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We present time-resolved photoluminescence (PL) spectroscopy of a series of colloidal CdSe/CdS core/shell nanoplatelets with different core and shell thicknesses. Exciton numbers are determined fro...

The most recent class of semiconductor nanocrystal to be synthesized colloidally is the quantum well, in which carriers are confined quantum mechanically in only one dimension. Electrons and holes in colloidal quantum wells undergo different dynamics than in either colloidal quantum dots or epitaxially grown quantum wells, providing new opportunities for applications. The opportunities presented by cadmium chalcogenide nanoplatelets are particularly exciting, because they can be grown with control over their thickness down to the single atomic layer and with all nanoplatelets in an ensemble having the same thickness. This Feature Article reviews the relaxation and recombination dynamics of electrons and holes, which are tightly bound into excitons, in nanoplatelets. These dynamics are favorable for optical gain and lasing, and this Article reviews the progress that has been made toward practical realization of nanoplatelet lasers, including the demonstration of low thresholds for room-temperature gain and...

Recent measurements on the gigahertz vibration of nanoparticles immersed in simple liquids, such as glycerol, show that the liquid’s viscoelastic properties can significantly affect the nanoparticle’s mechanical response. Here, we theoretically explore the high-frequency (elastic) limit of this phenomenon where the characteristic time scale for molecular relaxation in the liquid far exceeds the nanoparticle’s vibration period. Paradoxically, we find that the effects of liquid elasticity (and viscosity) may not be visible in the nanoparticle’s dynamic response in this high-frequency elastic limit—the response being identical to that of a macroscopic resonator in an inviscid fluid. A comprehensive mechanistic study reveals that the conditions for this unusual behavior are strongly dependent on the nanoparticle’s vibration mode and the liquid’s properties. A judicious choice of vibration mode is essential for interrogating the viscoelastic properties of simple liquids. Our findings explain recent measurement...

The dynamics of mechanical structures can be strongly affected by the fluid in which they are immersed. Ultrafast laser spectroscopy has recently provided fundamental insight into this fluid-structure interaction for nanoparticles immersed in a range of viscous fluids. In this article, we present results of a rigorous finite-element analysis and commensurate scaling theory that enable interpretation and analysis of these experiments, for the extensional vibrational modes of axisymmetric nanoparticles immersed in viscous fluids. Right circular, conical, and bipyramidal axisymmetric cylinder geometries are considered. We also develop an approximate analytical model that accounts for finite viscous penetration depth, which displays excellent agreement with finite-element results for particles of large aspect ratio. The finite-element results agree well with available measurements for particles in low-viscosity fluids such as water, but significant discrepancies exist at higher viscosities. Possible mechanisms for these differences are discussed.

We present measurements and theoretical analysis of the damping of high-frequency acoustic vibrations of metal nanoparticles immersed in solution. Building on our previous work [Pelton, M.; Sader, ...

Contact between aqueous solutions of silver nitrate (AgNO3) and pristine surfaces of p-type gallium arsenide (GaAs) wafers results in essentially no reaction at room temperature and in the dark. The galvanic reactions between the GaAs wafers and AgNO3 can be triggered under illumination of laser beams with power densities higher than a critical value (∼15 mW/cm2 for a 630 nm laser), resulting in the growth of silver (Ag) nanoplates on the GaAs surface. The density and dimensions (including both thickness and edge length) of the resulting nanoplates can be readily tuned by controlling the growth time and laser power density. The as-grown Ag nanoplates on the substrates significantly enhance Raman signals of interesting molecules and serve as a new class of promising surface-enhanced Raman scattering substrates for sensitive chemical detection.

The dispersion functions for the refractive index and the extinction coefficient of single- and multiple-layer graphene oxide samples were measured by imaging spectroscopic ellipsometry in the wavelength range of 350−1000 nm and were compared to previously reported results measured by confocal microscopy. The dispersion functions for thin platelets were also compared to those obtained by standard spectroscopic ellipsometry on a deposit consisting of many overlapping graphene oxide layers. Changes were observed in both the thickness of the deposits and the values of the dispersion parameters following heating. A model is proposed to explain these observations, based on the removal of water between the graphene-oxide layers upon thermal treatment.

Electron dynamics in Au nanorods are studied with femtosecond nonlinear spectroscopic techniques, by directly exciting and probing the longitudinal surface plasmon resonance. The dispersive and absorptive parts of the third-order signal are measured using optical heterodyne detected four-wave-mixing spectroscopy. These signals are used to describe dynamics in Au nanorods in terms of frequency shift and broadening of the plasmon resonance. Pump−probe experiments are performed with a series of pump intensities. The results are treated in two ways: (1) by calculating the temperature changes of electrons and phonons in the nanorods and the effects of these temperatures on the dielectric constant of Au; and (2) by a nonlinear least-squares fitting using a phenomenological response function. The first model agrees with the pump−probe experimental results for pump energies up to 2.0 nJ (2.5 GW/cm2) and for delays in the range of 150 fs to 150 ps, but does not reproduce three additional features present in the d...