  Debye temperature decrease, residual resistivity increase and electron-phonon
coupling constant increase as crystallite size decreases have been found from
electrical resistivity in temperature range 5 K to 300 K of well-characterized
Ag nanoparticles synthesized with oleylamine, trioctylphosphine and/
polyvinylpyrrolidone with Scherrer sizes ranging from 15.1 nm to 33.4 nm.
Notably, about 36 % reduction in Debye temperature in 15.1 nm compared to bulk
Ag is found. Remarkably, usual phonon drag peak found in Seebeck coefficient
for bulk Ag turned into dips or phonon drag minima in these NPs that gradually
gets suppressed and shifted towards lower temperature with decrease in
crystallite size in oleylamine and trioctylphosphine-induced NPs.
Contrastingly, it appears at higher temperature in trioctylphosphine-induced
nanoparticles. A broad hump between 125 K to 215 K, a slope change near 270 K
in resistivity and an additional dip-like feature near 172 K in Seebeck
coefficient are seen in oleylamine-polyvinylpyrrolidone-induced nanoparticles
with different shapes. They are attributed to spatial confinement of electrons
and phonons, varying barrier heights, different charge-transfer mechanisms
among metal nanoparticles and surfactant/s, enhanced disorders (grain
boundaries, increase in fraction of surface atoms, surfactant matrix and other
defects), leading to modifications in their overall electron and phonon
interactions. Finally, their thermoelectric power factor has also been
assessed.
