Developing an AFM-Based SECM System; Instrumental Setup,
SECM Simulation, Characterization, and Calibration
A. Davoodi,
a,c,
*
,z
A. Farzadi,
b
J. Pan,
a,
**
C. Leygraf,
a,
**
and Y. Zhu
d
a
Division of Corrosion Science, Department of Chemistry, and
b
Department of Mechanics,
School of Engineering Sciences, Royal Institute of Technology, SE-100 44 Stockholm, Sweden
c
Department of Materials Engineering, Tarbiat Moallem University of Sabzevar, 397, Sabzevar,
Khorasan, Iran
d
Windsor Scientific Limited, Slough, SL1 4HE, United Kingdom
An integrated atomic force microscopy/scanning electrochemical microscopy AFM/SECM system was developed as an in situ
local electrochemical probing technique. It consists of a dual-mode probe acting as an AFM cantilever and SECM microelectrode
to simultaneously obtain the topography and electrochemical current map of the same area. Two types of probes with different
geometries were used. The scan velocity and concentration profile of the redox mediator during the scan were simulated, using the
equations of convection–diffusion mass transport coupled with continuity and momentum in three dimensions under steady-state
and transient conditions. The temporal and spatial resolutions of the probes were investigated. It was found that, during a normal
scan rate around 1 Hz, the effect of convective transport is negligible and the SECM lateral resolution depends on the geometri-
cal parameters. With favorable geometry, a probe with a Pt microelectrode of 1 m diameter can distinguish two active sites with
a distance of at least 3–4 m. The paper also reports experiments for characterization and calibration of the AFM/SECM system.
Concurrent AFM and SECM images obtained on a gold band calibration sample verify the high-resolution capability of the SECM
of one or a few micrometers with optimized conditions.
© 2008 The Electrochemical Society. DOI: 10.1149/1.2943324 All rights reserved.
Manuscript submitted December 18, 2007; revised manuscript received May 19, 2008. Available electronically June 24, 2008.
In recent years, both atomic force microscopy AFM and scan-
ning electrochemical microscopy SECM have been applied to in-
vestigate localized electrochemical events such as biosensor, cata-
lytic activity, localized corrosion, etc.
1-23
AFM gives high-resolution
topographic information, and is able to precisely control the distance
between the sample and the probe, whereas SECM can provide de-
tailed information of local electrochemical activities due to anodic
and cathodic processes. SECM results can give valuable information
on chemical and electrochemical changes above and around local
active sites which can indirectly be an indication of local activity
variation related to local surface activities due to its heterogeneous
microstructure. Despite many successful applications, the spatial
resolution of SECM is lower than that of scanning probe
microscopy-based techniques such as AFM.
13
Therefore, a low lat-
eral resolution of SECM current imaging was a crucial issue. How-
ever, usually AFM lacks chemical specificity. Attempts were made
to solve the problem in different ways. Great efforts were made to
improve the spatial resolution of SECM by using AFM distance
control capability, which permits high-resolution data acquisition.
Different approaches have been reported for producing a bifunc-
tional probe acting simultaneously as an AFM tip and SECM micro-
electrode. Macpherson et al. reported the integration of an SECM/
AFM probe achieved by using a photolithography method.
13,14
In
this case, an oxide sharpened silicon nitride Si
3
N
4
AFM tip was
used and the gold-coated AFM cantilever was insulated with photo-
resist coating. Illuminating with an optical micropattern generator
opened the end point of the AFM tip for the SECM signal. This type
of probe was also used in dynamic force microscopy with magnetic
field excitation to recognize the enzymatic activities of a glucose
biosensor.
24
Moreover, the SECM/AFM Si
3
N
4
cantilever was com-
bined with integrated pyramidal tips.
18,19
The underside of the probe
was sputter coated with Cr and Pt to provide conducting AFM tips
capable of functioning as electrodes, as well as force sensors. The
electrochemical and electrical properties of dimensionally stable
Ti /TiO
2
/Pt electrodes have been investigated using this type of
probe.
18,19
In another approach, a combined SECM/AFM probe was fabri-
cated by a coating flattened and etched Pt microwire with electro-
phoretically deposited paint for insulation.
14-16
The steps involved in
the fabrication of SECM-AFM tips were i constructing a Pt micro-
wire electrode, ii etching the Pt to a small point, iii fabricating
the cantilever component of the SECM-AFM tip by compression,
iv insulating the electrode and cantilever by electrodeposition of a
paint, which retracts from the tip end during heat curing to expose a
small Pt electrode, and v fixating the probe to the AFM holder
using epoxy resin.
16-19
Meanwhile, Kranz et al. presented an ap-
proach to fabricate a microelectrode integrated in a standard AFM
tip.
20-22
The sequences of fabrication were i metal Cr and Au
coating of the Si
3
N
4
cantilever by sputtering, ii insulating with a
silicon nitride layer of the metal-coated AFM cantilever by plasma-
enhanced chemical vapor deposition, and iii focused ion beam
FIB cutting of the modified AFM cantilever. Simultaneous contact
mode imaging of a micropatterned sample with immobilized en-
zyme spots and imaging of enzyme activity was obtained using this
type of probe.
22
More recently, by using nanofabrication facilities
including several ionic and chemical etching steps of Si wafer and
following low-pressure chemical vapor deposition of silicon nitride
Si
3
N
4
and Pt, a platinum silicide Pt
x
Si
x
tip was produced for a
bifunctional AFM/SECM probe.
23
Moreover, taking advantage of
metal masking technology and FIB, a precise exposure of Pt-coated
silicon nitride AFM tip could be achieved to fabricate an array of
SECM/AFM probes.
25
To determine the benefits of integrated SECM/AFM measure-
ments, various numerical simulation methods have been employed
for the SECM part.
26-33
The simulation of SECM could assist in the
analysis of the influence of different electrochemical and geometri-
cal parameters on the experimental performance, such as the reso-
lution. For instance, to estimate and improve the resolution of
SECM, one can use simulations to study the influence of geometri-
cal parameters. Because the fabrication of bifunctional AFM/SECM
probes still is a challenging and expensive process, simulation could
also help to evaluate the performance and improve the probe design.
An adaptive finite element algorithm was used for simulation of
arbitrarily shaped SECM tips in two dimensions.
26,27
Steady-state
and transient amperometric SECM responses were simulated in
three dimensions with the boundary element method BEM.
28,30-33
The SECM amperometric experiments with a heptode microelec-
trode a Pt core microelectrode surrounded by quartz glass and six
microelectrodes at premier as a local probe were quantitatively ana-
lyzed by means of three-dimensional 3D numerical simulations
* Electrochemical Society Student Member.
** Electrochemical Society Active Member.
z
E-mail: adavoodi@kth.se
Journal of The Electrochemical Society, 155 8 C474-C485 2008
0013-4651/2008/1558/C474/12/$23.00 © The Electrochemical Society
C474
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