Study of Optical Properties Blue Emitting Co²⁺ Doped Zn₂SiO₄ Phosphor

Akash Sinha¹, Vinod Jena^1*, B. Verma², Kanchan Tiwari²

¹Department of Chemistry, Government Nagarjuna P.G. College of Science, Raipur (Chhattisgarh) - 492010, India.

²Department of Physics, Government Nagarjuna P.G. College of Science, Raipur (Chhattisgarh) - 492010, India.

Abstract

This study presents the synthesis and optical characterization of a blue-emitting Co²⁺-doped Zn₂SiO₄ phosphor, prepared via an economical solid-state reaction method. Co²⁺ doping concentrations were systematically varied (1, 2, 4, 6, 8, and 10 mol%) to explore their effect on the phosphor's properties. Powder X-ray diffraction (PXRD) analysis confirmed the orthorhombic crystal structure (ICCD No. 00-001-1975) of Zn₂SiO₄, with an average crystallite size of approximately 34.79 nm calculated using the Debye-Scherrer formula.  Photoluminescence (PL) measurements revealed a prominent blue emission peak at 460 nm, attributed to the ⁴A₂ → ⁴T₁ (⁴P) ⁺ transition of Co²⁺ ions under 334 nm excitation. Additionally, the color coordinates and correlated color temperature (CCT) of the emission were determined, underscoring the phosphor's potential as a robust, efficient blue emitter suitable for optoelectronic and lighting applications.

Keywords:  Solid State method, Phosphor, PXRD, Crystal Size, Photoluminescence,

Introduction

Phosphor materials are increasingly becoming essential in the advancement of optoelectronic technologies. Many modern devices rely on lighting units made from light-emitting diodes (LEDs), which incorporate one or more phosphor compounds (Samsudin et al., 2015). The key to these phosphors lies in the luminescent properties of transition metal or rare earth ions that are doped into specific host materials (Ren et al., 2021). Therefore, the choice of transition metals or rare earth ions and the characteristics of the host materials are crucial factors in phosphor synthesis (Gupta et al., 2021; Sohn et al., 2000).

Willemite has emerged as an ideal host material for phosphors due to its chemical stability and transparency in the UV-visible range(Rasdi et al., 2017). Phosphors that can crystallize into multiple phases or polymorphs, each with distinct space groups, are particularly valuable(Tarafder et al., 2014). Willemite, in particular, fluoresces a bright green when exposed to shortwave UV light. Among its polymorphs, the α-Zn₂SiO₄ phase is the most widely used in practical applications, owing to its rhombohedral lattice structure (R3 space group) and green light-emitting properties (Rao et al., 2014). Willemite ceramics are considered technologically important due to their excellent thermal conductivity, mechanical strength, and electrical properties (Zeng et al., 2009).

Transition metals are well-known as effective optically active dopants in crystalline hosts. The synthesis and optical properties of willemite phosphors have attracted significant interest due to their wide range of applications, including in displays, cathode ray tubes (CRT), plasma display panels (PDP), field emission displays (FED), fluorescent lamps, and solid-state lasers. These applications benefit from willemite’s highly saturated color, high luminescence, long lifespan, moisture resistance, and chemical stability. The development of solid-state lighting technology, particularly phosphor-converted white light-emitting diodes (pc-WLEDs), has also garnered significant attention due to its advantages over traditional incandescent and fluorescent lamps, such as lower energy consumption, higher efficiency, environmental benefits (mercury-free production), longer lifespan, compact design, and high brightness (Hossain et al., 2022., Cho et al. 2003, Hua et al. 2019, Babetto et al. 2020, Sivakumar et al. 2012). This table 1 shows Overview of Synthesis Methods for Phosphor Materials: Key Characteristics, Advantages, and Limitations.

This table highlights how the Solid-State Reaction (SSR) method remains a practical choice for large-scale phosphor production due to its simplicity, cost-efficiency, and ease of handling, despite requiring higher temperatures. Other methods may offer better control over particle size and morphology, but they tend to involve more complex setups or higher costs.

In this study, we employed the solid-state reaction technique to synthesize Zn₂SiO₄:Co²⁺ phosphors. We also investigated the structural, optical, and Colour properties (CIE) of the synthesized phosphor.

Material and methods

Zn₂SiO₄ phosphors were synthesized using a solid-state reaction (SSR) method and doped with various mol% of cobalt. High-purity zinc oxide (ZnO, 99%), silicon dioxide (SiO₂, 99%), and cobalt carbonate (CoCO₃, 99%) were accurately weighed according to their stoichiometric ratios. These materials were thoroughly ground together in an agate mortar and pestle to obtain a uniform powder. The powder was then placed in an aluminum crucible and sintered at 800°C for 4 hours in an air atmosphere. After the sintering process, the mixture was slowly cooled to room temperature in a furnace, resulting in the final product, which was subsequently used for further characterization. The powder X-ray diffraction (P-XRD) pattern was obtained using a Bruker D8 Advanced A25 Powder X-ray Diffractometer with a Cu-Kα radiation source (λ = 1.5406 Å) operating at 40 kV and 40 mA. The XRD pattern was recorded over a 2θ range of 10° to 80°. Photoluminescence excitation and emission spectra were measured at room temperature using a Horiba Fluorolog Fluorescence Spectrometer equipped with a time-correlated single photon counting (TCSPC) detector, with spectra collected in the 200 to 800 nm range

Results and Discussion  

PXRD Analysis

Figure 1 displays the XRD patterns for both pure Zn₂SiO₄ and Co-doped Zn₂SiO₄ phosphors. The observed patterns align well with the standard International Centre for Diffraction Data (ICDD) card no. 00-001-1076, indicating that the majority of diffraction peak positions and intensities for the synthesized phosphors are in good agreement with the standard, confirming the presence of a single-phase willemite crystal structure. The XRD pattern for pure zinc silicate (host) showed broad peaks, while the Co-doped samples exhibited sharper peaks, indicating enhanced crystallinity due to the solid-state synthesis method. This variation in peak sharpness suggests differences in crystallite particle size, which were further examined using the Debye-Scherrer formula

………….………………………………. (i)

Where, D was crystallite size, β was the full width at half maxima (FWHM), λ was Wavelength (1.54 Cu-Kα) and θ was Bragg angle [Mbule et al., 2017; Banjare et al., 2021; Bakr et al., 2020]. Using the above-mentioned formula calculated average crystallite size were 28.36 and 34.79 nm for host material and Co²⁺ doped Zn₂SiO₄ respectively.

Fig. 1 X-ray diffraction pattern of Pure Zn₂SiO₄ and 2 mol% Cobalt (II) Zinc Silicate

PL studies

The PL excitation and emission spectra of Zn₂SiO₄:xCo²⁺ phosphors were recorded at room temperature to explore their optical properties. Figure 2 displays the PL excitation spectra of Zn₂SiO₄ : xCo²⁺ (x = 1, 2, 4, 6, 8, and 10 mol%). When monitored at an emission wavelength of 460 nm, the excitation spectrum reveals a broad absorption band ranging from 200 to 400 nm, with a peak intensity at 334 nm.

Figure 3 depicts the PL emission spectra of Zn₂SiO₄: xCo²⁺ (x = 1, 2, 4, 6, 8, and 10 mol%) phosphors. Upon excitation at 334 nm, the samples exhibit a broad emission band ranging from 400 to 650 nm, centered at 460 nm, corresponding to a bright blue emission. This observation aligns with the findings of  (Rasdi et al. (2017), who reported PL spectra within the 400 to 600 nm range for cobalt-doped zinc silicate synthesized via the sol-gel method. Similarly, B. Chandra et al. (2014) noted PL spectra spanning 350 to 700 nm under excitation at 384 nm and 442 nm. The emission peaks observed can be attributed to the d–d transitions of Co²⁺ ions, specifically the ⁴A₂ → ⁴T₁ (⁴P) transition, which occurs via an electric dipole mechanism. During this process, some excited electrons may become trapped below the conduction band (CB) and later be released from these trapping centres during thermal activation, contributing to the observed luminescence. The Co²⁺ ions are likely positioned at high-symmetry sites within the Zn₂SiO₄ host lattice, which may influence this luminescent behaviour.

Fig. 2 Excitation Spectra of Zn₂SiO₄ : Co²⁺

Fig.3 Emission Spectra of Zn₂SiO₄: Co²⁺

Fig.4. Graph in between Emission intensity versus Co²⁺ Concentration of Zn₂SiO₄: xCo²⁺ (x = 1, 2, 4, 6, 8 and   10 mol%)

Figure 4 shows a graph illustrating the relationship between varying concentrations (mol%) and the emission intensity of Zn₂SiO_4: xCo²⁺ (x = 1, 2, 4, 6, 8, and 10 mol%) phosphors under 334 nm excitation. The data reveal that the emission intensity initially increases with higher Co²⁺ dopant concentrations, reaching its peak at x = 2 mol%. Beyond this point, further increases in Co²⁺ concentration result in a decline in intensity, which is attributed to the concentration quenching (CQ) effect.

Colour Chromaticity Analysis

Figure 5 shows the CIE colour chromaticity diagram for Zn₂SiO₄ : Co²⁺ phosphors at x = 2 mol%. Colourimetry analysis plays a crucial role in correlating PL emission with the x, y chromaticity coordinates. The chromaticity diagram, established by the Commission Internationale de l'Éclairage (CIE) in 1931, visually represents the color of the phosphor under appropriate lighting conditions. This diagram is utilized to define colours based on tristimulus responses, with CIE coordinates calculated using specific relationships involving the tristimulus values X, Y, and Z [Wahab, et al., 2023; Brunold et al., 1996].

For the Zn₂SiO₄ :2 mol% Co²⁺ phosphor, the CIE coordinates were determined to be (0.1495, 0.1695), indicating that the emitted colour under 334 nm excitation falls within the blue region.

             Fig.5. CIE chromaticity of  2 mol%  Co²⁺ doped in Zn₂SiO₄

These results highlight the effectiveness of Co²⁺ doping in achieving desirable blue emission in Zn₂SiO₄ phosphors, confirming the material's potential for applications in blue-light-emitting devices. Such applications include white LED (w-LED) technologies, where the blue emission can be combined with other colors to achieve broader spectral output. The placement of Zn₂SiO₄: 2 mol% Co²⁺ in the CIE chromaticity diagram as a blue emitter is significant, demonstrating how fine-tuning dopant concentrations can lead to tailored color emissions in photonic and optoelectronic devices.

Conclusion

This study presents the successful synthesis of Zn₂SiO₄ : Co²⁺ single-host phosphors through an economical and scalable solid-state reaction method at 800°C. By carefully varying Co²⁺ ion concentrations from 1 to 10 mol%, we explored their effects on the optical, luminescent, and structural properties of the phosphor material. The powder X-ray diffraction (PXRD) analysis confirmed an orthorhombic crystal structure, consistent with ICDD No. 00-001-1076, and an estimated crystallite size of approximately 34 nm calculated using the Debye-Scherrer formula. The photoluminescence (PL) study showed a prominent blue emission at 460 nm, attributed to ⁴A₂ → ⁴T₁ (⁴P)   transition, a characteristic yet underexplored feature for Co²⁺-doped Zn₂SiO₄ silicates. Color coordinates analysed, with the 2 mol% Co²⁺ -doped sample demonstrating ideal blue emission according to the CIE diagram, suggesting promising applications in light display systems, such as white LEDs (w-LEDs). This work offers new insights into the development of efficient blue phosphors for advanced lighting and display technologies.

Acknowledgement

SAIF MG University, Kottayam, IIT Bhilai and NIT Raipur, For Providing Instruments facilities.  

References