Presence Of Defect Centers Research Articles

Chemistry with F Centers: Reduction of Organic Molecules on the Defective Potassium Chloride(100) Surface

The local electronic structure and the resulting reactivity of crystalline surfaces are significantly influenced by the presence of defect centers. The most prominent type of point defects of alkal...

Formation of stable charge regions in an array of germanium nanocrystallites inside SiO2 using electrostatic force microscopy

The objects of investigations are germanium nanocrystallites formed inside a SiO2 layer by ion implantation followed by high-temperature annealing. Germanium nanocrystallites inside a thin (30 nm) layer of thermal SiO2 oxide are charged by means of scanning probe microscopy, and the resulting charged zones are visualized and analyzed. It is shown that stable charge regions up to 30 nm in diameter that consist of several charged germanium nanocrystallites can be formed under the probe of an atomic-force microscope. It turns out that the ability of the germanium nanocrystallites to keep the induced charge is extremely sensitive to the intercrystallite distance and the presence of defect centers in the SiO2 layer. This is explained by the fact that the charge escapes from the germanium nanocrystallites by tunneling either between adjacent crystallites or through defect centers in the SiO2.

A comparative study of ultraviolet photoconductivity relaxation in zinc oxide (ZnO) thin films deposited by different techniques

Photoresponse characteristics of ZnO thin films deposited by three different techniques namely rf diode sputtering, rf magnetron sputtering, and electrophoretic deposition has been investigated in the metal-semiconductor-metal (MSM) configuration. A significant variation in the crystallinity, surface morphology, and photoresponse characteristics of ZnO thin film with change in growth kinetics suggest that the presence of defect centers and their density govern the photodetector relaxation properties. A relatively low density of traps compared to the true quantum yield is found very crucial for the realization of practical ZnO thin film based ultraviolet (UV) photodetector.

Effect of interface states associated with transitional nanophase precipitates in the enhancement of red emission from SrAl12O19:Pr3+ by Ti4+ incorporation

SrAl12O19:Pr3+, Ti4+ phosphor suitable for field emission displays is prepared by the wet chemical gel-carbonate method and the mechanism of enhancement in red photoluminescence (PL) intensity with Ti4+ therein has been investigated. The PL spectra of Pr3+ show both 1D2–3H4 and 3P0–3H6 emission in the red region with very weak intensity when excited at 355nm. The emission intensity has increased by about 100 times at room temperature in the compositional range SrAl12−xTixO19+x/2:Pr3+, with 0.1≤x≤0.3 in comparison to Ti-free SrAl12O19:Pr3+. TEM investigations show the presence of exsolved nanophase of SrAl8Ti3O19, the precipitation of which is preceded by the presence of defect centers at the interfacial regions between the semicoherent transient phase and the parent SrAl12O19 matrix. The presence of transitional nanophase and the associated defects modify the excitation–emission process by way of formation of electronic sub-levels at lower energy (3.5eV) than the band gap of SrAl12O19 (∼7eV) followed by non-resonance energy transfer to Pr3+ level, leading to magnetic-dipole related red emission with enhanced intensity. The PL intensity of Pr3+ decreases at high Ti4+ concentrations (x>0.3) due to higher extent of segregation of non-emissive SrAl8Ti3O19:Pr3+ phase.

Scanning electron microscopy, cathodoluminescence, and Raman spectroscopy of experimentally shock‐metamorphosed quartzite

Abstract— We studied unshocked and experimentally (at 12, 25, and 28 GPa, with 25, 100, 450, and 750°C pre‐shock temperatures) shock‐metamorphosed Hospital Hill quartzite from South Africa using cathodoluminescence (CL) images and spectroscopy and Raman spectroscopy to document systematic pressure or temperature‐related effects that could be used in shock barometry. In general, CL images of all samples show CL‐bright luminescent patchy areas and bands in otherwise nonluminescent quartz, as well as CL‐dark irregular fractures. Fluid inclusions appear dominant in CL images of the 25 GPa sample shocked at 750°C and of the 28 GPa sample shocked at 450°C. Only the optical image of our 28 GPa sample shocked at 25°C exhibits distinct planar deformation features (PDFs). Cathodoluminescence spectra of unshocked and experimentally shocked samples show broad bands in the near‐ultraviolet range and the visible light range at all shock stages, indicating the presence of defect centers on, e.g., SiO4 groups. No systematic change in the appearance of the CL images was obvious, but the CL spectra do show changes between the shock stages. The Raman spectra are characteristic for quartz in the unshocked and 12 GPa samples. In the 25 and 28 GPa samples, broad bands indicate the presence of glassy SiO2, while high‐pressure polymorphs are not detected. Apparently, some of the CL and Raman spectral properties can be used in shock barometry.

Annealing radiation damage and the recovery of cathodoluminescence

The structural recovery upon heat treatment of a highly metamict, actinide-rich zircon (U≈6000 ppm) has been studied in detail using a range of techniques including X-ray powder diffraction, Raman spectroscopy, SHRIMP ion probe, electron microprobe, transmission electron microscopy and cathodoluminescence analysis. The structural regeneration of the amorphous starting material depends on random nucleation. It starts between 800 and 950 °C when amorphous ZrSiO 4 decomposes to form crystalline ZrO 2 and amorphous SiO 2. At around 1100 °C, well-crystallised ZrSiO 4 grows at the expense of the oxides. U has been retained in the newly grown zircon whereas Pb was evaporated during the heat treatment. This process is in marked opposition to the reconstitution of moderately metamict minerals, which experience a gradual recovery controlled by the epitaxial growth at the crystalline–amorphous boundaries. Both of these recovery processes are not the direct inverse of metamictisation. The structural regeneration was found to be connected with a significant increase in the emission of CL. In all cases (annealing heavily damaged zircon and moderately damaged zircon and monazite), we observe that the final, well-crystallised annealing products emit more intense CL than their radiation-damaged starting minerals, although having almost identical elemental composition. Our observations are taken as evidence that the CL is not only determined by the chemical composition of the sample but is also strongly controlled by structural parameters such as crystallinity or the presence of defect centres.

Photoluminescence of Eu2+-doped mullite (xAl2O3·ySiO2; x/y=3/2 and 2/1) prepared by a hydrothermal method

Mullite with different Al2O3/SiO2 ratios (3/2 and 2/1) is prepared by the decomposition of the precursor prepared by the hydrothermal method at above 1300°C. The phase relations and thermal events during the decomposition are studied by powder X-ray diffraction and thermal analyses. Presence of defect centres is revealed by electron paramagnetic resonance (EPR) spectroscopy and are nonemissive. Eu2+-doped mullite shows blue luminescence and the luminescence characteristics show composition dependence. The emission maximum of Eu2+-doped 3/2-mullite is around 460 nm and that of Eu2+-doped 2/1-mullite is around 420 nm. Shift in the emission maxima with change in the Al2O3/SiO2 ratio is explained based on the relative change in covalency. Based on photoluminescence studies, site preference of Eu2+-doped mullite is assigned to structural cavity rather than to 6- or 4-coordinated regular lattice sites.

ELECTRIC STRESS DISTRIBUTION IN POLYMERIC INSULATION CONTAINING DEFECT SITES AND SPACE CHARGE

Polymeric insulation is progressively replacing traditional materials in cables, switchgear and machines. The fundamental electricl properties of polymer dielectrics such as conduction and breakdown are strongly influenced by the presence of defect centres (voids and inclusions) and trapped charge. A thorough understanding of the role of trapped charge in the breakdown process is essential if the full potential of these new materials is to be realised.