Energy-momentum spectroscopy could improve optical devices
8 Mar 2013
A novel spectroscopic method could provide key insights into how light is emitted from layered nanomaterials.
A multi-university team has used a novel spectroscopic method to gain a better understanding of how light is emitted from layered nanomaterials and other thin films.
The technique, called energy-momentum spectroscopy, enables researchers 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.
Knowing the orientations of emitters could help engineers make better use of thin-film materials in optical devices like LEDs or solar cells.
The technique takes advantage of a fundamental property of thin films: interference. Scientists can analyse how light constructively and destructively interferes at different angles to draw conclusions about the film itself – how thick it is, for example.
“The key difference in our technique is we’re looking at the energy as well as the angle and polarisation at which light is emitted”
“The key difference in our technique is we’re looking at the energy as well as the angle and polarisation at which light is emitted,” said Rashid Zia, assistant professor of engineering at Brown University.
“We can relate these different angles to distinct orientations of emitters in the film. At some angles and polarisations, we see only the light emission from in-plane emitters, while at other angles and polarisations we see only light originating from out-of-plane emitters.”
The researchers demonstrated their technique on two thin-film materials, molybdenum disulfide (MoS2) and PTCDA.
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 says, 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.
“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 coverts more incoming light to electricity.
“One of the exciting things about this research is how it brought together people with different expertise,” Zia said.
The research was a collaborative effort by Brown University, Case Western Reserve University, Columbia University, and the University of California-Santa Barbara.
