SCAN-Lab Technical Notes 22: Approach Curve Topography Extrapolation Experiment

Introduction

In Scanning Electrochemical Microscopy (SECM), the current measured by the biased probe is dependent not only on the activity of the sample towards the redox mediator, but also on the probe to sample distance. In a constant height SECM measurement, when sample tilt exists, the probe to sample distance changes throughout the measurement, resulting in a change in current magnitude even for surface regions with the same activity. This effect can be counteracted prior to measurement through careful levelling of the sample, however this can be a lengthy process, and is often not enough to fully remove the sample tilt.

On the other hand, the Tilt Correction analysis function in the M470 software can be used to remove the effect of small amounts of sample tilt from the measurement upon its completion [1]. This is not without its own problems, for example in some cases the tilt is too large for the tilt correction to be applicable over the whole map. It is therefore beneficial to follow the tilt of the sample throughout the measurement.

While it is possible on the M470 to follow sample tilt using the Intermittent Contact (ic)-SECM option this technique is not always applicable, particularly for soft samples where the intermittent contact between the probe and sample can damage the sample, or for highly dynamic samples where the longer experiment time required by ic-SECM is restricting. In these cases, it is beneficial to first determine the background tilt of the sample and then use the height tracking option available on the M470 to follow this tilt during the measurement.

In the past determining the sample tilt for use in a height tracking measurement has required the use of another technique such as OSP [2], or even an initial ic-SECM measurement to determine sample tilt [3]. However, these have their own complications. Both techniques require users have access to a technique other than SECM to determine the sample tilt. When OSP is used the user must align the SECM probe and OSP laser, this can be non-trivial and lead to mismatch between the two positions.
While for soft samples even using ic-SECM to measure only an initial sample tilt to follow may not be possible. These complications can make the use of OSP and/or ic-SECM in this way poor choices.

In this application note, we introduce the newest experiment in the M470 software: The Approach Curve Topography Extrapolation Experiment (referred to throughout as Topography Extrapolation). Using this experiment a series of approach curves to the sample are used to determine the sample background tilt, allowing it to be followed during a height tracking measurement.