Tailoring electron vortex beams with customizable intensity patterns by electron diffraction holography

An electron vortex beam(EVB) carrying orbital angular momentum(OAM) plays a key role in a series of fundamental scientific researches, such as chiral energy-loss spectroscopy and magnetic dichroism spectroscopy. So far, almost all the experimentally created EVBs manifest isotropic doughnut intensity patterns. Here, based on the correlation between local divergence angle of electron beam and phase gradient along azimuthal direction, we show that free electrons can be tailored to EVBs with customizable intensity patterns independent of the carried OAM. As proof-of-concept, by using computer generated hologram and designing phase masks to shape the incident free electrons in the transmission electron microscope, three structured EVBs carrying identical OAM are tailored to exhibit completely different intensity patterns. Furthermore, through the modal decomposition, we quantitatively investigate their OAM spectral distributions and reveal that structured EVBs present a superposition of a series of different eigenstates induced by the locally varied geometries. These results not only generalize the concept of EVB, but also demonstrate an extra highly controllable degree of freedom for electron beam manipulation in addition to OAM.

Analysis of Orange Peel Defect in St14 Steel Sheet by Electron Backscattered Diffraction (EBSD)

In this paper, the orange peel defect in the surface range of the st14 steel sheet has been investigated using the electron backscattered diffraction (EBSD) technique. It has been found that the orange peel defect in the st14 steel sheet was resulted from the local coarse grains which were produced during hot-rolling due to the critical deformation in dual-phase zone. During deep drawing, the coarse grains with {100}<001> microtexture can slip on the {112}<111> slip system to form bulging and yields orange peel defects, while the coarse grains with {112}<110> orientation do not form the defect as the Schmid factor of {112}<111> slip system in it equals zero.

Surface segregation of InGaAs films by the evolution of reflection high-energy electron diffraction patterns

Surface segregation is studied via the evolution of reflection high-energy electron diffraction （RHEED） patterns under different values of As4 BEP for InGaAs films. When the As4 BEP is set to be zero, the RHEED pattern keeps a 4x3/（nx3） structure with increasing temperature, and surface segregation takes place until 470 ℃ The RHEED pattern develops into a metal-rich （4x2） structure as temperature increases to 495℃. The reason for this is that surface segregation makes the In inside the InGaAs film climb to its surface. With the temperature increasing up to 515℃, the RHEED pattern turns into a GaAs（2x4） structure due to In desorption. While the As4 BEP comes up to a specific value （1.33 x 10-4 Pa-1.33 x 10-3 Pa）, the surface temperature can delay the segregation and desorption. We find that As4 BEP has a big influence on surface desorption, while surface segregation is more strongly dependent on temperature than surface desorption.

Crystallographic Characteristic of Intermetallic Compounds in Al-Si-Mg Casting Alloys Using Electron Backscatter Diffraction

The Al-Si-Mg alloy which can be strengthened by heat treatment is widely applied to the key components of aerospace and aeronautics. Iron-rich intermetallic compounds are well known to be strongly influential on mechanical properties in Al-Si-Mg alloys. But intermetallic compounds in cast Al-Si-Mg alloy intermetallics are often misidentified in previous metallurgical studies. It was described as many different compounds, such as AlFeSi, Al8Fe2Si, Al5（Fe, Mn）3Si2 and so on. For the purpose of solving this problem, the intermetallic compounds in cast Al-Si alloys containing 0.5% Mg were investigated in this study. The iron-rich compounds in Al-Si-Mg casting alloys were characterized by optical microscope（OM）, scanning electron microscope（SEM）, energy dispersive X-ray spectrometer（EDS）, electron backscatter diffraction（EBSD） and X-ray powder diffraction（XRD）. The electron backscatter diffraction patterns were used to assess the crystallographic characteristics of intermetallic compounds. The compound which contains Fe/Mg-rich particles with coarse morphologies was Al8FeMg3Si6 in the alloy by using EBSD. The compound belongs to hexagonal system, space group P6_2m, with the lattice parameter a=0.662 nm, c=0.792 nm. The β-phase is indexed as tetragonal Al3FeSi2, space group I4/mcm, a=0.607 nm and c=0.950 nm. The XRD data indicate that Al8FeMg3Si6 and Al3FeSi2 are present in the microstructure of Al-7Si-Mg alloy, which confirms the identification result of EBSD. The present study identified the iron-rich compound in Al-Si-Mg alloy, which provides a reliable method to identify the intermetallic compounds in short time in Al-Si-Mg alloy. Study results are helpful for identification of complex compounds in alloys.

Determination of Space Group Pncm of Prehnite Using Electron Diffraction Method

Space symmetry of prehnite, which occurs in cavities and veins within Skarn from the Tieshan iron mineral deposit,Daye,Hubei province,Central China,has been determined using selected area electron diffraction (SAED) and convergent-beam electron diffraction (CBED) on the submicrometer scale.Our results confirm that the natural prehnite may have the structure with symmetry Pncm.The unit-cell parameters of investigated prehnite (a=0.458nm,b=0.555nm,and c=1.853nm) have been calculated by using the multicrystal diffraction rings of gold,the internal standard.

Reactive Deposition Epitaxial Growth ofβ-FeSi_(2) Film on Si(001):in situ Observation by Reflective High Energy Electron Diffraction

Reactive deposition epitaxial growth ofβ-FeSi_(2) film on Si(001)has been studied by in situ observation of reflective high energy electron diffraction.Metastable strained phase was observed at initial stages.Surface roughness due to the islanding was observed during the deposition.The existed great tendency to transform the alignment of the orientation of crystallites into random as the thickness of deposited iron increased.

X-Ray Diffraction, Electron Paramagnetic Resonance and Optical Absorption Study of Bauxite

The bauxite mineral obtained from Araku, Vishakapatnam district of Andhra Pradesh, India is used in the present work. Structural characterization was performed by X-ray diffraction (XRD). The mineral was found to be gibbsite in phase. The transitional metal ions present were investigated using electron paramagnetic resonance (EPR) and optical absorption spectra. The EPR results suggest that Fe3+ has replaced Al3+ in the unit cell of bauxite. The optical absorption spectrum is due to Fe3+ which indicates that it is in distorted octahedral environment. The near-infrared (NIR) spectrum is due to water fundamentals and combination overtones, which confirm the formula of the compound. The impurities in the mineral are identified using spectroscopic techniques.

TEXTURE DETERMINATION OF THIN FILM ZrO_2 BY ELECTRON DIFFRACTION

A method for semi-quantitative determination of thin film texture using electron diffrac- tion is described.The texture state of a zirconia thin film with a cubic structure has been determined in this way and is presented as an example.

Modification of Analytical Expression of Electron Dynamical Diffraction

Assuming that the wave function φ>=eη, the Schrodinger equation can be written as. Neglecting the last two terms, an analytical expression of electron dynamical diffraction was derived by Qibin YANG et al. In this paper, the analytical expression is modified by further considering thesecond-order differential term When the accelerating voltage is not very high, or the sample is not very thin, the reciprocal vector g is large, the modification of the second-order differential is necessary; otherwise it can be neglected.

Effect of Electron Beam Orientation on Exit Wave Function via Simulation of Electron Dynamic Diffraction

Based on the electron dynamic diffraction, phase shift of the exit wave function vs misorientation of the incident electron beam from the exact zone axis has been calculated for the [001] oriented copper. The result shows that the peak of phase shift is the maximum at the atom position as the electron beam along the exact [001] zone axis, and the peak value of phase shift decreases as increases of the misorientation. At small misorientation, i.e. less than 5 degree, change of the phase shift is minimal. The peak value of phase shift decreases significantly when the incident beam deviates form the zone axis over 10 degree and the exit wave has a planar configuration as the misoriention angle arrives -17 degree. The effect of this phase shift characteristics on the information extracted from the hologram has also been considered.

A Comparative Study on the Selected Area Electron Diffraction Pattern of Fe Oxide/Au Core-shell Structured Nanoparticles

The selected area electron diffraction （SAED） pattern of magnetic iron oxide core/gold shell nanoparticles has been studied. For the composite particles with mean size less than 10 nm, their SAED pattern is found to be different from either the pattern of pure Fe oxide nanoparticles or that of pure Au particles. Based on the fact that the ring diameters of these composite particles fit the characteristic relation for the fcc structure, the Au atoms on surfaces of the concerned particles are supposed to pack in a way more tightly than they usually do in pure Au nanoparticles. The driving force for this is the coherency strain which enables the shell material at the heterostructured interface to adapt the lattice parameters of the core.

Ultrafast electron diffraction

Ultrafast electron diffraction （UED） technique has proven to be an innovative tool for providing new insights in lattice dynamics with unprecedented temporal and spatial sensitivities. In this article, we give a brief introduction of this technique using the proposed UED station in the Synergetic Extreme Condition User Facility （SECUF） as a prototype. We briefly discussed UED＇s functionality, working principle, design consideration, and main components. We also briefly reviewed several pioneer works with UED to study structure-function correlations in several research areas. With these efforts, we endeavor to raise the awareness of this tool among those researchers, who may not yet have realized the emerging opportunities offered by this technique.

CRYSTAL STRUCTURE OF A LONG－PERIOD ORDERED PHASE IN Fe－C MARTENSITE AND COMPUTER SIMULATION OF ITS ELECTRON DIFFRACTION PATTERNS

Different structure models of a long-period ordered phase in Fe-C martenstie formed during aging have been checked by computer simulation of electron diffraction(ED) patterns based on these models.The results showed that the simulated ED pattern of γ'-FexC(Ⅱ) model proposed by the present authors is in good agreement with experimentally observed ED pattern.It was also confirmed that the incommensurate superperiod stems from the coexistence of several γ'-Fe_xC(H) phases with different superperiods.The Fe(144)C(24)(Fe6C) model proposed by Uwakweh et al.generated ED patterns remarkably different from the experimental ones.

Isolated attosecond electron wave packet diffraction

Wave-particle duality is one of the most fundamental and mysterious natures of matters. Here, we present an interesting scheme of isolated electron wave packet diffraction with a few-cycle laser pulse and an extreme ultraviolet （XUV） pulse. The diffraction fringes are clearly present in the laser dressed XUV photoelectron spectra, strongly resembling the Airy diffraction pattern of optical waves. This phenomenon suggests a great potential of attosecond diffractometry. According to this scheme we also propose a simple method to determine the XUV pulse duration from the photoelectron spectra with a rather high resolution.

DETERMINATION OF SPACE GROUP OF PrCo_(12)B_6 COMPOUND BY CONVERGENT BEAM ELECTRON DIFFRACTION

The space group of PrCo_(12)B_6,compound has been determined using the convergent beam elec- tron diffraction method.The space group is found to be R3m.

Uncovering the internal structure of five-fold twinned nanowires through 3D electron diffraction mapping

Five-fold twinned nanostructures are intrinsically strained or relaxed by extended defects to satisfy the space-filling requirement.Although both of metallic and semiconductor five-fold twinned nanostructures show inhomogeneity in their cross-sectional strain distribution,the evident strain concentration at twin boundaries in the semiconductor systems has been found in contrast to the metallic systems.Naturally,a problem is raised how the chemical bonding characteristics of various five-fold twinned nanosystems affects their strain-relieving defect structures.Here using three-dimensional(3D)electron diffraction mapping methodology,the intrinsic strain and the strain-relieving defects in a pentagonal Ag nanowire and a star-shaped boron carbide nanowire,both of them have basically equal radial twin-plane width about 30 nm,are nondestructively characterized.The non-uniform strain and defect distribution between the five single crystalline segments are found in both of the five-fold twinned nanowires.Diffraction intensity fine structure analysis for the boron carbide five-fold twinned nanowire indicates the presence of high-density of planar defects which are responsible for the accommodation of the intrinsic angular excess.However,for the Ag five-fold twinned nanowire,the star-disclination strain field is still present,although is partially relieved by the formation of localized stacking fault layers accompanied by partial dislocations.Energetic analysis suggests that the variety in the strain-relaxation ways for the two types of five-fold twinned nanowires could be ascribed to the large difference in shear modulus between the soft noble metal Ag and the superhard covalent compound boron carbide.

How to realize an ultrafast electron diffraction experiment with a terahertz pump:A theoretical study

To integrate a terahertz pump into an ultrafast electron diffraction(UED)experiment has attracted much attention due to its potential to initiate and detect the structural dynamics both directly.However,the deflection of the electron probe by the electromagnetic field of the terahertz pump alters the incident angle of the electron probe on the sample,impeding it from recording structural information afterwards.In this article,we studied this issue by a theoretical simulation of the terahertz-induced deflection effect on the electron probe,and came up with several possible schemes to reduce such effect.As a result,a terahertz-pump-electron-probe UED experiment with a temporal resolution comparable to the terahertz period is realized.We also found that Me V UED was more suitable for such terahertz pump experiment.

Ultrafast Molecular Movies:Probing Chemical Dynamics with Femtosecond Electron and X-Ray Diffraction

Understanding the process of chemical reactions has always been a relentless pursuit for chemists.The development of femtosecond pump-probe techniques since the 1980s has revolutionized the field of chemical dynamics,enabling the capture of timeresolved snapshots of reactions on the femtosecond timescale.Starting from the 2010s,breakthroughs in femtosecond electron and X-ray sources has enabled ultrafast electron and X-ray diffraction techniques,which is able to directly reveal the temporal evolution of atomic geometries of molecules,allowing for the creation of molecular movies.Gas-phase molecular movies reveal intrinsic intramolecular processes,while liquid-phase molecular movies provide insights for complicated solvent-solute interplay.This minireview focuses on the advances in studying gasphase and liquid-phase molecular dynamics(MD)using ultrafast electron and X-ray diffraction techniques on femtosecond and picosecond timescales.The fast-developing experimental capability of the direct observation of molecular structural evolution during chemical reactions,on its natural femtosecond timescale and subangstrom length scale,offers tremendous potential for the field of chemical kinetics and MD.

Design of an ultrafast electron diffractometer with multiple operation modes

Directly resolving structural changes in material on the atomic scales of time and space is desired in studies of many disciplines.Ultrafast electron diffraction(UED),which combines the temporal resolution of femtosecond-pulse laser and the spatial sensitivity of electron diffraction,is an advancing methodology serving such a goal.Here we present the design of a UED apparatus with multiple operation modes for observation of collective atomic motions in solid material of various morphologies.This multi-mode UED employs a pulsed electron beam with propagation trajectory of parallel and convergent incidences,and diffraction configurations of transmission and reflection,as well utilities of preparation and characterization of cleaned surface and adsorbates.We recorded the process of electron-phonon coupling in single crystal molybdenum ditelluride following excitation of femtosecond laser pulses,and diffraction patterns of polycrystalline graphite thin film under different settings of electron optics,to demonstrate the temporal characteristics and tunable probe spot of the built UED apparatus,respectively.

Phase Identification Using Series of Selected Area Diffraction Patterns and Energy Dispersive Spectrometry within TEM

Transmission electron microscopy (TEM) is a very powerful technique for materials characteriza-tion, providing information relating to morphology, composition, and crystal structure. Selected area diffraction patterns (SADPs) are crystallographic data that can be obtained using a TEM in-strument. Conventional identification through SADP/TEM is tricky and tedious, thereby increasing the difficulty of phase identification. To establish a procedure for phase identification of known and unknown phases, in this study we examined two samples: one, a known phase, was Si with alignment;the other, unknown, was the TixOy phase at the 96.4Au-3Ni-0.6Ti interlayer/ yttria-stabilized zirconia (YSZ) interface of a steel/96.4Au-3Ni-0.6Ti interlayer/YSZ joint. The procedures for phase identification of the known and unknown phases are described herein using a series of SADPs and energy dispersive spectrometry within TEM that would be useful for general researchers.