Measuring Local Charging and the Contact Potential Difference (CPD) of Surface Elements Using Kelvin Probe Force Microscopy

Topics Covered

Introduction to Kelvin Probe Force Microscopy
Sample Preparation
Experimental Procedure and Results
About NanoScan

Introduction to Kelvin Probe Force Microscopy

Kelvin Probe Force Microscopy (KPFM) is ideal for the measurement of local charging, and of the contact potential difference (CPD) between the sample and the tip. Besides providing true topography measurements, KPFM yields key insights into the work function of the features being imaged.

This article discusses the application of KPFM to measure local charging and CPD of surface elements. Compensating for the CPD in real time during scanning enables measuring the true topography of non-homogeneous surfaces.

Sample Preparation

The sample preparation involved the creation of topographical features out of various metals. At first, 30nm of aluminum was coated over a silicon substrate, followed by coating of 20nm of copper.

Then, an ion-sputtering technique was used to pattern the logo of NanoScan into the surface, etching unique parts of the logo in various metals. Schematic representation of the patterned sample is illustrated in Figure 1.

Figure 1. Schematic representation of the patterned sample

Experimental Procedure and Results

Scanning of the cantilever in KPFM is the same for a non-contact AFM image (Figure 2). At the same time, a bias was introduced dynamically between the sample and the tip at the frequency of the first overtone of the cantilever.

The cantilever oscillates owing to capacitive coupling with the surface directly underneath the tip.

Figure 2. Non-contact AFM topography image of the logo sample patterned by ion-sputtering. See below profile taken along dotted line. Image size is 32 x 10µm

The DC bias applied to the cantilever was continuously adapted by the KPFM feedback loop of the NanoScan Controller to negate this electrostatic oscillation. The feedback’s output is the CPD or KPFM signal.

This not only allows for performing comparative material contrast measurements on conducting as well as semiconducting surfaces, but also facilitates accessing true topography. The KPFM image depicted in Figure 3 presents the CPD between the surface and the tip material, showing clear contrast between silicon, aluminum and copper.

Figure 3. KPFM image representing the contact potential difference between the tip material and the surface. Image size is 32 x 10µm

Line profiles of topography (orange) and KPFM signal (blue) are illustrated in Figure 4, clearly revealing the difference in the contact potential of aluminum and copper. The inverted trend in the KPFM signal between the two materials cannot be due to the topography.

Figure 4. Line profiles of topography (orange) and KPFM signal (blue).


The results clearly demonstrate the ability of the KPFM to image CPDs of different materials on the nanoscale with millivolt resolution.

About NanoScan

With excellent know-how in the fast growing field of nanoscience, the main focus of NanoScan AG is on research and development of high-resolution and versatile magnetic and non-contact scanning force microscopes in high vacuum. These instruments are especially designed to fulfill the present and future analytical needs on nanometer-sized surface structures.

This information has been sourced, reviewed and adapted from materials provided by NanoScan.

For more information on this source, please visit NanoScan.

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