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Summary

Specialized imaging methods with high spatial resolution are essential for advancing the physical understanding of emergent materials in quantum-based devices and sensors. Photoemission electron microscope (PEEM) is a full-field imaging technique which can resolve surfaces in real space and momentum space providing rich information of its electronic structure. We will further advance our laboratory-based PEEM with novel photon sources to expand on new measurement modalities such as imaging electron dynamics and electron response to light polarization.

Description

Simplified schematic of the electron optics and instrument geometry of the PEEM at NIST. Adapted from R. Tromp et al, J. Phys. Condens. Matter 21, 314007

Simplified schematic of the electron optics and instrument geometry of the PEEM at NIST. Adapted from R. Tromp et al, J. Phys. Condens. Matter 21, 314007

With the rise of emergent material systems, nanoscale devices and components, there is a need to assess their electronic properties at similar length scales. Bulk-sensitive measurements provide characteristic information averaged over the sample or device, and these properties may not be uniform over macroscopic or mesoscopic length scales. There is a need to close the measurement gap between spatially-integrated photoemission techniques and atomically-resolved scanning probe-based techniques to tease out the interplay between microstructure and electronic structure, and how ensemble or collective properties impact quantum materials.

PEEM images are generated by photoelectrons that are ejected from a sample by photons. PEEM is an electron microscope with many contrasting mechanisms due to topography or electronic variation of a sample. Currently, real space imaging can reach a resolution of ~50 nm and can be coupled with energy filtering capability. The PEEM is also capable of μ-angle resolved photoemission (μ-ARPES) for samples with uniformity of ~10 μm with an energy resolution of < 300 meV. Other capabilities include in-situ heating (up to 1500 K) and cooling (~120 K).

 

Typical PEEM image of epitaxial graphene grown on silicon carbide at NIST where the contrast is due to regions of 1 to few layer graphene, graphite, and silicon carbide.
Typical PEEM image of epitaxial graphene grown on silicon carbide at NIST where the contrast is due to regions of 1 to few layer graphene, graphite, and silicon carbide.

Laboratory-based PEEM measurement capability will be advanced by integrating it with laser-based photon sources within the next (2) years. We will gain more measurement modalities -- expanded accessible “information depth,” polarization control and time resolved capability to alter systems from equilibrium. By extending the current PEEM instrument capability to beyond continuous wave plasma-based photon sources (arc, ~5 eV and helium, 21.2 eV and 40.8 eV) – it provides additional capability to tune the “information depth” by altering the kinetic energy (hv = Ebinding + Ekinetic) of the ejected photoelectrons. This can be an analytical parameter to discern between electronic properties or processes are at the surface or below the surface (i.e., in the bulk or buried interface). The other capabilities gained allows for dynamic or non-linear processes to be investigated which are becoming critical for understanding the microscopic physical processes underpinning the progress of quantum devices. Novel photon sourced coupled to a PEEM allow for us to advance our understanding in the physical processes behind emerging nanotechnologies with high spatial resolution, electronic contrast, and dynamics.

Major Accomplishments

2019

  • Arrival of NIST PEEM instrument and commissioning of instrument

2017

  • Collaboration with Center of Integrated Nanotechnologies (Sandia NL) for initial PEEM experiments (2017-2018)

 

μ-ARPES measurement of graphene on SiC using the PEEM. The π and σ bands are discerned with HeI and HeII excitation, yellow circles identify the Dirac point.
μ-ARPES measurement of graphene on SiC using the PEEM. The π and σ bands are discerned with HeI and HeII excitation, yellow circles identify the Dirac point.
Created October 21, 2019, Updated November 20, 2019