Luminescent Silicon Nanocrystals—From Single Quantum Dot to Light-Converting Applications

Fundamental photophysical properties of Si quantum dots (QDs) were investigated on a single-particle level to understand the mechanism of light conversion [1-3]. Emission, absorption and lifetime data were obtained for individual silicon nanocrystals with either oxide or ligand passivation. In comparison with theory we found that core-related luminescence from Si QDs emanates from an indirect-bandgap state for all practical size range of Si QDs. Small intermixing of direct-bandgap character due to quantum confinement effect takes place, but relevant only for higher energy states pertinent to the light absorption process. Quantum size effect from Si bandgap (1.1 eV) up to 2 eV has been clearly demonstrated by observation of sharp emission linewidths in a broad energy range [3].<br/><br/>Despite partly forbidden character of the recombination process, luminescence from Si QDs may still possess high quantum efficiency when a defect-free core with proper surface passivation realized. We have demonstrated a new synthesis method, which can reduce pre-cursor cost by an order of magnitude from the established HSQ-based method [4]. Si QDs prepared in this way have near-unity internal and &gt; 50% external (quantum yield) quantum efficiency. They also possess a large Stokes shift, which suppresses re-absorption of the emitted light of importance in a number of applications.<br/><br/>One such application is a semi-transparent photovoltaics for glazing in building-integration. It is based on a luminescent solar concentrator concept, where high efficiency and a large Stokes shift are necessary requirements for nanophosphors [5-6]. In this configuration absorbed solar light is re-emitted and a large fraction of it is guided by total internal reflection to the edges for collection by standard solar cells. As a proof-of-concept we fabricated 20x20 cm² prototypes, where Si QD-doped polymer layer is sandwiched between glass plates in a triplex geometry. Such “solar windows” feature high transparency (&gt;80%), low haze (&lt;3%), high color rendering index (~ 88) and, at the same time, deliver up to 0.6 W of electrical peak power under one sun [7-8]. Another application is in bio-labeling, where long lifetime of Si QD emission (~ us) makes them sensitive to surface chemistry. It was shown previously that the presence of nitrogen moieties on nanocrystal surface may introduce a fast, ~ ns, recombination channel, which can take over carrier recombination from the QD core. Here we used this effect to monitor amino acids in the cell, where their binding of Si QDs manifested in shifted luminescence from near-infrared to blue and drastically reduced the lifetime, acting as an efficient amino acid probe in live cells [9].<br/><br/>1. Luo, J. W., et al., Absence of Red-shift in the Direct Band Gap of Silicon Nanocrystals with Reduced Size. <i>Nat. Nanotechnol. </i><b>2017,</b> <i>12</i>, 930-932.<br/>2. Nestoklon, M. O., et al., Tight-binding calculations of optical properties of Si nanocrystals in SiO2 matrix. <i>Faraday Discuss</i>. <b>2020</b>, 222, 258-273.<br/>3. Zhou, J., et al., Wafer-scale fabrication of isolated luminescent silicon quantum dots using standard CMOS technology. <i>Nanotechnology </i><b>2020,</b> <i>31</i>, 505204.<br/>4. Zhou, J., et al., Low-Cost Synthesis of Silicon Quantum Dots with Near-unity Internal Quantum Efficiency. <i>J. Phys. Chem. Lett. </i><b>2021,</b> <i>12</i>, 8909−8916.<br/>5. Sychugov, I., Analytical Description of a Luminescent Solar Concentrator. <i>Optica </i><b>2019,</b> <i>6</i>, 1046-1049.<br/>6. Sychugov, I., Geometry Effects on Luminescence Solar Concentrator Efficiency: Analytical Treatment. <i>Appl. Opt.</i> <b>2020</b>, 59, 5715-5722.<br/>7. Huang, J., et al., Triplex Glass Laminates with Silicon Quantum Dots for Luminescent Solar Concentrators. <i>Solar RRL</i> <b>2020</b>, 4, 2000195<br/>8. Huang, J., et al., Large-area Transparent “Quantum Dots Glass” for Building Integrated Photovoltaics. <i>ACS Photonics </i><b>2022,</b> 9, 2499-2509.<br/>9. Chen, H., et al., Color-switchable nano-silicon fluorescent probes. <i>ACS Nano</i> <b>2022</b>, 16, 15450–15459.