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In recent years, blockchains have obtained so much attention from researchers, engineers, and institutions; and the implementation of blockchains has started to revive a large number of applications ranging from e-finance, e-healthcare, smart home, Internet of Things, social security, logistics and so forth. In the literature on blockchains, it is found that most articles focused on their engineering implementation, while little attention has been devoted to the exploration of theoretical aspects of the system; however, the existing work is limited to model the mining process only. In this paper, a queuing theory-based model is proposed for understanding the working and theoretical aspects of the blockchain. We validate our proposed model using the actual statistics of two popular cryptocurrencies, Bitcoin and Ethereum, by running simulations for two months of transactions. The obtained performance measures parameters such as the Number of Transactions per block, Mining Time of Each Block, System Throughput, Memorypool count, Waiting Time in Memorypool, Number of Unconfirmed Transactions in the Whole System, Total Number of Transactions, and Number of Generated Blocks; these values are compared with actual statistics. It was found that the results gained from our proposed model are in good agreement with actual statistics. Although the simulation in this paper presents the modeling of blockchain-based cryptocurrencies only, the proposed model can be used to represent a wide range of blockchain-based systems. [ABSTRACT FROM AUTHOR]

Producing stable semiconducting thin films with low band gap energy by a viable technique is challenging for solar energy harvesting. Herein, Bi-doped Sb 2 S 3 thin films have been deposited on glass substrates by chemical bath deposition (CBD) technique followed by annealing at 400 °C. X-ray diffraction (XRD) patterns confirmed the orthorhombic phase of Sb 2 S 3 and successful incorporation of Bi+3 in the host lattice. Raman spectroscopy revealed the characteristic vibrational modes of Sb 2 S 3. Surface of films became smoother and compact with increasing Bi contents. Samples showed wide absorption range in the visible region. Band gap energy (E g) values were estimated using Tauc's relation and a blue shift in the absorption edge was observed with addition of Bi, thus E g increased from 1.58 to 1.71 eV. Higher values of absorption coefficient and absorption edge in the visible region suggest that the prepared Sb 2 S 3 thin films can be a right choice of an absorber layer. [Display omitted] • A novel approach for very smooth and large area thin films. • The deposited thin films show very high value of absorption coefficient, 4.0 x 10-4 cm-1. • The prepared thin films show excellent absorption in visible region. [ABSTRACT FROM AUTHOR]

To develop potential phosphors used in ultraviolet (UV) based white light-emitting diodes (WLEDs), a novel red Na 3.6 Y 1.8 (PO 4) 3 :Eu3+ (NYP:Eu3+) phosphor was successfully prepared by a solid-state method. Rietveld refinement analysis determined that NYP has a cubic unit cell with space group Pm -3m. The structural properties were analyzed in detail by employing X-ray diffraction (XRD), transmission electron microscopy (TEM) and energy dispersive spectrometry. Scanning electron microscope (SEM) image revealed the irregular morphology of NYP:Eu3+ particles and elemental mapping verified that Eu3+ ions were uniformly distributed on Y3+ sites. Photoluminescence (PL) spectra exhibited strong red emission under the excitation of 394 nm and optimal Eu3+ doping concentration in the NYP host was determined to be 30 mol%. The critical distance was calculated to be about 13.954 Å and dipole-dipole interaction was responsible for the concentration quenching mechanism. NYP:Eu3+ phosphor demonstrated high thermal stability with emission intensity at 424 K was 97.4% of that at 298 K. Moreover, by employing the title red, green (Ba,Sr) 2 SiO 4 :Eu2+, blue BaMgAl 10 O 17 :Eu2+ (BAM:Eu2+) phosphors and near UV (NUV) light-emitting diode chip, a WLED device was fabricated, which possessed excellent CIE chromaticity coordinates of (0.3351, 0.3499) and high color rendering index (Ra = 87.5). Image 1 • Na 3.6 Y 1.8 (PO 4) 3 has a body-centered cubic unit cell with space group Pm -3m. • The NYP:Eu3+ has better emission intensity than commercial red phosphors. • The prepared phosphor exhibits excellent thermal stability. • WLED device possesses high Ra = 87.5 and excellent CIE (0.33, 0.34). [ABSTRACT FROM AUTHOR]

In this work, Mn 2+ -doped ZnS nanorods were synthesized by a facile hydrothermal method. The morphology, structure, and composition of the as-prepared samples were investigated. The temperature-dependent photoluminescence of ZnS:Mn nanorods was analyzed, and the corresponding activation energies were calculated by using a simple two-step rate equation. Mn 2+ -related orange emission ( 4 T 1 → 6 A 1 ) demonstrates high stability and is comparatively less affected by the temperature variations than the defect-related emission. A metal-semiconductor-metal junction ultraviolet photodetector based on the nanorod networks has been fabricated by a cost-effective method. The device exhibits visible blindness, superior ultraviolet photodetection with a responsivity of 1.62 A/W, and significantly fast photodetection response with the rise and decay times of 12 and 25 ms, respectively., Competing Interests: The authors declare no competing financial interest., (© 2021 The Authors. Published by American Chemical Society.)

A comprehensive strategy has been developed to construct nano-sized homogeneous and heterogeneous core/shell structures of NaYF 4 host. Synthesis conditions of cubic phase/α-NaYF 4 and hexagonal phase/β-NaYF 4 are discussed. Pure cubic NaYF 4 :Yb,Er nanocrystals were synthesized with different average sizes extending from 7 nm to 15 nm by varying the reaction time. Temperature and time thresholds of hexagonal nucleation were determined and utilized for controlled core/shell structures of different phases. α-NaYF 4 :Yb,Er@α-NaYF 4 , α-NaYF 4 :Yb,Er@β-NaYF 4 , β-NaYF 4 :Yb,Er@α-NaYF 4 , and β-NaYF 4 :Yb,Er@β-NaYF 4 core/shell structures were prepared by adopting the required conditions to achieve the desired phase. Excess sodium was used to grow hexagonal shell over metastable cubic core under controlled conditions of reaction time and temperature to prevent the structural transition of the core. Upconversion emission spectra have also been obtained. UCL integrated intensities demonstrated about 5-fold enhancement for α-shell over α-core as compared to the core alone and 22-fold enhancement with β-shell. On the other hand, α-shell over β-core exhibited 5-fold enhancement and β-shell over β-core exhibited 6-fold enhancement., Competing Interests: There are no conflicts to declare., (This journal is © The Royal Society of Chemistry.)

Wide-range optical thermal sensing is achieved here based on the two-photon upconversion luminescence of the high-temperature (HT) cubic phase NaYF 4 :Yb, Er. In the range of room temperature to 973 K, the single-phase sample exhibits two bands of green and red emission with different dependences on the temperature. The CIE chromaticity diagram shows that the color point moves from deep red (0.6357, 0.3501) at room temperature to the yellow region (0.4379, 0.475) at 600 K and then to the green region (0.318, 0.669) at 973 K. It reveals that HT cubic phase NaYF 4 :Yb, Er is the promising ratiometric and colorimetric luminescent thermometer. The relative sensitivity decreases slightly up to 673 K and then increases with the increasing temperature. The lattice expansion of the HT cubic phase alters the crystal symmetry around the activator ion and further increases the green-to-red emission ratio.