Tuneable effects of pyrrolic N and pyridinic N on the enhanced field emission properties of nitrogen-doped graphene.

Graphene is one of the most potential field emission cathode materials and a lot of work has been carried out to demonstrate the effectiveness of nitrogen doping (N doping) for the enhancement of field emission properties of graphene. However, the effect of N doping on graphene field emission is lacking systematic and thorough understanding. In this study, undoped graphene and N-doped graphene were prepared and characterized for measurements, and the field emission property dependence of the doping content was investigated and the tuneable effect was discussed. For the undoped graphene, the turn-on field was 7.95 V µm-1 and the current density was 7.3 µA cm-2, and for the 10 mg, 20 mg, and 30 mg N-doped graphene samples, the turn-on fields declined to 7.50 V µm-1, 6.38 V µm-1, and 7.28 V µm-1, and current densities increased to 21.0 µA cm-2, 42.6 µA cm-2, and 13.2 µA cm-2, respectively. Density functional theory (DFT) calculations revealed that N doping could bring about additional charge and then cause charge aggregation around the N atom. At the same time, it also lowered the work function, which further enhanced the field emission. The doping effect was determined by the content of the pyrrolic-type N and pyridinic-type N. Pyridinic-type N is more favourable for field emission because of its smaller work function, which is in good agreement with the experimental results. This study would be of great benefit to the understanding of N doping modulation for superior field emission properties.

Synthesis of violet phosphorus with large lateral sizes to facilitate nano-device fabrications.

Violet phosphorus has been proven to be the most stable phosphorus allotrope and has attracted much attention recently. The growth of violet phosphorus with large lateral sizes is crucial to obtain good quality violet phosphorene for nanodevice fabrication. Herein, a large number of violet phosphorus plates have been produced from molten lead using an optimized method to achieve red bronze luster. The crystal structure of the as-produced violet phosphorus was determined by single-crystal X-ray diffraction to be monoclinic with the space group P2/n (13) (CSD-2160375), identical to the one from the chemical vapor transport method (CSD-1935087). The as-produced violet phosphorus plates were found to have lateral sizes of 1.30 ± 0.41 mm2. The violet phosphorus plates were easily exfoliated and directly transferred to silicon substrates to facilitate building of a back-gate field-effect transistor. A hole mobility of 2.308 cm2 V-1 s-1 was obtained from a violet phosphorus nanosheet with a thickness of 52 nm under ambient conditions. The absolute responsivity of 130 mA W-1 with a fast response time of 27 ms was also obtained under the irradiation of a 530 nm laser.

Prediction of boridenes as high-performance anodes for alkaline metal and alkaline Earth metal ion batteries.

We conducted a comprehensive density functional theory investigation using the r2SCAN-rVV10 functional on the structural stability and electrochemical properties of boridenes for their use as anode materials in rechargeable alkaline (earth) metal-ion batteries (Li+, Na+, K+, Mg2+ and Ca2+). According to first-principles molecular dynamics simulations and reaction thermodynamic calculations, Mo4/3B2(OH)2 and Mo4/3B2F2 are unstable in the presence of alkaline (earth) metal ions due to the surface-conversion reactions between the surface terminations and adsorbates. Meanwhile, the bare Mo4/3B2 and Mo4/3B2O2 monolayers not only can accommodate alkaline (earth) metal ions, but also form stable multi-layer adsorption structures for most of the studied metal ions (Li+, Na+, K+, Mg2+ and Ca2+). The predicted gravimetric capacities of the bare Mo4/3B2 monolayer (Mo4/3B2O2) are 625.9 mA h g-1 (357.3 mA h g-1), 247.20 mA h g-1 (392.1 mA h g-1), 101.8 mA h g-1 (206.4 mA h g-1), 667.0 mA h g-1, and 413.0 mA h g-1 (485.4 mA h g-1) for Li+, Na+, K+, Mg2+ and Ca2+ ions, respectively. The bare Mo4/3B2 exhibits lower onset charging open circuit voltages for alkaline (earth) metal ions than that of Mo4/3B2O2. The diffusivities of the metal ions were revealed to be high on the boridene monolayer especially for the outer fully stable adsorption layers, where the migration energy barriers were found to be less than 0.10 eV. Similar to that of MXenes, the negative electron cloud (NEC) also plays a vital role in stabilizing the observed multi-layer adsorption structures for various metal ions on either the bare Mo4/3B2 or Mo4/3B2O2 monolayer.

Vapor-Liquid-Solid Growth of Morphology-Tailorable WS2 toward P-Type Monolayer Field-Effect Transistors.

Although substantial efforts have been made, controllable synthesis of p-type WS2 remains a challenge. In this work, we employ NaCl as a seeding promoter to realize vapor-liquid-solid (VLS) growth of p-type WS2. Morphological evolution, including a one-dimensional (1D) nanowire to two-dimensional (2D) planar domain and 2D shape transition of WS2 domains, can be well-controlled by the growth temperature and sulfur introduction time. A high growth temperature is required to enable planar growth of 2D WS2, and a sulfur-rich environment is found to facilitate the growth of high-quality WS2. Raman and photoluminescence (PL) mappings demonstrate uniform crystallinity and high quantum efficiency of VLS-grown WS2. Moreover, monolayer WS2-based field-effect transistors (FETs) are fabricated, showing p-type conducting behavior, which is different from previous reported n-type FETs from WS2 grown by other methods. First-principles calculations show that the p-type behavior originates from the substitution of Na at the W site, which will form an additional acceptor level above the valence band maximum (VBM). This facile VLS growth method opens the avenue to realize the p-n WS2 homojunctions and p/n-WS2-based heterojunctions for monolayer wearable electronic, photonic, optoelectronic, and biosensing devices and should also be a great benefit to the development of 2D complementary metal-oxide-semiconductor (CMOS) circuit applications.