Ultrafast Energy Flow and Structural Changes in Nanoscale Heterostructures

A central goal of nanotechnology is the precise construction of nanoscale heterostructures with optimized chemical, physical or biological functionalities. It is known that function stems from structure but, in addition, function always involves nonequilibrium conditions and energy flow. The central topic of this thesis is the ultrafast energy flow in nanoscale heterostructures and how this energy flow drives ultrafast structural changes. The main experimental technique of this work is femtosecond electron diffraction, which probes the lattice response to electronic excitations. The nanoscale heterostructures contain metallic (Au) nanostructures of well-defined 0D or 2D morphology, supported on 2D substrates. In photoexcited heterostructures, thermal equilibrium is restored by electron-lattice interactions, within each component, and electronic and vibrational coupling across their interface. A newly developed model of ultrafast energy flow is used to measure the microscopic couplings, like electron-phonon coupling and interfacial vibrational coupling in nanoscale heterostructures using the observed Debye-Waller dynamics. Ultrafast energy flow in supported metallic nanostructures can initiate a rich variety of real-space motions like anharmonic lattice expansion and surface premelting, which manifest as distinct and quantifiable observables in reciprocal-space. These phenomena have been studied for Au nanoclusters on amorphous thin-film substrates. Au nanoclusters are found to exhibit ultrafast surface premelting at atypically low lattice temperatures and pronounced electron-lattice nonequilibrium conditions. Femtosecond electron diffraction is mostly used to study ultrafast motions related with phonons but in ultrasmall nanocrystals a new observable arises: the motion of the phonons’ frame of reference, meaning the crystal itself. This has been demonstrated for Au nanoclusters attached on graphene using femtosecond electron diffraction experiments, molecular dynamics and electron diffraction simulations. The substrate has a significant effect on the energy flow and the structural motions of ultrasmall, adsorbed nanostructures and, inversely, metallic nanostructures can alter fundamental properties of semiconducting substrates. Surface decoration with plasmonic, quasi-2D nanoislands of Au sensitizes WSe2 to sub-band-gap photons, causes nonlinear lattice heating and accelerates electron-phonon equilibration times. Conclusively, nanoscale heterostructures have a rich variety of nonequilibrium phenomena that affect their structure at ultrafast timescales. Ultrafast diffractive probes, like femtosecond electron diffraction, can provide a detailed, quantitative understanding of this relationship.In dieser Doktorarbeit wird der ultraschnelle Energietransfer in nanoskaligen Heterostrukturen sowie die dadurch verursachten ultraschnellen Strukturänderungen untersucht. Die wichtigste Methode dieser Arbeit ist Femtosekunden-Elektronenbeugung. Diese Methode untersucht die Reaktion des Kristallgitters auf elektronische Anregung. Die Heterostrukturen bestehen aus Gold-Nanostrukturen mit wohldefinierten 0D oder 2D Strukturen, die auf 2D Substraten aufgebracht sind. In mit Licht angeregten Heterostrukturen wird das thermische Gleichgewicht durch Elektron-Phonon-Kopplung in den einzelnen Materialien sowie durch elektronische und phononische Kopplung zwischen den Materialien wiederhergestellt. Ein neu eingeführtes Modell für ultraschnellen Energietransfer wird verwendet, um die ultraschnellen Veränderungen der Gittertemperatur zu beschreiben. Das Modell ermöglicht es, aus der gemessenen Debye-Waller-Dynamik mikroskopische Größen wie Elektron-Phonon-Kopplung und Phonon-Phonon-Kopplung an der Grenzfläche der nanoskaligen Heterostrukturen zu extrahieren. Ultraschneller Energietransfer in metallischen Nanostrukturen können eine Vielzahl an Veränderungen im Kristallgitter hervorrufen, z.B. Gitterausdehnung und Schmelzen der Kristalloberfläche. Diese Veränderungen gemessen werden, für 0D Gold Nanostrukturen die auf 2D Substraten aufgebracht sind. Au-Nanocluster zeigen ultraschnelles Schmelzen der Kristalloberfläche bei außergewöhnlich niedrigen Gittertemperaturen und ausgeprägtem Nichtgleichgewichtszustand zwischen Elektronen und Gitter. Femtosekunden Elektronen Beugung ist eine Methode, die am häufigsten bei der Untersuchung durch Phonen induzierter ultraschneller Bewegungen von Atomen Anwendung findet. In ultrakleinen Nanokristallen stellt sich aber ein neue Herausforderung dar: der Referenzrahmen der Bewegung der Phononen, was der Kristall selber ist. Demonstriert wurde das für Gold 0D Nanostructuren, die auf Graphen. Das Substrat hat einen signifikanten Einfluss auf den Energiefluss und die strukturelle Bewegung von ultrakleinen, adsorbierten Nanostrukturen und in inverser Weise können metallische Nanostrukturen due fundamentalen Eigenschafter halbleitender Proben verändern. Wenn WSe2 mit plasmonische quasi-2D Gold-Nanoinseln bedeckt wird, ändern sich dessen Eigenschaften so, dass Photonen unterhalb der Bandlücke absorbiert werden können. Die resultierende Erwärmung des Gitters folgt einem nichtlinearen Zusammenhang mit der Fluenz des einkoppelnden Lasers und die Elektron-Gitter Relaxationszeit ist reduziert

Momentum-Resolved View of Electron-Phonon Coupling in Multilayer WSe2_2

We investigate the interactions of photoexcited carriers with lattice vibrations in thin films of the layered transition metal dichalcogenide (TMDC) WSe$_2$. Employing femtosecond electron diffraction with monocrystalline samples and first principle density functional theory calculations, we obtain a momentum-resolved picture of the energy-transfer from excited electrons to phonons. The measured momentum-dependent phonon population dynamics are compared to first principle calculations of the phonon linewidth and can be rationalized in terms of electronic phase-space arguments. The relaxation of excited states in the conduction band is dominated by intervalley scattering between $\Sigma$ valleys and the emission of zone-boundary phonons. Transiently, the momentum-dependent electron-phonon coupling leads to a non-thermal phonon distribution, which, on longer timescales, relaxes to a thermal distribution via electron-phonon and phonon-phonon collisions. Our results constitute a basis for monitoring and predicting out of equilibrium electrical and thermal transport properties for nanoscale applications of TMDCs

Photoinduced ultrafast transition of the local correlated structure in chalcogenide phase-change materials

Revealing the bonding and time-evolving atomic dynamics in functional materials with complex lattice structures can update the fundamental knowledge on rich physics therein, and also help to manipulate the material properties as desired. As the most prototypical chalcogenide phase change material, Ge2Sb2Te5 has been widely used in optical data storage and non-volatile electric memory due to the fast switching speed and the low energy consumption. However, the basic understanding of the structural dynamics on the atomic scale is still not clear. Using femtosecond electron diffraction, structure factor calculation and TDDFT-MD simulation, we reveal the photoinduced ultrafast transition of the local correlated structure in the averaged rock-salt phase of Ge2Sb2Te5. The randomly oriented Peierls distortion among unit cells in the averaged rock-salt phase of Ge2Sb2Te5 is termed as local correlated structures. The ultrafast suppression of the local Peierls distortions in individual unit cell gives rise to a local structure change from the rhombohedral to the cubic geometry within ~ 0.3 ps. In addition, the impact of the carrier relaxation and the large amount of vacancies to the ultrafast structural response is quantified and discussed. Our work provides new microscopic insights into contributions of the local correlated structure to the transient structural and optical responses in phase change materials. Moreover, we stress the significance of femtosecond electron diffraction in revealing the local correlated structure in the subunit cell and the link between the local correlated structure and physical properties in functional materials with complex microstructures

Thickness-dependent elastic softening of few-layer free-standing MoSe2

Few-layer van der Waals (vdW) materials have been extensively investigated in terms of their exceptional electronic, optoelectronic, optical, and thermal properties. Simultaneously, a complete evaluation of their mechanical properties remains an undeniable challenge due to the small lateral sizes of samples and the limitations of experimental tools. In particular, there is no systematic experimental study providing unambiguous evidence on whether the reduction of vdW thickness down to few layers results in elastic softening or stiffening with respect to the bulk. In this work, micro-Brillouin light scattering is employed to investigate the anisotropic elastic properties of single-crystal free-standing 2H-MoSe as a function of thickness, down to three molecular layers. The so-called elastic size effect, that is, significant and systematic elastic softening of the material with decreasing numbers of layers is reported. In addition, this approach allows for a complete mechanical examination of few-layer membranes, that is, their elasticity, residual stress, and thickness, which can be easily extended to other vdW materials. The presented results shed new light on the ongoing debate on the elastic size-effect and are relevant for performance and durability of implementation of vdW materials as resonators, optoelectronic, and thermoelectric devices

Traversing double-well potential energy surfaces: photoinduced concurrent intralayer and interlayer structural transitions in XTe2 (X=Mo, W)

Manipulating crystal structure and the corresponding electronic properties in quantum materials provides opportunities for the exploration of exotic physics and practical applications. Here, by ultrafast electron diffraction, structure factor calculation and TDDFT-MD simulations, we report the photoinduced concurrent intralayer and interlayer structural transitions in the Td and 1T' phase of XTe2 (X=Mo, W). Concomitant with the interlayer structural transition by shear displacement, the ultrafast suppression of the intralayer Peierls distortion within 0.3 ps is demonstrated and attributed to Mo-Mo (W-W) bond stretching. We discuss the modification of multiple quantum electronic states associated with the intralayer and interlayer structural transitions, such as the topological band inversion and the higher-order topological state. The twin structure and the stacking fault in XTe2 are identified by the ultrafast structural response. Our work elucidates the pathway of the photoinduced intralayer and interlayer structural transitions in atomic and femtosecond spatiotemporal scale. Moreover, the concurrent intralayer and interlayer structural transitions reveals the traversal of all double-well potential energy surfaces (DWPES) by laser excitation in material system, which may be an intrinsic mechanism in the field of photoexcitation-driven symmetry engineering, beyond the single DWPES transition model and the order-disorder transition model

Observation of Multi-Directional Energy Transfer in a Hybrid Plasmonic–Excitonic Nanostructure

Hybrid plasmonic devices involve a nanostructured metal supporting localized surface plasmons to amplify light–matter interaction, and a non-plasmonic material to functionalize charge excitations. Application-relevant epitaxial heterostructures, however, give rise to ballistic ultrafast dynamics that challenge the conventional semiclassical understanding of unidirectional nanometal-to-substrate energy transfer. Epitaxial Au nanoislands are studied on WSe2 with time- and angle-resolved photoemission spectroscopy and femtosecond electron diffraction: this combination of techniques resolves material, energy, and momentum of charge-carriers and phonons excited in the heterostructure. A strong non-linear plasmon–exciton interaction that transfers the energy of sub-bandgap photons very efficiently to the semiconductor is observed, leaving the metal cold until non-radiative exciton recombination heats the nanoparticles on hundreds of femtoseconds timescales. The results resolve a multi-directional energy exchange on timescales shorter than the electronic thermalization of the nanometal. Electron–phonon coupling and diffusive charge-transfer determine the subsequent energy flow. This complex dynamics opens perspectives for optoelectronic and photocatalytic applications, while providing a constraining experimental testbed for state-of-the-art modelling

Attosecond core-level spectroscopy reveals the flow of excitation in a material between light, carriers and phonons

© 2022 IEEE. Personal use of this material is permitted. Permission from IEEE must be obtained for all other uses, in any current or future media, including reprinting/republishing this material for advertising or promotional purposes,creating new collective works, for resale or redistribution to servers or lists, or reuse of any copyrighted component of this work in other works.We use attosecond core-level X-ray spectroscopy to disentangle the spectral and dynamical signatures of energy conversion pathways between photons, charge carriers and the lattice in graphite with attosecond precision and across a picosecond range.Peer ReviewedArticle signat per 19 autors/es: T.P.H. Sidiropoulos1*, N. Di Palo1, D.E. Rivas1,2, S. Severino1, M. Reduzzi1, B. Nandy1, B. Bauerhenne3, S. Krylow3, T. Vasileiadis4, T. Danz5, P. Elliott6,7, S. Sharma6, K. Dewhurst7, C. Ropers5, Y. Joly8, K. M. E. Garcia3, M. Wolf4, R. Ernstorfer4, J. Biegert1,9 // 1 ICFO - Institut de Ciencies Fotoniques, The Barcelona Institute of Science and Technology, 08860 Castelldefels (Barcelona), Spain; 2 European XFEL GmbH, Holzkoppel 4, 22869 Schenefeld, Germany; 3 Theoretische Physik, FB-10, Universität Kassel, 34132 Kassel, Germany; 4 Fritz Haber Institute of the Max Planck Society, Berlin, Germany; 5 4th Physical Institute - Solids and Nanostructures, University of Göttingen, Germany; 6 Max-Born-Institut für Nichtlineare Optik und Kurzzeitspektroskopie, 12489 Berlin, Germany; 7 Max-Planck-Institut für Mikrostrukturphysik, Weinberg 2, 06120 Halle, Germany; 8 Université Grenoble Alpes, CNRS, Grenoble INP, Institut Néel, 38000 Grenoble, France; 9 ICREA - Institució Catalana de Recerca i Estudis Avançats, Barcelona, Spain // * present address: Max-Born-Institut für Nichtlineare Optik und Kurzzeitspektroskopie, 12489 Berlin, GermanyPostprint (author's final draft

Lattice dynamics and ultrafast energy flow between electrons, spins, and phonons in a 3d ferromagnet

The ultrafast dynamics of magnetic order in a ferromagnet are governed by the interplay between electronic, magnetic, and lattice degrees of freedom. In order to obtain a microscopic understanding of ultrafast demagnetization, information on the response of all three subsystems is required. A consistent description of demagnetization and microscopic energy flow, however, is still missing. Here, we combine a femtosecond electron diffraction study of the ultrafast lattice response of nickel to laser excitation with ab initio calculations of the electron-phonon interaction and energy-conserving atomistic spin dynamics simulations. Our model is in agreement with the observed lattice dynamics and previously reported electron and magnetization dynamics. Our approach reveals that the spin system is the dominating heat sink in the initial few hundred femtoseconds and implies a transient nonthermal state of the spins. Our results provide a clear picture of the microscopic energy flow between electronic, magnetic, and lattice degrees of freedom on ultrafast timescales and constitute a foundation for theoretical descriptions of demagnetization that are consistent with the dynamics of all three subsystems

THz emission from Fe/Pt spintronic emitters with L10_{0}-FePt alloyed interface

Recent developments in nanomagnetism and spintronics have enabled the use of ultrafast spin physics for terahertz (THz) emission. Spintronic THz emitters, consisting of ferromagnetic FM / non-magnetic (NM) thin film heterostructures, have demonstrated impressive properties for the use in THz spectroscopy and have great potential in scientific and industrial applications. In this work, we focus on the impact of the FM/NM interface on the THz emission by investigating Fe/Pt bilayers with engineered interfaces. In particular, we intentionally modify the Fe/Pt interface by inserting an ordered L1$_{0}$-FePt alloy interlayer. Subsequently, we establish that a Fe/L1$_{0}$-FePt (2\,nm)/Pt configuration is significantly superior to a Fe/Pt bilayer structure, regarding THz emission amplitude. The latter depends on the extent of alloying on either side of the interface. The unique trilayer structure opens new perspectives in terms of material choices for the next generation of spintronic THz emitters