Electrostatic interactions at the nanoscale can lead to novel properties and functionalities that bulk materials and devices do not have. Here we used Kelvin probe force microscopy (KPFM) to study the work function (WF) of gold nanoparticles (NPs) deposited on a Si wafer covered by a monolayer of alkyl chains, which provide a tunnel junction. We find that the WF of Au NPs is size-dependent and deviates strongly from that of the bulk Au. We attribute the WF change to the charging of the NPs, which is a consequence of the difference in WF between Au and the substrate. For an NP with 10 nm diameter charged with ∼ 5 electrons, the WF is found to be only ∼ 3.6 eV. A classical electrostatic model is derived that explains the observations in a quantitative way. We also demonstrate that the WF and charge state of Au NPs are influenced by chemical changes of the underlying substrate. Therefore, Au NPs could be used for chemical and biological sensing, whose environmentally sensitive charge state can be read out by work function measurements.

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The use of nanoparticles for advanced applications critically depends on the control of the interfaces between the substrate, the intermediate/linking layer, and the nanostructures. While much work has been done to attach nanoparticles and to determine their properties, less effort has been devoted to the quality of starting surfaces and little is known about the impact of the attachment process on the pre-existing interfaces. In this study, the properties of interfaces of two model surfaces obtained by covalently grafting alkyl chains directly to oxide-free silicon surfaces, either via Si−O−C or Si−C bonds are compared with those currently obtained by attaching organic silane molecules (e.g., (aminopropyl)triethoxysilane) on oxidized silicon surfaces. Using FTIR, Raman spectroscopy, atomic force microscopy, and spectroscopic ellipsometry, we show that nanopatterned Si−O−C surfaces suffer some oxidation upon attaching nanoparticles, although they remain stable in ambient environments. In contrast, surfaces with Si−C bonds remain oxide free and remarkably stable during and after gold nanoparticle attachment. The attachment of AuNP to these oxide-free, stable semiconductor surfaces opens the way to applications sensitive to interface state quality.