Article pubs.acs.org/journal/apchd5
Silicon Nanostructures for Bright Field Full Color Prints Valentin Flauraud,† Miguel Reyes,†,‡ Ramón Paniagua-Domínguez,§ Arseniy I. Kuznetsov,§ and Juergen Brugger*,† Microsystems Laboratory, Institute of Microtechnique, École Polytechnique Fédérale de Lausanne, 1015 Lausanne, Switzerland Department of Materials Science and Engineering, National University of Singapore, 9 Engineering Drive 1, Blk EA, #03-09, Singapore 117575 § Data Storage Institute, A*STAR (Agency for Science, Technology and Research), 2 Fusionopolis Way, #08-01, Innovis 138634, Singapore † ‡
S Supporting Information *
ABSTRACT: Nanoscale color printing has recently emerged as a unique alternative to traditional pigments by providing record spatial resolution, angular independent, durable and single material colors. Widely based on plasmonic nanostructures, numerous efforts in the field have aimed at extending color range and saturation relying on a variety of designs and metals. Alternatively, silicon nanostructures support finely tunable electric and magnetic multipolar resonances, afford low absorption losses and benefit from well-established industrial fabrication processes, all features ideally suited to nanoscale color printing. Here we compare the properties of silicon nanodiscs with those of aluminum and silver plasmonic elements for the specific purpose of nanoscale color reproduction targeting the coverage of a broad and vivid color palette. We highlight the different properties of such metallic and dielectric resonators in various geometric and illumination conditions leading to the optimization of silicon nanodisc arrays for the fabrication of high resolution color features as well as millimetric paining replicas. The fabricated structures span a large, continuous color range with varying hue and saturation that is visible by conventional optical microscopy, photography as well as the bare eye under white light illumination. High-throughput electron beam lithography as well as color mixing schemes are discussed to further harness the unique properties of silicon nanodiscs as color elements, paving the way for a broader exploitation of nanoscale color printing. KEYWORDS: color printing, structural colors, nanoscale colors, nanophotonics, Mie resonances, dielectric nanoanetnnas
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the intrinsic properties of metals at visible wavelengths. Besides the prohibitive cost of gold (Au), plasmon resonances are restrained to a range from the green to red end of the spectrum. Aluminum (Al), although cost-efficient and supporting resonances throughout the visible, suffers from broad resonances that hinder color pureness and saturation.22 Silver (Ag) may provide the best optical properties but is plagued by a difficult manufacturing due to limited stability in ambient conditions. In this context, Si nanostructures hold a number of advantages over metallic ones for nanoscale color printing. Si nanostructures do indeed support optical resonances through the visible spectrum via so-called Mie resonances.23 In opposition to plasmonic antennas, Si structures are less affected by absorption losses24,25 and therefore possess improved resonance quality factors that may enable high color purity. Additionally, Si nanostructures support both electric and magnetic resonances26−29 that provide an additional handle
he unique absorption and scattering properties of metallic nanostructures, arising from the excitation of localized plasmon resonances,1,2 has been exploited for numerous applications for nanoscale waveguiding, focusing and imaging, sensing, heating, or enhancement of nonlinear processes.3−6 Beyond their intrinsic ability to manipulate near and far fields, these so-called optical antennas can be easily tuned to resonance by controlling their size and shape.1 In the recent years, the field of plasmonic color printing has emerged, leveraging the aforementioned properties of metallic nanostructures to print color at the nanoscale.7,8 Whereas most industrial applications to date rely on natural or synthetic pigments, nanoscale resonant antennas offer promising opportunities such as high stability and low photodegradation,9,10 record spatial resolution,7 and polarization sensitivity,11,12 as well as scalable, single material manufacturing13−15 in different geometric configurations.16−21 Although these potentials have been realized, the spread of nanoscale color printing has been partially limited by an inability to truly compete with saturated and wide gamut of traditional pigments. This inherently stems from manufacturing considerations as well as © 2017 American Chemical Society
Received: December 22, 2016 Published: June 29, 2017 1913
DOI: 10.1021/acsphotonics.6b01021 ACS Photonics 2017, 4, 1913−1919
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Figure 1. Experimental and simulated dark field spectra for Si, Ag, and Al nanodiscs. Dark field scattering spectra for (a) Si nanodiscs (100 nm height), (b) Ag nanodiscs (30 nm height), and (c) Al nanodiscs (35 nm height) and corresponding dark field micrographs. Experimental data is shown in full lines and simulations in dashed lines. All structures are on a 30 nm thick freestanding Si3N4 membrane, and isolated structures are considered.
to finely tune their response in comparison to plasmonic antennas that are mostly mediated by optically accessible electric dipoles. Finally, manufacturing of Si can rely on wellestablished and cost-efficient processes from CMOS technology. These properties of Si nanostructures have already been used successfully for color elements or antireflection coatings in linear grating,30−33 two-dimensional array,34−39 and thin-film40 configurations. Additionally, Si cylinders41 and optimized polarization sensitive designs42 have recently been exploited for the specific purpose of reproducing discrete color palettes using isolated elements observed in dark field illumination. In this Article, the performance of Si nanodiscs is investigated and optimized for fine nanoscale color reproduction throughout the visible spectrum. In contrast to previous reports, we specifically target bright field illumination conditions that are the most suited to a practical implementation of printing for applications beyond the laboratory. We first measure and compare the scattering of isolated Si, Ag, and Al nanodiscs to unveil their respective resonance tunability and practical line widths. By further comparing the performance of these three materials for discs fabricated in different geometric arrangements and under different illumination conditions, we benchmark and highlight the unique advantages of Si nanodiscs for their ability to produce continuously tunable colors both in hue and saturation. An optimized, broad Si-based color palette is then characterized and used as an input for the design of both single element prints as well as full color painting reproductions visible by traditional wide field microscopy or even naked eye for the largest structures. As an outlook, we discuss opportunities such as color mixing and large-scale fabrication that may play an important role in the widespread adoption of Si as nanoscale color elements.
implementations for color reproduction. Isolated and closepacked arrays of nanostructures with a circular footprint were produced on freestanding silicon nitride membranes. Briefly, double-side polished silicon wafers were coated with 30 nm Si3N4 and 100 nm amorphous silicon both by low pressure chemical vapor deposition. Electron beam lithography with negative tone hydrogen silsesquioxane (HSQ) resist followed by hydrogen bromide dry etching were used to respectively pattern and etch the Si structures. After removal of the HSQ mask by hydrofluoric acid and release of the silicon nitride membranes that support the Si nanodiscs, Al and Ag were patterned in two subsequent lift-off steps with a thickness of 35 and 30 nm, respectively (details in the methods in the Supporting Information). First, dark field scattering spectra were measured and simulated for isolated Si, Ag, and Al nanodiscs ranging from 80 to 200 nm in diameter (Figure 1). Dark field microscopy is initially used, as this enables the selective analysis of the nanodisc resonances, free from background illumination and substrate reflections. Consistently with previous reports,22 Al covers the largest resonance range, along with the broadest resonance line widths. Comparably, Ag exhibits improved quality factors as well as red-shift of the resonance peaks. Here we note that the quality factor of the fabricated Ag structures is lower than theoretical predictions, possibly due to the combination of size variations and partial sulfidation of Ag. Although exposure of the samples in ambient atmosphere was limited, this highlights the need for passivation layers and tailored fabrication processes if Ag is to be used. For Si, resonances are also tuned across the visible spectrum and provide the most gradual spectral tuning, while excellent agreement is found between measured and simulated structures. Noticeably, high quality factors, compared to metals, are achieved at the red end of the spectrum where Si presents limited absorption. Next, Si nanodisc arrays with a square lattice of 8 × 8 μm surface were measured in order to study uniformly colored macroscopic pixels under bright field illumination.
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RESULTS AND DISCUSSION Si, Ag, and Al nanostructures are fabricated with similar substrate and geometric arrangement in order to compare their optical response in various configurations and identify optimal 1914
DOI: 10.1021/acsphotonics.6b01021 ACS Photonics 2017, 4, 1913−1919
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Figure 2. Bright field measurements for Si, Al, and Ag nanodiscs in reflection, transmission, and reflection with back-reflector. (a) Schematic of a nanodisc on a freestanding membrane observed in bright field reflection along with experimental optical micrographs of Si, Al, and Ag nanodisc arrays. (b) Corresponding reflection spectra measured for disc diameters ranging from 80 to 200 nm and a constant 200 nm gap. (c) Schematic of a nanodisc on a freestanding membrane observed in bright field transmission, along with experimental optical micrographs of Si, Al, and Ag nanodisc arrays. (d) Corresponding reflection spectra measured for disc diameters ranging from 80 to 200 nm and a constant 200 nm gap. (e) Schematic of a nanodisc on a freestanding membrane observed in bright field reflection including a 60 nm thick Al back reflector along with experimental optical micrographs of Si, Al and Ag nanodisc arrays. (f) Corresponding reflection spectra measured for disc diameters ranging from 80 to 200 nm and a constant 80 nm gap. Spectra are displayed with arbitrary relative offsets.
first related to absorption at the blue end of the spectra for Si and absorption of Al at the red end, respectively, owing to their dielectric properties. Second, subtractive colors require broader or multiple resonances compared to reflective ones to provide well-defined colors. Indeed, if only a limited spectral range is absorbed from the transmitted or reflected white light continuum, only minimal effects are observable. A back reflector of 60 nm Al evaporated on the backside of the Si3N4 membrane of each sample provides comparable results as reported for similar structures in Al16 whereby noticeably more saturated hues were achieved. In plasmonic systems, the back reflector acts both as a mirror for the otherwise transmitted light but also provides a mirror charge that allows coupling and hybridization with the nanoparticle modes.45,46 Similar mechanisms, yet with a different mode coupling nature, have also been exploited for metal films evaporated on nanoposts.7,9,11,12,18 Back reflectors in plasmonic systems may therefore partially tune and enhance resonances.47 Such effects have also been studied for the Si structures in direct contact with metal films,48 and with dielectric spacers,49 on the back reflector and provide a strong modulation of scattering directionality and loss channels that effectively results here in
Nanodiscs of diameters ranging from 50 to 250 nm (10 nm step) and interparticle gaps from 40 to 680 nm are considered. A constant gap width, rather than disc pitch, is considered in order to minimize fill-fraction discrepancies. In bright field reflection images (Figure 2a), visual differences in the colors produced by Si and metal nanostructures are well evidenced. For varying interparticle gap widths, Si exhibits a gradual intensity modulation corresponding to the nanodisc surface filling fraction, whereas Al and Ag structures display abrupt hue and intensity modulations. This partially translates to the fact that electromagnetic near fields are more strongly confined to the volume of the Si particles,43 whereas important near field overlap, in the case of plasmonic systems, may result in significant coupling mechanisms44 that highly entangle color brightness and hue for varying nanogaps. Reflection spectra for these arrays, compared with 200 nm disc to disc gap to avoid excessive near field plasmonic coupling of Al and Ag structures, exhibit a gradual tuning of reflection maxima (Figure 2b) associated with the nanoparticle resonances (Figure 1). In transmission (Figure 2c), resonances are associated with transmission minima (Figure 2d), and none of the three materials display well-defined resonances and associated vivid colors from the entire blue to red end of the spectrum. This is 1915
DOI: 10.1021/acsphotonics.6b01021 ACS Photonics 2017, 4, 1913−1919
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Figure 3. Large scale bright field reflection color palette for Si nanodics on a quartz substrate. (a) Optical micrograph of a large-scale Si nanodisc color palette. Disc diameters span 35 to 250 nm in 5 nm step and disc to disc gap is varied from 60 to 430 nm in 10 nm steps. Experimental and calculated reflection spectra for Si nanodiscs, with a diameter ranging from 80 to 200 nm, with either 80 nm (b) or 200 nm (c) disc to disc gap. (d) sRGB Colorspace coverage of the fabricated nanodisc arrays and color evolution with varying disc diameter and gap. The size evolution is shown for a series of discs with a constant gap of 200 nm, and the gap series is shown for a constant 140 nm diameter.
lower color saturation compared to a case without backreflector. Overall, this comparison spanning a large range of previously studied plasmonic systems reveals that Si structures, measured in reflection without back reflector, exhibit the most gradual and monotonic brightness and hue modulation with varying nanodisc density and size, respectively. This configuration is additionally advantageous application-wise as it allows preserving the transparent nature of the substrate without the need of a back-reflector. We therefore produce a large color palette of such nanostructures on a robust quartz substrate rather than freestanding membranes. The Si structures are tuned from 35 to 250 nm disc diameter and 60 to 430 nm disc to disc distance (Figures 3a and S1). Both measured and simulated spectra are compared showing a consistent and gradual modulation of the maximal reflection peak that controls the hue of the pixel via the disc diameter as well as absolute intensity via the tuning of the nanodisc fill factor controlled by the gap size (Figure 3b,c). Overall, the peak observed at lower wavelength arises from an electric dipole dominant contribution whereas the peaks at longer wavelength result from a fine interplay between electric and magnetic dipolar contributions presented in detail in Figure S3. Data from the palette are displayed in a sRGB colorspace diagram showing the large extent a gradual tuning of Si colors (Figure 3d; details in methods and Figure S2 in the SI). Using this color palette as a reference lookup table, different example structures are written to demonstrate high-DPI, isolated nanometric color features. Isolated single discs with 500 nm pitch (>50000 DPI) provide sufficient optical contrast to print clearly distinguishable patterns in bright field microscopy (Figures 4a−c and S4a−d) showing that this approach relying on Si nanodiscs is also competitive with plasmonic alternatives when considering high resolution printing. Using as few as three silicon discs over the 1 μm width of a character, microscale features are written with a gradient covering the full visible spectrum (Figure 4d,e). The combination of high density features, well visible in bright field, along with isolated features provides an additional degree of
Figure 4. Single pixel silicon-based color structures. (a) Bright field micrograph of the word “colors” written with single Si discs with 500 nm pitch on a quartz wafer. Disc diameter evolves, left to right and top to bottom, from 40 to 150 nm (10 nm step). Corresponding SEM micrographs for 70 nm diameter (b) and 150 nm diameter discs (c). (d) SEM micrograph and optical micrograph (e) of a nanodisc array with a diameter gradient and the word “Silicon” written with a threedisc font line width.
freedom to produce dual tone images with distinct bright field and dark field results (Figure S4e−g). Beyond these features observed at high magnifications, the optimized, broad and gradual color palette is exploited to produce large image reproductions. Various bitmap images were reproduced by discretizing the original image color palette to the color palette of Figure 3, whereby closest neighbor colors were chosen as the minimal distance in LAB colorspace. Each pixel of the image was then written as a 2 × 2 μm arrays of Si discs with suitable gap and diameter. This is here demonstrated for two well-known paintings reproduced based on resized and processed low resolution images. A self-portrait by Vincent Van Gogh (1889) adapted and reproduced with permission from the copyright office of the Réunion des Musées Nationaux et du Grand Palais (RMNGP), Paris France, Copyright 2017 1916
DOI: 10.1021/acsphotonics.6b01021 ACS Photonics 2017, 4, 1913−1919
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RMNGP as well as the iconic painting The Scream by Edvard Munch (1910) also adapted and reproduced with permission from the copyright office of the Munch Museum, Oslo Norway, Copyright 2017 Munch Museum. These two paintings were chosen as they include fine hue variations covering a broad color space. The overall Van Gogh self-portrait (Figure 5a) and
Figure 6. Large scale painting reproduction. (a) Original image of the painting The Scream, by Edvard Munch; (b) same image after discretization of the color range to the colors produced by the color palette of Figure 3. (c) Optical micrograph of the Si based painting reproduction covering 545 × 700 μm. Painting adapted and reproduced with permission from the copyright office of the Munch Museum, Oslo Norway. Copyright 2017 Munch Museum.
the fine fracture writing scheme. This allows the macroscopic demonstration of the minimal angular dependence of such Si color-based palettes (Figure S7) and may provide a valuable approach in minimizing the effective write time of such structures either for their direct fabrication or the design of large stamps that may be used for further reproduction by nanoimprint lithography.50 A second investigation was focused on color mixing. In the original writing scheme, fine 5 nm diameter steps were used for the creating of the calibration color palette. Transitions over the blue/green color range prove to be the most limited and are highly sensitive to the geometric configuration. Different binary tilings were fabricated and imaged (Figures S8 and S9) whereby nanodiscs of different sizes are mixed at a subdiffraction scale in order to extend the achievable gradients. In analogy to color displays and prints, such concepts will be crucial to further extend nanoscale printable from the >1500 variations shown here to millions of colors routinely printed by traditional methods. In conclusion, Si and metal nanodisc based optical resonators were designed fabricated, measured and simulated to compare their respective response when applied to nanoscale color printing. Si nanodiscs were then used to create a large color palette with tunable color hue and intensity harnessing both electric and magnetic resonances. Visual elements were created, demonstrating the potential of Si nanostructures as color elements from few dot structures to large millimetric color replicas. The mixture of high-throughput lithography, bright field and dark field combination, as well as color mixing, was further discussed as additional opportunities to harness the abilities of Si-based nanoscale colors. Our investigations based on bright field microscopy provides structures conveniently visible under daylight illumination for the largest structures and we envision that this work will pave the way to a broader use of nanoscale color elements ranging from anticounterfeiting features to decorative elements in consumer products and packaging.
Figure 5. Large scale painting reproduction. (a) Bright field micrograph of a Si-based reproduction of the 1889 Vincent van Gogh self-portrait. The painting covers 825 × 465 μm. (b) Optical micrograph showing a detailed view of the eye region and (c) SEM micrograph of the individual pixel arrays. Painting adapted and reproduced with permission from the copyright office of the Réunion des Musées Nationaux et du Grand Palais (RMNGP), Paris France. Copyright 2017 RMNGP.
corresponding optical and electron microscopy details (Figure 5b,c) demonstrate the Si color printing scheme. The image is reproduced from the digital file with high accuracy (Figure S5) and may be observed either by optical microscopy in bright field or using a standard photographic camera and macrophotography lens (Figure S6). The painting of The Scream (Figure 6) further demonstrates the ability to reproduce colors spanning a large range covering the red, green, and blue fundamental colors. Overall, slight color variations are mainly associated with proximity and charging effects occurring during electron beam lithography, as the image layout may induce slightly different final nanodisc sizes when compared to the calibration palette. As an outlook, two further considerations were also investigated. First, concerning lithographic throughput, all the structures presented up to this point have been written using a fine 3 nA current and 5 nm diameter Gaussian electron beam while the nanodisc design was carefully fractured in fine trapezoids using a 2 nm electron beam lithography fracture grid. Disc structures are simple feature that may be reproduced using alternative lithography schemes, a so-called write on the fly technique was therefore investigated, whereby a single point is written by EBL for each disc, with a 100 nA beam, and diameter variations are solely based on a dose per shot variation. A total of 14 arrays of 7.5 × 7.5 mm each were consequently written at an average speed of 1.17 mm2/min presenting a 2 orders of magnitude speed improvement over 1917
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsphotonics.6b01021. Methods; Broad color palette color range under different illumination conditions; Comparison of RGB colors extracted from spectra and bright field images; Fewelement Si based color dark field imaging; Comparison from original image to printed structure; Macroscopic photograph of color reproductions; Si nanodiscs by write on the fly lithography scheme; Color mixing SEM images; Color mixing optical images (PDF).
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AUTHOR INFORMATION
Corresponding Author
*E-mail: juergen.brugger@epfl.ch. ORCID
Valentin Flauraud: 0000-0002-1393-3198 Juergen Brugger: 0000-0002-7710-5930 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The research leading to these results has received funding from the European Commission’s Seventh Framework Programme (FP7-ICT-2011-7) under Grant Agreement 288263 (NanoVista). Authors from DSI were supported by A*STAR SERC Pharos program, Grant 152 73 00025 (Singapore). The authors gratefully acknowledge the valuable support from the EPFL center of micro- and nanofabrication (CMi), Clemens Herkommer and Michail Zervas from the laboratory of photonics and quantum measurements for their advice on HBr dry etching, as well as Christian Santschi and Xiaolong Wang from the Nanophotonics and Metrology Laboratory (NAM-EPFL) for enabling the use of the DF/BF spectroscopy setup.
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REFERENCES
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DOI: 10.1021/acsphotonics.6b01021 ACS Photonics 2017, 4, 1913−1919
ACS Photonics
Article
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DOI: 10.1021/acsphotonics.6b01021 ACS Photonics 2017, 4, 1913−1919
