IBM researchers demo molecular switch

ZURICH, Switzerland - Scientists at the IBM Zurich Research Laboratory have demonstrated how a single molecule can be switched between two distinct conductive states, which allows it to store data.

The experiments, first published in the journal SMALL, show that certain types of molecules reveal intrinsic molecular functionalities that are comparable to devices used in today’s semiconductor technology.

Researchers Heike Riel and Emanuel Lörtscher reported that, using a sophisticated mechanical method, they were able to establish electrical contact with an individual molecule to demonstrate reversible and controllable switching between two distinct conductive states.

The investigation is part of their work to explore and characterize molecules to become possible building blocks for future memory and logic applications. By applying voltage pulses to the molecule, it can be controllably switched between two distinct “on” and “off” states. The researchers found that both states were stable and could therefore enable non-volatile memory. They documented more than 500 switching cycles and switching times in the microsecond range.

The SEM image is of a metallic bridge. Atomic-sized tips (which serve as electrodes) are created by stretching and breaking the bridge. The switching molecule is then “caught” between the electrodes by closing the bridge gradually until a single molecule reaches between both electrodes. Image courtesy of IBM
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In order to individually address the molecules, Riel and Lörtscher extended a method called the mechanically controllable break-junction. With this technique, a metallic bridge on an insulating substrate is carefully stretched by mechanical bending. Ultimately the bridge breaks, creating two separate electrodes that possess atomic-sized tips.

Then a solution of the organic molecules is deposited on top of the electrodes. As the junction closes, a molecule capable of chemically bonding to both metallic electrodes can bridge the gap. In this way, an individual molecule is “caught” between the electrodes, and measurements can be performed.

The molecules investigated are specially designed organic molecules measuring about 1.5 nm. The molecules were designed and synthesized by Professor James Tour and co-workers at Rice University.

Nano-etched cavity brightens up LEDs

GAITHERSBURG, Md. - Researchers at the National Institute of Standards and Technology (NIST) have made semiconductor light-emitting diodes (LEDs) more than seven times brighter by etching nanoscale grooves in a surrounding cavity to guide scattered light.

Semiconductor LEDs typically emit only about two percent of their light in the desired direction: perpendicular to the diode surface. Far more light skims uselessly below the surface of the LED, because of the extreme mismatch in refraction between air and the semiconductor. The NIST nanostructured cavity boosts useful LED emission to about 41 percent and may be cheaper and more effective for some applications than conventional post-processing LED shaping and packaging methods that attempt to redirect light.

Etched nanostructured rings around an LED can make it more than seven times brighter. Image courtesy of NIST
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The NIST team fabricated their own infrared LEDs consisting of gallium arsenide packed with quantum dots of assorted sizes made of indium gallium arsenide. The LEDs were backed with an alumina mirror to reflect the light emitted backwards. The periphery of each LED was turned into a cavity etched with circular grooves, in which the light reflects and interferes with itself in an optimal geometry.

The researchers experimented with different numbers and dimensions of grooves. The brightest output was attained with 10 grooves, each about 240 nm wide and 150 nm deep, and spaced 40 nm apart. The team spent several years developing the design principles and perfecting the manufacturing technique. The principles of the method are transferable to other LED materials and emission wavelengths, as well as other processing techniques, such as commercial photolithography, according to lead author Mark Su. The research was published in Applied Physics Letters.

Researchers pencil in plans for new composite

EVANSTON, Ill. - Northwestern University researchers have developed a process that could enable new composite, or graphene-based, materials.

The method uses graphite to produce individual graphene-based sheets with useful physical, chemical and barrier properties that could be mixed into materials such as polymers, glasses and ceramics.

“This research provides a basis for developing a new class of composite materials for many applications, through tuning of their electrical and thermal conductivity, their mechanical stiffness, toughness and strength, and their permeability to flow various gases through them,” said Rod Ruoff, professor of mechanical engineering in the McCormick School of Engineering and Applied Science, in a prepared statement. “We believe that manipulating the chemical and physical properties of individual graphene-based sheets and effectively mixing them into other materials will lead to discoveries of new materials in the future.”

The team’s approach was based on chemically treating and thereby “exfoliating” graphite to individual layers which are expected to display the in-plane properties of graphite. The research was published in the journal Nature.

Scientists build magnetic semiconductors one atom at a time

PRINCETON, N.J. - a team of scientists from Princeton University, the University of Illinois at Urbana-Champaign and the University of Iowa has turned semiconductors into magnets by the precise placement of metal atoms within a material from which chips are made. The team used their method to make a semiconductor magnetic, one atom at a time.

By incorporating manganese atoms into the gallium arsenide semiconductor, the team has created an atomic-scale laboratory in which researchers can explore the precise interactions among atoms and electrons in chip material. The team used their new technique to find the optimal arrangements for manganese atoms that can enhance the magnetic properties of gallium arsenide.

Substitution of magnetic atoms (manganese) into a semiconductor (gallium arsenide) creates a material for future electronics. Spins of the magnetic atoms interact via a cloud of electrons, which can be visualized using a scanning tunneling microscope. The image is a composite of microscopic visualization of electron cloud together with a model of the gallium arsenide crystal structure. Image courtesy of A. Yazdani/Princeton University
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“Using the tip of a scanning tunneling microscope, we can take out a single atom from the base material and replace it with a single metal that gives the semiconductor its magnetic properties,” said Yazdani, a Princeton professor of physics, in a prepared statement.

The arrangement of manganese atoms that exhibits magnetic properties is an important factor in developing spin-based electronics. The research was published in the journal Nature.

Study shows catalytic activity of gold can be tuned

ATLANTA - Researchers at the Georgia Institute of Technology have made a discovery that could allow scientists to exercise more control over the catalytic activity of gold nanoclusters.

The researchers found that the dimensionality and structure, and thus the catalytic activity, of gold nanoclusters changes as the thickness of their supporting metal-oxide films is varied.

“We’ve been searching for methods for controlling and tuning the nanocatalytic activity of gold nanoclusters,” said Uzi Landman, director of the Center for Computational Materials Science and Regents’ professor and Callaway chair of physics at Georgia Tech, in a prepared statement. “I believe the effect we discovered, whereby the structure and dimensionality of supported gold nanoclusters can be influenced and varied by the thickness of the underlying magnesium-oxide film may open new avenues for controlled nanocatalytic activity.”

Structures of a gold cluster (depicted by yellow spheres) containing 20 atoms, adsorbed on a magnesium oxide bed (magnesium in green and oxygen in red) which is itself supported on top of a molybdenum substrate (blue spheres). The excess electronic charge at the interface is depicted in pink and the charge depletion is shown in light blue. Image courtesy of Uzi Landman/Georgia Tech
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Landman’s group found that by using a thin catalytic bed with a thickness of up to 1 nm, or 4 to 5 layers, of magnesium oxide, one may activate the gold nanoclusters which may act then as catalysts even if the bed is defect-free. In the study, the researchers simulated the behavior of gold nanoclusters containing eight, sixteen and twenty atoms when placed on catalytic beds of magnesium oxide with a molybdenum substrate supporting the magnesium oxide film. Quantum mechanical calculations showed that when the magnesium oxide film was greater than 5 layers or 1 nm in thickness, the gold cluster kept its three-dimensional structure. However, when the film was less than 1nm, the cluster changed its structure and lied flat on the magnesia bed - wetting and adhering to it.

The gold flattens because the electronic charge from the molybdenum penetrates through the thin layer of magnesium oxide and accumulates at the region where the gold cluster is anchored to the magnesium oxide. With a negative charge underneath the gold nanocluster, its attraction to the molybdenum substrate, located under the magnesia film, causes the cluster to collapse. The research appeared in the journal Physical Review Letters.