Field emission from nanotubes and flakes of transition metal dichalcogenides

Transition metal dichalcogenides such as MoS₂ and WS₂ are low-dimensional semiconductor materials. MoS₂ and WS₂ nanotubes and flakes were grown by a chemical transport reaction under a temperature gradient. I₂ was used as a transport agent for previously synthesized MoS₂ and WS₂, respectively. These multilayered nanotubes are indirect bandgap semiconductors with a bandgap depending on their diameter. WS₂ flakes were prepared by the sulfurization of thin WO_x flakes. To increase the field enhancement of such low-dimensional structures by a higher aspect ratio, two approaches were examined: (a) the MoS₂ and WS₂ nanotubes were attached individually by a focused ion beam with Pt on dry etched n-type Si pillars and (b) the WS₂ flakes were grown directly on the surface of the (n-type and p-type) Si pillars. Integral field emission measurements were performed in a diode configuration with a 50 μm mica spacer in a vacuum chamber at pressures of about 10⁻⁹ mbar. At a voltage of 900 V (18 MV/m), the integral emission current from the nanotubes is up to 11 μA for the lateral mounted MoS₂ and about 1.3 μA (1.0 μA) for the upright mounted WS₂ (MoS₂). The onset voltage for a current of 1 nA is about 550 V for MoS₂ and 500 V for WS₂, respectively. The voltage conversion factor is in the range of 6 × 10⁴–8 × 10⁴ cm⁻¹ for the nanotubes. The mounted MoS₂ flakes show a field emission current of about 6 μA at 18 MV/m in contrast to the directly grown WS₂ flakes, which show a pronounced saturation regime and, therefore, a lower emission current of about 0.5 μA is reached at 1500 V (25 MV/m). The WS₂ flakes show a two times higher (1 × 10⁵ cm⁻¹) voltage conversion factor in comparison to the MoS₂ flakes (5 × 10⁴ cm⁻¹). The extracted characteristics of the current-limiting part show a difference in the behavior of the extracted current-limiting characteristics between the lateral (linear) and upright mounted (exponential) nanotubes and the MoS₂ flakes. In contrast, the WS₂ flakes show charge carrier depletion effects.