Quantum dots (QDs) are nanoscale semiconductor particles, typically ranging in diameter from just two to ten nanometers. Their composition and size are intricately linked, determining how they interact with light and enabling their use in advanced technology.
Defining Quantum Dots by Size and Function
The term “quantum” refers to quantum confinement, which dictates their light-emitting behavior. In bulk semiconductor materials, electrons move freely in continuous energy bands, but the extremely small size of a quantum dot restricts this movement in all three dimensions. This confinement forces the electrons into discrete, atom-like energy levels.
The energy required to excite an electron and the energy released as light are directly controlled by the dot’s diameter. Smaller dots have a larger energy gap, meaning they emit higher-energy, shorter-wavelength light (blue or green). Conversely, larger dots have a smaller energy gap and emit lower-energy, longer-wavelength light (orange or red). This size-dependent color tunability is a distinguishing feature.
The Core Composition Semiconductor Materials
The central part of a quantum dot is the semiconductor core, which is the component responsible for absorbing and emitting light. These cores are primarily composed of elements from the periodic table’s Groups II-VI or III-V, which form crystalline structures. The choice of core material is the first step in defining the quantum dot’s overall properties, including its efficiency and potential toxicity.
Historically, the most studied and highly efficient QDs have been Cadmium-based, utilizing compounds such as Cadmium Selenide (\(\text{CdSe}\)) or Cadmium Sulfide (\(\text{CdS}\)) as the core. These materials are known for their high photoluminescence quantum yield. However, the presence of cadmium, a heavy metal, raises concerns about toxicity and environmental impact, particularly as the core can slowly degrade and release \(\text{Cd}^{2+}\) ions.
Growing regulatory pressure and safety demands have driven a shift toward Cadmium-Free QDs, which are now widely adopted in consumer electronics and biomedical research. Indium Phosphide (\(\text{InP}\)) is the best-in-class material among these alternatives and is increasingly used for its low-toxicity profile. Other cadmium-free options include Gallium Arsenide (\(\text{GaAs}\)) and Copper Indium Sulfide (\(\text{CuInS}_2\)).
While \(\text{InP}\) and other cadmium-free cores are safer, they historically exhibited lower efficiency compared to their cadmium counterparts, though significant advancements have narrowed this gap. Researchers often pair these cores with a shell to boost their performance. The composition of the core ultimately determines the entire dot’s potential for brightness, stability, and suitability for specific applications like bioimaging or display technology.
The Protective Shell and Surface Ligands
The Protective Shell
The core is typically encapsulated by a protective layer called the shell, forming a core/shell structure. This shell is composed of a wider bandgap semiconductor material, such as Zinc Sulfide (\(\text{ZnS}\)) or Zinc Oxide (\(\text{ZnO}\)). The shell acts as a physical barrier that shields the core from oxidation and environmental degradation, which is important for maintaining efficiency over time.
The shell also improves the quantum efficiency by electronically passivating surface defects on the core, which are sites where energy can be wasted as heat instead of light. By reducing these defects, the shell allows the core to emit light more brightly and reliably.
Surface Ligands
The outermost layer consists of organic molecules known as surface ligands, which are chemically attached to the shell. These ligands are typically long-chain molecules, such as oleate or various polymers and fatty acids. Their primary purpose is to make the QD soluble and dispersible in a specific environment, such as organic solvents for display manufacturing or aqueous solutions for biological applications.
Ligands also allow for the functionalization of the quantum dot surface, enabling the attachment of other molecules like antibodies or peptides for targeted delivery.
Practical Applications Driven by Composition
For display technology, such as QLED televisions, the primary need is high efficiency, color purity, and long-term stability. The industry has largely shifted to using Cadmium-Free Indium Phosphide (\(\text{InP}\)) cores with a \(\text{ZnS}\) shell to achieve vivid colors while meeting stringent environmental and safety regulations. The pure, narrow-band light emission from these \(\text{InP}\)-based QDs allows televisions to achieve a wider range of colors than traditional LED screens.
In biomedical imaging, the material choice is driven by the requirement for low toxicity, favoring \(\text{InP}\) and \(\text{CuInS}_2\) cores. Here, the surface ligands are the most relevant component, as they are chemically engineered to bind to specific biomarkers or cancer cells. These functionalized, non-toxic QDs act as highly stable, brightly glowing probes that can be excited by a single light source to track multiple biological processes simultaneously. The ability to tune the core size for color and the ligand composition for targeting makes the quantum dot a versatile tool for both consumer technology and advanced medical diagnostics.