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.

     

Nanosphere lithography with variable deposition angle for the production of one‐directional transparent conductors

The production of high quality and cheap transparent electrodes is a fundamental step for a variety of optoelectronic devices. We present a method for the production of transparent conducting films optimised for electrical conduction in one direction. The deposition of a metal film through a perfectly aligned nanosphere‐lithography mask at variable incidence angle gave origin to parallel nanowires with thin interconnections. This structure showed excellent conductivity in one direction and high optical transparency.

Glass substrates under the crystalline areas of the polystyrene‐nanospheres mask.     

Delayed phase separation in growth of organic semiconductor blends with limited intermixing

Binary mixed thin films of picene (C₂₂H₁₄, PIC) and pentacene (C₂₂H₁₄, PEN) consist of crystallites with a statistical occupation of the lattice sites by either PEN or PIC and unit cell parameters continuously changing with the mixing ratio. For high PIC ratios a PIC phase forms which corresponds to a limited intermixing of the two compounds. The growth behavior of these mixtures is investigated in situ and in real‐time using grazing incidence X‐ray diffraction. We observe a delayed phase separation in PIC‐rich blends, i.e. complete intermixing in the monolayer range and the nucleation of a pure PIC‐phase in addition to the intermixed phase starting from the second monolayer.

Growth scenario of picene‐rich pentacene‐picene blends.     

Investigation of strain‐induced phase transformation in ferroelectric transistor using metal‐nitride gate electrode

In this work, we report a ferroelectric memory with strained‐gate engineering. The memory window of the high strain case was improved by ～71% at the same ferroelectric thickness. The orthorhombic phase transition (from ferroelectric to anti‐ferroelectric transition) plays a key role in realizing negative capacitance effect at high gate electric field. Based on a reliable first principles calculation, we clarify that the gate strain accelerates the phase transformation from metastable monoclinic to orthorhombic and thus largely enhances the ferroelectric polarization without increasing dielectric thickness. This ferroelectric strain technology shows the potential for emerging device application.

     

Ab initio prediction of a new allotrope of two‐dimensional silicon

Silicene, a promising candidate for future electronic devices, has been fabricated only on supporting substrates as silicon atom prefers to form the sp³ hybridization structure. Therefore, it's important to search more stable two‐dimensional (2D) silicon allotropes and several 2D silicon allotropes have been proposed recently. In this work, we predict a new type of 2D silicon allotrope (called OTDS) based on ab initio structure, phonon‐mode and molecular dynamics calculations. OTDS has the in‐plane octagonal tiling (OT) pattern with dumbbell‐like structures and silicon atoms in OTDS are four‐ and three‐coordinated. OTDS is a semiconductor with a large band gap (about 1.5 eV by HSE calculation) and the band gap can be tuned effectively by the in‐plane strain.

Perspective and side views of the atomic structure of OTDS.     

O– centers in LuAG:Ce,Mg ceramics

Electron traps are known to a have great influence on the carriers transport process in Ce based scintillators. On the other hand, the role of hole traps in the scintillation process has been less considered. By means of electron spin resonance (ESR), we detected hole traps in highly Ce‐doped LuAG:Ce,Mg ceramics in the form of σ‐type O^– centers. The g ‐tensor components turn out to be g_⊥ = 2.0103, and g_|| = 2.0023, revealing the axial symmetry of these defects. Mg‐perturbed variants of O^– centers are proposed to exist in LuAG:Ce,Mg. Their occurrence is related to an elongated Mg–O bond as evidenced by DFT calculations. Finally, the multiple role of O^– centers in the scintillation process is discussed.

     

Hole‐extraction layer dependence of defect formation and operation of planar CH3NH3PbI3 perovskite solar cells

Three planar CH₃NH₃PbI₃ (MAPbI₃) solar cells having the same structure except a hole‐extraction layer (HEL) showed distinctive difference in operation characteristics. Analysis of frequency‐dependent capacitance and dielectric‐loss spectra of the three MAPbI₃ devices showed two types of recombination‐loss channels with different time constants that we attributed respectively to interface and bulk defects. Discrepancy in defect formation among the three devices with a HEL of PEDOT:PSS, NiO_x, or Cu‐doped NiO_x was not surprising because grain‐size distribution and crystalline quality of MAPbI₃ can be affected by surface energy and morphology of underlying HELs. We were able to quantify interface and bulk defects in these MAPbI₃solar cells based on systematic and simultaneous simulations of capacitance and dielectric‐loss spectra, and current–voltage characteristics by using the device simulator SCAPS.

     

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.

     

Sulfur impregnation of multi‐walled carbon nanotubes via SF6/NH3 plasma exposure

Plasma treatments are established methods to functionalise carbon nanotubes (CNTs) and modify their surface structure. This paper presents a mild glow‐discharge plasma treatment of aligned arrays of multi‐walled carbon nanotubes employing sulfur hexafluoride (SF₆), ammonia (NH₃), and their mixtures as process gases. For the latter, sulfur was detected at the tip and sidewalls of the nanotubes via energy‐dispersive X‐ray spectroscopy, while electron microscopy served as method to verify the structural integrity of the CNTs after the plasma treatment. This approach provides the basis for an easy and quick alternative to existing sulfur functionalisation methods of MWCNTs. Furthermore, the proposed method can conveniently be applied to carbon nanotube arrays on substrate while preserving their structure and alignment.

SEM‐EDX map of SF₆/NH₃ plasma‐treated multi‐walled carbon nanotubes on substrate. Green, yellow and red correspond to silicon, carbon and sulfur signals, respectively.     

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.

     

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%.

     

Ag fractals formed on top of a porous TiO2 thin film

This work demonstrates the formation of Ag fractals on top of a Ag:TiO₂ thin film. The dendrite‐type objects emerged from a homogeneous and highly transparent Ag:TiO₂ nanocomposite, via the mechanism of diffusion‐limited‐aggregation of Ag atoms, during heat‐treatment at 500 °C. A porous TiO₂ matrix was also formed during this process, opening a wide range of possible applications, namely in sensing‐based ones, together with surface enhanced spectroscopies. Furthermore, fractals incorporate a wide range of shapes and spatial scales, inducing a potentially interesting optical response, over the whole visible range, presumably related with localized surface plasmon modes with very broad spectral distribution.

     

True nature of active layers in organic solar cells fabricated by sequential casting of donor and acceptor layers

The operation characteristics of nominal bilayer (BL) organic solar cells (OSCs), the active layers (ALs) of which consisted of sequentially casted bottom P3HT donor and top ICBA acceptor layers, resembled those of OSCs with bulk heterojunction (BHJ) ALs. Optical analysis and device simulations showed that such resemblance can be attributed to a similarity in the micromorphology of ALs; as‐deposited BL‐type ALs transformed spontaneously into BHJ‐type ALs. The inclusion of P3HT nanowires (NWs) in the donor layers resulted in different AL micromorphology and consequently a larger power conversion efficiency. Separate assessment of the exciton generation and charge–carrier transport and/or extraction showed that the contribution of P3HT NWs was more prominent in optical effects.

     

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).     

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.     

MOCVD synthesis of compositionally tuned topological insulator nanowires

Device applications involving topological insulators (TIs) will require the development of scalable methods for fabricating TI samples with sub‐micron dimensions, high quality surfaces, and controlled compositions. Here we use Bi‐, Se‐, and Te‐bearing metalorganic precursors to synthesize TIs in the form of nanowires. Single crystal nanowires can be grown with compositions ranging from Bi₂Se₃ to Bi₂Te₃, including the ternary compound Bi₂Te₂Se. These high quality nanostructured TI compounds are suitable platforms for on‐going searches for Majorana fermions (Mourik et al., Science 336 , 1003 (2012) and Cook et al., Rev. B 86 , 155431 (2012) [1, 2]).

     

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.