Owing to its low resistivity, high transmittance, and tunable optical band gap, ZnO is of great interest for optoelectronic applications. Herein, the sol–gel technique was used to synthesize un-doped and zirconium-doped zinc oxide (ZZO) nanostructures with different concentrations of Zirconium (Zr). X-ray diffraction (XRD), scanning electron microscope (SEM), Raman spectroscopy, Fourier transform infrared spectroscopy (FTIR), UV-Vis spectroscopy, and photoluminescence (PL) measurements were used to investigate the influence of Zr doping on the structural, optical, and electrical properties of developed nanostructures. XRD and SEM confirmed the increase in crystallite size with increasing concentrations of Zr. Raman analysis indicated the presence of oxygen vacancies in synthesized nanostructures. UV-Vis spectroscopy illustrated the blue shift of band gap and red shift of the absorption edge for ZZO nanostructures with increasing concentrations of Zr. For the measurement of electrical properties, the spin-coating technique was used to deposit un-doped and Zr-doped ZnO layers of ~165 nm thickness. The four-probe-point (4PP) method illustrated that the doping of Zr caused a reduction in electrical resistance. Hall Effect measurements showed a high value, 3.78 × 10²⁰ cm⁻³, of the carrier concentration and a low value, 10.2 cm²/Vs, of the carrier mobility for the Zr-doped layer. The high optical transmittance of ~80%, wide band gap of 3.51 eV, low electrical resistivity of 1.35 × 10⁻³ Ω·cm, and maximum carrier concentration of 3.78 × 10²⁰ cm⁻³ make ZZO nanostructures one of the most promising candidates for the application of transparent conductive oxide (TCO) in optoelectronic devices.

Zero-dimensional graphene quantum dots (GQDs) present unique optoelectronic properties in the large-spectrum range from UV to visible. However, the origin of luminescence in GQDs is still a debatable question. Therefore, the present work investigates the features of trap-mediated and edge-state-functionalized group-associated luminescence enhancement of GQDs. The attached functional groups’ involvement in the upsurge of photoluminescence has been discussed theoretically as well as experimentally. In addition, the role of the aromatic ring, the functional group attached, and their positions of attachment to the aromatic ring to tune the emission wavelength and Raman modes have been elucidated theoretically as well as experimentally. We found that in the case of the –OH group attached outside of the aromatic ring, the long-range π hybridization dominates, which suggests that the emission from this model can be dictated by long-range π hybridization. In particular, we found that oxygen-containing functional groups attached outside of the aromatic ring are the main source of the luminescence signature in GQDs. Furthermore, density functional theory (DFT) indicates that the –OH functional group attached outside of the aromatic ring perfectly matched with our experimental results, as the experimental bandgap (2.407 eV) is comparable with the theoretical simulated bandgap (2.399 eV) of the –OH group attached outside of the aromatic ring.