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.
Schematic and spectra showing the differences before and after oxygen plasma treatment
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.
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.
Dr Rohan Dhall - University of Southern California
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Reference * ACS Photonics, 2016, 3 (3), pp 310-314
DOI: 10.1021/acsphotonics.5b00593