Nikifor Rakov

Spectroscopic analysis of Eu3+ -and Eu3+:Yb3+-doped yttrium silicate crystalline powders prepared by combustion synthesis

Yttrium silicate powders doped with Eu3+ and codoped with Eu3+ and Yb3+ were prepared by combustion synthesis. The x-ray powder diffraction data showed the presence of Y2SiO5 and Y2Si2O7 crystalline phases. Singly doped (1 wt %) sample illuminated with ultraviolet light (λ=256 nm) showed the characteristic red luminescence corresponding to D50→F7J transitions of Eu3+. The Judd–Ofelt intensity parameters were calculated from experimental data and the radiative and nonradiative relaxation rates were estimated. The results showed that the nonradiative relaxation rate is smaller in yttrium silicate compared to yttrium oxide powder, a reference material, prepared under similar conditions. Codoped samples were exposed to near-infrared laser excitation (λ=975 nm) and the red luminescence of Eu3+ was also observed. In this case, the luminescence is achieved due to a cooperative upconversion (CUC) process involving energy transfer (ET) from pairs of ytterbium ions to europium ions. The ET rate was estimated by fit...

Strong upconversion from Er3Al5O12 ceramic powders prepared by low temperature direct combustion synthesis

Crystalline ceramic powders of Er3Al5O12 were obtained by low temperature direct combustion synthesis. Irradiating the sample with a low-power continuous-wave infrared (1.48 mu m) diode laser led t ...

Nd 3+ -Yb 3+ doped powder for near-infrared optical temperature sensing

Er³⁺ doped powders are generally used for fluorescence-based temperature sensing application when near-infrared lasers are the excitation sources of choice. The fluorescence of Er³⁺ is produced by nonlinear (upconversion) processes, which generate strong internal heat. Lowering the excitation power causes drastic reduction of the fluorescence signal, and as a consequence the sensor applicability of Er³⁺ doped powders becomes compromised. Here we propose the use of the downconverted fluorescence of Yb³⁺ produced by efficient energy transfer from Nd³⁺ as an alternative temperature sensing system. Our results are presented for yttrium silicate powders prepared by combustion synthesis.

Photon conversion in lanthanide-doped powder phosphors: concepts and applications

The investigation of luminescent lanthanide doped nanostructures (LDNs) has attracted a great deal of interest in the last decades. This is a multidisciplinary research field in which scientific challenges as well as societal needs are addressed by chemists, physicists, and materials scientists. In this scenario, research groups involved in developing novel LDNs have concentrated their efforts on engineering structures with superior luminescence quantum efficiencies for photonics applications. In this article, we briefly present the strategy that we have been using to develop nanocrystalline lanthanide-doped powders with superior luminescence yield using the combustion synthesis technique. Furthermore, the use of these lanthanide-doped powders as up-conversion phosphors in temperature sensing, lighting and solar cell technologies is discussed.

Europium luminescence enhancement in Al2O3:Eu3+ powders prepared by direct combustion synthesis

The luminescence properties of Eu3+:Al2O3 powders prepared via low temperature direct combustion synthesis was investigated. It was observed that the heat treatment of the powders modifies the dynamics of the radiative transition D05→F27 of Eu3+ (1.0mol%) and produces an enhancement of the luminescence intensity by nearly one order of magnitude. The luminescence enhancement is attributed to the presence of Eu3+ in α-Al2O3 crystalline phase as the heat treatment drastically reduces the amount of amorphous Al2O3 phases present in the powder.

Near-infrared quantum cutting in Ce3+, Er3+, and Yb3+ doped yttrium silicate powders prepared by combustion synthesis

Yttrium silicate (YS) powders doped with Ce3+, Er3+, and Yb3+ were prepared by combustion synthesis. The material was investigated for use as energy downconverters to reduce thermalization losses in crystalline Si solar cells. The powders were excited by UV light (355 nm), and near-infrared emission around 1 μm was observed corresponding to a quantum cutting (QC) effect. The QC process occurs via cooperative energy transfer from Ce3+ (sensitizer) to Yb3+ (activator) in Ce3+:Yb3+ co-doped YS powders. QC was also observed in Er3+:Yb3+ co-doped YS powder via sequential energy transfer. The idea of synergy by use of Ce3+:Er3+:Yb3+ triply doped system to enhance the QC efficiency was investigated. We observed that the QC performance of Ce3+:Er3+:Yb3+ triply doped YS powder is not superior to that of Ce3+:Yb3+ co-doped YS powder due to near-infrared luminescence quenching induced by energy back-transfer from Yb3+ to Er3+.

An efficient energy transfer process in Nd3+:Yb3+ co-doped SrF2 powders containing Al3+ and prepared by the combustion synthesis technique

Strontium fluoride (SrF2) powders containing aluminum (Al3+) and doped with rare-earth neodymium (Nd3+) and ytterbium (Yb3+) were synthesized using the combustion synthesis technique. Heat-treated (700 °C for 3 h) powder samples presented the SrF2 network arranged in a face-centered cubic phase. Under pulsed laser excitation at λ = 750 nm, at room temperature, near-infrared luminescence attributed to 4f–4f electronic relaxation 4F3/2 → 4I9/2 from Nd3+ was observed in the spectral range λ ∼ 850–930 nm for both Nd3+ doped and Nd3+:Yb3+ co-doped SrF2 powder samples. Nd3+:Yb3+ co-doped SrF2 powder samples presented an additional luminescence band peak at λ ∼ 980 nm assigned to the 2F5/2 → 2F7/2 transition of Yb3+. Quenching of the luminescence originating from the 4F3/2 state of Nd3+ was observed in co-doped samples and the phenomenon was assigned to energy transfer (ET). ET was investigated by analysing the near-infrared luminescence dynamics of Nd3+ using the Inokuti–Hirayama model. The energy transfer mechanism was found to be of a quadrupole–quadrupole type with efficiency as high as 62%. Our results show that this system is a potential phosphor for photonics applications involving near-infrared light sources.

Three- and four-photon excited upconversion luminescence in terbium doped lutetium silicate powders by femtosecond laser irradiation

Frequency upconversion (UC) luminescence was investigated in terbium (Tb3+) doped lutetium silicate powders when the samples were irradiated with femtosecond lasers operating either at 800 nm or 1500 nm. The samples with three different Tb3+ concentrations were prepared by the combustion synthesis method. Rietveld analysis of the X-ray powder diffraction data showed the predominance of monoclinic Lu2SiO5 phase. UC luminescence signals induced by three- and four-photon absorption were identified. The mechanisms that originate the anti-Stokes luminescence were discussed.

Anomalous up-conversion dynamics in rare-earth doped yttrium oxide powders

We performed up-conversion (UC) experiments in rare-earth (Er3+, Tm3+, Nd3+) doped yttrium oxide (Y2O3) powders using a high power (1.6 W) near-infrared (808 nm) diode laser modulated at 20 Hz as the excitation source. Below the threshold for incandescence, the analysis of the dynamics of the UC luminescence showed the presence of an unusual depletion of the luminescence signal. This anomalous effect could be controlled by the laser excitation power or the modulation frequency or the combination of rare-earth species doped in the samples. The phenomenon was associated with a thermo-optical effect generated by strong laser absorption, poor thermal dissipation of the powder and energy transfer (ET) between different rare-earth species. The experimental results also led us to believe that the ET mechanism cools down the rare-earth donor, Tm3+, and heats up the rare-earth acceptor, Er3+.

Upconversion fluorescence and its thermometric sensitivity of Er3+:Yb3+ co-doped SrF2 powders prepared by combustion synthesis

Upconversion fluorescence of co-doped Er3+:Yb3+:SrF2 powders prepared by combustion synthesis was investigated under near-infrared (λ = 980 nm) continuous wave laser excitation. Surface morphology of the samples and structures of the Er3+:Yb3+:SrF2 powders were studied with scanning electronic microscopy, energy dispersive x-ray, and x-ray powder diffraction. The spectrum of the fluorescence contains bands centered at ~410, ~522, ~545 and ~660 nm, corresponding respectively to transitions from upper levels 2H9/2, 2H11/2, 4S3/2 and 4F9/2 to the ground state 4I15/2, which can be identified as 4f-4f transitions from Er3+ excited states. In addition, the fluorescence is found sensitive to the temperature, which suggests that an optical temperature sensor would be feasible. The maximum sensitivity of the proposed sensor was found 0.00396 K−1.