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Abstract
1 Materials Science and Technology Division, Oak Ridge National Laboratory, USA.
2 Department of Physics and Astronomy, Vanderbilt University, USA.
The properties and performance of many materials are often strongly influenced by the presence of structural defects. Especially in nano-materials, the influence from structural defects can be dominant. Defects are not always bad; in some cases, defects could just be something desirable- they can help to improve the properties and induce new functionalities in the material. A deep understanding of the defect structure at the atomic scale, thus, can bring new insights into the material system, and help us to achieve the goal of “materials by design”.
In this presentation, I will show how aberration-corrected electron microscopy techniques can help us to understand the defect structure and the properties, atom-by-atom. Two examples will be discussed.
Single Atom Plasmonics on Graphene
Plasmons in graphene can be tuned by using electrostatic gating or chemical doping, and the ability to confine plasmons in very small regions could have applications in optoelectronics and nano-plasmonics. However, little is known about how atomic scale defects influence the plasmonic properties of graphene. Moreover, the smallest localized plasmon resonance observed in any material to date has been limited to the sub-10 nanometer scale. Here we show that surface plasmon resonances in graphene can be enhanced locally at the atomic scale. Using electron energy-loss (EEL) spectrum imaging in an aberration-corrected scanning transmission electron microscope (STEM), we find that a single point defect can act as an atomic antenna in the petaHertz (1015 Hz) frequency range, leading to surface plasmon resonances at the sub-nanometre scale [1]. In addition we observe a new one-dimensional plasmon mode at the open edge of monolayer graphene with a spatial extension of ~ 0.6 nm [2]. Our results suggest new possibilities for designing nanoscale optoelectronic and plasmonic devices based on graphene.
Anisotropic Defect Distribution in LiFePO4 Battery Material
Li-ion mobility in LiFePO4 occurs preferentially via one dimensional channels oriented along the [010] direction (b-axis). Such one-dimensional diffusion, however, can be impeded by the presence of immobile or low-mobility defects in the diffusion path. Using aberration-corrected STEM imaging, we observed that antisite defects (FeLi) in LiFePO4 cluster in some particular channels, leaving others open to Li ion diffusion. In addition, STEM-EELS analysis shows that FeLi in the b-axis channels has a slightly higher oxidation state than the nominal value of +2 for Fe in the bulk lattice sites. First-principles calculations reveal that the clustering of FeLi in select channels is driven by Li vacancies bound to the FeLi. Li vacancies (VLi) are confined in one-dimensional b-axis channels, shuttling between neighboring FeLi. Segregation in select channels results in shorter FeLi-FeLi spans, whereby the energy is lowered by the VLi’s spending more time bound to end-point FeLi’s. VLi-FeLi-VLi complexes also form, accounting for observed electron energy loss spectroscopy features [3].
[1] W. Zhou, et al. Atomically localized plasmon enhancement in monolayer graphene. Nature Nanotechnology, in press 2012.
[2] W. Zhou, et al. Localization of inelastic electron scattering in the low-loss energy regime. Ultramicroscopy doi:10.1016/j.ultramic.2011.11.013 (2011).
[3] J. Lee, et al. Vacancy-driven anisotropic defect distribution in the battery-cathode material LiFePO4. Physical Review Letters, 107, 085507 (2011). |
