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Keysight Technologies
Combining Atomic Force
Microscopy with Scanning
Electrochemical Microscopy
Application Note
Introduction
Scanning probe microscopy (SPM) techniques have found a broad range of applications in characterizing
the physical and chemical properties of the surface of interested materials. One technique that is
particularly useful for studying localized electrochemical activities at the solid/liquid and liquid/
liquid interface is Scanning Electrochemical Microscopy (SECM), which was introduced by Bard and
coworkers in 1986[1]. Laterally resolved, in situ electrochemical information of surface properties
can be obtained by scanning an ultramicroelectrode (UME) at a defined distance across the sample
surface. Even though it looks similar to other SPM techniques in the sense that it involves the control
of scanning a physical probe across a sample surface, the operation principle for a conventional SECM
is quite different. SECM characterizes the localized properties of the electrified solid/liquid and liquid/
liquid interfaces by monitoring the electrochemical current. When the electrode is swept across a
sample surface, changes in current allow imaging of insulating and conducting surfaces for topology and
reactivity information. A detailed introduction of SECM and its applications are found in a separate note.
PD Dr. Christine Kranz, Institute of Analytical and
Bioanalytical Chemistry, University of Ulm
Dr. Shijie Wu, Keysight Technologies, Inc.
The Bifunctional AFM-SECM Probe
SECM imaging is commonly operated in constant-height mode, where the tip
is held at a fixed position above the sample surface. The spatial resolution
of SECM depends significantly on the size and geometry of the UME and the
substrate-to-tip distance. One major drawback of constant height operation
in SECM is the lack of sufficient spatial resolution due to current dependent
positioning of the microelectrode and the convolution of topographical and
electrochemical information. The EC SmartCart probe offered by Keysight
Technologies, Inc. provides an innovative solution to this problem, which
directly integrates a micro- or nanoelectrode into an AFM probe.
This integrated probe (the AFM-SECM probe) maintains the functionality of
both AFM and SECM technique, by integrating a sub-microelectrode recessed
from the end of the AFM tip [2]. Consequently, the electrode is located at a pre-
Non-conductive
defined distance to the sample surface, determined by the height of the actual AFM tip
AFM tip (Fig. 1). Thus, in-situ (electro)chemical information on a wide range Exposed
Insulation electrode
of homogeneous or heterogeneous electron-transfer processes occurring at Metal layer
surfaces and interfaces can be simultaneously obtained during AFM imaging.
Figure 1. Microfabricated bifunctional AFM-SECM probe.
AFM-SECM Instrumentation The probe has a frame-shaped electrode recessed from the
AFM tip that governs the distance between the electrode
and the imaged surface.
The combined AFM-SECM system includes a standard Keysight AFM
(5500/7500) platform with a built-in bipotentiostat that controls the potential
of both the sample (for generation/collection mode) and the tip against the
same reference electrode. The bifunctional AFM-SECM tip is mounted to a
special SECM nosecone that plugs into a standard AFM scanner (Fig. 2). The tip
current, i.e., the current flowing through the tip is measured by a pre-amplifier
built into the SECM nosecone, which is located close to the tip itself to minimize
Figure 2. Combined AFM-SECM system includes an AFM control unit and a SECM unit. The EC SmartCart is pre-mounted on
a special nosecone that inserts into a standard AFM scanner from Keysight.
2
electromagnetic noises from the line. The major advantage of this setup is the
tip is pre-mounted onto a cartridge (bottom right picture of Fig.2), insulted and
tested in factory before it is delivered to the customer. The user only needs to
plug in the cartridge into the nosecone and is ready to start the experiment. This
allows the customer to be released from the challenging and time consuming
work that normally required in preparing for an SECM experiment, which
includes mounting the tip, making electric contact, insulating the tip, etc., thus
allowing the customer to focus on the research instead of on the setup.
AFM-SECM Probe Characterization and
Application Example
After a probe is mounted on the cartridge, the electrochemical performance of
the probe will be tested in electrolyte solution (0.1MKCl) containing a standard
redox mediator such as 10mM [Ru(NH 3) 6]3+. A typical CV of the probe (inset)
recorded in the AFM set-up is presented in Figure 3. The steady state current
varies with the actual size of the ring electrode. The noise of the measured
current is evaluated by measuring the redox current as a function of time at a
constant potential. Typical noise level of the measured current is about 10pA
(Fig. 3), allowing customers to perform low current experiments.
Figure 3. Combined AFM-SECM measurements based on AFM tip-integrated electrodes. Bottom: Simultaneously recorded
images showing the topography (left) of the Agilent logo deposited from platinum/carbon composite by an ion beam-
induced deposition (SEM image, middle) and the electrochemical image recorded in feedback mode SECM (right).
Results from a simultaneously recorded contact mode AFM and feedback mode
SECM experiment on a model sample are presented in Fig. 4 to demonstrate the
functionality of the combined AFM-SECM system. The model sample contains
Figure 4. Topography (left) and SECM (right) images of an Au/Si sample recorded in 1mM FeMethanol solution/0.1M KCl with a combined AFM-SECM
probe biased at 240 mV vs. Ag/AgCl. Topography image shows the deposited Au strip on Si substrate, and the SECM image shows a corresponding larger
current on the conductive Au surface.
3
conductive (gold stripes) and non-conductive (Si wafer) regions coexisting on
the surface. This Au/Si sample was imaged in contact mode AFM in a 1mM
FeMethanol solution/0.1MKCl solution with the AFM tip-integrated electrode
biased at 240mV vs. Ag/AgCl. The simultaneously recorded topography and
current images are shown in Figure 4. The SECM image was obtained in the so-
called feedback mode, while the sample was not biased during the experiment.
Due to the feedback effect, as explained above, the SECM current is smaller on
the insulating Si surface, and is larger on the conducting Au surface.
A second example of AFM-SECM imaging is present in Figure 5. The sample
is Pt-coated glass slide with FIB-structured patterns, an Agilent logo.
Images are recorded in AFM contact and SECM feedback mode in 10mM
[Ru(NH 3) 6]3+/0.1MKCl. The SECM image (left in Fig. 5) revealed some changes
in conductivity which are not clearly visible in the corresponding topography
image.
Figure 5. Images recorded in AFM contact and SECM feedback mode in 10 mM [Ru(NH3)6]3+/0.1M KCl;
sample: Micro-structured Platinum-coated glass slide with non-conductive star patterned.
The imaging power attainable by combining information on the surface
morphology with localized (electro)chemical data can be applied to a wide
variety of complex engineering and biological problems, ranging from
corrosion science to life sciences. For example, modification of the integrated
electrode surface with enzymatic biosensing interfaces results in imaging
amperometric nanobiosensors [3,4]. In addition, boron-doped diamond can be
used as electrode material [5], resulting in a combined probe with exceptional
properties in terms of robustness and potential window. The integrated
SECM functionality is not limited to amperometric experiments, also imaging
potentiometric microsensors (e.g. Ir/IrO x or Sb electrode for laterally resolved
pH measurements), or thin film amalgam microelectrodes (Au/Hg or Pt/Hg) for
imaging stripping voltammetry (e.g. for heavy metal detection) are envisaged.
4
Summary
This combined AFM-SECM approach with bifunctional probe provides
topographical and correlated electrochemical information with high spatial
and temporal resolution, thereby enabling the transformation of scanning
probe microscopic techniques into multifunctional devices useful in industrial
and academic environments for fundamental or applied interests. The
innovative probe design eliminates certain intrinsic drawbacks in conventional
SECM, providing high-resolution topographical information correlated with
electro(activity) information of the sample. The unique design of the SECM nose
cone with the pre-mounted probes on exchangeable cartridges allows users to
perform SECM measurements without having to deal with the time consuming
process of experimental setup.
References
1. A. J. Bard, F. R. F. Fan, D. T. Pierce, P. R. Unwin, D. O. Wipf, and F. Zhou, Science, 1991,
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