Heat treatment effects on the structural and optical properties of thin Bi2(Se1-xTex)3 films

Thin layers of Bi₂-chalcogenides, in the form of Bi₂(Se_1-xTe_x)₃ films, were evaporated on glass substrates by means of the vacuum thermal evaporation. Microstructure of the as prepared layers was investigated by x-ray diffraction (XRD) analysis. Identifications of the surface morphology and roughness were determined via scanning electron microscope (SEM). Optical transmissivity spectra proved that the as prepared films have low transparency with growing trend upon increasing the wavelength beyond the infra-red region. Low transmittance was observed for the as prepared films. Heat treatment, in the form of temperature annealing, was carried out aiming at boosting the structural features and the materials transmissivity. Structural properties and surface features of the annealed films were probed also via XRD and SEM analyses. It was found that the crystal size increases while the micro-strain and the dislocation density decrease obviously due to annealing. It was also observed that the annealing process significantly enhances the materials transmission especially in the range of higher wavelengths. Optical band gap was studied after annealing at various temperatures. Notable change in the band gap value was observed as a result of annealing. The band gap of the undoped (Bi₂Se₃) materials showed significant rise from 0.14 to 1.79 eV due to annealing. Similarly, the Te-doped samples exhibited notable increase in their band gap values after annealing. For example, the optical band gap of the sample doped at x = 0.20 increased from 0.03 to 0.41 eV by annealing. On the other hand, transmittance was also enhanced by annealing. For samples treated at 250 °C for 3 h, their optical transmissivity is enhanced to over 99% at the visible near-IR range. Such significant enhancement can be ascribed to structural enhancements. With such enhancement in the optical transmissivity, optoelectronic applications including transparent electrode can be met.     

Selective formation of crystal polymorphs in Er3+-doped Bi2O3 thin films and their photoluminescence properties

The process conditions for selectively forming crystal polymorphs in Er³⁺-doped Bi₂O₃ films deposited on Si, SiO₂, and C-plane sapphire substrates were systematically investigated. Bi₂O₃:Er films were deposited at either room temperature or 300 °C and subsequently post annealed to promote crystallization. The critical factor controlling the crystal polymorphs was Er content. When the Er content was less than 1.5 at.%, only α-Bi₂O₃ phase nucleated upon post annealing. Deposition at 300 °C somewhat lowered the oxidization state, under which β-Bi₂O₃ structure appeared at lower temperatures and α-Bi₂O₃ structure appeared at higher temperatures. When the films were doped with over 2 at.% Er³⁺ ions, the excess Er₂O₃ stabilized the δ-Bi₂O₃ structure as the lowest temperature phase. The universal phase transition scheme with increasing annealing temperature was δ-Bi₂O₃ → β-Bi₂O₃ → α-Bi₂O₃. The δ → β transition proceeded through splitting each diffraction peak of δ-Bi₂O₃ into two components of β-Bi₂O₃, indicating a correlation between the structures of β-Bi₂O₃ and δ-Bi₂O₃. The γ-Bi₂O₃ phase appeared only in films on Si(100) substrates and after vacuum annealing, suggesting the formation of sillenite (Bi₁₂SiO₂₀). Deposition on C-plane sapphire by using H₂O as the oxygen source gas produced a highly (111)-oriented δ-Bi₂O₃ structure, whereas deposition with O₂ yielded a randomly oriented δ-Bi₂O₃ structure. At Er content exceeding 4 at.%, δ-Bi₂O₃ was the primary phase in the films on SiO₂. The photoluminescence (PL) activity of dopant Er³⁺ under excitation at a wavelength of 532 nm strongly depended on the crystal polymorphs. α-Bi₂O₃:Er exhibited intense and stable PL spectra consisting of eight Stark splitting lines. PL from γ-Bi₂O₃:Er exhibited much weaker two emission lines. δ-Bi₂O₃:Er and β-Bi₂O₃:Er films were not emission-active at all. However, δ-Bi₂O₃:Er film on SiO₂ with an Er content of 4 at.% exhibited an intense and broad emission at 1530 and 1560 nm.     

Effect of substrate temperature on the physical properties of co-evaporated Sn2S3 thin films

We report the deposition of tin sulfide (Sn₂S₃) thin films by co-evaporation technique at different substrate temperatures. The influence of substrate temperature on the structural and optical properties of the thin films is investigated. X- ray diffraction (XRD) analysis and Micro-Raman studies confirm the formation of Sn₂S₃ phase. Scanning electron microscopy (SEM) and atomic force microscopy (AFM) are used to examine the surface morphology. The transmission spectra of the deposited Sn₂S₃ thin films have been recorded in the wavelength range of 200–3000 nm using UV–vis-NIR spectrometer. Film thickness (d) and optical constants such as refractive index (n), extinction coefficient (k), real (ε₁) and imaginary (ε₂) parts of the dielectric constants of thin films are estimated from the optical transmittance. The optical band gaps of the deposited films at different substrate temperatures are in the range of 1.46–1.64 eV. Hall effect measurements confirm the n-type nature of the as-prepared Sn₂S₃ thin films.     

Elaboration and characterization of PVP-assisted NiO thin films for enhanced sensitivity toward H2 and NO2 gases

It is widely demonstrated that the synthesis conditions of sol-gel films have a great impact on their gas sensing properties. In this work, transparent PVP-assisted nickel oxide thin films with an average grain size of ~5?nm were synthesized using two distinctive deposition procedures combining the sol-gel method with the spin-coating technique then tested as optical gas sensors for the detection of hazardous pollutant gases. The first method is ascribed to a typical spin-coating deposition followed by a thermal annealing, and the second method consisted on a multistep coating annealing process. Structural and morphological studies showed enhanced crystallization rate and homogeneous surface morphology using a multistep deposition. The as prepared films exhibit a clear and reversible response toward H₂, CO and NO₂ gases and the multistep deposition process enhanced the sensitivity of about 113% and 194% toward 1% of H₂ and 0.1% of NO₂ respectively. The shrinkage of the band gap from 4.07 to 3.91?eV and the increased PL intensity indicate the presence of higher rate of charge density and intrinsic defect states that promoted the sensitivity of the film. Furthermore, improved response intensity was detected in the near UV region and higher stability with fast response was obtained for hydrogen gas.     

Influence of Er doping on the structural,optical and luminescence properties of pulsed laser deposited Er: BaZrO3 thin films

Pure and erbium doped (1, 2, 3 and 5 at%) Barium zirconate (BZE) thin films have been deposited on Si (0 0 1) substrate via pulsed laser deposition using 100 mJ Nd: YAG laser operated at second harmonics (532 nm). Er doping significantly affects the surface morphology, microstructure and optical properties of grown thin films. All the films exhibit cubic BaZrO₃ structure and are polycrystalline in nature as extracted from XRD data. The optical band gap energies (3.75–3.63 eV) of doped (1, 2, 3, and 5 at%) BZE thin films are found to be less than that of pure BZO film (4.03 eV). PL spectra, excited at 328 nm, mainly consist of violet-blue (412 nm) and green (523–543 nm) emissions for all the doped films. The green emission increases with the increase in Er doping upto 3 at% and then concentration quenching effect appears at 5 at%. It is noted that the relative intensity of PL emission and the optical band gap can be tuned by varying Er concentration to alter the properties of the phosphor. The emission peaks in photoluminescence spectra makes the Er: BZO films potential candidates to be used in optoelectronic devices such as light emitting diodes (LEDs).     

Trap engineering in ZnGa2O4:Tb3+ through Bi3+ doping for multi-modal luminescence and advanced anti-counterfeiting strategy

Optical information encryption based on luminescence materials have received much attention recently. However, the single luminescence mode of the luminescence materials greatly limits its anti-counterfeiting application with high safety level. Here, a series of luminescence materials of Tb³⁺ and Bi³⁺ co-doped ZnGa₂O₄ phosphors with great correspondence in photoluminescence (PL), persistent luminescence (PersL), and thermoluminescence (TL) modes was synthesized by the conventional solid-phase method for the application in multi-modal anti-counterfeiting fields. Under the excitation of 254 nm, ZnGa_1.99O₄:0.01 Tb³⁺, yBi³⁺ (y = 0.001,0.002) sample exhibited a broad blue emission band (the transition from [GaO₆]) at 440 nm and the characteristic emission peaks of Tb³⁺ at 495 nm, 550 nm, 591 nm and 625 nm, corresponding to the transitions of ⁵D₄-⁷F_n (n = 6, 5, 4, 3), respectively. Interestingly, the co-doping of Bi³⁺ ions improve the crystallinity and particle size of the phosphor, subsequently enhanced the PL intensity of Tb³⁺ to 6 times that of Tb³⁺ singly doped ZnGa₂O₄ phosphor. Further, the flexible films with multi-modal luminescence properties have been fabricated through the unique TL and PersL characteristics of ZnGa₂O₄: Tb³⁺, Bi³⁺ phosphors, including “Optical information storage film”, “snowflake and characters” and “QR code”. Moreover, a set of optical information encryption is obtained by combining ZnGa₂O₄:Tb³⁺, Bi³⁺ phosphor and red emitting phosphor. The results indicate that ZnGa₂O₄:Tb³⁺, Bi³⁺ phosphor with multi-modal stimulus response can be expected to be potentially used in the applications of optical information storage and anti-counterfeiting fields.     

Unveiling the mechanisms of defects induced upconversion luminescence enhancement in Er3+-sensitized 2D Bi3O4Br nanosheets

Lanthanide (Ln³⁺) ions doped upconversion (UC) nanosheets have attracted tremendous attention such as displays, sensing, bioimaging and lasers etc, which was benefitting from the intriguing optical characters of Ln³⁺. However, the field of UC nanosheets has been hindered by low UC conversion efficiencies associate with nonradiative relation (NR) occurring by defect, the existence and influence of defects still cannot be eliminated completely. In this work, we design introduce the impurity energy level by doping Er³⁺in Bi₃O₄Br:Er³⁺ nanocrystal materials, which was closed with the intermediate band (IB) formed by oxygen vacancies defects. The density functional theory calculations confirm the IB energy level was closed with the intermediate excited states of Er³⁺, which provided the potential to tailored the ground state carriers transition from matrix semiconductor to Er³⁺ and thus tool to counteract the effect of NR and even enhance the UC luminescence performance. The photo-current results evidenced that the photocarrier success transition from IB to Er³⁺ intermediate excited states energy level leads to a sharp decrease in the surface carrier, on the contrary, the electron population on the excited state energy level of Er³⁺ have increased. As a result, compared with unmodified sample the UC emission intensity under excited by 980 nm of green and red is enhanced by 7 and 4 times respectively. This work paves the way to design efficient UC nanosheets through by energy transfer (ET) combine matrix semiconductor with RE and greatly enriches the understanding about the ET behavior of RE.     

Effects of In2O3 nanoparticles doping on the photoluminescent properties of Eu2+/Eu3+ ions in silica glasses

Eu²⁺/Eu³⁺ ions doped silica glasses contained In₂O₃ nanoparticles (NPs) have been fabricated by using nanoporous silica glasses. Interestingly, efficient energy transfer from In₂O₃ NPs to Eu²⁺/Eu³⁺ ions enhanced the photoluminescence (PL) emission of Eu²⁺/Eu³⁺ ions, which derives from lattice defects in In₂O₃ NPs. Our work has not only demonstrated a facile way to fabricate NPs and rare earth ions co-doped silica glasses, but also extended the applications of semiconductor oxide NPs such as In₂O₃ NPs.     

Mn-doped titania thin films prepared by spin coating

TiO₂ thin films doped with ≤7 mol% Mn (metal basis) were deposited on F-doped SnO₂-coated (FTO) glass substrates by spin coating. The structural, morphological, and optical properties of the films were investigated by glancing angle X-ray diffraction (GAXRD), laser Raman microspectroscopy, field emission scanning electron microscopy (FESEM), and ultraviolet–visible spectroscopy (UV–VIS). Mn doping of TiO₂ (anatase) extended the optical absorption edge to longer wavelengths (lower photon energies) significantly lowering the band gap from 3.32 eV (undoped) to 2.90 (7 mol% Mn). The absorption edges of all films were sharp and the transparencies in the visible region were in the range 60–75%. All of the films were homogeneous, fully dense, and essentially crack-free.     

Structural phase transition,electrical and photoluminescent properties of Pr3+-doped (1-x)Na0.5Bi0.5TiO3-xSrTiO3 lead-free ferroelectric thin films

Lead-free ferroelectric Pr³⁺-doped (1-x)Na_0.5Bi_0.5TiO₃-xSrTiO₃ (x?=?0–0.5) (hereafter abbreviated as Pr-NBT-xSTO) thin films were prepared on Pt/Ti/SiO₂/Si and fused silica substrates by a chemical solution deposition method combined with a rapid thermal annealing process at 700?°C, and their structural phase transition, dielectric, ferroelectric, and photoluminescent properties were investigated as a function of STO content. Raman analysis shows that with increasing STO content, the phase structures evolve from rhombohedral phase to coexistence of rhombohedral and tetragonal phases (i.e. morphotropic phase boundary), and then to tetragonal phase. The structural phase transition behavior has been well confirmed by temperature- and frequency- dependent dielectric measurements. Meanwhile, the variation in photoluminescence intensity of Pr³⁺ ions with different STO content in the NBT-xSTO thin films also indicates that there exists a clear structural phase transition when the film composition is close to the morphotropic phase boundary. Superior dielectric and ferroelectric properties are obtained in the Pr-NBT-0.24STO thin films due to the formation of morphotropic phase boundary. Our study suggests that Pr-NBT-xSTO thin films be promising multifunctional materials for optoelectronic device applications.     

Near-infrared photo-responsive Er3+-K+ co-doped Y2O3 phosphors with enhanced UC luminescence

Y₂O₃: x% Er³⁺ (x=5, 7, 10, 12, 15) and Y₂O₃: 10% Er³⁺,x% K⁺ (x=0, 1, 3, 5, 7, 10, 15) phosphors were successfully prepared by a low-temperature combustion method. The structure as well as the absorption/emission spectra of phosphors were investigated. The effect of doping concentration of K⁺ ions on the upconversion (UC) luminescence of Y₂O₃: 10% Er³⁺ phosphor was examined and the possible optical transitions were discussed. The results showed that K⁺ ion doping not only changed the microstructure and crystallinity of the phosphors, but also enhanced its UC luminescence intensity. The Y₂O₃: 10% Er³⁺, 7% K⁺ phosphor exhibit the strongest UC emission intensity. Compared with the Y₂O₃: 10% Er³⁺ phosphor, the UC luminescence intensity at 563 nm and 661 nm was enhanced by 67.8 and 27.3 times for the K-codoped samples, respectively. The phosphor with the optimal doping concentration was mixed with a polymer to form a composite film, which was employed for the fabrication of near-infrared (NIR) photo-responsive detection devices. The device exhibited strong photo-current response to NIR light at 980 nm, implying that our work could inspire new design strategy for the development of NIR photo-detection devices.     

Effects of Ta doping and irradiation with He+ ions on photoluminescence of MgAl2O4 spinel ceramics

Photoluminescence (PL) properties of pristine and Ta-doped MgAl₂O₄ spinel ceramics prepared via spark plasma sintering technique and irradiated with He⁺ ions were studied. The results indicate strong influence of the grain boundaries on PL spectra. Ta doping promotes the formation of O and Al vacancies at the grain boundaries leading to an increased number density of F⁺ centres. The ionised irradiation forms antisites preferentially at the grain boundaries, which inhibit excitonic PL and exciton energy transfer while do not affect proper PL of lattice defects. A weak PL excitation band at 7.25 ± 0.25 eV may belong to excitons localised near bulk antisites. In the Ta-doped ceramics, the electronic transitions between 5.75 eV and 7.0 eV belong to an intermediate state situated at the grain boundaries and structurally linked to Ta, which readily transfers energy to F and F⁺ centres; it was assigned to the nucleated Mg₄Ta₂O₉ phase.     

Environmental influence of high Na2O additions on microstructure and ligand field parameters of Cr3+ inside borosilicate glass

A glass system based on the Na₂O/B₂O₃-doped CrO₃ borosilicate has been prepared by the melt quenching technique. The structure, color, optical absorbance and ligand field parameters were investigated for a wide range of Na₂O additives (20–60 mol%). All X-ray photoelectron spectroscopy (XPS) profiles were used to study the chemical shift states of the glass-constituting elements. Fourier transform infrared (FTIR) analyses explored the internal structure and subnetwork units. Furthermore, from the FTIR results, we concluded the transformation of trigonal borate units (BO₃) to tetrahedral borate units (BO₄) and the possibility of transformation from B₃-O-Si linkages to B₄-O-Si linkages. Despite the fixed CrO₃ content, the doped glasses showed a color transition from green to yellow with additional Na₂O content. The increased intensity of the band at 451–427 nm and the decreased intensity of the band at 619–627 nm are the main reasons for this color transformation. The optical absorption spectra confirmed the existence of Cr³⁺ and Cr⁶⁺ states. A decreasing behavior for the crystal field splitting (10Dq) and an increasing behavior for Racah parameter (B) were obtained with further Na₂O additives. The decreasing behavior of 10Dq was attributed to reduced oxygen concentrations with more Na₂O/B₂O₃ substitutions. The increasing behavior of B reflects the tendency of the bond between the Cr cations and their oxygen ligands towards an ionic nature. Moreover, the Dq/B values indicated that Cr³⁺ cations are in high-field positions for the glass sample containing 20 mol% Na₂O, and Cr³⁺ cations are in intermediate field positions for the glass sample containing 30 mol% Na₂O. However, for the glass samples doped with 40, 50 and 60 mol% Na₂O glass samples, Cr³⁺-cations are in weak field positions. These results of (Dq/B) recommend the glass sample doped with 20 mol% Na₂O for tunable laser applications.     

Influence of Gd2O3 on the microstructure and properties of transparent MgAl2O4: Cr3+ ceramic

In this work, MgAl₂O₄: Cr³⁺ transparent ceramics have been synthesized by the hot press sintering techniques, and the effect of the sintering aid Gd₂O₃ and its content on the densification, microstructure, and optical, photoluminescence was studied and discussed. The relative density reached 99.29% with 0.8 wt% Gd₂O₃ as a sintering aid, and the optical transmittance at 686 nm and 1446 nm were approximately 76%. As Gd₂O₃ content continued to increase, the grain size of the ceramics became smaller and uniform, accompanied by some pores with the size of ～1 μm. The ceramics with 4.0 wt% Gd₂O₃ showed a higher transmittance, of 82% at 1446 nm. Additionally, Gd₂O₃ was helpful for Cr³⁺ in the sites of octahedral symmetry, which increased the quantum yield. The quantum yield of MgAl₂O₄: Cr³⁺ with 0.8 wt% Gd₂O₃ was about 0.175, which was 36% higher than that of ceramic without Gd₂O₃. In short, the sintering aid Gd₂O₃ not only contributed to improving the densification, homogenizing the grain size, and heightening the optical transmittance but also enhanced the quantum yield of Cr³⁺.     

Electro-mechano-optical properties of the Er3+ modified Bi0.5Na0.4K0.1TiO3 versatile ceramics

A series of Bi_0.5-xEr_xNa_0.4K_0.1TiO₃ (BNKT-xEr) ceramics were designed and fabricated, the dopant effects on dielectric, piezoelectric and photoluminescence properties were studied. The results show that the piezoelectric property of BNKT can be enhanced by a trace amount of Er dopant, which is also reflected in the large linear electrostrain (S_uni = 0.29%, under 55 kV/cm) achieved in BNKT-0.0025 Er. On the other hand, higher Er content can produce excellent dielectric temperature stability, with △?/?_{150 °C} < ±15% over temperature range of 90～510 °C. Meanwhile, all BNKT-xEr ceramics exhibit good photoluminescence properties, which may open new applications of these multifunctional ceramics.     

Li2B4O7-Bi2O3-WO3玻璃的光学性能研究

采用熔融淬冷方法制备了(100-x)Li2B4O7-x(Bi2O3·WO3)(5≤x≤20)玻璃.采用比重计测定了玻璃密度,分光光度计测量了玻璃的吸收光谱,V棱镜折射仪测量了玻璃折射率.结果表明,随着Bi2 O3·WO3含量的增加,玻璃样品的密度和摩尔体积增大,而氧堆积密度减小;玻璃的吸收光谱中截止波长逐渐向长波方向移动,玻璃的间接跃迁光学带隙、Urbach能和费米能逐渐减小,折射率增大.光学性能的变化和玻璃网络中部分桥氧转变为非桥氧有关.     

Up-conversion luminescence and temperature-sensing properties of Li+, K+ doped Na2Zn3Si2O8: Er3+ phosphors

In this work we detail the preparation of new luminescent Li⁺ and K⁺ doped Na₂Zn₃Si₂O₈: Er³⁺ up-conversion phosphors using the high-temperature solid-phase method. We investigate the phosphors phase structure, elemental distribution, up-conversion luminescence characteristics and temperature sensing properties. Our fabricated samples were found to be homogeneous and when excited using 980 nm light, they emitted wavelengths in the green and red visible wavelength bands, which correspond to two major emission bands of Er³⁺. Doping with Li⁺ and K⁺ increased the luminescence intensity of the Na₂Zn₃Si₂O₈: Er³⁺ phosphor at 661 nm by 36 and 21 times respectively. The highest relative temperature sensitivity (Sa) of the fabricated phosphor reached a value of 19.69% K^?1 and the highest absolute temperature sensitivity (Sr) reached 1.20% K^?1. These values are superior to other materials which utilize up-conversion by Er³⁺ ions as a tool for temperature sensing. We anticipate that these new phosphors will find significant application as components in optical temperature measurement systems.     

Highly efficient photocatalytic activity and mechanism of novel Er3+ and Tb3+ co-doped BiOBr/ g-C3N5 towards sulfamethoxazole degradation

Bismuth oxyhalides (BiOX (X = Cl, Br, I) are considered to be an important p-type semiconductors in the photocatalysis applications. In particular, tetragonal BiOBr is considered as a stable photocatalyst due to its resilient absorption in the visible region with an band gap energy of 2.8 eV. In the meantime, lanthanide ions (with 3+ oxidation state) implies as conversion catalyst gained huge impact and remain a serious topic in materials chemistry. Here we synthesized upconversion photocatalyst mainly consists of BiOBr with the Er ³⁺ and Tb ³⁺ ions along with low band gap g-C₃N₅ for the improved photocatalytic performances. The synthesized Er³⁺/Tb³⁺@BiOBr-g-C₃N₅ heterojunction was systematically characterized by XRD, and FT-IR for the confirmation of the composite and their morphology were analysed with FESEM and HR-TEM analysis which revealed that the sheets of g-C₃N₅ were decorated by Er³⁺/Tb³⁺ loaded BiOBr microspheres. The XPS analysis confirmed the suitable oxidation state of all the individual elements existing in the composite. As the UV-DRS analysis showed that the band gap of the Er³⁺/Tb³⁺ BiOBr-gC₃N₅ heterojunction was narrowed to 2.64 eV. To evaluate the photocatalytic efficiency of the synthesized g-C₃N₅, Er³⁺/Tb³⁺@BiOBr and Er³⁺/Tb³⁺@BiOBr-gC₃N₅ heterojunction under the simulated visible light irradiation source towards the aqueous sulfamethoxazole degradation. The Er³⁺/Tb³⁺@BiOBr-gC₃N₅ heterojunction shows maximum degradation efficiency of 94.2% after 60 min of visible light irradiation whereas the pure g-C₃N₅ provided about 43.8% and Er³⁺/Tb³⁺@BiOBr implies 55.2% degradation efficiency. The plausible degradation mechanism of pollutant removal was proposed.     

Order-disorder structural transition in Pr3+-doped Ba3Ga2O6 for rewritable and write-once-read-many optical data storage

The emerging challenges of the big data era, both in storage density and security, require the development of the next-generation optical data storage materials. Here, we report for the first time a photo-stimulated luminescence (PSL) material, Ba₃Ga₂O₆: Pr³⁺, for rewritable optical storage and write-once-read-many data preservation. Ba₃Ga₂O₆: Pr³⁺, with an isolated deep trap depth in the range 1.26–1.53eV, has been used for data encoding/decoding under the excitation of 254 nm UV light and by the simulation of an 808 nm NIR laser. Meanwhile, the phosphor allows for high-security write-once-read-many optical memory by taking advantage of the irreversible change of the photoluminescence (PL) color from blue to green (a binary blue ‘0’ and green ‘1’ code) irradiated by 365 nm UV light. The comprehensive investigations indicate that the irreversible PL switching is as a result of the order-disorder structural transition by thermal treatment. The new persistent luminescence material not only exhibits promising applications in sustainable rewritable data storage, but also paves the way for write-once-read-many optical information storage with a high level of security.