Flexible,Transparent And Conductive Metal Films Fabricated By Phase Separation Of Polymer Blends

Nowadays,transparent conductive electrodes have attracted tremendous interest in both scientific and industrial community because they serve as essential components for numerous optoelectronic devices such as touch-screen displays,solar cells,and OLEDs.Indium tin oxide(ITO)dominates the field at present because of its high optoelectronic performance.However,ITO suffers from several drawbacks,mainly centered on its scarcity of supply,high cost,brittleness,and instability.These disadvantages certainly limit its developments in the next generation of flexible optoelectronic devices.Recently,as rapid advances in nano-materials research,many emerging transparent conductive materials including conducting polymers,CNTs,graphene,metal nanowires,and patterned metal films have been widely studied as possible alternatives to traditional ITO,each of these materials with unique properties.Currently,it is hardly realized to fabricate transparent conductive materials with low sheet resistance(Rs),high optical transmittance(T),as well as flexibility and stability in a facile,low-cost,and large-scale way.Among these transparent conductive materials,patterned metal films show great application potential as they possess simultaneously good electrical conductivity,optical transmittance,stability,and flexibility.Manufacturing methods of patterned metal films mainly referred to conventional micro/nano-fabrication e.g.photolithography,nanoimprint and nanosphere lithography.Some of these patterning methods are complex and expensive.One key issue of the patterned metal films is that fabricated periodic micro/nano structures show different colors when the viewing angle or the light incident angle is altered,resulting in brilliant iridescence.The angle-dependent structural color is attributed to the optical diffraction of periodic structures and could limit the application of patterned metal films in optoelectronic devices that require high-performance and wide-viewing-angle displays.This problem can be solved by creating aperiodic structures with angle-independent structural color via molecular self-assembly,including spray-coating of colloidal particles,droplet casting of mixed polystyrene nanoparticles(NPs),and adding sodium chloride in colloidal solution,etc.Phase separation of polymer blends,a common self-assembly method,has not been reported to fabricate patterned metal films with disordered structures.Phase separation of PPSQ/PS polymer blend could be generated fast and simply by spin coating due to the high miscibility between them.The blend film consists of disordered and separated PPSQ nanopillars distributed in continuous PS phase.Then,we can easily selectively remove the PS phase by O2 reactive ion etching(RIE)with disordered PPSQ nanopillars remained due to the high O2 plasma etching resistance of silicon.We investigated the influence factors including polymer blend solution concentrations,PPSQ/P S weight ratios,and spin speeds which probably influence the morphology of phase separation.Also,the morphology of disordered nanopillars can be modified and transferred successfully by controlling the etching time to remove the residual PPSQ layer.Therefore,we can simply fabricate desired disordered nanopillars by altering experimental parameters above.In this paper,we developed a novel and maskless nano-patterning method based on the research results of phase separation of PPSQ and PS.We used the PMMA/SiO2/disordered PPSQ nanopillars triple-layer structure,and combined with common micro/nano-fabrication process such as etching,metal deposition and lift-off,to achieve metal films with disordered nanoholes in a facile,low-cost,and large-scale way.The structures of metal films mainly depend on the morphologies of phase separation.By varying experimental parameters,we successfully fabricated metal films with different nanoapertures,various metal types and thicknesses on different substrates.The as-fabricated Au nanomesh film on a flexible PET substrate presents good optoelectronic properties(Rs=41?/sq,T = 71.9%;Rs = 8.9?/sq,T = 61.5%)as well as benefits of high visibility,mechanical flexibility,stability,and scalability over a large area.Other than these performances,the aperiodic structures of disordered nanoholes in metal films can eliminate the optical diffraction effect to show constant color.The resultant metal films with disordered nanoholes could be applied to various high-performance and flexible optoelectronic devices with wide viewing angles.