Aug 13 2019
To better understand advanced materials such as graphene nanostructures and enhance them for devices in opto-, nano-, and quantum-technology, it is important to comprehend how phonons—the vibration of atoms in solids—impact the materials’ properties.
Schematic representation of local lattice vibrations in graphene excited by a wavefront of transmitted fast electrons. (Image credit: © Ryosuke Senga, AIST)
Scientists from the University of Vienna, the Advanced Institute of Science and Technology (AIST) in Japan, the company JEOL, and La Sapienza University in Rome have formulated a technique that can measure all phonons present in a nanostructured material. This is an innovation in the examination of nanoscale functional materials and devices.
With this pilot research using graphene nanostructures, these scientists have demonstrated the uniqueness of their method, which will be described in a recent issue of Nature.
Crucial thermal, optoelectronic, mechanical, and transport characteristics of materials are regulated by phonons: the propagating atomic vibrational waves. It is then inferable that the determination of such lengthy atomic vibrations is vital for the optimization of nanoelectronic devices. The presently available methods use optical techniques as well as inelastic electron-, X-ray- and neutron scattering.
Regardless of its scientific significance in the last ten years, none of these approaches has been able to establish all phonons of a freestanding monolayer of two dimensional (2D) materials such as graphene and their local variants within a graphene nanoribbon, which are in turn employed as active elements in nano- and optoelectronics.
The new limits of nanospectroscopy
An international research team of leading specialists in electron spectroscopy guided by Thomas Pichler at the University of Vienna, theoretical spectroscopy led by Francesco Mauri at La Sapienza University in Rome and electron microscopy led by Kazu Suenaga at the AIST Tsukuba in Japan, along with the Japanese company JEOL have demonstrated an original technique applying it to graphene nanostructures as model: "high resolution electron spectroscopy inside an electron microscope with enough sensitivity to measure even an atomic monolayer".
In this manner, they could for the first time establish all vibrational modes of freestanding graphene as well as the local extension of various vibrational modes in a graphene nanoribbon. This new technique, which they termed as "large q mapping" opens completely new possibilities to establish the spatial and momentum extension of phonons in all nanostructured as well as two-dimensional advanced materials.
These experiments push the boundaries of nanospectroscopy approaching the boundaries of Heisenbergs uncertainty principle and expose new potentials to explore local vibration modes at the nanometer scale down to specific monolayers.
New electron nanospectrometer as "table top" synchrotron
The direct experimental proof of the full spatial and momentum resolved mapping of local vibrations of all materials including even monolayer 2D materials and nanoribbons will enable us to fully disentangle different vibration modes and their momentum transfers at non-perfect structures such as edges or defects, which are extremely important to understand and optimize the local properties of a material.
Ryosuke Senga, Study Author, AIST
This study of "High q-Mapping of Vibrations" in the electron microscope paves a new pathway of nanospectroscopy of all materials integrating spatial and momentum resolved measurements. This has been the greatest challenge concerning the combination of spectroscopy and microscopy, as the spatial and momentum resolutions are compensated because of the limit of Heisenbergs uncertainty principle.
We believe that our methodology will boost vast research in material science and will push high resolution electron spectroscopy in electron microscopy to the next level, to be envisaged as a true table top synchrotron.
Thomas Pichler, Study Lead, University of Vienna