Papers by Keyword: High Resolution Transmission Electron Microscopy

Abstract: Abstract. New methods of electron crystallography, particularly modern methods of electron diffraction have opened new strategies for the structure analysis of nanostructured materials and materials systems. The possibilities and limitations of the combined use of electron crystallography methods will be demonstrated for a semi-automatic orientation determination of MnAs clusters in a GaAs matrix and structural investigations of ferecrystals.

Abstract: High resolution transmission electron microscopy (HRTEM) observation was performed to clarify the early stage of precipitation in Mg-Gd (-Zr) alloy and Mg-Y(-Zr) alloy aged at 423 K. At the early stage of agigng at 423 K, the intensity of the diffuse spots become higher at the 1/2 distance of 1100 or 2110 corresponding to magnesium matrix spots with aging time. Contrasts of mono layers on {100}_Mg planes, and the feature of D0₁₉ type structure, precipitates with five layers corresponding the feature of (020)_b' plane and b' phase co-existed and b' phase formed predominantly in the peak aged Mg-Gd (-Zr) alloy at 423 K, wheres in the peak aged Mg-Y(-Zr) alloy at 423 K, monolayer and precursor of precipitates with five layers corresponding the feature of (020)_b' plane formed predominantly.

Abstract: The precipitation process in an aged Cu-1.9wt%Ni-0.3wt%Be alloy has been examined by high-resolution transmission electron microscopy. The precipitation sequence found is: Guinier- Preston (G.P.) zones → γ'' → γ' → stable γ. The disk-shaped G.P. zones and the disk-shaped γ'', γ' and γ precipitated phases are composed of monolayers of Be atoms on {100}_α of the Cu matrix and alternative Be and Ni matrix layers parallel to {100}_α. The γ'' phases consisting of two to eight Be-layers has a body-centered tetragonal (bct) lattice with a=b=0.24 nm and c=0.28 nm. The γ' or γ phase is bct with a=b=0.24 nm and c=0.26 nm or a=b=0.26 nm and c=0.27 nm. The γ'', γ' or γ phase aligns with the matrix according to the Bain orientation relationship. The growth kinetics of disk-shaped γ precipitates on aging at 500°C has been also investigated. The {001}_α habit planes of the γ precipitates migrate by a ledge mechanism. The average thickness of the γ disks increases with aging time t as t^1/2. An analysis of experimental data using a kinetic model yields the diffusivity of solute in the Cu matrix, which is in agreement with the reported diffusivity of Ni in Cu.

Abstract: A high temperature HRTEM holder equipped with a W-coil heater was used to make insitu observation of high temperature behavior of Al2O3 very thin (about 1 nm in thickness) protective film on AlN particles. The film was used to prevent AlN particles from damages by moisture. Rapid melting and rapid solidification of very small Al2O3 particles of about 2 nm in diameter were found within about 0.2 seconds. Therefore we concluded that the Al2O3 protective film worked as the sintering additives in the high temperature heating process. In the present study, very small Al2O3 particles were identified by the space between observed lattice fringe images. It was found that a tilt boundary was instantaneously formed and annihilated in an Al2O3 particle. There was also evidence that showed the formation and annihilation of edge dislocations within seven seconds during sintering.

Abstract: The precipitates of Al-5Zn-0.02In-1Mg-0.05Ti-0.5Ce (wt %) anode alloy were studied by scanning electron microscopy, X-ray microanalysis, high resolution transmission electron microscopy and selected area electron diffraction analyses in the present work. The results show that the alloy mainly contains hexagonal structure MgZn2 and tetragonal structure Al2CeZn2 precipitates. From high resolution transmission electron microscopy and selected area electron diffraction, aluminium, Al2CeZn2 and MgZn2 phases have [0 1 -1] _Al || [1 -10] _Al2CeZn2 || [-1 1 0 1] _MgZn2 orientation relation, and Al2CeZn2 and MgZn2 phases have the [0 2 -1] _Al2CeZn2 || [0 1 -10] _MgZn2 orientation relation.

Abstract: A new type of 6H zigzag faults has been identified from high resolution transmission electron microscopy (HRTEM) measurements performed on low-doped 4H-SiC homoepitaxial layer grown on off-axis substrates in a hot-wall CVD reactor. They are made of half unit cells of 6H with corresponding low temperature photoluminescence (LTPL) response ranging from about 3 eV to 2.5 eV at liquid helium temperature.

Abstract: In-grown stacking faults (IGSFs) in thick 4H-SiC epilayers grown at high growth rates have been characterized by micro-photoluminescence (micro-PL) spectroscopy and its intensity mapping. Strong PL emissions from the IGSFs are observed even at room temperature. Three kinds of IGSFs have been identified in the samples based on the micro-PL spectra. Each IGSF shows the specific PL emission peak located at 460 nm, 480 nm, and 500 nm, respectively. The shapes, distributions, and densities of IGSFs in the epilayers are revealed by the micro-PL intensity mapping. The stacking sequences of three IGSFs have been determined as (4,4), (3,5), and (6,2) in the Zhadonov’s notation, respectively, by high-resolution transmission electron microscopy observations. Three identified IGSFs are then classified as quadruple Shockley SFs, triple Shockley SFs, and double Shockley SFs, respectively, based on the shear formation model.

Abstract: We have comparatively characterized the electrical characteristics of 4H-SiC and 2H-GaN MOS capacitors and FETs. While progressive refinement of gate oxide processes, notably with NO anneal, has resulted in better threshold voltage control, reduced subthreshold slope and higher field-effect mobility for 4H-SiC MOSFETs, we have recently reported more superior MOS parameters for 2H-GaN MOSFETs. In addition, we have performed MOS-gated Hall measurements to extract the intrinsic carrier concentration and MOS mobility, indicating that both less channel electron trapping and scattering take place in 2H-GaN MOSFETs.

Abstract: Multiwalled carbon nanotubes (MWNTs) were fabricated by thermal chemical vapor deposition (CVD) using monometallic and bimetallic Co and Ni on MgO as the catalyst. The mixture of H2/C2H2 gas was used as carbon source. The prepared CNTs have different sizes/shapes and morphologies with minimal formation of carbon particles. The maximum yield of CNTs was obtained with 50% Co catalyst at 600 oC. The morphology of the CNTs with 50% Co loading generates curved structure while 50% Ni results in the formation of linear structure with aligned graphene walls. Intensity ratio of D and G-peaks (ID/IG) was measured from Raman spectra. Scanning electron microscopy (SEM) and high resolution transmission electron microscopy (HRTEM) was done for the structural analysis of the prepared MWNTs.

Abstract: Potential application of Carbon Nanotubes as a drug delivery system is limited by their hydrophobity and their natural tendency to aggregate in the bundles. Dispersion and solubility of Singlewall Carbon Nanotubes (SWCNT) in Phosphate Buffered Saline (PBS) solution via non covalent and covalent interactions was investigated. Galactosyl-β1-Sphyngosine (glycolipid precursor of cerebrosides, structured with a hydrophobic chain, a hydrophilic head and an amine group between them) was used. Pristine SWCNTs were wrapped with Galactosyl-β1-Sphyngosine (Gal-Sphy), whereas the carboxylic groups of the functionalized CNTs were activated in order to interact with amine groups of Galactosylsphyngosine and render the coating stronger. Samples dispersion was characterized by optical absorption spectroscopy (OAS). The comparison and efficiency of the dispersion stability of the functionalized material in respect to the pristine SWCNTs will be presented. We exploited Raman Spectroscopy to evaluate relative purity of the samples, and the Infrared analysis to characterize the presence of the functional groups on the tubes surface. The morphology of the samples was studied using high resolution transmission electron microscopy (HR-TEM).