Recent progress in low-voltage cathodoluminescent materials: synthesis, improvement and emission properties

New Series of Red-Light Phosphor Ca9-xZnxGd0.9(PO4)7:0.1Eu3+ (x = 0-1)

In this study, a new series of phosphors, Ca9−xZnxGd0.9(PO4)7:0.1Eu3+ (x = 0.00−1.00, step dx 0.05), was synthesized, consisting of centro- and non-centrosymmetric phases with β-Ca3(PO4)2-type structure. Crystal structures with space groups R3c (0.00 ≤ x < 0.35) and R3¯c (x > 0.8) were determined using X-ray powder diffraction and the method of optical second harmonic generation. In the region 0.35 ≤ x ≤ 0.75, phases R3c and R3¯c were present simultaneously. Refinement of the Ca8ZnGd(PO4)7 crystal structure with the Rietveld method showed that 71% of Gd3+ ions are in M3 sites and 29% are in M1 sites. A luminescent spectroscopy study of Ca9−xZnxGd0.9(PO4)7:0.1Eu3+ indicated the energy transfer from the crystalline host to the Gd3+ and Eu3+ luminescent centers. The maximum Eu3+ luminescence intensity corresponds to the composition with x = 1.

Two d10 2D Cathode-Ray Scintillation Coordination Polymers with High Efficiency and High-Voltage Stability

Two examples of efficient cathode-ray scintillation coordination polymers with good stability at high voltage were prepared by conjugating luminescent groups with d¹⁰ metal ions. The synergistic effect of inorganic metal and organic ligand suppresses the self-quenching of the conjugated luminescent groups and enhances the scintillation performance. This work provides new ideas for the design of new field-emission displays and cathode-ray scintillation materials.

Self-Assembly of Au-Fe3O4 Hybrid Nanoparticles Using a Sol-Gel Pechini Method

The Pechini method has been used as a synthetic route for obtaining self-assembling magnetic and plasmonic nanoparticles in hybrid silica nanostructures. This manuscript evaluates the influence of shaking conditions, reaction time, and pH on the size and morphology of the nanostructures produced. The characterization of the nanomaterials was carried out by transmission electron microscopy (TEM) to evaluate the coating and size of the nanomaterials, Fourier-transform infrared spectroscopy (FT-IR) transmission spectra to evaluate the presence of the different coatings, and thermogravimetric analysis (TGA) curves to determine the amount of coating. The results obtained show that the best conditions to obtain core–satellite nanostructures with homogeneous silica shells and controlled sizes (<200 nm) include the use of slightly alkaline media, the ultrasound activation of silica condensation, and reaction times of around 2 h. These findings represent an important framework to establish a new general approach for the click chemistry assembling of inorganic nanostructures.

Cyan Broad-Band Emission Phosphor with Scandium Silicon Multiple-Ring Structure for White Light-Emitting Diodes and Field Emission Displays

Silicate phosphor KBaScSi₂O₇:Ce³⁺ (KBSS:Ce³⁺), as a novel material, has been prepared by the solid-state method in this study. The crystal structure, luminescent properties, thermal stability, and cathodoluminescence (CL) properties of this material were analyzed systematically. It concludes that the phosphor can emit cyan light with emission peak at 509 nm under n-UV light excitation (300-400 nm). By coating KBSS:Ce³⁺ with a red-emitting CaAlSiN₃:Eu²⁺ on a n-UV (365 nm) light-emitting diode (LED) chip, the intense warm white light with high R_A (83.4) and low CCT (3652) can be produced under a 350 mA forward bias current.In addition, the CL performance shows that KBSS:0.10Ce³⁺ has high saturation current and voltage and good color stability under low voltage conditions. All these results indicate that KBSS:Ce³⁺ phosphor will be very promising in LED and field emission display applications.

Fluorescent Carbon Dots: Fantastic Electroluminescent Materials for Light-Emitting Diodes

Fluorescent carbon dots (CDs) have emerged as fantastic luminescent nanomaterials with significant potentials on account of their unique photoluminescence properties, high stability, and low toxicity. The application of CDs in electroluminescent light-emitting diodes (LEDs) have aroused much interest in recent years. Herein, the state-of-the-art advances of CD-based electroluminescent LEDs are summarized, in which CDs act as active emission layer and interface transport layer materials is discussed and highlighted. Besides, the device structure of CD-based LEDs and preparation methods of CDs are also introduced. Furthermore, the opportunities and challenges for achieving high performance CD-based electroluminescent LED devices are presented. This review article is expected to stimulate more unprecedented achievements derived from CDs and CD-based electroluminescent LEDs, thus further promoting their practical applications in future solid-state lighting and flat-panel displays.

Efficient Energy Transfer from Trap Levels to Eu3+ Leads to Antithermal Quenching Effect in High-Power White Light-Emitting Diodes

The most critical aspect in the assembly of phosphor-converted white light-emitting diodes (pc-WLEDs) is how to stabilize the device in a practical environment. The high applied currents can generate enormous heat up to more than 100 °C, and such a continuous illumination process will lead to serious effects concerning the stability of the device. Therefore, the new search for examples to fully suppress thermal quenching effect is a real challenge. In this study, a novel Eu³⁺-activated CaMgGeO₄ (CMGO) phosphor of olivine type is developed via a conventional solid-state reaction. The results reveal that Eu³⁺ occupies the low symmetric Ca²⁺ site of this host. Upon visible-light sensitization at 464 nm, a dominant red emission band with maximum at 612 nm is witnessed. Its full width at half-maximum (fwhm) is merely ∼4.37 nm, and a high color purity of around 94% is achieved. Their corresponding Commission Internationale de L'Eclairage (CIE) coordinates are very close to standard red color coordinates (0.666, 0.333). The influence of concentration and temperature on the optical property has been explored. It has been discovered that the optimized sample (CMGO:0.01Eu³⁺) is not influenced by the thermal quenching effect and its fluorescent intensity is improved even up to 473 K, which is mainly attributed to the incorporation of abundant trap sites generated by the nonequivalent substitution Eu³⁺ for Ca²⁺. After it is integrated into commercially available YAG:Ce³⁺ phosphor-based pc-WLEDs, the excellent optical parameters of the fabricated WLEDs are evaluated. The correlated color temperature (CCT) varies from cool white (6458 K) to warm (4370 K), and the color rendering index (CRI) increases from 78 to 86 under a high flux operating current of 200 mA. Furthermore, the chromaticity coordinates remain almost stable with the increasing drive current from 200 mA to 1000 mA. It is highly expected that CaMgGeO₄:0.01Eu³⁺ will become a suitable red phosphor for the preparation of white LEDs with high efficiency.

Efficient photoluminescence of isotropic rare-earth oxychloride nanocrystals from a solvothermal route

Eu3+-doped sub-10 nm LaOCl nanocrystals with 43% photoluminescence quantum yield were prepared by solvothermal synthesis from hydrated rare-earth chlorides. As-obtained nanocrystals are nearly spherical, monodisperse and stable as colloidal dispersions. These combined features should intensify the interest for nanocrystalline rare-earth oxyhalides and their optical properties.

A novel germanate based red-emitting phosphor with high efficiency, high color purity and thermal stability for white light-emitting diodes and field emission displays

A series of Eu³⁺-activated K₂SrGe₈O₁₈ phosphors for white light emitting diodes and field emission displays were developed and explored.

Control of Eu Oxidation State in Y2O3-xSx:Eu Thin-Film Phosphors Prepared by Atomic Layer Deposition: A Structural and Photoluminescence Study

Structural and photoluminescence studies were carried out on Eu-doped Y₂O_3−xS_x thin films grown by atomic layer deposition at 300 °C. (CH₃Cp)₃Y, H₂O, and H₂S were used as yttrium, oxygen, and sulfur precursors, respectively, while Eu(thd)₃ was used as the europium precursor. The Eu oxidation state was controlled during the growth process by following the Eu(thd)₃ pulse with either a H₂S or O₃ pulse. The Eu(thd)₃/O₃ pulse sequence led to photoluminescence emission above 550 nm, whereas the Eu(thd)₃/H₂S pulse sequence resulted in emission below 500 nm.

Single-Composition White Light Emission from Dy3+ Doped Sr₂CaWO₆

A series of Dy³⁺ ion doped Sr₂CaWO₆ phosphors with double perovskite structure were synthesized by traditional high temperature solid-state method. It was found that there is significant energy transfer between Dy³⁺ and the host lattice, and the intensities of emission peaks at 449 nm (blue), 499 nm (cyan), 599 nm (orange), 670 nm (red), and 766 nm (infra-red) can be changed by adjusting the concentration of dopant amount of Dy³⁺ ion in Sr₂CaWO₆. The correlated color temperature of Dy³⁺ ion doped Sr₂CaWO₆ phosphors can be tuned by adjusting the concentration of Dy³⁺ ion. Upon optimal doping at 1.00 mol% Dy³⁺, white light with chromaticity coordinate (0.34, 0.33) was emitted under excitation at 310 nm. Thus, single composition white emission is realized in Dy³⁺ doped Sr₂CaWO₆.

Calcium-based efficient cathode-ray scintillating metal–organic frameworks constructed from π-conjugated luminescent motifs

π-Conjugated luminescent motifs that suffer from aggregation-caused quenching can respond well to ionizing cathode-rays after coordination with Ca(ii) ions.

(Ca0.8Mg0.2Cl2/SiO2):Eu2+: a violet-blue emitting phosphor with a low UV content for UV-LED based phototherapy illuminators

A violet-blue emitting phosphor with a low UV content in its emission spectrum has been fabricated through cation substitution for UV-LED based phototherapy illuminators.

Efficient Blue-emitting Phosphor SrLu2O4:Ce3+ with High Thermal Stability for Near Ultraviolet (~400 nm) LED-Chip based White LEDs

Blue-emitting phosphors for near ultraviolet (NUV) based tri-color RGB phosphor blend converted white light emitting diodes (LEDs) have been extensively investigated in the past few years. LED chip peaked near 400 nm is the most efficient among the NUV chips currently. However, most of blue phosphors show inefficient excitation around 400 nm. Herein, a novel blue phosphor SrLu2O4:Ce3+ matching well with near 400 nm chip and showing high thermal stability has been developed. The photoluminescence spectrum presents a broad emission band peaking at 460 nm with a bandwidth of nearly 90 nm. By optimizing the Ce3+ concentration, an internal quantum efficiency (IQE) as high as 76% was achieved. Furthermore, 86% of the room-temperature emission intensity is still maintained at 150 °C, indicating a good thermal stability and practicality. A series of white LEDs were fabricated based on 405 nm chips coated with a blend of the new blue phosphor with the commercial yellow and red phosphors. High color rendering indexes (≥90) were achieved while the correlated color temperature was tuneable in the range of 3094 to 8990 K. These results suggest that SrLu2O4:Ce3+ can be utilized as a blue-emitting phosphor in NUV based white LEDs.

Photonic structuring improves the colour purity of rare-earth nanophosphors

Nanophosphor integration in an optical cavity allows unprecedented control over both the chromaticity and the directionality of the emitted light, without modifying the chemical composition of the emitters or compromising their efficiency. Our approach opens a route towards the development of nanoscale photonics based solid state lighting.

Engineering triangular carbon quantum dots with unprecedented narrow bandwidth emission for multicolored LEDs

Carbon quantum dots (CQDs) have emerged as promising materials for optoelectronic applications on account of carbon's intrinsic merits of high stability, low cost, and environment-friendliness. However, the CQDs usually give broad emission with full width at half maximum exceeding 80 nm, which fundamentally limit their display applications. Here we demonstrate multicolored narrow bandwidth emission (full width at half maximum of 30 nm) from triangular CQDs with a quantum yield up to 54-72%. Detailed structural and optical characterizations together with theoretical calculations reveal that the molecular purity and crystalline perfection of the triangular CQDs are key to the high color-purity. Moreover, multicolored light-emitting diodes based on these CQDs display good stability, high color-purity, and high-performance with maximum luminance of 1882-4762 cd m-2 and current efficiency of 1.22-5.11 cd A-1. This work will set the stage for developing next-generation high-performance CQDs-based light-emitting diodes.

Down-Conversion Nitride Materials for Solid State Lighting: Recent Advances and Perspectives

Advances in solid state white lighting technologies witness the explosive development of phosphor materials (down-conversion luminescent materials). A large amount of evidence has demonstrated the revolutionary role of the emerging nitride phosphors in producing superior white light-emitting diodes for lighting and display applications. The structural and compositional versatility together with the unique local coordination environments enable nitride materials to have compelling luminescent properties such as abundant emission colors, controllable photoluminescence spectra, high conversion efficiency, and small thermal quenching/degradation. Here, we summarize the state-of-art progress on this novel family of luminescent materials and discuss the topics of materials discovery, crystal chemistry, structure-related luminescence, temperature-dependent luminescence, and spectral tailoring. We also overview different types of nitride phosphors and their applications in solid state lighting, including general illumination, backlighting, and laser-driven lighting. Finally, the challenges and outlooks in this type of promising down-conversion materials are highlighted.

White light emitting MgAl2O4:Dy3+,Eu3+ nanophosphor for multifunctional applications

An efficient energy transfer from Dy³⁺ to Eu³⁺ in MgAl₂O₄ offers white light emission and self-referencing thermal behaviour.

Morphology/phase controllable synthesis of monodisperse ScVO4 microcrystals and tunable multicolor luminescence properties in Sc(La)VO4(PO4):Bi3+,Ln3+ phosphors

A facile one-pot hydrothermal approach was employed to prepare novel chocolate-like ScVO₄ microcrystals using polyethylene glycol as an additive.

Visible emission from bismuth-doped yttrium oxide thin films for lighting and display applications

Due to the great development of light sources for several applications from displays to lighting, great efforts are devoted to find stable and efficient visible emitting materials. Moreover, the requirement of Si compatibility could enlarge the range of applications inside microelectronic chips. In this scenario, we have studied the emission properties of bismuth doped yttrium oxide thin films grown on crystalline silicon. Under optical pumping at room temperature a stable and strong visible luminescence has been observed. In particular, by the involvement of Bi ions in the two available lattice sites, the emission can be tuned from violet to green by changing the excitation wavelength. Moreover, under electron beam at low accelerating voltages (3 keV) a blue emission with high efficiency and excellent stability has been recorded. The color is generated by the involvement of Bi ions in both the lattice sites. These peculiarities make this material interesting as a luminescent medium for applications in light emitting devices and field emission displays by opening new perspectives for the realization of silicon-technology compatible light sources operating at room temperature.

Structure and photoluminescence properties of red-emitting apatite-type phosphor NaY9(SiO4)6O2:Sm3+ with excellent quantum efficiency and thermal stability for solid-state lighting

A novel red-emitting phosphor NaY₉(SiO₄)₆O₂:Sm³⁺ (NYS:Sm³⁺) was synthesized and the X-ray diffraction and high-resolution TEM testified that the NYS compound belongs to the apatite structure which crystallized in a hexagonal unit cell with space group P6₃/m. The novel phosphor boasts of such three advantageous properties as perfect compatible match with the commercial UV chips, 73.2% quantum efficiency and 90.9% thermal stability at 150 °C. Details are as follows. NYS:Sm³⁺ phosphor showed obvious absorption in the UV regions centered at 407 nm, which can be perfectly compatible with the commercial UV chips. The property investigations showed that NYS:Sm³⁺ phosphor emitted reddish emission with CIE coordination of (0.563, 0.417). The optimum quenching concentration of Sm³⁺ in NYS phosphor was about 10%mol, and the corresponding concentration quenching mechanism was verified to be the electric dipole–dipole interaction. Upon excitation at 407 nm, the composition-optimized NYS:0.10Sm³⁺ exhibited a high quantum efficiency of 73.2%, and its luminescence intensity at 150 °C decreased simply to 90.9% of the initial value at room temperature. All of the results indicated that NYS:Sm³⁺ is a promising candidate as a reddish-emitting UV convertible phosphor for application in white light emitting diodes (w-LEDs).

Enhanced emission of encaged-OH--free Ca12(1-x)Sr12xAl14O33:0.1%Gd3+ conductive phosphors via tuning the encaged-electron concentration for low-voltage FEDs

Encaged-OH^--free Ca_12(1-x)Sr_12xAl₁₄O₃₃:0.1%Gd³⁺ conductive phosphors were prepared through a melt-solidification process in combination with a subsequent heat treatment. Absorption spectra showed that the maximum encaged-electron concentration was increased to 1.08 × 10²¹ cm^-3 through optimizing the doping amount of Sr²⁺ (x = 0.005). Meanwhile, FTIR and Raman spectra indicated that pure Ca_11.94Sr_0.06Al₁₄O₃₃:0.1%Gd³⁺ conductive phosphor without encaged OH^- and C₂^2- anions was acquired. For the conductive powders heat-treated in air for different times, the encaged-electron concentrations were tuned from 1.02 × 10²¹ to 8.3 × 10²⁰ cm^-3. ESR, photoluminescence, and luminescence kinetics analyses indicated that the emission at 312 nm mainly originated from Gd³⁺ ions surrounded by encaged O^2- anions, while Gd³⁺ ions surrounded by encaged electrons had a negative contribution to the UV emission due to the existence of an energy transfer process. Under low-voltage electron-beam excitation (3 kV), enhanced cathodoluminescence (CL) of the conductive phosphors could be achieved by tuning the encaged-electron concentrations. In particular, for the encaged-OH^--free conductive phosphor, the emission intensity of the CL was about one order of magnitude higher than that of the conductive phosphor containing encaged OH^- anions. Our results suggested that the encaged-OH^--free conductive phosphors have potential application in low-voltage FEDs.

New Developments in Cathodoluminescence Spectroscopy for the Study of Luminescent Materials

Herein, we describe three advanced techniques for cathodoluminescence (CL) spectroscopy that have recently been developed in our laboratories. The first is a new method to accurately determine the CL-efficiency of thin layers of phosphor powders. When a wide band phosphor with a band gap (E_g > 5 eV) is bombarded with electrons, charging of the phosphor particles will occur, which eventually leads to erroneous results in the determination of the luminous efficacy. To overcome this problem of charging, a comparison method has been developed, which enables accurate measurement of the current density of the electron beam. The study of CL from phosphor specimens in a scanning electron microscope (SEM) is the second subject to be treated. A detailed description of a measuring method to determine the overall decay time of single phosphor crystals in a SEM without beam blanking is presented. The third technique is based on the unique combination of microscopy and spectrometry in the transmission electron microscope (TEM) of Brunel University London (UK). This combination enables the recording of CL-spectra of nanometre-sized specimens and determining spatial variations in CL emission across individual particles by superimposing the scanning TEM and CL-images.

Blue to magenta tunable luminescence from LaGaO3: Bi3+, Cr3+ doped phosphors for field emission display applications

A series of blue to magenta emitting color-tunable LaGaO₃: Bi³⁺/Cr³⁺ phosphors were prepared by chemical route, and their phase structure, morphology and photoluminescence (PL) properties were investigated.

A luminescent lanthanide approach towards direct visualization of primary cilia in living cells

A simple and direct imaging tool (HGEu001) for primary cilia based on long-lived europium luminescence is firstly presented.

All-Copper Nanocluster Based Down-Conversion White Light-Emitting Devices

Most of the present-day down-conversion white light-emitting devices (WLEDs) utilize rare-earth elements, which are expensive and facing the problem of shortage in supply. WLEDs based on the combination of orange and blue emitting copper nanoclusters are introduced, which are easy to produce and low in cost. Orange emitting Cu nanoclusters (NCs) are synthesized using glutathione as both the reduction agent and stabilizer, followed by solvent induced aggregation leading to the emission enhancement. Photoluminescence quantum yields (PL QY) of 24% and 43% in solution and solid state are achieved, respectively. Blue emitting Cu nanoclusters are synthesized by reduction of polyvinylpyrrolidone supported Cu(II) ions using ascorbic acid, followed by surface treatment with sodium citrate which improves both the emission intensity and stability of the clusters, resulting in the PL QY of 14% both in solution and solid state. All-copper nanocluster based down-conversion WLEDs are fabricated by integrating powdered orange and blue emitting Cu NC samples on a commercial GaN LED chip providing 370 nm excitation. They show favorable white light characteristics with Commission Internationale de l'Eclairage color coordinates, color rendering index, and correlated color temperature of (0.36, 0.31), 92, and 4163 K, respectively.

Local Field Enhancement-Induced Enriched Cathodoluminescence Behavior from CuI-RGO Nanophosphor Composite for Field-Emission Display Applications

Field-emission displays (FEDs) constitute one of the major foci of the cutting edge materials research because of the increasingly escalating demand for high-resolution display panels. However, poor efficiencies of the concurrent low voltage cathodoluminescence (CL) phosphors have created a serious bottleneck in the commercialization of such devices. Herein we report a novel CuI-RGO composite nanophosphor that exhibits bright red emission under low voltage electron beam excitation. Quantitative assessment of CL spectra reveals that CuI-RGO nanocomposite phosphor leads to the 4-fold enhancement in the CL intensity as compared to the pristine CuI counterpart. Addition of RGO in the CuI matrix facilitates efficient triggering of luminescence centers that are activated by local electric field enhancement at the CuI-RGO contact points. In addition, conducting RGO also reduces the negative loading problem on the surface of the nanophosphor composite. The concept presented here opens up a novel generic route for enhancing CL intensity of the existing (nano)phosphors as well as validates the bright prospects of the CuI-RGO composite nanophosphor in this rapidly growing field.

Tunable emissions via the white region from Sr2Gd8(SiO4)6O2:RE3+ (RE3+: Dy3+, Tm3+, Eu3+) phosphors

Sr₂Gd₈(SiO₄)₆O₂ is an excellent host lattice for tunable emissions via the white-light region when co-doped with suitable trivalent rare-earth ions.

Luminescence properties of Eu2+-doped BaSi2O5 as an efficient green phosphor for light-emitting devices and wide color gamut field emission displays

A green-emitting BaSi₂O₅:Eu²⁺ phosphor for white light emitting diodes and field emission displays.

Enhancement of encaged electron concentration by Sr2+ doping and improvement of Gd3+ emission through controlling encaged anions in conductive C12A7 phosphors

Conductive C12A7:0.1%Gd³⁺,y%Sr²⁺ phosphors have been prepared through hydrogen route in combination with subsequent UV-irradiation.

Versatile properties of CaGd₂ZnO₅:Eu³⁺ nanophosphor: its compatibility for lighting and optical display applications

Red color-emitting CaGd2ZnO5:Eu(3+) (CGZO:Eu(3+)) nanophosphors were synthesized by a facile sol-gel process. The structural and luminescent properties of these phosphors were investigated as a function of annealing temperature and Eu(3+) ion concentration. The orthorhombic phase was confirmed at different annealing temperatures, showing an irregular morphology within the nanoscale range. Photoluminescence (PL) excitation spectra of CGZO:Eu(3+) showed host absorption band (HAB), charge transfer band (CTB), and intense f-f transitions of Eu(3+) in the violet and blue wavelength regions. The CTB intensity increased and the HAB intensity decreased with increasing annealing temperature or Eu(3+) ion concentration. The CGZO:Eu(3+) exhibited a strong absorption in the blue region as compared to the CTB and had a superior property compared to available commercial phosphors. This feature facilitates the fabrication of high color rendering index white light-emitting diodes for display systems. In PL spectra, an intense red emission was observed due to the hypersensitive (5)D0→(7)F2 transition with good asymmetry ratio and chromaticity coordinates. Optimized annealing temperature and concentration of Eu(3+) ions were observed for CGZO host lattice based on the 466 nm excitation wavelength. The cathodoluminescent properties were also similar to the PL results.

Incorporation of Zn(2+) ions into BaTiO3:Er(3+)/Yb(3+) nanophosphor: an effective way to enhance upconversion, defect luminescence and temperature sensing

Ferroelectric BaTiO3 became a multifunctional material via doping of lanthanide ions (0.3 mol% Er(3+)/3.0 mol% Yb(3+)) and subsequently upconversion luminescence was enhanced by incorporation of Zn(2+) ions. Upconversion luminescence of BaTiO3:Er(3+)/Yb(3+) perovskite nanophosphor has been studied using 800 and 980 nm laser excitations. The emission dynamics is studied with respect to its dependence on input power and external temperature including lifetime. Based on time-resolved spectroscopy, it is inferred that two types of Er(3+) sites are present in the barium titanate lattice. The first one is a short lived component (minor species) present at 6-coordinated Ti-sites of low symmetry while the second one is a long lived component (major species), present at 12-coordinated Ba-sites with high symmetry. The influence of the introduction of Zn(2+) ions on the lifetime of (4)S3/2 and (4)F9/2 levels of Er(3+) ions is also investigated. Enhanced temperature sensing performance (120 K to 505 K) of the material is observed using the fluorescence intensity ratio technique, employing the emission from the thermally coupled, (2)H11/2 and (4)S3/2 energy levels of Er(3+) ions. The defect luminescence of the material is also found to increase upon Zn-doping.