Extensive study of magneto-optical and optical properties of Cd1−xMnxTe between 675 and 1025 nm

We determine Faraday rotations and measure the optical reflection and transmission from magneto-optical Cd1−xMnxTe crystals with various stoichiometric ratios. For wavelengths between 675 and 1025 nm, we derive Verdet constants, optical loss coefficients, and the complex indices of reflection that are relevant measures to find suitable stoichiometric ratios of Cd1−xMnxTe for the realization of miniaturized optical isolators. By reflection and transmission measurements, we determine the stoichiometric ratios of several different Cd1−xMnxTe crystals and discuss the observed dependence of the optical properties on the stoichiometric ratio with respect to their use in optical isolators. Finally, we show the relevant figure of merit, i.e., the ratio of Verdet constants and optical loss coefficients for Cd1−xMnxTe crystals with Mn contents ranging from x = 0.14 to x = 0.50

Crystal structure of polymeric carbon nitride and the determination of its process-temperature-induced modifications

Based on the arrangement of two-dimensional 'melon', we construct a unit cell for polymeric carbon nitride (PCN) synthesized via thermal polycondensation, whose theoretical diffraction powder pattern includes all major features measured in x-ray diffraction. With the help of this unit cell, we describe the process-temperature-induced crystallographic changes in PCN that occur within a temperature interval between 510 and 610 °C. We also discuss further potential modifications of the unit cell for PCN. It is found that both triazine- and heptazine-based g-C3N4 can only account for minor phases within the investigated synthesis products

Photophysics of polymeric carbon nitride: An optical quasimonomer

A comprehensive investigation of the luminescent properties of carbon nitride polymers, based on tri-s-triazine units, has been conducted. Steady-state temperature- and excitation-power-dependent as well as time-resolved measurements with near-UV excitation (λ=325 nm and 405 nm) yield strong photoluminescence, covering the visible spectrum. The spectral, thermal, and temporal features of the photoluminescence can be satisfactorily described by the excitation and radiative recombination of molecular excitons, localized at single tri-s-triazine units. The discussed model is in accordance with the recently reported absorption features of carbon nitride polymers. Thus, from the point of view of optical spectroscopy, the material effectively behaves as a monomer

An optical quasimonomer

A comprehensive investigation of the luminescent properties of carbon nitride polymers, based on tri-s-triazine units, has been conducted. Steady-state temperature- and excitation-power-dependent as well as time-resolved measurements with near-UV excitation (λ=325 nm and 405 nm) yield strong photoluminescence, covering the visible spectrum. The spectral, thermal, and temporal features of the photoluminescence can be satisfactorily described by the excitation and radiative recombination of molecular excitons, localized at single tri-s-triazine units. The discussed model is in accordance with the recently reported absorption features of carbon nitride polymers. Thus, from the point of view of optical spectroscopy, the material effectively behaves as a monomer

A hybrid MBE-based growth method for large-area synthesis of stacked hexagonal boron nitride/graphene heterostructures

Van der Waals heterostructures combining hexagonal boron nitride (h-BN) and graphene offer many potential advantages, but remain difficult to produce as continuous films over large areas. In particular, the growth of h-BN on graphene has proven to be challenging due to the inertness of the graphene surface. Here we exploit a scalable molecular beam epitaxy based method to allow both the h-BN and graphene to form in a stacked heterostructure in the favorable growth environment provided by a Ni(111) substrate. This involves first saturating a Ni film on MgO(111) with C, growing h-BN on the exposed metal surface, and precipitating the C back to the h-BN/Ni interface to form graphene. The resulting laterally continuous heterostructure is composed of a top layer of few-layer thick h-BN on an intermediate few-layer thick graphene, lying on top of Ni/MgO(111). Examinations by synchrotron-based grazing incidence diffraction, X-ray photoemission spectroscopy, and UV-Raman spectroscopy reveal that while the h-BN is relaxed, the lattice constant of graphene is significantly reduced, likely due to nitrogen doping. These results illustrate a different pathway for the production of h-BN/graphene heterostructures, and open a new perspective for the large-area preparation of heterosystems combining graphene and other 2D or 3D materials

Carbon nitrides: synthesis and characterization of a new class of functional materials

Carbon nitride compounds with high N[thin space (1/6-em)]:[thin space (1/6-em)]C ratios and graphitic to polymeric structures are being investigated as potential next-generation materials for incorporation in devices for energy conversion and storage as well as for optoelectronic and catalysis applications. The materials are built from C- and N-containing heterocycles with heptazine or triazine rings linked via sp2-bonded N atoms (N(C)3 units) or –NH– groups. The electronic, chemical and optical functionalities are determined by the nature of the local to extended structures as well as the chemical composition of the materials. Because of their typically amorphous to nanocrystalline nature and variable composition, significant challenges remain to fully assess and calibrate the structure–functionality relationships among carbon nitride materials. It is also important to devise a useful and consistent approach to naming the different classes of carbon nitride compounds that accurately describes their chemical and structural characteristics related to their functional performance. Here we evaluate the current state of understanding to highlight key issues in these areas and point out new directions in their development as advanced technological materials.Our work on carbon nitride materials has been supported by the EPSRC (EP/L017091/1) and the EU Graphene Flagship grant agreement No. 696656 - GrapheneCore1. Additional support to advance the science and technology of these materials was also received from the UCL Enterprise Fund and the Materials Innovation Impact Acceleration funding enabled by the UK EPSRC

Tunable optical transition in polymeric carbon nitrides synthesized via bulk thermal condensation

Polymeric derivatives of dicyandiamide were synthesized via a bulk thermal condensation method, using a range of process temperatures between 400 and 610 C. The obtained carbon nitride powders exhibit an optical transition in the UV green range that has been assigned to the direct bandgap of a semiconductor like material. Within this context, the apparent bandgap is linearly tunable with increasing process temperatures, showing a temperature coefficient of amp; 1048576;1 7.1 meV K amp; 1048576;1 between 2.5 and 3.0 eV. The obtained results show a predominant optical transition within the tri s triazine unit of the polymer, with a bathochromic shift originating from a gradually increasing degree of polymerizatio