Spectroscopy Method Could Lead to Better Optical Devices

A new spectroscopy method that takes advantage of a fundamental property of thin films — interference — could help make better use of these materials in optical devices like LEDs and solar cells.

The technique, called energy-momentum spectroscopy, was developed by a multi-university research team to gain insight into how light is emitted from layered nanomaterials and other thin films. The method allows investigators to look at the light emerging from a thin film and determine whether it is coming from emitters oriented along the plane of the film or from emitters oriented perpendicular to the film.

Devised by scientists at Brown University in collaboration with colleagues from Case Western Reserve and Columbia universities, and the University of California, Santa Barbara, the method takes advantage of a fundamental property of thin films: interference. Interference effects can be seen in the rainbow colors visible on the surface of soap bubbles or oil slicks. Scientists can analyze how light constructively and destructively interferes at different angles to draw conclusions about the film itself — how thick it is, for example. This new technique takes that kind of analysis one step further for light-emitting thin films.

Understanding the orientations of light emitters in layered nanomaterials and other thin films could lead to better optical devices. A new spectroscopy method developed by a multi-university research team allows scientists to distinguish these orientations. The angular distribution of light emission from monolayer MoS₂, left, closely matches the theoretical calculations for in-plane oriented emitters, right, indicating that light emission from the graphene-like material MoS₂ originates from in-plane oriented emitters. Courtesy of Zia lab/Brown University.

"The key difference in our technique is we're looking at the energy as well as the angle and polarization at which light is emitted," said Rashid Zia, assistant professor of engineering at Brown University and one of the study's lead authors. "We can relate these different angles to distinct orientations of emitters in the film. At some angles and polarizations, we see only the light emission from in-plane emitters, while at other angles and polarizations, we see only light originating from out-of-plane emitters."

The technique was demonstrated on two important thin-film materials: molybdenum disulfide (MoS₂), a 2-D material similar to graphene, and PTCDA, an organic semiconductor. Each represents a class of materials that shows promise for optical applications. The research showed that light emission from MoS₂ occurs only from in-plane emitters. In PTCDA, light comes from two distinct species of emitters, one in-plane and one out-of-plane.

Once the orientation of the emitters is known, Zia said, it may be possible to design structured devices that maximize those directional properties. In most applications, thin-film materials are layered on top of each other. The orientations of emitters in each layer indicate whether electronic excitations are happening within each layer or across layers, and that has implications for how such a device should be configured.

Rashid Zia, a Brown University assistant professor of engineering. Courtesy of Brown University.

"If you were making an LED using these layered materials and you knew that the electronic excitations were happening across an interface," Zia said, "then there's a specific way you want to design the structure to get all of that light out and increase its overall efficiency."

The same concept could apply to light-absorbing devices like solar cells. By understanding how the electronic excitations happen in the material, it could be possible to structure it in a way that converts more incoming light to electricity.

The research — funded by the Air Force Office of Scientific Research, the Department of Energy, the National Science Foundation and the Nanoelectronic Research Initiative of the Semiconductor Research Corp. — was published in Nature Nanotechnology (doi: 10.1038/nnano.2013.20).

For more information, visit: www.brown.edu