Ordered arrays of nanoporous gold nanoparticles

SEM images (false color) at 25° tilt of the perfectly
ordered array of the nanoporous gold nanoparticles
formed from the 15 nm Au/30 nm Ag bilayers.© 2012 Wang et al; licensee Beilstein-Institut.

A combination of a “top-down” approach (substrate-conformal imprint lithography) and two “bottom-up” approaches (dewetting and dealloying) enables fabrication of perfectly ordered 2-dimensional arrays of nanoporous gold nanoparticles. 

The dewetting of Au/Ag bilayers on the periodically prepatterned substrates leads to the interdiffusion of Au and Ag and the formation of an array of Au–Ag alloy nanoparticles. The array of alloy nanoparticles is transformed into an array of nanoporous gold nanoparticles by a following dealloying step. 

Large areas of this new type of material arrangement can be realized with this technique. In addition, this technique allows for the control of particle size, particle spacing, and ligament size (or pore size) by varying the period of the structure, total metal layer thickness, and the thickness ratio of the as-deposited bilayers.

Metallic nanoparticle arrays are attracting more and more attention due to their potential applications in plasmonics, magnetic memories, DNA detection, and catalytic nanowire growth. Nanoporous gold is very interesting for application in catalysi, for sensors, for actuators, and as electrodes for electrochemical supercapacitors. This is due to the unique structural, mechanical and chemical properties of this material. Nanoporous gold, already synthesized in the form of nanoparticles, possesses a much higher surface-to-volume ratio than bulk nanoporous gold films and gold nanoparticles. These nanoporous gold nanoparticles are expected to broaden the range of applications for both gold nanoparticles and nanoporous gold due to their two-level nanostructures (porosity of around 10 nm and particle size of a few hundreds of nanometers).

Solid-state dewetting of metal films is a simple “bottom-up” approach to fabricate nanoparticles. The dewetting of metal films is driven by reducing the surface energy of the film and the interface energy between the film and the substrate, and occurs by diffusion even well below the melting temperature of the film. In addition, alloy nanoparticles can be fabricated by exploiting the dewetting of metallic bilayers. By combining both, “top-down” approaches (such as lithography) and “bottom-up” approaches, an ordered array of metallic nanoparticles can be fabricated. The surface of the substrate is prepatterned into periodic structures by using laser interference lithography, focused ion beam (FIB), or substrate conformal imprint lithography (SCIL). During the dewetting of metal films onto prepatterned substrates, the periodic structure of the prepatterned substrates modulates the local excess chemical potential by the local curvature or by limiting the diffusion paths. This leads to the formation of 2-D nanoparticle arrays with well-defined particle size and particle spacing. Dealloying is a “bottom-up” approach to fabricate nanoporous gold by selectively removing or leaching the element Ag from the Au–Ag alloy in an Ag-corrosive environment. In this paper, perfectly ordered arrays of nanoporous gold nanoparticles are fabricated by using a combination of a “top-down” approach (SCIL) and two “bottom-up” approaches (dewetting and dealloying).

Full paper: http://www.beilstein-journals.org/bjnano/single/articleFullText.htm?publicId=2190-4286-3-74

Scientists scale terahertz peaks in nanotubes

Rice U. researchers find plasmonic root of terahertz signals in some carbon nanotubes 

Carbon nanotubes carry plasmonic signals in the terahertz range of the electromagnetic spectrum, but only if they’re metallic by nature or doped.

In new research, the Rice University laboratory of physicist Junichiro Kono disproved previous theories that dominant terahertz response comes from narrow-gap semiconducting nanotubes.

Knowing that metallic or doped nanotubes respond with plasmonic waves at terahertz frequencies opens up the possibility that the tubes can be used in a wide array of optoelectronic amplifiers, detectors, polarizers and antennas.

The work by Kono and his Rice colleagues appeared online recently in the American Chemical Society journal Nano Letters.

Scientists have long been aware of a terahertz peak in nanotubes, the tiny cylinders of rolled-up carbon that show so much promise for advanced materials. But experiments on batches of nanotubes, which generally grow in a willy-nilly array of types, failed to reveal why it was there.

The origin of the peak was not explainable because researchers were only able to experiment on mixed batches of nanotube types, said Qi Zhang, a graduate student in Kono’s group and lead author of the paper. “All the previous work was done with a mixture of semiconducting and metallic tubes. We are the first to clearly identify the plasmonic nature of this terahertz response,” he said.

Rice’s growing expertise in separating nanotubes by type allowed Kono and his group to test for terahertz peaks in batches of pure metallic nanotubes known as “armchairs” as well as nonmetallic, semiconducting tubes.

“Metallic carbon nanotubes are expected to show plasmon resonance in the terahertz and infrared range, but no group has clearly demonstrated the existence of plasmons in carbon nanotubes,” Zhang said. “Previously, people proposed one possible explanation — that the terahertz peak is due to interband absorption in the small band gaps in semiconducting nanotubes. We rejected that in this paper.”

Plasmons are free electrons on the surface of metals like gold, silver or even aluminum nanoparticles that, when triggered by a laser or other outside energy, ripple like waves in a pond. Strong waves can trigger plasmon responses in adjacent nanoparticles. They are being investigated at Rice and elsewhere for use in sophisticated electronic and medical applications.

The Kono group’s research showed plasmons rippling at terahertz frequencies only along the length of a nanotube, but not across its width. “The only way charge carriers can move around is in the long direction,” Kono said. The researchers previously used this fact to demonstrate that aligned carbon nanotubes act as an excellent terahertz polarizer with performance better than commercial polarizers based on metallic grids.

Nanotubes can be thousands of times longer than they are wide, and the ability to grow them (or cut them) to specific lengths or to dope semiconducting nanotubes to add free carriers would make the tubes highly tunable for terahertz frequencies, Kono said.

“This paper only clarifies the origin of this effect,” he said. “Now that we understand it, there’s so much to do. We will be making various terahertz devices, architectures and systems based on carbon nanotube plasmons.”

Rice alumni Erik Hároz, now a postdoctoral researcher at Los Alamos National Laboratory, and Lei Ren, a researcher at TGS, co-authored the paper with undergraduate student Zehua Jin, postdoctoral researcher Xuan Wang, senior research scientist Rolf Arvidson and Andreas Lüttge, a research professor of Earth science and chemistry, all of Rice. Kono is a professor of electrical and computer engineering and of physics and astronomy and of materials science and nanoengineering.

The Department of Energy, the National Science Foundation and the Robert A. Welch Foundation supported the research.

Source: http://news.rice.edu/2013/12/09/scientists-scale-terahertz-peaks-in-nanotubes/

Engineers make golden breakthrough to improve electronic devices based on molybdenum disulfide

A Kansas State University chemical engineer has discovered that a new member of the ultrathin materials family has great potential to improve electronic and thermal devices.

Vikas Berry, William H. Honstead professor of chemical engineering, and his research team have studied a new three-atom-thick material -- molybdenum disulfide -- and found that manipulating it with gold atoms improves its electrical characteristics. Their research appears in a recent issue of Nano Letters.

The research may advance transistors, photodetectors, sensors and thermally conductive coatings, Berry said. It could also produce ultrafast, ultrathin logic and plasmonics devices.

Berry's laboratory has been leading studies on synthesis and properties of several next-generation atomically thick nanomaterials, such as graphene and boron-nitride layers, which have been applied for sensitive detection, high-rectifying electronics, mechanically strong composites and novel bionanotechnology applications.

"Futuristically, these atomically thick structures have the potential to revolutionize electronics by evolving into devices that will be only a few atoms thick," Berry said.

For the latest research, Berry and his team focused on transistors based on molybdenum disulfide, or MoS2, which was isolated only two years ago. The material is made of three-atom-thick sheets and has recently shown to have transistor-rectification that is better than graphene, which is a single-atom-thick sheet of carbon atoms.

When Berry's team studied molybdenum disulfide's structure, they realized that the sulfur group on its surface had a strong chemistry with noble metals, including gold. By establishing a bond between molybdenum disulfide and gold nanostructures, they found that the bond acted as a highly coupled gate capacitor.

Berry's team enhanced several transistor characteristics of molybdenum disulfide by manipulating it with gold nanomaterials.

"The spontaneous, highly capacitive, lattice-driven and thermally-controlled interfacing of noble metals on metal-dichalcogenide layers can be employed to regulate their carrier concentration, pseudo-mobility, transport-barriers and phonon-transport for future devices," Berry said.

The work may greatly improve future electronics, which will be ultrathin, Berry said. The researchers have developed a way to reduce the power that is required to operate these ultrathin devices.

"The research will pave the way for atomically fusing layered heterostructures to leverage their capacitive interactions for next-generation electronics and photonics," Berry said. "For example, the gold nanoparticles can help launch 2-D plasmons on ultrathin materials, enabling their interference for plasmonic-logic devices."

The research also supports the current work on molybdenum disulfide-graphene-based electron-tunneling transistors by providing a route for direct electrode attachment on a molybdenum disulfide tunneling gate.

"The intimate, highly capacitive interaction of gold on molybdenum disulfide can induce enhanced pseudo-mobility and act as electrodes for heterostructure devices," said T.S. Sreeprasad, a postdoctoral researcher in Berry's group.

The researchers plan to create further complex nanoscale architectures on molybdenum disulfide to build logic devices and sensors.

"The incorporation of gold into molybdenum disulfide provides an avenue for transistors, biochemical sensors, plasmonic devices and catalytic substrate," said Phong Nguyen, a doctoral student in chemical engineering, Wichita, Kan., who is part of Berry's research team.

Namhoon Kim, master's student in grain science and industry, Korea,worked on the research as an undergraduate in chemical engineering.

http://www.k-state.edu/media/newsreleases/sept13/berry9513.html