8 research outputs found
Synthesis and characterization of biodegradable lignin nanoparticles with tunable surface properties
Lignin nanoparticles can serve as biodegradable carriers of biocidal actives with minimal environmental footprint. Here we describe the colloidal synthesis and interfacial design of nanoparticles with tunable surface properties using two different lignin precursors, Kraft (Indulin AT) lignin and Organosolv (high-purity lignin). The green synthesis process is based on flash precipitation of dissolved lignin polymer, which enabled the formation of nanoparticles in the size range of 45–250 nm. The size evolution of the two types of lignin particles is fitted on the basis of modified diffusive growth kinetics and mass balance dependencies. The surface properties of the nanoparticles are fine-tuned by coating them with a cationic polyelectrolyte, poly(diallyldimethylammonium chloride). We analyze how the colloidal stability and dispersion properties of these two types of nanoparticles vary as a function of pH and salinities. The data show that the properties of the nanoparticles are governed by the type of lignin used and the presence of polyelectrolyte surface coating. The coating allows the control of the nanoparticles’ surface charge and the extension of their stability into strongly basic regimes, facilitating their potential application at extreme pH conditions
Discrete Chromatic Aberrations Arising from Photoinduced Electron-Photon Interactions in Ultrafast Electron Microscopy
In femtosecond ultrafast
electron microscopy (UEM) experiments,
the initial excitation period is composed of spatiotemporal overlap
of the temporally commensurate pump photon pulse and probe photoelectron
packet. Generation of evanescent near-fields at the nanostructure
specimens produces a dispersion relation that enables coupling of
the photons (ℏω = 2.4 eV, for example) and freely propagating
electrons (200 keV, for example) in the near-field. Typically, this
manifests as discrete peaks occurring at integer multiples (<i>n</i>) of the photon energy in the low-loss/gain region of electron-energy
spectra (i.e., at 200 keV ± <i>n</i>ℏω
eV). Here, we examine the UEM imaging resolution implications of the
strong inelastic near-field interactions between the photons employed
in optical excitation and the probe photoelectrons. We find that the
additional photoinduced energy dispersion occurring when swift electrons
pass through intense evanescent near-fields results in a discrete
chromatic aberration that limits the spatial resolving power to several
angstroms during the excitation period
Defect-mediated phonon dynamics in TaS2 and WSe2
We report correlative crystallographic and morphological studies of defect-dependent
phonon
dynamics in
single flakes of 1T-TaS2 and 2H-WSe2 using selected-area diffraction and
bright-field imaging in an ultrafast electron microscope. In both materials, we observe
in-plane speed-of-sound acoustic-phonon wave trains, the dynamics of which (i.e.,
emergence, propagation, and interference) are strongly dependent upon discrete interfacial
features (e.g., vacuum/crystal and crystal/crystal interfaces). In TaS2, we observe
cross-propagating in-plane acoustic-phonon wave trains of differing frequencies that undergo
coherent interference approximately 200 ps after initial emergence from distinct
interfacial regions. With ultrafast bright-field imaging, the properties of the interfering
wave
trains are observed to correspond to the beat frequency of the individual oscillations,
while intensity oscillations of Bragg spots generated from selected areas within the
region of interest match well with the real-space dynamics. In
WSe2, distinct acoustic-phonon dynamics are observed emanating and propagating away
from structurally dissimilar morphological discontinuities (vacuum/crystal interface and crystal terrace),
and results of ultrafast selected-area diffraction reveal thickness-dependent phonon frequencies. The overall
observed dynamics are well-described using finite element analysis and
time-dependent linear-elastic continuum mechanics
Characterization of fast photoelectron packets in weak and strong laser fields in ultrafast electron microscopy
The development of ultrafast electron microscopy (UEM) and variants thereof (e.g., photon-induced near-field electron microscopy, PINEM) has made it possible to image atomic-scale dynamics on the femtosecond timescale. Accessing the femtosecond regime with UEM currently relies on the generation of photoelectrons with an ultrafast laser pulse and operation in a stroboscopic pump-probe fashion. With this approach, temporal resolution is limited mainly by the durations of the pump laser pulse and probe electron packet. The ability to accurately determine the duration of the electron packets, and thus the instrument response function, is critically important for interpretation of dynamics occurring near the temporal resolution limit, in addition to quantifying the effects of the imaging mode. Here, we describe a technique for in situ characterization of ultrashort electron packets that makes use of coupling with photons in the evanescent near-field of the specimen. We show that within the weakly-interacting (i.e., low laser fluence) regime, the zero-loss peak temporal cross-section is precisely the convolution of electron packet and photon pulse profiles. Beyond this regime, we outline the effects of non-linear processes and show that temporal cross-sections of high-order peaks explicitly reveal the electron packet profile, while use of the zero-loss peak becomes increasingly unreliable
Plasmonic Interactions through Chemical Bonds of Surface Ligands on PbSe Nanocrystals
When
functional films are cast from colloidal dispersions of semiconductor
nanocrystals, the length and structure of the ligands capping their
surfaces determine the electronic coupling between the nanocrystals.
Long chain oleic acid ligands on the surface of IV–VI semiconductor
nanocrystals such as PbSe are typically considered to be insulating.
Consequently, these ligands are either removed or replaced with short
ones to bring the nanocrystals closer to each other for increased
electronic coupling. Herein, using high-angle annular dark-field scanning
transmission electron microscopy imaging combined with electron energy
loss spectroscopy, we show that partial oxidation of PbSe nanocrystals
forms conjugated double bonds within the oleic ligands, which then
facilitates enhanced plasmonic interaction among the nanocrystals.
The changes in the geometric configurations of the ligands are imaged
directly and correlated with the changes in the surface plasmon intensities
as they oxidize and undergo structural modifications