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Research Alert: August 27, 2014

Scientists craft a semiconductor only three atoms thick

Scientists have developed what they believe is the thinnest-possible semiconductor, a new class of nanoscale materials made in sheets only three atoms thick.

As seen under an optical microscope, the heterostructures have a triangular shape. The two different monolayer semiconductors can be recognized through their different colors.

The University of Washington researchers have demonstrated that two of these single-layer semiconductor materials can be connected in an atomically seamless fashion known as a heterojunction. This result could be the basis for next-generation flexible and transparent computing, better light-emitting diodes, or LEDs, and solar technologies.

“Heterojunctions are fundamental elements of electronic and photonic devices,” said senior author Xiaodong Xu, a UW assistant professor of materials science and engineering and of physics. “Our experimental demonstration of such junctions between two-dimensional materials should enable new kinds of transistors, LEDs, nanolasers, and solar cells to be developed for highly integrated electronic and optical circuits within a single atomic plane.”

The research was published online this week in Nature Materials.

The researchers discovered that two flat semiconductor materials can be connected edge-to-edge with crystalline perfection. They worked with two single-layer, or monolayer, materials – molybdenum diselenide and tungsten diselenide – that have very similar structures, which was key to creating the composite two-dimensional semiconductor.

Collaborators from the electron microscopy center at the University of Warwick in England found that all the atoms in both materials formed a single honeycomb lattice structure, without any distortions or discontinuities. This provides the strongest possible link between two single-layer materials, necessary for flexible devices. Within the same family of materials it is feasible that researchers could bond other pairs together in the same way.

The researchers created the junctions in a small furnace at the UW. First, they inserted a powder mixture of the two materials into a chamber heated to 900 degrees Celsius (1,652 F). Hydrogen gas was then passed through the chamber and the evaporated atoms from one of the materials were carried toward a cooler region of the tube and deposited as single-layer crystals in the shape of triangles.

After a while, evaporated atoms from the second material then attached to the edges of the triangle to create a seamless semiconducting heterojunction.

“This is a scalable technique,” said Sanfeng Wu, a UW doctoral student in physics and one of the lead authors. “Because the materials have different properties, they evaporate and separate at different times automatically. The second material forms around the first triangle that just previously formed. That’s why these lattices are so beautifully connected.”

With a larger furnace, it would be possible to mass-produce sheets of these semiconductor heterostructures, the researchers said. On a small scale, it takes about five minutes to grow the crystals, with up to two hours of heating and cooling time.

A breakthrough in imaging gold nanoparticles to atomic resolution by electron microscopy

Nanometer-scale gold particles are intensively investigated for application as catalysts, sensors, drug delivery devices, biological contrast agents and components in photonics and molecular electronics. Gaining knowledge of their atomic-scale structures, fundamental for understanding physical and chemical properties, has been challenging. Now, researchers at Stanford University, USA, have demonstrated that high-resolution electron microscopy can be used to reveal a three-dimensional structure in which all gold atoms are observed. The results are in close agreement with a structure predicted at the University of Jyväskylä, Finland, on the basis of theoretical modelling and infrared spectroscopy. The research was published in Science on 22 August 2014.

The revealed gold nanoparticle is 1.1 nm in diameter and contains 68 gold atoms organised in a crystalline fashion at the center of the particle. The result was supported by small-angle X-ray scattering done in Lawrence Berkeley National Laboratory, USA, and by mass spectrometry done at Hokkaido University, Japan.

Electron microscopy is similar in principle to conventional light microscopy, with the exception that the wavelength of the electron beam used for imaging is close to the spacing of atoms in solid matter, about a tenth of a nanometer, in contrast with the wavelength of visible light, which is hundreds of nanometres. A crucial aspect of the new work is the irradiation of the nanoparticle with very few electrons to avoid perturbing the structure of the nanoparticle. The success of this approach opens the way to the determination of many more nanoparticle structures and to both fundamental understanding and practical applications.

The researchers involved in the work are Maia Azubel, Ai Leen Koh, David Bushnell and Roger D. Kornberg from Stanford University, Sami Malola, Jaakko Koivisto, Mika Pettersson and Hannu Häkkinen from the University of Jyväskylä, Greg L. Hura from Lawrence Berkeley National Laboratory, and Tatsuya Tsukuda and Hironori Tsunoyama from Hokkaido University. The work at the University of Jyväskylä was supported by the Academy of Finland. The computational work in Hannu Häkkinen’s group was done at the HLRS-GAUSS centre in Stuttgart as part of the PRACE project “Nano-gold at the bio-interface”.

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