Large Photothermal Effect in Sub‐40 nm h‐BN Nanostructures Patterned Via High‐Resolution Ion Beam

The controlled nanoscale patterning of 2D materials is a promising approach for engineering the optoelectronic, thermal, and mechanical properties of these materials to achieve novel functionalities and devices. Herein, high‐resolution patterning of hexagonal boron nitride (h‐BN) is demonstrated via both helium and neon ion beams and an optimal dosage range for both ions that serve as a baseline for insulating 2D materials is identified. Through this nanofabrication approach, a grating with a 35 nm pitch, individual structure sizes down to 20 nm, and additional nanostructures created by patterning crystal step edges are demonstrated. Raman spectroscopy is used to study the defects induced by the ion beam patterning and is correlated to scanning probe microscopy. Photothermal and scanning near‐field optical microscopy measure the resulting near‐field absorption and scattering of the nanostructures. These measurements reveal a large photothermal expansion of nanostructured h‐BN that is dependent on the height to width aspect ratio of the nanostructures. This effect is attributed to the large anisotropy of the thermal expansion coefficients of h‐BN and the nanostructuring implemented. The photothermal expansion should be present in other van der Waals materials with large anisotropy and can lead to applications such as nanomechanical switches driven by light.     

Anti‐Ambipolar Transport with Large Electrical Modulation in 2D Heterostructured Devices

Van der Waals materials and their heterostructures provide a versatile platform to explore new device architectures and functionalities beyond conventional semiconductors. Of particular interest is anti‐ambipolar behavior, which holds potentials for various digital electronic applications. However, most of the previously conducted studies are focused on hetero‐ or homo‐ p–n junctions, which suffer from a weak electrical modulation. Here, the anti‐ambipolar transport behavior and negative transconductance of MoTe₂ transistors are reported using a graphene/h‐BN floating‐gate structure to dynamically modulate the conduction polarity. Due to the asymmetric electrical field regulating effect on the recombination and diffusion currents, the anti‐ambipolar transport and negative transconductance feature can be systematically controlled. Consequently, the device shows an unprecedented peak resistance modulation factor (≈5 × 10³), and effective photoexcitation modulation with distinct threshold voltage shift and large photo on/off ratio (≈10⁴). Utilizing this large modulation effect, the voltage‐transfer characteristics of an inverter circuit variant are further studied and its applications in Schmitt triggers and multivalue output are further explored. These properties, in addition to their proven nonvolatile storage, suggest that such 2D heterostructured devices display promising perspectives toward future logic applications.