97 research outputs found
Structure and band edge energy of highly luminescent CdSe 1-xTex alloyed quantum dots
No full textCdSe1-xTex quantum dot (QD) alloys are characterized by high luminescence quantum yields and a strong band gap bowing as a function of the Se:Te ratio, featuring longer emission wavelengths than CdTe or CdSe dots of identical size. In this contribution, these properties are rationalized by examining the structure and band edge energy of CdSe1-xTex as functions of x. The QDs were synthesized employing the "hot- injection" method, in the presence of either trioctylphosphine oxide (TOPO) or octadecene (ODE) as the Cd precursor solvent. Elementary analysis of the QDs indicated that TOPO plays a crucial role in tuning the content of Se in the alloys, as only traces of this element were found when using ODE. Detailed studies based on X-ray diffraction (XRD), high-resolution transmission electron microscopy (HRTEM) and selected area electron diffraction (SAED) revealed a high degree of complexity in the structure of the alloyed dots. The analysis concluded that the structure of the QDs was essentially wurtzite, although features associated with zinc blende can be seen due to the presence of stacking faults and to a small population of nanocrystals with cubic structure. More importantly, these studies reveal a nonlinear expansion of the effective lattice constant with increasing Te content. The valence band edge energy of the alloys in solution was estimated from the first oxidation potential measured by linear sweep voltammetry at Au microelectrodes. The results show that the valence band edge exhibits a very weak dependence on x for values below 0.5, indicating that the decrease in the optical band gap is mainly linked to a decrease in the conduction band edge energy. For x textgreater 0.5, the conduction and valence band edges shift to higher values with an overall increase in the band gap. The experimental trends show, for the first time, that the characteristic red shift of the band gap with low to intermediate Te content is determined by relaxation of the lattice constant, whereas the contribution arising from the change in anion electronegativity becomes predominant for x textgreater 0.5. ? 2013 American Chemical Society
Structural and optical emission uniformity of m-plane InGaN single quantum wells in core-shell nanorods
No full textControlling the long-range homogeneity of core-shell InGaN/GaN layers is essential for their use in light-emitting devices. This paper demonstrates variations in optical emission energy as low as ~7 meV.µm-1 along the m-plane facets from core-shell InGaN/GaN single quantum wells as measured through high-resolution cathodoluminescence hyperspectral imaging. The layers were grown by metal organic vapor phase epitaxy on etched GaN nanorod arrays with a pitch of 2 µm. High-resolution transmission electron microscopy and spatially-resolved energy-dispersive X-ray spectroscopy measurements demonstrate a long-range InN-content and thickness homogeneity along the entire 1.2 μm length of the m-plane. Such homogeneous emission was found on the m-plane despite the observation of short range compositional fluctuations in the InGaN single quantum well. The ability to achieve this uniform optical emission from InGaN/GaN core-shell layers is critical to enable them to compete with and replace conventional planar light-emitting devices
NATO Advanced Research Workshop on the Evaluation of Advanced Semiconductor Materials by Electron Microscopy
No full textElectron microscopy and analysis 1995 proceedings of the Institute of Physics Electron Microscopy and Analysis Group Conference, University of Birmingham, 12 - 15 September 1995
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Nano-cracks in a synthetic graphite composite for nuclear applications
No full textMrozowski nano-cracks in nuclear graphite were studied by transmission electron microscopy and selected area diffraction. The material consisted of single crystal platelets typically 1–2 nm thick and stacked with large relative rotations around the c-axis; individual platelets had both hexagonal and cubic stacking order. The lattice spacing of the (0002) planes was about 3% larger at the platelet boundaries which were the source of a high fraction of the nano-cracks. Tilting experiments demonstrated that these cracks were empty, and not, as often suggested, filled by amorphous material. In addition to conventional Mrozowski cracks, a new type of nano-crack is reported, which originates from the termination of a graphite platelet due to crystallographic requirements. Both types are crucial to understanding the evolution of macro-scale graphite properties with neutron irradiation.</p
Microstructure of laterally overgrown GaN layers
No full textTransmission electron microscopy study of plan-view and cross-section samples of epitaxial laterally overgrown (ELOG) GaN samples is described. Two types of dislocation with the same type of Burgers vector but different line direction have been observed. It is shown that threading edge dislocations bend to form dislocation segments in the c-plane as a result of shear stresses developed in the wing material along the stripe direction. It is shown that migration of these dislocations involves both glide and climb. Propagation of threading parts over the wing area is an indication of high density of point defects present in the wing areas on the ELOG samples. This finding might shed new light on the optical properties of such samples
New method of reducing threading dislocation in epitaxial ZnO films grown on c-sapphire
No full textAbstractThe behavior of threading dislocations (TDs) in ZnO/(0001)sapphire heterostructures grown by a two-step method have been investigated. The initial template, grown by pulsed laser deposition, consisted of a continuous underlayer, which was O-polar and an overlayer comprising a high density of Zn-polar nanorods. High densities (∼7×1010cm−2) of TDs were found to be restricted to the underlayer, whereas the nanorods were almost defect-free. Subsequent treatments by either hydrothermal growth or chemical vapour deposition (CVD) achieved epitaxial lateral overgrowth of nanorods and led to continuous Zn-polar films. The low TD density of nanorods remained until misoriented grain boundaries and boundary dislocations were generated when neighbouring nanorods become coalesced. The lateral migration of TDs in the overgrowth led to dislocation interactions and reduction of TDs. The total TD density at the top of the overlayer was estimated to be ∼1×109cm−2for hydrothermal growth and ∼7×109cm−2for CVD growth.</jats:p
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