Argonne-UChicago researchers work to annihilate nanoscale defects in semiconductors

Researchers from the University of Chicago and Argonne use the supercomputing resources at the Argonne Leadership Computing Facility to predict the path molecules must follow to find defect-free states. They designed a process that delivers industry-standard nanocircuitry that can be scaled down to smaller densities without defects.
Courtesy of Argonne National Laboratory

Target dates are critical when the semiconductor industry adds small, enhanced features to consumer devices by integrating advanced materials onto the surfaces of computer chips. Missing a target means postponing a device’s release, which could cost a company millions of dollars or the loss of competitiveness and an entire industry.

But meeting target dates can be challenging because the final integrated devices, which include billions of transistors, must be flawless—less than one defect per 100 square centimeters.

Researchers at the University of Chicago and Argonne National Laboratory, led by Profs. Juan de Pablo and Paul Nealey, may have found a way for the semiconductor industry to hit miniaturization targets on time and without defects.

To make microchips, de Pablo and Nealey’s technique includes creating patterns on semiconductor surfaces that allow block copolymer molecules to self-assemble into specific shapes, but thinner and at much higher densities than those of the original pattern. The researchers can then use a lithography technique to create nano-trenches where conducting wire materials can be deposited.

This is a stark contrast to the industry practice of using homo-polymers in complex “photoresist” formulations, where researchers have “hit a wall,” unable to make the material smaller.

Before they could develop their new fabrication method, however, de Pablo and Nealey needed to understand exactly how block copolymers self-assemble when coated onto a patterned surface—their concern being that certain constraints cause copolymer nanostructures to assemble into undesired metastable states. To reach the level of perfection demanded to fabricate high-precision nanocircuitry, the team had to eliminate some of these metastable states.

Using the Argonne Leadership Computing Facility, UChicago and Argonne researchers have found a way miniaturize microchip components using a technique producing zero defects. This advance will allow semiconductor manufacturers to meet miniaturization target dates to produce smaller components with added functionality for consumer devices.
Courtesy of Argonne National Laboratory

To imagine how block copolymers assemble, it may be helpful to picture an energy landscape consisting of mountains and valleys, in which some valleys are deeper than others. The system prefers defect-free stability, which can be characterized by the deepest (low-energy) valleys, if they can be found. However, systems can get trapped inside higher (medium-energy) valleys, called metastable states, which have more defects.

To move from a metastable to stable state, block copolymer molecules must find ways to climb over the mountains and find lower energy valleys.

“Molecules in these metastable states are comfortable, and they can remain in that state for extraordinarily long periods of time,” said de Pablo.

“In order to escape such states and attain a perfect arrangement, they need to start rearranging themselves in a manner that allows the system to climb over local energy barriers, before reaching a lower energy minimum. What we have done in this work is predict the path these molecules must follow to find defect-free states and designed a process that delivers industry-standard nanocircuitry that can be scaled down to smaller densities without defects.”

Supported by a DOE leadership computing grant, de Pablo and his team used the Mira and Fusion supercomputers at the Argonne Leadership Computing Facility. The team generated molecular simulations of self-assembling block polymers along with sophisticated sampling algorithms to calculate where barriers to structural rearrangement would arise in the material. 

After all the calculations were done, the researchers could precisely predict the pathways of molecular rearrangement that block copolymers must take to move from a metastable to stable state. They could also experiment with temperatures, solvents and applied fields to further manipulate and decrease the barriers between these states.

To test these calculations, de Pablo and Nealey partnered with IMEC, an international consortium located in Belgium. Their commercial-grade fabrication and characterization instruments helped the researchers perform experiments under conditions that are not available in academic laboratories.

An individual defect measures only a handful of nanometers; “finding a defect in a 100-square centimeter area is like finding a needle in hay stack, and there are only a few places in the world where one has access to the necessary equipment to do so,” said de Pablo.

“Manufacturers have long been exploring the feasibility of using block copolymer assembly to reach the small critical dimensions that are demanded by modern computing and higher data storage densities,” de Pablo said. “Their biggest challenge involved evaluating defects; by following the strategies we have outlined, that challenge is greatly diminished.”

John Neuffer, president and CEO of the Semiconductor Industry Association, said industry is relentlessly focused on designing and building chips that are smaller, more powerful and more energy-efficient.

“The key to unlocking the next generation of semiconductor innovation is research,” he said. “SIA commends the work done by Argonne National Laboratory and the University of Chicago, as well as other critical scientific research being done across the United States.”

De Pablo, Nealey and their team will continue their investigations with a wider class of materials, increasing the complexity of patterns and characterizing materials in greater detail while also developing methods based on self-assembly for fabrication of three-dimensional structures.

Their long-term goal, with support from the DOE’s Office of Science, is to arrive at an understanding of directed self-assembly of polymeric molecules that will enable creation of wide classes of materials with exquisite control over their nanostructure and functionality for applications in energy harvesting, storage and transport.

University of Chicago

How to make ceramics that bend without breaking

New materials developed at MIT could lead to actuators on a chip and self-deploying medical devices.

Ceramics are not known for their flexibility: they tend to crack under stress. But researchers from MIT and Singapore have just found a way around that problem — for very tiny objects, at least.

The team has developed a way of making minuscule ceramic objects that are not only flexible, but also have a “memory” for shape: When bent and then heated, they return to their original shapes. The surprising discovery is reported this week in the journal Science, in a paper by MIT graduate student Alan Lai, professor Christopher Schuh, and two collaborators in Singapore. 

Shape-memory materials, which can bend and then snap back to their original configurations in response to a temperature change, have been known since the 1950s, explains Schuh, the Danae and Vasilis Salapatas Professor of Metallurgy and head of MIT’s Department of Materials Science and Engineering. “It’s been known in metals, and some polymers,” he says, “but not in ceramics.”

In principle, the molecular structure of ceramics should make shape memory possible, he says — but the materials’ brittleness and propensity for cracking has been a hurdle. “The concept has been there, but it’s never been realized,” Schuh says. “That’s why we were so excited.” 

The key to shape-memory ceramics, it turns out, was thinking small.

The team accomplished this in two key ways. First, they created tiny ceramic objects, invisible to the naked eye: “When you make things small, they are more resistant to cracking,” Schuh says. Then, the researchers concentrated on making the individual crystal grains span the entire small-scale structure, removing the crystal-grain boundaries where cracks are most likely to occur.

Those tactics resulted in tiny samples of ceramic material — samples with deformability equivalent to about 7 percent of their size. “Most things can only deform about 1 percent,” Lai says, adding that normal ceramics can’t even bend that much without cracking.

David Dunand, a professor of materials science and engineering at Northwestern University, says the MIT team “achieved something that was widely considered impossible,” finding “a clever solution, based on fundamental materials-science principles, to the Achilles’ heel of ceramics and other brittle materials.” 

“Usually if you bend a ceramic by 1 percent, it will shatter,” Schuh says. But these tiny filaments, with a diameter of just 1 micrometer — one millionth of a meter — can be bent by 7 to 8 percent repeatedly without any cracking, he says.

While a micrometer is pretty tiny by most standards, it’s actually not so small in the world of nanotechnology. “It’s large compared to a lot of what nanotech people work on,” Lai says. As such, these materials could be important tools for those developing micro- and nanodevices, such as for biomedical applications. For example, shape-memory ceramics could be used as microactuators to trigger actions within such devices — such as the release of drugs from tiny implants.

Compared to the materials currently used in microactuators, Schuh says, the strength of the ceramic would allow it to exert a stronger push in a microdevice. “Microactuation is something we think this might be very good for,” he says, because the ceramic material has “the ability to push things with a lot of force — the highest on record” for its size.

The ceramics used in this research were made of zirconia, but the same techniques should apply to other ceramic materials. Zirconia is “one of the most well-studied ceramics,” Lai says, and is already widely used in engineering. It is also used in fuel cells, considered a promising means of providing power for cars, homes and even for the electric grid. While there would be no need for elasticity in such applications, the material’s flexibility could make it more resistant to damage.

The material combines some of the best attributes of metals and ceramics, the researchers say: Metals have lower strength but are very deformable, while ceramics have much greater strength, but almost no ductility — the ability to bend or stretch without breaking. The newly developed ceramics, Schuh says, have “ceramiclike strength, but metallike ductility.”

Robert Ritchie, a professor of materials science and engineering at the University of California at Berkeley, says, “The very notion of superelastic ceramics is somewhat of a surprise. … We all know that ceramics invariably are extremely brittle.”

Ritchie, who was not connected with this work, points out that shape-memory metals are already used in satellite antennae and in self-expanding dental and cardiovascular prostheses. “Applying these concepts to ceramics, however,” he says, “is somewhat startling and raises many interesting possibilities.”

In addition to Schuh and Lai, the work was carried out by Zehui Du and Chee Lip Gan of Nanyang Technological University in Singapore.

Source: http://web.mit.edu/newsoffice/2013/how-to-make-ceramics-that-bend-without-breaking-0926.html

New Model Should Expedite Development of Temperature-Stable Nano-Alloys

Researchers from North Carolina State University have developed a new theoretical model that will speed the development of new nanomaterial alloys that retain their advantageous properties at elevated temperatures.

Nanoscale materials are made up of tiny crystals, or grains, that are less than 100 nanometers in diameter. These materials are of interest to researchers, designers and manufacturers because two materials can have the same chemical composition but very different mechanical properties depending on their grain size. For example, materials with nanoscale grains can be harder and stronger than chemically identical materials with larger grains.

But widespread use of nanoscale materials has long been handicapped by the tendency of nanoscale grains to grow when exposed to elevated temperatures – thereby losing their desired mechanical properties.

This is a problem because creating bulk materials from powdered nanomaterials involves exposure to high temperatures, and even nanomaterials made using other techniques may be exposed to elevated temperatures. The grains in some nanomaterials can even grow – and lose their desired properties – when exposed to room temperature for an extended period of time.

A team of NC State researchers decided to tackle the problem by exploring a concept that had been discussed in the research community for some time: stabilizing nanomaterials by introducing small amounts of an additional element. The idea is that this additional element would serve as a stabilizing agent, migrating to the grain boundaries – or interfaces between grains – and preventing the grains from growing at elevated temperatures. Implementing that concept had been daunting, since there are thousands of possible combinations of these elements.

To turn that idea into a practical solution, the researchers developed a theoretical model to identify suitable candidates that can be used as stabilizing agents.

The theoretical model focuses on alloys that consist of two elements, such as iron and chromium, then allows users to see what would happen if a third element is added to the mix. If users plug the atomic size and thermodynamic properties of each element into the model, the model predicts the grain size of the alloy at any given temperature.

“This model allows anyone to design alloys in a targeted and effective way without having to resort to a trial-and-error approach,” says Dr. Ron Scattergood, a professor of materials science and engineering at NC State and senior author of a paper describing the work. “And our experimental results confirm the accuracy of the model.”

“We are already using the model in our investigations into lightweight aluminum alloys and high-temperature alloys for nuclear energy applications,” says Dr. Mostafa Saber, lead author of the study and a postdoctoral research scholar in materials science and engineering at NC State.

The paper, “A Predictive Model for Thermodynamic Stability of Grain Size in Nanocrystalline Ternary Alloys,” was published online Sept. 12 in the Journal of Applied Physics. The paper was co-authored by Dr. Hasan Kotan, a former Ph.D. student and postdoctoral researcher at NC State, and Dr. Carl Koch, Kobe Steel Distinguished Professor of Materials Science and Engineering at NC State. The research was supported by the National Science Foundation and the U.S. Department of Energy.

Source: http://news.ncsu.edu/releases/wms-saber-model-2013/

The gold standard for cell penetration

Gold nanoparticles with special coatings can deliver drugs or biosensors to a cell’s interior without damaging it.

Cells are very good at protecting their precious contents — and as a result, it’s very difficult to penetrate their membrane walls to deliver drugs, nutrients or biosensors without damaging or destroying the cell. One effective way of doing so, discovered in 2008, is to use nanoparticles of pure gold, coated with a thin layer of a special polymer. But nobody knew exactly why this combination worked so well, or how it made it through the cell wall.

Now, researchers at MIT and the Ecole Polytechnique de Lausanne in Switzerland have figured out how the process works, and the limits on the sizes of particles that can be used. Their analysis appears in the journal Nano Letters, in a paper by graduate students Reid Van Lehn, Prabhani Atukorale, Yu-Sang Yang and Randy Carney and professors Alfredo Alexander-Katz, Darrell Irvine and Francesco Stellacci.

Until now, says Van Lehn, the paper’s lead author, “the mechanism was unknown. … In this work, we wanted to simplify the process and understand the forces” that allow gold nanoparticles to penetrate cell walls without permanently damaging the membranes or rupturing the cells. The researchers did so through a combination of lab experiments and computer simulations.

The team demonstrated that the crucial first step in the process is for coated gold nanoparticles to fuse with the lipids — a category of natural fats, waxes and vitamins — that form the cell wall. The scientists also demonstrated an upper limit on the size of such particles that can penetrate the cell wall — a limit that depends on the composition of the particle’s coating.

The coating applied to the gold particles consists of a mix of hydrophobic and hydrophilic components that form a monolayer — a layer just one molecule thick — on the particle’s surface. Any of several different compounds can be used, the researchers explain.

“Cells tend to engulf things on the surface,” says Alexander-Katz, an associate professor of materials science and engineering at MIT, but it’s “very unusual” for materials to cross that membrane into the cell’s interior without causing major damage. Irvine and Stellacci demonstrated in 2008 that monolayer-coated gold nanoparticles could do so; they have since been working to better understand why and how that works.

Since the nanoparticles themselves are completely coated, the fact that they are made of gold doesn’t have any direct effect, except that gold nanoparticles are an easily prepared model system, the researchers say. However, there is some evidence that the gold particles have therapeutic properties, which could be a side benefit. 

Gold particles are also very good at capturing X-rays — so if they could be made to penetrate cancer cells, and were then heated by a beam of X-rays, they could destroy those cells from within. “So the fact that it’s gold may be useful,” says Irvine, a professor of materials science and engineering and biological engineering and member of the 
Koch Institute for Integrative Cancer Research.

Significantly, the mechanism that allows the nanoparticles to pass through the membrane seems also to seal the opening as soon as the particle has passed. “They would go through without allowing even small molecules to leak through behind them,” Van Lehn says.

Irvine says that his lab is also interested in harnessing this cell-penetrating mechanism as a way of delivering drugs to the cell’s interior, by binding them to the surface coating material. One important step in making that a useful process, he says, is finding ways to allow the nanoparticle coatings to be selective about what types of cells they attach to. “If it’s all cells, that’s not very useful,” he says, but if the coatings can be targeted to a particular cell type that is the target of a drug, that could be a significant benefit.

Another potential application of this work could be in attaching or inserting biosensing molecules on or into certain cells, Van Lehn says. In this way, scientists could detect or monitor specific biochemical markers, such as proteins that indicate the onset or decline of a disease or a metabolic process.

In general, attachment to nanoparticles’ surface coatings could provide a key to cells’ interiors for “molecules that normally wouldn’t have any ability to get through the cell membrane,” Irvine says.

Vince Rotello, a professor of chemistry at the University of Massachusetts at Amherst who was not involved in this research, says this work is “careful, well thought out and elegantly presented.” He adds, “This study provides a very interesting alternative mechanism to cell uptake of nanomaterials that could open up new therapeutic pathways.”

http://web.mit.edu/newsoffice/2013/the-gold-standard-for-cell-penetration-0823.html

Two become one with the 3D NanoChemiscope

Unique surface analysis instrument

The 3D NanoChemiscope is a miracle of state-of-the-art analysis technology. As a further development of well-known microscopic and mass spectroscopic methods, it maps the physical and chemical surfaces of materials down to the atomic level. This instrument, which is unique in the world, not only delivers high-definition images; it also knows what it is "seeing".

The result of a combined three-dimensional ToF-SIMS-/SFM surface analysis of a PCBM/CyI-polymer blend used by Empa's Functional Polymers Laboratory to produce organic solar cells.

What do a penguin and the surface of a solar cell have in common? Not a lot concedes Empa physicist Laetitia Bernard. Yet she must have smiled when, while processing an image of a polymer blend required to produce a new type of organic solar cell, at a certain point she could make out more and more clearly the outline of a penguin. A small detail in the complex world of high-performance microscopy. The 3D NanoChemiscope, which was developed at Empa, not only maps samples with nanometre precision, but for the first time can also provide precise information about which chemical elements are arranged where in a sample. This enables both mechanical properties, such as hardness, elasticity or friction, and chemical properties of surfaces to be determined simultaneously in three dimensions. In the case of the "penguin” image, this means that the 3D NanoChemiscope not only captures the outline of the "penguin", but also detects which polymers are located at its "beak", at its "eye" and "around" it. Using this analysis technique, the solar cell researchers are able to efficiently control the mechanisms of their materials and adapt the composition or concentration of their polymer blend accordingly. This enables new structures and therefore leads to better performances of the solar cell to be created.

Some of the many individual images from which the 3D NanoChemiscope generated the 3D view. 

The SFM scans the topography of the surface (The image on the left shows a section 12µm x 12µm in size. The differences in height visible in the image measure 100-200nm). 

With the TOF-SIMS, it is possible to identify where the different materials or polymers in the polymer blend are located on the surface (The images in the middle and on the right show C-+C2- and CN-+I- ions).

Scanning force microscope and high-end mass spectrometer

This analysis is made possible by the 3D NanoChemiscope, which combines two previously independent techniques. The scanning force microscope (SFM) scans the surface with an ultra-fine tip, while the time-of-flight secondary ion mass spectrometer (ToF-SIMS) determines the material composition of the first surface mono-layer by "shooting" metallic ions at it.

Up to now, in order to study both the chemical and physical properties of surfaces, it was necessary to analyse the sample in two different instruments. However, when transporting the sample from one instrument to the other, there was always a danger of contamination or oxidation. In addition, it was practically impossible to find the exact location scanned by the SFM again. What, therefore, could be more appropriate than to "combine" the two instruments? In a four-year project sponsored by the EU, project leader Laetitia Bernard, together with Empa researchers and partners from academia and industry, has carried out meticulous work to develop a new instrument in which an SFM and a ToF-SIMS are placed in an ultra-high vacuum chamber as near to each other as possible.

Mechanical engineer Sasa Vranjkovic and Laetitia Bernard, leader of the 3D NanoChemiscope project, discussing the construction drawing of a component.

The microscope experts have also equipped the 3D NanoChemiscope with a novel transport system developed in-house, which uses piezomotors to move the sample gently back and forth on tracks coated with a diamond-like carbon layer (DLC). The sample holder can move along five axes, allowing the location under investigation to be analysed from any angle.

Following its construction, the prototype – a monster made of gleaming aluminium 1 metre long, 70 centimetres wide and 1.7 metres tall – has been in operation at project partner ION-TOF GmbH in Münster, Germany, where it is being used by industrial clients and research partners. The construction of more instruments is planned, customers having expressed a keen interest and being prepared to pay sums over one million Swiss francs.

http://www.empa.ch/plugin/template/empa/3/139442/---/l=2

Polymer Solar Cells Employing Förster Resonance Energy Transfer

Two crucial tasks exist for realizing high-efficiency polymer solar cells: increasing the range of the spectral absorption of light and efficiently harvesting photo-generated excitons.  In this work, Förster resonance energy transfer (FRET)-based heterojunction polymer solar cells that incorporate squaraine dye (SQ) were fabricated and investigated.  The high absorbance of squaraine in the near-infrared region broadens the spectral absorption of the solar cells and assists in developing an ordered nano-morphology for enhanced charge transport.  Femtosecond spectroscopic studies revealed highly efficient (up to 96%) excitation energy transfer from poly(3-hexylthiophene), also known as P3HT, to squaraine occurring on a picosecond timescale.  A 38% increase in power conversion efficiency was realized to reach 4.5%; this finding suggests that this system has improved exciton migration over long distances.  This architecture transcends traditional multiblend systems, allowing multiple donor materials with separate spectral responses to work synergistically, thereby enabling an improvement in light absorption and conversion.  This discovery opens up a new avenue for the development of high-efficiency polymer solar cells.

Why Does This Matter?

A new energy transfer mechanism has been exploited for the first time, allowing significantly more efficient energy harvesting in P3HT/dye solar cells compared to P3HT-alone solar cells.  Also, broadening the light absorption spectrum into the near-infrared region and developing nanoscale parts to the solar cell has improved the device.

Allowing different light-absorbing materials to work synergistically has led to well-ordered polymer networks without post-processing. 

What Are the Specifics?

• CFN Capability: CFN’s Advanced Optical Spectroscopy & Microscopy Facility was used to understand the energy conversion mechanism and rate of electronic transfer between the dye and polymer in the solar cells.
• The use of squaraine dye and FRET of charge carriers improved the efficiency of polymer solar cells.  Femtosecond spectroscopic studies revealed highly efficient excitation energy transfer from P3HT to SQ occurring on a picosecond timescale.  This suggested that this system has improved exciton migration over long distances.
• For the first time, FRET was exploited to enhance exciton harvesting in polymer bulk heterojunction solar cells.

Reference

Jing-Shun Huang1, Tenghooi Goh1, Xiaokai Li1, Matthew Y. Sfeir2, Elizabeth A. Bielinski3, Stephanie Tomasulo4, Minjoo L. Lee4, Nilay Hazari3, and André D. Taylor1, Polymer bulk heterojunction solar cells employing Förster resonance energy transfer, Nature Photonics 7, 479-485 (2013).

1. Department of Chemical and Environmental Engineering, Yale University, New Haven, Connecticut 06511, USA
2. Center for Functional Nanomaterials, Brookhaven National Laboratory, Upton, New York 11973, USA
3. Department of Chemistry, Yale University, New Haven, Connecticut 06511, USA
4. Department of Electrical Engineering, Yale University, New Haven, Connecticut 06511, USAhttp://www.bnl.gov/newsroom/news.php?a=24233