Photoluminescence Centers Research Articles

The one-pot fabrication of green-emitting composites based on PMMA

Photoluminescent composites consisting of a photoluminescent material dispersed in a suitable matrix have been applied in many applications, such as light-emitting diodes, solar concentrators, and anti-counterfeiting inks. The traditional method for the fabrication of composites by blending an as-synthesized photoluminescent material and a matrix is very challenging as it is difficult to obtain homogeneous composites. In this study, we have demonstrated a one-step method to prepare homogeneous composites by inducing the formation of in-situ photoluminescent centers in a stable matrix. Poly(methyl methacrylate) (PMMA) coated with o-phenylenediamine (oPD) was thermally annealed at 165oC for a duration of 5 minutes in an extruder to obtain green-emitting composites. The composites exhibited a broad absorption peak at 425 nm and an absorption shoulder at 495 nm. The emission spectrum of the composite was broad, ranging from 400 nm to 700 nm, and reached the maximum at 525 nm. The photoluminescent maximum position was independent of the excitation wavelength. The photoluminescent excitation spectrum of the composite resembled the absorption near 425 nm. Time-dependent density functional theory (TD-DFT) calculations suggested that 2,3-diaminophenazine and 3-amino-2-hydroxyphenazine are the main molecular fluorophores accounting for the optical properties of the composites. The synthetic method demonstrated in this study is transferable for preparing numerous photoluminescent thermoplastics.

Achieving Color‐Tunable Solid‐State Fluorescence by Adjusting the Molecular‐State Chromophores on Carbonized Polymer Dots

Carbon dots (CDs) have attracted great interest in recent years due to their excellent fluorescent properties, convenient synthetic methods, and wide availability of source materials. However, although most CDs exhibit excellent luminescent properties in solution, achieving solid‐state fluorescence (SSF) CDs is still difficult due to aggregation‐caused quenching, and realizing color‐tunable SSF‐CDs remains a serious challenge. Herein, the SSF‐carbonized polymer dots (CPDs) are successfully obtained by one‐step hydrothermal of polyethyleneimine (PEI) and citric acid. The photoluminescence centers of these CPDs are molecular‐state chromophores, which are proved as imidazo [1,2‐a] pyridine‐7‐carboxylic acid, 1,2,3,5‐tetrahydro‐5‐oxo‐ (IPCA) derivatives by NMR. Further investigation reveals that the formation and distribution of IPCA derivatives would affect the bandgaps when CPDs particles aggregate, which is also verified by density functional theory calculations. Based on the above conclusion, the blue, green, yellow, and orange SSF CPDs are successfully obtained, and the multicolor and white emission light emitting diodes are effectively fabricated by these CPDs.

Triethoxysilane-derived silicon quantum dots: A novel pathway to small size and high crystallinity

The crystalline fraction is a critical parameter for assessing the quality of silicon quantum dots (SiQDs), and its enhancement is anticipated to improve the optoelectronic performance of these materials. However, the crystalline fraction of small SiQDs produced through the pyrolysis of hydrogen silsesquioxane (HSQ) polymers still has significant potential for improvement. In this study, we successfully synthesized SiQDs with a diameter of approximately 3 nm and near-perfect crystallinity by optimizing the triethoxysilane (TES)/aqueous hydrochloric acid (HCl) volume ratio during the hydrolysis-condensation process of HSQ polymers. The SiQDs exhibited a photoluminescence (PL) center at 760 nm and an average PL quantum yield (PLQY) of 24.4%. Our findings demonstrate that the TES/aqueous HCl ratio significantly influences the proportion of cage structure and the cross-linking density of the network structure in HSQ polymers, which in turn governs SiQD size and crystallinity. A high proportion of cage structures in HSQ polymers promotes high crystallinity. Notably, an increased cross-linking density within the network structure results in elevated and uniform diffusion barriers. This phenomenon not only hinders the diffusion of silicon atoms, leading to smaller sizes but also facilitates the achievement of high crystallinity due to uniform diffusion. This work presents a novel approach to achieving exceptional crystalline in small SiQDs, with implications for advanced applications in lighting, display technologies, medical imaging, and photovoltaics.

Toward 95.5% Efficient Red Emissive Carbon Dots: Oxidation State Enhancing Radiative Electron-Transition of Indole Fluorophore.

Red fluorescence carbon dots (CDs) are promising for diverse applications and have attracted tremendous research interest. However, it is still challenging to achieve red fluorescence CDs with high fluorescence quantum yields (QYs > 50%). Herein, three kinds of red fluorescence CDs with QYs of 53.48, 85.21, and 59.18% are prepared. Benefiting from the oxidation induced by atmospheric-pressure O2 plasma processing, 95.5% efficient red fluorescence emission is achieved. It is revealed that the indole based fluorophores act as the red-emitting photoluminescence center. The synergistic effect between the C-O-C structure and indole based fluorophores plays a key role in promoting the efficiency of radiative electron transition and controlling the red fluorescence QYs. Additionally, the CDs show promising prospects for in vivo bioimaging and low in vivo toxicity. This work shows a new way for achieving high-efficiency red fluorescence CDs, and it may guide the development of high-performance CDs for diverse applications.

Achieving efficient self-trapped excitons emission by suppressing defect level in Sb3+ doped metal halide

The field of metal halide engineering has made considerable progress, yet additional studies are needed to completely grasp the intriguing principles that govern their luminescence. In this work, we report a novel zero-dimensional (0D) organic-inorganic metal halide hybrid (OIMH) (C3H12N2)2InCl7 (C3H12N2 = 1,3-propane diamine cation) and reveal its interesting luminescent properties through experimental research and theoretical calculations. The photoluminescence (PL) of (C3H12N2)2InCl7 is a synergistic combination of intrinsic self-trapped exciton (STE) emission and defect-bound exciton (DBE) emission. Doping with Sb3+ induces several changes in (C3H12N2)2InCl7, including defect passivation, a reduction in the band gap, and structural shrinkage. These modifications result in a transition of PL centers to external STE emission, with the photoluminescence quantum yield (PLQY) increasing from 4.44 % to 68.70 %. Our work proposes, for the first time, the synergistic emission of STE and DBE and reveals the effect of metal ion doping, providing new insights into the mechanisms of materials exhibiting multifunctional exciton emission.

Optical properties of SiO2 opal crystals decorated with silver nanoparticles

SiO2 opal crystals decorated with silver nanoparticles (AgNPs) have been successfully fabricated. SEM analysis revealed that the SiO2 opals have an ordered region of 25–30 μm2, and AgNPs are located into the gaps between the SiO2 nanospheres. The effect of the plasmons of AgNPs on the photoluminescence of SiO2 opal crystals were investigated. Under excitation of a 325 nm laser, we observed the quenching of the intrinsic defect emission in SiO2 as the concentration of AgNPs increased. The cause of the observed photoluminescence behavior is attributed to the interaction between surface plasmon induced by AgNPs and photoluminescence centers in SiO2 opal crystals. Then, we utilized SiO2 opals crystals decorated with AgNPs as a SERS substrate. The final enhancement factor of the Raman shift was 1.64 × 107, enabling the trace detection limit for a case of malachite green to be as low as 0.05 ppm, with a relative standard deviation of less than 1.75 %. This outcome suggests the potential for direct application of the prepared substrates in ultra-fast chemical analysis.

Thermally stable photoluminescence centers at 1240 nm in silicon obtained by irradiation of the SiO2/Si system

The study of light-emitting defects in silicon created by ion implantation has gained renewed interest with the development of quantum optical devices. Improving techniques for creating and optimizing these defects remains a major focus. This work presents a comprehensive analysis of a photoluminescence line at a wavelength of 1240 nm (1 eV) caused by defects arising from the ion irradiation of the SiO2/Si system and subsequent thermal annealing. It is assumed that this emission is due to the formation of defect complexes WM with trigonal symmetry similar to the well-known W-centers. A distinctive feature of these defects is their thermal resistance up to temperatures of 800 °C and less pronounced temperature quenching compared to the W-line. The difference in the properties of these defect centers and W-centers can be explained by their different defect environments, resulting from the larger spatial separation between vacancies and interstitial atoms diffusing from the irradiated layer. This, in turn, is associated with the difference in the distribution of primary radiation defects during irradiation of the SiO2/Si system and silicon not covered with a SiO2 film. The patterns of changes in the WM line depending on various factors, such as the thickness of the SiO2 film, type of conductivity and impurity concentration in the original silicon, irradiation parameters, and annealing regimes, is studied and explained in detail. These findings demonstrate the benefits of this new approach when compared to previous methods.

Photoluminescence and TL defect center analysis of radiation shielding Ba(25-x)La15Si60Nix glasses

Photoluminescence and TL defect center analysis of radiation shielding Ba(25-x)La15Si60Nix glasses

Photoluminescence Properties of Graphene Oxide in Non-Aqueous Solvents.

Detailed attention to the study of interaction between graphene oxide (GO) and various different organic fluorophores have been documented in literature as a result of which the impact of GO on the photophysical properties of the fluorophores is well known to the scientific community. However, the photoluminescence (PL) properties of GO in polar aprotic solvents is yet to be established. In this article we report the PL properties of GO in various polar aprotic solvents using various spectroscopic techniques. n-π* transition due to the C=O bonds in the sp3 hybrid regions and π-π* transition due to C=C bonds is the sp2 hybrid are prominent in GO. The presence of quasi-molecules within sp2-sp3 domains which acts as PL centres, and located in the sp3 matrixes of GO are responsible for the PL properties. The present article implies the presence of multiple emissive states of GO in polar aprotic solvents and conveys the fact that the PL properties of GO are very much wavelength dependent.

Formation of SiV centers by doping in bottom-up grown HPHT nanodiamonds and its implication for optical nanosensig

We investigate doping and formation of luminescent point defects in nanodiamond at high pressures and high temperatures (HPHT) in fluorine-containing hydrocarbon growth systems with an excessive content of dopant organosilicon component. It is shown experimentally that the average concentration of silicon-vacancy centers (SiV-) tends to increase proportionally to the diameter of the nanocrystals. Respectively, the local concentration of the dopant in a crystal linearly increases with the distance from the crystal center. This phenomenon is explained by competition between growth of a perfect crystal and doping. The kinetics of the crystal growth are determined by the diffusion of carbon containing precursors to the surface of the crystal and the doping takes place at a very high concentration of the dopant in the growth mixture and that is defined by the rate of the chemical reaction on the crystal surface. Electrodynamic effects in the nanocrystals affecting the observed intensity of the photoluminescence are also considered. Although these results are obtained using important for applications SiV- photo luminescent centers, they have much wider implications.

The Optical Properties of the Carbon Di-Vacancy-Antisite Complex in the Light of the TS Photoluminescence Center

The TS center is a promising temperature-stable photoluminescence center in 4H SiC. Here we investigate the carbon di-vacancy-antisite complex inthe framework of ab initio theory as a tentative model for the TS center. We identify optical transitionsof the basal complexes with the TS lines based on excitation energies, Stark shifts, and radiative char acteristics. Charge-state-control of the TS center in p- and n-type Schottky contacts is demonstrated. Our experimental findings are consistent with the positively charged complex.

Preparation of High‐Intensity Fluorescent Black Phosphorus Nanosheets

Black phosphorus (BP) nanosheets have been widely used in the field of electronic devices for their exceptional qualities, but the preparation and optical properties of black phosphorus nanosheets need to be further explored. Herein, high fluorescence intensity black phosphorus nanosheets are prepared by a simple and economical solvothermal method using red phosphorus (RP) and ethylenediamine as raw materials. The physical structure and morphology of black phosphorus nanosheets crystals are characterized by X‐ray diffractometer (XRD), Raman spectrometer, transmission electron microscope (TEM), and atomic force microscope (AFM), and the effect of the concentration of raw material to solvent ratio on the structure and optical properties of the prepared black phosphorus nanosheets are also investigated. Black phosphorus nanosheets containing mostly monolayer structures with transverse dimensions of about 200 nm are obtained using the solvothermal method. When the preparation parameter is RP/EDA = 30 g L−1, the room‐temperature photoluminescence center of the black phosphorus nanosheets is 470 nm with superior luminescence performance.

Communication of molecular fluorophores with other photoluminescence centres in carbon dots.

The establishment of structure-photoluminescence (PL) relationships remains an ultimate challenge in the field of carbon dots (CDs). It is now commonly understood that various structural domains may evolve during the preparation of CDs; nonetheless, we are still far from capturing the specific features that determine the overall PL of CDs. Although the core, surface and molecular states are usually considered the three main sources of PL, it is not known to which extent they interact and/or affect one another. Expectedly, the communication between the different PL centres depends on the mutual arrangement and the type of linking. To gain insights into such a communication, time-dependent density functional theory (TD-DFT) calculations were performed for several (N-doped/O-functionalized) polyaromatic hydrocarbons (PAHs) as representative models for the core/surfaces PL states and the prototypical molecular fluorophore (MF) 5-oxo-1,2,3,5-tetrahydroimidazo-[1,2-α]-pyridine-7-carboxylic acid (IPCA), considering different interaction modes, namely hydrogen bonded and stacked complexes as well as covalently bonded and fused structures. Our results revealed that each of the studied arrangements in some way supported the communication between the PL centres. The deactivation pathways typically involve multiple charge and energy transfer events that can promote the formation of charge separated states and/or lead to the activation of other PL centres in CDs. Depending on the arrangement, the doping pattern and surface functionalization, both the CD core and the MF can act as an electron donor or acceptor, which could help to design CDs with desirable hole-electron surface/core characteristics.

Люминесценция природных алмазов, индуцированная ультракороткими лазерными импульсами ульрафиолетового диапазона

Обнаружено, что УФ-излучение с длиной волны 257 и 320 nm, не вызывает свечение А-полосы, что связано, по-видимому, с прямым межзонным фотовозбуждением алмаза. In this paper nature crystals of the pink and brown colored diamonds with the evident plastic deformations were comprehensively studied with IR-Fourier spectroscopy and photoluminescence methods. A- and B-centers presence were revealed by IR-Fourier specters. Femtosecond (250 fs) UV-range laser pulses with wavelength of 257, 320, 352 and 365 nm inducted photoluminescence demonstrates N3 and H3 photoluminescence centers and broad structureless A-band luminescence. It was found that UV radiation with a wavelength of 257 and 320 nm does not cause A-band luminescence, which might be due to direct interband photoexcitation of diamond.

Synchrotron-Excited Luminescence and Converting of Defects and Quantum Dots in Modified Silica Films

Optically active defects in modified silicon oxide films on a silicon substrate have been studied by low-temperature photoluminescence (PL) spectroscopy using excitation by synchrotron radiation in the vacuum ultraviolet region. Films of dry thermal silicon oxide obtained by treatment of stoichiometric SiO2 in hydrogen plasma, films of wet thermal silicon oxide, and films with low dielectric constant (so called "low-k" dielectrics) have been studied. Investigations of various type SiO2 films detect the PL centers, which can be conditionally divided into two groups. The first group includes intrinsic point defects, the different variants of oxygen-deficient centers (ODCs). The second group contains centers like the spatially confined excitons in silicon quantum dots (SiQDs). It is shown that SiQDs differ in spectral characteristics, are formed in different ways, due to the transformation and clustering of point defects such as E'-centers, ODC(I) and ODC(II). In this case, the size and spectral properties of quantum dots depend on the mechanism of their formation. Schemes for the conversion of these defects are proposed.It was found that for wet films and nonstoichiometric SiOx films treated in hydrogen plasma, "metastable" centers ODC(I) and ODC(II) arise only under the action of synchrotron radiation in the region of interband or exciton absorption and then they decay because of radiative relaxation. The absence of stable ODC centers in these films is due to their clustering and the formation of SiQDs. In "low-k" mesoporous films, on the contrary, silicon quantum dots SiQDs are not formed, but ODC(I) and ODC(II) centers are clearly manifested. A scheme of electronic transitions upon indirect excitation of silicon quantum dots through exciton states of the SiO2 matrix is considered. The presented results demonstrate the possibility of controlling the optical properties of silicon thin-film structures by creating in them both oxygen-deficient point defects and quantum dots of various sizes.

Facile and cost-effective technique to control europium oxidation states in glassy fluorophosphate matrices with tunable photoluminescence

Inorganic fluorophosphate glasses doped with Eu^{2+}/Eu^{3+} are potential candidates for phosphors for commercial white LEDs. This report presents a fast, inexpensive and effective method of controlling the relative concentrations of Eu^{2+}/Eu^{3+} photoluminescent centers in these glasses. The technique consists of a fast quenching of the melt of initial reagents under appropriate conditions. Eu^{2+}/Eu^{3+} ratio was controlled by carrying out the melting under a reducing atmosphere at a temperature between 1000 and 1200 ^circC for periods of 5 to 15 minutes. The reducing atmosphere was provided by a ’double crucible’ technique and did not require special gas lines during the synthesis. The samples were studied by several complementary experimental methods (X-ray diffractometry—XRD, X-ray photoelectron spectroscopy—XPS, photoluminescence—PL—and photoluminescence excitation—PLE—spectroscopies as well as optical transmission spectroscopy). It was shown that the syntheses resulted in amorphous materials with different relative Eu^{2+}/Eu^{3+} concentration ratios, strongly dependent on the preparation conditions: the temperature and the time of melting in a reducing atmosphere. Moreover, changes in these ratios strongly affected the materials’ PL and PLE spectra. Demonstration of reproducible smooth transition from amaranth to blue luminescence color, with white in between, was the most spectacular result of this work.

Photoluminescence and defect centers in near violet emitting Pb2+ doped CaYAl3O7 phosphors obtained by urea-nitrate auto combustion synthesis

A series of CaYAl3O7 phosphors doped with different concentrations of Pb2+ were prepared by an auto solution combustion route. The structure, luminescence, and electron paramagnetic resonance (EPR) spectra were analysed. The X-ray diffraction (XRD) patterns identify the tetragonal phase formation of CaYAl3O7 with a crystallite size in a range of 37–42 nm. The scanning electron microscope (SEM) images were captured to study the surface morphology of the prepared samples. The emission spectra for the prepared phosphors exhibit a wide emission band centered at 362 nm under 270 nm excitation. Un-doped and Pb2+ doped CaYAl3O7 phosphor exhibit an EPR spectrum which arises from two noticeable defect centers. Center I with a g-value equal to 1.96 and a relatively broad line width is temporarily defined to an F+ - center (an electron trapped at an anion vacancy). Center II with g = 2.0 is also ascribed to an F+ - center.

Modified divacancies in 4H-SiC

Divacancies near or at lattice defects in SiC, the PL5–PL7 photoluminescence centers, are known to have more favorable optical and spin properties for applications in quantum technology compared to the usual divacancies. These centers were previously predicted to be divacancies near stacking faults. Using electron paramagnetic resonance, we observe PL5, PL6, and four other divacancy-like centers, labeled PLa–PLd, in electron-irradiated high-purity semi-insulating (HPSI) 4H-SiC. From the observed fine-structure D-tensors, we show that these centers including PL6, which has so far been believed to be an axial center, all have C1h symmetry. Among these, PLa, PLc, and PLd are basal divacancies and PL5 and PL6 are slightly deviated from axial symmetry, while PLb is different from others with the principal Dzz axis of the D-tensor aligning at ∼34° off the c-axis. We show that these modified divacancies are only detected in one type of HPSI materials but not in commercial n- and p-type substrates or n-type pure epitaxial layers irradiated by electrons regardless of surface treatments which are known to create stacking faults.

Electron–phonon coupling-assisted universal red luminescence of o-phenylenediamine-based carbon dots

Due to the complex core–shell structure and variety of surface functional groups, the photoluminescence (PL) mechanism of carbon dots (CDs) remain unclear. o-Phenylenediamine (oPD), as one of the most common precursors for preparing red emissive CDs, has been extensively studied. Interestingly, most of the red emission CDs based on oPD have similar PL emission characteristics. Herein, we prepared six different oPD-based CDs and found that they had almost the same PL emission and absorption spectra after purification. Structural and spectral characterization indicated that they had similar carbon core structures but different surface polymer shells. Furthermore, single-molecule PL spectroscopy confirmed that the multi-modal emission of those CDs originated from the transitions of different vibrational energy levels of the same PL center in the carbon core. In addition, the phenomenon of “spectral splitting” of single-particle CDs was observed at low temperature, which confirmed these oPD-based CDs were unique materials with properties of both organic molecules and quantum dots. Finally, theoretical calculations revealed their potential polymerization mode and carbon core structure. Moreover, we proposed the PL mechanism of red-emitting CDs based on oPD precursors; that is, the carbon core regulates the PL emission, and the polymer shell regulates the PL intensity. Our work resolves the controversy on the PL mechanism of oPD-based red CDs. These findings provide a general guide for the mechanism exploration and structural analysis of other types of CDs.

Atomic scale memristive photon source

Memristive devices are an emerging new type of devices operating at the scale of a few or even single atoms. They are currently used as storage elements and are investigated for performing in-memory and neuromorphic computing. Amongst these devices, Ag/amorphous-SiOx/Pt memristors are among the most studied systems, with the electrically induced filament growth and dynamics being thoroughly investigated both theoretically and experimentally. In this paper, we report the observation of a novel feature in these devices: The appearance of new photoluminescent centers in SiOx upon memristive switching, and photon emission correlated with the conductance changes. This observation might pave the way towards an intrinsically memristive atomic scale light source with applications in neural networks, optical interconnects, and quantum communication.