Dark Field-scanning Electron Microscopy Research Articles

Synthesis of AuAg@Ag core/shell bimetallic nanoparticles in titanium nitride thin films by sequential ion implantation

In this study, we demonstrated the formation of a new type of AuAg@Ag nanostructure in titanium nitride (TiN) thin films using sequential implantation of Au and Ag ions. The TiN films deposited by DC reactive sputtering were sequentially implanted with 200 keV gold ions at fluences of 0.5 × 1016 and 1 × 1016 ions/cm2 and 150 keV silver ions at fluences of 1 × 1016, 2 × 1016, 3 × 1016, and 4 × 1016 ions/cm2. Formation of spherical-like particles with dimensions below 10 nm, homogeneously distributed along the ion track of implanted ions is verified by high resolution transmission electron microscopy. Specific nanostructure containing AuAg alloy in the center of the particle and only silver on the periphery was found in the TiN sample sequentially implanted with 1.0 × 1016 ions/cm2 of Au and 4 × 1016 ions/cm2 of Ag using high angle annular dark field scanning electron microscopy together with elemental mapping analysis. On the other hand, for the sample implanted with 0.5 × 1016 ions/cm2 of Au and 4 × 1016 ions/cm2 of Ag only formation of AuAg alloy was observed. A mechanism for the formation of a new type core/shell nanostructure in ion implanted TiN thin films is proposed. By varying the implantation fluence of the gold and silver ions properly, the optimized experimental parameters for creation of a specific AuAg@Ag core/shell nanoparticles were achieved.

Concurrent defects of intrinsic tellurium and extrinsic silver in an n-type Bi2Te2.88Se0.15 thermoelectric material

We suggested a defect system consisting of Te- and Ag-related defects in n-type Bi2(Te,Se)3. We prepared the Te-rich Bi2Te2.88Se0.15 compound, which is expected to have Te antisite defects, via a conventional pulverization process. We devised a one-pot chemical process in which Ag nanoparticles were synthesized and instantly deposited onto the Bi2Te2.88Se0.15, followed by spark plasma sintering; thus, Ag atoms were inserted into the lattice structure of the Bi2Te2.88Se0.15 during the sintering process. Atomic layers of the Bi2Te2.88Se0.15 were resolved by transmission electron microscopy and dark-field scanning electron microscopy imaging with energy dispersive X-ray spectroscopy; the Ag atoms were thereby proven to occupy the interstitial sites in the Bi2Te2.88Se0.15 (i.e., Ag interstitial defects). The defect system composed of the Te antisites and Ag interstitials decoupled the electrical properties of the Bi2Te2.88Se0.15 from its thermal properties. We endeavored to adjust the defect system to maximize the decoupling effect, which resulted in the preeminent average figures of merit for operation below 150 °C (ZTave = 1.22 at 25–100 °C and 1.18 at 25–150 °C) among n-type Bi2(Te,Se)3 materials.

Regulating kinetics of deformation-induced phase transformation in amorphous alloy composite via tuning nano-scale compositional heterogeneity in crystalline phase

Nano-scale compositional heterogeneity of crystalline phase in Ti-based amorphous alloy composites (AACs) has been successfully tuned by tuning cooling rate in solidification process. And the effect of compositional heterogeneity on the kinetics of the deformation-induced phase transformation was investigated by in-situ synchrotron-based high-energy X-ray diffraction (HE-XRD) and ex-situ transmission electron microscopy. In-situ HEXRD experiments provide obvious evidence that with the decrease of the cooling rate during solidification, the critical stress of the deformation-induced phase transformation becomes lower, and the phase transformation rate becomes higher. Further high angle annular dark field-scanning electron microscopy investigation shows that the occurrence of the nano-scale Zr-lean compositional heterogeneity, which can favor the nucleation of the martensite, is the reason for the variation of the phase transformation kinetics.

Microstructural characterization of sulfur-doped Bi2Te3 crystals

In this study, the detailed microstructure of sulfur-doped Bi2Te3 and the distribution of sulfur dopants were investigated using X-ray diffraction (XRD) and transmission electron microscopy (TEM). XRD result indicates that the interplanar distances of Bi2Te3 are shortened after introducing sulfur dopants. HRTEM reveals that the unit cell lengths along [001, 100] are decreased as a result of the substitution of Te by smaller S atoms on the anion lattice. XRD and HRTEM analysis suggest that the distortion of the crystal lattice in Bi2Te3 is induced by doping sulfur. High annular angler dark field scanning electron microscopy (HAADF-STEM) image shows obvious contrast variations in the Te atomic columns, indicating that Te sites were unevenly substituted by S dopants. Te columns with least contrast intensity correspond to the preferential occupation sites of S atoms. The replacement of Te atoms by S can be attributed to the evaporated sulfur powder in the gas reaction procedure. The present work is not only contributed to obtain a better understanding of the distribution of sulfur dopants introduced by gas reaction, but also can help explore the structural-property relationship of sulfur-doped Bi2Te3.

Modulation doping of GaAs/AlGaAs core-shell nanowires with effective defect passivation and high electron mobility.

Reliable doping is required to realize many devices based on semiconductor nanowires. Group III-V nanowires show great promise as elements of high-speed optoelectronic devices, but for such applications it is important that the electron mobility is not compromised by the inclusion of dopants. Here we show that GaAs nanowires can be n-type doped with negligible loss of electron mobility. Molecular beam epitaxy was used to fabricate modulation-doped GaAs nanowires with Al0.33Ga0.67As shells that contained a layer of Si dopants. We identify the presence of the doped layer from a high-angle annular dark field scanning electron microscopy cross-section image. The doping density, carrier mobility, and charge carrier lifetimes of these n-type nanowires and nominally undoped reference samples were determined using the noncontact method of optical pump terahertz probe spectroscopy. An n-type extrinsic carrier concentration of 1.10 ± 0.06 × 10(16) cm(-3) was extracted, demonstrating the effectiveness of modulation doping in GaAs nanowires. The room-temperature electron mobility was also found to be high at 2200 ± 300 cm(2) V(-1) s(-1) and importantly minimal degradation was observed compared with undoped reference nanowires at similar electron densities. In addition, modulation doping significantly enhanced the room-temperature photoconductivity and photoluminescence lifetimes to 3.9 ± 0.3 and 2.4 ± 0.1 ns respectively, revealing that modulation doping can passivate interfacial trap states.

Strain Measurement with Nanometre Resolution by Transmission Electron Microscopy

Strain is routinely used in state-of-the-art semiconductor devices in order to improve their electrical performance. Here we present experimental strain measurements obtained by different transmission electron microscopy (TEM) based techniques. Dark field electron holography, nanobeam electron diffraction (NBED) and high angle annular dark field scanning electron microscopy (HAADF STEM) are demonstrated. In this paper we demonstrate the spatial resolution and sensitivity of these different techniques on a simple calibration specimen where the accuracy of the measurement can easily be assessed.

Strain mapping for the silicon-on-insulator generation of semiconductor devices by high-angle annular dark field scanning electron transmission microscopy

The strain in pMOS p-type metal-oxide-semiconductor devices grown on silicon-on-insulator substrates has been measured by using the geometrical phase analysis of high angle annular dark field scanning electron microscopy. We show that by using the latest generations of electron microscopes, the strain can now be quantitatively measured with a large field of view, a spatial resolution as low as 1 nm with a sensitivity as good as 0.15%. This technique is extremely flexible, provides both structural and strain information, and can be applied to all types of nanoscale materials both quickly and easily.

Magnetic Nanoparticles-Templated Assembly of Protein Subunits: A New Platform for Carbohydrate-Based MRI Nanoprobes

A new approach for the preparation of carbohydrate-coated magnetic nanoparticles is reported. In a first step, we show that the pH-driven assembly-disassembly natural process that occurs in apoferritin protein is effective for the encapsulation of maghemite nanoparticles of different sizes: 4 and 6 nm. In a second step, we demonstrate that the presence of functional amine groups in the outer shell of apoferritin allows functionalization with two carbohydrates, N-acetyl-D-glucosamine and d-mannose. High-resolution electron microscopy (HREM), high angle annular dark field scanning electron microscopy (HAADF-STEM), electron energy loss spectroscopy (EELS), X-ray diffraction (XRD), and SQUID technique have been used to characterize the magnetic samples, termed herein Apomaghemites. The in vivo magnetic resonance imaging (MRI) studies showed the efficiency in contrasting images for these samples; that is, the r(2) NMR relaxivities are comparable with Endorem (a commercial superparamagnetic MRI contrast agent). The r(2) relaxivity values as well as the pre-contrast and post-contrast T(2)*-weighted images suggested that our systems could be used as perspective superparamagnetic contrast agents for magnetic resonance imaging (MRI). The carbohydrate-functionalized Apomaghemite nanoparticles retained their recognition abilities, as demonstrated by the strong affinity with their corresponding carbohydrate-binding lectins.

A Bioinspired Approach to the Synthesis of Bimetallic CoNi Nanoparticles

Bimetallic CoNi nanoparticles have been prepared within the apoferritin cavity. The protein shell controls size, prevents aggregation, and makes nanoparticles water-soluble. The CoNi series prepared in this way were structurally and magnetically characterized, the resulting magnetic properties varying accordingly with composition (Co(75)/Ni(25), Co(50)/Ni(50), Co(25)/Ni(75)). Co and Ni metals were associated in each nanoparticle, as demonstrated by high-angle annular dark field scanning electron microscopy and electron energy loss spectroscopy (EELS). After intentional oxidation, the CoNi nanoparticles were characterized by EELS, X-ray absorption near edge structure (XANES), and SQUID measurements to evaluate the importance of the oxidation on magnetic properties.

Structural Characteristics of Lithium Nickel Phosphate Studied Using Analytical Electron Microscopy and Raman Spectroscopy

The structural characteristics of lithium nickel phosphate (LiNiPO4) prepared by solid-state chemical reaction have been studied in detail using the analytical electron microscopy and Raman spectroscopy measurements. The high-resolution transmission electron microscopy and selected area electron diffraction measurements indicate that the grown LiNiPO4 is well-crystallized in olivine structure without any indication of crystallographic defects such as dislocations or misfits. The energy-dispersive X-ray spectrometry coupled with the elemental compositional mapping using high-angle angular dark field scanning electron microscopy confirms the chemical quality of the grown LiNiPO4 in terms of homogeneity and uniform elemental distribution characteristics. The local structure and chemical bonding between NiO6 octahedral and (PO4)3- tetrahedral groups probed by Raman spectroscopy also indicate the high-quality of LiNiPO4. Structural analysis of the delithiated Li0.09NiPO4 phase indicates lattice contraction and distortion upon lithium extraction. A detailed analysis and comparison of the pristine and delithiated phases is also reported.

Nanoscale mineralogy of arsenic in a region of New Hampshire with elevated As-concentrations in the groundwater

Bedrock samples were examined from a region in south-central New Hampshire in order to un- derstand the factors responsible for elevated As-concentrations (up to 180 ppb) in the groundwater. Although the As is predominantly from natural sources, the precise mineralogy of the As-bearing phases was unknown. As-bearing samples were examined in detail using advanced electron micros- copy techniques, including high-angle annular dark field scanning electron microscopy (HAADF- STEM), STEM elemental mapping, and high resolution TEM. Numerous As-bearing minerals were observed, predominantly arsenopyrite (FeAsS) with some westerveldite (FeAs). The arsenopyrite was partially altered to nanocrystalline (~20 nm) magnetite and westerveldite, most likely during hydrothermal circulation of fluids following the emplacement of a nearby granitoid pluton. A reac- tion rim surrounds the pristine cores of arsenopyrite, with compositionally variable amorphous zones (containing Fe, As, K, and O), a Cu-sulfide, and a uranium-bearing phase, suggesting at least three distinct fluid compositions during alteration. Because the K-Fe-As-oxide is amorphous and the nanocrystalline FeAs has a high surface area per unit mass, the dissolution rate of As-bearing phases during recent low-temperature weathering is increased. The rapid dissolution of the reduced As phase (FeAs), which is unstable under oxidizing conditions, may be an important factor in the el- evated As-concentrations in the groundwaters of this region.

Three-dimensional structure of NADH-dehydrogenase from Neurospora crassa by electron microscopy and conical tilt reconstruction

NADH-dehydrogenase (Complex I) is the first complex of the mitochondrial respiratory chain. It is an amphipatic molecule located in the inner mitochondrial membrane and is composed of at least 35 unique subunits encoded by both mitochondrial and nuclear DNA. The whole complex was isolated in detergent from the fungus Neurospora crassa. It is very stable in its isolated form and was analysed as such by electron microscopy. Its mass, determined by dark-field scanning electron microscopy was estimated as 1.12 MDa. The complex was imaged by transmission electron microscopy, by negative staining and by cryo-electron microscopy. A three-dimensional model, with a resolution estimated at 35 Å, was calculated from images of negatively stained complexes by the random conical tilt reconstruction technique. This model confirms the general L-shape of the molecule, with arms of equal length and corroborates the hypothesis of a subdivision of the whole complex into three functional domains. Immuno-labelling of the 49 kDA subunit of the peripheral arm allowed its localization within the complex. This is a first step in the subunit mapping of Complex I and the understanding of its activity.

Chromatin architecture: investigation of a subunit of chromatin by dark field electron microscopy.

Dark field scanning electron microscopy of unstained, unfixed samples of chromatin, histone-1-depleted chromatin, and nucleohistone has been used to identify an apparent subunit of chromatin, namely a disk-shaped structure we term the unit particle, which is probably about 135 A wide and 50 A thick in the hydrated state. The unit particles are found at rather uniform intervals along thin DNA-like fibers. Histone 1 depletion leads to a bimodal distribution of these spacings. Our observations suggest that the unit particle consists of a loop of nucleoprotein, perhaps around a histone core.