Ultrafast electron microscopy in material science

Recent advances in the ultrafast transmission electron microscope(UTEM), with combined spatial and temporal resolutions, have made it possible to directly visualize the atomic, electronic, and magnetic structural dynamics of materials.In this review, we highlight the recent progress of UTEM techniques and their applications to a variety of material systems.It is emphasized that numerous significant ultrafast dynamic issues in material science can be solved by the integration of the pump–probe approach with the well-developed conventional transmission electron microscopy(TEM) techniques. For instance, UTEM diffraction experiments can be performed to investigate photoinduced atomic-scale dynamics, including the chemical reactions, non-equilibrium phase transition/melting, and lattice phonon coupling. UTEM imaging methods are invaluable for studying, in real space, the elementary processes of structural and morphological changes, as well as magnetic-domain evolution in the Lorentz TEM mode, at a high magnification. UTEM electron energy-loss spectroscopic techniques allow the examination of the ultrafast valence states and electronic structure dynamics, while photoinduced near-field electron microscopy extends the capability of the UTEM to the regime of electromagnetic-field imaging with a high real space resolution.     

Determination of Mean Inner Potential by Electron Holography Along with Electron Dynamic Simulation

Off-axis electron holography in a field-emission-gun transmission electron microscope and electron dynamic simulation is used to determine the mean inner potential of copper.The phase shift of object wave versus specimen thickness is calculated up to 30 nm using electron dynamic formula,and the sample thickness is decided by match of the experimental and calculated phase shift.Based on the measured phase shift the calculated mean inner potential of Cu is 21.2V,which agrees with the reported values within the experimental error.     

Quantum transport signatures of non-trivial topological edge states in a ring-shaped Su–Schrieffer–Heeger double-chain system

In the ring-shaped Su–Schrieffer–Heeger(SSH)double-chain,the quantum interference between the two different electron tunneling paths of the upper and lower chains has an important influence on the electron transport properties of non-trivial topological edge states.Here,we have studied the electron transport signatures of non-trivial topological edge states in a ring-shaped SSH double-chain system based on the wave-guide theory and transfer-matrix method.In the ringshaped SSH double-chain with the upper chain being different from the lower one,it is demonstrated that the electron transmission probability displays the four and two resonance peaks associated with the non-trivial topological edge states in the weak and strong coupling regimes,respectively.Whereas in the case of the upper chain being the same as the lower one,the two transmission resonance peaks associated with the non-trivial topological edge states in the weak coupling regime are only found,and that in the strong coupling regime disappear that originated from the destructive interference between the two different electron tunneling paths of the upper and lower chains.Consequently,the variation of the number of transmission resonance peaks associated with the non-trivial topological edge states in the weak and strong coupling regimes suggests that an alternative scheme for detecting non-trivial topological edge states in the ring-shaped SSH doublechain system.     

Hydrothermal synthesis and dielectric properties of chrysanthemum-like ZnO particles

By orthogonal design theory, technological parameters of chrysanthemum-like ZnO particles prepared in a hydrothermal process are optimized. This paper reports a set of technological parameters for growing chrysanthemum-like ZnO particles on a large scale. It investigates the morphologies and crystalline structures of the as-synthesized three-dimensional ZnO particles with a scanning electron microscope, x-ray diffractometer and transmission electron microscope, and the possible growth mechanism on the three-dimensional ZnO particles. The experimental results indicate that the values of ε', ε ' and \tan δ_e gradually increase in the X band with the improvement of the developmental level of chrysanthemum-like ZnO particles, implying that the electromagnetic wave absorbing property depends on the morphologies of three-dimensional ZnO particles.     

Influence of Oxygen Pressure on Structural and Sensing Properties of β-Ga2O3 Nanomaterial by Thermal Evaporation

We prepare the gallium oxide （β-Ga2O3） nanomaterials from gallium and oxygen by thermal evaporation in the argon atmosphere and research their oxygen sensing under UV illumination with different oxygen pressures. X-ray diffraction reveals that the synthesized product is monoclinic gallium oxide, it is further confirmed by electron diffraction of transmission electron microscope, and its morphology through the observation using scanning electron microscope reveals thatβ-Ga2O3 nanobelts with a breadth less than lOOnm and length of severai micrometers are synthesized under low oxygen pressure, while the nano/microbelts are synthesized under high oxygen pressure. Room-temperature oxygen sensing is tested under at 254 nm illumination and it is found that the current decreases quickly first and then slowly with oxygen pressure from low to high.     

High Refractive Index Ti_3O_5 Films for Dielectric Metasurfaces

Ti_3 O_5 films are deposited with the help of an electron beam evaporator for their applications in metasurfaces.The film of subwavelength(632 nm) thickness is deposited on a silicon substrate and annealed at 400°C. The ellipsometr.y result shows a high refractive index above 2.5 with the minimum absorption coefficient in the visible region, which is necessary for high efficiency of transparent metasurfaces. Atomic force microscopy analysis is employed to measure the roughness of the as-deposited films. It is seen from micrographs that the deposited films are very smooth with the minimum roughness to prevent scattering and absorption losses for metasurface devices. The absence of grains and cracks can be seen by scanning electron microscope analysis, which is favorable for electron beam lithography. Fourier transform infrared spectroscopy reveals the transmission and reflection obtained from the film deposited on glass substrates. The as-deposited film shows high transmission above 60%,whieh is in good agreement with metasurfaces.     

Preparation of ZnO Nanospheres and Their Applications in Dye-Sensitized Solar Cells

ZnO nanospheres are synthesized by a two-step self-assembly method. X-ray diffraction pattern and Raman scattering spectra measurements show that all the samples present a typical wurtzite structure. A regular sphere shape is inspected by field emission scanning electron microscope and transmission electron microscope for the samples. It is shown that the as-synthesized ZnO nanosphere is composed of numbers of primary particles with size of around 1Ohm. A possible growth mechanism for the two-step self-assembly ZnO nanosphere is proposed. After applying the ZnO nanospheres to dye-sensitized solar cells （DSSCs）, a 117% increase of the overall light to electricity conversion efficiency η is observed compared with that of the ZnO nanopartieles based DSSCs. Associated with the UV-vis results, light scattering is assigned to the great improvement of η.     

Synthesis of high-TC ferromagnetic Mn-doped ZnO nanorods by thermal evaporation

This paper reports that a large amount of Mn-doped ZnO nanorods have been synthesized through thermal evaporation. The morphologies and properties are studied with x-ray diffraction, a scanning electron microscope, transmission electron microscope and Raman spectroscope. The results indicate that the manganese atoms occupy the zinc vacancies in the wurtzite lattice of ZnO without forming secondary phases. The exact manganese content has been studied by the x-ray fluorescence spectrum. Meanwhile, the magnetic moment versus temperature result proves that the as-prepared Mn-doped ZnO nanorods show ferromagnetic properties at temperatures as high as 400 K. These studies provide a good understanding of the origin of magnetic properties in diluted magnetic semiconductors.     

Fabrication of high-quality colloidal photonic crystals with sharp band edges for ultrafast all-optical switching

Application of the pressure controlled isothermal heating vertical deposition method to the fabrication of colloidal photonic crystals is systematically investigated in this paper. The fabricated samples are characterized by scanning electron microscope and transmission spectrum. High-quality samples with large transmissions in the pass bands and the sharp band edges are obtained and the optimum growth condition is determined. For the best sample, the transmission in the pass bands approaches 0.9 while that in the band gap reaches 0.1. More importantly, the maximum differential transmission as high as 0.1/nm is achieved. In addition, it is found that the number of stacking layers does not increase linearly with concentration of PS spheres in a solution, and a gradual saturation occurs when the concentration of PS spheres exceeds 1.5 wt.%. The uniformity of the fabricated samples is examined by transmission measurements on areas with different sizes. Finally, the tolerance of the fabricated samples to baking was studied.     

Low-Field Emission from Iron Oxide-Filled Carbon Nanotube Arrays

Arrays of multi-walled carbon nanotubes (MWCNTs) filled with iron oxide have been fabricated by a one-step route based on the pyrolysis of ferrocene under a well-chosen synthesis condition. The MWCNT arrays were observed with a scanning electron microscope, with which an energy dispersive x-ray spectrum (EDXS) was also acquired, and they are analysed by x-ray diffraction. Furthermore, individual MWCNTs were studied by using selected area electron diffraction (SAED) and the EDXS in a transmission electron microscopy observation. All the observation and analysis confirmed that the MWCNTs were filled with iron oxide. Field emission from these arrays of iron oxide-filled MWCNTs was measured and the turn-on field was determined to range from 0.83-1.01 V/μm, appearing to be much lower than those of arrays of pure MWCNTs and arrays of nitrogen-doped MWCNTs fabricated in similar ways. The possible reasons of the observed low-field emission are discussed.     

High-resolution transmission electron microscopy and bulk magnetometry study of LaFe11.5Si1.5 compound

This paper studies the microstructural and magnetic properties of LaFe11.5Si1.5 compound by means of high-resolution transmission electron microscope and bulk magnetometry measurements. The crystalline structure is accompanied with the noncrystalline and nanocrystalline structures. This characteristic is the reflection of the crystalline process held by quenching. The inverse susceptibilities diverge and deviate from Curie-Weiss law under low applied magnetic fields. This paper proposes the possible mechanism between the anomalous susceptibilities and microstructure, and offers a perspective on the magnetic properties of metastable intermetallic compounds.     

Synthesis, structure and antiferromagnetic behaviour of brannerite MnV2O6

Brannerite MnV 2 O 6 with plate-like shape is successfully synthesized by hydrothermal method.Its crystal structure and morphology are investigated by x-ray diffraction (XRD),scanning electron microscopy (SEM),transmission electron microscope (TEM),high resolution transmission electron microscopy (HRTEM) and select area electronic diffraction (SAED).The results show that the brannerite MnV 2 O 6 with monoclinic structure has a uniform plate-like shape with a diameter of about 5-8 μm and a thickness of about 500 nm.SAED patterns further confirm the structure of the brannerite MnV 2 O 6 and the single crystalline character of the plate crystal.Magnetic properties are measured by superconducting quantum interference device (SQUID) in a temperature range of 2-300 K under a magnetic field of 1 T.The magnetic measurement results indicate that the material undergoes an antiferromagnetic transition with a N’eel temperature of 17 K.Above 50 K,the inverse susceptibility is fitted well to the Curie-Weiss law with a calculated moment of 5.98 μ B.Finally,the origin of antiferromagnetic behaviour in the brannerite MnV 2 O 6 is explained by means of Anderson model.     

Determination of Thickness and Optical Constants of ZnO Thin Films Prepared by Filtered Cathode Vacuum Arc Deposition

ZnO thin films are prepared on glass substrates by filtered cathode vacuum arc （FCVA） deposition technique. A new method is demonstrated to extract the refractive index, thickness and optical band gap of ZnO thin films from the transmission spectrum alone. The refractive index is calculated from the extremes of the interference fingers. The transmission spectrum is divided into two terms, non-interference term and interference effect term. The thickness of thin films is calculated by simulating the interference term, and the non-interference term is used to calculate optical band gap with the gained thickness. The results are compared with measurements by using an ellipsometry and a scanning electron microscope.     

Charge ordering modulations in a Bi_0.4Ca_0.6MnO₃ film with a thickness of 110 nm

The low temperature sample stage in a transmission electron microscope is used to investigate the charge ordering behaviours in a Bi 0.4 Ca 0.6 MnO 3 film with a thickness of 110 nm at 103 K.Six different types of superlattice structures are observed using the selected-area electron diffraction(SAED) technique,while three of them match well with the modulation stripes in high-resolution transmission electron microscopy(HRTEM) images.It is found that the modulation periodicity and direction are completely different in the region close to the Bi 0.4 Ca 0.6 MnO 3 /SrTiO 3 interface from those in the region a little further from the Bi 0.4 Ca 0.6 MnO 3 /SrTiO 3 interface,and the possible reasons for this are discussed.Based on the experimental results,structural models are proposed for these localized modulated structures.     

Charge ordering modulations in a Bi0.4Ca0.6MnO3 film with a thickness of 110 nm

The low temperature sample stage in a transmission electron microscope is used to investigate the charge ordering behaviours in a Bi0.4Ca0.6MnO3 film with a thickness of 110 nm at 103 K. Six different types of superlattice structures are observed using the selected-area electron diffraction （SAED） technique, while three of them match well with the modulation stripes in high-resolution transmission electron microscopy （HRTEM） images. It is found that the modulation periodicity and direction are completely different in the region close to the Bi0.4Ca0.6MnO3/SrTiO3 interface from those in the region a little further from the Bi0.4Ca0.6MnO3/SrTiO3 interface, and the possible reasons for this are discussed. Based on the experimental results, structural models are proposed for these localized modulated structures.     

Fabrication and electrochemical performance of graphene–ZnO nanocomposites

Graphene–ZnO nanocomposites were synthesized successfully through a one-step solvothermal approach. The morphology, structure, and composition of the prepared nanocomposites were investigated by scanning electron microscopy(SEM), transmission electron microscope(TEM), laser micro Raman spectroscopy, and Fourier transform infra-red spectroscopy(FT-IR). The outcomes confirmed that this approach is comparatively steady, practicable, and operable compared with other reported methods. The electrochemical performance of the graphene-ZnO electrodes was analyzed through cyclic voltammetry, altering-current(AC) impedance, and chronopotentiometry tests. The graphene–ZnO electrodes exhibited an improved electrode performance with higher specific capacitance(115 F·g-1), higher electrochemical stability, and higher energy density than the graphene electrodes and most reported graphene–ZnO electrodes. Graphene–ZnO nanocomposites have a steady reversible charge/discharge behavior, which makes them promising candidates for electrochemical capacitors(ECs).     

Synthesis of large-scale GaN nanowires by ammoniating Ga2O3 films on Co layer deposited on Si（111） substrates

A mass of GaN nanowires has been successfully synthesized on Si（111） substrates by magnetron sputtering through ammoniating Ga2O3/Co films at 950℃. X-ray diffraction, scanning electron microscopy, high resolution transmission electron microscope and Fourier transformed infrared spectra are used to characterize the samples. The results demonstrate that the nanowires are of single-crystal GaN with a hexagonal wurtzite structure and possess relatively smooth surfaces. The growth mechanism of GaN nanowires is also discussed.     

Effect of Sb-doping on the morphology and dielectric properties of chrysanthemum-like ZnO nanowire clusters

Chrysanthemum-like ZnO nanowire clusters with different Sb-doping concentrations were prepared using a hydrothermal process. The microstructures, morphologies, and dielectric properties of the as-prepared products were characterized by X-ray diffraction (XRD), high-resolution transmission electron microscopy (HRTEM), field emission environment scanning electron microscope (FEESEM), and microwave vector network analyzer respectively. The results indicate that the as-prepared products are Sb-doped ZnO single crystallines with a hexagonal wurtzite structure, the flower bud saturation degree F d is obviously different from that of the pure ZnO nanowire clusters, the good dielectric loss property is found in Sb-doped ZnO products with low density, and the dielectric loss tangent tanδ e increases with the increase of the Sb-doping concentration in a certain concentration range.     

Refit Silver Nanostructures Using a Convergent Electron Beam

Using a superionic conductor AgI thin film and a direct current electric field, we synthesize silver nanowires in diameter of about lOOnm. In order to refit the prepared nanowires, the samples are irradiated by a convergent electron beam （200 k V） inside a transmission electron microscope to prepare new small silver nanostructures. The new nanostructures are investigated in situ by high-resolution transmission electron microscope. This electron- induced crystal growth method is useful for technical applications in fabrication of nanodevices.     

Investigation of the growth process of Si nanowires using the vapour－liquid－solid mechanism

Silicon nanowires have been grown by the thermal decomposition of silane via the vapour－liquid－solid (VLS) mechanism. Three different stages of VLS growth (eutectic alloy formation, crystal nucleation and unidirectional growth) were studied separately using a scanning electron microscope and a high-resolution transmission electron microscope. Very short silicon nanowires prepared under particular conditions provide direct evidence of the VLS mechanism on a nanometre scale. Our results will be very helpful for the controllable synthesis of Si nanowires.