MoS2 nanosheets via electrochemical lithium-ion intercalation under ambient conditions

Abstract

Two-dimensional (2D) transition metal dichalcogenides (TMDs) are continuously attracting attention for both fundamental studies and technological applications. The physical and chemical properties of ultrathin TMD sheets are extraordinarily different from those of the corresponding bulk materials and for this reason their production is a stimulating topic, especially when the preparation method enables to obtain a remarkable yield of nanosheets with large area and high quality. Herein, we present a fast (<1 hour) electrochemical exfoliation of molybdenum disulfide (MoS 2) via lithium-ion intercalation, by using a solution of lithium chloride in dimethyl sulfoxide (DMSO). Unlike the conventional intercalation methods based on dangerous organolithium compounds, our approach leads to the possibility to obtain mono-, bi-and tri-layer thick MoS 2 nanosheets with a large fraction of the semiconducting 2H phase (~60%), as estimated by X-ray photoelectron spectroscopy (XPS). The electrical properties of the exfoliated material were investigated through the fabrication and characterization of back-gated field-effect transistors (FETs) based on individual MoS 2 nanosheets. As-fabricated devices displayed unipolar semiconducting behavior (n-type) with field-effect mobility µ FE ≤ 10-3 cm 2 V-1 s-1 and switching ratio I on /I off ≤ 10, likely limited by 1T/2H polymorphism and defects (e.g. sulfur vacancies) induced during the intercalation/exfoliation process. A significant enhancement of the electrical performances could be achieved through a combination of vacuum annealing (150 °C) and sulfur-vacancy healing with vapors of short-chain alkanethiols, resulting in µ FE up to 2×10-2 cm 2 V-1 s-1 and I on /I off ≈ 100. Our results pave the way towards the fast preparation ─ under ambient conditions ─ of semiconducting MoS 2 nanosheets, suitable for application in low cost (opto-)electronic devices.

Emerging 2D Materials Produced via Electrochemistry

“…A series of TMDs, such as MoS 2 , WS 2 , TiS 2 , TaS 2 , ZrS 2 , NbSe 2 , and WSe 2 , could be produced by this methodology and a large fraction of the exfoliated sheets were monolayers . To overcome the limited production from coin‐shaped batteries, in 2018, Garah et al achieved the scalable exfoliation of MoS 2 in a mixed solution of lithium chloride in DMSO . The exfoliated materials were rich in mono‐, bi‐ and trilayers and 60% of the sheets were semiconducting 2H‐MoS 2 .…”

“…However, lithium intercalation constantly transforms TMDs from original semiconducting phase to a distorted metallic phase . The mixed lattice structures resulted in significantly low mobility (e.g., 0.02 cm 2 V −1 s −1 ) and on/off ratio (e.g., 100), being a drawback for high‐performance electronic devices …”

Tailoring the physicochemical properties of solution-processed transition metal dichalcogenides via molecular approaches

“…Nevertheless, in order to tackle the costs concerning the use of lithium foils, new techniques exploiting different lithium sources, such as electrolytes, 85 have been explored. By taking advantage of that, the electrochemical exfoliation can be performed using aqueous solutions 86 and under ambient conditions 87 , thus representing an appealing and cutting-edge technique to produce thin TMD nanosheets.…”

“…121 Nevertheless, some works have also been published about the functionalization of substrate-supported solution-processed TMD nanosheets. 87 To date, the majority of works reported in Please do not adjust margins Please do not adjust margins literature envisages treating the exfoliated materials with the functionalizing molecular systems after the exfoliation steps.…”

“…In particular, transition metal disulfide have been extensively studied and processed with dithiolane derivatives and thiolated molecules, in order to produce hybrid systems characterized by new or enhanced physicochemical properties and superior performances, for applications spanning from biosensing 122 to electronics. 87 Nevertheless, the sulfur healing phenomenon is not always the most favorable process. Indeed, McDonald and co-workers demonstrated that the disappearance of the S-H vibration in the IR spectra, considered the main proof of the functionalization reaction, can also be due to a TMD-mediated dimerization of thiolated molecules to form disulfide species.…”