Molecular Beam Epitaxy of Highly Crystalline MoSe₂ on Hexagonal Boron Nitride

Molybdenum diselenide (MoSe₂) is a promising two-dimensional material for next-generation electronics and optoelectronics. However, its application has been hindered by a lack of large-scale synthesis. Although chemical vapor deposition (CVD) using laboratory furnaces has been applied to grow two-dimensional (2D) MoSe₂ cystals, no continuous film over macroscopically large area has been produced due to the lack of uniform control in these systems. Here, we investigate the molecular beam epitaxy (MBE) of 2D MoSe₂ on hexagonal boron nitride (hBN) substrate, where highly crystalline MoSe₂ film can be grown with electron mobility ∼15 cm²/(V s). Scanning transmission electron microscopy (STEM) shows that MoSe₂ grains grown at an optimum temperature of 500 °C are highly oriented and coalesced to form continuous film with predominantly mirror twin boundaries. Our work suggests that van der Waals epitaxy of 2D materials is tolerant of lattice mismatch but is facilitated by substrates with similar symmetry

Tuning and Persistent Switching of Graphene Plasmons on a Ferroelectric Substrate

We characterized plasmon propagation in graphene on thin films of the high-κ dielectric PbZr_0.3Ti_0.7O₃ (PZT). Significant modulation (up to ±75%) of the plasmon wavelength was achieved with application of ultrasmall voltages (< ±1 V) across PZT. Analysis of the observed plasmonic fringes at the graphene edge indicates that carriers in graphene on PZT behave as noninteracting Dirac Fermions approximated by a semiclassical Drude response, which may be attributed to strong dielectric screening at the graphene/PZT interface. Additionally, significant plasmon scattering occurs at the grain boundaries of PZT from topographic and/or polarization induced graphene conductivity variation in the interior of graphene, reducing the overall plasmon propagation length. Lastly, through application of 2 V across PZT, we demonstrate the capability to persistently modify the plasmonic response of graphene through transient voltage application

Ultrafast and Nanoscale Plasmonic Phenomena in Exfoliated Graphene Revealed by Infrared Pump–Probe Nanoscopy

Pump–probe spectroscopy is central for exploring ultrafast dynamics of fundamental excitations, collective modes, and energy transfer processes. Typically carried out using conventional diffraction-limited optics, pump–probe experiments inherently average over local chemical, compositional, and electronic inhomogeneities. Here, we circumvent this deficiency and introduce pump–probe infrared spectroscopy with ∼20 nm spatial resolution, far below the diffraction limit, which is accomplished using a scattering scanning near-field optical microscope (s-SNOM). This technique allows us to investigate exfoliated graphene single-layers on SiO₂ at technologically significant mid-infrared (MIR) frequencies where the local optical conductivity becomes experimentally accessible through the excitation of surface plasmons via the s-SNOM tip. Optical pumping at near-infrared (NIR) frequencies prompts distinct changes in the plasmonic behavior on 200 fs time scales. The origin of the pump-induced, enhanced plasmonic response is identified as an increase in the effective electron temperature up to several thousand Kelvin, as deduced directly from the Drude weight associated with the plasmonic resonances