Laser induced white emission of diamond.

Laser-induced white emission of diamond was investigated under irradiation with a focused beam of an infrared laser diode. It is a surface-related coherent emission, characterized by an excitation threshold and an exponential dependence on pumping laser power. The mechanism of white emission is discussed in terms of multiphoton ionization of carbon atoms in an irradiated spot. The excitation power dependence of white emission intensity has demonstrated hysteresis loop behavior. This phenomenon could be useful in new broadband laser sources and optical information storage.

Surface related laser induced white emission of Cr:YAG ceramic.

In this work we report the white light emission in transparent Cr:YAG ceramic pellet upon irradiation with focused beam of CW infrared laser diode. It was found that this phenomenon is specifically related to interaction of laser beam with a surface of the pellet. The white light was emitted outside an irradiated spot at the surface of the pellet and did not penetrate inside the pellet. Moreover, the red emission related to two-photon absorption along the laser beam penetrating the Cr3+:YAG pellet was observed. Interaction of the laser beam with the surface of the pellet leads to an efficient white light emission from an outer side of the pellet. The resulting white light emission did not entry back the pellet. Multiphoton ionization leading to intervalence charge transfer followed by light emission was proposed as the mechanism of experimentally observed white light emission.

Preparation and physical characteristics of graphene ceramics.

Graphene, a two-dimensional structure of carbon, due to its structure has unique physico-chemical properties that can be used in numerous research and industry areas. Although this structure is already well known, there are still technological (and cost) barriers which do not allow to produce this material in large quantities and hence prevent its use in various applications. For this reason, many technologies are currently being developed to obtain graphene in forms that would enable its widespread use. The graphene-like ceramics were fabricated by the high isostatic pressure method at different temperatures. This technique allows to obtain dense ceramics with various shapes. The structure and morphology of sintered graphene were investigated by XRD, SEM and the Raman spectroscopy. The hardness, thermal conductivity and electric transport measurements recorded in a wide range of temperatures were used to analyze the physical properties of the obtained ceramics.

Direct light-induced propulsion of vessels filled with a suspension of graphene particles and methanol.

The direct propulsion of glassy capsule filled with solution of methanol and disperse graphene foam (GF) particles under irradiation with infrared LED is reported. The vertical propulsion occurred after irradiation of transparent glassy bottom. The velocity of propulsion was dependent of light irradiation power. It was observed that with irradiation the GF particles moved violently and vertically with direction of lighting. It was found that upon light irradiation there is generated efficiently hydrogen upon solution surface. The mechanism of propulsion effect was discussed in terms of the explosive hydrogen-oxygen reaction.

Kinetics of Cr3+ to Cr4+ ion valence transformations and intra-lattice cation exchange of Cr4+ in Cr,Ca:YAG ceramics used as laser gain and passive Q-switching media.

This paper focuses on the kinetics of Cr4+ formation in Cr,Ca:YAG ceramics prepared by solid-state reaction sintering. The kinetics of Cr4+ formation was studied by annealing of Cr,Ca:YAG ceramics in ambient air under different temperatures at different times, resulting in the transformation of Cr3+ to Cr4+. The activation energy (Ea) of Cr3+ oxidation determined by the Jander model was 2.7 ± 0.2 eV, which is in good correlation with the activation energy of innergrain oxygen diffusion in the YAG lattice. It is concluded that Cr3+ to Cr4+ transformation in YAG ceramics is limited by oxygen diffusion through the grain body. It was established that in Cr,Ca:YAG ceramics, the intralattice cation exchange, in which the Cr4+ ions exchange positions with the Al3+ ions, switching from "A" to "D" sites, is faster than Cr3+ to Cr4+ oxidation. In the temperature range of 900-1300 °C, the reaction enthalpy of Al3+/Cr4+ ion exchange between octahedral "A" and tetrahedral "D" lattice sites is close to zero, and this exchange ratio is thermodynamically driven by entropy.

Structural and optical characterization of RbLaP4O12:Ln3+ (Ln3+ = Ce3+, Nd3+, Tm3+, or Yb3+).

In this work, for the first time, detailed structural and optical characterization of RbLaP4O12 doped with different concentrations of Ce3+, Nd3+, Tm3+, or Yb3+ ions is reported. The samples were obtained via a precipitation technique. Their structural characterization was performed using X-ray diffraction (XRD), and infrared and Raman spectroscopies. Following XRD data, the unit cell parameters of host lattices were calculated using Rietveld refinement. It was found that an increase in the dopant content leads to a decrease in the unit cell volume. The optical characterization of RbLaP4O12:Ln3+ was carried out by collecting absorption and emission spectra, as well as luminescence decay profiles. Following absorption spectra, the energy band gap of the studied matrix was determined. It was found that the broad absorption band located in the ultra-violet range, in most cases ascribed to charge transfer or f-d transitions, is in fact related to the absorption of the host lattice. The analysis of luminescence properties allowed us to investigate possible ways of depopulation emission levels of impurities.

Light-induced confinement of electrons in stacked distorted graphene layers - a (TD-)DFT study.

Recently observed white-light emission from graphene ceramics cannot be explained by black body radiation theory because of a relatively low temperature of the sample. Furthermore the intensity of the emission stays at the same level even at temperatures as low as 10 K, indicating the purely electronic nature of the observed emission. In this work a model of light emission from locally interacting stacked graphene layers after intense continuous wave laser excitation is proposed. After a light-induced sp2 to sp3 change of the hybridization sp2-nanodomains surrounded by sp3 carbon atoms could be created and the electrons can be confined. Using DFT and TD-DFT methods followed by a molecular-like approach we examine the electronic structure and the optical properties of graphene (sp2,sp3)-clusters. We show the quantized and well separated energy levels of electrons from the domain's interior and the possibility of emission in the NIR/VIS/UV range.

Laser induced white emission generated by infrared excitation from Eu3+:Sr2CeO4 nanocrystals.

The laser induced white emission (LIWE) was observed from Eu3+:Sr2CeO4 nanocrystals. The samples were obtained in form of powders by the modified sol-gel route. The structure and morphology of the phosphors were investigated by powder X-ray diffraction and transmission electron microscope techniques. The intense LIWE occurred under reduced pressure and focused beam of near infrared laser excitation. The power and pressure dependencies exhibit evident threshold character typical for the avalanche effect. The photoconductivity of the Eu3+:Sr2CeO4 nanocrystals measured as a function of different powers of excitation source was analyzed.

Optimization of highly sensitive YAG:Cr3+,Nd3+ nanocrystal-based luminescent thermometer operating in an optical window of biological tissues.

Luminescent and temperature sensitive properties of YAG:Cr3+,Nd3+ nanocrystals were analyzed as a function of temperature, nanoparticle size, and excitation wavelength. Due to numerous temperature-dependent phenomena (e.g. Boltzmann population, thermal quenching, and inter-ion energy transfer) occurring in this phosphor, four different thermometer definitions were evaluated with the target to achieve a high sensitivity and broad temperature sensitivity range. Using a Cr3+ to Nd3+ emission intensity ratio, the highest 3.48% K-1 sensitivity was obtained in the physiological temperature range. However, high sensitivity was compromised by a narrow sensitivity range or vice versa. The knowledge of the excitation and temperature susceptibility mechanisms enabled wise selection of the spectral features found in luminescence spectra for a temperature readout, which enabled the preservation of relatively high temperature sensitivity (>1.2% K-1 max) and extended the temperature sensitivity range from 100 K to 850 K. The size of the nanophosphors had negligible impact on the performance of the studied materials.

Broadband anti-Stokes white emission of Sr2CeO4 nanocrystals induced by laser irradiation.

The laser induced white emission (LIWE) from Sr2CeO4 nanocrystals upon irradiation with a focused IR laser beam was investigated. It was observed to be a threshold phenomenon with its intensity increasing exponentially with the excitation power density. This process was investigated under double laser beam simultaneous excitation in the UV range leading to Stokes emission in the visible range and in the IR range leading to anti-Stokes LIWE. With increasing LIWE intensity, the Stokes emission intensity strongly decreased. The LIWE is accompanied by efficient photocurrent generation depending on laser excitation density followed by multiphoton absorption and ionization processes. Photoimpedance measurements showed a sharp increase of the dielectric constant by several orders of magnitude in the Sr2CeO4 nanocrystals during the LIWE process demonstrating a metallic-like behaviour. The mechanisms of LIWE include multiphoton absorption and ionization that lead to the creation of a coupled pair of Ce3+ and Ce4+ ions that allow for the intervalence charge transfer (IVCT) emission transitions in the white light range. A strong decrease of absorption band intensity of Sr2CeO4 with increasing LIWE intensity confirms the creation of (Ce3+, Ce4+) pairs.

Silver Nanoforms as a Therapeutic Agent for Killing Escherichia coli and Certain ESKAPE Pathogens.

The scope of this study included the preparation of silver nanoforms with high antimicrobial efficacy, low cost, and ease of application. The term 'silver nanoforms' refers to silver located on the amorphous or crystalline titanium dioxide (TiO2). Silver nanoforms may be used as an alternative to antibiotics in killing bacteria. Pure and silver-incorporated titanium (used as a carrier) was prepared using the sol-gel-modified method. Physical and chemical properties of the samples were described, and the antibacterial activity was indicated using the following strains of bacteria: Staphylococcus aureus, Klebsiella pneumoniae (ESKAPE pathogens), and Escherichia coli. The results have shown that the antibacterial activity of silver nanoforms with amorphous TiO2 is much better than that in the samples based on anatase (crystalline TiO2). The sensitivity of the tested bacteria to silver nanoforms depends on physical and chemical properties of the nanoforms and individual characteristics of the bacteria. For the first time, significant participation of amorphous TiO2 in antibacterial compounds has been described through this study.

New photosensitive nanometric graphite oxide composites as antimicrobial material with prolonged action.

A new conjugate material based on partially reduced graphite oxide (rGO), silver nanoparticles (Ag), and bis(lysinato)zirconium(IV) phthalocyanine complex (ZrPc) was obtained. Its optical properties (absorption and photoluminescence) after dispersion in solvents were examined. The antimicrobial properties were tested to determine the effect of the composite on the following bacterial strains: Staphylococcus aureus, Pseudomonas aeruginosa, and Escherichia coli, which are responsible for many infections and are one of the pathogens the most difficult to treatment. The results obtained for rGO-ZrPc-Ag composite were compared with the properties of GO, GO-ZrPc, and rGO-Ag structures. The influence of the near-infrared irradiation on the antimicrobial activity of ZrPc- and Ag-doped materials against bacteria was observed for very low concentration (0.32mg/mL) of GO-ZrPc to stop the growth of P. aeruginosa in comparison to the nonirradiated sample (41mg/mL). The usefulness of this material in therapy, such as wound infection treatment or endodontic treatment, as antibacterial agent with sustained action was discussed.

Two blinking mechanisms in highly confined AgInS2 and AgInS2/ZnS quantum dots evaluated by single particle spectroscopy.

Ternary AgInS2 quantum dots (QDs) have been found as promising cadmium-free, red-shifted, and tunable luminescent bio-probes with efficient Stokes and anti-Stokes excitations and luminescence lifetimes (ca. 100 ns) convenient for time resolved techniques like fluorescence life-time imaging. Although the spectral properties of the AgInS2 QDs are encouraging, the complex recombination kinetics in the QDs being still far from understood, limits their full utility. In this paper we report on a model describing the recombination pathways responsible for large deviations from the first-order decay law observed commonly in the ternary chalcogenides. The presented results were evaluated by means of individual AgInS2 QD spectroscopy aided by first principles calculations including the electronic structure and structural reconstruction of the QDs. Special attention was devoted to study the impact of the surface charge state on the excited state relaxation and effect of its passivation by Zn(2+) ion alloying. Two different blinking mechanisms related to defect-assisted charge imbalance in the QD responsible for fast non-radiative relaxation of the excited states as well as surface recharging of the QD were found as the major causes of deviations from the first-order decay law. Careful optimization of the AgInS2 QDs would help to fabricate new red-shifted and tunable fluorescent bio-probes characterized by low-toxicity, high quantum yield, long luminescence lifetime, and time stability, leading to many novel in vitro and in vivo applications based on fluorescence lifetime imaging (FLIM) and time-gated detection.

Near infrared absorbing near infrared emitting highly-sensitive luminescent nanothermometer based on Nd(3+) to Yb(3+) energy transfer.

A new type of near infrared absorbing near infrared emitting (NANE) luminescent nanothermometer is presented, with a physical background that relies on efficient Nd(3+) to Yb(3+) energy transfer under 808 nm photo-excitation. The emission spectra of LiLa0.9-xNd0.1YbxP4O12 (x = 0.05, 0.1, 0.2, 0.3, 0.5) nanocrystals were measured in a wide 100-700 °C temperature range. The ratio between the Nd(3+) ((4)F3/2→(4)I9/2) and Yb(3+) ((2)F5/2→(2)F7/2) luminescence bands, and the thermometer sensitivity were found to be strongly dependent on the Yb(3+) concentration. These phenomenological relations were discussed in terms of the competition between three phenomena, namely (a) Nd(3+)→ Yb(3+) phonon assisted energy transfer, (b) Yb(3+)→ Nd(3+) back energy transfer and (c) energy diffusion between Yb(3+) ions. The highest sensitivity of the temperature measurement was found for x = 0.5 (LiLa0.4Nd0.1Yb0.5P4O12), which was equal to 4 × 10(-3) K(-1) at 330 K. In stark contrast to conventional approaches, the proposed phosphate host matrix allows for a high level of doping, and thus, owing to the negligible concentration quenching, the presented luminophores exhibit a high absorption cross section and bright emission. Moreover, such optical remote thermometers, whose excitation and emission wavelengths are weakly scattered or absorbed and fall into the optical transmission window of the skin, may therefore become a practical solution for biomedical applications, such as remote control of thermotherapy.

Synthesis and up-conversion luminescence of Er(3+) and Y b(3+) codoped nanocrystalline tetra- (KLaP4O12) and pentaphosphates (LaP5O14).

The up-converting nanocrystals of KLa0.95Er0.05Y bxP4O12 and La0.95-xEr0.05Y bxP5O14 were prepared using co-precipitation method. The spectroscopic properties of these materials were investigated in a function of Y b(3+) concentration. The up-conversion emission, power dependence of emission intensities, and the luminescence decay times were investigated. It was found that the green to red and (2)H11/2 → (4)I15/2 to (4)S3/2 → (4)I15/2 emission intensity ratio were strongly affected by the Y b(3+) concentration. Moreover, the order of up-conversion emission and threshold power rises up with Y b(3+) concentration for (4)S3/2 → (4)I15/2 transition. The luminescence decay time of the (4)S3/2 → (4)I15/2 emission increases with Y b(3+) concentration while the (4)F9/2 → (4)I15/2 emission is independent of dopant concentration. The influence of the Y b(3+) concentration on the up-conversion emission intensities was discussed in terms of concentration dependent hetero looped photon avalanche process. A comparison of the up-conversion properties of KLa0.95Er0.05Y bxP4O12 and La0.95-xEr0.05Y bxP5O14 nanocrystals was presented.

Influence of grain size on optical properties of Sr2CeO4 nanocrystals.

The absorption, excitation, and emission spectra of the Sr2CeO4 nanocrystals prepared by the modified sol-gel method were investigated. The impact of the average grain size of Sr2CeO4 nanocrystals on their optical properties was investigated. It was observed that with increasing the average grain size of Sr2CeO4 nanocrystals, the emission decay times decreased significantly. A similar behavior was observed for the emission quantum efficiencies and the Huang-Rhys factors. The grain size dependence of optical parameters of Sr2CeO4 nanocrystals was found well fitted by functions of the reciprocal of the grain diameter. It was shown that this dependence may be rationalized assuming that the correction for electric local field associated with effective refractive index affecting the spherical nanoparticle is governed by its shell.

Synthesis and luminescent properties of La(1-x)Nd(x)P5O14 nanocrystals.

La(1-x)Nd(x)P5O14 nanocrystals were synthesized using a coprecipitation method. Their structure and morphology were determined. The luminescence and excitation spectra of La(1-x)Nd(x)P5O14 nanocrystals were measured in the entire range of Nd(3+) concentration. It was found that the relative intensities of absorption transitions increased significantly with concentration due to the cooperative interactions. The effect of concentration on fluorescence transitions was investigated. It was found that the intensity of the (4)F3/2 → (4)I11/2 transition significantly increased with concentration relative to the resonant (4)F3/2 → (4)I9/2 transition almost three times due to strong reabsorption. The concentration quenching of fluorescence was discussed in terms of the Yokota-Tanimoto model.

Optimisation of ligand exchange towards stable water suspensions of crystalline NaYF4: Er3+, Yb3+ nanoluminophors.

NaYF4: 2%Er3+, 20%Yb3+ nanoparticles were synthesized through a wet chemical route. In order to transfer the nanoparticles from chloroform to aqueous solution an oleic acid to 3-mercaptopropionic acid ligand exchange process has been performed and optimized. The influence of temperature and atmosphere onto the NaYF4:2%Er3+, 20%Yb3+ nanoparticles water suspensions dispersibility and stability after replacing oleic acid with 3-mercaptopropionic acid ligands has been investigated. The results revealed an improvement of nanoparticles water suspension transparency and luminescence yield after ligand exchange process performed under inert atmosphere and at elevated temperature.

Giant enhancement of upconversion in ultra-small Er³âº/Yb³âº:NaYF4 nanoparticles via laser annealing.

Most of the synthesis routes of lanthanide-doped phosphors involve thermal processing which results in nanocrystallite growth, stabilization of the crystal structure and augmentation of luminescence intensity. It is of great interest to be able to transform the sample in a spatially localized manner, which may lead to many applications like 2D and 3D data storage, anti-counterfeiting protection, novel design bio-sensors and, potentially, to fabrication of metamaterials, 3D photonic crystals or plasmonic devices. Here we demonstrate irreversible spatially confined infrared-laser-induced annealing (LIA) achieved in a thin layer of dried colloidal solution of ultra-small ∼8 nm NaYF4 nanocrystals (NCs) co-doped with 2% Er³âº and 20% Yb³âº ions under a localized tightly focused beam from a continuous wave 976 nm medium power laser diode excitation. The LIA results from self-heating due to non-radiative relaxation accompanying the NIR laser energy upconversion in lanthanide ions. We notice that localized LIA appears at optical power densities as low as 15.5 kW cm⁻² (∼354 ± 29 mW) threshold in spots of 54 ± 3 µm diameter obtained with a 10 × microscope objective. In the course of detailed studies, a complete recrystallization to different phases and giant 2-3 order enhancement in luminescence yield is found. Our results are highly encouraging and let us conclude that the upconverting ultra-small lanthanide-doped nanophosphors are particularly promising for direct laser writing applications.