Seminars

Theoretical Perspectives on Proton-Coupled Electron Transfer

Proton-coupled electron transfer (PCET) reactions play a vital role in a wide range of chemical and biological processes. This talk will focus on the theory of PCET as well as illustrative applications to catalysis and energy conversion processes. [more]

TBA

  • TH Department Online Seminar
  • Date: Mar 18, 2021
  • Time: 17:00 - 18:00
  • Speaker: Professor Garnet Chan
  • Division of Chemistry and Chemical Engineering, California Institute of Technolog - Caltech, CA, USA
  • Location: TBA
  • Host: TH Department
TBA [more]

TBA

  • PC Online Talk
  • Date: Mar 18, 2021
  • Time: 11:00
  • Speaker: Stefano Calati
  • FHI Department PC
TBA [more]

Towards Predicting the Charge of Electrochemical Interfaces

The charge of an electrochemical interface helps to determine its chemical reactivity and macroscopic electrostatic properties. However, predicting the charge at an electrochemical interface is challenging because it generally requires both the electronic structure of the interface, and the thermodynamically averaged spatial distribution of the electrolyte. Due to this difficulty, interfacial charge prediction remains an open problem in computational electrochemistry. [more]

Excitonic Properties of Localized Emitters from Monolayer WSe2 an WSe2/MoSe2 Heterobilayer

  • PC Department Online Seminar
  • Date: Feb 23, 2021
  • Time: 16:00
  • Speaker: Dr. Xiu Lu
  • Tulane University, New Orleans, USA
Since the discovery of single photon emitters (SPEs) from monolayer WSe2 in 2015, the two-dimensional layered materials, including transition metal dichalcogenides (TMDCs) and hexagonal boron nitride (h-BN), has emerged to be a new playground for exploring quantum emitters and novel spin-photon interface1. A SPE in TMDC is believed to arise from trapping of an exciton by defects, localized strain or moirépotential which occurs in heterobilayer or twisted homobilayer. [more]

FHI Library Online Workshop: Information Resources & Open Access

This interactive online workshop will introduce available options for making your research outputs open access. Discover which publishing fees are paid centrally or by the FHI library and get practical tips on how to adopt open access practices in your research area. Furthermore, you will get an overview of useful search tools. More details on how to join the workshop will be announced by e-mail or contact the library team.

ZnO: Ultrafast photodoping

  • PC Online Talk
  • Date: Feb 18, 2021
  • Time: 11:00
  • Speaker: Lukas Gierster
  • FHI Department PC
Band bending at oxide surfaces induced by chemical doping or electric fields is known to create metallic surfaces with properties not found in the bulk, such as high electron mobility, magnetism or superconductivity. In this talk I show that photoexcitation of the ZnO(10-10) surface acts in direct analogy with chemical doping and induces band bending on ultrashort timescales. [more]

FHI Library Online Workshop: Databases

Our interactive online workshop will help you to select and apply the most appropriate resources and tools for your research. Learn more on how to quickly find and analyze relevant scientific literature. The two databases Web of Science and SciFinder will be presented and discussed in detail. More details on how to join the workshop will be announced by e-mail or contact the library team.

Reaching for the stars with density functional theory

Accurately modeling warm dense matter deep inside astrophysical objects is a grand challenge.The associated thermodynamic states are characterized by solid-state densities, temperatures ofthousands of Kelvin, and GPa pressures. The extreme of the conditions can vary gravely dependingon the mass, radius, and composition of the studied object ranging from several GPa in planetarymantles to millions of GPa at the center of stellar interiors. A method that has proven highlysuccessful in describing this peculiar state of matter is density functional theory moleculardynamics (DFT-MD). [more]

Sum Frequency Microscopy of Phonon Polaritons

Sum frequency generation (SFG) allows the study of surfaces and inversion broken systems. In a new approach we implemented a wide field sum-frequency microscope combining the FHI infrared free electron laser (IR FEL) as excitation source with visible upconversion. The IR FEL provides a powerful, narrow band, and tunable light source [1]. By direct imaging of the SFG light with a microscope in a wide field scheme without scanning the sample or the focus [2], we achieve a spatial resolution well beyond the infrared diffraction limit. [more]

Automatic topography of multidimensional probability densities

A Seminar of the NOMAD Laboratory
Unsupervised methods in data analysis aim at obtaining a synthetic description of high-dimensional data landscapes, revealing their structure and their salient features. We will describe an approach for charting complex and heterogeneous data spaces, providing a topography of the high-dimensional probability density from which the data are harvested. [more]

Vibrational Surface Spectroscopy of Liquid, Interfaces with Nanometer Depth Resolution

  • PC Online Talk
  • Date: Jan 28, 2021
  • Time: 11:00
  • Speaker: Dr. Martin Thämer
  • FHI Department PC
An important portion of chemical reactions in biology, heterogeneous catalysis, or electrochemistry exclusively happens at liquid interfaces. This selectivity originates from the special physical and chemical properties of these interfaces, which can largely deviate from the corresponding bulk properties. [more]

FHI Library Online Workshop: Databases

Our interactive online workshop will help you to select and apply the most appropriate resources and tools for your research. Learn more on how to quickly find and analyze relevant scientific literature. The two databases Web of Science and SciFinder will be presented and discussed in detail. More details on how to join the workshop will be announced by e-mail or contact the library team.

Ultrafast Electron Microscopy and Diffraction

  • PC Department Online Seminar
  • Date: Jan 25, 2021
  • Time: 11:00
  • Speaker: Prof. Jonas Weissenrieder
  • KTH Royal Institute of Technology, Stockholm
Ultrafast electron microscopy (UEM) facilitates microscopic imaging, diffraction, and spectroscopy at picosecond timescales. Electron probe bunches are generated through photoemission by a femtosecond UV laser pulse while a synchronized laser pulse excites a change of state in a sample of choice. I will show the design of the recently constructed UEM at KTH and some results from first model systems using both diffraction and imaging. [more]

Introduction to Approximate Bayesian Computation

The goal of statistical inference is to draw conclusions about properties of a population given a finite observed sample. This typically proceeds by first specifying a parametric statistical model (that identifies a likelihood function) for the data generating process which is indexed by parameters that need to be calibrated (estimated). There is always a trade-off between model simplicity / inferencial effort / prediction power. [more]

FHI Library Online Workshop: Information Resources & Open Access

This interactive online workshop will introduce available options for making your research outputs open access. Discover which publishing fees are paid centrally or by the FHI library and get practical tips on how to adopt open access practices in your research area. Furthermore, you will get an overview of useful search tools. More details on how to join the workshop will be announced by e-mail or contact the library team.

Topology, Molecular Simulation, and Machine Learning as Routes to Exploring Structure and Phase Behavior in Molecular and Atomic Crystals

Organic molecular crystals frequently exist in multiple forms known as polymorphs. Structural differences between crystal polymorphs can affect desired properties, such as bioavailability of active pharmaceutical formulations, lethality of pesticides, or electrical conductivity of organic semiconductors. [more]

Smart Sampling for Chemical Property Landscapes with BOSS

A Seminar of the NOMAD Laboratory
Atomistic structure search for organic/inorganic heterostructures is made complex by the many degrees of freedom and the need for accurate but costly density-functional theory (DFT) simulations. To accelerate and simplify structure determination in such heterogeneous functional materials, we developed the Bayesian Optimization Structure Search (BOSS) approach [1]. BOSS builds N-dimensional surrogate models for the energy or property landscapes to infer global optima. The models are iteratively refined by sequentially sampling DFT data points with high information content. The uncertainty-led exploration/exploitation sampling strategy delivers global minima with modest sampling, but also ensures visits to less favorable regions of phase space to gather information on rare events and energy barriers. [more]

FHI Library Online Workshop: Databases

Our interactive online workshop will help you to select and apply the most appropriate resources and tools for your research. Learn more on how to quickly find and analyze relevant scientific literature. The two databases Web of Science and SciFinder will be presented and discussed in detail. More details on how to join the workshop will be announced by e-mail or contact the library team.

Tracking Singlet Fission in Momentum Space

  • PC Online Talk
  • Date: Dec 10, 2020
  • Time: 10:00
  • Speaker: Alexander Neef
  • FHI Department PC
In organic materials with strong exchange interactions, the S = 0 (singlet) exciton might have twice the energy of the S = 1 (triplet) exciton. This allows the spin-conserving fission of the optically prepared singlet to two dark antiparallel triplets. Due to the many-body nature of this process, consensus over the underlying mechanism is still lacking. Here, we use trARPES to resolve the transient states involved in singlet fission in pentacene and provide new insights into the microscopic and ultrafast processes governing it. [more]

Datasets, Workflows, Models and Active Learning to Accelerate Catalyst Discovery

Machine learning accelerated catalyst discovery efforts has seen much progress in the last few years. Datasets of computational calculations have improved, models to connect surface structure with electronic structure or adsorption energies have gotten more sophisticated, and active learning exploration strategies are becoming routine in discovery efforts. [more]

FHI Library Online Workshop: Information Resources & Open Access

This interactive online workshop will introduce available options for making your research outputs open access. Discover which publishing fees are paid centrally or by the FHI library and get practical tips on how to adopt open access practices in your research area. Furthermore, you will get an overview of useful search tools. More details on how to join the workshop will be announced by e-mail or contact the library team.

Energy Conversion Pathways in Graphite from Attosecond Soft X-ray Spectroscopy

  • PC Department Online Seminar
  • Date: Nov 23, 2020
  • Time: 11:00
  • Speaker: Jens Biegert
  • ICFO, Barcelona, Spain
The conversion of light to fundamental excitations of matter is governed by the build-up of electronic coherences and their dephasing to excited quasiparticles due to scattering processes, which occur on atto- and femtosecond timescales. Disentangling the interplay of these mechanisms, and how they lead to a specific flow of energy inside a material, is extremely challenging since many of these effects occur on overlapping temporal scales. [more]

FHI Library Online Workshop: Information Resources & Open Access

This interactive online workshop will introduce available options for making your research outputs open access. Discover which publishing fees are paid centrally or by the FHI library and get practical tips on how to adopt open access practices in your research area. Furthermore, you will get an overview of useful search tools. More details on how to join the workshop will be announced by e-mail or contact the library team.

Second-Harmonic Phonon Spectroscopy Using an Infrared Free-Electron Laser

  • PC Online Talk
  • Date: Nov 19, 2020
  • Time: 11:00
  • Speaker: Christopher Winta
  • FHI Department PC
Nonlinear optical spectroscopy constitutes a powerful tool for the investigation of crystalline solids and their structure. Apart from improved sensitivity compared to linear techniques, it offers additional experimental degrees of freedom which can be used to selectively study different symmetry components of the detected signal. The mid-infrared spectral region is particularly interesting as it contains optical phonon resonances which themselves carry symmetry information. [more]

Vom Energie- zum Nachhaltigkeitsmanagement - Die Freie Universität auf dem Weg zur Klimaneutralität

Die Freie Universität Berlin betreibt im eigenen institutionellen Verantwortungsbereich bereits seit zwei Jahrzehnten aktiven Umwelt- und Klimaschutz. 2001 gründete sie ein betriebliches Energiemanagement und entwickelte diese Aktivitäten schrittweise weiter: vom zertifizierten Umweltmanagement zu einem holistisch ausgerichteten Nachhaltigkeitsmanagement. Im Dezember 2019 hat die Freie Universität Berlin als erste deutsche Hochschule den Klimanotstand erklärt. Dieser beinhaltet die Verpflichtung, Nachhaltigkeit und Klimaschutz bei allen Entscheidungen und Planungen zu berücksichtigen und die Universität bis 2025 klimaneutral zu gestalten. [more]

Reference Management Systems

This course explains what reference management systems (also known as bibliographic or citation management software) are, why they are useful for any kind of research, and what to look out for when considering the use of one of the numerous available bibliographic management applications. The two systems, EndNote and Mendeley, are demonstrated as examples. More details on how to join the workshop will be announced by e-mail or contact the library team.

Bright and Dark Excitons in 2D Semiconductors and Heterostructures

  • PC Department Online Seminar
  • Date: Nov 9, 2020
  • Time: 11:00
  • Speaker: Alexander Högele
  • Fakultät für Physik, Munich Quantum Center, and Center for NanoScience (CeNS), Ludwig-Maximilians-Universität, München
Van der Waals crystals of transition metal dichalcogenide (TMD) semiconductors have evolved as an increasingly significant material platform for condensed matter research. They can be routinely exfoliated down to the monolayer limit or assembled into rationally designed vertical heterostructures. In response to optical excitations, both semiconductor systems host strongly bound excitons in various configurations of spin and valley degrees of freedom. [more]

Der Wert der Biodiversität - Konsequenzen & Praxis

Im 21. Jahrhundert sterben Tierarten um ein vielfaches schneller aus als vor Beginn der Industrialisierung. Der Grund: menschliche Aktivitäten. Wir befinden uns in einer Biodiversitätskrise. Doch was bedeutet Biodiversität eigentlich? Braucht der Mensch Biodiversiät? Und hat Biodiversität auch einen Eigenwert? Im Vortrag werde ich diese Fragen beantworten, über die Konsequenzen sprechen, die sich aus der Biodiversitätskrise ergeben und wie man sich wissenschaftlich damit auseinandersetzt. [more]

FHI Library Online Workshop: Databases

Our interactive online workshop will help you to select and apply the most appropriate resources and tools for your research. Learn more on how to quickly find and analyze relevant scientific literature. The two databases Web of Science and SciFinder will be presented and discussed in detail. More details on how to join the workshop will be announced by e-mail or contact the library team.

Momentum-resolved Interlayer Charge and Energy Transfer in a Monolayer WSe2/Graphene Heterostructure

  • PC Online Talk
  • Date: Oct 29, 2020
  • Time: 11:00
  • Speaker: Dr. Shuo Dong
  • FHI Department PC
Atomically thin layered van der Waals (vdW) heterostructures feature exotic and emergent optoelectronic properties.[1] With growing interest in these novel quantum materials, the microscopic understanding of the fundamental interlayer coupling is of capital importance.[2] Here, using four-dimensional photoemission spectroscopy, we provide a layer- and momentum-resolved view on ultrafast interlayer electron and energy transfer across a monolayer (ML)-WSe2/graphene heterostructure. [more]

The Key Role of Lattice Anharmonicity for Heat and Charge Transport in Solids: Fundamental Concepts, Novel Methods, and Relevant Applications

A Joint Seminar of the NOMAD Laboratory and of the Ma group
Heat and charge transport play a key role in materials science and thus for many technological applications that are key to establish a sustainable energy economy and ecology. Examples include improving the fuel-efficiency of aeronautic turbines [1], for developing efficient thermoelectric devices able to recover useful voltage from otherwise wasted heat [2], and for designing novel battery materials for advancing e-mobility [3]. [more]

FHI Library Online Workshop: Databases

Our interactive online workshop will help you to select and apply the most appropriate resources and tools for your research. Learn more on how to quickly find and analyze relevant scientific literature. The two databases Web of Science and SciFinder will be presented and discussed in detail. More details on how to join the workshop will be announced by e-mail or contact the library team.

Charge-carrier dynamics and optoelectronic properties of metal halide perovskites for solar cells

  • Online Seminar
  • Date: Oct 12, 2020
  • Time: 11:00
  • Speaker: Adam D. Wright
  • University of Oxford
Over the past decade, metal halide perovskites have become renowned for their rapidly improving performance as photovoltaic active layers. This success has however often outpaced understanding of the electronic processes occurring within these materials. In this talk, I will give an overview of my work on the factors influencing electron motion and recombination in a series of metal halide perovskites, ranging from the archetypal CH3NH3PbI3 to the double-perovskite Cs2AgBiBr6. Using a combination of optical spectroscopy with computational techniques, I have investigated electron-phonon coupling [1], trap-mediated recombination [2], halide segregation [3] and intrinsic quantum confinement [4] within these materials, developing quantitative models to explain these phenomena. [more]

Spectroscopic Signatures of Edge States in the Quantum Spin Hall System Bismuthene

  • PC Online Talk
  • Date: Oct 8, 2020
  • Time: 11:00
  • Speaker: Julian Maklar
  • FHI Department PC
Quantum spin Hall (QSH) systems are two-dimensional topological insulators with promising device applications due to dissipationless spin currents in their edges. However, most QSH systems realized so far have required cryogenic temperatures due to their small bulk gap sizes. A candidate for a high-temperature QSH material is Bismuthene, i.e., a monolayer of bismuth arranged in a honeycomb lattice on a silicon carbide substrate, which features a semiconducting band structure with an indirect band gap of 0.8 eV. [more]

ARES: A Real-Space Electronic Structure Calculation Method

A Joint Seminar of the NOMAD Laboratory and of the Ma group
[more]

FHI Library Online Workshop: Databases

Our interactive online workshop will help you to select and apply the most appropriate resources and tools for your research. Learn more on how to quickly find and analyze relevant scientific literature. The two databases Web of Science and SciFinder will be presented and discussed in detail. More details on how to join the workshop will be announced by e-mail or contact the library team.

Structural dynamics of singlet fission in pentacene single crystals

Singlet fission in organic molecular semiconductors has attracted significant attention for its relevance in opto-electronic applications. It is known that molecular packing strongly impacts singlet fission properties, and the structural dynamics are thought to play an essential role as well. Yet to this day, a direct probe of the molecular motions involved in the singlet fission process has been lacking. Here we directly observe the structural dynamics accompanying the singlet fission process in single crystal pentacene using femtosecond electron diffraction. [more]

Mapping Ultrafast Chemical Reaction Dynamics with Femtosecond Time-Resolved Time-Domain Raman Spectroscopy

  • Online Seminar
  • Date: Sep 28, 2020
  • Time: 11:00
  • Speaker: Prof. Hikaru Kuramochi
  • Institute for Molecular Science, Japan
In ordinary Raman spectroscopy, the laser source illuminates the sample, and the scattered light is collected, dispersed, and detected by a multi-channel detector yielding a Raman spectrum directly in the frequency domain. On the other hand, it is also possible to obtain a Raman spectrum by observing coherent molecular vibrations directly in the time domain using ultrashort pulses. [more]

Wave-optical Properties and Spatial Resolution in Point-projection Microscopy and Holography

  • PC Online Talk
  • Date: Sep 24, 2020
  • Time: 11:00
  • Speaker: Dr. Faruk Krecinic
  • FHI Department PC
Point-projection microscopy is an electron microscopy technique that uses a sharp metallic tip as a point source of electrons to project a magnified image of a sample, without any additional electron-optical lens elements. At sufficiently large magnification the projected image becomes an in-line hologram, which can be inverted to retrieve a real-space image of the sample object. Due to the use of low-energy electrons (typically <200 eV) this technique was shown to be capable of imaging a single elementary charge adsorbed on graphene, making it a sensitive probe of electric fields at the nanometer scale [1]. Moreover, photo-emitting the imaging electrons with an ultrafast pulsed laser enables the extension of PPM to the femtosecond domain, where it has demonstrated it can visualize the ultrafast dynamics of charge carriers with a combined spatial and temporal resolution of better than 100 nm and 30 fs, respectively [2,3]. [more]

What you get for free with Euclidean Neural Networks

Equivariance to Euclidean symmetry is a simple assumption with many consequences. In this talk, we show that Euclidean symmetry equivariant Neural Networks naturally inherit these consequences. [more]

FHI Library Online Workshop: Databases

Our interactive online workshop will help you to select and apply the most appropriate resources and tools for your research. Learn more on how to quickly find and analyze relevant scientific literature. The two databases Web of Science and SciFinder will be presented and discussed in detail. More details on how to join the workshop will be announced by e-mail or contact the library team.

FHI Library Online Workshop: Databases

Our interactive online workshop will help you to select and apply the most appropriate resources and tools for your research. Learn more on how to quickly find and analyze relevant scientific literature. The two databases Web of Science and SciFinder will be presented and discussed in detail. More details on how to join the workshop will be announced by e-mail or contact the library team.

The Evolution of Off-axis Electron Holography towards a Versatile TEM-Method

  • AC/PC Joint Seminar
  • Date: Sep 7, 2020
  • Time: 11:00
  • Speaker: Michael Lehmann
  • TU-Berlin Institut für Optik und Atomare Physik Electron Microscopy and -Holography
According to the original proposal by Dennis Gabor, electron holography was invented to overcome the electron-optical aberrations in the transmission electron microscope (TEM) by a-posteriori light optical correction. In order to fulfill Gabor’s dream, it took many decades and important developments like e.g. coherent electron sources, stable microscopes and rooms, fast CCD-cameras as well as dedicated computer software for reconstruction and correction. Nowadays with hardware aberration correctors, however, atomic resolution electron holography with a-posteriori correction of aberrations plays a less significant role. Meanwhile, the phase of the electron wave as carrier of information of electric potentials and/or magnetic fields in or around samples comes into focus for real-world applications pushing the development off-axis electron holography towards a method for measurements of these quantities on the nanometer scale. An interesting alternative electron-optical setup is dark-field off-axis electron holography for measurements of strain fields in solids. More recently, gating the interference fringe contrast by deliberately introducing of noise has open new developments towards time-resolved electron holography with a time-resolution in the nanosecond range. The talk will cover this evolution of electron holography up to the latest developments. [more]

Ultrafast Optical and Low-frequency Spectroscopy of Semiconductor Nano-systems and Quantum Materials

  • Online Seminar
  • Date: Aug 18, 2020
  • Time: 10:00
  • Speaker: Jit Sarkar
  • Indian Institute of Science Education and Research Kolkata
Ultrafast spectroscopic studies are always instructive to understand the physics of light-matter interactions. Within a few pico-seconds after photo-excitation, the excess energy of photo-excited carriers get transferred to the lattice and eventually they attain a state of equilibrium, a process known as ultrafast thermalization. Electron-phonon (e-ph) coupling play a pivotal role in this process. The process of ultrafast thermalization was investigated in an assembly of ZnO nanorods and the analysis from the framework of Two Temperature Model (TTM) yields prolonged thermalization time due to reduced electron-phonon coupling at higher photo-excited carrier density [1]. [more]

Triggering and Watching Water Electrolysis on Ultrafast Timescales

  • PC Online Talk
  • Date: Aug 13, 2020
  • Time: 15:00
  • Speaker: Gregor Zwaschka
  • FHI Department PC
For a greenhouse gas emission free society, hydrogen from water electrolysis is fundamental. Despite decades of study, the mechanism of the hydrogen evolution reaction (HER) on the best available catalyst, Pt, remains controversial. At least in part, understanding is complicated by not being able to disentangle the involved timescales ranging from femtoseconds (interfacial charge transfer) to milliseconds (mass transport).I approach this problem by performing perturbation experiments with ultrashort laser pulses that drive the HER close to its reversible potential and induce charge transfer along the Pt-H bond in the underpotential deposition region [1]. I disentangle the involved timescales by performing both i) time averaging electrochemical measurements under femtosecond laser irradiation and ii) time resolved sum frequency generation spectroscopy (SFS) of Pt-H after laser excitation.Ultrafast charge transfer is found to be a function of interfacial structure (electrode and electrolyte) and trends on single crystals and a microelectrode correlate with HER activity and allow insight into the reactions rate determining step. SFS provides information on Pt-H as a function of potential, electrolyte composition and temporal evolution during and after ultrafast charge transfer. Implications for the HER are discussed.[1] ChemElectroChem 2019, 6, 2675-2682 [more]

Towards materials data science – where high-throughput computation will meet high-throughput experimentation

  • Online Seminar
  • Date: Aug 12, 2020
  • Time: 15:30
  • Speaker: Patrick Xian
  • Northwestern University, USA
Constructing a materials discovery platform requires concerted efforts between multiple domains, including theory, experiments and the computational methods for synergetic exchanges in between [1]. Firstly, these exchanges require domain-informed, compact data representations and metrics to facilitate the high-throughput methodology and unite disparate fields associated with materials science. I discuss corresponding examples from electronic structure [2] and crystal structure [3] data. Secondly, existing materials characterization methods are not designed to scale up to macroscopic samples and batches, and the correspondence between multimodal measurements are often not exact [4], I discuss these existing limitations and propose solutions by co-designing experimental and computational workflows [5]. [1] M. Aykol et al. Matter 1, 1 (2019).[2] R. P. Xian, V. Stimper et al. arXiv:2005.10210.[3] C. J. Bartel et al. J. Am. Chem. Soc. 142, 5135 (2020).[4] T. L. Burnett and P. J. Withers, Nat. Mater. 18, 1041 (2019).[5] M. Du et al. under review. [more]

Subsystem- and material-resolved view of nonequilibrium states innanostructured metal/2D semiconductor heterostructures

Noble metal nanostructures allow to enhance and tune light absorption to efficientlyproduce plasmonic excitations, which couple strongly to two subsystems of excitations in the semiconductor: hot carriers and phonons. These excitations relax following complex pathways, unlocking numerous nanoplasmonic applications ranging from photocatalysis to photovoltaic. In this work, we distinguish charge carrier and phonon dynamics in a plasmonic metal/semiconductor heterostructure, with the combined use of time- and angle-resolved photoemission spectroscopy (trARPES) and femtosecond electron diffraction (FED). [more]

Quasi-2D Perovskite Semiconductors: Physical & Electronic Structure

  • Online Seminar
  • Date: Jul 31, 2020
  • Time: 14:00
  • Speaker: Dr. Rafael Quintero-Bermudez
  • Electrical and Computer Engineering, University of Toronto, Canada
Ruddlesden-Popper phase metal-halide perovskites (RPPs) have attracted significant attention in recent years due to their promising light harvesting and emissive properties. In the past 5 years alone, RPPs have enabled LEDs and solar cells with competitive operational stabilities and efficiencies. In my talk I will discuss my PhD work on the physical and electronic structure of RPPs. I will briefly discuss how RPPs are formed; how their kinetics of formation determine their composition and orientation; the implications of their composition and orientation; and how this understanding could be used to improve optoelectronic devices. I will then discuss our work on the band alignment of RPPs in the context of a disagreement within the metal-halide perovskite community; and the implications of this band alignment on charge transfer and optoelectronic devices. Ultimately, our work could help improve understanding of this material class, and support research to improve device performance and stability. References:R. Quintero-Bermudez et al., J. Phys. Chem. Lett., 11, 4213 (2020).R. Quintero-Bermudez et al, J. Am. Chem. Soc., 141, 13459 (2019).R. Quintero-Bermudez et al., Nature Materials, 17, 900 (2018). [more]

Polarization Dressed Light Propagation in Lead Halide Perovskites in Lead Halide Perovskites

The ultrafast polarization response to incident light and ensuing electronic excitations are essential to the outstanding optoelectronic properties of lead halide perovskites (LHPs). In recent studies, a dynamically disordered structure and anharmonic crystal lattice was suggested to be a key component for LHPs’ complex polarization dynamics1,2. In this work, we develop a novel type of two-dimensional spectroscopy to spectrally resolve and disentangle contributions to the ultrafast Kerr-effect in MAPbBr3 and its all-inorganic counterpart CsPbBr3. This technique allows us to energetically dissect broadband light propagation and dispersive polarization responses in the vicinity of the electronic bandgap. Light propagation in LHPs is in particular technologically relevant for solar cell, light modulation and LED applications due to stimulated emission, polariton condensation and photon recycling which may take place in the investigated spectral region3,4. In both LHPs, we find intense nonlinear mixing of anistropically propagating light fields, resulting in an oscillatory polarization response, which strongly depends on the crystallographic phase and the position of the electronic bandgap. We further exploit temperature-dependent two-dimensional optical Kerr-effect (2D-OKE) fingerprints to quantify the dispersion anisotropy via an analytic model. In addition to revealing highly dispersive anisotropic light propagation and its nonlinear mixing, this study finally establishes a unified origin of ultrafast Kerr responses in single crystal LHPs near the optical bandgap. 1. Miyata, K. et al. Sci Adv 3, e1701217 (2017). 2. Zhu, H. et al. Science 353, 1409 (2016). 3. Pazos-Outón, L. M. et al. Science 351, 1430 (2016). 4. Su, R. et al. Nature Physics 16, 301–306 (2020). [more]

Ultrafast spectroscopy studies of hybrid nanomaterials: from lead halide perovskites to J-aggregate/gold nanoparticles

  • Special Seminar
  • Date: Jul 28, 2020
  • Time: 11:00
  • Speaker: Victoria C. A. Taylor
  • School of Chemistry, University of Bristol
The first part of my talk will focus on ultrafast two-dimensional infrared (2DIR) and transient infrared (TRIR) spectroscopic studies of formamidinium lead iodide perovskite films. High absorption coefficients, high carrier mobilities, and low charge recombination rates, despite low-cost solution-phase synthesis,1 have led to intense research into hybrid organic-inorganic halide perovskites for photovoltaic applications. Hybrid organic-inorganic halide perovskites consist of organic cations caged within an inorganic lattice of metal and halogen atoms. Several studies have proposed that the organic cations play a pivotal role in the high charge separation efficiencies of these materials2 by orientating within the inorganic lattice to form (anti)ferroelectric domains at room temperature.3 It has been proposed that these domains act to channel opposite charges away from one another, thereby reducing charge recombination. 2DIR anisotropy measurements were used to interrogate the reorientation of formamidinium cations (FA+, NH2CHNH2+) in formamidinium lead iodide perovskite films. These rotational anisotropy measurements returned 470 ± 50 fs and 2.8 ± 0.5 ps time constants, meaning that any initial alignment of FA+ molecules is very short lived, casting significant doubt on the presence of long-lived (anti)ferroelectric domains.4 TRIR measurements revealed a prominent vibrational transient feature arising from a vibrational Stark shift: photogenerated charge carriers increase the internal electric field of perovskite thin films, perturbing the FA+ antisymmetric stretching vibrational potential, resulting in an observed 5 cm–1 shift. This observation suggests the formation of large polarons, which may explain the observed long charge-carrier lifetimes despite inhomogeneous microscopic morphologies. In the second part of this talk I will present data from recent ultrafast transient absorption studies of J-aggregate-hollow gold nanoshells hybrids. These hybrid structures allow for the exploration of coupling between excitons (J-aggregates) and localised surface plasmon resonances (LSPR) on gold nanoparticles.5 Through my wavelength dependent studies of various samples, I have revealed that the coupling between the exciton and plasmon resonance is faster than my instrument response (25 fs) and the ‘true’ transient spectrum of the hybrid particle. (1) Wright, A. D.; Verdi, C.; Milot, R. L.; Eperon, G. E.; rez-Osorio, M. A. P. E.; Snaith, H. J.; Giustino, F.; Johnston, M. B.; Herz, L. M. Electron-Phonon Coupling in Hybrid Lead Halide Perovskites. Nat. Commun. 2016, 7, 1–9. (2) Gélvez-Rueda, M. C.; Cao, D. H.; Patwardhan, S.; Renaud, N.; Stoumpos, C. C.; Schatz, G. C.; Hupp, J. T.; Farha, O. K.; Savenije, T. J.; Kanatzidis, M. G.; et al. Effect of Cation Rotation on Charge Dynamics in Hybrid Lead Halide Perovskites. J. Phys. Chem. C 2016, 120 (30), 16577–16585. (3) Frost, J. M.; Butler, K. T.; Brivio, F.; Hendon, C. H.; van Schilfgaarde, M.; Walsh, A. Atomistic Origins of High-Performance in Hybrid Halide Perovskite Solar Cells. Nano Lett. 2014, 14 (5), 2584–2590. (4) Taylor, V. C. A.; Tiwari, D.; Duchi, M.; Donaldson, P. M.; Clark, I. P.; Fermin, D. J.; Oliver, T. A. A. Investigating the Role of the Organic Cation in Formamidinium Lead Iodide Perovskite Using Ultrafast Spectroscopy. J. Phys. Chem. Lett. 2018, 9 (4), 895–901. (5) Faucheaux, J. A.; Fu, J.; Jain, P. K. Unified Theoretical Framework for Realizing Diverse Regimes of Strong Coupling Between Plasmons and Electronic Transitions. J. Phys. Chem. C 2014, 118 (5), 2710–2717. [more]

Addressing the Surface Composition and Structure under Catalytic (T, p) Conditions

A Joint Seminar of the NOMAD Laboratory and of the Ma group
A prerequisite for reaching a microscopic understanding of heterogeneous catalytic is the identification of the catalyst surface composition and structure under catalytic (T, p) conditions. For decades, ab initio atomistic thermodynamic (aiAT) has been very successful in predicting phase diagrams for surfaces at realistic (T, p) conditions. [more]

Sailing transient k-space – Is there a perfect spectrometer for time-resolved ARPES

  • PC Online Talk
  • Date: Jul 9, 2020
  • Time: 15:00
  • Speaker: Dr. Laurenz Rettig
  • FHI Department PC
Time- and angle-resolved photoelectron spectroscopy (trARPES) is a very powerful technique to investigate the transient electronic band structure and the fundamental scattering processes in solid state materials, combining the direct momentum- and energy-resolved view on the electronic structure provided by ARPES with the additional dimension of femtosecond time resolution. This also opens up a new horizon compared to conventional ARPES, namely the spectroscopy of formerly unoccupied states above the chemical potential, which can be transiently populated and subsequently studied. Challenges posed by the necessary extreme ultraviolet (XUV) photon energy to cover the whole Brillouin zone (BZ) in electron momenta have been tackled by new laser developments, allowing for operation at high repetition rates and providing the necessary high sensitivity to such unoccupied states throughout the BZ [1,2]. Combining such advanced laser sources with the recently developed time-of-flight based momentum microscopes, which promise a huge improvement in parallel detection efficiency and allow for the simultaneous detection of multiple BZ without the need to rearrange the sample geometry [3] seems like an ideal match.Recently, we upgraded our OPCPA-driven high-repetition rate XUV setup at the FHI, which now combines both a hemispherical analyzer and a time-of-flight momentum microscope (SPECS Metis 1000) in the same experimental chamber. In my talk, I will present the various possibilities enabled by the new instrumentation, like movies of the whole transient Fermi surface, observation of anisotropic scattering dynamics, analysis of dichroism in the momentum distribution and more. In addition, I will quantify the advantages and limitations of both hemispherical analyzer and momentum microscope for certain use cases in the field of trARPES and discuss the advantage of combining both types of instruments within a single experimental apparatus. [1] M. Puppin, et al., Opt. Express 23, 1491 (2015)[2] M. Puppin, et al., Rev. Sci. Instrum. 90, 023104 (2019).[3] G. Schönhense, et al., J. Electron Spectrosc. Relat. Phenom. 200, 94–118 (2015). [more]

Ultrafast lattice dynamics of 3d ferromagnets

  • PC Online Talk
  • Date: Jul 2, 2020
  • Time: 15:00
  • Speaker: Daniela Zahn
  • FHI Department PC
The response of ferromagnets to laser excitation is governed by the interplay of electronic, magnetic and lattice degrees of freedom. In the case of 3d ferromagnets, strong coupling between electrons and spins leads to ultrafast demagnetization on femtosecond time scales. The lattice plays an important role in the magnetization dynamics, since it drains energy from the electrons on similar timescales and absorbs angular momentum from the spin system. Here, we study the lattice response of the 3d ferromagnets nickel, iron and cobalt directly using femtosecond electron diffraction (FED). To learn more about the energy flow between electrons, spins and the lattice, we compare the experimental results to spin-resolved DFT calculations combined with energy flow models. We incrementally increase the complexity of these models in 3 steps: While the commonly adopted two-temperature model (TTM) cannot describe our experimental results, we find excellent agreement using a modified TTM that assumesstrong coupling between electrons and spins. In the next step, we discuss how atomistic spin dynamics (ASD) simulations can be employed for a more accurate description of the spin system in out-of-equilibrium conditions. The ASD simulation results for nickel maintain the excellent agreement to the lattice dynamics while yielding a much more consistent description of the dynamics of the system. Our results suggest that the energy cost of ultrafast demagnetization has a strong effect on the lattice dynamics. [more]

Terahertz spin dynamics in Mn2Au driven by the Neel spin-orbit torque

Neel spin-orbit torque (NSOT) is a novel tool for spin manipulation in antiferromagnets with special symmetry (CuMnAs, Mn2Au). As shown experimentally, application of current through such materials can lead to switching of the Neel vector. With clear opportunity of high-speed control of antiferromagnetic ordering, there were yet no investigations of NSOT time-dynamics in the terahertz range.This talk will present our recent results in measuring ultrafast spin dynamics in Mn2Au following application of free-space terahertz pulses. The data indicates that the THz-induced NSOT acts on Mn2Au spins and launches an 0.6 THz antiferromagnetic resonance mode. [more]

Phonon Polaritons in Polar Dielectric Heterostructures

  • PC Online Talk
  • Date: Jun 18, 2020
  • Time: 15:00
  • Speaker: Nikolai Paßler
  • FHI Department PC
The field of nanophotonics aims at understanding and harnessing light-matter interaction in structures of dimensions far below the wavelength, enabling applications such as highly efficient sensing or all-optical integrated circuitry. The fundamental excitation driving nanophotonics is the surface polariton, arising in different types depending on the supporting material. A promising candidate for applications at infrared frequencies is the surface phonon polariton (SPhP) supported by polar crystals. However, a SPhP on a single polar crystal possesses several limitations that hinder the application in nanophotonic technologies.This work implements layered heterostructures built from various materials as a versatile platform for phonon polariton nanophotonics, overcoming the limitations of a conventional SPhP. By studying a variety of different polar crystal heterostructures, novel polariton modes with intriguing characteristics are discovered, such as ultra-thin film modes with immense field enhancements, strongly coupled polaritons at epsilon-near-zero frequencies, and waveguide modes with polariton-like properties. [more]

Dramatic enhancement of Raman scattering and Raman thermometryin electrically-fused junctions

The ability to precisely design Å-scale plasmonic cavities has boosted the sensitivity and spatial resolution of surface- and tip-enhanced Raman scattering (SERS and TERS). In this context, low-temperature scanning probe microscopy (LT-SPM) offers great advantages to perform nanoscale vibrational spectromiscroscopy (TER-SM). Along with nanofabrication techniques of plasmonic tips, LT-SPM now allows to examine light–matter interactions in plasmonic “picocavities” down to the sub-molecular level. However, the underlying mechanisms behind the large enhancement factors present in such cavities remain unclear. We reveal how TERS evolves at vanishing tip–sample distances including the transition from a tunneling to conductive coupled regime. Upon atomic-point contact (APC) formation, a dramatic TERS enhancement is observed. In order to shed light on the mechanisms behind, we examined different model systems: an Ag tip with ultrathin ZnO films and single C molecules on the Au(111), Ag(111), and Cu(111) surfaces at 10 K. A pronounced electromagnetic enhancement of Raman scattering is commonly observed for a few Å gaps. The sudden increase of the TERS intensity upon APC formation is attributedto the chemical interaction between the tip and the sample which provides additional charge transfer enhancement. Furthermore, intense anti-Stokes signals can be observed, allowing us to perform Raman thermometry in electrically-fused plasmonic junctions. The results reveal pronounced non-thermal contributions, which underlines the necessity to better understand atomic-scale light–matter interactions. [more]

Deterministic Control of an Antiferromagnetic Spin Arrangement using Ultrafast Optical Excitation

A central prospect of antiferromagnetic spintronics is to exploit magnetic properties that are unavailable with ferromagnets. However, this poses the challenge of accessing such properties for readout and control. To this end, light-induced manipulation of the transient ground state, e.g. by changing the magnetic anisotropy potential, opens promising pathways towards ultrafast deterministic control of antiferromagnetism. In this talk I will show how we use this approach to trigger a coherent rotation of the entire long-range antiferromagnetic spin arrangement about a crystalline axis in GdRh2Si2 and demonstrate deterministic control of this rotation upon ultrafast optical excitation. I will also show that our observations can be explained by a displacive excitation of the Gd spins' local anisotropy potential by the optical excitation, allowing for a full description of this transient magnetic anisotropy potential. See also: https://arxiv.org/abs/2002.01398 [more]

Reference Management Systems

This course explains what reference management systems (also known as bibliographic or citation management software) are, why they are useful for any kind of research, and what to look out for when considering the use of one of the numerous available bibliographic management applications. The two systems, EndNote and Mendeley, are demonstrated as examples. More details on how to join the workshop will be announced by e-mail or contact the library team.

Real-time Rotation of Encapsulated Ortho and Para Water in Fullerene-C60

Copious physical, chemical and thermodynamic properties make water a unique material. For instance, it is known that confined water in nano-cages behaves differently from bulk water. Recent studies even indicate on quantum behavior and incipient ferroelectricity of water in nano-cages. To further study the behavior of confined water molecules, we use H2O@C60 system: encapsulated single water molecule in fullerene-C60 and study the distinct rotational dynamics of water’s spin isomers at cryogenic temperatures. We employ single-cycle terahertz (THz) pulses to coherently excite the low-frequency rotational motion of ortho- and para-water. The excitation leads to the slight orientation of water’s permanent dipoles towards the field polarization and consequently to the emission of electromagnetic waves, which we resolve via the field-free electro-optic sampling technique. We discuss our results on the real-time conversion of ortho- to para-water at 4 K and further show the direct impact of temperature on rotational degrees of freedom of entrapped water inside its cage. [more]

Surface Chemistry Spanning Surface Composition and Surface Structure

Catalytic surface chemistry is determined, to a large extent, by catalyst surface composition and surface structure. In the case of metallic catalysts, this translates to alloy surface composition and crystallographic surface orientation. [more]

Next Generation Extended Lagrangian First Principles Molecular Dynamics

Quantum-based Born-Oppenheimer molecular dynamics (QMD) simulations, where the interatomic forces are calculated on the fly from a relaxed quantum-mechanical description of the electronic structure in each time step, is often considered the gold standard for molecular dynamics simulations. [more]

Phase-resolved Detection and Control of Ultrabroadband THz Pulses coupled to an STM Junction

Coupling phase-stable single-cycle terahertz (THz) pulses to scanning tunneling microscope (STM) junctions enables spatio-temporal imaging with femtosecond temporal and Ångstrom spatial resolution. The time resolution achieved in such THz-gated STM is ultimately limited by the sub-cycle temporal variation of the tip-enhanced THz field acting as an ultrafast voltage pulse, and hence by the ability to feed high-frequency, broadband THz pulses into the junction. In this talk, I will present our results on the coupling of ultrabroadband (1-30 THz) single-cycle THz pulses from a spintronic THz emitter (STE) into a metallic STM junction. We demonstrate broadband phase resolved detection of the tip-enhanced THz waveform via THz-field-induced modulation of ultrafast photocurrents across the junction. Comparison to the unperturbed far-field THz waveform reveals the antenna response of the STM tip. Despite tip-induced low-pass filtering, frequencies up to 15 THz can be detected in the enhanced near-field, resulting in THz transients with a half-cycle period of 115 fs. Moreover, versatile phase and polarity control of the THz waveform can be achieved via the STE excitation conditions and magnetization, and few Volts THz bias at 1 MHz repetition rate can be reached in the current setup. Finally, we find a nearly constant THz voltage and waveform over a wide range of tip-sample distances, which by comparison to numerical simulations confirms the quasi-static nature of the THz pulses. Our results demonstrate the suitability of spintronic THz emitters for ultrafast THz-STM and provide insight into the femtosecond response of defined nanoscale junctions. [more]

Ultrafast Light-Induced Lifshitz Transition

Fermi surface is at the heart of our understanding of the properties of metals and strongly correlated many-body systems. An abrupt change in the Fermi surface topology, also called Lifshitz transition, can leads to the emergence of fascinating phenomena like colossal magnetoresistance and superconductivity. While Lifshitz transitions have been demonstrated for a broad range of materials using equilibrium tuning of macroscopic parameters like strain, doping, pressure, and temperature, a nonequilibrium route toward ultrafast and transient switching of the Fermi surface topology has not been demonstratedyet. Using time-resolved multidimensional photoemission spectroscopy combined with TDDFT+U simulations, we demonstrate a scheme based on ultrafast laser-driven band renormalization that drives a Lifshitz transition in the topological type-II Weyl semimetal Td-MoTe2, due to transient modification of effective electron-electron interactions. [more]

Layered materials beyond graphene – new possibilities and applications

Beyond graphene, which is intensively studied over more than one decade, the other related materials remain almost unexplored. The research activities in the field of other layered materials like phosphorene, arsenene, silicene and germanene are rapidly growing in the last few years. Compare to graphene, all these materials are non-zero band-gap semiconductors. This property opens new application possibilities in electronic and optoelectronic devices. The properties of 2D materials can be further controlled by their functionalization. The chemistry of materials beyond graphene is none explored and shows high application potential in many fields. Compare to the graphene and pnictogen group, the chemical exfoliation method mast be applied for synthesis of silicene / germanene derivatives using Zintl phase compounds like CaGe2 and CaSi2. Various methods well know from organic chemistry can be applied for synthesis of tetrel derivatives reaching almost complete derivatization of 2D material skeleton. [more]

Tracking signatures of quantum geometry in time- and angle-resolved photoemission spectroscopy

Angle-resolved photoemission spectroscopy (ARPES) is one of the most powerful tools to study the electronic properties of solids. Besides providing a wealth of information on the momentum-dependent band structure, the impressive progress in high-resolution and multi-dimensional ARPES allows insights into the nature of the quantum states in the solid itself. [more]

Time-resolved ARPES at 88 MHz repetition rate with full 2π collection - Video Conference (for conference link write to Ralph Ernstorfer)

Angle-resolved photoemission spectroscopy (ARPES) is often considered the best way to experimentally determine the ground-state electronic structure of materials. However, although applying ARPES to short-lived excited states via the pump/probe method (tr-ARPES) demands orders of magnitude more data than ground-state ARPES studies, measurements have been forced to work with orders of magnitude lower data rates due to the limits imposed by the repetition rate of available short-pulse extreme-ultraviolet (XUV) light sources and the collection efficiency of photoelectron analyzers. [more]

Ultrafast Science and Technologies at ALLS

The Advanced Laser Light Source (ALLS) is located at INRS-ÉMT near Montreal. It is the national laser facility of Canada offering access to a variety of laser systems and secondary sources. [more]

Quantum sensor networks as exotic field telescopes for multi-messenger astronomy

Multi-messenger astronomy, the coordinated observation of different classes of signals originating from the same astrophysical event, provides a wealth of information about astrophysical processes with far-reaching implications. [more]

Establishing a New Canadian IR-FEL Program of Research

The Canadian research community is planning to create a new national program for IR-FEL-based research. [more]

Single-molecule chemistry via noncontact atomic force microscopy: ultrahigh resolution imaging and tip-induced switching

Single-molecule chemistry [1] has progressed together with the development of scanning probe microscopy and its related methods. Scanning tunneling microscopy (STM) has been widelyused for the observation and control of configurational changes and reactions for individual molecules on surfaces. [more]

Coffee Talk No. 10: Building automation at the FHI

PP&B provides the software and infrastructure for building automation at the FHI. Operation and maintenance is carried out by the building services department (HT). Examples are given of how a well networked building automation can contribute to sustainability (energy) and quality improvement in scientific experiments.

Nanomaterials with their specific properties for applications in biomedicine and electronics

The content of my lecture are the results of my research carried out in recent years and educational activities on the preservation of molecular matter condensed in nano-systems and their practical applications using their specific properties in biomedicine and electronics. [more]
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