37 research outputs found
Surface Effects on Anisotropic Photoluminescence in One-Dimensional Organic Metal Halide Hybrids
One-dimensional (1D) organic metal halide hybrids exhibit strongly
anisotropic optical properties, highly efficient light emission, and large
Stokes shift, holding promises for novel photodetection and lighting
applications. However, the fundamental mechanisms governing their unique
optical properties and in particular the impacts of surface effects are not
understood. Here, we investigate 1D C4N2H14PbBr4 by polarization-dependent
time-averaged and time-resolved photoluminescence (TRPL) spectroscopy, as a
function of photoexcitation energy. Surprisingly, we find that the emission
under photoexcitation polarized parallel to the 1D metal halide chains can be
either stronger or weaker than that under perpendicular polarization, depending
on the excitation energy. We attribute the excitation-energy-dependent
anisotropic emission to fast surface recombination, supported by
first-principles calculations of optical absorption in this material. The fast
surface recombination is directly confirmed by TRPL measurements, when the
excitation is polarized parallel to the chains. Our comprehensive studies
provide a more complete picture for a deeper understanding of the optical
anisotropy in 1D organic metal halide hybrids
Controllable Strain-driven Topological Phase Transition and Dominant Surface State Transport in High-Quality HfTe5 Samples
Controlling materials to create and tune topological phases of matter could
potentially be used to explore new phases of topological quantum matter and to
create novel devices where the carriers are topologically protected. It has
been demonstrated that a trivial insulator can be converted into a topological
state by modulating the spin-orbit interaction or the crystal lattice. However,
there are limited methods to controllably and efficiently tune the crystal
lattice and at the same time perform electronic measurements at cryogenic
temperatures. Here, we use large controllable strain to demonstrate the
topological phase transition from a weak topological insulator phase to a
strong topological insulator phase in high-quality HfTe5 samples. After
applying high strain to HfTe5 and converting it into a strong topological
insulator, we found that the sample's resistivity increased by more than two
orders of magnitude (24,000%) and that the electronic transport is dominated by
the topological surface states at cryogenic temperatures. Our findings show
that HfTe5 is an ideal material for engineering topological properties, and it
could be generalized to study topological phase transitions in van der Waals
materials and heterostructures. These results can pave the way to create novel
devices with applications ranging from spintronics to fault-tolerant
topologically protected quantum computers
Exceptional electronic transport and quantum oscillations in thin bismuth crystals grown inside van der Waals materials
Confining materials to two-dimensional forms changes the behavior of
electrons and enables new devices. However, most materials are challenging to
produce as uniform thin crystals. Here, we present a new synthesis approach
where crystals are grown in a nanoscale mold defined by atomically-flat van der
Waals (vdW) materials. By heating and compressing bismuth in a vdW mold made of
hexagonal boron nitride (hBN), we grow ultraflat bismuth crystals less than 10
nanometers thick. Due to quantum confinement, the bismuth bulk states are
gapped, isolating intrinsic Rashba surface states for transport studies. The
vdW-molded bismuth shows exceptional electronic transport, enabling the
observation of Shubnikov-de Haas quantum oscillations originating from the
(111) surface state Landau levels, which have eluded previous studies. By
measuring the gate-dependent magnetoresistance, we observe multi-carrier
quantum oscillations and Landau level splitting, with features originating from
both the top and bottom surfaces. Our vdW-mold growth technique establishes a
platform for electronic studies and control of bismuth's Rashba surface states
and topological boundary modes. Beyond bismuth, the vdW-molding approach
provides a low-cost way to synthesize ultrathin crystals and directly integrate
them into a vdW heterostructure
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Sulfurization Engineering of One-Step Low-Temperature MoS2 and WS2 Thin Films for Memristor Device Applications
2D materials have been of considerable interest as new materials for device applications. Non-volatile resistive switching applications of MoS2 and WS2 have been previously demonstrated; however, these applications are dramatically limited by high temperatures and extended times needed for the large-area synthesis of 2D materials on crystalline substrates. The experimental results demonstrate a one-step sulfurization method to synthesize MoS2 and WS2 at 550 °C in 15 min on sapphire wafers. Furthermore, a large area transfer of the synthesized thin films to SiO2/Si substrates is achieved. Following this, MoS2 and WS2 memristors are fabricated that exhibit stable non-volatile switching and a satisfactory large on/off current ratio (103–105) with good uniformity. Tuning the sulfurization parameters (temperature and metal precursor thickness) is found to be a straightforward and effective strategy to improve the performance of the memristors. The demonstration of large-scale MoS2 and WS2 memristors with a one-step low-temperature sulfurization method with simple strategy to tuning can lead to potential applications such as flexible memory and neuromorphic computing.This research was
primarily supported by the National Science Foundation through
the Center for Dynamics and Control of Materials: an NSF MRSEC
under Cooperative Agreement No. DMR-1720595. The work was partly
done at the Texas Nanofabrication Facility supported by NSF grant
NNCI-2025227. This work was performed in part at the Center for
Integrated Nanotechnologies, an Office of Science User Facility operated
for the U.S. Department of Energy (DOE) Office of Science. Los Alamos
National Laboratory, an affirmative action equal opportunity employer,
is managed by Triad National Security, LLC for the U.S. Department
of Energy’s NNSA, under contract 89233218CNA000001.Center for Dynamics and Control of Material
Correlated Excitonic Signatures in a Nanoscale van der Waals Antiferromagnet
Composite quasi-particles with emergent functionalities in spintronic and
quantum information science can be realized in correlated materials due to
entangled charge, spin, orbital, and lattice degrees of freedom. Here we show
that by reducing the lateral dimension of correlated antiferromagnet NiPS3
flakes to tens of nanometers, we can switch-off the bulk spin-orbit entangled
exciton in the near-infrared (1.47 eV) and activate visible-range (1.8 to 2.2
eV) transitions with charge-transfer character. These ultra-sharp lines (<120
ueV at 4.2 K) share the spin-correlated nature of the bulk exciton by
displaying a Neel temperature dependent linear polarization. Furthermore,
exciton photoluminescence lineshape analysis reveals a polaronic character via
coupling with at-least 3 phonon modes and a comb-like Stark effect through
discretization of charges in each layer. These findings augment the knowledge
on the many-body nature of excitonic quasi-particles in correlated
antiferromagnets and also establish the nanoscale platform as promising for
maturing integrated magneto-optic devices
Cobalt Doping as a Pathway To Stabilize the Solid-State Conversion Chemistry of Manganese Oxide Anodes in Li-Ion Batteries
Metal oxides have been widely studied in recent years to replace commercial graphite anodes in lithium ion batteries. Among the metal oxides, manganese oxide has a high theoretical capacity, low cost, and is environmentally friendly. However, many MnO materials have shown limited reaction reversibility and poor conversion kinetics. To understand why, in this paper we investigate the mechanism, kinetics, and reversibility for the solid-state conversion reaction of MnO with Li+. We definitively show, for the first time, that during repeated reaction cycles, multiple reaction pathways occur that lead not only to the reformation of MnO but also higher oxidation-state Mn3O4—which when combined with the poor intrinsic electronic conductivity of both manganese oxide species results in a rapid loss in the amount of charge that can be stored in these materials. Learning this, the approach in this study was to use cobalt doping to concomitantly stabilize the redox behavior of manganese (allowing for the gradual transformation of MnO to Mn3O4 over time) and to increase the intraparticle electronic conductivity of the active layer. The result is an active material, Mn0.9Co0.1O, that exhibits excellent charge stability and conversion kinetics (near 600 mAh/g at a rate of 400 mA/g), even over hundreds of reaction cycles
Reexamination of basal plane thermal conductivity of suspended graphene samples measured by electro-thermal micro-bridge methods
Thermal transport in suspended graphene samples has been measured in prior works and this work with the use of a suspended electro-thermal micro-bridge method. These measurement results are analyzed here to evaluate and eliminate the errors caused by the extrinsic thermal contact resistance. It is noted that the room-temperature thermal resistance measured in a recent work increases linearly with the suspended length of the single-layer graphene samples synthesized by chemical vapor deposition (CVD), and that such a feature does not reveal the failure of Fourier’s law despite the increase in the reported apparent thermal conductivity with length. The re-analyzed apparent thermal conductivity of a single-layer CVD graphene sample reaches about 1680 ± 180 W m−1 K−1 at room temperature, which is close to the highest value reported for highly oriented pyrolytic graphite. In comparison, the apparent thermal conductivity values measured for two suspended exfoliated bi-layer graphene samples are about 880 ± 60 and 730 ± 60 Wm−1K−1 at room temperature, and approach that of the natural graphite source above room temperature. However, the low-temperature thermal conductivities of these suspended graphene samples are still considerably lower than the graphite values, with the peak thermal conductivities shifted to much higher temperatures. Analysis of the thermal conductivity data reveals that the low temperature behavior is dominated by phonon scattering by polymer residue instead of by the lateral boundary
Ultrathin Graphite Foam: A Three-Dimensional Conductive Network for Battery Electrodes
We report the use of free-standing, lightweight, and highly conductive ultrathin graphite foam (UGF), loaded with lithium iron phosphate (LFP), as a cathode in a lithium ion battery. At a high charge/discharge current density of 1280 mA g(-1), the specific capacity of the LFP loaded on UGF was 70 mAh g(-1), while LFP loaded on Al foil failed. Accounting for the total mass of the electrode, the maximum specific capacity of the UGF/LFP cathode was 23% higher than that of the Al/LFP cathode and 170% higher than that of the Ni-foam/LFP cathode. Using UGF, both a higher rate capability and specific capacity can be achieved simultaneously, owing to its conductive (similar to 1.3 x 10(5) S m(-1) at room temperature) and three-dimensional lightweight (similar to 9.5 mg cm(-3)) graphitic structure. Meanwhile, UGF presents excellent electrochemical stability comparing to that of Al and Ni foils, which are generally used as conductive substrates in lithium ion batteries. Moreover, preparation of the UGF electrode was facile, cost-effective, and compatible with various electrochemically active materials