Nonradiative Deexcitation Research Articles

Spectroscopic characterization of sol–gel synthesized Tm:Lu2O3 nanocrystals

Thulium-doped Lu2O3 nanocrystals with \(\mathit{Ia}\overline{3}\) cubic phase and mean grain size of 17 nm were synthesized by using a modified Pechini method. Their spectroscopic properties have been characterized by Raman, optical absorption, photoluminescence and cathodoluminescence spectroscopies at low and room temperatures, with emphasis on blue and infrared emissions. The reduction of the crystal size induces an increase of the optical line widths and new non-radiative de-excitation paths ascribed to a surface effect.

Cooperative effect of La-doping and template on surface photoelectron characteristics of mesoporous nano-TiO 2

A mesoporous La-doped nano-TiO 2 was prepared by the sol–gel method using polyethylene glycol as the template. A combination of surface photovoltaic (SPV) and photoacoustic technologies, aided by laser Raman technology, was used to probe the photoexcited charge transfer transition behaviors in the surface space charge region of the sample. The results confirm that the lanthanum doping was responsible for inhibiting the transformation from the anatase to rutile form and grain growth, thus strengthening the microstructure that was formed after removal of the template clearly. The experiment reveals that appropriate La-doping resulted in an obvious increase in the SPV response. By contrast, the remainder of the template had a somewhat negative effect on the SPV response of the samples. The effect of both the La-doping and the removal of the template on the nonradiative de-excitation process of the main band-gap can be negligible, in spite of the nonradiative de-excitation processes of the sub-band-gap of the La-undoped sample, in which the surface states possessed the donor characteristic, being more obvious than that of the La-doped samples. The results showed that the electron–phonon interactions on the surfaces resulting from the nonradiative de-excitation process were closely dependent on the effect of quantum confinement of the mesoporous nano-TiO 2.

Microdynamics behavior of the photogenerated carriers in ligand-capped CdTe quantum dots

A microdynamics study of high-quality CdTe/ligand self-assembled quantum dots (QDs) is significant because it is closely related to the energy transformation among specific quantum states, such as the electron and vibration levels of an elementary reaction. Transient photovoltaic (TPV) technology was used to probe the microdynamics behavior of the photogenerated carriers in the QDs. The TPV polarity of the nanocrystalline CdTe particles depended on the type of capping ligand used [3-mercaptopropionic acid (MPA) or β-mercaptoethylamine (MA)] because of a quantum tunneling effect. This resulted in a negative TPV polarity of the MPA samples upon illumination with a laser pulse of 532 nm and excitation intensity of 50 μJ, the opposite to the situation for the MA sample. The effect of the concentration of Cd 2+ cations on the separation and recombination rates of the photo-induced free charge carriers of the CdTe/MPA self-assembled QDs was negligible. The different chemical bonds that formed at the interface between the core-CdTe nanoparticles and the capping ligand led to the change in the intensity of the photoacoustic signal in the 550–800 nm region caused by the non-radiative de-excitations as compared to the CdTe QDs capped by MA with that capped by MPA, and to the differences in the Raman scattering between the two samples. The property of those lattice vibration quantum states locating at the Brillouin zone boundary or near the Brillouin zone boundary was associated to a great extent with the formation of the shell-CdS in between the core-CdTe and the capping ligand according to the computer simulation results.

Phosphorescence of palladium and platinum complexes of benzo-fused hydroporphyrazines

Palladium and platinum complexes of benzo-fused hydroporphyrazines, analogs of phthalocyanine in whose molecules one or two isoindole fragments are substituted by hydrogenated pyrrole rings giving macrocycles of the chlorin, bacteriochlorin, and isobacteriochlorin types, have been synthesized. Phosphorescence in the near IR region has been detected at 291 and 77 K for complexes of tribenzotetraazachlorin and dibenzotetraazaisobacteriochlorin. The energy of the lowest triplet level T 1 has been determined from the positions of 0–0-bands in phosphorescence spectra. As compared to complexes of phthalocyanine, hydrogenation of a pyrrole ring has a stronger influence on the energy of the T 1 level (lowering) whereas hydrogenation of two adjacent pyrrole rings results in a larger S 1–T 1 gap. The phosphorescence quantum yield and lifetime have been measured. Based on the data obtained, the rate constants of the radiative and non-radiative de-excitation of the T 1 level have been calculated. The highest phosphorescence quantum yield is obtained for Pt dibenzotetraazaisobacteriochlorin (1.7% at 77 K). The influence of the nature of the metal atom on the electronic absorption spectra of metal complexes of benzohydroporphyrazines is analyzed taking into account literature data for bacteriochlorophyll a, metal complexes of bacteriopheophytin a, and chlorophyll-like molecules having less complex structures.

Fluorescent-magnetic nanocrystals: synthesis and property of YPxV1−xO4:Eu@GdPO4 core/shell structure

A fluorescent-magnetic YP(x)V(1-x)O(4):Eu@GdPO(4) core/shell nanostructure was prepared by a two-step method. The YP(x)V(1-x)O(4):Eu core was synthesized using a hydrothermal method, and it exhibits strong photoluminescence with the effective doping of phosphorus (P) and europium (Eu) into a YVO(4) matrix. The hydrothermal process provides a hydrophilic and fresh surface for coating GdPO(4) shell. As YP(x)V(1-x)O(4):Eu and GdPO(4) have the similar unit cell parameters, YP(x)V(1-x)O(4):Eu nanoparticles (NPs) were favorably coated by an epitaxial growth of GdPO(4) shell in aqueous phase. The core/shell nanostructure was identified by X-ray diffraction (XRD), transmission electron microscopy (TEM) and scanning transmission electron microscopy (STEM). The GdPO(4) shell not only possesses the paramagnetic character, but also enhances the photoluminescence efficiency by blocking the non-radiative de-excitation from the VO(4)(3-) groups to the surface quenching sites. These optical and magnetic properties promise outstanding fluorescent-magnetic bifunctional nanomaterials.

Eu 3+-doped CaF 2 nanocrystals in sol–gel derived glass–ceramics

Glass–ceramics containing Eu 3+-doped CaF 2 nanocrystals of about 16 nm size have been made using the controlled crystallization at higher temperatures of the Eu 3+-doped CaF 2–SiO 2 xerogels. In the glass–ceramic material the Eu 3+ ions are embedded in both silica network and in the non-centrosymmetric sites of the CaF 2 nanocrystals structure. Lower phonon energy of CaF 2 and the dehydratation processes reduce the probability of non-radiative de-excitations and as a result a luminescence enhancement and an increase of the PL lifetime is observed. Eu-doping modifies the traps distribution within the glass–ceramics and as a result an additional broad thermoluminescence peak at about 380 °C was observed. The peak was assigned to the recombination of the electrons thermal released from the Eu 3+-related traps in the glass matrix and in the CaF 2 nanocrystals.

High pressure spectroscopy study of SCF Tb3Al5O12:Mn

Single crystal film of Tb3Al5O12:Mn, with concentration of Mn approximately equal to 0.07 at%, has been grown by liquid phase epitaxy on Y3Al5O12 substrate. We have measured the absorption, luminescence, luminescence excitation spectra and luminescence kinetics of the film. To analyze the energetic structure and to estimate the crystal field strength we have measured the photoluminescence spectra under high hydrostatic pressure applied in diamond anvil cell up to 110 kbar. The Mn emission was consisted with the band peaked at 16800 cm−1 that shifts toward the lower energy with increasing pressure with the rate −9 cm−1/kbar. To characterize the Mn charge state we performed calculations of Tanabe -Sugano diagrams for 3d5, 3d4 and 3d3 system. The negative pressure shift of the 16800 cm−1 band yields attribution of this band to the 4T1 →6A1 transition in dodecahedrally coordinated Mn2+ ion. We have estimated quantity of Dq/B equal to 1.8 ±0.2. The configurational coordinate diagram of the Mn2+(3d5) has been calculated to analyze the nonradiative deexcitation processes 4T1 → 2T2 →6A1.

Understanding the differences in photochemical properties of substituted aminopyrimidines

The luminescent patterns of several members of the aminopyrimidine family are very different, showing not fluorescence at all, only a fluorescence band, normal or anomalous, or dual fluorescence, depending on the substituents and on the environment (gas phase vs. polar solvents). In this work, we study the lowest excited states of several members of this family that exhibit different fluorescence patterns to try to explain their photochemistry and to understand the effect of the substituents and the environment. We have found that several excited states (local excited (LE), charge transfer (CT) and nN–π* states) have minima on the lowest excited potential energy surface (S1), being their relative energy the determinant factor of the luminescent behavior. If the more stable S1 minima are of nN–π* character, a non-radiative deexcitation channel is the most efficient and the system shows no fluorescence. If the CT and/or LE states are the most stable, the non-radiative deactivation channel is not accessible and the system fluoresces. The relative energies of the CT and LE minima (affected by substituents and by the presence of a polar solvent) and the different magnitude of the oscillator strength for the radiative transition to the ground state determine which emission is more efficient, giving place to normal, anomalous or dual fluorescence. The study has been carried out by CASSCF/CASPT2 computations, including the solvent effect by means of the PCM model.

Spin-orbit effects on the photophysical properties of Ru(bpy)32+

We present in this article a detailed study of the photophysics of the Ru(bpy)(3)(2+) complex based on a formalism using density functional theory and an a posteriori treatment of the spin-orbit coupling. The absorption and emission spectra were computed and a very good agreement was obtained with the available experimental data. This work also reveals for the first time that the triplet metal-centered (MC) dd state cannot deactivate radiatively and corresponds to a transient structure for the nonradiative de-excitation of the triplet metal-to-ligand charge transfer (MLCT) state. Thus, a competition takes place between the luminescence of the MLCT state and nonradiative decay back to the ground state via population of the MC state. This radiationless return to the ground state occurs via a deactivation funnel which has been characterized for the first time by optimizing the minimum energy crossing point between the triplet MC and singlet ground state potential energy surfaces. Its low-lying energy position relative to the energy necessary to access the MC minimum suggests that this funnel will be readily accessible.

Listening to Nanomaterials: Photoacoustic Spectroscopy

An efficient and low-cost spectroscopic tool, photoacoustic spectroscopy (PAS), is described. Although this tool is suitable for all levels of chemistry and physics laboratories, it is particularly useful in an undergraduate laboratory. PAS is an excellent technique for studying the optical absorption spectra of opaque and highly light-scattering substances such as nanomaterials. It also has the capability of obtaining the depth profiles of crystals and thin films. Unlike in the conventional absorption and fluorescence spectroscopic techniques, PAS monitors the pressure changes in the gas medium surrounding the sample owing to the heat generated as a result of nonradiative deexcitation following optical absorption. The technique virtually requires no sample preparation. A few examples to demonstrate the usefulness of this tool are given. Fundamental knowledge on spectroscopic techniques like PAS will be an asset to those students who opt for graduate studies or those students who go on to a career in industry.

Band-to-band and direct optical excitation of Er in silicon: Comparison of kinetics, temperature dependence of erbium PL

Photoluminescence excitation spectra of erbium ions in the epitaxial Si:Er/Si structures have been measured in a wide range of excitation wavelengths ( λ ex =800–1580 nm) including the range of direct optical excitation of Er ( λ ex =1460–1580 nm). The temperature quenching of erbium PL have been compared for the regimes of direct optical excitation and excitation by photo-generated carriers in silicon matrix. It has been demonstrated that in case of direct optical excitation of Er ions the temperature quenching of Er PL was much weaker than under band-to-band pumping and the major processes of erbium non-radiative deexcitation such as Auger-deexcitation with equilibrium free carriers and “back-transfer” are substantially suppressed.

Optical properties of Er 3+ in fullerenes and in β-PbF 2 single-crystals

With the aim of providing a thorough description of the optical properties of Er 3+-doped endohedral fullerenes, we studied their characteristics in the light of those of well-known Er 3+-doped β-PbF 2 single-crystals. Various Er 3+-doped endohedral fullerenes were considered: Er 2C 2@C 82, where the Er 2C 2 group is encapsulated inside a cage of 82 carbon atoms and the Er 3− x Sc x N@C 80 ( x = 0, 1 and 2) family, where the Er 3N, Er 2ScN and ErSc 2N clusters are trapped in a 80 carbon atom cage. In this article, we discuss the absorption and photoluminescence of trivalent erbium ions in fullerenes and in β-PbF 2 crystals. The extinction coefficient of Er 3N@C 80 was found to be 4.8 (±0.5) × 10 3 mol/l −1 cm −1 at 540 nm, due to the C 80 cage absorbance. Even in a saturated fullerene solution, the absorption of Er 3+ encapsulated inside a C 80 cage cannot be observed at room temperature. We suggest that this is due to an insufficient number of Er 3+ ions in the solution and their low absorption cross-section. Low temperature photoluminescence measurements show that the line width of Er 3+ in a carbon cage, dissolved in a polycrystalline solvent, is similar to the one of Er 3+ in β-PbF 2 single-crystals. The quantum efficiency of Er 3+ at 1.5 μm in fullerenes is four orders of magnitude lower than that for Er 3+ in crystals, due to very efficient non-radiative decay processes. Molecular vibrations of the cage might be responsible for those rapid non-radiative de-excitations.

Nonradiative De-excitation Mechanisms in Long-Lived Erbium(III) Organic Compounds ErxY1-x[(p-CF3-C6F4)2PO2]3

We have performed a spectroscopic study of ErxY1-x[(p-CF3-C6F4)2PO2]3 aimed at understanding nonradiative de-excitation mechanisms. These fluorinated compounds have a long lifetime for the erbium 4I13/2-->4I15/2 emission at lambda approximately 1540 nm, but the lifetime increases with decreasing x. We have studied the lifetime as a function of morphology, temperature, and high hydrostatic pressure. We have demonstrated the occurrence of energy migration and calculated the corresponding activation energy. Moreover, using high pressure techniques, we provide evidence that cross-relaxation involving energy transfer from an excited erbium in the 4I13/2 promoting a neighbor in the same state to 4I9/2 is the dominant mechanism at ambient conditions for short erbium-erbium distances. The model explains the observed dynamics of excited states in the series and is tested against the Yb[(p-CF3-C6F4)2PO2]3 compound.

Er-based materials for Si microphotonics

We have investigated the role of the Si excess on the photoluminescence properties of Er-doped Si nanoclusters (Si nc). We demonstrate that the Si excess has two competing roles: when agglomerated to form Si nc it enhances the Er excitation efficiency but it also introduces new non-radiative decay channels. When Er is excited through an energy transfer from Si nc, the beneficial effect on the enhanced excitation efficiency prevails and the Er emission increases with increasing Si content. The luminescence quenching processes limiting quantum efficiency in Er-doped Si nc light emitting devices are investigated and identified. It is found that carrier injection, while needed to excite Er ions through electron–hole recombination, at the same time produces an efficient non-radiative Auger de-excitation with trapped carriers. The maximum excited Er fraction in this system is only of the order of percent. In order to increase the concentration of excited Er ions, a different approach based on Er silicate thin films has been explored. Under proper annealing conditions, an efficient luminescence at 1535 nm is found and all the Er ions in the material are optically active. The possibility to efficiently excite Er ions also through electron–hole mediated processes is demonstrated in nanometer-scale Er–Si–O/Si multilayers.

New approaches for enhancing light emission from Er-based materials and devices

In this work, we present some approaches recently developed for enhancing light emission from Er-based materials and devices. We have investigated the luminescence quenching processes limiting quantum efficiency in light-emitting devices based on Si nanoclusters (Si nc) or Er-doped Si nc. It is found that carrier injection, while needed to excite Si nc or Er ions through electron–hole recombination, at the same time produces an efficient non-radiative Auger de-excitation with trapped carriers. A strong light confinement and enhancement of Er emission at 1.54 μm in planar silicon-on-insulator waveguides containing a thin layer (slot) of SiO 2 with Er-doped Si nc at the center of the Si core has been obtained. By measuring the guided photoluminescence from the cleaved edge of the sample, we have observed a more than fivefold enhancement of emission for the transverse magnetic mode over the transverse electric one at room temperature. Slot waveguides have also been integrated with a photonic crystal (PhC), consisting of a triangular lattice of holes. An enhancement by more than two orders of magnitude of the Er near-normal emission is observed when the transition is in resonance with an appropriate mode of the PhC slab. Finally, in order to increase the concentration of excitable Er ions, a completely different approach, based on Er disilicate thin films, has been explored. Under proper annealing conditions crystalline and chemically stable Er 2Si 2O 7 films are obtained; these films exhibit a strong luminescence at 1.54 μm owing to the efficient reduction of the defect density.

Features of erbium nonradiative deexcitation and electroluminescence temperature quenching in sublimation MBE-grown Si:Er/Si diode structures

Temperature dependencies of electroluminescence intensity and decay kinetics from n-Si:Er:O/p-Si diodes, grown by sublimation molecular-beam epitaxy, have been studied. Radiative lifetime of excited Er 3+ ion (1.1 ms) and activation energy of the nonradiative deexcitation process (70 meV) have been measured. Contribution of nonradiative deexcitation of erbium ions to the temperature quenching of electroluminescence has been determined. It is revealed that considerable part of luminescence temperature quenching is due to the decrease in erbium excitation efficiency with an increase in temperature.

Strong coupling between adenine nucleobases in DNA single strands revealed by circular dichroism using synchrotron radiation

Circular dichroism (CD) experiments on DNA single strands (dA(n)) at the ASTRID synchrotron radiation facility reveal that eight adenine (A) bases electronically couple upon 190 nm excitation. After n=8, the CD signal increases linearly with n with a slope equal to the sum of the coupling terms. Nearest neighbor interactions account for only 24% of the CD signal whereas electronic communication is limited to nearest neighbors for two other exciton bands observed at 218 and 251 nm (i.e., dimer excited states). Electronic coupling between bases in DNA is important for nonradiative deexcitation of electronically excited states since the hazardous energy is spread over a larger spatial region.

Solvent Effects on Internal Conversions and Intersystem Crossings: The Radiationless De-Excitation of Acrolein in Water

An extended version of the averaged solvent electrostatic potential from molecular dynamics data (ASEP/MD) method oriented to the study of the solvent effects on internal conversion and intersystem crossing processes is presented. The method allows for the location of crossing points between free energy surfaces both in equilibrium and in frozen solvent conditions. The ground and excited states of the solute molecule are described at the complete active space self-consistent field (CASSCF) level while the solvent structure is obtained from molecular dynamics simulations. As an application, we studied the nonradiative de-excitation of s-trans-acrolein 1(n --> pi*) in aqueous solution. We found that the solvent modifies the relative stability of the different crossing points but not enough as to alter the relative order of stability with respect to the in vacuo situation. The relaxation through an equilibrium path involves a strong solvent reorganization. On the contrary, the nonequilibrium path does not involve solvent motion and the de-excitation could proceed with the same speed as in vacuo.

Thulium environment in a silica doped optical fibre

Thulium-doped optical fibre amplifiers (TDFA) are developed to extend the optical telecommunication wavelength division multiplexing (WDM) bandwidth in the so-called S-band (1460–1530nm). The radiative transition at 1.47μm (3H4→3F4) competes with a non-radiative multi-phonon de-excitation (3H4→3H5). The quantum efficiency of the transition of interest is then highly affected by the phonon energy (Ep) of the material. For reliability reasons, oxide glasses are preferred but suffer from high phonon energy. In the case of silica glass, Ep is around 1100cm−1 and quantum efficiency is as low as 2%. To improve it, phonon energy in the thulium environment must be lowered. For that reason, aluminium is added and we explore three different core compositions: pure silica, and silica slightly modified with germanium or phosphorus. The role of aluminium is studied through fluorescence decay curves, fitted according to the continuous function decay analysis. From this analysis, modification of the thulium local environment due to aluminium is evidenced.

The influence of carotene and bacteriopheophytin c addition on paths of nonradiative and radiative deexcitation of the oil from Amarantus seeds

The absorption, fluorescence emission and excitation as well as steady state photoacoustic spectra (PAS) of the oil from Amarantus seeds, without and with carotene and bacteriopheophytin c (BPhe c) addition, have been measured. The oil investigated originally contains only some amount of carotene but no admixtures of chlorophyll-like pigments. As follows from the dependence of the photoacoustic signal multiplied by the frequency of light modulation on this frequency, the kinetics of thermal deactivation (TD) of oil depends on the pigment contents. The relative values of thermal deactivation calculated for the samples with pigments added to that of the original oil depend on the type of the pigment added. The relative yields of fluorescence have been also calculated. The results suggest that also the photochemical reactions taking place in various illuminated samples could be different. This suggestion has been experimentally confirmed. The photochemical stability of the oil sample increases when carotene is added. The information about the oil stability is important in order to establish proper conditions for oil storage and transportation.