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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 edgesite states are inferred to make dominated contributions to the defect states in the selenization of platinum films.

In this work, by employing ultrafast optical pump-probe spectroscopy, we investigated 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 the frequency variation and the related parameters (interlayer force constant and sound velocity) of the two modes are investigated.

In this study, we have reported a newly ultrafast optically modulated terahertz (THz) switch based on the transition metal dichalcogenide (TMD) material platinum diselenide (${{\rm PtSe}_2}$) with different thicknesses. The high-quality ${{\rm PtSe}_2}$ thin films with centimeter scale are fabricated on sapphire substrate by the chemical vapor deposition method. The optical pump and THz probe (OPTP) spectroscopy reveals that the THz response of the thin films is as fast as ${\sim} 2.0 \; {\rm ps}$ after photoexcitation of a 780 nm pulse. Interestingly, we found that the THz response time of the ${{\rm PtSe}_2}$ semimetal phase is faster than that of the semiconducting phase. In addition, the THz response time becomes faster when increasing the film thickness for the semimetal phase ${{\rm PtSe}_2}$, while for the semiconducting phase, the response time becomes slower with film thickness. Moreover, degenerate optical pump and optical probe spectroscopy (OPOP) demonstrated that the ultrafast photoinduced negative absorption (photoinduced bleaching) occurs after photoexcitation of 780 nm, and the subsequent recovery consists of two relaxation processes: the fast component with more than 85% of weight has a lifetime of ${\sim}{1.5}\;{\rm ps}$ for semiconducting-phase films and less than 1 ps for the semimetal phase, similar to the response time obtained from OPTP measurement. The slow component with less than 15% of weight has a lifetime of a few hundred picoseconds. The subpicosecond response time observed in both OPTP and OPOP is ascribed to the carrier trapping by defect states, and the slow relaxation process appearing in OPOP arises from the defect state relevant relaxation that is insensitive to the THz photoconductivity due to the frozen carrier mobility in defect states. Our experimental results demonstrate a new application of TMD materials such as ${{\rm PtSe}_2}$ in THz technology, for instance, the design and fabrication of THz modulators and THz switches.

As a newly emergent type-II Dirac semimetal, Platinum Telluride (PtTe2) 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 PtTe2 a good candidate for applications in optoelectronics, valleytronics and far infrared detectors. Although the transport properties of PtTe2 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 photocarrier dynamics of PtTe2 thin films with varying pump fluence, temperature, and film thickness. Upon photoexcitation the THz photoconductivity (PC) of 5 nm PtTe2 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 PtTe2 film was found to be absent in a 2 nm PtTe2 film. We assign the unexpected negative THz PC as the small polaron formation due to the strong electron-Eg-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 PtTe2-based optoelectronic devices., 18 pages, 4 figures

We report the ultrafast photocarrier dynamics and coherent phonon excitation in type-II Dirac semimetal platinum ditelluride ( PtTe 2 ) thin films via femtosecond (fs) pump-probe spectroscopy at room temperature. Quantitative analysis revealed that the incoherent electronic relaxation consists of two components: a subpicosecond fast relaxation process and a slow component with a time constant of hundreds of picoseconds (ps). Furthermore, the launch of a coherent acoustic phonon (CAP) in the 20 nm film but absence in the 6.8 nm film uncovers the dominant role of temperature gradient in producing a strain wave. The sound velocity and Young’s modulus in the thick PtTe 2 are determined to be 1.736 km/s and 29.5 GPa, respectively. In addition, the coherent optical phonon (COP) with a frequency of 4.7 THz corresponding to Te atoms out-of-plane A 1 g vibration has been well resolved in all films, which is ascribed to displacive excitation of coherent phonon (DECP). The observation of a strong probe-wavelength dependent COP amplitude reveals the resonant feature of the optical excitation-induced atomic displacement in PtTe 2 . Our findings provide deep insight into the excitation and dynamics of CAP and COP as well as the photocarriers’ recovery pathway and lifetimes in PtTe 2 . Our study also demonstrates that the COP spectroscopy is a powerful tool to reveal the modulation of frequency-dependent optical constants induced by atomic vibrations, which may find applications in the fields of optoelectronics and ultrafast photonics.

In this study, we have reported a newly ultrafast optically modulated terahertz (THz) switch based on the transition metal dichalcogenide (TMD) material platinum diselenide ( P t S e 2 ) with different thicknesses. The high-quality P t S e 2 thin films with centimeter scale are fabricated on sapphire substrate by the chemical vapor deposition method. The optical pump and THz probe (OPTP) spectroscopy reveals that the THz response of the thin films is as fast as ∼ 2.0 p s after photoexcitation of a 780 nm pulse. Interestingly, we found that the THz response time of the P t S e 2 semimetal phase is faster than that of the semiconducting phase. In addition, the THz response time becomes faster when increasing the film thickness for the semimetal phase P t S e 2 , while for the semiconducting phase, the response time becomes slower with film thickness. Moreover, degenerate optical pump and optical probe spectroscopy (OPOP) demonstrated that the ultrafast photoinduced negative absorption (photoinduced bleaching) occurs after photoexcitation of 780 nm, and the subsequent recovery consists of two relaxation processes: the fast component with more than 85% of weight has a lifetime of ∼ 1.5 p s for semiconducting-phase films and less than 1 ps for the semimetal phase, similar to the response time obtained from OPTP measurement. The slow component with less than 15% of weight has a lifetime of a few hundred picoseconds. The subpicosecond response time observed in both OPTP and OPOP is ascribed to the carrier trapping by defect states, and the slow relaxation process appearing in OPOP arises from the defect state relevant relaxation that is insensitive to the THz photoconductivity due to the frozen carrier mobility in defect states. Our experimental results demonstrate a new application of TMD materials such as P t S e 2 in THz technology, for instance, the design and fabrication of THz modulators and THz switches.

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.