Phonons play a key role in the physical properties of materials, and have long been a topic of study in physics. Nanoscale experimental methods to excite and detect phonons are imperative for their application.

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.

Programmable photovoltaic photodetectors based on 2D materials can modulate optical and electronic signals in parallel, making them particularly well‐suited for optoelectronic hybrid dual‐channel communication. This work presents a programmable non‐volatile bipolar semi‐floating gate photovoltaic photodetector (SFG‐PD) constructed using tungsten diselenide (WSe ₂ ), hexagonal boron nitride (h‐BN), and graphene (Gra). By controlling the voltage pulses applied to the control gate, the device generates opposing built‐in electric field junctions (p ⁺ ‐p and n‐p junctions), enabling reversible switching between positive and negative light responses with a rapid response time of up to 2.02 µs. Moreover, the application of this device is demonstrated in dual‐channel optoelectronic hybrid communication, offering a practical solution for achieving high‐speed, large‐capacity, low‐loss, and secure multi‐channel communication.

Artificially engineered 2D materials offer unique physical properties for thermal management, surpassing naturally occurring materials. Here, using van der Waals epitaxy, we demonstrate the ability to engineer extremely insulating thermal metamaterials based on atomically thin lattice-mismatched Bi₂Se₃/MoSe₂ superlattices and graphene/PdSe₂ heterostructures with exceptional thermal resistances (70-202 m² K/GW) and ultralow cross-plane thermal conductivities (0.012-0.07 W/mK) at room temperature, comparable to those of amorphous materials. Experimental data obtained using frequency-domain thermoreflectance and low-frequency Raman spectroscopy, supported by tight-binding phonon calculations, reveal the impact of lattice mismatch, phonon-interface scattering, size effects, temperature, and interface thermal resistance on cross-plane heat dissipation, uncovering different thermal transport regimes and the dominant role of long-wavelength phonons. Our findings provide essential insights into emerging synthesis and thermal characterization methods and valuable guidance for the development of large-area heteroepitaxial van der Waals films of dissimilar materials with tailored thermal transport characteristics.

We present a comparative investigation of the influence of crystallinity and film thickness on the acoustic and thermal properties of layered PtSe₂ films of varying thickness (1–40 layers) using frequency-domain thermo-reflectance, low-frequency Raman, and pump-probe coherent phonon spectroscopy. We find ballistic cross-plane heat transport up to ~30 layers PtSe₂ and a 35% reduction in the cross-plane thermal conductivity of polycrystalline films with thickness larger than 20 layers compared to the crystalline films of the same thickness. First-principles calculations further reveal a high degree of thermal conductivity anisotropy and a remarkable large contribution of the optical phonons to the thermal conductivity in bulk (~20%) and thin PtSe₂ films (~30%). Moreover, we show strong interlayer interactions in PtSe₂, short acoustic phonon lifetimes in the range of picoseconds, an out-of-plane elastic constant of 31.8 GPa, and a layer-dependent group velocity ranging from 1340 ms⁻¹ in bilayer to 1873 ms⁻¹ in eight layers of PtSe₂. The potential of tuning the lattice thermal conductivity of layered materials with the level of crystallinity and the real-time observation of coherent phonon dynamics open a new playground for research in 2D thermoelectric devices and provides guidelines for thermal management in 2D electronics.