Photoluminescence in quantum dots arises from complex interactions between confined charge carriers and their nanoscale environment. The emission properties are governed by exciton dynamics, non-radiative pathways, and surface effects that collectively determine the quantum yield and stability of these semiconductor nanocrystals. Understanding these mechanisms is essential for optimizing quantum dot performance in applications ranging from displays to biological imaging.

The primary photoluminescence mechanism involves the formation and recombination of excitons, which are electron-hole pairs bound by Coulombic attraction. In quantum dots, quantum confinement effects dominate due to their small size, typically 2-10 nm in diameter. This confinement creates discrete energy levels analogous to atomic orbitals, leading to size-tunable emission wavelengths. The bandgap energy increases as particle size decreases following the Brus equation, which accounts for both quantum confinement and Coulombic interactions. Exciton recombination occurs through either radiative pathways producing photons or non-radiative processes that dissipate energy as heat.

Auger recombination represents a major non-radiative pathway that becomes significant under high excitation densities or in charged quantum dots. This three-particle process involves the recombination of one electron-hole pair transferring its energy to a third carrier instead of emitting light. The Auger recombination rate scales inversely with quantum dot volume, making smaller dots particularly susceptible. In biexciton systems where two electron-hole pairs coexist, Auger recombination can occur on timescales of 10-100 ps, orders of magnitude faster than radiative biexciton recombination. This effect limits the performance of quantum dots in light-emitting devices operating at high currents.

Quantum dot blinking, the random intermittency in photoluminescence intensity, stems from charge carrier trapping at surface defects. The off-states occur when either the electron or hole becomes trapped at surface sites, preventing radiative recombination. Two primary models explain this phenomenon. The charging model proposes that ionization of the quantum dot creates a net charge that quenches emission through Auger recombination. The alternative neutral model suggests that non-ionizing traps can also quench luminescence by providing non-radiative recombination pathways. Blinking statistics typically follow power-law distributions for both on- and off-times, indicating a complex energy landscape of trap states.

Surface chemistry plays a critical role in determining photoluminescence quantum yield through the passivation of dangling bonds. In II-VI semiconductor quantum dots like CdSe, an inorganic shell of wider bandgap material such as ZnS reduces surface trap density by terminating unsatisfied bonds. Core-shell structures can achieve quantum yields exceeding 80% compared to 5-15% for bare cores. Organic ligands also passivate surface states while maintaining colloidal stability, with thiolates, phosphines, and amines commonly employed. However, ligand binding is dynamic, and desorption can create temporary trap states that reduce emission efficiency.

Defect states within the quantum dot lattice or at interfaces introduce mid-gap energy levels that facilitate non-radiative recombination. These defects may arise from stoichiometric imbalances, lattice vacancies, or interfacial strain in core-shell structures. For instance, selenium vacancies in CdSe quantum dots create electron traps that reduce luminescence efficiency. The defect density correlates with synthetic methods and post-processing treatments, with high-temperature reactions generally producing fewer bulk defects but potentially more surface disorder.

Temperature dependence studies reveal the activation energies for various quenching mechanisms. Below 100 K, photoluminescence quantum yield typically increases as thermal energy becomes insufficient to activate non-radiative pathways. The Arrhenius plots often show multiple activation energies corresponding to different trap states. At room temperature, the competition between radiative and non-radiative processes determines the overall quantum yield, with values varying from near-zero for poorly passivated dots to near-unity for optimized structures.

Quantum confinement also affects the exciton fine structure due to enhanced electron-hole exchange interactions. The dark exciton states with spin-forbidden transitions lie slightly below the bright states in CdSe quantum dots, typically by 5-25 meV depending on size. Thermal population of these dark states reduces luminescence efficiency at low temperatures. External fields or lattice strain can mix dark and bright states, modifying the polarization and decay dynamics of the emission.

The photoluminescence decay kinetics provide insight into the dominant recombination pathways. Monoexponential decays indicate a single dominant process, while multiexponential behavior reflects multiple recombination channels. Core-shell quantum dots often exhibit stretched exponential decays due to dispersive charge carrier dynamics in the shell material. Typical radiative lifetimes range from 10-30 ns for CdSe quantum dots, with non-radiative processes shortening the observed lifetime.

Advanced surface engineering strategies continue to improve quantum dot photoluminescence properties. Graded shell structures minimize lattice strain while providing effective passivation. Novel ligand chemistries enhance binding stability and reduce trap state density. Doping with transition metals or rare earth ions creates alternative emission pathways that bypass surface recombination channels. These developments have pushed photoluminescence quantum yields to near-theoretical limits while significantly reducing blinking behavior in state-of-the-art quantum dots.

The interplay between quantum confinement, surface chemistry, and defect physics creates both challenges and opportunities in quantum dot photoluminescence. Precise control over these factors enables tuning of emission properties for specific applications while maintaining high quantum efficiency. Future progress in understanding and manipulating these nanoscale phenomena will further expand the technological applications of quantum-confined semiconductor systems.