  An important question in the physics of superconducting nanostructures is the
role of thermal fluctuations on superconductivity in the zero-dimensional
limit. Here, we probe the evolution of superconductivity as a function of
temperature and particle size in single, isolated Pb nanoparticles. Accurate
determination of the size and shape of each nanoparticle makes our system a
good model to quantitatively compare the experimental findings with theoretical
predictions. In particular, we study the role of thermal fluctuations (TF) on
the tunneling density of states (DOS) and the superconducting energy gap (D) in
these nanoparticles. For the smallest particles, h < 13nm, we clearly observe a
finite energy gap beyond Tc giving rise to a "critical region". We show
explicitly through quantitative theoretical calculations that these deviations
from mean-field predictions are caused by TF. Moreover, for T << Tc, where TF
are negligible, and typical sizes below 20 nm, we show that D gradually
decreases with reduction in particle size. This result is described by a
theoretical model that includes finite size effects and zero temperature
leading order corrections to the mean field formalism.
