Divalent Lanthanide Ions Research Articles

Enhancing two-photon spontaneous emission in rare earths using graphene and graphene nanoribbons

The enhancement of two-photon spontaneous emission (2PSE) from trivalent and divalent rare earth ions in proximity to graphene and graphene nanoribbons is calculated for achievable experimental conditions using a combination of finite difference time domain simulations and direct computation of transition rates between energy levels in rare earths. For Er$^{3+}$, we find that the 2PSE rate is initially 8 orders lower than the single-photon spontaneous emission rate but that, with enhancement, 2PSE can reach 2.5% of the overall decay. When graphene nanoribbons are used, we also show that the emission of free-space photon pairs from Er$^{3+}$ at 3 - 3.2 $\mu$m via 2PSE can be increased by $\sim 400$. Our calculations show significantly less relative graphene-enhanced 2PSE than previous works, and we attribute this variation to differences in emitter size and assumed graphene mobility. We also show that the internal energy structure of the ion can have an impact on degree of 2PSE enhancement achievable and find that divalent rare earths are more favorable.

A Trinuclear Gadolinium Cluster with a Three-Center One-Electron Bond and an S = 11 Ground State.

The recent discovery of metal-metal bonding and valence delocalization in the dilanthanide complexes (CpiPr5)2Ln2I3 (CpiPr5 = pentaisopropylcyclopentadienyl; Ln = Y, Gd, Tb, Dy) opened up the prospect of harnessing the 4fn5dz21 electron configurations of non-traditional divalent lanthanide ions to access molecules with novel bonding motifs and magnetism. Here, we report the trinuclear mixed-valence clusters (CpiPr5)3Ln3H3I2 (1-Ln, Ln = Y, Gd), which were synthesized via potassium graphite reduction of the trivalent clusters (CpiPr5)3Ln3H3I3. Structural, computational, and spectroscopic analyses support valence delocalization in 1-Ln resulting from a three-center, one-electron σ bond formed from the 4dz2 and 5dz2 orbitals on Y and Gd, respectively. Dc magnetic susceptibility data obtained for 1-Gd reveal that valence delocalization engenders strong parallel alignment of the σ-bonding electron and the 4f electrons of each gadolinium center to afford a high-spin ground state of S = 11. Notably, this represents the first clear instance of metal-metal bonding in a molecular trilanthanide complex, and the large spin-spin exchange constant of J = 168(1) cm-1 determined for 1-Gd is only the second largest coupling constant characterized to date for a molecular lanthanide compound.

HRBE diagrams for lanthanides and the temperature dependence of Tb3+ luminescence in zinc-sodium-lanthanum-borate glass

Zinc-sodium-lanthanum-borate (ZNLB) glasses undoped and doped with trivalent lanthanide ions (Ce3+, Eu3+ and Tb3+) ions have been synthesized by the melt-quenching method. The lowest energies of the f-d transition for divalent and trivalent lanthanide ions in ZNLB glass have been predicted based on the redshift model and the excitation spectrum of Ce3+ ions. The 4f and 5d ground-state energy diagrams of the Ln2+ and Ln3+ ions within the bandgap of ZNLB glass have been constructed based on the excitation spectra of Ce3+ and Eu3+ ions by using the chemical shift model. The luminescence quenching of Tb3+ ions due to temperature as well as energy transfer from Ce3+ to Tb3+ has been also predicted from the host referred binding energy (HRBE) diagrams of the lanthanide ions and confirmed by the experimental data. The energy transfer process from charged intrinsic defects to Tb3+ ions in ZNLB glass has also been discussed in detail.

Toward Opto-Structural Correlation to Investigate Luminescence Thermometry in an Organometallic Eu(II) Complex

Lanthanide-based luminescent materials have unique properties and are well-studied for many potential applications. In particular, the characteristic 5d → 4f emission of divalent lanthanide ions such as EuII allows for tunability of the emissive properties via modulation of the coordination environment. We report the synthesis and photoluminescence investigation of pentamethylcyclopentadienyleuropium(II) tetrahydroborate bis(tetrahydrofuran) dimer (1), the first example of an organometallic, discrete molecular EuII band-shift luminescence thermometer. Complex 1 exhibits an absolute sensitivity of 8.2 cm-1 K-1 at 320 K, the highest thus far observed for a lanthanide-based band-shift thermometer. Opto-structural correlation via variable-temperature single-crystal X-ray diffraction and fluorescence spectroscopy allows rationalization of the remarkable thermometric luminescence of complex 1 and reveals the significant potential of molecular EuII compounds in luminescence thermometry.

Study of spectroscopy of Eu3+ and energy transfer from Ce3+ to Eu3+ in sodium-zinc-lead-borate glass

Eu3+ and Ce3+ singly doped as well as co-doped into sodium-zinc-lead-borate (NZPB) glass were prepared by the melt-quenching technique. Optical spectra including excitation, emission spectra of Ce3+ and Eu3+ as well as decay curves for the excitation levels were measured at room temperature. The optical properties and the application ability of Eu3+-doped NZPB glass were analyzed by Judd-Ofelt theory. The host referred binding energy (HRBE) schemes of divalent and trivalent rare earth ions in NZPB glass were constructed using the luminescence excitation spectra of Eu3+ and Ce3+ ions. In the NZPB glass co-doped with Ce3+ and Eu3+ ions, the luminescence intensity and lifetime of the Ce3+:5d1 level decrease with the increasing Eu3+concentration. The luminescence quenching and lifetime of the Ce3+:5d1 level are related to two mechanisms of the resonance energy transfer and electron transfer. These transfer mechanisms were discussed in detail by using the 4fn:Ln3+ energy diagrams and the HRBE schemes.

Afterglow enhancement of AlMgGaO4:Cr3+, Ln3+ (Ln=Eu, Sm, Yb) guiding by VRBE diagram

The afterglow intensity of AlMgGaO4: Cr3+ has been enhanced by co-doping rare earth ions. The HRBE and VRBE diagram of the rare earth ions are constructed to determine the ground state energy level position of 4f in the forbidden band. The activation energy Ex is calculated to be 6.41 eV according to the diffuse reflection spectrum of AlMgGaO4, and the 4f level position of Eu2+ is obtained by the charge transfer band of Eu3+, whereby the ground state energy level position of all the divalent rare earth ions are determined. The energy difference between the ground state energy level of Eu3+ ion and Eu2+ ion are obtained by the centroids shift of the 5d energy level of Ce3+, and the ground state energy level position of all the trivalent rare earth ions are determined. The 4f energy levels of Eu2+, Sm2+ and Yb2+ are calculated to be 1.65, 0.4 and 1.21 eV below the conduction band, respectively, which can be used as energy storage levels of Cr3+. Then the near-infrared afterglow phosphors AlMgGaO4:0.03Cr3+, Ln3+ (Ln = Eu, Sm, Yb) are synthesized, and the afterglow properties are studied carefully. With Eu3+/Sm3+/Yb3+ concentrations increasing, the afterglow intensity is enhanced, and the intensity of thermoluminescence increases gradually and shifts to the high temperature, which indicates that the trap energy level of the phosphor is enriched with the increase of rare earth ion concentration. The results prove that constructing VRBE diagram is an effective way to select rare earth ions to enhance the afterglow intensity.

VUV spectroscopy of lanthanide doped fluoride crystals K2YF5

K2Y0.99Ln0.01F5 (Ln = Ce, Eu and Sm) single crystals have been synthesized by the hydrothermal technology. The optical properties of Ce3+ and Eu3+ ions doped into K2YF5 matrix have been studied under excitation in the deep UV and vacuum UV (VUV) region. The obtained data from excitation spectra and Dorenbos models have been used to predict the energies of the f-d and charge transfer transitions as well as the energy levels location of divalent (Ln2+) and trivalent (Ln3+) lanthanide ions within the band gap of K2YF5. In particular, the f-d and charge-transfer transitions of Sm3+ in the luminescence excitation spectrum of K2YF5:Sm3+ have been identified. The host referred binding energy (HRBE) and vacuum referred binding energy (VRBE) schemes have been constructed for Ln2+ and Ln3+ ions doped K2YF5 crystal which were used for the analysis of possible applications of K2YF5 doped with Ln3+ ions as thermoluminescence materials.

Luminescence of divalent lanthanide doped BaBrI single crystal under synchrotron radiation excitations

Luminescence excitation spectra of BaBrI single crystals doped by divalent lanthanide ions are studied using synchrotron radiation excitations from the MAX IV 1.5 GeV storage ring. The energy of the edge and the formation of core cation exciton as well as the energy threshold of the multiplications of electronic excitations is found. It was clearly established the energy transfer from intrinsic luminescence centers to Sm2+ and Eu2+ ions.

Synthesis and characterization of luminescent Ln3+ (Ln = Eu, Tb and Dy)-doped LiYF4 microcrystals produced by a facile microwave-assisted hydrothermal method

This paper reports microstructural and luminescent properties of lithium yttrium fluoride (LiYF4) doped with rare earth ions. Submicrometric crystals of LiYF4 were successfully produced using the microwave-assisted hydrothermal method, which allowed the formation of single crystalline phase at temperatures as low as 140 °C for a reaction time of 15 min. The structural study was carried out by X-ray diffraction (XRD), revealing single tetragonal phase. The electronic structure and microstructure formation of LiYF4 was investigated through the binding energy of the atoms, measured via X-ray photoemission spectroscopy (XPS) and scanning electron microscopy (SEM). The luminescent properties as scintillator were investigated through X-ray excited luminescence (XEL) and time-resolved luminescence. Spectroscopy studies were done via photoluminescence (PL) technique in the visible (VIS) and vacuum ultraviolet (VUV) regions, from ∼1.0 to 15.0 eV. PL excitation results showed that the band gap (Eg) of the LiYF4 is at around 11.0 (±0.1) eV. 4f-5d transitions were identified for Eu3+, Tb3+ and Dy3+. The experimental data were used to predict the energy levels of all divalent and trivalent lanthanide (Ln) ions relative to the valence and conduction bands of LiYF4 and to construct the host referred binding energy (HRBE) and vacuum referred binding energy (VRBE) schemes.

Optical absorption spectra of X-ray irradiated alkaline earth fluoride crystals doped with divalent rare-earth ions studied by thermal bleaching

The absorption spectra of X-ray irradiated alkaline earth fluoride (CaF2, SrF2, BaF2) crystals doped with Sm, Нo, Er, Tm ions are investigated by thermal bleaching of the sample in the 300–700 K temperature range. X-ray irradiation creates absorption bands of inter-configuration 4fn-4fn−1-5 d1 transitions of divalent impurity ions and F3− and (F2−)ii hole centers. The thermal oxidation of the divalent RE ions have been observed. Two possible mechanism of thermal oxidation of divalent lanthanide ions in alkaline-earth fluorides are discussed.

Synthesis, spectroscopic properties and applications of divalent lanthanides apart from Eu2+

Abstract In this review, modern insights into the synthesis and fundamental optical properties as well as applications of divalent lanthanides as spectral energy converters are discussed, which have especially evolved within the last two decades. An emphasis is put on the state-of-the-art knowledge about the luminescence of other divalent lanthanides than the most commonly applied representative Eu2+ and what special features in their luminescence arises due to the different number of 4fn electrons. In particular, the luminescence of Sm2+, Tm2+, Yb2+ and the less stable ions Nd2+ and Dy2+ will be regarded. A broad overview over the synthetic approaches to inorganic compounds based on divalent lanthanide ions will be conveyed as well and shall demonstrate the fascinating developments in the field of inorganic solid-state chemistry of the lanthanides. Besides their fundamental optical properties, also applications of the presented divalent lanthanide activators are discussed. Sm2+ is a well-known temperature and pressure sensor due to its interconvertible 4f‐4f and 5d-4f transitions. Moreover, nanocrystalline BaFCl:Sm2+ has an appreciable potential as novel X-ray storage phosphor. Tm2+ is interesting for upconversion in photovoltaic and lasing applications, whereas Yb2+ in halides may be a promising alternative to Eu2+ for scintillation materials and phosphors for light-emitting diodes (LEDs). Nd2+ and Dy2+ are currently emerging as novel near infrared (NIR)-absorbing and emitting activators that might become interesting as lasing ions. The spectroscopy of these unusual lanthanide ions provides many non-trivial features due to the excited two-open shell 4fn-15d1 configuration overlapping with the excited 4fn levels. Finally, a perspective for future developments will be given.

Synthesis of homoleptic, divalent lanthanide (Sm, Eu) complexes via oxidative transmetallation.

The direct synthesis of neutral, divalent samarium and europium complexes supported by the bulky bis(tris-tert-butoxysilyl)amide (BTTSA) ligand via oxidative transmetallation is reported. Through the use of a copper(i) ligand complex, conventional lanthanide halide starting materials for complex formation are circumvented and the clean formation of divalent complexes is achieved directly from the bulk metal. The structures of the [Ln(BTTSA)2] (Ln = Sm, Eu) complexes are isotypic, presenting divalent lanthanide ions with distorted, six-coordinate geometries.

Optical temperature sensing properties of a phosphor mixture of Sr2Mg3P4O15:Eu2+ and SrB4O7:Sm2+

In this work, a mixture of a Eu2+-doped phosphor (Sr2Mg3P4O15:Eu2+) and a Sm2+-doped phosphor (SrB4O7:Sm2+), noted as ESMix, was prepared to investigate its fluorescence intensity ratio (FIR) for the use of temperature sensing. Temperature-dependent luminescence of ESMix were measured under 365 nm excitation and the blue band emission of Eu2+ (centered around 450 nm) together with the orange band emission of Sm2+ (centered around 580 nm) were observed to have different trends with the change of temperature. FIR between the blue band and the orange band were proved to have high sensitivity from 380 K to 700 K. This method may be a promising FIR-type optical thermometry using divalent rare earth (RE) ions whose emissions are broad bands which is different from traditional FIR scheme using trivalent RE ions whose emissions are sharp peaks. Changing peaks into bands makes ESMix insensitive of peak shift and has great potential of reducing the requirements on the instruments thus saving costs.

Tertiary phosphine oxide complexes of lanthanide diiodides and dibromides

The reaction of OPR3 (R = Me, Ph) with YbI2, EuI2 and EuBr2 in rigorously anhydrous MeCN under N2 produces the divalent lanthanide complexes [LnX2(OPR3)4] (Ln = Yb, Eu, X = I; Ln = Eu, X = Br) in moderate to good yield, whilst [SmI2(OPR3)4] were obtained from SmI2 and OPR3 in dry, degassed thf. These are the first examples involving divalent lanthanide ions and the complexes have been characterised by microanalysis, IR, UV–Vis and 31P{1H} NMR spectroscopy. The X-ray crystal structure of [EuI2(OPPh3)4]·MeCN confirmed the six-coordinate Eu(II) species with a cis-octahedral geometry, which IR spectroscopy suggests is present in all of the OPPh3 complexes. In contrast the [LnX2(OPMe3)4] complexes appear to be trans isomers. The OPPh3 complexes are readily oxidised by dry O2 or I2, yielding the corresponding trivalent cations, [LnX2(OPPh3)4]+; a crystal structure of the product formed by oxidation of [EuI2(OPPh3)4] in MeCN solution confirms this to be [EuI2(OPPh3)4]I3·1.5MeCN, containing a trans-octahedral cation.

Development of persistent phosphor of Eu2+ doped Ba2SiO4 by Er3+ codoping based on vacuum referred binding energy diagram

We have developed the Ba2SiO4: Eu2+ -Er3+ persistent phosphor. The vacuum referred binding energy (VRBE) diagram of the Ba2SiO4 host with the 4f ground states of divalent and trivalent lanthanide ions in the host band gap was constructed from the obtained spectroscopic data and the Dorenbos's procedure. From the constructed diagram, the VRBE values of the conduction band (CB) bottom and valence band (VB) top were estimated to be −0.66 and −8.27 eV, respectively. From the energy gap between the CB bottom and 4f ground states of divalent lanthanide ions, we selected the Er3+ ion as an efficient co-dopant because the trap depth was estimated to be 0.64 eV, suitable for improving the persistent luminescence duration. The Eu2+-Er3+-codoped sample showed persistent luminescence intensity of over 0.32 mcd/m2 for 7 h duration. The initial persistent luminance intensity of the Eu2+-2%Er3+-codoped sample was found to be 250 times stronger than that of the only Eu2+-doped sample.

Self-reduction of Yb3+ to Yb2+ and red persistent luminescence of Na2CaGe2O6:Yb2+ phosphor

Phosphors are luminescent materials that are widely used in the lighting and displays of today's world. A reduction-atmosphere is commonly used to synthesize efficient broadband phosphors activated by divalent rare earth ions. In this work, we first report the unusual self-reduction of Yb3+ to Yb2+ in air in Na2CaGe2O6 (NCGO), which shows red broadband persistent luminescence (PersL). The self-reduction effect is revealed to be associated with the appropriate structure of NCGO. On the basis of the investigations, we summarize the basic structural conditions and mechanisms of the self-reduction effect. This important finding will enlighten us to a new approach for designing broadband germanate PersL phosphors. In addition, the PersL properties and mechanisms of the NCGO:Yb2+ phosphors are also investigated.

Enhanced persistent red luminescence in Mn2+-doped (Mg,Zn)GeO3 by electron trap and conduction band engineering

The effect of Zn substitution on the persistent luminescence properties of MgGeO3:Mn2+-Ln3+ (Ln = Eu and Yb) red phosphors was investigated. The intensity of the persistent luminescence of the Eu3+ co-doped phosphors increased with increasing Zn content, whereas that of the Yb3+ co-doped samples decreased. For both series of lanthanide co-doped samples, the thermoluminescence (TL) glow peak shifted to the lower temperature side with increasing Zn content. These persistent luminescence properties were well explained in terms of lowering of the bottom of the conduction band relative to the ground state of the divalent lanthanide ions. Especially, in Eu3+ co-doped system, TL peak shifted from 520 K to 318 K by 50% Zn substitution. The persistent radiance of the (Mg0.5 Zn0.5)GeO3: Mn2+-Eu3+ sample at 1 h after ceasing UV light was 46 times stronger than that of MgGeO3:Mn2+-Eu3+, and 11 times stronger than that of ZnGa2O4: Cr3+ standard deep red persistent phosphor.

Optical Properties of Ce-Doped Li4SrCa(SiO4)2: A Combined Experimental and Theoretical Study.

Investigation of optical properties of Ce3+-activated phosphors is not only of practical importance for various applications but also of fundamental importance for providing a basis to understand relevant properties of other lanthanide ions in the same host. We report herein a combined experimental and theoretical study of optical properties of Ce3+ in Li4SrCa(SiO4)2. Photoluminescence properties of the material prepared by a solid-state reaction method are investigated with excitation energies in the vacuum-ultraviolet (VUV) to ultraviolet (UV) range at low temperatures. The band maxima in the excitation spectra are assigned with respect to 4f → 5d transitions of Ce3+ at the Sr and Ca sites, from comparison between experimental and ab initio predicted transition energies. As a result of the two-site occupation, the material displays luminescence at 300-500 nm with a high thermal quenching temperature (>500 K), consistent with the calculated large gaps (∼1.40 eV) between the emitting 5d levels and the bottom of the host conduction band. On the basis of experimental and calculated results for Ce3+ in Li4SrCa(SiO4)2, the energy-level diagram for the 4f ground states and the lowest 5d states of all trivalent and divalent lanthanide ions at the Sr and Ca sites of the same host is constructed and discussed in association with experimental findings.

Reduction of a Cerium(III) Siloxide Complex To Afford a Quadruple‐Decker Arene‐Bridged Cerium(II) Sandwich

AbstractOrganometallic multi‐decker sandwich complexes containing f‐elements remain rare, despite their attractive magnetic and electronic properties. The reduction of the CeIII siloxide complex, [KCeL4] (1; L=OSi(OtBu)3), with excess potassium in a THF/toluene mixture afforded a quadruple‐decker arene‐bridged complex, [K(2.2.2‐crypt)]2[{(KL3Ce)(μ‐η6:η6‐C7H8)}2Ce] (3). The structure of 3 features a [Ce(C7H8)2] sandwich capped by [KL3Ce] moieties with a linear arrangement of the Ce ions. Structural parameters, UV/Vis/NIR data, and DFT studies indicate the presence of CeII ions involved in δ bonding between the Ce cations and toluene dianions. Complex 3 is a rare lanthanide multi‐decker complex and the first containing non‐classical divalent lanthanide ions. Moreover, oxidation of 1 by AgOTf (OTf=O3SCF3) yielded the CeIV complex, [CeL4] (2), showing that siloxide ligands can stabilize Ce in three oxidation states.

Reduction of a Cerium(III) Siloxide Complex To Afford a Quadruple-Decker Arene-Bridged Cerium(II) Sandwich.

Organometallic multi-decker sandwich complexes containing f-elements remain rare, despite their attractive magnetic and electronic properties. The reduction of the CeIII siloxide complex, [KCeL4 ] (1; L=OSi(OtBu)3 ), with excess potassium in a THF/toluene mixture afforded a quadruple-decker arene-bridged complex, [K(2.2.2-crypt)]2 [{(KL3 Ce)(μ-η6 :η6 -C7 H8 )}2 Ce] (3). The structure of 3 features a [Ce(C7 H8 )2 ] sandwich capped by [KL3 Ce] moieties with a linear arrangement of the Ce ions. Structural parameters, UV/Vis/NIR data, and DFT studies indicate the presence of CeII ions involved in δ bonding between the Ce cations and toluene dianions. Complex 3 is a rare lanthanide multi-decker complex and the first containing non-classical divalent lanthanide ions. Moreover, oxidation of 1 by AgOTf (OTf=O3 SCF3 ) yielded the CeIV complex, [CeL4 ] (2), showing that siloxide ligands can stabilize Ce in three oxidation states.