National Institute Of Materials Physics - Romania

In a milestone paper [F. D. M. Haldane, Phys. Rev. Lett. 61, 2015 (1988)], Haldane elaborated a model of graphene within the time -reversal symmetry breaking is achieved by next -nearest -neighbors imaginary counterrotating hopping, hence conferring topological properties. In recent years, the time -reversal symmetry turned out to be broken also by light irradiation in so-called Floquet topological insulators (FTIs). On the other hand, Kane and Mele introduced a spin -orbit coupling (SOC) model [C. L. Kane et al., Phys. Rev. Lett. 95, 226801 (2005)] inspired by the Haldane's mechanism. In this paper, we present the topological properties of a FTI possessing SOC, using graphene as the playground. It was found that the interplay between sublattice subspace and the spin one triggers interesting topological phase transitions. Basically, in a FTI with SOC, two topological phases may be excited: charge quantum Hall effect (CQHE) and, respectively, spin quantum Hall effect (SQHE) phases. Also, it was demonstrated that the CQHE and SQHE coexistence is forbidden by the topology of the system. As well, it was identified a special driving regime of spin filter (SF), in which only one spin state is topological and, consequently, will be filtered in quantum transport.

SiGeSn nanocrystals (NCs) in oxides are of considerable interest for photo-effect applications due to the fine-tuning of the optical bandgap by quantum confinement in NCs. We present a detailed study regarding the silicon germanium tin (SiGeSn) NCs embedded in a nanocrystalline hafnium oxide (HfO2) matrix fabricated by using magnetron co-sputtering deposition at room temperature and rapid thermal annealing (RTA). The NCs were formed at temperatures in the range of 500-800 degrees C. RTA was performed to obtain SiGeSn NCs with surfaces passivated by the embedding HfO2 matrix. The formation of NCs and beta-Sn segregation were discussed in relation to the deposition and processing conditions by employing HRTEM, XRD and Raman spectroscopy studies. The spectral photosensitivity exhibited up to 2000 nm in short-wavelength infrared (SWIR) depending on the Sn composition was obtained. Comparing to similar results on GeSn NCs in SiO2 matrix, the addition of Si offers a better thermal stability of SiGeSn NCs, while the use of HfO2 matrix results in better passivation of NCs increasing the SWIR photosensitivity at room temperature. These results suggest that SiGeSn NCs embedded in an HfO2 matrix are a promising material for SWIR optoelectronic devices.

In this work, we demonstrate, for the first time, that coupling together the pyroelectric effect, the photovoltaic effect and the plasmonic effect is a novel method to significantly enhance the performance of self-powered photodetectors in the visible region. Photodetectors based on tri-layered heterojunction of n-Si/p-SnO/n-ZnO through the inclusion of silver (Ag) nanoparticles (NPs) at the SnO/ZnO interface were fabricated. The photo-response of the device, with excitation from a chopped 650 nm wavelength laser, was carefully investigated, and it was shown that the photodetector performance is enhanced the most with the inclusion of spheroidal Ag NPs with similar to 70 nm diameter. The Al/Si/SnO/Ag NPs/ZnO/ITO device exhibited an optimum responsivity, detectivity and sensitivity of 210.2 mA W-1, 5.47 x 109 Jones and 15.0 x 104, respectively, together with a rise and fall time of 2.3 and 51.3 mu s, respectively, at a laser power density of 317 mW cm-2 and at a chopper frequency of 10 Hz. The present photodetectors are more than twice as responsive as the current best-performing ZnO-based pyro-phototronic photodetectors and they also exhibit other competitive features, such as detectivity, and fall and rise times. Therefore, by exploiting the plasmonic effect of the Ag NPs together with the pyroelectric effect in a ZnO film, and the photovoltaic effect at a Si/SnO junction, all in a single device, photodetectors were developed with state-of-the-art performance for the visible region. Coupling together the pyroelectric effect, the photovoltaic effect and the plasmonic effect is a novel method to significantly enhance the performance of self-powered photodetectors in the visible region.

Synapses play a vital role in information processing, learning, and memory formation in the brain. By emulating the behavior of biological synapses, electronic synaptic devices hold the promise of enabling high-performance, energy-efficient, and scalable neuromorphic computing. Ferroelectric memristive devices integrate the characteristics of both ferroelectric and memristive materials and present a far-reaching potential as artificial synapses. Here, it is reported on a new ferroelectric device on silicon, a field-effect memristor, consisting of an epitaxial ultrathin ferroelectric Hf(0.5)Z(r0.5)O(2) film sandwiched between an epitaxial highly doped oxide semiconductor SrTiO3-delta and a top metal. Upon a low voltage of less than 2 V, the field-effect modulation in the semiconductor enables to access multiple states. The device works in a large time domain ranging from milliseconds down to tens of nanoseconds. By gradually switching the polarization by identical pulses, the ferroelectric diode devices can dynamically adjust the synaptic strength to mimic short- and long-term memory plasticity. Ionic contributions due to redox processes in the oxide semiconductor beneficially influence the device operation and retention.

Ti-aluminosilicate gels were used as supports for the immobilization of Fe, Co, and Ni oxides (5%) by impregnation and synthesis of efficient photocatalysts for the degradation of beta-lactam antibiotics from water. Titanium oxide (1 and 2%) was incorporated into the zeolite network by modifying the gel during the zeolitization process. The formation of the zeolite Y structure and its microporous structure were evidenced by X-ray diffraction and N-2 physisorption. The structure, composition, reduction, and optical properties were studied by X-ray diffraction, H-2-TPR, XPS, Raman, photoluminescence, and UV-Vis spectroscopy. The obtained results indicated a zeolite Y structure for all photocatalysts with tetracoordinated Ti4+ sites. The second transitional metals supported by the post-synthesis method were obtained in various forms, such as oxides and/or in the metallic state. A red shift of the absorption edge was observed in the UV-Vis spectra of photocatalysts upon the addition of Fe, Co, or Ni species. The photocatalytic performances were evaluated for the degradation of cefuroxime in water under visible light irradiation. The best results were obtained for iron-immobilized photocatalysts. Scavenger experiments explained the photocatalytic results and their mechanisms. A different contribution of the active species to the photocatalytic reactions was evidenced.

For the development of memory devices for the continuous advancement of IT engineering, a good understanding of the charging-discharging mechanisms in nanocrystalline floating gate memories is crucial to overcoming the current limitations. The charging-discharging mechanism in Al2O3/Ge/Al2O3 trilayer memory structures obtained by magnetron sputtering deposition is investigated as a function of the postdeposition annealing temperature, up to 900 degrees C. The change by annealing of C-V hysteresis curves from a clockwise type at low temperatures to counterclockwise one in a sample annealed within the intermediary temperature range of 550 to 650 degrees C, and then, a return to a clockwise type for annealing within the higher temperature range of 800-900 degrees C was observed. Up to 700 degrees C, memory performances are constantly improved reaching for 600 degrees C annealed samples, a memory window of 5.6 V for voltage sweep in the range -1 to +15 V, and good retention characteristics for 650 degrees C annealed structures, in which the charge loss is only similar to 2% after 10(8) s. When the annealing temperature was increased above 700 degrees C, a rapid decrease in the memory performance takes place. The annealing-induced changes are explained based on the Ge fast diffusion and nanocrystallization process, in correlation with morphological and structural high-resolution transmission electron microscopy results.

Rhombohedral phase HfxZr1-xO2 (HZO, x from 0 to 1) films are promising for achieving robust ferroelectric polarization without the need for an initial wake-up pre-cycling, as is normally the case for the more commonly studied orthorhombic phase. However, a large spontaneous polarization observed in rhombohedral films is not fully understood, and there are also large discrepancies between experimental and theoretical predictions. In this work, in rhombohedral ZrO2 thin films, we show that oxygen vacancies are not only a key factor for stabilizing the phase, but they are also a source of ferroelectric polarization in the films. This is shown experimentally through the investigation of the structural properties, chemical composition and the ferroelectric properties of the films before and after an annealing at moderate temperature (400 degrees C) in an oxygen environment to reduce the V-O concentration compared. The experimental work is supported by density functional theory (DFT) calculations which show that the rhombohedral phase is the most stable one in highly oxygen defective ZrO2 films. The DFT calculations also show that V-O contribute to the ferroelectric polarization. Our findings reveal the importance of V-O for stabilizing rhombohedral ZrO2 thin films with superior ferroelectric properties.

. The study on efflorescence in salts collected from Curtea de Arges cathedral's exterior wall during restorations aimed to characterize compounds and lithic material using SEM-EDX, XRD, Raman, FTIR. Radiocarbon measurements using AMS method and FTIR results demonstrate decarbonation/recarbonation at the compound-lithic interface but further research is required.

In this paper, short-wave infrared (SWIR) photosensing of GeSn-HfO2 films with small Si amount is studied in correlation with structure and composition of films. SiGeSnHfO2 films are deposited by magnetron sputtering and nanostructured by subsequent rapid thermal annealing. XRD and Raman spectroscopy investigations are carried out revealing the SiGeSn nanocrystallization in annealed films. Spectral responsivity shows enhanced sensitivity up to 2 mu m due to SiGeSn nanocrystals (NCs) and clusters with contribution from disorder.

This study examines the opto-electric characteristics of Ge:SiO2 composite films produced via magnetron sputtering at substrate temperatures of 300 degrees C, 400 degrees C, and 500 degrees C, with varying Ge concentrations. We employed x-ray diffraction, current-voltage measurements, and spectral photocurrent analysis to investigate the films structural, optical, and opto-electrical properties. Illumination of the samples resulted in a marked increase in current compared to dark conditions. Spectral photocurrent measurements revealed cutoff wavelengths of 1300 nm for films with 25:75 vol% Ge:SiO2 ratio, extending to 1320 nm for compositions with higher Ge content (60:40 vol%). These findings align with observations from I-V curve analyses. Our research highlights the potential of Ge:SiO2 composites for enhancing optoelectronic device performance. The results underscore the importance of continued investigation and innovative applications in this field to drive technological advancements.