56 research outputs found
Towards fabrication of ordered gallium nanostructures by laser manipulation of neutral atoms: study of self-assembling phenomena
Surface diffusion has an impact on the lateral resolution of nanostructures
in bottom-up atom nanofabrication. In this paper we study the effects of the
gallium atoms self-assembled on silicon surfaces (100) patterned with trenches
at different slopes. These particular substrate morphologies have been made to
enable an effective deposition rate variation along the surface. In this way we
experimentally mimic the effect of the atomic flux modulation created by
standing wave during an atom nanofabrication experiment. Even if we observe
self organization of gallium atoms on the surface, we conclude that the
nano-islands are not affected by surface diffusion processes and the effective
variation of the deposition rate per unit area is the dominant factor affecting
the growth differences along the surface. This result demonstrates that the
gallium atoms self-organization should not prevent the observation of a
periodic nano-patterning created by atom nano-fabrication techniques.Comment: 7 pages, 5 figures, EMRS conference procee
Correlating electron trapping and structural defects in Al2O3 thin films deposited by plasma enhanced atomic layer deposition
In this article, electron trapping in aluminum oxide (Al2O3) thin films grown by plasma enhanced atomic layer deposition on AlGaN/GaN heterostructures has been studied and a correlation with the presence of oxygen defects in the film has been provided. Capacitance–voltage measurements revealed the occurrence of a negative charge trapping effect upon bias stress, able to fill an amount of charge traps in the bulk Al2O3 in the order of 5 × 1012 cm−2. A structural analysis based on electron energy-loss spectroscopy demonstrated the presence of low-coordinated Al cations in the Al2O3 film, which is an indication of oxygen vacancies, and can explain the electrical behavior of the film. These charge trapping effects were used for achieving thermally stable (up to 100 °C) enhancement mode operation in AlGaN/GaN transistors, by controlling the two-dimensional electron gas depletion
Heteroepitaxial Growth of Ge Nanowires on Si Substrates
Electron beam evaporation has been used to prepare Ge nanowires (NWs) on top of (111) Si substrates. Despite the non-UHV growth conditions, scanning and transmission electron microscopies demonstrate that NWs are single crystal with specific crystallographic growth directions ([111], [110], and [112]). NWs are faceted, exhibiting the lower energy plans on the surface. The faceting depends on the growth direction. Moreover, the detrimental effects for Ge NWs growth of O atoms contamination are discussed. Finally, we describe how a proper preparation of the Au catalyst is able to increase the Ge NW density by a factor of 4, while heteroepitaxy and faceting features are maintained
CdSe/CdS/ZnS Double Shell Nanorods with High Photoluminescence Efficiency and Their Exploitation As Biolabeling Probes
We report the synthesis, the structural and optical characterization of CdSe/CdS/ZnS "double shell" nanorods and their exploitation in cell labeling experiments. To synthesize such nanorods, first "dot-in-a-rod" CdSe(dot)/CdS(rod) core/shell nanocrystals were prepared. Then a ZnS shell was grown epitaxially over these CdSe/CdS nanorods, which led to a fluorescence quantum yield of the final core-shell-shell nanorods that could be as high as 75%. The quantum efficiency was correlated with the aspect ratio of the nanorods and with the thickness of the ZnS shell around the starting CdSe/CdS rods, which varied from 1 to 4 monolayers (as supported by a combination of X-ray diffraction, elemental analysis with inductively coupled plasma atomic emission spectroscopy and high resolution transmission electron microscopy analysis). Pump-probe and time-resolved photoluminescence measurements confirmed the reduction of trapping at CdS surface due to the presence of the ZnS shell, which resulted in more efficient photoluminescence. These double shell nanorods have potential applications as fluorescent biological labels, as we found that they are brighter in cell imaging as compared to the starting CdSe/CdS nanorods and to the CdSe/ZnS quantum dots, therefore a lower amount of material is required to label the cells. Concerning their cytotoxicity, according to the MTT assay, the double shell nanorods were less toxic than the starting core/shell nanorods and than the CdSe/ZnS quantum dots, although the latter still exhibited a lower intracellular toxicity than both nanorod samples
Double role of HMTA in ZnO nanorods grown by chemical bath deposition
ZnO nanorods (NRs) grown by chemical bath deposition (CBD) are among the most promising semiconducting nanostructures currently investigated for a variety of applications. Still, contrasting experimental results appear in the literature on the microscopic mechanisms leading to high aspect ratio and vertically aligned ZnO NRs. Here, we report on CBD of ZnO NRs using Zn nitrate salt and hexamethylenetetramine (HMTA), evidencing a double role of HMTA in the NRs growth mechanism. Beyond the well-established pH buffering activity, HMTA is shown to introduce a strong steric hindrance effect, biasing growth along the c-axis and ensuring the vertical arrangement. This twofold function of HMTA should be taken into account for avoiding detrimental phenomena such as merging or suppression of NRs, which occur at low HMTA concentration
Self-formed Micro-Membranes
Oxide heterostructures represent a unique playground for triggering the
emergence of novel electronic states and for implementing new device concepts.
The discovery of 2D conductivity at the interface has been
linking for over a decade two of the major current research fields in Materials
Science: correlated transition-metal-oxide systems and low-dimensional systems.
A full merging of these two fields requires nevertheless the realization of
heterostructures in the form of freestanding membranes. Here
we show a completely new method for obtaining oxide hetero-membranes with
micrometer lateral dimensions. Unlike traditional thin-film-based techniques
developed for semiconductors and recently extended to oxides, the concept we
demonstrate does not rely on any sacrificial layer and is based instead on pure
strain engineering. We monitor through both real-time and post-deposition
analyses, performed at different stages of growth, the strain relaxation
mechanism leading to the spontaneous formation of curved hetero-membranes.
Detailed transmission electron microscopy investigations show that the
membranes are fully epitaxial and that their curvature results in a huge strain
gradient, each of the layers showing a mixed compressive/tensile strain state.
Electronic devices are fabricated by realizing ad hoc circuits for individual
micro-membranes transferred on silicon chips. Our samples exhibit metallic
conductivity and electrostatic field effect similar to 2D-electron systems in
bulk heterostructures. Our results open a new path for adding oxide
functionality into semiconductor electronics, potentially allowing for
ultra-low voltage gating of a superconducting transistors, micromechanical
control of the 2D electron gas mediated by ferroelectricity and
flexoelectricity, and on-chip straintronics.Comment: 8 pages, 4 figure
Covid-19 And Rheumatic Autoimmune Systemic Diseases: Role of Pre-Existing Lung Involvement and Ongoing Treatments
The Covid-19 pandemic may have a deleterious impact on patients with autoimmune systemic diseases (ASD) due to their deep immune-system alterations
Geographical heterogeneity of clinical and serological phenotypes of systemic sclerosis observed at tertiary referral centres. The experience of the Italian SIR-SPRING registry and review of the world literature
Introduction: Systemic sclerosis (SSc) is characterized by a complex etiopathogenesis encompassing both host genetic and environmental -infectious/toxic- factors responsible for altered fibrogenesis and diffuse microangiopathy. A wide spectrum of clinical phenotypes may be observed in patients' populations from different geographical areas. We investigated the prevalence of specific clinical and serological phenotypes in patients with definite SSc enrolled at tertiary referral centres in different Italian geographical macro-areas. The observed findings were compared with those reported in the world literature.Materials and methods: The clinical features of 1538 patients (161 M, 10.5%; mean age 59.8 +/- 26.9 yrs.; mean disease duration 8.9 +/- 7.7 yrs) with definite SSc recruited in 38 tertiary referral centres of the SPRING (Systemic sclerosis Progression INvestiGation Group) registry promoted by Italian Society of Rheumatology (SIR) were obtained and clustered according to Italian geographical macroareas.Results: Patients living in Southern Italy were characterized by more severe clinical and/or serological SSc phenotypes compared to those in Northern and Central Italy; namely, they show increased percentages of diffuse cutaneous SSc, digital ulcers, sicca syndrome, muscle involvement, arthritis, cardiopulmonary symptoms, interstitial lung involvement at HRCT, as well increased prevalence of serum anti-Scl70 autoantibodies. In the same SSc population immunusppressive drugs were frequently employed. The review of the literature underlined the geographical heterogeneity of SSc phenotypes, even if the observed findings are scarcely comparable due to the variability of methodological approaches.Conclusion: The phenotypical differences among SSc patients' subgroups from Italian macro-areas might be correlated to genetic/environmental co-factors, and possibly to a not equally distributed national network of information and healthcare facilities
Effect of VLS and VS mechanisms during shell growth of GaAs/AlGaAs core-shell nanowires investigated by transmission electron microscopy
We investigated by means of transmission electron microscopy (TEM) the final growth stage of GaAs/AlGaAs core-shell nanowires (NWs) self-assembled by Au-catalyst assisted metalorganic vapor phase epitaxy (MOVPE). TEM observations and energy dispersive x-ray spectroscopy revealed the presence of an AlGaAs tapered region of varying chemical composition nearby the NW extreme end (i.e. between the core-shell NW trunk and the Au nanoparticle catalyst). Our findings evidence that this region exhibits an unintentional AlyGa1−yAs/AlxGa1−xAs core-shell structure, a result of the combined axial (vapor-liquid-solid, VLS) self-assembly and conventional (vapor-solid, VS) overgrowth of the material. While the VS-grown AlxGa1−xAs alloy retains the Al composition (x=0.3) of the AlGaAs shell along the NW trunk, the central AlyGa1−yAs section is made of an Al-rich (y≈0.8–0.9) alloy segment formed during AlGaAs shell overgrowth, followed by a graded-alloy segment formed upon deposition of the terminating GaAs cap layer, the latter segment due to the effect of the Al reservoir left in the Au catalyst nanoparticle (NP)
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