Ambient aging has substantially hindered the development of electronic and optoelectronic devices made of two-dimensional (2D) semiconducting layered materials because the origin of oxidation, degradation, and aging effects remains largely unexplored. This study unveils the mechanism and process of ambient aging in 2D-layered GaSe crystals by exploring the evolution of the ambient aging process in a snapshot fashion. Through the detailed examinations on surface morphology, lattice structure, and elemental compositions, three major stages of the aging process in GaSe crystals are identified: the formation of a defective GaSe top layer, a crystalline Ga 2 Se 3 layer, and an amorphous Ga 2 O 3 layer evolving on top of the Ga 2 Se 3 layer. In particular, it is suggestive that the formation of the crystalline Ga 2 Se 3 layer plays a crucial role in the entire oxidation process. The present results may be also applicable to other 2D semiconducting layered materials.
Tin sulfide (SnS) is one of the promising materials for the applications of optoelectronics and photovoltaics. This study determines the nematic dynamics of photoexcited electrons and phonons in SnS single crystals using polarization-dependent pump–probe spectroscopy at various temperatures. As well as the fast (0.21–1.38 ps) and slow (>5 ps) relaxation processes, a 36–41 GHz coherent acoustic phonon with a sound velocity of 4883 m/s that is generated by the thermoelastic effect is also observed in the transient reflectivity change (Δ R/ R) spectra. Electrons and coherent acoustic phonons show significant in-plane anisotropy from 330 to 430 K due to strong electron–phonon coupling. However, this in-plane anisotropy weakens dramatically in the low-temperature (<330 K) and high-temperature (>430 K) phases. These results add to the knowledge about the anisotropy of electrons and coherent acoustic phonons that give SnS applications in photovoltaic or optoelectronic devices.
Charge density waves (CDWs) involved with both electronic and phononic subsystems simultaneously are a common quantum state in solid-state physics, especially in low-dimensional materials. However, there is no complete CDW phase diagram in various dimensions and the phase transition mechanism is currently moot. Using the distinct temperature evolution of orientation-dependent ultrafast electron and phonon dynamics, variously dimensional CDW phases are verified in CuTe and the electronic subsystem in CuTe is also demonstrated to drive the formation of one-dimensional (1D) CDW chain phase at Tc of 335 K. At T=280 K, electron-phonon coupling creates collective modes along the a-axis, which synchronize via an interchain interaction to establish a two-dimensional (2D) CDW phase on the ab-plane while T<250 K. The 2D CDW phase planes are finally locked with each other in anti-phase to form a three-dimensional (3D) CDW phase at temperatures of less than 220 K. This study shows that the hidden CDW phases with various dimensions and their transition mechanisms are critical for CDW materials.
A complete temperature-dependent scheme of the Mn3+ on-site d-d transitions in multiferroic hexagonal HoMnO3 (h-HoMnO3) thin films was unveiled by energy-resolved ultrafast spectroscopy. The results unambiguously revealed that the ultrafast responses of the e1g and e2g states differed significantly in the hexagonal HoMnO3. We demonstrated that the short-range antiferromagnetic and ferroelectric orderings are more relevant to the e2g state, whereas the long-range antiferromagnetic ordering is intimately coupled to both the e2g and e1g states. Moreover, the primary thermalization times of the e2g and e1g states were 0.34 ± 0.08 ps and 0.38 ± 0.08 ps, respectively.
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