Room-temperature Emission Research Articles

Influence of π -conjugated cations and halogen substitution on the optoelectronic and excitonic properties of layered hybrid perovskites

Low-cost chemical engineering of two-dimensional layered hybrid halide perovskite structures allows for the design of hybrid semiconductor quantum wells with tailored room-temperature excitonic optical absorption, emission, and charge carrier transport properties. Here density functional theory and the Bethe-Salpeter equation are used to predict the electronic structure and optical response of layered perovskites with two representative single-ring conjugated organic spacers, ammonium-propyl-imidazole (API) and 2-phenethylammonium (PEA). The inorganic perovskite quantum well properties are further tuned by analyzing the effect of halogen (X = I, Br, Cl) substitution. We found that visible light absorption occurs primarily within the perovskite layer and that UV light absorption induces partial electron-hole separation between layers. In addition, a strong exciton binding energy and influence on absorption spectrum is found by solving the Bethe-Salpeter equation. Our results suggest that further engineering is necessary beyond the single-ring limit, by introducing more conjugated rings and/or heavier nuclei into the organic spacer. This is a promising future direction to achieve photoinduced charge separation and more generally hybrid heterostructures with attractive optoelectronic properties.

Plasma-assisted synthesis of ZnSe hollow microspheres with strong red emission

We demonstrate the growth of ZnSe microspheres in large quantities by using a mixture of Zn and Se as precursors via direct current arc discharge method without the addition of any catalyst or template. The as-obtained samples were characterized by X-ray diffraction, Raman, energy-dispersive X-ray spectrometry, scanning and transmission electron microscopy, and ultraviolet-visible reflectance spectroscopy. The majority of the ZnSe microspheres which were composed of ZnSe nanoparticles exhibited perfectly spherical morphology and a distinct hollow structure with a relatively rough surface. The growth mechanism of ZnSe hollow microspheres has been discussed. Room-temperature excitation and emission spectra as well as the fluorescence lifetime of ZnSe hollow microspheres were measured and the results showed ZnSe hollow microspheres exhibit an intensive wide broad emission bands from 550 to 750 nm upon 470 nm excitation with fast decay time 1.09 ms and CIE chromaticity coordinates (0.65, 0.34), which was ascribed to characteristic structure defects (vacancies and interstitials) in ZnSe. Additionally, the temperature dependent emission behavior was measured and discussed.

Mid-Infrared Photonic-Crystal Surface-Emitting Lasers with InGaAs/GaAsSb ‘W’-Type Quantum Wells Grown on InP Substrate

InP-based InGaAs/GaAsSb ‘W’-type quantum well (QW) photonic-crystal (PC) surface- emitting lasers (SELs) of 2.2 μm wavelength range are fabricated and room-temperature lasing emissions by optical pumping are demonstrated for the first time. Photonic-crystal surface-emitting laser (PCSEL) devices are investigated in terms of PC parameters of etch depth, lattice period, and filling factor. The lasing emissions cover wavelengths from 2182 nm to 2253 nm. The temperature-dependent lasing characteristics are also studied in terms of lattice period. All PCSELs show consistent lasing wavelength shift against temperature at a rate of 0.17 nm/K. The characteristic temperatures of PCSELs are extracted and discussed with respect to wavelength detuning between QW gain peak and PC cavity resonance.

Narrow-band single-photon emission through selective aryl functionalization of zigzag carbon nanotubes.

The introduction of sp3 defects into single-walled carbon nanotubes through covalent functionalization can generate new light-emitting states and thus dramatically expand their optical functionality. This may open up routes to enhanced imaging, photon upconversion, and room-temperature single-photon emission at telecom wavelengths. However, a significant challenge in harnessing this potential is that the nominally simple reaction chemistry of nanotube functionalization introduces a broad diversity of emitting states. Precisely defining a narrow band of emission energies necessitates constraining these states, which requires extreme selectivity in molecular binding configuration on the nanotube surface. We show here that such selectivity can be obtained through aryl functionalization of so-called 'zigzag' nanotube structures to achieve a threefold narrowing in emission bandwidth. Accompanying density functional theory modelling reveals that, because of the associated structural symmetry, the defect states become degenerate, thus limiting emission energies to a single narrow band. We show that this behaviour can only result from a predominant selectivity for ortho binding configurations of the aryl groups on the nanotube lattice.

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

AbstractSpectroscopic 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.

Effect of size and shape on the excitonic stimulated emission process in ZnO microstructures

There has been some (partly controversial) discussion about the role of excitons in room-temperature laser emission of ZnO. Recently, we have demonstrated from temperature and excitation-density-dependent photoluminescence measurements on a ZnO film consisting of well crystallized micrometer-sized grains that the mechanism of optical gain at temperatures below ∼150 K is the exciton-exciton (ex-ex) scattering, whereas at temperatures from ∼150 K to room temperature, the gain results from the exciton-electron (ex-el) scattering without any contribution from electron-hole plasma lasing [R. Matsuzaki et al., Phys. Rev. B 96, 125306 (2017)]. However, the mechanism of optical feedback inside these ZnO microstructures is not fully understood. In this work, we investigate the emission properties of various ZnO microcrystals with different size and shape in the temperature region from 3 to 300 K using a nanosecond pulsed laser as an excitation source. We found that room temperature stimulated emission is observed only from the sphere-like ZnO particles with the size of a few micrometers in diameter. We also found the temperature-induced transition between ex-ex and ex-el scattering processes at a temperature of ∼150 K, similar to the case of the ZnO film consisting of micrometer-sized grains reported previously. The close similarity observed between the two different types of ZnO microstructures allows us to deduce that the temperature-dependent excitonic stimulated emission characteristics are common in micrometer-sized ZnO crystals with a low-loss feedback mechanism.

Room-temperature excitonic emission with a phonon replica from graphene nanosheets deposited on Ni-nanocrystallites/Si-nanoporous pillar array.

Graphene nanosheets (GNSs) were grown on a Si nanoporous pillar array (Si-NPA) via chemical vapour deposition, using a thin layer of pre-deposited Ni nanocrystallites as catalyst. GNSs were determined to be of high quality and good dispersivity, with a typical diameter size of 15 × 8 nm. Light absorption measurements showed that GNSs had an absorption band edge at 3.3 eV. They also showed sharp and regular excitonic emitting peaks in the ultraviolet and visible region (2.06–3.6 eV). Moreover, phonon replicas with long-term stability appeared with the excitonic peaks at room temperature. Temperature-dependent photoluminescence from the GNSs revealed that the excitonic emission derived from free and bound excitonic recombination. A physical model based on band energy theory was constructed to analyse the carrier transport of GNSs. The Ni nanocrystallites on Si-NPA, which acted as a metal-enhanced fluorescence substrate, were supposed to accelerate the excitonic recombination of GNSs and enhanced the measured emission intensity. Results of this study would be valuable in determining the luminescence mechanism of GNSs and could be applied in real-world optoelectronic devices.

InAs QDs on (111)-faceted Si (001) hollow substrates with strong emission at 1300 nm and 1550 nm

Highly uniform (111)-faceted Si sawtooth with underlying hollow structures is formed by homo-epitaxy on a U-shaped patterned Si (001) substrate. With in-situ III-V growth on such substrates by the III-V/IV dual chamber molecular beam epitaxy, a high-quality GaAs film is obtained. The (111)-faceted sawtooth structures are found effectively annihilating the antiphase boundaries and terminating mostly the lattice mismatch induced dislocations at the III-V/Si interface, while the hollow structures can effectively reduce the thermal stress. The high-quality GaAs layers on (111)-faceted hollow Si (001) are measured with a threading dislocation density of ∼106 cm−2 via the electron channeling contrast image method. By implementing the designed structure, strong room-temperature emission of InAs/GaAs and InAs/InGaAs quantum dots (QDs) at both O-band (1300 nm) and C/L-band (1550 nm) telecommunication wavelengths are achieved on Si (001) substrates. The O-band emission of InAs/GaAs QDs on Si (001) shows similar intensity to that on the GaAs substrate.

Realization of Lasing Emission from One Step Fabricated WSe₂ Quantum Dots.

Two-dimensional (2D) transition-metal dichalcogenides (TMDCs) quantum dots (QDs) are the vanguard due to their unique properties. In this work, WSe2 QDs were fabricated via one step ultrasonic probe sonication. Excitation wavelength dependent photoluminescence (PL) is observed from WSe2 QDs. Room-temperature lasing emission which benefits from 3.7 times enhancement of PL intensity by thermal treatment at ~470 nm was achieved with an excitation threshold value of ~3.5 kW/cm2 in a Fabry–Perot laser cavity. To the best of our knowledge, this is the first demonstration of lasing emission from TMDCs QDs. This indicates that TMDCs QDs are a superior candidate as a new type of laser gain medium.

Wideband Ge-Rich SiGe Polarization-Insensitive Waveguides for Mid-Infrared Free-Space Communications

The recent development of quantum cascade lasers, with room-temperature emission in the mid-infrared range, opened new opportunities for the implementation of ultra-wideband communication systems. Specifically, the mid-infrared atmospheric transparency windows, comprising wavelengths between 3–5 µm and 8–14 µm, have great potential for free-space communications, as they provide a wide unregulated spectrum with low Mie and Rayleigh scattering and reduced background noise. Despite the great efforts devoted to the development of mid-infrared sources and detectors, little attention is dedicated to the management of polarization for signal processing. In this work, we used Ge-rich SiGe alloys to build a wideband and polarization-insensitive mid-infrared photonic platform. We showed that the gradual index change in the SiGe alloys enabled the design of waveguides with remarkably low birefringence, below 2 × 10−4, over ultra-wide wavelength ranges within both atmospheric transparency windows, near wavelengths of 3.5 µm and 9 µm. We also report on the design of a polarization-independent multimode interference device achieving efficient power splitting in an unprecedented 4.5-µm bandwidth at around 10-µm wavelength. The ultra-wideband polarization-insensitive building blocks presented here pave the way for the development of high-performance on-chip photonic circuits for next-generation mid-infrared free-space communication systems.

Hydrothermal Addition Polymerization for Ultrahigh-Yield Carbonized Polymer Dots with Room Temperature Phosphorescence via Nanocomposite.

Hydrothermal/solvothermal treatments have been widely used to prepare carbonized polymer dots (CPDs) through the condensation and carbonization of small molecules and/or polymers. However, the basic scientific issues, such as the nucleation and growth process, morphology and size control, yield increase, and photoluminescence (PL) mechanism have not been well investigated. In this work, enlightened by the principle of soap-free emulsion polymerization, CPDs with ultrahigh yields (ca. 85 %) were obtained by hydrothermal addition polymerization and carbonization (HAPC) of monomers. The unprecedented initiator-induced addition polymerization was exploited to synthesize CPDs for the first time. As expected in typical emulsion polymerization processes, the developed HAPC method can produce CPDs with designed sizes by systematically regulating the HAPC parameter, uncovering an unprecedented strategy for regulating the size of CPDs. In addition, the obtained CPDs were provided with high photoluminescence quantum yields (PLQY) up to 45.58 %, while the relationship between the photoluminescence (PL) mechanism and chemical structure was investigated. The viscosity parameter was first adopted to measure the polymer property of CPDs. Moreover, the ultrahigh yield and low-cost CPDs elicited the high-performance CPDs/PVA nanocomposite (PVA=poly(vinyl alcohol)) with fluorescence and room-temperature phosphorescence dual-mode emission, demonstrating potential for advanced anti-counterfeit applications.

Tunable emission with excellent thermal stability in single-phased SrY2O4:Bi3+,Eu3+ phosphors for UV-LEDs

Novel single-phased phosphors SrY2O4:Bi3+,Eu3+ (SYO:Bi3+,Eu3+) with tunable emitting were successfully synthesized by the conventional solid-state method. X-ray diffraction, excitation and emission spectra, decay curves as well as temperature-dependent luminescence were applied to characterize the as-obtained phosphors. Under ultraviolet (UV) excitation, blue luminescence centered at 410 nm was found in SYO:Bi3+. By introducing Eu3+ into SYO:Bi3+, highly efficient energy transfer (ET) process with ET efficiency of 92.65% from Bi3+ to Eu3+ was obtained before concentration quenching. The possible ET mechanism from Bi3+ to Eu3+ were investigated systematically. By altering Eu3+ content, tunable emission from blue to red was realized. More importantly, investigation of the thermal stability showed that over 98% of the room-temperature emission intensity was still preserved at 155 °C, which is superior to that of most recently reported Bi3+,Eu3+ co-doped phosphors. These results indicate that this kind of easy fabrication, low-cost and highly stable SYO:Bi3+,Eu3+ are potential candidates for blue-red phosphors for application in UV chip based w-LEDs.

Efficient Blue-emitting Phosphor SrLu2O4:Ce3+ with High Thermal Stability for Near Ultraviolet (~400\u2009nm) LED-Chip based White LEDs

Blue-emitting phosphors for near ultraviolet (NUV) based tri-color RGB phosphor blend converted white light emitting diodes (LEDs) have been extensively investigated in the past few years. LED chip peaked near 400 nm is the most efficient among the NUV chips currently. However, most of blue phosphors show inefficient excitation around 400 nm. Herein, a novel blue phosphor SrLu2O4:Ce3+ matching well with near 400 nm chip and showing high thermal stability has been developed. The photoluminescence spectrum presents a broad emission band peaking at 460 nm with a bandwidth of nearly 90 nm. By optimizing the Ce3+ concentration, an internal quantum efficiency (IQE) as high as 76% was achieved. Furthermore, 86% of the room-temperature emission intensity is still maintained at 150 °C, indicating a good thermal stability and practicality. A series of white LEDs were fabricated based on 405 nm chips coated with a blend of the new blue phosphor with the commercial yellow and red phosphors. High color rendering indexes (≥90) were achieved while the correlated color temperature was tuneable in the range of 3094 to 8990 K. These results suggest that SrLu2O4:Ce3+ can be utilized as a blue-emitting phosphor in NUV based white LEDs.

Femtosecond laser-induced size reduction and emission quantum yield enhancement of colloidal silicon nanocrystals: effect of laser ablation time

Colloidal silicon (Si) nanocrystals (NCs) with different sizes were successfully prepared by femtosecond laser ablation under different laser ablation time (LAT). The mean size decreases from 4.23 to 1.42 nm by increasing the LAT from 30 to 120 min. In combination with structural characterization, temperature-dependent photoluminescence (PL), time-resolved PL and PL excitation spectra, we attribute room-temperature blue emissions peaked at 405 and 430 nm to the radiative recombination of electron–hole pairs via the oxygen-deficient centers related to Si–C–H2 and Si–O–Si bonds of colloidal Si NCs prepared in 1-octene, respectively. In particular, the measured PL quantum yield of colloidal Si NCs has been enhanced significantly from 23.6% to 55.8% by prolonging the LAT from 30 to 120 min.

IR Emission of Single-Crystal Silicon Excited by Broadband Light

Room-temperature mid-IR emission from single-crystal silicon exposed to broadband (320 to 700 nm) light with power up to 150 m W has been revealed by Fourier transform IR emission spectroscopy. It is shown that broadband optical excitation significantly enhances IR emission in comparison with thermal emission. Spectral lines of lattice vibrations caused by interaction between combinations of longitudinal and transverse optical or acoustic phonons are identified in experimental IR emission spectra.

Magnetoluminescence in a Photostable, Brightly Luminescent Organic Radical in a Rigid Environment

We investigated the emission properties of a photostable luminescent organic radical, (3,5-dichloro-4-pyridyl)bis(2,4,6-trichlorophenyl)methyl radical (PyBTM), doped into host molecular crystals. The 0.05 wt %-doped crystals displayed luminescence attributed to a PyBTM monomer with a room-temperature emission quantum yield of 89 %, which is exceptionally high among organic radicals. The 10 wt %-doped crystals exhibited both PyBTM monomer and excimer-centered emission bands, and the intensity ratio of these two bands was modulated drastically by applying a magnetic field of up to 18 T at 4.2 K. This is the first observation of a magnetic field affecting the luminescence of organic radicals, and we also proposed a mechanism for this effect.

Catalyst-free growth of lateral InAs nanowires

We demonstrate a catalyst-free method to grow lateral InAs nanowires on GaAs (1 0 0) substrate by means of molecular beam epitaxy. By applying pre-surface treatment under oxygen plasma, lateral InAs nanowires with lengths of 1–2 μm and width of approximately 30–80 nm are epitaxial grown along [11¯0] direction. Stacking faults are not observed in the epitaxial process, which is usually an issue for InAs nanowires grown vertically on (1 1 1) substrates. Photo-luminescent measurements were performed for both single and multiple layers of InAs nanowires. A spectrum peak at the wavelength of 1625 nm is observed for a single wire at 5 K and room temperature emission is obtained for three layers of InAs nanowires. In addition, InAsSb nanowires are achieved along [1 1 0] direction, with a length of 0.4–0.8 μm and a width of 60–80 nm.

Hollow-sphere SrTiO3 nanocube assemblies with enhanced room-temperature photoluminescence

A porous SrTiO3 (STO) three-dimensional (3D) hierarchical structure is fabricated by a rapid “TEG-sol” method under atmospheric pressure. The STO architecture is characterized by the hollow-sphere assemblies of well-crystallized sub-10 nm cubes. The formation kinetics of the STO hierarchical structure reveals that alkalinity of the reaction solvent plays a key role in the formation and self-assembly of the nanocubes. Owing to the large and highly-active specific surface area, the nanocubes present significantly enhanced room-temperature white photoluminescence (PL) emission. Moreover, the hollow-sphere STO nanocube assemblies with unique hierarchical structure show enhanced room-temperature PL emission, which provides a high-efficiency white PL material for the oxide-based optoelectronic devices. This method possesses mild reaction condition, rapid formation (2 h) and high efficiency, and the as-obtained nanopowders have a high purity, which is promising to be enlarged for the low-cost industrial production of perovskite oxides with well-controlled morphology and fine nanostructures.

Coupling a Single Nitrogen-Vacancy Center in Nanodiamond to Superparamagnetic Nanoparticles

Single nitrogen-vacancy (NV) defect centers in diamond have been exploited as single photon sources and spin qubits due to their room-temperature robust quantum light emission and long electron spin coherence times. They were coupled to a manifold of structures, such as optical cavities, plasmonic waveguides, and even injected into living cells to study fundamental interactions of various nature at the nanoscale. Of particular interest are applications of NVs as quantum sensors for local nanomagnetometry. Here, we employ a nanomanipulation approach to couple a single NV center in a nanodiamond to a single few-nm superparamagnetic iron oxide nanoparticle in a controlled way. After measuring via relaxometry the magnetic particle spin-noise, we take advantage of the crystal strain ms = ± 1 spin level separation to detect the superparamagnetic particle’s effect in presence of a driving AC magnetic field. Our experiments provide detailed insight in the behavior of such particles with respect to high frequency fields. The approach can be extended to the investigation of increasingly complex, but controlled nanomagnetic hybrid particle assemblies. Moreover, our results suggest that superparamagnetic nanoparticles can amplify local magnetic interactions in order to improve the sensitivity of diamond nanosensors for specific measurement scenarios.

Spectroscopic investigation of Er3+ doped BaF2 single crystal

We report on the spectroscopic investigation of Er3+ doped BaF2 single crystals. High-quality crystals have been gown by the Bridgman-Stockbarger pulling technique. The room temperature absorption, excitation, emission and fluorescence decay spectra of the luminescent Er3+ ions inserted in BaF2 fluoride single crystals have been investigated. Using the Judd-Ofelt (JO) theory, the intensity parameters of Er3+ ions have been calculated to be Ω2 = 0.949 × 10−20 cm2, Ω4 = 0.975 × 10−20 cm2 and Ω6 = 1.258 × 10−20 cm2. These parameters were then used to calculate the radiative transition probabilities (AJJ′), branching ratios (βJJ′) and radiative lifetimes (τrad) of the main laser emitting levels of Er3+ ions. The obtained spectroscopic properties are compared to those of Er3+ transitions in other fluoride and oxide hosts. The excitation spectrum in the UV–Visible spectral range is very close to the absorption spectrum indicating that all observed absorption levels can excite the Erbium green emission. The emission spectrum is mainly dominated by the green emission alongside a weak red emission. For the main transitions, there is a good agreement between the emission spectrum and the spontaneous emission probabilities given by the JO analysis. Using the Fuchtbauer–Ladenburg method, the emission cross-section of the three main visible emission were determined in addition to other important laser parameters such radiative quantum efficiency and optical gain.