Using the Franck-Hertz Experiment To Illustrate Quantization Energy States of the Neon Atom by Electron Impact

na provide students with motivation for the study of quantum mechanics. That microscopic matter exists in quantized states can be demonstrated with modem versions of historic experiments: atomic line spectra (I), resonance potentials, and blackbody radiation. The resonance potentials of mercury were discovered by Franck and Hertz in 1914 (2). Their experiment consisted of bombarding atoms by electrons, and detecting the kinetic energy loss of the scattered electrons (3). Prior to the Franck-Hertz experiment, spectroscopic work bv Balmer and Rvdbere revealed that atoms emitted radiatibn at discrete ekergiis. The Franck-Hertz experiment showed directly that auantized enerm levels in an atom are real, not jist optiEal artifacts. atom can be raised to excited states by inelastic collisions with electrons as well as lowered from excited states by emission of photons. The classic Franck-Hertz experiment is carried out with mercury (4-7). Here we present an experiment for the study of resonance potentials using neon.

Synthesis and Surface Modification of CdSe and CdS Quantum Dots Exhibiting High Quantum Yield

The thesis investigates the effect of surface treatment with various reducing and oxidizing agents on the quantum yield (QY) of CdSe and CdS quantum dots (QDs). The QDs, as synthesized by the organometallic method, contained defect sites on their surface that trapped photons and prevented their radiative recombination, therefore resulting in adecreased QY. To passivate these defect sites and enhance the QY, the QDs were treated with various reducing and oxidizing agents, including: sodium borohydride (NaBH4), calcium hydride (CaH2), hydrazine (N2H4), benzoyl peroxide (C14H10O4), and tert-butylhydroperoxide (C4H10O2). It was hypothesized that the reducing/oxidizing agents reduced the ligands on the QD surface, causing them to detach, thereby allowing oxygen from atmospheric air to bind to the exposed cadmium. This cadmium oxdide (CdO) layeraround the QD surface satisfied the defect sites and resulted in an increased QY. To correlate what effect the reducing and oxidizing agents were having on the optical properties of the QDs, we investigated these treatments on the following factors:chalcogenide (Se vs. S), ligand (oleylamine vs. OA), coordinating solvent (ODE vs.TOA), and dispersant solvent (chloroform vs. toluene) on the overall optical properties of the QDs. The QY of each sample was calculated before and after the various surface treatments from ultra-violet visible spectroscopy (UV-Vis) and fluorescence spectroscopy data to determine if the treatment was successful.From our results, we found that sodium borohydride was the most effective surface treatment, with 10 of the 12 treatments resulting in an increased QY. Hydrazine, on the other hand, was the least effective treatments, as it quenched the QD fluorescence in every case. From these observations, we hypothesize that the effectiveness of the QD surface treatments was dependent on reaction rate. More specifically, when the surface treatment reaction happened too quickly, we hypothesize that the QDs began to aggregate, resulting in a quenched fluorescence. Furthermore, we believe that the reactionrate is dependent on concentration of the reducing/oxidizing agents, solubility of the agents in each solvent, and reactivity of the agents with water. The quantum yield of the QDs can therefore be maximized by slowing the reaction rate of each surface treatment toa rate that allows for the proper passivation of defect sites.