Laser Dynamics Laboratory, School of Chemistry and Biochemistry, Georgia Institute of Technology, Atlanta, Georgia 30332-0400, USA.
J Am Chem Soc. 2010 Mar 3;132(8):2633-41. doi: 10.1021/ja907657j.
Using the Langmuir-Blodgett (LB) technique, a poly(paraphenyleneethynylene) (PPE) fluorescent conjugated polymer was assembled on either a quartz substrate (system I) or on the surface of silver nanocube (AgNC) monolayers (system II). The fluorescence intensity of the polymer was studied in system I as a function of the surface density of the polymer sample when deposited on quartz substrates and in system II on the surface coverage of the underlying AgNC monolayers. In system I, a continual increase in the fluorescence intensity is observed as the surface density of excited polymer is increased. In system II, the fluorescence intensity of the polymer first increased until a threshold surface coverage of AgNC was reached, after which it decreased rapidly with increasing surface coverage in the AgNC monolayer. The exciting light intensity dependence is studied before and after this threshold in system II. The results suggest that one-photon processes were responsible for the increased intensity before the threshold, and two-photon processes were responsible for the rapid decrease in the polymer fluorescence intensity after the threshold. These observations are explained by the increase of the surface plasmon enhancement of the exciting light intensity as the nanoparticle surface coverage is increased. In turn, this increases the polymer absorption rate, which results in a continuous increase in the exciton density and is evident by an increase in the fluorescence intensity. At the threshold, the increased exciton density leads to an increase in the rate of exciton-exciton collisions, which leads to exciton-exciton annihilations. When this phenomenon becomes faster than the rate of fluorescence emission, an intensity decrease is observed. By exploiting the surface plasmon enhancement of absorption processes, we have observed the first exciton-exciton annihilation using a low-intensity Hg-lamp continuous wave source.
使用 Langmuir-Blodgett（LB）技术，将聚对苯乙炔（PPE）荧光共轭聚合物组装在石英基底上（系统 I）或银纳米立方体（AgNC）单层表面上（系统 II）。当聚合物样品沉积在石英基底上时，研究了系统 I 中聚合物的荧光强度作为聚合物表面密度的函数；当研究系统 II 中聚合物的荧光强度时，研究了其作为底层 AgNC 单层表面覆盖率的函数。在系统 I 中，随着激发态聚合物表面密度的增加，荧光强度持续增加。在系统 II 中，聚合物的荧光强度最初增加，直到达到 AgNC 的表面覆盖率阈值，之后随着 AgNC 单层表面覆盖率的增加，荧光强度迅速下降。在系统 II 中，研究了阈值前后的激发光强度依赖性。结果表明，在阈值之前，单光子过程负责增加强度，而在阈值之后，双光子过程负责聚合物荧光强度的快速下降。这些观察结果可以通过随着纳米颗粒表面覆盖率的增加，激发光强度的表面等离子体增强来解释。反过来，这增加了聚合物的吸收速率，从而导致激子密度连续增加，这可以通过荧光强度的增加来证明。在阈值处，增加的激子密度导致激子-激子碰撞速率增加，从而导致激子-激子湮灭。当这种现象变得比荧光发射速率更快时，就会观察到强度下降。通过利用吸收过程的表面等离子体增强，我们使用低强度汞灯连续波源观察到了第一个激子-激子湮灭。
