Thin 2D Mo ₂ C/graphene vertical heterostructures have attracted significant attention due to their potential application as electrodes in the hydrogen evolution reaction (HER) and energy storage. A common drawback in the chemical vapor deposition synthesis of these structures is the demand for high temperature growth, which should be higher than the melting temperature of the metal catalyst. The most common metallic catalyst is Cu, which has a melting temperature of 1084 °C. Here, we report the growth of thin, ∼200 nm in thickness, semitransparent micrometer-sized Mo ₂ C domains and Mo ₂ C/graphene heterostructures at lower temperatures using liquid Sn–Cu alloys. No Sn-associated defects are observed, making the alloy an appealing growth substrate. Raman spectroscopy reveals the vertical interaction between graphene and Mo ₂ C, as shown by the variation in the strain of the graphene film. The results demonstrate the capability to grow continuous nanometer-thin Mo ₂ C films at temperatures as low as 880 °C, without sacrificing the growth rate. Mo ₂ C films are proven to be efficient electrocatalysts for the HER. Moreover, we demonstrate the beneficial role of graphene overgrown on Mo ₂ C in reducing the HER overpotential values, which is attributed to more efficient charge transfer kinetics, compared to pure Mo ₂ C films.

The degree of thermal anisotropy affects critically key device-relevant properties of layered two-dimensional materials. Here, we systematically study the in-plane and cross-plane thermal conductivity of crystalline SnSe₂ films of varying thickness (16–190 nm) and uncover a thickness-independent thermal conductivity anisotropy ratio of about ∼8.4. Experimental data obtained using Raman thermometry and frequency domain thermoreflectance showed that the in-plane and cross-plane thermal conductivities monotonically decrease by a factor of 2.5 with decreasing film thickness compared to the bulk values. Moreover, we find that the temperature-dependence of the in-plane component gradually decreases as the film becomes thinner, and in the range from 300 to 473 K it drops by more than a factor of 2. Using the mean free path reconstruction method, we found that phonons with MFP ranging from ∼1 to 53 and from 1 to 30 nm contribute to 50% of the total in-plane and cross-plane thermal conductivity, respectively.

As ferroelectric Hf_0.5Zr_0.5O₂(HZO) thickness scales below 10 nm, the switching characteristics are severely distorted typically showing an antiferroelectric-like behavior (pinched hysteresis) with reduced remanent polarization. Using Landau-Ginsburg-Devonshire (LGD) theory for the analysis of the experimental results, it is shown here that, in thin (5 nm) HZO, depolarization fields drive the system in a stable paraelectric phase coexisting with a metastable ferroelectric one, which explains the pinched hysteresis. This state of matter resembles a first order ferroelectric above the Curie temperature which is known to result in similar double-loop behavior. Here, based on the analysis of experimental data in the framework of LGD theory, it is reported that charge injection and trapping at pre-existing interface defects during field cycling (“wake-up”) screens the depolarization field stabilizing ferroelectricity. It is found in particular that a sufficiently large energy density of interface states is beneficial for the recovery of fully open ferroelectric loops.

Plasma assisted atomic oxygen deposition was used to grow polycrystalline ferroelectric Hf_1-xZr_xO₂ (x = 0.5–0.7) on technologically important (100) Germanium substrates showing sharp crystalline interfaces free of interfacial amorphous layers and strong evidence for the presence of a predominately orthorhombic phase. The electrical properties, evaluated using metal-ferroelectric-semiconductor (MFS) capacitors, show symmetric and robust ferroelectric hysteresis with weak or no wake-up effects. The MFS capacitors with x = 0.58 show very large remanent polarization up to 34.4 μC/cm² or 30.6 μC/cm² after correction for leakage and parasitics, combined with good endurance reaching 10⁵ cycles at a cycling field of 2.3 MV/cm. The results show good prospects for the fabrication of Ge ferroelectric field effect transistors (FeFETs) for use in 1 T FeFET embedded nonvolatile memory cells with improved endurance.