Deep Emission Band Research Articles

Exploring innovative applications of Sr2MgSi2O7:Fe3+ luminescent system in surface-triggered fingerprint divergence and cheiloscopy screening

In this study, we employ a cost-effective combustion method to synthesize (1–11 mol %) Fe3+ doped Sr2MgSi2O7 nanophosphors (SMSO NPs) and conduct a comprehensive investigation into their structural and luminescent properties. The impact of iron concentration on the structural and optical characteristics is thoroughly analysed, revealing a tetragonal crystal structure with space group P-421m through Rietveld refinement and X-ray Diffraction (XRD) patterns. Scanning electron microscopy showcases the random dispersion of agglomerated particles with non-uniform dimensionality. Energy Dispersive X-ray analysis (EDS) assesses the nominal composition of Sr, Mg, Si, Fe, and O, while X-ray Photon Spectroscopy (XPS) confirms the direct incorporation of Fe3+ ions into the SMSO lattice. Photoluminescence (PL) emission spectra exhibit a deep red emission band at 718 nm. The residual luminescence intensity of SMSO:7Fe3+ at 200 °C is 91 %. Under 365 nm UV light, the optimized SMSO:7Fe3+ NPs displays easily identifiable levels I-III fingerprints (FPs) features with good resolution and contrast. The resilient luminescence and nano-regime of the phosphor reveal well-defined type I–V groove patterns in latent lip prints (LLPs) on various non-porous surfaces, unaffected by background interference or colour distraction. This study highlights SMSO:7Fe3+ NPs as a promising option for individualizing and detecting latent fingerprints (LFPs) and lip prints (LPs).

Ionoluminescence and photoluminescence study of annealing and ion irradiation effects on zinc oxide

The effects of annealing at 473 K and 800 K and of 2 MeV H+ irradiation on the behaviour of defects in ZnO crystals were studied via ionoluminescence (IL) and photoluminescence (PL). The PL spectra of samples annealed in vacuum at 473 K and 800 K showed stronger visible emission and weaker UV emission than virgin samples. The stronger visible emission corresponded to an increase in the concentration of point defects in the samples. The PL spectra of the sample annealed in vacuum at 473 K revealed stronger visible emission than that of the sample annealed in vacuum at 800 K; hence, the concentration of point defects in the sample annealed at 473 K was higher. The IL spectrum of ZnO annealed in vacuum at 473 K showed a weaker intensity than the spectrum of the sample annealed at 800 K, which seemed inconsistent with the PL spectra results. This inconsistency was attributed to different mechanism of PL and IL. IL and PL in some way link the ZnO DBE to different point defect related electronic transfer processes. The PL spectra of samples showed that deep band emission (DBE) is a superposition of 2.1 eV band emission and 2.4 eV band emission and that near band emission (NBE) is a superposition of 3.1 eV band emission and 3.3 eV band emission. However, the DBE of IL is a superposition of 1.8 eV band emission and 2.1 eV band emission, and the NBE exhibited overall weak intensity caused by an increase in the number of defects induced during irradiation. Comparing the effects of annealing in an oxygen atmosphere and in vacuum, the enhanced peak area ratio of 2.1 eV band emission to 2.4 eV band emission in the PL spectra and the enhanced peak area ratio of 2.1 eV band emission to 1.8 eV band emission in the IL spectra of the sample annealed in an oxygen atmosphere demonstrate a correspondence between the ~1.8 eV band and Zn interstitials (Zni), between the ~2.1 eV band and O interstitials (Oi) and between the ~2.4 eV band and O vacancies (VO). The increase in the UV emission of the sample annealed in an oxygen atmosphere suggested that the NBE is a superposition of 3.1 eV band emission and 3.3 eV band emission, which are associated with Zn vacancies (VZn) and self-trapped excitons, respectively.

Ions beam induced luminescence study of variation of defects in zinc oxide during ion implant and after annealing

<sec>The optical and electrical properties of ZnO related on the type and the concentration of defects in ZnO crystal. Ion implantation and annealing can change the type and the concentration of defects in ZnO. To understand the variation of defects in ZnO during ion implantation and after different temperature annealing, in situ luminescence measurements of ZnO crystal samples were carried out by ion beam induced luminescence (IBIL) during ion implantation of 2 MeV H<sup>+</sup> and then after annealing at 473 K and 800 K in vacuum on the GIC4117 tandem accelerator in Beijing Normal University.</sec><sec>IBIL spectra of ZnO showtwo emission peaks: UV emission, which is called near band emission (NBE), and visible emission, which is called deep band emission (DBE).The high-intensity of DBE and weak NBE of IBIL spectra of ZnOmay be due to the NBE is intrinsic to ZnO samples and therefore is just visibly observed from samples that are virtually defect-free. With the ion implantation, the destruction of the crystal structure and the arising of a mass of defects, inducing the weak intensity NBE and intense DBE.In addition, the overall IBIL spectra of ZnOreveal decrease intensity with the ion fluence,which indicates that the concentration of luminescence centersdecreases duringion implantation.With the H<sup>+</sup> fluence, the concentration of the point defects increases. The point defects migrate and subsequently agglomerate into larger defect clusters. These defect clusters serve as traps for catching electrons and holes, which result in the quenching of luminescence centres. Annealing can help todecompose the defect clusters and repair the defects of crystal. However, amounts of defects and clusters still remain in the irradiated sample annealed at 473 K in vacuum, which acted as nonradiative center and suppress the luminescence induced weak intensity of IBIL. Annealing the sample at 800 K in vacuum may facilitate the decomposition of defect clusters during ion irradiation to point defects and the point defect return to the lattice position that can reduce the nonequilibrium defects inside the crystal and improve the crystallinity of the crystal, which increase the intensity of its IBIL.</sec>

Fabrication of ZnO nanostructures with tunable luminescence, electrical properties: effect of annealing reaction temperature, ligand and seed layer

In present manuscript, the optical and electrical properties of ZnO thin films under the effect of ligand, synthesis & annealing temperature, seed layer have been studied in detail. ZnO thin film grown at 80 °C reaction temperature tuned transmittance (at 550 nm) to ∼37% (post-annealed sample) as compared to pre-annealed samples’ ∼5%. Observed Refractive index (n) found inversely proportional to their transmittance & refractive index (n) for corresponding sample has been tuned from 1.75 to 1.26. The intensity ratio of near band emission (INBE) to deep band emission (IDBE) is drastically enhanced after annealing of ZnO thin films which is confirmed in photoluminesecnce study. The uniform & dense ZnO nanorods’ growth, optimum transparency & prominent UV range dominated emission bring resistivity down to 1012 ohm-cm. The synthesized ZnO thin films having best suited structural, optical and electrical properties which can be utilized in fabrications of low cost luminesent devices.

Origin of optical absorption in AlN with air voids

We deposited a 4-inch crack-free aluminum nitride (AlN) layer on a flat sapphire substrate and a patterned sapphire substrate grown using metal-organic vapor phase epitaxy method. To use the 4-inch sapphire substrate, we introduce air voids in the AlN layer in the initial growth region. Moreover, we discovered that optical absorption exists in the AlN layer with air voids. The secondary ion mass spectrometry (SIMS) results show that the impurity concentrations are high at the regions where the air voids exist, though the growth conditions are same throughout the AlN layer. We considered that it is most probable that the optical absorption is mainly attributed to the complexes of Al and O. The origin of the optical absorption was identified as a deep emission band due to impurities such as carbon and oxygen around the air voids. In this study, it was possible to obtain a transparent AlN template without air voids by changing the growth conditions and the layer structure. In addition, we could further suppress the underlying layer with the air voids.

Synthesis and photoluminescence of Mn4+ in M4Al14O25 (M = Sr or Mg) compounds as red-light phosphors for white LED

Red-light-emitting phosphors Mn4+-doped M4Al14O25 (M = Sr or Mg) were synthesized by solid-state reaction at 1400 °C in air, and their luminescent properties were initially investigated for their applications in warm white-light-diodes (W-LEDs). The excitation spectra of Mn4+-doped M4Al14O25 (M = Sr or Mg) showed that those phosphors were efficiently excited by near ultraviolet (UV) or blue LED chips, and those phosphors exhibited deep red emission band around 650 nm arising from the spin-forbidden 2E → 4A2 transition of Mn4+. The energy transfer mechanism in Mn4+-doped Mg4Al14O25 phosphor was found to be the electric dipole-dipole interaction and the critical distance of energy transfer between the Mn4+ ions was obtained to be about 1.384 nm. The Commission International de L’Eclairage (CIE) chromaticity coordinates of 0.5at.% Mn4+-doped Mg4Al14O25 were (0.631, 0.252). The temperature-dependent luminescence indicated Mn4+-doped M4Al14O25 phosphor was poor thermal-stable. Further work related to improve its thermal stability need be executed for Mn4+-doped M4Al14O25 (M = Sr or Mg) as a promising candidate of red-component for warm W-LEDs.

Homo-epitaxial growth of high crystal quality GaN thin films by plasma assisted–molecular beam epitaxy

Gallium nitride (GaN) films homo-epitaxially grown on GaN/sapphire templates and free standing GaN substrates by plasma-assisted molecular beam epitaxy (PA-MBE) have been investigated. After optimizations of growth temperature, the Ga/N flux ratio, surface preparation as well as initial growth process, the high quality MBE-GaN films were obtained. Combined with X-ray diffraction (XRD) and transmission electron microscopy (TEM) measurements, the TD densities were measured ∼4.2 × 108 cm−2 and ∼5 × 105 cm−2 for MBE-grown GaN films grown on GaN/sapphire templates and free standing GaN substrates, respectively. The observations of TEM prove the clear interface and well-arranged atomic lattice between substrates and as-grown layers. Besides, a flat and smooth surface with step-flow growth mode is observed by atomic force microscopy (AFM). Photoluminescence (PL) measurements exhibit that the linewidth for free excitons is as narrow as 4 meV at low temperature of 8 K and the deep level related yellow band emission ∼550 nm have been effectively suppressed at room temperature. The process optimization and obtained results give us wide latitude to fabricate high performance devices with lower tolerance to dislocation density.

A novel rare-earth free red-emitting Li3Mg2SbO6:Mn4+ phosphor-in-glass for warm w-LEDs: Synthesis, structure, and luminescence properties

Developing a novel non-rare-earth activated oxide red phosphor with admirable luminescent performance and low cost is still of great interest in the field of energy-saving warm white light-emitting diodes (w-LEDs) illumination. Here, Li3Mg2SbO6:Mn4+ (LMSO:Mn4+) red-emitting phosphor has been successfully prepared with cheap raw materials via a traditional solid-state method. Crystal structure, morphology, composition as well as optical properties are studied in detail. The prepared LMSO:Mn4+ phosphor exhibits a broad and deep red emission band in the range of 600–750 nm with the maximum centered at 666 nm, which is ascribed to the Mn4+:2Eg→4A2g transition in [SbO6] octahedral environment. Moreover, the nephelauxetic effect of Mn4+ in the LMSO matrix is evaluated by investigating the crystal field strength (Dq) and Racah parameters (B and C). Importantly, LMSO:0.01Mn4+ phosphor demonstrates good thermal stability and high quantum efficiency. By embedding LMSO:Mn4+ and Y3Al5O12:Ce3+ phosphors with various ratios in the TeO2-based glass host, a series of phosphors-in-glass(PiG) composites are achieved and demonstrate to remain the original optical properties of phosphors. Remarkably, by coupling PiG color converters with a blue LED chip, high-power w-LEDs devices with tunable optoelectronic parameters are constructed, with the correlated color temperature evolving from cool white (6783 K) to warm one (4014 K) and the color rendering index increasing from 72.2 to 88.7.

A deep red-light-emitting phosphor Mn4+:CaAl4O7 for warm white LEDs

Red-light-emitting phosphors Mn4+-doped CaAl4O7 were synthesized by high temperature solid-state reaction, and we investigated their photoluminescence properties for their applications in white-light-diodes (W-LEDs). Mn4+-doped CaAl4O7 exhibited deep red emission band peaking at 635, 641 and 655 nm arising from the spin-forbidden 2E→4A2 transition of Mn4+ under near ultraviolet (UV) or blue light excitation. The Commission International de L’Eclairage (CIE) chromaticity coordinates of 0.6at.% Mn4+-doped CaAl4O7 are (0.714, 0.285) under 470 nm excitation. The electric quadrupole-quadrupole interaction was the energy transfer mechanism in Mn4+-doped CaAl4O7 phosphor, and the critical distance of energy transfer between the Mn4+ ions was estimated to be about 1.533 nm. Mn4+-doped CaAl4O7 show poor thermal quenching properties based on the temperature-dependent fluorescent spectra. Further work to improve thermal stability need be carried out for Mn4+-doped CaAl4O7 as a promising red-light-emitting phosphor.

Effects of HfO2 dopant on characteristics of Li2MgTiO4-based red phosphors: Thermal stability, photoluminescence intensity and quantum efficiency improvement

To produce natural and vivid color, the color rendering index of white light-emitting diodes (WLEDs) with single phosphors is usually lower than 70, which is problematic for LED applications. A commonly used method to resolve this issue is to enhance the red component of WLEDs. In the present study, Hf4+ and Mn4+ co-doped Li2MgTiO4 red phosphors are synthesized using a solid-state reaction method. When this red phosphor is excited at 397 and 468 nm, it exhibits weak reabsorption in the blue region and emits a broad and deep red emission band in the range of 640–750 nm, which is attributed to the 2Eg → 4A2 g transition. With 5 mol% HfO2 dopant, the photoluminescence intensity is enhanced by 1.45-fold and thermal stability is increased by 7.7%. Moreover, this red phosphor was applied to a red phosphor-in-glass (RPiG) optical device with a low-melting TeO2-B2O3-ZnO-Na2O-WO3 glass system. In the RPiG melting process, Li2MgTiO4:Mn4+, Hf4+ red phosphor triggered neither a chemical reaction nor severe degradation, indicating good thermal stability. Li2MgTiO4:Mn4+, Hf4+ has potential as a red emission material for warm WLED applications.

Effect of Sb incorporation on the structural, optical, morphological and electrical properties of CdSe thin films deposited by electron beam evaporation technique

Cadmium Selenide (CdSe) and antimony (Sb) incorporated CdSe (CdSe:Sb) thin films were deposited by electron beam evaporation technique at various concentration of Sb (5 wt%, 10 wt%, 15 wt% and 20 wt%). The effect of Sb concentration on the properties of CdSe thin films was investigated. Formation of hexagonal crystal structure with preferred growth along [001] direction was confirmed by X-ray diffraction analysis. The interplanar spacing, lattice constant, average crystallite size, strain and dislocation density of CdSe and CdSe:Sb thin films were calculated from X-ray diffraction studies. The crystallite size of CdSe thin film is 40 nm, which is decreased to ~25 nm with increasing Sb concentration in CdSe from 5 wt% to 20 wt% in the source material. Optical transmittance of the CdSe films obtained from UV–Vis spectra is decreased with various amount of Sb incorporation in CdSe film. Direct optical band gap of the film varies in the range of 1.74–2.3 eV as a function of Sb concentration in the source material. Photoluminescence spectra of CdSe:Sb films show near band edge emission at 605 nm and deep trap related emission band at 743 nm. The intensity of the emission bands varies and shows shifts towards the lower wavelength side with increase in Sb concentration. The electrical resistivity is relatively minimum for CdSe:Sb (5 wt%) and further increase in Sb concentration increases the resistivity of the CdSe films.

An evaluation of structural, optical and electrical characteristics of Ag/ZnO rods/SnO2/In–Ga Schottky diode

ZnO rods were successfully deposited on ZnO seeded SnO2 coated glass substrates by a spray pyrolysis route. It was seen that the grown ZnO sample had a wurtzite structure with a well-defined hexagons of ~ 1–3 µm in diameter. Photoluminescence results measured at 300 K indicated that ZnO rods had a ultra-violet peak located at 383 nm without a deep level band emission. ZnO rods were subsequently used to realize a Ag/ZnO rods/SnO2/In–Ga Schottky diode which displayed rectifying current–voltage (I–V) characteristic with a turn on voltage of 1.2 V at 300 K. Electrical parameters of the Ag/ZnO rods/SnO2/In–Ga Schottky diode were further examined by (I–V) characteristics at the temperature range of 125–300 K with a step of 25 K. Using thermionic emission theory (TET), it was found that the ideality factor reduced with the increase of temperature whereas the zero bias barrier height increased with the increment of the temperature. Using ln(I0)/T2 versus q/nkT plot, Richardson constant (A*) and zero bias barrier height (ΦB0) was obtained as 3.27 × 10−5 A m−2 K−2 and 0.32 eV, respectively. After applying TET with the assumption of Gaussian distribution, a mean barrier height $$(\overline {{{\phi _{B0}}}})$$ and the modified Richardson constant (A**) were determined to be 0.88 eV and 2.75 × 105 A m−2 K−2.

Optoelectronic Characterization of ZnO Nanorod Arrays Obtained by Pulse Electrodeposition

ZnO nanorod arrays have been grown by potentiostatic pulse electrodeposition between a reduction potential and a “rest” potential. The effect of the duty cycle and pulse frequency as well as the heat-treatment in air on the properties of the electrodeposits has been studied. Surface morphology, structural, optical and electrical properties were evaluated. Absorption spectra reveal a high energy bandgap Burstein-Moss shift for the as-grown nanorods, in line with the donor density (1.1 × 1019 and 9.5 × 1019 cm−3) determined from electrochemical impedance spectroscopy. After annealing, the carrier concentration decreases to 1017―1018 cm−3, which is accompanied by an increase of the optical quality of the samples, assessed by the narrowing of the full width at half maximum of the near band edge recombination and steeper absorption at ∼3.3 eV. The donor density and the flatband potential are dependent of the applied duty cycle and pulse frequency. All the analyzed samples evidence deep broad emission bands in the visible region, whose intensity is enhanced after annealing. The defect luminescence is due to an overlap of emitting centers in the red, yellow and green spectral regions, as evidenced and discussed by comparing the steady-state and transient spectroscopies.

Analysis of the structure and abnormal photoluminescence of a red-emitting LiMgBO3:Mn2+ phosphor.

Mn2+ is attracting increasing attention due to its potential application as an activated ion in the phosphors. In this study, a rare-earth free phosphor, LiMgBO3:Mn2+, was synthesized using a high temperature solid-state reaction. Combined with X-ray diffraction and PL/PLE as well as first-principles calculation, the coordination environment of Mn2+ was disclosed, revealing that Mn2+ ions were inclined to occupy two partially filled Li sites. Under 427 nm-excitation, an abnormal deep red emission band centered at 705 nm was observed. This was due to the strong crystal field environment of Mn2+ induced by the extremely distorted five-coordinated sites, which was proved by the electronic band structure obtained from the density functional theory calculations. In addition, the luminescence intensity and thermal stability of LiMgBO3:Mn2+ was significantly enhanced when Li+ was replaced by Na+ and K+. This study disclosed the origin of the abnormal luminescence of Mn2+ in LiMgBO3 and further enriched our understanding of the abnormal luminescence of Mn2+ in special crystal structures (five-coordinated configuration).

Enhancement of external quantum efficiency and quality of heterojunction white LEDs by varying the size of ZnO nanorods

The size of ZnO nanorods (NRs) plays an important role in tuning the external quantum efficiency (EQE) and quality of light generated by white light emitting diodes (LEDs). In this work, we report on the enhancement of EQE and the quality of ZnO NR-based hetrojunction white LEDs fabricated on a p-GaN substrate using a low temperature solution method. Cathodoluminescence spectra demonstrate that ultraviolet (UV) emission decreases and visible deep band emission increases with an increase in the length of the ZnO NRs. The UV emission could be internally reabsorbed by the ZnO NR excitation, thus enhancing the emission intensity of the visible deep band. Photocurrent measurements validated the fact that the EQE depends on the size of ZnO NRs, increasing by 87% with an increase in the length of the ZnO NRs. Furthermore, the quality of white light was measured and clearly indicated an increase in the color rendering indices of the LEDs with an increase in the length of the ZnO NRs, confirming that the quality of light generated by LEDs can be tuned by varying the length of the ZnO NRs. These results suggest that the EQE and visible deep band emission from n-ZnONRs/p-GaN heterojunction LEDs can be effectively controlled by adjusting the length of the ZnO NRs, which can be useful for realizing tunable white LEDs.

H2PO4-- and Solvent-Induced Polymorphism of an Amide-Functionalized [Pt(N^C^N)Cl] Complex.

A simple [Pt(N^C^N)Cl] complex functionalized with an amide group was prepared, and its absorption and emission properties were examined in different solvents in response to various anions. On the one hand, in the presence of H2PO4-, the solution of the complex shows distinct color changes in CH3CN, together with a ratiometric emission change from a green emission band at 537 nm to a deep red emission band at 680 nm. On the other hand, two-step spectral changes were observed in response to H2PO4- in CH2Cl2, with the green emission being attenuated first followed by the appearance of enhanced and yellow-green emissions at a lower-energy region. These recognition processes are highly selective for H2PO4- against other common anions including F-, Cl-, Br-, I-, OAc-, NO3-, and HSO4-. In addition, the platinum complex displays multistage emission polymorphism in mixed CH3CN/H2O solvent of various ratios. The hydrogen-bonding interaction between H2PO4- and the amide unit was confirmed by NMR analysis. In the solid state, this platinum complex emits red light. However, the composite material of the platinum complex with H2PO4- shows purely monomeric yellow emissions. The solid-state materials were further analyzed by single-crystal X-ray and Fourier-transform IR analysis. These studies suggest that this simple platinum complex is useful for the selective recognition of H2PO4- and as solid-state emitting materials with tunable emission colors.

Ionoluminescence study of Zn− and O− implanted ZnO crystals: An additional perspective

An investigation into the role of native point defects on the optical properties of ZnO using ion implantation, rapid thermal annealing (RTA) and ionoluminescence (IL) is presented. Low-energy (60keV) fixed-fluence (8×1015cm−2) Zn− and O− implantation has been used to directly introduce native point defects into ZnO single crystals. It is shown that annealing of implanted samples in Ar at T=1000°C for 2min amplifies the deep band emission (DBE) peak centered around 2.4eV while at the same time revealing subtle differences not clearly resolved in similar implanted samples treated under prolonged annealing. Particularly, a relative shift in the DBE peak maxima of the O and Zn doped samples subjected to RTA is observed. Gaussian decomposition of the IL spectra show distinct enhancements of the red (1.62eV) and yellow (2.15eV) emission bands in the O− implanted sample and the green (2.36eV) emission band in the Zn− implanted sample. Based on these results, and recent density functional theory (DFT) calculations, we have proposed a possible energy level scheme for some common ZnO native point defects.

Photoluminescence of high‐resistivity ZnTe crystals doped with gallium and indium

AbstractWe have measured photoluminescence spectra of high‐resistivity ZnTe crystals doped with gallium and indium in the wide energy range from near‐band‐edge energy to deep energy region up to 0.78 eV. In the near‐band‐edge energy, we detected a characteristic excitonic peak at 2.363 eV in both gallium and indium doped high resistivity ZnTe. By comparison with a similar PL peak observed in high resistivity CdTe, we identified the characteristic peak at 2.363 eV as emission from excitons bound to defects complex arising from a Zn vacancy and gallium or indium atoms (VZn‐2D). In lower energy range, a newly strong deep emission band near 1.0 eV has been observed for the first time in high‐resistivity ZnTe crystals. The intensity of the deep emission is two order magnitude stronger than that in undoped one. Furthermore, the intensity and peak position of the deep emission band are almost unchanged by Zn‐saturated anneal at 600‐680 °C. We specualte that this deep emission band is responsible for the high resistivity of ZnTe and prevent conversion of n‐type low resistivity. (© 2016 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Defect segregation and optical emission in ZnO nano- and microwires.

The spatial distribution of defect related deep band emission has been studied in zinc oxide (ZnO) nano- and microwires using depth resolved cathodoluminescence spectroscopy (DRCLS) in a hyperspectral imaging (HSI) mode within a UHV scanning electron microscope (SEM). Three sets of wires were examined that had been grown by pulsed laser deposition or vapor transport methods and ranged in diameter from 200 nm-2.7 μm. This data was analyzed by developing a 3D DRCLS simulation and using it to estimate the segregation depth and decay profile of the near surface defects. We observed different dominant defects from each growth process as well as diameter-dependent defect segregation behavior.

ChemInform Abstract: Preparation, Structural and Photoluminescence Characteristics of Novel Red Emitting Mg7Ga2GeO12:Mn4+ Phosphor.

AbstractMg7Ga2GeO12:Mn4+ is prepared by firing stoichiometric amounts of (MgCO3)4Mg(OH)2·5H2O, Ga2O3, GeO2, and MnCO3 in air (alumina crucible, 1400 °C, 10 h).