基于水凝胶的颜色动态可调的金属-介质-金属结构

Hydrogel-Based Metal-Insulator-Metal Structures with Dynamically Tunable Colors

提出一种基于聚乙烯醇水凝胶(PVA)的金属-介质-金属结构,用于实现颜色动态可调的结构色传感器。利用PVA薄膜对相对湿度的溶胀/收缩和折射率变化响应,并结合法布里-珀罗(F-P)共振效应,实现了F-P共振波长随相对湿度变化而发生偏移,结构色随环境相对湿度的改变而变化。实验结果表明,当环境相对湿度在3.0%~84.2%范围内变化时,结构的颜色会发生显著变化,在CIE 1931色度空间中显示为坐标顺时针旋转,平均相对湿度灵敏度可达0.998 nm/%。该传感器色彩显示丰富,颜色随相对湿度变化明显,灵敏度较高,响应迅速,在湿度传感结合色彩显示的环境检测研究领域具有良好的应用前景。

Objective Structural color is produced by the interaction between light and micro-nano structures.Under the constantly unchanged refractive index and lattice of the material,the color will remain stable.Therefore,structural color is more stable than chemical dyes,and structural color devices play a crucial role in color printing,medical diagnosis,and information storage.In particular,dynamic structural color sensing devices surpass the limitations of static structural color devices,possessing superior flexibility.Consequently,they have more applications in such areas as anti-counterfeiting,dynamic full-color displays,and security encryption.In summary,research on dynamic and adjustable structural color sensing devices holds practical significance.Some researchers have yielded dynamic structural color display by employing periodic micro-nano structural devices,such as metal hole arrays and metal disk arrays,in combination with adjustable insulating materials based on electrochemical activity or temperature/humidity sensitivity.However,the complex fabrication process relying on surface plasmon resonance(SPR)morphology for such structural color devices is cumbersome and involves multiple processing techniques,making it impractical for production.We propose and implement a metal-insulator-metal(MIM)structure based on hydrogel films.By leveraging the humidity-sensitive properties of the insulating layer material and the Fabry-Perot(F-P)resonance phenomenon,we realize dynamic,rapid,and easily fabricated structural color humidity sensing.Methods In hydrogel-based metal-insulator-metal structures with dynamically tunable colors,an Ag-PVA-Ag structure is formed to create an asymmetric F-P cavity.When incident light shines on the structure’s surface,the thin silver film on top allows light to be both transmitted and reflected,while the 200 nm thick bottom silver film prevents the incident light from passing through.The middle insulating layer of PVA forms the cavity of the F-P cavity and controls light waves through interference by combining with the upper and lower silver films.The upper and lower silver films are prepared via magnetron sputtering,while the middle insulating layer is created by spin-coating a PVA aqueous solution.The thickness of the PVA film layer and the top silver film affects the resonance wavelength,half-width at half-maximum(FWHM),and resonant intensity of the F-P resonance dip.By adjusting the thickness of the PVA film layer and the top silver film,light of specific frequencies can be localized within the metal microstructure,while that of other frequencies is reflected to bring different colors displayed by the structure and achieve a wide range of static color displays.As the relative humidity in the external environment increases,the refractive index of the PVA film layer decreases,with rising thickness.This phenomenon alters the cavity length of the F-P cavity,thus changing the optical path of the incident light within the cavity and realizing humidity-sensitive dynamic color displays.Results and Discussions When the bottom silver film thickness is 200 nm and the top silver film thickness is 10 nm,we study the influence of PVA film thickness on resonance wavelength and color(Fig.6).As the PVA film thickness increases,the resonance wavelength gradually shifts towards longer wavelengths(red shift),while the FWHM remains almost unchanged.The coordinates in the CIE 1931 chromaticity diagram rotate clockwise.Then,when the bottom silver film thickness is 200 nm and the PVA film thickness is 110 nm,we investigate the influence of top silver film thickness on resonance wavelength and color(Fig.7).Increasing the top silver film thickness causes a gradual shift towards shorter wavelengths(blue shift),with a narrowing FWHM.The resonance intensity first increases and then decreases,and the coordinates in the CIE 1931 chromaticity diagram slowly move clockwise.Under another scenario of a bottom silver film thickness of 200 nm,a top silver film thickness of 10 nm,and PVA film thicknesses of 90 nm and 110 nm,we examine the variation in resonance wavelength and color as relative humidity increases from 3.0%to 84.2%(Figs.10 and 11).Due to the self-driven swelling phenomenon of the PVA film,both samples experience a red shift in resonance wavelength.The coordinates in the CIE 1931 chromaticity diagram rotate clockwise by one lap.The average relative humidity sensitivity is 0.998 nm/%and 0.862 nm/%respectively.Conclusions We present and investigate hydrogel-based MIM structures with dynamically tunable colors,and employ the unique property of the PVA film which undergoes a self-driven swelling/contraction phenomenon in different relative humidity environments and has a refractive index decreasing with the rising relative humidity.Combined with the Fabry-Perot(F-P)resonance phenomenon,the F-P resonance wavelength can shift with relative humidity changes.This enables the absorption or reflection of light at different wavelengths in varying relative humidity environments,leading to corresponding changes in the structural color.Experimental results demonstrate that by controlling the thickness of the PVA film and the top Ag film in the MIM structure,the displayed color can be influenced.Furthermore,as the relative humidity gradually increases from 3.0%to 84.2%,the resonance wavelength of the two samples shifts from 474 nm to 555 nm and from 526 nm to 596 nm respectively.The average relative humidity sensitivities are 0.998 nm/%and 0.862 nm/%.Additionally,the chromaticity coordinates of the samples rotate clockwise,indicating a color change.Compared to existing structural color devices,the fabricated MIM structural humidity sensor can achieve dual-parameter sensing of relative humidity and color,with rapid response,simple fabrication,low cost,and small size.Considering the deformability,self-driven nature,and biocompatibility of hydrogels,hydrogel-based structural color devices can further be applied to complex dynamic nano-photonic coloration devices.