Todd A. Major

Time-Resolved Studies of the Acoustic Vibrational Modes of Metal and Semiconductor Nano-objects.

Over the past decade, there have been a number of transient absorption studies of the acoustic vibrational modes of metal and semiconductor nanoparticles. This Perspective provides an overview of this work. The way that the frequencies of the observed modes depend on the size and shape of the particles is described, along with their damping. Future research directions are also discussed, especially how these measurements provide information about the way nano-objects interact with their environment.

Charge Carrier Trapping and Acoustic Phonon Modes in Single CdTe Nanowires

Semiconductor nanostructures produced by wet chemical synthesis are extremely heterogeneous, which makes single particle techniques a useful way to interrogate their properties. In this paper the ultrafast dynamics of single CdTe nanowires are studied by transient absorption microscopy. The wires have lengths of several micrometers and lateral dimensions on the order of 30 nm. The transient absorption traces show very fast decays, which are assigned to charge carrier trapping into surface defects. The time constants vary for different wires due to differences in the energetics and/or density of surface trap sites. Measurements performed at the band edge compared to the near-IR give slightly different time constants, implying that the dynamics for electron and hole trapping are different. The rate of charge carrier trapping was observed to slow down at high carrier densities, which was attributed to trap-state filling. Modulations due to the fundamental and first overtone of the acoustic breathing mode were also observed in the transient absorption traces. The quality factors for these modes were similar to those measured for metal nanostructures, and indicate a complex interaction with the environment.

Compressible Viscoelastic Liquid Effects Generated by the Breathing Modes of Isolated Metal Nanowires.

Transient absorption microscopy is used to examine the breathing modes of single gold nanowires in highly viscous liquids. By performing measurements on the same wire in air and liquid, the damping contribution from the liquid can be separated from the intrinsic damping of the nanowire. The results show that viscous liquids strongly reduce the vibrational lifetimes but not to the extent predicted by standard models for nanomaterial-liquid interactions. To explain these results a general theory for compressible viscoelastic fluid-structure interactions is developed. The theory results are in good agreement with experiment, which confirms that compressible non-Newtonian flow phenomena are important for vibrating nanostructures. This is the first theoretical study and experimental measurement of the compressible viscoelastic properties of simple liquids.

Detection of single gold nanoparticles using spatial modulation spectroscopy implemented with a galvo-scanning mirror system

The optical extinction of single nanoparticles can be sensitively detected by spatial modulation spectroscopy (SMS), where the particle is moved in and out of a tightly focused laser beam with a piezo-device. Here we show that high sensitivity can be obtained by modulating the beam with a galvo-mirror system, rather than by moving the sample. This work demonstrates an inexpensive method for making a SMS microscope, and shows how an existing laser scanning microscope can be adapted for SMS measurements. The galvo-mirror technique also allows SMS measurements to be performed in a liquid, which is difficult to do with piezo-modulation.