Gregory Hartland | University of Notre Dame (original) (raw)

Papers by Gregory Hartland

Physical Chemistry Chemical Physics, 2006

The scattering spectra of single gold nanorods with aspect ratios between 2 and 4 have been exami... more The scattering spectra of single gold nanorods with aspect ratios between 2 and 4 have been examined by dark field microscopy. The results show that the longitudinal plasmon resonance (electron oscillation along the long axis of the rod) broadens as the width of the rods decreases from 14 to 8 nm. This is attributed to electron surface scattering. Analysis of the data using gamma = gamma(bulk) + Anu(F)/L(eff), where L(eff) is the effective path length of the electrons and nu(F) is the Fermi velocity, allows us to determine a value for the surface scattering parameter of A = 0.3. Larger rods with widths of 19 and 30 nm were also examined. These samples also show spectral broadening, which is attributed to radiation damping. The relative strengths of the surface scattering and radiation damping effects are in excellent agreement with recent work on spherical gold nanoparticles by Sönnichsen et al., Phys. Rev. Lett., 2002, 88, 077402; and by Berciaud et al., Nano Lett., 2005, 5, 515.

The Journal of Physical Chemistry B, 2000

The preparation of gold-silver nanoparticles with a core-shell structure by radiation chemistry i... more The preparation of gold-silver nanoparticles with a core-shell structure by radiation chemistry is described. The optical properties of particles containing Au cores and Ag shells are compared to those of the reverse system for a variety of overall particle compositions. Nanosecond and picosecond laser-induced heating (at 532 nm) is used to melt the Au core Ag shell particles into homogeneous alloyed nanoparticles. The transition from the kinetically stable core-shell structure to the alloy is demonstrated by TEM and by the spectral changes accompanying melting. It is found that the particles must accumulate many laser pulses to completely mix into the alloy. In the case of nanosecond excitation, alloying and reshaping from faceted and irregular particles into smooth spheres occurs at absorbed energies of 5-6 mJ/pulse, and fragmentation takes place at higher energies, >10 mJ/pulse. In the case of 30 ps laser excitation, the thresholds for alloying/reshaping and fragmentation are lower: 1 and 4 mJ/pulse, respectively. The higher energy threshold for nanosecond excitation compared to the picosecond case is due to dissipation of the absorbed energy to the solvent during excitation, which is estimated to occur on a 100-200 ps time scale. Thus, the temperatures reached in the particles by nanosecond excitation are lower than those achieved by picosecond excitation for equal pulse energies.

Laser Techniques for Condensed-Phase and Biological Systems, 1998

ABSTRACT The electron transfer dynamics of 9-anthracene carboxylic acid bound to TiO2 nanoparticl... more ABSTRACT The electron transfer dynamics of 9-anthracene carboxylic acid bound to TiO2 nanoparticles in ethanol has been examined by a combination of transient absorption and time- resolved anistropy measurements. The results from these experiments show that the forward electron transfer reaction is very fast,

Physical Chemistry of Interfaces and Nanomaterials, 2002

ABSTRACT Ultrafast laser excitation of metal nanoparticles can create temperature increases of ma... more ABSTRACT Ultrafast laser excitation of metal nanoparticles can create temperature increases of many hundreds of Kelvin. The aim of this paper is to provide an overview of our recent experimental studies of heat dissipation and the coherent generation of acoustic phonon modes in the particles. Our results show that the rate of heat dissipation depends on the surface area of the particles, and that both impulsive lattice heating and hot-electron pressure contribute to phonon excitation. The measured periods also depend on the pump laser intensity: higher intensities yield slower periods. This softening of the coherently excited phonon modes is due to the temperature dependence of the elastic constants of the particles.

Nano letters, Jan 10, 2011

Correlated transient absorption and atomic force microscopy (AFM) measurements have been performe... more Correlated transient absorption and atomic force microscopy (AFM) measurements have been performed for monolayer graphene, both free-standing and supported on a glass substrate. The AFM images allow us to locate regions of the suspended graphene. The transient absorption traces show a fast instrument response limited decay, followed by a slower intensity dependent decay. The fast decay is assigned to a combination of coupling between the excited charge carriers and the optical phonon modes of graphene and the substrate, and diffusion of the charge carrier out of the probe region. The slow decay is due to the hot phonon effect and reflects the lifetime of the intrinsic optical phonons of graphene. The time constant for the slow decay is longer for suspended graphene compared to substrate-supported graphene. This is attributed to interactions between the excited charge carriers and the surface optical phonon modes of the substrate, which supplies an additional relaxation channel for supported graphene.

Transient absorption microscopy was employed to image charge carrier dynamics in epitaxial multil... more Transient absorption microscopy was employed to image charge carrier dynamics in epitaxial multilayer graphene. The carrier cooling exhibited a biexponential decay that showed a significant dependence on carrier density. The fast and slow relaxation times were assigned to coupling between electrons and optical phonon modes and the hot phonon effect, respectively. The limiting value of the slow relaxation time at high pump intensity reflects the lifetime of the optical phonons. Significant spatial heterogeneity in the dynamics was observed due to differences in coupling between graphene layers and the substrate.

Ultrafast Phenomena in Semiconductors and Nanostructure Materials XIV, 2010

ABSTRACT Single particle transient absorption experiments have been used to study metallic and se... more ABSTRACT Single particle transient absorption experiments have been used to study metallic and semiconducting nanowires. For the metal wires the major result is the observation of modulations in the transient absorption traces due to coherently excited breathing modes. The vibrational periods depend on the dimensions of the nanowire, and the decay times are sensitive to the environment. The nanowires in our experiments are spin coated from a polymer solution onto a glass substrate, and experience a range of different environments. This causes large variations in the quality factor of the breathing mode for different wires. Semiconducting nanowires of CdTe and CdSe were also examined. The CdTe wires show fast picosecond time scale dynamics, which are assigned to charge carrier trapping at surface states of the wires. In contrast, CdSe nanowires show no dynamics on the time scale of our measurements. For the CdTe nanowires the charge carrier trapping times vary from wire-to-wire, and also vary with position in a single wire. This is attributed to differences in surface chemistry. Overall these experiments illustrate the important of single particle techniques for studying nanomaterials, especially for elucidating how differences in local environment and structure affect dynamics.

Surface Science, 2011

Hot carrier cooling in few-layer and multilayer epitaxial graphene on SiC, and chemical vapor dep... more Hot carrier cooling in few-layer and multilayer epitaxial graphene on SiC, and chemical vapor deposition (CVD) grown graphene transferred onto a glass substrate was investigated by transient absorption spectroscopy and imaging. Coupling to the substrate was found to play a critical role in charge carrier cooling. For both multilayer epitaxial graphene and monolayer CVD graphene, charge carriers transfer heat predominantly to intrinsic in-plane optical phonons of graphene. At high pump intensity, a significant number of optical phonons are accumulated, and the optical phonon lifetime presents a bottleneck for charge carrier cooling. This hot phonon effect did not occur in few-layer epitaxial graphene because of strong coupling to the substrate, which provided additional cooling channels. The limiting charge carrier lifetimes at high excitation densities were 1.8 ± 0.1 ps and 1.4 ± 0.1 ps for multilayer epitaxial graphene and monolayer CVD graphene, respectively. These values represent lower limits on the optical phonon lifetime for the graphene samples.

Nanoscale, 2014

The surface plasmon polariton (SPP) modes of gold nanobars (nanowires with rectangular dimensions... more The surface plasmon polariton (SPP) modes of gold nanobars (nanowires with rectangular dimensions) have been investigated by scanning pump-probe microscopy. In these experiments the nanobars were suspended over trenches cut in glass coverslips, and propagating SPP modes were launched in the supported portion of the nanobar by focusing a near-IR pump laser beam at the end of the nanobar. Transient absorption images were then collected by scanning the probe laser over the nanobar using a galvo-mirror system. The images show that the trench has a large effect on the SPP modes, specifically, for approximately half the nanowires the propagation length is significantly reduced after the trench. Finite element calculations were performed to understand this effect. The calculations show that the pump laser excites bound and leaky modes (modes that have their fields localized at the nanobar/glass or nanobar/air interfaces, respectively) in the supported portions of the nanobars. These modes propagate along the nanobar. When they meet the trench their field distributions are altered. The modes that derive from the bound mode are strongly damped over the trench. Thus, the bound mode is not reconstituted on the opposite side of the trench, and only the leaky mode contributes to the signal. Because the bound and leaky modes can have different propagation lengths, the propagation lengths measured in our experiments can change from one side of the trench to the other. The results show how the substrate can be engineered to control the SPP modes in metal nanostructures.

The Journal of Physical Chemistry C, 2009

CdTe and CdSe nanowires grown by chemical vapor deposition. The traces for the CdTe nanowires sho... more CdTe and CdSe nanowires grown by chemical vapor deposition. The traces for the CdTe nanowires show fast picosecond timescale dynamics, which is attributed to charge carrier trapping to surface states. The time constants vary for different nanowires, presumably due to differences in surface chemistry, highlighting the importance of single particle measurements for these systems. This fast decay component is absent in experiments on CdSe nanowires performed under identical conditions. This is consistent with the much lower emission quantum yield of CdTe nanowires compared to CdSe nanowires.

The Journal of Physical Chemistry C, 2011

The Journal of Physical Chemistry C, 2013

ABSTRACT Single crystal, micrometer-sized nanoplates were formed by reducing Au(III) in the prese... more ABSTRACT Single crystal, micrometer-sized nanoplates were formed by reducing Au(III) in the presence of surfactants using a modified polyol protocol. The shapes of the plates range from triangular to hexagonal. The nanoplates have {111} surfaces with an average edge length of 5 ± 2 μm and an average width of 107 ± 30 nm. Scanning electron microscopy (SEM) images reveal that the plates grow through a re-entrant groove created by twinning. The optical properties of the plates were studied by scattered light and transient absorption experiments. The scattered light measurements show that propagating surface plasmon polariton (SPP) modes of the Au nanoplates can be excited when a laser beam is focused at the edge of the nanoplate. We also demonstrate that the direction of propagation of the SPP modes can be controlled through the polarization of the laser beam. The transient absorption traces for single suspended nanoplates show oscillations, which are assigned to thickness vibrations of the plates. The quality factors for the oscillations are smaller than those recently measured for suspended gold nanowires, indicating possible contributions to the vibrational damping from surface bound molecules or from the crystal structure.

The Journal of Physical Chemistry B, 1998

Photoinduced electron transfer to TiO 2 nanoparticles has been examined for the 1-, 2-, and 9-iso... more Photoinduced electron transfer to TiO 2 nanoparticles has been examined for the 1-, 2-, and 9-isomers of anthracenecarboxylic acid. TiO 2 samples with either anatase or amorphous crystal structures were used for these experiments. The results from time-resolved transient absorption measurements show that the rates of the forward (dye-to-semiconductor) and reverse (semiconductor-to-dye) electron transfer reactions depend on the chemical structure of the dye and the method used to synthesize the particles. These effects arise from differences in both the energetics and the coupling elements for the reactions. Specifically, the reverse electron transfer reactions for the 1-and 2-isomers are significantly faster than that for the 9-isomer due to differences in the oxidation potentials of the dye molecules. In addition, both the forward and reverse electrontransfer times are faster for the anatase TiO 2 particles compared to the amorphous particles. For example, the forward electron transfer time for the anatase particles is e200 fs, whereas it is ca. 1.5 ps for the amorphous particles. This is due to a difference in the coupling elements for the forward electron transfer reaction. Finally, all the anthracenecarboxylate dyes examined show red shifts in their UV-vis absorption spectra when they are attached to the semiconductor particles. Experiments with ZrO 2 show that these shifts are not due to a charge-transfer band. The spectra are more strongly perturbed when the dye molecules are attached to the anatase particles, which shows that for this series of compounds there is a correlation between the spectral shifts and the time scale for electron transfer.

The Journal of Physical Chemistry B, 2005

A signal due to coherently excited vibrational motion has been observed in polydisperse silver na... more A signal due to coherently excited vibrational motion has been observed in polydisperse silver nanoparticle samples. The particles were synthesized via a wet chemistry seed mediated method, which yields different particle shapes, including spheres, rods, and irregular triangular-shaped particles. The measured vibrational periods were compared to the results from continuum mechanics calculations. This analysis shows that the observed signal arises from the triangular-shaped particles, rather than the rods or spheres. The period of vibration increases as the dimensions of the triangular-shaped particles increase; specifically, we find that the period is given by 2h/c l , where h is the bisector of the triangle and c l is the longitudinal speed of sound in silver.

The Journal of Physical Chemistry B, 2013

Coupling between excitons of CdSe nanocrystal quantum dots (NQDs) and surface plasmon polaritons ... more 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.

The Analyst, 2013

In recent years there has been considerable effort in developing ultra-sensitive imaging techniqu... more In recent years there has been considerable effort in developing ultra-sensitive imaging techniques based on absorption. This mini-review describes recent results from our laboratory on detecting single nano-objects using transient absorption microscopy. This technique is extremely flexible, allowing the detection of single semiconductor and metal nanostructures with high sensitivity. The goal of this review is to illustrate key points in implementing transient absorption microscopy for ultra-sensitive detection, as well as to discuss the advantages and disadvantages of this technique compared to other optical absorption based methods.

Physical Review Letters, 1999

Physical Chemistry Chemical Physics, 2009

Transient absorption experiments with diffraction-limited spatial resolution have been used to st... more Transient absorption experiments with diffraction-limited spatial resolution have been used to study the optical absorption properties and dynamics of isolated, single silver nanowires. The images and polarization analysis show that the near-IR pump and near-UV probe beams couple to fundamentally different electron motions. The near-IR pump laser excites the propagating surface plasmon polariton (SPP) modes of the wires when focused at the ends, and multipolar plasmon modes (antenna modes) for medial excitation. The images show that these two modes have comparable optical absorption cross-sections. In contrast, the near-UV probe couples to the transverse plasmon resonance of the wire independent of the spatial position. For either end-on or medial excitation, pump laser absorption causes lattice heating and coherently excites the breathing vibrational mode of the nanowires. The vibrational quality factors depend on the acoustic impedance mismatch between the nanowire and the environment, and are similar to those recently measured for silver nanocubes. Experiments performed with spatially separated pump and probe beams, with the pump beam focused at one end to excite the propagating SPP modes, show that the amplitudes of the initial transient absorption signal and the breathing motion decrease with distance along the wire. This arises because the propagating SPP mode decays as it moves down the wire, which reduces the number of electronic excitations and, therefore, the signal level in the experiments. The measured length scale for the SPP decay is similar to that obtained in previous light scattering experiments.

Optics Letters, 2013

Knowledge of how energy and charge carriers move in nanoscale systems is essential for engineerin... more 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.