|Foresight Update 31 - Table of Contents|
More than a quarter of the papers presented at the conference, including Nobel Laureate Richard E. Smalley's keynote address, described experimental or theoretical work on carbon nanotubes. Carbon nanotubes are essentially cylinders of graphite. Single walled nanotubes (SWNTs) consist of a single layer, while multi-walled nanotubes consist of several nested layers. These nanotubes are important for nanotechnology because of their well-defined structures, stiffness (1 terapascal), strength, ease of formation (up to 70%-90% yield), stability, and (probably) their range of electronic properties.
Smalley put considerable emphasis on the stability of nanotubes. He described purifying them by boiling a mixture of nanotubes and amorphous carbon in nitric acid. The nanotubes are the only part of the mixture which survives this treatment, the rest oxidizing away. This would not be too surprising in a bulk material, but SWNTs are only a single atom thick, so each atom is exposed directly to this environment. Smalley described nanotubes as "first class molecules" (in terms of stability), and said that he expected them to become available by the megaton, and to become available with precisely controlled terminating groups.
The stability and strength of nanotubes is similar to that of graphite itself and to C60. A unique feature of certain classes of nanotubes is their combination of chemical stability with efficient conductivity. Graphite's electrons are delocalized, and it acts as a conductor to some extent. Unfortunately, the hexagonal symmetry of a graphite sheet makes its valence band just barely touch its conduction band at one point, so it has a vanishingly small concentration of charge carriers. In one class of nanotubes (the armchair nanotubes those where two edges of each hexagon lie on a circumference of the tube) symmetry forces the bands closest to the Fermi energy to cross. This produces a large concentration of mobile carriers, and nanotubes have indeed been shown experimentally to behave as perfect quantum wires. Before this development, we either had good, stable molecules (such as graphite or polyacetylene) which were poor conductors, or "poor molecules" (in the sense of being reactive) which were good conductors (such as doped graphite). The armchair SWNTs conduct without special protection even "in boiling coffee," as Smalley put it.
Deepak Srivastava presented a paper on several types of analysis of nanotube systems. He displayed molecular dynamics simulations of stressed tubes and of gears and motors built around nanotubes. The dynamics simulations contained up to 500,000 atoms. He also showed results from quantum mechanical analysis of multi-tube junctions.
The molecular dynamics calculations represented the forces between the carbon atoms in the tube with Brenner's potential. This model of the forces allows the bonds between carbons to form and break during the course of a simulation. This is more accurate than older models, which required that bonds between atoms all be specified before a simulation begins and be held constant during the simulation. The simulations of stressed nanotubes exploit this new accuracy because they can create localized stress concentrations that can break bonds. Srivastava displayed SWNTs and MWNTs being compressed and bent. One striking result was to show that a SWNT could be bent through 180 degrees, severely flattening the tube at the bend, yet recover completely when allowed to relax. MWNTs were less distorted by deformation, because the internal tubes supported the outer one. The MWNTs were, however, more likely to break than the SWNTs.
Srivastava also presented atomistic dynamics calculations on a laser-pumped molecular motor. The motor consisted of a nanotube with two embedded charges immersed in an oscillating electric field. An intrinsic rotational rate for the tube was found by examining linearized equations of motions for it. An oscillating field at this frequency produced unidirectional rotation, while previous attempts with the field oscillating at much lower frequency had only produced back and forth oscillations. His group also compared pulsing the driving field with having it on continuously. They found that the rotation remained unidirectional even when the field was pulsed, but that this reduced the temperature rise produced by the field.
The quantum mechanical calculations looked at the electronic structure of 2-, 3-, and 4-tube junctions. These can be important as electronic devices, exhibiting rectification at metal-semiconductor junctions, for instance. The type of tight-binding approximation used is "transferable", applying not just to purely carbon structures, but also able to calculate electronic structures of networks containing boron or nitrogen atoms.
A few papers described current or planned experimental work on the chemistry of nanotubes. One paper described work on controlling the catalytic metal particles that speed the formation of nanotubes, fabricating precise arrays of metal particles with a controlled sequence of layers by lithographic techniques. Several presentations described the chemical activation expected at stress concentrations in buckled nanotubes, and suggested performing selective reactions at these sites. One suggested that reactions might occur at bends at tubes that could later be reversed on relaxing the stress, providing a true mechanochemical catalyst. Diels-Alder addition of cyclopentadiene to stressed nanotubes is a candidate for this process.
There is some ambiguity in describing what kinds of nanotube structures can be built today. A great many structures have been produced experimentally: SWNTs; MWNTs; C60; and larger near-spherical fullerenes, including multilayer fullerene "onions," helices, structures with partial buckling, and even toroids. Each of these observations proves that the structure seen is stable (or metastable with a long lifetime). It doesn't, however, show that the structure can be produced reliably or in quantity today.
Several experimental papers described mechanical or electromechanical properties of nanotubes or C60. The use of C60 molecules as counters on a mechanical abacus was shown. A C60 molecule was used as the active element in an electromechanical amplifier, increasing the current it transmitted by 100X when compressed by 0.1 nm. Nanotubes have been used as AFM tips. Nanotubes have been embedded in plastic, buckled by heating the film, and the ripples on the tubes measured.
One group is building a mechanical stage to allow very precise stressing of nanotubes while observing their structure with transmission electron microscopy (TEM). The stage will have a 50 nm gap for imaging while keeping the two sides coplanar to a few nm. The stage will be stressed with a piezoelectric bimorph. See also "Recent Progress" for AFM measurements of nanotube stiffness.
One experimental paper probed the electronic properties of nanotubes. The basic technique was to drive an STM tip 100 nm into a mat of SWNTs, then retract it in 2nm steps, taking I(V) conduction measurements after each step, until contact failed (in some cases after retraction of over 2 microns). SWNTs are extremely sticky. This experiment relies on their stickiness to maintain electrical contact to the STM tip. The ropes of SWNTs were found to have consistent I(V) characteristics extending over hundreds of nm, with a few very localized defects abruptly changing the conduction characteristics over "mere nm of tip motion." In particular, changes from a mildly nonlinear bidirectional conduction to a strongly rectifying I(V) curve were seen. More precisely, each 2nm step produces a change in the I(V) curves, but 99.9% of the time these changes just scale the current up or down (albeit by as much as a factor of 2), leaving the shape of the curve unchanged. A very few times per experiment the curve changes shape, and this is interpreted as passing an electronic defect. This experiment showed that electronic devices in nanotubes can indeed be as small as theory suggests, so circuits using them can be very dense.
A great deal of theoretical work on nanotubes was presented at the conference. Papers were presented on quantum mechanical calculations of electronic properties, generally using tight binding approximations. The electronic structures of uniform tubes seem to have been covered in previous work. Current papers included structures of tube junctions, structures of model defect systems, including various combinations of 5, 7, and 8-membered rings. At the finest grain level, one author described calibration of a density functional theory calculation using an ab initio calculation as the standard and a spectrum of small defect structures (5 and 7-membered ring combinations, surrounded by hexagons) as calibration points.
Some authors displayed localization of the electronic effects of defects by showing how conduction band wavefunction amplitudes (presumably at the fermi level) dropped as one went past a junction into a semiconducting tube. One calculation introduced a junction between 4 nanotubes and predicts anisotropic current flow in that geometry. One calculation looked at electronic properties of tubes with lines of adducts extending radially outwards from the tube and found that with one pattern of adducts the tube had a band gap while with another the gap vanished. External adducts may be more controllable with wet chemistry than embedding pentagons and heptagons in the network of the tube itself. This could make intentionally building electronically active structures from nanotubes easier than the intra-tube defect structures would suggest. One paper used electronic calculations to compute vibrational mode frequencies (specifically for Raman-active modes). This seems to be the current limit for combining electronic quantum mechanical calculations with mechanical deformations of the nanotubes.
A great many papers presented atomistic molecular dynamics calculations. Simulated tubes were bent, twisted, buckled, bound into gears, stressed as parts of diamond-nanotube composites, pushed into surfaces as probe tips, cooled with simulated helium atoms, compressed under crossovers on surfaces, wrapped into toroids, and collapsed under their own Van der Waals attraction. (Or, as one speaker wryly put it, "all kinds of mean things being done to nanotubes"). One of the simulations shown (on buckling modes of SWNTs and MWNTs) used 100,000 atoms simulated on a 32-processor computer. A number of general conclusions seem to emerge from many of these simulations:
The onset of many mechanical instabilities is quite sharp, changing the shape of the tube dramatically when loading changes slightly. Small nanotubes, notably the heavily studied (10,10) armchair tubes, maintain their cylindrical forms quite well, recovering from severe bending. The larger tubes, particularly the SWNTs larger than 6nm in diameter, collapse due to just Van der Waals attraction between opposite sides of the tube and can also collapse easily due to forces between the tube and a surface.
The nanotubes, as tubes, can behave very nonlinearly while the bonds within them are still very stable. Many simulations showed extreme deformations of the tubes, with a great deal of buckling and dramatic stress concentrations, yet (even though reactive potentials were used) very few of these simulations permanently changed carbon bond topologies. A ninety degree bend in a SWNT does not imply that a C-C bond somewhere is about to snap. As one author emphasized, nanotubes follow classical thin-shell theory quite closely. At least one group is planning to abstract atomistic models to continuum models based on this.
Overall, the work presented on nanotubes at this conference looked very promising, with valuable machine components becoming both experimentally accessible and theoretically understood. Both theory and experiment are tackling more complex systems than in the past, and are checking each other at more points.
Ralph Merkle presented a paper on the metabolism of a hydrocarbon assembler. This study looks at what reactions would suffice to build an assembler capable of replicating itself and of building stiff hydrocarbons without getting bogged down in questions of what constitutes a "universal" assembler or how many bond types it must construct. This study fits within a system model of an assembler floating in a feedstock solution, powered and controlled by externally supplied acoustic signals, and containing highly reactive positionally controlled molecular tools (such as free radicals and carbenes) in an inert internal environment. The overall material and energy flow is for a main carbon-bearing feedstock and a "vitamin" feedstock (containing silicon, tin, and a transition metal) plus the energy in the acoustic waves to be converted by an assembler into more assemblers (and possibly some small amount of hydrogen-rich or carbon-rich waste immobilized in solid form in a compartment of the assembler or of its products).
Butadiyne, H-CC-CC-H, was picked as the carbon-rich feedstock because its simple linear shape permits simple binding sites (truncated buckytubes); it can be converted into useful tools with simple reactions; and its ratio of carbon to hydrogen is fairly high, plausibly matched to that of desirable structures. Since the butadiyne is initially a separate molecule, the first step in using it must be to covalently bind it so that its position can be controlled. Merkle immobilizes the butadiyne by reaction with a pair of R3Si· radicals at the terminal carbons, yielding R3Si-CH=C=C=CH-SiR3. The R groups attached to the Si atoms are part of the polycyclic tool structure, solid machinery controlling the positions of the radicals. Even for a stepwise addition of the Si radicals, quantum chemical calculations gave an activation barrier of only 3kT. In general, many of the reactions in this paper use multiple radicals as reactants, allowing very favorable thermodynamics since more bonds are formed than broken. The energy for this comes proximately from formation of free radicals by tearing bonds apart mechanically and ultimately from externally supplied acoustic energy.
Merkle shows a series of reactions converting the bound butadiyne into ethynyl radicals, R3C-CC·, useful as hydrogen abstraction tools. These reactions again use attack pairs of R3Si· radicals to cut single carbon-carbon bonds. Mechanical stress on the resulting R3Si-CC-CR3 structures then selectively breaks the weaker Si-C bond, regenerating the R3Si· radicals. Carbene insertion tools, carbon dimer insertion tools, hydrogen addition tools, and silicon and tin radicals are covered by this paper. It is shown that, starting with butadiyne, an arbitrary number of each of the carbon-rich tools can be made without "running short" of any starting tools. The regeneration of the silicon and tin radicals is also shown, but their synthesis from a "vitamin" feedstock is left for future work.
This paper shows a full set of reactions for going from a feedstock to a set of molecular tools sufficient to build stiff hydrocarbons and demonstrates numerical parts closure for this system.
James Gimzewski of IBM Zurich described experimental work on manipulating and imaging molecules with an STM at room temperature under ultrahigh vacuum (UHV). His lab emphasizes work with medium sized molecules such as the 173 atom modified porphyrin that they were able to position on a Cu(100) surface. They treat a molecule "as a prefab unit," with carefully chosen features incorporated during synthesis to enable them to manipulate it with the STM. In the case of the porphyrin, they bound 4 "legs" (di-tertiary-butyl-phenyl groups) to the molecule to give it just the amount of mobility on the surface that they wanted. An unmodified porphyrin can form a strong pi bond to a metal surface. Too strong a bond can make it impossible to push a molecule around on a surface without damaging it. The legs of the substituted porphyrin lift the porphyrin pi system far enough above the surface that this pi bond doesn't form. The saturated tertiary butyl groups of the legs bind the molecule to the metal surface just weakly enough to permit movement. The legs can rotate with respect to the porphyrin plane, and this permits the molecule to accommodate itself to the surface (and therefore bind more tightly) than a more rigid molecule could. When the molecule is pushed with an STM tip, the flexibility also permits the molecule to move one leg at a time, each building up strain as the tip is moved, then jumping to a new local minimum. The molecule's shape is also important in its interaction with the STM tip. During these experiments the molecule is pushed, and a smooth, near-spherical molecule like C60 tends to slide if it is on a flat surface. The modified porphyrin, however, can be pushed between two of its legs. Gimzewski is now also trying annular molecules, with the tip in the center of the ring.
Gimzewski also described the molecular abacus experiment, using C60 counters at a step on a Cu surface (see Update 28). He described the C60 electromechanical amplifier as well: a system where an STM tip sitting on top of a C60 molecule, which is in turn on a metal surface, can change the current through the C60 by 100 fold by compressing it only 0.1 nm with the piezoelectric actuator driving the STM tip. The amplifier's current bandwidth is only 0.6Hz, but the fundamental limit to the system is in the terahertz range (set by mechanical resonances of C60). In this experiment the tunneling mode was "virtual resonant tunneling" (VRT), where the fermi levels of the two metals are between the lowest unoccupied molecular orbital (LUMO) and the highest occupied molecular orbital (HOMO) of the C60. This is a desirable mode for molecular electronics to operate in because in VRT electrons don't actually jump on to orbitals in the active molecule. If they actually jumped on, they would dissipate energy in the molecule, heating it and possibly damaging it.
Gimzewski displayed images of a "propeller" molecule, similar to the modified porphyrin but with different symmetry (triangular rather than square if I recall correctly). This molecule can rotate freely when isolated on a surface, merging the lobes of its legs into a ring in the STM image. When packed into a 2D lattice, its rotation is frozen. Near a hole in a lattice, they were able to move a molecule back and forth between locked and rotating conditions, observing the change with the STM. They also simulated the system (presumably with atomistic molecular mechanics) and the simulations were consistent with their interpretation of the STM images.
K. Eric Drexler spoke on the lessons of several years of hands-on experience designing diamondoid machines. If we view the landscape of possible structures along two dimensions, synthetic difficulty and design difficulty, we find that we have experimental access to the regions with low synthetic difficulty but there is also an interesting region with high synthetic difficulty but low design difficulty. We have computational access to this region now and hope to eventually obtain experimental access. A number of patterns become apparent as we explore this region:
1) While our computers have become more powerful over time, processing power still stringently limits how many atoms we can simulate. Thus, there is pressure to keep device designs small, even when optimized designs would be larger.
2) Thousands of atoms are often needed in order to make an interesting structure. For example, it is often desirable to enclose an object. This can require a surprising increase in the atom count. For example, to wrap a single fat atom requires a C60 layer, raising the atom count from 1 to 61.
3) Highly symmetrical structures are desirable for several reasons: a) Symmetry can be used to suppress undesirable potential barriers. The combination of an n-fold symmetric shaft with an m-fold symmetric sleeve can give as high as an nm-fold symmetric potential (when n and m are relatively prime). In a 3000 atom bearing, this can give barriers as low as 1/1000 kT. b) symmetry reduces the amount of design work, reducing the number of separate choices that must be made. c) Symmetry enables specific types of design exploration. For instance it allows one to change all of the atom types at symmetry-equivalent positions simultaneously and see how that change affects the quality of the design.
An alternative to exploiting point group symmetry which applies to a machine part is to use lattice symmetry. Drexler displayed an example of a universal joint where the vast majority of the atoms lie on diamond lattice sites (surfaces were terminated mostly with nitrogen). The geometrical constraints were fairly weak in this design.
Drexler also displayed a series of designs in order of increasing complexity (and decreasing symmetry), from the bearing, through a planetary gear, then a differential gear, up to the fine motion controller. In each of these the atoms that are most directly affected by broken symmetries absorbed disproportionate amounts of design effort. For example the small irregular patches in the differential gear where the bases of the conical bevel gears approach the cylindrical casing consumed much design effort. The fine motion controller appears to be at the limit of today's molecular CAD software. The cam interface in this structure has actually been designed, but the surrounding structures are mock ups.
Enhancements to molecular CAD software can extend our ability to design functional structures. Amongst desirable enhancements are:
Al Globus described some of the excitement in NASA over the potential of nanotechnology to support NASA's missions and lead eventually to the colonization of the solar system ("Fullerene nanotechnology, swarms, and space colonies"). The focus of current efforts is on fullerenes and nanotubes, which can not only be modelled computationally, but are also chemically plausible candidates for relatively near term fabrication. Globus pointed out that benzyne is known to add to C60 under mild conditions, and that simulations suggest that it should also make a stable addition product with 14,0 SWNT's. If some way could be found to target the benzyne additions to the proper places on the nanotubes, gears could be made with the benzynes forming teeth upon a nanotube shaft. Computational work by Globus and his colleagues has established that such gears could operate at 100 GHz. At higher speeds the teeth slip but are not damaged, so that the gears would resume functioning if the speeds again decreased. Further simulations showed that if two gears were unequal in radius [made from (18,0) and (10,0) nanotubes], then powering the larger one would turn the smaller gear, but not vice versa. Other simulations explored using noble gases to cool the gears, and using alternating electric fields supplied by a laser to power the gears.
Globus also described a preliminary concept for a system architecture for forming active materials from "swarms" of nanomachines. Swarms would consist of large numbers of interconnected robots, each of which would have internal computers, sense force, and transmit power and data. Robots would be of two types: "nodes" and "edges." Nodes could move four attached edges in pitch and yaw, and edges could rotate on their long axis, change length, and attach and detach from nodes. Free ends of edges could manipulate objects. Major problems to be considered are how to build and assemble the robots from fullerene components, and how to make the software to control the swarms.
Ned Seeman presented a paper on "DNA Nanotechnology". A good deal of the conference paper overlaps with a paper covered in this issue's "Recent Progress" column. Seeman covered the evolution of his syntheses of DNA topologies from 4-branch junctions to cubes, truncated octahedra with 14 linked DNA cycles, Borromean rings, and recently to more rigid structures built from antiparallel double crossover structures. One can't get additional rigidity by simply crosslinking with small molecules like glutaraldehyde because the resulting links are too nonspecific, yielding an irregular structure.
At the conference, Seeman made the point that, despite the existence of a number of related systems such as peptide nucleic acid (PNA) and RNA, there are substantial advantages to working with DNA. He explained that the set of enzymatic tools available for DNA is much better than for the other systems, including ligases to join strands, restriction enzymes to cut strands, and topoisomerases (also active on RNA) to pass one strand through another. Seeman described three goals for this work:
Seeman described adding a moving part to his structures by shifting the location of a 4-branch junction with mechanical force. He joined two of the branches into a cycle, then applied torque to the cycle by adding ethidium (which changes the amount of supercoiling in an unconstrained DNA strand). This makes the DNA cycle act as an actuator, pulling the other two DNA arms into the 4-branch junction. "This molecule represents the very first step in using DNA structural transitions to achieve a nanomechanical result." Work has also been proceeding since 1991 on using DNA's interconversion between B and Z forms as an actuator.
Avouris emphasized that nanotubes on surfaces are significantly different from the idealized tubes, isolated in vacuum, on which band structure calculations have been done. The mere Van der Waals attraction of a tube for a surface deforms the structure substantially. While the energy of the Van der Waals interaction between two atoms is rather small, 100 meV for a pair of argon atoms, for instance, it becomes much larger for structures the size of nanotubes, around 0.5 eV/angstrom. When one nanotube passes under another one, the forces can reach 5.5 nN and the pressures can reach 30 GPa, close to the graphite/diamond transformation. These deformations break the symmetry that allows metallic conduction in the (n,n) nanotubes. An analysis of a mild (30 degree) bend in a nanotube showed that the band gap was substantially changed in the bent region, that the hybridization of the carbon atoms was changed from sp2 (as in graphite) to sp3 (as in diamond), and that the whole structure acted like a semiconductor heterostructure. The attraction to a surface is sufficiently strong that, even though a nanotube has a TPa Young's modulus, it still conforms to the shape of the substrate. These bends inhibit the ballistic transport that calculations on isolated tubes would lead one to expect.
Avouris also described experiments using an STM to break Si-H bonds on an Si(100) surface. The operation of an STM breaks Si-H bonds in two quite different ways. At high voltages (a bias of -7 volts), an electron in an attractive sigma orbital in an Si-H bond can be excited into a repulsive sigma* orbital, breaking the bond. This excitation takes about 6 eV. About one hydrogen atom is ejected for every million electrons that flow through the tip. Spatially, this process has around a 2 nm scatter, because the STM tip must be kept fairly far from the substrate at such high biases. This process is also hard on the STM tip, since the high fields (~10 volt/nm) affect it as well as the Si-H bonds. At lower voltages the tunneling electrons don't have enough energy to directly excite electronic transitions, but they can excite vibrations. If a sufficient amount of vibrational energy is added to an Si-H bond, this can also break the bond. Spatially, this process has atomically precise resolution. A number of factors enhance the resolution:
The vibrational process is less efficient at removing hydrogen atoms, removing only one for every hundred billion electrons that flow through the STM tip. Nonetheless, this isn't the limiting factor. Avouris said that this isn't a feasible manufacturing technology because of reproducibility problems. The tip characteristics typically change every 15 minutes or so. The patterns of dehydrogenation which are produced by this process are as stable as the vacuum allows. On an Si surface the barriers to migration are typically 50% of desorption energies (while on metals they can be 10-15% of desorption energies), so hydrogen migration is thermally inaccessible. Hydrogen can, however, migrate as a result of STM-induced events.
Petr N. Luskinovich presented a poster session paper on a "Universal Probe". The unique feature of the probe is that it is drawn with a scanning probe, and is useful as a scanning probe. The fabricating scanning probe lays down a wire (currently of carbon, but other materials are possible) on a pyramidal substrate with an SiO2 surface similar to the tips used in AFMs. The substrate is built with conventional lithography and anisotropic etching. The wire can be up to 10 microns long, and can be fabricated with a gap (for lateral tunneling measurements), an interelectrode dot (for single electron transistor operation), or with a weakly conducting bridge (for FET-like operation). The wire has a diameter of 40 nm, but the tunneling gap in the wire can be made as small as 1 nm. This gap is made by depositing a series of successively closer overlapping dots while monitoring the interelectrode tunneling current.
In the sensor with a gap, the lateral tunneling current is sensitive to the presence of a sample that partially bridges the gap. This current does not require that the sample as a whole be conducting, which is an advantage over conventional STM. At the moment the resolution of these probes is about 10 nm, but the physics should permit finer resolution with smaller gaps. The group was also able to build a structure with a cluster of 4 electrodes, permitting nanometer scale experiments with 3 independently controllable potential differences within a few nm of each other.
James Tour presented a paper primarily on results from electronic experiments on conjugated systems. The target for these experiments is to measure the properties of a single molecule bridging two probe electrodes.
In Tour's experiments the molecules are generally poly(phenylene ethynylene)s, (-C6H4-CC-)n and poly(thiophene ethynylene)s, (-C5H2S-CC-)n. These molecules are synthesized by a precise doubling strategy. Monomers are combined into dimers, then into tetramers and so on. At each stage the oligomers are atomically precise. 16mers more than 10 nm long have been built. These structures have been verified with 13C NMR.
In several kinds of experiments, terminal -SH groups have been added to these structures, yeilding thiols. The -SH groups bind to gold electrodes, forming self-assembled monolayers (SAMs). The -SH groups can be thought of as "molecular alligator clips" to grab on to the gold conductors.
In one of these experiments, microlithography was used to form a gold conductor covered by insulator except for a 35 nm hole, on which a SAM was grown. The hole was needed to avoid the shorts that form in large area SAMs. The SAM was covered with a titanium conductor and the electronic properties of the sandwich measured. Diode behavior could be seen, with asymmetrical injection of current from the Au and Ti conductors. Around 30 types of molecules were investigated in this geometry. One of the molecules had a saturated section, forming a potential barrier and exhibiting a resonance peak in its conduction. The peak was sufficiently sharp that the current dropped with increasing voltage in one section of the I(V) curve. This negative differential resistance is an experimental demonstration of the same electronic amplification mechanism as in a tunnel diode, but on a molecular scale. Other experiments done with these molecules included measuring resistance as a function of distance using a mechanical break junction (see the current Recent Rrogress column) and the measurement of the conduction of individual conjugated thiols embedded in a saturated thiol SAM with an STM (see the Recent Progress column in Update 27).
Another structure that was described briefly was a tetrahedral compound with four conjugated molecular wires protruding from a central carbon. Because of its overall tetrahedral structure, this molecule might bind to a scanning probe tip with a radius of a few nanometers, yet provide an atomically sharp, chemically well-defined tip. The legs of this molecule span 2.8 nm. Because of the symmetry of the molecule, any binding orientation will yield the same structure.
Tour also described some work based on fullerenes. He described binding an ammonium group to C60, thus giving it a positive charge. This compound is attracted to the negatively charged phosphate groups on a DNA backbone. It forms an outer spiral coil around the DNA. While all of the atoms involved are light, and normally don't show up well in transmission electron microscopy (TEM), the fullerenes do show up in TEM, and so does DNA surrounded by this fullerene helix. Tour suggested that this arrangement of C60 might be used as another type of molecular wire.
Geoff Leach and Robert Tuzun presented a paper analyzing diamond and graphite struts with molecular dynamics calculations and displaying the results on the web. Struts are important parts of a variety of machines, notably including positioning devices like Stewart platforms.
The calculations looked at struts with varying aspect ratios, cross-sections, terminating atoms (for diamond), potential energy functions, and temperatures. The most dramatic differences were seen between struts with 100:1 aspect ratios and struts with lower (10:1 and 1:1) aspect ratios. The diamond strut with a 100:1 aspect ratio and a unit cell cross-section actually starts to curl (with the end-to-end distance dropping by 1.2 nm after 20 psec) due to thermal vibration (both at 150K and at 300K), while a similar strut with an aspect ratio of 10:1 showed end-to-end length fluctuations of only 0.1 nm.
The other variables in the simulations made surprisingly little difference. Switching the simulations between 150K and 300K appears to change the transverse displacements and end-to-end distance fluctuations by perhaps a factor of sqrt(2), as one might expect. The presence or absence of terminating atoms doesn't change results much. Changing the potential function by switching non-bonded interactions on or off "also appears to have little effect." This is encouraging, since this part of the potential requires a lot of computation. Omitting it in exploratory studies will allow examining more structures when optimizing them.
One choice that proved important was setting initial conditions carefully. These simulations were done with an initial annealing step, during which the structure was allowed to approach an equilibrium geometry while kinetic energy was gradually removed. In one simulation where this wasn't done completely, the strut vibrated in a low order flexing mode for the remainder of the simulation.
The structural conclusions from this study were that support struts should have a cross section of at least 1 nm square. This was sufficient to keep both end-to-end distance and transverse fluctuations below 0.1 nm in struts with 10:1 aspect ratios.
The dynamics shown in these simulations are being distributed in the virtual reality modeling language, VRML 2.0. The trajectories of the atoms are represented by (x,y,z) coordinates at 1 psec intervals. Unlike the mpeg movie format, this allows a viewer to look at the moving structure from whatever angle they wish.
Reza Ghadiri presented a paper on self-reproducing molecular systems. The talk centered on the chemical kinetics of reacting populations of molecules. He described one example where a template aligns two reactants with surfaces complementary to that of the template to form a copy of itself, but the bulk of the talk was not on creating new covalent or weakly bonded geometric structures. The focus was on the information held by concentrations of reactants in a network of interdependent reactions. He outlined a classification scheme for some of the possible patterns of reaction networks.
Richard Jaffe presented a paper on electronic quantum mechanical calculations of the structure of a diamond (100) surface. It is important to know how diamond surfaces react because this affects how one might build them. The (100) surface has an ambiguous structure. Adjacent carbons pair up into dimers, but it is unclear whether these pairs act like single bonded diradicals or double bonded pairs. A diradical structure would cause higher reactivity than a double bonded structure, possibly requiring more passivation of this surface during mechanosynthesis to avoid undesirable side reactions.
Jaffe did ab initio calculations on a model system using the 6-31G* basis set. The model system is a C9H12 cluster. It contains atoms from 4 layers in the crystal: 2 surface carbons from a dimer, 4 carbons from the first buried layer, 2 carbons from the second buried layer, and 1 carbon from the third buried layer. All of the carbons from the buried layers are bonded to hydrogens. In this system, the calculated C-C bond length is 0.136 nm. By comparison, in single-bonded ethane it is 0.153 nm and in double-bonded ethylene it is 0.134 nm.
A previous analysis of this system by Weiner examined orbital density plots and concluded that it acted like a diradical. Jaffe examined the quantum mechanical calculation results by examining the Laplacian of the electron density. The Laplacian is a second derivative, and highlights the places where the electron density changes rapidly. Jaffe says that this highlights the chemically important valence electrons and deemphasizes the chemically unimportant core electrons. Some of the surfaces of constant Laplacian density seemed to show lobes on the two surface carbons, pointing roughly away from the bond between them. This is consistent with a view of the carbons as diradicals, with a tetrahedral arrangement of the bonds around them. Jaffe's overall conclusion was that the dimers showed enough diradical character to make them reactive.
Jaffe also looked at changes in the electron spin density as a surface hydrogen is added to this cluster. When the bond to the hydrogen is completely broken, this density becomes 1, and when the bond is fully formed, this measure becomes 0.
Personally speaking, I think that it would have been clearer just to show the energy changes as a function of the reaction coordinate as a hydrogen atom is brought towards a C9H12 cluster, and compare this graph with the equivalents for adding a hydrogen atom to ethylene and to a methyl radical. The latter would represent the double bonded and radical cases. In analyzing reactivity, the electron densities are mostly important insofar as they affect the potential energy for an incoming reactant.
Stephen Walch presented a paper on quantum mechanical simulations of tools for adding carbon atoms to diamond surfaces. This work follows in the tradition of Musgrave's calculations of the energetics of a hydrogen abstraction tool. The present work is more ambitious, following more active atoms in more complex tools. Walch examines the use of two tools, a carbon atom deposition tool and a carbon dimer deposition tool on two surfaces, the diamond (111) surface and the reconstructed diamond (100) surface. Walch examined the stability of each deposition tool in isolation, then examined the feasibility of adding carbon to diamond with these tools.
Both tools could potentially isomerize to different structures. The carbon atom deposition tool is a carbene attached to a cyclopropene ring with a double bond. The carbene moeity is the carbon atom that is eventually delivered to the workpiece. The remainder of the tool stays attached to the actuator. The four carbons in this tool might plausibly form a four membered ring instead of a three membered ring plus an attached carbene. Walch calculated that the desired carbenecyclopropene structure is more stable than the undesired cyclobutane, so isomerization won't destroy this tool. The carbon dimer deposition tool is a six membered ring with two carbons and two oxygens permanently attached to the actuator and two carbons triple bonded together. These last carbons are deposited on the workpiece. This dimer deposition tool is less stable than an isomer, but the barrier to rearrangement is high enough (0.11 aJ) to prevent it at room temperature. These tools are both stable enough to be usable.
Walch examined both single carbon and carbon dimer deposition on the diamond (111) surface. He concluded that single carbon deposition won't work on (111), but dimer deposition will work. The failure of the single carbon deposition is fairly subtle. The carbene moeity does form a bond with the surface. Unfortunately, it only bonds to a single preexisting carbon. In order for this tool to work, the carbon it deposits must bond to two pre-existing atoms. This allows the tool to drop the new carbon by breaking the bond to it (which is a double bond) in a two step process, twisting the bond to reduce it to a single bond, then pulling away to break the single bond. The surface carbons on (111) are too far apart for a single carbon to bridge them, so it bonds to just one. Pulling the carbene deposition tool away from this structure would just pull the new carbon back off the surface. The dimer deposition tool is able to span two surface sites on (111), and it is able to leave a pair of carbons on the surface successfully. On reconstructed diamond (100) the preexisting atoms are present as dimers so they are more closely spaced (0.14 nm) than on (111) (0.252 nm). The closer spacing on (100) allows the carbon atom deposition tool to bond to two adjacent atoms, so the bond/twist/pull sequence works here.
Walch also examined using a pair of tools on the diamond (111) surface. First he showed that the dimer deposition tool could add a pair of atoms on top of the surface, then he showed that the carbene tool can deposit a single carbon on top of this new dimer. When a third neighboring site on the original (111) surface is also open, these three new carbons can flip down to lie parallel to the original surface.
From Foresight Update 31, originally published 15 December 97.
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