7 research outputs found
Enhanced Photoresponse of SnSe-Nanocrystals-Decorated WS<sub>2</sub> Monolayer Phototransistor
Single-layer WS<sub>2</sub> has shown
excellent photoresponse properties, but its promising applications
in high-sensitivity photodetection suffer from the atomic-thickness-limited
adsorption and band-gap-limited spectral selectivity. Here we have
carried out investigations on WS<sub>2</sub> monolayer based phototransistors
with and without decoration of SnSe nanocrystals (NCs) for comparison.
Compared to the solely WS<sub>2</sub> monolayer, SnSe NCs decoration
leads to not only huge enhancement of photoresponse in visible spectrum
but also extension to near-infrared. Under excitation of visible light
in a vacuum, the responsivity at zero gate bias can be enhanced by
more than 45 times to ∼99 mA/W, and the response time is retained
in millisecond level. Particularly, with extension of photoresponse
to near-infrared (1064 nm), a responsivity of 6.6 mA/W can be still
achieved. The excellent photoresponse from visible to near-infrared
is considered to benefit from synergism of p-type SnSe NCs and n-type
WS<sub>2</sub> monolayer, or in other words, the formed p-n heterojunctions
between p-type SnSe NCs and n-type WS<sub>2</sub> monolayer
Ultrahigh-Gain and Fast Photodetectors Built on Atomically Thin Bilayer Tungsten Disulfide Grown by Chemical Vapor Deposition
The
low responsivity observed in photodetectors based on monolayer transition-metal
dichalcogenides has encouraged the pursuit of approaches that can
efficiently enhance the external quantum efficiency, which relies
predominantly on the light absorption, the lifetime of the excess
carriers, and the charge collection efficiency. Here, we demonstrate
that phototransistors fabricated on large-area bilayer tungsten disulfide
(WS<sub>2</sub>) grown by chemical vapor deposition exhibit remarkable
performance with photoresponsivity, photogain, and detectivity of
up to ∼3 × 10<sup>3</sup> A/W, 1.4 × 10<sup>4</sup>, and ∼5 × 10<sup>12</sup> Jones, respectively. These
figures of merit of bilayer WS<sub>2</sub> provide a significant
advantage over monolayer WS<sub>2</sub> due to the greatly improved
carrier mobility and significantly reduced contact resistance. The
photoresponsivity of bilayer WS<sub>2</sub> phototransistor can be
further improved to up to 1 × 10<sup>4</sup> A/W upon biasing
a gate voltage of 60 V, without evident reduction in detectivity.
Moreover, the bilayer WS<sub>2</sub> phototransistor exhibits a high
response speed of less than 100 μs, large bandwidth of 4 kHz,
high cycling reliability of over 10<sup>5</sup> cycles, and spatially
homogeneous photoresponse. These outstanding figures of merit make
WS<sub>2</sub> bilayer a highly promising candidate for the design
of high-performance optoelectronics in the visible regime
Unique Lead Adsorption Behavior of Activated Hydroxyl Group in Two-Dimensional Titanium Carbide
The
functional groups and site interactions on the surfaces of
two-dimensional (2D) layered titanium carbide can be tailored to attain
some extraordinary physical properties. Herein a 2D alk-MXene (Ti<sub>3</sub>C<sub>2</sub>(OH/ONa)<sub><i>x</i></sub>F<sub>2–<i>x</i></sub>) material, prepared by chemical exfoliation followed
by alkalization intercalation, exhibits preferential PbÂ(II) sorption
behavior when competing cations (CaÂ(II)/MgÂ(II)) coexisted at high
levels. Kinetic tests show that the sorption equilibrium is achieved
in as short a time as 120 s. Attractively, the alk-MXene presents
efficient PbÂ(II) uptake performance with the applied sorption capacities
of 4500 kg water per alk-MXene, and the effluent PbÂ(II) contents are
below the drinking water standard recommended by the World Health
Organization (10 μg/L). Experimental and computational studies
suggest that the sorption behavior is related to the hydroxyl groups
in activated Ti sites, where PbÂ(II) ion exchange is facilitated by
the formation of a hexagonal potential trap
Flexible Black-Phosphorus Nanoflake/Carbon Nanotube Composite Paper for High-Performance All-Solid-State Supercapacitors
We
proposed a simple route for fabrication of the flexible BP nanoflake/carbon
nanotube (CNT) composite paper as flexible electrodes in all-solid-state
supercapacitors. The highly conductive CNTs not only play a role as
active materials but also increase conductivity of the hybrid electrode,
enhance electrolyte shuttling and prevent the restacking between BP
nanoflakes. The fabricated flexible all-solid-state supercapacitor
(ASSP) device at the mass proportion of BP/CNTs 1:4 was found to deliver
the highest volumetric capacitance of up to 41.1 F/cm<sup>3</sup> at
0.005 V/s, superior to the ASSP based on the bare graphene or BP.
The BP/CNTs (1:4) device delivers a rapid charging/discharging up
to 500 V/s, which exhibits the characteristic of a high power density
of 821.62 W/cm<sup>3</sup>, while having outstanding mechanical flexibility
and high cycling stability over 10 000 cycles (91.5% capacitance retained).
Moreover the BP/CNTs (1:4) ASSP device still retains large volumetric
capacitance (35.7 F/cm<sup>3</sup> at the scan rate of 0.005 V/s)
even after 11 months. In addition, the ASSP of BP/CNTs (1:4) exhibits
high energy density of 5.71 mWh/cm<sup>3</sup> and high power density
of 821.62 W/cm<sup>3</sup>. As indicated in our work, the strategy
of assembling stacked-layer composites films will open up novel possibility
for realizing BP and CNTs in new-concept thin-film energy storage
devices
Sulfur-Doped Black Phosphorus Field-Effect Transistors with Enhanced Stability
Black phosphorus
(BP) has drawn great attention owing to its tunable band gap depending
on thickness, high mobility, and large <i>I</i><sub>on</sub>/<i>I</i><sub>off</sub> ratio, which makes BP attractive
for using in future two-dimensional electronic and optoelectronic
devices. However, its instability under ambient conditions poses challenge
to the research and limits its practical applications. In this work,
we present a feasible approach to suppress the degradation of BP by
sulfur (S) doping. The fabricated S-doped BP few-layer field-effect
transistors (FETs) show more stable transistor performance under ambient
conditions. After exposing to air for 21 days, the charge-carrier
mobility of a representative S-doped BP FETs device decreases from
607 to 470 cm<sup>2</sup> V<sup>–1</sup> s<sup>–1</sup> (remained as high as 77.4%) under ambient conditions and a large <i>I</i><sub>on</sub>/<i>I</i><sub>off</sub> ratio of
∼10<sup>3</sup> is still retained. The atomic force microscopy
analysis, including surface morphology, thickness, and roughness,
also indicates the lower degradation rate of S-doped BP compared to
BP. First-principles calculations show that the dopant S atom energetically
prefers to chemisorb on the BP surface in a dangling form and the
enhanced stability of S-doped BP can be ascribed to the downshift
of the conduction band minimum of BP below the redox potential of
O<sub>2</sub>/O<sub>2</sub><sup>–</sup>. Our work suggests
that S doping is an effective way to enhance the stability of black
phosphorus
Strain Release Induced Novel Fluorescence Variation in CVD-Grown Monolayer WS<sub>2</sub> Crystals
Tensile
strain is intrinsic to monolayer crystals of transition
metal disulfides such as MoÂ(W)ÂS<sub>2</sub> grown on oxidized silicon
substrates by chemical vapor deposition (CVD) owing to the much larger
thermal expansion coefficient of MoÂ(W)ÂS<sub>2</sub> than that of silica.
Here we report fascinating fluorescent variation in intensity with
aging time in CVD-grown triangular monolayer WS<sub>2</sub> crystals
on SiO<sub>2</sub> (300 nm)/Si substrates and formation of interesting
concentric triangular fluorescence patterns in monolayer crystals
of large size. The novel fluorescence aging behavior is recognized
to be induced by the partial release of intrinsic tensile strain after
CVD growth and the induced localized variations or gradients of strain
in the monolayer crystals. The results demonstrate that strain has
a dramatic impact on the fluorescence and photoluminescence of monolayer
WS<sub>2</sub> crystals and thus could potentially be utilized to
tune electronic and optoelectronic properties of monolayer transition
metal disulfides
Low-Temperature Diffusion of Oxygen through Ordered Carbon Vacancies in Zr<sub>2</sub>C<sub><i>x</i></sub>: The Formation of Ordered Zr<sub>2</sub>C<sub><i>x</i></sub>O<sub><i>y</i></sub>
Investigations are performed on low-temperature oxygen
diffusion in the carbon vacancy ordered ZrC<sub>0.6</sub> and thus
induced formation of the oxygen atom ordered ZrC<sub>0.6</sub>O<sub>0.4</sub>. Theoretically, a superstructure of Zr<sub>2</sub>CO can
be constructed via the complete substitution of carbon vacancies with
O atoms in the Zr<sub>2</sub>C model. In the ordered ZrC<sub>0.6</sub>, the consecutive arrangement of vacancies forms the vacancy channels
along some zone axes in the C sublattice. Through these vacancy channels,
the thermally activated oxygen diffusion is significantly facilitated.
The oxygen atoms diffuse directly into and occupy the vacancies, producing
the ordered ZrC<sub>0.6</sub>O<sub>0.4</sub>. Relative to the ordered
ZrC<sub>0.6</sub>, the Zr positions are finely tuned in the ordered
ZrC<sub>0.6</sub>O<sub>0.4</sub> because of the ionic Zr–O
bonds. Because of this fine adjustment of Zr positions and the presence
of oxygen atoms, the superstructural reflections are always observable
in a selected area electron diffraction (SAED) pattern, despite the
invisibility of superstructural reflections in ZrC<sub>0.6</sub> along
some special zone axes. Similar to the vacancies in ordered ZrC<sub>0.6</sub>, the ordering arrangement of O atoms in the ordered ZrC<sub>0.6</sub>O<sub>0.4</sub> is in nanoscale length, thus forming the
nano superstructural domains with irregular shapes