Plasmon-Related Modification of Spectral Kinetic Properties of Copper Phthalocyanine Thin Films in the Presence of Silver Nanoparticles*

Abstract

We have studied the relaxation dynamics for hot electrons in hybrid layered nanocomposites based on island o lms of silver and copper phthalocyanine. Using femtosecond kinetic spectroscopy, we have shown that the fast component of the bleaching relaxation kinetics in the spectral region corresponding to the absorption bands of copper phthalocyanine correlates with the recovery kinetics for surface plasmon resonance absorption of the silver nanoparticles. This suggests that the fast relaxing bleaching recorded in the absorption bands of copper phthalocyanine and its dynamics are not due to a change in the populations of the energy states of copper phthalocyanine but rather to a change in the state of the plasmon nanoparticles, as a result of their excitation by femtosecond pulses and subsequent relaxation. Introduction. The ultrafast optical response of semiconductor thin o lms and nanostructures is one of the important properties explaining why these materials are in demand and promising for designing hardware components for nanophotonics and optoelectronics. Until recently, the effect of plasmon nanoparticles on the properties of semiconductor thin-o lm components were analyzed mainly with a view toward the possibility of changing their spectrally selective characteristics, in particular in order to increase the efo ciency of active elements in solar batteries. It has been established that doping functional semiconductor layers with plasmon nanoparticles can substantially change their absorption and conductivity because of the near-o eld effects arising in such systems and also as a result of additional possibilities for energy and charge carrier transfer in such structures (1, 2). At the same time, we know (3, 4) that the surface plasmon resonance (SPR) absorption band is substantially transformed when the nanoparticles are excited by ultrashort laser pulses, acting on the electronic subsystem of metallic nanoparticles. It is logical to assume that the change in state of the nanoparticles should be re? ected in fast dynamics of the near-o eld distribution pattern and consequently in fast dynamics of surface-enhanced effects, in particular the effect of modio cation of the absorption spectrum of copper phthalocyanine (CuPc) in the presence of Ag nanoparticles. This may prove to be useful in development of new types of dynamic devices: modulators, switches. The aim of this work was to study the characteristic features of the appearance of fast charge carrier relaxation processes in non-steady state absorption spectra of layered hybrid nanocomposites based on island o lms of silver and CuPc when the Ag nanoparticles are excited by femtosecond laser pulses in the SPR absorption region. Sample Preparation Procedure. The experimental samples were prepared by successive thermal vaporization under vacuum of Ag and CuPc materials on glass and quartz substrates at room temperature. The residual gas pressure was 2·10 -3 Pa. The thicknesses of the deposited layers were monitored with a quartz sensor. The spectral characteristics of the experimental samples were recorded on a Cary 500 spectrophotometer. Three groups of samples were prepared: 1) planar Ag nanostructures (close packed monolayers); 2) CuPc thin o lms; 3) two-layer Ag/CuPc nanostructures (a close packed monolayer of Ag nanoparticles, covered with a CuPc o lm). Steady-State Absorption Spectra. In the absorption spectrum of an Ag monolayer with metal surface density