Tailoring ZnO Emission for UV Devices: Enhanced Photoluminescence through Thiophenol Surface Capping

Abstract

In this research, the optical characteristics of ZnO nanoparticles modified with 4-aminothiophenol were thoroughly examined. The surface modification was achieved through colloidal synthesis, employing a wet chemical precipitation method with carefully selected precursors. The attachment of 4-aminothiophenol molecules to the ZnO nanoparticle surfaces resulted in significant changes to their optical behaviour. These surface-modified nanoparticles were characterized using X-ray diffraction (XRD), scanning electron microscopy (SEM), and Fourier-transform infrared (FTIR) spectroscopy. Optical analysis was conducted using UV-Visible absorption and photoluminescence (PL) spectroscopy. The study focused on understanding the underlying mechanisms behind the observed changes in fluorescence and absorption properties. Notably, the surface capping led to a marked enhancement of near-band-edge ultraviolet (UV) emission and a significant suppression of the defect-related green luminescence in the PL spectra. This enhancement indicates that surface defects, such as oxygen vacancies, were effectively passivated by the capping agent. The improved optical performance highlights the potential of these modified ZnO nanoparticles for advanced optoelectronic applications, including UV lasers and LEDs. Overall, the use of 4-aminothiophenol as a capping agent was shown to play a critical role in tuning the optical properties of ZnO nanoparticles.

Introduction

Group II–VI semiconductor nanoparticles, particularly ZnO nanoparticles, are widely studied for their unique optical and electronic properties arising from quantum confinement and high surface-to-volume ratios. These properties make them suitable for applications in optoelectronics, such as LEDs, sensors, and bio-imaging. Control over nanoparticle size, morphology, and surface chemistry is crucial, with surface capping agents playing a key role in modulating surface defects and growth.

In this study, ZnO nanoparticles were synthesized via a wet chemical method and surface-modified using 4-aminothiophenol as a capping agent. ZnO’s wide bandgap and strong exciton binding energy make it ideal for UV emission applications. The 4-aminothiophenol capping effectively reduced particle size, prevented agglomeration, and passivated surface defects, which typically cause visible defect-related luminescence.

Characterization by XRD confirmed the hexagonal wurtzite structure for both capped and uncapped nanoparticles, with capped samples showing smaller crystallite sizes (~21 nm) compared to uncapped (~34 nm). SEM imaging revealed that capped nanoparticles were less agglomerated and smaller in physical size (~57 nm vs. 92 nm).

Photoluminescence studies showed uncapped ZnO nanoparticles exhibited strong visible emission due to surface defects, while capped nanoparticles displayed enhanced UV emission and nearly complete suppression of visible defect luminescence, attributed to effective defect passivation by 4-aminothiophenol. Longer annealing (reflux) times increased nanoparticle size and UV emission intensity.

UV-Vis absorption spectra demonstrated quantum confinement effects, with a blue shift in absorption peaks for smaller particles and a red shift as particle size increased with longer annealing. The bandgap narrowed from 3.54 eV (smallest particles) to 3.42 eV (largest), confirming size-dependent optical properties.

Overall, the study highlights the effectiveness of 4-aminothiophenol as a capping agent in producing high-quality ZnO nanoparticles with optimized optical properties for advanced optoelectronic applications.

Conclusion

Capped ZnO nanoparticles were successfully synthesized using the wet chemical precipitation method, with 4-aminothiophenol employed as the capping agent. Their optical characteristics were explored through both photoluminescence and UV-Visible absorption spectroscopy. The photoluminescence results clearly demonstrated the impact of the capping process, with a significant suppression of defect-related visible emissions and a marked enhancement in ultraviolet emission intensity. Additionally, UV-Visible absorption spectra revealed a noticeable blue shift in the absorption edge for the capped nanoparticles compared to their uncapped counterparts. This shift is attributed to the quantum confinement effect, which indicates that the capped ZnO nanoparticles are smaller in size and absorb light at shorter wavelengths than the uncapped ones. These observations confirm the effectiveness of the capping agent in tailoring both the emission and absorption properties of ZnO nanoparticles at the nanoscale.

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Copyright © 2025 Shashi Bhushan Rana. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.