Scanning probe workstations

SVET/Vibrating Probe – M470.

A non-intrusive scanning probe electrochemistry technique particularly useful technique for mapping local electrochemical events in real-time.

SVP is a technique often exploited by researchers to investigate corrosion processes. For corrosion it is often referred to as SVET. SVET has been used extensively to look at the effect of exposure time on the corrosion processes occurring in real time.

Scanning Vibrating Electrode Technique (SVET), Vibrating Probe Technique, and the SVP470

Easily perform Scanning Vibrating Electrode Technique / Vibrating Probe measurements on BioLogic’s modular M470 with the SVP470 option.

Map local electrochemical events in situ: SVP/SVET a valuable technique for both corrosion and biological applications

Scanning Vibrating Probe (SVP) also known as Scanning Vibrating Electrode Technique (SVET) is a non-intrusive scanning probe electrochemistry technique. In SVP the probe is vibrated at a known amplitude perpendicular to the plane of the sample, to measure the local current distribution in the electrolyte above a sample. While SVP does not directly measure sample activity, it is influenced by the sample activity. The increased electrical sensitivity and system stability of the SVP470 makes it a particularly useful technique for mapping local electrochemical events in real-time.

With SVP, events occurring in dynamic samples, can be followed in real time: a factor that is highly advantageous for researchers wishing to follow electrochemical processes as they occur. This includes following corrosion propagation, coating efficiency, biological growth and healing, and more.

SVP has regularly been used by researchers to investigate corrosion processes. In this field it is commonly referred to as SVET. In particular, SVET has been used to look at the effect of exposure time on the corrosion processes occurring in real time. In the related field of coatings for corrosion protection, it has been successfully used to look at the effectiveness of protective coatings, including self-healing coatings, and Self Assembled Monolayers (SAM).

SVP is also well established in the field of biology and electrophysiology, for which it was originally developed to investigate current distribution at living cells. In these fields SVP is often referred to as vibrating probe. While SVET/SVP is well-established in corrosion and biology it also has potential in other fields and has also been used in energy research to investigate battery materials as well as photocurrents at semiconductors.

SVP measurement of gold point in space measurement in water.

 

 

A short introduction to the Scanning Vibrating Probe/Scanning Vibrating Electrode Technique

SVP uses the ohmic drop (iR drop) above an active surface to measure the local current in the electrolyte above the surface. The local current is related to the electrochemical activity; therefore, SVP measurements can be used to visualize electrochemical processes such as corrosion.

Real-time measurements of dynamic samples

Scanning vibrating probe experiments benefit from the ability to implement a sweep scan mode, meaning the probe does not need to pause at each measurement point. Repeated upgrades to the BioLogic SVP470 module have ensured that it now has excellent electrical sensitivity and system stability ensuring that experiments can be performed quickly and reliably, to clearly visualize anodic and cathodic regions of a sample. Scanning vibrating probe techniques with the SVP470, therefore, offer clear advantages for the measurement of dynamic samples in real time, as the electrochemical processes occur in situ and in vivo. It is even possible to sequence multiple SVP scans to automatically follow these processes as they develop, using the M470’s proprietary software.

Auto-tune capability for faster experimental setup

SVP depends on the vibration of the probe perpendicular to the sample to produce a sinusoidal current signal. This AC current is converted to a DC current by the application of a demodulation signal by the lock-in amplifier of the M470. The demodulation signal applied by the lock-in amplifier must be selected so that its phase maximizes the DC response. To remove the difficulties sometimes associated with determining the best reference phase it is possible to use Auto-Tuning with the SVP experiment. Aside from ensuring the maximum DC signal, the use of Auto-Tuning also reduces the setup time of the experiment.

How does SVP/SVET work?

A current can be produced above a surface either through natural activity, like that of a corroding sample, or by forced activity when a sample is biased galvanostatically. In both situations, a potential gradient exists above the active region due to the electrical double layer of the system. The potential gradient which exists, will be related to the sample activity and electrolyte conductivity. When dealing with a Point In Space (PIS) sample, the double layer diffuses out hemispherically and evenly around the active region. This gives rise to a situation like that shown below, where equipotential regions exist around the PIS.

Over a point in space, equipotential lines radiate out as shown. The probe vibration in z allows the probe to sample the electrolyte potential at different distances from the point in space.

In SVP the probe is vibrated perpendicularly to the plane of the sample by a piezoelectric actuator. Due to the existence of the potential gradient as the probe vibrates, it measures a different potential depending on its position in its vibration above the sample. The result of this is the measurement of an AC potential with a DC offset, which can be measured by the electrometer. The lock-in amplifier of the SVP system demodulates the sinusoidal current of the AC potential to allow the DC potential to be determined and plotted.

Handling SVP/SVET results

SVP results are presented in a number of ways. In the literature it is common for results from SVP measurements to appear as the raw potential data, qualitative plots showing the most cathodic to most anodic regions, and as a converted current density plot.

The potential measured by the SVP tip is theoretically related to the current flowing in the system by Ohm’s law: V = IR, where V is potential, I is current, and R is the resistance. However, the resistance of the system can only be roughly estimated giving a poor potential conversion. To practically convert the potential measured to the current flowing a calibration is performed using a PIS sample with the same experimental setup as the experiment of interest. This is measured at a number of different applied currents to produce a calibration curve for that particular experimental setup. Through this calibration the measured SVP voltage can be converted to the current density at the sample surface. Detailed information on how to perform this conversion can be found in SCAN-Lab TN#2: Practical methods to correlate the SVP voltage to a current at a sample’s surface.

An example calibration curve used in SVP/SVET experiments is shown.

Factors Affecting SVP/SVET

There are a number of factors which contribute to the quality of the resulting SVP measurement. These include:

  • Probe to sample distance: The current from the sample in solution radiates out, so that the measured current changes with changing z position. When the probe is too far from the surface, the effect of the sample will be negligible in solution. As the probe moves towards the surface the signal measured becomes stronger. Therefore, the probe should be positioned near the surface; however, care must be taken to ensure topographic changes do not result in probe crash.
  • Solution conductivity: In a low conductivity electrolyte, the decrease in potential difference with increasing probe to sample distance is slower than in a high conductivity electrolyte. This means that lower conductivity electrolytes result in a stronger measurement signal than high conductivity electrolytes. Furthermore, when a low conductivity electrolyte is used the probe can be positioned further from the surface than when a high conductivity electrolyte is used.
  • Sample activity: The signal measured by SVP is influenced by the sample activity. This naturally leads to samples with a stronger activity having a stronger SVP signal. In cases where the sample activity is low it may be difficult to measure a strong signal if used as is. In these cases, it is possible to bias the sample galvanostatically to increase the activity, and therefore increase the SVP signal. This is the approach typically used with point in space samples used to test and calibrate the SVP setup.

Glossary of terms

  • Ohmic drop (iR drop): The iR drop is related to the resistance of the solution in which the sample (electrode) is situated. The solution resistance results in different potentials being experienced at different distances from the sample. This can be demonstrated by equipotential lines from a point in space.
  • Reference phase: The vibration of the SVET probe results in an AC signal. The AC signal can be converted to the final DC signal using a lock-in amplifier. The reference phase is the phase applied by the lock-in amplifier for the demodulation of the measured signal.
  • Scanning Reference Electrode Technique (SRET): SRET is an early evolution of SVET. It is a non-invasive technique which maps the potential of a system through the use of a reference electrode probe. Unlike SVET the probe does not vibrate during the measurement, meaning it has reduced sensitivity compared to SVET.

Workstation Software

The Scanning Electrochemical Workstation software provides unique capabilities and interactivity in support of the Model 370 and Model 470 nanometer-resolution scanning probe microscopes. This highly ergonomic software has been designed to facilitate and improve the user experience and render work flows more efficient:

  • Improved data analysis, manipulation and interactivity
  •  Automatic measurement and sequencing functionalities.

Over 40 discrete experiments provided throughout, each with their own individual variations

Specifications

SVP Electronics

  • Signal chain:
    Phase-sensitive detection using microprocessor-controlled lock-in amplifier with a digital dual-phase oscillator and differential electrometer input.
  • Lock-in amplifier
    Software controllable gain range (1 – 105).
    Output time constant: 0.1, 1, 10 s
  • Differential electrometer
    1015 Ohms input impedance.
    Decade gain ranges: 0 to 80 dB.
    Common mode range: ±12 V
  • Vibration actuator
    One dimensional low voltage
    piezo-electric actuator
  • Vibration amplitude
    Software set from 1 – 30 μm
    perpendicular to sample surface
  • Electrochemical sensitivity
    Better than 5 μA/cm²
    (using standard PIS test approach)

Positioning system specifications

Stepper Motors

  • Scan Range (x,y,z): 110 mm x 110 mm x 110 mm
  • Minimal step size on all axes: 20 nm
  • Closed-loop positioning linear zero hysteresis encoder with direct real-time readout of displacement in x, y and z
  • Linear position encoder resolution: 20 nm.
  • Max. scan speed: 10 mm/s
  • Measurement resolution: 32-bit decoder @ up to 40 MHz

Piezoelectric element (for z-axis only)

  • Vibration range 20 nm – 2 µm peak to peak with 1 nm increments
  • Min. vibration resolution: 0.12 nm calculated (16-bit DAC on 4 µm)
  • Piezo crystal extension: 100 µm
  • Positioning resolution: 0.09 nm calculated (20-bit DAC on 100 µm)

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