20 research outputs found
Revealing Surface States in In-Doped SnTe Nanoplates with Low Bulk Mobility
Indium (In) doping in topological
crystalline insulator SnTe induces superconductivity, making In-doped
SnTe a candidate for a topological superconductor. SnTe nanostructures
offer well-defined nanoscale morphology and high surface-to-volume
ratios to enhance surface effects. Here, we study In-doped SnTe nanoplates,
In<sub><i>x</i></sub>Sn<sub>1–<i>x</i></sub>Te, with <i>x</i> ranging from 0 to 0.1 and show they superconduct.
More importantly, we show that In doping reduces the bulk mobility
of In<sub><i>x</i></sub>Sn<sub>1–<i>x</i></sub>Te such that the surface states are revealed in magnetotransport
despite the high bulk carrier density. This is manifested by two-dimensional
linear magnetoresistance in high magnetic fields, which is independent
of temperature up to 10 K. Aging experiments show that the linear
magnetoresistance is sensitive to ambient conditions, further confirming
its surface origin. We also show that the weak antilocalization observed
in In<sub><i>x</i></sub>Sn<sub>1–<i>x</i></sub>Te nanoplates is a bulk effect. Thus, we show that nanostructures
and reducing the bulk mobility are effective strategies to reveal
the surface states and test for topological superconductors
Ambipolar Field Effect in Sb-Doped Bi<sub>2</sub>Se<sub>3</sub> Nanoplates by Solvothermal Synthesis
A topological insulator is a new phase of quantum matter
with a
bulk band gap and spin-polarized surface states, which might find
use in applications ranging from electronics to energy conversion.
Despite much exciting progress in the field, high-yield solution synthesis
has not been widely used for the study of topological insulator behavior.
Here, we demonstrate that solvothermally synthesized Bi<sub>2</sub>Se<sub>3</sub> nanoplates are attractive for topological insulator
studies. The carrier concentration of these Bi<sub>2</sub>Se<sub>3</sub> nanoplates is controlled by compensational Sb doping during the
synthesis. In low-carrier-density, Sb-doped Bi<sub>2</sub>Se<sub>3</sub> nanoplates, we observe pronounced ambipolar field effect that demonstrates
the flexible manipulation of carrier type and concentration for these
nanostructures. Solvothermal synthesis offers an affordable, facile
approach to produce high-quality nanomaterials to explore the properties
of topological insulators
Semipolar (202Ì…1Ì…) GaN and InGaN Light-Emitting Diodes Grown on Sapphire
We have
demonstrated growing uniform and purely nitrogen
polar semipolar (202Ì…1Ì…) GaN epilayers on 2 in. patterned
sapphire substrates. The as-grown surface of (202Ì…1Ì…)
GaN is composed of two stable facets: (101Ì…0) and (101Ì…1Ì…).
A chemical mechanical polishing process was further used to planarize
the surface with a final surface root-mean-square roughness of less
than 1.5 nm over an area of 10 × 10 μm<sup>2</sup>. InGaN
light-emitting diodes were grown on a polished (202Ì…1Ì…)
GaN/sapphire template with an electroluminescence emission at around
490 nm. Our work exhibits the potential to produce high-quality nitrogen-polar
semipolar GaN templates and optoelectronic devices on large-area sapphire
substrates with economical feasibility
Metal Seed Layer Thickness-Induced Transition From Vertical to Horizontal Growth of MoS<sub>2</sub> and WS<sub>2</sub>
Two-dimensional
(2D), layered transition metal dichalcogenides (TMDCs) can grow in
two different growth directions, that is, horizontal and vertical.
In the horizontal growth, 2D TMDC layers grow in planar direction
with their basal planes parallel to growth substrates. In the vertical
growth, 2D TMDC layers grow standing upright on growth substrates
exposing their edge sites rather than their basal planes. The two
distinct morphologies present unique materials properties suitable
for specific applications, such as horizontal TMDCs for optoelectronics
and vertical TMDCs for electrochemical reactions. Precise control
of the growth orientation is essential for realizing the true potential
of these 2D materials for large-scale, practical applications. In
this Letter, we investigate the transition of vertical-to-horizontal
growth directions in 2D molybdenum (or tungsten) disulfide and study
the underlying growth mechanisms and parameters that dictate such
transition. We reveal that the thickness of metal seed layers plays
a critical role in determining their growth directions. With thick
(>∼3 nm) seed layers, the vertical growth is dominant, while
the horizontal growth occurs with thinner seed layers. This finding
enables the synthesis of novel 2D TMDC heterostructures with anisotropic
layer orientations and transport properties. The present study paves
a way for developing a new class of 2D TMDCs with unconventional materials
properties
High-Density Chemical Intercalation of Zero-Valent Copper into Bi<sub>2</sub>Se<sub>3</sub> Nanoribbons
A major goal of intercalation chemistry is to intercalate
high
densities of guest species without disrupting the host lattice. Many
intercalant concentrations, however, are limited by the charge of
the guest species. Here we have developed a general solution-based
chemical method for intercalating extraordinarily high densities of
zero-valent copper metal into layered Bi<sub>2</sub>Se<sub>3</sub> nanoribbons. Up to 60 atom % copper (Cu<sub>7.5</sub>Bi<sub>2</sub>Se<sub>3</sub>) can be intercalated with no disruption to the host
lattice using a solution disproportionation redox reaction
Role of Heterointerface in Lithium-Induced Phase Transition in <i>T</i><sub>d</sub>‑WTe<sub>2</sub> Nanoflakes
A new
polytype of WTe2 with a bandgap has been recently
discovered through the intercalation of lithium into the van der Waals
gaps of Td-WTe2. Here, we report
the effects of reduced thicknesses and heterointerfaces on the intercalation-induced
phase transition in WTe2. Using in situ Raman spectroscopy during the electrochemical lithiation of WTe2 flakes as a function of flake thickness, we observe that
additional electrochemical energy is required for the phase transition
of WTe2 from the Td phase to
the new lithiated Td′ phase, going
from 0.8 V of the applied electrochemical voltage for a thick flake
to 0.5 V and 0.3 V for 7- and 5-layered samples, respectively. We
ascribe this suppression of the phase transition to the interfacial
interaction between the nanoflake and SiO2/Si substrate,
which plays an increasing role as the sample thickness is reduced.
The suppressed kinetics of the phase transition can be mitigated by
placing the WTe2 flake on a hexagonal boron nitride (hBN)
flake, which facilitates the release of the in-plane strain induced
by the phase transition. Our study underscores the significance of
interfacial effects in modulating phase transitions in two-dimensional
(2D) materials, suggesting heterogeneous transition pathways, as well
as interfacial engineering to control these phase transitions
Synthesis of MoS<sub>2</sub> and MoSe<sub>2</sub> Films with Vertically Aligned Layers
Layered materials consist of molecular layers stacked
together
by weak interlayer interactions. They often crystallize to form atomically
smooth thin films, nanotubes, and platelet or fullerene-like nanoparticles
due to the anisotropic bonding. Structures that predominately expose
edges of the layers exhibit high surface energy and are often considered
unstable. In this communication, we present a synthesis process to
grow MoS<sub>2</sub> and MoSe<sub>2</sub> thin films with vertically
aligned layers, thereby maximally exposing the edges on the film surface.
Such edge-terminated films are metastable structures of MoS<sub>2</sub> and MoSe<sub>2</sub>, which may find applications in diverse catalytic
reactions. We have confirmed their catalytic activity in a hydrogen
evolution reaction (HER), in which the exchange current density correlates
directly with the density of the exposed edge sites
Chemical Intercalation of Zerovalent Metals into 2D Layered Bi<sub>2</sub>Se<sub>3</sub> Nanoribbons
We have developed a chemical method to intercalate a
variety of
zerovalent metal atoms into two-dimensional (2D) layered Bi<sub>2</sub>Se<sub>3</sub> chalcogenide nanoribbons. We use a chemical reaction,
such as a disproportionation redox reaction, to generate dilute zerovalent
metal atoms in a refluxing solution, which intercalate into the layered
Bi<sub>2</sub>Se<sub>3</sub> structure. The zerovalent nature of the
intercalant allows superstoichiometric intercalation of metal atoms
such as Ag, Au, Co, Cu, Fe, In, Ni, and Sn. We foresee the impact
of this methodology in establishing novel fundamental physical behaviors
and in possible energy applications
Synthesis of SnTe Nanoplates with {100} and {111} Surfaces
SnTe is a topological crystalline
insulator that possesses spin-polarized,
Dirac-dispersive surface states protected by crystal symmetry. Multiple
surface states exist on the {100}, {110}, and {111} surfaces of SnTe,
with the band structure of surface states depending on the mirror
symmetry of a particular surface. Thus, to access surface states selectively,
it is critical to control the morphology of SnTe such that only desired
crystallographic surfaces are present. Here, we grow SnTe nanostructures
using vapor–liquid–solid and vapor–solid growth
mechanisms. Previously, SnTe nanowires and nanocrystals have been
grown [Saghir et al. <i>Cryst. Growth Des.</i> <b>2014</b>, <i>14</i>, 2009–2013; Safdar et al. <i>Cryst.
Growth Des.</i> <b>2014</b>, <i>14</i>, 2502–2509;
Safdar et al. <i>Nano Lett.</i> <b>2013</b>, <i>13</i>, 5344–5349; Li et al. <i>Nano Lett.</i> <b>2013</b>, <i>13</i>, 5443–5448]. In this
report, we demonstrate the synthesis of SnTe nanoplates with lateral
dimensions spanning tens of micrometers and thicknesses of a few hundred
nanometers. The top and bottom surfaces are either (100) or (111),
maximizing topological surface states on these surfaces. Magnetotransport
on these SnTe nanoplates shows a high bulk carrier density, consistent
with bulk SnTe crystals arising due to defects such as Sn vacancies.
In addition, we observe a structural phase transition in these nanoplates
from the high-temperature rock salt to a low-temperature rhombohedral
structure. For nanoplates with a very high carrier density, we observe
a slight upturn in resistance at low temperatures, indicating electron–electron
interactions
General Facet-Controlled Synthesis of Single-Crystalline {010}-Oriented LiMPO<sub>4</sub> (M = Mn, Fe, Co) Nanosheets
Facet-controlled
synthesis of phospho-olivine (LiMPO<sub>4</sub>, M = Mn, Fe, Co) cathode
materials is of particular interest to
manipulate their electrochemical properties because of their anisotropic
ionic transport behavior. This study provides a general facet-controlled
synthesis of single-crystalline LiMPO<sub>4</sub> (M = Mn, Fe, Co)
nanosheets with significantly large exposure of (010)-facets, which
has not been readily achieved by conventional solution-based coprecipitation
or solid-reaction methods. The as-obtained nanosheets show controllable
thickness with the thinnest thickness down to 15–20 nm and
lateral dimension up to ∼5 μm. Due to the shortened lithium
ion diffusion pathway and high ratio of active surface enabled by
the thin thickness, the as-prepared LiFePO<sub>4</sub> nanosheets,
as a model material, demonstrate greatly improved rate capability
and cycling stability, with a reversible capacity of ∼80 mA
h g<sup>–1</sup> at a current rate of 30 C and a stable capacity
retention of ∼93% after 500 cycles at a current rate of 5 C.
Further electrochemical analysis reveals an enhanced interfacial lithium
ion diffusion of the nanosheets, suggesting that facet-controlled
2D LiMPO<sub>4</sub> nanosheets are a promising material platform
for next-generation high-rate lithium-ion batteries