Current Highlights

2023 MRS Spring Meeting & Exhibit

Call for Papers

Prof. Molina-Luna is co- organizer for Symposium CH02-Advances in Cryogenic Transmission Electron Microscopy and Spectroscopy for Quantum and Energy Materials

Call for papers

Congratulations to Dr. rer. nat. Alexander Zintler for his succesful PhD graduation!

On 04.07.2022 Alexander Zintler successfully defended his Thesis

“Investigating the influence of microstructure and grain boundaries on electric properties in thin film oxide RRAM devices – A component specific approach”

DOI: 10.26083/tuprints-00021657

The doctoral supervisor was Prof. Dr. Leopoldo Molina-Luna Molina-Luna and the 2nd examiner Prof. Dr. Lambert Alff

We congratulate Hui Ding on her succesful PhD graduation!

Hui Ding is a PhD student at Prof. Dr. Hans-Joachim Kleebe's research group Geomaterial Science and at Prof. Dr. Leopoldo Molina-Luna's research group AEM. She is also a member of the LOEWE project FLAME.

We wish her all the best for her future!

DOI: 10.26083/tuprints-00021768

In-situ/Operando TEM Techniques

for Advanced Nanomaterial Characterization Workshop

Prof. Molina-Luna is an invited speaker at the In-situ/Operando TEM Techniques for Advanced Nanomaterial Characterization Workshop of the Canadian Center for Electron Microscopy

https://ccem.mcmaster.ca/events/in-situ-operando-tem-techniques-for-advanced-nanomaterial-characterization-workshop/ link

Collaborative study

Our collaborative study between the groups of Advanced Electron Microscopy (AEM), Advanced Thin Film Technology (ATFT) and Physical Metallurgy (PhM) investigates the influence of nitrogen deficiency on presence and type of grain boundaries in superconducting Titanium Nitride thin films. Zintler et. al. performed the study within the ERC starting grant FOXON and the ECSEL Joint Undertaking project StorAIge. The results will be published in the journal ACS Omega.

URLs

AEM: https://www.mawi.tu-darmstadt.de/aem/ aem

ATFT: https://www.mawi.tu-darmstadt.de/ds/ ds

PhM: https://www.mawi.tu-darmstadt.de/phm/ phm

StorAlge: https://www.elektronikforschung.de/projekte/storaigeStorAIge

StorAIge: FOXON: https://cordis.europa.eu/project/id/805359/deFOXON

ACS Omega: https://pubs.acs.org/journal/acsodfascodf

MRS 2022 Spring Meeting

Call for Papers

Prof. Molina-Luna is the lead organizer for Symposium CH03—Advances in In Situ and Operando TEM Methods for the Study of Dynamic Processes in Materials

link

MRS 2021 Virtual Spring Meeting & Exhibit, April 17 – 23, 2021

Professor Leopoldo Molina-Luna will be chairing sessions CT02.01: Structural Evolution and Structure-Property Correlation and CT02.02: Phase Transformation and Structure-Property Correlation of Symposium CT02 : In Situ TEM Characterization of Dynamic Processes During Materials Synthesis and Processing at the next MRS Spring 2021 Meeting. link

MIT-Germany Lockheed Martin Seed Fund for Professor Molina-Luna

08 December 2020

TU Professor Leopoldo Molina-Luna from the Department of Materials and Geosciences is being funded by the MIT-Germany Lockheed Martin Seed Fund 2019/2020 with 22,000 US dollars for his project “Investigation of Oxygen Diffusion in HfO2 Based Memristive Devices by Using iDPC STEM”. The Seed Fund was first announced in 2019 and supports research projects between MIT scientists and German, mostly technical, universities. A total of five projects from Germany were funded, including the project of Professor Molina-Luna. He is collaborating with an MIT working group in the Department of Materials Science and Engineering (DMSE). The project started this year and will be extended until 2022 due to the current corona pandemic. It deals with a new method to image light elements on the atomic level. mho

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Co-Organization of Symposium P10 for M&M 2021 in Pittsburgh from August 1-5, 2021, titled “Investigating Phase Transitions in Functional Materials and Devices by In Situ / Operando TEM.”

ORGANIZERS:

Michele Conroy, University of Limerick

Trevor Almeida, University of Glasgow

Leopoldo Molina-Luna, TU Darmstadt

Judy Cha, Yale

The possibility to control phase transitions in functional materials and devices within the TEM provides fundamental insight into dynamic, localised processes that were previously inaccessible. The development of in-situ TEM capabilities (heating, biasing, cooling, magnetic fields, etc.) and their combination with advanced TEM techniques (phase-related, spectroscopy, 4D-STEM etc.) enables operando studies to characterize the physical properties of materials at the highest resolution while simultaneously measuring their functional performance. These innovative investigations provide a wealth of information that opens a plethora of opportunities to study functional materials and devices in a range of new applications. This proposed symposium invites in-situ (S)TEM experiments that utilise not only applied stimulus via in-situ TEM holders, but also controlled electron-beam-induced transitions. The main goal is to bring together experimental and theoretical TEM researchers that employ a range of in-situ/operando methods to understand the fundamental physics governing the nano- to atomic-scale phase transitions of functional materials and devices.

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In Situ Transmission Electron Microscopy Analysis of Thermally Decaying Polycrystalline Platinum Nanowires

Owing to their large surface area, continuous conduction paths, high activity, and pronounced anisotropy, nanowires are pivotal for a wide range of applications, yet far from thermodynamic equilibrium. Their susceptibility toward degradation necessitates an in-depth understanding of the underlying failure mechanisms to ensure reliable performance under operating conditions. In this study, we present an in-depth analysis of the thermally triggered Plateau–Rayleigh-like morphological instabilities of electrodeposited, polycrystalline, 20–40 nm thin platinum nanowires using in situ transmission electron microscopy in a controlled temperature regime, ranging from 25 to 1100 °C. Nanowire disintegration is heavily governed by defects, while the initially present, frequent but small thickness variations do not play an important role and are overridden later during reshaping. Changes of the exterior wire morphology are preceded by shifts in the internal nanostructure, including grain boundary straightening, grain growth, and the formation of faceted voids. Surprisingly, the nanowires segregate into two domain types, one being single-crystalline and essentially void-free, while the other preserves void-pinned grain boundaries. While the single-crystalline domains exhibit fast Pt transport, the void-containing domains are unexpectedly stable, accumulate platinum by surface diffusion, and act as nuclei for the subsequent nanowire splitting. This study highlights the vital role of defects in Plateau–Rayleigh-like thermal transformations, whose evolution not only accompanies but guides the wire reshaping. Thus, defects represent strong parameters for controlling the nanowire decay and must be considered for devising accurate models and simulations.

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Neuromorphic Computing: Tailoring the Switching Dynamics in Yttrium Oxide‐Based RRAM Devices by Oxygen Engineering: From Digital to Multi‐Level Quantization toward Analog Switching (Adv. Electron. Mater. 11/2020)

For neuromorphic computing that mimics the human brain, digital memory has to turn analogue. A key scientific question is how to teach materials to adopt synaptic plasticity. In article 2000439, Stefan Petzold, Eszter Piros, and co‐workers use the novel technique of oxygen engineering on yttrium oxide and show that this particular material is a prominent candidate to allow for controlling and fine‐tuning switching dynamics.

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Forming-Free Grain Boundary

Forming-Free Grain Boundary Engineered Hafnium Oxide Resistive Random Access Memory Devices Our recent publication on a new type of grain boundary engineering in electronic devices was chosen as a back cover of the renowned journal “Advanced Electronic Materials”. The novel method presented yielded forming free devices with a narrow distribution of forming voltages. This makes this approach a powerful tool to overcome one of the main challenges for resistive switching devices; which is device-to-device variation. The combination of atomic-resolution scanning transmission electron microscopy imaging along with DFT-based simulations were fundamental in order to decipher the induced low-energy grain boundary structures, furthermore, electron transparent lamellae of the actual devices were successfully operated inside our Cs-probe corrected transmission electron microscope.

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Forming-Free Grain Boundary Engineered Hafnium Oxide Resistive Random Access Memory Devices

In this study, a new method of direct defect engineering, namely grain boundary engineering, has been developed for hafnia based resistive random access memory (RRAM) devices in close collaboration with the Advanced Thin Film Technology division of Prof. Alff. This new method tackles the major challenges of RRAM being high energy consumption and large device-to-device variation. By growing highly textured thin films with low energy grain boundaries (GBs) of high symmetry threading trough the hole dielectric layer (hafnia), it was possible to achieve forming-free RRAM devices with a narrow distribution of forming voltages caused by predefined pathways along GBs for formation of the conductive filament. Further, transmission electron microscopy has revealed properties of the GBs by analysing the atomically resolved microstructure as shown by this exemplary image.

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Analysis and simulation of the multiple resistive switching modes occurring in HfOx-based resistive random access memories using memdiodes

In this study, analysis and simulation of all experimentally observed switching modes in hafnium oxide based resistive random access memories are carried out using a simplified electrical conduction model. The impact of the oxygen content of the dielectric layer on the electronic structure was investigated by low-loss electron energy-loss spectroscopy, which revealed a clear difference between deficient and stoichiometric films. The electronic structure fingerprints of stoichiometric m-HfO2 and oxygen deficient t-HfO1.5 could be identified.

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Atomically interface engineered micrometer-thick SrMoO3 oxide electrodes for thin-film BaxSr1-xTiO3 ferroelectric varactors tunable at low voltages

This publication was chosen by the Editors of APL Materials as one of the Editor’s Picks. In the paper we contributed high-resolution high-angle annular dark field STEM imaging to the characterization of micrometer thick, yet atomically sharp interfaces in stacks of ferroelectric varactor materials for high frequency applications. These devices work at Li-ion battery voltage levels, are highly tunable in their frequency range and are key part to the emerging 5th generation (5G) mobile communication networks.

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Gradual Reset and Set Characteristics in Yttrium Oxide based Resistive Random Access Memory

In this study, we report for the first time the coexistence of bipolar resistive switching and unipolar resistive switching within one stack combination based on yttrium oxide. The BRS is characterized by low variability, good data retention, high DC endurance, and average on-off-ratio above 100. The obtained concurrent gradual transitions during BRS set and reset which have been employed in pulse depression and potentiation operations (see figure) with shown plasticity over about two orders of magnitude make RRAM based on yttrium oxide an interesting alternative candidate for neuromorphic synapses. Combined with its characteristic of being forming-free and having low operation voltages, RRAM based on yttrium oxide is highlighted as a suitable material candidate for new high density nonvolatile memory technologies.

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Enabling nanoscale flexoelectricity at extreme temperature by tuning cation diffusion

Electro-thermal chip sample carrier for in situ transmission electron microscopy. a Schematics of the electro-thermal chip, including the set of biasing electrodes surrounded by the encapsulated microheater colored in green that is temperature controlled by Joule heating. b Corresponding simulated temperature distribution profile generated by the microheater. c Magnified view of the biasing wires region, showing a close-up of the 20 nm thick electron transparent window and the four surrounding biasing wires. The green plane represents the cross-section where the electric field magnitude is plotted. d Finite element simulation of local electric field magnitude and the electric field lines over the cross-sectional plane indicated in c. A nanoparticle was placed in the window area between the electrodes for modeling

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Lightning featured on Nature Communications

Materials with switchable polarization are indispensable in memory devices, sensors, actuators, and transducers. In this publication Leopoldo Molina-Luna et al reported an unexpected phenomenon, the appearance of domain-like nanoregions (DLNRs) at extreme temperatures. The origin of the phenomenon can be ascribed to flexoelectricity, an intrinsic property of dielectric materials that can generate polarization under a strain induced by compositional gradients. To examine the origin of the DLNRs experimentally, the authors applied electric fields up to ±22 kV/mm at 800 °C and observed the dynamics by means of high resolution imaging. This discovery opens up a new exciting science and will certainly motivate the study and development of other high-temperature flexoelectric nanomaterials.

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