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RESEARCH NEWS
Nanomanipulation by Atomic Force
Microscopy**
By F. Javier Rubio-Sierra, Wolfgang M. Heckl, Robert W. Stark*
The linking of our macroscopic world to the nanoscopic world of single molecules, nanoparticles and
functional nanostructures is a technological challenge. Researchers in nanobiotechnology face the
questions ªHow extract and analyze a single nano-object?º, ªHow to pick and place nano-objects?º and
ªHow to prototype a functional nanostructure?º. Here, nanomanipulation by an atomic force microscope
(AFM) in combination with optical manipulation by a microbeam laser offers a practicable solution. In
such a system, the AFM can be operated as a nanorobot for manipulation purposes allowing for nanometer
precision. A contact free manipulation is achieved by the laser microbeam.
1. Introduction
Technologies to control and manipulate objects on the
nanometer scale are thought to be key technologies for the
upcoming decades or even century. The problems we are
currently confronted with were already foreseen by Richard
Feynman in 1959 as he described the challenges of small scale
manufacturing in his famous and visionary talk Plenty of
Room at the Bottom' at Caltech. Nearly fifty years later, there
are two fundamental questions that are not yet fully answered:
ªHow do we write small?º and ªHow do we manipulate and
control things on a small scale?º. Science and technology have
made tremendous progress in providing us with various
methods to write on a small scale. Manipulating and controlling things on the nanometer scale, however, turned out to be
far more difficult. Handling nanoscale objects includes finding
these objects, tracking and moving them. Technologies are
sought after that allow one to pick and place or cut and fuse in a
completely alien environment. Thus, there are two important
lines of investigation towards nanoscale manipulation: nanopatterning, where strategies are developed that allow us to
generate arbitrary nanostructures, and nanotelerobotics,
where the nanoworld is translated into a virtual reality that
allows us to interact with smallest objects.
Current research towards nanopatterning includes nanostructured surface functionalization and directed self-assembly of nanoscale objects. Surface properties such as wettability,
adhesion, friction and specific chemical interaction can be
controlled with nanometer resolution.[1] Controlled positioning of particles, clusters or single molecules allows for the
investigation of new chemical processes, predefined molecule
interaction and the direct assembly of new chemical compounds.[2]
ADVANCED ENGINEERING MATERIALS 2005, 7, No. 4
Although various nanomanipulation methods have been
developed, systems based on scanning probe microscopes offer
the most versatile approach.[3] Among the other instruments of
the scanning probe microscope family the AFM has the widest
range of application, both for imaging and for nanomanipulation. However, standard AFM systems are restricted to small
working areas and operating speed efficiency is quite limited.
To enhance and expand the capabilities of the AFM for
manipulation we have developed a combined system. In this
system, we conjoin an AFM for high resolution imaging and
manipulation with video microscopy and non-contact ablation
by a ultraviolet (UV) microbeam laser for large scale manipulation of biological specimen.[4] This laser ablation method
relies on a locally restricted ablative photodecomposition
process without heating.
±
[*] F. J. Rubio-Sierra, Prof. W. M. Heckl, Dr. R. W. Stark
Ludwig-Maximilians-Universität München,
Kristallographie and Center for Nanoscience CeNS,
Theresienstr. 41, 80333 Munich, Germany
E-mail: [email protected]
Prof. W. M. Heckl
Deutsches Museum, Museumsinsel 1,
80538 Munich, Germany.
[**] We thank the German Federal Ministry of Education and
Research BMBF for financial support within the framework
program ªYoung Scientist Competition in Nanotechnologyº
(03N8706). FRS thanks the Cusanuswerk for financial support.
SEM image courtesy of Prof. G. Wanner, München. Chromosome samples courtesy of Dr. S. Thalhammer, München.
DOI: 10.1002/adem.200400174
2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
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Rubio-Sierra et al./Nanomanipulation by Atomic Force Microscopy
2. AFM as a Nanorobot
The main elements of an AFM are depicted in Figure 1a. The
AFM probe consists of a high aspect ratio tip attached to the end
of a micromachined cantilever. To obtain high-resolution
images, the tip is brought into mechanical contact with the
sample surface and the cantilever is deflected by the tip-sample
loading forces. The deflection of the cantilever can be measured
with high accuracy (about 0.1 Š), the lateral resolution is
limited by tip radius to less than 1 nm depending on the
imaging conditions. The relative position between probe and
sample is adjusted by three orthogonal piezo actuators. A laser
beam is focused on the cantilever and reflected onto a quadrant
photodiode in order to detect the deflection. The electronic
control unit reads the photo diode signal and runs a feedback
loop to control the tip deflection. Data are transmitted to a
personal computer for graphical representation of the sample
surface.
Unfortunately, the performance of the open loop controlled
piezo actuator is limited in an AFM. The dynamic response
restricts the operation speed of the microscope and nonlinearities such as hysteresis and creep limit the position reproducibility. These issues severely affect the microscope's nanomanipulation performance. A straightforward solution to
compensate for the nonlinearities is to operate the AFM in
closed-loop in all three dimensions. However, this does not
suppress the mechanical resonances of the actuator. Recently it
was demonstrated that modern control feedback designs can
dramatically increase the imaging speed of the AFM.[5] The
operation of several cantilevers in parallel is another promising
approach to increase the manipulation speed.[6]
A simple and straightforward manipulation procedure
includes the localisation and imaging of the area of interest,
followed by the blind' feed-forward execution of the manipulation task as defined by the user. Finally, success or failure of
the manipulation is determined by re-imaging the manipulated
area. However, this procedure has some drawbacks. First, the
state of the system may change during the manipulation
process, mainly due to drift processes. Secondly, due to the
strong non-linearities dominating the tip-sample interaction it
is difficult to know a priori the manipulation results. Frequently, time-consuming trial-and-error experiments are required. Different elements of virtual reality can assist the
human user to explore and manipulate nanoscale objects.
Haptic interfaces that generate a feed back of the tip-sample
forces provide the user with a ªfeelingº for surface structure
and forces during the manipulation process. [7±9] Entire
synthetic worlds facilitate the orientation within the nanoworld.[10]
3. Nanoscale Modification Methods by AFM
Several methods have been proposed for the local modification of surfaces by AFM. One powerful patterning strategy is
local anodic oxidation. The local growth of an oxide layer is
induced by the application of an electric current between tip
and sample surface. Under ambient conditions, the water
meniscus acts as an electrolyte. Electronic devices such as
nanowires, metal-oxide field effect transistors (MOSFET),
single-electron transistors and quantum point contacts have
been fabricated.[3] The control of the growth direction of
organic molecule monolayers with conducting properties is a
basic issue for the development of molecular electronics
devices that could be solved using a silicon oxide template
fabricated by means of local anodic oxidation.[11]
Chemical surface modification by AFM nanolithography
can be achieved by adding molecules to the surface, e.g. by dip
pen nanolithography,[12] or by substitution of surface components. In substitution lithography (nanografting and nanoshaving), self-assembled monolayers are removed by means
of mechanical desorption by the AFM tip followed by in-situ
replacement of a second component.[13]
Another prominent example for AFM nanomanipulation is
mechanical nanomachining.[14] In this manipulation mode the
loading force is increased beyond the limit for plastic
deformation of the material. The main parameters influencing
the machining process are the applied normal force and the
patterning speed. For a better control of the machining process,
the machining force can be modulated in the so-called dynamic
plowing lithography.[15] AFM nanomachining was demonstrated on a wide variety of materials including metals,[16]
semiconductors,[17] polymers,[18] and biomaterials.[19] Selective
removal of superficial layers to study the internal structure of
biological specimens has been achieved revealing the internal
structure of collagen[20] or a bacterial cell wall.[21] Mechanical
nanomanipulation was also used to extract small amounts of
DNA from human metaphase chromosomes.[22]
Fig. 1. (a) Scheme of an atomic force microscope (AFM). The sample is
raster scanned and a small nanotip probes the sample surface. A light
lever detection scheme is used to measure the deflection of the
cantilever due to the surface topography. (b) Actual design of the AFM
system placed on top of an inverted optical microscope.
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Rubio-Sierra et al./Nanomanipulation by Atomic Force Microscopy
We have built a nanomanipulator with a tailor-made
controlling system that includes robotic operation of the
system and haptic interfacing. Currently, the system can
accomplish the following tasks: (i) identification of region of
interest for manipulation; (ii) surface modification beyond light
diffraction limit; (iii) manipulation of nanoscale components;
(iv) material extraction in the micron and nanometer scale; (v)
high efficiency operation by user-friendly design. To fulfil
these requirements in a single instrument, different imaging
and manipulation techniques were combined.
Figure 2 shows a sketch of the nanomanipulation system for
the combination of AFM and UV-laser photoablation. The
tailor-made AFM system (Fig. 1b) is placed on top of an
inverted optical video-microscope (Axiovert 100S, Carl Zeiss).
An UV-laser microbeam (P.A.L.M. Mikrolaser Technologies
AG) is focused on the sample through the microscope objective
using the fluorescence illumination path. The high power
nitrogen laser is specified with a laser output of 337 nm, a pulse
width of 3 ns, a pulse repetition rate between 0 and 60 Hz and a
maximum pulse energy of 300 lJ. A digital signal processor
module (Adwin-Gold, Jäger Computergesteuerte Meûtechnik)
generates the waveform for AFM operation and nanomanipulation tasks. Two joysticks coupled to the system enable
interactive control by a human user. The first one is a springloaded joystick used to position the sample in the planar
coordinates. The second is a force-feedback joystick used to set
the vertical probe position while delivering a haptic signal to
the user. A Pentium PC with Windows 2000 communicates
with the digital signal processor, manages the joysticks and
runs a graphical user interface.
Figure 1b shows the design of the AFM system. To avoid
undesired coupling of the lateral and horizontal motion, planar
and vertical actuators are separated. The sample is positioned
by a piezoelectrically driven closed-loop XY positioner (P-517
PZT Flexure Stage, Physik Instrumente, Karlsruhe) with a
resolution of 1 nm over a 100 lm range. An additional Z
positioner (P-753 LISA NanoAutomation Stage Actuator,
Physik Instrumente) controls the vertical probe positioning
with a resolution of 0.1 nm over a range of 25 lm. Thus, position
repeatability of 5 nm for sample planar motion and 1 nm for the
probe vertical motion can be achieved over the full range.
The real-time routines programmed in the DSP allow timeefficient operation of the system for both imaging and
manipulation with a sample rate of 50 kHz. AFM routines
generate the scanning waveform, regulate the tip-sample
interaction, and communicate with the user interface for online
representation of the data. The manipulation routines generate
the waveform for the tasks predefined by the user and deliver
the signals from the joysticks for interactive user control.
All functionalities that are necessary for standard AFM
operation are available to the user. The capabilities of the
systems are extended by nanomanipulation modules. A
lithography module is used for surface modification experiments by predefining manipulation parameters and the
trajectory of the tip or laser focus. Alternatively, an interactive
manipulation module gives control of the system to the user
through the force-feedback joystick system. Thus, the user can
set the tip position and interaction with the sample while
receiving visual feedback of the tip position and directly feeling
the applied forces.
In order to compare the effects of mechanical and non-contact
laser ablation, two adjacent human metaphase chromosomes
were microdissected. As first step, the AFM cantilever was
placed on top of a spread of chromosomes using videomicroscopy. Subsequently, the chromosomes were imaged with
high-resolution to select two specimens for manipulation. After
the image was acquired, the operator took control of the system
using the joysticks, and both chromosomes were dissected.
The chromosome situated on the right was dissected six
times using the laser beam with a laser output energy of 0.7 lJ
and a pulse repetition rate of 60 Hz. The chromosome on the
left was microdissected three times using the AFM tip at
different load forces. As a last step, the chromosomes were reimaged to check dissection results as shown in Figure 3.[23] The
Fig. 2. Diagram of the nanomanipulator combining non-contact laser ablation, AFM,
video- microscopy and robotic control.
Fig. 3. Pseudo 3-dimensional representation of topographic AFM data showing human
metaphase chromosomes dissected by means of mechanical AFM dissection (left
chromosome, cuts A1-A3) and UV-laser photo ablation (right chromosome, cuts L1L6). Image range is 12x8 lm.
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2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
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4. Combined Laser and AFM Nanomanipulator
Rubio-Sierra et al./Nanomanipulation by Atomic Force Microscopy
RESEARCH NEWS
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Fig. 4. Topographic AFM image showing the dissection of a single chromatide of a human
metaphase chromosome (arrow). The insert shows a scanning electron microscope image
of an AFM tip after manipulation on a human metaphase chromosome. Biomaterial
adhered to the tip and could be used for subsequent biochemical analysis.
chromosome on the right was successfully dissected into seven
different pieces. The minimal cut width (full-width-at-halfmaximum-height) was 380 nm. The smallest isolated chromosomal piece had a width of 420 nm corresponding to a volume
of approximately 0.2 lm3. This remaining isolated volume
contains about 4000 DNA base pairs. The cut depth achieved by
mechanical dissection depends on the applied loading force.
The first two attempts did not completely dissect the chromosome. The third cut completely dissected the chromosome
yielding a cut width of 280 nm. The dimensions of the cut
correspond to the mechanical dimensions of the AFM tip. In the
AFM image of Figure 4, a single cut was performed on a human
metaphase chromosome by means of AFM dissection;[19] the
cut was so precise that only one chromatide of the chromosome
was dissected. After AFM dissection of chromosomes, usually
some biomaterial adheres to the tip (inset of Fig. 4). DNA thus
extracted by AFM nanomanipulation experiments has been
biochemically analyzed by means of a degenerate oligonucleotide-primed polymerase chain reaction.[22] Thus, mechanical
dissection of chromosomes by an AFM tip represents a
straightforward method to generate genetic probes.
Conclusions: Nanomanipulation by atomic force microscopy
provides a versatile technology for nanopatterning and
nanomanipulation. For the extraction of DNA from human
metaphase chromosomes an instrument that combines AFM
nanomanipulation with UV laser surgery proved to be of high
value. This experiment demonstrates that especially for
applications in nanobiotechnology nanomanipulation devices
are needed to provide additional feedback to the user. Haptic
interfaces or even an entire virtual world help the human
operator to explore and manipulate objects in an environment
that is very distant in scale. Future developments in nanorobotics will certainly facilitate to bridge this distance leading
to a deeper understanding of the nanocosmos.
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Received: November 12, 2004
Final version: November 29, 2004
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2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
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