Shun Shang Lo

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.

Imaging the extent of plasmon excitation in Au nanowires using pump-probe microscopy

Knowledge of how energy and charge carriers move in nanoscale systems is essential for engineering efficient devices. In this Letter, we demonstrate a technique to directly image dynamics in nanostructures based on laser scanning transient absorption microscopy, which provides near diffraction-limited spatial resolution and ultrafast time resolution. The capabilities of the technique are demonstrated by experiments on propagating surface plasmon polariton modes of Au nanowires, although these measurements can be used to study a variety of fluorescent and nonfluorescent systems.

Picosecond Kinetics of Strongly Coupled Excitons and Surface Plasmon Polaritons

Coupling between excitons of CdSe nanocrystal quantum dots (NQDs) and surface plasmon polaritons (SPPs) of an Ag film attached to a prism have been studied by steady-state and transient reflectivity measurements in the Kretschmann geometry. In these experiments, the angle of incidence of the probe beam selects hybrid exciton/SPP states with different wavevectors and exciton/SPP compositions. The dynamics measured in the transient reflectivity experiments are sensitive to the composition of the hybrid states. Specifically, fast dynamics are observed at probe wavevectors where the lower hybrid state has predominant SPP character. In contrast, at probe wavevectors where the lower hybrid state is predominantly excitonic, the dynamics are similar to that measured for CdSe NQDs on glass.

Near-Infrared Croconaine Rotaxanes and Doped Nanoparticles for Enhanced Aqueous Photothermal Heating

The photothermal effect is the generation of heat by molecules or particles upon high-energy laser irradiation, and near-infrared absorbers such as gold nanoparticles and organic dyes have a range of potential photothermal applications. The favourable photothermal properties of thiophene-functionalised croconaine dyes were recently discovered. The synthesis and properties of novel croconaine rotaxane and pseudorotaxane architectures capable of efficient photothermal performance in both organic and aqueous environments are reported. The versatility of this dye-encapsulation strategy was demonstrated by the preparation of two organic croconaine rotaxanes using different synthetic methods: the formation of an aqueous pseudorotaxane association complex, and the synthesis of water-soluble, croconaine-doped silicated micelle nanoparticles. All of these near-infrared-absorbing systems exhibit excellent photothermal behaviour, with pseudorotaxane and rotaxane formation vital for effective aqueous heat generation. Dye encapsulation provides steric protection to enhance the stability of a water-sensitive croconaine dye, while rotaxane-doped nanoparticles avoid detrimental band broadening caused by chromophore coupling.

Ultrafast Spatial Imaging of Charge Dynamics in Heterogeneous Polymer Blends.

Proof-of-concept transient absorption microscopy (TAM) with simultaneously high spatial and temporal resolution was demonstrated to image charge generation and recombination in model systems of poly(3-hexylthiophene) (P3HT) and [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) blends upon extended thermal annealing. Significant spatial heterogeneity in charge generation and recombination dynamics was revealed on the length scale of hundreds of nanometers near the micrometer-sized PCBM crystallites, suggesting that information obtained in ensemble measurements by integrating over microscopically inhomogeneous areas could be misleading. In contrast to previous studies, high sensitivity of our instrumentation allows us to employ low excitation intensities to minimize higher-order recombination processes. TAM provides a unique noncontact tool to probe local functionality in microscopically heterogeneous energy harvesting systems.

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.

Spatial modulation spectroscopy of graphene sheets.

Two different samples of graphene, multi-layer flakes on Si/SiO2 substrates and single layer graphene on glass, have been examined by reflectivity contrast and spatial modulation spectroscopy measurements. For the multi-layer graphene flakes, the reflectivity contrast and spatial modulation spectroscopy measurements are in good agreement, validating the application of spatial modulation spectroscopy to two-dimensional samples. The measurements for single layer graphene on glass show features that correspond to increases and decreases in reflectivity. The features with increased reflectivity are assigned to small regions of multilayer graphene or polymer, and the features with decreased reflectivity are assigned to holes in the graphene film. Using a model for thin film reflectivity we calculate the size dependent spatial modulation signal for the holes, and find that a significant number of holes have a larger than expected signal. This could arise from the presence of multi-layers of graphene in the sample, or because of optical resonance effects for the holes.

Spectroscopy Beyond the Single-Particle Limit

Ultrafast x-ray diffraction studies reveal the lattice vibrations of single gold nanoparticles. [Also see Report by Clark et al.] Scientists can now routinely detect and study single molecules and nanoparticles (1). However, direct observation of chemical processes and/or atomic motions in real time remains a challenge, primarily because ultrafast (subpicosecond) time resolution is needed. Optical techniques have recently been developed to study the dynamics of individual molecules or nanoparticles where two laser beams (a pump and a probe) are focused onto a single nano-object under a microscope (2, 3). The spatial resolution of these measurements is limited by the diffraction of light, so that the movements of the individual atoms can only be inferred. On page 56 of this issue, Clark et al. (4) present a study of the lattice motions of individual gold nanoparticles recorded using ultrafast coherent x-ray pulses as a probe. These measurements yield three-dimensional images of the atomic displacements in the particles as a function of time, with a spatial resolution that is orders of magnitude better than what can be achieved with optical microscopes.