Clear Photoluminescence Research Articles

Low-temperature photoluminescence measurement with a micromachined Joule-Thomson cooler

This study evaluates the effectiveness of micromachined Joule-Thomson (MJT) cooling for photoluminescence (PL) materials. Achieving low temperatures is crucial for enhancing PL performance in semiconductors. However, the commonly used liquid nitrogen (LN2) cryostats require frequent refills, hindering their long-term operation. The MJT cooler offers a potential solution by enabling integration with devices and longer operating time. To validate its effectiveness, this study conducted low-temperature PL measurements using a nitrogen MJT cooler. A MAPbI3 thin film was used as the characterization sample owing to its clear PL mechanism. The experiment successfully preserved its temperature-dependent PL property, with an observed orthorhombic phase-change phenomenon between 155-165 K. Furthermore, the system demonstrated short cool-down time (<1 h), minimal temperature impact from laser stimulus (<±0.1 K), sample storage stability, and low coolant consumption.

Fractionation of beech wood cell walls into digestible cellulose-rich residues and photoluminescent lignin-rich precipitates via semi-flow hot-compressed water treatment with 2-naphthol.

Utilization of cell wall components of woody biomass has attracted attention as alternatives for fossil fuels towards a sustainable society. A semi-flow hydrothermal treatment was used to fractionate the beech (Fagus crenata) wood into cellulose-rich residues and lignin-rich precipitates. The enzymatic saccharification of the cellulose component in the residue was enhanced significantly because the preferential delignification from the secondary wall increased enzyme accessibility. Meanwhile, the precipitated lignin was soluble in organic solvent and exhibited clear photoluminescence (PL) according to the chromophore distances. Furthermore, the carbocation scavenger, 2-naphthol, was impregnated into the beech wood to inhibit the lignin re-condensation reaction. As a result, the digestibility of the cellulose component in the residue increased because unproductive enzymatic binding of lignin and lignin re-condensation were both suppressed. In addition, the PL intensity of the precipitates was significantly enhanced, indicating that 2-naphthol bound to the lignin molecules influenced the PL properties. Overall, fractionation using a semi-flow hydrothermal treatment efficiently uses both polysaccharides and lignin, especially the impregnation of 2-naphthol provided advantages for both saccharides and lignin. Monosaccharides can be converted into valuable products via a sugar platform, and the lignin precipitates exhibit useful PL properties that give them significant potential as a feedstock for numerous valuable materials, such as fluorescence reagents and spectral conversion agents. The results presented herein provide insights that are crucial for the comprehensive utilization of cell wall components for sustainable biorefinery systems.

Scintillation and dosimeter properties of Pr2O3-doped Ga2O3–K2O–La2O3 glasses

Rare-earth-activated glasses have attracted research attention for radiation measurements for decades because of several practical merits, including their amenability towards mass production, large-volume fabrication, easing shaping, and their low cost. Various glass materials (e.g., borate and silicate glasses) have been characterized. In this study, photoluminescence, scintillation, and thermally stimulated luminescence (TSL) characteristics were observed for Pr3+-activated gallate glasses (69Ga2O3–20K2O–(11-x)La2O3–xPr2O3). Clear photoluminescence and scintillation peaks derived from the 4f-4f transitions of Pr3+ appeared in the range of 480–670 nm, where the photoluminescence and scintillation decay time constants attributable to the 4f-4f transitions of Pr3+ were on the order of microseconds. The thermoluminescence of these glasses was also investigated, where a broad TSL glow peak was detected at approximately 90°C. The lowest measurable dose limit is approximately 1 mGy. Moreover, isothermal decay curve analysis suggested that nine trap levels are associated with the TSL.

Membrane potential sensing: Material design and method development for single particle optical electrophysiology.

We review the development of "single" nanoparticle-based inorganic and organic voltage sensors, which can eventually become a viable tool for "non-genetic optogenetics." The voltage sensing is accomplished with optical imaging at the fast temporal response and high spatial resolutions in a large field of view. Inorganic voltage nanosensors utilize the Quantum Confined Stark Effect (QCSE) to sense local electric fields. Engineered nanoparticles achieve substantial single-particle voltage sensitivity (∼2% Δλ spectral Stark shift up to ∼30% ΔF/F per 160 mV) at room temperature due to enhanced charge separation. A dedicated home-built fluorescence microscope records spectrally resolved images to measure the QCSE induced spectral shift at the single-particle level. Biomaterial based surface ligands are designed and developed based on theoretical simulations. The hybrid nanobiomaterials satisfy anisotropic facet-selective coating, enabling effective compartmentalization beyond non-specific staining. Self-spiking- and patched-HEK293 cells and cortical neurons, when stained with hybrid nanobiomaterials, show clear photoluminescence intensity changes in response to membrane potential (MP) changes. Organic voltage nanosensors based on polystyrene beads and nanodisk technology utilize Fluorescence (Förster) Resonance Energy Transfer (FRET) to sense local electric fields. Voltage sensing FRET pairs achieve voltage sensitivity up to ∼35% ΔF/F per 120 mV in cultures. Non-invasive MP recording from individual targeted sites (synapses and spines) with nanodisks has been realized. However, both of these QCSE- and FRET-based voltage nanosensors yet need to reach the milestone of recording individual action potentials from individual targeted sites.

Singlet and Triplet Excited-State Dynamics of a Nonfullerene Electron Acceptor Y6

Understanding the excited-state dynamics of nonfullerene electron acceptors is essential for further improvement of organic solar cells. Herein, we investigated the singlet and triplet excited-state dynamics in Y6, a novel nonfullerene acceptor, using transient absorption spectroscopy. We found that pristine Y6 films show biphasic singlet exciton decay kinetics with decay constants of ~220 ps and ~1200 ps, which is the origin of the large discrepancies in the previously reported exciton lifetimes in the solid state. The majority of the Y6 singlet excitons decayed with the faster (~220 ps) component, whereas a clear photoluminescence with the slower (~1200 ps) component was observed. Y6 singlet excitons undergo fast diffusion in the crystalline domains, resulting in fast singlet–singlet exciton annihilation, after which ultrafast triplet formation, assigned to singlet fission from higher excited singlet states, is observed.

Manganese Fluoride Nanoparticles Synthesized by Microwave Irradiation Using Ionic Liquid–Ethylene Glycol Mixtures: Room-Temperature Photoluminescence, Crystalline Phase, and Morphology

The facile and rapid microwave-assisted method has been demonstrated to be effective in synthesizing manganese fluoride (MnF2) nanoparticles in binary mixtures of ethylene glycol and imidazolium-based ionic liquid containing fluorine, within a minute, without any surfactant and stabilizer. The effect of volume ratio of an ionic liquid in the binary mixtures with ethylene glycol and the length of the alkyl chain of imidazolium on the morphologies and crystalline structures of MnF2 was investigated. Two types of crystalline phases MnF2, P42/mnm rutile and P4̅2m space group, were synthesized depending on the volume ratio of EMIM BF4. The morphologies of rutile MnF2 clearly varied depending on what kind of ionic liquid was used in the synthesis. MnF2 nanoparticles and spherical nanoclusters assembled by nanospheres were synthesized when the used ionic liquid was OMIM BF4 and EMIM BF4, respectively. Clear room temperature photoluminescence (PL) spectra of MnF2 nanoparticles were observed at ambient pressure. The change of PL spectra depending on the crystalline phase was also observed. In this study, we report that the microwave-assisted method using ethylene glycol–ionic liquid binary mixtures is proper for synthesizing high-quality MnF2 nanoparticles with controllable crystalline phase and morphology.

Quantum-confined stark effect in the ensemble of phase-pure CdSe/CdS quantum dots.

Colloidal semiconductor quantum dots (QDs) have recently attracted great attention in electric field sensing via the quantum-confined Stark effect (QCSE), but they suffer from the random local electric field around the charged QDs through the Auger process or defect traps. Here, QCSE in the ensemble of phase-pure wurtzite CdSe/CdS QDs was studied by applying a uniform external electric field. We observed clear field-dependent photoluminescence (PL) and absorption characteristics in thick-shell CdSe/CdS QDs with 11 CdS monolayers (11 MLs) including a pronounced spectral redshift in PL of ∼2.3 nm and absorption of ∼2.1 nm. The time-dependent PL intensity traces implied that the thick-shell QDs were conducive to realize the Stark shift in QD ensembles due to the effective suppression of the main sources of the local field. These findings were in stark contrast to those of moderate-shell (5 MLs) and ultrathick-shell (15 MLs) CdSe/CdS QDs. The measurement value of exciton polarizability was smaller than the theoretical value, which may be influenced by very few exciton traps. Moreover, the amplified stimulated emission also exhibited obvious optical modulations under the electric field with decreased emission intensity and an increased ultrafast lifetime. Finally, the temporal evolution of the multiexciton process in thick-shell CdSe/CdS QDs indicated that the multiexciton state induced a higher energy state near the band edge, which may weaken the QCSE of a single exciton. Therefore, it was demonstrated that efficient field control over the optical properties of these nanomaterials is feasible and this can open up potential applications in field-controlled electro-optic modulators.

Size Fractionation of Fluorescent Graphene Quantum Dots Using a Cross-Flow Membrane Filtration System.

Graphene quantum dots (GQDs) have received great attention as optical agents because of their low toxicity, stable photoluminescence (PL) in moderate pH solutions, and size-dependent optical properties. Although many synthetic routes have been proposed for producing GQD solutions, the broad size distribution in GQD solutions limits its use as an efficient optical agent. Here, we present a straightforward method for size fractionation of GQDs dispersed in water using a cross-flow filtration system and a track-etched membrane with cylindrical uniform nanopores. The GQD aqueous suspension, which primarily contained blue-emitting GQDs (B-GQDs) and green-emitting GQDs (G-GQDs), was introduced to the membrane in tangential flow and was fractionated with a constant permeate flow of about 800 L m−2 h−1 bar−1. After filtration, we observed a clear blue PL spectrum from the permeate side, which can be attributed to selective permeation of relatively small B-GQDs. The process provided a separation factor (B-GQDs/G-GQDs) of 0.74. In the cross-flow filtration system, size-dependent permeation through cylindrical nanochannels was confirmed by simulation. Our results demonstrate a feasible method facilitating size fractionation of two-dimensional nanostructures using a cross-flow membrane filtration system. Since membrane filtration is simple, cost-effective, and scalable, our approach can be applied to prepare a large amount of size-controlled GQDs required for high performance opto-electronics and bio-imaging applications.

Exploring the optimum growth conditions for InAs/GaSb and GaAs/GaSb superlattices on InAs substrates by metalorganic chemical vapor deposition

The effect of growth parameters in metalorganic chemical vapor deposition (MOCVD) on the crystal quality and surface morphology of the InAs/GaSb superlattices (SLs) on InAs substrates was systematically investigated by X-ray diffraction, scanning transmission electron microscopy, atomic force microscopy and photoluminescence. It was found that InAs/GaSb SLs grown at 530 °C with intentionally-formed GaAs-type interfaces exhibited lattice mismatch as low as −0.01%, abrupt SL interfaces and atomically smooth surface with a root mean square roughness of only 0.136 nm. Clear photoluminescence was observed around 8 μm at 77 K for a long-wavelength SL structure. The nominal thickness of the GaAs-type interfacial layers and the overall SL strain can be effectively controlled by tuning the AsH3 flow rate. In addition, high-quality GaAs/GaSb SLs on InAs substrate were demonstrated as a replacement of the GaAsSb alloys. An AsH3-purge step was introduced after GaSb growth to form GaAs layers via As-Sb exchange. An optimal condition has resulted in lattice-mismatch of 0.21% and surface roughness of 0.122 nm for the GaAs/GaSb SLs.

Atomically uniform Sn-rich GeSn semiconductors with 3.0–3.5 μm room-temperature optical emission

The simultaneous control of lattice strain, composition, and microstructure is crucial to establish high-quality, direct bandgap GeSn semiconductors. Herein, we demonstrate that multilayer growth with a gradual increase in composition is an effective process to minimize bulk and surface segregation and eliminate phase separation during epitaxy yielding a uniform Sn incorporation up to ∼18 at. %. Detailed atomistic studies using atom probe tomography reveal the presence of abrupt interfaces between monocrystalline GeSn layers with interfacial widths in the 1.5–2.5 nm range. Statistical analyses of 3-D atom-by-atom maps confirmed the absence of Sn precipitates and short-range atomic ordering. Despite the residual compressive strain of −1.3 %, the grown layers show clear room-temperature photoluminescence in the 3.0–3.5 μm wavelength range originating from the upper GeSn layer with the highest Sn content. This finding lays the groundwork to develop silicon-compatible mid-infrared photonic devices.

Langmuir-Blodgett self-assembly of ultrathin graphene quantum dot films with modulated optical properties.

Multiple-color-emissive graphene quantum dots (GQDs) have great potential in diverse applications such as bioimaging, light emission, and photocatalysis. Growing interest in GQDs is largely focused on their macro-scale aggregations that could inherit the unique properties of individual dots. However, the lack of advanced fabrication methods limits the practical applications of GQDs. Here, we employed a Langmuir-Blodgett (LB) technique to fabricate ultrathin, high-quality GQD aggregated films with well-modulated optical properties in a wide range of wavelengths. Through the combination of a bottom-up synthesis of GQDs and the LB assembly method, uniform, closely packed, and ultra-thin GQD films can be self-assembled with a well-controlled thickness on different substrates. The photoluminescence (PL) spectra of ultra-thin GQD films have an obvious red-shift compared with isolated GQD solution. We then elucidate remarkably strong energy transfer in self-assembled GQDs. Furthermore, the ultra-thin GQD films exhibit a clear excitation-dependent PL that could almost cover the entire visible light. This convenient self-assembly method and systematic optical and physical studies of ultra-thin GQD films may provide a new direction for developing low-cost, GQD film-based light-emitting devices.

Supramolecular Cross-Link-Regulated Emission and Related Applications in Polymer Carbon Dots.

Involvement of clear photoluminescence (PL) mechanism in specific chemical structure is at the forefront of carbon dots (CDs). Supramolecular interaction exists in plenty of materials, offering an inherent way to administrate the optical and photophysical properties, especially in terms of newly developed polymer carbon dots (PCDs). However, supramolecular-interaction-derived PL regulation is always ignored in the shadow of many kinds of PL factors, and we still have a limited understanding on the distinct chemical structure and mechanism of supramolecular effect in PCDs. Herein, several distinct photoluminescent phenomena of PCDs under aqueous and solid state are reviewed in terms of supramolecular cross-linking, with highly emphasizing the importance of supramolecular cross-link-enhanced emission (SCEE) effects, and the regulated function of supramolecular interaction's intensity and types between PCDs for special PL behaviors of PCDs. In addition, we categorize the photoluminescent phenomena in PCDs into the following aspects: supramolecular cross-link-enhanced dilute-solution-state emission, concentration-controlled multicolor emission, supramolecular regulation for quenching-resistant solid-state fluorescence, as well as supramolecular cross-link-assisted room-temperature- phosphorescence (RTP) under solid states. Furthermore, the applications of PCDs in light-emitting diodes (LED), solar cells, and anticounterfeiting and data encryption, etc., are presented, based on the distinct supramolecular cross-link-regulated photoluminescent phenomena, especially the solid-state emission. Finally, a brief outlook is given, highlighting the currently existing problems and development direction of supramolecular cross-link-regulated emission in PCDs.

Tuning the Photoluminescence of Graphene Quantum Dots by Photochemical Doping with Nitrogen.

Nitrogen-doped graphene quantum dots (NGQDs) were synthesized by irradiating graphene quantum dots (GQDs) in an NH3 atmosphere. The photoluminescence (PL) properties of the GQDs and the NGQDs samples were investigated. Compared with GQDs, a clear PL blue-shift of NGQDs could be achieved by regulating the irradiating time. The NGQDs obtained by irradiation of GQDs for 70 min had a high N content of 15.34 at % and a PL blue-shift of about 47 nm. This may be due to the fact that photochemical doping of GQDs with nitrogen can significantly enhance the contents of pyridine-like nitrogen, and also effectively decrease the contents of oxygen functional groups of NGQDs, thus leading to the observed obvious PL blue-shift.

Optical studies on a single GaAs laterally coupled quantum dot in comparison with an uncoupled quantum dot

Abstract We performed spectroscopy studies on a single GaAs laterally coupled quantum dot and an uncoupled quantum dot. Photoluminescence spectra confirmed the presence of optical coupling in the coupled quantum dot through dipole–dipole interactions. The optical coupling was investigated in terms of the integrated photoluminescence intensities and redshift of emission energies as the excitation power was increased. The excitation intensity was increased with linearly polarized light in the lateral coupling direction 1 1 ¯ 0 , which resulted in excitons X 1 and X 2 of the coupled quantum dot showing a clear photoluminescence peak shift to lower energy with drastically different power factors. These results were obtained by integrating the PL spectrum and comparing it to that of a single quantum dot. We also found that the decay rates of X 1 and X 2 in the coupled quantum dot increased significantly as a consequence of overlap of electron and hole wavefunctions extended via optical coupling in individual dots of the coupled quantum dot system.

Electrical and optical properties improvement of GeSn layers formed at high temperature under well-controlled Sn migration

Electrical and optical properties of GeSn layers formed at various growth conditions under changing deposition temperature (Td) and deposition speed (vd) were systematically investigated. A high Sn content of 3.0% leads to high electron mobility and electron concentration for an as-deposited sample compared with the Sn content of 1.9%. Subsequent annealing at 550°C after the growth is effective for improving the mobility and the activated carrier concentration. Moreover, the high vd and Td growth makes it possible to get clear photoluminescence (PL) signal from the band-to-band transition of the GeSn layers. The annealing at 550°C leads to the high and sharp PL spectra compared with those for the as-deposited samples. Consequently, we found that the high vd and Td growth of the GeSn layers suppressing the Sn migration is quite important for getting electrical and optical properties to realize future electrical and optical devices.

Enhancement of photoluminescence from nanocrystal β-FeSi2/SiO2 composite and relaxation of thermal quenching

We have investigated the thermal quenching behavior of photoluminescence (PL) from β-FeSi2 (β-NC) embedded in Si (β-NC/Si) and SiO2 (β-NC/SiO2). The β-NC/SiO2 composite was prepared directly from the β-NC/Si composite by selective oxidation. In the β-NC/SiO2 composite, we found an increase in the critical temperature, which indicates the relaxation of thermal quenching for PL intensity. Furthermore, we observed a clear PL spectrum including the intrinsic A band PL at 300 K; however, the PL intensity was extremely low. Rutherford backscattering spectrometry (RBS) and photocarrier injection PL (PCI-PL) measurements revealed the reason why the β-NC/Si composites were maintained after oxidation. We discussed the thermal quenching behavior of both samples on the basis of a thermal activation model of holes from valence band wells at the heterointerface and confirmed that this model was appropriate for understanding the thermal quenching of these composites.

High-temperature photoluminescence and photoluminescence excitation spectroscopy of Al0.60Ga0.40N/Al0.70Ga0.30N multiple quantum wells

The excitonic optical properties of an Al0.60Ga0.40N/Al0.70Ga0.30N multiple quantum well (MQW) structure were studied using photoluminescence (PL) and PL excitation (PLE) spectroscopy at high temperatures. Clear excitonic PL was observed at temperatures up to 750 K. Biexciton luminescence was clearly observed even at this high temperature. These observations unambiguously demonstrated the extremely high thermal stability of biexcitons in this MQW. Furthermore, additional PL peaks were observed on the low-energy side of the biexciton luminescence. The observation of biexciton two-photon resonance in the PLE spectra of these peaks indicates that these peaks can be explained by processes involving inelastic scattering of excitons and biexcitons.

Electrical and optical properties improvement of GeSn layers formed at high temperature under well-controlled Sn migration

Electrical and optical properties of GeSn layers formed at various growth conditions under changing deposition temperature (Td) and deposition speed (vd) were systematically investigated. A high Sn content of 3.0% leads to high electron mobility and electron concentration for an as-deposited sample compared with the Sn content of 1.9%. Subsequent annealing at 550°C after the growth is effective for improving the mobility and the activated carrier concentration. Moreover, the high vd and Td growth makes it possible to get clear photoluminescence (PL) signal from the band-to-band transition of the GeSn layers. The annealing at 550°C leads to the high and sharp PL spectra compared with those for the as-deposited samples. Consequently, we found that the high vd and Td growth of the GeSn layers suppressing the Sn migration is quite important for getting electrical and optical properties to realize future electrical and optical devices.

Defects as a factor limiting carrier mobility in WSe2: A spectroscopic investigation

The electrical performance of two-dimensional transition metal dichalcogenides (TMDs) is strongly affected by the number of structural defects. In this work, we provide an optical spectroscopic characterization approach to correlate the number of structural defects and the electrical performance of WSe2 devices. Low-temperature photoluminescence (PL) spectra of electron-beam-lithographyprocessed WSe2 exhibit a clear defect-induced PL emission due to excitons bound to defects, which would strongly degrade the electrical performance. By adopting an electron-beam-free transfer-electrode technique, we successfully prepared a backgated WSe2 device containing a limited amount of defects. A maximum hole mobility of approximately 200 cm2·V–1·s–1 was achieved because of the reduced scattering sources, which is the highest reported value for this type of device. This work provides not only a versatile and nondestructive method to monitor the defects in TMDs but also a new route to approach the room-temperature phonon-limited mobility in high-performance TMD devices.

Si/SiGe Heterointerfaces in One-, Two-, and Three-Dimensional Nanostructures: Their Effect on SiGe Light Emission

The development of a light emitter compatible with Si based complementary metal-oxide- semiconductor (CMOS) circuit technology and fast optical interconnects is important for the new generations of microprocessors and computers. Self-assembled Si/Si1-xGex nanostructures (NSs) with light emission in the important optical communication wavelength range of 1.3 – 1.55 μm are compatible with conventional CMOS processes. However, the predicted and experimentally confirmed long carrier radiative lifetimes in Si and Si/Si1-xGex NSs impede the demonstration of efficient light-emitting devices and lasers. Thus, engineering of Si/Si1-xGex heterostructures with controlled composition and interface abruptness is critical in producing the desired fast and efficient photoluminescence (PL) peaked at around 0.8-0.9 eV. In this paper we assess how the nature of the interfaces between SiGe NSs and Si in heterostructures strongly affects carrier mobility and recombination for physical confinement in one dimension (corresponding to the case of quantum wells), two dimensions (quantum wires), and three dimensions (quantum dots). The interface sharpness is influenced by many factors such as growth conditions, strain, and thermal processing, which in practice can make it difficult to attain the ideal structures required. This is certainly the case for NS confinement in one dimension. However, we demonstrate that axial Si/Ge nanowire (NW) heterojunctions (HJs) with a Si/Ge NW diameter in the range 50 – 120 nm produce a clear PL signal associated with band-to-band electron-hole recombination at the NW HJ that is attributed to a specific interfacial SiGe alloy composition. For three-dimensional confinement, the experiments outlined here show that two quite different Si1-xGex NSs incorporated into a Si0.6Ge0.4 wavy structure exhibit an intense PL signal with a characteristic decay time as much as 1000 times shorter than that observed in conventional Si/SiGe NSs. The experimentally observed non-exponential PL decay in Si/SiGe NSs is explained as being due to variations of the distances separating electrons and holes at the Si/SiGe heterointerface. The results demonstrate that an abrupt Si/SiGe heterointerface reduces the carrier radiative recombination lifetime and increases the PL quantum efficiency making these SiGe NSs promising candidates for applications in CMOS compatible light-emitting devices.