Mary Sajini Devadas

Absorption Spectroscopy of Single Optically Trapped Gold Nanorods.

Extinction spectra of single gold nanorods optically trapped in water were measured by spatial modulation spectroscopy. Comparison of the extinction cross sections and resonance frequencies to finite element calculations allows us to determine the dimensions of the nanorod and estimate the contribution of radiation damping to the LSPR line width. Subtracting the radiation damping and bulk contributions from the measured line widths yields the electron-surface scattering contribution. The results show that the surfactant coating for the nanorods causes large electron-surface scattering effects with significant particle-to-particle variations. These effects are more pronounced than those seen for substrate-supported particles in previous single particle studies. Indeed, the measured line widths are only slightly narrower than that of the ensemble spectrum. These results show the importance of removing surfactant for sensing applications of these materials.

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.

Imaging and Analysis of Single Optically Trapped Gold Nanoparticles Using Spatial Modulation Spectroscopy.

The extinction cross sections and spectra of single nanoparticles can be directly measured by moving the particle in and out of a tightly focused laser beam. This technique, known as spatial modulation spectroscopy, yields detailed information about the size, shape, and environment of the particles. These experiments are typically done on particles immobilized on a substrate. Here we demonstrate for the first time the use of spatial modulation spectroscopy to interrogate single, optically trapped nanoparticles in solution. Gold nanoparticles as small as 15 nm were trapped and imaged. The experiments were performed by modulating the position of the probe laser beam while scanning it over the trapped particle with a galvo-scanning mirror system. This technique opens up the possibility of precisely measuring the optical properties of single nanoparticles in liquid environments, free from the influence of a surface.

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.

Role of Resonances in the Transmission of Surface Plasmon Polaritons between Nanostructures.

Understanding how surface plasmon polaritons (SPPs) propagate in metal nanostructures is important for the development of plasmonic devices. In this paper, we study the transmission of SPPs between single-crystal gold nanobars on a glass substrate using transient absorption microscopy. The coupled structures were produced by creating gaps in single nanobars by focused ion beam milling. SPPs were launched by focusing the pump laser at the end of the nanobar, and the transmission across the gaps was imaged by scanning the probe laser over the nanostructure. The results show larger losses at small gap sizes. Finite element method calculations were used to investigate this effect. The calculations show two main modes for nanobars on a glass surface: a leaky mode localized at the air-gold interface, and a bound mode localized at the glass-gold interface. At specific gap sizes (approximately 50 nm for our system), these SPP modes can excite localized surface plasmon modes associated with the gap, which dissipate energy. This increases the energy losses at small gap sizes. Experiments and simulations were also performed for the nanobars in microscope immersion oil, which creates a more homogeneous optical environment, and consistent results were observed.

Spatial modulation spectroscopy imaging of nano-objects of different sizes and shapes

Spatial modulation spectroscopy (SMS) is a powerful method for interrogating single nanoparticles. In these experiments optical extinction is measured by moving the particle in and out of a tightly focused laser beam. SMS is typically used for particles that are much smaller than the laser spot size. In this paper, we extend the analysis of the SMS signal to particles with sizes comparable to or larger than the laser spot, where the shape of the particle matters. These results are important for the analysis of polydisperse samples that have a wide range of sizes. The presented example images and analysis of a carbon microparticle sample show the utility of the derived expressions. In particular, we show that SMS can be used to generate extinction cross-section information about micrometer-sized particles with complex shapes.

A New Field Emission Scanning Electron Microscopy Facility with STEM and EDS Capabilities for Interdisciplinary Research and Education at Towson University, Fisher College of Science and Mathematics

Recently, Towson University (TU) was successful in acquiring a new state-of-the-art Thermo Fisher Scientific APREO LoVac Schottky Field Emission Gun (FEG) Scanning Electron Microscope (SEM). This microscope is a high-performance SEM that combines highand low-voltage ultra-high resolution (~1 nm) capabilities with an electrostatic lens design. In addition, it features beam deceleration and unique in-lens detection providing unprecedented contrast and versatility of samples for research, education, and training activities across multiple disciplines, in the Fisher College of Science and Mathematics (FCSM). The instrument uses an entirely oil-free vacuum system and is standardly equipped with a five-axes motorized x-y-z-tilt-rotate stage and features an Everhart-Thornley highvacuum secondary electron detector (SED) optimized for use across the available kV, current, and pressure range. It bears a Trinity detection system, comprised of a lower in-lens detector (T1), upper inlens detector (T2), and in-column detector (T3). The microscope includes a dedicated low-vacuum (LV) SED to provide charge-free topographic contrast imaging of non-conductive samples and offers an immersion lens, providing an additional magnetic objective lens. This lens, when combined with the electrostatic column, creates a compound final lens, which boosts the low-kV resolution performance to 1.0 nm at 1 kV without requiring beam deceleration. The Directional Back-Scatter (DBS) detector enables separate detection of electrons emitted at different angles. This instrument comes with dispersive X-ray Spectroscopy (EDS) and Scanning Transmission Electron Microscopy (S-TEM) capabilities as well.

Imaging Surface Plasmon Polaritons in Nanostructures with Transient Absorption Microscopy

Surface Plasmon Polaritons (SPPs) are electromagnetic waves that propagate at metal-dielectric interfaces. In this paper transient absorption microscopy is used to image SPPs in gold nanowires, and study how they couple between nanostructures.