7 research outputs found
Room-Temperature Pressure Synthesis of Layered Black PhosphorusāGraphene Composite for Sodium-Ion Battery Anodes
Sodium-ion
batteries offer an attractive option for grid-level
energy storage due to the high natural abundance of sodium and low
material cost of sodium compounds. Phosphorus (P) is a promising anode
material for sodium-ion batteries, with a theoretical capacity of
2596 mAh/g. The red phosphorus (RP) form has worse electronic conductivity
and lower initial Coulombic efficiency than black phosphorus (BP),
but high material cost and limited production capacity have slowed
the development of BP anodes. To address these challenges, we have
developed a simple and scalable method to synthesize layered BP/graphene
composite (BP/rGO) by pressurization at room temperature. A carbon-black-free
and binder-free BP/rGO anode prepared with this method achieved specific
charge capacities of 1460.1, 1401.2, 1377.6, 1339.7, 1277.8, 1123.78,
and 720.8 mAh/g in a rate capability test at charge and discharge
current densities of 0.1, 0.5, 1, 5, 10, 20, and 40 A/g, respectively.
In a cycling performance test, after 500 deep cycles, the capacity
of BP/rGO anodes stabilized at 1250 and 640 mAh/g at 1 and 40 A/g,
respectively, which marks a significant performance improvement for
sodium-ion battery anodes
Hierarchical Carbon-Coated Ball-Milled Silicon: Synthesis and Applications in Free-Standing Electrodes and High-Voltage Full Lithium-Ion Batteries
Lithium-ion batteries
have been regarded as one of the most promising
energy storage devices, and development of low-cost batteries with
high energy density is highly desired so that the cost per watt-hour
($/Wh) can be minimized. In this work, we report using ball-milled
low-cost silicon (Si) as the starting material and subsequent carbon
coating to produce low-cost hierarchical carbon-coated (HCC) Si. The
obtained particles prepared from different Si sources all show excellent
cycling performance of over 1000 mAh/g after 1000 cycles. Interestingly,
we observed <i>in situ</i> formation of porous Si, and it
is well confined in the carbon shell based on postcycling characterization
of the hierarchical carbon-coated metallurgical Si (HCC-M-Si) particles.
In addition, lightweight and free-standing electrodes consisting of
the HCC-M-Si particles and carbon nanofibers were fabricated, which
achieved 1015 mAh/g after 100 cycles based on the total mass of the
electrodes. Compared with conventional electrodes, the lightweight
and free-standing electrodes significantly improve the energy density
by 745%. Furthermore, LiCoO<sub>2</sub> and LiNi<sub>0.5</sub>Mn<sub>1.5</sub>O<sub>4</sub> cathodes were used to pair up with the HCC-M-Si
anode to fabricate full cells. With LiNi<sub>0.5</sub>Mn<sub>1.5</sub>O<sub>4</sub> as cathode, an energy density up to 547 Wh/kg was achieved
by the high-voltage full cell. After 100 cycles, the full cell with
a LiNi<sub>0.5</sub>Mn<sub>1.5</sub>O<sub>4</sub> cathode delivers
46% more energy density than that of the full cell with a LiCoO<sub>2</sub> cathode. The systematic investigation on low-cost Si anodes
together with their applications in lightweight free-standing electrodes
and high-voltage full cells will shed light on the development of
high-energy Si-based lithium-ion batteries for real applications
Synthesis, Characterization, and Device Application of Antimony-Substituted Violet Phosphorus: A Layered Material
Two-dimensional
(2D) nanoflakes have emerged as a class of materials
that may impact electronic technologies in the near future. A challenging
but rewarding work is to experimentally identify 2D materials and
explore their properties. Here, we report the synthesis of a layered
material, P<sub>20.56(1)</sub>Sb<sub>0.44(1)</sub>, with a systematic
study on characterizations and device applications. This material
demonstrates a direct band gap of around 1.67 eV. Using a laser-cutting
method, the thin flakes of this material can be separated into multiple
segments. We have also fabricated field effect transistors based on
few-layer P<sub>20.56(1)</sub>Sb<sub>0.44(1)</sub> flakes with a thickness
down to a few nanometers. Interestingly, these field effect transistors
show strong photoresponse within the wavelength range of visible light.
At room temperature, we have achieved good mobility values (up to
58.96 cm<sup>2</sup>/VĀ·s), a reasonably high on/off current ratio
(ā¼10<sup>3</sup>), and intrinsic responsivity up to 10 Ī¼A/W.
Our results demonstrate the potential of P<sub>20.56(1)</sub>Sb<sub>0.44(1)</sub> thin flakes as a two-dimensional material for applications
in visible light detectors
High-Performance WSe<sub>2</sub> Field-Effect Transistors <i>via</i> Controlled Formation of In-Plane Heterojunctions
Monolayer
WSe<sub>2</sub> is a two-dimensional (2D) semiconductor with a direct
band gap, and it has been recently explored as a promising material
for electronics and optoelectronics. Low field-effect mobility is
the main constraint preventing WSe<sub>2</sub> from becoming one of
the competing channel materials for field-effect transistors (FETs).
Recent results have demonstrated that chemical treatments can modify
the electrical properties of transition metal dichalcogenides (TMDCs),
including MoS<sub>2</sub> and WSe<sub>2</sub>. Here, we report that
controlled heating in air significantly improves device performance
of WSe<sub>2</sub> FETs in terms of on-state currents and field-effect
mobilities. Specifically, after being heated at optimized conditions,
chemical vapor deposition grown monolayer WSe<sub>2</sub> FETs showed
an average FET mobility of 31 cm<sup>2</sup>Ā·V<sup>ā1</sup>Ā·s<sup>ā1</sup> and on/off current ratios up to 5 Ć
10<sup>8</sup>. For few-layer WSe<sub>2</sub> FETs, after the same
treatment applied, we achieved a high mobility up to 92 cm<sup>2</sup>Ā·V<sup>ā1</sup>Ā·s<sup>ā1</sup>. These values
are significantly higher than FETs fabricated using as-grown WSe<sub>2</sub> flakes without heating treatment, demonstrating the effectiveness
of air heating on the performance improvements of WSe<sub>2</sub> FETs.
The underlying chemical processes involved during air heating and
the formation of in-plane heterojunctions of WSe<sub>2</sub> and WO<sub>3ā<i>x</i></sub>, which is believed to be the reason
for the improved FET performance, were studied by spectroscopy and
transmission electron microscopy. We further demonstrated that, by
combining the air heating method developed in this work with supporting
2D materials on the BN substrate, we achieved a noteworthy field-effect
mobility of 83 cm<sup>2</sup>Ā·V<sup>ā1</sup>Ā·s<sup>ā1</sup> for monolayer WSe<sub>2</sub> FETs. This work is
a step toward controlled modification of the properties of WSe<sub>2</sub> and potentially other TMDCs and may greatly improve device
performance for future applications of 2D materials in electronics
and optoelectronics
Reversible Semiconducting-to-Metallic Phase Transition in Chemical Vapor Deposition Grown Monolayer WSe<sub>2</sub> and Applications for Devices
Two-dimensional (2D) semiconducting monolayer transition metal dichalcogenides (TMDCs) have stimulated lots of interest because they are direct bandgap materials that have reasonably good mobility values. However, contact between most metals and semiconducting TMDCs like 2H phase WSe<sub>2</sub> are highly resistive, thus degrading the performance of field effect transistors (FETs) fabricated with WSe<sub>2</sub> as active channel materials. Recently, a phase engineering concept of 2D MoS<sub>2</sub> materials was developed, with improved device performance. Here, we applied this method to chemical vapor deposition (CVD) grown monolayer 2H-WSe<sub>2</sub> and demonstrated semiconducting-to-metallic phase transition in atomically thin WSe<sub>2</sub>. We have also shown that metallic phase WSe<sub>2</sub> can be converted back to semiconducting phase, demonstrating the reversibility of this phase transition. In addition, we fabricated FETs based on these CVD-grown WSe<sub>2</sub> flakes with phase-engineered metallic 1T-WSe<sub>2</sub> as contact regions and intact semiconducting 2H-WSe<sub>2</sub> as active channel materials. The device performance is substantially improved with metallic phase source/drain electrodes, showing on/off current ratios of 10<sup>7</sup> and mobilities up to 66 cm<sup>2</sup>/VĀ·s for monolayer WSe<sub>2</sub>. These results further suggest that phase engineering can be a generic strategy to improve device performance for many kinds of 2D TMDC materials
Red Phosphorus Nanodots on Reduced Graphene Oxide as a Flexible and Ultra-Fast Anode for Sodium-Ion Batteries
Sodium-ion batteries
offer an attractive option for potential low
cost and large scale energy storage due to the earth abundance of
sodium. Red phosphorus is considered as a high capacity anode for
sodium-ion batteries with a theoretical capacity of 2596 mAh/g. However,
similar to silicon in lithium-ion batteries, several limitations,
such as large volume expansion upon sodiation/desodiation and low
electronic conductance, have severely limited the performance of red
phosphorus anodes. In order to address the above challenges, we have
developed a method to deposit red phosphorus nanodots densely and
uniformly onto reduced graphene oxide sheets (P@RGO) to minimize the
sodium ion diffusion length and the sodiation/desodiation stresses,
and the RGO network also serves as electron pathway and creates free
space to accommodate the volume variation of phosphorus particles.
The resulted P@RGO flexible anode achieved 1165.4, 510.6, and 135.3
mAh/g specific charge capacity at 159.4, 31878.9, and 47818.3 mA/g
charge/discharge current density in rate capability test, and a 914
mAh/g capacity after 300 deep cycles in cycling stability test at
1593.9 mA/g current density, which marks a significant performance
improvement for red phosphorus anodes for sodium-ion chemistry and
flexible power sources for wearable electronics
High-Performance Sub-Micrometer Channel WSe<sub>2</sub> Field-Effect Transistors Prepared Using a FloodāDike Printing Method
Printing technology has potential
to offer a cost-effective and
scalable way to fabricate electronic devices based on two-dimensional
(2D) transition metal dichalcogenides (TMDCs). However, limited by
the registration accuracy and resolution of printing, the previously
reported printed TMDC field-effect transistors (FETs) have relatively
long channel lengths (13ā200 Ī¼m), thus suffering low
current-driving capabilities (ā¤0.02 Ī¼A/Ī¼m). Here,
we report a āfloodādikeā self-aligned printing
technique that allows the formation of source/drain metal contacts
on TMDC materials with sub-micrometer channel lengths in a reliable
way. This self-aligned printing technique involves three steps: (i)
printing of gold ink on a WSe<sub>2</sub> flake to form the first
gold electrode, (ii) modifying the surface of the first gold electrode
with a self-assembled monolayer (SAM) to lower the surface tension
and render the surface hydrophobic, and (iii) printing of gold ink
close to the SAM-treated first electrode at a certain distance. During
the third step, the gold ink would first spread toward the edge of
the first electrode and then get stopped by the hydrophobic SAM coating,
ending up forming a sub-micrometer channel. With this printing technique,
we have successfully downscaled the channel length to ā¼750
nm and achieved enhanced on-state current densities of ā¼0.64
Ī¼A/Ī¼m (average) and high on/off current ratios of ā¼3
Ć 10<sup>5</sup> (average). Furthermore, with our high-performance
printed WSe<sub>2</sub> FETs, driving capabilities for quantum-dot
light-emitting diodes (LEDs), inorganic LEDs, and organic LEDs have
been demonstrated, which reveals the potential of using printed TMDC
electronics for display backplane applications