1.1 Definition and basic properties of quantum dots

Quantum dots are tiny semiconductor crystals with unique properties due to their nanoscale size. They exhibit quantum confinement effects, which give them atom-like behavior and tunable optical and electronic characteristics.

This section dives into the definition and basic properties of quantum dots. We'll explore their composition, size-dependent features, and synthesis methods, setting the stage for understanding their applications in various fields.

Quantum dots: Definition and Characteristics

Composition and Size

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• Quantum dots consist of nanoscale semiconductor crystals (CdSe, CdS, InP, PbS)
• Typical sizes range from 2 to 10 nanometers in diameter
• High surface-to-volume ratio makes properties sensitive to surface modifications and surrounding environment

Unique Optical and Electronic Properties

• Exhibit properties different from bulk semiconductor materials due to small size and quantum confinement effects
• Electronic structure characterized by discrete energy levels, similar to individual atoms or molecules
• Properties can be tuned by controlling size, shape, and composition

Morphology and Shape

• Common morphologies include spherical, ellipsoidal, and rod-like structures
• Shape influences electronic structure and optical properties
• Anisotropic shapes (nanorods) exhibit polarization-dependent absorption and emission

Surface Passivation and Functionalization

• Surface passivation maintains optical properties and stability by coating with wider bandgap material (ZnS, CdS)
• Surface functionalization enables solubility, biocompatibility, and targeting for specific applications
• Ligands and polymers used for surface modification affect charge carrier dynamics and photoluminescence efficiency

Quantum Confinement Effect in Quantum Dots

Confinement of Charge Carriers

• Occurs when semiconductor size is reduced to nanoscale, confining charge carriers (electrons and holes) in all three spatial dimensions
• Leads to discretization of energy levels within the quantum dot
• Energy level spacing increases as quantum dot size decreases

Enhanced Electron-Hole Interactions

• Confinement results in enhanced electron-hole interactions and increased overlap of wave functions
• Gives rise to unique optical and electronic properties
• Enables phenomena such as enhanced exciton binding energy and altered carrier dynamics

Tunable Bandgap Energy

• Quantum confinement enables tuning of bandgap energy by controlling quantum dot size
• Allows manipulation of absorption and emission spectra
• Enables creation of quantum dots with emission colors spanning visible and near-infrared regions

Size-Dependent Phenomena

• Confinement-induced changes in electronic structure give rise to size-dependent phenomena
• Examples include size-dependent photoluminescence, enhanced exciton binding energy, and altered carrier dynamics
• Enables applications in optoelectronics, photovoltaics, and biological imaging

Size-Dependent Properties of Quantum Dots

Optical Properties

• Absorption and photoluminescence spectra strongly dependent on size due to quantum confinement effect
• Decreasing quantum dot size results in blue-shift of absorption and emission spectra
• Emission wavelength can be precisely tuned by controlling size (visible to near-infrared)
• High photoluminescence quantum yields (often >50%) due to enhanced electron-hole overlap and reduced non-radiative recombination

Electronic Properties

• Carrier mobility and conductivity influenced by size
• Smaller quantum dots exhibit reduced carrier mobility due to increased quantum confinement and surface effects
• Size-dependent electronic properties impact performance in optoelectronic devices (LEDs, solar cells)

Exciton Dynamics

• Exciton (bound electron-hole pair) dynamics affected by quantum dot size
• Smaller quantum dots exhibit increased exciton binding energy and reduced exciton Bohr radius
• Size-dependent exciton dynamics influence optical gain, lasing, and energy transfer processes

Applications Enabled by Size-Dependent Properties

• Optoelectronics: LEDs, lasers, photodetectors
• Photovoltaics: solar cells, light-harvesting devices
• Biological imaging: fluorescent probes, biosensors
• Quantum computing and cryptography: single-photon sources, qubits

Synthesis Methods for Quantum Dots

Colloidal Synthesis

• Widely used method for producing high-quality quantum dots
• Involves reaction of precursor materials in coordinating solvent at elevated temperatures
• Precursor materials chosen based on desired composition (CdSe, CdS, InP, PbS)
• Size and shape controlled by adjusting reaction conditions (temperature, time, precursor ratio)

Epitaxial Growth and Chemical Vapor Deposition

• Epitaxial growth techniques (MBE, MOCVD) used for creating quantum dot arrays and superlattices
• Chemical vapor deposition enables synthesis of quantum dots on various substrates
• Provides control over size, density, and spatial arrangement of quantum dots

Alternative Synthesis Methods

• Hot-injection and heat-up techniques developed for improved size distribution and optical properties
• Microwave-assisted synthesis offers rapid and uniform heating for quantum dot production
• Biosynthesis using bacteria or fungi explores eco-friendly and sustainable approaches

Environmentally Friendly and Less Toxic Materials

• Recent advancements focus on developing quantum dots with reduced toxicity
• Examples include silicon, carbon, and metal chalcogenide-based quantum dots (CuInS2, AgInS2)
• Aim to address concerns regarding the use of heavy metals (Cd, Pb) in conventional quantum dots
• Enables broader application potential, especially in biomedical and consumer products