CIE Chromaticity Diagram Research Articles

Photoluminescence Studies of Europium Doped Di-Strontium Magnesium Silicate Phosphors

This article reports the study of photoluminescence spectra of Di Strontium Magnesium Silicate (Sr2MgSi2O7) doped with various concentration of Europium (Eu3+) prepared using solid-state reaction technique. The doping concentration (s) of Eu3+ were 0.2, 0.5, 1.0, 1.5, 2.0, 2.5 mol% respectively. Excitation spectrum monitored at 620 nm wavelength that exhibited two prominent peaks centered at 256 nm and 277 nm. Peak observed at 277 nm was more intense therefore the emission spectra were monitored at 277. Emission Spectra of all the samples revealed intense peaks centred at 607, 618 and 637 are attributed to 5D0 7F2 of Europium (III) ions accommodated at various lattice sites having different energies. Overall emission was found in the red colour region which was confirmed using a CIE chromaticity diagram with coordinate (0.4805, 0.3763). Critical distance for energy transfer in the concentration, beyond which concentration quenching occurred in PL spectra, was calculated. In this case, the critical distance was found to be 19.87 Å, therefore the mechanism involved in concentration quenching of Sr2MgSi2O7 doped with 2.0 mol % of Europium (III) must be only multipole-multipole exchange whereas the exchange interaction is ineffective.

Transparent Gd3Al2Ga3O12 nanocrystals/glass system: Al2O3 enhanced photoluminescence and optical nonlinearity

Nanocrystals doped transparent materials have shown promising enhanced optical absorption and luminescence properties. In this study, we report the in-situ crystallization of Gd3Al2Ga3O12 in tellurite glass under the modification of Al2O3, and the influence on glass network structure and optical performances as a function of the size, coordination states, crystal structure, and defects of nanocrystals Gd3Al2Ga3O12 garnet (GAGG). 10-20 nm-cubic Gd3Al2Ga3O12 (Ia3d space group) nanocrystals formed in glass whose size increased with the Al2O3 content. Due to the existence of nanocrystals Gd3Al2Ga3O12, Sm3+ containing glass showed enhanced 9 absorption bands due to electron transitions of activator ions. In addition, Al2O3-activated GAGG crystallization decreased the Eg of glass, resulting in greatly improved red luminescence at 600 nm. The CIE chromaticity diagram revealed good luminescence stability. The polarizability was significantly increased which contributed to the huge third-order nonlinear behaviors. As Al2O3 increased, the χ(3) and n2 increased while the optical limiting threshold decreased. Glass A75 exhibited excellent optical linear, nonlinear, and emission properties which are promising for photonics device applications.

Structural features and optical-luminescent properties of the Pb-containing germanate and silicate oxyfluoride glasses

The XRD, IR transmission, EPR, optical absorption, luminescence spectra and decay kinetics in new un-doped 0.5PbO–0.3GeO2–0.2AlF3 and 0.53PbO–0.226SiO2–0.244AlF3 glasses were investigated. The XRD patterns were analysed in order to obtain the radial distribution functions and define the average interatomic distances and coordination numbers in the network of investigated glasses. The IR transmission, EPR, and UV–vis optical absorption spectra of the studied glasses were interpreted. Optical band gap and Urbach energy were obtained from analysis of the fundamental absorption edge. Under UV photoexcitation the 0.5PbO–0.3GeO2–0.2AlF3 and 0.53PbO–0.226SiO2–0.244AlF3 glasses demonstrate broad emission band at 493 nm and 499 nm, respectively. Fast (band-to-band recombination) and slow (recombination with a contribution of defects levels) mechanisms of intrinsic luminescence have been proposed from a detailed analysis of luminescence spectra and decay kinetics curves. The CIE chromaticity diagram shows a variation in emission colour of studied glasses with the change in their composition.

Luminescence properties and energy transfer studies in thermally stable Bi2O3: Sm3+, Eu3+ phosphor

The advent of innovative rare-earth based metal oxide phosphors has prompted increasing interest in the search for highly efficient red emitting phosphors for various optoelectronic devices, lighting and display applications and motivated the research community to develop some promising, state-of-the-art, and potential red phosphors to meet the necessary requirements. In this study, a series of Sm3+ (1–7 mol%) doped, Sm3+ (5%) doped and Eu3+ (2–5 mol%) co-doped Bi2O3 red phosphors were synthesized by a facile urea assisted auto-combustion process. The X-ray diffraction (XRD) analysis of the phosphors ascertains the tetragonal crystal structure without any trace of impurity. The UV-Vis-NIR absorption study of the phosphors was carried out in 200–1000 nm range and the estimated optical band gap of the phosphors was lying in the range of 2.778–2.831 eV. The photoluminescence (PL) emission studies were performed at an excitation wavelength of 481 nm in the 550–720 nm range. The effective energy transfer mechanism from Sm3+ to Eu3+ in the Bi2O3 phosphor has been ascribed to the dipole-dipole (d-d) interaction. The energy transfer process was further validated by PL decay time measurements. The CIE color coordinates were obtained using PL emission data. The CIE chromaticity diagram ascertains that the emission color of Bi2O3: Sm3+ phosphors were tuned from reddish-orange to the red region by varying the concentration of Eu3+ ions. The temperature dependent PL study of the phosphors was taken out in the range of 293–453 K and was found thermally stable at elevated temperatures.

Correlation of Emulsion Chemistry, Film Morphology, and Device Performance in Polyfluorene LEDs Deposited by RIR-MAPLE

Thin films of polyfluorene (PFO) were deposited using emulsion-based resonant infrared, matrix-assisted pulsed laser evaporation (RIR-MAPLE). Here, it is shown that properly selected surfactant chemistry in the emulsion can increase crystalline β phase (β-PFO) content and consequently improve the color purity of light emission. To determine the impact of surfactant on the device performance of resulting films, blue light-emitting diodes (LEDs) with PFO as an active region were fabricated and compared. Molecular dynamics (MD) simulations were used to explain the physical and chemical changes in the emulsion properties as a function of the surfactant. The results indicate that the experimental film morphology and device performance are highly correlated to the emulsion droplet micelle structure and interaction energy among PFO, primary solvent, and water obtained from MD simulations. While the champion device performance was lower than other reported devices (luminous flux ∼0.0206 lm, brightness ∼725.58 cd/m2, luminous efficacy ∼0.0548 lm/W, and luminous efficiency ∼0.174 cd/A), deep blue emission with good color purity (CIE chromaticity diagram coordinate of (0.177,0.141)) was achieved for low operating voltages around 3 V. Furthermore, a much higher β-phase content of 21% was achieved in annealed films (without the pinholes typically found in β-PFO deposited by other techniques) by using sodium dodecyl sulfate (SDS) as the surfactant.

Unprecedented investigation on luminescence property of proton conductive samarium incorporated barium cerate nanoparticles

Single phase perovskite structured BaCe0.98Sm0.02O2.99 (BCS) were synthesized through solution-combustion method followed by calcination in air atmosphere. Orthorhombic BCS perovskite crystal formation was confirmed by XRD analysis. Sm3+ incorporation causes emissions with a dominant 4G5/2 → 6H5/2 transition centered at 574 nm and 4G5/2 → 6H7/2 at 615 nm. Chromaticity values were calculated using CIE chromaticity diagram. The calibre of emitted light is assessed by color purity (CP) and correlated color temperature (CCT). The obtained results indicate that the synthesized BCS perovskites have excellent luminescence properties suitable for lighting technology.

Synchrotron, luminescence, and XPS studies of Gd3+:Dy3+:Ce3+ tri-rare-earth oxides in borate glasses

Synchrotron studies were employed to understand the oxidation state and coordination chemistry of the Ce3+ ions which when added with Dy3+- co-doped calcium zinc gadolinium borate glasses prepared by conventional melt quenching technique. X-ray photoelectron spectroscopy was employed to understand the coordination chemistry of the prepared glass samples. Detailed analysis was investigated to understand the glass's optical properties such as density, refractive index, molar volume, field strength, inter-ionic radius, and average boron-boron distance. Using optical absorption studies, the Judd-Ofelt parameters were obtained, and radiative properties were evaluated and compared with other reported literature through the JO parameters. Photoluminescence and X-ray luminescence studies show similar results and found that the 575 nm corresponds to the electric-dipole transition of 4F9/2 → 6H13/2. Asymmetry ratio (Y/B) was evaluated to understand their covalency around the rare-earth ion. Lifetime analysis for Dy3+ and Gd3+ ions were able to probe and analyzed with a comparison of other reported glasses. CIE chromaticity diagram was able to plot for the prepared glass samples to bring about the (x, y) significance which provides their color coordinate temperature (CCT) values and compared with other glasses.

Luminescence in Er3+ co-doped bismuth germinate glass–ceramics for blue and green emitting applications

Glass samples with the composition 40 Bi2O3-60 GeO2-x Er2O3 are prepared using the melt quenching procedure. Fourier-transform infrared spectroscopy (FTIR), differential thermal analysis (DTA), Ultraviolet/Visible/Near-infrared (UV/Vis–NIR) absorption, photoluminescence spectroscopy (PL), and X-ray diffraction (XRD) are used to characterize the glass and glass–ceramic materials. The glass transition and crystallization temperatures are determined. Crystallization kinetics was studied in non-isothermal conditions. The oscillator strengths and Judd–Ofelt parameters (Ω2, Ω4, Ω6) for reported Er3+ absorption transitions are estimated, and they follow the trend Ω2 > Ω6 > Ω4 for observed Er3+ absorption transitions. The iconicity of glass samples was unaffected by an increase in Er3+ ions. XRD and FTIR confirmed the formation of the Bi4Ge3O12 phase after heat treatment of glass samples at the crystallization temperature. The CIE chromaticity diagram computes the CIE chromatic coordinates. The values for all glass and crystal samples are close to bright blue and green. Glass and glass ceramic samples are suitable for green and blue optoelectronics device applications.

High contrast green luminescence from spin-coated Mo-doped β-Ga2O3 thin films

High-contrast green luminescence was obtained from molybdenum (Mo)-doped beta-phase gallium oxide (β-Ga2O3) by a sol-gel spin coating method. The X-ray diffraction, atomic force microscopy, and ultraviolet–visible–near infrared transmission results revealed that the Mo dopants did not significantly affect the fabricated films’ crystal structure, surface morphology, and energy bandgap (Eg). All spin-coated Mo-doped films formed a beta phase structure and exhibited smooth surface morphology with a root-mean-square surface roughness of less than 7 nm. The thickness of the coated film increased gradually with Mo doping concentration. In the photoluminescence measurements, a green luminescence band associated with the Mo ion dxz-dyz band transition slowly appeared, which hindered the original emission from the recombination of electron–hole pairs in β-Ga2O3 but increased overlapped green color luminescence. Compared with the undoped β-Ga2O3, the CIE chromaticity diagram showed that luminescence changed from blue to green, and the contrast of the green luminescence significantly improved from 0.33 to 0.73.

Coactivated cyan emitting phosphors in optical thermometry using thermally and non- thermally coupled levels

The tunability in upconversion (UC) emission has been obtained with varying dopants concentration, visualized with a CIE chromaticity diagram and optimized in the cyan emitting region. The optical bistability has been observed in the Tm3+/Yb3+: MgWO4 phosphors. The CIE coordinates migrated towards the green region after doping Er3+ in the Tm3+/Yb3+: MgWO4 phosphors. The decay curve analysis of the developed phosphors has been performed. Thermally and non-thermally coupled levels of Er3+ and Tm3+ ions in the coactivated MgWO4 phosphors have been considered in the optical thermometry using the fluorescence intensity ratio (FIR) approach. The addition of Er3+ and Li+ in Tm3+/Yb3+: MgWO4 phosphors improves the sensitivity of luminescence thermometry. The absolute and relative sensitivity in the non-thermally coupled levels is found maximum as compared to the system containing thermally coupled levels.

A Color-Mixing Setup for a Physics Teaching Laboratory

In this paper, an experimental setup composed of a color-mixing device, power supply, and spectrometer is developed. This setup is available for students to do hands-on color mixing with red, green, and blue light in undergraduate labs. LED volt-ampere and spectrum characteristics are investigated. The apparatus was used to perform additive color mixing, and visual colorimetric parameters such as color coordinates, dominant wavelength, and color purity were obtained. This will help students understand the physics of LEDs and chromaticity. In addition, this experiment is useful in introducing topics such as the CIE chromaticity diagram and human perception of colors.

Synthesis, structure and luminescence properties of bifunctional KCaF3 phosphor influenced by incorporating Eu3+ ions for solid state lighting and TL dosimetry applications

Europium doped KCaF3 phosphors (KCaF3:Eu3+) were prepared using various concentrations of Eu3+ by conventional solid-state reaction process. The X-ray diffraction (XRD) studies confirmed the formation of orthorhombic structured KCaF3:Eu3+ phosphors. Scanning Electron Microscopy (SEM) image of the synthesized phosphor exhibits agglomerated particles with irregular shapes. The composition of the synthesized sample was determined by Energy Dispersive Spectroscopy (EDS) spectrum and elemental mapping showed the homogeneous dispersion of Eu3+ ions into the synthesized KCaF3:Eu3+ phosphor. The emission peak intensity at 594 nm from photoluminescence (PL) spectra was found to increase with the increase of Eu3+ concentrations from 0.02 mol% to 0.06 mol% and decreased with the further increase of Eu3+ concentration up to 0.1 mol%. CIE1931 chromaticity diagram coordinates (x, y) of KCaF3:(0.06 mol%) Eu3+ phosphors were positioned in the reddish-orange region (x = 0.5736, y = 0.4224). Photoluminescence property confirms the suitability of KCaF3:Eu3+ phosphors for Solid state lighting application. X-ray induced luminescence (radioluminescence, RL) is recorded for KCaF3:Eu3+ phosphors showing the characteristic emission of Eu2+ and Eu3+. ESR study on KCaF3:Eu3+ phosphors confirm the presence of Eu2+ ions. Beta irradiated thermoluminescence (TL) glow curve of Eu3+ doped KCaF3 phosphors is measured and deconvoluted using Gaussian fitting. TL kinetic parameters like activation energy (Ea) and frequency factor (s) are calculated for all the deconvoluted peaks using peak shape method which shows the synthesized KCaF3:Eu3+ phosphors is suitable for dosimetry application.

Facile and eco-friendly PVA nanocomposites as a photo-luminescent: Anti-counterfeiting, LED and wettability applications

Novel MoO3:Eu3+ nanopowder (NP) were synthesised by the propellant chemical route. Polymer nanocomposites (PNCs) were prepared by embedding NPs effectively into the mixture of Poly vinyl alcohol (PVA) and Chitosan solution through the solution casting technique (SCT). This process was extensively used owing to certain benefits such as uniform dispersion of NPs and easiness of the process. Eu3+ doped MoO3 (2–5 wt %) nanocomposites (NCs) were fabricated. Powder X-ray diffraction analysis indicated single orthorhombic crystal structure of the synthesised NCs and confirmed the integration of the nanophosphor into the polymer blend. Crystallite size was estimated to be ∼17nm using Scherrer's formula. The Fourier transform infrared spectrometer technique was employed to reveal the vibrational bands as well as the intermolecular bonding between the polymer blend and MoO3:Eu3+ NPs. Thermogravemetric analysis showed the enhanced thermal stability of the PNCs with the incorporation of NPs. The nature of electronic transitions was studied by optical bandgap energy from Tauc's relation. The photoluminescence emission spectra were monitored at excitation wavelength of 394 nm and noticed the transitions at wavelengths of 599 nm, 613 nm, 630 nm, 652 nm, and 694 nm belongs to the intense orange-red emissions. The emission colour of the NC films was confirmed by CIE chromaticity diagram. Especially, the NCs complex not only can be uniformly dispersed in the monomer solution and can be acted as both the coordination crosslinking agent and luminophores. Besides, upon the incorporation of Eu3+ into the polymer matrix comprising ammonium group, the adhesive strength as well as the mechanical behaviour of the hydrogel are considerably enhanced. The Eu3+ doped NCs that were developed have a great potential for fabricating efficient optoelectronic devices and luminescent hydrogels. This mechanism was further validated and utilised for application in relief printing and screen printing.

An All-Dielectric Color Filter, with a Wider Color Gamut

Due to their extraordinary abilities to manipulate light propagation at the nanoscale, dielectric resonators that generate electric and magnetic Mie resonances for minimal optical loss have recently attracted great interest. Based on an all-dielectric metasurface, made of H-type silicon nanoarrays, this study proposed and constructed a visible-wavelength-range color filter, with high-quality Mie resonance and the ability to synthesize new colors. Using the finite-difference time-domain (FDTD) approach, we can create a larger color gamut by modifying the H-type array’s structural properties. The all-dielectric color filter suggested has a high color saturation and narrow bandwidth. The Mie resonance can be adjusted by manipulating the structural characteristics. By translating the reflectance spectrum into color coordinates and using the CIE1931 chromaticity diagram, a wide range of colors can be generated. This color filter offers a larger color range and saturation than other color filters. We produced color passband filters that span the visible spectrum using Mie resonator arrays, based on an H-type nanoresonator. This technology could have many applications, including high-resolution color printing, color-tunable switches, and sensing systems.

Radio and photo luminescence properties of Dy3+ ion doped bismuth barium gadolinium borate glass

Investigations on physical, optical and photo – radio luminescence properties of dysprosium doped bismuth barium gadolinium borate glass (BiBaGdBDy) have been prepared for the melt at 1200 °C and rapidly cooling in the casting process. The glasses were doped by Dy2O3 with six concentrations which varied from 0.00 to 1.50 mol%. The cut and polished glass samples were measured density and refractive index at room temperature. The absorption spectra show at the wavelength 802, 891, 1083, 1265 and 1679 nm that that are matched with the transition of Dy3+ from 6H15/2 (ground state) to various excited state 6F5/2, 6F7/2, 6F9/2, 6F11/2 and 6H11/2 respectively. The 387 nm flash lamp was used to excite the sample lead to display the five emission bands at 481, 576, 661 and 753 nm which matched with the emission transitions 4F9/2→6HJ (J = 15/2, 13/2, 11/2 and 9/2) respectively. The prominent blue light from 4F9/2→6H15/2 and yellow light from 4F9/2→6H13/2 emissions was combined and produces white light emission, which is confirmed by the CIE chromaticity diagram. The certain Lifetime of important laser-level is 4F3/2, which shows a decreasing trend with the increase of Dy2O3 concentration. The Judd-Ofelt (J-O) intensity parameters found the trend Ω2 > Ω4 > Ω6. The branching ratios (βR) transition probabilities (AR) and stimulated emission cross-sections (σe) were estimated for 4F9/2→6H13/2 transition they are the radiative properties that were used to figure out the potential benefit of these glasses for laser action in the visible region along these lines suggests that the present Dy3+ doped bismuth barium gadolinium borate glasses are suitable for white light applications and scintillator material.

Enhancement of the red emission of the Y2O3:Yb3+-Er3+ nanocrystals using Li+ as a dopant and their incorporation into PMMA pellets

In this study, the visible up-conversion (UC) radiation in (Yb3+-Er3+) co-activated Y2O3 nanophosphors by tri-doping with Li+ were prepared via hydrothermal-precipitation method. The synthesized nanophosphors were also embedded in poly methyl methacrylate (PMMA) to fabricate composite polymer materials. The morphology and elemental mapping of the chemical composition for the synthesized nanophosphors were examined through scanning electron microscopy (SEM) and energy-dispersive spectroscopy (EDS). The cubic crystalline structure of the Y2O3 nanomaterials containing (211), (222), (400), (411), (420), (134), (440), (123), (622), (136), and (444) planes was determined using X-ray diffractometer (XRD) with an average size of 19 nm. The vibrational groups of the phosphor samples were evaluated via infrared (FTIR) spectroscopy. Under the irradiation of 980 nm, the UC emission spectra of the nanophosphors were measured in the range of 450–750 nm and compared with the results obtained for Y2O3:(Yb3+-Er3+-Li+) in PMMA. All the phosphors prepared in this work had UC emissions and it increases about a factor of nearly 5 times for green (4S3/2 → 4I15/2) and 27 times for red (4F9/2 → 4I15/2) emissions at 5 mol% of Li+. Furthermore, the obtained color quality parameters that allows a bright intensity of 4F9/2 → 4I15/2 emission transition present in the redish-orange region in CIE chromaticity diagram.

Crystallization behavior and enhanced fluorescence properties of Yb3+/Ho3+/Tb3+ co-doped transparent glass-ceramics containing oxyapatite-type Na3YSi2O7 crystals

The Yb3+/Ho3+/Tb3+ co-doped Na2O-Y2O3-SiO2 system transparent glass-ceramics containing oxyapatite-type Na3YSi2O7 (NYS) crystals were prepared. The structure of the crystal was characterized by XRD and TEM. The most suitable heat treatment process was confirmed by DSC and TEM results. The strongest emission peak in the fluorescence spectra relates to the Tb3+: 5D4→7F5 transition. In the CIE chromaticity diagram, the luminescence is concentrated in the green area. The emission intensity of the glass-ceramic is increased by 7 times compared with the precursor glass. The increase in emission intensity is caused by the entry of rare-earth ions into the Na3YSi2O7 grains to replace the positions of Y3+ ions. The fluorescence properties of the glass-ceramics containing Na3YSi2O7 crystals were tested at different temperatures. It is demonstrated that the luminescent materials show good thermal stability. The obtained glass-ceramic would be a candidate material for green diode elements.

A Self-Color-Changing Film with Periodic Nanostructure for Anti-Counterfeit Application

A self-color-changing film aimed at enhanced security and anti-counterfeit packaging is presented. Its function is to change color automatically when flipped under visible light. It is low-cost, takes a few seconds to check by the naked eye, and does not need any special tools to evaluate. The design of the color-changing, anti-counterfeiting film is based on a frequency-selective surface (FSS). The film is designed with aluminum nanocubes. They are laid out as an array in a plane with equal distance from one another. This arrangement allows us to select certain wavelengths of light to pass through by the size of the cubes and the separation distance between them. The performance is evaluated by a finite element analysis (FEA) method. The results show that the intersection of transmittance and the reflectance curves cause the film to change its color automatically when flipped. We also propose a method to predict the color of the film based on the transmittance values. The accuracy of this method is verified by actual colors from experiments with an error of no more than 12.8%, analyzed by the CIE chromaticity diagram.

Luminescent Manganese/Europium doped ZnS quantum dots: Tunable emission and their application as fluorescent sensor

Manganese and europium doped zinc sulphide (Mn2+/Eu3+@ZnS) quantum dots (QDs) were prepared via direct aqueous route in order to broaden tunable emission. We report a new strategy featuring enhanced photoluminescence through Mn2+/Eu3+@ZnS QDs leading to an intense orange-red emission and thus remarkably enhancing PL QY up to 41.35%. The X-ray diffraction (XRD) and Transmission Electron Microscope (TEM) studies revealed the cubic crystalline structure with average crystalline sizes in the range of 1.5–2.5 nm. The Mn2+/Eu3+@ ZnS QDs was applied as an exceptional chemical sensor for Cu2+ via quenching mechanism due to competitive coordination between QDs and Cu2+ with the limit of detection (LOD) of 0.036 μM. Furthermore, the proposed approach is capable of detecting Cu2+ in water samples with satisfactory recoveries highlighting viability and prospective applications of the sensing platform. We finally demonstrate from CIE Chromaticity diagram that doped QDs show meliorated colorimetric properties which render this new class of QDs a promising prospect to be employed in white LEDs.

Physical, structural and spectroscopic studies of Al2O3-B2O3-Sm2O3 scintillating glass doped with heavy metal oxides

In the present paper, physical, structural, optical and luminescence properties of Sm3+ activated scintillating glasses with variation in Bi2O3 concentration have been studied. The amorphous nature of the glass samples was ascertained by the presence of a broad hump in XRD spectra. FTIR examinations advocated that Bi2O3 exists primarily as [BiO6] units and BO4 units enhanced with surging bismuth oxide content. The values of density increase continuously with the addition of Bi2O3, whereas Vm first decreases till 25 mol% Bi2O3 content, after that it is found to increase. Optical characterization of samples affirms strong sensitivity towards the absorbance of UV–visible radiation. Among the prepared samples, 35 mol% of Bi2O3 doped glass showed the lowest bandgap, high refractive index and Urbach energy. Also, it contains more localized states that confirm its suitability as a protection material from UV radiation. PL results reveal the strong emission occurs at 562, 598, 644, 704 nm wavelength and Bi15 glass exhibits maximum intensity in both excitation and emission spectrum. The intense emission peak originated from 4G5/2 → 6H7/2 transition centered at 600 nm in XRL which discloses the scintillation properties of the material. The CIE chromaticity diagram demonstrated the emission located in the reddish-orange region, which suggests prepared glasses could be useful for reddish-orange laser and LED. Thus, 15 mol% Bi2O3 doped glass becomes more efficient for the use of scintillators in high-energy physics experiments.