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We call our research group the Micro-Femto Energetics Lab. This means we investigate how (and if) electrons in new nanoscale devices and materials can be harvested for energy generation, light emission, or electronic applications. We do this by effectively filming what happens to electrons after they get excited by light. By 'filming' how electrons move and relax in nanomaterials, we find out how to avoid bottlenecks that would ultimately limit the efficiency of a solar cell produced. Since electrons are both very small and move very fast, in order to film electrons new solar cell materials, we require sub-micrometer (<10-6 m) spatial resolution and femtosecond (10-15 s) time resolution. Hence the our lab's name, "the Micro-Femto Energetics Lab" or simply the μ f E Lab.

How do the physical properties of matter change as materials get small? The μ f E lab is able ‘to film’ the excited electron’s energetic journey from light absorption to photocurrent generation with sub-micron (<10-6 m) spatial resolution and femtosecond (<10-15 s) time-resolution.  By 'filming' how electrons move and relax at nanoscale interfaces, we identify the bottlenecks that inherently limit the efficiency of solar voltaics and photosensors.


We also investigate energy/current generation and transport at novel material interfaces. With micro-femto precision, we can resolve the electron's journey from initial excitation until current generation. The μ f E Lab will apply these unique measurement tools to a host of emerging materials including 1D/2D nanostructures,organic photovoltaics and biological systems. Our goal is to understand and control the processes drive light, energy and current extraction at nanoscale interfaces. Through this we can intelligently engineer next-generation optoelectronics and solar voltaics that fully exploit the unique physical properties of nanomaterials.


Strongly bound excitons dominate electronic relaxation in resonantly excited twisted bilayer graphene

Graham lab resolves electron dynamics in single grains of graphene . . . but there is a twist! How do the physical properties of bulk materials change as they get small? To better answer such questions Professor Matt Graham’s Micro-Femto Energetics lab at Oregon State is able ‘to film’ the photoexcited electronic journey with both micron (<10-6 m) spatial resolution and femtosecond (10-15 s) time-resolution.  By 'filming' the journey of photoexcited electrons from light absorption to electron extraction, we can identify electronic recombination bottlenecks that control the photoconversion efficiency of next-generation solar voltaic and fast photosensor materials.
            Recently Graham lab graduate student Hiral Patel and collaborators have applied such “micro-femto” approaches to examine the interlayer electronic interactions in bilayer 2D materials such as graphene.   In her paper published in Nano Letters, Hiral Patel looks at photoexcitations in bilayer graphene when it is stacked at a twisted angle.  Previously it was known that the interlayer 2p orbitals will rehybridize to give absorption resonances that are tunable with the bilayer stacking angle.   However, Hiral proved that these optical resonances are actually strongly bound exciton states.  Even more surprising, she shows how the remarkable symmetry of this material results in decoupling of lowest lying excitons from the graphene electron continuum through a physical phenomenon known as a “ghost-Fano” resonance effect. The is the first known 2D (or  3D)  metallic material that can also form stable, strongly bound excitons. Hiral presented this work orally at APS and Graphene Week in Manchester, UK.  She was also awarded the OSA Optical Materials Group Best Poster Prize at the CLEO Conference in San Jose. Hiral has further discoveries for this remarkable material under peer review.  Stay tuned!

Exciton-exciton annihilation and relaxation pathways in semiconducting carbon nanotubes

 

Some older works:

Absorptive movies of suspended graphene captuere a new kinetic relaxation route


Our ultrafast ‘movies’ give a time-space resolved microscopy map of the electronic population in materials such as graphene, carbon nanotubes and single atomic layer dichalcogenides devices.


Photocurrent measurements of supercollision cooling in graphene.

a. Materials Views
b. Pro-Physik
c. Cornell Chronicle
d. IOP: NanotechWeb

Two-dimensional electronic spectroscopy reveals the dynamics of phonon mediated excitation pathways in semiconducting single-walled carbon nanotubes