Linear-scaling algorithm for rapid computation of inelastic transitions in the presence of multiple electron scattering

Strong multiple scattering of the probe in scanning transmission electron microscopy (STEM) means image simulations are usually required for quantitative interpretation and analysis of elemental maps produced by electron energy-loss spectroscopy (EELS). These simulations require a full quantum-mechanical treatment of multiple scattering of the electron beam, both before and after a core-level inelastic transition. Current algorithms scale quadratically and can take up to a week to calculate on desktop machines even for simple crystal unit cells and do not scale well to the nanoscale heterogeneous systems that are often of interest to materials science researchers. We introduce an algorithm with linear scaling that typically results in an order of magnitude reduction in computation time for these calculations without introducing additional error and discuss approximations that further improve computational scaling for larger-scale objects with modest penalties in calculation error. We demonstrate these speedups by calculating the atomic resolution STEM-EELS map using the L-edge transition of Fe, for a nanoparticle 80 Å in diameter, in 16 hours, a calculation that would have taken at least 80 days using a conventional multislice approach

Capturing 3D atomic defects and phonon localization at the 2D heterostructure interface

The three-dimensional (3D) local atomic structures and crystal defects at the interfaces of heterostructures control their electronic, magnetic, optical, catalytic, and topological quantum properties but have thus far eluded any direct experimental determination. Here, we use atomic electron tomography to determine the 3D local atomic positions at the interface of a MoS2-WSe2 heterojunction with picometer precision and correlate 3D atomic defects with localized vibrational properties at the epitaxial interface. We observe point defects, bond distortion, and atomic-scale ripples and measure the full 3D strain tensor at the heterointerface. By using the experimental 3D atomic coordinates as direct input to first-principles calculations, we reveal new phonon modes localized at the interface, which are corroborated by spatially resolved electron energy-loss spectroscopy. We expect that this work will pave the way for correlating structure-property relationships of a wide range of heterostructure interfaces at the single-atom level

Exploring the 3D structure of defects and electron-beam induced dynamics in graphene

When an electron’s propagation direction in a material is restricted in one or multiple dimensions, the material’s properties deviate heavily from its bulk counterpart. These properties were intensively studied theoretically decades before the experimental synthesis of these ”low-dimensional” materials was realized. Carbon is one of the major elements in organic chemistry and can form a large variety of allotropes in different dimensions. Graphene, an atomically thin single layer of carbon, is the 2D component of the carbon allotropes and can be regarded as the building block for most of the other allotropes. It is the thinnest material in the world and the first 2D material to be successfully isolated. Stacking graphene layers results in graphite, which is the 3D crystalline carbon allotrope besides diamond. Rolling up a graphene sheet can form a carbon nanotube, which can have different phases and therefore can have different properties. The nanotubes are regarded as 1D materials and were described in 1991 by Sumio Iijima. Fullerenes, where 60 carbon atoms build the smallest soccer balls in the world, are regarded as a 0D material and were successfully synthesized already in 1985. Graphene has been successfully isolated in 2003, recently after the development of aberration correctors in transmission electron microscopes which enabled resolving single atoms in very beam-sensitive materials. This coincidence made it possible to intensively study graphene as well as the dynamics of single atoms. This enormous effort is not only justified by its unique properties which are very interesting for industrial applications, but also for the possibility of a fundamental understanding low-dimensional physics experimentally at the atomic scale. Despite the huge attention paid to graphene, there is still limited information about its actual 3D structure, especially at defect sites. This knowledge gap is due to the limited information in 2D using aberration corrected transmission electron microscopy techniques, because their images are essentially projections of an object. This cumulative thesis focuses on filling this missing knowledge gap and delivers a relevant contribution for the understanding of the (3D) structural properties of defects in graphene. The cumulative dissertation is based on 3 peer-reviewed first-author publications and one relevant peer-reviewed non-first-author publication, which present a new method of reconstructing atomically-thin structures using only 2 atomically resolved images. They reveal insights into the 3D structure of grain boundaries, heteroatom impurities and van-der-Waals heterostructures. Not just static properties, but also out-of-plane dynamics induced by the electron beam are studied. In addition, scanning transmission electron microscopy is used to unambiguously identify single oxygen and nitrogen atoms in defective graphene. The data set allows statistical assessment of all the bonding configurations and comparison of oxygen with nitrogen configurations. Remarkably, graphitic oxygen substitutions with three carbon neighbors are observed. This cumulative thesis is clustered in 4 different chapters. Chapter 1 presents an introduction on the studied material and a motivation of this work. Chapter 2 summarizes the experimental methods and introduces their principles and basic physics. Chapter 3 discusses the novel reconstruction method in detail. Chapter 4 briefly summarizes the papers and the author’s contributions. Each summary follows the corresponding original publication

In Situ Study of Ultrafast Carrier Transport Dynamics in Perovskite Thin-Films

Perovskites are a novel class of materials that have piqued the interest of researchers in photovoltaics, photodetectors, and optoelectronics. In this study, we measure and elucidate in situ ultrafast carrier dynamics in both organic and inorganic, lead, and non-lead-based halide perovskite thin films using ultrafast photocurrent spectroscopy (UPCS) with a sub-25 ps time resolution. The UPCS technique enables us to define carrier transport dynamics in spatial, temporal, and energy landscapes via measurements at different electric fields, laser intensities, and temperatures. Here we explore and analyze solution-processed bulk MAPbI3 and nanocrystalline CsPbI3-based devices and novel non-lead double-layered perovskite devices through UPCS. In particular, we elucidate carrier transport dynamics focusing on early-time phonon interactions and the role played by the material defects. From our data and analysis, we successfully elucidated the role of ultra-shallow trap levels in MAPbI3 for the first time, while in nanocrystalline CsPbI3, we identified the optical phonon responsible for carrier phonon scattering through the analysis of the transport index number. Regarding double-layered perovskites, spectroscopic analysis of their charge carrier dynamics revealed the feasibility of using them in future optoelectronic applications. The design of lead-free, environmentally friendly perovskites can potentially replace current lead-based materials

Photophysical and Structural Properties of Tin-Lead Alloyed Perovskite Nanocrystals

電気通信大学博士（工学）2024doctoral thesi

First- and second-order Raman spectra of carbonaceous material through successive contact and regional metamorphic events (Ryoke belt, SW Japan)

The evolution of both the first- and second-order Raman spectra of carbonaceous material (CM) through successive contact and regional metamorphic events is explored in the western part of the Ryoke belt (Iwakuni-Yanai area, SW Japan). Thirty-two metasedimentary rock samples were collected along a N-S gradient locally affected by contact metamorphism before and after the main regional tectono-metamorphic event (DP1). First-order spectra document a decreasing peak area ratio R2 and an increasing temperature TCM towards the south and the surrounding granitoids. Domains with intermediate (535–600 °C) TCM values match the extent of the pre-DP1 contact aureole but also image a so far unclear post-DP1 aureole. The axial part of the belt, likely unaffected by granite intrusions, preserves southward increasing TCM from 425 to 660 °C. Second-order spectra show a single S1 band that splits into two peaks (S1- and S1+) whose frequency difference ΔS1 increases stepwise towards the south. The spatial distribution of ΔS1 follows that of the E–W trending regional metamorphic zones. The splitting of S1 indicates a transition from two-dimensional to three-dimensional CM and occurs at ~500 °C, which seems to be common to all metamorphic belts worldwide. Despite regional metamorphism CM was able to record the post-DP1 contact overprint and there is no clear observation of delayed CM recrystallization, which likely depends on the crystallinity of the CM precursor. A discrepancy between first- and second-order Raman parameters suggests that they partly record the influence of different factors; R2 gives an account of thermal events, particularly those related to localized contact metamorphism, whereas ΔS1 potentially yields information on regional variations in heating duration and pressure. This demonstrates the potential of the full Raman spectrum of CM for deciphering the complex thermal history of orogenic systems