Electronic properties probed by scanning tunneling spectroscopy: From isolated gold nanocrystal to well-defined supracrystals

Abstract

Scanning tunneling microscopy and spectroscopy at 5 K have been used to determine the electronic properties of 7-nm dodecanethiol-passivated Au nanocrystals in three different configurations: isolated nanocrystal, self-organized thin films (few nanocrystal layers), and large three-dimensional well-defined thick films (over 30 nanocrystal layers) called supracrystals. The electronic properties of both thin and thick well-ordered supracrystals are analyzed in scanning tunneling spectroscopy geometry through $dI/dV$ curves and conductance mapping at different bias voltages. The single particles exhibit a typical $dI/dV$ curve with a Coulomb gap of \ensuremath{\sim}360 meV and a Coulomb staircase. The $dI/dV$ curve of the thin supracrystals presents a Coulomb blockade feature \ensuremath{\sim}100 meV narrower in width than that of the single nanocrystal but without well-defined staircase. On the contrary, the thick supracrystals exhibit a $dI/dV$ curve showing a large Coulomb gap with a Coulomb-staircase-like structure. Generally, the conductance mapping is found to be very homogeneous for both supracrystals. Nevertheless, for some bias voltages, inhomogeneities across individual nanocrystals appear. Additionally, some of these inhomogeneities seem to be related to the supracrystal surface morphology. Finally, these slight variations in the conductance mapping across individual nanocrystals embedded in the supracrystal are discussed in terms of high degree of nanocrystal ordering, low nanocrystal size distribution, and nanocrystal crystallinity.