Tailored Langmuir-Schaefer Deposition of Few-Layer MoS2 Nanosheet Films for Electronic Applications.

Few-layer MoS2 films stay at the forefront of current research of two-dimensional materials. At present, continuous MoS2 films are prepared by chemical vapor deposition (CVD) techniques. Herein, we present a cost-effective fabrication of the large-area spatially uniform films of few-layer MoS2 flakes using a modified Langmuir-Schaefer technique. The compression of the liquid-phase exfoliated MoS2 flakes on the water subphase was used to form a continuous layer, which was subsequently transferred onto a submerged substrate by removing the subphase. After vacuum annealing, the electrical sheet resistance dropped to a level of 10 kΩ/sq, being highly competitive with that of CVD-deposited MoS2 nanosheet films. In addition, a consistent fabrication protocol of the large-area conductive MoS2 films was established. The morphology and electrical properties predetermine these films to advanced detecting, sensing, and catalytic applications. A large number of experimental techniques were used to characterize the exfoliated few-layer MoS2 flakes and to elucidate the formation of the few-layer MoS2 Langmuir film.

An elevated concentration of MoS2 lowers the efficacy of liquid-phase exfoliation and triggers the production of MoOx nanoparticles.

It is generally accepted that liquid-phase exfoliation (LPE) enables large-scale production of few-layer MoS2 flakes. In our work, we studied in detail few-layer MoS2 oxidation in the course of standard LPE in a water/ethanol solution. We demonstrate that an increase of the initial MoS2 concentration above a certain threshold triggers a pronounced oxidation and the exfoliation process starts to produce MoOx nanoparticles. A subsequent decrease of the water pH along with an increased content of SO42- suggests an oxidation scenario of few-layer MoS2 oxidation towards MoOx nanoparticles. Moreover, the lowered pH leads to agglomeration and sedimentation of the few-layer MoS2 flakes, which significantly lowers their production yield. We employed a large number of physico-chemical techniques to study the MoS2-to-MoOx transformation and found a threshold value of 10 mg ml-1 of the initial MoS2 concentration to trigger this transformation.