Combination Of Electron Diffraction Research Articles

Structural analysis of nanocrystals by pair distribution function combining electron diffraction with crystal tilting.

As an important characterization method, pair distribution function (PDF) has been extensively used in structural analysis of nanomaterials, providing key insights into the degree of crystallinity, atomic structure, local disorder etc. The collection of scattering signals with good statistics is necessary for a reliable structural analysis. However, current conventional electron diffraction experiments using PDF (ePDF) are limited in their ability to acquire continuous diffraction rings for large nanoparticles. Herein, a new method - tilt-ePDF - is proposed to improve the data quality and compatibility of ePDF by a combination of electron diffraction and specimen tilting. In the present work, a tilt-series of electron diffraction patterns was collected from gold nanoparticles with three different sizes and a standard sample polycrystalline aluminium film for ePDF analysis. The results show that tilt-ePDF can not only enhance the continuity of diffraction rings, but can also improve the signal-to-noise ratio in the high scattering angle range. As a result, compared with conventional ePDF data, tilt-ePDF data provide structure parameters with a better accuracy and lower residual factors in the refinement against the crystal structure. This method provides a new way of utilizing ePDF to obtain accurate local structure information from nanoparticles.

Monitoring the Evolution of Relative Product Populations at Early Times during a Photochemical Reaction.

Identifying multiple rival reaction products and transient species formed during ultrafast photochemical reactions and determining their time-evolving relative populations are key steps toward understanding and predicting photochemical outcomes. Yet, most contemporary ultrafast studies struggle with clearly identifying and quantifying competing molecular structures/species among the emerging reaction products. Here, we show that mega-electronvolt ultrafast electron diffraction in combination with ab initio molecular dynamics calculations offer a powerful route to determining time-resolved populations of the various isomeric products formed after UV (266 nm) excitation of the five-membered heterocyclic molecule 2(5H)-thiophenone. This strategy provides experimental validation of the predicted high (∼50%) yield of an episulfide isomer containing a strained three-membered ring within ∼1 ps of photoexcitation and highlights the rapidity of interconversion between the rival highly vibrationally excited photoproducts in their ground electronic state.

Structural and electronic properties of MoS2 and MoSe2 monolayers grown by chemical vapor deposition on Au(111).

The exceptional electronic and photonic properties of the monolayers of transition metal dichalcogenides including the spin-orbit splitting of the valence and conduction bands at the K points of the Brillouin zone make them promising for novel applications in electronics, photonics and optoelectronics. Scalable growth of these materials and understanding of their interaction with the substrate is crucial for these applications. Here we report the growth of MoS2 and MoSe2 monolayers on Au(111) by chemical vapor deposition at ambient pressure as well as the analysis of their structural and electronic properties down to the atomic scale. To this aim, we apply ultrahigh vacuum surface sensitive techniques including scanning tunneling microscopy and spectroscopy, low-energy electron diffraction, X-ray and angle-resolved ultraviolet photoelectron spectroscopy in combination with Raman spectroscopy at ambient conditions. We demonstrate the growth of high-quality epitaxial single crystalline MoS2 and MoSe2 monolayers on Au(111) and show the impact of annealing on the monolayer/substrate interaction. Thus, as-grown and moderately annealed (<100 °C) MoSe2 monolayers are decoupled from the substrate by excess Se atoms, whereas annealing at higher temperatures (>250 °C) results in their strong coupling with the substrate caused by desorption of the excess Se. The MoS2 monolayers are strongly coupled to the substrate and the interaction remains almost unchanged even after annealing up to 450 °C.

Interplay of Kinetic Limitations and Disintegration: Selective Growth of Hexagonal Boron Nitride and Borophene Monolayers on Metal Substrates.

The CVD growth of bielemental 2D-materials by using molecular precursors involves complex formation kinetics taking place at the surface and sometimes also subsurface regions of the substrate. Competing microscopic processes fundamentally limit the parameter space for optimal growth of the desired material. Kinetic limitations for diffusion and nucleation cause a high density of small domains and grain boundaries. These are usually overcome by increasing the growth temperature and decreasing the growth rate. In contrast, the nature of molecular precursors with limited thermal stability can result in dissociation and preferential desorption, leading to an undesired or ill-defined composition of the 2D-material. Here we demonstrate these constraints in a combined low-energy electron diffraction and low-energy electron microscopy study by examining the selective formation of single-layer hexagonal boron nitride (hBN) and borophene on Ir(111) using a borazine precursor. We derive a temperature-pressure phase diagram and apply classical nucleation theory to describe our results. By considering the competing processes, we find an optimum growth temperature for hBN of 950 °C. At lower temperatures, the hBN island density is increased, while at higher temperatures the precursor disintegrates and borophene is formed. Our results introduce an additional aspect that must be considered in any high-temperature growth of bielemental 2D-materials from single molecular precursors.

Molecular structure of 3,3-pentamethylenediaziridine in gas and solution phases

The equilibrium molecular structure of the 3,3-penta-methylenediaziridine (PMDA) molecule in the gas phase was determined by gas-phase electron diffraction in combination with quantum chemical calculations up to the CCSD(T)-AE/cc-pwCVQZ level and estimation of the mean amplitudes and anharmonic vibrational corrections using quadratic and cubic force fields. 1D and 2D 1H and 13C NMR spectra of PMDA in a CDCl3 solution recorded at different temperatures revealed the temperature dependence of the chemical shifts of NH protons. Nonempirical simulations of small PMDA clusters, carried out at the DFT-B3LYP/6-31++G(2d,p) level, enabled us to suggest an interpretation of the NMR data

Can NbO Keep nbo Topology under Electrons? -Unveiling Novel Aspects of Niobium Monoxide at the Atomic Scale.

A precise investigation of NbO has been carried out by advanced electron microscopy combined with powder and single crystal X-ray diffraction (XRD). The structure of pristine NbO has been determined as Pm-3 m space group (SG) with a = 4.211 Å and the positions of Nb and O at the 3c and 3d Wyckoff positions, respectively, which is consistent with previous report based on powder XRD data. Electron beams induced a structural transition, which was investigated and explained by combining electron diffraction and atomic-resolution imaging. The results revealed that the electron beam stimulated both Nb and O atom-migrations within each fcc sublattice, and that the final structure was SG Fm-3 m with a = 4.29 Å, Nb and O at the 4a and 4b with 75 % occupancy and same chemical composition. Antiphase planar defects were discovered in the pristine NbO and related to the structural transformation. Theoretical calculations performed by density functional theory (DFT) supported the experimental conclusions.

Structural Evolution in an Annealed (Eu, Tb)-Doped ZnO/Si Nanoscale Junction: Implication for Red LED Development

Codoped (Tb,Eu) ZnO films grown by magnetron sputtering on a silicon substrate and annealed up to 1200 °C showed intense photoluminescence (PL) emission from Eu3+ ions. The high-temperature annealing led to diffusion and segregation of rare earth (RE) elements toward the bottom of the film, which induced the formation of nanometric Zn-free inclusions responsible for remarkable PL emission intensity. Combined electron diffraction, chemical contrast imaging, and optical studies of these nanometric phases have been carried out. Large inclusions of zinc silicates and RE silicates a few hundreds of nanometers in size were observed, embedded in a silica phase. The structural determination of these RE-rich inclusions was carried out by combining atomic Z contrast imaging (high-angle annular dark-field imaging) and precession electron diffraction data. Upon annealing at 1200 °C, it appeared that the structure was related to an F-type disilicate structure. Energy-dispersive X-ray spectroscopy and electron energy loss spectroscopy experiments were carried out to determine the ratios between elements and the oxidation states of the RE elements in the abovementioned inclusions. A (Tb,Eu)2Si2O7 formulation is proposed from dynamical precession electron diffraction tomography refinements, leading to a +III valence state for the RE species in agreement with spectroscopic results. PL modeling is also in good agreement with the experimental data. These results complete those obtained at 1100 °C for which the inclusions were identified as some RE10–x(SiO4)6O2–x oxyapatite structures and pointed out a combined structural and PL properties’ evolution between 1100 and 1200 °C annealing temperatures.

Resistance against Abrasive Wear and Corrosion of Laser Powder Bed Alloyed High Chromium Tool Steels

Additive manufacturing by laser-based powder bed fusion of metals (PBF-LB/M) enables the production of complex shaped components. High-carbon tool steels tend to cracking during PBF-LB/M due to internal stresses caused by the rapid solidification. Expensive atomization and long lead times for powder generate high costs in this processing route. In situ alloying during PBF-LB/M of powder blends from conventionally available powders enables a more flexible approach of alloy design. For industrial use, the mechanical properties of in situ alloyed parts must be comparable to those of conventionally manufactured parts. In some cutting and forming applications, high wear resistance and corrosion resistance are required simultaneously. High alloyed cold work tool steels with sufficient chromium solved in the metal matrix fulfill these demands. Herein, AISI H13 has been modified by Cr3C2 and elemental Cr to suit these requirements. Two novel alloys are modeled thermodynamically and processed by PBF-LB/M. In-depth microstructural investigations by backscatter electron imaging and diffraction in combination with abrasive wear tests and potentiodynamic polarization curves allow microstructure property correlations for different heat-treated conditions. Partial crack-free processing, hardenability, formation of Cr-rich carbides, and residual Cr-rich inclusions are observed and their influence on the wear and corrosion resistance is discussed.

Strain modulation in small molecule physisorption in two dimensions: LEED structure analysis and DFT modeling of the system.

The structure of the system was investigated experimentally by means of quantitative LEED I(V) analysis and computationally using dispersion corrected density functional theory (DFT-D). Three different structure models with four, five, and six molecules were considered. The lowest reliability factors and thus best agreement of measured and calculated I(V) curves was found for the structure model containing five molecules per surface unit cell. Essential features of the experimental best-fit adlayer structure are supported by DFT. A slight inclination and lateral shift of twofold coordinated molecules away from the on-top position over Na+ adsorption sites is interpreted as compensation of strain between substrate and adlayer.

Molecular Structure of Nickel Octamethylporphyrin-Rare Experimental Evidence of a Ruffling Effect in Gas Phase.

The structure of a free nickel (II) octamethylporphyrin (NiOMP) molecule was determined for the first time through a combined gas-phase electron diffraction (GED) and mass spectrometry (MS) experiment, as well as through quantum chemical (QC) calculations. Density functional theory (DFT) calculations do not provide an unambiguous answer about the planarity or non-planar distortion of the NiOMP skeleton. The GED refinement in such cases is non-trivial. Several approaches to the inverse problem solution were used. The obtained results allow us to argue that the ruffling effect is manifested in the NiOMP molecule. The minimal critical distance between the central atom of the metal and nitrogen atoms of the coordination cavity that provokes ruffling distortion in metal porphyrins is about 1.96 Å.

Electron Microscopy of Nanoporous Crystals

ConspectusNanoporous crystals, such as silica mesoporous crystals (SMCs), zeolites, metal–organic frameworks (MOFs) and covalent organic frameworks (COFs), usually have unique geometrical features of periodically arranged pores from micro-, meso- to macro-scale. They have great potential to be structurally designed toward novel materials for various applications, such as storage, separation, and catalysis. In addition to the synthesis and applications of these materials, the structural analysis of nanoporous crystals is also critical for obtaining novel materials and unravelling their structure–property relationships. The crystal structure determines the pore size and connectivity of cages/channels, which are closely related to the performance of these materials in adsorption and catalysis. However, there are significant challenges in structural analysis due to small crystal size, electron-beam sensitivity, and local defects. Due to the strong interaction between electrons and matter, electron microscopy (EM) can provide rich structural information, which at the same time can also cause structural damage to samples. As early as 1960s, EM had been used to directly observe the lattice fringes of zeolites. Subsequently, the structures of many nanoporous crystals were solved and structural details such as defects, intergrowths, and surface structures were revealed by exploiting EM during the past decades. With the development of three-dimensional electron diffraction (3D ED) and low-dose high-resolution electron microscopy imaging, these approaches become more routine for solving the structure of nanocrystals and for revealing the local structural details. In this Account, we present our previous work on EM studies of several typical nanoporous materials, including SMCs, zeolites, MOFs, the TiO2@MIL-101 composite, and COFs. Various methods and strategies were developed and applied to different materials based on the characteristics of their structures. For example, 3D electrostatic potential maps of SMCs can be reconstructed by Fourier synthesis of crystal structure factors obtained from high-resolution TEM images, leading to the discovery of many 3D mesostructures. New zeolites/MOFs/COFs structures, which were not previously solved due to their structural complexity and small crystal size, have been determined using 3D ED data from nanosized crystals or using a combination of electron diffraction and high-resolution imaging. In addition, EM methods for determining the handedness of nanocrystals have also been developed and successfully applied to SMCs and STW zeolite based on high-resolution EM imaging or dynamical electron diffraction. Recently, a new strategy involving a combination of 3D ED and cryogenic protocol was proposed to study the structure, dynamics, and the host–guest interactions in a COF material. The direct-space strategy for structure solution, implemented using a genetic algorithm, was also demonstrated to be a successful approach for solving structures from low-resolution 3D ED data. By now, EM has become one of the most widely used methods in the study of nanoporous materials. We hope this Account will promote further developments of EM and stimulate the design and synthesis of new functional nanoporous materials.

Machine learning for laser-induced electron diffraction imaging of molecular structures

Ultrafast diffraction imaging is a powerful tool to retrieve the geometric structure of gas-phase molecules with combined picometre spatial and attosecond temporal resolution. However, structural retrieval becomes progressively difficult with increasing structural complexity, given that a global extremum must be found in a multi-dimensional solution space. Worse, pre-calculating many thousands of molecular configurations for all orientations becomes simply intractable. As a remedy, here, we propose a machine learning algorithm with a convolutional neural network which can be trained with a limited set of molecular configurations. We demonstrate structural retrieval of a complex and large molecule, Fenchone (C10H16O), from laser-induced electron diffraction (LIED) data without fitting algorithms or ab initio calculations. Retrieval of such a large molecular structure is not possible with other variants of LIED or ultrafast electron diffraction. Combining electron diffraction with machine learning presents new opportunities to image complex and larger molecules in static and time-resolved studies.

Identification of (Tb,Eu)9.43(SiO4)6O2-δ Oxy-Apatite Structures as Nanometric Inclusions in Annealed (Eu,Tb)-Doped ZnO/Si Junctions: Combined Electron Diffraction and Chemical Contrast Imaging Studies.

(Tb,Eu)-doped ZnO-annealed films at 1100 °C showed intense photoluminescense (PL) emission from Eu and Tb ions. The high-temperature annealing led to a chemical segregation and a secondary Zn-free phase formation that is suspected to be responsible for the high PL intensity. Large faceted inclusions of rare-earth (RE) silicates of a size of few hundred nanometers were observed. Owing to various advanced electron microscopy techniques, a detailed microstructural study of these nanometric inclusions combining atomic Z contrast imaging (STEM) and precession electron diffraction tomography (PEDT) data was carried out and resulted in the determination of a hexagonal P63/m-type (Tb,Eu)9.43(SiO4)6O2-δ structure related to an oxy-apatite structure. Chemical analyses from spectroscopic data (energy-dispersive X-ray mapping and electron energy loss spectroscopy) at the atomic scale showed that both RE elements sitting on two independent (4f) and (6h) atomic sites have three-fold oxidation states, while refinements of their occupancy sites from PEDT data have evidenced preferential deficiency for the first one. The deduced RE-O distances and their corresponding bond valences are listed and discussed with the efficient energy transfer from Tb3+ toward Eu3+.

Exotic Long-Range Surface Reconstruction on La0.7Sr0.3MnO3 Thin Films.

Due to an extremely diverse phase space, La1-xSrxMnO3, as with other manganites, offers a wide range of tunability and applications including colossal magnetoresistance and use as spin-polarized electrodes. Here, we study an unprecedented, exotic surface reconstruction (6 × 6) in La1-xSrxMnO3 (x = 0.3) observed via low-energy electron diffraction (LEED). Scanning tunneling microscopy (STM) shows the surface is relatively flat, with unit-cell step heights, and X-ray photoelectron spectroscopy (XPS) reveals a strong degree of Sr segregation at the surface. By combining electron diffraction and first-principles computations, we propose that the long-range surface reconstruction consists of a Sr-segregated surface with La (6 × 6) ordering. This study expands our understanding of manganite systems and underscores their ability to form interesting surface reconstructions, driven largely by cation segregation that can potentially be controlled for tuning surface ordering.

Nickel-containing nanophases as the carriers of catalytic active sites in the ethylene oligomerization in the presence of systems based on Ni(acac)2 and organoaluminum compounds

The nature and size of particles, formed under conditions of the ethylene oligomerization in the systems based on nickel bis(acetylacetonate) with Et2AlCl or Et2AlCl·Cl2AlEt (Al: Ni = 50: 1), have been determined using ex situ HRTEM, electron diffraction and EDAX in combination with EPR spectroscopy and kinetic studies. It was revealed that the average sizes of Ni-containing particles depend on the nature of cocatalyst and are ∼3.5 and ∼6 nm for the systems derived from Et2AlCl and Et2AlCl·Cl2AlEt, respectively. It was shown that these nanoparticles act as the carriers of catalytically active sites during the C2H4 conversion.

Combined gas electron diffraction and mass spectrometric experimental setup at Bielefeld University.

We have designed and constructed a combined experimental setup for synchronous measurements of electron diffraction patterns and mass-spectra of gas samples. Test measurements have been performed for acetic acid at two temperatures, 296 K and 457 K. Electron diffraction data have been analyzed taking into account mass spectra measured in the same experiments. From the diffraction intensities, molecular structures and mole fractions of the acetic acid monomer and dimer have been refined. The obtained results demonstrate the importance of measuring mass spectra in gas electron diffraction experiments. In particular, it is possible to detect the sample decomposition, which can be used for the optimization of experimental conditions and for the data interpretation. The length of the hydrogen bond in the acetic acid dimer determined in this work, re(O⋯H) = 1.657(9) Å, is in good agreement with modern theoretical predictions. We recommend measuring the diffraction patterns of acetic acid for the calibration of the sample pressure in the diffraction volume.

Electronic and nuclear dynamics in one

Chemical Physics Because of the complex, ultrafast interplay between nuclear and electronic degrees of freedom, probing both nuclear and electronic dynamics in excited electronic states within a single time-resolved experiment is a great challenge. Yang et al. used ultrafast electron diffraction in combination with ab initio nonadiabatic molecular dynamics and diffraction simulations to study the relaxation dynamics of isolated pyridine molecules after photoexcitation to the S1 state (see the Perspective by Domcke and Sobolewski). They showed that electronic state evolution and molecular structural changes can be recorded simultaneously and independently by tracing a transient signal in small-angle inelastic scattering and large-angle elastic diffraction, respectively. Science , this issue p. [885][1]; see also p. [820][2] [1]: /lookup/doi/10.1126/science.abb2235 [2]: /lookup/doi/10.1126/science.abb9937

Molecular structure of cobalt(II) etioporphyrin-II determined by combined gas-phase electron diffraction/mass-spectrometry and quantum chemical calculations: Searching a ruffling and saddling effects

The structure of a free cobalt (II) etioporphyrin-II (CoEP-II) molecule was investigated for the first time by combined gas-phase electron diffraction and mass spectrometry (GED/MS) method. Selected experimental parameters are as follows: rh1(Co-N) = 1.976(5) Å, rh1(N-Cα) = 1.383(4) Å, rh1(Cα-Cβ) = 1.455(4) Å, rh1(Cα-Cm) = 1.388(3) Å, rh1(Cβ-Cβ) = 1.371(3) Å. Despite the significant shortening of the internuclear distance between metal and nitrogen atoms compared to CuEP-II and ZnEP-II, the macrocycle of CoEP-II was determined to be quasiplanar. The ground state of CoEP-II is of the low spin and possesses 2A symmetry. These results correlate with the structural features of cobalt (II) porphyrin (CoP) molecule which is planar in the ground state (2A1g).

Nonequilibrium Thermodynamics of Colloidal Gold Nanocrystals Monitored by Ultrafast Electron Diffraction and Optical Scattering Microscopy.

Metal nanocrystals exhibit important optoelectronic and photocatalytic functionalities in response to light. These dynamic energy conversion processes have been commonly studied by transient optical probes to date, but an understanding of the atomistic response following photoexcitation has remained elusive. Here, we use femtosecond resolution electron diffraction to investigate transient lattice responses in optically excited colloidal gold nanocrystals, revealing the effects of nanocrystal size and surface ligands on the electron-phonon coupling and thermal relaxation dynamics. First, we uncover a strong size effect on the electron-phonon coupling, which arises from reduced dielectric screening at the nanocrystal surfaces and prevails independent of the optical excitation mechanism (i.e., inter- and intraband). Second, we find that surface ligands act as a tuning parameter for hot carrier cooling. Particularly, gold nanocrystals with thiol-based ligands show significantly slower carrier cooling as compared to amine-based ligands under intraband optical excitation due to electronic coupling at the nanocrystal/ligand interfaces. Finally, we spatiotemporally resolve thermal transport and heat dissipation in photoexcited nanocrystal films by combining electron diffraction with stroboscopic elastic scattering microscopy. Taken together, we resolve the distinct thermal relaxation time scales ranging from 1 ps to 100 ns associated with the multiple interfaces through which heat flows at the nanoscale. Our findings provide insights into optimization of gold nanocrystals and their thin films for photocatalysis and thermoelectric applications.

Interfacial synthesis of crystalline two-dimensional cyano-graphdiyne.

A well-defined crystalline cyano-functionalized graphdiyne (CN-GDY) is synthesized at a liquid/liquid interface through alkyne-alkyne coupling reactions. The configurations and nanostructures of CN-GDY were well characterized by TEM, SEM, AFM, XPS, and Raman spectroscopy. HR-TEM and selected area electron diffraction (SAED) in combination with structure simulation firmly revealed a 9-fold stacking mode for CN-GDY.