Polarization anisotropy dynamics for thin films of a conjugated polymer aligned by nanoimprinting
Polarization anisotropy dynamics for thin films of a conjugated polymer aligned by nanoimprinting
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DOI：
10.1103/physrevb.77.115338
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发表时间：
2008-03-01
影响因子：
3.7
通讯作者：
Herz, L. M.
中科院分区：
文献类型：
--
作者：
Schmid, S. A.;Yim, K. H.;Herz, L. M.
用时间积分和飞秒时间分辨光致发光光谱研究了通过纳米印迹技术排列链的共轭聚合物薄膜的动态发射偏振各向异性。结果表明，在远离印迹表面的薄膜区域中，存在少量未对齐的链结构域，具有高度的链取向。与更无序的薄膜区域相比，取向磁区的时间平均发射略向更高的光子能量移动。这一效应归因于薄膜中排列较多的区域中微妙不同的链堆积几何结构，这导致了激子扩散率的降低，并抑制了激子在非均匀展宽的态密度中的能量弛豫。而对于未取向的参照膜，激子迁移导致发射在最初300ps内几乎完全退极化，而对于取向薄膜，在相同的时间尺度上，从未取向的磁区到取向的磁区的链间激子跳跃增加了各向异性。此外，研究还发现，在取向薄膜磁区中产生的激子对非辐射猝灭效应的敏感性比在薄膜内部更深的无序区中产生的激子更敏感，这表明在取向薄膜的纳米印迹表面附近有较高的缺陷密度。
Time-integrated and femtosecond time-resolved photoluminescence spectroscopy has been used to study the dynamic emission polarization anisotropy for thin films of a conjugated polymer whose chains had been aligned through a nanoimprinting technique. The results indicate a high degree of chain alignment, with the presence of a small fraction of unaligned chain domains in film regions far from the imprinted surface. The time-averaged emission from aligned domains is found to be slightly shifted to higher photon energies compared to that from more disordered film regions. This effect is attributed to a subtly different chain packing geometry in the more aligned regions of the film, which leads to a reduced exciton diffusivity and inhibits energetic relaxation of the exciton in the inhomogeneously broadened density of states. While for an unaligned reference film, exciton migration results in a nearly complete depolarization of the emission over the first 300 ps, for the aligned films, interchain exciton hopping from unaligned to aligned domains is found to increase the anisotropy over the same time scale. In addition, excitons generated in aligned film domains were found to be slightly more susceptible to nonradiative quenching effects than those in disordered regions deeper inside the film, suggesting a marginally higher defect density near the nanoimprinted surface of the aligned film.