Saturday 13th August 2022

Dynamic plasmonic color generation enabled by functional materials

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DYNAMIC PLASMONIC COLOR GENERATION

The realization of dynamic structural color generation is challenging but indispensable for functional display devices. Basically, the vibrant but static plasmonic colors require a reversible modification after fabrication. Dynamic plasmonic color generation has been demonstrated using various concepts. Each coloration scheme consists of two essential parts: the coloration mechanism and the coloration control. The coloration mechanism underlines how a broad range of colors is dynamically generated, e.g., by size variations of the nanostructures or changing the dielectric properties of the nanostructure itself or the surrounding media. It also involves the technical implementation of pixels, e.g., as monopixels or as sub-pixels. Ideally, one dynamic pixel composed of one or more nanostructures can exhibit any desired color. The experimental realization of such a monopixel design is challenging, because it requires plasmonic resonance shifts over the entire spectral range. In sub-pixel designs, well known from the current display technology, the perceived color of a pixel is generated by additive or subtractive mixing the plasmonic colors provided by the constituent sub-pixels. The coloration control encompasses how variations of the size or dielectric properties are experimentally realized. This is usually accomplished by a functional medium, e.g., EC materials and LCs, controlled by external stimuli including electric fields, light, gases, and pH changes.

The most straightforward coloration mechanism is to directly tune the plasmonic excitation. It is accomplished by reversibly tailoring the intrinsic properties of the nanostructure, e.g., the material dielectric properties, size, or shape (9). For example, the electron density Ne of a metal determines its plasma frequency ωp (eigenfrequency of the electron density oscillations) and thus the plasmonic resonance frequency of the metal nanostructures (Fig. 1A). As a result, the visual appearance associated with the plasmonic excitation can be directly adjusted by the metal’s properties. While this appealing coloration mechanism allows a direct color control without any additional functional material, it is usually difficult to implement for metals due to an effective Debye screening. Phase change materials, such as metal hydrides, offer a solution to it. The optical properties of magnesium (Mg) and magnesium hydride (MgH2), respectively, for example, can be reversibly controlled by hydrogenation and dehydrogenation, suggesting Mg as a plasmonically active (26), functional material for dynamic color control (14, 2729). In analogy to plasmonic coloration, structural colors produced by dielectric metasurfaces can be actively controlled by the intrinsic optical properties, e.g., the absorption of the dielectric material (30).

<a rel="nofollow" href="https://advances.sciencemag.org/content/advances/6/36/eabc2709/F1.large.jpg?width=800&height=600&carousel=1" title="Dynamic plasmonic color generation enabled by functional materials and the related key performance indicators. Among others, the intensity and resonance frequency of a plasmonic excitation determine the perceived plasmonic colors (middle panel). Both quantities can be effectively tuned through electrochemically induced size modulations of the constituent metal nanostructures, functional plasmonic materials themselves, or functional media surrounding the passive plasmonic elements. (A) Reversible transformations between metallic magnesium (Mg) and dielectric magnesium hydride (MgH2) can take place upon hydrogen (H2) and oxygen (O2) exposures, respectively. The metal to insulator phase transition induces a change in the electron density or more generally in the complex refractive index (n + i∙k), with n and k being the refractive index and the absorption coefficient, respectively. (B) Electrochemical deposition is applied to reversibly modulate the sizes of plasmonic nanoparticles. (C) LCs allow the control of the polarization state of the incident or scattered light, the anisotropic refractive index n of the LCs, and the orientation of anisotropic nanoparticles embedded in the LCs. (D) Switchable EC materials surrounding the plasmonic nanostructures offer an efficient control of the complex refractive index. Selected key performance indicators, such as (E) lifetime (cycling number), (F) switching time, and (G) reflectance/transmittance, strongly depend on the coloration concepts." class="fragment-images colorbox-load" rel="gallery-fragment-images-1189693127" data-figure-caption="

Fig. 1 Dynamic plasmonic color generation enabled by functional materials and the related key performance indicators.

Among others, the intensity and resonance frequency of a plasmonic excitation determine the perceived plasmonic colors (middle panel). Both quantities can be effectively tuned through electrochemically induced size modulations of the constituent metal nanostructures, functional plasmonic materials themselves, or functional media surrounding the passive plasmonic elements. (A) Reversible transformations between metallic magnesium (Mg) and dielectric magnesium hydride (MgH2) can take place upon hydrogen (H2) and oxygen (O2) exposures, respectively. The metal to insulator phase transition induces a change in the electron density or more generally in the complex refractive index (n + i∙k), with n and k being the refractive index and the absorption coefficient, respectively. (B) Electrochemical deposition is applied to reversibly modulate the sizes of plasmonic nanoparticles. (C) LCs allow the control of the polarization state of the incident or scattered light, the anisotropic refractive index n of the LCs, and the orientation of anisotropic nanoparticles embedded in the LCs. (D) Switchable EC materials surrounding the plasmonic nanostructures offer an efficient control of the complex refractive index. Selected key performance indicators, such as (E) lifetime (cycling number), (F) switching time, and (G) reflectance/transmittance, strongly depend on the coloration concepts.

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Fig. 1 Dynamic plasmonic color generation enabled by functional materials and the related key performance indicators.

Among others, the intensity and resonance frequency of a plasmonic excitation determine the perceived plasmonic colors (middle panel). Both quantities can be effectively tuned through electrochemically induced size modulations of the constituent metal nanostructures, functional plasmonic materials themselves, or functional media surrounding the passive plasmonic elements. (A) Reversible transformations between metallic magnesium (Mg) and dielectric magnesium hydride (MgH2) can take place upon hydrogen (H2) and oxygen (O2) exposures, respectively. The metal to insulator phase transition induces a change in the electron density or more generally in the complex refractive index (n + i∙k), with n and k being the refractive index and the absorption coefficient, respectively. (B) Electrochemical deposition is applied to reversibly modulate the sizes of plasmonic nanoparticles. (C) LCs allow the control of the polarization state of the incident or scattered light, the anisotropic refractive index n of the LCs, and the orientation of anisotropic nanoparticles embedded in the LCs. (D) Switchable EC materials surrounding the plasmonic nanostructures offer an efficient control of the complex refractive index. Selected key performance indicators, such as (E) lifetime (cycling number), (F) switching time, and (G) reflectance/transmittance, strongly depend on the coloration concepts.

Other than the intrinsic electronic properties, the size of the nanoparticle determines the perceived color as well (Fig. 1B). It is well known that the LSPR and thereby the perceived color strongly depend on the charge distribution on the particle’s surface (31, 32). For small particles, the resonances are dominated by the excitation of dipolar modes. As the particle size increases, the restoring force between the opposite charges decreases and the plasmonic band appears at longer wavelengths. Thus, the size of the particle offers a direct control of plasmonic color. In addition, if arranged in arrays, in particular closely spaced arrays, a modification of the particle size is inevitably accompanied by a change of the interparticle distances of adjacent nanoparticles. Depending on the particle separation, different effects such as near-field or far-field coupling promote a variety of coupled plasmonic modes (33). Because the plasmonic properties are highly sensitive to interparticle distances of closely spaced nanostructures, e.g., nanoparticle dimers, already minute modifications give rise to marked color changes. On the one hand, these coupled plasmonic systems open a pathway to continuously adjust the plasmonic color over a broad spectral range, which goes far beyond the mere size tunability of noninteracting nanoparticles. On the other hand, the coloration based on coupled plasmonic modes comes along with demanding challenges. The interparticle separation requires an excellent control with nanometer precision over the entire plasmonic color pixel, usually composed of several nanoparticles, to ensure homogenous and vibrant colors. Basically, these dynamic distance modifications between adjacent nanoparticles as well as size changes of nanoparticles are rather difficult to achieve after fabrication. Reversible electrochemical deposition of metals onto predefined nanostructures using reduction-oxidation-chemistry (redox-chemistry) offers a practical solution to size control (3440), whereas mechanical strain (41, 42) and configurational changes of molecules (43) can be used to efficiently modulate the interparticle distances.

Resonantly excited, metal nanostructures offer strongly confined electromagnetic fields. These highly confined near fields markedly increase the light matter interactions on the nanoscale, giving rise to various applications, including ultrahigh-sensitivity spectroscopy and biosensing, super-resolution imaging, and subwavelength optics (44). In refractive…

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