Ultrafast Electron Crystallography Research Articles

Theoretical Simulation of the Temporal Behavior of Bragg Diffraction Derived from Lattice DeformationSupported by the National Key Research and Development Program of China under Grant Nos. 2016YFA0300303, 2017YFA0504703, and 2017YFA0302904, the National Basic Research Program of China under Grant No. 2015CB921304, the National Natural Science Foundation of China under Grant Nos. 11604372, 11474323, 11774391, 11774403, and 61575085, the Strategic Priority

A theoretical study on the structural dynamics of the temporal behavior of Bragg diffraction is presented and compared with experimental results obtained via ultrafast electron crystallography. In order to describe the time-dependent lattices and calculate the Bragg diffraction intensity, we introduce the basic vector offset matrix, which can be used to quantify the shortening, lengthening and rotation of the three lattice vectors (i.e., lattice deformation). Extensive simulations are performed to evaluate the four-dimensional electron crystallography model. The results elucidate the connection between structural deformations and changes in diffraction peaks, and sheds light on the quantitative analysis and comprehensive understanding of the structural dynamics.

Relativistic Modeling of Ultra-Short Electron Pulse Propagation

The ultrafast electron microscopy, electron diffraction, electron crystallography, and nanocrystallography methods opened the possibility of studying the coherent structural dynamics of matter. The time resolution of the ultrafast electron microscopy and electron diffraction methods determined by the duration of electron pulses is the key parameter of experimental setups. This paper treats electron pulse dynamics in the field free drift region specifically for applications in atomic imaging. The electron beam is modeled as a system of particles (N) with N = 1000 and N = 10 000 electrons. The beam propagates for a certain period of time (1–4 ns); during its propagation, electron distribution parameters (over coordinates and velocities) are calculated to characterize the temporal profile and uncertainty in the electron wavelength at the sample. The results of applying relativistic dynamic equations show that nonrelativistic results are satisfactorily applicable (with 15 per cent or better accuracy) for modeling short electron pulse elongation and broadening at 30 keV and lower energies. However, the results of such modeling may be significantly in error for intermediate energies (300 keV), and for the fast relativistic beams (3 MeV) they become completely wrong. The relative reduction in Coulomb repulsion effects at higher energies is known, however; we give a comprehensive treatment that allows a quantitative picture. Using high-energy electron pulses results in almost complete elimination of the repulsive Coulomb effect. Dispersion of electron velocities becomes much lower at higher energies. For 3 MeV electrons, electron pulse duration as well as its radius does not noticeably change even after traveling for 4 ns (1.2 m). Even at 300 keV, the pulse duration increase is negligible until 1 ns (0.2 m). A simple mean-field model suggested in [13] has been extended to arbitrarily fast relativistic electron pulses with good correspondence to direct dynamic modeling.

Generation of attosecond electron beams in relativistic ionization by short laser pulses

Ionization by relativistically intense short laser pulses is studied in the framework of strong-field quantum electrodynamics. Distinctive patterns are found in the energy probability distributions of photoelectrons, which are sensitive to the properties of a driving laser field. It is demonstrated that these electrons are generated in the form of solitary attosecond wave packets. This is particularly important in light of various applications of attosecond electron beams such as in ultrafast electron diffraction and crystallography, or in time-resolved electron microscopy of physical, chemical, and biological processes. We also show that, for intense laser pulses, high-energy ionization takes place in narrow regions surrounding the momentum spiral, the exact form of which is determined by the shape of a driving pulse. The self-intersections of the spiral define the momenta for which the interference patterns in the energy distributions of photoelectrons are observed. Furthermore, these interference regions lead to the synthesis of single-electron wave packets characterized by coherent double-hump structures.

ULTRAFAST ELECTRON CRYSTALLOGRAPHY AND NANOCRYSTALLOGRAPHY: FOR CHEMISTRY, BIOLOGY AND MATERIALS SCIENCE. PART II. ULTRAFAST ELECTRON NANOCRYSTALLOGRAPHY

This article shows the development of the Ultrafast Electron Nanocrystallography (UEnC), a specific implementation of Ultrafast Electron Crystallography (UEC), optimized for high sensitivity and high data acquisition efficiency and trained on the quantitative studies of different solid nanostructures with high temporal resolution ranging from pico- to femtoseconds. These achievements have opened up new possibilities for studying the coherent structural dynamics of the nanoparticles. Nanostructures are characterized by a number of unusual properties compared to their bulk counterparts. First of all, this is due to the manifestation of the quantum-size effect and the corresponding relatively-large percentage of structural units of the nanoparticle being on its surface. One of the structural features is the emergence of the so-called nano-crystallographic structure types, which have a tendency to form closed shells and are connected with the appearance of magic numbers in the size distribution. The morphology and lattice parameters of nanocrystals are strongly dependent on the substrate and the surface modification, which results in the contraction, and the formation of twin boundaries due to the relaxation of the surface deformations. The article describes UEnC experiment, the theory and data analysis, and sample preparation. The presented examples include the investigation of the structural dynamics of condensed phase, surfaces, and nanocrystals, the melting dynamics of gold nanoparticles and direct observation of coherent optical phonons generation in nanofilms. The results of several internationally renowned research groups are included and cited.Forcitation:Schafer L., Tarasov Yu.I., Sharonova N.V., Ischenko A.A. Ultrafast electron crystallography and nanocrystallography: for chemistry, biology and materials science. Part II. Ultrafast electron nanocrystallography. Izv. Vyssh. Uchebn. Zaved. Khim. Khim. Tekhnol. 2017. V. 60. N 6. P. 4-27.

ULTRAFAST ELECTRON CRYSTALLOGRAPHY AND NANOCRYSTALLOGRAPHY: FOR CHEMISTRY, BIOLOGY AND MATERIALS SCIENCE. PART I. ULTRAFAST ELECTRON CRYSTALLOGRAPHY

The direct probing and understanding of the dynamics of chemical and biological processes occurring in condensed matter, is currently in its early stages. Progress in this field has been pushed by the development of methods for the study of the structural dynamics of matter in a state far from equilibrium, including extreme states. The forthcoming information serves as the basis for testing new theoretical approaches to the description of the substance in casually connected triad "structure-dynamics-function". Observation of the dynamic behavior of matter in the space-time continuum on ultrashort time scales is a necessary first step in the explanation and, ultimately, control of far from equilibrium processes, and functionality of the systems studied. The method of ultrafast electron crystallography (UEC) makes it possible to investigate transient nonequilibrium structures, which yield decisive information about the structural dynamics of the phase transitions and coherent dynamics of the nuclei in the solid state, on the surface, and in macromolecular systems. In recent years, the electron bunch path length in the UEC apparatus diminished significantly, while the accelerating voltage increased considerably. Therefore, femtosecond electron pulses were obtained. A technique of radio frequency grouping of electrons was proposed to increase the electron pulse brightness. The method of electron field emission was used to increase the spatial coherence, and ponderomotive wave front acceleration was applied to reduce the mismatch between the velocities of the light and electron pulses and to contract the electron bunches. These achievements have opened up new possibilities for studying the coherent structural dynamics – atomic and molecular movie with femtosecond temporal resolution. The results of several internationally renowned research groups are included and cited.

Ahmed Hassan Zewail

Reference EPFL-ARTICLE-224968doi:10.1063/Pt.3.3406Afficher la publication dans Web of Science Notice creee le 2017-01-24, modifiee le 2017-05-12

Rippling ultrafast dynamics of suspended 2D monolayers, graphene

Here, using ultrafast electron crystallography (UEC), we report the observation of rippling dynamics in suspended monolayer graphene, the prototypical and most-studied 2D material. The high scattering cross-section for electron/matter interaction, the atomic-scale spatial resolution, and the ultrafast temporal resolution of UEC represent the key elements that make this technique a unique tool for the dynamic investigation of 2D materials, and nanostructures in general. We find that, at early time after the ultrafast optical excitation, graphene undergoes a lattice expansion on a time scale of 5 ps, which is due to the excitation of short-wavelength in-plane acoustic phonon modes that stretch the graphene plane. On a longer time scale, a slower thermal contraction with a time constant of 50 ps is observed and associated with the excitation of out-of-plane phonon modes, which drive the lattice toward thermal equilibrium with the well-known negative thermal expansion coefficient of graphene. From our results and first-principles lattice dynamics and out-of-equilibrium relaxation calculations, we quantitatively elucidate the deformation dynamics of the graphene unit cell.

Ultrafast electron crystallography of the cooperative reaction path in vanadium dioxide.

Time-resolved electron diffraction with atomic-scale spatial and temporal resolution was used to unravel the transformation pathway in the photoinduced structural phase transition of vanadium dioxide. Results from bulk crystals and single-crystalline thin-films reveal a common, stepwise mechanism: First, there is a femtosecond V−V bond dilation within 300 fs, second, an intracell adjustment in picoseconds and, third, a nanoscale shear motion within tens of picoseconds. Experiments at different ambient temperatures and pump laser fluences reveal a temperature-dependent excitation threshold required to trigger the transitional reaction path of the atomic motions.

Transient Structures and Possible Limits of Data Recording in Phase-Change Materials.

Phase-change materials (PCMs) represent the leading candidates for universal data storage devices, which exploit the large difference in the physical properties of their transitional lattice structures. On a nanoscale, it is fundamental to determine their performance, which is ultimately controlled by the speed limit of transformation among the different structures involved. Here, we report observation with atomic-scale resolution of transient structures of nanofilms of crystalline germanium telluride, a prototypical PCM, using ultrafast electron crystallography. A nonthermal transformation from the initial rhombohedral phase to the cubic structure was found to occur in 12 ps. On a much longer time scale, hundreds of picoseconds, equilibrium heating of the nanofilm is reached, driving the system toward amorphization, provided that high excitation energy is invoked. These results elucidate the elementary steps defining the structural pathway in the transformation of crystalline-to-amorphous phase transitions and describe the essential atomic motions involved when driven by an ultrafast excitation. The establishment of the time scales of the different transient structures, as reported here, permits determination of the possible limit of performance, which is crucial for high-speed recording applications of PCMs.

Anisotropic electron-phonon coupling investigated by ultrafast electron crystallography: Three-temperature model

In low-dimensional electronic materials, the charge or spin ordering can be subtly controlled by specific mode or modes, giving rise to functioning states such as charge- and spin-density waves, Mott insulators, and superconductors. The coupling between the electrons and the atomic lattice can be effectively investigated by ultrafast optical, photoemission, and electron diffraction techniques providing detailed description of microscopic and collective state evolutions in separate electronic and lattice subsystems. However, the electronic and phononic relaxation time scales obtained from these techniques are often distinctly different in low-dimensional electronic materials, even in a system as simple as graphite. Here, we seek to understand their origins from examining the nonequilibrium scenarios considering anisotropic electron-phonon coupling leading to hot phonons, which can be investigated directly from the momentum-dependent scattering changes in the transmission ultrafast electron crystallography. A three-temperature model is constructed to achieve unified understandings combining ultrafast spectroscopy and diffraction results of the nonadiabatic optically driven dynamics in graphite, charge-density waves in CeTe${}_{3}$, and the Mott insulator VO${}_{2}$.

Ultrafast electron crystallography of heterogeneous structures: Gold-graphene bilayer and ligand-encapsulated nanogold on graphene

Here, we report studies of structures comprising a nanoscale gold-graphene bilayer and ligand-encapsulated nanogold on graphene multilayers. The observed time scale for the heating dynamics of the gold layer is significantly slower, when compared to previous results on free-standing gold films, and is independent of the level of carrier excitation. A model is proposed which incorporates the local carrier excitation in the gold layer, carrier relaxation in the graphene layer and heating of the gold layer by thermal conduction. When gold is isolated from graphene by ligand encapsulation, the carriers become again localized, consistent with the two phase description.

Ultrafast electron crystallography of monolayer adsorbates on clean surfaces: Structural dynamics

By combining the surface sensitivity and ultrafast temporal resolution of ultrafast electron crystallography (UEC) with in situ surface preparation, adsorbate deposition and surface characterization, we extend the capability of UEC to monitor structural dynamics of monolayer adsorbates. Here, we report investigation of the Ni(100)–c(2×2)–S, Ni(100)–p(2×2)–O, and Ni(100)–c(2×2)–CO systems. Their structural dynamics are quantified by comparing the temporal change of the experimental electron diffraction patterns to the theoretically-obtained multi-scattering simulations. This Letter paves the way to future exploration of monolayer reactivity under controlled conditions of ultrahigh vacuum and with spatiotemporal resolutions at the atomic scale.

Structural dynamics of surfaces by ultrafast electron crystallography: Experimental and multiple scattering theory

Recent studies in ultrafast electron crystallography (UEC) using a reflection diffraction geometry have enabled the investigation of a wide range of phenomena on the femtosecond and picosecond time scales. In all these studies, the analysis of the diffraction patterns and their temporal change after excitation was performed within the kinematical scattering theory. In this contribution, we address the question, to what extent dynamical scattering effects have to be included in order to obtain quantitative information about structural dynamics. We discuss different scattering regimes and provide diffraction maps that describe all essential features of scatterings and observables. The effects are quantified by dynamical scattering simulations and examined by direct comparison to the results of ultrafast electron diffraction experiments on an in situ prepared Ni(100) surface, for which structural dynamics can be well described by a two-temperature model. We also report calculations for graphite surfaces. The theoretical framework provided here allows for further UEC studies of surfaces especially at larger penetration depths and for those of heavy-atom materials.

Structural dynamics of nanoscale gold by ultrafast electron crystallography

By employing ultrafast electron crystallography in a transmission geometry for ultra-thin (2–3 nm) gold, here we show that structural dynamics of the transverse atomic motions and the atomic displacements around the equilibrium position can be separated from the measured change in Bragg diffraction, the positions and intensities of the peaks, respectively. The rate of intensity change provides the electron-lattice equilibration time whereas the observed lattice expansion, which occurs on a slower time scale, maps the delayed response of transverse lattice strain. These textbook-type results provide the microscopic stress–strain profile that is critical for understanding dynamical deformations and the effect of morphological structures at surfaces.

Methods for studying the coherent 4D structural dynamics of free molecules and condensed state of matter

Studies in the coupled 4D spatial and temporal continuum are necessary for understanding the dynamic features of molecular systems with a complex profile of the potential energy surface. The introduction of time sweep into diffraction methods and the development of principles for studying coherent processes have revealed new approaches to the analysis of the dynamics of wave packets, the intermediate products and the transition state of the reaction center, and short-lived compounds in gaseous and condensed media. The use of picosecond and femtosecond electron probe pulses, synchronized with excitation laser pulses, determined the development of ultrafast electron crystallography, time-resolved X-ray diffraction, and dynamic transmission electron microscopy (DTEM). One of the most promising applications of the developed diffraction methods is the characterization and visualization of the processes occurring upon the photoexcitation of free molecules and biological objects and the analysis of surface and thin films. The whole set of spectral and diffraction methods based on different physical principles, which are complementary and make it possible to perform the photoexcitation of nuclei and electrons and carry out diagnostics of their dynamics at ultrashort time sequences, reveal new possibilities for studies with the necessary integration of the “structure-dynamics-function” triad in chemistry, biology, and materials science.

Primary structural dynamics in graphite

The structural dynamics of graphite and graphene are unique, because of the selective coupling between electron and lattice motions and hence the limit on electric and electro-optic properties. Here, we report on the femtosecond probing of graphite films (1–3 nm) using ultrafast electron crystallography in the transmission mode. Two time scales are observed for the dynamics: a 700 fs initial decrease in diffraction intensity due to lattice phonons in optically dark regions of the Brillouin zone, followed by a 12 ps decrease due to phonon thermalization near the Γ and K regions. These results indicate the non-equilibrium distortion of the unit cells at early time and the subsequent role of long-wavelength atomic motions in the thermalization process. Theory and experiment are now in agreement regarding the nature of nuclear motions, but the results suggest that potential change plays a role in the lateral dynamics of the lattice.

Erratum: Structural Preablation Dynamics of Graphite Observed by Ultrafast Electron Crystallography [Phys. Rev. Lett.100, 035501 (2008)

Erratum: Structural Preablation Dynamics of Graphite Observed by Ultrafast Electron Crystallography [Phys. Rev. Lett.<b>100</b>, 035501 (2008)

Comment on “Structural Preablation Dynamics of Graphite Observed by Ultrafast Electron Crystallography”

In a recent letter (F. Carbone, P. Baum, P. Rudolf, et al., Physical Review Letters 100, 2008), graphite is reported to undergo a c-axis contraction on the time scale of few picoseconds (ps) after ultrafast pulsed laser excitation. The velocity of lattice contraction depended on the laser fluence. Furthermore, the lattice contraction is followed by large, non-thermal, lattice expansion of several picometers (pm) after few hundreds of ps. These results were interpreted based on the position of the (0014) diffraction spot in the glancing angle diffraction geometry off the graphite surface using ultrafast electron diffraction (UED). The lattice contraction and expansion corresponded to upward and downward movements of the diffraction spot respectively. Here, we show that ultrafast pulsed laser excitation of graphite produces large transient electric fields (TEFs). The TEFs deflect the electron beam in a manner similar to these reported and the neglect of TEFs likely led to erroneous conclusions.

Quantitative nanoparticle structures from electron crystallography data

We describe the quantitative refinement of nanoparticle structures from gold nanoparticles probed by ultrafast electron crystallography (UEC). We establish the equivalence between the modified radial distribution function employed in UEC and the atomic pair distribution function (PDF) used in x-ray and neutron powder diffraction analysis. By leveraging PDF refinement techniques, we demonstrate that UEC data are of sufficient quality to differentiate between cuboctahedral, decahedral and icosahedral nanoparticle models. Furthermore, we identify the signatures of systematic errors that may occur during data reduction and show that atomic positions refined from UEC are robust to these errors. This work serves as a foundation for reliable quantitative structural analysis of time-resolved laser-excited nanoparticle states.

Electronically Driven Fragmentation of Silver Nanocrystals Revealed by Ultrafast Electron Crystallography

We report an ultrafast electron diffraction study of silver nanocrystals under surface plasmon resonance excitation, leading to a concerted fragmentation. By examining simultaneously transient structural, thermal, and Coulombic signatures of the prefragmented state, an electronically driven nonthermal fragmentation scenario is proposed.