Forum for Science, Industry and Business

Sponsored by:     3M 
Search our Site:

 

Physicists Discover Structures of Gold Nanoclusters

18.01.2007
Using different experimental techniques, two separate and independent research groups in collaboration with a team from the Center for Computational Materials Science (CCMS) at the Georgia Institute of Technology, have unveiled the size-dependent evolution of structural and electronic structural motifs of gold nanoclusters ranging in size from 11 to 24 atoms. The experiments, in conjunction with the theoretical analysis performed by the Georgia Tech team, show near perfect agreement pertaining to the cluster structures occurring in the experiments.
Understanding the electronic and geometric structures of gold nanoclusters is a key step towards understanding their behavior under different conditions, such as their use as nanocatalysts or in certain medical applications. The results appear in separate papers in The Physical Review B and in the journal ChemPhysChem.

In its bulk form, gold is treasured for its property as a non-reactive metal. Its use in electronics, dentistry, jewelry and art, depends on this inertness. But at the nano scale, when gold clusters contain only a small number of atoms, gold shows very different properties, which exhibit chemical reactivity that make them potent catalysts. Because their chemical and physical properties depend greatly on their physical structures, significant efforts have been invested by scientists to determine what the most stable configurations of gold clusters are in this size range. Understanding this is of great importance for elucidating the chemical properties of these clusters and in research aiming to discover the physical patterns that govern how the clusters are put together.

Between 2000 and 2002, a Georgia Tech team, led by Uzi Landman, director of CCMS, Regents’ and Institute professor, and Callaway chair of physics at Georgia Tech, predicted that negatively charged gold nanoclusters, up to 13 atoms in size, would exhibit two-dimensional, flat structures. The appearance of two-dimensional structures for such relatively large metal clusters is unique to gold, and the researchers showed that it is related to the strong relativistic effects for this metal. When these predictions were verified experimentally, research in Landman’s group and in other places focused on what happens when the nanoclusters are even larger.

"We wanted to know, what happens after 13 atoms,” said Landman. “What happens when these clusters become three-dimensional and what is their structural motif?” For the past few years, scientists at the CCMS have made theoretical predictions about the structures of gold nanoclusters in the larger size range. Now, working with two independent experimental groups, Landman and his collaborators have found firm evidence pertaining to the size-dependent structural development of these nanoclusters.

One of these collaborations involved researchers from the University of Freiburg and the Fraunhofer Institute for Mechanics of Materials, both in Germany, and a scientist from the University of Jväskylä in Finland. The Freiburg team performed photoemission experiments, in which a laser is shot at the gas-phase cluster causing it to eject an electron. Measuring the energy profiles of the emitted electrons using lasers of different wavelengths allowed the researchers to gain knowledge about the occupied electronic energy levels in the clusters. The distribution of these levels depends on the specific geometric arrangement of atoms in the clusters. Indeed, the theoretical analysis of the correlation between the distributions of the electronic energy levels and the atomic spatial arrangements allowed the researchers to determine the clusters’ electronic properties, as well as geometric structures.

In the other collaboration, the Georgia Tech researchers worked with a team from the Rowland Institute at Harvard University. They used electron diffraction, a technique in which a beam of electrons is fired at the clusters, causing the electrons to scatter. By measuring the intensity of the scattered electrons and comparing it to the change in momenta of the electrons caused by their collisions with the atoms of the clusters, they obtaines information about the spatial arrangements of the atoms in the clusters. Theoretical analysis of the interference patterns in these measured intensities allowed them to determine the clusters’ structures.

"It turns out that close to all the stable structures that were found through our theoretical analysis of the photoemission measurements were the same as those that emerged from analysis of the electron scattering experiments,” said Landman. “In our analysis we have used first-principles electronic structure calculations based on density-functional theory, in conjunction with structural optimization techniques. This is likely the first time that two separate and independent experimental tools, in conjunction with a common theoretical analysis, have shown such a high degree of agreement in the challenging area of structural determination of nano clusters.”

To avoid any bias, and ensure that the groups’ analyses weren’t being unintentionally influenced by knowledge of each other, neither experimental group saw the results of the other until the publication of their respective papers.

The results of the Georgia Tech collaborative investigations with the European group are published in the journal ChemPhysChem Volume 8, (2007), and those obtained from the collaboration with the Rowland Institute are published in The Physical Review B volume 74, (2006).

Through this comparison between experiment and theory, the teams found that the clusters start out as two-dimensional structures till 13 or 14 atoms in size, changing to three-dimensional hollow cages from about 16 atoms, and developing a face-centered-cubic tetrahedral structure at 20 atoms, resembling the bulk gold crystalline structure. However, at 24 atoms the gold clusters take an unexpected capped tubular cigar shape.

"These results assist us not only in determining the structures of the clusters, but also provide insight into the factors that underlie their self-assembly,” said Landman. “In some ways, we are determining the ‘structural grammar’ of these gold nanoclusters and by understanding that, we may better understand what motifs appear as we continue to search for the structures of clusters larger than 24 atoms.

The Georgia Tech team consisted of Uzi Landman and research scientist Bokwon Yoon. The collaborations consisted of Pekka Kosiken, Bernd Huber and Michael Moseler from the Fraunhofer Institute for Mechanics of Materials and the University of Freiburg, Oleg Kostko and Bernd von Issendorff from the University of Freiburg and Hannu Hakkinen from the University of Jyvaskla. The Rowland Institute team was made up of Xiaopeng Xing and Joel H. Parks.

David Terraso | EurekAlert!
Further information:
http://www.gatech.edu

More articles from Physics and Astronomy:

nachricht NASA detects solar flare pulses at Sun and Earth
17.11.2017 | NASA/Goddard Space Flight Center

nachricht Pluto's hydrocarbon haze keeps dwarf planet colder than expected
16.11.2017 | University of California - Santa Cruz

All articles from Physics and Astronomy >>>

The most recent press releases about innovation >>>

Die letzten 5 Focus-News des innovations-reports im Überblick:

Im Focus: A “cosmic snake” reveals the structure of remote galaxies

The formation of stars in distant galaxies is still largely unexplored. For the first time, astron-omers at the University of Geneva have now been able to closely observe a star system six billion light-years away. In doing so, they are confirming earlier simulations made by the University of Zurich. One special effect is made possible by the multiple reflections of images that run through the cosmos like a snake.

Today, astronomers have a pretty accurate idea of how stars were formed in the recent cosmic past. But do these laws also apply to older galaxies? For around a...

Im Focus: Visual intelligence is not the same as IQ

Just because someone is smart and well-motivated doesn't mean he or she can learn the visual skills needed to excel at tasks like matching fingerprints, interpreting medical X-rays, keeping track of aircraft on radar displays or forensic face matching.

That is the implication of a new study which shows for the first time that there is a broad range of differences in people's visual ability and that these...

Im Focus: Novel Nano-CT device creates high-resolution 3D-X-rays of tiny velvet worm legs

Computer Tomography (CT) is a standard procedure in hospitals, but so far, the technology has not been suitable for imaging extremely small objects. In PNAS, a team from the Technical University of Munich (TUM) describes a Nano-CT device that creates three-dimensional x-ray images at resolutions up to 100 nanometers. The first test application: Together with colleagues from the University of Kassel and Helmholtz-Zentrum Geesthacht the researchers analyzed the locomotory system of a velvet worm.

During a CT analysis, the object under investigation is x-rayed and a detector measures the respective amount of radiation absorbed from various angles....

Im Focus: Researchers Develop Data Bus for Quantum Computer

The quantum world is fragile; error correction codes are needed to protect the information stored in a quantum object from the deteriorating effects of noise. Quantum physicists in Innsbruck have developed a protocol to pass quantum information between differently encoded building blocks of a future quantum computer, such as processors and memories. Scientists may use this protocol in the future to build a data bus for quantum computers. The researchers have published their work in the journal Nature Communications.

Future quantum computers will be able to solve problems where conventional computers fail today. We are still far away from any large-scale implementation,...

Im Focus: Wrinkles give heat a jolt in pillared graphene

Rice University researchers test 3-D carbon nanostructures' thermal transport abilities

Pillared graphene would transfer heat better if the theoretical material had a few asymmetric junctions that caused wrinkles, according to Rice University...

All Focus news of the innovation-report >>>

Anzeige

Anzeige

Event News

Ecology Across Borders: International conference brings together 1,500 ecologists

15.11.2017 | Event News

Road into laboratory: Users discuss biaxial fatigue-testing for car and truck wheel

15.11.2017 | Event News

#Berlin5GWeek: The right network for Industry 4.0

30.10.2017 | Event News

 
Latest News

NASA detects solar flare pulses at Sun and Earth

17.11.2017 | Physics and Astronomy

NIST scientists discover how to switch liver cancer cell growth from 2-D to 3-D structures

17.11.2017 | Health and Medicine

The importance of biodiversity in forests could increase due to climate change

17.11.2017 | Studies and Analyses

VideoLinks
B2B-VideoLinks
More VideoLinks >>>