Effect of nanoscale size and medium on metal work function in oleylamine-capped gold nanocrystals

• Gold nanocrystals coated with oleylamine surfactant have been drop casted onto gold.
• Surface potential measurements of gold clusters allowed to extract the work function.
• The dependence of the work function on the cluster size was characterized.
• The effect of a change in surrounding medium upon annealing was assessed.
• A general agreement with theory was observed in terms of energy band modifications.

The work function is an important material property with several applications in photonics and optoelectronics. We aimed to characterize the work function of clusters resulting from gold nanocrystals capped with oleylamine surfactant and drop-casted onto gold substrate. We used scanning Kelvin probe microscopy to investigate the work function, and complemented our study mainly with X-ray diffraction and X-ray photoelectron spectroscopy. The oleylamine works as an electron blocking layer through which the electrical conduction takes place by tunneling effect. The surface potential appears to depend on the size of the clusters, which can be ascribed to their difference in effective work function with the substrate. The charge state of gold clusters is discussed in comparison with theory, and their capacitance is calculated from a semi-analytical equation. The results suggest that at the nanoscale the work function is not an intrinsic property of a material but rather depends on the size and morphology of the clusters, including also effects of the surrounding materials.

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