Local strain at wrinkles dramatically modulates the localized excited electron density and photoluminescence (PL) energy in curled bilayer 2D transition metal dichalcogenides, culminating in an excited electron funneling phenomenon. Due to their poor spatial resolution and reduced sensitivity, standard analytical methods are incapable of exploring such excited electron characteristics, particularly at nanoscale length scales.
Study: Probing Nanoscale Exciton Funneling at Wrinkles of Twisted Bilayer MoS2 Using Tip-Enhanced Photoluminescence Microscopy. Image Credit: Syda Productions/Shutterstock.com
In a recent study, published in The Journal of Physical Chemistry Letters, researchers addressed this problem using elevated resolution tip-enhanced photoluminescence (TEPL) microimaging to analyze excited electron funneling at a crease in a twisted bilayer MoS2 specimen with a tiny angular position of 0.5°.
What are Twisted Bilayer TMDs?
Twisted 2D nanomaterials are created by layering a single layer 2D substance over another at a specific angular position (θt). TMDs, including MoS2, WS2, and others are single-layer and multi-layer semiconductors. Their curled bilayer equivalents with θt values of 4-5.2° exhibit superconductivity and Mott insulation identical to that of magic-angle curled bilayer graphene. Moreover, curled bilayer TMDs exhibit a t-dependent coupling effect between the layers, valley polarization, and coupling excited electron photoluminescence between the layers that differs significantly from that of properly mounted bilayers.
These amazing features enable the development of innovative twistronics based on 2D nanomaterials. In thin sheet TMD specimens produced using a transferring approach, nanoparticle-sized creases are often found. The strain field is projected to affect the band edge energy at these creases, resulting in a gap narrower than the neighboring regions. The excited electrons created in the neighboring regions migrate towards the crease, leading to increased PL intensity, a phenomenon called the exciton funneling effect.
The excitonic distortions of curled bilayer specimens would significantly impair the optical-electronic efficiency of prospective twistronic systems.
Recognizing the excitonic variations surrounding creases is critical for improving the optical-electronic characteristics of prospective curled bilayer systems. Yet, owing to the optical diffraction threshold, traditional PL spectroscopy offers no real nano-scaled observation of the regional excited electron funneling since the spatial breadth of the creases is on the magnitude of just a few nanometers.
Why is Tip-Enhanced Photoluminescence Microscopy Important?
TEPL micro-imaging generates a greatly confined and powerful electromagnetic field at the peak of a plasmonic probing device by combining regional surface plasmon resonance with a lightning rod phenomenon. This nanosized hyperspectral imaging technology eliminates the constraints of traditional PL spectroscopy and may give extensive knowledge of local excitonic dynamics.
TEPL microimaging and tip-enhanced Raman spectroscopy (TERS) have been used effectively in recent times for nanosized characterization of many 2D substances such as WSe2, WS2, MoS2, graphene, and others. However, thus far, the investigation of regional excited electrons in twisted bilayer TMDs nanomaterials has been restricted to far-field optical spectroscopy, with no observation of excitonic funneling at the nanometer scale feasible.
How the Samples were prepared for the Study
In a tubular furnace with multi heating zones, 1L MoS2 specimens were generated using a chemical vapor deposition technique. A semi-wet transferring technique was used to create t2L MoS2 specimens with varying θt values. The acquired MoS2 specimen was split into two fairly similar portions.
Important Results of the Study
In conclusion, the team used a hyperspectral optical nanospectroscopy approach to analyze regional excited electron funneling in a crease of twisted bilayer MoS2. Elevated resolution TEPL imagery permitted viewing of excitonic activity at MoS2 creases with a spatial resolution of less than 10 nm, which had previously been impossible to obtain using any spectroscopic approach. For the very first time, a nano-sized excited electron funneling phenomenon was reported on a curled bilayer MoS2 crease with a considerable distortion potential in curled bilayer MoS2.
Due to the absence of sensitivity and diffraction-limited spatial resolution, such nano-sized excitonic processes cannot be identified or observed using traditional analytical methods. These findings showed that TEPL has the promise to be a strong nanospectroscopy method for full structural and physical characterization of twisted bilayer TMDs for prospective uses in 2D substance-based twistronic systems.
Shao, J., Chen, F., Su, W., Kumar, N., Zeng, Y., Wu, L., and Lu, H.-W. (2022). Probing Nanoscale Exciton Funneling at Wrinkles of Twisted Bilayer MoS2 Using Tip-Enhanced Photoluminescence Microscopy. The Journal of Physical Chemistry Letters. Available at: https://pubs.acs.org/doi/10.1021/acs.jpclett.2c00815