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

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

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

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

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

Measured intensity with tungsten halogen lamp Control solution Ferritin Solution

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

New indirect band gap plot Measured absorption Best linear fit Point out that 1.5-1.8eV 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.

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

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

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.

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.

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

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