National Institute Of Materials Physics - Romania

In this study, we detailed the fabrication and characterization of photovoltaic structures based on PTB7:ICBA (1:1, wt.%) bulk-heterojunction on optical glass substrates by spin-coating. Some samples were deposited on a flat substrate, and others were placed on a patterned substrate obtained by nano-imprinting lithography; the induced effects were analyzed. We demonstrated that using a patterned substrate enhanced the maximum output power, primarily because the short-circuit current density increased. This can be considered a direct consequence of reduced optical reflection and improved optical absorption. The topological parameters evaluated by atomic force microscopy, namely, the root mean square, Skewness, and Kurtosis, had small values of around 2 nm and 1 nm, respectively. This proves that the mixture of a conductive polymer and a fullerene derivative creates a thin film network with a high flatness degree. The samples discussed in this paper were fabricated and characterized in air; we can admit that the results are encouraging, but further optimization is needed.

The present research explores for the first time the intricate relationship between sulfurization temperature at unusual high temperatures (up to 700 degrees C) and the structural/optoelectronic properties of Cu-2(Zn,Cd)SnS4 (CZCTS) thin films, synthesized via a two-step sequential process involving the precursor film deposition using aprotic molecular ink followed by thermal treatment in sulfur atmosphere. X-ray diffraction patterns confirms the tetragonal structure. Scanning Electron Micrographs revealed significant grain growth, with grain sizes increasing from similar to 0.3 mu m at 620 degrees C to similar to 1.5 mu m at 680 degrees C, effectively reducing grain boundary recombination. Energy dispersive X-ray spectroscopy demonstrated a Cu-poor and Zn-rich composition, with a consistent Cd incorporation of similar to 3.7 at%. Raman spectroscopy showcases the homogeneity and purity of the CZCTS crystalline structure. Precise control of the sulfurization temperature plays a crucial role in determining the photovoltaic characteristics of CZCTS-based solar cells. By increasing the grain size and preventing the thermal decomposition of the CZTS phase, the photovoltaic performance peaked at a sulfurization temperature of 680 degrees C, achieving a power conversion efficiency (PCE) of 10.4%, with an open-circuit voltage of 0.701 V, a short-circuit current density of 24.3 mA/cm(2) and a fill factor of 60.8%. External quantum efficiency reached a maximum of 83.3% at 580 nm. The bandgap of the CZCTS absorber was determined to be 1.48 eV, optimal for photovoltaic applications. However, further increasing the sulfurization temperature to 700 degrees C resulted in a lower PCE of 8.5%, attributed to interface degradation and secondary phase formation. Temperature-dependent current-voltage measurements revealed a reduction in recombination losses, with an activation energy of 1.24 eV at the CZCTS/CdS interface, indicating effective defect passivation by Cd incorporation. The optimized films, sulfurized at 680 degrees C, displayed an absorber thickness of similar to 1.2 mu m after sulfurization, providing efficient light absorption and charge transport. The findings not only emphasize the critical role of sulfurization temperature in engineering CZCTS film and subsequently their functionality but also provide valuable insights for fine tuning their performance in the field of photovoltaic applications.

A multiphase high-entropy diboride (Ti0.25Ta0.25Hf0.25Zr0.25)B2 is obtained by spark plasma sintering from a mixture of single-metal diborides. The as-prepared material at the microscale can be defined as a composite where grains of a Ta-rich/Ti-poor complex diboride phase are the reinforcement and grains of Ta-poor/Ti-rich complex diboride are the matrix. However, at the nanoscale, the grains are heterogeneous, composed of regions with a multitude of complex diboride compositions. The interface between nanoregions is compositionally graded and has an irregular shape. The four-metal diboride shows a deformation-resistant mechanism under bending load. A strengthening process is active, increasing the room temperature bending strength (326 MPa) by approximate to 50% at 1800 degrees C (488 MPa). A ductile behavior with a deformation strain of approximate to 7.5% is observed at 2000 degrees C while bending strength (407 MPa) is approximate to 25% above the value at room temperature. At 2000 degrees C, observation of dislocations propagating from one compositional nanoregion to another and with a different density suggests dislocation contribution, first of all, to plasticity. The peculiar heterogeneity of this material at nano- and microscales is considered the reason for the remarkable mechanical response to bending load at different temperatures.

We investigate the second-order Floquet topological (SOFT) phase transitions, from the perspective of spatial symmetries. In this respect, we consider a generic honeycomb lattice Floquet topological insulator (FTI), realized by circularly polarized light irradiation, in the presence of spin-orbit coupling (Kane-Mele model). We find that our studied FTI presents chiral symmetry on a preferential direction in Fourier space, the same property that protects the topological phases in the Su-Schrieffer-Heeger (SSH) model. Thus we were allowed to characterize the SOFT phases in terms of mirror-graded winding numbers (Zak phase). Moreover, our model exhibits C2 and C3 symmetry in Fourier space, a property which lead us to investigate two finite structures having the aforementioned symmetries, namely, rhombic and triangular shapes. Indeed, we find that both of them undergo SOFT phase transitions, characterized by the appearance of 0D corner states symmetrically localized over the whole sheet. Finally, we investigate the C2 and C3 symmetry breaking. Interestingly, we reveal that the corner states are not destroyed, but localize at preferential corners instead, giving rise to a corner polarization.

The gas sensitivity of field-effect structures with 2D-MoS2 channels selectively grown between Mo electrodes using the Mo-CVD method was investigated by measuring the effect of molecular adsorption from air on the device source-drain current (Isd). The channels were composed of interconnected atomically thin MoS2 grains, with their density and average thickness varied by choosing two different distances (15 and 20 mu m) between the Mo contacts. A high response to the tested stimuli, including molecule adsorption, illumination and gate voltage changes, was observed. A significant, persistent photoconduction was induced by positive charge accumulation on traps, most likely at grain boundaries and associated defects. Isd increased under high vacuum, both in the dark and under illumination. The relative dark current response to the transition from air to high vacuum reached up to 1000% at the turn-on voltage. When monitored during the gradual change in air pressure, Isd exhibited a non-monotonic function, sharply peaking at about 10-2 mbar, suggesting molecular adsorption on different defect sites and orientations of adsorbed H2O molecules, which were capable of inducing electron accumulation or depletion. Despite the screening of disorder by extra electrons, the #20 mu m sample remained more sensitive to air molecules on its surface. The high vacuum state was also investigated by annealing devices at temperatures up to 340 K in high vacuum, followed by measurements down to 100 K. This revealed thermally stimulated currents and activation energies of trapping electronic states assigned to sulfur vacancies (230 meV) and other shallow levels (85-120 meV), possibly due to natural impurities, grain boundaries or disorder defects. The results demonstrate the high sensitivity of these devices to molecular adsorption, making the technology promising for the easy fabrication of chemical sensors.

We present herein the synthesis of novel [2 + 2] and [3 + 3] N-acylhydrazone-based macrocycles using a pool of dialdehydes and dihydrazides, under thermodynamic control. The resulting macrocycles, which were assigned triangular and rectangular shapes, were characterised in the solid state. The rectangular macrocycle was further investigated in solution by absorption and emission spectroscopy. Additionally, the rectangular macrocycle was used in various assays to investigate the hosting capacity towards various guests using NMR, HRMS, UV-visible and fluorescence spectroscopy. Theoretical calculations regarding structure and properties of the novel compounds were in agreement with the experimental details. The experimental data showed intriguing results toward tetra-n-butylammonium fluoride.

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.