The unique optical properties of semiconductor nanocrystals, or quantum dots, are well suited to a range of needs in biomedical imaging and sensing, but work so far has focused on toxic materials like cadmium selenide. Silicon, in contrast, is an abundant nontoxic element that can be readily synthesized into luminescent nanocrystals in nonthermal plasmas. Like all quantum dots, silicon nanocrystals have limited photoluminescent stability. Motivated by preliminary data that demonstrate a clear link between nanoparticle packing and ensemble photostability, we will perform a combined experimental and computational study of the influence of particle size distribution, particle packing order, and local environment on the photostability of films and clusters assembled from purified silicon quantum dots. The plasma-synthesized nanocrystals will be (i) dispersed in organic solvents using ligands of varied length, (ii) separated by size in a scalable fashion through density-gradient ultracentrifugation into highly monodisperse fractions, and (iii) assembled into ordered films and clusters through controlled drying. Correlations between packing density, crystalline order, local environment, and photostability will be established by independently measuring the microstructure with optical and electron microscopy while simultaneously measuring the time-dependent spectral response. To interpret these experiments, we will combine numerical simulations of non-equilibrium self-assembly in drying colloidal suspensions with a quantum-mechanical model that accounts for the influence of inter-particle interactions on the photophysics of closely packed nanoparticle ensembles.
This project represents the first comprehensive scientific study of the influence of particle interactions on the photostability of silicon quantum dots, and it will have significant impact on a number of diverse fronts. The striking visual signal of photoluminescence is a particularly accessible manifestation of nanotechnology at macroscopic scales, and we will exploit this to engage the public, including high school students from across the state of North Dakota. We also have a strong record of mentoring undergraduates, including women and underrepresented minorities, through challenging projects that culminate in high-impact publications. A significant portion of the preliminary data in this proposal were collected and analyzed by undergraduates. Motivated by the interdisciplinary scope of the research, the PI is also developing a new graduate course specifically designed to train doctoral researchers at the ?soft-hard? interface, which represents a paradigm for nanotechnology in the 21st century. Finally, the potential technological impact of the knowledge gained is tremendously broad, ranging from efficient solid-state lighting and solution-processed photovoltaics to fiber-optic cryothermometers and stable non-toxic fluorophores for live cell diagnostics.
