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Unformatted text preview: 1 Revised June 2010 CHEMISTRY 461 Experiment 5 – SPECTRA OF POLYENES AND NANOPARTICLES (by Bryant Fujimoto, Philip Reid, and David Ginger) I. HAZARDS Ethanol is flammable and should be kept away from flames. Zinc Acetate, Zinc Oxide and Tetramethyl Ammonium Hydroxide are toxic, and should be disposed of as described below. II. WASTE DISPOSAL The product of the reactions is ZnO in ethanol. It should be placed in the marked bottle in the hood. If you are not working in a hood, take a 250 ml erlenmeyer flask and label it nanoparticle waste. Collect the waste solution in the flask until you have a chance to empty the flask into the waste bottle in the hood. III. THEORY Particle In a Box Model Semiconductor nanoparticles are nanometer-sized crystals of semiconductors such as CdSe or ZnO. The electronic absorption spectra of these particles is strongly dependent on size for nanoparticles with diameters ranging from 1-20. Because the size dependence of the absorption spectra arises from quantum-mechanical “particle-in-a-box” confinement effects, semiconductor nanoparticles are often called “quantum dots” and their size-dependent absorption spectra provide a simple and convenient way to monitor particle size during growth. In this laboratory you will investigate the effects of quantum confinement on the optical properties of both one- dimension systems (linear alkenes) and three-dimension systems (semiconductor nanoparticles). The dependence of an absorption spectrum on particle size can be understood using the particle- in-a-box model introduced in undergraduate quantum mechanics. For a particle in a one dimensional box of length L , the potential V(x) is given by: ( ) x V x x L x L ∞ < = ≤ ≤ ∞ > Figure 1 . The potential for a one dimensional particle in a box. 2 The time-independent Schrödinger equation for a particle in this potential is: 2 2 2 2 E x L m x ψ ψ ∂- = ≤ ≤ ∂ h Where h is Planck’s constant divided by 2 π , m is the mass of the particle, ψ is the wavefunction describing the particle, and E is the total energy of the particle. Imposing boundary conditions and normalizing, the solutions to the above equation are the familiar particle-in-a-box wavefunctions and associated energy levels: 2 2 2 2 ( ) sin 8 n n n x x L L n h E mL π ψ = = A few of these wavefunctions are shown in Figure 2. The number n is called the quantum number and is constrained to integer values beginning at one (i.e., n = 1, 2, 3, … ). Different wavefunctions or energy levels are specified using their corresponding quantum number ( 1 2 , , ψ ψ K 1 2 , , E E K ). Notice that the number of nodes in the wavefunctions (points where the wavefunction equals zero) increases as the energy increases....
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- Summer '08
- pH, Atomic orbital, spectrum, ZnO Nanoparticles, reference spectrum, tetramethyl ammonium hydroxide