National Institute Of Materials Physics - Romania

We report the synthesis of a room temperature ferromagnetic Mn-Ge system obtained by simple deposition of manganese on Ge(001), heated at relatively high temperature (starting with 250 degrees C). The samples were characterized by low energy electron diffraction (LEED), scanning tunneling microscopy (STM), high resolution transmission electron microscopy (HRTEM), X-ray photoelectron spectroscopy (XPS), superconducting quantum interference device (SQUID), and magneto-optical Kerr effect (MOKE). Samples deposited at relatively elevated temperature (350 degrees C) exhibited the formation of similar to 5-8 nm diameter Mn5Ge3 and Mn11Ge8 agglomerates by HRTEM, while XPS identified at least two Mn-containing phases: the agglomerates, together with a Ge-rich MnGe similar to 2.5 phase, or manganese diluted into the Ge(001) crystal. LEED revealed the persistence of long range order after a relatively high amount of Mn (100 nm) deposited on the single crystal substrate. STM probed the existence of dimer rows on the surface, slightly elongated as compared with Ge-Ge dimers on Ge(001). The films exhibited a clear ferromagnetism at room temperature, opening the possibility of forming a magnetic phase behind a nearly ideally terminated Ge surface, which could find applications in integration of magnetic functionalities on semiconductor bases. SQUID probed the co-existence of a superparamagnetic phase, with one phase which may be attributed to a diluted magnetic semiconductor. The hypothesis that the room temperature ferromagnetic phase might be the one with manganese diluted into the Ge crystal is formulated and discussed.

A genuine GaAs surface is covered with a relatively thick layer (similar to 10 nm) of native oxide pinning the surface Fermi level within the band gap of semiconductor. The method presented in this work is related to the sulphur passivation by treating n-GaAs(100) and (110) in sulphide solutions (e.g pure ammonium sulphide and sulphur monochloride) that combines both chemical electronic passivation by reducing the surface state density. The effects of passivation were put into evidence by Photoluminescence measurements where it was observed the variation of state density. The presence of sulphur dipoles at surface was presented in a diagram of Second Harmonic Generation analysis and the general aspect of sulphur compound was identified by SEM images. The presence of the covalent bond Ga-S and the variation of surface barrier as well as the effect of crystal orientation were put into evidence by micro-Raman Spectroscopy. At the surface of n-GaAs it is developed an adherent layer of sulphur compound as a resul of chemical interaction of sulphur ions with n-GaAs surface.

Monte-Carlo estimations of the integrals of Bose-Einstein distribution function for magnons are performed, by avoiding number of approximations involved in the usual derivation of the Bloch T-3/2 law, describing the dependence of magnetization with temperature in the low temperature regime, where the main mechanism of magnetization decrease is due to spin waves. More precisely: (i) the magnon dispersion law is considered the exact one (depending on the crystal structure) and not the quadratic approximation epsilon(k) alpha k(2); (ii) the integrals are performed over the first Brillouin zone and the limit of the integral is not extended to infinity; (iii) the dimensionality of the system is taken into account: 1D, 2D and 3D systems are considered; (iv) finite size magnetic systems are treated by rigorous summation of the Bose-Einstein distribution function, without using the continuous limit. The results can be summarized as follows: (a) Infinite three-dimensional (3D) systems exhibit deviations from the Bloch T-3/2 law, namely the deviation from zero Kelvin magnetization vertical bar Delta M vertical bar/M(0) behaves as T-gamma, where gamma is sensibly (by some 10 %) larger than 3/2 and, also, depends on the crystal structure considered. (b) Finite 3D systems exhibit more drastic deviation from the Bloch law, in that the gamma exponent deviates seriously from 3/2, going up to 2.5 for small systems of about 1,000 atoms. (d) Finite 2D systems also show serious deviations from the Bloch-like T law. As described a few years ago [C.M. Teodorescu, Surf. Sci. 601, 4292 (2007)], finite temperature magnetization may be considered only in finite size 2D systems. Several recent examples from literature are analyzed in the framework of the actual theoretical results. It is shown that M(T) analysis in the low temperature regime allows one to extract the average nanoparticle size of the magnetic moieties, and as well the exchange interaction and average spin quantum number.

X-ray photoelectron spectroscopy (XPS) analyses of lead zirco-titanate Pb(Zr,Ti)O-3(001) single crystal thin layers as a function of the time spent between sample preparation by pulsed laser deposition and introduction in to a ultrahigh vacuum revealed the fact that freshly prepared samples showed a shift of the C 1s towards a higher binding energy, together with shifts of core levels originating from the substrate (particularly Ti 2p and O 1s) towards a lower binding energy. This behaviour is explained by considering that the molecules of contaminants (fatty acids, alcohols, esters) are adsorbed preferentially on areas exhibiting outwards polarization P(+). Thus, photoelectrons originating from contaminants will have larger binding energies because of the charge state of the P(+) areas, whereas the substrate XPS signals from these P(+) areas are attenuated by the contaminants, with the consequence of a prevalence of XPS substrate signals originating from the P(-) areas, shifted towards lower binding energies. Piezoresponse force microscopy confirmed the assumptions derived from XPS results and suggests the existence of an interplay between the adsorption of contaminants and the surface polarization state.

We report on structure-property relationships in Pr-doped CeO2 and ZrO2 using X-ray diffraction (XRD), Raman, UV to Vis Diffuse Reflectance (DR-UV/Vis), X-ray Photoelectron (XPS), and luminescence (PL) spectroscopies. Both 3+ and 4+ valence states of Pr are evidenced, irrespective of the host and calcination temperature, T (T = 500 and 1000 degrees C) with consequences on absorption, surface, vibrational and luminescence properties. Only zirconia represents a suitable host for Pr3+ luminescence. The distinct trivalent Pr centers and their excitation mechanism are identified in relation to the tetragonal and monoclinic phases of ZrO2. A near-infrared to visible up-conversion (UPC) emission of Pr3+ is observed upon excitation at 959 nm which occurs, most probably, via a two-photon excited state process. By using a multi-wavelength, time-gated excitation, the UPC process is established as phase selective, i.e. only Pr3+ located in the monoclinic sites of the mixed phase, monoclinic and tetragonal ZrO2 (T = 1000 degrees C) contribute to the UPC emission. We believe that, besides the local symmetry, a key role in phase selective UPC is played by the presence of Pr3+ low-lying 4f 5d levels. To the best of our knowledge, this is the first report of phase selective up-conversion emission in a lanthanide doped multi-phase host.

This work reports on the use of X-ray photoelectron spectroscopy to quantify band bending at ferroelectric free surfaces and at their interfaces with metals. Surfaces exhibiting out-of-plane ferroelectric polarization are characterized by a band bending, due to the formation of a dipole layer at the surface, composed by the uncompensated polarization charges (due to ionic displacement) and to the depolarization charge sheet of opposite sign, composed by mobile charge carriers, which migrate near surface, owing to the depolarization electric field. To this surface band bending due to out-of-plane polarization states, metal-semiconductor Schottky barriers must be considered additionally when ferroelectrics are covered by metal layers. It is found that the net band bending is not always an algebraic sum of the two effects discussed above, since sometimes the metal is able to provide additional charge carriers, which are able to fully compensate the surface charge of the ferroelectric, up to the vanishing of the ferroelectric band bending. The two cases which will be discussed in more detail are Au and Cu deposited by molecular beam epitaxy on PbZr0.2Ti0.8O3(001) single crystal thin layers, prepared by pulsed laser deposition. Gold forms unconnected nanoparticles, and their effect on the band bending is the apparition of a Schottky band bending additional to the band bending due to the out-of-plane polarization. Copper, starting with a given thickness, forms continuous metal layers connected to the ground of the system, and provide electrons in sufficient quantity to compensate the band bending due to the out-of-plane polarization.

The porous alumina (Al2O3) layer obtained at the interface between polydimethylsiloxane/hydrogen peroxide medium and aluminum substrate under charged and neutral species injection produced in negative corona discharges in air at atmospheric pressure is analyzed by different methods in this paper. The scanning electron microscopy investigations showed the uniform distribution of the pores formed in the alumina layer and their columnar structures. Both energy dispersive X-ray spectroscopy (EDS) and X-ray photoelectron spectroscopy (XPS) measurements indicate that during the anodization process of the aluminum in the polydimethylsiloxane/hydrogen peroxide medium in corona discharge the incorporation of silicon in the structure of the alumina layer is possible.

The interplay between magnetic properties and structure and interdifussion at interfaces is analyzed mainly by surface science methods and by core level spectroscopies (based on X-ray absorption XAS and X-ray photoelectron spectroscopy XPS) on model systems implying deposition of magnetic metals (Fe, Co, Sm) on usual semiconductors (Si, GaAs, InAs). The Chapters begins with a review of the core level spectroscopies and of derived techniques, such as X-ray photoelectron diffraction (XPD) from XPS and X-ray magnetic circular dichroism (XMCD) from XAS, together with a brief description of other surface science techniques employed, such as low energy electron diffraction (LEED), reflection high energy electron diffraction (RHEED), magneto-optical Kerr effect (MOKE), and with some basics of the molecular beam epitaxy (MBE) method. The examples are organized in pairs, namely one analyzes comparatively (i) interfaces formed by Fe or by Sm on Si(001), (ii) by Fe on GaAs(001) and on InAs(001), (iii) and by Co on bare GaAs(011) and on GaAs(011) passivated with Sb, in order to outline how small changes of the nature of the metal deposited, of the substrate (though identical from the chemical point of view) and of its initial state may result in strong deviations concerning both the quality of structures obtained and their magnetic properties. For instance, it is shown that Sb/Si(001) yields better properties than Fe/Si(001), Fe/InAs(001) exhibits lower reactivity and enhanced Fe magnetic moments than Fe/GaAs(001), and that passivation with antimony of GaAs(011) substrates yields to a reduced As out-diffusion into the metal layer, yielding lower tetragonal distortion and enhanced Co magnetic moments.

Phenylboronic-acid-modified nanoparticles (NPs) are attracting considerable attention for biological and biomedical applications. We describe here a convenient and general protocol for attaching multiple copies of para-substituted phenylboronic acid moieties onto either iron-oxide-, silica-or diamond-derived NPs. The boronic acid fiinctionalized NPs are all fabricated by first modifying the surface of each particle type with 4-azidobenzoic ester functions. These azide-terminated nanostructures were then reacted with 4-[1-ozo-4-pentyn-1-yl) amino]phenylboronic acid units via a Cu(I) catalyzed Huisgen cycloaddition to furnish, conveniently, the corresponding boronic-acid modified NPs (or "borono-lectins") targeted in this work. The potential of these novel "borono-lectins" as antiviral inhibitors was investigated against the Hepatitis C virus (HCV) exploiting a bioassay that measures the potential of drugs to interfere with the ability of cell-culture-derived JFH1 virus particles to infect healthy hepatocytes. As far as we are aware, this is the first report that describes NP-derived viral entry inhibitors and thus serves as a "proof-of-concept" study. The novel viral entry activity demonstrated, and the fact that the described boronic-acid-functionalized NPs all display much reduced cellular toxicities compared with alternate NPs, sets the stage for their further investigation. The data supports that NP-derived borono-lectins should be pursued as a potential therapeutic strategy for blocking viral entry of HCV.

Radiation damage in silicon, caused by the creation of point and cluster defects due to energetic charged hadrons and neutrons, results in a serious degradation of silicon-sensor performance and limits their lifetime. So far not all the defects are understood. The work presented here focuses on the study of radiation damage by electrons of different kinetic energies, from 1.5 MeV to 15 MeV, in order to study the differences between point- and cluster-related defects. The introduction rate of vacancy-related point defects and of so-called clustered regions was investigated as a function of electron energy. It is shown that the ratio between cluster dominated and point defect formation increases with increasing electron energy. 1.5 MeV electrons create only point defects, and the formation of cluster defects starts already at 3.5 MeV. To study the defect kinetics, isothermal annealing at 80 degrees C and isochronal annealing measurements were performed. (C) 2013 Elsevier B.V. All rights reserved.