Nanotechnology Now - Press Release: XEI Scientific and the University of Southern California collaborate on the use of downstream plasma cleaning in sample preparation and publish a paper in ACS Photonics

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Schematic and spectra showing the differences before and after oxygen plasma treatment

Abstract:
XEI Scientific Inc. reports on the latest publication from their user group at the Ming Hsieh Department of Electrical Engineering at the University of Southern California, Los Angeles. The paper released in the ACS Photonics Journal. The authors describe their work with multilayer molybdenum disulfide and their use of XEI's Soft Clean plasma cleaning system for sample preparation.

XEI Scientific and the University of Southern California collaborate on the use of downstream plasma cleaning in sample preparation and publish a paper in ACS Photonics

Redwood City, CA | Posted on April 26th, 2016

The research team of Associate Professor Stephen Cronin is located in the Ming Hsieh Department of Electrical Engineering at USC Los Angeles. They focus mainly on optical spectroscopy and electron transport at the nanometer scale. Professor Cronin's research spans a broad range of topics including electrical and spectroscopic characterization of carbon nanotubes, graphene, and other novel low dimensional materials.

Following an earlier publication in Advanced Materials, the group's latest paper is entitled “Strong Circularly Polarized Photoluminescence from Multilayer MoS2 through Plasma Driven Direct-Gap Transition.” The lead author is Dr Rohan Dhall and he describes the work: “Our paper reports circularly polarized photoluminescence spectra taken from few-layer MoS2 after treatment with a remotely generated oxygen plasma. Here, the oxygen plasma decouples the individual layers in the MoS2 by perturbing the weak interlayer van der Waals forces without damaging the lattice structure. This decoupling causes a transition from an indirect to a direct band gap material which, in turn, causes a strong enhancement of the photoluminescence intensity. This work is important because, firstly, direct band gap materials are much better at absorbing and emitting light. So, converting the indirect gap bulk MoS2 to a direct gap is great for potential LED and solar cell applications. Further, this transition also changes the symmetry of crystal and allows us to make an efficient source of photons with a specified angular momentum (analogous to the spin of an electron). This is important for "multiplexing" of information. If information is being stored or transmitted by photons, the ability to know their angular momentum offers one more degree of freedom to encode information.”

The XEI Evactron® Soft Clean plasma cleaner is used for the process. Dr Dhall found it really convenient to use. “The plasma generation parameters were adjustable over a large span, and the system still worked reliably. In contrast, other machines have trouble igniting plasma at lower RF powers (which generates the plasma). This allowed us to finely tune the plasma concentration in our work. This is very important where our material of interest (MoS2) is a weakly bonded crystal held together by van der Waals forces (much weaker than covalent bonds). The low plasma concentration ensured that even these weak bonds are not completely broken, although they are somewhat perturbed. The perturbation itself was a good thing because it makes for better opto-electronic device properties.”

XEI has now sold more than 2000 Evactron systems worldwide solving contamination problems in many different environments using high vacuum including electron microscopes, FIBs and other vacuum sample chambers. Each system comes with a 5 year limited warranty. Please visit the XEI web site for the latest details: www.evactron.com. .

Reference * ACS Photonics, 2016, 3 (3), pp 310-314
DOI: 10.1021/acsphotonics.5b00593

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About XEI Scientific Inc.
XEI Scientific Inc. invented the Evactron De-Contaminator in 1999 as the first plasma cleaner to use a downstream cleaning process to remove carbon from electron microscopes. A proprietary plasma source uses air to produce oxygen radicals for oxidation of carbon compounds for removal by the pumps. Carbon-free-vacuum produces the highest quality images and analytical results from SEMs and other vacuum analytical instruments. XEI innovations also include a unique RF plasma generator, a patented RF electrode, and easy start programmed plasma cleaning. All XEI products come with a 5 year limited warranty and are compliant with CE, NRTL, and Semi-S2 safety standards. XEI offers a variety of Evactron® decontamination systems to meet user needs and >2000 installations around the world.

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