Showing papers in "Physica Status Solidi B-basic Solid State Physics in 2016"

Cu–Zn disorder and band gap fluctuations in Cu2ZnSn(S,Se)4: Theoretical and experimental investigations

Abstract: Cu2ZnSn(S,Se)4 (CZTS(e)) solar cells suffer from low-open-circuit voltages that have been blamed on the existence of band gap fluctuations, with different possible origins. In this paper, we show from both theoretical and experimental standpoints that disorder of Cu and Zn atoms is in all probability the primary cause of these fluctuations. First, quantification of Cu–Zn disorder in CZTS thin films is presented. The results indicate that disorder is prevalent in the majority of practical samples used for solar cells. Then, ab initio calculations for different arrangements and densities of disorder-induced [CuZn + ZnCu] defect pairs are presented and it is shown that spatial variations in band gap of the order of 200 meV can easily be caused by Cu–Zn disorder, which would cause large voltage losses in solar cells. Experiments using Raman spectroscopy and room temperature photoluminescence combined with in situ heat-treatments show that a shift in the energy of the dominant band-to-band recombination pathway correlates perfectly to the order-disorder transition, which clearly implicates Cu–Zn disorder as the cause of band gap fluctuations in CZTS. Our results suggest that elimination or passivation of Cu–Zn disorder could be very important for future improvements in the efficiency of CZTS(e)-based solar cells.

Chiral three‐dimensional isotropic lattices with negative Poisson's ratio

Abstract: Chiral three-dimensional isotropic cubic lattices with rigid cubical nodules and multiple deformable ribs are developed and analyzed via finite element analysis. The lattices exhibit geometry-dependent Poisson's ratio that can be tuned to negative values. Poisson's ratio decreases from positive to negative values as the number of cells increases. Isotropy is obtained by adjustment of aspect ratio. The lattices exhibit significant size effects. Such a phenomenon cannot occur in a classical elastic continuum but it can occur in a Cosserat solid.

A 3D auxetic material based on intersecting double arrowheads

Abstract: Recent years have seen a consistent trend in the extension of 2D auxetic models towards 3D ones. This article introduces a 3D auxetic linkage structure by extending the double arrowhead honeycomb structure to an intersecting double arrowhead structure. A geometrical analysis on the basis of rigid linkages with rotational joints shows that auxeticity is manifested in all three orthogonal planes. Results reveal that the geometry, in terms of either the relative linkage lengths or the half angles, can be changed to substantially control the Poisson's ratio. It is herein suggested that functionally graded beams and plates that exhibit gradual change in Poisson's ratio from negative to positive, as well as functionally graded solids with opposing auxeticity trends on different planes, are easily achievable by 3D printing using the intersecting double arrowhead structure, and that further mechanistic analysis be performed.

First‐principles prediction of mechanical and bonding characteristics of new T2 superconductor Ta5GeB2

Abstract: In the present paper, density functional theory (DFT) based first-principles methods are applied to investigate the mechanical and bonding properties of the newly synthesized T2 phase superconductor Ta5GeB2 for the first time. The calculated lattice constants are in reasonable agreement with the experiment. The elastic constants (Cij), bulk modulus (B), shear modulus (G), Young's modulus (Y), Poisson's ratio (v), Pugh ratio (G/B), and elastic anisotropy factor A of Ta5GeB2 are calculated and used to explore the mechanical behavior of the compound. To give an explanation of the bonding nature of this new ternary tetragonal system, the band structure, density of states, and Mulliken atomic population are investigated. The estimated Debye temperature and Vickers hardness are also used to justify both the mechanical and bonding properties of Ta5GeB2.

Large conduction band offset at SiO2/β-Ga2O3 heterojunction determined by X-ray photoelectron spectroscopy

Abstract: We evaluated the band offsets at the SiO2/β-Ga2O3 (010) interface by X-ray photoelectron spectroscopy (XPS). Plasma chemical vapor deposition with a liquid tetraethyl orthosilicate source for SiO2 was used to prepare an SiO2(40 nm)/Ga2O3 sample and an SiO2(3 nm)/Ga2O3 sample for XPS analyses. The bandgap of SiO2 was determined to be 8.7 ± 0.2 eV. A large conduction band offset of 3.1 ± 0.2 eV and a corresponding valence band offset of 1.0 ± 0.2 eV were determined for the SiO2/Ga2O3 interface. These results suggest that an SiO2 gate insulator is favorable for Ga2O3 field effect transistors operating under high temperatures.

Negative Poisson's ratio in periodic porous graphene structures

Abstract: We conducted molecular statics simulations to investigate the negative Poisson's ratio (auxetic behavior) of periodic porous graphene structures based on the rotating rigid unit mechanism. To obtain a negative Poisson's ratio, simple voids were periodically introduced into graphene. We showed that the Poisson's ratio of the designed graphene structure is strongly dependent on the aspect ratio of the voids, and it can approach the theoretical limit of −1.0. More importantly, the graphene periodic structure maintains its auxetic behavior even under large strains (ϵ ∼ 0.20). Hence, it can be employed in a wide range of applications requiring structures that can endure large deformation. In addition, we found that the key factor in the auxeticity of the investigated structures is the deformation occurring at the void tips.

Auxetic composites made of 3D textile structure and polyurethane foam

Abstract: Auxetic composites have attracted considerable attention in recent years and have demonstrated a high potential of applications in different areas due to their wonderful properties as compared to non-auxetic composites. In this study, a three-dimensional (3D) auxetic textile structure previously developed was used as reinforcement to fabricate auxetic composites with conventional polyurethane (PU) foam. Both the deformation behavior and mechanical properties of the auxetic composites under compression were analyzed and compared with those of the pure PU foam and non-auxetic composites made with the same materials and structural parameters but with different yarn arrangement. The results show that the negative Poisson's ratio of composites can be obtained when suitable yarn arrangement in a 3D textile structure is adopted. The results also show that the auxetic composites and non-auxetic composites have different mechanical behaviors due to different yarn arrangements in 3D textile structure. While the auxetic composites behave more like damping material with lower compression stress, the non-auxetic composite behaves more like stiffer material with higher compression stress. It is expected that this study could pave a way to the development of innovative 3D auxetic textile composites for different potential applications such as impact protection.

Design and properties of 3D‐printed chiral auxetic metamaterials by reconfigurable connections

Abstract: This article describes the rules and procedures for designing a three-dimensional (3D) antitetrachiral structure that achieves a negative Poisson's ratio. Numerical analysis and experimental validation showed good agreement between the deformations and Poisson's ratios. Moreover, the proposed auxetic structure displayed anisotropic behavior in x- and y-directional compression as well as isotropic behavior in z-directional compression. The parametric study and design examples presented in this article demonstrate the relationships among the Poisson's ratio, effective Young's modulus, porosity, and geometry of the proposed structure, and provide insights into the potential applications for future auxetic designs.

Tailoring Poisson's ratio by introducing auxetic layers

Abstract: Monte Carlo simulations in the isobaric–isothermal ensemble with variable shape of the periodic box are used to study elastic properties of two-dimensional (2D) model systems of hard discs with parallel layers of hard cyclic hexamers. Both the particles in their pure phases form elastically isotropic crystals. However, the crystal of discs shows positive and that of hexamers – negative Poisson's ratio (PR). The studied systems with layers are of low symmetry. Hence, a general analytic formula for the orientational dependence of PR in 2D systems is applied to such systems. It is shown that, by changing parameters of the particles and their relative concentration, as well as orientation of the layers, one can obtain systems which are non-auxetic (their PR is positive), auxetic (their PR is negative), or partially auxetic ones (their PR is negative for some directions and positive for other ones). If the disks and hexamers are interpreted as small molecules, the obtained results indicate that by introducing auxetic nano-layers one can strongly modify PR of crystals or even thin layers. The periodic box with particles can be thought of as a model of a thin layer or as a model crystalline structure with parallel horizontal layers built of four hard cyclic hexamer rows in the hard disc system.

Computational design of two-phase auxetic structures

Abstract: Optimization of structures with complex shapes is a big challenge for computational physics. Results of numerical calculations show that composite or sandwich panel structures have a great influence on their effective properties. This article presents numerical results of optimization of sandwich panel properties. Calculations were provided for a three-layer sandwich two-phase composite. Optimization techniques were used for minimization of the effective Poisson's ratio of the core. The resultant composite structure exhibits a negative Poisson's ratio (NPR), although all its constituents are characterized by positive values of the Poisson's ratio. The structure of the composite is completely filled with solid materials, hence no voids appear within its whole volume. To find a solution, the finite-element method combined with an optimization algorithm MMA (method of moving asymptotes) was used. For the purpose of analysis, the material parameters were written by means of the shape interpolation SIMP (solid isotropic material with penalization) scheme.

Equilibrium diamond-like carbon nanostructures with cubic anisotropy: Elastic properties

Abstract: Diamond-like carbon nanostructures with cubic anisotropy made by joining fullerene-like molecules of different types via valence bonds are studied by means of molecular dynamics simulations. The considered structures are interesting because they include both - and -hybridized carbon atoms, which lead to their distinct properties compared to the structures with one type of hybridization. Seven diamond-like carbon phases having different shapes of structural units and/or different ways of their connection are studied in the present work. For the relaxed equilibrium structures, the engineering elastic constants (Poisson's ratio, Young's modulus, and shear modulus) are calculated as the functions of the crystal orientation angles. Extreme values of the elastic constants are reported. It is shown that two of the considered diamond-like structures have negative Poisson's ratio and can be regarded as the partial auxetics. According to the results of the present study, elastic properties of the bulk diamond-like carbon structures can vary considerably depending on their structure. Diamond-like carbon nanostructures

Temperature dependency of Cu/Zn ordering in CZTSe kesterites determined by anomalous diffraction

Abstract: Low-temperature cation ordering is emerging as a critical factor limiting the efficiency of CZTSSe kesterite photovoltaic materials. By means of direct determination of the site occupancies from anomalous X-ray powder diffraction data at the Cu- and Zn-absorption edges, the ordering of Cu+ and Zn2+ in B-type Cu2ZnSnSe4 (CZTSe) kesterite upon annealing at temperatures below 203 (6) °C is demonstrated. Anomalous X-ray diffraction on the Cu- and Zn-K absorption edges allows determination of the distribution of isoelectric Cu1+ and Zn2+ over the crystallographic sites in a B-type CZTSe kesterite (Cu1.949(20)Zn1.059(10)Sn0.983(10)Se4) powder. By Rietveld refinement the quantitative determination of both Cu- and Zn-occupancy for all relevant sites is achieved. From this, the temperature dependency of a structure-based, quantitative order parameter is determined. The critical temperature of the phase transition is confirmed at 203 (6) °C. The ordering mechanism is in agreement with a transition from disordered to ordered kesterite. The photoluminescence band maximum shows a closely related temperature dependency, directly demonstrating the effect of cation ordering on the optical properties of CZTSe.

Correlation of sapphire off‐cut and reduction of defect density in MOVPE grown AlN

Abstract: X-ray diffraction and TEM investigations of MOVPE grown AlN on sapphire with small off-cuts to a- and m-plane reveal the influence of the off-cut direction and angle on the reduction of threading dislocation density by annihilation during growth. Higher off-cut angles as well as off-cut to a-plane seem to facilitate the annihilation, with the main reduction taking place within the first 300 nm layer thickness. On planar substrate the thickness is limited by cracking to below 2 μm which also limits the ability to further reduce the defect density. By epitaxial lateral overgrowth on stripe patterned substrates the crack-free thickness is increased and further reduction of the defect density is possible. This process is effective up to 3–5 μm layer thickness. Using templates with off-cuts ≥0.2° to m-plane, step bunching perpendicular to the stripe direction occurs and bends the vertically directed threading dislocations into inclined grain boundaries starting from the point of coalescence. These partially block/incline threading dislocations over the ridge areas and thus further reduce the dislocation density. The dislocations are concentrated in stripes over the ridges and the coalescence areas. For smaller off-cut to m or especially for off-cut to a-plane, the dislocation distribution is more homogeneous but nevertheless stripe-like with alternating densities of low 108 cm−2 in the laterally overgrown areas and low 109 cm−2 in the areas over the ridges and the coalescence lines.

Piezoelectric properties of monolayer II–VI group oxides by first‐principles calculations

Abstract: Piezoelectric properties of group II–VI oxide structures is carried out by first principles methods. Piezoelectric effect is the ability of the certain materials to generate an electric charge in response to applied mechanical stress, and it is also reversible. Two-dimensional IIA/IIB–VI group oxides are expected to have great potential due to their non-centrosymmetric structure and intrinsic large band gaps. PBE formulation and HSE06 hybrid functionals were used to get realistic forbidden bandgap values. Density functional theory-based first-principles calculations (DFT and DFPT) are used to investigate the piezoelectric stress, e11, and strain, d11, coefficients of monolayer IIA/IIB–VI group oxides (MO where M = Be, Mg, Ca, Sr, Ba, Zn, and Cd). We predicted that 2D II–VI family compounds exhibit highly promising piezoelectric properties. BeO has the highest e11 and the BaO has the highest d11 relaxed-ion coefficients. All piezoelectricity calculation results give close values for the CaO structure. Calculation results show increasing trend for relaxed ion piezoelectric coefficients for IIA group and decreasing trend for IIB group, according to row number of the atoms. Our calculations reveal that II–VI family oxide structures are strong candidates for future atomically thin piezoelectric applications. Calculated DFT (clamped-ion and relaxed-ion) and DFPT values of (a) piezoelectric stress (e11) and (b) piezoelectric strain (d11) coefficients.

EPR study of nanocrystalline CeO2 exhibiting ferromagnetism at room temperature

Abstract: Electron paramagnetic resonance (EPR) spectroscopy complemented with X-ray diffraction, X-ray fluorescence, and optical spectroscopy was used to study nanocrystalline CeO2 powder samples that exhibit weak room-temperature ferromagnetism. EPR lines assigned to the Ce3+ trigonal sites were found for the first time in cerium dioxide that contains a trace impurity of Mn2+. This finding indicates that manganese dopant facilitates the conversion of the oxidation state of Ce4+ to Ce3+ in nanocrystalline CeO2. Our results support the view that Ce3+/Ce4+ pairs along with defects on the surface of nanoparticles are responsible for the ferromagnetism in CeO2. The EPR study reveals that the charge-transfer mechanism proposed recently is more suitable to explain the origin of room-temperature ferromagnetism in CeO2 than the F+-centers exchange interactions.

Experimental and numerical analysis of a novel three-dimensional auxetic metamaterial

Abstract: An innovative three-dimensional (3D) auxetic structure based on 2D re-entrant honeycombs was developed in this article. A sample of the proposed structure made of acrylonitrile–butadiene–styrene (ABS) was fabricated through 3D printing technology, and a compression test was performed on it. Meanwhile, the unit cell required for the finite-element (FE) analysis was determined to simulate the auxetic behavior of the proposed structure. Both experimental and numerical results showed that the 3D structure can achieve a negative Poisson's ratio (NPR) in two principal orthogonal coordinate directions when it was compressed along the other principal axis. A systematic numerical simulation was performed through the experimentally validated finite element models to analyze the effects of structural parameters (i.e., top-view re-entrant angle and lateral-view re-entrant angle) and deformation on the auxetic behavior. Numerical results revealed that this structure can achieve a negative Poisson's ratio (NPR) under large deformation and the structural parameters and compression strain have significant effects on the auxetic behavior. Our structure, with large flexibility in design, is based on very simple initial geometric shapes. This analysis can provide useful information for the design, fabrication of this kind of 3D auxetic structure, which is promising in some technical applications.

Negative linear compressibility from rotating rigid units

Abstract: The possibility of achieving the anomalous property of negative linear compressibility through a mechanism involving the rotations of rigid units is presented and discussed through analytical modelling. It is shown that some rotating rigid units with particular geometric features and connectivities will expand rather than contract in certain directions when a hydrostatic pressure is applied. The conditions required for this anomalous property to be achieved via rotations are elucidated in terms of specific geometric and structural requirements as well as their Poisson's ratio and stiffness properties.

Negative Poisson's ratio in cubic materials along principal directions

Abstract: This report employed molecular statics simulation and density-functional-theory calculation to study the Poisson's ratios of face-centered-cubic materials. We provide numerical and theoretical evidences to show that cubic materials can exhibit auxetic behavior in a principal direction under proper loading conditions. When a stress perpendicular to the loading direction is applied, cubic materials can exhibit a negative Poisson's ratio at finite strain. The negative Poisson's ratio behavior, including its direction and value, is highly dependent on the direction and magnitude of the transversely applied stresses. As a result, we show that it is possible to tune the direction and magnitude of the negative Poisson's ratio behavior of cubic materials by controlling the transverse loadings.

Enhanced photocatalytic activity of ZnO nanostructure for water purification

Abstract: A study of the ZnO nanowire array photocatalysis performance has been carried out under UV radiation for the degradation of the toxic organic compounds such as methylene blue (MB), methyl orange (MO), and acid red 14 (AR14) dyes, which are commonly used in textile and pharmaceutics industries. UV–Vis spectrometry has been used to follow the dye degradation characterization. The degradation mechanism has been proposed for each dye, which will give a reasonable explanation about different degradation degree under same experimental conditions. MB and AR14 showed a better degradation performance than MO.

Synthesis, crystal structure, and magnetic properties of novel 2D kagome materials RE3Sb3Mg2O14 (RE = La, Pr, Sm, Eu, Tb, Ho): Comparison to RE3Sb3Zn2O14 family

Abstract: The crystal structures and magnetic properties of RE3Sb3Mg2O14 (RE = La, Pr, Sm, Eu, Tb, Ho) with a perfect kagome lattice are presented and compared to RE3Sb3Zn2O14. Rietveld structure refinements were performed using X-ray diffraction data, indicating that the layered compounds are fully structurally ordered. The compounds crystallize in a rhombohedral supercell of the cubic pyrochlore structure, in the space group R-3m. Magnetic susceptibility measurements show no signs of magnetic ordering above 2 K. The RE3Sb3Mg2O14 family is similar to that of RE3Sb3Zn2O14; however, the series reported here features a fully ordered distribution of cations in both the nonmagnetic antimony and magnetic rare earth kagome lattices. Unlike the offsite disorder that Zn2+ experiences in RE3Sb3Zn2O14, the magnesium sites in RE3Sb3Mg2O14 are completely ordered. Here we compare the magnetic properties in both series of kagome compounds to determine how significant Zn2+'s positional ordering is within this structure type. The compounds reported here appear to be relatively defect-free and are therefore model systems for investigating magnetic frustration on an ideal 2D rare earth kagome lattice.

New description of metastable hcp phase for unaries Fe and Mn: Coupling between first-principles calculations and CALPHAD modeling

Abstract: The main focus in developing the third generation of CALPHADdatabases is to model thermodynamic properties of materialsby using models which are more physically based andvalid down to 0K. First-pri ...

Optical spectrum and excitons in bulk and monolayer MX2 (M=Zr, Hf; X=S, Se)

Abstract: authoren We present first principles calculations of the electronic and optical properties of bulk and monolayer structures for four transition metal chalcogenides, MXMZr and Hf; XS, Se), using a post density functional many-body perturbation GW approximation in conjunction with the Bethe–Selpeter approximation (GW-BSE). Optical absorption spectra predict the presence of a strongly bound exciton that lies below the direct band gap in two bulk (HfS and ZrSe) and in all the monolayer structures. The binding energy of the excitons are predicted to lie between 0.11 and 0.96 eV.

Stability, bonding, and electronic properties of silicon and germanium arsenides

Abstract: First-principles calculations were carried out to study the stability, structural, and electronic properties of compounds of silicon arsenides (SiAs and SiAs2) and germanium arsenides (GeAs and GeAs2). The group IV atom (Si or Ge atom) is four-coordinated and the As atom is three-coordinated in both monoclinic and orthorhombic structures, which are energetically favored based on the calculated formation enthalpies for Si/Ge monoarsenides and diarsenides, respectively. The calculated Si-As bond lengths are slightly smaller than the Ge-As bond lengths. In agreement with the experimental results, our calculations show both silicon arsenides and germanium arsenides are semiconductors. The calculated bandgaps of germanium arsenides are slightly smaller than those of silicon arsenides, which may be related to the lower ionicity of the Ge-As bonds in germaniumarsenides. The calculated density of states (DOS) of As atoms in the four compounds are somewhat different from each other indicating the difference of their local atomic environments. The calculations show that Si/Ge arsenides with Si/Ge vacancies are metals, which may be due to the unsaturated As atoms near the vacancy sites. (C) 2016 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

Quantitative measurements of internal electric fields with differential phase contrast microscopy on InGaN/GaN quantum well structures

Abstract: Piezoelectric and spontaneous polarization play an essential role in GaN-based devices. InGaN quantum wells (QWs) in GaN host material, especially grown along the polar c-direction, exhibit strong internal fields in the QW region due to the indium-induced strain. An exact knowledge of the electric fields is essential, since they are one of the factors limiting the performance of green LDs and LEDs. Differential phase contrast in a scanning transmission electron microscope enables direct, local, and quantitative measurements of these electric fields. For a multiQW sample, it was possible to determine the piezoelectric field in the range of 43-67MVm(-1) with a resolution of 10MVm(-1) (= 10mVnm(-1)). (C) 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

The cluster architecture of carbon in polymer nanocomposites observed by impulse acoustic microscopy

Abstract: The high-resolution ultrasonic technique (impulse acoustic microscopy) was applied for observing the bulk microstructure of nanocomposites with various types of carbon nanoparticles as filler. It was shown that ultrasonic methods are excellent tools for studying nanoparticle distribution over the material bulk and for revealing probable nonuniformity. The acoustic microscopy technique allows for looking through the bulk microstructure of objects by means of layer-by-layer visualization and object cross-sectional imaging. In composites based on dispersed micrometer-sized particles of exfoliated graphite, the particle distribution over the composite material bulk was observed. In nanocomposites with various kinds of low-dimensional carbon nanofillers, i.e., nanoflakes, nanoplatelets, and nanotubes, the technique allowed the cluster architecture of the nanoparticle distribution to be revealed. Contact conjugation of low-dimensionality nanoparticles led to fractal clusters despite significant technological efforts for providing homogeneity to the nanocomposite materials and uniformity to their properties. A pronounced tendency to form micrometer-sized fractal agglomerates was found for 2D carbon nanoparticles: nanoflakes and nanoplatelets. The impulse acoustic microscopy technique provides visualization of the agglomerate distribution over the nanocomposite material bulk. Another kind of nanoparticle distribution was observed with carbon nanotubes (CNTs). The latter formed CNT packings having different densities. Such regions were seen in acoustical images as small-sized areas of various brightness.

Absorption edges of black phosphorus: A comparative analysis

Abstract: We have studied the excited states of black phosphorus through the analysis of K and L absorption edges of black phosphorus with energy-filtered transmission electron microscopy. Energy-loss spectra exhibit several peaks reflecting features in the density of conduction-band states at corresponding energies above the vacuum level. We have reproduced the experimental electron energy-loss near-edge structures by means of ab initio self-consistent real-space multiple-scattering theory. The comparison of the calculated symmetry-projected density of unoccupied states and electron energy loss spectra allowed us to assign the spectral features to transitions to specific electronic states.

Surfactant‐aided exfoliation of molybdenum disulfide for ultrafast pulse generation through edge‐state saturable absorption

Abstract: We use liquid phase exfoliation to produce dispersions of molybdenum disulfide (MoS2) nanoflakes in aqueous surfactant solutions. The chemical structures of the bile salt surfactants play a crucial role in the exfoliation and stabilization of MoS2. The resultant MoS2 dispersions are heavily enriched in single and few (6) layer flakes with large edge to surface area ratio. We use the dispersions to fabricate free-standing polymer composite wide-band saturable absorbers to develop mode-locked and Q-switched fiber lasers, tunable from 1535 to 1565 and 1030 to 1070 nm, respectively. We attribute this sub-bandgap optical absorption and its nonlinear saturation behavior to edge-mediated states introduced within the material band-gap of the exfoliated MoS2 nanoflakes.

First-principles lattice dynamics and Raman scattering in ionic conductor β-Ag2S

Abstract: The phonon structure of the silver sulfide Ag2S was investigated, experimentally using Raman spectroscopy, and theoretically using the density-functional perturbation theory for the first time. Seven Raman-active modes were observed and identified at 23, 39, 42, 44, 62, 65, and 243 cm−1. Symmetry assignments of all the vibrational modes were derived from considerations of point group symmetry. The phonon band structure and the relative Raman intensities were also investigated by ab initio calculations and compared with the experimental data. The temperature, laser power, and illumination time dependencies of frequency, linewidth, and intensity of the Raman-active modes are discussed. In the Raman spectra at higher frequencies 1300–1700 cm−1, additional broad Raman modes observed in all samples at higher laser powers 8–10 mW were ascribed to luminescence from β-Ag2S. The phonon and Raman spectra of the β-Ag2S provide a useful insight into the β-Ag2S → α-Ag2S phase transition. Finally, calculated infrared vibrational mode frequencies were compared with measured infrared mode frequencies.

Encapsulated graphene-based Hall sensors on foil with increased sensitivity

Abstract: The encapsulation of graphene-based Hall sensors on foil is shown to be an effective method for improving the performance in terms of higher sensitivity for magnetic field detection. Two types of encapsulation were investigated: a simple encapsulation of graphene with polymethyl methacrylate (PMMA) as a proof of concept and an encapsulation with mechanically exfoliated hexagonal boron nitride (hBN). The Hall sensor with PMMA encapsulation already shows higher sensitivity compared to the one without encapsulation. However, the Hall sensor with graphene encapsulated between two stacks of hBN shows a current and a voltage normalized sensitivity of up to 2270 V/AT and 0.68 V/VT, respectively, which are the highest reported sensitivity values for Hall sensors on foil so far.

Review on carrier multiplication in graphene

Abstract: The remarkable gapless and linear band structure of graphene opens up new carrier relaxation channels bridging the valence and the conduction band. These Auger scattering processes change the number of charge carriers and can give rise to a significant multiplication of optically excited carriers in graphene. This is an ultrafast many-particle phenomenon that is of great interest both for fundamental many-particle physics as well as technological applications. Here, we review the research on carrier multiplication in graphene and Landau-quantized graphene including theoretical modeling and experimental demonstration. Illustration of the electronic band structure of graphene including Auger scattering channels that can lead to a carrier multiplication (CM). Figure adapted from Ref. [2].