Electrical Properties of ZnO Nanoparticle-Embedded/Polyethylenimine-Functionalized Nitrogen-Doped Graphene Quantum Dot Nanocomposites

In this work, a sustainable solution-based route was employed to synthesize a ZnONPs/PEI N-GQDs nanocomposite by integrating zinc oxide nanoparticles into polyethylenimine-functionalized nitrogen-doped graphene quantum dots. The primary optoelectronic properties of the synthesized material in liquid phase were evaluated by UV-Vis spectroscopy, and Tauc analysis indicated a direct optical band gap of approximately 3.0 eV. Photoluminescence (PL) spectroscopy further revealed a red-shifted visible emission centered at ∼518 nm, attributed to defect- and interface-mediated radiative recombination between ZnONPs and PEI-functionalized N-GQDs. Complementary structural characterization using FTIR, XPS, and TEM confirmed the successful formation of the hybrid nanocomposite, evidencing the coexistence of ZnO nanoparticles and PEI-functionalized N-GQDs. Ultraviolet photoelectron spectroscopy (UPS) was employed to elucidate the interfacial energy-level alignment, yielding a work function of ∼3.17 eV and indicating the formation of a Schottky contact at the metal/nanocomposite interface together with a type-II-like band alignment at the ZnONPs/PEI N-GQDs/n-Si junction. Two diode configurations were fabricated: a conventional Au/n-type Si Schottky diode and a heterojunction diode based on Au/ZnONPs/PEI N-GQDs/n-type Si. Their semilogarithmic current-voltage characteristics were systematically investigated under dark and illuminated conditions at room temperature. Compared with the Au/n-type Si reference, the heterojunction diode exhibited pronounced rectification and a significantly enhanced photoresponse. Illumination induced a substantial increase in reverse current due to photogenerated carriers, resulting in a decrease in the rectification ratio from 3.25 × 10³ (dark) to 6.64 × 10¹ (light) at ±5 V, reflecting a trade-off between rectification and photosensitivity. While the Au/n-type Si diode showed a higher rectification ratio in the dark (2.17 × 10⁴ at ±5 V) and a lower ideality factor (<i>n</i> = 5.06), the heterojunction device demonstrated improved illuminated performance with a reduced ideality factor of 3.17 and an increased barrier height of 0.76 eV, underscoring its potential for silicon-based optoelectronic applications.