The color, visible absorption, and photoluminescence of Zn0.2Cu0.9In0.9S2, which can also be written (ZnS)0.2(CuInS2)0.9, nanoparticles depend on the size of the particle. A high temperature is needed to decompose dodecanethiol to give sulfide. Octadecene is used as a non-coordinating, high-boiling solvent. Oleic acid and steric acid are used as ligands to dissolve the metal salts in the solvent at about 150 °C. Once seed crystals have formed at a high temperature (above 230 °C), a lower temperature (200 °C) is used for crystal growth to obtain uniform particles. Samples are withdrawn from the hot solution and quenched at room temperature
to produce a series of increasing particle sizes.
Weigh out 0.0267 g CuI (0.14 mol), 0.0409 g In(CH3COO)3 (0.14 mol), and 0.0759 g Zn(CH3(CH2)16COO)2 (0.12 mol).
Avoid plastic containers to reduce static problems. This step may have been done for you with all three solids combined in one container.
In a hood (potential fire hazard), preheat a stirring hotplate to around 200 °C using an assigned Corning heat setting of 335, 340, 345, 350, 355, or 360.
Wait for the temperature to stabilize. You can use a cooking thermometer to verify. This will take at least ten minutes.
Transfer the weighed sample(s) to a 25 mL Erlenmeyer flask. Add a 1/4" stir bar.
Add 1 mL dodecanthiol, CH3(CH2)11SH (4.175 mol),
16 mL octadecene, CH3(CH2)15CH=CH2,
and 0.23 ml oleic acid, cis-CH3(CH2)7CH=CH(CH2)7COOH (0.8 mol), to the Erlenmeyer flask.
Place the flask on the preheated hot plate. Begin stirring and turn the heat setting to maximum to heat as quickly as possible.
Add a thermometer capable of measuring 250 °C to the flask and position it out of way of the stir bar using a clamp at the top of the thermometer.
As the solution rapidly heats up, the starting materials will dissolve. (Check with your instructor if the sample is still cloudy by 160°C. The actual elapsed time is longer than shown in the video; the heating takes about 5 minutes.)
As the thermometer reading passes 190 °C turn the heat setting down to the assigned value. Start timing the reaction.
The temperature will continue to rise about 30 °C more as required for nucleation of the reaction and then drop back to the desired set temperature for the growth of the nanoparticles. (The hotter the solution, the quicker the particles will grow, and taking samples fast enough becomes difficult. At 190 °C the reaction will proceed at about half the rate as at 200 °C.)
As the nanoparticles grow in size, use a 9 inch glass pipet with a 2 mlL bulb to transfer approximately 1-2 mL samples every minute or so (when the color changes) and quench in individual dry containers. A rack of test tubes is convenient. Fully squeeze the bulb before inserting the pipet into the solution then release to withdraw solution over a short time period. Repeat for subsequent samples when the color has changed. Have a partner record the times and temperature for each withdrawal. WARNING: the solution and glass will be hot!
Record the visible absorbance spectra of the solutions using octadecene as a blank and a small volume (1 mL with 1 cm path length) PMMA sample cell. (It is efficient to load the cuvet, measure the visible absorbance spectrum and then measure the emission wavelength in the next step before returning the sample to its original container.) You will eventually graph the band edge wavelength (see the calculation method shown below) as a function of growth time.
Use a 395 nm excitation and find the wavelength for maximum emission using a small volume (1 mL with 1 cm path length) PMMA sample cell. Excite in the thin direction and observe the full path length. (It is efficient to load the cuvet, measure the visible absorbance spectrum in the previous step and then measure the emission wavelength before returning the sample to its original container.) You will eventually graph the emission wavelength as a function of growth time. What is the evidence for band gap excitation rather than molecular absorbance?
Reaction time, sec
Band edge wavelength, nm
Band gap energy, J
Emission wavelength, nm
Use the oxidation state of each of the elements in the quantum dots to show how the overall charge is neutral.
Graph solution temperature as a function of time. Identify when nucleation happens and when growth happens. What was your best estimate of the growth temperature?
Extrapolate the linear portions of the lowest energy absorbance as a function of wavelength to find the band edge wavelength for each sample.
(One option is to use the band edge tab on the Beers Law template. Change the wavelength values in the colored shaded boxes to choose the ends of the linear portions of your absorbance graph. The program will least squares fit the interval and show the intersection of the two lines in the table. Print the sheet to show your work. Another option is to transfer the wavelength and absorption data to the band edge spreadsheet and do the same analysis.)
Finding the band edge wavelength for a semiconductor.
Graph the band edge wavelength as a function of growth time.
Graph the emission peak wavelength as a function of growth time.
How do the absorption band edge wavelengths compare with the emission peak wavelengths? Make another graph to illustrate your answer.
Convert the band edge wavelength to the band gap energy, Egnano. The effective mass model suggests
where r is the radius of the nanoparticle. The second term is the particle-in-a-box confinement energy for an electron-hole pair in a spherical quantum dot
and the third term is the Coulomb attraction between an electron and hole modified by the screening of charges by the crystal.
Eg = h c / λ h = 6.626x10-34 J s c = 2.998x108 m/s e = 1.602x10-19 C
ε0 = 8.854x10-12 C2/N/m2
m0 = 9.110x10-31 kg CdS
λbulk = 512 nm
ε = 5.7
me* = 0.19
mh* = 0.80
λbulk = 3400 nm
ε = 17.2
me* = 0.25
mh* = 0.25
After multiplying by r2, rearranging, and using the quadratic formula to find r,
What is the band gap energy and diameter of each nanoparticle sample?
Extraction (optional extension)
To remove the nanoparticles from solution, add a five-fold excess of ethanol, mix well and centrifuge at 3500 rpm. Remove the top layer (retain the colored substance at the bottom of the tube) and repeat the ethanol extraction several times. When only a gooey dark substance remains, dissolve it in hexane. Repeat the ethanol extraction several more times. The quantum dots will require less hexane to re-dissolve after the first few extractions.
XRD (alternative synthesis)
To prepare samples for x-ray powder diffraction (XRD), prepare a 5-fold batch using 20 mL dodecanethiol as the sulfur source and the solvent (no octadecene.) Quench the entire batch at the same time to get enough sample. Remove the solution as in the previous step.
Dissolve the purified sample in hexane and transfer to an evaporating dish. As the hexane evaporates, sample will dry on the sides of the dish. Swirl the dish to re-dissolve the sample back into the hexane.
Transfer the hexane sample onto the XRD holder a drop at a time. Since the sample will spread away from where it is initially placed, add the sample a few drops at a time and then wait for the hexane to evaporate before adding more.