Doped Single Crystals Research Articles

Non-thermal to thermal electric field effects induced by DC and current pulses in pure and doped VO2 single crystals

Pure VO2, GexV1−xO2 (x < 0.2), and AlxV1−xO2 (x ≤ 0.02) exhibit above room temperature the well-known insulator-metal-transition (IMT) (at TIMT ≅ 340 K). In addition, AlxV1−xO2 exhibits an insulator-insulator-transition (IIT) at TIIT < TIMT. Both transitions can be induced either by external or by Joule heating. Both leave prominent marks on the I–V characteristics of the investigated samples, IIT at low currents and IMT at high currents. Reported herein is a comparative study of the I–V characteristics of needle-like pure and doped VO2 single crystals obtained with pulsed currents of various durations and with DC, in an attempt to separate thermal from non-thermal electric field effects. Identification of the Poole–Frenkel effect (a non-thermal electric field effect) in the I–V characteristics of the doped crystals may play an important role in a long-standing debate on the mechanism of IMT. Such an identification was obtained in this investigation for several Ge- and Al-doped VO2 crystals, so far only in narrow ranges of electric fields (<2000 V/cm) and pulse durations (≤1 ms) above which the non-linear conductivity of the crystals is dominated by Joule heating.

Original Na5(Lu1–x–yGdxYy)9F32: Pr3+/Ho3+ Hybrid Single Crystals for Efficient ∼2.9 μm Mid-Infrared Emission

A novel Ho3+/Pr3+/Y3+/Gd3+ multiply doped Na5Lu9F32 mixed single crystal was successfully grown via a modified Bridgman technique. The structure of the mixed crystal was characterized by means of X-ray diffraction and Raman spectra. The impacts of Y3+ and Gd3+ ions on the 2.9 μm emission of Na5Lu9F32 single crystals, which incorporated fixed Ho3+/Pr3+, were investigated systematically with the aid of the optical spectra and decay curves of fluorescence. The 2.9 μm emission intensity (Ho3+: 5I6 → 5I7) was further heightened upon gradually introducing both Y3+ and Gd3+ ions into Ho3+/Pr3+-incorporated single crystals pumped at 640 nm. The brokenness of ionic quenching clusters of [Ho3+–Ho3+] and the release of more luminescent ions are in charge of the augment of emission at 2.9 μm by introducing Gd3+ or Y3+ ions into the Ho3+/Pr3+ codoped single crystals. A value of ∼8.2 × 10–21 cm2 was estimated for the maximum emission cross section at 2.9 μm of the mixed crystal. The processing of energy transfer among Ho3+, Pr3+, Y3+, and Gd3+ ions was investigated, and the energy transfer efficiency for Ho3+: 5I7 → Pr3+: 3F2 was approximately 57.32%. The Na5Lu9F32-doped Ho3+/Pr3+/Y3+/Gd3+ mixed crystals may be a new laser gain medium for 2.9 μm generation because of their special spectral and physicochemical properties.

Crystal growth and spectral properties of Eu3+-doped La2CaB10O19 single crystals

The invention and commercial application of InGaN/GaN-laser diodes have made it feasible to directly obtain the visible lasing radiation by pumping rare earth (RE) doped laser crystals. Hence searching for new laser crystals with efficient luminescence and applicable optical and physical properties in the visible range is an essential work. Here, the crystal growth and spectral properties of a series of large-size Eu3+ doped La2CaB10O19 (Eu3+-LCB) single crystals with the nominal atomic ratios of 3%, 7%, and 10% have been well studied. High-quality Eu3+-LCB single crystals with the size of 45 × 30 × 17 mm3, was grown by top-seeded solution growth (TSSG) method. The luminescence probe was employed as the tool to identify occupation sites, demonstrating that Eu3+ ions enter into La3+ sites first, then occupy Ca2+ sites after the nominal atomic ratio of Eu3+ ions exceeds 7%. Based on the absorption and emission spectra, the intensity parameters, spontaneous transition probability, radiation lifetime, and fluorescence branch ratio for Eu3+-LCB single crystals are studied with the Judd–Ofelt theory. From the transient spectra, the emission cross-section, fluorescence decay lifetime and luminescence quantum efficiency are deduced, which exhibit the maximum performance at the nominal atomic ratio of 7% Eu3+. Combining the luminescence probe and spectral properties, it can be concluded that Eu3+ doped crystals will have the best spectral properties when Eu3+ occupation at the single La3+ sites reaches the maximum in LCB single crystals, which may be universal for RE-doped crystal with two possible occupation sites. Besides, our work demonstrates that the 7% Eu3+-LCB single crystal may be a promising medium for laser radiation in the visible region in comparison with other Eu3+ doped single crystals.

Structure and spectral properties of Dy3+ doped CaYAlO4 single crystal

A 2 at.% Dy3+: CaYAlO4 single crystal was grown successfully. The electronic structures of Ca2+/Y3+ mixed sites in CaYAlO4 were investigated using first-principles based on density functional theory. The effects of Dy3+ doping on the structural parameters of host crystal were studied using XRD pattern. The optical properties including absorption spectrum, excitation spectrum, emission spectra and fluorescence decay curves were thoroughly investigated. The results show that the Dy3+: CaYAlO4 crystal could be pumped by the blue InGaN and AlGaAs or 1281 nm laser diodes. Furthermore, an intense 578 nm yellow emission was obtained directly under excitation at 453 nm, meanwhile, evident mid-infrared light emitting was observed by 808 or 1281 nm laser excitation. The fitted fluorescence lifetimes of 4F9/2 and 6H13/2 levels were about 0.316 ms and 0.038 ms, respectively. It can be concluded that this Dy3+: CaYAlO4 crystal could simultaneously act as a promising medium for both solid-state yellow and mid-infrared laser outputs.

Growth Process and Optical Properties of Cs2ZrCl6 Doped with Ce3+ and Li+ Crystals

Cs2ZrCl6 (CZC) has very promising scintillation properties in the application of gamma-ray spectroscopy. So far, undoped crystals of this composition have been studied, but the growth of the large-scale doped single crystals has not been reported so far. In this paper, a process improvement strategy is reported. Large-scale Cs2ZrCl6:Ce,Li (CZC:Ce,Li) single crystals with excellent energy resolution are obtained by appropriately adjusting the temperature gradient and growth rate. CZC:Ce,Li single crystals doped with 0.5, 1.5, and 3.0 mol % Ce3+ were grown by the Bridgman method. After the phase purity of the sample was determined by X-ray diffraction analysis, the crystal defects were determined by the energy-dispersive X-ray spectroscopy, and the crystal growth process was improved on this basis. The transmission spectrum confirmed the improvement of crystal quality. The energy resolution of CZC:Ce,Li crystals grown by the improved process is 4.3%, which confirmed that the improved process is conducive to the growth of bulk doped CZC single crystals, and hence, our results provide a basis for the growth of large-scale doped CZC single crystals.

Rejuvenation of giant electrostrain in doped barium titanate single crystals

Engineering materials through the introduction of point defects has resulted in significant advances in semiconductor processing and, more recently, the observation of novel phenomena such as large reconfigurable strains in ferroelectrics as a result of defect dipole complexes. Up to 0.8% strain has been demonstrated in BaTiO3 crystals dilutely doped with iron. However, the defect dipole pinning sites and the corresponding achievable strains are found to degrade as the crystal is electrically cycled as part of the measurement process. The strain degradation rate is dependent on the applied field values but shows an exponential change in materials properties regardless of the electric field. This behavior, plus a change in impedance with number of times cycled, suggests these changes are due to electric field induced oxygen migration—similar to the cause of the resistance degradation effect. Despite this, effective piezoelectric coefficients of over 4700 pm/V were recorded with 1.5 kV/cm fields, one of the largest values for a lead-free piezoelectric material thus far. In addition, the defect dipole-aligned state and the high strains can be repeatably recovered by a subsequent heat treatment step after cycling. Potential paths to exploiting the defect dipole induced effects and large piezoelectric coefficient in these dilutely doped systems are proposed.

Synthesis and characterization of pure and l-proline doped copper sulphate single crystals

Synthesis and characterization of pure and l-proline doped copper sulphate single crystals

Effective enhancement of ∼2.86 µm mid-infrared emission of Na5Y9F32 single crystal co-doped Ho3+/Pr3+ introduced by Gd3+ ions

The optical spectral properties of high-quality Na5Y9F32 single crystals triply incorporated Ho3+/Pr3+/Gd3+ ions prepared successfully via a Bridgman technique were characterized with the help of their absorption spectra, emission spectra, and fluorescence lifetimes. It was discovered that the emission intensity at ∼2.86 µm of Na5Y9F32 co-doped Pr3+/Ho3+ single crystal was boosted obviously by introducing of Gd3+ ions, and it reached its maximum when the concentration of Gd3+ ion dopant was ∼0.7 mol%. The release of more luminescent centers due to the destruction of [Ho3+-Ho3+] ion quenching clusters and variation of the symmetry of crystal field around Ho3+ ions are responsible for the augmentation of ∼2.86 µm fluorescence emission. In addition, the introduction of Gd3+ ion reduced the fluorescence lifetime of Ho3+: 5I7 level, while the Ho3+:5I6 level remained slightly changed, which is beneficial to improve the particle inversion of Ho3+ ion between the 5I6 and 5I7 energy levels to enhance the ∼2.86 µm emission. The results showed that the triply doped fluoride single crystal can become a potentially prospective material for ∼2.86 µm mid-infrared lasers.

Thermally induced effects in the optical absorption of pure and doped Bi12SiO20 single crystals

Thermally induced effects in the optical absorption of pure and doped Bi12SiO20 single crystals

Photoconductivity of pure and rare-earth doped p-GaSe single crystals

Photoconductivity characteristic of undoped and gadolinium (Gd)/dysprosium (Dy) co-doped layered [Formula: see text]-GaSe semiconductor has been experimentally investigated. A model is proposed to explain the obtained results. The creating possibility of multi-band photo-detectors with stable and reproducible parameters is shown and characteristics based on single crystals of [Formula: see text]-GaSe are activated with mentioned rare-earth ions with concentration of [Formula: see text] at.%.

Dilution of the magnetic lattice in the Kitaev candidate α−RuCl3 by Rh3+ doping

Magnetic dilution of a well-established Kitaev candidate system is realized in the substitutional ${\mathrm{Ru}}_{1\ensuremath{-}x}{\mathrm{Rh}}_{x}{\mathrm{Cl}}_{3}$ series $(x=0.02--0.6)$. Optimized syntheses protocols yield uniformly doped single crystals and polycrystalline powders that are isostructural to the parental $\ensuremath{\alpha}\text{\ensuremath{-}}{\mathrm{RuCl}}_{3}$ as per x-ray diffraction. The Rh content $x$ is accurately determined by the quantitative energy-dispersive x-ray spectroscopy technique with standards. We determine the magnetic phase diagram of ${\mathrm{Ru}}_{1\ensuremath{-}x}{\mathrm{Rh}}_{x}{\mathrm{Cl}}_{3}$ for in-plane magnetic fields from magnetization and specific-heat measurements as a function of $x$ and stacking periodicity and identify the suppression of the magnetic order at $x\ensuremath{\approx}0.2$ towards a disordered phase, which does not show any clear signature of freezing into a spin glass. Comparing with previous studies on the substitution series ${\mathrm{Ru}}_{1\ensuremath{-}x}{\mathrm{Ir}}_{x}{\mathrm{Cl}}_{3}$, we propose that chemical pressure would contribute to the suppression of magnetic order, especially in ${\mathrm{Ru}}_{1\ensuremath{-}x}{\mathrm{Ir}}_{x}{\mathrm{Cl}}_{3}$, and that the zigzag magnetic ground state appears to be relatively robust with respect to the dilution of the Kitaev-$\mathrm{\ensuremath{\Gamma}}$-Heisenberg magnetic lattice. We also discovered a slight dependence of the magnetic properties on thermal cycling, which would be due to an incomplete structural transition.

Synthesis, growth and characterization of magnesium chloride doped L-alanine cadmium chloride single crystal: For nonlinear optical application

The aim of this research was to synthesize and characterize pure and magnesium chloride (MgCl2) doped L–alanine cadmium chloride (LACC) single crystals. Pure and MgCl2 doped LACC single crystals were synthesized by solution method with slow evaporation solution growth technique at room temperature. The single crystal X-ray diffraction studies of pure, 1 and 2 mol% MgCl2 doped LACC single crystals revealed monoclinic crystal structure with C2 space group. The optical properties of pure and MgCl2 doped LACC single crystals investigated by UV–VIS/NIR spectrometer confirmed that the crystals were transparent in the wavelength range of 230-1100 nm. The optical band gap energy of pure and doped LACC single crystals were found to have the same value of 5.4 eV. The energy-dispersive X-ray analysis indicated the incorporation of magnesium and chlorine atoms in LACC single crystal. The second harmonic generation efficiency of 1 and 2 mol% MgCl2 doped LACC crystals were analyzed by Kurtz-Perry powder technique and found to be 1.75 and 2 times greater than that of the standard potassium dihydrogen phosphate crystal, respectively.

Optical spectroscopy and thermally stable characteristics of Ce3+/Tb3+/Eu3+ triply doped Na5Y9F32 single crystals for white light emission

Na5Y9F32 single crystals incorporated with fixed 0.4 mol% Ce3+, 0.5 mol% Eu3+ and various Tb3+ ion concentrations from 0.3 to 0.9 mol% were synthesized via Bridgman method. The effects of Tb3+ ion concentration and exciting wavelength on the fluorescence spectra of the single crystals were investigated in detail. Upon excitation of 320 nm light, as Tb3+ concentration increasing from 0.3 to 0.9 mol%, the 420 and 441 nm emission intensities of Ce3+ ion decrease continuously but the 544 nm of Tb3+, and 591 nm and 613 nm of Eu3+ increase. The energy transfers among Ce3+, Tb3+ and Eu3+ ions were responsible for the above changes of emissions, which was studied with help of the measured optical spectra and decay curves. Upon excitation of 320 nm, the emission of the triply doped crystal from the combination of 441, 544, 591 and 613 nm was changed from blue gamut with color coordinates (0.2993,0.2759）to white with color coordinates (0.2986,0.3208) as Tb3+ ion incorporating concentration change from 0.3 to 0.9 mol%. Moreover, the internal quantum yield of practical down-conversion was determined by integrating sphere to be ∼15.32% under excited by 320 nm light. The optically thermal stability was studied by the temperature dependence emission spectra, and the result showed that as the ambient temperature increases from 293 K to 423 K, the integrated emission intensity remains around 89%. Therefore, the prepared single crystal is a promising optical device material at present.

Enhanced scintillation performance of β-Ga2O3 single crystals by Al3+ doping and its physical mechanism

β-Ga2O3 is a potential fast semiconductor scintillator with no significant self-absorption and excellent stability. However, the relatively low light yield of β-Ga2O3 at room temperature limits its practical application. In order to improve its scintillation performance, Al3+ doped β-Ga2O3 single crystals are investigated. By doping 5%Al3+ (atomic concentration), the light yield of β-Ga2O3 is increased from 4394 to 6816 ph/MeV. The increased light yield may be attributed to the decreased free electron concentration and inhibition of Auger nonradiative recombination. The thermal quenching effect can also be moderated by the increase in the thermal activation energy induced by Al3+ doping. The results indicate that Al3+ doping is an effective method to increase the light yield of β-Ga2O3.

Luminescence properties and time-resolved spectroscopy of rare-earth doped SrMoO4 single crystals

Luminescence properties of nominally pure and doped with Eu3+ and Pr3+ ions SrMoO4 single crystals grown by the Czochralski method have been studied. Thermal quenching of intrinsic emission of pure and doped SrMoO4 single crystals has been observed, as well as a correlation of thermal quenching activation energies with rare-earth ion concentration has been observed. Tunable laser was used to study time-resolved luminescence in a range from 10 K to room temperature. The effect of dopant nature and concentration on intrinsic emission and decay kinetics has been elucidated.

Crystal growth and scintillation properties of Tm3+ doped LaCl3 single crystal for radiation detection

Single crystal of Tm-doped LaCl3 (LaCl3:Tm3+) with the dimension of diameter 7 mm and length 55 mm was successfully grown using an in-house-developed Bridgman furnace. The grown crystal has the self-trapped exciton (STE) emission in the range of 300–500 nm along with the characteristic emissions corresponding to the 4f-4f transition of Tm3+ ions under X-ray excitation. The scintillation characterizations of the grown crystal were performed by pulse height spectra, scintillation decay time, as well as pulse shape discrimination studies under γ-ray and α-particle excitation sources. The LaCl3:Tm3+ crystal was found to have a high light yield (42,000 ± 4,200 ph/MeV) with 4.2% energy resolution under γ-rays excitation. Moreover, the pulse shape discrimination was studied using 137Cs and 241Am as the radiation sources with a good FOM value. The results conclude the LaCl3:Tm3+ single crystal to be a promising candidate for X-ray and γ-ray detection purposes.

Highly sensitive photodetector of Zn/Bi doped MAPbBr3 single crystals formed homojunction

The double doped MAPbBr3 single crystals (SCs) based homojunction is prepared by a modified solution-processed epitaxy growth method to tune the bandgap and form a larger energy band barrier for homojunction. The optoelectronics of a homojunction formed by epitaxial growing Zn/Bi doped MAPbBr3 SCs shows relatively lower trapped carrier density and higher built-in potential. Compared with pure MAPbBr3 SCs, the bandgap of Zn doped MAPbBr3 SCs become bigger while that of Bi doped MAPbBr3 SCs become smaller, which lead to a larger energy band barrier for the homojunction. The as-fabricated MAPbBr3 SCs based homojunction shows smaller dark current and excellent current stability compared to the pure MAPbBr3 SCs under external reverse voltage of 4 V. The doped MAPbBr3 SCs based homojunction displays highly sensitive characteristics, with a responsivity of 43 mA/W and a specific detectivity of 3.41×1013 Jones. These results confirm that the double doped MAPbBr3 SCs based homojunction is promising photodetector frameworks, which may be the key to realizing superior perovskite single crystal sensitive photodetector in the future.

ZnO under Pressure: From Nanoparticles to Single Crystals

In the present review, new approaches for the stabilization of metastable phases of zinc oxide and the growth of ZnO single crystals under high pressures and high temperatures are considered. The problems of the stabilization of the cubic modification of ZnO as well as solid solutions on its basis are discussed. A thermodynamic approach to the description of zinc oxide melting at high pressures is described which opens up new possibilities for the growth of both undoped and doped (for example, with elements of group V) single crystals of zinc oxide. The possibilities of using high pressure to vary phase and elemental composition in order to create ZnO-based materials are demonstrated.

Solid-state single-crystal growth of YAG and Nd: YAG by spark plasma sintering

• Growth of YAG and Nd:YAG single crystals achieved during the spark plasma sintering. • Porosity of the single crystals reduced due to the application of external pressure. • High Nd 3+ doping had a positive effect on the single-crystal conversion kinetics. Recent studies have shown that many challenges encountered in conventional single crystal growth methods, including high production costs, can be overcome by using the solid-state single-crystal growth (SSCG) approach, which has been recognized as a simple and cost-effective alternative for obtaining single crystals. In this work, Y 3 Al 5 O 12 (YAG) and Nd 3+ -doped YAG (Nd:YAG) single crystals were grown via the SSCG method using spark plasma sintering (SPS). The growth of single crystals was initiated at the surface of (110) YAG single-crystal seeds embedded inside YAG and Nd:YAG powder beds, and this growth continued as the surrounding polycrystalline matrix was converted into a single crystal. The application of external pressure during the SPS process has been found beneficial for reducing the porosity of the grown single crystals. Moreover, high Nd 3+ doping levels had a positive effect on the conversion kinetics, with a growth rate of almost 50 µm/h, which increased the driving force for single-crystal growth through the solute drag effect. EDS elemental mapping and line scans confirmed the compositional uniformity of the grown single crystals, while EBSD images verified their crystallization in the (110) direction. The obtained results confirm the strong potential of the SSCG technique coupled with SPS for the growth of undoped and highly doped YAG single crystals with excellent quality.

Ultraviolet C random lasing at 230–280 nm from Pr3+ doped bulk crystal resonators by two-photon absorption

Ultraviolet solid-state light sources, especially in the short-wave ultraviolet band, are attracting great attention for potential applications in high-density optical data storage, biomedical research, and water and air purification and sterilization. As one of the means to generate solid-state short-wavelength lasers, upconversion technology, which can transform a low-energy photon to a high-energy photon, has a simple structure and does not require strict phase-matching conditions. However, because of the high non-radiative decay rate during multiphoton absorption process, the short-wave upconversion ultraviolet lasing is still difficult to achieve. Here, under 450 nm blue laser excitation, we have successfully obtained ultraviolet C random lasing from a Pr3+ doped YPO4 single crystal via two-photon absorption. Presently, ultraviolet C random lasing with the wavelength range of 230–280 nm is the shortest wavelength for upconversion lasing emission.