Resonant Energy Transfer from Porous Silicon to Iodine Molecules

Abstract

The electronic excitation energy transfer between excitons in porous silicon and iodine molecules in an organic solution is studied. From the time-resolved photoluminescence the rate of the energy transfer is increased with approaching a wavelength region where the photoluminescence spectrum of porous silicon overlaps the absorption spectrum of iodine molecules, and with increasing the radiative recombination rate of porous silicon. We show that the dependence of the rate is well explained by Förster type dipoledipole interaction mechanism in which the diffusion of the assemblies and molecules is taken into consideration. Furthermore, it is found that the efficiency of the energy transfer strongly depends on the emission wavelength at low iodine concentration. IntroductionSilicon (Si) nanocrystal is a promising material for opt-electronic devices because of their strong visible and near infrared photoluminescence (PL) at room temperature. The efficient luminescence is due to the recombination of quantum confined excitons (1)-(3). The typical system that contains luminescent Si nanocrystal assemblies is porous Si. Because Porous Si is easily prepared by the anodic electrochemical etching in a hydrofluoric acid based solution and has high luminescent efficiency, the structural and optical properties of porous Si are intensively investigated (4).The radiative lifetime of Si nanocrystals is rather long (~ µs range) because of the inheritance of indirect bandgap nature of bulk Si. Then, it is difficult to realize the light emitting devices based on the Si nanocrystal assemblies. It is demonstrated that Si nanocrystals used as efficient photosensitizer for rare earth ions (5)-(6), molecular oxygen (7). Particularly, in the photosensitized effect for the molecular oxygen, the long lifetime of Si nanocrystals is an important role since the decay time of excitons is smaller than the transfer time of electronic excitation energy to molecular oxygen. Additionally the large surface area in porous Si is an advantage for the energy transfer.Si nanocrystals can be used as the photosensitizer for other materials such as organic molecules. Recently, the energy transfer from Si nanocrystals to organic molecules is demonstrated (8). In that work, the energy transfer takes place via the direct electron exchange interaction, Dexter process, which is the same mechanism as that between Si nanocrystals and molecular oxygen. Basically, the electronic excitation energy of donor can non-radiatively transfer to acceptor via the dipole-dipole Columbic interaction, Förster process (9), in addition to Dexter process. In colloidal semiconductor nanocrystals such as CdSe quantum dots, the excitation energy transfer to organic dye molecules or nearby quantum dots via Förster process is observed (10)-(11). Then, Si nanocrystals also expect for use of the energy donor in Förster type energy transfer. It is important to investigate Förster process in Si nanocrystals, because Si nanocrystals have a longer lifetime compared to direct bangap semicondu...