Phonon confinement and plasmon-phonon interaction in nanowire based quantum wells

Resonant Raman spectroscopy is realized on closely spaced nanowire based quantum wells. Phonon quantization consistent with 2.4 nm thick quantum wells is observed, in agreement with cross-section transmission electron microscopy measurements and photoluminescence experiments. The creation of a high density plasma within the quantized structures is demonstrated by the observation of coupled plasmon-phonon modes. The density of the plasma and thereby the plasmon-phonon interaction is controlled with the excitation power. This work represents a base for further studies on confined high density charge systems in nanowires

Exciton Footprint of Self-assembled AlGaAs Quantum Dots in Core-Shell Nanowires

Quantum-dot-in-nanowire systems constitute building blocks for advanced photonics and sensing applications. The electronic symmetry of the emitters impacts their function capabilities. Here, we study the fine structure of gallium-rich quantum dots nested in the shell of GaAs-AlGaAs core-shell nanowires. We used optical spectroscopy to resolve the splitting resulting from the exchange terms and extract the main parameters of the emitters. Our results indicate that the quantum dots can host neutral as well as charges excitonic complexes and that the excitons exhibit a slightly elongated footprint, with the main axis tilted with respect to the growth axis. GaAs-AlGaAs emitters in a nanowire are particularly promising for overcoming the limitations set by strain in other systems, with the benefit of being integrated in a versatile photonic structure

Short-course thrombolysis as the first line of therapy for cardiac valve thrombosis

AbstractObjective: To retrospectively evaluate the clinical and echocardiographic criteria of thrombolytic therapy for mechanical heart valve thrombosis. Methods: Nineteen consecutive patients with 22 instances of prosthetic heart valve thrombosis (14 mitral, 2 aortic, 3 tricuspid, and 3 pulmonary) were treated with short-course thrombolytic therapy as first option of treatment in absence of contraindications. The thrombolytic therapy protocol consisted of streptokinase (1,500,000 IU in 90 minutes) (n = 18) in one (n = 7) or two (n = 11) cycles or recombinant tissue-type plasminogen activator (100 mg in 90 minutes) (n = 4). Results: Overall success was seen in 82%, immediate complete success in 59%, and partial success in 23%. Six patients without total response to thrombolytic therapy underwent surgery, and pannus was observed in 83%. Six patients showed complications: allergy, stroke, transient ischemic attack, coronary embolism, minor bleeding, and one death. At diagnosis, 10 patients evidenced atrial thrombus by transesophageal echocardiography, 3 of whom experienced peripheral embolism during thrombolysis. Four episodes of rethrombosis were observed (16%). The survivorship was 84% with a mean follow-up of 42.6 months. Conclusions: A short-course of thrombolytic therapy may be considered first-line therapy for prosthetic heart valve thrombosis. The risk of peripheral embolism may be evaluated for the presence of atrial thrombus by transesophageal echocardiography at diagnosis. (J Thorac Cardiobasc Surg 1998;115:780-4

Direct correlation of crystal structure and optical properties in wurtzite/zinc-blende GaAs nanowire heterostructures

A novel method for the direct correlation at the nanoscale of structural and optical properties of single GaAs nanowires is reported. Nanowires consisting of 100% wurtzite and nanowires presenting zinc-blende/wurtzite polytypism are investigated by photoluminescence spectroscopy and transmission electron microscopy. The photoluminescence of wurtzite GaAs is consistent with a band gap of 1.5 eV. In the polytypic nanowires, it is shown that the regions that are predominantly composed of either zinc-blende or wurtzite phase show photoluminescence emission close to the bulk GaAs band gap, while regions composed of a nonperiodic superlattice of wurtzite and zinc-blende phases exhibit a redshift of the photoluminescence spectra as low as 1.455 eV. The dimensions of the quantum heterostructures are correlated with the light emission, allowing us to determine the band alignment between these two crystalline phases. Our first-principles electronic structure calculations within density functional theory, employing a hybrid-exchange functional, predict band offsets and effective masses in good agreement with experimental results

Structural and functional characterization of (110)-oriented epitaxial La2/3Ca1/3MnO3 electrodes and SrTiO3 tunnel barriers

La2/3Ca1/3MnO3 (LCMO) films have been deposited on (110)-oriented SrTiO3 (STO) substrates. X-ray diffraction and high-resolution electron microscopy reveal that the (110) LCMO films are epitaxial and anisotropically in-plane strained, with higher relaxation along the [1¿10] direction than along the [001] direction; x-ray absorption spectroscopy data signaled the existence of a single intermediate Mn3+/4+ 3d-state at the film surface. Their magnetic properties are compared to those of (001) LCMO films grown simultaneously on (001) STO substrates It is found that (110) LCMO films present a higher Curie temperature (TC) and a weaker decay of magnetization when approaching TC than their (001) LCMO counterparts. These improved films have been subsequently covered by nanometric STO layers. Conducting atomic-force experiments have shown that STO layers, as thin as 0.8 nm, grown on top of the (110) LCMO electrode, display good insulating properties. We will show that the electric conductance across (110) STO layers, exponentially depending on the barrier thickness, is tunnel-like. The barrier height in STO (110) is found to be similar to that of STO (001). These results show that the (110) LCMO electrodes can be better electrodes than (001) LCMO for magnetic tunnel junctions, and that (110) STO are suitable insulating barriers

Bandgap engineering in a nanowire: self-assembled 0, 1 and 2D quantum structures

Inherent to the nanowire morphology is the exciting possibility of fabricating materials organized at the nanoscale in three dimensions. Composition and structure can be varied along and across the nanowire, as well as within coaxial shells. This opens up a manifold of possibilities in nanoscale materials science and engineering which is only possible with a nanowire as a starting structure. As the variation in composition and structure is accompanied by a change in the band structure, it is possible to confine carriers within the nanowire. Interestingly, this results in the formation of local two, one and zero-dimensional structures from an electronic point of view within the nanowire. This novel palette of nanostructures paves the way toward novel applications in many engineering domains such as lasers, high-mobility transistors, quantum information and energy harvesting. In the present review we summarize and give an overview on recent achievements in the design and growth of advanced quantum structures starting from nanowire templates. The quantum structures presented have been grown by molecular beam epitaxy and correspond to different confinement approaches: quantum wells (2D), quantum wires (1D) and quantum dots (0D)

Synthesis parameter space of bismuth catalyzed germanium nanowires

The synthesis parameter space of bismuth catalyzed germanium nanowires by chemical vapor deposition is determined. The process window for high aspect ratio nanowires is found to be extremely narrow. The optimal conditions are found to be 300 degrees C and 150 Torr gas pressure. For lower temperatures, the solubility of Ge in Bi is too low for the nucleation of Ge nanowires to occur. For higher temperatures, small Bi droplets tend to evaporate leading to an extreme reduction in the nanowire density. The extremely low process temperature makes Bi a good candidate for its growth on low cost and low thermal budget substrates such as plastics

Archivos 2015: Archivos en abierto

Nota editorial año 201

In(Ga)As quantum dot formation on group-III assisted catalyst-free InGaAs nanowires

Growth of GaAs and InxGa1-xAs nanowires by the group-III assisted molecular beam epitaxy growth method on (001)GaAs/SiO2 substrates is studied in dependence on growth temperature, with the objective of maximizing the indium incorporation. Nanowire growth was achieved for growth temperatures as low as 550 degrees C. The incorporation of indium was studied by low temperature micro-photoluminescence spectroscopy, Raman spectroscopy and electron energy loss spectroscopy. The results show that the incorporation of indium achieved by lowering the growth temperature does not have the effect of increasing the indium concentration in the bulk of the nanowire, which is limited to 3-5%. For growth temperatures below 575 degrees C, indium rich regions form at the surface of the nanowires as a consequence of the radial growth. This results in the formation of quantum dots, which exhibit spectrally narrow luminescence