Tag Archives: Technische Universität Wien

Pure gold nanostructures

This Nov. 4, 2016 news item on ScienceDaily features another ‘alchemy’ story although this one is truer to the source material than some of the other stories,

The idea is reminiscent of the ancient alchemists’ attempts to create gold from worthless substances: Researchers have discovered a novel way to fabricate pure gold nanostructures using an additive direct-write lithography technique. An electron beam is used to turn an auriferous organic compound into pure gold. This new technique can now be used to create nanostructures, which are needed for many applications in electronics and sensor technology. Just like with a 3D-printer on the nanoscale, almost any arbitrary shape can be created.

Caption: Nanostructure made of gold. Credit: TU Wien

Caption: Nanostructure made of gold. Credit: TU Wien

A Nov. 3, 2016 Technical University of Vienna (Technische Universität Wien) press release (also so on EurekAlert), which originated the news item, expands on the theme,

“Gold is not only a noble metal of exceptional beauty, but also a highly desired material for functional nanostructures”, says Professor Heinz Wanzenböck from TU Wien. Especially patterned gold nanostructures are key enabling structures in plasmonic devices, for biosensors with immobilized antibodies and as electrical contacts. For decades the fabrication of pure gold nanostructures on non-planar surfaces as well as of 3-dimensional gold nanostructures has been the bottleneck. Up to now, only 2-dimensional gold nanostructures on planar surfaces were achievable by resist based lithography.

The new technology, developed at TU Wien, can now solve this problem. The principle is the local decomposition of a metalorganic precursor by the focused electron beam of an electron microscope. With extremely high precision, the electron beam can decompose the organic compound at exactly the right position, leaving behind a 3D-trail of solid gold.

The final obstacle was getting the material purity right, as the electron-induced decomposition of metalorganic precursors has typically yielded metals with high carbon contaminations. This last bottleneck on the road to custom-designed, pure gold nanostructures has now been overcome as described in the work on “Highly conductive and pure gold nanostructures grown by electron beam induced deposition” published in Scientific Reports.

While conventional gold deposition usually contains about 70 atomic % carbon and only 30 atomic % gold, the new approach developed by a research group lead by Dr. Heinz Wanzenboeck at TU Wien has allowed to fabricate pure gold structures by in-situ addition of an oxidizing agent during the gold deposition. “The whole community has been working hard for the last 10 years to directly deposit pure gold nanostructures”, says Heinz Wanzenböck. At last, the group’s expertise in engineering and chemical reactions paid off and direct deposition of pure gold was successful. “It’s a bit like discovering the legendary philosopher’s stone that turns common, ignoble material into gold” joked Wanzenboeck.

This deposited pure gold structure exhibits extremely low resistivity near that of bulk gold. Generally, a FEBID gold structure has a resistivity around 1-Ohm-cm which is about 1 million times worse than the resistivity of purest bulk gold. However, this specially enhanced FEBID process produces a resistivity of 8.8 micro-Ohm-cm which is only a factor 4 away from the bulk resistivity of purest gold (2.4 micro-Ohm-cm).

The authors of the paper Dr. Mostafa Moonir Shawrav and Dipl.Ing. Philipp Taus stated, “This highly conductive and pure gold structure will open a new door for novel nanoelectronic devices. For example, it will be easier to produce pure gold structures for nanoantennas and biomolecule immobilization which will change our everyday life”. Dr. Shawrav added “it is remarkable how a regular SEM (Scanning Electron Microscope) nowadays can deposit nanostructures compared to 20 years back when it was only a characterization device”. And with pure gold direct deposition available now, he expects nanodevices to be deposited directly and utilized in many different applications for technological revolution. Concluding, this work is a giant leap forward for 3D nano-printing of gold structures which will be the core part of nanoplasmonics and bioelectronics devices.

Here’s  a link to and a citation for the paper,

Highly conductive and pure gold nanostructures grown by electron beam induced deposition by Mostafa M. Shawrav, Philipp Taus, Heinz D. Wanzenboeck, M. Schinnerl, M. Stöger-Pollach, S. Schwarz, A. Steiger-Thirsfeld, & Emmerich Bertagnolli. Scientific Reports 6, Article number: 34003 (2016)  doi:10.1038/srep34003 Published online: 26 September 2016

This paper is open access.

Creating quantum dots (artificial atoms) in graphene

An Aug. 22, 2016 news item on phys.org describes some recent work on artificial atoms and graphene from the Technical University of Vienna (Austria) and partners in Germany and the UK,

In a tiny quantum prison, electrons behave quite differently as compared to their counterparts in free space. They can only occupy discrete energy levels, much like the electrons in an atom – for this reason, such electron prisons are often called “artificial atoms”. Artificial atoms may also feature properties beyond those of conventional ones, with the potential for many applications for example in quantum computing. Such additional properties have now been shown for artificial atoms in the carbon material graphene. The results have been published in the journal Nano Letters, the project was a collaboration of scientists from TU Wien (Vienna, Austria), RWTH Aachen (Germany) and the University of Manchester (GB).

“Artificial atoms open up new, exciting possibilities, because we can directly tune their properties”, says Professor Joachim Burgdörfer (TU Wien, Vienna). In semiconductor materials such as gallium arsenide, trapping electrons in tiny confinements has already been shown to be possible. These structures are often referred to as “quantum dots”. Just like in an atom, where the electrons can only circle the nucleus on certain orbits, electrons in these quantum dots are forced into discrete quantum states.

Even more interesting possibilities are opened up by using graphene, a material consisting of a single layer of carbon atoms, which has attracted a lot of attention in the last few years. “In most materials, electrons may occupy two different quantum states at a given energy. The high symmetry of the graphene lattice allows for four different quantum states. This opens up new pathways for quantum information processing and storage” explains Florian Libisch from TU Wien. However, creating well-controlled artificial atoms in graphene turned out to be extremely challenging.

Florian Libisch, explaining the structure of graphene. Courtesy Technical University of Vienna

Florian Libisch, explaining the structure of graphene. Courtesy Technical University of Vienna

An Aug. 22, 2016 Technical University of Vienna press release (also on EurekAlert), which originated the news item, provides more detail,

There are different ways of creating artificial atoms: The simplest one is putting electrons into tiny flakes, cut out of a thin layer of the material. While this works for graphene, the symmetry of the material is broken by the edges of the flake which can never be perfectly smooth. Consequently, the special four-fold multiplicity of states in graphene is reduced to the conventional two-fold one.

Therefore, different ways had to be found: It is not necessary to use small graphene flakes to capture electrons. Using clever combinations of electrical and magnetic fields is a much better option. With the tip of a scanning tunnelling microscope, an electric field can be applied locally. That way, a tiny region is created within the graphene surface, in which low energy electrons can be trapped. At the same time, the electrons are forced into tiny circular orbits by applying a magnetic field. “If we would only use an electric field, quantum effects allow the electrons to quickly leave the trap” explains Libisch.

The artificial atoms were measured at the RWTH Aachen by Nils Freitag and Peter Nemes-Incze in the group of Professor Markus Morgenstern. Simulations and theoretical models were developed at TU Wien (Vienna) by Larisa Chizhova, Florian Libisch and Joachim Burgdörfer. The exceptionally clean graphene sample came from the team around Andre Geim and Kostya Novoselov from Manchester (GB) – these two researchers were awarded the Nobel Prize in 2010 for creating graphene sheets for the first time.

The new artificial atoms now open up new possibilities for many quantum technological experiments: “Four localized electron states with the same energy allow for switching between different quantum states to store information”, says Joachim Burgdörfer. The electrons can preserve arbitrary superpositions for a long time, ideal properties for quantum computers. In addition, the new method has the big advantage of scalability: it should be possible to fit many such artificial atoms on a small chip in order to use them for quantum information applications.

Here’s a link to and a citation for the paper,

Electrostatically Confined Monolayer Graphene Quantum Dots with Orbital and Valley Splittings by Nils M. Freitag, Larisa A. Chizhova, Peter Nemes-Incze, Colin R. Woods, Roman V. Gorbachev, Yang Cao, Andre K. Geim, Kostya S. Novoselov, Joachim Burgdörfer, Florian Libisch, and Markus Morgenstern. Nano Lett., Article ASAP DOI: 10.1021/acs.nanolett.6b02548 Publication Date (Web): July 28, 2016

Copyright © 2016 American Chemical Society

This paper is behind a paywall.

Dexter Johnson in an Aug. 23, 2016 post on his Nanoclast blog (on the IEEE [Institute of Electrical and Electronics Engineers] website) provides some additional insight into the world of quantum dots,

Quantum dots made from semiconductor materials, like silicon, are beginning to transform the display market. While it is their optoelectronic properties that are being leveraged in displays, the peculiar property of quantum dots that allows their electrons to be forced into discrete quantum states has long held out the promise of enabling quantum computing.

If you have time to read it, Dexter’s post features an email interview with Florian Libisch where they further discuss quantum dots and quantum computing.