National Institute Of Materials Physics - Romania

Artificial complex-oxide heterostructures containing ultrathin buried layers grown along the pseudocubic [111] direction have been predicted to host a plethora of exotic quantum states arising from the graphene-like lattice geometry and the interplay between strong electronic correlations and band topology. To date, however, electronic-structural investigations of such atomic layers remain an immense challenge due to the shortcomings of conventional surface-sensitive probes with typical information depths of a few angstroms. Here, we use a combination of bulk-sensitive soft X-ray angle-resolved photoelectron spectroscopy (SX-ARPES), hard X-ray photoelectron spectroscopy (HAXPES), and state-of-the-art first-principles calculations to demonstrate a direct and robust method for extracting momentum-resolved and angle-integrated valence-band electronic structure of an ultrathin buckled graphene-like layer of NdNiO3 confined between two 4-unit cell-thick layers of insulating LaAlO3. The momentum-resolved dispersion of the buried Ni d states near the Fermi level obtained via SX-ARPES is in excellent agreement with the first-principles calculations and establishes the realization of an antiferro-orbital order in this artificial lattice. The HAXPES measurements reveal the presence of a valence-band bandgap of 265 meV. Our findings open a promising avenue for designing and investigating quantum states of matter with exotic order and topology in a few buried layers.

It is shown that quantum electromagnetic transitions to high orders are essential to describe the time-dependent path of a nanoscale electron system in a Coulomb blockade regime when coupled to external leads and placed in a 3D rectangular photon cavity. The electronic system consists of two quantum dots embedded asymmetrically in a short quantum wire. The two lowest in energy spin degenerate electron states are mostly localized in each dot with only a tiny probability in the other dot. In the presence of the leads, a slow high-order transition between the ground states of the two quantum dots is identified. The Fourier power spectrum for photon-photon correlations in the steady state shows a Fano type of resonance for the frequency of the slow transition. Full account is taken of the geometry of the multilevel electronic system, and the electron-electron Coulomb interactions together with the para- and diamagnetic electron-photon interactions are treated with step-wise exact numerical diagonalization and truncation of appropriate many-body Fock spaces. The matrix elements for all interactions are computed analytically or numerically exactly.

The main objective of the tissue engineering field is to regenerate the damaged parts of the body by developing biological substitutes that maintain, restore, or improve original tissue function. In this context, by using the electrospinning technique, composite scaffolds based on polycaprolactone (PCL) and inorganic powders were successfully obtained, namely: zinc oxide (ZnO), titanium dioxide (TiO2) and hydroxyapatite (HAp). The novelty of this approach consists in the production of fibrous membranes based on a biodegradable polymer and loaded with different types of mineral powders, each of them having a particular function in the resulting composite. Subsequently, the precursor powders and the resulting composite materials were characterized by the structural and morphological point of view in order to determine their applicability in the field of bone regeneration. The biological assays demonstrated that the obtained scaffolds represent support that is accepted by the cell cultures. Through simulated body fluid immersion, the biodegradability of the composites was highlighted, with fiber fragmentation and surface degradation within the testing period.

This paper describes an alternative active layer for the solar cells based on the organometallic compounds in two configurations: bulk heterojunction and donor/acceptor junction between the organometallic compounds as the electron donor and carbon-based layer as the electron acceptor. Both configurations depend on the band alignment which ensures optimal charge transport towards electrodes in the sandwich structures of these active layers, but the optimization also depends by the exciton diffusion length which limits the thicknesses of the active layer. In the bulk heterojunctions, the exciton diffusion length could be extended to 100 nm which allows a better efficiency then bilayer structures. The photoconductive behaviors of these two configurations have shown the superiority of the bulk heterojunctions, increasing the intensity of the measured photocurrent. The redshift of the photoluminescence of Alq3-5Cl in the bulk heterojunctions reveals a better charge transfer towards the acceptor layer, in this case, formed from graphene oxide. The alternative of organometallic compounds as donor materials ensures a better thermal and chemical stability compared with other organic materials like perovskites.

The fundamental phenomena at ferroelectric interfaces have been the subject of thorough theoretical and computational studies due to their usefulness in a large variety of emergent electronic devices, solar cells and catalysts. Ferroelectricity determines interface band-bending and shifts in electron energies, which can be beneficial or detrimental to device performance. However, the underlying mechanisms are still the subject of debate and investigation, as a deeper understanding of the electrochemistry is required to develop bona fide design principles for functional ferroelectric surfaces and interfaces. Here, using first principles calculations within the GGA + U formalism, we investigate the problem of band alignment in non-defective, asymmetric SrRuO3/PbTiO3/SrRuO3 capacitors with ultra-thin ferroelectric layers. The effects of the dielectric size on the polar distortion stability and interface-specific properties are analyzed. It is shown that the critical size of the dielectric for polarization switching is approximate to 2 nm (5 PbTiO3 u.c.). Below this limit there is no bulk-like region in the dielectric, the space charge accumulated at interfaces leads to the presence of gap states in the whole PbTiO3 layer and ferroelectricity vanishes. We draw the band alignment diagrams as given by the band line-up and band structure terms, as well as by taking Ti 3s semi-core states as reference. In the ferroelectric structures, both approaches predict a strong effect of band-bending on the type of contact, Schottky or Ohmic, at the asymmetric interfaces. The effect of interface states on the interface dipole amplitude and band alignment is discussed.

Ferroelectrics are intensively studied materials due to their unique properties with high potential for applications. Despite all efforts devoted to obtain the values of ferroelectric material constants, the problem of the magnitude of static dielectric constant remains unsolved. In this article it is shown that the value of the static dielectric constant at zero electric field and with negligible contribution from the ferroelectric polarization (also called static background dielectric constant, or just background dielectric constant) can be very low (between 10 and 15), possibly converging towards the value in the optical domain. It is also found that the natural state of an ideal, mono-domain, epitaxial ferroelectric is that of full depletion with constant capacitance at voltages outside the switching domain. The findings are based on experimental results obtained from a new custom method designed to measure the capacitance-voltage characteristic in static conditions, as well from Rayleigh analysis. These results have important implications in future analysis of conduction mechanisms in ferroelectrics and theoretical modeling of ferroelectric-based devices.

As a new class of sustainable carbon material, "carbon dots" is an umbrella term covering many types of materials. Herein, a broad range of techniques was used to develop the understanding of hydrothermally synthesized carbon dots, and it is shown how fine-tuning the structural features by simple reduction/oxidation reactions can drastically affect their excited-state properties. Structural and spectroscopic studies found that photoluminescence originates from direct excitation of localized fluorophores involving oxygen functional groups, whereas excitation at graphene-like features leads to ultrafast phonon-assisted relaxation and largely quenches the fluorescent quantum yields. This is arguably the first study to identify the dynamics of photoluminescence including Stokes shift and allow the relaxation pathways in these carbon dots to be fully resolved. This comprehensive investigation sheds light on how understanding the excited-state relaxation processes in different carbon structures is crucial for tuning the optical properties for any potential commercial applications.

Electronic phase separation is crucial for the fascinating macroscopic properties of the LaAlO3/SrTiO3 (LAO/STO) paradigm oxide interface, including the coexistence of superconductivity and ferromagnetism. We investigate this phenomenon using angle-resolved photoelectron spectroscopy (ARPES) in the soft-x-ray energy range, where the enhanced probing depth combined with resonant photoexcitation allow us access to fundamental electronic structure characteristics - momentum-resolved spectral function, dispersions and ordering of energy bands, Fermi surface - of buried interfaces. Our experiment uses x-ray irradiation of the LAO/STO interface to tune its oxygen deficiency, building up a dichotomic system where mobile weakly correlated Ti t(2g) electrons coexist with localized strongly correlated Ti e(g) ones. The ARPES spectra dynamics under x-ray irradiation shows a gradual intensity increase under constant Luttinger count of the Fermi surface. This fact identifies electronic phase separation (EPS) where the mobile electrons accumulate in conducting puddles with fixed electronic structure embedded in an insulating host phase, and allows us to estimate the lateral fraction of these puddles. We discuss the physics of EPS invoking a theoretical picture of oxygen-vacancy clustering, promoted by the magnetism of the localized Ti e(g) electrons, and repelling of the mobile t(2g) electrons from these clusters. Our results on the irradiation-tuned EPS elucidate the intrinsic one taking place at the stoichiometric LAO/STO interfaces.

Using UV-VIS absorption spectroscopy, photoluminescence (PL) and photoluminescence excitation (PLE), the photodegradation reactions of folic acid (FA) in phosphate buffer (PB) solutions were studied. Regardless of the PB solution's pH, the UV-VIS spectra showed a gradual decrease in absorbance at 284 nm simultaneous with an increase in the absorbance of another band in the spectral range of 320-380 nm, which was downshifted under UV irradiation. The relative intensity of the FA PL band, situated in the spectral range 375-600 nm, was dependent on the pH of the PB solution. The FA PL intensity increased as increasing UV irradiation time up to 281 min. in PB solutions with pH values of 6.4 and 5.4. Under an emission wavelength of 500 nm, the position of the FA PLE spectrum changed as the PB solution pH varied from 7 to 5.4 and the irradiation time increased to 317 min. These changes were correlated with the formation of two photodegradation products, namely, pterine-6-carboxylic acid and p-amino-benzoyl-L-glutamic acid. According to UV-VIS spectroscopy and PL and PLE studies, the presence of various excipients in commercial pharmaceutical tablets does not affect the photodegradation of FA in PB solutions. Using IR spectroscopy, new evidences for the formation of the two photodegradation products of FA in PB solutions are shown.

The work is dedicated to the investigation of optical, structural, magnetic and magneto-optical properties of an aluminophosphate glass doped with Dy3+ ions, for specific applications as Faraday rotators in the visible spectral domain. The vitreous material belongs to the 16Li(2)O-8Al(2)O(3)-6BaO center dot 60P(2)O(5)-10Dy(2)O(3) system. Optical homogeneity measured by a polariscopic method, as well as by polarimetry and interferometry revealed an optical quality glass. Time dependent electrical conductance measurements have shown a high chemical strength of the glass. Optical absorption of the doped glass in the visible domain evidenced the specific absorption lines of dysprosium ions, whereas structural investigations made by FTIR and Raman spectroscopy put in evidence the vitreous network forming role of phosphorous pentoxide. Magnetic and magneto-optical measurements demonstrated paramagnetic features of the doped glass, as well as a Verdet constant of about -0.05 min/Oe/cm at 600 nm wavelength.