Investigation of Fluorescence Emission from CdSe Nanorods in PMMA and P3HT/PMMA Films

Abstract

Complementary fluorescence microscopy and ultrafast transient absorption spectroscopy measurements spanning a range of time scales from seconds to femtoseconds probe the interfacial dynamics of charge carriers in CdSe nanorod/polymer blends. Together, these very different techniques provide new information about the origin and dynamics of below-band-edge emission from CdSe nanorods in CdSe/PMMA and CdSe/P3HT/PMMA films [PMMA = poly(methyl methacrylate); P3HT = poly(3-hexylthiophene)]. Emission below the band edge of the CdSe nanorods is associated with surface defects (traps) at the nanoparticle/polymer interface, where conduction band electrons radiatively relax to the intraband defect sites. The fluorescence microscopy experiments simultaneously monitor both the trap emission and the band edge emission from single nanoparticles, and reveal that the two emission channels are distinct. Transitions between the two emissive states occur on time scales longer than ∼20 ms, and always involve an intermediate dark state in which no emission is observed. The presence of P3HT increases the relative band edge emission intensity and reduces the fluorescence intermittency (blinking) of both emissive states. The ultrafast transient absorption experiments monitor the evolution of a stimulated emission band below the CdSe band edge following excitation of P3HT. The measurements reveal ultrafast electron transfer from photoexcited P3HT to the CdSe nanorods within the instrument response time of approximately 140 fs, and confirm that there is strong coupling between the nanorods and P3HT in these dilute blends. Analysis of separate CdSe nanorod etching experiments suggests that the trap states are formed by the removal of atoms from the ends of the nanorods in the presence of chloroform. Mechanisms for charge trapping at the nanoparticle/polymer interface are discussed.