30th GREEN Open Seminar

1. Research purpose
　I am not doubtful of the ability of optical measurements such as photoluminescence in evaluation of luminescent materials. However, the optical measurements observe a small minority, if enormous non-radiative processes hide in the materials. Although we are always attracted by a novel luminescence mechanism, the key to fabricate truly valuable materials should be identification and minimization of the non-radiative processes.
　Obviously, it is inconsistent to analyze non-radiative processes using optical measurements. The purpose of this research is establishment of a quantitative analytical method of the non-radiative processes and its application to development of luminescent materials.

2. Electric response measurement of luminescent materials
　In order to analyze the radiative and non-radiative processes equally, we adopt an electric measurement. Considering that the emission is induced by injected charges and that the non-radiative process is thermal loss and reflection of the injected charges, the electric measurement sensitive to the charges can cover the research purpose. Photocurrent response to a pulsed excitation light may be a typical example of the electric measurements. In this study, however, by using the mathematical equivalency between time- and frequency-responses through the Fourier transform, the processes are investigated with a spectroscopy. The advantages of the spectroscopy are as follows:

1. Windowing key frequency band improves the accuracy of the measurement.
2. Since non-radiative processes respond at a specific frequency, the spectra can be deconvolved and analyzed individually.
3. From phase analysis of the frequency response, the origin of the non-radiative processes, such as thermal loss and reflection can be quantified.
4. Anomalous behavior is possibly reflected in the spectral shape.

3. Analysis of the non-radiative processes
We have analyzed non-radiative processes in Si nano-crystals (Si-nc, Si-nc:P) and rare-earth doped semiconductors (TiO₂:Sm, Si-nc:Er, GaAs:Er).1-4) The common procedure of the analyses is
　(i)　 Measurement of current response spectra by applying AC electric field to photo-excited sample and scanning AC frequency,
　(ii)　 Identification of the origin of the non-radiative processes using thermal quenching, and
　(iii)  Comparison between photo-excited and ground states to evaluate contributions of photon and intrinsic property of the materials.
From (i), an equivalent circuit (parallel connection of resistance R and capacitance C) that represents the spectrum can be deduced, and it quantifies the thermal loss and reflection in the non-radiative processes. For (ii), variation in R and C with respect to temperature clarifies the origin of the quenching.
　Figure 1 shows electrically evaluated a thermal quenching process of luminescence for TiO₂:Sm. We confirmed that the response frequency 0 (that corresponds to a response time constant 0 = 1/0 through the inverse Fourier transform) correlates with the quenching of Sm³⁺ luminescence, and proposed dynamics of non-radiative process from a spectral feature i.e., broadening spectrum with increasing temperature. The broadening can be explained by hopping charges in a sub-band of TiO₂.
　Thus, the electric measurement can quantify the non-radiative processes. The result is expected to be used as a criterion for development truly valuable luminescent materials.
Fig. 1 Electrically evaluated thermal quenching of Sm³⁺ luminescence of TiO₂:Sm