1,721,528 research outputs found
Templated growth of europium-doped sulfide thin films by a solvothermal method
No full textRare-earth doped sulfides are an important class of luminescent materials with applications as colour-conversion material in white-light emitting diodes and for use in thin film electroluminescent displays.
Here we report on the solvothermal deposition of europium-doped (Ca,Sr)S thin films. The deposition of luminescent and crystalline alkaline-earth sulfide thin films is far from straightforward, and post-deposition treatments at relatively high temperature are most of the time required, often involving the use of H2S. With the novel method, we are able to deposit luminescent, uniform and textured thin films with a thickness of a few micrometers at a temperature of only 200°C, without the need of post-deposition thermal treatments or the use of H2S. Furthermore, the method has good scalability and potentially allows the deposition onto flexible substrates, paving the road for new (luminescence) applications.
Substrates covered with a thin sulfide layer (typical thickness of 20 to 50nm, deposited with electron-beam evaporation) were inserted into an autoclave, filled with ethylenediamine, capping agents, alkaline-earth and rare-earth chlorides and elemental sulfur. This solvothermal synthesis method was used before to synthesize nano-to-micron-sized luminescent sulfide particles [1,2]. Then the mixture was heated to 200°C and thin film growth occurred on the SrS or CaS template layer. After recuperation, bright and homogeneous luminescence is observed from the deposited layer. For SrS:Eu this emission peaks at 615nm and at 650nm for CaS:Eu, which is nearly identical to the emission in bulk material [2].
X-ray diffraction revealed a fully crystallized layer, with strong preferential orientation. On Si, Ge and Corning Vycor substrates, a full (200) out-of-plane orientation was detected for the CaS:Eu solvothermally deposited thin films, which could be related to the crystallographic orientation of the template layers. Upon solvothermal growth on silicon and on germanium substrates, a partial to a full in-plane crystallographic orientation was observed respectively. The latter could be attributed to the fact that the lattice mismatch between CaS and Ge is only 0.5%.
Scanning electron microscopy was used to characterize the morphology of the thin films. Depending on the synthesis conditions, a rough surface can be obtained due to the surface being terminated by micron-sized pyramids. This type of surface is advantageous for increased light outcoupling from the thin film. Alternatively, a relatively flat surface can also be obtained, with a more columnar thin film growth perpendicular to the substrate surface. Finally, growth only occurs on the template sulfide layer, which allows a controlled, selective-area deposition.
[1] C. Wang, K. Tang, Q. Yang, C. An, B. Hai, G. Shen, and Y. Qian, Chem. Phys. Lett. 351 (2002) 385
[2] J.E. Van Haecke, P.F. Smet, K. De Keyser, and D. Poelman, J. Electrochem. Soc. 154 (2007) J27
New challenges in radiation dosimetry and possible materials
No full textThe measurement of ionizing radiation and its response to living beings is subject to great scientific uncertainty and intense controversy. It seems likely that even small doses of radiation might do some harm. So, there is a need to measure both small doses in the environment and very high doses at the time of accidents like radiation leakage, and moreover for the treatment of cancer. Already, a lot of work has been done to find the best dosimeter to measure the absorbed dose. Continuous efforts have been made by the research community worldwide to develop new materials with improved dosimetric properties, but still, there are many new challenges that the research community needs to focus upon. The present chapter provides an outline of the new challenges in radiation dosimetry and proposed materials
Principle, mechanism, and models of radiation dosimetry
No full textThe chapter describes some basic principles of radiation that are relevant for an understanding of radiation dosimetry. It describes various characteristics of γ-rays, β-particles, X‐rays, electrons, neutrons, etc., including their interaction with matter. In this chapter, the physical phenomena of emission and absorption of radiation are discussed. The interaction of ionizing radiation with matter brings some changes in the matter, therefore the basic physical mechanisms of different kinds of radiation interaction with matter are discussed. A number of quantities and units are defined for describing radiation beams and the most commonly used dosimetric quantities and their units are defined. The chapter also focuses on the advancements, processes, technologies, techniques, and principles involved in radiation dosimetry
Sol-gel preparation and characterization of titanium dioxide films for degradation of organic pollutants
Full text linkThe Effect of Glass Structure on the Luminescence Spectra of Sm3+-Doped Aluminosilicate Glasses
Full text linkPeralkaline Sm3+-doped aluminosilicate glasses with different network modifier ions (Mg2+, Ca2+, Sr2+, Ba2+, Zn2+) were investigated to clarify the effect of glass composition and glass structure on the optical properties of the doped Sm3+ ions. For this purpose, the Sm3+ luminescence emission spectra were correlated with the molecular structure of the glasses derived by molecular dynamics (MD) simulations. The different network modifier ions have a clear and systematic effect on the peak area ratio of the Sm3+ emission peaks which correlates with the average rare earth site symmetry in the glasses. The highest site symmetry is found for the calcium aluminosilicate glass. Glasses with network modifier ions of lower and higher ionic radii show a notably lower average site symmetry. The symmetry could be correlated to the rare earth coordination number with oxygen atoms derived by MD simulations. A coordination number of 6 seems to offer the highest average site symmetry. Higher rare earth coordination probabilities with non-bridging oxygen result in an increased splitting of the emission peaks and a notable broadening of the peaks. The zinc containing glass seems to play a special role. The Zn2+ ions notably modify the glass structure and especially the rare earth coordination in comparison to the other network modifier ions in the other investigated glasses. The knowledge on how glass structure affects the optical properties of doped rare earth ions can be used to tailor the rare earth absorption and emission spectra for specific applications
Advanced Luminescent Materials and Devices
No full textThis text presents recent developments in a wide range of applications of luminescent materials. It compiles contributions from a number of leading scientists in the field, and should be an inspiration for both established and new researchers in this exciting area of research. Thanks to developments in materials research, new applications, requiring specific phosphor properties, have emerged. Wavelength ranges have expanded towards UV and near-IR emission; highly performant persistent phosphors have been developed for safety illumination and bio-imaging; phosphors have been proposed for anti-counterfeiting, energy storage, photocatalysis, thermometry, scintillators and dosimetry; and new synthesis and characterization techniques for luminescent materials have been developed
Pair distribution function analysis of ZrO2 nanocrystals and insights in the formation of ZrO2-YBa2Cu3O7 nanocomposites
Full text linkThe formation of superconducting nanocomposites from preformed nanocrystals is still not well understood. Here, we examine the case of ZrO2 nanocrystals in a YBa2Cu3O7-x matrix. First we analyzed the preformed ZrO2 nanocrystals via atomic pair distribution function analysis and found that the nanocrystals have a distorted tetragonal crystal structure. Second, we investigated the influence of various surface ligands attached to the ZrO2 nanocrystals on the distribution of metal ions in the pyrolyzed matrix via secondary ion mass spectroscopy technique. The choice of stabilizing ligand is crucial in order to obtain good superconducting nanocomposite films with vortex pinning. Short, carboxylate based ligands lead to poor superconducting properties due to the inhomogeneity of metal content in the pyrolyzed matrix. Counter-intuitively, a phosphonate ligand with long chains does not disturb the growth of YBa2Cu3O7-x. Even more surprisingly, bisphosphonate polymeric ligands provide good colloidal stability in solution but do not prevent coagulation in the final film, resulting in poor pinning. These results thus shed light on the various stages of the superconducting nanocomposite formation
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
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