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Uncovering new thermal and mechanical behavior at the nanoscale using coherent extreme ultraviolet light

TitleUncovering new thermal and mechanical behavior at the nanoscale using coherent extreme ultraviolet light
Publication TypeThesis
Year of Publication2015
AuthorsHoogeboom-Pot, KM
Academic DepartmentPhysics
Number of Pages188
Date Published05-06-2015
UniversityUniversity of Colorado

Tremendous recent progress in nanofabrication capabilities has made high-quality single atomic layers and nanostructures with dimensions well below 50 nm common place, enabling unprecedented access to materials at the nanoscale. However, tools and techniques capable of characterizing the properties and function of nanosystems are still quite limited, leaving much of the fundamental physics that dominates material behavior in the deep nano-regime still unknown. Further understanding gained by studying nanoscale materials is critical both to fundamental science and to continued technological development. This thesis applies coherent extreme ultraviolet (EUV) light from tabletop high harmonic generation to study nanoscale systems on their intrinsic length and time scales (nanometers and femtoseconds, and above), specifically following thermal transport and acoustic dynamics. These studies have shown where and how nanostructured material properties can be quite different from their bulk counterparts. This has in turn allowed us to develop new theoretical descriptions to guide further work.

By observing heat dissipation from the smallest nanostructure heat sources measured to date (at 20 nm in lateral size), this work uncovers a previously unobserved and unpredicted nanoscale thermal transport regime where both size and spacing of heat sources play a role in determining the heat dissipation efficiency. Surprisingly, this shows that nanoscale heat sources can cool more quickly when spaced close together than when far apart. This discovery is significant to the engineering of thermal management in nanoscale systems and devices while also revealing new insight into the fundamental nature of thermal transport. Furthermore, we harness this new regime to demonstrate the first experimental measurement of the differential contributions of phonons with  different mean free paths to thermal conductivity, down to mean free paths as short as 14 nm for the first time.

The same technique is then applied to the study of acoustic waves in nanostructured materials, where they are used to characterize mechanical properties at the nanoscale. This thesis demonstrates the application of EUV nanometrology for the complete characterization of isotropic ultrathin films down to 50 nm in thickness across a broad range of stiffnesses. By simultaneously measuring both longitudinal and transverse waves, we are able to study trends in elastic properties that are normally assumed to be constant because it is difficult to measure them. This work also extends the technique to study anisotropic materials.

Finally, by observing the acoustic resonances of nanostructured ultrathin bilayers, this work is the first to apply EUV nanometrology to layers with sub-10nm thickness and to measure the mechanical properties of nanostructures down to single monolayer levels. Here it is shown that the density ratio of the ultrathin layers is not substantially altered from the bulk material counterpart, but the nanoscale elastic properties do deviate significantly and follow opposing trends for two different metallic materials.