Ultrafast and Real-Time Dynamics of Nanomaterials Studied by Advanced Spectroscopic Techniques

Ultrafast and nonlinear spectroscopies are used to study excited-state dynamics and monitor real-time growth dynamics of different types of nanomaterials. In the first study, the growth dynamics of colloidal gold-silver core-shell nanoparticles are studied using in situ second harmonic generation and extinction spectroscopy. The growth lifetimes are studied under different reaction conditions, resulting in different silver shell thicknesses, with spectral comparisons to finite-difference time-domain calculations. The results are consistent with a three-step growth process. During the first step of the nanoparticle growth reaction, rough and uneven surfaces are formed rapidly giving rise to plasmonic hot spots with corresponding broad, red-shifted plasmon spectra. In the second step, the nanoparticle surface becomes smoother, reaching a thermodynamic equilibrium. The Au@Ag nanoparticle growth process has a third, slower step where the nanoparticle surface charge density changes due to chemical reactions resulting in a decreasing SHG signal while the extinction spectrum remains constant. In another study, transient absorption spectroscopy is used to study the ultrafast dynamics of porphyrin- and zinc porphyrin-based colloidal nanomaterials for potential applications in light-harvesting devices. Porphyrin and zinc porphyrin dyes in water exhibit long-lived excited states on the order of several nanoseconds. However, these porphyrin excited-state lifetimes are significantly faster when in the nanoparticle environment due to energy transfer, enhanced intersystem crossing, and electronic delocalization. Finally, the excited-state heating and melting dynamics of aluminum thin film samples at different thicknesses are investigated using ultrafast pump-probe reflectivity with 800 nm light under varying laser pump pulse powers. The data reveal a dramatic change in the carrier relaxation mechanism below and above 150 nm in thickness under varying laser powers due to the role of the characteristic heat penetration depth. Samples with thickness below this length scale show faster heating dynamics and a lower power threshold for melting. Overall, the synthesis, characterization, nonlinear spectroscopy, and ultrafast spectroscopy of a wide variety of nanomaterials are studied for the development of potential nanomedicine, optoelectronic, molecular sensing, and laser-based additive manufacturing applications