László Forró

Fast Lead-Free Humidity Sensor Based on Hybrid Halide Perovskite

Pavao Roko Andricevic, Alla Arakcheeva, Dragan Damjanovic, László Forró, Endre Horvath, Trpimir Ivsic, Márton Kollár

An environmentally friendly analog of the prominent methylammonium lead halide perovskite, methylammonium bismuth bromide (MA3Bi2Br9), was prepared and investigated in the form of powder, single crystals and nanowires. Complete characterization via synchrotron X-ray diffraction data showed that the bulk crystal does not incorporate water into the structure. At the same time, water is absorbed on the surface of the crystal, and this modification leads to the changes in the resistivity of the material, thus making MA3Bi2Br9 an excellent candidate for use as a humidity sensor. The novel sensor was prepared from powder-pressed pellets with attached carbon electrodes and was characterized by being able to detect relative humidity over the full range (0.7–96% RH) at ambient temperature. Compared to commercial and literature values, the response and recovery times are very fast (down to 1.5 s/1.5 s).

2022

Solar water purification with photocatalytic nanocomposite filter based on TiO2 nanowires and carbon nanotubes

Florian Frédéric Vincent Breider, László Forró, Miriam Gaál, Dominique Grandjean, Endre Horvath, Luca Rossi, Andrzej Sienkiewicz

Water contamination due to environmental conditions and poor waste management in certain areas of the world represents a serious problem in accessing clean and safe drinking water. This problem is especially critical in electricity-poor regions, where advanced water purification methods are absent. Here, we demonstrate that titanium dioxide nanowires (TiO2NWs)-based photocatalytic filters assisted only with sunlight can efficiently decontaminate water. Moreover, interweaving TiO2NWs with carbon nanotubes (CNTs) leads to the formation of a TiO2NWs/CNTs composite material and offers an additional water decontamination channel, that is of pasteurization with the visible part of the solar emission spectrum. Our results demonstrate that this nanoporous filter can successfully intercept various types of microbial pathogens, including bacteria and large viruses. In addition, photo-catalytically generated reactive oxygen species (ROS) on the surface of the TiO2NWs/CNTs-based filter material under exposure to sunlight contribute to an efficient removal of a broad range of organic compounds and infective microbes. A pilot study also yielded encouraging results in reducing traces of drugs and pesticides in drinking water.

2022

Decoupling of lattice and orbital degrees of freedom in an iron-pnictide superconductor

Johan Juul Chang, László Forró, Sergiy Katrych, Qian Wang

The interplay between structural and electronic phases in iron-based superconductors is a central theme in the search for the superconducting pairing mechanism. While electronic nematicity is competing with superconductivity, the effect of purely structural orthorhombic order is unexplored. Here, using x-ray diffraction and angle-resolved photoemission spectroscopy, we reveal a structural orthorhombic phase in the electrondoped iron-pnictide superconductor Pr4Fe2As2Te0.88O4 (T-c = 25 K), which is distinct from orthorhombicity in the nematic phase in underdoped pnictides. Despite the high electron doping we find an exceptionally high orthorhombic onset temperature (T-ort similar to 250 K), no signatures of phase competition with superconductivity, and absence of electronic nematic order as the driving mechanism for orthorhombicity. Combined, our results establish a high-temperature phase in the phase diagram of iron-pnictide superconductors and impose strong constraints for the modeling of their superconducting pairing mechanism.

AMER PHYSICAL SOC2021

Fighting Health Hazards in Lead Halide Perovskite Optoelectronic Devices with Transparent Phosphate Salts

Pavao Roko Andricevic, László Forró, Endre Horvath, Márton Kollár, Xavier François Mettan, Lidia Rossi

Organic-inorganic lead halide perovskite (CH3NH3PbI3) solar cells have surpassed 25% power conversion efficiency, being ready for industrial-scale production of cheap photovoltaic (PV) panels. In this action, the major hurdle is its lead content, which in case of device failure, could be washed into the soil, entering the food chain. Since there is a zero tolerance on lead in the human organism, this health hazard is a critical obstacle for commercialization. Here, we propose a solution to this problem by incorporating phosphate salts (e.g., (NH4)(2)HPO4) in PV and other perovskite-based optoelectronic devices in various architectures. Phosphate salts do not react with CH3NH3PbI3 and do not alter its advantageous optoelectronic properties, but in a wet environment, they react immediately with lead, forming a highly insoluble compound, precluding this way the spread of lead into the environment. It is expected that this study will stimulate research, enabling lead halide perovskite solar cells to reach a similar environmental risk category as the commercially available, nonwater-soluble heavy metal-containing CdTe and gallium diselenide technologies.

AMER CHEMICAL SOC2021

One-dimensional composite host-guest structure in BaVS3

Alla Arakcheeva, Neven Barisic, Helmuth Berger, Gervais Chapuis, László Forró, Philip Pattison

A detailed synchrotron X-ray diffraction (XRD) study performed with a single crystal of BaVS3 (barium vanadium trisulfide) in the temperature range between 10 and 295 K is reported. Aside from the known tetragonal–orthorhombic (240 K) and orthorhombic–monoclinic (69 K) phase transitions, in the 130 < T ≤ 295 K range the overall structure can be viewed as a host–guest (H–G) composite. The BaS3 matrix is the host, while the V-chains form the guest. The two subsystems lock in at TLOCK = 130 ± 20 K. This temperature is marked by a symmetry change from orthorhombic to monoclinic. This results in the formation of twins, implying a structural phase transition identified here for the first time. From the refined structural data, it is possible to follow, starting already at 295 K downwards, the stepwise transformation of VS6 octahedra into VS5 tetragonal pyramids as the origin of the structure evolution. The new findings will yield a better understanding of the complex electronic phase diagram of BaVS3.

2021

Two-fold symmetric superconductivity in few-layer NbSe2

Helmuth Berger, László Forró, Xi Zhang

The strong Ising spin-orbit coupling in certain two-dimensional transition metal dichalcogenides can profoundly affect the superconducting state in few-layer samples. For example, in NbSe2, this effect combines with the reduced dimensionality to stabilize the superconducting state against magnetic fields up to similar to 35 T, and could lead to topological superconductivity. Here we report a two-fold rotational symmetry of the superconducting state in few-layer NbSe2 under in-plane external magnetic fields, in contrast to the three-fold symmetry of the lattice. Both the magnetoresistance and critical field exhibit this two-fold symmetry, and it also manifests deep inside the superconducting state in NbSe2/CrBr3 superconductor-magnet tunnel junctions. In both cases, the anisotropy vanishes in the normal state, demonstrating that it is an intrinsic property of the superconducting phase. We attribute the behaviour to the mixing between two closely competing pairing instabilities, namely the conventional s-wave instability typical of bulk NbSe2 and an unconventional d- or p-wave channel that emerges in few-layer NbSe2. Our results demonstrate the unconventional character of the pairing interaction in few-layer transition metal dichalcogenides and highlight the exotic superconductivity in this family of two-dimensional materials.

NATURE RESEARCH2021

Viscoelastic behavior of suspensions of reduced graphene oxide nanoparticles in epoxy

László Forró, Yves Leterrier, Arnaud Magrez

The viscoelastic behavior of dilute suspensions of reduced graphene oxide (RGO) nanoparticles in a liquid SU8 epoxy dissolved in gamma butyrolactone was investigated at RGO concentrations in the range from 0.16 to 1.07 vol. %. Our results revealed that the addition of only 1 vol. % of RGO can increase the complex viscosity of the composites by up to 3 orders of magnitude, which is crucial for processing purposes. A marked shear thinning behavior with a yield stress was, moreover, found to emerge at low RGO concentrations for sonicated homogenized suspensions. This behavior was attributed to the partial polymerization of the epoxy and bonding between the epoxy rings and the residual functional groups on the RGO induced by the energy intensive ultrasound process that is key to achieve a good dispersion of the nanoparticles, in combination with the formation of a gel-like, percolated nanocomposite network. The percolation threshold and the aspect ratio of the RGO particles were derived from the analysis of the elastic shear modulus of the suspensions and found to be equal to 9 x 10(-4) and 500, respectively. We attribute this extremely with low percolation threshold to the restriction of the polymer chain mobility within the well dispersed, high aspect ratio RGO network.

AMER INST PHYSICS2021

Filamentous and step-like behavior of gelling coarse fibrin networks revealed by high-frequency microrheology

László Forró, Sylvia Jeney

By a micro-experimental methodology, we study the ongoing molecular process inside coarse fibrin networks by means of microrheology. We made these networks gelate around a probe microbead, allowing us to observe a temporal evolution compatible with the well-known molecular formation of fibrin networks in four steps: monomer, protofibril, fiber and network. Thanks to the access that optical-trapping interferometry provides to the short-time scale on the bead's Brownian motion, we observe a Kelvin-Voigt mechanical behavior from low to high frequencies, range not available in conventional rheometry. We exploit that mechanical model for obtaining the characteristic lengths of the filamentous structures composing these fibrin networks, whose obtained values are compatible with a non-affine behavior characterized by bending modes. At very long gelation times, a omega (7/8) power-law is observed in the loss modulus, theoretically related with the longitudinal response of the molecular structures.

ROYAL SOC CHEMISTRY2020

Giant anomalous Hall effect in quasi-two-dimensional layered antiferromagnet Co1/3NbS2

Helmuth Berger, László Forró, Edoardo Martino

The discovery of the anomalous Hall effect (AHE) in bulk metallic antiferromagnets (AFMs) motivates the search of the same phenomenon in two-dimensional (2D) systems, where a quantized anomalous Hall conductance can, in principle, be observed. Here we present experiments on microfabricated devices based on Co1/3NbS2, a layered AFM that was recently found to exhibit AHE in bulk crystals below the Neel temperature T-N = 29 K. Transport measurements reveal a pronounced resistivity anisotropy, indicating that upon lowering temperature the electronic coupling between individual atomic layers is increasingly suppressed. The experiments also show an extremely large anomalous Hall conductivity of approximately 400 S/cm, more than one order of magnitude larger than in the bulk, which demonstrates the importance of studying the AHE in small exfoliated crystals, less affected by crystalline defects. Interestingly, the corresponding anomalous Hall conductance, when normalized to the number of contributing atomic planes, is similar to 0.6 e(2)/h per layer, approaching the value expected for the quantized anomalous Hall effect. The observed strong anisotropy of transport and the very large anomalous Hall conductance per layer make the properties of Co1/3NbS2 compatible with the presence of partially filled topologically nontrivial 2D bands originating from the magnetic superstructure of the antiferromagnetic state. Isolating atomically thin layers of this material and controlling their charge density may therefore provide a viable route to reveal the occurrence of the quantized AHE in a 2D AFM.

AMER PHYSICAL SOC2020