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Many semiconductors exhibit the property of phosphorescence, the phenomenon whereby a material excited by visible or ultraviolet radiation glows (i.e., emits visible light) for a substantial time after the excitation source has been removed. Absorption of visible or ultraviolet radiation, from sunlight or room lights, causes excitation of electrons and produces nonequilibrium concentrations of electrons and holes in the solid. Light is emitted as the electrons recombine with the holes, a process that occurs at a rate dependent on the material and the temperature. One phosphorescent semiconductor commonly used in glow-in-the dark toys and emergency signs is copper-doped zinc sulfide (ZnS:Cu), a material that produces a yellow-green glow. We have found the study of this light emission to be a suitable introduction to kinetics and to semiconductors.
Experimental Procedure A Glow-Yo yo-yo (Duncan #3057NP) and an Amazing Glow Ball (Noteworthy #GB0565B Baseball) were obtained from Toys-R-Us. We also obtained copper-doped zinc sulfide Glow-in-the-Dark Phosphorescent Pigment (P-1000) and Glo-Vinyl Tape (such as used in theaters) from Hanovia Division of Conrad, 100 Chestnut St., Newark, NJ 07105.
Samples were placed in a Shimadzu RF-5000 Spectrofluorophotometer and excited with ambient light or a 120-W flood lamp. The sample compartment was then closed and either an emission spectrum was recorded or the kinetics were followed at 530 nm by recording the emission intensity every second for several minutes. A 3-nm slit width was used on the emission monochromator and the spectrometer was operated at low sensitivity. Data recording began when light emission was on scale.
For variable temperature experiments the sample was placed in a glass cell and the temperature controlled by circulating water over the sample from a GCA Precision R40 constant-temperature bath. A dark cloth was used to cover the glass cell in the instrument compartment in order to avoid the interference of outside light. For data taken below 25(deg(C, nitrogen gas was used to purge the sample compartment to avoid water condensation. The yo-yo was cut on a band saw to fit snugly in the glass cell. The Hanovia P-1000 pigment was well mixed with DER 324 resin and DEH 24 hardener (Dow Chemical Co.); the epoxy hardened into a pellet, which was anchored in the glass cell by the use of small weights.
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