Quenching of Molecular Fluorescence near the Surface 0f.a Semiconductor

The possibility of energy transfer from an excited molecule to a nearby semiconductor was recently proposed by Dexter [l] , who thus extended previous work [2-51 on energy transfer from an excited molecule to a metal Dexter’s idea was that energy produced in the form of excitons in a strongly absorbing organic coating could be transferred to a semiconductor producing free electrons and holes in the semiconductor. ‘Ike energy transfer occurs via a non-radiative process from the excited molecule when the excitation energy of the molecules is greater than the band gap of the semiconductor_ Below we report the initial resuits of an experimental study of energy transfer from an organic thin.film (tetracene) to a semiconductor (Si or GaAs). Extensive theoretical and experimental investigations have been made on the interaction of an emitting dipole with a nearby metal surface [2,5 ] _ The lifetime of an emitting dipole changes with the distance between the dipole and the metal. The dependence of lifetime on the separation has been well explained using classical electromagnetic theory [2]. Non-radiative decay of a dipole emitter at a separation well below the wavelength of emitted light is attributed to absorption of virtual photons by the metal [3,4] _ Recently, Stsvola et al. [6] have performed a quantum mechanical calculation of the interaction between excited molecules and semiconductors based on the dipole-dipole interaction mechanism_ They concluded that, if the distance d of the excited molecule from the semiconductor surface is much larger than the semiconductor lattice constant, the energy transfer probability is proportional to d --3. This is the same result as obtained from the classical analysis [2]. If the energy of the emitted photon is in the indirect absorption region of the semiconductor, the probability is small because it is proportional to the absarption coefficient of the semiconductor_ However, for distances d of the order of 10 lattice spa&q or less the momentum conservation rule between photons and interband transitions of the semiconductor is relaxed and the transfer rate deviates from the de3 rule. For the indirect band gap case, a probability proportional to d -4 is predicted. In this paper new results are given of an experimental study of energy transfer from organic molecules to Si and GaAs single crystals. Thin evaporated films of * Permanent address: College of General Education, Kyoto tetracene were used as the sensitizer. The fluorescence University. Kyoto 606. Japan. efficiency of tetracene was measured as a function of