General Strategy toward Dual-Scale-Controlled Metallic Micro−Nano Hybrid Structures with Ultralow Reflectance Peixun Fan,*,†,‡ Benfeng Bai,‡ Minlin Zhong,*,† Hongjun Zhang,† Jiangyou Long,† Jinpeng Han,† Weiqi Wang,‡ and Guofan Jin‡ †
Laser Materials Processing Research Centre, School of Materials Science and Engineering, and ‡State Key Laboratory of Precision Measurement Technology and Instruments, Department of Precision Instrument, Tsinghua University, Beijing 100084, People’s Republic of China S Supporting Information *
ABSTRACT: Functional metal surfaces with minimum optical reflection over a broadband spectrum have essential importance for optical and optoelectronic devices. However, the intrinsically large optical impedance mismatch between metals and the free space causes a huge obstacle in achieving such a purpose. We propose and experimentally demonstrate a general pulse injection controlled ultrafast laser direct writing strategy for fabricating highly effective antireflection structures on metal surfaces. The presented strategy can implement separate and flexible modifications on both microscale frame structures and nanoscale particles, a benefit from which is that optimized geometrical light trapping and enhanced effective medium effect reducing the surface reflection can be simultaneously achieved within one hybrid structure. Thus, comprehensively improved antireflection performances can be realized. Hybrid structures with substantial nanoparticles hierarchically attached on regularly arrayed microcones are generally constructed on different metal surfaces, achieving highly efficient light absorption over ultraviolet to near-infrared broadband spectrum regions. Reflectance minimums of 1.4%, 0.29%, and 2.5% are reached on Cu, Ti, and W surfaces, respectively. The presented strategy is simple in process, adaptable for different kinds of metals, reproduceable in dual-scale structural features, and feasible for large-area production. All these advantages make the strategy as well as the prepared antireflection structures excellent candidates for practical applications. KEYWORDS: broadband, antireflection, ultralow reflectance, metal, micro−nano structure, ultrafast laser
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nano structures with elaborately designed and well-defined geometries can stimulate resonance light absorption mechanisms through which ultralow surface reflectance can be reached. However, the absorption spectral range strongly depends on the pattern and size of their component structural units and is inherently limited to a narrow and specific wavelength range, no matter whether they are surface plasmon based13,14 or metamaterial based.15−17 In contrast to that, metallic micro−nano structures with undefined topographies and imprecise dimensions, particularly the micro−nano dualscale hierarchical structures formed by intense laser irradiation, can suppress optical reflection over a broadband spectrum through geometrical light trapping and multiple light scattering.18−20 However, the lack of design and controllability
limination of metal surface reflection is of both fundamental interest and realistic value for various fields. However, due to the large optical impedance mismatch between them and the free space, there usually exist great challenges in achieving perfect antireflection performances on metals. For the most developed antireflection coating strategies, few candidate materials are able to bridge the refractive index gaps between metals and the free space, making conventional destructive interference coatings1−3 and gradient refractive index films4−9 less effective. For the recently developed antireflection micro−nano structure strategies, although landmark advancements on antireflection properties with broadband effectiveness (from UV to infrared) and ultralow reflectance (500 mm s−1) are applied, the surface reflectance rises over that of the base structure, resulting from the smoother cone surfaces, which are caused by laser reablation of preformed nanoscale features. Figure S2 shows that when arrayed microcones are fabricated, the intrinsic shiny copper surface can turn dark; however the red color can still be recognized in its appearance. When nanoparticles are further introduced onto microcones and thus hybrid structures are constructed, the copper surface turns pitch black. On the basis of these results, the vital significance of nanoparticles in eliminating metal surface reflection is clearly evidenced. In addition to the nanoparticles, the dimensions of arrayed microcones also have essential importance to surface antireflection properties and can be facilely tuned through simply control process 1. For instance, we produced base structures with varied periodicities, namely, P30 Stru.1, P40 7404
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Figure 5. SEM images of surface structures formed on Ti: (a) P30 Stru.1, (c) P30 Stru.2, (e) (g) (i) P30 Hybrid Stru. The scanning velocities in process 2 for (e), (g), and (i) are 25, 50, and 100 mm s−1, respectively. The corresponding magnified SEM images are shown at the bottom. (k) Surface reflectance of different micro−nano structures. (l) Evolution of surface reflectance of the micro−nano structures in (e), (g), and (i).
filling factors to obviously lower levels. Therefore, the comprehensive effect of both microscale frame structures and nanoscale features becomes clarified. The nanoscale features can induce lower reflectance for each single bounce of light via rough and porous morphologies, which is then enhanced by the amplification effect of the microcones via multi-internal reflections. It should be noted that the nanoscale metallic features can also localize light energy via the excitation of surface plasmons, which can be reasonably regarded as part of the contribution of the above-mentioned effective medium effect. The varied dimensions of those features are beneficial for broadening the effective wavelength range of the light trapping and surface plasmon resonances, which can result in a more efficient effective medium. Considering that ultrafast laser direct writing is a technique with no apparent material dependence, we anticipate the pulse injection controlled ultrafast laser processing strategy to also be applicable in creating the regular microcone arrays and abundant nanoparticle hybrid structures on other metal surfaces. Particularly, the residual surface reflection of these metals with small real parts of relative permittivities via conventional methods is expected to be eliminated.13,28,29 As an illustration, Figure 5a−j demonstrate that arrayed microcones with regular outer shapes and decorated by abundant nanoparticles are successfully formed on titanium surfaces via the pulse injection controlled ultrafast laser processing strategy. The formed hybrid structures exactly resemble those produced on copper. Accordingly, Figure 5k shows that the hybrid structure exhibits the most effective antireflection properties compared to both Stru.1 and Stru.2. Besides, the gradual
1100 nm and was 4.4%. As for the overall antireflection performance throughout the UV−vis−NIR ranges, an average reflectance of 4.1% and correspondingly an average absorptance of ∼95.9% are achieved. In order to further reveal the antireflection mechanism of the dual-scale-controlled micro−nano hybrid structures, theoretical calculations have been conducted for Cu as well as other metals, whose experimental results will be shown in the next parts. Briefly, the microcone arrays are modeled as twodimensional sinusoidal gratings, whose surfaces have effective refractive indices induced by the nanostructured porous features. The refractive indices of the effective medium layers are estimated by neff = fnmetal + (1 − f)nair, where f is a filling factor of metal in the range [0, 1]. Since the spacing between the adjacent microcones is much larger than the wavelength of incident light investigated in this research, multiple internal reflections of light can be aroused in the grooves of the microcone array, which further enhances the antireflection effect of the nanostructured surface. The reflectance for one single bounce of light at the structure surface is determined by the Fresnel equation. The filling factor f in our calculation can be an indicator of the surface roughness conditions. Rougher and more porous structures can result in smaller filling factors. By calculations, different multi-internal reflection times and different filling factors have been tested, with typical spectra shown in Figure S3. It can be seen that, as the filling factor gets smaller, the surface reflectance is reduced gradually, which is consistent with the evolution trend of the experimentally measured spectra in Figure 3j. Then, more multi-internal reflection times can bring all calculated curves with different 7405
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Figure 6. Microstructure characterization of the modified micro−nano structures. SEM images for (a, b) P30 Stru.1-Deep and (c, d) P30 Hybrid Stru.-Deep. (e) Reflection spectra of different micro−nano structures. Long and short cyan arrows indicate the reflectance decrease from P30 Stru.1 to P30 Stru.1-Deep and from P30 Hybrid Stru. to P30 Hybrid Stru.-Deep, respectively.
to previous reports has been achieved on Ti via our strategy. As for Cu, which is a representative of metals with large optical constants, its surface reflection is harder to eliminate by the previous methods, especially in the infrared spectral range. In spite of the difficulty, our strategy has successfully made the copper surface significantly antireflective over a broad range from UV to infrared. In addition to copper and titanium, which are typical representatives of metals with large and small optical constants, respectively, we have further applied our approach to produce surface structures on metals with higher hardness and higher melting point (tungsten, for instance), from the standpoint of practical application. Figure S6 shows that a dual-scalecontrolled micro−nano hybrid structure can also be successfully fabricated on tungsten surfaces with our pulse injection controlled ultrafast laser direct writing strategy, which exhibits the most effective antireflection properties compared to both Stru.1 and Stru.2. A steady, low-surface reflectance around 3% throughout the UV−vis−NIR spectrum is displayed, with a minimum appearing at around 300 nm of ∼2.5%. Its average reflectance in the wavelength ranges of 400−750 nm, 1000− 1800 nm, and 250−2250 nm is 3.0%, 3.2%, and 3.2%, respectively, further confirming the wide applicability of our approach in forming desired micro−nano structures and significantly suppressing reflection on metal surfaces.
decreasing tendency of surface reflectance with scanning velocities in process 2 is also well presented, as depicted in Figure 5l. It can be found from the photographs in Figure S2 that the intrinsic bright white titanium surface turns gray when arrayed microcones are fabricated and then to pitch black when nanoparticles are further introduced and thus hybrid structures are constructed. Also, base structures and corresponding hybrid structures with varied periodicities have been fabricated on titanium surfaces, as can be seen in Figure S4. Similar decreasing trends of surface reflection spectra from Stru.1 and Stru.2 to hybrid structures are produced (see Figure S5), just as observed on copper surfaces. Therefore, the pulse injection controlled ultrafast laser processing strategy is validated to be generally effective in creating the hybrid structures on metal surfaces and giving them highly efficient antireflection properties. The depth of microscale structures is another factor influencing the light-trapping effect in addition to their periodicities. In order to further improve the antireflection performances, we fabricated deeper microcone arrays on titanium surfaces by applying more repeats in process 1, as shown in Figure 6a,b. Resulting from the enhanced lighttrapping effect, surface reflectance of titanium is apparently reduced, as indicated by the long cyan arrow in Figure 6e. After hybrid structures are constructed, the one based on deeper microcone arrays also exhibits better antireflection properties, as shown by the short cyan arrow in Figure 6e. A steady low surface reflectance around 2% through the spectrum from the UV to near-infrared is displayed, with average values in the wavelength regions of 400−750 nm, 1000−1800 nm, and 250− 2250 nm of 2.2%, 2.0%, and 2.4%, respectively. Actually, a minimum reflectance of 0.29% has been measured at around 1640 nm (Figure S7b), which is the lowest reflectance experimentally achieved on metal surfaces as far as we know. The above results illustrate that the pulse injection controlled ultrafast laser processing strategy can indeed offer separate and flexible modification on both microscale and nanoscale construction features of the surface hybrid structures. Through such a process, highly efficient elimination of optical reflection can be realized on metal surfaces. We further compare the antireflection effect of our dual-scale-controlled micro−nano hybrid structures with several typical reported results (Table S1). It can be seen that surface reflection of Ti, which is representative of metals with small optical constants, can be relatively easier to eliminate, as discussed at the beginning of this paper. Encouragingly, lower surface reflectance compared
CONCLUSION We propose and experimentally demonstrate a general pulse injection controlled ultrafast laser processing strategy for fabricating highly effective antireflection micro−nano structures on metal surfaces, which has the potential to outperform the state-of-the-art antireflection methods for metals. The presented strategy can implement separate and flexible modifications on both microscale frame structures and nanoscale particles, a benefit from which is that optimized geometrical light trapping and enhanced effective medium antireflection effects can be simultaneously achieved within one hybrid structure. As a result, comprehensively improved antireflection performances can be realized. Hybrid structures with substantial nanoparticles hierarchically attached on regularly arrayed microcones are constructed, achieving an average reflectance of 4.1%, 2.4%, and 3.2% in the broadband spectrum from ultraviolet to near-infrared on Cu, Ti, and W surfaces, respectively. The antireflection structures are naturally and in situ formed via ultrafast laser interaction with metal surfaces, with no extra materials needing to be added. The 7406
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ACS Nano ORCID
developed ultrafast laser micro−nano processing is capable of producing the antireflection structures over large areas. All these advantages make the prepared antireflection structures excellent candidates for practical applications.
Peixun Fan: 0000-0002-8600-7179 Minlin Zhong: 0000-0002-7897-2593 Notes
The authors declare no competing financial interest.
METHODS Micro−Nano Structure Fabrication. A Trumpf TruMicro 5000 ultrafast laser system, which can generate 800 fs pulses at a central wavelength of 1030 nm and a repetition rate of 200k Hz, was utilized for the micro−nano fabrication procedure. Before laser processing, the copper, titanium, and tungsten samples (99.9% purity) were mechanically polished to mirror finish and cleaned ultrasonically with ethanol to remove the oxide and grease on their surfaces. An x−y galvo was used to focus and scan the laser beam on the metal surfaces in a pattern of crossed lines in an atmospheric environment. The diameter of the focused spot, defined by an intensity drop to 1/e2 of the maximum value, was approximately 30 μm. Specific laser processing parameters for fabricating different micro−nano structures can be found in Table S2. Characterization and Measurements. The surface micro−nano structures of the laser-processed samples were studied with a Tescan Mira 3 LMH field emission scanning electron microscope (SEM). The wavelength dependence of the hemispherical reflectance in the UV, vis, and NIR regions (250−2250 nm) was characterized with a Lambda 950 spectrophotometer incorporated with an integrating sphere of 150 mm in diameter. Different light sources as well as different detectors are utilized in this spectrophotometer for different wavelength ranges. Among them, a preadjusted D2 lamp for the UV range and a preadjusted halogen lamp for the vis−NIR ranges are used as light sources, while a photomultiplier of the type R 6872 for the UV−vis ranges and a Peltier-stabilized PbS detector type OTC-21-73 for the NIR range are used as the detectors. During broadband measurement, the system is automatically switched from R 6872 to OTC-21-73 when getting into the NIR range. The detector switching occurs at around 850−860 nm. Accordingly, fluctuations on measured spectra will be displayed. In order to reduce noise and fluctuations induced by the spectrophotometer system itself and more clearly show the change in the levels of surface reflectance of different micro−nano structures, smoothing by Origin software has been done to the raw measured reflection curves. Figure S7 presents the comparison of several representative raw measured curves with their smoothed counterparts, showing that the smoothed curves well preserved the evolution trend of raw measured reflection curves with wavelength.
ACKNOWLEDGMENTS We acknowledge the support by the National Natural Science Foundation of China (Grant Nos. 51210009, 51575309, 11474180). REFERENCES (1) John, S. Why Trap Light? Nat. Mater. 2012, 11, 997−999. (2) Raut, H. K.; Ganesh, V. A.; Nair, A. S.; Ramakrishna, S. AntiReflective Coatings: A Critical, In-Depth Review. Energy Environ. Sci. 2011, 4, 3779−3804. (3) Kats, M. A.; Blanchard, R.; Genevet, P.; Capasso, F. Nanometre Optical Coatings Based on Strong Interference Effects in Highly Absorbing Media. Nat. Mater. 2013, 12, 20−24. (4) Huang, Y. F.; Chattopadhyay, S.; Jen, Y. J.; Peng, C. Y.; Liu, T. A.; Hsu, Y. K.; Pan, C. L.; Lo, H. C.; Hsu, C. H.; Chang, Y. H.; Lee, C. S.; Chen, K. H.; Chen, L. C. Improved Broadband and Quasiomnidirectional Anti-Reflection Properties with Biomimetic Silicon Nanostructures. Nat. Nanotechnol. 2007, 2, 770−774. (5) Xi, J. Q.; Schubert, M. F.; Kim, J. K.; Schubert, E. F.; Chen, M.; Lin, S.; Liu, W.; Smart, J. A. Optical Thin-Film Materials with Low Refractive Index for Broadband Elimination of Fresnel Reflection. Nat. Photonics 2007, 1, 176−179. (6) Lin, Q.; Hua, B.; Leung, S. F.; Duan, X.; Fan, Z. Efficient Light Absorption with Integrated Nanopillar/Nanowell Arrays for ThreeDimensional Thin-Film Photovoltaic Applications. ACS Nano 2013, 7, 2725−2732. (7) Tavakoli, M. M.; Tsui, K. H.; Zhang, Q.; He, J.; Yao, Y.; Li, D.; Fan, Z. Highly Efficient Flexible Perovskite Solar Cells with Antireflection and Self-Cleaning Nanostructures. ACS Nano 2015, 9, 10287−10295. (8) Tsui, K. H.; Lin, Q.; Chou, H.; Zhang, Q.; Fu, H.; Qi, P.; Fan, Z. Low-Cost, Flexible, and Self-Cleaning 3D Nanocone Anti-Reflection Films for High-Efficiency Photovoltaics. Adv. Mater. 2014, 26, 2805− 2811. (9) Lin, H.; Xiu, F.; Fang, M.; Yip, S. P.; Cheung, H. Y.; Wang, F.; Han, N.; Chan, K. S.; Wong, C. Y.; Ho, J. C. Rational Design of Inverted Nanopencil Arrays for Cost-Effective, Broadband, and Omnidirectional Light Harvesting. ACS Nano 2014, 8, 3752−3760. (10) Mizuno, K.; Ishii, J.; Kishida, H.; Hayamizu, Y.; Yasuda, S.; Futaba, D. N.; Yumura, M.; Hata, K. A Black Body Absorber from Vertically Aligned Single-Walled Carbon Nanotubes. Proc. Natl. Acad. Sci. U. S. A. 2009, 106, 6044−6047. (11) Lehman, J.; Sanders, A.; Hanssen, L.; Wilthan, B.; Zeng, J.; Jensen, C. Very Black Infrared Detector from Vertically Aligned Carbon Nanotubes and Electric-Field Poling of Lithium Tantalate. Nano Lett. 2010, 10, 3261−3266. (12) Yang, J.; Luo, F.; Kao, T. S.; Li, X.; Ho, G. W.; Teng, J.; Luo, F.; Hong, M. Design and Fabrication of Broadband Ultralow Reflectivity Black Si Surfaces by Laser Micro/Nanoprocessing. Light: Sci. Appl. 2014, 3, e185. (13) Teperik, T. V.; Garcia De Abajo, F. J.; Borisov, A. G.; Abdelsalam, M.; Bartlett, P. N.; Sugawara, Y.; Baumberg, J. J. Omnidirectional Absorption in Nanostructured Metal Surfaces. Nat. Photonics 2008, 2, 299−301. (14) Guo, C. F.; Sun, T.; Cao, F.; Liu, Q.; Ren, Z. Metallic Nanostructures for Light Trapping in Energy-Harvesting Devices. Light: Sci. Appl. 2014, 3, e161. (15) Landy, N. I.; Sajuyigbe, S.; Mock, J. J.; Smith, D. R.; Padilla, W. J. Perfect Metamaterial Absorber. Phys. Rev. Lett. 2008, 100, 207402. (16) Watts, C. M.; Liu, X.; Padilla, W. J. Metamaterial Electromagnetic Wave Absorbers. Adv. Mater. 2012, 24, OP98−OP120.
ASSOCIATED CONTENT S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsnano.7b03673. Relative permittivity for different metals, photographs of Cu and Ti surfaces after different treatment, calculated reflection spectra for different metal surface structures, SEM images of Stru.1 and hybrid structures with different periodicities on Ti surfaces, surface reflectance of Stru.1, Stru.2, and hybrid structures with different periodicities on Ti surfaces, comparison of antireflection effect with typical previous reports, SEM images of Stru.1, Stru.2, and hybrid structures on W surfaces as well as their reflection spectra, comparison of smoothed reflection curves with raw measured ones, ultrafast laser processing parameters for different metal samples (PDF)
AUTHOR INFORMATION Corresponding Authors
*E-mail (P. Fan):
[email protected]. *E-mail (M. Zhong):
[email protected]. 7407
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