Red photoluminescence and blue-shift caused by phase transformation in multilayer films of titanium dioxide and zinc sulfide

The most versatile methods for altering the properties and behavior of materials involve a phase transformation in the solid state. In this article, we report multilayered films of ZnS/TiO2/ZnS on amorphous SiOx/Si substrates by pulsed laser deposition (PLD). After sequential vacuum annealing at various temperatures, we investigated the effects of TiO2 on the phase transformation of ZnS films and the consequential changes in photoluminescence (PL) property. PL spectra of the film revealed red emission centered at 686 nm after annealing at 600°C, however, this emission disappears, and the color shifts to blue after annealing at 700°C. Detailed analysis identified that TiO2 acts as a catalytic agent for the phase transformation of ZnS at this temperature, and that the color shift to blue resulted from decreased red emissions attributed to oxygen in the film. The present results show that catalytic agent-mediated phase transformation has strong potential for the modification of material properties. © 2015 Optical Society of America.1

Electroluminescent devices with function of electro-optic shutter

The polymer-dispersed liquid crystal (PDLC) was used as a dielectric layer of electroluminescent (EL) device to provide multi-function of electroluminescence and electro-optic shutter. A 50 μm-thick PDLC layer was formed between a transparent electrode and a ZnS:Cu phosphor layer. The electro-optic properties of the EL device were not distorted by the introduction of the PDLC layer. The extraction efficiency of luminescence was improved by more than 14% by PDLC layer. The transmittance of the PDLC was also founded not to be degraded significantly by excitation frequency. Therefore, the electroluminescence of the device was ignited by excitation frequency at a given voltage for full transparency of the PDLC. This device has great potential for applications in transparent displays with the function of a privacy window. © 2012 Optical Society of America.1

Electrically tunable birefringence of a polymer composite with long-range orientational ordering of liquid crystals

We report an optical film with electrically tunable birefringence in which the liquid crystals (LCs), mixed with the host polymer, form longrange ordering. The film was prepared through polymerization without phase separation between the LCs and polymers. Driving voltage below 30 V for full switching of birefringence is achieved in a 6 μm-thick film. Electro-optical investigations for the film suggest that the long-range ordering of the LCs mixed in the film caused by polymerization lead to rotations of the LCs as well as optical anisotropy in the film. These films with electrically tunable birefringence could have applications as flexible light modulators and phase retardation films for 2D-3D image switching. © 2014 Optical Society of America.1

Machine Learning Feature Extraction Based on Binary Pixel Quantification Using Low-Resolution Images for Application of Unmanned Ground Vehicles in Apple Orchards

Deep learning and machine learning (ML) technologies have been implemented in various applications, and various agriculture technologies are being developed based on image-based object recognition technology. We propose an orchard environment free space recognition technology suitable for developing small-scale agricultural unmanned ground vehicle (UGV) autonomous mobile equipment using a low-cost lightweight processor. We designed an algorithm to minimize the amount of input data to be processed by the ML algorithm through low-resolution grayscale images and image binarization. In addition, we propose an ML feature extraction method based on binary pixel quantification that can be applied to an ML classifier to detect free space for autonomous movement of UGVs from binary images. Here, the ML feature is extracted by detecting the local-lowest points in segments of a binarized image and by defining 33 variables, including local-lowest points, to detect the bottom of a tree trunk. We trained six ML models to select a suitable ML model for trunk bottom detection among various ML models, and we analyzed and compared the performance of the trained models. The ensemble model demonstrated the best performance, and a test was performed using this ML model to detect apple tree trunks from 100 new images. Experimental results indicate that it is possible to recognize free space in an apple orchard environment by learning using approximately 100 low-resolution grayscale images. © 2020 by the authors.1

Enhanced blue photoluminescence realized by copper diffusion doping of ZnO thin films

ZnO thin films with blue photoluminescence (PL) have been fabricated through Cu diffusion doping. A CuOx-ZnO mixture, and Cu/ZnO double layer, films were prepared on amorphous SiOx/Si substrates by pulsed laser deposition (PLD), and electron beam (e-beam) deposition, respectively. After sequential oxygen annealing, CuOx-ZnO mixture films exhibited green emission centered at 523 nm. However, Cu/ZnO double layer films differed in producing a blue emission centered at 480 nm. Detailed analysis identified that this blue shift in the emission center resulted from increased blue emissions attributed to Cu dopants in the film by e-beam deposition. Luminescence intensity was increased to 6 cd/m2 for a sample annealed at 700 deg;C. Color points were close to the locus of points following the line of a black-body-radiator on the CIE 1931 XY chromaticity diagram. The present results show that Cu-doped ZnO has strong potential as a cost effective phosphor for use in down converting LEDs. © 2013 Optical Society of America.1

Luminance enhancement of electroluminescent devices using highly dielectric UV-curable polymer and oxide nanoparticle composite

A flexible hybrid structure electroluminescent (HSEL) device was fabricated from ZnS:Cu phosphor microparticles dispersed in a UVcurable polymer matrix. We observed a maximum luminance of 111 cd/m2 at 10 kHz and 170 V from a device wherein the mixing ratio between the phosphor particles and highly dielectric polymer binder was 70:30 wt%. Furthermore, by uniformly dispersing highly dielectric BaTiO3 nanoparticles within the polymer matrix, we were able to obtain a luminance of up to 211 cd/m2 in the HSEL device. Compared to the conventional thermal curing process, this UV process greatly simplifies the fabrication steps by combining phosphors and dielectric materials at room temperature. This process also demonstrates a promising pathway toward creating flexible and printed EL devices in the future. © 2014 Optical Society of America.1

Optical and photoelectric properties of Mn-doped ZnS thin film on a flexible indium-tin-oxide/polyethylene terephthalate substrate prepared by pulsed laser deposition

Optical and photoelectric properties of Mn-doped ZnS thin films on indium-tin-oxide (ITO)/polyethylene terephthalate (PET) substrates by pulsed laser deposition (PLD) were investigated. The XRD patterns revealed that the thin film deposited at room temperature (RT) had a wurtzite phase, which changed to a sphalerite phase at a substrate temperature of approximately 100°C. The transmittance of the films was approximately 87% in the visible range. The optical bandgap of the film deposited at RT was 3.29 eV, which increased to 3.361 eV with increasing substrate temperature to 200°C. The photoluminescence (PL) intensity at 468 nm and the photocurrent by UV irradiation increased in proportion to the substrate temperature. The present results imply that Mn-doped ZnS films deposited on flexible PET substrates are useful for fabricating flexible optoelectronic devices such as flexible UV detectors. © 2016 Optical Society of America.1

Thermoelectric materials by using two-dimensional materials with negative correlation between electrical and thermal conductivity

In general, in thermoelectric materials the electrical conductivity σ and thermal conductivity κ are related and thus cannot be controlled independently. Previously, to maximize the thermoelectric figure of merit in state-of-the-art materials, differences in relative scaling between σ and κ as dimensions are reduced to approach the nanoscale were utilized. Here we present an approach to thermoelectric materials using tin disulfide, SnS2, nanosheets that demonstrated a negative correlation between σ and κ. In other words, as the thickness of SnS2 decreased, σ increased whereas κ decreased. This approach leads to a thermoelectric figure of merit increase to 0.13 at 300 K, a factor ~1,000 times greater than previously reported bulk single-crystal SnS2. The Seebeck coefficient obtained for our two-dimensional SnS2 nanosheets was 34.7 mV K-1 for 16-nm-thick samples at 300 K.1

Highly Contrast Image Correction for Dim Boundary Separation of Image Semantic Segmentation

The efficiency and accuracy of the image semantic segmentation algorithm represent a trade-off relationship, and the loss of accuracy tends to increase as the model structure simplifies to improve efficiency. Developing more efficient and accurate algorithms requires methods to complement them. In this study, we applied the logarithmic-exponential mixture (LEM) function for gamma correction of images to improve the accuracy of image semantic segmentation. The basic model used in this work was produced by constructing a full convolution neural network based on MobileNetV2. To avoid the noise of input compression, we corrected training and validation images with gamma from 1/8 to 8 (7 different levels) before doing convolution. We evaluated models using Tensorflow deep-learning library based on Python. We compared models using LEM function to models using conventional gamma function. The prediction masks of the proposed model using the LEM function had relatively small fluctuations of accuracy upon gamma change. For images that have shadows overlapped on the object, the object was better distinguished in small gamma values. For dark images, the increase in accuracy was more effective. The results indicated that the proposed gamma correction could improve image segmentation accuracy in images with unclear edges. We believe that the presented results will guide further studies for accuracy improvement of image recognition algorithms applicable to future devices, such as autonomous vehicles and mobile robots. © IEEE. CCBYTRU

Metal-Insulator Transition Detection of Vanadium Dioxide Thin Films by Visible Light Reflection

Vanadium dioxide (VO2)-based thin films have received considerable attention in recent years due to their superior performance in creating next-generation color-rendering materials. The near-room-temperature metal-insulator transition of VO2 promises the advantage of active color tuning in the visible wavelength range. Although various results of dynamic color generation combined with plasmonic nanostructures are currently being investigated, so far, very few studies have addressed the visible-light optical performance of pure VO2 thin films prepared on conventional substrates. This article shows in detail the phase transition behavior of VO2 thin films in the visible wavelength range of 400-750 nm prepared on glass with subsequent annealing at temperatures of 350, 400, 450, and 500 degrees C. The results show an anomalous phase transition reducing the overall RGB reflectivity correlated with the crystallization behavior of the VO2 phase and scattering effect. The sample annealed at 350 degrees C shows the smallest phase transition at 47 degrees C, correlating with a crystallite size of 7 nm. The blue band reflectivity anomaly after annealing at 450 degrees C was considered an effect of the secondary reflection. The results of this research could play a huge role in the production of active switching photonic devices, color-managed reflectors, and temperature indicators. © 2022 American Chemical Society. All rights reserved.FALS