Recent progress of zeolitic imidazolate frameworks (ZIFs) in superhydrophobic and anticorrosive coatings for metals and their alloys

Metal–organic frameworks (MOFs) have gained interest in recent years for anticorrosion applications owing to their inimitable structures and excellent properties. As a prominent subclass of MOFs, zeolite imidazolate frameworks (ZIFs) exhibited hydrophobicity, corrosion inhibition, and positive water stability, making them frequently appear in the field of metallic anticorrosion. This review presents the establishment and development of a theoretical model for superhydrophobicity, followed by the reported applications of different kinds of ZIFs in the field of metallic anticorrosion with particular emphasis on ZIF-8, which is the most extensively researched structure among ZIFs. In addition, the applications of superhydrophobic coatings in many aspects other than anticorrosion are also summarized. Finally, the existing challenging problems and future development trends of this kind of coating are discussed. It is hoped that this review will contribute to further development in the field of superhydrophobic anticorrosion coatings for metallic materials

Design of ZIF-8-based PVB composite coating on AZ31B Mg alloy surface with superhydrophobic, wear, corrosion protection and durability

Magnesium (Mg) alloys are susceptible to being attacked by corrosive ions in practical application environments, which limits their widespread application as the most promising engineering material. Herein, the superhydrophobic composite coating PVB/STA/ZIF-8@SiO2 (PSZS) has been designed on the surface of Mg alloy by using a drop coating method to improve its corrosion resistance. In this study, we provide an approachable decoration strategy for hydrophobic and hierarchical modification of zeolite imidazolate frameworks (ZIF-8) with stearic acid (STA) and SiO2, and combined with polyvinyl butyral (PVB) to form a composite coating with excellent performance on the substrate surface. Notably, the PSZS composite coating exhibited superhydrophobic and the contact angle is greater than 150° after 300 cm of mechanical friction. The electrochemical testing of the PSZS composite coating indicated that it greatly protected the Mg alloys from corrosion. Moreover, the PSZS coating displayed highly effective corrosion resistance even after being immersed in 3.5 wt% NaCl aqueous solution for 7 days. Overall, the PSZS composite coating with the advantages of corrosion resistance, corrosion durability, superhydrophobic, abrasion resistance and simple preparation process broadens the horizons of Mg alloy corrosion resistance research

Transient THz Study of Phonon-Assisted Carrier Scattering between Bulk and Surface States in Bi₂Te₃

Ultrafast photocarrier dynamics of the topological insulator Bi2Te3 nanocrystal film was investigated using time-resolved terahertz (THz) spectroscopy. Unlike reducing the film thickness to increase the proportion of surface states, the influence of surface states on the bulk photocarrier dynamics remains important in Bi2Te3 nanocrystal films of a few hundred nanometers. After photoexcitation at 780 nm, the transient THz transmission of the Bi2Te3 nanocrystal film shows a slow dropping process with a typical time of about 3 ps, and the maximum modulation depth of THz transmission shows a nonlinear pump fluence dependence. Coupling rate equations involving both surface and bulk states were used to simulate the photocarrier dynamics of the Bi2Te3 film, which can reproduce the experimental data nicely. We conclude that the delayed dropping process for THz transmission is caused by phonon-assisted scattering of bulk carriers to the surface states. The present study provides new insight into the photocarrier dynamics in topological insulators and directions for technological applications at the nanoscale

Disorder-assisted Robustness of Ultrafast Cooling in High Doped CVD-Graphene

Dirac Fermion, which is the low energy collective excitation near the Dirac cone in monolayer graphene, have gained great attention by low energy Terahertz probe. In the case of undoped graphene, it has been generally understood that the ultrafast terahertz thermal relaxation is mostly driven by the electron-phonon coupling (EOP), which can be prolonged to tens and hundreds of picoseconds. However, for the high doped graphene, which manifests the negative photoinduced terahertz conductivity, there is still no consensus on the dominant aspects of the cooling process on a time scale of a few picoseconds. Here, the competition between the disorders assisted defect scattering and the electron-phonon coupling process in the cooling process of the graphene terahertz dynamics is systematically studied and disentangled. We verify experimentally that the ultrafast disorder assisted lattice-phonon interaction, rather than the electron-phonon coupling process, would play the key role in the ultrafast thermal relaxation of the terahertz dynamics. Furthermore, the cooling process features robustness which is independent on the pump wavelength and external temperature. Our finding is expected to propose a considerable possible cooling channel in CVD-graphene and to increase the hot electron extracting efficiency for the design of graphene-based photoconversion devices

Role of the Optical–Acoustic Phonon Interaction in the Ultrafast Cooling Process of CVD Graphene

The Dirac fermion, a high-mobility electron in the Dirac cone of monolayer graphene, has significant potential for use in the terahertz probing technique. For undoped graphene, ultrafast terahertz conductivity relaxation is mostly driven by electron–acoustic phonon supercollision coupling. The decay time of this coupling can be increased to tens or hundreds of picoseconds by decreasing the temperature. However, for chemical vapor deposition (CVD)-grown graphene, which exhibits negative photoinduced terahertz conductivity, there is currently no consensus on the dominant aspects of the terahertz conductivity relaxation process on time scales of less than 10 ps. In this study, the competition between electron–acoustic and optical–acoustic phonon coupling processes during the cooling of CVD graphene was systematically investigated. We experimentally verified that the ultrafast disorder-assisted optical–acoustic phonon interaction plays a key role in ultrafast terahertz conductivity relaxation. Furthermore, the ultrafast cooling process was found to be robust under different pump wavelengths and external temperatures, and it could be modulated by substrate doping. These findings are expected to contribute to a possible cooling channel in CVD graphene and to increase hot electron extraction efficiency for the design of graphene-based photoconversion devices

Dynamical Response of Nonlinear Optical Anisotropy in a Tin Sulfide Crystal under Ultrafast Photoexcitation

Analogous to black phosphorus, SnS processes folded structure that shows a strongly anisotropic optical absorption. Herein, by using ultrafast two-color pump and probe spectroscopy, the azimuthal angle dependence of nonlinear optical anisotropy in SnS is investigated. After 390 nm photoexcitation, the reflectivity of the 780 nm probe beam is first reduced significantly, followed by a complex alternation with the rotation of the sample along the c-axis. The relaxation of reflectivity consisted of two components: a 1–3 ps fast process that shows azimuthal angle and pump fluence dependence, which arises from electron–phonon coupling. The slow process shows strongly azimuthal angle dependence, which arises from the recovery of a photoinduced structural change, i.e., from the photoinduced metastable state with Cmcm-like symmetry to the initial state with Pnma symmetry. In addition, a coherent acoustic phonon with a frequency of 40 GHz is also identified, which originates from the temperature gradient-induced strain wave in the SnS crystal

Dynamical Response of Nonlinear Optical Anisotropy in a Tin Sulfide Crystal under Ultrafast Photoexcitation

Analogous to black phosphorus, SnS processes folded structure that shows a strongly anisotropic optical absorption. Herein, by using ultrafast two-color pump and probe spectroscopy, the azimuthal angle dependence of nonlinear optical anisotropy in SnS is investigated. After 390 nm photoexcitation, the reflectivity of the 780 nm probe beam is first reduced significantly, followed by a complex alternation with the rotation of the sample along the c-axis. The relaxation of reflectivity consisted of two components: a 1–3 ps fast process that shows azimuthal angle and pump fluence dependence, which arises from electron–phonon coupling. The slow process shows strongly azimuthal angle dependence, which arises from the recovery of a photoinduced structural change, i.e., from the photoinduced metastable state with Cmcm-like symmetry to the initial state with Pnma symmetry. In addition, a coherent acoustic phonon with a frequency of 40 GHz is also identified, which originates from the temperature gradient-induced strain wave in the SnS crystal

Magnetically Tuned Continuous Transition from Weak to Strong Coupling in Terahertz Magnon Polaritons

Depending on the relative rates of coupling and dissipation, a light-matter coupled system is either in the weak- or strong-coupling regime. Here, we present a unique system where the coupling rate continuously increases with an externally applied magnetic field while the dissipation rate remains constant, allowing us to monitor a weak-to-strong coupling transition as a function of magnetic field. We observed a Rabi splitting of a terahertz magnon mode in yttrium orthoferrite above a threshold magnetic field of ~14 T. Based on a microscopic theoretical model, we show that with increasing magnetic field the magnons transition into magnon polaritons through an exceptional point, which will open up new opportunities for in situ control of non-Hermitian systems

Ultrafast Dynamics of Defect-Assisted Auger Process in PdSe₂ Films: Synergistic Interaction between Defect Trapping and Auger Effect

By using optical pump and terahertz probe spectroscopy, we have investigated the photocarrier dynamics in PdSe2 films with different thicknesses. The experimental results reveal that the photocarrier relaxation consists of two components: a fast component of 2.5 ps that shows the layer-thickness independence and a slow component that has typical lifetime of 7.3 ps decreasing with the layer thickness. Interestingly, the relaxation times for both fast and slow components exhibited both pump fluence and temperature independence, which suggests that synergistic interactions between defect trapping and Auger effect dominate the photocarrier dynamics in PdSe2 films. A model involving a defect-assisted Auger process is proposed, which can reproduce the experimental results well. The fitting results reveal that the layer-dependent lifetime is determined by the defect density rather than carrier occupancy rate after photoexcitation. Our results underscore the interplay between the Auger process and defects in two-dimensional semiconductors

Temperature-Dependent Terahertz Emission from Co/Mn2Au Spintronic Bilayers

Recently, ferromagnetic/nonmagnetic heavy metal heterostructures have been intensively investigated as terahertz (THz) emitters. The interconversion of spin-to-charge dynamics plays a central role for efficient emission of THz electromagnetic pulses. However, a direct observation of spin–charge interconversion in antiferromagnetic (AFM) materials occurring on the sub-picosecond time scale remains a challenge. Herein, the magnetic-field-, pump-fluence-, and polarization-dependent THz emission behaviors by a femtosecond optical pump in cobalt (Co)/Mn2Au nanometer heterostructure are experimentally investigated. The Co/Mn2Au bilayer generates sizable THz signals, whereas the Mn2Au/Pt bilayer does not show any THz emission. In addition, the thickness- and temperature-dependent THz emission measurements indicate a direct relation between the THz amplitude and the conductivity of AFM Mn2Au layer. The results obtained will not only promote the fundamental understanding of ultrafast spin–charge interconversion in Co/Mn2Au heterostructures, but also provide a possibility of spectroscopic-based spin current detector at THz-frequency range