We report, for the first time, the fabrication of single-crystal In2Se3 thin layers using mechanical exfoliation and studies of crystalline-crystalline (α → β) phase transformations as well as the corresponding changes of the electrical properties in these thin layers. Particularly, using electron microscopy and correlative in situ micro-Raman and electrical measurements, we show that, in contrast to bulk single crystals, the β phase can persist in single-crystal thin layers at room temperature (RT). The single-crystal nature of the layers before and after the phase transition allows for unambiguous determination of changes in the electrical resistivity. Specifically, the β phase has an electrical resistivity about 1-2 orders of magnitude lower than the α phase. Furthermore, we find that the temperature of the α → β phase transformation increases by as much as 130 K with the layer thickness decreasing from ~87 nm to ~4 nm. These single-crystal thin layers are ideal for studying the scaling behavior of the phase transformations and associated changes of the electrical properties. For these In2Se3 thin layers, the accessibility of the β phase at RT, with distinct electrical properties than the α phase, provides the basis for multilevel phase-change memories in a single material system.

Simultaneous observations of the middle atmosphere and lower thermosphere by Na Wind/Temperature (W/T) and Rayleigh/Raman lidar and other ground‐based measurements were conducted the night of October 21, during the ALOHA‐93 campaign. Rayleigh/Raman lidar measurements provide density from 25 to 90 km and temperature from 25 to 85 km. From the same location, Na W/T lidar measurements were also obtained between ∼83 and 103 km. The combined data provides continuous temperature from 25 to 103 km. Strong perturbations which may be associated with the diurnal tide are observed in the temperature and wind profiles.

Using micro-Raman spectroscopy and finite-element simulations, we determine the in-plane thermal conductivity of suspended two-dimensional single-crystal In2Se3 grown by chemical vapor deposition. The thermal conductivity shows a strong dependence on the layer thickness: it reaches ∼60 W/m·K at the thickness of 35 nm, and it decreases to ∼4 W/m·K for the 5 nm thick layer. This dependence demonstrates the significance of phonon surface scattering and also indicates changes to the phonon dispersion relations as the layer thickness decreases. The determination of the thickness-dependent thermal conductivity provides an important practical basis for advancing 2D In2Se3-based device technologies and, more generally, also enables fundamental insight into the limiting mechanisms for 2D thermal transport.

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Horizontal and vertical heat fluxes between 85 and 100 km were computed using temperature and wind profiles observed on 21 October 1993 by the Na Wind/Temperature lidar operated at Haleakala during the ALOHA‐93 Campaign. On this night a warm layer developed between 85 and 90 km. The temperature increased by almost 40 K near 88 km, reaching a maximum of approximately 235 K at 1030 UT before decreasing throughout the remainder of the 5 h observation period. This substantial temperature increase was associated with a large amplitude wave, possibly the diurnal tide, and with a sporadic Na layer. The meridional wind field exhibited a strong vertical shear zone within the warm layer and sporadic Na layer which briefly exceeded the shear instability limit of ≈ 42 m/s/km at the time the temperature reached maximum. The vertical heat flux was directed upward within the warm layer reaching maximum at 88 km and downward above resulting in strong flux convergence. The average heat fluxes between 85 and 100 km were 2.5±2.4 K m/s (vertical), 77±25 K m/s (zonal), and −19±20 K m/s (meridional). The maximum heating rate associated with the convergence of vertical heat flux was 11±4.1 K/h at 91 km. The average heating rate between 85 and 100 km was 1.9±0.8 K/h.