bluuebunny/crossref_metadata_2025 huggingface dataset:
Near-Field Scanning Optical Microscopy Studies of Individual Dislocations in Relaxed GeSi Films
Authors: Hsu Julia W P , Gray M H , Xu Q
Published: July 2020

Due to the submicron size of crystallographic defects, characterization of dislocations has been done. mostly by electron microscopy techniques. Transmission electron microscopy has generated invaluable structural information at the atomic scale. However, the influence of these electrically active defects on carrier transport can only be learned from lower resolution (∼ 1 μm) techniques such as electron beam induced current (EBIC) and photocurrent measurements. Near-field scanning optical microscopy (NSOM) is a novel optical technique that circumvents the diffraction limit. In this talk, we will present the application of NSOM to perform near-field photocurrent (NPC) measurements on strain-relaxed GeSi films on Si substrates to study the electrical activity of individual threading dislocations. Photoexcited carriers are generated locally by NSOM light and are collected by the builtin p-n junctions in the sample resulting in an external photocurrent. As the tip moves across the sample, topographic and NPC images were acquired simultaneously.
Holography Measurement of Mean Inner Potential of Germanium
Authors: Li Jing , McCartney MR , Dunin-Borkowski RE , Smith David J
Published: July 2020

There is increasing interest in the quantitative characterization of nanometer-scale semiconductor alloy layers, such as those of SiGe or AlGaAs. Electron holography, which has the ability to detect small changes in mean inner potential Vo, can in principle provide information about local changes in composition. However, the measurement of absolute values of Vo is not always straightforward and values for only a few materials are known with confidence. In this study, we measure Vo for Ge using electron holography, and we compare our experimental data with calculations. The holographic approach that we use to measure Vo is described in the paper of Gajdardziska-Josifovska et al. The Ge samples were nominally 90° cleaved wedges from a <100> wafer. Holograms were recorded on a Philips CM200 FEG equipped with a Gatan 794 CCD camera, and data were obtained for several crystal tilts (determined from diffraction patterns) to assess dynamical contributions to the phases.
Electronic Structure and Conductivity Mechanism in Manganite Thin Films Exhibiting Colossal Magnetoresistance
Authors: Krishnan Kannan M , Ju Honglyoul , Nelson C
Published: July 2020

Perovskite manganites, which have the the general formula R1-xAxMnO3 (R = La, Pr, or Nd and A = Ca, Sr, Ba, or Pb), have generated much recent interest because they exhibit “colossal magnetoresistance” (CMR), i.e. a small change in an applied magnetic field dramatically changes the electrical resistance of the material. Materials that exhibit this effect are being developed for various field-sensing applications but currently, the mechanism by which CMR occurs is not known. Conduction in these materials is explained by the “double exchange” mechanism, where the conductivity is attributed to electrons hopping back and forth between neighboring manganes ions. Such hopping is a maximum when the magnetic moments of the magnese ions are aligned parallel and a minimum when they are aligned antiparallel. In short, these materials show metallic conductivity when they are ferromagnetic and insulating behavior when they are antiferromagnetic. Moreover, they change from metallic to insulating behavior as a function of temperature.
Inversion Domain Boundaries in Ain and GaN Thin Films
Authors: Dovidenko K , Oktyabrsky S , Narayan J
Published: July 2020

High-resolution transmission electron microscopy (HRTEM), multislice image simulation and multiple dark field TEM imaging techniques have been used to investigate the structure of extended defects in AlN and GaN thin films grown on (0001) α-Al2O3 by metal-organic chemical vapor deposition (MOCVD). AlN layers were grown directly on the (0001) sapphire. In the case of GaN thin films, 300-500 Å AlN buffer was deposited first.

Cross-sectional TEM revealed the presence of domain boundaries in these Ill-nitride films. In this study we investigated these defects by multiple dark field imaging technique and proved some of them to be IDBs lying in planes. The multiple dark field images of several adjacent domains of AlN film are shown in Fig. 1 (a, b). The images were obtained exactly in [110] zone of AlN using (0002) and (000) reflections.
Ain Films Grown by Electric Field Induced Flux of al Cations
Authors: Johnson Matthew T , Mao Zhigang , Barry Carter C
Published: July 2020

The properties of aluminum nitride (A1N) make it an interesting material for applications in the microelectronics industry. These properties include: a large bandgap, good thermal conductivity, high-temperature stability and chemical inertness. Additionally, thin A1N films are used as buffer layers for the growth of GaN on alumina. In the present study, thin-films of A1N have been grown on (000l)-oriented α-Al2O3 through the use of an electric field. At elevated temperatures an electric field applied across a normally insulating oxide material will typically induce a flux of ions that carry current. In the case of Al2O3, since the more mobile species are the Al3+ cations, by applying an electric-field across the material, with an appropriate electrode material, a flux of 3+ cations can be induced toward the cathode surface (negative electrode).
Prospects For Imaging of Single Dopant Atoms in Silicon by ADF Stem
Authors: Vanfleet Richard R , Silcox John
Published: July 2020

The demands of the National Technology Roadmap for Semiconductors will necessitate measurement of dopant concentrations with greater spatial resolution than now possible. Current experimental and simulation experience indicate that Annular Dark Field (ADF) imaging in a Scanning Transmission Electron Microscope (STEM) should be able to determine dopant distributions with near atomic resolution. The ADF signal is derived from electrons diffusely scattered to high angles, resulting in contrast due to atomic number (Z-contrast) and defects in the crystal lattice. Thus, heavy atoms can be imaged by their Z-contrast and small atoms by the misfit strain induced in the silicon lattice. Atomic number scattering is proportional to Zn where n is between 1.5 and 1.9 depending upon the inner detector angle of the ADF detector.
Direct Transmission Electron Microscope Observations of Doping Variations in InP-Based Semiconductor Laser Diodes
Authors: Hull R
Published: July 2020

The ability to map semiconductor doping distributions with high spatial resolution (∽10 nm) and high compositional sensitivity (of order 1 ppm) is of enormous importance to the microelectronics and optoelectronics industry. Although several methods (e.g. scanning capacitance microscopy, masked secondary ion mass spectroscopy, secondary electron microscopy, electron holography) are under development, no technique currently offers the combination of sufficient resolution, sensitivity and reproducibility to fully address characterization requirements. In this paper, we describe development of a technique which utilizes focused ion beam (FIB) sputtering and transmission electron microscopy (TEM) to enable direct imaging of dopant distributions in InP-based semiconductor devices, with spatial resolution of order 10 - 30 ran, and compositional sensitivity of order 1017 cm-3 (i.e. 5 ppm).

Laser diode samples are prepared for TEM imaging using a FEI 200 30 kV Ga+ focused ion beam system. A schematic of the relevant structure is shown in Figure 1(a).
A New Method for Pin Point Failure Analysis Using Fib Combined Analytical Tem
Authors: Kamino T , Yaguchi T , Matsumoto H , Tomita M , Koike H
Published: July 2020

Recently, we developed the dedicated FIB system(FB-2000A) for the TEM specimen preparation. The system has ion optics with the maximum ion beam current density of 15 A/cm2 and the minimum ion probe size of l0nm. In the system, the side entry type specimen stage originally designed for high resolution TEM was employed, and the drift rate of the stage during FIB milling is less than lnm/min. The FIB-TEM compatible specimen holder has been developed it made milling of specimen in FIB system and observation of milled specimen in TEM without remounting the specimen possible.. The holder was designed to rotate the specimen around the axis of specimen holder so that both side of milled surfaces can be faced to incident electron beam in TEM.

To improve positional resolution in pin point TEM specimen preparation, it is important to know the inside structure of specimen before starting the milling.
Microstructural Characterization of Heteroepitaxial SiGeC Alloys
Authors: Smith David J , Chandrasekhar D , Laursen T , Mayer JW , Huffman J , Robinson McD , Croke ET , Hunter AT
Published: July 2020

Heteroepitaxial growth of Group IV elements on Si is attracting increased attention because of the possibility of strain compensation in addition to bandgap engineering. The incorporation of the smaller C atom into Si1-xGex binary alloys to compensate for the larger size of the Ge atom offers the prospect of lattice matching and hence strain-free growth. In our early work, ternary SiGeC alloy films with up to ∽ 2% C were epitaxial with excellent crystallinity and very few interfacial defects. However, with increased C content, the films developed considerable disorder away from the substrate and finally became amorphous. Moreover, even at low C contents (∽2-3%), it appears that substantial C is being incorporated interstitially rather than substitutionally into the covalent lattice.2 Our recent work has therefore been aimed at establishing growth conditions that enable the amount of substitutional C to be maximized while still maintaining high crystal quality.
Direct Observation of Threading Dislocations in Gan by High Resolution z-contrast imaging
Authors: Xin Y , Browning ND , Pennycook SJ , Nellist PD , Sivananthan S , Faurie J-P , Gibart P
Published: July 2020

Wide gap nitride semiconductors have attracted significant attention recently due to their promising performance as short-wavelength light emitting diodes (LEDs) and blue lasers. One interesting issue concerning GaN is that the material is relatively insensitive to the presence of a density of dislocations which is six orders of magnitude higher than that for III-V arsenide and phosphide based LEDs. Although it is well known that these dislocations originate at the film-substrate interface during film growth, thread through the whole epilayer with line direction along <0001> and are perfect dislocations with Burgers vectors of a, c, or c+a, the reason why they have such a small effect on the properties of GaN is unclear.

To develop a fundamental understanding of the properties of these dislocations, the core structures are studied here by high resolution Z-contrast imaging in a 300kV VG HB603 scanning transmission electron microscope (STEM) with a resolution of 0.13nm.
Atomic Structure of Twinned As Precipitates in Lt-GaAs
Authors: Ruvimov S , Dicker Ch , Washburn J , Liliental-Weber Z
Published: July 2020

Low-temperature GaAs (LT-GaAs) of a high resistivity and a short carrier lifetime (0.2-20 ps) is promising for device applications. High temperature annealing of LT-GaAs often leads to formation of twinned As precipitates in the GaAs matrix. Here high resolution electron microscopy has been applied to study the structure of As precipitates formed in MBE grown LT-GaAs layers during rapid thermal annealing at 850 and 950 °C. Twinning in the rhombohedral As was a special focus of this study.

Fig. 1 shows a HREM image of a typical As precipitate formed in a LT-GaAs layer after annealing at 850 °C. The precipitate has a rhombohedral structure with a=0.376 nm and c= 1.055 nm and a polyhedral shape with distinct facets. The long facets bounded by the {111}A planes of the GaAs are planar while the others have a high density of steps.
Microstructure Of Au/Ti Ohmic Contacts On n-GaN
Authors: Liu R-J , Smith L L , Kim M J , Carpenter R W , Davis R F
Published: July 2020

GaN is a semiconductor with a direct band-gap of 3.4 eV which has a potential application in optoelectronic devices such as UV-emitting lasers and blue light emitting diodes. Extensive efforts have been made to develop low-resistance, thermally stable and uniform ohmic contacts on GaN. In this study, n-GaN films (Si-doped) with an intermediate A1N buffer layer were grown on 6H-SiC substrates via MOVPE. The contacts were formed by depositing Ti and then Au films on GaN by e-beam evaporation at room temperature. Current-voltage measurements as a function of temperature showed that the ohmicity of the Au/Ti contact improved upon annealing, corresponding to changes in the microstructure of the contact interfaces involving a redistribution of nitrogen and gold.

Fig. 1 shows the microstructure of the as-deposited Au/Ti/GaN films. The nanocrystalline Ti film was about 50 nm thick. Au film was polycrystalline, containing many growth twins. The thickness of Au film varied from 260 to 350 nm.
Structure of Low- And High-Angle Grain Boundaries In YBCO/MgO Films
Authors: Oktyabrsky S , Kalyanaraman R , Jagannadham K , Narayan J
Published: July 2020

Grain boundaries (GBs) in laser deposited YB2Cu3O7-δ/MgO(001) thin films have been investigated by high-resolution transmission electron microscopy (TEM) and scanning TEM (STEM). We report both statistics and atomic structure of low-angle and high-angle [001] tilt grain boundaries resulting from almost perfect c-axis textured YBCO films.

Atomic structure of low-angle GBs was analyzed using a dislocation model. These boundaries have been found to be aligned primarily along (100) and (110) interface planes. For (100) boundary plane, the GB consists of a periodic array of [100] dislocations (Fig.l). For (110) boundary plane, the array is also periodic but every [110] dislocation is split by ∼ 1.5 nm into two [100] and [010] dislocations (Fig.2). We have calculated energy of various configurations and shown that the energy of the (110) boundary with dissociated dislocations is comparable to that of (100) boundary, which explains the coexistence of (100) and (110) interface facets along the boundary.
The Atomic-Scale Origins of Grain Boundary Superconducting Properties
Authors: J. Pennycook S , Buban J , Prouteau C , Chisholm M F , Nellist P D , Browning N D
Published: July 2020

Due to the extemely short coherence lengths of the high-Tc superconductors (around 30 Å in the a-b plane), defects such as grain boundaries are obvious barriers to the flow of supercurrent. Within a few months of the discovery of these materials, it was shown how the critical current dropped four orders of magnitude as the grain boundary misorientaion increased from zero to 45°. Even today, there is no quantitative understanding of this behavior. A qualitative understanding is however possible through atomic resolution Z-contrast imaging on YBa2cu3O7-δ and SrTiO3 bicrystal grain boundaries, combined with bond-valence-sum analysis.

The Z-contrast image of a YBa2cu3O7-δ low angle grain boundary in Fig. 1 shows the same kind of reconstructed dislocation cores as seen in SrTiO3, containing reconstructions on both the Cu and Y/Ba sublattices.
Investigation of the Local Superconducting Properties at Grain Boundaries in High-Tc Superconductors
Authors: Prouteau C , Duscher G , Browning N D , Pennycook S J , Verebelyi D , Christen D K , Chisholm M F , Norton D P
Published: July 2020

Developing an atomic scale study of the structure-property relationships of grain boundaries in high-Tc superconductors is essential to understand their current dissipation mechanism and for incorporating these materials into viable devices. Thin YBa2Cu3O7-δ films have been deposited by pulsed laser deposition (PLD) on SrTiO3 symmetric bicrystals. Transport measurements in a magnetic field have been conducted across the grain boundaries through a wide bridge. The data obtained are consistent with microstructural observation in a VG Microscopes HB603 U and a VG HB501 UX dedicated STEM. Of particular interest in the study of high-Tc materials is the use of EELS, which can highlight the presence of non-superconducting regions through interpretation of the onset positions and finestructure (ELNES) of characteristic core-edge features.

The V(I) curves recorded across a 24° boundary for several magnetic fields (fig. 1 - left) show an onset critical current density followed by a linear differential ohmic behavior which gives a negative intercept.
Anisotropy of Electronic Structure and Transport Properties of Oxide Superconductors
Authors: Dravid Vinayak P
Published: July 2020

High temperature superconductivity in cuprates has become a fairly mature field now that more than a decade has passed by since its discovery. While scientific appeal of the field continues, the technological implications have faced harsh realities. Although the transition temperatures are high, the coherence length in oxide superconductors is awfully short and thus inhomegeneity or perturbation at the length scale of a few nanometers proves extremely detrimental to their properties, especially transport. Further, slight changes in the oxygen stoichiometry in cuprates have a profound effect on their superconducting properties and it is often not clear whether the nonstoichiometry at defect sites (e.g. grain boundaries- GBs) is intrinsic or extrinsic.

The anisotropy of electronic structure is well reflected in low loss EELS spectra of these materials. Figure 1 shows low loss EELS spectra from Y124: one taken with momentum transfer confined to the Cu-02 planes, and the other normal to them. These observations are in accord with the fact that conduction is predominantly along the ab-planes, within single crystals.
Structure in Amorphous Network Solids and its Evidence in Electron Diffraction
Authors: Hobbs Linn W
Published: July 2020

A cubic millimeter (10-9 m3) of condensed matter contains typically ∼1020 atoms, and a complete solution to the general structure problem requires finding ∼3 ×l020 atom coordinates. For crystals, long-range translational periodicity immensely simplifies the structure elucidation problem to something like a 101-102 problem (further reduced by the symmetry accompanying orientational order), because only the contents of the unit cell (< 100 atoms for most inorganic structures) and the unit cell geometry need be determined. Amorphous is the epithet traditionally bestowed on structures with neither orientation nor translational order, a more precise term for which is topologically disordered [1], A structural average for an amorphous solid can be established with a good many fewer than 1020 atoms, because many atom-atom correlations nevertheless exist in topologically-disordered structures due to bonding, packing and associated topological constraints. In fact, assemblies with less than 200 atoms can usually adequately represent characteristic structural topologies [1].
Experimental and Theoretical Characterisation of Structure in Thin Disordered Films
Authors: McCulloch DG , McKenzie DR , Cockayne DJH , Goringe CM
Published: July 2020

The electron microscope provides an ideal environment for the structural analysis of small volumes of amorphous and polycrystalline materials by collecting scattering information as a function of energy loss and momentum transfer. The scattering intensity at zero energy loss can be readily processed to a reduced density function G(r), providing information on nearest neighbour distances and bond angles[l]. Figure 1(a) shows the G(r) for glassy carbon, a turbostratic form of graphite. The three nearest neighbours in glassy carbon (labelled 1-3 in figure 1) are at 1.42 Å, 2.44 Å and 3.75 Å respectively. These distances correspond to the first three nearest neighbours in a graphite sheet and are expected in glassy carbon which is know to have good in-plane graphitic order. In figure 1(b) the G(r) of cathodic arc deposited tetrahedral amorphous carbon is shown. This material contains a high fraction of diamond-like bonding[2] and has a 1st nearest neighbour peak at 1.52 Å.
An Investigation Into Beam Damage of Mesoporous Materials
Authors: Blanford C F , Bentley J , Stein A , Carter C B
Published: July 2020

In 1992, researchers at Mobil Research and Development created a new class of porous silicates, most notably mesoporous MCM-41.2-3 This material features a hexagonal arrangement of linear pores and surface areas in excess of 1000 m2 g-1. MCM-41 exhibits narrow pore size distributions in the nanometer range. The walls of MCM-41 are essentially amorphous silica, but its porous nature makes it about 3 kJ mol-1 less stable than the collapsed form. Particles of MCM-41 are beam sensitive and it appears that they cannot withstand the large current densities required to obtain reliable analytical data at the nanometer scale in the transmission electron microscope (TEM). For example, low beam currents were used to preserve the pore structure but resulted in energy-filtered TEM elemental maps of oxygen K and Ti L2,3 edge intensities that were too noisy to reveal structure at 5 nm resolution.
A BCC Superlattice of Passivated Gold Nanocrystals
Authors: Harfenist SA , Wang ZL , Schaaff TG , Whettent RL
Published: July 2020

A recent development in the study of nanocrystalline materials has been the self-assembly of passivated nanometer scale building blocks into larger, well ordered structures reaching the micron scale. Nanocrystal supercrystals (NCS) have been observed in metallic, semiconductor, and magnetic materials. In most cases the nanocrystals (NXs) are encapsulated in some inert medium that effectively protects the nanocrystal core and its unique electronic and optical properties. Here we describe the self-assembly of gold nanocrystals (∼4.5 nm core diameter), passivated with hexanethiol self-assembled-monolayers into ordered regions exhibiting a body-centered-cubic (bcc) superstructure. Transmission Electron Microscopy (TEM) imaging and Electron Diffraction (ED) experiments were used to characterize the NCSs and their resulting superstructures.

A large agglomeration of NCSs can be seen in figure 1. One can clearly see regions of periodicity within the nanocrystal aggregation.
Growth Mechanism of Ag Nanocrystal Supercrystals
Authors: Harfenist SA , Wang ZL , Vezmar I , Alvarez MM , Whetten RL
Published: July 2020

Of recent interest in the science of nano-scale materials is the production and characterization of substances composed of ordered lattices of up to millions of identical nanometer scale particles. Silver nanocrystals of ∼ 4.5 nm diameter passivated with dodecanthiol surfactant were produced in the aerosol phase. Characterization of the individual particles and the subsequent supercrystals (SXs) resulting from their periodic packing was brought about using Mass Spectrometry, Transmission Electron Microscopy (TEM)/Electron Diffraction (ED), Scanning Electron Microscopy and Atomic Force Microscopy (AFM). It was determined that the micrometer and greater sized faceted SXs exhibited an hexagonal-close-packed (HCP) superstructure (fig.l) resulting in triangular shapes bounded by ﹛1100﹜ facets (fig. la); their platlet-like nature (∼20 nanocrystal layers thick) owing to their particular growth pattern. Knowledge of the growth mechanism is important to production of these materials on a macroscopic scale. Here we report a proposed growth model for these supercrystals.
Energy-Filtered Imaging of Zirconia Pillars in Montmorillonite
Authors: Crazier P A , Pan M , Bateman C , Alcaraz J J , Holmgren J S
Published: July 2020

The ability to vary the plane spacing in different clays is becoming increasingly important in catalysis and other technological applications [Baker and Murrell, 1990]. Considerable effort is now being devoted to understanding the mechanisms involved in pillaring such materials with a view to being able to fine-tune parameters such as the plane spacing and active sites for the application of interest. We have been able to use analytical electron microscopy to directly image the pillars in zirconia pillared montmorillonite. By using a combination of TEM techniques, we are able to correlate morphological information with elemental composition and conclusively identify the location of the zirconia pillars.

A sample of natural montmorillonite clay, trade name HPM-20, was obtained from American Colloid and used as received. Zirconia pillared clays were prepared following published procedures [Bartley, 1988]. The Z1O2 pillars were prepared by using hydroxyzirconium cations produced by the solvation of zironyl chloride.
Kinetically Controlled Growth And Shape Formation Mechanism Of Platinum Nanoparticles
Authors: Petroski Janet M , Wang Zhong L , Green Travis C , El-Sayed Mostafa A
Published: July 2020

Many studies on colloidal nanoparticles have focused on the control of nanoparticle size and correlated this to the catalytic activity. Recently, our group has reported for the first time a technique that controlled the shape distribution of Pt nanoparticles. This was done by varying the concentration of the capping polymer and the platinum ion ratio used in the reductive synthesis of colloidal nanoparticles at room temperature. Cubic, tetrahedral and truncated octahedral (TO) particles have been prepared, making it possible to study the catalytic activities of nanoparticles with different shapes and facets.

Using transmission electron microscopy (TEM), we imaged the shapes and determined the shape distribution of platinum nanoparticles at different stages of their growth as a function of time. The small nanoparticles formed during the early stages of growth or at high polymer concentration displayed distributions with a dominance of tetrahedral shapes (see Figure la).
Cu Nanoparticle Formation: Copper Redistribution During NaCl Solution Corrosion of Al-Cu-Mg Alloys
Authors: Ford R G , Carpenter R W , Sieradzki K
Published: July 2020

Aluminum-copper-magnesium alloys also containing manganese and iron (commercial designation 2024) are susceptible to marine corrosion and stress corrosion cracking. Susceptibility depends on heat treatment, and is thought to involve redistribution of copper from within the microstructure onto the surface of the corroding alloy, but few direct observations of the mechanism have been made. Copper can be distributed in several ways throughout the microstructure, which complicates mechanism studies. The primary age-hardening phase is Al2CuMg (orthorhombic; a=4.01, b=9.25, c=7.15 Å) which appears as more or less large equiaxed S particles at equilibrium, and as metastable S’ plates after aging for shorter times at lower temperature (∼190°C). In addition, ubiquitous so-called “dispersoids” containing copper and manganese (prolate spheroid morphology) or iron (irregular “blocky” morphology) do not go into solution when the alloy is solid state homogenized (-495 °C) and are always present in the microstructure. All of these phases are copper-rich sources for surface redistribution relative to the matrix during corrosion.
Z-Contrast Scanning Transmission Electron Microscopy Of Nanometer-Scale Coated Particulate Materials
Authors: Gao HJ , Yan Y , Fitz-Gerald J , Kumar D , Singh RK , Pennycook SJ
Published: July 2020

Particulate materials with unique functional properties have been the focus of much attention in recent years. Of particular interest, due to their considerable scientific and technological importance, are particles coated with nanoparticles. These have greatly stimulated interest for their novel structure and properties. In these kinds of particulate materials, the interface structures between the support particle and the nanoparticle play a crucial role in controlling their properties. Consequently, imaging of the atomic structures at the interfaces can provide deep understanding of the relationship between the particulate and the corresponding properties. Z-contrast scanning transmission electron microscope (STEM) provides a new view of materials on the atomic scale, a direct image of atomic structure composition which can be interpreted without the need for any preconceived model structure. Therefore it is a powerful tool in the study of particulate materials. In this report, we will present the structures of 18 micron diameter alumina particles coated with Ag nanoparticles.
Microanalysis of Alloy Nanoparticles
Authors: Scott John Henry J
Published: July 2020

Nanoparticles and other nanostructured samples are developing into an increasingly important class of materials. Because of their small size (diameters less than 100 nm), reduced dimensionality, and large surface areas, nanoparticles often exhibit unusual physical, optical, mechanical, and electronic properties. In addition to providing valuable model systems in the pure sciences, nanoparticles are proving useful in technological applications, showing promise as biomedical imaging contrast agents, data storage media, magnetic inks and toners, ferrofluids, and as precursors for advanced compacted materials. Fulfilling this potential depends on characterizing the chemical inhomogeneities of the nanoparticles (and their surfaces) with very high spatial resolution. In this paper I examine the quantitative microanalysis of nanoparticles— electron energy-loss spectroscopy (EELS) and energy-dispersive x-ray spectrometry (EDS)— using transition metal alloy particles (Fig. 1) as a case study.

The microanalysis of nanoparticles presents unique challenges to the analyst because of the very small length scales in the system.
Complex Atomic-Scale Structures in Solids by a Combination of Theory and Microscopy
Authors: Pantelides S T , Pennycook S J , Chisholm M F , Maiti A , Yan Y , Reboredo F , Ferconi M
Published: July 2020

Modern high-performance computers are now capable of calculations that can be used to determine the preferred atomic arrangements in complex systems. Both first-principles and semiempirical approaches have been developed with complementary capabilities. At the same time, powerful transmission electron microscopes have been developed that yield direct atomic-scale images of crystals and extended defects such as dislocations, grain boundaries and buried interfaces. This paper presents several examples where a synergistic approach combining theoretical results, Z-contrast scanning transmission electron microscopy, and spatially resolved electron energy loss spectroscopy (EELS) have led to the elucidation of complex atomic structures. In some cases, theory predicts, experiment confirms and expands and theory revisits; in other cases, observations come first and theory helps put together a comprehensive picture or goes beyond the original observations with new predictions. In both cases, equilibrium structures, impurity or stressinduced structural transformations, and dynamical processes such as diffusion, segregation or precipitation are elucidated in great detail.
Bonding and Stability of Metal/Ceramic Interfaces
Authors: Alber U , Schweinfest R , Riihle M
Published: July 2020

Metal/ceramic interfaces play a crucial role in materials science and for various industrial purposes. In technical applications these interfaces are often exposed to high temperatures and different atmospheres. This often results in a change of the mechanical stability via the morphology and electronic structure of the interfaces. We present a comprehensive analytical electron microcopy (AEM) and fracture mechanics study of this connection on a metal/ceramic-interface model system: Cu/ α-Al2O3.

The specimens were produced by UHV diffusion bonding of bulk Cu to (α-Al2O3 single crystals. Two different Cu materials were used, either pure Cu (noted: Cu) or Cu containing 83±12 ppm oxygen (noted: Cu(O)). After bonding the interfaces were annealed in an oxygen partial pressure at 1000°C between 20 and 120 h. Four point bending tests showed an increase of the fracture energy for the Cu(O)/α -Al2O3-interfaces compared to the Cu(O)/α-Al2O3-interfaces by a factor of 5±2.
Microanalysis of CaWO4 And ErTaO4 Coatings In Oxide Fiber Reinforced Alumina Matrix Composites
Authors: Kim S T , Dravid V P , Sambasivan S , Goettler R W
Published: July 2020

Ceramic matrix composites (CMC's) rely on designed weaknesses in order to provide toughening mechanisms during failure. Specifically, the fiber/matrix interface is well established as the parameter having the most influence on CMC fracture behavior, as first noted by Sambell. With proper design, this interface can allow fracture energy to dissipate by two mechanisms: an increase in crack path tortuosity and frictional energy dissipation through fiber pullout.

One design approach in the area of oxide-based CMC's which has seen considerable success recendy has been in the use of “weakly bonding” oxide coating materials placed between the fiber and the matrix phases. Such CMC systems rely on the formation of a weak interface between the fiber and the coating itself to promote the desired fracture behavior. The most notable example of such a coating material is monazite, LaPO4, as observed by Morgan et al.
Local Dielectric Function Of Biogenic and Geological Polymorphs of CaCO3 Via Transmission Eels
Authors: Katti Kalpana S , Frech Daniel , Qian Maoxu , Sarikaya Mehmet
Published: July 2020

This paper focuses on the differences of physical properties, specifically local dielectric function, of biogenic and geological mineral CaCO3. The goal is to assess the role of organism in forming biogenic inorganic materials. Local dielectric functions of biogenic and geological minerals were determined by transmission electron energy loss spectroscopy using a previously developed strategy.

Previous work on microstructural characterization of biological hard tissues, e.g., abalone shells, has shown variations in terms of defects, morphology, crystallography, and organization of nano- and microstructures in two biological polymorphs of calcium carbonate, calcite and aragonite, in the prismatic and nacreous regions, respectively. In the abalone, the outer (1-5 mm thick) region of the shell is composed of calcite crystallites (1-5 μm diameter) with columnar organization perpendicular to shell plane. On the inner region, 1-10 mm thick nacre is composed of aragonite crystallites (0.5 μm thick and 5 -10 μm edge-length) forming flat platelets, parallel to the shell plane.
Quantitative Hrtem of Twin Boundaries in Compound Semiconductors and Metals Using Non-Linear Least-Squares Methods
Authors: Cohen Dov , Campbell Geoffrey H , King Wayne E , Barry Carter C
Published: July 2020

The analysis of the atomic structure of grain boundaries is often performed through the use of high-resolution transmission electron microscopy (HRTEM). A complication of the HRTEM technique is the inability to analyze directly the experimental images in order to determine projected atomic models of lattice defects. Since contrast features in HRTEM images, in general, do not correspond directly to atomic positions, experimental images are typically evaluated qualitatively through comparison with image simulation. Recently, the interest in quantitatively measuring the atomic structure of internal interfaces for comparison with theoretical calculations has motivated the development of computational methodologies to analyze HRTEM images.123 In this paper, the quantitative analysis of HRTEM images of twin boundaries in semiconductors and metals is described.

AΣ=3 coherent twin boundary in GaP was imaged along the <110> zone in a JEOL-4000EX HRTEM at Sandia National Laboratories, Livermore.
Determination Of Rigid-Body Lattice Translations Across Antiphase and Twin Boundaries in Compound Semiconductors
Authors: Cohen Dov , Medlin D L , Barry Carter C
Published: July 2020

The physical properties of crystalline materials are dependent on the presence and nature of planar defects such as grain boundaries within the crystalline lattice. A comprehensive understanding of a material's properties requires a detailed knowledge of the atomic structure and chemistry of such defects. In general, the crystal orientation relationship at grain boundaries and related planar defects are described in geometric terms, such as the coincidence-site lattice (CSL) model. However, geometric models do not take account of the atomic bonding across grain boundaries whose characteristics differ from that in the bulk. In general, defect structures may relax from the exact CSL orientation in order to minimize the energy of the interface. Lattice relaxations can manifest as a local rearrangement of the atoms close to the boundary and/or rigid-body lattice translations which perturb the relative positions of the grains far from the interface.
Atomic Structures of Inversion Domain Boundaries and Dislocations in Sintered ALN
Authors: Yan Yanfa , Chisholm M F , Pennycook S J
Published: July 2020

A1N is an attractive material because of its mechanical, thermal and electronic properties. However, these properties are often adversely altered by extended defects such as inversion domain boundaries (IDBs), and through impurities associated with the defects, especially oxygen. Thus to improve the properties of the material it is important to understand the structures of the defects. A number of studies have been carried out to characterize these defects using fringe contrast, weak-beam, convergent-beam electron diffraction and conventional high-resolution phase contrast imaging. These techniques all require the use of model structures for data interpretation. In this paper, we report a direct determination of the structures of IDBs and dislocation cores in A1N by highresolution Z-contrast imaging using a 300 kV VG HB603U scanning transmission electron microscope, with a probe size of 1.3 Å.
Bandstructure Near Misfit Dislocations in Si Quantum Wells
Authors: Batson PE
Published: July 2020

High electron mobility structures have been built for several years now using strained silicon layers grown on SixGe(1-x) with x in the 25-40% range. In these structures, a thin layer of silicon is grown between layers of unstrained GeSi alloy. Matching of the two lattices in the plane of growth produces a bi-axial strain in the silicon, splitting the conduction band and providing light electron levels for enhanced mobility. If the silicon channel becomes too thick, strain relaxation can occur by injection of misfit dislocations at the growth interface between the silicon and GeSi alloy. The strain field of these dislocations then gives rise to a local potential variation that limits electron mobility in the strained Si channel. This study seeks to verify this mechanism by measuring the absolute conduction band shifts which track the local potential near the misfit dislocations.
In Situ Electron Holography of Grain Boundary Schottky Barrier Dynamics in 0.5 Wt% NB Doped SrTiO3 36.8° Symmetric Tilt Bicrystals
Authors: Johnson KD , Dravid VP
Published: July 2020

Electrical phenomena such as positive temperature coefficient of resistance (PTCR), grain boundary layer capacitance, and varistors are controlled by Schottky barriers to current transport formed in grain boundary regions. These grain boundary potential barriers are a result of charged defect accumulation, either through equilibrium segregation or kinetic diffusion processes. Recent work on Nb and Mn doped BaTiO3 highlights the importance of substitutional defects which activate grain boundary trap states and give rise to the PTCR effect. The dynamic interaction of charge carriers with such trap states also produces the rapid electrical breakdown characteristic of varistors. This study explores the issues of defect species, space charge, and electrical transport in donor doped SrTi03 by electron holography and electron energy loss spectroscopy.

The specific defect species and mechanisms of current transport through donor doped SrTiO3 remain unresolved because of the difficulty in connecting defect distributions (structural information) and resultant Schottky barrier effects (chemical information).
Dose-Rate Dependence of Radiation Damage in Polymers
Authors: Egerton RF , Rauf I
Published: July 2020

Three aspects of radiation damage are of concern to electron microscopists: changes in crystallographic or molecular structure, mass loss and change in chemical composition. Structural change can be monitored from the fading of diffraction patterns or from loss of fine structure in an energy-loss spectrum. Total mass loss, in the form of a reduction in inelastic-scattering power, can be observed from the low-loss spectrum. Mass loss can also be monitored from energy-loss ionization edges, with the advantage that the loss of particular elements can be studied separately. It is possible to assign a characteristic dose De for the disappearance of a particular element.

At room temperature, the amount of damage usually depends on the accumulated dose (exposure) but not on the dose rate (current density). However, cooling the specimen tends to reduce mass loss, probably because of the reduced diffusion coefficients.
Dose Limited Resolution
Authors: Van Dyck D , den Dekker AJ , Sijbers J , Bettens E
Published: July 2020

1. Introduction

The definition of resolution as introduced by Lord Rayleigh [1] is related to the width of the point spread function of the imaging device. In this definition, noise has not been taken into account. Another definition of resolution has been introduced by Rose [2] in the field of radar and TV. Here the resolution is defined in terms of the dose (D) (i.e. number of imaging particles per unit area) and the signal to noise ratio SNR (i.e. the minimal contrast) A third definition of resolution is based on the idea that the microscope is a communication channel between the object and the observer. The resolution can then be rephrased as the amount of information that is transmitted by the channel in the sense as defined by Shannon [3] as a number of bits per unit area. This definition however does not describe how this information can be deduced and what its precision is.
Effects of Fast Secondary Electrons on Spatiallyresolved Low-Loss Eels of Polystyrene
Authors: Siangchaew K , Libera M
Published: July 2020

A good understanding of the effect of electron irradiation on polymers is necessary in order to optimally utilize the spectroscopic information and resolution of spatially-resolved electron energy-loss spectroscopy (EELS). This investigation studies the effect of electron irradiation on the low-loss spectroscopic signal and spatial resolution obtainable from polystyrene (PS) homopolymer. Because of the conjugated valence electron distribution associated with its pendant phenyl ring, polystyrene is relatively stable under electron irradiation and has well characterized spectroscopic fingerprints including a notable π—π* transition circa 7eV (1). In addition, polystyrene is used as a positive photoresist because it can cross-link effectively when exposed to an electron irradiation (2).

The critical dose characterizing degradation of aromatic polymers is of the order 1-10 C/cm2 (3). In practice, the dose delivered to a specimen is determined both by electron probe size and probe current.
Morphology Study of Polystyrene-Polybutadiene-polycaprolactone (PS-b-PB-b-PCL), Polybutadiene-Polycaprolactone (PB-b-PCL), and Polystyrene-Polycaprolactone (PS-b-PCL) Semicrystalline Block Copolymers
Authors: Kim G , Jackson C L , Gyldenfeldt F v , Balsamo V , Libera M , Stadler R , Han C C
Published: July 2020

Semicrystalline block copolymers containing one crystalline block, such as PCL, show more complicated morphologies than amorphous block copolymers. This is because the microstructure of semicrystalline block copolymers is determined by the interplay between the microphase-separation temperature, crystallization temperature, and glass transition temperatures of the respective blocks. This paper describes initial results of thermodynamic and kinetic effects associated with microphase separation and crystallization in as-cast semicrystalline (PS)0.62(PCL)0.38, (PB)0.2(PCL)0.8 and (PS)0.35(PB)0.15(PCL)0.5 block copolymers, where the subscripts denote the mass fraction of each block.

Semicrystalline block copolymers (PS-b-PCL with Mn=209,000 g/mol, PB-b-PCL with Mn=146,000 g/mol and PS-b-PB-b-PCL with Mn= 150,000 g/mol) were prepared by anionic polymerization. Films were cast from 5% (mass fraction) solutions in toluene into ∼lmm thick films. A relatively slow solvent evaporation over approximately a 10 d period was achieved by partially covering the dishes containing polymer solutions.
Quantitation of the Lateral Orientational Order in Poly(p-Phenylene Terephthalamide) Fibers with X-ray Linear Dichroism Microscopy
Authors: Smith A P , Garcia A , Ade H
Published: July 2020

Determination of internal structure of complex polymeric materials often requires a variety of analytical techniques. One technique that we have developed and recently employed for the first time for quantitative analysis is x-ray linear dichroism microscopy. With sensitivity to the presence and orientation of specific chemical functionalities within polymer molecules, x-ray linear dichroism microscopy provides a powerful tool to probe the internal structure of such molecules. We have used this ability to quantitatively analyze the lateral orientational order (tangential and radial orientation of transverse sections) within different grades of poly(p-phenylene terephthalamide) (Kevlar) fibers.

Kevlar fibers are highly crystalline and possess a complicated internal structure that exhibits lateral radial orientation. Figure 1 shows x-ray micrographs of (a) Kevlar 149, (b) Kevlar 49 and (c) Kevlar 29 fibers imaged at a photon energy specific to the aromatic functionality.
Analysis of the Spatial Variation of Crosslink Density in Superabsorbent Polymers
Authors: Urquhart S G , Ade H W , Mitchell G E , Wilson L , Rightor E G , Dineen M , Hitchcock A P , Neuhaeusler U
Published: July 2020

Superabsorbent polymers are often designed with increased density of crosslinking in the outer layer of the particles in order to improve liquid retention under load. For efficient product design, it is desirable to directly measure the spatial variation in crosslink density. Typically employed techniques (such as solvent uptake or measuring the changes in various mechanical properties such as the modulus) do not provide spatially resolved crosslink density information. We have applied Scanning Transmission X-ray Microscopy to examine the swelling of inhomogenously crosslinked superabsorbent polymers in deionized water and salt water solution.

Scanning Transmission X-ray Microscopy (STXM) is an effective way to study the chemical and morphological character of polymers on a sub micron spatial scale.1 STXM image contrast is based on core electron excitation by x-ray absorption; an interaction that has remarkable chemical sensitivity. Beam damage is less than in TEM microscopes and samples can be examined in wet and in ambient conditions.
Atomic Force Microscopy and Related Techniques: Introduction, Instrumentation and Application to Polymeric Materials
Authors: Musselman Inga Holl
Published: July 2020

Atomic force microscopy (AFM) was introduced in 1986 by Binnig, Quate and Gerber. In this method, a sample is scanned beneath a small, sharp silicon or silicon nitride probe attached to the apex of a flexible cantilever. Cantilever deflection is measured to give height information corresponding to the sample topography. Since AFM relies on tip-sample force interaction, the technique can be applied to insulators as well as to conducting and semiconducting materials. AFM therefore extends local probe studies to an important class of materials which can be difficult to investigate by electron microscopy and spectroscopy techniques owing to problems with sample charging. Among other materials, AFM has been used extensively to characterize the morphology, roughness, nanostructure, chain packing and conformation of polymer surfaces at the nanometer scale.

Early AFM studies of polymers were conducted in the contact mode and included the investigation of polymerized monolayers of n-(2-aminoethyl)-10,12-tricosadiynamide (AE-TDA) and poly(octadecylacrylate) (PODA) at submonolayer coverage.