Using magnetic levitation for 2D and 3D self‐assembly of cubic silicon macroparticles

Today's micro‐ and nano‐fabrication is essentially two‐dimensional, with very limited possibilities of accessing the third dimension. The most viable way to mass‐fabricate functional structures at the nano‐scale, such as electronics or MEMS, with equal feature sizes in all directions, is by three‐dimensional self‐assembly. Up to now, three‐dimensional self‐assembly has mainly been restricted to crystals of polymer spheres. We report on two‐ and three‐dimensional self‐assembly of silicon cubes, levitated in a paramagnetic fluid. We demonstrate the benefits of templating and study the effect of a change in hydrophilicity of the cubes. These experiments bring us one step closer to three‐dimensional self‐assembly of anisotropic, semiconducting units, which is a crucial milestone in overcoming the scaling limits imposed by contemporary 2D microfabrication.

     

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.

     

First‐principle investigations of the peculiar magnetic behavior of Sr2FeOsO6

Structural, electronic and magnetic properties of Sr₂FeOsO₆ have been revisited by using the first‐principle calculations. Semiconducting behavior is reproduced. The band gap is 0.09 eV from generalized gradient approximation (GGA) and 0.30 eV by considering both SOC and U, a bit larger than the experimental observed 0.125 eV. In the C‐type antiferromagnetic configuration, spin frustration is found by analysing the magnetic exchange parameters, explaining the experimental observed magnetic complexity.

     

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.

     

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.

     

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.     

Microstructural identification of Cu in solar cells sensitive to light‐induced degradation

Light‐induced degradation (mc‐LID or LeTID) can lead to a severe efficiency loss in multi‐crystalline solar cells. The underlying mechanism clearly distinguishes from known mechanisms as B‐O‐LID and Fe‐B‐LID. Various defect models have been suggested for mc‐LID mainly based on metal impurities, including Cu which is known to cause light‐induced degradation. We investigate mc‐LID sensitive PERC cells that show an efficiency degradation of 15%_rel. The weaker degradation of the grain boundaries (GBs) typical for mc‐LID is identified and further investigated from front and rear side with respect to recombination activities. The combination of local electrical measurements (LBIC), target preparation (REM, FIB) and element analysis (EDX, TEM) unveil Cu‐containing precipitates at the rear side of the solar cells. They accumulate at grain boundaries and at the rear surface of the Si‐bulk material where the passivation stack is damaged. We conclude that Cu originates from the cell material and discuss its relation to mc‐LID.

LBIC mapping (EQE at fixed wavelength) of a degraded mc‐Si PERC cell from front and rear side results in qualitatively different appearance of GBs.     

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.

     

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.

     

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.

     

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.

     

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.

     

A high efficiency 3D photovoltaic microwire with carbon nanotubes (CNT)‐quantum dot (QD) hybrid interface

An innovative hybrid QD sensitized photovoltaic carbon nanotubes microyarn has been developed using thermally‐stable and highly conductive carbon nanotubes yarns (CNYs). These CNYs are highly inter‐aligned, ultrastrong and flexible with excellent electrical conductivity, mechanical integrity and catalytic properties. The CNYs are coated with a QD‐incorporated TiO₂ microfilm and intertwined with a second set of CNYs as a counter electrode (CE). The maximum photon to current conversion efficiency (η_AM1.5) achieved with prolonged‐time stability was 5.93%. These cells are capable of efficiently harvesting incident photons regardless of direction and generating photocurrents with high efficiency and operational stability.

     

Whispering gallery modes in intrinsic TiO2 microspheres coupling to the defect‐related photoluminescence after visible excitation

Defect‐caused visible photoluminescence after visible excitation in anatase TiO₂ microresonators couples to whispering gallery modes (WGMs). Spherical anatase TiO₂ of a radius between 1.5 µm and 4 µm have been prepared by a sol–gel technique based on hydrolysis of titanium tetrabutoxide. The observation of WGMs in intrinsic anatase TiO₂ without additional dopant offers new perspectives for the localisation of light at TiO₂ surfaces for the design of photocatalysts.

WGMs show up as narrow peaks in the photoluminescence spectra of TiO₂ microparticles after visible excitation.     

Photo‐enhanced catalytic activity of spray‐coated Cu2SnSe3 nanoparticle counter electrode for dye‐sensitised solar cells

Cu₂SnSe₃ nanoparticles are synthesised using oleylamine as both a solvent and capping agent and spray coated to form dye‐sensitised solar cell (DSSC) counter electrodes (CEs) using earth‐abundant elements. The film requires annealing at only 400 °C in nitrogen, which is a lower temperature than previous reports of both Cu₂SnSe₃and Cu₂ZnSnSe₄ films, also avoiding the use of Se gas. The composition and phase of the material is confirmed to be kesterite Cu₂SnSe₃. DSSCs using Cu₂SnSe₃ CEs give a power conversion efficiency of 4.87%, compared to 5.35% when using Pt. Electrochemical impedance spectroscopy indicates that the performance of the Cu₂SnSe₃ CE is enhanced under illumination, leading to a drop in the charge transfer resistance. This illumination‐induced enhancement of the catalytic activity provides a novel mechanism for the enhancement of CE performance in DSSCs.

     

Probing Dy3+ magnetic moments in multiferroic perovskite DyMnO3 by optical spectroscopy

We present a detailed temperature‐dependent (4–300 K) spectroscopic study of DyMnO₃ single crystals with distorted perovskite structure. Energies of 36 crystal‐field levels of Dy³⁺ in paramagnetic DyMnO₃ were determined. The Dy³⁺ ground Kramers doublet does not split at and splits below T_lock = 18 K. The splitting grows fast at temperatures near and reaches Δ₀ ≈ 11 ± 2 cm^–1 at 4 K. Using the experimental temperature dependence Δ₀(T), we calculate the dysprosium magnetic moment m_Dy(T) and the dysprosium contribution into specific heat and magnetic susceptibility. Analysing all the experimental data, we conclude that the Dy–Mn interaction is of the Dzyaloshinskii–Moriya type.

Intensity map in the temperature–wave number coordinates for a spectral line corresponding to the f–f transition of Dy³⁺ in DyMnO₃ and a scheme of the splitting of the Dy ground Kramers doublet. Arrows represent Dy magnetic moments.     

Nonhydrogenic exciton spectrum in perovskite CH3NH3PbI3

The excitons in the orthorhombic phase of the perovskite CH₃NH₃PbI₃ are studied using the effective mass approximation. The electron–hole interaction is screened by a distance‐dependent dielectric function, as described by the Haken potential or the Pollmann–Büttner potential. The energy spectrum and the eigenfunctions are calculated for both cases. The results show that the Pollmann–Büttner model, using the corresponding parameters obtained from ab initio calculations, provides better agreement with the experimental results.

     

Non‐magnetic defects in the bulk of two‐dimensional topological insulators

We found that non‐magnetic defects in two‐dimensional topological insulators induce bound states of two kinds for each spin orientation: electron‐ and hole‐like states. Depending on the sign of the defect potential these states can be also of two kinds with different distribution of the electron density. The density has a maximum or minimum in the center. A surprising effect caused by the topological order is a singular dependence of the bound‐state energy on the defect potential.

     

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.