Synthesis of Quantum Dots: Cadmium Selenide Nanocrystals | Lab Guide

This section discusses surface passivation strategies to improve quantum efficiency. Surface traps (selenium/sulfur defects) cause non-radiative recombination and reduce fluorescence. CdS shells provide good passivation but allow electron delocalization due to small conduction band offset. ZnS shells maintain type-I heterostructure with larger conduction band offset, preserving quantum confinement. Growing ZnS shells around CdSe nanoplatelets initially shifts emission from green to red due to reduced quantum confinement. Despite significant shell growth, fluorescence decay remains unchanged, confirming type-I heterostructure preservation.

The hot injection method, developed by Chris Murray in 1993, revolutionized quantum dot synthesis by producing monodisperse samples where all particles share identical sizes and properties. The technique involves rapidly injecting precursor solutions into a hot reaction mixture, triggering immediate burst nucleation followed by controlled growth. Rapid stirring ensures uniform nucleation events, and reaction termination at precisely timed intervals produces quantum dots of targeted sizes. Surface passivation using appropriate ligands fills trap states that would otherwise compete with radiative recombination. Core-shell structures (e.g., CdSe/ZnS) provide additional benefits by isolating excited states from surface defects through type-I band alignment. This synthesis chemistry, refined over decades, enabled the transition from laboratory curiosity to commercial technology, demonstrating how materials science bridges fundamental understanding with practical application.

Surface passivation eliminates trap states from unsatisfied surface atoms, enhancing quantum yield. Amphiphilic ligands bind to surfaces while stabilizing in solvents. Core-shell structures (e.g., ZnS on CdSe) provide superior passivation. Supersaturation-controlled growth enables precise size control through careful management of nucleation and growth kinetics, producing high-quality quantum dots for device applications.

This lecture covers the synthesis of colloidal semiconductor nanoparticles, explaining how quantum confinement allows size-tunable optical properties, with the hot injection method being the primary technique; the Lamer model describes size control through separated nucleation and growth phases, while core-shell structures improve photoluminescence quantum yield by passivating surface defects; various materials including cadmium selenide, indium phosphide, copper indium sulfide, and perovskites are discussed, each with specific synthesis challenges and advantages, particularly emphasizing non-toxic alternatives and aqueous synthesis methods.

Quantum dots include CdSe (size-dependent emission), PbS (near-infrared, biocompatible via ligand exchange), CdS/ZnS core-shell (400-460 nm emission, >75% quantum yield), CIS/CIGS/CZTS (non-toxic, stable, 500-800 nm emission), and ZnS/ZnS (400-440 nm). Colloidal synthesis involves heating cadmium compounds, injecting selenium precursors, controlling growth duration, and coating with zinc sulfide. Applications span biomedical imaging (superior to organic dyes), cellular tracking, drug delivery, cancer therapy, and quantum computing as qubits.

Various methods exist to stabilize luminescence properties of semiconductor quantum dots in aqueous media. These include surface passivation with protective layers such as proteins and coating quantum dots with protecting silicon oxide films or polymer films. An additional method relies on exchanging native organic ligands linked to quantum dots (such as trio til phosphino or TOPO) with other ligands. Common binding groups include thiophosphines and amines. Surface modifications make quantum dots water-soluble and suitable for biological applications such as molecular imaging and medical diagnosis. Organic molecules including derivatives of mercapto molecules, aliphatic ligands, phospholipids, dendrimers, and anilic polymers have been used for surface modifications.

After forming the core of quantum dots, a shell layer is added around them to ensure proper electron confinement. In the example shown, dissolved sulfur is injected to react with zinc from earlier, creating zinc sulfide shells around the cores. This shell prevents the excited electron from escaping before emitting the desired photon. Additionally, ligands are added to enable the quantum dots to interact with specific mediums—for example, making them soluble in water or forming quantum dot films for different applications.

Quantum dots can be made water-soluble by coating them with amphiphilic polymers such as 40% octylamine-modified polyacrylic acid, where the hydrophobic groups of the polymer bind to the quantum dot ligands while the hydrophilic groups interact with water, enabling the formation of stable aqueous dispersions suitable for biological applications like cell imaging.

Quantum dots are thermodynamically unstable and tend to aggregate, requiring organic ligands for surface stabilization. These ligands form an organic shell around the inorganic core. For biomedical applications, quantum dots must be hydrophilic (water-soluble) rather than hydrophobic. The ligand shell can be modified to attach biologically active molecules like transferrin, enabling selective targeting of cancer cells. This transforms quantum dots into potential drug delivery vehicles that can navigate to specific locations in the body.

Liquid crystal templating uses self-assembled lamellar structures as templates for quantum dot assembly, creating 2D nanosheet arrays. Adding surfactants expands interlayer spacing, and ultrasound exfoliates individual nanosheets. Doping introduces foreign atoms into the quantum dot lattice, modifying electronic structure and optical properties—for example, copper doping in CdSe quantum dots eliminates the dominant emission peak and enhances a secondary peak. Ligand exchange allows modification of quantum dot properties after synthesis by replacing original stabilizers with different molecules. Silicon nanocrystals are synthesized by laser pyrolysis of silane in methane, producing highly oxidized particles that rapidly agglomerate. Surface passivation converts silanol groups (-SiOH) to hydrophobic groups (e.g., phenyl groups) using ultrasound-assisted reactions, enabling stable colloidal dispersions in non-polar solvents. Chemical etching removes surface oxide layers while preserving crystalline cores, with HF acid rapidly dissolving SiO2 but stopped by hydrogen passivation.

QLED devices employ quantum dots as emissive layers between electron and hole injection/transport layers, achieving external quantum efficiencies >10% and luminance >100,000 cd/m². Nanoplatelets represent a breakthrough as two-dimensional structures offering one-order-of-magnitude greater absorption cross-sections than spherical quantum dots. By tuning thickness, researchers achieve sub-10 nm emission linewidths. External quantum efficiencies of 19.2% demonstrate their potential as next-generation optoelectronic building blocks, addressing limitations of conventional quantum dots in light harvesting and emission control.

Quantum dots enable diverse optoelectronic applications: displays and LEDs use size-tunable band gaps for precise color control; solar cells benefit from broader light absorption; sensors detect molecules through fluorescence changes. Quantum cascade lasers use intraband transitions in quantum wells for wavelength-tunable infrared light. Quantum dot lasers use quantum dots as active medium, achieving population inversion more easily and enabling room-temperature operation with lower threshold currents.

Three main approaches integrate quantum dots into solar cells: (1) Quantum dot sensitized solar cells use quantum dots on TiO₂ nanocrystals, facing electrolyte infiltration and corrosion challenges; (2) Bulk heterojunctions combine quantum dots with conducting polymers like P3HT, leveraging strong light absorption; (3) Quantum dot solids are conductive films of pure quantum dots sandwiched between contacts. Quantum dot solids offer advantages: highest quantum dot loading, fewest heterojunctions, all-inorganic construction, and direct comparison with conventional semiconductors. Effective quantum dot solar cell films require five essential properties: (1) Light absorption - sufficient thickness (~1 micron) to capture all sunlight; (2) Electrical conductivity - achieved through electronic coupling between dots via tunneling or mini-band conduction; (3) Charge separation mechanism - typically built-in electric fields or donor-acceptor junctions; (4) Long minority carrier diffusion length - dependent on mobility, lifetime, and thermal voltage; (5) Long-term stability - critical for practical deployment.

Quantum dot enhanced backlight units (QD-LED TVs) represent a transformative display technology leveraging quantum dot tunability for color generation. Unlike conventional LCD displays requiring colored filters, QD TVs use blue LED backlighting combined with quantum dot layers that convert blue light into green and red emissions, achieving broader color gamut coverage. Stacked quantum dot layers (blue, green, orange, red) enable white light emission with nearly complete coverage of the HDTV color coordinate space. Beyond displays, quantum dots enable flexible optoelectronic devices including touch sensors, wearable electronics, and biometric monitoring systems. Quantum dot solar cells utilize nano crystal semiconductors with tunable band gaps for optimized light absorption, typically employing titanium dioxide n-type layers with quantum dot active layers. Luminescent solar concentrators embed quantum dots within transparent substrates, absorbing sunlight and re-emitting it at longer wavelengths that propagate through the substrate via total internal reflection to edge-mounted solar cells, enabling distributed light collection across large surface areas while concentrating energy at compact solar cell locations.

Quantum dots offer significant advantages over traditional organic dyes in biological imaging: broad absorption for efficient light collection, narrow emission for clear spectral separation of multiple labeled cellular components, surface functionalization for targeted delivery, and infrared emission for deep-tissue imaging. In optoelectronics, quantum dots enable tunable light-emitting diodes where emission color is controlled by particle size, and solar cells where absorption profiles can be optimized through size adjustment. The colloidal nature of quantum dots allows low-temperature processing via printing, spraying, or dip coating, simplifying device fabrication compared to conventional semiconductor manufacturing.