Transition metal dichalcogenides (TMDs) have emerged as a new class of semiconductors that display distinctive properties in monolayer thicknesses.
Stady: Ultracentric ferroelectric properties of nanobubbles in two-dimensional semiconductors. Image credit: Love Employee / Shutterstock.com
Although modulation of their optical properties at the nanoscale has been previously reported by electrical and mechanical nanostructures, elucidation of the local electronic structure with corresponding emission properties is unprecedented.
Article published in a magazine nano messages Use a combination of near-field photoluminescence (nano-PL) and scanning tunneling microscopy (STM) to investigate the optical and electronic properties of nanobubbles in heterogeneous bilayer structures of tungsten selenide (WSe)2) on molybdenum selenide (MoSe2).
Here, the electronic states were localized at the edge of these nanobubbles, as shown by STM. These electronic states were independent of chemical defects in the layers. A significant change in the local bandgap of the nanobubble is observed with continuous evolution to the bubble edge on a length scale of approximately 20 nm.
Nano-PL measurements showed a consistent redshift of the interlayer exciton upon entering the nanobubble, which is in agreement with the band-to-band transitions measured by STM. In addition, self-consistent Schrödinger-Poisson simulations revealed that strong doping in the nanobubble region were important in achieving localized states and mechanical stress.
Nanobubbles in TMDs
Two-dimensional (2D) TMDs are very attractive for fundamental studies of new physical phenomena and applications, ranging from nanoelectronics and nanophotonics to sensing and operation at the nanoscale. Single-layer TMDs with large direct bandgaps are promising materials for optoelectronic, field-effect transistors, and photovoltaic applications.
The band gap in single-layer TMDs is sensitive to the dielectric environment, doping, and mechanical stress, providing an ideal platform for establishing mechanical or electrical field-based quantum confinement.
Nanobubbles are created when elements trapped between the atomic layer and the substrate condense into a local pocket of pressure, one of the most common defects. Although it is often undesirable for device applications, its ability to modify the electronic structure of the atomic layer has sparked great interest in nanobubble physics.
In the case of the molybdenum and tungsten families of TMDs, the nanobubbles dramatically change the optical bandgap, and the excitatory photoluminescence energy increases by up to several hundred millielectronvolts.
Determining the size and strain distribution within individual nanobubbles is critical to understanding their fundamental properties. This characterization is often performed by combining atomic force microscopy (AFM) to measure the topological parameters of nanobubbles with models based on elasticity theory and treating a single-layer TMD as a thin plate subjected to a transverse load.
According to previous studies, quantum emission can combine with the nanobubble, and strain models developed from the connected elastic plate theory suggest that the top of the nanobubble will contain one region of the highest strain.
Photoelectric properties of nanobubbles
Although previous theoretical calculations have predicted significant changes in the band gap at the edges of nanobubbles and wrinkles, these predictions have not been experimentally verified. Moreover, the main contributors remained unknown, as they investigated localized optical emitters in TMD materials.
In the present work, STM and local imaging techniques were used to measure the local scale pattern in nanobubbles that was associated with near-field optics measurements with a spatial resolution of 10 nm. WSe alignment approx2/ The Ministry of Education2 A heterogeneous bilayer stacked on a graphite/hBN electrode was used to perform the STM measurements.
The STM terrain was obtained with a constant tunneling current and a specific voltage bias via a feedback loop. Here, the tunnel current depended on the integrated local density of states between the bias voltage and the Fermi sample level.
Moreover, nanobubbles were observed between the TMD layers, which were formed during the stacking process via AFM imaging. The results showed that the nanobubbles ranged from 20 to 400 nm in size and height ranged between 2 to 50 nm.
Furthermore, STM imaging revealed a moiré pattern inside and outside the nanobubbles, indicating the contact between MoSe.2 and WSe2 Layering and presence of nanobubbles under the heterogeneous bilayer. Thus, room temperature manufacturing often results in the trapping of water molecules between these layers.
In general, the local scale pattern across the nanobubbles was measured using local imaging and STM, and the results of the measurements were correlated with near-field optical measurements at nanometer resolution.
WSe alignment approx2/ The Ministry of Education2 Heterogeneous bilayers were placed on a graphite/hBN conductive electrode for STM measurements. Here, the relative orientation of two monolayers of TMD via second harmonic generation (SHG) was determined. The nanobubbles were created during the stacking process in the interlayer spaces.
The nanobubble sizes of 20 and 400 nm and heights of 2 to 50 nm were confirmed by AFM images. The results indicate that light emitters with arbitrarily desired shapes can be made at the nanoscale by combining interfacial geometry and nanostructure.
Shabani, S and others. (2022). Ultracentric ferroelectric properties of nanobubbles in two-dimensional semiconductors. nano messages. https://doi.org/10.1021/acs.nanolett.2c02265