The Effect of Electron Beam Irradiation on Electron Diffraction Patterns of Bi-Sr-Ca-Cu-O High- Tc Superconductors

The effect of electron irradiation having the energy of 75, 100, and 200 keV on structural modifications of Bi-2212 superconducting samples has been studied. For the last energy, the irradiation time from zero to 150 min was used. At a constant energy of the electrons, the observed phenomena consist in the disappearance of the incommensurate unidimensional modulation, in the decreasing of spots' intensity and their elongation along the equivalent crystallographic axis a, and even spot splitting with the occurrence of double extra spots, with the increase of the irradiation time.After electron irradiation with energy of 75 and 100 keV, the structural modifications lead to some spot patterns consisting of some planar lattices (in some cases a pseudo-tetragonal one) that are twisted on each other at different angles (8°, 13.6°, 19°, etc.) around the axis of the incident electron beam. For the irradiation at increased doses of thin microcrystals having reduced lateral dimensions, the electron diffraction spots were arranged in discrete or partial continuous Debye rings or continuous concentric Debye rings characteristic for the polycrystalline state.After electron irradiation with energy of 200 keV, the effects of electron irradiation on Bi-2212 samples depend strongly on irradiation fluence rate and time and consisted in the following: disordering defects in the diffraction patterns (disappearance of some spots, spot intensity modification, streaks occurrence, spot elongation); alteration and disappearance of incommensurate structural modification; conversion of single crystal particle areas into polycrystalline material; and quasi-amorphization.A simple approach based on the evaluation of the displacement yield of in-plane oxygen atoms vs. irradiation time for the different incident energy and electron fluence rates could explain the general trend of irradiation damage in HTS materials.     

Organic High Electron Mobility Transistors Realized by 2D Electron Gas

A key breakthrough in inorganic modern electronics is the energy‐band engineering that plays important role to improve device performance or develop novel functional devices. A typical application is high electron mobility transistors (HEMTs), which utilizes 2D electron gas (2DEG) as transport channel and exhibits very high electron mobility over traditional field‐effect transistors (FETs). Recently, organic electronics have made very rapid progress and the band transport model is demonstrated to be more suitable for explaining carrier behavior in high‐mobility crystalline organic materials. Therefore, there emerges a chance for applying energy‐band engineering in organic semiconductors to tailor their optoelectronic properties. Here, the idea of energy‐band engineering is introduced and a novel device configuration is constructed, i.e., using quantum well structures as active layers in organic FETs, to realize organic 2DEG. Under the control of gate voltage, electron carriers are accumulated and confined at quantized energy levels, and show efficient 2D transport. The electron mobility is up to 10 cm² V^?1 s^?1, and the operation mechanisms of organic HEMTs are also argued. Our results demonstrate the validity of tailoring optoelectronic properties of organic semiconductors by energy‐band engineering, offering a promising way for the step forward of organic electronics.     

Dependence of Electron Density on Fermi Energy in N-Type Gallium Antimonide

The majority electron density as a function of the Fermi energy is calculated in zinc blende, n-type GaSb for donor densities between 10¹⁶ cm⁻³ and 10¹⁹ cm⁻³. These calculations solve the charge neutrality equation self-consistently for a four-band model (three conduction sub-bands at Γ, L, and X and one equivalent valence band at Γ) of GaSb. Our calculations assume parabolic densities of states and thus do not treat the density-of-states modifications due to high concentrations of dopants, many body effects, and non-parabolicity of the bands. Even with these assumptions, the results are important for interpreting optical measurements such as Raman measurements that are proposed as a nondestructive method for wafer acceptance tests.     

Concentration dependence of lipopolymer self-diffusion in supported bilayer membranes

Self-diffusion coefficients of poly(ethylene glycol)2k-derivatized lipids (DSPE-PEG2k-CF) in glass-supported DOPC phospholipid bilayers are ascertained from quantitative fluorescence recovery after photobleaching (FRAP). We developed a first-order reaction–diffusion model to ascertain the bleaching constant, mobile fraction and lipopolymer self-diffusion coefficient D_s at concentrations in the range c ≈ 0.5–5 mol%. In contrast to control experiments with 1,2-dioleoyl-sn-glycero-3-phosphoethanolamine-N-(7-nitro-2-1,3-benzoxadiazol-4-yl) (ammonium salt) (DOPE-NBD) in 1,2-dioleoyl-sn-glycero-3-phosphocholine (DOPC), the lipopolymer self-diffusion coefficient decreases monotonically with increasing concentration, without a distinguishing mushroom-to-brush transition. Our data yield a correlation D_s = D₀/(1 + αc), where D₀ ≈ 3.36 µm² s⁻¹ and α ≈ 0.56 (with c expressed as a mole percent). Interpreting the dilute limit with the Scalettar–Abney–Owicki statistical mechanical theory for transmembrane proteins yields an effective disc radius a_e ≈ 2.41 nm. On the other hand, the Bussell–Koch–Hammer theory, which includes hydrodynamic interactions, yields a_e ≈ 2.92 nm. As expected, both measures are smaller than the Flory radius of the 2 kDa poly(ethylene glycol) (PEG) chains, R_F ≈ 3.83 nm, and significantly larger than the nominal radius of the phospholipid heads, a_l ≈ 0.46 nm. The diffusion coefficient at infinite dilution D₀ was interpreted using the Evans–Sackmann theory, furnishing an inter-leaflet frictional drag coefficient b_s ≈ 1.33 × 10⁸ N s m⁻³. Our results suggest that lipopolymer interactions are dominated by the excluded volume of the PEG-chain segments, with frictional drag dominated by the two-dimensional bilayer hydrodynamics.     

Low Voltage Transmission Electron Microscopy of Graphene

The initial isolation of graphene in 2004 spawned massive interest in this two‐dimensional pure sp² carbon structure due to its incredible electrical, optical, mechanical, and thermal effects. This in turn led to the rapid development of various characterization tools for graphene. Examples include Raman spectroscopy and scanning tunneling microscopy. However, the one tool with the greatest prowess for characterizing and studying graphene is the transmission electron microscope. State‐of‐the‐art (scanning) transmission electron microscopes enable one to image graphene with atomic resolution, and also to conduct various other characterizations simultaneously. The advent of aberration correctors was timely in that it allowed transmission electron microscopes to operate with reduced acceleration voltages, so that damage to graphene is avoided while still providing atomic resolution. In this comprehensive review, a brief introduction is provided to the technical aspects of transmission electron microscopes relevant to graphene. The reader is then introduced to different specimen preparation techniques for graphene. The different characterization approaches in both transmission electron microscopy and scanning transmission electron microscopy are then discussed, along with the different aspects of electron diffraction and electron energy loss spectroscopy. The use of graphene for other electron microscopy approaches such as in‐situ investigations is also presented.     

A Systematic Approach for Multidimensional,Closed-Form Analytic Modeling: Minority Electron Mobilities in Ga1?xAlxAs Heterostructures

A significant, practical challenge, which arises in developing computationally efficient physical models for use in computer simulations of microelectronic and optoelectronic devices (for example, transistors in digital cellular phones and lasers in optical networks, respectively), is to represent vast amounts of numerical data for transport properties in two or more dimensions in terms of closed form analytic expressions. In this paper, we present a general methodology to achieve the above goal for a class of numerical data in a bounded two-dimensional space. We then apply this methodology to obtain a closed-form analytic expression for the minority electron mobilities at 300 K in p-type Ga_1−xAl_xAs as functions of the acceptor density N_A between 10¹⁶ cm⁻³ and 10²⁰ cm⁻³ and the mole fraction of AlAs x between 0.0 and 0.3. This methodology and its associated principles, strategies, regression analyses, and graphics are expected to be applicable to other problems beyond the specific case of minority mobilities addressed in this paper.     

Electron microscopy study of striation contrast in Al-Cu-Co-Si decagonal quasicrystals

Detailed transmission electron microscopy study was carried out in single crystals of a decagonal phase in the Al-Cu-Co-Si quaternary system. X-ray diffraction and convergent beam electron diffraction patterns of the powder samples confirmed the structures to be decagonal quasicrystals. No microcrystalline nor crystalline phases could be identified. Thin slices normal to the 10-fold directions were prepared for transmission electron microscopy. Diffuse streaks along symmetric directions around the fundamental spots were observed in the diffraction patterns. Bright field images and dark field images showed discontinuous lines or striations lying perpendicular to the direction of diffuse streaking. The striation contrast appears to be originating from anti-phase boundary (APB) in the decagonal superstructures. The diffuse streaks seem to be a characteristic feature of a partially ordered decagonal superlattice structure. The atomic rearrangement or phasonic movement in certain symmetric directions along the pentagrids or Ammann lines in the structure has obviously caused the type of contrast observed in the images. The evolution of rhombic domains consisting of APBs in localized regions can be understood as one of the signature of an intermediate structural state formed prior to a superstructure formation.     

Extracting Electron Densities in N-Type GaAs From Raman Spectra: Theory

In this paper, we present the theory for calculating Raman line shapes as functions of the Fermi energy and finite temperatures in zinc blende, n-type GaAs for donor densities between 10¹⁶ cm⁻³ and 10¹⁹ cm⁻³. Compared to other theories, this theory is unique in two respects: 1) the many-body effects are treated self-consistently and 2) the theory is valid at room temperature for arbitrary values of the ratio R = (Q²/α), where Q is the magnitude of the normalized wave vector and α is the normalized frequency used in the Raman measurements. These calculations solve the charge neutrality equation self-consistently for a two-band model of GaAs at 300 K that includes the effects of high carrier concentrations and dopant densities on the perturbed densities of states used to calculate the Fermi energy as a function of temperature. The results are then applied to obtain the carrier concentrations from Fermi energies in the context of line shapes in Raman spectra due to the coupling between longitudinal optical phonons and plasmons. Raman measurements have been proposed as a non-destructive method for wafer acceptance tests of carrier density in semiconductor epilayers. The interpretation of Raman spectra to determine the majority electron density in n-type semiconductors requires an interdisciplinary effort involving experiments, theory, and computer-based simulations and visualizations of the theoretical calculations.     

Interlaboratory Study on the Lithographically Produced Scanning Electron Microscope Magnification Standard Prototype

NIST is in the process of developing a new scanning electron microscope (SEM) magnification calibration reference standard useful at both high and low accelerating voltages. This standard will be useful for all applications to which the SEM is currently being used, but it has been specifically tailored to meet many of the particular needs of the semiconductor industry. A small number of test samples with the pattern were prepared on silicon substrates using electron beam lithography at the National Nanofabrication Facility at Cornell University. The structures were patterned in titanium/palladium with maximum nominal pitch structures of approximately 3000 μm scaling down to structures with minimum nominal pitch of 0.4 (μm. Eighteen of these samples were sent out to a total of 35 university, research, semiconductor and other industrial laboratories in an interlaboratory study. The purpose of the study was to test the SEM instrumentation and to review the suitability of the sample design. The laboratories were asked to take a series of micrographs at various magnifications and accelerating voltages designed to test several of the aspects of instrument performance related to general SEM operation and metrology. If the instrument in the laboratory was used for metrology, the laboratory was also asked to make specific measurements of the sample. In the first round of the study (representing 18 laboratories), data from 35 instruments from several manufacturers were obtained and the second round yielded information from 14 more instruments. The results of the analysis of the data obtained in this study are presented in this paper.     

Green synthesis of silver nanoparticles and their applications as colorimetric probe for determination of Fe3+ and Hg2+ ions

Silver nanoparticles (AgNPs) were prepared by a green method using Cordia myxa leaf extract. They were characterised by UV–vis spectroscopy, Fourier transform infrared spectroscopy and their X‐ray diffraction pattern. Their sizes were determined by scanning electron micrographs, transmission electron micrographs imaging and dynamic light scattering analysis. The shapes of nanoparticles were spherical or truncated triangular and their average size was determined to be 51.6 nm. Their solution was stable at least for one month. The prepared AgNPs were used as a selective chemical sensor for determination of iron(III) (only when Cl⁻ ions were present in the medium) and mercury(II) ions with detection limits of 0.084 and 0.037 nM, respectively. It was shown that the mechanism of these detections is through oxidation of Ag atoms by Fe³⁺ and Hg²⁺ ions.Inspec keywords: visible spectra, nanoparticles, transmission electron microscopy, nanofabrication, ultraviolet spectra, chemical sensors, scanning electron microscopy, silver, X‐ray diffraction, Fourier transform infrared spectra, oxidationOther keywords: Ag, Cordia myxa leaf extract, iron(III) ions, mercury(II) ions, oxidation, scanning electron micrographs, Fourier transform infrared spectroscopy, silver nanoparticles, chemical sensor, dynamic light scattering analysis, transmission electron micrographs, X‐ray diffraction pattern, UV–vis spectroscopy, colorimetric probe, green synthesis     

Study of Fullerenes,Nanotubes and Nanowires by Transmission Electron Microscopy

Abstract

The arc discharge reactor developped by Kratschmer et al^l produces a variety of new carbonaceous solids, including fullerenes and different kinds of graphitic cages of nanometric size, which can be hollow or filled with various materials. Complementary to neutron and X ray diffraction techniques, transmission electron microscopy rapidly appeared among the most important techniques for studying the structures of fullerenes and fullerites^2?6. Very generally, it allows us to obtain diffraction patterns of micronic size crystallites and the different imaging modes provide an unique tool to investigate in real space the morphology of the crystallites, structural defects faults at microscopic and nanometric scales^6?8. It is possible to perform very local chemical analysis and to study chemical environments in order to determine the nature of chemical bonds⁹. Futhermore, it is possible to study phase transitions through in situ experiments^5,10–12 and the behaviour of fullerites upon electron irradiation¹³. The interest of this technique has appeared even more crucial with the discovery of narotubes¹⁴ and their ability to be filled¹⁵ since, because of their nanometric size, the electron microscopy is at the moment the unique tool allowing their study and is therefore a challenge in the development of these new materials for applications in nanotechnology.     

SUSANS With Polarized Neutrons

Super Ultra-Small Angle Neutron Scattering (SUSANS) studies over wave vector transfers of 10^–4 nm^–1 to 10^–3 nm^–1 afford information on micrometer-size agglomerates in samples. Using a right-angled magnetic air prism, we have achieved a separation of ≈10 arcsec between ≈2 arcsec wide up- and down-spin peaks of 0.54 nm neutrons. The SUSANS instrument has thus been equipped with the polarized neutron option. The samples are placed in a uniform vertical field of 8.8 × 10⁴ A/m (1.1 kOe). Several magnetic alloy ribbon samples broaden the up-spin neutron peak significantly over the ±1.3 × 10^–3 nm^–1 range, while leaving the down-spin peak essentially unaltered. Fourier transforms of these SUSANS spectra corrected for the instrument resolution, yield micrometer-range pair distribution functions for up- and down-spin neutrons as well as the nuclear and magnetic scattering length density distributions in the samples.     

Electron Transport in Cryocrystals

We review our recent experimental studies of the excess electron transport in cryocrystals and cryoliquids. We use a muon spin relaxation technique to explore the phenomenon of delayed muonium formation: excess electrons liberated in the ⁺ ionization track converge upon the positive muons and form Mu (⁺e^–) atoms in which the ⁺ polarization is partially lost. The spatial distribution of such electrons with respect to the moon is shown to be highly anisotropic: the ⁺ thermalizes well downstream from the center of the electron distribution. Measurements in electric fields up to 30 kV/cm allow one to estimate the characteristic muon-electron distance in different insulators: the results range from 10^–6 cm to 10^–4 cm. This circumstance makes the basis of a recently developed new technique for electron transport studies on microscopic scale: electron mobility can be extracted when both the characteristic muon-electron distance and characteristic time for muonium atom formation are determined. The microscopic length scale enables the electron to sometimes spend its entire free lifetime in a state which may not be detected by conventional macroscopic techniques. The muonium formation process in condensed matter is shown to depend critically upon whether the excess electron forms a polaron or remains in a delocalized state. Different mechanisms of electron transport in insulators are discussed.     

A Backscatter Suppressed Electron Detector for the Measurement of “a”

A new method of measuring the electron-antineutrino angular correlation coefficient, little “a”, from neutron decay—to be performed at the National Institute of Standards and Technology—will require an electron spectrometer that strongly suppresses backscattered electrons. A prototype consisting of six trapezoidal veto detectors arranged around a plastic scintillator has been tested with an electron beam produced by a Van de Graaff accelerator. The results of this test and its implications for the little “a” measurement are discussed.     

Properties of Nanostructured Hydroxyapatite Prepared by a Spray Drying Technique

In previous studies nano sized hydroxyapatite (HA) particles were prepared by solgel or precipitation methods, in which the products were washed by aqueous or non-aqueous liquids to remove impurities or undesired components. The washing is know to modify the surfaces of the cystalline particles. This study evaluated properties of nano HA materials prepared by a spray drying method in which the HA product was not exposed to any liquid after its formation. The spray drying apparatus consisted of a nozzle that sprayed an acidic calcium phosphate solution in the form of a fine mist into a stream of filtered air flowing through a heated glass column. The water and volatile acid were evaporated by the time the mist reached the end of the column, and the fine particles were collected by an electrostatic precipitator. Powder x ray diffraction patterns suggested the material was amorphous, exhibiting a single broad peak at 30.5° 2θ. However, high resolution transmission electron microscopic analysis showed that the particles, some of which were 5 nm in size, exhibited well ordered HA lattice fringes. Small area diffraction patterns were indicative of HA. Fourier transfer infrared spectroscopy showed patterns of typical of HA with small amounts of HPO₄²⁻. The thermodynamic solubility product of the nano HA was 3.3 × 10⁻⁹⁴ compared to 1 × 10⁻¹¹⁷ for macro scale crystalline HA. These results showed that a spray drying technique can be used to prepare nanometer sized crystalline HA that have significantly different physicochemical properties than those of its bulk-scale counterpart.     

Ultrafast and Stable Li-(De)intercalation in a Large Single Crystal H-Nb2O5 Anode via Optimizing the Homogeneity of Electron and Ion Transport

Exploring anode materials with fast, safe, and stable Li-(de)intercalation is of great significance for developing next-generation lithium-ion batteries. Monoclinic H-type niobium pentoxide possesses outstanding intrinsic fast Li-(de)intercalation kinetics, high specific capacity, and safety; however, its practical rate capability and cycling stability are still limited, ascribed to the asynchronism of phase change throughout the crystals. Herein this problem is addressed by homogenizing the electron and Li-ion conductivity surrounding the crystals. An amorphous N-doped carbon layer is introduced on the micrometer single-crystal H-Nb₂O₅ particle to optimize the homogeneity of electron and Li-ion transport. As a result, the as-prepared H-Nb₂O₅ exhibits high reversible capacity (>250 mAh g⁻¹ at 50 mA g⁻¹), unprecedented high-rate performance (≈120 mAh g⁻¹ at 16.0 A g⁻¹) and excellent cycling stability (≈170 mAh g⁻¹ at 2.0 A g⁻¹ after 1000 cycles), which is by far the highest performance among the H-Nb₂O₅ materials. The inherent principle is further confirmed via operando transmission electron microscopy and X-ray diffraction. A novel insight into the further development of electrode materials forlithium-ion batteries is thus provided.     

X-Ray Microanalysis in the Variable Pressure (Environmental) Scanning Electron Microscope

Electron-excited x-ray microanalysis performed in the variable pressure and environmental scanning electron microscopes is subject to additional artifacts beyond those encountered in the conventional scanning electron microscope. Gas scattering leads to direct contributions to the spectrum from the environmental gas, as well as remote generation of x rays by electrons scattered out of the focussed beam. The analyst can exert some degree of control over these artifacts, but depending on the exact situation, spurious elements can appear at the trace (< 0.01 mass fraction), minor (0.01 mass fraction to 0.1 mass fraction), or even major (> 0.1 mass fraction) levels. Dispersed particle samples give the least compromised results, while fine scale microstructures are the most severely compromised. Procedures to optimize the situation based upon specimen preparation as well as spectral processing are described.     

金相学史话(6):电子显微镜在材料科学中的应用

Ruska在三十年代研制出第一台电子显微镜 ,战后 (195 4年 )又在极端困难条件下发展出带有电子衍射功能的高分辨电镜ElmiskopI。但是 ,从专利优先权角度看 ,他不是电镜的发明人。直到半个世纪后 ,有关的争议人都已过世 ,他才在 1986年获得这个迟到的但却是当之无愧的诺贝尔物理奖。材料科学的几次突破性进展充分说明电子显微镜的重要性。首先是电子衍射与成像的结合使位错的直接观察得以实现。在双束 (透射束与一个强衍射束 )条件下 ,位错产生的畸变区的衍射强度与基体不同从而显示衬度差异 (衍衬像 )。位错等晶体缺陷因此得以成为六、七十年代的研究热点。选区衍射使晶体结构分析进入到微米甚至到纳米层次。迄今为止 ,八十年代发现的各种类型的准晶 (五重、八重、十重、十二重旋转对称准晶 )都是使用这种手段实现的 ,从而扩大了晶体的范围 ,把无周期性的准晶也包括进去。高分辨电镜已发展到分辨单个原子的水平 ,这就为九十年代发现和研究纳米碳管创造了条件 ,开辟了纳米技术的新纪元     

Assessment of different techniques for quantification of α-Al(FeMn)Si and β-AlFeSi intermetallics in AA 6xxx alloys

During homogenisation of AA 6xxx aluminium alloys, the platelike β-AlFeSi intermetallic phase will transform to a less Si-rich and more spheroidised α-Al(FeMn)Si phase which is more favourable for extrusion. In this study, several quantitative methods, which determine the relative volume fraction of α-Al(FeMn)Si and β-AlFeSi, are compared and an assessment of each method is made. The methods used are optical microscopy, scanning electron microscopy (SEM) in combination with electron dispersive X-ray (EDX) using polished samples, and X-ray diffraction (XRD) on intermetallics, extracted through selective dissolution of the Al matrix. The highest accuracy is obtained by using SEM/EDX analysis and applying two criteria.     

The preparation, surface modification, and characterization of metallic α-Fe nanoparticles

α-Fe nanoparticles were prepared by reduction of Fe²⁺ using potassium borohydride in a simple ethanol/water system in the presence of surfactant. The in-situ modification of particles was carried out by taking advantage of a modifying solution containing Ni²⁺. The structure and size of the particles were investigated by X-ray diffraction (XRD), X-ray photoelectron spectrum (XPS), transmission electron microscopy (TEM) and electron diffraction (ED). Results showed that the in-situ electrochemical reaction between α-Fe nanoparticles and Ni²⁺ resulted in the formation of stable multilayer composite nanostructure. The cores of composite nanostructure were α-Fe.