Nanocars!

Researchers at the Rice University embarked on a 'fantastic voyage' toward miniaturizing the beloved car to a scale that would be considered science fiction 20 years ago.  In 1998, principal investigator James Tour, professor of organic chemistry at Rice, began this journey.  It took his group eight years to synthesize a functional nanocar.  The goal was to create a nanomachine capable of converting energy-inputs (such as heat or electric fields) into controlled motion on a surface and transport cargo from one place to another on a surface.  Future nanomachine development depends on this because the bottom-up constructions may occur on the surface of semiconductors, metals, or biological and artificial membranes.  Thus, acquring better data on the chemistry and physics on nano-transporters on surfaces is critical; the creation of nanocars is an initial, but significant, step in this scientific frontier.  Nanocars allow us find out basic rules for their design and operation. 

The basic design of a nanocar is much like a real car: it has a  chassis and four axles, except that it's made out of organic groups with pivoting suspension and the axles freely rotate.  The wheels are buckyballs, The entire car measures just 3-4 nm across, just wider than a strand of DNA. 

Though laborious measurements on the STM, the researchers proved that the cars rolled on the surface rather than slide across as previously predicted.  Of course, they had to raise the temperature of the gold surface to 200 ºC, but they were still able to determine that it was easier to move the car forward or backward rather than sideways.   

Molecular Machinery: A Brief Overview

Functional molecular devices have been designed and fabricated for several years now:

Gold nanorotor

Nanoswitch

Such nano versions of macroscopic machines are in part inspired by the visionary physicist Richard Feynman 50 years ago.  Also, fabrication and synthesis technologies has advanced enough to initialize and perpetuate the quest to miniaturize machinery in the current science and engineering disciplines. 

One of the most advanced current fabrication meths, the "top-down approach to miniturization in the semiconductor industry, is approaching the limits in scaling. For example, the band structure of silicon disappears when silicon layers are just a few atoms thick.  What about photolithography?  Well, its operational wavelengths are too limited for making nanomachines with moving parts.  

Recently, organic chemists in Rice University realized they can synthesize 3 × 4 nm nanocars in 100 mL laboratory reaction flasks–30 mg worth (that translates into 3 × 10¹⁸ nanocars–far exceeding the number of automobiles made in the history of the world.).  30 nanocars, side by side, can span the 90 nm width of a small in line in the most advanced logic chip being made today.  Tiny objects with many moving parts like nanocars have to be made differently, from the "bottom-up".  This follows more naturally in line with Darwinian evolution at the biochemical level, although we're nowhere near the sophistication of Nature's handiwork.  Consequently, molecular scientists have turned their attention to much simpler systems, where the machinery may be easier to construct and understand, perhaps serving as a base for eventual progression toward sophisticated and useful nanomachines.  

Most of the molecular machines shown above were designed and synthesized to operate in the liquid phase because of the abundant tools available to analyze products in solution.  However, useful nanomachines with mechanical functions require eventual integration into devices that interface surrounding systems (likely to operate in the solid phase).  For example, the biological nanomachine ATPase is anchored in a biological membrane in order to connect with its surroundings.  Overcoming this interface problem ranks among the most intractable of problems in nanomachine design.  

Nevertheless, there are several viable designs that (1) work in a crystalline solid, or (2) be mounted on and operated on surface, or (3) move around and operate on a surface.  

 

Machines operating in crystalline solid state

UCLA researchers have made considerable progress in the formulation, design, and preparation of crystalline molecular machines.  Examples of their work include molecular compasses and gyroscopes.  

The center component of the molecule above (red) has a magnetic moment that responds via rotation to external magnetic fields.  The plot qualitatively indicates the energy required for the rotation to occur. 

 Conventional crystalline materials have their molecular components tightly bonded and restricts their motion, which contrasts to another class of materials known as amphidynamic crystals.  These exotic crystals possess rigid lattices connected to moving parts (See pic below).   

Example of an amphidynamic crystal

Successful utilization of such crystals will lead to photonics materials whose components  stem from the bulk (not from solution or thin films), which increases the density, which leads to greater advances.  

Machines mounted and operating on a surface

Molecular machines heavily depend on positioning to be to pump, push, lift materials, or other useful work.  Examples of such machines already produced in the lab are: (see captions of following pictures)

cyclodextrin necklaces. 
J. Am. Chem. Soc. 2000, 122, 5411-5412

Molecular Shuttles,     http://www.ch.nagasaki-u.ac.jp/mol/research_e.html

Light powered molecular motor.  The absorption of sunlight by one of the two stoppers, a light-harvesting one, causes the transfer of one electron to station A, which is deactivated as far as wanting the ring to encircle it. As a consequence, the ring moves to its second port of call, station B. Station A is subsequently reactivated by the return of the transferred electron to the light-harvesting stopper, and the ring moves back to this station. Copyright © UCLA

Machines moving and operating on a surface

As if mounting nanomachines on a surface weren't hard enough, people are also trying to make them move and do stuff.  That may call for zealous usage of scanning probes such as the scanning tunneling microscope (STM).  In fact, the STM has been used to push these molecules on a surface laterally, with atomic scale precision.  Perhaps the most famous molecule used for this purpose is the molecular wheelbarrow:

(a) macroscopic wheelbarrow.  Its colors correspond to the chemical structure in (b).  (c) shows the molecule in the gas phase. 

Mechanical interactions between molecule and surface determines the molecule's motion on a variety of surfaces.  For macroscopic objects, the theory and basic rules for the design of machinery is well-known and justly exploited.  Skiing works for snow, but not for asphalt.  On water, wheels only float on their own if they're made of rubber; recommended solution, get a sailboat.  These situations don't require much brainpower to imagine, but rigorous mathematical and physical theories are necessary to explain them.  But on the atomic level, the rules are different, and therefore the normal human intuitions for working machinery do not apply.  Movement on surfaces might not follow Newtonian physics, but rather quantum mechanics.  Quantum calculations are notoriously difficult and computationally costly to obtain exact answers with the currently available tools.  For now, the development of molecular machinery depends mostly on dogged trial-and-error.  

DNA Robospider

The nanites are here, and they're made of DNA.  Well, not really, but researchers at 4 institutions across the country have created and programmed DNA spider-bots that walk independently across a a nano-scale track.  A baby step (spider step?), but also a milestone, in the nascent fields of molecular computing and and robotics.  Eric Drexler had long envisioned molecular robots in his seminal book Engines of Creation, now manifesting into reality. 
Normally, a robot would have a set of instructions to follow specific tasks repeatedly, from vacuuming carpets to welding cars, but programming biomolecules?  Not a simple task, and requires researchers from many disciplines (CS, chemistry, engineering, biology) to collaborate, which did occur.  The result are DNA walkers with 'legs' that allow them to briefly walk autonomously on a fabricated 2D landscape.  They can begin and continue walking, turn, then stop.  Rinse and repeat.  The walkers are only ~ 4 nm in diameter, and walk very slowly, covering 100 nm in 30 minutes to an hour by only 100 steps.   Compare that to previous walkers took that long to walk only 3 nm.

Researchers have created and observed a molecular robot capable of many steps, and of making decisions where to step and how long to stay. As the robot walks on the substrate, it changes each piece by cleaving off a part. If it touches a spot that has been cleaved already, it does not linger as long. The end of the track glows red and captures the robot, letting the researchers know when it has completed its walk. The robot glows green, allowing for the researchers to see it better. Credit: Zina Deretsky, National Science Foundation

This shows high promise for the emerging nanomedicine field, which promises localized cancer treatment.  Imagine a time when we can look back and consider chemotherapy to be barbaric and inhumane.  Nanomedicine could be our ticket to that future. 

Plasmonics at the bottom: Nanoshells and Invisibility Cloaks

Plasmonics may have roles to play aside from computing.  Researchers Naomi Halas and Peter Nordlander at Rice University have developed structures called nanoshells that consist of a thin layer of gold coating (usually 10 nm thick) around the entire surface of a silica particle about (100 nm in diameter).  Placing this coated particle in an electromagnetic (EM) field generates electron oscillations in the gold shell; this is due to the coupling interaction between the fields on the shell's inner and outer surfaces.  Varying the silica particle size and the gold layer thickness changes the wavelength at which the particle resonantly absorbs energy.  In short, nanoshells can be designed to selectively absorb wavelengths from the blue end of the visible spectrum (approx. 300 nm), and down to the near infrared (about 10 microns).

Surprisingly enough, nanoshells are now being considered for cancer treatment.  In 2004, Halas, working with Rice colleague Jennifer West, injected plasmonic nanoshells into the bloodstream of mice with malignant tumors and found that the particles were nontoxic.  Moreover, the nanoshells tended to embed themselves in the rodents' cancerous tissues rather than healthy ones due to higher blood circulation in the fast-growing tumors.

Additionally, human and animal tissues are transparent to radiation at certain infared wavelengths.  When near-infrared laser light is aimed at the tumors through the mice's skin, the resonant absorption of energy in the embedded nanoshells raised the temperature of the cancerous tissues from 37 ºC to 45 ºC.

This photothermal heating killed the cancer cells while leaving the surrounding healthy issue intact.  In the mice treated with nanoshells, the tumors virtually disappeared within 10 days; in the control groups, the tumors grew unabated.  As of 2009, practitioners are seeking FDA approval for clinical trials of nanoshell therapy in patients with head and neck cancer.

Plasmonics may also revolutionize the lighting industry by making LEDs bright enough to compete with incandescent bulbs.  In the early '80s, researchers found that the plasmonic enhancement of the electric field at the metal-insulator boundary could make certain dyes more luminescent if placed near the metal's surface.  This kind of plasmonic enhancement can also raise the radiation rate of quantum dots and quantum wells (tiny semiconductor structures that absorb and emit light), which improves the efficiency and brightness of solid-state LEDs.  A recent collaboration between Caltech and Nichia Corporation in Japan demonstrated that coating the surface of a gallium nitride LED with dense arrays of plasmonic nanoparticles (made of gold, silver, or aluminum) could intensify the emitted light 14-fold. 

Additionally, plasmonic nanoparticles may one day lead to LEDs made of silicon.  Silicon-based LEDs would be much cheaper than conventional LEDs composed of gallium nitride or gallium arsenide, but such devices currently emit too little light.  Research has shown that coupling silver or gold plasmonic nanostructures to silicon-dot arrays could boost their photon intensity by 10 times.  Moreover, the frequency of the enhanced emissions can be tuned by adjusting the geometries of the nanoparticles.  Simulations have indicated that careful tuning of the plasmonic resonance frequency combined with precise control of the separation between the metallic particles and the semiconductor materials may allow emission rates to rise more than 100-fold, allowing silicon LEDs to shine just as brightly as traditional devices.

Then there's the plasmonic analogue to a laser.  Mark Stockman of Georgia State University and David Bergman of Tel Aviv University theorized a new device called a SPASER (surface plasmon amplification of stimulated emission of radiation).  They have suggested fabrication methods via semiconductor quantum dots and metal particles.  The radiative energy from the quantum dots would be transformed into plasmons, which would then be amplified in a plasmonic resonator.  The plasmons generated by a SPASER  would be much more tightly localized than a conventional laser beam; this would allow the device to operate at very low power and selectively excite very small objects.  The result: SPASERs may further sensitize spectroscopy to more phenomena and to more sensitive hazardous material-detectors for minute amounts of chemicals or viruses.

Speaking of excitement, plasmonics are a potential candidate for the proverbial invisibility cloak of Hollywood films.  What is taught again and again in freshman physics is the refractive index, which is the ratio of the speed of light in a vacuum to the speed of light in the material.  If the refractive index is made equal to that of air, the object is rendered nearly invisible.  For plasmonic materials, this could be done with two things:
1) Use radiation that is close to the resonant frequency of the structure; it would neither bend nor reflect light.  The light is absorbed, never to bounce back to the observer's eye.
2) Laminate the structure with a material that produces optical gain (amplifying the transmitted signal just as the resonator in a SPASER would); the resulting higher intensity would offset the absorption losses.  The structure is thus rendered invisible, at least to radiation in a selected range of frequencies.

In theory, plasmonic materials could render objects invisible.  In one proposal, the cloaking device would be a thick shell constructed of metamaterials, which exhibit unusual optical properties.  This shell could bend radiation around its central cavity, where a spaceship could be hidden.  A space telescope pointed at the shell would see only the galaxy behind it. 

Now, it's one thing to render an object invisible for a set frequency, but it's quite another to hide it for the entire visible spectrum.  Some physicists say it is possible.  John B. Pendry of Imperial College London and colleagues showed that a shell of metamaterials might, in theory, reroute the electromagnetic waves traveling through it, diverting them around a spherical shell, as seen in the figure above.

Invisibility cloaks may never see the light of day (ha!), but such ideas highlight the wealth of optical properties that inspire scientists working in plasmonics.  By investigating the sophisticated coupling between electrons unbounded to atoms, and electromagnetic waves, researchers have identified new possibilities for transmitting data in our microchips, illuminating our homes, and fighting cancer.  Indeed, there's still a lot of 'room' to explore in plasmonics.

Plasmonics at the bottom: shrinking wavelengths

Plasmonics may be older than you think.  Alchemists and glassmakers spent millennia taking advantage of plasmonic effects when they created stained-glass windows and colorful goblets that incorporated small metallic particles in the glass. By analogy, ignorance of modern biology did not stop Stone-Age humans from genetically engineering wolves and various plants (via selective breeding) to become dogs and farming crops.   Best known example of alchemical plasmonics is the Lycurgus cup, a Roman goblet dating from the 300s A.D.; now held in the British museum.  Thanks to plasmonic excitation of electrons in the metallic particles suspended in the glass, the cup absorbs and scatters blue and green light (relatively short wavelengths in the visible spectrum).  When viewed in reflected light, the plasmonic scattering gives the cup a greenish hue, but if a normal light source (emits white light) is placed inside the goblet, the glass appears red because it transmits only the longer wavelengths (red) and absorbs the shorter ones (green/blue).

Lycurgus Cup: a Roman goblet dating from the 4th century A.D.  It changes color because of the plasmonic excitation of metallic particles within the glass matrix.  When a light source is placed inside the normally green goblet, it looks red.

 Surface plasmon research began with a bang in the 1980s, as chemists studied the phenomenon using Raman spectroscopy: the scattering of laser light off a sample to determine its structure from molecular vibrations.  In 1989, Thomas Ebbesen, then at the NEC Research Institute in Japan, discovered that when he illuminated a thin but opaque gold film imprinted with millions of microscopic holes, the foil somehow transmitted more light than was expected from the number and size of the holes.  This phenomenon, now known as extraordinary optical transmission, was eventually found to be caused by surface plasmons that intensified the transmission of electromagnetic energy.

Then came the discovery of novel "metamaterials": materials in which electron oscillations can result in some weird optical effects.  Their optical effects are so complex (due to plasmons) they require astronomically powerful computers for accurate simulation, but studying them has become less daunting by novel methods for constructing nanoscale structures that allow researchers to build and test ultrasmall plasmonic devices and circuits.

In most circumstances, it would be unwise to use metallic structures to transmit light signals because metals are known for high optical losses (signal weakens after each bounce off the metallic surface until evanescence).  However, when a thin film is combined with an electrical insulator, optical losses decrease due to the electromagnetic (EM) field spreading into the insulating material, where there are no conducting electrons  to oscillate, thus no energy-dissipating collisions.  This property naturally confines plasmons to the metallic surface adjoining the insulator; e.g., in the the top portion of the figure below (PLANAR WAVEGUIDE), the surface plasmons propagate only in the thin plane at the interface.  Note that in this example, the insulator is air. 

With this planar structure acting as a waveguide, shepherding the EM waves along the metal-dielectric boundary, it may be useful in routing signals on a chip.  Most optical signals may attenuate rapidly in metals, but the exception here is a plasmon traveling in a thin-film metal waveguide, whose journey can last for several cm.  The plasmon signal can be made to last longer  if the waveguide employs an asymmetric mode,  meaning the EM energy disperses more intensely in a certain direction than others.  This mode pushes a greater portion of the EM energy away from the guiding metal film and into the surrounding insulator, thus delaying attenuation a little longer.  The EM fields at the top and bottom surfaces of the metal film are known to interact with each other, and this interaction can be manipulated by changing the thickness of the film, which manifests as different frequencies and wavelengths for the plasmons.  In fact, this has already been done in the late '90s by a Danish/Canadian collaboration; they created a planar plasmon waveguide like in the top of the figure, but the EM fields it generated were too large to convey signals through the nanoscale innards of a processor.

Plasmons can propagate through nanoscale wires, but they require more complex waveguide geometries than that of the planar form; plasmons can shrink the wavelength of the optical signal by squeezing it into a narrow space.  This is seen in the middle illustration of the above figure.  This device is called a plasmon slot waveguide; the plasmon wavelength changes with respect to the thickness of the insulator core in the middle.  This is capable of transmitting a signal 10s of microns long (1 micron = 1 µm ≈ diameter of a blood cell).  Now comes the juicy part: a Japanese researcher had managed to squeeze red light (wavelength of 651 nm) into a plasmon slot waveguide that was only 3 nm thick and 55 nm wide.  The wavelength of the surface plasmon propagating through the device was 51 nm, or 8% of the original red light wavelength.  But the frequency remains the same, which means information gets transmitted.  This striking ability to shrink the wavelength opens the floodgates for nanoscale plasmonic structures to perhaps replace purely electronic circuits containing wires and transistors. 

Mass-production of plasmon slot waveguides is another story, but it should be similar to lithography (used to imprint circuit patterns on silicon chips).  This process could mass-produce miniuscule plasmonic devices with arrays of narrow insulating stripes and gaps.  These arrays would guide the waves of positive and negative charges on the metal surface; their behavior is similar to alternating current traveling along an ordinary wire.  However, since the frequency of an optical signal is so much higher than that of an electrical signal (400,000 GHz vs. 60 Hz, respectively),  the plasmonic circuit can carry tons more data.  Additionally, electrons  don't travel from one end of a plasmonic circuit to another; rather, they clump together and spread apart (thus no net directional surface current).  This fact accounts for the device's immunity to resistance and capacitance effects that limit the data-carrying capacity of integrated circuits with electrical interconnects.

Plasmonics at the bottom? Part one.

According to Thomas Friedman, one reason why the world got flat is the mass installation of optical fibers that now span the globe; fibers that guide light signals conveying voluminous streams of voice communications and gigantic amounts of data.  Some researchers believe that such colossal capacity in photonic devices (able to channel and manipulate electromagnetic radiation like light) could someday replace electronic circuits in microprocessors and other computer chips.  Why haven't they already?  Because of the diffraction limit, which constrains the size and performance of photonic devices due to interference between closely-spaced light waves.  The width of an optical fiber carrying closely space light waves must be at least half the light's wavelength inside the material.  Chip-based optical signals usually employ near infared wavelengths of 1.5 µm (micrometers, or millionths of meter–a common unit in cell biology), which far exceeds typical dimensions of base components in most electronic devices being used today–certainly in my computer.

However, scientists have kept busy.  In the 1980s, researhers experimentally confirmed that directing light waves at the interface between a meter and a dielectric (a nonconductive material such as air, glass, or highly pure water) can, if conditions are right, induce a resonant interaction between the waves and the mobile electrons at the metal's surface.  This means that the oscillations of electrons at the surface match those of the electromagnetic field (read: waves) outside the metal.  We now have surface plasmons:  density waves of electrons that propagate along the interface like the ripples that spread across the surface of a pond after a rock splashes into it. 

Recently, scientists discovered that a fine-tuned metal-dielectric interface can generate surface plasmons with the same frequency as the outside electromagnetic (EM) waves, but with a much shorter wavelength.  This effect may enable the plasmons to travel along nanoscale wires called interconnects, carrying information from one section of a microprocessor to another.  I could sense chip designers salivating over these interconnects; they're still making ever smaller and faster transistors, but it's now harder to to build minute electronic circuits that can move data quickly across the chip.

Ten years ago, Professor Atwater and colleagues at Caltech named this emerging discipline "plasmonics", sensing a research pathway may lead to a new class of devices.  One day, plasmonic components might be vital in a diverse set of instruments, using them to improve the resolution of microscopes, LED efficiency, and the sensitivity for chemical and biological detectors.  Plasmonics are even being considered for medical applications; e.g., tiny particles designed for plasmon resonance absorbion to kill cancerous tissues.  Then there's the invisibility cloaks, but that'll be discussed in a future entry, which will be quite soon.

In meantime, here's a nice artist's rendering of a light beam striking a metal surface, thereby generating a plasmon, which is also awkwardly called an electron density wave.  If the light beam is focused on a surface with a circular grove, as shown here, it produces concentric waves, organizing the electrons into high- and low-density rings.

Boron Nitride nanotubes

I wrote about CNTs in my last post; figured it was inevitable since they get so much press.  I've been binging on Skeptics Guide to the Universe recently.  They're a weekly podcast that covers pseudoscience and various shenanigans in the world of nonsense.  Each podcast concludes with a segment known as "Science, or, Fiction", in which the SGU members guess which of the three science news stories is fiction (I should use this when I'm the tabletopics master in a Toastmasters meeting).  One podcast from earlier this year had a S or F story about boron nitride nanotubes, which I've never heard before.  I correctly guessed this is science, since I haven't yet heard a materials science article of their selection that turned out to be fiction. 


So, what the heck are boron nitride nanotubes? Well, to start, boron nitride (BN) is a binary chemical compound consisting of equal proportions of boron and nitrogen. It's the white material used in clown makeup and face powder.   It's isoelectronic (shares the number of electrons) with carbon, and can morph into similar physical structures: graphene-like, and diamond-like.  The latter  is the only material nearly as hard as diamond.  It remains a solid at temperatures up to 1000ºC for air, and up to 1800ºC in an inert gas atmosphere.  That's twice as high for carbon nanotubes. 

If you look for carbon on the periodic table, you may see boron is on its left, and nitrogen is on its right.  Carbon has four electrons in its valence shell, which means another four is needed to satisfy the octet rule.  Thus, boron has three valence electrons, and nitrogen has five.  Combining these two atoms satisfies the octet rule.  By extension, BN can form a graphene (1 atomic layer of graphite) lattice.  But what makes BN different from its carbon analogues is its wide band gap of approx. 5.5 eV, making a wide-gap semiconductor (effectively an insulator). 

The high bandgap for BN nanotubes (BNNT) is a factor behind their high thermal and chemical stability.  They also have excellent mechanical properties and high thermal conductivity, much like carbon nanotubes. 

Structural models of a BN monoatomic sheet and a single-shelled BN nanotube. Alternating B and N atoms are shown in blue and red.

Boron nitrides are now being used in a rather ancient technique, the spinning of textiles into yarns.  They've already done this with CNTs as demonstrated in this video here:





Until recently, high-qualtiy BNNTs could be synthesized only up to a micron ( 1 µm) long. Longer nanotubes were riddled with crystalline defects. A team of materials scientists from four institutions
(The paper is found here) spun a ~1 mm thick yarn that looks like cotton:

This yarn is 3 cm long.  "They're big and fluffy, textile-like" said Kevin Jordan, a staff electrical engineer at Jefferson Lab.  "This means you use commercial textile manufacturing and handling techniques to blend them into things like body armor and solar cells and other applications."

How'd they spin this stuff?  By using a laser aimed at a cake of boron inside a chamber filled with nitrogen gas.  This forms a plume of boron gas that shoots upward.  They then insert a cooled metal wire into the gas, causing the gas to condense into liquid droplets.  The droplets combine with nitrogen to self-assemble into BNNTs.  It's a violent reaction that yields the long nanotubes in milliseconds. 

Mass-production of cheap BNNTs may lead to lighter, faster car frames, affordable space vehicles,  and ultralightweight armor; all promised by CNTs, but with the implication that BNNTs are simply easier to make en masse.  What separates CNTs from BNNTs in terms of utility comes down to electrical properties.  The geometry of CNTs drastically changes their conductivity. whereas BNNTs experience very little conductivity alternations with respect to geometry.  In addition, thanks to their chemical inertness, BNNTs promise "pinpoint precision to attack cancer cells by sticking to tumors, absorbing neutrons from a targeted beam, and generating local alpha radiation to kill cancer cells." (See ScienceNOW )