Tip‐Induced Nano‐Engineering of Strain, Bandgap, and Exciton Funneling in 2D Semiconductors

Abstract

The tunability of the bandgap, absorption and emission energies, photoluminescence (PL) quantum yield, exciton transport, and energy transfer in transition metal dichalcogenide (TMD) monolayers provides a new class of functions for a wide range of ultrathin photonic devices. Recent strain‐engineering approaches have enabled to tune some of these properties, yet dynamic control at the nanoscale with real‐time and ‐space characterizations remains a challenge. Here, a dynamic nano‐mechanical strain‐engineering of naturally‐formed wrinkles in a WSe2 monolayer, with real‐time investigation of nano‐spectroscopic properties is demonstrated using hyperspectral adaptive tip‐enhanced PL (a‐TEPL) spectroscopy. First, nanoscale wrinkles are characterized through hyperspectral a‐TEPL nano‐imaging with <15 nm spatial resolution, which reveals the modified nano‐excitonic properties by the induced tensile strain at the wrinkle apex, for example, an increase in the quantum yield due to the exciton funneling, decrease in PL energy up to ≈10 meV, and a symmetry change in the TEPL spectra caused by the reconfigured electronic bandstructure. Then the local strain is dynamically engineered by pressing and releasing the wrinkle apex through an atomic force tip control. This nano‐mechanical strain‐engineering allows to tune the exciton dynamics and emission properties at the nanoscale in a reversible fashion. In addition, a systematic switching and modulation platform of the wrinkle emission is demonstrated, which provides a new strategy for robust, tunable, and ultracompact nano‐optical sources in atomically thin semiconductors.

Low-threshold exciton transport and control in atomically thin semiconductors

“…The ideal nanoscale strain-gradient structure for effective X 0 funnelling and X-conversion exists in nature, i.e., the nanoscale wrinkle in 2D TMDs. Our previous study revealed the exciton funnelling behaviors of the naturally-formed wrinkles in a WSe 2 ML [12]. As experimentally confirmed again in this study, the uniaxial tensile strain is induced in the wrinkle structure with the highest strain at the apex region, which facilitates the exciton funnelling to the lowest bandgap energy region, spatially at the nanoscale.…”

“…The specific details for the physical mechanism are depicted with a theoretical model in Fig. 5, which was not considered in our recent report [12]. In contrast, the TEPL intensity decreases with distinctly increased linewidth at the wrinkle for a MoS 2 ML compared to the crystal face region, as shown in Fig.…”

Imaging Charged Exciton Localization in van der Waals WSe2/MoSe2 Heterobilayers

“…with δ, h, λ, and σ being the bubble height, layer thickness, bubble width, and Poisson's ratio of WSe 2 , respectively. 21 With a base width and height of ∼125 nm and ∼11.5 nm, respectively, and with a Poisson's ratio of 0.19, 23 ε t of this bubble is ∼0.75%. This tensile strain leads to a reduction of ∼37.5 meV in the band gap (i.e., ΔE g ≈ −37.5 meV), assuming a linear relation between ΔE g and ε t with a proportionality ratio of ∼−50 meV/%.…”

“…20 Similarly, the manipulation of local strain fields and heterostructure architectures was used to control exciton localization and emission. 21,22 In this work, we use hyperspectral nanophotoluminescence (nano-PL) to image excitonic emission from WSe 2 (top layer)/MoSe 2 (bottom layer) heterobilayer structures. We observe strong localization of positively charged trions of the WSe 2 layer inside individual heterostructure nanobubbles.…”

Electron Dynamics in a Two-Dimensional Nanobubble: A Two-Level System Based on Spatial Density

“…Localized potentials can be either deterministically induced, for example, by patterned substrates − and helium irradiation, or they build up naturally in the form of defects and disorder ,, or nanobubbles. − Bubbles are naturally formed in experiments when depositing the 2D material on a substrate, ,,,− similar to the everyday process of air bubbles forming when putting a plastic foil on glass. Bubbles in various monolayers and of different dimensions can also be obtained via bottom-up approaches such as ionic irradiations. , Nanobubbles are able to induce an energy confinement due to the interplay of strain and local screening variations, resulting in a nontrivial circular shape recently observed in experiments. , Besides being extensively studied via optical response, − ,− localized potentials can impact also the transport and associated recombination in different hosting 2D materials ,, and even lead to excitonic funneling for larger potentials. − …”