Visible Spectral Range Research Articles

Photocatalytic Reactions over TiO2-Based Interfacial Charge Transfer Complexes

The present review is related to the novel approach for improvement of the optical properties of wide bandgap metal oxides, in particular TiO2, based on the formation of the inorganic–organic hybrids that display absorption in the visible spectral range due to the formation of interfacial charge transfer (ICT) complexes. We outlined the property requirements of TiO2-based ICT complexes for efficient photo-induced catalytic reactions, emphasizing the simplicity of the synthetic procedure, the possibility of the fine-tuning of the optical properties supported by the density functional theory (DFT) calculations, and the formation of a covalent linkage between the inorganic and organic components of hybrids, i.e., the nature of the interface. In addition, this study provides a comprehensive insight into the potential applications of TiO2-based ICT complexes in photo-driven catalytic reactions (water splitting and degradation of organic molecules), including the identification of the reactive species that participate in photocatalytic reactions by the spin-trapping electron paramagnetic resonance (EPR) technique. Considering the practically limitless number of combinations between the inorganic and organic components capable of forming oxide-based ICT complexes and with the knowledge that this research area is unexplored, we are confident it is worth studying, and we emphasized some further perspectives.

Enhanced Coplanarity and Giant Birefringence in Hydroxypyridinium Nitrate via Hydrogen Bonding between Planar Donors and Planar Acceptors.

Birefringent crystals, which possess optical anisotropy, are important optical components. However, designing and synthesizing birefringent crystals faces the challenge of achieving anisotropic structures, especially coplanar geometries. Herein, we achieve a significant birefringence in an ionic compound (C5H6ON)+(NO3)-, (4-hydroxypyridinium nitrate, 4HPN) by hydrogen bonding between planar donors and planar acceptors. We demonstrate that the interactions between the planar hydrogen bond donor ((C5H6ON)+) and planar hydrogen bond acceptor ((NO3)-) ensure the coplanarity during the crystal packing, generating the desired giant optical anisotropy. On two manually cut crystal chips, we observe a = 0.494 (= 0.593), which is the largest among nitrates, or hydroxypyridinium derivatives. This value already surpasses those of the benchmark crystals, e.g., YVO4 and CaCO3, commonly used in the UV to visible and near IR spectral range. 4HPN also exhibits a strong second harmonic generation response (9.55 × KDP). This strategy offers a promising avenue for the design and development of birefringent crystals with potential applications in optical communication, sensing and signal processing devices.

All-dielectric structural coloration empowered by bound states in the continuum.

The technological requirements of low-power and high-fidelity color displays have been instrumental in driving research into advanced coloration technologies. At the forefront of these developments is the implementation of dye-free approaches, which overcome previous constraints related to color resolution and fading. Resonant dielectric nanostructures have emerged as a promising paradigm, showing great potential for high efficiency, high color saturation, wide gamut palette, and image reproduction. However, they still face limitations related to color accuracy, purity, and simultaneous brightness tunability. Here, we demonstrate an all-dielectric metasurface empowered by photonic bound states in the continuum (BICs), which supports sharp resonances throughout the visible spectral range, ideally suited for producing a wide range of structural colors. The metasurface design consists of TiO2 ellipses with carefully controlled sizes and geometrical asymmetry, allowing versatile and on-demand variation of the brightness and hue of the output colors, respectively.

Shrink effects on nanoscale MOS capacitor in visible and NIR spectral Ranges

Shrink effects on nanoscale MOS capacitor in visible and NIR spectral Ranges

Crystallization and Optical Behaviour of Nanocomposite Sol-Gel TiO2:Ag Films.

Sol-gel spin coating method was employed for depositing TiO2 and Ag-doped TiO2 films. The effects of Ag doping and the annealing temperatures (300-600 °C) were studied with respect to their structural, morphological, vibrational, and optical properties. Field Emission Scanning Electron microscopy (FESEM) investigation exhibited the grained, compact structures of TiO2-based films. Ag incorporation resulted in a rougher film surface. X-ray diffraction (XRD) results confirmed the formation of Ag nanoparticles and AgO phase, along with anatase and rutile TiO2, strongly depending on Ag concentration and technological conditions. AgO fraction diminished after high temperature annealing above 500 °C. The vibrational properties were characterized by Fourier Transform Infrared (FTIR) spectroscopy. It was found that silver presence induced changes in IR bands of TiO2 films. UV-VIS spectroscopy revealed that the embedment of Ag NPs in titania matrix resulted in higher absorbance across the visible spectral range due to local surface plasmon resonance (LSPR). Ag doping reduced the optical band gap of sol-gel TiO2 films. The optical and plasmonic modifications of TiO2:Ag thin films by the number of layers and different technological conditions (thermal and UV treatment) are discussed.

Yb:YSAG ceramics: An attractive thin-disk laser material alternative to a single crystal?

The 8.3 at.% Yb-doped yttrium-scandium-aluminum garnet (YSAG) ceramics were fabricated by a modified reverse co-precipitation method followed by uniaxial and cold isostatic pressing, annealing in air and high-temperature sintering in vacuum. Spectroscopic and lasing properties were investigated in dependence on Y/Sc/Al ratio in ceramic composition. Optical transmission of Yb:YSAG ceramics in visible and near-infrared spectral range exceeds 80 % that indicates a high quality of the Yb:YSAG samples. Among all the ceramics, the Y2.35Yb0.25Sc1.00Al4.40O12 composition demonstrates the best lasing performance and the highest slope efficiency up to 75 %.

Mass-Producible Transparent Flexible Passive-Cooling Film.

Passive cooling is regarded as the desired method for interior temperature regulation due to its advantage of energy consumption reduction. An ideal passive-cooling window is expected to exhibit a high emissivity in the mid-infrared (MIR, 8-13 μm) spectral range for cooling and a low transmittance in the near-infrared (NIR, 780-2100 nm) spectral range for reducing heat flow, while presenting a high transmittance in the visible (VIS, 400-780 nm) spectral range for daylighting. However, material structures that meet these requirements often come with high demands for precision in manufacturing and elevated processing costs, which have limited their potential for large-scale mass production. Here, we propose a mass-producible transparent flexible passive-cooling film that is relatively easy to process and low-cost and meets all of the requirements mentioned above. The film is made of poly(methyl methacrylate) mixed with Cs0.33WO3 nanoparticles, and it shows a high absorptance (80%) in NIR for blocking solar radiation penetration and a high emissivity (93%) in MIR for radiative cooling as well as a reasonable transmittance (40%) in VIS for visibility. Under solar intensity of ∼900 W/m2, a maximum temperature reduction of 8.4 and 7.8 °C has been achieved for a window coated by the film compared to the uncoated window in the condition of the absorbing chamber and car, respectively. Such a mass-producible transparent flexible passive-cooling film holds promising applications in large windows, such as those used in automobiles and buildings, where there is a high demand for both daylighting and cooling.

Accurate Determination of the Uniaxial Complex Refractive Index and the Optical Band Gap of Polymer Thin Films to Correlate Their Absorption Strength and Onset of Absorption.

The advanced development of optoelectronic devices requires a methodical knowledge of the fundamental material properties of the key active components. Systematic investigations and correlations of such basic optical properties can lead to new insights for the design of more potent materials. In this perspective, we provide a systematic overview of the uniaxial anisotropic complex refractive indices and the absorption coefficients obtained by ellipsometry as well as the optical band gap energies derived from Tauc plots of six selected solution-processed polymer thin films. While the optical band gap energies are intentionally distributed over the visible spectral range, we found that the absorption strength of all polymer samples are grouped in a random distribution within a rather uniform range of values.

Investigation of fullerene and non-fullerene materials in organic photocatalysts on the efficiency of photocatalytic degradation of polychlorinated biphenyls

The photocatalytic degradation of polychlorinated biphenyls (PCBs) is advancing, yet the efficiency of degradation within the visible spectral range continues to encounter significant challenges. In this study, two biochar-based organic semiconductor photocatalysts, Active Carbon@PTQ10 (5,8-Dibromo-6,7-difluoro-2-(2-hexyldecoxy)quinoxaline; trimethyl-(5-trimethylstannylthiophen-2-yl)stannane): ITIC-Th (Propanedinitrile,2,2′-[[6,6,12,12-tetrakis(5-hexyl-2-thienyl)-6,12-dihydrodithieno[2,3-d: 2′,3′-d’] −s-indaceno[1,2-b:5,6-b’] dithiophene-2,8-diyl] bis[methylidyne(3-oxo-1H-indene-2,1(3H)-diylidene)]] bis-) (AC@PI) and Active Carbon@PTQ10: PC71BM (6,6)-phenyl C71 butyric acid methyl ester), were synthesized using a wide bandgap material, PTQ10, as the electron donor, along with a non-fullerene material, ITIC-Th, and a fullerene material, PC71BM, as the acceptors, respectively. Under optimized conditions, AC@PI degraded 93.4 % of 2,2 ’,4,4 ’-tetrachlorobiphenyl (PCB 47) within 60 min. By incorporating a non-fullerene acceptor (ITIC-Th), AC@PI exhibits a larger surface photopressure, a lower hole-electron transfer ratio, a broader absorption spectrum (400 – 1000 nm), and enhanced structural stability. AC@PI can generate photogenerated electrons and holes, as well as superoxide anions (O2−) and hydroxyl radicals (OH), through type II heterojunctions, which contributes to its exceptional properties. This study synthesized novel organic semiconductor catalysts that offer a green, efficient, and non-toxic method for the degradation of aromatic pollutants, such as polychlorinated biphenyls.

Comparison of PMMA layers doped with methyl red and CdSe quantum dots for application in fibre optic

The paper presents a study of the luminescence properties in the visible spectral range of poly(methyl methacrylate) (PMMA) doped with organic dye-methyl red and CdSe quantum dots. The emission spectra of polymer-doped composite structures were investigated and compared with pure PMMA. Optical characterisation was carried out, focused on the measurement and analysis of excitation and luminescence spectra. Comparison of the spectroscopic characteristics of the developed materials allowed comparing the effect of doping on the luminescence properties of the obtained materials and considering their potential application as luminescent materials in fibre optic sensors.

Photon shifting and trapping in perovskite solar cells for improved efficiency and stability

Advanced light management techniques can enhance the sunlight absorption of perovskite solar cells (PSCs). When located at the front, they may act as a UV barrier, which is paramount for protecting the perovskite layer against UV-enabled degradation. Although it was recently shown that photonic structures such as Escher-like patterns could approach the theoretical Lambertian-limit of light trapping, it remains challenging to also implement UV protection properties for these diffractive structures while maintaining broadband absorption gains. Here, we propose a checkerboard (CB) tile pattern with designated UV photon conversion capability. Through a combined optical and electrical modeling approach, this photonic structure can increase photocurrent and power conversion efficiency in ultrathin PSCs by 25.9% and 28.2%, respectively. We further introduce a luminescent down-shifting encapsulant that converts the UV irradiation into Visible photons matching the solar cell absorption spectrum. To this end, experimentally obtained absorption and emission profiles of state-of-the-art down-shifting materials (i.e., lanthanide-based organic-inorganic hybrids) are used to predict potential gains from harnessing the UV energy. We demonstrate that at least 94% of the impinging UV radiation can be effectively converted into the Visible spectral range. Photonic protection from high-energy photons contributes to the market deployment of perovskite solar cell technology, and may become crucial for Space applications under AM0 illumination. By combining light trapping with luminescent downshifting layers, this work unravels a potential photonic solution to overcome UV degradation in PSCs while circumventing optical losses in ultrathin cells, thus improving both performance and stability.

Single-Shot Multispectral Encoding: Advancing Optical Lithography for Encryption and Spectroscopy.

Most modern optical display and sensing devices utilize a limited number of spectral units within the visible range, based on human color perception. In contrast, the rapid advancement of machine-based pattern recognition and spectral analysis could facilitate the use of multispectral functional units, yet the challenge of creating complex, high-definition, and reproducible patterns with an increasing number of spectral units limits their widespread application. Here, we report a technique for optical lithography that employs a single-shot exposure to reproduce perovskite films with spatially controlled optical band gaps through light-induced compositional modulations. Luminescent patterns are designed to program correlations between spatial and spectral information, covering the entire visible spectral range. Using this platform, we demonstrate multispectral encoding patterns for encryption and multivariate optical converters for dispersive optics-free spectroscopy with high spectral resolution. The fabrication process is conducted at room temperature and can be extended to other material and device platforms.

Measurement of bidirectional transmittance distribution function in the visible and near-infrared spectral range

This work characterizes a facility for bidirectional transmittance distribution function (BTDF) measurements in the visible and near-infrared (NIR) wavelength range. The facility includes an absolute reference gonioreflectometer, with an extended capability for BTDF measurements, and a commercial instrument, Cary 7000, using a goniometer extension, Universal Measurement Accessory. The facility characterization includes measurements of two quasi-Lambertian diffusers for their BTDF. The diffuser samples include a piece of porous polytetrafluoroethylene (PTFE) and a piece of fused synthetic silica, HOD-500. The samples are measured for their spectral BTDF in the wavelength range from 450 nm to 1650 nm in 50 nm steps, and in the viewing zenith angle range from −35° to 35° in 5° steps. Measurements are performed in-plane, using an incident zenith angle of 0°, while rotating the samples around their surface normal from 0° to 180° in 90° steps. Rotation of the sample serves the dual purpose of checking the sample rotational symmetry in azimuth angles and for averaging the measurement results. The characterization results show that both samples exhibit Lambertian characteristics across the visible and NIR wavelength range. Furthermore, both samples show a smooth increase in their spectral BTDF as a function of wavelength. Notably, the PTFE sample demonstrates a steeper slope in its spectral BTDF and a higher signal above 650 nm. The facility capability for measurements of spectral BTDF is validated by a rigorous uncertainty analysis and by comparing the difference in measurement results between the absolute gonioreflectometer and the commercial instrument. Characterization results indicate that the absolute reference gonioreflectometer has a standard uncertainty ranging from 0.29% to 0.42%, dependent on the BTDF measurement wavelength. Experimental results show that the devices of the facility deviate in their BTDF within their combined expanded uncertainty, affirming the capability and reliability of the facility for BTDF measurements.

(Keynote) Engineering Lead Halide Perovskite Nanocrystals for Fast Single-Particle and Collective Light Emission

Colloidal lead halide perovskite (LHP) nanocrystals (NCs), with bright and spectrally narrow photoluminescence (PL) tunable over the entire visible spectral range, are of immense interest as classical and quantum light sources. Fast (bright) and pure single-photon emission is key for many quantum technologies, from optical quantum computing to quantum key distribution and quantum imaging.The brightness of an emitter is ultimately described by Fermi’s golden rule, with a radiative rate proportional to its oscillator strength (intrinsic emitter property) times the local density of photonic states (photonic engineering, i.e. cavity). With perovskite NCs, we present a record-low sub-100 ps radiative decay time for CsPb(Br/Cl)3, almost as short as the reported exciton coherence time, by the NC size increase to 30 nm. The characteristic dependence of radiative rates on QD size, composition, and temperature suggests the formation of giant transition dipoles, as confirmed by effective-mass calculations for the case of the giant oscillator strength. Importantly, the fast radiative rate is achieved along with the single-photon emission despite the NC size being ten times larger than the exciton Bohr radius. NC self-assembly is a versatile platform for materials engineering, particularly for attaining collective phenomena with perovskite NCs, such as superfluorescence in perovskite NC superlattices. Thus far, LHP NCs have been co-assembled with building blocks that acted solely as spacers to promote the tuning of the mutual arrangement of LHP nanocubes [2]. However, the functionality of the second SL component can give rise to the enhancement of the LHP NCs properties or the emergence of new collective effects. We present the formation of multicomponent SLs made from the CsPbBr3 NCs of two different sizes. The diversity of obtained SLs encompassed the binary ABO6-, ABO3-, and NaCl-type structures, all of which contained orientationally and positionally confined NCs. For the selected model system, the ABO6-type SL, we observed efficient NC coupling and Förster-like energy transfer from strongly confined 5.3 nm CsPbBr3 NCs to weakly confined 17.6 nm CsPbBr3 NCs. Exciton spatiotemporal dynamics measurements reveal that binary SLs exhibit enhanced exciton diffusivity compared to one-component SLs across the entire temperature range (from 5 K to 298 K). Observed incoherent NC coupling and controllable excitonic transport within the solid NC SLs hold promise for potential applications in optoelectronic devices.[1]C. Zhu, S. C. Boehme, L. G. Feld, A. Moskalenko, D. N. Dirin, R. F. Mahrt, T. Stöferle, M. I. Bodnarchuk, A. L. Efros, P. C. Sercel, M. V. Kovalenko, G. Rainò. In review.[2] Ihor Cherniukh et al. Nature, 2021, 593, 535–542. [3] T. Sekh et al. Submitted

A single phased full color emitting pyrochlore type phosphor La2Y0.66Sn0.66Sb0.66O7: Bi3+, Eu3+ for white light emitting diode applications (WLEDs)

The phosphor converted white light emitting diodes (pc-WLEDs) are advancing rapidly in replacing the conventional fluorescent and incandescent light sources. However, the current technology of pc-WLEDs limits their large scale indoor lighting applications due to their high correlated color temperature (CCT) and poor color rendering index (CRI). In this work, a single phased full color emitting phosphor in the pyrochlore oxide system, La2Y0.66Sn0.66Sb0.66O7: Bi3+, Eu3+ has been developed by the high temperature ceramic route. The developed phosphors are characterized by powder X-ray diffraction, luminescence and lifetime measurements. Upon near UV excitation wavelength, the singly doped phosphor, La2Y0.66Sn0.66Sb0.66O7: Bi3+ show intense blue green light in the wavelength region 400–600 nm due to the characteristic transitions (3P1 → 1S0) of Bi3+. The emission spectral analysis suggests that there exist two luminescence centers of Bi3+ due to occupation of different crystallographic sites in the pyrochlore lattice of La2Y0.66Sn0.66Sb0.66O7. The deficit of red component in the emission spectra is overcome by the Eu3+ co-doping in the system, La2Y0.66Sn0.66Sb0.66O7: Bi3+, Eu3+ which results in the full color emission covering the visible spectral range up to 700 nm with broad peaks along with sharp narrow characteristic peaks of Eu3+. The full color emission of the La2Y0.66Sn0.66Sb0.66O7: 0.04Bi3+, yEu3+ phosphors can be tuned by adjusting the suitable Eu3+ dopant concentration and using appropriate energy transfer from Bi3+ to Eu3+. The full color emitting phosphor, La2Y0.66Sn0.66Sb0.66O7:0.04Bi3+, 0.06Eu3+ can be realized with the color coordinates (0.30, 0.36), and correlated color temperature (4383K). All these results demonstrate that La2Y0.66Sn0.66Sb0.66O7:xBi3+, yEu3+ are potential single phased full color emitting phosphors for the development of pc-WLEDs.

Impact of oxygen vacancy on luminescence properties of dysprosium-doped zinc oxide thin films

Pristine and dysprosium-doped ZnO thin films, highly c-axis oriented, were grown on a c-Al2O3 substrate using the pulsed laser deposition technique. During the deposition process, the growth was impacted by changes in the oxygen partial pressure. The structural, optical, and compositional properties of the pristine and dysprosium-doped ZnO thin films were systematically characterized using various state-of-the-art experimental techniques. The crystallinity associated with these films was observed to be highly dependent on the oxygen partial pressure maintained during the growth process. An average optical transparency of more than ∼90% in the visible spectral range (i.e., 400–800 nm) was observed for all specimens. For doped specimens, expected intra-4fn emissions of (dysprosium) Dy3+ ions that emanated from the 4F9/2 energy state were observed in room-temperature photoluminescence spectra under direct excitation. The Jacobian-transformed wavelength-to-energy plot for Dy-doped ZnO thin films in the UV-Visible region revealed intriguing findings. Specifically, it depicted a prominent peak at 2.82 eV, indicative of the energy state of Dy3+ ions, alongside intense band-edge emission and various defect-related emissions. The intensity of these transitions was observed to depend on the oxygen partial pressure kept during deposition. An x-ray photoelectron spectroscopy analysis of the doped specimen confirmed the +3 oxidation state of dysprosium into the ZnO matrix. These results provide a promising approach for controlling the doping and growth strategy of Dy-doped ZnO thin films, thereby opening up a wide range of applications and enhancement for next-generation optoelectronic devices.

Devices and Methods for Dosimetry of Personalized Photodynamic Therapy of Tumors: A Review on Recent Trends.

Significance: Despite the widespread use of photodynamic therapy in clinical practice, there is a lack of personalized methods for assessing the sufficiency of photodynamic exposure on tumors, depending on tissue parameters that change during light irradiation. This can lead to different treatment results. Aim: The objective of this article was to conduct a comprehensive review of devices and methods employed for the implicit dosimetric monitoring of personalized photodynamic therapy for tumors. Methods: The review included 88 peer-reviewed research articles published between January 2010 and April 2024 that employed implicit monitoring methods, such as fluorescence imaging and diffuse reflectance spectroscopy. Additionally, it encompassed computer modeling methods that are most often and successfully used in preclinical and clinical practice to predict treatment outcomes. The Internet search engine Google Scholar and the Scopus database were used to search the literature for relevant articles. Results: The review analyzed and compared the results of 88 peer-reviewed research articles presenting various methods of implicit dosimetry during photodynamic therapy. The most prominent wavelengths for PDT are in the visible and near-infrared spectral range such as 405, 630, 660, and 690 nm. Conclusions: The problem of developing an accurate, reliable, and easily implemented dosimetry method for photodynamic therapy remains a current problem, since determining the effective light dose for a specific tumor is a decisive factor in achieving a positive treatment outcome.

Effect of Nitrogen Doping on the Photocatalytic Properties and Antibiofilm Efficacy of Reduced TiO2 Nanoparticles.

Nanomaterial-mediated antibacterial photodynamic therapy (aPDT) emerges as a promising treatment against antibiotic-resistant bacterial biofilms. Specifically, titanium dioxide nanoparticles (TiO2 NPs) are being investigated as photosensitizers in aPDT to address biofilm related diseases. To enhance their photocatalytic performance in the visible spectral range for biomedical applications, various strategies have been adopted, including reduction of TiO2 NPs. However, despite improvements in visible-light photoactivity, reduced TiO2 NPs have yet to reach their expected performance primarily due to the instability of oxygen vacancies and their tendency to reoxidize easily. To address this, we present a two-step approach to fabricate highly visible-light active and stable TiO2 NP photocatalysts, involving nitrogen doping followed by a magnesium-assisted reductive annealing process. X-ray photoelectron spectroscopy analysis of the synthesized reduced nitrogen-doped TiO2 NPs (H:Mg-N-TiO2 NPs) reveals that the presence of nitrogen stabilizes oxygen vacancies and reduced Ti species, leading to increased production of reactive oxygen species under visible-light excitation. The improved aPDT efficiency translates to a 3-fold enhancement in the antibiofilm activity of nitrogen-doped compared to undoped reduced TiO2 NPs against both Gram-positive (Streptococcus mutans) and Gram-negative (Porphyromonas gingivalis, Fusobacterium nucleatum) oral pathogens. These results underscore the potential of H:Mg-N-TiO2 NPs in aPDT for combating bacterial biofilms effectively.

Thin-Film Polarizers in Visible Spectral Range Dopped with Quartz Nanoparticles: a Possible Violation of Microhardness Increase Trend During Nanostructuring of Polymer Matrix

Thin-Film Polarizers in Visible Spectral Range Dopped with Quartz Nanoparticles: a Possible Violation of Microhardness Increase Trend During Nanostructuring of Polymer Matrix

Anisotropic Optical Response of Ti-Doped VO2 Single Crystals.

This study delves into the effects of titanium (Ti) doping on the optical properties of vanadium dioxide (VO2), a material well known for its metal-to-insulator transition (MIT) near room temperature. By incorporating Ti into VO2's crystal lattice, we aim to uncover the resultant changes in its physical properties, crucial for enhancing its application in smart devices. Utilizing polarized infrared micro-spectroscopy, we examined TixV1-xO2 single crystals with varying Ti concentrations (x = 0.059, x = 0.082, and x = 0.187) across different crystal phases (the conductive rutile phase and insulating monoclinic phases M1 and M2) from the far-infrared to the visible spectral range. Our findings reveal that Ti doping significantly influences the phononic spectra, introducing absorption peaks not attributed to pure VO2 or TiO2. This is especially notable with polarization along the crystal growth axis, mainly in the x = 0.187 sample. Furthermore, we demonstrate that the electronic contribution to optical conductivity in the metallic phase exhibits strong anisotropy, higher along the c axis than the a-b plane. This anisotropy, coupled with the progressive broadening of the zone center infrared active phonon modes with increasing doping, highlights the complex interplay between structural and electronic dynamics in doped VO2. Our results underscore the potential of Ti doping in fine-tuning VO2's electronic and thermochromic properties, paving the way for its enhanced application in optoelectronic devices and technologies.