Time-resolved Diffraction Research Articles

Selective Reduction Mechanism of Graphene Oxide Driven by the Photon Mode versus the Thermal Mode.

A two-dimensional nanocarbon, graphene, has attracted substantial interest due to its excellent properties. The reduction of graphene oxide (GO) has been investigated for the mass production of graphene used in practical applications. Different reduction processes produce different properties in graphene, affecting the performance of the final materials or devices. Therefore, an understanding of the mechanisms of GO reduction is important for controlling the properties of functional two-dimensional systems. Here, we determined the average structure of reduced GO prepared via heating and photoexcitation and clearly distinguished their reduction mechanisms using ultrafast time-resolved electron diffraction, time-resolved infrared vibrational spectroscopy, and time-dependent density functional theory calculations. The oxygen atoms of epoxy groups are selectively removed from the basal plane of GO by photoexcitation (photon mode), in stark contrast to the behavior observed for the thermal reduction of hydroxyl and epoxy groups (thermal mode). The difference originates from the selective excitation of epoxy bonds via an electronic transition due to their antibonding character. This work will enable the preparation of the optimum GO for the intended applications and expands the application scope of two-dimensional systems.

Sintering kinetics of nanocrystalline Ce0.9Zr0.1O2 prepared by gel combustion method

The sintering process of nanometric Ce0.9Zr0.1O2 mixed oxides prepared by the amino acid–nitrate gel combustion method was analyzed by dilatometry using the constant heating rate (CHR) and stepwise isothermal dilatometry (SID) methods. The morphology of the mixed oxides was studied by scanning and transmission electron microscopies. The evolution of crystallite size with temperature was analyzed by time-resolved X-ray powder diffraction with synchrotron radiation. The sintering behavior resulted dependent on the morphology of the powders. Samples with nanometric crystallite size start to sinter even 500 K before than micrometric sample. We concluded that sintering process of nanometric Ce0.9Zr0.1O2 is controlled by grain-boundary diffusion for temperatures below 1373–1573 K, depending on the morphology of the sample. At high temperature, the grain-boundary and lattice diffusion become parallel controlling mechanism for sintering.

Ultrafast electron diffraction from a Bi(111) surface: Impulsive lattice excitation and Debye-Waller analysis at large momentum transfer.

The lattice response of a Bi(111) surface upon impulsive femtosecond laser excitation is studied with time-resolved reflection high-energy electron diffraction. We employ a Debye–Waller analysis at large momentum transfer of 9.3 Å−1 ≤ Δ k ≤ 21.8 Å−1 in order to study the lattice excitation dynamics of the Bi surface under conditions of weak optical excitation up to 2 mJ/cm2 incident pump fluence. The observed time constants τint of decay of diffraction spot intensity depend on the momentum transfer Δk and range from 5 to 12 ps. This large variation of τint is caused by the nonlinearity of the exponential function in the Debye–Waller factor and has to be taken into account for an intensity drop ΔI > 0.2. An analysis of more than 20 diffraction spots with a large variation in Δk gave a consistent value for the time constant τT of vibrational excitation of the surface lattice of 12 ± 1 ps independent on the excitation density. We found no evidence for a deviation from an isotropic Debye–Waller effect and conclude that the primary laser excitation leads to thermal lattice excitation, i.e., heating of the Bi surface.

Violation of centrosymmetry in time-resolved coherent x-ray diffraction from rovibrational states of diatomic molecules

Owing to increasing applications of time-resolved coherent x-ray scattering for the investigation of molecular reaction dynamics, we develop a theoretical model for time-dependent x-ray diffraction from molecular and/or electronic motion in molecules. Our model shows that the violation of centrosymmetry (VOC) is a general phenomenon in time-resolved diffraction patterns. We employ our theoretical model to illustrate the VOC in time-resolved coherent x-ray diffraction from two oriented diatomic molecules undergoing rovibrational motion: lithium hydride (LiD) and hydrogen (HD). Our simulations show asymmetric x-ray diffraction images that reflect the directions of the molecular motions.

Sub-lattice polarization states in anti-ferroelectrics and their relaxation process

We report studies of quasi-remanent polarization states in Pb0.99Nb0.02[(Zr0.57Sn0.43)0.94Ti0.06]0.98O3 (PNZST) anti-ferroelectric ceramics and investigation of their relaxation effects using unique in-situ electrically activated time-resolved Synchrotron X-ray powder diffraction (SXPD) and 119Sn Mössbauer Spectroscopy (MS). The SXPD patterns are consistent with a phase transition from quasi-tetragonal perovskite in 0 V relaxed anti-ferroelectric state to rhombohedral distortion in ferroelectric state under saturating applied voltages of ±2 kV. The observed quasi-remanent polarization relaxation processes are due to the fact that tetragonal to rhombohedral distortion does not occur at the applied voltage required to access the quasi-remanent polarization states, and the tetragonal symmetry restored after the removal of the applied electric field is preserved. Since these quasi-remanent polarization states were seen as possibly suitable for memory applications, the implications of this study are that anti-ferroelectrics are more feasible for multi-state dynamic random access memories (DRAM), while their application to non-volatile memories requires development of more sophisticated “read-out” protocols, possibly involving dc electrical biasing.

Measurement of low-energy and low-charge ultrashort bunches using an S-band RF deflector

Ultrafast Electron Diffraction (UED) system, using an electron bunch of the duration with less than 100 fs and of the energy with 3 MeV, has been developed as a tool probing the ultrafast dynamics in pump–probe experiments. To keep the temporal and transverse characteristics of the electron bunch for the time-resolved electron diffraction, the bunch charge is limited to be the pico-Coulomb or less. S-band transverse deflecting cavity working on TM120 mode is designed, fabricated, and installed in the UED system to measure directly the fs-scaled pulse duration of low-energy and low-charge electron bunches. We describe the design and the expected performance of a single-cell RF deflector. For the electron beam of 3 MeV in energy and 1.88 pC in charge, we could measure the bunch duration of 69 fs and the timing jitter of 62 fs, both in rms.

Ultrafast X-ray Photoelectron Imaging of Attosecond Electron Dynamics in Molecular Coherent Excitation.

Ultrafast photoelectron imaging allows to measure information about coherent electron dynamic processes in materials or chemical compounds on femtosecond to attosecond time scales. We show that molecular time-resolved photoelectron diffraction produced by a time-delayed soft X-ray attosecond pulse can be used to monitor the ultrafast coherent excitation induced by a resonant UV pump pulse with variable carrier-envelope phases. Asymmetric diffraction angular patterns illustrate coherent electron dynamics of charge migration with spatiotemporal resolution on the attosecond and ångström scale. This allows the temporal reconstruction of phases and amplitudes of electronic states and geometry of molecules as a function of time delay of the probe pulse and carrier-envelope phases of the pump pulse. Results are obtained from solutions of time-dependent Schrödinger equations of the hydrogen molecular ion, and analyzed by ultrafast photoelectron diffraction models for coherent superposition of electronic states. The present demonstration provides a guiding principle for monitoring ultrafast spatiotemporal coherent electron dynamics and imaging molecular electronic structure in complex systems by ultrafast pump-probe experiments and their dependence on carrier-envelope phases and time delays.

Performance of an Indium-sealed S-band RF Photoelectron Gun for Time-resolved Electron Diffraction Experiments

We have developed a one-and-half -cell S-band radio-frequency (RF) photoelectron gun (photogun) fed by a coaxial coupler. The RF photogun is dedicated to ultrafast-electron-diffraction experiments by generating electron bunches of 3-MeV energy and a-few-pC charge, which is not strict condition compared to those for X-ray free-electron lasers. Brazing of RF cavities is welldeveloped process for making RF guns or RF accelerators. Sometimes, however, a failure occurs in the brazing process, causing the entire electron gun or accelerating cavity to spoil. Axial-symmetric design of the RF photogun permits indium sealing for cavity cells, a photocathode plate, and a coupling RF part. We firstly report that the indium-sealed RF photogun successfully meets the required performance and long-term stability for ultrafast electron diffraction experiments. We have stably operated the RF photogun for more than three years with the electron beam conditions of ~ 3-MeV energy, up-to-10-pC charge, and a repetition rate of 50 Hz. The quantum efficiency of the copper photocathode had improved from 10−6 to 10−5 depending on vacuum condition from 10−8 to 5 × 10−10 Torr, respectively. Measured emittance and energy spread of the generated electron beam showed 0.3 mm-mrad and less than 0.25%, respectively, for a bunch charge of ~ 2 pC, which agree well with those obtained by ASTRA simulation.

Optical fiber-driven low-energy electron gun for time-resolved streak diffraction

The wave guiding feature of the optical fibre optical fibres is specifically exploited to construct a novel type of electron gun to realize single-shot low-energy electron diffraction experiments with the sub-picosecond resolution for studying irreversible samples.

Monitoring nonadiabatic dynamics in molecules by ultrafast X-Ray diffraction

We theoretically examine time-resolved diffraction from molecules which undergo non-adiabatic dynamics and identify contributions from inelastic scattering that indicate the presence of an avoided crossing and the corresponding nuclear configuration.

Pressure-induced amorphization in plagioclase feldspars: A time-resolved powder diffraction study during rapid compression

The pressure-induced amorphization of the two endmembers of the plagioclase ((Na1−xCax)Al1+xSi3−xO8) solid-solution, anorthite (CaAl2Si2O8) and albite (NaAlSi3O8), has been studied as a function of compression rate by means of time-resolved powder diffraction. Anorthite and albite were compressed in a diamond anvil cell to 80 GPa at multiple rates from 0.05 GPa/s to 80 GPa/s. The amorphization pressure decreases with increasing compression rate. This negative strain rate sensitivity indicates a change in deformation mechanism in the plagioclase solid-solution endmembers from brittle to ductile with increasing compression rate. The presented data support the previously proposed shear deformation mechanism for the amorphization of plagioclase. Furthermore, amorphization progresses over a wide pressure range suggesting heterogeneous amorphization, similar to observations based on recovered material from shock-compression experiments of plagioclase. Our experiments support the contention that amorphization pressures for plagioclase may occur at lower pressures than usually considered.

Bond Dissociation Triggering Molecular Disorder in Amorphous H2O.

We developed a system to deposit H2O molecules onto ultrathin silicon nitride substrates in situ using time-resolved transmission electron diffraction apparatus and performed ultrafast time-resolved electron diffraction measurements in the noncrystalline (amorphous) H2O under near-ultraviolet photoexcitation. The observed dynamics directly represent O-H bond dissociation via multiphoton absorption and charge transfer, which trigger ionization and intermolecular disorder in the amorphous H2O. Our results illustrate the intriguing nature of light-matter and matter-matter interactions in H2O molecules.

Determination of the electron-lattice coupling strength of copper with ultrafast MeV electron diffraction.

Electron-lattice coupling strength governs the energy transfer between electrons and the lattice and is important for understanding the material behavior under highly non-equilibrium conditions. Here we report the results of employing time-resolved electron diffraction at MeV energies to directly study the electron-lattice coupling strength in 40-nm-thick polycrystalline copper excited by femtosecond optical lasers. The temporal evolution of lattice temperature at various pump fluence conditions were obtained from the measurements of the Debye-Waller decay of multiple diffraction peaks. We observed the temperature dependence of the electron-lattice relaxation time which is a result of the temperature dependence of electron heat capacity. Comparison with two-temperature model simulations reveals an electron-lattice coupling strength of (0.9 ± 0.1) × 1017 W/m3/K for copper.

Time filtering of event based neutron scattering data: A pathway to study the dynamic structural responses of materials.

Time-resolved diffraction has become a vital tool for probing dynamic responses to an applied stimulus. Such experiments traditionally use hardware solutions to histogram measured data into their respective bin. We will show that a major advantage of event-based data acquisition, which time-stamps measured diffraction data with 100 ns accuracy, is much preferred over hardware histogramming of the data by enabling postprocessing for advanced custom binning using a software solution. This approach is made even more powerful by coupling measured diffraction data with metadata about the applied stimuli and material response. In this work, we present a time-filter approach that leverages the power of event-based diffraction collection to reduce stroboscopic data measured over many hours into equally weighted segments that represent subsets of the response to a single cycle of the applied stimulus. We demonstrate this approach by observing ferroelectric/ferroelastic domain wall motion during electric field cycling of BaTiO3. The developed approach can readily be expanded to investigate other dynamic phenomena using complex sample environments.

Cation migrations and Bronsted site formation in NH4 form of zeolite omega: a combined in situ time-resolved synchrotron and neutron powder diffraction study

Cation migrations and Bronsted site formation in NH₄ form of zeolite omega: a combined <i>in situ</i> time-resolved synchrotron and neutron powder diffraction study

Sub-na and sub-ps time-resolved diffraction at CRISTAL beamline – SOLEIL synchrotron

Sub-na and sub-ps time-resolved diffraction at CRISTAL beamline – SOLEIL synchrotron

Thermal stability of the layered modification of Cu0.5ZrTe2 in the temperature range 25–900 °C

The thermal stability of the layered modification of the Cu0.5ZrTe2 polycrystalline intercalation compound, synthesized at room temperature, has been studied in the temperature range 25-900 °C. A change in the occupation of the octahedral and tetrahedral coordinated sites in the interlayer space of the zirconium ditelluride was observed using in-situ time-resolved synchrotron X-ray powder diffraction experiments. The formation of the rhombohedral CuZr2Te4 phase, which is stable in the temperature range 300-700 °C, has been observed. The copper intercalation at room temperature leads to the formation of a phase in which the Cu atoms occupy only octahedral sites in the interlayer space. At temperatures above the decay temperature of the rhombohedral CuZr2Te4, a layered phase with Cu atoms uniformly distributed between octahedral and tetrahedral sites in the interlayer space is stable. The changes in the crystal structure independent of temperature are in agreement with the previously proposed model, according to which the stability of the layered or the rhombohedral phase is determined by the entropy factor associated with the distribution of the intercalated atoms between the octahedral and tetrahedral sites in the interlayer space.

VO2(A) nanorods: One-pot synthesis, formation mechanism and thermal transformation to VO2(M)

The monoclinic VO2(M) has promising applications in intelligent devices but its preparation still requires improvement to permit cost-effective mass production. In this work, we report a 2-stage approach for producing VO2(M) nanorods by (1) hydrothermal reduction of vanadium pentoxide by sodium bisulfate at 220 °C to form VO2(A), and (2) subsequent thermal activated phase transformation of VO2(A) to VO2(M) at 350–450 °C in vacuum. The obtained VO2(M) nanorods showed a reversible phase transition temperature at about 62.5 °C and a narrow thermal hysteresis width of 10 °C. The mechanism of the hydrothermal reduction was studied by combined ex situ microscopic and diffraction characterization of cooled samples as well as in situ PXRD experiments, in which the hydrothermal synthesis was monitored in real time by time-resolved diffraction datasets. It was found that the hydrothermal synthesis of VO2(A) is a 4-step process: (1) reduction of V2O5 to form VO2(B) nanoparticles, (2) oriented attachment of VO2(B) nanoparticles along the [110] direction, (3) formation of VO2(B) nanorods as a results of oriented attachments, and (4) hydrothermal transformation of the metastable intermediate VO2(B) nanorods to VO2(A) nanorods. This clear understanding of the mechanism will help the further optimization of synthesis temperature and time for preparing VO2(A). This method provides a low temperature thermal treatment alternative and hence helps the reduction of cost for the production of VO2(M), bring the mass application of VO2(M) one step closer.

Electrically Conducting Ceramics Based on Al–AlN–TiB2

The high-temperature conductivity of ceramics based on Al–AlN–TiB2 obtained by self-propagating high-temperature synthesis has been studied. The dependences of DC resistivity were measured in vacuum (1 Pa) by a four-point technique in a temperature range of 293–1273 K. The evolution of the phase composition of the material upon heating was studied by time-resolved diffraction in real-time mode. The measurements show a “metallic” behavior of conductivity when the Al–AlN–TiB2 composite is heated, and there is also an abrupt change of resistivity in a temperature range of 870–970 K that is linked with Al melting. In this case, the crystalline structure of ceramic phases does not undergo changes in the studied temperature range. The same temperature coefficient of resistance has been observed for all studied compositions up to the melting point of aluminum.

Structural dynamics probed by high-coherence electron pulses

Abstract