Generation of Reflection Colors from Metal-Insulator-Metal Cavity

Subscriber access provided by Nottingham Trent University

Article

Generation of Reflection Colors from Metal-Insulator-Metal Cavity Structure Enabled by Thickness-Dependent Refractive Indices of Metal Thin Film Jaeyong Kim, Harim Oh, Minseok Seo, and Myeongkyu Lee ACS Photonics, Just Accepted Manuscript • DOI: 10.1021/acsphotonics.9b00894 • Publication Date (Web): 29 Aug 2019 Downloaded from pubs.acs.org on August 29, 2019

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 41 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Photonics

Generation of Reflection Colors from MetalInsulator-Metal Cavity Structure Enabled by Thickness-Dependent Refractive Indices of Metal Thin Film Jaeyong Kim, Harim Oh, Minseok Seo, and Myeongkyu Lee*

Department of Materials Science and Engineering, Yonsei University, Seoul 120-749, Korea

ABSTRACT: Tunable structural colors have diverse applications ranging from displays and photovoltaics to surface decoration and art. A metal-insulator-metal (MIM) cavity structure formed by thin continuous layers has drawn great interest as a lithography-free and scalable optical structure to control light transmission and reflection at the surface of a material. However, the production of distinct reflection colors from the structure is challenging because the typical MIM cavity absorbs a narrow wavelength range and reflects the rest of the spectrum. This study shows that the MIM structure can exhibit a reflection peak instead of a reflection dip if the metal layer has proper optical constants.

ACS Paragon Plus Environment

1

ACS Photonics 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 2 of 41

Vivid reflection colors are generated by using thermally evaporated Au and Ag thin films whose refractive indices are much different from the standard handbook data. The strong thickness dependence of the refractive indices also enables color tuning by varying the thickness of the metal layer only. Consequently, color images can be printed by locally controlling the thickness of either the insulating spacer or the metal layer. The results of the study are attractive and useful for both practical and artistic purposes.

Keywords: Reflection colors; Metal-insulator-metal structure; Color printing; Structural colors The interaction of light with surface structures can produce structural colors through fundamental optical phenomena such as interference, diffraction, scattering, and resonance.1-10 Brilliant structural colors are easily observed in nature, such as in bird feathers and butterfly wings. The engineering of structural colors has attracted great interest in a variety of fields, including light-emitting diodes, photovoltaics, flat panel displays, sensors, product identification, anti-forgery, and art. Surfaces decorated with structural colors are also receiving tremendous attention owing to their widespread use.


2

Page 3 of 41 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Photonics

Artificial structures such as photonic crystals, multiscale structures, diffraction gratings, and selective mirrors have been extensively investigated to mimic the structural colors of living creatures.2,6,7,11-13 However, the practical applications of these biomimetic structures are still limited due to complicated fabrication processes and durability issues. Plasmonic nanostructures can manipulate lightmatter interactions at the nanoscale and enable the spectral information (transmission, reflection, and absorption) of light to be controlled.4,9,14,15 Since the spectral information depends strongly on the size, shape, and periodicity of metallic or dielectric nanostructures, plasmonic colors are very sensitive to these factors. This means that nanostructures with different sizes or shapes are needed to produce different colors. In addition, the plasmonic nanostructures operating at visible wavelengths require subwavelength features. These features are typically fabricated by nanofabrication techniques such as e-beam lithography and focused ion-beam etching, which are neither scalable nor economical.

Meanwhile, planar and multilayered structures provide an alternative opportunity in controlling and manipulating lightmatter interactions. A metal-insulator-metal (MIM)


3

ACS Photonics 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 4 of 41

cavity structure formed by thin continuous layers is of special interest because its fabrication is lithography free and involves only thin-film deposition.16-28 The MIM structure is a Fabry-Perot (FP) cavity that enhances resonant absorption within the structure. Due to their advantages and promising applications, FP-cavity-based spectral filters have been extensively studied. Li et al.26 demonstrated a narrow bandwidth (~17 nm) super absorber with 97% maximum absorption using an Ag/SiO2/Ag cavity structure. They also investigated transmission color filters using ultrathin metallic films, in which different colors can be obtained by controlling the dielectric spacer thickness. An inherent characteristic of the conventional FP resonator is that it acts as a band-stop filter in reflection mode, absorbing a narrow wavelength range and reflecting the rest of the spectrum.19,26 This consequently leads to weak reflection colors. Yang et al.21 obtained fairly vivid reflection colors using a Ni/SiO2/Al structure. The use of a highly absorbing ultrathin Ni film as the top metal layer and a highly reflecting thick Al film as the bottom layer resulted in a rippled spectrum consisting of alternating reflectance maxima and minima. In the rippled spectrum, reflection peaks and dips alternately appear as the wavelength varies. Thus, if the width of a certain visible peak is somehow


4

Page 5 of 41 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Photonics

reduced to improve the saturation of the corresponding color, new peaks come into the visible spectral range, deteriorating the color purity. Park et al.22 realized trans-reflective color filters by adding an additional dielectric layer over a conventional FP structure (TiO2/Ag/TiO2/Ag). While primary R/G/B (red, green, and blue) colors were obtained in the transmission mode, the reflection colors were subtractive colors (cyan, magenta, and yellow). Recently, Ghobadi et al.17 reported the generation of R/G/B reflection colors using a series connection of cavities, but it required a complicated structure consisting of metal-insulator-metal-insulator-semiconductor layers. To produce distinct reflection colors without the restrictions mentioned above, the reflection spectrum should exhibit a peak, rather than a dip. Herein, we show that the reflection spectrum of the MIM cavity structure can be modified to have a peak if the metal layer has proper optical constants.

RESULTS AND DISCUSSION


5

ACS Photonics 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 6 of 41

Fig. S1(a) (Supporting Information) shows the refractive indices (n, k) of Ag and Au adopted from a standard reference handbook.29 The reflectivity (R) of a bulk material is given by R = {(n  1)2  k2}/{(n  1)2  k2}. R approaches unity when either the real index

n is very close to zero or the imaginary index k is much larger than n. Ag has n > n in the whole visible range and thus exhibits reflectivity R  90% for all visible wavelengths. The color of an object gives information about which wavelengths it reflects. The fact that Au has a reddishyellow color indicates that it does not reflect all visible wavelengths equally. Au exhibits interband absorption at wavelengths below 500 nm, resulting in a dramatic decrease in reflectivity (R  30% for wavelengths < 500 nm). That is why the refractive indices of Au are much different from those of Ag at wavelengths below 500 nm. Many studies on the MIM cavity structure have been performed using metal thin films deposited by e-beam evaporation. Ag and Au films were deposited on glass substrates by e-beam evaporation in the thickness range from 10 to 50 nm and their frequency-dependent refractive indices were measured by ellipsometry. The refractive indices of the e-beam-evaporated Ag and Au films were very similar to the handbook data (Fig. S1(b)). Both n and k were nearly independent of


6

Page 7 of 41 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Photonics

the film thickness. Ag and Au films were also thermally evaporated onto glass substrates at the same deposition rate of 1 Å/s used for the e-beam evaporation.

Figure 1 (a) Refractive indices of thermally evaporated Ag films. (b) Refractive indices of thermally evaporated Au films. (c) Cross-sectional TEM images of a 10-nm-thick Ag film. (d) SEM image showing the surface morphology of a 10-nm-thick Ag film. (e) TEM images of a 30-nm-thick Ag film.


7

ACS Photonics 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 8 of 41

Fig. 1(a) shows the refractive indices measured for the thermally evaporated Ag films. A 10-nm-thick Ag film exhibited significantly different indices from the handbook data. The film had n > k at wavelengths longer than 500 nm; then, it acts like a lossy dielectric film in the spectral range over 500 nm. The difference between the measured and handbook indices decreased as the film thickness increased. When it reached 40 nm, both n and k became almost equal to the handbook data. The refractive indices of the thermally evaporated Au films were also thickness dependent, coinciding with the handbook data only for thicknesses > 30 nm (Fig. 1(b)). The refractive indices of a thin film are influenced by its microstructure, crystallinity, and density. These factors may also depend on the film thickness, deposition method, and underlying material.30,31 Ultrathin metal films may not be continuous due to a potential island-growth mode. However, no perforation was observed for both Ag and Au films even when their thickness was 10 nm. Fig. 1(c) shows transmission electron microscopy (TEM) images of a thermally evaporated 10-nm-thick Ag film. The film was continuous although its thickness was locally fluctuated. Fig. 1(d) is a scanning electron microscopy (SEM) image showing the surface morphology. The 10-nm-thick film consisted of fine


8

Page 9 of 41 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Photonics

particulate grains with sizes of a few tens of nanometers. This particulate structure accounts for the local thickness fluctuation. As the film thickness increased, the fine particulate grains coalesced into larger grains, increasing the grain sizes. Consequently, the film surface became more smooth. A 30-nm-thick Ag film exhibited fairly uniform thickness, as shown in Fig. 1(e). Au films exhibited denser structures than Ag films at the same thickness. Although the exact cause of the thickness-dependent refractive indices has yet to identified, it is likely to be related to the thickness-dependent microstructures.


9

ACS Photonics 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 10 of 41

Figure 2 (a) Reflection spectra simulated using the handbook refractive indices of Ag. Inset is the MIM structure used for simulation. (b) Variation of the reflection spectra with SiO2 thickness at tAg = 30 nm. (c) Simulation colors obtained for different SiO2 thicknesses (tAg = 30 nm). (d) Spectra simulated using the thickness-dependent refractive indices measured for thermally evaporated Ag films. (e) Variation of the reflection spectra with SiO2 thickness at tAg = 20 nm. (f) Simulation colors obtained for different SiO2 thicknesses (tAg = 20 nm). (g) Reflection spectra simulated using the measured refractive indices. Top metal layer is Au. (h) Variation of the reflection spectra with SiO2 thickness at tAu = 20 nm. (i) Simulation colors obtained for different SiO2 thicknesses (tAu = 20 nm).


10

Page 11 of 41 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Photonics

The experimental Ag and Au films used in this study were deposited by thermal evaporation. As shown, the refractive indices of these films were strongly thickness dependent, coinciding with the standard handbook data only when the films were sufficiently thick (> 40 nm for Ag and > 30 nm for Au). This indicates that the reflective and absorptive characteristics of a thermally evaporated Ag (or Au) thin film may be much different from those of an e-beam-evaporated film with the same thickness. The effects of the thickness-dependent Ag and Au refractive indices were thus analyzed by finite-difference time-domain (FDTD) simulation using a MIM cavity formed on Si. The inset of Fig. 2(a) schematically illustrates the cavity structure; it consists of a 125-nmthick SiO2 film sandwiched between a thick Ag layer-coated Si substrate and a thin Ag layer. Since the bottom Ag layer coated on the Si substrate is optically thick (100 nm), no light transmission occurs. Fig. 2(a) shows the reflection spectra simulated using the handbook refractive indices of Ag. When the thickness (tAg) of the top Ag layer is 10 nm, a shallow and broad reflection dip is observed. As tAg increases from 10 to 30 nm, the reflection dip becomes deeper and narrower. The dip position also blue-shifts due to the


11

ACS Photonics 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 12 of 41

gradual phase shift of the light reflected from the airAg interface relative to the internally reflected light. For tAg = 30 nm, the reflection dip is located at 525 nm, where the light reflected from the airAg interface interferes destructively with the internally reflected light, maximizing resonant absorption by the cavity. As the thickness further increases, the reflection dip begins to shrink. For very large thicknesses, the reflection spectrum resembles that of bulk Ag (not shown in Fig. 2(a)). The overall reflectance of the FP cavity can be derived using the transfer matrix method, i.e., by multiplying the transfer matrix of each layer.22,32-35 The resonance condition is given by the following equation,32-34

 4   res

  ni di  b  t  2m 

(1)

where res is the resonance wavelength, ni and di are the refractive index and thickness of the dielectric layer, respectively, and m is an integer determining the order of cavity mode. The first term in the left side of Eq. (1) represents the round-trip propagation phase shift within the cavity. b and t are the phase shifts upon reflection at the bottom and top metaldielectric interfaces, respectively. As the optical thickness (nidi) of the


12

Page 13 of 41 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Photonics

dielectric layer increases, res increases, and thus, the resonance peak red-shifts. The reflection phase shift (b and t) depends on the thickness of the respective metal layer. In this study, the thickness of the bottom Ag layer is fixed, leading to a fixed b value. The variation of t with the top-layer thickness is opposite to the way that the propagation phase shift varies with the dielectric-layer thickness.22,26,27 This accounts for the counter-intuitive blue-shift of the resonance peak with increasing tAg. Fig. 2(b) shows the variation of the reflection spectrum with the SiO2 thickness at tAg = 30 nm. The simulated spectra were mapped to points on the CIE-1931 chromaticity diagram using colorimetric transformations to display the corresponding colors. Fig. 2(c) shows the colors obtained at different SiO2 thicknesses (tAg = 30 nm). Since the cavity has a narrow absorption peak, the colors produced are very weak. All these results are consistent with the literature.19,26

In comparison to Fig. 2(a), Fig. 2(d) shows the spectra simulated using the thickness-dependent refractive indices of the thermally evaporated Ag films. The two spectra for tAg = 50 nm are very similar to each other because the Ag refractive indices


13

ACS Photonics 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 14 of 41

measured at this thickness are almost identical to the bulk-state handbook data. For tAg  30 nm, however, the spectra simulated using the measured refractive indices show much lower reflectance at long visible wavelengths than those simulated using the handbook data. This causes the spectra to exhibit an asymmetric peak (Fig. 2(e)), consequently producing stronger and more vivid colors (Fig. 2(f)). The reflection spectra are more significantly changed when an Au film is used as the top metal layer, as shown in Fig. 2(g); the bottom layer is a 100-nm-thick Ag film. Compared to the spectra obtained with an Ag top layer (Fig. 2(d)), the reflectance is reduced at short visible wavelengths due to the interband absorption of Au. Another remarkable change is that a thin Au layer (1020 nm) significantly suppresses light reflection at long visible wavelengths. This may be explained by the thickness-dependent refractive indices of the thermally evaporated Au films. As shown in Fig. 1(b), the 10-nm- and 20-nm-thick Au films exhibit comparable n and k values. Then, the films have much lower reflectivity than the case with k >> n. Fig. 2(h) shows the variation of the spectra with the SiO2 thickness at a fixed top-layer thickness of 20 nm. The reflection spectra reveal a nearly symmetric peak instead of a dip. This consequently leads to colors that are much more


14

Page 15 of 41 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Photonics

distinct, as illustrated in Fig. 2(i). Fig. S2(a) shows the reflection spectra and colors simulated using the handbook refractive indices, where the top Au layer thickness is fixed at 20 nm and the SiO2 thickness is varied from 35 to 195 nm with an interval of 10 nm. As expected, the spectra exhibit a reflection dip whose width varies with the dip position. By using a thermally evaporated Au film as the top layer, the spectra are modified to exhibit a peak instead of a dip, although the peak is wider than the dip (Fig. S2(b)). This produces more distinct reflection colors and allows a wider tuning range of color hues. If the Au film has bulk-state refractive indices, blue colors are not generated from the cavity, as shown in Fig. S2(a).


15

ACS Photonics 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 16 of 41

Figure 3 (a) Reflection spectrum of Au(20 nm)/SiO2(100 nm)/Ag(100 nm)/Si structure simulated using the handbook refractive indices of Au. (b) Electric field and absorbed power profiles at 520 nm. (c) Electric field and absorbed power profiles at 700 nm. (d) Reflection spectrum of Au(20 nm)/SiO2(100 nm)/Ag(100 nm)/Si structure simulated using the measured refractive indices of Au. (e) Electric field and absorbed power profiles at 570 nm. (f) Electric field and absorbed power profiles at 700 nm. (g) Reflection spectra of the cavity samples fabricated with SiO2 thicknesses of 65, 105, 125, and 175 nm, where the top Au layer is 20 nm thick. (h) Camera images of the samples.


16

Page 17 of 41 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Photonics

In order to better understand why the use of a thermally evaporated Au film as the top layer produces vivid reflection colors, we calculated the electric field intensity and absorbed power distributions within the MIM cavity. Fig. 3(a) is the reflection spectrum calculated using the handbook indices of Au for the structure of Au(20 nm)/SiO2(100 nm)/Ag(100 nm)/Si. Fig. 3(b) shows the electric field and absorbed power profiles at  = 520 nm, where resonance absorption occurs. Under this resonance condition, the wave reflected from the top metallic layer interferes destructively with the internally reflected waves. Then, the electric field is highly confined at the dielectric SiO2 layer, and most of the optical power is absorbed by the top metallic layer due to the enhanced electric field resulting from the cavity effect. The resonance condition is associated with the multiple round-trip phase shifts of light inside the cavity.26,27 At non-resonance wavelengths, the wave reflected from the top metallic layer interferes constructively with the internally reflected waves, strongly reflecting the incident light. As an example, Fig. 3(c) represents the electric field and absorbed power distributions calculated at 700 nm. With the measured refractive indices of Au, the resonance wavelength is shifted to 570 nm, and the reflectance is significantly reduced at long visible wavelengths (Fig. 3(d)).


17

ACS Photonics 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 18 of 41

Fig. 3(e) shows that the incident light at 570 nm is also mostly absorbed by the top Au layer. However, the intensity of the electric field confined at the SiO2 layer is lower than the case when the handbook indices of Au are used. This indicates that the incident light is substantially absorbed by the Au layer before entering the cavity. That is, the perfect absorption obtained at 570 nm is a combined result of the direct absorption of the incident light and the enhanced electric field due to the cavity effect. Fig. 3(f) shows the electric field and absorbed power distributions observed at 700 nm. In comparison to Fig. 3(c), the Au layer more absorbs and less reflect the incident light. Fig. 3(g) shows the reflection spectra of the cavity samples fabricated with SiO2 thicknesses of 65, 105, 125, and 175 nm, where the top Au layer is 20 nm thick. Fig. 3(h) shows the camera images of the samples. The experimental spectra and colors were highly consistent with the simulation results shown in Fig. S2(b). Different colors appearing with different SiO2 thicknesses made it possible to print color images by locally controlling the thickness of the SiO2 spacer (Fig. S3). Since SiO2 is a low-index material (n  1.5), the colors were sensitive to the viewing angle (Fig. S4). The angle dependence of the colors became stronger as the thickness of the SiO2 layer increased. A higher-index dielectric spacer


18

Page 19 of 41 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Photonics

can mitigate the angle dependence of the colors because the refraction angle in a highindex film remains small over a wide range of incidence angles owing to the large difference in indices between the film and air.

In a recent report,36 we showed that metal substrates such as stainless steel (STS) and Al can be colored by coating a metal-dielectric double layer. Thermally evaporated Ag and Au films were used for the metal layers. Although color tuning was possible by varying the thickness of the metal layer only, the experimental colors were not consistent with the colors predicted by simulation based on the optical constants of bulk Ag and Au. The discrepancy between the experimental and simulation colors may be explained by the findings of the current study, that is, the thickness-dependent refractive indices of the thermally evaporated films. In an optical sense, a Si substrate coated with a thick metal layer can be regarded as a metal substrate. A metaldielectric double layer coated on a metal substrate naturally forms a MIM structure. Herein, we also investigate whether it is possible to generate predictable colors from metal substrates. STS, which is well known for its resistance to corrosion, is widely used in home and


19

ACS Photonics 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 20 of 41

industry applications. STS substrates (size = 3  3 cm2) were prepared by cutting polished STS plates, and the substrate was coated with a metaldielectric double layer. Si3N4 and indium-tin oxide (ITO) were used for the high-index dielectric spacers. The real-part refractive index of Si3N4 was n = 2.02.1 in the visible wavelength range from 400 to 700 nm. ITO showed slightly lower n values (1.882.05) in the same range. Various reflection colors, including blue, green, pink, purple, scarlet, and yellow, could be produced by combining the dielectric and metal layers. Fig. S5(a) shows some colors obtained from the Au/Si3N4/STS structure. The color changed from yellow to orange, pink, and blue as the Si3N4 thickness varied from 30 to 40, 50, and 75 nm, respectively. The angle dependence of the colors was negligible in the range from 0 to 45o once the thickness of the Si3N4 spacer remained less than 100 nm. In Fig. S5(a), the thickness of the top Au layer is 15 nm, while the Au thickness in Fig. 2(h) and (i) is 20 nm. STS has lower reflectivity (6070%) than a 100-nm-thick Ag layer coated on Si. On the other hand, Si3N4 has higher reflectivity than SiO2. This reduces the thickness of the top metal layer required for destructive interference. At specific Si3N4 thicknesses, the hue of the color varied significantly with the Au layer thickness. Fig. S5(b) shows the


20

Page 21 of 41 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Photonics

variation of the experimental spectra with the Au layer thickness at a fixed Si3N4 thickness of 100 nm. Because the reflectance changed mostly at near-infrared wavelengths (> 700 nm), only the saturation of the color changed slightly (Fig. S5(c)). Reducing the Si3N4 thickness to 60 nm shifted the spectra (Fig. S5(d)), making the reflectance of visible wavelengths (> 550 nm) change also. Interestingly, a 5-nm-thick Au layer completely suppressed light reflection in the yellowred range, resulting in blue coloration. As the Au thickness increased, the reflectance of light in this range gradually increased. Consequently, the color progressively changed to purple and pink (Fig. S5(e)). Simulation analysis confirms that the tuning of the color hues with the top-layer thickness is possible due to the thickness-dependent refractive indices (Fig. S6).


21

ACS Photonics 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 22 of 41

Figure 4 (a) Schematic of color printing method. (b) Image printed by selectively depositing an Au layer on a 50-nm-thick uniform Si3N4 film; the blue network-like pattern has a 5-nm-thick Au layer, and the Au thicknesses in the lower-left, center, and upperright background regions are 10, 15, and 20 nm, respectively. (c) Image obtained after coating a 5-nm-thick Au layer uniformly over the image “(b)”. (d) Image obtained after coating a 5-nm-thick Au layer uniformly over the image “(c)”. (e) and (f) Images printed by coating an Au layer conformally over selectively deposited Si3N4 films. (g) Printed color image and SEM images. Scale bars are 0.5 mm.


22

Page 23 of 41 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Photonics

Metal surfaces finished in color images are attractive for decoration and art as well as for product identification, anti-counterfeiting, and reflective color filtering. Fig. 4(a) illustrates two different ways to print color images. Color images can be printed by depositing a uniform dielectric film and then coating a metal layer selectively on top of it. They may also be printed by coating a metal layer conformally over a selectively deposited dielectric film. Fig. 4(b) shows an image printed by selectively depositing an Au layer on a 50-nm-thick uniform Si3N4 film. The blue network-like pattern has a 5-nmthick Au layer. The Au thicknesses in the lower-left, center, and upper-right background regions are 10, 15, and 20 nm, respectively. Fig. 4(c) shows the image obtained after coating a 5-nm-thick Au layer uniformly over the image shown in Fig. 4(b). The coating of an additional 5 nm Au layer produced the image shown in Fig. 4(d). Color images were also printed by coating an Au layer conformally over a selectively deposited dielectric film. The yellow, blue, and light-blue regions in Fig. 4(e) consist of Si3N4 films with thicknesses of 30, 75, and 110 nm, respectively, where the top Au layer is 15 nm thick. Fig. 4(f) shows another image printed in the same way, where the scarlet network region consists of a 15 nm Au layer coated on a 40 nm Si3N4 film. Fig. 4(e) and (f) show


23

ACS Photonics 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 24 of 41

that differently colored regions do not have sharp boundaries. This is an engineering issue rather than a scientific one. The Si3N4 film was selectively deposited using a shadow mask by plasma-enhanced chemical vapor deposition. It is likely that the mask edge and center regions had not been coated to the same thickness. This problem was not observed when the top metal layer was selectively deposited, probably because the metal layer was thinner than the underlying dielectric film and was coated by a physical deposition method. Fig. 4(g) shows a color image printed by selectively depositing an Au layer on a 50-nm-thick uniform ITO film. The Au thicknesses of the upper and lower flowers are 20 and 15 nm, respectively. The blue background region has a 5-nm-thick Au layer. SEM images agreed well with the color images, although the contrast of the SEM images was not as high as expected. In Fig. 4, the top metal layers of some color images are 5-nm-thick Au films. Au films could be formed at such a small thickness without any noticeable perforation (Fig. S7). A higher-index, thinner dielectric layer and a lower-index, thicker layer may have the same optical thickness. However, the former is preferred over the latter when angle-insensitive reflection colors are to be generated. This was experimentally verified by comparing the results obtained using SiO2 and


24

Page 25 of 41 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Photonics

Si3N4/ITO as the dielectric spacers. Unlike SiO2 and ITO, Si3N4 requires a high deposition temperature of ~250°C. In Fig. 4, Fig. S5, and Fig. S6, Si3N4 films were deposited directly on STS substrates by plasma-enhanced chemical vapor deposition (PE-CVD). A practical consideration when using nonmetallic substrates is that the bottom metal layer should not be dewetted at the deposition temperature of the dielectric spacer. A Si substrate coated with a 100-nm-thick Ag film was annealed for 30 min under the deposition conditions of the Si3N4 film (250°C and 1.5  10-5 Torr). No dewetting in the Ag film was observed.

In this study, we showed that vivid reflection colors can be produced from the conventional MIM structure. The presented coloration and printing schemes are also planar, scalable, and lithography free. However, the reflection peaks still have large widths (full width at half maximum = 150200 nm). To improve the saturation of the colors, the peak width should be further reduced. The Au and Ag films used in the study were evaporated at a fixed deposition rate of 1 Å/s. There is room for enhancing the color saturation further because metal films deposited under different conditions may


25

ACS Photonics 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 26 of 41

lead to different refractive indices. The dependence of the refractive indices on the deposition condition, along with their influence on the MIM characteristics, will be an important topic of our future work. The vivid reflection colors from the MIM structure are useful for a variety of colorimetric sensors. Mechanochromic sensors are of special interest, which detect color changes in response to mechanical stimuli such as strain and pressure and have a wide range of applications.37-40 The presented MIM cavity structure consists of planar thin films only and are thus suitable for wearable applications if the cavity is made flexible or stretchable. We plan to construct mechanically flexible cavities by employing elastomeric polydimethylsiloxane for the dielectric spacer and using metal-nanoparticle-dispersed polymers for the top and bottom reflecting layers.

EXPERIMENTAL SECTION

The Si substrates (2.5  2.5 cm2) were cut from a single-crystalline Si wafer (LG Siltron: (001)-oriented, p-type, resistivity = 130 ·cm, thickness = 675700 μm). Before


26

Page 27 of 41 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Photonics

deposition, the native oxide layer was removed by immersing the substrate in a diluted HF solution for 30 s. The STS substrates were prepared from commercially available STS plates (type = STS 304, thickness = 1 mm, one side super-mirror polished). They were cut to lateral dimensions of 30 mm  30 mm. SiO2 and Si3N4 films were deposited via PE-CVD (Oxford PlasmaPro 800Plus). ITO films were deposited via direct-current magnetron sputtering using a 10 wt% SnO2-doped In2O3 target. Deposition was performed without heating the substrate, under a working pressure of 7  10-3 Torr and gas flow rates of 90 sccm Ar and 0.1 sccm O2. Si3N4 and ITO films were also prepared on glass substrates, and their frequency-dependent refractive indices were measured via ellipsometry. Ag and Au thin films were deposited using a thermal evaporator. The deposition chamber was evacuated to

Generation of Reflection Colors from Metal-Insulator-Metal Cavity

Recommend Documents