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The atomically thin layered transition metal dichalcogenide (TMDCs) PtSe2 is a new emerging two-dimension (2D) material, which has a transition from indirect-gap semiconductor to semimetal with increasing thickness. Defects in 2D TMDCs are very ubiquitous and play a crucial role in understanding many electronics and optoelectronics in TMDCs. In this article, PtSe2 films with different thickness are obtained by selenizing the variously thick Pt films. Extrapolation of the onset absorption from the visible-infrared spectrum demonstrates that the selenization of 1 and 3 nm Pt films shows semiconductor-like character, while those of thick Pt films (with thickness of 10 and 15 nm) show metallic behavior. The transient absorption (TA) spectroscopy reveals that all films show immediately photobleaching signals after photoexcitation at 800 nm, and the subsequent relaxation process shows strongly thickness dependence. The thinnest film shows biexponential relaxation with typical time constants of 1.28 and 101.2 ps. In contrast, the photobleaching signals are developed into photoinduced absorption signals in thicker films, and the absorption magnitude increases with the thickness of the films. The optical pump and terahertz (THz) probe spectroscopy reveals that all samples show positive photoconductivity after photoexcitation of 800 nm, the subsequent recovery is completed within 2.0 ps, and the recovery time constant decreases with the increase of the films’ thickness. Our TA and THz spectroscopy results reveal that the defect states of the films play dominated role in the relaxation of photocarrier after photoexcitation, and edge-site states are inferred to make dominated contributions to the defect states in the selenization of platinum films.

As a newly emergent type-II Dirac semimetal, Platinum Telluride (PtTe 2 ) stands out from other 2D noble-transition-metal dichalcogenides for the unique structure and novel physical properties, such as high carrier mobility, strong electron-phonon coupling and tunable bandgap, which make the PtTe 2 a good candidate for applications in optoelectronics, valleytronics and far infrared detectors. Although the transport properties of PtTe 2 have been studied extensively, the dynamics of the nonequilibrium carriers remain nearly uninvestigated. Herein we employ optical pump-terahertz (THz) probe spectroscopy (OPTP) to systematically study the 2 photocarrier dynamics of PtTe 2 thin films with varying pump fluence, temperature, and film thickness. Upon photoexcitation the THz photoconductivity (PC) of 5 nm PtTe 2 film shows abrupt increase initially, while the THz PC changes into negative value in a subpicosecond time scale, followed by a prolonged recovery process that lasted hundreds of picoseconds (ps). This unusual THz PC response observed in the 5 nm PtTe 2 film was found to be absent in a 2 nm PtTe 2 film. We assign the unexpected negative THz PC as the small polaron formation due to the strong electron-E g -mode phonon coupling, which is further substantiated by pump fluence-and temperature-dependent measurements as well as the Raman spectroscopy. Moreover, our investigations give a subpicosecond time scale of sequential carrier cooling and polaron formation. The present study provides deep insights into the underlying dynamics evolution mechanisms of photocarrier in type-II Dirac semimetal upon photoexcitation, which is fundamental importance for designing PtTe 2 -based optoelectronic devices.

The layered transition metal dichalcogenide has attracted tremendous attention for its unique structure and electrical and optoelectronic properties. As an emerging two-dimensional material, PdSe2 plays a key role in optoelectronic applications due to its distinct optical and tunable electrical properties. The carrier dynamic and low-frequency phonon modes and how they evolve with the number of layers are important for future device fabrication in photonics, optoelectronics, and nanomechanics. Here, by employing ultrafast optical pump–probe spectroscopy, we have investigated systematically the photocarrier dynamics as well as the thickness dependent interlayer coherent phonon modes in PdSe2 films. Two low-frequency phonon modes in PdSe2 films are identified after photoexcitation at 780 nm. The higher-frequency mode is ascribed to the interlayer breathing mode, and the lower one is assigned to the standing wave mode, and both of the mode frequencies decrease with increasing the number of layers of films. Analysis based on simple one-dimensional chain model produces interlayer force constant K = 5.74 × 1019 N/m3 for the interlayer breathing mode, and sound velocity of v = 8.27 × 104 cm/s for the standing wave mode in PdSe2 film. Our experimental finding paves the way for designing and developing PdSe2-based optoelectronic and nanomechanic devices.

The van der Waals magnet CrSiTe 3 has captured immense interest because it is capable of retaining the long-range ferromagnetic order even in its monolayer form, thus offering potential use in spintronic devices. Bulk CrSiTe 3 crystal has inversion symmetry that is broken on the crystal surface with thickness of a few quintuple layers. Here, ultrafast terahertz (THz) emission spectroscopy and time resolved THz spectroscopy are employed to investigate the THz emission and the dynamics of photocarrier respectively on the surface of the CrSiTe 3 crystal, a strong THz emission from the surface of CrSiTe 3 crystal is observed under femtosecond pulse excitation at 800 nm. Theoretical analysis based on space symmetry of CrSiTe 3 suggests the dominant role of shift current occurring on the surface with thickness of a few quintuple layers in producing the THz emission, in consistence with the experimental observation that the emitted THz amplitude strongly dependents on the azimuthal and pumping polarization angles. The present study offers a new efficient THz emitter as well as a better understanding of the nonlinear optical response of CrSiTe 3 . It hopefully will open a window toward the investigation on the nonlinear optical response in the mono-/few-layer van der Waals crystals with low-dimensional magnetism.