Phonon Spectroscopy at Atomic Resolution Hage, F S; Kepaptsoglou, D M; Ramasse, Q M ...
Physical review letters,
2019-Jan-11, Volume:
122, Issue:
1
Journal Article
Peer reviewed
Open access
Advances in source monochromation in transmission electron microscopy have opened up new possibilities for investigations of condensed matter using the phonon-loss sector of the energy-loss spectrum. ...Here, we explore the spatial variations of the spectrum as an atomic-sized probe is scanned across a thin flake of hexagonal boron nitride. We demonstrate that phonon spectral mapping of atomic structure is possible. These results are consistent with a model for the quantum excitation of phonons and confirm that Z-contrast imaging is based on inelastic scattering associated with phonon excitation.
The spatial distributions of antibonding π^{*} and σ^{*} states in epitaxial graphene multilayers are mapped using electron energy-loss spectroscopy in a scanning transmission electron microscope. ...Inelastic channeling simulations validate the interpretation of the spatially resolved signals in terms of electronic orbitals, and demonstrate the crucial effect of the material thickness on the experimental capability to resolve the distribution of unoccupied states. This work illustrates the current potential of core-level electron energy-loss spectroscopy towards the direct visualization of electronic orbitals in a wide range of materials, of huge interest to better understand chemical bonding among many other properties at interfaces and defects in solids.
The control of material interfaces at the atomic level has led to novel interfacial properties and functionalities. In particular, the study of polar discontinuities at interfaces between complex ...oxides lies at the frontier of modern condensed matter research. Here we employ a combination of experimental measurements and theoretical calculations to demonstrate the control of a bulk property, namely ferroelectric polarization, of a heteroepitaxial bilayer by precise atomic-scale interface engineering. More specifically, the control is achieved by exploiting the interfacial valence mismatch to influence the electrostatic potential step across the interface, which manifests itself as the biased-voltage in ferroelectric hysteresis loops and determines the ferroelectric state. A broad study of diverse systems comprising different ferroelectrics and conducting perovskite underlayers extends the generality of this phenomenon.
Hot deformed Nd–Fe–Co–B–Ga magnets were infiltrated with a Nd–Cu eutectic liquid, resulting in a 71% increase in coercivity to μ0Hc=2.4T without the use of Dy, and a 22% decrease in remanence, ...attributed to the dilution effect. Aberration-corrected scanning transmission electron microscopy and electron energy loss spectroscopy have been used to reveal the structure and chemical composition of phase boundaries in the magnets on the atomic scale. The results showed that the Nd–Cu liquid penetrated the ≈1nm thick intergranular regions. The coercivity increase following infiltration was therefore attributed to improved volume fraction and distribution of the intergranular phases. Co enrichment in the outermost 1–2 unit cells at several {001} and {110} surfaces of the Nd2(Fe,Co)14B crystals was shown for the infiltrated sample. The as-deformed sample did not appear to show this Co enrichment. Molecular dynamics simulations indicated that the distorted layer at an {001} surface of a Nd2(Fe,Co)14B grain was significantly thicker with higher Co surface enrichment. The magnetocrystalline anisotropy may be reduced in such distorted regions, which could have a detrimental effect on coercivity. Such features may therefore play a role in limiting coercivity to a fraction of the anisotropy field. Interfacial segregation of Cu between Nd2(Fe,Co)14B and the Nd-rich intergranular phase occurred in the infiltrated sample. Step defects in Nd2(Fe,Co)14B {001} surfaces, a half or a whole unit cell in height, were also observed.
Materials with the pyrochlore/fluorite structure have diverse technological applications, from magnetism to nuclear waste disposal. Here we report the observation of structural instability present in ...the pyrochlores A
Zr
O
O' (A = Pr, La) and Yb
Ti
O
O', that exists despite ideal stoichiometry, ideal cation-ordering, the absence of lone pair effects, and a lack of magnetic order. Though these materials appear to have good long-range order, local structure probes find displacements, of the order of 0.01 nm, within the pyrochlore framework. The pattern of displacements of the A
O' sublattice mimics the entropically-driven fluxional motions characteristic of and well-known in the silica mineral β-cristobalite. The universality of such displacements within the pyrochlore structure adds to the known structural diversity and explains the extreme sensitivity to composition found in quantum spin ices and the lack of ferroelectric behavior in pyrochlores.
The concept of electronic orbitals has enabled the understanding of a wide range of physical and chemical properties of solids through the definition of, for example, chemical bonding between atoms. ...In the transmission electron microscope, which is one of the most used and powerful analytical tools for high-spatial-resolution analysis of solids, the accessible quantity is the local distribution of electronic states. However, the interpretation of electronic state maps at atomic resolution in terms of electronic orbitals is far from obvious, not always possible, and often remains a major hurdle preventing a better understanding of the properties of the system of interest. In this review, the current state of the art of the experimental aspects for electronic state mapping and its interpretation as electronic orbitals is presented, considering approaches that rely on elastic and inelastic scattering, in real and reciprocal spaces. This work goes beyond resolving spectral variations between adjacent atomic columns, as it aims at providing deeper information about, for example, the spatial or momentum distributions of the states involved. The advantages and disadvantages of existing experimental approaches are discussed, while the challenges to overcome and future perspectives are explored in an effort to establish the current state of knowledge in this field. The aims of this review are also to foster the interest of the scientific community and to trigger a global effort to further enhance the current analytical capabilities of transmission electron microscopy for chemical bonding and electronic structure analysis.
Ion irradiation has been observed to induce a macroscopic flattening and in-plane shrinkage of graphene sheets without a complete loss of crystallinity. Electron diffraction studies performed during ...simultaneous in-situ ion irradiation have allowed identification of the fluence at which the graphene sheet loses long-range order. This approach has facilitated complementary ex-situ investigations, allowing the first atomic resolution scanning transmission electron microscopy images of ion-irradiation induced graphene defect structures together with quantitative analysis of defect densities using Raman spectroscopy.
Al-Ti-B based master alloys have been widely used for grain refining of Al-alloys in industry for many decades. However, the effectiveness of such grain refiners is severely compromised when a few ...hundred ppm of Zr is present in the Al melt, and this phenomenon is referred to as Zr poisoning in the literature. So far the exact mechanisms for Zr poisoning are not clear albeit significant research effort on the subject in the last few decades. In this work we investigated the mechanism for Zr poisoning through extensive examinations of the Al/TiB2 interface using the state-of-the-art electron microscopy and ab initio molecular dynamics simulations. We found that the presence of Zr in Al melts leads to (i) the dissolution of the Al3Ti 2-dimensional compound (2DC) formed on the (0 0 0 1) TiB2 surface during the grain refiner production process; and (ii) the formation of an atomic monolayer of Ti2Zr 2DC on the (0 0 0 1) TiB2 surface, which replaces the original Ti-terminated TiB2 basal surface. This monolayer of Ti2Zr not only has large lattice misfit (4.2%) with α-Al, but also is atomically rough, rendering the TiB2 particles impotent for heterogeneous nucleation of α-Al. This work, in combination of our previous work, demonstrates that heterogeneous nucleation can be effectively manipulated, either enhanced or impeded, by chemical segregation of selected alloying/impurity elements at the liquid/substrate interface.
(a, b) High resolution STEM HAADF images showing an atomic monolayer of Ti2Zr 2-dimensional compound (2DC) on (0 0 0 1) surface of TiB2 being viewed along (a) 1 1 -2 0TiB2 and (b) 1 0 -1 0TiB2 direction respectively, and (c) 3D construction of the Ti2Zr 2DC on top of TiB2. Display omitted
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