Photoluminescence (PL) in quantum-confined semiconductor structures such as quantum dots, wells, and wires is a fundamental optical phenomenon with significant implications for optoelectronics and quantum technologies. These nanostructures exhibit unique electronic and optical properties due to quantum confinement effects, which modify the density of states and carrier dynamics compared to bulk materials. The emission characteristics, exciton behavior, and surface interactions in these systems are critical for designing advanced devices.

Quantum confinement arises when the physical dimensions of a semiconductor structure are smaller than the exciton Bohr radius, leading to discrete energy levels. In quantum wells, carriers are confined in one dimension, resulting in a step-like density of states. Quantum wires confine carriers in two dimensions, producing a more pronounced quantization effect. Quantum dots, with three-dimensional confinement, exhibit atomic-like discrete energy levels, making them particularly interesting for tunable light emission.

The size-dependent emission shift is a hallmark of quantum-confined systems. For quantum dots, the bandgap energy increases as the dot size decreases due to the quantum confinement effect. For example, CdSe quantum dots exhibit a tunable emission wavelength from approximately 500 nm (2.48 eV) for 2 nm dots to 650 nm (1.91 eV) for 6 nm dots. This tunability is described by the particle-in-a-box model, where the energy levels scale inversely with the square of the confinement dimension. In quantum wells, the emission energy depends on the well width, with narrower wells yielding higher energy transitions. For GaAs/AlGaAs quantum wells, the ground state transition energy can shift from 1.42 eV (bulk GaAs) to over 1.6 eV for wells thinner than 5 nm.

Exciton binding energies are significantly enhanced in quantum-confined structures due to the spatial restriction of electron-hole pairs. In bulk semiconductors, exciton binding energies are typically a few meV, but in quantum dots, they can exceed 100 meV due to strong confinement. For instance, InAs quantum dots exhibit binding energies around 20-30 meV, while CdSe dots can reach 50-100 meV depending on size and surface passivation. The increased binding energy stabilizes excitons at room temperature, enabling efficient light emission even under thermal excitation.

Surface effects play a crucial role in the PL properties of quantum-confined structures. Surface states can act as non-radiative recombination centers, reducing PL efficiency. In quantum dots, unpassivated surface dangling bonds lead to trap states that quench luminescence. Proper surface passivation with organic ligands or inorganic shells (e.g., ZnS on CdSe) can significantly improve PL quantum yield, often exceeding 80%. For quantum wells and wires, interface roughness and defects at heterojunctions can broaden PL linewidths. High-quality epitaxial growth techniques like molecular beam epitaxy (MBE) minimize these effects, achieving narrow linewidths below 10 meV in optimized structures.

The PL lifetime in quantum-confined systems reflects the recombination dynamics. Radiative lifetimes in quantum dots range from nanoseconds to microseconds, depending on material and size. For example, CdSe quantum dots exhibit lifetimes around 10-30 ns, while InAs dots show longer lifetimes due to weaker oscillator strengths. Non-radiative processes, such as Auger recombination, become prominent at high carrier densities or in charged exciton states, shortening the effective lifetime. In quantum wells, the radiative lifetime is typically shorter (sub-nanosecond) due to the higher density of states and weaker confinement.

Applications of quantum-confined PL span optoelectronics and quantum technologies. In light-emitting diodes (LEDs), quantum dots enable precise color tuning and high color purity. Quantum dot LEDs (QLEDs) achieve external quantum efficiencies exceeding 20% for red and green emitters. Quantum wells are the active regions in semiconductor lasers, providing high gain and low threshold currents. Vertical-cavity surface-emitting lasers (VCSELs) leverage quantum wells for efficient emission in telecommunications and sensing.

In quantum technologies, quantum dots serve as single-photon sources for quantum cryptography and computing. The discrete energy levels allow deterministic photon emission with high indistinguishability. Colloidal quantum dots have demonstrated single-photon emission with purity above 99%, while epitaxial dots enable on-demand photon generation with g(2)(0) values below 0.01. Quantum wires and wells are explored for entangled photon generation via biexciton cascades, relevant for quantum communication networks.

Thermal effects on PL in quantum-confined systems are also notable. The emission energy typically redshifts with temperature due to lattice expansion and electron-phonon coupling. However, the shift is less pronounced than in bulk materials due to the confinement-induced stabilization of excitons. The PL intensity quenching at elevated temperatures follows an Arrhenius behavior, with activation energies corresponding to the exciton binding energy or trap state depths. For instance, CdSe quantum dots show thermal quenching activation energies of 50-150 meV, depending on surface passivation.

Advanced PL techniques like time-resolved and spatially resolved PL provide deeper insights into quantum-confined systems. Time-resolved PL reveals recombination pathways and carrier trapping dynamics, while confocal PL mapping resolves inhomogeneities in single nanostructures. Micro-PL studies on individual quantum dots show spectral diffusion and blinking behavior, linked to charge fluctuations in the local environment. These phenomena are critical for optimizing device performance and stability.

In summary, photoluminescence in quantum-confined semiconductor structures is governed by size-dependent quantization, enhanced exciton effects, and surface interactions. These properties enable tailored light emission for applications ranging from high-efficiency displays to quantum light sources. Continued advancements in synthesis, passivation, and characterization will further unlock the potential of these materials for next-generation technologies.