Quantitative and Nanoscale Surface Potential Tracking of Ionic and Organic Adsorbates at sub-Monolayer Coverage
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Quantitative and Nanoscale Surface Potential Tracking of Ionic and Organic Adsorbates at sub-Monolayer Coverage L.M. Eng #, Ch. Loppacher, and U. Zerweck Institute of Applied Photophysics, University of Technology Dresden, Germany ABSTRACT We use an improved setup for deducing quantitative surface potential values by means of frequency modulated Kelvin-probe force microscopy (FM-KPFM). This method is sensitive to the electrostatic force gradient rather than the absolute force probed in KPFM so far, and therefore provides both a higher lateral resolution and quantitative values. Furthermore, FMKPFM allows using cantilevers with high spring constants which even favors both the stability and increased topographic resolution. Here, we apply FM-KPFM to deduce interfacial electrical properties of the sub-monolayer coverage of three adsorbates on metal substrates: lithium chloride films, Copper-porphyrines, and C60 molecules. INTRODUCTION Kelvin Probe Force Microscopy (KPFM) has proven to be a valuable method to measure local surface potential differences on the nanometer scale [1,2]. Measuring local potential values or their changes are of essence for any molecular or atomic adsorbate on top of a sample surface, or when inspecting interstitials and local variations due to defects and voids. They all result in a locally distorted electric field distribution giving rise to surface potential variations. Adapting the basic idea after Sir William Thomson later named Lord Kelvin of Largs [3] by vibrating a tapered conductor above the sample of interest, KPFM allows to record local potential differences while mapping the sample topography independently using a modified scanning force microscope setup. In such an experiment, the tip acts as the reference electrode yielding surface potential values between neighboring regions of interest down to ~10 mV resolution. Furthermore, calibrating the tip on a good test sample such as a flat gold sample partially covered with an alkanethiol monolayer [4] allows us not only to deduce relative potential values, but even to specify quantitative data. So far, the most widely used KPFM technique [1,5] bases on modulating the net force interaction between tip and sample by an externally applied voltage of the form U ≈ = U mod ⋅ sin(2π ⋅ f modt ) . The voltage U ≈ is added to an adjustable DC voltage U DC . With ∆Φ the workfunction difference between tip and sample and e the elementary charge, the overall tip-sample bias voltage U is given as follows: U = U DC − ∆Φ e + U mod sin(2π ⋅ f modt ) . The goal of the Kelvin controller is to balance any electrostatic force between tip and sample by adjusting U DC . The value of the contact potential difference U CPD between two areas 1 and 2 then is simply obtained by U CPD = U DC ,1 − U DC ,2 . The spectrum in fig. 1 shows the frequency distribution of these signals. In force amplitude modulated KPFM (AM-KPFM), U CPD is demodulated by a lock-in amplifier at f mod due to the variable electrostatic force interaction. The net DC force between tip
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