Principles of Nanomanipulation
The Manipulation Mechanism
Fabrication of 2D and
Moving and Manipulating
Cutting and Bending
Materials With An AFM Tip
for Prototyping Devices
and Layered Fabrication
Embedding of Nanostructures
Atomic force microscopes (AFMs)
are most often used for high-resolution imaging and detailed surface
characterization, but soon after their invention it was recognized that
they could also be used to change, interact with, and control nanoscale
matter. A well-known early example of this was the IBM logo written
with Xenon atoms by Don Eigler‘s group at the IBM Almadén Research
Center. Lars Samuelson’s group at the University of Lund
suggested it would be possible to build nano-objects with larger,
molecular-sized building blocks and assemble them with an AFM in
A group at the University of Southern California’s Laboratory for Molecular Robotics (LMR) headed by
Aristides Requicha and Bruce Koel has been investigating this approach
for several years. Their research focused on the development of
high-level systems for programming an AFM as a sensory robot, and the
application of these systems to challenging nanomanipulation problems,
such as building prototypes for nanosystems.
AFMs are designed to work as an
imaging tool based on feedback control. A special software solution was
needed to minimize the interaction between the tip and the substrate,
thereby allowing AFM manipulation. The LMR researchers developed
NanoMove manipulation software based on the Application Programming
Interface (API) for Bruker's AFM
systems. NanoMove allows AFM manipulation using a variety of
protocols and the acquisition of various signals.
This article reviews the
research conducted by the LMR group. Their work shows that
nanomanipulation offers great advantages for computer scientists
studying nanorobotics issues, as well as by chemists and physicists
An AFM tip can be used via
different mechanisms to modify surfaces with nanometer resolution.
Tasks such as pushing and pulling or cutting and indenting can be
performed, and nanoscale objects can be mechanically moved by the AFM
probe tip. The AFM tip can serve as a robotic hand to precisely
position nano-objects and assemble them under computer control.
AFM manipulation does pose an
interesting problem in robotics, however. It can be likened to a mobile
robot (e.g., a helicopter) mapping a terrain and navigating over it by
using only altitude radar and dead reckoning in the presence of large
spatial uncertainties. The nanorobotic system includes substrates that
serve as nano-workbenches on which to place the objects to be
manipulated (analogous to fixtures in the macrorobotics world); tips,
probes, and molecules that serve to grasp others, and function as
grippers or end-effectors; chemical and physical nanoassembly
processes; primitive nanoassembly operations (analogous to
macroassembly operations like peg-in-hole insertions); methods for
exploiting self-assembly to combat spatial uncertainty (analogous to
mechanical compliance in the macroworld); hardware primitives for
building nanostructures; and software for sensory interpretation,
motion planning, and acting (i.e., driving the AFM).
The LMR researchers developed
methods for positioning colloidal nanoparticles (typically gold
colloids with diameters 5–30nm) accurately and reliably on mica and
silicon substrates, in ambient air or liquid environments. The
experiments were conducted with a CP-Research AFM n TappingMode using
triangularly shaped silicon cantilevers with a spring constant of
approximately 13.0 N/m-1 and a resonance frequency around 340 kHz. These
relatively stiff cantilevers showed the best results for mechanical
Manipulation of the gold
nanoparticles was performed by utilizing the LMR group’s NanoMove
software. The software adds several unique features to the instrument
to enable manipulation. NanoMove gives the user the ability to 1)
control the feedback operation, 2) perform one-line scans in any
arbitrary direction in the X-Y plan; and 3) acquire various signals
simultaneously with the manipulation.
Figure 1 shows the NanoMove
user interface. The upper right menu shows the main control and
operation window for imaging and manipulation.
The NanoMove software user interface.
After recording a topography
image, the user can draw an arrow on the image (see the red arrow in
the upper left window) to determine the manipulation trajectory. The
arrow dictates the direction and length of the scan line, and can be
moved by the operator in the X and Y direction until the displayed
topography indicates that its path is centered over the particle. Two
bars are positioned along the scan line, showing the range of
alternative operating conditions for the AFM, where the ”start” and
”end” points of the manipulation can be selected. The feedback is
turned off just before the tip scans across the particle, and is
switched back on after reaching the desired lateral position (see
Imaging takes place with the feedback control on. Turning off the
feedback allows the mechanical pushing of the nanoparticle.
A contact mode setup is
recommended in case the particles and structures one wants to
manipulate are strongly attached to the underlying surface. There are
several different manipulation protocols in the NanoMove software. One
can select either a feedback-off protocol, with or without additional
direct movement of the scanner, or a feedback-on protocol, with
indirect movement of the scanner. In these experiments, the TappingMode
setup was chosen because the nanoparticles cannot be imaged accurately
in contact mode, and are replaced during imaging due to the presence of
lateral shear forces.
Figure 3 shows the manipulation
of a 30nm diameter nanoparticle by using the appropriate operation
parameters, the particles can even be pushed up a 10nm step on the
surface. The step height and the particle size are of the same order,
and thus the experiment is a first step towards mechanical construction
of three-dimensional structures.
Figure 3. A
30nm gold particle (a) before and (b) after being pushed over a 10nm
high step along the direction indicated by the arrow. Image sizes are
both 1μm x 0.5μm.
When going down to the
nanometer scale the physical forces that are dominant in the macroscale
become negligible. AFMs provide the ability to study the mechanics in
nanoscale. By looking at various tip signals (e.g., the amplitude and
the deflection signals) during the manipulation operation and analyzing
the changes, it is possible to study the mechanism of the manipulation.
In a series of papers the LMR
group studied the AFM manipulation phenomena involved in pushing a
nanoparticle. They observed that when the tip is oscillating relatively
far from the surface, the amplitude decreases as the tip approaches the
particle but the particle does not move. When the tip is sufficiently
close to the surface, the vibration amplitude goes to zero as the
particle is approached. The DC cantilever deflection becomes non-zero,
and the particle moves, as long as the deflection is above a certain
threshold dependent on the cantilever and various other characteristics
of the setup. The changes in vibration amplitude and cantilever DC
deflection can be used to monitor the manipulation in real time, and
without further imaging verify with a high degree of confidence that it
is successful. The studies showed that the manipulation of
nanoparticles takes place by sliding and not by a rolling mechanism.
Fabrication of 2D and 3D Nanostructures
Nanoparticles are attractive
building blocks for nanostructures because 1) there are many known
methods for synthesizing nanoparticles with a variety of
characteristics (e.g., metallic, semiconducting, or magnetic) and the
state of the art is steadily improving; 2) the particles have more
uniform sizes (i.e., are more monodisperse) than structures of
comparable sizes made by competing techniques such as electron-beam
lithography; and 3) arbitrary planar patterns of nanoparticles can be
built by nanomanipulation using the protocols discussed above.
Moving and Manipulating Nanoparticles
AFM manipulation can be a tool
for the fabrication of nanoparticles patterns. Figure 4 shows an
example of manipulation of randomly deposited gold particles on a mica
substrate. The 15nm diameter gold particles were pushed from an initial
random position to form the USC logo.
Figure 4. A
random pattern of 15nm gold balls that was converted into the "USC"
logo pattern by a sequence of pushing commands.
The LMR researchers also
studied the possibility of using nanomanipulation for data storage.
Figure 5 shows the construction of a pattern that encodes ASCII
characters in horizontal rows of nanoparticles on a surface. The
presence of a particle at a node of a regular 2D grid is interpreted as
a "1," and its absence as a "0." The pattern, read from top to bottom
encodes "LMR." The particles have diameters of 15nm, and the grid nodes
are spaced with a 100nm pitch. The real density is on the order of 60
Gb/in2 and it should be possible to increase this density by
over an order of magnitude using smaller particles and tighter spacing.
This would give entities approaching the Tb/in2. This digital storage
technique is a candidate for an editable NanoCD. However, there are
obstacles that must be overcome for it to be practical.
Figure 5. a)
The characters "LMR" ASCII encoded in rows of nanobits, and b) the
trace obtained by reading the second row with an AFM.
The manipulation approach can
be extended to 3D fabrication. The LMR group demonstrated the
construction of a 3D structure by controlled manipulation of single
nanoparticles (see Figure 6).
Figure 6. 3D
image projection of a pyramid like structure. The preparation took
place by pushing a 30nm nanoparticle up between two others.
However, the manipulation of
asymmetrically shaped features is more complicated. Gold nanorods,
100nm in length and 10nm in diameter, were used by the LMR researchers
to study the manipulation of elongated nanoobjects (see Figure 7).
Figure 7. A
sequence of SFM images (500nm x 500nm scan size) displaying the
manipulation of four gold nanorods. The arrows in each image show the
manipulation direction that results in the rod configuration in the
next image: (a) Initial arrangement of the rods; (b) result of
translational manipulations of "1" along it s longitudinal axis and "2"
acrossthis axis; (c) results of rotational manipulation operations of
all four rods by 45°, relative to their original orientation. The
height scale from black to white is 10nm.
Translational manipulation of a
nanorod without rotation takes place whenever the tip hits the nanorod
at its center. In these experiments, it was found that it was easier to
translate the rods when the pushing direction was along the
longitudinal axis than when pushing was done transverse to this axis.
This is because it is easy to locate the highest point across the width
of the rod (which is the center of the rod). Therefore, the result of
longitudinal manipulation was often perfect translation while
transverse manipulation often caused a combination of translation and
rotation. This information on rod manipulation is important for
assembling a functional nanostructure. It will be somewhat complicated
to build such structures with nanorods, because after the rods are
roughly positioned; subsequent movements may have to specify both
position and angle of the nanorods.
Cutting and Bending Materials With An AFM Tip
As mentioned above, an AFM tip
can also be used to cut or bend soft materials, such as polymers, DNA,
and nanotubes. Figure 8 shows the cutting operation of DNA plasmid. It
can be seen that the cutting result is too coarse. A better approach
would be to use enzymes to cut the biological sample in combination
with an AFM probe to select the exact site for the modification. Figure
9 shows an AFM tip that is under NanoMove control bending a nanotube.
Using an AFM tip to cut plasmid.
Using an AFM tip to bend nanotubes.
Using Nanomanipulation for Prototyping Devices
Manipulation of nanoparticles
can also be used to build prototypes of electronic and optoelectronic
devices. In fact, many of the existing nanoelectronic devices have
either relied on chance to place an element in the desired relationship
with others or have used AFM manipulation. For example, placing a
nanoparticle at tunneling distances between two electrodes (source and
drain) can be used to make a single-electron transistor (SET). Figure
10 shows AFM images that were taken during the manipulation of two
particles into a gap of a SET structure.
Steps in the manipulation of two gold particles into a single electron
transistor (SET) junction.
A similar approach was used for
another prototype system. The LMR group and the Atwater
group at Caltech collaborated on the fabrication of a “plasmonic”
waveguide by placing colloidal 30nm diameter gold nanoparticles at
equal distances from each other in a chain, with a 100nm fluorescent
latex particle at the end of the chain. Energy at a wavelength in the
visible range is injected into the gold particle at one end of the
chain, and propagates through the chain by exploiting near-field
effects. The propagation is detected by observing the fluorescence of
the latex ball. The waveguide can also be constructed by using e-beam
lithography to fabricate gold nanostructures, but the AFM tip is still
needed to manipulate the latex fluorescent bead to the end of the
structure (see Figure 11). Therefore, the usage of the AFM manipulation
is crucial for the construction of the prototype. This nanowaveguide is
unique because it has transverse dimensions much smaller than the
diffraction limit for the wavelengths (hundreds of nm) that are being
studied. It may also serve to feed light to individual molecular
machines without exciting other machines in the same neighborhood.
Plasmonic waveguide: (a) Schematic of a plasmonic waveguide, (b) SEM
micrographs of e-beam lithography fabricated gold nanostructures, and
(c) AFM image of a latex bead (marked by yellow arrow) that was
manipulated to the end of the gold nanostructures matrix.
Solid Nanostructures and Layered Fabrication
Though patterns of unlinked
nanoparticles can be useful, many applications require “solid”
nanostructures of specific shapes. These can be approximated by groups
of suitably positioned and linked nanoparticles. The LMR group
investigated several approaches to linking. The first uses covalent
bonding to a linker. For example, gold particles can be connected with
dithiols (organic molecules with sulfur at both ends). The dithiols
self-assemble to the gold and serve as chemical glue. Two variants of
this approach were demonstrated: 1) depositing the particles,
positioning them, and then immersing the sample in the dithiol solution
to link them; or 2) depositing the particles, applying the dithiols,
and then manipulating the particles into linked contact. It was found
that it is indeed possible to push a group of nanoparticles linked by
dithiols as a whole. These results demonstrate hierarchical assembly at
the nanoscale (i.e., the construction of assemblies of components,
which are themselves subassemblies of other components or of primitive
The second approach to linking
also uses selective self-assembly. Additional material is deposited on
the particles until they become connected. The material and
experimental conditions must be selected to ensure that the material
assembles to the particles but not to the remainder of the sample. For
example, a pattern of gold nanoparticles can be used as a template for
the electroless deposition of additional gold (see Figure 12).
SFM images (1μm x 1μm scan size) displaying 8nm gold colloidal
particles on SiO2 as randomly deposited (left), after manipulation of
thirteen particles to form a wire nanotemplate (center), and after 5
minutes in the seeding solution (right).
Gold wires of arbitrary
geometry can be built by first manipulating the particles into the
desired geometry and then linking them by immersion of the sample in
the electroless solution with a specific set of parameters, such as
immersion time, concentration, and so forth.
A third approach discovered
very recently uses sintering to connect fluorescent latex
nanoparticles. The particles are first manipulated to form a desired
template. The template is then heated, melting the particles together
into a single nanostructure (see Figure 13).
Sequence of AFM images showing the construction of a 3D nanostructure:
a) randomly deposited particles, b) after manipulation, c) after
sintering at 160 ± 2ºC for 10 minutes, d) after a single particle is
‘pushed’ on top of the island.
For certain applications it is
necessary to ensure that nanocomponents are fixed on the substrate.
This can also be done by selective self-assembly. A material that
assembles to the substrate but not the particles is used, thus
embedding the particles in a thin layer. The LMR group demonstrated
particle embedding in a silicon oxide layer by first depositing
particles and manipulating them, then depositing a monolayer of a
silane (an organic molecule containing silicon atoms that attaches only
to the substrate), and finally oxidizing the silane layer. Successive
layers were used to embed particles for a proposed new rapid
prototyping technique at the nanoscale, called layered nanofabrication
or LNF (see Figure 14). Three-dimensional objects were fabricated by
nanoparticle manipulation, and each layer was planarized by adding a
molecular sacrificial layer whose top surface served as support for the
next processing step. The sacrificial layers were removed in a final
step. Thus, the researchers demonstrated that it is possible to build
sacrificial layers and to manipulate gold nanoparticles on top of them
(see Figure 15).
Schematic view of the embedding procedure of nanoparticles in a SiO2 matrix.
AFM images and corresponding line scans displaying the successful
construction of a two-particle, upright column by pushing particle ”1”
on top of particle ”2”. The scan size is 600nm x 600nm, and the height
scale is 6nm from black to white.
AFMs provide effective tools
for fabricating nanodevice and nanosystem prototypes and products in
small quantities. By using the NanoMove software developed by the
Laboratory for Molecular Robotics (commercially available from Bruker),
it is possible to use an AFM for manipulation. AFM manipulation can be
used to accurately and reliably position molecular-sized components.
Unlike its macroscopic counterparts, which are primarily governed by
classical mechanics, nanomanipulation phenomena fall mostly in the
realm of chemistry. The linking and assembling of nanoscale objects can
be accomplished via chemical and physical means using such techniques
as ”gluing” with suitable compounds, chemical deposition, or simple
heating. Demonstrations that may lead to useful applications of
nanoassembly are beginning to appear. However, increased levels of
automation in nanomanipulation are needed to prototype more complex and
useful devices and systems. Pick-and-place operations and the
construction of three-dimensional nanostructures are still very
primitive and need further development. Clearly, AFMs will have a
crucial role in the further investigation of these processes.
This information has been sourced, reviewed and adapted from materials provided by Bruker AXS.
For more information on this source please visit Bruker AXS.