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Plasma bullets in atmospheric pressure plasma jets have drawn much scholarly attention in the past decade, especially its shape. This Letter reports the formation and evolution mechanism of a solid-in-hollow structured bullet. At high pulse repetition frequency (≥20 kHz) and long rising time (≥200 ns), the traditional hollow ring bullet turns into the solid-in-hollow structure, and the plasma plume correspondingly appears as a purple external plume with an extra internal filamentary red core under bare eyes. An image process method based on color recognition and decoupling is designed to separate the images of the external plume and the internal filamentary core and is then used to analyze the discharge mechanism together with the intensified charge-coupled device imaging and spectrum measurement. Analyses reveal that the dominant ionization sources for external ring and internal core are N2 and He, respectively, and the internal discharge can be enhanced by increasing the pulse repetition frequency, which might be explained by the electron oscillating heating in high frequency. Finally, the evolution mechanism of the internal filamentary core is studied, and the internal discharge is initially a hollow ring as well when it just exits the tube end, making the bullet a hollow-in-hollow structure, but quickly contracts into a solid core due to the Coulomb repulsion between the two plasma rings. [ABSTRACT FROM AUTHOR]

Heterojunctions and multi-gated structures facilitate the fabrication of high-performance and multifunctional transistors. Here, a WSe2/MoS2 heterojunction structure transistor with a back gate and two top gates is proposed. The back gate controls the carrier transport of the entire heterojunction channel, and the top gates independently control the carrier transports of MoS2 or WSe2 channels. The rectification direction of the heterojunction device could be reversed, and the rectification ratio could be modulated from 10−4 to 104 by changing the back-gate voltage. In addition, an evident negative-differential transconductance phenomenon with a current peak and a current valley are observed in the back-gate transfer characteristic curve, which results from the different control ability of the same gate voltage to the Fermi levels in MoS2 and WSe2. The current peak can be obviously modulated and eliminated by the MoS2 top gate, while the WSe2 top gate can control the position of the current valley from −8 to +12 V, which clearly supports the heterostructure energy band model. Moreover, the diversity of output states under multi-gate modulation makes applications in logic circuits possible. These results demonstrate the potential of this approach for the development of next-generation electronic functional devices. [ABSTRACT FROM AUTHOR]