Local Crystal Field Research Articles

Doping Lanthanide Nanocrystals With Non-lanthanide Ions to Simultaneously Enhance Up- and Down-Conversion Luminescence.

The rare-earth nanocrystals containing Er3+ emitters offer very promising tools for imaging applications, as they can not only exhibit up-conversion luminescence but also down-conversion luminescence in the second near-infrared window (NIR II). Doping non-lanthanide cations into host matrix was demonstrated to be an effective measure for improving the luminescence efficiency of Er3+ ions, while still awaiting in-depth investigations on the effects of dopants especially those with high valence states on the optical properties of lanthanide nanocrystals. To address this issue, tetravalent Zr4+ doped hexagonal NaGdF4:Yb,Er nanocrystals were prepared, and the enhancement effects of the Zr4+ doping level on both up-conversion luminescence in the visible window and down-conversion luminescence in NIR II window were investigated, with steady-state and transient luminescence spectroscopies. The key role of the local crystal field distortions around Er3+ emitters was elucidated in combination with the results based on both of Zr4+ and its lower valence counterparts, e.g., Sc3+, Mg2+, Mn2+. Univalent ions such as Li+ was utilized to substitute Na+ ion rather than Gd3+, and the synergistic effects of Zr4+ and Li+ ions by co-doping them into NaGdF4:Yb,Er nanocrystals were investigated toward optimal enhancement. Upon optimization, the up-conversion emission of co-doped NaGdF4:Yb,Er nanocrystals was enhanced by more than one order of magnitude compared with undoped nanocrystals. The current studies thus demonstrate that the local crystal field surrounding emitters is an effective parameter for manipulating the luminescence of lanthanide emitters.

Morphology Manipulation and Upconversion Luminescence Enhancement of β-NaLuF4: Er3+ Hexagonal Microtubes via Sr2+ Doping

A new and facile strategy to enhance the upconversion luminescence (UCL) emission of NaLuF4: Er3+ microcrystals (MCs) using strontium (Sr) as a dopant has been reported. With the introduction of Sr2+, the products change from long NaLuF4: Er3+ hexagonal microtubes to short hexagonal microtubes and finally to hexagonal microprisms. The growth mechanism is profoundly discussed according to the different reaction time-dependent morphologies. More importantly, the total fluorescence intensity is significantly reinforced by doping Sr2+ ions. When 18% Sr2+ is doped into NaLuF4: Er3+ hexagonal microtubes, the maximum green and red luminescence intensities are about 5.8 and 4.4 times higher than those of Sr2+-free samples, respectively. The influences of Sr2+ ion doping content on the phase, the morphology, and the local crystal field symmetry of the as-synthesized NaLuF4 crystals are investigated.

Structure analysis, tuning photoluminescence and enhancing thermal stability on Mn4+-doped La2-xYxMgTiO6 red phosphor for agricultural lighting

Currently, phosphor-converted LEDs (pc-LEDs) are revolutionizing the industry of plant growth lighting. To meet the requirements of this technology, phosphors with tunable photoluminescence, high thermal stability and luminous intensity are required. Herein, we found that the simple substitution of yttrium for lanthanum in La2MgTiO6:Mn4+ (LMT:Mn4+) system could satisfy above three criteria simultaneously. The photoluminescence properties can be regulated by continuously controlling the chemical composition of La2-xYxMgTiO6:Mn4+ solid solution. The La sites are occupied by Y ions, causing a significant blue shift in the emission spectra which owing to the change of local crystal field strengthen. Meanwhile, the thermal stability and decay lifetimes are also varied due to the variation of local structure and band gap energy. The thermal stability of the material reaches 83.5% at 150 °C, which is better than the reported La2MgTiO6:Mn4+ and Y2MgTiO6:Mn4+ phosphors. The electronic luminescence (EL) of pc-LED devices using La2-xYxMgTiO6:Mn4+ red phosphor is evaluated, which matching the absorption regions of plant pigments well, reflecting the superiority of the studied phosphors in plant growth lighting areas.

EPR spectroscopy of 53Cr monoisotopic impurity ions in a single crystal of yttrium orthosilicate Y2SiO5

The electron paramagnetic resonance spectra of 53Cr monoisotopic impurity ions in yttrium orthosilicate single crystals (Y2SiO5) were studied by electron paramagnetic resonance (EPR) in the X-band. The directions of the principal magnetic axes and the parameters of the effective spin Hamiltonian, which describes magnetic characteristics of the impurity centers of chromium are determined. It is shown that the orientation dependencies of the EPR spectra are described by the second-order spin Hamiltonian corresponding to the rhombic symmetry of the local crystal field acting on the impurity ion. It was assumed that the g-tensor and A-tensor describing the Zeeman energy of electron levels in a magnetic field and the hyperfine interaction of electron and nuclear spins are isotropic, and the entire anisotropy of the EPR spectra is due to the anisotropy of the D-tensor, which describes the fine structure of electron levels in crystal electric field. A strong dependence of the probability of forbidden transitions between hyperfine sublevels of electron levels on the orientation of an external magnetic field is established. Moreover, for some orientations, the probability of forbidden transitions becomes comparable to the probability of allowed transitions.

Effect of Ni2+ ions concentration on the local crystal field of Zn1-xNixTe nanocrystals

In this work, we report the study of Zn1-xNixTe nanocrystals in a glass matrix, synthesized by a new protocol based on the fusion-nucleation method. Optical absorption spectra, analyzed based on the crystalline field theory, confirm the substitutional incorporation of Ni2+ ions in the ZnTe semiconductor lattice with tetrahedral symmetry. Transmission electron microscopy images show invariance in the lattice parameter of these nanocrystals, with the incorporation of Ni2+ ions. Electron paramagnetic resonance spectra display resonance lines that confirm the substitutional incorporation (g = 2.0991) and also the presence of Ni2+ ions at Zn vacancy sites (g ≈ 2.0023) of the ZnTe lattice.

Magnetic frustration in a metallic fcc lattice

Magnetic frustration in metals is scarce and hard to pinpoint, but exciting due to the possibility of the emergence of fascinating novel phases. The cubic intermetallic compound HoInCu$_4$ with all holmium atoms on an fcc lattice, exhibits partial magnetic frustration, yielding a ground state where half of the Ho moments remain without long-range order, as evidenced by our neutron scattering experiments. The substitution of In with Cd results in HoCdCu$_4$ in a full breakdown of magnetic frustration. Consequently we found a fully ordered magnetic structure in our neutron diffraction experiments. These findings are in agreement with the local energy scales and crystal electric field excitations, which we determined from specific heat and inelastic neutron scattering data. The electronic density of states for the itinerant bands acts as tuning parameter for the ratio between nearest-neighbor and next-nearest-neighbor interactions and thus for magnetic frustration.

Effects of Ca2+ doping on upconversion luminescence intensity and crystal field asymmetry of β-NaYF4:Yb3+/Er3+ microcrystals

The green and red upconversion (UC) emissions of 35 mol% Ca2+ doped β-NaYF4:Yb3+/Er3+ microcrystals were improved to 102.8 and 43.1 times higher than those of Ca2+-free counterparts. X-ray powder diffraction spectra, scanning electron microscopy, Fourier transform infrared spectroscopy, upconversion luminescence (UCL) spectroscopy and Judd-Ofelt analysis were performed to investigate the mechanism of UCL intensity enhancement caused by Ca2+ doping. The hexagonal-to-cubic phase transition was observed when Ca2+ doping concentration exceeded 35 mol%. The incorporation of Ca2+ into hexagonal microcrystals lattice induced unit cell volume enhancement and microcrystals surface to volume ratio variation, which affected the UCL intensity by hindering the non-radiative energy loss process via Yb3+/Er3+ ions and changing the amount of surface quenchers adsorbed on the microcrystals surface. The hypersensitive transition related local crystal field asymmetry around Er3+ in the grown hexagonal microcrystals was also investigated by Judd-Ofelt analysis but unexpectedly found to be the less dominant factor for the UCL intensity variation. This study helps to realize the mechanism of impurity ion doping resulted UCL intensity enhancement, which is instructive for high-performance UC microcrystals synthesis.

Highly bright and sensitive thermometric LiYF4:Yb, Er upconversion nanocrystals through Mg2+ tridoping

A new strategy is reported to enhance the upconversion (UC) luminescence emission of LiYF4:Yb, Er nanocrystals (NCs) using magnesium as a dopant. We carried out systematic experimental studies on the crystal structure, grain size, and UC emitting property of the tetragonal LiYF4:Yb, Er with varied concentrations of Mg2+. The UC luminescence properties were examined under 980 nm laser illumination with various excitation power densities. At a proper doping concentration, co-doping of Mg2+ ions into LiYF4:Yb, Er is found to result in efficient reinforcement in both the green and red upconverted emissions. Remarkably, the maximum green and red luminescence intensities were reinforced by sevenfold and fivefold, respectively, when 7 mol% Mg2+ was co-doped into tetragonal LiYF4. The possible origin and mechanism for boosting UC emission were explained according to the alteration of the cell volume and the local crystal field surrounding the Er3+ ions by co-doping of Mg2+. Moreover, the emission-optimized LiYF4 UCNCs were further investigated to understand thermal-sensing behaviors employing the fluorescence intensity ratio (FIR) approach from the two neighboring thermal coupled states (2H11/2/4S3/2). The optimization of Mg2+ co-doping in LiYF4:Yb, Er allowed the resultant UCNCs to be an excellent luminescent thermometer over a wide range of temperature. Applying the optimized UCNCs as an optical thermometer, a maximum thermal sensitivity (S) of 5.43 × 10–2 K−1 was achieved at room temperature and a low-power excited upconversion (1 W cm−2). The achieved S value is more advanced than most of the Er-based nanophosphors reported heretofore. This paper provides a perspective scheme to design and grow high-quality upconversion nanomaterials for achieving the preconditions of the pragmatic application in temperature sensing, optically heating, and color display devices.

Elucidation of site symmetry, location, bond, ground state of Cu2+ ions in lithium sulphate monohydrate single crystal through EPR and optical studies

The highly efficient non linear optical (NLO) lithium suphate monohydrate (LSMH) single crystal with well-developed faces is obtained in a few days by slow evaporation of aqueous solutions at room temperature. Small amount of copper II sulphate pentahydrate is added as the paramagnetic impurity into the system. The powder XRD study shows the monoclinic cell dimension of Cu2+ doped LSMH single crystal. Also FTIR analysis reveals the existence of host in a crystal. The single crystal EPR investigation talks about the presence of copper ion. Even though more than one site is noticed, two are followed and reported. The principle values g and A parameters were calculated for both the sites. The spin Hamiltonian parameters found for both the sites, indicate the rhombic local crystal field symmetry around the impurity ion Cu2+, which is further confirmed by optical absorption studies. Interestingly, both the paramagnetic ions enter the lattice interstitially. The type of bonding between the metal ligand bands for site I and site II are found to be moderate ionic and covalent nature. The ground state wave function of both the sites are observed as dx2−y2, which is also supported by Rache parameter and optical studies.

Screen printing of upconversion NaYF4:Yb3+/Eu3+ with Li+ doped for anti-counterfeiting application

Li ions affect the upconversion efficiency by changing the local crystal field of the luminescent center. Herein, in order to improve the upconversion efficiency of NaYF4:Yb3+/Eu3+, a series of NaYF4:Yb3+/Eu3+ micro-particles with different Li+ doping concentrations were synthesized by the hydrothermal synthesis method, respectively. Firstly, the structure and morphology of NaYF4:Yb3+/Eu3+ upconversion micro-particles (UCMPs) with different doping concentrations were analyzed by X-ray diffraction and a scanning electron microscope (SEM). SEM results show that the UCMPs are not only highly crystallized, but also have hexagons with different Li+ concentrations of NaYF4:Yb3+/Eu3+. X-ray diffraction shows that the crystal field around Eu3+ changes with the increase of Li+ concentration. Then, the fluorescence spectrum of NaYF4:Yb3+/Eu3+ was studied under the irradiation of a 980 nm laser. The results show that the fluorescence intensity of NaYF4:Yb3+/Eu3+ with 2% Li+ is the strongest, which is twice the intensity of NaYF4:Yb3+/Eu3+ without Li+. Finally, the fluorescence imaging analysis of NaYF4:Yb3+/Eu3+ with 2% Li+ concentration was carried out. The UCMPs are used to screen printing to evaluate the imaging effect on different sample surfaces. The results show NaYF4:Yb3+/Eu3+ (with 2% Li+) has great application prospects in anti-counterfeiting recognition.

A new method to synthesize Sub-10 nm CaF2: Nd3+ nanoparticles and fluorescent enhancement via Li+ ions or Ce3+ ions doping

In this contribution, we provided a new, simple, cheap, repeatable and easily tunable method to synthesize CaF2:Nd3+ NPs using coprecipitation strategy. By controlling reaction time, temperature and solute/solvent, CaF2:Nd3+ NPs with dimeters ranging from 2.2–4.6 nm were synthesized. Then we optimized the Nd3+ concentration and found CaF2:6Nd(mol%) NPs with strongest luminescence. Furthermore, to further enhance the fluorescence of the optimized NPs, we explored both Li+ doping and Ce3+ doping to afford highly fluorescent CaF2:Nd3+ NPs. For Li+ doping CaF2:6Nd,xLi(mol%) NPs, under the excitation of 800 nm, a ~4-fold fluorescence enhancement was observed, while for Ce3+ doping CaF2:6Nd,xCe(mol%) NPs, a 10.4-fold fluorescence enhancement was observed. Li+ doping leads to two fluorescence maxima at 4% and 30%, mainly dominated by charge compensation, and increased crystallity/grain size, separately. As comparison, Ce3+ doping resulted in the monotonical increase of the fluorescence, dominated by lowered crystal symmetry, changed local crystal field and dissociated Nd3+ - Nd3+ clusters.

Influence of Nd3+ Doping on the Structure, Thermal Evolution and Photoluminescence Properties of Nanoparticulate TiO2 Xerogels

Undoped and Nd3+-doped xerogels (Nd3+-dopant content in the range 0.5–10 mol%) were obtained from colloidal sols in turn prepared by a “green sol-gel route” in aqueous media. The as-synthesized xerogels were characterized by X-ray diffractometry (XRD), high resolution electron microscopy (HREM) together with selected area electron diffractometry (SAED), and Raman spectroscopy (RS), finding that they crystallize as tiny nanocrystals of anatase with traces of brookite and rutile. The thermal stability of the as-synthesized xerogels was next studied in-situ by X-ray thermo-diffractometry (XRTD) as well as ex-situ by XRD, HREM/SAED, and RS on xerogels calcined at different temperatures (400–900 °C), concluding that Nd3+ doping results in the formation of substitutional solid solutions with enlarged unit-cell volume that (i) stabilizes the anatase phase retarding the onset of its transformation to rutile to ever higher temperatures and (ii) provides the anatase nanocrystals with superior resistance to crystal growth. It was also found that Nd4Ti9O24 eventually precipitates at high temperatures when the Nd3+-dopant content is equal to or higher than 3 mol%, which leaves the still-existing anatase nanocrystals without stabilizer cations therefore causing their immediate transformation to rutile. Finally, the photoluminescence (PL) behaviour of the Nd3+-doped TiO2 xerogels with both anatase and rutile crystal structures was investigated and critically compared, observing that the PL emission (i) increases with increasing crystallinity of the host nanocrystals, (ii) decreases above a certain optimal Nd3+-dopant content due to concentration quenching, and (iii) is much greater for rutile host matrices. In this scenario, the 3 mol% Nd3+-doped rutile xerogel maximized the PL emission, emitting at both ∼915 and 925 nm due to the existence of two distinct local crystal fields for the Nd3+ cations.

A novel scheme to acquire enhanced up-conversion emissions of Ho3+ and Yb3+ co-doped Sc2O3

A detailed investigation about the effect of Sc2O3: 1 mol%Ho3+/5 mol%Yb3+ co-doped with Ce4+ ions prepared by sol-gel methods was performed systematically. Under the excitation of 980 nm laser diode, both green emission (553 nm, 5F4/5S2→5I8) and red emission (672 nm, 5F5→5I8) were both observed in the emission spectra of the samples, which were found to be two-photon process and sensitized by Yb3+ ions. With the increasing of Ce4+ ions, the up-conversion green emission intensity are increased by 6.52, 8.69, 10.85, 13.92 and 16.66 fold, corresponding to the Ce4+ ions concentrations from 5 mol% to 13 mol%, respectively. The number of photons are necessary to populate the upper emitting state decreases to 2 and the infrared absorption coefficient is reduced, when the Ce4+ ions concentration increase to 13 mol%. Ce4+ ions play an important role in tailoring the local crystal field around Ho3+ ions, lowering the highest phonon cut-off energy of matrix and reducing the infrared absorption coefficient, thus hindering the non-radiative processes, which contribute to the increased emission intensity. The excellent enhancement makes it a promising multifunctional optical luminescence material.

Mechanically controlled reversible photoluminescence response in all-inorganic flexible transparent ferroelectric/mica heterostructures

The ability to reversibly control the luminescent properties of functional materials with diverse external stimuli, such as an electric field, strain, and temperature, is crucial for designing high-performance optical devices. Here, we demonstrate that a purely mechanical strain in a flexible mica substrate triggered by bending can be used to dramatically modify the photoluminescence response of a Pr-doped Ba0.85Ca0.15Ti0.9Zr0.1O3 epitaxial thin film in a stable and repeatable manner with a large gauge factor of up to 6853. The strong dependence of the photoluminescence performance on the mechanical bending arises from strain-induced variations in the lattice symmetry of the host film and the local crystal field around the Pr3+. In particular, because of the nature of mica, the film structure exhibits excellent antifatigue characteristics after 104 bending cycles as well as high optical transparency in the range of 450–780 nm. This study provides a viable route for exploring the correlation between structural symmetry and photoluminescence in ferroelectric thin-film systems and offers new possibilities for developing all-inorganic, reconfigurable, transparent and flexible light sources, photodetectors, and wearable sensors.

Role of Er3+ concentration on the photoluminescence and temperature sensing properties in Er3+ doped Y2O3 based transparent ceramics

The photoluminescence and temperature sensing properties of Er3+ doped Y2O3 based transparent ceramics have been studied as a function of Er3+ doping concentration. The samples exhibit high transparency thanks to the homogeneous microstructure with no obvious impurities or pores. Under 980 nm excitation, strong visible upconversion (UC) and near infrared (NIR) emissions of Er3+ ions are observed. By using the fluorescent intensity ratio (FIR) technique, the temperature sensing properties are investigated based on the thermally coupled levels 2H11/2 and 4S3/2 (FIR-1 = I544/I569) and non-thermally coupled levels 4F9/2 and 4S3/2 (FIR-2 = I666/I569) of Er3+ ions, respectively. The Judd-Ofelt (J-O) theory analysis reveals that the local crystal field around Er3+ ion changes with the doping concentration, leading to the tuned absolute sensing sensitivity (Sa) based on FIR-1. As for FIR-2, the sensing sensitivities of the samples enhance with the increasing Er3+ content due to the promoted nonradiative processes (including 4S3/2 → 4F9/2 and 4F7/2 + 4I11/2 → 4F9/2 + 4F9/2). The optimal Sa reaches ~371.8 × 10−4 K−1 in 6 mol% Er3+ doped sample, ranking among the highest ones for the luminescent materials.

Broad band emission near-infrared material Mg3Ga2GeO8:Cr3+: Substitution of Ga-In, structural modification, luminescence property and application for high efficiency LED

Series of Cr3+ doped Mg3Ga2GeO8 magnesium gallogermanate near-infrared (NIR) materials were synthesized by solid state method. Mg3Ga2GeO8 with a spinel solid solution structure can provide three different sites for Cr3+, and the priority of Cr3+ entering the three sites has been clearly determined by analyzing the distortions of the three polyhedral sites and local crystal field strength. The emission of the phonon vibration line from the non-forbidden transition 2E→4A2 is observed at T = 10k in sub-peak 1, however, at room temperature, the spectrum presents a broad emission band 700–1200 nm superimposed on 2E→4A2 emission. When introduced In3+ into Mg3Ga2GeO8 to substitute for Ga3+, the emission intensity can be greatly enhanced and a red shift of emission spectra is observed, which may be ascribed to the crystal field environment getting weakened with increasing In3+. A novel NIR phosphor converted light emitting diode was fabricated by combining Mg3Ga1.87In0.1GeO8:0.03Cr3+ with 455 nm blue chip, of which an optical power is 6.143 mW, a photoelectric efficiency is 8.5%, importantly, the spectrum range of device covers 650–1200 nm with a full width at half maximum (FWHM) of 244 nm.

Small non-uniform basal crystal fields in HVPE free-standing GaN:Mg as evidenced by angular dependent and frequency-dependent EPR

We studied thin-film and free-standing Mg-doped GaN using multi-frequency electron paramagnetic resonance (EPR) at 3–3.5 K and 9.4–130 GHz. Free-standing samples exhibit a highly anisotropic intensity, varying by a factor of 20 from 0° to 60°. In contrast, the intensity of the thin-film samples is significantly more isotropic, varying by no more than 10% over the same range of angles. The angular dependent intensity can be modeled in both free-standing and thin-film samples similarly to the g-factor anisotropy reported for thin films, supporting the theoretical predictions that the hole is on a basal site around the Mg acceptor. In addition, frequency-dependent transmission EPR measurements reveal a distribution of in free-standing samples, indicating that the local basal crystal field is non-uniform.

Colossal photovoltages in strain-driven crystal field transition and symmetry breaking of superconducting epitaxial systems

Materials showing light-induced voltages (LIVs) are a flourishing class characterized by anisotropic charge transport. The LIVs' functionality could be tuned by local perturbations and structural symmetry, but achieving large LIVs is still challenging and a clear guiding principle to this end thus remains to be established. Here, a long-range lattice strain approach that enables manipulating the crystal field and symmetry breaking is proposed to realize colossal LIVs. The tailored tensile and compressive strains are introduced into superconducting $(\mathrm{L}{\mathrm{a}}_{1.45}\mathrm{N}{\mathrm{d}}_{0.4})\mathrm{S}{\mathrm{r}}_{0.15}\mathrm{Cu}{\mathrm{O}}_{4}$ epitaxial films that are fabricated by the pulsed laser deposition method. The experimental characterization indicates that long-range lattice compressive strain results in the epitaxially mosaic structures and the local crystal field transition from tetragonal to octahedral configurations that spontaneously breaks the symmetry. Such a structural transition and change through modulated compressive strain could greatly favor in-plane transport that contributes to a large anisotropy of the Seebeck coefficient $\mathrm{\ensuremath{\Delta}}S$ of over $100\phantom{\rule{0.16em}{0ex}}\ensuremath{\mu}\mathrm{V}\phantom{\rule{0.16em}{0ex}}{\mathrm{K}}^{\ensuremath{-}1}$, achieving strikingly colossal LIVs of over 70 V, many times larger than those of existing materials reported. This would broaden LIVs' promising application in light or photovoltage response and also exemplifies a steady route to exploring new high-performance LIV materials with structural or transport anisotropy.

Preparation of Ca2SnO4:Tb3+, R3+ phosphors (R3+ = B3+, Al3+ or Ga3+) and the dependence of their luminescence on R3+ cations

Ca2SnO4:Tb3+ phosphors co-doped by R3+ cations (R3+ = B3+, Al3+, Ga3+) can be prepared by a solid-state reaction. They are fine solid solutions, which have been studied in detail by XRD, photoluminescence spectra, and decay curves. The results show that their luminescence can be effectively improved by co-doping R3+ cations and adding Al3+ cation is superior to that with other two dopants whether in fluorescence or phosphorescence. The concentration quenching effect will weaken their luminescence beyond m = 0.07 when Al3+ cation is co-doped. The distortion of host lattice and change of local crystal field symmetry around Tb3+ due to co-doping R3+ can enhance their fluorescence. The formation and increase of oxygen vacancy (V••O) predominate in improving their phosphorescence, while the single bond strength of R-O also plays an important role.

Spin-orbital order in LaMnO3 : d−p model study

Using the multiband $d-p$ model and unrestricted Hartree-Fock approximation we investigate the electronic structure and spin-orbital order in three-dimensional MnO$_3$ lattice such as realized in LaMnO$_3$. The orbital order is induced and stabilized by particular checkerboard pattern of oxygen distortions arising from the Jahn-Teller effect in the presence of strong Coulomb interactions on $e_g$ orbitals of Mn ions. We show that the spin-orbital order can be modeled using a simple \textit{Ansatz} for local crystal fields alternating between two sublattices on Mn ions, which have non-equivalent neighboring oxygen distortions in $ab$ planes. The simple and computationally very inexpensive $d-p$ model reproduces correctly nontrivial spin-orbital order observed in undoped LaMnO$_3$. Orbital order is very robust and is reduced by $\sim 3$ \% for large self-doping in the metallic regime.