Lateral Force Microscopy
Nanosurf is a leading provider of easy-to-use atomic force
microscopes (AFM) and scanning tunneling microscopes (STM). Our products and
services are trusted by professionals worldwide to help them measure, analyze,
and present 3D surface information. Our microscopes excel through their compact
and elegant design, their easy handling, and their absolute reliability.
Several different lithography techniques exist that allow modification of
material surfaces during or after their microfabrication. One of the most
versatile of these techniques probably is the so-called Dip-Pen Nanolithography®
(DPN). DPN® is the nanoscale equivalent to writing with a fountain pen, in which
the tip of an atomic force microscope (AFM) cantilever acts as the pen (Figure
1). The "ink", which can consist of a wide variety of nanoscale materials, is
transferred from the tip to the sample surface through a water meniscus that
automatically forms between tip and surface at ambient humidity.
Figure 1. Principle of Dip-Pen Nanolithography® (DPN).
(Left) Loading: A cantilever is dipped into a nano-well of "ink" and is
retracted. (Right) Writing: The loaded cantilever is brought into contact with
the writing surface, and "ink" is being deposited through a self-forming water
meniscus. Images courtesy of Nanoink Inc.
The strength of DPN® lies in its high patterning resolution (15 nm) and
accuracy (5 nm). This way, it is possible to deposit new substances (e.g. Thiols
or other chemicals) onto a surface in a highly controlled manner and on a tiny
scale, resulting in exciting new applications. The technique of Dip-Pen
Nanolithography® was reported by Professor Chad Mirkin at Northwestern in 1999,
who was awarded the patents for the process. The exclusive license for the DPN®
technology resides with NanoInk, Inc., which is the sole provider for DPN®
equipment. The characteristics of materials deposited by DPN® are usually
studied by Lateral Force Microscopy (LFM), as it is one of the few techniques
capable of detecting material differences at such high resolutions. The Nanosurf
easyScan 2 FlexAFM offers LFM in combination with easy handling, making it
an obvious choice for DPN® analysis.
All measurements were performed with a Nanosurf
easyScan 2 FlexAFM Large Scan (100 µm scan range) scan head equipped with a
CONTR type cantilever and operated in Lateral Force mode in air.
Lateral Force Microscopy
Lateral Force Microscopy allows areas with different frictional attributes to
be distinguished. Differences in frictional attributes can arise through
differences in viscosity, elasticity, adhesion, capillary forces, surface
chemistry, or electrostatic interactions of the materials involved. When a
cantilever is scanned statically and perpendicularly to its longitudinal axis, a
torsional bending of the cantilever occurs. The angle of torsion is proportional
to the lateral force acting on the tip. When moving over a flat surface with
regions showing different frictional attributes, the angle of torsion will be
different for each region. These regions with different friction attributes can
therefore be mapped, and their properties analyzed (Figure 2).
Figure 2. AFM recordings on Alkanethiol molecules
deposited on gold using Dip-Pen Nanolithography® (DPN), NanoInk's patented
process for deposition of nanoscale materials onto a substrate. (Left)
Topography data. (Right) Lateral force data. The combined scan area of the two
image halves corresponds to 1.0 µm x 1.0 µm.
As the cantilever load normal to the surface has a lateral component at
inclined surface features, surface topography has an influence on the lateral
force measurement. Fortunately, it is possible to distinguish between lateral
deflection due to topographic features of the surface and due to frictional
forces by simply comparing the forward and backward scan of the AFM images. The
lateral deflection due to frictional forces changes sign while the one produced
by topography does not (compare Figure 3, left and right).
Figure 3. Differences between lateral forces caused by
friction and the ones caused by topographic features of the scanned surface.
(Left) Mirroring of lateral deflection due to frictional forces. (Right) No
mirroring with topographically induced lateral deflection. All forward scan
traces are in blue, backward scan traces in red.
Depending on the elasticity of the sample, the sample surface can be deformed
during measurements in lateral force mode, especially when it has steep inclines
or high topographic features. This deformation may lead to artifacts in the
measurement or even to damage of the sample due to the excessive lateral forces
that arise from these features. To avoid these deleterious effects, make sure
the effective stiffness of the surface is higher than the cantilever's torsional
LFM has been successfully used to investigate surface contaminations,
chemical specifications, and frictional characteristics of materials such as
semiconductors, polymers, thin films, and data storage devices.
For more information on this source please visit Nanosurf