10 research outputs found
Enhancing the Domain Wall Conductivity in Lithium Niobate Single Crystals
Domain
walls (DWs) in ferroelectric/ferroic materials have been
a central research focus for the last 50 years; DWs bear a multitude
of extraordinary physical parameters within a unit-cell-sized lateral
confinement. Especially, one outstanding feature has recently attracted
a lot of attention for room-temperature applications, which is the
potential to use DWs as two-dimensional (2D) conducting channels that
completely penetrate bulk compounds. Domain wall currents in lithium
niobate (LNO) so far lie in the lower pA regime. In this work, we
report on an easy-to-use and reliable protocol that allows enhancing
domain wall conductivity (DWC) in single-crystalline LNO (sc-LNO)
by 3 to 4 orders of magnitude. sc-LNO thus has become one of the most
prospective candidates to engineer DWC applications, notably for domain
wall transport both with and without photoexcitation. DWs were investigated
here for several days to weeks, both before and after DWC enhancement.
2D local-scale inspections were carried out using adequate local-probe
techniques, <i>i</i>.<i>e</i>., piezoresponse
force microscopy and conductive atomic force microscopy, while Cerenkov
second-harmonic generation was applied for mapping the DW constitution
in three-dimensional space across the full LNO single crystal. The
comparison between these nano- and microscale inspections allows us
to unambiguously correlate the DW inclination angle α close
to the sample surface to the measured domain wall current distribution.
Moreover, ohmic or diode-like electronic transport characteristics
along such DWs can be readily interpreted when analyzing the DW inclination
profile
Enhancing the Domain Wall Conductivity in Lithium Niobate Single Crystals
Domain
walls (DWs) in ferroelectric/ferroic materials have been
a central research focus for the last 50 years; DWs bear a multitude
of extraordinary physical parameters within a unit-cell-sized lateral
confinement. Especially, one outstanding feature has recently attracted
a lot of attention for room-temperature applications, which is the
potential to use DWs as two-dimensional (2D) conducting channels that
completely penetrate bulk compounds. Domain wall currents in lithium
niobate (LNO) so far lie in the lower pA regime. In this work, we
report on an easy-to-use and reliable protocol that allows enhancing
domain wall conductivity (DWC) in single-crystalline LNO (sc-LNO)
by 3 to 4 orders of magnitude. sc-LNO thus has become one of the most
prospective candidates to engineer DWC applications, notably for domain
wall transport both with and without photoexcitation. DWs were investigated
here for several days to weeks, both before and after DWC enhancement.
2D local-scale inspections were carried out using adequate local-probe
techniques, <i>i</i>.<i>e</i>., piezoresponse
force microscopy and conductive atomic force microscopy, while Cerenkov
second-harmonic generation was applied for mapping the DW constitution
in three-dimensional space across the full LNO single crystal. The
comparison between these nano- and microscale inspections allows us
to unambiguously correlate the DW inclination angle α close
to the sample surface to the measured domain wall current distribution.
Moreover, ohmic or diode-like electronic transport characteristics
along such DWs can be readily interpreted when analyzing the DW inclination
profile
Heuristic Description of Magnetoelectricity of Cu<sub>2</sub>OSeO<sub>3</sub>
CuO<sub>2</sub>SeO<sub>3</sub> is
an insulating material that hosts topologically nontrivial spin whirls,
so-called skyrmions, and exhibits magnetoelectric coupling allowing
to manipulate these skyrmions by means of electric fields. We report
magnetic force microscopy imaging of the real-space spin structure
on the surface of a bulk single crystal of CuO<sub>2</sub>SeO<sub>3</sub>. Based on measurements of the electric polarization using
Kelvin-probe force microscopy, we develop a heuristic description
of the magnetoelectric properties in CuO<sub>2</sub>SeO<sub>3</sub>. The model successfully describes the dependency of the electric
polarization on the magnetization in all magnetically modulated phases
Immobilized Multifunctional Polymersomes on Solid Surfaces: Infrared Light-Induced Selective Photochemical Reactions, pH Responsive Behavior, and Probing Mechanical Properties under Liquid Phase
Fixing polymersomes onto surfaces
is in high demand not only for the characterization with advanced
microscopy techniques but also for designing specific compartments
in microsystem devices in the scope of nanobiotechnology. For this
purpose, this study reports the immobilization of multifunctional,
responsive, and photo-cross-linked polymersomes on solid substrates
by utilizing strong adamantane−β-cyclodextrin host–guest
interactions. To reduce nonspecific binding and retain better spherical
shape, the level of attractive forces acting on the immobilized polymersomes
was tuned through poly(ethylene glycol) passivation as well as decreased β-cyclodextrin
content on the corresponding substrates. One significant feature of
this system is the pH responsivity of the polymersomes which has been
demonstrated by swelling of the immobilized vesicles at acidic condition
through in situ AFM measurements. Also, light responsivity has been
provided by introducing nitroveratryloxycarbonyl (NVOC) protected
amine molecules as photocleavable groups to the polymersome surface
before immobilization. The subsequent low-energy femtosecond pulsed
laser irradiation resulted in the cleavage of NVOC groups on immobilized
polymersomes which in turn led to free amino groups as an additional
functionality. The freed amines were further conjugated with a fluorescent
dye having an activated ester that illustrates the concept of bio/chemo
recognition for a potential binding of biological compounds. In addition
to the responsive nature, the mechanical stability of the analyzed
polymersomes was supported by computing Young’s modulus and
bending modulus of the membrane through force curves obtained by atomic
force microscopy measurements. Overall, polymersomes with a robust
and pH-swellable membrane combined with effective light responsive
behavior are promising tools to design smart and stable compartments
on surfaces for the development of microsystem devices such as chemo/biosensors
Plasmonic Biosensor Based on Vertical Arrays of Gold Nanoantennas
Implementing
large arrays of gold nanowires as functional elements
of a plasmonic biosensor is an important task for future medical diagnostic
applications. Here we present a microfluidic-channel-integrated sensor
for the label-free detection of biomolecules, relying on localized
surface plasmon resonances. Large arrays (∼1 cm<sup>2</sup>) of vertically aligned and densely packed gold nanorods to receive,
locally confine, and amplify the external optical signal are used
to allow for reliable biosensing. We accomplish this by monitoring
the change of the optical nanostructure resonance in the presence
of biomolecules within the tight focus area above the nanoantennas,
combined with a surface treatment of the nanowires for a specific
binding of the target molecules. As a first application, we detect
the binding kinetics of two distinct DNA strands as well as the following
hybridization of two complementary strands (cDNA) with different lengths
(25 and 100 bp). Upon immobilization, a redshift of 1 nm was detected;
further backfilling and hybridization led to a peak shift of additional
2 and 5 nm for 25 and 100 bp, respectively. We believe that this work
gives deeper insight into the functional understanding and technical
implementation of a large array of gold nanowires for future medical
applications
Second Harmonic Generation from Metal Nano-Particle Resonators: Numerical Analysis On the Basis of the Hydrodynamic Drude Model
A detailed computational study of
the wavelength-dependent efficiency
of optical second-harmonic generation in plasmonic nanostructures
is presented. The computations are based on a discontinuous Galerkin
Maxwell solver that utilizes a hydrodynamic material model to calculate
the free-electron dynamics in metals without any further approximations.
Besides wave-mixing effects, the material model thus contains the
full nonlocal characteristics of the electromagnetic response, as
well as intensity-dependent phenomena such as the Kerr effect. To
be specific, two prototypical nanostructures are studied in depth
with the help of two independent computer codes. For an infinitely
long metal cylinder, it is found that the spectral position of linear
particle plasmon modes (dipolar modes, higher-order modes, and, for
frequencies above the plasma frequency also bulk plasmon modes) and
their associated relative strengths for scattering and absorption
both at the fundamental and second-harmonic wavelengths largely control
the conversion efficiency. Notably, Fabry–Perot resonances
associated with longitudinal bulk plasmons may be detectable via background-free
second-harmonic spectroscopy. For a more complex V-groove nanostructure,
it becomes possible to engineer a doubly resonant scenario at the
fundamental and the second-harmonic wavelength. This leads to an efficient
enhancement of second-harmonic emission. Our work thus demonstrates
that the careful design of nanostructures on the nonlocal linear level
facilitates highly efficient nanoantennas for second-harmonic emission
with applications in background-free imaging and frequency conversion
systems
Germanium Monosulfide as a Natural Platform for Highly Anisotropic THz Polaritons
Terahertz (THz) electromagnetic
radiation is key to access collective
excitations such as magnons (spins), plasmons (electrons), or phonons
(atomic vibrations), thus bridging topics between optics and solid-state
physics. Confinement of THz light to the nanometer length scale is
desirable for local probing of such excitations in low-dimensional
systems, thereby circumventing the large footprint and inherently
low spectral power density of far-field THz radiation. For that purpose,
phonon polaritons (PhPs) in anisotropic van der Waals (vdW) materials
have recently emerged as a promising platform for THz nanooptics.
Hence, there is a demand for the exploration of materials that feature
not only THz PhPs at different spectral regimes but also host anisotropic
(directional) electrical, thermoelectric, and vibronic properties.
To that end, we introduce here the semiconducting vdW-material alpha-germanium(II)
sulfide (GeS) as an intriguing candidate. By employing THz nanospectroscopy
supported by theoretical analysis, we provide a thorough characterization
of the different in-plane hyperbolic and elliptical PhP modes in GeS.
We find not only PhPs with long lifetimes (τ > 2 ps) and
excellent
THz light confinement (λ0/λ 45) but also an
intrinsic, phonon-induced anomalous dispersion as well as signatures
of naturally occurring, substrate-mediated PhP canalization within
a single GeS slab
Flexible Heteroepitaxy of CoFe<sub>2</sub>O<sub>4</sub>/Muscovite Bimorph with Large Magnetostriction
A bimorph
composed of ferrimagnetic cobalt ferrite (CoFe<sub>2</sub>O<sub>4</sub>, CFO) and flexible muscovite was fabricated via van der Waals epitaxy.
The combination of X-ray diffraction and transmission electron microscopy
was conducted to reveal the heteroepitaxy of the CFO/muscovite system.
The robust magnetic behaviors against mechanical bending were characterized
by hysteresis measurements and magnetic force microscopy, which maintain
a saturation magnetization (<i>M</i><sub>s</sub>) of ∼120–150
emu/cm<sup>3</sup> under different bending states. The large magnetostrictive
response of the CFO film was then determined by digital holographic
microscopy, where the difference of magnetostrction coefficient (Δλ)
is −104 ppm. The superior performance of this bimorph is attributed
to the nature of weak interaction between film and substrate. Such
a flexible CFO/muscovite bimorph provides a new platform to develop
next-generation flexible magnetic devices
Terahertz Twistoptics–Engineering Canalized Phonon Polaritons
The terahertz (THz) frequency range is key to studying
collective
excitations in many crystals and organic molecules. However, due to
the large wavelength of THz radiation, the local probing of these
excitations in smaller crystalline structures or few-molecule arrangements
requires sophisticated methods to confine THz light down to the nanometer
length scale, as well as to manipulate such a confined radiation.
For this purpose, in recent years, taking advantage of hyperbolic
phonon polaritons (HPhPs) in highly anisotropic van der Waals (vdW)
materials has emerged as a promising approach, offering a multitude
of manipulation options, such as control over the wavefront shape
and propagation direction. Here, we demonstrate the THz application
of twist-angle-induced HPhP manipulation, designing the propagation
of confined THz radiation between 8.39 and 8.98 THz in the vdW material
α-molybdenum trioxide (α-MoO3), hence extending
twistoptics to this intriguing frequency range. Our images, recorded
by near-field optical microscopy, show the frequency- and twist-angle-dependent
changes between hyperbolic and elliptic polariton propagation, revealing
a polaritonic transition at THz frequencies. As a result, we are able
to allocate canalization (highly collimated propagation) of confined
THz radiation by carefully adjusting these two parameters, i.e. frequency
and twist angle. Specifically, we report polariton canalization in
α-MoO3 at 8.67 THz for a twist angle of 50°.
Our results demonstrate the precise control and manipulation of confined
collective excitations at THz frequencies, particularly offering possibilities
for nanophotonic applications
Heteroepitaxy of Fe<sub>3</sub>O<sub>4</sub>/Muscovite: A New Perspective for Flexible Spintronics
Spintronics
has captured a lot of attention since it was proposed. It has been
triggering numerous research groups to make their efforts on pursuing
spin-related electronic devices. Recently, flexible and wearable devices
are in a high demand due to their outstanding potential in practical
applications. In order to introduce spintronics into the realm of
flexible devices, we demonstrate that it is feasible to grow epitaxial
Fe<sub>3</sub>O<sub>4</sub> film, a promising candidate for realizing
spintronic devices based on tunneling magnetoresistance, on flexible
muscovite. In this study, the heteroepitaxy of Fe<sub>3</sub>O<sub>4</sub>/muscovite is characterized by X-ray diffraction, high-resolution
transmission electron microscopy, and Raman spectroscopy. The chemical
composition and magnetic feature are investigated by a combination
of X-ray photoelectron spectroscopy and X-ray magnetic circular dichroism.
The electrical and magnetic properties are examined to show the preservation
of the primitive properties of Fe<sub>3</sub>O<sub>4</sub>. Furthermore,
various bending tests are performed to show the tunability of functionalities
and to confirm that the heterostructures retain the physical properties
under repeated cycles. These results illustrate that the Fe<sub>3</sub>O<sub>4</sub>/muscovite heterostructure can be a potential candidate
for the applications in flexible spintronics