2009 Advanced Laboratories Conference Abstract Detail Page

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Abstract Title: Study of the relationship between radius and emission wavelength of indium phosphide quantum dots
Abstract: Quantum dots are particles, with dimensions in the range of 1-30nm, composed of a semiconductor material whose electron-holes are thus confined to a small volume.  The ones investigated here are spherical, with radius about 3nm, and consist of indium phosphide cores surrounded by thin zinc sulfide shells.  They are suspended in the liquid, tetradecane.  When a photon of high enough frequency is incident, it can form an electron-hole pair.  The electron and hole exist almost independently in the quantum mechanical ground state determined by the small spherical volume.  Upon recombination, a photon is emitted with energy dependent on the radius of the dot.  Smaller dots emit higher energy photons.  The elementary quantum mechanical calculation of the ground state for a particle confined to a sphere yields the energy of the photon as the sum of band gap energy, kinetic energy of the particle, and kinetic energy of the hole. The kinetic energy parts are inversely proportional to the square of the radius of the sphere.

This experiment is about verifying the quantitative relationship between radius and emission wavelength predicted by the elementary quantum mechanical theory.  A set of four vials of dots was easily obtained from the Cenco company.  Emissions are in the red, orange, yellow, and green regions of the spectrum.  All of the samples were examined, but special focus was given to the one emitting orange.

The emission spectra were obtained with an Ocean Optics Spectrometer.  Each sample produces a single peak of about 60nm width.  This suggests a ststistical distribution of particles according to radius.  The theoretical formula was used to calculate the radius from the wavelength, and then the spectra were plotted against radius.  The graphs obtained were fitted with a Gaussian curve, and mean radius and standard deviation were obtained.  For orange, these are 2.7nm and 0.3nm.  To check this prediction,  an image was obtained using a Transmission Electron Microscope at the Center for Electron Microscopy and Nanofabrication at Portland State University.  For orange, the directly measured mean radius and standard deviation are 2.6nm and 0.3nm.  This result serves well as an initial check of the theoretical formula.
Abstract Type: Poster
Session: Session IX - Effective Experiments Poster Session

Author/Organizer Information

Primary Contact: Donald Schnitzler
Linfield college
900 SE Baker St.
McMinnville, OR 97128
Phone: 971-237-9977
and Co-Presenter(s)
Ajeeta Khatiwada, Department of Physics, Linfield College,
McMinnville, OR

Tyler Harmon, Department of Physics, Linfield college,
McMinnville, OR