Fast Dynamic Color Switching in Temperature‐Responsive Plasmonic Films

Abstract

COMMUNICATIONmembranes, biosensing, tissue engineering, and drug delivery. [ 19,20 ] Due to its strong volume shrinkage (up to 90%) when the temperature rises above a critical hydration temperature ( T c ) and its full reversibility across this phase transition, pNIPAM has been utilized to tune plasmons in Au/Ag nanoparticles (NPs) and nanorods. [21][22][23][24][25][26][27] However, because these plasmonic assemblies are either randomly decorated around the surface of pNIPAM microgels or aggregates with undefi ned number of particles and confi gurations, it is hard to understand the physics of the mixture of different plasmonic constructs properly. Moreover, it takes time for water to diffuse inside the microgels to fully swell them, thus the speed of volume changes has been relatively slow (typically a few seconds).Here, we graft pNIPAM layers ( d = 70 nm thick) onto Au mirror by applying surface-initiated atom transfer radical polymerization (SI-ATRP) protocols, and place scattering Au NPs on top. By utilizing the phase transition of the pNIPAM, the separation between Au NP and Au fi lm is widely tunable, thereby leading to a dynamic and reversible plasmon tuning system based on modulating the resonant modes. The resulting fi lms exhibit strong color changes, are simple to fabricate, can be fl exible, are low cost, can easily scale to large areas, and respond faster than video rates. In addition, the plasmonic nanoparticles can also be optically irradiated to locally switch the surrounding pNIPAM layers, creating video-rate all-optical switching over large areas.The Au mirrors are deposited via electron beam evaporation with roughness controlled below 1.5 ± 0.5 nm. [ 28 ] The pNIPAM fi lms are then prepared on these Au fi lms by SI-ATRP ( Figure 1 , Experimental Section). [ 29 ] The thickness of the as-grown dry pNIPAM fi lms is determined to be 67 ± 1 nm via ellipsometry ( Figure S1, Supporting Information). The molecular weight estimated from the free pNIPAM synthesized under the same condition is 26.3 kDa as determined by size-exclusion chromatography. The transition temperature of this pNIPAM fi lm determined through contact angle measurement is around 30 °C ( Figure S2, Supporting Information). This value is slightly lower than the normal lower critical solution temperature (LCST) (32 °C) of pNIPAM mainly because of the residue Cu 2+ salts and polar end groups introduced during the ATRP process. The D = 100 nm diameter Au NPs (from BBI Solutions with 10% monodispersity, Figure S3, Supporting Information) deposited on the fi lms from solution appear slightly embedded inside the pNIPAM brushes as seen in scanning electron microscopy (SEM) images (inset Figure 1 ). Nanoparticle densities are varied up to 1 nanoparticle µm -2 to ensure that there is minimal coupling between nanoparticles, while the total scattering is maximized. Though this Au NP on mirror (NPoM) is related to the recent advances in small-gap constructs, [ 22 ] here the gap remains larger than the radius, reducing the plasmonic This is an ope...

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Tunable, Cost‐Effective, and Scalable Structural Colors for Sensing and Consumer Products

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