Effect of Water Adsorption on the Photoluminescence of Silicon Quantum Dots

The luminescence mechanism of ligand-induced interface states in silicon quantum dots

Over decades of research on photoluminescence (PL) of silicon quantum dots (Si-QDs), extensive exploratory experiments have been conducted to find ways to improve the photoluminescence quantum yield. However, the complete physical picture of Si-QD luminescence is not yet clear and needs to be studied in depth. In this work, which considers the quantum size effect and surface effect, the optical properties of Si-QDs with different sizes and surface terminated ligands were calculated based on first principles calculations. The results show that there are significant differences in the emission wavelength and emission intensity of Si-QD interface states connected by different ligands, among which the emission of silicon-oxygen double bonds is the strongest. When the size of the Si-QD increases, the influence of the surface effect weakens, and only the silicon-oxygen double bonds still localize the charge near the ligand, maintaining a high-intensity luminescence. In addition, the presence of surface dangling bonds also affects luminescence. This study deepens the understanding of the photoluminescence mechanism of Si-QDs, and provides a direction for both future improvement of the photoluminescence quantum efficiency of silicon nanocrystals and for fabricating silicon-based photonic devices.

Degradation of Losartan Potassium Highlighted by Correlated Studies of Photoluminescence, Infrared Absorption Spectroscopy and Dielectric Spectroscopy

In this paper, new results on the degradation of losartan potassium (LP, (1)), in the absence and presence of excipients, which was induced by UV light, the acid character of phosphate buffer solution (PBS) and alkaline medium, respectively, are reported through correlated studies of FTIR spectroscopy, photoluminescence and dielectric spectroscopy. The photoluminescence (PL) spectra of LP and the drug marked under the name Lorista (LO) are characterized by intense emission bands, peaking at 378 nm and 380 nm, respectively, accompanied by low intensity bands with a maximum at ~450–460 nm. Photodegradation of LO in a solid state is evidenced by a decrease in the intensity of the PL band at 380 nm, a variation that originates both in the adsorption of water vapors from the air and in the interaction of LP with excipients such as cornstarch, silicon dioxide and cellulose. The LP-water interaction is described, taking into account the main electrical parameters, i.e., complex dielectric permittivity and electrical conductivity. Photodegradation of LP and LO also induces an increase in the intensity of the emission band, at ~450–460 nm. The influence of acid and alkaline medium on the LO degradation is analyzed using phosphate buffer (PBS) and NaOH solutions, respectively. In both cases, a decrease in the intensity of the PL band, at 380 nm, is reported. The intensity diminution of the PL spectra of NaOH-reacted LP and LO is the result of the formation of the photodegradation product N-methanolamide-{[2′-(1H-tetrazol-5-yl)(1,1′-biphenyl)-4-yl]methyl} (2). This compound was proven by the studies of FTIR spectroscopy achieved on LP and NaOH-reacted LP. The appearance of the IR band at 1740 cm−1 and the increase in the absorbance in the IR band at 1423 cm−1 indicate that the photodegradation product (2) contains the C=O and C-OH functional groups.

Si quantum dots enhanced hydrogen bonds networks of liquid water in a stimulated Raman scattering process

Stimulated Raman scattering (SRS) of silicon quantum dots (Si QD) water solutions of different sizes (2 and 5 nm) are investigated using Nd:YAG laser. Since strong and weak hydrogen bonds are formed by the charge transfer between water molecules and Si QDs, two SRS peaks of OH stretching vibrations of Si QDs solutions are observed in the forward direction. Simultaneously, characteristic feature peaks related to the interaction between OH groups and excess electrons are obtained in the backward SRS of 2 nm Si QDs solutions. The excess electrons induce a strong electrostatic field, leading to the transformation from water to an ice-VIII structure.

Oriented attachment growth of monocrystalline cuprous oxide nanowires in pure water

As a crucial mechanism of non-classical crystallization, the oriented attachment (OA) growth of nanocrystals is of great interest in nanoscience and materials science. The OA process occurring in aqueous solution with chemical reagents has been reported many times, but there are limited studies reporting the OA growth in pure water. In this work, we report the temperature-dependent OA growth of cuprous oxide (Cu₂O) nanowires in pure water through a reagent-free electrophoretic method. Our experiments demonstrate that Cu₂O quantum dots randomly coalesced to form polycrystalline nanowires at room temperature, while they form monocrystalline nanowires at higher temperatures by the OA mechanism. DFT modeling and computations indicate that the water coverage on the Cu₂O nanoparticles could affect the particle attachment mechanisms. This study sheds light on the understanding of the effects of water molecules on the OA mechanism and shows new approaches for better controllable non-classical crystallization in pure water.

A dipole-dipole interaction tuning the photoluminescence of silicon quantum dots in a water vapor environment

The optical properties of silicon quantum dots (Si QDs) depend on the working conditions, which are critical for their application in optoelectronic devices and fluorescent tags. However, how a humid environment, most common in daily life, influences the photoluminescence (PL) of Si QDs has not been fully understood yet. Herein, we applied time-dependent density functional calculations to show that the adsorption of water molecules would exhibit distinct effects on the PL spectra of Si QDs as a function of size. In particular, the PL of Si QDs presents dual band emission with the adsorption of the cyclic water trimer (H2O)3 under common humid conditions, completely different from the PL of Si QDs under other conditions. The transition dipole moment decomposition analysis shows that the additional emission peak originates from the single Si-Si stretched bond of Si QDs induced by the dipole-dipole interaction between the cyclic water trimer and Si QDs. Moreover, the PL characteristics are size dependent. As the size increases from Si17H24 (the diameter of 0.6 nm) to Si52H52 (1.4 nm), the dipole-dipole interaction energy between (H2O)3 and Si QDs rapidly decreases from 19.1 × 10-22 J to 6.0 × 10-26 J, resulting in a single peak of PL of (H2O)3 adsorption on Si52H52. This study not only gives a deep understanding of PL of Si QDs under humid conditions, but also provides a new perspective on the development of optical devices based on Si QDs.

Surface Dynamics and Ligand-Core Interactions of Quantum Sized Photoluminescent Gold Nanoclusters

Quantum-sized metallic clusters protected by biological ligands represent a new class of luminescent materials; yet the understanding of structural information and photoluminescence origin of these ultrasmall clusters remains a challenge. Herein we systematically study the surface ligand dynamics and ligand-metal core interactions of peptide-protected gold nanoclusters (AuNCs) with combined experimental characterizations and theoretical molecular simulations. We show that the peptide sequence plays an important role in determining the surface peptide structuring, interfacial water dynamics and ligand-Au core interaction, which can be tailored by controlling peptide acetylation, constituent amino acid electron donating/withdrawing capacity, aromaticity/hydrophobicity and by adjusting environmental pH. Specifically, emission enhancement is achieved through increasing the electron density of surface ligands in proximity to the Au core, discouraging photoinduced quenching, and by reducing the amount of surface-bound water molecules. These findings provide key design principles for understanding the surface dynamics of peptide-protected nanoparticles and maximizing the photoluminescence of metallic clusters through the exploitation of biologically relevant ligand properties.

Polyethylene glycol-coated blue-emitting silicon dots with improved properties for uses in aqueous and biological environments

Grafting of polyethylene glycol (PEG) to ultrasmall photoluminescent silicon dots (SiDs) is expected to improve and expand the applications of these particles to aqueous environments and biological systems. Herein we report a novel one-pot synthesis of robust, highly water compatible PEG-coated SiDs (denoted as PEG-SiDs) of (3.3 ± 0.5) nm size. The nanoparticles' synthesis is based on the liquid phase oxidation of magnesium silicide using PEG as reaction media and leading to high PEG density grafting. PEG-SiDs enhanced photophysical, photosensitising, and solution properties in aqueous environments are described and compared to those of 2 nm size PEG-coated SiDs with low PEG density grafting (denoted as PEG-NHSiDs) obtained from a multistep synthesis strategy. PEG-SiDs form highly dispersed suspensions in water showing stable photoluminescence and quantum yields of Φ = 0.13 ± 0.04 at 370 nm excitation in air-saturated suspensions. These particles exhibited the capacity of photosensitising the formation of singlet molecular oxygen, not observed for PEG-NHSiDs. PEG robust shielding of the silicon core luminescent properties is further demonstrated in bio-imaging experiments stressing the strong interaction between PEG-SiDs and Staphylococcus aureus smears by observing the photoluminescence of particles. PEG-SiDs were found to be nontoxic to S. aureus cells at concentrations of 100 mg ml^-1, though a bacteriostatic effect on S. aureus biofilms was observed upon UV-A irradiation under conditions where light alone has no effect.