Synthesis and Characterization of CdSe Nanocrystals ( ) ( , , ) 8 h ...

Synthesis and Characterization of CdSeNanocrystalsSynthesisAdd 75 mg of Li 4 [Cd 10 Se 4 (SPh) 16 ] to a crimp topvial containing a magnetic stir bar. Seal the vial andpurge with argon for 2-5 minutes. Transfer 5 mL ofmolten hexadecylamine 1 that is stirring under anargon atmosphere via syringe to the vial. Place thevial in an aluminum heating block and raise thetemperature to 200 o C. Over the course of severalhours the solution will change from colorless toyellow. You may remove the vial at this time andallow to cool. If larger diameter particles are desired,the temperature is raised to 220oC and allowed to stirovernight. Raising the temperature further (230 o C)and allowing the mixture to stir for up to a week willresult in larger particles. The solution will changefrom yellow to orange to red as the particles grow. 2If the solution turns cloudy at any time, decrease thetemperature to 190 o C until the solution becomesclear. The temperature is raised to the previous pointand the growth is allowed to continue.Remove 0.5 grams of the waxy material andplace it in a test tube. Heat the test tube in a waterbath until the wax has melted. Add 2-3 mL ofmethanol to the liquid. The CdSe nanocrystalsshould precipitate at this point. Place the test tube incentrifuge and spin the solutions for ~5 minutes.Note: The centrifuge must be balanced for safe use.Tubes opposite each other in the centrifuge rotormust have the same mass. Check the tubes beforeyour turn on the centrifuge. After spinning, decantthe liquid and then add 1-2 mL of toluene to theremaining solid. The result should be a suspension ofnanocrystals in toluene.Figure 1. Optical absorbance spectrum of CdSe. Thearrow indicates the λ max for the exciton.Theoretically, the exciton absorbance shouldcorrespond to the confinement energy of a particle ina three dimensional box. This is indeed the case asshown in figure 2 where the lowest energy excitonabsorption shifts to longer wavelength withincreasing particle size. The confinement energy of acharged particle in a cubic box is given by theequation below. The potential energy is defined asinfinite outside the box and zero inside. This is athree dimensional extension of a particle on a line inan infinite potential well.2 2 2 2h ⎡∂ ψ ∂ ψ ∂ ψ ⎤E( ψ) = U( x, y, z)ψ −2 2 2 28πm⎢ + +dx dy dz⎥⎣⎦CharacterizationRecord the absorbance spectrum of yournanocrystal solution. Determine the wavelength(λ max ) of the exciton peak. The exciton peak isidentified in the CdSe optical spectrum shown belowin figure 1. In a semiconductor absorption of light ofa sufficient energy causes an electron to move fromthe valance band to the conduction band. In theprocess a hole (positive charge) is created in thevalence band. An exciton is analogous to an excitedstate in a molecule.Figure 2. Absorbance spectra of various sizes ofCdSe nanocyrystals.The exciton in a semiconductor nanocrystal can bethought of as a real world example of the particle in abox problem, however there are some significantdifferences. First the nanocrystal is not a cubic shape.1

Experiments reveal that the shape is better thought ofas ellipsoidal. The equation above can be modifiedto account for this geometry. Another importantdifference is that the optical transition we observe asthe exciton band results from the creation of twocharged particles; an electron(-) and a hole(+). Thistoo can be accounted for and the Hamiltonian can bewritten for the electron and hole 3h hH =− ( , )2 28π∇ − 8∇ +2 22 2e hU Se Shmeπ mhwhere m e and m h are the effective masses of anelectron and a hole in CdSe and S e and S h are thepositions of the electron and hole inside thenanocrystal. Now the potential energy is consideredinfinite outside of the nanocrystal but inside coulombattraction between the oppositely charged particlesmust be taken into account. Including the appropriateterms leads to the following equation known as theeffective mass model2 2h ⎡ 1 1 ⎤ 1.8eE*≅ Egap+2 ⎢ + ⎥−8R ⎣memh⎦4πεRwhere E gap is the bulk CdSe bandgap energy and ε isthe dielectric constant of CdSe. The effective massmodel is good for particles above ~7 nm in diameter.For smaller particles the approximations made inreaching the effective mass model are no longervalid. Research is currently underway to develop amore general theory. Optical characterizationmethods can still be utilized to determine the size ofnanocrystals. Extensive data have been collected tocorrelate the exciton energy and the particle size.Particle sizes are determined by SEM images and x-ray powder diffraction techniques (see figure 3).The line through the data is an empirical fit and canbe used to estimate the diameter of CdSenanocrystals using the following equation1−BλD = A + C λwhere D is the diameter of the nanocrystal (nm), λ isthe exciton wavelength (nm) and A = -0.79196, B =3.8121 × 10 -3 and C = 9.5125 × 10 -4 .Use the equation above to determine the size ofyour nanocrystals. To explore the idea of particle ina box further we will look at the emission energy as afunction of particle size. Record the emissionspectrum of your nanocrystal solution. Use the lmaxof the lowest energy exciton as the excitationwavelength. To avoid saturating the detector startcollecting data 10 nm to the red (longer wavelength)of your excitation wavelength. Record the emissionmaximum in your laboratory notebook and on thechalk board in the lab. The class data will be postedto the web at the end of the week. The particle in abox model predicts that the emission energy shouldbe proportional to 1/R 2 . Therefore a plot of theemission energy (in eV) vs. 1/R 2 should result in astraight line. Plot the class data and discuss theresults.1 Hexadecylamine is a waxy substance at roomtemperature (m.p. = 60 o C). Melt the solid in a threeneck round bottom flask fitted with a rubber septum,a thermometer and a reflux condenser. Heat theHDA until the temperature is ~140 o C. Evacuate theflask several times to remove any residual water.2 The cooled vials will turn much lighter in color asthe cool. An orange solution will result in a yellowsolid.⎡∂ ψ ∂ ψ ∂ ψ ⎤⎢dx dy dz⎥⎣⎦2 2 23 2∇ = + +2 2 2Figure 3. Plot of CdSe diameter vs. excitonabsorbance.2