Thermomechanical Nanostraining of Two-Dimensional Materials

Abstract

Local bandgap tuning in two-dimensional (2D) materials is of significant importance for electronic and optoelectronic devices but achieving controllable and reproducible strain engineering at the nanoscale remains a challenge. Here, we report on thermomechanical nanoindentation with a scanning probe to create strain nanopatterns in 2D transition metal dichalcogenides and graphene, enabling arbitrary patterns with a modulated bandgap at a spatial resolution down to 20 nm. The 2D material is in contact via van der Waals interactions with a thin polymer layer underneath that deforms due to the heat and indentation force from the heated probe. Specifically, we demonstrate that the local bandgap of molybdenum disulfide (MoS 2 ) is spatially modulated up to 10% and is tunable up to 180 meV in magnitude at a linear rate of about −70 meV per percent of strain. The technique provides a versatile tool for investigating the localized strain engineering of 2D materials with nanometer-scale resolution.

Local strain and tunneling current modulated excitonic luminescence in MoS2 monolayers

“…The modulation rate of strain on A exciton energy shift is estimated to -70 ± 13 meV/% from linear fitting. This value is close to the results obtained by diffraction-limited photoluminescence of monolayer MoS2 on patterned substrates [19,22]. Moreover, spectra acquired on other monolayer MoS2 flakes transferred from the same CVDsynthesis have closely matching values for the strain modulation rates (Figure S4).…”

“…Thus, the spectral shifts in excitonic luminescence in monolayer MoS2 are directly related to alterations of the electronic 10 bandgap induced by strain. In strain engineered 2D materials, thermal scanning probe lithography has recently achieved a strain pattern with 20 nm resolution [22]. It is challenging to adequately resolve such a fine pattern by far-field optical excitation methods.…”

“…The electronic band structures of monolayer TMDCs can be tuned by applying mechanical strain [17,18]. Thus, strain engineering has been reported to modify the optoelectronic responses of TMDCs, not only by controlling the magnitude of the strain [19,20] but also by controlling the spatial distribution [21,22], as proven by photoluminescence and Raman spectroscopy. However, far-field optical excitation methods are limited in providing information on the nanometer scale due to the diffraction of light.…”

Mechanical, Elastic, and Adhesive Properties of Two‐Dimensional Materials: From Straining Techniques to State‐of‐the‐Art Local Probe Measurements

“…It is worth mentioning that indentation can also be employed on 2D crystals deposited on soft substrates made of polymeric materials. [ 79 ] Indeed, it was shown that permanent deformations of monolayers over nanometer‐sized regions can be achieved after indentation, resulting in strain‐modulated patterns [ 81 ] or in the formation of single‐photon emitters. [ 82 ]…”

Low-Background Tip-Enhanced Raman Spectroscopy Enabled by a Plasmon Thin-Film Waveguide Probe

“…In recent years, theoretical insights and technical breakthroughs in the fields of nanophotonics and nanoplasmonics have significantly improved nanoscale spectroscopic techniques by identifying physical and chemical properties of materials and devices. − When Raman spectroscopy is combined with a scanning probe microscopy-controlled plasmonic tip, it generates a powerful nanoscale spectroscopic technique, termed tip-enhanced Raman spectroscopy (TERS). − TERS, as an effective tool to provide single-molecule-level chemical fingerprint information, has been widely used to study the interaction and geometric arrangement of organic molecules, − physical property of functional inorganic materials, − and composition of biological systems. − …”