National Institute Of Materials Physics - Romania

Synchrotron-based small- and wide-angle X-ray scattering is used to elucidate the structure of low-dimensional lepidocrocite-titanate-based nanofilaments. In the colloidal state, they consist of quantum-confined 1D NFs, loosely associated into nanoribbons, one lepidocrocite sheet thick (about 4 & Aring;), 30-40 & Aring; wide (5-8 NFs), and more than 300 & Aring; long. In the dry state, they reach a final state of extended sheets, stacked three to about twenty high, whose crystallinity increases with stack height, in parallel with a decrease in photocatalytic activity. These findings suggest a kinetic pathway for the self-assembly of initially 1D titanate nanoribbons into 2D and ultimately 3D structures, providing context for a recent body of work on these low-dimensional materials.

Self-powered near-infrared (NIR) photodetectors are essential for surveillance systems, sensing in IoT electronics, facial recognition, health monitoring, optical communication networks, night vision, and biomedical imaging. However, silicon commercial detectors need external power to operate and cooling to suppress large dark currents. This work demonstrates a new class of CMOS-compatible self-powered NIR photodetector based on ferroelectric 5-nm thick ZrO2 films which do not require cooling and therefore have two key advantages over Si, and at the same time have comparable performance metrics. At room-temperature, under 940 nm wavelength illumination (1.4 mW cm-2 power density, 10 Hz repetition rate), and without any power applied, fast rise and fall times of approximate to 2 and 4 mu s, respectively, are achieved in Al/Si/SiOx/ZrO2/ITO devices, along with responsivity, detectivity and sensitivity values of up to approximate to 3.4 A W-1, 1.2 x 1010 Jones and 4.2 x 103, respectively, far exceeding all other emerging self-powered systems. Furthermore, dual-band NIR detection is shown for different NIR wavelengths, proof-of-concept feasibility being demonstrated for the smart identification of NIR targets. Therefore, it is demonstrated, for the first time, that coupling together the pyroelectric effect, the photovoltaic effect, and the ferroelectric effect is a novel method to significantly enhance the performance of CMOS-compatible ZrO2-based self-powered photodetectors in the NIR region.

We report on the increase of luminescence of ZnS:Ag after exposure to a radio-frequency (RF) argon plasma at low powers ranging from 5W to 50W. The best luminescence enhancement was achieved at 5W RF power, when the increase was approximately 57.02 % over a 35-min exposure. The luminescence is measured in-situ by excitation with an electron beam with energy 13 keV and a fiber coupled to a spectrometer. The increase in luminescence is attributed to the cleaning effect of surface defects of the crystal caused by argon ions accelerated in the plasma sheath. Surface impurities were highlighted by TEM and XPS analysis. Zn2p(3/2), Zn2p(1/2) and S2p(3/2) peaks show initially high oxidation state and after plasma treatment they shifted to lower value which indicated a decontamination of trapped oxygen. At higher RF powers up to 50W, the trend of increased luminescence continues, but it is mitigated by the thermal quenching effect and sulfur depletion observed in EDS analysis. Calculations based on power deposition indicate a thermoquenching point of approximately 130-150 degrees C.

Most photocatalytic processes involve physicochemical phenomena occurring at the semiconductor-water interface. The interfacial charge transfer strongly depends on the charge carrier self-trapping or defect-based trapping mechanisms active in the crystal lattice of the photocatalyst. Thus, the crystal lattice distortion is expected to influence the photocatalytic efficiency during polymorphic phase transformations (PPT). A simple synthesis method involving the ultrasound-assisted excess hydrolysis of titanium tetra-isopropoxide (TTIP) (hydrolysis ratio (number of moles of water/number of moles of TTIP) r = 245) was used to obtain multiphase titanium dioxide (TiO2) nanomaterials with complex defect structures. Electron paramagnetic resonance (EPR) spectroscopy was employed to characterize the paramagnetic centers in the synthesized TiO2 and their behavior during incipient PPT. The calcined samples showed a complex defect structure comprising three types of paramagnetic centers: F+-centers (an electron trapped in an oxygen vacancy (Ov)), V-centers (oxygen ions with trapped holes) and paramagnetic centers involving Ti3+ such as Ti3+- Ov. The sample obtained at 600 degrees C, temperature marking the onset of a massive mixed transformation of anatase into rutile and brookite, composed of approximately 81 % anatase, 10 % brookite, and 9 % rutile, exhibited an intense and broadened EPR signal and enhanced photocatalytic activity for hydroxyl radical generation and hydrogen production by water splitting, despite its rather low specific surface area of 34 m2/g. The results revealed the synergistic effects of charge carrier trapping mechanisms in the early stages of PPT, boosting the photocatalytic performance of TiO2. The present study supports the design of facile synthesis methods for better TiO2 photocatalysts and promotes the development of further studies regarding lattice defect engineering during phase transformations in nanomaterials.

In this work, we investigated the influence of reduced graphene oxide (RGO) on the electrical performances of ZnO/Ag multilayer transparent conductive electrodes. RGO flakes were successfully laser transferred by matrix assisted pulsed laser evaporation (MAPLE) using frozen targets obtained from suspensions of commercially chemically RGO powder dispersed in dimethyl sulfoxide (DMSO). The main deposition parameters such as RGO concentration, laser fluence, number of the laser pulses and deposition pressure were varied to identify the optimal morphological and optical characteristics. The laser transfer conditions for the RGO structures with the best transmittance were further employed in preparation of RGO/Ag/ZnO multilayer transparent conductive electrodes (MTCE). Thus, the MAPLE deposited RGO structures were covered with metal (Ag) by vacuum thermal evaporation (VTE) and subsequently with metal oxide (ZnO) by radio frequency magnetron sputtering (RF-MS). In comparison to the optical and electrical properties of a ZnO/Ag/ZnO reference structure, the results emphasize that the RGO/Ag/ZnO are featured by a similar transmittance (similar to 82-85 %) and improved sheet resistance (similar to 10.6 Omega/square, meaning up to 2-fold smaller).

Oxygen in the excited state is essential for organic synthesis and medical treatment. Herein, a novel phenomenon is reported in which the magnetic ground state of molecular oxygen undergoes a transition at room temperature from S = 1 to S = 1/2, corresponding to the transition of O2 from a triplet to a doublet state after stable physical adsorption on the defect-free surface of bulk VSe2. This density functional theory (DFT) calculations demonstrate the stable physical adsorption of O2 on both 1T- and 2H-VSe2 surfaces without further decomposition. Electron spin resonance (ESR) measurements confirm the spin state transition. Theoretical simulations reveal the charge transfer from entangled V-3d and Se-4p bands to oxygen as the leading cause of the spin state transition. This mechanism has not been previously proposed and offers multiple potential applications, from organic synthesis to medicine. Moreover, this approach can be extended to reveal new aspects of known catalytic materials and to design novel catalysts.

We theoretically study the 2D Su-Schrieffer-Heeger model in the context of Floquet topological insulators (FTIs). FTIs are systems which undergo topological phase transitions, governed by Chern numbers, as a result of time reversal symmetry (TRS) breaking by a time periodic process. In our proposed model, the condition of TRS breaking is achieved by circularly polarized light irradiation. We analytically show that TRS breaking is forbidden in the absence of second order neighbors hopping. In the absence of light irradiation, we identify a symmetry-protected degeneracy and prove the appearance of a flat band along a specific direction in the momentum space. Furthermore, we employ a novel method to show that the four unit cell atoms, in the absence of irradiation, can be interpreted as conserved spin states. With the breaking of TRS via light irradiation, these spin states are no longer conserved, leading to the emergence of chiral edge states. We also show how the interplay between the TRS breaking and dimerization leads to some complex topological phase transitions. The validity of our findings is substantiated through Chern numbers, spectral properties, localization of chiral edge states and simulations of quantum Hall transport. Our model is suitable not only for condensed matter (materials), but also for cold gases trapped in optical lattices or topolectrical circuits.

Herein we present a comparative study among different spray-coated nanometric mesoporous electron transporting layers (ETLs) in perovskite solar cells (PSC), namely m-TiO2, 2 , m-SnO2 2 and m-SnO2 2 quantum dots (mSnO2QDs). 2 QDs). The solutions used for deposition were prepared from commercial pastes and colloidal suspensions for m-TiO2 2 and m-SnO2. 2 . For m-SnO2QDs 2 QDs in-house QDs solutions were prepared. The formamidiniummethylamonium-potassium (FAMA@10 K) has been used as light absorber material in the fabricated PSCs. The structural, compositional and morphological studies, correlated with the photovoltaic performance of PSCs, indicate that the m-SnO2 2 QDs layer is the best candidate among the three investigated mesoporous ETLs. Compared with the suspensions used for the other two ETLs, the in-house prepared SnO2 2 QDs solution presents smaller agglomerates of nanoparticles and results in the formation of a thinner, more uniform and compact mesoporous ETL. The FAMA@10 K perovskite deposited on m-SnO2 2 QDs ETL presents a lower roughness, better uniformity and a higher amount of PbI2. 2 . Our work unveils that the SnO2 2 QDs solution can be easily produced in laboratory and when is deposited as mesoporous scaffold in a PSC with FAMA@10 K perovskite, the power conversion efficiency increases up to 14.90 %, being with up to 27 % larger than in the PSCs with m-TiO2 2 and mSnO2 2 ETLs prepared from commercial solutions. By modeling the J-V dynamic hysteresis with more than 90 % match between the calculated and experimental J-V data, for all three types of mesoporous ETLs, the relevant parameters that explain the hysteresis magnitude and account for ionic-induced recombination processes in PSCs were determined.

Polymorphism determines significant variations in materials' properties by lattice symmetry variation. If they are stacked together into multilayers, polymorphs may work as an alternative approach to the sequential deposition of layers with different chemical compositions. However, selective polymorph crystallization during conventional thin film synthesis is not trivial; changes of temperature or pressure when switching from one polymorph to another during synthesis may cause degradation of the structural quality. The present work reports on the single-step ion-beam-assisted fabrication of multilayered polymorph structures while applying the disorder-induced ordering approach. The dynamic annealing of disorder may be tuned, during ion irradiation, toward self-assembling of several polymorph interfaces. Gallium oxide multilayers with two polymorph interface repetitions are obtained. The single-crystal structure of the polymorphs is maintained between interfaces, exhibiting repeatable crystallographic relationships and optical properties. These data pave the way for enhancing materials' functionalities using not previously conceived capabilities of ion beam technology.

This study investigates the fabrication of short-wavelength infrared (SWIR) photosensitive amorphous and nanocrystalline Ge1-xSnx:H thin films by magnetron sputtering from separate Ge and Sn targets using different Ar: H mixing ratios as working gas. Amorphous Ge1-xSnx:H films have been obtained on both c-Si and fused quartz substrates at ambient temperature, while dynamic nanocrystallization occurs in-situ when the substrate temperature during deposition is raised to 200 degrees C. Fourier-transform infrared spectroscopy has shown the hydrogen incorporation by detecting an absorption line at 1873 cm(-1), close to the value corresponding to Ge-H bonding, only in the room temperature amorphous films. Based on that, we infer that the hydrogen concentration is very low in the films deposited at high temperature. The higher concentration of hydrogen in the amorphous samples is associated with an increase of the absorption gap to 0.5 eV compared to 0.3 eV in the 200 degrees C samples. In-situ (during deposition) and ex-situ (by subsequent rapid thermal annealing) nanocrystallization have been analyzed by high-resolution transmission electron microscopy, X-ray diffraction and micro-Raman spectroscopy. SWIR spectral photosensitivity up to 2.4 mu m was found to be more than two orders of magnitude improved in hydrogenated amorphous films with high hydrogen content, compared to the nanocrystalline ones that are weakly hydrogenated. These findings demonstrate the potential of hydrogenation to enhance the photoelectric properties of GeSn sputtering films for optoelectronic SWIR infrared applications.