Physicochemical properties of ultrasmall colloidal silver nanoparticles: an experimental and computational approach.

Silver nanoparticles (AgNPs) exhibit very interesting properties that have been employed in several kinds of applications spanning from antibacterial activity to plasmon-polaritons generation. Nanoparticle size strongly influences these applications. However, the characterization of ultrasmall AgNPs (particle diameter < 10 nm) encompassing different aspects such as average size, polydispersion, shape (anisotropy), concentration, and density remains a challenging task. To address these challenges, we combined TEM measurements with a computational framework based on Mie-Gans theory. This allowed us to describe the aforementioned AgNP features accurately. The synthesis of AgNPs in an aqueous medium involved the use of silver nitrate as a chemical precursor and sodium borohydride as a reducing agent, with polyvinylpyrrolidone acting as a stabilizing agent. Our outcomes showed that increasing the concentration of the precursor and reducing agent with a fixed 1 : 2 molar ratio tends to yield ultrasmall AgNPs with low to moderate polydispersion, a nearly spherical shape (low anisotropy), concentration in the nanomolar range and density close to silver bulk. Also, we established an analytical expression that correlates the extinction molar absorptivity to AgNP size considering the nanoparticle shape. Notably, the computational framework proved to be highly effective in extracting crucial information about the AgNPs from UV-vis spectroscopy data. In conclusion, our study sheds light on the unique properties of ultrasmall AgNPs and presents a comprehensive approach for properly characterizing these nanoparticles, paving the way for further advancements in their applications.

Size-Dependent Photobleaching Mechanism and Kinetics Induced by Nanosecond Laser Pulses in Colloidal Semiconductor Quantum Dots.

An experimental-theoretical approach is proposed to investigate the size-dependent photobleaching of colloidal semiconductor quantum dots (QDs) excited by a nanosecond pulsed laser. In the experimental background, the ground-state absorption and photoluminescence (PL) spectra of chemically prepared QDs are monitored over an excitation time at distinct laser irradiances. The magnitude of photobleaching in the QD solution is quantified by the decay rate of the PL signal as a function of the excitation time and the laser power. A theoretical spectroscopy model is then used to estimate the particle size distribution (PSD) in colloidal solution from the absorption data generated at different laser powers. The resulting evolution of the PSD of the QD ensemble under irradiation is analyzed in terms of classical crystallization theories dealing with the formation, growth, and dissolution of colloidal particles in a supersaturated medium. The QD response to laser irradiation is also interpreted by a simple mechanical model that correlates the photoinduced hydrostatic strain at the solid/liquid interface and the predicted variation of the mean particle size. The reported experimental and theoretical methods are used to completely elucidate the basic physico-chemical processes responsible for the laser-induced photobleaching kinetics of glutathione-capped CdTe aqueous QDs with very small mean sizes. For this purpose, we synthesized a series of colloidal QD samples with mean particle diameters ranging from 1.95 to 2.68 nm. Our results indicate that a faster photobleaching rate occurs in QD samples with smaller sizes in which particle dissolution under laser irradiation is predominant. On the other hand, the photobleaching rate becomes slower in samples with larger dot sizes, possibly due to the formation of core/shell structures in solution via thermal degradation of thiol ligands either during the chemical synthesis or as a consequence of the subsequent interaction with the excitation laser.

Probing the cw-Laser-Induced Fluorescence Enhancement in CsPbBr3 Nanocrystal Thin Films: An Interplay between Photo and Thermal Activation.

Perovskite nanocrystals hold significant promise for a wide range of applications, including solar cells, LEDs, photocatalysts, humidity and temperature sensors, memory devices, and low-cost photodetectors. Such technological potential stems from their exceptional quantum efficiency and charge carrier conduction capability. Nevertheless, the underlying mechanisms of photoexcitation, such as phase segregation, annealing, and ionic diffusion, remain insufficiently understood. In this context, we harnessed hyperspectral fluorescence microspectroscopy to advance our comprehension of fluorescence enhancement triggered by UV continuous-wave (cw) laser irradiation of CsPbBr3 colloidal nanocrystal thin films. Initially, we explored the kinetics of fluorescence enhancement and observed that its efficiency (φph) correlates with the laser power (P), following the relationship φph = 7.7⟨P⟩0.47±0.02. Subsequently, we estimated the local temperature induced by the laser, utilizing the finite-difference method framework, and calculated the activation energy (Ea) required for fluorescence enhancement to occur. Our findings revealed a very low activation energy, Ea ∼ 9 kJ/mol. Moreover, we mapped the fluorescence photoenhancement by spatial scanning and real-time static mode to determine its microscale length. Below a laser power of 60 µW, the photothermal diffusion length exhibited nearly constant values of approximately (22 ± 5) µm, while a significant increase was observed at higher laser power levels. These results were ascribed to the formation of nanocrystal superclusters within the film, which involves the interparticle spacing reduction, creating the so-called quantum dot solid configuration along with laser-induced annealing for higher laser powers.