The electrical performance of suspended few-layer MoTe2 field-effect-transistors with ionic liquid gating has been investigated. The suspended structure not only enhances the mobility of MoTe2 by removing the influence of the substrate but also allows ions to accumulate on both the top and the bottom surface of MoTe2. The consequent increase of the gate capacitance resulted in an improved subthreshold swing (~73 mV/dec) and on-off ratio (10 6 ) at room temperature for suspended MoTe2 compared to substrate-supported devices. Suspended transistors with ionic liquid gating enable larger charge density compared to ionic liquid gated supported devices, and may provide a useful platform to study screening physics in 2D materials. Two-dimensional transition metal dichalcogenide compound (TMDC) materials have a variety of properties depending on the crystal structure and constituent elements and attract broad interests for applications in nanoelectronics and optics. 1,2 Among them, MoTe2 has received increasing attention owing to its low phase transition barrier 3,4 and its sizeable bandgap close to that of Si. [5][6][7] MoTe2 has an indirect electronic bandgap of 0.88 increasing to 1.0 eV going from bulk to few-layer, with a direct bandgap of about 1.1 eV for the monolayer. [7][8][9][10][11] Field-effect-transistors (FETs) based on α-MoTe2 have been reported 8,12-20 and applied to logic circuits 21 and sensors. 19,22 However, much lower mobilities have been reported than the theoretically predicted phonon-limited mobility at room temperature of ~240 cm 2 V -1 s -1 for bulk, 23 and ~100-2500 cm 2 V -1 s -1 for monolayer, 10,24 mainly due to charged traps at the MoTe2/substrate interface and Schottky barriers at the MoTe2/metal contacts. 13 High-κ screening can reduce the influence of the charged traps, and an enhanced electron mobility (80 cm 2 V -1 s -1 at room temperature) has been reported for a MoTe2 device capped with Al2O3. 15 On the other hand, ionic liquid (IL) gating is an attractive doping method for TMDC because of the high gate capacitance of the electric double layer (EDL) formed on the sample surface. Heavy doping of the semiconductor can result in the reduction of the effective Schottky barrier, and in addition the high dielectric constant of ionic liquids can improve the performance of MoTe2 FETs as shown by previous studies on substrate-supported multilayer devices. 8,25

MoS2 nanoscrolls that have inner core radii of ∼250 nm are generated from MoS2 monolayers, and the optical and transport band gaps of the nanoscrolls are investigated. Photoluminescence spectroscopy reveals that a MoS2 monolayer, originally a direct gap semiconductor (∼1.85 eV (optical)), changes into an indirect gap semiconductor (∼1.6 eV) upon scrolling. The size of the indirect gap for the MoS2 nanoscroll is larger than that of a MoS2 bilayer (∼1.54 eV), implying a weaker interlayer interaction between concentric layers of the MoS2 nanoscroll compared to Bernal-stacked MoS2 few-layers. Transport measurements on MoS2 nanoscrolls incorporated into ambipolar ionic-liquid-gated transistors yielded a band gap of ∼1.9 eV. The difference between the transport and optical gaps indicates an exciton binding energy of 0.3 eV for the MoS2 nanoscrolls. The rolling up of 2D atomic layers into nanoscrolls introduces a new type of quasi-1D nanostructure and provides another way to modify the band gap of 2D materials.

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