Exploring Functional Materials by Understanding Beam‐Sample Interactions

Abstract

Ultra-low-dose electron diffraction is performed with a double metal cyanide catalyst (DMC) to understand how electron irradiation stimulates structural alterations in functional materials. The commonly fading diffraction patterns with dose accumulation depend on the irradiated area and the beam current even when below 50 femto Amperes. Heat generation is observed and modeled by statistical, inelastic scattering events to describe how phonon excitations modulate radiation hardness. Specifically, the characteristic 1/e-decay of Bragg intensities from DMC is delayed from 6 to 30 eÅ −2 at room temperature, which is comparable to the effect of embedding radiation soft matter in ice. DMC's radiation hardness is enhanced by a latency dose that forms during a phase transformation. This unifying model predicts that a critical dose rate exists for any material that varies between 0.1 and 10 4 eÅ −2 s −1 because of a material dependent competition of heat generation and spread. It shows that Brillouin scattering causes time dependent perturbations in electron irradiated solids that trigger time-temperature-transformations on a time scale of nanoseconds to microseconds at room temperature, which is not included in traditional models describing the decay of Bragg intensities by radiolysis.

Probing the Boundary between Classical and Quantum Mechanics by Analyzing the Energy Dependence of Single-Electron Scattering Events at the Nanoscale

“…Moreover, electron scattering is commonly perceived as either coherent-elastic or incoherent-inelastic [ 1 ]. However, there is growing evidence that Coulomb interactions in solids are always inelastic [ 15 , 17 ], and definitions of coherence or decoherence remain ambiguous. Consequently, the description of electron scattering must be considered carefully with the aim to explain already-diverging interpretations (for example, compare [ 29 , 30 ] or [ 9 , 31 ]) that often relate to the excitation of structural dynamics by the electron beam itself.…”

“…Based on the implemented approximations, state-of-the-art MS calculations of amplitude images were insufficient to simulate evanescent intensity ( Figure 4 a, inset) [ 5 ]. At present, the MS calculations make use of Huygen’s principle for coherent–elastic interferences of wave functions [ 3 ], but do not include coherent–inelastic Coulomb scattering processes [ 12 , 17 ]. Certainly, the literature which exists to describe electron excitation in solids, including imaging in the optical frequency range [ 25 ] and aloof spectroscopy [ 26 , 27 , 28 ], is extensive.…”

“…Surprisingly, we found that our observed delocalization of information in recorded amplitude images was dependent only on the energy loss during the interaction of wave functions and tracks in the Heisenberg relation (1), while the wave character of the scattered electrons was always maintained by the formation of propagating wave packets (pulses) with energy-dependent widths. As an example of the impact of our results on science at the nanoscale, we point out that it recently became possible to estimate and manipulate the critical accumulated electron dose for the irradiation-induced decay of crystal structures by better understanding time-dependent phonon scattering in both radiation-hard and radiation-soft materials [ 17 ].…”

Coherence and Inelastic Scattering in Electron Microscopy

“…An outstanding contribution of the approach includes the ability to address beam-sample interactions in the limit of single electron self-interferences to better understand the dose and dose rate dependences of irradiated matter. [5][6][7] For this purpose we assume that quantum theory is universally valid so that observable properties emerge from inelastic interactions among wave functions with energy loss ΔE that degrade phase relations because they change the Broglie wavelength by Δλ and create wave packages of finite size. Specifically, we consider the case of self-interferences where a phase shift φ = 0.5 rad equals (2π/λ -2π/(λ+Δλ))*l [5] to show in Figure 2 how the resulting coherence lengths l (ΔE) relate to the coherence times l t = l (ΔE) / c, where c is the speed of light.…”

Latency Dose Formation In DMC By Inelastic Electron Scattering

“…Recent advancements of detection techniques in electron microscopy [1] enable approaching the ultimate detection limit while exploiting time resolution by operating with beam currents below 50 femto Amperes [2] or by using pulsed electron sources. [3] The TEAM I microscope is uniquely suited for such experiments because it features a Cc corrector, a Nelsonian illumination scheme, and direct electron detectors.…”