Fabrication of MoS2 Nanotube Based Humidity Sensors and Investigation of Their Electrode Formation Position
Authors: Matsumura Tana , Hirono Shoichi , Ito Takeshi , Shingubara Shoso , Shimizu Tomohiro 
Published: December 2024

Transition metal dichalcogenides (TMDCs), which have a layered thin crystal structure, mechanical strength, and excellent flexibility, have attracted attention as a new material for electronic devices. Molybdenum disulfide (MoS2), a type of transition metal dichalcogenide, is a flexible and chemically and thermally stable material with a band gap of about 1.8 eV in a single layer. MoS2 is also expected to be a flexible and sensitive material for wearable humidity sensor due to its inherent defects and excellent humidity response. To date, several humidity sensors using MoS2 thin films have been reported, but they all suffer from a narrow relative humidity range, long response and recovery times, and low sensitivity. The general working principle of humidity sensors is adsorption/desorption, and one approach to improve the sensitivity is to physically increase the contact surface area for adsorption/desorption by structure. In this study, we attempted to fabricate MoS2 nanostructures to improve the sensitivity of MoS2-based humidity sensors. Furthermore, the humidity response of MoS2 nanotubes was evaluated by comparing the humidity response of the nanotubes in the plane direction and in the perpendicular direction (longitudinal direction of the tube) with the electrodes attached in different ways.
An anodic aluminum oxide (AAO) template with a through-hole structure was first prepared by applying 40 V for 3 hours using 0.3 M aqueous oxalic acid solution as the first anode oxidation, followed by AAO selective etching to obtain hole traces. The voltage was then applied for 24 hours under the same conditions as the second anodic oxidation. The hole diameter was increased by widening, and the Al and barrier layers were removed to obtain the through-hole structure. In this study, an AAO through-hole template with a diameter of about 50 nm and a depth of about 40 μm was prepared. In order to coat the MoS2 precursor solution in the AAO template, MoS2 nanotubes were prepared by repeated decompression filtration, followed by a two-step heat treatment.
From the scanning electron microscope (SEM) observation of AAO nanoholes before decompression filtration, the average diameter was about 50 nm. The average diameters after 4 and 8 cycles of decompression filtration were about 45 nm and 30 nm, respectively. It is estimated that a MoS2 film of about 2.5 nm was deposited after 4 cycles of decompression filtration, and about 10 nm was deposited after 8 cycles of decompression filtration. The thickness of the MoS2 film also increases with the number of cycles of decompression filtration, indicating that the thickness of the MoS2 layer can be controlled by the number of the cycles.
We placed two different pairs of electrodes on the AAO surface and on the back of the AAO surface so that electrical measurements could be made both in the plane direction and in the perpendicular direction (longitudinal direction of the tube). We performed humidity response measurements in each of the two different electrode pairs, respectively. As a result, it was found that the humidity response in the perpendicular direction was about 6.8 times higher than that in the in-plane direction. In addition, compared with the humidity sensor without nanostructure of MoS2 thin film prepared using the same precursor, the humidity response range is wider and the response is about 11 times higher, indicating that the nanostructured sensor is expected to have higher sensitivity. In the presentation, the detailed structure, electrical properties, and humidity response will be discussed.

Figure 1