570 research outputs found
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Vibrational exciton nanoimaging of phases and domains in porphyrin nanocrystals.
Much of the electronic transport, photophysical, or biological functions of molecular materials emerge from intermolecular interactions and associated nanoscale structure and morphology. However, competing phases, defects, and disorder give rise to confinement and many-body localization of the associated wavefunction, disturbing the performance of the material. Here, we employ vibrational excitons as a sensitive local probe of intermolecular coupling in hyperspectral infrared scattering scanning near-field optical microscopy (IR s-SNOM) with complementary small-angle X-ray scattering to map multiscale structure from molecular coupling to long-range order. In the model organic electronic material octaethyl porphyrin ruthenium(II) carbonyl (RuOEP), we observe the evolution of competing ordered and disordered phases, in nucleation, growth, and ripening of porphyrin nanocrystals. From measurement of vibrational exciton delocalization, we identify coexistence of ordered and disordered phases in RuOEP that extend down to the molecular scale. Even when reaching a high degree of macroscopic crystallinity, identify significant local disorder with correlation lengths of only a few nanometers. This minimally invasive approach of vibrational exciton nanospectroscopy and -imaging is generally applicable to provide the molecular-level insight into photoresponse and energy transport in organic photovoltaics, electronics, or proteins
Ultrafast Dynamics of Vibrational Symmetry Breaking in a Charge-ordered Nickelate
The ability to probe symmetry breaking transitions on their natural time
scales is one of the key challenges in nonequilibrium physics. Stripe ordering
represents an intriguing type of broken symmetry, where complex interactions
result in atomic-scale lines of charge and spin density. Although phonon
anomalies and periodic distortions attest the importance of electron-phonon
coupling in the formation of stripe phases, a direct time-domain view of
vibrational symmetry breaking is lacking. We report experiments that track the
transient multi-THz response of the model stripe compound
LaSrNiO, yielding novel insight into its electronic and
structural dynamics following an ultrafast optical quench. We find that
although electronic carriers are immediately delocalized, the crystal symmetry
remains initially frozen - as witnessed by time-delayed suppression of
zone-folded Ni-O bending modes acting as a fingerprint of lattice symmetry.
Longitudinal and transverse vibrations react with different speeds, indicating
a strong directionality and an important role of polar interactions. The hidden
complexity of electronic and structural coupling during stripe melting and
formation, captured here within a single terahertz spectrum, opens new paths to
understanding symmetry breaking dynamics in solids.Comment: 21 pages, 4 figures; updated version with journal re
Liquid Heterostructures: Generation of Liquid-Liquid Interfaces in Free-Flowing Liquid Sheets
Chemical reactions and biological processes are often governed by the
structure and transport dynamics of the interface between two liquid phases.
Despite their importance, our microscopic understanding of liquid-liquid
interfaces has been severely hindered by difficulty in accessing the interface
through the bulk liquid. Here we demonstrate a method for generating large-area
liquid-liquid interfaces within free-flowing liquid sheets, which we call
liquid heterostructures. These sheets can be made thin enough to transmit
photons from across the spectrum, which also minimizes the amount of bulk
liquid relative to the interface and makes them ideal targets for a wide range
of spectroscopies and scattering experiments. The sheets are produced with a
microfluidic nozzle that impinges two converging jets of one liquid onto two
sides of a third jet of another liquid. The hydrodynamic forces provided by the
colliding jets both produce a multilayered laminar liquid sheet with the
central jet is flattened in the middle. Infrared microscopy, white light
reflectivity, and imaging ellipsometry measurements demonstrate that the buried
layer has a tunable thickness and displays well-defined liquid-liquid
interfaces, and that the inner layer can be thinner than 100 nm.Comment: 30 pages, 8 figures, 1 table. Supplement: 19 pages, 8 figure
Highly Quantum-Confined InAs Nanoscale Membranes
Nanoscale size-effects drastically alter the fundamental properties of
semiconductors. Here, we investigate the dominant role of quantum confinement
in the field-effect device properties of free-standing InAs nanomembranes with
varied thicknesses of 5-50 nm. First, optical absorption studies are performed
by transferring InAs "quantum membranes" (QMs) onto transparent substrates,
from which the quantized sub-bands are directly visualized. These sub-bands
determine the contact resistance of the system with the experimental values
consistent with the expected number of quantum transport modes available for a
given thickness. Finally, the effective electron mobility of InAs QMs is shown
to exhibit anomalous field- and thickness-dependences that are in distinct
contrast to the conventional MOSFET models, arising from the strong quantum
confinement of carriers. The results provide an important advance towards
establishing the fundamental device physics of 2-D semiconductors
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Reconciling FTIR Spectroscopy with Top-off Operations at the Advanced Light Source
Top-off operations is a quasi-continuous injection mode that increases the flux and brightness of a synchrotron source and improves thermal stability of optical components by maintaining a constant current in the storage ring. Although the increased and constant flux is advantageous for FTIR measurements, the frequent injections (about one every 30 seconds in the ALS case) introduce artifacts into the spectrum by creating spikes in the interferogram data. These spikes are caused by brief beam motion during the injection event. Here, we describe our efforts to minimize the effects of top-off generated interferogram spikes on several FTIR spectrometers. They include using a fast feedback mirror system to correct for beam motion and a gating signal to inhibit interferogram collection during a top-off injection
Tunable intraband optical conductivity and polarization-dependent epsilon-near-zero behavior in black phosphorus
Black phosphorus (BP) offers considerable promise for infrared and visible
photonics. Efficient tuning of the bandgap and higher subbands in BP by
modulation of the Fermi level or application of vertical electric fields has
been previously demonstrated, allowing electrical control of its above bandgap
optical properties. Here, we report modulation of the optical conductivity
below the band-gap (5-15 um) by tuning the charge density in a two-dimensional
electron gas (2DEG) induced in BP, thereby modifying its free carrier dominated
intraband response. With a moderate doping density of 7x10^12/cm2 we were able
to observe a polarization dependent epsilon-near-zero behavior in the
dielectric permittivity of BP. The intraband polarization sensitivity is
intimately linked to the difference in effective fermionic masses along the two
crystallographic directions, as confirmed by our measurements. Our results
suggest the potential of multilayer BP to allow new optical functions for
emerging photonics applications.Comment: 17 pages, 4 figure
Nanoscale infrared spectroscopy as a non-destructive probe of extraterrestrial samples
Advances in the spatial resolution of modern analytical techniques have tremendously augmented the scientific insight gained from the analysis of natural samples. Yet, while techniques for the elemental and structural characterization of samples have achieved sub-nanometre spatial resolution, infrared spectral mapping of geochemical samples at vibrational 'fingerprint' wavelengths has remained restricted to spatial scales >10 mu m. Nevertheless, infrared spectroscopy remains an invaluable contactless probe of chemical structure, details of which offer clues to the formation history of minerals. Here we report on the successful implementation of infrared near-field imaging, spectroscopy and analysis techniques capable of sub-micron scale mineral identification within natural samples, including a chondrule from the Murchison meteorite and a cometary dust grain (Iris) from NASA's Stardust mission. Complementary to scanning electron microscopy, energy-dispersive X-ray spectroscopy and transmission electron microscopy probes, this work evidences a similarity between chondritic and cometary materials, and inaugurates a new era of infrared nano-spectroscopy applied to small and invaluable extraterrestrial samples
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