Transient Grating Studies of Femtosecond Processes in Ultra‐Thin Layers of PTCDA

Abstract

Elementary processes like energy transfer, charge transport, and exciton diffusion in thin films occur on time scales of femtoseconds. Time-resolved photo-electron spectroscopy, a technique limited to ultra-high vacuum environment and the proper choice of a substrate, has been used to study ultrafast processes in sub-nanometer thin films so far. Herein we show that a transient (population) grating created by the interference of laser pulses can be used to study ultrafast processes in such films under ambient conditions. Our investigations of exciton dynamics in 1.4±0.2 nm and 0.4±0.2 nm thin films, formed by nanocrystals of 3,4,9,10-Perylenetetracarboxylic dianhydride (PTCDA) on glass and mica, show that the dynamics differ with the crystal size, possibly due to the confinement induced changes in the electronic structure. The technique is sensitive enough to investigate the dynamics in systems, where only 20 % of the surface is covered by nano-crystals. We expect such an optical technique that is sensitive enough to study dynamics in few to sub-nanometer thin layers under ambient conditions to become important in investigating ultrafast dynamics on surfaces, interfaces, functionalized materials, organic semiconductors, and quantum phenomena in ordered structures of reduced dimensions, such as quantum dots and graphene sheets.

Generalized lock-in amplifier for precision measurement of high frequency signals

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Femtosecond Time-Resolved Transient Absorption Spectroscopy with Sub-Diffraction-Limited Spatial Resolution Reveals Accelerated Exciton Loss at Gold-Poly(3-Hexylthiophene) Interface

“…They demonstrated that the observation of how lateral carrier diffusion affects the exciton dynamics is possible because of the increased spatial resolution. In our own work, we have studied very thin layers of an organic semiconductor and have also investigated dynamics using SNOM in combination with different nonlinear spectroscopy techniques. , …”

“…In continuation of our previous works, ,, the work presented in the following combines scanning near-field optical microscopy with femtosecond time-resolved pump–probe spectroscopy (PP-SNOM) to study concurrent charge pair dynamics at the Au–P3HT interfaces, thereby revealing the elementary quenching process localized at the metal surface.…”