In 1981, two research scientists from
IBM’s European research facility
in Rueschlikon, Switzerland, made
a discovery that would help change
the face of the world today. With the
invention of the scanning tunneling
microscope (STM),1 Gerd Binnig and
Heinrich Rohrer gave birth to a family
of techniques that have allowed
scientists to study materials from the
atomic level upward. Five years later
came an even more powerful invention,
the atomic force microscope (AFM).2 Binnig, now collaborating
with Cal Quate and Christoph Gerber,
produced the instrument that
was to become the father of scanning probe microscopy (SPM), spawning
a number of metrological tools that
were referred to as “the picks and
shovels of nanotechnology.” Unlike
the usual timelines of instrumentation
development, where an idea may
take 15 or more years to transition
from a concept to development tool
and finally to a routine analytical
solution, the AFM came along at
the start of the explosive growth
of the semiconductor industry
defined by Moore’s Law. In 1975,
Gordon Moore of Intel (Santa
Clara, CA) predicted that data
storage capacity and processing
power would double every two
years. To keep up with this model,
and sometimes exceed it, industry
required a means to locate and
measure the size and performance
of chips that were being packed
more and more densely onto silicon
wafers. The AFM had found
its first niche.
The advantage of the AFM
was its ability to study all types
of material in a variety of environments,
from air to liquid, ambient
pressure to ultrahigh vacuum. This
has led to AFM now being found in
the laboratories of multiple scientific
disciplines. Some of the most
exciting steps forward in recent
years have been seen in the life
sciences, where AFM techniques
have been used to enhance the
knowledge of molecules in vitro,
with the AFM probe being able
to look at individual or moleculeto-
There are many great scientists
who have spent their lives working
in SPM research, but few have
had the impact in applying these
technologies as Kumar Wickramasinghe.
In the 1980s and ’90s,
Dr. Wickramasinghe was working
in the T.J. Watson Research
Center (Yorktown Heights, NY),
where he pioneered many applied
techniques that were to benefit
IBM’s work in semiconductor
manufacture. Having been one of
the first scientists to apply tapping
mode for imaging surfaces, Dr. Wickramasinghe
developed it to invent
other now established techniques
such as magnetic force microscopy,3
electrostatic force microscopy,4 Kelvin
probe microscopy,5 scanning thermal
microscopy,6 and the apertureless near-field optical microscope.7
Freed now from project work, Dr.
Wickramasinghe has been able to
go back to his first love, practical
research. For the past few years, he
has been able to focus on new areas
in which to apply scanning probe
techniques. Working with postdoctoral
research fellow Kerem Udal,
Dr. Wickramasinghe has published
a paper that details his work using
AFM to sort and deliver molecules at
extremely high speeds.8
Rather than the AFM being applied
as a microscope, it is now being used
as a tool that acts like a printer,
writing onto a surface. In the late
1990s, the Mirkin group at Northwestern
University (Evanston, IL)
first applied AFM in a technique called dip-pen nanolithography
(DPN),9,10 which was developed to
put down a variety of chemical compounds
onto a variety of substrates.
According to Dr. Wickramasinghe,
DPN was analogous to a quill pen,
with little control of the deposition
rate since the mechanism of writing
was through diffusion. Also, control
was related to the speed at which
the pen was moved over the substrate,
and the process was
stopped and started by the
pen being lifted to and from
This latest approach, however,
works more like an inkjet
printer. A new type of probe is
used that is conical in shape;
where it connects to the cantilever,
IBM’s design incorporates
a reservoir to supply the
molecular ink. Not only can
the device write, but it can
also remove molecules from
Figure 1 a)- Scheme of an AFM probe used in
this study with the associated electric field. b) SEM
images of a modified cantilever.
Figure 2- Depending on the voltage pulse and
polarity, molecules (red) may be released from the tip
and deposited on the substrate.
Figure 3- The different mobilities of 5- and 16-
base DNA fragments enable their separation using
this AFM-based electrophoretic technique.
Figure 4- Topographic (a) and lateral force (b)
images showing controlled surface patterning with
5-base-long DNA fragments. Line scans (c and
d) show a mean height of 2.4 nm and line width of
between 59 and 79 nm.
The technique works because of
the thin film of water found on
the substrate and on the probe
itself (Figures 1–4 display the
ultrafast molecule sorting story). By
applying a field between a conducting
cantilever and probe and the
conducting substrate, it is possible to
exploit the electrophoretic mobilities
of molecules. Thus, by varying
field strength and polarity, it is possible
to precisely control deposition
and removal of the molecules
from the surface. When this is combined
with the positioning capabilities
of a modern AFM system, it is
possible to envision a method that
writes features five times smaller than
today’s e-beam lithography and 10
times smaller than photolithography.
It is the control of the thin film of
water through control of the humidity
of the experiment that makes this
Understanding the chemistry of
the process is also extremely important.
Close control of the probe
and surface chemistries is required
to ensure the molecules are immobilized to prevent diffusion. In
the experiments used to illustrate
this inkjet concept of writing, Dr.
Wickramasinghe’s team investigated
single-stranded DNA fragments.
Deposition was confirmed
using lateral force microscopy
(LFM) because it is particularly
sensitive to frictional change, thus
readily “seeing” the sticky molecules
on the smooth substrate.
The electrophoretic AFM method was
compared to traditional capillary electrophoresis.
The transfer times using
AFM were dramatically reduced: A
15-base-long strand of DNA could
be transferred in 5 msec compared to
170 sec in a conventional 8.5-cm-long
The potential impact of this work
ranks alongside some of Dr. Wickramasinghe’s
The method has enabled the acceleration
of molecular separation,
and transfer has been speeded up
by several orders of magnitude.
Dr. Wickramasinghe believes this
wi l l have an impa c t on futur e
research in biology and medicine.
For example, DNA sequencing will
be speeded up through reduction
in sample size. The method is very
scaleable using either multiple-tip
arrays or multiple reservoirs with
It is work like this that makes
exploring the field of SPM so exciting.
With nanotechnology already
being part of today’s world, it is the
work of scientists like Dr. Wickramasinghe
that emphasizes the benefits
of working on the nanoscale
to enhance the quality of life today
and for generations to come. So
what will the next 25 years bring?
The only restriction is the power of
Atomic Force Microscopy
AFM, the most common of the SPM techniques, is now routinely used for metrology
and surface characterization of a multitude of materials. Invented in 1986 by
Binnig et al.,2 AFM has developed beyond a tool to produce high-resolution topographic
information to one that is applied to many scientific disciplines, from semiconductors
to the life sciences. While most measurements are made in air, systems
are available that enable the user to study samples in different environments—
liquids to gases, from ambient to high vacuum.
Like all other SPM techniques, AFM uses a sharp probe (usually made from silicon or silicon nitride) moving over the surface
of a sample in a raster scan. The tip is at the end of a cantilever, which bends in response to the change of force between the
tip and the sample.
Most AFMs use an optical lever technique to detect flexure of the cantilever. A laser is focused onto the back of the cantilever
and is reflected onto a four-quadrant photodetector. This enables topography to be measured by the up–down movement
of the cantilever, while frictional or lateral forces may be measured by following the twisting of the cantilever (see diagram).
There are many different modes of AFM. The topographic modes use the
force interaction between tip and sample. In contact
mode, the tip is working in the repulsive region very close to the
surface where the cantilever is pushed away from the
surface. In the various noncontact modes, the cantilever is oscillated
at its resonant frequency and is attracted to the surface.
Change of amplitude or shift in frequency may be monitored to measure
the force interactions.
Specific properties may also be measured using AFM. For example, the tip may be coated with an appropriate metal to enable
properties such as magnetic or electrostatic field changes emanating from the sample surface. Again, vibrating techniques are
used, yielding simultaneous spatial topographic and material properties.
- Binnig, G.; Rohrer, H. Appl. Phys.
Lett. 1982, 40, 178–80.
- Binnig, G.; Quate, C.F.; Gerber, C.
Phys. Rev. Lett. 1986, 56, 930.
- Martin, Y.; Wickramasinghe, H.K.
Appl. Phys. Lett. 1987, 50(20), 1455.
- Martin, Y.; Abraham, D.W.; Wickramasinghe,
H.K. Appl. Phys. Lett.
1988, 52(13), 1103.
- Nonnenmacher, N.; O’Boyle, M.P.;
Wickramasinghe, H.K. Appl. Phys.
Lett. 1991, 58(25), 2921.
- Williams, C.C.; Wickramasinghe, H.K.
Appl. Phys. Lett. 1986, 49(23), 1587.
- Zenhausen, F.; O’Boyle, M.P.; Wickramasinghe,
H.K. Appl. Phys. Lett.
1994, 65(13), 1623.
- Unal, K.; Frommer, J.; Wickramasinghe,
H.K. Appl. Phys. Lett. 2006, 88, 183105.
- Piner, R.D.; Zhu, J.; Xu, F.; Hong, S.;
Mirkin, C.A. Science 1999, 283, 661–3.
- Hong, S.; Zhu, J.; Mirkin, C.A. Langmuir
1999, 15, 7897–900.
A native of Colombo, Sri Lanka, Hemantha Kumar Wickramasinghe was
educated at the University of London (B.Sc. and Ph.D. degrees
in electrical engineering in 1970 and 1974, respectively). After a
postdoctoral appointment in the Applied Physics Department at Stanford
University, California, he joined the Electrical Engineering Department
at University College London in 1978, gaining tenure in 1982. In
1984, he joined the IBM T.J. Watson Research Center (Yorktown Heights,
NY), where he held management positions in the Manufacturing
Research Department and the Physical Sciences Department. He was made an
IBM Fellow in 2000, and in 2001 moved to the IBM
Almaden Research Center (San Jose, CA) on assignment before accepting a
position as Senior Manager of Nanoscale Science and Technology
in the Science and Technology Department in September 2002. His final
role at Almaden was to serve as CTO of Science and Technology
before moving to an academic post at the University of California,
Irvine, where he now holds an Endowed Chair in Bio-Nano research.
His goal is to build a world-class activity in nanotechnology in the
areas of Bio-Nano (with a focus on developing instrumentation to study
chemistry of living cells at the molecular level) and in novel
instrumentation development for protein and gene sequencing.
goal further applies his interests in novel scanning probe microscopes,
near-field optics, storage technologies, and in situ measurements
that improve the yield and/or throughput of manufacturing lines to the
opportunities for bio uses of SPMs.
Dr. Wickramasinghe is a member of the National Academy of Engineering, and is a Fellow of the APS, Institute of Physics, IEEE,
and Royal Microscopical Society. He received the IEEE Best Paper Award (G-SU Transactions) in 1982, the V.K. Zworykin
Premium of the IEEE in 1983, the IEEE Morris E. Leeds Award in 1992, and the Distinguished Corporate Inventor Award,
National Inventors Hall of Fame, in 1998. He was chosen Centennial Lecturer for the APS in 1999. In 2000, Dr. Wickramasinghe
and Prof. Calvin Quate of Stanford University received the American Physical Society’s Joseph F. Keithley Award for their
“pioneering contributions to nanoscale measurement science through their leadership in the development of a range of nanoscale
force microscopes that have had major impact in many areas of physics.” Dr. Wickramasinghe has published over 150 papers and
has been awarded 50 patents.
Mr. Leckenby runs IMS Europe, a company specializing in the marketing of
science & technology instrumentation (De Bohun Ct., Saffron Walden,
CB10 2BA, U.K.; tel.: +44 1799 521881; e-mail:email@example.com. ; www.ims-europe.net). He first came across the world of scanning probe
microscopes in the late 1980s and has been fascinated by the growth and
potential of applications ever since. Mr. Leckenby has previously
articles to American Laboratory on subjects ranging from thermal
analysis to applications of SPM. Figures 1–4 of this article are
reprinted with permission
from Unal, K.; Frommer, J.; Wickramasinghe, H.K. Ultrafast molecule
sorting and delivery by atomic force microscopy. Appl. Phys. Lett.
2006, 88, 183105. Copyright 2006, American Institute of Physics. The
diagram shown in the “Atomic Force Microscopy” sidebar is courtesy of
Pacific Nanotechnology (Santa Clara, CA).