As a materials scientist, I am interested in structure-property relationships in a complex materials systems, including functional oxides and nanocomposites. These systems are fascinating because of their exotic physics and importance for computing, energy storage, and light harvesting technologies. My work combines analytical electron microscopy with complementary ion and X-ray scattering methods, as well as electronic structure calculations to control crystal growth, chemistry, and emergent properties in nanostructured systems.
Many of today's most important materials possess multiple degrees of order spanning the atomic- to macroscale. How can we understand and control this order to achieve desired properties? While traditional indirect probes can provide detailed volume-averaged information about a sample, a direct local analysis is increasingly needed to understand the hierarchy of order in complex materials systems.
By combining analytical scanning transmission electron microscopy (STEM) with atom probe tomography (APT), we are able to visualize the real space structure of these materials in three dimensions. This unprecedented level of understanding can greatly inform the design of compounds such as double perovskites, which show promise for use in thermoelectrics and spintronics.
Electron spectroscopy mapping provides direct insight into the cation ordering process in these compounds, while APT has revealed a previously unknown nanoscale phase separation in this system. Ab initio calculations show that double perovskites possess several competing energetic pathways as a result of the low oxygen growth pressure used in their synthesis. Taken together our results illustrate the powerful way that microscopy can inform theory and structure-property models.
APT model of NiO secondary phases (green) in a La2MnNiO6 thin film (blue). Lateral size ~ 30 nm
Over the past decade, advances in scanning transmission electron microscopy have enabled imaging and spectroscopy at near atomic-resolution. These new techniques have yielded unprecedented insights into a range of useful materials, including functional oxides and 2-dimensional layered structures such as graphene. However, the interpretation of data from electron spectroscopy is not trivial because of the strong beam-specimen interactions.
In the case of atomic-scale energy-dispersive X-ray spectroscopy (STEM-EDS), it is found that delocalization and channeling of the electron beam can greatly influence the information encoded in chemical maps. Combining experiment and theory, it is possible to identify thickness regimes and the particular delocalization trends for different ionization edges. This information can guide sample preparation and establish limits for the accuracy of STEM-EDS mapping of atomistic structure.
Similar challenges are encountered in atomic-scale electron energy loss spectroscopy (STEM-EELS), whose exceptional energy resolution permits direct mapping of local chemistry and bonding. Understanding the limitations of signal broadening affects the use and interpretation of STEM-EELS for the characterization of nanostructured systems.
STEM-EDS signal delocalization across an oxide interface.
Magnetoelectrics are a class of materials exhibiting coupled magnetic and electronic order. These materials are very promising for solid-state memory applications, where an electric field might be used to switch magnetization. Such devices are predicted to be more energy-efficient, since they possess no moving parts and no large magnetic fields are required to magnetize a bit.
Unfortunately, single-phase magnetoelectrics are exceedingly rare in nature, so we have turned to alternative magnetoelectric heterostructures, in which we layer very thin (several nm) layers of a piezoelectric with a ferromagnet. Ferroelectric polarization and strain are then transferred from the piezoelectric to the ferromagnet when an electric field is applied. This results in a reversible change in the magnetization of the ferromagnet.
The La1-xSrxMnO3 (LSMO) / PbZrxTi1-xO3 (PZT) system is particularly attractive for such an application, since LSMO has the highest ferromagnetic Curie temperature of any of the manganites (~360 K) and PZT has a large piezoelectric coefficient. To better understand the nature of strain coupling I conducted local measurements of strain using transmission electron microscopy. In contrast to traditional X-ray techniques, I was able to visualize strain directly and determine its spatial relationship relative to the LSMO / PZT interface using geometric phase analysis. I then directly correlated this information to magnetic depth profiles measured by polarized neutron reflectometry.
The results of my work have revealed the presence of significant local strain fluctuations across the LSMO / PZT interface, which actually redistribute electrons in the dz2 and dx2-y2 orbitals. This results in a local suppression of ferromagnetic ordering. Moreover, my colleagues and I found that different substrate geometries may be used to reshape these strains and tune ferromagnetic ordering for different applications.
High-angle annular dark field (STEM-HAADF) image of PZT.
Local strain map of a PZT / LSMO interface generated using geometric phase analysis.
© 2015–2018 STEVEN R. SPURGEON
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