Oxygen vacancies induced photoluminescence in SrZnO2 nanophosphors probed by theoretical and experimental analysis

We report, for the first time, the influence of oxygen vacancies on band structure and local electronic structure of SrZnO2 (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 vacancies on near edge structure

Дослідження сплавів Fe1 – xAlx, подрібнених у кульовому млині, за допомогою TEM та XPS

Техніка подрібнення в кульовому млині широко використовується для отримання різних метастабільних станів з нанокристалічними мікроструктурами з інтерметалічних сполук. У статті автори систематично досліджували структурні та електронні властивості серії механічно легованих зразків Fe1 – xAlx (0.3 ≤ x ≤ 0.6) за допомогою просвічуючої електронної мікроскопії (TEM) та рентгенівської фотоелектронної спектроскопії (XPS). Процес подрібнення в кульовому млині викликає протікання твердотільних реакцій, яким сприяє сильна пластична деформація, що призводить до зменшення розміру кристалітів і цікавих мікроструктурних та електронних змін у отриманій системі. Результати TEM показують, що розмір кристалітів зменшується до нанометрового діапазону (між 6-8 нм) як функція x, і метали розчиняються на межах нанозерен. Через нанометричні розміри реакційна здатність підвищується в результаті збільшення співвідношення поверхні до об'єму, що призводить до утворення фази сплаву FeAl. Оглядове сканування XPS показує, що зразки не мають серйозного забруднення, а спектри рівня ядра демонструють невеликий зсув піків Fe2p та Al2p у бік вищої енергії зв'язку (BE), що доводить, що різні багаті на Fe та Al фази сплаву FeAl утворилися після 5 годин подрібнення.The ball milling technique has been extensively used to prepare different metastable states with nanocrystalline microstructures from intermetallic compounds. In the present manuscript, the authors have systematically investigated the structural and electronic properties of a series of mechanically alloyed Fe1 – xAlx (0.3 ≤ x ≤ 0.6) samples using transmission electron microscopy (TEM) and X-ray photoelectron spectroscopy (XPS). The ball milling process causes the formation of solid-state reactions that are aided by severe plastic deformation, resulting in reduced crystallite size and interesting microstructural and electronic changes in the resulting system. The TEM results show that the crystallite size decreases to the nanometer range (between 6-8 nm) as a function of x and the metals dissolve at the nanograin boundaries. As a result of nanometric dimensions, the reactivity increases as a result of the increased surface-to-volume ratio, which leads to the FeAl alloy phase formation. The XPS survey scan shows that the samples do not have any major contamination, and the core level spectra show a slight shift of Fe2p and Al2p peaks toward higher binding energy (BE), which proves that different Fe- and Al-rich phases of FeAl alloy have formed after 5 h of milling

Mechanistic insights into defect generation and tuning of optical properties in Zn1-xFexAl2O4 (0.01 <= x <= 0.40) nanocrystals

The correlation of several defects and optical and magnetic properties with Fe content in Zn1-xFexAl2O4 (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 Fe-i(center dot center dot center dot), Al-Zn(center dot) and Fe-Al(x) 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 sites. A transition is observed from paramagnetism to superparamagnetism at higher Fe concentrations. A visual colour change from pearly white to orange-brown is observed in Zn1-xFexAl2O4 nanocrystals with increasing Fe content, revealing its potential candidature for pigments in the paint and dye industries

Enhanced near-infrared luminescence in zinc aluminate bestowed by fuel-blended combustion approach

Fuel-blend combustion method using urea and monoethanolamine (MEA) fuels, is employed to tune the defect states and hence luminescence properties of nanocrystalline ZnAl2O4. Prepared nanocrystals flaunt a ravishing blue and intensified near-infrared (NIR) emission with varying intensities. Sample prepared using 80% MEA blended with 20% urea exhibits highest NIR emission, seven-fold intense than that prepared by 100% MEA, attributed to larger amount of deep band gap defects. Presence of multiple defects viz. vacancies of zinc, aluminium and oxygen is probed by means of Rietveld refinement, X-ray absorption near edge spectroscopy and photoluminescence spectroscopy. Band tailing and presence of shallow defect states are indicated by optical band gap and Urbach energy values. Broad range visible to NIR emission envisages application of ZnAl2O4 nanocrystals for bioimaging purposes. Radiative transitions among various defect levels contributing toward the emission have been depicted through a band model diagram

X-ray spectroscopy study of ZnxSn1-xO2 nanorods synthesized by hydrothermal technique

An environment friendly, inexpensive solvothermal route is used to synthesize polycrystalline ZnxSn1-xO2 (0.0 &lt;= x &lt;= 0.07) nanorods with nanoflower morphology without using any organic solvents, surfactant or any catalytic agent under low hydrothermal pressure. Scanning electron microscopy (SEM) equipped with energy-dispersive x-ray spectra depicts the formation of near stoichiometric ZnxSn1-xO2 nanorods. SEM analysis reveals that Zn incorporation in SnO2 decreases the nanorod diameter from similar to 20 nm to similar to 10 nm. High-resolution x-ray diffraction analysis reveals the rutile structure for x &lt;= 0.04 samples, while for x = 0.07 a Zn2SnO4 phase is observed. Near edge x-ray absorption fine structure and x-ray photoemission spectroscopy indicate that Zn atoms have been substituted for Sn sites in SnO2 lattice without forming secondary phases and also create numerous oxygen vacancy in the structure