Exfoliation procedure-dependent optical properties of solution deposited MoS2 films

Abstract

The development of high-precision large-area optical coatings and devices comprising low-dimensional materials hinges on scalable solution-based manufacturability with control over exfoliation procedure-dependent effects. As such, it is critical to understand the influence of technique-induced transition metal dichalcogenide (TMDC) optical properties that impact the design, performance, and integration of advanced optical coatings and devices. Here, we examine the optical properties of semiconducting MoS2 films from the exfoliation formulations of four prominent approaches: solvent-mediated exfoliation, chemical exfoliation with phase reconversion, redox exfoliation, and native redox exfoliation. The resulting MoS2 films exhibit distinct refractive indices (n), extinction coefficients (k), dielectric functions (ε1 and ε2), and absorption coefficients (α). For example, a large index contrast of Δn ≈ 2.3 is observed. These exfoliation procedures and related chemistries produce different exfoliated flake dimensions, chemical impurities, carrier doping, and lattice strain that influence the resulting optical properties. First-principles calculations further confirm the impact of lattice defects and doping characteristics on MoS2 optical properties. Overall, incomplete phase reconfiguration (from 1T to mixed crystalline 2H and amorphous phases), lattice vacancies, intraflake strain, and Mo oxidation largely contribute to the observed differences in the reported MoS2 optical properties. These findings highlight the need for controlled technique-induced effects as well as the opportunity for continued development of, and improvement to, liquid phase exfoliation methodologies. Such chemical and processing-induced effects present compelling routes to engineer exfoliated TMDC optical properties toward the development of next-generation high-performance mirrors, narrow bandpass filters, and wavelength-tailored absorbers.

Recent Advances in Layered MX2-Based Materials (M = Mo, W and X = S, Se, Te) for Emerging Optoelectronic and Photo(electro)catalytic Applications

“…The nature of the chemical bonds between the metal cations and the chalcogen anions (M-X) is mainly covalent [19], while the weaker intermolecular bonds between the triple sheets occur through van der Waals (vdW) bonds, i.e., making the X-X junction (Figure 1A) [20]. As we know, this unique structure allows for easy exfoliation into single-or few-layered flakes, offering tunable electronic properties and surface functionalities [21][22][23]. Additionally, TMDCs even with the same composition can be synthesized in different polymorphs and polytypes.…”

“…properties and surface functionalities [21][22][23]. Additionally, TMDCs even with the same composition can be synthesized in different polymorphs and polytypes.…”

Electrochemistry of Thin Films and Nanostructured Materials

“…For instance, Au films less than 300 nm thick covering copper and bronze statues and jewelry have been found in ancient tombs in the Pyramid of Djoser (Netjerykhet), built by the second King of the Third Dynasty, Old Kingdom (from 2667 to 2648 BC) in Saqqara [3]. Since then, humanity has learned not only how to obtain thin films of metals, metal oxides, various nitrides, carbides, semiconductors, and polymers [4][5][6][7][8], but also to study and control their chemical, optical, electrical, and mechanical properties, which differ considerably from the corresponding bulk materials [9][10][11][12][13]. The film properties can be precisely controlled by changing the thickness, which is a great benefit for thin film technologies and applications.…”