Abstract
We report on topography-induced changes of the localized surface plasmon resonance (LSPR) enhanced luminescence of gold tip on SrTiO3 nanostructures with apertureless scanning near-field optical microscopy (aSNOM) in tip-enhanced Raman spectroscopy (TERS) configuration. Our experimental and simulated results indicate that the averaged refractive index of the dielectric environment of the tip apex containing both air and SrTiO3 in variable volume ratios, is dependent on the topography of the sample. This reveals that the local topography has to be taken into consideration as an additional contribution to the position of the LSPR.
Highlights
The first contribution is the luminescence of the gold tip, which is an intrinsic property that can be attributed to the gold inter-band d-sp transitions. [8,12,13] This gold luminescence has a broad feature extending to high wavenumbers, broad enough to be considered locally flat within the spectral range of interest
In the spectral range under consideration, the localized surface plasmon resonance (LSPR) position is linked to the average permittivity of the environment at the very vicinity of the tip apex. [11,18,19] Imaging the LSPR position allows the study of changes in optical properties of materials at the nanoscale that are correlated with changes in the dielectric properties (ε) of the materials, which translates to changes in refractive index (n). [20,21] While our previous studies have shown that the LSPR shift can be exploited to image changes in refractive index of a material, they suggested an influence of the topography, which remained unexplained
In order to verify the quality of the SrTiO3 sample, grazing-incidence X-ray diffraction (GIXRD) was carried to characterize the crystallographic structure as can be seen in the Fig. 3
Summary
With the rapid growth in the field of nano-technology, there has been an increasing need for optical imaging techniques capable of investigating the material properties with a spatial resolution in the nanometer range.[1,2,3] Conventional spectroscopy techniques such as confocal Raman and infrared spectroscopy are limited by the diffraction limit to a couple hundred nanometers.[4,5] In order to overcome this limitation, techniques such as apertureless scanning near-field optical microscopy (aSNOM) were developed.[6] aSNOM combines the high spatial resolution of conventional scanning probe microscopy (SPM) with the high sensitivity of surface enhanced optical analysis.[7,8]. [9] As topography has previously been demonstrated to provide crucial contributions to the interpretation of scanning probe data e.g. in the case of piezoresponse force microscopy, [22] understanding its effect in refractive index imaging is crucial to avoid any misinterpretation of the results To assess this topography influence, we image the LSPR position on chemically homogeneous strontium titanate (SrTiO3) nanostructures deposited on platinized MgO (100) substrate. The quality of the SrTiO3 thin films obtained under these conditions is reported elsewhere.[24]
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