Your browser doesn't support javascript.
loading
Mostrar: 20 | 50 | 100
Resultados 1 - 20 de 29
Filtrar
Más filtros










Base de datos
Intervalo de año de publicación
1.
RSC Adv ; 11(19): 11513-11518, 2021 Mar 16.
Artículo en Inglés | MEDLINE | ID: mdl-35423610

RESUMEN

The reduction and oxidation of epitaxial Fe3O4 films grown by reactive deposition on a Fe-p(1 × 1)O surface have been investigated by means of Auger electron spectroscopy (AES), low energy electron diffraction (LEED) and scanning tunneling microcopy (STM). The as-grown iron oxide samples display a square LEED pattern with a lattice constant compatible with a p(1 × 1) bulk terminated Fe3O4(001) surface. STM topographic images of Fe3O4 are characterized by atomically flat terraces separated by highly oriented steps running along the (010) and (100) crystallographic directions of the substrate. Upon annealing at 800 K in an ultra-high vacuum, AES reveals that magnetite transforms to FeO. The sample exposes the (001) surface of the rock salt structure, with a lattice parameter close to that of bulk wüstite. The Fe3O4 phase can be recovered by oxidation at 10-6 mbar of molecular oxygen.

2.
J Microsc ; 280(3): 222-228, 2020 Dec.
Artículo en Inglés | MEDLINE | ID: mdl-32472560

RESUMEN

The intercalation of graphite by electrochemical methods is an efficient strategy to produce massive graphene flakes. In fact, when graphite is biased inside an acidic solution, anions enter inside the stratified structure of the electrode and reduce the layer-to-layer interaction. Consequently, a gentle sonication is sufficient to disperse the graphene flakes inside the electrolyte. In view of an optimisation of the production protocol, a detailed analysis of the intercalation mechanism at the molecular length scale is mandatory. In the last 30 years, electrochemical (EC) scanning probe microscopies (e.g. EC-STM and in situ AFM) have been widely exploited in this research topic. In fact, these techniques have the possibility of combining the EC characterisation (e.g. cyclic-voltammetry, CV) with mechanical characterisation (e.g. adhesion and friction) and topography acquisition with high (molecular) lateral resolution. In this work, we investigate the tribological properties of the basal surface of graphite before and after the anion intercalation. By comparing the results acquired after the extraction of the graphite electrode from the EC cell with those collected inside the EC cell during the CV by an in situ AFM, we show how some features deriving from anisotropic friction can be exploited to unveil the very early stage of graphite exfoliation.

3.
PLoS One ; 13(7): e0200523, 2018.
Artículo en Inglés | MEDLINE | ID: mdl-30024890

RESUMEN

BACKGROUND: Tuberculosis (TB) represents the ninth leading cause of death worldwide. In 2016 are estimated 1.3 million TB deaths among HIV negative people and an additional 374,000 deaths among HIV positive people. In 2016 are estimated 1.4 million new cases of TB in people living with HIV (PLHIV), 74% of whom were living in Africa. In light of these data, the reduction of mortality caused by TB in PLHIV is strongly required specially in low-income countries as Mozambique. According to international guidelines, the initial TB screening in HIV+ patients should be done with the four symptoms screening (4SS: fever, current cough, night sweats and weight loss). The diagnostic test more used in resource-limited countries is smear microscopy (SMEAR). World Health Organization (WHO) recommended Lateral Flow urine LipoArabinoMannan assay (LF-LAM) in immunocompromised patients; in 2010 WHO endorsed the use of Xpert Mycobacterium Tuberculosis/Rifampicin (MTB/RIF) test for rapid TB diagnosis but the assay is not used as screening test in all HIV+ patients irrespectively of symptoms due to cost and logistical barriers. The paper aims to evaluate the cost-effectiveness of three screening protocols: standard (4SS and SMEAR in positive patients to 4SS); MTB/RIF; LF-LAM / MTB/RIF. METHODS: We developed a model to assess the cost-effectiveness of the MTB/RIF protocol versus the common standard and LF-LAM / MTB/RIF protocol. The model considered a sample of 1,000 HIV+ antiretroviral treatment naïve patients in Mozambique. We evaluated disability-adjusted life year (DALY) averted for each protocol, cost per DALY, and incremental cost-effectiveness ratio (ICER), over 1-year, assuming a national healthcare system perspective. The model considered the delayed diagnosis as the time elapsed between a false negative test and the diagnosis and treatment of TB. Additional health system organization delay is defined as the time interval between positive test and treatment initiation caused by a delay in the delivery of results due organization of services. We conducted a sensitivity analysis on more relevant variables. RESULTS: The MTB/RIF protocol was cost-effective as compared to the standard protocol with an ICER of $56.54 per DALY saved. In a cohort of 1,000 patients MTB/RIF and LF-LAM / MTB/RIF protocol generated 1,281 and 1,254 DALY's saved respectively, with a difference of 174 and 147 DALY respect to the standard protocol. The total cost of MTB/RIF protocol was lower ($92,263) than the standard ($147,226) and the LF-LAM / MTB/RIF ($113,196). Therefore, the cost per DALY saved including new infections due to delayed diagnosis with the standard protocol was $79.06, about 5 fold higher than MTB/RIF and LF-LAM / MTB/RIF protocols. The cost of additional TB infections due to delays in diagnosis plus health system delay seemed the more relevant costs. The low sensibility and sensitivity of the standard protocol led to a high number of false negatives, thus delayed TB diagnoses and treatment lead to the development of newly transmitted TB infections. CONCLUSIONS: Our study shows that the MTB/RIF adoption could lead to an increasing of TB case-finding and a reduction in costs compared with standard and LF-LAM / MTB/RIF protocols.


Asunto(s)
Pruebas Diagnósticas de Rutina/economía , Infecciones por VIH/epidemiología , Tamizaje Masivo/economía , Tuberculosis/epidemiología , Adulto , Comorbilidad , Análisis Costo-Beneficio , Diagnóstico Tardío , Pruebas Diagnósticas de Rutina/métodos , Femenino , Infecciones por VIH/diagnóstico , Infecciones por VIH/terapia , Humanos , Masculino , Tamizaje Masivo/métodos , Modelos Económicos , Mozambique/epidemiología , Reproducibilidad de los Resultados , Sensibilidad y Especificidad , Tuberculosis/diagnóstico , Tuberculosis/terapia
4.
Phys Rev Lett ; 118(16): 167402, 2017 Apr 21.
Artículo en Inglés | MEDLINE | ID: mdl-28474919

RESUMEN

We exploit the spin-Hall effect to generate a uniform pure spin current in an epitaxial n-doped Ge channel, and we detect the electrically induced spin accumulation, transverse to the injected charge current density, with polar magneto-optical Kerr microscopy at a low temperature. We show that a large spin density up to 400 µm^{-3} can be achieved at the edges of the 100-µm-wide Ge channel for an applied electric field lower than 5 mV/µm. We find that the spin density linearly decreases toward the center of the Ge bar, due to the large spin diffusion length, and such a decay is much slower than the exponential one observed in III-V semiconductors, allowing very large spin accumulations over a length scale of tens of micrometers. This lays the foundation for multiterminal spintronic devices, where different spin voltages can be exploited as inputs for magnetologic gates on the same Ge platform.

5.
Phys Chem Chem Phys ; 19(21): 13855-13859, 2017 May 31.
Artículo en Inglés | MEDLINE | ID: mdl-28513671

RESUMEN

In the currently accepted picture, when graphite is immersed and polarized in a diluted sulfuric acid electrolyte, the surface undergoes an invasive process due to the intercalation of solvated sulphate anions inside the crystal. The following evolution of CO, CO2 and O2 promotes the surface swelling and the growth of blisters. Here, we give evidence that the appearance of blisters affects the graphite surface as soon as the oxygen potential is reached, i.e. before the traditionally accepted anion intercalation stage, which instead is demonstrated slowing the blister development. These results suggest a new picture of the solvated anion intercalation in graphite with respect to the current interpretative model.

7.
J Phys Condens Matter ; 28(19): 195001, 2016 May 18.
Artículo en Inglés | MEDLINE | ID: mdl-27073190

RESUMEN

The spin features of surface resonance bands in single layer Bi on Ge(1 1 1) are studied by means of spin- and angle-resolved photoemission spectroscopy and inverse photoemission spectroscopy. We characterize the occupied and empty surface states of Ge(1 1 1) and show that the deposition of one monolayer of Bi on Ge(1 1 1) leads to the appearance of spin-polarized surface resonance bands. In particular, the C 3v symmetry, which Bi adatoms adopt on Ge(1 1 1), allows for the presence of Rashba-like occupied and unoccupied electronic states around the [Formula: see text] point of the Bi surface Brillouin zone with a giant spin-orbit constant [Formula: see text] eV · Å.

8.
J Phys Condens Matter ; 28(16): 165801, 2016 Apr 27.
Artículo en Inglés | MEDLINE | ID: mdl-26988255

RESUMEN

We report on the spin transport properties in p-doped germanium (Ge-p) using low temperature magnetoresistance measurements, electrical spin injection from a ferromagnetic metal and the spin pumping-inverse spin Hall effect method. Electrical spin injection is carried out using three-terminal measurements and the Hanle effect. In the 2-20 K temperature range, weak antilocalization and the Hanle effect provide the same spin lifetime in the germanium valence band (≈1 ps) in agreement with predicted values and previous optical measurements. These results, combined with dynamical spin injection by spin pumping and the inverse spin Hall effect, demonstrate successful spin accumulation in Ge. We also estimate the spin Hall angle θ(SHE) in Ge-p (6-7 x 10(-4) at room temperature, pointing out the essential role of ionized impurities in spin dependent scattering.

9.
J Phys Condens Matter ; 26(44): 445001, 2014 Nov 05.
Artículo en Inglés | MEDLINE | ID: mdl-25213009

RESUMEN

We investigated the room temperature oxidation of ultra-thin Ni and Cr films grown on Fe(0 0 1). In particular, we characterized the degree of crystallinity and the stoichiometry of the oxide layers and addressed the chemical stability of the interface with the highly reactive Fe substrate by means of low-energy electron diffraction and x-ray and UV photoemission spectroscopy. In the Ni case we detected, upon oxidation, the formation of a Fe(3)O(4) layer covering the Ni oxide, due to the diffusion of Fe cations towards the surface. At high temperature and in ultra-high vacuum conditions, the Ni oxide dissolved and the Fe oxide layer was reduced to FeO. In the Cr case, we observed the formation of a thin Cr(2)O(3) oxide layer, showing a diffraction pattern compatible with a defective γ-Cr(2)O(3) phase. A thicker Cr oxide layer could be produced by oxidizing the sample at 300 °C, at the expense of the incorporation of trace amounts of Fe cations.

10.
Phys Rev Lett ; 113(4): 046102, 2014 Jul 25.
Artículo en Inglés | MEDLINE | ID: mdl-25105635

RESUMEN

A remarkable enhancement of atomic diffusion is highlighted by scanning tunneling microscopy performed on ultrathin metastable body-centered tetragonal Co films grown on Fe(001). The films follow a nearly perfect layer-by-layer growth mode with a saturation island density strongly dependent on the layer on which the nucleation occurs, indicating a lowering of the diffusion barrier. Density functional theory calculations reveal that this phenomenon is driven by the increasing capability of the film to accommodate large deformations as the thickness approaches the limit at which a structural transition occurs. These results disclose the possibility of tuning surface diffusion dynamics and controlling cluster nucleation and self-organization.

11.
Rev Sci Instrum ; 85(7): 073901, 2014 Jul.
Artículo en Inglés | MEDLINE | ID: mdl-25085146

RESUMEN

We present a versatile apparatus for the study of ferromagnetic surfaces, which combines spin-polarized photoemission and inverse photoemission spectroscopies. Samples can be grown by molecular beam epitaxy and analyzed in situ. Spin-resolved photoemission spectroscopy analysis is done with a hemispherical electron analyzer coupled to a 25 kV-Mott detector. Inverse photoemission spectroscopy experiments are performed with GaAs crystals as spin-polarized electron sources and a UV bandpass photon detector. As an example, measurements on the oxygen passivated Fe(100)-p(1×1)O surface are presented.

12.
Langmuir ; 29(26): 8302-10, 2013 Jul 02.
Artículo en Inglés | MEDLINE | ID: mdl-23725023

RESUMEN

In polymer-metal oxide hybrid solar cells, an extremely careful engineering of the interface is required to ensure good device performances. Recently, very promising results have been obtained by functionalizing titanium dioxide (TiO2) by means of 4-mercaptopyridine (4-MPy) molecules, showing the beneficial effect of these molecules on the interface morphology. This study investigates the nature of the interaction of 4-MPy molecules with the TiO2 surface by means of X-ray photoemission spectroscopy. In order to mimic the device processing conditions, our analysis is carried out on molecules adsorbed from solution on a nanocrystalline surface. According to our analysis, 4-MPy molecules (C5H5NS) are likely bound with the oxide through the nitrogen atom. The bonding precedes either via a covalent interaction with Lewis surface sites, or via hydrogen mediation, possibly in the form of hydrogen bonds. Interestingly, in the latter case, we also observe strong changes in the spectroscopic features attributed to the thiol group.


Asunto(s)
Piridinas/química , Titanio/química , Adsorción , Cristalización , Espectroscopía de Fotoelectrones , Propiedades de Superficie
13.
Phys Rev Lett ; 108(15): 156603, 2012 Apr 13.
Artículo en Inglés | MEDLINE | ID: mdl-22587272

RESUMEN

We demonstrate optical orientation in Ge/SiGe quantum wells and study their spin properties. The ultrafast electron transfer from the center of the Brillouin zone to its edge allows us to achieve high spin polarizations and to resolve the spin dynamics of holes and electrons. The circular polarization degree of the direct gap photoluminescence exceeds the theoretical bulk limit, yielding ∼37% and ∼85% for transitions with heavy and light holes states, respectively. The spin lifetime of holes at the top of the valence band is estimated to be ∼0.5 ps and it is governed by transitions between light and heavy hole states. Electrons at the bottom of the conduction band, on the other hand, have a spin lifetime that exceeds 5 ns below 150 K. Theoretical analysis of the spin relaxation indicates that phonon-induced intervalley scattering dictates the spin lifetime of electrons.

14.
Rev Sci Instrum ; 82(2): 023709, 2011 Feb.
Artículo en Inglés | MEDLINE | ID: mdl-21361603

RESUMEN

We present a confocal microscopy setup that is able to record magneto-optical hysteresis cycles separating the in-plane and out-of-plane magnetization components. This apparatus is based on a modified commercial microscope, where the light beam has been deviated from the cylindrical symmetry axis of the objective lenses by inserting a translating plate in the optical path. The instrument allows for the magneto-optical imaging with a lateral resolution of 600 nm at λ = 635 nm light wavelength.

15.
Phys Rev Lett ; 95(13): 137203, 2005 Sep 23.
Artículo en Inglés | MEDLINE | ID: mdl-16197170

RESUMEN

We have studied the magnetoresistance (TMR) of tunnel junctions with electrodes of La(2/3)Sr(1/3)MnO3 and we show how the variation of the conductance and TMR with the bias voltage can be exploited to obtain precise information on the spin and energy dependence of the density of states. Our analysis leads to a quantitative description of the band structure of La(2/3)Sr(1/3)MnO3 including the energy gap delta between the Fermi level and the bottom of the t(2g) minority-spin band, in good agreement with data from spin-polarized inverse photoemission experiments. This shows the potential of magnetic tunnel junctions with half-metallic electrodes for spin-resolved spectroscopic studies.

17.
Phys Rev Lett ; 77(5): 908-911, 1996 Jul 29.
Artículo en Inglés | MEDLINE | ID: mdl-10062937
18.
Phys Rev B Condens Matter ; 53(20): 13817-13823, 1996 May 15.
Artículo en Inglés | MEDLINE | ID: mdl-9983136
19.
Phys Rev B Condens Matter ; 52(5): 3063-3066, 1995 Aug 01.
Artículo en Inglés | MEDLINE | ID: mdl-9981415
20.
SELECCIÓN DE REFERENCIAS
DETALLE DE LA BÚSQUEDA
...