Nano Materials

01.03.2004
 

 

 


 

Introducing fullerenes, nanotubes and other mini-marvels

 

 

Introducing fullerenes, nanotubes and other mini-marvels

 

A land whose stones are iron, and out of whose hills you can dig copper. - Moses, describing the material richness of the Promised Land, Deuteronomy 8:9
 
They sound like characters in a miniature fantasyland: fullerene, nanotube and quantum dot. But these and other nanosized particles may give rise to a host of real-life technologies.
 
Fullerenes are soccer-ball shaped molecules made of 60 carbon atoms, discovered in 1985 by Richard Smalley and colleagues at Houston’s Rice University. While commercial applications have been slow to materialize - primarily due to their high production costs - scientists believe that fullerenes may serve the basis for future insulators, semiconductors and novel drug delivery systems that might, thanks to their diminutive size, interact directly with target cells, proteins and viruses.
 
Nanotubes rolled onto the scene a few years later. Roughly one hundred times stronger than steel and able to withstand temperatures of up to 6,500 0 F, these cylinder-shaped carbon molecules are a promising material for diverse engineering targets, from electronics to ultrastrong lightweight cars, airplanes, ropes and fabrics.
 
And the quantum dot (QD), while still largely a laboratory curiosity, is a nanosized “box” that allows researchers to trap individual electrons and monitor their movements. Future applications might feature QDs in solid-state lasers emitting light at wavelengths never before achieved.
 
The immense potential of these tiny materials lies in the nano niche they occupy, where the classical Newtonian laws of physics play second fiddle to quantum phenomena. On this scale, electrons and other sub-atomic particles can act as either waves or particles, allowing them to pull off such surprising feats as being in two places at the same time and traveling through otherwise imper-meable layers in a semiconductor. Taking advantage of the electron’s dual nature, future computers might have quantum bits capable of existing simultaneously in the 0 or 1 state (the language used by computers to represent information), thus dramatically fast-forwarding information processing.
 
Another important property of nanoscopic structures emerges from their high surface-to-volume ratio: as structures get smaller, the percentage of their atoms located at or near the surface increases. This feature has major implications since so many of the interactions occurring in the material or biological world take place at the surface (such as immune responses triggered by the surface-borne antigens carried by invading foes).
 
Collectively, future devices made of nano-materials will be aimed at cashing in on the unique behavior of electrons in a quantum setting to usher in some of the coveted goals of miniaturization: better, faster, cheaper.
 

 

Fullerenes

 

illustration:"Buckyball" discovery

 

 
Nanotechnology is the builder’s final frontier - Richard Smalley, co-discoverer of fullerenes
 
In the early 1980s British chemist Sir Harold Kroto was spending much of his time gazing into space, studying strange-looking carbon molecules found near red giant stars. Thinking that the conditions near these stars might be critical for the molecules’ formation, Kroto decided to contact Rice University scientists, Robert Curl and Richard Smalley, who were studying similar clusters using microwave spectroscopy.
 
By the end of 1985 the three had made history, discovering a new form of pure carbon containing 60 carbon atoms, which proved to be incredibly stable. While the molecule’s structure was not initially clear, the team suspected that its immense stability could arise only from a spherical shape that remarkably resembles a soccer ball - hence their chosen name for it: Buckminsterfullerene, in honor of R. Buckminster Fuller, an architect with a passion for building geodesic domes.
 
This newly discovered molecule (popularly known as buckyball), revolutionized the understanding of carbon, giving rise to a new field of chemistry. It earned its discoverers the 1996 Nobel Prize in Chemistry.
 
 

Life on the dangling edge

 
For a while after the discovery of fullerenes, people thought that these super-tough molecules could be formed only by carbon, or perhaps by other carbon-containing materials. But in 1992, researchers at the Weizmann Institute led by Prof. Reshef Tenne discovered an inorganic (carbon-lacking) compound that also forms fullerene-like balls and nanotubes under certain conditions. This finding opened a promising field of research, currently pursued by around 50 groups around the world.
 
“The key question we asked ourselves,” says Tenne, who heads the Institute’s Materials and Interfaces Department, “is what happens when you scale down a material, bringing at least two of its dimensions (say, length and width) to a scale of roughly 10 - 100 nanometers.” Tenne’s hypothesis was that it would be unstable and immediately fold into the hollow, cage-like structure of fullerenes.
 
Experimentation proved him right. While preparing thin sheets of tungsten-disulfide for a new kind of solar cells, Tenne’s team noticed that exposing the sheets to high temperatures caused them to acquire strange configurations resembling fullerenes and nanotubes. Subsequent research showed that a multitude of materials shaped in a planar, sheet-like form, behave similarly under the same conditions.
 
Any material shaped like a sheet of paper is subject to a phenomenon known as the “edge effect,” Tenne explains. The chemical bonds at the edges, called dangling bonds, are unpaired, making them chemically reactive - they seek to pair up with neighboring molecules. This reactivity is too small to have an effect, as long as the material is large enough, but once scaled down to nano dimensions, nearly 50 percent of its molecules are at the edge, causing it to fold.
 
 

Slip sliding away

 
In related research, Prof. Reshef Tenne’s team soon realized that these seamless fullerene-like structures might make an ideal lubricant, a critical requirement of all machinery, from car engines to space ships.
 
Working with Prof. Lev Rapaport of the Holon Institute of Technology, the team found that inorganic fullerene-like materials made of tungsten disulfide reduced friction in machinery by more than 50 percent, particularly under extreme conditions where conventional lubricants fail. Their study was published in Nature.
 
The new material was patented by Yeda Research and Development, the Institute’s technology transfer arm, and has recently been transferred from the laboratory to commercial development by NanoMaterials Ltd. Potential applications range from bullet-proof vests and safety-enhanced vehicles to improved hip replacements and machinery lubricants. Even the sporting world might benefit, with precious milliseconds shaved off world records thanks to reduced-friction skis, swimsuits or bikes.
 
“Basically, everything that moves is subject to friction. This, in a nutshell, explains the importance of effective lubricants,” says Tenne.
 
According to Tenne, conventional lubricants contain crystallites shaped like flat platelets, with chemically reactive edges. Under working conditions, they stick to machinery parts and undergo chemical reactions that cause them to decompose and rub off. The new molecules, in contrast, are round and inert. They have no dangling bonds at the edges that can catch on to metal surfaces, so they just roll against each other and against the machinery parts, reducing friction between surfaces. They also have an odd, onion-like makeup, which enhances their durability. As the top layers wear off, those underneath maintain a lubricating action. An analysis of the conditions leading to the peeling of layers was undertaken in collaboration with Prof. Sam Safran of the same department.
 

 

Nanotubes

 
illustration: "space elevators" made of nanotubes

When asked about the practical value of carbon nanotubes, I respond with the same reply given by the great English scientist Michael Faraday to the then Minister of Finance: "One day, Sir, you may tax it." - Sumio Iijima, discoverer of nanotubes
 
They're incredibly tiny - a millionth of a millimeter in width - and yet are expected to accomplish a multitude of feats. Discovered in 1991, the first nanotubes were made of carbon and captured the attention of scientists worldwide when they proved to be the strongest material ever made as well as being excellent conductors of electricity and heat.
 
Scientists estimate that a fiber about the width of a human hair made of carbon nanotubes could support around 2 tons. The tubes’ phenomenal strength has triggered research targeting applications ranging from classic engineering quests - such as taller buildings and a potpourri of other structures - to science-fiction-like missions, including cables that would tether a satellite in orbit or even make possible “space elevators” carrying people or equipment into outer space.
 
Significant challenges remain in carting these tinker tubes from the lab to industry. For starters, their production process results in a random mix of both metallic and semi-conducting tubes. Scientists have yet to figure out how to selectively prepare one or the other, or even how to separate the two types after production.
 
 

A nanotube nursery

 
Tackling one of the hurdles facing the nanotube industry, Dr. Ernesto Joselevich of the Institute’s Materials and Interfaces Department recently chanced upon a possible solution to sort the newly formed tubes according to their potential function: metallic or semiconducting.
 
“My hypothesis was that I could control nanotube production using an aligning force, such as an electrical field,” says Joselevich. “Just like a compass aligns to the north, the tube’s direction of growth would be controlled by an electrical field, whereas its length would be determined by its rate and duration of growth.”
 
The approach proved successful, producing nanotubes at a record length of nearly 100 microns. Moreover, Joselevich found that in applying an electrical field to control the direction of growth, he had inadvertently discovered a way of separating the tubes according to function. As long as they were longer than one micron, the tubes were aligned to the electrical field, but once short of this mark, only the metallic tubes were aligned; the semiconducting ones lay in completely random orientations.
 
Published in Nano Letters, these findings are the first case of a nanotube production strategy that separates metallic from semiconducting tubes as they are formed and may be a launch pad to controlled production. A streamlined production line of this sort would play a critical role in integrating these uncanny tubes into electronic circuits. The method could also be applied to control the growth direction of any material that can be used as a wire, from metals to protein fibers, thus offering potential industrial as well as biomedical applications.
 
 

The force

 
A new study of how carbon nanotubes bind to different materials may advance the development of some of the most effective composites ever.
 
The study, appearing in Physical Review Letters, was performed by Prof. Daniel Wagner and Dr. Asa Barber of the Institute’s Materials and Interfaces Department, together with Dr. Sidney Cohen of the Chemical Research Support. The team showed that the strength of adhesion between individual carbon nanotubes and an assortment of polymers is between 5-10 times stronger than that between these polymers and the leading composite wonder - carbon fiber.
 
The principle of forming a composite is to combine two materials to create a new material that is generally far superior to both its individual components.
 
Researchers first raised the idea of using nanotubes to create stronger composites after studies (including one by Wagner in the late 1990s) showed that they were phenomenally strong - checking in at 100 times stronger than steel and even 30 times stronger than carbon fiber.
 
But for their idea to succeed, it was necessary to determine that nanotubes would bind effectively to their proposed partners, since a key factor to creating high-performance composites is good adhesion. Only through a successful “merger” could the tubes serve as an efficient reinforcing element to absorb stresses incurred by the material.
 
Previous studies had attempted to evaluate nanotube adhesion using indirect measures, by examining the adhesion formed between many nanotubes and a polymer and then calculating the average. But these results offered only a limited understanding of the adhesion process. To overcome this problem, the Institute team set out to perform an experimental first - to measure the adhesive force between a single nanotube (where each tube is only a millionth of a millimeter wide) and an embedding polymer. Using an imaging technique known as atomic force micro-scopy (AFM) they first introduced the tube to the polymer and then detached it, since the force required to detach this tube would precisely indicate the force of adhesion.
 
“This finding raises the hope of creating a new generation of substantially stronger composite materials,” says Wagner. “Potential applications would range from novel aircraft and space technologies to unsurpassed bridges and new nanodevices.”
 
 

Breaking the scales

 
In 1997 Prof. Wagner performed the first experiment aimed at determining the mechanical properties of nanotubes, a new kid on the materials science block.
 
In measuring how much force could be applied to the tubes before they collapsed, the team’s scales hit 150 gigapascals - thirty times higher than carbon fiber, one of the strongest materials previously known. “Instead of being thrilled I was rather worried,” Wagner recalled with a smile. “How do you publish such extraordinary figures?”
 
The study, published in Physical Review Letters (1998), was confirmed a year later in computer simulations at NASA and a subsequent experimental study at Northwestern University.
 
 

Tubes with a twist

 
Having set out to study the passage of biological molecules through different membranes, Institute scientists recently came up with something entirely different - a new type of nanotube built of gold, silver and other nanoparticles.
 
Made at room temperature - a first time achievement, the tubes exhibit unique electrical, optical and other properties, depending on their components, and as such, may form the basis for future nanosensors, catalysts and chemistry-on-a-chip systems.
 
The study, published in Angewandte Chemie, was performed by Prof. Israel Rubinstein, Dr. Alexander Vaskevich, postdoctoral associate Dr. Michal Lahav and doctoral student Tali Sehayek, all of the Institute’s Department of Materials and Interfaces.
 
“We were amazed when we discovered the beautifully formed tubes,” says Rubinstein. “The construction of nanotubes out of nanoparticles is unprecedented.
 
The new nanotube created at the WIS lacks the mechanical strength of carbon nanotubes. Its advantages lie instead in its use of nanoparticles as building blocks, which makes it possible to tailor the tube’s properties for diverse applications. The properties can be altered by choosing different types of nanoparticles or even a mixture, thus creating composite tubes. Moreover, the nanoparticle building blocks can serve as a scaffold for various add-ons, such as metallic, semiconducting or polymeric materials - thus further expanding the available properties.
 
The resulting tube is porous and has a high surface area, distinct optical properties and electrical conductivity. Collectively, the tubes’ unusual properties may enable the design of new catalysts as well as sensors capable of detecting diverse substances present in minuscule amounts.
 
A key feature of their success would be the ability, due to the tube’s room-temperature production, to add on biological molecules that would otherwise be destroyed by high production temperatures. These would then perform their natural function of recognizing other molecules in nature, in a key-fits-lock manner. Other tube applications might include lab-on-a-chip systems used in biotechnology, such as DNA chips that detect genetic mutations or evaluate drug performance. Yeda, the Institute's technology transfer arm, has filed a patent application for the tubes.
 

 

Self Assembling Films

 

Illustration: self-assemble into functional structures

 


The elevator to success is out of order. You'll have to use the stairs... one step at a time. - Joe Girard
 
Scientists spent years developing the tools needed to observe individual atoms or everyday housekeeping events occurring in the living cell. But the key challenge of nanoscience is not just how to observe these minuscule settings, but how to manipulate them to design nano-structures with novel applications.
 
In the early 1980s, the Institute’s Prof. Jacob Sagiv made a pioneering contribution to this field, with an approach known as planned or guided self-assembly. The idea is to allow atoms and molecules - the tiniest building blocks of matter - to self-assemble into functional structures. The approach introduces chemical changes to organic molecules, causing them to vertically bind additional molecules. Working with Dr. Rivka Maoz, Sagiv later expanded this approach to create 3-dimensional designs, in a strategy called constructive nanolithography.
 
Today, numerous groups around the world are pursuing the goal of self assembly. Some are turning to DNA, proteins and other biological compounds - master self-assemblers in the body - in the hope of incorporating them into transistors and other nanodevices.
 
 

Constructive nanolithography - The art of bottom-up

 
Prof. Jacob Sagiv aims to build data from the bottom up, out of atoms and molecules - much as a builder uses bricks to construct a brick wall. The key idea is that an artificial molecular system may control its own construction, similar to the way a biological system controls its development.
 
Beginning with a smooth silicon surface consisting of a one-molecule-thick layer in which the exposed ends of the molecules are rendered chemically inert, the team, headed by Sagiv and Dr. Rivka Maoz, has devised methods to chemically activate a select portion of these molecules. Since the properties of the activated molecules then differ from those of surrounding molecules, they can encode diverse data - from text to images or even music.
 
This kind of information can be inscribed using an atomic force microscope (AFM) as a pencil. Equipped with an ultrasharp needle probe that transmits electrical signals, the AFM writes the information by electrochemically modifying the ends of molecules it touches. These modified molecules are later detected by an AFM operating in its reading mode.
 
Once the molecular ends are activated, they are capable of binding other atoms and molecules, making possible the deposition of additional molecular floors.
 
Molecules in the first layer have binding sites to which the second layer adheres as it is added.
 
Unlike the "destructive" information development in conventional photo- and electron-beam lithographies, achieved by etching into the underlying substrate material, in this approach, named constructive nanolithography, the initial information stored in the first layer guides the assembly pattern of the higher data “floors.”
Cashing in on this feature, the research team was able to create double-deck or even taller information packs, including a nanosized molecular replica of the Weizmann Institute’s logo tree with leaves one-thousandth the width of a human hair.
 
This novel bottom-up approach could offer precise control over the structure and chemical composition of future nano devices, paving the way to chemically synthesized electronics with significantly enhanced data density. For starters, we may enjoy a Beethoven symphony from a thumbnail-sized chip.
 

 

Thin Films

 

Illustration: The quest for quality films

 


To see a world in a grain of sand - William Blake, Auguries of Innocence
 
The quest for quality films is at the forefront of nanotechnology research. The distinguishing characteristic of these films is their thinness - they are built of one or several layers of molecules, where each layer may range in thickness from just a few nanometers to several microns.
 
Current applications include computer and other electronic chips, sensors, flame-retardant films and liquid crystal displays (which alone have an estimated market of nearly $12 billion).
 
Key challenges in this field include the intro-duction of better characterization and quality control tools with which to analyze these tiny materials, and the development of films with enhanced organization, thermostability and complexity.
 
 

Thin films on a scale

 
Time equals money. But so does weight - when it comes to the films used in computers and optical telecommunications. Shaving off pounds from these devices could mean huge benefits for microelectronics as well as for satellites or spacecraft, where launching costs around $50,000 per kilogram (2.2 pounds).
 
A new recruit to the Institute, Dr. Milko van der Boom of the Organic Chemistry Department, is working to create thin films with such desirable qualities as low weight and long-term thermostability. He is targeting an “all-organic” product, which he hopes will replace today’s inorganic materials. The rationale is simple. Organic films would be much easier to modify, offering far better, cheaper devices that could even be introduced into home appliances, revolutionizing the electronics industry.
The challenges of creating these films, however, are considerable - from effectively integrating organic molecules into thin films, to creating films that are thick enough to efficiently convey the light signal.
 
To address these challenges, Van der Boom and groups led by Prof. Tobin J. Marks and Prof. Pulak Dutta at Northwestern University have created a novel bottom-up growth method. The teams begin by producing custom-designed organic molecules, which they then integrate into the film, building it up layer by layer (each layer is only 2.5 nanometers thick).
 
They had to “trick” nature to do so, organizing the molecules in a novel arrangement in which the molecules are all aligned in one direction. “Nature prefers a random orientation,” says Van der Boom.
 
Another innovation is the introduction of polymers that help to organize the films, creating smoother materials. Using this approach, the teams have created highly organized films consisting of 100 layers - a marked improvement over the average 10-layer films achieved to date. The team has recently created the first prototype electro-optic modulators based on these films.
 
 

Beneficial defects

 
Just as gourmet chefs chop carrots and onions into admirably fine slices, scientists working with nano-materials hope to create ever-thinner crystals - films so thin they can't be seen by the naked eye. The goal: to determine at what point these films exhibit one of the remarkable quirks of nanochemistry - taking on properties that are dramatically different from those of their original crystals.
 
The stumbling block in this endeavor is the films' tendency to collapse at a thickness of 5 microns (five millionths of a meter). Scientists have tried stabilizing films by placing them on various supporting surfaces, however, this process often affects their properties.
 
Targeting this challenge, Dr. Igor Lubomirsky of the Materials and Interfaces Department is studying an unusual form of freestanding films - also known as substrate-free films - in which the films are supported only along their edges, like drums.
 
Working with Vera Lyahovitsky of the same department, Lubomirsky has developed a production technique that allows control of grain size, shape and dispersion - all key mechanical factors in achieving film stability. He is also developing a new characterization technique designed to uncover the precise link between the way a film is prepared and its resulting properties, which might advance the production of new films with predictable, hence controllable, properties.
 
Lubomirsky has applied his technique to create nanofilms from a variety of materials, one of which was recently described in Europhysics Letters. The grains in these films are significantly smaller than those in the original material, resulting in an increased surface-to-volume ratio, richer interactions between neighboring grains and a much higher bonding energy. This in turn leads to new optical, mechanical and conductivity properties and even to new types of defects. “Some of these qualities, even those perceived as defects, may turn out to be very useful,” says Lubomirsky.
 
Future nanocrystalline films might be used to produce tiny, highly efficient fuel cells, advanced infrared detectors and high-frequency microelectronics. Other films might find their way into medical applications, such as microsensors or microinjectors that would be positioned within the body, delivering medications directly on target.
 
 

Why choose a smaller diamond

 
Diamonds are far more than sparkling gems meant to complete an evening gown. Roughly 80 percent of the diamonds mined annually are used for industrial applications - all designed around the stone’s unique chemical and mechanical properties, which have fascinated scientists for centuries.
 
Made of carbon, diamonds are one of the strongest materials known, have the highest thermal conductivity in nature, and are chemically inert and optically transparent. Their mechanical strength makes them an obvious choice for wear-resistant coatings used in cutting and polishing tools, eyeglasses, computer chips and blades used in high-precision surgery.
 
Synthetic diamonds lead the way in most of these applications. They are cheaper and far easier to work with than natural diamonds. Yet despite significant advances in production since their discovery in the 1950s (see box), key challenges remain. Two of these were recently tackled by Weizmann Institute researchers.
 
One technique for producing synthetic diamonds, known as chemical vapor deposition (CVD), involves a gas-phase reaction in which small diamonds are deposited onto a solid surface. These diamonds serve as “seeds” that promote nucleation centers around which individual crystals form and eventually connect to create a closed layer. Attempts to commercially produce high-quality CVD films are hindered by a painfully slow growth rate and the need for extremely high deposition temperatures.
 
Aiming to enhance the process, Prof. Yehiam Prior of the Institute’s Chemical Physics Department replaced the traditional diamond particles used for nucleation with particles half their size - around 50 nanometers. This subtle change in protocol resulted in significantly improved production rates and quality, making possible the synthesis of uniform films less than 500 nanometers thick. It also reduced the temperatures necessary by roughly 40%. “The minuscule size of these nucleating particles makes the ratio of their surface area to volume unusually large, promoting better bonding and thus better, more uniform films,” Prior explains.
 
 

From nature’s depths to a factory production line

 
In 1796 British chemist Smithson Tennant established that diamonds are a crystalline form of carbon (according to some reports, burning his wife’s diamonds to do so) - the same material of which coal, graphite and other combustible materials are made.
 
The race to produce synthetic diamonds began, but it would be more than 150 years before the secret of converting graphite into the coveted diamond was discovered. The “recipe” called for temperatures of 1,400 degrees C and pressures of over 55,000 atmospheres. The recent development of CVD, characterized by much lower temperature and pressure production levels, may advance entirely new applications, including hip or other body part replacements, made possible due to the chemical inertness of diamonds.
 
 

The effect of the defect

 
Iris Visoly-Fisher knew exactly what she wanted to work on when starting her Ph.D - to follow up on a hunch she had that defects in a certain material used in solar cells would actually improve their performance.
 
But something was bugging her. She couldn’t understand why so few people were working on this topic. “It seemed so clear that something was up, but there was virtually no literature about it,” she says.
 
She soon found out why. The technical difficulties in examining this puzzle were significant. To effectively tackle the problem, she would need to figure out how to zoom in on solar cell performance at the nanoscopic scale.
 
An unexplained finding from years earlier was what had triggered this headache of a challenge. Most commercial solar cells are made of single crystal semiconductors, such as silicon, Visoly-Fisher explains. But researchers had been looking around for alternative materials for a while, since silicon’s manufacture costs are high. Then, nearly 15 years ago, a finding came up that was out of sync with all predictions. Certain solar cells made of polycrystalline (multi-grained) films were systematically outperforming their counterpart single crystal cells.
 
Nobody understood why. Scientists working to improve solar cells had traditionally shied away from multi-grain films since they contained numerous structural defects - a property believed to impair the light conversion process.
 
“In examining and reexamining the multi-grained films that had proven so successful, we could come up with only a key difference between them and the single crystal films: the presence of a leading defect, known as the grain boundary defect,” says Visoly-Fisher, who performed the study under the guidance of Prof. David Cahen of the Institute’s Materials and Interfaces Department, in collaboration with Dr. Sidney Cohen of the Chemical Research Support.
 
The team decided to find a way of studying the electrical properties of a single defect - in other words, the meeting point between two of these microscopic grains. Only after combining 3 different high-resolution imaging techniques did they have an irrefutable answer: Contrary to earlier notions, grain boundary defects significantly enhance the efficiency of certain solar cells.
 
As the Institute team has now shown, the grain boundaries improve the solar cell’s light-to-electricity conversion because they provide a path where free electrons in the semiconductor are efficiently collected and channeled.
 
“The grain boundaries essentially function as a freeway for electrons to exit, without traffic lights or roundabouts,” says Prof. Cahen. “This finding offers a promising direction for improving solar cell performance while cutting production costs.”
 
 

Going magnetic

 
When Itai Carmeli first came to his Ph.D. adviser with his results, he was gently told to get back to work. “I told him there was no such physics,” recalls a smiling Prof. Ron Naaman of the Institute’s Department of Chemical Physics. “A year later he was back - with similar findings.”
 
Carmeli had been tinkering around with organic molecules, which he used to create extremely thin, single-layered films on a gold substrate. The surprise was that the films were behaving like a powerful magnet. There are different types of magnets, Naaman explains, from the common fridge magnets we all played with as kids, that always display magnetic behavior; to temporary magnets such as paperclips and nails, which only work when exposed to a strong magnetic field; to magnets powered by an electric current. Nearly all of these contain one or more components with magnetic properties. The twist in our case was that our films lacked any magnetic materials.
 
In their study, recently published in Physical Review Letters and the Journal of Chemical Physics, the Institute team experimented with films made of three types of organic molecules. The molecules each had a positive and negative pole, and they were tightly packed, such that their negative poles faced the gold substrate, while their positive poles faced away.
 
And this, says Naaman, might have been the trick: while opposite charges are known to attract, like charges repel, particularly when in close proximity on the surface of the film. The team - which included physicist Prof. Zeev Vager and materials scientists Prof. Shimon Reich and Dr. Gregory Leitus - believes that this repulsion force causes electrons to flow from the gold substrate to neutralize charged sites on the molecules, in an attempt to stabilize the system. This extremely thin layer of electrons in turn induces an electric current - forming a leading type of magnet dubbed an electromagnet (see box).
 
“We believe that the electrons are behaving as if in a co-op,” says Vager. “Electrons usually orbit in small circles, around individual molecules; but in this case they may be orbiting domains containing hundreds of thousand of molecules in the film, creating an electric current that transforms the system into a powerful magnet. Films of this sort might feature in electromagnets used in futuristic high-density discs and other electronics.”
 
 

Magnetic moments

 
Birds do it, bees do it, so do whales, salmon and, according to a new study, even Caribbean spiny lobsters - all use the earth’s magnetic field as a navigating compass.
 
Magnetism was first discovered by the ancient Greeks and Chinese. Experimenting with the materials of their natural environment, they found that certain rare stones, called lodestones, attract small pieces of iron. Adding to their “magic,” these stones were found to always point in a north-south direction when suspended on a string. They quickly became invaluable to navigators, fortune-tellers and builders.
 
During the 13th century Frenchman Pierre de Maricourt discovered that magnets had two magnetic poles - north and south - and in the 1600s, England’s Sir William Gilbert concluded that Earth itself is a giant magnet, with north and south poles - which explains the wonder of animal migration treks.
 
The 1800s saw the first connection made between electricity and magnetism, when Danish physicist Hans Christian discovered that running an electric current through a wire creates a magnetic field - a phenomenon that quickly became known as electromagnetism. And today, this form of magnetism is everywhere - used in designing the electric motors in refrigerators, washing machines and racecars; the read/write heads of discs and videotape players, and far more.
 

A mismatch made in heaven

 
Size is everything… when it comes to materials science. For instance, the properties of crystals smaller than roughly 10 nanometers can be altered by merely changing their dimensions. This phenomenon, known as the quantum size effect, makes it possible to derive materials with dramatically different properties from chemically identical compounds.
 
Researchers at the Institute’s Materials and Interfaces Department achieved an important step toward this aim using a technique called electrodeposition. Prof. Gary Hodes, Prof. Israel Rubinstein and then doctoral student Yuval Golan laid down crystals of the semiconductor cadmium selenide, each measuring four to five nanometers, onto a gold substrate. The crystals were found to be oriented in a uniform manner. This configuration, which is highly beneficial for controlling semiconductor properties, occurs because the atoms of the cadmium selenide crystals tend to align themselves with the atoms in the surface layer of the gold substrate.
 
But then the researchers discovered a paradoxical fact: the match had a slight imperfection, and whereas uniform configuration generally improves the control of semiconductor properties, in this case the mismatch allowed for the precise control of crystal size.
 
It turned out that the gold substrate “stretches” the growing crystal in an attempt to minimize the mismatch, creating a strain within the crystal that eventually causes it to stop growing altogether. By fine-tuning the mismatch the scientists were able to control when the crystal would stop growing, in other words, its size. They showed that by adding small amounts of a material called tellurium they could control the degree of mismatch, thus producing uniformly oriented crystals of varying sizes. Recent results have shown that not only the size of crystals but also their shape and phase can be controlled in this manner.
 

Please share if you found this interesting:

 

 

 

 

Instagram