arXiv subjects

Bálint Náfrádi

Publications and source records attributed to Bálint Náfrádi.

15 recordsLinked to original sources

arXiv2021

Three-dimensionally Enlarged Photoelectrodes by a Protogenetic Inclusion of Vertically Aligned Carbon Nanotubes into CH3NH3PbBr3 Single Crystals

We demonstrate that single crystals of methylammonium lead bromide (MAPbBr3) could be grown directly on vertically aligned carbon nanotube (VACNT) forests. The fast-growing MAPbBr3 single crystals engulfed the protogenetic inclusions in the form of individual CNTs, thus resulting in a three-dimensionally enlarged photosensitive interface. Photodetector devices were obtained, detecting low light intensities (~20 nW) from UV range to 550 nm. Moreover, a photocurrent was recorded at zero external bias voltage which points to the plausible formation of a p-n junction resulting from interpenetration of MAPbBr3 single crystals into the VACNT forest. This reveals that vertically aligned CNTs can be used as electrodes in operationally stable perovskite-based optoelectronic devices and can serve as a versatile platform for future selective electrode development.

cond-mat.mtrl-sci↗

arXiv2021

Ultrasensitive 3D Aerosol-Jet-Printed Perovskite X-Ray Photodetector

X-ray photon detection is important for a wide range of applications. The highest demand, however, comes from medical imaging, which requires cost-effective, high-resolution detectors operating at low photon flux, therefore stimulating the search for novel materials and new approaches. Recently, hybrid halide perovskite CH3NH3PbI3 (MAPbI3) has attracted considerable attention due to its advantageous optoelectronic properties and low fabrication costs. The presence of heavy atoms, providing a high scattering cross-section for photons, makes this material a perfect candidate for X-ray detection. Despite the already-successful demonstrations of efficiency in detection, its integration into standard microelectronics fabrication processes is still pending. Here, we demonstrate a promising method for building X-ray detector units by 3D aerosol jet printing with a record sensitivity of 2.2 x 108 μC Gyair-1cm-2 when detecting 8 keV photons at dose-rates below 1 Gy/s (detection limit 0.12 Gy/s), a four-fold improvement on the best-in-class devices. An introduction of MAPbI3-based detection into medical imaging would significantly reduce health hazards related to the strongly ionizing X-rays photons.

cond-mat.mtrl-sci↗

arXiv2021

Light-Emitting Electrochemical Cells of Single Crystal Hybrid Halide Perovskite with Vertically Aligned Carbon Nanotubes Contacts

Based on the reported ion migration under electric field in hybrid lead halide perovskites we have developed a bright, light emitting electrochemical cell with CH3NH3PbBr3 single crystals directly grown on vertically aligned carbon nanotube (VACNT) forests as contact electrodes. Under the applied electric field, charged ions in the crystal drift and accumulate in the vicinity of the electrodes, resulting in an in operando formed p-i-n heterojunction. The decreased interface energy barrier and the strong charge injection due to the CNT tip enhanced electric field, result in a bright green light emission up to 1800 cd/m2 at room temperature (average = 60 cd/m2). Beyond the light emission, this original device architecture points to the possibility of implementing vertically aligned CNTs as electrodes in operationally-stable perovskite-based optoelectronic devices.

cond-mat.mtrl-sci↗

arXiv2021

Influence of the organic cation disorder on photoconductivity in ethylenediammonium lead iodide, NH3CH2CH2NH3PbI4

We report the synthesis and crystal structure of an organic inorganic compound, ethylenediammonium lead iodide, NH3CH2CH2NH3PbI4. Synchrotron based single crystal X-ray diffraction experiments revealed that the pristine and thermally treated crystals differ in the organic cation behaviour, which is characterized by a partial disorder in the thermally treated crystal. Based on current voltage measurements, increased disorder of the organic cation is associated with enhanced photoconductivity. This compound could be a potential candidate for interface engineering in lead halide perovskite-based optoelectronic devices.

cond-mat.mtrl-sci↗

arXiv2016

Controlled growth of CH$_3$NH$_3$PbI$_3$ nanowires in arrays of open nanofluidic channels

Spatial positioning of nanocrystal building blocks on a solid surface is a prerequisite for assembling individual nanoparticles into functional devices. Here, we report on the graphoepitaxial liquid-solid growth of nanowires of the photovoltaic compound CH$_3$NH$_3$PbI$_3$ in open nanofluidic channels. The guided growth, visualized in real-time with a simple optical microscope, undergoes through a metastable solvatomorph formation in polar aprotic solvents. The presently discovered crystallization leads to the fabrication of mm2-sized surfaces composed of perovskite nanowires having controlled sizes, cross-sectional shapes, aspect ratios and orientation which have not been achieved thus far by other deposition methods. The automation of this general strategy paves the way towards fabrication of wafer-scale perovskite nanowire thin films well-suited for various optoelectronic devices, e.g. solar cells, lasers, light-emitting diodes and photodetectors.

cond-mat.mes-hall↗

arXiv2016

Room temperature manipulation of long lifetime spins in metallic-like carbon nanospheres

The time-window for processing electron spin information (spintronics) in solid-state quantum electronic devices is determined by the spin-lattice (T1) and spin-spin (T2) relaxation times of electrons. Minimising the effects of spin-orbit coupling and the local magnetic contributions of neighbouring atoms on T1 and T2 at room temperature remain substantial challenges to practical spintronics. Here, we report a record-high conduction electron T1=T2 of 175 ns at 300 K in 37 nm +/- 7 nm carbon spheres, which exceeds by far the highest values observed for any conducting solid state material of comparable size. The long T1=T2 is due to quantum confinement effects, to the intrinsically weak spin-orbit coupling of carbon, and to the protecting nature of the outer shells of the inner spins from the influences of environmental disturbances. Following the observation of spin polarization by electron spin resonance, we controlled the quantum state of the electron spin by applying short bursts of an oscillating magnetic field and observed coherent oscillations of the spin state. These results demonstrate the feasibility of operating electron spins in conducting carbon nanospheres as quantum bits at room temperature.

cond-mat.mtrl-sci↗

arXiv2016

Electron spin dynamics of two-dimensional layered materials

The growing library of two-dimensional layered materials is providing researchers with a wealth of opportunity to explore and tune physical phenomena at the nanoscale. Here, we review the experimental and theoretical state-of-art concerning the electron spin dynamics in graphene, silicene, phosphorene, transition metal dichalcogenides, covalent heterostructures of organic molecules and topological materials. The spin transport, chemical and defect induced magnetic moments, and the effect of spin-orbit coupling and spin relaxation, are also discussed in relation to the field of spintronics.

cond-mat.mtrl-sci↗

arXiv2016

Anisotropic Elliott-Yafet Theory and Application to KC$_8$ Potassium Intercalated Graphite

We report Electron Spin Resonance (ESR) measurements on stage-I potassium intercalated graphite (KC$_8$). Angular dependent measurements show that the spin-lattice relaxation time is longer when the magnetic field is perpendicular to the graphene layer as compared to when the magnetic field is in the plane. This anisotropy is analyzed in the framework of the Elliott-Yafet theory of spin-relaxation in metals. The analysis considers an anisotropic spin-orbit Hamiltonian and the first order perturbative treatment of Elliott is reproduced for this model Hamiltonian. The result provides an experimental input for the first-principles theories of spin-orbit interaction in layered carbon and thus to a better understanding of spin-relaxation phenomena in graphene and in other layered materials as well.

cond-mat.str-el↗

arXiv2015

Tuning of the Thermoelectric Figure of Merit of CH$_3$NH$_3$MI$_3$ (M=Pb,Sn) Photovoltaic Perovskites

The hybrid halide perovskites, the very performant compounds in photovoltaic applications, possess large Seebeck coefficient and low thermal conductivity making them potentially interesting high figure of merit ($ZT$) materials. For this purpose one needs to tune the electrical conductivity of these semiconductors to higher values. We have studied the CH$_3$NH$_3$MI$_3$ (M=Pb,Sn) samples in pristine form showing very low $ZT$ values for both materials; however, photoinduced doping (in M=Pb) and chemical doping (in M=Sn) indicate that, by further doping optimization, $ZT$ can be enhanced toward unity and reach the performance level of the presently most efficient thermoelectric materials.

cond-mat.mtrl-sci↗

arXiv2015

Transport, Magnetic and Vibrational Properties of Chemically Exfoliated Few Layer Graphene

We study the vibrational, magnetic and transport properties of Few Layer Graphene (FLG) using Raman and electron spin resonance spectroscopy and microwave conductivity measurements. FLG samples were produced using wet chemical exfoliation with different post-processing, namely ultrasound treatment, shear mixing, and magnetic stirring. Raman spectroscopy shows a low intensity D mode which attests a high sample quality. The G mode is present at $1580$ cm$^{-1}$ as expected for graphene. The 2D mode consists of 2 components with varying intensities among the different samples. This is assigned to the presence of single and few layer graphene in the samples. ESR spectroscopy shows a main line in all types of materials with a width of about $1$ mT and and a $g$-factor in the range of $2.005-2.010$. Paramagnetic defect centers with a uniaxial $g$-factor anisotropy are identified, which shows that these are related to the local sp$^2$ bonds of the material. All kinds of investigated FLGs have a temperature dependent resistance which is compatible with a small gap semiconductor. The difference in resistance is related to the different grain size of the samples.

cond-mat.mtrl-sci↗

arXiv2015

The Role of Transport Agents in MoS2 Single Crystals

We report resistivity, thermoelectric power and thermal conductivity of MoS2 single crystals prepared by chemical vapour transport (CVT) method using I2, Br2 and TeCl4 as transport agents. The material presents low-lying donor and acceptor levels, which dominate the in-plane charge transport. Intercalates into the Van der Waals gap strongly influence the inter-plane resistivity. Thermoelectric power displays the characteristics of strong electron-phonon interaction. Detailed theoretical model of thermal conductivity reveals the presence of high number of defects in the MoS2 structure. We show that these defects are inherent to CVT growth method, coming mostly from the transport agent molecules inclusion as identified by Total Reflection X-ray Fluorescence analysis (TXRF) and in-beam activation analysis (IBAA).

cond-mat.mtrl-sci↗

arXiv2013

Synthesis of Homogeneous Manganese-Doped Titanium Oxide Nanotubes from Titanate Precursors

We report a novel synthesis route of homogeneously manganese-doped titanium dioxide nanotubes in a broad concentration range. The scroll-type trititanate (H(2)Ti(3)O(7)) nanotubes prepared by hydrothermal synthesis were used as precursors. Mn2+ ions were introduced by an ion exchange method resulting Mn(x)H(2-x)Ti(3)O(7). In a subsequent heat-treatment they were transformed into Mn(y)Ti(1-y)O(2) where y=x/(3+x). The state and the local environment of the Mn2+ ions in the precursor and final products were studied by Electron Spin Resonance (ESR) technique. It was found that the Mn2+ ions occupy two positions: the first having an almost perfect cubic symmetry while the other is in a strongly distorted octahedral site. The ratio of the two Mn2+ sites is independent of the doping level and amounts to 15:85 in Mn(x)H(2-x)Ti(3)O(7) and to 5:95 in Mn(y)Ti(1-y)O(2). SQUID magnetometry does not show long-range magnetic order in the homogeneously Mn2+-doped nanotubes.

cond-mat.mtrl-sci↗

arXiv2013

Observation of conduction electron spin resonance in boron doped diamond

We observe the electron spin resonance of conduction electrons in boron doped (6400 ppm) superconducting diamond (Tc =3.8 K). We clearly identify the benchmarks of conduction electron spin resonance (CESR): the nearly temperature independent ESR signal intensity and its magnitude which is in good agreement with that expected from the density of states through the Pauli spin-susceptibility. The temperature dependent CESR linewidth weakly increases with increasing temperature which can be understood in the framework of the Elliott-Yafet theory of spin-relaxation. An anomalous and yet unexplained relation is observed between the g-factor, CESR linewidth, and the resistivity using the empirical Elliott-Yafet relation.

cond-mat.mtrl-sci↗

arXiv2012

Two-dimensional magnetism in kappa-(BEDT-TTF)2Cu[N(CN)2]Cl, a spin-1/2 Heisenberg antiferromagnet with Dzyaloshinskii-Moriya interaction

Field-induced antiferromagnetic (AF) fluctuations and magnetization are observed above the (zero-field) ordering temperature, T_N = 23 K by electron spin resonance in kappa-(BEDT-TTF)_2Cu[N(CN)_2]Cl, a quasi two-dimensional antiferromagnet with a large isotropic Heisenberg exchange interaction. The Dzyaloshinskii-Moriya (DM) interaction is the main source of anisotropy, the exchange anisotropy and the interlayer coupling are very weak. The AF magnetization is induced by magnetic fields perpendicular to the DM vector; parallel fields have no effect. The different orientation of the DM vectors and the g factor tensors in adjacent layers allows the distinction between interlayer and intralayer correlations. Magnetic fields induce the AF magnetization independently in adjacent layers. We suggest that the phase transition temperature, T_N is determined by intralayer interactions alone.

cond-mat.str-el↗

arXiv2010

Spin dynamics in S = 1/2 antiferromagnetic chain compounds delta-(EDT-TTF-CONMe_2)_2X (X=AsF_6, Br): a multi-frequency Electron Spin Resonance study

We present a multi-frequency Electron Spin Resonance (ESR) study in the range of 4 GHz to 420 GHz of the quasi-one-dimensional, non-dimerized, quarter-filled Mott insulators, delta-(EDT-TTF-CONMe_2)_2X (X=AsF_6, Br). In the high temperature orthorhombic phase above T~190 K, the magnitude and the temperature dependence of the high temperature spin susceptibility are described by a S = 1/2 Heisenberg antiferromagnetic chain with J_AsF6=298 K and J_Br=474 K coupling constants for X=AsF_6 and Br respectively. We estimate from the temperature dependence of the line width an exchange anisotropy, J'/J of ~2 * 10^{-3}. The frequency dependence of the line width and the g-shift have an unusual quadratic dependence in all crystallographic orientations that we attribute to an antisymmetric exchange (Dzyaloshinskii--Moriya) interaction.

cond-mat.str-el↗