CHEM 146 Experiment 6 A Visual Demonstration of Particle in a Box theory Multicolor CdSe Quantum Dots Yat Li Department of Chemistry Biochemistry University of California Santa Cruz Objective In this laboratory experiment we will learn 1 The principle of interband transition and quantum confinement effect in zero dimensional quantum dots 2 Synthesis of CdSe nanocrystals 3 Absorption and Emission properties of CdSe nanocrystals Semiconductor nanocrystals Nanocrystals are zero dimensional nanomaterials which exhibit strong quantum confinement in all three dimensions and thus they are also called quantum dots UV light Ambient light Size dependent optical properties Particle in a Box theory The particle is restricted to the region 0 x a the probability that the particle is found outside the region is zero 0 x 0 a 0 a Free particle in a one dimensional box Schr dinger equation d2 dx2 2m E V x x 0 2 h 2 E total energy of the particle V x potential energy of the particle and x wavefunction of the particles Free particle the particle experience no potential energy V x 0 d2 dx2 2mE 2 x 0 Particle in a Box theory The general solution of Schr dinger equation 2mE 1 2 k x A cos kx B sin kx when 0 0 cos 0 1 sin 0 0 A 0 when a 0 a B sin ka 0 B 0 rejected or ka n n 1 2 3 Substitute k n a back to equation for k En h2n2 8ma2 E h2 8ma2 n 1 2 3 nf ni 2 2 2mE 1 2 h Quantum dots A quantum dot is in analogy to the particle in a box model where E increases with decreasing a E h2 8ma 2 nf ni 2 CdSe has a Bohr exciton radius of 56 so for nanocrystals smaller than 112 in diameter the electron and hole cannot achieve their desired distance and become particles trapped in a box Free exciton Synthesis of CdSe nanoparticles Preparation of Se precursors 1 2 30 mg of Se and 5 mL octadecene 3 0 4 mL trioctylphosphine completely dissolve the selenium Preparation of Cd precursors 1 2 a b Add 13 mg of CdO to a 25 mL round bottom flask add by pipet 0 6 mL oleic acid and 10 mL octadecene Heat the cadmium solution to 225 C http mrsec wisc edu Edetc nanolab CdSe index html Synthesis of CdSe nanoparticles Preparation of CdSe nanocrystals 1 2 Transfer 1 mL of the room temperature Se solution to the 225 C Cd solution and start timing Remove approximately 1 mL samples at 10s intervals for the first five samples 3 Ten samples should be removed within 3 minutes of the initial injection http mrsec wisc edu Edetc nanolab CdSe index html Spectroscopy Spectroscopic techniques all work on the principle of that under certain conditions materials absorb or emit energy Quantized energy photon E h E h hc X axis Frequency or wavelength UV vis Spectroscopy Transitions in the electronic energy levels of the bonds of a molecule and results in excitation of electrons from ground state to excited state Energy changes 104 to 105 cm 1 or 100 to 1000 kJ mol 1 Four types of transitions i Within the same atom e g d d or f f transition ii To adjacent atom charge transfer iii To a delocalized energy band conduction band photoconductivity iv Promotion of an electron from valence band to conduction band bandgap in semiconductors A powerful technique to study the interband electronic transition in semiconductors Interband absorption Electrons are excited between the bands of a solid by making optical transition Ef Ei h Direct bandgap h Eg h 0 h Eg h h Eg Indirect bandgap Relative position of conduction band and valence band is not matched The transition involve phonon to conserve momentum Ef Ei h indirect h Eg 2 Beer Lambert law log I0 I cl A cl extinction coefficient I0 incident radiation c concentration I transmitted radiation l path length A absorbance value determine transition is allowed or forbidden Luminescence Spontaneous emission when electron in excited states drop down to a lower level by radiative emission Spontaneous emission rate R A 1 Non radiative emission Electron in excited states will relax rapidly to lowest level in the excited band Sharp emission peak If R NR R 1 maximum light will be emitted Interband luminescence Direct bandgap materials Allowed transition short lifetime ns Narrow emission line close to bandgap e g GaN CdS ZnS Indirect bandgap materials Second order process involve phonon Low emission efficiency e g Si Ge