New method for analysing crack progression

English

Weizmann Institute scientists develop a method for analysing crack progression


Could engineers have known ahead of time exactly how much pressure the levees protecting New Orleans could withstand before giving way? Is it possible to predict when and under what conditions material wear and tear will become critical, causing planes to crash or bridges to collapse? A study by Weizmann Institute scientists takes a new and original approach to the study of how materials fracture and split apart.

When force is applied to a material (say, a rock hitting a pane of glass), a crack starts to form in the interior layers of that material. In the glass, for example, the force of the striking rock will cause the fracture to progress through the material with gradually increasing speed until the structure of the glass splits apart.

The path the forming crack follows and the direction it takes are influenced by the nature of the force and by its shape. As cracking continues, microscopic ridges form along the advancing front of the crack and the fracture path repeatedly branches, creating a lightning bolt or herringbone pattern.

Physicists attempting to find a formula for the dynamics of cracking, to allow them to predict how a crack will advance in a given material, have faced a serious obstacle. The difficulty lies in pinning down, objectively, the fundamental directionality of the cracking process: From any given angle of observation or starting point of measurement, the crack will look different and yield different results from any other. Scientists all over the world have experimented with cracking, but, until now, no one has successfully managed to come up with a method for analyzing the progression of a forming crack.

Prof. Itamar Procaccia and research students Eran Bouchbinder and Shani Sela of the Chemical Physics Department set out to find a way of analyzing data from experiments in cracking that would avoid the direction problem. First, they divided the cracks’ ridged surfaces up into mathematically-determined sectors. For each sector they were able to measure and evaluate different aspects of the crack’s formation and to assign it simple directional properties. After some complex data analysis of the combined information from all the sectors, the team found their method allowed them to gain a deeper understanding of the process of cracking, no matter which direction the measurements started from. The team then successfully applied the method to a variety of materials – plastic, glass and metal.

From the concrete in dams and buildings, to the metal alloys and composites in airplane wings, to the glass in windshields, many of the materials we depend on daily are subject to cracking. The team’s method will give engineers and materials scientists new tools to understand how all of these basic materials act under different stresses, to predict how and when microscopic or internal, unseen fractures might turn life-threatening, or to improve these materials to make them more resistant to cracks’ formation or spread.      

Prof. Itamar Procaccia’s research is supported by the Minerva Center for Nonlinear Physics of Complex Systems; and the Naftali and Anna Backenroth-Broniki Fund for Complexity.  Prof. Procaccia is the incumbent of the Barbara and Morris L. Levinson Chair in Chemical Physics.
Chemistry
English
  • Chemical Physics
  • Itamar Procaccia

Switching to Chemistry

English
New kind of electrical switch formed from organic molecules could be used in the future in nanoscale electronic components

Researchers at the Weizmann Institute of Science have demonstrated a new kind of electrical switch, formed of organic molecules, that could be used in the future in nanoscale electronic components.

Their approach involved rethinking a phenomenon that drives many of today’s high-speed semiconductors. Negative differential resistance (NDR), as the phenomenon is called, works contrary to the normal laws of electricity, in which an increase in voltage translates into a direct increase in current. In NDR, as the voltage steadily increases, the current peaks and then drops off, essentially allowing one to create a switch with no moving parts. But until now, those attempting to recreate NDR at the molecular scale had only managed it at extremely low temperatures.

Prof. David Cahen of the Institute’s Materials and Interfaces Department and graduate student Adi Salomon thought research carried out by Salomon and others in Cahen’s lab during her M.Sc. studies on connections between metal wires and organic (carbon based) molecules might hold part of the key to usable nanoscale NDR. They had found that, like people, molecules and metal wires need chemistry between them for barriers to be lowered and the juice to really flow. For a given voltage, if the molecules are held to the wire by chemical bonds (in which the two are linked by shared electrons), the current flowing through them will be many times higher than if they are only touching – a mere physical bond.

Using this insight, the team designed organic molecules that pass electricity through chemical bonds at a lower voltage, but through physical bonds at a higher voltage. As the voltage approaches the higher level, sulfur atoms at one end of the molecule loosen their chemical bonds with the wire, and the current drops off as the switchover occurs.

But the molecules, once the chemical bond to the wire was broken, tended to move apart, preventing them from switching back to the chemically-bonded state. Prof. Abraham Shanzer of the Organic Chemistry Department, who had worked with the team on the original molecular design, now helped them to create long add-on tails to hold the molecules in place with a weak attraction. Now, the NDR in their molecules was stable, reversible and reproducible at room temperature.

Possible applications include nanoscale electronic memory and heat-sensing switches. The future of miniaturized electronics may lie in methods that combine chemistry with nanoscience, say the scientists. “We don’t take human-sized objects and try to scale them down, but create new things from the universe of possibilities open to chemists that are specifically designed to function in the nanoworld.”

Prof. David Cahen’s research is supported by the Minerva Stiftung Gesellschaft fuer die Forschun M. B. H.; the Wolfson Advanced Research Center; the Philip M. Klutznick Fund for Research; Delores and Eugene M. Zemsky; and the Weizmann-Johns Hopkins Research Program. Prof. Cahen is the incumbent of the Rowland Schaefer Professorial Chair in Energy Research.
Chemistry
English
  • David Cahen
  • Materials and Interfaces
  • Nanoelectronics
  • Organic molecule

Nanotubes Form Along Atomic Steps

English
The Weizmann Institute of Science today announced that a research group headed by Dr. Ernesto Joselevich has developed a new approach to create patterns of carbon nanotubes by formation along atomic steps on sapphire surfaces. Carbon nanotubes are excellent candidates for the production of nanoelectronic circuits, but their assembly into ordered arrays remains a major obstacle toward this application.

The team was initially researching in a different direction: they were trying to give carbon nanotubes (structures reminiscent of rolled-up sheets of graphite) a preferred orientation on a wafer by applying an electrical field as the tubes were being formed. This works very well with silicon dioxide wafers. On a sapphire support (sapphire is a form of aluminum oxide), on the other hand, it didn’t work: the nanotubes were beautifully arranged in parallel, but with an orientation that was completely independent of the electrical field - even when no field was applied at all!

Closer examination of the sapphire surface solved the mystery: commercial sapphire wafers are generally not cut exactly along the plane of the crystal. Their surface is thus not completely smooth; instead, it has parallel steps - of atomic dimensions - between the different planes of the crystal. The nanotubes wind up lying along these steps. The researchers explain it like this: the nanotubes form from a catalyst of iron nanoparticles and are attracted to a local field created by the steps. It is clear that these iron particles don’t like “climbing stairs;” instead, they “glide” along the inner edge of the step, like on a track. They thus remain continuously in contact with two surfaces, rather than just one, which seems to stabilize the catalyst. Just as an airplane leaves behind a condensation trail, the iron particles leave the newly formed nanotubes lying along their “tracks.” The nanotubes even follow kinks in the steps, which are caused by defects in the crystal. This results in either straight or zigzag-shaped tubes, which are expected to have particularly interesting electronic properties.

“The orientation and form of the atomic steps on a crystal surface can be controlled by the cutting process, and defects can be created artificially,” says Joselevich. “It should thus be possible to produce different nanowire arrangements in a controlled fashion.”

Dr. Joselevich’s findings appear this week as the cover story of Angewandte Chemie.

Dr. Joselvich's research is supported by the The Asher and Jeannette Alhadeff Research Award, the Ilse Katz Institute for Material Sciences and Magnetic Resonance Research, the Philip M. Klutznick Fund for Research, Sir Harry A.S. Djanogly, CBE, UK and Sylvia and Henry Legrain, Spain. He is the incumbent of the Dr. Victor L. Ehrlich Career Development Chair.
Chemistry
English
  • Carbon nanotubes
  • Ernesto Joselevich
  • Materials science
  • Nanoelectronics

How the Sea Urchin Grows New Spines

English

A team of scientists at the Weizmann Institute of Science has shown how sea urchins grow new spines.


The sea urchin’s tough, brittle spines are an engineering wonder. Composed of a single crystal from base to needle-sharp tip, they grow back within a few days after being broken off. Now, a team of scientists at the Weizmann Institute of Science has shown how they do it.


While many crystals grow from component atoms or molecules that are dissolved in liquid, sugar and salt being the most familiar examples, the team of Profs. Lia Addadi and Steve Weiner, of the Institute’s Structural Biology Department, found that the sea urchin uses another strategy. The material of the spines is first amassed in a non-crystalline form, termed amorphous calcium carbonate (ACC). Packets of ACC are shoveled out of the cells surrounding the base of the broken spine and up to the growing end. Within hours of arriving in place, the amorphous material, which is composed of densely packed, but disorganized molecules, turns to calcite crystal in which the molecules line up evenly in lattice formations.


Working with graduate student Yael Politi and Eugenia Klein and Talmon Arad of the Chemical Research Support Unit, they used four different methods of investigation, including two kinds of electron microscopy, to look for the ACC as it was being deposited and turning to crystal. “The question,” says Weiner “is why it should be so difficult to observe a process that seems to be so basic. Scientists have been studying it for over a hundred years. In fact, because the ACC is a transient phase, we had to develop new methods to catch it while it exists.”


The captured images show microscopic needles that grow first straight out from the stump of the old spine, and then branch out to form a lacy structure that is hard but light. The crystalline structure of the old spine provides the template for the alignment of the molecules in the crystal, and thus controls the intricate, yet precise growth pattern.


Though previous studies by the Weizmann group have shown the same strategy is used by immature sea urchins and mollusks in the larval stage to build internal skeletons, this is the first time that the process was observed in adult marine animals. It is far from obvious that larva and adult would use the same methods - their lifestyles are very different, and this can translate into differences in biological processes, as well. (For instance, the tiny sea urchin larva is transparent and swims around, while the round, spiky adult lives on the sea floor.)


Because it works for both, Addadi and Weiner believe this method is probably a basic strategy used by not only close relatives of the sea urchin such as sea stars, but by a wide variety of spiny and shelled sea creatures like mollusks and corals. In addition, the idea of growing single crystals by first creating the material in an amorphous phase might prove useful to material scientists and engineers wanting to produce and shape sophisticated synthetic materials that have the properties of single crystals.


 
Prof. Addadi's research is supported by the J & R Center for Scientific Research, the Ilse Katz Institute for Material Sciences and Magnetic Resonance Research, the Helen and Milton A. Kimmelman Center for Biomolecular Structure and Assembly, the Philip M. Klutznick Fund for Research, the Minerva Stiftung Gesellschaft fuer die Forschung m.b.H., the Women's Health Research Center and the Ziegler Family Trust, Encino, CA. She holds the Dorothy and Patrick Gorman Professorial Chair.

Prof. Weiner's research is supported by the Helen and Martin Kimmel Center for Archaeological Science, the Philip M. Klutznick Fund for Research, the Alfried Krupp von Bohlen und Halbach Foundation, Women's Health Research Center, and George Schwartzman, Sarasota, FL. He is the incumbent of the Dr. Walter and Dr. Trude Borchardt Professorial Chair in Structural Biology.

 

 

The broken tip of a sea urchin spine shows new growth. From base to tip, the spine is made up of a single crystal. The images were taken by Yael Politi with a scanning electron microscope as a part of the research, which was conducted together with Profs. Lia Addadi and Steve Weiner of the Structural Biology Department of the Weizmann Institute of Science.

 

Chemistry
English
  • Lia Addadi
  • Regeneration
  • Sea urchin
  • Stephen Weiner
  • Structural Biology
Yes

Wet Scans

English

New invention allows scientists to view intact biological samples under the electron microscope

The scanning electron microscope (SEM) has been a basic research tool for fifty years, and for those fifty years, scientists have been looking for better ways to observe biological samples under its beam.

 

The problem is that the viewing chamber of the SEM must contain a vacuum (in which liquid water in tissues “boils” away). To overcome this difficulty, scientists have had to resort to all sorts of complicated procedures, including coating the specimens with an ultra-fine layer of gold, quick-freezing samples in special deep-freezes, or treating them with drying solvents.

 

Now, scientists at the Weizmann Institute of Science have found a way to view samples of biological materials in their natural, “wet” state. Their secret lies in the production of a very thin but tough polymer capsule to enclose the sample, allowing it to withstand the force of the vacuum. Says Dr. Ory Zik, who worked on the capsule with Professor Elisha Moses of the Physics of Complex Systems Department: “The material for the capsule is a result of advances in the area of semiconductors. We came across it while researching ways to apply automation techniques used in the semiconductor industry to the life sciences’ scanning electron microscopes.”

 

The capsule’s polymer is unique in that it is allows the electrons with which a SEM works to pass through unobstructed, giving scientists a clear view of what lies within, without the use of tricky, tissue-distorting procedures. Researchers hope the new method will advance the studies of biological materials, such as the lipids that make up fat, which are easily destroyed by the old sample preparation methods.

 

Since the discovery was made, Zik, in cooperation with Yeda, the business arm of the Weizmann Institute, has founded a company, called QuantomiX, based on this technology. The findings of the team were published in the March 9 Proceedings of the National Academy of Sciences, USA (PNAS).

 

Prof. Elisha Moses's research is supported by the Clore Center for Biological Physics and the Rosa and Emilio Segre Research Award.

 

Chemistry
English
  • Elisha Moses
  • Microscopy
  • Scanning electron microscope
  • Yeda

Brittlestars Use Crystal Lenses To Spot Approaching Predator

English

Brittlestars of the species Ophiocoma wendtii form crystal lenses in their skeletons that allow them to spot approaching predators, according to a study reported in Nature on August 23.

 

This unique 'visual' system is the first of its kind to be discovered in animals inhabiting the earth today. The discovery is a result of a collaborative study conducted by researchers from the Weizmann Institute of Science in Rehovot, Israel, from Bell Laboratories/Lucent Technologies in New Jersey, and from the Natural History Museum of Los Angeles County in Los Angeles, California.

 

Brittlestars, also known as serpent stars, are marine invertebrates that usually have five thin long arms emanating from a small, disk-shaped body. They belong to the phylum of echinoderms, which also includes sea urchins, sea cucumbers, starfish, and several other classes of marine animals.

 

Over the past few years, Prof. Lia Addadi, Dean of the Weizmann Institute's Chemistry Faculty, and Prof. Steve Weiner, of the Institute's Structural Biology Department, have conducted a series of studies examining ways in which animals build their skeletons. The scientists have revealed that animals produce different types of proteins, some of which control crystal formation.

 

The idea for the current study was born when the Institute scientists met with Dr. Gordon Hendler of the Natural History Museum of Los Angeles County. Dr. Hendler brought their attention to one particular species of brittlestars, Ophiocoma wendtii; he had found that this species, which appears to be particularly sensitive to light, can change its color.

 

Even though these animals have no specialized eyes, they are capable of detecting shadows and escaping quickly from predators into dark crevices. Hendler suspected that the arrays of spherical crystal structures on the surface of its outer skeleton serve as lenses that transmit light to the brittlestar's nervous system. This hypothesis was reinforced by the fact that within their skeletons, brittlestars indeed have relatively extensive nerve networks. Moreover, the movement of pigmented cells between the crystal structures and the nerves appear to alter the brittlestar's response to light.

 

Addadi and Weiner, together with their then graduate student Joanna Aizenberg who now works at Bell Laboratories, began to study the phenomenon. They discovered that each skeletal element with its hundreds of lenses is a single calcite crystal; the crystal's optic axis is roughly perpendicular to the plane of the lens array. This means that the calcite lens array is capable of transmitting light without splitting it in different directions. But does the lens's geometrical shape place its optical focus precisely over the area where the brittlestar's nerves are located under the skeleton? In other words, do the lenses guide and focus light and transmit the concentrated rays inside the tissue, to the nervous system?

 

These questions remained unanswered for almost 10 years until recently the scientists found a way to examine them experimentally in a controlled manner. The experiment was conducted at Bell Laboratories with the help of lithography, a semiconductor technology.

 

Dr. Aizenberg removed a calcite crystal array from the skeletal element of the brittlestar species Ophiocoma wendtii, placed it above a layer of photosensitive material, and exposed the system to light. She found that light had reached the photosensitive material in spots directly underneath the calcite crystals. By altering the distance between the lenses and the material, she found that the estimated focal distance of each lens - at which the lens concentrates the light by about 50 times - coincided with the depth at which nerve bundles that presumably serve as photoreceptors are located in the bodies of brittlestars.

 

Thus the crystalline lenses and the pigmented cells in the skeletons of Ophiocoma wendtii brittlestars act as 'corrective glasses, ' filtering and focusing light on the photoreceptors. This type of 'visual' system has never before been described in animals living on our planet today, but Prof. Weiner notes that calcite crystals were also used in the compound eyes of trilobites, the now extinct marine animals that inhabited the earth some 350 million years ago.

 

In their Nature report, the scientists conclude: 'The demonstrated use of calcite by brittlestars, both as an optical element and as a mechanical support, illustrates the remarkable ability of organisms, through the process of evolution, to optimize one material for several functions, and provides new ideas for the fabrication of 'smart' materials.'

 

Prof. Stephen Weiner holds the Dr. Walter and Dr. Trude Borchardt Professorial Chair in Stuctural Biology. His research is supported by the Helen and Martin Kimmel Center for Archaeological Science, Mr. George Schwartzman of Sarasota, Fl. and the Angel Faivovich Foundation for Ecological Research. Prof. Lia Addadi holds the Dorothy and Patrick Gorman Professorial Chair. Her research is supported by the Minerva Stiftung Gesellschaft f?r die Forschung m.b.H.

 

 

The Weizmann Institute of Science is a major center of scientific research and graduate study located in Rehovot, Israel. Its 2,500 scientists, students and support staff are engaged in more than 1,000 research projects across the spectrum of contemporary science.

Chemistry
English
  • Lia Addadi
  • Smart material
  • Stephen Weiner
  • Structural Biology

Delving In To The Nanoscopic

English
 
 
Ever tried determining what's inside a layered chocolate cake without slicing it? Now, how about tackling a similar task, yet on a nanometer-scale?
 

For decades, thinking big has frequently meant pursuing smaller and smaller goals. Take ultrathin films for instance. Often less than 10-15 nanometers in width, ultrathin films are used in diverse applications, from optoelectronics to biological sensors. (A nanometer is roughly one 100,000th the width of a human hair.)

 

A central requirement for performing these Lilliputian feats is accurate composition and structural analysis. Yet, 'looking inside' these films, which are often multi-layered, calls for highly sensitive probes. Most available techniques do not provide the depth information essential for evaluating layered structures. (Similarly, X-rays offer a spectacular glimpse into the human body, however determining the relative depth of individual structures is highly difficult.) Techniques devised to solve this problem are generally complicated and frequently damage the sample, distorting the results.

 

Now, Dr. Hagai Cohen of the Weizmann Institute Chemical Services and Prof. Israel Rubinstein of the Materials and Interfaces Department have developed a novel method for evaluating ultrathin films, specifically, non-conducting films on conducting substrates. Recently appearing in Nature, their study builds upon X-ray Photoelectron Spectroscopy (XPS), a common surface analysis technique.

 

In XPS, the sample is irradiated with X-rays, causing photoelectrons to be ejected. By measuring the photoelectrons' energy, it is possible to determine the atoms from which they originated. Researchers have routinely used an electron flood gun to neutralize the positive surface charge formed in non-conducting samples as a natural consequence of the photoelectron ejection, since the charging affects the photoelectrons' energy, distorting the measurements.

 

However, proving that one person's stumbling block may be another's stepping stone, Cohen and Rubinstein realized that the charging effect actually provides structural information - the magnitude of the photoelectron energy change correlates directly with the atoms' depth within the film (the deeper the atom the smaller the change). They decided to turn things around, using the electron gun to flood the sample with low energy electrons, thus negatively charging the surface and causing controlled, easily detectable changes in the energy of the ejected photoelectrons. By measuring these changes, the researchers were able to determine both the atom type and its depth within the film.

 

To evaluate their approach, the scientists used one of their previous research accomplishments - a highly organized ultrathin film, which they laced with marker atoms at different depths. When tested on this system, the new method provided depth information with a superior resolution of about one nanometer while causing minimal damage to the sample. It also offered a unique side-benefit, yielding information regarding the film's electrical properties.

 

The Weizmann innovation should prove beneficial in developing a wide range of microelectronic applications as well as in studying various chemical and biological systems.

 

This research was conducted together with Prof. Abraham Shanzer of the Organic Chemistry Department, Dr. Alexander Vaskevich of the Materials and Interfaces Department, and doctoral students Ilanit Doron-Mor, Anat Hatzor and Tamar van der Boom-Moav.

 

Prof. Israel Rubinstein's research is supported by the Philip Klutznick Endowed Scientific Research Fund, the Minerva Foundation, the Henri Gutwirth Fund for the Promotion of Research and the Fritz Haber Center for Physical Chemistry.

 

The Weizmann Institute of Science is a major center of scientific research and graduate study located in Rehovot, Israel. Its 2,500 scientists, students and support staff are engaged in more than 1,000 research projects across the spectrum of contemporary science.

Chemistry
English
  • Hagai Cohen
  • Israel Rubinstein
  • Spectroscopy
  • Thin film
  • X-ray

Molecular Footprints And Memory Squeeze-Downs

English

Is your music collection taking up too much space? How would you like to pack all of your music onto a single CD? Weizmann Institute scientists have recently taken a large step toward this miniaturization target.

 

Reported in the March and May issues of Advanced Materials, a team of Weizmann scientists headed by Prof. Jacob Sagiv of the Institute's Materials and Interfaces Department has developed a new strategy for high-density, long-term data storage using a unique molecular approach. Sagiv worked together with Dr. Rivka Maoz and Eli Frydman of the same department, as well as with Dr. Sidney Cohen of the Chemical Services Unit.

 

Current CD and microchip technologies are based on etching data onto existing materials -- the smaller the data-encryption markers used, the more information on a given surface. Engineers have excelled at this task over the last few decades -- with the bacchanalia of microelectronic gadgets on the market as ticking proof. However, it was clear that the party couldn't last. The race toward ever-smaller data-storage technologies on limited physical surfaces would eventually hit a stone wall.

 

The Institute scientists decided to skirt this obstacle, using a refreshing 'build from scratch' approach. Instead of etching data on existing surfaces, they actually construct it out of atoms and molecules, which they bind to one another much like a builder constructs a brick wall.

 

The construction work starts out with a smooth silicon surface covered with a one-molecule-thick layer, in which the exposed ends of these molecules are chemically inert. The researchers succeeded in activating a selected portion of these molecules while leaving others inert. Having different properties, the activated molecules can serve as minute footprints of information -- encoding diverse data, from text to images, or even music.

 

To achieve this, the researchers used an atomic force microscope (AFM) as their 'pencil.' Equipped with an ultra-sharp needle that can transmit electrical signals, the AFM 'writes' information by electrochemically modifying the ends of the molecules touched by the needle. Such modified molecules can later be detected by an AFM operated in its 'reading' mode. Using a computer, this molecular information is decoded to recreate the original letter, image, or sound. In a following study, the scientists took advantage of the same process to double-deck the information packs: once the molecular ends are activated, they are also capable of binding other atoms and molecules, thus enabling the researchers to add additional 'molecular floors' according to a predefined plan. This bottom-up approach could offer precise control over the structure and chemical composition of future nano-devices, thereby enabling dramatically increased data density, and potentially paving the way to breakthrough nano-electronic tools.

 

Prof. Jacob Sagiv's research is supported by the Verband der Chemischen Industrie and the German Society of Friends of the Weizmann Institute of Science, Germany.

 

The Weizmann Institute of Science is a major center of scientific research and graduate study located in Rehovot, Israel. Its 2,500 scientists, students and support staff are engaged in more than 1,000 research projects across the spectrum of contemporary science.

Chemistry
English
  • Atomic force microscopy
  • Jacob Sagiv

A First: Scientists Control The Properties of Semiconductor Devices Using Organic Molecules

English

Weizmann Institute scientists have made an important step towards harnessing organic molecules to future electronics. Reported in the March 9th issue of Nature, their approach places common semiconductor-based devices -- for the first time ever -- under the control of organic molecules.


The functions of organic molecules are so diverse that their inclusion in electronics would provide an extensive range of possibilities. However, the observation of these molecules' electrical properties has up until now been impeded by incongruities in the structure of organic molecules themselves. Layers of organic molecules that are used in this kind of research contain 'pinholes' -- small defects that are very difficult to detect but radically sway conductance. Scientists were unable to determine whether their measurements resulted from the passage of the current through the organic molecules or through a pinhole. But the new approach circumvents this problem.


The Weizmann scientists chose to analyze the molecules indirectly -- by focusing on the influence that the molecules were suspected to have on semiconductors. Using a series of molecules synthesized by Prof. Abraham Shanzer of the Weizmann Institute's Organic Chemistry Department, Ayelet Vilan, a graduate student working with Prof. David Cahen of the Materials and Interfaces Department, constructed a one-molecule-thick layer (monolayer) of very short organic molecules.


Vilan placed the monolayer on a common semiconductor, GaAs, and directed an electric current through it. The monolayer was so thin that, for the most part, the electric current passed by the molecules without interacting with them. This fact meant that it was of minimal importance if the electrons went via a molecule or a pinhole. (However, it is important to note that while the organic molecules barely affect the passage of the electrical current through them, they very much influence the electric properties of the semiconductor.)


The decision to work with monolayers of organic molecules compelled Vilan to develop a new method for preparing semiconductor devices. The technique is founded on a widely used semiconductor device (diode), which is comprised of a semiconductor connected to a metal. She inserted the organic monolayer between these two components. Since the organic molecules were 'sandwiched' between the semiconductor and the metal sheet, it was critically important to ensure that the delicate monolayer would not be crushed underneath the metal sheet. Vilan, building on the findings of Ellen Moons, one of Cahen's former students, reworked a method used in other fields to suit the device. She used a thin gold leaf as the metal sheet and gently floated it onto the monolayer. Thus, the monolayer remained intact.


The scientists found that changing the type of organic molecules in a monolayer led to a predictable, systematic change in the electrical characteristics of the device. Thus, not only were they able to control the properties of the semiconductor, but they also were able to predict the kind of control that would be exerted by different types of organic molecules.


'This study introduces a feasible way to incorporate organic molecules into electronic devices,' says Vilan. 'But mainly, it provides new insights into the emerging field of molecular electronics. So little is known about the effects that occur between molecules and the electric conductors we normally use. This approach may provide a basis for the design of novel types of semiconductor-based devices, from improvements in relatively simple devices such as solar cells, to possible new types of computer chips.'


Professor Abraham Shanzer holds the Siegfried and Irma Ullman Professorial Chair.


The Weizmann Institute of Science is a major center of scientific research and graduate study located in Rehovot, Israel.

Chemistry
English
  • Abraham Shanzer
  • David Cahen
  • Materials and Interfaces
  • Microelectronics
  • Organic Chemistry
  • Semiconductor

New Class of Molecular Magnets May Advance Microelectronics

English
REHOVOT, Israel -- September 24, 1998 -- Weizmann Institute scientists have created a new class of magnetic materials made of clusters of inorganic molecules. These molecular magnets, described in the September 24 issue of Nature, display an unusual combination of properties that open up new research possibilities and may lead to a broad range of future applications in the microelectronics industry.

The molecules of nickel dichloride that make up the new magnets are much smaller in size than the metal-organic compounds used previously to create most molecular magnets.

Another distinguishing feature of the new molecules is their shape. Some have a cage-like structure that resembles fullerenes (the soccer-ball like molecules named after the architect R. Buckminster Fuller), while others are shaped like tubes, called nanotubes.
 
The method by which the nickel dichloride clusters have been created is also unusual. Rather than producing chunks of magnetic material, the scientists build the magnetic molecules from individual atoms. The molecules then self-assemble into a spherical layer one molecule thick. This method of creating a magnetic material, known as the "bottom-up" approach, gives scientists precise control over the size and structure of the magnet's molecules and the number of their layers. This, in turn, allows them to tailor the material to specific needs.

"The bottom-up approach gives us enormous flexibility," says research leader Prof. Reshef Tenne of the Weizmann Institute's Materials and Interfaces Department. "It's like constructing a building from individual bricks as opposed to moving around the walls within a prefabricated house." Tenne conducted this research with graduate student Yaron Rosenfeld Hacohen, in collaboration with Dr. Enrique Grunbaum of Weizmann and Drs. Jeremy Sloan and John Hutchison of the University of Oxford.

The study of fullerenes and nanotubes in inorganic materials was pioneered by Tenne and his Weizmann Institute colleagues in the early 1990s, creating a new avenue of research in materials science. The production of the nickel dichloride molecular magnets further expands this field, by introducing to it a totally new family of compounds.


Potential Future Uses


Molecular magnets are being developed because they represent the ultimate in miniaturization for the microelectronics industry, which is looking for ways to create smaller and smaller devices. In particular, such magnets are intended to allow as much computer memory as possible to be packed into a limited space.

Hard-disk memory is usually built up of a multitude of magnetic switches, in which a change between the "on" and "off" positions is performed by altering the switch's magnetic polarity. An ideal magnetic switch must be operated by a relatively weak magnetic force, so that its polarity can be altered with relative ease. Yet at the same time it must be sufficiently stable so as to preserve its polarity long-term.

Numerous molecular switches are currently being developed, but scientists run into a problem when they place them next to one another. Magnetic forces work over a relatively long range, so when the minuscule magnets are packaged tightly, interference results. Thus, when the polarity of one of these magnets is switched, the orientation of neighboring magnets changes as well. Such interference makes it impossible to store information reliably over a long period of time.

The new nickel-dichloride structures created by Tenne and his team promise to offer a solution to this problem. They are expected to be much less influenced by the magnetic fields of their neighbors and to be relatively "indifferent" to other environmental influences, such as temperature. Their seamless structure also suggests they should not be sensitive to "hostile" chemical effects of the environment, such as oxidation.
 
In addition, because these structures contain no impurities and because their spatial structure is well defined, their magnetic properties can be defined in a precise manner according to predetermined needs.
 
Apart from switches for computer memory, Tenne's molecular magnets in the form of nanotubes may have a multitude of other industrial applications. Thanks to their small size, they may be used for extremely fine "etching" of information onto magnetic disks, the process known as lithography, and for "reading" this information. Such "reading" is performed, for example, during the quality control of computer chips, and a nickel dichloride nanostructure could do this at a far greater resolution than any existing device.
 
Since the magnets are new and are expected to display intriguing magnetic behaviors which have not yet been fully investigated, they may find other, unexpected applications in the future.
 
Additional uses may arise from the fact that the nickel-dichloride magnetic materials -- unlike most other molecular magnets -- are semiconductors. This means that they can be used to create switches operated by an electrical current, and not only a magnetic field. Furthermore, they can also be operated optically because they selectively absorb light of certain wavelength. This combination of properties makes the new molecular magnets extremely versatile.

Tenne's team is now developing methods for synthesizing large quantities of nickel dichloride magnetic materials in order to study their magnetic properties in greater detail and pave the way for industrial testing.

This research was funded by Israel's Ministry of Science and the Israel Science Foundation.
 

The Weizmann Institute of Science is a major center of scientific research and graduate study located in Rehovot, Israel.
Chemistry
English
  • Fullerene
  • Nanomaterial
  • Reshef Tenne
  • Self assembly

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