Semiconducting molybdenum ditelluride (2H-MoTe 2 ), a fast-emerging 2D material with an appropriate band gap and decent carrier mobility, is configured as field-effect transistors and is the focus of substantial research interest, showing hole-dominated ambipolar characteristics. Here, carrier modulation of ambipolar few-layer MoTe 2 transistors is demonstrated utilizing magnesium oxide (MgO) surface charge transfer doping. By carefully adjusting the thickness of MgO film and the number of MoTe 2 layers, the carrier polarity of MoTe 2 transistors from p-type to n-type can be reversely controlled. The electron mobility of MoTe 2 is significantly enhanced from 0.1 to 20 cm 2 V −1 s −1 after 37 nm MgO film doping, indicating a greatly improved electron transport. The effective carrier modulation enables to achieve high-performance complementary inverters with high DC gain of >25 and photodetectors based on few-layer MoTe 2 flakes. The results present an important advance toward the realization of electronic and optoelectronic devices based on 2D transition-metal dichalcogenide semiconductors.

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Phase evolution, sintering behavior, and microwave dielectric properties of compounds within the Bi 2 O 3 -B 2 O 3 phase diagram have been investigated. Two promising ceramics, Bi 6 B 10 O 24 and Bi 4 B 2 O 9 , were fabricated which densified at 700°C and 660°C with permittivity, e r = 10 and 39, microwave quality factor, Qf 0 = 10800 and 2600 GHz, and temperature coefficient of resonant frequency, τ f = À41 and À203 ppm/ o C, respectively. The unusually large -ve τ f , high e r and reasonable values of Qf 0 of Bi 4 B 2 O 9 coupled with its low sintering temperature suggested that it may be useful in the fabrication of temperature-stable composites with high e r , +ve τ f end members. Composites were, therefore, fabricated of xBi 4 B 2 O 9 with (1Àx)BaNd 2 Ti 4 O 12 (xBB-BNT), and (1Àx)[0.2CaTiO 3 À0.8(Li 0.5 Nd 0.5 )TiO 3 ] (xBB -CTLNT), respectively (x is in wt%). In each case, the sintering temperature to achieve > 95% theoretical density was reduced by ! 150°C to 1200°C. Optimum properties were achieved for 0.075BB-BNT with e r = 86, Qf 0 = 5400 GHz, and τ f = 4 ppm/°C, and for 0.04BB-CTLNT with e r = 127, Qf 0 = 2700 GHz, and τ f = À4 ppm/°C. Unusually for composites, Qf 0 did not significantly deteriorate and e r increased markedly with respect to the high-permittivity end-member for composites with x < 0.1. Optimized compositions are suitable for applications in dielectrically loaded antennas and in C0G high-frequency filter applications. C. A. Randall-contributing editor

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