VS-SECM (DC and AC) Scanning Electrochemical Microscope

• Most popular technique of VersaSCAN Scanning Electrochemical Workstation
• SECM controls and monitors electrochemistry at a sample and probe with 2-channel potentiostat
• Low Current Interface provides improved current measurement and ultimate resolution
• Includes AC-SECM technique
• Optional STYLUS as a soft probe for constant-distance imaging and biological applications

The VersaSCAN Scanning Electrochemical Workstation is a single platform capable of providing spatial resolution to both electrochemical and materials-based measurements.  This system integrates a positioning system, measurement instrumentation and probe to provide local information.  Traditional electrochemical experiments measure an average response over the entire electrode, electrolyte interface.  Rarely is a sample homogenous.  Samples often consist of local sites of passivate or active nature and sites of anodic or cathodic character.  This need to investigate localized phenomenon led to the emergence of scanning probe electrochemistry.

Positioning System:

• VersaSCAN is unique in the use of piezoelectric motors to drive the stages on all axes.
• Piezos are materials that move with very high precision, accuracy, repeatability but over small distances.  We use them to drive stages; therefore we reach a combination of scan area and scan resolution that is unmatched.
• AFMs do not move with the same scan range (usually piezo-element only; without the stage)
• Alternative positioning system designs use step motors, exclusive or as a basis for a hybrid system, and cannot move with the same resolution.

System Installation and Training:

This is full ‘white-glove’ service. An experienced team member arrives on site – unpacked the instrumentation, assembles, levels the system.  A developed checkout procedure is executed on a test sample to verify the system (installation).  The fundamentals of the technique are covered.  The users are trained to execute the same test (training).

Multi-Technique platform:

The VersaSCAN platform offers the ability to leverage the positioning system and software as common components for other scanning probe techniques.  The best-in-class measurement characteristics of AMETEK equipment combines with this unique positioning system for high-resolution measurements even at large areas.

The SECM integrates a positioning system, a bipotentiostat, and an ultramicroelectrode probe or tip.  The positioning system moves the probe close to the surface of the sample, within the local imaging zone. The bipotentiostat can polarize the probe only (feedback mode) or the sample and the probe independently (generator-collector mode), while measuring the resulting current(s). The probe is specially designed to have a specific tapered polish (per the RG ratio) and active radius below 100 microns. The positioning system scans the probe and charts position with measured electrochemical parameters, creating a data map of local current.

Approach Curves (feedback mode) are used as an electrochemical means by which to position the probe in Z sufficiently close to the sample to be in the local imaging zone. A DC voltage is applied to the probe, while the DC current response is plotted as the probe is incremented towards the sample. When the probe is sufficiently close to the sample (a distance of 2-4 times that of the probe diameter) the measured current at the tip transitions from a bulk response to a local response. Over a region of high conductivity, a Nernstian response provides a local current enhanced over that of the bulk current. However, local current is decreased relative to the bulk value over areas of low conductivity of the sample as mass-transport is hindered to the probe.

The common Generator-Collector Mode experiments are similar to a Rotating – Ring Disk Electrode (RRDE) experiment.  In both cases, one electrode undergoes a redox reaction generating an electroactive product that is subsequently measured (“collected”) at the other electrode where it undergoes another redox reaction.  A SECM monitors these reactions while adding the extra dimension of spatial resolution to the data.  Additionally, the system can easily change from Sample Generation – Tip Collection (SG-TC) to Tip Generation –Sample Collection (TG-SC) often by simply changing polarization levels.

Standard SECM probes are rigid and made of a noble metal, such as Pt, imbedded in glass.  The Soft Stylus Probe technology from LEPA-EPFL, Switzerland, offers a convenient method of controlling the probe to sample distance without the need for complicated feedback electronics.  These constant distance measurements are especially important when sample topography can affect the electrochemical response, such as in the case of corrugated surfaces.  In addition, the soft probe technique has been shown to be ideal for soft tissue samples and has been applied to the detection and staging of cancer.

A new feature of the VS-SECM is the capability to perform AC-SECM measurements.  The basic operation is the same as with traditional DC-SECM except that an AC signal, rather than a DC only signal, is applied to the probe.  The resulting AC response is measured and the impedance magnitude or phase is plotted as a function of probe position.  While DC-SECM always requires the presence of an electroactive species, often in the form of an added mediator, AC-SECM can be operated with or without such a species.  This technique has been used to map local electrochemical activity (e.g. formation of corrosion pits), determine sample topography of insulators, and probe local surface conductivity (e.g. surface defects).

Since the imaging mechanism of all forms of SECM is electrochemistry, the applications of a SECM are as varied as the applications of electrochemistry itself.  Some key applications include biological sensors, reaction kinetics, porous membrane studies, fuel cell catalyst evaluation, and corrosion mechanisms.

• For the determination of localized electrochemical events such as electrode kinetics, metal dissolution and studies of bio-logical samples

• Capable of Approach Curves experiments and both Feedback Mode and Generator-Collector Mode imaging modes

• AC-SECM technique operates without need for electrochemical mediator, Useful for corroding systems

• Constant-Distance operation in conjunction with a topographic measurement technique (e.g. OSP) or Constant Distance SECM with EPFL Soft Stylus Probe technology

 

VersaSCAN Positioning System

Specifications	VersaSCAN Base (VS-BASE)
Positioning System Range	X-axis:  100 mm Y-axis:  100 mm  Z-axis:  100 mm
Positioning Motor Technology	X-axis:  Piezoelectric motors,            no stepper-motor Y-axis:  Piezoelectric motors,            no stepper-motor Z-axis:  Piezoelectric motors,            no stepper-motor

SECM Measurement Technology

Specifications	VersaSTAT3	VersaSTAT3F
Max Current Output	±2 A (standard) up to ±20 A	±2 A (standard) up to ±20 A
Compliance Voltage	±12 V	±12 V
EIS Capability	1 MHz, Option	1 MHz, Option
Min Current Range	± 200 nA (6 pA resolution) down to ±4 pA (122 aA)	± 4 nA (122 fA resolution) down to ±4 pA (122 aA)
E and I Filters	200 kHz to 1 kHz,  including None	200 kHz to 1 Hz,  including None
Grounding	Grounded, Earthed	Floating, Isolated