Abstract
Temperature dependence of the energy gap of CdSe quantum dots: a sophomore-level nanomaterials experimentThis paper presents a sophomore-level experiment that allows students to see the“particle-in-a-box” behavior of a real system (quantum dots of different sizes) andexplores the temperature dependence of the quantum dots’ energy gap. Quantum dots arenanometer-sized clusters of atoms that contain anywhere from a few to a few thousandatoms. Because of their size, quantum effects become important, which makes theminteresting to study and potentially useful for technological applications. Forsemiconductor quantum dots of a given material, the size of the quantum dot determinesthe energy gap (the energy difference between the HOMO – highest occupied molecularorbital, and the LUMO – lowest unoccupied molecular orbital) of that particular quantumdot. As a result of their different sizes, the cadmium selenide quantum dots studied(which we obtain from Cenco Physics) emit light of different wavelength (and hencecolor) when excited by ultraviolet light. In addition to being size dependent, the energygap also depends on temperature. In the experiment, the cadmium selenide quantum dotsare exposed to different temperatures and, by using a spectrometer, the wavelengths atwhich they emit light when excited by an ultraviolet source are recorded and compared.Using the model of an electron and a hole inside a spherical infinite potential well, therecorded wavelengths can be related to the size of the quantum dots at varioustemperatures. The experimental data is then compared to the changes that would beexpected from classical volume expansion and from the Varshni equation. Both methodsprovide an adequate explanation of the temperature dependence of the energy gap of thecadmium selenide quantum dots studied.