Broadband near-infrared emission enhancement in K2Ga2Sn6O16:Cr3+ phosphor by electron-lattice coupling regulation

Cr³⁺-doped phosphors have recently gained attention for their application in broadband near-infrared phosphor-converted light-emitting diodes (pc-LEDs), but generally exhibit low efficiency. In this work, K₂Ga₂Sn₆O₁₆:Cr³⁺ (KGSO:Cr) phosphor was designed and synthesized. The experimental results show that the Cr³⁺-doped phosphor exhibited broadband emissivity in the range 650-1300 nm, with a full width at half maximum (FWHM) of approximately 220-230 nm excited by a wavelength of 450 nm. With the co-doping of Gd³⁺ ions, the internal quantum efficiency (IQE) of the KGSO:Cr phosphor increased from 34% to 48%. The Gd³⁺ ions acted neither as activators nor sensitizers, but to justify the crystal field environment for efficient Cr³⁺ ions broad emission. The Huang-Rhys factor decreased as the co-doping of Gd³⁺ ions increased, demonstrating that the nonradiative transitions were suppressed. An efficient strategy for enhancing the luminescence properties of Cr³⁺ ions is proposed for the first time. The Gd³⁺–co-doped KGSO:Cr phosphor is a promising candidate for broadband NIR pc-LEDs.     

Field emission enhancement of Au-Si nano-particle-decorated silicon nanowires

Au-Si nano-particle-decorated silicon nanowire arrays have been fabricated by Au film deposition on silicon nanowire array substrates and then post-thermal annealing under hydrogen atmosphere. Field emission measurements illustrated that the turn-on fields of the non-annealed Au-coated SiNWs were 6.02 to 7.51 V/μm, higher than that of the as-grown silicon nanowires, which is about 5.01 V/μm. Meanwhile, after being annealed above 650°C, Au-Si nano-particles were synthesized on the top surface of the silicon nanowire arrays and the one-dimensional Au-Si nano-particle-decorated SiNWs had a much lower turn-on field, 1.95 V/μm. The results demonstrated that annealed composite silicon nanowire array-based electron field emitters may have great advantages over many other emitters.     

Broadband emission and flat optical gain glass containing Ag aggregates for tunable laser

Spectroscopic properties of Sm³⁺-doped borate glass with silver aggregates were investigated, and it was found that the glass presented broad excitation and emission band covering violet-infrared region. Furthermore, the optical gain coefficients at various wavelengths were measured via an amplified spontaneous emission technique, it was confirmed that the glass displayed flat net optical gain coefficient in almost full visible region. In addition, the fluorescent temperature quenching of the glass was examined, and it was seen that about 50% of room temperature emission intensity was kept when the sample was heated up to 325°C.     

Field emission enhancement of amorphous carbon films by nitrogen-implantation

Effect of nitrogen-implantation on electron field emission properties of amorphous carbon films has been examined. Raman and X-ray photoelectron spectroscopy measurements reveal different types of C-N bonds formed upon nitrogen-implantation. The threshold field is lowered from 14 to 4 V/μm with increasing the dose of implantation from 0 to 5 × 10¹⁷ cm⁻² and the corresponding effective work function is estimated to be in the range of 0.01-0.1 eV. From the perspective of tetrahedron bond formation, a mechanism for the nitrogen-lowered work function is proposed, suggesting that both the nitrogen nonbonding (lone pair) and the lone-pair-induced carbon antiboding (dipole) states are responsible for lowering the work function and hence the threshold field.     

Field emission enhancement of diamond tips utilizing boron doping and surface treatment

A practical field emission enhancement technique for diamond tips with sp² content utilizing boron doping and surface treatment, achieving a very low turn-on electric field of 1 V/μm, has been developed. The effects of surface treatment and boron doping on electron field emission from an array of micropatterned polycrystalline diamond microtips with sp² content have been systematically investigated. Regardless of doping, the field emission characteristics of diamond tips are significantly enhanced and the turn-on electric field is reduced more than 60% after surface treatment. Likewise, regardless of surface treatment, the turn-on electric field of the diamond tips with sp² content decreases substantially with boron doping. Possible mechanisms responsible for the field emission enhancement are an increase in the field enhancement factor due to hole accumulation via the formation of cascaded sp²–diamond–sp² embedded microstructures and field forming process with enhanced hole accumulation after surface treatment.     

Broadband deep-red-to-near-infrared emission from Mn2+ in strong crystal-field of nitride MgAlSiN3

Broadband near-infrared (NIR) phosphors have received increasing attention for fabricating phosphor-converted light-emitting diodes (pc-LEDs) as NIR light source. Most of the reported broadband NIR phosphors originate from Cr³⁺ in weak crystal field environments. Herein, we report a luminescent material, MgAlSiN₃:Mn²⁺ with CaAlSiN₃-type structure, demonstrating that broadband deep-red-to-NIR emission can be achieved via doping Mn²⁺ into crystallographic sites with strong crystal field in inorganic solids. This phosphor is synthesized via easy-handle solid-state reaction, and the optimized sample, (Mg_0.93Mn_0.07) AlSiN₃ shows an emission band with peak at ~754 nm, FWHM of 150 nm, and internal quantum efficiency of 70.1%. The photoluminescence intensity can further be enhanced by co-doping Eu²⁺ as sensitizer. This work provides a new strategy for discovering new broadband NIR phosphors using Mn²⁺ in strong crystal field as luminescence center.     

Energy transfer and luminescence study of Dy3+ doped zinc-aluminoborosilicate glasses for white light emission

A series of Dy³⁺ doped aluminoborosilicate glasses with general formula 20SiO₂-(40-x) B₂O₃–10Al₂O₃–20NaF₂–10ZnO-xDy₂O₃ (x = 0, 0.1, 0.5, 1.0. 1.5, 2.0 and 2.5 mol %) were prepared by melt-quenching method. The FTIR analysis confirms the presence of stretching and bending vibrations of BO₄, BO₃ and SiO₄ units in the prepared glasses. DTA results show that Tg is decreasing with addition of Dy³⁺ ions in the glass system. Theoretically calculated mechanical properties such as elastic modulus, bulk modulus, shear modulus and Poisson's ratio suggest the increase in mechanical stability of glasses with dysprosium addition. Also experimental analysis carried out using Vicker's microhardness suggests glass stability with applied loads. Absorption spectrum shows twelve bands that exist due to transition from ⁶H_15/2 level to different excited levels. Nephelauxetic ratio and bonding parameter calculated shows decreasing ionicity of glasses with increase in Dy³⁺ ions. Judd-Oflet parameter calculated for all the glasses follow the trend Ω₆ > Ω₄ > Ω₄. Luminescence study shows three emission peaks having transitions from ⁴F_9/2 → ⁶H_15/2 (blue), ⁴F_9/2 → ⁶H_13/2 (yellow) and ⁴F_9/2 → ⁶H_11/2 (red). Radiative parameters calculated suggest higher stimulated emission cross-section for present glasses having ⁴F_9/2 → ⁶H_13/2 transition. The decay measurement for all the glass samples were recorded with an excitation at 350 nm and monitoring emission at 575 nm corresponding to the ⁴F_9/2 → ⁶H_13/2 transition and decay curves were fitted to bi-exponential fit. The CIE colour chromaticity coordinates were determined using CIE chromaticity diagram and the values were found to be in close proximity with the standard white light (0.33. 0.33) for all the glasses. Further colour correlated temperature values were found to lie in the near bright white region with CCT around ~4000 K.     

Field emission enhancement and microstructural changes of carbon films by single pulse laser irradiation

The effect of single nanosecond laser pulse irradiation on the microstructure and field emission (FE) properties of carbon films is studied. Amorphous carbon films were exposed to a single pulse of a 248 nm Excimer laser with pulse width of 23 ns. Microstructural changes of the films were investigated by Raman spectroscopy, transmission electron microscopy and electron energy loss spectroscopy. FE study was conducted in a parallel plate configuration. It was found that the landscape of the FE properties is not directly correlated to the laser energy in a simple way, whereas low energy laser irradiation (<117 mJ/cm²) leads to a lower emission threshold field due to the formation of sub-nanometer conductive sp² clusters within the insulating sp³ matrix. A medium energy range (117–362.5 mJ/cm²) would actually reduce field enhancement and increase the threshold field because of the increased size of the same sp² clusters. Interestingly, a much higher laser energy (>362.5 mJ/cm²) would reverse this effect by forming multiple continuous conductive sp² channels and thereby reduce the threshold field sharply again.     

Broadband mid-infrared emission from Cr2+ in crystal-in-glass composite glasses by Hot Uniaxial Pressing

Cr²⁺-doped II-VI crystals have witnessed an excellent gain media for continuously tunable and femtosecond-pulsed lasers. Despite this, major challenges persist toward realizing ultrabroad emission bandwidth and efficient Cr²⁺-doped fiber due to the valence diversity of Cr, especially in chalcogenide glasses. Here, we propose to prepare Cr²⁺:ZnSe/As₂S_3-xSe_x composite glasses by Hot Uniaxial Pressing (HUP), a method that sinters uniformly mixed crystal and glass powders into geometrically designed composite chalcogenide glasses. The densification of the composite glasses reached 99.88%, indicating that a few or none pores remain. Our research shows that Cr²⁺:ZnSe crystals have good performance in chalcogenide glasses, and the composite glasses have the potential to be made into mid-infrared–doped fibers. It was demonstrated by scanning electron microscopy (SEM) and X-ray diffraction (XRD) that the composite glasses have a uniform Cr²⁺:ZnSe distribution and no crystal disintegration. The transmittance of the composite glasses was significantly improved by tailoring the refraction index. The mid-infrared (MIR) fluorescence and decay of the glasses were measured. The lattice constant was measured, calculated, and discussed to reveal the influence of sintering process on lifetime.     

Simultaneous enhancement of quantum cutting luminescence in Er-doped NaBaPO4 phosphors by crystal field control and plasmonic modulation

It is of great significance to enhance the quantum-cutting (QC) luminescence for practical applications due to the narrow absorption cross-section and low luminescence efficiency of rare earth ions. In this work, NaBaPO₄:Er³⁺ phosphors doped with Li⁺ were synthesized through a solid-state reaction. The QC luminescence of NaBaPO₄:Er³⁺ phosphor was enhanced 5.71 times by doping Li⁺. XRD patterns and Judd-Ofelt calculations demonstrated the crystal field distortion when introduced Li⁺, which would increase the transition probability of Er³⁺. Furthermore, NaBaPO₄:Er³⁺, Li⁺ phosphors were decorated with silver nanoparticles (Ag NPs). The effect of Ag NPs on QC luminescence was studied, and the results showed that QC luminescence was further enhanced up to 1.95 times by Ag NPs. FDTD simulations revealed that Ag NPs generated substantial surface plasmons, which would boost the excitation rate of Er³⁺. Our studies would provide a useful strategy to enhance QC luminescence, which has potential application in germanium-based solar cells.     

Evolution of the sensitized Er3+ emission by silicon nanoclusters and luminescence centers in silicon-rich silica

The structural and optical properties of erbium-doped silicon-rich silica samples containing different Si concentrations are studied. Intense photoluminescence (PL) from luminescence centers (LCs) and silicon nanoclusters (Si NCs), which evolves with annealing temperatures, is obtained. By modulating the silicon concentrations in samples, the main sensitizers of Er³⁺ ions can be tuned from Si NCs to LCs. Optimum Er³⁺ PL, with an enhancement of more than two, is obtained in the samples with a medium Si concentration, where the sensitization from Si NCs and LCs coexists.     

An effective oxidation approach for luminescence enhancement in CdS quantum dots by H2O2

The effects of surface passivation on the photoluminescence (PL) properties of CdS nanoparticles oxidized by straightforward H₂O₂ injection were examined. Compared to pristine cadmium sulfide nanocrystals (quantum efficiency ≅ 0.1%), the surface-passivated CdS nanoparticles showed significantly enhanced luminescence properties (quantum efficiency ≅ 20%). The surface passivation by H₂O₂ injection was characterized using X-ray photoelectron spectroscopy, X-ray diffraction, and time-resolved PL. The photoluminescence enhancement is due to the two-order increase in the radiative recombination rate by the sulfate passivation layer.     

Au nanoparticles embedded inverse opal photonic crystals as substrates for upconversion emission enhancement

Photonic crystals (PCs) with periodic dielectric structures are capable to control the propagation of photons when photon energy is in the region of photonic band gap. The upconversion luminescence (UCL) of nanocrystals coated on the PCs surface can be enhanced by the PCs effects. While surface plasmon resonance (SPR) of noble metal nanoparticles (NPs) is being extensively applied to enhance the UCL properties of nanocrystals. However, the PCs or SPR effect are developed separately for the UCL enhancement. In this work, we present a facile preparation method of the Au NPs embedded inverse opals, which was used as substrates to improve the UCL properties of NaYF₄:Yb³⁺, Er³⁺ NPs. The significant luminescence enhancement of NaYF₄:Yb³⁺, Er³⁺ upconverting NPs was obtained by the coupling between the SPR of Au NPs and PCs effects from Au NPs embedded inverse opals substrates. The finding demonstrated that the Au NPs embedded inverse opals as substrates may be useful for the enhanced UCL of other phosphors, producing novel photonic devices.     

Tunable solid‐state fluorescence emission and red upconversion luminescence of novel anthracene‐based fluorophores

Novel, tunable solid‐state emitters based on anthracene groups were synthesised and characterised by spectroscopy and elemental analysis. Their solid‐state photoluminescence properties were studied. These fluorophores display interesting solid‐state emission properties with an emission at wavelengths ranging from 550 to 650 nm when excited by a 325 nm helium–cadmium laser at room temperature. In particular, among them, 1,6‐di(9‐anthryl)hexa‐1,5‐diene‐3,4‐dione, 2‐[4‐(2‐benzoxazolyl)phenyl]‐4,5‐bis[2‐(9‐anthryl)vinyl]‐1H‐imidazole and 2,3‐bis[2‐(9‐anthryl)vinyl]quinoxaline show red, yellow and green emission, respectively, at 650, 584 and 550 nm. The results demonstrated that the luminescent colours can be tuned from red to yellow and green by simply varying molecular structure. Besides, 1,6‐di(9‐anthryl)hexa‐1,5‐diene‐3,4‐dione also exhibited an upconverted red fluorescent emission peak at around 675 nm under femtosecond excitation at 800 nm.     

Near-infrared luminescence enhancement in Yb3+/Ho3+ co-doped bismuth-tellurite glass by tailoring glass network

In this work, the glass network has been tailored by introducing the modifier WO₃ when a Ho³⁺/Yb³⁺ co-doped bismuth-tellurite glass composition was designed. The physical, absorption, emission, structure, and gain characteristics of the glasses with the different contents of WO₃ were investigated comprehensively based on various tests and analytical methods such as absorption spectra, fluorescence spectra, Raman spectra, and J-O theory. The results show that both the optical band gap and the bridge oxygen content are enhanced remarkably while the substitution of TeO₂ by WO₃, accelerating the transition from [TeO₃] to [TeO₄] units. Simultaneously, the value of Ω₆ reached the maximum of 2.26×10^-20 cm² when the WO₃ is equal to 10 mol%, indicating that the Stark splitting of Ho³⁺: ⁵I₇ energy level is the weakest. The optimal FWHM and the gain coefficient are 143.3 nm and 2.22 cm^-1, respectively. Furthermore, a blue shift of the central wavelength in absorption and emission peaks can be observed at the 2 μm band. As mentioned above, the bismuth-tellurite glass prepared is an ideal gain material that can realize a wider spectra span.     

Significant enhancement of yellow-green light emission of ZnO nanorod arrays using Ag island films

Surface plasmon (SP) mediated emission from ZnO nanorod arrays (NRAs)/Ag/Si structures has been investigated. The ratio of visible emission to UV emission can be increased by over 30 times via coupling with SP without deterioration of the crystal quality. The fact that the effect of SP crucially depends on the size of Ag island films provides the feasibility to significantly enhance the yellow-green emission of the ZnO nanostructures without sacrificing the crystallinity of ZnO.     

Mechanism of diamond nucleation enhancement by electron emission via hot filament chemical vapor deposition

The mechanism of diamond nucleation enhancement by electron emission in the hot filament chemical vapor deposition process has been investigated by scanning electron microscopy, Raman spectroscopy and infrared (IR) absorption spectroscopy. The maximum value of the nucleation density was found to be 10¹¹ cm⁻² with a −300 V and 250 mA bias. The electron emission from the diamond coating on the electrode excites a plasma, and greatly increases the chemical species, as we have seen by in situ IR absorption. The experimental studies showed that the diamond and chemical species were transported and scattered from the diamond coating on the electrode and through the plasma towards the substrate surface, where they caused enhanced nucleation.     

Low-threshold green and red random lasing emission in inorganic halide lead perovskite microcrystals with plasmonic and interferential enhancement

Herein, low-threshold green and red random lasing actions were investigated in inorganic caesium lead bromide (CsPbBr₃) and caesium lead iodide (CsPbI₃) perovskite microcrystals, which were grown using metal organic chemical vapor deposition. Sharp random lasing emission peaks were visible on the right side of the broad photoluminescence emission spectra. Interferential patterned sapphire substrates and novel metal nanoparticles were used to enhance the lasing emission intensity and reduce the lasing emission thresholds. Fast-decayed random lasing oscillation dynamics and backscattering angular distribution of light confirmed that the multi-scattering of light was important in the reduction of the threshold in thin films on patterned sapphire substrates. Plasmonic enhancement between gold or silver nanoparticles and the perovskite microcrystals contributed similarly to low threshold random lasing emission, which was verified using the simulation result from the finite difference time domain (FDTD) method. This work has a wide range of applications in sensing, speckle-free imaging, illumination, and medical diagnosis.     

Extracting independently the work function and field enhancement factor from thermal-field emission of multi-walled carbon nanotube tips

We have characterized the emission properties of multi-walled carbon nanotubes (MWCNTs) grown by thermal chemical vapor deposition (CVD). Our characterizations have been conducted in the thermal-field (T-F) zone, a narrow transition zone between field emission and thermionic emission. The characteristic I-V curve in T-F zone, designated to Christov plot, is advantageous to extract independently the field enhancement factor and work function at the emitting site. Our approach is more reliable than Fowler-Nordheim (F-N) plot whose linear region provides the solution with two unknowns. On the other hand, the Christov plot in the T-F zone provides the solution with only the field enhancement factor. Using the general theory of electron emission, we find that the work function and field enhancement factor of the CVD-grown MWCNTs are 4.7 ± 0.4 eV and 3300 ± 200, respectively.