High Temperature Photoluminescence Research Articles

High temperature synthesis, surface characterization, and photoluminescence property of boron carbon nitride

The present study demonstrates the synthesis of boron carbon nitride (BCN) by molten salt method which utilizes melamine (source of carbon and nitrogen), boric acid (source of boron) and potassium chloride as the salt. The synthesis is performed at 915 °C for 4 h under N2-atmosphere. The synthesized material is further acid-treated. The physicochemical characterization data for as-synthesized and acid-treated materials confirm successful synthesis of BCN. SEM and AFM data display that synthesized BCN pre and post acid-treatment exhibit sheet-type morphology. Thermogravimetric analysis data confirm that the acid-treated material is thermally stable up to 800 °C, with only 2 % mass-loss is observed till 800 °C. Water contact angle measurement study demonstrates that acid-treatment significantly improves the surface hydrophilicity. Additionally, the low photoluminescence peak intensity of the acid-treated material indicates low electron-hole pair recombination rate. The synthesized material may be useful for the photocatalytic degradation of potentially toxic chemicals from water.

High-temperature photoluminescence properties of various defects in hydrothermally grown ZnO microrods

High-temperature photoluminescence properties of hydrothermally grown ZnO microrods were reported. Photoluminescence spectra of ZnO microrods deconvoluted into three Gaussian components: near band edge (NBE) emission, Violet emission (VL) and Yellow emission (YL) peaks corresponding to free excitons, zinc vacancies and interstitial oxygen atoms, respectively. Increasing the temperature caused a redshift of the energy position of the NBE and VL peaks and a blueshift of the energy position of the YL peak were observed. In the temperature range from 300K to 473K, the intensities of the NBE and VL bands varied based on a nonlinear pattern. Intensity of these emission bands decreases from temperature 473K–673K. As the temperature increases, changes in the energy and crystal structures of the material due to decrease in the concentration of defects cause a blueshift of the emission bands related to interstitial atoms, and a redshift of the emission bands related to vacancies.

High temperature anomalous Raman and photoluminescence response of molybdenum disulfide with sulfur vacancies

We report an intriguing anomalous behavior observed in the temperature-dependent Raman spectra of mono-, bi-, and trilayer molybdenum disulfide samples with sulfur vacancies, measured at high temperatures ranging from room temperature to 463 K. In contrast to existing reports, we observed a decrease in the FWHM of the A_{1g} phonon mode, along with an increase in the relative intensity of the A_{1g} mode to the E_{2g}^1 mode, as the temperature increased. This trend becomes less prominent as the layer number increases from monolayer, disappearing entirely in few-layer samples. Additionally, we observed an intensity enhancement in the photoluminescence spectra of MoS2 samples at high temperatures (up to 550 K), which depends on the layer number. These observations are explained by considering the presence of sulfur vacancies, their interaction with the environment, electron density reduction, and a phonon-mediated intervalley charge transfer at elevated temperatures. Our results unambiguously establish that the effect of defects (sulfur vacancies) is more prominently reflected in the temperature dependence of FWHM and the relative intensity of the Raman modes rather than in the Raman peak positions.

Crystal structure and temperature sensing performance of the novel Sr2Y8(1-x-y)(SiO4)6O2:xCe3+,yTb3+ thermometric phosphors

For the research of the thermometric performance, a series of Sr2Y8(1-x-y) (SiO4)6O2:xCe3+,yTb3+ phosphors were synthesized with the solid-state reaction method at N2/H2 atmosphere. The crystal structure, room and high temperature photoluminescence spectra, luminescence decay, CIE variation, energy transfer and thermometric performance have been investigated. With the ultraviolet light excitation, both the Ce3+ broadband spectrum and Tb3+ line spectra have been observed. With the increase of the Tb3+ concentration, the emission color chromaticity varies from dark blue to bright green for the series of Sr2Y8(1–0.07-y)(SiO4)6O2:0.07Ce3+,yTb3+ phosphors. Under high temperature, the Ce3+ emission intensity quenches faster than that of Tb3+. Based on the different temperature quenching speeds of Ce3+ and Tb3+, the fluorescence intensity ratio technique was used to evaluate the thermometric performance of the series of Sr2Y8(1–0.07-y) (SiO4)6O2:0.07Ce3+,yTb3+ phosphors. During the temperature range of 298–498 K, the maximum relative sensitivity values of the phosphors of yTb = 0.05, 0.08, 0.1, 0.15, 0.2, 0.25 were calculated as 0.65, 0.69, 0.74, 0.71, 0.66, 0.6% K−1, which are much better than the recommended value of 0.30% K−1 for the practical application. The physical mechanisms for the Ce3+/Tb3+ excitation and emission, energy transfer and temperature quenching have been discussed. Sr2Y8(1-x-y) (SiO4)6O2:xCe3+,yTb3+ phosphors are the candidate thermometric probes for the non-contact temperature sensing applications.

Thermal stability and high-temperature photoluminescence of chemical vapor deposited MoS2 in different atmosphere

Understanding the high-temperature behaviors of two-dimensional (2D) transition metal dichalcogenides under different environments will provide a directive guide for the fabrication and application of their nanoelectronic devices. In this study, we investigate the high-temperature properties of chemical vapor deposition grown molybdenum disulfide (MoS2) in different atmospheres through the ex situ and in situ morphological and spectroscopical characterizations. When the MoS2 is exposed to Ar, dry O2, and wet O2 at the temperature of 500, 350, and 300 °C, the etching process starts at the edge and inner regions simultaneously, and the etched trenches are opposite to the outline of the MoS2 triangular shape. We observe a clear redshift of MoS2 in the Raman peak position and the photoluminescence peak position by in situ spectroscopical characterizations as the temperature increases. Moreover, a strong photoluminescence enhancement of 24-fold of MoS2 is observed when exposing to O2 at 300 °C. This should be due to the chemical and physical adsorbed oxygen on the MoS2 surface. Moreover, we find that physical adsorption can be eliminated after vacuum annealing. This study provides a real-time strategy to study the morphology and property evolution of 2D materials at high temperature in different atmospheres. These results will contribute to the applications in future electronic and optoelectronic functional devices of 2D materials.

Photoluminescence Spectroscopy of the InAsSb-Based p-i-n Heterostructure.

Photoluminescence in a double heterostructure based on a ternary InAsSb solid solution was observed in the mid-infrared range of 2.5–4 μm. A range of compositions of the InAs1−ySby ternary solid solution has been established, where the energy resonance between the band gap and the splitting-off band in the valence band of the semiconductor can be achieved. Due to the impact of nonradiative Auger recombination processes, different temperature dependence of photoluminescence intensity was found for the barrier layer and the narrow-gap active region, respectively. It was shown that efficient high-temperature photoluminescence can be achieved by suppressing the nonradiative Auger recombination (CHHS) process. Increased temperature, for which the energy gap is lower than the split-off band energy, leads to violation of the resonance condition in narrow gap antimonide compounds, which explains the observed phenomenon. This finding might influence future application of the investigated material systems in mid-infrared emitters used for, e.g., optical gas sensing.

Synthesis and Comparison of Eu3+ Doped M-Gd2(MoO4)3 and O-Gd2(MoO4)3 Phosphors

The Eu3+ doped monoclinic phase of Gd2(MoO4)3 (M-Gd2(MoO4)3) and the Eu3+ doped orthorhombic phase of Gd2(MoO4)3 (O-Gd2(MoO4)3) red phosphors were synthesized by adjusting the synthesis temperatures. The XRD (X-ray diffraction) patterns, the EDS (energy-dispersive spectra), and the FE-SEM (field emission scanning electron microscope) images were used to comparatively study crystalline structures, compositions, and morphologies of the two materials. The DRS (diffuse reflection spectra), the room and high temperature PL (photoluminescence) spectra, and the fluorescence decay curves were recorded to compare the PL properties of the two phosphors. The characterization results suggest that the M-Gd2(MoO4)3 phosphor has lower synthesis temperature, while the O-Gd2(MoO4)3 phosphor has bigger particles, larger energy band gap value, stronger PL intensities, longer lifetimes, and better thermal stabilities. For Eu3+, the research results prove that the O-Gd2(MoO4)3 is a better host material choice than that of the M-Gd2(MoO4)3.

High-Temperature-Induced Intervalley Carrier Transfer in Two-Dimensional Semiconductors: WSe2 versus WS2

Thin layer transition metal dichalcogenides (TMDs) have shown great potential in the field of electronics and optoelectronics devices due to their unique electronic and optical properties. Multilayer WSe2 is an indirect bandgap semiconductor and generally optically inactive. To improve the optical properties of multilayer TMDs, heating as a simple and effective method was widely chosen. Herein, we analyze high-temperature photoluminescence (PL) enhancement results on excitons in three-layer and four-layer WS2 and WSe2 with a theoretical analysis of their spectral behavior. Both direct and indirect exciton emissions in WSe2 are relatively enhanced, which is different from the high-temperature exciton emission behavior of WS2 in the same layer. The PL enhancement in WSe2 could be attributed to the transfer of thermally activated electrons from the Λ valley to K valley involving K → K and K → Γ transition at high temperature, where the conduction band extreme is located at the Λ valley. We quantitatively describe this enhancement phenomenon and demonstrate that the observed spectral behavior reflects the competition between intervalley carrier transfer and high-temperature PL quenching. An excellent agreement between calculated and measured intensities is obtained. This research provides a deeper understanding of the thermally induced intervalley carrier transfer model in multilayer TMDs.

Synthesis, luminescent properties, and thermal stabilities of Gd2(1-x)Eu2x(MoyW1-yO4)3 (0≤x≤0.2, 0≤y≤1) solid solution phosphors

Through a solid-state reaction method, Gd2(1-x)Eu2x(MoyW1-yO4)3 (0 ≤ x ≤ 0.2, 0 ≤ y ≤ 1) solid solution phosphors were synthesized. The crystalline structures, the morphologies, and the compositions of the solid solution phosphors were characterized by X-ray diffraction (XRD) patterns, field emission scanning electron microscope (FE-SEM) images, energy-dispersive spectra (EDS), respectively. The diffuse reflection spectra (DRS), room temperature photoluminescence (PL) excitation and emission spectra, luminescence decay curves, and high temperature PL spectra were recorded to characterize the luminescent properties, the luminescent dynamics, and the thermal behaviors of the phosphors. The synthetic temperatures were explored to determine the optimal synthetic temperature. The Eu3+ concentration quenching effects were observed in the orthorhombic phase of Gd2(1-x)Eu2x(MoO4)3 phosphors and the monoclinic phase of Gd2(1-x)Eu2x(WO4)3 phosphors. By introducing Mo6+ to Gd2(1-x)Eu2x(MoyW1-yO4)3 phosphors, crystalline phase variations and red shift excitation bands were recorded. Enhancing Eu3+ excitation and emission intensities were recorded in the Gd2(1-x)Eu2x(MoyW1-yO4)3 solid solution phosphors with the concentration of W6+ increasing. The obtained Gd2(1-x)Eu2x(MoyW1-yO4)3 phosphors can be used in phosphor converted based ultraviolet (UV) LEDs.

Selective area growth of cubic gallium nitride on silicon (001) and 3C-silicon carbide (001)

Selective area growth of cubic gallium nitride is investigated in a plasma assisted molecular beam epitaxy setup. 380 μm thick silicon (001) and 10 μm thick 3C-silicon carbide (001), grown on 500 μm silicon (001), were used as substrates and structured with silicon dioxide masks. Selective area growth on silicon and 3C-silicon carbide was tested for both thermal and plasma deposited oxides. Multiple growth series showed that gallium nitride coverage of silicon dioxide vanished at growth temperatures of 870 °C for silicon substrates and at a surface temperature of 930 °C for 3C-silicon carbide substrates. Whereas gallium nitride is grown in its hexagonal form on silicon substrates, phase pure cubic gallium nitride could selectively be grown on the 3C-silicon carbide template. The cubic phase is verified by high resolution x-ray diffraction and low temperature photoluminescence measurements. The photoluminescence measurements prove that gallium nitride condensed selectively on the 3C-silicon carbide surfaces uncovered by silicon dioxide.

Stabilization of Chemical-Vapor-Deposition-Grown WS2 Monolayers at Elevated Temperature with Hexagonal Boron Nitride Encapsulation.

Chemical vapor deposition (CVD)-grown flakes of high-quality monolayers of WS2 can be stabilized at elevated temperatures by encapsulation with several layer hexagonal boron nitride (h-BN), but to different degrees in the presence of ambient air, flowing N2, and flowing forming gas (95% N2, 5% H2). The best passivation of WS2 at elevated temperature occurs for h-BN-covered samples with flowing N2 (after heating to 873 K), as judged by optical microscopy and photoluminescence (PL) intensity after a heating/cooling cycle. Stability is worse for uncovered samples, but best with flowing forming gas. PL from trions, in addition to that from excitons, is seen for covered WS2 only for forming gas, during cooling below ∼323 K; the trion has an estimated binding energy of ∼28 meV. It might occur because of doping level changes caused by charge defect generation by H2 molecules diffusing between the h-BN and the SiO2/Si substrate. The decomposition of uncovered WS2 flakes in air suggests a dissociation and chemisorption energy barrier of O2 on the WS2 surface of ∼1.6 eV. Fitting the high-temperature PL intensities in air gives a binding energy of a free exciton of ∼229 meV.

Energy transfer from Tb3+ to Eu3+ in ZnLaB5O10: A candidate for near ultraviolet LED pumped phosphor

ZnLaB5O10 is a good photoluminescent host for a high concentration doping of rare earth (RE) activators and the f-f transitions in near ultraviolet range could be enhanced since RE3+ locates at an irregular cavity, in which the parity-forbidden law is partially suppressed. Two solid solutions of ZnLa1-xTbxB5O10 (0 ≤ x ≤ 1) and ZnLa0.8-yEuyTb0.2B5O10 (0.01 ≤ y ≤ 0.1) were prepared by high temperature solid state reactions and the phase purity was confirmed by powder X-ray diffraction. Under the excitation at 364 nm (the strongest f-f absorption), ZnLa1-xTbxB5O10 exhibits a monotonic increase of the bright green emission without any concentration quenching. Tb3+ to Eu3+ energy transfer in ZnLa0.8-yEuyTb0.2B5O10 was confirmed by the variation of the compositional dependent emission intensity as well as the fluorescence decay curves of Tb3+. Color-tunable emissions from green to yellow and to orange were realized in these phosphors. High thermal stability and low thermal quenching is proved by the in-situ high temperature photoluminescence spectra of ZnLa0.4Tb0.6B5O10 and ZnLa0.75Eu0.05Tb0.2B5O10.

Achieving white-light/tunable emissions via the controllable energy transfer in the single Ba9La2Si6O24:Eu3+,Bi3+ phosphor for UV converted white LEDs

Nowadays, ultraviolet (UV) and blue converted phosphors have played a key role in the solid-state lighting field, and the white light is usually produced by mixing multiple phosphors. In this work, we report a type of UV converted single white phosphor, i.e., Ba9La2Si6O24:Eu3+,Bi3+.Various techniques, such as powder X-ray diffraction (XRD), scanning electron microscopic (SEM), photoluminescence (PL) spectra, and quantum efficiency (QE), have been used to characterize the samples. In Bi3+ doped samples, upon excitation within the wavelength range of 250–380 nm can lead to three Bi3+ emission bands (i.e., 358 nm, 415 nm, and 516 nm), but only one emission band peaking at 415 nm appears when the excitation wavelength exceeds 380 nm. Structural analysis and discussion on the PL intensity variation reveal there are three Bi3+ luminescent centers at two different La3+ sites and one Ba2+ site. The Bi3+ concentration-induced luminescent quenching is dominated by the d-d interaction. In view of the spectral overlap between the Bi3+ emissions and the Eu3+ excitation spectra, the energy transfer from Bi3+ to Eu3+ of the Ba9La2Si6O24:Eu3+,Bi3+ is studied, and it is dominated by a dipole-quadrupole (d-q) interaction. Moreover, upon excitation at 365 nm, the emissions broadly tuning from blue, white and to red are achieved by varying the Eu3+ content. For the white Ba9La2Si6O24:Eu3+,Bi3+ sample, high temperature PL results show that it does not have severe degradation of emission intensity after increasing the temperature from 25 to 300 °C, exhibiting a good thermal stability. To show the potential application in the lighting fields, we fabricate a white light LEDs device by depositing this white phosphor on a 365 nm UV LED chip. As a result, the values, i.e., a color rendering index (CRI) of 95, luminous efficiency of 80 lm/W, and color temperature (CT) of ~4215 K at chromaticity (0.382, 0.398), are achieved.

Eu3+-based efficient red phosphors Y1-xEuxGa3(BO3)4 (0

The current white light emitting diode (WLED) employs an InGaN chip together with the yellow phosphor Y3Al5O12:Ce3+, however it has a weakness of insufficient red component. In addition to traditional Eu2+ or Mn4+ based red phosphors, we propose that Eu3+ in a noncentrosymmetric coordination could be an alternative for pure red emission and most importantly, it could be efficiently excited by f-f transitions in the near ultraviolet (NUV) region. Complete solid solutions of Y1-xEuxGa3(BO3)4 were prepared by typical solid state reactions, and the crystallographic refinements proved the successful cationic substitutions, where Eu3+ locates at the Y3+ site exclusively. Eu3+ is coordinated by oxygen atoms in a triangular prism symmetry, thus emits a strong and pure red light with the CIE coordinate (0.667, 0.333) upon 393 nm excitation. The internal quantum yields for the samples with x = 0.1, 0.5 and 1 are as high as 81%, 80% and 75%, respectively. In-situ high temperature photoluminescence study confirmed that Y0.1Eu0.9Ga3(BO3)4 retained 84% of the emission intensity at 423 K compared to the value at room temperature, which is sufficiently good for NUV LED applications.

Eu3+-doped ZnLaB5O10: A suitable candidate for near ultraviolet LED pumped red phosphor

It is important to develop red phosphors which can be efficiently excited by near ultraviolet (NUV) irradiation, matching the wavelength of commercial LED chips. ZnLaB5O10 possesses a significantly distorted coordination of La3+ and suitable for the Eu3+ incorporation, whose parity-forbidden law could be suppressed in such a coordination and be efficiently excited by f-f transitions in the NUV range. ZnLa1-xEuxB5O10 (0 ≤ x ≤ 1.0) were prepared by high temperature solid state reactions. Excitation and emission spectra prove that the concentration saturation occurs at a high Eu3+ concentration (∼60 atm%). All the phosphors emit the bright red light with a high red-to-orange ratio (2.7–3.1) and CIE coordinate (0.64, 0.36). In-situ high temperature photoluminescence spectra of ZnLa0.4Eu0.6B5O10 prove its high thermal stability, i.e. the emission intensity at 150 °C remains 90% of the value at room temperature, thus being a suitable candidate for NUV-LED pumped red phosphor.

Enhancement of direct and indirect exciton emissions in few-layer WSe2 at high temperatures

We present a temperature-dependent and time-resolved photo-luminescence (PL) study for few-layer tungsten diselenide (WSe2) that is exfoliated from the bulk crystals. The PL intensities of direct and indirect exciton emissions in monolayer and bilayer WSe2 decrease while temperature increases. However, abnormal enhancement of PL emission from direct and indirect excitons in ≥3-layer WSe2 is observed at high temperatures, ranging from 300 K to 400 K. A crossover of Λ → Γ and Κ → Γ indirect transitions occurs in few-layer WSe2 at elevated temperature and this phenomenon is verified by comparing the temperature-dependent shift of indirect exciton PL peak and ab initio band structure calculations. The enhancement of high-temperature PL from direct and indirect excitons can be attributed to the rapid intervalley transfer of thermally activated carriers from Λ point to K point and the crossover of Λ → Γ and Κ → Γ indirect transitions. The distinct behavior of temperature dependence of the PL intensities at a high temperature between bilayer and trilayer WSe2 reflects their band structure discrepancy. This work provides a thorough understanding of the origin of the observed indirect optical transition in few-layer WSe2 and suggests a possible means of improving the luminescence efficiency of multilayer WSe2.

Highly Photoluminescent Sulfide Clusters Confined in Zeolites

The design, synthesis and photoluminescence characterization of sulfide cluster-based zeolitic materials with the general formula Na8[SiAlO4]6SxCl2–2x is presented in this report. The employed experimental conditions, which included the thermal treatment of sodium Linde type A (Na-LTA) zeolites in the presence of Na2SO4 and NaCl, yielded highly photoluminescent sulfur-zeolites with remarkably large Stokes shifts. An emission band at 650 nm for samples with low sulfur contents is observed when excited at 380 nm, which is fully consistent with S2– clusters as light-emitting centers. These orange emitting materials display external quantum efficiencies (EQEs) reaching 53% with peculiar high temperature photoluminescence stability up to 450 °C. For higher S/Cl molar ratios, a gradual conversion to near-infrared (NIR) light-emitting centers is observed with a maximum at 780 nm. This novel NIR luminescent species with EQEs of about 10% are attributed to polysulfide clusters (tentatively S42– species). Given the...

High-Temperature Photoluminescence in Colloidal “Quasi” 2D Materials

The recent development of quasi-two-dimensional (2D) semiconducting nanomaterials [nanoplatelets (NPLs)] prepared through solution-based chemistry has attracted significant attention in the field of color conversion materials. Herein, we demonstrate the efficient high-temperature photoluminescence of the as-prepared cadmium telluride NPLs from 300 to 500 K with visually observable emission at temperatures as high as 500 K. Indeed, the thermal-emission quenching of the as-prepared NPLs in solution was found to be not only reversible following thermal cycling up to 475 K, but also comparable to more chemically complex core/shell nanostructures. To the best of our knowledge, this is the first example of the as-synthesized colloidal nanoparticles capable of high-temperature photoluminescence. This discovery highlights previously unexplored emission properties of NPLs that have the potential to be utilized for the efficient design of high-quality light emitters.

Analysis of the Efficient High-Temperature in Situ Photoluminescence from GaN Layers during Epitaxial Growth

Photoluminescence (PL) in GaN or InGaN layers monitored during epitaxial growth at high temperatures permits a quasi-continuous in situ characterization of opto-electronic properties. Therefore, epitaxial parameters can now be optimized at the earliest possible stage. A pulsed and high-power UV laser was required for PL excitation at high temperatures. Herein, the underlying nonlinear mechanism was studied via time-resolved PL experiments and rate equation-based modeling. A temperature-activated and saturable path for quenching over defects was identified. Beyond the saturation threshold, reasonably-intensive PL sets in. At high temperatures not only is the near band gap-PL present, but also—as a new observation—a defect-assisted PL emerges. Apart from these specific electronic transitions in high-temperature PL of GaN, a simple, but reasonably predictive model of the luminescent thin film has been set up to track down interference fringes in the PL spectra. It is worth mentioning that the spectral PL modulation (aiming at the Purcell effect) is often mixed up with ordinary Fabry–Pérot interference. A distinction has become key to properly analyze the spectral signatures of high-temperature PL in order to provide a reliable in situ characterization of GaN layers during epitaxial growth.

Multifunctional role of dysprosium in HfO2: stabilization of the high temperature cubic phase, and magnetic and photoluminescence properties.

Hafnium oxide (HfO2) can exist in different crystalline structures such as monoclinic at room temperature, tetragonal at 1700 °C and cubic at 2600 °C. In the present study, nanocrystalline powders of HfO2 synthesized by a Pechini type sol-gel technique show a monoclinic phase, P21/c, at room temperature. By incorporating Dy into the HfO2 lattice, the intensity of all diffraction peaks corresponding to P21/c decreases and at a concentration of 11 at% of Dy, the monoclinic phase transforms completely to the cubic phase, Fm3[combining macron]m, showing a mixed phase of monoclinic and cubic at intermediate concentrations (5-9 at%) of Dy. For the first time, we have stabilized the high temperature cubic phase of HfO2 at room temperature by incorporating Dy. Selected area electron diffraction patterns confirm the monoclinic and the cubic phase as observed from the X-ray diffraction patterns. A mechanism for stabilization of the high temperature cubic phase in Hf1-xDyxO2 has been analyzed based on the substitution of dysprosium for hafnium ions and the formation of oxygen vacancies. While ferromagnetic ordering at room temperature observed in HfO2 nanoparticles is quenched after incorporating 1 at% of Dy, photoluminescence (PL) studies demonstrate excellent emissions in the blue and yellow region after exciting with UV light of wavelength 352 nm. Combining excitation and emission profiles, we have proposed a tentative energy band diagram illustrating the energetic processes taking place in Hf1-xDyxO2.