In another paper, they relate their discovery that gold in this size regime can be made magnetic through oxygenation of gold nanowires. They also found that up to a certain length, oxygenated gold nanowires behave as a conducting metal, but beyond that, they become insulators.
This marks the first time on the nanoscale that such a metal-to-insulation transition has been found on the nanoscale. Both findings are important predictions that could some day be implemented as control parameters governing the chemical and physical material properties employed in nanotechnology.
The researchers focused their theoretical investigations on gold nanostructures because of the well known chemical inertness of gold in the bulk form, allowing one to maintain samples with minimal influence on the environment.
“However, we again find that small is different,” said Uzi Landman, Regents’ and Institute Professor, holder of the F.E. Callaway Chair, and director of the Center for Computational Materials Science, repeating a phrase that he coined and has used often for close to two decades. “On the nanoscale, even gold becomes a potent catalyst, exhibiting new and surprising, chemical, mechanical, electrical and magnetic behavior, which could not have been extrapolated or predicted on the basis of our knowledge about this substance in the bulk form. Some of these systems may find technological uses in nanocatalysis and as chemical and electrical sensors,” Landman added.
For the first study, which appears in the February 8 edition of Physical Review Letters, Landman and Research Scientist Bokwon Yoon performed first-principles quantum mechanical computer simulations of a 20-atom gold nanocluster adsorbed on the surface of a film of magnesium oxide (MgO), an insulator which itself is supported by an underlying metallic silver substrate. The optimal configuration of the adsorbed gold nanocluster depends on the thickness of the underlying MgO film, and for an eight-layer film it was found to be a three-dimensional four-sided pyramid, with one of the sides contacting the magnesium oxide surface.
However, the researchers discovered that under the influence of an externally applied electric field, the aforementioned three-dimensional shape of the gold nanocluster is no longer the energetically favored structure. Instead, the optimal structure becomes a flat planar 20-atom gold island spread on the MgO surface.
Turning off the applied field or reversing it’s direction results in reverting the structure back to the pyramidal one. The origin of the nanocluster morphological change was found to relate to accumulation of excess electronic charge at the interface between the cluster and the magnesium oxide film. This excess charge, which stabilizes the planar nanocluster structure, originates from the underlying silver substrate, and it’s ability to penetrate to the cluster interface through the eight-layer thick MgO film depends on the presence of the externally applied driving electric field.
The researchers also discovered that the chemical activity of the adsorbed gold nanocluster varied significantly under the influence of the applied field, enhancing the low-temperature catalytic oxidation of CO to carbon dioxide.
“We found that we can change in a controllable manner the physical as well as the chemical properties of the adsorbed nanostructure by applying an external voltage across the supported gold nanocluster,” said Landman. “ I believe that this finding may introduce a potent control parameter into the chemistry of materials. The newly proposed method for tuning and controlling the structure and reactivity of nanostructures through the application of external electric fields may open novel directions and increase the range and applications of nanocatalytic systems and chemically based sensors and catalytic switches.”
The second finding, which appears in the February 1 edition of the same journal, answers the question of what happens when a nanowire of gold is pulled in the presence of oxygen. In these studies Landman, Postdoctoral Fellow Chun Zhang and Senior Research Scientist Robert Barnett used first-principles simulations and quantum electrical transport calculations. They found that oxygenated gold nanowires exhibit different properties, depending on whether the oxygen is incorporated in a molecular form or as individual atoms. Indeed, some of these theoretical results offer a new interpretation of recent laboratory experiments on oxygenated gold nanowires.
In the case of incorporation of molecular oxygen into the gold nanowire, the simulations revealed that the nanowires can be stretched to a significantly longer extent than pure gold nanowires – in other words, the adsorbed oxygen molecule serves as a reinforcing clamp.
Furthermore, the simulations predict that up to a certain stretching distance (typically up to wires that resemble a stretched necklace of about six gold atoms and an embedded oxygen molecule), such nanowires will conduct electrons similar to a pure gold nanowire. These results have been confirmed experimentally. Moreover, the simulations predict that oxygenated gold nanowires extended beyond a length of about six gold atoms become insulating. The conducting state can be restored by a slight contraction of the wire, thus allowing distance - dependent sensitive metal-to-insulator nano-switching.
When individual oxygen atoms, rather than an oxygen molecule, are incorporated in the gold nanowire, the elongation range was found to be limited and the electrical conductance was predicted to be lower than in the previous case of molecular oxygen incorporation. However, a surprising finding was made for such wires, predicting the emergence of magnetism with the magnetic moments localized on the embedded oxygens and on the neighboring gold nanowire atoms.
“It’s a very exotic thing,” said Landman of the phenomenon. “Finding materials that have magnetic properties when their bulk form doesn’t have those properties is very interesting from a fundamental point of view, and may have certain future technological applications.”
The Georgia Institute of Technology is one of the nation's premiere research universities. Ranked seventh among U.S. News & World Report's top public universities, Georgia Tech's more than 18,000 students are enrolled in its Colleges of Architecture, Computing, Engineering, Liberal Arts, Management and Sciences. Tech is among the nation's top producers of women and African-American engineers. The Institute offers research opportunities to both undergraduate and graduate students and is home to more than 100 interdisciplinary units plus the Georgia Tech Research Institute.
David Terraso | EurekAlert!
Study offers new theoretical approach to describing non-equilibrium phase transitions
27.04.2017 | DOE/Argonne National Laboratory
SwRI-led team discovers lull in Mars' giant impact history
26.04.2017 | Southwest Research Institute
More and more automobile companies are focusing on body parts made of carbon fiber reinforced plastics (CFRP). However, manufacturing and repair costs must be further reduced in order to make CFRP more economical in use. Together with the Volkswagen AG and five other partners in the project HolQueSt 3D, the Laser Zentrum Hannover e.V. (LZH) has developed laser processes for the automatic trimming, drilling and repair of three-dimensional components.
Automated manufacturing processes are the basis for ultimately establishing the series production of CFRP components. In the project HolQueSt 3D, the LZH has...
Reflecting the structure of composites found in nature and the ancient world, researchers at the University of Illinois at Urbana-Champaign have synthesized thin carbon nanotube (CNT) textiles that exhibit both high electrical conductivity and a level of toughness that is about fifty times higher than copper films, currently used in electronics.
"The structural robustness of thin metal films has significant importance for the reliable operation of smart skin and flexible electronics including...
The nearby, giant radio galaxy M87 hosts a supermassive black hole (BH) and is well-known for its bright jet dominating the spectrum over ten orders of magnitude in frequency. Due to its proximity, jet prominence, and the large black hole mass, M87 is the best laboratory for investigating the formation, acceleration, and collimation of relativistic jets. A research team led by Silke Britzen from the Max Planck Institute for Radio Astronomy in Bonn, Germany, has found strong indication for turbulent processes connecting the accretion disk and the jet of that galaxy providing insights into the longstanding problem of the origin of astrophysical jets.
Supermassive black holes form some of the most enigmatic phenomena in astrophysics. Their enormous energy output is supposed to be generated by the...
The probability to find a certain number of photons inside a laser pulse usually corresponds to a classical distribution of independent events, the so-called...
Microprocessors based on atomically thin materials hold the promise of the evolution of traditional processors as well as new applications in the field of flexible electronics. Now, a TU Wien research team led by Thomas Müller has made a breakthrough in this field as part of an ongoing research project.
Two-dimensional materials, or 2D materials for short, are extremely versatile, although – or often more precisely because – they are made up of just one or a...
20.04.2017 | Event News
18.04.2017 | Event News
03.04.2017 | Event News
27.04.2017 | Life Sciences
27.04.2017 | Physics and Astronomy
27.04.2017 | Earth Sciences