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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 10(7) and mobilities up to 66 cm(2)/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.

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.

MonolayerTwo dimensional (2D) semiconducting transition metal dichalcogenides (TMDCs) have ignited substantial scientific interest in the past few years owning to their unique structures and properties, which satisfy requests of many electronic devices. One of the primary advantages of field effect transistors (FETs) based on atomically thin TMDCs is the absence of dangling bonds on the top and bottom surfaces of these 3 materials, which prevent devices from performance degradation due to interface states. [1][2][3][4] In addition, the ultrathin body of TMDCs can result in a strong electrostatic gating of these materials, making them attractive for electronics and optoelectronics. 5 Another advantage for TMDCs is that by tuning compositions, number of layers, and strains, scientists can modify the electronic properties of these materials. [6][7][8][9][10][11][12][13][14][15] Accordingly, direct or indirect bandgap materials, and n-type, p-type or ambipolar FET devices, have been fabricated. 2,8,[15][16][17][18][19][20] The enhanced spin-orbit coupling in TMDCs also makes it possible to use spin and valley information of carriers for novel concept devices. 21 Meanwhile, investigations of monolayer TMDCs have also indicated new physics like many-body quasiparticles including trions and biexcitons, which have no analogue in bulk semiconductors. 22 Thanks to these extraordinary merits, devices fabricated from either mechanically-exfoliated or vapor-phase-grown TMDCs have been proven to show interesting electronic, optical, catalytic, and energy storage properties. 2,13,17,23,24 Among all TMDCs, semiconducting MoS2 is the one which has received the most attention while research interest in WSe2 has shown rapid increase recently. 4,13 Compared with monolayer MoS2, monolayer WSe2 possesses a smaller bandgap (~1.6 eV in monolayer WSe2 and ~1.8 eV in monolayer MoS2), and in average, the field effect mobilities are higher in WSe2 FETs than in MoS2 FETs. Recent experiments have also showed that both n-type FETs, 18 p-type FETs, 16 as well as complementary inverters 17 were fabricated based on WSe2, which has been proven to be difficult in MoS2 due to Fermi level pinning effects. Therefore, 4 from the aforementioned comparisons, WSe2 appears promising for future FET applications.Because of large amount of surface atoms, TMDCs and other 2D materials have shown to be very susceptible to environments. [25][26][27][28] This feature benefits their applications in chemical and biomedicine sensing, but on the other hand, raises a crucial concern about stability of TMDC devices. Recent results have indicated that TMDCs can be oxidized at moderate temperatures in oxygen or ozone environment, leading to the formation of metal oxides which have distinct electronic properties with TMDC themselves. [29][30][31][32] Therefore, the stability of TMDC devices in ambient air is of particular importance concerning the practical use of these materials. Moreover, recent reports also indicate that properties of TMDCs are tunable, by mea...