Prediction of hyperbolic exciton-polaritons in monolayer black phosphorus

Hyperbolic polaritons exhibit large photonic density of states and can be collimated in certain propagation directions. The majority of hyperbolic polaritons are sustained in man-made metamaterials. However, natural-occurring hyperbolic materials also exist. Particularly, natural in-plane hyperbolic polaritons in layered materials have been demonstrated in MoO3 and WTe2, which are based on phonon and plasmon resonances respectively. Here, by determining the anisotropic optical conductivity (dielectric function) through optical spectroscopy, we predict that monolayer black phosphorus naturally hosts hyperbolic exciton-polaritons due to the pronounced in-plane anisotropy and strong exciton resonances. We simultaneously observe a strong and sharp ground state exciton peak and weaker excited states in high quality monolayer samples in the reflection spectrum, which enables us to determine the exciton binding energy of ~452 meV. Our work provides another appealing platform for the in-plane natural hyperbolic polaritons, which is based on excitons rather than phonons or plasmons.

Influence of Colloidal Au on the Growth of ZnO Nanostructures

Vapor-liquid-solid processes allow growing high-quality nanowires from a catalyst. An alternative to the conventional use of catalyst thin films, colloidal nanoparticles offer advantages not only in terms of cost, but also in terms of controlling the location, size, density, and morphology of the grown nanowires. In this work, we report on the influence of different parameters of a colloidal Au nanoparticle suspension on the catalyst-assisted growth of ZnO nanostructures by a vapor-transport method. Modifying colloid parameters such as solvent and concentration, and growth parameters such as temperature, pressure, and Ar gas flow, ZnO nanowires, nanosheets, nanotubes and branched-nanowires can be grown over silica on silicon and alumina substrates. High-resolution transmission electron microscopy reveals the high-crystal quality of the ZnO nanostructures obtained. The photoluminescence results show a predominant emission in the ultraviolet range corresponding to the exciton peak, and a very broad emission band in the visible range related to different defect recombination processes. The growth parameters and mechanisms that control the shape of the ZnO nanostructures are here analyzed and discussed. The ZnO-branched nanowires were grown spontaneously through catalyst migration. Furthermore, the substrate is shown to play a significant role in determining the diameters of the ZnO nanowires by affecting the surface mobility of the metal nanoparticles.

СOLLOIDAL SYNTHESIS AND CHARACTERIZATION HYDROPHILIC CDTE QUANTUM DOTS FOR MEDICAL DIAGNOSTICS

The present article deals with colloidal quantum dots (QDs) synthesized in an aqueous medium using thioglycolic acid (TGA), L-cysteine (L-cys) and mercaptoethylamine (MEA) as stabilizers. In contrast to high-temperature synthesis in an organic medium, this method of synthesis enables us to skip an additional time-consuming stage of hydrophilization of QDs. The resulting CdTe QDs were studied by spectroscopic methods of analysis. In the absorption spectra of the QDs there is an exciton peak. The average size of the QDs, which is about 3-4 nm, was calculated based on the position of the exciton peak, the zeta potential of the QDs was measured.

Characterizing temperature-dependent optical properties of (MA0.13FA0.87) PbI3 single crystals using spectroscopic ellipsometry

In this paper, we present spectroscopic ellipsometry measurements of (MA0.13FA0.87)PbI3 single crystals assessed at photon energies of 0.73–6.42 eV and at temperatures between 4.4 and 400 K. At room temperature, the refractive index was dispersed as a function of frequency, which is typical of a semiconductor. The absorption spectrum exhibited several electronic transitions. We estimated a room temperature direct band gap of 1.66 ± 0.02 eV and exciton binding energy of 40 meV. With decreasing temperature, the refractive index increased. The room-temperature thermo-optic coefficients were −1.7 × 10−4 and −2.5 × 10−4 K−1 at wavelength of 600 and 1200 nm. The exciton peak position and bandgap energy exhibited a redshift, which was attributed to a reverse ordering of the band structures. Additionally, an anomaly in exciton peak position and bandgap occurred at approximately 100–200 K due to the structural phase transition. This phenomenon was associated with the coexistence of MA/FA-disordered and MA/FA-ordered domains. Our results provide a foundation for the technological development of lead halide perovskites-based photonic devices at various temperatures.

Probing the Optical Properties of MoS2 on SiO2/Si and Sapphire Substrates

As an important supplementary material to graphene in the optoelectronics field, molybdenum disulfide (MoS2) has attracted attention from researchers due to its good light absorption capacity and adjustable bandgap. In this paper, MoS2 layers are respectively grown on SiO2/Si and sapphire substrates by atmospheric pressure chemical vapor deposition (APCVD). Atomic force microscopy, optical microscopy, and Raman and photoluminescence spectroscopy are used to probe the optical properties of MoS2 on SiO2/Si and sapphire substrates systematically. The peak shift between the characteristic A1g and E12g peaks increases, and the I peak of the PL spectrum on the SiO2/Si substrate redshifts slightly when the layer numbers were increased, which can help in obtaining the layer number and peak position of MoS2. Moreover, the difference from monolayer MoS2 on the SiO2/Si substrate is that the B peak of the PL spectrum has a blueshift of 56 meV and the characteristic E12g peak of the Raman spectrum has no blueshift. The 1- and 2-layer MoS2 on a sapphire substrate had a higher PL peak intensity than that of the SiO2/Si substrate. When the laser wavelength is transformed from 532 to 633 nm, the position of I exciton peak has a blueshift of 16 meV, and the PL intensity of monolayer MoS2 on the SiO2/Si substrate increases. The optical properties of MoS2 can be obtained, which is helpful for the fabrication of optoelectronic devices.

Rheological effect of the concentration of nanoparticles in cassava starch

ABSTRACTBiodegradable material was prepared from cassava starch in combination with zinc oxide nanoparticles (ZnO NPs) to give the properties of microbial growth resistance, glycerin concentrations were varied to 5%, 10%, and 20% (w/v) for the study of the rheological properties. The nanoparticles were characterized by a spectrophotometer where an exciton peak at 370 nm was obtained. The different samples were subjected to a thermomechanical study through an AR-G2 hybrid rheometer, using a parallel plane geometry of 20 mm, a weak gel behavior is observed, it is a slimming material and it is thermostable, it is also established that the zinc oxide concentration nanoparticles do not affect the mechanical behavior of the material.

Люминесцентные свойства тонких пленок Cd-=SUB=-x-=/SUB=-Zn-=SUB=-1-x-=/SUB=-O

Thin Cd_ x Zn_1 – x O films with a Cd content in the range from zero to 35 at % are synthesized by pulsed laser deposition. A record-breaking solubility limit of 30 at % of Cd in wurtzite-structured Cd_ x Zn_1 – x O thin films is attained. Apart from the exciton peak, additional peaks associated with an inhomogeneous distribution of Cd in the samples are observed in the low-temperature (10 K) photoluminescence spectra of Cd_0.15Zn_0.85O and Cd_0.3Zn_0.7O films. An unsteady ( S -like) temperature dependence of the spectral position of the exciton photoluminescence peak in Cd_ x Zn_1 – x O films is observed. Such a dependence is associated with the effect of the localization of charge carriers.

Development and Characterization of Tungsten Disulfide Ink for Ink-jet Printing

ABSTRACTIn this work, a printable tungsten disulfide (WS2) based ink is developed from readily available WS2 powder (0.6 µm average particle size), and an ink-jet printing based deposition method for a tungsten disulfide film is presented. WS2 flake coverage and bulk electrical characteristics under three different irradiance conditions are examined and discussed. Presence of excitons in the absorbance of the inks is performed by optical UV-Vis spectrometry. Metrics using the A exciton peak generated by the few-layered flakes are used to calculate the average flake lateral dimensions, the concentration of WS2 in the inks after size selection and filtering, as well as the average monolayer count of the flakes. After printing, scanning electron microscopy is used to confirm average flake lateral size and average flake area coverage, while an atomic force microscope is used to confirm flake thickness.

High Uniformity of ZnO Nanoparticles Synthesized by Surfactant-Assisted Solvothermal Technique

The aim of this study is to develop the synthetic procedure of Zinc Oxide (ZnO) nanoparticles by using surfactant-assisted solvothermal technique in order to produce highly uniform nanosize of ZnO particles. The solvothermal reaction evidently produces smaller ZnO particle sizes compared with those obtained from hydrothermal reaction. The zwitterionic surfactant is employed in this work and it typically works well under extremely conditions i.e. high pH levels, strong electrolytes, and high temperature. The key success of surfactant utilization in the solvothermal reaction is to create reversed micelles which act as nanoreactors or templates. Because micelle consist of polar cores that may occupy a finite amount of water forming a water pool for ZnO nanomaterial synthesis. Synthesized ZnO nanoparticles were obtained from solvothermal reaction at 180°C and 18 hours in a hydrothermal reactor. The ZnO colloidal particles were separated by paper filter and cellulose nitrate membrane, respectively. The XRD pattern shows that the structure of the synthesized ZnO nanoparticles is hexagonal wurtzite and the use of surfactant does not interfere the crystal growth and structure. The particle size distribution reveals a high uniform ZnO nanoparticles obtained via this method. The UV absorption spectrum of ZnO nanoparticles synthesized by this method presents exciton peak at approximate value of 365 nanometers. The energy band gap determined by Tauc plot is 3.31 eV. Moreover, TEM images confirm the particle size consistency showing the morphology of the prepared ZnO nanoparticles.