Control of light-valley interactions in 2D transition metal dichalcogenides with nanophotonic structures

Directing valley-polarized emission of 3 L WS2 by photonic crystal with directional circular dichroism

The valley degree of freedom that results from broken inversion symmetry in two-dimensional (2D) transition-metal dichalcogenides (TMDCs) has sparked a lot of interest due to its huge potential in information processing. In this experimental work, to optically address the valley-polarized emission from three-layer (3 L) thick WS2 at room temperature, we employ a SiN photonic crystal slab that has two sets of holes in a square lattice that supports directional circular dichroism engendered by delocalized guided mode resonances. By perturbatively breaking the inversion symmetry of the photonic crystal slab, we can simultaneously manipulate s and p components of the radiating field so that these resonances correspond to circularly polarized emission. The emission of excitons from distinct valleys is coupled into different radiative channels and hence separated in the farfield. This directional exciton emission from selective valleys provides a potential route for valley-polarized light emitters, which lays the groundwork for future valleytronic devices.

Ultrafast carrier dynamics in vanadium-doped MoS2 alloys

Substitutional doping is a most promising approach to manipulate the electronic and optical properties of two-dimensional (2D) transition metal dichalcogenides (TMDCs). In addition to inducing magnetism, vanadium (V) doping can lead to semiconductor-metal transition in TMDCs. However, the dynamics of charge carriers that governs the optoelectronic properties of doped TMDCs has been rarely revealed. In this work, we have investigated the dynamics of photocarriers in pristine and V-doped monolayer (ML) MoS2. Comparison of the transient absorption (TA) spectra of ML MoS2 with lightly (≤1%) and heavily (3.62%) V-doped MoS2 infers the induction of additional energy states in the doped materials giving rise to new low energy bleach features in the TA spectra. The quasiparticle band structure of MoS2 is found to disappear at sufficiently high V doping due to the presence of impurity bands. An attempt has also been made to study the manipulation of the carrier lifetime with V doping in MoS2. Our TA kinetic measurements suggest that the decay kinetics of the carriers becomes slower with increasing doping percentage and at a higher doping level the carriers survive for a much longer time compared to pristine MoS2. Furthermore, we have identified a new electronic transition (NET) in heavily V-doped MoS2 at high pump fluences.

Functionalized MoS2: circular economy SERS substrate for label-free detection of bilirubin in clinical diagnosis

A one-pot hydrothermal synthesis of Fe-doped MoS₂ nanoflowers (Fe-MoS₂ NFs) has been developed as a surface-enhanced Raman spectroscopy (SERS) substrate. The Fe-MoS₂ NFs display high reproducibility, stability, and recyclability, which is beneficial for the development of the sustainable ecological environment. The SERS substrate provides a high enhancement factor of 10⁵, which can be ascribed to the inducing defects by doping Fe that can improve the charge transfer between probe molecules and MoS₂. The Fe-MoS₂ NFs have been used to detect bilirubin in serum. The Fe-MoS₂ NF SERS substrate exhibits a linear detection range from 10^-3 to 10^-9 M with a low limit of detection (LOD) of 10^-8 M. The substrate displays an excellent selectivity to bilirubin in the presence of other potentially interfering molecules (dextrose and phosphate). These results provide a novel concept to synthesize ultra-sensitive SERS substrates and open up a wide range of possibilities for new applications of MoS₂ in clinical diagnosis.

Transport properties of MoS2/V7(Bz)8 and graphene/V7(Bz)8 vdW junctions tuned by bias and gate voltages

The MoS2/V7(Bz)8 and graphene/V7(Bz)8 vdW junctions are designed and the transport properties of their four-terminal devices are comparatively investigated based on density functional theory (DFT) and the nonequilibrium Green's function (NEGF) technique. The MoS2 and graphene nanoribbons act as the source-to-drain channel and the spin-polarized one-dimensional (1D) benzene-V multidecker complex nanowire (V7(Bz)8) serves as the gate channel. Gate voltages applied on V7(Bz)8 exert different influences of electron transport on MoS2/V7(Bz)8 and graphene/V7(Bz)8. In MoS2/V7(Bz)8, the interplay of source and gate bias potentials could induce promising properties such as negative differential resistance (NDR) behavior, output/input current switching, and spin-polarized currents. In contrast, the gate bias plays an insignificant effect on the transport along graphene in graphene/V7(Bz)8. This dissimilarity is attributed to the fact that the conductivity follows the sequence of MoS2 < V7(Bz)8 < graphene. These transport characteristics are examined by analyzing the conductivity, the currents, the local density of states (LDOS), and the transmission spectra. These results are valuable in designing multi-terminal nanoelectronic devices.