Electronic devices: Can be scaled and integrated but limited in signal processing speed

Photonic devices : Fast signal processing speed but large device dimensions

Excitonic devices: Combine best of the two – fast, energy-efficient, and can be integrated



Two-dimensional transition metal dichalcogenides (TMDs) exhibit excitons, optically excited coulomb bound electron-hole pair, stable even at room temperature, and are potential candidates for emerging excitonic devices. Several excitonic states with distinct properties can coexist in these materials. Depending on the application, some of these excitonic states may be more suitable than others and hence need to be selectively enhanced or suppressed.



In this work, excitonic properties of monolayer (1L), bilayer (BL), and multilayer (ML) TMDs have been explored experimentally. Photoluminescence (PL) spectroscopy was used to understand different types of excitonic states and their properties present in these systems and devise simple methods to selectively either enhance or suppress these states. Based on the type of excitonic states and the TMD system, the work has been broadly divided into the following three parts:



i. Bright and Dark excitons in monolayer TMDs

ii. Interlayer excitons in bilayer TMDs

iii. Momentum indirect excitons in multilayer TMDs



In the case of 1L-TMDs, the excitons can be optically bright (Xo) or dark (XD) depending on the conduction band splitting and spin ordering. In the case of molybdenum dichalcogenides, the lowest transition is optically bright while in the case of tungsten dichalcogenides it is dark. These states can be clearly distinguished using temperature-dependent PL measurements. The lifetime of XD is longer when compared to that of Xo, however, they cannot be probed optically. In this work, gold nanoparticles (Au-NPs) were used to achieve plasmon-induced brightening of XD in 1L-WSe2. This was attributed to the out-of-plane electric field induced by the scattering from Au-NPs, which results in out-of-plane dipole moment and spin-flip of conduction band electrons in Au-NPs/1L-WSe2 making XD bright [1].



In the case of BL-TMDs, the constituent electron and hole of the exciton can be in different layers and hence are known as interlayer (IL) excitons. In this work, two different BL systems were analyzed, where different excitonic states emerged in BL which were absent in the constituent monolayers. Firstly, a homo-bilayer WSe2 system, where different higher-order excitonic states were observed in BL systems which were absent in 1L-WSe2. The relative stacking angle between the two layers selectively enhanced or suppressed different excitonic states, as observed from experiments [2]. Secondly, a hetero-bilayer MoSe2/MoS2 system, where two new peaks corresponding IL excitons were observed in heterostructures which were attributed to type-2 band alignment and band hybridization in these HSs [3].



ML-TMDs have advantages over 1L-TMDs in terms of increased absorption and high optical density of states, however, the emission of ML-TMDs is dominated by the momentum indirect excitons (IX) which makes them optically inefficient. Therefore, in order to exploit the properties of ML systems, either IX should be selectively suppressed making the dominant emission momentum direct or it should be enhanced to compensate for its poor efficiency. In this work, both these strategies have been explored. Firstly, with the incorporation of Au islands to ML-MoS2 using sputtering and electron-beam evaporation, complete suppression of the IX emission was observed. The direct-to-indirect intensity with Au was 80 times when compared to that in pristine samples [4]. Secondly, the IX emission was enhanced in ML-MoS2 by integrating it with Zinc oxide (ZnO) thin films to form ML-MoS2/ZnO heterostructures (HS). The emission intensity was further enhanced when Au islands were introduced at the MoS2-ZnO interface. For MoS2 flakes of thickness > 200 nm, these HS exhibited room temperature, continuous wave, random lasing in the NIR region. This was attributed to the enhanced multiple scattering and exciton-plasmon coupling because of gold nanoparticles [5].



These results can be used to selectively enhance or suppress specific excitonic states and may help in engineering excitonic devices for optoelectronic applications.



Finally, thickness-dependent electrical properties of Muscovite mica (MuM) flakes were analyzed. The in-plane dc electrical conductivity of few-layer MuM flakes was found to be thickness dependent and was three orders of magnitude larger in 10 nm thick flake compared to that in 20 nm thick flake. These results indicate that there is a possibility of using few-layer mica as a wide bandgap semiconductor and can be a promising candidate for harsh environment electronic devices.



References:

[1] A. Arora, et al., Appl. Phys. Lett., 114(20), 201101 (2019).

[2] A. Arora, et al., Nanophotonics, 9(12), 3881-3887 (2020).

[3] A. Arora, et al., Phys. Rev. B, 103(20), 205400 (2021).

[4] T. Dixit, A. Arora, et al., IEEE Photonics Journal, 11(5), 4501106 (2019).

[5] T. Dixit, A. Arora, et al., ACS Omega, 3(10), 14097-14102 (2018).