Colorimetric characterization of blue semiconductor, Zn2(1-x)Co2xGeO4 (x = 0.001)

This paper reports the optical spectroscopy of a semiconductor—zinc germanate Zn2(1-x)Co2xGeO4 (x = 0.001)—prepared using the solid-state method. The sample exhibited a deep blue color characteristic of Co2 + in a low-strength crystalline field (tetrahedral sites). Photoluminescence two emission bands at 750 and 825 nm, assigned to the 4T1(4P) → 4A2(4F) and 4T1(4P) → 4T2 (4F) transitions, respectively. Reflectance data revealed excited-state transitions at 547, 588, and 643 nm, corresponding to 4A2(4F) → 2T2(2G), 4A2(4F) → 4T1(4P), and 4A2(4F) → 2E(2G), respectively. The crystal-field parameter Dq = 463 cm-¹ and the Racah parameter B = 772 cm-¹ were determined from the optical data. The transitions and energy parameters were assigned to the Co²⁺ ions in tetrahedral sites, where the bonds exhibit a covalent nature. The optical bandgaps associated with direct and indirect transitions were determined to be 4.48 and 4.36 eV, respectively, using the Kubelka–Munk formalism from diffuse reflectance spectra. Reflectance data were used to determine the dominant wavelength (478 nm) and the complementary color (576 nm). The correlated color temperature (10,220 K) and colorimetric parameters (L* = 75.59, a* = –2.15, and b* = –16.50) were obtained from the chromaticity diagram.