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.

     

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.     

On the assessment of CIGS surface passivation by photoluminescence

An optimized test structure to study rear surface passivation in Cu(In,Ga)Se₂ (CIGS) solar cells by means of photoluminescence (PL) is developed and tested. The structure – illustrated in the abstract figure – is examined from the rear side. To enable such rear PL assessment, a semi‐transparent ultra‐thin Mo layer has been developed and integrated in place of the normal rear contact. The main advantages of this approach are (i) a simplified representation of a rear surface passivated CIGS solar cell is possible, (ii) it is possible to assess PL responses originating close to the probed rear surface, and (iii) a stable PL response as a function of air exposure time is obtained. In this work, PL measurements of such structures with and without rear surface passivation layers have been compared, and the measured improvement in PL intensity for the passivated structures is associated with enhanced CIGS rear interface properties.

     

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.

     

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.

     

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.

     

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.

     

Casimir interactions in strained graphene systems

We theoretically study the strain effect on the Casimir interactions in graphene based systems. We found that the interactions between two strained graphene sheets are strongly dependent on the direction of stretching. The influence of the strain on the dispersion interactions is still strong in the presence of dielectric substrates but is relatively weak when the substrate is metallic. Our studies would suggest new ways to design next generation devices.

     

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.

     

High transmission nanowire contact arrays with subwavelength spacing

We demonstrate high optical transmission in solar cell contacts based on nanowire arrays with subwavelength spacing. The photocurrent results obtained from fabricated devices are compared with numerical simulations. The proposed contact design leads to optical losses significantly smaller than the fraction of the top surface taken up by the metallic contact (<10% vs. 36%). The resulting sheet resistance of the contact is 46.7 Ω/square, comparing favourably with transparent conductive oxides.

     

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.

     

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.     

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.     

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.

     

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.

     

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.

     

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.