Exploring Periodic Properties using Light Emitting Diodes (LEDs)

This experiment has been included in introductory chemistry courses at Lawrence since 2000, and is also used periodically in inorganic, materials chemistry, and non-majors chemistry courses.

This popular experiment, in which students explore the colors of various LEDs based on their semiconductor compositions, has been augmented to include making measurements of the spectra of each LED using the Ocean Optics spectrometers. Students can measure the change in intensity and color shift of the emitted light as the LEDs are cooled in liquid nitrogen. These spectral changes provide a dramatic visual confirmation of the changes in the band gap of a semiconductor as it is cooled. Using specially designed tricolor LED color mixers, red, green and blue, students can mix the light to generate colors and use the spectrometer to analyze their mixed colors. Students will also examine the spectra of conventional light sources and compare the 'wasted' infrared heat emission of these light sources through a comparison with the 'cool', nearly monochromatic light of LEDs. While the LED chip is roughly 300 microns3, some of the semiconductor layers are less than 10 nm thick. [ref] Helping students understand the connection of the composition, band gap energy, and the color emitted by the LED will complement the study of semiconducting quantum dots (QD) described below where the size of the dot itself, not the composition, determines the band gap and other electronic properties.

References: M.R. Krames, M. Ochiai-Holcomb, G.E. Hofler, C. Carter-Coman, E.I.Chen, I.H. Tan, P. Grillot, N.F. Gardner, H.C. Chui, J.W. Huang, S.A. Stockman, F.A. Kish, M.G. Craford, T.S. Tan, C.P. Kocot, M. Hueschen, J. Posselt, B. Loh, G. Sasser, D. Collins, "High-power truncated-inverted-pyramid (AlxGa1-x)(0.5)In0.5P/GaP light-emitting diodes exhibiting > 50% external quantum efficiency", Appl. Phys. Lett., 1999, 75, 2365.

Links: http://mrsec.wisc.edu/edetc/cineplex/LED/index.html

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Last updated June , 2005

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