Intense Visible Photoluminescence Research Articles

Ultra-Long Lived Luminescent Triplet Excited States in Cyclic (Alkyl)(amino)carbene Complexes of Zn(II) Halides.

The high element abundance and d10 electron configuration make ZnII‐based compounds attractive candidates for the development of novel photoactive molecules. Although a large library of purely fluorescent compounds exists, emission involving triplet excited states is a rare phenomenon for zinc complexes. We have investigated the photophysical and ‐chemical properties of a series of dimeric and monomeric ZnII halide complexes bearing a cyclic (alkyl)(amino)carbene (cAAC) as chromophore unit. Specifically, [(cAAC)XZn(μ‐X)2ZnX(cAAC)] (X=Cl (1), Br (2), I (3)) and [ZnX2(cAAC)(NCMe)] (X=Br (4), I (5)) were isolated and fully characterized, showing intense visible light photoluminescence under UV irradiation at 297 K and fast photo‐induced transformation. At 77 K, the compounds exhibit improved stability allowing to record ultra‐long lifetimes in the millisecond regime. DFT/MRCI calculations confirm that the emission stems from 3XCT/LEcAAC states and indicate the phototransformation to be related to asymmetric distortion of the complexes by cAAC ligand rotation. This study enhances our understanding of the excited state properties for future development and application of new classes of ZnII phosphorescent complexes.

Виготовлення та дослідження властивостей бактеріальної наноцелюлози

Nanocellulose is a biopolymer that due to its attractive physicochemical properties has been intensively studied as a material for use in biomedicine, food industry, electronics etc. Modern chemical methods of nanocellulose production from wood raw materials require the use of acids, alkalis and solvents. This is a disadvantage from both economic and environmental points of view. The biomass that is obtained as a result of microbial processes can be regarded as an alternative source of nanocellulose. This paper deals with the application of the method based on Kombucha membranes for the preparation of bacterial nanocellulose. The structure and optical properties of the obtained films of bacterial nanocellulose have been studied by X-ray diffraction analysis and luminescence spectroscopy. The difference in the sizes of the regions on which X-ray scattering occurs was established from the analysis of diffraction patterns of nanocellulose films obtained by microbial and chemical methods. These regions are much larger in the case of bacterial nanocellulose. The redistribution of the peaks intensity in the diffraction patterns with a change in the manufacturing method reflects, probably, the difference in the ratio between crystalline and amorphous content for cellulose samples of various types. Samples of bacterial cellulose both "pure" and with the addition of the Rhodamine C dye are characterized by intense visible photoluminescence at room temperature. The treatment of samples with a NaOH solution leads to a decrease in the intensity of the red band (with a maximum at 670 nm) of cellulose luminescence, while the addition of a dye enhances the band in the yellow (maximum at 570 nm) spectral range. Thus, the method used in this work to made bacterial nanocellulose makes it possible to create luminescent films which emission spectra can be easily modified with alkalis or dyes treatment.

Porous Silica as a Nanotemplate for the Solid State and Liquid Phase Synthesis of Luminescent Carbon Dots

Samples of nanoporous silicon oxide (pore size of 40 nm and 6 nm with specific are of 90 m2/g and 233 m2/g respectively) were synthesised by sol-gel method and used as nanotemplate for solid stated and liquid phase synthesis of lluminescent carbon nanodots (nanoagregates). Solid state method was two step process of chemical treatment of porous silicas by toluene solution of phenylstrimethoxisilane followed by pyrolysis at temperature in range of 400-600 0C for 60-180 minutes. Three characteristic emission bands in UV and VIS spectral range (360-380 nm, 410-420 nm, 440-450 nm) and their combinations were found in annealed por-SiO2:C depending on annealing time and temperature. The nature of light emission of pyrolytic carbon moieties in SiO2:C nanopowder was analysed and discussed in frame of the concepts of polycyclic carbon clusters and their aggregates based on the analogy with polycyclic organic molecules and luminescent carbon nanodots. It is suggested to assign the characteristic emission bands at about 380 nm and 420 nm to polycyclic carbon fluorophores with 2- and 3-ring structure respectively. The other low energy characteristic emission bands at wavelength 440-450 nm and higher are assigned to amorphous nanoaggregates. It is found that PL intensity of por-SiO2:C nanocomposite is sensitive to the absorption of the air. Response of the PL on absorption/desorption of air moieties was studied and discussed.Hydrophylic carbon nanodots were synthesized by liquid phase thermolysis of sucrose in dimethyl sulfoxide solution at 200 0C that resulted in formation of luminescent colloid. Porous silica were infiltrated by this colloid and then dried. Luminescent moieties absorbed in porous silicas were then extracted in water resulting in aqueous solution that exhibited intense visible photoluminescence with maximum intensity at 430-440 nm and well pronounced excitation band at 360-400 nm. This emission is suggested to be assigned to hydrophilic amorphous carbon nanodots (nanoagregates). Examination of Z-parameter of the colloids gave a value of the size of carbonnanodots below 1 nm. Spectral parameters (maximum position and spectral width) of the emission and excitation of photoluminescence were found to be dependent on the size of the pores in porous silica template that can be assigned to the effect of the size and size distribution of carbon nanodots (nanoagregates).As a summary, it was shown that nanoporous silica with high specific surface area can be used as solid state “container” for both solid state and liquid phase synthesis of carbon nanoemitters with emission spectrum variable from the near UV to the visible.

A cost-effective and facile approach for realization of black silicon nanostructures on flexible substrate

In this paper, we report a cost effective and facile approach to produce uniform nanostructural black silicon over large-area by metal assist chemical etching followed by its transfer over pressure sensitive flexible substrate. Structural and optical properties of black silicon over flexible substrate were investigated. Field emission scanning electronic microscope (FESEM) reveals textured surface and dense morphology of silicon nanostructures; that appeared to be black. An intense and broadband UV and Visible photoluminescence spectra from these nanostructures was observed and suggested the optically active nature of black silicon. Broadening and asymmetric shifting of Raman line shape, corresponding to black silicon, confirmed quantum confinement of phonon. The crystal sizes of silicon (Si) nanostructures calculated using frequency shift in Raman spectra analytical model were 3.4 nm to 4.7 nm. Significant reduction of reflection below (1%) over the wide UV–Vis spectrum was recorded due to presence of textured surface as observed by FESEM. The present work aids our understanding of tuning the optical properties of black silicon and its’ realization on flexible substrate could be beneficial for flexible electronics including bio-sensing and optoelectronics devices. More interestingly low reflectance of black silicon could pave the way for realization of highly efficient flexible solar cells.

Metal organic framework doped Spiro-OMeTAD with increased conductivity for improving perovskite solar cell performance

In this work, a metal–organic framework, namely, [In0.5K(3-qlc)Cl1.5(H2O)0.5]2n, (In10) is first incorporated into 2,2′,7,7′-tetrakis(N,N′-di-p-methoxyphenylamine)-9,9′-spirobifluorene (Spiro-OMeTAD). Through the simple and effective strategy, Spiro-OMeTAD oxidation is achieved and the conductivity of corresponding hole transport layer is increased, which favors the charge transport and suppresses charge recombination of perovskite solar cells (PSCs). Besides, due to the intense visible photoluminescence feature of In10, its addition can enhance the light response of perovskite solar cell. Furthermore, the modified PSCs exhibit an obviously improved device performance with short-current density (Jsc), open circuit voltage (Voc), and fill factor (FF) increasing from 22.7 to 24.3 mA cm−2, 0.95 to 1.0 V, and 0.65 to 0.70, respectively. As a result, the power conversion efficiency (PCE) exhibits 20% enhancement from 14.1 to 17.0%.

Yb3+, Er3+ and Tm3+ doped α-Sialon as upconversion phosphor

α-Sialon ceramics are mostly known for high-temperature and high-endurance applications due to their ability to withstand high structural load and their excellent chemical and thermal stability. Here we demonstrate functional α-Sialon ceramics with photon upconversion properties. Intense visible and near-infrared upconversion photoluminescence is observed in Yb3+, Er3+, and Tm3+ doped α-Sialon ceramics prepared by hot press sintering. Clear evidence of α-Sialon formation with the incorporation of Yb3+, Er3+ and Tm3+ ions in the lattice has been established. Emission bands at 498 nm, 551 nm, 662 nm and 802 nm are observed as a result of the two-photon absorption process. The decay behavior of the emission bands suggests the existence of efficient energy transfer from Yb3+ to Er3+ and Tm3+ ions. The possibility of temperature sensing properties has been investigated via high-power laser excitation. These findings could possibly extend the usefulness of Yb3+, Er3+ and Tm3+ doped α-Sialon as an upconversion phosphor material.

1.2 µm and 1.5 µm near-infrared photoluminescence and visible upconversion photoluminescence in GeGaS:Er3+/Ho3+ glasses under 980 nm excitation

We report on the Er3+ ↔ Ho3+ optical interaction on the 1.2 and 1.5 µm near-infrared and visible upconversion photoluminescence in Er3+/Ho3+-co-doped GeGaS glasses at pumping wavelength of 980 nm. Since the 980 nm pumping wavelength is off resonance for Ho3+ intra-4f electronic transitions, we have utilized the Er3+ sensitization with observed efficient Er3+ → Ho3+ energy transfer. As a result, the Ho3+ 1.2 µm emission in Er3+/Ho3+-co-doped GeGaS glass has been notably improved while the intense Er3+ 1.5 µm emission has been maintained. The visible upconversion photoluminescence has been observed at 530, 550 and 660 nm and was assigned mainly to intra-4f electronic transitions within Er3+ ions. The overall upconversion photoluminescence intensity significantly increases by addition of Ho3+ ions into GeGaS:Er glass which may be explained by energy transfer processes between Er3+ and Ho3+ ions leading to population of the Er3+ upper energy levels. The Ho3+ 1.2 µm and Er3+ 1.5 µm emission has much potential in telecommunication while the intense visible upconversion photoluminescence in sensing.

Time-gated triplet-state optical spectroscopy to decipher organic luminophores embedded in rigid matrices.

Wet-chemically synthesized inorganic materials often exhibit luminescence behavior. We have recently shown that the amorphous yttrium-aluminium-borate (a-YAB) powders obtained by sol-gel and modified Pechini methods exhibit organic impurities, responsible for their intense visible photoluminescence and phosphorescence afterglow. However, the heterogeneity of impurity organic compounds and difficulties in their intact extraction from the solid inorganic host matrix limit the extraction-based chemical analysis for luminophore identification. Here, we propose a photo-physical route based on time-gated triplet-state optical spectroscopy (TGTSS) to construct the electronic structures of the trapped unknown luminophores, which successfully illustrates the luminescence properties of a-YAB powders in more detail and also provides important insights intrinsic to the nature of the luminophores. The experimental results accompanied with TD-DFT calculations of the theoretical electronic structures thus help us to propose the probable luminophore compounds trapped in rigid a-YAB matrices. We anticipate that the present approach will open new opportunities for analyzing similar complex luminescent materials, including carbon dots, graphene oxides, etc., which is vital for their improvement.

Afterglow Luminescence in Wet-Chemically Synthesized Inorganic Materials: Ultra-Long Room Temperature Phosphorescence Instead of Persistent Luminescence.

Wet-chemically synthesized amorphous yttrium-aluminum-borates (a-YAB) exhibit intense visible photoluminescence (PL). Preliminary investigations revealed a correlation of PL with the presence of carbon-related impurities; however, their exact nature is still under investigation. These powders also exhibit afterglow luminescence that lasts for several seconds at room-temperature (RT). A comparison with persistent phosphors and phosphorescent dye revealed that the afterglow in a-YAB is a phosphorescence phenomenon and not the persistence luminescence, which is more common in inorganic solids. The unusual RT phosphorescence in a-YAB could be achieved due to triplet-state stabilization of trapped luminescent organic moieties in glassy matrix. This is indeed an important step forward in understanding the complex luminescence mechanism in such promising wet-chemically synthesized functional materials. Moreover, phosphorescence is detectable for over 10 s at RT, suggesting rigid glassy inorganic matrix is more efficient in preserving phosphorescence at elevated temperatures, opening the path for several attractive applications including time-resolved bioimaging and thermometry.

Red-orange photoluminescence and dielectric relaxation of Eu3+-doped Bi2Ti2O7 pyrochlore structure thin films

Eu3+-doped Bi2Ti2O7 pyrochlore structure thin films were prepared by a chemical solution deposition method. The red-orange photoluminescence and dielectric relaxation of Bi2−xEuxTi2O7 thin films have been investigated in terms of Eu3+ doping content (x = 0, 0.1, 0.2, 0.3, 0.4, 0.5, and 0.6) and annealing temperature. The crystallinity and surface morphology of the thin films were evaluated by X-ray diffractometer and field emission scanning electron microscopy. It was shown that the thin films annealed at 600 °C were well crystallized in pyrochlore structure without other impurity phases. Intense visible red-orange photoluminescence, originated from doubly split electric-dipole 5D0→7F2 and magnetic-dipole 5D0→7F1 transitions of Eu3+ ions, could be observed under ultraviolet excitation. The quenching concentration for photoluminescence of Bi2−xEuxTi2O7 thin films is about x = 0.5. In addition, the Bi2−xEuxTi2O7 thin films had a relatively low dielectric loss and a high dielectric constant as well as highly stable bias electric field properties. Meanwhile, the thin films exhibited dielectric relaxation based on the temperature and frequency dependence of dielectric properties. Our study suggested that Eu3+-doped Bi2Ti2O7 thin films have potential applications in new multifunctional optoelectronic thin-film devices.

Self-assembled nc-Si/a-SiNx: H quantum dots thin films: An alternative solid-state light emitting material

Nanocrystalline silicon quantum dots (QDs) of varying size from ~5.4 to 2.2nm embedded in amorphous silicon-nitride matrix (nc-Si-QDs/a-SiNx:H) were prepared via ICP-CVD (13.56MHz), using a mixture of silane (SiH4), ammonia (NH3) gases in H2-diluted plasma, by changing the low deposition pressure in a narrow range, 10–50mTorr at low substrate temperature (250°C). Strong visible photoluminescence (PL) tunable over a significantly wide range (1.67–3.02eV) has been observed. The origin of the PL is mostly attributed to band-to-band recombination due to the quantum confinement effect (QCE) in the nanocrystalline silicon QDs. A large amount of atomic hydrogen flux that originates due to the high degree dissociation of the gas molecules in high-density inductively coupled plasma (ICP) passivates well the nonradiative dangling bonds and helps in growing plenty of ultra-nanocrystallites that demonstrate intense visible photoluminescence. The red-shift of Raman peak and the corresponding line broadening have been associated to the confinement of optical phonons within the nc-Si QDs. The widening of band gap and tune-ability of visible photoluminescence over a notably wide range along with significantly high electrical conductivity of the material demonstrates enormous promise for its utilization in the fabrication of effective solid-state light emitting devices.

Fabrication of Porous Cubic Architecture of ZnO Using Zn-terephthalate MOFs with Characteristic Microstructures

A method for synthesizing porous cubic-shaped ZnO particles a few tens of micrometers in size is described on the basis of a pyrolytic conversion of Zn-terephthalate metal-organic frameworks (MOFs). MOF crystals were initially grown in solutions containing Zn(NO3)2·6H2O and terephthalic acid as solutes and N,N-dimethylformamide (DMF) or N,N-diethylformamide (DEF) as a solvent under a solvothermal condition. It was the key to controlling the microstructure of MOF cuboids for their use as an intermediate compound for ZnO. Actually, many cracks were formed and hence the cubic microstructure was somewhat destroyed in the pyrolytic conversion from dense MOF crystals (grown in the DMF solution) to ZnO. In contrast, mesocrystal-like MOF cuboids (grown in the DEF solution) could maintain their shape during the pyrolysis because of the relaxation against a MOF-to-ZnO volume change. The resultant ZnO with a highly porous cubic structure showed intense visible photoluminescence upon irradiation with ultraviolet light.

Characterization of hollow BaTiO3 nanofibers and intense visible photoluminescence

BaTiO3 hollow nanofibers were fabricated by electrospinning and then subsequent calcination of as-spun nanofibers with a heating rate of 2.5 °C/min. Scanning electron microscope and transmission electron microscope (TEM) results indicated that the heating rate had a significant effect on the morphology of the BaTiO3 hollow nanofibers. The X-ray diffraction, Raman spectroscopy, and TEM results indicate the prepared BaTiO3 hollow nanofibers have tetragonal phases. From the results of the X-ray photoelectron spectroscopy analysis, in the amorphous BaTiO3 nanofiber, peaks at 457.2 eV for Ti 2p3/2 were also found, which corresponded to the Ti3+ ions. However, in the crystalline BaTiO3 nanofibers, peaks of Ti 2p3/2 showed the Ti4+ ions. Intense visible photoluminescence was observed in the amorphous BaTiO3 nanofiber, which is calcined below a temperature of 500 °C. The observed intense photoluminescence was ascribed to a multiphonon process with localized states within the band gap of the highly disordered states. In the crystalline BaTiO3 hollow fiber, low intensity of photoluminescence showed at the visible region, which is originated from an intrinsic Ba defect.

Green Up-Conversion Photoluminescence and Dielectric Relaxation of Ho3+ /Yb3+ -Codoped Bi2 Ti2 O7 Pyrochlore Thin Films

Ho3+/Yb3+-codoped Bi2Ti2O7 pyrochlore thin films were prepared by a chemical solution deposition method, and their visible up-conversion (UC) photoluminescence and dielectric relaxation were studied. Ho and Yb can be doped into Bi2Ti2O7 lattice and single pyrochlore phase is maintained. Intense visible UC photoluminescence can be observed under the excitation of a 980-nm diode laser. Two UC emission bands centered at 551 nm and 665 nm in the spectra can be assigned to 5F4, 5S2→5I8 and 5F5→5I8 transitions of Ho3+ ions, respectively. The dependence of their UC emission intensity on pumping power indicates that both the green and red emissions of the thin films are two-photon process. In addition, a Stokes near-infrared emission centered at 1200 nm can be detected, which is due to 5I6→5I8 transition of Ho3+ ions. The thin films prepared on indium tin oxide–coated glass substrates exhibit a relatively high dielectric constant and a low dielectric loss as well as a good bias voltage stability. The dielectric relaxation of the thin films was also analyzed based on the temperature- and frequency-dependent dielectric properties. This study suggests that Ho3+/Yb3+-codoped Bi2Ti2O7 thin films are promising materials for developing multifunctional optoelectronic thin film devices.

Soft solution synthesis and intense visible photoluminescence of lamellar zinc oxide hybrids

In this study, we demonstrate the synthesis of layered zinc oxide films intercalated with dodecyl sulphate ions by a simple soft solution process. The presence of potassium (K+) and lithium (Li+) ions in the precursor solution of layered zinc hydroxide resulted in lamellar hybrid zinc oxide films instead of layered zinc hydroxides. On the other hand, the addition of nickel phthalocyanine induces zinc hydroxide host layers which exhibit an intense blue emission. This is also promoted by K+ and Li+ ions.

Tunable photoluminescence from nc-Si/a-SiNx:H quantum dot thin films prepared by ICP-CVD

Intense visible photoluminescence (PL) tunable within 1.66-2.47 eV, under UV 325 nm excitation, was obtained from nanocrystalline silicon quantum dots (∼5.72-1.67 nm in diameter) embedded in amorphous silicon-nitride matrix (nc-Si/a-SiN(x):H) prepared in RF-ICPCVD (13.56 MHz) at substrate temperatures between 400 to 150 °C. The dominant component of PL, having a narrow band width of ∼0.16-0.45 eV, originates from quasi-direct band-to-band recombination due to quantum confinement effect (QCE) in the nanocrystalline silicon quantum dots (nc-Si QDs) of appropriate size; however, the contribution of defects arose at lower substrate temperatures leading to asymmetric broadening. Intense atomic hydrogen flux in high-density inductively coupled plasmas (ICPs) provides a very high surface coverage, passivates well the nonradiative dangling bonds, and thereby favors the PL intensity. The average size of nc-Si QDs measured by HR-TEM appears consistent with similar estimates from Raman studies. The red shift of the Raman line and corresponding line broadening originates from the confinement of optical phonons within nc-Si QDs. Photoluminescence emerging from nc-Si/a-SiN(x):H quantum dots obtained from the low temperature and single-step plasma processing holds great promise for the fabrication of light-emitting devices and flexible flat panel displays.

Triangular prism-shaped p-type 6H-SiC nanowires

We report, for the first time, the growth of p-type 6H-SiC nanowires with a unique triangular prism shape via the pyrolysis of polymeric precursors. The resultant wires were systematically characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM) and high-resolution transmission electron microscopy (HRTEM). The triangular prism-shaped SiC nanowires are averagely sized at ∼700 nm in edge widths with lengths of up to tens of microns. The wires are single-crystalline with 1 at% Al dopants and grown along the direction of [102]. A mechanism based on a typical vapor–liquid–solid (VLS) process was discussed for the growth of the 6H-SiC wires. Intense visible photoluminescence was observed with two respective strong emission peaks centered at 2.4 and 3.1 eV, suggesting that the p-type 6H-SiC wires could be useful in the fabrication of optoelectronic nanodevices.

Carbon-assisted growth and high visible-light optical reflectivity of amorphous silicon oxynitride nanowires

Large amounts of amorphous silicon oxynitride nanowires have been synthesized on silicon wafer through carbon-assisted vapor-solid growth avoiding the contamination from metallic catalysts. These nanowires have the length of up to 100 μm, with a diameter ranging from 50 to 150 nm. Around 3-nm-sized nanostructures are observed to be homogeneously distributed within a nanowire cross-section matrix. The unique configuration might determine the growth of ternary amorphous structure and its special splitting behavior. Optical properties of the nanowires have also been investigated. The obtained nanowires were attractive for their exceptional whiteness, perceived brightness, and optical brilliance. These nanowires display greatly enhanced reflection over the whole visible wavelength, with more than 80% of light reflected on most of the wavelength ranging from 400 to 700 nm and the lowest reflectivity exceeding 70%, exhibiting performance superior to that of the reported white beetle. Intense visible photoluminescence is also observed over a broad spectrum ranging from 320 to 500 nm with two shoulders centered at around 444 and 468 nm, respectively.

The Role of Short-Range Disorder in BaWO4 Crystals in the Intense Green Photoluminescence

An intense and broad visible photoluminescence (PL) band was observed at room temperature in short-range disordered BaWO4 crystals. The scheelite crystalline BaWO4 powders prepared by the polymeric precursor method and annealed at different temperatures were structurally characterized by means of X-ray diffraction and Fourier transform (FT) Raman spectroscopy measurements. Quantum-mechanical calculations indicated that the disorder at short range only of Ba atoms in the BaWO4 lattice has a very important role in the charge transfer involved in the intense green PL emission. This indication was detected and confirmed by means of FT Raman spectroscopy measurements. The experimental and theoretical results are in good agreement, both showing the specific type of structural disorder that is the most favorable condition for generating the most intense green PL emission in the scheelite BaWO4 lattice.

Intense visible and near-infrared upconversion photoluminescence in colloidal LiYF₄:Er³+ nanocrystals under excitation at 1490 nm.

We report intense upconversion photoluminescence (PL) in colloidal LiYF(4):Er(3+) nanocrystals under excitation with telecom-wavelength at 1490 nm. The intensities of two- and three-photon anti-Stokes upconversion PL bands are higher than or comparable to that of the Stokes emission under excitation with low power density in the range 5-120 W/cm(2). The quantum yield of the upconversion PL was measured to be as high as ∼1.2 ± 0.1%, which is almost 4 times higher than the highest upconversion PL quantum yield reported to date for lanthanide-doped nanocrystals in 100 nm sized hexagonal NaYF(4):Yb(3+)20%, Er(3+)2% using excitation at ∼980 nm. A power dependence study revealed that the intensities of all PL bands have linear dependence on the excitation power density, which was explained by saturation effects in the intermediate energy states.