Defect Induced high-color-rendering white light emissions from Ag-doped ZnO

It is crucial to find high-quality white-light phosphors that are simple to synthesize in order to create efficient lighting. In this pursuit, the focus has been on achieving the emission of white light in ZnO and Ag-doped ZnO emerging as a promising phosphor candidate. The optimized ZnO nano phosphors demonstrated visible-light emission with ideal Commission international de I E “Eclairage frequently abbreviated as coordinates (x = 0.33, y = 0.33 ) and a correlated color temperature of 6200 K, and a color rendering index of 100 for equivalent to day white light. Furthermore, under certain ideal processing conditions, neutral white-light emission (x = 0.33, y = 0.33 at λex = 280 and 370 nm) with a Color correlated temperature (CCT) of 5500 K(approx.) was achieved. The Color Rendering Index (CRI) of the ZnO and Ag:ZnO nanoparticles exceeded 90, with high values approaching 96%, which is necessary for accurately portraying object colors in comparison to natural sunshine. The research findings revealed a decrease in the FS emission intensity with all the UV excitation when Ag doped in ZnO, which aligns well with the principles of Stern-Volmer quenching. A reduction in intensity was observed which may be due to the Ag dopant interacting with the luminescent nanoparticles and causing a reduction in their emitted light. The study successfully develops and optimizes ZnO-based nano phosphors with tailored white-light emission characteristics, presenting a promising solution for achieving energy-efficient and high-quality white-light sources for various lighting applications.


Introduction
ZnO is a semiconductor material that exhibits remarkable properties, making it suitable for white light emitting diode(WLED) It is a semiconductor of having a wide band gap with a high level of exciton binding energy, which allows it to emit ultraviolet (UV) light efficiently when it is excited by an 1285 (2024) 012034 IOP Publishing doi:10.1088/1755-1315/1285/1/012034 2 electric current [1][2].In (WLEDs, ZnO acts as a key component in the blue or near-UV emission spectrum [3][4][5].To generate white light in WLEDs, a phosphor layer is used over the ZnO layer.The blue or near-UV light emitted by the ZnO excites the phosphor, which then emits light in the visible spectrum, thus producing white light.The use of ZnO in WLEDs allows for precise control of the color temperature, leading to different shades of white light according to the specific requirements of the application [6].Furthermore, ZnO's unique properties, such as less toxicity, thermal stability and chemical inertness.make it an ideal candidate for environmentally friendly lighting solutions.Its wide bandgap also contributes to the stability and durability of the WLEDs, ensuring a longer operational life.As technology continues to advance, researchers are exploring new ways to improve ZnO's performance in WLEDs, such as enhancing its light extraction efficiency and optimizing its electrical properties.These ongoing efforts aim to further enhance the efficiency and luminous output of Silver (Ag) is an essential element for a variety of applications, including electronics, and optical devices, due to its distinctive optical and electrical characteristics.Some key optical and electrical properties of silver include high reflectivity, high transparency, and very low resistivity.Silver nanoparticles exhibit plasmonic behavior, where the collective oscillations of electrons on the metal's surface can interact with light, resulting in enhanced light absorption and scattering

Experimental detail
The auto-combustion methodology is an effective and easy technique for the preparation of Zinc Oxide (ZnO).This method yields ZnO nanoparticles through the exothermic reaction of metal nitrates with organic fuel.Due to its less cost, this methodology is commonly employed in research and industrial applications.In this article, we will explore the steps involved in the preparation of ZnO using the solgel auto-combustion method.Zinc nitrate hexahydrate (Zn(NO3)2•6H2O) is the primary source of zinc ions.During the Combustion process, an organic fuel polyvinyl alcohol (PVA, (C2H4O)n) behaves as a capping and reducing agent, and the concentration taken was 0.1M.Deionized water is used as a solvent to dissolve the Zinc nitrates and the fuel.Accurately measure the required amount of zinc nitrate hexahydrate and the organic fuel in the stoichiometric ratio(1:1).Dissolve the Zinc nitrates and the fuel in deionized water under constant stirring to ensure a homogeneous solution.Place the solution in a suitable container and allow it to dry at room temperature.This will result in the formation of a gel-like precursor.Spontaneous combustion is brought on by the exothermic interaction of the metal nitrates with the organic fuel.Thus, ZnO nanoparticle production is driven by this combustion process.After the auto-combustion process, the prepared samples were sintered at 400°C for a 4hrs duration.
WLEDs, making them even more energy-efficient and cost-effective for widespread use in various lighting applications.Defects are essential in shaping the luminescent behavior of ZnO and offer exciting opportunities for various optoelectronic applications.The optical characteristics of ZnO are mainly due to defect states in ZnO.The visible emission spectra in ZnO are due to the defect center in the energy band such as vacancies of oxygen, doubly ionized Zn vacancy, and Zn interstitial and antisite oxygen in ZnO. Green emission around 500 nm is due to the singly ionized oxygen vacancy which is a donor state [7][8].The yellow emission is frequently thermally unstable and is usually attributable to an absorbed hydroxyl group and interstitial oxygen.Excess oxygen and interstitial zinc have been identified as the sources of the orange-red fluorescence seen in air-annealed nanostructures [9].
Where "λ" represent the wavelength ,2θ is the diffracted angle, β represent full width half maxima,K is a crystallite constant and its value is 0.94.The average crystallite size of Silver doped ZnO is ~22nm more as compare to pure ZnO calculated by using Scherer"s formula (2) .From the XRD spectra , it was found that there was an average shift of 0.16 o in 2θ.This is due to ionic radius of Ag + (1.26Ȧ) is larger as correlate the ionic radius of Zn +2 (0.74Ȧ) .Large ionic radius of Ag + create the strain in the ZnO lattice on doping.The fact is supported by bond length and Volume of unit cell which is increased on doping.
In the XRD , By using the Scherrer equation we can calculate the average crystallite size "S" of the material [11]

Fourier Transform Infra-Red Spectroscopy
The spectra were recorded for ZnO and Ag + : ZnO samples sintered at 400°C in the range of 400 to 4000 cm -1 by using FT-IR, Model, Perkin Elmer.The observed broad peak around 400-600 cm - 1 is assigned to the stretching of Zn-O and Ag-O metal-oxygen bonds [12].Additionally, a weak peak at approximately 3500 cm -1 due to the of moisture in the sample.Surprisingly, both the FT-IR spectra exhibit identical characteristics, leading to the conclusion that the lattice structure was not affected by the doping process.This result aligns with the results of X-ray diffraction (XRD) investigation, which likewise showed that doping had not significantly altered the crystal structure.

Compositional analysis
As illustrated in Figure 6, Energy Dispersive X-ray Spectroscopy(EDS) was utilized to examine the chemical makeup of the samples of Zinc oxide and Silver doped Zinc oxide.The EDS analysis definitively identified the presence of elements such as zinc (Zn), oxygen (O), and Ag in both samples.Importantly, the EDS profiles of Silver doped and ZnO showed no observable evidence of any impurity elements.

UV-VIS Spectroscopy
A UV-visible Shimadzu spectrometer was used to analyze the optical properties of the material between the wavelength of 200 and 800 nm.The samples were sintered at 400°C, and the results revealed that they exhibited over 80% absorption in the range of 200-380 nm, with an edge of absorption observed at 375 nm as shown in Figure 7 (a) [14].From the absorption results, the optical band gap was determined to be roughly 3.31 eV, which was quite similar to the values published in the literature [15] .This sharp peak shows that the distribution of nanoparticle monodisperse and majority of the particle are present in the nanoscale form.There were no further peaks in the spectrum, confirming the XRD result described in section 2.1 that the only material produced during synthesis is the wurtzite hexagonal phase of Zinc oxide .The reflectance spectra in Figure 7(b) show a sharp increase in the visible region, which is indicative of electron transitions taking place in the optical band gap as a result of an oxygen vacancy defect.

Fluorescence Spectroscopy(FS)
The primary properties of Zinc Oxide that make it an appealing material for researching its luminescence capabilities are its broad absorption and configurable band gap.In ZnO, defects create energy levels within the band gap Figure 7. (a,b,c) are the emission spectra acquired with excitation of UV wavelengths of 252 280 and 370 nm.The Fluorescence spectrum (FS) reveals characteristic emission peaks at 423, 442,460,486,500, 532,590, 685, and 732 nm, all associated with ZnO and ZnO with Ag doped when excited with UV light.ZnO is known to contain six major types of defects namely oxygen and zinc: vacancies, interstitials, and antisites.The peak at 423 nm is due to interstitial defects of Zinc, the strong peak at 442 nm [16] and the peak around 500 nm are induced due to vacancies created by oxygen, and the peak around 532nm might be caused by antisite defects in the material.This is a wellestablished cause for white light emission by ZnO is due to the defects like Zn interstitials and oxygen vacancies [17].In particular, the green emission in ZnO is typically attributed to the recombination of the hole and single ionized atom to exciton, while visible emission is mainly influenced by the presence of oxygen vacancies of this defect.Additionally, the emissions at 442 nm and 500 nm can be attributed to surface defects in the ZnO material.The emission peaks were centered at 423 nm and 500 nm with sub-peaks at 460 nm and 500 and 532 nm at the acceptor Level.Ionized atom of oxygen at void and at interstitials emit at 590 nm, 685 nm, and 723 nm, respectively.These findings highlight the importance of controlling the size, morphology, and surface properties of ZnO, as well as considering external doping, to modulate and optimize the emission characteristics in practical applications.
The research findings revealed a decrease in the FS emission intensity with all the UV excitation when Ag was doped in ZnO, which aligns well with the principles of Stern -Volmer quenching [18][19].Stern-Volmer quenching is a well-known phenomenon in spectroscopy that describes the reduction in fluorescence or emission intensity of a luminescent material when it interacts with a quencher.In this case, the quencher could be the Ag dopant that interacts with the luminescent nanoparticles and causes a reduction in their emitted light.In order to showcase a desired color sample, specifically the study of white light, The initial stage in this study determined the physiologically observed color of the samples.This is accomplished by determining the xyz chromaticity coordinates of the photoluminescent emission originating from the materials.The xyz chromaticity coordinates adhere to a standard set by the International Commission on Illumination (CIE) and are represented as x, y, and z.These CIE coordinated can be described as normalized fractions that establish a quantitative relationship between the emission of the material and the color perception by the three different receptors in the human eye.[20][21] These coordinates offer a standardized way to assess and characterize how the human eye perceives colors based on the emitted light from the sample [22].
The Correlated Colour Temperature (CCT) is an indicator used to define a light source's color appearance, notably in connection to the color of a black-body radiator.(An ideal light source).It is measured in Kelvin (K) and helps characterize whether a light source appears warm, neutral, or cool.Lower CCT values (e.g., 2700K) indicate warmer, more yellowish light associated with incandescent bulbs, while higher CCT values (e.g., 5000K) correspond to cooler, bluish light akin to daylight.CCT is essential in choosing lighting for different applications, as it influences the atmosphere, visual comfort, and even our perception of colors under the light source [23][24].When comparing a light source to a refractive light source with the same CCT, the CRI( Color Rendering Index) is a measure of quantity utilized to assess and evaluate how precisely a light source reproduces the colors of objects.The CRI value is expressed on a scale of 0 to 100, with higher values indicating better color rendering capabilities.A CRI of 100 explains perfect color rendering, as seen in natural daylight.CIE, CCT, and CRI are vital concepts in the world of lighting, ensuring that we have a better understanding of light sources, their color characteristics, and their impact on our environment and visual experiences.By following these standards, we can make informed decisions in lighting design and applications to create better and more efficient lighting solutions for various needs

Conclusion
This study involves white light emission characteristics of ZnO and Ag-doped ZnO under UV excitations produced by auto-combustion sol-gel methodology at room temperature.The XRD patterns revealed that both pure Zinc oxide and Silver doped Zinc oxide possess the structure of hexagonal wurtzite.Notably, a significant peak at 101 for doped Zinc oxide and Silver doped pure Zinc Oxide was observed, indicating an average crystalline size of 40 and 61 nm respectively.UV-vis measurements exhibited over 80% absorption in the 200-380 nm range, with an absorption edge observed at 375 nm with an absorption edge at 370nm.Moreover, by carefully controlling specific processing conditions, the study achieved neutral white-light emission (x = 0.33, y = 0.33 at λex = 280 and 370 nm) with a correlated color temperature (CCT) of approximately 5500 K.The ZnO nanoparticles demonstrated an outstanding color rendering index (CRI) exceeding 90, along with high R9 values up to 96%, essential for accurately reproducing object colors comparable to natural sunlight.Through this research, ZnObased nano phosphors were successfully developed and optimized, exhibiting tailored white-light emission characteristics.This achievement represents a promising solution to attain energy-efficient and high-quality white-light sources for diverse lighting applications.

Figure 1 .
Figure 1.Flow Chart explaining the preparation of Ag-ZnO Powder3.Result and Analysis3.1 Phase AnalysisXRD is an effective analytical methodology for the identification of the structure of the material.XRD spectra of ZnO and Ag doped ZnO is shown in figure2.Spectra of X-ray diffraction was mapped with JCPDS card 36-1451 and concluded that ZnO has a hexagonal wurtzite structure.No extra phase was found in Ag + doped ZnO spectra which conclude that Ag + doping did not change the crystal structure.Calculated bond length,d spacing, and the lattice parameters (a and c) by using equations 1 and results are explained in table 1.The calculated parameters were well resembled with the literature.The Miller indices (hkl), plane spacing (d) the lattice constants (c), and in the ZnO hexagonal structure are linked together in the equation(1)[10]

Figure 2 .
Figure 2. XRD pattern of silver doped ZnO powder sintered at 400 o C. Peaks are indexed with JCPDS file No. 36-1451.

Figure 3 .Figure 5 .
Figure 3. FTIR Spectrum of nanoparticle of Zinc Oxide and Ag Doped Zinc oxide.

Figure 6 .
Figure 6.It shows the EDX Spectrum of ZnO and Ag Doped ZnO nanoparticles.

Figure 7 .
Figure 7. UV-VIS (a) absorption spectra and (b) Reflectance graph of Zinc oxide and Silver doped ZnO sintered at 400 o C

Figure 8 .
Figure 8. shows the Fluorescence emission spectrum of ZnO and Silver doped ZnO (a) The excitation wavelength of (a) 252 nm (b) 280 nm and (c) 370nm

Table 1 .
Lattice parameters "c" & "a", ratio c/a, crystallite size (S) ,bond length and d spacing of ZnO and Ag + doped ZnO for major peak (101) calculated from XRD data.S.