Silicon quantum dots (SiQDs) represent a fascinating class of nanoscale semiconductors where quantum confinement effects dominate their optical behavior. Unlike bulk silicon, which is an indirect bandgap material with weak luminescence, SiQDs exhibit tunable photoluminescence (PL) across the visible and near-infrared (NIR) spectrum due to their size-dependent electronic structure. The optical properties of SiQDs are primarily governed by quantum confinement, surface chemistry, and defect states, making them a compelling alternative to traditional II-VI quantum dots (QDs) like CdSe or CdTe.

The origin of size-dependent emission in SiQDs stems from the quantum confinement effect, which arises when the physical dimensions of the nanocrystal become smaller than the excitonic Bohr radius of silicon (approximately 5 nm). In this regime, the energy levels of electrons and holes become discretized, leading to a widening of the bandgap as the dot size decreases. For example, SiQDs with diameters around 2 nm emit blue light (∼450 nm), while larger dots around 5 nm emit red or NIR light (∼700–900 nm). This tunability is a direct consequence of the inverse relationship between particle size and bandgap energy, described by the Brus equation for effective mass approximation.

A notable feature of SiQDs is the Stokes shift, the energy difference between the absorption onset and the PL peak. This shift occurs due to excitonic relaxation processes, including surface trapping and phonon-assisted recombination. In SiQDs, the Stokes shift can range from 50 to 300 meV, depending on surface passivation and crystallinity. Poorly passivated surfaces introduce trap states that non-radiatively quench PL, while well-passivated SiQDs (e.g., with hydrogen or organic ligands) exhibit higher PL quantum yields (QYs). The highest reported PL QYs for colloidal SiQDs approach 60–70% in the red to NIR range, though achieving such efficiency requires meticulous control over surface chemistry and defect minimization.

Absorption characteristics of SiQDs differ significantly from bulk silicon. While bulk Si absorbs weakly in the visible range due to its indirect bandgap, SiQDs exhibit stronger absorption coefficients at higher energies, particularly in the UV and blue regions. The absorption profile shows distinct features corresponding to transitions between quantized energy levels, with the first excitonic peak becoming more pronounced as size decreases. However, the molar extinction coefficients of SiQDs are generally lower than those of II-VI QDs, which can limit their brightness in certain applications.

Comparing SiQDs with II-VI QDs reveals trade-offs in brightness and stability. II-VI QDs like CdSe exhibit higher PL QYs (up to 90%) and narrower emission linewidths due to their direct bandgap nature. However, they suffer from photobleaching and toxicity concerns. In contrast, SiQDs are biocompatible and photostable, with negligible blinking under continuous excitation. Their emission stability in physiological environments makes them attractive for bioimaging, though their lower brightness remains a challenge. Recent advances in surface engineering, such as oxide shell encapsulation or ligand exchange, have improved SiQD brightness while retaining stability.

Recent breakthroughs in SiQD research have expanded their emission range. Traditional SiQDs emitted primarily in the red to NIR region, but advances in synthesis and passivation have enabled visible emission (blue, green) by reducing size and minimizing surface oxidation. For NIR emission, larger SiQDs (5–10 nm) with optimized surface passivation have achieved PL peaks beyond 1000 nm, useful for deep-tissue imaging. Additionally, doping SiQDs with elements like phosphorus or boron has introduced mid-gap states that enable tailored emission wavelengths and enhanced QYs.

The role of surface chemistry in SiQD optical properties cannot be overstated. Hydride-terminated SiQDs exhibit efficient PL but are prone to oxidation, leading to PL quenching. Organic ligands (e.g., alkyl chains, polymers) enhance stability but may introduce insulating barriers that hinder charge transport. Recent work on chlorine or nitrogen-terminated SiQDs has shown improved air stability and higher QYs by reducing surface defects. Additionally, hybrid passivation strategies combining organic and inorganic layers (e.g., SiO2 shells) have further boosted performance.

Another key aspect is the exciton dynamics in SiQDs. Due to the indirect bandgap, radiative recombination in SiQDs involves phonon assistance, resulting in longer PL lifetimes (microseconds) compared to direct bandgap QDs (nanoseconds). This slow recombination can be advantageous for applications requiring long-lived excited states but may limit high-speed optoelectronic applications. Auger recombination, a non-radiative process, is also significant in smaller SiQDs and can reduce PL efficiency at high excitation densities.

The environmental sensitivity of SiQDs presents both challenges and opportunities. Unlike II-VI QDs, SiQDs are less prone to heavy-metal leaching, making them environmentally friendly. However, their PL is highly sensitive to ambient conditions (oxygen, moisture), necessitating careful encapsulation for practical use. Recent studies have demonstrated that embedding SiQDs in robust matrices (e.g., polymers, glasses) preserves their optical properties while enhancing durability.

In summary, the optical properties of SiQDs are intricately linked to their quantum-confined structure and surface environment. Their tunable PL, biocompatibility, and stability offer unique advantages over traditional QDs, though brightness and absorption efficiency remain areas for improvement. Advances in surface passivation, doping, and hybrid architectures continue to push the boundaries of SiQD performance, enabling new applications in lighting, sensing, and bioimaging. The ongoing exploration of SiQD photophysics promises to unlock further potential in this versatile nanomaterial.