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Nanoscale aluminum concaves for light-trapping in organic thin-films
Optics Communications 370 (2016) 135–139
Contents lists available at ScienceDirect
Optics Communications journal homepage: www.elsevier.com/locate/optcom
Nanoscale aluminum concaves for light-trapping in organic thin-films Arkadiusz Jarosław Goszczak n, Jost Adam, Paweł Piotr Cielecki, Jacek Fiutowski, Horst-Günter Rubahn, Morten Madsen Mads Clausen Institute, NanoSYD, University of Southern Denmark, Alsion 2, DK-6400 Sønderborg, Denmark
art ic l e i nf o
a b s t r a c t
Article history: Received 18 December 2015 Received in revised form 3 February 2016 Accepted 10 February 2016 Available online 9 March 2016
Anodic aluminum oxide (AAO) templates, fabricated from oxalic acid and phosphoric acid, lead to nonperiodic nanoscale concave structures in their underlying aluminum layer, which are investigated for their field-enhancement properties by applying a thin-film polymer coating based laser ablation technique. Local ablation spots, corresponding to field enhancement on the ridge edges of the aluminum concave nanostructures, are observed in surface-covering polymer films, and confirmed with FDTD studies. The field enhancement leads to improved light absorption in the applied polymer layers, which may be used as an efficient method for enhancing the power conversion efficiency of organic solar cells. & 2016 Elsevier B.V. All rights reserved.
Keywords: AAO Nanostructures Light-trapping Plasmonic Ablation
1. Introduction Light-trapping has been shown to have a positive effect on the performance of several devices such as sensors [1,2], where device sensitivity improvements due to enhanced absorption at a specific wavelength range have been demonstrated, and solar cells [3–5], where production of additional charge carriers arises due to enhanced light absorption in the active layers. Many mechanisms that have been introduced for light enhancement in inorganic thin films, however, are not applicable to organic films, due to the typical active layer thickness limitation of ∼100 nm (e.g., in solar cells or light-emitting devices) [6–8]. Field enhancement in organic films by the use of plasmonic nanostructures has attracted much focus, as nanoparticles [9], nanomeshes [10], nanowires [11] etc. can be used to enhance light absorption in the organic films. The aforementioned enhancement methods, however, come at the price of expensive materials and usually slow and large-scale-incompatible fabrication processes. Anodization of aluminum for the fabrication of anodic aluminum oxide (AAO) templates is a well-known and established technique [12–16] that can be used for fabrication of nanostructures such as nanowires [17,18], nanoparticles [18,19], nanoimprinting and patterning of materials [20,21] etc. and has attracted much attention as a scalable and easily reproducible template method. The pore formation during the anodization n
Corresponding author. E-mail address: [email protected] (A.J. Goszczak).
http://dx.doi.org/10.1016/j.optcom.2016.02.021 0030-4018/& 2016 Elsevier B.V. All rights reserved.
process leads to nanopatterning of the underlying aluminum layer resulting in a structured Al surface after the selective removal of the AAO template, typically used as a guide for ordered pore formation in a double anodization process [12,22–24]. Moreover the influence of the operating conditions during the anodization process on the fabrication of AAO and the underlying aluminum pattern has been reported in literature [25–27]. However aluminum is an abundant and cheap material with interesting plasmonic properties [28,29] and little attention has been drawn on the optical properties of the underlying concave Al patterned surface resulting from the fabrication of AAO templates. Here, we report an alternative, rapid and easily scalable fabrication method for light trapping aluminum nanostructures, using AAO templates on supported silicon (Si) substrates, avoiding thus the time consuming lithographic techniques [30] and long anodization processes of thick aluminum foils [31] as patterning methods. To demonstrate the light enhancement effect, we conduct a non-destructive dry topographic modification method of near-field mapping, based on local laser ablation of an “imaging” polymer layer deposited on the aluminum nanostructures [32].
2. Experimental We used p-type silicon wafers (1–100 Ωcm, (100) crystal orientation) as substrates. After an initial cleaning process (15 min sonication in acetone, followed by 15 min sonication in isopropanol and nitrogen-based drying), 10 nm titanium (rate
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Fig. 2. In the left column, AFM characterization of aluminum concaves exposed after removal of the AAO in a mixture of chromic acid and phosphoric acid is shown. Samples have been anodized in oxalic acid in (a) and phosphoric acid in (b), while in the right column the respective concave profiles are shown for the two aformentioned electrolytes.
Fig. 1. Schematic illustration of the experimental process. (a) SEM image of an AAO template fabricated from phosphoric acid. (b) SEM image of the Al concaves of the sample in (a) after selective etching of the AAO. (c) PMMA spin-coating and (d) irradiation and direct observation via laser scanning microscopy.
0.05 nm/s) and subsequently 800 nm of aluminum (rate 0.1 nm/s) were deposited via e-beam evaporation. The titanium layer serves as adhesion layer between the silicon and the aluminum layer. The silicon wafers were hand-cut with a diamond cutter into 20 20 mm2 samples, followed by a rinse with isopropanol and blown dry with nitrogen prior to anodization. We performed the anodization of the aluminum substrates in a home-built Teflon container, with a cooling system (Brinkmann LAUDA RM 6), under potentiostatic control (Keithley 2400). The temperature of the substrate was maintained constant at 5 °C for all samples during anodization, while the potentials applied for samples in different electrolytes were 40 V for the 0.3 M oxalic acid and 120 V for the 5% phosphoric acid. Anodization times were t¼ 16 min. for anodization in oxalic acid and t¼ 26 min. for anodization in phosphoric acid. After the anodization of Al in different electrolytes, the AAO was etched selectively in a mixture of chromic acid (20 g/l) and phosphoric acid (70 ml/l) at constant temperature of 60 °C. Directly after the removal of the AAO, a 200 nm thick poly (methyl methacrylate) (PMMA) layer was spin-coated on top of each sample's concave surface. This layer thickness turned out to be sufficient to fully cover all concave structures, while still remaining thin enough to be sensitive to the laser ablation experiments [32]. In order to obtain the required robustness for a dry ablation treatment, all samples were baked for 90 s at 200 °C. Directly after PMMA coating, samples were mounted on a custom built laser scanning microscope (LSM) and raster scanned with a Ti:Sapphire laser beam (Spectra Physics Tsunami, sub 100 femtosecond pulses, rep. 82 MHz, max pulse energy ∼8 nJ) with the central wavelength (λ) set at 790 nm. It is worth to notice that, due to the femtosecond regime of the used light pulses, we have eliminated all thermal effects, which could otherwise appear for longer pulses [33]. In Fig. 1, the overall fabrication process is presented, first the deposited aluminum is anodized to form the AAO template (a) and then the AAO template is etched away in a mixture of chromic acid and phosphoric acid to reveal the underlying aluminum nanostructures (b). A thin layer of PMMA is
spin-coated onto the samples as shown in the schematic in (c) followed by the laser ablation experiments (d). The ablation experiments were performed at normal incidence with linearly polarized light illumination. We scanned a focused laser beam via an objective lens ( 40, Nikon) with a numerical aperture of 0.6, to ensure a nearly diffraction limited laser spot. The laser treatment results were monitored by a CCD camera system. The laser fluence and number of pulses per spot were precisely controlled and set below the PMMA ablation threshold, such that only on structured aluminum areas, exhibiting fieldenhancing nanostructures, coatings would be ablated. For subsequent, precise imaging with a Scanning Electron Microscope (SEM – Hitachi S-4800), all samples were sputter-coated with a 3 nm thin layer of Au:Pd. Topography analysis of the nanostructured aluminum samples without any PMMA was conducted with an Atomic Force Microscope (AFM – Veeco Dimension 3100).
3. Results and discussion Fig. 2 shows 2D AFM images of the aluminum concaves after selective removal of the AAO fabricated in different electrolytes.. The surface shows arrays of hemispherical dimples randomly arranged over the sample area, denoting that the height of the structures does not exceed the thickness of the spin coated PMMA layer. Samples fabricated from oxalic acid have an average concave size of 110 7 20 nm, while samples fabricated from phosphoric acid have an average concave size of 300 7 20 nm. Fig. 3 shows, SEM images of aluminum concave samples after etching the AAO template fabricated from oxalic acid in (a) and from phosphoric acid in (b), respectively. It is worth mentioning that the dimple structures follow the same concave pattern as the bottom structure of the formed AAO. In (c) and (d) the ablated SEM images of the correspondent concaves from the different electrolytes in (a) and (b) are shown. It can be observed that ablation occurs on both samples and that the ablated area is located at a distinctive area of each concave structure, more clearly visible in (d) for concaves fabricated from AAO in phosphoric acid as larger structures give a better contrast. From (c) and (d) cracks and small voids on the PMMA layer are evident that are not related to the
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Fig. 4. (a) Close up to ablated spot with a concave perimeter indicated in red. (b) After laser overexposure. (c) and (d) SEM images after ablation for samples from phosphoric acid with different polarization indicated by the red arrows.
Fig. 3. (a) and (b) SEM images of Aluminum concaves from AAO, fabricated from oxalic and phosphoric acid, respectively. (c) and (d) SEM images of PMMA covered aluminum concave samples after ablation with a pulsed laser of the samples in (a) and (b), respectively. (e) SEM image of a PMMA covered sample with the dotted red line denoting the border between a laser scanned area where ablation occurred (bottom left) and where no laser scan occurred (top right).
clearly presented ablation spots. In order to rule out that the ablation could occur due to inhomogeneous coverage of the PMMA on the aluminum patterned surface, Fig. 3–(e) shows an image of a sample where only part was scanned with the laser beam while the rest was not exposed to the laser beam. The red dotted line serves as an indication of the border between the ablated area, bottom left, and the non-ablated area, top right. The PMMA layer on the top right shows no presence of any defect areas of structures penetrating the PMMA film. To call attention to the area where ablation happens on a concave structure, a zoom-in SEM image of an ablated area is shown in Fig. 4(a). The red hexagon represents a perimeter area of a concave feature, where it shown that the ablation of the PMMA layer occurs exactly at the ridge area. In addition it is evident from the ablated images in Fig. 3, that the ablation spots do not cover all the ridge places. This stems from the fact that the height of the ridge differs from area to area, hence PMMA will ablate first on ridges of the same height. Further exposure results in complete ablation of PMMA making the visibility of the effect impossible (Fig. 4(b)). Fig. 4(c) and (d) presents SEM images for ablated phosphoric concaves where the polarization direction has been altered. As can be seen the ablation is parallel
to the polarization direction indicating strong polarization dependence. In order to further investigate the origin of the laser ablation effect and to support our experimental findings, we performed three-dimensional (3D) finite-difference time-domain (FDTD) [34] simulations, with a commercial FDTD solver (Lumerical FDTD solutions [35]). To this end, we imported the AFM height profile for arbitrarily chosen oxalic acid and phosphoric acid concaves (as shown in Fig. 2) as the top surface of an aluminum layer (dispersion characteristics adopted from Palik [36]), covered by a 200 nm thick PMMA layer (modeled with constant refractive index n ¼1.49). Perfectly matched layers (PMLs) surround our simulation region to implement the necessary absorbing boundary conditions. According to the experiments, we excited the simulation cell by a plane of normal incidence, with a spectrum centered at 790 nm wavelength and variable linear polarizations. Our simulation results underline the ablation effect seen experimentally. They further reveal a pronounced polarization dependence: plasmonic field enhancements are clearly visible, occurring (for the specific excitation wavelength) at steep edges of prominent ridges, where the structure gradient’s lateral component matches the polarization direction (Fig. 5). For a more detailed and broadband view on the resonance behavior we refer to the supplementary material and the supplementary video provided with this manuscript.
4. Conclusion In conclusion, we demonstrated that aluminum concave nanostructures of different dimensions and of non-periodic arrangement arise from a simple fabrication process on supported substrates. Via an experimental laser ablation technique we furthermore demonstrated polarization dependent field enhancements inside an organic polymer layer above the aluminum patterned surface, close to the structures' ridge areas. The
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Fig. 5. Finite-difference time-domain simulated electrical field intensity inside a PMMA layer, on top of an arbitrarily chosen, 800 nm times 800 nm wide aluminum concave structure, fabricated from oxalic acid (a) and phosphoric acid (b), excited by a normally incident plane wave at 790 nm wavelength, with (c, e) y-directed and (d, f) x-directed polarization. Plots (c–f) show those x–y slices through the mixed Al-PMMA region, where the maximum relative field intensity (for each polarization and patterned surface) is taken. The simulation results clearly show plasmonic field enhancements, occurring at prominent, vertical edges matching the polarization direction.
experimental hypothesis is supported by FDTD simulations, confirming that the field enhancement is of plasmonic nature, as it arises at ridge edge areas with a gradient, whose lateral part is parallel to the impinging light's polarization direction. Since the organic polymer layer's refractive index used in this work is similar to the refractive index of the most common organic materials/layers in organic solar cells devices, our experimental and numerical setups also mimic actual physical conditions arising in such devices. As the structural features easily can be controlled by slight modifications to the AAO fabrication process, such nanostructures can potentially be used for improving light absorption in a variety of organic films, i.e., for enhancing the power conversion efficiency in organic solar cell devices. Additionally, such substrates comprise potential for straightforward patterning and upscaling, which makes them compatible with large area device fabrication processes.
Acknowledgments AJG, HGR and MM thank the Mads Clausen Foundation for a
research grant supporting this work. The work is supported by the SDU2020 project “Production of Next-Generation Energy Devices”.
Appendix A. Supplementary material Supplementary data associated with this article can be found in the online version at http://dx.doi.org/10.1016/j.optcom.2016.02. 021.
References [1] J. Homola, S.S. Yee, G. Gaulitz, Surface plasmon resonance sensors: review, Sens. Actuators B 54 (1999) 3–15. [2] M.A. Matoian, R. Sweetman, E.C. Hall, S. Albanese, W.B. Euler, Light trapping to amplify metal enhanced fluorescence with application for sensing TNT, J. Fluoresc. 23 (2013) 877–880. [3] E. Garnett, P. Yang, Light trapping in silicon nanowire solar cells, Nano Lett. 10 (2010) 1082–1087. [4] R. Yu, K.L. Ching, Q. Lin, S.F. Leung, D. Arcrossito, Z. Fan, Strong light absorption of self-organized 3-D nanospike arrays for photovoltaic applications, ACS Nano 5 (2011) 9291–9298.
A.J. Goszczak et al. / Optics Communications 370 (2016) 135–139
[5] A. Peer, R. Biswas, Nanophotonic organic solar cell architecture for advnced light trapping with dual photonic crystals, ACS Photonics 1 (2014) 840–847. [6] W. Wang, S. Wu, K. Reinhardt, Y. Lu, S. Chen, Broadband light absorption enhancement in thin-film silicon solar cells, Nano Lett. 10 (2010) 2012–2018. [7] J. Rao, S. Varlamov, Light trapping in thin film polycrystalline silicon solar cell, Energy Procedia 33 (2013) 129–136. [8] S.-F. Leung, Q. Zhang, F. Xiu, D. Yu, J.C. Ho, D. Li, Z. Fan, Light management with nanostructures for optoelectronic devices, J. Phys. Chem. Lett. 5 (2014) 1479–1495. [9] M. Notarianni, K. Vernon, A. Chou, M. Aljada, J. Liu, N. Motta, Plasmonic effect of gold nanoparticles in organic solar cells, J. Sol. Energy 106 (2014) 23–27. [10] K. Jung, H.J. Song, G. Lee, Y. Ko, K.J. Ahn, H. Choi, J.Y. Kim, K. Ha, J. Song, J.K. Lee, C. Lee, M. Choi, Plasmonic organic solar cells employing nanobump assembly via aerosol-delivered nanoparticles, ACS Nano 8 (2014) 2590–2601. [11] M.G. Kang, T. Xu, H.J. Park, X. Luo, L.J. Guo, Efficiency enhancement of organic solar cells using transparent plasmonic Ag nanowire electrodes, Adv. Mater. 22 (2010) 4378–4383. [12] H. Masuda, K. Fukuda, Ordered metal nanohole arrays made by a two-step replication of honeycombstructures of anodic alumina, Science 268 (1995) 1466–1468. [13] A. Eftekhari, Nanostrcutured materials in Electrochemistry, Wiley-VCH, 2008. [14] G.E.J. Poinern, N. Ali, D. Faweett, Progress in nano-engineered anodic aluminum oxide membrane development, Materials 4 (2011) 487–526. [15] M. Stepanova, S. Dew, Nanofabrication Techniques and Principles, Springer, 2012. [16] W. Lee, R. Ji, U. Gösele, K. Nielsch, Fast fabrication of long-range ordered porous alumina membranes by hard anodization, Nature Materials 5 (2006) 741–747. [17] R. Yang, C. Sui, J. Gong, L. Qu, Silver nanowires prepared by modified AAO template method, Mater. Lett. 3 (2007) 900–903. [18] S. Mátéfi-Tempfli, M. Mátéfi-Tempfli, Vertically aligned nanowires on flexible silicone using a supported alumina template prepared by pulsed anodization, Adv. Mater. 21 (2009) 4005–4010. [19] H. Masuda, K. Fukuda, Fabrication of Gold nanodot array using anodic porous alumina as an evaporation mask, Jpn. J. Appl. Phys. 35 (1996) L126–L129. [20] D. Chen, W. Zhao, T.P. Russell, P3HT Nanopillars for organic photovoltaic devices nanoimprinted by AAO Templates, ACS Nano 6 (2012) 1479–1484. [21] J.B. Park, T.S. Bae, J.I. Sohn, S. Cha, J. Lee, S.K. Kim, W.-K. Hong, Fabrication of Ag Nanorods-embedded P3HT/PCBM films for the Enhancement of Light Absorption, ECS Solid State Lett 4 (2015) Q5–Q9.
139
[22] W. Lee, K. Nielsch, U. Gösele, Self-ordering behavior of nanoporous anodic aluminum oxide (AAO) in malonic acid anodization, Nanotechnology 18 (2007) 475713 8pp. [23] X.Y. Han, W.Z. Shen, Improved two-step anodization technique for ordered porous anodic aluminium membranes, J. Electroanal. Chem. 655 (2011) 56–64. [24] L. Zaraska, E. Kurowska, G.D. Sulka, I. Senyk, M. Jaskula, The effect of anode surface area on nanoporous oxide formation during anodizing of low purity aluminum (AA1050 alloy), J. Solid State Eletrochem 18 (2014) 361–368. [25] S. Ono, M. Saito, H. Asoh, Self-ordering of anodic porous alumina formed in organic acid electrolytes, Electrochim. Acta 51 (2005) 827–833. [26] M. Norek, M. Dopierała, W.J. Stępniowski, Ethanol influence on arrangement and geometrical parameters of aluminium concaves prepared in a modified hard anodization for fabrication of highly ordered nanoporous alumina, J. Electroanal. Chem. 750 (2015) 79–88. [27] M. Norek, M. Włodarski, W.J. Stępniowski, Tailoring of UV/violet plasmonic properties in Ag, and Cu coated Al concaves arrays, Appl. Surf. Sci. 314 (2014) 807–814. [28] M.W. Knight, N.S. King, L. Liu, H.O. Everitt, P. Nordlander, N.J. Halas, Aluminum for plasmonics, ACS Nano 8 (2014) 834–840. [29] M.W. Knight, L. Liu, Y. Wang, L. Brown, S. Mukherjee, N.S. King, H.O. Everitt, P. Nordlander, N.J. Halas, Aluminum plasmonic nanoantennas, Nano Lett. 12 (2012) 6000–6004. [30] V.E. Ferry, M.A. Verschuuren, H.B.T. Li, R.E.I. Schropp, H.A. Atwater, A. Polman, Improved redresponse in thin film a-Si:H solar cells with soft-imprinted plasmonic back reflectors, Appl. Phys. Lett. 95 (2009) 183503-1–183503-3. [31] H. Huang, L. Lu, J. Wang, S.-F. Leung, Y. Wang, D. Chen, X. Ghen, G. Shen, D. Li, Z. Fan, Performance enhancement of thin-film amorphous silicon solar cells with low cost nanodent plasmonic substrates, Energy Environ. Sci. 6 (2013) 2965–2971. [32] J. Fiutowski, C. Mailohm, J. Kjelstrup-Hansen, H.-G. Rubahn, Laser ablation of polymer coating allows for electromagnetic field enhancement mapping around nanostructures, Appl. Phys. Lett. 98 (2011) 193117-1–193117-3. [33] P. Leiderer, C. Bartels, J. Konig-Birk, M. Mosbacher, J. Boneberg, Imaging optical near-fields of nanostructures, Appl. Phys. Lett. 85 (2004) 5370. [34] A. Taflove, S.C. Hagness, Computational Electrodynamics: The Finite-Difference Time-Domain Method, Third Edition Artech House, 2004. [35] See 〈〈https://www.lumerical.com/tcad-products/fdtd/〉〉 for more information about FDTD Solutions, accessed 16 August 2015. [36] E.D. Palik, Handbook of Optical Constants of Solids, Elsevier, 1997.
Contents lists available at ScienceDirect
Optics Communications journal homepage: www.elsevier.com/locate/optcom
Nanoscale aluminum concaves for light-trapping in organic thin-films Arkadiusz Jarosław Goszczak n, Jost Adam, Paweł Piotr Cielecki, Jacek Fiutowski, Horst-Günter Rubahn, Morten Madsen Mads Clausen Institute, NanoSYD, University of Southern Denmark, Alsion 2, DK-6400 Sønderborg, Denmark
art ic l e i nf o
a b s t r a c t
Article history: Received 18 December 2015 Received in revised form 3 February 2016 Accepted 10 February 2016 Available online 9 March 2016
Anodic aluminum oxide (AAO) templates, fabricated from oxalic acid and phosphoric acid, lead to nonperiodic nanoscale concave structures in their underlying aluminum layer, which are investigated for their field-enhancement properties by applying a thin-film polymer coating based laser ablation technique. Local ablation spots, corresponding to field enhancement on the ridge edges of the aluminum concave nanostructures, are observed in surface-covering polymer films, and confirmed with FDTD studies. The field enhancement leads to improved light absorption in the applied polymer layers, which may be used as an efficient method for enhancing the power conversion efficiency of organic solar cells. & 2016 Elsevier B.V. All rights reserved.
Keywords: AAO Nanostructures Light-trapping Plasmonic Ablation
1. Introduction Light-trapping has been shown to have a positive effect on the performance of several devices such as sensors [1,2], where device sensitivity improvements due to enhanced absorption at a specific wavelength range have been demonstrated, and solar cells [3–5], where production of additional charge carriers arises due to enhanced light absorption in the active layers. Many mechanisms that have been introduced for light enhancement in inorganic thin films, however, are not applicable to organic films, due to the typical active layer thickness limitation of ∼100 nm (e.g., in solar cells or light-emitting devices) [6–8]. Field enhancement in organic films by the use of plasmonic nanostructures has attracted much focus, as nanoparticles [9], nanomeshes [10], nanowires [11] etc. can be used to enhance light absorption in the organic films. The aforementioned enhancement methods, however, come at the price of expensive materials and usually slow and large-scale-incompatible fabrication processes. Anodization of aluminum for the fabrication of anodic aluminum oxide (AAO) templates is a well-known and established technique [12–16] that can be used for fabrication of nanostructures such as nanowires [17,18], nanoparticles [18,19], nanoimprinting and patterning of materials [20,21] etc. and has attracted much attention as a scalable and easily reproducible template method. The pore formation during the anodization n
Corresponding author. E-mail address: [email protected] (A.J. Goszczak).
http://dx.doi.org/10.1016/j.optcom.2016.02.021 0030-4018/& 2016 Elsevier B.V. All rights reserved.
process leads to nanopatterning of the underlying aluminum layer resulting in a structured Al surface after the selective removal of the AAO template, typically used as a guide for ordered pore formation in a double anodization process [12,22–24]. Moreover the influence of the operating conditions during the anodization process on the fabrication of AAO and the underlying aluminum pattern has been reported in literature [25–27]. However aluminum is an abundant and cheap material with interesting plasmonic properties [28,29] and little attention has been drawn on the optical properties of the underlying concave Al patterned surface resulting from the fabrication of AAO templates. Here, we report an alternative, rapid and easily scalable fabrication method for light trapping aluminum nanostructures, using AAO templates on supported silicon (Si) substrates, avoiding thus the time consuming lithographic techniques [30] and long anodization processes of thick aluminum foils [31] as patterning methods. To demonstrate the light enhancement effect, we conduct a non-destructive dry topographic modification method of near-field mapping, based on local laser ablation of an “imaging” polymer layer deposited on the aluminum nanostructures [32].
2. Experimental We used p-type silicon wafers (1–100 Ωcm, (100) crystal orientation) as substrates. After an initial cleaning process (15 min sonication in acetone, followed by 15 min sonication in isopropanol and nitrogen-based drying), 10 nm titanium (rate
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Fig. 2. In the left column, AFM characterization of aluminum concaves exposed after removal of the AAO in a mixture of chromic acid and phosphoric acid is shown. Samples have been anodized in oxalic acid in (a) and phosphoric acid in (b), while in the right column the respective concave profiles are shown for the two aformentioned electrolytes.
Fig. 1. Schematic illustration of the experimental process. (a) SEM image of an AAO template fabricated from phosphoric acid. (b) SEM image of the Al concaves of the sample in (a) after selective etching of the AAO. (c) PMMA spin-coating and (d) irradiation and direct observation via laser scanning microscopy.
0.05 nm/s) and subsequently 800 nm of aluminum (rate 0.1 nm/s) were deposited via e-beam evaporation. The titanium layer serves as adhesion layer between the silicon and the aluminum layer. The silicon wafers were hand-cut with a diamond cutter into 20 20 mm2 samples, followed by a rinse with isopropanol and blown dry with nitrogen prior to anodization. We performed the anodization of the aluminum substrates in a home-built Teflon container, with a cooling system (Brinkmann LAUDA RM 6), under potentiostatic control (Keithley 2400). The temperature of the substrate was maintained constant at 5 °C for all samples during anodization, while the potentials applied for samples in different electrolytes were 40 V for the 0.3 M oxalic acid and 120 V for the 5% phosphoric acid. Anodization times were t¼ 16 min. for anodization in oxalic acid and t¼ 26 min. for anodization in phosphoric acid. After the anodization of Al in different electrolytes, the AAO was etched selectively in a mixture of chromic acid (20 g/l) and phosphoric acid (70 ml/l) at constant temperature of 60 °C. Directly after the removal of the AAO, a 200 nm thick poly (methyl methacrylate) (PMMA) layer was spin-coated on top of each sample's concave surface. This layer thickness turned out to be sufficient to fully cover all concave structures, while still remaining thin enough to be sensitive to the laser ablation experiments [32]. In order to obtain the required robustness for a dry ablation treatment, all samples were baked for 90 s at 200 °C. Directly after PMMA coating, samples were mounted on a custom built laser scanning microscope (LSM) and raster scanned with a Ti:Sapphire laser beam (Spectra Physics Tsunami, sub 100 femtosecond pulses, rep. 82 MHz, max pulse energy ∼8 nJ) with the central wavelength (λ) set at 790 nm. It is worth to notice that, due to the femtosecond regime of the used light pulses, we have eliminated all thermal effects, which could otherwise appear for longer pulses [33]. In Fig. 1, the overall fabrication process is presented, first the deposited aluminum is anodized to form the AAO template (a) and then the AAO template is etched away in a mixture of chromic acid and phosphoric acid to reveal the underlying aluminum nanostructures (b). A thin layer of PMMA is
spin-coated onto the samples as shown in the schematic in (c) followed by the laser ablation experiments (d). The ablation experiments were performed at normal incidence with linearly polarized light illumination. We scanned a focused laser beam via an objective lens ( 40, Nikon) with a numerical aperture of 0.6, to ensure a nearly diffraction limited laser spot. The laser treatment results were monitored by a CCD camera system. The laser fluence and number of pulses per spot were precisely controlled and set below the PMMA ablation threshold, such that only on structured aluminum areas, exhibiting fieldenhancing nanostructures, coatings would be ablated. For subsequent, precise imaging with a Scanning Electron Microscope (SEM – Hitachi S-4800), all samples were sputter-coated with a 3 nm thin layer of Au:Pd. Topography analysis of the nanostructured aluminum samples without any PMMA was conducted with an Atomic Force Microscope (AFM – Veeco Dimension 3100).
3. Results and discussion Fig. 2 shows 2D AFM images of the aluminum concaves after selective removal of the AAO fabricated in different electrolytes.. The surface shows arrays of hemispherical dimples randomly arranged over the sample area, denoting that the height of the structures does not exceed the thickness of the spin coated PMMA layer. Samples fabricated from oxalic acid have an average concave size of 110 7 20 nm, while samples fabricated from phosphoric acid have an average concave size of 300 7 20 nm. Fig. 3 shows, SEM images of aluminum concave samples after etching the AAO template fabricated from oxalic acid in (a) and from phosphoric acid in (b), respectively. It is worth mentioning that the dimple structures follow the same concave pattern as the bottom structure of the formed AAO. In (c) and (d) the ablated SEM images of the correspondent concaves from the different electrolytes in (a) and (b) are shown. It can be observed that ablation occurs on both samples and that the ablated area is located at a distinctive area of each concave structure, more clearly visible in (d) for concaves fabricated from AAO in phosphoric acid as larger structures give a better contrast. From (c) and (d) cracks and small voids on the PMMA layer are evident that are not related to the
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Fig. 4. (a) Close up to ablated spot with a concave perimeter indicated in red. (b) After laser overexposure. (c) and (d) SEM images after ablation for samples from phosphoric acid with different polarization indicated by the red arrows.
Fig. 3. (a) and (b) SEM images of Aluminum concaves from AAO, fabricated from oxalic and phosphoric acid, respectively. (c) and (d) SEM images of PMMA covered aluminum concave samples after ablation with a pulsed laser of the samples in (a) and (b), respectively. (e) SEM image of a PMMA covered sample with the dotted red line denoting the border between a laser scanned area where ablation occurred (bottom left) and where no laser scan occurred (top right).
clearly presented ablation spots. In order to rule out that the ablation could occur due to inhomogeneous coverage of the PMMA on the aluminum patterned surface, Fig. 3–(e) shows an image of a sample where only part was scanned with the laser beam while the rest was not exposed to the laser beam. The red dotted line serves as an indication of the border between the ablated area, bottom left, and the non-ablated area, top right. The PMMA layer on the top right shows no presence of any defect areas of structures penetrating the PMMA film. To call attention to the area where ablation happens on a concave structure, a zoom-in SEM image of an ablated area is shown in Fig. 4(a). The red hexagon represents a perimeter area of a concave feature, where it shown that the ablation of the PMMA layer occurs exactly at the ridge area. In addition it is evident from the ablated images in Fig. 3, that the ablation spots do not cover all the ridge places. This stems from the fact that the height of the ridge differs from area to area, hence PMMA will ablate first on ridges of the same height. Further exposure results in complete ablation of PMMA making the visibility of the effect impossible (Fig. 4(b)). Fig. 4(c) and (d) presents SEM images for ablated phosphoric concaves where the polarization direction has been altered. As can be seen the ablation is parallel
to the polarization direction indicating strong polarization dependence. In order to further investigate the origin of the laser ablation effect and to support our experimental findings, we performed three-dimensional (3D) finite-difference time-domain (FDTD) [34] simulations, with a commercial FDTD solver (Lumerical FDTD solutions [35]). To this end, we imported the AFM height profile for arbitrarily chosen oxalic acid and phosphoric acid concaves (as shown in Fig. 2) as the top surface of an aluminum layer (dispersion characteristics adopted from Palik [36]), covered by a 200 nm thick PMMA layer (modeled with constant refractive index n ¼1.49). Perfectly matched layers (PMLs) surround our simulation region to implement the necessary absorbing boundary conditions. According to the experiments, we excited the simulation cell by a plane of normal incidence, with a spectrum centered at 790 nm wavelength and variable linear polarizations. Our simulation results underline the ablation effect seen experimentally. They further reveal a pronounced polarization dependence: plasmonic field enhancements are clearly visible, occurring (for the specific excitation wavelength) at steep edges of prominent ridges, where the structure gradient’s lateral component matches the polarization direction (Fig. 5). For a more detailed and broadband view on the resonance behavior we refer to the supplementary material and the supplementary video provided with this manuscript.
4. Conclusion In conclusion, we demonstrated that aluminum concave nanostructures of different dimensions and of non-periodic arrangement arise from a simple fabrication process on supported substrates. Via an experimental laser ablation technique we furthermore demonstrated polarization dependent field enhancements inside an organic polymer layer above the aluminum patterned surface, close to the structures' ridge areas. The
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Fig. 5. Finite-difference time-domain simulated electrical field intensity inside a PMMA layer, on top of an arbitrarily chosen, 800 nm times 800 nm wide aluminum concave structure, fabricated from oxalic acid (a) and phosphoric acid (b), excited by a normally incident plane wave at 790 nm wavelength, with (c, e) y-directed and (d, f) x-directed polarization. Plots (c–f) show those x–y slices through the mixed Al-PMMA region, where the maximum relative field intensity (for each polarization and patterned surface) is taken. The simulation results clearly show plasmonic field enhancements, occurring at prominent, vertical edges matching the polarization direction.
experimental hypothesis is supported by FDTD simulations, confirming that the field enhancement is of plasmonic nature, as it arises at ridge edge areas with a gradient, whose lateral part is parallel to the impinging light's polarization direction. Since the organic polymer layer's refractive index used in this work is similar to the refractive index of the most common organic materials/layers in organic solar cells devices, our experimental and numerical setups also mimic actual physical conditions arising in such devices. As the structural features easily can be controlled by slight modifications to the AAO fabrication process, such nanostructures can potentially be used for improving light absorption in a variety of organic films, i.e., for enhancing the power conversion efficiency in organic solar cell devices. Additionally, such substrates comprise potential for straightforward patterning and upscaling, which makes them compatible with large area device fabrication processes.
Acknowledgments AJG, HGR and MM thank the Mads Clausen Foundation for a
research grant supporting this work. The work is supported by the SDU2020 project “Production of Next-Generation Energy Devices”.
Appendix A. Supplementary material Supplementary data associated with this article can be found in the online version at http://dx.doi.org/10.1016/j.optcom.2016.02. 021.
References [1] J. Homola, S.S. Yee, G. Gaulitz, Surface plasmon resonance sensors: review, Sens. Actuators B 54 (1999) 3–15. [2] M.A. Matoian, R. Sweetman, E.C. Hall, S. Albanese, W.B. Euler, Light trapping to amplify metal enhanced fluorescence with application for sensing TNT, J. Fluoresc. 23 (2013) 877–880. [3] E. Garnett, P. Yang, Light trapping in silicon nanowire solar cells, Nano Lett. 10 (2010) 1082–1087. [4] R. Yu, K.L. Ching, Q. Lin, S.F. Leung, D. Arcrossito, Z. Fan, Strong light absorption of self-organized 3-D nanospike arrays for photovoltaic applications, ACS Nano 5 (2011) 9291–9298.
A.J. Goszczak et al. / Optics Communications 370 (2016) 135–139
[5] A. Peer, R. Biswas, Nanophotonic organic solar cell architecture for advnced light trapping with dual photonic crystals, ACS Photonics 1 (2014) 840–847. [6] W. Wang, S. Wu, K. Reinhardt, Y. Lu, S. Chen, Broadband light absorption enhancement in thin-film silicon solar cells, Nano Lett. 10 (2010) 2012–2018. [7] J. Rao, S. Varlamov, Light trapping in thin film polycrystalline silicon solar cell, Energy Procedia 33 (2013) 129–136. [8] S.-F. Leung, Q. Zhang, F. Xiu, D. Yu, J.C. Ho, D. Li, Z. Fan, Light management with nanostructures for optoelectronic devices, J. Phys. Chem. Lett. 5 (2014) 1479–1495. [9] M. Notarianni, K. Vernon, A. Chou, M. Aljada, J. Liu, N. Motta, Plasmonic effect of gold nanoparticles in organic solar cells, J. Sol. Energy 106 (2014) 23–27. [10] K. Jung, H.J. Song, G. Lee, Y. Ko, K.J. Ahn, H. Choi, J.Y. Kim, K. Ha, J. Song, J.K. Lee, C. Lee, M. Choi, Plasmonic organic solar cells employing nanobump assembly via aerosol-delivered nanoparticles, ACS Nano 8 (2014) 2590–2601. [11] M.G. Kang, T. Xu, H.J. Park, X. Luo, L.J. Guo, Efficiency enhancement of organic solar cells using transparent plasmonic Ag nanowire electrodes, Adv. Mater. 22 (2010) 4378–4383. [12] H. Masuda, K. Fukuda, Ordered metal nanohole arrays made by a two-step replication of honeycombstructures of anodic alumina, Science 268 (1995) 1466–1468. [13] A. Eftekhari, Nanostrcutured materials in Electrochemistry, Wiley-VCH, 2008. [14] G.E.J. Poinern, N. Ali, D. Faweett, Progress in nano-engineered anodic aluminum oxide membrane development, Materials 4 (2011) 487–526. [15] M. Stepanova, S. Dew, Nanofabrication Techniques and Principles, Springer, 2012. [16] W. Lee, R. Ji, U. Gösele, K. Nielsch, Fast fabrication of long-range ordered porous alumina membranes by hard anodization, Nature Materials 5 (2006) 741–747. [17] R. Yang, C. Sui, J. Gong, L. Qu, Silver nanowires prepared by modified AAO template method, Mater. Lett. 3 (2007) 900–903. [18] S. Mátéfi-Tempfli, M. Mátéfi-Tempfli, Vertically aligned nanowires on flexible silicone using a supported alumina template prepared by pulsed anodization, Adv. Mater. 21 (2009) 4005–4010. [19] H. Masuda, K. Fukuda, Fabrication of Gold nanodot array using anodic porous alumina as an evaporation mask, Jpn. J. Appl. Phys. 35 (1996) L126–L129. [20] D. Chen, W. Zhao, T.P. Russell, P3HT Nanopillars for organic photovoltaic devices nanoimprinted by AAO Templates, ACS Nano 6 (2012) 1479–1484. [21] J.B. Park, T.S. Bae, J.I. Sohn, S. Cha, J. Lee, S.K. Kim, W.-K. Hong, Fabrication of Ag Nanorods-embedded P3HT/PCBM films for the Enhancement of Light Absorption, ECS Solid State Lett 4 (2015) Q5–Q9.
139
[22] W. Lee, K. Nielsch, U. Gösele, Self-ordering behavior of nanoporous anodic aluminum oxide (AAO) in malonic acid anodization, Nanotechnology 18 (2007) 475713 8pp. [23] X.Y. Han, W.Z. Shen, Improved two-step anodization technique for ordered porous anodic aluminium membranes, J. Electroanal. Chem. 655 (2011) 56–64. [24] L. Zaraska, E. Kurowska, G.D. Sulka, I. Senyk, M. Jaskula, The effect of anode surface area on nanoporous oxide formation during anodizing of low purity aluminum (AA1050 alloy), J. Solid State Eletrochem 18 (2014) 361–368. [25] S. Ono, M. Saito, H. Asoh, Self-ordering of anodic porous alumina formed in organic acid electrolytes, Electrochim. Acta 51 (2005) 827–833. [26] M. Norek, M. Dopierała, W.J. Stępniowski, Ethanol influence on arrangement and geometrical parameters of aluminium concaves prepared in a modified hard anodization for fabrication of highly ordered nanoporous alumina, J. Electroanal. Chem. 750 (2015) 79–88. [27] M. Norek, M. Włodarski, W.J. Stępniowski, Tailoring of UV/violet plasmonic properties in Ag, and Cu coated Al concaves arrays, Appl. Surf. Sci. 314 (2014) 807–814. [28] M.W. Knight, N.S. King, L. Liu, H.O. Everitt, P. Nordlander, N.J. Halas, Aluminum for plasmonics, ACS Nano 8 (2014) 834–840. [29] M.W. Knight, L. Liu, Y. Wang, L. Brown, S. Mukherjee, N.S. King, H.O. Everitt, P. Nordlander, N.J. Halas, Aluminum plasmonic nanoantennas, Nano Lett. 12 (2012) 6000–6004. [30] V.E. Ferry, M.A. Verschuuren, H.B.T. Li, R.E.I. Schropp, H.A. Atwater, A. Polman, Improved redresponse in thin film a-Si:H solar cells with soft-imprinted plasmonic back reflectors, Appl. Phys. Lett. 95 (2009) 183503-1–183503-3. [31] H. Huang, L. Lu, J. Wang, S.-F. Leung, Y. Wang, D. Chen, X. Ghen, G. Shen, D. Li, Z. Fan, Performance enhancement of thin-film amorphous silicon solar cells with low cost nanodent plasmonic substrates, Energy Environ. Sci. 6 (2013) 2965–2971. [32] J. Fiutowski, C. Mailohm, J. Kjelstrup-Hansen, H.-G. Rubahn, Laser ablation of polymer coating allows for electromagnetic field enhancement mapping around nanostructures, Appl. Phys. Lett. 98 (2011) 193117-1–193117-3. [33] P. Leiderer, C. Bartels, J. Konig-Birk, M. Mosbacher, J. Boneberg, Imaging optical near-fields of nanostructures, Appl. Phys. Lett. 85 (2004) 5370. [34] A. Taflove, S.C. Hagness, Computational Electrodynamics: The Finite-Difference Time-Domain Method, Third Edition Artech House, 2004. [35] See 〈〈https://www.lumerical.com/tcad-products/fdtd/〉〉 for more information about FDTD Solutions, accessed 16 August 2015. [36] E.D. Palik, Handbook of Optical Constants of Solids, Elsevier, 1997.