Strained 2D TMD lateral heterojunctions via grayscale thermal-Scanning Probe Lithography

Nanoscale tailoring of the optoelectronic response of 2D Transition Metal Dichalcogenides semiconductor layers (TMDs) has been achieved thanks to a novel strain engineering approach based on the grayscale thermal-Scanning Probe Lithography (t-SPL). This method allows the maskless nanofabrication of locally strained 2D MoS2-Au lateral heterojunction nanoarrays that are characterized by asymmetric electrical behavior. 2D MoS2 layers are conformally transferred onto grayscale t-SPL templates characterized by periodic nanoarrays of deterministic faceted nanoridges. This peculiar morphology induces asymmetric and uniaxial strain accumulation in the 2D TMD material allowing to tailor their electrical work-function at the nanoscale level, as demonstrated by Kelvin Probe Force Microscopy (KPFM). The modulation of the electronic response has been exploited to develop periodic nanoarrays of lateral heterojunctions endowed with asymmetric electrical response by simple maskless deposition of Au nanocontacts onto the strained 2D TMD layers. The locally strained Au-MoS2 layers show asymmetric lateral heterojunctions with engineered carrier extraction functionalities, thus representing a promising platform in view of tunable ultrathin nanoelectronic, nanophotonic and sensing applications.

💡 Research Summary

The authors present a novel strain‑engineering platform for two‑dimensional transition‑metal dichalcogenides (TMDs) that combines grayscale thermal‑Scanning Probe Lithography (t‑SPL) with mask‑less metal deposition to create asymmetric lateral heterojunctions. First, a high‑resolution grayscale t‑SPL process is used to write periodic faceted nanoridges (530 nm pitch, 60 nm height, facet angles of –29° and +37°) into a 140 nm thick polyphthalaldehyde (PPA) layer on an ITO‑coated glass substrate. The tip can control depth with 1 nm resolution and lateral resolution down to 10 nm, enabling deterministic 3‑D nanostructures over hundreds of µm². Monolayer and few‑layer MoS₂ flakes are then transferred onto these templates using a PDMS stamp. Because the PDMS (Young’s modulus ≈ 8 × 10⁻⁴ GPa) is much softer than the PPA (≈ 0.3 GPa), the 2D crystal conforms to the right‑hand facet without deformation but is stretched over the left‑hand facet, leaving a periodic, uniaxial tensile strain localized on the left side of each ridge. Raman spectroscopy confirms the monolayer nature of the transferred flakes and shows a two‑fold increase in Raman intensity for the strained regions, while photoluminescence (PL) measurements reveal a 2.9‑fold enhancement of the A‑exciton emission, indicating strain‑induced band‑structure modification. Kelvin Probe Force Microscopy (KPFM) provides nanoscale mapping of the surface work function: the contact potential difference (CPD) drops by ~130 mV (monolayer) and ~150 mV (few‑layer) on the strained facets, reflecting an increase in the local work function due to tensile strain. The CPD modulation follows the underlying periodicity with a spatial resolution of a few tens of nanometers. To exploit this strain‑engineered landscape, the authors perform glancing‑angle Au evaporation (≈ 80° from the surface normal) which deposits Au nanowires only on the flat tops of the ridges, leaving the strained side of the MoS₂ exposed. This mask‑less approach creates a periodic array of Au‑MoS₂ contacts that alternate between strained and unstrained regions. KPFM of the hybrid structures shows a strong, periodic CPD contrast: Au nanowires appear at higher potential while the underlying strained MoS₂ exhibits lower potential, indicating that the Schottky barrier height at the Au‑MoS₂ interface is modulated by the local strain. Consequently, the device exhibits asymmetric electrical behavior—Ohmic conduction when current flows from Au into the unstrained side and Schottky‑type rectification when it encounters the strained side. The work demonstrates three key advances: (i) deterministic grayscale t‑SPL fabrication of faceted nanostructures with sub‑10 nm lateral precision, (ii) controlled, periodic tensile strain in 2D TMDs that tunes both electronic work function and optical response, and (iii) a simple, mask‑free method to pattern metal contacts that selectively address strained versus unstrained regions, enabling engineered lateral heterojunctions with built‑in asymmetry. This platform opens pathways for ultrathin nanoelectronics, nanophotonics, and strain‑tunable sensing technologies, where precise nanoscale control of band alignment and carrier extraction is essential.