The fabrication of stoichiometric (BixSb1- x)2Te3 thermoelectric films comprised of nanostructured building blocks were fabricated using solution processing compatible methods. Nanostructured films of n-type Bi2Te3, (Bi0.75Sb0.25)2Te3, and (Bi0.50Sb0.50)2Te3 and p-type (Bi0.25Sb0.75)2Te3 and Sb2Te3 TE thermoelectric materials were fabricated using intercalated and exfoliated stoichiometric bulk materials. Fine control was exerted over the composition and reaction to maintain the purity and effectiveness of the nanostructured alloys composition of the materials as they were deposited, pressed, and annealed in a Te-rich ambient. A Seebeck coefficient was measured to be -235 μVK-1 for the n-type Bi2Te3 films, and 262 μVK-1 for the p-type (Bi0.25S0.75)2Te3 films. Although limited by high resistance, due to cracking of the films, ZT was estimated to be between 0.8 and 1.69 for the n-type films and an order of magnitude lower for the p-type films.
Nanocrystal quantum dot photovoltaics and photodetectors with performance optimized by engineering the nanocrystals
size and the optoelectronic properties of the nanocrystal's chemical coating are reported. Due to the large surface-to-volume
ratio inherent to nanocrystals, the surface effects of ligands used to chemically coat and passivate nanocrystals
play a significant role in device performance. However, the optoelectronic properties of ligands are difficult to ascertain,
as the band structure of the ligand-capped nanoparticle system is complex and difficult to model. Using density-of-states
measurements, we demonstrate that modeling of electropositive and electronegative substituents and use of the Hammett
equation, are useful tools in optimizing nanocrystal detector performance. A new particle, the Janus-II nanoparticles,
developed using 'charge-donating' and 'charge-withdrawing' ligands distributed over opposite surfaces of the
nanocrystal, is described. The polarizing ligands of the Janus-II nanoparticle form a degeneracy-splitting dipole, which
reduces the overlap integral between excitonic states, and thus reduces the probability of carrier recombination, allowing
carrier extraction to take place more efficiently. This is shown to allow increased photodetection efficiencies and to
allow the capture of multiple exciton events in working photodetectors.
A series of nanocrystal and nanocrystal quantum dot taggant technologies were developed for covertly tagging and
tracking objects of interest. Homogeneous and heterogeneous nanocrystal taggant designs were developed and optimized
for ultraviolet through infrared emissions, utilizing either Dexter energy transfer or Förster resonant energy transfer
(FRET) between specific absorbing and emitting functionalities. The conversion efficiency, target-specific identification,
and adhesion properties of the taggants were engineered by means of various surface ligand chemistries. The ability to
engineer poly-functional ligands was shown effective in the detection of a biological agent simulant, detected through a
NC photoluminescence that is altered in the presence of the agent of interest; the technique has broad potential
applicability to chemical, biological, and explosive (CBE) agent detection. The NC photoluminescence can be detected
by a remote LIDAR system; the performance of a taggant system has been modeled and subsequently verified in a series
of controlled field tests. LIDAR detection of visible-emitting taggants was shown to exceed 2.8 km in calibrated field
tests, and from these field data and calibrated laboratory measurements we predict >5 km range in the covert shortwavelength
infrared (SWIR) spectral region.
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