Room-Temperature Pressure Synthesis of Layered Black Phosphorus–Graphene 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, LiCoO₂ and LiNi_0.5Mn_1.5O₄ cathodes were used to pair up with the HCC-M-Si anode to fabricate full cells. With LiNi_0.5Mn_1.5O₄ 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_0.5Mn_1.5O₄ cathode delivers 46% more energy density than that of the full cell with a LiCoO₂ 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_20.56(1)Sb_0.44(1), 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_20.56(1)Sb_0.44(1) 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 cm²/V·s), a reasonably high on/off current ratio (∼10³), and intrinsic responsivity up to 10 μA/W. Our results demonstrate the potential of P_20.56(1)Sb_0.44(1) thin flakes as a two-dimensional material for applications in visible light detectors

High-Performance WSe₂ Field-Effect Transistors <i>via</i> Controlled Formation of In-Plane Heterojunctions

Monolayer WSe₂ 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 WSe₂ 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 MoS₂ and WSe₂. Here, we report that controlled heating in air significantly improves device performance of WSe₂ FETs in terms of on-state currents and field-effect mobilities. Specifically, after being heated at optimized conditions, chemical vapor deposition grown monolayer WSe₂ FETs showed an average FET mobility of 31 cm²·V^–1·s^–1 and on/off current ratios up to 5 × 10⁸. For few-layer WSe₂ FETs, after the same treatment applied, we achieved a high mobility up to 92 cm²·V^–1·s^–1. These values are significantly higher than FETs fabricated using as-grown WSe₂ flakes without heating treatment, demonstrating the effectiveness of air heating on the performance improvements of WSe₂ FETs. The underlying chemical processes involved during air heating and the formation of in-plane heterojunctions of WSe₂ and WO_3–<i>x</i>, 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 cm²·V^–1·s^–1 for monolayer WSe₂ FETs. This work is a step toward controlled modification of the properties of WSe₂ 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 WSe₂ 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 WSe₂ are highly resistive, thus degrading the performance of field effect transistors (FETs) fabricated with WSe₂ as active channel materials. Recently, a phase engineering concept of 2D MoS₂ materials was developed, with improved device performance. Here, we applied this method to chemical vapor deposition (CVD) grown monolayer 2H-WSe₂ and demonstrated semiconducting-to-metallic phase transition in atomically thin WSe₂. We have also shown that metallic phase WSe₂ can be converted back to semiconducting phase, demonstrating the reversibility of this phase transition. In addition, we fabricated FETs based on these CVD-grown WSe₂ flakes with phase-engineered metallic 1T-WSe₂ as contact regions and intact semiconducting 2H-WSe₂ as active channel materials. The device performance is substantially improved with metallic phase source/drain electrodes, showing on/off current ratios of 10⁷ and mobilities up to 66 cm²/V·s for monolayer WSe₂. 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 WSe₂ Field-Effect Transistors Prepared Using a Flood–Dike 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 (13–200 μm), thus suffering low current-driving capabilities (≤0.02 μA/μm). Here, we report a “flood–dike” 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 WSe₂ 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 ∼750 nm and achieved enhanced on-state current densities of ∼0.64 μA/μm (average) and high on/off current ratios of ∼3 × 10⁵ (average). Furthermore, with our high-performance printed WSe₂ 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