Field emission from nanotubes and flakes of transition metal dichalcogenides

Abstract

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 WOx 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.

Confinement Related Phenomena in MoS2 Tubular Structures Grown from Vapour Phase

“…The results were in good agreement with electrostatic finite element method simulations using COMSOL. [15] As expected from the geometry, a much larger value S for the emission area is obtained for the lateral nanowire (S = 5.5 • 10 À 12 cm 2 ) compared and lateral (blue circles) MoS 2 nanotubes and regression curves using the FN-model for field emission of electrons (solid lines). The regression curve for the lateral nanotube (blue solid line) perfectly fits the measurement data, whereas in the vertical arrangement, a strong deviation occurs at higher voltages (red solid line).…”

“…In order to investigate the field emission properties of nanotubes, they can be mounted on several micrometres' high silicon columns, which can be pre-prepared on highly conductive n-Si substrates. [15] Such a geometry ensured that the measured field emission was actually from the nanowire located there and not from other objects located on the substrate surface, e. g. scratches or particles. The silicon column was flattened at the highest point, so that no significant field emission occurred there even at voltages of 1000 V or higher, and only after the nanotubes were mounted, a measurable field emission current was recorded.…”

“…This arrangement was used to study the field emission properties of the MoS 2 structures. [15] Two directions of the tubes with regards to electric field were tested, parallel with emission from end of the nanotube, and perpendicular with emission from the lateral surface of another nanotube (Figure 5). [15] The samples were prepared inside the FEI Helios Nanolab 650 FIB SEM system.…”

“…Recently, interesting phenomena were observed in these tubular structures such as single electron conductance [12] and appearance of whispering gallery modes. [13] In addition, the designs of field-effect transistors [14] and field emitters [15] based on MoS 2 tubes were demonstrated. These recent discoveries revitalize research of MoS 2 curved structures grown from vapour phase several decades ago.…”

Fabrication of Eutectic Ga-In Nanowire Arrays Based on Plateau–Rayleigh Instability

“…In order to further expand the range of applications, it is necessary to improve the shape and position controllability. If EGaIn can be fabricated into quasi-one-dimensional anisotropic nanowires (NWs) with a sub-micron diameter, and if the NWs can be arranged in two-dimensional arrays on a flat substrate surface, they can be used for wiring in integrated circuits, tips for scanning probe microscopes [ 16 , 17 , 18 ], and field electron emission devices [ 19 ]. Previously, Ladd et al fabricated EGaIn microwires with a diameter of approximately 200 μm by changing the distance between a syringe and a stage while injecting liquid metal into the stage from the syringe [ 20 ].…”