Gold nanomembranes resist bending in new experiment

The first direct measurement of resistance to bending in a nanoscale membrane has been made by scientists from the University of Chicago, Peking University, the Weizmann Institute of Science and the Department of Energy's (DOE) Argonne National Laboratory.

Their research provides researchers with a new, simpler method to measure nanomaterials' resistance to bending and stretching, and opens new possibilities for creating nano-sized objects and machines by controlling and tailoring that resistance. (A nanometer is one-billionth of a meter, about as long as your fingernails grow in one second.)

The research team worked with a gold nanomembrane. "It's like a sheet of paper, only ten thousand times thinner," said Heinrich Jaeger of the University of Chicago. "If you slide a piece of paper over the edge of a table, it bends down. The gold nanomembrane behaves the same way, but it's a hundred times stiffer than the paper if scaled to the same thickness — a hundred times more resistant to bending.

"Researchers around the world are seeking ways to manipulate ultrathin nanomaterials into stable three-dimensional objects," Jaeger said. "The challenge is how to make a two-dimensional film into a three-dimensional shape when the film is so thin and flexible. It's like nano-origami: how do you get it to hold a stable shape? You need something stiffer than you would expect. It turns out that many nanomembranes may already possess that property."

"We were surprised to find that the gold nanomembrane was over a hundred times more resistant to bending than we predicted, based on standard elasticity theory and our experience with thin sheets, such as paper," said Xiao-Min Lin, who fabricated the gold nanoparticles in specialized facilities at the Center for Nanoscale Materials, a DOE Office of Science User Facility located at Argonne. "We believe it's related to membrane's internal structure. The membrane is only one nanoparticle thick, so it's essentially all surface with very little interior volume.  Minor structural disorder along its surface would significantly increase its resistant to bending. We also think molecular packing between nanoparticles might strongly affect its ability to bend." 

Critical to the team's discovery were a new method for creating gold membranes that roll themselves into nano-sized scrolls and a new technique for measuring the scroll's resistance to bending. Both were developed by Yifan Wang of the University of Chicago using CNM's facilities.

The gold nanoscrolls were self-assembled by suspending a fluid containing gold nanoparticles on a carbon screen. As the fluid dried, it left a gold membrane suspended like a nano-drumhead across the screen's circular holes. As the membranes continued to dry and tighten, one edge pulled loose from the screen, and the membrane spontaneously rolled up to form a hollow tube.

"There are many ways to make nanoparticle tubes," said Wang, "but they involve things like exposing membranes to electron beams, which can alter physical properties, such as their resistance to bending and stretching – the very things we wanted to measure. We needed a non-invasive way to make nanoparticle tubes without changing those properties."

The team found that a nanomembrane's resistance to both bending and stretching can be calculated from a single experiment that uses atomic force microscopy to measure bending resistance along the length of a monolayer membrane that has been rolled into a hollow cylinder.

(Atomic force microscopy uses a physical probe to measure surface details as small as a fraction of a nanometer.) Previous methods required two separate experiments on nanoscale membranes — one to measure stretching resistance and another to measure bending resistance.

"The tube's response to small local indentations is a signature of contributions to both bending and stretching," said Wang. "As a result, a single set of measurements of the resistance to indentation along the length of the tube provides direct access to its bending modulus and stretching modulus — key parameters needed to calculate resistance to both bending and stretching."

Since the measurement is based only on elasticity theory and the tube's geometry, Wang explained, it should have general applicability across a wide range of materials and size scales, from nano- and microtubules to truly macroscopic objects. 

"Ultrathin sheets just one nanoparticle thick have unique mechanical properties," Wang said. "This experiment provides new input for independent control of resistance to bending and stretching at the nanoscale. It should be possible to tailor bending and stretching parameters and to develop new nanomaterials and nano-objects with specific desirable properties."

The paper, "Strong Resistance to Bending Observed for Nanoparticle Membranes," was published in the online journal NanoLetters.

Other members of the research team were Jianhui Liao of Peking University in China, who visited Jaeger's lab at the University of Chicago for an extended research stay and performed the first experiments and measurements on the tubes; postdoc Sean McBride of the University of Chicago; and theory postdoc Efi Efrati of the University of Chicago, who helped with the modeling and who is now on the faculty at the Weizmann Institute of Science, Rehovot, Israel.

This research was supported by the National Science Foundation and the University of Chicago's Materials Research Science and Engineering Center. The Center for Nanoscale Materials is a DOE Office of Science User Facility.  

Argonne National Laboratory seeks solutions to pressing national problems in science and technology. The nation's first national laboratory, Argonne conducts leading-edge basic and applied scientific research in virtually every scientific discipline. Argonne researchers work closely with researchers from hundreds of companies, universities, and federal, state and municipal agencies to help them solve their specific problems, advance America's scientific leadership and prepare the nation for a better future. With employees from more than 60 nations, Argonne is managed by UChicago Argonne, LLC for the U.S. Department of Energy's Office of Science.

Argonne National Laboratory

Thinnest feasible membrane produced

Artist’s rendering of the two-layered graphene membrane (grey honeycomb
structure) with molecules (blue) being able – as a function of their
size – to pass the pores. (Illustration: Ben Newton / ETH Zurich)

A new nano-membrane made out of the “super material” graphene is extremely light and breathable. Not only can this open the door to a new generation of functional waterproof clothing, but also to ultra-rapid filtration. 

The membrane produced by the researchers at ETH Zurich is as thin as is technologically possible.

Researchers have produced a stable porous membrane that is thinner than a nanometre. This is a 100,000 times thinner than the diameter of a human hair. The membrane consists of two layers of the much exalted ”super material” graphene, a two-dimensional film made of carbon atoms, on which the team of researchers, led by Professor Hyung Gyu Park at the Department of Mechanical and Process Engineering at ETH Zurich, etched tiny pores of a precisely defined size.

The membrane can thus permeate tiny molecules. Larger molecules or particles, on the other hand, can pass only slowly or not at all. “With a thickness of just two carbon atoms, this is the thinnest porous membrane that is technologically possible to make,” says PhD student Jakob Buchheim, one of the two lead authors of the study, which was conducted by ETH-Zurich researchers in collaboration with scientists from Empa and a research laboratory of LG Electronics. The study has just been published in journal Science.

The ultra-thin graphene membrane may one day be used for a range of different purposes, including waterproof clothing. “Our membrane is not only very light and flexible, but it is also a thousand fold more breathable than Goretex,” says Kemal Celebi, a postdoc in Park’s laboratory and also one of the lead authors of the study. The membrane could also potentially be used to separate gaseous mixtures into their constituent parts or to filter impurities from fluids. The researchers were able to demonstrate for the first time that graphene membranes could be suitable for water filtration. The researchers also see a potential use for the membrane in devices used for the accurate measurement of gas and fluid flow rates that are crucial to unveiling the physics around mass transfer at nanoscales and separation of chemical mixtures.

Breakthrough in nanofabrication

Part of a graphene membrane with a multiplicity of pores
(black) of precisely defined size (in this case with a diameter of
50 nanometres; photomicrograph).
(Photo: Celebi K. et al. Science 2014)

The researchers not only succeeded in producing the starting material, a double-layer graphene film with a high level of purity, but they also mastered a technique called focused ion beam milling to etch pores into the graphene film. In this process, which is also used in the production of semiconductors, a beam of helium or gallium ions is controlled with a high level of precision in order to etch away material. The researchers were able to etch pores of a specified number and size into the graphene with unprecedented precision. This process, which could easily take days to complete, took only a few hours in the current work. “This is a breakthrough that enables the nanofabrication of the porous graphene membranes,” explains Ivan Shorubalko, a scientist at Empa that also contributed to the study.

In order to achieve this level of precision, the researchers had to work with double-layer graphene. “It wouldn’t have been possible for this method to create such a membrane with only one layer because graphene in practice isn't perfect,” says Park. The material can exhibit certain irregularities in the honeycomb structure of the carbon atoms. Now and again, individual atoms are missing from the structure, which not only impairs the stability of the material but also makes it impossible to etch a high-precision pore onto such a defect. The researchers solved this problem by laying two graphene layers on top of each other. The probability of two defects settling directly above one another is extremely low, explains Park.
Fastest possible filtration

A key advantage of the tiny dimensions is that the thinner a membrane, the lower its permeation resistance. The lower the resistance, the higher the energy-efficiency of the filtration process. “With such atomically thin membranes we can reach maximal permeation for a membrane of a given pore size and we believe that they allow the fastest feasible rate of permeation,” says Celebi. However, before these applications are ready for use on an industrial scale or for the production of functional waterproof clothing, the manufacturing process needs to be further developed. To investigate the fundamental science, the researchers worked with tiny pieces of membrane with a surface area of less than one hundredth of a square millimetre. Objectives from now on will be to produce larger membrane surfaces and impose various filtering mechanisms.

https://www.ethz.ch/en/news-and-events/eth-news/news/2014/04/thinnest-feasible-membrane-produced.html

Process devised for ultrathin carbon membranes

Physicists from Bielefeld University have developed a new method of fabrication

In the future, carbon nanomembranes are expected to be able to filter out very fine materials. These separating layers are ultrathin, consisting of just one layer of molecules. In the long term, they could allow to separate gases from one another, for example, filtering toxins from the air. At present, the basic research is concerned with the production of nanomembranes. A research team working with Professor Dr. Armin Gölzhäuser of Bielefeld University has succeeded in developing a new path to produce such membranes. The advantage of this procedure is that it allows a variety of different carbon nanomembranes to be generated which are much thinner than conventional membranes. The upcoming issue of the renowned research journal ‘ACS Nano’ reports on this research success.  

More than ten years ago, Professor Gölzhäuser and his then team created the groundwork for the current development, producing a carbon nanomembrane from biphenyl molecules. In the new study, the process was altered so as to allow the use of other starting materials. The prerequisite is that these molecules are also equipped with several so-called phenyl rings. For their new method, the researchers use the starting material in powder form. They dissolve the powder to pure alcohol and immerse very thin metal layer in this solution. After a short time the dissolved molecules settle themselves on the metal layer to form a monolayer of molecules. After being exposed to electron irradiation, the monolayer becomes a cross-linked nanomembrane. Subsequently the researchers ensure that the metal layer disintegrates, leav-ing only the nanomembrane remaining. ‘Up until now, we have produced small samples which are are a few centimetres square’, says Gölzhäuser. ‘However, with this process it is possible to make nanomembranes that are as big as square metres.’  

This new method is so special because the researchers can produce made-to-measure nanomembranes. ‘Every starting material has a different property, from thickness or trans-parency to elasticity. By using our process, these characteristics are transferred onto the nanomembrane.’ In this way, carbon nanomembranes can be produced to address many dif-ferent needs. ‘That was not possible before now’, says Gölzhäuser. 

Furthermore, graphene can be made from nanomembranes. Researchers worldwide are expecting graphene to have technically revolutionising properties, as it has an extremely high tensile strength and can conduct electricity and heat very well.  The conversion from nanomembranes into graphene is simple for the Bielefeld researchers: The membranes have to be heated in a vacuum at a temperature of about 700 degrees Celsius.   Gölzhäuser’s team is working on the project with physicists from Ulm University, Frankfurt University and the Max Planck Institute for Polymer Research. The study has been funded by the Federal Ministry of Education and Research (BMBF) and the German Research Foundation (DFG).

http://ekvv.uni-bielefeld.de/blog/uninews/entry/process_devised_for_ultrathin_carbon