Cooperative Energy Transfer Mechanism Research Articles

Near infrared fluorescence properties of Tb4+ and Tb3+ inter-conversion states in CaF2:Yb3+nanocrystals

Near infra-red emission was enhanced in calcium fluoride (CaF2) co-doped with terbium ions and ytterbium (Yb3+) under a 325 nm laser excitation wavelength, during investigations performed for possible application in Si solar cells. Tb3+ ions were oxidized to Tb4+ ions and emission enhancement occurred due to a cooperative energy transfer mechanism between the Tb4+-Yb3+ couple. The oxidation of Tb3+ ions in CaF2 was achieved by annealing the nanocrystals at 450 °C in air. X-ray diffraction analysis confirmed the cubic face of the CaF2 crystal without any impurity peaks. UV–VIS luminescence and X-ray photoelectron spectroscopy were conducted to probe the conversion of Tb3+ to Tb4+ ions as a function of the annealing time. The main conclusion was that the NIR emission from the Tb4+-Yb3+ couple revealed that down-conversion occurred through a cooperative energy transfer mechanism.

A systematic interpretation of the quantum cutting effect by a cooperative energy transfer mechanism in Te4+/Yb3+ co-doped tellurite glasses

The near-infrared downconversion (DC) mechanism in Te4+/Yb3+ co-doped 75TeO2–25Li2O tellurite glasses (amounts in mol%) was closely followed using optical and thermal spectroscopy techniques. The glasses were prepared by the conventional melt-quenching method, with a melting temperature of 800 °C, in an ambient atmosphere, which were the best synthesis conditions for observing Te4+ in the glass. The results indicated that excitation in the ultraviolet region led to an intense emission of two NIR photons (at around 978 nm) in the co-doped tellurite glasses. This effect revealed the occurrence of a cooperative energy transfer (CET) mechanism in the system, where a Te4+ ion was responsible for the excitation of two Yb3+ ions. The CET efficiency (ηCET) was calculated from the Te4+ lifetime, obtaining a maximum of 74% for the tellurite glass prepared with the highest Yb3+ concentration. The use of thermal lens spectroscopy confirmed the quantum cutting effect, by observing the dependence of the thermal properties of the glass on the Yb3+ concentration. A maximum DC efficiency of 137% was measured for the sample with 4 mol% of Yb3+.

Energy Transfer Studies in Tb3+-Yb3+ Co-Doped Phosphate Glasses.

Detailed optical properties of Tb3+-Yb3+ co-doped phosphate glasses were performed based on their emission spectra and decay measurements. Under blue excitation of Tb3+ at 488 nm, the intensity of Yb3+ emissions gradually enhanced upon increasing the Yb3+ content until 1 mol% indicated an energy transfer from Tb3+ to Yb3+. Otherwise, under near infrared excitation of Yb3+ at 980 nm, these glasses exhibit intense green luminescence, which led to cooperative sensitization of the 5D4 level of Tb3+ ions. A cooperative energy transfer mechanism was proposed on the basis of the study on the influence of Yb3+ concentration on up-conversion emission intensity, as well as the dependence of this up-conversion intensity on near infrared excitation power. Moreover, the temporal evolution of the up-conversion emissions have been studied, which was in positive agreement with a theoretical model of cooperative up-conversion luminescence that showed a temporal emission curve with rise and decay times of the involved levels.

Cooperative emission mediated by cooperative energy transfer to a plasmonic antenna

We develop a theory of cooperative emission mediated by cooperative energy transfer (CET) from an ensemble of quantum emitters (QEs) to plasmonic antenna at a rate equal to the sum of individual QE-plasmon energy transfer rates. If the antenna radiation efficiency is sufficiently high, the transferred energy is radiated away at approximately the same cooperative rate that scales with the ensemble size. We derive explicit expressions, in terms of local fields, for cooperative Purcell factor and enhancement factor for power spectrum valid for plasmonic structures of any shape with characteristic sizes smaller than the radiation wavelength. The radiated power spectrum retains the plasmon resonance lineshape with overall amplitude scaling with the ensemble size. If QEs are located in a region with nearly constant plasmon local density of states (LDOS), e.g., inside a plasmonic nanocavity, we demonstrate that the CET rate scales linearly with the number of excited QEs, consistent with the experiment, and can be tuned in a wide range by varying the excitation power. For QEs distributed in an extended region saturating the plasmon mode volume, we show that the cooperative Purcell factor has universal form independent of the system size. The CET mechanism incorporates the plasmon LDOS enhancement as well, giving rise to possibilities of controlling the emission rate beyond field enhancement limits.

On the efficient Te4+→Yb3+ cooperative energy transfer mechanism in tellurite glasses: A potential material for luminescent solar concentrators

Te4+/Yb3+ co-doped 80%TeO220%Li2O glasses (amounts in mol%) were prepared by the conventional melt-quenching method in an ambient atmosphere, in order to investigate their optical characteristics for application in solar cells. An efficient near-infrared down-conversion mechanism was observed in these samples, involving the emission of two near infrared photons (at around 978 nm) after the absorption of one photon in the ultraviolet-blue region. This effect occurred by means of a process of cooperative energy transfer (CET) from Te4+ to Yb3+ ions. The CET efficiency (ηCET) was calculated from radiative transitions (lifetime) and also by a new method, developed in this work, based on rate equations quantifying the non-radiative transitions (thermal effect) involved in the system. The two methods showed very good agreement, with ηCET between 82 and 100% obtained for the sample prepared using a higher Yb3+ concentration (4 mol%). The results suggested that these tellurite materials could have potential applications in improving the efficiency of silicon-based solar cells.

Near-Infrared Quantum Cutting Luminescence and Energy Transfer Mechanism of Ba2Y(BO3)2Cl:Bi3+, Yb3+Phosphors

The quantum cutting luminescent materials have attracted a great of interest in the fields of solar cells. In this paper, the novel Ba2Y(BO3)2Cl:Bi3+, Yb3+ quantum cutting phosphors were prepared, and their quantum cutting luminescence and energy transfer mechanism were investigated. The visual emission of Bi3+ was observed ranging from 350 to 650 nm, which is attributed to the 3P1→1S0 transition of Bi3+. The near infrared emission ranging from 950 to 1100 nm is due to the 2F7/2→2 F5/2 transition of Yb3+. The cooperative energy transfer mechanism from the Bi 3+ to Yb3+ in Ba2Y(BO3)2Cl:Bi3+, Yb 3+ phosphor was investigated, and their cooperative energy transfer process could be expressed as the Bi 3+(3P1)→Yb3+(2F5/2) + Yb3+( 2F5/2). The maximum quantum cutting efficiency was about 170.2%, which suggested the Bi3+ and Yb3+ co-doped Ba2Y(BO3)2Cl phosphors can act as a promising candidate for the silicon solar cells.

Infrared cascade and cooperative multicolor upconversion emissions in Y_8V_2O_17:Eu:Yb nanophosphors

This work reports on the efficient cooperative upconversion and infrared cascade downconversion emissions in a novel Y₈V₂O₁₇:Eu:Yb nanophosphor. The excitation with UV light produces emission in the 950-1000 nm region, corresponding to the Yb³⁺:²F₅/₂-->²F₇/₂ transition, as well as visible emissions of the Eu³⁺ ion. Time-resolved spectroscopy measurements revealed that the mechanism responsible for this transition is the efficient cascade nonresonant energy transfer from VO₄³-->Eu-->Yb. When the same nanophosphor is excited with 976 nm radiation, bright reddish upconversion emission of the Eu³⁺:⁵DJ-->⁷FJ transition is observed as consequence of the Yb + Yb-->Eu cooperative energy transfer mechanism, which was established by analyzing the emission power dependence and the time-resolved spectroscopy of radiative transitions.

Broadband downconversion based near-infrared quantum cutting via cooperative energy transfer in YNbO4:Bi3+, Yb3+ phosphor

YNbO4:Bi3+, Yb3+ phosphor was prepared to study the quantum cutting process of converting one ultraviolet photon into two near-infrared photons. An intense near-infrared emission of Yb3+:2F5/2 → 2F7/2 around 1 μm was observed under the ultraviolet excitation belonging to the broadband absorption of the [NbO4]3- group and the Bi3+ ion. The photoluminescence spectra and decay lifetime measurements indicate efficient energy transfer from Bi3+ to Yb3+ ions, which is attributed to be of a cooperative energy transfer mechanism. The YNbO4:Bi3+, Yb3+ phosphor with optimized doping concentration may be applicable in improving the efficiency of silicon-based solar cells.

Cooperative energy transfer in Tb 3+/Yb 3+- and Nd 3+/Yb 3+/Tb 3+-codoped oxyfluoride glasses

This paper reports on cooperative energy transfer and upconversion luminescence properties of Tb 3+/Yb 3+- and Nd 3+/Yb 3+/Tb 3+-codoped oxyfluoride glasses. Upon excitation with a 980 nm laser diode, an intense green upconversion luminescence along with weak ultraviolet (UV)–visible emissions has been observed in Yb 3+/Tb 3+-codoped oxyfluoride glasses. Power dependence of UV–visible upconversion luminescence intensity has been examined, revealing that a cooperative energy transfer mechanism from Yb 3+ ions is responsible for the excitation of Tb 3+ ions. Meanwhile, it is noticed that Tb 3+ upconversion emission bands have also been clearly detected at 487, 542, 587 and 620 nm in Nd 3+/Yb 3+/Tb 3+-codoped oxyfluoride glasses upon excitation with a 808 nm laser diode. The quadratic dependence of the upconversion luminescence on the pump-laser power indicates two-photon process for the population of Tb 3+: 5D 4 state via Nd 3+→Yb 3+→Tb 3+ energy transfer. However, no emission has been observed in the oxyfluoride glasses codoped with Yb 3+/Tb 3+ or Nd 3+/Tb 3+, respectively, upon excited at a 808 nm laser diode. A proposed upconversion mechanism involving energy transfer from Nd 3+ to Yb 3+, and then a cooperative energy transfer process from two excited Yb 3+ to Tb 3+ has been presented.

Synthesis and Spectroscopic Properties of GdAl3(BO3)4 Poly-crystals Codoped with Yb3+ and Eu3+

GdAl3(BO3)4 polycrystals co-doped with Yb3+ and Eu3+ has been synthesised by combustion method with urea. Upon the excitation at 465 nm (Eu3+/7F6-->5D2 transition), emission bands centered at 590, 613, 697 and 702 nm in the wavelength region of 550-750 nm have clearly been observed, assigned to the electronic transitions of 5D0-->7FJ (J=1, 2, 4 and 5) of Eu3+ ions, respectively. Meanwhile, an intense emission centred at 980 nm along with a shoulder at 1,040 nm has also been observed by exploiting a cross-relaxation process between the transitions of Eu3+/5D0-->7F6 and Yb3+/2F7/2-->2F5/2. On the contrary, an intense red up-conversion emission centred at 613 nm originated from the 5D0-->7F2 transition of Eu3+ has been observed upon excitation with 980 nm laser diode. The quadratic dependence of the red up-conversion intensity on the pump-laser power reveals a cooperative energy transfer mechanism from a pair of Yb3+ ions to one Eu3+ ion.

Energy transfer mechanisms in Tb3+, Yb3+ codoped Y2O3 downconversion phosphor

Tb3+ and Yb3+ co-activated luminescent material that can cut one photon of around 483 nm into two NIR photons of around 1000 nm could be used as a downconversion luminescent convertor in front of crystalline silicon solar cell panels to reduce thermalization loss of the solar cell. The Tb3+ → Yb3+ energy transfer mechanisms in the UV–blue region in Y2O3 phosphor were studied by PL excitation spectra and time-resolved luminescence, from which the charge transfer mechanism and the cooperative transfer mechanism were identified. Tb3+ ions in the 4f75d1 state relax down to the 5D4 level and cooperatively transfer energy to two Yb3+ ions, which is followed by the emission of two photons (λ ∼ 1000 nm). It was found in the (Y0.79Tb0.01Yb0.20)2O3 sample that 37% of the Tb3+ ions at the 5D4 level transfer energy to two neighbouring Yb3+ ions by the cooperative energy transfer mechanism Tb3+ (5D4) → 2Yb3+ (2F5/2). Unfortunately, the high Yb3+ concentration leads to severe concentration quenching that significantly reduces the external quantum efficiency. Moreover, the energy of the Tb3+ 4f75d1 state can also be lost non-radiatively or transferred to the Yb3+ 2F5/2 state via the charge transfer state Tb4+–Yb2+. In conclusion, RE3+ (RE = Ce, Pr, Tb) with strong absorption in the UV region is not an appropriate sensitizer of Tb3+ in Tb3+–Yb3+ codoped downconversion phosphor.