Fluorescent photoswitches are highly attractive, because they hold great promises for photonic devices and imaging. However, a limited number of reversible switches with a response to light have been achieved in the solid state. Here, we report reversible dual fluorescent photoswitching characteristics in the solid state of spiropyran (SP)−functionalized tetraphenylethene (TPE) derivatives. These photoswitches exhibit two distinct and selectively addressable states, a cyan fluorescence and a red fluorescence, which can be conveyed into each other in a reversible feature upon irradiation with alternating UV and visible light. Detailed spectroscopic and theoretical studies suggest that the nonplanar molecular conformation of TPE moieties leads to large free volumes, which facilitates the reversible photoisomerization of SP. The excellent reversibility and high-contrast fluorescence of solid-state photoswitches enable great applications in multimodality anticounterfeiting and optical writing and erasing fluorescent devices.

Solid-state fluorescent switches with reversible luminescence characteristics have attracted considerable attention because of their broad applications in advanced photonics, such as anticounterfeiting inks, optical writing and erasing, and biological imaging. Herein, we have fabricated a solid-state reversible fluorescent switch under alternating UV (365 nm) and visible light treatments based on a fulgide (FUL)-functionalized tetraphenylethylene (TPE) derivative (TPE-FUL) containing a photochromic group FUL and aggregation-induced emission (AIE) luminogen TPE. TPE-FUL exhibited excellent reversible absorption and luminescence owing to the interconversion between open TPE-FUL (O-TPE-FUL) and closed TPE-FUL (C-TPE-FUL). Photophysical and theoretical investigations revealed that the luminescence of O-TPE-FUL is based on the local excited state of the TPE moiety, whereas the fluorescence quenching of C-TPE-FUL originates from the intramolecular charge transfer from the TPE to the FUL moiety. The excellent reversible photoswitching properties of TPE-FUL in the solid state allows for its potential use in advanced optical memory applications, such as anticounterfeiting, optical writing and erasing, and information encryption.

In photoswitches that undergo fluorescence switching upon ultraviolet irradiation, photoluminescence and photoisomerization often occur simultaneously, leading to unstable fluorescence properties. Here, we successfully demonstrated reversible solid-state triple fluorescence switching through "Pump-Trigger" multiphoton manipulation. A novel fluorescence photoswitch, BOSA-SP, achieved green, yellow, and red fluorescence under excitation by pump light and isomerization induced by trigger light. The energy ranges of photoexcitation and photoisomerization did not overlap, enabling appropriate selection of the multiphoton light for "pump" and "trigger" photoswitching, respectively. Additionally, the large free volume of the spiropyran (SP) moiety in the solid state promoted reversible photoisomerization. Switching between "pump" and "trigger" light is useful for three-color tunable switching cell imaging, which can be exploited in programmable fluorescence switching. Furthermore, we exploited reversible dual-fluorescence switching in a single molecular system to successfully achieve two-color superresolution imaging.

In this paper, we examine the application of an ideal phonon-hydrodynamic material as the heat transfer medium between two diffuse-gray boundaries with a finite temperature difference. We use the integral-equation approach to solve a modified phonon Boltzmann transport equation with the displaced Bose-Einstein distribution as the equilibrium distribution between two boundaries perpendicular to the heat transfer direction. When the distance between the boundaries is smaller than the phonon normal scattering mean free path, our solution converges to the ballistic limit as expected. In the other limit, we find that, although the local thermal conductivity in the bulk of the hydrodynamic material approaches infinity, the thermal boundary resistance at the interfaces becomes dominant. Our study provides insights to both the steady-state thermal characterization of phonon-hydrodynamic materials and the practical application of phonon-hydrodynamic materials for thermal management.

Phonon scattering by electrons, or "phonon-electron scattering", has been recognized as a significant scattering channel for phonons in materials with high electron concentration, such as thermoelectrics and nanoelectronics, even at room temperature.Despite the abundant previous studies of phonon-electron scattering in different types of three-dimensional (3D) bulk materials, its impact on the phonon transport, and thus the heat transfer properties, of two-dimensional (2D) materials has not been understood. In this work, we apply ab initio methods to calculate the phonon-electron scattering rates in two representative 2D materials, silicene and phosphorene, and examine the potential of controlling the thermal conductivity of these materials via externally induced phonon-electron scattering by electrostatic gating. We also develop an analytical model to explain the impact of reduced dimensionality and distinct electron and phonon dispersions in 2D on phonon-electron scattering processes. We find that over 40% reduction of the lattice thermal conductivity can be achieved in silicene with an induced charge carrier concentration in the range of 10 13 cm −2 , which is experimentally achievable. Our study not only generates new fundamental insights into 1 arXiv:1904.11011v1 [cond-mat.mtrl-sci] 24 Apr 2019 phonon transport in 2D materials but also provides practical guidelines to search for 2D materials with strong phonon-electron scattering for potential thermal switching applications.Keywords phonon-electron scattering, 2D materials, phonon transport, lattice thermal conductivity, thermal switch Electron-phonon interactions play a major role in determining the electronic properties of materials since they are the major contributors to electrical resistance and also mediate conventional superconductivity. 1,2 Due to these reasons, the influence of electron-phonon interaction on the transport of electrons has been intensively studied and well understood.On the other hand, the scattering of phonons due to electron-phonon interactions (hereafter "phonon-electron scattering") and its impact on thermal transport of solids have received limited research interest, due to the long belief that it is only important at cryogenic temperatures. [3][4][5] The main reason is that most of the previous studies and practical interests were limited to devices with a low or moderate electron concentration, typically below 10 19 cm −3 .Recent technological developments have led to important applications with electron concentration as high as 10 20 to 10 21 cm −3 , such as in heavily-doped thermoelectric materials 6 and nanoelectronic devices. 7 In this regime, however, the impact of phonon-electron scattering on thermal transport in largely unknown. Recently, Liao et al. used ab initio calculations 8 to show that the lattice thermal conductivity of silicon with a high electron concentration can be suppressed by as much as 50% even at room temperature due to phonon-electron scattering. 9 Significant suppression of phonon propagation by p...