基于无序纳米颗粒的光子晶体有机薄膜

Organic Films with Photonic Crystals Based on Disordered N anoparticles

无序光子晶体有机物薄膜的制备是实现光阻隔材料量产的关键。设计了无序多级光子晶体结构有机物薄膜,以热塑性聚氨酯作为膜材基质,将二氧化钛颗粒作为反射隔光材料填入基质中。采用时域有限差分法进行了模拟,结果表明,增加填充因子导致了反射峰和光谱禁带的蓝移,而增加颗粒直径造成了光谱禁带红移。将单一直径均匀分布的阵列作为对比,分析了粒径多级性和结构聚结性对光谱的影响。对于横电波和横磁波,在0°~70°的宽入射角范围内实现了高效禁带效应。这种无序光子晶体结构的有机薄膜为宽角度光阻隔材料提供了参考。

Objective The random arrangement of dielectrics and nanostructures in disordered photonic crystals produces strong Anderson localization effect and does not require high-precision nanomaterials and structures.In the previous study,we reported photonic crystal organic films with disordered nanoparticles,which were a roll-to-roll expandable material for various applications in aerospace,automotive,construction,and apparel.However,the design parameters of disordered photonic crystals still need to be optimized.This paper aims to investigate the effect of fill factor,particle size distribution,and structural symmetry of nanoparticles on the light-insulation properties by using a finite-difference time-domain(FDTD)method.The discrete nanoparticle system of organic polymer films should be further designed by analyzing the light transport properties in the microstructure.Methods Organic film samples of discrete nanoparticles are prepared by using the tape casting method.The microstructure of TiOz particles is observed by scanning electron microscopy and modeled in the FDTD.The nanostructure is simplified to a two-dimensional(2D)disordered photonic crystal in the non-polarized plane,with the particle size and the fil factor set according to experimental measurements.Electromagnetic field calculation is carried out by the FDTD method to analyze the microscopic electric field spatial distribution and macroscopic optical characteristic curves.Then the effect of different design parameters on the optical transmission characteristics is investigated.On the basis of SEM photographs,models of hierarchical size and agglomerated structures are established,and the electric field distribution of light waves in three typical structures is calculated and compared with the experimental transmittance curves.Floquet periodicity boundary conditions are set to investigate the propagation characteristics of polarized lights at different incident angles,and the wide-angle light-blocking capability is verified.ResultsandDiscussionsThe effects of the fill factor and particle diameter of TiOz particles on the film spectra are investigated,and the spatial distribution of the electric field is used to describe the transmission characteristics of light waves in disordered TiO photonic crystals.At a fill factor of M,=10%,the forbidden band width is wider in the range of 200-1500 nm,and the transmittance is higher.However,when M,is larger than 45%,the forbidden band width is narrower in the wavelength range of 200-1100 nm,and the reflectance is higher at 1000 nm compared with that at a low fill factor.The results suggest that the optimal fill factor for TiOz particles shall fall in the range of 35%-45%,so as to produce the best spectral forbidden band effect.As the nanoparticle diameter increases,the forbidden band region shifts towards the long wavelength band,and the reflection peaks become redshifted,in contrast to the blue shift observed when the fill factor increases.For a wavelength of 200 nm,the spatial electric field distribution is confined to the upper region of the array for nanoparticle diameter of 100 nm.The light of 800 nm propagates to the bottom surface of the film in the array with three particle sizes,but the phases are not synchronous.For electromagnetic waves with a length of 1600 nm,their propagation is unobstructed in the array of 1o0 nm,and the phase of the light reaching the bottom surface almost always reaches the wave peak,while the light reaching the bottom surface in the arrays of 200 nm and 300 nm undergoes multiple scattering and results in reduced transmittance.When the light wavelength is 3000 nm,much larger than the nanoparticle diameter,the light propagates through the film without scattering effects,the spatial distribution of the electric field is no longer influenced by the TiOz particles,and the light transmission properties are consistent with those in a homogeneous polymer matrix.The effects of hierarchical particle size and structural aggregation on the light transmission properties are further investigated and compared with experimental results,which result in a 54%reduction in near-infrared(NIR)light transmission.Finally,the wide-angle light-blocking capability of the films is evaluated.Then the transmittance profiles and electric field spatial distribution at polarized waves are analyzed.For TE-and TM-mode polarized light,efficient bandblocking properties are achieved over a wide incidence angle range of 0°-70°for wavelengths of 200-600 nm.For a wavelength of 300 nm similar to the particle size,the light propagates only to the shallow layers of the film over a wide range of incidence angles.For a wavelength of 2000 nm,which is well beyond the particle size,TiOz particles have difficulty in blocking light.This study provides theoretical support for the optimal design of parameters for disordered photonic crystals,especially for the preparation of organic films of TiO,nanoparticles.Conclusions The preparation of disordered photonic crystal organic films is the key to achieving mass production of light-insulation materials.In this paper,hierarchically disordered photonic crystal structured organic films are designed by using thermoplastic polyurethane as the film substrate and titanium dioxide particles as the reflective barrier material filling the substrate.Simulations are carried out by using the FDTD method,and the results show that the increase in fill factor causes a blue shift in the reflection peak and spectral forbidden band,while the increase in particle diameter causes a red shift in the spectral forbidden band.Compared with arrays with a uniform distribution and a single diameter,the effects of hierarchical particle size and structural aggregation on NIR waves are analyzed.For both transverse electric and transverse magnetic waves,efficient forbidden band effects are achieved over a wide incidence angle range of 0°-70°.Such organic films containing disordered photonic crystal structures provide a reference for wide-angle light-insulation materials.