by Dan Krauss
Gone are the days when materials scientists resorted to "heating and beating" stubborn substances into useable forms. Today's chemists and chemical engineers are at the next stage in materials development, building materials with advanced properties from the ground up, molecule by molecule.
THE DISCOVERY OF NEW MATERIALS is often as fortuitous as it is fortunate. With so many possible arrangements of elements and so many useful physical properties still unexplored, it's no wonder that chance is frequently a significant factor in such research.
Theory was once thought to be a useful tool in the quest, but partly because of the increasing complexity of modern materials, it can no longer provide a reliable way to predict what structures will exhibit practical properties.
"There's a huge opportunity in materials discovery, if we consider the fraction of the periodic table that we have explored to date. It's going to take a long time to exploit it using existing approaches," says Chemistry Professor Peter Schultz.
Instead, Schultz suggests taking a cue from nature. Particular biological systems have evolved a strategy of building huge "libraries" of molecules first and selecting the best ones for the job later. The immune system, for example, derives its potency by filtering as many as 1012 unique antibodies through a biological sieve to isolate the one antibody that can best recognize a foreign pathogen.
Combinatorial library used to discover
Motivated by the incredible specificity of this phenomenon, Schultz is bringing the concept to the lab, developing chemical techniques to synthesize and screen immense combinations of potentially useful molecules.
The technique, known as "combinatorial chemistry," has already yielded several successes, most notably the generation of antibodies with catalytic abilities--a feat which won Schultz the 1995 Wolf Prize in Chemistry.
Combinatorial chemistry is also revolutionizing the field of drug discovery, allowing scientists to quickly screen vast libraries of small organic molecules for those with possible pharmaceutical applications.
Now Schultz and his collaborator at Lawrence Berkeley National Laboratory, Xiaodong Xiang, are introducing the approach to their colleagues in the field of materials discovery, thereby generating a great deal of interest and anticipation.
"It's clear that most of materials science is involved in the characterization of materials," Schultz says. "And it's clear that the combinatorial approach makes sense for the synthesis and processing of solid state materials."
Schultz and his collaborators have recently developed a working method to systematically explore new combinations of these materials at lightning speed.
The scientists deposit thin films of solid state compounds using a set of "binary masks," essentially spraying the molecules through cleverly arranged stencils to form a grid representing all possible combinations and stoichiometries of their starting materials. The squares in the grid, which measure down to a scant 200 micrometers in size, are individualized swatches of new materials whose properties can be easily scanned.
"We started just to illustrate the idea and the feasibility of this combinatorial approach in the thin film area, showing that we could make thin film libraries of high Tc superconductors," Schultz says of his team's initial work, featured in the June issue of Science magazine.
A mere four months later, Science reported the group's first use of the method to actually discover new materials, in this case novel magnetoresistant materials, substances that change their electrical resistivity in magnetic fields.
Next, Schultz will tackle luminescent materials, key to the development of flat panel displays. But first the group is going back to the drawing board to perfect the technology used in the experiments.
"I think in another year, we'll routinely be able to generate, process and screen libraries of 104 materials per square inch per day," Schultz predicts.
"Realistically, this is 10,000 times faster than conventional methods." Smiling, he adds, "With one graduate student, we hope to be able to do what it what would usually take 10,000 graduate students to do."
FINDING A SOUGHT-AFTER PROPERTY, whether it be super strength or superconductivity, is the biggest challenge in discovering new materials. But, what happens when you discover a material that doesn't fit any established mold? That's precisely the question that the Department of Chemistry's Paul Alivisatos is addressing as he learns how to exploit completely new traits discovered in minuscule materials the size of a single molecule.
Alivisatos is the acknowledged guru in the realm of "nanocrystals," where tiny clusters consisting of only hundreds or thousands of atoms exhibit unusual behavior somewhere between that of an atom and that of a bulk solid.
Alivisatos has spent the last decade finding ways to master the size and composition of the nanocrystals, allowing him in turn to manipulate their unique properties.
"You can tune the bandgap, you can tune how it conducts charge, you can change what crystal structure it resides in, you can change its melting temperature," says Alivisatos, rattling off physical properties like features of a new car. "All of these properties can be altered in a systematic way, so it seems very promising that there might be a practical use for [the nanocrystals]."
With fabrication of the clusters now well in hand, Alivisatos says it is finally time to take another step in the long journey toward realizing the materials' full potential.
"The nanocrystals have reached a certain level of sophistication so that now we can try to use them to build more sophisticated structures out of them," Alivisatos says.
Electron micrograph image of
The trick to building nanocrystal structures is finding a way to link them together firmly without sacrificing the unique properties that the nanocrystals derive by being independent clusters.
"In a sense, what we have in this business is the ability to make all the little building blocks," Alivisatos says. "But, what we don't have is the chemistry that tells them where they should all go."
Alivisatos' current strategy employs synthetic organic molecules as glue to stick nanocrystals to one another. Here, Alivisatos recruits the synthetic expertise of Schultz.
"Mainly, I really knew I wanted to make these kinds of [molecules] and Pete more or less knew that he could do it," Alivisatos recalls of the start of his collaboration with Schultz.
So far, the team has managed to construct and purify pairs of nanocrystals--a small but important first step towards building larger nanocrystal arrays.
Such arrays could find their way into a myriad of applications, the most promising of which is in solid state electronics. Indeed, many of the nanocrystals that Alivisatos creates are composed of industry standard semiconductors like cadmium sulfide and cadmium salenide.
However, Alivisatos maintains that his work is still at the level of basic research. That hasn't stopped industry from showing its support for nanocrystals, though. Corporate giants like Hewlett-Packard have substantial investments in nanocrystal research.
"There's a big investment in technology that's still 10 or 20 years down the line, so they must think there's something there," says Alivisatos.
IF THE DISCOVERERS BEAR MATERIALS and see them through their infancy, then synthetic chemists handle the materials during their adolescence, taking charge of their growth during the unsteady period between creation and use.
Where other solid state synthetic chemists prefer to crush and cook compounds to build new molecular structures, Professor Angelica Stacy approaches the task from a more fundamental chemical standpoint.
"If you look at the rest of chemistry, the synthetic chemists work at low temperatures. The big advantage is that they have a solvent; they can dissolve things into molecules or ions. Things can move around freely and come together," says Stacy, gesturing molecular motions with her hands. "You can grow nice crystals out of solutions, as opposed to grinding a lump of this rock and a lump of that rock together and shoving it in a furnace."
This methodological attitude may be what gives Stacy an edge in the race to develop high-tech superconductors that reach their zero-resistance capabilities at relatively high temperatures.
Copper oxide superconductor
The leading materials in this race contain copper and oxygen combined with other metals and exhibit superconductivity at temperatures as high as -140 degrees Celsius--"warm" enough to be sustained by common refrigerants like liquid nitrogen.
Since their sensationalized introduction in the mid-1980s, copper oxide superconductors have become the focus of hundreds of research efforts. But Stacy has a new synthetic approach that may revolutionize creation of these wondrous structures as well as open doors to other materials.
"We've developed a method where you can dissolve everything into a liquid and then directly form crystals or films of the material," says Stacy. "[This method] lets us make some new things, it lets us make better quality material, and it lets us understand some more about these systems."
The centerpiece of Stacy's innovations is a molten mixture composed of sodium hydroxide and potassium hydroxide that allows her to work in an unusually low temperature range where there exist new possibilities for synthesis as well as discovery.
"Liquid hydroxide is a wonderful solvent for dissolving the oxide reactants," says Stacy. "We get nice clear solutions which look very much like aqueous solutions and we've learned how to modify the conditions to directly crystallize these materials."
This chemical technique is proving to be a startlingly versatile synthetic method for virtually all metal oxides, many of which have practical applications. Stacy is now using this approach to generate manganese oxide materials with magnetoresistant properties.
"We're excited about these manganese oxides," Stacy says. "I see us expanding to show how versatile this method is for oxides in general while keeping an eye on how we can continue to contribute in the superconductivity area."
TO FIND AN ARCHETYPE FOR ADVANCED materials, one must look no further than the human body. Polymer synthesist Doug Gin uses biological materials such as bone and collagen as structural blueprints for the design of next generation materials.
"We want to make structured materials that mimic the degree of order found in these natural materials," says Gin, one of the Department of Chemistry's newest Assistant Professors.
Ordering solids at a molecular level is the key to optimizing many important properties in materials, including strength.
"It's the difference between putting a couple of 2 by 4s together and putting up an A-frame," analogizes Gin.
Nylon is a perfect example. In a disordered state, such as one might find in textiles or clothing, nylon is floppy. But, dial in a highly organized structure and the material stiffens up, gaining several orders of strength.
"It's not necessarily the advanced nature of the components which makes a good material; it's how the material is put together on the small scale," notes Gin.
Lending order to a material has proven to be more difficult a task than most imagined, though. Today, most materials are literally pulled and stretched into a cooperative alignment after a material has been synthesized. This technique is not always effective.
Like Stacy, Gin advocates a chemist's approach to the dilemma: build order into the basic design of the material.
This is perhaps an obvious solution, but constructing such a solid is not obvious in the least.
Nonetheless, Gin has developed a promising new technique using liquid crystals (LCs) as a molecular scaffolding. LCs work well in this role because of their tendency to assemble spontaneously into an organized phase that seems to be partially solid and partially liquid in nature.
At left, a cross section of self-assembled liquid crystal
By piggy-backing polymerizable components on LC backbones, Gin intends to let the modified LCs do the organizational work and then fix them in position by triggering polymerization of the ordered molecules.
The Gin group has had its first taste of success now that a graduate student has managed to produce a soap-like, self-organizing monomer which can be polymerized to retain its ordered structure.
"The monomers self-organize into what appears to the naked eye as a transparent, greasy matrix," says Gin. "And," he notes proudly, "it's stable in the presence of a variety of chemical additives."
At its cross section, the material looks like a honeycomb, with water filling the channels formed by the geometric matrix. The existence of the water channels is a ripe opportunity for Gin, who will use them to incorporate inorganic solids into the ordered matrix to form truly advanced composites with controllable small-scale architectures.
"Nature makes its materials on the basis of what's most abundant," says Gin. "We'd like to be able to pick and choose what we use in our materials."
The fact that the newly synthesized matrix is transparent is doubly convenient. Gin uses light to polymerize and lock in the organized organic structure. He also plans to use the same method to initiate precipitation of solids from water-soluble precursors incorporated in the water channels.
Synthesis of ultra-high-strength composites using the LC method is still a distant goal. However, other properties amplified by ordering may see uses in the near future, according to Gin.
In particular, Gin is interested in designing new piezoelectric materials, ordered structures like quartz that can convert mechanical movement into electricity and vice versa.
"We're taking a designer approach to these materials," says Gin, "but, our inspiration is drawn from natural and man-made materials already in use."
SYNTHESISTS LIKE GIN DREAM OF THE DAY when materials will leave their labs ready for use in the outside world. Despite their efforts, though, that day may still be a long while in coming. Therefore, processing of crude materials remains now and in the future a pivotal step in conferring properties to a newly-generated substance.
Polyethylene, for example, is a common long-chain hydrocarbon and is the guinea pig of choice for polymer processing work. It's the main ingredient of plastic bags; with the right kind of processing, it becomes one of the strongest fibers known to man.
"Polymers are long molecules," says Chemical Engineering Professor Morton Denn. "The properties of a polymeric material come about largely from the way in which you have oriented those molecules.
"The basic exercise in [polymer] processing is to come up with methodologies that allow you to work on a macroscopic scale and, through some clever way, grab the ends of a polymer molecule and pull it out," says Denn, a veteran in materials science who also heads a tightly-knit group of polymer scientists and engineers at Lawrence Berkeley National Laboratory.
"Sharkskin" surface distortion in a
In the case of polyethylene and most other polymers, the molecules are aligned while the material is in its liquid state where the polymer molecules resemble loosely tangled spaghetti.
"If you take a spoon and stir up a bowl of spaghetti, there is a certain degree of order that's imposed on the strands," says Denn. "It's much more ordered than if you take the strands and simply dump them into the bowl."
Finding ways to impose such order in polymers, however, is not as straightforward. One technique commonly known as "extrusion" coaxes the stringy molecules into alignment by corralling them through small, carefully-shaped holes to form shaped objects. Unfortunately, as the polymer is pushed through the die at higher and higher rates, the molecules falter and the shape distorts.
"We now believe that these distortions have to do with the way in which the polymer in the liquid state interacts with the surface of the die that you're using," says Denn. "From the point of view of traditional fluid flow studies, this proposal is heresy--the equivalent of a round world being proposed."
But, just as they're sure that the world is round, scientists now generally agree that surface chemistry and physics are pivotal aspects of processing. Trying to understand and exploit these phenomena has practically created a new field of study.
What is the shape of polymer molecules near a surface? What are the forces one would expect to exist there? What role do adhesion and lubrication play? These are among many questions concerning polymers at surfaces that Denn and his departmental colleagues, such as Arup Chakroborty and Jeffrey Reimer, address using experimental, theoretical and computational methods.
Like Gin, Denn has turned to the ordered condition of the liquid crystal state as a way to help align polymer strands so that they resemble something closer to a pack of uncooked spaghetti than a bowl of tangled noodles. The use of liquid crystalline polymers is key to the development of "high-performance" materials such as DuPont's Kevlar and Hoechst Celanese's Vectra, which are extremely strong and resistant to heat and chemicals.
"The problem is that the technology for making something like Kevlar is only in place for fibers, not for large, three dimensional objects" Denn says. "We're still struggling to learn how to exploit these wonderful properties that exist on micron scales in objects that are measured in centimeters, not microns."
WHERE DENN'S YARDSTICK OF PROGRESS could quite literally be a yardstick, Assistant Professor Roya Maboudian would need a ruler far smaller than the diameter of a human hair to judge the quality of the materials she strives to make.
Maboudian is aiming to optimize the natural attributes of semiconductors, particularly silicon, to perform remarkable tasks in a miniaturized setting.
Silicon arose as the workhorse of the computer and electronics industry because of the rapid development of chemical methods for etching astoundingly small patterns onto the surface of the material--patterns essential for controlling properties on the small scale.
But while industry has come a long way in its ability to carve more and more minute features onto silicon, it lacks the detailed understanding of the chemistry needed to master even more precise manipulations. As part of the new generation of silicon blacksmiths, Maboudian is meeting this challenge using advanced surface science techniques to investigate chemical phenomena in silicon.
"It's incredibly important to know how to process and manipulate with unbelievable precision the surface of silicon, as well as other semiconductors," says Maboudian, who, along with graduate student Jon Kluth, is carefully probing the surface chemistry of the materials.
Such control will ultimately allow the scientists to harness silicon's renowned electrical properties on an even smaller level. But Maboudian and a handful of other pioneering scientists are now starting to realize that electrical properties are only a small portion of the many physical characteristics of silicon that can be brought to the micron scale.
Moveable microscopic mirror can precisely
Most notably, Maboudian's group is exploring the material's mechanical properties at this size, using etching technology to construct "micromachines"--tiny robotic devices made of chemically-machined silicon components.
"While the electronics industry is now a mature field both scientifically and commercially," says Maboudian, "it is widely believed that micromachining will be the technological revolution of the next decade."
However, tiny devices that can aim lasers down fiber optic cables or serve as sensors and actuators in miniaturized instruments are on hold until scientists solve one substantial problem: "stiction."
"When the machines' surfaces come into contact, they stick together and remain engaged," says Maboudian. "When I shake your hand, we don't remain engaged because we are heavy bodies. But, surface forces which are negligible in the macroscopic world become overwhelming in the microscopic world."
The Maboudian group is devising new fabrication techniques that reduce the energy of the materials' surface while simultaneously lowering its contact area--achievements that attenuated "stiction" by nearly four orders of magnitude.
"Some of our solutions to the problem incorporate materials which are cutting-edge themselves," says graduate student Mike Houston, referring to the advanced thin films the group has developed to mask the sticky surfaces of the miniaturized materials.
Developing specialized materials to control properties of other materials is perhaps one of the most striking examples of how College researchers are using today's understanding of chemical phenomena to create tomorrow's technological wonders.