EXPERIMENT 4. This lab will meet on Monday October 22 nd and Tuesday October 23 rd in Science Center 210.

Transcription

1 EXPERIMENT 4 Synthesis and Analysis of CdSe Quantum Dot Nanocrystals This lab will meet on Monday October 22 nd and Tuesday October 23 rd in Science Center 210. Before You Come to Lab: Read the entire lab, paying careful attention to the safety precautions. Print a copy and bring it with you to lab. Complete the pre-lab, which is the last page of the lab report, and give the pre-lab to your TF as you enter the lab. On Your Assigned Lab Day Arrive on time! You will not be admitted to lab if the Head TF has already started the safety training. Wear proper attire and bring your goggles (see safety manual). Listen carefully to the pre-lab talk given by your TF. Before You Leave the Lab Have your TF check your lab bench for cleanup. Submit your data and lab report to your TF. Wash your hands thoroughly before leaving the lab. Grading: Prelab: / 10 Lab Report: / 20 Safety: / 3 Cleanup: / 2 Total: / 35 1

2 EXPERIMENT 4 Synthesis and Analysis of CdSe Quantum Dot Nanocrystals This experiment is adapted from E. M. Boatman, G. C. Lisensky, and K. J. Nordell, "A Safer, Easier, Faster Synthesis for CdSe Quantum Dot Nanocrystals," J. Chem. Educ., 82, (2005) Introduction Quantum dots are semiconducting nanocrystals whose properties vary with size. CdSe nanocrystals have a crystal lattice of alternating cadmium and selenium atoms that exhibits the same structure as bulk CdSe. When a photon is absorbed by the bulk crystal, an electron-hole pair is created and the particles maintain a characteristic distance apart. However, when the size of the crystal is small enough (less than 112 Å in diameter for CdSe), the electron-hole pair can no longer achieve the desired distance and the particles become confined to a box. As a result, these nanocrystals demonstrate quantum mechanical behavior and their transition energies can be explained using the particle in a box model, where the electron is the particle and the nanocrystal is the box. The particle in a box model, modified for the case of a sphere, is shown below, E n hc max h 2 (n f 2 n i 2 ) 8m e * R 2 where h = Planck s constant R = radius of the nanocrystal n f = the final energy level n i = the initial energy level m e * = effective mass of an electron. The effective mass of an electron must be used, because conduction electrons have a smaller mass than regular electrons. Effective mass varies by material and is 0.13m 0 (m 0 = mass of an electron) for CdSe. In this experiment, you will synthesize CdSe quantum dots, varying in diameter from approximately 1.8 nm to 4.0 nm. You will then measure the absorbance and emission spectra of the quantum dots and use max to calculate their size. 2

3 Safety Precautions Proper PPE Wear proper attire You must wear a lab coat, chemical splash goggles, and gloves while in the lab. Wear proper heat-resistant gloves for transferring QD solution Chemicals Several of the chemicals used in this experiment can be hazardous if handled improperly. It is extremely important that all work be carried out in the hood and that the sash be kept as low as possible while you are working. Take your time to do the experiment and always think carefully about what you are doing. If you have any questions, ask your TF. Octadecene - Octadecene vapor should not be inhaled, and contact with skin and eyes should be avoided. Trioctylphosphine - Trioctylphosphine is corrosive and causes burns. Cadmium oxide - Cadmium oxide is highly toxic and cadmium compounds are known to be human carcinogens if inhaled or swallowed. Oleic acid - Oleic acid is air, light, and heat sensitive and irritating to eyes, the respiratory system, and the skin. Elemental selenium is toxic by inhalation, by absorption through the skin, and the effects of ingestion are cumulative. Heat You will heat your reaction mixture to 225C (more than double the boiling point of water!) and transfer hot solution from one vessel to another. You should wear nitrile gloves and heat resistant gloves during this step and focus on transferring the solution in a slow and controlled manner to avoid spilling any hot solution on yourself or others. 3

4 Procedure Practice using a Pasteur Pipet In this experiment, you will be using a Pasteur pipet to transfer your reaction mixture (at very high temperature!) from a round bottom flask to a test tube. It is important to have the flask clamped at a height that makes it comfortable and safe to transfer liquid from the flask to the test tube. Set up your apparatus as shown below using a clean 25 ml round bottom flask. 25 ml round bottom flask, clamped to rod. Heating mantle Hot-plate stirrer Only use for stirring Lab Jack Fill the flask with ~ 10 ml of distilled water. Place a test tube holder and one test tube next to the set up. Using a Pasteur pipet, practice pipetting from the flask to the test tube. Adjust the height of the flask until it is at the most comfortable height for pipetting. Add a clamp above the rbf for holding a thermometer. When you are done practicing, remove the test flask by lowering the lab jack (Do not change the height of the clamp!). Dump out the water and put the flask on a rack to dry. Preparation of CdSe Quantum Dots Obtain from your TF a 25 ml round bottom flask containing approximately 13 mg of CdO. Clamp this flask above the heating mantle. Using a 1 ml syringe, add 0.6 ml oleic acid to the flask. Then add 10 ml octadecene using a graduated cylinder. Insert a thermometer capable of measuring 225 C and turn the variac to 70 to begin heating the flask. While you are waiting for the reaction mixture to heat up, get a test tube rack and fill it with ten test tubes. Obtain a clean Pasteur pipet. 4

5 A M selenium stock solution has been prepared for you by dissolving elemental selenium in octadecene and trioctylphosphine. As soon as the temperature of your reaction mixture reaches 225 C, notify your TF. Using a syringe, your TF will transfer 1 ml of the room temperature selenium stock solution to the 225 C cadmium solution. Because the size of the nanoparticles depends on reaction time, start timing when the selenium solution is added. Use a 9-inch Pasteur pipet to carefully remove approximately 1 ml samples at frequent intervals and quench by squirting into separate test tubes. You should obtain nine or ten samples within two to three minutes. Observing Fluorescence Place your test tubes in the UV box and record the color of the emitted light for each of your samples. Note the intensity of the light. Measuring Absorbance and Emission Obtain a cuvette holder and 5 cuvettes and caps. Choose your five most distinctly colored samples and, using clean pipettes, transfer your quantum dots to the cuvettes and cap them. Record a set of absorbance spectra and emission spectra for your samples using a Vernier spectrometer (your TF will demonstrate) and note the maximum wavelength. Print a copy of each set of spectra and attach it to your lab report. Sample Absorbance max Emission max 5

6 Clean-up and Waste Disposal Dump all waste into the designated waste container in the hood. Thoroughly clean all glassware with soapy water over a waste beaker. Rinse with distilled water. When you are done, dump the contents of waste beaker into the designated waste container. Place washed glassware into the blue bins. Thoroughly clean the surface of your bench using paper towels. Dispose of all solid waste (contaminated gloves and paper towels, empty cuvettes and caps) in the solid waste container. 6

7 Lab Report 25 points 1. What colors did you observe using visible light? Using ultraviolet light? 2. Do the wavelengths get longer or shorter as the particles grow? Why is this the case? 3. What relationship do you observe between the absorption wavelength and the emission wavelength for each sample? Briefly explain. 4. How can solutions containing particles with identical chemical composition (all of the samples are CdSe) have different colors? 7

8 5. Use the particle in a box model and the wavelength of maximum absorbance to calculate the diameter of your quantum dots. Do the calculations correspond to the expected size range of the particles? Sample Particle Size 6. A major goal in synthesizing nanocrystals is the production of monodisperse particles (monodisperse particles exhibit a relatively narrow size distribution.). Were your samples monodisperse? How do you know? 8

9 Prelab 1. Briefly describe one application of quantum dots that you learned about in class or on your own. 2. Using the particle in a box model, calculate the size of a CdS quantum dot that has max = 500 nm. (m e * = 0.19 for CdS) 3. Name three safety hazards in this experiment and the precautions you will take to minimize the risk of an accident. 9