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Quantum Dot Fluorescence

Analyzing the lifetimes of fluorescence of quantum dots

Chris Eddy

on 6 November 2012

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Transcript of Quantum Dot Fluorescence

Advisor: Dr. James Butler Outline Background
What I have accomplished WHAT ARE QUANTUM DOTS? Fluorescence
wavelength emitted is typically lower in energy, higher in wavelength--more red than absorbed light
electron jumps from the valence band to the conduction band, deexcites by nonradiative, vibrational, heat, etc. means, and then makes the move back to the valence band by emitting a photon of light. This photon is then captured by our photo-diodes, which are connected to an oscilloscope that yields images like this. [show which photo diode--scatterlight vs fluorescence is to each curve. "as you can see, there is some properties of an exponential decay in the observed fluorescence. From the image, explain that this curve is important because we can determine the lifetime of fluorescence, which is roughly the amount of time the decay lasts for. The lifetime is characteristic of decay processes of the quantum dot, which is further dependent on the chemistry of the quantum dot, say whats attached to it, its environment, etc. It is very well possible that we might be able to determine whether or not a quantum dot has bound itself to a molecule in a solution by tracking changes in its lifetime. As I said in my last talk, it's not a simple matter of just looking at the curve and saying "oh, yeah, sure 50ns." The observed curve is a combination of the pump beam, as well as an exponential decay curve (perhaps show equation with the lifetime there!). So since we need the lifetime, we need to be able to pull out the exponential decay curve. To do this, we do deconvolution. There are many methods such as[]. Frequently, people assume just a single exponential, since that is the easiest case, and thus a single lifetime. I tried that with our data and was yielded with this as a best fit [show picture from maple]. As you can see the tail doesn't fit, so when this happens people have to assume a multiexponential decay. I chose to do the method of moments because a) it was said to be computationally not difficult, b) it allows for multiexponentials. The method of moments does some numerical integrals and other things to compute the lifetimes. Its six pages, but let me tell you six pages is a lot of code (picture perhaps?) So when I run my program with a guassian pulse, and a set of convoluted data that matches my pulse (with lifetime values that I put in), I get this back, a near perfect fit. However if I add noise to the signal, which makes the data more realistic, the program has issues and yields this. This is exactly what we have been seeing with our real data[image]. So I have been trying to eliminate as much noise as possible using FFT and linear regressions, but still there seems to be a problem. I am still troubleshooting it, so for those out there who are good at code and might think I missed something, please talk to be after the talks today. Anyway, this is where I am at. For the rest of the summer, I plan to start taking data on the fluorescence emission, which includes the lifetimes, the wavelength, and the intensity. I also plan to look at how strong the QD's absorbance is. QUESTIONS?? You need to add the relative sizes, 10's to 100's of atoms in diameter. You should also add a picture of the bands of the conductor and insulator, so the explanation is easier, as well as a pic of what they are, such as metal and plastic. Guassian, signal with noise Guassian, signal with no noise 435 nm Real Data Single Exponential 10's to 100's of atoms
in diameter Nano-crystals Observed Fluorescence Pump Beam Notes:
Fluorescence has exponential decay characteristics
Fluorescence also has characteristics of the pump beam This is termed convolution What is the lifetime?
Roughly the amount of time the decay lasts for
Characteristic of the decay processes of the QD
Changes based on the QD chemistry

Why do we care?
Possible to track what the QD is bound to, based on changes in its lifetime. Deconvolution I(t)=C*e^(-t/τ)+... Roughly Guassian WE HAVE DATA
FOR THIS --- artificial
--- from program ---real data Multiple exponentials=multiple lifetimes What's Next? Debug program
Take data to analyze
Fluorescence wavelength
Fluorescence intensity
Absorption spectrums
Lifetimes Conduction Band

Valence Band Band Gap Energy Energy gap is within optical excitation range THE SETUP References http://www.assaymetrics.com/fluorescence_lifetime.htm
J. N. Demas, Excited State Lifetime Measurements; Academic Press, New York, 1983.
http://iupac.org/publications/pac/pdf/1990/pdf/6208x1631.pdf Acknowledgments Drs. James Butler, Stephen Hall, Andrew Dawes, David Cordes
Alec Bowcock for his entertainment
Yuliya Panfilova for her quantum dots
Pacific University Chemistry and Physics Departments
National Science Foundation, Holce Endowment, and Pacific University for funding Method of Moments Why Care? Quantum dots can be used in medical imaging, and even televisions.
Long term: use quantum dots as a biomarker to determine virulence of cancerous cells, based on use of nutrients (glucose).
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