Revealing nanoscale optical properties and morphology in perfluoropentacene films by confocal and tip-enhanced near-field optical microscopy and spectroscopy.
Revealing nanoscale optical properties and morphology in perfluoropentacene films by confocal and tip-enhanced near-field optical microscopy and spectroscopy.
复制标题
DOI：
10.1039/c6cp01153e
复制
发表时间：
2016-06
期刊：
影响因子：
--
通讯作者：
Xiao Wang;K. Broch;F. Schreiber;A. Meixner;Dai Zhang
中科院分区：
文献类型：
--
作者：
Xiao Wang;K. Broch;F. Schreiber;A. Meixner;Dai Zhang
结合高分辨率光学显微镜和光谱学，我们提出了一种新颖、普遍适用且高度灵敏的方法来确定有机半导体薄膜（例如全氟并五苯（PFP））的局部形貌。方位角或径向偏振的环形模式（APDM 或 RPDM）激光束由高数值孔径抛物面镜聚焦，以完全面内或主要面外（相对于基材）偏振的电场激发 PFP 薄膜的衍射极限体积。我们发现 PFP 薄膜有两种不同的形态：分子聚集体和结晶梯田。从分子聚集体观察到的明确的偶极子发射模式强烈表明局部激发的存在。对于两种激光模式，我们观察到，对于 PFP 聚集体，与分子平台相比，主要电子跃迁的光致发光 (PL) 发射蓝移了约 10 meV。对于 C-C 弯曲模式，1581 cm(-1) (ν1) 处的 B3g 和 1316 cm(-1) (ν0) 处的 Ag，我们观察到强度比 (Iν1/Iν0) 从 0.6（梯田）下降到 0.15（聚集）。此外，不同偏振激发的ν1的强度比（IAPDM/IRPDM）从0.12（梯田）增加到0.73（聚合）。这些结果表明，PFP 分子在聚集体中的取向相当平行于基底，而在阶地中则更直立。受益于尖端增强近场光学方法提供的纳米级光学分辨率，我们观察到分子聚集体和平台以及平台内各个层之间清晰的光学对比度。从分子平台和聚集体局部获取的尖端增强光谱显示出与远场组装测量相似的主 PL 峰的蓝移和不同偏振的拉曼强度的变化，这进一步证实了这两种形态中不同的分子取向。
Combining high resolution optical microscopy and spectroscopy, we propose a novel, generally applicable and highly sensitive method for determining the local morphology in organic semiconductor thin films (e.g. perfluoropentacene (PFP)). An azimuthally or radially polarized doughnut mode (APDM or RPDM) laser beam is focused by a high numerical aperture parabolic-mirror to excite a diffraction limited volume of the PFP film with an electric field polarized either exclusively in-plane or dominantly out-of-plane (relative to the substrate). We find two distinct morphologies of thin PFP films: molecular aggregates and crystalline terraces. The well-defined dipole emission patterns observed from the molecular aggregates strongly suggest the presence of localized excitations. For both laser modes, we observe that for the PFP aggregates, the photoluminescence (PL) emission from the main electronic transition is blue-shifted by about 10 meV, as compared to that from the molecular terraces. For the C-C bending modes, the B3g at 1581 cm(-1) (ν1) and the Ag at 1316 cm(-1) (ν0), we observe a decrease of the intensity ratio (Iν1/Iν0) from 0.6 (terrace) to 0.15 (aggregate). Furthermore, the intensity ratios (IAPDM/IRPDM) of ν1 excited by different polarizations increase from 0.12 (terrace) to 0.73 (aggregate). These results indicate that the PFP molecules orient rather parallel to the substrate in the aggregates, whilst more upright in the terraces. Benefiting from the nanometer scale optical resolution offered by the tip-enhanced near-field optical method, we observe clear optical contrasts between the molecular aggregate and the terrace as well as individual layers within a terrace. Tip-enhanced optical spectra locally taken from the molecular terrace and the aggregate show similar blue-shift of the main PL peak and change in the Raman intensity with different polarizations as from the far-field assemble-measurements, which further confirms the different molecular orientations in these two morphologies.