Research proposal
Pavel Chvykov
Environmental sensing using exciton decoherence time in quantum dots
Potential (V)
Semiconductor nanocrystal quantum dots have recently become popular for use in many areas of modern research including quantum computing, biology, and photovoltaics. The primary biological application is non-invasive tagging and imaging of the dynamics of organic processes, which provides insight on the functionality of organisms. The goal of this work was to devise a method, using only fluorescence spectroscopy of quantum dots, to detect the chemical composition of the crystal’s environment. This could vastly expand the applicability of quantum dots, and allow significant further insight into the biological experiments that already use dots for tagging. Additionally, this work could provide new information about the fundamental concepts of quantum decoherence and wavefunction collapse that are central to the proposed method. The quantum dots used consist of a ~10 nm diameter core of CdSe, with a thin shell of ZnS around it, which has a larger band-gap than the core1. This creates a spherical potential well that traps the exciton, thus resulting in a discrete excited energy level, and a single emission line2. Now, we place the dot in a chemical environment and continually excite it with a pump laser, causing it to continually (with exciton lifetime on the order of nanoseconds) emit fluorescence photons. When a sporadic electron from the environment is electron and Band gap hole temporarily (usually on the order of seconds) trapped wavefunction near the dot, the resulting electric field shifts the potential well of the dot2, thus increasing the portion of the exciton wavefunction outside of the crystal and Position (x) exposed to the environment (figure 1). Then, depending on the chemical activity of the environment, FIG. 1. The Stark shifted q. dot potential well with the electron and hole wavefunctions this exposed wavefunction should have a shortened decohrence time. This dependence can then be used for sensing. Therefore, the idea is to examine the correlation between decoherence time and the applied electric field in different media. This can be done via fluorescence spectroscopy on single quantum dots – the center of the emission line gives the electric field via the Stark effect2, while the width of the homogeneously broadened peak gives the decoherence time via the uncertainty principle. Thus, the first step is to filter out the non-homogeneously broadened spectra, and then to plot the spectral width vs. the central wavelength for ~500 spectra for each quantum dot, producing a linear regression plot. Such plots are then made for ~10 crystals in each medium, and the slopes of the linear regressions are graphed vs. the media. This way it can be expected that more chemically active environments will yield greater slopes. In order to measure the fluorescence spectra of a single q. dot, a solution of the crystals is drastically diluted, and then spin cast onto a glass substrate. In my experiments, gaseous media were used for simplicity. Figure 2 shows some of the results. It can be seen that the last graph shows no correlation between the environment and the slopes, and thus, further elaborations on the technique are necessary. There are many possible explanations for the lack of the expected correlation, as there were many parameters that were not controlled, and whose effect turned out to
Research proposal
Pavel Chvykov
FWHM
Slope
be important. In particular, the error bars on the second graph in Fig. 2 show that the measured slopes were conclusively different even for crystals in the same environment, thus indicating that the local anomalies of the environment of the individual crystals played a more important role than the overall medium. Additionally, the potential well of the dots slightly changes shape as the molecules of the medium diffuse into them, and thus the history of the particular nanocrystal also plays a role in the measured slope.
Central wavelength FIG. 2. The left graph is representative of the results for the fluorescence spectra of a single dot. The right graph shows the slopes of the regressions such as that on the left, measured for various crystals in the (left to right, separated by lines) Oxigen, air, Nitrogen, Argon and Helium gas environments.
There are many further directions for this work that could produce the desired results. One such direction is looking at liquid media, which are more chemically active and dense, and are also closer to actual biological applications. However, this would require another method for sample preparation, as the liquid washes the crystals off the substrate in the current setup. Such new method could also help make the local environment of the individual crystals more uniform and medium-dependent. Additionally, other properties of q. dots, such as blinking rate, could be explored for sensing purposes. Due to the importance of the practical applications of this work, and due to the scientific value of the fundamental concepts that could be further studies in the course of this investigation, this would be a very exciting project for me to pursue during my graduate studies. Additionally, the required equipment is relatively simple and inexpensive.
1
Anonymous Nanoparticles: from theory to application, Wiley-VCH; Wiley distributor, Weinheim; Chichester 2004. 2 S. Empedocles, M. Bawendi, 1999, Acc. Chem. Res., 32, 389.
