Exfoliation procedure-dependent optical properties of solution deposited MoS2 films.

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.

Microwave Facilitated Covalent Organic Framework/Transition Metal Dichalcogenide Heterostructures.

Organic/inorganic heterostructures present a versatile platform for creating materials with new functionalities and hybrid properties. In particular, junctions between two dimensional materials have demonstrated utility in next generation electronic, optical, and optoelectronic devices. This work pioneers a microwave facilitated synthesis process to readily incorporate few-layer covalent organic framework (COF) films onto monolayer transition metal dichalcogenides (TMDC). Preferential microwave excitation of the monolayer TMDC flakes result in selective attachment of COFs onto the van der Waals surface with film thicknesses between 1 and 4 nm. The flexible process is extended to multiple TMDCs (MoS2, MoSe2, MoSSe) and several well-known COFs (TAPA-PDA COF, TPT-TFA-COF, and COF-5). Photoluminescence studies reveal a power-dependent defect formation in the TMDC layer, which facilitates electronic coupling between the materials at higher TMDC defect densities. This coupling results in a shift in the A-exciton peak location of MoSe2, with a red or blue shift of 50 or 19 meV, respectively, depending upon the electron donating character of the few-layer COF films. Moreover, optoelectronic devices fabricated from the COF-5/TMDC heterostructure present an opportunity to tune the PL intensity and control the interaction dynamics within inorganic/organic heterostructures.

Laser-Fabricated 2D Molybdenum Disulfide Electronic Sensor Arrays for Rapid, Low-Cost, Ultrasensitive Detection of Influenza A and SARS-Cov-2.

Multiplex electronic antigen sensors for detection of SARS-Cov-2 spike glycoproteins and hemagglutinin from influenza A are fabricated using scalable processes for straightforward transition to economical mass-production. The sensors utilize the sensitivity and surface chemistry of a 2D MoS2 transducer for attachment of antibody fragments in a conformation favorable for antigen binding with no need for additional linker molecules. To make the devices, ultra-thin layers (3 nm) of amorphous MoS2 are sputtered over pre-patterned metal electrical contacts on a glass chip at room temperature. The amorphous MoS2 is then laser annealed to create an array of semiconducting 2H-MoS2 transducer regions between metal contacts. The semiconducting crystalline MoS2 region is functionalized with monoclonal antibody fragments complementary to either SARS-CoV-2 S1 spike protein or influenza A hemagglutinin. Quartz crystal microbalance experiments indicate strong binding and maintenance of antigen avidity for antibody fragments bound to MoS2. Electrical resistance measurements of sensors exposed to antigen concentrations ranging from 2-20 000 pg mL-1 reveal selective responses. Sensor architecture is adjusted to produce an array of sensors on a single chip suited for detection of analyte concentrations spanning six orders of magnitude from pg mL-1 to µg mL-1.