Investigation of gold nanocrystals by ultrahigh vacuum cryogenic scanning tunneling microscopy.

摘要

Alkane thiol stabilized 28,000 amu Au nanocrystals (28k Au) supported on graphite and Au(111) substrates have been investigated using a newly-constructed cryogenic STM. STM measurements revealed that isolated nanocrystals are highly mobile on bare Au(111) and graphite, and cannot be imaged by STM even at cryogenic temperatures unless they are stabilized by inclusion in a nanocrystal film or by some surface defect. Using a Au substrate which was coated with a SAM of xylenedithiol to support the nanocrystals reduced their surface mobility to the point they could be imaged in isolation, but also reduced their overall binding to the surface.; Tunneling spectroscopy performed in conjunction with the STM imaging revealed that the charging energy of adding a single electron to a 28k Au nanocrystal, which has a diameter of 1.7 nm, plays an important role in the nanocrystal's electronic properties, and leads to a series of steps in the measured current-versus-voltage spectra. The semi-classical model of double-junction tunneling through a small center conductor was used to model this phenomenon which is known as the Coulomb staircase.; In some cases, a nanocrystal transferred to the tip and stabilized at its apex. Under these conditions, exceptionally clear Coulomb staircase tunneling behavior was observed. These spectra showed multiple charging steps and were in excellent agreement with the semi-classical model. Examination of the first derivative of the nanocrystal-on-tip spectra revealed small peaks in addition to the charging peaks. This additional structure was attributed to quantum size effect splitting of the electronic energy levels of the nanocrystal.