CuNi binary alloy catalyst for growth of nitrogen‐doped graphene by low pressure chemical vapor deposition

The CuNi binary alloy can be significant as a catalyst for nitrogen‐doped (N‐doped) graphene growth considering controllable solubility of both carbon and nitrogen atoms. Here, we report for the first time the possibility of synthesizing substitutional N‐doped bilayer graphene on the binary alloy catalyst. Raman spectroscopy, atomic force microscopy and transmission electron microscopy analysis confirm the growth of bilayer and few‐layer graphene domains. X‐ray photoelectron spectroscopy analysis shows the presence of around 5.8 at% of nitrogen. Our finding shows that large N‐doped bilayer graphene domains can be synthesized on the CuNi binary alloy.

     

High‐temperature ferroelectricity and strong magnetoelectric effects in a hybrid organic–inorganic perovskite framework

A Cu‐based organic–inorganic perovskite framework exhibits high‐temperature ferroelectricity with strong magnetoelectric effects. Both electric field control of magnetization and magnetic field control of polarization are realized. Theoretical calculations suggest that a new mechanism of hybrid improper ferroelectricity arising from the Jahn–Teller distortions of magnetic metal ions and tilting of the organic cations are responsible for the peculiar multiferroic behaviors.

     

Localization of π‐electron density in twisted bilayer graphene

In bilayer graphene, mutual rotation of layers has strong effect on the electronic structure. We theoretically study the distribution of electron density in twisted bilayer graphene with the rotation angle of 21.8° and find that regions with AA‐like and AB‐like stacking patterns separately contribute to the interlayer low‐energy van Hove singularities. In order to investigate the peculiarities of interlayer coupling, the charge density map between the layers is examined. The presented results reveal localization of π‐electrons between carbon atoms belonging to different graphene layers when they have AA‐like stacking environment, while the interlayer coupling is stronger within AB‐stacked regions.

Charge density map for bilayer graphene with a layer twist of 21.8° (interlayer region).     

Observation of negative refraction in the graphene/ferrite composite

An observation of negative refraction in the naturally obtained composition of graphene and barium ferrite is reported. The capacitance and inductance measurements revealed the electric and magnetic resonances accompanied with the negative values of permittivity and permeability in the overlapped frequency range. According to the “left‐handed” media approach such a material is characterized by negative refraction. The derived values of the real part of refractive index are negative at the frequencies above 500 MHz.

     

First‐principles prediction of a novel hexagonal phosphorene allotrope

Phosphorus prefers three‐connected configurations due to its inequivalent sp³‐hybridization. In the past year, many quasi two‐dimensional three‐connected networks were proposed as possible phosphorene allotropes. In this Letter, a new quasi two‐dimensional three‐connected network is proposed as a new potential phosphorene allotrope (Hex‐star). Based on first‐principles method calculations, the structure, stability and electronic properties of Hex‐star were systematically investigated. Our results indicate that Hex‐star is dynamically stable and it is a semiconductor with quasi‐direct band gap of 1.81 eV based on HSE06 method. Perspective top view (left) and Magen–David‐like orthographic top view (right) of Hex‐star phosphorene.

     

Low leakage stoichiometric SrTiO3 dielectric for advanced metal–insulator–metal capacitors

Metal–insulator–metal capacitors (MIMCAP) with stoichiometric SrTiO₃ dielectric were deposited stacking two strontium titanate (STO) layers, followed by intermixing the grain determining Sr‐rich STO seed layer, with the Ti‐rich STO top layer. The resulted stoichiometric SrTiO₃ would have a structure with less defects as demonstrated by internal photoemission experiments. Consequently, the leakage current density is lower compared to Sr‐rich STO which allow further equivalent oxide thickness downscaling.

Schematic of MIMCAP with stoichiometric STO dielectric formed from bottom Sr‐rich STO and top Ti‐rich STO after intermixing during crystallization anneal.     

Photocatalytic activity of α‐PbO2‐type TiO2

The α‐PbO₂‐type TiO₂ is synthesized under high‐pressure and high‐temperature environment and it shows higher photocatalytic activity as compared to rutile and anatase under UV irradiation. The reduction in α‐PbO₂‐type TiO₂ induces visible‐light photocatalytic activity. These results indicate that α‐PbO₂‐type TiO₂ is an important candidate material for use in a photocatalytic matrix.

     

Numerical modeling of photovoltaic efficiency of n‐type GaN nanowires on p‐type Si heterojunction

In this Letter, we investigate the photovoltaic properties of heterojunction solar cells based on n‐GaN nanowire (NW)/ p‐Si substrate heterostructures by means of numerical modeling. Antireflection properties of the NW array on the top of Si substrate were studied theoretically to show an order of magnitude enhancement in antireflection properties in comparison to the pure Si surface (2.5% vs. 33.8%). In order to determine the optimal morphology and doping levels of the structure with maximum possible efficiency we simulated its properties. The carried out simulation showed that the maximum efficiency should be more than 20% under AM1.5D illumination. The proposed design opens new perspectives and opportunities in the field of heterojunction tandem solar cell researches.

     

Flexible X‐ray detector based on sliced lead iodide crystal

A promising flexible X‐ray detector based on inorganic semiconductor PbI₂ crystal is reported. The sliced crystals mechanically cleaved from an as‐grown PbI₂ crystal act as the absorber directly converting the impinging X‐ray photons to electron hole pairs. Due to the ductile feature of the PbI₂ crystal, the detector can be operated under a highly curved state with the strain on the top surface up to 1.03% and still maintaining effective detection performance. The stable photocurrent and fast response were obtained with the detector repeated bending to a strain of 1.03% for 100 cycles. This work presents an approach for developing efficient and cost‐effective PbI₂‐based flexible X‐ray detector.

     

P‐type tin‐oxide thin film transistors for blue‐light detection application

We reported the characteristics of p‐type tin‐oxide (SnO) thin film transistors (TFTs) upon illumination with visible light. Our p‐type TFT device using the SnO film as the active channel layer exhibits high sensitivity toward the blue‐light with a high light/dark read current ratio (I_light/I_dark) of 8.2 × 10³ at a very low driven voltage of <3 V. Since sensing of blue‐light radiation is very critical to our eyes, the proposed p‐type SnO TFTs with high sensitivity toward the blue‐light show great potential for future blue‐light detection applications.

     

Lead‐free mesoscopic Cs2SnI6 perovskite solar cells using different nanostructured ZnO nanorods as electron transport layers

Lead‐free and more air‐stable perovskite Cs₂SnI₆ absorber with a direct bandgap of 1.48 eV is synthesized via a modified solution process. Different nanostructured ZnO nanorod arrays as electron transport layers and hole blocking layers are grown by controlling the seed layer and used to fabricate mesoscopic perovskite solar cells with Cs₂SnI₆ as light absorber layer. The influences of ZnO seed layers and nanorod morphology on the device photovoltaic performance were also investigated. With careful control of ZnO nanorod length and pore size to ensure high loading of the Cs₂SnI₆ absorber, we achieved power conversion efficiency of near 1%.

     

Thermally‐activated post‐growth dewetting of fullerene C60 on mica

Understanding and controlling the growth and stability of molecular thin films on solid surfaces is necessary to develop nanomaterials with well‐defined physical properties. As a prominent model system in organic electronics, we investigate the post‐growth dewetting kinetics of the fullerene C₆₀ on mica with real‐time and in situ X‐ray scattering. After layer‐by‐layer growth of C₆₀, we find a thermally‐activated post‐growth dewetting, where the smooth C₆₀‐layer breaks up into islands. This clearly shows that growth is kinetically limited before the system moves over an activation barrier into an energetically favored configuration. From the temperature‐dependent dewetting kinetics we find an effective activation barrier of 0.33 eV, which describes both the temperature‐dependent macroscopic changes in the surface morphology and the microscopic processes of inter‐ and intralayer diffusion during dewetting.

     

Spectral dependence of light extraction efficiency of high‐power III‐nitride light‐emitting diodes

Using the recently suggested method of processing the data on external quantum efficiency as a function of output optical power, we have estimated the dependence of light extraction efficiency of high‐power light‐emitting diodes (LEDs) on their emission wavelength varied between 425 nm and 540 nm. The extraction efficiency is found to increase with the wavelength from ～80% to ～85% in this spectral range and to correlate with the wavelength dependence of reflectivity of the large‐area p‐electrode being the essential unit of the LED chip design. The correlation found identifies the incomplete reflection of emitted light from the electrode as the major mechanism eventually controlling the spectral dependence of the efficiency of light extraction from the LEDs.

     

Bulk superconductivity in single‐phase Bi3O2S3

We report the synthesis of single‐phase Bi₃O₂S₃ sample and confirm the occurrence of bulk superconductivity with transition temperature at 5.8 K. The Bi₃O₂S₃ superconductor is categorized as typical type‐II superconductor based on the results of both temperature and magnetic field dependences of magnetization. Hall coefficient measurements give evidence of a multiband character, with a dominant conduction mainly by electron‐like charge carriers. The charge carrier density is about 1.45 × 10¹⁹ cm^–3, suggesting that the system has very low charge carrier density.

     

Efficient synthesis and tailoring of magnetic and dielectric properties of Pb‐free perovskite‐like ABX3 metal‐organic frameworks

Perovskite‐like metal‐organic frameworks (MOFs) are hybrid materials of high interest for their potential in information storage technology, as Pb‐free substitutes for the widely used lead zirconate titanate (PZT) family of multiferroics. We report here a new, microwave‐assisted method of synthesis for perovskite‐like MOFs, which exploits the advantages of rapid and volumetric heating by microwaves in order to achieve synthesis within minutes, compared to days required by previously reported methods. The preliminary results demonstrate a broad control over the size and morphology of the products, by minor changes in the reaction conditions. An investigation of the effects of size and morphology on the magnetic and dielectric properties is presented here.

     

Transition absorption as a mechanism of surface photoelectron emission from metals

Transition absorption of a photon by an electron passing through a boundary between two media with different permittivities is described both classically and quantum mechanically. Transition absorption is shown to make a substantial contribution to photoelectron emission at a metal/semicon‐ductor interface in nanoplasmonic systems, and is put forth as a possible microscopic mechanism of the surface photoelectric effect in photodetectors and solar cells containing plasmonic nanoparticles.

     

Direct correlation of microstructure and device performance of liquid phase crystallized Si thin film solar cells on glass

Si thin films on glass grown by liquid phase crystallization (LPC) exhibit large grains resembling those in multicrystalline Si wafers. The present work gives direct insight into how planar defects in LPC‐Si thin films influence the device performance of the corresponding solar cells by acquiring electron‐backscatter diffraction maps and measuring solar cell parameters on the same identical positions. By this approach, it was possible to demonstrate how low scanning velocities of the laser line during the crystallization lead to lower densities of grain boundaries, to improved charge‐carrier diffusion lengths, and hence to improved device performances.

     

Impact of strain induced by polymer curing in benzocyclobutene embedded semiconductor nanostructures

Polymers such as benzocyclobutene are commonly used as embedding materials for semiconductor nanostructures. During the curing process of the polymer up to 250 °C, a significant impact of strain can be induced on the embedded semiconductor material due to different thermal expansion coefficients. This strain has been revealed by X‐ray diffraction in free‐standing GaAs nanowires grown on a silicon substrate, embedded in a polymer matrix. It will be shown that this strain is released during the X‐ray irradiation if additionally an external static electric field is applied.

     

Exchange bias at low fields exhibited by the interface between epitaxial layers of ferromagnetic and charge‐ordered rare‐earth manganites

Epitaxial thin films of ferromagnetic La_0.7Sr_0.3MnO₃ (LSMO) and charge‐ordered, antiferromagnetic Y_0.5Ca_0.5MnO₃ (YCMO) were deposited on SrTiO₃ (100) substrates by pulsed laser deposition (PLD). The heterostructure undergoes tetragonal distortion due to strong biaxial tensile strain imposed by the substrate. The LSMO–YCMO bilayers exhibit significant exchange bias (EB) across the interface even in a very small remnant magnetic field (～5 Oe) present in the superconductor magnet. The unidirectional exchange anisotropy at the interface can be switched by reversing the polarity of the remnant magnetic field.

     

Delta‐layer doping profile in diamond providing high carrier mobility

We show that the nano‐scale delta‐layer doping profile in diamond can significantly influence both the carrier mobility and two‐dimensional conductivity. We numerically considered and compared a simple boron doping profile with one maximum at the delta‐layer center and a more complicated profile with two maxima inside the delta layer and a minimum at its center. As a result we concluded that in the last case the hole mobility and the two‐dimensional conductivity are higher by more than 3 times and 60% respectively than in the first case. The physical reason for the improvement is that for the two‐maxima doping profile the peaks of the carrier and ionized dopant densities are spatially separated, whereas for the simple one‐maximum doping profile they coincide. So, the carrier scattering on ionized dopants for the two‐maxima profile significantly decreases in comparison with the simple one‐maximum profile. The proposed two‐maxima delta‐layer doping profile can be used for the creation of diamond‐based micro‐ and nanoelectronics devices, e.g. high‐frequency field effect transistors.