Synthesis, Optical, and Magnetic Properties of Graphene Quantum Dots and Iron Oxide Nanocomposites

The combination of nanomaterial graphene quantum dots (GQDs) with magnetic nanoparticles offers a unique set of optical and magnetic properties for future energy and medical applications. We report on the synthesis and engineering of GQDs and iron oxide (Fe3O4) nanocomposites (NCs) by using a pulsed laser discharge technique. High-resolution transmission electron microscopy (HRTEM) images showed a high yield of pure GQDs with 2–10 nm diameter. The hexagonal structures and lattice fringes associated with the C–C bond in GQDs were clearly identifiable. The structural and optical changes in GQDs and GQDs-Fe3O4 NC samples induced by UV light were investigated by the absorption and emission spectroscopy over the deep UV–visible spectral range. The photoluminescence spectra have shown subband π→π∗ transitions in GQDs-Fe3O4 NC. Magnetic properties of the GQDs-Fe3O4 NC samples have shown room temperature ferromagnetism induced by pure Fe3O4 nanoparticles and from the substantial spin polarized edges of GQD nanoparticles. It is concluded that the observed optical and magnetic properties could be further tailored in the studied nanocomposites for prospective medical applications

Enhancing Blue Emission in Ce Doped Silicon Oxynitrides Based Electroluminescent Devices

International audienceCe-doped SiO x N y and SiAlON matrices are promising materials for blue LED applications. The uniqueness of this approach stems from the fact that SiO x N y , as a host, combines specific properties of individual SiO x and SiN y matrices like solubility, efficient emission, 5 eV gap, with a broad excitation range from 400 nm to 500 nm of Ce 3+ ions due to the 4f-5d transitions. Furthermore, the co-doping with aluminum enhances the Ce 3+ emission. In this work, we fabricated electroluminescent devices using SiO x N y :Ce 3+ and SiAlON:Ce 3+ as active layers and investigated the resulting emission under optical and electrical excitations as a function of nitrogen, cerium and aluminum concentrations. I-V measurements were conducted to determine the SiO x N y :Ce 3+ layer electrical parameters. Charge transport through the devices obeys the Poole-Frenkel conduction mechanism. It was demonstrated that by optimizing the SiO x N y :Ce 3+ growth parameters, an improvement of electroluminescence yield can be achieved with a maximum intensity obtained for devices with cerium content of 4 at.%. Rare earth (RE) doped silicon based materials have been extensively investigated in the past few years. Various hosts were doped with Er 3+ ions that emit at 1.5 μm corresponding to the maximum transparency of silica used in telecommunications. 1 For the Ce 3+ ion, up to date, only a few studies have been reported on its electrolu-minescence (EL). 2 Among the silicon-based matrices, silica (SiO 2) and silicon nitride (Si 3 N 4) have been explored; however, each of them present certain advantages and drawbacks. In the case of silica matrices , achievement of strong RE 3+ ions emission is limited by a low excitation cross section, 3 a low RE solubility as well as RE clusters formation. 4,5 However, the main drawback limiting SiO 2 : RE 3+ light emitting applications comes from the large bandgap of the matrix (∼9 eV) resulting in low electrical conductivity. On the other hand, Si 3 N 4 with a smaller energy bandgap (4 eV) and reduced tendency of the RE to form clusters, seems to be more suitable for RE doping. 6-8 However, despite these advantages, the emission efficiency from RE 3+ ions in a nitride matrix is much lower than in silica matrix. 9 To capitalize on the RE doping advantages offered by both oxide and nitride silicon matrices, a Ce-doped SiO x N y matrix has been explored by Ramirez et al. 10 It was reported that the maximum EL peak from Ce 3+ ion shifted from 400 nm to 476 nm as function of the nitrogen concentration (i.e. the nephelauxetic effect). 10,11 Koao et al. showed that aluminum co-doping Ce-doped SiO 2 glasses lead to an enhancement of photoluminescence emission. 12 In this work, Ce-doped Si(Al)O x N y layers with a typical thickness of 50 nm were grown by sputter de-position. Photoluminescence (PL) from the SiO x N y :Ce 3+ layers and EL from this active layer were examined for device performance as a function of growth parameters and material composition. Experimental Active layer preparation.-The devices were fabricated in a few step processes. First, the Ce-doped SiO x N y active layer was grown by magnetron reactive sputtering with 8 sccm for Ar and 2 sccm for N 2 on 2-inch diameter (001) p-type silicon wafers. During the growth, the chamber base pressure was fixed at 3 mTorr and the substrate temperature was at room temperature (RT). Additional information on the growth process can be found in the following Reference 11. z Samples were deposited from CeO 2 , Al and Si targets with density of power varied from 0 to 2.1 W.cm −2 , 0.3 to 0.75 W.cm −2 and fixed at 4.5 W/cm 2 , respectively. As-deposited films were then thermally annealed in the 600°C to 1200°C temperature range for 1 h, in nitrogen atmosphere at ambient pressure. Device fabrication.-Figure 1 illustrates the typical fabricated device structure with the indicated specific layers thicknesses. The individual indium tin oxide (ITO) top electrical contacts were deposited on the SiO x N y :Ce 3+ layer by electron beam evaporation using a shadow mask having a set of circular holes with a diameter of 200 μm. In 2 O 3 /SnO 2 (90%/10%) pellets with a diameter of 1 mm or 2 mm were used as sputtering targets. An oxygen flux was maintained in the sputtering chamber during deposition cycle to prevent the formation of oxygen defects in the transparent conducting layer, which would potentially cause EL from ITO layer. The ITO layer thickness was 200 nm. The SiO x N y :Ce 3+ /ITO structure was heated up at a ramp rate of 15°C/min from RT to 600°C and annealed for 1 h in a nitrogen atmosphere at ambient pressure. The bottom metal contact, a 200-nm-thick Al layer, was deposited at RT on the back of the silicon substrate. Figure 1. Schematic layout of fabricated Al/p-Si/SiO x N y :Ce 3+ /ITO device

The nitrogen concentration effect on Ce doped SiO x N y emission: towards optimized Ce 3+ for LED applications

International audienceCe-Doped SiOxNy films are deposited by magnetron reactive sputtering from a CeO2 target under a nitrogen reactive gas atmosphere. Visible photoluminescence measurements regarding the nitrogen gas flow reveal a large emission band centered at 450 nm for a sample deposited under a 2 sccm flow. Special attention is paid to the origin of such an emission at high nitrogen concentration. Different emitting centers are suggested in Ce doped SiOxNy films (e.g. band tails, CeO2, Ce clusters, Ce3+ ions), with different activation scenarios to explain the luminescence. X-ray photoelectron spectroscopy (XPS) reveals the exclusive presence of Ce3+ ions whatever the nitrogen or Ce concentrations, while transmission electron microscopy (TEM) shows no clusters or silicates upon high temperature annealing. With the help of photoluminescence excitation spectroscopy (PLE), a wide excitation range from 250 nm up to 400 nm is revealed and various excitations of Ce3+ ions are proposed involving direct or indirect mechanisms. Nitrogen concentration plays an important role in Ce3+ emission by modifying Ce surroundings, reducing the Si phase volume in SiOxNy and causing a nephelauxetic effect. Taking into account the optimized nitrogen growth parameters, the Ce concentration is analyzed as a new parameter. Under UV excitation, a strong emission is visible to the naked eye with high Ce3+ concentration (6 at%). No saturation of the photoluminescence intensity is observed, confirming again the lack of Ce cluster or silicate phase formation due to the nitrogen presence

First rare earth ions doped Si based down converter layers integration in an industrial Si solar cell

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