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Metallic nanosieves formed by ultra-short-pulse laser ablation
Applied Surface Science 255 (2009) 4246–4249
Contents lists available at ScienceDirect
Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Metallic nanosieves formed by ultra-short-pulse laser ablation C. Smith a,b, B.H. Christensen a, J. Chevallier a, P. Balling a,b,* a b
Department of Physics and Astronomy, University of Aarhus, DK-8000 Aarhus C, Denmark Interdisciplinary Nanoscience Center iNANO, University of Aarhus, DK-8000 Aarhus C, Denmark
A R T I C L E I N F O
A B S T R A C T
Article history: Received 1 September 2008 Received in revised form 6 November 2008 Accepted 6 November 2008 Available online 17 November 2008
The formation of free-standing gold nanosieves by ablation with ultra-short laser pulses is demonstrated. Macroscopic areas are generated fast and efficiently by the application of a parallel production technique. The technique is based on a lens array formed by self-assembling quartz microspheres on a thin metal foil. The evaporated foils have a final thickness of 400 nm, and the hole spacing is set by the diameter of the microspheres ( 7 mm) while the pore size is 700 nm. The characteristic spacing of the generated hole structure is verified by an optical diffraction technique. ß 2008 Elsevier B.V. All rights reserved.
PACS: 78.67. n 79.20.Ds 81.16. c Keywords: Laser ablation Ultra-short laser pulses Nanosieves Structured metal foils
1. Introduction The formation of nano- and microsieves has received significant interest in recent years [1–7]. They have been demonstrated to be of interest in such diverse areas as filters and sensor technology. The filters are, e.g., used in water purification [1], filtration of lager beers [2] and separation of hydrogen from helium [3]. Metallic nanosieves have also been investigated with interest in their optical properties. The pioneering work by Ebbesen et al. has demonstrated many interesting effects, including the transmission of light through holes with diameters below the classical transmission limit [4] and resonant transmission of specific wavelengths through periodic arrays of 100-nm-sized holes [5–7]. In this paper, a fast and inexpensive method for the production of metallic nanosieves will be demonstrated. Previous investigations have shown that the application of a self-assembled microsphere lens array in combination with illumination by ultra-short laser pulses provide very efficient nanostructuring of an extended area [8,9]. The technique provides an alternative to
* Corresponding author at: Department of Physics and Astronomy, University of Aarhus, Ny Munkegade, Building 1520, DK-8000 Aarhus C, Denmark. Tel.: +45 8942 1111; fax: +45 8612 0740. E-mail address: [email protected] (P. Balling). 0169-4332/$ – see front matter ß 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2008.11.016
the application of a lens array in close proximity to the surface [10], and the self-assembly method is less expensive and less critical to align. The present investigation combines the method described in Refs. [8,9] with state-of-the-art thin film technology, to produce free-standing metallic nanosieves. The manufactured structures are characterized by scanning-electron microscopy and optical diffraction. 2. Experimental Ultra-short laser pulses of 100-fs pulse duration and a wavelength of 800 nm are sent through a 10 microscope objective onto the thin-film samples, see Fig. 1. As described previously in Refs. [8,9], the laser beam is shaped into a multitude of foci by a selfassembled array of 6.8- mm-diameter quartz spheres deposited on the thin-film samples. The sample preparation will be described in more detail below. Here it is stressed that the use of the on-sampledeposited lens array permits the application of a comparatively large laser spot on the sample thereby allowing the production of typically 1000 holes per laser pulse. A schematic of the experimental setup is shown in Fig. 1. In order to obtain a reasonably constant intensity distribution over the laser spot, the Gaussian laser beam is telescoped down to a diameter of 4 mm and apertured by a 2.5-mm-diameter diaphragm. This truncated Gaussian profile is then imaged onto
C. Smith et al. / Applied Surface Science 255 (2009) 4246–4249
Fig. 1. A schematic of the experimental setup for producing the nano-structured metal films showing the optical setup for the laser beam and a CCD-based built-in inspection system, see discussion in the text.
the sample by the microscope objective to provide a 250- mmdiameter laser spot on the sample. Slight variation in the laser intensity is still observed as a spatial dependence of the hole diameter in the produced hole array with larger holes being formed near the middle of the laser beam. Larger surface areas are laser structured by translating the sample perpendicularly to the laser beam by 250 mm between laser pulses. In the present setup, this sample translation is done by hand, so the laser is set to run at a 10 Hz repetition rate, and a mechanical shutter is used to subject each position on the sample to a single laser pulse, see Fig. 1. A camera looking through one of the dielectric mirrors allows inspection of the laser-irradiated area. In principle, striking the sample by a few pulses will not induce significant changes, since the quartz spheres are ejected after a single laser pulse, and the laser is operated at a fluence, which is chosen so that there is no or very little ablation of the sample in the absence of the focussing effect of the spheres. The average laser fluence applied for ablation of the thin foils is typically 0.75 J/cm2, which is around the threshold for (single-shot) ablation [11]. Since the optimum focus provided by the quartz spheres is at some distance behind the sphere, the sample preparation incorporates a transparent spacer layer of quartz (see below). As discussed in more detail in Ref. [8], the optical properties of the quartz spheres can be described either by the Mie theory or to a reasonable approximation also by geometrical optics, since their dimensions exceed the wavelength sufficiently. While the first in its simplest form neglects the presence of the surface, the latter fails to include diffraction effects. However, both methods predict an intensity maximum roughly 3 mm behind the spheres, which dictates the thickness of the applied spacer layer. The Mie theory predicts an intensity enhancement relative to the impinging light of 50. This brings the peak fluence to 35 J/cm2, which shows that the hole formation is performed in the high-fluence limit of short-pulse ablation. Consequently, heat transport is expected to play a decisive role for the geometry of ablated structures. This is supported by numerical simulations of the ablation process in the two-temperature model [9,12]. The thin-film samples are prepared by vacuum evaporation and radio-frequency magnetron sputtering. For the present investigation, a gold foil is produced, but with small changes to the production technique, a broad range of metal foils can be structured. The thickness of the foil is chosen as a compromise between two constraints. The depth of the laser-ablated holes must exceed the foil thickness to ensure the generation of through holes, but on the other hand the thickness of the foil must be significantly greater than the optical penetration depth in order to facilitate a measurement of the optical properties of the nanosieve.
4247
Based on measured hole depths in bulk samples, a thickness of 400 nm is chosen, which corresponds to the attainable hole depths due to the significant heat transport [9]. The foil thickness is still more than an order of magnitude larger than the optical skin depth of gold in the visible range. As described briefly above, to obtain the optimum focussing conditions of the quartz spheres, a transparent spacer layer must be applied. This puts constraints on the foil-preparation procedure. Initially, foils were produced on a glass substrate with a quartz spacer layer. This leads to the generation of foils possessing a high optical quality yet are difficult to remove from the substrate. In order to overcome this problem, an attempt was made to apply a thin sodiumchloride (NaCl) layer below the gold foil for easy subsequent release in water. This procedure had, however, to be discarded since the application of the thin quartz layer led to a release of the gold foil from the NaCl layer because of the mechanical stresses in the quartz film. The solution, which was finally chosen, applied a sequence of five layers on the glass substrate. First 10 nm Ti is deposited by electron-gun evaporation at a rate of 1 nm/s followed in the same run by the deposition of 200 nm Cu at 2.5 nm/s and finally 400 nm Au at 2.5 nm/s. Then, 5 nm chromium is deposited by RF magnetron sputtering at a rate of 0.5 nm/s followed by 3 mm quartz at 0.39 nm/ s. This sequence ensured optimum bonding between the substrate and the gold film (by using Ti and Cu) and between the gold and quartz (facilitated by the thin Cr layer). This technique facilitates the production of free-standing gold nano sieves by subsequent removal of the quartz layer by hydrofluoric acid and of the copper layer by nitric acid. A drawback of this method is that after laser ablation and before release, the combined metallic multi layer is not penetrated by the laser ablation. The ablated holes penetrate the gold and extends partly into the copper. Consequently, the nanosieve remains opaque until removal of the copper layer. In addition, the method can only be applied to metals that are stable in hydrofluoric acid. The final sample preparation amounts to the deposition of a layer of quartz microspheres. The spheres are dispensed in distilled water and pipetted onto the thin-film sample. The concentration of the spheres is adjusted so that it is appropriate for the formation of close to a monolayer within the boundary formed by the water droplet. The sample is left to dry on a vibrating bench at a slightly elevated temperature. This leads to the formation of a layer of spheres with only small bare areas and small areas with multilayer coverage. 3. Discussion A scanning-electron microscope image of one of the generated nanosieves is shown in Fig. 2. The characteristic hole size is 700 nm with a weak dependence on the location of the hole relative to the intensity envelope of the laser radiation, as described in the previous section. It shows how the close-packed structure formed by the quartz microspheres leads to a characteristic nearestneighbor separation. However, due to the non-perfect packing and a coverage slightly below one mono layer, the holes lack longrange order. In order to achieve a better long-range order, different techniques may possibly be applied. For instance the application of a colloidal suspension has previously been demonstrated to produce a more regular pattern, albeit typically for slightly smaller sphere sizes [10]. The lower right corner of Fig. 2 corresponds to the center of the intensity distribution, and it shows that the maximum fluence in the absence of the focussing quartz spheres is quite close to the threshold fluence, since the surface of the foil shows signs of significant heating. Fig. 3 is an optical microscope image showing the transmission of light through the multitude of holes. The image shows the
4248
C. Smith et al. / Applied Surface Science 255 (2009) 4246–4249
Fig. 2. A scanning-electron microscope image of a free-standing gold nanosieve. The scale bar represents 20 mm. The laser-ablated holes have a characteristic spacing of 6.8 mm and an individual hole size of 700 nm as described in the text.
clusters of holes corresponding to the motion of the laser spot between laser pulses. In Fig. 3 this translation was diagonally topleft to bottom right, and the image shows the characteristic distance between the clusters, 250 mm. The ablation of the multilayered metal film does not deviate significantly from previous results on bulk metal samples. The thermal and optical properties of the two main metal constituents, Cu and Au, are so similar that hemispherical holes may still be expected. The lower heat conductivity in the glass substrate may, however, change the boundary conditions of the heat flow making it slightly less isotropic. The two thin layers introduced to achieve optimal bonding, Ti and Cr, will not have any influence on the heat propagation. In order to illustrate the large-scale validity of the properties seen by microscopy, a light diffraction experiment was conducted. A parallel laser beam of 632.8 nm was sent at normal incidence onto the nanosieve. The diffraction pattern is collected on a chargecoupled device (CCD) sensor. The generated light pattern at a distance of 13 mm behind the nanosieve is shown in Fig. 4, panel
Fig. 3. A back-illuminated optical microscope image of the same sample as in Fig. 2 at 10 times less magnification. The clusters of holes are separated by 250 mm as described in the text.
Fig. 4. (a) Optical diffraction image resulting from the transmission of 632.8-nm light through a gold nanosieve. Diffraction rings corresponding to the two lowest orders are clearly visible. The radii of diffraction rings at different distances from the nanosieve are obtained from radial plots as shown in the inset of panel (b). The resulting measurement provides the characteristic diffraction angles for the two observed orders and the associated characteristic hole spacing of the nanosieve, see discussion in the text. The error bars on individual measurements are smaller than the size of the marker.
(a). Two diffraction rings are clearly observed. They originate from the coherent addition of light from the individual nanoholes. If the nanosieve had exhibited long-range order, diffraction spots characteristic of the closely-packed lattice would have been seen. This is clearly not the case when illuminating with a 1 mm2laser beam. However, the observed rings signify a characteristic distance between the individual sources of light. The phenomenon is analogous to the rings observed in an X-ray diffraction experiment on amorphous materials [13]. The radii of the rings in the diffraction pattern at different positions from the nanosieve are obtained from plots of the average radial intensity [see the inset of Fig. 4(b)]. This provides the diffraction angles corresponding to the two orders without knowledge of the absolute position of the CCD relative to the sample, which is an experimental advantage; the angle is determined form the radius of the circles versus the relative motion of the CCD sensor. The diffraction angles for the two orders are determined from the slope of the curves as 5:84 0:02 and 10:71 0:10 ,
C. Smith et al. / Applied Surface Science 255 (2009) 4246–4249
respectively. These angles correspond to a phase difference of 2p and 4p between neighboring holes, so two determinations of the characteristic hole separation, 6:21 0:02 mm from the first-order diffraction and 6:81 0:06 mm from the second-order diffraction, are obtained. The quoted uncertainties represent one standard deviation on the fitted slope, but the actual uncertainty (including possible systematic effects) must be larger since the two measurements, which differ by 10%, should in principle give the same result. The second-order determination is in perfect agreement with the nominal (and observed) diameter of the microspheres applied, 6.8 mm. 4. Summary A combination of advanced thin-film technology with an onsample-deposited lens array provides a method for efficient generation of large-area laser-micro-structured thin films. The method is used for the production of a free-standing gold nanosieve with pore size 700 nm and a characteristic hole spacing of 7 mm. Such structured films may potentially be of interest for both sensors and filters. The versatility of the
4249
production method gives much freedom in the choice of foil material. A sensor could for instance be based on the plasmonic resonances given by the periodic hole array. The resonance frequency is highly sensitive to the effective dielectric constant near the surface [14] and appropriate functioning of the surface may allow an optical read out of the degree of surface attachment. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14]
C.J.M. van Rijn, G.J. Veldhuis, S. Kuiper, Nanotechnology 9 (1998) 343. S. Kuiper, et al. J. Membr. Sci. 196 (2002) 159. H.D. Tong, et al. J. Microelectromech. Syst. 14 (2005) 113. A. Degiron, H.J. Lezec, N. Yamamoto, T.W. Ebbesen, Opt. Commun. 239 (2004) 61. A. Degiron, T.W. Ebbesen, J. Opt. A: Pure Appl. Opt. 7 (2005) 90. R.J. Reeves, L. Lin, R.J. Blaikie, Phys. Rev. B 74 (2006) 155407. Z. Ruan, M. Qiu, Phys. Rev. Lett. 96 (2006) 233901. K. Vestentoft, J.A. Olesen, B.H. Christensen, P. Balling, Appl. Phys. A 80 (2005) 493, Rapid communication. K. Vestentoft, P. Balling, Appl. Phys. A 84 (2006) 207. K. Piglmayer, R. Denk, D. Ba¨uerle, Appl. Phys. Lett. 80 (2002) 4693. J. Kru¨ger, D. Dufft, R. Koter, A. Hertwig, Appl. Surf. Sci. 253 (2007) 7815. B.H. Christensen, K. Vestentoft, P. Balling, Appl. Surf. Sci. 253 (2007) 6347. B.E. Warren, X-ray Diffraction, Dover Publications, New York, 1990. J. Homola, Anal. Bioanal. Chem. 377 (2003) 528.
Contents lists available at ScienceDirect
Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Metallic nanosieves formed by ultra-short-pulse laser ablation C. Smith a,b, B.H. Christensen a, J. Chevallier a, P. Balling a,b,* a b
Department of Physics and Astronomy, University of Aarhus, DK-8000 Aarhus C, Denmark Interdisciplinary Nanoscience Center iNANO, University of Aarhus, DK-8000 Aarhus C, Denmark
A R T I C L E I N F O
A B S T R A C T
Article history: Received 1 September 2008 Received in revised form 6 November 2008 Accepted 6 November 2008 Available online 17 November 2008
The formation of free-standing gold nanosieves by ablation with ultra-short laser pulses is demonstrated. Macroscopic areas are generated fast and efficiently by the application of a parallel production technique. The technique is based on a lens array formed by self-assembling quartz microspheres on a thin metal foil. The evaporated foils have a final thickness of 400 nm, and the hole spacing is set by the diameter of the microspheres ( 7 mm) while the pore size is 700 nm. The characteristic spacing of the generated hole structure is verified by an optical diffraction technique. ß 2008 Elsevier B.V. All rights reserved.
PACS: 78.67. n 79.20.Ds 81.16. c Keywords: Laser ablation Ultra-short laser pulses Nanosieves Structured metal foils
1. Introduction The formation of nano- and microsieves has received significant interest in recent years [1–7]. They have been demonstrated to be of interest in such diverse areas as filters and sensor technology. The filters are, e.g., used in water purification [1], filtration of lager beers [2] and separation of hydrogen from helium [3]. Metallic nanosieves have also been investigated with interest in their optical properties. The pioneering work by Ebbesen et al. has demonstrated many interesting effects, including the transmission of light through holes with diameters below the classical transmission limit [4] and resonant transmission of specific wavelengths through periodic arrays of 100-nm-sized holes [5–7]. In this paper, a fast and inexpensive method for the production of metallic nanosieves will be demonstrated. Previous investigations have shown that the application of a self-assembled microsphere lens array in combination with illumination by ultra-short laser pulses provide very efficient nanostructuring of an extended area [8,9]. The technique provides an alternative to
* Corresponding author at: Department of Physics and Astronomy, University of Aarhus, Ny Munkegade, Building 1520, DK-8000 Aarhus C, Denmark. Tel.: +45 8942 1111; fax: +45 8612 0740. E-mail address: [email protected] (P. Balling). 0169-4332/$ – see front matter ß 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2008.11.016
the application of a lens array in close proximity to the surface [10], and the self-assembly method is less expensive and less critical to align. The present investigation combines the method described in Refs. [8,9] with state-of-the-art thin film technology, to produce free-standing metallic nanosieves. The manufactured structures are characterized by scanning-electron microscopy and optical diffraction. 2. Experimental Ultra-short laser pulses of 100-fs pulse duration and a wavelength of 800 nm are sent through a 10 microscope objective onto the thin-film samples, see Fig. 1. As described previously in Refs. [8,9], the laser beam is shaped into a multitude of foci by a selfassembled array of 6.8- mm-diameter quartz spheres deposited on the thin-film samples. The sample preparation will be described in more detail below. Here it is stressed that the use of the on-sampledeposited lens array permits the application of a comparatively large laser spot on the sample thereby allowing the production of typically 1000 holes per laser pulse. A schematic of the experimental setup is shown in Fig. 1. In order to obtain a reasonably constant intensity distribution over the laser spot, the Gaussian laser beam is telescoped down to a diameter of 4 mm and apertured by a 2.5-mm-diameter diaphragm. This truncated Gaussian profile is then imaged onto
C. Smith et al. / Applied Surface Science 255 (2009) 4246–4249
Fig. 1. A schematic of the experimental setup for producing the nano-structured metal films showing the optical setup for the laser beam and a CCD-based built-in inspection system, see discussion in the text.
the sample by the microscope objective to provide a 250- mmdiameter laser spot on the sample. Slight variation in the laser intensity is still observed as a spatial dependence of the hole diameter in the produced hole array with larger holes being formed near the middle of the laser beam. Larger surface areas are laser structured by translating the sample perpendicularly to the laser beam by 250 mm between laser pulses. In the present setup, this sample translation is done by hand, so the laser is set to run at a 10 Hz repetition rate, and a mechanical shutter is used to subject each position on the sample to a single laser pulse, see Fig. 1. A camera looking through one of the dielectric mirrors allows inspection of the laser-irradiated area. In principle, striking the sample by a few pulses will not induce significant changes, since the quartz spheres are ejected after a single laser pulse, and the laser is operated at a fluence, which is chosen so that there is no or very little ablation of the sample in the absence of the focussing effect of the spheres. The average laser fluence applied for ablation of the thin foils is typically 0.75 J/cm2, which is around the threshold for (single-shot) ablation [11]. Since the optimum focus provided by the quartz spheres is at some distance behind the sphere, the sample preparation incorporates a transparent spacer layer of quartz (see below). As discussed in more detail in Ref. [8], the optical properties of the quartz spheres can be described either by the Mie theory or to a reasonable approximation also by geometrical optics, since their dimensions exceed the wavelength sufficiently. While the first in its simplest form neglects the presence of the surface, the latter fails to include diffraction effects. However, both methods predict an intensity maximum roughly 3 mm behind the spheres, which dictates the thickness of the applied spacer layer. The Mie theory predicts an intensity enhancement relative to the impinging light of 50. This brings the peak fluence to 35 J/cm2, which shows that the hole formation is performed in the high-fluence limit of short-pulse ablation. Consequently, heat transport is expected to play a decisive role for the geometry of ablated structures. This is supported by numerical simulations of the ablation process in the two-temperature model [9,12]. The thin-film samples are prepared by vacuum evaporation and radio-frequency magnetron sputtering. For the present investigation, a gold foil is produced, but with small changes to the production technique, a broad range of metal foils can be structured. The thickness of the foil is chosen as a compromise between two constraints. The depth of the laser-ablated holes must exceed the foil thickness to ensure the generation of through holes, but on the other hand the thickness of the foil must be significantly greater than the optical penetration depth in order to facilitate a measurement of the optical properties of the nanosieve.
4247
Based on measured hole depths in bulk samples, a thickness of 400 nm is chosen, which corresponds to the attainable hole depths due to the significant heat transport [9]. The foil thickness is still more than an order of magnitude larger than the optical skin depth of gold in the visible range. As described briefly above, to obtain the optimum focussing conditions of the quartz spheres, a transparent spacer layer must be applied. This puts constraints on the foil-preparation procedure. Initially, foils were produced on a glass substrate with a quartz spacer layer. This leads to the generation of foils possessing a high optical quality yet are difficult to remove from the substrate. In order to overcome this problem, an attempt was made to apply a thin sodiumchloride (NaCl) layer below the gold foil for easy subsequent release in water. This procedure had, however, to be discarded since the application of the thin quartz layer led to a release of the gold foil from the NaCl layer because of the mechanical stresses in the quartz film. The solution, which was finally chosen, applied a sequence of five layers on the glass substrate. First 10 nm Ti is deposited by electron-gun evaporation at a rate of 1 nm/s followed in the same run by the deposition of 200 nm Cu at 2.5 nm/s and finally 400 nm Au at 2.5 nm/s. Then, 5 nm chromium is deposited by RF magnetron sputtering at a rate of 0.5 nm/s followed by 3 mm quartz at 0.39 nm/ s. This sequence ensured optimum bonding between the substrate and the gold film (by using Ti and Cu) and between the gold and quartz (facilitated by the thin Cr layer). This technique facilitates the production of free-standing gold nano sieves by subsequent removal of the quartz layer by hydrofluoric acid and of the copper layer by nitric acid. A drawback of this method is that after laser ablation and before release, the combined metallic multi layer is not penetrated by the laser ablation. The ablated holes penetrate the gold and extends partly into the copper. Consequently, the nanosieve remains opaque until removal of the copper layer. In addition, the method can only be applied to metals that are stable in hydrofluoric acid. The final sample preparation amounts to the deposition of a layer of quartz microspheres. The spheres are dispensed in distilled water and pipetted onto the thin-film sample. The concentration of the spheres is adjusted so that it is appropriate for the formation of close to a monolayer within the boundary formed by the water droplet. The sample is left to dry on a vibrating bench at a slightly elevated temperature. This leads to the formation of a layer of spheres with only small bare areas and small areas with multilayer coverage. 3. Discussion A scanning-electron microscope image of one of the generated nanosieves is shown in Fig. 2. The characteristic hole size is 700 nm with a weak dependence on the location of the hole relative to the intensity envelope of the laser radiation, as described in the previous section. It shows how the close-packed structure formed by the quartz microspheres leads to a characteristic nearestneighbor separation. However, due to the non-perfect packing and a coverage slightly below one mono layer, the holes lack longrange order. In order to achieve a better long-range order, different techniques may possibly be applied. For instance the application of a colloidal suspension has previously been demonstrated to produce a more regular pattern, albeit typically for slightly smaller sphere sizes [10]. The lower right corner of Fig. 2 corresponds to the center of the intensity distribution, and it shows that the maximum fluence in the absence of the focussing quartz spheres is quite close to the threshold fluence, since the surface of the foil shows signs of significant heating. Fig. 3 is an optical microscope image showing the transmission of light through the multitude of holes. The image shows the
4248
C. Smith et al. / Applied Surface Science 255 (2009) 4246–4249
Fig. 2. A scanning-electron microscope image of a free-standing gold nanosieve. The scale bar represents 20 mm. The laser-ablated holes have a characteristic spacing of 6.8 mm and an individual hole size of 700 nm as described in the text.
clusters of holes corresponding to the motion of the laser spot between laser pulses. In Fig. 3 this translation was diagonally topleft to bottom right, and the image shows the characteristic distance between the clusters, 250 mm. The ablation of the multilayered metal film does not deviate significantly from previous results on bulk metal samples. The thermal and optical properties of the two main metal constituents, Cu and Au, are so similar that hemispherical holes may still be expected. The lower heat conductivity in the glass substrate may, however, change the boundary conditions of the heat flow making it slightly less isotropic. The two thin layers introduced to achieve optimal bonding, Ti and Cr, will not have any influence on the heat propagation. In order to illustrate the large-scale validity of the properties seen by microscopy, a light diffraction experiment was conducted. A parallel laser beam of 632.8 nm was sent at normal incidence onto the nanosieve. The diffraction pattern is collected on a chargecoupled device (CCD) sensor. The generated light pattern at a distance of 13 mm behind the nanosieve is shown in Fig. 4, panel
Fig. 3. A back-illuminated optical microscope image of the same sample as in Fig. 2 at 10 times less magnification. The clusters of holes are separated by 250 mm as described in the text.
Fig. 4. (a) Optical diffraction image resulting from the transmission of 632.8-nm light through a gold nanosieve. Diffraction rings corresponding to the two lowest orders are clearly visible. The radii of diffraction rings at different distances from the nanosieve are obtained from radial plots as shown in the inset of panel (b). The resulting measurement provides the characteristic diffraction angles for the two observed orders and the associated characteristic hole spacing of the nanosieve, see discussion in the text. The error bars on individual measurements are smaller than the size of the marker.
(a). Two diffraction rings are clearly observed. They originate from the coherent addition of light from the individual nanoholes. If the nanosieve had exhibited long-range order, diffraction spots characteristic of the closely-packed lattice would have been seen. This is clearly not the case when illuminating with a 1 mm2laser beam. However, the observed rings signify a characteristic distance between the individual sources of light. The phenomenon is analogous to the rings observed in an X-ray diffraction experiment on amorphous materials [13]. The radii of the rings in the diffraction pattern at different positions from the nanosieve are obtained from plots of the average radial intensity [see the inset of Fig. 4(b)]. This provides the diffraction angles corresponding to the two orders without knowledge of the absolute position of the CCD relative to the sample, which is an experimental advantage; the angle is determined form the radius of the circles versus the relative motion of the CCD sensor. The diffraction angles for the two orders are determined from the slope of the curves as 5:84 0:02 and 10:71 0:10 ,
C. Smith et al. / Applied Surface Science 255 (2009) 4246–4249
respectively. These angles correspond to a phase difference of 2p and 4p between neighboring holes, so two determinations of the characteristic hole separation, 6:21 0:02 mm from the first-order diffraction and 6:81 0:06 mm from the second-order diffraction, are obtained. The quoted uncertainties represent one standard deviation on the fitted slope, but the actual uncertainty (including possible systematic effects) must be larger since the two measurements, which differ by 10%, should in principle give the same result. The second-order determination is in perfect agreement with the nominal (and observed) diameter of the microspheres applied, 6.8 mm. 4. Summary A combination of advanced thin-film technology with an onsample-deposited lens array provides a method for efficient generation of large-area laser-micro-structured thin films. The method is used for the production of a free-standing gold nanosieve with pore size 700 nm and a characteristic hole spacing of 7 mm. Such structured films may potentially be of interest for both sensors and filters. The versatility of the
4249
production method gives much freedom in the choice of foil material. A sensor could for instance be based on the plasmonic resonances given by the periodic hole array. The resonance frequency is highly sensitive to the effective dielectric constant near the surface [14] and appropriate functioning of the surface may allow an optical read out of the degree of surface attachment. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14]
C.J.M. van Rijn, G.J. Veldhuis, S. Kuiper, Nanotechnology 9 (1998) 343. S. Kuiper, et al. J. Membr. Sci. 196 (2002) 159. H.D. Tong, et al. J. Microelectromech. Syst. 14 (2005) 113. A. Degiron, H.J. Lezec, N. Yamamoto, T.W. Ebbesen, Opt. Commun. 239 (2004) 61. A. Degiron, T.W. Ebbesen, J. Opt. A: Pure Appl. Opt. 7 (2005) 90. R.J. Reeves, L. Lin, R.J. Blaikie, Phys. Rev. B 74 (2006) 155407. Z. Ruan, M. Qiu, Phys. Rev. Lett. 96 (2006) 233901. K. Vestentoft, J.A. Olesen, B.H. Christensen, P. Balling, Appl. Phys. A 80 (2005) 493, Rapid communication. K. Vestentoft, P. Balling, Appl. Phys. A 84 (2006) 207. K. Piglmayer, R. Denk, D. Ba¨uerle, Appl. Phys. Lett. 80 (2002) 4693. J. Kru¨ger, D. Dufft, R. Koter, A. Hertwig, Appl. Surf. Sci. 253 (2007) 7815. B.H. Christensen, K. Vestentoft, P. Balling, Appl. Surf. Sci. 253 (2007) 6347. B.E. Warren, X-ray Diffraction, Dover Publications, New York, 1990. J. Homola, Anal. Bioanal. Chem. 377 (2003) 528.