Abstract
Nanotechnology and modern materials science demand reliable local probing techniques on the nanoscopic length scale. Most commonly, scanning probe microscopy methods are applied in numerous variants and shades, for probing the different sample properties. Scattering scanning near-field optical microscopy (s-SNOM), in particular, is sensitive to the local optical response of a sample, by scattering light off an atomic force microscopy (AFM) tip, yielding a wavelength-independent lateral resolution in the order of ∼10 nm. However, local electric potential variations on the sample surface may severely affect the probe–sample interaction, thereby introducing artifacts into both the optical near-field signal and the AFM topography. On the other hand, Kelvin-probe force microscopy (KPFM) is capable of both probing and compensating such local electric potentials by applying a combination of ac and dc-voltages to the AFM tip. Here, we propose to combine s-SNOM with KPFM in order to compensate for undesirable electrostatic interaction, enabling the in situ probing of local electric potentials along with pristine optical responses and topography of sample surfaces. We demonstrate the suitability of this method for different types of materials, namely, metals (Au), semiconductors (Si), dielectrics (SiO2), and ferroelectrics (BaTiO3), by exploring the influence of charges in the systems as well as the capability of KPFM to compensate for the resulting electric force interactions.
Highlights
We propose to combine Scattering scanning near-field optical microscopy (s-SNOM) with Kelvin-probe force microscopy (KPFM) in order to compensate for undesirable electrostatic interaction, enabling the in situ probing of local electric potentials along with pristine optical responses and topography of sample surfaces
We have demonstrated and quantified the benefits of combining s-SNOM with KPFM that enables both probing and compensating for local electric potential differences
Retract curves on contacted Au with varying US were taken to illustrate the influence of the electric potential difference (EPD) on the near-field signal, as the cantilever oscillation is damped by the electric force
Summary
Scattering-type scanning near-field optical microscopy (sSNOM) is a widely established technique sensitive to the sample’s optical properties, with a lateral resolution greatly exceeding the diffraction limit of conventional far-field microscopy. In sSNOM, the lateral resolution is characterized by the strong localization of the illuminated atomic force microscopy (AFM) tip’s hotspot, being in the order of ∼10 nm. KPFM witnesses a large variety of applications such as the investigation of local material contrast at grain boundaries or nanocrystal facets, local doping concentrations, charge transfer in metallic nanostructures, surface defects in Si (111), various quantum dots, and mono- and bilayer graphene quantum capacitance.37 As both KPFM and s-SNOM are based on an AFM operating in tapping mode, both methods, in principle, may work. Combining KPFM with s-SNOM so far has mainly been reported by our group, for instance, when optically inspecting ferroelectric domain structures in GaV4S838 and BaTiO325,39,40 at the nanometer length scale, as these materials show considerably large local surface charges due to their pyroelectric and insulating behavior Applying this combination of local electrical and optical analysis is recommended for any heterogeneous sample, in order to compensate for artifacts in the near-field signal signatures, as will be shown in this article. We thereby confirm that s-SNOM only probes the net near-field responses
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