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30th GREEN Open Seminar

Schedules 2013.11.28 Finished


The 30th GREEN Open Seminar will be held at NIMS on November 28, 2013, to have a lecture by Dr. Masashi Ishii,Surface Physics and Structure Unit (SPSU), Surface Physics Group, NIMS.

Abstract

An inquiry into “Non-radiative processes” for development of truly valuable luminescent materials

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


Fig.1




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Summary

Event Title
30th GREEN Open Seminar
Venue
Meeting room, 3F NanoGREEN Bldg., Namiki site, NIMS
Schedules Hours
2013.11.28
15:00-17:00
Language
English

Contact

GREEN

E-Mail: Email Address
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