Lithium Co-doping Research Articles

Oxygen vacancy sensitized energy transfer and tunable emission in Li+ codoped CaWO4:Bi3+

In the present study, Li+ codoped CaWO4:2 % Bi3+ was synthesized by solid state reaction. The synthesized luminescent materials were characterized using x-ray diffraction (XRD), particle induced gamma-ray emission (PIGE), positron annihilation lifetime spectroscopy (PALS) and time resolved photoluminescence (TRPL). Photoluminescence emission studies showed that Lithium co-doping aids in the energy transfer processes and improves emission significantly. This study also shows that emission is tunable via careful choice of excitation scheme and Li+ sensitization to tune the energy transfer from WO42- to Bi3+. Through PALS, it was unequivocally proved that Li+codoping is not charge compensating for the created cation vacancies and improved emission with Li+ codoping is due to sensitization by oxygen vacancies.

Investigation on the valence state stability and optical properties of Mg2GeO4:Cr

In this work, the valence states and optical properties of chromium-doped Mg2GeO4 are investigated via first-principles methods. The formation energies and thermal charge transition levels show that the site occupation and valence states of chromium are strongly related to the growth condition: Cr3+ dominates in the Mg sites under Mg-poor conditions, and Cr4+ dominates in the Ge sites under Ge-poor conditions. With the lithium co-doping, the Fermi-level is pinned below the midgap, and the formation energies of chromium-doped Mg sites become the lowest either Mg-poor or Ge-poor conditions, suggesting that the co-doping of lithium ions promotes the chromium to preferentially occupy Mg sites, in good agreement with experiments. Furthermore, the excitation and emission processes of chromium ions are investigated, and our results recommend that the near-infrared emission is related to the 4T2 to 4A2 transition of Cr3+ at the Mg sites and the infrared emission is related to the 3T2-3A2 transition of Cr4+ at the Ge site. Our results help to understand the stability of the valence states of chromium ions doped Mg2GeO4 and the effects of ions co-doping in regulating the chromium valence states, as well as the corresponding optical properties of different types of chromium ions.

Achieving high energy storage performance and efficiency in lead-free SrTiO3 ceramics via neodymium and lithium co-doping technique

ABSTRACT There is an immediate demand for eco-friendly, high-performance, and highly stable energy storage materials for pulse power systems. Ceramics based on SrTiO3 have a high breakdown strength (BDS) and dielectric constant. In this work, we fabricated a polycrystalline of Sr(1-x)(Nd, Li)xTiO3 ceramics via microwave-assisted heating of the starting materials. X-ray diffraction analysis reveals a pure perovskite phase without any secondary phase. Scanning electron microscopy images exhibit dense grain morphology with a decrease in grain size as the dopant concentration increases. The frequency dependences of the dielectric properties were studied in the frequency range of 100 Hz-1 MHz at room temperature. The polarization-electric field hysteresis loops were examined to ascertain the effect of co-doping on the energy-storage capability of SrTiO3 ceramics. After increasing the co-dopants from 0 to 8%, the energy density increased nine times (from 0.11 J/cm3 to 0.952 J/cm3), and the energy storage efficiency increased from 80.71% to 95.98%, respectively. In addition, the samples demonstrate excellent thermal stability, and their energy storage properties are stable up to 80°C. We may infer from this discovery that the bulk Nd3+ and Li+ co-doped SrTiO3 materials are good candidates for high-energy-density capacitor applications.

Effect of lithium codoping on the structural, morphological and photocatalytic properties of Nd-doped ZnO

Nanostructured ZnONd nanoparticles (NPs) codoped with lithium at low temperature and short synthesis time were prepared using the polyol method, and the influence of different lithium contents at constant atomic concentration of Nd was evaluated. The resulting materials were analyzed by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), Brunauer–Emmett–Teller analysis (BET), X-ray photoelectron spectroscopy (XPS), and optical spectroscopies (photoluminescence (PL), cathodoluminescence (CL) and Raman). X-ray diffraction patterns demonstrate that the materials are polycrystalline with a wurtzite structure with no secondary phases in the concentrations evaluated. The unit cell parameters were determined, and the crystallite size was calculated considering the three most intense diffraction peaks. TEM shows that the particles are polycrystalline with no amorphization. Raman analysis further confirms the wurtzite hexagonal structure of the particles. Morphological and size studies using SEM and TEM show that the most remarkable change is the evolution from hemispherical ZnO NPs to spindle-shaped particles for Li–Nd doped ZnO. The surface chemistry, as studied by X-ray photoelectron spectroscopy (XPS), confirms the incorporation of Nd3+. The study of the photocatalytic behavior of the Li–Nd codoped ZnO NPs reveals that the ZNL0.5 sample exhibits the highest photocatalytic activity using a solution of Rhodamine B (2.5 ppm) as a reference.

Lithium/Boron Co-doped Micrometer SiOx as Promising Anode Materials for High-Energy-Density Li-Ion Batteries.

The carbon-coated silicon monoxide (SiOx@C) has been considered as one of the most promising high-capacity anodes for the next-generation high-energy-density lithium-ion batteries (LIBs). However, the relatively low initial Coulombic efficiency (ICE) and the still existing huge volume expansion during repeated lithiation/delithiation cycling remain the greatest challenges to its practical application. Here, we developed a lithium and boron (Li/B) co-doping strategy to efficiently enhance the ICE and alleviate the volume expansion or pulverization of SiOx@C anodes. The in situ generated Li silicates (LixSiOy) by Li doping will reduce the active Li loss during the initial cycling and enhance the ICE of SiOx@C anodes. Meanwhile, B doping works to promote the Li+ diffusion and strengthen the internal bonding networks within SiOx@C, enhancing its resistance to cracking and pulverization during cycling. As a result, the enhanced ICE (83.28%), suppressed volume expansion, and greatly improved cycling (85.4% capacity retention after 200 cycles) and rate performance could be achieved for the Li/B co-doped SiOx@C (Li/B-SiOx@C) anodes. Especially, the Li/B-SiOx@C and graphite composite anodes with a capacity of 531.5 mA h g-1 were demonstrated to show an ICE of 90.1% and superior cycling stability (90.1% capacity retention after 250 cycles), which is significant for the practical application of high-energy-density LIBs.

A strategy of synergistic optimization: gold and lithium co-doped vanadium oxide as a hole-injection layer for high-performance OLEDs

A gold and lithium co-doping strategy has been employed to fabricate a VOx hole-injection layer for a matched energy level alignment between the ITO anode and TAPC (hole-transport layer) and improved performance of OLEDs.

Role of Lithium Codoping in Enhancing the Scintillation Yield of Aluminate Garnets

The aim of this work is to clarify the scintillation-yield enhancement in $\mathrm{Lu}\mathrm{YAG}:\mathrm{Pr}$ scintillators obtained by $\mathrm{Li}$ codoping via integrated study of the valence state of activators, the preferential site occupancy of $\mathrm{Li}$ codopants, and defect structures from experimental and theoretical insights. With $\mathrm{Li}$ codoping, the light yield and energy resolution of $10\ifmmode\times\else\texttimes\fi{}10\ifmmode\times\else\texttimes\fi{}10\phantom{\rule{0.1em}{0ex}}{\mathrm{mm}}^{3}$ $\mathrm{Lu}\mathrm{YAG}:\mathrm{Pr}$ samples are improved from 15 600 to 24 800 photons/MeV, and 5.3 to 4.3% at 662 keV, respectively. The optical absorption spectra indicate that $\mathrm{Li}$ codoping does not induce conversion of stable ${\mathrm{Pr}}^{3+}$ to ${\mathrm{Pr}}^{4+}$ in $\mathrm{Lu}\mathrm{YAG}:\mathrm{Pr}$ single crystals. Based on the formation energies of substitutional and interstitial $\mathrm{Li}$ sites using density-functional-theory (DFT) calculations and the ${}^{7}\mathrm{Li}$ nuclear magnetic resonance results, it is shown that the $\mathrm{Li}$ ions prefer to dominantly occupy the fourfold coordinated interstitial sites and fourfold coordinated $\mathrm{Al}$ sites. The systematic analysis of thermoluminescence glow curves, positron annihilation lifetime spectroscopies, and defect formation energies derived from DFT calculations reveals that the concentration of isolated $\mathrm{Lu}$ and $\mathrm{Al}$ vacancies as dominant acceptor defects is reduced by $\mathrm{Li}$ codoping, whilst the shallow ${\mathrm{Li}}_{i}$ interstitial defects and the deep ${V}_{O}$ oxygen vacancies are introduced simultaneously. We propose that the lowering of hole trapping at defects resulting from $\mathrm{Li}$ codoping contributes to the scintillation-yield enhancement.

Influence of Li+ co-doping on the luminescence of MgO:Eu3+ nanocrystals: Probing asymmetry, energy transfer and defects

The influence of Li+ co-doping on the photophysical properties of MgO:Eu3+ nanocrystals is examined. The MgO nanocrystals displayed defect emission which enhances on Li co-doping as a result of enhanced defect density. On Eu3+doping, host to dopant energy transfer (HDET) takes place leading to red emission in MgO:Eu3+along with fractional host emission as well. On Li+ doping of 1.0 mol %, there is further improvement in HDET, but, beyond that, there is quenching owing to enhanced Eu–O covalency and higher non-radiative pathways. Li ion seems to have no influence on the asymmetry ratio in the emission from Eu3+, when its concentration is less than that of Eu3+, but, there is significant increase in asymmetry at higher concentration of co-dopant. The creation of oxygen vacancies and increase in Stark splitting of magnetic and electric dipole transitions seem to enhance the asymmetry when co-dopant concentration is high. Photoluminescence lifetime increases on lithium co-doping which is attributed to the removal of charge compensating cation vacancies, as confirmed using positron annihilation lifetime spectroscopy, and the homogenous distribution of dopants within the nanoparticles due to synergic effects.

Enhanced electrochromic performance on anodic nickel oxide inorganic device via lithium and aluminum co-doping

Anodic materials like nickel oxide play a very important role in lithium-ion battery, solar cells and especially electrochromic devices (ECDs). While the optical modulation and cycle stability performance of nickel oxide (NiOx) for electrochromic devices still require improvement. Here, lithium-doping (LiNiOx) and lithium-aluminum co-doping nickel oxide (Al-LiNiOx) thin films are prepared through co-sputtering. Compared with the un-doped nickel oxide films, the optical modulation and cycle stability of the nickel oxide film containing both lithium and aluminum were significantly improved. LiNiOx films show improvement in the transmittance contrast while the transmittance of bleached state decreases. Al-LiNiOx films not only increase optical modulation, but also improve the transmittance of bleached state. Furthermore, ITO/WO3/LiNbO3/Al-LiNiOx/ITO inorganic all-solid-state ECD with good performance was prepared. The transmittance of the ECD in the bleached state reaches 74.5% at 550 nm, the transmittance contrast could be 44%, and the stability of ECD is good. These indicate that lithium-aluminum co-doping could be a good way to improve electrochromic performance.

Effect of lithium codopant concentration on the luminescence properties of (Lu0.75Y0.25)3Al5O12: Pr3+ single crystals: Before and after air annealing

In this paper, we present the photoluminescence and scintillation performance of Pr3+ doped (Lu0.75Y0.25)3Al5O12 single crystals as a function of Li+ concentration. Lithium concentration had no effect on the optical properties of LuYAG: Pr single crystal scintillators regardless of concentration. On the other hand, scintillation light yield increased by as much as 94% with the incorporation of lithium as a codopant, and the energy resolution at 662 keV was improved to 4.1%, which is an outstanding energy resolution for an oxide single crystal scintillator. Lithium codoping slightly lengthened the fast component of the scintillation decay time, and slightly decreased the contribution from the slow component. Additionally, the samples were annealed in air and scintillation properties were compared before and after this annealing treatment. The change in scintillation properties is explained through the analysis of thermoluminescence glow curves.

Unraveling the Critical Role of Site Occupancy of Lithium Codopants in Lu2SiO5:Ce3+ Single-Crystalline Scintillators.

Lithium codoping has emerged as an effective strategy to enhance the light yield of oxide scintillators for radiation detection applications, but the understanding of the actual role played by Li+ remains unclear. In this work, we comprehensively study the effects of Li codoping on optical and scintillation properties of Lu2SiO5:Ce (LSO:Ce) single crystals and reveal the critical role of site occupancy of Li. High-quality LSO:Ce single crystals codoped with 0.05, 0.1, and 0.3 at. % Li ions were grown by the Czochralski method. The optical absorption spectra confirm nonconversion of stable Ce3+ to Ce4+ in Li-codoped LSO:Ce regardless of the Li codoping concentration. The photoluminescence decay kinetics suggest an enhanced ionization of the excited 5d1 state of Ce3+ centers in highly codoped samples. A simultaneous improvement of scintillation light yield, decay time, and afterglow is achieved in LSO:Ce codoped with low concentrations of Li. The preferential occupation of Li at interstitial spaces and lutetium sites is proven to rely on its codoping concentration by using the 7Li nuclear magnetic resonance technique. The concentration-dependent site occupancy of Li alters the defect structures of LSO:Ce, in particular resulting in a distinct change in the number of cerium spatially correlated oxygen vacancies confirmed by thermoluminescence and afterglow measurements.

Low temperature densification by lithium co-doping and its effect on ionic conductivity of samarium doped ceria electrolyte

Low temperature densification and improving the ionic conductivity of doped ceria electrolyte is important for the realization of efficient intermediate temperature solid oxide fuel cell system. Herein, we report the effect of lithium co-doping (1, 3, 5 and 7 mol%) in 20 mol% samarium doped ceria on the low temperature sinterability and conductivity. The synthesized nanoparticles by citrate-nitrate combustion method showed a decrease in lattice parameter and increase in oxygen vacancy with lithium content after calcination due to the substitution of Li+ into CeO2 lattice. Upon sintering at 900 °C, the density improved and reached a maximum value of 98.6% for 5% Li which exhibited a dense microstructure than at 7% Li. 5%Li co-doping exhibited the best conductivity of 3.65 × 10−04–1.81 × 10−3 S cm−1 in the operative temperature range of IT-SOFC (550–700 °C).Our results demonstrate the significance of lithium as co-dopant for efficient low temperature sintering as well as improving the electrolyte conductivity.

Lithium and Silver Co-Doped Nickel Oxide Hole-Transporting Layer Boosting the Efficiency and Stability of Inverted Planar Perovskite Solar Cells.

In this work, a lithium and silver co-doping strategy has been successfully implied to prepare NiO x films for high performance inverted planar perovskite solar cells (PSCs). Compared to the pristine and single-doped NiO x, the Li and Ag co-doping approach exhibits the synergistic effect and can endow NiO x films with higher electrical conductivity, higher hole mobility and better interface energy band alignment with perovskite active layers. Moreover, the perovskite film with enhanced crystallinity can be obtained induced by the Li,Ag:NiO x film. The PSC with Li,Ag:NiO x HTL shows a high power conversion efficiency (PCE) up to 19.24% and less hysteresis effect, which outperforms the devices with the pristine NiO x or single-doped NiO x HTLs. Meanwhile, the Li,Ag:NiO x device can retain 95% of its initial PCE after storage at the relative humidity of 30 ± 2% in 30 days without encapsulation. Our work demonstrates that lithium and silver co-doping is a promising route for realizing efficient p-type NiO x HTL, which provides a simple way to boost the efficient and stable of inverted planar PSCs.

On the Role of Li+ Codoping in Simultaneous Improvement of Light Yield, Decay Time, and Afterglow of Lu2SiO5:Ce3+ Scintillation Detectors

Commercially available Lu2SiO5:Ce (LSO:Ce) scintillators for the nuclear medical imaging applications, such as positron electron tomography (PET), normally have a light yield of 30 000–32 000 photons/MeV and a scintillation decay time of 43–45 ns. We demonstrate a simultaneous improvement of light yield and decay time of LSO:Ce single crystal scintillators with lithium codoping. Li codoping significantly enhances the light yield of LSO:Ce from 32 500 to 39 000 photons/MeV, shortens the scintillation decay time from 45.4 to 42.1 ns, and reduces the room temperature afterglow by around one order of magnitude. The physical insights on the role of Li codopants in scintillation mechanism are provided by studying cerium oxidation state, luminescence properties of cerium activator centers, defect structure, and preferential occupation of lithium ions. The improved light yield and afterglow are explained in terms of the dissociation of spatially correlated oxygen vacancies (VO) and Ce centers or the suppression of VO formation, allowing a more efficient electron migration to Ce centers. We attribute the decay time shortening upon Li+ codoping to the reduction of slow Ce2 emissions via a non‐radiative energy transfer from six‐coordinated Ce2 to seven‐coordinated Ce1 centers.

The role of compensation defects in Eu3+ stabilization under reductive atmosphere in Sr2SiO4 matrix

Luminescent materials based on strontium orthosilicate doped with a fixed concentration of europium and co-doped with various concentrations of sodium, lithium and boron were synthesized in order to study the role of co-dopants in luminescent properties of europium ions. The obtained materials were characterized using powder X-ray diffraction (XRD) technique. The results of the optical measurements indicate that the incorporation of sodium or lithium increases emission intensity of Eu3+, but does not stabilize Eu3+ under reductive (5% H2/N2) atmosphere, which was observed for the boron co-doping. Moreover, it is shown that the sodium or lithium co-doping facilitates the reduction process of Eu3+ → Eu2+. The obtained results suggest that diffusion limited processes are crucial for the stabilization of Eu3+ during the reduction step of synthesis.

Improved electrochromic performance of NiO-based thin films by lithium and tantalum co-doping

Nickel oxide (NiO) is one of the most promising counter electrode materials for complementary electrochromic (EC) devices. Its charge storage capacity affects the coloration depth of the device. However, NiO films still suffer from some problems like inferior optical modulation and poor bleached-state transparency. Doping of multiple additives is a typical way to enhance the EC performance of NiO films. Here, Li-Ta co-doped NiO films were fabricated by direct-current and radio-frequency magnetron co-sputtering from Ni metallic and LiTaO3 ceramic targets. The influences of different LiTaO3 deposition power on film structure, morphology, composition, optical and EC characteristics were investigated. It was found that the crystal structure and surface morphology of the NiO films could be readily tuned by varying LiTaO3 sputtering deposition power level. The charge capacity and EC performance of NiO films showed a strong dependence on crystal structure variation and surface morphology. When the LiTaO3 deposition power was 100 W, Li-Ta co-doped NiO films exhibited the optimal EC performance with an optical modulation of 62% and coloration efficiency of 35 cm2/C at 550 nm.

Photodetectors for weak-signal detection fabricated from ZnO:(Li,N) films

ZnO films with carrier concentration as low as 5.0×1013cm−3 have been prepared via a lithium and nitrogen codoping method, and ultraviolet photodetectors have been fabricated from the films. The photodetectors can be used to detect weak signals with power density as low as 20 nw/cm2, and the detectivity and noise equivalent power of the photodetector can reach 3.60×1015cmHz1/2/W and 6.67×10−18W−1, respectively, both of which are amongst the best values ever reported for ZnO based photodetectors. The high-performance of the photodetector can be attributed to the relatively low carrier concentration of the ZnO:(Li,N) films.

Enhanced down and upconversion emission for Li+ co-doped Gd2O3:Er3+ nanostructures

A phosphor with higher emission efficiency in the visible region can be very helpful in various optoelectronics applications such as solar cells. Effective conversion of UV and NIR radiations to more useful visible and red emissions were carried out through Gd2O3:Er3+ nanostructures synthesized by conventional co-precipitation method. The spectroscopic study of Gd2O3:Er3+ samples were carried out as a function of dopant concentration under the excitation wavelengths of 378, 975 and 1540 nm, respectively. The doping of erbium show downconversion emission at 561 and 660 nm under 378 nm excitation while upconversion emission under 975 and 1540 nm excitations shows upconversion of NIR photons to 561 and 660 nm visible emissions. The enhanced red emission (660 nm) at higher erbium concentration was observed due to the dominant cross-relaxation processes. The efficiency of upconversion luminescence is highly dependent on the various relaxation and energy transfer processes involved in different energy levels of erbium. The intensity of downconversion and upconversion emission was enhanced by lithium co-doping in Gd2O3:Er3+ nanostructures. The incorporation of alkali metal ions in Gd2O3:Er3+ shows considerable enhancement in downconversion and upconversion emission intensity making the phosphor more suitable for optoelectronic applications.

Enhancing the white light emission of SrAl2O4:Ce3+ phosphors by codoping with Li+ ions

The structural, morphological and luminescent properties of SrAl2O4:Ce3+(SAO:Ce) phosphors doped with Ce3+ concentrations from 0.5at% up to 7at% and synthesized by combustion synthesis are reported. The morphology of the particles changes from coalesced grains (with average size of 121nm) for a cerium concentration of 0.5at% to needle-like shape when the cerium concentration is above 1% (with an average length of 1–2µm). XRD patterns also show a change in the crystalline structure from monoclinic to hexagonal when the Li ion is introduced to improve the luminescent properties. The sample doped with 0.5at% of Ce3+ presents the highest white luminescence. When this sample was codoped with 0.2at% of Li+, there was an increase of 100% in the white light emission. The luminance values of the samples with and without Li+, under UV-LED excitation (365nm commercial Star LED) were 1630cd/m2 and 3760cd/m2, respectively. That is, the luminance enhancement achieved by lithium co-doping was 130%. Thus, our results indicate that Li codoped SrAl2O4:Ce3+ phosphors could be useful for applications in displays and solid state lighting.

DFT study of electronic and optical properties of anatase titanium dioxide tuned by nitrogen and lithium co-doping

The electronic and optical properties of anatase titanium dioxide (TiO2), co-doped by nitrogen (N) and lithium (Li), have been investigated by density functional theory plus Hubbard correction term U, namely DFT+U. It is found that Li-dopants can effectively balance the net charges brought by N-dopants and shift the local state to the top of valence band. Depending on the distribution of dopants, the adsorption edges of TiO2 may be red- or blue-shifted, being consistent with recent experimental observations.