Excitation energy transfer in porous silicon/laser dye composites

Abstract

Excited state relaxation and energy transfer in porous silicon (PS)/laser dye [oxazine 1 (Ox1), rhodamine 6G (Rh6G)] composites have been studied by means of steady-state and time-resolved fluorescence. Fluorescence decay kinetics reveals the nonradiative energy transfer from PS to the dye. Increased decay rate of the dye luminescence in the composite indicates that opposite energy transfer is also likely. Analysis of the time-resolved fluorescence of the PS/R6G composite also shows that there is no energy transfer from silicon oxide responsible for the "blue" fluorescence band to silicon nanocrystalites, and that interaction between Si nanocrystals responsible for the "red" PS fluorescence is absent or weak. 1 Introduction Since its discovery, porous silicon (PS) proved to be a non-trivial fluorescent system. Having many different energy levels, each with its own relaxation time and at least two distinct fluorescence mechanisms (surface oxide and Si nanocrystallites) PS is still not fully understood.PS/organic material composites were first investigated few years after the discovery of the visible luminescence of PS. Polymers were spin-coated onto PS in order to produce hybrid LEDs with variable fluorescence spectra [1-3], while laser dye impregnation into the pores was motivated by possible application of such composites in laser physics [4][5][6][7]. One of the key processes in such systems -energy transfer, attracted most of the researcher's attention because of its natural complexity resulting from intricate fluorescence of PS.In 1993 Canham [8] reported impregnation of PS with laser dye in order to get a solid state host for optically active media -a goal of great interest still. In his work Canham examined possible energy transfer in such composites. Since then, driven by possible applications of such materials in optoelectronics, a number of researchers investigated PS/ laser dye composites using different techniques and reported energy transfer from PS to the dye as their main findings [9][10][11][12][13].Time-gated fluorescence of PS reveals two bands -high energy "blue" fluorescence, which is intensive but decays in less than 5 ns, and low energy "red" fluorescence, which is