Optical Active Centers Research Articles

Optical and luminescent properties of ceria nanoparticles produced by gas phase method

The study on luminescent and optical properties of ceria nanoparticles allow one to indirectly assess the defectiveness of the material crystal lattice. The decomposition of the luminescence spectrum in the Gaussian bands shows the presence of Ce3+ ions in the CeO2 crystal lattice, as well as optical active centers, probably associated with oxygen vacancies. These characteristics permit us to estimate the relative amount of Ce3+ ions in the crystal lattice and could be considered as an important prerequisite for the use of nanoparticles as redox agents in industry and medicine.

Designing a sensitive luminescent probe for organophosphorus insecticides detection based on post-synthetic modification of IRMOF-3

Metal organic frameworks (MOFs) represent an excellent material for chemical species sensors because they contain various optically-active centers, different ligands, variable metals, porosity and various guest species. This may be rationally chosen to afford the desired light emission properties. Unlike other porous crystalline solids, MOFs may be post-synthetically modified (PSM) with various types of chemical compounds embedded in the pore walls. Here, Eu-IRMOF-3-EBA is built up via post-synthetic modification of IRMOF-3 with ethylbenzoylacetate followed by coordination with Eu3+ ions. The main point of this work is not to report yet another MOF sensor. Rather, it is to show, for the first time, that PSM is an interesting route to tune the optically-active organophosphorus insecticides (prothiofos and profenofos) sensors. The complexation behavior of ethion, prothiofos, or profenofos with Eu-IRMOF-3-EBA has been computationally investigated.

The effect of interaction between surface plasmons of gold nanoparticles and optical active centers on luminescence of Eu3+- doped Zn2SnO4 nanocrystals

The enhancement and quenching of Eu3+ ion emission were investigated in Zn2SnO4:Eu3+@Au (ZTO:Eu3+@Au) nanocomposites. Under 361 nm excitation we revealed the extinction of the intrinsic defect emission and the enhancement of Eu3+ ion emission when Au content in samples is increased, but under excitation wavelength of 394 nm we observed only the suppression of Eu3+ ion emission. The cause of the observed PL behavior is related to the interaction between surface plasmon induced by gold nanoparticles and luminescence centers in the samples.

Structuring of Amide Cross-Linked Non-Bridged and Bridged Alkyl-Based Silsesquioxanes.

The development of sophisticated organized materials exhibiting enhanced properties is a challenging topic of the domain of organic/inorganic hybrid materials. This review, composed of four sections, reports the work we have carried out over the last 10 years on the synthesis of amide cross-linked alkyl/siloxane hybrids by means of sol-gel chemistry and self-directed assembly/self-organization routes relying on weak interactions (hydrophobic interactions and hydrogen bonding). The various as-produced lamellar structures displaying a myriad of morphologies, often closely resembling those found in natural materials, are discussed. The major role played by the synthetic conditions (pH, water content, co-solvent(s) nature/concentration and dopant presence/concentration), the alkyl chains (length and presence of ramification or not) and the number of the amide cross-links present in the precursor, is evidenced. Examples of highly organized hybrids structures incorporating ionic species (alkali and alkaline earth metal salts) and optically-active centers (organic dyes and lanthanide ions) are described. A useful qualitative relationship deduced between the emission quantum yield of the ordered hybrid materials and the degree of order of the hydrogen-bonded network is highlighted.

Surface localization of the Er-related optical active centers in Er doped zinc oxide films

Er-doped ZnO films were grown on c-plane sapphire by employing the pulsed laser deposition method. In accordance with the previously reported literature studies, post-growth annealing was required to activate the optical emission originated from the intra-shell transitions of the Er atoms. Importantly, the present systematic studies revealed that the thermal activation of the optical activity is due to the atomistic rearrangements occurring only in the vicinity of the film surface. The processes exhibit a distinct signature of changing oxygen coordination with the Er atom, as observed by the x-ray photoemission study.

Luminescence properties of calcium doped zinc oxide nanoparticles

Aerogel nanopowder of calcium-doped zinc oxide (ZnO:Ca) was synthesized by modified sol-gel method. In this process, hydrolyses was slowly released and followed by a thermal drying in supercritical conditions or ethyl alcohol. The obtained nanopowder was characterized by various techniques such as X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), Cathodoluminescence (CL) and photoluminescence (PL). XRD data showed that Ca-doped ZnO sample has a hexagonal wurtzite structure with a slight distortion of ZnO lattice and no extra secondary phases, suggesting the substitution of Ca ions in the ZnO structure. SEM micrograph shows spherical microparticles having a rough porous fine-grained. From TEM micrograph, the samples are composed by single particles having an inhomogeneous size distribution, with most of them having a dimension in the range between 20 and 50nm. This powder presents a strong photoluminescence band in the visible range. From photoluminescence excitation (PLE) the energy position of the obtained PL band depends on the wavelength of excitation. The luminescence results are also confirmed by cathodoluminescence technique and suggests the presence of photo-active centers in ZnO:Ca as deduced from new published works for visible photo-activated gas sensors and photo-catalysis of dyes degradation. We hope that this work provides some answers to the scientific community concerning the effect of doping in the creation of optical active centers in ZnO, promising for many technological applications.

Luminescence of rare-earth ions and intrinsic defects in Gd2O3 matrix

The X-ray luminescence and photoluminescence of the ultradispersed Gd2O3 powders with different purity were investigated in 90-360 K temperature range. Both impurity and intrinsic optical active centers were detected. The effect of energy transfer from Gd3+ to RE3+ ions was observed.

White luminescence from sol–gel silica doped with silver

Undoped silica and Ag-doped silica (SiO2) were synthesized by the sol–gel method. The photoluminescence properties as a function of annealing temperature were investigated in detail. The addition of Ag introduced new optical active centres in the sample annealed at 1000 °C, responsible for emission bands at 2.25 eV (550 nm) and 3.26 eV (380 nm) in the SiO2 luminescence spectrum. These new emission bands were related to excess oxygen due to decomposition of Ag2O at high temperatures. The additional luminescence band at 2.25 eV changed the blue emission from pure SiO2 to near white light from Ag-doped SiO2, suggesting that the SiO2:Ag system may be suitable for solid-state lighting applications. The stability of this phosphor under ultraviolet irradiation was investigated.

A Visible‐Light‐Driven Photocatalyst of NASICON Li2Ni2(MoO4)3 Nanoparticles

A visible‐light‐driven photocatalyst based on the well‐known cathode material of NASICON‐type Li2Ni2(MoO4)3 was prepared by a modified Pechini method. The sample was characterized by X‐ray diffraction, scanning electron microscope, transmission electron microscopy, and UV‐vis absorption spectrum. The average size of Li2Ni2(MoO4)3 particle is below 50 nm. NASICON nanoparticles Li2Ni2(MoO4)3 has an efficient absorption in the UV‐visible light wavelength region with a direct allowed electronic transition of 2.07 eV. The photocatalytic properties of Li2Ni2(MoO4)3 were evaluated by the photodegradation of methylene blue (MB). Li2Ni2(MoO4)3 has an efficient photocatalytic activity and could be a potential photocatalyst driven by visible‐light. The photocatalytic activity was discussed on the optical absorption and special hexagonal tunnel structure connected by optical active centers of MoO4 and NiO6 and its good conductivity.

Luminescence studies on SnO2 and SnO2:Eu nanocrystals grown by laser assisted flow deposition.

Transparent conductive tin oxide materials have been a research topic extensively studied in recent years due to the great interest for many applications. However, in most of them, the pure form is rarely used, being usually modified by the incorporation of dopants. Selecting the most appropriate technique to develop nanocrystals of doped tin oxide and understanding the influence of dopant on the optical properties are the challenges that need to be addressed when envisaging devices. To fulfill this objective, the recently developed laser assisted flow deposition (LAFD) method is explored to grow SnO2 and SnO2:Eu nanocrystals. The morphology of these nanocrystals was investigated by scanning electron microscopy and well defined prismatic nanocrystals with sizes of ∼60 nm were identified. The crystalline quality assessed by X-ray diffraction measurements and Raman spectroscopy indicates that the produced nanocrystals are monophasic and crystallize in the tetragonal rutile structure. Steady state luminescence studies provide the information on the optical active centres in the SnO2 and SnO2:Eu nanocrystals. For the undoped samples only broad emission bands were observed by pumping the samples in the ultraviolet region. The broad emission was found to be an overlap of green and red optical centres as identified by temperature and excitation intensity dependent luminescence. The latter was found to exhibit an excitonic-related behaviour and the green emission was found to be of utmost importance to discuss the intraionic luminescence in the doped samples. For the SnO2:Eu nanocrystals the luminescence is dominated by the magnetic allowed (5)D0 → (7)F1 transition with the ions in almost undistorted centrosymmetric sites. The ion luminescence integrated intensity is found to increase with increasing temperatures being well accounted for a thermal population provided by the thermal quenching of the green band.

Nanometric Resolved Cathodoluminescence on Few-Layer h-BN Flakes

Within the latest years number of layered materials at reduced dimensions have demonstrated remarkable optical properties. However most studies focused on perfect system and the role of defects as optical active centers remain still largely unexplored. Hexagonal boron nitride (h-BN) is one of the most promising candidates for light emitting devices in the far UV region, presenting a single strong excitonic emission at 5.8 eV. However, a single line appears only in extremely pure monocrystals that can hardly be obtained only though complex synthesis processes. Common h-BN samples present more complex emission spectra that have been generally attributed to the presence of structural defects. Despite a large number of experimental studies up to now it was not possible to attribute specific emission features to well identify defective structures. Here we address this fundamental questions by adopting a theoretical and experimental approach combining few nanometer resolved cathodoluminescence techniques with high resolution transmission electron microscopy images and state of the art quantum mechanical simulations. Very recently, the Orsay team has developed a cathodoluminescence detection system integrated within a scanning transmission electron microscope. This unique experimental set up is now able to provide full emission spectra with a resolution as low as few tens of meV associated with an electron probe size of one nanometer. A cathodoluminescence spectrum-image can thus be recorded in parallel with an HAADF image. Nanometric resolved cathodoluminescence on few-layer chemically exfoliated h-BN crystals have shown that emission spectra are strongly inhomogeneous within individual flakes. Emission peaks close to the free exciton appear in extended regions. Complementary investigations through high resolution transmission electron microscopy allow to associate these emission lines with extended crystal deformation such as stacking faults and folds of the planes. By means of ab-initio calculations in the framework of Many Body Perturbation Theory (GW approximation and Bethe-Salpeter equation) we provide an in-depth description of the electronic structure and spectroscopic response of bulk hexagonal boron nitride in the presence of extended morphological modifications. In particular we show that, in a good agreement with the experimental results, additional excitons are associated to local symmetry changes occurring at crystal stacking faults. Additional features appearing within the band gap present a high spatial localization, typically less than 100 nm, and thus they can be related to individual point defects. When addressed individually through a highly focused electron probe they might have a single photon emitter quantum character. This hypothesis has been recently confirmed by experiments combining our cathodoluminescence system with an Hanbury Brown and Twiss (HBT) interferometer [4].

Photoluminescence in Ga/Bi co-doped silica glass

Bismuth-Gallium co-doped silica glass fiber preform was prepared from nano-porous silica xerogels using a conventional solution doping technique with a heterotrinuclear complex and subsequent sintering. Ga-connected optical Bismuth active center (BAC) was identified as the analogue of Al-connected BAC. Visible and infrared photoluminescence (PL) were investigated in a wide temperature range of 1.46 - 300 K. Based on the results of the continuous wave (CW) and time resolved (TR) spectroscopy we identify the centers emitting in the spectral region of 480 - 820 nm as Bi(+) ions. The near infrared (NIR) PL around 1100 nm consists of two bands. While the first one can be ascribed to the transition in Bi(+) ion, the second band is presumably associated to defects. We put in evidence the energy transfer (ET) between Bi(+) ions and the second NIR emitting center via quadrupole-quadrupole and dipole-quadrupole mechanisms of interactions. Finally, we propose the energy level diagram of Bi(+) ion interacting with this defect.

Influence of Annealing on the Structure and 1.54 µm Photoluminescence of Er-Doped ZnO Thin Films Prepared by Sol–Gel Method

We have investigated the effects of Er concentration, post-annealing time and temperature through a sol–gel preparation method on the structure and 1.54-µm-related photoluminescence (PL) of ZnO:Er thin films. The results illustrated that the 1.54 µm emission was greatly influenced by the local structure of Er–O complex and ZnO host. The active oxygen movement during annealing process resulted in the formation of optical active center of Er ions, which probably attributed to the formation of a similar pseudo-octahedron with C4v structure around Er. The preferential orientation of ZnO host had more effect on the 1.54 µm PL intensity than the crystallinity of ZnO host. Therefore, the optimum annealing condition was about 800 °C/2 h and the appropriate concentration was about 0.05 at. % Er. A low-cost and fast formation of highly efficient Er centers in ZnO host for strong luminescence at near-infared region should be benefit for both fundamental research and also applications of light-emitting devices.

Electrochemical oxidation-induced polymerization of 5,10,15,20-tetrakis[3-( N-ethylcarbazoyl)]porphyrin. Formation and characterization of a novel electroactive porphyrin thin film

The formation and characterization of novel polymer modified Pt and ITO electrodes obtained by electropolymerization is depicted. The presences of porphyrin, a powerful optical and redox active center, together with carbazole, a well-known hole-transporting material, confer to the polymer electric and optical activity, with potential application in the development of organic optoelectronic devices. 5,10,15,20-Tetrakis[3-( N-ethylcarbazoyl)]porphyrin form conductive, stable and reproducible electropolymer films. Combined electrochemical and spectroscopic studies show that the electropolymerization mechanism involves the dimerization of carbazole units. During coupling of carbazole radicals, protons are released to the media and porphyrin film is protonated, generating the porphyrin dication. The observation of the characteristic porphyrin electronic spectrum after reduction of the film indicates that the tetrapyrrolic macrocycle remains unaltered in the electrogenerated polymer. The formation of a broad band that extends into the near-IR region upon polymer oxidation is in agreement with the presence of a conducting polymer with good charge transport capability.

Effect of V/III flux ratio on luminescence properties and defect formation of Er-doped GaN

Erbium-doped GaN samples grown with different V/III ratios through gas source molecular beam epitaxy were prepared to investigate the influence of the V/III ratio on the defect formation and the optical activity of the Er-related luminescence center. Obvious V/III ratio dependence was observed in photoluminescence measurement. Positron annihilation spectroscopy measurements suggested that VGa-VN vacancy-complexes formed in these samples and that the VN/VGa ratio depended on the V/III ratio. The generation of Er-VN defect complexes, which act as high optical active luminescence centers, is suggested as the cause of improved optical properties of Er-doped GaN grown with a lower V/III ratio.

Luminescence and optical gain in Pb-doped silica-based optical fibers

IR luminescence and optical gain in a Pb-doped fiber have been observed for the first time. Absorption, luminescence and pump on/pump off optical gain spectra, as well as luminescence decay time, have been measured in these fibers. Comparison of optical active center characteristics in Pb-doped and Bi-doped fibers of the same composition indicates an essential difference of optical active centers in these two types of fibers.

Far-Action Radiation Defects and Gettering Effects in 4H-SiC Implanted with Al Ions

Structural features of 4H-SiC structures with CVD epitaxial layers, subjected to high-dose Al ion implantation and short high-temperature pulse annealing, have been studied using secondary-ion mass-spectroscopy, transmission electron spectroscopy, local cathodoluminescence and cathodoluminescence imaging on cross-sectionally cleaved surfaces of the structures. An accelerated diffusion of radiation defects, a “long-range action effect”, with a diffusion coefficient of 10 -9 cm2 s-1 after high-dose Al ion implantation and the gettering effect after subsequent pulsed thermal annealing have been observed for the first time. After a short high-temperature annealing, the quality of the starting material is improved in the course of formation of implantation-doped p+-n junctions due to defect gettering. As a result of the decrease in the concentration of optical active defect centers as well of deep centers by an order of magnitude in CVD layer, an increase in the diffusion length of minority carriers (Lp) by a factor of 1.5-2 was obtained.

Intrinsic <I>p</I> Type ZnO Films Deposited by rf Magnetron Sputtering

ZnO films were deposited on c-plane sapphire substrates in Ar atmosphere by rf magnetron sputtering and were post-annealed at 400 degrees C in green gas (95% N2 + 5% H2). The properties of the as-grown and annealed films have been characterized by X-ray diffraction (XRD), Rutherford backscattering (RBS), elastic recoil detection analysis (ERDA), Hall measurement and photoluminescence spectra. XRD studies confirmed the variation in strain and an improvement in crystallinity. From RBS and ERDA analysis, the presence of H atoms on the surface of the as-grown ZnO films was evidenced. Annealing in green gas increased the amount of H in the film. Compared with the as-grown films, the ultra exciting intensity obviously decreases in the annealed films and new optical active centres in the blue/violet (approximately 3.0 eV) and red (approximately 1.9) regions are emerged in the PL spectrum. The positive sign of Hall coefficient confirmed the low p-type conductivity in the as grown films, which was improved after annealing. However, the p-type conductivity was not stable, especially for the annealed sample it changes from p type to n type after 9 days.

Spectroscopic Study of $\gamma$-Ray and Pulsed X-Ray Radiation-Induced Point Defects in Pure-Silica-Core Optical Fibers

We investigate the radiation-induced effects on pure-silica-core (PSC) optical fibers. For this, we measured the radiation-induced attenuation (RIA) growth and decay kinetics in four fibers with different hydroxyl and chorine contents. Our results show that PSC fibers exhibit different transient and continuous radiation responses depending of the silica-glass composition. Self-trapped charges [self-trapped excitons (STEs) and self-trapped holes (STHs)] seem mainly responsible for the fiber transient responses (times shorter than 1 s after the X-ray pulse) in the ultraviolet to near infrared part of the spectrum (300-900 nm). As these defects are unstable at room temperature, the contribution of other defects, like non-bridging oxygen hole centers (NBOHC) increases with time. Finally, these stable defects mainly explain the fiber permanent responses. To complete our online RIA measurements, we also studied by confocal microscopy luminescence (CML) the spatial distribution of the stable radiation-induced emitting centers in both pristine and irradiated fibers. Our CML results, obtained with 514 and 633 nm laser excitation, showed that several optically-active centers are non-uniformly generated in the fiber cross-sections. The generation of NBOHC by breakage of Si-O-Si strained bonds is clearly enhanced in high-OH fibers, especially at their core and cladding interfaces. The generation mechanisms of other unidentified defects emitting around 600 nm seem to be affected by both the hydroxyl content in the fiber core and by another factor that is not related to the silica glass composition.

Optical active centres in ZnO samples

In recent years, there has been a resurgence in the interest in the use of ZnO (Eg∼3.37eV) as a material for a wide range of opto-emitter applications spanning visible and short wavelengths. Bulk, thin films and nanomaterials obtained using different synthesis methods have been investigated for optoelectronic and biotechnological device applications. Nominally undoped bulk samples typically present a myriad-structured near-band-edge recombination, mainly due to free/bound excitons and donor–acceptor pair transitions. Furthermore, deep level emission due to intrinsic defects and extrinsic impurities, such as transition metal ions, are commonly observed in different grades of bulk ZnO samples. Undoped thin film and ZnO nanocrystal samples also present optically-active centres due to the presence of native and extrinsic defects. Continuing improvement in device performance hinges on improved understanding of the role of these defects present in ZnO samples. In this work a correlation between the optical centres was observed between nominally-undoped bulk, thin films and nanocrystal ZnO. We also observed a correlation between the structural properties and ion optical activation for single crystal samples which were intentionally-doped with rare earth ions (Tm, Er, Eu and Tb) either (a) by ion-implantation or (b) during synthesis. For the doped ZnO nanocrystals, intra-ionic recombination and XRD data suggest that the ions are in a crystalline environment.