Electronic Structure Engineering Achieved via Organic Ligands in Silicon Nanocrystals

Abstract

A class of important semiconductors, such as Si, Ge, or C, has an indirect band gap, which critically limits their optical properties. Lack of efficient emission is especially unfortunate for silicon, where Si light sources could enable realization of the long-awaited on-chip-integrated Si laser for an integrated optical computing CPU architecture. Hence, methods toward the improvement of optical properties of Si-based materials are in high demand. Unlike most of the applied light-emitting semiconductor nanocrystals (NCs) with a direct band gap, the radiative rate in covalent silicon NCs (SiNCs) is size-dependent but remains low even for the smallest SiNCs. Additionally, the radiative rate is also ligand-sensitive, and the covalent bond with ligands is very rigid and static and could be, in principle, used for straining via steric hindrance, further influencing the radiative rates. In this work, we use the self-consistent density functional theory (DFT) simulation together with a “fuzzy” band-structure concept to show the effect of covalently bonded ligands on the electronic structure of NCs and their k⃗-space projection. For instance, in 2 nm large SiNCs with C-linked organic ligands, we demonstrate that radiative rates can be manipulated by ligands to a considerable extent through an intricate interplay between charge transfer from the core to the ligand, orbital delocalization, and strain by steric hindrance. We propose that the tunability of electronic properties achieved via ligands in covalent systems offers a possible direction toward the design of an ideal Si light-emitting system.

Cost-Effective Synthesis of Silicon Quantum Dots

“…Namely, the series 1 and 3 samples are poorly and highly crystalline, respectively. Thus, the QY of the series 1 sample was reduced by its poor crystallinity that led to a higher density of defect sites that act as electron and hole traps, e.g., distortion, strain, and charge transfer with ligands . In fact, the PLE spectrum of series 1 shows a shoulder on the lower energy side at approximately 3.4 eV in Figure d, which can be considered to be due to defects such as the disorder of amorphous components of SiQDs or an extremely small amount of carbon from the methoxy group in SiQDs.…”

“…Finally, let us mention another important insight related to the optical properties and structures of SiQDs. Analysis of the Raman spectral profile of SiQDs, i.e., the asymmetric profile and peak position, quantifies the amount of stress for the freestanding SiQDs and the SiQDs embedded in the matrices based on the consideration of the phonon confinement effect. , The stress of the former is an indicator that can be used to improve the electronic structure, PLQY, and PL wavelength of SiQDs, whereas the stress of the latter would characterize the local reaction sites of the SiQDs in the matrix during the pyrolysis of the HSQ polymers. In future work, the analysis of these stresses will be crucial for obtaining a further understanding of the SiQDs, the pyrolysis of the HSQ cage cluster and the HSQ polymers, and their synthesis mechanisms.…”

Band-Gap Tunability in Partially Amorphous Silicon Nanoparticles Using Single-Dot Correlative Microscopy

“…This is partially improved in small Si nanoparticles (Si-NPs) via quantum confinement effect (QCE) and surface engineering. 1 − 3 Similar to other NPs, Si-NPs with a radius smaller than the excitonic Bohr radius have size-tunable band-gap energy. 2 , 4 − 9 According to the effective mass approximation (EMA), the band-gap energy of a spherical crystalline NP deviates from its bulk value in inverse proportion to the square of the NP’s radius (the localization term) plus the term that scales inverse with the radius (polarization term), 10 and a similar trend is also predicted by the first-principles methods.…”

Trap-State-Induced Becquerel Type of Photoluminescence Decay in DPA-Activated Silicon Nanocrystals

“…However, other experiments based on sophisticated time-tagged time-resolved single-photon counting techniques prove that the PL dynamics does not necessarily need to be single-exponential even in single direct-bandgap NCs . These results indicate that more than a single process can be responsible for the light emission even in a single nanoparticle, which can, in principle, also be explained simply by the thermal population of states . Thus, not surprisingly, non-single-exponential PL dynamics are often observed even in systems of well-defined direct semiconductor NCs, especially in their densely packed systems, as thoroughly described in ref and references therein.…”

“…5 These results indicate that more than a single process can be responsible for the light emission even in a single nanoparticle, which can, in principle, also be explained simply by the thermal population of states. 6 Thus, not surprisingly, non-single-exponential PL dynamics are often observed even in systems of well-defined direct semiconductor NCs, especially in their densely packed systems, as thoroughly described in ref 7 and references therein. The complexity of the decay dynamics can also be seen in elaborate experiments involving the characterization of both the intrinsic PL decay and blinking-induced delayed PL on very broad time scales.…”