Hot Injection Synthesis of CdSe Nanocrystals: Structural, Optical, and Electronic Properties

Abstract

Cadmium selenide (CdSe) nanocrystals, commonly known as quantum dots (QDs), have emerged as pivotal materials in the realm of nanotechnology due to their unique size-dependent optical and electronic properties, primarily governed by quantum confinement effects. Among various synthesis routes, the hot-injection method has proven to be one of the most reliable techniques for producing high-quality, monodisperse CdSe nanocrystals with controlled morphology, narrow size distributions, and tunable emission spectra. This paper presents a comprehensive investigation into the synthesis of CdSe nanocrystals using the hot-injection method, with a particular focus on understanding how reaction parameters—such as precursor concentration, ligand selection, injection temperature, and growth duration—affect the final structural, optical, and electronic properties of the resulting nanocrystals. We begin by analyzing the nucleation and growth dynamics inherent to the hot-injection process, emphasizing the importance of rapid precursor mixing and thermal stability to initiate controlled nucleation while minimizing secondary nucleation events. The role of coordinating ligands, such as trioctylphosphine (TOP), trioctylphosphine oxide (TOPO), oleic acid, and hexadecylamine, is explored in depth, particularly regarding their function in surface passivation, crystal phase control (e.g., wurtzite vs. zinc blende), and colloidal stability. The impact of reaction temperature is also examined, as it significantly influences not only the size and shape of nanocrystals but also their crystallinity and defect density. The study delves into the optical properties of CdSe quantum dots, highlighting how variations in particle size and surface chemistry lead to tunable photoluminescence across the visible spectrum. Spectroscopic techniques such as UV-Vis absorption, photoluminescence (PL) spectroscopy, and time-resolved fluorescence are discussed as critical tools for characterizing bandgap energy, exciton recombination dynamics, and quantum yield. Special attention is given to quantum confinement-induced blue shifts in smaller particles, as well as red shifts and reduced bandgaps in larger nanocrystals. The influence of surface defects and trap states on the optical performance is analyzed, along with approaches for surface passivation and core/shell engineering (e.g., CdSe/ZnS structures) to enhance quantum yield and photostability.