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Posted: Mar 12, 2015

Chemically enhanced 2D material makes excellent tunable nanoscale light source

(Nanowerk Spotlight) Molybdenum disulfide's (MoS2) semiconducting ability, strong light-matter interaction and similarity to the carbon-based graphene makes it of interest to scientists as a viable alternative to graphene in the manufacture of electronics, particularly photoelectronics (read more: "2D molybdenum disulfide: a promising new optical material for ultra-fast photonics"). In particular, MoS2 has excellent optical properties when deposited as a single, atom-thick layer – unlike graphene, it emits light when excited; albeit relatively poorly.
"While a suspended monolayer MoS2 flake shows photoluminescence efficiencies 100 times higher compared to a suspended bilayer flake, its overall quantum yield is still below 1%, and excitonic emission is spectrally broad at room temperature, Jason C Reed, a PhD student in Professor Ertugrul Cubukcu's Nanoengineered Photonics Group at the University of Pennsylvania, tells Nanowerk. "In contrast, when transferred onto substrates for device integration, quantum yield for MoS2 monolayers is even lower and only 3.5 times higher than that of a bilayer."
Therefore, in order to realize the potential of atomically thin MoS2 as a nanoscale active material in a light source, a considerable enhancement of its emission efficiency is necessary.
And that's exactly what the Penn researchers have done. In new work, published in the February 27, 2015 online edition of Nano Letters ("Wavelength Tunable Microdisk Cavity Light Source with a Chemically Enhanced MoS2 Emitter"), they showed both experimentally and theoretically that two-dimensional (2D) MoS2 can be chemically enhanced and used to make a tunable light source with high spatial and temporal coherence when integrated with a notched microdisk optical cavity.
notched microdisk light source with integrated MoS2 layer
(a) SEM image of a MoS2-coupled microdisk cavity showing a false-colored, tilted view and with low accelerating voltage. The shaded, lighter orange area denotes the area with the MoS2 flake coverage on the microdisk. Scale bar is 2 µm. (b) SEM image of the same microdisk with top-down view and with higher accelerating voltage, showing the free-standing portion of the microdisk supported by a silicon pillar. The white dotted area shows the location of the MoS2. Scale bar is 5 µm. Field intensity profiles at 664 nm (on resonance) for a (c) perfect 3.8 µm radius disk as well as a (d) notched disk. (e) FDTD simulations showing dipole excitation at 664 nm and the resulting notch emission from the cavity. (f) A plot from FDTD simulations showing the spectral dependence of the excitation wavelength on the power absorbed by the MoS2 flake coupled to the microdisk cavity. (Reprinted with permission by American Chemical Society)
"What we have done is found a way to chemically enhance the emission intensity of the MoS2 and incorporate it into a nanoscale structure," Reed, who is the paper's first author, explains. "By enhancing the MoS2, we made it much brighter. By incorporating it into our nanostructure, we made it more functional." Other authors of the paper are Alexander Y. Zhu, Hai Zhu, Fei Yi and Ertugrul Cubukcu all from the University of Pennsylvania.
He notes that the added functionality comes from the nanostructure: the light from the MoS2 can resonate with the nanostructure, lingering around for much longer than if it was on a flat surface.
The result of this is that when this light is extracted from the defect incorporated into the nanostructure, it can be tuned from light that is 'spectrally broad' to light that is very 'spectrally narrow'.
While it might seem counter-intuitive to modify a light source from having 'more colors' to 'fewer colors', this light is now higher quality and is also very useful for detecting very small changes in the wavelength of the emission.
"We can also tune the wavelength of light by adding as little as 100 femtograms of material – roughly equal to one seventh the mass of an E. coli bacterium – to our nanostructure," says Reed. "This, on the other hand, means that we can also detect equally small additions of material, opening up our work to the great potential for very sensitive detection applications."
The researchers point out that no other work has been done to modify the light emission from MoS2 to this degree. Not only were they able to chemically enhance the emission of MoS2, they also achieved a very high level of contrast between the emission peaks and the background.
In terms of pure science, this microdisk cavity light source allows researchers to study the optical properties of a wide range of different 2D materials.
Potential practical application of this work appear to be promising in the realm of biological sensors, e.g. for early detection of certain diseases that may show low concentrations of biomarkers in the early stages.
"Because we can detect very small changes on the surface of our nanostructure, we can also detect small biomolecules interacting with the surface as well," says Reed. "By attaching the correct proteins or antibodies to the surface, we can tailor specific biomolecule attachment at potentially very low detection limits."
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