Pandey Rajiv K, Yadav Swatantra K, Upadhyay Chandan, Prakash Rajiv, Mishra Hirdyesh
School of Materials Science and Technology, Indian Institute of Technology (BHU), Varanasi-221005, India.
Nanoscale. 2015 Apr 14;7(14):6083-92. doi: 10.1039/c5nr00682a.
The coupling of organic molecule excitons with metal nano-structure surface plasmons can improve the performance of optoelectronic devices. This paper presents the effect of localized silver metal surface plasmons on spectral as well as charge transport properties of ordered molecular Langmuir Schaefer (LS) films of a fluorescent conducting multifunctional organic polymer: poly (3,3'''-dialkylquarterthiophene) [PQT-12]. The stability and thickness of the PQT-12 LS film were studied by the pressure vs. area isotherm curve. Atomic force microscopy images indicate the formation of a smooth ordered polymer thin LS film of PQT-12 over silver nanostructure island films [SNIF] (∼40 to 50 nm in size). Raman, electronic absorption and fluorescence spectral measurements of the PQT-12 LS film, near SNIF i.e. the near field, show a plasmon coupled enhancement of ∼13 fold in the intensity of Raman bands along with a two-fold enhancement in the absorption band (531 nm) and a six-fold enhancement in the fluorescence band (665 nm) coupled with a decrease in fluorescence decay time with improved photostability as compared to an identical control sample containing no SNIF i.e. the far field condition. These results indicate the formation of a plasmon coupled unified fluorophore system due to adsorption of the PQT-12 LS film over SNIF. The effect of plasmonic coupling is also studied by applying an electric field in sandwiched structures of Al/PQT-12 LS/SNIF/ITO with respect to Al/PQT-12 LS/ITO. Nearly three orders of magnitude enhancement in the current density (J-V plot) of the PQT-12 LS film is observed in the presence of SNIF, which further increases, on illuminating the film by green laser light [532 nm], while the fluorescence intensity and decay time decrease. X-ray photoelectron spectroscopic measurements of SNIF also show a red shift in 3d3/2 and 3d5/2 transitions of silver in the PQT-12 coated LS film, which indicates partial charge transfer from the PQT-12 polymer backbone to SNIF and causes an enhancement in conductivity. This again supports the formation of a field controlled radiating plasmon coupled fluorophore unified system. These findings show greater potential in developing a voltage controlled high photon flux electroluminescent material for multifarious applications.
有机分子激子与金属纳米结构表面等离子体的耦合可以提高光电器件的性能。本文介绍了局域银金属表面等离子体对一种荧光导电多功能有机聚合物：聚（3,3'''-二烷基四噻吩）[PQT-12]的有序分子朗缪尔-谢弗（LS）膜的光谱以及电荷传输特性的影响。通过压力-面积等温线曲线研究了PQT-12 LS膜的稳定性和厚度。原子力显微镜图像表明在银纳米结构岛膜[SNIF]（尺寸约为40至50纳米）上形成了光滑有序的PQT-实12聚合物薄LS膜。在靠近SNIF即近场处对PQT-12 LS膜进行拉曼、电子吸收和荧光光谱测量，结果显示拉曼带强度有~13倍的等离子体耦合增强，吸收带（531纳米）增强两倍，荧光带（665纳米）增强六倍，与不含SNIF的相同对照样品即远场条件相比，荧光衰减时间缩短且光稳定性提高。这些结果表明由于PQT-12 LS膜吸附在SNIF上形成了等离子体耦合统一荧光团系统。通过在Al/PQT-12 LS/SNIF/ITO与Al/PQT-12 LS/ITO的夹心结构中施加电场，也研究了等离子体耦合的影响。在存在SNIF的情况下，观察到PQT-12 LS膜的电流密度（J-V曲线）增强了近三个数量级，在用绿色激光[532纳米]照射该膜时，电流密度进一步增加，而荧光强度和衰减时间减小。对SNIF的X射线光电子能谱测量还显示，在PQT-12包覆的LS膜中银的3d3/2和3d5/2跃迁发生红移，这表明部分电荷从PQT-12聚合物主链转移到SNIF并导致电导率增强。这再次支持了场控辐射等离子体耦合荧光团统一系统的形成。这些发现对于开发用于多种应用的电压控制高光子通量电致发光材料具有更大的潜力。
