Monolayer MoS2 and plasmonic heterostructure for molecular sensor

Abstract

In recent years, artificially engineered materials, known as meta structures, have demonstrated tuneable optoelectronic properties, making meta-coupled devices highly advantageous for a wide range of technological applications. These devices offer novel functionalities when integrated with various nanomaterials such as graphene, transition metal dichalcogenides (TMDCs), and black phosphorus.

Two-dimensional (2D) TMDCs offer unique characteristics that are not present in their bulk counterpart. For example, tuneable density of states presents unprecedented coupling with other artificially engineered materials, such as metamaterials and photonic crystals. These devices often find applications in tuneable colour filters, security and medical testing, food testing, environmental testing, etc. The detection technologies mostly operate in microwave regime and the detection using the visible radiation is still a challenge due to the limitations in the fabrication processes. For example, the liquid detection in drug and biomedical applications often requires materials which respond to the ultraviolet-visible (UV-visible) radiations. The aim of the thesis is to detect molecules present in different mediums, like liquids and gases, by monitoring ultra-small changes in the refractive index of the medium under UV-visible light illumination.

Meta-FET (field effect transistor) device is fabricated by coupling 2D monolayer of molybdenum disulfide (MoS2) with the metal plasmonic structure of dimers and its coupling efficiency under light illumination has been investigated. The optical coupling efficiency of two opposite polarity of gases like electron withdrawing e.g. sulfur dioxide (SO2) and electron donating e.g. ammonia (NH3) is evaluated through normalized reflection spectra using a confocal microscope in the visible range. Both the A-and B-excitonic responses are recorded in the presence of gases. The electrical transport properties of meta-FET are also measured in each case. We investigated the electrical response in the subthreshold region of the plasmonic meta-FET device. The turn-on voltage of the meta-FET device results in a significant shift of 483% to a higher negative potential upon optical illumination. We investigated the response of the device with different concentrations of gases under dark and illuminated conditions. The transfer characteristics of meta-FET device showed an excellent variation of 518% and 2506% in the maximum drain current upon interaction with NH3 and SO2 molecules in the presence of optical excitations. The meta-FET device paves the way to provide a novel mechanism for the molecular interaction with extremely improved sensitivity and selectivity.

Further, the optical responses are recorded in liquid mediums such as isopropyl alcohol (IPA) and glycerol as well as air, using two different geometries of plasmonic surface−one being a gold (Au) dimer and the other a dumbbell-shaped nano disk. The dumbbell structure demonstrated improved sensitivity, reaching 60.7 nm/RIU compared to 24.49 nm/RIU in dimer structure. The concept of shape engineering techniques to improve the sensitivity, has drawn attention for potential biomedical applications, such as health monitoring using biomedicine that contains IPA and glycerol. The optical response of two plasmonic heterostructures—plasmonic MoS2 incorporating these geometries—are also recorded. The shift of peak position of A- and B- excitons are modified through the geometric effect of plasmonic structures. Additionally, an improvement in excitonic intensity in plasmonic MoS2 is observed, offering a promising platform for opto-chemical sensing applications using enhanced optoelectronic properties through refractive index variation.