Foresight Update 35
page 4
A publication of the Foresight Institute
 Scanning Probe Microscopy
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MinFeng Yu |
Santosh Devasia |
MinFeng Yu on manipulating nanotubes using SPMs
Graduate student MinFeng Yu of Prof. Rodney S. Ruoff's group at Washington University presented work done in collaboration with Zyvex L L C on "Manipulation of Carbon nanotubes using Scanning Probe Microscopes." Their four degree of freedom nanomanipulator performs 3-dimensional nanomanipulation studies under vacuum inside a scanning electron microscope, to permit visualization of the results of the nanomanipulation. For details:
Abstract that Mr. Yu submitted to the conference. Full paper, which contains link to movies showing the manipulation of nanotubes.
Tuan Vo-Dinh on optical nanosensors and nanoprobes
Dr. Tuan Vo-Dinh of Oak Ridge National Laboratory described the development of an antibody-based nanoprobe capable of monitoring biochemicals within single cells. A fiberoptic sensor is pulled to fine tip and coated with metal except at the tip, to which is bound 102 to 103 molecules of antibody to the molecule to be detected, in this case benzo [a] pyrene tetrol (BPT). The probe is inserted inside a cell, illuminated by a laser, and the fluorescence due to BPT molecules bound to the probe tip measured. Sensitivity was estimated at 40 attomoles of BPT.
Abstract that Dr. Vo-Dinh submitted to the conference.
Fumiya Watanabe on diamond tip arrays for parallel processing of microelectromechanical systems
Dr. Fumiya Watanabe of Kyushu University described his work on a "poor man's" multitip processor based upon 30 year-old field emitter array technology. The tip arrays produced various patterns etched onto a Si wafer, with 300 µm spacing between each mark, and at best 150 nm line width.
Abstract that Dr. Watanabe submitted to the conference.
Pavel Krecmer on light-actuated AFM
Dr. Pavel Krecmer of the University of Cambridge described how polarized light could cause an AFM cantilever made of a chalcogenide glass to move up and down by about 1 µm in a reversible fashion. He speculated that this effect could be the basis of very small atomic force microscopes, about 1 µm in size. However, the response time seems very slow, about 5 minutes.
Abstract that Dr. Krecmer submitted to the conference.
Santosh Devasia on optimal tracking of piezo-based nano-positioners
Prof. Santosh Devasia of the University of Utah described how applying an inverse function of the known dynamics of piezo-positioners to compensate for vibrations permits optimizing how the probes are tracked, leading to an order of magnitude increase in scanning speed, from 50 Hz to 445 Hz.
Abstract that Dr. Devasia submitted to the conference. Full paper
Modeling Diamonoid and System Architectures
Ralph C. Merkle on casing an assembler
Dr. Ralph C. Merkle of Xerox PARC presented a design study on a graphite casing to contain a simple replicating assembler. The simplified assembler would be composed of stiff hydrocarbons and would use highly reactive tools (like radicals and carbenes) in a neon atmosphere. There would be room to build within the assembler two copies of itself so that net replication is achieved even with the destruction of the original assembler during release of the products. A new proposal was presented for using acoustic control to provide both power and information for operation.
Abstract that Dr. Merkle submitted to the conference. Full paper
J. Storrs Hall on system architectures for self replicating systems
Dr. J. Storrs Hall of the Institute for Molecular Manufacturing analyzed the tradeoffs between a system architecture with simple assemblers that self-replicate to grow to macroscopic size, and an architecture composed of many different types of assemblers working together to make complex objects. He concludes that it is desirable to design a replicator to reproduce itself only for a few generations, and then build something else. "Furthermore, it is crucial to design replicators that can cooperate in the construction of objects larger and more complex than themselves."
Abstract that Dr. Hall submitted to the conference. Full paper
Tahir Cagin on friction and wear of diamond
Dr. Tahir Cagin of the California Institute of Technology reported both simulation and experimental work on friction and wear in diamond as a material for MEMS and NEMS applications. He noted that the bond strength in diamond is 80% greater than in silicon, which translates into 10 times the hardness, 5 times the toughness, and a wear rate 104 times less.
Abstract that Dr. Cagin submitted to the conference. Full paper
Stephen P. Walch on the interaction of hydrogen with diamond and silicon surfaces
Dr. Stephen P. Walch of ELORET at NASA Ames Research Center presented computational studies of the interaction of hydrogen with diamond and silicon surfaces. These calculations address the issue of using gas phase hydrogen to satisfy dangling bonds on the surfaces of diamondoid nanodevices, rather than having to add individual hydrogen atoms one at a time.
Abstract that Dr. Walch submitted to the conference. Full paper
Enabling Technologies Tutorial
The Tutorial on Critical Enabling Technologies for Nanotechnology was held Nov. 12, 1998. Tutorial Chair, Prof. Jan H. Hoh, Department of Physiology, Johns Hopkins University School of Medicine defined nanotechnology for the purposes of the tutorial as atomically precise manufacturing from the bottom up. The pathway to develop such technology is not yet clear so research in this area is extraordinarily interdisciplinary, ranging from computer science to biology. Sampling the relevant enabling technologies, four topics were presented in the hope of cross-educating researchers on some of the other approaches in use. Dr. James C. Ellenbogen of MITRE discussed molecular electronics both in the tutorial and during the Conference. More detailed summaries are available on his Web site. The discussions of simulation methods, scanning probe microscopy, and self-assembly follow.
At the end of the day, Dr. Ralph Merkle put the talks in perspective, pointing out that each of the four talks covered a specific area in the journey from present-day theoretical studies and experimental techniques to future molecular manufacturing technology. A few comments he made about the path to molecular manufacturing:
- the first devices capable of doing positional chemistry will likely be self-assembling
- self-replication of manufacturing devices is the key to low cost
- success will require funding directed to the goal of molecular manufacturing.
Donald W. Brenner on molecular dynamics
--reported by Jeffrey Soreff
Professor Donald W. Brenner (from NCSU) gave a talk on simulating
dynamics on the molecular scale.
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Donald Brenner |
Professor Brenner began his talk with a description of some application
areas for atomistic simulations: Predicting the mechanical limits of
nanostructures, the chemical reactivity of nanostructures, the
electronic properties of nanostructures. Both small molecules and
macroscopic amounts of materials can be analyzed without massive
computation, but simulations are critical at the intermediate scales.
He used a nanogear analysis as an example, showing how simulation
turned local knowledge about bonding into structural level knowledge
on what speeds and accelerations would make gear teeth slip or tear
themselves apart.
Approximations
Professor Brenner then described the series of approximations needed to
simulate a structure like the nanogear with ~100-200 atoms.
Ideally one would solve the time-dependent Schroedinger's equation for
electrons and nuclei. In practice, this is too hard, and there are also
useful insights from the approximate models.
The first approximation is the Born-Oppenheimer approximation.
This separates the electronic and nuclear degrees of freedom.
It finds the solution to Schroedinger's equation for the electrons
in the presence of fixed nuclei. It then treats the energy from
the electron's states as setting a new potential energy for the
nuclei. The wavefunctions for the movement of the nuclei are
now found from Schroedinger's equation using the new potential.
This approximation effectively assumes that the electrons move
fast enough that they can always keep up with nuclear motion.
This can break down when there are low energy electronic states
or when molecules are colliding rapidly in comparison to
electronic movement. This approximation drastically reduces the
number of particles, hiding the electrons in the potential energy
of the nuclei.
The second approximation is to treat the nuclei as classical
particles. This loses quantum effects such as nuclear tunneling
or zero-point vibrational energy. There is some work on wave packet
dynamics which retrieves some of these effects.
The third approximation is to approximate the potential energy
from the electrons' motion with some function of the nuclear
coordinates which is easier to calculate.
Professor Brenner touched on the complex history of atomistic
simulations, extending from 1936 to the present, and involving
at least 4 different communities with heterogeneous goals:
- Chemistry: Calculating reaction rates for small molecules accurately
- Statistical Mechanics: Modeling correlated many-body motion
- Materials Scientists: Modeling solids and many-body motion
- Biologists: Predicting structure, as in protein folding
Separate development of techniques by several communities has sometimes
given several names (e.g. "Huckel" and "tight binding") to the same
technique.
Components of a Simulations
Professor Brenner then described three components of an atomistic
simulation:
- The integrator
- The thermostat
- The interatomic forces (aka the potential energy function)
The integrator treats the differential equations of motion for the nuclei
as difference equations with a finite step size. Typical step sizes are
10-15 - 10-14 seconds. Important criteria for an
integrator are its accuracy and stability.
In an ideal simulation of a large enough system, a heat bath would form
a natural part of the system, and the dynamics of the interface between,
for instance, a nanogear and the heat bath would hold the nanostructure
at nearly constant temperature, with only physically accurate excursions
in temperature. Unfortunately, a heat bath would require many atoms,
so its effect is simulated by less computationally expensive methods.
Typically the velocities of atoms in the model are modified in some way
or fictitious forces are added to the model. Many types of thermostats
have been used. The Nose' thermostat incorporated many of the
innovations of its predecessors and is currently the only one that
generates the correct thermal fluctuations in small systems.
Interatomic force calculations have taken a number of approaches.
The most CPU intensive approach is to directly calculate them from
a quantum mechanical calculation of the electrons' state. Most
development effect is currently going to this approach.
Historically, they have been calculated with ad-hoc functional
forms fitted to known molecular properties. Some functional forms
have also been derived from quantum mechanical arguments.
Pairwise interatomic potentials have well known properties. The
1/r repulsion of the nuclei, the exponential attractions and
repulsions of the core and valence electrons, and the 1/r6
attractions of Van der Waals forces are well known functions.
Three-body (e.g. bond bending) and higher order terms have been
used extensively, but they do not have a solid grounding in the
electrons' quantum mechanics.
"Molecular Mechanics" generally refers to a popular potential
function like Allinger's MM2 that includes bond stretches and
angle bends between bonded atoms and pairwise forces between
non-bonded atoms. It requires fixed bonding topology during
a simulation.
Potential functions that can accommodate dynamic changes in
bonding topology calculate effective bond order between
nearby atoms on the fly. While they do not do a full quantum
mechanical treatment of the electrons, they rely on a theorem
that calculates the local density of electronic states from
the paths with fixed numbers of "hops" between atoms. In
particular, the binding energy of an atom is largely dependent
on the width of the electronic orbital energy distribution.
This width is largely dependent on the number of paths with
two "hops", one away from the atom in question and one back to
it.
In general, potential functions of some sort have been done with
some success with all elements. What hasn't been done well yet
are mixed metallic/covalent systems.
Professor Brenner concluded by touching on the major challenges
in atomistic simulations today:
- Expanding the number of atoms in simulations.
Today 107 is routine, but simulations of cracks and
of structures with mixed pore sizes are pushing the limit
towards 109 atoms.
- Parallel processing, possibly including exotic architectures
such as cellular automata
- On-the-fly quantum calculation of forces
- Mixed levels of simulations: molecular mechanics + quantum mechanics,
finite element analysis + molecular mechanics + quantum mechanics
Jason P. Cleveland on scanning probe microscopes
--reported by Jeffrey Soreff
Dr. Jason P. Cleveland (from Digital Instruments) gave a talk on
scanning probe microscopes, with an emphasis on force microscopes.
The fundamental physics of force microscopy are very similar to what one
would expect from exploring a surface by touch manually, or to the physics
of Edison phonographs. The major changes are in the tip radii, now
5-10 nm, and in the size and speed of the cantilevers on which they
are mounted, now ~100 µm long, with resonant frequencies of ~1 MHz.
The cantilever deflection (and hence tip position) is typically
measured by reflecting light from the cantilever and measuring the
light beam's movement. The noise in this measurement is only
about 0.1 Å in any 10 kHz bandwidth (above the base
10 kHz bandwidth which contains 1/f noise).
The cantilevers (and their attached tips) are typically moved
with piezoelectric actuators. An actuator a few inches in size
provides a few µms of travel.
Modes of AFM Operation
There are three primary modes of AFM operation:
- Contact mode
- Non-contact mode
- TappingModeTM
Contact Mode
In contact mode a feedback loop maintains constant deflection
(constant force) by varying the height of the tip while the tip is
scanned. Ideally, one would prefer use tip-sample forces of ~ < 1 nN,
but they range from 100 nN - 0.01 nN. In air, capillary and
electrostatic forces add to the tip-sample force. Operating AFMs
under fluids partially cancel capillary, electrostatic, and Van der
Waals forces, allowing tip-sample forces of 0.1 nN - 0.01 nN.
Contact mode imaging can be destructive. Dr. Cleveland showed us an
example of a (1 µm)2 image of epitaxial silicon,
followed by zooming out to see a (2 µm)2 image of the
same area. The second image had a blank square where the first image
has been scanned, showing that the tip had leveled the (1
µm)2 area as it measured it.
Non-contact Mode
In non-contact mode the cantilever is vibrated near its resonant
frequency, and small shifts in the resonant frequency due to force
gradients from the sample are detected. In this mode the tip is
typically kept out of any adsorbed liquid layer. It typically senses
forces 1 nm - 10 nm from the sample. This mode is nondestructive, but
it has limited resolution because of the tip-sample distance. It also
is sensitive to gunk and must "scan slowly to avoid contacting and
getting stuck in [an] adsorbed layer."
TappingModeTM
In TappingModeTM the AFM tip is oscillated vertically with
a typical amplitude of > 20 nm. A feedback loop maintains constant
amplitude as the tip is scanned across the surface. Hitting the
surface makes the amplitude drop. Surprisingly, even though the
tip-surface interaction is very nonlinear on this scale, the tip
oscillation stays close to sinusoidal.
The main advantage to TappingModeTM is that it eliminates
the large lateral forces that are generated during contact mode
scanning. Dr. Cleveland made an analogy to the macroscopic behavior
of sandpaper. One can push on sandpaper with one's finger all day
without damage, but sliding across it under pressure does
damage. The large oscillation amplitude of this mode lets the tip
pull back to where scanning does no damage. It also allows the tip to
pull out of adsorbed layers. It "allows imaging of soft, fragile, and
adhesive surfaces without risk of sample damage."
Typical oscillation frequencies in air are 50 kHz-500 kHz. At these
frequencies, many soft surfaces act stiffer than under DC probing,
so more structural details are visible.
Dr. Cleveland displayed some TappingModeTM images made
in aqueous solution, including some in DNA. He said that base pairs
were not visible, that resolution was limited to helix turns, around
30 Å. The outlines of base pairs are also rather similar.
Hansma is working on imaging bases on strands separated by a polymerase,
using micron-scale cantilevers to operate at higher frequencies.
Dr. Cleveland touched on a number of specialized techniques:
- magnetic force microscopy
- electric force microscopy
- scanning capacitance microscopy
- lateral force microscopy
- force modulation microscopy (elastic modulus imaging)
- scanning thermal microscopy and
- phase imaging (this last mode covered in detail)
Most of these techniques involved weaker or higher-order interactions
than AFM itself, and appeared to have somewhat coarser resolution.
One notable technique (called LiftModeTM operation),
involved scanning a line for topographic information, then using
the topographic information to rescan it at constant height above
the measured profile of the sample while measuring some other
interaction.
Phase Imaging
In phase imaging, the relative phase of the drive signal on the
piezoelectric actuator and of the cantilever deflection is
measured. This turns out to be sensitive to materials differences,
and is good for seeing material phase boundaries in samples where
topography isn't informative. Dr. Cleveland went through the
modeling of the cantilever oscillation and showed how amplitude
and phase information could be combined to yield power dissipation
in the sample, which is a good probe of viscoelastic properties.
Nanoindentation
Dr. Cleveland described the use of diamond tips mounted on stiff
cantilevers to do destructive measurements of materials properties.
These tips are currently made by polishing 3 facets by hand.
Radii of 10 nm can be reached. Unlike the macroscopic case, where
indentation is the better standardized measurement, films are better
measured by scratching. Indentation tends to go through films,
measuring substrate properties rather than film properties.
Force Curves
Dr. Cleveland went through the various regimes that a tip on a spring
experiences as it approaches and retracts from a sample:
- (approaching)
- low interaction far from the sample
- stable deflection from attractive forces
- jump to contact
- repulsion from sample at close contact
- (backing away)
- stable deflection, first from repulsive forces, then from attractive forces
- jump out of contact
- low interaction far from the sample
For very stiff springs the mechanical instability disappears and the
whole force curve is measurable.
For very weak springs the jump from contact moves the tip very far
from the sample and only an adhesion force is measurable.
In one experiment a calcite sample under water had a complex force curve
with multiple minima. It was possible to trace it, not by directly moving
the tip, but by letting the tip hop between the minima (and nearby positions)
thermally. The statistics of tip positions gave the force curve.
Joseph A. Zasadzinski on self-assembly in Langmuir-Blodgett films
--reported by Jeffrey Soreff
Professor Joseph A. Zasadzinski (from UCSB) gave a talk on
Langmuir-Blodgett films.
Langmuir-Blodgett films are monolayers of amphiphiles (soap-like
molecules with hydrophobic groups at one end and hydrophilic groups at
the other end) which are formed on the surface of a water trough and
transferred to a solid substrate. A sequence of layers can be
deposited, giving one-dimensional molecular scale control of
structure. Langmuir-Blodgett films have been known for seven decades.
They have been studied intensively, with over 2700 papers published in
the last 4-5 years. There are a long list of potential applications,
which have thus far remained potential.
Langmuir-Blodgett films are made by
- dissolving an amphiphile in a volatile organic solvent
- depositing the solution on a water surface
- evaporating the solvent to leave a floating monolayer of the amphiphile
- compressing the monolayer with a movable barrier which extends
through the water surface
- dipping the solid substrate through the water surface (possibly
repeatedly, for depositing multiple layers)
Langmuir-Blodgett films can be deposited with either the hydrophobic
end or the hydrophilic end of the amphiphile touching the substrate,
depending on whether the substrate is hydrophobic or hydrophilic.
Zasadzinski described a great deal of work his group had done on the
structure of Langmuir-Blodgett films. The films were imaged with
AFMs, which was able to see individual methyl groups at the
hydrophobic ends of individual molecules. The films were made of
divalent metal (Cd2+, Mn2+, Pb2+,
Mg2+, Ca2+, Ba2+, Zn2+)
salts of arachidic acid
(CH3(CH2)18CO2H).
Roughly speaking, small metal ions gave a simple, nearly hexagonal
packing (though the unit cell is rectangular), very similar to the
packing in polyethylene. Larger metal ions gave much more complex
structures. These ions force the hydrocarbon tails to tilt in order
to contact their neighbors. The spacing of CH2 groups
along the tails favors certain tilt angles. The net effect is to
generate complex structures with long repeat distances - even chiral
packings.
Langmuir-Blodgett films haven't found applications because they are
unstable. Professor Zasadzinski showed a dramatic series of images
showing micron-scale cracks appear in a film within 15 minutes of
formation. Over 48 hours, the film reconstructs to yield islands with
roughly polygonal edges - analogs to facets in a 3D crystal.
The problem appears to be in the vertical structure of the film. A 3D
cadmium arachidate crystal has the two arachidate groups bound to the
metal going off in opposite directions ("splayed"). A
Langmuir-Blodgett film of cadmium arachidate has to have all of the
hydrophilic groups (the metal and both CO2H groups) on one
side of the film and the hydrocarbon tails on the other (a "hairpin"
structure). The hairpin structure has an area of 19.4
Å2 per molecule, while the splayed structure has an
area of 18.0 Å2. The transformation "means holes.
Lots of holes."
Films have been made stronger by putting in double bonds and
crosslinking. The problem with this is that the act of crosslinking
shrinks the film and shreds it. A new approach is to add one -OH
group to the cadmium arachidate. This ties up one bond to the
cadmium, so only one arachidate group is needed. This allows a
monolayer structure that is more similar to the stable 3D crystal
structure, and is less prone to reorganize.
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Foresight staff Elaine Tschorn (left) and Tanya Jones (center) are supported by volunteer Norma Peterson (right) which keeps the conference running smoothly. (Not shown in picture Harriet Hillyer) |
Institute for Molecular Manufacturing Report
The portion of Update 35 that constitutes the IMM Report is on the IMM Web site: http://www.imm.org/.
From Foresight Update 35, originally
published 30 January 1999.
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