11 research outputs found
Photocurrent Response in Multiwalled Carbon Nanotube Core–Molybdenum Disulfide Shell Heterostructures
In this report, a few-layer molybdenum
disulfide (MoS<sub>2</sub>) shell was coated on core multiwalled carbon
nanotube (CNT) by a facile solvothermal method. The morphology and
high crystallinity of this structure were demonstrated and verified
by transmission electron microscopy (TEM), X-ray diffraction (XRD),
Raman spectroscopy, and X-ray photoelectron spectroscopy (XPS). After
being integrated into a planar device, the CNT–MoS<sub>2</sub> core–shell structure exhibits clear photoresponse and a wide
response range upon laser illumination. In addition, the device shows
a bias-dependent and position-sensitive photocurrent effect. Further
experiments show that larger photocurrent was obtained under laser
illumination with longer wavelength. Both the photocurrent and response
speed are enhanced when the device is placed under vacuum condition.
The simple material synthesis and device fabrication method used in
this work may provide a practical strategy for low-cost and large-scale
optical applications
NIR Schottky Photodetectors Based on Individual Single-Crystalline GeSe Nanosheet
We have synthesized high-quality,
micrometer-sized, single-crystal GeSe nanosheets using vapor transport
and deposition techniques. Photoresponse is investigated based on
mechanically exfoliated GeSe nanosheet combined with Au contacts under
a global laser irradiation scheme. The nonlinearship, asymmetric,
and unsaturated characteristics of the <i>I</i>–<i>V</i> curves reveal that two uneven back-to-back Schottky contacts
are formed. First-principles calculations indicate that the occurrence
of defects-induced in-gap defective states, which are responsible
for the slow decay of the current in the OFF state and for the weak
light intensity dependence of photocurrent. The Schottky photodetector
exhibits a marked photoresponse to NIR light illumination (maximum
photoconductive gain ∼5.3 × 10<sup>2</sup> % at 4 V) at
a wavelength of 808 nm. The significant photoresponse and good responsitivity
(∼3.5 A W<sup>–1</sup>) suggests its potential applications
as photodetectors
Plasmon-Enhanced Photocatalytic Properties of Cu<sub>2</sub>O Nanowire–Au Nanoparticle Assemblies
Cu<sub>2</sub>O–Au nanocomposites (NCs) with tunable coverage
of Au were prepared by a facile method of mixing gold nanoparticles
(Au NPs) with copper(I) oxide nanowires (Cu<sub>2</sub>O NWs) in various
ratios. These Cu<sub>2</sub>O–Au NCs display tunable optical
properties, and their photocatalytic properties were dependent on
the coverage density of Au NPs. The photocatalytic activity of Cu<sub>2</sub>O–Au NCs was examined by photodegradation of methylene
blue. The presence of Au NPs enhanced the photodegradation efficiency
of Cu<sub>2</sub>O NCs. The photocatalytic efficiency of Cu<sub>2</sub>O–Au NCs initially increased with the increasing coverage
density of Au NPs and then decreased as the surface of Cu<sub>2</sub>O became densely covered by Au NPs. The enhanced photocatalytic efficiency
was ascribed to enhanced light absorption (by the surface plasmon
resonance) and the electron sink effect of the Au NPs
Desorption of Ambient Gas Molecules and Phase Transformation of α‑Fe<sub>2</sub>O<sub>3</sub> Nanostructures during Ultrahigh Vacuum Annealing
Desorption and readsorption of gas molecules from ambient
air onto
the surface of α-Fe<sub>2</sub>O<sub>3</sub> quasi-1D nanostructures
(nanoflakes and nanostrips) were studied in situ by X-ray photoelectron
spectroscopy (XPS) and ex situ by X-ray diffraction (XRD) and scanning
electron microscopy (SEM). XPS revealed that carbon and oxygen species
were physisorbed and chemisorbed as C–C/C–H, C–O,
O–CO, and O<sub>2</sub> states on both surfaces of
α-Fe<sub>2</sub>O<sub>3</sub> quasi-1D nanostructures upon exposure
in air. The physisorbed carbon species (C–C/C–H) and
O<sub>2</sub> desorbed from the surfaces when the two nanostructures
were heated to 100 °C inside the vacuum chamber of XPS. Significant
desorption of chemisorbed O–CO and O–C occurred
above 200 °C, which resulted in a reduction of Fe<sub>2</sub>O<sub>3</sub> into Fe<sub>3</sub>O<sub>4</sub> for both samples between
200 and 300 °C. Complete desorption of carbon and O–C/O–CO/O<sub>2</sub> species in O1s occurred at 400 °C, where Fe<sub>3</sub>O<sub>4</sub> in nanoflakes (sample 1) was reduced further into FeO
by excess metallic Fe from the bulk, while Fe<sub>3</sub>O<sub>4</sub> in nanostrips (sample 2) was largely oxidized into Fe<sub>2</sub>O<sub>3</sub> by the oxygen from the bulk of Fe<sub>2</sub>O<sub>3</sub>. Although no band bending was observed during the annealing
and desorption of ambient gases, the valence band changed as the phase
transformation occurred. After the annealed samples were exposed to
air for two days, the same chemical states associated with C and O
species were again detected on the surfaces of the two nanostructures.
In addition, FeO (sample 1) was found to be oxidized into a mixture
of Fe<sub>2</sub>O<sub>3</sub> and Fe<sub>3</sub>O<sub>4</sub> on
the surface. The adsorption of gas molecules from ambient environment
thus has a strong influence on the chemical and physical properties
of nanostructures with large surface to volume ratio
Ultrasensitive Phototransistor Based on K‑Enriched MoO<sub>3</sub> Single Nanowires
An ultrasensitive phototransistor was fabricated based
on K-intercalated
MoO<sub>3</sub> single nanowire. Devices with ultrafast photoresponse
rate, high responsivity, and broad spectral response range were demonstrated.
Detailed analysis of the charge transport in the device revealed the
coexistence of both thermal-activation and photoactivation mechanisms.
The promising results are expected to promote the potential of this
material in nano/micro-scaled photoelectronic applications
Microsteganography on WS<sub>2</sub> Monolayers Tailored by Direct Laser Painting
We
present scanning focused laser beam as a multipurpose tool to
engineer the physical and chemical properties of WS<sub>2</sub> microflakes.
For monolayers, the laser modification integrates oxygen into the
WS<sub>2</sub> microflake, resulting in ∼9 times enhancement
in the intensity of the fluorescence emission. This modification does
not cause any morphology change, allowing “micro-encryption”
of information that is only observable as fluorescence under excitation.
The same focused laser also facilitates on demand thinning down of
WS<sub>2</sub> multilayers into monolayers, turning them into fluorescence
active components. With a scanning focused laser beam, micropatterns
are readily created on WS<sub>2</sub> multilayers through selective
thinning of specific regions on the flake
Improved Photoelectrical Properties of MoS<sub>2</sub> Films after Laser Micromachining
Direct patterning of ultrathin MoS<sub>2</sub> films with well-defined structures and controllable thickness is appealing since the properties of MoS<sub>2</sub> sheets are sensitive to the number of layer and surface properties. In this work, we employed a facile, effective, and well-controlled technique to achieve micropatterning of MoS<sub>2</sub> films with a focused laser beam. We demonstrated that a direct focused laser beam irradiation was able to achieve localized modification and thinning of as-synthesized MoS<sub>2</sub> films. With a scanning laser beam, microdomains with well-defined structures and controllable thickness were created on the same film. We found that laser modification altered the photoelectrical property of the MoS<sub>2</sub> films, and subsequently, photodetectors with improved performance have been fabricated and demonstrated using laser modified films
Reduced Graphene Oxide Conjugated Cu<sub>2</sub>O Nanowire Mesocrystals for High-Performance NO<sub>2</sub> Gas Sensor
Reduced graphene oxide (rGO)-conjugated Cu<sub>2</sub>O nanowire
mesocrystals were formed by nonclassical crystallization in the presence
of GO and <i>o</i>-anisidine under hydrothermal conditions.
The resultant mesocrystals are comprised of highly anisotropic nanowires
as building blocks and possess a distinct octahedral morphology with
eight {111} equivalent crystal faces. The mechanisms underlying the
sequential formation of the mesocrystals are as follows: first, GO-promoted
agglomeration of amorphous spherical Cu<sub>2</sub>O nanoparticles
at the initial stage, leading to the transition of growth mechanism
from conventional ion-by-ion growth to particle-mediated crystallization;
second, the evolution of the amorphous microspheres into hierarchical
structure, and finally to nanowire mesocrystals through mesoscale
transformation, where Ostwald ripening is responsible for the growth
of the nanowire building blocks; third, large-scale self-organization
of the mesocrystals and the reduction of GO (at high GO concentration)
occur simultaneously, resulting in an integrated hybrid architecture
where porous three-dimensional (3D) framework structures interspersed
among two-dimensional (2D) rGO sheets. Interestingly, “super-mesocrystals”
formed by 3D oriented attachment of mesocrystals are also formed judging
from the voided Sierpinski polyhedrons observed. Furthermore, the
interior nanowire architecture of these mesocrystals can be kinetically
controlled by careful variation of growth conditions. Owing to high
specific surface area and improved conductivity, the rGO-Cu<sub>2</sub>O mesocrystals achieved a higher sensitivity toward NO<sub>2</sub> at room temperature, surpassing the performance of standalone systems
of Cu<sub>2</sub>O nanowires networks and rGO sheets. The unique characteristics
of rGO-Cu<sub>2</sub>O mesocrystal point to its promising applications
in ultrasensitive environmental sensors
Fluorescence Concentric Triangles: A Case of Chemical Heterogeneity in WS<sub>2</sub> Atomic Monolayer
We report a novel optical property in WS<sub>2</sub> monolayer.
The monolayer naturally exhibits beautiful in-plane periodical and
lateral homojunctions by way of alternate dark and bright band in
the fluorescence images of these monolayers. The interface between
different fluorescence species within the sample is distinct and sharp.
This gives rise to intriguing concentric triangular fluorescence patterns
in the monolayer. The novel optical property of this special WS<sub>2</sub> monolayer is facilitated by chemical heterogeneity. The photoluminescence
of the bright band is dominated by emissions from trion and biexciton
while the emission from defect-bound exciton dominates the photoluminescence
at the dark band. The discovery of such concentric fluorescence patterns
represents a potentially new form of optoelectronic or photonic functionality
Atomic Healing of Defects in Transition Metal Dichalcogenides
As-grown transition metal dichalcogenides
are usually chalcogen deficient and therefore contain a high density
of chalcogen vacancies, deep electron traps which can act as charged
scattering centers, reducing the electron mobility. However, we show
that chalcogen vacancies can be effectively passivated by oxygen,
healing the electronic structure of the material. We proposed that
this can be achieved by means of surface laser modification and demonstrate
the efficiency of this processing technique, which can enhance the
conductivity of monolayer WSe<sub>2</sub> by ∼400 times and
its photoconductivity by ∼150 times