer is removed by selective etching and (4) subsequently sputter-coated with gold. The gold particles are in the bottom of the figure illustrated as spheres, although the shape of the actually fabricated structures depends on the amount of sputter-coated gold.
applying a mixture of 1:5 w/w perchloric acid and ethanol. Electropolishing (35 s) was done using a voltage of 48 V, 9 on the flow rate scale of the instrument and at a temperature of 20 °C. The initial anodization (step 1 in Figure 1) was carried out with the purpose of creating a 200-400-nm-thick protective porous oxide layer on the aluminum surface. The layer was formed under MA conditions in oxalic acid with a molar concentration of 0.30 M and a constant applied voltage of 40 V. For all anodizations, the temperature of the electrolyte was kept at 0-2 °C using an Ultrathermostat type 03E 623 from Heto, and a vigorous stirring of the electrolyte was maintained during the 8 min of anodization. Only the electropolished part of the aluminum was exposed to the electrolyte during the anodization. As illustrated in Figure 1, the anodization results in vertically aligned pores in the oxide, with the bottom of the pores separated from the aluminum by a thin alumina layer known as the barrier oxide. An instantaneous switch-on of the HA voltage of 120 V in oxalic acid resulted in immediate breakdown in the electrolyte, causing serious damage to the oxide film (so-called “burning”12) in the shape of film thickening and black, protruding areas. This
Controlling Gold Nanostructure Interparticle Gaps
Figure 2. Graphs showing time-related developments of the anodization voltage and the resulting current density in the MA and HA regimes. The resulting measured current density is plotted for the corresponding voltage intervals with a close-up of the MA interval (the first 8 min). The marked regions correspond to (1) MA in 0.30 M oxalic acid at 40 V for 8 min; (2) 1 min break at 0 V; (3) ramping the voltage from 40 to 110 V at a rate of 7.0 V/min, with an observed exponential increase in the current density; (4) 3 min break at 0 V; (5) further ramping from 110 to 120 V at a rate of 1.7 V/min; and (6) holding the voltage at 120 V for 10 min.
reaction has been linked to the high current density caused by the large electric field at the barrier oxide. The resulting heavy evolution of heat will lead to burning if insufficient cooling of the substrate is applied. Consequently, cooling of the substrate with an efficient cooling stage in direct thermal contact with the substrate, such as by a Peltier element, is therefore necessary. However, no such setup was available, and therefore, a systematic development of a more gentle and reproducible procedure including ramping of the voltage from 40 to 120 V was conducted. Direct continuous ramping of the voltage from 40 to 120 V caused an escalating increase in the current density, calling for voltage ramping in a stepwise fashion. The voltage was applied with a Power Supply EA-PS 9300-02, from Elektroautomatik, which is capable of supplying voltages and currents in the range of 0-300 V and 0-2 A, respectively, and controlled by a Labview computer program with a voltage ramping option. The developed HA-voltage profile capable of averting catastrophic breakdown in the electrolyte is shown in Figure 2. The anodization current was continuously logged with a Fluke 189 True rms Multimeter, and the resulting current density as a function of time is also plotted in Figure 2 for the corresponding voltage intervals. The 1 min break separating the mild and hard anodization intervals in the depicted voltage profile was added for practical reasons, but has no influence on the final outcome and could be omitted. Initially, the voltage is increased steadily from 40 to 110 V at a rate of 7.0 V/min. The current density increases exponentially when the voltage reaches approximately 70-80 V, accompanied by a heavy discharge of hydrogen at the cathode. By adding a 3 min break to the anodization procedure before proceeding with the ramping from 110 to 120 V, the gas bubbles accumulated in the electrolyte container are allowed to diffuse away while the generated heat at the pore bottom is dissipated in the cooled electrolyte. The need for the additional ramping from 110 to 120 V is due to the fact that the homogeneity of
J. Phys. Chem. C, Vol. 114, No. 8, 2010 3461 the porous layer and the size of the hexagonally ordered domains increase dramatically when raising the voltage above 110 V (ideally in the range of 120-150 V, in which case the domain size is insensitive to the anodization voltage).16 The ramping of the voltage from 110 to 120 V at a rate of 1.7 V/min after the break causes a significant drop in the current density, as compared with before the break. During the following 6 min of ramping, the current density continuously decreases (region 5 in Figure 2) until 120 V is reached, at which point the rate of decrease becomes larger. For HA, the exponential decrease in the current density in region 6 has been related to the continuous growth of a porous layer at a constant voltage, where diffusion of the electrolyte to the barrier oxide layer at the pore bottom through the rapidly growing length of the pores limits the current density.16 This in turn results in a decreasing growth rate of the porous layer; however, the rate is still much larger than during the MA process. The upper left inset in Figure 2 shows a close-up of the logged current density during the MA process (region 1), where the current density initially drops to a minimum after ∼100 s due to the formation of a nonporous barrier oxide layer. This decrease in current density is followed by an increase caused by pore initiation on the surface until steady state in the current density is reached when, for MA, the continuous growth of pores at a constant rate is started. This happens typically within the first 5-10 min, depending on the electrolyte temperature.20 The interpore distance for MA with various electrolytes has on several occasions been reported to be linearly dependent on the applied anodization voltage, with a constant of proportionality of ζMA ) 2.5 nm/V,21 and similarly, a constant of proportionality for HA has been reported: ζHA ) 2.0 nm/V.16 For oxalic acid, the ζMA constant of proportionality is valid around 40 V; while ζHA is valid in the interval 120-160 V, although this linear relationship recently has been shown to be valid only for small changes in the anodization voltage.22 As a consequence of the applied voltage ramp, the interpore distance of the porous layer will change as a function of depth through the layer, starting with the topmost layer created under MA, having an interpore distance of ∼100 nm, continuously increasing to a distance of 270 nm at the bottom of the pores. This final interpore distance at the bottom of the pores can be tuned simply by adjusting the anodization voltage in the range of 110-160 V. Etching of the topmost porous alumina layer was carried out in a mixture of 5% w/w phosphoric acid and 1.8% w/w chromic acid at 70 °C for 16 h. The etching process revealed the selforganized hexagonally ordered pore bottom. Due to a very high selectivity of the etchant, the alumina will be removed from the substrate without modifying the underlying aluminum structures, even for prolonged etching times. Formation of gold particles on top of the hexagonally ordered pore bottom was realized by using a Fine Coat Ion Sputter model JFC-1100 from JEOL. An ionization voltage of 1200 V and a sputter current of 5 mA was applied with the sample placed at an angle of 60° between the normals of the sputter target and the sample. A 2.5 nm/min sputter-coating rate was applied for all fabricated surfaces. A Cressington Sputter Coater 208 HR equipped with a Thickness Controller MTM 20 was, when stated, used to enhance the contrast of the imaged oxide in the SEM micrographs by sputter-coating a thin gold/palladium layer on the otherwise insulating alumina.
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Figure 3. SEM micrograph top view of the porous alumina layer seen after MA in 0.30 M oxalic acid at 40 V and an opening of the topmost nonporous layer with phosphoric acid. The surface was sputter-coated with 3 nm gold/palladium (80/20) to avoid charging of the otherwise insulating alumina. The pores extend into the plane of the micrograph and are seen as black areas, whereas the white areas are the pore walls separating the individual pores. From this SEM micrograph, the interpore distance in the ordered domains is estimated to be 103 ( 5 nm. Domains with hexagonally ordered pores are encircled by boundaries with merging pores.
The surface morphology of the nanostructures was investigated with SEM: JEOL JSM-6490LV with a maximum resolution of 3 nm at 30 kV and a Hitachi S-4800 FE-SEM with a maximum resolution of 1.4 nm at 1 kV. The optical properties of the fabricated structures were investigated by reflection measurements, using a UV-vis-NIR lightsource DH-2000 from Ocean Optics. The light was focused onto the substrate and collected by a 0.85 numerical aperture objective. Reflection spectra were recorded with an Avaspec2048 Fiber Optic Spectrometer from Avantes. A flat gold reference surface for the reflection measurements was fabricated by evaporating 100 nm of gold onto a piece of Si wafer with a Cryofox 600 Explorer from Polyteknik. Results and Discussion SEM Characterization. Generally, when terminating the anodization process, the top of the pores will tend to close, and the surface will appear nonporous when investigated from a top view in an SEM. Therefore, to investigate the pores, a 30 min etching of the top oxide layer in 5% w/w phosphoric acid at 30 °C is necessary. Figure 3 shows a top view of a porous layer fabricated under MA conditions in 0.30 M oxalic acid at 40 V, corresponding to step 1 in Figure 1. The surface was sputter-coated with 3 nm gold/palladium (80/20) to avoid charging of the otherwise insulating alumina when viewed in the SEM. For this SEM micrograph, as well as for all other presented micrographs in this work, the illustrated structures constitute a representative selection of the entire anodized surface. The interpore distance of the porous layer in Figure 3 is from the SEM micrograph estimated to be 103 ( 5 nm. Hexagonally ordered pore domains are encircled by boundaries with merging pores. This merging is a part of the selforganization process in which the hexagonally ordered domains at the aluminum-alumina interface will enlarge as the anodization time is prolonged or repeated.8 This oxide film with a thickness of ∼200-400 nm is in step 2 acting as a means of promoting a uniform oxide film growth during the HA by evading breakdown related effects, such as locally confined enormous current densities, film thickening, burning, and heavy gas evolution at the cathode.12,13,16 A relatively thick, yellowish oxide layer is created by the HA step (step 2 in Figure 1). An SEM micrograph taken at an
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Figure 4. SEM micrograph taken at an angled view of a crack in the oxide fabricated by anodization employing the voltage profile presented in Figure 2. The oxide layer appears brighter in the SEM micrograph, as compared to the aluminum. The thickness of the oxide layer, as indicated by the double arrow, is estimated as ∼30 µm. The pores, clearly seen in the inset, run in parallel with the arrow.
Figure 5. Top view of the scallop-shaped aluminum template revealed after removal of the approximately 30-µm-thick porous oxide layer by selective oxide etching. The porous oxide was formed by HA in 0.30 M oxalic acid applying the voltage profile presented in Figure 2. Perfect hexagonal order is observed within the domain, and each pore bottom is seen to be surrounded by six small white apexes. The interpore distance is in the SEM micrograph estimated to be 270 ( 10 nm.
angled view of an oxide crack, is shown in Figure 4, where the porous oxide layer, indicated by a double arrow, appears brighter than the underlying aluminum. The thickness of the oxide is estimated (by also considering the tilting angle) as 30 µm ( 5 µm. Due to the selectivity of the subsequent etching (step 3), a precise evaluation of the thickness is not vital. The inset SEM micrograph shows the porous layer near the alumina-aluminum interface, where the pores have self-organized into a hexagonal pattern with an interpore distance of 270 ( 10 nm. By performing the etching procedure (step 3 in Figure 1) for 16 h, the oxide layer will be completely removed, and a hexagonally ordered scallop-shaped aluminum bottom is revealed, as shown in Figure 5. At each joining of three adjacent pores, a small, white apex protrudes. From the SEM micrograph, the interpore distance is estimated to be 270 ( 10 nm. From these results it is deduced that HA in oxalic acid can be performed with such a relatively small and simple setup as used here with no need for direct cooling of the substrate by, for example, a Peltier element. By carrying out the ramping of the voltage in intervals, the generated heat and gas can dissipate. This simple stepwise voltage sequence could be proven successful in achieving burnfree anodizations at even higher voltages with other setups, as well, thereby rendering possible the tuning of the interpore distances at even larger intervals than has so far been reported with HA in oxalic acid.16
Controlling Gold Nanostructure Interparticle Gaps
Figure 6. SEM micrograph of hexagonally ordered and triangularly shaped Au nanostructures with sub-10 nm gaps. The underlying alumina template was fabricated by MA and has an interpore distance of 103 ( 5 nm.19
Earlier published work has revealed that sputter-coating of the pore bottom made with MA in oxalic acid under the right circumstances could result in hexagonally ordered self-organizing gold nanostructures with interparticle gaps less than 10 nm wide.19 An example of this is shown in Figure 6. No systematic investigation of the growth of these nanostructures was reported earlier, whereas in the present article, the amount of gold sputtercoated onto the aluminum pore bottom was varied systematically in the range of 25-137.5 nm. A representative number of SEM micrographs illustrating the resulting structures can be seen in Figure 7. It is inferred from Figure 7a) that the gold will commence its growth on the apexes between adjacent pores with a gap size typically in the range of 80-100 nm, and as the amount of gold is increased, a protruding hemispherical gold structure will emerge. Although the interparticle distance on a given substrate is mostly within a rather narrow range, a few spots, where two adjacent apexes have almost merged, will
J. Phys. Chem. C, Vol. 114, No. 8, 2010 3463 always be present. This phenomenon is present at the domain boundaries and is related to the finite domain sizes. By optimizing the anodization parameters (anodization time, voltage, temperature, electrolyte concentration, etc.), utilizing a twostep procedure,8 or using various prepatterning techniques prior to anodization,23-26 the hexagonal order of the pores has been reported perfect over as much as 50-100 µm2. The gap separating the individual gold particles will change its shape, as the size of the particles increases, and the gap widths tend to get smaller as the amount of sputter-coated gold is further increased until the sub-10 nm gap regime is reached. The shape of the particles is modified from hemispherical shells into increasingly sharper triangular nanoprisms as the amount of gold is increased. The diameter of the remaining pore opening (seen as the dark spot surrounded by six prisms) will gradually shrink as the gold amount is increased. Although it is difficult to estimate the topography of the individual particles with an SEM, it seems as if the top surface of the particles (parallel to the substrate surface) flattens as the particles change from hemispherical shells into the triangular prism shape. This flattening might have an essential influence on the local electromagnetic fields. Figure 8 shows SEM micrographs with a higher magnification of the substrates from Figure 7d, f, and h. The change in the shape of the gold particles and the narrowing of the gaps as the amount of gold increases is more clearly seen. It should be mentioned that the fabricated gold nanostructures in Figure 7 cover the entire anodized area, which in this case is approximately 5 cm2. Thus, a method for fabrication of substrates with self-organizing, easily controllable interparticle gaps over large areas has been established. Further tuning of the particle and gap size can be accomplished simply by tuning the interpore distance with the applied HA voltage as well as the amount of sputter-coated gold. Comparing the fabricated structures in Figure 7 with the earlier published structures fabricated by MA (Figure 6), it is found, that the
Figure 7. SEM micrographs of the bottom of the underlying aluminum templates after gold sputter-coating the substrates anodized under HA conditions in 0.30 M oxalic acid. The amount of gold has been systematically varied according to the table at the lower right to investigate the growth of the gold nanostructures. The formed hemispherical gold protrusions (a-c) will evolve into a triangular prism shape (gradually from c to h) when the amount of deposited gold is increased, and the interparticle gap will narrow, until the sub-10 nm regime is reached.
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Figure 8. (a-c) SEM micrographs of the substrates shown in Figure 7d, f, and h, respectively, at an increased magnification.
This change in the spectral position of the resonances could possibly also be affected by the transformation of the gold particles into triangular nanoprisms. Thorough investigations of the optical properties of metal nanoprisms and the change in the spectral position of the resonance as the tip of the prism gets more and more truncated have been carried out both in simulations and in experiments.29-32 Increasing the edge length of a 20-nm-thick gold prism in the interval 50-200 nm gives rise to optical resonances red-shifting from 650 to 1000 nm.31 As the tip of a truncated prism is made sharper, a red shift will again be introduced, and the resonance band will become broader.29,31 Examining the structures in Figure 7, it is reckoned that the red shift could partly be attributed to the increasing outer diameter of the hemispherical shells while the growing edge length and sharpening of the prism tips could be responsible for the widening of the reflection spectra toward longer wavelengths. Additionally, the size of the gap between the individual gold particles will influence the shape of the spectra and possibly lead to electromagnetic hot-spots.
Figure 9. Reflection measurements on samples sputter-coated with gold deposited with thicknesses indicated in the top right corner of the figure. The resonance dip at approximately 550 nm for the sample sputter-coated with 25 nm gold is seen to red-shift as the amount of gold is increased. The inset illustrates a truncated nanoprism, where the edge length and the size of the snip have been shown to be important factors in determining the spectral position of the optical resonances.29-32
hexagonal order now is maintained over significantly larger areas. This can be utilized in experiments, where the geometrical order, for symmetry reasons, may need to be extended to an area larger than a focused laser spot. Optical Properties of Substrates. UV/vis reflection measurements were conducted in order to obtain the localized surface plasmon resonance spectra. The results for a representative number of samples are presented in Figure 9. The reflectance spectrum recorded from the substrate sputter-coated with 25 nm gold exhibits a pronounced minimum around 550 nm. A distinct resonance at this position could be assigned to the presence of the small protruding gold semishells with diameters of approximately 100 nm formed on the apexes between adjacent pores. It is well-known that increasing the size of metal nanoparticles beyond the quasi-static limit will lead to a red shift of the localized surface plasmon resonance and an increase in line width.27-29 For gold shells with a 50 nm radius, Mie calculations show that increasing the gold shell thickness will similarly result in a red shift, given that the shell thickness is larger than approximately 30 nm. As the amount of gold and, consequently, the radii of the gold semishells are increased, the resonances shown in Figure 9 gradually become broader, and a dip at increasingly longer wavelengths also appears.
Systematic investigations of the influence of the gap size on extinction spectra have been carried out for solid gold dimers of 100 nm diameter on a glass substrate, showing that the extinction peak will be red-shifted as the gap narrows.33 In fact, in the cited paper, the extinction maximum when varying the gap from 94 nm to almost 0 nm is demonstrated to shift approximately 90 nm, stressing that the gap width indeed has a substantial effect on the optical properties of nanostructures while at the same time giving rise to a large increase in the molecular detection sensitivity. In our case, a similar effect could be responsible for the shift in the resonance on the fabricated structures. Apart from the fact that the shape, size, and interparticle distance of the nanostructures influence the optical properties of the substrate, these parameters also affect the applicability of the substrates within sensor technology. Although enormous enhancements of the exciting electromagnetic field close to the tip of prisms or in the gap between nearly touching particles can be present on a substrate, there is no guarantee that a probing molecule would be located at this position. This is influenced by the dimensions and orientation of the specific molecule as well as its chemisorption affinity to the gold. In a recent paper, it was shown that the amount of molecular information that can be extracted from an SERS experiment in cases that the molecule is adsorbed in a site with highly anisotropic field distribution is drastically reduced.34 The results obtained in the present paper open up new possibilities for the design and realization of nanostructures, where the molecules can be trapped in sites in which electric fields with different spatial field distributions may be excited and thereby increase the amount of molecular information. With the possibility of tuning the optical properties, these substrates could evidently serve as versatile molecular sensors.
Controlling Gold Nanostructure Interparticle Gaps Conclusion We have invented and explored a stepwise hard anodization procedure without the need for direct cooling of the substrate. This was made possible by initially forming a protective porous alumina layer with mild anodization prior to entering the hard anodization processing conditions, as detailed in the text. The formation of the alumina causes it to self-organize into a porous layer with an interpore distance of ∼270 nm, which can be tuned continuously by adjusting the applied anodization voltage. By applying a stepwise voltage profile, hard anodization at higher potentials without burning might be possible, enabling an increased processing versatility with increased tunability of the pore dimensions. Following a selective etching of the porous alumina, the revealed aluminum/barrier oxide templates were sputter-coated with gold, systematically varying the thickness. Over large sample areas, this procedure created a new type of self-organizing triangular nanostructures with easily controllable interparticle gaps. The structures were investigated by optical extinction measurements in reflection, from which it could be concluded that a change in the morphology of the gold structures and the width of the interparticle gap had a direct effect on the optical properties of the structures. The tunability of the fabricated structures will be optimized and exploited with special reference to surface enhanced molecular sensing techniques. Acknowledgment. The authors thank Danny Kyrping for his help with the Labview computer program for the programmed voltage ramping. References and Notes (1) Kneipp, K.; Wang, Y.; Kneipp, H.; Pearlman, L. T.; Itzkan, I.; Dasari, R. R.; Feld, M. Phys. ReV. Lett. 1997, 78, 1667–1670. (2) Jeanmaire, D. J.; Van Duyne, R. P. J. Electroanal. Chem. 1977, 84, 1. (3) Fleischmann, M.; Hendra, P. J.; McQuillan, A. J. Chem. Phys. Lett. 1974, 26, 163. (4) Albrecht, M. G.; Creighton, J. A. J. Am. Chem. Soc. 1977, 99, 5215. (5) Willets, K. A.; Van Duyne, R. P. Annu. ReV. Phys. Chem. 2007, 58, 267. (6) Fursina, A.; Lee, S.; Sofin, R. G. S.; Shvets, I. V.; Natelson, D. Appl. Phys. Lett. 2008, 92, 113102.
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