van der Waals epitaxy of TMDCs and their applications.
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We report a MoS2/GaN heterojunction-based gas sensor by depositing MoS2 over a GaN substrate via a highly controllable and scalable sputtering technique coupled with a post sulfurization process in a sulfur-rich environment. The... more
We report a MoS2/GaN heterojunction-based gas sensor by depositing MoS2 over a GaN substrate via a highly controllable and scalable sputtering technique coupled with a post sulfurization process in a sulfur-rich environment. The microscopic and spectroscopic measurements expose the presence of highly crystalline and homogenous few atomic layer MoS2 on top of molecular beam epitaxially grown GaN film. Upon hydrogen exposure, the molecular adsorption tuned the barrier height at the MoS2/GaN interface under the reverse biased condition, thus resulting in high sensitivity. Our results reveal that temperature strongly affects the sensitivity of the device and it increases from 21% to 157% for 1% hydrogen with an increase in temperature (25-150 °C). For a deeper understanding of carrier dynamics at the heterointerface, we visualized the band alignment across the MoS2/GaN heterojunction having valence band and conduction band offset values of 1.75 and 0.28 eV. The sensing mechanism was demonstrated based on an energy band diagram at the MoS2/GaN interface in the presence and absence of hydrogen exposure. The proposed methodology can be readily applied to other combinations of heterostructures for sensing different gas analytes.
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We demonstrate a highly selective and reversible NO2 resistive gas sensor using vertically aligned MoS2 (VA-MoS2) flake networks. We synthesized horizontally and vertically aligned MoS2 flakes on SiO2/Si substrate using kinetically... more
We demonstrate a highly selective and reversible NO2 resistive gas sensor using vertically aligned MoS2 (VA-MoS2) flake networks. We synthesized horizontally and vertically aligned MoS2 flakes on SiO2/Si substrate using kinetically controlled rapid growth method of CVD process. Uniformly interconnected MoS2 flakes and their orientation were confirmed by scanning electron microscopy, X-ray diffraction, Raman spectroscopy, and X-ray photoelectron spectroscopy. The VA-MoS2 gas sensor showed two times higher response to NO2 as compared to horizontally aligned MoS2 at room temperature. Moreover, the sensors exhibited a dramatically improved complete recovery upon NO2 exposure at its low optimum operating temperatures (100°C). In addition, sensing performance of the sensors was investigated with exposure to various gases such as NH3, CO2, H2, CH4, and H2S. It was observed that high response to gas directly correlates the strong interaction of gas molecules on edge sites of the VA-MoS2. The VA-MoS2 gas sensor exhibited high response with good reversibility and selectivity towards NO2 as a result of high aspect ratio as well as high adsorption energy on exposed edge sites.
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The possibility to synergise two-dimensional (2D) materials with 0D nanoparticles has sparked a surge in high performance futuristic electronic devices. Here, we decorated plasmonic Au nanoparticles on surface of chemical vapor deposition... more
The possibility to synergise two-dimensional (2D) materials with 0D nanoparticles has sparked a surge in high performance futuristic electronic devices. Here, we decorated plasmonic Au nanoparticles on surface of chemical vapor deposition (CVD) grown 2D MoS<sub>2</sub> nanosheet and demonstrated bifunctional sensing behaviour within a single device. The plasmonic Au nanoparticles functionalized MoS<sub>2</sub> device showed about ~5 times higher sensitivity to NO<sub>2</sub> than that of pristine MoS<sub>2</sub> at room temperature. The enhanced gas sensing performance was attributed to a combination of Schottky barriers modulation at Au/MoS<sub>2</sub> nanointerfaces and catalytic effects upon exposing the gas analyte. In addition, the device also exhibited enhanced photoresponse with a high photo-responsivity of ~17.6 A/W and a moderate detectivity of ~<inline-formula> <tex-math notation="LaTeX">${6.6} \times {10}^{11}$ </tex-math></inline-formula> Jones due to enhanced local plasmonic effects. Finally, photons and gas molecules are detected in sequence, which proved that only a single Au-MoS<sub>2</sub> device exhibited remarkable bifunctional sensing characteristics. Such excellent bifunctional sensing ability of a single Au-MoS<sub>2</sub> device paves the way to integrate the 2D material with plasmonic nanostructures for developing an advanced multifunctional sensor.
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The slow evaporation solution growth technique (SEST) was used to successfully grow inorganic single crystals of sodium sulphamate (NaS) in ambient conditions. The titled crystal’s structural and optical characteristics were evaluated... more
The slow evaporation solution growth technique (SEST) was used to successfully grow inorganic single crystals of sodium sulphamate (NaS) in ambient conditions. The titled crystal’s structural and optical characteristics were evaluated using a variety of spectroscopic techniques. Powder X-ray diffraction (PXRD) analysis and Fourier transform infrared (FT-IR) spectroscopy were used to examine the crystal structure characteristics. Using UV-Vis spectroscopy, the optical properties of the NaS crystal were carefully examined, and the optical bandgap was determined using Tauc’s plot. The HR-XRD analysis examined crystalline perfection. Additionally, photoluminescence spectroscopy determined the emission spectra, and time-resolved photoluminescence measured the decay rate. The [Formula: see text]-scan technique was also used to examine the titled crystal’s nonlinear optical (NLO) performance.
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This work focuses on the bulk growth of Itaconic acid (ITA) single crystal from aqueous solution by slow evaporation solution growth technique. The detailed structural, optical, laser studies have been carried out for the title compound... more
This work focuses on the bulk growth of Itaconic acid (ITA) single crystal from aqueous solution by slow evaporation solution growth technique. The detailed structural, optical, laser studies have been carried out for the title compound by adopting different instrumentation techniques. The cut-off wavelength of ITA crystal was obtained at 268 nm shows good nonlinear response. From the PL measurement, the high intense emission peak was observed at 442 nm corresponds to blue emission. The dominant emission colour of the grown specimen was estimated by CIE plot. The fast and slow decay of ITA single crystal were studied by time resolved PL. The laser resistive nature was examined by Nd-YAG laser and found that it is having good laser damage threshold (LDT) property. The third-order nonlinear optical behaviour of ITA crystal has been analyzed with the help of a femto-second laser using Z-scan technique. The value of various parameters like nonlinear refractive index (n2) and nonlinear absorption coefficient (β) is estimated using the theoretical calculations. The values of n2 and β were found to be 3.13 ×\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\times$$\end{document} 10–17 cm2/W and 1.57 ×\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\times$$\end{document} 10–12 cm/W, respectively. Such values of nonlinear parameters shows that grown crystal can be useful in optoelectronics applications.
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The fabrication of unique taper-ended GaN-Nanotowers structure based highly efficient ultraviolet photodetector is demonstrated. Hexagonally stacked, single crystalline GaN nanocolumnar structure (nanotowers) grown on AlN buffer layer... more
The fabrication of unique taper-ended GaN-Nanotowers structure based highly efficient ultraviolet photodetector is demonstrated. Hexagonally stacked, single crystalline GaN nanocolumnar structure (nanotowers) grown on AlN buffer layer exhibits higher photocurrent generation due to high quality nanotowers morphology and increased surface/volume ratio which significantly enhances its responsivity upon ultraviolet exposure leading to outstanding performance from the developed detection device. The fabricated detector display low dark current (~ 12 nA), high ILight/IDark ratio (> 104), fast time-correlated transient response (~ 433 µs) upon ultraviolet (325 nm) illumination. A high photoresponsivity of 2.47 A/W is achieved in self-powered mode of operation. The reason behind such high performance could be attributed to built-in electric field developed from a difference in Schottky barrier heights will be discussed in detail. While in photoconductive mode, the responsivity is observe...
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This study reports host-controlled luminescence from 4f0 and 4f 1 Ce levels in SrZnO2, probed through x-ray absorption near edge and photoluminescence spectroscopies. O K-edge perceived the presence of Ce 4f states at the bottom of... more
This study reports host-controlled luminescence from 4f0 and 4f 1 Ce levels in SrZnO2, probed through x-ray absorption near edge and photoluminescence spectroscopies. O K-edge perceived the presence of Ce 4f states at the bottom of conduction band and 5d states merging with conduction band. Due to such an arrangement, a fast radiative emission (11.15 ns, average) is observed through charge transfer excitation from host to empty 4f0 Ce levels. Ce content is found to vary the site occupancy, which is then modifying the population of 4f0 and 4f 1 energy states by altering Ce–O bond covalency. This study proposes the Ce-doped SrZnO2 system as a potential system for scintillators and lighting materials.
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Facile syntheses of 3D-triangular GaN nano prism islands (TGNPI) were grown on Si(553) surface by sputter-gun technique. TGNPI exhibit ultra-violet luminescence emission at 366 nm upon 325 nm excitation wavelength.
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A systematic study has been performed to correlate structural, optical and electrical properties with defect states in the GaN films grown on a-plane (112̄0) sapphire substrate via rf-plasma molecular beam epitaxy.
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Epitaxial GaN nanostructures are developed, and the influence of the AlN buffer layer (temperature modulation) on material characteristics and optoelectronic device application is assessed. The AlN buffer layer was grown on a Si (111)... more
Epitaxial GaN nanostructures are developed, and the influence of the AlN buffer layer (temperature modulation) on material characteristics and optoelectronic device application is assessed. The AlN buffer layer was grown on a Si (111) substrate at varying temperatures (770–830 °C), followed by GaN growth using plasma-assisted molecular beam epitaxy. The investigation revealed that the comparatively lower temperature AlN buffer layer was responsible for stress and lattice strain relaxation and was realized as the GaN nano-obelisk structures. Contrarily, the increased temperature of the AlN growth led to the formation of GaN nanopyramidal and nanowax/wane structures. These grown GaN/AlN/Si heterostructures were utilized to develop photodetectors in a metal–semiconductor–metal geometry format. The performance of these fabricated optoelectronic devices was examined under ultraviolet illumination (UVA), where the GaN nano-obelisks-based device attained the highest responsivity of 118 AW−...
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Broadband photodetectors operable under harsh temperature conditions are crucial optoelectronic components to support ongoing and futuristic technological advancement. Conventional photodetectors are limited to room temperature operation... more
Broadband photodetectors operable under harsh temperature conditions are crucial optoelectronic components to support ongoing and futuristic technological advancement. Conventional photodetectors are limited to room temperature operation due to the thermal instability of semiconductors under harsh conditions and incapable of covering the ultraviolet (UV) spectrum due to narrow bandgap properties. Gallium nitride (GaN) is a wide bandgap and thermally stable semiconductor, ideal for addressing the abovementioned limitations. Here, epitaxial honeycomb nanostructured GaN film is grown via a plasma‐assisted molecular beam epitaxy system and deployed for stable broadband photodetectors, which can be operated from −75 to 250 °C. Further, spectral response is investigated for a broad spectrum from UV (280 nm) to near‐infrared (850 nm) region. It displays a peak responsivity at 365 nm associated to the bandgap energy of GaN. Fabricated photodetectors with honeycomb‐like nanostructures drive peak responsivity and external quantum efficiency of 2.41 × 106 AW−1 and 8.18 × 108%, respectively, when illuminated at a power density of 1 mWcm−2 and 365 nm wavelength source under 1 V bias. Temperature‐correlated spectral response presents a quenching of responsivity at higher temperatures in visible spectrum associated with the thermal quenching of defect states. The thermally stable and efficient broadband photodetector based on honeycomb‐like nanostructured GaN is promising for the combustion industry, arctic science, and space explorations.
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A bulk size transparent nonlinear organic single crystal of Iminodiacetic acid was harvested using slow evaporation solution growth technique in controlled atmosphere with a span of 4 weeks. The structural properties of titled crystal... more
A bulk size transparent nonlinear organic single crystal of Iminodiacetic acid was harvested using slow evaporation solution growth technique in controlled atmosphere with a span of 4 weeks. The structural properties of titled crystal were examined by single-crystal X-ray diffraction. The quality of the ingot was determined by high-resolution X-ray diffraction and found that the sample is free from grain boundaries. From UV–Vis analysis, one can understand that there is less absorption in the entire wavelength range of visible region and observed cut-off wavelength is 242 nm. From photoluminescence analysis, the emission of wavelength is identified at 421 nm. Mechanical strength and various parameters like work-hardening coefficient and stiffness constant of ingot was measured using Vickers microhardness test and corresponding experimental data had been explained using distinct theoretical models. The variation measurement of dielectric constant and dielectric loss in respect of frequency was carried out to compute the electronic polarizability of titled compound using Penn model. The detailed analysis of third-harmonic generation was executed using Z-scan method. The values of nonlinear refractive index ( n 2 ) and two photon absorption coefficient ( β ) were obtained to be 2.19 × 10 −17 cm 2 W −1 and 4.25 × 10 −12 cm W −1 , respectively. The higher value of coupling factor suggests nonlinear absorption is more dominant and the crystal is suitable for optical limiting application.
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The correlation of several defects and optical and magnetic properties with Fe content in Zn1−x Fe x Al2O4 (0.01 ≤ x ≤ 0.40) nanocrystals has been scrutinized through X-ray diffraction, O K-edge X-ray absorption near-edge structure,... more
The correlation of several defects and optical and magnetic properties with Fe content in Zn1−x Fe x Al2O4 (0.01 ≤ x ≤ 0.40) nanocrystals has been scrutinized through X-ray diffraction, O K-edge X-ray absorption near-edge structure, FT–IR, diffuse reflectance, photoluminescence and electron spin-resonance spectroscopies, and vibrating sample magnetometry. Increasing Fe content causes elongation in the octahedral units of the lattice, accompanied by distortion in the octahedral coordination. Fe introduces non-radiative centres in the forbidden gap, thereby tuning the band gap from 4.37 to 3.88 eV and eliminating emission in the visible region. Zn vacancies are found to tail off, while {\rm Fe}_i^{\bullet \bullet \bullet}, {\rm Al}_{\rm Zn}^\bullet and FeAl × antisite defects increase in concentration with increasing Fe content. Inhomogeneous broadening of spin-resonance signals infers strong spin-lattice interactions of Fe3+ ions at distorted octahedral and non-symmetric tetrahedral ...
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This article highlights the emerging demand for gallium nitride (GaN) semiconductor technology that offers superior optoelectronic properties making it suitable for futuristic ultraviolet (UV) photodetection applications.
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Abstract Gallium oxide (Ga2O3) has emerged as a fourth-generation semiconductor for futuristic device requirements. Integration of Ga2O3 with industry-viable gallium nitride (GaN) can provide a pathway to design efficient device... more
Abstract Gallium oxide (Ga2O3) has emerged as a fourth-generation semiconductor for futuristic device requirements. Integration of Ga2O3 with industry-viable gallium nitride (GaN) can provide a pathway to design efficient device technology. In this paper, we design Ga2O3/GaN heterointerface by using a simple thermal annealing method under an oxygen-rich environment. Thermal annealing at 850 °C for 5 h results in a good yield of Ga2O3 from surficial GaN which was grown on sapphire and Si substrates. Surface morphology revealed a nanorod structure-based Ga2O3 on the GaN surface grown on industrial compatible Si substrate. XPS measurements provide a quantitative understanding of the heterostructure, where 84.62% and 70.92% of surficial GaN is converted into Ga2O3. Besides, the valence band maximum is shifted to a higher energy side in comparison to bare GaN samples. This helps in understanding the device physics of the grown heterostructures. Current-Voltage (I–V) characteristics revealed Schottky behaviour where the Schottky barrier height is increased after thermal annealing of the GaN films. Temperature correlated I–V characteristics suggest that the thermally annealed GaN films are stable up to 300 °C. These material structures can potentially be used in the high-temperature applications for power devices, optoelectronics, and sensing applications.
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Introduction H2S is colorless, highly flammable and a toxic gas. It has a short (10 minutes) and long term (8 hours) exposure limit of 15 and 10 ppm respectively. Although, H2S odor can be detected by human nose at a very low... more
Introduction H2S is colorless, highly flammable and a toxic gas. It has a short (10 minutes) and long term (8 hours) exposure limit of 15 and 10 ppm respectively. Although, H2S odor can be detected by human nose at a very low concentration (0.02 ppm) but the sense of smell is lost in just a few minutes after exposure due to olfactory fatigue [1]. This makes it very difficult to sense dangerous concentrations of H2S which could be sometimes lethal. Hence, it is crucial to detect H2S in ppm or sub ppm range. Various metal oxides-based materials have been employed for H2S detection owing to their simplicity, ease of production and low cost [2-6]. Among such oxide materials, WO3-x is versatile material which has been used for detection of various toxic gases including H2S owing to mixed oxidation state of W (W4+, W5+, W6+) [7,8]. The optimization of the sensing material has to go through a series of steps in which film thickness, morphology, grain size etc. are the key aspects which req...
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Methylammonium lead bromide (CH3NH3PbBr3) colloidal quantum dots (QDs) exhibit strong green photoluminescence (PL) with high photoluminescence quantum yield (PLQY) making it valuable for various optoelectronic applications. Under the... more
Methylammonium lead bromide (CH3NH3PbBr3) colloidal quantum dots (QDs) exhibit strong green photoluminescence (PL) with high photoluminescence quantum yield (PLQY) making it valuable for various optoelectronic applications. Under the influence of polar gaseous molecules, hybrid halide perovskites show changes in its structural and electrical properties. We, for the first time, have investigated the influence of NH3 gas molecules on the optical properties of CH3NH3PbBr3 colloidal QDs. The investigations carried out under a controlled environment reveal that even the presence of 37 ppm of ammonia (NH3) gas molecules causes a significant reduction in the PL intensity of CH3NH3PbBr3 colloidal QDs. The reduction rate of PL intensity can be tuned with the concentration of NH3 gas molecules. We propose that the decrease in PL intensity is because of the formation of a non-luminescent NH4PbBr3 phase under the presence of NH3 gas molecules. Further, the non-luminescent NH4PbBr3 retransformed into luminescent CH3NH3PbBr3 on the introduction of methylamine (CH3NH2) gas molecules. This reversible alternation in PL properties enables us to demonstrate its application for (NH3) gas sensing. The advantage of using CH3NH3PbBr3 colloidal QDs for luminescence-based sensing is that its green emission is visible with the naked eye even under daylight, which is easy to detect.
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Abstract The effect of partial (1–3%) rare-earth Gadolinium doping in D 0 22 Mn 3 Ga hard magnet has been investigated in this work. The Mn 3 Ga undergoes magnetic transition at T c ∼ 745 K, which decreases down to T c ∼ 715 K upon 1%... more
Abstract The effect of partial (1–3%) rare-earth Gadolinium doping in D 0 22 Mn 3 Ga hard magnet has been investigated in this work. The Mn 3 Ga undergoes magnetic transition at T c ∼ 745 K, which decreases down to T c ∼ 715 K upon 1% Gd-doping with introduction of mixed Mn valence states, as revealed from XPS study. The hard magnetic behavior of Mn 3 Ga has been observed in a wide temperature range (up to T c ). More importantly, the partial Gd-doping significantly enhances the room temperature hard magnetic properties i.e. squareness ratio ( m r / m s ), coercivity ( H c ) and energy product ( BH max ) from 0.43, 2.73 kOe and 0.39 MGOe ( Mn 3 Ga ) to 0.57, 4.25 kOe and 0.5 MGOe ( Mn 2.97 Gd 0.03 Ga ), respectively. The modified Stoner-Wohlfarth model and low-field minor loop analysis reveal the nucleation hardening mechanism for the magnetization reversal. The study proposes the partial rare-earth doping as a new approach to enhance the hard magnetic properties of rare-earth free hard magnets.
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In this study, the use of the novel 1,6,7,10-tetramethylfluoranthene as an acceptor in the organic solar cell has been demonstrated.
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Abstract Homogenous Ag–Au alloy nanoparticles having an average size of 12±2 nm were successfully prepared by the exploding wire technique comprising of a wire–plate system and using 12 V batteries. The X-ray photoelectron spectroscopy... more
Abstract Homogenous Ag–Au alloy nanoparticles having an average size of 12±2 nm were successfully prepared by the exploding wire technique comprising of a wire–plate system and using 12 V batteries. The X-ray photoelectron spectroscopy data reveal the formation of alloy nanoparticles with Ag80-Au20 composition, which agrees with the absorption data, obtained using UV-Visible spectroscopy. XPS also reveals a thin metal-oxide shell on the metallic alloy core. These alloy nanoparticles show visible fluorescence emission that was ...
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We report, for the first time, the influence of oxygen vacancies on band structure and local electronic structure of $$\hbox {SrZnO}_2$$ SrZnO 2 (SZO) nanophosphors by combined first principle calculations based on density functional... more
We report, for the first time, the influence of oxygen vacancies on band structure and local electronic structure of $$\hbox {SrZnO}_2$$ SrZnO 2 (SZO) nanophosphors by combined first principle calculations based on density functional theory and full multiple scattering theory, correlated with experimental results obtained from X-ray absorption and photoluminescence spectroscopies. The band structure analysis from density functional theory revealed the formation of new energy states in the forbidden gap due to introduction of oxygen vacancies in the system, thereby causing disruption in intrinsic symmetry and altering bond lengths in SZO system. These defect states are anticipated as origin of observed photoluminescence in SZO nanophosphors. The experimental X-ray absorption near edge structure (XANES) at Zn and Sr K-edges were successfully imitated by simulated XANES obtained after removing oxygen atoms around Zn and Sr cores, which affirmed the presence and signature of oxygen vaca...
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A good optical quality semi-organic single crystal of Potassium Acid Phthalate (KAP) was harvested from aqueous solution by using slow evaporation solution growth technique (SEST) at ambient condition. The defect free single crystal of... more
A good optical quality semi-organic single crystal of Potassium Acid Phthalate (KAP) was harvested from aqueous solution by using slow evaporation solution growth technique (SEST) at ambient condition. The defect free single crystal of KAP was analyzed by different instrumentation techniques, for checking its compatibility for different applications. Its unit cell dimensions and phase purity was examined by powder X-ray diffraction (PXRD) and found that it crystallizes in orthorhombic with non-centrosymmetric in nature. The quality of the grown ingot was assessed by high resolution X-ray diffraction (HRXRD) technique and found that, the crystalline quality is better with less grain boundaries. Its optical properties were scrutinized by UV–vis., photoluminescence (PL) and time resolved photoluminescence (TRPL) measurements respectively. The mechanical stability of the grown crystal was tested by Vickers microhardness method and found that the grown crystal shows indentation size effe...