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A8.01 - Carrier Dynamics of Composite Silicon Thin Films: Silicon Quantum Dots Encapsulated within an Amorphous Silicon Matrix 
April 24, 2014   1:30pm - 2:00pm

Recent progress in understanding electronic wave functions in condensed matter nanostructures has led to an ability to synthesize isolated, quantum confined building blocks with a variety of tailored optical properties. No matter what optical gap is engineered and how cleverly exciton energy is redistributed, though, novel materials composed of such nanostructures need to also exhibit efficient carrier dynamics and energy transport-now the central issue in harnessing the true power of quantum dot materials for solar cells, light emission and many other uses. This has led to the consideration of quantum dots encapsulated within amorphous matrices, but such environments fundamentally change the nature of quantum confinement and so the optoelectronic properties of the dots. The relationship between amorphous matrix and the character of quantum confinement is computationally elucidated here with particular emphasis paid to the location and shape of electronic states near the effective valence and conduction band edges. For instance, valence band edge states tend to be localized within nanocrystals while conduction band edge states tend to reside at the interface between nanocrystals and the surrounding amorphous matrix. In addition, confined states within nanocrystals exhibit a ribbon-like electronic structure that can be explained in terms of crystalline symmetry and interface curvature. Finally, there exists a critical nanocrystal size below which quantum confinement is not possible. Understanding and designing to such properties is critical for optimizing device performance with respect to carrier injection, internal conversion and carrier transport. These key aspects of carrier dynamics are explored using an incoherent (Fermi Golden Rule) hopping model. As part of this analysis, hole and electron mobilities are estimated in the absence of phonon assistance, showing the significant role of the amorphous matrix in improving both.

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Keynote Address
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Fred Kavli Distinguished Lectureship in Nanoscience
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