Temporally probing the thermal phonon and charge transfer induced out-of-plane acoustical displacement of monolayer and bi-layer MoS2/GaN heterojunction.

Acoustical behavior of semiconducting transition metal dichalcogenides determines the heat transfer pathway, and thus plays a crucial role in the electronics and optoelectronics design. In this research, van der Waals heterojunctions (vdWHs) consisting of transferred monolayer and bi-layer MoS2 on GaN substrate were studied. We observed an asymmetric bipolar acoustic strain wave with ∼5 ps duration, which describes the surface of substrate undergoing strong compressive deformation after weak tensile deformation in the out-of-plane direction. We developed a theory to explain the mechanisms responsible for the observed strain waveform in the vdWHs elastic system, and obtained the critical parameters of the carrier dynamics by temporal fitting. Our results not only report a coherent acoustic phonon generated in the vdWHs, which will complement our understanding of the thermal transfer at the 2D/substrate interface, but also provide information about the intrinsic properties in the vdWHs, which would benefit the design of the 2D-based devices in the future.

Revealing the interlayer van der Waals coupling of bi-layer and tri-layer MoS2 using terahertz coherent phonon spectroscopy.

In this research, we applied THz coherent phonon spectroscopy to optically probe the vibrational modes of the epitaxially-grown bi-layer and tri-layer MoS2 on sapphire substrate. The layers' THz vibration is displacively stimulated and temporally retrieved by near-UV femtosecond laser pulses, revealing Raman-active and Raman-inactive modes in one measurement. With the complete breathing modes revealed, here we extend the linear chain model by considering the elastic contact with the substrate and vdWs coupling of the next nearest MoS2 layer to analyze the effective spring constants. We further considered the intralayer stiffness as a correction term to acquire the actual interlayer vdWs coupling. Our THz phonon spectroscopy results indicate the interlayer spring constants of 9.03 × 1019 N/m3 and 9.86 × 1019 N/m3 for bi-layer and tri-layer respectively. The extended model further suggests that a non-negligible substrate mechanical coupling and next nearest neighbor vdWs coupling of 1.48 × 1019 N/m3 and 1.04 × 1019 N/m3 have to be considered.

Microwave resonant absorption of SARS-CoV-2 viruses.

Low power microwave can effectively deactivate influenza type A virus through the nonthermal structure-resonant energy transfer effect, at a frequency matching the confined-acoustic dipolar mode frequency of the virus. Currently, aerosol is considered the major route for SARS-CoV-2 transmission. For the potential microwave-based sterilization, the microwave-resonant frequency of SARS-CoV-2 must be unraveled. Here we report a microwave absorption spectroscopy study of the SARS-CoV-2 and HCoV-229E viruses through devising a coplanar-waveguide-based sensor. Noticeable microwave absorption can be observed, while we identified the resonant frequencies of the 1st and 2nd dipolar modes of SARS-CoV-2 virus as 4 and 7.5 GHz respectively. We further found that the resonant frequencies are invariant to the virus titer, and we also studied the microwave absorption of HCoV-229E in weak acidity medium to simulate the common pH value in fluid secretion. Our results suggest the possible radiation frequency for the recently proposed microwave sterilization devices to inactivate SARS-CoV-2 virus through a nonthermal mechanism so as to control the disease transmission in the post-pandemic era.