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Nanoscale materials are currently being exploited as active components in a wide range of
applications in various fields, such as chemical sensing, biomedicine, and optoelectronics.
While conventional spherical colloidal nanocrystals have shown promise in these fields due to
their ease of fabrication, processibility and salient optical properties, it may be envisaged that
more applications may emerge if nanocrystals can be synthesized in shapes of higher
complexity and therefore increased functionality. Shape control in both single- and multicomponent
systems, which greatly impacts their physical and chemical properties, however,
remains empirical and challenging. In this talk we will presents a systematic, surfactantdriven
hot injection method to synthesize CdSe seeded CdS nanoheterostructures with very
high yield. This was extended to other systems such as CdSe/CdTe and CdTe/CdS or
PbSe/PbS and Ag2Se/Ag2S via cation exchange techniques. In order to elucidate the reactivity
of the facets at the tips of such branched structures as a function of the shape of the arms, we
exposed the structures of various arm dimensions to controlled amounts of metal precursors
and discovered conditions in which the metal nanoparticle can be deposited precisely at the
tip of one of four arms with symmetric reactivity. In next, we will showcase the utility of such
branched heterostructures in applications such as light emitting diodes, photodetectors and
solar cells.
In the second part of this talk we will display thermoelectric power conversion based on
semiconductor nanowires. More than half of the energy generated worldwide is lost as heat.
Such ‘waste heat’ can originate from large point sources (e.g., industrial processes) or smaller
distributed sources (e.g., automobiles). Even partial recovery of this lost energy would have a
dramatic impact on the economic and environmental costs associated with the increasing
global appetite for energy. Nanoscale semiconductors are increasingly viewed as viable TE
materials capable of realizing a high TE figure-of-merit, or ZT, where nanostructuring can be
used to reduce phonon transport and, ideally, to simultaneously increase another key ZT
parameter, the Seebeck coefficient, via direct modify of the semiconductor electronic densityof-states
(DOS). In this part, we will discuss our synthetic efforts to fabricate lead
chalcogenide nanowires (NWs) with structures designed for optimized ZT. In an attempt to
combine these influences on ZT, we synthesize branching and diameter-controlled PbS, PbSe,
PbSeS and PbTe NWs. We utilize the catalyzed solution-phase growth method known as
solution-liquid-solid (SLS) growth that affords diameter control by tuning of the size of the
metal nanoparticle catalyst. Novel TE device platforms are fabricated and characterized to
enable correlating NW structure – diameter and branching – with measurements of Seebeck
coefficient, electrical conductivity, and thermal conductivity.
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