Sunday, July 31, 2011

Writing nanostructures: Heated AFM tip allows direct fabrication of ferroelectric nanostructures on plastic

Using a technique known as thermochemical nanolithography (TCNL), researchers have developed a new way to fabricate nanometer-scale ferroelectric structures directly on flexible plastic substrates that would be unable to withstand the processing temperatures normally required to create such nanostructures.

The technique, which uses a heated atomic force microscope (AFM) tip to produce patterns, could facilitate high-density, low-cost production of complex ferroelectric structures for energy harvesting arrays, sensors and actuators in nano-electromechanical systems (NEMS) and micro-electromechanical systems (MEMS). The research was reported July 15 in the journal Advanced Materials.

"We can directly create piezoelectric materials of the shape we want, where we want them, on flexible substrates for use in energy harvesting and other applications," said Nazanin Bassiri-Gharb, co-author of the paper and an assistant professor in the School of Mechanical Engineering at the Georgia Institute of Technology. "This is the first time that structures like these have been directly grown with a CMOS-compatible process at such a small resolution. Not only have we been able to grow these ferroelectric structures at low substrate temperatures, but we have also been able to pattern them at very small scales."

The research was sponsored by the National Science Foundation and the U.S. Department of Energy. In addition to the Georgia Tech researchers, the work also involved scientists from the University of Illinois Urbana-Champaign and the University of Nebraska Lincoln.

The researchers have produced wires approximately 30 nanometers wide and spheres with diameters of approximately 10 nanometers using the patterning technique. Spheres with potential application as ferroelectric memory were fabricated at densities exceeding 200 gigabytes per square inch -- currently the record for this perovskite-type ferroelectric material, said Suenne Kim, the paper's first author and a postdoctoral fellow in laboratory of Professor Elisa Riedo in Georgia Tech's School of Physics.

Ferroelectric materials are attractive because they exhibit charge-generating piezoelectric responses an order of magnitude larger than those of materials such as aluminum nitride or zinc oxide. The polarization of the materials can be easily and rapidly changed, giving them potential application as random access memory elements.

But the materials can be difficult to fabricate, requiring temperatures greater than 600 degrees Celsius for crystallization. Chemical etching techniques produce grain sizes as large as the nanoscale features researchers would like to produce, while physical etching processes damage the structures and reduce their attractive properties. Until now, these challenges required that ferroelectric structures be grown on a single-crystal substrate compatible with high temperatures, then transferred to a flexible substrate for use in energy-harvesting.

The thermochemical nanolithography process, which was developed at Georgia Tech in 2007, addresses those challenges by using extremely localized heating to form structures only where the resistively-heated AFM tip contacts a precursor material. A computer controls the AFM writing, allowing the researchers to create patterns of crystallized material where desired. To create energy-harvesting structures, for example, lines corresponding to ferroelectric nanowires can be drawn along the direction in which strain would be applied.

"The heat from the AFM tip crystallizes the amorphous precursor to make the structure," Bassiri-Gharb explained. "The patterns are formed only where the crystallization occurs."

To begin the fabrication, the sol-gel precursor material is first applied to a substrate with a standard spin-coating method, then briefly heated to approximately 250 degrees Celsius to drive off the organic solvents. The researchers have used polyimide, glass and silicon substrates, but in principle, any material able to withstand the 250-degree heating step could be used. Structures have been made from Pb(ZrTi)O3 -- known as PZT, and PbTiO3 -- known as PTO.

"We still heat the precursor at the temperatures required to crystallize the structure, but the heating is so localized that it does not affect the substrate," explained Riedo, a co-author of the paper and an associate professor in the Georgia Tech School of Physics.

The heated AFM tips were provided by William King, a professor in the Department of Mechanical Science and Engineering at the University of Illinois at Urbana-Champaign.

As a next step, the researchers plan to use arrays of AFM tips to produce larger patterned areas, and improve the heated AFM tips to operate for longer periods of time. The researchers also hope to understand the basic science behind ferroelectric materials, including properties at the nanoscale.

"We need to look at the growth thermodynamics of these ferroelectric materials," said Bassiri-Gharb. "We also need to see how the properties change when you move from the bulk to the micron scale and then to the nanometer scale. We need to understand what really happens to the extrinsic and intrinsic responses of the materials at these small scales."

Ultimately, arrays of AFM tips under computer control could produce complete devices, providing an alternative to current fabrication techniques.

"Thermochemical nanolithography is a very powerful nanofabrication technique that, through heating, is like a nanoscale pen that can create nanostructures useful in a variety of applications, including protein arrays, DNA arrays, and graphene-like nanowires," Riedo explained. "We are really addressing the problem caused by the existing limitations of photolithography at these size scales. We can envision creating a full device based on the same fabrication technique without the requirements of costly clean rooms and vacuum-based equipment. We are moving toward a process in which multiple steps are done using the same tool to pattern at the small scale."

In addition to those already mentioned, the research team included Yaser Bastani from the G.W. Woodruff School of Mechanical Engineering at Georgia Tech, Seth Marder and Kenneth Sandhage, both from Georgia Tech's School of Chemistry and Biochemistry and School of Materials Science and Engineering, and Alexei Gruverman and Haidong Lu from the Department of Physics and Astronomy at the University of Nebraska-Lincoln.

Story Source:

The above story is reprinted (with editorial adaptations ) from materials provided by Georgia Institute of Technology Research News. The original article was written by John Toon.

Journal Reference:

Suenne Kim, Yaser Bastani, Haidong Lu, William P. King, Seth Marder, Kenneth H. Sandhage, Alexei Gruverman, Elisa Riedo, Nazanin Bassiri-Gharb. Direct Fabrication of Arbitrary-Shaped Ferroelectric Nanostructures on Plastic, Glass, and Silicon Substrates. Advanced Materials, 2011; DOI: 10.1002/adma.201101991

Modified metals change color in the presence of particular gases

 Modified metals that change colour in the presence of particular gases could warn consumers if packaged food has been exposed to air or if there's a carbon monoxide leak at home. This finding could potentially influence the production of both industrial and commercial air quality sensors.

"We initially found out by accident that modified rhodium reacts in a colourful way to different gases," says Cathleen Crudden, a professor in the Department of Chemistry. "That happy accident has become a driving force in our work with rhodium."

Rhodium that is modified using carbon, nitrogen or hydrogen-based complexes changes to yellow in the presence of nitrogen, deep blue in the presence of oxygen, and brown in the presence of carbon monoxide. This colour change occurs because of the way that the gases bind to the compound's central metal, according to the researchers.

Another remarkable aspect of this discovery is that the chemical changes take place without disrupting the exact placement of each individual atom in the compound's crystalline lattice. Dr. Crudden notes that this type of transformation is virtually unprecedented.

Rhodium is the main metal used in the production of catalytic convertors to reduce the toxicity of car exhaust emissions. Dr. Crudden's team, including graduate student Eric Keske and postdoctoral fellow Dr. Olena Zenkina, is currently investigating whether cobalt, a significantly cheaper metal than rhodium, reacts similarly.

Story Source:

The above story is reprinted (with editorial adaptations) from materials provided by Queen's University.

Note: If no author is given, the source is cited instead.

Disclaimer: Views expressed in this article do not necessarily reflect those of ScienceDaily or its staff.

Chemists create molecular polyhedron -- and potential to enhance industrial and consumer products

Chemists have created a molecular polyhedron, a ground-breaking assembly that has the potential to impact a range of industrial and consumer products, including magnetic and optical materials.

The work, reported in the latest issue of the journal Science, was conducted by researchers at New York University's Department of Chemistry and its Molecular Design Institute and the University of Milan's Department of Materials Science.

Researchers have sought to coerce molecules to form regular polyhedra -- three-dimensional objects in which each side, or face, is a polygon -- but without sustained success. Archimedean solids, discovered by the ancient Greek mathematician Archimedes, have attracted considerable attention in this regard. These 13 solids are those in which each face is a regular polygon and in which around every vertex -- the corner at which its geometric shapes meet -- the same polygons appear in the same sequences. For instance, in a truncated tetrahedron, the pattern forming at every vertex is hexagon-hexagon-triangle. The synthesis of such structures from molecules is an intellectual challenge.

The work by the NYU and University of Milan chemists forms a quasi-truncated octahedron, which also constitutes one of the 13 Archimedean solids. Moreover, as a polyhedron, the structure has the potential to serve as a cage-like framework to trap other molecular species, which can jointly serve as building blocks for new and enhanced materials.

"We've demonstrated how to coerce molecules to assemble into a polyhedron by design," explained Michael Ward, chair of NYU's Department of Chemistry and one of the study's co-authors. "The next step will be to expand on the work by making other polyhedra using similar design principles, which can lead to new materials with unusual properties."

The research team's creation relies on a remarkably high number of hydrogen bonds -- 72 -- to assemble two kinds of hexagonal molecular tiles, four each, into a truncated octahedron, which consists of eight molecular tiles. Although chemists often use hydrogen bonds because of their versatility in building complex structures, these bonds are weaker than those holding atoms together within the molecules themselves, which often makes larger scale structures constructed with hydrogen bonds less predictable and less sustainable. The truncated octahedron discovered by the NYU team proved to be remarkably stable, however, because the hydrogen bonds are stabilized by the ionic nature of the molecules and because no other outcomes are possible. In fact, the truncated octahedra assemble further into crystals that have nanoscale pores, resembling a class of well-known compounds called zeolites, which are made from inorganic components.

Because the structure also serves as a molecular cage, it can house, or encapsulate, other molecular components, giving future chemists a vehicle for developing a range of new compounds.

The study's other co-authors were Yuzhou Liu, a graduate student, and Chunhua Hu, a researcher professor, in NYU's Department of Chemistry and Molecular Design Institute and Professor Angiolina Comotti of the University of Milan's Department of Materials Science.

Story Source:

The above story is reprinted (with editorial adaptations) from materials provided by New York University, via EurekAlert!, a service of AAAS.

Journal Reference:

Yuzhou Liu, Chunhua Hu, Angiolina Comotti, Michael D. Ward. Supramolecular Archimedean Cages Assembled with 72 Hydrogen Bonds. Science, 2011; 333 (6041): 436-440 DOI: 10.1126/science.1204369

Cadmium selenide quantum dots degrade in soil, releasing their toxic guts, study finds

Quantum dots made from cadmium and selenium degrade in soil, unleashing toxic cadmium and selenium ions into their surroundings, a University at Buffalo study has found.

The research, accepted for publication in the journal Environmental Science and Technology, demonstrates the importance of learning more about how quantum dots -- and other nanomaterials -- interact with the environment after disposal, said Diana Aga, the chemistry professor who led the study.

Quantum dots are semiconductor nanocrystals with diameters of about 2 to 100 nanometers. Though quantum dots are not yet commonly used in consumer products, scientists are exploring the particles' applications in technologies ranging from solar panels to biomedical imaging.

"Quantum dots are not yet used widely, but they have a lot of potential and we can anticipate that the use of this nanomaterial will increase," said Aga, who presented the findings in late June at a National Science Foundation-funded workshop on nanomaterials in the environment. "We can also anticipate that their occurrence in the environment will also increase, and we need to be proactive and learn more about whether these materials will be a problem when they enter the environment."

"We can conclude from our research that there is potential for some negative impacts, since the quantum dots biodegrade. But there is also a possibility to modify the chemistry, the surface of the nanomaterials, to prevent degradation in the future," she said.

Aga's research into the afterlife of quantum dots is funded by a $400,000 Environmental Protection Agency grant to investigate the environmental transport, biodegradation and bioaccumulation of quantum dots and oxide nanoparticles.

Her collaborators on the new study in Environmental Science and Technology include PhD student Divina Navarro, Assistant Professor Sarbajit Banerjee and Associate Professor David Watson, all of the UB Department of Chemistry.

Working in the laboratory, the team tested two kinds of quantum dots: Cadmium selenide quantum dots, and cadmium-selenide quantum dots with a protective, zinc-sulfide shell. Though the shelled quantum dots are known in scientific literature to be more stable, Aga's team found that both varieties of quantum dot leaked toxic elements within 15 days of entering soil.

In a related experiment designed to predict the likelihood that discarded quantum dots would leach into groundwater, the scientists placed a sample of each type of quantum dot at the top of a narrow soil column. The researchers then added calcium chloride solution to mimic rain.

What they observed: Almost all the cadmium and selenium detected in each of the two columns -- more than 90 percent of that in the column holding unshelled quantum dots, and more than 70 percent of that in the column holding shelled quantum dots -- -remained in the top 1.5 centimeters of the soil.

But how the nanomaterials moved depended on what else was in the soil. When the team added ethylenediaminetetraacetic acid (EDTA) to test columns instead of calcium chloride, the quantum dots traveled through the soil more quickly. EDTA is a chelating agent, similar to the citric acid often found in soaps and laundry detergents.

The data suggest that under normal circumstances, quantum dots resting in top soil are unlikely to burrow their way down into underground water tables, unless chelating agents such as EDTA are introduced on purpose, or naturally-occurring organic acids (such as plant exudates) are present.

Aga said that even if the quantum dots remain in top soil, without contaminating underground aquifers, the particles' degradation still poses a risk to the environment.

In a separate study submitted for publication in a different journal, she and her colleagues tested the reaction of Arabidopsis plants to quantum dots with zinc sulfide shells. The team found that while the plants did not absorb the nanocrystals into their root systems, the plants still displayed a typical phytotoxic reaction upon coming into contact with the foreign matter; in other words, the plants treated the quantum dots as a poison.

Story Source:

The above story is reprinted (with editorial adaptations ) from materials provided by University at Buffalo.