21 research outputs found
Band Gap Tuning of Twinned GaAsP Ternary Nanowires
GaAs<sub>1ā<i>x</i></sub>P<sub><i>x</i></sub> ternary
alloy nanowires have drawn much interest because their
tunable band gaps, which range from the near-infrared to visible region,
are promising for advanced and integrated nanoscale optoelectronic
devices. In this study, we synthesized compositionally tuned GaAs<sub>1<i>āx</i></sub>P<sub><i>x</i></sub> (0
ā¤ <i>x</i> ā¤ 1) alloy nanowires with two average
diameters of 60 and 120 nm by vapor transport method. The nanowires
exhibit exclusively twinned superlattice structures, consisting of
zinc blende phase twinned octahedral slice segments between wurtzite
phase planes. Smaller diameter and higher P content (<i>x</i>) result in shorter periodic superlattice structures. The band gap
of the smaller diameter nanowires is larger than that of the larger
diameter nanowires by about 90 meV, suggesting that the twinned superlattice
structure increases the band gap. The increase in band gap is ascribed
to the higher band gap of the wurtzite phase than that of the zinc
blende phase
Transition-Metal Doping of Oxide Nanocrystals for Enhanced Catalytic Oxygen Evolution
Catalysts for the oxygen reduction
and evolution reactions are
central to key renewable-energy technologies including fuel cells
and water splitting. Despite tremendous effort, the development of
oxygen electrode catalysts with high activity at low cost remains
a great challenge. In this study, we report a generalized solāgel
method for the synthesis of various oxide nanocrystals (TiO<sub>2</sub>, ZnO, Nb<sub>2</sub>O<sub>5</sub>, In<sub>2</sub>O<sub>3</sub>,
SnO<sub>2</sub>, and Ta<sub>2</sub>O<sub>5</sub>) with appropriate
transition metal dopants for an efficient electrocatalytic oxygen
evolution reaction (OER). Although TiO<sub>2</sub> and ZnO nanocrystals
alone have little activity, all the Mn-, Fe-, Co-, and Ni-doped nanocrystals
exhibit greatly enhanced OER activity. A remarkable finding is that
Co dopant produces higher OER activity than the other doped metals.
X-ray photoelectron and X-ray absorption spectroscopies revealed the
highly oxidized metal ions that are responsible for the enhanced catalytic
reactivity. The excellent OER activity of the Co-doped nanocrystals
was explained by a synergistic effect in which the oxide matrix effectively
guards the most active Co dopants at higher oxidation states by withdrawing
the electrons from the metal dopants. The metal-doped NCs exhibit
enhanced catalytic activity under visible light irradiation, suggesting
their potential as efficient solar-driven OER photoelectrocatalysts
Germanium and Tin Selenide Nanocrystals for High-Capacity Lithium Ion Batteries: Comparative Phase Conversion of Germanium and Tin
Germanium
and tin sulfide nanostructures are considered the most
promising candidates for useful alternative materials in commercial
Liāgraphite anodes of lithium ion batteries. Selenides have
received less attention, but the electrochemical reaction mechanism
is still being debated. We report the novel synthesis of GeSe<sub><i>x</i></sub> and SnSe<sub><i>x</i></sub> (<i>x</i> = 1 and 2) nanocrystals by a gas-phase laser photolysis
reaction and their excellent reversible capacity for lithium ion batteries.
The capacity was 400ā800 (mA h)/g after 70 cycles, which is
close to the theoretical capacity (Li<sub>4.4</sub>Ge or Li<sub>4.4</sub>Sn). Remarkably, SnSe<sub><i>x</i></sub> exhibited higher
rate capabilities than GeSe<sub><i>x</i></sub>. Ex situ
X-ray diffraction and Raman spectroscopy revealed the <i>cubic</i>ā<i>tetragonal</i> phase conversion of Ge and Sn
upon lithiation/delithiation to support their distinctive lithium
ion battery capacities. First-principles calculations of the Li intercalation
volume change indicate that the smallest volume expansion in the cubic
Sn phase can guarantee the enhanced cycling capability of the Sn compounds
GaPāZnS Pseudobinary Alloy Nanowires
Multicomponent nanowires (NWs) are
of great interest for integrated
nanoscale optoelectronic devices owing to their widely tunable band
gaps. In this study, we synthesize a series of (GaP)<sub>1ā<i>x</i></sub>(ZnS)<sub><i>x</i></sub> (0 ā¤ <i>x</i> ā¤ 1) pseudobinary alloy NWs using the vapor transport
method. Compositional tuning results in the phase evolution from the
zinc blende (ZB) (<i>x</i> < 0.4) to the wurtzite (WZ)
phase (<i>x</i> > 0.7). A coexistence of ZB and WZ phases
(<i>x</i> = 0.4ā0.7) is also observed. In the intermediate
phase coexistence range, a coreāshell structure is produced
with a composition of <i>x</i> = 0.4 and 0.7 for the core
and shell, respectively. The band gap (2.4ā3.7 eV) increases
nonlinearly with increasing <i>x</i>, showing a significant
bowing phenomenon. The phase evolution leads to enhanced photoluminescence
emission. Strikingly, the photoluminescence spectrum shows a blue-shift
(70 meV for <i>x</i> = 0.9) with increasing excitation power,
and a wavelength-dependent decay time. Based on the photoluminescence
data, we propose a type-II pseudobinary heterojunction band structure
for the single-crystalline WZ phase ZnS-rich NWs. The slight incorporation
of GaP into the ZnS induces a higher photocurrent and excellent photocurrent
stability, which opens up a new strategy for enhancing the performance
of photodetectors
Strain Mapping and Raman Spectroscopy of Bent GaP and GaAs Nanowires
Strain engineering
of nanowires (NWs) has been recognized as a
powerful strategy for tuning the optical and electronic properties
of nanoscale semiconductors. Therefore, the characterization of the
strains with nanometer-scale spatial resolution is of great importance
for various promising applications. In the present work, we synthesized
single-crystalline zinc blende phase GaP and GaAs NWs using the chemical
vapor transport method and visualized their bending strains (up to
3%) with high precision using the nanobeam electron diffraction technique.
The strain mapping at all crystallographic axes revealed that (i)
maximum strain exists along the growth direction ([111]) with the
tensile and compressive strains at the outer and inner parts, respectively;
(ii) the opposite strains appeared along the perpendicular direction
([2Ģ
11]); and (iii) the tensile strain was larger than the coexisting
compressive strain at all axes. The Raman spectrum collected for individual
bent NWs showed the peak broadening and red shift of the transverse
optical modes that were well-correlated with the strain maps. These
results are consistent with the larger mechanical modulus of GaP than
that of GaAs. Our work provides new insight into the bending strain
of IIIāV semiconductors, which is of paramount importance in
the performance of flexible or bendable electronics
IrO<sub>2</sub>āZnO Hybrid Nanoparticles as Highly Efficient Trifunctional Electrocatalysts
Development of high-performance
catalysts is very crucial for the
commercialization of sustainable energy conversion technologies. Searching
for stable, highly active, and low-cost multifunctional catalysts
has become a critical issue. In this study, we report the synthesis
of IrO<sub>2</sub>āZnO hybrid nanoparticles and their highly
efficient electrocatalytic activities toward oxygen/hydrogen evolution
reaction (OER/HER) as well as oxygen reduction reaction (ORR). For
comparison, we synthesized RuO<sub>2</sub>āZnO, showing a smaller
catalytic activity than IrO<sub>2</sub>āZnO, which provides
robust evidence for the unique synergic effect of these hybrid structures.
IrO<sub>2</sub>āZnO and RuO<sub>2</sub>āZnO exhibit
excellent OER catalytic performance with Tafel slopes of 57 and 59
mV decade<sup>ā1</sup>, respectively. For HER, IrO<sub>2</sub>āZnO shows a higher catalytic activity than RuO<sub>2</sub>āZnO. The numbers of electrons involved in the ORR were 3.7
and 2.8, respectively, for IrO<sub>2</sub>āZnO and RuO<sub>2</sub>āZnO. The remarkable catalytic performance of IrO<sub>2</sub>āZnO would be ascribed to the abundant oxygen vacancies
and the metallic states of Ir, which ensure excellent catalytic activity
and stability
Zn<sub>3</sub>P<sub>2</sub>āZn<sub>3</sub>As<sub>2</sub> Solid Solution Nanowires
Semiconductor alloy nanowires (NWs)
have recently attracted considerable attention for applications in
optoelectronic nanodevices because of many notable properties, including
band gap tunability. Zinc phosphide (Zn<sub>3</sub>P<sub>2</sub>)
and zinc arsenide (Zn<sub>3</sub>As<sub>2</sub>) belong to a unique
pseudocubic tetragonal system, but their solid solution has rarely
been studied. Here In this study, we synthesized composition-tuned
Zn<sub>3</sub>(P<sub>1ā<i>x</i></sub>As<sub><i>x</i></sub>)<sub>2</sub> NWs with different crystal structures
by controlling the growth conditions during chemical vapor deposition.
A first type of synthesized NWs were single-crystalline and grew uniformly
along the [110] direction (in a cubic unit cell) over the entire compositional
range (0 ā¤ <i>x</i> ā¤ 1) explored. The use
of an indium source enabled the growth of a second type of NWs, with
remarkable cubic-hexagonal polytypic twinned superlattice and bicrystalline
structures. The growth direction of the Zn<sub>3</sub>P<sub>2</sub> and Zn<sub>3</sub>As<sub>2</sub> NWs was also switched to [111]
and [112], respectively. These structural changes are attributable
to the Zn-depleted indium catalytic nanoparticles which favor the
growth of hexagonal phases. The formation of a solid solution at all
compositions allowed the continuous tuning of the band gap (1.0ā1.5
eV). Photocurrent measurements were performed on individual NWs by
fabricating photodetector devices; the single-crystalline NWs with
[110] growth direction exhibit a higher photoconversion efficiency
compared to the twinned crystalline NWs with [111] or [112] growth
direction
LightāMatter Interactions in Cesium Lead Halide Perovskite Nanowire Lasers
Lightāmatter
interactions in inorganic perovskite nanolasers
are investigated using single-crystalline cesium lead halide (CsPbX<sub>3</sub>, X = Cl, Br, and I) nanowires synthesized by the chemical
vapor transport method. The perovskite nanowires exhibit a uniform
growth direction, smooth surfaces, straight end facets, and homogeneous
composition distributions. Lasing occurs in the perovskite nanowires
at low thresholds (3 Ī¼J/cm<sup>2</sup>) with high quality factors
(<i>Q</i> = 1200ā1400) under ambient atmospheric
environments. The wavelengths of the nanowire lasers are tunable by
controlling the stoichiometry of the halide, allowing the lasing of
the inorganic perovskite nanowires from blue to red. The unusual spacing
of the FabryāPeĢrot modes suggests strong lightāmatter
interactions in the reduced mode volume of the nanowires, while the
polarization of the lasing indicates that the FabryāPeĢrot
modes belong to the same fundamental transverse mode. The dispersion
curve of the excitonāpolariton model suggests that the group
refractive index of the polariton is significantly enhanced
Surface-Modified Ta<sub>3</sub>N<sub>5</sub> Nanocrystals with Boron for Enhanced Visible-Light-Driven Photoelectrochemical Water Splitting
Photocatalysts for
water splitting are the core of renewable energy technologies, such
as hydrogen fuel cells. The development of photoelectrode materials
with high efficiency and low corrosivity has great challenges. In
this study, we report new strategy to improve performance of tantalum
nitride (Ta<sub>3</sub>N<sub>5</sub>) nanocrystals as promising photoanode
materials for visible-light-driven photoelectrochemical (PEC) water
splitting cells. The surface of Ta<sub>3</sub>N<sub>5</sub> nanocrystals
was modified with boron whose content was controlled, with up to 30%
substitution of Ta. X-ray photoelectron spectroscopy revealed that
boron was mainly incorporated into the surface oxide layers of the
Ta<sub>3</sub>N<sub>5</sub> nanocrystals. The surface modification
with boron increases significantly the solar energy conversion efficiency
of the water-splitting PEC cells by shifting the onset potential cathodically
and increasing the photocurrents. It reduces the interfacial charge-transfer
resistance and increases the electrical conductivity, which could
cause the higher photocurrents at lower potential. The onset potential
shift of the PEC cell with the boron incorporation can be attributed
to the negative shift of the flat band potential. We suggest that
the boron-modified surface acts as a protection layer for the Ta<sub>3</sub>N<sub>5</sub> nanocrystals, by catalyzing effectively the
water splitting reaction
Red-to-Ultraviolet Emission Tuning of Two-Dimensional Gallium Sulfide/Selenide
Graphene-like two-dimensional (2D) nanostructures have attracted significant attention because of their unique quantum confinement effect at the 2D limit. Multilayer nanosheets of GaSāGaSe alloy are found to have a band gap (<i>E</i><sub>g</sub>) of 2.0ā2.5 eV that linearly tunes the emission in red-to-green. However, the epitaxial growth of monolayers produces a drastic increase in this <i>E</i><sub>g</sub> to 3.3ā3.4 eV, which blue-shifts the emission to the UV region. First-principles calculations predict that the <i>E</i><sub>g</sub> of these GaS and GaSe monolayers should be 3.325 and 3.001 eV, respectively. As the number of layers is increased to three, both the direct/indirect <i>E</i><sub>g</sub> decrease significantly; the indirect <i>E</i><sub>g</sub> approaches that of the multilayers. Oxygen adsorption can cause the direct/indirect <i>E</i><sub>g</sub> of GaS to converge, resulting in monolayers with a strong emission. This wide <i>E</i><sub>g</sub> tuning over the visible-to-UV range could provide an insight for the realization of full-colored flexible and transparent light emitters and displays