1. Quantum Dot Band Gap Measurements
J. Ryan Peterson
Thanks to:
Dr. John S. Colton (advisor)
Kameron Hansen, Luis Perez, Cameron Olsen
label scans for control and sample intensity
Explain line (or make it dashed) at 1.0 transmission
Talk about obtaining the tungsten halogen lamp

2. Quantum Dot Synthesis
Band gap distribution due to core size distribution
Empty ferritin
Uranyl Acetate halo
Different core sizes
Explain how image was taken
Explain PL briefly
Explain that having a distributin of band gaps means absorption will not be perfectly linear

4. Measured intensity through Mn Ferritin solution
Control solution
Ferritin Solution
Explain what the orange and blue lines are
Add labels to the curves
Clearer, bigger axis labels

5. Transmission of Mn Ferritin solution
Normalization
𝐼 𝑓𝑒𝑟𝑟𝑖𝑡𝑖𝑛 𝐼 𝑐𝑜𝑛𝑡𝑟𝑜𝑙
Explain how absorption coefficient is obtained
Make connection between band gap being on the right side because there is no absorption there

6. Indirect band gap plot Indirect Transition: 𝛼 ~ 𝐸 𝛾 − 𝐸 𝑔𝑎𝑝
𝛼 ~ 𝐸 𝛾 − 𝐸 𝑔𝑎𝑝
Measured absorption
Best linear fit
Explain fitting (red line) and how the noise on the bottom is problematic
Explain that the band gap is the onset of the absorption
Circle the x-intercept

7. Measured intensity with tungsten halogen lamp
Control solution
Ferritin Solution

8. New transmission Normalization 𝐼 𝑓𝑒𝑟𝑟𝑖𝑡𝑖𝑛 𝐼 𝑐𝑜𝑛𝑡𝑟𝑜𝑙
𝐼 𝑓𝑒𝑟𝑟𝑖𝑡𝑖𝑛 𝐼 𝑐𝑜𝑛𝑡𝑟𝑜𝑙
Show which sample this is (and do so on all other absorption curves)

9. New indirect band gap plot
Measured absorption
Best linear fit
Point out that eV is now decent and the graph goes even lower than the previous ones, exposing another absorption edge.
Explain that first, we thought we should report the lower edge, since that is usually the indirect band gap. But then we noticed that many of those lower edges fell off before the main edge took over.

10. Second possible fit for band gap
Measured absorption
Best linear fit
We needed to know if that bump on the left side was actual absorption or just error in our experiment or due to actual absorption.
Have arrow pointing to value from other fit

11. Defect Binding: explaining the first edge
Explain what a defect is; missing/extra atom, dislocation, atom replaced with another, and explain that perfect crystal creates these bands but defects create additional states
Don’t say “middle band gap”; explain that it is the gap corresponding to middle part of line

12. Conclusions Old New Band gap can be varied continuously
Indirect gap close to Silicon band gap
Implications: we can vary the band gap continuously. Also, this band gap is near silicon, which is what we had been looking for. However, it is an indirect absorber, so we’re looking for direct absorbers to use instead (aka PbS) since they will absorb much much more light.

13. Direct and Indirect Transitions
This method is much much easier if it can be done, but it doesn’t work for most of our samples. It reveals a problem, though; the nanoparticles have varying band gaps.
Say explicitly that there is no absorption below the band gap
Explain or have pic of indirect band gaps and show how they can have direct transitions.

14. Possible explanation: Scattering
Explain how we gathered this data
Don’t forget to explain why the graph falls off on the right
Can cut off the right side
Can use percent on y axis

15. Direct band gap plot Direct Transition: 𝛼 2 ~ 𝐸 𝛾 − 𝐸 𝑔𝑎𝑝
𝛼 2 ~ 𝐸 𝛾 − 𝐸 𝑔𝑎𝑝
Since direct band gaps absorb strongly, small changes at low energies do not matter since the result is squared.
Explain that the distribution of band gaps causes a gradual rise in the slope instead of a step function slope
Explain that the direct gap measurements worked fine because they were high energy but xenon did not work well for the lower energy indirect gaps.
Cut off the right side of the graph where it falls down again