National Institute Of Materials Physics - Romania

Magnesium and zinc are important additives or substituents in melt-quenched Na2O-CaO-SiO2-P2O5 bioactive glasses, due to their beneficial influence on glass stability and processability. They modify the dissolution kinetics of these glasses and impart favorable biological properties and functions to them. For rational bioactive glass design, it is essential to understand the local environments and distributions of these ions. While diffraction studies suggest MgO and ZnO to be close-to-four-coordinate in silicate glasses, their structural roles in the network are still under debate. Traditionally these oxides have been viewed as network modifiers, converting Si-O-Si linkages to anionic non-bridging oxygen atoms, whose charges are compensated by the divalent cations. It has been suggested that MgO4/2|(2-) or ZnO4/2|(2-) network-forming units (NFUs) may be present, forming Mg-O-Si or Zn-O-Si linkages. If such units are formed, they would attract modifier cations for charge compensation, which would in turn result in an increased degree of polymerization of the silicate species, the main network-forming component. This study explores the roles of MgO and ZnO in bioactive glasses with approximate composition 50SiO(2)-(50-x)[MO,M'O-2]-xP(2)O(5) in mol% (2 <= x <= 6), where M = Ca, Sr, Mg, Zn; M' = Na, K. Quantitative estimates of the various silicate and phosphate NFUs were obtained from Si-29 and P-31 solid-state nuclear magnetic resonance (NMR) spectroscopic techniques and molecular dynamics (MD) simulations, allowing the determination of the degree of network polymerization in terms of the average Si and P connectivities (-values). Both NMR spectroscopy and MD studies consistently revealed that the extent of Si polymerization increases, as expected, with P2O5 addition, reflecting the well-documented preferential cation attraction by the phosphate species. On the other hand, data obtained from a set of comparative samples containing either Mg, Zn, or a mixture of both showed no significant changes in the degree of silicate network polymerization. This result strongly supports a network-modifying role of both magnesium and zinc oxide, as suggested by previous works.

HfZrO2(HZO) is a promising ferroelectric material compatible with CMOS technology, retaining functionality at nanometer-scale thicknesses. Its ferroelectricity arises from metastable polar phases-orthorhombic (Pca21) and rhombohedral (R3m)-coexisting with the thermodynamically stable, non-polar monoclinic phase (P21/c). Accurate quantification of these coexisting phases is essential for optimizing device performance. Here, we use first-principles calculations to estimate core-level shifts in the O 1s X-ray photoelectron spectroscopy (XPS) peak, revealing a distinct, up to 0.84 eV higher binding energy component for oxygen atoms in the polar sublattice compared to those in non-polar environments. These shifts incorporate both initial and final state effects in photoemission. Experimental validation through X-ray diffraction (XRD) and XPS on HZO films with varying phase composition confirms our predictions. Our findings provide a clear, spectroscopically accessible fingerprint to distinguish polar and non-polar phases in HZO via O 1s XPS analysis, offering a practical tool for phase quantification and enabling targeted integration of ferroelectricity in advanced nanoelectronic devices.

Palladium diselenide (PdSe2) is a two-dimensional (2D) transition metal dichalcogenide van der Waals material that exhibits a unique wrinkled pentagonal structure that results in strong anisotropic layer-dependent properties. Owing to these unique properties the material has a great potential for applications in electronic, optoelectronic, photonic and thermoelectric devices. Here, we study the electronic properties of monolayer (ML) PdSe2 in a Schottky junction by contacting the material with a tungsten scanning tunneling microscope (STM) tip. The STM-induced lifting of the 2D layer leads to structural warping of the top layer, which causes significant changes in the electronic properties of PdSe2 as well as the electrostatic potential across the junction. We compare the STM-determined work function of the warped PdSe2 ML with the work function of bulk PdSe2 as obtained by Kelvin probe force microscopy and electronic structure obtained by synchrotron-based angle-resolved photoemission spectroscopy, and x-ray photoelectron spectroscopy.

The elusive X-Defect, a defect found in low-resistivity p-type Silicon after irradiation, observed as a low-temperature shoulder of the BiOi defect (Boron-interstitial-Oxygen-interstitial complex) in Thermally Stimulated Current (TSC) measurements, was investigated to determine its properties, matching them with those of a previously identified defect. Through a combination of TSC, Deep-Level Transient Spectroscopy (DLTS), Difference-DLTS (DDLTS), numerical simulations of field-enhanced charge carrier emissions in TSC measurements and a comparison to literature, the X-Defect was identified as the singly positively charged Silicon di-vacancy V2(+/0). This assignment is supported by an agreement in activation energy, capture cross-section, trap type and charge emission process, as well as simulations comparing the effects of phonon-assisted tunnelling (PAT) and Poole-Frenkel (PF) mechanisms on TSC spectra. DDTLS measurements revealed a quadratic dependence of the activation energy on the electric field strength, confirming PAT as the prevailing mechanism over PF in the case of the radiation-induced X-Defect. Assigning the X-Defect to an electrically neutral defect in the space charge region resolves previous contradictions regarding its lack of impact on the effective doping concentration Neff.

A new mechanism is proposed for polarization switching in metal-ferroelectric-metal (MFM) structures. The switching is triggered by charge injection at electrode or grain interfaces under an external field, opposite to the initial direction of polarization. Such injection destabilizes the compensation of the polarization, and is favored by disappearance of Schottky barriers at interfaces, leading to a resistive like behavior. At the coercive voltage the depolarization field is no longer compensated and the polarization change orientation parallel to the applied field. After switching, the Schottky barriers are restored and the current start to decrease although the voltage continues to increase. This leads to a differential negative resistance until the barriers are totally restored, polarization saturated on the new direction and the depolarization field annihilated by the injected charges. In a perfect structure the switching takes place homogeneously, while the defects presence induces domains via local nucleation and fields. The model is supported by theory, showing that at switching the MFM structure has a resistive like behavior, and by experiments, showing that the voltage dependence of the current during switching is linear and that the phase change at switching, in PFM measurements performed on high quality epitaxial films, is abrupt. The analysis was performed on MFM structures based on different ferroelectric layers (e.g. Pb(Zr,Ti)O3, BaTiO3, BiFeO3 or (HfZr)O2), different metal electrodes and different structural qualities. It appears that polarization switching triggered by charge injection at interfaces is a mechanism that can be applied to all MFM structures based on oxide ferroelectrics.

This study explores strategies to enhance the ionic conductivity of LiTi2(PO4)(3) (LTP), a promising NASICON-type solid electrolyte for next-generation solid-state batteries. Although LTP exhibits an open-framework structure favorable for Li+ diffusion, its intrinsic low ionic conductivity limits practical application. To address this, this research investigates the impact of co-doping titanium (Ti4+) with iron (Fe3+) and yttrium (Y3+) in Li1+xFeyYx-yTi2-x(PO4)(3) (where x = 0.3 and y = 0.1, 0.15, 0.2), aiming to increase charge carrier concentration and induce structural distortion beneficial for ion transport. Samples were synthesized via solid-state reaction and characterized by X-ray diffraction, Raman spectroscopy, and scanning electron microscopy. The local environment of Fe3+ was further analyzed using electron paramagnetic resonance and Mossbauer spectroscopy. Spark Plasma Sintering (SPS) was employed to obtain dense ceramics and suppress secondary phase formation. The electrical properties were investigated by impedance spectroscopy over the 150K-400K temperature range, and the relaxation dynamics and conduction mechanisms were analyzed using the modulus formalism and a.c. methods conductivity. The Li1.3Fe0.15Y0.15Ti1.7(PO4)(3) composition exhibited the highest room-temperature conductivity of 8.21 x 10(-4) S/cm, confirming the combined effect of Fe3+/Y3+ co-doping and SPS densification in enhancing the ionic transport properties of LTP-based solid electrolytes.

ZnO is recognized as one of the most versatile semiconductor materials, due to its adjustable properties, making it ideal for various applications. Herein, C-doped ZnO nanoparticles were obtained by thermal decomposition and tested as photocatalysts for water depollution and electrode materials in a symmetric supercapacitor device. These nanoparticles exhibited polyhedral shapes with sizes ranging from 95 to 104 nm. The impact of the doping level on the evolution of defect centers was analyzed using EPR and PL spectroscopy. Under visible light, the photocatalytic performance was tested against two contaminants, Rhodamine B (RhB) dye and oxytetracycline (OTC) antibiotic. A 1% C doping level provided the optimal photocatalytic performance, achieving degradation rates of 80% for RhB and 50% for OTC. The photodegradation process was elucidated by examining ROS species generated, linked to the measured valence band position and defect levels. Additionally, the same sample showed superior electrochemical performance when tested in a symmetric supercapacitor device, achieving a maximum specific capacitance of 104.16 F/g at a 2 mV/s scan rate, with an energy density of 14.46 Wh/kg and excellent cyclic stability, retaining 99% capacity after 2000 cycles. These findings highlight the versatility of C-doped ZnO nanoparticles, making them effective in environmental remediation and energy storage applications.

Boron-doped silicon detectors used in high radiation environments like the future HL-LHC show a degradation in device performance due to the radiation induced deactivation of the active boron dopant. This effect, known as the so-called Acceptor Removal Effect (ARE), depends on particle type, particle energy and radiation dose and is usually explained by the formation of boron-interstitial-oxygen-interstitial (BiOi) defects that induce a donor-type defect level in the upper part of the Si band gap. Here we present defect characterization studies using Thermally Stimulated Current technique (TSC) and Deep Level Transient Spectroscopy (DLTS) on a set of epitaxially grown p-type silicon diodes of different resistivity, irradiated with 60Co gamma-rays. We used the defect parameters (activation energy, charge carrier capture cross sections and defect concentration) obtained from DLTS experiments for modeling the corresponding TSC spectra, and subsequently compared those with the experimental TSC results. This approach shows that the di-vacancy which is well characterized by DLTS correlates with the so-far unspecified charge emission signal of the X-defect that partially overlaps with the BiOi peak in TSC spectra. Additionally, in order to evaluate the impact of BiOi defect formation on the macroscopic properties of the device, we compared the BiOi defect concentration with the change in the effective carrier concentration Neff obtained from C-V measurements. It shows that the variations in Neff are about twice the changes in the BiOi concentration, which corroborate the assumption of boron deactivation by the formation of the BiOi donor in irradiated p-type Si.

Layers of HfO2 and (Hf,Zr)O2 crystalline nano-particles are synthesized via direct liquid injection atomic layer deposition, and a comprehensive set of structural, chemical, and electrical characterizations is employed to elucidate their phase composition and functional behavior. X-ray photoelectron spectroscopy revealed a compositional contrast between the films: (Hf,Zr)O2 layers contained up to 45% stoichiometric oxide, while pure HfO2 films are dominated by sub-oxides, especially under strongly reducing conditions, in which exclusively sub-oxide phases and p-type semiconducting behavior is revealed. Electrical measurements indicated room-temperature stabilization of polar phases and tetragonal-to-orthorhombic phase transition with a Curie temperature near 200 K. FTIR spectroscopy confirmed the presence of tetragonal and orthorhombic HfO2 phases, providing insight into minor features observed approximate to 30 degrees (2 theta) in X-ray diffraction patterns. Notably, devices incorporating an AlN interlayer demonstrated a significant enhancement in pyroelectric performance, suggesting this strategy to advance the pyroelectric performance of HfO2-based materials, supporting their development for lead-free sensor technologies.

In this work, we investigate the electrical properties of a ternary biocomposite material based on a vinyl resin (VR) matrix filled with two types of fillers, namely green microcrystalline cellulose (MCC) and carbon nanotubes (CNT), over a frequency range of 10-1 to 107 Hz and a temperature range from 10 to 130 degrees C. Thermal analyses were carried out using differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA), which revealed significant changes in the glass transition and degradation temperatures of the biocomposite, respectively. The electrical measurements were analyzed using two formalisms: (i) Electrical conductivity, which showed the presence of two distinct low-frequency behaviors associated with quasi-dc conductivity below the glass transition temperature and dc conductivity above it, along with high-frequency dispersion, which allows determining the ac conductivity mechanisms using Jonscher's power law, and (ii) Electric modulus, which revealed a low-temperature relaxation originating from water dipole polarization and two high-temperature dielectric relaxation processes. The first high-temperature relaxation, observed at low frequencies, was attributed to the Maxwell-Wagner-Sillars (MWS) effect, while the second, appearing at high frequencies, was associated with the alpha-relaxation. These relaxations were modeled using Bergman's equation. Furthermore, the temperature dependence of the relaxation time and dc conductivity were analysed using an Arrhenius representation.